Impact of diesel engine oil additives-soot interactions on

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Impact of diesel engine oil additives-soot interactions on physiochemical, oxidation and wear characteristics of soot Kimaya Vyavhare, Sujay Bagi, Mihir Patel, and Pranesh B Aswath Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03841 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Energy & Fuels

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Impact of diesel engine oil additives-soot interactions on

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physiochemical, oxidation and wear characteristics of soot

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Kimaya Vyavhare†, Sujay Bagi†1, Mihir Patel†2 and Pranesh B. Aswath†*

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† Department of Materials Science and Engineering, University of Texas at Arlington, Arlington,

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TX 76019, USA

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†1 Department of Mechanical Engineering, Massachusetts Institute of Technology, 77

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Massachusetts Avenue, Cambridge, Massachusetts, 02139, United States.

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†2 Product Development, Chevron Lubricants, San Ramon, California, 94583, United States.

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*Corresponding author: [email protected]

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1 ACS Paragon Plus Environment

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ABSTRACT

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Carbonaceous soot accumulated in crankcase oil is known to have adverse effect on diesel engine

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performance, durability and fuel efficiency. The current study is focused on determining the

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influence of engine oil additive package and soot interactions on crankcase soot chemistry,

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structure and oxidation. Soot was extracted from the crankcase oil of Mack T-12 dynamometer

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diesel engine tests and characterized using X-ray absorption near edge structure (XANES)

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spectroscopy, high-resolution transmission electron microscopy (HR-TEM), high-temperature X-

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ray diffraction (HT-XRD) and energy dispersive spectroscopy (EDS). Additionally, four-ball wear

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bench tests were conducted to study the effect of several interactions like antiwear additive-soot,

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dispersant-soot on lubricity of formulated engine base oils. XANES and HR-TEM analysis suggest

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presence of chemical compounds originating from engine oil chemistry as either

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adsorbed/embedded into the turbostratic nano-structure of soot. HT-XRD method was employed

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to map variations in interplanar spacing of basal plane (002) of turbostratic soot with temperature,

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indicative of degree of disorder and ease of oxidation. Oxidative reactivity of crankcase soot is

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found to be strongly dependent on the changes in physical and chemical makeup of carbonaceous

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soot structure. Wear assessment proved that increase of soot concentration in formulated oils,

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significantly increases wear of sliding surfaces. SEM-EDS worn surface analysis suggest that soot

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induced wear occurs through abrasive wear mechanism, where soot antagonistically interact with

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protective antiwear additive formed tribofilms and exacerbate wear of engine components.

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Keywords: Diesel Soot; Soot structure; Soot Chemistry; Soot Induced Wear; XANES; Ionic

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liquids; Ashless Antiwear Additives.

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1. Introduction

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Soot is a nanoscopic carbonaceous material produced due to incomplete combustion of

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hydrocarbon fuel in an internal combustion engine. Excessive soot formation occurs in diesel

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engines as compared to the gasoline engines because of the difference in the way fuel/air mixture

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is injected and ignited in them. Of the soot produced during in-cylinder fuel combustion, only 29%

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reaches the atmosphere through the exhaust tail-pipe 2, with the remainder being deposited on the

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cylinder walls and piston crown, which eventually ends up in the crankcase oil via blow-by

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mechanisms 3-5. Rapid soot production with extended drain interval in a heavy-duty diesel engine

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can lead to high soot levels of 8 or 10 vol% accumulating in crankcase engine oil 6. It has been

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shown that the crankcase soot decreases engine efficiency by deteriorating physical and chemical

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lubrication oil properties of the engine oil and inducing severe friction and wear of engine

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components 7-11,12,13. However, a thorough understanding is required to apprehend the interaction

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of crankcase soot and, lubrication oil additive chemistries and its potential effects on engine wear.

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Soot is found to be in the form of necklace-like agglomerates, which are around 100 nm in size.

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These agglomerates are composed of collections of smaller, spherical basic particle units called as

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“primary soot particles”, with diameter around 10-80 nm and the cluster or chain-like soot

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aggregates defined as “secondary particles”, which are composed of several tens to hundreds of

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primary soot particles. The primary soot particle has a turbostratic structure wherein crystalline

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graphite-like layers (known as platelets) are arranged in a concentric manner surrounding an inner

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core. The factors like fuel composition, engine operating conditions, vehicle mileage can change

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or modify the physical structure and bulk/surface chemistry of soot and are expected to play an

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important role in controlling adverse effect of soot. Many studies have been directed to explore

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the impact of these factors, Sharma et al. 12 used Raman spectroscopy, high resolution transmission


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electron microscopy (HR-TEM), X-ray absorption near edge spectroscopy (XANES), X-ray

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diffraction (XRD) techniques to study soot extracted from crankcase and exhaust and reported that

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crankcase soot exhibited turbostratic structure with the presence of some nanocrystalline species

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originating from lubricant chemistry, while exhaust soot showed perfectly ordered graphitic

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crystalline structure. Uy et al. 14 compared the morphology and chemistry of soot produced through

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different combustion modes (gasoline soot and diesel soot) using X-ray photoelectron

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spectroscopy (XPS), Auger electron spectroscopy (AES), Raman spectroscopy, HR-TEM, and X-

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ray fluorescence (XRF). They reported that gasoline soot contains more lubricant additive

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elements and wear debris than the diesel soot. Ishiguro et al. 15 studied the structure of soot using

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HR-TEM and found that primary particle of soot comprised of an inner core of 10nm which is

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more amorphous and disordered and an outer shell which is more graphitic. Outer shell composed

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of crystallites with polycyclic aromatic hydrocarbon layers oriented concentrically with the inner

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core. The effect of fuel composition on the soot structure and physiochemical properties was

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studied by Lu et al.

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low-sulfur diesel, and ultra-low sulfur diesel fuel and reported that biodiesel soot has more

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disordered nanostructure.

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The oxidative reactivity of soot i.e. the ease of soot burning by downstream after-treatment

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components (vis-à-vis DPF regeneration) closely depends on the structure and morphology of soot

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12,21.

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the catalytic oxygen groups present on primary soot particle generated from oxygenated fuel.

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Yehliu et al.

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carbonaceous nanostructure of soot. Extensive work of Vander Wal et al.

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the effect of soot synthesis conditions (temperature, time and fuel content) on soot particle

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They characterized soot extracted from engines operated with biodiesel,

Song et al. 22 and Lu et al. 16,23,24 showed that high oxidative reactivity of soot is attributed to

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reported that soot oxidative reactivity depends on the degree of disorder of 26-28

on understanding


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nanostructure proved that structural changes in the graphene layer (like planar elongation of the

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individual carbon layers) alters the ratio between basal plane and edge site carbon atoms which

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increases the rate of soot oxidation. There is active research in the area of soot oxidation that

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focuses on understanding structure-oxidative reactivity relationship of soot, which evokes us to

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elucidate the oxidation behavior of crankcase soot. In this research work, the influence of

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interaction of crankcase soot with oil additives on soot’s chemistry, structure, oxidation reactivity

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and wear properties is examined which will provide benefit to engine manufacturers and catalyst

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industry in resolving soot related issues.

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Soot in the diesel engine oil can lead to wear and premature engine failure 29. Several researchers

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have addressed the effect of soot on engine wear and various soot caused wear mechanisms have

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been proposed such as 1) competition of soot with anti-wear additives for adsorption sites at metal

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surfaces 30, 2) adsorption of active anti-wear component from the oil phase by soot 31,32, 3) abrasion

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of protective anti-wear films by soot through three body wear mechanism where soot acts as the

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intermediate body 33-37 and 4) accumulation of soot in the contact inlet, ultimately restricting oil

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supply at the contact 38-41. Although various wear mechanisms have been suggested, there is still

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no consensus on the mechanism contributing to high wear rates caused due to high soot loaded

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lubricating oils, thereby a better understanding of the underlying soot caused wear mechanism is

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required.

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In light of the above considerations, in this research work, diesel engine soot extracted from Mack

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T-12 engine dynamometer crankcase oil was characterized using XANES and HR-TEM. XANES

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is used in determining the local coordination, valence and chemical bonding of individual

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elements, coupled with HR-TEM it is possible to characterize the crystalline and amorphous

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phases in diesel soot. In addition, the oxidation behavior of soot was evaluated using high 5 ACS Paragon Plus Environment

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temperature X-ray diffraction (HT-XRD), Brunner-Emmett-Teller (BET), and energy dispersive

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spectroscopy (EDS). The secondary objective of this study was to understand the effects of

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crankcase soot induced engine wear. A series of experiments were designed to assess the

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tribological performance of model test oil containing soot, dispersant and antiwear additives.

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Detailed evaluation of the effect of diesel engine soot on the tribological performance of the three

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different anti-wear additives, zinc dialkyl dithiophosphate, phosphonium based ionic liquid, and

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metal free dithiophosphate was performed using four-ball tribological test. An in-depth

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characterization of the wear surface, as well as chemical properties of formed tribofilms using

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scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), was performed

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and various phenomenological models for wear mechanisms were proposed. The findings of this

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paper will help engine oil formulators and lubricant additive manufacturers to develop improved

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engine lubricants or and optimize existing lubrication oil additive technology capable of mitigating

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the adverse effect of soot accumulation and enhancing diesel engine efficiency, durability, fuel

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efficiency, and, simultaneously, reducing harmful emissions.

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2. Experimental Methodology

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2.1 Diesel Soot Extraction

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Crankcase soot was extracted from the used diesel engine oil sample acquired from Mack-T12

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lubrication qualification dynamometer engine test. At first, the collected sump oil was diluted with

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hexane in a 50-50 wt% ratio followed by bath ultrasonication for 15 minutes. The oil-hexane

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mixture was then centrifuged at 12,000 rpm for a period of 2 hours to collect soot. The supernatant

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was discarded, and the remaining residue of soot was washed with hexane and ultrasonicated,

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followed by centrifugal process. This process was repeated for three times until the oil-free thick

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black residue of soot particles was obtained. To further ensure the removal of any trapped oil


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molecules a Soxhlet extraction method was used for 24 hours using hexane as the solvent. The

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extracted black residue was then dried and ground using a mortar and pestle to break up the

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agglomerates for further analysis.

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2.2 Characterization of Diesel Soot – Chemistry, Structure and Oxidation Attributes

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XANES was used to study the chemical composition of the extracted diesel engine oil soot. This

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characterization technique helps to understand the local co-ordination of the elements present in

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the soot structure from near surface to the bulk. The photo-absorption spectra were obtained using

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a 2.9 GeV storage ring at the Canadian Light Source (CLS), Saskaton, Canada. Three beam lines,

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soft X-ray micro-characterization (SXRM), variable line grating-plane grating monochromator

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(VGM-PGM), and spherical grating monochromator (SGM) were used at CLS to acquire K and L

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shell absorption edge spectra. The phosphorus K-edge (P K-edge), sulfur K-edge (S K-edge), and

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calcium K-edge (Ca K-edge) spectra were recorded using the SXRM beam line with a photon

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energy range of 1700-10,000 eV, resolution of 0.2 eV and beam spot size of 4mm × 300µm.

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Phosphorus L-edge (P L-edge) and sulfur L-edge (S L-edge) spectra were collected using the

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VGM-PGM beam line operating at the energy range of 5-250 eV with photon resolution of 0.2 eV

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and beam spot size of 500 µm × 500 µm. Zinc L-edge spectra (Zn L-edge) was acquired at SGM

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beam station covering energy range between 250 and 2000 eV with photon resolution of 0.2 eV.

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A beam spot size of 100 µm × 100 µm was chosen to collect spectra at SGM beamline. XANES

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sample preparation was carried out by placing soot particles using a spatula on the indium foil and

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gently tapping on the rear of the foil to remove any loose particles. As a result, a very fine layer of

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particles with a thickness of around 4-5 microns was left on the foil. These foils were then

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immediately placed in the sample chamber for characterization. Spectra were acquired using total

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electron yield (TEY) and fluorescence yield (FY) mode. TEY spectra give near-surface sensitive


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information of the sample, whereas FY spectra provide information from the bulk of the sample.

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In the case of P L-edge, the sampling depth of TEY and FY modes is 50 Å and up to 600 Å

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respectively.

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HR-TEM was used to characterize the structure of extracted carbonaceous soot particles. High

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resolution lattice imaging was acquired using a Hitachi H-9500 microscope at an accelerating

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voltage of 300 kV with a lattice resolution of 0.18 nm.

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Oxidation characteristics of the soot particles were studied using HT-XRD. The diffraction pattern

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of the sample was recorded using a Rigaku Smart Lab Diffractometer with CuKα radiation of

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wavelength 1.5406 ˚A. The diffractometer was operated in Bragg-Brentano focusing geometry

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with tube voltage at 40 kV, current at 44 mA and 2θ scan range at 10-90˚. The soot sample was

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placed on a fused silica glass inserted in a heating furnace apparatus, co-planarity of the soot

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particles surface with the sample holder surface and setting of the sample holder at the position of

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symmetric reflection geometry was ensured. The heating unit of the furnace was set to raise

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temperature from room temperature to 700˚C at a 30˚C/min. Phase or composition changes were

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mapped for each X-ray scan taken at the temperature increments of 50˚C. After cooling the sample

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from 700˚C, an additional scan at 50˚C was taken to study the final structure of the soot. Interplanar

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spacing (d002) was calculated using Bragg’s law for each temperature step from the position of the

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(002) peak. The weight percentage of the residue left behind after soot oxidation was calculated

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by weighing initial and final weight of the sample. Phase identification of residue was carried out

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by matching the peak positions and peak RIR (reference intensity ratio) values of model

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compounds in the ICDD (international center for diffraction data) crystal structure database. In

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addition, the elemental analysis of the incombustible residue obtained after HT-XRD studies was


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performed using a Vega 35B electron microscope with a fully integrated EDS microanalyzer from

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TESCAN Instruments.

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2.3 Bench Test Oil Formulations and Procedure

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In this study, a total of sixteen test oil formulations were prepared to determine the effect of certain

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interactions like soot - antiwear additive, soot - dispersant on the wear of tribological contact.

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Table 1 details the test oil formulations and the respective code names that would be used further

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as a reference. Oil formulations were composed of base oil, antiwear additives, dispersant, and

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extracted soot particles. Dispersant – sorbitan monoleate was purchased from Sigma Aldrich.

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Antiwear additives – zinc dialkyl dithiophosphate (ZDDP) and ashless neutral dialkyl

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dithiophosphate (DDP) were acquired from the Chevron and BASF respectively. The phosphorus-

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containing ionic liquid, tetra-n-butyl-phosphonium O,O-diethyl-dithiophosphate (DEDTP) was

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provided from AC2T research GmbH. Oil formulations were prepared by adding all the antiwear

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additives at 0.07 wt% phosphorus treat rate and dispersant at 5 wt%. A thorough mixing of all the

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components in the oil formulations was ensured by using pulse ultrasonication for 15 min in de-

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ionized water bath. The tribological tests were conducted for each oil formulation immediately

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after mixing. The tribological performances of the formulated oil were evaluated using Plint TE92

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four-ball Tribometer. ASTM D2266 was used for testing the antiwear properties of oil

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formulations in continuous sliding mode under boundary lubrication regime. According to ASTM

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standard, three AISI E52100 chrome steel balls of ½ inch diameter were clamped together at the

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bottom and immersed in the formulation to be evaluated. A fourth steel ball held in ball holder was

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pressed with force of 40 kg into the cavity formed by the three clamped balls. The temperature of

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the test was regulated at 75˚C and the top ball was rotated at 1200 rpm for 60 min and 72000

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cycles. All the tests were repeated three times to avoid any discrepancy in the data. After


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Table 1. Overview of test oil formulations. Code Name

Test Oil Formulation

B. O

Group III Base oil

ZDDP_D

B.O + ZDDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%)

ZDDP_D_Soot_2 wt%

B.O + ZDDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (2 wt%)

ZDDP _D_Soot _5 wt%

B.O + ZDDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (5 wt%)

ZDDP_D_Soot_10 wt%

B.O + ZDDP (0.0 7wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (10 wt%)

DEDTP_D

B.O + DEDTP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%)

DEDTP_D_Soot_2 wt%

B.O + DEDTP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (2 wt%)

DEDTP_D_Soot_5 wt%


DEDTP_D_Soot_10 wt%


DDP_D

B.O + DDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%)

DDP_D_Soot_2 wt%

B.O + DDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (2 wt%)

DDP_D_Soot_5 wt%

B.O + DDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt%) + Soot (5 wt%)

DDP_D_Soot_10 wt%

B.O + DDP (0.07 wt% of Phosphorus treat rate) + Dispersant (5 wt %) + Soot (10 wt%)

D_Soot _2 wt%

B.O + Dispersant (5 wt%) + Soot (2 wt%)

D_Soot_5 wt%

B.O + Dispersant (5 wt%) + Soot (5 wt%)

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D_Soot_10 wt% B.O + Dispersant (5 wt%) + Soot (10 wt%) termination of each test, three balls were removed and cleaned thoroughly using hexane to observe

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wear scar.

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2.4 Analysis of the Wear Surfaces and Formed Tribofilms 10 ACS Paragon Plus Environment

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Stereo-optical microscope (model type: Nikon SMZ 1500) was used to image the wear scars

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formed on the three stationary balls after the four-ball tribological test. The optical images were

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obtained at 100X magnification and were further analyzed using image J software to calculate the

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Wear Scar Diameter (WSD). A precise measurement of the WSD for each test oil formulation

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was carried out using an average of 12 readings on 6 ball samples from a total of 2 repeat runs.

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Measured WSD for each ball is the average of longest horizontal striation and longest vertical

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striation.

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The nature of the wear occurring on the steel ball specimen surface was determined by carefully

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examining the surface of the wear tracks using a SEM. A Hitachi S-3000N SEM equipped with an

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energy dispersive X-ray spectrometer was used in the secondary electron mode to acquire the

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images of the wear surface at highest magnification of upto 1000X with an accelerating voltage of

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15-25 kV. Higher resolution images of the specific area of interest helped to understand the wear

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mechanism in play on the surface. The corresponding elemental maps and composition data

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collected by EDS micro-analyzer unit were used to study the tribochemistry of the films generated

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on the rubbing surfaces.

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3. Results and Discussion

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3.1 XANES

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3.1.1 Phosphorus Characterization

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The P L-edge TEY spectra as shown in figure 1 provides information about local coordination of

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phosphorus near the surface of the soot particles. To understand the interaction of the lubricating

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oil additive chemistries with the soot, it is essential to compare the XANES spectra with the

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different model compounds of the known local chemical environment representing the

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characteristics of typical P and S based antiwear films as well as the decomposition products of


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lubricating oil. As a result, the photo-absorption spectra of the soot are compared with iron

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phosphate (FePO4), zinc phosphate (ZnPO4), calcium phosphate (Ca3(PO4)2) and calcium

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pyrophosphate (Ca2P2O7). Phosphorus L-edge spectra is characterized by spin-orbit splitting of

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phosphorus 2p electrons and excitation to the antibonding orbitals. Transition of spin orbit split of

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2p electrons between 2p3/2 (L3 edge) and 2p1/2 (L2 edge) is shown by peaks a and b. Peak labelled

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as c is assigned to transitions to 3p orbitals, which are sensitive to the presence of other elements

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such as oxygen and cations like Fe or Zn. Peak d is the shape resonance peak characteristic of all

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phosphates regardless of structure, whether crystalline or glassy 42-44. The presence of peak d in P

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L-edge spectra of soot suggests that the phosphorous is present as phosphates in soot.

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Figure 1. (a) Phosphorus L-edge TEY (b) phosphorus K-edge FY spectra of crankcase soot

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compared with reference compounds.

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Comparison of the soot absorption spectra with the model compounds in TEY mode indicates that

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the pre-edge absorption peak a closely matches with pre-edge of Fe, Zn and Ca phosphates. The

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characteristic main absorption peak c of the soot aligns with that of Ca3(PO4)2 and rules out the

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possibility of presence of Fe and Zn phosphates in the soot structure. The misalignment of the

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main absorption edge peak c and d of the soot to that of undecomposed ZDDP eliminates the

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possibility of any trapped oil in the soot structure.

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The P K-edge FY spectra is presented and compared with the model compounds in figure 1. The

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P K-edge spectra gives more adequate information of the local coordination of phosphorus in the

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bulk of the soot because the sampling depth of FY (> 8000 Å) at P K-edge is greater than the

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sampling depth of TEY (50 Å) at P L-edge 50. Excitation of electrons from phosphorus 1s orbital

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to unoccupied 2p orbital results into a main absorption peak of P k-edge spectra as marked in figure

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1. The energy at main absorption peak matches with main absorption edge of Zn3(PO4)2 and

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Ca3(PO4)2, suggesting that Zn or Ca or a mixture of Zn and Ca phosphates is present. Zn3(PO4)2

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is the decomposition product of lubrication anti-wear additive, ZDDP, while Ca3(PO4)2 arises from

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the detergent chemistry. Possibility of FePO4 is not likely due to absence of pre-edge of FePO4 in

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soot spectrum. Presence of short chain polyphosphate was identified by calculating a/c ratio

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through phosphorus L-edge spectra. The method to calculate a/c ratio and estimate chain length of

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polyphosphate glass is explained by various authors in the literature. 13,45-52,54.

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The P K-edge and P L-edge XANES results indicate the variation of phosphorus local co-

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ordination in the extracted soot at different depths. At near surface (P L-edge TEY), Ca phosphates

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are present, whereas, in the bulk (P K-edge TEY and FY) P is primarily associated with Zn or Ca 13 ACS Paragon Plus Environment

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phosphates. Based on these results, it can be suggested that as soot particles get trapped in the

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crankcase oil, they interact with reactive degradation compounds or polar additive species present

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in the engine oil additive package. This potential interaction results in the adsorption of inorganic

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decomposition products like Ca phosphates from detergent chemistry and Zn phosphates from

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decomposed ZDDP on the soot structure. In addition, the presence of Zn phosphates raises the

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possibility of direct interaction of soot particles with the phosphorus based tribofilms formed by

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ZDDP antiwear additives. During engine operation, soot particles can get adsorbed on the engine

271

part surfaces like cylinder walls and abrades the protective antiwear Zn phosphate tribofilms

272

formed on the engine parts. Such abrasive action along the surfaces in motion results in

273

incorporation of Zn phosphates in the soot structure.

274

3.1.2 Sulfur Characterization

275

Figure 2 shows the S K-edge TEY and FY spectra compared with sulfates of zinc, calcium and

276

iron and sulfides of zinc and iron. Sulfur has rich K-edge spectra with sharp linewidths and

277

chemical shift range (of nearly 14 eV) over its range of oxidation states of -2 to +6 54. The Sulfates

278

(+6) have absorption peak at higher energy clearly distinguishing them from sulfides (-2). ZnS has

279

characteristic low energy strong peak at 2470 eV while FeS has absorption edge at 2482 eV. FeSO4

280

has higher energy peak at 2482.3 eV which differentiate it from zinc and calcium sulfate. Zinc

281

sulfate and calcium sulfate have quite similar main absorption peak at ~2481 eV, however, a post

282

edge at 2484.8 eV of CaSO4 mark it off from ZnSO4.

283

Upon comparing S K-edge TEY and FY spectra of soot with reference compounds, main

284

absorption peak of soot matches with the characteristic peak of ZnSO4 and CaSO4. But the

285

possibility of CaSO4 in soot structure is nullified because of the absence of post-edge peak of

286

CaSO4 at 2484.8 eV. It is important to note that pre-edge and main absorption peak in TEY soot 14 ACS Paragon Plus Environment

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spectra slightly matches with the characteristic peaks of FeS, however, FY spectra of FeS has

288

distinctive features indicating the absence of FeS in the bulk of soot. Characteristic peak a in both

289

TEY and FY clearly indicates presence of ZnS in near-surface and bulk of soot. S K-edge spectra

290

can also be used to quantify the relative proportion of sulfur compounds in diesel soot. This

291

quantification was carried out by deconvoluting the sulfur K edge spectra in TEY mode. Analysis

292

of relative amount of sulfide and sulfate compound was done by calculating height ratio of sulfide

293

to sulfate peak. The sulfide/sulfate ratio for diesel soot is 0.37.

294

The presence of Zn sulfur compounds strongly indicates the interaction of the soot with the

295

antiwear tribofilms formed by ZDDP on the engine parts. In addition, the presence of CaSO4 and

296

FeS indicates the adhesion of decomposed Ca-based detergent compounds and wear debris with

297

the engine soot carried into the crankcase oil.


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Page 16 of 54

298 299

Figure 2. Sulfur K-edge (a) TEY spectra (b) FY spectra of crankcase soot and model compounds.

300

3.1.3 Calcium Characterization

301

The Ca K-edge TEY spectra for the extracted soot is presented in the figure 3. Spectra of soot is

302

compared with model compounds of CaSO4, Ca3(PO4)2, CaCO3 and CaO.

303

For the Ca K-edge, the pre-edge peak around 4042 eV is attributed to transition from 1s to bound

304

3d orbitals. The main absorption peak region consists of three peak structure, a shoulder on the

305

lower side (around 4047 eV) resulted from 1s to 4s quadrupole transition and the principal double

306

peaks attributed to 1s to 4p1/2 and 1s to 4p3/2 dipole transition. As the occurrence of dipole transition

307

is likelier than quadrupole, the K-edge spectra is primarily dominated by high intensity main peak

308

arising due 1s to 4p transition 55. CaO has a distinctive pre-edge at 4043 eV and strong peak at 16 ACS Paragon Plus Environment

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4049.8 eV. CaSO4 and Ca3(PO4)2 has an intense absorption edge at 4050 eV and 4049.4 eV

310

respectively. The CaSO4 has more fine structure compared to Ca3(PO4)2.

311 312

Figure 3. Calcium K-edge TEY spectra of crankcase soot and model compounds.

313

The main absorption peak of soot around 4049 eV is clearly matching with the characteristic edges

314

of CaSO4, Ca3(PO4)2 and CaO, but, the pre-edge (4043 eV) and pre-shoulder (4047 eV) peak of

315

CaO is not aligned with the absorption peak of soot. These results indicate that Ca is primarily

316

associated with Ca sulfate and phosphate compounds which corroborates the earlier observations

317

of the P, S L-edge and P, S K-edge results.

318

Additionally, P L-edge FY, P K-edge TEY, S L-edge TEY and FY and Zn L-edge TEY and FY

319

spectra were also recorded to gain detailed chemical composition analysis. These spectra can be


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Page 18 of 54

320

found in the supporting information section of this paper. Table 2 summarizes chemistry of the

321

soot identified through XANES detection modes.

Table 2. Overview of different compounds identified via XANES Chemical Compounds

XANES Measuring Modes PL TEY

PL FY

P K P K S L S L S K S K Zn L TEY FY TEY FY TEY FY TEY   N/A N/A N/A N/A N/A

Zn L FY N/A

Ca K TEY

Ca K FY

Ca3(PO4)2









Zn3(PO4)2

-

-





N/A

N/A

N/A

N/A





N/A

N/A

ZnSO4

N/A

N/A

N/A

N/A

CaSO4

N/A

N/A

N/A N/A

 

N/A

N/A

N/A

N/A

FeS

N/A

N/A

N/A

N/A

  

 

FeSO4



N/A

N/A

N/A

N/A





ZnS

N/A

N/A

N/A

N/A

-

-





N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

-

-

N/A

N/A

CaCO3

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

 

 

CaO

N/A

N/A

N/A

N/A

N/A

N/A

322 323

3.2 High Resolution Transmission Electron Microscopy (HR-TEM)

324

HR-TEM was employed to examine structure and morphology of soot particles. Figure 4 is a bright

325

field HR-TEM image illustrating a typical carbonaceous turbostratic structure of primary soot

326

particle comprised of both crystalline and amorphous regions.

327

A crystalline domain of primary soot particle can be identified in the figure 4 with several layers

328

stacked a top of each other in a circular fashion. Such a closed packed array suggests that the

329

crystalline domain has a turbostratic configuration. It has been reported in the literature that a 18 ACS Paragon Plus Environment

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turbostratic arrangement comprised of 4-5 graphene layers called platelets stacked in the circular

331

manner to form crystallite with interlayer distance of about 3.5 Å 12,56,57. Marked lattice fringe area

332

on the image can be identified as the nanocrystalline particles embedded in turbostratic structure

333

of soot. The size of these particles is in the range of 3-6 nm.

334 335

Figure 4. High-resolution bright field transmission electron micrograph of crankcase soot

336

showing nanocrystalline particles, turbostratic and amorphous regions.

337

Careful observation of the HR-TEM image of the soot imply that the crystalline region is present

338

on the outer shell of the primary soot particle and is tightly anchored. Nanocrystalline particles are

339

continuous with crystalline domain of a turbostratic soot structure. These observations indicate the

340

possibility of mechanical embedding or chemical bonding of nanocrystalline particles on surface

341

of diesel soot primary particles which cannot be removed by washing, ultra-sonication and

342

centrifuging the soot. Table 3. Interplanar spacing of the marked nanocrystalline particles. 19 ACS Paragon Plus Environment

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Marked

Page 20 of 54

Interplanar Lattice

Interplanar Lattice

Nanocrystalline

Spacing d (A˚)

Spacing d (A˚)

Particles

Standard Value

Measured Value

BD

2.81

2.823

BE

2.778/2.811

2.77

BB

2.778/2.811

2.789

BA

2.81

2.892

Ca5(PO4)3OH (112)

BC

2.81

2.852

Ca5(PO4)3OH (112)

BG

2.77

2.74

Ca5(PO4)3OH (112)

Miller indices of crystallographic plane (hkl) Ca5(PO4)3OH (112) Ca5(PO4)3OH (112)/ Ca5(PO4,CO3)3OH (211) Ca5(PO4)3OH (112)/ Ca5(PO4,CO3)3OH (211)

343 344

Careful observation of the HR-TEM image of the soot indicates that the crystalline region is

345

present on the outer shell of the primary soot particle and is tightly anchored. The crystalline phase

346

of nanocrystalline particles is continuous with crystalline domain of turbostratic soot structure.

347

These observations indicate the possibility of mechanical embedding or chemical bonding of

348

nanocrystalline particle on surface of diesel soot primary particles which are incapable of removal

349

by washing, ultra-sonication and centrifuging the soot.

350

To correlate XANES results with HR-TEM and gain detailed insight of embedded compounds,

351

interplanar spacing of nanocrystalline region is measured and detailed in table 3. Calculated values

352

were compared with similar d-spacing of compounds possible from XANES results. Based on the

353

XANES findings, it was speculated to detect various compounds such as ZnS, ZnSO4, Zn3(PO4)2,

354

CaO, CaSO4, Ca3(PO4)2, FeS, Ca(OH)2, but no apparent match was found. Matching of interplanar

355

spacing indicates the presence of only Ca based compounds, hydroxyapatite (Ca5(PO4)3OH) and

356

carbonate hydroxyl apatite (Ca5(PO4,CO3)3OH) in the soot structure. It appears that Zn phosphates,

357

sulfates or sulfides are present in amorphous form while crystalline particles are dominated by Ca 20 ACS Paragon Plus Environment

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Energy & Fuels

358

based compounds. The origin of Ca compounds is from the detergents such as alkyl benzene

359

sulfonates wherein Ca occurs as CaCO3 used in engine oil formulations. These detergents are used

360

to suspend insoluble polar debris and neutralize acidic byproducts of combustion. Also, they

361

contribute to protective tribofilms formed on engine parts. Many researchers have studied

362

influence of Ca detergent chemistry on tribofilm formation and have reported the formation of

363

small chain calcium phosphate films on the rubbing surfaces

364

bonded or mechanically incorporated calcium phosphates in the turbostratic soot structure

365

indicates either possibility of cross-interaction of soot particles with detergent decomposition

366

products in engine oil or removal of Ca based tribofilms by trapped soot during engine operation.

367

One of the mechanism responsible for embedment of these nanocrystalline particles can be three

368

body wear which is a proposed wear model in several studies 36,37,62,63,93. It can be postulated that

369

soot particles/ agglomerates in crankcase film can get trapped between two surfaces in motion and

370

can experience three body wear condition at extreme pressure and temperature, leading to

371

incorporation of nanocrystalline particles originating from calcium detergent chemistry in soot

372

structure. These nanocrystalline particles had been reported to have higher hardness 13,51,58. Thus,

373

presence of such mechanically embedded hard nanocrystalline particles in soot structure will make

374

it more abrasive in nature and might promote soot induced abrasive engine wear.

375

3.3 Oxidation Characteristics of Soot (HT-XRD, BET surface analysis)

376

Several studies have investigated oxidation behavior of soot extracted from flame or engine

377

exhaust stream using thermogravimetric analysis; however less importance has been given to

378

understanding oxidation characteristics of soot extracted from engine oils 64-68. This research work

379

focuses on evaluating oxidation behavior of soot sampled from engine oil using a temperature

380

resolved X-ray diffractometry.

53, 59-61.

Detection of chemically


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Page 22 of 54

381

3.3.1 High Temperature X-ray Diffractometry (HT-XRD)

382

HT-XRD is crucial in analyzing phase transformations, crystal structure, incombustible residue

383

content and oxidative stability of materials with an increase in temperature. HT-XRD experimental

384

analysis of engine soot helps to assess the influence of physical-chemical properties of turbostratic

385

soot on its oxidation reactivity. Figure 5 and 6 displays the XRD spectra for soot and carbon black

386

respectively. Both figures are divided into two sections, left graphs represent the XRD data

387

recorded for the entire scan range of 10˚-90˚ 2θ and right graphs shows “d002 range” of 18˚-30˚ 2θ.

388

The peak originating at ~25˚ 2θ represents a characteristic peak arising from the basal plane (002)

389

of the disordered graphitic lattice in the soot structure 69,70. Therefore, the data in the range of 18˚-

390

30˚ was separately plotted for easier visualization of this peak on the figures. Spectra marked in

391

the blue color shows the XRD data acquired before complete oxidation of carbonaceous material,

392

while the spectra in orange color is from the oxidized sample. Even though XRD scans were

393

recorded at intervals of 50˚C increase in temperature starting from 50˚C to 700˚C, the data was

394

plotted for only those temperatures where changes in lattice spacing, phase transition or peaks

395

from crystalline species are noticeable. For this reason, XRD scans for carbon black are only

396

shown at 50ºC and 700ºC in figure 6.


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Energy & Fuels

397 398

Figure 5. Variations in crankcase soot HT-XRD spectra with temperature for 10˚-90˚ 2θ scan

399

range (left) and 18˚-30˚ 2θ d002 scan range (right).

400 401

Figure 6. Variations in carbon black HT-XRD spectra with temperature for 10˚-90˚ 2θ scan range

402

(left) and 18˚-30˚ 2θ d002 scan range (right).


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Page 24 of 54

403

The data in the range of 18˚-30˚ provides information on diffraction peaks originating from

404

nanocrystallites embedded in the soot turbostratic structure. Result shows the changes in full width

405

half maxima, peak intensity and peak shifts with an increase in temperature. It is observed that the

406

d002 peak diminishes beyond 550˚C. A careful examination of d002 range of 18˚-30˚C 2θ in figure

407

5 indicates that with increase in temperature, intensity maxima (represented by red dotted lines) of

408

diffraction peaks gradually shifts towards lower 2θ values signifying increase in interplanar lattice

409

spacing. Expansion of lattice in [002] direction eases access of oxygen to soot core structure

410

helping soot particles to oxidize at relatively lower temperature.

411

Comparison of d002 range (18˚-30˚C 2θ) of soot and carbon black shows absence of any diffraction

412

peak originating from carbon black sample after oxidation. However, multiple diffraction peaks

413

are observed for soot sample at 700ºC possibly originating from polycrystalline material (i.e.

414

incombustible residue in the current sample) which may be adhered or embedded in the

415

carbonaceous turbostratic soot structure. These results imply that interaction of soot particles with

416

decomposition products of oil additives (metallic species from detergents, antiwear components

417

etc.) and wear debris in the engine oil, at extreme pressure and temperature conditions create higher

418

disorder in the soot graphitic structure, thereby leading to increase in oxidative reactivity. In

419

addition, the adsorbed wear debris in the form of Fe, Al oxides can catalyze the oxidation of

420

carbonaceous soot at higher temperature (above 300˚C) 71,72. Most recently, Bagi et al. 4,5,21,73 and

421

co-workers examined correlation between the oxidation behavior and chemical composition of

422

engine oil soot with changes in accumulated vehicle mileage. They proposed that with the engine

423

age, there was increase in the amount of wear debris, tribofilm compounds and engine oil species

424

embedded in the soot structure which would influence ease of oxidation of soot.


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Energy & Fuels

425

Mapping changes in interplanar lattice spacing (d002) with temperature provides significant

426

information about degree of disorder, number of graphene layers per crystallite, crystallite height,

427

width, which can be correlated to fuel composition, engine operating conditions, combustion

428

mechanism, ease of oxidation, etc. Figure 7 shows variation in interplanar spacing (d002) with

429

increase in temperature for soot and carbon black.

430 431

Figure 7. Mapping variations in interplanar spacing (d002) with increase in temperature for

432

crankcase soot and carbon black.

433

To calculate d (002) values, baseline corrections and intensity maxima were applied to diffraction

434

spectra and then Bragg’s law was used. Interplanar lattice spacing increases in soot and carbon

435

black samples with increase in temperature. In both cases, carbonaceous soot structure remains

436

intact till 250 ˚C, beyond which the d (002) planes expands in the case of carbon black but remains

437

constant in the case of soot. The turbostratic structure of soot falls apart after 550˚C and that of

438

carbon black after 600˚C. Oxidation reaction rate can be attributed to induced higher disorder of 25 ACS Paragon Plus Environment

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Page 26 of 54

439

carbonaceous soot structure featuring increased number of graphene edges on the surfaces i.e. a

440

greater number of active sites for oxygen attack

441

changed soot turbostratic structure (HR-TEM results) and chemical composition (XANES results)

442

on soot oxidation characteristics.

443

3.3.2 Analysis of Incombustible Residue

444

Th residue was collected for both soot and carbon black sample after completion of their HT-XRD

445

studies at 700ºC. This residue has a yellowish to gray appearance as shown in figure 8. Quantitative

446

analysis of remaining residue was carried out by normalizing them as a percentage of their initial

447

weights and by using following formula.

71,93.

HT-XRD analysis ascribe the influence of

448

Residue (wt %) = {(Wsoot − Wresidue)/Wsoot} × (100)

449

“Wsoot” represents the weight of the soot sample before oxidation while “Wresidue” corresponds to

450

the weight of residue left behind after oxidation; they were measured on an electronic balance with

451

precision scale in milligrams. Soot and carbon black sample showed 64 wt% and 42 wt% of the

452

residue left at 700ºC respectively. The higher amount of incombustibles remained after combustion

453

of soot also supports postulate of possible interaction of crankcase soot particles with

454

wear/corrosion byproducts, decomposed additive compounds and polar species in engine oil,

455

leading to adsorption of inorganic compounds on soot particles. On the other hand, carbon black

456

experienced no interaction with engine oil and hence had less amount of residue.

457

Energy dispersive spectroscopy (EDS) was used to analyze the elemental composition of the

458

residue; EDS uses the ZAF method for quantification of elements in the sample. The acronym

459

'ZAF' describes a procedure in which corrections for atomic number effects (Z), absorption (A)

460

and fluorescence (F) are calculated separately. The atomic number (Z), absorption coefficient (A)

461

for every element is calculated. Figure 8 shows the EDS spectra for residue acquired from


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Energy & Fuels

462

crankcase soot. Elements such as Zn, P, and S originate from antiwear additives, whereas Fe

463

originates from the wear of engine components. In addition, S can come from the fuel and

464

detergents like sulfonates and phenates. As mentioned before, source of Ca is from the calcium

465

sulfonate based detergents. To compliment EDS results and gain detailed knowledge on actual

466

composition and structure, XRD spectra of residue were acquired for soot and carbon black.

467

Figure 9 displays XRD spectra obtained from crankcase soot and carbon black samples after being

468

heated up to 700˚C. The residue left behind after oxidation of soot shows the presence of

469

polycrystalline material whereas carbon black spectra does not show any diffraction peaks. Even

470

though 42wt% of residue was left after oxidation of carbon black, compounds in the residue are

471

likely amorphous in nature, and hence were not detected in XRD spectra. Diffraction peak

472

positions and Miller indices are marked on the XRD spectra of crankcase soot along with the

473

reference XRD spectra of CaSO4 compound. Identification of possible polycrystalline compounds

474

were carried out using the search-match function in the ICDD database and majority of the peaks

475

were matched with CaSO4 spectra as shown in figure 9. CaSO4 in the diesel engine soot originates

476

from the interaction of soot with detergents dispersed in crankcase oil. EDS spectra of crankcase

477

soot (figure 8) shows strong peaks of Zn, P, S elements, however, these elements were not

478

identified in XRD spectra, thereby indicating the presence of amorphous zinc phosphates or

479

sulfates compounds which are well known to be a part of ZDDP based antiwear tribofilms

480

Presence of these compounds supports our analysis made from XANES of interaction of soot

481

particles with the protective antiwear tribofilms during three body wear condition where crankcase

482

oil soot gets trapped between two surfaces in motion and results in extreme wear and tear of

483

contacting surfaces 57. Thus, the results suggest the detrimental effect of interaction of soot with

78-80.


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Page 28 of 54

484

components of lubricant additive package on soot’s oxidation characteristics and the amount of

485

incombustible residue left behind.

486 487

Figure 8. EDS spectra of residue left behind after oxidation of crankcase soot.

488

489 490

Figure 9. XRD spectra of oxidized soot and phase identification 28 ACS Paragon Plus Environment

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Energy & Fuels

491

3.3.3 Surface Area Analysis

492

The surface area of particles within the material can greatly influence its surface stability and

493

performance characteristics

494

determined by the BET nitrogen physisorption method described in the experimental section (2.2).

495

The BET physisorption studies were subjected to carbon black for baseline comparisons as it is

496

considered to be surrogate of diesel engine soot 35, having similar structure and morphology, but

497

different composition than diesel engine soot. The measured surface area of crankcase soot is 69

498

m2/g while carbon black exhibited higher surface area of 124 m2/g. Coupled with agglomeration,

499

the adsorption of lubricant additive compounds, decomposition products, wear debris and polar

500

impurities from engine oil on the soot particles results in lower surface area for crankcase soot.

501

The carbon black samples displayed higher surface area as they did not experience any interactions

502

with engine oil components. Patel et al. 13 also used the BET nitrogen adsorption method to study

503

the differences in surface area of the carbon black and diesel engine soot. Owing to similar primary

504

particle sizes between diesel soot and carbon black, it was proposed that size differences between

505

the two samples were not responsible for large variation in measured surface area. A previous

506

study by Bagi et al.

507

adhesion of wear debris and engine oil inorganic species with accumulated vehicle mileage. BET

508

results can be correlated with HT-XRD oxidation studies. It can be seen that the amount of residue

509

left after oxidation of soot and carbon black are inversely proportional to the available surface area

510

determined by BET analysis.

511

3.3.4 Oxidation Modes

512

The soot reactivity towards oxidation is strongly dominated by soot nanostructure and thus, the

513

data acquired through HR-TEM and HT-XRD should be used simultaneously to examine oxidation

21

25,64,81.

The surface area of the extracted crankcase soot was

reported a lower surface area of diesel engine soot due to increase in the


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Page 30 of 54

514

modes of soot. The primary particle of soot exhibits an inner core and an outer shell structure,

515

comprised of crystalline and amorphous domain 50,83-85. The HR-TEM study (Section 3.2) indicates

516

the presence of Ca based nano-crystalline particles embedded in the outer periphery (crystalline

517

domain) of the soot primary particle. Embedment of such particles in the outer shell possibly result

518

in smaller fringe length (measure of the physical extent of carbon layer planes)

519

fringe separation (mean distance between adjacent carbon layer planes, also known as d(002)) 70 of

520

soot crystallite domain. Smaller fringe length indicates smaller graphene layer dimensions and

521

more edge-site carbon atoms i.e. higher degree of carbon structural disorder

522

present at edge site have greater possibility to interact with oxygen than those located in basal

523

plane (002)

524

enhancing oxidation of the primary soot particle 88. The higher surface energy and lower activation

525

energy available due to structural disorder in the outer shell of the soot particle may possibly lead

526

to higher reactivity of outer shell. In addition, by referring to d(002) variations in figure 10, it can be

527

proposed that oxidation of primary soot particle initially starts with the slow external burning

528

(outside-in) of an outer shell till 550˚C. The inner core is considered to be highly reactive 70,71,89,

529

initial stage of combustion is followed by fast internal burning (inside-out) of inner core between

530

550˚C-700˚C. The HT-XRD results suggests likelihood of primary soot particles to follow “Dual

531

oxidation mode” of combustion. However, data from HR-TEM and BET obtained at different

532

temperatures need to be correlated with HT-XRD data to have a concrete understanding of

533

oxidation modes. In addition to the structural disorder factor, the possibility of adsorption of

534

catalytic metallic compounds on soot particles (that may catalyze soot oxidation) also advocates

535

the higher reactivity of outer shell and dual mode of primary soot particle oxidation.

536

3.4 Evaluation of Tribological Test Wear Results

87,89.

86,87

81,88.

and larger

Carbon atoms

Additionally, larger fringe separation can ease oxygen access to carbon layers


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Energy & Fuels

537

XANES, HR-TEM, HT-XRD and BET studies confirm the interaction of trapped soot

538

particles/agglomerates with the protective tribofilms and decomposition products of P and S based

539

lubricant antiwear additives and Ca sulfonate based detergents; wear debris and combustion

540

byproducts in the engine oil used for MackT12 dynamometer test. Additionally, results indicate

541

the potential of crankcase soot to promote severe wear of engine components through wear

542

conditions like three body wear. To get comprehensive understanding of soot induced wear

543

mechanisms, four ball tribological bench tests were performed. Test oil formulations with different

544

concentrations of soot were prepared to examine the influence of various soot loading conditions

545

on wear mechanism, whereas, various antiwear additives were used to elucidate the effect of soot-

546

antiwear additive interaction on the wear rate of tribological contact. WSD was obtained for three

547

stationary balls using the method described in the section 2.3. Figure 10 is a comparative bar chart

548

detailing the WSD results for test oil formulations having three different antiwear additives and

549

soot in increment amount. Result for base oil was added for baseline comparison. WSD values are

550

reported in µm with error bars representing corresponding variation in the scar diameter values for

551

each test. In comparison with base oil, all three additives showed improvement in wear

552

performance of oil formulations.

553

ZDDP exhibited best antiwear performance with 618 µm WSD, while, DEDTP and DDP had

554

slightly higher wear losses. Addition of 2 wt% soot resulted in decrement of wear performance of

555

all oil formulations. Changes in antiwear behavior of the respective additives with an increase in

556

soot concentration from 2 wt% to 10 wt% are of prominent importance. ZDDP oil formulations

557

showed a stable WSD with increase in soot levels from 2 wt% to 5 wt%, however, a sudden

558

increase in WSD is observed at 10 wt% soot. DEDTP also displayed gradual decrease in wear

559

performance with a significant increase in WSD value by 870 µm from 0 to 10 wt% soot. In the 31 ACS Paragon Plus Environment

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Page 32 of 54

560

case of DDP formulations, notable rise in WSD is observed at 5 wt% as well as at of wt% soot,

561

unlike DEDTP and ZDDP formulations. These observations indicate, compared to ZDDP, both

562

DEDTP and DDP exhibit better wear performance at a higher soot concentration of 10 wt%.

563 564

Figure 10. Effect of increase in soot concentration on wear scar diameter of all oil formulations.

565

3.5 Analysis of Rubbed Surfaces (SEM-EDS)

566

Topographical and elemental composition analysis of worn surfaces of tribological test samples

567

were carried out using SEM-EDS. Figure 11 to 13 show low magnification (70X) and high

568

magnification (1000X) images of wear scars along with the corresponding surface elemental

569

composition spectra of wear scars acquired from test oil formulations containing different antiwear

570

additives and no soot. The boxes marked on low magnification images are the representative of


Page 33 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

571

the area under consideration, which is further magnified to 1000X and subjected to EDS spectrum

572

analysis.

573

SEM images at 70X show scratches in the direction of sliding which occurred during initial stage

574

of test, however, at higher magnification it is clearly seen that scratches are covered with tribofilm

575

patches (darker regions in the SEM images represent the presence of phosphate tribofilms). SEM

576

images demonstrate that the rubbed surfaces generated using ZDDP lubrication have wider

577

coverage of tribofilms than DEDTP and DDP, hence indicating better wear performance of ZDDP

578

(i.e. low WSD value) as compared to DEDTP and DDP. The corresponding EDS spectra

579

determines the type and amount of elements present on the worn surfaces. ZDDP is well known to

580

form antiwear tribofilms consisting of zinc phosphates, zinc sulfides/sulfates, iron phosphate and

581

iron sulfides/sulfates 83,84.

582

EDS spectra of ZDDP shown in figure 11.(A) confirms the presence of Zn, P, S, O, and Fe

583

elements. Zhang et al.

584

XANES and found that predominant cations were of Zn as compared to Fe. However, in this study

585

use of EDS can’t help to differentiate between Zn and Fe abundance. As the penetration depth of

586

the electron beam is deeper than the thickness of the tribofilm on the surface, the majority amount

587

of Fe as seen in the figure may have been originated from the substrate. The identification of P, S,

588

O, and Fe elements on the worn surfaces developed by DEDTP and DDP oil formulation indicates

589

that these tribofilms are comprised of iron phosphates and iron sulfides/sulfates. This is in

590

accordance with the study by Sharma et al. 43,54 and Kim et al. 47, who reported that phosphorus

591

and sulfur based ionic liquid and ashless dithiophosphate additives form pad like tribofilms very

592

similar to ZDDP with only one difference of presence of compounds coordinated by Fe cations

593

instead of Zn cations.

45,80

examined the low load and friction tribofilm formed by ZDDP using


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 54

594


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Energy & Fuels

595

Figure 11. SEM-EDS results for test formulations having ZDDP as antiwear additive (A) ZDDP

596

(B) ZDDP_D_Soot_2wt% (C) ZDDP_D_Soot_5wt% (D) ZDDP_D_Soot_10wt%.

597

Figure 11.(B), 12.(B) and 13.(B) represents the SEM-EDS results for the oil formulations

598

containing 2 wt% soot. Abrasive wear along with some polishing wear tracks and smooth patches

599

of tribofilms can be observed in the direction of sliding for the rubbed surfaces generated by all

600

oil formulations containing 2wt% soot. Higher magnification SEM image of wear scar created

601

through ZDDP lubrication with 5wt% (Figure 11.C) soot display furrowed wear tracks with fine

602

cracks indicating significant amount of abrasive wear, while that of DEDTP at 5 wt% (Figure

603

12.C) soot show combination of adhesive and abrasive wear with some deep cuts. Worn surface

604

lubricated by DDP formulation having 5 wt% soot (Figure 13.C) has pitting and abrasive tracks,

605

indicating the severe wear of the substrate through corrosive aided abrasive mechanism. Various

606

studies in the literature have well-documented the combination of corrosive and abrasive processes

607

leading to high rates of wear 33,34,90, which justifies a significant increase in WSD (Figure 10) for

608

DDP formulations at 5 wt% as opposed to ZDDP and DEDTP at 5 wt%. The corresponding EDS

609

spectra shown in figure 12.(C) and 13.(C) show low intensity peak of tribofilm forming elements

610

like P and S which suggests the depletion of antiwear phosphate and sulfides tribofilms on rubbed

611

surfaces. Figure 11.(D), 12.(D) and 13.(D) corresponds to the SEM-EDS results of the wear

612

surfaces developed during tribological testing of formulations containing antiwear additives,

613

dispersant and 10 wt% soot. Representative SEM images of rubbed surfaces display larger wear

614

scars with deep ploughing grooves in the direction of sliding, which is sign of severe abrasive

615

wear. In addition to abrasive wear, there are evidences of scuffing, adhesive-abrasive wear, deep

616

metal cut and corrosive-abrasive wear. These observations suggest the possibility of adverse

617

engine wear induced due to high level of soot contamination of lubricating engine oil. 35 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 54

618


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Energy & Fuels

619

Figure 12. SEM-EDS results for test formulations having DEDTP as antiwear additive (A)

620

DEDTP (B) DEDTP_D_Soot_2wt% (C) DEDTP_D_Soot_5wt% (D) DEDTP_D_Soot_10wt%.

621 37 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 54

622

Figure 13. SEM-EDS results for test formulations having DDP as antiwear additive (A) DDP (B)

623

DDP_D_Soot_2wt% (C) DDP_D_Soot_5wt% (D) DDP_D_Soot_10wt%.

624


Page 39 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

625

Figure 14. Correlation of soot content with WSD and concentration of P and S elements present

626

on tribosurfaces generated due to lubrication with formulations containing (A) ZDDP antiwear

627

additive (B) DEDTP antiwear additive (C) DDP antiwear additive.

628

Figure 14 summarizes the correlation between decrement of major elements corresponding to

629

tribofilm compounds (P, S) and wear scar diameter (WSD) of all the test formulations under study.

630

It can be observed in the graph 14(A), with the increase in soot concentration, wear scar diameter

631

increases and ZDDP formed tribofilm elements P and S decreases. Similar trend is also seen for

632

the formulations containing DEDTP and DDP as antiwear additives (figure 14(B) and 14(C)).

633

Therefore, the correlation between WSD and elements corresponding to antiwear tribofilms

634

indicates that the soot particles promotes high wear of surfaces in contact through removal of the

635

protective tribofilms formed by antiwear additives present in lubricating oil.

636

3.6 Antagonistic Interaction of Soot and Antiwear Additives

637

From the above discussion, negative impact of soot particles on the wear of tribological contacts

638

operating under boundary lubrication regime is evident. To understand the effect of antiwear

639

additives soot interaction on the wear of steel substrate, tests were conducted using base oil

640

containing only soot and dispersant.

641

Figure 15 shows comparison of wear results obtained with formulations containing only soot and

642

no antiwear additives to that of formulations having anti-wear additives and soot. Dispersant

643

concentration was kept constant at 5 wt% so as to directly correlate influence of presence of soot

644

and antiwear additives in oil with the wear outcomes.

645

For lubrication containing only soot and no antiwear additives, WSD increases from 756 μm to

646

776 μm as the soot concentration increases from 2 wt% to 5 wt%. In both cases, it can be speculated

647

that soot is acting as the third body abrasive particles generating local wear to the substrate through 39 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 54

648

sliding or rolling motion at the interface. For D_Soot_10 wt% sudden increase in the WSD is

649

observed. This can be explained with the loss in suspension of soot particles, wherein amount of

650

dispersant is not sufficient enough to suspend high soot concentration of 10 wt%. In such systems,

651

secondary or primary soot particles can coalesce to form agglomerates, large enough in creating

652

lubrication starvation at the interface and inducing direct two body wear.

653 654

Figure 15. Effect of soot-antiwear additive interaction on wear scar diameter.

655

In figure 15, it can be seen that all formulations containing anti-wear additives exhibits poor wear

656

performance (i.e. higher WSD values) as compared to the formulations containing only soot and

657

no anti-wear additives. Considering statistical significance, it is safe to say that this behavior is 40 ACS Paragon Plus Environment

Page 41 of 54 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

658

constant at all soot concentrations of 2, 5 and 10 wt%. There are two possible inferences from this

659

observation, ⅰ) soot particles are directly abrading off the formed tribofilms by mechanical action

660

or ⅱ) soot particles are disrupting the formation of tribofilms after initial removal by adsorbing

661

film forming species or accumulating on active steel substrate. This study clearly suggests the

662

prevailing wear mechanism as the direct removal of protective tribofilms by the abrasive soot

663

particles. Salehi et al. 33 also suggested corrosive-abrasive wear mechanism is responsible for high

664

wear observed in fully formulated oils containing carbon black as surrogate for diesel engine soot.

665

In their study, corrosive-abrasive wear was attributed to the removal of protective tribofilms

666

formed on steel substrate through abrasion by carbon black particles.

667

Generation of tribofilms in boundary lubrication regime is a complicated process. In our previous

668

work on ZDDP tribofilms, we have detailed two step tribofilm formation mechanism 78,79. Initially,

669

films (generally iron free) are formed on the steel surface due to deposition/ adsorption of

670

tribochemical reaction products produced by degradation /thermo-oxidative decomposition of

671

additive chemistries. During continued friction and wear process, this film can get disrupted and

672

the active elements (like P, S) present in lubricant chemically reacts with the nascent surface (Fe)

673

generating iron phosphates, iron sulfides/sulfates. These compounds are incorporated in the top

674

layer and are strongly bonded to the nascent steel surface; thus, a sacrificial protective surface film

675

is formed which is commonly known as a tribofilm. Tribofilms are reported to have less hardness

676

than the steel substrate 34,79,91 and hence, can get easily removed by abrasive soot particles. A new

677

tribofilm forms on the newly generated surface and is then again removed by soot particles. Rapid

678

removal of antiwear films can overcome the rate of film formation by antiwear additives and

679

results in severe wear of steel substrate. Therefore, the rate of wear can be said to be indirectly

680

dependent on the availability of a film formed by antiwear additives. 41 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 54

681

The proposed corrosive-abrasive wear mechanism induced due to presence of anti-wear additives

682

and soot particles is also explained with the help of phenomenological model shown in figure 16

683

(B), where soot particles are shown to create deep abrasive grooves (marked by red line) by

684

removal of tribofilms (marked by yellow line). Figure 16(A) shows the three body soot induced

685

wear taking place when soot particles and dispersant based lubrication is present at the interface.

686 687

Figure 16. Schematic diagram explaining soot induced wear mechanism with lubrication

688

containing (A) dispersant and soot; (B) anti-wear additives, dispersant and soot.

689

4. Conclusion

690

The variations in the carbonaceous structure and chemical composition of crankcase soot were

691

mapped using XANES and HR-TEM. XANES analysis demonstrated the presence of zinc

692

polyphosphate, calcium sulfate, calcium phosphate, and iron sulfate and phosphate. Identification

693

of short chain zinc polyphosphate suggested the interaction of soot with the antiwear tribofilm. 42 ACS Paragon Plus Environment

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Energy & Fuels

694

HR-TEM results showed the nanocrystalline particles of hydroxyapatite and carbonate hydroxyl

695

apatite embedded in the periphery of the turbostratic carbon structure of the soot. It was concluded

696

that the mechanical embedment of the hard nanocrystalline particles during the three-body contact

697

of soot particles in the crankcase oil altered its structure and could make it more abrasive in nature.

698

In this study, a different approach using high temperature XRD was used to depict and correlate

699

the oxidation stability with interplanar lattice spacing changes of soot. Results from XANES, HR-

700

TEM, HT-XRD, BET techniques and EDS analysis of residue (left behind after oxidation)

701

indicated that interaction of lubricant additives and crankcase soot extracted from Mack-T12

702

dynamometer engine test influences its structure and chemistry, which in turn affects its ease of

703

oxidation.

704

This research work also focused on understanding the detrimental effect of increased levels of soot

705

in lubricating oil on antiwear properties of additives and elucidating the mechanism by which

706

abrasive soot promotes high wear in boundary lubrication conditions. The proposed dominant wear

707

mechanism is the abrasion wear due to rapid removal of the protective antiwear tribofilms by the

708

abrasive soot particles/agglomerates. ZDDP exhibited the worst performance at higher level of

709

soot while ionic liquid, DEDTP exhibited better anti-wear performance. This indicates that ionic

710

liquids may serve as a secondary antiwear protection for fully formulated diesel engine lubricants.

711 712

Acknowledgments

713

The authors appreciate Dr. Ewa Bardasz assistance in providing drained oil from a Mack T-12

714

engine test. We also appreciate Chevron, BASF, Sigma Aldrich and AC2T research GmbH

715

(Austrian COMET-Program K2 XTribology, project no. 849109) for providing with required

716

dispersants and anti-wear additives for this study. The XANES experiments were conducted at the 43 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 54

717

Canadian Light Source, which is supported by NSERC, NRC, CIHR, and the University of

718

Saskatchewan. Experimental facilities for HR-TEM, SEM and EDS provided by the Center of

719

characterization for materials and biology (CCMB), University of Texas at Arlington are gratefully

720

acknowledged.

721

Supporting Information

722

XANES P L-edge FY, P K-edge TEY, S L-edge TEY and FY and Zn L-edge TEY and FY spectra

723

are supplied as the supporting information. Additionally, EDS mapping results exhibiting removal

724

of antiwear tribofilms due to soot induced wear are available for reference.

725

References

726

[1] Xi J, Zhong BJ. Soot in Diesel Combustion Systems. Chem Eng Technol 2006;29:665-73.

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[2] Daido S, Kodama Y, Inohara T, Ohyama N, Sugiyama T. Analysis of soot accumulation

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inside diesel engines. JSAE Review 2000;21:303-8.

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[3] Green D.A, Lewis R, Dwyer-Joyce R.S. Wear effects and mechanisms of soot-

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contaminated automotive lubricants. Proceedings of the Institution of Mechanical

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Engineers, Part J: Journal of Engineering Tribology 2006;220(3):159-69.

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[4] Bagi S, Bowker R, Andrew R. Understanding chemical composition and phase transitions

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of ash from field returned DPF units and their correlation with filter operating conditions.

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[5] Bagi S, Singh N, Andrew R. Investigation into Ash from Field Returned DPF Units:

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Composition, Distribution, Cleaning Ability and DPF Performance Recovery. No. 2016-

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01-0928. SAE Technical Paper, 2016.

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[6] Chiñas-Castillo F, Spikes H. The behavior of diluted sooted oils in lubricated contacts. Tribol Lett 2004;16(4):317-22. 44 ACS Paragon Plus Environment

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a heavy-duty diesel engine using analytical ferrography. SAE Technical paper

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[9] Colacicco P, Mazuyer D. Role of soot aggregation on the lubrication of diesel engines. Tribology Transactions 1995;38:959-65. [10] Gautam M, Durbha M, Chitoor K, Jaraiedi Mea. Contributions of Soot Contaminated Oils to Wear. SAE Technical Paper 1998. [11] Green D.A, Lewis R. The effects of soot-contaminated engine oil on wear and friction: A review. Proc Inst Mech Engineering Pt D: J Automobile Engineering 2008;222:1669-89.

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[12] Sharma V, Uy D, Gangopadhyay A, O'Neill A, Paxton W, Sammut A, Ford M, Aswath P.

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Structure and chemistry of crankcase and exhaust soot extracted from diesel engines.

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Carbon 2016;103:327-38.

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[13] Patel M, Aswath P. Structure and chemistry of crankcase and cylinder soot and tribofilms on piston rings from a Mack T-12 dynamometer engine test. Tribol Int 2014; 77:111-21.

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[14] Uy D, Ford MA, Jayne DT, O’Neill AE, Haack LP, Hangas J. Characterization of gasoline

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soot and comparison to diesel soot: Morphology, chemistry, and wear. Tribol Int

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[15] Ishiguro T, Takatori Y, Akihama K. Microstructure of diesel soot particles probed by

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and Emissions of a Heavy-Duty Diesel Engine. Society of automotive engineers 2003.

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[18] Ajayi O, Erck R, Erdemir A, Fenske G, Goldblatt I. Effect of exhaust gas recirculation

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(EGR) on diesel engine oil- Impact on wear. 14th DEER Conference, MI, 2008;4-7.

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[19] Sasaki M, Kishi Y, Hyuga T, Okazaki K, Tanaka M, Kurihara I. Effect of EGR on diesel

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engine oil, and its countermeasures. Proceedings of the 1997 International Spring Fuels &

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[20] Nagai I, Endo H, Nakamura H, Yano H. Soot and valve train wear in passenger car diesel engines. SAE Technical Paper 1983;831757.

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[21] Bagi S, Sharma V, Patel M, Aswath P. Effects of diesel soot composition and accumulated

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vehicle mileage on soot oxidation characteristics. Energy and Fuels 2016; 30(10):8479-90.

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[22] Song J, Alam M, Boehman A, Kim U. Examination of the Oxidation Behavior of Biodiesel

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Soot. Combust Flame 2006;146:589-604. [23] Lu T, Cheung CS, Huang Z. Effects of Engine Operating Conditions on the Size and Nanostructure of Diesel Engine Particles. J Aerosol Sci 2012;47:27-38.

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Emissions from a Diesel Engine Fueled with Three Fuels. Sci Total Environ 2012;438:33-

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[26] Vander Wal RL, Tomasek AJ. Soot Oxidation: Dependence Upon Initial Nanostructure. Combust Flame 2003;134(1):1–9. [27] Vander Wal RL, Tomasek AJ. Soot nanostructure: Dependence upon synthesis conditions. Combust Flame 2004;136:129-140.

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nanostructure of soot from a direct-injection diesel engine. Energ Fuels 2006;20(6):2364-

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[33] Salehi F, Khaemba D, Morina A, Neville A. Corrosive–Abrasive Wear Induced by Soot in

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Boundary Lubrication Regime. Tribol Lett 2016; 63-19. [34] Olomolehin Y, Kapadia R, Spikes H. Antagonistic Interaction of Antiwear Additives and Carbon Black. Tribol Lett 2010; 37:49-58. [35] Uy D, O'Neill A. E, Simko S. J, Gangopadhyay, A. K. Soot–additive interactions in engine oils. Lubrication Science 2010; 22:19–36. [36] George S, Balla S, Gautam M. Effect of diesel soot contaminated oil on engine wear. Wear 2007; 262:1113-22.


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of XANES spectroscopy to tribology. Canadian Journal of Chemistry 2007; 85:816-30.

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