Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model

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Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model Glasses Fabiana Spadaro, Antonella Rossi, Shivaprakash N. Ramakrishna, Emmanuel Lainé, Philip Woodward, and Nicholas D. Spencer Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01795 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model Glasses F. Spadaro1, A. Rossi1,2, Shivaprakash N. Ramakrishna1, E. Lainé3, P. Woodward3 and N. D. Spencer1* 1

Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-

Prelog-Weg 5, CH-8093 Zurich, Switzerland 2

Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, Cittadella

Universitaria di Monserrato, I – 09100 Cagliari, Italy 3

Enabling Research, Infineum UK Ltd., Milton Hill, Steventon, Oxfordshire OX13 6BD, UK

* Corresponding author, email: [email protected]

Abstract The discovery of the spontaneous reaction of boric oxides with moisture in the air to form lubricious H3BO3 films has led to great interest in the tribology of boron compounds in general. Despite this, a study of the growth kinetics of H3BO3 on a B2O3 substrate under controlled relative humidity (RH) has not yet been reported in the literature. Here, we describe the tribological properties of H3BO3-B2O3 glass systems after aging under controlled relative humidity over different lengths of time. A series of tribological tests has been performed applying a normal load of 15 N, at both room temperature and 100°C in Yubase 4 oil. In addition, the cause of H3BO3 film failure under high-pressure and hightemperature conditions has been studied to find out whether the temperature, the tribostress or both influence the removal of the lubricious film from the contact points. The following techniques were exploited: confocal Raman spectroscopy to characterize the structure and chemical nature of the glass systems; environmental scanning electron microscopy (ESEM) to examine the morphology of the H3BO3 films developed; atomic force microscopy (AFM) to monitor changes in roughness as a consequence of the air exposure; focused-ion-beam scanning electron microscopy (FIB-SEM) to measure the average thickness of the H3BO3 films grown over various times on B2O3 glass substrates and to reveal the morphology of the sample in vertical section, tribological tests, to shed light on the system’s lubricating properties and, finally, small-area X-ray photoelectron spectroscopy to investigate the composition of the transfer film formed on the steel ball while tribotesting.

Keywords: H3BO3-B2O3 glass systems, ESEM, FIB-SEM, AFM, XPS, tribological tests

ACS Paragon Plus Environment

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1. Introduction Boron-containing compounds, thanks to their versatile chemistry and multifunctional character have the potential to overcome many challenges in current lubrication technologies. They are suitable for use as solid lubricants, liquid lubricants, lubricant additives, and coatings. In addition, they are economical and environmentally friendly [1,2]. These characteristics have spurred on the investigation of boron compounds for automotive and industrial lubrication systems. Boron oxides mainly consist of boroxol rings (Figure 1a)) [3,4], but as a consequence of their exposure to atmospheric moisture, a spontaneous reaction between B2O3 and water occurs, leading to the formation of superficial boric acid films (see Equation 1). ½ B2O3 + 3/2 H2O H3BO3

∆H298= -45.1 kJmol-1

Equation 1

The boric acid is characterized by a lamellar structure, the oxygen, boron and hydrogen in-plane atoms being covalently bonded. The layers themselves are widely spaced and held together by van der Waals’ forces (Figure 1b)) [5,6]. O

a) Boron oxide (B2O3) structure

B O O

B

O

B O O

Boroxol rings b) Boric acid (H3BO3) structure Oxygen Boron

Weak interactions between lamellae

Hydrogen Hydrogen bonds

Lamellar solid

Figure 1. Structures of a) boron oxide and b) boric acid, adapted from [1]

Boron-containing industrial coatings, such as boron carbide (B4C) and boron suboxides (BxO, x>1) are generally hard and find numerous technical applications [7,8]. Boron carbides are used in grinding wheels for sharpening cutting tools, as super-abrasives for polishing and grinding, and as fibers to reinforce ceramic composites, while B22O even has the capability to scratch diamond (111) faces [9]. Erdemir et al. [5], have reported that the surface of B4C undergoes an oxidation process upon annealing


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in air at 800°C for 1 h. This occurs because the thermally activated boron and carbon atoms react with oxygen to form B2O3 and CO2. B2O3, when exposed to air, spontaneously reacts with moisture and forms a H3BO3 film. In previous studies, the same authors demonstrated the self-lubricating mechanism of H3BO3 films formed on B2O3 and the low friction coefficient that these H3BO3 films provide when sliding against metallic and ceramic surfaces [10]. Tribological tests carried out on annealed B4C with a H3BO3 film displayed significantly lower friction (0.04) and wear (3×10-7 mm3N1

m-1) coefficients [10] than those performed on pristine B4C, which exhibited a substantial increase in

the coefficient of friction during the sliding process up to a value of 0.7. Additionally very high wear values (∼2.9×10-5 mm3N-1m-1) were observed on the 440C steel countersurfaces. Cuong et al. [11] studied the effect of relative humidity on the lubricating properties of boron carbide against a steel ball. The formation of a boric acid coating on boron carbide under tribological conditions in the presence of 45% and 85% relative humidity conditions led to low friction and reduction of wear loss on the steel ball and on the boron carbide coating [11]. Boron sub-oxides (BxO, x>1) have been poorly investigated as abrasion-resistant hard coatings, although they merit attention [12]. Elemental boron can oxidize to B2O3 and react with moisture to form H3BO3. Thus, they form attractive tribological, self-renewing systems consisting of a hard coating with an integral superficial lubricating layer. Boriding, similar to nitriding, is a common surface-diffusion treatment for ferrous alloys, which is able to increase the hardness and the adhesive-, abrasive- and corrosive-wear resistance of the material. Boriding can be performed using techniques such as plasma boriding, low energy-ion implantation, or spark plasma sintering [13,14]. Boron atoms, because of their small size, can readily diffuse in ferrous

alloys to form FeB and Fe2B. Borided steel surfaces are generally characterized by improved wear resistance, but still display high coefficients of friction against steel and other engineering alloys [15]. In 1995, Bindal and Erdemir [16] proposed “flash annealing” for borided steel, which consisted of the high-temperature (600-800 °C) exposure of the sample for 3 to 8 min, followed by cooling to room temperature in air. This process provides enough energy to the boron atoms in the borided layer to diffuse towards the surface, react with oxygen in the air and form a thin boron oxide layer. This layer, during cooling, reacts with moisture and forms a thin, lubricious boric acid film. Tribological tests using Si3N4 on borided samples show that the friction coefficient is initially low (about 0.1), but increases with sliding distance, to reach 0.5. On flash-annealed borided samples, however, the


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coefficient of friction is initially 0.07 and during sliding it decreases further, leveling off at 0.06. These results confirm that the presence of a boric acid film between the two sliding surfaces is able to provide low friction [16]. In X-ray photoelectron spectroscopy (XPS), the B 1s photoelectron signals attributed to boric oxide and boric acid fall at very similar binding-energy values, and thus it is not possible to distinguish between the two species [11]. Raman spectroscopy, on the other hand, is an ideal technique to characterize the structure and the chemical composition of B2O3-H3BO3 systems, because it readily allows the discrimination between the two major components. Atomic force microscopy (AFM) allows us to follow topographical changes occurring as a consequence of exposure to moisture for different time intervals. Focused-ion-beam scanning electron microscopy (FIB-SEM) [17] has also been adopted for the investigation of B2O3-H3BO3 systems. It allows the evolution in the thickness and in the morphology of the grown interface H3BO3/B2O3 glass to be monitored following different incubation times at controlled relative humidity. Environmental scanning electron microscopy (ESEM) is another appealing technique to study insulating hydrated samples [18-21]. We have exploited this technique to collect high-resolution images of the surface, avoiding the conductive coating and high vacuum necessary for conventional SEM, which could both influence the morphology of B2O3-H3BO3 systems. In analogy with previous studies performed on zinc and iron polyphosphates [22,23], to investigate the lubricating properties of B2O3-H3BO3 systems, friction tests have been carried out at room temperature and at 100°C in Yubase 4 oil. The tests were performed in the presence of base oil in order to limit the growth of H3BO3 during the tests themselves, due to the humid-air exposure, and to simulate the situation occurring at the component interfaces (e.g. during forming of aluminum alloy sheets [24]). We report an increase in the roughness of B2O3 glass due to the formation of H3BO3, the morphology of the developed H3BO3 film, the growth kinetics of H3BO3 and the tribological properties of these systems in oil at room temperature and at 100°C. Moreover, by means of a series of tribological tests performed inside and outside a glovebox at room temperature and at high temperature, we have investigated whether the failure of H3BO3 is caused by temperature, by tribological effects or by a combination of both effects.


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2. Experimental 2.1 Synthesis B2O3 glass discs were prepared by the classical melt-quenching technique using pure reagent-grade compound H3BO3 (Fluka, Chemika, >99.8% ). B2O3 glass

2H3BO3 B2O3+ 3H2O

T= 1200 °C, time= 2 hours

The powdered H3BO3 was melted in air in a platinum crucible (Fine-Grain Stabilized Marvel Pt/Au 95/5% produced by Ögussa, Wien, Austria) in a RHF 16/3 furnace (Carbolite®, Hope Valley, UK). The furnace temperature was increased at a rate of 2 °C/min to 1200 °C and maintained at this value for a heating time of 2 hours. The 2 °C/min heating rate was chosen, in order to avoid the formation of small gas bubbles on the glass surface. Then, the melt was quenched in a copper tray that had been previously cooled to liquid-nitrogen temperature. After quenching, the glass samples were annealed for at least 12 hours at 200 °C to avoid the glass splintering during the subsequent treatment steps (mechanical polishing and tribological tests). All glass samples were in the shape of round discs, 2 cm in diameter and 5 mm in thickness. B2O3 glasses are colorless and transparent as synthesized, but become dull and cloudy after a few minutes of environmental air exposure.

2.2 X-ray diffraction (XRD) The X-ray diffraction measurements of the as-prepared borate glasses were carried out with a Stoe Stadi P Powder Diffractometer (STOE & Cie GmbH, Germany) using Cu Kα radiation (λ =1.54056 Å) at room temperature.

2.3 Mechanical polishing The B2O3-H3BO3 glass discs were ground using grit 320, 600, 1200 and 2000 silicon carbide papers (Struers GmbH, Birmensdorf, Switzerland) on a rotating polishing wheel (Jean Wirtz Phoenix 4000, Wirtz-Buehler GmbH, Düsseldorf, Germany). Subsequent polishing was performed using diamond paste (Struers GmbH, Birmensdorf, Switzerland) of different grain sizes (3, 1 and ¼ µm) on polishing cloths (Struers GmbH, Birmensdorf, Switzerland). The solvent adopted for cooling, lubricating and cleaning the samples was (p.a.) ethanol. Mechanical polishing was performed in order to obtain a


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smoother surface and a reproducible starting roughness. 2.4 Controlled-humidity chamber In order to monitor the growth of H3BO3 films on B2O3 glass discs, the samples were incubated after polishing in a well-sealed chamber and exposed to a controlled humidity of 48%. A saturated salt solution of K2CO3 (Sigma-Aldrich, >99.0%) was used to maintain a constant relative humidity inside the chamber over a wide range of temperatures (T = 20-30 °C) [25].

2.5 Atomic force microscopy (AFM) AFM analysis was performed on freshly polished B2O3 glass samples exposed to RH= 48%, in order to observe the topographical changes occurring on its surface as a consequence of the moisture exposure. AFM tapping-mode images were acquired using a NaioAFM (Nanosurf, Switzerland) with a Tap 190 Al-G silicon cantilever (BudgetSensors, Bulgaria) with a spring constant of ∼48 N/m (manufacturer’s value). The humidity inside the analysis chamber was maintained at RH= 48% by means of a saturated K2CO3 solution (see further details in Section 2.4). AFM images were continuously acquired over a time interval of 8 hours over the selected area, but in the following only the images acquired after each 90 minutes are presented. In this way, it was possible to follow the topographical evolution on the sample surface with time of humidity exposure. 2.6 Confocal Raman microscopy

A WITec Confocal Raman Microscope 200 (WITec, Ulm, Germany) was used, with a laser as a light source (wavelength: 532.14 nm) and a spatial resolution down to 200 nm laterally and 780 nm vertically. The measurements were performed at room temperature and under ambient conditions, with a laser power on the sample of 1.95 mW and using a 100x objective with 0.8 N.A. (numerical aperture).

2.7 Environmental scanning electron microscopy (ESEM)

An FEI Quanta 200 FEG environmental scanning electron microscope (ESEM) (Thermo Fisher Scientific, USA) was used to collect high-resolution, low-vacuum (130 Pa) secondary-electron (SE) images of an aged B2O3-H3BO3 glass sample. The ESEM was operated while introducing controlled amounts of gases, such as water vapor, into the sampling chamber and avoiding high-vacuum


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conditions. Moreover, it was possible to analyze both non-conductive and hydrated specimens in a nondestructive manner, without resorting to the use of a conductive coating.

2.8 Focused-ion-beam scanning electron microscopy (FIB-SEM)

Cross sections of the glass sample surface were obtained by means of a NVision 40 FIB-SEM workstation (Carl Zeiss, Germany) to measure the average H3BO3 thickness developed on different samples incubated for different aging times at a controlled relative humidity (RH = 48%). Each glass sample was coated with a gold layer of 10 nm to avoid sample charging. Moreover, in the vacuum chamber, before the milling operation, a second protective platinum coating was deposited by in situ FIB-assisted chemical vapor deposition over the area of interest for the cut. This latter operation is of fundamental importance to better preserve the sample surface during cutting. Cross sections of the sample surface were obtained by using a focused gallium ion beam. At the end of the cutting process, high-resolution SEM micrographs were collected using a conventional secondary-electron detector.

2.9 Tribological tests A UMT-2 tribometer (Bruker (CETR), Campbell, CA, USA) was used in ball-on-disc configuration to investigate the tribological properties of the glasses. The tribological tests were performed in rotational motion. Prior to each tribological test, a running-in of the 100Cr6 hardened-steel ball (radius= 2 mm) was carried out, by applying a 15 N load at a speed of 56.5 mm/min for 60 min on 100Cr6 steel discs (hardened) at room temperature in Yubase 4. The aim of the running-in is to obtain conformal surfaces on both sides of the contact: the disk and the steel ball. The radius of the worn area on the steel ball at the end of the running-in was measured by optical microscopy (average radius = 85(5) µm). Afterwards, the steel ball was used for carrying out the tribotest on a polished B2O3-H3BO3glass disc in Yubase 4. All tribological tests were performed at a normal load of 15N (average applied pressure 660MPa), at a sliding speed of 26.4 mm/min, for 120 min, at two different temperatures: ambient temperature (∼25°C) and 100°C, on B2O3-H3BO3 glass discs incubated for different aging times. The tests at room temperature were performed on samples aged for 1 day, 15 days and 60 days, whereas the tests at 100°C were carried out on samples aged for 1 day, 25 days and 60 days. The relative humidity was recorded during each test and always was between 32 and 50%. At the end of the test, the disc sample and the steel ball were washed in n-hexane in an ultrasonic bath for 5 minutes and gently dried


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with a powder-free tissue paper.

2.10 Heating tests

In order to evaluate the temperature effect on B2O3-H3BO3 glass, after being incubated at a relative humidity of 48% for 30 days, a 30-day-old sample was immersed for two hours in preheated (100°C) Yubase 4 oil, without stirring or purging. At the end of this time, the sample was removed from the oil, cleaned with n-hexane as described in the previous section (2.9 Tribological tests) and characterized by Raman spectro-microscopy. Afterwards, the sample was incubated for another week at RH= 48% and at the end of this time reanalyzed by Raman spectro-microscopy. A similar heating test was performed in the oven, in order to avoid the immersion of the glass sample in the oil. In this case, the sample was heated for 2 hours at 100°C in the oven. The Raman characterization was carried out before and after the heating test. 2.11 Viscosity measurements

The oil used for the tribological tests is Yubase 4 oil—a highly refined mineral oil composed of petroleum distillates and hydro-treated heavy paraffinics [26]. Viscosity measurements were performed with PHYSICA MCR 501 rheometer (Anton Paar, Switzerland), using concentric cylinders in a double-gap configuration.

2.12 Optical microscopy Optical-microscopy images were taken using an AX10 Imager M1m (Carl Zeiss, Oberkochen, Germany) with objectives from 5 X to 40 X and equipped with a CCD camera.

2.13 Small-area and imaging X-ray Photoelectron Spectroscopy (small-area XPS, i-XPS) XPS analysis was carried out by means of a PHI QuanteraSXM spectrometer (ULVAC-PHI, Chanhassen, MN, USA) equipped with an AlKα monochromatic source, whose beam size ranges from 9 to 200µm. In standard mode, the photoelectrons were collected at an emission angle of 45° and directed to the 32channel detector system. The spectrometer is also equipped with a low-voltage argon ion gun and a sample neutralizer for charge compensation. Calibration is regularly carried out using sputter-cleaned gold, silver, and copper as reference materials according to ISO 15472:2009.


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Survey spectra were acquired in fixed-analyzer-transmission mode, selecting a pass energy of 280 eV, while the high-resolution spectra were collected with a pass energy of 69 eV; the full-width at half maximum of the peak-height of the Ag 3d5/2 signal for high-resolution spectra was found to be equal to 0.72 eV under these analysis conditions. Thanks to the external sample-positioning station, it was possible to select and record analysis areas at high magnification before placing the sample into the vacuum system. Afterwards, X-ray-excited, secondary-electron images (SXI) were collected, in order to determine the topography and to enable the selection of the points where small-area XP-spectra should be acquired on the steel ball. The beam diameter employed for the analysis of the flattened area on the steel ball after tribotesting was of 20 µm. An electron-beam neutralizer was used to compensate possible sample charging. The position of the adventitious aliphatic carbon was checked to be at 285.0 eV. The spectra were processed using CasaXPS (v2.3.17PR1.1, Casa Software Ltd., Wilmslow, Cheshire, UK). Imaging-XPS (i-XPS) was carried out with a beam size of 20 µm and a PE of 112 eV. The full width at half-maximum of the peak height, FWHM, of the silver Ag 3d5/2 signal for the pass energy of 112eV was 0.9 eV. The maps were processed using Multi-PakTM software (version 9.0, ULVACPHI, Chanhassen, MN, USA). Each pixel in the map corresponds to a complete acquired spectrum; the intensity of each spectrum corresponding to the color intensity in the map. A linear-least-squares (LLS) routine was adopted to reconstruct the different chemical-state maps.


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3. Results 3.1 Characterization of H3BO3-B2O3 glass systems

3.1.1 X-ray Diffraction (XRD)

All XRD patterns collected on borate samples that were prepared as previously described in the experimental section, exhibited two characteristic wide broad signals, sometimes referred as halos, with dhkl= 3.9 and 2.0 Å, confirming the amorphous nature of these materials (Supporting Information Figure S1) [27].

3.1.2 Raman Spectroscopy on H3BO3-B2O3 glass systems The Raman spectrum collected on fresh polished B2O3 glass is shown in Figure 2a). The band at 808 cm-1 is assigned to the symmetric breathing vibration of boroxol rings, whereas the weak, broad band around 1260 cm-1 arises from the delocalized B-O stretch involving both the ring and network contributions [10,28,3,29]. The signal at 880 cm-1 indicates the contribution of H3BO3, which can be observed even on fresh polished B2O3 glass samples. Figure 2b) displays the Raman spectrum acquired on a 5-day-old sample. In this case, the bands at 880 cm-1 and 499 cm-1 are quite intense and are attributed to H3BO3. In addition, a weak band at 1170 cm-1 is detected and assigned to H3BO3, while the B2O3 contributions are already negligible at this incubation time [30]. Raman spectra were also obtained for bulk boric acid for comparison (see Figure 2c)) and the bands of this spectrum overlap perfectly with those detected on the 5-day-old sample, Figure 2b).


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100

Raman Intensity (Arb. Unit)

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B2O3 Boroxol rings 480 cm-1

H3BO3 499 cm-1

200


a) B-O stretching 1260 cm-1

H3BO3 880 cm-1

b)

c)

20.103 H3BO3 1170 cm-1

500

1000

Wavenumber (cm-1)

Figure 2. Raman spectra acquired on a) a freshly polished B2O3-H3BO3 glass sample, b) a B2O3-H3BO3 glass sample after 5 days of exposure to RH= 48% and c) boric acid powder (reference spectrum).

3.1.3 Topography of H3BO3-B2O3 glass systems The root-mean-square-roughness (Rq) of the glass surface, immediately after mechanical polishing, was measured by atomic force microscopy. The root-mean-square (Rq) initial surface roughness of the boric oxide glass was found to be 34 (7) nm, Figure 3a, and significantly increased over time, Figures 3 b-g. Immediately after polishing, the surface was characterized by some protrusions, Figure 3a [31], which became larger until they coalesced (see Figures 3 b-f). Figure 3g shows the measured roughness versus the time of exposure to the RH=48%. After 450 minutes, an average roughness value of 114.4(15.2) nm was reached.


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Figure 3. AFM images on B2O3 glass surface a) immediately after the polishing treatment and after b) 90 minutes, c) 180 minutes, d) 270 minutes, e) 360 minutes and f) 450 minutes of exposure at RH%= 48%. In g) measured roughness (Rq) versus time

3.1.4 Morphology of H3BO3-B2O3 glass systems The morphology of the sample surface was investigated by means of environmental scanning electron microscopy (ESEM). This technique allowed the observation of the polished surface of a B2O3-H3BO3 glass sample after being incubated for 19 days at a controlled relative humidity of 48%, avoiding possible artifacts due to conductive coatings or other requirements for sample preparation for highvacuum SEM. Figure 4 shows the presence of a porous layer, formed on top of the glass sample.

Figure 4. ESEM images of a polished H3BO3-B2O3 glass system stored at a controlled relative humidity (RH= 48%) for 19 days a) low magnification, b) and c) high magnification


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3.1.5 Kinetics of H3BO3 growth on B2O3 glass The FIB-SEM technique allows the investigation of three-dimensional (3D) microstructures: the FIB is employed to remove layers of material, while the electron beam is exploited to observe the freshly exposed surface [32]. In the present case, in order to evaluate the H3BO3 thickness and, thus, its kinetics of growth on B2O3 glass, incubated at a specific relative humidity (RH= 48%) for different intervals of time, a series of FIB-SEM measurements was carried out. Figure 5a) displays the FIB sectioning of a 1-day-old sample and allows the observation of the 3D structure. It is evident that already at this stage a thin porous structure attributed to H3BO3 with an average thickness of 0.37(0.25) µm had developed on top of the glass. In Figure 5b), the FIB-SEM image of a 5-day-old sample is shown. The average thickness of the H3BO3 layer is higher (1.00(0.49) µm) and the voids are larger and better defined. These pores seem to be surrounded by a lamellar structure (indicated by red arrows in Fig. 5b), where each lamella is separated by a certain volume from the others. These cavities appear to continue growing and penetrating into the glass below the surface. The H3BO3 films on top of the B2O3 glass have not developed homogeneously. There are points at which they are thick, with holes extended towards greater depth, while in other regions the porous structure is more localized at the surface and is thin. These phenomena are already clearly observable from the FIB-SEM image acquired on a 5-day-old sample (Fig. 5b)), but can be better visualized from the images collected on older samples (Fig. 5c-e)).

Figure 5. FIB-SEM images of polished H3BO3-B2O3 glass samples after being exposed at a controlled relative humidity of 48% for a) 1-day, b) 5-days, c) 15-days, d) 27-days and e) 120-days. The section shown in the pictures allows the identification of the following layers: Pt, Au, H3BO3 and B2O3. A gold layer was applied ex situ to avoid sample charging, while a platinum coating was deposited in situ to better preserve the sample surface during cutting.


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The average values of thickness for the developed H3BO3 film are summarized in Table 1. The inhomogeneous growth of the H3BO3 films results in a high standard deviation.

Time of exposure to RH= 48% (day) 1 5 15 27 120

Average H3BO3 thickness (µm) 0.4 (0.3) 1.0 (0.5) 1.7 (0.5) 2.8 (0.8) 3.2 (0.6)

Table 1. Average thickness of H3BO3 grown on B2O3 glass sample after different aging times incubated at RH= 48%

While the possibility cannot be ruled out that the high-vacuum conditions are influencing the morphology, the fact that these structures are also evident in the environmental SEM (Figure 4) suggests that this is not a major issue. The kinetics of H3BO3 growth is higher within the first 30 days, but as soon as a layer of ∼3µm is grown, the process appeared to slow down. After 120 days of exposure at RH= 48%, only a small increment in thickness (3.23(0.64) µm) was detected in comparison to the 30-day-old sample. In Figure 6, the log of the average thickness values of H3BO3 is plotted versus the log of exposure time of the samples to a relative humidity of 48%. The power fit of these experimental points displays a direct dependence of the H3BO3 thickness on the square root (exponent 0.46) of the exposure time to RH= 48%. This suggests that a diffusion process governs the growth of the H3BO3 layer.

Figure 6. Kinetics of H3BO3 growth on B2O3 glass at RH= 48%. Plot of the log of average H3BO3 thickness versus the log of time of exposure to RH=48%.

3.2 Tribological properties 3.2.1 Tribotests performed at room temperature (∼25°C) and at 100°C


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In order to investigate the lubricating properties of the H3BO3-B2O3 glass samples, after being exposed to a controlled RH= 48% for different intervals of time, tribological tests were carried out at ∼25°C and at 100°C applying a normal load of 15N in Yubase 4 oil, using a 100Cr6 steel ball with a flat contact produced during running-in against a 100Cr6 steel sample. The running in was carried out against a

steel disc, in order to produce a conformal contact. This allows us to run experiments under constant conditions of pressure. In standard ball-on-disk tests the pressure continuously drops as the ball wears. In Figure 7 (left), the curves of the coefficient of friction are displayed for tests carried out at room temperature in Yubase 4 oil for a steel ball against a borate glass disc, previously incubated at RH= 48% for specific intervals of time (a) 1-day, b) 15-days and c) 60-days). Each test was repeated at least 3 times and no significant changes in friction were observed during the 2-hours tests. Since, at room temperature, the coefficient of friction did not exhibit significant changes with sliding time, the average coefficient of friction was calculated and is reported in Table 2. The coefficients of friction recorded for the tribological tests at room temperature on H3BO3-B2O3 glass systems of different ages were very similar within the experimental uncertainty and in the order of 0.07.

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Figure 7. Tribological tests performed at a normal load of 15N , at room temperature (a-c) and at 100°C (d-f) on polished H3BO3-B2O3 glass systems after being incubated at RH= 48% for a), d) 1 day, b), e) 15 days and c), f) 60 days


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The results of the tribological tests performed at 100°C in Yubase 4 using steel ball-versus-borate glass disc tribopairs are shown in Figure 7 (right). Before tribotesting, the borate glass discs were incubated at RH= 48% for d) 1 day, e) 25 days or f) 60 days. The tribological tests performed at 100°C exhibited a substantial increase of the CoF with sliding time. On the basis of the CoF values, different stages for the 2-hour tests can be identified: the experiments carried out on the 1-day-old sample at 100°C (Figure 7c) revealed CoFs 2000 mm and the COFs reached values of 0.6 at the end of the tests.

Tribotests at ∼25°C

Steel versus Borate 1-day-old tribopairs



CoF values 0.07(0.02) 0.07(0.03) 0.06(0.02) Table 2. Average CoF values obtained at room temperature on H3BO3-B2O3 glass samples of different ages: 1 day old, 15 days old and 60 days old

The tribological test carried out on 25- and 60-day-old samples (Figures 7b and c) displayed a similar behaviour to the 1-day-old sample within the first 600 mm of sliding distance, with CoF values 2000 mm, whereas the 25-day-old (H3BO3 thickness 2.83(0.82) µm) and 60-day-old (H3BO3 thickness 2.83(0.82)3.23(0.64) µm) samples displayed CoF values ∼0.4 at a sliding distance of 3100 mm. Moreover, it was observed that a partial dehydration of H3BO3 to HBO2-III occurs during the tribotests at 100°C (see Raman characterization after tribotesting at 100°C, Figure 9). The influence of HBO2-III formation on the CoF has been investigated and the data are provided in Supporting Information, Section S2. The HBO2-III layer was formed following heat treatment of the glass sample at 100°C for 2 hours. Following the Raman characterization to ascertain the presence of HBO2-III, a tribotest at room temperature was carried out on it. The increase in the CoF was negligible and occurred at the end of the test (see Supporting Information, Section S2). Thus, this phenomenon can have an influence but it cannot be the only determining factor to explain these observations. Furthermore, the structural similarities between H3BO3 and HBO2-III were studied by Zachariasen [52] and Bertoluzza et al. [35]; all boron atoms in both structures are three-coordinated, and the BO3 groups are arranged in pseudohexagonal layers with van der Waals interactions between the layers. A plausible explanation to justify the failure of H3BO3 coating might be related to a significant decrease in the adhesion of this soft coating due to the high temperature. This is supported by the Raman spectra collected on the glass samples before and after the tribotests outside the contact areas (Figures 9a-c before the tribotest 1) and out of contact 3)). It is evident that, before the tribotests, the intensity of the B2O3 signal was negligible, while at the end of the tribotest, due to the exposure at 100°C for 2 hours, this contribution appears to be detectable. This situation occurs similarly within the tribostressed regions, where not only the temperature effects, but also the mechanical stress encourage this phenomenon (Figures 8a-c, contact area1)).


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The heat generated in the contact during sliding may raise the temperature to values even higher than 100°C, which might encourage not only the detachment of the H3BO3, but also its dehydration. It might be possible that the detached H3BO3, localized at the interface of the rubbing contacts, experiences, under these conditions, complete dehydration to B2O3. The B2O3 does not present a layered structure. In addition, another parameter that cannot be neglected is the decrease in oil viscosity at the high temperature, rendering the experimental conditions more severe (Table 3). The decrease of the viscosity from 29.83(0.06) mPa⋅s at 25°C to 3.39(0.05) mPa⋅s at 100°C suggests that at this temperature the liquid may be easily squeezed out and asperity-asperity contact may occur, causing the detachment of the coating. Although experimental evidence is absent, the formation of HBO2-II and HBO2-I cannot be ruled out. These are harder and denser than the previously described HBO2-III, because the boron atoms are partially or totally present in a tetrahedral configuration and the hydrogen bonds can form links between chains of different layers as well as between chains of the same layer. Bertoluzza et al. [35] obtained pure HBO2-II by dehydrating recrystallized H3BO3 at 140°C for 78 h, using a partially open tube to prevent evaporation of water, while the pure HBO2-I was obtained by melting HBO2-II in a sealed tube at a temperature of around 200°C, and a few HBO2-I crystallites were formed in the melt. Five months were required for the complete crystallization of the melt into HBO2-I. These experimental conditions are quite different to that within the contact points while tribotesting at 100°C; furthermore, no experimental evidence confirms their formation; nevertheless we cannot exclude the possible formation of these compounds in small amounts. Thicker H3BO3 coatings exhibit lower friction values than thinner H3BO3 coatings (CoF values of about 0.4 for 25-day-old and 60-day-old samples instead of about 0.6 measured on a 1-day-old sample at the end of the tribotest). These results indicate the poorer tribological performance of H3BO3-B2O3 glass in oil at high temperature, compared to its behavior at room temperature.

4.3.1 Mechanism acting at the contact interface at 100°C The formation of a transfer film of iron borate is observed when tribotesting at 100°C. The durability and stability of this film on the steel ball are weakened by the high temperature and the rubbing process (Figure 17).


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Furthermore, the high temperature encourages the conversion of H3BO3 into HBO2-III, but also the detachment and the squeezing out of H3BO3 from the contact points (higher CoF). These phenomena hinder the effective lubricating mechanism that acts at room temperature. Higher initial thickness of H3BO3 guarantees better tribological performance than thinner H3BO3 layer (see Figures 7d-f)).

Figure 17. Possible mechanism acting at the interfaces of H3BO3-B2O3 glass system and steel ball in presence of Yubase 4 oil at 100°C

The results obtained on the model borate glasses allow us to explain tribological performance when tribotests are performed in the presence of borate additives at 100 °C [42,51,53]. In this case the in situ formation of iron borate hinders the growth of a boric acid layer. This might explain the higher resistance to failure of the borate films formed in situ vs that measured on the bulk borate glasses, where the amount iron ions is insufficient to avoid the formation of boric acid.

4. Conclusions The results confirm the relevance of H3BO3-B2O3 glasses as model compounds to aid in the understanding of the properties of protective and lubricating superficial films developed on boronbased materials. They also allow the interpretation of tribochemistry of complex tribofilms formed under mechanical conditions in the presence of boron-based engine oil formulations. The conclusions can be summarized as follows: -

The H3BO3 layer developed on polished B2O3 glass following its spontaneous reaction with moisture induces an increase in roughness (AFM analysis). This layer is characterized by a


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porous structure (ESEM analysis), where each pore is surrounded by lamellar walls (FIBSEM images). The H3BO3 growth kinetics at RH= 48% are faster up to 30 days exposure, at which point the film reaches a thickness of about 2.83(0.82) µm. Afterwards, the diffusion process slows down (after 120 days, the thickness is about 3.23(0.64) µm). Moreover, it was observed that the growth of the soft, fluffy H3BO3 layer on top of the B2O3 glass does not develop uniformly within the sample; this may be due to the existence of preferential diffusion paths for the H2O molecules in the porous layer. -

Tribological tests between H3BO3-B2O3 glasses and steel balls, carried out at room temperature, showed low CoFs. The formation of a protective iron borate film within the contact area on the steel ball and the availability of the H3BO3 on the glass disc constitute a system able to ensure low friction.

-

The tribological tests performed at 100°C exhibited a worse behavior in terms of CoF. These results were caused by a partial surface detachment of H3BO3, assisted by the rubbing process at high temperature. The dehydration of H3BO3 together with the possible formation of harder HBO2–II, HBO2–I, species under tribological conditions may be the origin of the poor lubricating performance of the systems at high temperature. Nevertheless, it appeared that thick (about 3 µm) H3BO3 layers are better able to survive these conditions, possibly due to their ability to partially embed the by products generated during the rubbing process.

These results corroborate the importance of the H3BO3 layer characteristics (thickness, porosity, surface roughness) developed naturally on boron-based materials in air, and the relevance of the experimental conditions (temperature, lubricating oil, thickness of the lubricious film) for the tribological performance of H3BO3 films. The understanding of the poor tribological performance of H3BO3 films in oil at high temperature and under severe tribological stress was worthwhile to exclude the presence of H3BO3 in all tribofilm compositions formed in presence of boron-base engine oil formulations and characterized by good lubricating performance. Furthermore, these findings pointed out how a smart control of the compositions of the boron-based materials allows the achievement of outstanding lubricating performance under different experimental conditions.

Acknowledgments


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The authors would like to thank Infineum UK Ltd. for financial support of this project. The authors thank Prof. R. Spolenak and his collaborator A. Wyss for the providing access to their Raman microscopy instrument, Dr. P. Fischer, from the Institute for Food Science and Nutrition at ETH Zurich, for giving us the possibility to carried out the viscosity measurements and the service facilities of the Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich for help in performing ESEM and FIB-SEM measurements.

Supporting Information Available: XRD patterns collected on B2O3 glass sample, the CoF values on 30-days-old H3BO3-B2O3 glass sample after being heating treated in Yubase 4 at 100°C for 2 hours, the Raman spectra and the protocol for the X-ray photoelectron spectroscopy characterization of the B2O3 glass system as a reference sample.

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a) Boron oxide (B2O3) structure

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808 cm

-1

B-O stretching 1260 cm-1

H3BO3 880 cm-1

b)

c)

20.103 H3BO3 1170 cm-1

ACS Paragon Plus Environment 500

1000

Wavenumber (cm-1)

Langmuir

m

x:

m

23

3

:2

m

m

x:

m

23

m

y:

23

m

Page 42 of 57m

m

x:

m

23

m

Roughness (Rq, nm)

y 1 a) b) c) 2 3 m m 4 m m x x5 : m :2 x: 23 m 3 23 3 3 2 m 2 y: 6 m m y: 7 d) f) e) 8 140 9 120 10 100 11 80 12 60 13 40 14 20 0 15 ACS Paragon Plus Environment 0 100 200 300 400 500 16 Time (min) 17 g)

3 :2

m

y

m m

y:

23

m

Page 43 of 57 1 2 3 4 5

Langmuir

10 m

a)


1 m

3 m

b)

c)

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

2 m

2 m

a)

Page 44 of 57

b)

2 m

d)

ACS Paragon Plus Environment 2 m e)

2 m

c)

Log (Average H3BO3 Thickness, µm)

10 45 of 57 Page

Langmuir

6 4

1 2 2 1 3 6 4 4 5 2 60.1 7 1 8

y = 0.46x0.46 R2 = 0.94


4

6 8

10

2

4

6 8

Log (time, days)

100

2

4

6 8

1000

Tribotests at 25°C 0.8


Langmuir

0.8

a)

CoF

0.6

Tribotests at 100°C

Page 46 of 57

0.6

CoF

2800

CoF

e)

2800

f)

CoF

CoF

10.4 0.4 2 0.2 0.2 3 0.0 40.0 400 1200 2000 2800 400 1200 2000 5 Sliding distance (mm) Sliding distance (mm) 60.8 0.8 Incubated 25-days at RH= 48% b) 7 Incubated 15-days at RH= 48% 0.6 80.6 0.4 90.4 10 0.2 0.2 11 0.0 0.0 12 400 1200 2000 400 1200 2000 2800 Sliding distance (mm) 13 Sliding distance (mm) 14 0.8 0.8 Incubated 60-days at RH= 48% 15 Incubated 60-days at RH= 48% c) 0.6 0.6 16 0.4 0.4 17 18 0.2 0.2 ACS Paragon Plus Environment 19 0.0 0.0 20 400 1200 2000 2800 400 1200 2000 Sliding distance (mm) Sliding distance (mm) 21

CoF

d)


2800

Page 47 of 57 500

Raman Intensity

Raman Intensity

Raman Intensity

1 2 3 4 5 500 6 7 8 9 10 11 12 13 500 14 15 16 17 18 19 20 500 21 22 23 24 25 26 27 28 29200 30 31 32 33 34 35500 36 37 38 39 40 41 42

Langmuir 1)


B-O stretching 1260 cm-1

Contact area

2)

H3BO3 880 cm-1

H3BO3 500 cm-1

a) 1-day

Out of contact H3BO3 1170 cm-1

500

750

1000

1250

Wavenumber (cm-1)

1)

H3BO3 880 cm-1 B2O3 H3BO3 500 cm-1 Boroxol rings 808 cm-1

b) 15-days Contact area

2) Out of contact H3BO3 1170 cm-1 500

750

1000

B2O3 Boroxol rings 808 cm-1 B2O3 -1 480 cm H BO

H3BO3 880 cm-1

Wavenumber (cm-1)

3

1250

1) B-O stretching 1260 cm-1

3 -1

500 cm

c) 60-days Contact area

2) Out of contact H3BO3 1170 cm-1


750

1000

Wavenumber (cm-1)

1250

1000

HBO2-III 600 cm-1 B2O3 480 cm-1

Raman Intensity

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

B2O3 808 cm-1

H3BO3 880 cm-1

Langmuir1)

Contact area

2)

1000

Out of contact

3)

1000 H3BO3 500 cm-1

500

1000

Raman Intensity

Page 48 of 57 a) 1-day

HBO2-III 600 cm-1

Before the tribotest

H3BO3 1170 cm-1

750

1000

Wavenumber (cm-1)

1250

1)

B2O3 808 cm-1

b) 25-days Contact area

2)

1000

Contact area 3)

1000

Out of contact 1000

H3BO3 880 cm-1

H3BO3 500 cm-1

500

HBO2-III 600 cm-1

750

B2O3 808 cm-1

1000

Raman Intensity

1000

Wavenumber (cm-1)

H3BO3 1170 cm-1

4) Before the tribotest

1250

1)

H3BO3 880 cm-1

c) 60-days Contact area

2)

1000 H3BO3

Out of contact

499 cm-1

3)

1000 H3BO3 1170 cm-1


750

1000

Wavenumber (cm-1)

1250

Before the tribotest

Page 500 49 of 57

H BO Langmuir880 cm


3

c)

3 -1

Raman Intensity

1 2 3 500 b) HBO -III 4 600 cm 5 6 7 a) 1000 8 H BO 9 500 cm H BO 1170 cm 10 ACS Paragon Plus Environment 11 500 750 1000 1250 12 Wavenumber (cm-1) 13 2

3

-1

3 -1

3

3

-1

LangmuirH BO Page 50 ofb)57

400

3

3

880 cm-1 HBO2-III 600 cm-1


Raman Intensity

1 2 3 4 a) 5 500 6 7 H BO 500 cm H BO 8 1170 cm 9 ACS Paragon Plus Environment 10 500 750 1000 1250 11 Wavenumber (cm-1) 12 3

3 -1

3

3

-1

Intensity (CPS)

Carbon 1s SXI C-O-C PageFlattened 51 of 57area on the steel ball Langmuir 400 Out of contact

C-CO-C -CO-O-

C-C Carbide

Contact area

1 2 3 Out of contact 295 290 285 280 4 Binding Energy (eV) 5 a) Boron 1s Oxygen 1s 6 Iron 2p3/2 Fe borate/FeOOH borate BO/C-CO-C 7 Fe O B Fe(III)oxide Borate 1000 100 400 borate NBO/FeOOH Fe(II)oxide 8 H O/-CO-OFe met FeO/Fe O 9 Contact area 10 CO /-CO-O-/C-CO-C Fe(III)oxide 11 FeOOH FeOOH Fe(II)oxide H O/-CO-OFe met FeO/Fe O 12 ACS Paragon Plus Environment Out of contact 540 535 530 525 200 195 190 185 13 740 730 720 710 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) 14 Contact area

2-

3+

Intensity (CPS)

Intensity (CPS)

Intensity (CPS)

3+

2

2

2-

3

2

b)

c)

3

2

d)

3

a)

1 2 3 4 5 6 b) 7 8 9 10 11 12 13 14 15 16

Langmuir

Page 52 of 57

LSS fit

LSS fit


Carbon 1s LangmuirC-O-C

400

Intensity (CPS)

SXI Page 53 of 57 Flattened area on the steel ball Out of contact

C-CO-C -CO-O-

C-C Carbide

Contact area

1 2 3 Contact area Out of contact 295 290 285 280 4 Binding Energy (eV) 5 a) 6 Iron 2p3/2 Oxygen 1s Boron 1s Fe borate/FeOOH 7 Fe(III)oxide borate BO/-CO-O-/C-CO-C Fe O B 500 Fe(II)oxide 500 200 Borate 8 borate NBO/FeOOH -CO-OFe met 9 FeO/Fe O Contact area 10 11 12 ACS Paragon Plus Environment Out of contact 13740 730 720 710 540 535 530 525 200 195 190 185 Binding Energy (eV) Binding Energy (eV) 14 Binding Energy (eV) 2-

3+

b)

Intensity (CPS)

Intensity (CPS)

Intensity (CPS)

3+

c)

2

d)

3

a)

1 2 3 4 5 6 b)7 8 9 10 11 12 13 14 15 16

Langmuir

Page 54 of 57

LSS fit

LSS fit


Page 55 of 57 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Langmuir


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Page 56 of 57


Page 57 of 57 Tribotests at H RT BO3 3 Low CoF

Steel ball

Oil

Langmuir

H3BO3

Fe

Fe borate

lubricious H3BO 3

film of H3BO3 1 B2O3 ball glass disc Steel 2 Yuba 3 Steel ball H3BO3 Oil 4Tribotests at 100°C H3BO 3 Fe b ACS Paragon Plus Environment 5 High CoF squeezing Fe borate Hout BO 6 3 of H3 BO 3 3 7 B O glass disc 2

3

Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model

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