Article pubs.acs.org/IECR
Investigation of the Tribological Properties of Two Different Layered Sodium Silicates Utilized as Solid Lubrication Additives in Lithium Grease Xiaosheng Zhang, Weichao Sun, Huijuan Ma, Hong Xu,* and Jinxiang Dong* Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, Shanxi, People’s Republic of China S Supporting Information *
ABSTRACT: Layered sodium silicates β-Na2Si2O5 and kanemite were synthesized via facile methods under mild conditions. The tribological properties of β-Na2Si2O5 and kanemite utilized as additives in lithium grease were evaluated with a four-ball tester under different experimental conditions. The maximum nonseizure load value of 5.0 wt % β-Na2Si2O5 grease jumped from 353 N (the base grease) to 1568 N. However, 5.0 wt % MoS2 grease could only reach 617 N under the same conditions. The SEM and EDS results confirm that a protective film mainly composed of sodium silicates was formed on the worn surface during the friction process. The structural stability of β-Na2Si2O5 and kanemite after the wear test was studied by XRD. It was found that a loss of interlamellar water causes the layer structure of kanemite to collapse during a long-duration wear test. The layered structure of β-Na2Si2O5 is stable, and its tribological properties are better than those of kanemite.
1. INTRODUCTION Layered sodium silicates are a group of materials with complex polymorphisms. At least eight different modifications have been reported, as a function of temperature, pressure, and synthesis conditions.1 Of these modifications, the most well-known layered sodium silicates are α-Na2Si2O5, δ-Na2Si2O5, βNa2Si2O5, and kanemite (NaHSi2O5·3H2O). In these compounds, the metal atom is sandwiched between two anionic planes with strong covalent metal−anion bonds. Weak van der Waals interactions are responsible for the stacking of the molecular layers into a crystal. These compounds may be used for nonphosphate detergent builders in washing powders,2 as precursor materials for zeolite synthesis,3 and as solid lubricants.4 Layered sodium silicates are inexpensive and environmentally friendly, and they are currently utilized as nonphosphate detergent builders, because of their ion exchange properties. However, the scope for their industrial applications is somewhat narrow. Therefore, it is worthwhile to develop other highly value-added commercial utilizations for them that utilize their structural features. Friction causes wear and energy dissipation, and it is responsible (directly or indirectly) for about one-third of the world’s energy resource consumption.5 Therefore, friction and wear reduction are key factors in any future energy conservation plan. δ-Na2Si2O5, α-Na2Si2O5, β-Na2Si2O5, and kanemite, a group of relatively inexpensive white powders compared to other solid additives, have been reported to provide effective wear protection, as a result of layer characteristics similar to typical solid lubricants MoS2 and graphite.4,6−8 Using oleic acid as a dispersant, α-Na2Si2O5 as additives in liquid paraffin and 150 SN base oil (mineral base oil) can considerably improve the maximum nonseizure loads (PB) from 392 N to 470 N and from 392 N to 647 N, respectively.4,6 Ca2+ and Mg2+ kanemite products obtained by the freezing titration ion exchange method performed better in wear testing. After adding 1.0 wt % Ca-kanemite or 1.0 wt % © 2013 American Chemical Society
Mg-kanemite to 100 SN base oil (mineral base oil), the PB values reached 598 N, while the wear scar diameter (WSD) decreased by over 50% and 25%, respectively.7 δ-Na2Si2O5, αNa2Si2O5, and β-Na2Si2O5 also have been investigated in lithium-based grease by using a four-ball tester, and the PB values increased with increasing additive amounts. The PB value reached its maximum value when 2.0 wt % layered disilicate was added. The worn steel ball surface lubricated by the grease containing 2.0 wt % of the layered silicates was smooth.8 However, the authors only present the tribological properties of the target layered silicates. The relationship between the structures and the tribological properties is still unclear. In previous research, δ-Na2Si2O5, α-Na2Si2O5, and βNa2Si2O5 were obtained by a high-temperature roasting method; the synthesis temperatures for these layered sodium silicates are usually in the range of 600−780 °C.4,9 The traditional production has high energy consumption and encounters difficulty in controlling the crystal size in largescale industry production. A method of preparing these products under milder conditions seems necessary. Taking these factors into account, β-Na2Si2O5 and kanemite were selected for examination of their solid lubricating performance, because they can be obtained via mild synthesis methods and their different layered crystal structures are sufficient for probing the structure−property relationship. β-Na2Si2O5 and kanemite have similar layers of SiO4 tetrahedra with different cations existing between their layers. The layers of β-Na2Si2O5 are joined together through Na atoms,9 while kanemite has Na ions and water (H2O) molecules in the interlayer space.10 According to the available literature, β-Na2Si2O5 can be Received: Revised: Accepted: Published: 182
July 31, 2013 December 11, 2013 December 15, 2013 December 16, 2013 dx.doi.org/10.1021/ie402481u | Ind. Eng. Chem. Res. 2014, 53, 182−188
Industrial & Engineering Chemistry Research
Article
Table 1. Raw Materials in the Experimentsa material
description
amorphous sodium silicate
SiO2/Na2O molar ratio = 1.9−2.3 in the presence of 10−25 wt % water SiO2/Na2O molar ratio = 1.9−2.1 AR, 98% 98% AR, boiling range (60−90 °C)
δ-Na2Si2O5 LiOH·H2O stearic acid petroleum ether MoS2 12-hydroxyoctadecanoic acid poly(α-olefin) PAO9 a
manufacturer Shandong Shengtong Group Share Co,. Ltd.
99% >75.0% (GC)
Shandong Shengtong Group Share Co,. Ltd. Aladdin Chemistry Co., Ltd. Aladdin Chemistry Co., Ltd. Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. SCM Industrial Chemical Co., Ltd. Tokyo Chemical Industry Co., Ltd.
viscosity of 53.11 mm2/s at 40 °C, viscosity index of 133
Chevron Corporation
All chemicals were purchased from commercial sources and used in all syntheses without further purification.
Figure 1. Experimental and simulated X-ray diffraction (XRD) patterns of β-Na2Si2O5.
Figure 2. Experimental and simulated XRD patterns of kanemite.
prepared at 200−400 °C,11 and kanemite may be obtained from the hydrolysis product of δ-Na2Si2O5.12 In this paper, two different crystalline structures of layered sodium silicates, β-Na2Si2O5 and kanemite, are synthesized under mild conditions. Their tribological performances as additives in lithium grease are evaluated using a four-ball test. The rubbed surface is investigated by means of scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) in an attempt to study the lubricating mechanism. The relationship between the structures and lubricating properties is discussed.
cooling the autoclave to room temperature, the solid products were obtained. According to the literature method,12 kanemite was prepared at δ-Na2Si2O5/H2O (mass ratio) = 1:15−1:30 in an unsealed beaker for 30 min under constant stirring (700 rpm). After filtration, the sample was air-dried and kanemite was obtained. Characterization of Preparation Products. The crystallinity and phase purity of the products was analyzed by powder X-ray diffraction (XRD) on a Rigaku MiniflexII diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 30 kV and 15 mA. Because the raw material and reactor adopted in this work are different from those disclosed in DE 4,421,850, the synthesis of β-Na2Si2O5 was studied accordingly in the temperature range of 240−300 °C for 45 min. Figure 1 shows the experimental and simulated XRD patterns of β-Na2Si2O5. β-Na2Si2O5 can be obtained at calcination temperatures from 260 to 300 °C, and the crystallinity of β-Na2Si2O5 almost remains the same. However, the amorphous phase was obtained when the calcination temperature was lowered to 240 °C; as a result, a
2. EXPERIMENTAL SECTION Materials. In our experiments, the chemicals that have been used are listed in Table 1. Preparation of β-Na2Si2O5 and Kanemite. β-Na2Si2O5 was synthesized according to German patent DE 4,421,850.11 Silicates powder (5 g) was put into a stainless-steel autoclave and heated at 240−300 °C for 45 min in a muffle furnace. After 183
dx.doi.org/10.1021/ie402481u | Ind. Eng. Chem. Res. 2014, 53, 182−188
Industrial & Engineering Chemistry Research
Article
temperature of 260 °C was chosen as the suitable calcination temperature to form the crystalline phase. The powder XRD method was employed to identify the framework structure of the synthesized samples in the present study. The diffraction peak positions and relative diffraction intensities of β-Na2Si2O5 are in good agreement with those of previously reported β-Na2Si2O5.9 Therefore, the unit-cell parameters of β-Na2Si2O5 have been refined with Powder Cell for Windows (PCW). [PCW is a GUI (Graphical User Interface)-based program that is used to represent the unit cells and calculated powder pattern.] The resulting powder pattern can be directly compared to experimental data13 from the powder XRD pattern, and the cell dimensions were a = 12.3076 Å, b = 4.8538 Å, c = 8.1175 Å, which are very similar to those reported for β-Na2Si2O5 (a = 12.329(4) Å, b = 4.848(4) Å, c = 8.133(3) Å). The powder XRD pattern was simulated by PCW. The simulated and experimental powder XRD patterns of βNa2Si2O5 (Figure 1) are identical, indicating that they possess the same structure. The experimental and simulated XRD patterns of kanemite are shown in Figure 2. Kanemite can be obtained in the range of δ-Na2Si2O5/H2O (mass ratio) = 1:15−1:30. Taking into account the crystallinity and solid yield, the suitable mass ratio was chosen to be 1:15. The diffraction peak positions and relative diffraction intensities of kanemite are in good agreement with those of previously reported kanemite.10 Therefore, the unit-cell parameters of kanemite have been refined with PCW13 from the powder XRD pattern and the cell dimensions were as follows: a = 4.9455 Å, b = 20.4830 Å, c = 7.2742 Å, which are very similar to those reported for kanemite (a = 4.946(1) Å, b = 20.510(10) Å, c = 7.277(1) Å). The powder XRD pattern was simulated by PCW. The simulated and experimental powder XRD patterns of kanemite (Figure 2) are identical, indicating that they possess the same structure. Preparation of Lithium Grease. Lithium-based grease was prepared as follows. First, the predetermined amounts of PAO9, 12-hydroxystearic acid, and stearic acid were added to a grease kettle and the mixture were stirred and heated to 85 °C until it became homogeneous; second, a water slurry of lithium hydroxide initially heated to ∼80 °C was slowly added into the above mixture, then heated to 120 °C and kept at this temperature for ∼3 h until the lithium soap formed. Additional base oil was slowly added into the lithium soap at 90 °C and once again heated to 130 °C until the water completely evaporated. The mixture was kept at 205 °C for 10 min, then cooled to room temperature and ground on a triple-roller mill three times to form the desired grease. The typical properties of lithium grease follows: soap content, 11.5%; dropping point, 207 °C; cone penetration (0.1 mm), 285. To investigate the effects of β-Na2Si2O5, kanemite, and MoS2 on the tribological behavior of lithium grease, the three additives were added at concentrations of 1.0, 3.0, 5.0, and 7.0 wt %, respectively. Each variation of lithium grease was mixed by mechanical stirring and ground three times in a triple-roller mill. Tribological Test. The tribological behavior of β-Na2Si2O5, kanemite, and MoS2 as additives in lithium grease at different concentrations were evaluated on a Model MS-10J four-ball tester (Xiamen Tenkey Co., Ltd.). The experimental setup of the tribotester is shown in Figure S1 in the Supporting Information, along with a schematic view of the four-ball assembly. The tester is operated with one steel ball, referred to as the “top ball”, under a load rotating against three steel balls
that are held stationary and clamped together for three-point contact. All of the balls (diameter = 12.7 mm, HRC 59−61, grade 25 EP (Extra Polish)) are made of AISI 52100 steel (composition: 0.95%−1.05% C, 0.15%−0.35% Si, 0.24%− 0.40% Mn, 0.027% P, < 0.020% S, 1.30%−1.67% Cr, < 0.30% Ni, < 0.025% Cu). The maximum nonseizure loads (PB value, which represents the load-carrying capacity) were determined according to China Petrochemical Industry Standard Method SH/T 0202-92, which is similar to ASTM Standard D2596-82 (as shown in Table 2). Table 2. Tribological Test (Maximum Non-Seizure Loads, Friction and Wear Tests) Conditionsa Maximum Nonseizure Loads Test: Test Conditions temperature, T (°C)
time, t (s)
rotate speed (rpm)
ambient temperature 10 Friction and Wear Test: Test Conditions
1770
temperature, T (°C)
time, t (s)
rotate speed (rpm)
load (N)
75 75 75 75 75 75 75
1800 1800 1800 1800 1800 1800 1800
1450 1450 1450 1450 1450 1450 1450
98 196 294 392 490 588 686
a
The friction and wear tests were also determined in Table 2. The experiment was interrupted when the friction coefficient reached the value of 0.2. The wear scar diameter (WSD) representing the antiwear ability on the three lower balls were measured using an optical microscope to an accuracy of ±0.01 mm and the friction coefficients reflecting the friction reduction property were recorded automatically with a strain gauge equipped with the four-ball tester. Three identical tests were performed for an average, to minimize data scattering. The method for how the wear scar diameter was obtained with the optical microscope is shown in Figure S2 in the Supporting Information.
Characterization of Samples and Worn Surface after Antiwear Test. XRD was used to characterize the change of βNa2Si2O5 and kanemite samples before and after the antiwear test to investigate whether the typical samples were able to maintain their structure through the test. After the antiwear test, β-Na2Si2O5 and kanemite greases were adequately diluted with petroleum ether and oleic acid to remove the lithium soap and oil, then they were magnetically stirred for 15 min, filtered with petroleum ether, and dried at room temperature overnight. A Model TM3000 scanning electron microscopy (SEM) system that was equipped with Bruker Quantax 70 energy-dispersive spectroscopy (EDS) were used to analyze the morphology and the element distribution of the rubbed surface. The stains of sludge or varnish found on the ball surface in the vicinity of the wear scar were removed with cotton and then cleaned with petroleum ether for 20 min in an ultrasonic bath (Model KQ50DB, Kun Shan Ultrasonic Instruments Co., Ltd.) with 50 W of power and 40 kHz of frequency before the SEM and EDS analyses.
3. RESULTS AND DISCUSSION Lubrication Properties. The present study investigates the tribological properties of β-Na2Si2O5 and kanemite as the grease additives, compared with those of MoS2. β-Na2Si2O5, kanemite, and MoS2, which were studied as solid lubricants, are effective load-bearing lubricant additives, because of their 184
dx.doi.org/10.1021/ie402481u | Ind. Eng. Chem. Res. 2014, 53, 182−188
Industrial & Engineering Chemistry Research
Article
lamellar structure. This characteristic makes them necessary for processes with extreme contact pressures. So the load carrying capacity factor PB measure is an important parameter in this research study. Figure 3 discloses variation of the maximum
Figure 4. Wear scar diameter (WSD), as a function of applied load with the lubrication of lithium grease containing 5.0 wt % of βNa2Si2O5, kanemite, or MoS2 additives (four-ball, 75 °C, 1450 rpm, 30 min).
Figure 5 shows the relationship between the friction coefficient and the applied load with the greases containing
Figure 3. Load-carrying capacity of grease as a function of additive concentration of β-Na2Si2O5, kanemite, and MoS2 (four-ball, 1770 rpm, 10 s).
nonseizure load (PB) with the concentration of additives. The PB value of β-Na2Si2O5 jumps from 353 N (the PB value of the base grease) to 1568 N when the additive concentration jumps from 1.0 wt % to 7.0 wt %. Kanemite exhibits the same PB values as β-Na2Si2O5, except that β-Na2Si2O5 possesses a higher value than that of kanemite at the 5.0 wt % additive concentration. The PB values of MoS2 slowly increase from 353 N to 617 N when the additive concentration increases from 1.0 wt % to 7.0 wt %, affording lower PB values than those of βNa2Si2O5 and kanemite. These results, overall, show that βNa2Si2O5 has excellent load-carrying capacity and perform the best among the three additives. The PB test is a process with a very short run time (10 s), according to China Petrochemical Standard SH/T 0202-92. The run time is too short to reflect actual working conditions. Therefore, a wear test under different loads for a long run time (1800 s) was adopted to evaluate the antiwear properties of greases with β-Na2Si2O5, kanemite, and MoS2 as additives. The WSD and friction coefficient were used to evaluate the antiwear and friction reduction properties of lubricant additives, respectively. Figures 4 and 5 display the friction coefficient and WSD as functions of applied load with the lubrication of lithium grease containing 5.0 wt % of β-Na2Si2O5, kanemite, or MoS2. Figure 4 shows that the WSD of the three additives vary differently with load. The WSD for β-Na2Si2O5 at first increases from 0.33 mm to 0.46 mm steadily as the applied load varies from 98 N to 392 N, and the WSD values fluctuate slightly in the range of 0.45−0.46 mm when the applied load increases from 392 N to 686 N. For kanemite, the WSD increases almost linearly from 0.42 mm to 0.64 mm when the applied load rises from 98 N to 588 N. In contrast, the WSD for MoS2 and base grease changes insignificantly from 0.49 mm to 0.48 mm and from 0.35 mm to 0.45 mm in the applied load range of 98−196 N. Further increasing the applied load to 294 N elevates the WSD value to 0.52 mm and 0.5 mm. When increasing the applied load further, the experiments are interrupted when the friction coefficient reached the cutoff value of 0.2. Taking into account the WSD values for the three additives in the entire range of applied loads, β-Na2Si2O5 performs the best.
Figure 5. Friction coefficient as a function of applied load with the lubrication of lithium grease containing 5.0 wt % of β-Na2Si2O5, kanemite, or MoS2 additives (four-ball, 75 °C, 1450 rpm, 30 min).
the three additives. As an additive, β-Na2Si2O5 is found to considerably reduce the friction coefficient from 0.096 to 0.057 as the applied load increases from 98 N to 686 N. With regard to kanemite, the friction coefficient increases gradually and reaches a plateau, then decreases slightly as the applied loads increases from 490 N to 588 N. As for MoS2, the friction coefficient is larger than that of kanemite and lower than that of β-Na2Si2O5 when the applied load increases from 98 N to 196 N. However, the friction coefficient of MoS2 surpasses those of kanemite and β-Na2Si2O5 at an applied load of 294 N. With regard to base grease, overall, the friction coefficient seems to surpass that of grease containing the other three additives under the same loads (from 98 N to 294 N). After further increasing the applied load, the experiments were interrupted because the friction coefficient reached 0.2. The friction coefficient vs time graphs for each of the lithium greases under different loads are shown in Figure S3 in the Supporting Information. The friction coefficient results demonstrate that β-Na2Si2O5 is the most effective additive for the friction-reduction properties. The performance of kanemite lies between that of MoS2 and βNa2Si2O5. SEM and XRD Observations of Worn Surfaces. In order to understand the behavior of β-Na2Si2O5, kanemite, and MoS2 as additives in lithium grease, an analysis of the worn surfaces of friction pairs was carried out. For this purpose, scanning 185
dx.doi.org/10.1021/ie402481u | Ind. Eng. Chem. Res. 2014, 53, 182−188
Industrial & Engineering Chemistry Research
Article
wear appearance and elemental composition of wear surfaces after being lubricated by each grease for 60, 300, 600, and 1800 s. Under limited loads, β-Na2Si2O5 and kanemite could form a protective film in a short time (within
