tribological study of nano lubricant integrated soybean...

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Transactions of NAMRI/SME, Vol. 38, 2010, pp. 137-144TP10PUB30

Tribological Study of Nano LubricantIntegrated Soybean Oil for MinimumQuantity Lubrication (MQL) Grinding

authors

PARASH KALITA and AJAY P. MALSHEMaterials and Manufacturing Research Laboratories (MMRL)Mechanical Engineering Dept.University of Arkansas, Fayetteville, AR

WENPING JIANG ALBERT J. SHIHNanoMech, LLC, Springdale, AR Dept. of Mechanical Engineering

University of Michigan, Ann Arbor, MI

abstract

The application of minimum quantity lubrication (MQL) in high energy abrasive machiningprocesses requires lubricants with enhanced tribological properties to survive severethermo-mechanical effects. Nano lubricants’ containing inorganic MoS2 nanoparticlesencapsulated by organic molecular layers of triglycerides and phospholipids is onesuch novel formulation, as demonstrated by successful experiments in high energyMQL grinding process in the authors’ previous research (Shen et al., 2008). Nanolubricants in MQL application showed superior grinding performance by reducing thetangential grinding force and the specific grinding energy and by delivering high grinding(G)-ratios as compared to that of MQL applications of pure base oils and flood coolingusing a typical water based grinding fluid.

In the current study, the performance enhancement by nano lubricants in MQL grindingis investigated by systematic tribological testing under a simulated machining interactionbetween abrasive crystals and a workpiece in a surface grinding process. It is foundthat nano lubricants effectively reduce sliding frictional losses by a continuous supplyof active lubricant additives and by forming a stable, low friction tribofilm at the slidinginterface of the abrasive grit and the workpiece surface.

terms

Grinding, Minimum quantity lubrication (MQL), Molybdenum disulphide (MoS2),Nano lubricant, Nanoparticles, Tribotesting

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TRIBOLOGICAL STUDY OF NANO LUBRICANT INTEGRATED SOYBEAN OIL FOR

MINIMUM QUANTITY LUBRICATION (MQL) GRINDING

Parash Kalita and Ajay P. Malshe Materials and Manufacturing Research Laboratories (MMRL)

Mechanical Engineering Department University of Arkansas

Fayetteville, AR

Wenping Jiang NanoMech, LLC Springdale, AR

Albert J. Shih Department of

Mechanical Engineering University of Michigan

Ann Arbor, MI

KEYWORDS Grinding, Minimum quantity lubrication (MQL), Molybdenum disulphide (MoS2), Nano lubricant, Nanoparticles, Tribotesting ABSTRACT The application of minimum quantity lubrication (MQL) in high energy abrasive machining processes requires lubricants with enhanced tribological properties to survive severe thermo-mechanical effects. Nano lubricants’ containing inorganic MoS2

nanoparticles encapsulated by organic molecular layers of triglycerides and phospholipids is one such novel formulation, as demonstrated by successful experiments in high energy MQL grinding process in the authors’ previous research (Shen et al., 2008). Nano lubricants in MQL application showed superior grinding performance by reducing the tangential grinding force and the specific grinding energy and by delivering high grinding (G)-ratios as compared to that of MQL applications of pure base oils and flood cooling using a typical water based grinding fluid.

In the current study, the performance enhancement by nano lubricants in MQL grinding is investigated by systematic tribological testing under a simulated machining interaction between abrasive crystals and a workpiece in a surface grinding process. It is found that nano lubricants effectively reduce sliding frictional losses by a continuous supply of active lubricant additives and by forming a stable, low friction tribofilm at the sliding interface of the abrasive grit and the workpiece surface. BACKGROUND There is an increasing interest in developing highly efficient, low cost and environmentally friendly material removal processes. MQL machining with a minute supply of lubricants/coolants with a flow rate of 5-500 ml/hour to the machining interface is one such technology (Autret et al., 2003). It has a high production efficiency and a minimal consumption of lubricants, which results in low application and recycling costs along with low environmental pollution compared to that of conventional flood cooling. Research has shown promising results of MQL application in grinding with some challenges

Transactions of NAMRI/SME, Vol. 38, 2010, pp. 137-144(TP10PUB30)

(Baheti et al., 1998; Hafenbraedl and Malkin, 2000; Silva et al., 2005). Thermo-mechanical survivability of liquid lubricant additives at the tool-workpiece interface is one of the major challenges of high-energy abrasive machining processes such as grinding. ‘Stochasticity or process variability’ is an inevitable feature of an abrasive material removal process due to random multi-point cutting tool and abrasive grain wear and fracture. The selection of an appropriate process fluid with enhanced tribological properties is critical to the performance of MQL machining as a minimal amount of fluid has to satisfy the core functionalities of a lubricant, such as cooling and lubricating the grinding zone in addition to removing of chips from the machining interface. MoS2 nanoparticle integrated grinding fluid is a novel solution to address the severe machining challenges of MQL grinding (Shen et al., 2008). With a hexagonal close packed (HCP) layered structure, these nanoparticles exhibit exceptional tribological and extreme pressure properties (Winer, 1967; Rapoport et al., 2005; Malshe et al. 2006; Wu et al., 2006; Verma et al., 2008). It was hypothesized that nanoparticles of MoS2 suspended in grinding fluid would navigate into the narrow contact zone between the grinding wheel and the workpiece due to their size and high surface energy. This would provide significant lubrication and related value addition when the crystals/grit from the grinding wheel plunge and remove cast iron workpiece material. To test that hypothesis, the MQL grinding performance of MoS2 nanoparticle-based lubricant additives in three commercially available base oils--CANMIST, paraffin oil and soybean oil--were compared with that of the MQL application of the three base oils (without MoS2 nanoparticles) and with that of conventional flood cooling using water-based commercial synthetic grinding fluid. The performance comparison was made using two key factors, grinding force and grinding wheel wear. Nano lubricants significantly reduced grinding forces (27% for the CANMIST oil group, 21% for the paraffin oil based group and 9% for the soybean oil group), and most distinctively increased the life of expensive grinding wheels by increasing G-ratio (46% in the CANMIST oil group, 35% in the paraffin oil group and 15% in

the soybean oil group). More details can be found in Shen et al. (2008). The objective of this research is to develop a fundamental understanding of the mechanisms related to the nano lubricants responsible for enhancing the productivity of MQL grinding process. Influence of Lubrication on Grinding (Abrasive Machining) Process Parameters During grinding, a significant portion of energy is expended by the sliding of wearflats against the workpiece surface. ‘Wearflats’ are the flattened, worn out edges of abrasive grains sliding against the workpiece without removing any material (Malkin, 1989). With fixed machine/equipment settings, the grinding forces, namely, specific normal force (Fn) and tangential grinding force (Ft), increase with wear-flat area (Aa) (Malkin, 1989) and are expressed as: Ft = Ft,c + Ft,sl = Ft,c + µ*p* Aa ……………(1) Fn = Fn,c + Fn,sl = Fn.c + p*Aa ……………(2) where Ft,c and Fn,c are tangential and normal cutting forces, and Ft,sl and Fn,sl are tangential and normal sliding forces, respectively. Ft,sl = µ*p* Aa …………….(3) where µ is coefficient of sliding friction, p is contact stress and Aa is wearflat area. For given grinding conditions, the cutting force components (Ft,c and Fn,c) are constant (Malkin, 1989). Thus, by using a lubricant with superior tribological properties, particularly if a tribofilm can be deposited over wearflats, the friction coefficient (μ) can be significantly reduced. This will result in much smaller sliding friction losses, and hence much lower heat flux as well as grinding temperature, without sacrificing the material removal rate and while enhancing the operational life of the abrasive wheel. The tangential grinding force and G-ratio results for the soybean oil group (Shen et al., 2008) are shown in Figure 1 and 2, respectively. The test results showed a significant reduction in the tangential grinding force with MQL application of soybean nano lubricant, compared to that of flood lubrication. However, compared to MQL with pure soybean oil, the reduction in tangential force was moderate with nano lubricants. On the other hand, in the grinding


ratio (G-Ratio) results, soybean oil containing nano lubricants showed significant improvement with a distinctively high G-ratio.

FIGURE 1. TANGENTIAL GRINDING FORCE RESULTS OF MQL TESTING USING SOYBEAN OIL BASED LUBRICANTS AND FLOOD COOLING USING CIMTECH 500 [NOTE: HIGH CONC.- 20% OF NANOADDITIVE BY WEIGHT IN BASE OIL].

FIGURE 2. G-RATIO RESULTS OF MQL TESTING USING SOYBEAN OIL BASED LUBRICANTS AND FLOOD COOLING USING CIMTECH 500 [NOTE: HIGH CONC. - 20% OF NANOADDITIVE BY WEIGHT IN BASE OIL].

In light of these interesting observations, the objective of current research is to understand this variation in the grinding test results by studying the nano and micro scale mechanisms occurring at the abrasive grit and workpiece interface under MQL applications of soybean oil containing nano lubricants.

Pin-on-flat reciprocating tribotests were performed using alumina (Al2O3) abrasive pins and polished cast iron samples under MQL and flood lubrication conditions. The tribological performance of pure soybean oil was measured and compared under MQL and flood lubrication conditions with that of MQL with MoS2 based nano lubricants. Tests were also performed under dry conditions without supplying any kind of lubrication. The coefficient of friction (COF), the wear tracks generated on the surface of the workpiece and the chemistry and morphology of the tribofilm generated at the grinding interface were analyzed after each tribotest. EXPERIMENTAL DETAILS MQL Grinding Fluid- Nano lubricant The composition of the nano lubricants was kept the same as those used during the MQL grinding tests (Shen et al., 2008). MoS2 nanoparticles based grinding lubricant additive consisted of MoS2 nanoparticles (<100 nm) intercalated with triglyceride chemistry and phospholipids (lecithin- emulsifier). This additive was dispensed in soybean oil (a vegetable-based oil) at 20% concentration by weight and will be referred to as the high concentration nano lubricant from here onward. The preparation procedure of the described nanoengineered MoS2 additive is published elsewhere (Malshe et al., 2006 and Shen et al., 2008). A high resolution transmission electron microscope (TEM) image of a resultant nanoengineered MoS2 particle, which is shaped like an “elongated coconut”, is shown in Figure 3. The average size of the MoS2 particle repetitively measured by HRTEM was about 70 nm and 40 nm along the major and minor axis, respectively.

FIGURE 3. TEM IMAGE OF NANOENGINEERED MoS2 PARTICLE.


Particle size analysis of the nano additive by the HORIBA laser scattering particle size distribution analyzer confirmed that the MoS2 particle size was less than 100 nm. Pin-on-Flat Tribotest Setup A CSM Instruments tribometer was used in the research for measuring the coefficient of friction values. The reciprocating pin-on-flat test setup is shown in Figure 4.

FIGURE 4. PIN-ON-FLAT TRIBOTEST SETUP. During a tribotest, the flat surface of the workpiece was reciprocated linearly relative to a stationary pin (shown in the inset of Figure 4). A linear reciprocating module was attached to the tribometer to obtain a linear-reciprocating motion of the sample relative to the pin. A precisely known load/weight (also known as the normal force) was applied normally on the pin. The normal force, along with the amount of strain measured on the deflection arm was used to calculate the coefficient of sliding friction (COF) between the pin and the workpiece. A mounting cup was designed to supply and contain the desired amount of lubricant for in situ tribological testing. The tribotests were performed in a glass enclosed chamber of the CSM tribometer to prevent any contamination and to maintain a constant ambient temperature and humidity during the tests. The pin-on-flat tribotest parameters are listed in Table 1. A relatively high workpiece linear speed and linear sliding cycles were selected for the tribotests in order to study the effect of lubrication under high machining pressure and wear conditions, respectively. Pin-on-flat tribotests were performed under four different lubrication conditions:

1. Dry condition 2. Flood with pure base oil (soybean) 3. MQL with pure base oil (soybean) 4. MQL with nano lubricant.

The base oil without a nano additive will be referred as ‘pure base oil’, from here onwards. For MQL tests, nanolubricants were sonicated for 30 minutes before application. This was performed to homogeneously disperse MoS2 nanoparticles in the soybean base oil. During MQL tribotesting, liquid lubricants were periodically supplied in the form of fine droplets placed directly into the tool-workpiece abrasion zone by using a semi-automated fluid delivery system. For flood tribotesting, the lubricant volume was selected in an order of magnitude higher than MQL to simulate the conditions of flood machining. TABLE 1. EXPERIMENTAL PARAMETERS.

Grinding Parameters Grinding wheel peripheral speed (mm/s) 30,000

Workpiece velocity (mm/s) 40 Total depth of cut (mm) 2

Number of grinding passes 200 Flow rate- MQL (ml/min) 5 Flow rate- Flood (ml/min) 5400

Related Pin-on-Flat Tribotesting Parameters

Workpiece linear speed (mm/s) 200 and 300 Normal load (N) 20

Number of linear cycles 6000 Test duration (s) 3600

Volume of lubricants- MQL (ml) 3 Volume of lubricants- Flood (ml) 30

Abrasive Pin and Workpiece Material

A Norton-vitrified bonded aluminum oxide (Al2O3) 32A mounted abrasive pin was used for the pin-on-flat tests (shown in Figure 5). These pins were selected to better replicate the morphology and chemistry of the grinding wheel and the process in which the Norton 32A46-HVBEP vitrified bonded Al2O3 wheel was used (Shen et al., 2008). The cylindrical mounted grinding wheels were ground to a cuboidal shape [Figure 5(b)] to provide a small rectangular contact area between the abrasive pin and the substrate, which closely resembled the contact area between an abrasive wheel and a workpiece in grinding experiments. Dura-Bar 100-70-02 Ductile Iron (HRC 50) was selected as the substrate material for pin-on-flat tests. It


is the same workpiece material used in the authors’ previous grinding experiments (Shen et al., 2008). The samples were polished to an average surface roughness (Ra) value of 0.3 µm.

FIGURE 5. Al2O3 ABRASIVE MOUNTED PIN (RIGHT) AND ITS MICROSTRUCTURE (LEFT).

The tool (abrasive pin) and the workpiece materials, the linear sliding contact of abrasive grains and the workpiece surface and the experimental parameters were carefully chosen in tribotesting to approximately simulate the desired interaction between the abrasive crystals and the workpiece during the surface grinding process. Analytical Techniques The coefficient of friction (COF) data were acquired and plotted using CSM-InstrumX software. FEI XL-30 environmental scanning electron microscope (ESEM) was used to analyze the microstructure of the wear tracks. The elemental chemistry of the tribofilm generated on the workpiece surface was analyzed using Energy Dispersive X-Ray Spectroscopy (EDS). To excite X-ray signals of “molybdenum (Mo)-K shell” (characteristic peaks at 17.478 and 19.603 keV), a 20 keV- accelerating voltage was applied during EDS. This step was also necessary for distinct elemental peak identification of molybdenum (Mo) and sulfur (S), as “Mo-L” peak and “S-K” peak overlaps at the accelerating voltage of 2.3 keV. Before SEM and EDS analysis, the cast iron samples were cleaned using organic solvents to eliminate excess and loosely bonded oil and additive molecules.

RESULTS AND DISCUSSION Coefficient of Friction (COF) Results Figure 6 represents the COF results along with standard error after 6000 cycles of pin-on-flat tests at linear speed conditions of 200 mm/s. Each COF result is the average of three tribotests performed at constant normal load and linear speed.

FIGURE 6. AVERAGE COEFFICIENT OF FRICTION RESULTS AFTER 6000 CYCLES OF TRIBOTEST (TEST CONDITIONS: NORMAL LOAD= 20 N, LINEAR SPEED= 200 mm/s). The highest COF value was recorded for dry conditions. With the application of lubricant (pure soybean oil) under flood and MQL conditions, the COF values dropped as expected. The reduction in the friction coefficient with MQL compared to flood conditions can be explained by the enhanced lubrication capability of the MQL fluid delivery system. A further reduction in the COF values was observed with the MQL application of nano lubricants, which is consistent with the reduction in tangential cutting forces in grinding (Figure 1). Compared to flood lubrication with pure soybean oil, the MQL with the nano lubricant showed a 25% reduction in COF values; compared to pure soybean oil MQL, nano lubricant additives further reduced the COF by 10%. These results assist in explaining that when inorganic MoS2 nanoparticles were added to soybean oil, significant reduction occurred in the sliding friction between abrasive wearflats and the cast iron workpiece. This data raises the possibility of the formation of low friction-lubricating transfer film by shearing nanoparticles entrapped between the surfaces of abrasive grit crystals and the workpiece.


Figure 7 represents the average COF results after 6000 cycles of tribotests at higher linear speed conditions of 300 mm/s. The highest COF value was recorded for dry conditions. The variation in the COF values with the MQL application of pure soybean oil compared to its flood application was suggestive of the limited lubricity of MQL at amplified conditions of machining speed and load. A significant reduction in the COF values were observed with the MQL application of nano lubricants compared to other cases. Compared to flood lubrication with pure soybean oil, MQL with nanolubricants showed a 13% reduction in COF values, and compared to pure soybean oil MQL, the nano lubricant additive further reduced COF by 23%, which is much higher than was observed at lower speeds (Figure 6). This reduction in sliding friction can be attributed to the formation of a stable and continuous tribofilm, withstanding extreme pressures at the grinding interface of the abrasive grit crystal and the workpiece surface. To understand this further, wear tracks were studied for the morphology and the chemistry of tribofilm under various process conditions.

FIGURE 7. AVERAGE COEFFICIENT OF FRICTION RESULTS AFTER 6000 CYCLES OF TRIBOTEST (TEST CONDITIONS: NORMAL LOAD= 20 N, LINEAR SPEED= 300 mm/s) Surface Analysis of Wear tracks Figure 8 highlights typical SEM micrographs of wear tracks generated on a workpiece after 6000 cycles of tribotesting for four different cases, (a) dry condition, (b) flood condition-soybean oil, (c) MQL-soybean oil, and (d) MQL-nano lubricant, tested under a normal load of 20 N and a linear speed of 200 mm/s. Figure 8 (a)

to (d) clearly demonstrates the decrease in the density of aggressive wear tracks caused by the abrasive action of alumina grit crystals. Under MQL conditions, material removal due to abrasive actions is observed to be more uniform when compared to that of flood lubrication and dry abrasion. Further reduction in the density of wear tracks is observed when comparing micrographs in (c) for MQL with pure soybean oil and (d) for MQL with nano lubricant. This is attributed to the reduced sliding friction due to effective lubrication at the grit and workpiece interfaces.

FIGURE 8. SEM MICROGRAPHS OF WEAR TRACKS AFTER 6000 CYCLES OF TRIBOTEST (PIN-ON-FLAT TEST CONDITIONS: LOAD= 20 N, LINEAR SPEED = 200 mm/s)

Figure 9 represents the SEM images of wear tracks generated on the cast iron workpiece after 6000 reciprocating cycles of tribotest, with a load condition of 20 N and a higher linear speed of 300 mm/s, for the soybean oil group. At higher speed conditions, the use of nano lubricants showed exceptional results in reducing the COF and decreasing the aggressiveness of wear tracks with uniform material removal. This can be attributed to the continuous entrapment and replenishment of the nano lubricant, which enabled the formation of low-friction tribofilm even under severe machining conditions.

In summary, the COF and wear track data of the nano lubricants corroborate with the reduction in tangential force data observed in grinding tests, and they show improvement in the performance of MQL, not only under low but


also high speed machining conditions, where the thermo-mechanical effects are comparatively severe.

FIGURE 9. SEM MICROGRAPHS OF WEAR TRACKS AFTER 6000 CYCLES OF TRIBOTEST (PIN-ON-FLAT TEST CONDITIONS: NORMAL LOAD= 20 N, LINEAR SPEED = 300 mm/s). Tribofilm Analysis

The SEM micrographs and EDS-chemical analysis of the wear track surface are shown in Figure 10. A low vacuum SEM analysis at higher magnification confirmed the formation of a continuous tribofilm. Further, EDS-chemical analysis showed elemental signatures of molybdenum-Mo, sulphur-S (from MoS2) and phosphorus-P (from phospholipids) in the wear track in the form of a tribofilm. It is proposed that the continuous sliding action of multi-point abrasive grains over the workpiece surface at the machining interface results in the shearing of nanoparticulate MoS2

and the delivery of organic molecules. As a result of thermo-chemical reactions, multilayered Mo-S-P composite tribofilm is delivered, which reduces frictional losses when wearflats slide against the workpiece surface. These tribofilms are known to withstand extreme pressures (Verma et al., 2008). It is also important to note that these films are formed in the sliding interface of the wearflats and the workpiece and does not hinder the machining efficiency in terms of the material removal rate. Further, the formation of tribofilm and the reduction of wear are expected to contribute significantly to enhancing the life of an abrasive wheel, which

was evident in earlier MQL grinding experimental results of G-ratio.

FIGURE 10. SEM MICROGRAPHS AND EDS- CHEMICAL ANALYSIS OF THE WEAR TRACK GENERATED AFTER 6000 CYCLES OF TRIBOTEST, UNDER MQL APPLICATION OF NANO LUBRICANT (TEST CONDITIONS: NORMAL LOAD= 20 N AND LINEAR SPEED= 300 mm/s). CONCLUSIONS The authors have successfully studied the role of nano lubricants in MQL grinding process through friction and wear analysis at the grinding grit and the workpiece interface. The conditions of the machining interaction between abrasive grit crystals and the workpiece in surface grinding were simulated using a pin-on-flat tribotester for two different speed conditions. Sliding friction analysis at different speed conditions showed a notable reduction in the COF values when soybean oil was used for MQL with nano lubricant additive. This reduction was consistent with the reduction in tangential grinding forces. Further, microstructural and chemical analysis showed the formation of a uniform tribofilm in the wear tracks as well as the synthesis of Mo-S-P composition chemistries known to reduce friction and wear. These results cumulatively aid in enhancing the G-ratio without sacrificing the material removal rate. Overall, these findings are important to understand the role and to optimize the concentration of nano


lubricant additive in soybean oil for further value addition. Future research will address stability, chemistry and consistency of tribofilm formation for various lubricant additives and base oil combinations for a range of machining applications. ACKNOWLEDGMENTS The authors thank NSF-CMMI GOALI project (Grant # 0927541), for partial financial support, for this research. Support from NORTON/Saint-Gobain Abrasives and AMCOL Corporation are greatly appreciated. REFERENCES Autret, R., and S.Y. Liang (2003). “Minimum Quantity Lubrication in Finish Hard Turning,” Proceedings of International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment, and Management (HNICEM), pp. 1-9. Baheti, U., C. Guo, and S. Malkin (1998). “Environmentally Conscious Cooling and Lubrication for Grinding,” Proceedings of the International Seminar on Improving Machine Tool Performance, Vol. 2, pp. 643-654. Ganesan, M., C. Guo, and S. Malkin (1995). “Measurement of Hydrodynamic Forces in Grinding,” Transactions of NAMRI/SME, Vol. 22, pp. 103-107. Guo, C., and S. Malkin (1996). “Cooling Effectiveness in Grinding.” Transactions of NAMRI/SME, Vol. 23, pp. 111-116. Hafenbraedl, D., and S. Malkin (2000). “Environmentally-Conscious Minimum Quantity Lubrication (MQL) for Internal Cylindrical Grinding.” Transactions of NAMRI/SME, Vol. 28, pp. 149-154.

Malkin, S. (1989). Theory and Applications of Machining with Abrasives. Ellis Horwood, Chichester, and John Wiley & Sons, New York. Malshe, A.P., and A. Verma (2006). “Nanoparticle Compositions and Methods for Making and Using the Same.” International Application No. PCT/US07/60506. Rapoport, L., N. Fleischer, and R. Tenne (2005). “Applications of WS2 (MoS2) Inorganic Nanotubes and Fullerene-Like Nanoparticles for Solid Lubrication and for Structural Nanocomposites,” Journal of Materials Chemistry, Vol. 15, pp. 1782-1788. Silva, L.R., E.C. Bianchi, R.E. Catai, R.Y. Fusse, T.V. França, and P.R. Aguiar (2005). “Study on the Behavior of the Minimum Quantity Lubricant – MQL Technique under Different Lubricating and Cooling Conditions when Grinding ABNT 4340 steel.” J. of the Braz. Soc. of Mech. Sci. and Engg., Vol. XXVII, 2, pp. 193-198. Shen, B., P. Kalita, A.J. Shih, and A.P. Malshe (2008). “Performance of Novel MoS2 Nanoparticles Based Grinding Fluids in Minimum Quantity Lubrication Grinding,” Transactions of NAMRI/SME, Vol. 36, pp. 357-364. Verma, A., W. Jiang, H.H. Abu-Safe, and A.P. Malshe (2008). “Tribological Behavior of the Deagglomerated Active Inorganic Nanoparticles for Advanced Lubrication,” Tribology Transactions, Vol. 51, Issue 5, pp. 673-678. Winer, W.O., (1967), “Molybdenum Disulphide as a Lubricant: A Review of Fundamental Knowledge,” Wear, Vol. 10, pp. 422-452. Wu, J.H., B.S. Phillips, W. Jiang, J.H. Sanders, J.S. Zabinski, and A.P. Malshe (2006). “Bio-Inspired Surface Engineering and Tribology of MoS2 Overcoated CBN-TiN Composite Coating,” Wear, Vol. 261, pp. 592-599.


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