Low Friction and Anti-wear Mechanism of Molybdenum Based FMs on Boron-doped DLC

K. Onodera¹, H. Watanabe¹, T. Sato¹, S. Ogano¹, K. Manabe², T, Kaneko², K. Yamamori², A. Jaishankar³, A. Jusufi³,

J.L. Vreeland³, A. M. Schilowitz³

¹EMG Lubricants GodoKaisha, ²Toyota Motor Corporation, ³ExxonMobil Research and Engineering

INTRODUCTION: Friction reduction is one of the most important factors for improving energy efficiency of mechanical components. Various studies have been carried out on lubricants containing friction reducing additives, called friction modifiers (FMs). For example, molybdenum containing FMs (Mo-FMs) react to form molybdenum disulfide (MoS₂) films on steel surfaces.  They have been widely used in low friction lubricants, especially engine oils.

Development of low friction surface materials matched with low friction lubricants can be an important route for attaining further friction reduction. Diamond-like carbon (DLC) is a coating of interest because it can combine the durability of diamond with the low friction properties of graphite. In this study friction performance of lubricants with Mo-FMs on boron-doped DLC (B-DLC) was examined.

It is well known that FMs must be surface active. However, few studies have been published on the fundamental adsorption properties of FMs on surfaces. In this work we use a quartz crystal microbalance (QCM) to determine the adsorption properties of FMs. We seek to identify the surface adsorption properties of Mo-FMs on a B-DLC surface and determine the properties that enable performance. Towards this end we have measured the friction and wear performance and adsorption isotherms of Mo-FMs on B-DLC. 

METHODS: Three Mo-FMs, Mo-trimer, Mo dithiophosphate (MoDTP) and Mo dithiocarbamate (MoDTC) were used in this study. B-DLC was prepared by the unbalanced magnetron sputtering method.

Friction and wear were measured with a block on ring (LFW-1) machine with a load of 294N (Hertzian contact pressure of about 300MPa), a sliding velocity of 0.3m/s, an oil temperature of 80°C, and a sliding duration of 30 min. The lowest friction coefficient during each individual test is reported. Wear depth was measured after each test with a surface profilometer. The ring was SAE4620 steel while the block was steel coated with B-DLC. All tests were made on the same engine oil formulation containing varying Mo-FMs.

QCM experiments were performed on a Biolin Scientific Q-Sense QCM-D. The quartz sensors were coated with B-DLC. The injection flow rate was maintained at 150 mL/min. During measurements, the pure polyalpha olefin solvent was first injected for 18 - 20 hours to determine the solvent contribution to the total frequency shift. Subsequently, additive solutions of varying concentration of Mo-FM in polyalpha olefin were injected sequentially. Each solution was injected until a stable frequency shift was established.  Data were converted to mass vs. time using the method of Voinova.

RESULTS:  Friction coefficients and wear depths of the lubricants with the three Mo-FMs in block on ring tests were measured over a range of concentrations as depicted in Figure 1. All three Mo-FMs were equally effective and reduced the friction coefficient to about 0.04. This is likely due to the fact that they all form MoS₂ in the contact. Wear depth of MoDTC and Mo-trimer increased with concentration.  A similar trend was also observed with  MoDTP over the concentration range tested, but the amount of wear remained relatively stable as Mo concentration increased, i.e., the DLC coating did not wear through.

QCM measurements were made on varying concentrations of the three FMs  and adsorption isotherms were constructed. Values for free energy of adsorption were calculated from the isotherms based on fitting the data to either Langmuir or BET isotherm models. MoDTP showed the highest absolute value of free energy of adsorption (-35 kJ/mol) followed by Mo-trimer and MoDTC.

Figure 1- Friction and wear data measured in block on ring test for three Mo-FMs.

DISCUSSION: The data in Figure 1 demonstrate that all three Mo-FMs showed similar friction reduction on B-DLC. In contrast, wear performance was different. The wear depths increased for MoDTC and Mo-trimer as concentration increased, while it remained relatively stable for MoDTP.  Based on the results of both the block on ring tests and QCM experiments, it can be seen that a more negative free energy of adsorption reduces wear of the B-DLC surface.  Wear was especially high for MoDTC which also had the least negative free energy of adsorption.  

A number of mechanisms have been offered for the effect of Mo on DLC wear. Shinyoshi et al.¹ attributed the wear to a reaction between DLC and MoO₃ formed as a decomposition product from the parent Mo compound. More recently, De Feo et al.² proposed a multi-step reaction scheme where Mo in the tribofilm - formed during rubbing on the steel side of a contact - interacts with dangling bonds on the DLC side to form molybdenum carbide species resulting in increased friction and wear. In our QCM study strong adsorption energy of MoDTP was observed. A strongly bound protective layer is a feature that can mitigate these mechanistic approaches. The protective layer could reduce diffusion of oxygen and inhibit MoO₃ formation or prevent dangling bonds in the DLC from reacting with Mo in the tribofilm covering the steel counter face and slow down chemistry needed for breaking down DLC. The strongly bound Mo-FMs can inhibit interaction between the DLC surface and Mo containing tribofilm on the counter surface. It might also inhibit surface oxidation of the Mo to form MoO₃. An FM with a high free energy of adsorption might also serve to quickly passivate any dangling bonds formed on the DLC surface arising from mechanical rubbing.

REFERENCES:    1. T. Shinyoshi, Y. Fuwa and Y Ozaki, Wear analysis of DLC coating in oil containing Mo-DTC,  Japan Society of Automotive Engineers, 20077103,  (2007) pp. 956. 

2. M. De Feo, M.I. De Barros Bouchet, C. Minfray, Th. Le Mogne, F. Meunier, L. Yang, B. Thiebaut and J.M. Martin, MoDTC lubrication of DLC-involving contacts. Impact of MoDTC Degradation, Wear, 348 (2016)  pp.116.