ICMCTF2012 Session E1-1: Friction Wear Lubrication Effects & Modeling

Solid lubricants could be used in dry bearings, gears, seals, sliding electrical contacts and retainers in rolling element bearings and many kinds of sliding contacts [1] to reduce wear and to increase durability of the contacts, where the liquid lubricant is undesirable. Molybdenum disulphide (MoS₂) is one of the most widely used solid lubricants to reduce friction and mitigate wear in tribological applications mainly because it is easy to shear in the direction of motion, resulting in a low friction. At present, many efforts have been made to predict its durability under different working conditions such as normal force, displacement amplitude and contact configuration, while there is a lack of a quantitative evaluation of the effect of each factor [2-5].

In this paper, fretting experiments have been carried out under various values of contact pressure, displacement amplitude and two contact configurations (ball-on-flat and cylinder-on-flat). In addition, the method of factorial design has been used to evaluate the effect of each factor and the interaction between each factor.

According to the experimental results, it could be found that the coating presents a longer lifetime in mixed slip regime than that of gross slip regime in both contact configurations, because the central stick-slip will enclose the third layer at the contact center which aims to prolong the coating lifetime. In addition, its durability under ball-on-flat configuration is better than that of cylinder-on-flat.

By the results of factorial design, the contact configuration and displacement amplitude are the controlling factors for the coating lifetime, while the normal force is the controlling factor for the initial friction coefficient. The two factors “interaction” have also strong effect on the coating lifetime especially the interaction between the displacement amplitude and normal force and the interaction between the displacement amplitude and contact configuration. Selection of coatings should consider the contact configuration firstly, the displacement amplitude secondly. Based on the two parameters, the contact force should be then considered.

[1]E. Richard Booser.Handbook of Lubrication (Theory and Practice of Tribology), Vol. II, ed., CRC Press, 1989.

[3]V. Fridrici, S. Fouvry, P. Kapsa, P. Perruchaut. Wear 255 (2003) 875.

[4]D. B. Luo, V. Fridrici, Ph. Kapsa, T. Murakami. Lubrication Science 21(2009) 193.

The selection of low friction coatings is of great interest to industrial applications. Nevertheless, regarding the lifetime of DLC coatings, the selection criteria often depend on the experimental apparatus and contact configuration and then cannot be directly applied to real cases. In this study, we use a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the normal force and the sliding amplitude. Hence by plotting the “lifetime” versus the “local energy density” a single master curve defining the intrinsic endurance of the coating can be defined. To identify the intrinsic “energy density capacity” variable, characterising the coating fretting wear endurance (i.e. used to model the endurance master curve), a flat on flat contact configuration, allowing constant pressure conditions, is applied. This approach is considered to investigate various DLC systems used in aeronautical applications to protect titanium surfaces in contact. The effects of contact pressure, displacement amplitude, frequency and temperature are investigated. The results show that, the local energy wear approach is suitable to characterize the coating endurance for variable mechanical loading conditions (i.e. sliding amplitude and pressure). By contrast, elevate sliding frequencies and temperatures, by activating severe tribo oxidation processes, sharply modify the wear processes so that the endurance values, which are significantly reduced, can not be transposed on the fretting wear master curve. Using this friction energy concept, a first quantitative description is nevertheless provided to formalise the lubricant endurance of DLC coatings subjected to severe thermal exposure.

Optical, mechanical and thermal properties are among the most important properties that govern ophthalmic lens functionality. The functional ‘Mirror’ surface of ophthalmic plastics lenses is actually produced by multi-processes, turning and polishing. The polishing stage has the important role to remove tool marks and to hold the required level of transparency.

This paper examines the effects of polishing cycle at the nanoscale on the surface modifications of ophthalmic plastic. AFM is used to explore topographical and thermal nanoscale imaging as function of polishing time. Simultaneously a multiscale decomposition, based on ridgelets transform, is introduced to explore the produced surface texture from nano to micro scale. This enables a complete description of surface aspect including scratches density, spacing, orientation and distribution. Also, the process map that characterizes the evolution of polished surface features was derived. This mapping plays a tangible role in visualizing processes of abrasive fluid – plastic surface interaction which could previously only be studied indirectly.

Several works on the friction coefficient during abrasive wear tests are available in the literature, but only a few were dedicated to the friction coefficient in micro-abrasive wear tests conducted with rotating ball. This work aims to study the influence of titanium nitride (TiN) and titanium carbide (TiC) coatings hardness on the friction coefficient and wear coefficient in ball-cratering micro-abrasive wear tests. A ball of AISI 52100 steel and two specimens of AISI D2 tool steel, one coated with TiN and another coated with TiC, were used in the experiments. The abrasive slurry was prepared with black silicon carbide (SiC) particles (average particle size of 3 μm) and distilled water. Two normal forces and six sliding distances were defined, and both normal and tangential forces were monitored constantly during all tests. The movement of the specimen in the direction parallel to the applied force was also constantly monitored with the help of a Linear Ruler. This procedure allowed the calculation of crater geometry, and thus the wear coefficient for the different sliding distances without the need to stop the test. The friction coefficient was determined by the ratio between the tangential and the normal forces, and for both TiN and TiC coatings, the values remained, approximately, in the same range (from μ = 0.4 to μ = 0.9). On the other hand, the wear coefficient decreased with the increase in coating hardness.