Experimental Study on the Effect of DLC Coating Inslider-on-Disc Oil Film Lubrication

The effects of solid/liquid interface wettability on the thickness and friction of hydrodynamic lubricant films were investigated using a slider bearing test rig, where hydrodynamic lubrication films were generated by a stationary slider with fixed inclination and a rotating glass disc. DLC coatings with various surface energies were prepared on the slider surface by varying the coating bias voltages and doping elements using the DC magnetron sputtering technique. Raman spectroscopy and contact angle measurements indicated that both decrease in bias voltage and increase in the doping elements could reduce the surface energy of the DLC coating. Ti/TiN/(Ti,N)−DLC/Ti−DLC/DLC coating was deposited on the slider surface at a bias voltage of 0 V (denoted as 4TDLC−1), Ti/TiN/(Ti,N)−DLC/Ti−DLC/DLC coating at a bias voltage of −200 V (denoted as 4TDLC−2), and Ti/TiN/(Ti,N)−DLC coating at a bias voltage of −100 V denoted as (Ti, N)−DLC. Using the original steel slider and the three coated sliders, film thickness and friction were measured with PAO base oil and aqueous glycerin solution of the same viscosity at various speeds and loads. The results showed that all the four sliders produced nearly the same film thickness and friction by PAO10. And when the lubricant was 93.5% glycerol aqueous solution, the 4TDLC−1 slider produced the largest film thickness and friction. The 4TDLC−2 slider and the original steel slider presented similar film thickness and friction. And the (Ti,N)−DLC slider gave the smallest film thickness and friction. At the same time, the contact angle and contact angle hysteresis of PAO base oil and glycerin aqueous solution respectively on the four sliders were also measured using a contact angle meter, and the corresponding adhesion work and potential energy barrier were calculated. It was found that the adhesion work of PAO10 to the four sliders was basically the same. And when the lubricant was 93.5% glycerol aqueous solution, its adhesion work to the 4TDLC−1 slider was the largest, and the adhesion work to (Ti,N)−DLC slider was the smallest, and the 4TDLC−2 slider and the original steel slider presented the similar adhesion work. The results showed that the film thickness and friction demonstrated close correlation to the adhesion work when one lubricant and various sliders were employed. However, when several lubricants and one slider were employed, for example, the film thickness and friction of four lubricants, PAO10/PAO20 and 89% glycerol aqueous solution/93.5% glycerol aqueous solution, on 4TDLC−2 slider were measured, and the adhesion work was inconsistent with the experimental results. Fortunately, it was found that in these cases the potential energy barrier presents good correlation to the film thickness and the friction. It was evident that the adhesion work was mainly determined by the surface tension and the contact angle. When the difference in the surface tension of the two lubricants was large, and the contact angle did not accurately reflect the interfacial wettability. Therefore, the potential energy barrier could reflect well the effect of the interfacial wettability on the film lubrication under the present test conditions. The test results showed that the load has some influence on the solid/liquid interface wettability. In addition, the experiments in the present study were carried out under conditions of low pressure and low sliding speed, which effectively suppressed the thermal effect, indicating that the difference in solid/liquid interfacial wettability play a dominating role in the film thickness and friction of full−film lubrication.