Influence of Wettability and Mechanical Properties on Tribological Performance of DLC Coatings under Water Lubrication

DLC coatings are currently used in a wide variety of industrial fields because of their outstanding properties such as high hardness, high wear resistance, low friction and so on under oil lubrication as well as no lubrication. In order to evaluate the tribological performance of DLC coatings under water lubrication, eight types of DLC coatings were evaluated by water-lubricated sliding tests at sliding velocities of 0.02 m/s and 0.1 m/s. The different DLC coating types comprised both hydrogenated and hydrogen-free DLC films that were deposited using different techniques: amorphous hydrogenated DLC (a-C:H), that doped with tungsten (a-C:H:W), that doped with silicon (a-C:H:Si) and hydrogen-free DLC (ta-C). DLC coatings showed low friction coefficient and excellent wear resistance. Friction coefficient and wear rate decreased with increasing sliding velocity. The effect of surface wettability and mechanical properties on tribological characteristics was investigated. Friction coefficient decreased as the contact angle increased. Hydrogen-free DLC coating with the highest hardness showed the highest friction and a larger wear rate. Furthermore, DLC coatings deposited with interlayer showed longer lifetimes.

Tribological Behavior of Thermally Sprayed WC Coatings under Water Lubrication

Thermally sprayed coatings have been used in various fields of industry for enhancing surface characteristics of materials and extending their service life. The contact surface of some mechanical equipment such as the fine pulverization equipment which is used in the woody biomass production process is required to have wear resistance in the water environment. Thermally sprayed coatings would be a good candidate to improve surface wear resistance under water lubrication. The objective of this study was to evaluate the tribological performance of thermally sprayed coatings under water lubrication. Thermally sprayed coatings which were classified into WC, WB and Ni spraying of three categories were compared with water-lubricated sliding test at a sliding velocity of 0.02 m/s and mean pressure of p0 = 10 MPa with a ring-on-disk apparatus. Thermally sprayed coatings showed comparatively high friction coefficient and well wear resistance under water lubrication. WC contained coatings showed better wear resistance than WB and Ni coatings. Thermally sprayed coatings showed obviously different mechanical properties and tribological behaviors, and the effect of wettability and hardness on tribological characteristics was discussed under water lubrication. Friction coefficient increased as the surface contact angle of thermally sprayed coatings increased. The wear rate decreased as the surface hardness of thermally sprayed coatings increased. Wear resistance of thermally sprayed coatings was excellent under water lubrication. WC contained coatings showed lower wear rate than WB and Ni coatings. WC-14CoCr coating showed the lowest wear rate.

Influence of Surface Properties Modified with Fine Shot Peening on Scuffing

Recently, gears of high strength, reliability, and surface-damage-resistant under severe service conditions are required to achieve the weight saving and downsizing of a product. For the high-speed condition in particular, it is important to understand the influence of the surface properties on the scuffing resistance. If the effective surface profile to improve the lubrication property was found, the metal surfaces could be obtained with both surface strength and surface lubricity. Herein, the influence of surface properties modified with fine shot peening, which can form the arbitrary surface profile, on the scuffing resistance in the rolling-sliding contact machine element, was investigated. The scuffing test was performed using a two-cylinder rolling contact test machine. In a specific sliding, a faster roller of 60% and a sliding velocity of 1.75 m/s were utilized. The scuffing test results with shot-peened test rollers and those with non-shot-peened test roller were compared. The influence of the surface roughness of the shot-peened test roller was also discussed. We found that the shot-peened roller had a better scuffing resistance compared with the roller without the shot-peening process.

Tribological Characteristics of DLC Coatings in Vacuum under Sliding Contact

Diamond-like Carbon (DLC) coatings of thickness 3 μm and 10 μm were deposited with and without radical nitriding on stainless steel disk and ring specimens. Plasma CVD method was used to deposit the DLC coatings with silicon as dopant. The specimens were tested in vacuum under sliding contact condition to evaluate the tribological characteristics with reference to the DLC coating thickness and sliding distance. The results revealed that wear resistance was more for the highest coating thickness. The changes on the worn surfaces were observed and wear mechanism is discussed using ex situ analysis.