Tribological comparison of different C-based coatings in lubricated and unlubricated conditions

The use of carbon-based coatings (hydrogenated and non-hydrogenated DLC, doped and alloyed-DLC) is of wide interest due to its applications in mechanical components submitted to friction and wear including sliding parts in automotive engines. 

A tribological comparative analysis using a reciprocating (SRV) tester in lubricated and unlubricated conditions with a 4-stroke motor oil has been carried out on six currently relevant state-of-the-art coatings (namely WC/a-C, TiBC/a-C and TiC/a-C:H nanocomposites, Ti-doped DLC, BCN film and a crystalline monolithic TiC film as reference). The quantification of the fraction of the sp2-bonded matrix has been done by fitting of C 1s XPS peak and the mechanical properties evaluated by nanoindentation. The comparative analysis has allowed us to identify the capabilities of each system depending on the testing conditions and the possible synergies as a function of the chemical composition and film nature. Under lubricated harsh conditions (max. contact pressure 1.7 GPa) only coatings displaying hardness superior to 20 GPa could stand the sliding motion without failure. At lower contact pressures, a significant fraction of sp2 carbon (≥ 75%) is advantageous for reducing wear in boundary lubrication. WC/a-C, BCN and Ti-DLC films showed the best tribological response in dry sliding conditions. This fundamental information would be of relevance for assisting engineers in selecting best partnership for lubrication systems.