Study Of Tribological Compatibility Between Diamond-like Carbon Coatings For Engine Application And Lubricant Additives

Energy consumption,environmental pollution and climate warming are the three major challenges faced by the sustainable development of global industry.Mining and drilling industries are the main fields of energy consumption.As the main power source of heavy-duty vehicles,e.g.drilling rig,diesel engines usually work under demanding conditions,which leads to the exhaust emissions and the friction and wear losses during the working process.Therefore,the energy conservation and emissions reduction in drilling diesel engines is significant for the green development of mining and drilling industries.Diamond-like carbon(DLC)coating is well known as a solid lubricant material owing to its high hardness,good friction and wear resistance and chemical inertness.Solid-liquid composite lubrication technology using DLC coatings and lubricant additives is an important means to improve the fuel economy and durability of engines.Therefore,it is of great significance to understand the compatibility between DLC coatings and lubricant additives.This will be helpful for improving the coating protection effect,optimizing the lubricant formulation and achieving energy conservation and emission reduction of drilling diesel engines.In this paper,four types of DLC coatings were deposited on steel substrates used as engine component materials by means of unbalanced magnetron sputtering and multi-arc ion plating deposition technologies.The differences between the DLC coatings on surface morphology,microstructure and mechanical properties were investigated.Moreover,the compatibility properties of DLC coatings and lubricant additives,such as antiwear additive(zinc dialkyldithio-phosphate,ZDDP),friction modifiers(MoDTC and Moly trimer),dispersant and detergent were systematically investigated under boundary lubrication conditions.The solid-liquid composite lubrication mechanism of DLC coatings and lubricant additives,as well as the effect of the interaction between additives on the tribological properties of DLC coatings were revealed.The results show as following:(1)ZDDP-derived tribofilms consist of phosphate and ZnS formed on the steel and DLC surfaces,which presented good antiwear properties on the steel,but had little effect on the DLC coatings.(2)Mo-compounds presented good friction reduction properties by forming MoS₂-containing tribofilms on the steel and DLC surfaces,but they accelerated the wear of amorphous DLC coatings(a-C,a-C:H and a-C:W).In addition,MoDTC showed better friction reduction properties than Moly trimer,wheras Moly trimer was less harmful for the amorphous DLC coatings.(3)ZDDP tribofilm can prevent DLC coatings from the wear acceleration caused by Mo-compounds.However,the decomposion produces of Mo-compounds were negative for the antiwear properties of ZDDP.Therefore,the friction reduction and antiwear properties of ZDDP and Mo-compounds were weakened by combining the two addtives with each other.(4)ZDDP combined dispersant presented better friction reduction and antiwear properties for the steel,rather than for the DLC coatings.In addition,both ZDDP and detergent decomposed to form phosphate/carbonate-containing tribofilm during the rubbing process,which prevented the rubbing surfaces from direact contact.Therefore,ZDDP combined detergent presented better antiwear properties for the DLC coatings.(5)In general,the hydrogen-free DLC coatings(a-C and ta-C)showed better tribological compatibility with additives,but the antiwear properties of a-C:H varioued with the lubrication conditions.Furthermore,the study of a-C:W confirmed that the low hardness of steel substrate may lead to sever wear under high load,but it had a negligible effect on friction.