Multi-Scale Tribological Mechanisms Of Diamond-Like Carbon Under Water Environment

As one of the main forms of mechanical failure for the key components in vehicles,friction and wear are common phenomena during the motion of machines.Currently,two methods are normally applied to reduce the friction and wear of tribo-pairs,surface modification and interfacial lubrication.Diamond-like carbon(DLC),as surface coating,has excellent anti-wear and-friction properties;Water,as liquid lubricant,hold good fluidity and thermal conductivity;the tribological performance of tribo-pairs can be effectively improved when they work together.However,due to the many factors during the sliding process,the interfacial evolution is ambiguous,and tribological mechanisms remain unclear.In the present thesis,the multi-scale tribological mechanisms of DLC under water environment are carefully studied,according to the multi-scale simulations and experiments.The main conclusions are listed as follows:(1)In the nanoscale,the load-induced interfacial C-C covalent bonds mainly contribute to the increase of interfacial adhesion,which is the major cause of increased friction force.However,the high temperature and wear during the friction lead to the increase of graphite phase carbon atoms and the loose carbon networks,which prevents the continual increase of friction force in the running-in period.The surface morphology influences the distribution of water molecules and the direct contact between DLC films.For cases with smooth surface,the friction interfaces are gradually merging and water molecules are diffusing toward the films.For cases with surface textures,the wear of textures results in the interfacial diffusion of water molecules which are originally accumulated in the surface grooves,and thus reducing the direct contact of DLC films.(2)In the mesoscale simulations,the coarse-grained DLC reduces the simulation computation to 1/8 comparing to that of nanoscale simulation,and the results from both scales are similar.The normal load determines the increase of friction force through changing the real contact areas,but water behaviors cause the complicated variation of friction force.Under a small load,the behavioral change of water from accumulation to diffusion makes that the initially low friction force dramatically increases in the late sliding process.In contrast,the good diffusion of water under a large load leads to the gradually decrease of the initially high friction force.(3)The friction curves obtained from experiments show the similar variation tendencies with that obtained from nano-scale and meso-scale simulations,which demonstrates that the simulation methods can well predict the variation of friction force of DLC under water environment.However,due to the different water moving environment,there are slight variations in tribological mechanisms at different scales.