Evaluation of hardness, tribological behaviour and impact load of carbon-based hard composite coatings exposed to the influence of humidity

Diamond-like composite coatings were deposited by PACVD with an interface magnetron sputtered metallic layer prepared by d.c. unbalanced magnetron sputtering onto HSS substrates. The focus of the present work was on the study of the humidity effect on the tribological properties, hardness and impact resistance of the deposited coatings. The coatings were tested under both dry and humid conditions using several testing methods, like the pin-on-disc test equipped with linear reciprocating movement, impact test and depth sensing indentation test. It was found that the humidity may substantially influence not only the results of tribological tests, but surprisingly, also the results of the dynamic wear and hardness tests. By increasing the relative humidity, the value of dynamic impact resistance increased at an equivalent stress of 1.6 GPa more than three times. On the other hand, the wear rate of the coated part increased by more than four orders of magnitude. The results of the indentation test were influenced by humidity mainly at nanometre scales. The hardness measured at dry conditions was higher by 25% than the values obtained by the measurements under humid conditions.