Substrate and bonding layer effects on performance of DLC and TiN biomedical coatings in Hank's solution under cyclic impact-sliding loads

A cyclic inclined impact-sliding test was operated in an unlubricated, ambient environment and Hank's balanced salt solution (HBSS) to study the contact fatigue wear behavior of DLC and TiN biomedical coatings on relatively soft but corrosion resistant Ti alloy (Ti6Al4V) and hard but corrodible AISI M2 steel as model systems. The test was designed to simulate coating wear under combined impact and sliding motion. In each impact-sliding cycle, the forces comprised a dynamic impact load, F_i (140N) and a "pressing" load, F_p (300N). As expected, both coatings performed better on hard M2 substrates than Ti substrates under ambient test conditions. In the HBSS-lubricated solution test conditions, no obvious corrosion degradation occurred when either the bonding layers or substrates were Ti-based; instead, the solution provided a lubricating effect and enhanced coating performance. When the bonding layer for the DLC coating case was Si-based, it could not prevent crack propagation into the substrate after a certain number of test cycles. The crack opening allowed the HBSS solution to contact the substrates, which should only cause a minor problem when the substrate was a corrosion-resistant Ti alloy. However, when the substrate was corrodible M2, a severe corrosion-induced weakening of the interface occurred. When the coating bonding layer was a Ti layer (within the TiN coating), it could function to some extent as a corrosion and crack barrier to protect the M2 steel from interface degradation. Thus, a corrosion-resistant bonding layer and its ability to impede extension of cracking under cyclic dynamic loads can have a positive influence on the coating performance when the substrate has inferior anti-corrosion properties.