Finite element analysis of the initial yielding behavior of a hard coating/substrate system with functionally graded interface under indentation and friction

The initial yielding behavior due to the indentation and friction process between an elastic cylindrical surface and hard coating/functionally graded substrate system is analyzed by finite element modeling. A thin hard DLC film deposited on a soft Ti-6Al-4V alloy substrate is considered as a model system. Two functional gradient substrate conditions are considered: (i) a gradient in yield strength and (ii) a gradient in elastic modulus. In both cases, appropriate gradients result in significant benefits to the reliability of the coated system compared to the case of an ungraded substrate. The results indicate that systems with an appropriate gradient in yield strength: (i) can withstand significantly higher applied contact stresses (3-12 times higher for the present model system), (ii) shift the location of the initial yield point deeper into the substrate (at least by a factor of 2 for the present model system), and (iii) can use coatings of greater thickness. Finally, the results indicate that an appropriate gradient in elastic modulus results in a dramatic reduction in equivalent stress on the contact surface and at the interface as compared to the ungraded case. The present results suggest distinct benefits to the durability of coated systems when using a substrate with functionally graded properties.