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

The initial yielding behavior due to the indentation and friction process b etween an elastic cylindrical surface and hard coating/functionally graded substrate system is analyzed by finite element modeling. A thin hard DLC fi lm deposited on a soft Ti-6Al-4V alloy substrate is considered as a model s ystem. Two functional gradient substrate conditions are considered: (i) a g radient in yield strength and (ii) a gradient in elastic modulus. In both c ases, appropriate gradients result in significant benefits to the reliabili ty of the coated system compared to the case of an ungraded substrate. The results indicate that systems with an appropriate gradient in yield strengt h: (i) carl withstand significantly higher applied contact stresses (3-12 t imes higher for the present model system), (ii) shift the location of the i nitial 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 mod ulus results in a dramatic reduction in equivalent stress an the contact su rface and at the interface as compared to the ungraded case. The present re sults suggest distinct benefits to the durability of coated systems when us ing a substrate with functionally graded properties. [S0742-4787(00)02002-6 ].