Functionally graded materials with a soft surface for improved indentation resistance

We report the finding of an optimal layout of functionally graded materials (FGM) towards indentation resistance. This optimum is characterized by a minimum in tensile surface stresses that can lead to a belated onset of cracking compared to homogeneous materials of uniform stiffness. The parameters influencing the tensile surface stresses in a FGM consisting of a soft surface layer, a stiff base material and a graded region between them have been investigated by finite element analysis and an optimum is reported for the first time. The results in general units can be used to design the gradient in any FGM from plastics to ceramics to result in low tensile surface stresses for a given load.