Analysis of coating fracture and substrate plasticity induced by spherical indenters: diamond and diamond-like carbon layers on steel substrates

Hard coatings as diamond or diamond-like carbon (DLC) layers are widely used as protective coatings on metal substrates, such as steel or hard metal. Failure mechanisms of the substrate/coating composite are studied in this paper through a parametric elastic-plastic finite element analysis, for the common load case of the indentation of spherical bodies into a layered surface considering a wide range of coating thicknesses. For the case of typical DLC layers on tool steel, the first damage of the layer/substrate composite is found to occur by substrate plasticity at or below the interface for most geometries. The fracture mode of the coating changes with varying ratio of layer thickness to indentor radius. If this ratio decreases from large to small values, radial and/or circumferential cracks are initiated first at the surface at the edge of the contact, then preferentially cracking occurrs at the interface on the symmetry axis or at the surface near to the contact edge, and finally again at the interface below the contact area, but without preference for the symmetry axis. Indentation experiments with DLC films on tool steel validate the fracture mechanisms deduced from calculations. For typical DLC layers on tool steel and diamond layers on tool steel, the critical forces for the onset of plastic deformation in the substrate and coating fracture are calculated by means of finite element analysis and analytical approximations of the contact problem. The results can be subsumed in normalised failure maps, from which the optimal coating thickness for the special load case of a spherical indentor can be estimated. Approximate analytical solutions to the results from finite element calculations are derived from simple mechanical analogues. They give more insight into the role of materials and geometry parameters and allow the extrapolation of the results to similar substrate coating systems. For instance, for coating thicknesses equal to or larger than the indentor radius, the load necessary to induce plasticity is shown to vary linearly with the substrate yield strength and the square of the layer thickness. Similarly, the load to initiate coating fracture varies with the square of the layer thickness and was linear with the coating fracture strength if the coating thickness is in the order of the indentor radius. (C) 2001 Published by Elsevier Science B.V. All rights reserved.