Assessment of thin-film hardness through elastic/plastic stress analysis in a microindentation test

The true hardness of thin film is assessed through micromechanical analysis of the composite hardness measured by a microindentation tester. In partitioning the respective contributions of the film and the substrate to the composite hardness, we apply the modified plastic-zone volume-law-of-mixtures theory by analyzing the indentation stress field of a film/substrate system. At this time, while the substrate is assumed to undergo radial deformation, the film is assumed to be deformed mainly parallel to the surface. On the basis of these stress analyses, the influence of interface is incorporated; the deformation in the softer material is constrained by interface bonding and the plastic-zone radii are modified by considering the respective virtual pressures that the film and the substrate actually support. When an interface has perfect bonding, the hardness value for a film can be calculated analytically from the condition of strain matching at the interface-approximately 2200 kgf/mm(2) for diamondlike carbon (DLC) films deposited on WC-Co substrates. On the other hand, when the interface is weak, an interface parameter K is introduced to describe the partial release of film constraint due to interface delamination. For DLC films on Si substrates, the expansion rate of the extent of this partial release is constant regardless of both the film thickness and indentation load, so that it can be used as a measure of adhesion loss. (C) 1997 American Institute of Physics.