THE NANOINDENTATION RESPONSE OF SYSTEMS WITH THIN HARD CARBON COATINGS

In order to further our interpretation of nanoindentation data from co ated systems, and to learn more about the systems themselves, several hydrogenated amorphous carbon (a-C:H) coated systems have been investi gated with systematic variations in coating thickness and substrate ch aracteristics. Three different substrates have been used: a tool steel (M42), a semi-plastic ceramic single crystal (MgO) and a polymer (pol ytetrafluorethylene (PTFE)), each. with chemically vapour-deposited a- C:H coatings of thickness 0.1-1 mu m. Low load indentation experiments were performed with a Nano IndenterTM II with indenter displacements typically less than, or of the order of, the nominal coating thickness es. The resultant data have been analysed in terms of load-displacemen t curves and various comparative parameters (e.g. hardness, total disp lacements and elastic recovery) derived from them. High resolution sca nning electron microscopy has also been used to examine the deformatio n structures at the indentation sites, the occurrence of various crack types in the coating being of particular interest. Our results clearl y show how the change in near-surface elastic and plastic properties c onferred by the coating can be characterized for a given system. Gener ally, the coatings were found to support part of the load and to delay the elastic-plastic transition in surface deformation behaviour to hi gher loads. The MgO studies clearly demonstrate that ''pop-ins'' in th e load-displacement curves can arise from dislocation generation in th e substrate as well as crack propagation in the coatings. The PTFE stu dies demonstrate how the viscoelastic response of the substrate can be modified by the coating.