NANOINDENTATION STUDIES OF CHEMOMECHANICAL EFFECTS IN THIN-FILM COATED SYSTEMS

Recently, nanoindentation techniques have been successfully used to st udy chemomechanical effects on the very near-surface mechanical proper ties of single-crystal sapphire. In order to investigate whether such chemomechanical effects also occur in ceramic-coated systems, a series of experiments have been performed on a 100 nm TiN coating (over a 10 0 nm Ti interlayer) on a silicon substrate. Indentation loads were cho sen to produce displacements less than and greater than the coating th ickness. The specimens were annealed at low temperature in a vacuum ov en for 24 h to desorb any physi- or chemisorbed surface species and th en exposed to several environments including water and a series of alc ohols and tested dry. Three distinct effects were observed. At displac ements greater than the coating thickness, some environments (especial ly water) produce fracture of the coating around indentations although no differences in the load-displacement responses of the different en vironments were evident. At very low loads (1 mN) some evidence was fo und for both dislocation nucleation and plasticity being environmental ly sensitive with ethanol and methanol extending the load range over w hich ''elastic-only'' response occurs on surface contact. Further, all low load results displayed pronounced irreproducibility and ''wavines s'' at a spatial scale commensurate with the microstructure suggesting that even the low temperature anneal had relaxed the stresses providi ng structural cohesion and allowed ready ingress of liquids etc.