Effect of work hardening on the critical indentation limit in spherical nano-indentation of thin film/substrate systems

A finite element analysis has been carried out to investigate the effect of work hardening of materials on the critical indentation limit (the ratio of indentation depth to the film thickness) to ensure the determination of the ‘film-only’ properties in spherical nano-indentation of thin film/substrate layered systems. The work hardening of the film decreased the critical limit whilst that of substrate did not show any appreciable influence. The film hardening effect was more apparent when a smaller indenter was used. For the case when the film is harder than substrate, the influence of film hardening was more apparent especially when the film/substrate yield stress ratio was low. However, for soft film/hard substrate system, the influence of work hardening was more or less similar regardless of the yield stress ratio. Thus, it is suggested that care has to be taken to include the influence of work hardening when drawing out a critical indentation limit.