Microstructural changes due to making a multilayer

There is considerable experimental work in both metallic and ceramic films, showing that such internal stresses can greatly increase the measured hardness. The simplest explanations are in terms of superimposing an inplane stress on the overall maximum shear stress under the indenter, although Pharr (Bolshakov eta ., 1996 Tsui et al 1996), looking at nanoindentation of A1 films, considers the effect to arise due to pile-up around the indenter, causing the actual depth of penetration, and hence area of the indentation, to be greater than that calculated. 

5 Variation of the internal stress and measured hardness for a variety of multilayer films compared with monolithic films. The multilayer films are shown with filled symbols and the monolithic films with unfilled ones. The TiN was made by either ion assisted arc deposition (IAAD) or filtered arc deposition (FAD). Note that hardnesses in the monolithic materials of up to 50 GPa appear to be achievable simply by internal stresses. 

In uniformly strained materials, deformation structures can be readily observed using transmission electron microscopy. However, it is much more difficult to prepare a similar sample where the deformation is more localized, as is the case of nanoindentation. Recently this situation has been revolutionized by the development of focused ion beam techniques for semiconductor processing, so that it is possible to select the region to be thinned to within 100 nm (Overwijk et al., 1993 Saka, 1998). 

7 Cross-sectional TEM images of (a) a CrN-AIN multilayer (the AIN is the lighter phase) and (b) monolithic CrN. Note that there are long pores present in the monolithic CrN, marked by the black arrow, but these are not present in the multilayer. 

A rather extreme example of this is given by Wang et al. in a study of TiN/ AIN multilayers (Wang et al., 1998). For samples made with pulsed d.c. substrate bias, the improved hardness was retained down to the lowest values of A. However, when r.f. substrate bias was used the structure at A 2 nm 

---