A nano-indentation study of the mechanical properties of thin surface layers affected by manufacturing processes

Nano-indentation has become an important tool in the study of mechanical properties of solids at small length scales, ever since its formulation as a technique in the early 1980s [1]. The small size of an indentation, typically one micrometer or less, makes it a potentially attractive tool also for the quantitative study of surface layers in monolithic solids. Here, we report results from a study in which nano-indentation has been combined with taper-sectioning to analyze the mechanical properties of thin surface layers affected by manufacturing processes. The so-called white etching layer (WL) produced in steel surfaces by machining and abrasion has been characterized. The WL is found to have a hardness in the range of 11.5 GPa - 16.2 GPa, which is significantly greater than that of untempered martensite produced by heat treatment processes. The grain size within the WL is shown to be in the sub-micrometer range with values ranging typically between 30 nm and 500 nm, making the layer a nano-crystalline structure. The so-called "bum-layer" produced on ground surfaces of steels is found to have a hardness distribution very similar to that of a white layer, suggesting that the two layers are of the same type. Localized hardening and softening of surface layers, over spatial extents of a few micro-meters, caused by material removal processes are accurately resolved by the combined use of nano-indentation and taper-sectioning. The taper-sectioning/nano-indentation approach is also shown to be a good procedure for characterizing the hardness and Young's modulus of PVD-TiN films deposited on hard material substrates.