How to deal with roughness in nanoindentation measurements

The nanoindentation technique has primarily been developed for use on thin layers and coatings because it allows measurements of layers with micrometer and sub-micrometer thickness. The demand came from the thin film industry and research because hardness of thin films and coatings provides fast estimation of their quality. Since then, the method has found wide use also in other areas of material research such as biomaterials or metallurgy where hardness of different phases or grains reveals information about their microstructure. With the growing demand for measurement of surface and near-surface properties the surface morphology became an important factor. Many thin films and coatings are smooth with roughness below 1 nm. However, there are many thin films whose surface roughness is in tens and hundreds of nanometers. The same applies for metallic samples: some are perfectly smooth but many samples exhibit considerable surface roughness. In nanoindentation the depth involved in the mechanical response is often in the tens and hundreds of nanometers, which is in the same order of magnitude as the surface roughness. The effects of surface roughness are mentioned in the ISO 14577 standard which specifies that the Ra of the measured surface shall be less than 5 % the maximum indentation depth. In real life, this criterion is sometimes difficult to achieve and it is therefore good to know the effects of roughness on the nanoindentation results. In this application report we demonstrate the influence of roughness on the hardness results on two typical nanoindentation samples: 4.2 μm thick AlTiN hard coating and stainless steel. Both materials were available with different surface states: in the as-deposited state (AlTiN) and in a polished state (stainless steel). The surface roughness was then modified by either roughening with sand paper (stainless steel) or by polishing with 3 μm diamond paste (AlTiN).