On the factors affecting the critical indenter penetration for measurement of coating hardness

The nanoindentation test is the only viable approach to assess the properties of very thin coatings (<1 μm) since it can operate at the required scale and provides a fingerprint of the indentation response of the coating/substrate system. To measure the hardness of the coating only it is traditionally assumed that, as a rule-of-thumb, when the relative indentation depth (RID, i.e. the penetration divided by the coating thickness) is less than 0.1, the substrate will not affect the measured hardness of the coating. However, it is found that this rule is too strict for some and too loose for other coated systems. In this paper we present a comprehensive investigation of the factors influencing the critical relative indentation depth (CRID) using finite element simulation. The CRID is very sensitive to tip radius for soft coatings on hard substrates. For most coating/substrate combinations at reasonable penetration depths the 0.1 rule-of-thumb is a safe estimate. It is shown that the elastic property mismatch between coating and substrate also has an important effect on the measured hardness and this means that the Oliver and Pharr method generally used to extract hardness from nanoindentation data may give inaccurate results in coating/substrate systems with significant elastic mismatch.