Direct measurement of residual contact area and volume during the nanoindentation of coated materials as an alternative method of calculating hardness

Abstract

The mechanical properties of thin films can be measured by a variety of different techniques, with nanoindentation being one of the most recent developments in this growing field. By using a depth-sensing indentation method it is possible to obtain quantitative values for the hardness and modulus and thus to gain better insight into the response of a material to controlled deformation at such small scales. However, previous work by Tsui and Pharr has shown that the effects of pile-up, particularly in soft films deposited on hard substrates, can produce significant overestimation of the hardness and modulus due to an underestimation of the true contact area by common nanoindentation analysis procedures. By measuring the topography of the residual indent using scanning force microscopy (SFM) and combining this information with the indentation data, it is possible to gain a fuller understanding of the indentation method and its effects on the material being tested. In addition, the true contact area can be directly measured from the SFM images and subsequently used to recalculate the hardness of the material more accurately. Moreover, the SFM allows the plastic volume of indentation to be measured, from which hardness can also be calculated in terms of plastic work. Experimental results are presented for two types of thin film deposited on hard substrates where SFM analysis of indentations at various depths gives significant additional information concerning the true response of the system to instrumented indentation at a nanometric scale. Pile-up effects can be precisely monitored as a function of depth and correlated to hardness variations encountered across the coating-substrate interface.