A criterion to identify sinking-in and piling-up in indentation of materials

• The elastic modulus of various metals and alloys are determined by instrumented indentation.
• Frame compliance of the instrument is taking into account for accurate measurements.
• Piling-up and sinking-in are considered for the contact area calculation.
• A simple criterion is proposed for identifying the deformation mode.
• Piling-up and sinking-in both coexist and their relative importance depends on this criterion.

The instrumented indentation test is usually used to determine the mechanical properties of materials. Depending on the nature of the material, the way the matter flows under the indenter by piling-up or sinking-in affects the calculation of these mechanical properties. Consequently, corrections proposed by Oliver and Pharr and Loubet et al. should be done according to these two behaviors in addition to other corrections associated with the indenter tip defect as well as the compliance of the instrument. In this work we tested different materials having supposedly piling-up or sinking-in behavior: low-carbon steel, aluminum, brass, copper, beta tricalcium phosphate (β-TCP) bioceramic, rolled or sintered stainless steel and ceramic composite TiB2–60% B4C by using two types of indenter, i.e. Vickers and Berkovich ones. From the corrected load–indenter displacement curve, we showed that a criterion, defined as the ratio between the residual indentation depth and the maximum indentation depth reached at the maximum load, is able to identify the predominant deformation mode. For materials for which this ratio is higher than 0.83 piling-up prevails while it is sinking-in when it is lower than 0.83. When the ratio equals 0.83, the two modes of deformation should coexist since the calculations made using either correction of Oliver and Pharr or Loubet et al. give the same results. This novel way of considering the instrumented indentation measurements renders more accurate the determination of the hardness and the elastic modulus since the observation of the indent is then not required for identifying the deformation mode which affects the contact area calculation.