Modeling of elastic modulus and hardness determination by indentation of porous yttria stabilized zirconia coatings

The mechanical properties of materials can be determined by means of the instrumented indentation experiment. However, when the indentation test leads to a regular load-depth curve for homogeneous massive materials, the presence of defects such as roughness, porosity or cracks present in the region close to the indented zone can greatly modify the shape of this curve. To avoid influence of such defects on the indentation measurement, a cautious polishing is generally performed to obtain a smooth surface. However, for the study of coated materials when polishing is not possible, the defects can interfere with the results of the mechanical property determination and they must be definitely taken into consideration for the indentation data analysis. In this work, the indentation test is employed to characterize suspension plasma sprayed porous ZrO2 + 8 wt.% Y2O3 (8YSZ) coatings. By comparing different models for hardness analysis, we selected the most appropriate one allowing the calculation of the macrohardness taking into account the influence of roughness, porosity and cracks. Afterwards, we showed how the roughness interferes with the depth measurement at the beginning of the indentation test where a fast depth increase is observed. We also showed how the presence of confine defects leads to an abnormal effect during the loading, such as a horizontal plate or a jump in depth. Finally, we propose a methodology to avoid the influence of the roughness and of the porosity on the hardness determination and to model a selected part of the loading curve for the determination of the macrohardness and a parameter representative of the indentation size effect. (C) 2012 Elsevier B.V. All rights reserved.