| In this paper, plasma sprayed Cr3C2-NiCr coatings were prepared and thennanoindentation tests were conducted to achieve elastic modulus and load-depth curves. Afterthat, the nanoindentation process was modeled by finite element method (FEM). Themathematical model between loading-depth curves and elastoplastic properties of coating wasestablished through dimension analysis, inverse analysis and data fitting. The mechanicalproperties of coating were calculated through combination of this model and experimentalloading-depth curve. Finally, digital image processing technology (DIP) was used to extractmeso-structure of coating. By using grayscale threshold, image segmentation was processed.After that, the edges of pores approximate to the original micrograph were obtained viawavelet transform and geometry vectorization algorithm. Then the influence of pores on stressdistribution along spray direction and transverse direction was calculated.The results indicate that the error between calculated elastic modulus through FEM andexperimental value is small and the yield strength is27.6MPa. When indentation depth is notexceeding13%of coating thickness, substrate has little effect on elastic modulus calculationand reaction force increases as coating thickness decreases. Otherwise, substrate has a greatinfluence on elastic modulus calculation and small scale pile up of coating emerges. Sothicker coating with shallower penetration depth should be selected to conductnanoindentation tests for plasma sprayed Cr3C2-NiCr coatings. During loading process, stressdistribution in homogeneous coating shows semicircle shape and maximum stress existsaround indentor and coating-substrate interface. Interface stripping may come up aspenetration depth increases. During unloading process, residual stress around indentor and interface increases as the indentation depth increases. So it is easy to form circumferentialcrack in coating and interface stripping. Thus, the calculated stress distribution can predictcrack initiation areas. When modeling nanoindentation process in consideration of pores, theresults show a greater elastic modulus along transverse direction than along spraying direction.And also pores directly disturb stress propagation and thus the stress distribution showsrandomness. Stress concentration also exists around pores. As the penetration depth increases,stress propagates easily toward to areas with higher porosity. |
