Development of an indentation method for material surface mechanical properties measurement [electronic resource] /

by Yao, Ziheng

Development of an Indentation Method for Material Surface Mechanical Properties Measurement Ziheng Yao With the utilizing structural materials with small size dimensions (such as MEMS devices), there are growing need to characterize the mechanical properties of the small volumes of materials. Tensile test is not possible to be applied to these small volumes. Indentation technique has emerged as a cost-effective, convenient and non-destructive method to solve this problem. It is also a promising means of obtaining mechanical properties for thin films, which were widely used in various industries nowadays. In this work, finite element simulations of spherical indentation on various material models were performed to develop a series of methodology, by which the material surface mechanical properties are to be determined through indentation test. Finite element mesh and modeling were first verified by both Hertz analytical solution and experiment results of surface displacement fields from combined Moiré Interferometry and Twyman-Green Interferometry. Method to calculate elastic properties through initial unloading indentation stiffness obtained in load depth curve is verified. Special attention was put to the relationship between the surface displacement fields (U (in-plane radial displacement) and W (out-of-plane displacement)) and material mechanical properties. It is discovered that each point on the surface experienced same equivalent stress strain history governed by input uniaxial stress strain curve. Elasticplastic boundary corresponds to characteristic point in the W field variation, which indicated an experimental method to capture the E-P boundary. The yield strength could be calculated from Hooke’s law by strain fields derived from displacements using kinematic equations. The strain-hardening exponent is to be obtained by modified Meyer’s law or Tabor’s relation. Hence, the uniaxial stress strain relation of the material could be reestablished using indentation method. Then the micro spherical indentations on thin film – substrate systems were simulated. The influence of substrate on the load depth curve, surface deformation was studied for both soft film on hard substrate and hard film on soft substrate cases. Then different thickness of thin film was simulated and it is concluded that the effect of substrate is negligible when the indentation depth is less than 10-20% of film thickness.