Theoretical Modeling Developed and Applied to Evaluate the Mechanical and Interfacial Properties for the Multilayer Composite Film Produced in Nanoindentation Tests

• Main Authors: Chang-Fu Han, 韓長富
• Other Authors: Jen-Fin Lin
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/70188687637525558257

博士 === 國立成功大學 === 機械工程學系碩博士班 === 97 === The present studies are theoretical modeling developed and applied to evaluate the mechanical and interfacial properties for the multilayer composite film produced in nanoindentation tests. The present studies can consist of three subjects. In the first subject (in the Chapter 2), a new mechanical model is developed in the present study for hard materials to investigate the behavior arising during the loading/unloading process of a nanoindentation test. Two governing differential equations are derived for the depth solution of the indenter tip and the depth solution formed at the separation point expressed in a power form. The exponent value in either the loading process or unloading process is considered to be a variable as a function of the indentation depth in the governing differential equation. All coefficients shown in these governing differential equations associated with the spring and damping behavior are determined by the real-coded genetic algorithm. Quartz and silicon were used as the examples of hard materials, and the contact projected area predicted by the present model is quite close to the solution predicted by the area function of Oliver and Pharr [1]. The phase lagging behavior demonstrated in the nanoindentation test at two different loading/unloading rates was investigated, and it is enhanced by increasing the loading/unloading rate. No restriction for hard materials in the loading rate is needed in the nanoindentation test if the present model is employed. The contact area is decreased by increasing the loading rate. In the second subject (in the Chapter 3) and according the above modeling, a general mechanical model employed to describe the contact behaviors and deformations arising at all layers (including the substrate), is successfully developed in the present study for multilayer specimens in order to evaluate the contact projected area by a theoretical model, and thus the hardness and reduced modulus, using nano-indentation tests. The governing differential equations for the depth solutions of the indenter tip formed at all layers of the specimen under their contact load are developed individually. The influence of the material properties of the substrate on a multilayer specimen’s hardness and reduced modulus at various indentation depths can thus be evaluated. Using the present analysis in the C (top layer)/a-Si (buffer layer)/Si(substrate) specimen, the depths corresponding to the transition and pop-in behaviors can be predicted effectively. In the present study, the indentation depth corresponding to the pop-in arising in the loading process is found to be quite close to the C/a-Si composite film thickness. This load-depth behavior gives a clue that the occurrence of pop-in is perhaps related to the buckling of the composite film which had already delaminated from the silicon substrate. In the third subject (in the Chapter 4), the membrane theory was first applied to develop the internal compression stress arising at the nanoindentation as a function of the normal load and the composite film thickness t*. A deflection model for a rectangular plate fixed at its two end and sides was developed equivalent to the indentation behavior using a Berkovich indenter. This model is convenient for us to determine the mean compression stress and thus its value occurring the first-mode buckling. The equality of and ( )buckling allows to determine the indentation depth of having buckling in the composite film. This indentation depth of buckling predicted by the present model is quite close to the pop-in depth obtained from experimental results, regardless of the change in the C-film thickness. This characteristic reveals that the present model is developed successfully to predict the pop-in depth of a specimen; and the pop-in is created due to the buckling of the composite film under a compression stress. From the above studies in multilayer specimen, the mechanical properties and delamination of the composite film can be predicted more efficiently.