Plastic zone and pileup around large indentations
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I.
INTRODUCTION
INDENTATIONS have been used to measure the mechanical properties of bulk materials for many years. Conventional hardness testing, such as Rockwell hardness tests, is based on measuring the depth of penetration of an indenter of known shape at a given load into a material. Conical indenters, such as the Brale indenter, are often used in Rockwell hardness tests. Vickers hardness testing relies on measuring the diagonals of the residual indentation made with a pyramidal indenter at a given load. Both methods are used to test bulk materials and are well-established test methods.[1] More recently, there has been a significant amount of research based on light load indentations using depth sensing devices.[2,3,4] These indentations are used in an attempt to measure the mechanical properties of very small volumes of materials, particularly thin films and coatings on substrates. With this work has come a discussion regarding the amount of plastic deformation around indentations and the morphology of the deformation during indentation. The extent of the plastic zone and the amount of material upheaval at the edge of the indentation are significant features in attempts to extract the yield strength of the tested material from the indentation response. When testing film-substrate systems, the adhesion of the film to the substrate is of particular interest. Indentations have been used to investigate this property in many ways. Early studies by Marshall and Evans[5] on the adhesion of films relied on a sharp indenter to penetrate only into the film, causing the film to be placed in radial compression around the indentation. The volume of the indentation was assumed to be displaced radially into the film, creating residual stresses in the film. Due to the compressive stresses, the film would buckle and delaminate from the substrate, providing a measure of the adhesion of the film. More reD.F. BAHR, Graduate Student, and W.W. GERBERICH, Professor, are with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. Manuscript submitted November 1, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
cently, this method has been extended to light load indentations using a depth sensing device.[6] Testing the adhesion of diamond films presents a difficulty not present in other film-substrate systems. Testing diamond films with sharp diamond indenters may cause the indenter to fail before the film that is being tested. Therefore, several researchers have approached this problem with a variation of the initial indentation adhesion tests.[7] Instead of using a small, sharp indenter, a Brale indenter was used. Loads of up to 60 kg were applied to this indenter causing the indenter to penetrate through the diamond films and into the substrate up to depths 100 times deeper than the film. This method has produced delamination and spallation around the indentation of diamond films (and, in fact, other hard coatings on softer substrates[8]). It has been anticipated that variat
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