Continuous Microindentation and Microscratch Measurements of Metal-Ceramic Adhesive strengths
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CONTINUOUS MICROINDENTATION AND MICROSCRATCH MEASUREMENTS OF METAL-CERAMIC ADHESIVE STRENGTHS S. VENKATARAMAN°, D.L. KOHLSTEDT° AND W.W. GERBERICH° " Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. ** Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455.
ABSTRACT To investigate the effect of heat-treatment on the adhesion of Pt thin films to NiO substrates, the strain energy release rate, interfacial fracture toughness and interfacial shear strength were determined from continuous microscratch and continuous microindentation experiments. Samples were prepared by sputtering Pt onto single crystals of NiO, followed by a heat-treatment at temperatures of 300, 500 and 800'C and an oxygen partial pressure of either 0.21 or 105 atm. Continuous microscratch tests were performed by driving a conical indenter with either a 1 or 5 jim tip radius simultaneously into and across the Pt film. From the magnitude of the critical load at the point of film delamination and the area of the delaminated piece of the thin film, the strain energy release rate (practical work of adhesion) and interfacial fracture toughness were calculated. The practical work of adhesion and interfacial fracture toughness increased from 0.2 J/m 2 and 0.2 MPa4 m, respectively, for as-sputtered samples to 4.6 0 J/m2 and 1 MPa'Im for samples heat-treated at 500 C and 10"- atm. Preliminary analysis of continuous microindentation results for Pt/NiO samples yielded interfacial shear strengths of 270 MPa for as-sputtered samples and 725 MPa for samples heat-treated at 500'C and 10-5 atm. While these values are in good agreement with those determined by other methods for metalceramic systems, there are sufficient differences in test method for a single system to require additional analysis of the proposed models.
INTRODUCTION As the number of applications for high-temperature materials increases, a fundamental understanding of the bonding at metal-ceramic interfaces is required to predict their mechanical and adhesive strengths, so that their properties and processing can be optimized. Although many techniques for evaluating mechanical and adhesive properties of interfaces are available, the microindentation and microscratch techniques have assumed special importance in the last few years [1-2]. These techniques are simple, and samples can be tested in the same configuration as they are used in service conditions. Moreover, these techniques enable the determination of various properties such as hardness, elastic moduli and adhesion strength. This paper addresses the adhesion of metal-ceramic systems, with special reference to heat-treated thin films of Pt on NiO. For this system it is known that heat-treatments at reduced oxygen partial pressures result in diffusion of Ni into the Pt, thus producing an intermetallic zone at the interface [3]. Hence the adhesion strength can be increased enormously and systematically by controlling the time, temperature, and oxygen partial p
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