Fracture Behavior of CVD Diamond
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FRACTURE BEHAVIOR OF CVD DIAMOND M. D. DRORY, C. F. GARDINIER, AND J. M. PINNEO Crystallume, 125 Constitution Drive, Menlo Park, CA 94025 ABSTRACT The fracture behavior of CVD diamond is examined by indentation experiments in which slow crack growth has been observed. Diamond films of 400grm thickness were prepared by plasma-enhanced CVD on a silicon substrate, subsequently removed from the substrate, and polished for indentation testing. A microhardness tester was used to produce Vickers indentations under low load. Crack length measurements over time revealed susceptibility to slow crack growth in ambient testing conditions. The stress-corrosion susceptibility coefficient, N, was measured as 9.3. INTRODUCTION The extreme Young's modulus, hardness, and high thermal conductivity of CVD diamond make it an attractive material for a number of applications, including: detector windows, tooling, and heat sinks. Despite the broad interest in CVD diamond only a few of the mechanical properties have been reported. The elastic and plastic properties have been examined with a nanoindenter to determine Young's modulus and hardness at room temperature [1]. Strength has been measured for thin windows by burst testing [2], while the fracture toughness was measured with the Vickers indentation method [3]. The extreme properties of diamond such the highest Young's modulus and hardness makes mechanical properties measurements difficult. For example, fracture toughness is determined at reduced load to avoid indenter failure, thereby producing radial crack length which can be difficult to resolve [3]. Despite this difficulty, the indentation test provides a useful tool for first order property measurements, and is used here to examine the long term reliability of CVD diamond. Slow crack growth is observed in many ceramic materials [4-6]. In particular, slow crack growth, or static fatigue of glass has been the most characterized of these materials in which the effects of environment on crack growth velocity, dc/dt, have been found to be represented by a power law function of the mode I stress intensity factor [4]:
dc =N dt
(1)
N is the stress corrosion susceptibility coefficient that is an environmentally dependent material parameter, and P3is a constant. Slow crack growth in glass has been shown to be strongly influenced by temperature, humidity and the presence of reactive species. The focus here is to examine the susceptibility of
Mat. Res. Soc. Symp. Proc. Vol. 239. @1992 Materials Research Society
562
CVD diamond to slow crack growth at room temperature under ambient humidity. EXPERIMENTAL PROCEDURE Diamond films were grown using plasma-enhanced chemical vapor deposition at 2.45 GHz. Diamond deposition was achieved in the range of 1.52.0 KW and a total gas pressure of 50-100 Torr. Reactant gases, H 2 and carbon precursors were metered with flow controllers at flow rates of 200-300 sccm. The substrates were two inch diameter, [100] silicon wafers with a surface roughness of
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