Fracture strength measurement of filament assisted CVD polycrystalline diamond films
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The fracture strength of polycrystalline diamond films deposited by filament assisted chemical vapor deposition in the thickness range of 3.5 to 160 fim is investigated. Using a burst pressure technique, the fracture strengths of circular diamond film specimens are calculated. An average fracture strength of 730 MPa for nine samples was computed. This value is in good agreement with published strengths of microwave plasma deposited diamond films, comparable to other high strength materials, and within an order of magnitude of the fracture strength of bulk natural diamond. The average fracture strength of the fine-grained substrate interface appears consistently higher than that of the coarse-grained diamond growth surface.
I. INTRODUCTION Polycrystalline diamond produced by chemical vapor deposition (CVD) is emerging as one of the most important materials of the 1990's. Its desirable properties include high hardness, strength and stiffness, high electrical resistivity, high thermal conductivity, and optical transparency from the ultraviolet to infrared wavelengths. 12 These properties, combined with its chemical inertness, make diamond an excellent material of choice for optical applications in a wide variety of environments. Published data on most of these properties are abundant although diamond film fracture strength data are not so voluminous.3 Windischmann et al. have reported4 the ultimate tensile strength of microwave plasma chemical vapor deposition (MPCVD) diamond films, and the strength of bulk natural diamond is found in Refs. 1 and 2. A thorough characterization of the mechanical properties of thick (>1 /im) diamond films for use in thick film applications involves the evaluation of their strength. In this study, we report the fracture strength of free standing diamond films in the thickness range 3.5—161 /im prepared by a hot filament assisted CVD method. We calculate the strength of both the substrate interface and growth surfaces of the diamond and compare our results with published values of natural diamond and other infrared dome materials. The columnar growth mechanism in diamond films produces a microstructure and morphology of the growth surface which differs from that of the initial interface.5 The mechanical properties related to the structure are therefore likely to be significantly different for one side of the freestanding membranes compared with the other side. As the diamond film grows the grain size increases ^Present address: Department of Mechanical Engineering, University of California, Davis, California 95616. 1432
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 6, Jun 1992
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and morphology changes so that a difference in mechanical strength is expected. Applications such as x-ray lithography masks and infrared dome coatings require a broad understanding of the mechanical properties of diamond films. In applications where unsupported diamond membranes are utilized, such as detector windows, differences in the mechanical properties of one surface c
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