Fracture surface analysis of free-standing diamond films
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P. Klocek, J. T. Hoggins, and J. M. Trombetta Texas Instruments, Dallas, Texas 75265 (Received 23 August 1993; accepted 3 February 1994)
Free-standing CVD diamond disk-shaped films of varying thicknesses between ~ 4 0 and 1000 fxra were fractured in ball-on-ring flexure using a high-precision hydraulic testing machine. Some of the films were smooth on the side formerly in contact with the substrate and rough (large crystallites) on the reverse side and some of the films were polished. Raman spectroscopic examination showed some of the films to be of high quality and others to be of lower quality (higher levels of non-diamond carbon mixed with diamond phase). All fracture surfaces were examined to identify the origin of failure. Strength varied between 650 and 971 MPa, and the fracture toughness was determined from fracture surface analysis to average 8 ± 1 MPam1/2.
I. INTRODUCTION
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Several studies have appeared in the literature on strength and toughness measurement of CVD diamond films, including results based on indentation techniques1 and burst tests.2 The purpose of this study is to demonstrate that fracture surface analysis can be used to obtain fracture toughness values for diamond films. The main advantage of the fracture surface analysis technique is that the critical flaws that are measured are of the type that would exist in service; i.e., the fracture-initiating flaws are caused by the finishing or fabrication process. In addition, we can measure the strength and toughness with one method.
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1/cm FIG. 1. Typical Raman spectrum of the diamond disks.
II. MATERIALS AND METHODS All of the samples were provided by Texas Instruments Inc. Some of the samples were polished and others were not. The polished disks were polished to optical quality on both sides, whereas the unpolished specimens were relatively smooth on the side adjacent to the growth substrate but rough and irregular on the opposite side. The unpolished specimens were broken such that the smooth side was the tensile surface. Some of the disks were analyzed using Raman spectroscopy in order to determine their quality and purity. Figure 1 shows a typical spectrum collected. The full-width-half-maximum (fwhm) peak width of the first order Raman peak at 1332 cm"1 is an indicator of the quality of the diamond and for the disks it was found to be approximately 10 cm"1 as compared to 2 cm"1 (Knight and White7) and 3.3 cm"1 (Solin and Ramdas10) reported for single-crystal diamond. The broad shoulder centered at approximately 1550 cm"1 has been shown to be due to disordered sp2 type hybrid bonded carbon and graphite.7"9 The cross section for Raman scattering 1540
J. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org
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from sp2 bonded carbon is approximately 50 times that of sp3 bonded carbon7'11; thus, an assessment of the disks would be that they are composed mostly of diamond phase with additional amounts of non-diamond carbon phases pr
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