Residual Stress Determination and Subsurface Microstructure in Ground and Polished Alumina/Silicon Carbide Nanocomposite
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Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
25 jim grit size diamond slurry. This was followed by the removal of at least 100 Pjm using an 8 jim slurry, and 50 jim by a 3 jtm and a 1 jim slurry in turn if required. All polishing was carried out on a Kemet polishing machine (Engis Ltd., UK) with a rotating rate of about 60 rpm and an external load of 1.5 kg. The 3 and 8 jim polishing operations were done on hard plates and 1 jim finishing was done on a soft cloth. Hertzian indentation tests were carried out using a CK10 testing machine (Engineering Systems, Nottingham, U.K.). The indenter is a 5 mm diameter polycrystalline alumina ball. For every specimen, at least 40 tests were made. After chemical etching and light lapping, optical microscopy was used to check for the presence of a Hertzian "ring" crack around each site; data from tests with no visible crack were not used. Fluorescence spectroscopy was carried out on the same surfaces as used for Hertzian indentation. Fluorescence spectra were obtained using an unmodified Raman microprobe system [3]. The fluorescence spectra were obtained using the 632.8 nm red line of a 15 mW He-Ne laser with an intensity of about 1.0 mW. This beam intensity was found to cause minimum shift in the wavenumber of the fluorescence lines due to heating effects. Measurements were made using a x40 microscope objective lens with a numerical aperture of 0.65, giving an approximate beam diameter on the specimen of 2 jim. 30 probing sites were randomly selected for measurement on each specimen. The details of TEM cross-section specimen preparation are described in a companion paper in these proceedings [4]. The TEM observations were carried out using JEM 200CX and CM20 microscopes operated at 200 kV. RESULTS Surface Residual Stress Contact mechanics analysis shows that a given test material/indenter ball combination, there is a minimur n indentation load required to produce Hertzian fracture [5,6]. If indenter and test material are of elastically identical materials, and the test surface is stress IP 3P 8P G free, this minimum fracture load 0 depends only on the K1c, Young's modulus and Poisson's ratio of the material [6]. Thus measuring the .- 200 minimum fracture load can be used to determine K1c. If there is a residual S-400 stress in the test surface, the minimum load to fracture will change [5]. The Dalumina stress can be calculated by using the nanocomposite * -600 "apparent fracture toughness" (Ksc) derived from the minimum load for -800 Hertzian fracture on the stressed surface, and the real fracture toughness (K%) as Figure 1 Surface residual stress detected by Hertzian irndentation. Surface finish conditions follows: (Kor_ K0) include: machine grinding (G), 8 jtm polishing ( )c rr 1.2 (8P), 3 jim npolishing (3P) and 1 jtm polishing 1.12t;rfc (1P). where c* is the surface crack depth
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giving rise to fracture at the minimum load; for a 5 mm diameter ball indenter on alumina c* is 7 jim [5]. The calculated surface residual stress values are shown in
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