Crack Healing in an Alumina/Silicon Carbide Nanocomposite After Grinding and Annealing
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CARBIDE NANOCOMPOSITE
H.Z. WU*, J.M. TITCHMARSH*, S.G. ROBERTS*, B. DERBY** *Department of Materials, University of Oxford, Parks Road, Oxford OX I 3PH, UK "**Manchester Materials Science Centre, Grosvenor Street, Manchester Ml 7HS, UK ABSTRACT Alumina/silicon carbide nanocomposites are known to show their highest strength levels after surface grinding followed by annealing. After annealing in flowing argon, nanocomposites with very coarsely ground surfaces have strengths exceeding those with a finely polished surface. Specimens with lapped surfaces also show a small improvement in strength on annealing. TEM investigations of annealed cross-sections show that the annealing process leads to surface crack healing. The chemical composition of the subsurface region has been studied, and reactive products on and close to the nanocomposite surface after annealing have been investigated by energy dispersive X-ray analysis in the STEM. INTRODUCTON Strengthening of A120 3/SiC nanocomposites after annealing was reported by Niihara et al. [1]. They found that after annealing at 1300°C for 2 hrs in argon or air, the A120 3/SiC nanocomposite increased in bending strength from 1000 MPa to 1500 MPa. They attributed this strengthening effect to the following possible mechanisms: healing of subsurface cracks, relaxation of local residual stress originating from different thermal expansion coefficients, and the formation of sub-grain boundaries. Later Zhao et al. [2] also reported a strengthening effect after annealing but with a smaller strength increase. They argued that annealing had the double effect of diminishing the compressive surface residual stress after surface machining while also healing the surface flaws. Thompson et al. [3] investigated whether machining flaws could be healed in A120 3/SiC nanocomposites during annealing by comparing the healing behaviour of indentation cracks in A120 3/SiC and A120 3. Wu et al. [4] found that a machine ground nanocomposite, with a severely damaged surface, could be strengthened to a level greater than that of a finely lapped nanocomposite. By measuring the surface Rayleigh wave velocity with line-focus acoustic microscopy, the results showed that annealing increases the surface elastic modulus of the ground nanocomposite. This surface stiffness improvement is thought to be an indication of subsurface crack healing during annealing. Further study indicated that chemical reactions occured on the surfaces during annealing although the precise mechanisms leading to strengthening were unclear. In this paper we investigate further the mechanisms of strengthening of A120 3/SiC nanocomposites after grinding and annealing, using TEM of surface cross-sections. EXPERIMENT The A120 3/SiC nanocomposites used in this investigation were prepared by hot-pressing with a standard freezing-drying powder preparation method explained in detail elsewhere [5]. c-A12 0 3 powder (AKP53 Sumitoma, Japan) is mixed with 5% by volume SiC particles (UF45 Lonza, Germany) of diameter ; 90 nm by attrition milling an
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