Thermal shock-induced fracture of ion-implanted LiF crystals
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J. S. Williams Microelectronics and Materials Technology Centre, RMIT, Melbourne 3001, and Department of Electronic Materials, Engineering Research School of Physical Sciences, ANU, Canberra 2600, Australia (Received 10 May 1989; accepted 15 February 1990)
Monocrystals of LiF, ion implanted with Ar+, were exposed to thermal shock in a plasma of different intensities. Ion implantation substantially alters the fracture pattern and characteristics of the material, particularly in reducing the thermal shock resistance parameter, S', and in increasing the damage resistance parameter, 5"'. The former parameter indicates that ion implantation allows fracture to be initiated at lower thermal shock temperature differences and the latter parameter is associated with higher crack densities and lower crack penetration depths. The increase in the parameter 5"' indicates that ion implantation can result in a higher mechanical stability and greater durability of the crystals damaged by thermal shock. Surface melting at very high heat fluxes eliminates any effect of ion implantation.
I. INTRODUCTION
The low mechanical stability of ceramic materials operating under conditions of severe thermal shock considerably limits their use in many technological applications. It is known that thermal shock-induced fracture mainly originates at the surface, where maximum tensile stresses first appear, and then it propagates into the crystal interior.1'2 The response to thermal shock essentially depends on the state of the surface and material surface properties. Cracks initiate at surface defects such as cleavage steps, microcracks, scratches, twins, grain boundaries, and other surface inhomogeneities.3'4 This means that resistance to thermal shock can be altered by modifying the state of the surface and material surface properties. The subject of the present article is to investigate the effect of surface modification produced by ion implantation on material stability under thermal shock conditions. Material response to thermal shock is characterized by two major parameters. (a) The thermal shock resistance parameter, S', represents the minimum temperature variation necessary to initiate fracture. For thermal shock produced by rapid quenching of uniformly heated brittle samples5 the parameter is S' = (1 - ix)o-f/aE, where /JL is Poisson's coefficient, o-f is the fracture stress, a is the heat expansion coefficient, and E is the elastic modulus. For thermal shock produced by pulse heating of brittle materials,6 S' is obtained from the following thermomechanical model. During the heating period a cold rigid matrix prohibits expansion of the surface J. Mater, Res., Vol. 5, No. 6, Jun 1990
layer, producing compressive stresses first elastically and then plastically. Fracture cannot occur on cooling unless plastic deformation takes place during the heating cycle since tensile stresses are needed to initiate fracture. The heating stage is represented by the negative stresses on the stress-temperature diagram in Fig. 1, where ay is the compressiv
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