Crack Nucleation in ion Beam Irradiated Magnesium Oxide and Sapphire Crystals
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V.N. GURARIE*, D.N. JAMIESON*, R. SZYMANSKI*, A.V. ORLOV*, J.S. WILLIAMS** *School of Physics, MARC, University of Melbourne, Parkville VIC. 3052 Australia "**Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, ANU, Canberra, 0200, Australia ABSTRACT Monocrystals of magnesium oxide and sapphire have been subjected to ion implantation with 86 keV Si- ions to a dose of 5x106 cm-2 and with 3 MeV H÷ ions with a dose of 4.8x107 cm-2 prior to thermal stress testing in a pulsed plasma. Fracture and deformation characteristics of the surface layer were measured in ion implanted and unimplanted samples using optical and scanning electron microscopy. Ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The effect of ion dose on the thermal stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance is determined for sapphire crystals implanted with 86 keV Si. In comparison with 86 keV Si ions the high energy implantation of sapphire and magnesium oxide crystals with 3 MeV H÷ ions results in the formation of large-scale defects, which produce a low density crack system and cause a considerable reduction in the resistance to damage. Fracture mechanics principles are applied to evaluate the size of the implantation-induced microcracks which are shown to be comparable with the ion range and the damage range in the crystals tested. Possible mechanisms of crack nucleation for a low and high energy ion implantation are discussed. INTRODUCTION
Surface modification by ion implantation has been previously shown to alter a number of strength and fracture characteristics of lithium fluoride, magnesium oxide and glass samples [1,2]. In particular, thermal shock testing of LiF and MgO crystals implanted with Ar+ or Si ions has revealed that the fracture threshold is lowered by ion implantation, allowing fracture to be initiated at lower surface temperatures. At the same time, ion implantation produces a higher density of cracks, but such cracks penetrate smaller distances into the material. This effectively raises the damage resistance parameter [3,4]. The observed modification of fracture behaviour is due to the formation of surface energy- absorbing layers which are known to improve the impact and thermal shock resistance of ceramic materials. Fine cracks developed in such layers limit the strength of the material, but provide an effective mechanism for absorbing strain energy during thermal shock and preventing catastrophic crack propagation [5]. Ion implantation has been shown to be effective in generating numerous crack nucleating centers in lithium fluoride, magnesium oxide and glass samples [1,2]. These centers are readily activated to develop multiple microcracking and thus to effectively absorb the strain energy under the applied stress. Ion implantation using 86 keV Si- and Cr÷ ions is shown to be capable of producing h
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