Changing the Indentation Behaviour of MgO by Ion Implantation
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CHANGING THE INDENTATION BEHAVIOUR OF Mgo BY ION IMPLANTATION
P.J. BURNETT AND T.F. PAGE University of Cambridge, Department of Metallurgy and Materials Science, Pembroke Street, Cambridge, CB2 3QZ, U.K.
ABSTRACT Cleaved {100} MgO single crystal surfaces have been implanted 16 8 with 300keV Ti+ and Cr+ ions to doses in the range 10 -101 ions cm- 2 . Localised plasticity and fracture behaviour has been investigated using microhardness indentation tests at loads of between 10gf and 500gf. Significant surface hardening has been observed at lower doses with amorphisation related softening occurring at higher doses. Post-implantation heat treatments produce precipitation reactions with associated hardness changes.
INTRODUCTION By changing the chemistry, structure and properties of a highly-localised thin surface layer ion-implantation is capable of modifying materials behaviour in areas such as corrosion, fatigue and wear [e.g. 1]. This paper arises from a study of the means by which implantation might affect the triWe bological properties (e.g. hardness, friction, wear) of hard ceramics. have already reported [2-4] that implantation-induced stresses can modify near-surface crack genesis and growth, thus altering the indentation fracture behaviour of brittle solids. Also, implantation can lead to significant solid solution and/or radiation hardening effects, while at higher doses 7 2 (typically k4 x 101 ions cm- ) amorphisation of the surface can result in surface softening. Further we have proposed a simple quantitative model which allows the position and extent of this amorphous layer to be predicted [3]. Thus, it has been shown that amorphisation begins at the peak of the 2 damage profile (e.g. for MgO this occurs at '\8x 10O6Ti+cm- @ 300keV [5]), the initially forming a sub-surface amorphous layer. With increasing dose 7 layer extends to the surface (e.g. for MgO this occurs at "'5 x 101 Ti+ cm-2 @ 300keV [5]). Precipitation hardening has also been reported during the post-implantation heat-treatment of sapphire [4,6]. This paper presents the results of an exploration of the extent to which similar phenomena might be induced by implanting Mgo with aliovalent species.
EXPERIMENTAL High purity (99.99%) {100} habit MgO single crystals, supplied by W & C Spicer, were cleaved into (100) surface slices "'10 x 5 x 2mm and imImplantation with 300keV Ti+ and Cr+ ions planted in the as-cleaved state. 16 8 2 to doses in the range 10 -101 ions cm- were carried out in the CockcroftThese ion species were chosen on the critWalton facility at AERE Harwell. eria of ionic radius and, more importantly, valency as those likely to cause solid-solution hardening in MgO [7]. The doses were determined by postimplantation Rutherford backscattering (RBS). Microhardness tests (Leitz "Miniload") were used to determine materials response to surface contacts. All tests were performed under ambient conKnoop microhardness tests at loads of 10gf and ditions at room temperature. 25gf were performed on all specimens, the long axis of the indenter
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