V.N. Gurarie

Thermal shock-induced fracture of ion-implanted LiF crystals

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, S ″. 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 S ″ 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.

Raman-based analysis of implantation-induced expansion and stresses in sapphire crystals

Raman spectroscopy was used to determine the lattice expansion and stress distribution within the ion implanted layer in sapphire crystals. The crystals with the (11 2 0) facewere implanted with 3.0 MeV H + ions to doses of 3.3 × 10 17 cm −2 and 4.8 × 10 17 cm −2 .The strain components and their variation with depth were analyzed by measuring the shift of the Raman peak on the cross-sectional basal plane. A continuum mechanics approach considered a model of a semi-infinite anisotropic elastic space subjected to the implantation-induced lattice expansion. The expansion and resulting compressive stresses were found to increase with depth, reaching a sharp maximum at the end of the ion range. The implantation-induced expansion coefficient was shown to be independent of the ion energy loss and implantation depth in sapphire. Such behavior was discussed in light of stopping and range of ions in matter data and defect production by nuclear collisions and ionization processes.

SURFACE ENERGY ABSORBING LAYERS PRODUCED BY ION IMPLANTATION

Abstract Single crystals of magnesia have been ion implanted with 80 keV Si − and Cr + ions at variable doses and then subjected to testing in a shock plasma. The peak surface temperature has been calibrated by measuring the size and temperature deformation of the fragments formed by multiple microcracking during thermal shock. The crack density curves for MgO crystals demonstrate that in a wide range of thermal shock intensity the ion implanted crystals develop a system of microcracks of a considerably higher density than the unimplanted ones. The high density of cracks nucleated in the ion implanted samples results in the formation of a surface energy absorbing layer which effectively absorbs elastic strain energy induced by thermal shock. As a consequence the depth of crack penetration in the layer and hence the degree of fracture damage are decreased. The results indicate that a Si implant decreases the temperature threshold of cracking and simultaneously increases the crack density in MgO crystals. However, in MgO crystals implanted with Cr a substantial increase in the crack density is achieved without a noticable decrease in the temperatutre threshold of fracture. This effect is interpreted in terms of different Cr and Si implantation conditions and damage. The mechanical properties of the energy-absorbing layer and the relation to implantation-induced lattice damage are discussed.

Synthesis of Carbo-Nitride Films Using High-Energy Shock Plasma Deposition

The present work reports on the properties of nitrogen rich carbon films produced by an intense gas discharge between carbon electrodes in a nitrogen atmosphere. The energy of the discharge, initial nitrogen pressure, number of discharges and geometry are varied to establish their effect on the nitrogen content and the mechanical, structural and morphological characteristics of the deposited carbon-nitride films. The structural diagnostics include optical and scanning electron microscopy, as well as Auger and Raman Spectroscopes and Rutherford Backscattering. The C-N films formed fell into two categories, differing in morphology and mechanical properties. Type I are C-N films, containing up to 35 at. % nitrogen, and which have an amorphous structure. These films are formed at relatively low plasma shock pressure and exhibit relatively low microhardness, ~ 2 GPa. In a relatively narrow range of the plasma shock pressure and temperature the second type of C-N deposition is observed consisting of high density, closely-packed crystal-like grains growing perpendicular to the substrate surface and displaying a cauliflower-like morphology. The microhardness of these films reaches 15 GPa, as measured by the Vickers method.

Crack Nucleation in ion Beam Irradiated Magnesium Oxide and Sapphire Crystals

Monocrystals of magnesium oxide and sapphire have been subjected to ion implantation with 86 keV Si − ions to a dose of 5×10 16 cm −2 and with 3 MeV H + ions with a dose of 4.8×10 17 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.