J.T.A. Pollock

Ion-implanted graphitic carbons

Abstract Ion implantation of glassy carbon and other graphitic forms of carbon leads to significantly increased resistance to wear. The effect is observed for a variety of ion species. Nitrogen ion doses as low as 5 × 10 15 ions/cm 2 at 50 keV are effective and the enhancement is related to the damage produced by the incident ions. New and previous data show the importance of ion energy and dose in determining the modified (damaged) zone. The general applicability of implantation as a modification tool for graphite-based carbon is supported by work with highly ordered pyrolytic graphite (HOPG), electrode carbon and carbon fibre. Aspects of possible mechanisms responsible for the enhanced wear resistance are discussed.

Nitrogen implanted steels-aspects of oxidation and diffusion during wear

An assumption of many implanted wear studies has been the diffusion of nitrogen during the wear process; this migration was proposed to explain the retention of wear resistance at depths beyond that of the implant zone. Recent ion implantation studies have suggested a favourable role for modified oxidation wear, some associated with nitrogen implants, others with the implantation of known oxide-forming elements. Using Auger electron spectroscopy concentration/depth profiling data, aspects of oxidation and possible diffusion in mild steel are examined. It is shown that extensive diffusion does not occur but there is a modification of oxidation-wear in the presence of significant nitrogen.

Neutron Activation Measurements of As and Ga Loss During Transient Annealing of Gaas

Significant dissociation is normally detected under non-optimised transient annealing of GaAs. We have utilised neutron activation to measure As and Ga loss from virgin and implanted material annealed under various transient conditions. Complementary RBS data are reported. In particular, surface dissociation has been measured as a function of pulsed ruby laser power and for several combinations of time and surface temperature using an incoherent light source and a vitreous carbon strip heater. The results indicate that neutron activation analysis offers a powerful tool to identify the conditions required to minimise GaAs dissociation during annealing. For examplq ruby laser pulses of energy 0.29–1.38 J cm −2 caused As loss of 4 – 90 × 10 cm −2 .

Wear reduction by recoil implantation of aluminium into steel

Abstract The wear resistance of medium carbon steel has been increased by up to 10 × by 84 keV N+ recoil implantation of 30–100 nm thick Al films thermally deposited onto the steel. Low energy ion scattering (LEIS) and X-ray diffraction were used to identify the mechanisms responsible for the improvements seen during ball-on-disc wear testing. Nitrogen implantation of 66 nm thick Al layers produced the lowest wear rate on the steel and the WC ball. This improvement is due to the formation of a surface iron rich layer supported by hard iron and aluminium nitrides well bonded to the substrate.

Ion Beam Modification of Glassy Carbon

Implanted glassy carbon (GC; 50 keV, 10 16 N + cm -2 ) is reported to be 400 times more wear-resistant to 1 μm diamond polishing than unimplanted GC. A number of samples were implanted with 15 N to produce the modified surface layer and to allow a very high sensitivity measurement of wear, using the 15 N(p, α) 12 C nuclear reaction. Optical interferometric wear measurements are also reported. Samples implanted with 1 MeV N to 10 15 ions cm -2 indicated that the enhanced wear-resistance is correlated with the nuclear energy loss density within the ion range. Mechanisms likely to be responsible for the enhanced wear resistance are also discussed.

Wear of Ion Implanted Glassy Carbon

Significantly improved wear properties are described for glassy carbon following implantation with 2 MeV helium and 50 keV nitrogen to doses in the range 10 15 –10 17 ions cm −2 . Implanted material is up to 100 times more wear resistant to diamond abrasion than unimplanted material. Enhanced wear resistance is available at the surface with nitrogen but lies below the surface with helium, reflecting the difference in modified depth associated with implant energy and ion mass. Unusually for ion implantation, dose related surface compaction is observed for both nitrogen and helium. Changes in microstructure during implantation with particular regard to collision processes and amorphisation of the graphitic fraction of glassy carbon are discussed.

Concentration/depth profiling and wear resistance of nitrogen implanted steels

Abstract The migration of implanted nitrogen during wear has been investigated. Nitrogen depth profiles measured by Auger Electron Spectroscopy (AES) and argon sputter milling within wear tracks made in mild steel are reported. These in-track AES profiles are compared with profiles for as-implanted nitrogen and wear-track roughness. Evidence in favour of nitrogen migration beyond the implant depth is qualified by the effect of surface roughness on the AES data. Some preliminary data and thoughts on the role of oxygen in the wear process are presented.

ION IRRADIATION OF POLYMER-DERIVED GRAPHITIC CARBONS

Carbon atoms exist in the natural state as graphite and diamond, two of the most interesting allotropes found among the elements. Diamond, the metastable, highly crystalline structure, is the hardest known substance and prized for its beauty as well as its utility. Graphite has a layer structure and although the bonds within the layers are probably stronger than those of diamond, weak layer-linking bonds produce a material which slips easily and exhibits high lubricating properties. Each of these carbon structures may be formed by industrial processing.

Surface Temperatures and Dissociation Loss During the Pulsed Laser Annealing of GaAs

From reported equilibrium partial and total dissociation pressure data for GaAs and melt times derived from reported time resolved reflectivity experiments, estimates have been made of the anticipated rate of As loss. Good agreement was found with experimentally determined As loss. A similar approach using experimentally determined Ga loss data allowed estimates of the maximum temperatures reached during pulsed laser annealing. These temperatures are considerably higher than suggested in thermal modelling studies. The boiling point of Ga gould be exceeded at incident laser energies >0.8 J cm −2 .