Expanded austenite (γN), or S-phase, is a special phase of low-temperature nitrided austenite containing highly super-saturated nitrogen in the form of heterogeneous Cr-N nano-clusters. A nitrided layer of singe phase γN is known to provide austenitic stainless steel with combined high hardness, good wear resistance and superior corrosion resistance. This paper reports recent experiments on a comparative study of the sliding wear properties and wear mechanisms of nitrided austenite stainless steel AISI 316, with a special attention paid on worn surface structural evolutions induced by frictional heating and sliding deformation. The samples were prepared by DC pulsed plasma nitriding treatments of various time at a fixed power. Knoop micro-indentation has revealed hardening behaviour of the nitrided samples. The reciprocating ball-on-disc sliding wear and friction properties were investigated at ambient environment conditions using an alumina counterpart ball. The worn surfaces have been analysed by XRD,FEG-SEM and EDX to show wear induced changes in the crystalline characteristics and the wear mechanisms of tribo-oxidation, cracking, abrasive wear and ploughing deformation. Moreover, longitudinal cross-sectional foils of the worn samples have been prepared and analysed using TEM, to investigate the wear induced structural changes, including tribofilm formation, plastic deformation and delamination in depths of nano-scale.

Ultrahigh strength steel 56NiCrMoV7 was austempered at 270 • C for different durations in order to investigate the microstructure evolution, carbon partitioning behaviour and hardness property. Detailed microstructure has been characterised using optical microscopy and field emission gun scanning electron microscopy. A newly developed X-ray diffraction method has been employed to dissolve the bainitic/martensitic ferrite phase as two sub-phases of different tetragonal ratios, which provides quantitative analyses of the carbon partitioning between the resultant ferrites and the retained austenite. The results show that, a short-term austempering treatment was in the incubation period of the bainite transformation, which resulted in maximum hardness being equivalent to the oil-quenching treatment. The associated microstructure comprises fine carbide-free martensitic and bainitic ferrites of supersaturated carbon contents as well as carbon-rich retained austenite. In particular, the short-term austempering treatment helped prevent the formation of lengthy martensitic laths as those being found in the microstructure of oil-quenched sample. When the austempering time was increased from 20 to 80 min, progressive decrease of the hardness was associated with the evolution of the microstructure, including progressive coarsening of bainitic ferrite, carbide precipitating inside high-carbon bainitic ferrite and its subsequent decarbonisation.

Background: Telescopic rods in the management of Osteogenesis Imperfecta (OI) fail frequently. This could be attributed to technical errors, rod design and rod structure. We aimed to analyse the mechanical properties and tribology of explanted male and female components to identify effects of in vivo telescoping by assessment of clinical, radiological, mechanical and tribological features.Methods: Recruitment took place at 3 of the 4 English centres for OI. 25 rods explanted for growth or failure during revision to a new rod were analysed in terms of clinical indication and pre-revision imaging to identify if there was a technical mode of failure. Laboratory analysis was performed using optical and scanning electrical microscopy, radiograph diffraction analysis, hardness test, bending test and energy dispersive X-ray spectroscopy.Results: All implants tested were of high grade stainless steel. Female components had inferior strength (mean Vickers hardness property (HV0.3) at 0.3kg-313) in comparison to male components (HV0.3 406) due to different techniques of manufacture. Female rods also had a higher wear coefficient (7.89 10 -12 m 3 N -1 m -3 ) than the male rods (6.46 10 -12 m 3 N -1 m -3 ). Abrasive wear, shear deformation, scratches and wear debris were identified in all rods. Male and female components displayed biomedical deposits. Intra-operatively cut rods, particularly the female components, had irregular ends leading to more wear. Conclusion:Current manufacturing techniques result in inferior material strength in female components compared to males, which combined with wear patterns is likely to lead to implant failure. Intra operative cutting of rods may increase risk of failure due to wear. Considering techniques to improve strength as well as design in new implants may lead to better outcomes.

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