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Swavesey, United Kingdom

Manfridini A.P.A.,Federal University of Minas Gerais | Godoy C.,Federal University of Minas Gerais | Avelar-Batista Wilson J.C.,Wallwork Tecvac R and D | Auad M.V.,Auad Godoy Consultants
Surface and Coatings Technology

Shot peening and plasma processes are widely used to improve surface properties of several alloys. In this work, triode plasma nitriding (TPN) was applied to Ti-stabilized interstitial free (IF) steels in an attempt to increase their hardness without compromising their excellent conformability. Shot peening was also trialed before triode plasma nitriding in an attempt to enhance nitriding kinetics and achieve deeper case depths. Triode plasma nitriding was performed at 450. °C, 475. °C and 500. °C for 4. h on Ti-stabilized IF steel. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the steel structure. Instrumented indentation hardness-depth profiles indicated that a significant hardening effect was achieved after plasma nitriding at 500. °C for 4. h. These nitriding conditions doubled the near-surface hardness of the parent IF steel and led to a case depth of 500. μm. Instrumented indentation hardness-profile data also indicated that the shot peening pre-treatment did not have any beneficial effect on nitriding kinetics as a reduction in case depth occurred after this mechanical pre-treatment. Increased surface roughening promoted by shot peening reduced the nitrogen uptake during nitriding. Dry sliding wear tests also corroborated the benefits of plasma nitriding on Ti-stabilized IF steels, as significantly lower wear volumes resulted after this surface hardening treatment. Although oxidative wear was found to occur in all IF steel samples, wear performance was found to be influenced by load support provided by underlying steel substrates and thickness of compound layers. The best wear performance of solely plasma nitrided samples could be attributed to thicker compound layers and deeper hardened cases compared to shot-peened + plasma nitrided samples, which exhibited shallower case depths and thinner compound layers. © 2014 Elsevier B.V. Source

Duarte M.C.S.,Federal University of Minas Gerais | Godoy C.,Federal University of Minas Gerais | Avelar-Batista Wilson J.C.,Wallwork Tecvac R and D
Surface and Coatings Technology

In this paper, the influence of different plasma treatments on wear regime transitions during sliding wear tests carried out on AISI 316L steel was investigated. Selected plasma treatments included plasma nitriding, plasma carburizing and a sequential process involving plasma carburizing followed by plasma nitriding. During sliding wear tests, sharp transitions in frictional force were detected after the initial running-in period. These transitions in frictional force, which were measured as a function of sliding distance, could be correlated to different plasma-modified layers at the surface of the AISI 316L steel. They were also associated with concentrations of nitrogen and/or carbon along sample depths and also with instrumented indentation hardness-depth profiles. Once sliding distances that corresponded to such transitions were determined, additional tests were performed and then interrupted at these distances to record EDS (energy dispersive spectroscopy) composition maps of wear tracks. Additionally, both depth and wear volume of wear tracks were determined by stylus profilometry. Significant reductions in nitrogen and/or carbon contents were detected for each wear track analyzed, indicating that plasma modified layers were almost worn out for these sliding distances at which sharp transitions in frictional force were recorded. The smallest overall wear volume was measured for the sequentially treated sample, which exhibited the best wear resistance among all tested samples. Results indicated that transitions in friction coefficient were distance-dependent during sliding wear tests; this dependence could be attributed to variations in material hardness and/or concentration of interstitial solutes (carbon, nitrogen) in the austenite phase. © 2014 Elsevier B.V. Source

de Villiers D.,Queen Mary, University of London | Traynor A.,Corin Ltd. | Collins S.N.,Corin Ltd. | Banfield S.,Wallwork Tecvac R and D | And 2 more authors.

Large diameter total hip replacements using polyethylene liners have been proposed due to low wear and oxidative stability observed in the latest generation of this material. Concerns exist with large diameter metal bearing surfaces and ceramic heads are generally expensive to manufacture. A ceramic chromium nitride (CrN) coating on a metal head may be an alternative bearing surface, maintaining low polyethylene wear and minimising cobalt release. Vitamin-E blended highly crosslinked polyethylene liners (52mm diameter) paired with electron beam physical vapour deposited (EBPVD) CrN coated and uncoated CoCrMo heads were tested in a hip simulator. Under standard conditions no difference was observed in polyethylene wear rates (9.2 and 9.5mm3/mc) but the coating prevented cobalt release. Alumina particles produced substantial damage on the uncoated heads but did not damage the coated heads. Further testing without abrasive particles increased the polyethylene wear (469mm3/mc) and cobalt release (847ppb/mc) in the uncoated bearings yet remained low in the coated components (13mm3/mc wear, 17ppb/mc cobalt). Additionally, the coating reduced the generation of nanometre sized polyethylene particles by an order of magnitude under all adverse test conditions. This CrN coating may have the potential to reduce clinical wear allowing for large diameter components. © 2015 Elsevier B.V. Source

Bonello T.,University of Malta | Avelar-Batista Wilson J.C.,Wallwork Tecvac R and D | Housden J.,Wallwork Tecvac R and D | Gutmanas E.Y.,Technion - Israel Institute of Technology | And 4 more authors.
Materials Science and Engineering A

Powder Immersion Reaction Assisted Coating (PIRAC) is a relatively simple nitrogen diffusion based process which has been proposed as a technique capable of considerable improvements in the tribological performance of ceramics and metals alike; however, the necessary exposure of the substrate material to high temperatures for several hours may have an adverse effect on the bulk properties of materials such as titanium alloys. The effect of PIRAC treatments on the bulk metallography and mechanical properties of Ti-6Al-4V has been studied. Following PIRAC nitrogen-diffusion treatment, studies using X-ray diffraction and cross-sectional microscopy have shown evidence of the formation of a thin (~1.4μm) TiN/Ti2N layer, together with the presence of some Ti3Al intermetallic phase. Semi-logarithmic S-N plots show a deleterious effect after PIRAC treatment in terms of material cyclic fatigue strength, particularly at higher treatment temperatures. Samples processed at 800°C for 4h however exhibit better fatigue performance than others treated at lower temperatures for longer nitriding times. Fractographic inspection has shown that fatigue cracks originate at (or near) the surface for the untreated Ti-alloy and from the subsurface regions following diffusion treatment, owing to the build-up of compressive stresses in the latter, which hinder crack propagation. © 2014 Elsevier B.V. Source

Avelar-Batista Wilson J.C.,Wallwork Tecvac R and D | Banfield S.,Wallwork Tecvac R and D | Housden J.,Wallwork Tecvac R and D | Olivero C.,HORIBA Jobin Yvon S.A.S. | Chapon P.,HORIBA Jobin Yvon S.A.S.
Surface and Coatings Technology

Titanium alloys have been widely used in automotive, biomedical and aerospace industries due to their high strength-to-weight ratio, outstanding corrosion resistance and biocompatibility. Although α-β alloys such as Ti-6Al-4V (workhorse of aerospace industry) and Ti-6Al-7Nb (much used for surgical implants) exhibit a good combination of mechanical properties, their tribological properties are still limited as they tend to seize and gall when in contact with other surfaces. In this paper, the response of Ti-6Al-4V and Ti-6Al-7Nb α-β alloys to a Nitron-100 treatment is investigated. This treatment, which enhances the load-bearing capacity of titanium alloys, consists of two sequential processes: plasma nitriding using a glow discharge under triode configuration and deposition of a TiN coating. Scanning electron microscopy (SEM), optical surface profilometry (OSP), Knoop microhardness measurements and glow discharge optical emission spectroscopy (GDOES) were used to characterise both titanium alloy materials prior to and after the Nitron-100 treatment. Although the Nitron-100 treatment significantly improved the load-bearing capacity of both alloys, the Ti-6Al-4V alloy exhibited a deeper hardened case but lower surface hardness than the Ti-6Al-7Nb alloy for identical processing conditions. Their different response to the Nitron-100 treatment could be attributed to the different chemical compositions (replacement of vanadium by niobium) of these two-phase (α-β) titanium alloys containing the same amount (i.e. 3.6. at.%) of niobium or vanadium. Due to the larger β-phase stabilising effect of niobium, a higher fraction of β phase resulted in the Ti-6Al-7Nb alloy at the nitriding temperature (750. °C). Under the Nitron-100 treatment conditions used in this work, results indicate that Nitron 100-treated Ti-6Al-4V alloy (evidenced by its higher load-bearing capacity) may be preferred to be used in engineering applications involving high contact loads (e.g. aerospace applications such as aero-bearings). Nitron 100-treated Ti-6Al-7Nb alloy may be more suitable in applications requiring high surface hardness but relatively lower contact loads (e.g. biomedical applications). © 2014 Published by Elsevier B.V. Source

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