Andrew Bloyce

Surface modification of titanium alloys for combined improvements in corrosion and wear resistance

Abstract A simple and effective surface modification technique, namely palladium-treated thermal oxidation (PTO), has been developed in the present research. Comparative investigations on both corrosion and wear resistance have been carried out on surface-engineered titanium-based materials by conventional plasma nitriding (PN), thermal oxidation (TO), and the newly developed palladium-treated thermal oxidation (PTO). Both the TO- and PTO-treated materials have a significantly superior corrosion resistance in boiling HCl solutions compared to the PN-treated and untreated materials. The lifetime for the protective surface layer breakdown of the TO-treated titanium in boiling 20% HCl solution is about 13 times that of the PN-treated titanium, whereas the lifetime of the PTO-treated material has been increased further by a factor of 2.6 over the TO-treated material. The PTO-treated material has shown a better anti-scuffing capacity than the TO-treated material under oil-lubricated conditions. Characterisation of both the TO- and PTO-treated surface layers was performed using glow discharge spectrometry (GDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Finite element analysis of plastic deformation of various TiN coating/ substrate systems under normal contact with a rigid sphere

The elastic-plastic axisymmetric contact between a rigid ball and TiN coatings of varying thickness on various substrates has been simulated using the finite element method. Yield strengths and modulus values representing typical values for high-speed steel, titanium and aluminium alloys differentiated between the substrates. The effects of such important parameters as the thickness of the TiN coating and properties of the substrate material on the initiation and development of plastic deformation and the load bearing capacity of the TiN coating/substrate systems have been investigated. The results show that in most of the coating/substrate systems, plastic deformation is initiated in the substrate at the coating/substrate interface and plastic deformation does not initiate in the TiN coating until a large plastic zone has been developed in the substrate. Substrate strength and coating thickness have a significant influence on the plastic deformation behaviour and the load bearing capacity of the composite. In light of the finite element analysis results, the mechanisms of coating failure are also briefly discussed.

Towards the design of dynamically loaded titanium engineering components

Abstract Surface engineering research centred at the University of Birmingham has, for several years, been directed at producing wear-resistant surfaces on titanium alloy substrates in order to facilitate the use of titanium alloys in general engineering applications, usually as direct replacements for steel components. More specifically, bearings and gears have been identified as an area in which this could be accomplished, where a low coefficient of friction, low wear rate and load-bearing capacity are all necessary. The processes investigated have included plasma nitriding, physical vapour deposition, laser surface alloying and electron beam surface alloying and currently ion implantation and plasma immersion ion implantation. Some results from plasma nitriding and energy beam surface alloying processing and evaluation achieved to date are included which have culminated in the use of a duplex surface engineering process. This process has allowed a Ti6Al4V alloy (where the compositions is in approximate weight percent) to be tested at 1300 MPa maximum hertzian contact pressure and 50% slip. This represents an improvement of 1200% in contact pressure and 500% in slip ratio survival over untreated Ti6Al4V.

Unlubricated rolling-sliding wear mechanisms of complex aluminium bronze against steel

Unlubricated rolling-sliding wear tests of as-received and electron beam surface melted complex aluminium bronze, CA104, against hardened En19 steel have been carried out. Test samples have been examined using optical microscopy, scanning electron microscopy and microhardness measurements. It is found that both adhesive wear and delamination wear occur in the wear process and the wear debris forms in two ways. Two types of structures exist in the wear debris, which are related to a deformed and a highly deformed subsurface structure in the tested samples. Electron beam surface melting improves the wear resistance of the material but the wear mechanisms involved have not been fundamentally changed.

The effects of plasma nitriding process parameters on the wear characteristics of AISI M2 tool steel

The main aim of this work is to evaluate the effects of the plasma nitriding process on AISI M2 tool steel. In previous work, treatment time and temperature were varied to identify the treatment conditions for good wear behaviour. In the present work, the treatment time was fixed while temperature and gas pressure were varied. Samples were characterised by glow discharge optical spectroscopy, scanning electron microscopy, X‐ray diffraction, surface microhardness and wear test. The specimens nitrided at 400 and 900 Pa showed the best wear performance, which is possibly due to reduction of the friction coefficient and the low adhesive wear observed. Samples processed at 200 Pa showed spalling during the wear test, indicating a brittle surface.

Influence of surface melting on dry rolling-sliding wear of aluminium bronze against steel

Abstract The frictional and wear properties of a complex aluminium bronze against a hardened steel have been studied using dry rolling-sliding wear testing. The wear of the samples and the counterpart wheels has been investigated, noting the changes in behaviour conferred by the electron beam surface melting treatment. It was found that the friction coefficient and the wear loss were reduced by surface melting and that the increase in hardness and the subsequent decrease in friction coefficient were the main causes for the improvement of wear resistance resulting from surface melting. Other beneficial changes came from the resultant homogeneous martensitic structure in the surface melted layer. The elimination of the large κ particles due to surface melting eliminates one of the possible crack nucleation sites, and the layered α + κ structure formed in the as-received material due to wear deformation provides an easier crack propagation path.

Slurry abrasion response of surface engineered Ti6Al4VELI

Abstract Titanium alloys, by virtue of their outstanding combination of properties, continue to evolve as direct replacements for steels in offshore production systems to meet the challenging offshore application conditions. However, the wider use of titanium and its alloys is frequently retarded by their reputation for poor tribological behaviour. It is because of this simple engineering scenario that comparative tribological behaviour of surface engineered Ti6Al4VELI in abrasive slurry has been evaluated to identify treatments capable of improving its wear behaviour. To best simulate potential application conditions, sliding wear tests were carried out using a block-on-wheel test configuration in abrasive mud slurry. The wear volumes lost from the surface engineered Ti6Al4VELI test blocks ranged from negligibly small for substrates plasma sprayed with either WC–Co, Ni–Cr or Mo, to approximately twice that measured for a steel test block in the cases of untreated, shot peened, electroless Ni plated and anodised Ti6Al4VELI. Hard chrome plated material and thermochemically treated material demonstrated a certain degree of wear, which however, was significantly less than that found for the steel block.

Characterization of Plasma Nitrided and PVD-TiN Duplex Treated Armco Iron and En40B Steel by Nanoindentation

Abstract The nanoindentation test has been applied to evaluate the mechanical properties such as hardness, elastic moduli and deformation behaviors of Fe4N iron nitride layers produced on Armco iron and En40B steel by plasma nitriding, and PVD-TiN coatings deposited on En40B with or without prior plasma nitriding treatment. Results showed that the Fe4N layer produced on En40B exhibits higher hardness than that on Armco iron. This is attributed to the effect of the alloy compositions, especially Cr element. However, similar elastic modulus values to that of bulk ferrous alloys have been found for Fe4N layers produced on both Armco iron and En40B. Under lower loads, TiN coatings on nitrided substrate behave quite the same in hardness and elastic modulus as TiN coatings on untreated En40B. Whilst with increasing indentation load and depth, duplex treated (i.e., combined plasma nitriding and PVD-TiN coating) En40B possesses higher composite hardness, elastic modulus and load bearing capacity than TiN coated b...

Nickel diffusion treatment of Timetal 550 titanium alloy

AbstractA series of nickel diffusion treatments have been carried out on Timetal 550 titanium alloy at temperatures ranging from 650 to 900°C in order to achieve deep case hardening. Systematic characterisation of the nickel diffusion treated surface layers was performed using GDS, XRD, EDS, SEM, and optical microscopy. Microhardness and compositional profiles have been carried out in order to reveal the hardening effect and the nickel penetration. The sliding wear resistance against hardened 709M40 steel was evaluated using an Amsler wear tester. The results have shown that by choosing proper treatment parameters, different combinations of intermetallics from the system Ti–Ni can be formed in a selective manner. In addition, with specific conditions it is possible to form diffusion zones without intermetallics on the surface. Finally, a significant improvement in the sliding wear resistance was observed for treated specimens under unlubricated and lightly loaded conditions.