Research Projects: Surface Engineering and Tribology

132 WEAR PROPERTIES AND THERMAL STABILITY OF NEW PVD COATING COMPOSITIONS BASED ON TITANIUM WITH COMBINED METALLIC, METALLOID AND NON-METALLIC ALLOYING ADDITIONS
Supervisors: Dr A Leyland and Professor A Matthews

Titanium nitride and chromium nitride PVD ceramic coatings are now used widely in manufacturing industry to protect cutting and forming tools against wear. Both coatings possess certain technical advantages and disadvantages, which lend themselves to specific (and different) applications. For example, TiN behaves well in sliding wear against steel and against mild abrasion; CrN often performs well in impact wear and when some measure of corrosion protection is also needed (eg. in polymer injection moulding). However, with new requirements for dry, or minimally-lubricated, machining (where contact temperatures are high) and for machining and forming of non-ferrous alloys (where tribochemical interactions can be severe), more sophisticated (and preferably ‘adaptive’) coating systems are needed. Alloyed coatings such as TiAlN and CrAlN are now used commercially, where the aluminium additions provide improved oxidation resistance by promoting the formation of a protective alumina film in service. Such coatings can also, with careful selection of composition and/or processing route, exhibit ‘nanocomposite’ structures with two- or multi-phase compositions that are claimed to improve both hardness and toughness. For extreme hardness and high temperature stability, there has for many years been an interest in boron based coatings (eg. cubic boron nitride, boron carbide and transition-metal borides such as TiB2 and, more recently, CrB2). However, such films tend to be brittle and exhibit poor adhesion to many substrate materials. On the other hand, the addition of metalloid elements such as boron or silicon to TiN coatings has shown that ‘pseudo-binary’ nanocomposite structures (eg. TiN/TiB2, TiN/BN, TiN/Si3N4) can be generated, with exciting combinations of high hardness, improved toughness, chemical inertness and – particularly in the case of TiSiN – impressive thermal stability, to temperatures in excess of 1200°C. This project aims to explore the combined addition of metallic (eg. Cr, Al) and metalloid (eg. Si, B) elements to the Ti-N metal/non-metal binary system, with a view to developing new, adaptive, nanostructured coatings to satisfy future industrial requirements for the machining and forming of non-ferrous alloys and composites.

133 LOW TEMPERATURE DIFFUSION TREATMENTS FOR STAINLESS STEEL
Supervisors: Dr A Leyland and Professor A Matthews

Austenitic stainless steels are used widely in structural applications and for functional devices where good corrosion resistance is required. The tribological behaviour of such materials is however poor, preventing their use in mechanical devices where (for example) sliding or impact wear may occur.

Attempts to apply conventional diffusion treatments (such as gas nitriding) on such materials to improve wear behaviour show limited success - due both to the surface oxide film present and to the reduction in corrosion resistance which chromium nitride precipitation tends to cause. Recent attempts to apply plasma nitriding processes at treatment temperatures below 450°C have been shown to suppress nitride formation and reveal the development of a so-called 'expanded austenite' phase (with extreme interstitial nitrogen supersaturation) - which exhibits high hardness and wear resistance. The exact structure and composition of this phase is a matter of some debate; further detailed analytical work is required to understand more about both the nanostructure of this phase and the influence of process parameters on its formation. There is also literature evidence to suggest that carbon-expanded austenite is similarly wear-resistant and can be obtained at higher temperatures (ie. 500°C+) whilst still avoiding the formation of other phases damaging to corrosion resistance - with positive commercial implications for the depth and rapidity of treatment attainable. Low-pressure, high-intensity plasma processing techniques (pioneered in Physical Vapour Deposition of wear-resistant thin films) show excellent promise for the application of such diffusion layers on stainless steels and other candidate materials; further studies of the treatment parameters are however required to achieve process optimisation.

134 PHYSICAL VAPOUR DEPOSITION OF METAL NANOCOMPOSITE FILMS FOR WEAR AND CORROSION PROTECTION
Supervisors: Dr A Leyland and Professor A Matthews

Chemical inertness, they are difficult to deposit at a thickness sufficient to provide adequate corrosion protection of a metal substrate. This is often due to a combination of high compressive stress within the deposited film and practical/commercial considerations - whereby the synthesis of stoichiometric films of high structural integrity is difficult to achieve at rates of more that 2-3µm/hr, whilst maintaining an acceptable substrate temperature (ie. <500°C).

Both factors tend to impose a practical thickness limit of ~5-7µm on many PVD ceramic films, such that through-coating porosity remains high - and corrosive media can rapidly attack the coating-substrate interface. The relative chemical inertness of the PVD ceramic film serves only to accentuate this effect. PVD metallic coatings are however generally not subject to the thickness limitations described above; lightly-stressed films in excess of 10µm thick, with negligible porosity, can be produced at a rate of 10µm/hr, or higher. Taking advantage of the low miscibility of certain transition metal alloy pairings - eg. chromium/copper - and the ability, with the selection of appropriate deposition parameters, to supersaturate the chromium with nitrogen (or other interstitially-locating elements), thick metallic nanocomposite coatings can be produced which exhibit 'ceramic' hardness (ie. 20GPa+) and thus excellent wear resistance, yet possess many other potentially desirable properties (eg. toughness, corrosion protection) inherent to a metallic film. An improved understanding of how deposition parameters influence the structure-property relationships in such films is required; studies of coating composition and structure 'as-deposited' (and, for example, heat-treated at various temperatures above the deposition temperature) will provide valuable information which may be correlated to wear and corrosion behaviour.

135 HYBRID AND DUPLEX PROCESSES FOR IMPACT RESISTANCE
Supervisors: Professor A Matthews and Dr A Leyland

Many practical situations of contact between surface involve condition of repetitive impact. These include automotive (engine and transmission) systems, and many other applications in fields as diverse as printer technologies to metal forming processes. In order to resist surface failure under such impact conditions, coatings are often used. However, it has been found that in such situations fatigue-related failure mechanisms ensue, leading to coating cracking and delamination. Several approaches exist to try to mitigate such failures. For example, the mechanical properties of the coating can be modified, by structural and compositional changes, in order to prevent cracking, and to ensure that the coating can accommodate substrate deformations induced by the loading. Also, the substrate itself can be modified, to provide greater support to the coating. These approaches usually involve plasma-based coating and treatment methods, and the project aims to further develop such processes and to characterise the coatings and treatments produced, and evaluate their performance under repetitive dynamic impact conditions.

136 THERMAL BARRIER AND BOND COAT TECHNOLOGIES USING PHYSICAL VAPOUR DEPOSITION
Supervisors: Professor A Matthews and Dr A Leyland

Gas turbine engines are used in jet engines and in land-based power generation plant. The application of advanced thermal barrier coatings (TBCs) to components within the engine allow it to operate hotter, under controlled conditions of corrosion and thermal degradation, and thus with greater fuel efficiency and power output. Also, the turbine can last longer, thus reducing the frequency of maintenance and overhaul. However, the demands placed on the TBC are extreme, and include a need for excellent adhesion even after extended thermal cycling and hot corrosion conditions. Usually “bond coats” are applied between the TBC and the turbine component (eg the turbine blade) to try to mitigate these effects. A range of processes exist to apply the TBC and the bond coat. These often differ in terms of process characteristics and hardware, and are often carried out separately. The project aims to investigate a sequential bond coat and TBC deposition system to be carried out in a single coating cycle, and based on Physical Vapour Deposition (PVD) methods. The project involves coating characterisation and testing, as well as process development research.

137 PULSED PLASMA ELECTROLYTIC PROCESSES FOR COATING AND SURFACE TREATMENTS
Supervisors: Professor A Matthews and Dr A Yerokhin

Plasma electrolytic processes combine conventional electrolysis with a plasma discharge which can under certain conditions be generated at the metal-electrolyte interface. This allows effective modification of metal surfaces and deposition of coatings with unique properties. The project objectives are to develop novel processes of plasma electrolytic surface treatment, which can be used in a wide range of industrial applications. These include oxide ceramic coatings for wear and corrosion protection of lightweight metals (e.g. Mg, Al, Ti, and their alloys), environmentally friendly surface cleaning/coating processes for Cd replacement, new coating materials and structures for biomedical applications, etc. The investigations are focused on achieving an improved insight into the nature of electrolytic plasma discharges, with both plasma-metal and plasma-electrolyte interactions being considered. Various spectroscopic methods are employed for the discharge characterisation, along with current/voltage probing, video imaging and other techniques. Particular emphasis is given to the studies of pulsed bipolar modes of electrolysis, for which the effects of electrolyte composition and current pulse parameters are investigated, with an outlook to the process control and optimisation.

The coatings are characterised using advanced methods of SEM, TEM, EDX, XRD, etc. Testing will also be carried out to evaluate the performance of the modified surfaces.

138 WEAR BEHAVIOUR OF HIGH PERFORMANCE MATERIALS AND COATINGS
Supervisor: Professor W M Rainforth

Replacement of worn out components is a major cost to the world economy. In addition, it is being increasingly recognised that a reduction in friction between contacting components can make a direct contribution to a reduction in energy consumption, reducing environmental impact. Failure of components through wear can have major consequences, for example, a failed hip prosthetic results in severe pain and the requirement for immediate replacement. Wear behaviour depends on the dynamic changes to surface microstructure, which can either enhance or degrade wear resistance. Therefore, to understand how to improve the wear behaviour it is essential that evolution of surface microstructure as a result of frictional contact be understood. A number of projects can be run depending on the exact interest. Projects can be concerned with (a) friction and wear behaviour of biomedical ceramic materials (b) wear of advanced hard coatings, (c) high temperature degradation mechanisms. A number of advanced techniques will be required in order to characterise surface microstructure, including high resolution SEM, state-of-the-art TEM, focused ion beam (FIB) microscopy, atomic force microscopy (AFM) nanoindentation and Glow Discharge Optical Emission Spectroscopy (GDOES).

139 CHARACTERISATION AND IN-VITRO EVALUATION OF NOVEL BIOACTIVE COATINGS FOR Ti INTRABONE IMPLANTS
Supervisors: Dr A Yerokhin and Dr G Reilly

There are many clinical situations which necessitate implantation of an orthopaedic device including osteoporosis, osteoarthritis, sports injuries and congenital defects. Because many of these diseases are related to age and the increasing average age of the population is rising the numbers of patients needing implants are increasing rapidly, leading to a growing market for orthopaedic materials and devices. However a routine problem is that the patient’s own bone does not integrate well with the implant material and so the implant loosens and fails. 10-20% of hip and knee replacement operations worldwide are revisions due to implant failure through loosening. Considerable research is ongoing on modifications of the implant surface aimed at improving how the patient’s bone grows around the implant (osseointegration) to reduce the need for implant revision surgery. Integration of an implant with bone involves several process, first bone-forming cells (osteoblasts) must be induced to differentiate from mesenchymal stem cells (MSCs) present in the bone marrow. Then the osteoblasts synthesise a mixture of matrix proteins (mostly collagen) and bone mineral (carbonated hydroxyapatite). Therefore, for an implant to induce bone formation at its surface MSCs and osteoblasts should be able to attach and differentiate ultimately producing a matrix that adheres well to the implant. To address these requirements a new type of coating has been developed based on plasma electrolytic oxidation process of Ti alloys. Specially formulated electrolytes and current regimes employed allow formation of 15 to 30 micron thick ceramic surface layers that are hard, uniform and well adhered to the Ti substrate. The coating phase composition comprises titania matrix nesting various Ca and P containing compounds (both amorphous and crystalline) to ensure accelerated osseoinduction and integration on the implant surface. In depth characterisation of the coating morphology, chemical and phase composition will be carried out using optical microscopy, XRD, SEM, TEM, EDX and GDOES techniques. In-vitro tests will include seeding osteoblasts and MSCs onto the coated surfaces and examining cell proliferation and matrix production using histological staining methods. Cell morphology and attachment will be examined by immunostaining for matrix proteins and integrins and imaging the cells by confocal microscopy. A new method of testing osseointegration will be used in which cylindrical test samples of coated implants will be placed into a 3 dimensional tissue engineered bone matrix. The tissue engineered bone will be grown in a bioreactor and the growth of bone-like matrix around the implant analysed with histology confocal microscopy and CT scanning.