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Detonation gun coatings of tungsten carbide have been widely recognised as one of the most effective anti-wear coatings for oilfield applications. However, very little fundamental tribological information exists for the material, which hinders coating development and the evolution of correct specifications. This study redresses this problem by conducting adhesive, abrasive and erosive tests upon the coating and relating the findings to the coated microstructure. The intention has been to simulate the in-service behaviour of parallel gate valves, which are used primarily to control flow in remote locations where reliability and freedom from maintenance are essential. Although problems with such valves are rare, costs associated with replacement are exceptionally high and therefore a high research priority has been placed on valves of this type. Currently, new designs of valve are tested using a pipe loop rig at BP Research Centre. However, such tests are both expensive and time consuming and with the increasing desire to bring products to market more quickly an alternative is sought. Probably, the area offering most scope for improvement is in material specification of the sealing surfaces and this work sets out to produce a first stage selection procedure for candidate materials. Uniquely, the study has taken one component, systematically categorised its failure mechanisms using non-destructive replication techniques and then reproduced them in the laboratory. The failure analysis has pointed to three-body abrasion, erosion and adhesion being the dominant failure modes and therefore, a suite of tribo-test methods have been developed to replicate them These are namely reciprocating diamond-on-flat, slurry erosion and reciprocating pin-on-plate tests. The material studied was a proprietary detonation gun coating of tungsten carbide, LW45, which is currently the most popular seal facing material specified for gate valves. A conformal contact geometry was chosen for the reciprocating pin-on-plate tests and problems with alignment were overcome by using a pre-test running-in procedure with 1 μm metallographic paste. Wear of LW45 occurring during the pin-on-plate test was not affected by test speed over the range selected, but was highly dependent upon load. Four different categories ranging from minimal wear to catastrophic wear have been identified. Extensive post test analysis using optical and scanning electron microscopy has further classified the failure that occurs into two groups, termed mild and severe. In the mild regime wear occurs by preferential removal of the binder phase, which is minimised on further sliding by protruding carbide particles. Eventually sufficient binder is removed for carbide fall-out to occur, upon which the cycle is repeated. A greater wear volume is produced by the severe wear mechanism which is caused by the interlinking of cracks present within the microstructure of the coating. To ensure operation in the mild regime, continuous sliding under operating pressures of above 7.84 MPa should be avoided. Abrasive wear simulated by the diamond-on-flat test increased with load. However, the failure mechanisms produced were independent of load and consist of a combination of plastic deformation and brittle fracture with plastic deformation representing the rate controlling step in the wear process. Slurry erosion tests have shown that LW45 wears by a brittle erosive mechanism and is therefore best able to resist erosion at low impingement angles. The volume loss per particle impact for LW45 is proportional to the kinetic energy of the impinging particles. The failure mechanism involved the growth of cracks in the microstructure by a fatigue action eventually leading to crack interlinking and material fall out. For all wear conditions, it is suggested that the removal of microcracking from the coating microstructure will lead to significant improvements in wear performance. A simplified design guide has been produced that gives a weighted importance to the various failure modes attributable to the respective tests. A significant improvement in performance was recorded by LW45 in comparison to typical substrate materials such as AISI 410 and Ferralium F255 stainless steels.

The unlubricated sliding wear behaviour of gas nitrided, plasma nitrided, and ferritic nitrocarburized BS970,905M39 (EN41B) steels was investigated systematically from an engineering point of view. Commercial nitriding processes were employed, and the wear studies were carried out using a pin-on-disc machine over a wide range of sliding speeds and applied loads. The worn specimens and wear debris were examined by several techniques; including optical and electron microscopy and X-ray analysis. A sharp wear rate transition between mild and severe regimes was identified by varying the load for both untreated and gas nitrided steels. The wear rate was reduced by gas nitriding by up to two orders of magnitude, depending on the sliding condition. Moreover, gas nitriding expanded the mild wear region toward higher loads and sliding speeds. In the mild regime an oxidative wear mechanism operated, contrasting with the metallic wear in the severe regime. Wear maps for untreated and gas nitrided steels have been constructed, which show the dominant regimes of the wear mechanisms. A "wear-face-limited" gas nitrided pin test showed that the benefit of the treatment was lost once the effective surface layer was completely worn away. It is suggested that hardness has a crucial role in determining the wear rate through nitrided diffusion layers. A thick and porous compound layer produced by gas nitriding showed a poor wear behaviour owing to its brittleness, while a thin nitrocarburized E-Fe3N compound layer exhibited a low wear rate. There was no significant difference between the nitride steels in terms of transition load and wear rate. However, the wear lives of the nitrided layers were dependent on their case depth. Severe wear should be avoided in engineering component design, and operating conditions should ensure that only mild wear occurs. Both the hardness profiles and the cost performance of nitriding processes should be considered in the selection of nitriding treatments.

Abrasive wear occurs when a contact associated with stress between a metal surface and a herd particle (frequently of mineral origin) leads to friction between the two. In a very wide range of industrial applications, abrasive wear is the main reason for component and equipment repair or replacement. In most of these applications, especially those of earth moving, construction and mining equipment, digger teeth are used to improve equipment performance. Digger teeth can be produced in different shapes and sizes (mainly by casting) and a wide range of materials are used. This project is concerned with both a field trial of the wear of digger teeth fixed to the front of a bucket used in a gravel pit, and also a laboratory investigation of abrasive wear mechanisms. It was found that the wear of digger teeth increased with increasing working hours, but the wear rate eventually decreased. The dimensions and shape of the front of the tooth changed and gravel removal became more inefficient. Plastic deformation and phase transformation were observed in the worn surfaces of the teeth. In the laboratory study, many parameters were investigated utilising a pin-on disc technique. Wear rate increases linearly with load and decreases with sliding distance. The effect of attack angle on abrasive wear showed that wear volume increases with increasing attack angle up to a certain value (90°) and then decreases. Corrosion increases the initial wear rate, and the amount of material removed in the wet corrosive test was higher than the corresponding dry test. It was difficult to reproduce the same results from the field trial in the laboratory because of the difference in the conditions in the two cases. Optical and scanning electron microscopy were used to study the worn surfaces, abrasive papers and wear debris. Different abrasive wear mechanisms were observed throughout this investigation. A cutting mechanism associated with spiral debris was observed during short pin-on disc tests and with higher attack angles. A ploughing action associated with plate-like debris was observed during longer tests and at lower attack angles. Fragmentation was observed in brittle materials.

The present work reports the effect of heat input, cooling rate, parent plate deformation and restraining conditions on the formation and morphology of delta ferrite in welds on AISI 318L and 321 steels. The experiments were carried out on commercially produced plates in the following conditions: (i) as received condition (ii) further deformed by cold rolling. (iii) further deformed by hot rolling. The effect of heat input and cooling rates were examined using bead-onplate Submerged Arc welds on the same parent plate material. The parent plate condition was assessed using bead-on-plate metal Inert Gas (MIG) and Subm rged Arc butt welds. The results suggest that 1. The weld metal solidification proceeds epitaxially from the existing unmelted base metal. The weld exhibited surface marking i.e. deformation bands or close packed plane, in the austenite matrix, but not passing through delta ferrite phase. 2. The solidification substructure, the ferrite content, and morphology are influenced by tile (i) thermal stress induced during welding, (ii) parent plate chemical composition, (iii) the parent plate microstructural and deformation characteristics i.e. strain energy. 3. Rapidly cooled welds have lower ferrite content than welds produced with slower cooling rates. The randomly distributed elongeed ferrite with some lath type ferrite morphology was predominantly attributed with the welds produced with high cooling rates. 4. The ferrite is the first solidified phase to form and austenite is formed from the liquid rather than by solid phase transformation of primary ferrite to austenite. 5. The ferrite formation is a diffusion controlled phase transformation, the degree of its dendrites development depends upon the welding parameters i.e. heat input, cooling rates. 8. Two types of ferrite morphology were observed: (a) elongated type dendrites identified as vermicular by previous investigators and (b) the cellular type morphology which has not been classified in previous investigations.

The introduction of overhead camshaft valve train systems some 25 years ago has resulted in improved engine performance and efficiency, this in turn however, has caused increased wear problems. In order to understand the tribological behaviour of overhead camshaft systems, test engines and dynameter test methods have been extensively used. These are however, time consuming and expensive, a cheaper and quicker method of camshaft material and lubricant appraisal is therefore required and this has resulted in the design and construction of a simulative overhead camshaft test rig. A range of commercially available camshaft and follower materials have been tested using both a fully formulated and a -mineral base -equivalent. The most commonly encountered cam and follower wear failures of scuffing, pitting and polishing were reproduced. Results showed scuffing wear to be associated with all of the camshaft materials using the mineral base oil under high load, boundary lubrication conditions. Scuffing also occurred under low load, mixed lubrication conditions using the case-hardened steel cam and chilled white iron follower combinations. The remaining cam materials, carbonitrided and induction hardened grey flake iron did not fail under similar test conditions due to the presence of free graphite in the icrostructure. The fully formulated oil generally suppressed the onset of scuffing under all test conditions due to the presence of the zinc-dialkyldithiophosphate (Z.D.D.P) anti-wear additive in the oil. Pitting failure was associated with the chilled white iron follower using the fully formulated oil under high load conditions. This failure could have been initiated by either a surface, sub-surface or stress - corrosion mechanism. Analysis of this type of failure is complicated by the difference in both chill depth and hardness and also the presence. of cracks within, samples prior to examination and testing. Standardisation of chilled white irons therefore needs to be carried out in order to understand fully the wear behaviour of such materials. Polishing wear was associated with the fully formulated oil at low loads, and appeared to occur by a chemical reaction between the contacting surfaces and the additives present in the oil. The use of the acetate replica technique proved a satisfactory method of analysing the change in surface topography of cam surfaces. Using the fully formulated oil the cam surface 'ran in' satisfactorily, by a process of plastic deformation and fracture, whilst the mineral base oil caused instanteous scuffing of the cam surface. Once the surfaces had 'run in' satisfactorily by using either the additives in the oil or by a diamond lapping operation, the cams continued to run without scuffing in the mineral base oil under continuous, non stop test conditions. An intermittent, stop/start operating cycle however results in eventual scuffing due to additive film depletion and repeated surface interaction under boundary lubrication conditions. From a range of 'new' materials tested for follower application the ceramics, with the exception of the toughened zirconia, exhibited the best wear characteristics using a case hardened steel cam as a standard and tested under mineral base oil conditions. The silicon carbide and sialon ceramics both exhibited a low frictional resistance and wear rate, whilst the metallic follower materials of cobalt and nickel base hard facing alloys and a ceramic fibre reinforced aluminium/silicon alloy all suffered some form of material.

Hard anodising on aluminium and its alloys has been widely practised for many years in order to improve the resistance of the otherwise poor wear characteristics of aluminium. In recent years there has been an increasing interest in other treatments and coatings, on both aluminium and other base metals. The aim of this investigation is to explain the tribological performance and wear mechanism(s) of an uncoated aluminium alloy, four anodic coated alloys, and also an electroless nickel alloy. All of the coatings are produced on three different aluminium alloys. The thickness of the anodic films is 30-35 micron, as this thickness falls within the range commonly used by industry. In an endeavour to explain the role of coating thickness on wear life, electroless nickel alloy has been produced in a range of thicknesses of 10, 20 and 30 micron. To evaluate abrasive and adhesive wear, the samples were rubbed against a single point diamond and steel ball, respectively, in a reciprocating movement at room temperature and 65-75% relative humidity, under a wide range of load and sliding distance. Some tests continued to run until a breakdown of the coatings occurred, whilst other tests were interrupted at intermediate stages. This enabled the initiation and propagation of failure mechanisms to be studied. Abrasive wear was performed under dry conditions, whereas, adhesive wear was evaluated under both dry and lubricated conditions. Wear of these coatings was proportional to the applied load and sliding distance, but there was no direct relationship between wear and hardness. The tribological performance of these coatings appears to be dictated by a) the composition of the substrate, b) the chemical and physical nature of the coatings and c) the test conditions. Under boundary lubricated conditions there was a considerable increase in the wear life of the coatings. A three dimensional surface texture is superior to a machined surface, in controlling contact conditions. There is an approximate linear relationship between coating thickness and wear life for electroless nickel alloys. These coatings predominantly fail by adhesion, plastic deformation and brittle fracture. A microscopic model for fracture of brittle materials, under both static and dynamic conditions for abrasive and adhesive wear correlates very well with the behaviour of these coatings. Analytical interpretation of adhesive wear was made by separately calculating the coefficient of wear "K" of the counterbodies. This information enables an improved understanding of the wear test itself to be added to the model of the wear mechanisms involved.

There are two important omissions in the literature on bore polishing, firstly there is no evidence of the successful development of a reliable tribo test device to simulate bore polishing and secondly, the mechanism of bore polishing has not been fully defined. The aims of this study were: 1. To establish the principal characteristics of bore polishing In engines. 2. To produce bore polishing in the laboratory. 3. Differentiate between two reference oils in a laboratory tribo test. 4. To understand the mechanism of bore polishing. The principal characteristics of bore polishing have been identified by the examination of Tornado cylinder bores from an engine test. The graphite structure is visible on the surface which has a surface finish of less then 0.125 micro-m in C.L.A. value. The components used in these tests were a grey cast iron piston ring running on a grey cast iron cylinder bore typically used in commercial engines. A reciprocating tribo test was used to distinguish between the two reference oils. The result showed higher friction, wear and a smoother surface with the oil causing bore polishing compared to the other oil which did not produce bore polishing. Adding carbon, taken from the wall of a piston used in an engine test, to the lubricant in the laboratory tribo test produced a phenomenon resembling bore polishing. Comparisons have been made between the tribo test results and service engines and a good correlation has been obtained. Several analytical techniques have been used and the knowledge of bore polishing has been advanced. In particular, it is suggested that a combination of two processes, one mechanical and the other chemical, are associated with bore polishing. Four wear mechanisms were identified during this investigation; abrasion, delamination, corrosion and adhesion.