This paper evaluates the cavitation erosion wear rate and failure modes of WC-10Co-4Cr coatings. These coatings are used in various industrial applications to protect against erosive, abrasive, sliding and cavitation wear in corrosive environments. Cavitation erosion tests were performed using a modified ASTM G-32 cavitation test rig. Thermally sprayed High Velocity Oxy-Fuel (HVOF) WC-Co-Cr coatings were deposited using industrially optimised coating process parameters on carbon steel and stainless-steel substrate coupons. Coatings were tested to simulate the cavitation bubbles occurring in valves, pumps, and ship propellers. Indirect cavitation was used to impact the cavitation bubbles on the test specimen at a fixed offset distance from the vibrator end. Test specimens were immersed in natural seawater. A water circulation cooling system was used to control the temperature of the water. The cumulative mass cavitation erosion and erosion rate results were evaluated. The coating microstructure was analysed using Scanning Electron Microscopy (SEM) and x-ray diffraction. Post-test evaluations included SEM observation in combination with energy dispersive x-ray analysis (EDX) to understand the failure modes. Results are discussed in terms of the factors controlling the cavitation erosion rate.

WC-Co coatings were deposited using conventional High Velocity Oxy-Fuel Jet-Kote (HVOF-JK) and Suspension HVOF (S-HVOF) methods. Microstructural and mechanical properties along with the wear resistance of coatings were investigated. Reciprocating sliding wear tests were conducted against sintered Si 3 N 4 counter-body with a normal load of 25N and total sliding distance of 500m following ASTM G133-2 standard. Coatings were characterised by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD) and nano-Indentation techniques. HVOF-JK coating showed good retention of WC whereas S-HVOF coating showed the presence of W, W2C and amorphous/nanocrystalline phases. Nano-indentation of HVOF-JK and S-HVOF showed that the relative hardness of the HVOF-JK coating was higher but their elastic modulus was lower. The lower total wear rate was exhibited by the HVOF-JK coating. This difference in wear performance is attributed to the difference in hardness of the coatings and decarburisation of WC particles.

This work investigates the sliding wear resistance of alumina coatings deposited on stainless steel substrates by HVOF and air plasma spraying, using fine (1-5 μm) and conventional (10-45 μm) powders. Sliding wear tests were carried out using a sintered WC-Co ball as the counter-body and the wear tracks were examined to obtain a better understanding of wear mechanisms. HVOF coatings showed an order of magnitude improvement in wear resistance compared to their APS counterparts. The disparity in wear performance is correlated to differences in phase composition, porosity, hardness, and fracture toughness as revealed by SEM and XRD analysis and nanoindentation testing. The development of tribofilms and their role in wear behavior is also discussed.

This research delivers an integrated performance of generic coating-substrate systems under impact loading, and comprehends the understanding of underpinning failure mechanisms. This investigation thus benchmarks the coating design process for high impact stress applications. Repeated severe impacts to the coatings often result in poor performance by cracking and delamination from the coating-substrate interface. The durability and delamination resistance of coatings depend on the choice of coating and substrate materials, coating deposition process and service conditions. The design of overlay thermal spray coatings thus requires an optimization of these parameters. A thorough understanding of the underpinning failure mechanisms is thus critical for future coating developments. This investigation thus provides an insight to the role coating and substrate properties on the impact resistance of coated materials, and maps the relationship between the impact resistance of WC-Co and WC-CrC-Ni coatings on a variety of substrates. Results indicate that coating’s delamination resistance during impact loading not only depends upon the hardness and roughness of the substrate material, but more importantly, substrates with higher work-hardening coefficient indicate a higher delamination resistance during impact loading.

Whilst innovative advancements in thermal spray technology, especially in terms of characterization of starting powders, coating processes and optimisation of coating process parameters have resulted in coatings of improved quality, there is an ever increasing demand to push the frontiers of coating applications. Post-treatment of thermal spray coatings either by HIPing (Hot Isostatic Pressing) or vacuum heating can thus offer one such opportunity by presenting a combination of coating and substrate properties not achievable by individual processes. Hence the aim of this study was to investigate the potential of two integrated process technologies of thermal spraying and HIPing. Tribo-mechanical properties of WC-Co coatings deposited by the HVOF process in the as-sprayed and post-treated conditions were thus investigated in this study. Results are discussed in terms of coating microstructure, sliding wear resistance, elastic modulus, hardness, residual strain and rolling contact fatigue resistance. These results indicate that significant improvements in coating performance can be achieved by appropriate design of post-treated components.

In the case of thermal spraying of cermet powders using HVOF with longer barrel, such as a JP-5000 and a Diamond Jet 2700, spitting and clogging troubles tend to be brought about on the inner wall of the barrel. This phenomenon comes from the existence of fine particles (in spray powders) which tend to fully melt through barrel. This investigation has been carried out to realize the influence of powder particle strength and particle size distribution of agglomerated and sintered Cr 3 C 2 -25%NiCr powders during HVOF spraying. It is found that the finer and weaker the powder particles, the higher the deposit efficiency. However, the finer and weaker the powder particles, the easier is the onset of spitting and clogging phenomena. On the contrary, it is found that the coarser and stronger the powder particles, the lower the deposit efficiency with less spitting and clogging phenomena. Finally optimum powder particle strength and particle size distribution of Cr 3 C 2 -25%NiCr powders have been found in this study using a JP-5000 system.

The aim of this experimental study was to comprehend the relative performance and failure modes of WC-NiCrBSi Thermal Spray coatings in As–Sprayed and HIPed (Hot Isostatically Pressed) conditions in rolling/sliding contact. Recently a number of scientific studies have addressed the fatigue performance and durability of Thermal spray coatings in rolling/sliding contact, but as of yet there have been no investigations on Thermal Spray Coatings which have undergone the post treatment HIPing. The understanding of the mechanisms of failure in rolling /sliding contact after HIPing is therefore critical in optimising the parameters associated with this post treatment to achieve superior performance. Coatings were deposited by a JP5000 system and HIPing was carried out at two different furnace temperatures of 1123K and 1473K. At both HIPing temperatures the rate of cooling was kept constant at 8°C/minute. Rolling Contact Fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress, configuration and lubrication. Results are discussed in terms of as-sprayed and HIPed surface examination of rolling elements using Scanning Electron Microscope (SEM) and Light Microscope.

The aim of this preliminary investigation was to ascertain the synergetic potential of two process technologies of thermally spraying and HIPing (Hot Isostatic Pressing) for tribological applications and address the key design factors, which need to be considered for successful applications of HIPed thermal spray WC-NiCrBSi coatings. The relative performance of the as-sprayed and hot isostatically pressed WC-NiCrBSi functionally graded coatings was investigated in sliding wear conditions. Results indicate that HIPing post-treatment can improve the sliding wear resistance of WC-NiCrBSi coatings. These coatings were deposited by a High Velocity Oxy-Fuel - JP5000 system and HIPing process was carried out at two different temperatures of 850°C and 1200°C. This study shows that un-capsulated HIPing can be successfully applied to functionally graded WC-NiCrBSi coatings, which has economical as well as technical incentives for industrial applications. Sliding wear tests were carried out using a high frequency reciprocating ball on plate rig using steel and ceramic balls. Results are discussed in terms of powder manufacture method, microstructural investigations, phase transformation, mechanical properties and residual stress investigations. Phase analysis by X-ray diffraction revealed transformations, which altered the phase composition such as the elimination of secondary phase W2C and metallic W and the formation of new phases containing Ni, Si and B after the post-treatment. The measurements of hardness, Young’s modulus and residual stress indicate that substantial improvements can be achieved due to simultaneous application of temperature and pressure during the HIPing post-treatment. Hardness and Young’s modulus measured by indentation method, increased after the HIPing process due to the transformations in the morphology and phase composition of the coatings. The residual stress evaluations by sin2Ψ technique using synchrotron x-ray diffraction showed a relaxation of residual stress fields in the coating with increasing temperature of the HIPing process.

Hot isostatic pressing has been shown to be an effective post treatment for thermal spray coatings, improving hardness, density, and microstructure as well as metallurgical bonding between splats. In this study, the sliding wear resistance of as-sprayed and post-treated WC-Co deposited by HVOF is evaluated by means of ball-on-disk testing and the effects of HIPing are assessed based on SEM and XRD analysis, hardness measurements, and fracture toughness tests. Changes observed in the WC-Co layers, including the precipitation of carbides and the elimination of secondary phase W 2 C, are also discussed. Paper includes a German-language abstract.

Fatigue and delamination resistance of overlay coatings is critical to their performance in tribological applications involving Hertzian loading. This study addresses the influence of coating thickness and contact stress fields on the fatigue/delamination resistance of thermal spray (WC-12%Co) coatings, deposited by a JP-5000 system. These coatings were deposited in three different thicknesses on the surface of 440-C steel substrate cones. Fatigue tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration. Results are discussed in terms of Hertzian contact stress fields, coating thickness and Scanning Electron Microscope (SEM) observations to comprehend the performance and ascertain the fatigue failure modes of coated rolling elements. These results indicate that by appropriate control of coating thickness and tribological conditions, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure. Further studies in this field can thus trigger an area of new novel applications of thermal spray coatings.

A non-destructive experimental approach was adapted to investigate the variations in residual stress fields within thermal spray coatings. WC-Co coatings produced by a HVOF technique were considered for concentrated rolling sliding contacts in this study. These coatings were produced in various thicknesses on various substrates. Residual stress measurements were made using an x-ray diffraction technique, along and across the rolling direction. A modified four-ball machine was used to conduct rolling contact fatigue tests under various tribological conditions of contact stress, lubrication and contact configuration. Residual stress measurements were made before and after the tribological tests. Failed rolling elements were analyzed using scanning electron microscopy, electron probe microscopy and surface interferometry. Results indicate that the magnitude of compressive residual stress attenuates during fatigue failure. The magnitude of attenuated residual stress was dependent upon the type of tribological failure. This attenuation of residual stress was attributed to the microcracking of coating under the influence of contact stress.

The full potential of rolling element bearings operating in specialised conditions such as high speed and corrosive environments are realised using surface coatings. Tungsten Carbide coating by thermal spray HVOF and D-Gim processes are considered for these applications. An experimental approach using a modified four-ball machine simulates the tribological conditions within a rolling element bearing. The fatigue failure modes of the tungsten carbide coating in rolling contact with steel and silicon nitride are examined using conventional surface analysis techniques. The stress fields within the coating are examined using traditional contact theory and residual stress measurement by X-ray diffraction. The residual stress measurements of the pre-test coating, the contacting surface and the fatigue failures are described. Results of residual stress relating to orientation, failure depth, coating thickness are discussed along with the fatigue failure mode.