Cavitation is a wear process in engineering systems caused by the energy release of collapsing bubbles leading to the failure of critical components such as valves, pumps, and propellers. Thermally sprayed coatings can be applied to improve the wear resistance of these components. This investigation considers a WC-NiCrBSi coating composition under cavitation wear, where the WC phase provides the strength and the NiCrBSi matrix offers corrosion resistance in seawater. Coatings were deposited on AISI 440C stainless steel discs of 32mm diameter and 8mm thickness using industrially optimized parameters for the HVOF JP5000 system. Indirect cavitation tests were conducted using a modified ASTM G32 testing procedure on coated test coupons in as-sprayed and Hot Isostatic Pressed (HIPed) conditions. Two tests were performed for each coating using natural seawater of pH 8.19 at room temperature, and averaged wear values are reported to compare the cavitation rate and cumulative mass loss of the coatings. Coating microstructural phases in the as-sprayed and HIPed conditions were identified using X-ray diffraction. The microstructure of the coating substrate system and post-cavitation test wear scars were investigated using Scanning Electron Microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). This investigation provides an understanding of the corrosive-cavitation wear behavior and failure modes of coatings. The cavitation erosion rate and cumulative mass loss results showed that the as-sprayed WC-NiCrBSi coatings improve the cavitation wear resistance of the substrate.

Gas-fuel HVOF for thermal spraying of WC-CoCr powder is widely known and well described in literature. Focus are the various influencing factors like fuel-to-oxygen ratio, standoff-distance and powder feed rate on the coating characteristics like hardness and porosity. However, the total gas flow is usually not being described in this context despite its wide influence on particle characteristics and therefore on coating properties. In this study, the characteristic influence of the total gas flow on roughness, hardness and porosity is described as well as its effect on the particle characteristics. The study performed was based on technical standard values for thermally spraying WC-Co-Cr via gas-fuel HVOF (DJ2700 hybrid) and additional trials for increased and decreased total gas flow. It was possible to determine that with higher gas flow the deposition rate increases while the roughness and porosity decrease. However, these results cannot be viewed in isolation as other factors, such as the fuel-to-oxygen ratio, are affecting the particle and coating characteristics at the same time. Therefore, the total gas flow is also considered in combination with other factors.

The Ti-6Al-4V alloy is widely used in aerospace applications for its excellent mechanical properties, however, it presents low wear resistance. It is often coated with a cermet using high-velocity oxy-fuel (HVOF) spraying to improve its wear performance. The Cr3C2-NiCr cermet becomes particularly interesting since it is non-carcinogenic, compared to traditional cermet coatings containing tungsten-cobalt compounds. While the improvement in wear resistance of Ti-6Al-4V with this coating has been demonstrated, its impact on the fatigue performance of the alloy remains to be studied. This is precisely the aim of this study, which focuses on the fatigue life of a Cr3C2-25NiCr-coated Ti-6Al-4V alloy. Among the various influencing factors, surface preparation represents a significant source of crack initiation, particularly in the case of sandblasted surfaces. Indeed, the inclusion of fragmented alumina particles can produce stress concentration zones. Thus, laser texturing, which is a method involving the creation of anchoring points through controlled ablation, can be considered today as a less harmful surface preparation technique. The results obtained from cyclic tensile fatigue tests with a stress ratio of 0.1 for these two surface preparation methods are presented in this paper.

Light alloys are being investigated as an alternative to ferrous-based engineering components. The manufacturing of such components requires a surface modification step necessary to eliminate the top surface's poor wear and corrosion response for improved functionality. Thermally sprayed cermet coatings offer improved surface resistance to wear and/or corrosion. This work presents a customized composition of WC-CoCr feedstock cut in fine and coarse powder size distribution (PSD) to fabricate different coatings on aluminium alloy and steel substrates using two high velocity spray techniques. The WC-CoCr coatings sprayed using the high velocity air-fuel (HVAF) technique at varied parameters consist of six different coatings (four thick, ~ 200 μm and two thin ones, 60-80 μm) to investigate the relationship between processing conditions, microstructure, and performance. Using scanning electron microscopy (SEM) and electro-dispersive X-ray spectroscopy (EDX) offered a comprehensive characterization of the respective coatings. Micro indentation, dry sliding wear, dry sand abrasion, and cavitation erosion tests conducted on the samples show the performance of the coatings based on the processing techniques and spray conditions. However, despite the similarities in the microstructural makeup of the coatings and the measured micro indentation hardness of the coatings (1000-1300 HV0.1), their respective specific wear rate (SWR) varied based on spray processing techniques and the substrate on which the coatings were deposited. Three of the HVAF coatings showed ~ 60 % more wear on the aluminium alloy substrate compared to the same coating deposited on a steel substrate. However, irrespective of the substrate used the HVAF coatings showed better wear resistance than the HVOF coating. The dry sand abrasion wear results of the two thick HVAF coatings show them superior to the HVOF coating in the three-body wear experiment conducted. The cavitation erosion resistance of the coatings varied based on the processing conditions and the driving mechanisms but the best two were the AF-2 and AF-6 samples.

Recently, laser deposition technologies have made significant advancements in their ability to manufacture high temperature metals and ceramics. One of these technologies, known as Direct Energy Deposition (DED), has the potential to deposit a wide range of materials from polymers to refractory materials, ceramics and functionally graded materials. This study evaluates major microstructural characteristics of WC-Co additively manufactured by DED technology. This material is commonly used for deposition of protective coatings due to its high hardness and excellent wear resistance. To this end, hardness and wear resistance of the DED processed samples were also investigated in this study. WC-Co coatings are generally deposited using various thermal spray technologies. However, it is speculated that DED deposited WC-Co could provide superior properties such as higher hardness and wear resistance. A DED manufactured WC-Co sample was examined by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). Those studies could provide information about important microstructural features, chemical compositions and phase distribution. All the tests were also repeated on High-Velocity Oxygen Fuel (HVOF) deposited WC-Co with the same composition. Both DED and HVOF produced WC-Co coatings experience decomposition of the carbides into compound phases; however, the DED deposited sample displays unique dendritic and eutectic structures that improve the hardness and wear properties compared to the homogenous HVOF coating. In addition, DED produced samples show higher hardness and relatively better wear resistance compared to the HVOF deposited ones. The obtained results could establish a relationship between microstructural characteristics with hardness and wear properties of both samples.

Nowadays, Cr 3 C 2 -based cermet coatings by HVOF process are widely recognized for their corrosion and erosion resistance, particularly at high temperatures. These coatings also offer the advantage of being lightweight and exhibiting superior wear, corrosion and cavitation resistance in room-temperature applications. Their lightweight nature and high temperature capability make them an attractive alternative to WC-based alloy coatings and hard Cr plating coatings. The objective of this study is to develop optimal Cr 3 C 2 -NiCr coatings by comparing different feedstock materials, including feedstock with nanocrystalline and/or submicron sized Cr 3 C 2 phases. The focus of the investigation is on understanding the impact of feedstock features such as particle size, morphology, and carbide sizes, as well as sliding abrasive wear conditions (specifically SiC grit size and working load), on the coating properties and sliding wear performance. The results of the study indicate that the sliding wear resistance of the Cr 3 C 2 -NiCr coatings is highly influenced by the features of the Cr 3 C 2 carbides. The presence of nano, submicron and few microns sized carbides in the coatings improves their density and hardness, leading to a significant reduction in wear rates under test conditions. Furthermore, the size of the abrasive SiC grit on the counter surface plays a significant role in determining the sliding wear behavior of these coatings. Based on the analysis of the test data, the mechanisms behind the performance of the Cr3C2-NiCr coatings have been investigated and used to interpret their sliding wear behaviors. A high microhardness in the coating is considered a reliable indicator of high quality, full density, and satisfactory wear resistance. This study has identified and recommended optimized materials for improved coating properties based on the key findings. These findings contribute to the understanding of the relationship between feedstock features, sliding abrasive wear conditions, and the wear rates of HVOF-sprayed Cr 3 C 2 -NiCr coatings.

Thermally sprayed wear resistant coatings have proven their effectiveness in many applications. Their benefit is unquestionable in the case of mutual sliding contact or abrasive stress caused by hard particles. However, for the case of dynamic impact loading, either single or cyclic, the lifetime of different types of coatings is rarely described, probably due to the complex influence of many parameters. The paper deals with the evaluation of resistance to dynamic impact loading of two types of HVOF-sprayed Cr3C2-rich binary hardmetal coatings (Cr3C2-42%WC-16%Ni and Cr3C2-37%WC-18%NiCoCr) with respect to the variation of their deposition parameters and compares them to a well established Cr3C2-25%NiCr coating. For each coating, a Wohler-like curve was constructed based on a failure criterion of sudden increase in impact crater volume. Besides, coatings deposition rate, residual stress, microstructure and hardness were evaluated. Differences in the coatings dynamic impact wear resistance was found, related to their residual stress. The failure mechanism and crack propagation mode are analyzed using SEM of impact surface and cross-sections. Deformation and related stress changes in coated systems during dynamic impact loading are described using FEA analyzes.

High-pressure die casting (HPDC) is a well-established manufacturing process used in the automotive sector to make high-precision components. The necessity to reduce fuel consumption increases the use of low-density components in the automotive industry. Corrosion induced by molten metal is one of many failure modes for dies, changing the die's geometry and surface roughness. All combined wear changes the dimensional precision of the manufactured parts but also the surface quality of the components. Many additive deposition methods are applied to decrease wear and recover the surface. Thermally sprayed coatings can improve the surface properties and recover the geometry of the die caused by the aluminum attack. The main objective of this work is to observe the behavior of the H13, Cr3C2-25NiCr, and WC10Co4Cr coatings deposited by HVOF and HVAF, tested against Aluminum corrosion and Die-soldering tests. After dissolution, the chromium carbide reacts with the aluminum, creating a tough intermetallic interface, and raising the extraction tensile stress. After Aluminum corrosion tests, it was observed that the WC 10Co 4Cr HVAF coating presented low adhesion to the aluminum with no observed coating failure due to the formation of intermetallic. Die soldering tests indicated that the WC 10Co 4Cr protects the substrate, resulting in lower extraction tensile stress than H13 base material and other HVOF coatings. It was possible to observe that WC 10Co 4Cr HVAF coating showed results comparable to AlCrN PVD coating.

In this study, microstructural characterization is conducted on WC-17Co coatings produced via High Velocity Oxygen Fuel (HVOF), High Velocity Air Fuel (HVAF), and Cold Spraying (CS). All coatings prepared were observed to be of good quality and with relatively low porosity content. SEM study showed important microstructural features and grain morphologies of each coating. While composition of feedstock material was approximately similar, elemental composition using EDS showed higher Co content and lower WC in the CS deposited coating. XRD experiment identified formation of more complex oxides and tungsten phases in coatings deposited technologies involving melting of powders such as HVOF and HVAF. These phases consisted mainly of cobalt oxides and brittle phases such as W 3 Co 3 C or W 2 C caused by decarburization of the tungsten carbide particles. Hardness of all coating samples were examined and CS deposited coating exhibited considerably lower hardness compared to the other two coating samples instead of having significantly lower porosity content. It could be contributed to dissociation and physical loss of hard carbide phase during high velocity impact of particles in CS process. It is in good agreement with detection of higher amount of cobalt in CS deposited coating material. It is strongly believed that results obtained from this study can be used for future investigation in thermo-mechanical properties of WC-Co coatings.

The cavitation performance of wear resistant cermet coatings can deteriorate in a corrosive environment. This investigation therefore considered the cavitation resistance in seawater of thermally sprayed High Velocity Oxy Fuel (HVOF) WC-10Co-4Cr coatings deposited on two different substrate materials of carbon steel and austenitic stainless steel. Coatings were deposited using industrially optimised parameters. Cavitation tests were conducted following the ASTM G32 test method in indirect mode, where there was a gap of 0.5 mm between the sonicator and the test surface. A submersed copper cooling coil controlled the temperature of the seawater. The cumulative cavitation erosion mass loss and cavitation erosion rate results are reported. The eroded substrate and coating surfaces were analysed using Scanning Electron Microscopy (SEM) in combination with energy dispersive x-ray analysis (EDX) to understand the failure modes. Coating phases were identified using x-ray diffraction. Results are discussed in terms of the cavitation failure modes and cavitation erosion rates for both the substrate and coated surfaces.

The performance of two distinct coating materials under alumina particle impingement was tested in this study. CrMnFeCoNi and WC-Ni coatings were applied to 2205 duplex stainless steel substrates using cold spray method with nitrogen as the process gas. In between the substrate and the high entropy alloy coating, an interlayer coating of 316 stainless steel was used. The presence of WC particles in the WC-Ni composite coatings was confirmed by SEM cross sectional inspection. Following deposition, the coatings were heat treated in an air furnace. The influence of heat treatment holding time on the WC-Ni coatings was studied using chemical analysis by X-ray diffraction. Heat treatments peak temperatures for the WC/Ni- Ni and high entropy alloy coatings were 600°C and 550°C, respectively. Coatings microhardness and porosity volume fraction were measured for all the samples. The HEA coating outperformed the WC/Ni-Ni hardness but exhibited a higher level of porosity. The coatings were then subjected to erosion experiments using alumina particles with variable impact angles (30°, 60°, and 90°). To compare the different materials, an average erosion value was calculated for each target specimen. The WC/Ni-Ni as-sprayed coating was the most effective against a 60° impingement angle. The HEA coating, on the other hand, demonstrated greater resistance to impact angles of 30° and 90°. SEM was utilized to examine the eroded areas and determine the main mechanisms of erosion.

Ni/Co-based alloys have been widely employed as bond coats (BCs) in thermal barrier coatings (TBCs) to provide oxidation resistance through the formation of a dense thermally grown oxide (TGO) layer. TGO thickening is a major contributor to TBC failure. Conventional approaches to minimize its growth have included refinement/optimization of the BC composition, deposition techniques, and post-treatments. However, these approaches have only led to incremental improvements in TBC performance and do not directly address the effect of the thin interfacial oxide layer on the TBC lifetime. In a shift from conventional thinking, the development of an Al 4 C 3 -Ni alloy composite BC aims to overcome the challenges generated by current TGOs. Post-deposition heat treatment tailors the coating microstructure to form a continuous internal carbide network. At elevated temperatures, the Al 4 C 3 preferentially oxidizes to form an interlacing protective Al 2 O 3 “root” that provides better TGO anchoring and reduces TBC thermal mismatch with the substrate. In this paper, the coatings were manufactured through gas-shrouded plasma spraying using various parameters to optimize the degree of inflight carbide dissolution and minimize the extent of coating porosity and cracking. XRD and carbon analysis were performed on the coatings and the microstructure was observed using SEM. Differences between coatings are discussed in relation to the spraying parameters.

Conventionally, bulk WC and Cr 3 C 2 -based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hardmetal coating system. This research builds on earlier work characterising 42%wt% WC-42%wt% Cr 3 C 2 - 16%wt% Ni coatings sprayed under “low”, “medium” and “high” thermal input conditions, to assess their compositions and microstructures after heat treatment in air at 900°C for up to 30 days. Coatings were deposited by HVOF, Ar-He and Ar- H 2 shrouded plasmas respectively, onto Alloy 625 substrates with Ni20Cr bond-coats and top-coats. The coating compositions and lattice parameters were quantified by Rietveld peak fitting of XRD patterns. The microstructures were analysed from cross sectional backscatter electron micrographs. Rapid phase development occurred within the first five days, beyond which the compositions and microstructures remained stable. The microstructures retained extremely fine, sub-micron grain sizes, while the carbide phases exhibited high degrees of metastable alloying, even after 30 days at 900°C. The coating compositions are discussed, and a mechanism proposed to account for the rate of development and overall metastable microstructure.

There are several challenges when designing components exposed to harsh environments. Cases such as hydraulic turbines and marine propellers are classic examples of demands for materials capable of withstanding erosion and corrosion wear. To enhance and recover worn surfaces, it is usual the use of coatings. This study proposes a new series of coatings based on diffusional effects observed for thermally sprayed chromium carbide coating. A columnar morphology was observed, due to the diffusional gradient perpendicular to the surface. The coating has also shown an absence of porosity and peculiar properties.

Thermally sprayed WC/CoCr coatings are the most established coatings in the valve industry. However, due to the high wear resistance and as-sprayed surface roughness, the surface post processing costs are very high. Near-net-shaped fine powder coatings have the possibility to reduce the costs effectively. Due to the high specific surface to volume ratio of the powders, undesired phase transformations can occur during the spraying process. To avoid such phase transformations, the novel thermal spraying process Ultra-HVOF (UHVOF) is used in this study. An extensive parameter study is carried out on the influences of the process parameters on microhardness, porosity, as-sprayed surface roughness, phase composition and wear resistance. With suitable process parameters, near-netshaped and almost pore-free coatings can be applied. Compared to a conventional HVOF sprayed WC/CoCr coating, a wear reduction by a factor of three can be achieved in a pin-on-disktest against Al 2 O 3 at a load of F = 15 N. Due to the pore-free and highly wear-resistant coatings, significantly thinner coatings can be used for the protection against corrosion and wear in valves. In addition, the required surface quality of the near-net-shape coatings can be achieved by polishing only. Thus, the novel UHVOF coatings represent a cost-effective alternative to conventionally used valve coatings.

In thermal spraying, one of the fundamental elements to achieving good bonding strength of the applied coating is surface preparation. Traditionally grit blasting using hard particles such as corundum is used to achieve suitable roughness on the substrate. Lately, there is an effort to find a suitable alternative from ecological and economical aspects. A promising possibility is laser texturing which enables the preparation of defined structures on the surface. Within a research project, procedures are developed to texture various substrates to direct application of HVOF coatings. The main goal is to achieve speeds of texturing comparable to grit blasting – more than 500 mm 2 /s while ensuring good bonding strength of the applied coating. This study focuses on HVOF spraying of Stellite 6 and WC-CoCr Coating. Selected substrates are steel, and then materials that cannot be traditionally grit blasted – nitrided steel and alumina ceramics. The study presents the analysis of laser textures on substrates, analysis of coating substrate-coating interface, and adhesion tests by tensile test. The most suitable textures – regarding the processing speed and achieved adhesion are selected.

In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. This contribution proposes a comparative study of the corrosion properties of the S-HVOF WC-12Co coatings and conventional sprayed HVOF coatings. The corrosion properties were evaluated at room temperature in NaCl electrolytes with different pH values and in a pH neutral 0.5 M Na 2 SO 4 solution. By varying the pH value, the corrosion mechanism of the cemented carbide coatings should be assessed more precisely, since the two components, WC and Co, show strongly different pH dependencies. The electrochemical properties of the sprayed coatings were investigated using open circuit potential measurements, linear sweep voltammetry and potentiodynamic polarization methods. Before and after corrosion tests, microstructural evaluations of the coatings were performed. Moreover, element analyses of the eluates have been performed to determine soluble corrosion products. The S-HVOF coatings show a similarly good corrosion resistance as the conventional HVOF WC-Co coatings. Generally, the coating properties, i.e. microstructure and phase compositions, as well as the electrolyte significantly influence the corrosion performance of the sprayed coatings.

Thermal spray WC-NiCr coatings generally requires grinding processing to meet the surface finish requirements. The cost associated with grinding can potentially be reduced through the deposition of finer (– 30 + 5 μm) feedstock rather than the more conventional commercial (– 45 + 15 or – 53 + 20 μm) feedstock. Additionally, such a fine powder is likely to require lower energy spray parameters, resulting in less heat input to the substrate, which could be beneficial in application on heat sensitive substrates. However, the spray parameters need to be optimised to mitigate increased degradation of coatings due to unacceptable brittleness caused by decarburisation and oxidation, and to produce defect-free dense coatings. In the present work, a theoretical model to optimise spray parameters was developed, which suggested an oxygen-to-fuel ratio slightly more than 3.3 and a shorter barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long barrel. Scanning electron microscopy, X-ray diffraction, microhardness, and 3Dprofilometer were used to analyse the produced coatings. The coatings deposited using optimised parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, and roughness. Coating deposited at lower kerosine and oxygen flow rates, with reduced stand-off distance, was denser, crack-free, and ductile. Hence, the fine-cut powder can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts. Additionally, successful deposition of coatings using low energy parameters, making this an attractive option for thermally sensitive substrates.

Cermet double carbide coatings (WC-Cr 3 C 2 -Ni) were HVOF sprayed onto magnesium substrate. The variable parameter was spray distance (320, 360 and 400 mm). The microstructure of the coatings has been characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). Additional, porosity and residual stress have been estimated. Phase composition of WC-Cr 3 C 2 -Ni cermet coatings consists of hexagonal WC carbide, as well as the Cr 3 C 2 and Cr 7 C 3 carbides. For the longest spray distance, minor presence of WC 6 O 6 was detected, most likely as an effect of higher spraying distance, leading to partially oxidation of WC at powders particles boundaries. Comparing lattice parameters with model data it should be noted that no significant contribution of stress is present, due to minor changes in WC lattice parameters in comparison to ICDD data. It also should be noted that Cr 7 C 3 carbide in WC-Cr 3 C 2 -Ni coating has different lattice parameters than ICDD data what shows its reactive nature. In obtained results it is clearly seen, that residual stress have the lowest values for coating sprayed from the shorter distance. This tendency is visible for both, linear and shear stress. The crystallite sizes are also the smallest for the shorter spray distance. Such fine structure shows a tendency to good redistribute of the thermal stress in the sprayed coating and also on the coating-substrate interface.

Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. Significant work was devoted to the development of appropriate aqueous hardmetal suspensions starting from commercially available fine WC and Co raw powders feedstock. This contribution proposes a step forward in the development of the S-HVOF WC-12Co coatings and evaluation of their microstructural and tribological properties. Suspension spraying trials were carried out using gas-fuelled HVOF TopGun system. For comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercial available spray powder. Microstructural characterization, X-Ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered Al 2 O 3 and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of the microstructure, phase composition and coating-ball test couples.