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.

Thermally sprayed WC/CoCr coatings are established in the valve industry for wear protection. However, conventional coatings have to be cost-intensively postprocessed. Therefore, the aim of this study is to develop near net shaped (nns) WC/CoCr-coatings with a high wear resistance in order to avoid the expensive grinding postprocess. For the development of the nns coatings a parameter study is used to investigate the influence of the stand-off distance and hydrogen volume flow rate in the HVAF process. The parameter study indicates the influence of the hydrogen volume flow and the stand-off distance on the porosity, microhardness and wear resistance of the coatings. The developed coatings exhibit a low porosity and high wear resistance. Through the correlation of the coating properties with the process parameters, promising parameter ranges for a further development of HVAF-sprayed nns coatings of WC/CoCr can be identified. With these results, first benchmarks for HVAF process parameters, hydrogen flow rate and stand-off distance, could be delivered, advancing the overall goal of reducing manufacturing costs of valves.

This study investigates the solid particle erosion performance of cold sprayed tungsten carbide-nickel coatings using alumina particles as erodent material. After coating fabrication, specimens were annealed in an electric furnace at a temperature of 600 °C for 1 hour. The coatings were examined in terms of microhardness and microstructure in the as-sprayed (AS) and annealed (AN) conditions. Subsequently, the erosion tests were carried out using a General Full Factorial Design with two control factors and two replicates for each experimental run. The effect of the annealing on the erosion behavior of the coating was investigated at the two levels (AS and AN conditions), along with the impact angle of the erodents at three levels (30°, 60°, 90°). Finally, two regression models that relate the impact angle to the mass loss were separately obtained for the two cold spray coatings.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate of Ti6Al4V deposited by cold spray using nitrogen was studied. For that, flat deposits were tested with stress acting in the in-plane directions and tubular deposits were tested in the out-of-plane stress directions. In all tests, unified small-size specimens were used. It was shown that for the in-plane stress, the deposits can be considered isotropic, whereas the out-of-plane stress led to significantly lower values of the measured properties. The obtained results were related to fractography and microstructural analysis. While a combination of trans-particle and inter-particle fracture determined the fatigue properties in the near-threshold regime, at higher loads, inter-particle fracture was dominant. It was also shown that the different particle-to-stress orientations influenced the resulting fatigue and static properties.

Deposition of hybrid plasma-sprayed coatings employing both dry powder and liquid feedstocks enables preparation of innovative coating architectures. Using this technique, miniature domains of additional (secondary) material may be introduced via the liquid feedstock route into the more conventional powder-deposited coating, providing potential benefits for the coating functionality. In this contribution, we have explored the tribological properties of hybrid coatings sprayed from alumina powder with additions of chromia (Cr 2 O 3 ), zirconia (ZrO 2 ), yttria-stabilized zirconia (YSZ), and titania (TiO 2 ) delivered from liquid feedstocks. The coatings were subjected to dry sliding wear testing and a subsequent analysis of the wear tracks to determine their wear resistance and coefficient of friction, as well as a qualitative assessment of the wear mechanisms. The hybrid coating doped with the chromia addition matched the remarkable wear resistance of highly-dense suspension-sprayed coatings. This is a significant result, especially when considering the order of magnitude better production efficiency of the hybrid coatings.

An important parameter that affects the protection from wear and tear, is the geometric structure of the abrasive causing the destruction. The equivalent diameter of the particles of the primary influence on protection from wear and tear must be greater than the width of the particle of the crushed abrasive. Here also belongs the requirement for the distribution of the particles inside the matrix was more uniform and dense. The use of solid materials gives this possibility only in the certain circumstances, therefore, it is necessary to use pseudoalloys. Pseudoalloys can be obtained by means of thermomechanical processing of the conventional layers deposited by means of spraying. The particles of the solid material charged with kinetic energy are applied to the heated layers, and they penetrate into inside. Under the influence of the heat energy inside the layers suitable for this, the additional reaction between the substrate and the layer takes place, thus improving the adhesion of the layers. The appropriate regulations were developed, and the reproducible stratification appeared caused by spraying, that is pseudoalloys. A positive consequence of the inclusion of the particles of a solid ceramic material into the spayed layer is the emergence of the current when compressed. The result of the research of the parameters are the optimal performance indexes, optimization criterion served as an indicator of the intensity of the wear. Manufactured pseudoalloys underwent checking in terms of spray wear. The study of spray wear was carried out at the temperatures up to 750 ° C and speed load amounted to 37.5 m/s. The silica sand fractions + 150-212 µm was used as an abrasive. As the covering material the alloys based on aluminum were used. In the places of transition to the substrate the multiphase alloys of the system Fe-Al are formed. It is advisable to use layers with a high iron content to prevent the loose and destruction.

Cavitation erosion (CE) damage, which occurs in the main parts (made of high chromium cast steel) of hydroelectric power generation machine, is one of the serious problems. It is expected that life time of those parts would be prolonged if the suitable CE-resistant coating is applied on the surface of the cast steel. In this study, WC-cermet coatings (WC-CoCr and WC-Cr 3 C 2 -Ni), which were fabricated by high-velocity oxygen-fuel (HVOF) thermal spraying process, was interested in protecting CE attack to the cast steel. To clarify CE property of the WC-cermet coatings, the ultrasonic vibration tests were conducted, and the amount of volume loss characterized as CE damage was measured. The microstructure and the fracture toughness, which was evaluated by the indentation test method, of the coatings were related with the CE damage. As the results obtained in this study, the fragment which was spalled from the surface after CE test was almost flake-like shape, and its size was from 2µm to 50µm. SEM observation indicated that this fragment included both WC particle and metal binder, which means that WC particle and metal binder was still strongly bonded together. It was also confirmed that the amount of volume loss could relate directly with the fracture toughness KIC rather than Vickers hardness. It was considered that CE damage was progressed into the depth by throwing out the fragment originated from micro crack initiation. Thus, it was required that the CE resistance of the developed coatings could be labelled through the fracture toughness.

In this study, ball-milled Al 2 O 3 powder was used as a feedstock material for vacuum kinetic spray to deposit hard ceramic Al 2 O 3 coating on a relatively soft polycarbonate substrate. Microstructural and X-ray diffraction analysis of powders and coatings were performed. The results shows that the ball-milled powder has more unstable state than the primary powder. Compared to primary Al 2 O 3 coating the crystallite size and coating thickness of ball-milled Al 2 O 3 coating are smaller and thicker, respectively. Since the ball-milled Al 2 O 3 particles are more easily fragmented during the VKS coating process, it is possible to deposit hard Al 2 O 3 coating on the soft polycarbonate substrate.

HVOF processes represent the state of art for the spray deposition of wear and corrosion resistant coatings since their supersonic gas velocities in combination with moderate flame temperatures allow the deposition of optimal coatings with very high bond strengths, fine surface finishes and low oxide levels. However, new generation coating materials (fine powders), stringent quality requirements and the high productivity demanded by the industry, push the HVOF technologies to their limits. Recently, a novel air-oxygen controlled high velocity combustion process has been development by Tecnalia. The system operates within the supersonic regime using a broad range of fuel/oxidant ratios thanks to the use of air-oxygen mixtures and a carefully optimized gun design. Extremely low flame temperatures can be achieved while keeping a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant metal alloys with superior deposit qualities. In this work, a systematic investigation of the influence of the powder particle size and gun configuration on resulting coating microstructural features has been performed. For comparison, two fine structured commercially available WC10Co4Cr powders with different particle size distributions have been investigated. The coating structure has been characterized with by high resolution SEM cross-section imaging and X-ray diffraction analysis. Resulting coatings are characterized by highly dense structures, a high retention of the primary carbides, average microhardness of up 1885 HV0.3 and fracture toughness varying between 3 and 7 MPa.m -1/2 depending on the powder particle size distribution and the process conditions used.

The super-elasticity behavior of a NiTi-shape memory alloy (SMA) is very promising regarding cavitation resistance. The need of high vacuum conditions by thermal spraying processes, to avoid oxidation, has always been and still is the main obstacle for the widespread of NiTi as a coating material. This work deals with studying the effect of the different shroud concepts on the obtained oxide content and the phases of the obtained twin wire arc sprayed (TWAS) coatings. The concepts include the use of argon as a shield in gas shroud (GS) as well as the use of an extended air cap attachment as a massive shroud (MS). The use of MS-concept led to a significant decrease in oxide content and therefore was selected to spray pre-alloyed NiTi-SMA wires. The standoff distance between the MS-outlet and the substrate surface shows also an effect on the obtained phases and thus on the behavior of the obtained coatings. At lower standoff distance a pseudo-elastic behavior was obtained and therefore a higher cavitation and wear resistance. The use of argon as atomization and shield gas with a massive shroud could be a cost-effective alternative for vacuum process in case of spraying NiTi-SMA pre-alloyed feedstock materials.

This study assesses the effect of yttrium additions on plasma sprayed MoS 2 /Ni-SiC-Y coatings produced from particle-reinforced composite powders. It is shown that the microstructure of the self-lubricating coatings improves with the addition of yttrium, resulting in increased hardness and cohesive strength. The tribological properties of the coatings were also studied, showing that the ideal amount of yttrium is 12 wt% based on wear loss measurements and that the fractal dimension of sliding wear debris depends on the friction load as well as the mass fraction of yttrium.

This study evaluates a new iron-based hardfacing powder alloy. The powder, a FeWCrCB tool steel, is applied to mild steel substrates by plasma transferred arc (PTA) and laser cladding. The clad specimens are examined and tested for weldability, impact and abrasive wear resistance, and wear life. It is shown that the alloy solidifies in a narrow temperature range, first forming a fcc phase followed by a eutectic structure consisting of austenite, carbides, and borides. After solidification, the austenite is transformed to martensite. Impact wear testing shows that the new alloy offers approximately ten times longer life than tungsten-based nickel-matrix composites, but it was outperformed by 50% in abrasive wear tests.

This study assesses the microstructure and mechanical properties of tungsten boride (WB) powder and cemented carbide coatings with WB additions. HVOF-sprayed layers produced from 60WC-30WB-10Co composite powders are compared with conventional 88WC-22Co and 86WC-10Co-4Cr coatings based on phase composition, hardness, wear resistance, and wear surface structure. The results indicate that Co reacts with WB during spraying, forming ternary phases (WCoB, W 2 CoB 2 ) that increase hardness as well as sliding wear resistance.

This study assesses the sliding wear and impact behavior of thick carbide coatings deposited on hot-rolled steel by high-velocity airfuel (HVAF) spraying. Coating samples are evaluated based on scratch, ball-on-disc sliding, normal impact, and compound sliding impact tests and efforts are made to rank materials according to tribological criteria including coating failure mode, friction response, and wear. The approach is intended to provide insights for product designers specifying thermal spray coatings for steel components and structures from a wear performance perspective.

This paper describes the development of a high-velocity oxyfuel (HVOF) spray process optimized for hardfacing components in aircraft engines and industrial gas turbines. Experiments show that the new process can produce WC-CoCr coatings with near-net-shape, ultrasmooth as-sprayed surfaces, and improved erosion resistance. Information obtained through particle diagnostics and other analytical methods is presented, showing how different process parameters influence coating microstructures, surface morphologies, and physical and mechanical properties. With appropriate process settings, hardface coatings can be produced that require only a surface finishing step.

In this study, TiB 2 -40Ni and TiB 2 -50Ni powders are deposited on mild steel substrates by HVOF spraying in order to investigate the influence of Ni on coating hardness and corrosion, wear, and thermal shock resistance. The surface morphology and cross-sectional microstructure of the ball-milled powders and composite coatings are examined, and various tests are conducted to measure properties of interest. The findings are presented and discussed in the paper.

Superplastic forming (SPF) is an advanced sheet manufacturing process typically used for low- volume, high-value products. SPF dies made from refractory castables have a lower production cost than metal dies, but their brittle nature is a limiting factor. This work investigates surface degradation mechanisms in ceramic dies and how they are affected by the application of thermal spray coatings. Among the more notable accomplishments of the study is the development of a test rig that simulates die-part interface conditions during superplastic forming and monitors die wear, making it possible to predict ceramic die lifetime for different coatings.

HVOF-sprayed WC–10wt%Co–4wt.%Cr coatings were obtained using experimental feedstock powders (manufactured by spray-drying + sintering), containing nanometric carbide particles. Three reference coatings were also deposited using commercially-available powders containing sub-micrometric carbide particles. The coatings obtained from nanostructured powders, although affected by decarburisation phenomena, contained very fine carbide particles (~200 nm size). Those obtained from commercially-available powders simultaneously exhibited sub-micrometric (~400 nm size) and micrometric carbide particles, and were much less decarburised. Sliding wear tests performed at room temperature against sintered Al 2 O 3 balls showed the occurrence of brittle fracture wear (detachment of near-surface material by local brittle cracking) on the nanostructured coatings, which were embrittled by decarburisation. The reference coatings, by contrast, exhibited either ductile wear behaviour (plastic deformation, pull-out of single carbide particles) or a mix of both ductile and brittle wear mechanisms. When the decarburisation of the nanostructured coatings was not too extensive, their wear loss was comparable to that of the reference ones. At 500 °C, the wear behaviour of all coatings was dominated by abrasive grooving, on account of thermal softening. The most decarburised nanostructured coatings, however, still experienced brittle cracking as well.

The dry sliding wear behaviour of two HVOF-sprayed Fe-Cr-Ni-Si-B-C (Colferoloy) alloy coatings was studied by ball-on-disk tests performed at room temperature (against alumina and 100Cr6 steel balls), at 400 °C and at 700 °C (against alumina balls only). HVOF-sprayed Ni-Cr-Fe-B-Si-C and Cr 3 C 2 -NiCr layers were also tested for comparison. Under all test conditions, the wear rate of the Colferoloy coatings is lower than that of the Ni-Cr-Fe-B-Si-C coating but larger than that of the Cr 3 C 2 -NiCr cermet. Specifically, at room temperature, the Colferoloy coatings exhibit a combination of mild abrasion, delamination and tribo-oxidative wear against alumina, whereas, against steel, they undergo very limited delamination with negligible wear loss. By contrast, the Ni-Cr-Fe-B-Si-C coating suffers larger wear against steel and undergoes more severe abrasive grooving against alumina. Although the Colferoloy and Ni- Cr-Fe-B-Si-C coatings possess similar microstructure and micro-hardness, their scratch behaviours, which depend on cracking resistance and plastic deformability, differ, thus explaining the micromechanical reason for the different wear mechanisms. At 400°C and 700°C, all of the metal alloy coatings are softened and suffer more severe abrasive grooving; by contrast, the behaviour of the Cr 3 C 2 -NiCr layer at 700 °C is controlled by the formation and delamination of an oxidised layer.

Microstructure and mechanical properties of 7075 Al alloy matrix and SiC reinforced composite coatings deposited on a T6 6061 Al alloy by cold spray are investigated. Feedstocks are prepared as mixtures of 7075 Al alloy and SiC powders with SiC content varying between 0 to 40 vol. %. Microstructural characterization is carried out by optical and scanning electron microscopic examinations and X-ray diffraction (XRD) analysis. The coatings mechanical behavior is evaluated using hardness measurements and wear tests. Wear tests are conducted under dry conditions using a ball-on disc tester under atmospheric conditions. The presence of SiC improves the coatings hardness and wear resistance when compared to pure 7075 Al coatings. The coating hardness increases with increasing SiC content; however SiC content higher than 10 vol.% does not lead to further increase in wear resistance. In this respect, the optimum composition of the coating is determined to be 92 vol. % 7075 Al + 8 vol. % SiC.