Among hardfacing processes using welding, laser cladding is nowadays one of the most efficient surface coating techniques. It is widely used to increase wear and corrosion resistance of machine parts as a result of the unique process characteristics such as low heat input (smaller heat affected zone), distortion free clad layers, lower dilution rate, finer coating microstructure as well as good metallurgical bonding at the coating/substrate interface. A wide range of new hardfacing materials and corrosion-resistant alloys are available on the market and in this study, different coatings of Ni-, Co- and Fe-based alloys as well as carbide-based metal matrix composites have been deposited by laser cladding for benchmarking purposes. Coatings were deposited onto mild steel substrates using a high-power diode laser. Coating microstructure and hardness were investigated as well as their tribological properties such as 2-body and 3-body abrasion, slurry abrasion and cavitation erosion resistance. Corrosion performance of coatings was also investigated with the salt spray test. Coatings are ranked according to their performance in the different tests and relationships between microstructure and coating properties are discussed.

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.

Despite their light weight, 2.3 times lighter than Al, polymers are limited to application with low thermal, wear, and abrasion demands. The enhancement of the functional surfaces of the polymers using thermal spraying techniques is a challenging task due to the thermal degradation of polymers, the low wettability, and the disparate atomic properties. The twin-wire arc spraying (TWAS) process comprises two contradictory features. Almost all spraying particles are in a molten state on the one hand, and on the other hand, the spray plume has the lowest heat output among the different thermal spraying techniques. Therefore, it is a promising spraying technique for the required surface improvement. The surface of the 3D-printed parts was metalized using two successive layers. The first layer is a TWAS coating made of low-melting ZnAl 4 to avoid thermal degradation and provide a bond coat. The topcoat is also applied using a TWAS process and was made out of Ni-WC-Co as cored wires. The top hard coating has improved the wear resistance of the polymers by 14.6 times. The erosion of the coated and uncoated specimens was determined using a low-pressure cold gas spray gun. Ni-WC-Co coating led to more than five times higher erosion resistance.

The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.

Thermally sprayed Al 2 O 3 -TiO 2 ceramic coatings provide exceptional hardness and corrosion and wear resistance, but the high velocities at which they are applied result in an inherently porous structure that requires some type of remediation. This study evaluates the effectiveness of ultrasonic aluminum phosphate sealing treatments on plasma sprayed Al 2 O 3 -40TiO 2 ceramic coatings. The sealants were applied with and without ultrasonication (20-40 kHz) and were assessed using SEM/EDX analysis, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Test data indicate that optimum sealing, corresponding to the highest values of corrosion protection and erosion resistance, are achieved under ultrasonication at 30 kHz for 5 hours.

FeMnCrSi alloys have been developed and studied over the past several years with an emphasis on their use as coatings on CA6NM stainless steel hydroturbine components. Much of the work conducted has focused on the optimization of cavitation resistance through chemical composition changes, the use of different thermal spraying (ASP, HVOF, HVAF) and welding (PTA) processes, and post-treatments such as shot-peening, cold working, and PTA remelting. The aim of this current work is to present a compilation of published articles that report on the research that has been done. Among the trends observed is that coating density and cavitation resistance improve with increasing particle velocity, particularly for HVOF-kerosene spraying. In regard to post-treatments, cold working was found to most effective, reducing cavitation mass loss (in PTA FeMnCrSi coatings) by a factor of nearly two.

The present study compares needed prerequisites for the application of cavitation resistant bronzes by applying different coating techniques, such as cold spraying, HVOF spraying, warm spraying and arc spraying. By optimization to optimum cavitation resistance, the deposited coatings can increase the service life of ship rudders significantly and even serve as repair processes for ship propellers. The given overview aims to support the selection of processes when specifying the target properties to be set with regard to cavitation protection. By using high-pressure warm spraying and cold spraying, properties similar to those of cast nickel aluminum bronze were achieved, however at relatively high costs. In contrast, coatings produced by using HVOF and arc spraying have erosion rates that are only about four respectively three times higher as compared to cast nickel aluminum bronze, while far outperforming bulk shipbuilding steel. Hence, their properties should be sufficient for acceptable service life or docking intervals for ship rudder applications. Propeller repair might demand for better coating properties as obtained by cold spraying. With respect to costs, HVOF and arc spraying in summary might represent a good compromise to reach coating properties needed in application.

Developing effective heating systems to prevent ice accretion on the surface of wind turbine blades and aircraft wings is of great significance for extreme cold environments. However, due to high velocity impingement of water droplets and solid particles on the surface of these components, an appreciable degree of surface material degradation may occur. In this study, nickel-chromium-aluminum-yttrium (NiCrAlY) was chosen as a metal matrix material for a coating-based heating system. Pure ceramic powders, namely, alumina and titania, and a cermet powder, tungsten carbide-cobalt (WC-12Co), were mechanically admixed with NiCrAlY powder and deposited to fabricate reinforced metal matrix composite (MMC) coatings. The powders were deposited on cylindrical low carbon steel bars by using flame spraying. The specimens were placed in a wind tunnel to conduct a comparative investigation of their erosive wear resistance under water droplet impact. A cold spraying unit was used for solid particle impact erosion tests. The erosive wear rates were quantified by measuring mass loss. The experimentally obtained results showed noticeably lower wear rate in NiCrAlY-WC-12Co and NiCrAlY-titania coatings compared to the other coatings. The results suggest that certain MMC coatings could be effectively employed to decrease the erosion rate of coating-based heating elements.

Hydroelectric turbines are strongly affected by cavitation and the damage it can cause to critical part surfaces and profiles. The study of thermal spray processes and materials is thus relevant to improving turbine performance. The main objective of this work is to evaluate the influence of fuel-oxygen ratio on tungsten- and chromium-carbide cermet coatings deposited by HVOF. Particle velocity and temperature were measured as were coating hardness, porosity, and cavitation resistance. Higher particle velocities were obtained at higher fuel ratios, producing harder, denser coatings with better cavitation resistance. Based on test results, the wear mechanism starts with the nucleation of the cavitation that occurs in the pores, resulting in the formation of craters and the eventual detachment of lamellae as indicated by the smoothness of the surface.

Two kinds of cermet powders, WC-10Co4Cr and WC-20CrC-7Ni, were deposited on 1040 steel via high velocity air fuel (HVAF) spraying to evaluate resistance in cavitation erosion conditions with additional electrochemical effects. Coating microstructure, phase composition, and microhardness were examined along with the topography of eroded surface layers. The cavitation resistance of the WC-20CrC-7Ni coating was found to be approximately 1.3 times greater than that of the other coating, which can be attributed to its finer grain structure, lower pore density, and the presence of high Cr and Ni content in the feedstock powder which serves to strengthen the matrix.

Thermal spray coatings are an effective means for improving the cavitation resistance of hydroturbine components, especially in on-site repair situations. In this study, WC-CoCr cermet coatings are deposited by HVOF spraying and their microstructure, hardness, and cavitation behavior are assessed. The coatings exhibit better cavitation resistance than stainless steel with an erosion mechanism that leaves wrinkles and craters that correspond to specific erosion stages.

Hydraulic turbines, valves, and pumps operate in environments where they are exposed to cavitation phenomena and corrosion, which can result in mass loss, leading to reduced performance and failure. HVOF spraying has been used to repair eroded surfaces on such components and new alloys are being developed to reduce repair costs. This investigation assesses the cavitation resistance of FeMnCrSiNiB alloy coatings deposited by HVOF spraying. Corrosion rates and oxidation potentials are measured under different conditions and compared to stainless steel coatings normally used on water turbine runners.

This study investigates the cavitation erosion (CE) behavior and fracture morphology of tungsten carbide thermal spray coatings. WC-CoCr and WC-CrC-Ni powders of various sizes were deposited on stainless steel substrates by HVOF spraying using different combustion pressures. Coating samples and Cr steel reference specimens were subjected to vibratory cavitation erosion tests, volume loss was measured, and erosion damages were examined by SEM to assess fracture morphology. The results indicate that CE resistance can be improved by reducing porosity and increasing interparticle bonding strength.

Thermal spray materials used in power generation applications are required to function within a challenging array of requirements. The coatings must be applied over typically large surface areas cost effectively, the coating must be resistant to extreme erosion and/or corrosion, the coatings must function at high temperatures and, if possible, the thickness of the coatings should be readable with standard equipment such as an Elcometer gauge. Simultaneously meeting all these requirements and advancing the alloy technology is a daunting if not impossible task if done via experiments alone or through scientific intuition. However, the design of new alloys which must possess a variety of attributes simultaneously is well suited for big data techniques. The calculation of millions of alloys and advanced data mining techniques help to quickly identify the best alloy for the application. This paper details how this process was used to design Metco 8294, a proprietary alloy. Metco 8294 is unique in that it is a Fe-based alloy of high hardness, coating adhesion, and erosion resistance, and is readable in the as-sprayed condition and after high temperature exposure.

This study aims at evaluating the erosion resistance at temperature of several hard coatings, including: CrC-NiCr by HVOF, Fe-based alloy by Arc Spray, NiCrBSiFe by powder flame spraying. These coatings are to be used for the recovery of highly eroded walls (above 10 mm thickness) of gray cast iron in the exhaust ducts in heavy-fuel engines. The erosion test consists of erosive particles thrown through a high temperature gas jet, for 5 cycles of 5 minutes, according to ASTM G211-14 (modified). Coated samples are subjected to a fuel gas-torch reaching a front temperature of 450ºC and a back temperature of 90ºC (water cooled), simulating the actual application. The eroded samples are characterized using EDS, and SEM. The results show the erosion rate of each material/system, and their corresponding erosion mechanisms. Thus, the results allows for the selection of an optimum coating for this surface recovery application.

Cavitation and corrosion on hydrodynamic components and systems reduces the operational efficiency. The use of wear resistant coatings have been studied as a solution to the problem of corrosion and cavitation in the industrial environment. Thermal spray processes are recognized as excellent technique to deposit coatings. The high velocity oxy-fuel process (HVOF) can produce high density and bond strength coatings. High velocity air-fuel process (HVAF) is an alternative process, shown to be superior regarding corrosion protection and production costs. HVAF can deposit coating with shorter dwell time and lower temperature, producing coating with lower oxide content. This paper presents the use of HVOF and HVAF process to deposit FeCrMnSiNi and FeCrMnSiB coatings, studying the resistance against corrosion and cavitation in comparison to 316L HVOF coating. Microstructure was analyzed by XRD, microscopic means and mechanical testing. Cavitation and corrosion behavior of the coatings were also studied comparatively. HVAF coatings presented lower porosity and oxide levels, as well as higher hardness values, compared with the coatings deposited by HVOF process. The HVAF process presented better cavitation resistance than HVOF coatings. The FeCrMnSiNi HVAF coating had the best corrosion protection performance between the developed alloys.

Inter-lamellae bonding within thermal sprayed coatings is one of the most important factors influencing the properties and performance of coatings. It has been revealed that there exists a critical bonding temperature for a molten ceramic splat to form the bonding to the same splat surface. The erosion behaviors of thermal sprayed coatings are significantly influenced by the interface bonding between lamellae. In this study, the erosion behavior of plasma-sprayed TiO 2 , Al 2 O 3 and YSZ coatings deposited at different deposition temperatures was investigated. The cross section of plasma sprayed coatings was characterized by the scanning electron microscope. It was revealed that the coatings deposited at room temperature exhibit a typical lamellar structure with numerous unbonded interfaces, whereas the coatings prepared at the temperature above the critical bonding temperature present a dense structure with well bonded lamellae. The erosion rate significantly decreases with the improvement of interface bonding. In addition, the erosion mechanisms of the conventional coatings and the lamellae well bonded coatings were further discussed. The unbonded interfaces act as precracks accelerating the erosion of plasma-sprayed coatings. Thus, controlling inter-lamellae bonding based on the critical bonding temperature is conducive to the improvement of erosion resistance of plasma-sprayed ceramic coatings.

The highly cavitation erosion resistant propeller alloys CuAl9Ni5Fe4Mn and CuMn13Al8Fe3Ni2 were arc sprayed with different traverse speeds by using a mixture of nitrogen and 2 % of hydrogen as atomising gas. Residual stresses were measured by the modified hole-drilling method using ESPI. Microstructural, chemical and mechanical analyses were realised to examine adhesive and cohesive properties. Additionally, the cavitation erosion behaviour was investigated. In comparison to coatings sprayed with pressurised air, the results of the study show superior coating qualities with regard to microstructure, cavitation erosion resistance and residual stresses.

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.

NiCr-Mo composite coating was prepared by plasma spraying of shell-core-structured NiCr-Mo powders. The morphologies of the NiCr-Mo powders and microstructure of the corresponding NiCr-Mo coating were characterized by SEM. Furthermore, the erosion behavior of the NiCr-Mo coating at impact angles of both 30° and 90° was investigated, and was further compared with that of the Ni20Cr coating and the In-738 alloy bulk. Results showed that fully-dense and homogenous NiCr-Mo coating with excellent interface bonding and no pure Mo inclusions was obtained. Furthermore, the erosion test results showed that the erosion rate of the optimized NiCr-Mo coating is lower than that of NiCr coating at both impact angles. Moreover, the NiCr-Mo coating presented excellent erosion resistance which was comparable as that of In-738 alloy bulk, attributing to the fully-dense microstructure and metallurgical interface bonding within coating.