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.

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.

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.

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.

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.

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.

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.

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.

Fe-based coatings, such as novel FeCrMnBC alloys, have both economic and ecological advantages compared to other coatings like Ni-based or Co-based coatings. In recent years, high performance Fe-based wear and corrosion resistant coating systems have been developed. Some of them have even been introduced into the market. However, the suitability of the FeCrMnBC alloy as coating for cast iron under complex erosive and corrosive stresses in particle-loaded fluids for pump parts has not been investigated yet. Especially the impact of the process robustness of three-cathode plasma spraying coatings applied with variable process parameters like stand-off distance and spray angle is in the focus of interest. The objective of the present work has been the characterization of novel FeCrMnBC alloys, for the first time deposited via Thermal Spray processes. The corrosion resistances as well as the cavitation and erosion properties were separately evaluated by current density-potential measurements and supersonic cavitation in artificial sea water. Erosion corrosion behavior has been investigated in a pump test rig with 10 wt.-% corundum (Al 2 O 3 ) particles. The results show that the reduction of spray angle and the variation of stand-off distance limit the corrosion and cavitation resistance in different ways. The erosion behavior shows only small variations for the tested parameters. The results reveal that the FeCrMnBC coatings exhibit high process robustness for the chosen parameter variations and a large potential to improve the protection of cast iron even for not optimized conditions.

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.

Bronze materials such as Ni-Al-bronze show exceptional performance against corrosion, erosive wear, and cavitation erosion due to their high fatigue strength and resistance to plastic deformation, and are thus used for ship propellers and in turbines, pumps, and other equipment where alternating stresses occur. Usually, the respective parts are cast, but in this study, a number of opportunities are evaluated to apply bronze as a coating to critical part surfaces. Initial experiments with cold gas spraying were promising enough to assess the use of warm spraying, a nitrogen-cooled HVOF process that provides similar particle impact velocities but higher particle temperatures, while still minimizing the effects of oxidation. The formation and performance of warm sprayed Ni-Al-bronze coatings was systematically investigated for different combustion pressures and nitrogen flow rates. Substrate preheating was also used to improve coating adhesion. The coatings obtained show low porosities, high strengths, and in some cases, cavitation resistance similar to that of the bulk material.

Within the scope of a current research project, aluminum bronze alloy wires were arc sprayed at different traverse speeds in order to influence heat transfer and hence the stress state of the coating. Microstructural, chemical, and mechanical analyses were conducted to evaluate adhesive and cohesive properties. The materials used are highly cavitation erosion resistant propeller alloys, CuAl9Ni5Fe4Mn and CuMn13Al8Fe3Ni2. Cavitation erosion tests were carried out and residual stress distribution was measured using a modified hole-drilling method.

Fast streaming fluid media causes cavitation-erosion in pumps, ship-propellers and rudders. To avoid severe damage, materials with high resistance against plastic deformation and a high fatigue strength should be used. Bronzes fulfill these criteria, but as-cast bulk parts are rather costly. A promising alternative is cold-spray deposition of dense and oxide-free coatings onto exposed surfaces. To achieve high quality bronze coatings by cold-spraying, parameter optimization has to tackle the high hardness of the feedstock powder materials. Additionally, practical limits due to nozzle clogging have to be considered, which may occur at gas temperatures above 700 °C. The present study investigates possible solutions by systematic process parameter and feedstock material optimization, including variation of bronze compositions. Thus, dense coating microstructures and - in consequence - high hardness and good cavitation resistance were obtained. Cold-spray coatings reach up to 8 times better cavitation resistance as compared to conventional ship-building steel (GL-A).

This study investigates microscale features in plasma sprayed alumina and how they correlate with mechanical property differences at the macroscale. Thick alumina coatings were deposited on aluminum and titanium substrates by atmospheric and low-pressure plasma spraying using α-Al 2 O 3 powder as the feedstock. Coating surfaces and cross-sections were examined by FE-SEM, phase distributions were identified by means of EBSD, and porosity was measured via image analysis. Mechanical properties at the micro and macroscale were assessed by nanoindentation, Vickers hardness, and cavitation erosion testing. Relevant test results and observations are presented and discussed in the paper.

Combined damage of cavitation and abrasion is one of the serious problems that affect lifetime and performance on hydraulic machineries and its components. Thermal spraying as a surface hardening and protection technology is required to protect hydraulic components, such as water turbine and radial flow pump, in general. Detonation sprayed coating with high abrasion resistance is one of the promising solutions as the protective coating against combined damage of cavitation and abrasion under solid-liquid double phase liquid conditions. In this study, NiCr-Cr 3 C 2 and WC12Co coatings on the No.45 steel substrate surface have been formed by detonation spraying with the same technological parameter and their properties such as microstructure and microhardness have been studied. The cavitation and abrasion behavior of coatings and the substrate has been investigated using a self-manufactured rotating disk system. The resistance properties of cavitation erosion and abrasion have been illustrated and analyzed with the substrate by the cumulative mass loss curves. The results indicate that, the resistance properties of cavitation erosion and abrasion of NiCr-Cr 3 C 2 coating are the best.