High entropy alloys (HEAs) constitute a new class of advanced metallic alloys that exhibit exceptional properties due to their unique microstructural characteristics. HEAs contain multiple (five or more) elements in equimolar or nearly equimolar fractions compared to traditional alloy counterparts. Due to their potential benefits, HEAs can be fabricated with thermal spray manufacturing technologies to provide protective coatings for extreme environments. In this study, the AlCoCrFeMoW and AlCoCrFeMoV coatings were successfully developed using flame spraying. The effect of W and V on the HEA coatings were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness testing. Furthermore, performance of the coating under abrasive loading was investigated as per ASTM Standard G65. Microstructural studies showed different oxides with solid-solution phases for all the HEA coatings. Hardness results were higher for the AlCoCrFeMoV coatings followed by AlCoCrFeMoW and AlCoCrFeMo coatings. Lower wear rates were achieved for the AlCoCrFeMoV coatings compared to AlCoCrFeMoW and AlCoCrFeMo coatings. The evolution of multiple oxide phases and underlying microstructural features improved the resistance to abrasive damage for the AlCoCrFeMoV coatings compared to other HEA coatings. These results suggest that the flame-sprayed HEA coatings can be potential candidates for different tribological interfaces while concurrently opening new avenues for HEA coating utilization.

This study developed microstructure-based finite element (FE) models to investigate the behavior of cold-sprayed aluminum-alumina (Al-Al2O3) metal matrix composite (MMCs) coatings subject to indentation and quasi-static compression. Based on microstructural features (i.e., particle weight fraction, particle size, and porosity) of the MMC coatings, representative volume elements (RVEs) were generated by using Digimat software and then imported into ABAQUS/Explicit. State-of-the-art physics-based modelling approaches were incorporated into the model to account for particle cracking, interface debonding, and ductile failure of the matrix. This allowed for analysis and informing on the deformation and failure responses. The model was validated with experimental results for cold-sprayed Al-18 wt.% Al2O3, Al-34 wt.% Al2O3, and Al-46 wt.% Al2O3 metal matrix composite coatings under quasi-static compression by comparing the stress versus strain histories and observed failure mechanisms (e.g., matrix ductile failure). The results showed that the computational framework is able to capture the response of this cold-sprayed material system under compression and indentation, both qualitatively and quantitatively. The outcomes of this work have implications for extending the model to materials design and under different types of loading (e.g., erosion and fatigue).

This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

This study evaluates the friction and wear behavior of iron-base coatings produced by arc spraying using experimental cored wires. Coating microstructure was analyzed and various wear tests were performed. The results show that the tribological properties of the ferrous coating materials are greatly affected by porosity, oxide inclusions, particle shape, and microhardness.

This study evaluates an internal diameter HVAF spray system and compares coatings characteristics obtained with WC and Cr 3 C 2 based powders with those achieved via standard HVAF spraying. Coating microstructure, phase composition, hardness, roughness, and corrosion resistance are investigated and the potential for further optimization is discussed. It is also shown that the new system can be used for grit-blasting as well as spraying.

In this investigation, 5083 aluminum alloy coatings were deposited on substrates of the same material by high-pressure cold spraying. Spray trials were carried out using powders with size ranges of 5-20 µm and 20-44 µm, gas temperatures of 673 K and 773 K, and nitrogen and helium process gases. Coatings and coating-substrate interfaces were evaluated primarily by SEM and EDS, while XRD was used to examine coating stresses and oxidation effects. Corrosion protection was assessed by electrochemical potentiodynamic measurements in synthetic seawater and Knoop indentations tests were conducted as a measure of work-hardening and mechanical integrity of the coatings. Test results are presented and correlated with spray parameters.

Adhesion strength of thermally sprayed coatings is usually measured in accordance with the tensile method specified by ISO 14916. A major limitation of the method, however, is that it cannot measure adhesion strengths greater than that of the glue used to prepare the test specimen. Indentation testing, by virtue of its simplicity and practicality, is a promising alternative in such cases. Collaborative work has been conducted by members of the Japan Thermal Spray Society (JTSS) to establish a standard method for measuring coating adhesion using a conventional Vickers indenter. This paper provides an overview of the experimental and theoretical work that was done and describes the criteria proposed to quantify adhesion strength based on standardized test procedures.

Adhesive strength of the plasma-sprayed thermal barrier coating (TBC) is one of the most important parameters which influence the reliability during service. In the past, numerous test methods were reported to measure the coating adhesion. However, most of them require careful and time consuming preparation. Consequently, limited information could be obtained to establish the relationship between the processing conditions and the adhesive property. To produce more measurements using a simpler procedure, the interfacial indentation test and the modified tensile adhesive test are examined. In this paper, the interfacial fracture toughness of the plasma-sprayed ZrO 2 coatings, deposited on Al substrates, were evaluated by these two tests. In order to study the effects of the powder injection, samples were sprayed with various carrier gas flow rates. The test results show a certain correlation between the melting index and the interfacial fracture toughness. In addition, variations between the results obtained from the two different methods are discussed.

Cold gas dynamic spraying (“cold spraying”) at low pressure (150 psig) was used to fabricate Al-Al 2 O 3 metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained -45 µm Al stock powder admixed with -10 Al 2 O 3 in fractions ranging from 0-90 wt%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted on the as-sprayed coatings. The coatings were then friction-stir processed (FSP) using a milling machine and a 12 mm diameter cylindrical tool. Microhardness testing, SEM, and image analysis were then repeated to study the effect that FSP had on the MMC coating hardness. Hardness increased with increasing fraction of Al 2 O 3 in the feedstock powder, resulting in a maximum as-sprayed coating hardness of 85 HV when 90 wt% Al 2 O 3 is used. After FSP, the hardness of the MMC fabricated from a 90 wt% Al 2 O 3 powder blend increased to a maximum of 140 HV. SEM micrographs showed that the as-sprayed MMC coatings contained Al 2 O 3 particles that had been trapped between the larger Al particles. FSP succeeded in redistributing the Al 2 O 3 particles, decreasing the mean free interparticle distance and increasing the probability of load sharing between the reinforcing particles. It was suggested that this redistribution may be the primary reason for hardness improvement in the MMC coatings.

C-BNp/NiCrAl composite coating was deposited by cold spraying using a mechanically alloyed composite powder. To modify coating microstructure, especially the bonding at the interfaces between c-BN particles and NiCrAl alloy matrix, and bonding at the sprayed particle/particle interface, annealing treatment at series of temperatures in Ar atmosphere was carried out. The results show that a zigzag interface layer is formed at the interface between c-BN particle and NiCrAl matrix after annealing at 825°C for 300 min through reaction of c-BN with NiCrAl. It is also observed that the thickness of the interface reaction layer increases with the increasing annealing temperature. Moreover, the interface between spray particles and the plastic deformation ability of the cermet coating can be improved through post-spray annealing. Vickers microhardness test shows that the hardness decreases with increasing annealing temperature due to the reduction of work hardening effect and grain growth of NiCrAl alloy matrix resulting from recovery and recrystallization during annealing treatment.

This study assesses the effectiveness of nickel-coated diamond powder for producing metal-diamond composite coatings by cold spraying. The results of the investigation show that diamond fracturing was mitigated by the protective nickel coating. In general, the softer the metal matrix and the finer the diamond, the less fracturing that occurs and the greater the diamond fraction in the composite layer. It is also shown, however, that deposition efficiency and diamond fraction must be improved especially for diamond sizes of 50 μm and above.

In this investigation, flame spraying is used to deposit polyether ether ketone (PEEK) layers on stainless steel substrates and CO 2 and Nd:YAG laser remelting treatments are performed to densify the deposited material. Microstructural analysis of the as-sprayed and remelted coatings shows that both lasers are suitable for densifying PEEK polymer layers on stainless steel and that the resulting crystalline structure depends on laser processing parameters. Hardness measurements and tribological and scratch tests are also carried out and the results are correlated with microstructure.

HVO/AF (High-Velocity-Oxygen/Air-Fuel) WC-17Co and WC-10Co4Cr coatings exhibit great potential in the replacement of electrolytic hard chrome (EHC) coating, and comprehensive properties of such coatings should be not worse than those of electrolytic hard chrome coating. The impingement-resistance of HVAF coatings sprayed on 300M ultra-high strength steel was studied in this paper. As an important property index, the fracture toughness of HVAF WC coatings was measured using micro-indentation method at the load of 9.8, 19.6, 24.5, 29.4 and 49.0N respectively. The cracks resulted from stress concentration in the micro-indentation were analyzed. The impingement-resistance for two HVAF WC coatings and EHC was evaluated according to the ASTM D3170 standard, and steel ball dropping experimentation was performed at the height of 0.61, 1.52, 1.83, 2.36 and 2.59m respectively. The cracks caused by both impingements were analyzed using SEM and optical microscopy in comparison with cracks in micro-indentation test.

In this paper, the effects of ceria addition to HVOF thermal sprayed NiAl intermetallic coatings were investigated through micro-indentation, thermal shock testing, and microstructural analysis techniques including SEM with EDX, and XRD analysis. It has been found that the addition of ceria to NiAl coatings reduces the tendency of brittle peeling during thermal spraying. This reduction in peeling is presumably due to the improved wetting of the substrate by the molten coating material, which ultimately leads to better coating adhesion. The addition of ceria also resulted in higher coating hardness and elastic modulus as determined by micro-indentation. The coatings containing ceria also exhibited significant increases in thermal shock resistance when compared with that of the pure NiAl coatings. The possible mechanisms responsible for the improvement of the above mentioned properties upon the addition of ceria are also addressed in this paper.

For constant conditions concerning substrate state, feedstock and environment properties of thermal spray coatings depend only on temperature and velocity of particles at impact on the substrate. Two different HVOF spraying guns, the kerosene fuel system Tafa JP5000 with radial powder injection and the ethylene fuel system Sulzer Metco Diamond Jet Hybrid 2700 with axial powder feeding, are characterized concerning the evolution of space resolved velocity and density of particles by LDA. Also influence of process parameter variations is examined. The region of shock diamonds is studied specifically. The influence of different characteristics concerning impact velocity and trajectories on the coatings microstructure is determined by means of optical microscopy and microhardness testing.

This study investigates the effect of mounting materials on microstructure and property measurements obtained from thermal spray coatings. Various epoxies and embedding techniques are used and a wide range of layers are examined, including HVOF sprayed WC-Co, Cr 3 C 2 -NiCr, and Al 2 O 3 -TiO 2 ; plasma sprayed Cr 2 O 3 , YSZ, WC-Co, and Ni-Al; and arc sprayed copper and silicon bronze. Image analysis measurements of area percent porosity, thickness, lamellar spacing, and unmelted particles and the results of hardness tests show substantial variation relative to the method used to encapsulate soft and porous coatings. Results indicate that the ideal mounting system for thermal spray coatings would consist of a low viscosity epoxy, to maximize penetration depth, and a high cured hardness, for adequate protection of surfaces and open porosity of hard coating materials. Paper includes a German-language abstract.

Hard surfacing layers of WC-Co/Ni-based self-fusing alloy (SFA), Ni-based SFA and Cr 3 C 2 -NiCr alloy were formed using powder and an electron beam. When the layers were examined using the Vickers hardness test, a sand erosion test and an immersion corrosion test, they were found to display high erosion and corrosion resistance. The WC-Co/Ni-based SFA, Ni-based SFA, and Cr 3 C 2 -NiCr alloy layers displayed high hardness of 1400HV, 780HV and 900HV, respectively.

The nanoindentation technique has been applied to thermal-sprayed metal, cermet and ceramic deposits. The hardness and elastic modulus were determined from the load-displacement curves. Each test was implemented by varying the penetration depth (100, 200, 300 and 400 nm) in the same test location and at least 20 tests were performed. The results were compared to those from microindentation tests. The nanoindentation test, essentially, measured the submicrometer scale properties of thermal spray deposits, which can be considered as "near-intrinsic" properties of the coatings. Thus, these measurements exclude most of the microstructural factors that influence the "macroscale" properties. The nanoindentation test exhibits significantly greater hardness and elastic modulus values than the microindentation test.

Experimental studies of the subsonic combustion process have been conducted in order to determine the quality and economics of polyester, epoxy, urethane, and hybrid polyester-epoxy coatings. Thermally sprayed polymer coatings are of interest to several industries for anti-corrosion applications, including the infrastructural, chemical, automotive, and aircraft industries. Classical experiments were conducted, from which a substantial range of thermal processing conditions and their effect on the resultant coating were obtained. The coatings were characterized and evaluated by a number of techniques, including Knoop microhardness tests, optical metallography, image analysis, and bond strength. Characterization of the coatings yielded thickness, bond strength, hardness, and porosity.