Material’s tensile strength can be improved by the presence of a body-centered cubic (BCC) phase, which is essential in highstrength applications and highly corrosive environments. Thus, synthesizing a BCC single-phase, equiatomic AlCoCrFeNi high-entropy alloy (HEA) feedstock particle using a highenergy mechanical alloying (HE-MA) method was investigated. The transient alloy particles were developed using a planetary mill at a constant rotational speed of 580 rpm employing milling times in the range of 4 to 24 hours. During the process, stearic acid of 3 wt.% of the precursor composition was used as a process-controlling agent (PCA). Two HE-MA manufacturing regimes were utilized: i) conventional (milling constituent elements simultaneously), and ii) sequential (progressive milling while adding elements in a certain order). In addition to the conventional method, a sequential regime was employed to develop FeNiCoCrAl, wherein individual elements were added every 4 hours to the starting/milled Fe + Ni mixture. Based on the results, the HE-MA FeNiCoCrAl showed a BCC single-phase formation after 24 hours, with no intermetallic or contamination traceability. Finally, a nanoindentation hardness measurement was carried out to support the observed phase transformation before and after the HE-MA process.

Surface treatments and coatings are widely used to protect components from wear and corrosion. Of all available methods, thermal spraying is arguably the most versatile with regard to coating material and morphology. Surface roughness and porosity can be adjusted in a wide range depending on the requirements. However, as-sprayed coating surfaces inevitably exhibit a certain roughness necessitating post-treatment if a smooth surface is required. The surface roughness of thermal spray coatings is usually determined by the used powder fraction and the particles’ melting degree. Using wires as feedstock material allows for a certain influence on the particle size distribution by adjusting process parameters. In this study, the influence of nozzle geometry and atomizing gas pressure on coating quality, surface roughness and cost-efficient post-treatments of wire-arc sprayed Fe-based alloys with a wide hardness-range is investigated. To allow for easy transfer to real components, the sample geometry is based on real world examples of coatings for new components and repair of worn parts. Using adapted process parameters and air-flow, the surface roughness could be decreased to allow for a less time-consuming post-treatment by grinding. Especially in repair coatings for large area applications requiring a smooth surface finish, significant runtime and cost reductions are feasible.

Hybrid aerosol deposition (HAD) is a new coating method to deposit homogeneous nano-structured ceramic coatings. An accurate evaluation of the fabricated coating properties is required. In this study, α-Al 2 O 3 fine powder was sprayed by HAD. The obtained coatings were dense and uniform with a nanocrystalline structure. An X-ray diffraction measurement revealed that the fabricated HAD Al 2 O 3 coatings mainly consisted of α-Al 2 O 3 phase. The hardness and Young's modulus of the HAD Al 2 O 3 coatings were evaluated by a micro-Vickers method and a nanoindentation method using the Weibull distribution. The hardness of HAD Al 2 O 3 coatings measured by micro-Vickers was ~1400 HV (~15 GPa). The variation of mechanical properties of HAD coatings measured by the nanoindentation method was extremely small compared to those of plasma-sprayed coatings, which also indicates that HAD coatings contain less pores and cracks than plasma-sprayed coatings.

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).

In this study, a water-based corundum suspension was used to deposit 60 μm alumina coatings onto carbon steel substrates by HVOF spraying. The aim was to develop thin coatings with superior wear properties. Hydrogen was used as a fuel gas and process parameters were varied to determine their effect on microstructure and properties. Coating microstructure was examined by SEM to assess particle melting and morphology and XRD was used to study the phase transformation of the feedstock suspension. At higher combustion flame energy, the coating transformed primarily to gamma alumina, while at lower energy, it was found to be a mixture of alpha and gamma alumina. Nanoindentation tests were used to measure the hardness and elastic modulus of individual phases. Ball-on-plate wear tests helped reveal the relationship between wear performance and the alpha-gamma ratios in the coatings.

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.

For well over a hundred years, hardness testing has provided engineers a quick measure of the mechanical properties of a material or coating. However, the technique has also been fraught with potential artifacts, many of which are related to a phenomenon known as the “indentation size effect”. Unlike bulk materials, experimental studies on the hardness measurements of cold spray coatings in different load regimes shows strong dependency on the indentation size in a manner different from the Nix–Gao model. In cold spray coating additional parameters such as porosity and cohesive strength between cold sprayed particles affect the hardness measurements. As a result the hardness loss was observed by increasing the indentation load. To interpret the experimental observation, a two dimensional model was developed taking into consideration the inter particle damage. Ductile damage initiation in combination with the linear damage evolution model has been used. The deviation of load-displacement curves in the material with inter particle defects in comparison to bulk material was studied to explain the mechanism involved in hardness loss.

Very low pressure plasma spraying (VLPPS) is an emerging process allowing manufacturing oxide and metallic coatings by condensation of vapors generated by feedstock powder vaporization. This process operates at unusually low pressures, typically between 100 and 1000 Pa. This paper aims at presenting recent developments for manufacturing TixAlyN coatings via a reactive mode. At first, nitrogen was used as the primary plasma forming gas to enrich spraying surrounding with nitriding species. Plasma jet mass enthalpy and substrate surface temperature were varied to evidence nitride phase formation during spraying. Then, a secondary nitrogen injection was implemented and located close to the surface to be covered in view of creating a continuous nitrogen supply to promote the nitriding mechanisms on the surface. SEM, XRD, GDOES and NHT were implemented to characterize coatings structure. This study highlights the nitrides formation versus spray operating conditions. The microstructural and mechanical features as well as the chemical composition are presented.

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.

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.

Silicon based coatings are showing great promise for power applications in the semiconductor, target, and solar industries. In order for thermal plasma spraying of silicon to continue to have an industrial impact in these industries, careful investigations into the deposition properties must be undertaken. We used a Thermach gun and powder feeder, to deposit silicon onto 100 mm x 50 mm x 1.6 mm steel samples. Coating cross-sectioning and image analysis was performed in order to evaluate the coating’s microstructure and porosity. Mechanical property measurements consisted of hardness testing on the coating cross sections. In addition, scanning electron microscopy and optical microscopy were conducted. These results combined for an analysis into the deposition properties of silicon coatings using various particle sizings, plasma power, and spray distances. Correlations between these input parameters and their effect on the microstructure are critical to semiconductor depositions of silicon.

This is the first of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeCSiMn coatings. In part 1 the elastic and plastic forming behavior of the layers are investigated by indentation, bending and tensile tests. They were performed on coated mild steel substrates as well as freestanding as-sprayed samples with different geometries. Considering the coatings microstructure, element and pore distribution, as well as the local microhardness the results of the indentation, bending, and tensile tests were evaluated. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by SEM after the indentation and tensile tests. In-situ surface observation by optical 3D-microscopy was used to study the onset of cracking during the 3-point bending test.

The characterization of thermal sprayed coatings is often limited to microstructural analysis to evaluate the coatings morphology. Indentation is commonly used to determine the mechanical properties of different kind of engineering materials. However, due to the complex structure of thermal sprayed coatings few results have been obtained so far. This is the second of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeSiCMn coatings. In part 2 experimental nanoindentation tests and simulation results are compared. The experimental indentation tests show scattering in the force-deformation data due to the complex structure of the arc-sprayed coating which is investigated by means of an indentation test simulation. Based on these results the effective Young's modulus as well as further properties are identified. A general procedure is presented to predict the effective mechanical properties of different coating composites based on the microstructure, porosity and properties of the chemical composition after thermal spraying.

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.

This study investigates the mechanical response of plasma-sprayed ceramic coatings to different levels of mechanical and thermal loading. Test samples were subjected to four-point bending and thermal cycling loads. Nonlinear behavior and significant hysteresis were observed, indicative of inelastic phenomena. Previous tests were complemented by structural examinations and bonded-interface testing. Relevant structural features and possible mechanisms underlying this behavior are discussed.