Optimized high-velocity oxy-fuel spraying of fine-cut WC-NiCr powder results in dense, crack-free, and ductile coatings, with a smoother as-sprayed surface that is expected to reduce grinding efforts and enhance coating performance. These coatings are candidates for replacing hexavalent chromium plating in high-performance applications.

Eutectic high-entropy alloys (EHEAs) feature multiple distinct phases, unlike the single-phase solid solutions typical of conventional high-entropy alloys (HEAs). While EHEAs inherently consist of multiple phases, their performance as thermal spray coatings is directly influenced by their relative phase proportion and microstructural distribution, making phase control crucial for property optimization. While CALPHAD has been used to predict phases in HEAs, there remains a critical gap in its quantitative phase predictive capabilities, particularly for non-equilibrium cooling processes imitated by Scheil simulation. This study investigates Scheil simulation as a tool for both qualitative and quantitative phase prediction in the AlCoCrFeNi2.1 EHEA HVOF (high velocity oxygen fuel) processed coating. The HVOF coating exhibited a deformed zone with unmelted and partially melted regions retaining the eutectic structure of the original powder, as well as a rapid solidified zone displaying a single-phase FCC solid solution. Phase fractions, determined by Rietveld analysis, closely matched those predicted by Scheil simulation - 66.7 wt% FCC and 32.3 wt% B2 versus experimental values of 72.7 +/- 1.1 wt% FCC and 27.3 +/- 1.1 wt% BCC/B2. Furthermore, the coating showed low microhardness and wear resistance, underscoring the limitations of FCC-rich microstructures for tribological applications. These results suggest that Scheil simulations can effectively aid the materials engineering design of EHEA thermal spray coatings.

The conventional vacuum-based methods used to fabricate all-solid-state lithium-ion battery (ASSLIB) components are typically expensive, characterized by slow deposition rates and yield mostly amorphous coatings that demand postprocessing to enhance performance. This research explores the advanced fabrication of Li6.5La3Zr1.5Ta0.5O12 (LLZTO) solid electrolyte coatings using suspension plasma spray (SPS) for ASSLIBs. The LLZTO feedstock suspension was stabilised by identifying most suitable suspension parameters such as zeta potential, particle size, and pH values to obtain uniform flowability. Coatings with thicknesses ranging from 15 to 30 mu m were developed by varying stand-off-distance (SOD) and torch surface speed. X-ray diffraction (XRD) reveals the retention of high amounts of 68 % of the cubic-LLZTO phase content in the coatings compared to the raw materials used for making the suspension. Coatings exhibit around 75 % of crystallinity and 7 % porosity, which is favourable for better phase stability at high operating temperatures and restrict the dendritic formation respectively. Moreover, the coatings retained around 4.5 wt% of Li content from an initial 5 wt% in the powder. Furthermore, SPS'ed. LLZTO coatings resulted in ion-conductivity of the order of 10-8 S cm-1, which is five times the conductivity of the same material obtained by magnetron sputtering, hence, demonstrating the potential of SPS to fabricate ASSLIB components.

A novel cemented carbide composite coating was developed using high-velocity oxygen fuel (HVOF) spraying with an agglomerated and sintered WC-20 wt.% CoCrFeNi high-entropy alloy (HEA) powder. The coating exhibited a complex microstructure of WC, W2C, W, Cr2O3, spinel oxide, and two FCC phases. CALPHAD simulations provided insights into phase stability but were not fully accurate due to decarburization during deposition. Nano- and micro-indentation revealed hardness variability between the carbide-rich and FCC phases, attributed to decarburization and dissolution mechanisms. The WC-20CoCrFeNi coating demonstrated superior hardness and wear resistance compared to HEAs but did not exceed those of conventional cemented carbide thermal spray coatings. Electrochemical testing revealed enhanced seawater corrosion resistance for the WC20CoCrFeNi coating than both HEAs and conventional cemented carbides. This study highlights the potential of HEAs as binder materials in cemented carbides processed via agglomeration and sintering, offering a promising balance of properties for engineering applications.

Composite coatings composed of blended nickel alloy (IN625) and WC-Co cermet were applied via the extreme high-speed laser cladding (EHLA) process to investigate its feasibility as a coating replacement for hard chrome plating. A range of laser powers was investigated. These cermet coatings were benchmarked against an IN625only coating, in the analysis of their macrostructure, microstructure, phase composition, Vickers microhardness and sliding wear resistance by the pin-on-disc test. The EHLA process resulted in crack-free coatings, with a good metallurgical bonding to the substrate and homogenous distribution of cermet particles within the coatings. The microhardness of the EHLA composite coatings increased by 81-102 % as compared to the IN625 EHLA coating. The wear rates of the composite coatings were only 0.5-1.4 % that of the IN625 coating, and only 1-4 % that of hard chrome coating. A decrease in laser power demonstrated an increase in the carbide-occupied cross-sectional area from 11.2 % to 17.2 %, which corresponded to a 64 % enhancement in wear resistance. This study highlights the critical balance required between laser power, carbide area fraction and microstructural characteristics, on the performance of EHLA-deposited composite coatings.

In this research, the microstructural and mechanical aspects of a CoCrFeNiMo0.5 high-entropy alloy coating (HEAC) deposited via high-velocity oxygen-fuel (HVOF) spraying are analysed. The investigation explores the cross-sectional and plan-view properties of the coating, which consists of unmelted splats (USs), semi-molten splats (SMSs), fully-molten splats (FMSs), oxides, and pores. Within the USs, Ni-Co-Fe-rich face-centred cubic (FCC) dendrites containing deformation twins are identified, accompanied by Mo-rich sigma interdendrites. In SMSs, heavily deformed impact regions lead to ultra-fine elongated grains, while dynamic recrystallization (DRX) promotes (ultra-)fine equiaxed grains in their interior sections, both adopting a FCC structure. FMSs, on the other hand, exhibit fine columnar FCC grains with a completely uniform elemental distribution. Beyond the extremely hard sigma phase, strengthening mechanisms, such as lattice distortion, grain refinement, and twinning-related strengthening, contribute to the mechanical strength of the coating. Notably, oxide dispersion strengthening occurs predominantly through spinels formed during the HVOF spraying process, playing a crucial role in improving the coating's strength further. In contrast, the presence of small amounts of pores has a detrimental effect on mechanical properties, though this effect remains localized.

CrFeNi-based medium-entropy alloys (MEAs) not only suffer from the strength-ductility trade-off, but also raise multiple concerns regarding cost-benefit analysis in their bulk form. The current study overcomes these issues by introducing Al and Ti into an Al0.3CrFeNiTi0.3 medium-entropy alloy coating (MEAC) deposited by high-velocity oxygen-fuel (HVOF) spraying. Accordingly, the extremely chaotic distributions of the principal microstructural components of this HVOF-MEAC, i.e., (i) the unmelted splats (USs), (ii) semi-molten splats (SMSs), (iii) fullymolten splats (FMSs), and (iv) oxides, lead to the generation of dense coating layers together with low amounts of (v) pores. Influenced by dissimilar non-equilibrium cooling rates and heat transfer mechanisms, various solidification morphologies, i.e., (i) the sunflower-like eutectic, (ii) heterostructured, and (iii) columnar grains, are observed within the three types of splats having different melting index (MI), flattening degree (xi), and phase formation kinetics. As a result of the unidirectional deposition conditions, some degree of microstructural and mechanical anisotropy is encountered without strong texture development at the micron-scale, while nano-scale phases contribute to the overall mechanical strength through several mechanisms. Oxide dispersion strengthening is noted due to the presence of crystalline (AB2O4-type) and (possibly) amorphous (AO2type) oxides, which are the products of in-flight oxidation (IFO) and post-impact oxidation (PIO) periods, respectively. Twinned structures in FMSs are anticipated to enhance plasticity. Compared to many 3d transition metal high- or medium-entropy alloy coatings (HEACs/MEACs) with distinct compositions deposited by similar processing methods, the excellent nano-hardness properties of this Al0.3CrFeNiTi0.3 HVOF-MEAC make it an ideal candidate for many engineering applications.

In surface engineering, thermal spray (TS) processing offers significant advantages over bulk production methods, particularly with the use of high-velocity oxygen-fuel (HVOF) spraying. This study investigates, for the first time, the microstructural and mechanical evolution of an AlCrFeMnNi high-entropy alloy coating (HEAC) deposited via HVOF spraying. As part of the investigation, the cross-sectional and plan-view features of the coating were studied in detail after determining the morphological and solidification characteristics of gas atomized feedstock powders. Accordingly, the coating comprises unmelted splats (USs), semi-molten splats (SMSs, possibly), fully-molten splats (FMSs), in-flight and post-impact oxides, as well as different types of defects, including pores and cracks. USs exhibit Fe-Cr-rich BCC dendrites and Ni-rich B2 interdendrites, while FMSs are composed of fine columnar grains of the B2 phase, in which the constituent elements are homogeneously distributed. In the severely deformed region of the USs, dynamic recrystallization (DRX) results in the transformation of dendrites and interdendrites into (ultra-)fine, equiaxed B2-structured grains. Continuous dynamic recrystallization (CDRX), as opposed to discontinuous dynamic recrystallization (DDRX), appears to play a key role, where progressive lattice rotation contributes to the lack of crystallographic texture. Cracks propagate along the upper surfaces of FMSs, parallel to the substrate, leading to mechanical anisotropy and concerns with coating spallation. Despite this challenge, a robust coating-substrate interface was established through mechanical interlocking of splats, while oxide dispersion strengthening, lattice distortion, grain size refinement, and B2-type ordering serve to enhance the micro-hardness and nano-indentation properties of the coating.

The present study investigates the deformation behavior of an equiatomic CrMoNbTiW refractory high entropy alloy (RHEA) synthesized via powder metallurgy. The alloy displayed a dominant BCC phase with secondary TiC and Cr2Nb phases. Uniaxial compression tests were performed at temperatures ranging from 1000 to 1350 degrees C and strain rates of 10-1 to 10-3 s-1. The flow stress data were analyzed using an Arrhenius-type power law, revealing a strong dependence of the apparent activation energy and stress exponent on temperature, strain, and strain rate. The ultrafine-grained microstructure and secondary phases facilitated continuous dynamic recrystallization (CDRX). However, flow localization was observed under all deformation conditions. Nanoindentation analysis indicated distinct deformation behaviors among the different phases, with the W-rich BCC phase exhibiting higher deformation resistance compared to TiC. These findings highlight the importance of analyzing the post- deformation microstructure of multiphase materials to elucidate the underlying deformation mechanisms.

Thermal sprayed AlxCrFeNiTiy high-(or medium-)entropy alloy coatings (HEACs/MEACs) are an emerging class of materials used in surface engineering applications. Post-deposition treatments are anticipated to modify their microstructures and expand their range of uses. This investigation implements a two-step annealing process at 860 degrees C (for 2 h) and then at 700 degrees C (for 2 h) to induce distinct phase transformations in the three splat types, i.e., unmelted, semi-molten, and fully-molten splats, of a high-velocity oxygen-fuel (HVOF) sprayed Al0.3CrFeNiTi0.3 MEAC. Accordingly, it was determined that the annealed coating featured BCC, L21-Ni2TiAl, FCC, sigma-CrFe, L12- Ni3(Ti,Al), and D024-Ni3Ti phases, together with oxides such as TiO2, Al2O3, and Al2TiO5. Unlike the as-sprayed coating that showed non-uniform grain sizes and morphologies, the annealing treatment yielded more uniform grain characteristics. In addition, a number of specific orientation relationships were developed between the constituent phases, including {110}L21 //{111}L12 and (111)L21 //(110)L12 , {0001}D024//{111}L12 and (1120)D024//(110)L12, and {001}sigma//{111}L12 and (110)sigma//(110)L12. Oxygen atoms released from the in-flight oxides (IFOs) accumulated at splat boundaries, to where high oxygen-affinity Al atoms were attracted. This led to the concurrent formation of Al2O3 oxides and Kirkendall voids through bulk Al diffusion, causing further phase transformations in nearby areas that were depleted of Al. These microstructural developments resulted in changes in the mechanical behaviour of the coating, such that the annealed coating exhibited excellent micro- hardness and nano-hardness properties, with respective enhancements of 33 % and 40 % compared to the as- sprayed condition.

Slag deposition and high-temperature corrosion are major ash-related issues in biomass-fired boilers. Protective coatings are used as countermeasures to these challenges. The coatings are produced by methods such as slurry spray and thermal spray. Nevertheless, studies on the comparative performance of these coatings on slag deposition and corrosion resistance are limited. Furthermore, there is currently no standard method to evaluate the anti-slagging capabilities of boiler coatings. Therefore, this study investigated and compared the slag and high-temperature corrosion resistance of a slurry spray coating and its thermally sprayed counterparts: atmospheric plasma spray and suspension plasma spray coatings. Anti-slagging evaluation of the coatings was conducted using an in-house “slag testing rig”, which was designed to investigate coating performance under simulated boiler conditions. A suspension plasma spray coating with a cauliflower-like surface exhibited the largest biomass fly ash reduction of 12

Thermal spray processing parameters need to be optimised to mitigate decarburisation and oxidation of carbidebased coatings that causes unacceptable brittleness. In the present work, a theoretical model was developed to optimise kerosene fuelled high-velocity oxy-fuel (HVOF) flame characteristics and spray parameters for producing high-quality WC-NiCr coatings using finer feedstock of particle size distribution of - 30 + 5 mu m. The model suggested an oxygen-to-kerosene ratio of 3.3 and a shorter torch barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long torch barrel. The developed theoretical framework was successful to provide optimised set of HVOF spray parameters to deposit high-quality fine carbide coatings with less decarburisation. Also, the coatings deposited using the optimised set of parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, as-spray surface roughness, and corrosion resistance. Hence, the optimised parameters can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts.

Industrial boilers operate in extreme environments and are prone to degradation caused by high temperature oxidation, corrosion, and erosion. Protective coatings provide boiler components with protection to prolong their operation lifetime. Thus, a glass-ceramic coating has been developed using a slurry spray technique on carbon steel AS/NZS 3678 substrates. Microstructure, mechanical characteristics, and high temperature performance of the coating were investigated. The coating composed of chemically inert oxide ceramics dispersed in a glassy silicate binder matrix. The adhesion strength of the coating was 10.79 +/- 0.86 MPa. Coating mass loss after abrasion test was 0.013 % of the tested specimen. The coating withstood at least 75 air quenched thermal shock cycles from 800 degrees C to the room temperature without any visible spallation and cracks. The coating mostly remained undamaged after cyclic oxidation at 800 degrees C for 10 cycles. However, minor iron oxide formation was observed, specifically, in relatively thin sections of the coating. The oxidation resistance of the boiler steel has been improved with the developed coating. The linear oxidation rate constant of the boiler steel has been statistically reduced from 6.90 x 10(-4) mg cm(-2) s(-1) to 1.95 x 10(-4) mg cm(-2) s(-1) for coated steel (p = 0.007). The developed slurry spray glass-ceramic coating is a potential candidate for boiler environments.

The addition of larger atomic size elements, such as Al and Ti, to CrFeNi medium-entropy alloys (MEAs) offers a strategic approach to overcome the strength-ductility trade-off. This study focuses on examining the morphological, solidification, phase, and elemental characteristics of Al0.3CrFeNiTi0.3 feedstock powders produced through gas atomization (GA). The rapid cooling rate inherent in GA results in sunflower-like eutectic solidification. This structure exhibits firstly solidified Cr-rich BCC phases filled with cuboidal-shaped L21-type precipitates, each measuring up to similar to 20 nm. Their peripheral regions display outward and radial growth of L21 and Fe-rich BCC eutectic phases, while FCC phase develops at the remaining sites. Intermediate B2 layers between BCC and L21 phases are considered as kinetic buffers, which enhance the interfacial stability. The size and shapes of powders determine their cooling rates, leading to a transition from comparably large sunflower-like eutectic structures to finer equiaxed grains alongside reduced elemental segregation with decreasing powder size. Nanohardness measurements demonstrate superior mechanical strength in the peripheral regions due to the higher volume fraction of the L21 phase and its optimal interfacial coherency with the Fe-rich BCC phase. Furthermore, the slightly higher concentrations of Al and Ti in Fe-rich BCC phase (compared to the Cr-rich BCC phase) induce a better lattice distortion effect.

Thermal spray high-entropy alloy (HEA) coatings have demonstrated potential for improving the wear resistance of conventional materials used in extreme engineering environments. In the present work, an equiatomic AlCoCrFeNi HEA coating was manufactured using the high velocity air fuel (HVAF) process. The phase and microstructural transformations in gas-atomized (GA) powder during HVAF spraying were analyzed using SEM, EDS and EBSD techniques. The tribological properties of this HEA coating sliding against an Al2O3 ball at both room temperature (RT) and 600 °C were also evaluated. The GA powder was composed of Body Centred Cubic (BCC) + ordered BCC (B2) phases, which transformed to BCC + B2 + minor Face Centred Cubic (FCC) phases during the HVAF coating process, validating the thermodynamic phase prediction projected by the Scheil simulation for non-equilibrium processing conditions. The rapid solidification and high velocity impact-assisted deformation of GA powder resulted in significant grain refinement in the HVAF coating, which ultimately improved the mechanical properties at both micro and nanoscale levels. The wear resistance of the HEA coating at RT was severely impacted by the relatively brittle BCC/B2 phase structure, leading to susceptibility to abrasive wear and surface fatigue. The wear resistance at 600 °C was slightly lower at RT due to the formation of a brittle oxide layer on the worn surface, which induced surface fatigue and aggravated mass loss of the coating.

High temperature corrosion and slag deposition significantly reduce the thermal efficiency and lifespan of biomass-fired boilers. Surface modification with protective coatings can enhance boiler performance and prevent commercial losses due to maintenance and damage. This review focuses on the development of corrosion-resistant coatings (CRCs) and anti-slagging coatings (ASCs) over the past decade. CRCs are explored through thermal spray processes that include arc spray, atmospheric plasma spray (APS), high-velocity oxygen fuel (HVOF), detonation gun (D-gun™), and cold spray. Studies on alloys, ceramics, and ceramic–metal composites are summarised, highlighting the high temperature corrosion prevention mechanisms and discussing new coating materials. ASCs are reviewed in the context of advancements via thermal spray and slurry spray methods. The mechanisms for slag reduction, testing methods to evaluate ASC effectiveness, and the necessary architecture for preventing slag deposition are examined. A lab-based rig simulating fly ash deposition onto water-cooled coating coupons for anti-slagging investigations is also presented. Further research is needed to develop and evaluate materials for ASCs effectively.

Thermal spray chromium carbide-based composites have been widely used industrially because of their superior oxidation resistance at elevated temperatures compared to other metal matrix composite coatings. In this work, the serviceability of HVOF sprayed coatings from an atomized chromium carbide-based feedstock (Cr23C6- 40NiCr) was compared with those from a conventional Cr3C2-25NiCr agglomerated and sintered feedstock. Coatings were exposed to 540 degrees C and 610 degrees C for 168 h in both air and steam environments. These conditions are indicative of industrial power plant working conditions to assess their real-time performance. Quantitative Rietveld analysis of the Cr23C6-40NiCr coating XRD patterns indicated a comparable concentration of Cr23C6 in both the powder and coating, implying minimal carbide dissolution. However, the Cr3C2 content in the conventional Cr3C2-25NiCr coating was markedly reduced compared to the initial powder composition, indicating that carbide dissolution occurred to a greater extent. Air oxidation led to the formation of coarse surface oxide, due to the initial formation of Ni-based oxides before a continuous Cr2O3 layer could develop underneath. A notably finer and more uniform oxidized surface composed only of Cr2O3 was formed during steam treatment across both coating types. More importantly, the oxide scale was intact and crack-free in Cr23C6-40NiCr compared to Cr3C2-25NiCr. Moreover, despite Cr23C6 being the main carbide phase and the high binder content (40%NiCr), the microhardness of the Cr23C6-40NiCr coating was comparable to that of the conventional Cr3C2- 25NiCr coating, both in the as-sprayed and heat exposed states.

Thermal spray WC-NiCr coatings generally requires grinding processing to meet the surface finish requirements. The cost associated with grinding can potentially be reduced through the deposition of finer (– 30 + 5 μm) feedstock rather than the more conventional commercial (– 45 + 15 or – 53 + 20 μm) feedstock. Additionally, such a fine powder is likely to require lower energy spray parameters, resulting in less heat input to the substrate, which could be beneficial in application on heat sensitive substrates. However, the spray parameters need to be optimised to mitigate increased degradation of coatings due to unacceptable brittleness caused by decarburisation and oxidation, and to produce defect-free dense coatings. In the present work, a theoretical model to optimise spray parameters was developed, which suggested an oxygen-to-fuel ratio slightly more than 3.3 and a shorter barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long barrel. Scanning electron microscopy, X-ray diffraction, microhardness, and 3Dprofilometer were used to analyse the produced coatings. The coatings deposited using optimised parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, and roughness. Coating deposited at lower kerosine and oxygen flow rates, with reduced stand-off distance, was denser, crack-free, and ductile. Hence, the fine-cut powder can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts. Additionally, successful deposition of coatings using low energy parameters, making this an attractive option for thermally sensitive substrates.

The steel making processes involves extreme and harsh operating conditions; hence, the production hardware is exposed to degradation mechanisms under high temperature oxidation, erosion, wear, impact, and corrosive environments. These adverse factors affect the product quality and efficiency of the steel making industry, which contributes to production downtime and maintenance costs. Thermal spray technologies that circumvent surface degradation mechanisms are also attractive for their environmental safety, effectiveness and ease of use. The need of thermal spray coatings and advancement in terms of materials and spray processes are reviewed in this article. Application and development of thermal spray coatings for steel making hardware from the molten metal processing stages such as electric arc and basic oxygen furnaces, through to continuous casting, annealing, and the galvanizing line; to the final shaping process such as cold and hot rolling of the steel strips are highlighted. Specifically, thermal spray feedstock materials and processes that have potential to replace hazardous hard chrome plating are discussed. It is projected that novel coating solutions will be incorporated as awareness and acceptance of thermal spray technology grows in the steel making sectors, which will improve the productivity of the industry.