Journal of Thermal Spray Technology最新文献

Cold spray (CS) is a progressive method for the deposition of metals and alloys whose principles involve considerable plastic deformation of the produced material at extreme strain rates. Positron annihilation spectroscopy (PAS) is an analytical technique capable of studying deformation on the atomic scale level, even in extremely deformed materials. In our study, the PAS method was used to characterize the microstructure and quantify the open-volume defects in four cold sprayed metals: Al, Cu, Ni, and Ti. As counterparts, bulk samples of these materials with ultra-fine-grained structures were also produced by high-pressure torsion (HPT), a process exceeding cold spray in the total deformation, but having several orders of magnitude smaller strain rates, and by a traditional cold rolling process. The results show that the CS and HPT processes lead to the formation of similar lattice defects (dislocations and vacancy clusters), and both exhibit significantly higher dislocation densities than conventionally cold-rolled materials. Further, the vacancy clusters present in CS and HPT materials were not present in the rolled counterparts due to the lower vacancy production rate.

WC-based metal-ceramic coatings are widely used in cavitation erosion environments, where their service performance is primarily determined by their microstructural characteristics. Sintering serves as a crucial method for optimizing the coating microstructure. This study employed an innovative short-time vacuum sintering process for post-treating HVAF-sprayed WC-Ni60-Co-Cr coatings. Results showed that the coating treated at 1120 °C exhibited the best cavitation erosion resistance, demonstrating a cumulative mass loss of only 22.5% compared to the as-sprayed state, with the maximum erosion depth decreasing from 29.08 μm to 1.26 μm. This significant improvement was attributed to temperature-driven diffusion acceleration and phase transformation, which were critical for achieving rapid coating densification. These findings provide a novel approach for developing efficient coating post-treatment processes while offering valuable insights into the cavitation erosion mechanisms of thermally post-treated WC-based coatings.

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To investigate the effect of the preheating temperature of the substrate, the preheating temperature of Q235 steel was varied, and then, it was observed what changes it produced on the organization and properties of iron-based amorphous composite coatings prepared by laser melting and cladding. XRD, microanalysis, hardness testing, friction and wear, and electrochemical corrosion experiments were used for comprehensive evaluation. The results show that the preheating temperature significantly affects the coating amorphous phase content, crack formation, and microstructure. Under the preheating condition of 250 °C, the coating achieved a crack-free state with the highest amorphous phase content, the finest grain refinement, and precipitation of hard phases such as WC, Cr₇C₃, and M₂₃ (C, B)₆. This is attributed to the fact that the preheating effectively reduced the temperature gradient and thermal stress and optimized the solidification behavior of the melt pool. The coating exhibits an optimal overall performance with a microhardness of 813.7 HV_0.3, minimal wear volume, and the lowest corrosion current density. Too high a preheating temperature leads to grain coarsening, dissolution of hard phases such as WC, and property degradation. The proportion of the amorphous phase and the number of cracks together determine the performance of the coating.

To improve the hardness and wear resistance of aluminum alloys, 434 stainless steel coatings on aluminum substrates were deposited with different spraying parameters (number of sprayed-layers and spray distance) via high-velocity oxygen–fuel (HVOF) flame spraying. The Young’s modulus (E) of the SS coatings was calculated via three-point bending and measured the hardness by Vickers indentation tests. The tribological properties of SS coatings were discussed by the H/E and H³/E², as well as the influence of different iron oxide types on the abrasion of stainless steel coatings was investigated via thermodynamics and kinetics. The results showed that best match of a hard phase (martensitic transformation provides) with Young’s modulus of the coating, high H/E and H³/E² ratio, SS1-2 coated sample presented better wear resistance, which have a lowest average friction coefficient, and the wear rate is reduced from 11.21 × 10⁻⁵ mm³/(N m) − 6.01 × 10⁻⁵ mm³/(N m) to 7.67 × 10⁻⁵ mm³/(N m) − 4.96 × 10⁻⁵ mm³/(N m). In addition, compared with the first set of deposition conditions, SS2-2 coated sample have a higher average friction coefficient as spray distance increased, and the wear rate is increased by nearly two times, due to formation the hard and brittle Fe₃O₄ phase. The tribological properties of SS coatings can well revealed by calculating the ratio of Young’s modulus to hardness (H/E and H³/E²).

To analyze the effect of spray trajectories on the temperature and residual stress of plasma-sprayed coatings, this study employed finite element method coupled with the element birth and death technique to simulate the deposition of CrMnFeCoNi high-entropy alloy coatings on 45 steel substrates. The focus was on comparing the influence of “N” shaped and “S” shaped spray paths on the distribution and magnitude of residual stress in the coatings. Under the same spray process parameters, the residual stresses in the coating of the two paths are similar in magnitude and distribution. However, on the substrate surface, particularly in the transition regions between adjacent coatings, there are significant differences in residual stress. The shear residual stress at the interface between the substrate and the coating for the coating sprayed with the “S” shaped path is significantly lower compared to that sprayed with the “N” shaped path. Overall, the comparison revealed that the “S” shaped spray path effectively reduces shear residual stress at the edges of adjacent coating layers, thereby enhancing the bonding strength between the coating and the substrate. Therefore, to optimize the coating quality, it is recommended that the planning of adjacent spray passes be in opposite directions.

The increasing demand for higher operating temperatures and improved efficiency has highlighted the limitations of conventional bond coat materials, such as MCrAlX, which are prone to oxidation and degradation. This study investigates FeCoNiCrAl high-entropy alloy (HEA) as an alternative bond coat material to enhance thermal barrier system performance. Using the high-velocity oxygen fuel (HVOF) spray technique, FeCoNiCrAl HEA and industry-standard MCrAlX coatings were deposited and evaluated in terms of deposition efficiency, thermally grown oxide (TGO) growth, and oxidation resistance. The results revealed that HEA bond coats achieved a higher deposition efficiency and formed a dense and protective Al₂O₃ TGO layer, comparable to conventional bond coat alloys. Notably, the HEA bond coat exhibited no mixed oxide formation, even after 150 h of exposure at 1150 °C. Additionally, the TGO thickness remained controlled, demonstrating the superior oxidation resistance of FeCoNiCrAl HEA. These findings suggest that HEA coatings deposited via HVOF spraying offer a promising alternative to traditional MCrAlX bond coats for high-temperature applications.

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% by weight, which was contributed by the formation of air pockets at the interface between the biomass ash deposit and the coating. The slurry spray coating provided the highest corrosion resistance capability against molten salts with a linear corrosion rate constant of 3.40 × 10^-4 mg/cm²·s (p = 0.003).

At present, high-entropy alloy coating reinforced by addition of the principal elements has gained increasing attention for their mechanical property and wear and corrosion resistance. In this study, AlCoCrFeNi and AlCoCrFeNiB high-entropy alloy coatings were fabricated using HVOF spraying. The effects of boron addition on the microstructure, phase structure, hardness, wear and corrosion resistance of AlCoCrFeNi high-entropy alloy coating were investigated, and the wear mechanisms were studied. The results show that the addition of boron refines the microstructure of the coating, increases lattice distortion, promotes the formation of the Cr₂B phase, reduces porosity, causes a shift in the BCC phase, decreases the diffraction peak intensity, and increases hardness. Furthermore, adding boron to the AlCoCrFeNi high-entropy alloy coating results in a decrease in the coefficient of friction and wear rate, changes the wear mechanism, and improves its wear resistance. Additionally, the AlCoCrFeNiB high-entropy alloy coating has higher corrosion potential and lower corrosion current density, thereby enhancing its corrosion resistance.

Thermal barrier coatings (TBCs) are essential for insulating and protecting components in high-temperature environments, with applications traditionally focused on external surfaces. However, the development of TBCs for internal diameters (ID), particularly for small bores below 200 mm, poses significant challenges due to the limited understanding of coating formation mechanisms in confined geometries and the constraints of process conditions. Therefore, this study aims to elucidate the characteristics of plasma-sprayed ceramic TBCs on ID surfaces, with the goal of establishing a relationship between process parameters, microstructure, and coating properties for 8 wt.% yttria-stabilized zirconia (8YSZ) topcoats applied via atmospheric plasma spraying (APS) using the SM-F100 CONNEX ID torch. The key APS parameters, namely the hydrogen flow rate, current, and spray distance, were investigated using a design of experiments (DOE) methodology to systematically study both the individual and interactive effects of these parameters on coating systems such as porosity and deposition efficiency, which are critical features of TBC performance. Since ID coatings are constrained by limited space for spray distance and torch placement inside the tubular component, the selection of the appropriate feedstock becomes challenging. Therefore, feedstocks with different morphologies and sizes were investigated and evaluated based on the resulting porosity and deposition efficiency. An agglomerated and sintered 8YSZ feedstock showed the most promising results. In addition, this study also examined the impact of substrate geometry (flat versus curved surfaces) on coating properties and process control. The results contribute to the broader scientific understanding of plasma spray technology for internal diameter (ID) surfaces.

This work investigates the relationship between the cavitation resistance of WC-Co-Cr coatings and the high-velocity oxy-fuel (HVOF) spraying parameters utilized during their deposition. The coatings were applied using a fully automated spraying process with hydrogen and oxygen as combustion gases. A Design of Experiments (DoE) methodology was implemented, incorporating two factors—Total Combustion Flow (TCF) and Combustion Ratio (CR) —with three repetitions to ensure experimental repeatability. Cavitation resistance was evaluated according to the ASTM G32 standard. The results indicate that total combustion flow significantly influenced key coating characteristics, including roughness, microstructure, hardness, and WC retention index, whereas the combustion ratio exhibited no statistically significant effect. It was possible to improve the cavitation resistance of the WC-Co-Cr coatings using high levels of total combustion flow and a combustion ratio close to the stoichiometric point of the gas mixture.