Journal of Thermal Spray Technology最新文献

In this study, TiB₂-CoTi composite coatings were fabricated on AISI 304 stainless steel (SS) substrate through argon arc cladding (AAC) technique. The effects of AAC processing currents and weight percentage of titanium (Ti) content on mechanical and wear rate of the coatings have been examined. Microstructural and element distribution maps, as well as phase analysis of the produced coating, were determined using field emission scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. Results revealed that the coating exhibited good metallurgical bond to the substrate with columnar and network-shaped dendrite structure. The top surface of composite coatings was mainly comprised of TiB₂, NiTi, TiB, Co₃Ti, Co₂B, CoTi, and α-Ti phases. Components of the composite phases were beneficial for improved microhardness and reduced the wear rates. The maximum average microhardness of TiB₂-CoTi composite coating was achieved as 1582 HV_0.1. This is significantly seven times higher than that of AISI 304SS substrate hardness (223 HV_0.1). The wear rate of TiB₂-CoTi coating was determined to be 2.53 × 10⁻⁸ g/N m, whereas average wear rate of AISI 304SS substrate was 24.39 × 10⁻⁸ g/N m. The wear resistance of the TiB₂-CoTi coating was 9 times higher than that of AISI 304 SS substrate. Its durability and performance under challenging conditions suggest that it is suitable for applications that demand superior durability and performance.

Steel molds used for aluminum die-casting often fail due to excessive wear or cracking phenomena associated with the soldering effect in contact with molten aluminum, which leads to the formation of iron-based intermetallic compounds and causes problems in the cast components. One solution is to apply protective coatings whose composition is less reactive with the molten aluminum and improve its hardness, toughness, wear, and corrosion resistance, thus prolonging its service life. This work evaluates the effectiveness of Ti-6Al-4V coatings deposited by twin wire arc spraying in an air or nitrogen atmosphere. Nitrogen was used as the carrier and shielding gas for the in-flight molten particles. Coatings were deposited by varying the stand-off distance, the nitrogen gas pressures, and the substrate temperature. The microstructure of the coatings is interlayered, one porous layer of dendrites and one highly densified layer. The presence of TiN, TiO₂, α-Ti, and β-Ti phases was confirmed by different characterization methods. For instance, x-ray photoelectron spectroscopy measurements confirmed the presence of N-Ti, O-Ti, and N-O bonds, with the oxygen/nitrogen/titanium percentage associated with the formation of a non-stoichiometric (Ti, Al, V)N_xO_y phase. Finally, the reactivity of selected oxynitrided Ti-6Al-4V coating in contact with molten aluminum showed a low reaction rate compared to the coarse reaction layer suffered by the uncoated steel substrates.

In the process of detonation spraying, the deposition state and the distribution of powder particles determine the performance of the coatings. High-performance coatings require powder particles to be sprayed onto the substrate at high velocity, in a molten state, and in a concentrated distribution. The effects of particle diameter, right loading distance(L), and stand-off distance(S) on the flight velocity, temperature, and distribution of alumina particles during the spraying process were studied in this paper, which was achieved through two-way coupling between the particle and fluid phases. The simulation results indicated that differences among the diameters of the various powder particles led to velocity and temperature stratification within particle stream during the spraying process. As the right loading distance increased, the deposition velocity and temperature of particles increased; while, the powder particles were more concentrated. And as the stand-off distance increased, the deposition velocity of powder particles increased; however, it might make the temperature of particles fell below melting point and the divergence of the stream of powder particles increased.

CSAM deposition of Ti6Al4V is a challenging task, and high-quality deposits conforming to the AM application standards have not been developed so far. In our study, two distinct feedstock Ti6Al4V powders with different morphology and microstructure were used, and their influence on the key CSAM deposition parameters was investigated. The deposits were analyzed in terms of microstructure (including porosity), electrical conductivity, as well as tensile properties before and after heat treatment. The process gas temperature was found as the most influencing parameter, and the spherical plasma atomized powder resulted in a higher deposit density and electrical conductivity than the crystalline powder at 1100 °C and 50 bars. Using the optimal deposition parameters and heat treatment, unprecedented tensile properties for CSAM Ti6Al6V deposits were achieved, exhibiting 887 MPa yield strength and 929 MPa ultimate tensile strength, values that satisfy the ASTM B381 requirements for Ti6Al4V forgings, and a substantial 4.74% elongation.

Continuous monitoring of spray velocity during the cold spray process would be desirable to support quality control, as spray velocity is the key process parameter determining the deposit quality. This study explores the feasibility of utilising Airborne Acoustic Emission (AAE) for real-time monitoring of spray velocity. Six spray tests were conducted, varying pressure and temperature to achieve different velocities. Optical means were used to measure velocity; while, the signal from the AAE was captured during deposition via a microphone. Features demonstrating a strong correlation with velocity were extracted from the acoustic signals. Both rule-based and machine learning models were employed to identify the moments where the nozzle was engaged with the substrate and diagnose the velocity. The results indicate that monitoring the spray velocity of the cold spray process using AAE is feasible.

It is of great significance to better clarify the failure mechanism of thermal barrier coatings (TBCs) with film cooling hole edges to prolong the TBCs’ service life. In this paper, a numerical study of the film cooling model of flat plate with TBCs is carried out. The temperature field distribution of the TBCs is obtained by conjugate heat transfer, and the stresses of the TBCs at the edge of the film cooling hole are analyzed under real operating conditions, taking into account the TGO growth, plasticity, and creep properties of the material. The results show that due to the lower temperature of the film cooling hole edge, the TGO growth is slowed down, and the free-edge effect brought by the film cooling hole will play a large influence on the coating stress. The TBCs will have a large interfacial stress at the room temperature stage, and it will be more prone to flaking failure. The substrate, on the other hand, bears a larger stress in the high temperature stage.

Plasma spraying, an important coating and film preparation technology, provides a crucial method for depositing ceramic-based interconnects on tubular solid oxide fuel cells. However, traditional plasma-sprayed ceramic coatings exhibit typical lamellar porous structures with numerous unbound interfaces and gas-permeable channels, which makes it difficult to meet the microstructural requirements to prevent the leakage of fuel gas and oxidizing gas. This leads to the lower conductivity of the coating compared to the sintered bulk, and results in increased ohmic resistance and reduced output performance. To improve the interface bonding and conductivity of plasma-sprayed La_0.2Sr_0.8MnO₃-La_0.3Sr_0.7TiO₃ (LSM-LST) bilayer interconnects for tubular cells, a Co₃O₄ healing additive was added to the gap interfaces within the LST and LSM coatings. Results showed that metallurgical healing occurred at unbound interfaces and microcracks due to the liquid-phase sintering mechanism, and a bulk-like dense microstructure was obtained at a lower temperature (1200 °C) compared to the dense sintering temperature of the bulk. Moreover, the gas leakage rates of the stable coatings after interface healing were > 1 order of magnitude lower than that of the as-sprayed coatings. Additionally, their electrical conductivity was more than twice that of the as-sprayed coatings, which meets the microstructural and performance requirements of tubular cell interconnects.

The performance of the Al_0.5CoCrFeNi₂Ti HEA atmospheric plasma-sprayed coating was extended from characterizing the properties of its powder prepared via the gas atomization method. It was observed that the gas-atomized HEA powders possessed a solid solution BCC phase, while a major phase transformation to a FCC-L2₁ intermetallic phase occurred during the annealing process. The formation of the intermetallic phase resulted in an increase in average hardness from 6.28 to 7.64 GPa after annealing at 900 °C for 1 h. Afterward, HEA powders were applied in the atmospheric plasma spray technology. The phase constitution of Al_0.5CoCrFeNi₂Ti HEA coatings was investigated by varying powder size and applied current. It was observed that the smaller powder sizes prone to oxidation, whereas higher applied current facilitated the phase transformation from BCC to FCC phase. The nanoindentation test indicated distinct average microhardness values for the interlamellar oxide region, BCC unmelted particle and FCC phase lamellar region, which was measured at 12.35, 8.68 and 5.97 GPa, respectively. As a result, the adjustability of coating hardness was achieved by manipulating the relative phase ratio.

The effect of main arc current on the microstructure, electrochemical properties, and cavitation erosion resistance of NiCrBSi-WC alloy coating by plasma transfer arc welding was studied. The results show that the coating is composed of γ-Ni, WC, W₂C, Cr₇C₃, CrB, and FeNi₃ phases. With the decrease in main arc current, the microstructure is gradually refined, and the grain size and Schmidt factor are reduced, which are 12.62 ± 2.6 μm and 0.42, respectively. At the same time, the coating has a strong γ-Ni texture in (111) orientation. The refinement of the coating microstructure and the existence of surface passivation film slow down the electrochemical corrosion in boric acid solution and effectively improve the corrosion resistance of the coating. The increase in microhardness is due to the second phase strengthening mechanism of WC and Cr₇C₃, and the maximum value can reach 663.5 ± 6.2 HV. The damage and repair process of the passive film of the coating was studied by the synergistic method of cavitation erosion. With the increase in the main arc current, the average corrosion depth rate (MDER) of the coating can be reduced to 1.02 ± 0.03 μm/h, which is due to the improvement of the independent repair ability of the passive film, and the microhardness test results are inversely related to MDER. According to the above analysis results, the synergistic mechanism of cavitation corrosion of NiCrBSi-WC alloy coating is put forward. NiCrBSi-WC alloy coating prepared by plasma transfer arc welding technology in this study has great application prospects in the nuclear industry.

Double ceramic layer thermal barrier coatings (DLC-TBCs) are favored for combining the benefits of top and bottom ceramic materials. The thickness ratio of the top and bottom ceramic layers significantly impacts the performance of the DLC-TBCs. In the design process, it is generally desired to balance its thermal insulation properties with a long service life. Therefore, this study establishes a multi-objective parameter optimization design method based on NSGA-II to optimize the thickness of the CeYSZ/Al₂O₃ DCL-TBCs. Experimental verification of the coating performance was conducted based on the optimization results. Firstly, based on theoretical and numerical models, a quantitative analysis was conducted on the effects of the thickness of each material in the CeYSZ/Al₂O₃ DCL-TBCs system on thermal insulation and thermal stress. Space parameters were obtained using optimal Latin hypercube sampling, and a radial basis function (RBF) neural network surrogate model was constructed based on the numerical calculation results. Sensitivity analysis was employed to evaluate the impact of the total thickness of the TBCs and the thickness of the Al₂O₃ ceramic layer on the objective function. Finally, NSGA-II was utilized for optimization. The obtained Pareto optimal solution set was validated, showing that the performance of the CeYSZ 190 μm/Al₂O₃ 120 μm DLC-TBCs satisfied the requirements. Therefore, TBCs of different thicknesses were sprayed and subjected to thermal insulation and thermal shock experiments. The results demonstrated that the optimized TBCs significantly improved service life without compromising thermal insulation, providing a new approach for the subsequent design of DLC-TBCs structures.