Journal of Thermal Spray Technology最新文献

In this experimental study, diamond-reinforced Ni-P coatings were developed by flame and high-velocity air fuel (HVAF) spraying techniques using Ni-P capped diamond powder. Further, effect of heat-treatment on microstructure, structural, hardness and high temperature wear characteristics of the above coatings was investigated. After heat-treatment, high hardness was observed in HVAF coating compared to flame sprayed which is attributed to the high porosity of the latter as evident from the microstructure. Extensive diamond particle fragmentation was observed in the HVAF sprayed coating, providing motivation for including the lower velocity flame spraying in this work. It is interesting to note from the wear tests that coatings deposited by flame spraying exhibited superior wear resistance and low friction coefficient at high temperature, i.e., under dominated oxidative wear conditions, which is attributed to the soft matrix leading to diamond particles’ exposure and graphitization. However, hard and dense heat-treated HVAF sprayed coatings exhibited highest wear resistance in room temperature tests dominated by abrasive wear mechanism as evident from the wear track morphology. Raman spectroscopy and energy dispersive spectroscopy analysis (EDS) confirmed the graphitization for the flame sprayed coatings and formation of oxides in the wear tracks.

Cold spray has been extensively applied to deposit a range of materials in many industries. In the recent times, such a method has also shown its potential to deposit nickel-based superalloys, which currently are in demand due to their high tensile strength and corrosion resistance (especially at elevated temperatures); however, cold sprayed nickel super alloy coatings have poor mechanical properties due to the materials’ limited ability to undergo plastic deformation. Regarding this, numerous cold spray process modifications have been experimented, including preheating substrate and feedstock powder, applying laser irradiation, heat treating coatings post deposition, and heat treating feedstock powder, to promote plastic deformation, eliminate porosity and enhance inter particle bonding. Specifically, the important influence of external heat input on the underlying substrate and/or the incoming particles during cold spray deposition was highlighted in multiple studies. These studies indicated that the addition of external heat during cold spray increased the adhesion strength of the coatings due to an increase in the thermal softening effect of the deposited particles. In general, an attempt is made here to systematically review the influence of cold spray process modifications on the microstructure, mechanical properties and residual stresses of nickel super alloy coatings.

In the process of High velocity oxygen fuel (HVOF) spraying, micron-sprayed particles are bound to oxidize under high temperature oxygen-containing environment, particle oxidation and burning are the key factors affecting coating quality. However, how to quantitatively evaluate and control particle oxidation is the bottleneck problem faced by the industry. In this paper, a three-dimensional transient calculation model of flame flow during HVOF thermal spraying WC-12Co process was established, and the computational fluid dynamics and discrete phase surface reaction model were combined to calculate and reveal the distribution characteristics of flame flow and the oxidation degree for particles during the spraying process. The calculation showed that the oxide layer thickness of particles varies greatly with different particle sizes. The oxide layer thickness of particles with 5 μm size is about 90 Å, and the oxide layer thickness of particles with 60 μm size is only about 8 Å. By adjusting the process parameters of oxygen/fuel ratio, particle size and nitrogen mass flow rate in the model, the output samples of sprayed particle flight temperature, velocity and oxide layer thickness can be obtained. On this basis, the sample data were statistically analyzed based on Genetic Algorithm-Back Propagation (GA-BP) neural network model, and the optimal process parameters for preparing the optimized coating were determined: particle size 27 μm, oxygen/fuel ratio 3.1, nitrogen mass flow rate 0.000363 kg/s. Experiments were carried out with optimized parameters, the results show that the optimized coating has fewer defects, lower oxide content and higher hardness and wear resistance. This study provides an important theoretical basis for quantitative preparation of high quality HVOF spray coatings.

Laser cladding provides the feasibility for fabricating various ceramic-reinforced metal–matrix composite coatings, in which the particle size and content of the reinforcing particles play a crucial role in the properties of the cladding. In this paper, to improve the hardness and wear resistance of Inconel 718 coatings, a series of composite coatings were prepared by laser cladding and the effects of WC-Co grain size and content on the macrostructure, microstructure, phase composition, and mechanical properties of Inconel 718 coatings were revealed in detail. The results showed that with the increase in WC-Co content, multiple carbides coexisted in the coating, the grain size became smaller, and the cracking susceptibility increased. The coatings containing 10-55 μm WC-Co particles have higher grain refinement and higher hardness and better plasticity, but also higher coefficient of friction (COF) values. Based on the increased hardness, the coatings containing 40-120 μm WC-Co particles showed better wear resistance, with a 70.3% reduction in wear volume compared to pure Inconel 718 with 30% WC-Co content. The wear mechanism of each coating was dominated by abrasive wear.

This study investigates the influence of different heat treatment processes on the microstructure and mechanical properties of cold spraying coatings deposited on TIG joints of 2219 Al alloy. The results indicate that there were no significant changes in the microstructure of the joint before and after cold spraying. However, there was noticeable plastic deformation at the interface between the joint and the coating. The porosity rates of coatings subjected to room temperature, direct aging, and solution aging are 1.94%, 0.67%, and 4.57%, respectively. After direct aging, the extent of plastic deformation in the coating decreases. Following solution aging, recrystallization occurs in the coating leading to grain growth. A comparison of the microstructures of the three types of coatings revealed a gradual reduction in dislocation density and the quantity of low angle grain boundaries (LAGBs) within the coatings. Following solution aging, a significant amount of needle-like long-period transition phase θ′ precipitated. The coating hardness transitioned from a step-like distribution to a more uniform distribution, with an approximate 5HV increase in hardness within 2.5mm depth from the surface of the substrate. The tensile strength of joints with coatings improved by 5.6% and 30.9%, respectively, while the elongation increased by 15.4% and 93.3%, resulting in significantly enhanced ductility. The coating fracture mode transitions from predominantly brittle fracture to primarily ductile fracture.

Metallizing polymer composite via cold spray is an attractive process for producing lightning strike protection (LSP) in composite-based aircraft. However, direct deposition of copper, the most common conductive metal, on carbon fiber-reinforced thermoplastic polymers (CFRTP) remains challenging due to the severe polymer erosion and carbon fiber fracture induced by cold-sprayed multi-particle impact. In this work, the copper direct deposition window was explored by varying copper particle size and operating gas temperature, which are primary parameters determining particle impact energy. Following this, a lightning strike test was performed to evaluate the protective effectiveness of the deposited copper coating in reducing damage to the underlying CFRTP substrate. It was revealed that fine copper particles approximately 5 µm in diameter are effective for direct deposition due to their minimal induction of polymer erosion. Sufficient heating of the particle spray area above the glass transition temperature promoted continuous coating formation. The lightning strike tests demonstrated that the cold-sprayed copper coating effectively protects the underlying CFRTP by acting as a major electric current path and a sacrificial protective layer. These findings highlight the promising potential of cold spray as a new LSP technology for polymer composite aircraft.

Cold spray was employed to apply CuCrZr-based composite coatings onto CuCrZr substrates, aiming to enhance both their mechanical and thermal properties. A comparison was drawn between wear-resistant SiC and high thermal conductivity AlN reinforcements in terms of shear strength, tribological behavior, and thermal conductivity. Results show that the 30vol.% AlN/CuCrZr and 45vol.% SiC/CuCrZr coatings have similar grain structures, ceramic content and particle size distribution, but a distinct mean free path of the ceramics. The 30vol.% AlN/CuCrZr coating exhibits superior hardness and shear strength compared to the 45vol.% SiC/CuCrZr coating, attributed to the greater dispersion strengthening effect arising from the closely spaced AlN particles while the sparse distribution of SiC particles facilitates formation of the CuO lubricating film, resulting in the better wear resistance of the SiC/CuCrZr coating. Finally, the 30vol.% AlN/CuCrZr coating demonstrates superior thermal performance due to the higher thermal conductivity inherent in AlN ceramic. Therefore, the inherent thermal conductivity and distribution of the ceramic particles could be crucial factors in achieving comprehensive thermal and wear performance for the CuCrZr composite coating.