Journal of Thermal Spray Technology

To study the effect of deposition behavior of the reinforcement on microstructure and property of cold-sprayed metal matrix composite coating, four Al5056 composite powders containing 15 wt.%, 30 wt.%, 45 wt.% and 60 wt.% SiC were mixed to prepare the composite coatings by cold spraying. Effect of the SiC deposition behavior on microstructure, SiC distribution, micro-/nanohardness, surface residual stress and shear strength of the coatings were studied. Experimental results show that the increase in SiC content in the powder increased the SiC collision with the deposited SiC particles, which caused the difference in SiC content and distribution in the composite coatings. The SiC deposition behavior caused the SiC morphology to evolve from integrity to small cracks and complete fragmentation. The SiC deposition behavior effectively influenced its peening effect on the deformed Al5056 particles, that is, as the SiC fragmentation threshold arrived, the nanohardness, surface residual stress and shear strength of the coating showed the corresponding transition point. The surface residual stress state of the deformed Al5056 matrix could in turn affect the SiC deposition by increasing the SiC deposition efficiency at the minimum compressive stress.

Iron aluminides have been proposed lately as promising materials for wear applications. Many authors have focused their investigations on the friction behavior of FeAl coatings, emphasizing the role of this intermetallic phase as a new matrix to embed ceramic particles and replace the extensively studied WC-Co cermet system for high temperature. However, few of these studies deal with the evaluation of the different tribological properties and their relationship with the coating microstructure. In the present study, the near stoichometric Fe40Al was successfully sprayed by means of high velocity oxy-fuel using different particle size distribution and the tribological behavior was assessed through solid particle erosion, abrasive and dry sliding tests. The wear mechanisms of the deposited coatings are discussed with regard to the observed results. In addition, oxidized samples were tested to evaluate the role of the oxide top layer; also, the powder was previously annealed to produce a coating with an almost fully ordered FeAl structure.

Thermal spraying with liquid-based feedstocks demonstrated a potential to produce coatings with new and enhanced characteristics. A liquid delivery system prototype was developed and tested in this study. The feeder is based on the 5MPE platform and uses a pressure setup to optimally inject and atomize liquid feedstock into a plasma plume. A novel self-cleaning apparatus is incorporated into the system to greatly reduce problems associated with clogging and agglomeration of liquid suspensions. This approach also allows the liquid feedstock line to the gun to remain charged for quick on-off operation. Experiments on aqueous and ethanol-based suspensions of titania, alumina, and YSZ were performed through this liquid delivery system using a 9MB plasma gun. Coatings with ultrafine splat microstructures were obtained by plasma spraying of those suspensions. Phase composition and microstructure of the as-sprayed coatings were investigated.

Incineration in boilers is an environment-friendly treatment for industrial and civil sewage sludge. However, due to the aggressive nature of the sludge, the boiler fireside-surface is subjected to severe wear, erosive, and high temperature corrosion problems during incineration. In this study, we developed an economical FeCrBSi wire material with iron weight content as high as 80%. The coating was prepared by twin-wire arc spraying processing. The chemical compositions of the coating, as well as phase components were analyzed by energy-dispersive spectroscopy and x-ray diffraction method. The surface roughness, porosity, and cross-sectional morphology were further characterized. The coating hardness is close to that of the commercial Armacor M and Armacor C materials. In-boiler test was also carried out. The low thickness loss of the tube indicates a promising application future in sludge boilers.

The low-temperature oxygen-fuel (LTOF) spray is a modification of high velocity oxygen fuel spray. In this process, the high-temperature gas is accelerated to supersonic speed through a Laval nozzle followed by a straight barrel. By injecting room temperature gas into the mixing chamber, the temperature of the gas can be controlled in a range of about 1000-2500 K, so that some oxygen and temperature-sensitive materials, such as titanium and copper, can avoid oxidation or decomposition during the spraying process. The purpose of this paper is to establish a 2-D mathematical model to simulate the supersonic gas dynamics and particles behavior in LTOF process. The temperature and velocity of the flow fields, and the trajectory and heating of in-flight particles are predicted for different operating parameters. The model is validated by experimental data in the literature. Effects of the mixing gas flow rates, particle sizes, and injection conditions on this process were investigated as well.

The thermal barrier coatings investigated in this paper included a TiAl3 bond coat and a yttria partially stabilized zirconia (YSZ) layer. The TiAl3 bond coat was prepared by deposition of aluminum by cold spray, followed by a heat-treatment. The YSZ layer was prepared by air plasma spray. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 500 cycles to test the oxidation resistance of the thermal barrier coatings. The microstructure and composition of the γ-TiAl alloy with and without the thermal barrier coatings after oxidation were investigated. The results showed that a dense TGO layer about 5 μm had grown between the YSZ layer and the TiAl3 bond coat. The TGO had good adhesion to both the YSZ layer and the bond coat even after the TiAl3 bond coat entirely degraded into the TiAl2 phase, which decreased the inward oxygen diffusion. Thus, the thermal barrier coatings improved the oxidation resistance of γ-TiAl alloy effectively.

This work presents a systematic study of the lanthanum tungstate (LaWO) ceramic layers formation on porous metallic substrates as a function of the PS-PVD processing parameters including plasma characteristics, support type and temperature, as well as addition of O2 during the spraying. Through precise control of the PS-PVD parameters, a set of processing conditions were found that led to He gas-tight purely cubic LaWO layers with negligible secondary phase precipitations. Being dependent on process conditioning, the formation and evolution of the cubic La6−xWO12−δ (x = 0.3-0.6) as the main phase of functional importance and of the undesired secondary phases (La2O3 and La6W2O15) was strongly affected by the cation and oxygen stoichiometries. The rapid cooling of the feedstock at particle impact on the substrate led to the formation of highly La-saturated compositions which exhibited significant lattice expansion in comparison with conventionally processed LaWO and is considered beneficial in terms of material performance. And indeed, the H2 permeation performance of the PS-PVD processed LaWO ceramic layers shown earlier by our group was 0.4 ml/min∙cm2 at 825 °C for 60 µm thickness of the functional layer, the highest value reported for this type of proton conducting ceramics, so far.