Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

NiCr-Mo composite coating was prepared by plasma spraying of shell-core-structured NiCr-Mo powders. The morphologies of the NiCr-Mo powders and microstructure of the corresponding NiCr-Mo coating were characterized by SEM. Furthermore, the erosion behavior of the NiCr-Mo coating at impact angles of both 30° and 90° was investigated, and was further compared with that of the Ni20Cr coating and the In-738 alloy bulk. Results showed that fully-dense and homogenous NiCr-Mo coating with excellent interface bonding and no pure Mo inclusions was obtained. Furthermore, the erosion test results showed that the erosion rate of the optimized NiCr-Mo coating is lower than that of NiCr coating at both impact angles. Moreover, the NiCr-Mo coating presented excellent erosion resistance which was comparable as that of In-738 alloy bulk, attributing to the fully-dense microstructure and metallurgical interface bonding within coating.

The microstructure of thermal barrier coatings (TBC) plays an important role in the thermal cycling behavior of TBCs. In this study, ceramic coatings with different pore structures were prepared by atmospheric plasma spraying (APS). Graphite with different morphology was used as pore former to adjust the pore structure of the coatings. Then, the thermal cycling behavior of TBCs with different structure was characterized. By depositing a porous 8YSZ layer on the conventional 8YSZ layer, the thermal cycling life of TBCs can be improved. However, when the porosity of the porous layer increased to about 30%, the porous layer detached from the under layer after several cycles. An elastic energy model was applied to explain the thermal cycling behavior of TBCs

The growth behavior of thermally grown oxide (TGO) on the bond coats of thermal barrier coatings (TBCs) has a significant impact on the TBCs lifetime. The splashed particles on the thermally sprayed MCrAlY bond coat surface have weak bonding with the underlying bulk coating, which will lead to the formation of mixed oxides and promote the failure of TBCs. In this study, the interface between the splashed particles and underlying bond coating was healed by vacuum pre-treatment (pre-diffusion). The bond coats were divided into three groups by the ways of pre-treatments before deposition of the ceramic top coating. Two groups of bond coats were subjected to two different pre-treatments (pre-diffusion-oxidation and pre-oxidation) prior to cyclic oxidation tests. The third bond coats were directly subjected to the same cyclic oxidation tests. Results show that the different TBCs present different cyclic oxidation behavior. The TBCs with pre-diffusion-oxidation exhibited the best cyclic oxidation resistance, and the ones without pre-treatment had the worst cyclic oxidation resistance. It is revealed that the healing of interface between the splashed particles and underlying bulk coating can effectively improve the cyclic oxidation resistance. This study will benefit the development of advanced TBCs with long lifetime and high reliability.

It has been well accepted that thermal sprayings present a typical lamellar structure with limited lamellar interface bonding. It has been a great challenge to deposit a fully dense coating with fully bonded lamellae. In this report, three different novel approaches are introduced to deposit fully dense ceramic coatings and metal alloy coatings. With the deposition of a specific ceramic coating, it was found that there exists an intrinsic bonding temperature corresponding to the glass transient temperature of spray material. A chemical bonding is formed at the interface upon splatting of a molten ceramic droplet, as far as the maximum interface temperature between the spreading splat and the solid splat reaches over the intrinsic bonding temperature. Moreover, it will be presented that a simple critical bonding temperature in a linear relation with the melting point of coating materials can be utilized to deposit fully dense ceramic coatings by controlling the deposition temperature. Furthermore, with metal alloy coatings, a self-bonding mechanism is proposed utilizing the ultrahigh temperature molten droplet for dense coating with fully bonded lamellae. Using specially designed core-shell structured powders, the investigators demonstrated that a bulk-like metal coating is deposited by creating ultra-high temperature molten droplet. It will be found that such coatings present excellent properties and performance comparable to bulk materials. Moreover, it will be shown that, for ductile metal alloys, the solution-impermeable dense metal coatings can be deposited by using the novel in-situ shot-peening assisted cold spraying.

Lanthanum gallate doped with strontium and magnesium (LSGM) is a good electrolyte candidate for Intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, low-temperature sintering is used to increase the density of LSGM coatings prepared by vacuum cold spraying (VCS). LSGM layers with different thickness were deposited by VCS on NiO-YSZ substrates. In order to suppress chemical reactions between Ni and LSGM, the substrates were coated with gadolinium-doped ceria by tape casting. After sintering at 1200 °C, the coatings were found to be denser in most regions due to grain growth, which appears to be accompanied by cracking, particularly in thicker layers. A second layer was deposited on the annealed coatings to seal the cracks and the two-layer structure was further sintered. Gas permeability test results show that the multilayer films are dense enough to consider their use as electrolyte membranes in IT-SOFCs.

Vacuum cold spray is a promising method to deposit nanocrystalline ceramic coating. The effective control of porous structure within nanostructured coating is essentially important to enhance the performance of the mesoporous nanocrystalline coatings for applications to catalyst and photo-electrode. In this study, the ceramic-polymer composite powders were employed as spray feedstocks for vacuum cold spray to control the pore structure in the deposits. The ceramic-polymer composite powders were made from nano-sized TiO 2 (25nm), ZrO 2 (30nm) and Al 2 O 3 (30nm) and polyethylene glycol (PEG). The surface morphologies and the cross-sectional microstructures of the coatings were characterized using scanning electron microscope (SEM). The pore size distribution was measured using a nitrogen adsorption approach. The results showed that the deposition during spraying was implemented through the composite particles in a size ranging from submicrometers to several micrometers. Through post-spray heat treatment of the deposit, the PEG can be completely removed to increase the porosity in the deposit. The pores exhibited a bimodal distribution. The small pores present the size from several nanometers to tens of nanometers. Moreover, the size of large pores is in micrometer scale. The porosity and pore size distribution can be controlled by the composition of the composite powder.

Micro-structural design had attracted increasing interests in modern developments of hard coatings. The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to design coating microstructure through feedstocks for development of different coating properties. In this study, a multi-size modal WC-12Co powder containing nano-sized WC particles was designed to deposit WC-Co deposition with multi-sized WC. Multimodal WC-12Co powders were prepared with ball-milling of a commercial WC-12Co powder, cold-compacting, sintering in hydrogen atmosphere and crushing. WC particle size in the powder exhibits a distribution with two peaks in tens of nanometers and several micrometers. The multimodal WC-12Co deposition was prepared by cold spraying using helium as driving gas. The multimodal size of WC particles in the powders was retained into the deposit. The micro-hardness and fracture toughness of the multimodal structured WC-12Co deposit was compared with bulk WC-12Co. It was found that the multimodal deposition exhibits a comparable hardness to nano-sized WC-12Co and a high fracture toughness compared with micro-sized WC-12Co. The simultaneous strengthening and toughening of WC-12Co can be realized through the bimodal microstructure design of WC-Co.

Ni/Al alloy powders were synthesized by ball milling of nickel-aluminum powder mixture with a Ni/Al atomic ratio of 1:1. Ni/Al alloy coating was deposited by cold spraying using N 2 as accelerating gas. NiAl intermetallic compound was evolved in-situ through post-spray annealing treatment of cold-sprayed Ni/Al alloy coating. The effect of annealing temperature on the phase transformation behavior from Ni/Al mechanical alloy to intermetallics was investigated. The microstructure of the mechanically alloying Ni/Al powder and NiAl coatings was characterized by scanning electron microscopy and X-ray diffraction analysis. The results show that a dense Ni/Al alloy coating can successfully be deposited by cold spraying using the mechanically alloyed powder as feedstock. The as-sprayed alloy coating exhibited a laminated microstructure retained from the mechanically alloying powder. The annealing of the subsequent Ni/Al alloy coating at a temperature higher than 850°C leads to the complete transformation from Ni/Al alloy to NiAl intermetallic compound.

FeAl Intermetallic compounds have excellent wear resistance and high temperature oxidation resistances. The low temperature brittleness makes intermetallic compound materials more suitable to be applied in the form of coating to protect materials from high temperature oxidation and wear. In the present study, a iron/aluminum composite coating was produced by cold spraying of iron and aluminum powder mixtures and then was annealed at different temperatures to aim at forming an iron aluminide intermetallic based coating. The deposition behavior of iron and aluminum powder mixtures and microstructural characteristics of the as-sprayed deposit were examined by scanning electron microscopy (SEM). The kinetics of the phase transformation of the as-sprayed iron/aluminum composite deposit to iron aluminide was characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results showed that after heat treatment at a temperature of 600°C, intermediate phase Al 5 Fe 2 coexisted in the deposit with remaining Fe and Al. With increasing heat treatment temperature to 900°C, the deposits consisted of mainly FeAl phase and a trace of remaining Fe phase.

The significant erosion of the boiler tube at high temperature becomes an important problem for the safe operation of circulating fluidized bed boiler. This paper investigated the erosion behavior of the HVOF sprayed Cr 3 C 2 -NiCr coating at high temperature comparing with the typical mild steel for boiler tube. Results showed that the erosion rate of the mild steel increased with the increase of temperature. The erosion rate of the mild steel at 800°C was 4 times that at 300°C at an erosion angle of 30°. However, the erosion rate of the HVOF sprayed Cr 3 C 2 -NiCr coating was not influenced by the temperature in the range of 300 to 800°C. It is found that the erosion resistance of HVOF sprayed Cr 3 C 2 -NiCr coating was more than 3 time higher than that of the mild steel at 700 to 800°C. In addition to the ploughing on the surface of the worn coating, the cracking along splats interfaces in the coating was clearly observed on the cross-sectional microstructure of the coating. The results indicate that the erosion performance of the HVOF sprayed Cr 3 C 2 -NiCr coating is controlled by the cohesion between splats in the coating and can be further enhanced by improving splat cohesion.

Instabilities in plasma spray jets can result in coatings with inconsistent properties. The arc root fluctuation and shear layer instability due to strong gradients are of foremost concern. The shear layer instabilities result from shear between the high velocity, low density hot core gas, the intermediate density and velocity boundary layer, and the high density quiescent environment. A cold-flow facility with density gradients similar to a plasma torch has been used for implementation of traditional fluid dynamics measurements such as hot-wire anemometry. Methods to control these instabilities are developed and tested using both the plasma torch and the cold flow facility. Through nozzle design modifications the instabilities resulting from arc root fluctuations and high density gradients have been reduced. The effectiveness of the control on the plasma jet is determined using in-flight particle characterization along with high speed imaging and photodiode measurements of the jet.

Instabilities of plasma spray jets have been a source of inconsistencies in coating properties. These instabilities can be minimized through the use of central injection torches or torches with fixed anode attachment. However, any low density ( < ~0.7) jet is globally unstable to small disturbances. Globally unstable jets are characterized by a short potential core, rapid spreading, and high entrainment, all of which are present in a plasma jet. Plasma jets have ratios of jet density to density of the surrounding gas on the order of 0.01, as well as rather low Reynolds numbers and thick boundary layers. In the present work, the instabilities are investigated through analysis of the disturbance growth in the shear layer between the plasma and the cold surrounding gas. These investigations are using two types of experiments, one consisting of a SG 100 spray torch with several optical diagnostic methods being applied to the shear layer analysis. The other experiment simulates the plasma jet at low temperatures by using a helium core jet exhausting into a sulfur hexafluoride (SF6) environment. The simulated plasma jet (SPJ) has a density ratio of 0.03. The simulated plasma jet (SPJ) allows controlled variation of the boundary layer through different fluid dynamic arrangements. It further allows use of diagnostics such as hot wire anemometry and PIV to clearly characterize the shear layer. Some of the characteristics of the shear layer are presented and comparisons between the plasma jet and the simulated plasma jet, and initial results on controlling the jet instability, are discussed. Abstract only; no full-text paper available.