High temperature protection requires full coating density, high adhesion, minor oxide inclusions, and preferably fine grains, which is not achievable in most thermal spray processes. High velocity oxygen-fuel (HVOF) thermal spray process has been applied extensively for making such coatings with the highest density and adhesion strength, but the existence of not or partially melted particles are usually observed in HVOF-formed coatings because of relative low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using a liquid-state suspension/slurry containing small alloy powders. The advantages of using small particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. Process investigations have proven the benefits for making alloy coatings with full density and high bond strength attributing to increased melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800°C have demonstrated that the alloy coatings made by the novel process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths.

High velocity oxygen-fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which is not achievable in most other thermal spray processes. Like other thermal spray processes, however, a normal HVOF process is not able to apply fine powders less than 10 µm via a conventional powder feeder. The advantages of using smaller and even nano-sized particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. A new process has been developed to realize HVOF forming of fine-grained alloy layers by using liquid precursors containing fine metallic particles. Process investigations have shown the benefits for making single and duplex layered coatings with full density and high bond strength attributing to the very high kinetic energy of particles striking on the substrate surface and the better melting of the small particles. One of the targeted applications is for the water walls of a fossil-fired boiler that operate in a high temperature and corrosive environment. The new coating system is based on material selection, structure design, process innovation and diagnostics, microstructure, and property evaluation. It is promising to provide better protection of the boilers against various types of degradations like corrosion, oxidation, erosion and interfacial failure.

A typical high-pressure fuel pump for direct injection (DI) engines operates with fuels such as petroleum-based hydrocarbons that have inherent lubricating properties. However, environmental requirements put the thrush to use cleaner fuels that don’t contain lubricants and consequently an increasing abrasion problem presents with the surface of high-pressure fuel pump for direct injection engines. To alleviate this problem, the alternative solution is to promote wear and corrosion resistance of DI engines by applying high-lubricity coatings onto surfaces of engine components such as pump plungers. In this work, self-lubricating nanocomposites with nano-Al 2 O 3 /TiO 2 matrix and Fe 3 O 4 additive as solid lubricant was the first applied. The nanocomposites had been fabricated into lubricant coatings with a single layer or a functionally graded structure in plasma spray process. Tribological test results for the nanocomposite coatings demonstrated 4 times increase in sliding wear resistance and 3-5 times increase in abrasive wear resistance in under the tested conditions. The lowest coefficient of friction about 0.18 was measured on the nanocomposite coating with an optimal Fe 3 O 4 content in pin-on-disk test in ethanol. Based on morphologies and wear behavior analyses, the wear mechanism was proposed for the nanocomposites. The nanocomposite coatings have exhibited the advantages of cleavability, chemical stability, low friction and high wear resistance, and will have a potential for various applications that require high lubricity at ambient and elevated temperature.

Thermal barrier coatings (TBCs) are capable of protecting hot-section engine components from the hot gas stream, and thereby can provide improvements in component durability and engine efficiency. Thick TBCs can provide further improvements in durability and efficiency, especially for static components. The main commercial coating methods for TBCs are electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). These processes have limitations for depositing thick TBCs: for EB-PVD, the deposition rates are low and the cost is high; for APS, durability is reduced with increased thickness. Inframat Corporation, in collaboration with the University of Connecticut, is developing a new plasma spray process, namely, solution precursor plasma spray (SPPS), for the formation of TBCs and also functional films from liquid precursor feedstock, instead of the solid powder feedstock used in conventional APS. SPPS TBCs have many unique microstructural features, including: ultra-fine splats, vertical micro- and macrocracks, micrometer- and nanometer-size porosity. These unique microstructural features provide a TBC with high thermal cycling spallation life and bond strength. These coatings have been made in thickness up to 2 mm and show excellent durability. In this paper we present microstructural characteristics and thermal cycling performance of SPPS-formed 7YSZ thick coatings varying in the range of 0.5-2 mm.

Ceramic oxide coatings were made with a unique microstructure using solution plasma spray (“SPS”), a novel variant on the conventional powder plasma spray techniques. In the SPS process, a precursor solution is fed into an air plasma torch using a liquid injector, and a nanocrystalline ceramic coating is formed directly on various substrates without post heat treatment. It was found that the microstructure of the SPS coating depended on control of process parameters for liquid feed and plasma spray to a large extent. This study deals with the formation of SPS deposited yttria stabilized zirconia coatings with a well-controlled microstructure addressing porosity, cracking and adhesion. The SPS-deposited YSZ coatings have demonstrated unique microstructural characteristics including adjustable porosity, vertical microcracks and the absence of splat boundaries. Such zirconia-base coatings show great potential for the applications of high-density electrolyte layers in solid oxide fuel cells (“SOFC”s) and high porosity/low conductivity thermal barrier coatings for industrial and gas turbines.

This work addresses the fabrication of membrane-type SOFCs, operating at an intermediate temperature using all components fabricated by plasma spray technology, and to evaluate the performance of the SOFC single unit at a temperature range of 500-800 0C. Single cells composed of a LaSrMnO 3 (“LSM”) cathode, LaSrGaMgO 3 (“LSGM”) electrolyte and a Ni/YSZ anode, were fabricated in successive atmospheric plasma spraying processes. Plasma spraying processes have been optimized and tailored to each layer in order to achieve a high porosity cathode or anode layer as well as a high density electrolyte layer. Major effort has been devoted to the production of the LSGM electrolyte film with high density and free-cracking. Electrochemical impedance spectroscopy was used to investigate the conductivity of the electrode layers and particularly the resistance of the electrolyte layer. It was revealed that the heat treatment had a great influence on the specific conductivity of the sprayed electrolyte layer, and that the specific conductivity of the heat-treated one was dramatically increased to the same magnitude as that of a sintered LSGM pellet. The experimental results have demonstrated that the plasma spray process has great potential for the integrated fabrication of the medium temperature SOFC units.

This work assesses the properties of vacuum plasma sprayed YSZ coatings for potential use in solid oxide fuel cells. The results of the investigation show that low-porosity layers of yttria-stabilized zirconia can be produced by using a fine powder and by adjusting plasma gas composition. Under optimized spraying conditions, YSZ layers with a thickness of 10-20 μm, a porosity less than 1%, and an average roughness of 1 μm are achievable. Paper includes a German-language abstract.

In the present work thin Al 2 O 3 coating was obtained mainly by low pressure plasma spray (LPPS). Low-porosity and low-roughness deposits resulted from the optimized spray conditions, i.e. plasma parameters, grit blasting, powder feed rate and specimen rotation speed. Results showed that LPPS processing was highly beneficial for densifying the ceramic coatings, especially when coupled to a moderate powder feed rate. Coating average surface roughness (Ra) ranged from 1.5 to 2.5 µm for a coating thickness of less than 30µm and an original substrate Ra of 1.1µm. The spray conditions were optimized particularly for a low feed rate and a high specimen rotating speed to lower surface roughness. Moreover, a specific atmosphere/temperature control device was developed (using local gas injection close to the specimen to be coated). This resulted in improving cooling efficiency, which reduced microcracking in the deposits. Mechanical pull-off adhesion test was also carried out to evaluate these low-roughness thin coatings. Adhesion was shown to be satisfactory for direct coating (i.e. without any bond coat) of a low-roughness (Ra=1.1µm) AISI 316L substrate.