A CoNiCrAlY-hBN/Polyester material has recently been developed for clearance control applications in gas turbine compressors that use titanium alloy blades. While engine tests serve as the final evaluation of the coating performance, quality assurance laboratories and production shops would rely upon the more readily available coating hardness values to predict performance. This paper will focus on the reproducibility of coating macrohardness with a plasma spray process. It is shown that plasma spray parameters affect the hardness of CoNiCrAlY-hBN/Polyester coatings by changing the level of polyesters retained in the coating and the volume percentage of metallic matrix. The correlation between hardness, retained polyester level and microstructure of these coatings is captured in a coating hardness map from which desired microstructure and polyester entrapment are determined. Based on the understanding of the correlation between coating hardness and microstructural features, the use of additional criteria other than hardness such as retained polyester level and non-metallic portion of the coating is recommended in order to assure the quality of the coating more effectively.

Plasma spraying was used to produce continuously graded and layered structures of molybdenum disilicide and alumina. These microstructures were achieved by manipulating the powder hoppers and plasma torch translation via in-house created computer software. The resultant microstructures sprayed uniformly and were crack free. The mechanical and thermal performance of these sprayed materials will be evaluated through C-ring tests and thermal cycling experiments respectively. The purpose of this study is two fold; firstly to demonstrate the ability of produce such composite ceramic microstructures using a conventional plasma spraying process, and secondly to quantify the improvements in thermo-mechanical performance provided by these composite microstructures over conventional monolithic microstructures.

In cold spraying, coatings are formed by a high velocity impact of solid particles. The particles are accelerated in a supersonic gas jet at temperatures of only a few hundred degrees centigrade. In contrast to thermal spray processes no melting of the particles and negligible heating of the substrate occurs. A series of spray experiments with copper powders of different particle size ranges were performed to study the effect of various process parameters on microstructure and properties of the coatings. The coatings have been evaluated for their microstructure, density, oxygen content, hardness and bond strength. With nitrogen as process gas and a -25 +5µm powder, dense coatings were obtained within a broad range of gas inlet pressure and gas inlet temperature.

Cold work and heat treatment influence the mechanical properties, residual stress-state, and corrosion resistance of austenitic stainless steels. In this study we have examined changes in the defect substructure and microstructure of Type 304 stainless steel resulting from surface preparation, and deposition of bond coats and thick ceramic coatings using plasma spray methods. The structure of the stainless steel was examined as a function of depth from the coating surface using optical and transmission electron microscopy, and x-ray diffraction. Grit blasting was found to severely cold work the material to a depth of tens of microns, and the amount of cold work varied with measured abrasive particle velocity. The heat input to the surface as a result of depositing a metallic bond coat or thick ceramic coating resulted in substantial annealing of the cold work imparted into the substrate by surface preparation. There was, however, no evidence of change in grain size near the substrate-coating interface that could be attributed to recrystallization or grain growth in the substrate.

Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Plasma spraying is a very complex process, controlled by a large number of process parameters. The spray gun parameters control the plasma plume and thereby the velocity and temperature of the particles in the plasma. Some of the spray gun parameters are difficult or impossible to control, but variations of them give rise to fluctuations in the microstructure of the sprayed thermal barrier coating and thereby low reproducibility. By movement of the control from the spray gun to direct control of the particle properties in the plasma this problem will be avoided, and it should result in better process control, higher quality of the final coating and thus improved reproducibility. In this study, the influence of the plasma spray process on the coating microstructure was investigated. An orthogonal factorial designed experiment was performed, where eight process parameters were varied, resulting in 16 different coatings. The particle properties were observed in-situ with the optical measurement system DPV 2000. The microstructure of the coatings was studied using optical microscopy and the amount of different features, i.e. cracks and pores, was quantified. Multiple linear regression was used to find models describing the relation between the spray gun parameters and the particle properties, between the spray gun parameters and the microstructure, and between the particle properties and the microstructure. The results showed that the spray gun parameters well describe the variation in particle velocity and particle temperature. Further, it was found that particle velocity, particle temperature, spray angle, and substrate temperature are the most important parameters concerning influence on the coating microstructure. However, their influence on the different microstructure features varied. The study implies that focus can be set on one or two particle properties measured in the plasma, instead of the numerous spray gun parameters.

Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.

Improved understanding of microstructure-property relationship can help to shift from experiment-based to science-based development of thermally spray deposits. This should result in shorter and less expensive development as well as in higher functionality and reliability of the deposits. Significant amount of work has been done, however, nearly always studying deposits manufactured by only one of the thermal spray techniques. Results are therefore often spray technique specific. A broad study with samples manufactured by a number of different thermal spray techniques seems to be missing yet. Relationships valid across different techniques should provide better understanding of the generic relationships. This research employs number of different techniques - flame, HVOF, plasma (APS, VPS, WSP), to generate a wide range of microstructures. Various Ni-based alloys are studied starting from a simple chemistry (Ni) and ending with complex NiCrAlY alloys. Presented results were obtained with NiCr (80% Ni, 20% Cr) feedstock. Microstructures are characterized by various techniques-OM, SEM, XRD, small-angle neutron scattering (SANS) and others - to obtain the most comprehensive set of macro to micro structural parameters available today. The wear and corrosion properties of these deposits are measured together with internal coating stresses and the most generic microstructure-property relationships are sought.

316L stainless steel and Hastelloy C alloy powders were sprayed by an HVOF apparatus onto mild steel substrates. The microstructure, pore size distribution, composition and corrosion resistance of thus obtained coatings were evaluated experimentally. Corrosion resistance in sea-water was examined by monitoring the impedance and corrosion potential of samples immersed in artificial sea-water at 300 K over a period of more than 3 months and also by polarization measurement. It was found that the stainless coatings composed mainly of plastically deformed particles and some splats which were molten at the impact. By increasing the combustion pressure, the porosity as measured by mercury porosimeter could be reduced to below 1%. In comparison, Hastelloy C deposits sprayed under the standard condition were so dense that its porosity could not be measured by the porosimeter. The polarization curve and the results of impedance monitoring both exemplified that the Hastelloy C coatings possess much superior corrosion resistance to the stainless coatings in sea-water, which was attributed to the higher density and better adhesion of the Ni-base alloy coatings.

In cutting tool technologies WC-Co based materials are increasingly replaced by composites containing TiC as hard phase and Ni or Co based metallic binders to improve life time and the performance at higher temperatures. These new light-weight materials are also promising for wear resistant coatings. However, while the production of WC-Co coatings by thermal spray techniques, especially high velocity oxy-fuel flame (HVOF) spraying, is well-established, thermal spraying of TiC-based powders did not lead to satisfactory results so far. This could be attributed to the oxidation during the spray process and the insufficient bond between hard phases and the metallic binder. Strategies to improve the properties of TiC-based coatings aim for microstructural modifications, especially by alloying additives into the thermal spray powder. By HVOF and vacuum plasma spraying (VPS), modified TiC-based coatings are produced, which globally show similar microstructures but significantly differ in their oxide contents. Investigations of mechano-technological properties and wear mechanisms demonstrate that alloying Mo into the hard phases or the metallic binder of thermal spray powders can improve the adhesion between hard phases and metallic binder of the coatings. In addition, properties of the metallic matrix can be tuned up for specific applications by solution hardening. In case of HVOF-coatings these effects are partially compensated by high oxygen contents. The overall better performance of coatings produced by VPS demonstrates that the high potential to improve properties of TiC-based composites by alloying additives can only be attained by minimizing the oxidation during the spray process.