Abstract This paper presents a novel process that uses RF plasma spraying and premixed elemental powders to produce intermetallic matrix composites in situ without having to add reinforcement fibers or particles. Splats were collected on a stainless steel substrate and were analyzed to determine if nitrides had formed in metal droplets during flight and how it affected splat morphology. The typical splat morphology of impinged Ti droplets is disk-type with an outer peripheral fringe. Aluminum splats, on the other hand, are classified into two categories: a disk-type with an irregular outer periphery and a semi-massive-type. Other composites produced and examined include TiAl, AlN, and Ti2AlN.

Abstract Oxide-bonded silicon nitride (OBSN) powders have been developed to address thermal spray problems associated with high temperatures. This paper examines how such powders perform when applied via detonation gun (DGS) and atmospheric plasma spraying (APS) with axial powder injection. All coatings were characterized using optical microscopy and X-ray diffraction with additional tests being performed on DGS coatings. For the first time, relatively dense Si3N4-rich coatings with an oxide binder phase were produced, and some of the DGS coatings were found to be sufficiently wear resistance for industrial use.

Abstract The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material, using optical microscopy and scanning electron microscopy (SEM), has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Vickers microhardness has been measured on several sprayed forms. In order to complete the study of the different systems, abrasion (Rubber Wheel Test), friction (Ball on Disk Test) and erosion wear results have been obtained. These wear results have been used in order to evaluate the behaviour of the sprayed samples with a different powder percentage in the blends. Corrosion tests have been done to evaluate the corrosion resistance of the sprayed samples (ASTM D-1411).

Abstract This study was aimed at the production of SiC-MoSi2 composite powders through a high-temperature plasma reaction route. The addition of SiC appears to be the best second phase reinforcement for improving the mechanical properties of MoSi2 material for high-temperature structural application. The in-flight carbonization of MoSi2 powders was carried out in an Ar-H2-CH4 induction plasma process. Using methane served as both the powder carrier gas and the "precursor" to react with the MoSi2 powders forming the SiC phase in-situ . Under the experimental conditions employed in this investigation, up to about 8.0 wt. % of carbon was incorporated into the MoSi2 powder particles. The chemical composition, phase content and the microstructure of the composite powder products were examined by XRD, SEM, EDS etc. analysis methods. The reaction mechanisms are discussed in terms of the calculated thermodynamic equilibria.

Abstract The given article presents some results of the scientific research devoted to the development of a new class of scale-resistant powder materials of the Si-Ti-Mo-B system for thermal spraying and using these materials for the creation of heat-resistant coatings on the niobium base alloys by means of various methods of thermal spraying. Also under consideration are problems relating to the theory and practice of obtaining reliable protective coatings on high-melting metals and their alloys, niobium ones included, intended for operation in high-enthalpy oxygen-containing gas flows. Hazard in commencing an oxidation reaction of the base material under coating is connected with density of open pores and cracks, and partial pressure of the oxidizer. Powdered multicomponent heterophase materials for gas-thermal spraying of protective coating with a self-healing ability and controlled properties are proposed. Finally the results of some properties of new silicide-type heterophase powders containing silicide and boride phases for a thermal spraying process and some properties of protective coating deposited on the niobium base alloys by means of a thermal spraying technique are presented.

Abstract In this study, ethylene methacrylic acid copolymer (EMAA) was used as the matrix to produce EMAA/Al2O3 and EMAA/NiCr composite coatings from dry-blended powder mixtures. This work was conducted to determine processing concerns when using similar sized reinforcement particles of different density in a flame-spray process. This work has utility for applications that require a reduction in mechanical wear and/or to confer upon a polymeric deposit a certain functional property by the introduction of value-added powder. Free-standing coatings were produced to test the mechanical properties of the sprayed deposit. The effects of the filler content on the secant modulus, yield stress, and tensile strength are discussed. The differences in deposition efficiencies among the EMAA, Al2O3, and NiCr are highlighted with respect to particle size and density.

Abstract Plasma spraying is known to be one of the main promising processes for the manufacturing of Ti/SiC long fiber composites. However, some improvements remain to be done for this process to be applied in a routine industrial route. These include: oxygen contamination of the sprayed material through that of Ti particles before and during spraying damaging of fibers due to a high level of thermal stresses induced at the spraying stage adequate deposition of Ti-based powder to achieve a low-porosity matrix and good impregnation of the fiber array. This contribution deals with work in the 3 previously mentioned fields, which resulted in a whole 3-fold study of the process. Oxidation was studied using electron microprobe analysis of elementary particles quenched and trapped into a closed box at various given flight distances. Oxygen diffusion phenomena within the particles are discussed from a preliminary theoretical approach coupled with experimental data. Isothermal and thermo-mechanical calculations were made using ABAQUS code to determine stresses arising from contact of a liquid Ti-6Al-4V particle onto a SiC fiber. On a higher scale, i.e. that of the sprayed powder flow, a 2-dimensional original model simulating the deposition of droplets onto a substrate was developed. This model is based on a lattice-gas automaton which reproduces the hydrodynamical behavior of fluids.

Abstract Abradable coatings are used in gas-turbine engines to optimize compressor performance by maintaining tight blade tip clearances. The most common such coatings are thermally sprayed Al-Si/polyester, Al-Si/graphite, and Ni/graphite. Al-Si/graphite coatings have performed well in terms of wear but are prone to corrosion, which can lead to spalling and a reduction in engine efficiency. In this paper, we chart the development of a powder-based Al-Si/BN abradable material designed to overcome in-service corrosion and analyze laboratory and engine testing results.

Abstract A concurrent fibre winding and low pressure plasma spraying (LPPS) process has been developed to manufacture multiple fibre reinforced titanium matrix composite (TMC) rings in a single spraying operation. Optimisation of the LPPS parameters has been successively performed for two different sizes of Ti-6Al-4V powders by experimental design and statistical analysis, which provided minimum porosity and surface roughness for both powders. The most important LPPS parameters affecting porosity and surface roughness of deposits were Ar gas flow rate and chamber pressure. During TMC manufacture, the coarse Ti-6Al-4V powder spraying provided enhanced infiltration between fibres but caused degradation of fibre tensile strength, as well as a rough ring surface. The fine Ti-6Al-4V powder gave no significant degradation of fibre strength and a relatively smooth ring surface. Four-ply SiC fibre reinforced TMCs manufactured by the spray-wind process have also been evaluated in terms of porosity, fibre distribution and fibre damage.

Abstract The intermetallic compound, molybdenum disilicide (MoSi2), is being considered for high temperature structural applications because of its high melting point and superior oxidation resistance at elevated temperatures. The lack of high temperature strength, creep resistance and low temperature ductility has hindered its progress for structural applications. Plasma spraying of coatings and structural components of MoSi2-based composites offers an exciting processing alternative to conventional powder processing methods due to superior flexibility and the ability to tailor properties. Laminate, discontinuous and in situ reinforced composites have been produced with secondary reinforcements of Ta, Al203, SiC, Si3N4 and Mo5Si3. Laminate composites, in particular, have been shown to improve the damage tolerance of MoSi2 during high temperature melting operations. A review of research which has been performed at Los Alamos National Laboratory on plasma spraying of MoSi2-based composites to improve low temperature fracture toughness, thermal shock resistance, high temperature strength and creep resistance will be discussed.

Abstract Directed light fabrication (DLF) is a rapid fabrication process that fuses gas delivered metal powders within a focal zone of a laser beam to produce fully dense, near-net shape, 3D metal components from a computer generated solid model. Computer controls dictate the metal deposition pathways, and no preforms or molds are required to generate complex sample geometries with accurate and precise tolerances. The DLF technique offers unique advantages over conventional thermomechanical processes or thermal spray processes in that many labor and equipment intensive steps can be avoided to produce components with fully dense microstructures. Moreover, owing to the flexibility in power distributions of lasers, a variety of materials have been processed, ranging from aluminum alloys to tungsten, and including intermetallics such as M05Si3. Since DLF processing offers unique capabilities and advantages for the rapid fabrication of complex metal components, an examination of the microstructural development hhas been performed in order to define and optimize the processed materials. Solidification studies of DLF processing have demonstrated that a continuous liquid/solid interface is maintained while achieving high constant cooling rates that can be varied between 10 to 10 5 Ks-1 and solidification growth rates ranging up to 10-2 ms-1.

Abstract Future X-ray space missions will require telescopes with very large effective area, good angular resolution and low weight. To obtain these characteristics it is necessary to produce very thin and light mirror shells that, once integrated into a suitable mechanical structure, will fulfil the requested specifications. To meet these requirements, one approach which seems promising is the use of ceramic materials combined with the plasma forming technology to produce ceramic carriers that are subsequently used to manufacture the x-ray mirror shells by means of the epoxy replication technology on superpolished mandrels. The CEA/Le Ripault has shown the feasibility of producing ceramic carriers (diameter: 600 mm, height: 239 mm). The mass saving is substantial in comparison with the nickel electroforming technology which is now used to manufacture high throughput, medium angular resolution x-ray optics. The achieved out of roundness of the carriers is ± 70 microns; an effort must still be made to meet the requested specification of ± 20 microns.

Abstract Plasma spraying, commonly used for wear and heat resistant barriers, can be used to produce free-standing bulk ceramic parts as well. In this work the microstructure and phase development of a bulk plasma-sprayed alumina material with about 14 % porosity and splat like grains is investigated in the as-sprayed and various annealed material conditions, using electron microscopic and x-ray diffraction techniques. The fracture characteristics are investigated using standard CT specimens in in-situ SEM experiments. The mechanical response of the material is clearly a result of two features: the pronounced alignment of microstructure itself, and the occurrence of a splat-internal microcrack sub-structure in the as-sprayed condition. This microcrack substructure is a consequence of a splat internal columnar subgrain structure which occurs as a result of the rapid cooling conditions on deposition. The morphology of this subgrain structure and its phase composition is seen to change extensively on annealing. It is found that the mechanical behaviour of the as sprayed material is dominated by this internal subgrain structure, but the behaviour of sufficiently annealed material is dominated by the morphology and mechanical stability of the splat like grains themselves. The biggest change on annealing is not an overall sintering effect, but rather the recrystallisation of the splat internal substructure

Abstract The manufacture of tooling using the electric arc spray process to spray steel directly onto a master pattern offers substantial reductions in the lead times required to make complex tooling for polymer injection moulding and other applications. The process of spray forming is fast, efficient, and low cost, and has been shown to be dimensionally accurate with proper control over the residual stresses that develop during spraying. Poor dimensional control because of high internal stresses in thick arc sprayed steel coatings is well known, but these problems can be avoided by the use of correct spraying conditions. This paper describes the steps of the spray forming process used to make tooling for polymer injection moulding. The spray forming route competed directly with a traditional method for toolmaking and considerably reduced the lead time from order to completion. The tooling produced by spray forming has been operating commercially in production in the U.S. and has to date produced in the region of half a million parts without appreciable wear. The incorporation of contoured cooling channels during spraying has enabled plastic injection moulding cycle times to be decreased by 15%.

Abstract Many of the recent improvements in gas turbine engines have been attributed to the introduction of thermal barrier coatings (TBC) for superalloy components. There exists, however, some limitations in current fabrication methods for closed hot-section components: less than ideal coating quality, the need for welding, and limited choice of superalloy material. This paper describes a vacuum plasma near-net-shape process that overcomes these limitations. The process is used to fabricate closed components from yttria-stabilized-zirconia with a CoNiCrAlY bond coat and IN-738LC outer layer. The results from the study show that it is possible to produce near-net-shape superalloy parts with good coating properties and the absence of welds. The mold was reusable after minor reconditioning and the coatings were uniform in thickness and microstructure with a smooth surface finish. The bond coat and structural superalloy layers were very dense with no signs of oxidation at the interface. After heat treatment, the mechanical properties of the IN-738LC compare favorably to cast materials.

Abstract Zirconia can induce enhanced fracture toughness to a number of ceramics when introduced as a reinforcement either in the form of particulates, dispersed phase or whiskers because of its unique tetragonal-monoclinic transformation. This paper presents the preparation of ZrO2 reinforced mullite by plasma spraying a mixture of zircon and alumina. The dissociation of zircon into zirconia and silica in a plasma flame is well-known. Pre-mixed powders of zircon and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon and alumina. It was found that fine, mostly amorphous and chemically homogeneous composite powders can be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The amount of amorphous phases was much higher in plasma sprayed coatings than in spheroidized powders, and the relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders.

Abstract The aim of this study was to investigate potential weight savings using multi-layer blade containment systems for turboengines. The association of an external ductile layer with an internal hard layer could provide a good ductility of the armor with the capability to withstand the perforation of high kinetic projectiles. Comparisons between several thick deposits obtained by the vacuum plasma spray process were performed using a Charpy impact testing machine. Mechanical and structural characterisations of these two-layer structures were performed and compared to the behavior of monolithic ones. Heat treatment effects were also considered.

Abstract A new family of spherical powders produced by the spray drying route has been developed. This paper describes as an example the manufacturing method of an Y2O3-coated aluminum powder. Atmospheric Plasma Spraying (APS) was used to test the corresponding coatings. Morphology and phases of powders and coatings were investigated by optical and scanning electron microscopy while the level of porosity was evaluated using image analysis. Results show that homogenous composite coatings can be obtained from cladded spray dried powders.

Abstract Thermally sprayed Fe-based coatings can be applied in conditions ranging from almost solid to complete molten droplets. While spraying under atmospheric conditions, the oxygen content in the coating varies depending on the spray parameters and the portion of molten phases in the droplets. Using vacuum-plasma technology, Fe-based alloys can be sprayed with a significant amount of molten phase without oxidation. This capability can also be used for alloying Fe-based sprays with nitrogen as is done during reactive vacuum plasma spraying. Such alloying promotes the formation of dispersed vanadium-nitride which greatly improves corrosion and wear resistance.

Abstract Fiber-reinforced polymer composites are an important class of structural materials, offering high strength-to-weight ratios and high rigidities. For many applications, however, their wear resistance is less than desirable. Wear-resistant thermal spray coatings have the potential to improve the surface properties of fiber-reinforced polymer composites, although some require the application of a bond coat to achieve sufficient adhesion. The present study was conducted to find acceptable bond coat materials and compare their performance. Materials such as polyamides, polyimides, polyether-ether-ketone, or simply aluminum or nickel were found to be suitable bond coats for many composite substrates.