Abstract Some of the challenges to expanding the successes of Metal Additive Manufacturing (Metal AM) are associated with the manufacturing of refractory and high temperature alloy powders. Wire atomization processes are used to manufacture refractory and high temperature alloy powders. Typical wire atomization processes are more expensive than conventional atomization due to the cost of feedstock; the specific energy consumption being higher; and possibly higher specific gas consumption. This paper introduces Plasma Theiaization Process; a Transferred Arc Wire Atomization (TAWA) technology with the potential to overcome some of the deficiencies of conventional non-transferred arc plasma wire atomization systems. Hydrogen generation combined with this innovative plasma wire arc atomization process can offer new opportunities for several specific alloys that can be more easily processed by plasma wire atomization. Development and commercialization has begun on this breakthrough new metal powder production technique using a plasma wire arc to create atomized metal particles with highly desirable characteristics. This new approach shows promise to reduce both the fixed costs and the variable costs for certain refractory and high temperature materials.

Abstract Spectroscopic and electrostatic probe measurements were carried out to understand the plasma feature inside and outside a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle. Ammonia and a mixture of nitrogen and hydrogen were used as the working gas. The NH3 and N2+3H2 plasmas in the throat were expected to be nearly in a temperature-equilibrium condition, although the plasmas in the expansion nozzle and in the downstream plume without substrate plates were in thermodynamical nonequilibrium states. As a result, the H-atom excitation temperature and the N2 rotational excitation temperature decreased from 7000-11000 K in the throat to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit at 0.1-0.2 g/s, although the NH rotational temperature did not show an axial decrease even in the nozzle. On the other hand, each temperature was almost kept a small range in the downstream plume without substrate plates under an ambient pressure of 130 Pa except for the NH rotational temperature for NH, working gas, although in the case with a titanium plate the nonequilibrium plasma came to a temperature-equilibrium one as approaching the plate.

Abstract This paper compares two types of hydroxyapatite (HA) composite coatings, HA/Ti-6Al-4V and HA/Y-ZrO2. The powders used in the study were prepared using a slurry process then deposited by plasma spraying. The resulting coatings were characterized based on their microstructure, mechanical properties, and biocompatibility. Both composite coatings performed better than pure HA coatings in tensile adhesion and indentation tests. Testing also revealed that the HA/Y-ZrO2 coatings had favorable strength and fracture toughness and that the HA/Ti-6Al-4V coatings had good affinity to living tissue and sufficient mechanical strength.

Abstract A brief feasibility study was performed to produce thermal spray coatings using gas atomized powders of Cu47Ti34-xZr11Ni8Six, where x=0 and 1. These alloys have previously been shown to be capable of forming metallic glasses having thick (1-2 cm) cross sections because they can be cooled from the melt at relatively low cooling rates (e.g., 100-102Ks-1). The properties of these metallic glasses include high strength, high elasticity and high fracture toughness. Amorphous plasma arc sprayed coatings were produced which were close in composition to the starting powders, and exhibited comparable glass transition and crystallization behavior. The amorphous structure of the as-sprayed coatings was used as a source for forming a range of partially devitrified and fully crystallized structures. The average hardness of the coatings increased from around 6 GPa to near 10 GPa as the degree of crystallization increased.

Abstract Three actual applications of spray plume sensors for industrial coating operations are presented. Two of these are in the aeronautic industry and one in the automotive industry. Depending on the application, different types of sensors were used ranging from plume shape monitoring (particle trajectories) to full plume characterization including in-flight measurement of individual particle properties. In each case, the process monitoring scheme developed depended on the manufacturer's requirements as well as the nature of the information provided by the sensor. The significant differences in usage and working conditions compared to an R&D environment are examined. Advantages and drawbacks in each of the three situations are detailed. A tentative forecast of coming developments in this field is also given.

Abstract Commercially available coating techniques such as "open arc" and "spray & fuse" methods were used to compare the microstructural development with the plasma transferred arc (PTA) coating process for the hardfacing of NiCrBSi and Stellite 6 alloys. Denser eutectic structure was observed in the case of PTA coated layers of the Stellite 6 alloys than those of open arc weld-surfacing process. The shape of both carbides and borides in the 16C alloy coated by PTA processing were also obtained to have coarse morphology of carbides and borides while the "spray & fused" layers show a needle shape with finer distributions. Possible thermal history during each coating process is discussed. Based on microstructural observation, the hardness, wear resistance, and corrosion behaviors are reported. As expected, the alloy properties are directly related to their constituents of microstructure.

Abstract The Thermal Barrier Coating (TBC) used to improve the heat barrier and wear resistant property in high temperature of the aircraft engine and the automobile engine, usually has a two layer structure. One is a ceramic top layer for heat insulation and the other is a metal bond layer to facilitate the bond strength between the top ceramic layer and the substrate. But, the coated layers can be peeled off because of the accumulation of the thermal stress by the difference of the thermal expansion coefficient between metal and ceramics in a heat cyclic environment. In this study, the intermediate layer produced by plasma spray process was introduced to reduce the thermal stress. The powders of plasma spray coating were Yttria Stabilized Zirconia (YSZ), Magnesia Stabilized Zirconia (MSZ) and NiCrAIY. The intermediate layer was sprayed with the powders of partially stabilized zirconia with 50wt% NiCrAIY between the ceramics top coat and the bond coat for the purpose of alleviating heat expansion. The high temperature wear and thermal shock test were conducted. The high temperature wear resistance of the YSZ TBC was better than that of the MSZ TBC. The wear resistance decreased with increasing temperature between 400°C to 600°C. The 3 layers TBC with YSZ top coating showed the best thermal shock resistance. This means that the intermediate layer played an important roll to alleviate the difference of the thermal expansion between metallic layer and ceramics layer. SEM and OM were examined. The bond strength, hardness test, and wear test were also studied.

Abstract The tribological behavior of Mo/NiCrBSi coatings obtained by atmospheric plasma spray was studied under dry conditions using an Amsler machine. Discussion is made on the effects of the plasma gas mixture and of the preheating of the substrate. The wear process of Mo/NiCrBSi in situation of dry rolling-sliding contact versus cast iron was observed using a profilometer, an optical microscope, and a scanning electron microscope. It was found that the wear mechanism could be divided into two steps : the first one corresponds to an abrasive wear; it results from the difference in hardness between the counter-specimen and the Mo/NiCrBSi coating. The second one corresponds to an adhesive wear resulting from the transfer of cast iron from the counter-specimen to the coating.

Abstract This paper presents the results of a study showing how isothermal aging affects the wear properties of Cr-Ni overlay alloy coatings with dispersed NbC particles. High Cr-high Ni coatings, with and without niobium carbides, were deposited on mild steel substrates via plasma transferred arc welding then age-hardened at temperatures from 773 to 1023 K. The precipitation behavior and wear properties of the coating samples were examined using Vickers hardness testing, SEM, TEM, EDX, XRD, and Ohgoshi wear testing. The results showed that isothermal aging significantly improved the hardness and wear resistance of the NbC-dispersed alloy but had little effect on the NbC-free samples. The difference in precipitation behaviors is probably due to the presence of niobium atoms in the alloy matrix, resulting in a continuous precipitation of α' phase.

Abstract Wear of plasma sprayed Cr 2 O 3 TiO 2 , Cr 3 C 2 -NiCr and WC-Co coatings have been evaluated with a block-on-ring arrangement under dry and lubricated conditions. The results indicated that the wear of the coatings was interpreted in term of subsurface grain fracture which was related to the special microstructure of the coatings such as size, shape and distribution of pore as well as crack. Among the four kinds of coating, Cr 2 O 3 coating possesses the lowest wear coefficient. Water accelerated cracking and fracturing and deteriorated the wear resistance of both Cr 3 C 2 -NiCr and TiO 2 coatings. Ethanol reduced the wear of Cr 3 C 2 -NiCr coating, which was attributed to the formation of a smooth surface film mainly consisting of Cr 2 O 3 . Wear of Cr 2 O 3 , coating against an Al 2 O 3 ball at high temperature has been also produced. The wear of Cr 2 O 3 coating against Al 2 O 3 ceramics decreased with increase in temperature and load. The reaction between Cr 2 O 3 coating and Al 2 O 3 , ceramics at high temperature and the formation of a protective film consisting of Cr 2 O 3 , Al 2 O 3 and SiO2 glass on the surface of Cr 2 O 3 coating improved the wear resistance of Cr 2 O 3 coating.

Abstract Plasma transferred arc (PTA) is now currently used for reclamation of worn materials or to provide wear or corrosion resistant coatings welded to the base material. However, the powder injection in the molten pool created at the coated part surface is a critical parameter. In order to avoid coating reproducibility problems induced by the powder feed rate, the way to coat substrate surface with powder before the PTA treatment has been studied. As the powder cannot simply be deposited onto the substrate because of the plasma flow which would blow it off before melting it, tape casting process was used to obtain an adherent powder layer on the material surface. In this paper, tape casting of NiCu particles is described and the different organic additives used to obtain a homogeneous nickel copper film on cast iron and AG3 aluminum alloys are presented. The first results of the treatment of these films by PTA reclamation are then shown.

Abstract Plasma sprayed deposits consist of multitude of flattened lamellar particles - 'splats' - which as basic building elements form their structure and determine the deposit properties. Therefore, knowledge of the mechanism of their formation and characteristics is important for understanding the processing property relationships. Although extensive studies have been done on splat formation, there is a lack of correlation to macroscopic deposit properties. Among factors influencing the deposit properties and performance is residual stress, originating from splat quenching and thermal mismatch between the substrate and coating. This phenomenon has been studied mostly on macroscopic level. In the present study, an attempt is made to establish a connection between these two approaches. In the focus of this study is the effect of selected processing variables on splat characteristics, deposit properties and residual stress in a single-splat layer. The processing variables of primary interest were deposition temperature and substrate material. Molybdenum as a representative material of practical interest was used throughout this study. The correlation between stresses and processing conditions is discussed with regards to microstructure and relevant coating properties.

Abstract The main purpose of this work is the development of mathematical and computer models for the integrated simulation of all stages of the atmospheric plasma spraying process (APS) with temperature dependent thermophysical and mechanical properties of the used materials and gases and experimental verification of the simulated results. The following mathematical models of APS were created: particle heating and movement in the plasma jet; coating structure formation; heat transfer and residual stresses in the coating-substrate system. The computer realization of these models enables us to model all stages of APS (integrated or separately). Databases of coating, substrate and plasma-gas substances include the temperature dependent properties. The model of APS is divided in 3 parts, which are connected by continuous data interface. Two dimensional approximation of plasma-gas velocity and temperature in the free plasma jet was used for computation of particle velocity, trajectory and temperature. This information was created with a special Graphic program module and included in database. Computer experiments for plasma spraying of Ah03 and ZrO 2 +8%Y 2 O 3 in Ar/H 2 plasma were carried out. The experimental verification of developed models with High-Velocity-Pyrometry (HVP) and Laser-Doppler-Anemometry (LDA) have shown the satisfactory precision of simulated results.

Abstract A study was performed to examine the effects of starting powder composition, substrate thermal conductivity, and substrate temperature on the composition and structure of individual Al-Cu-Fe splats formed during thermal spraying. The fraction of quasicrystalline phase which formed was found to depend on the chemistry and solidification history of the splats. Due to evaporative loss of Al during spraying, an initial powder composition higher in Al produced splats closer to the desired composition, which yielded more of the quasicrystalline phase. Deposition onto lower thermal conductivity surfaces resulted in an increase in the quasicrystalline phase, as did solidification onto higher temperature substrates.

Abstract The structure and morphology of plasma sprayed splats are experimentally investigated using different droplet materials and substrate materials. Droplet materials include aluminum, copper, nickel and refractory metals such as molybdenum and tungsten, and substrate materials include aluminum, stainless steel, and molybdenum plates. The results show that the splashing occurs during the splatting of a completely molten droplet. Most splats formed by droplets molten completely are only central part of the ideal disk type ones, which are defined as the annulus-ringed disk-like splat. It is found that the morphology of such annulus-ringed disk-like splat is greatly influenced by the combination of droplet and substrate materials depending on whether substrate melting occurs. With the combinations of droplet and substrate materials which are of similar thermal properties the splashing of central area of splat tends to occur to present a honeycomb structure at the center of splat. When droplet impacting can cause melting of substrate annulus-ringed splat prefers to present a split type. The flattening ratio of an annulus-ringed disk splat is typically less than 2.

Abstract PTA (Plasma Transferred Arc) reclamation of aluminum alloys by hard materials with a much higher melting temperature is very difficult. This is due to the high thermal diffusivity of these al1oys. Below a critical heat flux φc nothing happens and over φc the substrate melts very rapidly contrarily to what is observed with steel substrates. That explains probably why PTA is mainly used for steel reclamation. Thus the knowledge of heat flux transferred to the anode is a critical point to develop PTA reclamation on aluminum alloys and this is the aim of this paper. An experimental set-up was built to study the heat transferred to three substrates made of different materials : cast iron for reference, aluminum alloy and copper for its high thermal conductivity. The plasma torch was a Castolin Eutectic gun and allowed to inject a sheath gas around the plasma column. The copper, aluminum alloy and cast iron substrates, easily interchangeable, were the top of a water-cooled calorimeter allowing to determine the variation of the received heat flux with the working parameters : arc current, stand off distance, plasma forming gas momentum, sheath gas composition and momentum. The determination of the arc electric field allowed to calculate the arc diameter which was compared first with pictures taken with a video camera and second, with wear traces left on the anode material. Several correlations have been established to characterize the arc voltage and the anode heat flux.

Abstract The thermomechanical properties of plasma-sprayed deposits strongly depend on residual stress distribution. This latter is mainly attributed to the relative torch/substrate velocity as well as to the cooling system location and efficiency. The determining of both quenching and thermal stresses, which are generated respectively during spraying stage and cooling stage, is then required to improve coatings quality. A rather simple apparatus, which consists in monitoring the curvature of a beam substrate during the whole deposition process, has been developed to work under industrial conditions. It has been applied to partially stabilized zirconia coatings performed onto stainless steel and cast iron substrates. Spraying temperature and plasma gun velocity have been selected as relevant parameters for this study about stress generation and mechanical release. Finally, four point bend tests have been performed on deposited samples to measure coating mechanical properties and to evaluate damage level.

Abstract A novel method for characterization of microstructure of coatings is presented. Properties of plasma spray coatings are affected by their microstructure, which depends on the spraying conditions. Therefore, a detailed knowledge of microstructure is very important in order to know the coating formation mechanism and the properties of the coatings. There are many studies to characterize the microstructure of coatings. In most of those studies, the microstructures are characterized from the polished cross-section of the coatings, and the results strongly depend on the preparation methods. In this study, a new method for the characterization of coating microstructures by means of surface morphology is proposed. The distribution of shape and dimensions of splats were examined using quantitative analysis of scanning electron microscope images from the surface of the coatings. The results indicate that the surface morphology strongly depends on the spraying conditions. Keywords: characterization, coating property, splat morphology, equivalent diameter, shape factor

Abstract CrNiAlTi, NiCrBSi and WC-Ni coatings have been thermal and plasma sprayed projected over a stainless steel surface in order to protect it against heat and erosion actions encountered in power plant boilers. Their microstructure, porosity and microhardness have been measured. High temperature oxidation under an atmosphere similar to service conditions in power plants and thermal fatigue tests have also been performed in our experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer has been evaluated by means of tensile tests. The obtained results have been discussed paying especial attention to the microstructural materials evolution due to thermal effects and coating projection methods.

Abstract Impact performance of plasma spray coatings is usually evaluated by means of surface observation after impact action. As a matter of fact, the dynamic response characteristics of coatings in the course of impact action are also very important. In this paper, a method of response frequency spectrum analysis is developed for the impact evaluation of plasma spray coatings. An impact test machine, in which the impact load is generated by a pivot-rod-lever system, is specially designed, allowing both single impact test and repeated impact test. The frequency spectra of Cr2O3 ceramic coating and WC-Co17% alloy coating under single and repeated impact action are analyzed. The results show that there is an obvious relationship between the impact performance and the impact response frequency spectrum. Abrupt changes in the coating, such as appearance of surface cracks and surface damage, correspond the sudden changes of the response frequency spectrum.