The metallic bond coat is generally utilized to increase the coating adhesion and the adhesion of thermal spray bond coat is of essential importance to applications. However, it usually depends on mechanical bonding with a low adhesive strength. In this study, a novel metal bond coat with high cohesion strength is proposed by plasma-spraying Mo-clad Ni-based or Fe-based spherical powder particles. Mo-cladding ensures the heating of spray particles to a high temperature higher than the melting point of Mo and prevents metal core from oxidation during spraying. Theoretical analysis on the splatsubstrate/ splat interface temperature and experimental examination into coating-substrate interface microstructure were performed to reveal the metallurgical bonding formation mechanism. The local melting of substrate surface and resultant bond coating by impacting high temperature droplets creates metallurgical bonding throughout the interfaces between substrate and bond coat, and within bond coat. The experiments were conducted with different substrates in different surface processing conditions including Ni-based alloy, stainless steel and low carbon steel. All pull-off tests yielded strong adhesion higher than the adhesives strength of 80 MPa. The present results revealed that Mo-clad metal powders can be used as new bond coat materials and high performance bond coat can be deposited by atmospheric plasma spraying.

In this study, pure spherical Ta powders made by induced plasma sphero technique were used in the laser cladding process. The powders were sent into high-energy laser zone and were melted at the surface of a steel substrate to create a Ta layer. The microstructure development in the Ta layer was investigated in a scanning electron microscope. The results showed that the layer was basically dense with some pore/crack defects. In the layer, typical dendritic crystalline structures were formed. With the help of an energy dispersive spectroscope, Fe was detected in the Ta layer. The top surface had about 5% Fe while at the bottom of the cladded layer 15% Fe was detected. So, the diffusion of Fe upwards occurred. With the participant of Fe, the microstructure of the Ta layer was changed. Thermocalc software was used to simulate the phase constitution at different Ta-Fe compositions. The results by the simulation basically agreed with the experimental observations.

In nuclear fusion reactors, the first wall is the name given to the surface which is in direct contact with the plasma. A part of it is the divertor which is a device that removes fusion products from the plasma and impurities that have entered into it from the vessel lining. It is covered with water cooled tiles which have to withstand high temperatures and high heat fluxes. Moreover, resistance to neutron bombardment, low tritium absorption and low hydrogen permeation are additional demands. One materials concept under research is the application of a Reduced Activation Ferritic Martensitic Steel (RAFM) as a structural material with a tungsten protective coating. Since there is a considerable thermal mismatch between, a functional graded materials (FGM) concept was proposed. As the formation of undesired intermetallic Fe-W phases as well as oxidation should be avoided, cold gas spraying was chosen as manufacturing process. Two powder blends of EUROFER97 RAFM steel and a fine tungsten powder cut on the one hand and a coarser one on the other hand were tested in different ratios. The coatings were characterized with respect to their porosity and surface structure. Furthermore, the deposition efficiencies for steel and tungsten were determined each. It turned out, that the deposition process is a complex mixed situation of bonding and erosion mechanisms as the deposition windows of these very different materials obviously diverge. Thus, a lower working gas temperature and pressure was advantageous in some cases. Unexpectedly, the coarser tungsten powder in general enabled to achieve better results.

This study investigates the potential of cold-sprayed tungsten coatings for use in nuclear fusion reactors. Three commercially available tungsten powders were selected from which six series of feedstock were prepared. The feedstocks were deposited on aluminum, steel, and stainless steel substrates using high-pressure nitrogen cold spraying. The coatings produced were characterized based on SEM, EDX, and XRD analysis and were found to be free of oxides with levels of tungsten that were previously unachieved. The results indicate that cold spraying is a viable technology for applying tungsten-base coatings to critical components in nuclear fusion equipment.

Wire atomization processes used to make refractory and high temperature alloy powders are relatively expensive due to the cost of feedstock, energy, and gas. A new process based on Transferred Arc Wire Atomization technology, however, has the potential to overcome these problems. This paper introduces the innovative process which, in combination with hydrogen generation, presents new opportunities for several alloys that can be more easily processed by plasma wire atomization. The new approach shows promise to reduce both fixed and variable costs for certain refractory and high temperature materials.

The purpose of this work is to study the effect of laser radiation on powder particles transported by gas during laser cladding. The temperature and velocity of particles entering the light field of a CO 2 laser were determined by measuring particle radiation as well as the scattered radiation of the diode laser, two independent methods. It is shown that under the action of laser radiation, the particles acquire additional acceleration due to the vapor pressure from the irradiated part of the particle surface. This sonic recoil vapor pressure can significantly affect the in-flight characteristics of powder particles in a gas jet. Particle velocities due to laser acceleration exceeded 100 m/s in a carrier gas with a flow rate less than 30 m/s. Particle temperature depends on several factors and was found to vary from ambient temperature to the boiling point of the powder.

This work investigates the effects of sulfidation on plasma-sprayed Al-Mo and Mo coatings. Pure Mo powder and Al-Mo powder mixtures were sprayed on Inconel substrates with either a NiCrAlY or Mo bond coat. Oxidation and sulfidation tests were carried out in air and Ar-S 2 atmospheres, respectively, at temperatures of 973, 1073, and 1173 K. Coating samples were evaluated before and after testing via SEM and XRD analysis and weight measurements. The results show that Al-Mo coatings with a Mo bond layer provided the best protection against high-temperature oxidation and sulfidation corrosion.

In the present study, X-ray microtomography is used to examine cold-sprayed tantalum splats on copper substrates. To resolve tantalum splats intermeshed with other splats of the same chemical composition, a contrasting medium of some sort is required. For this purpose, the feedstock powder is coated with an iron layer by means of fluidized-bed chemical vapor deposition. Experimental tests were coupled with finite element simulations to determine how stresses generated during the impact of a spherical iron-coated particle affect the integrity of the added contrasting layer.

The aim of this work is to optimize molybdenum disilicide coatings for high-temperature oxidation protection of metallic surfaces. Agglomerated and sintered MoSi2 powder was deposited on test substrates by atmospheric plasma spraying. The powders and coatings were characterized by means of optical and scanning electron microscopy. Various tests were carried out to determine the influence of powder size and spray parameters on coating porosity, hardness, and adhesive pull strength.

In this study, porous molybdenum (Mo) materials were prepared by flame spraying semi-molten particles to low velocity levels. The influence of spray particle state, including particle velocity and melting degree, on microstructure and porosity was investigated to understand the formation mechanism of the pore structure and connection between particles. The results showed that Mo sprayed particles at low velocities (<20 m/s) and limited semi-molten state can be generated by flame spraying. The annealed Mo deposits with the porosity ranged from 39% to 61% were deposited. High porosity in the deposit was achieved through the shielding effect of deposited particles, bonded by the settled melt in the particle/particle interface. Moreover, the porosity generally decreased with the increase of melting degree of spray particles prior to impact.

The adhesion mechanisms involved in the cold spray coatings are not still well elucidated. The quality of the deposit does depend mainly on particles and dynamic characteristics (which result from nozzle type, nozzle-substrate distance, etc.). The present work is based on the study of particle-substrate and particle-particle interfaces in the tantalum-copper coating-substrate system. The content focuses on the influence of the oxygen content in the starting powder on interface features, consequently on coating properties. Tantalum powders with different oxygen levels were studied using SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis). Laser shock spallation of cold-sprayed Ta coatings was developed as a reliable and flexible process to achieve Ta spalls to be deposited at a high-velocity onto Cu targets. The velocity due to the laser shock could be controlled to be similar to that of particles in conventional cold spray. This results in Ta-Cu interfaces, the study of which was carried out to go into interface phenomena involved in cold spray, using TEM (Transmission Electron Microscopy) in particular. Results were compared to those obtained from laser shock spallation of Ta bulk specimens (i.e. made of a conventional Ta sheet). The role of powder oxidation on interface soundness was exhibited. Adhesion was shown to be all the lower as powder oxygen content was higher, using LASAT (“ Laser Shock Adhesion Test”) in addition to direct observation of interfaces. Results were exploited to discuss properties of the corresponding Ta coatings onto Cu, i.e. which were cold sprayed using powders with different oxygen contents.

Cold spraying enables the production of pure and dense metallic coatings with very little porosity and oxygen content, all of which play an important role in corrosion resistance. This study investigates the microstructure, denseness (impermeability), and corrosion properties of tantalum coatings produced by high-pressure cold spraying. Various tests were conducted to assess corrosion behaviors in different aqueous solutions. Microstructural studies showed that interfaces in the coatings were practically free of voids and confirmed that high levels of localized plastic deformation occurred as expected during spraying. The study also confirmed that the polarization behavior of the cold-sprayed tantalum is similar to that of the corresponding tantalum bulk in saline as well as sulfuric and hydrochloric acid solutions.

Molybdenum disulfide (MoS 2 ) films are widely used to improve friction performance, but they are difficult to fabricate using conventional thermal spray processes due to thermal decomposition of the feedstock powder. In this study, Cu-MoS 2 composite coatings are fabricated by cold spraying using mechanically milled powders containing different concentrations of MoS 2 . Investigators found that increasing the concentration of MoS 2 in the powder improved some coating properties while degrading others. Through testing it was determined that the ideal concentration of MoS 2 is 5wt%. Increasing the milling time of the powder mixture also provided benefits in terms of hardness and wear resistance.

Bead blasting and thermal spray coatings are often applied on process kits used in vacuum deposition chambers to improve adhesion between kit surfaces and deposited films. This study shows that in order to maximize chamber service time and reduce processing defects, thermal expansion mismatches must be considered between chamber components, sprayed coatings, and vacuum deposited films. When a titanium sheet coated with arc sprayed aluminum was placed in a titanium nitride deposition chamber, significant particle spiking was observed. However, during the same period of chamber service time, particle performance was stable for titanium coated with arc sprayed molybdenum. It should be noted that the thermal expansion coefficients of Ti and Mo are much closer than those of Ti and Al. By further optimizing the cohesion strength of the arc-sprayed Mo coating, even lower particle counts have been achieved, corresponding to fewer processing defects and prolonged chamber kit lifetime.

A new plasma chemical process has been found that produces tungsten thin films. Using fine powders or precursors as feedstocks, the process vaporizes the feedstocks and then deposits nanometer size grains. The deposition kinetics of structures produced with this technique vary greatly from classical plasma spraying methods. Equiaxed and columnar grains (30-150 nm) are formed instead of splat structures, although the grains may continue to grow after spraying.

The goal of this study is to find applicable spray conditions for producing tungsten (W), zirconium carbide (ZrC), and W-ZrC cermet layers. In the experiments, W and ZrC powder mixtures were fed into the plasma of a water-stabilized plasma gun and coatings approximately 1 mm thick were sprayed on graphite substrates. Pure W and pure ZrC were deposited under similar conditions. Microhardness, surface roughness, XRD, XRF, dilatometry, and spectroscopic techniques were used to characterize the coatings. The resulting coatings were found to be hard with a high elastic modulus.

Plasma-sprayed, molten molybdenum particles (~55 µm diameter) were photographed during impact on grit-blasted glass surfaces that were maintained at either room temperature or at 350°C. Droplets approaching the surface were sensed using a photodetector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from behind the glass. A rapid two-color pyrometer was used to collect the thermal radiation from the spreading droplets to follow the evolution of their temperature and calculate the splat cooling rates. It was found that as the surface roughness increased, the maximum spread diameters of the molten molybdenum droplets decreased, while the splat cooling rates increased. Impact on non-heated and heated roughened glass with similar roughness values produced splats with approximately the same maximum spread diameters, skewed morphologies, and cooling rates. On smooth glass, the splat morphologies were circular, with larger maximum spread diameters and smaller cooling rates on non-heated smooth glass. An established model was used to estimate the splat-substrate thermal contact resistances. On highly roughened glass, the thermal contact resistance decreased as the glass roughness increased, suggesting that splat-substrate contact was improved as the molten metal penetrated the spaces between the large asperities.

Thick pure tungsten coatings were deposited on molybdenum and copper substrate by vacuum plasma spray for different purpose. The microstructures of tungsten coatings were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of preheating temperature of the substrate and introduction of gradient bond layer on the adhesive strength of the coating was investigated. High heat load was tested by high power electron beam bombardment only for tungsten coating on copper. All the results show that the properties of tungsten coating were strongly influenced by different processes, and the density of the coatings is close to 95% of theory density.

Vacuum plasma sprayed (VPS) tungsten (W) coatings hold great promise for plasma facing components in future fusion devices. However, the large coefficient of thermal expansion (CTE) mismatch between W and underlying structural steels poses a significant problem for manufacturing and service life because of the evolution of large thermally induced stresses leading to failure. In this paper both the concept of functionally graded material (FGM) W/steel interlayers and the use of steel substrate surfaces with regular surface sculptures of millimetre scale created by e-beam surface manipulation, termed surfi-sculpt and developed by TWI of the UK are investigated. The objective of these approaches is to enhance coating adhesion and to engineer macroscopic variations in the effective CTE through the thickness of the subsequently VPS deposited W coating. The effects of surface geometry on coating adhesion and microstructure have been investigated, and preliminary conclusions on the key surface sculpture geometrical features required for high adhesion dense W coatings have been identified.

Impacting of a molten droplet with melting point much higher than substrate results in melting of substrate around the impact area. The melting of the substrate surface to certain depth alters the flow direction of droplet fluid. The significant change of fluid flow direction leads to detaching of fluid from contact with the substrate. Consequently, splashing occurs during droplet spreading process. In the present study, Mo splats were formed on stainless steel substrate under different plasma spraying conditions. For comparison, Mo splats were also deposited on Mo surface. The substrate surface was polished prior to deposition. The powders used have a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was the split type presenting a small split structure on stainless steel substrate surface. With Mo particles, the preheating of steel substrate has no effect on splat morphology. On the other hand, disk-like type Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrate and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface-melting- induced splashing model was proposed to explain the formation mechanism of Mo splat on steel surface.