The addition of refractory metals represents a promising development approach for future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic alloys with their homogeneous property profile are of particular interest. In the present work, two eutectic HEAs (EHEAs) were developed for the starting Al 0.3 CoCrFeNi using electric arc furnace. Following mechanical and microstructural characterization, the two alloys Al 0.3 CoCrFeNiMo 0.75 and Al 0.3 CoCrFeNiNb 0.5 were identified. For thermal spray processing, powders were prepared by inert gas atomization. The coatings produced by high velocity oxy-fuel (HVOF) spraying were characterized and evaluated comparatively to the castings, allowing process-structure-property relationships to be derived. Based on the results, statements on possible application potential can be made.

Disk splats are usually observed when the deposition temperature exceeds the transition temperature, whereas thick oxide layer will reduce the adhesion resulting from high deposition temperature. In present study, single molybdenum splats were sprayed onto polished molybdenum substrates with different preheating processes to clarify the effect of surface oxidation on the splat formation. Three preheating processes included heating the substrate to 350 °C, 550 °C, and cooling the substrate from 550 °C to 350 °C, which were performed in argon atmosphere. The chemistry and compositions of substrate surface was examined by XPS. The cross sections of splats were prepared by focus-ion-beam (FIB), and then characterized by SEM. Nearly disc-shaped splat with small fingers in the periphery was observed on the substrate preheated to 350 °C. Perfect disc-shape splat was deposited at 550 °C. Flower-shaped splat exhibited a central core and discrete periphery detached by some voids on the substrate preheated to 350 °C (cooling down from 550 °C). The results of peeling off splats by carbon tape and morphology of FIB sampled cross-sections indicated that no effective bonding formed in the splat-substrate interface for the substrate ever heated to 550 °C, due to the increasing content of MoO 3 on preheated molybdenum surface.

Molybdenum disilicide (MoSi 2 ) has been applied as protective coating material on various substrates fabricated by different methods due to its good oxidation resistance at elevated temperature, relatively low density and coefficient of thermal expansion and high thermal conductivity. In this work, MoSi 2 coatings were fabricated by low pressure plasma spraying technology (LPPS). Their morphology, composition and microstructure characteristics were intensively investigated by SEM, XRD, EDS and TEM. The oxidation behaviors of MoSi 2 coatings were also explored. The results showed that the MoSi 2 coating was compact with porosity less than 5%. Its microstructure exhibited typical lamellar character. The MoSi 2 coating was made up of grains with irregular shapes and different sizes of 0.1-0.2 µm. It was mainly composed of tetragonal and hexagonal MoSi 2 phases. A small amount of tetragonal Mo 5 Si 3 phase formed and randomly distributed in the matrix of MoSi 2 . The MoSi 2 coating exhibited excellent oxidation-resistant behavior at 1773K, which resulted from the continuous dense glassy SiO 2 film formed on its surface.

Solution precursor plasma spray has been shown capable of depositing high surface area transition metal oxide coatings of interest as ultra-capacitor electrodes. These materials exhibit mixed double layer and pseudo-capacitive properties, enabling larger charge storage capacity than electrical double layer capacitor electrodes such as carbon. This investigation explored potential of SPPS to deposit molybdenum oxide with microstructures suitable for use as pseudo-capacitive electrodes. It further identified a two-step temperature-programmed heat treatment that resulted in the topotactic phase transformation of the α-MoO 3 deposits into high specific surface area molybdenum nitrides exhibiting a higher electrochemical stability window (i.e. a higher specific area capacitance). The electrochemical behavior of molybdenum oxide and molybdenum nitride deposits formed under different deposition conditions was studied using cyclic voltammetry in order to assess the influence of the resulting microstructure on the charge storage behavior and potential for use in ultra-capacitors.

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.

In twin wire arc spraying process it is possible to use feedstock wires of two different compositions at the same time. As a result of this procedure it can be achieved composite coatings called also as pseudo alloys with modified physical properties. In this study nickel and cobalt based super alloy materials were arc sprayed with pure molybdenum wire to tailor corrosion and wear resistance of the coatings. Coatings for the tests were sprayed using two different twin wire Sulzer Metco arc-spraying units, Smart Arc and OSU 300, operating with suitable spray parameters to produce coatings of good quality. It was already known that these twin wire configurations are producing coatings with differing microstructures. Coating sprayed with the OSU system was clearly finer in structure and one purpose of this study was to measure the effect of the micro structural size on the corrosion and wear properties of the final coatings. Microstructures of the coating materials were studied and analyzed from cross-sectional specimens. Volume fraction of pure molybdenum in the coating matrices was evaluated with simple line method and according to the results volume fraction of pure molybdenum metal is over 50 volume-% in all of these tested composite coatings and higher in materials sprayed with OSU unit. Also the microstructure of the coatings was seen to be finer when OSU was used as was expected. Wear resistance was measured with modified ASTM G65 rubber wheel sand abrasion wear test and corrosion resistance was tested in low pH values and chlorine containing environment according to the ASTM G48 corrosion testing standard. Corrosion testing was carried out at room temperature 22°C and also at higher 50°C temperature. Molybdenum addition is clearly improving the abrasion wear resistance of the tested coating systems. At room temperature also the corrosion resistance is getting better with molybdenum addition but at higher temperature this effect is not so clear.

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.

The impact of plasma-sprayed molybdenum particles on glass surfaces held at 25 and 400°C was photographed. A two-color pyrometer was used to collect thermal radiation from the particles to follow their temperature evolution and to calculate the splat cooling rate. Significant fragmentation of the splat on the surface at 25°C was observed. A 3D model of droplet impact and solidification was used to estimate the thermal contact resistances between the splat and glass. It was found that the thermal contact resistance was approximately two orders of magnitude smaller on the surface at 400°C, indicating faster solidification, which reduced splashing. The larger thermal contact resistance between the non-heated glass and splat was attributed to the presence of a gas barrier at the surface.

Plasma-sprayed, molten molybdenum particles (~50 µm diameter) were photographed during impact (with velocity ~135 m/s) on a glass surface that was maintained at either room temperature or 400°C. A droplet approaching the surface was sensed using a photodetector and after a known delay, a laser was triggered to illuminate the spreading splat and photograph it with a CCD camera. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on a surface at room temperature splashed and broke up after impact leaving only a small portion adhering to the substrate. On a surface held at 400°C, there was no splashing and a circular splat remained on the surface. Splats on a glass surface held at room temperature had a large maximum spread diameter, approximately 2.7 times that on a hot surface. The cooling rate on a cold surface was an order-of-magnitude lower than that on a hot surface, suggesting that thermal contact resistance was much greater.

Thermal spray coatings exhibit a wide variety of microstructural characteristics that lead to variation in their functional properties. A complete understanding of the plasma spray process includes examination of the particle-flame interaction, particle impact (to form the splats), and the particle-substrate interaction during coating deposition. The links between these process parameters and coating properties has been established by using diagnostic tools in conjunction with a splat collection shutter and an in-situ curvature measurement instrument. In this study, a commercial grade molybdenum (Mo) powder was plasma sprayed; the spray stream was characterized in relation to the resulting particle state. A "splat map" was deposited through a "spray stream guillotine" to capture the fingerprint of the plume cross section. Subsequently, coatings were deposited at these spray conditions on a newly developed in-situ curvature measurement instrument to measure coating stresses and to estimate the coating modulus. Splats and coatings were subsequently characterized by micro-diffraction (for splat residual stresses), by nano and micro-indentation for elastic and elastic-plastic properties, and by electron microscopy. This complete history of the process followed by splat and coating characterization provides insight into the correlation between processing parameters, resultant particle states, and final coating properties. The role of particle temperature and velocity on the splat (and coating) morphology and residual stress is explained in the results.

Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

A microplasma spraying torch with a hollow cathode electrode is designed to melt completely the refractory materials and deposit coatings at plasma power level up to several kilowatts. The designed torch permits spray material to be fed into plasma arc jet through axial powder injection. In the present study, molybdenum is used as a typical refractory spray material. The effects of the main processing parameters including plasma arc power, plasma gas flow and spray distance on the particle velocity during spraying, and the microstructure and properties of the coatings are investigated. The microstructure of coating is characterized with optical microscopy and scanning electron microscopy. The properties of the coating are characterized by microhardness and abrasive wear tests. The particle velocity during in-flight is carried out using a particle velocity/temperature measurement system based on thermal radiation. The comparison of the microstructure and property of micro-plasma sprayed Mo coatings with those of the coating deposited by the conventional plasma spraying operated at a power of 42 kW is performed. The results show that the abrasive wear loss of the Mo coatings deposited by the micro-plasma spray torch is comparable to that of the coating deposited by the conventional plasma spraying disregarding the one order difference in the plasma operating power.

Molybdenum powder has been plasma sprayed on stainless steel, brass and aluminum substrates. The substrate melting phenomenon is observed and investigated by means of scanning electron microscopy (SEM) and scanning white light interferometery (SWLI). It is found that the flower-shape splat morphology is typical for molybdenum on all three substrate materials when the substrate is at room temperature. Notable substrate melting is manifested through the energy dispersion analysis of X-ray (EDAX) map and Robinson back-scattered image of cross-sections of splats. It has been shown that the substrate material plays an important role in substrate melting phenomenon. The lift angle of the petals of splats and the maximum crater depth have been characterized and compared. Both of these increase in the sequence, from stainless steel, brass to aluminum. A ‘volume of fluid’ (VOF) based model coupled with rapid solidification has been used to simulate splat deformation, solidification, substrate melting and resolidification. The numerical & analytical results agree quite well with the experimental data. A substrate melting mechanism is proposed based on the time scales of the droplet solidification and substrate melting to explain the formation of flower like splat morphologies.

To improve wear resistance of the atmospheric thermal plasma sprayed molybdenum coating, diamond deposition on the molybdenum plate and the atmospheric plasma sprayed molybdenum coating by the combustion flame chemical vapor deposition (CVD) was carried out. Diamond has excellent properties such as low surface energy, hardness, chemical corrosion resistance ability and so on. Besides, since the combustion flame CVD is the process carried out in the air, diamond/ molybdenum complex coating can be deposited without any vacuum facilities by using this technique if molybdenum coating is deposited by atmospheric thermal spray. In this study, acetylene welding torch was used as diamond synthesis apparatus and mass flow ratio C 2 H 2 /O 2 was varied from 0.9 to 1.3. Consequently, many diamond particles which were 10 micrometer in diameter respectively were deposited on the molybdenum plate by only 20 minutes combustion flame irradiation in the case of 1.2 in mass flow ratio of C 2 H 2 /O 2 . Especially, the molybdenum coating was covered with diamond films consists of 10 micrometer diameter particles in the case of over 1373K in deposition temperature. Besides, according to the results of wear testing, wear mass loss of diamond deposited coatings were much lower than that of original thermal sprayed molybdenum coatings. From these results, this process was found to have a high potential in order to improve wear resistance of thermal sprayed coating.

Molybdenum splats were produced at three plasma conditions on steel substrates preheated to three temperatures. Morphology of splats and corresponding craters formed on substrates were observed; dimensions of splats and craters were measured with an optical non-contact interferometer. It is found that substrate is significantly melted and deformed upon impact of the droplet, which leads to the formation of flower like splats and craters. On average, only about 36 to 53 % of the areas covered by splats were in good metallurgical/mechanical contact with substrate. Normalized crater volume increases with droplet size and the contact is improved for the high particle energy/high substrate temperature condition as compared with low particle energy/medium substrate energy condition. Splat morphology and crater formation is explained based on impinging jet heat transfer model.

Wire flame sprayed molybdenum is a wide used procedure for manufacturing of wear resistance coatings. The properties of thermal sprayed coatings depend mainly on the kinetic and thermal energy of sprayed particles, i.e., a higher particle velocity causes an increase of coating quality. The now available high velocity spray system from Praxair which is used within this work is capable to realise the aim of high particle velocities. The coating properties presented in this work are analysed in comparison to conventional wire and powder plasma spray processes. HVWFS molybdenum coatings show lower porosity, higher adhesion and cohesion and better wear properties. To explain the results, particle size distribution, oxygen/carbon content and structure are analysed. Hardening mechanisms of coatings and their adhesion/cohesion properties are discussed based on light microscopy, SEM, XRD and TEM investigations.

A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

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.

This paper investigates the stability of molybdenum base silicides, which are located in combustion chambers and in an endothermic environment, for use in radiant tubes for heat treatment. The subject matter was plasma-sprayed molybdenum disilicide, pentamolybdenum trisilicide, hot-pressed molybdenum disilicide, and molybdenum disilicide composites containing SiC and silicon nitride reinforcements. Results of the investigation show that the oxidation resistance of plasma sprayed molybdenum disilicide can be detrimentally effected due to the silicon loss that occurs during the high temperature plasma spray process. Paper includes a German-language abstract.