Amorphous alloys have attracted extensive attention due to their unique atomic arrangement and excellent properties. However, the application in practical engineering is seriously limited due to the size, crystallization and other problems. Laser additive manufacturing technology has the characteristics of high heating, cooling rate and point by point melting deposition, which provides a new idea for the preparation of amorphous alloys. Zr 50 Ti 5 Cu 27 Ni 10 Al 8 amorphous alloy was prepared on the surface of pure zirconium substrate by selective laser melting technology. The composition and structure of the samples were characterized. The results show that the samples are mainly composed of amorphous phase, and the crystallization mainly occurs in the superimposed zone of heat affected zone. With the decrease of laser power, the area of crystallization zone and the number of crystallization particles decrease. However, if the laser power is too low, there will be non-fusion defects and cracks, which will seriously affect the forming quality and amorphous rate of amorphous alloy.

Thermal barrier coatings are generally produced one of two ways, depending on the thermomechanical loading expected. This study assesses an alternative approach in which the output of an air plasma torch is directed through two chambers connected by an expansion nozzle. In the first chamber, the particles evaporate under high pressure and temperature conditions. The vapor then passes through a supersonic nozzle into a low-pressure chamber where it condenses on the target substrate. A number of models are developed and used in order to assess the effects of process geometry and operating conditions on gas flows, powder vaporization efficiency, and nucleation and growth kinetics. Numerical simulations also informed various design decisions such as the length of the high-pressure chamber and the diameter of the expansion nozzle.

ZnO films were deposited by solution precursor plasma spray (SPPS) process with different substrate preheating temperatures and torch powers, which were used to study the effects on crystallizations and microstructures. With increasing substrate preheating temperature from 0 °C to 400 °C, ZnO films were always preferential orientation along (002) plane with much higher crystallinity. And more apparent crystallized particles appeared with higher agglomeration degree forming cauliflower-like microstructure under higher preheating temperature. For adjusting hydrogen flow rate, the moderate hydrogen flow rate was the suitable condition for obtaining oriented growth along (002). Besides, all ZnO films under different hydrogen flow rates with a constant preheating temperature as 400 °C were always combined with crystallized particles. Moreover, the increment of torch power makes microstructure becomes denser with less interspace between neighbouring particles. Moreover, it is found that crystallinity and crystallized particles is more dependent on preheating temperature and torch power plays a more important role on densification by two staggered experiments. Taking applications of metal oxides films via SPPS into consideration, choosing moderate substrate preheating temperature and hydrogen flow rate will obtain crystallized particles, unusual preferentially oriented planes and high specific surface area, which is very favourable for optical, electrical, electrochemical properties.

In plasma spray-physical vapor deposition (PS-PVD), deposition takes place not only from liquid splats, but also from nanosized clusters as well as the vapor phase. As a result, thin, dense, and porous ceramic coatings can be produced for special applications using this method. In this study, columnar-structured YSZ coatings were deposited by PS-PVD on graphite and zirconia substrates and the effect of substrate temperature on coating microstructure was investigated. A deposition mechanism of heterogeneous nucleation is presented based on the observations and findings of the study.

Tungsten carbide coatings are often applied to improve surface properties such as wear, high temperature degradation, and corrosion resistance. Zirconia coatings have also been used extensively in various industries due to their excellent tribological and insulation properties combined with high stiffness. It is speculated that adding zirconia to tungsten carbide may result in a coating with combination of excellent thermal and mechanical properties of constituents. In the current study, a powder mixture of 50 wt. % WC-Ni and 50 wt. % ZrO 2 -Y 2 O 3 deposited on a low carbon steel substrate using atmospheric plasma spray technique. The microstructural evolution of deposited sample was investigated. Splat boundaries, micro cracks, pore morphology conversion, and grain growth mechanism were elucidated comprehensively. Results indicated a good adhesion between two different major components. No porosity formed due to mismatch between zirconia and tungsten carbide. This study pays special attention to the dependency of the microstructural characteristics to the phase distribution within the coating.

The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y 2 O 3 ) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.

In earlier experiments, plasma-sprayed Al 2 O 3 coatings were deposited on preheated Al 2 O 3 substrates to study the effect of substrate temperature on splat formation and phase transformations. The aim of the present work is to develop a model to better understand the factors that affect phase selection during the solidification of Al 2 O 3 splats. A model based on one-dimensional heat transfer and classic nucleation theory is presented and used to simulate the rapid solidification process and the influence of process parameters on phase selection. The model accounts for under-cooling phenomena, heterogeneous nucleation, and nucleation kinetics. The findings indicate that the relationship between initial substrate temperature and phase selection is primarily based on the catalytic effect of the alumina substrate on the nucleation of Al 2 O 3 phases as a function of contact angle.

This study investigates the effects of spark plasma sintering (SPS) on the microstructure and properties of cold-sprayed metallic coatings. Water-atomized Cu powder was deposited on Al 5052 substrates by high-pressure cold gas spraying, and the resulting coatings were treated by spark plasma sintering and annealing heat treatment (AHT) at 200°C, 300°C, and 400 °C. To assess the effects of diffusion generated by pulsed dc power, a vertical load was not applied in the SPS system. In addition, a short duration time was used to inhibit crystal grain growth. Treated specimens were evaluated by SEM, EBSD, and hardness and tensile testing. The findings show that the microstructure and hardness of SPS specimens treated at 300 °C are close to that of AHT specimens treated at 400 °C. Tensile strength, however, is clearly higher in the SPS300 specimens, indicating that pulsed dc power accelerates particle interdiffusion due to Joule heating and electromigration, thereby increasing adhesion strength between particles in the coating.

This study investigates isothermal transformation kinetics in Ti-6Al-4V alloys structures produced by vacuum plasma spray forming. As-sprayed samples were homogenized in the β phase followed by fast cooling to the two-phase temperature region, then quenching to suppress further transformation. The microstructure of heat-treated specimens was examined by optical microscopy and equilibrium phases were measured using image analysis. The kinetics of the β → α+β phase transformation are revealed by plotting the amount of α-phase obtained over a 10 to 60 s interval at isothermal temperatures of 800, 850, 900, and 950 °C. Corresponding phase transformation rates are also calculated based on Johnson-Mehl-Avrami (JMA) theory. At temperatures below 900 °C, the main phase transformation mechanism is homogeneous nucleation and growth of α-phase. At higher temperatures, phase transformations are driven by two mechanisms: the formation of α-phase in grain boundaries and α-plate nucleation and growth.

Alumina splats were deposited on the polished single crystal alumina substrates with two different crystalline facet orientations of [001] and [110] by atmospheric plasma spraying (APS) at a substrate preheating temperature of 900°C to examine the epitaxy during splat cooling. The cross-sectional samples for high resolution transmission electron microscopy examination was prepared by focused ion beam assisted scanning electron microscopy (FIB-SEM). The results show that the whole splats with a thickness ranging from ~600 to ~1000nm exhibited the same crystalline structure as the substrate. Moreover, the splat deposited on the single crystalline alumina substrates exhibited exactly the same orientation as the substrate. The results evidently indicate that the epitaxial grain growth occurs after alumina droplets impact on single crystal alumina substrate. The present results suggest that the crystalline structure of alumina deposit formed by plasma spraying can be possibly controlled by the substrate preheating temperature.

Plasma Transferred Arc is the only thermal spray process that results in a metallurgical bond with the substrate. When compared with other welded coatings finer microstructures are observed after PTA processing, which have been associated with the faster solidification rate imposed by this technique. However, the powdered/atomized material used that forms the thermal spray, can play an important role in the solidification of coatings depending on their chemical composition and their grain size. This study analyzed the solidification of PTA coatings processed with an atomized cobalt based alloy (Stellite 6) with different average grain sizes. Typically, solidification of a welded coating follows solidification principles regarding the nucleation and growth of their microstructure determined mainly by the solidification rate. The role of the powdered feedstock in the solidification of coatings is analyzed based on the assumption that solidification is influenced by the initial interface energy of the atomized grains that melt in the plasma arc before reaching the melt pool. A commercial atomized Stellite 6 alloy was divided in two groups according to their grain size, below and over 125 microns, and deposited with the same processing parameters. Coatings were characterized by laser confocal microscopy and Vickers microhardness. Differences in coatings hardness and microstructure of coatings were associated with the grain size of the deposited alloy and subsequent solidification.

A simple and efficient numerical model describing the processes of nucleation, growth, and transport of multicomponent nanoparticles is developed. The approach is similar to the classical method of moments but can be applied to co-condensation of several substances. The processes of homogeneous nucleation, heterogeneous growth, and coagulations due to Brownian collisions are considered in combination with the convective and diffusive transport of particles and reacting gases within multidimensional geometries. The model is applied to the analysis of multi-component co-condensation of TaC nanoparticles within a DC plasma reactor.

The high cooling rate (>105 - 106 K/s) achieved in thermal spraying makes it possible to deposit coatings that exhibit a high content of amorphous phase and can be, afterwards, heated above the crystallization temperature to form nano-structured coatings. In this study, the relationship between the characteristics (temperature and velocity) of the sprayed particles at impact and degree of amorphization of the as-sprayed coatings was investigated for both plasma and wire-arc spraying techniques A Fe-Cr-based alloy that exhibits a high glass forming ability and is commercially available in the form of powder and wire with close chemical composition was used. The amorphous phase content was higher in plasma sprayed coatings than in wire arc sprayed ones and both types of coatings presented an increase in microhardness after heat treatment.

Stoichiometric cordierite (2MgO-2Al2O3-5SiO2) with additions of titania have been investigated for use as coatings on low thermal expansion refractory concretes. These concretes have coefficients of thermal expansion on the order of 2 ppm/°C. Titania additions of up to 8 mole percent were investigated and the effect of titania in the crystallization of cordierite was examined. Cordierite coatings were air plasma sprayed and both glass and crystalline coatings were produced. The crystalline structure of the coating was found to be dependent upon the preheat temperature of the substrate. Preheats greater than 700 °C produced a mixture of a quartz solid solution and indialite whereas glass coatings were produced at preheats less than 700 °C. Coefficients of thermal expansion for the cordierite materials were dependent upon titania addition and generally increased with addition of titania. In the glass state, the thermal expansion was modestly increased (4.6 to 4.9 ppm/°C) with titania additions, but the quartz and indialite forms of the cordierite increased from 1.2 to 4.7 ppm/°C as the titania addition increased from 0 to 8 mole percent.

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.

Layers of high-purity copper and iron produced by cold gas-dynamic spraying have been thermally processed to induce recrystallization and grain growth. In the case of copper deposits, the as-sprayed structure could be "pinned" by arrays of Cu2O particles present on the surfaces of the feedstock powder, however copper powders of higher purity and sphericity yielded sprayed structures which could be annealed to induce recrystallization and grain growth. The higher purity copper compacts exhibited a morphological change in fracture from a brittle, intraparticle mode in the as-deposited condition, to a ductile, "cup-and-cone" morphology in the annealed condition. For compacts produced from water atomized iron, annealing at sub-critical temperatures produced recrystallization and grain growth as found with copper, and thermal processing in the austenitic region resulted in altogether new and coarser grain structures upon cooling. Ease of thermal processing of cold-sprayed materials may offer additional processing routes for engineered surfaces and functional devices produced in this manner.

This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection of the crystalline phase. Calculations were performed for various substrate materials at different initial temperatures. Results are expressed in terms of nucleation temperature, nucleation rate, density and grain size distribution.

Spinel powders of different compositions were fabricated for their good properties of chemical resistance. These powders were plasma sprayed on steels and their microstructure was investigated by scanning electron microscopy (SEM), microanalysis, X-ray diffraction and transmission electron microscopy (TEM). Due to the powder fabrication process, coatings were very heterogeneous in composition, but had the spinel structure. TEM observations pointed out that splat solidification occurred with a cooling rate gradient leading to different crystallization inside a lamella. Young's moduli by the coatings were measured by the resonant frequency method and the correlation with coating microstructure was discussed.

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce Inconel 718 coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled Inconel 718 powders, as well as the HVOF Inconel 718 coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth as evidenced by a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good correspondence with the experimental results.