A shrouded-plasma spray process is described for the production of a metastable powder, deposit or preform. A high enthalpy atmospheric DC arc-plasma torch is used as heat source and a solution precursor as feed material. An aerosol-or liquid-jet of solution precursor is delivered to a steady-state reaction zone within the shrouded-plasma flame, where rapid and controlled precursor decomposition occurs. Depending on the operating conditions, the precursor material is pyrolyzed, melted or vaporized, prior to quenching to form a metastable nano-sized powder. This method is capable of processing a host of metastable materials, including the difficult-to-process refractory metals, oxide and non-oxide ceramics, as well as their composites. In this paper, we will discuss our results on multi-component oxide systems.

Cold Gas Dynamic Spray (CGDS), is a relatively new coating process in which small powder particles are accelerated to form coatings. Ductile metal powders are injected into a supersonic jet to achieve speeds of up-to approximately 1000m/s. Upon impact the powder particles dissipate their kinetic energy and form a coating without melting of the powder and unwanted heating of the substrate. In this process high deposition rates, at atmospheric conditions, of a wide range of ductile powders without phase change, grain growth or high residual stresses is possible. CGDS is ideal for production of coatings with bioactive materials, where it is desirable to maintain the chemical integrity of the powder in the coating. A twin powder feeder, that can feed two different materials in a cold gas dynamic spray system has been developed. The layered coatings of copper and aluminum are produced by this method are described.

Several compositions of agglomerated ZrO 2 (Y 2 O 3 ), Al 2 O 3 and MgAl 2 O 4 powders were plasma sprayed through an axial injection torch into a water quenching chamber in order to obtain a fully melted and homogenized, metastable particle nanostructure. This metastable extended solid solution morphology allows the powder to be subsequently heat treated and superplastically formed into ceramic parts with potentially excellent optical and mechanical properties. Light microscopy, SEM and XRD analyses were used to evaluate the melt-quenched powder properties, and some comparison was made to previous work performed with traditional radial injection plasma torches. Thick build-ups were also sprayed from the melt-quenched powders to test their ability to superplastically flow. Results showed that during a single processing through the torch the axial injection equipment produced powders with a significantly higher fraction of fully homogenized powders than previously attainable. Homogenization of powders comprised of a larger particle size range was also achieved.

A four inch external diameter copper reaction synthesis tube for attachment to an air plasma spray gun has been designed and constructed. This tube enables the entire hot plasma zone to be enclosed by flowing gas. The tube is water cooled, and enables control of the reaction/melting conditions for the materials that are injected into the plasma as dry powder, slurry or solutions. A supersonic nozzle is also incorporated into the design to promote powder production. Provision for capturing the reaction products in water is made and separation gives the final ceramic powders.

A method is described wherein a ceramic oxide nanocomposite coating has been produced via a co-precipitation route. The first step in the process consists of forming a solid solution by use of plasma melting, homogenization, and rapid quenching of two ceramics that are normally immiscible. In the best case, the resulting structure is a true solid solution in a new, metastable crystalline structure. The quenched particles can be deposited as a coating or sprayed into water or onto a chill block to form a powder. When the material is sintered via a pressureless process, such as that for a coating, the phase decomposition proceeds quickly, resulting in a homogeneously distributed two-phase structure of micron sized particles. However, when the powders are compacted and sintered at very high pressures (1-8 GPa) and low temperatures (T ~ 0.3 - 0.5 Tm), the increased nucleation of the precipitates results in a composite in which both phases have grain sizes less than 25nm. This paper will review the underlying phenomenon behind the method and the resulting kinetics.

This paper compares the wear properties of HVOF-sprayed WC/12Co hardcoatings produced from different powder feedstock materials, including conventional, nanophase, and mixed powders. The mixed or multimodal feed powder is designed to minimize the amount of material that goes through a high temperature cycle during spraying, thus potentially limiting the extent of decarburization in the resulting coating. As will be shown, decarburization is indeed minimal in a multimodal coating, which translates into exceptional resistance to abrasive and sliding wear. Another favorable factor is the ability to increase the volume fraction of hard WC phase in such a coating, thereby further enhancing its wear resistance.