This paper describes the basic design and operation of a low-pressure plasma spraying (LPPS) system in use at Sandia National Laboratories. To demonstrate the versatility of the system, Sandia engineers, working in collaboration with the New Mexico Institute of Mining and Technology, produced thin (< 100 μm), dense yttria-stabilized zirconia coatings using three deposition mechanisms: liquid droplet, vapor, and mixed mode (vapor and droplet). Despite slight differences in equipment configuration, the work duplicates many of the results obtained in previous investigations, confirming the advantages of LPPS over other thin film deposition techniques.

An earlier study reported an investigation of the mechanical properties of cold sprayed aluminum and the effect of annealing on those properties. In that study, cold spray coatings approximately one centimeter thick were prepared using three different feedstock powders: Valimet H-10, Valimet H-20, and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings. Each material was tested in two conditions: as-sprayed and after a 300°C, 22 hour air anneal. The as-sprayed material showed a high ultimate strength and low ductility, < 1% elongation. The annealed samples showed a reduction in the ultimate strength but a dramatic increase in ductility, up to 10% elongation. Microstructural examinations and fractography clearly showed a change in the fracture mechanism between the as-sprayed and annealed material, but insufficient data was available to conclusively explain the ductility increase at that time. Since then, Kikuchi mapping of the Valimet H-10 material in the as-sprayed and annealed conditions has been conducted. Kikuchi mapping allows indexing of grains, identification of grain boundaries, and phase identification using backscattered diffraction patterns in an SEM. The data shows that significant recrystallization within the splats upon annealing has occurred. No significant crystal growth across splat boundaries is observed. The data demonstrate that the mechanism of ductility increase in annealed cold spray deposits is recrystallization of the base aluminum material.

Use of graded coatings is a well-known strategy for creating materials with continuously changing physical properties. The stiffness (modulus of elasticity) and density of flyer plates used in light gas gun testing directly influences the shape of the shock wave produced by the flyer plate. Many strategies exist for creating flyer plates that produce variable shock profiles, including stacked foils and powder compaction. We have investigated graded thermal spray coatings as an alternative method for creating flyer plates that produce variable shock profiles. An initial proof of concept demonstration has been completed by air plasma spraying a graded coating of Cu & Al onto a copper substrate. This composite flyer plate was tested in a light gas gun to demonstrate that a non-linear shock profile can be created. The plasma spray strategies used to create a group of similar graded density impactors are discussed. Initial light gas gun testing shows that graded density impactors can be created using thermal spray coatings.

The effect of process conditions on flame spraying of titania (TiO 2 ) and magnetite (Fe 3 O 4 ) was investigated. Designed experiments were conducted to determine spraying conditions, specifically total combustible gas flow, stand off distance, and oxygen/acetylene ratio that produce high deposition efficiency (DE) and dense coatings. Along with DE, particle temperature and velocity were determined and correlated with process conditions. Results indicate that for both titania and magnetite, hot and high velocity molten particles result in higher DE and lower porosity coatings. Micrographs of coating cross-sections and surfaces were taken with both field emission scanning electron microscope (FESEM) and optical microscope. X-ray diffraction analysis shows that the titania coating retained its rutile structure while the magnetite coating had small amounts of magnetite (γ-Fe 2 O 3 ) in addition to magnetite.

The morphology and chemistry of low pressure plasma sprayed (LPPS) yttria coatings was investigated. White yttria powder used as the coating source material was 99.99% pure and contained particles from 1 to 5 microns in size. This yttria powder was plasma sprayed onto 8.9 cm diameter, 0.635 cm thick aluminum substrates using Ar-He as the arc gas. The powder gas was varied from Ar, to air to O 2 between samples. The sample sprayed using Ar as the powder gas used a chamber pressure of 2.5 kPa and resulted in a very dark colored coating. Subsequent samples were sprayed using air and then oxygen as the powder gas, both using a chamber pressure of 4.0 kPa. The resulting coating colors were medium gray and very light gray, respectively. A duplicate spraying using oxygen as the powder gas was performed to demonstrate repeatability. Porosity, splat shape, and the presence of amorphous phases were determined using secondary and backscattered electron microscopy imaging and related to processing conditions. Energy dispersive spectrometry was used to examine the chemistry of the coating and substrate. X-ray photoelectron spectroscopy (XPS) was used to determine the oxide stoichiometry of each of the coatings and this was correlated to coating color.

The effect of torch hardware, operating parameters, and powder type on substrate surface heat flux was quantitatively investigated using calorimeters. The Sulzer-Metco 6P oxyacetylene torch with two nozzles and two air caps and the Alamo PG-550 torch were studied using designed experiments to show the effects of total combustible gas flow, oxy-fuel ratio, air flow, and standoff distance on surface heat flux. Air caps which directed cooling air toward the flame produced lower heat flux than air caps providing gun cooling. For the 6P torch, nozzle geometry did not have a significant effect on heat flux. With low air flow rates, both torches exhibited similar heat fluxes. At high air flows, the surface heat flux of the PG-550 was larger than that of the 6P.

The effect of hardware on operating parameters and the resultant coating are qualitatively known; however, the quantitative effects have not been well defined. This study quantitatively characterizes particle temperature and velocity for the Sulzer-Metco 6P oxy-acetylene torch with 3 different nozzles and 3 air caps and also, the Alamo PG-550 then relates those data to particle diagnostics, deposition efficiency and coating microstructure. Both torches were evaluated using statistically designed experiments where the process inputs of oxy-fuel ratio, total combustible gas flow, and standoff distance were varied. Both torches can access similar regions of particle temperature - particle velocity space. Increasing total combustible gas flow increased particle velocity with little effect on particle temperature. Increasing oxy-fuel ratio decreased particle temperature with little effect on particle velocity. Higher particle velocity and particle temperature conditions yielded denser, less porous coatings. Flame cooling air caps increase the particle speed while decreasing particle temperature. Nozzles which inject powder directly into the flame jets significantly increase particle temperature as compared to nozzles which do not. Deposition efficiency is shown to not only be affected by particle temperature and particle velocity where hotter and faster usually increase efficiency, but is also dependent on the distribution of particles within the plume.

Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. We have investigated the mechanical properties of cold sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately one centimeter thick were prepared using three different feedstock powders: Valimet H-10, Valimet H-20, and Broadman Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed and after a 300°C, 22 h air anneal. The as-sprayed material showed high ultimate strength and low ductility, < 1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, up to 10% elongation. The annealed samples exhibited mechanical properties similar to wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed material. These results indicate good potential for cold spray as a bulk forming process.