A noncryogenic milling process was recently developed to produce equiaxed nanostructured NiCrAlY powders. In this study, the powders are used to deposit metal bond coats, with and without thermal barrier topcoats, via HVOF and low-pressure plasma spraying. TBCs with bond coats derived from non-cryogenically milled nanopowder show a reduction in porosity and TGO growth rate, delayed formation of mixed oxides, and a 50% increase in cycles to failure during thermal cycle testing. The presence of very fine alumina in the powder and bond coats plays a significant role in these improvements.

Gas atomized feedstock particles of an Al-13Co-26Ce alloy system were sprayed using the Cold Spray deposition technique. The microstructures of the coatings produced are examined and the mechanical characteristics, in particular the bending fatigue and the bond strength, of the Al-Co-Ce coatings are reported. The results show that the Al-Co-Ce coating improved the fatigue behavior of AA 2024-T3 specimens when compared to uncoated and Al clad specimens. During the bond strength tests, the bonding agent failed and no delamination of the coating from the substrate occurred. The microstructural features of the feedstock powder were also found in the coatings. The coatings contained amorphous and crystalline phase contents similar to the ones found in the feedstock powder. It is suggested that the increase in the fatigue properties can be attributed to the residual compressive stresses induced in the coatings and to the high adhesion strength of the coatings to the substrates.

Thermal barrier coatings (TBCs) with nanostructured bond coats have shown significant thermal cycling enhancements over their conventional microstructured counterparts; however, the high cost inherent to the cryomilling processing of the MCrAlY powder limits commercial application. Hence, this study characterizes and evaluates nanostructured bond coats derived from non-cryogenically milled MCrAlY powder with emphasis placed on reduced processing cost and scale-up capability. After extensive development of both a high-energy planetary mill and its operating parameters, fine-grained equiaxed NiCrAlY powder has been produced. XRD and SEM characterization of the milled powder will be presented. Microstructural analyses of the coatings sprayed via the HVOF and cold spray processes will also be carried out, in addition to some preliminary static oxidation test results of the conventional and milled NiCrAlY.

Nickel based alloys used in coating applications have been the focus of many studies, particularly in the aerospace industry. Their inherent corrosion and oxidation resistant properties have made them especially attractive for use as the metallic bond coat found in thermal barrier coating systems. Cold Spray is an emerging coating technology in which fine powder particles are accelerated in a supersonic flow and then deposited onto a substrate by means of plastic deformation. In this study, conventional CoNiCrAlY coatings and nanocrystalline nickel coatings are produced using the Cold Spray deposition technique. The coating quality is evaluated using scanning electron microscopy as well as porosity and microhardness measurements.

Since its inception, the Office of Naval Research (ONR) program entitled, “Thermal Spray Processing of Nanostructured Coatings”, has produced numerous successes relating to military and industrial applications. Commercially available thermal spray processes have been used to apply ceramic coatings from nanostructured powders, producing dramatically improved wear performance over coatings from conventional non-nanostructured powder. The nanostructured alumina-titania coating has been instrumental in extending the life of a critical Navy need, the protection of the main propulsion shaft of mine countermeasures ships. The advancement in nanostructured oxide coatings has led to the development of the first industrial application of a thermal sprayed nanostructured coating. A nanostructured titanium oxide coating has been developed and successfully incorporated into severe-service ball valves used in hydrometallurgical extraction of gold, nickel, and cobalt. This paper will also present promising results from current development work relating to nanostructured coatings for thermal barrier, biomaterial, and reconditioning applications.

Thermal cycle lifetimes of two thermal barrier coating (TBC) systems with the same plasma sprayed yttria-stabilized- zirconia (YSZ) topcoat but different low pressure plasma sprayed (LPPS) bond coats, conventional and cryomilled NiCrAlY feedstock powder, were studied. Thermal cycling tests consisted of 50 min at 1121C followed by 10 min air-cooling to room temperature. The coating produced with the cryomilled powder showed a 300% increase in lifetime when compared to the conventional one. Both TBCs failed as a result of delamination and spallation of the ceramic top coats. Several factors like thermally grown oxide (TGO) thickness, TGO composition, CTE mismatch, creep resistance of the NiCrAlY bond coat, and others that affected the thermal cycling life of the system, were analyzed in this work. Abstract only; no full-text paper available.

A mathematical model was developed and used to design and optimize a supersonic nozzle for a cold spray system. The objective was to spray 20 micron-size aluminum particles. Conventional and agglomerated nanostructured powders were used and successfully sprayed using a radial injection port. The microstructure of the coatings revealed that the nanocrystalline structure is preserved. An increase of 100% of the coating hardness was found for nanostructured coatings compared to conventional coatings. Further work needs to be done to improve the porosity of the coatings by changing some of the process parameter.

The growing need for new materials and material combinations with superior properties for severe service applications has led to the development of near net-shape forming techniques for certain materials, such as superalloys, refractory metals (Ta, W, and Mo) and highly reactive metals (Ti and its alloys). Vacuum plasma spray (VPS) was used to produce dense Ti-6Al-4V deposits for mechanical properties evaluation. Spherical Ti-6Al-4V powder, produced by Plasma Atomization (PA), a novel patented powder fabrication technique, was used as the starting powder. Plasma atomized Ti-6Al-4V powder characteristics include: high purity, tight particle size range, highly spherical with no attached satellites, and excellent flowability. The resulting as-sprayed Ti-6Al-4V deposits were dense and low in oxygen content. Thermal treatment was conducted after spraying in order to improve the structure and the properties of the spray formed material. The mechanical properties of the material, including tensile strength, elongation and hardness, in both the as-sprayed and the heat treated conditions were compared. The mechanical properties of these preliminary VPS Ti-6Al-4V specimens indicate that the combination of high purity starting powder and controlled environment deposition can be used to produce dense spray formed Ti-6Al-4V structures with properties comparable to those of cast or sintered powder metallurgy parts.

In order to properly characterize the entire deposition process, evaluation of the coating, including a reliable metallographic preparation technique which reveals the true microstructure, must be performed. Often, recommended metallographic sample preparation methods for thermally sprayed coatings are generic and are not tailored to specific materials. They are time-consuming and, in some cases, may provide inaccurate details (pull-outs, smearing, etc). This could lead to a wrong interpretation of the coating quality. The aim of the investigation was to develop new metallographic sample preparation procedures tailored to different types of coatings (metallic, ceramic, multilayer and composites), in order to reveal a more representative microstructure. A comparative study of different preparation procedures for the examination of various as-sprayed coatings is presented using an optical microscope. The coatings were deposited by atmospheric and vacuum plasma spray (APS and VPS) and high velocity oxygen fuel (HVOF) processes. A separate approach is recommended for choosing the right metallographic preparation procedure for ceramic, metallic, or composite coatings. Applied load and positioning of the mounted sample during preparation are identified as key factors in developing proper procedures. The microhardness of the coating must be considered when determining the applied load. Interesting practical trends in preparation procedures that may lead to superior coating representation and, in some instances, cost and time savings are presented.

Pressure acid leaching (PAL) of lateritic nickel ores requires the use of extremely severe processing conditions (250 °C, > 4000 kPa, 98 % H2SO4). In addition to the severe corrosive nature of the acid solution, up to 30% of abrasive solids are present in the slurry. PyroGenesis Inc. has applied its expertise in materials science and thermal spray technologies into developing and commercially applying coatings for the protection of ball valve components used in PAL autoclaves. Vacuum plasma spray (VPS) and atmospheric plasma spray (APS) processes are used to apply coatings of metals and ceramics for corrosion and wear resistance, respectively. A comparative study on the microstructure and mechanical properties of different coatings, applied with the two processes, will be presented. Although APS coatings provide enhanced abrasive resistance, VPS coatings have shown the potential for superior properties. The extreme temperatures and pressures associated with the actual PAL conditions are too severe to simulate in laboratory conditions, hence, corrosion testing was not possible. Microstructural analysis, microhardness, adhesion, and abrasion testing were determined for each coating/processing combination.

Many of the recent improvements in gas turbine engines have been attributed to the introduction of thermal barrier coatings (TBC) for superalloy components. There exists, however, some limitations in current fabrication methods for closed hot-section components: less than ideal coating quality, the need for welding, and limited choice of superalloy material. This paper describes a vacuum plasma near-net-shape process that overcomes these limitations. The process is used to fabricate closed components from yttria-stabilized-zirconia with a CoNiCrAlY bond coat and IN-738LC outer layer. The results from the study show that it is possible to produce near-net-shape superalloy parts with good coating properties and the absence of welds. The mold was reusable after minor reconditioning and the coatings were uniform in thickness and microstructure with a smooth surface finish. The bond coat and structural superalloy layers were very dense with no signs of oxidation at the interface. After heat treatment, the mechanical properties of the IN-738LC compare favorably to cast materials.