Particle melting is one of the key issues in air plasma spraying of high-temperature ceramics such as YSZ. This study aims to estimate the molten content of the spray stream from in-flight particle temperature measurements. Particle temperature distribution is delineated into particle states (unmolten, partially molten, completely molten) as a first approximation, which is then used to estimate the molten content in the spray stream. The estimated percentage of molten content is shown to correlate well with deposition efficiency measurements for a wide range of process conditions and feedstock characteristics. The use of this estimation technique for other materials and processes is also discussed.

The microstructure of thermally sprayed ceramic coatings is characterized by the existence of various pores and microcracks. The porous microstructure makes coating desirable for thermal insulation, but this unique microstructural feature also gives rise to anelastic response under tension and compression loads. Detail investigations of curvature measurements of ceramic coated substrate indicate the coatings to exhibit anelastic behavior composed of nonlinear and hysteresis characteristics. In this paper, the mechanisms of such behaviors were studied from curvature-temperature measurements and finite element analysis through modeling the microstructure of yttria stabilized zirconia (YSZ) coating. Computational models contain numerous randomly distributed pores and microcracks with various sizes, aspect ratios, locations and orientations. The effects of such attributes of pores and microcracks on coating anelastic behavior were studied by simulations of curvature change during thermal cycles.

Controlling particle state is important to not only achieve the required microstructure and properties in coatings but also to clearly isolate and understand the role of other clusters of variables (such as the various substrate and deposition conditions) on the aforementioned attributes. This is important to design coatings for high performance applications and in the ongoing efforts towards achieving prime reliance. This study examines the variabilities in particle state and explores a few strategies to control them for improved reproducibility with the aid of in-flight particle and plume sensors. The particle state can be controlled by controlling the torch parameters or by directly controlling the particle state itself via feedback from particle and plume sensors such as DPV2000 & TDS. There exist at least a few control protocols to control the particle state (predominantly temperature and velocity) with judicious choice of critical parameters. In the present case the particle state has been controlled by varying the critical torch parameters in a narrow range using 8% YSZ of angular morphology (fused and crushed) with 10-75 microns size distributions in conjunction with a N 2 -H 2 laminar (non-swirl) plasma. Two important results emerge. (1) The particle state resulting from averaged individual particle measurements (DPV 2000) is surprisingly stable with variabilities in T < 1% and variability in V of < 4%. Ensemble approaches yield a somewhat higher variability (5%). In spite of this the variability in basic coating attributes such as a thickness and weight is surprisingly large. (2) Applying a much simpler control strategy to only control the particle injection and hence the particle trajectory results in reduced variabilities in coating attributes.

Over the last decade there has been an explosion in terms of available tools for sensing the particle spray stream during thermal spray processes. This has led to considerable enhancement in our understanding of process reproducibility and process reliability. However, in spite of these advances, the linkage to coating properties has continued to be an enigma. This is partially due to the complex nature of the build-up process and the associated issues with measuring properties of these complex coatings. In this paper, we present an integrated strategy, one that combines process sensing, with process modeling and extracting coating properties in situ through the development of robust and advanced curvature based techniques. These techniques allow estimation of coating modulus, residual stress and non-linear response of thermal sprayed ceramic coatings all within minutes of the deposition process. Finally, the integrated strategy examines the role of process maps for control of the spray stream as well as design of thermal spray coatings. Examples of such studies for both MCrAlY and YSZ coatings will be presented.

It is known that particles injected in a plasma stream follow differing trajectories which in turn leads to different thermal and kinetic history dependent on the location of particle in the plume. The variation in particle characteristics (temperature and velocity) across the plume has been the focus of research over the years. The corresponding variation in impacting particles, particularly in terms of their splat characteristics have not been explored as systematically. This is important for a complete understanding of the coating build-up phenomena and the variations in coating properties. This paper presents the results of a study in which the spatial variation in particle properties is mapped to the spatial variation in splat properties. This has been accomplished using a procedure to collect splats using a shutter mechanism that allows us to expose the substrate for approximately 50 milliseconds. Splats of Alumina and a Ni- Cr-B-Si-Mo have been collected on polished substrates maintained at 250ºC and studied. Micrographs reveal differing splat morphologies across the spray plume – from missing-cores in one part to complete disc-shaped splats in the other. Extent of flattening and fragmentation have been quantified and found to vary within the ‘splat map’. Correlation between the location of particle in the plume and the resulting splat has been constructed using this data. Abstract only; no full-text paper available.