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.

Thermal spray coatings exhibit a wide variety of microstructural characteristics that lead to variation in their functional properties. A complete understanding of the plasma spray process includes examination of the particle-flame interaction, particle impact (to form the splats), and the particle-substrate interaction during coating deposition. The links between these process parameters and coating properties has been established by using diagnostic tools in conjunction with a splat collection shutter and an in-situ curvature measurement instrument. In this study, a commercial grade molybdenum (Mo) powder was plasma sprayed; the spray stream was characterized in relation to the resulting particle state. A "splat map" was deposited through a "spray stream guillotine" to capture the fingerprint of the plume cross section. Subsequently, coatings were deposited at these spray conditions on a newly developed in-situ curvature measurement instrument to measure coating stresses and to estimate the coating modulus. Splats and coatings were subsequently characterized by micro-diffraction (for splat residual stresses), by nano and micro-indentation for elastic and elastic-plastic properties, and by electron microscopy. This complete history of the process followed by splat and coating characterization provides insight into the correlation between processing parameters, resultant particle states, and final coating properties. The role of particle temperature and velocity on the splat (and coating) morphology and residual stress is explained in the results.

A wide range of manufacturing processes are used to supply yttria stabilized zirconia powders for plasma sprayed TBC applications. From previous studies it is known that the difference in coating properties can potentially result from variations in powder feedstock as a consequence of particle inflight behavior and particle impact. An additional strong contribution to splat variation results from changes in the particle in flight behavior. In order to understand the variation in particle condition as a consequence of different powder morphologies, a detailed diagnostic analysis was carried out for plasma densified (PD), fused and crushed (FC) and agglomerated & sintered (A&S) powders. In this study a “3D multiple sensors” based integrated approach was used to evaluate these differences. Direct feed back sensors were used for optimization and combined with sophisticated diagnostics for in-depth studies. To obtain comparable results, three batches of commercial powders were sized to the same specification. For a given set of spraying parameters the recorded spray stream characteristics such as plume position, particle temperature, size and velocity deviated strongly for the given morphologies. By optimizing injection, the different powders can be made to follow nominally similar trajectories. This study reveals the sensitivity of each powder to process parameters and the variability in particle state that can result from it. Some techniques are suggested to optimally inject the different powders and to achieve similar particle states for these morphologies

The melting behavior of in-flight particle and its impact on splat morphology are studied. A group parameter, “melting index”, has been derived to correlate the melting status of inflight particles with particle size, velocity, and temperature which can be measured experimentally. Numerical simulations have been used to determine the unknown parameters in the melting index. The effect of particle size on its melting behavior has been investigated.

Molybdenum powder has been plasma sprayed on stainless steel, brass and aluminum substrates. The substrate melting phenomenon is observed and investigated by means of scanning electron microscopy (SEM) and scanning white light interferometery (SWLI). It is found that the flower-shape splat morphology is typical for molybdenum on all three substrate materials when the substrate is at room temperature. Notable substrate melting is manifested through the energy dispersion analysis of X-ray (EDAX) map and Robinson back-scattered image of cross-sections of splats. It has been shown that the substrate material plays an important role in substrate melting phenomenon. The lift angle of the petals of splats and the maximum crater depth have been characterized and compared. Both of these increase in the sequence, from stainless steel, brass to aluminum. A ‘volume of fluid’ (VOF) based model coupled with rapid solidification has been used to simulate splat deformation, solidification, substrate melting and resolidification. The numerical & analytical results agree quite well with the experimental data. A substrate melting mechanism is proposed based on the time scales of the droplet solidification and substrate melting to explain the formation of flower like splat morphologies.

HVOF-sprayed alumina appears to be well suited for applications in semiconductor devices. This paper investigates the influence of HVOF spraying parameters on the electrical properties of alumina layers. Diagnostic tests show that small changes in gas ratios and flow rates can significantly alter particle and splat characteristics as well as the dielectric breakdown strength of the coatings. A large number of parameters are changed in order to assess the extent to which electrical properties can be controlled. Paper includes a German-language abstract.

The current study was undertaken as part of an ongoing effort at CTSR towards development of 'Process Maps' for thermal spray coatings of molybdenum. Extensive study of the spray stream of molybdenum powders in a plasma jet was carried out. The study was undertaken for three distinct nozzle (anode) geometries and different spray conditions for each nozzle. The results demonstrate that the particle velocities and temperatures vary in an interdependent manner over a narrow range for any single nozzle. When nozzles with different geometeries are used the velocity and temperature range can be extended considerably. By estimating the residence time of particles in the plasma jet and defining a 'melting index' for the particles, a cross comparison of nozzles provides insight into the particle state achieved in each case.