In this investigation, particle image velocimetry (PIV) diagnostics were employed to analyze the spray produced by a two-fluid atomizer as used in suspension plasma spraying (SPS). This led to a change in the design of the atomizing nozzle in order to achieve a high-speed spray with narrow distributions in droplet size. The resultant spray was characterized and the diagnostic was adapted accordingly. Various suspensions of YSZ powders were then injected into the plasma under different conditions and particle velocities were determined and correlated with the coating morphologies obtained.

This work investigates the effect of atmospheric plasma spraying (APS) parameters on in-flight particle properties, splat morphology, and coating microstructure for conventional and nano-size YSZ powders. Particle temperature and velocity were measured using a dual-slit velocimeter and individual splats and coating microstructures were examined in a scanning electron microscope. The results show that total porosity increases with decreasing arc current and increasing stand-off distance and that conventional powder coatings have higher total porosity at higher arc currents than coatings made from nanopowder. The effect of substrate temperature on splat formation was also assessed. Splat flattening and circularity increase with increasing substrate temperature, particularly for nanopowders.

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 peculiar microstructure of plasma-sprayed coatings affects their physical properties. Many of these properties are, to a great extent, dominated by the influence of the spraying process on in-flight particle parameters. In order to improve the quality of plasma-sprayed coatings and to enhance the efficiency of plasma spray deposition, a wide range of anode nozzles have been developed for air plasma torches. The objective of this work is to investigate the influence of two convergent-divergent nozzle designs on plasma-sprayed YSZ particles at impact and the permeability, porosity, and microstructure of the resulting coatings.

This paper discusses the different ways plume sensors can be used in thermal spraying and presents examples highlighting the use of a commercial plume sensor in actual production situations.

The aim of this work is to investigate the effect of substrate surface chemistry (e.g., oxidation and atom diffusion) on the flattening of a single millimeter-sized alumina drop. To that end, a new technique to produce such drops with different impact velocities has been developed. It consists of a rotating crucible heated by a transferred plasma arc and a piston that controls substrate velocity and, as a result, the impact velocity of the drop. A fast camera working in concert with a fast pyrometer precisely records drop flattening and cooling. This system makes it possible to study interface phenomena, such as desorption and wettability, as well as the effects, at impact, of the kinetic energy or Weber number of the flattening drop.

This paper introduces a gun monitoring device that measures thermal spray process variables directly inside the gun. It explains the advantages of measuring operating variables inside the gun and why it is important to have gun-relevant data stored within the gun itself.

In this study, computational methods are used to examine the in-flight dynamics of non-spherical WC-Co particles sprayed from HVOF guns and their impingement on substrates. Two sets of computational models are developed. First, the in-flight particles are simulated in a CFD-based combusting gas flow. Particle information prior to impact is extracted from the CFD results and fed into a FEA model to dynamically track the impingement of particles on the substrate. Particle morphology is examined for both spherical and non-spherical powders to establish critical particle impact parameters for bonding.

This work assesses the potential of using an ultrasonic probe attached to the back of the substrate to monitor the cold spraying process. While this is only a preliminary study, focusing more on presenting the results than analyzing them, a few conclusions may be drawn. With acoustic sensing, not only can the final value of thickness be estimated, it is also possible to see the dynamics of how the buildup takes place in real time. As shown in the data plots, the buildup process for aluminum-alumina composites is fairly universal across the spray with slower buildup at the outer edges of the coating. More importantly, it is shown that nozzle speed, spray diameter, and thickness estimates fit well with measured values.

Low pressure plasma spraying (LPPS) and LPPS-Thin Film (LPPS-TF) processes cover a broad operational pressure range from typically 200 mbar down to a few millibars, filling the gap between conventional thermal spray processes, where coatings are made from the liquid phase, and conventional thin film technologies such as PVD or CVD, where coatings are produced from precursors species in the vapor phase. Using some specific parameters of the LPPS-TF process, the injected material can be partially or even completely in gaseous phase, disqualifying diagnostics based on the detection of solid or liquid particles such as the DPV-2000 (Tecnar, St-Bruno, QC, CA). In this case, other optical diagnostic tools have to be used, such as optical emission spectroscopy (OES) to characterize the LPPS-TF process. In this paper, a qualitative study of the properties of the injected material in the plasma jet using DPV-2000 and optical emission spectroscopy is presented by varying specific plasma parameters. Moreover, in some particular cases, it is shown that the combination of DPV measurements and OES can help to monitor the coating process and to improve the basic understanding of the LPPSTF technology.

For a decade now, industrial sensors have been commercially available to both academia and industry. In general, these sensors measure individual and/or bulk properties of the powders being sprayed. Experience has shown that normally, researchers will tend to favor sensors with high spatial resolution like the DPV 2000, because of the fundamental information they give about the plume structure. Such information is vital for proper gun design and spray parameter optimization. However, for process monitoring applications typically performed with a sensor like the AccuraSpray, it is often more convenient to measure global properties over a wider volume inside the plume. In this case, there is always a tradeoff to be made between spatial resolution and fundamental process understanding. This paper illustrates this point by comparing two optical configurations, one with high spatial resolution and another one with medium resolution. This latter configuration makes use of a cylindrical lens to expand the sensor field of view in a direction perpendicular to the spray direction. Results clearly show that with minor optical modifications such sensors can be tailored to precise industrial requirements.

To use the manifold possibilities that the arc spraying offers to deposit carbide layers, the knowledge of the particle characteristics are necessary. This work is focused on understanding the particle formation during arc spraying with cored wires. The influence of primary and secondary atomization gas on the particle formation as well as on the particle shape, particle dimension and grain size are analyzed. Melting behaviors of different cored wires under various spraying parameters are investigated. Correlations between coating properties and particle characteristics are established. Experimental approaches to determine the in-flight properties of the sprayed particles in combination with metallographic analyses were employed to establish these correlations.

Graphitization behavior of water-atomized cast iron powder at each thermal spraying step, such as droplet flight, droplet impingement and splat layering, was successively examined. Both as-atomized cast iron powder and coatings sprayed with the powder contain no graphite structure owing to their rapid solidification. A short period of pre-annealing at 1173 K allows the formation of graphite structure in the cast iron powder, in which there exist precipitated graphite of 3.58 mass%. The microstructure observation exhibits that pre-existed pores in the as-atomized powder strongly affect the precipitating sites of graphite, that is, mainly inside the individual powder instead of the surface. However, marked reduction in graphite structure occurs to coatings sprayed with the pre-annealed powder because of in-flight burning and dissolution into molten iron. In-process post-annealing at 773 K for 60 s reveals the formation of graphite structure resulted from the decomposition of iron based metastable carbide in splats and coatings sprayed with the as-atomized powder. Chemical analysis demonstrates that graphitization level of post-annealed cast iron coatings is higher than that of coatings sprayed with the pre-annealed powder. Precipitated intersplat graphite structure of 1.68 mass% appears in cast iron coatings when introducing methane as a powder feeding carrier gas which is liable to decompose in plasma flame. The resultant coatings with graphite structure embedded in hard matrix are anticipated to offer superior wear resistance in comparison to centrifugally cast iron containing flaky graphite of 1.76 mass%.

Wear at the electrode surfaces of a one-cathode plasma torch changes the characteristic fluctuation pattern of the plasma jet. This affects the trajectory of the particles injected into the plasma jet in a non-controllable way, which degrades the reproducibility of the process. Time-based voltage measurements and Fourier analysis were carried out on a one-cathode F4 torch at different wear conditions to determine the evolution of wear-dependent characteristics. A significant correlation is observed between increasing torch wear and decreasing voltage roughness and high frequency noise. Furthermore, by means of particle diagnostic systems, the change in the particle velocity and temperature has been measured. The variations of the particle characteristics are significant and thus an influence on the sprayed coating microstructure is to be expected.

After having established an offline process control based on the optical process diagnostic system PFI (Particle Flux Imaging) the process robustness has been increased by experimental identification of the impact of the noise factors „electrode wear“ and „injector wear“ in the APS process. Neural networks were used for implementing a process controller into a GTV APS process control center. In combination with the PFI system a tool was installed in the process center that is able to predict coating quality by analyzing some characteristics of the plasma and the particle plume. The neural network can be trained for all applications and all feedstock materials. An offline controller is instructing the operator through a desktop in order to train the network. The training, that means the monitoring of the process through different parameter setups and its reactions, is generated and executed automatically. Due to the fact that controlling the process parameters cannot influence every aspect of the coating quality, noise factors have to be regarded. For the APS process the electrode wear and the injector wear were identified as the most influencing noise factors. Both were analyzed by means of Design of Experiment (DOE) and long-term monitoring (200 hrs). The samples were characterized by light microscopy and different coating test methods that were chosen with respect to the coatings functions (e.g. wear resistance). The result of this work is a set of parameters that are as robust against both noise factors as possible and that are adapted to certain changes by a neural network process control.

Numerous efforts have been carried out over the years to improve process control in the thermal spray industry. Among those, the need for online monitoring of the thickness of the coating is unquestionable as it leads directly to better uniformity, less powder usage and increased productivity. In 2006, a new technology based on 3D profiling has been adapted to the thermal spraying environment and has shown conclusive results for online real-time monitoring of the thickness of single layers as they were being sprayed. A micron level resolution was achieved independently of the process, the powder and the thermal effects related to thermal spraying. In this paper, we present the latest results from early industrial implementations. New applications of this technology for substrate geometries with increased complexity are also investigated.

Impressive gains have been made in thermal spray coatings through material development and coating parameter selection. However, it is usually up to shop personnel to develop a robot program to achieve the final coating. Coating programs are often developed using trial and error. Some motion and deposit analysis programs are available. However, they can be costly and require a level of expertise that is generally not available to the typical coating shop. This paper discusses how Microsoft Excel 2003 software can be used as an aid for off-line development of a robot program for coating applications. It starts with a discussion of the Normal Distribution characteristic of thermal spray patterns. It then goes through development of an example using standard Excel function and chart tools for interactive feedback of the coating layers. Also included is a discussion on the use of the program for sensitivity analysis regarding changes to the spray pattern.

Coating damages, such as cracks and spallation, always occur under high temperature gradient and its resulting residual stress level. This is why it is important and necessary to monitor and control the coating temperature during atmospheric plasma spray process. In this paper, a novel measurement approach for substrate and coating temperatures is presented. It is based on IR pyrometery combined to specific robot scanning trajectories in order to avoid harsh environment of spray workshops. The temperature-time evolution was continually detected and recorded during preheating, spraying and cooling stages and the two specific factors, periodic average temperature and standard deviation were adopted to evaluate the temperature variation and the fluctuation of the thermal cycle relevant to one robot scanning cycle based on the statistical method. These two factors were successful in describing the temperature variation during experimental processing sets. Finally, on the basis of experimental results of the influence of Z typed robot spray trajectory parameters, including spray distance, spray velocity and scanning step on coating temperature characterized by the two factors, an intelligent adaptive control of sprayed coating temperature was developed to obtain constant temperature cycles by means of adjusting robot trajectories, cooling, among other considered operating parameters. Excellent control performance is observed.