In order to broader thermal spraying applications and reduce constraints and time due to conventional surface preparation before thermal spraying, the PROTAL process was developed over the last decade. This process integrates laser ablation, using a Q-switched Nd-YAG laser to prepare the surface simultaneously to the coating build-up during Plasma or HVOF Spraying. The present paper aimed at evaluating the feasibility to use the Protal technology in conjunction with the Twin Wire Arc deposition (Arc spraying) onto the 7075 Aluminum based alloy and to define the optimal Protal process parameters. Coating adhesion, micro-gap at coating-substrate interface and the coating residual stresses are evaluated. The Protal parameters investigated are: the laser energy density onto the substrate and the coating layers, the number of laser passes and the time delay between the laser impact. This study results indicate that it is feasible to use the Protal technology in-situ with the Arc spray process in spite of the challenging overspray and the large plume size of the Arc process. Good adherence and absence of interface micro-gap is obtained with an appropriate range of laser energy density to ablate the substrate. The use of a low energy ablation prior each coating layers reduces coating tensile residual stresses and improves furthermore the bond strength. The elimination of the process ablation dust and overspray dust prior the coating deposition onto the plane substrate was found critical to obtain a good bond strength. This research results from collaboration between the LERMPS in France and National Research Council Canada. Abstract only; no full-text paper available.

The particle parameters including particle size, velocity and temperature influence significantly splat formation process in thermal spraying. The flattening degree of subsequent splat determines the coating structure and properties. Both theoretical analysis and simulation of splatting process indicate that the flattening degree depends on Reynolds number (Re) of spray particles. The experimental correlations suggest that the theoretical models overestimate the flattening degree. In the present study, with careful control of particle size and measurement of particle velocity and temperature, the relationship between the flattening degree and particle Reynolds number is examined experimentally. Copper powders of small size range are used to ensure valid of mean particle size. Plasma spraying is carried out under different conditions to change particle velocity and temperature. The particle velocity and temperature are measured using DPV- 2000. Splats were deposited on preheated polished stainless substrate surface. The diameter of individual splat was measured. The flattening degree was estimated using average diameter of splats and spray particles for individual spray condition. Using the exponential formula of Re with a power of 0.2, it was found that experimental correlation yielded a coefficient about half of that given by Madjeski’s model.

A thermal spray coating is formed through successive impact, flattening, rapid cooling and solidification processes of a stream of spray droplets. Splashing may occur during droplet flattening process. Recent studies suggested that splashing can be suppressed when a molten droplet impacts on a preheated flat substrate. In this study, the splatting behavior in plasma spray is examined using molten spray droplets of different Reynolds number. Splats are deposited on preheated flat stainless steel surface. The morphology of splats is examined using optical microscopy and scanning electron microscopy. To adjust Reynolds number of spray droplets, copper droplets are produced using both Ar-H 2 and Ar-He-H 2 plasma jets under different operating conditions. As a result, the Reynolds number of spray droplets have been varied from about 18,000 to 90,000. It has been found that Reynolds number will influence splashing phenomena during splatting and consequent splat morphology. At low Reynolds number, splats present a regular disc morphology. However, when Reynolds number was increased up to about 5x104, the severe splashing around periphery of splat droplet was clearly observed despite the preheating of substrate. Based on the morphology of splats, a model for the spreading of molten droplet is proposed to explain the effect of Reynolds number on the flattening behavior of molten spray droplet.

In this study, a neural network is used to model the complex relationships associated with dc plasma spraying. The paper describes how training and test data were experimentally obtained for alumina-titania particles processed under different conditions in order to assess various learning approaches and the predictive ability of the model. As reported, 80% of the test database was successfully recognized and the misclassifications are the result of not having enough data sets to adequately cover the wide range of values obtained during the experiments. This work considers in-flight particle characteristics as a first step of a more global approach that includes coating microstructure and mechanical properties as well. Paper includes a German-language abstract.

Recent studies have demonstrated that WC-12Co and WC- 10Co-4Cr coatings were the best performing HVOF coatings against erosion. This paper looks at the influences of the HVOF process parameters for WC-12Co and WC-10Co-4Cr materials on the erosion resistance of the coatings. The effect of powder morphology, matrix chemistry and HVOF process parameters with respect to both silica slurry erosion and alumina dry erosion has been studied. All coatings were produced using the HVOF JP-5000 system with kerosene-oxygen flame. The spraying parameters were analyzed in term of sprayed particle velocity and temperature as measured with the DFV2000 optical diagnostic system. Simultaneously with in-flight particle measurements, the substrate-coating temperature was monitored by infrared pyrometry during coating deposition. The resulting coating microstructure was evaluated in terms of microhardness, porosity type and extent of wear damage after dry and slurry erosion. The material volume loss under various erosion conditions was related to the coating properties and microstructure. According to the experimental results, the following conclusions are drawn: 1) the kerosene flow rate affects the inflight particle state (velocity and temperature) and the coating porosity. 2) Cobalt-chrome matrix cermet performs better in slurry erosion while denser and harder cobalt matrix cermet performs better in dry erosion. 3) The use of kerosene-rich flame with lower oxygen stoichiometry reduces the carbide degradation and optimizes the wear performance of WC-12Co coatings in both dry and slurry erosion.

In this paper, two long-term experiments are conducted in order to investigate the evolution of the arc root fluctuations and the evolution of the in-flight particle state during plasma spraying. Voltage as well as the acoustic fluctuations measured at three different angles are characterized while particle state was monitored using an optical integrated system, the DPV2000. A detailed study of the evolution of the gun power, in-flight particle state (temperature, velocity, diameter, particle flux) and coating microstructure is was carried out. Results showed that the microstructure of the deposited coating significantly changed during the forty-hour spraying period. Paper includes a German-language abstract.

Plasma sprayed MCrAlY bondcoats play a major role in thermal barrier coatings. During service, oxide forms on both sides of the bond coat and must be minimized to prevent coating failures. Along with powder chemistry, coating microstructure significantly influences oxide growth. It is known that both coating microstructure and coating strength are strongly related to plasma spraying parameters. This present work examines the effect of inflight particle properties on the adhesion strength and microstructure of NiCrAlY and NiCoCrAlY bondcoats. The relation between particle velocity and temperature and coating properties is particularly important. Relatively small changes in spray parameters such as arc current and gas flows can have a major impact on sprayed particles and consequently coating microstructure. Through online control of particle states it is expected that the quality of plasma-sprayed MCrAlY coatings can be significantly improved.

In plasma spraying temperature and velocity of the sprayed particles are among the most important parameters influencing the microstructure and properties of the deposited coatings. However, the sprayed particle state is influenced by uncontrollable parameters such as the wear state of the electrodes. In order to investigate the influence of the electrode wear state on sprayed particles, a long-term experiment was conducted during which on-line measurements of plasma sprayed yttria-zirconia powder were performed. Results show that even though input parameters were kept constant during the experiment the state of the sprayed particle changed significantly and coatings prepared at different spraying times have different microstructures and can have different properties. However, by changing some input spray parameters it was possible to retrieve the initial sprayed particle state and coating microstructure.

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed 2-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 microseconds after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 microseconds.

The correlation between particle temperature and velocity and the structure of plasma sprayed zirconia coatings is studied to determine which parameter most strongly influences the coating structure. The particle temperature and velocity are measured using an integrated optical monitoring system positioned normal to the spraying axis. The total porosity, angular crack distribution, crack size distribution and thermal diffusivity are correlated with the particle temperature and velocity. Results show that the temperature of the sprayed particles has a larger effect on the coating properties than the velocity in the conditions investigated.

Although very frequently used, traditional (low velocity) flame spraying is a much forgotten process. No major research has been performed during the last decade. This paper focuses on the problem of reproducibility of a typical flame gun used with modern automated process equipment. An on-line diagnostic process control tool measuring the temperature, velocity, size and position of the powder in the flame was applied during spraying of abradable coatings of NiCrAl/Bentonite, coatings which are commonly applied to fan and compressor housings of gas turbines. An automated closed loop flame spray unit with mass flowmeters for the oxygen and acetylene gases was used. Influence of different process parameters on the sprayed particles, such as nozzle design, gas flows, and powder feed rate is discussed. Coating properties, such as erosivity, porosity, microstructure and tensile strength, are evaluated. It is demonstrated that even fairly small process changes influence the flame sprayed particles as well as the properties of the resultant coating.