The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure properties of yttria-stabilized zirconia (YSZ) coatings was evaluated. In order to better understand the influence of the spray process on coating consistency, an YSZ powder, -125+44 µm, was sprayed with nitrogen/hydrogen parameters and a 9MB gun. DE and coating porosity produced using two different spray gun conditions yielding the same input power were compared. Amperage was allowed to vary between 500 A and 560 A and hydrogen was adjusted in order to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries, i.e., radial with and without a backward component, were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional photomicrographs. In-flight particle temperature and velocity measurements were also carried out with the DPV-2000 for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used in order to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain constant the in-flight particle temperature instead of keeping the input power constant by adjusting the secondary hydrogen flow rate for obtaining more consistent and reproducible DE and microstructures.

Two 7-8 wt% yttria-stabilized zirconia powders of similar size and chemistry but having different microstructure properties and manufacturing routes were studied. One significant difference was the density and internal porosity of the starting powders. Deposition efficiency (DE) of the low density (LD) powder was found to be higher and less sensitive to changes in the spray process parameters than the high density (HD) powder. Probing the in-flight particle characteristics with the DPV- 2000 made it possible to link the observed DE values with the in-flight particle temperature. For each powder, DE was found to depend mainly on a single variable, the in-flight particle temperature. DE was found to vary strongly with particle temperature for temperatures under 2700°C, whereas the dependence with particle temperature was much less important above 2700°C. Variations in DE seemed to evolve according to variations of the melted fraction of the sprayed material. Since the LD powder was found to achieve higher particle temperatures at given spray conditions, DE was found to be higher for the LD material and the range of variations in DE was found to be much less than that observed with the HD material. Examination of the coating microstructures revealed that a coating produced with the LD powder had slightly higher porosity than that produced with the HD powder at similar inflight parameters. Spraying at higher in-flight particle temperature or velocity, which resulted in higher and more robust DE values, tended to yield coatings with lower porosity, resulting in coating density exceeding the tolerance range specified by some end-users. Increasing the powder feed rate and using conditions that produced a higher in-flight particle temperature were found to increase the porosity up to an acceptable level without significantly degrading DE. Therefore, a solution was found that not only reduced the sensitivity of DE to changes in the spraying conditions but also increased the rate of production by reducing the time required to spray the part.

This paper investigates the influence of plasma variations on the microstructure and application rate of aluminum coatings and compares layers obtained using parameters that result in the same particle properties but different changes in stress. Layers produced in the less stable plasma state were found to be more porous and contained a significantly higher number of unfused particles than those produced under relatively stable conditions. These differences are due to the physical properties of the particle beam as revealed by laser illumination. Paper includes a German-language abstract.

Innovation and improvements are described which yield a 20 fold increase in the signal-to-noise level of a two fiber, twin wavelength high speed pyrometer used for in-flight particle diagnostics. Examples are given of how these developments extend the application range of the technology to low temperature processes such as flame spraying of low emissivity materials.

Thermal spraying of a nanograined WC-12Co cermet powder using high velocity oxy-fuel was employed to produce nanostructured coatings. The spray conditions were varied by employing a wide range of thermal spray parameter settings and using either hydrogen or propylene as fuel gas. By determining the characteristics of the spray jet for each set of spray conditions and by studying various aspects of the coatings, including the microstructure, properties, and performance in dry abrasion tests, conclusions were drawn regarding the effect of the spray parameters on the properties and performance. When comparing the effect of using hydrogen or propylene as fuel on in-flight particle characteristics, the results indicated that, for a given particle temperature, the particle velocity tended to be higher with hydrogen than propylene. As well, it was found that the coatings produced using hydrogen tended to have a higher microhardness and a lesser degree of carbide degradation. The resistance to wear in dry abrasion was significantly higher for coatings produced using hydrogen as fuel. For both series of coatings, it was found that the abrasion resistance increased with the particle temperature at the point of impact during thermal spraying and with the hardness of the coating. The abrasion resistance of coatings produced using propylene appeared to be much more sensitive to changes in hardness. For the thermal spray system studied in this work, the results indicate that nanostructured WC-12Co coatings having a maximum abrasion resistance are obtained by using hydrogen as fuel under conditions such that the particles achieve temperatures above approximately 1850-1900°C and a speed greater than 575-600 m/s.

The Accuraspray is a new in-flight particle sensor that provides information on the average in-flight particle temperature, using two-color pyrometry, and velocity, using a cross-correlation calculation. Various aspects influencing the reliability of the sensor estimates are studied. First, the sensitivity of the temperature and velocity estimates to the positioning of the sensor with respect to the particle jet, such as the angular orientation of the fibers and the working distance to the spray plume, is evaluated. Then, the influence of the plasma radiation on the temperature measurement is estimated. This influence can be reduced significantly by filtering out the low frequency components of the pyrometric signals, which contain most of the plasma fluctuations. Finally, a lower limit in the signal-to-noise ratio (SNR), for which an acceptable temperature estimate is obtained, is evaluated. A valid velocity estimate can still be obtained with a lower SNR. All these studies were performed under various spraying conditions, including plasma spraying and HVOF, using various feedstock materials (YSZ, Al-Si, cermets).

The influence of arc root fluctuations in DC plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involves two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations is performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, that can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Unlike what has been predicted by numerical modeling, time-dependent particle temperature and velocity due to power fluctuations induced by the arc movement can be very important. Periodic variations of the mean particle temperature and velocity, reaching ΔT = 600°C and Δv = 200m/s, are recorded during a voltage cycle. Moreover, very few particles are detected during some part of the cycle. The existence of quiet periods suggests that particles that are injected at some specific moments in the plasma are neither heated nor accelerated efficiently. To our knowledge, this is the first time large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated with quantitative measurements.