No abstract

The structure and properties of thermally sprayed coatings depend directly on the state of the particles before impact on the substrate or the already deposited coating layers. Monitoring particle parameters can thus provide a unique tool for optimising coating properties as well as controlling spraying processes during coating deposition. In this paper, a new optical sensing device for on line monitoring of temperature, velocity, diameter, and trajectory of in flight particles during coating production is described. Thermal radiation emitted by in flight particles is collected by a sensing head that can be attached to the gun providing continuous monitoring of the spray process. The collected radiation is transmitted through optical fibres to a detection cabinet located away from the dusty environment around the operating spray gun. Tests were carried out on two identical plasma torches with significant differences in deposition efficiency. Measured particle flowrates were in agreement with efficiency differences.

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).

Suspension plasma spray (SPS) is going through a transition phase from research and development to daily use on production lines. Improving repeatability and reproducibility of coating elements and parameters makes SPS a replacement of former well-developed processes. This transition can be achieved by using in-flight particles diagnostic systems to monitor and control key parameters that influence the coating microstructure. Temperature and velocity of the in-flight particles are among the most critical parameters that should be monitored. However, accurately characterizing the in-flight particles in SPS is particularly challenging due to the small particle size of coating materials, harsh spray conditions, considerably shorter spray distances compared to APS, possible interference from the solvent, and limitations of previous measurement systems. In this study, different strategies were investigated to improve the accuracy of temperature measurements of in-flight particles in SPS. For this purpose, two light collection configurations (double-point and single-point measurement) were investigated along with the influence of plasma radiation. The results were evaluated by collecting and studying splats. The size and shape of splats were correlated with the temperature of in-flight particles in order to confirm the accuracy of the sensor's temperature measurements. In addition, the sensitivity of temperature measurements to the optical filter used for two-color pyrometry, reflection of plasma radiation from surrounding objects, and direct radiation from plasma were investigated. The results showed that the single-point measurement configuration was well adapted for SPS.

We report a new measurement approach to recover temporally overlapping incident and reflected signals near a discontinuity using time-resolved electrooptic sampling. The technique involves measurement at two closely spaced locations and enables decomposition of the measured waveforms into components propagating toward and away from a discontinuity. We show experimental results for a simple coplanar structure.