No abstract

This study compares the performance of different measurement methods for characterizing particles during thermal spraying. The accuracy of double-point and single-point measurements is assessed in the context of HVOF and suspension plasma spraying (SPS) where in-flight particle temperature and velocity are recorded for different powders and suspensions. The results are evaluated by analyzing splats and correlating their size and shape to in-flight particle temperature measurements. It is shown that particle diagnostic systems based on single-point measurements are well suited for SPS and HVOF processes.

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.

A spray of suspension, forms mainly solid particles in a liquid phase from the atomization of two or multiphase flow, mainly solid particles in a liquid phase, and its transport phenomena by a gaseous crossflow have many natural and industrial applications. For example, injection of suspension jet in a high-speed flow is used in the emerging surface engineering process called suspension plasma spray. Typically, submicron ceramic oxide particles are mixed with water or ethanol to form a suspension that is injected in a plasma plume using different types of injectors. Injection parameters such as the type of injector and momentum flux influence the size, velocity, and trajectory of suspension droplets in the plasma and the microstructure of the deposited coatings. Using an effervescent atomizer, due to its capability in transporting flows with various rheological properties is promising for injection of suspension into the gaseous crossflow. In this study, an effervescent atomizer was employed to introduce a suspension radially into the flow of gas at room temperature. The spray of suspensions with different concentrations of glass particles in water was investigated in the crossflow by phase Doppler particle analyzer. The results were validated and supported by studying the spray by shadowgraph and light diffraction techniques. The results of this study provide a better understanding of the suspension spray generated by an effervescent atomizer in a crossflow configuration. It was found that the solid concentration of the suspension (up 10 wt.%) causes a slight decrease in size and brings the penetration of the suspension droplets in the gas flow.

Flash atomization is a technique to generate fine spray which can be used for effective heat and momentum transfer. Pressure and temperature of liquid are among the main parameters to control the process. Nucleation, bubble formation, and atomization are stages of this process. Rapid heating or rapid depressurizing is used to make the metastable fluid and eventually the spray. Comprehensive understanding of characteristics of the spray is essential for applications such as heating and fuel injection. Therefore, in this research the effect of superheat degree on atomization was evaluated. Moreover, the effect of pressure and temperature of the fluid on spray characteristics were studied. The spray shape, breakup length, and spray angles were experimentally investigated to provide a better understanding and interpretation of observations. The result showed that a distinct dense core was surrounded with a sheath of less dense spray. An interesting finding from high-speed imaging is that the spray pulsation is negligible. It seems a high-quality spray is achievable as a result of flash-boiling atomization at high pressure and high temperature.