In cold spray and thermal spray applications, one of the primary factors affecting coating deposition is the location where particles are injected into the gas jet. Therefore, a detailed knowledge of the gas flow distribution is required. Non-resonant laser scattering allows to spatially resolve the distribution of drift velocity and mass density within the flow, particularly at locations close to the injector. Based on laser scattering, this paper presents a new diagnostic that locally measures drift velocity, as well as a relative mass density distribution of a gas stream. Its application is mainly focused on cold gas flows, where velocity measurements in a supersonic nozzle, obtained by means of laser scattering, correlate well with theoretical calculations and particle image velocimetry (PIV) experimental results.

In thermal spray processes fast gas flows are used to accelerate and melt the coating compound materials. The kinetic energy transferred to the particles by the gas flow depends on the velocity of the jet, which thus becomes an important parameter for coating formation. This paper describes a non-contact technique for measuring the drift velocity in those flows. This technique is based on the additional wavelength shift induced by the gas flow for non resonant light scattering. This shift of the order 1 pm is detected by an interferometer composed of two mechanical tilted plates, such that the wavelength of the scattered light can be easily obtained from the location of the maxima in the corresponding Airy profile paired with an MCP-CCD camera, to enhance sensitivity and processing speed. The experimental setup will be described and first results shown.

Increasing erosion of the anode wall of a single-cathode F4 torch changes the fluctuation pattern of the plasma jet. Trajectories and temperature history of particles injected into the plasma become accordingly modified. In this work such fluctuations are characterised through spectral analysis of the torch voltage as well as of the light intensity of the jet outside the torch. Instead of considering the evolution of the main peaks in the frequency spectrum for both voltage and optical signals, the variation with increasing torch wear of the spectrum’s higher frequency fraction (above 20kHz) is focused on. The evolution of the fractal dimension in this high frequency range exhibits a reliable correlation with the erosion of the anode wall. This is confirmed further by the development of a simple stochastic model for the motion of the arc root along the anode wall. This model yields, depending on the probability for a pronounced arc root jump, a fractal dimension and an evolution just within the range observed in the measurements. Additionally, tracking the fluctuations at different locations outside the torch enables the isolation of the fluctuation dissipation due to jet viscosity and velocity.