A comparison of two laser-based diagnostics for analysis of particles in thermal spray streams

This paper discusses two commercially-available laser diagnostics that have been used in thermal spray research at Sandia National Laboratories: (1) a Phase Doppler Particle Analyzer (PDPA) and (2) a Laser Two-Focus (L2F) velocimeter. The PDPA provides simultaneous, correlated measurements of particle velocity and particle size distributions; but, particle sizing doesn`t work well with non-spherical particles or particles with rough surfaces. The L2F is used to collect particle velocity and number density distributions, and it can readily distinguish and separately measure particles with off-axis velocity vectors. PDPA and L2F principles of operation are presented along with potential advantages and limitations for thermal spray research. Four experiments were conducted to validate and compare measurement results with the PDPA and L2F instruments: (1) spinning wire, (2) powder in a High-Velocity Oxy-Fuel (HVOF) jet, (3) powder in a cold jet, and (4) droplets in a wire-fed HVOF jet. TWO DIFFERENT TYPES of commercially-available laser velocimeter systems, a Phase Doppler Particle Analyzer and a Laser-Two-Focus velocimeter have been used in the Thermal Spray Research Laboratory at Sandia National Laboratories. Each of these techniques has inherent advantages and limitations for thermal spray, and each involves assumptions that may not be valid for some experimental conditions. This paper describes operating principles and possible sources of measurement error for these two diagnostic systems. Some potential advantages and limitations are also presented. Four types of experiments were also conducted to validate and compare PDPA and L2F measurement results: (1) spinning wire, (2) powder in a High-Velocity Oxy-Fuel (HVOF) jet, (3) powder in a cold jet, and (4) droplets in a wire-fed HVOF jet. We also offer a few observations related to practical issues such as ease-of-use, reliability, and effects of dust and vibration in a thermal spray lab