Abstract:Three guns based on different thermal spray technologies-namely, gas flame spray, wire arc spray, and wire plasma spray-were operated at each best cost-performance condition, and the resulting spray droplets and deposited coating qualities were investigated. For the former, a simple optical monitoring system was used to measure temperatures and velocities of spray droplets ejected from the guns. On the other hand, for the latter, qualities of coating layers on substrates-namely, surface roughness, atomic composition, hardness, adhesive strength, and porosity-were characterized. Then, these coating qualities were discussed with respect to the measured temperatures and velocities of spray droplets, which revealed novel features in the coatings that have not been seen before, such as atomic composition and hardness strongly dependent on temperature and environments of droplets towards the substrates, and porosity on velocity of droplets impinging onto the substrates.

A: This article describes the first experimental result of a simple but reliable optical monitoring system for optimization of thermal spray droplets. The system, which was used in the present experiment and the authors call as "a pre-proto instrument", consisted of a light collecting lens followed by a part for separating the light into two wavelengths, with the light of each wavelength guided into an avalanche photo-diode (APD) to be electronically detected. First, it was calibrated using a special purpose-built calibration system. Then, it was taken to a plasma spray gun for a field test, yielding a satisfactory first result. Based on this positive result, future plans of the project are discussed. K: Plasma diagnostics -high speed photography; Plasma diagnostics -interferometry, spectroscopy and imaging 1Corresponding author.

Thermal spray is a technique to form molten droplets using either plasma-or combustionheating, which impinge upon substrates to form coating layers for various purposes, such as anticorrosion and anti-wear layers. Although it is an established technique having a history of more than a century, operations of spray guns together with preparing suitable substrate surfaces for obtaining good coating layers still rely on experienced technicians. Because of the necessity of meeting more and more stringent requirements for coating quality and cost from customers, there has been a strong need to try to monitor spray processes, so as to obtain the best possible spray coating layers. The basic requirements for such monitoring systems are *reasonably cheap, *easy operation for laypersons, *easy access to targets to be investigated, and *an in-situ capability. The purpose of the present work is to provide suitable optical monitoring systems for (1) droplets behavior and (2) substrate pre-treatments. For the former (1), the first result was already presented at the 17th laseraided plasma diagnostics meeting (LAPD17) in 2015 in Sapporo, and the results of its subsequent applications into real spray environments are shown in this article in order to validate the previous proposal. Topic (2) is new in the research program, and the proof-of-principle experiment for the proposed method yielded a favorable result. Based on this positive result, an overall strategy is being planned to fulfill the final objective of the optical monitoring of substrate pre-treatments. Details of these two programs (1) and (2) together with the present status are described. K: Plasma diagnostics -high speed photography; Plasma diagnostics -interferometry, spectroscopy and imaging 1Corresponding author.

This article first describes plazwire technology, a novel method of plasma spraying of molten metals. It has shown to be an effective and efficient means of preparing anti-corrosive coatings on metal surfaces exposed to outside air/water for extended periods of many decades. For its further development, there exist certain behavior of droplets in a plasma flame which must be elucidated, and these data needs are then enumerated. Against this background, the potential of laser-aided diagnostics for droplets are assessed, in particular laser-shadowgraphy, laser in-line holography, and laser-Doppler velocimetry/phase Doppler particle analysis. In addition to these diagnostics design studies, two proof-of-principle (POP) experiments were conducted on laser inline holography for droplet size determination and on laser-Doppler velocimetry for the droplet velocity relaxation with background gas flows, with the results clearly demonstrating the validity of these methods for the above-mentioned purpose.

Droplet velocities of thermal spray are known to have profound effects on important coating qualities, such as adhesive strength, porosity, and hardness, for various applications. For obtaining the droplet velocities, therefore, the TOF (time-of-flight) technique has been widely used, which relies on observations of emitted radiation from the droplets, where all droplets along the line-of-sight contribute to signals. Because droplets at and near the flow axis mostly contribute coating layers, it has been hoped to get spatially resolved velocities. For this purpose, a velocity-divided Abel inversion was devised from CMOS photographic data. From this result, it has turned out that the central velocity is about 25% higher than that obtained from the TOF technique for the case studied (at the position 150 mm downstream of the plasma spray gun, where substrates for spray coatings are usually placed). Further implications of the obtained results are discussed.