A plasma torch with a converging-diverging nozzle has been developed for low pressure plasma spraying. This study investigates the current-voltage characteristics of the plasma arc for argon and argon-hydrogen plasma gases and the effect of hydrogen volume percentage on the plasma jet. Emission spectroscopy is used to analyze the plasma spectra and electron temperatures in the center of the plasma jet are determined based on a Boltzmann distribution. The results show that an increase in input power considerably increases electron temperature and that gas composition has a significant effect on current-voltage characteristics. The impact of detection distance is also addressed.

A novel fully nano-equiaxed-structured thermal insulation Y 2 O 3 -stabilised ZrO 2 (YSZ) coating was first successfully deposited using very-low-pressure plasma spray (VLPPS) and the agglomerates feedstock from nanometer-sized powders. The microstructure of fully nano-equiaxed-structured coating is different from that of a “bimodal microstructure”, which is currently the dominant nanostructured coating. This desirable structure in the new TBCs appears to be responsible for their improved thermal cycling lifetimes and reduced thermal conductivity relative to those of conventional plasma-sprayed TBCs with lamellar structures or bimodal structures. Preliminary results from experiments aimed at understanding the VLPPS deposition mechanisms suggest that the nanometre-scale equiaxed in the coating are formed from original nano-particles, they underwent melted in the low pressure plasma plume and solidification onto the substrate. The VLPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

This study investigates the spraying characteristics of low-pressure plasma torches with different nozzle sizes. YSZ feedstock powders were sprayed with each torch at different stand-off distances and gas pressures. The plasma jets created were photographed and measured, showing that low-pressure spraying significantly increases plume length and diameter compared to atmospheric conditions. The coatings obtained were examined and microhardness was determined. It was found that the longer nozzle increases the temperature of the plasma jet, and with a longer dwell time, the particles heat more efficiently and evaporate more fully. At a spraying distance of 300 mm, the coatings were mostly composed of equiaxed grains, which were much larger in the coatings produced with the long anode nozzle. At longer spraying distances, more unmelted particles appeared in the coatings, leading to a reduction in hardness.

In contrast with the atmosphere plasma spraying (APS), the very low pressure plasma spraying (VLPPS) shows lower ability for heating spray materials at the plasma free region exit of nozzle. Most very low pressure plasma spraying at the present usually use a high power plasma gun which operated at great arc currents up to 2500A to meet lower ability for heating materials, thus result in a series of problems not only for the plasma torch but also for associated facility. According to the plasma spray characteristic in the low pressure environment we designed a plasma torch with separating anode and nozzle, and the powder injected to the plasma jets is from inside nozzle intake. In this study, the pressures in the plasma gases intake, nozzle intake and out of the plasma torch were measured respectively. For the practice, SUS 316 stainless steel coatings were prepared at plasma currents of 500-600A and arc voltage of 50V and the chamber pressure of 1000 Pa, the results indicated that the coatings with the equiaxed microstructure could deposited in the appropriate conditions.

The FeAl intermetallic compound coatings were sprayed by low pressure plasma spraying (LPPS), air plasma spraying (APS) and high velocity oxy-fuel spraying (HVOF). The influence of three kinds of thermal spraying processes on the microstructure, microhardness, elastic modulus and fracture toughness of coatings were investigated. The results showed that the APS and HVOF coatings exhibited similar microhardness, about 540HV 0.3 , which is much lower than that of LPPS coatings, 860HV 0.3 . The elastic modulus measured for APS, HVOF and LPPS coatings using Knoop indentation were 96, 84, 176GPa, respectively. The APS coatings were also observed to have lower elastic modulus values in the in-plane direction than those in the perpendicular direction, as a result of microcracks scattered within the coatings. In fracture toughness tests, the LPPS coatings revealed the lowest fracture toughness, as compared with other two spraying processes, indicating low porosity and crack levels are related to low fracture toughness. From these results, it appeared that potential improvements to certain mechanical properties can be achieved using low pressure plasma spraying process.

Plasma spraying with axial powder injection inside the anode nozzle is a method considered to markedly enhance deposition efficiency and reduce plasma power compared to the most general method of injecting powder at the nozzle exit. However, powder injecting inside the nozzle will also likely cause problems from powder deposition on the nozzle wall and clogging of the particle feed channels. In this study, spherical stainless steel 316 and angular alumina powders with a mean size of 20-40 μm are used to deposit coatings via axial powder injection. The effect of powder feed rate, gas flow rate, and plasma power on deposition efficiency and particle clogging are investigated. The results show that particle clogging can be avoided by reducing powder feed rate and increasing the velocity of the plasma jet.