The impact of plasma sprayed sieved Ni-5%Al particles on a titanium substrate was investigated. The particles had a narrow diameter range (-65 + 75 µm), and a speed and temperature just prior to impact of about 100m/s and 2400°C, respectively. These powder particles were sprayed on two sets of polished titanium alloy surfaces. One set was a non-oxidized surface and the other one was a previously oxidized surface at 600°C for two hours. Resulting splats were characterised experimentally by infrared pyrometry and scanning electron microscopy. The effects of the substrate’s oxide layer on the shape of the splat, also on the cooling rate and flattening speed during impact were studied and discussed. This problem is also being investigated numerically. A first step was devoted to the selection of relevant simulation parameters. Test cases to study qualitatively the effect of surface oxidation through parameters such as the contact angle and thermal contact resistance are in progress. They will be compared to the experimental results.

Plasma spraying operations performed with high carrier gas flow rate may improve the coating properties but they can also lead to lump formation and thus coating defects. The damaged work piece must then be stripped and recoated, which implies a considerable waste in terms of coating powder, energy and time. The aim of this study was to determine the cause of the lumps, and propose process modifications for avoiding their formation while keeping the coating quality. Numerical simulations based on 3D turbulent Navier-Stokes equations in local thermal and chemical equilibrium were carried out to understand the problem and estimate the feasibility of the proposed solutions. The computational results were supplemented by experiments for validation. A first set of investigations was focused on the location and orientation of the powder port injector. It turned out that it was not possible to keep the coating quality while avoiding lump formation by simply moving the powder injector. A new geometry of the nozzle exit was then designed and successfully tested for a first application with Ni-5Al powder used in production.

This paper presents a method to increase deposition rate of thermal plasma spray operations through the use of multiple injection ports. Numerical simulations indeed revealed a major energy loss in the process when using only one port. The influence of the carrier gas and the particle stream on the heat flow coming from the plasma torch was found very local and small compared to the total amount of energy. To take as much as possible advantage of the energy available in the plume, we thus propose to use a multiple number of injectors around the flame. Computational simulations are carried out to estimate the feasibility. They are based on the 3D Navier-Stokes equations coupled with a turbulence model. The gases (plasma gas, surrounding air and carrier gas) are supposed to be in local thermal and chemical equilibrium and loading effects are accounted for. The numerical results are supplemented by experimental results showing that multiple injectors can significantly increase deposition rate while preserving or even slightly improving the deposition efficiency. Characterisation of the microstructure, evaluated for all tests, is similar and no obvious differences can be detected apart from the porosity. This method thus results in a substantial reduction of the production cost.