Electric arc dynamics in plasma torch affects plasma jet stability and consequently, coating properties. Depending on plasma torch design, voltage fluctuations can vary from 100 % to only a few percent of the mean voltage. Particularly, cascaded-anode plasma torch leads to very low voltage fluctuation owing to the presence of neutrodes that limit the amplitude of arc fluctuations. However, electric arc dynamics and electrode erosion process in this type of plasma torch are still poorly understood. The aim of this work is to deepen the knowledge on the influence of nozzle diameter on electric arc dynamics for two plasma forming gas compositions by means of several diagnostics devices (end-on imaging, current and voltage time monitoring, plasma jet brightness fluctuations and thermal balance determination). Reducing nozzle diameter from 9 mm to 6.5 mm results in higher voltage fluctuations, lower mean voltage and lower plasma torch thermal efficiency, probably due to a more evenly distributed warm plasma gas in the anode nozzle volume, as suggested by the higher plasma brightness. Nozzle observations after testing show significant wear in a 6.5 mm diameter nozzle, which may be evidence of a longitudinal movement of the electric arc on the anode surface, leading to high voltage fluctuations.

Thermal barrier coatings are generally produced one of two ways, depending on the thermomechanical loading expected. This study assesses an alternative approach in which the output of an air plasma torch is directed through two chambers connected by an expansion nozzle. In the first chamber, the particles evaporate under high pressure and temperature conditions. The vapor then passes through a supersonic nozzle into a low-pressure chamber where it condenses on the target substrate. A number of models are developed and used in order to assess the effects of process geometry and operating conditions on gas flows, powder vaporization efficiency, and nucleation and growth kinetics. Numerical simulations also informed various design decisions such as the length of the high-pressure chamber and the diameter of the expansion nozzle.

Fiber-reinforced polymer composites (CFRP) are increasingly used in aerospace for weight-sensitive applications. However, they are subjected to degradation from the erosive forces of solid particles and water droplets. This degradation results in a decreased service life of composite components and increased repair costs. A coating can protect the CFRP surface against wear and plasma spraying could be a candidate technique to achieve this coating. However, an issue is the thermal and mechanical damage to the composite surface by the plasma-sprayed particles. Another issue is the coating adhesion, because of the low wettability of polymer surface to liquid metal and ceramic and different atomistic properties between substrate and coating material. A possible solution to both issues is the use of a primary layer deposited by a “softer” technique than thermal spraying. This study deals with the deposition of this primary layer by three methods (magnetron sputtering, air gun spraying and sol-gel) and the deposition of topcoat layer by plasma spraying. The effectiveness of the protection of the CRFP by the primary layer during topcoat plasma spraying is investigated as well as the interfaces of the duplex coatings.