Developments in direct current plasma spraying

Thermal spray processing is used to confer specific in-service properties to components via the production of a coating between 50 μm (minimum value) to a few millimeters thick. Thermal spray represents a global market of about 4.8 Billion Euros (i.e., ∼ US$5 billion) in 2004; 30% of which is European based. 50% of this activity is devoted to plasma spray processing with about 90% dedicated to direct current (DC) plasma torches. Several developments of new torch architectures, among which three-cathode torches, have evolved recently. However, most of the recent progress has been applied to conventional DC torches. The advances were related to two prime factors: (i) the development of industrial sensors permitting to diagnose the processes during spray operation (especially in-flight particle characteristics in terms of their surface temperature and velocity) and additionally the monitoring of the substrate and coating temperatures with the objective of controlling the operating parameters via a close-loop controller; (ii) the adaptation of plasma spray systems to manufacture nano-structured coatings via the development of suspension plasma spray and solution plasma spray. As well, there has been an enhanced understanding of the mechanisms controlling the coating formation and of the effects of the arc root fluctuations; thereby permitting a more robust process. This paper develops the above points by presenting focused examples.