Vacuum plasma spray quality control

Abstract

The change in the technology of plasma spraying from air to vacuum has led to substantial improvements in spraying parameters and coating properties. A great variety of reactive and oxygen-sensitive materials with interesting properties can now be applied by vacuum plasma spraying and used as coatings. However, these materials can only be used in connection with a very time-consuming optimization of coating parameters. Until now this optimization process has been the link between spraying experiments with parameter variations and the resulting microstructure. A new method of quality control, the wipe test, can accelerate the process of optimization and results in additional information. The plasma jet with the injected powder particles is moved across the substrate surface at a very high speed like a wiper. This produces a pictorial distribution of the powder particles in the plasma jet and, if a scanning electron microscope is also used, provides a description of the degree of melting that occurs during spraying The test demonstrates the manner in which the quality of a coating is influenced by the degree of melting of the plasma spray powder. The number of unmelted powder particles and their distribution across the plasma jet cross section are determined by the plasma energy and the powder injection parameters. Control of these parameters using the wipe test results in optimal coating structures for specific applications. Titanium and TiB 2 are given as different examples to illustrate this.