Porosity and Its Significance in Plasma Sprayed Coating: A Review

Porosity in plasma sprayed coatings is vital for most engineering applications. It is either advantageous or disadvantageous depending on the functionality of the coating and the immediate working environment. Consequently, the formation mechanisms and development of porosity has been extensively explored to find out modes of controlling porosity in plasma sprayed coatings. In this work, a comprehensive review of porosity on plasma sprayed coatings is established. The formation and development of porosity on plasma sprayed coatings are governed by set spraying parameters. Optimized set spraying parameters have been used to achieve the most favorable coatings with minimum defects. Even with the optimized set spraying parameters, defects like porosity still occur. Here, we discuss other ways that can be used to control porosity in plasma sprayed coating with emphasis to atmospheric plasma sprayed chromium oxide coatings. Techniques like multi-layer coatings, nano-structured coatings, doping with rare earth elements, laser surface re-melting and a combination of the above methods have been suggested in adjusting porosity. The influences of porosity on properties of plasma sprayed coatings and the measurement methods of porosity have also been reviewed.