The Plasma Spraying of Nitride Coatings

Ceramics without oxides (i.e. nitrides) are vulnerable to oxidation in the presence of plasmagenic gases or oxygen from atmosphere during plasma spraying. Initially there were two ways of oxidation’s avoidance: first, modification of powder material and second, limitation of an oxygen’s stream flowing into plasma. The modification of the powder material consisted in covering of ceramic granules with material limiting contact with oxygen or usage of easily oxidizing material getting oxygen away (i.e. a graphite thin layer over carbide’s granule). This method seemed to be less economic due to additional increase in powder’s price. Eventually the second conception of limiting of oxidation was considered as worth developing. Originally the process of spraying was carried out in vacuum (VPS – Vacuum Plasma Spraying). Then application of atmosphere under control was examined (CAPS – Controlled Atmosphere Plasma Spraying). It includes process of spraying with application of different gaseous controlled atmospheres under different pressure (decreased, normal and increased). There are three kinds of atmosphere: neutral (argon, nitrogen), reductive (carbon monoxide, methane) and acting with spraying material. Eventually the process was carried out under atmospheric pressure with gaseous ring-shaped protective gas jet. Moreover the experiments with spraying of nitrides were described (AlN, TiN, Si₃N₄). The most suitable characteristics of the process was chosen and some properties of coatings were examined as well.