Influence of Transient Liquid Phase Promoting Additives upon Reactive Plasma Spraying of AlN Coatings and Its Properties

The feasibility of using a transient liquid phase promoting agent upon reactive plasma spraying (RPS) of AlN coatings in atmospheric ambient is successfully investigated. Several sprayable fine AlN-Al2O3-Y2O3 mixtures with different compositions are prepared by spray-drying. Upon plasma spraying in N-2/H-2 plasma, thick and homogeneous AlN based coatings (h-AlN, c-AlN, -Al2O3, Al5O6N, -Al2O3, and YAG) are fabricated. Fine Y2O3 acts as transient liquid phase promoting additive, and reacts with Al2O3 phase (from feedstock mixture and on AlN particles surface) to create wetting agent of yttrium aluminate Y-Al-O around solid AlN particles. However, due to rapid solidification rates of plasma, it is difficult to react with the interspersed oxygen in AlN grains. The Y2O3 addition significantly reduces the coatings oxide content due to the consumption of starting Al2O3 to form the YAG. Influence of Y2O3 significantly observes with increasing AlN feedstock and high AlN content coatings with small oxides are fabricated by RPS of 80AlN-10Al(2)O(3)-10Y(2)O(3) mixture. Thermal conductivity is improved with using Y2O3 and increasing AlN feedstock. However, the conductivity is not so high compared to the AlN compacts, due to increased porosity, incomplete sintering of the formed initial liquid phase state, remaining oxide content, and low density of the coatings.