Characterization of Aluminum Phosphate Sealed Alumina and Chromia Coatings

Abstract

In this study as-sprayed and aluminum phosphate sealed alumina and chromia coatings were characterized. The effect of aluminum phosphate sealing on the mechanical properties was studied with abrasion and erosion wear tests. The microstructural characterization was carried out to find out the microstructural features, which could explain the sealing mechanism in the coatings as well as the effect of the sealant on the coating properties. Microstructural characterization was carried out by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Aluminum phosphate sealant had penetrated into the coatings via the structural defects such as gaps and voids between the lamellas and microcracks in the lamellas and had improved mechanical properties by sealing and bonding the lamellar coating structure. In the sealed alumina coatings the aluminum phosphate had reacted with the alumina lamellas forming a reaction layer at the interface between the sealant and the coating lamellas and thus the bonding mechanism of the sealant is based on chemical reaction. In the sealed chromia coatings the interface between the sealant and the coating indicated no reaction layer and thus only attractive forces bind the sealant to the coating. Despite the different bonding mechanisms, the effect of the sealing on coating properties was similar in both the alumina and chromia coatings.