Microstructure, Processing, Performance Relationships for High Temperature Coatings

High velocity oxy-fuel (HVOF)coating have shown high resistance to corrosion in fossil energy applications and it is generally accepted that mechanical failure, e.g. cracking or spalling, ultimately will determine coating lifetime. The use of HVOF thermal spray to apply coatings is one of the most commercially viable and allows the control of various parameters including powder particle velocity and temperature which influence coating properties, such as residual stress, bond coat strength and microstructure. Methods of assessing the mechanical durability of coatings are being developed in order to explore the relationship between HVOF spraying parameters and the mechanical properties of the coating and coating bond strength. The room temperature mechanical strength, as well as the resistance of the coating to cracking/spalling during thermal transients, is of considerable importance. Eddy current, acoustic emission and thermal imaging methods are being developed to detect coating failure during thermal cycling tests and room temperature tensile tests. Preliminary results on coating failure of HVOF FeAl coatings on carbon steel, as detected by eddy current measurements during thermal cycling, are presented. The influence of HVOF coating parameters of iron aluminides applied to more relevant structural steels, like 316 SS and Grade 91 steel, on coating durability will be explored once reliable methods for identification of coating failure have been developed.