Study and Comparison of Plasma Sprayed Mullite Coatings Properties Using Mullite and a Mixture of Alumina and Silica Powders Mahdi Hosseini Mahdi_hosseini83@yahoo.com 2011.04.11 Department of Material Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc Language:Persian Supervisor’s name: Hamid Reza Salimi Jazi Plasma sprayed ceramic coatings are widely used for thermal barrier coating applications. Commercially available mullite powder particles and a mixture of mechanically alloyed alumina and silica powder particles were used to deposit mullite ceramic coatings by plasma spraying. The coatings were deposited at three different substrate temperatures (25°C, 300°C, and 600°C) on stainless steel substrates. Microstructure and morphology of both powder particles as well as coatings were investigated by using scanning electron microscopy (SEM). Phase formation and degree of crystallization of coatings were analyzed and estimated by using X-ray diffraction technique. Differential thermal analysis (DTA) method was used to study the phase transformation of coatings. Results indicated that the porosity level in the coatings deposited using mullite as initial powder particles were lower than that deposited using the mixed powder particles at the same substrate temperatures. In general, coatings porosity level decreased with increasing substrate temperatures. The crystallization degree of the coatings deposited using the mixed powder particles are higher than that deposited using mullite powder particles at substrate temperatures, 25°C and 300°C. At 600°C, the crystallization degree of the coatings deposited using the mixed powder particles is less. DTA curves of coatings deposited using the mixed powders have showed some phase transformation due to the crystallization of the retained amorphous phases such as mullite, alumina and silica in the coatings. The degree of crystallization of both as sprayed coatings were significantly increased after post deposition heat treatments. Results showed that the mechanically alloyed mixed powder can be used as initial powder particles for deposition of mullite coatings with lower amorphous degree by setting substrate temperature close 300°C. Keywords : Plasma Spray, Mullite, Alumina-Silica, Crystallization, Microstructure
