Characterization Studies of Mullite Coatings on Cast Aluminum

Abstract

Thermal barrier coating of mullite was plasma sprayed on cast aluminum A 356.0, in the T6 condition (solution treated, quenched and artificially aged) for use in internal combustion engine applications. This study pertains to the mechanical, thermal, wear, corrosion and micro structural characterization of the coating. An average coating tensile strength of 50 MPa and average adhesive bond strength of 20 MPa was measured in the mechanical tests. A wear factor of 0.7 × 10−3 mg/Nm was measured in the wear studies using a pin on disk apparatus. An average value of 0.151 W/m K was measured in the thermal conductivity test. The coating withstood 100 cycles in the thermal shock test, without any sign of spallation. Corrosion tests showed no signs of corrosion even after 500 h. The microstructural and porosity studies were conducted using an optical microscope and scanning electron microscope with an energy dispersive spectroscopy (EDS) attachment. The studies revealed a crystalline microstructure of the substrate, which correspond to the splat structure of the coating. The porosity of the coated layers ranged from 6 to 21% by volume, at an average of 12-16%. EDS studies showed the elements present in the coating. X-ray diffraction patterns taken on coated specimens, showed the phases present in the coating, and indicated a crystalline structure of the coating along with some amorphous matter.