Experimental study on heat insulation performance of functionally graded metal/ceramic coatings and their fracture behavior at high surface temperatures

The objective of this research was to study heat insulation performance and fracture behavior of thermal barrier-type functionally graded material (FGM) coatings under high heat flux. The FGM coatings with thicknesses varying from 0.75 to 2.1 mm were designed and deposited onto a steel substrate by plasma spraying. With the high heat flux of 0.68 MW m−2, the thickest specimen exhibits a high surface temperature of 1690 °C and the corresponding temperature difference between the top surface and the bottom surface of the coating is high up to 760 °C. The results of thermal cycling test with a surface temperature of 1400 °C (during heating) showed that there existed a great difference in fracture location between different specimens. Fracture behavior of the coatings at surface temperatures of 1500, 1600 and 1700 °C was also investigated. The coating thickness and surface temperature were found to influence the number and the length of cracks. The specimen with the coating thickness of 1.1 mm is the most susceptible to local spallation at the top ceramic coat, while the thickest FGM coating is the most susceptible to cracking at the interface close to the bond coat. Heating time also had an effect on cracking behavior. These results provided some necessary information for appropriate design of FGM coatings to sustain higher temperatures and gradients than those in today's applications.