The relationship between microstructure and Young's modulus of thermally sprayed ceramic coatings

An idealized model for the microstructure of thermally sprayed ceramic coatings, consisting of the stacking of lamellae a few micrometres thick, has been used to estimate Young's modulus of the coating perpendicular to the coating plane. A theoretical relationship between Young's modulus and the microstructural parameters has been established. There are two components of elastic strain of the coating under tensile stress, one arising from localized elastic strain at the regions of real-bonded area between lamellae, and the other arising from elastic bending of the lamellae between bonded regions. The bending component only becomes significant for a percentage bonding ratio between lamellae of less than 40%. The bending strain contribution depends strongly upon geometrical parameters of the coating microstructure. The estimated Young's modulus for a typical alumina coating, based on quantitative microstructural data, was about 24% of that for the fully dense material. Taking into account the variable proportion of alpha-Al2O3 and gamma-Al2O3 forms in an alumina coating, the comparison of the estimated Young's modulus with published data gives reasonable agreement for the coating prepared over a wide range of processes and experimental conditions.