Irreversible Changes in the Linear Expansion of Aluminum Coatings during Annealing

A large irreversible change in the linear expansion of Al-coatings by flame spraying during annealing has been measured by dilatometry. Long free standing samples (4x4x25 mm) undergo an irreversible expansion in the first heating cycle. It is well known that thermally sprayed coatings experience annealing of defects, including: residual stress, recrystallization, and grain growth. Experimentally, such defect annealing would result in an irreversible contraction of the samples and could be measured only if the number of defects is quite large. However, in the present case, the sample expands irreversibly in a ~0.03% magnitude. This abnormal deformation is measured after the first heating and cooling cycle. This effect is analyzed in this study, by two strategies: 1) analyzing the microstructure of the Al-coating by FEA using OOF to simulate the residual stresses occurring at the microscale between the Al-Al₂O₃ interfaces in the first heating cycle; 2) observing individual splats by microscopy (optical and SEM) using an in-situ heating stage, and measuring their deformation during annealing by the DIC-digital image correlation technique. The residual stress at the microscale, and localized deformations that protrude over the splats surfaces are probably responsible for the overall effect. The magnitude of both are discussed.