Pronounced Strain Relief due to Defect Annealing in Thermal Spray Coatings

In this study, a pronounced and rapid strain relaxation is shown in coatings subjected to temperature, as a consequence of annealing of defects. The response is evaluated using Digital Image Correlation (DIC) on splats heated on a hot stage using microscopy. The images confirm dramatic deformations of the material when it is exposed to heat.  This effect is also proven by in-situ heating and x-rays diffraction monitoring, electrical resistivity, and thermal expansion measurements. This significant deformation during the first heating after processing is a consequence of the accommodation of defects and relief of the strain energy stored in the coatings. Evaluation of the kinetics during strain relaxation is studied by control of the heating rate, inert vs. oxidizing atmospheres, processing effects, and coating orientation (in-plane, and out-of-plane). Experimental results from various materials and processes, such as: Al, Ni, NiAl, and others; sprayed by arc-spray, flame spray, cold spray, etc., are presented.