Dear all: Let me make you the following question.
Suppose a polycrystalline coating deposited onto a substrate in such a way that the coating is subjected to in-plane compressive residual stresses after the deposition process. The coating has a cubic fcc crystal structure before the deposition process, and also appears to be cubic fcc with increased out-of-plane lattice parameter after the deposition. I understand the increased out-of-plane lattice parameter because the stresses are compressive in nature and the XRD experiment collects information on the diffracting planes that are parallel to the sample surface. However, I have the following question: Since the residual stresses are compressive in nature, the fcc crystal structure will become distorted after the deposition process, and strictly will be no longer cubic. The change in the shape of the unit cell is most likely a function of the orientation of the unit cell with respect to the direction of the residual stresses. Thus, the cube can change to tetrahedron, rhombohedral, etc. If the stresses are large enough in magnitude (bout 1000 MPa), why the XRD pattern still shows a cubic crystal structure (there are no additional peaks, peak splitting, etc). Thanks in advance Angel
