[EMAIL PROTECTED] wrote: > 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).
Because the diffracting planes are all parallel to the sample surface, you are only measuring d spacings perpendicular to the surface. As a result, every such plane sees the same stress and you get a cubic pattern with no splitting. If you could measure the diffraction pattern in transmission you would see the other component of the stress (assuming that your stress distribution is an ellipsoid of revolution). If the normal to the planes of diffraction were in the plane of the coating, the pattern would again be cubic with a different lattice parameter. Only if the normal were neither parallel nor perpendicular to the plane of the coating would you see splitting. Larry
