Dear Larry: Thanks for shearing for oppinion. Your explanation sounds great.
Best reagrds, Angel > [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 >
