Because you are collecting just in Bragg-Brentano condition, or if
not for every collection you just see only one direction in the
sample, not all together. And if you homogenize all together you will
only observe a broadening of the peaks.
Best regards,
Luca Lutterotti
On Jan 20, 2007, at 8:02 PM, [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).
Thanks in advance
Angel
