Scientific Advisory Board - Erich Schmid Institute

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE
Fig . 3 The relationship between the lattice constant ratio and N vacancy concentration is plotted.
The open symbols represent the calculated values using DFT, and a linear fit curve is attached.
the structure transformation may occur when the n vacancy concentration exceeds a certain
amount, which implies that the linear relationship between the lattice constant and vacancy
concentration become invalidated (Zaoli Zhang et al., Physical Review B 87, 014104-9(2013)).
Residual Stress Development in Metastable Ti 1-xAl xN Hard Coatings
metastable face-centered cubic (fcc) ti 1-xAl xn hard coatings deposited by physical vapor
deposition (pVd) undergo a phase transformation at high-temperatures which may be
accompanied by coating self-strengthening. the films undergo spinodal decomposition and
form nano-sized cubic (c) c-tin and c-Aln domains where the latter may further transform
into stable wurtzite (w) w-Aln at temperatures above ~900°c. the first transformation is
accompanied by a significant hardness increase, whereby the formation of w-Aln results in
stress relaxation and hardness decay. the effect of the phase transformation on the hardness
and residual stress changes in the films is still not fully understood.
We have performed in situ high-temperature synchrotron XRd characterization of residual
stress changes in a ti 0.52Al 0.48n coating on hard metal in the range 25-1003°c at the eddi
beamline of BeSSY in Berlin (fig. 4). the experiments revealed that heating above the deposition
temperature first results in the compressive residual stress relaxation (and hardness decrease)
due to the structure recovery caused by the remove of crystallographic defects. further
annealing above about 700°c stimulate the spinodal decomposition, a precipitation of nanosized
c-tin, c-Aln and w-Aln domains, which results in the compressive stress increase and a
dramatic hardness increase from 35 to 47gpa. finally our experimental results (combined with
complementary hardness, tem, Raman and Sem investigations) were used to understand
the correlation between thermal conditions, microstructural changes, stress development and
hardness in ti 1-xAl xn Hard coatings.
page 18 Scientific RepoRt 2012