Structure and composition of
titanium spark erosion powder obtained in liquid nitrogen
Chem.
Met. Alloys 4 (2011) 126-142
https://doi.org/10.30970/cma4.0178
Gennady MONASTYRSKY, Patrick OCHIN,
Guillaume Y. WANG, Victor KOLOMYTSEV, Yuri KOVAL, Vitaliy TINKOV, Anatoliy SHCHERBA, Sergey ZAHARCHENKO
Titanium powders were obtained by the
spark-erosion method in liquid nitrogen. As-processed powder particles with
typical sizes between 1 and 50 microns have spherical shape. The surface of the
Ti powder particles has irregular structure. Many of the micron-sized particles
contain irregular holes inside. The inner surface of the holes has well
arranged bubble-like structure. The chemical analysis, made by the Kjeldahl method, showed that the powder contains 11.24±0.06 wt.% of nitrogen, while the more reliable XRD
method gave 12.4±0.2 wt.%. Backscattering scanning electron microscopy
images of cross sections of powder particles, as well as EDX
analyses, showed a cellular structure of the particles with nitrogen-rich areas
of several microns, separated by relatively thin boundaries of α-Ti(N). The XRD study confirmed
that the powder contains about 85 wt.% of the δ-TiNx nitride (osbornite, Fm-3m), 10 wt.% of
α-Ti(N) (P63/mmc) and minor quantities of α-
and b-Ti (no more than 4 % in
total). The TEM investigation showed that particles with sizes between 10 and
100 nm are mainly δ-TiNx nitride, although the quantity of oxygen in the
nanoparticles is high. The proposed model for
solidification of Ti powder particles considers multiple formation
of nuclei of the solid phase on the surface of liquid droplets. It makes it
possible to estimate, on the one hand the upper and lower temperatures of the
molten droplets that were reached during the spark erosion, and on the other
hand the cooling rate, and eventually to specify the mechanism of pore
formation inside the powder particles.
Typical
XRD pattern of a Ti-N powder with grain size
less than 32 microns. The inset
shows an enlargement of the pattern between
40o and 48o.
Keywords
Powder / Spark-erosion method / Ti-N / Titanium
nitride