Nanocomposites and nanomaterials

Solid-phase consolidation of ultrafine grain WC–25%Co alloy with at impact sintering
Tolochyn O.I., Laptiev A.V., Yevyh Ya.I., Okun I.Yu.
Institute for Problems in Materials Science of the NAS of Ukraine. Krzhyzhanovskogo str., 3, Kyiv-03142, Ukraine. 
E-mail: tau@ipms.kiev.ua

The creation of hardmetals with ultrafine-grained or nanocrystalline structure is a rather complicated problem due to the difficulties of controlling the growth of ultrafine carbide particles. However, when using very fine powders of WC, less than 0.3 microns, the method of introducing inhibitors becomes ineffective. The growth of particles of the carbide phase can be inhibited by reducing the compaction temperature of the powders, what during sintering leads to an increased porosity of the samples. Therefore, the transition from classical sintering to a method of short-term sintering under the action of an impact will allow, at lower sintering temperatures, to solve the problem of both obtaining minimum porosity in samples and preserving more fine-grained structure in them without using inhibitors of growth of particles of the carbide phase.
The density, structure and mechanical properties of the WC – 25 wt% Co hardmetal produced from ultrafine WC and Co powder with impact sintering and conventional sintering in the temperature range of 1150–1350 °C was studied. 
Conventional sintering of the ultrafine-grained mixture WC–25%Co showed that a significant part of compaction occurs in the solid phase and at temperatures of 1150 and 1250 °C, the porosity is 36 and 22%, respectively (at the initial 50%), and when a liquid phase appears (1300 and 1350 °C) porosity decreases to 1–2%. Impact sintering allows obtaining samples with a porosity of 4% at 1150 °С and 0.6% at 1300 °С. The structure of the composite during compaction in the solid phase region is characterized by a uniform distribution of carbide particles (0.3–0.6 µm) in cobalt. In the liquid phase, carbide particles grow to 0.5-0.8 µm with the formation of cluster of particles. Tests for bending strength showed that with an increase in the temperature of impact sintering from 1150 to 1300 °C, the strength increases from 2700 to 3300 MPa, respectively. The conventional sintering of the samples at 1300 and 1350 °С ensures the obtaining of samples with bending strength at the level of 2400 and 2900 MPa. 
Thus, the solid-phase consolidation of ultra-fine WC+25%Co powders ensures an ultrafine-grained structure with a uniform distribution of the solid and plastic phases, and also allows obtaining samples with low porosity and high mechanical characteristics.