Thermal stability of nanocrystalline WC-Co powder synthesized by using mechanical milling at low temperature

Nanostructured WC-18% Co powder was synthesized by using cryogenic mechanic al milling, and the thermal stability of the nanostructured powder was inve stigated in detail. The results indicated that the as-synthesized WC-18% Co powder had an average WC particle size of 25 nm. Growth of WC particles oc curred above 873 K; however, the average WC particle size remained smaller than 100 nm in the powder isothermally heated for 4 h at 1273 K. Thermal ex posure in air at T < 623 K did not result in significant oxidation of the c ryomilled powder. The thermal exposure did promote the formation of WO2 and WO3 oxides. The Co6W6C phase was detected by x-ray diffraction in the powd er heated in nitrogen at 1273 K, and the phases associated with decarburiza tion of WC, such as W2C, W3C phases, were not observed. With increasing tem perature, the dissolution of W and C elements in the Co matrix led to a gra dual increase in {111} crystallographic plane spacing, eventually leading t o the formation of an amorphous phase.