Effects of powder processing on densification of titanium diboride based cermets

摘要

The sinterability of TiB_2-Ni_3(Al,Ti) based cermets has been significantly improved by aggressive milling of the starting TiB_2-Ni-TiAl_3 powder mixtures. This technique improves not only liquid spreading by reducing TiAl_3 particle size but also eliminates alumina agglomerates and the associated porosity found after vacuum sintering. Liquid phase sintering of TiB_2-Ni-TiAl_3 powder mixtures involves the presence of Ni based secondary borides at low temperatures (1200 deg C), which react afterwards with TiAl_3 particles leading to the formation of the final TiB_2-Ni_3(Al,Ti) eutectic liquid. Apart from improving liquid spreading around TiB_2 grains, aggressive milling is also found to disperse alumina agglomerates, which reduces the porosity associated to these particles. By this refined procedure, the amount of binder phase required for full densification of TiB_2 cermets by sinter hipping has been reduced from a previous limit of 16 vol.- percent to 10 vol.- percent. The hardness of these TiB_2-10 vol.- percent Ni_3(Al,Ti) cermets is in the range of ultra-fine WC-Co hardmetals in spite of their much coarser microstructure.