Effect of structural factors on the mechanical properties of superhard materials based on boron nitride

Conclusions With increasing grain size a transition takes place from intercrystallite to dual-type (inter- and transcrystallite) fracture. The two-phase Geksanit-R is characterised by the smallest grain size, a uniform grain size, and the lowest inclusion and pore contents. It is to these characteristics that the material owes its high hardness and excellent fracture toughness (K_c50–55 kgf/mm^3/2); its effective energy of fracture, y (6.2–7.5)·10⁴ ergs/cm⁴, is an order greater than its true surface enrgy o. The hardness of Geksanit-R sinterings is virtually independent of the ratio between the relative amounts of BN_w and BN_s. The presence in Geksanit-R specimens of 2–7% of graphitelike boron nitride embrittles the material, without substantially altering its hardness. The other BN_s-base superhard materials (including Elber-R, Belbor, and PTNB-IB-1) are less hard than Geksanit-R, which is attributable to their larger grain size and the presence of foreign substances and phases, and exhibit a marked tendency toward brittle rupture.Translated from Poroshkovaya Metallurgiya, No. 10(202), pp. 61–69, October, 1979.