Abstract
Evaluating methods for the fracture toughness of brittle hard materials were investigated. For measuring the fracture toughness, (1) Vickers indentation method and (2) three-point bending method were adopted. For the latter method, precracks of different shape were introduced by various techniques, such as (2-a) mechanical machining by a thin disc diamond wheel, (2-b) knoop indentation, (2-c) thermal machining by CO2 laser or (2-d) spark wire cut.
The results obtained are as follows.
(1) Each method has its own merits and demerits. To obtain least scattering and better reproducibility in K1c. data, however, three-point bending method using spark wire cut technique was found to be most appropriate. For non-electrified materials, Vickers indentation method or three-point bending method using mechanical machining with a very thin diamond wheel (15μm thickness) seems relevant from the same view point.
(2) Apparent K1c value of hard materials from this work are as follows. (K1c, MN/m3/2)
(a) 70% Al2O2-30% TiC
K1c=3.6-3.9 (Vickers indentation method)
K1c=3.2-3.4 (three-point bend, 15μm thickness thin diamond wheel)
(b) (Ti, Ta, Mo, W) (C, N)-Co, Ni Cermet
K1c=7.4-7.7 (Vickers indentation method)
K1c=6.2 (three-point bend, 15μm thickness thin diamond wheel)
(c) WC-12% Co cemented carbide
K1c=12.4-12.9 (Vickers indentation method)
K1c=11.5-12.3 (three-point bend, 15μm thickness thin diamond wheel)
(3) As for the three-point bending method, apparent K1c values depend on √ρ strongly and we found that ρo is smaller than 7.5μm for ceramics, cermet or cemented carbide used for this work.
