Evaluations of fatigue crack growth characteristics of sintered and thermal-sprayed WC-Co materials

Fatigue crack growth characteristics were investigated for sintered and thermal-sprayed tungsten carbide with cobalt (WC-Co) cermets. Acoustic emission (AE) techniques were applied to study the crack growth mechanism. The materials used were two types of sintered WC-Co materials, commercially available as G5 and KD20 with the WC grain sizes of 1.0-8.0/μm and 0.5-1.5μm, respectively, and 7mm thick sprayed layer with the WC grain size of 0.5-1.5μm. The Co content (mass %) of these materials were 12-13%. Compact type (CT) specimens were employed for fracture mechanics experiments in accordance with ASTM E647-95a. AE source wave analysis enabled us to estimate the crack volume dynamically developed near the crack tip. The fatigue crack growth rate, da/dN, depended on the WC grain size as well as the stress ratio for the sintered materials, and it was accelerated in sprayed material because of the existence of micro-pores. When these da/dN were compared with structural steels and aluminum alloy as a function of effective stress intensity factor, ΔK_eff, normalized by Young's modulus, E, they were much higher than those of the structural materials. This behavior was due to the brittle inter-granular (IG) fracture of WC particle, whose contributions were estimated by AE analysis and fracture surface observations. AE source wave analysis revealed that fracture volume of the IG fracture corresponded to the WC grain size and thus the IG fracture assumed to occur at the individual WC particles. The results of maximum stress intensity factor, K_max, constant, ΔK decreasing tests suggested that the frequency of the IG fracture of the WC particle was controlled by K_max. In the threshold region, the contribution of the IG fracture decreased and the threshold value of ΔK_eff tended to be constant for sintered materials, while it was lower for sprayed material due to the micro-pores.