Influence of graphite on the low-frequency fatigue behavior of zirconium diboride ceramics

The fatigue behavior of a ZrB2-based ceramic containing SiC and graphite was compared to a ZrB2-SiC reference material based on bending testing, quantitative calculations as well as crack growth and fracture characterization. The addition of graphite flake makes ZrB2-SiC-Graphite ceramics exhibit fatigue failure behavior at very high stress level (93% of the characteristic strength, σ0), owing to the increased KIc promoted by crack deflection, bridging, bifurcation and pull-out of graphite, while the fatigue behavior of ZrB2-SiC appears when the maximum stress is below ~86%σ0. However, both the slow crack growth exponents of the graphite containing ceramic, n and nc values, which reflect the fatigue resistance in static and cyclic fatigue conditions, respectively, are only 1/4 as compared to the reference graphite-free ceramic. This may be due to the weak boride/graphite interfaces, which lead to the decrease of the initial critical stress intensity factor (Kc-initial) value from 2.6 to 2.0 MPa m1/2.