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

The fatigue behavior of a ZrB 2 -based ceramic containing SiC and graphite was compared to a ZrB 2 -SiC reference material based on bending testing, quantitative calculations as well as crack growth and fracture characterization. The addition of graphite flake makes ZrB 2 -SiC-Graphite ceramics exhibit fatigue failure behavior at very high stress level (93% of the characteristic strength , σ 0 ), owing to the increased K Ic promoted by crack deflection, bridging, bifurcation and pull-out of graphite, while the fatigue behavior of ZrB 2 -SiC appears when the maximum stress is below ~86% σ 0 . 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 ( K c-initial ) value from 2.6 to 2.0 MPa m 1/2 .