Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB-SiC composites

A study has been carried out to examine the effect of LaB6 addition on the compressive creep behavior of ZrB2-SiC composites at 1300-1400 degrees C under stresses between 47 and 78 MPa in laboratory air. The ZrB2-20 vol% SiC composites containing LaB6 (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B4C and 4.8% C as additives were prepared by spark plasma sintering at 1600 degrees C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300 degrees C than the ZSBCL-10. The obtained stress exponent (n similar to 2 +/- 0.1) along with cracking at ZrB2 grain boundaries and ZrB,-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n similar to 1 and apparent activation energy similar to 700 kJ/mol obtained for the ZSBCL-14 composite at 1300-1400 degrees C suggest that ZrB2 grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of similar to 280 mu m, indicates the predominance of creep-based deformation and damage at the inner core of samples.