(V) ZrB2-SiC composites: Effect of SiC particle size on multi-length scale wear and oxidation resistance of ZrB2

ZrB₂-based composites are considered as a potential candidate for hypersonic vehicles because of their extraordinary properties, e.g., high hardness (15-29 GPa), fracture toughness (2.8-4 MPam^1/2), high thermal conductivity (58-60 Wm^-1k^-1), and low density (6.1 gcm^-3). Herein, reinforcement of varied SiC particle size (D₅₀ ~70 and ~3 m) is assessed towards resulting wear and high-temperature oxidation performance of ZrB₂ is investigated. Finer SiC reinforced ZrB₂ has displayed superior wear resistance at three different length scales of testing, as compared to that of coarse SiC reinforced ZrB₂ and un-reinforced ZrB₂. In addition, oxidation behavior of the composite samples tested at 1500 °C for 3 hours under two pO₂ (0.21 atm and 210^-5 atm) showed easier oxygen influx through coarse SiC reinforced ZrB₂ forming thicker silica layer (~ 9-94 µm as compared to that of ~ 4 µm in finer SiC reinforced ZrB₂), which could protect the bulk for longer duration, especially at 0.21 atm. Thickness of oxide-layers in both the composites increased with decreasing pO₂, signifying rapid oxidation taking place at lower pO₂. However, finer SiC reinforced ZrB₂ at 210^-5 atm resulted in the formation of fine ZrO₂ particles on the surface improving the oxidation resistance. The least tribological damage in the case of finer SiC addition at all the three length scales, complemented with superior oxidation resistance under both pO₂ makes its reinforcement a preferred choice for potential ZrB₂-based ultra high temperature ceramics for hypersonic and re-entry conditions.