Innovative powder-based wettability evaluation of HfB2-ZrB2-SiC-B4C-CNT composite: Effect of surface roughness and ambient conditions

Active metal brazing, the most common method for joining UHTCs, necessitates the wettability of diborides in contact with filler layer, which is evaluated in present work using a powder-based approach. Ni powder is placed on pre-spark-plasma-sintered HfB2-ZrB2-SiC-B4C-CNT (HZSBC) composites with varying surface roughness (0.1 and 0.7 μm) and heated to 1450°C in Ar and Ar+H2 atmosphere. Formation of gas-liquid-Ni interface for rough surface (Ra:0.7 µm) restricted the spreading of molten Ni on HZSBC surface and increased contact angle (by ∼30°) compared to smooth surface (Ra:0.1 µm). Addition of 5% H2 into Ar atmosphere suppressed the oxide formation and improved molten Ni spreading at a contact angle of ∼10°. Cross-sectional analysis of droplets supports dissolutive wetting where enhanced surface roughness has reduced the dissolution depth (by ∼50% for Ar and ∼12% for Ar+H2 atmospheres). Furthermore, brazing of HZSBC composites with Ni interlayer is performed at 1450°C, in Ar and Ar+H2 atmosphere. Lower wettability of Ni with HZSBC in Ar resulted in detachment of composites during polishing, thus, indicating poor interfacial bonding. Successful brazing of these composites in Ar+H2 atmosphere resulted in microstructural inhomogeneity with decreased hardness and Young's modulus by ∼68% and ∼58%, respectively, from HZSBC region to Ni-Si rich filler layer. Lower interfacial residual stresses (∼0.4 GPa) have provided the structural integrity between ceramic and metal layer, thus making them suitable for applications involving complex shapes. [Display omitted]