Spark plasma joining of HfB2-ZrB2 based Ultra High Temperature Ceramics using Ni interlayer

Real-life applications of ultra-high temperature ceramics (UHTCs) mandates the joining processes to realize the final complex shapes, often resulting in the compromised performance of metallic interlayer. In the present work, two pre-sintered HfB2-ZrB2 (HZ) and HfB2-ZrB2-SiC (HZS) composites were joined using interlayer of Ni powders via spark plasma sintering (1100 degrees C, 18 MPa). Solid-state diffusion of Ni into UHTCs has resulted in the formation of continuous and crack free interface in both HZ/Ni/HZ and HZS/Ni/HZS joints. Microstructure and phase analysis revealed the presence of nearly equal Ni-rich joint seam (-100 mu m) and diffusion zone width (-150 mu m) in HZ/Ni/HZ joint. However, prominent reaction of Ni with SiC in HZS composite has formed Ni2Si, while exhibiting wider diffusion zone (-200 mu m) and thinner Ni-rich joint seam (-30 mu m). Thus, microstructural inhomogeneity in HZ/Ni/HZ joint resulted in an overall reduction of -38% in hardness from parent HZ region (15.6 GPa) to Ni-rich joint (-10 GPa), whereas for HZS, a uniform micro-hardness of -16 GPa was obtained in all the three regions owing to microstructural homogeneity across the interface. Damage tolerance of the joints has also been investigated by micro-scratch testing, revealing dominant abrasive wear in the parent UHTC region, whereas plastic deformation along with the tensile cracks is observed in the Ni-rich joint seam of HZ/Ni/HZ joint. In contrast, microstructural homogeneity in HZS/Ni/HZS joint has resulted in similar abrasive wear along with limited tensile cracks in all the regions, including that of Ni-joint seam, making it suitable for attaining superior structural integrity in joining complex shapes.