Effects of surface roughness and atmosphere on wetting of HfB2-ZrB2 based composites using a powder-based method
BAJPAI S. 1, SINGH K. 1, UPADHYAYA A. 1, VENKATESWARAN T. 2, BALANI K. 1
1 Indian Institute of Technology Kanpur, Kanpur, India; 2 Vikram Sarabhai Space Centre, , Thiruvananthapuram , India
Hafnium and Zirconium based di-borides are good choices as ultra-high temperature ceramics owing to their high melting point (3380 °C and 3245 °C, respectively), high thermal conductivity (104 Wm-1K-1 and 60 Wm-1K-1, respectively), chemical inertness etc. Real-life applications of HfB2 and ZrB2 based diborides require them to be joined to themselves or sometimes to other ceramics. In the present work a novel powder-based approach has been utilized to estimate the wettability of HfB2-ZrB2 based composites in contact with Ni. Herein Ni powder is sprinkled on the pre-sintered HfB2-ZrB2-SiC-B4C-CNT (HZSBC) composites with different surface roughness (0.1 and 0.7 µm) and heated to 1450 °C in Ar and Ar+H2 atmosphere. Equal surface energy (~52 mJ/m2) of both the surface roughness reveals its independence on roughness parameters. However, the higher contact angle values (by ~30°) are observed for rougher surface owing to the formation of gas-liquid Ni interface. Introduction of 5% H2 into the atmosphere resulted in the water equilibrium reactions, thus, lesser surface oxide formation and improved wettability (↓CA by ~10°). Good adhesion of droplet on the rough surface (Ra:0.7 µm) in Ar+H2 atmosphere is confirmed with maximum penetration depth of ~125 µm. Furthermore, the cross-sectional analysis of the droplets has revealed the presence of 3 different layers, i.e., Ni-rich layer, diboride rich layer and (Hf, Zr, C) layer thus confirming the wetting to be dissolutive one. Based on these, brazing of HZSBC composites with Ni interlayer is carried out at the same condition where lower wettability and poor adhesion of Ni with HZSBC in Ar resulted in poor interfacial bonding and thus detachment of the composites during polishing. Successful joining of these composites was observed in Ar+H2 atmosphere where microstructural inhomogeneity resulted in decreased hardness and elastic modulus by 68% and 58%, respectively from HZSBC region to Ni layer, thus making them suitable for applications involving complex shapes.