Introduction ZrB is the leading material

Introduction ZrB2 is the leading material among ultra-high-temperature ceramics due to its high melting point (3245 °C), high hardness (23 GPa), high thermal conductivity (∼60 W m−1 ·k−1), electrical conductivity (∼107 S m−1), and relatively low density (6.09 g cm−3), high strength at elevated temperatures, and stability in extreme environments [1]. The addition of SiC and carbon can inhibit the grain growth, improve the sinterability, and increase the thermal and mechanical properties and oxidation resistance [2,3]. ZrB2–SiC based ceramics are attractive for aerospace applications such as thermal protection systems, leading edges, trailing edges, and propulsion components for hypersonic flight vehicles [4,5]. The components of ZrB2–SiC composite are generally made by pressureless sintering (PS) or hot pressing (HP). The difficulty in fabricating the large size or complex shapes limits the application of ZrB2–SiC composites. Joining is an alternative and cost effective method for fabricating the large size or complicated shape components of ceramics. Joining of engineering ceramics such as Al2O3, SiC, C/C, C/SiC has been widely studied [6–11]. Recently, several investigations on joining of ZrB2–SiC composites have been reported in the literature. Bellosi et al. [12] joined ZrB2–SiC composites using Ca–Al–Si–O, and Y–Al–Si–O glass powders as bonding inter layers at 1440 °C and reported a bending strength of 277 MPa at room temperature. Among other reports, brazing is the most common method. Muolo et al. [13] studied the wetting behavior of ZrB2 with different metals (Cu, Ag, Au) and their alloys and optimal results were obtained with silver-based alloys. Due to low melting point of these solders, the joints cannot be useful at temperature above 1000 °C. Singh et al. [14] studied the joining of ZrB2–SiC9–SiC, ZrB2–SiC–C and ZrB2–SiC to themselves and to commercially pure Ti using boron containing Ni buy zip braze alloys (MBF-20 and MBF-30). Asthana et al. [15] studied the joining of similar composites to themselves using Pd-based brazes Palco (65% Pd-35% Co) and Palni (60% Pd-40% Ni). Hair line cracks, substantial chemical interaction and interfacial cracking due to residual stresses were observed. Practical applications also require joining of ZrB2–SiC composites to refractory metals like Nb and its alloys. Bo et al. [16] joined the monolithic ZrB2 and ZrB2–SiC composites to themselves using pure Ni powder. The maximum shear strengths of 59.7 ± 5.3 MPa and 43.4 ± 7.5 MPa were obtained for ZrB2 and ZrB2–SiC joints, respectively. Peng et al. [17] diffusionally bonded ZrB2–SiC composites to Nb using Ti interlayer to synthesize TiB whiskers array insitu and reported a maximum shear strength of 158 MPa. Yang et al. [18] diffusionally bonded ZrB2–SiC to Nb using dynamically compressed Ni foam interlayer and reported a maximum shear strength of 155.6 MPa. Though arc welding of ZrB2 is possible due to its electrical conductivity [19], the research on joining by gas tungsten arc welding (GTAW) or plasma arc welding is very limited. Brown et al. [20] reported fusion welding of ZrB2-20 vol% SiC and ZrB2–SiC–B4C composites. By preheating and controlled cooling under protective atmosphere the 3 mm-thick parts were joined by GTAW. The strength of joints was ¼ of the strength of parent material. Thermal conductivity of joints was higher than that of parent material. Very recently King et al. [21] reported the plasma arc welding of TiB2 – 20 vol % TiC composites and ZrB2-20 vol % ZrC composites [22] achieved by preheating the weld coupons to 150 °C. In the present work, a filler material of (ZrB2–SiC–B4C–YAG) composite with oxidation resistance and thermal shock resistance has been produced in the form of welding rods. The filler was used in GTAW of HP (ZrB2 – 20 vol.% SiC), and PS (ZrB2 – 20 vol.% SiC – 8 vol.% B4C – 7 vol.% YAG) composites. Without any preheating, post controlled cooling, and extraneous protective gas shield, GTAW was performed manually.