Silicon carbide (SiC) is commonly used in industrial applications requiring chemical inertness, high hardness and excellent strength at elevated temperatures. However, its application scope is limited by its low fracture toughness. One way of addressing this limitation is by toughening SiC with whiskers or fibers to form SiC/SiC composites, endowing it with remarkable mechanical properties that can be modified to desired applications. Importantly, SiC and SiC/SiC ceramics also exhibit neutron irradiation tolerance, making them potential candidates for structural applications in nuclear reactors.
Ceramic structures, especially large ones, require highly effective and dependable joining techniques. While brazing is the most common joining method, most fillers and brazes have different irradiation tolerance and fail to remain inert in nuclear environments. Fusion welding of ceramics has been adopted to overcome these limitations. It requires materials to form melts during welding, which resolidify into one part. However, SiC and SiC/SiC components do not melt but rather dissociate at elevated temperatures around 2500 °C.
To form a more irradiation-tolerant joint containing SiC, a second material able to produce liquid upon heating with SiC is required. Such candidates include zirconium diboride (ZrB2) and zirconium carbide (ZrC) which can be welded with SiC ceramics in the SiC-ZrB2-ZrC ternary system. While joining SiC ceramics via fusion welding is an important step toward the implementation of welded ceramic components, the evolution of the mechanical properties of ceramic welds is still poorly understood. Analogous studies to ceramic welding in the SiC-ZrB2-ZrC system have been performed using arc-melting and induction heating techniques to measure mechanical properties of solidified melt patties, but these studies are scarce and focus on room temperature properties of produced samples. Additionally, there are very few studies on the mechanical properties of SiC-ZrB2-ZrC materials at elevated temperatures, none of which are systematic. “We’re really missing a big piece of the engineering picture here without higher temperature data,” states author Dr. Jecee Jarman. “We have very little data to rely on for modelling and only guesses as to how high in temperature a ceramic weld can go before losing its strength and failing.” “Within this ternary system, and other ultra-high temperature ceramic systems, our work at Missouri S&T aims to explore the high temperature behavior of these types of materials (both unwelded and welded, when applicable) and begin to fill in the gaps of missing high temperature mechanical knowledge which is not readily available in the associated literature or databases.”
Herein, then PhD candidate Jecee Jarman, Dr. William Fahrenholtz, Dr. Greg Hilmas, Dr. Jeremy Watts and Dr. Derek King from Missouri University of Science and Technology investigated the mechanical properties of fusion welded SiC-ZrB2 and SiC-ZrB2-ZrC ceramic systems. In particular, coupons from 60 SiC-40 ZrB2 vol% compositions, binary eutectic of SiC-ZrB2 system and ternary eutectic of SiC-ZrB2-ZrC systems were synthesized, welded using either plasma arc or tungsten inert gas welding and tested at elevated temperatures. The strength and strength retention of the welded ceramics were measured. Their work is currently published in the Journal of the European Ceramic Society.
The research team showed that all the investigated compositions were weldable, and the composition consisting of 40 vol% ZrB2 and 60 vol% SiC had the highest SiC content that could be effectively joined. The strength of SiC-ZrB2 binary eutectic, 40 vol% ZrB2 and 60 vol% SiC was 650 MPa when tested at 25 °C – 1700 °C. When ZrC was added to SiC-ZrB2 ceramics, the elevated temperature strength decreased to 300 MPa at 1700 °C. Consequently, the toughness property of the parent materials decreased from to after welding.
Typically, the welded strength of the ceramic compositions was 150 – 200, suggesting that they retained about 30 – 40% of their original room temperature strength upon welding at elevated temperatures up to 1700 °C. At elevated temperatures, weld strength retention was highest for SiC-ZrB2-ZrC ternary eutectic due to the effect of ZrC. During welding, ZrC played a vital role in reducing the melting temperature of the melt, resulting in a reduction of the sizes of the trapped pores upon solidification of the fusion zones.
In summary, the new study revealed that the strength of the welds and the strength of the parent materials were controlled by the size of the pores in the fusion welds and the size of the SiC clusters, respectively. Specifically, the low elevated temperature strength of the ceramic was attributed to the presence of pores in the fusion zones. To this end, the authors concluded that the strength of ceramic welds can be improved by removing the pores in the fusion zones.
In a statement to Advances in Engineering, Dr. Jecee Jarman first and corresponding author stated that their findings would contribute to better understanding and further improvement of the mechanical properties of fusion welded ceramics. Dr. Jarman noted “Our work sets the stage for joining more types of ceramics to themselves and other materials like refractory metals. Studying this relatively unexplored area of research teaching us about the cutting-edge capabilities of these materials when welded.” There is still more welding research to explore in the SiC-ZrB2-ZrC system, but the system is a likely candidate in many developing systems for industrial high temperature applications. “It’s been a one heck of a hands-on experience to investigate this system, and I’m looking forward to where our next generation systems incorporate these types of joints to achieve hotter and more efficient performance!” says Jarman.
Jarman, J. D., Fahrenholtz, W. G., Hilmas, G. E., Watts, J. L., & King, D. S. (2022). Mechanical properties of fusion welded ceramics in the SIC-zrb2 and SIC-zrb2-zrc systems. Journal of the European Ceramic Society, 42(5), 2107-2117.