Zirconium helps create super-strong composite materials

Introducing a layer of zirconium atoms between sheets of aluminium oxide and tungsten carbide creates exceptionally strong composite materials, according to Japanese scientists.

Ceramic matrix composites (CMCs) are incredibly strong materials used in jet engines, gas turbines and cutting tools for nickel superalloys. Aluminium oxide (Al₂O₃) is hard and chemically inert, and tungsten carbide (WC) is used as a superhard material, but past efforts to create an Al₂O₃-WC CMC have yielded unsatisfactory results. Researchers around the world have tested several formulations to identify one with a high bending strength — a measure of the physical stress a material can be subjected to before it becomes permanently bent or broken — but no group had developed an Al₂O₃-WC CMC with a bending strength greater than 1 gigapascal (GPa).

In an attempt to achieve a greater bending strength, scientists from Nagoya University, in collaboration with NGK Spark Plug Co, experimented with adding in small amounts of zirconium dioxide (ZrO₂) during the creation of Al₂O₃-WC CMCs. This addition yielded superhard Al₂O₃-WC CMCs with bending strengths greater than 2 GPa. According to lead investigators Dr Tomohiro Nishi and Dr Katsuyuki Matsunaga, who published their research in the journal Scientific Reports, “This is an all-time high in the field.”

Notably, the investigators achieved these considerable bending strength improvements with a relatively modest addition of ZrO₂. The additive represented less than 5% of the mass of the finished Al₂O₃-WC CMCs, which is less than the amount of additive usually present in additive-enhanced CMCs. When the investigators studied the structures of their superhard ZrO₂-enhanced Al₂O₃-WC CMCs using a method called atomic-resolution scanning transmission electron microscopy, they found that the Zr atoms were located in thin layers between sheets of Al₂O₃ and WC.

With respect to the interfaces between the Al₂O₃ and WC sheets, Dr Nishi and Dr Matsunaga said, “Such interfaces are generally weak points with respect to mechanical properties.” Therefore, the interface between the sheets is a plausible location for Zr atoms to exert effects that strengthen the Al₂O₃-WC CMCs. Indeed, when the investigators modelled the effects of the Zr atoms using techniques from a field of mathematical physics known as density functional theory, their results indicated that an interfacial layer of Zr atoms would enhance the stability of their CMCs.

The investigators foresee a bright future for their novel CMCs, with Dr Matsunaga stating, “The materials we developed can be used as superhard materials in metalworking devices for cutting hard metallic components to be used in aeroplanes and automobiles.” In fact, they note that engineers at NGK Spark Plug Co have already commercialised the materials as components in cutting tools. The creation of the enhanced Al₂O₃-WC CMCs also serves as an example of the considerable improvements in physical properties that can be achieved with relatively minor additions to a material.

Image credit: ©stock.adobe.com/au/Björn Wylezich

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