Tools for cutting hard materials, such as steel and cast iron, must be able to withstand high temperatures and pressures. Mikinori Hotta and Takashi Goto at Tohoku University in Japan1 are developing composite materials for these applications with an emphasis on reducing fracture susceptibility and the expense of production.

Cubic boron nitride (cBN) is the second hardest known compound after diamond carbon. Boron nitride has the same crystal structure as carbon, and the boron and nitrogen atoms in cBN are packed in the same arrangement as carbon atoms in diamond. Hotta and Goto were therefore keen to incorporate cBN into their composites. However, cBN is expensive to produce due to the ultrahigh pressures needed in the sintering process to prevent cBN from changing to the softer phase of hexagonal boron nitride, which is structurally analogous to graphite. Another difficulty in applying cBN in tools is its propensity to fracture.

“An economical process using a moderate pressure of less than 100 megapascals would be useful,” says Hotta. “To realize cutting tools having high hardness and high fracture toughness, a combination of ceramics and cBN would appear to be promising.”

The ceramic composite β-SiAlON, composed of silicon, aluminum, oxygen and nitrogen, is slightly softer than cBN and is also used in cutting tools. The researchers investigated the possibility of using a low-pressure method for synthesizing composites containing both β-SiAlON and cBN.

The composites were made using spark plasma sintering, a modern technique in which an electric current is used to provide the heat required to sinter (or fuse) powders of β-SiAlON and cBN. This speedy heating process provides shorter sintering times compared to traditional sintering methods, helping to suppress the phase transformation of cBN to the hexagonal phase. This means lower pressures can be used.

At the pressure of 100 MPa, the team investigated the effect of sintering temperature (1,600–1,900 °C) and β-SiAlON/cBN ratio on the microstructure of the composites using X-ray diffractometry. So far, the researchers have found that composites containing 10–30% cBN are denser — and therefore stronger — than β-SiAlON alone, and that the addition of β-SiAlON inhibits the phase change of cBN to the hexagonal form.