Chinese scientists create 'unbreakable' diamond — six times tougher than tungsten alloys

Scientists from the Institute of Physics of the Chinese Academy of Sciences, Beihang University, and other research centers have successfully embedded a three-dimensional network of multi-walled carbon nanotubes (MWCNTs) into a polycrystalline diamond matrix. The resulting composite maintains Vickers hardness up to 91.6 GPa, comparable to single-crystal diamond, while achieving average fracture toughness of 31.9 MPa·m1/2 and a maximum measured value of 36.4 MPa·m1/2 — about five to six times higher than typical single-crystal diamond and significantly exceeding the fracture toughness of several tungsten alloys. The researchers used diamond powder with grain sizes of 0.8–1.3 μm and MWCNTs with outer diameter 10–20 nm, inner diameter 5–10 nm, and length 10–20 μm. Final sintering was performed at 15 GPa and about 2000 °C, producing cylindrical samples about 3 mm in diameter and 2 mm thick. The surviving nanotubes occupied only about 0.35% by volume but formed a continuous network between diamond grains, acting like reinforcing fibers. Under high pressure, some nanotubes partially converted to diamond, covalently bonding to the matrix. When cracks form, they are forced to divert, branch, and change direction, expending more energy; nanotubes are pulled out and form bridges between separating grains. This combination of diamond-like hardness and high crack resistance makes the material promising for wear-resistant cutting tools, superhard ceramics, and parts subject to high pressure and impact loads, such as in aerospace.