当用在工具中来对最硬的材料进行切割和成形时，即便是金刚石也有其局限性。因此材料学家寻求合成比天然金刚石更硬、最好还有更好热稳定性的材料。在空气中，天然金刚石在大约800 °C开始氧化，导致在高温下产生严重磨损。通过降低其颗粒大小来提高金刚石硬度的努力已获成功，但代价是热稳定性变得更差了。Yongjun Tian及同事报告了他们获得的合成金刚石，它不但超硬，而且热稳定性也大大增强，氧化温度超过1,000 °C。该材料是用类似洋葱的碳纳米颗粒作为前体合成的，其硬度的增强是由于一种纳米结构——该结构不是由微小颗粒组成的，而是由孪晶(通过对称性相关联的晶格域)组成的。这一结果是在用纳米孪晶氮化硼立方体获得类似的成功之后取得的，它为制备具有优异特性的新型先进碳基材料提供了一个普遍性方法。
Although diamond is the hardest material for cutting tools, poor thermal stability has limited its applications, especially at high temperatures. Simultaneous improvement of the hardness and thermal stability of diamond has long been desirable. According to the Hall−Petch effect, the hardness of diamond can be enhanced by nanostructuring (by means of nanograined and nanotwinned microstructures), as shown in previous studies. However, for well-sintered nanograined diamonds, the grain sizes are technically limited to 10−30 nm (ref. 3), with degraded thermal stability compared with that of natural diamond. Recent success in synthesizing nanotwinned cubic boron nitride (nt-cBN) with a twin thickness down to ~3.8 nm makes it feasible to simultaneously achieve smaller nanosize, ultrahardness and superior thermal stability. At present, nanotwinned diamond (nt-diamond) has not been fabricated successfully through direct conversions of various carbon precursors (such as graphite, amorphous carbon, glassy carbon and C60). Here we report the direct synthesis of nt-diamond with an average twin thickness of ~5 nm, using a precursor of onion carbon nanoparticles at high pressure and high temperature, and the observation of a new monoclinic crystalline form of diamond coexisting with nt-diamond. The pure synthetic bulk nt-diamond material shows unprecedented hardness and thermal stability, with Vickers hardness up to ~200 GPa and an in-air oxidization temperature more than 200 °C higher than that of natural diamond. The creation of nanotwinned microstructures offers a general pathway for manufacturing new advanced carbon-based materials with exceptional thermal stability and mechanical properties.