Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp² to sp³ chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.

原子稀薄的石墨烯表现出令人着迷的机械性能，尽管其硬度和横向刚度低于金刚石。到目前为止，还没有关于将多层石墨烯转变成类金刚石超硬结构的实际证明。在这里，我们表明，在室温和纳米压痕之后，SiC（0001）上的两层石墨烯表现出的横向刚度和硬度与金刚石相当，对金刚石压头的穿孔具有抵抗力，并且压痕后导电性可逆地下降。密度泛函理论计算表明，压缩后，两层石墨烯薄膜会转变为类金刚石薄膜，同时产生弹性变形和sp ²至sp ³化学变化。实验和计算表明，对于厚度大于三到五层的SiC或石墨烯薄膜上的单个缓冲层，未观察到这种可逆相变。实际上，计算表明，尽管在两层石墨烯层堆叠结构中控制类金刚石膜的构象，但在多层膜中却阻碍了相变。