Interatomic Description of Clustered Structure for Strength of Diamond-like-carbon Film

类金刚石(DLC)薄膜主要由金刚石中的sp3键原子和石墨中的sp2键原子组成。关于其力学性能和摩擦学性能的研究已有较多报道，但对其变形机理的研究还不够深入。在本研究中，用分子动力学方法研究了单轴载荷下成键形式的变化。由随机游动方法得到的DLC薄膜的单轴拉伸分析表明，在张力作用下，sp3键原子的含量比sp2键原子的含量减少。观察到伴随着键的湮灭和成键形式的产生而产生的成键形态的变化。通过这些变化，包含背键相互作用的局域原子环境变得能量稳定。并得出结论：在形变过程中，原子间的弯曲和扭转效应比键的拉伸效应更为明显。然而，已有的特尔索夫势能并不能描述其扭转效应，当考虑非晶态非均匀结构的形变时，上述事实可能是致命的缺陷。因此，在下一阶段，我们将采用(N)阶紧束缚分子动力学方法(O(N)TB-MD)来解释这些问题。采用N阶尺度的密度矩阵法，将原子间作用力的合理精度作为势能的一阶梯度，将弹性刚度作为二阶梯度，从而大大减少了计算时间。结果表明，采用淬火工艺得到的初始类金刚石模型中的sp3含量较高，与实验结果吻合较好。扭转阻力对DLC薄膜的变形起着重要作用。

A diamond-like carbon (DLC) film is mainly composed of both sp^3 bonded atoms as in diamond and sp^2 bonded atoms as in graphite. Many reports concerning their mechanical and tribological properties have been published, while there are not enough studies on deformation mechanism yet. In the present research, the change of bonding form under uniaxial loading is investigated using the molecular dynamics. From uniaxial tension analyses of the DLC film obtained by the random walk method, the content rate of sp^3 bonded atom decreases as opposed to the one of sp^2 bonded atom under the tension. The morphological changes of bonding which are accompanied by the annihilation and the generation of the bonding forms are observed. By these changes, local atomic environment which includes the back-bond interaction comes to stabilize energetically. And, it is concluded that bending and torsion effects between atoms are more predominant rather than bond-stretching under the deformation. However, the well-established Tersoff type potential can not represent its torsion effect.The above-mentioned fact may be fatal defect when considering the deformation of the amorphous-like inhomogeneous structure. Therefore, Order (N) tight-binding molecular dynamics scheme (O(N)TB-MD) is applied to elucidate these problems at the next stage. The density matrix method as an Order (N) scaling is adopted to reduce the drastic computational time with keeping the reasonable accuracy of an interatomic force as the first-order gradient of the potential energy and elastic stiffness as the second-order gradient. It is found that the high content of sp^3 in the initial DLC model by quenching process is obtained, which is in good agreement with the experimental results. And the torsional resistance plays an important role in the deformation of the DLC film.