Yang Rong, Zhang Qun, Xiao Pan, Wang Jun, Bai Yilong
State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing, 100190, People's Republic of China.
Sci Rep. 2017 Nov 22;7(1):16053. doi: 10.1038/s41598-017-14734-w.
Although it has been well known that hardness of metals obtained with conical indenter remains a constant of about 3 times yield strength in conventional tests, and hardness will show a size effect of increasing hardness with decreasing indentation depth in micro-scale beyond 100 nm, the nano-indentation hardness experiments within 100 nm indentation depth usually show a large deviation and unclear trends. We report the cross-validated experimental and numerical results of two opposite depth-dependences of hardness at real nano-scale. That is to say, the indentation size effect (ISE) of hardness of single-crystal copper shows a rapid increase and then a slow decrease with increasing indentation depth within 100 nm depth. All of the results were coss-checked by means of both elaborated nano-indentation experiments with calibrated indenter tips and large scale molecular dynamics (MD) simulations. Further analysis of the MD results and experimental data reveal that the two opposite ISE of nano-hardness should be attributed to the finite roundness of the indenter tip and the intrinsic transition governing property of the material.
尽管众所周知，在传统测试中，使用锥形压头获得的金属硬度在屈服强度的约3倍左右保持恒定，并且在超过100纳米的微观尺度上，硬度会呈现出随着压痕深度减小而增加的尺寸效应，但在100纳米压痕深度内的纳米压痕硬度实验通常显示出较大偏差且趋势不明确。我们报告了在实际纳米尺度下硬度的两种相反深度依赖性的交叉验证实验和数值结果。也就是说，单晶铜硬度的压痕尺寸效应（ISE）在100纳米深度内随着压痕深度增加先快速增加然后缓慢减小。所有结果都通过使用校准压头尖端的精细纳米压痕实验和大规模分子动力学（MD）模拟进行了交叉验证。对MD结果和实验数据的进一步分析表明，纳米硬度的两种相反ISE应归因于压头尖端的有限圆度和材料的本征转变控制特性。
