AlMgB_(14)三元超硬硼化物的实验制备与理论研究

Experimental and therotical study on the mechanical properties of AlMgB_(14)

三元硼化物AlMgB14材料具有高硬度,高韧性,低密度,低摩擦系数以及抗高温氧化腐蚀等众多优异性能,在工具、模具、微机械制造及航空航天关键零部件等领域具有重要的应用前景.本文采用多靶射频磁控共溅射技术,以高纯度Al（99.99%）,Mg（99.95%）,B（99.9%）材料为溅射靶源,在单晶Si（100）衬底上溅射沉积得到了表面均匀致密,不同化学组分的Al-Mg-B三元非晶态薄膜,通过调控溅射参数,可以实现原子比接近Al：Mg：B=1：1：14.对于所制备的薄膜样品,在三元相图上沿Al-Mg等含量线,Al-Mg-B材料的硬度,随着B元素含量增加由13GPa增大到32GPa,其中AlMgB14附近成分点的硬度达到25~32GPa.同时,采用第一性原理计算,得到AlMgB14晶体的维氏硬度为27.6GPa,与实验值接近.通过电子结构分析,我们进一步探讨了AlMgB14晶体和其他相似结构富B三元化合物硬度的起源,发现他们共同具有的B12二十面体骨架是决定硬度的主要因素.Al,Mg等金属元素主要通过向B12的电荷转移对材料硬度进行微调,Al-B之间形成了弱的共价键而Mg-B之间形成离子键,这点同时也为XPS实验谱图所证实.采用准简谐近似德拜模型,我们还研究了AlMgB14晶体的热学性质以及温度对体弹性模量的影响,发现体模量随温度增加明显的减小,同实验上提高衬底温度对硬度的增强呈现了相反的变化趋势.这从侧面说明了衬底温度提高导致的硬度增强并不是改变了材料的本征硬度,而可能由于加强了薄膜同衬底间附着力.

Al-Mg-B films have been the subject of numerous investigations because their attractive properties, including high hardness, high melting point, low density, chemical inertness and thermal stability at high temperature. These materials may be useful in the fields of mechanical, aerospace and military. In this paper, using multi-target magnetron sputtering, we prepared Al-Mg-B amorphous film with atomic ratio approach to Al：Mg：B =1：1：14 on the silicon （100） substrate at room temperature by tuning the sputtering conditions. For the Al-Mg-B films along the AlMg isocontent line, nanoindentation test indicated that the hardness of films increase from 13 GPa to 32 GPa with the contents of B increasing, and the value would reached to 25–32 GPa for AlMgB14. Meanwhile, using first-principles calculations, we estimated the Vickers hardness of AlMgB14 crystal is 27.6 GPa, which agreed to the experimental value. Based on the electron density of states and Mulliken population analysis, the origination of hardness of AlMgB14 crystal other B12-based ternary compounds were discussed. The crystal hardness is primarily determined by the B12 icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms, showing weak covalent bond of Al–B and the ionic characteristics of Mg–B bonds, which is also demonstrated by our experimental XPS core level spectra. Moreover, we also investigated the thermodynamic property of AlMgB14 crystal by Quasi-harmonic Debye model, and found that the bulk modulus decrease with the temperature rising. Such tendency was opposite to the experiment results of Al-Mg-B films have higher hardness under high substrate temperature, which implying the role of temperature is to strengthen the adhesion between film and substrate rather than improving the true hardness of the materials.