氮流量对六方氮化硼(hBN)薄膜结构和力学性能的影响

目的 研究磁控溅射过程氮气(N2)流量对六方氮化硼(hBN)薄膜结构和力学性能的影响，为设计新型结构hBN薄膜提供新思路。方法 利用中频电源磁控溅射硼靶，调节不同N2流量(6、12、18、24、30、36 mL/min)，沉积4 h后得到hBN薄膜，使用表征工具分析N2流量对hBN薄膜的组成、微观结构、表面形貌以及力学性能的影响。最后使用透射电子显微镜和选区电子衍射对所制备薄膜的纳米晶粒尺寸和晶体点阵结构进行分析。结果 XRD结果显示，薄膜物相主要为hBN。XPS结果说明，所制备薄膜为富硼hBN薄膜。薄膜的硬度和弹性模量随N2引入量的增加呈现先下降、后上升的趋势，最大硬度可达7.21 GPa，对应弹性模量为116.78 GPa。薄膜最低的硬度值为1.2 GPa，弹性模量为32.68 GPa。薄膜弹性破坏应变(H/E^*)和塑性变形抗力(H³/E^*2)随N2引入量的增加也呈现先上升、后下降的趋势，硬度最高薄膜对应的H/E^*值为6.414 ´ 10^–2，H³/E^*2值为29.27 ´ 10^–3 GPa，最低硬度值对应的H/E^*值为3.819 ´ 10^–2，H³/E^*2值为1.77 ´ 10^–3 GPa。透射结果显示，当N2引入量从6 mL/min逐渐增加到36 mL/min时，薄膜微观结构由结晶较差的卷曲结构过渡到局部纳米晶结构，最后形成结晶性较好的卷曲结构，并再次证明所制备薄膜为hBN。结论 在中频磁控溅射沉积hBN薄膜时，通过调整N2流量可以有效调节薄膜的特殊组成，使结构发生转变，进而影响薄膜的力学性能。

Effect of N2 Fluxes on Structure and Mechanical Properties of Hexagonal Boron Nitride (hBN) Thin Films

The reports on deposition of hexagonal boron nitride (hBN) thin films by intermediate medium-frequency (MF) magnetron sputtering technique and the impact of these structures on mechanical properties are not comprehensive enough. In this paper, hBN thin films with special structures were synthesized by MF magnetron sputtering and the ways the structures changed mechanical properties of the films were investigated. Thin BN films were deposited under different N2 fluxes (6, 12, 18, 24, 30 and 36 mL/min, respectively) on Si. Subsequently, the effects of N2 fluxes on the composition, microstructure, surface morphology and mechanical properties of the as-obtained BN films were investigated with characterization tools. Finally, a transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were used to analyze the nanocrystalline grain size and crystal lattice structure of the films. The samples were sequentially named N6, N12, N18, N24, N30 and N36, according to the different N2 introduced during film deposition. According to the SEM, the B and N atoms in the film were uniformly distributed. The film''s B and N ratio was shown by XPS to be larger than 1, showing the presence of B-B bonds. The XRD result showed that the as-obtained films were mainly composed of hexagonal BN (hBN). It was observed that the film with the lowest hardness value had the least amount of surface roughness, which was associated with B oxidation. The content of boron oxide (B2O3) was the highest in N24, corresponding to the lowest hardness and surface roughness. This was related to the surface smoothing effect of B2O3. In addition, the hardness and elastic modulus of the films exhibited a trend of first dropping and then rising, and the maximum hardness and elastic modulus could be respectively up to 7.21 GPa and 116.78 GPa, while the lowest hardness and elastic modulus were only 1.2 GPa and 32.68 GPa, respectively. The elastoplasticity of thin films, the elastic failure strain (H/E) and the plastic deformation resistance (H3/E2) were also used to evaluate the mechanical properties of thin films. It could be seen that their change trend was the same as that of hardness and elastic modulus. N6 and N36 exhibited the highest H3/E2 and H/E, with N6 having H3/E2 up to 29.77 × 10–3 GPa and H/E up to 6.414 × 10–2. In contrast, N24 exhibited the lowest H3/E2 and H/E values, with H3/E2 reaching 1.77 × 10–3 GPa and H/E reaching 3.819 × 10–2. Then, for TEM analysis, N6, N18 and N36 were selected respectively. The result demonstrated that the structure of these films were mostly curly structure. The heart of these curly structures started to crystallize when N2 introduction was 18 sccm, while the outside layer still had a curly shape. The ultimate formation for N36 had a more clearly curly structure. The films with these curly structures had a high load bearing capacity and hardness. This indicated that thin films deposited with certain N2 content during the deposition process had a strong ability to redistribute load, and therefore could exhibit strong resistance to plastic deformation. Obviously, the mechanical properties of the hBN films can be regulated through adjusting the N2 fluxes during the MF magnetron sputtering process. This result offers a new strategy to regulate the microstructure and mechanical strength of hBN films in future.