材料研究学报, 2024, 38(6): 410-422 DOI: 10.11901/1005.3093.2023.383

研究论文

氟化六方氮化硼的制备及其作为水基添加剂的摩擦学性能

王伟,1, 常文娟1, 吕凡凡1, 解泽磊1, 于呈呈2

1.西安建筑科技大学冶金工程学院 西安 710055

2.季华实验室 佛山 528200

Preparation and Tribological Properties of Fluorinated Boron Nitride Nanosheets Water-based Additive

WANG Wei,1, CHANG Wenjuan1, LV Fanfan1, XIE Zelei1, YU Chengcheng2

1.Xi'an University of Architecture and Technology College of Metallurgical Engineering, Xi'an 710055, China

2.Jihua Laboratory, Foshan 528200, China

通讯作者: 王伟,教授,gackmol@163.com,研究方向为材料加工中的摩擦与润滑

收稿日期: 2023-08-11   修回日期: 2024-02-03  

基金资助: 国家自然科学基金(51975450)
陕西省科技新星基金(2021KJXX-32)
西安市创新能力强基计划-先进制造技术攻关项目(21XJZZ0031)
陕西省教育厅服务地方专项(22JC047)
陕西省重点研发计划(2023-YBGY-383)

Corresponding authors: WANG Wei, Tel: 13609264618, E-mail:gackmol@163.com

Received: 2023-08-11   Revised: 2024-02-03  

Fund supported: National Natural Science Foundation of China(51975450)
Science and Technology New Star Foundation of Shaanxi Provincial(2021KJXX-32)
Xi'an Innovation Capability Strong Foundation Plan - Advanced Manufacturing Technology Research Project(21XJZZ0031)
Shaanxi Provincial Department of Education Service Local Special Project(22JC047)
Key R&D Plan Projects in Shaanxi Province(2023-YBGY-383)

作者简介 About authors

王 伟,男,1985年生,博士

摘要

对NH4F氟源进行恒温磁力搅拌和球磨辅助氟化,制备出氟化氮化硼纳米片(F-BNNSs),在接触Ti6-Al-4V(TC4)/GCr15的条件下研究了六方氮化硼纳米片(h-BNNSs)和氟化氮化硼纳米片(F-BNNSs)作为水基润滑添加剂的摩擦学行为。结果表明,随着h-BN与NH4F的质量比从1∶2提高到1∶4这种水基润滑添加剂的平均摩擦系数先由0.3135降低到0.1435然后提高到0.2177。h-BN与NH4F的质量比为1∶3时的F-BNNSs12其摩擦系数和磨损率比h-BNNSs分别降低了55%和75%。基于磨损痕迹的分析结果表明,F-BNNSs12良好的减磨耐磨性能可归因于:1、纳米片层间相互作用的减弱使相对滑动变得容易;2、F-BNNSs12的不相称纳米棒-纳米片结构形成滚动摩擦使其润滑性能良好;3、F-BNNSs12沉积在摩擦副表面生成不连续的润滑膜,避免了与摩擦副的直接接触。

关键词: 无机非金属材料; 六方氮化硼; 氟化氮化硼纳米片; 水基润滑添加剂; 摩擦学性能

Abstract

Hexagonal boron nitride (h-BN) is a typical layered structure material with enormous potential in the field of friction and lubrication. Fluorinated boron nitride nanosheets (F-BNNSs) were prepared by constant temperature magnetic stirring and ball mill-assisted fluorination using NH4F as fluorine sourced, while h-BN as raw material. Tribological behavior of hexagonal boron nitride nanosheets (h-BNNSs) and F-BNNSs as water-based lubricant additives were evaluated under Ti6-Al-4V(TC4)/GCr15 contact conditions. The results showed that when as water based liblicant additives, the F-BNNSs prepared with the mass ratio of h-BN to NH4F increased from 1:2 to 1:4, the resulted average coefficients of friction (COFs) varied from 0.3135 to 0.1435 to 0.2177. When the mass ratio of h-BN and NH4F was 1:3, the COFs and wear rate of F-BNNSs12 prepared were 55% and 75% lower than that of h-BNNSs, respectively. Based on the analysis of wear scars, it were found that the excellent friction reduction and anti-wear performance of F-BNNSs12 can be attributed to three aspects: The weakening polar interactions between the nanosheet layers may facilitate the relative slidding; F-BNNSs12 incommensurate nanorod-nanosheet structure may cause rolling friction and good lubrication performance; F-BNNSs12 deposited on the surface of the friction pair favour forminga discontinuous lubricating film, to alleviate the direct contact for the friction pair.

Keywords: inorganic non-metallic materials; hexagonal boron nitride; fluorinated boron nitride nanosheets; water-based lubrication additives; tribological properties

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本文引用格式

王伟, 常文娟, 吕凡凡, 解泽磊, 于呈呈. 氟化六方氮化硼的制备及其作为水基添加剂的摩擦学性能[J]. 材料研究学报, 2024, 38(6): 410-422 DOI:10.11901/1005.3093.2023.383

WANG Wei, CHANG Wenjuan, LV Fanfan, XIE Zelei, YU Chengcheng. Preparation and Tribological Properties of Fluorinated Boron Nitride Nanosheets Water-based Additive[J]. Chinese Journal of Materials Research, 2024, 38(6): 410-422 DOI:10.11901/1005.3093.2023.383

钛合金的比强度高、抗腐蚀性强和耐高温性能优异,可用于制造航空薄壁部件[1]。在可切削加工钛制薄壁部件的高速切削过程中易发生“颤振”和“弯曲”,产生的切削热使刀具变形、切削刃断裂、切屑粘附或刀具磨损失控[2]。同时,在切削过程中工件与刀具之间较大的摩擦力严重影响工件表面的质量和降低刀具的寿命。使用切削液能润滑、冷却工件和切削刀具并降低切削过程中的温度、切削载荷、延长刀具寿命和提高工件表面的质量[1,3~5]

钛合金切削液,分为油基切削液和水基切削液[6]。传统的油基切削液可减少刀具的磨损,但是其冷却性能较差和不易降解[7]。因此,研发高效和环保的钛合金切削液极为迫切。水资源储量丰富和绿色环保,因此可在摩擦过程中提供流体动力膜,但是其单独作为润滑剂时润滑性能和成膜能力较低[6,8,9]。将添加剂加到水中生成的水基润滑剂,能减摩和提高耐磨性能[8,10],尤其是将纳米材料作为润滑添加剂其润滑性能更为优异。目前作为润滑相的纳米材料,有石墨烯[11],二氧化硅[12],黑磷[13],二硫化钼[14]和六方氮化硼[15]等。Liang[16]等研究了原位剥离石墨烯作为水基润滑添加剂的摩擦学行为。结果表明,石墨烯具有良好的分散稳定性和减磨耐磨性能,其在5~15N载荷下的摩擦系数为0.05~0.2。Guo[17]等用自组装法制备黑磷-氧化石墨烯纳米复合材料,研究了其作为水基润滑添加剂的摩擦学特性。结果表明,在5N负载下几乎实现了超滑状态。Wang[18]等用自组装法制备黑磷/二硫化钼复合材料,研究了其作为水基润滑添加剂的摩擦学特性。结果表明,黑磷与二硫化钼的质量比为1∶4时润滑效果最佳,摩擦系数为0.2272。Liang[19]等制备出一种GO@ SiO2纳米离子液体,使用球-盘摩擦机进行了室温摩擦实验。结果表明,载荷为15N、浓度为4%时减磨耐磨效果最佳,摩擦系数约为0.3。但是,虽然以上这几类纳米润滑材料的润滑效果较好,但是使用成本较高。h-BN是一种典型的二维材料,与石墨的晶体结构和晶格参数相似,具有优于石墨的良好的热稳定性、优异的机械强度和润滑性能[20]。同时,h-BN具有良好的生物相容性、优良的电绝缘和高化学稳定性,是水基润滑添加剂的理想候选材料[21]。Cho等[22]发现,向水中加入h-BN能提高耐磨性能和降低摩擦系数。但是,h-BN纳米片具有化学惰性和较高的表面能,其在润滑介质中的分散性较差且在滑动过程中易发生团聚,使其润滑性能降低[22,23]。同时,硼原子和氮原子的电负性不同,使h-BN的片层之间产生了极强的极性,能抑制层间的滑移,使h-BN作为润滑添加剂的减摩和抗磨性能降低[24,25]。用电化学剥离[26]、化学气相沉积[27]、液体超声处理[28]、化学反应[29]和水热法[30]等方法将块状h-BN剥离,可制备出单层或多层氮化硼纳米片(BNNS)。但是,这些方法的生产条件苛刻、生产率较低、成本较高和产生环境污染。特别是剥离的BNNS生成很少的官能团,且在润滑介质中的分散稳定性较差而产生团聚,严重降低其减摩作用[24,30]。表面功能化(例如氟化和氧化),是改变和监测二维材料的力学和物理性能的有效方法[31~33]。氟化物修饰能降低h-BN的表面极性和抑制其在溶剂中的团聚,掺杂到h-BNNSs上的原子可使电子再分配,从而减少其层间摩擦。鉴于此,本文以氟化铵(NH4F)作为氟源用恒温磁力搅拌和高能球磨工艺制备F-BNNSs[34~36],研究其作为水基添加剂的摩擦学性能。

1 实验方法

1.1 实验用材料

无水乙醇(C2H5OH,分析纯),h-BN(1~2 μm),氟化钠,石油醚。

1.2 h-BN的氟化

为了找出h-BN与NH4F的最佳比例,进行了3组实验,质量比分别为1∶2、1∶3和1∶4,分别命名为F-BNNSs8、F-BNNSs12和F-BNNSs16图1给出了制备F-BNNSs的流程图。先将h-BNNSs与NH4F混合在一个150 mL的烧杯中放入超声波清洗机中超声15 min使其混合均匀,然后在170℃进行6 h的磁力搅拌。磁力搅拌结束后将溶液与氧化锆小球(ϕ5 mm和ϕ10 mm)放入100 mL的球磨罐中,用球磨工艺进一步对h-BNNSs氟化与剥离,球料比为1∶50,球磨转速为350 r/min,球磨时间20 h。球磨结束后,反复用去离子水洗涤产物使上清液的pH值接近中性。最后,将以6000 r/min速度离心分离出的沉淀物干燥得到F-BNNSs。

图1

图1   F-BNNSs制备过程的示意图

Fig.1   Schematic illustration of F-BNNSs preparation process


1.3 摩擦磨损实验

使用旋转式摩擦磨损实验仪(UMT-5)进行球-盘实验,测定F-BNNSs润滑添加剂的摩擦磨损性能。上试样GCr15钢球的直径为6 mm,下试样TC4圆盘的直径为25 mm厚度为8 mm。实验参数列于表1

表1   摩擦实验参数

Table 1  Friction experiment parameters

Radius / mmLoad / NLinear velocity / mm·s-1Time / minConcentration / mg·mL-1
68, 10,12, 1550100.5, 1, 1.5, 2

新窗口打开| 下载CSV


在外部负载作用下,球-盘接触点处是椭圆接触区[34]。相应的赫兹接触应力(p)为

p=4Wπa2

其中

a=223WRE'1/3
1E'=121-μ12E1+1-μ22E2

E′为有效弹性模量[37]W位施加的载荷,R为球半径。E1 (TC4-113 GPa)和E2 (GCr15-207 GPa)分别为摩擦副的弹性模量,μ1 (TC4 0.34)和μ2 (GCr15 0.30)分别为摩擦副的泊松比。于是可计算出最大赫兹接触应力为1.3~1.6 GPa。

GCr15钢球的磨损率为

K=VNS

其中

V=πl63d24+l2
l=r-r2-d24

V为磨损体积,N为球上施加的载荷,S为滑动距离[38]l为磨损轨迹的深度,r为球的半径,d为磨损直径。

TC4盘的磨损率为

K=VNS

其中

V=πh2r-h3

r为GCr15小球的半径,h为TC4盘的磨损深度,N为施加的载荷,S为滑动距离[39,40]

1.4 微结构观测和相组成分析

用X射线衍射仪(XRD,Bruker D8),红外光谱仪(FTIR,Nicolet Is50)和X射线光电子能谱仪(XPS,Thermo Scientific K-Alpha)观察h-BNNS和F-BNNS的化学组成和相结构。用透射电子显微镜(TEM,Talos F200X)和扫描电子显微镜(SEM,Gemini 300)观察h-BNNS和F-BNNS的微观结构。使用自组装接触角测量仪和Zeta电位分析仪(Malvern Zetasizer Nano ZS90)检测F-BNNS的润湿性和分散稳定性。用配有X射线能谱仪(EDS)的扫描电子显微镜表征Ti-6Al-4V盘磨损表面的形态和化学复合物。用SEM测量GCr15球的磨痕直径,用激光共聚焦显微镜(HYBRID L7)分析磨损深度和磨损轨迹轮廓。用拉曼光谱仪(Raman,LabRAM HR Evolution)识别磨损表面上残留物的结构。用XPS鉴定磨损痕迹上的元素复合物和化学价。

2 结果和讨论

2.1 材料的组成和微结构

制备F-BNNSs时,因NH4F的分解温度为168℃,设定磁力搅拌温度为170℃[41]。h-BNNSs的化学惰性很强和稳定性较高,因此很难将小分子和杂原子共价接枝到h-BNNSs上。以NH4F作为氟源,F与B原子的结合能约为-4.17 eV。结合能较低有利于F原子的吸附,并且NH+和F-在剥离和氟化BNNSs中起关键作用[42]

图2a给出了h-BNNSs和F-BNNSs(F-BNNSs8、F-BNNSs12和F-BNNSs16)在20°~80°的完整XRD谱,可见所有样品的出峰位置一致,衍射角度(2θ)均为26.6°、41.5°和54.9°。图2b给出了h-BNNSs和F-BNNSs在(002)晶面的局部放大XRD谱,用以进一步分析2θ为26.6°处峰的结构。根据Bragg方程(2dsinθ = λ),XRD峰的衍射角(θ)随着纳米片层间间距(d)的变化而变化。X射线的波长为0.15406、2θ为26.6°,根据Bragg方程计算出h-BNNSs的平面间距大约为0.33 nm。B-F和N-F的键长分别为0.131 nm和0.138 nm,小于h-BNNSs的平面间距,因此(002)晶面并没有偏移[42]图2c给出了h-BNNSs、F-BNNSs8、F-BNNSs12和F-BNNSs16的FTIR光谱。h-BNNSs在780 cm-1和1362 cm-1处的峰可归因于B-N-B弯曲和B-N伸缩振动[43]。F-BNNSs12和F-BNNSs16在1080 cm-1和1253 cm-1处的特征峰,对应B-F振动[21]。没有发现N-F键振动的原因是,B-N-B平面外弯曲振动重叠[44]。F-BNNSs8并没有B-F和N-F的特征峰出现,其原因可能是NH4F的使用较少,嫁接在h-BNNSs的表面上F原子含量较低。

图2

图2   F-BNNSs的XRD谱以及局部放大和F-BNNSs的红外光谱

Fig.2   XRD of F-BNNSs and local magnification (a, b) and FTIR of F-BNNSs (c)


图3给出了F-BNNSs粉末的XPS谱。图3a、b和c分别给出了F-BNNSs8、F-BNNSs12和-BNNSs16的XPS全谱,谱中出现了B 1s、C 1s、N 1s和O 1s峰以及F1s和F KLL信号。NH4F的使用量不同,使F元素的峰强不同,NH4F的用量多因此F元素的强度高,反之强度低。对F-BNNSs12的窄谱进行了分峰,结果在图3d~f中给出。图3d中的190.3 eV和191 eV处的峰对应B-N和B-F键。图3e中397.9 eV和398.7 eV处的峰分别对应N-B和N-F键。与B和N元素相比,F的高电负性使B-F和N-F键对应的峰都出现在较高的结合能处。图3f中出现在688.8 eV处的峰对应F 1s能级的电子结合能。上述对XPS的分析证明,F原子已经接枝到h-BN表面。

图3

图3   F-BNNS的XPS图和F-BNNSs12的窄谱

Fig.3   XPS of F-BNNS and XPS narrow spectra of F-BNNSs12 (a~c) XPS full spectra of F-BNNSs8, F-BNNSs12, and F-BNNSs16; (d~f) B 1s, N 1s, F 1s


图4给出了h-BNNSs、F-BNNSs8、F-BNNSs12和F-BNNSs16的SEM照片以及相应的元素含量。图4a给出了h-BNNSs的高倍SEM照片,可见主要形态是圆形片层堆叠而成,EDS元素分析结果表明主要元素是B、N,质量比接近1∶1。图4b、c、d分别给出了F-BNNSs8、F-BNNSs12和F-BNNSs16的高倍SEM照片,可见与嫁接F原子的h-BNNSs的微观形貌相似,F-BNNSs比h-BNNSs的层数有所减少。同时,氟化物的修饰使F-BNNSs的边缘卷曲。随着氟源用量的增加,F-BNNSs8、F-BNNSs12和F-BNNSs16相应元素分布图片中的F元素含量随之提高。以上结果表明,氟源使用量的增加使h-BNNSs片层上嫁接的F原子增多,与预期的结果一致。

图4

图4   h-BNNSs和F-BNNSs的SEM照片

Fig.4   SEM images of h-BNNSs and F-BNNSs


图5给出了样品的TEM图像、选定区域电子衍射(SAED图像)以及粒径统计。图5a、b分别给出了不同区域F-BNNS12的TEM照片,可见纳米片边缘明显的卷曲结构。图5d给出了图5a白色边框内样品的放大示意图,可见卷曲结构且因弯曲较大形成了纳米棒结构[40]图5c中的SAED图表现出典型h-BN的6倍对称特征,表明得到的F-BNNSs在球磨过程中结晶良好,没有发生损伤[17]图5e给出了图5d中红框区域内的高分辨TEM照片,可见十个晶面的间距为2.189 nm,一个晶面间距大约为0.22 nm,对应于F-BNNSs12的(001)平面,与原始的h-BNNSs相同。对F-BNNSs12粒度分析(图5g、h)结果表明,最小直径为46.41 nm,最大直径为191.15 nm,平均直径为91.78 nm,大部分纳米片的直径集中在40~120 nm。

图5

图5   F-BNNSs12的TEM、SAED图像和粒径统计

Fig.5   TEM、SAED images and particle size statistical of F-BNNSs12 (a, b) images of different regions; (c) SAED image; (d) high magnification TEM image; (e, f) lattice stripes; (g, h) particle size statistical image


F-BNNSs是由恒温磁力搅拌与球磨辅助氟化与剥离制备的。在第一个制备过程中,氟化铵的高温分解有利于氟化h-BNNSs。N原子与B原子的电荷性不同,使其更倾向于与B原子结合,如图6所示。边缘的B原子嫁接了更多的F原子,且掺杂F原子使h-BNNSs表面弯曲[31,38]。随后在球磨过程中纳米片发生了剥离,使F原子有更多的嫁接位点,促进h-BNNSs的氟化形成了纳米棒结构[45]

图6

图6   NH4F氟化h-BN原理图

Fig.6   NH4F fluorinated h-BN schematic


2.2 F-BNNSs的分散稳定性

润滑液在金属基体表面的铺展能力越强,其润湿性能越好。因此,纳米粒子的润湿性是度量纳米流体稳定性的关键。图7给出了F-BNNSs的Zeta电位和接触角测试结果。纳米颗粒之间的团聚可用Zeta电位值表征(图7a),绝对值越大意味着纳米颗粒分散得越均匀[46,47]。F-BNNSs8、F-BNNSs12和F-BNNSs16的电位绝对值分别为13.1 mV、19.6 mV和37.0 mV,均大于h-BNNSs的绝对电位值。F-BNNSs的静电排斥性比h-BNNSs更强,从而分散性更好,随着NH4F使用量的增多分散性提高。图7b~f给出了纯水、h-BNNSs和F-BNNSs的接触角测试结果。可以看出,纯水与TC4盘的接触角为47.523°,向水溶液中加入少量的h-BNNSs接触角即增大为48.531°,表明其在TC4盘的表面铺展性不好。氟化改性的h-BNNSs,随着NH4F使用量的增多F-BNNSs与TC4盘的接触角逐渐减小,F-BNNSs16的接触角最小为36.613°。以上分析均表明,F-BNNSs润滑添加剂具有良好的分散稳定性和润湿性,并且氟化程度越高分散性和润湿性越好。

图7

图7   F-BNNSs水基润滑添加剂的 Zeta 电位值和接触角

Fig.7   Zeta potential values and contact angle of water-based lubricant additives


2.3 摩擦磨损性能

图8a、b给出了载荷为10 N、线速度为50 mm/s条件下h-BNNSs和不同氟化效果的F-BNNSS(F-BNNSs8、F-BNNSs12、F-BNNSs16)的摩擦曲线和磨损率。图8a表明,添加了h-BNNSs的水基润滑剂的摩擦系数最大,平均摩擦系数可达0.3135。h-BNNSs在水中的分散性最差,h-BNNSs的纳米片之间发生了团聚,且其润湿性较差使摩擦系数比较大。添加少量的F-BNNSs作为润滑剂,使摩擦系数显著降低。F-BNNSs8作为水基润滑添加剂,摩擦曲线在开始100 s内的磨合阶段摩擦系数较高,经过一段时间后饱和摩擦系数趋于平缓稳定,平均摩擦系数为0.1512,表明其润滑性能良好。以F-BNNS16作为润滑添加剂,摩擦系数随时间呈现缓慢的下降趋势且波动较小,相应的平均摩擦系数为0.2177。F-BNNSs12作为水基润滑添加剂时摩擦系数最低,摩擦曲线的变化趋势与F-BNNSs8相似,但是磨合阶段较短,摩擦系数略低。F-BNNSs12的平均摩擦系数为0.1435,比h-BNNSs水基润滑平均摩擦系数降低55%。虽然F-BNNSs16片层表面嫁接了更多的F原子,但是其摩擦系数在0.2附近波动。其原因是,随着F原子掺杂的进一步增加分子层之间的滑动变化缓慢[48]图8b给出了h-BNNSs和F-BNNSs作为水基添加剂的GCr15磨损率。可以看出,随着氟化强度的增强磨损率逐渐降低,最小的磨损率为1.41 × 10-6 mm3·N-1·m-1,比h-BNNSs的磨损率降低了75%。这表明,h-BNNSs表面接枝氟原子,减小了摩擦力且具有一定的抗磨损能力。

图8

图8   h-BNNSs和F-BNNSs的摩擦学性能

Fig.8   Tribological properties of h-BNNSs and F-BNNSs (a, b) h-BNNSs, F-BNNSs8, F-BNNSs12, F-BNNSs16 friction curves; (c) friction curves of different concentrations of F-BNNSs12; (d) average friction coefficient of pure water and F-BNNSs12 at 8~15N


图8c给出了不同质量的F-BNNSs12分散在水溶液中得到浓度分别为0.5 mg/mL、1 mg/mL、1.5 mg/mL、2 mg/mL的水基润滑剂的摩擦曲线,并且在载荷为10 N和线速度为50 mm/s的条件下验证上述4种浓度润滑剂的摩擦学性能。结果表明,F-BNNSs12的浓度为1 mg/mL、1.5 mg/mL时摩擦系数均低于0.2,浓度为1 mg/mL时平均摩擦系数为0.1435。浓度过低时难以形成连续的润滑膜,浓度过高时纳米片在摩擦副接触微区容易发生团聚,使摩擦系数过大[49,50]图8d给出了纯水和1 mg/mL F-BNNSs12在8 N、10 N、12 N和15 N负载下的平均摩擦系数。可以看出,在负载为8~15 N时纯水的平均摩擦系数均高于添加浓度为1 mg/mL的F-BNNSs12平均摩擦系数。随着载荷的增大添加1 mg/mL F-BNNSs12水基润滑剂的平均摩擦系数呈现先减小后增大的变化趋势,载荷为10 N和20 N时平均摩擦系数分别达到最小和最大。调整h-BNNSs与NH4F的质量比、浓度和载荷,发现最佳氟化样品为F-BNNSs12,最佳浓度和载荷分别为1 mg/mL、10 N。

2.4 磨损表面的形貌

图9给出了在10 N和50 mm/s条件下,经过h-BNNSs和F-BNNSs (F-BNNSs8、F-BNNSs12、F-BNNSs16)润滑后磨损表面的SEM照片。图9a给出了h-BNNSs润滑后的TC4表面,可见较深的犁沟和较多的磨屑。EDS分析结果表明,产生这些磨屑的原因是在一定压力下长时间摩擦使TC4盘表面发生剥落。图9b、c给出了F-BNNSs润滑后的表面,可见较浅的犁沟和少量磨屑。EDS分析结果表明,磨损表面白色框内的黑色区域聚集了B、N和F元素,是在TC4盘的磨损表面残留的润滑剂。F-BNNSs12的磨损表面最平整,基本上没有磨屑只有一些残留的润滑剂。这表明,F-BNNSs12具有良好的润滑性能和抗磨性能。

图9

图9   h-BNNSs和F-BNNSs的磨损表面SEM图像

Fig.9   SEM images of the wear surfaces of h-BNNSs and F-BNNSs (a) h-BNNSs; (b) F-BNNSs8; (c) F-BNNSs12; (d)F-BNNSs16


图10给出了负载分别为8 N、10 N、12 N和15 N,使用F-BNNSs12水基润滑剂磨损表面的SEM照片。图10a、d分别给出了负载分别为8 N和15 N的磨损表面,可见表面有较深的犁沟和剥落,剥落的磨屑中有少量润滑剂。这表明,润滑不足使TC4磨损表面产生了裂纹。图10b、c分别给出了负载分别为10 N和12 N的磨损表面,可见表面的磨损痕迹较浅,白色区域有大面积的黑色物质。EDS分析结果表明黑色物质是聚集的B、N和F元素,证明有润滑剂残留在磨损表面。对比分析不同载荷下的磨损表面,发现用F-BNNSs12润滑的磨损表面更为平整,证明其润滑性能较好。

图10

图10   不同载荷下磨损表面的SEM照片

Fig.10   SEM images of the wear surface for different loads (a) 8 N; (b) 10 N; (c) 12 N; (d) 15 N


图11给出了用纯水与浓度都为0.1 mg/mL的h-BNNSs和F-BNNSs12水基润滑后的磨损轨道、三维轮廓图像和对应的二维高度轮廓曲线。图11c给出了F-BNNSs12水基润滑后TC4盘的磨损轨道和三维轮廓图像,可见其表面比用纯水与h-BNNSs润滑(图11a、b)更平整光滑,也得到了二维高度轮廓曲线的进一步验证。纯水、h-BNNSs与F-BNNSs12的磨损轨道宽度分别为414.5 μm、524.15 μm、374.9 μm,磨损深度分别为12.2 μm、17.4 μm、11.4 μm。定量计算结果表明,对应的磨损率分别为4.5 × 10-6 mm3·N-1·m-1、9.2 × 10-6 mm3·N-1·m-1、3.9 × 10-6 mm3·N-1·m-1。h-BNNSs比纯水与F-BNNSs水基润滑的磨损量较大,其原因是h-BN的硬度(莫氏硬度为2)和断裂韧性(1.09 ± 0.01 MPa·m1/2)较低[51]。同时,硼原子与氮原子的电负性不同,使h-BNNSs的片层之间除了有极弱的范德华力还有极强的极性,抑制了层间滑移和降低了h-BNNSs作为润滑添加剂的性能[23,24]。F-BNNSs12的磨损轨道的磨损率最低,因为F-BNNSs12优异的抗裂能力使层状的平面结构免受平面内刚度和外应力的破坏,并使层间在外力的作用下相对滑动(起减磨润滑作用),与前文的实验验证结果一致[52]

图11

图11   TC4盘磨痕表面的磨损轨道、三维轮廓图和二维高度轮廓曲线

Fig.11   Wear scar, 3D contour map and 2D height profile of TC4 disk wear surface (a) pure water; (b) h-BNNSs; (c) F-BNNSs12


图12a、b分别给出了在10 N和50 mm/s条件下使用1 mg/mL F-BNNSs12水基润滑剂的TC4圆盘磨损表面的拉曼谱和检测位置。图12a表明,拉曼检测位置在磨痕的中间,保证了检测结果准确性。从拉曼谱可见,表面出现了大面积的暗斑,可能产生了不连续的润滑膜。在拉曼光谱(图12b)的1366 cm-1处出现一个F-BNNSs12特征峰,表明磨损表面残留有润滑剂[45]。为了进一步证明存在化学反应膜并得到其化学成分,进行了XPS分析 (图12c~g)。图12c给出了用1 mg/mL F-BNNSs12水基润滑后TC4盘磨损疤痕的全谱,检测到F 1s、O 1s、Ti 2p、N 1s、C 1s和B 1s。图12d表明,位于188.5 eV和190.4 eV处的B 1s峰主要以B-N和B-F的形式存在;还检测了F 1s XPS谱(图12f),证明摩擦实验后残留的润滑剂紧密结合在磨损处的表面。N 1s位于396.6 eV和399.5 eV处的峰对应N-B和Ti-N,但是没有检测到N-F键,可能是F原子与B原子很大的电负性差异使其更倾向于彼此结合,而不是与N原子结合,摩擦实验后N-F键的量更少[34];Ti 2p XPS谱中396.6 eV和399.5 eV处出现了双峰。结合图7b的EDS分析结果,在磨损痕迹处聚集了大量的N、B、F元素,证明大部分磨损痕迹都有残留的润滑剂。根据前述对磨痕的分析,磨损痕迹处有润滑剂的残留,在一定的力和滑动的作用下润滑剂与TC4盘发生了化学反应,在磨损表面生成了由F-BNNSs和TiN组成的不连续润滑薄膜。生成不连续润滑膜的原因,可能是TC4金属盘表面不十分平整,每个接触点受力的不同使有的位置没有产生润滑膜。本文的研究结果表明,磨损表面形成的润滑膜是降低钛合金/钢接触点的摩擦和磨损的关键。

图12

图12   1 mg/mL F-BNNSs12水基润滑磨损表面的Raman谱和XPS谱

Fig.12   Raman spectra and XPS spectra of wear surface of 1 mg/mL F-BNNSs12 water-based lubrication


2.5 F-BNNSs的润滑机理

根据Hamrock-Dowson理论,可使用公式

hmin=2.69G0.53U0.67W0.0671-0.61e-0.73k
λ=hminσ12+σ22

计算理论最小薄膜厚度和比率。式中U = ηV/ERG = αE′,W = F/ER2R (3 mm)为GCr15球的半径,V (50 mm/s)为线速度,η为润滑油粘度,α为粘度压力系数为,E′ (164 GPa)为有限弹性模量,F (10 N)为应用载荷,k ≈ 1为椭圆参数。σ1 (2.850 μm)与σ2 (4.424 μm)分别为上摩擦副球与下摩擦副TC4盘的粗糙度[38]。其中σ1 (2.850 μm)和σ2 (4.424 μm)分别为上摩擦副球与下摩擦副TC4盘的粗糙度。计算结果表明,hmin约为12.9 nm,λ约为0.25,小于1,表明润滑状态处于边界润滑。在边界润滑状态下,上下摩擦副之间产生一层极薄的润滑膜,有助于实现良好的润滑性能。法向载荷的不断增大和摩擦时间的延长,使润滑薄膜的完整性受到破坏,但是F-BNNSs12提供了更多的接触位点,避免了上下摩擦副之间较多的接触和实现了自身优异的润滑性能,从而产生了优良的减磨耐磨作用[53]

基于上述结果和对磨损表面的分析,提出了F-BNNSs12的润滑机理。与h-BNNSs相比,F-BNNSs12作为水基润滑添加剂使摩擦系数和磨损率大幅度降低。h-BNNSs作为水基润滑剂时,由于其在摩擦微区内发生团聚和较低的硬度与断裂韧性,在外力长时间作用下容易发生破损、断裂,使润滑效果降低[16,22,47]。F-BNNSs12作为水基润滑添加剂时,纳米片上较大的负电荷使层间的极性相互作用减弱,与h-BNNSs相比更小的剪切力就能使片层间发生相对滑动,从而使摩擦力更小。图6也表明,F-BNNSs12不相称的纳米棒-纳米片结构在摩擦过程中呈现出滚动摩擦。同时,F-BNNSs12还能填充磨损表面的凹槽使摩擦副之间的接触面积减小。这些因素,都有助于实现良好的润滑性能[16,21,45,54]。同时,在外力的作用下F-BNNSs12逐渐沉积在摩擦副的表面,在边界润滑状态下摩擦副的表面形成了一层稳定且不连续的润滑膜,减少了钛合金/钢接触点而使摩擦和磨损降低[55]

3 结论

用高效简单的恒温磁力搅拌和球磨辅助氟化可制备F-BNNSs12。与h-BNNSs相比F-BNNSs12在水中的分散性和稳定性大幅度提高,其分散稳定性(Zeta电位值)提高4倍,接触角度减少20%。h-BNNSs与NH4F配比为1∶3制备的F-BNNSs12实现了超低摩擦,其摩擦系数与TC4盘的磨损率分别为0.1435和1.41 × 10-6 mm3·N-1·m-1,比h-BNNSs的摩擦系数和磨损率降低了55%和58%。超低摩擦的实现,可归因于:1、F-BNNSs12在水中优异的分散性,使其在摩擦微区内不发生团聚。2、纳米片层间更易发生相对滑动,F-BNNSs12可以填充磨损表面存在的凹槽,避免摩擦副的大面积接触。3、嫁接F原子使h-BNNSs形成了不对称的纳米棒-纳米片结构而产生滚动摩擦,并且在摩擦副表面生成了不连续的润滑膜而使其摩擦系数降低。

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Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This "pre-filling" of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process, which is also relevant to other 2D materials and their heterostructures.

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Here, we present a simple method for preparing thin few-layer sheets of hexagonal BN with micrometer-sized dimensions using chemical exfoliation in the solvent 1,2-dichloroethane. The atomic structure of both few-layer and monolayer BN sheets is directly imaged using aberration-corrected high-resolution transmission electron microscopy. Electron beam induced sputtering effects are examined in real time. The removal of layers of BN by electron beam irradiation leads to the exposure of a step edge between a monolayer and bilayer region. We use HRTEM imaging combined with image simulations to show that BN bilayers can have AB stacking and are not limited to just AA stacking.

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Cubic boron nitride (c-BN) possesses a number of extreme properties rivaling or surpassing those of diamond. Especially, owing to the high chemical stability, c-BN is desired for fabricating electronic devices that can stand up to harsh environments. However, realization of c-BN-based functional devices is still a challenging task due largely to the subtlety in the preparation of high-quality c-BN films with uniform thickness and controllable properties. Here, we present a simple synthetic strategy by surface fluorination of few-layered hexagonal boron nitride (h-BN) sheets to produce thermodynamically favorable F-terminated c-BN nanofilms with an embedded N-N bond layer and strong inbuilt electric polarization. Due to these specific features, the fluorinated c-BN nanofilms have controllable band gap by thickness or inbuilt and applied electric fields. Especially, the produced nanofilms can be tuned into substantial ferromagnetism through electron doping within a reasonable level. The electron-doping-induced deformation ratio of the c-BN nanofilms is found to be 1 order of magnitude higher than those of carbon nanotubes and graphene. At sufficient high doping levels, the nanofilm can be cleaved peculiarly along the N-N bond layer into diamond-like BN films. As the proposed synthesis strategy of the fluorinated c-BN nanofilms is well within the reach of current technologies, our results represent an extremely cost-effective approach for producing high-quality c-BN nanofilms with tunable electronic, magnetic, and electromechanical properties for versatile applications.

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DOI      PMID     

Two-dimensional (2D) hexagonal boron-nitride oxide (h-BNO) is a structural analogue of graphene oxide. Motivated by recent experimental studies of graphene oxide, we have investigated the chemical oxidation of 2D h-BN sheet and the associated electronic properties of h-BNO. Particular emphasis has been placed on the most favorable site(s) for chemisorption of atomic oxygen, and on the migration barrier for an oxygen atom hopping to the top, bridge, or hollow site on the h-BN surface, as well as the most likely pathway for the dissociation of an oxygen molecule on the h-BN surface. We find that when an oxygen atom migrates on the h-BN surface, it is most likely to be over an N atom, but confined by three neighbor B atoms (forming a triangle ring). In general, chemisorption of an oxygen atom will stretch the B-N bond, and under certain conditions may even break the B-N bond. Depending on the initial location of the first chemisorbed O atom, subsequent oxidation tends to form an O domain or O chain on the h-BN sheet. The latter may lead to a synthetic strategy for the unzipping of the h-BN sheet along a zigzag direction. A better understanding of the oxidation of h-BN sheet has important implications for tailoring the properties of the h-BN sheet for applications.This journal is © the Owner Societies 2012

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The reduction of interfacial friction in commensurately stacked two-dimensional layered materials is important for their application in nanoelectromechanical systems. Our first-principles calculations on the sliding energy corrugation and friction at the interfaces of commensurate fluorinated-graphene/h-BN and oxidized-graphene/h-BN heterostructures show that the sliding energy barriers and shear strengths for these heterostructures are approximately decreased to 50% of those of commensurate graphene/h-BN. The adsorbed F and O atoms significantly suppress the interlayer electrostatic and van der Waals energy corrugations by modifying the geometry and charge redistribution of the graphene layers. Our empirical registry index models further reveal the difference between the roles of the F and O atoms in affecting the sliding energy landscapes, and are also utilized to predict the interlayer superlubricity in a large-scale oxidized-graphene/h-BN system. Surface functionalization is a valid way to control and reduce the interlayer friction in commensurate graphene/h-BN heterostructures.

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