Graphene/SiO2 纳米复合材料作为水基润滑添加剂的摩擦学性能

钛合金轻质、耐高温、耐腐蚀、生物相容性好且无磁性，在航空、航天、兵器、舰船、医疗等领域得到了广泛的应用[1,2] 但是，钛合金的导热系数低、高温化学活性高和弹性模量小，在切削加工过程中工件与刀具的粘连使其磨损严重、加工后的工件表面质量较差、加工成本提高，限制了钛合金的应用[3~5] 提高钛合金切削性能的关键，在于改善切削界面摩擦状态，实现高效润滑 但是，钛合金独特的摩擦学特性使传统金属加工润滑液难以在钛合金表面有效润滑 在基础液中添加纳米材料，是提高润滑介质加工性的主要手段[6~8] 石墨烯(Graphene)是一种典型的二维材料，层与层之间依靠弱范德华力连接，具有较弱的剪切力、优异的机械性能、大比表面积和较高的热导率，在润滑领域受到了极大的关注[9,10] Ming等[11]在植物油中添加石墨烯用于TC4合金切削加工润滑，可增强铣削区域油膜的润滑性能 Ning等[12]将Graphene、磷酸盐、纳米ZrO2等按一定比例混合制备石墨烯水基润滑剂应用于钛合金热轧，降低了热轧过程的摩擦磨损和氧化 Ibrahim等[13]将石墨烯加入棕榈油中，摩擦系数和切削能耗比Acculube LB2000商用润滑油大幅降低 但是，结构完整的Graphene因化学稳定性高而难以在溶剂中稳定分散，容易产生不可逆团聚使摩擦过程中难以进入工况表面，无法发挥抗磨减磨的作用[14,15]

纳米复合材料在基础液中的分散性高，且不同纳米材料之间的协同作用可进一步提高润滑性能 Meng[16,17]等在氧化石墨烯(GO)表面沉积Au或Cu，降低了石墨烯片层间的π-π键的相互作用减少了团聚 与单一纳米材料(GO、Au和Cu)相比，复合材料之间的协同作用使其具有更优异的润滑性能 Li等[18]用激光辐射制备的Ag/Graphene复合材料可稳定在油中悬浮60 d以上，这种润滑添加剂不会产生金属腐蚀和环境污染 Graphene与金属纳米材料复合的成本高，回收难，因此难以推广 SiO2中的Si-O亲水性和耐磨性较好，且成本较低[19,20] Na等[21]用原位引发聚合法制备的PTFE/SiO2复合材料，提高了PTFE在纯水中的分散性和摩擦性能 Zhang等[22]用溶胶-凝胶法制备Fe3O4@SiO2纳米复合材料，提高了Fe3O4在环氧树脂中的分散性 在Graphene表面原位生成SiO2制备Graphene/SiO2纳米复合材料，可提高Graphene在超纯水中的分散性且降低成本 鉴于此，本文用溶胶凝胶法在Graphene表面原位生成SiO2制备Graphene/SiO2纳米复合材料，以提高Graphene在超纯水中的分散性且降低成本，并将其作为水基润滑添加剂研究GCr15/TC4接触下的摩擦学性能并揭示其润滑机理

1 实验方法1.1 实验用材料

无水乙醇(C2H5OH，分析纯)，氨水(NH3·H2O，分析纯)，石油醚(PE)和正硅酸乙酯(TEOS)，Graphene和工业SiO2

1.2 纳米复合材料Graphene/SiO2 的制备

使用溶胶-凝胶法中的St?ber法制备Graphene/SiO2[23]，其工艺示意图如图1所示 将0.2 g的Graphene添加到50 mL无水乙醇和50 mL超纯水的混合溶液中，使用超声波破碎30 min 然后加入1 mL氨水和2 mL TEOS并对混合溶液磁力搅拌12 h 对产物进行离心分离后收集胶状固体产物，用无水乙醇多次清洗以除去氨水和未反应的TEOS 将所得胶状固体在75℃真空环境干燥12 h后达到纳米复合材料Graphene/SiO2

图1



图1制备Graphene/SiO2纳米复合材料的示意图

Fig.1Schematic diagram of preparation of Graphene/SiO2 composite nanomaterials

1.3 摩擦磨损实验

使用旋转式摩擦磨损试验仪(UMT-5)测试试样的摩擦磨损性能，上试样是直径为6 mm的GCr15钢球，下试样是厚度为8 mm直径为25 mm的TC4圆盘 摩擦实验中的润滑剂，是超纯水中添加不同质量分数的Graphene/SiO2 实验的线速度为0.047 m/s，载荷为8~15 N，时间为30 min，根据赫兹理论计算赫兹接触压力

πP=4Fπa2

(1)

a=2(23×FRE')3

(2)

1E'=12(1-μ12E1+1-μ22E2)

(3)

其中P为赫兹接触应力，a为接触直径、F为摩擦试验机施加载荷(8~15 N)、R为GCr15球半径、E'为有效弹性模量[24,25] E1(TC4 113 GPa)和E2(GCr15 207 GPa)为摩擦试样的弹性模量，μ1(0.34)和μ2(0.30)为泊松比 最大接触应力范围为1.04~1.29 GPa GCr15球的磨损率为[26,27]

h=r-r2-d24

(4)

V=πl63d24+h2

(5)

WB=VPS

(6)

式中d为等效圆的直径，r为GCr15球的直径，P为载荷，S为总滑动距离 实验前用无水乙醇超声清洗GCr15球和TC4圆盘30 min以去除污染，摩擦实验开始前滴加80 μL的润滑剂 实验结束后用棉球擦拭表面，干燥后保存

1.4 性能表征

用沉降法评估Graphene/SiO2在超纯水中的分散稳定性[35] 将0.2%(质量分数)的Graphene和Graphene/SiO2分别加到超纯水中，超声1 h静置适当时间后拍摄光学图像

用X射线衍射仪(XRD，D/MAX-RB)测试Graphene/SiO2纳米材料的晶体结构 用扫描电子显微镜(SEM，JSM-5610LV)观察Graphene/SiO2复合材料的微观组织形貌，用SEM附带的EDS分析复合材料和磨损表面元素的成分 用拉曼光谱仪(LabRam HR Evolution)测试Graphene和Graphene/SiO2纳米复合材料的拉曼光谱 用金相显微镜(OM，GX51) 测量钢球磨斑的直径，用三维白光扫描仪(TDWS，MicroXAM-800)测量TC4圆盘磨损体积 用扫描电镜分析实验后TC4圆盘磨痕的微观组织形貌和元素的分布 用X射线光电子能谱仪(XPS，PHI 5000) 分析磨损表面的特征元素

2 结果和讨论2.1 Graphene/SiO2 纳米复合材料的形貌与表征

图2给出了Graphene和SiO2的扫描电镜照片，可见片层之间褶皱，边缘处卷曲，SiO2球状颗粒的直径约为300 nm 图2c给出了Graphene/SiO2纳米复合材料的扫面电镜照片，可见Graphene的卷曲结构，表面均为小球颗粒，能谱分析表明主要元素为Si、O和C，即Graphene表面生成了SiO2纳米颗粒 与单一的纳米SiO2相比，Graphene表面的SiO2颗粒尺寸差异较大(图2b和c) 其原因是，在TEOS发生化学反应形成SiO2的过程中Graphene和部分SiO2颗粒都是形核位点，生成了较大的SiO2颗粒[28]

图2



图2不同试样的SEM照片、Graphene/SiO2的能谱、以及Graphene/SiO2、Graphene、Amorphous SiO2和SiO2的XRD谱

Fig.2SEM images of different samples (a) graphene, (b) SiO2, (c) graphene/SiO2; (d) energy spectrum of Graphene/SiO2;(e) XRD patterns of Graphene/SiO2, Graphene, Amorphous SiO2 and SiO2

图2e给出了XRD谱 可以看出，Graphene在谱中的26.34°和42.68°出现了两个特征衍射峰[29]，低矮的衍射峰对应非晶态SiO2，明显的衍射峰对应晶体SiO2[30] Graphene/SiO2纳米复合材料的衍射峰与非晶SiO2一致，没有出现Graphene的衍射峰特征

图3给出了Graphene和Graphene/SiO2的拉曼光谱，可见Graphene的衍射峰位于1333.2 cm-1、1567.1 cm-1和2671.8 cm-1处，分别对应D峰、G峰和2D峰 G峰的强度高于2D峰，表明材料具有多层结构[31,32] 与Graphene的特征峰相比Graphene/SiO2的特征峰正向偏移，表明Graphene表面原位生成了SiO2[33,34] 以上结果表明，已制备出Graphene/SiO2纳米复合材料

图3



图3Graphene/SiO2和Graphene的Raman谱

Fig.3Raman spectra of Graphene/SiO2 and Graphene

2.2 Graphene/SiO2 的分散性

图4给出了不同润滑剂放置不同时间的光学图像 Graphene在超纯水中分散性差，放置24 h就完全分层 而含有Graphene/SiO2的超纯水溶液的分散较为稳定，静止48 h后开始出现沉淀，上层溶液变浅，表明其分散性优于Graphene

图4



图4不同润滑剂在不同时间的光学图像

Fig.4Optical images of different lubricants at different time:(a) 0.2% Graphene; (b) 0.2% Graphene/SiO2

2.3 Graphene/SiO2 纳米复合材料作为水基润滑添加剂的摩擦学性能

图5给出了不同含量的Graphene/SiO2的平均摩擦系数和磨损率曲线 可以看出，平均摩擦系数和磨损率均呈现先下降后上升，0.2%(质量分数)的Graphene/SiO2摩擦系数最低，比超纯水工况降低17.9%，钢球磨损率降低61.7% 添加剂含量超过0.2%(质量分数)，则摩擦性能开始降低

图5



图5不同含量的Graphene/SiO2的平均摩擦系数和磨损率曲线

Fig.5Curves of average coefficient of friction and wear rate of Graphene/SiO2 with different contents

图6给出了在不同载荷下0.2%(质量分数)Graphene/SiO2润滑剂的摩擦系数 从图6可见，在相同的载荷下超纯水的摩擦系数曲线均在润滑添加剂上方 在8 N载荷工况下超纯水的摩擦系数先上升到0.36然后降到0.28，最终在0.29~0.32之间波动，而Graphene/SiO2的摩擦系数明显降低 在12 N载荷工况下，超纯水和Graphene/SiO2的摩擦系数接近，而Graphene/SiO2的摩擦曲线有升高的趋势 在15 N载荷工况下5 min后超纯水的摩擦系数保持在0.29，而Graphene/SiO2的摩擦系数保持在0.24 在总体上，在载荷相同的工况下Graphene/SiO2的摩擦系数曲线始终在超纯水之下

图6



图6超纯水和Graphene/SiO2在不同载荷条件下的摩擦系数

Fig.6Coefficient of friction curve of water and Graphene/SiO2 under different loads (a) 8 N, (b) 10 N, (c) 12 N, (d) 15 N

图7给出了超纯水和含量为0.2%(质量分数)的Graphene/SiO2在不同载荷下的平均摩擦系数和磨损率 可以看出，载荷由8 N增大到12 N时Graphene/SiO2的摩擦系数和磨损率均增大，而超纯水的摩擦系数降低、磨损率提高 载荷为10 N时Graphene/SiO2的平均摩擦系数比超纯水的平均摩擦系数降低了5.9%而磨损率降低了34.4% 载荷从12 N增大到15 N，Graphene/SiO2和超纯水的平均摩擦系数和磨损率都降低 在载荷相同条件下，Graphene/SiO2的平均摩擦系数和磨损率均低于超纯水 在15 N载荷工况下Graphene/SiO2的摩擦系数和磨损率最低，摩擦系数为0.2399，磨损率为3.75×10-8 mm3/N·m 与超纯水相比，摩擦系数降低17.9%，磨损率降低了61.7%

图7



图7超纯水和Graphene/SiO2不同载荷条件下的摩擦系数和磨损率

Fig.7Coefficient of Friction(a) and wear rates of water and Graphene/SiO2 (b) under various load conditions

图8给出了三维白光测量数据 计算结果表明，超纯水和0.2%(质量分数)润滑添加剂的磨痕磨损体积分别为0.017 mm3和0.019 mm3，但0.2%(质量分数)润滑添加剂的摩擦系数和钢球磨损率的实验结果均低于超纯水 其原因是，较高载荷产生更多的磨屑，使TC4盘磨损体积增大 同时，磨屑和SiO2颗粒对磨损表面共同修复提高了耐磨性，使摩擦系数降低[36]

图8



图8TC4圆盘的三维白光和磨痕剖面

Fig.83D Micrographs and profiles of wear tracks of TC4 discs (a) Ultra-pure water (b) Graphene/SiO2

2.4 磨损表面

图9给出了超纯水和Graphene/SiO2润滑下磨痕表面的OM图 可以看出，GCr15钢球磨痕均为椭圆状，在载荷作用下接触区域不是理想状态的刚体，因此使局部变形成椭圆状的接触区(图10)[37,38] 用超纯水润滑(图9a~d)则钢球表面沿滑动方向有深而宽的磨痕，划痕和凹坑较多，磨损量大 在超纯水中加入Graphene/SiO2润滑剂(图9e~h)使磨痕变浅变窄，磨斑明显变小

图9



图9不同载荷下GCr15的磨痕OM图

Fig.9OM images of GCr15 wear scars at different loads (a~d) Ultra-pure water (e~h) 0.2% Graphene/SiO2

图10



图10Hertz球盘接触模型

Fig.10Hertz contact model of sphere-on-disc

图11a~d给出了经超纯水润滑的TC4圆盘磨痕的SEM照片和EDS谱 可以看出，超纯水润滑的磨损表面有明显的脱屑且出现细小颗粒磨损 表面上的元素主要是TC4的主要元素而未发现氧元素，表明未发生氧化 在15 N载荷工况下表面出现片层状脱落、磨屑和犁沟，表明磨损机制为磨粒磨损和黏着磨损 图11e~h给出了经Graphene/SiO2润滑后的表面 可以看出，在8 N和10 N载荷下磨损表面上的残留物质较多 图12给出了对残留物质的能谱分析，可见磨损表面的物质主要为TC4和Graphene/SiO2 Fe元素来自于GCr15小球，表明发生了材料转移 在15 N载荷工况下磨损表面出现坑洞和裂缝，还出现颗粒和脱屑，表明磨损形式主要为疲劳磨损、磨粒磨损和黏着磨损 图11g~h给出了12 N和15 N载荷工况表面的EDS分析结果 可以看出，磨损表面出现Si元素，C元素的含量较低 这表明，在高载荷下润滑剂难以进入摩擦表面 图13给出了在15 N载荷工况下的面扫描结果 可以看出，表面出现均匀的Si元素，高分辨SEM图像证明磨损表面有SiO2颗粒

图11



图11不同载荷下TC4盘的磨痕SEM照片

Fig.11SEM images of wear scars of TC4 discs under different loads: (a~d) Ultra-pure water, (e~h) 0.2% Graphene/SiO2 lubricant

图12



图1210 N载荷下0.2%Graphene/SiO2润滑添加剂的TC4盘磨痕能谱

Fig.12EDS spectra of the wear scar of the TC4 disc lubricated by 0.2%Graphene/SiO2 lubrication additive under 10 N load (a) high resolution SEM image (b) area I EDS (c) area II EDS

图13



图1315N载荷下0.2% Graphene/SiO2润滑添加剂的 TC4盘磨痕能谱

Fig.13EDS spectra of the wear scar of the TC4 disc lubricated by 0.2%Graphene/SiO2 lubrication additive under 15 N load (a) The high resolution SEM image, (b) the spectra, (c~e) the distribution of Ti, C, Si elements

图14给出了对磨损表面特征元素的XPS分析，以揭示Graphene/SiO2添加剂的润滑机理 由图14a中的C1s谱峰对应磨损表面的C-C、C-O、C=O键可见，磨损表面存在Graphene，而SiC是切割圆盘制取XPS试样时引入的 Si2p的谱峰(图14c)也证实了SiC的存在[39] 从图14b中的O1s谱峰可知，Ti和Al金属在空气中易生成一层致密的氧化薄膜，磨损表面出现TiO2和Al2O3[40,41] 而SiO2的存在，证明磨损表面Graphene/SiO2润滑添加剂的存在 磨损表面并未发生复杂的化学反应，而在15 N载荷条件下Si元素在磨损表面均匀分布，表明在摩擦过程中Graphene/SiO2水基润滑剂在摩擦界面生成了一定厚度的物理吸附膜

图14



图14Graphene/SiO2润滑的TC4圆盘磨损表面的XPS分析

Fig.14XPS analysis of worn surface of TC4 disc lubricated by Graphene/SiO2: (a) C1s (b) O1s (c) Si2p (d) Al2p (e) Ti2p

根据润滑理论，润滑的状态可用润滑状态图中的两个分量

gV=GW3u2

(7)

gE=W8/3u2

(8)

表示 其中u=ηV/E'R，G=αE'，W=F/E'R2，R为GCr15球的半径(3 mm)，V为摩擦过程中的摩擦副的相对线速度(47.1 mm/s)，η为润滑剂的粘度，α为粘度-压力系数，E'(163 GPa)为有效弹性模量，F(8~15 N)施加的载荷，k(≈1.03)为椭圆参数 根据hamrock-dowson理论，薄膜的最小理论厚度和比率分别为[42]

hmin=2.69G0.53U0.67W0.067(1-0.61e-0.73k)

(9)

和

λ=hminσ12+σ22

(10)

其中σ1和σ2分别为球和盘的粗糙度(σ1=20 nm，σ2=40 nm) 计算结果表明，hmin约为8.08 nm，λ约为0.18，表明润滑状态处于边界润滑[43]

根据计算出的边界润滑状态提出相应的磨损机理(图15)：添加在超纯水中的Graphene/SiO2吸附或沉积在摩擦表面生成润滑膜，将摩擦副和磨损表面凹凸点接触转变为摩擦副-润滑膜-磨损面的接触，减少了磨损[44~46] 由图11h和图13可见，在摩擦实验过程中从Graphene表面脱落的SiO2修补了磨损表面，部分SiO2在接触面产生微轴承作用，将滑动摩擦转变为滚动摩擦[47] 在高载荷情况下摩擦副表面上的凸峰折断产生了更多的细小磨屑，磨屑与部分润滑添加剂相结合修复了磨损表面[48] 因此，与其他载荷相比15 N载荷情况下的摩擦系数更低 另一方面，Graphene片层间依靠范德华力结合，在滑动过程中摩擦副之间的低剪切力使片层产生相对滑动，Graphene给接触区域补充水而避免了直接接触[49] 这表明，Graphene/SiO2润滑添加剂的加入提高了超纯水的摩擦学性能

图15



图15Graphene/SiO2的润滑机理示意图

Fig.15Schematic diagram of lubrication mechanism of Graphene/SiO2

3 结论

(1) 使用溶胶-凝胶St?ber法制备的Graphene/SiO2复合材料，Graphene为软质内核，SiO2在其表面形成一层硬质外壳，外壳粒子的直径约为100 nm并能在水中稳定分散

(2) 在15 N载荷工况下，0.2% Graphene/SiO2水基润滑剂摩擦系数比超纯水降低17.9%，钢球磨损率降低了61.7%

(3) 在高载荷作用下Graphene/SiO2润滑剂的润滑效果更好，主要原因是Graphene/SiO2纳米复合材料在磨损表面生成了物理吸附膜、Graphene的层状剪切作用以及SiO2对磨损表面的修复和滚珠轴承作用

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Application of titanium alloy in airplane

1

2009