鈦合金輕質(zhì)、耐高溫、耐腐蝕、生物相容性好且無磁性，在航空、航天、兵器、艦船、醫(yī)療等領域得到了廣泛的應用[1,2] 但是，鈦合金的導熱系數(shù)低、高溫化學活性高和彈性模量小，在切削加工過程中工件與刀具的粘連使其磨損嚴重、加工后的工件表面質(zhì)量較差、加工成本提高，限制了鈦合金的應用[3~5] 提高鈦合金切削性能的關(guān)鍵，在于改善切削界面摩擦狀態(tài)，實現(xiàn)高效潤滑 但是，鈦合金獨特的摩擦學特性使傳統(tǒng)金屬加工潤滑液難以在鈦合金表面有效潤滑 在基礎液中添加納米材料，是提高潤滑介質(zhì)加工性的主要手段[6~8] 石墨烯(Graphene)是一種典型的二維材料，層與層之間依靠弱范德華力連接，具有較弱的剪切力、優(yōu)異的機械性能、大比表面積和較高的熱導率，在潤滑領域受到了極大的關(guān)注[9,10] Ming等[11]在植物油中添加石墨烯用于TC4合金切削加工潤滑，可增強銑削區(qū)域油膜的潤滑性能 Ning等[12]將Graphene、磷酸鹽、納米ZrO2等按一定比例混合制備石墨烯水基潤滑劑應用于鈦合金熱軋，降低了熱軋過程的摩擦磨損和氧化 Ibrahim等[13]將石墨烯加入棕櫚油中，摩擦系數(shù)和切削能耗比Acculube LB2000商用潤滑油大幅降低 但是，結(jié)構(gòu)完整的Graphene因化學穩(wěn)定性高而難以在溶劑中穩(wěn)定分散，容易產(chǎn)生不可逆團聚使摩擦過程中難以進入工況表面，無法發(fā)揮抗磨減磨的作用[14,15]

納米復合材料在基礎液中的分散性高，且不同納米材料之間的協(xié)同作用可進一步提高潤滑性能 Meng[16,17]等在氧化石墨烯(GO)表面沉積Au或Cu，降低了石墨烯片層間的π-π鍵的相互作用減少了團聚 與單一納米材料(GO、Au和Cu)相比，復合材料之間的協(xié)同作用使其具有更優(yōu)異的潤滑性能 Li等[18]用激光輻射制備的Ag/Graphene復合材料可穩(wěn)定在油中懸浮60 d以上，這種潤滑添加劑不會產(chǎn)生金屬腐蝕和環(huán)境污染 Graphene與金屬納米材料復合的成本高，回收難，因此難以推廣 SiO2中的Si-O親水性和耐磨性較好，且成本較低[19,20] Na等[21]用原位引發(fā)聚合法制備的PTFE/SiO2復合材料，提高了PTFE在純水中的分散性和摩擦性能 Zhang等[22]用溶膠-凝膠法制備Fe3O4@SiO2納米復合材料，提高了Fe3O4在環(huán)氧樹脂中的分散性 在Graphene表面原位生成SiO2制備Graphene/SiO2納米復合材料，可提高Graphene在超純水中的分散性且降低成本 鑒于此，本文用溶膠凝膠法在Graphene表面原位生成SiO2制備Graphene/SiO2納米復合材料，以提高Graphene在超純水中的分散性且降低成本，并將其作為水基潤滑添加劑研究GCr15/TC4接觸下的摩擦學性能并揭示其潤滑機理

1 實驗方法1.1 實驗用材料

無水乙醇(C2H5OH，分析純)，氨水(NH3·H2O，分析純)，石油醚(PE)和正硅酸乙酯(TEOS)，Graphene和工業(yè)SiO2

1.2 納米復合材料Graphene/SiO2 的制備

使用溶膠-凝膠法中的St?ber法制備Graphene/SiO2[23]，其工藝示意圖如圖1所示 將0.2 g的Graphene添加到50 mL無水乙醇和50 mL超純水的混合溶液中，使用超聲波破碎30 min 然后加入1 mL氨水和2 mL TEOS并對混合溶液磁力攪拌12 h 對產(chǎn)物進行離心分離后收集膠狀固體產(chǎn)物，用無水乙醇多次清洗以除去氨水和未反應的TEOS 將所得膠狀固體在75℃真空環(huán)境干燥12 h后達到納米復合材料Graphene/SiO2

圖1



圖1制備Graphene/SiO2納米復合材料的示意圖

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

1.3 摩擦磨損實驗

使用旋轉(zhuǎn)式摩擦磨損試驗儀(UMT-5)測試試樣的摩擦磨損性能，上試樣是直徑為6 mm的GCr15鋼球，下試樣是厚度為8 mm直徑為25 mm的TC4圓盤 摩擦實驗中的潤滑劑，是超純水中添加不同質(zhì)量分數(shù)的Graphene/SiO2 實驗的線速度為0.047 m/s，載荷為8~15 N，時間為30 min，根據(jù)赫茲理論計算赫茲接觸壓力

πP=4Fπa2

(1)

a=2(23×FRE')3

(2)

1E'=12(1-μ12E1+1-μ22E2)

(3)

其中P為赫茲接觸應力，a為接觸直徑、F為摩擦試驗機施加載荷(8~15 N)、R為GCr15球半徑、E'為有效彈性模量[24,25] E1(TC4 113 GPa)和E2(GCr15 207 GPa)為摩擦試樣的彈性模量，μ1(0.34)和μ2(0.30)為泊松比 最大接觸應力范圍為1.04~1.29 GPa GCr15球的磨損率為[26,27]

h=r-r2-d24

(4)

V=πl(wèi)63d24+h2

(5)

WB=VPS

(6)

式中d為等效圓的直徑，r為GCr15球的直徑，P為載荷，S為總滑動距離 實驗前用無水乙醇超聲清洗GCr15球和TC4圓盤30 min以去除污染，摩擦實驗開始前滴加80 μL的潤滑劑 實驗結(jié)束后用棉球擦拭表面，干燥后保存

1.4 性能表征

用沉降法評估Graphene/SiO2在超純水中的分散穩(wěn)定性[35] 將0.2%(質(zhì)量分數(shù))的Graphene和Graphene/SiO2分別加到超純水中，超聲1 h靜置適當時間后拍攝光學圖像

用X射線衍射儀(XRD，D/MAX-RB)測試Graphene/SiO2納米材料的晶體結(jié)構(gòu) 用掃描電子顯微鏡(SEM，JSM-5610LV)觀察Graphene/SiO2復合材料的微觀組織形貌，用SEM附帶的EDS分析復合材料和磨損表面元素的成分 用拉曼光譜儀(LabRam HR Evolution)測試Graphene和Graphene/SiO2納米復合材料的拉曼光譜 用金相顯微鏡(OM，GX51) 測量鋼球磨斑的直徑，用三維白光掃描儀(TDWS，MicroXAM-800)測量TC4圓盤磨損體積 用掃描電鏡分析實驗后TC4圓盤磨痕的微觀組織形貌和元素的分布 用X射線光電子能譜儀(XPS，PHI 5000) 分析磨損表面的特征元素

2 結(jié)果和討論2.1 Graphene/SiO2 納米復合材料的形貌與表征

圖2給出了Graphene和SiO2的掃描電鏡照片，可見片層之間褶皺，邊緣處卷曲，SiO2球狀顆粒的直徑約為300 nm 圖2c給出了Graphene/SiO2納米復合材料的掃面電鏡照片，可見Graphene的卷曲結(jié)構(gòu)，表面均為小球顆粒，能譜分析表明主要元素為Si、O和C，即Graphene表面生成了SiO2納米顆粒 與單一的納米SiO2相比，Graphene表面的SiO2顆粒尺寸差異較大(圖2b和c) 其原因是，在TEOS發(fā)生化學反應形成SiO2的過程中Graphene和部分SiO2顆粒都是形核位點，生成了較大的SiO2顆粒[28]

圖2



圖2不同試樣的SEM照片、Graphene/SiO2的能譜、以及Graphene/SiO2、Graphene、Amorphous SiO2和SiO2的XRD譜

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

圖2e給出了XRD譜 可以看出，Graphene在譜中的26.34°和42.68°出現(xiàn)了兩個特征衍射峰[29]，低矮的衍射峰對應非晶態(tài)SiO2，明顯的衍射峰對應晶體SiO2[30] Graphene/SiO2納米復合材料的衍射峰與非晶SiO2一致，沒有出現(xiàn)Graphene的衍射峰特征

圖3給出了Graphene和Graphene/SiO2的拉曼光譜，可見Graphene的衍射峰位于1333.2 cm-1、1567.1 cm-1和2671.8 cm-1處，分別對應D峰、G峰和2D峰 G峰的強度高于2D峰，表明材料具有多層結(jié)構(gòu)[31,32] 與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] 以上結(jié)果表明，已制備出Graphene/SiO2納米復合材料

圖3



圖3Graphene/SiO2和Graphene的Raman譜

Fig.3Raman spectra of Graphene/SiO2 and Graphene

2.2 Graphene/SiO2 的分散性

圖4給出了不同潤滑劑放置不同時間的光學圖像 Graphene在超純水中分散性差，放置24 h就完全分層 而含有Graphene/SiO2的超純水溶液的分散較為穩(wěn)定，靜止48 h后開始出現(xiàn)沉淀，上層溶液變淺，表明其分散性優(yōu)于Graphene

圖4



圖4不同潤滑劑在不同時間的光學圖像

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

2.3 Graphene/SiO2 納米復合材料作為水基潤滑添加劑的摩擦學性能

圖5給出了不同含量的Graphene/SiO2的平均摩擦系數(shù)和磨損率曲線 可以看出，平均摩擦系數(shù)和磨損率均呈現(xiàn)先下降后上升，0.2%(質(zhì)量分數(shù))的Graphene/SiO2摩擦系數(shù)最低，比超純水工況降低17.9%，鋼球磨損率降低61.7% 添加劑含量超過0.2%(質(zhì)量分數(shù))，則摩擦性能開始降低

圖5



圖5不同含量的Graphene/SiO2的平均摩擦系數(shù)和磨損率曲線

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

圖6給出了在不同載荷下0.2%(質(zhì)量分數(shù))Graphene/SiO2潤滑劑的摩擦系數(shù) 從圖6可見，在相同的載荷下超純水的摩擦系數(shù)曲線均在潤滑添加劑上方 在8 N載荷工況下超純水的摩擦系數(shù)先上升到0.36然后降到0.28，最終在0.29~0.32之間波動，而Graphene/SiO2的摩擦系數(shù)明顯降低 在12 N載荷工況下，超純水和Graphene/SiO2的摩擦系數(shù)接近，而Graphene/SiO2的摩擦曲線有升高的趨勢 在15 N載荷工況下5 min后超純水的摩擦系數(shù)保持在0.29，而Graphene/SiO2的摩擦系數(shù)保持在0.24 在總體上，在載荷相同的工況下Graphene/SiO2的摩擦系數(shù)曲線始終在超純水之下

圖6



圖6超純水和Graphene/SiO2在不同載荷條件下的摩擦系數(shù)

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

圖7給出了超純水和含量為0.2%(質(zhì)量分數(shù))的Graphene/SiO2在不同載荷下的平均摩擦系數(shù)和磨損率 可以看出，載荷由8 N增大到12 N時Graphene/SiO2的摩擦系數(shù)和磨損率均增大，而超純水的摩擦系數(shù)降低、磨損率提高 載荷為10 N時Graphene/SiO2的平均摩擦系數(shù)比超純水的平均摩擦系數(shù)降低了5.9%而磨損率降低了34.4% 載荷從12 N增大到15 N，Graphene/SiO2和超純水的平均摩擦系數(shù)和磨損率都降低 在載荷相同條件下，Graphene/SiO2的平均摩擦系數(shù)和磨損率均低于超純水 在15 N載荷工況下Graphene/SiO2的摩擦系數(shù)和磨損率最低，摩擦系數(shù)為0.2399，磨損率為3.75×10-8 mm3/N·m 與超純水相比，摩擦系數(shù)降低17.9%，磨損率降低了61.7%

圖7



圖7超純水和Graphene/SiO2不同載荷條件下的摩擦系數(shù)和磨損率

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

圖8給出了三維白光測量數(shù)據(jù) 計算結(jié)果表明，超純水和0.2%(質(zhì)量分數(shù))潤滑添加劑的磨痕磨損體積分別為0.017 mm3和0.019 mm3，但0.2%(質(zhì)量分數(shù))潤滑添加劑的摩擦系數(shù)和鋼球磨損率的實驗結(jié)果均低于超純水 其原因是，較高載荷產(chǎn)生更多的磨屑，使TC4盤磨損體積增大 同時，磨屑和SiO2顆粒對磨損表面共同修復提高了耐磨性，使摩擦系數(shù)降低[36]

圖8



圖8TC4圓盤的三維白光和磨痕剖面

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

2.4 磨損表面

圖9給出了超純水和Graphene/SiO2潤滑下磨痕表面的OM圖 可以看出，GCr15鋼球磨痕均為橢圓狀，在載荷作用下接觸區(qū)域不是理想狀態(tài)的剛體，因此使局部變形成橢圓狀的接觸區(qū)(圖10)[37,38] 用超純水潤滑(圖9a~d)則鋼球表面沿滑動方向有深而寬的磨痕，劃痕和凹坑較多，磨損量大 在超純水中加入Graphene/SiO2潤滑劑(圖9e~h)使磨痕變淺變窄，磨斑明顯變小

圖9



圖9不同載荷下GCr15的磨痕OM圖

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

圖10



圖10Hertz球盤接觸模型

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

圖11a~d給出了經(jīng)超純水潤滑的TC4圓盤磨痕的SEM照片和EDS譜 可以看出，超純水潤滑的磨損表面有明顯的脫屑且出現(xiàn)細小顆粒磨損 表面上的元素主要是TC4的主要元素而未發(fā)現(xiàn)氧元素，表明未發(fā)生氧化 在15 N載荷工況下表面出現(xiàn)片層狀脫落、磨屑和犁溝，表明磨損機制為磨粒磨損和黏著磨損 圖11e~h給出了經(jīng)Graphene/SiO2潤滑后的表面 可以看出，在8 N和10 N載荷下磨損表面上的殘留物質(zhì)較多 圖12給出了對殘留物質(zhì)的能譜分析，可見磨損表面的物質(zhì)主要為TC4和Graphene/SiO2 Fe元素來自于GCr15小球，表明發(fā)生了材料轉(zhuǎn)移 在15 N載荷工況下磨損表面出現(xiàn)坑洞和裂縫，還出現(xiàn)顆粒和脫屑，表明磨損形式主要為疲勞磨損、磨粒磨損和黏著磨損 圖11g~h給出了12 N和15 N載荷工況表面的EDS分析結(jié)果 可以看出，磨損表面出現(xiàn)Si元素，C元素的含量較低 這表明，在高載荷下潤滑劑難以進入摩擦表面 圖13給出了在15 N載荷工況下的面掃描結(jié)果 可以看出，表面出現(xiàn)均勻的Si元素，高分辨SEM圖像證明磨損表面有SiO2顆粒

圖11



圖11不同載荷下TC4盤的磨痕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

圖12



圖1210 N載荷下0.2%Graphene/SiO2潤滑添加劑的TC4盤磨痕能譜

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

圖13



圖1315N載荷下0.2% Graphene/SiO2潤滑添加劑的 TC4盤磨痕能譜

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

圖14給出了對磨損表面特征元素的XPS分析，以揭示Graphene/SiO2添加劑的潤滑機理 由圖14a中的C1s譜峰對應磨損表面的C-C、C-O、C=O鍵可見，磨損表面存在Graphene，而SiC是切割圓盤制取XPS試樣時引入的 Si2p的譜峰(圖14c)也證實了SiC的存在[39] 從圖14b中的O1s譜峰可知，Ti和Al金屬在空氣中易生成一層致密的氧化薄膜，磨損表面出現(xiàn)TiO2和Al2O3[40,41] 而SiO2的存在，證明磨損表面Graphene/SiO2潤滑添加劑的存在 磨損表面并未發(fā)生復雜的化學反應，而在15 N載荷條件下Si元素在磨損表面均勻分布，表明在摩擦過程中Graphene/SiO2水基潤滑劑在摩擦界面生成了一定厚度的物理吸附膜

圖14



圖14Graphene/SiO2潤滑的TC4圓盤磨損表面的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

根據(jù)潤滑理論，潤滑的狀態(tài)可用潤滑狀態(tài)圖中的兩個分量

gV=GW3u2

(7)

gE=W8/3u2

(8)

表示 其中u=ηV/E'R，G=αE'，W=F/E'R2，R為GCr15球的半徑(3 mm)，V為摩擦過程中的摩擦副的相對線速度(47.1 mm/s)，η為潤滑劑的粘度，α為粘度-壓力系數(shù)，E'(163 GPa)為有效彈性模量，F(xiàn)(8~15 N)施加的載荷，k(≈1.03)為橢圓參數(shù) 根據(jù)hamrock-dowson理論，薄膜的最小理論厚度和比率分別為[42]

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

(9)

和

λ=hminσ12+σ22

(10)

其中σ1和σ2分別為球和盤的粗糙度(σ1=20 nm，σ2=40 nm) 計算結(jié)果表明，hmin約為8.08 nm，λ約為0.18，表明潤滑狀態(tài)處于邊界潤滑[43]

根據(jù)計算出的邊界潤滑狀態(tài)提出相應的磨損機理(圖15)：添加在超純水中的Graphene/SiO2吸附或沉積在摩擦表面生成潤滑膜，將摩擦副和磨損表面凹凸點接觸轉(zhuǎn)變?yōu)槟Σ粮?潤滑膜-磨損面的接觸，減少了磨損[44~46] 由圖11h和圖13可見，在摩擦實驗過程中從Graphene表面脫落的SiO2修補了磨損表面，部分SiO2在接觸面產(chǎn)生微軸承作用，將滑動摩擦轉(zhuǎn)變?yōu)闈L動摩擦[47] 在高載荷情況下摩擦副表面上的凸峰折斷產(chǎn)生了更多的細小磨屑，磨屑與部分潤滑添加劑相結(jié)合修復了磨損表面[48] 因此，與其他載荷相比15 N載荷情況下的摩擦系數(shù)更低 另一方面，Graphene片層間依靠范德華力結(jié)合，在滑動過程中摩擦副之間的低剪切力使片層產(chǎn)生相對滑動，Graphene給接觸區(qū)域補充水而避免了直接接觸[49] 這表明，Graphene/SiO2潤滑添加劑的加入提高了超純水的摩擦學性能

圖15



圖15Graphene/SiO2的潤滑機理示意圖

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

3 結(jié)論

(1) 使用溶膠-凝膠St?ber法制備的Graphene/SiO2復合材料，Graphene為軟質(zhì)內(nèi)核，SiO2在其表面形成一層硬質(zhì)外殼，外殼粒子的直徑約為100 nm并能在水中穩(wěn)定分散

(2) 在15 N載荷工況下，0.2% Graphene/SiO2水基潤滑劑摩擦系數(shù)比超純水降低17.9%，鋼球磨損率降低了61.7%

(3) 在高載荷作用下Graphene/SiO2潤滑劑的潤滑效果更好，主要原因是Graphene/SiO2納米復合材料在磨損表面生成了物理吸附膜、Graphene的層狀剪切作用以及SiO2對磨損表面的修復和滾珠軸承作用

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" />

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