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
图1
图1
制备Graphene/SiO2纳米复合材料的示意图
Fig.1
Schematic 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
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
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.2
SEM 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3
图3
Graphene/SiO2和Graphene的Raman谱
Fig.3
Raman 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.4
Optical 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.5
Curves 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.6
Coefficient 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.7
Coefficient of Friction(a) and wear rates of water and Graphene/SiO2 (b) under various load conditions
图8
图8
TC4圆盘的三维白光和磨痕剖面
Fig.8
3D Micrographs and profiles of wear tracks of TC4 discs (a) Ultra-pure water (b) Graphene/SiO2
2.4 磨损表面
图9
图9
不同载荷下GCr15的磨痕OM图
Fig.9
OM images of GCr15 wear scars at different loads (a~d) Ultra-pure water (e~h) 0.2% Graphene/SiO2
图10
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.11
SEM 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12
10 N载荷下0.2%Graphene/SiO2润滑添加剂的TC4盘磨痕能谱
Fig.12
EDS 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13
15N载荷下0.2% Graphene/SiO2润滑添加剂的 TC4盘磨痕能谱
Fig.13
EDS 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14
Graphene/SiO2润滑的TC4圆盘磨损表面的XPS分析
Fig.14
XPS 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
表示。其中u=ηV/E'R，G=αE'，
和
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15
Graphene/SiO2的润滑机理示意图
Fig.15
Schematic diagram of lubrication mechanism of Graphene/SiO2
3 结论
(1) 使用溶胶-凝胶Stöber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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[J].Crystals are formed via nucleation and subsequent growth. In many cases, it is not easy to decide if nucleation occurs in the volume (homogeneous nucleation) or on a foreign surface, structure defects etc. (heterogeneous nucleation). This work is focused on crystal nucleation in a small supercooled liquid droplet when nuclei are formed in the volume or on the droplet surface simultaneously. The kinetic equations of homogeneous and heterogeneous nucleation are solved numerically to determine the size distribution of crystal nuclei and nucleation rate of both processes in Ni liquid droplet. The decrease of the number of atoms within the droplet volume in consequence of homogeneous and heterogeneous nucleation is taken into account. The number of nucleation sites on droplet surface decreases as new heterogeneous nuclei are formed. It is shown how both nucleation processes occur simultaneously.
Correlation between X-ray diffraction and Raman spectra of 16 commercial graphene-based materials and their resulting classification
[J].Structural properties of sixteen (16) commercial samples of graphene-based materials (GBM) labelled as graphene, graphene oxide or reduced graphene oxide are investigated at room temperature using X-ray diffraction (XRD) and Raman spectroscopy. Based on the observed correlation between the results obtained with these two techniques, these samples are classified into three groups: Group A of seven samples consisting of graphitic nanosheets with evaluated thickness ≃20 nm and exhibiting both the 2H and 3R phases in XRD; Group B of six samples exhibiting XRD spectra characteristic of either graphene oxides (GO) or carbons with some order; and Group C of three samples with XRD spectra characteristic of disordered carbons. The relative intensities and widths of D, G, D', 2D and (D + D') bands in the Raman spectra are equally distinguishable between the samples in groups A, B and C. The width of the D-band is the smallest for Group A samples, intermediate for group B and the largest for group C samples. The intensity ratio I(D)/I(G) of the D and G bands in the Raman spectra of the samples is used to quantify the Raman-active defects whose concentration increases in going from samples in Group A to those in Group C.
Influence of silica-core structure on polishing characteristics of core/shell structured composite particles of SiO2/CeO2
[J].
氧化硅内核结构对核/壳包覆型SiO2/CeO2复合颗粒抛光性能的影响
[J].设计合成了以具有放射状介孔孔道(孔径约2.6 nm)的介孔氧化硅(mSiO<sub>2</sub>)微球(粒径约300 nm)为内核、以CeO<sub>2</sub>纳米颗粒为包覆层(壳厚为15~20 nm)的mSiO<sub>2</sub>/CeO<sub>2</sub>复合颗粒(粒径在330~340 nm),使用场发射扫描电镜、透射电镜、X射线衍射、傅里叶转换红外光谱和氮气吸脱附等手段表征了样品的结构。结果表明,使用以实心氧化硅(sSiO<sub>2</sub>)为内核的sSiO<sub>2</sub>/CeO<sub>2</sub>复合颗粒抛光的热氧化硅片其表面粗糙度均方根值(Root-mean-square roughness, RMS)为0.309 nm,材料的去除率(Material removal rate, MRR)为24 nm/min)。mSiO<sub>2</sub>/CeO<sub>2</sub>复合颗粒有利于得到更低的氧化硅片抛光表面粗糙度(RMS=0.267 nm)和更高的抛光速率(MRR=45 nm/min),且能避免出现划痕等机械损伤。SiO<sub>2</sub>/CeO<sub>2</sub>复合颗粒中的氧化硅内核结构,对其抛光特性有明显的影响。
Process and properties of graphene reinforced Mg-based composite prepared by in-situ method
[J].
原位自生法制备石墨烯增强镁基复合材料的工艺和性能
[J].
Raman spectrum of graphene with its versatile future perspectives
[J].
Covalent functionalization of chemically converted graphene sheets via silane and its reinforcement
[J].
Mechanical properties of epoxy composites filled with silane-functionalized graphene oxide
[J].
Tribological performance of nanoparticles as lubricating oil additives
[J].
The Application of nano-MoS2 quantum dots as liquid lubricant additive for tribological behavior improvement
[J].One of the prerequisites of successful address delivery is controlling the release of encapsulated drugs. The new method of bacterial spore encapsulation in polyelectrolyte microcapsules allows for degrading the nanoscale membrane shell of microcapsules. The possibility of encapsulating spore forms of Bacillus subtilis in polystyrenesulfonate sodium/ polyallylamine hydrochloride (PSS/PAH) polyelectrolyte microcapsules was demonstrated. The activation and growth on a nutrient medium of encapsulated bacterial spores led to 60% degradation of the microcapsules nanoscale membrane shell. As a result, 18.5% of Fluorescein isothiocyanatedextran was encapsulated into polyelectrolyte microcapsules, and 28.6% of the encapsulated concentration of FITC-dextran was released into the solution.
Biotribology of artificial hip joints
[J].Hip arthroplasty can be considered one of the major successes of orthopedic surgery, with more than 350000 replacements performed every year in the United States with a constantly increasing rate. The main limitations to the lifespan of these devices are due to tribological aspects, in particular the wear of mating surfaces, which implies a loss of matter and modification of surface geometry. However, wear is a complex phenomenon, also involving lubrication and friction. The present paper deals with the tribological performance of hip implants and is organized in to three main sections. Firstly, the basic elements of tribology are presented, from contact mechanics of ball-in-socket joints to ultra high molecular weight polyethylene wear laws. Some fundamental equations are also reported, with the aim of providing the reader with some simple tools for tribological investigations. In the second section, the focus moves to artificial hip joints, defining materials and geometrical properties and discussing their friction, lubrication and wear characteristics. In particular, the features of different couplings, from metal-on-plastic to metal-on-metal and ceramic-on-ceramic, are discussed as well as the role of the head radius and clearance. How friction, lubrication and wear are interconnected and most of all how they are specific for each loading and kinematic condition is highlighted. Thus, the significant differences in patients and their lifestyles account for the high dispersion of clinical data. Furthermore, such consideration has raised a new discussion on the most suitable in vitro tests for hip implants as simplified gait cycles can be too far from effective implant working conditions. In the third section, the trends of hip implants in the years from 2003 to 2012 provided by the National Joint Registry of England, Wales and Northern Ireland are summarized and commented on in a discussion.
Ultralow concentration of graphene oxide nanosheets as oil-based lubricant additives
[J].
Microstructures and photocatalytic properties of Biocl-rgo nanocomposites prepared by two-step hydrothermal method
[J].Composites of BiOCl-RGO were synthesized via a two-step hydrothermal method. Firstly plain BiOCl was synthesized in the mixed solution of ethylene glycol and deionized water, the acquired nanosphere-like BiOCl of about 400 nm in diameter composed of many nanosheets. Then the RGO carrier was deposited onto the plain BiOCl to prepare BiOCl-RGO nanocomposites. The composites were characterized by Raman spectroscopy, XRD, XPS, SEM and TEM. The photocatalytic property of the composites was evaluated by degrading methyl orange. The results show that the temperature of hydrothermal process significantly affects the photocatalytic property of the composites. The composite of BiOCl -graphene prepared at 140°C shows the highest photocatalytic performance.
秦艳利, 杨 艳, 赵鹏羽
Preparation and performance of graphite/TiO2 composite photocatalyst
[J].
石墨/TiO2复合光催化剂的制备和性能
[J].
Tribological performance of PTFE and its composite in wide temperature range
[J].
宽温域环境中聚四氟乙烯及其复合材料摩擦学性能研究
[J].
Isothermal elastohydrodynamic lubrication of point contacts: part III—fully flooded results
[J].Utilizing the theory developed by the authors in an earlier publication, the influence of the ellipticity parameter, the dimensionless speed, load, and material parameters on minimum film thickness was investigated. The ellipticity parameter was varied from one (a ball on a plate configuration) to eight (a configuration approaching a line contact). The dimensionless speed parameter was varied over a range of nearly two orders of magnitude. The dimensionless load parameter was varied over a range of one order of magnitude. Conditions corresponding to the use of solid materials of bronze, steel, and silicon nitride and lubricants of paraffinic and naphthenic mineral oils were considered in obtaining the exponent in the dimensionless material parameter. Thirty-four different cases were used in obtaining the minimum film thickness formula given below as H¯min=3.63U0.68G0.49W−0.073(1−e−0.68k) A simplified expression for the ellipticity parameter was found where k=1.03RyRx0.64 Contour plots were also shown which indicate in detail the pressure spike and two side lobes in which the minimum film thickness occurs. These theoretical solutions of film thickness have all the essential features of the previously reported experimental observations based upon optical interferometry.
A quantum chemical study of the unimolecular decomposition mechanisms of zinc dialkyldithiophosphate antiwear additives
[J].
Synthesis of colloidal 2D/3D MoS2 nanostructures by pulsed laser ablation in an organic liquid environment
[J].
Synthesis and tribological properties of copper matrix solid self-lubricant composites reinforced with NbSe2 nanoparticles
[J].
Tribological and electrical behavior of Cu-based composites with addition of Ti-doped NbSe2 nanoplatelets
[J].This paper aims to clarify the size and morphology of transition metal dichalcogenides has an impact on lubrication performance of Cu-based composites. This study is intended to show that Cu-based electrical contact materials containing Nb0.91Ti0.09Se2 have better electrical and tribological properties than those containing NbSe2. The tribological properties of Cu-based with different Ti-dopped NbSe2 content were also discussed.
Friction and wear mechanisms of castor oil with addition of hexagonal boron nitride nanoparticles
[J].
Research on tribological behavior of TiN nanoparticles as lubricating additive
[J].
纳米TiN润滑油添加剂的摩擦学性能研究
[J].
A molecular dynamics study on the synergistic lubrication mechanisms of graphene/water-based lubricant systems
[J].
