仿生超滑涂层研究进展
邱豪楠, 刘威, 唐悦, 王胡军, 郑靖    
西南交通大学 机械工程学院, 摩擦学研究所, 成都 610031
摘要:仿生超滑涂层因具有优异的拒液性、自愈性和高压稳定性, 在防污、抗黏附和防结冰等应用领域受到广泛关注。将润滑油注入多孔基体获得的注液超滑涂层或在光滑平面接枝润滑分子获得的类液体超滑涂层均可获得上述优异性能。然而, 超滑涂层面临润滑层易损耗、机械稳定性不足等问题, 在实际应用中仍存在一定局限性。该文在总结注液超滑表面仿生设计原理的基础上, 详细介绍了注液超滑涂层和类液体超滑涂层的特点与研究进展, 并指出当前面临的问题。此外, 从高可靠性、长寿命超滑涂层的优化设计制造角度, 评述了超滑涂层的发展趋势。
关键词超滑涂层    仿生设计    表面结构    润滑油    稳定性    
Research progress in bioinspired slippery coatings
QIU Haonan, LIU Wei, TANG Yue, WANG Hujun, ZHENG Jing    
Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China
Abstract: [Significance] Bioinspired slippery coatings have attracted extensive attention in antifouling, anti-adhesion, and anti-icing applications because of their excellent liquid repellency, self-healing properties, and high-pressure stability. The slippery liquid-infused coating obtained by infusing lubricating oil into porous matrixes and the slippery liquid-like coating afforded by grafting lubricating molecules onto smooth surfaces exhibit the aforementioned properties. However, some limitations still hinder the practical applications of these coatings, such as easy loss of the lubrication layer and insufficient mechanical stability. Therefore, this study introduces the characteristics and research progress of slippery liquid-infused and slippery liquid-like coatings in detail by summarizing the bionic design principles of slippery liquid-infused surfaces. Furthermore, the existing problems related to coatings are highlighted. [Progress] According to the oil fixation mechanism and lubrication layer thickness, slippery coatings could be divided into three categories. Type 1D slippery coatings, known as slippery liquid-like coatings, mainly stabilize the lubrication layer by chemical grafting; thus, they showed good stability when subjected to gravity, shear force, and water scouring. However, they easily lost their slippery performance when subjected to mechanical wear due to their low thickness and poor wear resistance. The fabrication of type 1D-slippery coatings involved complex preparation processes, harsh preparation conditions, and high costs, limiting their large-scale applications. Type 2D- and 3D-slippery coatings stabilized the lubrication layer through their porous structures. Type 2D-slippery coatings exhibited good mechanical stability and could be easily prepared. However, due to their poor oil-fixing performance, the lubricating oil was easily lost, and they could not recover the oil themselves. Therefore, maintaining their slippery properties for a long time under harsh conditions was challenging. To solve this problem, researchers had conducted several studies on structural design and chemical modification. Despite their effective efforts, the porous structures of type 2D-slippery coatings could only store a small amount of lubricating oil, and the timely replenishment of oil after oil loss remained difficult. Type 3D-slippery coatings included gel and nongel coatings. Gel cross-linked networks and 3D porous physical structures could store/release lubricating oil, thereby improving the slippery stability of these coatings. With the introduction of smart materials, type 3D-slippery coatings could actively adjust the release of lubricating oil according to changes in the environment and coating states. However, the 3D-gel and -nongel slippery coatings exhibited insufficient mechanical stability and weaked oil control-release ability, respectively. [Conclusions and Prospects] To prepare highly reliable and long-life slippery coatings for large-scale industrial applications, further research is required. First, we need to understand the storage, fixation, and release mechanisms of the lubricating oil in slippery coatings, introduce intelligent materials, and systematically study the influence of structural characteristics, chemical compositions, and preparation methods on the stability of coatings. Second, the influence of lubricating oil on the adhesive strength of coatings must be further investigated because the lubricating oil may affect the bonding properties between the coatings and substrates. Additionally, the coating preparation methods should be simplified, and costs must be reduced to promote the applications of bioinspired slippery coatings. To achieve green production, more attention should be paid to the use of environmentally friendly materials in coating preparation processes. Finally, new slippery coatings need to be developed according to practical application environments by mimicking multiple biological templates.
Key words: slippery coating    bionic design    surface structure    lubricating oil    stability    

在机械、航空、海洋和光电等领域,由固-液摩擦引起的装备零部件表面污损、黏附和结冰等问题广泛存在,严重影响零部件的使役性能,造成能源浪费和资源损耗[1-6]。例如,藤壶、贻贝等生物附着于船舶表面,这会增大航行阻力[7-9];在高空低温、高湿环境下,表面结冰可能会促使机翼形状发生改变,进而导致升力不足、阻力增大,甚至可能引发坠机事故[10-11]。针对以上问题,现阶段的解决策略主要是利用低表面能聚合物涂层降低固-液摩擦[12]和通过机械方式进行表面清洁[13]。低表面能聚合物涂层能从根源上防止上述问题发生,但实际应用中涂层的清洁效果难以持续,且成本较高;而利用机械装置清洁不仅难以实时解决问题,还容易损伤装备零部件表面。因此,亟需寻找高效、低成本和操作简单的方法解决固-液摩擦引起的一系列问题。

固-固摩擦可以通过引入润滑油降低摩擦阻力,即通过“润”而达到“滑”的目的。与固-固摩擦不同,固-液摩擦的特性则是“润”而“黏”,需要通过去“润”而达到“滑”的目的,此处去“润”指的是去润湿[14]。生物经过长期进化,逐渐形成了精巧的结构和独特的材料,二者耦合可实现去“润”而“滑”,其中最典型的是“荷叶效应”[15]。文[15]研究发现,超疏水荷叶表面分布的微纳乳突结构和蜡质层可以吸附空气,将固-液接触转换为气-液接触,降低了表面摩擦,使水滴自由滚动,从而带走灰尘以保持表面清洁。因此,能有效降低固-液摩擦的仿荷叶超疏水表面被大量应用于防污[16-18]、抗结冰和减阻等领域[19],但仍存在局限性。超疏水表面依靠微观结构和捕获的空气共同支撑液滴呈Cassie状态,但是这种低黏附超疏水状态不稳定,在机械摩擦、外界冲击和高压环境等条件下,微观结构或空气层容易被破坏,造成液滴由Cassie状态转变为Wenzel状态,疏水性能显著降低,最终导致清洁功能弱化甚至丧失[20-22]

2011年,受猪笼草开口外缘捕虫机制启发,Wong等[23]开发了注液多孔超滑表面(slippery liquid-infused surface, SLIPS),将疏水润滑油注入微纳结构中,在基体表面形成连续均匀的动态液膜,从而将固-液摩擦转换为液-液摩擦,大幅度降低了表面摩擦。此外,以Wong等[23]为代表的多位研究人员提出了仿生超滑表面的设计理念,仿生超滑表面包括仿生注液超滑表面和仿生类液体超滑表面。将润滑油注入多孔结构,可制备仿生注液超滑表面;通过接枝将润滑分子固定在光滑表面即可获得仿生类液体超滑表面。仿生超滑表面的接触角θ通常小于超疏水表面(θ>150°),但仿生超滑表面的滑动角极小,具有出色的拒液性、自修复性和压力稳定性,在防污、减阻、防冰、防腐蚀、生物医学检测、液体操控和能量收集等领域具有广阔的应用前景[24-27],受到研究人员的广泛关注。迄今,国内外研究人员已提出多种可实现仿生超滑的设计与制造方法。然而,如何提升超滑表面的稳定性仍存在巨大挑战。首先,本文概述了仿生注液超滑表面的设计原理;其次,将仿生超滑涂层分为3类,分别阐述了各类涂层的设计和制备技术与研究现状;最后,从高可靠性、长寿命超滑涂层的优化设计制造角度,评述了仿生超滑涂层的未来发展趋势。

1 仿生注液超滑表面的设计原理

早期研究[28]是将润滑油注入粗糙的疏水表面制备超滑表面,但超滑表面的性能并不稳定,润滑油极易流失。实际上,仿生注液超滑表面具有复杂的多相界面,基材、粗糙结构和润滑油的简单组合难以实现超滑功能。Wong等[23]最早提出了超滑表面的设计标准:1) 润滑油能够完全润湿基底并被锁定在基底;2) 固体基底必须优先被润滑油而非被拒液体润湿;3) 润滑油必须与被拒液体互不相溶。

基于此,Preston等[29]总结了5种超滑失效情况,分别如下:1) 低表面能润滑油包裹被拒液体,即发生Cloak包裹现象,致使润滑油逐渐损耗;2) 被拒液体在高表面能润滑油层上铺展,难以凝聚成小液滴并滑离;3) 润滑油无法完全浸润粗糙结构;4) 粗糙表面对被拒液体更具亲和力,部分润滑油被置换;5) 润滑油与被拒液体可互溶。超滑表面的5种“失效模型”如图 1所示。基于上述失效情况,超滑表面设计制备必须满足以下5个条件:1) 润滑油层不包裹被拒液体;2) 被拒液体不能在润滑油层表面完全铺展;3) 润滑油层能完全浸润固体基底;4) 在被拒液体中,润滑油能在固体基底粗糙结构中铺展而不被取代;5) 润滑油与被拒液体不互溶。5个条件对应的相关计算分别表示如下:

$ S_{\mathrm{ol}(\mathrm{a})}=\gamma_{\mathrm{la}}-\gamma_{\mathrm{ol}}-\gamma_{\mathrm{oa}}<0, $ (1)
$ S_{\mathrm{lo(a)}}=\gamma_{\mathrm{oa}}-\gamma_{\mathrm{lo}}-\gamma_{\mathrm{la}}<0, $ (2)
$ S_{\mathrm{os}(\mathrm{a})}=\gamma_{\mathrm{sa}}-\gamma_{\mathrm{os}}-\gamma_{\mathrm{oa}}>-\gamma_{\mathrm{oa}} R, $ (3)
$ S_{\mathrm{os}(\mathrm{l})}=\gamma_{\mathrm{sl}}-\gamma_{\mathrm{os}}-\gamma_{\mathrm{ol}}>-\gamma_{\mathrm{ol}} R, $ (4)
$ \gamma_{\mathrm{ol}}>0 . $ (5)
图 1 超滑表面的5种“失效模型”[29]

其中:s、o、a、l分别表示界面处的固体基底(solid)、润滑油(oil)、气体(air)和被拒液体(liquid)等不同相态;Sol(a)为气体环境下润滑油在被拒液体表面的铺展系数;Slo(a)为气体环境下被拒液体在润滑油表面的铺展系数;Sos(a)为气体环境下润滑油在固体基底表面的铺展系数,Sos(l)为液体环境下润滑油在固体基底表面的铺展系数;γla为被拒液体与气体界面处的表面张力;γol为润滑油与被拒液体界面处的表面张力;γlo为被拒液体与润滑油界面处的表面张力,大小与γol相同,方向与γol相反;γoa为润滑油与气体界面处的表面张力;γsa为固体基底与气体界面处的表面张力;γos为润滑油与固体基底界面处的表面张力;γsl为固体基底与被拒液体界面处的表面张力;R为粗糙指数。

Smith等[30]总结了液滴在超滑表面可能出现的12种热力学状态,如图 2所示。其中:w表示相态为水;θc为特殊临界接触角;θos(w)为水环境下润滑油与固体基底间的接触角;θos(a)为气体环境下润滑油与固体基底间的接触角;Sow(a)为气体环境下润滑油在水表面的铺展系数;Sos(w)为水环境下润滑油在固体基底表面的铺展系数。θc的计算表示如下:

$ \cos \theta_{\mathrm{c}}=\frac{1-\phi}{r-\phi} . $ (6)
图 2 超滑表面的12种热力学状态[30]

其中:r为固体表面总面积与固体表面投影面积之比;ϕ为固体顶部面积与固体表面投影面积之比。

润滑油与被拒液体的相互作用会产生Cloak和非Cloak这2类润湿状态,每一类润湿状态均可能存在6种热力学状态。发生Cloak现象时,液滴受较大钉扎力,不容易实现超滑;不发生Cloak现象时,无论是空气环境还是液体环境,润滑油均能完全浸润基材粗糙结构并形成润滑油膜时(A3-W3状态),液滴容易以较小的滑动角滑离表面,实现超滑。虽然超滑表面通过在粗糙结构中注入润滑油将固-液摩擦转变为液-液摩擦,减小了摩擦力,但实现超滑还与特殊的四相作用状态密切相关。超滑表面的稳定性相较于超疏水表面明显提升,但仍受机械损伤和润滑油损失制约,其中润滑油损失是影响超滑表面稳定性的主要因素。在超滑表面服役过程中,强大的外力(离心力、剪切力和重力)作用、润滑油蒸发、润湿脊形成、高速液滴冲击等因素会使润滑油不可避免地产生流失,导致超滑表面最终失效。因此,设计超滑表面不仅要考虑超滑功能的可实现性,还要兼顾超滑表面的稳定性。近年来,已出现较多制备超滑表面的方法,其中涂层法因加工成本低、施工简便和适合规模化制备等特点极具应用前景。因此,本文围绕仿生超滑涂层的设计与制备进行综述。

2 仿生超滑涂层设计

根据固油机制和润滑层厚度,仿生超滑涂层分为1D型、2D型和3D型3类, 如图 3所示。

图 3 3类超滑涂层的示意图

2.1 1D型仿生超滑涂层

1D型仿生超滑涂层无需具备特殊的微纳结构,而是将润滑分子化学接枝到基材表面,具有类似液体的特征[31-32]。Wang等[33]利用酸催化二甲基二甲氧基硅烷的接枝缩聚,在光滑基材表面获得了一种全疏共价附着液体(slippery omniphobic covalently attached liquid, SOCAL), 如图 4a所示。SOCAL不仅具有优异的超滑特性,而且被锚定在基材表面,不会因重力、剪切力和离心力等作用而流失。Zhu等[34]研究发现,相比硅油溶胀的聚二甲基硅氧烷(swollen-polydimethylsiloxane, S-PDMS),厚度仅为4 nm的SOCAL的接触角滞后更低、抗上皮葡萄球菌和铜绿假单胞菌的稳定性更优;在水介质中流动处理2~7 d后,S-PDMS抗铜绿假单胞菌的性能比SOCAL降低了2个数量级。Hao等[35]通过在无机二氧化硅表面接枝环状PDMS制备了一种有机-无机杂化超滑涂层,如图 4b所示,其中无机二氧化硅层与柔性PDMS链间的强共价键赋予了涂层表面优异的耐久性,当经过除冰循环、紫外线照射和有机溶剂处理后,涂层仍保持稳定的低黏附性。2020年,Zhao等[36]提出了一种在任意基体上构筑SOCAL的方法,通过电泳沉积二氧化硅中间层使SOCAL附着在各种基体表面,这有助于促进1D型仿生超滑涂层的应用。

图 4 1D型超滑涂层

需要指出的是,1D型仿生超滑涂层较薄(厚度通常为2~10 nm), 易发生机械磨损失效或因撞击导致基材暴露而失效[37]。针对机械磨损问题,Zhao等[38]通过溶胶-凝胶法制备纳米涂层并接枝柔性硅烷分子,获得一种具有纳米结构的类液体超滑涂层,该涂层在湍流、机械磨损和腐蚀性环境中展现了优异的耐久性和优于常规SOCAL的机械稳定性。鉴于部分润滑分子在水环境中易从基材脱附,Huang等[37]通过碱催化烷氧基硅烷快速水解和缩聚,制备了一种厚度可调的液态聚硅氧烷涂层,增厚的该涂层展现了良好的耐水解性、化学损伤自愈性、耐热性和防污性。与仅接枝润滑分子不同,Leslie等[39]使用全氟化碳分子层对基材进行接枝,随后注入分子量较小的全氟萘烷作为液体层以增强全氟化碳在水中的稳定性,从而大幅提高了涂层疏水性能。Ma等[40]将惰性硅油与聚有机硅氮烷(PSZ)的混合物旋涂并加热,制备了一种几乎无润滑油损失的超滑涂层(slippery coatings without loss of lubricant, SCLL),如图 4c所示,SCLL的稳定性主要源于PSZ和硅油分子间的强吸引力。

2.2 2D型仿生超滑涂层

2D型仿生超滑涂层主要以具有表面多孔结构的疏水化涂层为主体,润滑油为客体,客体受毛细管力稳定在主体的微纳结构中,并通过分子间相互作用在涂层表面形成薄层油膜[41-42]。构筑2D型仿生超滑涂层通常包括多孔涂层构建、疏水化处理和润滑油注入3个步骤。其中,前2个制备步骤与常规超疏水涂层一致,因此超疏水涂层的制备策略能够轻易地拓展至超滑涂层,这也使2D型仿生超滑涂层最常见[1]

目前,研究人员主要通过多级结构设计、结构形状或尺寸调控和化学处理等手段对2D型超滑涂层进行优化。赵书瑞等[43]利用呼吸图法制备了多孔环氧树脂基体,并通过控制环境湿度改变基体孔径和孔隙率,优化了涂层的超滑性能。Kim等[44]制备了3种具有不同突起高度和层次结构的超滑涂层并表征了涂层的油膜稳定性,如图 5a所示,发现均匀纳米结构的固油能力优于多级微纳结构与单级微米结构。Zhang等[45]使用实心微球、杨梅状空心微球和海胆状微球制备了3种超滑涂层,如图 5b所示,发现利用海胆状微球制备的超滑涂层具有低接触角滞后、良好的抗剪切性和耐腐蚀性。这是因为海胆状微球独特的中空结构有利于储存润滑油,而且微球外壳上的纳米孔能够输送润滑油并减少润滑油损失。Yan等[46]发现,将润滑油注入通过阳极氧化法制备的孔径和高度不同的二氧化钛纳米管阵列后,所形成涂层的超滑性能相似,但大直径、大长度纳米管所对应的超滑涂层在水流冲击测试中的稳定性最佳,扫描电子显微镜(scanning electron microscope, SEM)图像如图 5c所示。Long等[47]选取具有微纳多孔结构的天然凹凸棒石为原料,以无机磷酸铝作为黏结剂,制备的超滑涂层具有多级结构(花瓣状与微孔结构),如图 5d所示,该超滑涂层具有十分优异的拒液性、耐腐蚀性、防结冰性和耐久性。

图 5 2D型超滑涂层

2D型仿生超滑涂层通常需要进行表面疏水化处理,但有研究发现亲水涂层注入润滑油后也表现出良好的超滑特性。Maji等[48]在基材表面喷涂支链聚乙烯酰胺和聚二季戊四醇五丙烯酸酯的戊醇溶液,固化成膜后利用伯胺改性,获得了亲水多孔的聚合物涂层,注入润滑油后可实现超滑。这种亲水性聚合物基超滑涂层表面残留的疏水性丙烯酸酯对涂层的非常规超滑特性起关键作用。

需要指出的是,2D型仿生超滑涂层依靠表面多孔结构储存的润滑油保持超滑特性的能力十分有限,且润滑油层会在重力、剪切力和离心力等作用下被破坏,从而失去润滑油自补充功能,因此2D型仿生超滑涂层仅适用于常规服役环境。

2.3 3D型仿生超滑涂层

相比于2D型仿生超滑涂层,3D型仿生超滑涂层的基体具有凝胶交联聚合物网络或三维多孔结构。因此,3D型仿生超滑涂层不仅可以利用表面结构固定润滑油,还可通过内部结构储存或释放润滑油,表层润滑油损失后能够得到及时补充,从而长期保持超滑功能。目前,PDMS是最常见的可溶胀润滑油的聚合物,硅油在PDMS交联网络中扩散会使PDMS溶胀,并在表面形成硅油覆盖层,当因剪切、挤压等作用而失去覆盖层时,硅油可以从PDMS内部得到补充[49]。这类可溶胀润滑油的交联聚合物也被称为有机凝胶,需要注意的是,凝胶不能过量释放润滑油,否则超滑涂层会因油膜过厚而丧失超滑效果。凝胶释放润滑油的能力通常与凝胶刚度相关,刚度越大,越不易释放润滑油。Sotiri等[49]通过调控PDMS预聚物成分和润滑油黏度制备了几种凝胶,将制备的凝胶循环浸入水中后检测接触角滞后的变化与超滑损伤的恢复时间,发现当PDMS的2种预聚物的质量比为20∶1和10∶1时,涂层刚度低,表面油膜过厚、润滑油易丢失,难以实现超滑;当质量比为5∶1时,虽然润滑油补给速率较慢,但涂层表面的油膜厚度合适且稳定吸附,并且润滑油能够得到持续补充,涂层的超滑性能与稳定性显著提升。可以看出,润滑油的储存与按需释放是提升超滑涂层稳定性的重要手段,如何通过调控刚度实现润滑油按需补充还需要进一步探索。

凝胶类涂层能够储存大量润滑油,但相较于2D型仿生超滑涂层,凝胶类涂层刚度小,易因机械磨损而被破坏。因此,在2D型仿生超滑涂层基体的基础上,引入三维多孔结构是制备具有耐磨、耐剪切和润滑油储存/释放功能的3D型仿生超滑涂层的有效方法。Yong等[50]利用飞秒激光直写在聚酰胺6(PA6)内部制备三维多孔网络结构,并通过化学改性和注入润滑油获得了3D型仿生超滑涂层,如图 6a所示。由于PA6的耐磨性好和润滑油的减磨作用,因此涂层在100次磨损循环后表面水滴滑动角仍小于3°。Han等[51]通过在基板上沉积含有聚乙二醇和硅油的乳液制备了类柑橘皮微腔结构,经过疏水化处理并灌注润滑油后得到一种3D型仿生超滑涂层,如图 6b所示。由于存在具有凹口和封闭侧壁的微腔结构,因此该涂层稳定性显著提升;而且在水流冲刷实验中,该涂层经液滴滑动数百次后仍具有超滑功能,涂层耐用性比微柱阵列的超滑涂层高2个数量级。

图 6 3D型超滑涂层

单层涂层的超滑和润滑油储存/释放功能往往难以兼顾,因此,研究人员提出通过构建多层膜实现超滑与润滑油储存/释放。Zhang等[52]先通过溶胶-凝胶法在铝板上制备花瓣状多孔微结构底层,再利用水热法制备均匀的疏水棒状氧化锌顶层,得到了具有润滑油储存/释放功能的双层超滑涂层,如图 6c所示。Sun等[53]通过喷涂磷酸锆/环氧树脂涂料(底层)与氟化石墨/聚四氟乙烯涂料(顶层)制备了一种双层超滑涂层,实现了底层牢固黏附、润滑油稳定吸附和润滑油持续释放,具有良好的润滑性能与抗腐蚀性。然而,这种多层超滑涂层的制备过程较复杂,需要考虑各层结构功能的差异和相邻层理化性质的相互影响,并需要解决层间分离与剥落问题。

润滑油的响应性和控释性是上述多层超滑涂层性能提升的关键。Damle等[54]设计了一种三明治结构有机凝胶,以此控制润滑油的释放,先将棉和聚酯织物封装到PDMS中,通过织物内部的中空结构储存润滑油,再利用PDMS吸收润滑油并补充表面油层,如图 6d所示。Cui等[55]总结了超滑涂层液体释放的调节方法,提出了一种可自我调节液体释放的动态聚合物系统,如图 6e所示。该系统由尿素和PDMS交联的聚合物基体、储油空腔和表面润滑油层三者组成,三者之间能够实现润滑油重复交换。其中,空腔中的润滑油以无壳液滴形式储存于由动态聚合物组成的交联网络,在液滴和凝胶相之间进行持续的液体交换。一旦表面油层被去除,基体就会释放润滑油并恢复油膜厚度,同时基体通过尿素和PDMS之间的可逆氢键的解离与缔合重新构建交联网络,释放因无壳液滴收缩而积聚的机械应力。这种通过聚合物交联、液滴收缩和液体传输之间动态反馈诱导的液体释放机制,为研发智能3D型仿生超滑涂层提供了新思路。Rao等[56]使用4-苯基偶氮苯酚、异佛尔酮二异氰酸酯和羟丙基封端的二甲基硅氧烷制备了一种共价交联的紫外响应聚合物,并以此为基础获得了具有紫外线响应、自修复功能和高耐用度的超滑涂层,可通过紫外线照射诱导偶氮苯基团的构象从反式变为顺式,使块状多孔基体轻微收缩,从而释放润滑油。Yao等[57]设计和制备了一种润滑油自补充有机凝胶涂层,由于熵增益主导的交联网络膨胀,因此低摩尔质量组分会优先膨胀至交联网络,当该涂层应用于石油运输管道防蜡时,可以通过吸收原油中的小分子连续补充有机凝胶表层润滑油。

综上,3D型仿生超滑涂层的凝胶聚合物交联网络和三维多孔结构,均有利于润滑油固定与储存,增强了超滑涂层的稳定性,而且具有响应、控释功能的智能3D型仿生超滑涂层已经得到初步发展,极具应用潜力。然而,凝胶类3D型仿生超滑涂层存在成本高、力学与耐磨性能差等问题,而非凝胶类的三维多孔结构3D型仿生超滑涂层虽然耐磨性能较好,但润滑油的控释性能较差。

3 仿生超滑涂层的制备技术 3.1 1D型仿生超滑涂层

1D型仿生超滑涂层主要通过Grafting to和Grafting from这2种化学接枝法制备[58-59]。Grafting to是指将聚合物链末端或侧链上的活性位点通过物理吸附或化学键合作用固定在基材表面,形成一层平整均匀的薄膜[60-61]。Krumpfer等[62]通过将三甲基封端的PDMS接枝到光滑硅晶表面,形成类液体超滑涂层。虽然三甲基封端的PDMS通常被认为是惰性的,但是可通过水解形成2个较短的羟基封端链,从而引发自反应附着。Sarma等[63]将带羟基的玻璃基板浸入硅氧烷基封端硅油与正庚烷(质量比为1∶10)的混合溶液中,玻璃基板上的羟基主动引发了硅氧烷基封端硅油接枝,从而获得了类液体超滑涂层。Gresham等[64]的研究表明,具有硫醇基团的聚合物分子可以接枝到金块表面。Singh等[65]进一步研究发现,通过Grafting to可在其他金属基材(铝、钛、铁、镍和氧化铜)表面制备超滑涂层。此外,分别对基材与聚合物进行预处理后,还可利用环氧基与羧基[66-67]、环氧基与氨基[68-69]及羧基与氨基[70-71]之间的反应将聚合物接枝到基材表面。Grafting to的优点是不涉及复杂的合成程序,但存在接枝层较薄、接枝分子数量受反应条件影响较大等局限性[72]

Grafting from是在引发剂修饰的基材表面原位聚合化学组成、厚度和接枝密度可控的聚合物薄膜[72]。Monga等[73]对硅片进行氧等离子处理以产生羟基,利用气相与液相接枝法在硅片上附着了不同分子量的氯封端PDMS,获得了2种1D型仿生超滑涂层。Zhang等[74]选取分子量更大的氯封端PDMS并通过同样的方法获得了厚度为30 nm的类液体超滑涂层,具有优异的拒液、除冰和耐磨性能。此外,卤代烃(α-溴代酯,α-卤代酮,α-卤代腈等)[75]和聚多巴胺[76]也可引发Grafting from反应。其中,聚多巴胺可以轻松黏附在各种材料表面,从而生长类液体超滑涂层,这拓展了Grafting from的适用范围[60]。Chiera等[77]在基材表面沉积聚多巴胺黏合剂,将单氨基丙基封端PDMS接枝在多巴胺层上,即可获得超滑涂层。Grafting from的优点是能控制聚合物均匀生长,缺点是需要选择特定基材或提前将引发剂锚定在基材表面。

3.2 2D型仿生超滑涂层

2D型仿生超滑涂层多孔基体的主要制备技术包括旋涂、浸涂、喷涂、气相沉积、电化学沉积和溶胶-凝胶等。浸涂是将基材浸入涂料一定时间后取出,而旋涂是在离心力和重力作用下,将涂料沉积在基材表面[78-79]。这2种技术操作简单,施工成本低,应用广泛。Liu等[80]将聚四氟乙烯的水悬浮液通过旋涂技术沉积在多种基材(硅片、玻璃、铜板和钛板)表面,注入全氟聚醚润滑油后得到一种超滑涂层,该涂层透明坚固且性能稳定。Li等[81]利用正硅酸乙酯、乙烯基三乙氧基硅烷、全氟辛基硅氧烷和二氧化硅纳米粒子制得纳米复合溶液,并通过旋涂复合溶液得到疏水涂层基体。Wang等[82]通过将载玻片浸入树枝状二氧化硅纳米粒子的乙醇悬浮液中,制备了具有中孔和集聚孔的纳米涂层基体。此外,旋涂和浸涂技术也常用于润滑油注入[83-87]。需要指出的是,旋涂技术存在大量浪费涂料和不能在曲面加工等缺点;浸涂技术虽然对基材种类限制小,但浸涂过程中溶液黏度、溶剂蒸发速率和基材取出角度等因素会影响成膜质量,不易精准控制涂层厚度。

喷涂是指通过喷枪或雾化器先将涂料分散成均匀的雾状液滴,再沉积在基材表面[88-89]。喷涂是制备多孔涂层基体最常用的技术。Liang等[90]提出了一种简单易用的制备粗糙基底的方法,先将氟癸基三乙氧基硅烷修饰的二氧化硅微球与醇类溶剂混合形成乳浊液,再将乳浊液喷涂在预加热的虫胶表面,待乙醇挥发后形成粗糙结构基体。Agarwal等[91]通过在柔性平面上交替喷涂或同时喷涂聚醚酰亚胺溶液和2-乙烯基-4, 4二甲基内酯,制备了具有微纳孔隙的弹性聚合物涂层,疏水化处理后注液,获得了能够卷曲的超滑涂层。Wang等[92]利用喷涂技术制备了由羟基磷灰石纳米纤维组成的多孔基体,注入油酸后获得了一种透明超滑涂层,该涂层通过羧基和钙原子配位,形成了均匀稳定的润滑层。为避免烦琐的制备步骤,Lu等[93]提出了一种由聚丙烯、硅油和十六烷组成的三元热致相分离系统,将该三元热致相分离系统加热至180 ℃后喷涂于基材表面,常温冷却后即可获得具有树枝状结构的超滑涂层,该涂层具有优异的稳定性和耐久性,经过21 d的水流冲刷仍表现出低于15°的滑动角。此外,热喷涂技术制造超滑涂层近年来受到广泛关注。Koivuluoto等[94]通过火焰喷涂聚乙烯制备多孔涂层,浸渍硅油后该多孔涂层展现了优异的疏冰性。热喷涂技术能够避免使用有毒、有害溶剂溶解聚丙烯、聚乙烯等材料,是一种对环境友好的喷涂技术。当采用热喷涂技术时,需考虑喷涂参数对涂层结合强度、孔隙率等的影响。Khammas等[95]使用热喷涂与冷喷涂技术分别制备了以聚醚醚酮粉末为填料的共聚物涂层。通过热喷涂技术获得的涂层表面存在一些球形的半熔融颗粒,孔隙率较低,而由冷喷涂技术制备的涂层表面存在与粉末原料相似的颗粒,润滑油注入后该涂层的超滑性能更佳。因此,针对不同的原材料、基材和施工环境,需要选取适合的喷涂技术[96]

电化学沉积技术可以便捷地控制基材表面特征的生长动力学,能够简单、快速地在基材表面制备管状、针状、树突状和片状等多种形态的微观结构[97]。Kim等[98]在铝板上采用电化学沉积技术沉积纳米结构聚吡咯,经过氟化处理后注入润滑油,获得了超滑涂层。Yan等[46]通过阳极氧化法在钛基板上生长半径和长度不同的二氧化钛纳米管阵列,注入润滑油后获得超滑涂层,润滑油被单腔管阵列吸附于结构内,稳定的润滑油液膜可使水滴在极低温度(-15 ℃)下保持液相长达24 h,具有极优异的延迟结冰性能。通过电化学沉积技术获得的超滑涂层通常具有优异的机械稳定性和高结合强度,但基材仅限于导电材料,并且电解液容易导致环境污染。

溶胶-凝胶技术制备多孔基体的过程如下:将烷氧基或金属醇盐等物质溶于溶剂中形成前驱体溶液,前驱体溶液在一定条件下发生水解和缩聚等化学反应并形成溶胶,将溶胶涂覆至基材表面,溶胶粒子不断生长,形成具有交联网络结构的凝胶,凝胶中的溶剂挥发后,即可得到多孔涂层基体[99]。Wei等[100]通过溶胶-凝胶技术制备了一种纳米复合涂层,该复合涂层具有丰富的内部交联网络和纳米结构,浸渍硅油后获得透明、稳定的超滑涂层。通常,采用溶胶-凝胶技术制备的涂层与基材之间的结合强度较低。Li等[101]针对涂层黏附性差等问题,先在基材表面预沉积聚多巴胺,再采用溶胶-凝胶技术制备了坚固、稳定的超滑涂层。溶胶-凝胶技术的优点是涂层化学成分容易控制、微观结构平坦均匀,缺点则是工艺复杂、耗时长和成本高等。

3.3 3D型仿生超滑涂层

3D型仿生超滑涂层多采用喷涂、旋涂、浸涂、静电纺丝和逐层组装等技术进行制备。喷涂、旋涂和浸涂技术是将涂料涂布在基材表面以制备超滑涂层。Zhu等[102]采用浸涂技术将氯化钠、氧化锌和聚四氟乙烯的混合乳液涂覆至载玻片表面,高温固化成膜后用乙酸除去氯化钠和氧化锌,获得了具有三维多孔结构的超滑涂层,该涂层具有良好的机械稳定性和耐久性。这种依靠内部三维多孔结构增强润滑油储存能力的3D型仿生超滑涂层相较2D型仿生超滑涂层具有更好的稳定性,但其固油能力弱于凝胶类3D型仿生超滑涂层,在强剪切和水流冲刷的条件下稳定性相对较弱。Zhu等[103]将PDMS预聚物浸涂于基材表面,经过固化和硅油溶胀后获得了凝胶类3D型仿生超滑涂层,溶胀的硅油会被逐步释放并修复润滑层,使涂层在空气中放置60 d不失效,水流冲刷失效后静置8 h恢复超滑效果,表现出良好的稳定性与自恢复性。Zhu等[104]则是将硅油与PDMS预聚物共混,通过旋涂技术制备了凝胶类3D型仿生超滑涂层。

逐层自组装技术是通过静电力、氢键、配位键和化学键等分子层间作用力,将带相反电荷的物质逐层交替组装,层与层之间自发缔合形成微纳结构[105]。Sunny等[106]将带负电荷的二氧化硅纳米颗粒和带正电荷的聚二甲基二烯丙基氯化铵逐层沉积在基材表面后,用氟化硅烷处理并注入氟化润滑油,获得超滑涂层,涂层的拒液性随着二氧化硅纳米颗粒层厚度增大而提升。Manabe等[107]先以静电力和氢键驱动壳聚糖、海藻酸钠和聚乙烯吡咯烷酮等材料进行逐层自组装成膜,再浸入缓冲液中去除聚乙烯吡咯烷酮,获得了可储存润滑油的3D多孔结构。Manna等[108]利用“反应性”或“共价”材料逐层自组装,制备了纳米多孔聚合物涂层,涂层经疏水化处理和注油后得到超滑表面,该超滑表面在海水中浸泡2个月或被砂纸严重磨损后仍具有超滑功能。Zhu等[109]提出了一种仅涉及逐层自组装和润滑油注入这2个步骤而不包含疏水化处理的超滑涂层制备方法,制备过程如下:先利用全氟磺酸基聚合物和支链聚乙烯亚胺在甲醇溶液中进行自组装,获得了具有分级三维多孔结构和低表面张力的超疏水表面,再经氟硅油渗透后形成超滑涂层。逐层自组装制备技术的优点是效率高、能耗低、操作简单,可应用于多种基材表面,缺点是需要消耗大量有毒溶液,易导致环境污染和危害健康,且不同逐层自组装体系难以相互借鉴。

静电纺丝技术通常用于制备聚合物微纳米纤维,通过注射器喷嘴和导电收集器对聚合物溶液施加静电作用,形成带电液体射流,射流中的溶剂在喷射过程中蒸发而聚合物沉积在收集器表面,最终形成固体纤维膜[42]。Wu等[110]利用静电纺丝技术将聚偏二氟乙烯和聚醋酸乙烯制备为多孔纤维,疏水化处理后灌注润滑油,获得超滑涂层。Wang等[111]通过静电纺丝技术制备了热塑性聚氨酯纳米纤维膜,灌注全氟聚醚后得到超滑涂层,该涂层可通过施加拉应力改变表面形貌,从而控制液滴滑动性能。Agarwal等[112]以聚己内酯作为纺丝材料,制备了具有互通三维多孔结构的纳米纤维膜,该纳米纤维膜具有良好的生物相容性与生物可降解性。Su等[113]将聚偏氟乙烯与疏水性二氧化硅纳米粒子混合,通过静电纺丝技术构筑了一种复合薄膜,其中二氧化硅纳米粒子增加了表面粗糙度,使薄膜表面和内部孔隙均具有粗糙层次结构,储油能力增强。通过改变静电纺丝参数可获得串珠状纤维[114]、多孔纤维[115]、带状纤维[116]和多通道微管结构纤维[117]等多种结构纤维,这为超滑涂层的制备提供了多种可能。静电纺丝技术具有操作简单、原材料丰富、纤维形状可调等优点,但涂层黏附性较弱、机械强度低,且易残留有毒溶剂。

4 结论与展望

仿生超滑涂层因优异的拒液性和广泛的应用潜力而受到研究人员的极大关注。本文根据固油机制和润滑层厚度,将超滑涂层分为3类,并综述了各类超滑涂层的固油机制、优化措施和制备方法。1D型仿生超滑涂层主要通过化学接枝稳定润滑层,因此在重力、剪切力和水流冲刷等作用下具有良好的超滑稳定性,但厚度较低、耐磨性较差,遭受机械磨损时易彻底失去超滑性能。此外,在制备方法方面,1D型仿生超滑涂层存在制备流程复杂、制备条件苛刻和成本高等局限性,不利于大规模应用。2D和3D型仿生超滑涂层均通过多孔结构稳定润滑油层。其中,2D型仿生超滑涂层的机械稳定性较强、制备方法简单,但固油能力较差,润滑油易损失且无法自恢复,苛刻条件下难以长久保持超滑功能。为解决2D型仿生超滑涂层固油能力较差的问题,研究人员在结构设计和化学改性方面开展了大量研究,设计和制备了一系列具有强固油能力的2D型仿生超滑涂层。然而,2D型仿生超滑涂层的表面多孔结构仅能储存少量润滑油,润滑油损耗后难以得到有效补充。3D型仿生超滑涂层包括凝胶类和非凝胶类2种涂层,凝胶交联网络结构和三维多孔物理结构均能够储存/释放润滑油,涂层的超滑稳定性大幅提升。此外,随着智能材料引入,3D型仿生超滑涂层可根据环境或涂层状态变化主动调节润滑油释放,因此能够更好地适应恶劣工况。不足的是,凝胶类超滑涂层的力学性能和耐磨性能表现欠佳,而非凝胶类涂层的润滑油控释效果较差。

综上,制备高可靠性、长寿命的仿生超滑涂层,并进行规模化工业应用,还需要进行深入研究,具体可从以下几方面考虑:

1) 深入理解超滑涂层的润滑油储存、固定和释放机制,引入智能材料体系,系统研究结构特征、化学组成和制备方法对涂层稳定性的影响规律,通过优化设计获得稳定的智能型超滑涂层。

2) 开展润滑油注入对膜基结合性能的影响规律研究。润滑油注入可能影响涂层与基材之间的结合性能,但目前研究主要集中于超滑界面,而鲜有关注润滑油注入后膜基结合性能的演化规律。

3) 从设计策略着手,简化制备方法、降低成本,促使仿生超滑涂层走向应用。

4) 使用环保材料,实现绿色制造。超滑涂层的制备通常涉及有害且无法降解的含氟材料,在生物医学与海洋装备等领域的应用受到限制。

5) 面向超滑涂层的实际应用环境,针对性地模仿不同自然环境下多种生物模本的功能原理,进而开发新型超滑涂层。

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