2. 哈尔滨工业大学 先进结构功能一体化材料与绿色制造技术工业和信息化部重点实验室,哈尔滨 150001
2. Key Laboratory of Advanced Structure-Function Integrated Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin 150001
铝、镁、钛、锆、铌、钽等金属合金及其复合材料表面微弧氧化(Microarc oxidation, MAO)技术,又称等离子体氧化(Plasma electrolytic oxidation,PEO)技术,是将金属浸入碱性电解液中,通过施加高电压使金属表面发生击穿微弧放电,进而微弧放电微区的局部高温高压作用使基体金属发生氧化,在基体金属表面形成以基体元素氧化物为主、电解液所含元素参与掺杂/混合改性的功能化陶瓷涂层[1-6]。金属表面微弧氧化涂层的组成、结构与性能,可通过电解液成分选择与电参数的匹配进行优化设计,进而可获得一系列高技术需求的特殊功能化涂层(如抗磨减摩、抗腐蚀、热防护、热控、介电绝缘、催化、生物活性等),在汽车、航空航天(航空发动机、舰载飞机、卫星、火箭、导弹等)、船舶、纺织、电子(3C产品)、医疗器械与环保净化等轻量化装备与高附加值涂层关键技术领域有广阔应用前景。
文中首先阐述了微弧氧化技术的发展概况。微弧氧化涂层相对于传统阳极氧化膜、硬质阳极氧化膜具有很强涂层结构可设计性、工艺简单环保及其特殊功能性优势,从而迅速引起学术界及工程界[7-9]的研究兴趣。2000年以来,微弧氧化技术已在诸多领域(抗磨减摩,抗腐蚀、热防护、热控、介电绝缘、催化、生物等)得到了广泛关注。通过Web of Science数据库检索(检索词microarc oxidation & plasma electrolytic oxidation)分析,图1给出该领域发表文章及各功能化涂层方向所占比例分布情况。由图1可见,自2000年以来,微弧氧化涂层研究受到了持续的关注;尤其从2005年开始,微弧氧化相关论文发表数量快速增加。由各功能化涂层方向发表论文比例可见:初期研究主要集中在抗磨减摩涂层,抗腐蚀涂层则逐年平稳上升;而自2005年起,生物、催化、热防护、热控等新功能涂层的论文数量显著提升。可见,微弧氧化功能化涂层的设计、制备及使役性能等研究是本领域持续的关注热点。
除学术研究外,工程界对微弧氧化功能涂层的应用开发给予了极大关注[10-13],已有许多企业(如英国的Keronite公司、德国的Magoxid-coat公司、美国Microplasmic公司、俄罗斯与乌克兰的诸多公司等)及国内诸多科研院所正在推进微弧氧化技术的工程应用。国际表面涂层技术(Surface &Coatings Technology)主编曾撰写评述文章总结了1999年前微弧氧化技术理论研究与发展情况 [7];为总结在这之后的研究成果,英国Woodhead出版社在其出版的“轻金属表面工程”一书中,蒋百灵和王亚明撰写第5章“铝与钛合金的微弧氧化”[14],同时德国材料研究所(Institute of Materials Research)C. Blawert和B. Srinivasan撰写第6章“镁合金的微弧氧化”,英国Keronite公司S. Shrestha与欧洲航天局(European Space Agency)的B. D. Dunn撰写“微弧氧化技术的航天应用”一章。
由此可见,微弧氧化技术的学术与应用研究在表面工程领域已经受到足够的重视,文中旨从微弧氧化技术的基本原理、功能陶瓷涂层设计制备与使役性能角度综述最新的研究与发展概况,并给出最新的工艺探索与可能的发展方向。
1 微弧氧化基本原理金属表面微弧氧化涂层制备装置,主要由高压脉冲电源、电解槽、搅拌系统和水冷系统等组成。通过调整脉冲电源两半周期的电容,可以对正负电参数(电压、频率、占空比等)的幅值的比率进行单独调节,从而拓展了微弧氧化涂层生长与涂层微观结构的调控范围。试样(工件)作为阳极,与电解槽的不锈钢内衬形成对等电极。用去离子水配制溶液,溶液温度一般控制在50 ℃以内;对于涂层生长要求严格的电解液,也可通过冷却系统精确控温。
微弧氧化过程伴随有火花放电现象及涂层组织结构的变化。研究者主要针对火花放电的本质提出了各种微弧氧化机理模型,如电子“雪崩”[15]、电子隧道效应[16 - 17]、氧化膜中电解质为放电中心[18]及接触辉光放电电解[19]模型等,目前对火花放电的本质仍存在争议。Yerokhin等描述了电压-电流开放式氧化时放电现象[20]及阳极产生气体与电流效率的关系[21]等。实验表明,不同金属表面微弧氧化现象与涂层形成过程相似。王亚明[14]以钛合金为例分析了微弧氧化涂层的组织结构与基体/涂层/电解液界面的化学反应,并提出击穿-通道-熔凝效应与涂层形成机制模型示于图2,对应典型电压-时间变化曲线示于图3。
在图3中区域I,电压线性增长对应于传统的阳极化阶段,此时按传统的阳极氧化方式形成非常薄的TiO2绝缘膜(如图2(b)所示),膜生长遵从法拉第定律,其电流效率为100%。在区域II中电压增加变得缓慢,氧化膜生长速率减小,此时阳极化涂层生长与阳极溶解同时存在,两种作用产生竞争。在区域III中电压值增加较快,在已形成的绝缘膜局部缺陷位置(如膜内气孔等)发生介质失稳,产生击穿放电并形成放电通道(如图2所示),同时伴随有大量的氧气释放,大量氧气的产生是电流效率降低的主要原因。在区域IV中电压值保持稳定,为稳定氧化阶段。
每一次火花放电对应一个贯穿于涂层的放电通道,导致击穿-通道-熔凝效应。
(1) 放电诱发离子“短路”迁移。通过分析不同电解液体系涂层沿截面的元素分布认为,含磷酸盐如
(2) 放电诱发向基体侧生长。放电微区内基体溶解(以钛合金为例)、熔化或溅射进入通道,并发生氧化:
膜基界面处产生的氧化产物在基体快速冷却下,形成新生涂层为细小均匀纳米晶层,此层在反复放电过程中发生冶金过程,晶粒不断长大,同时新生层也不断向基体侧推移,实现涂层的增厚。研究表明O2−的浓度及扩散速率是影响涂层生长速率的关键因素[22]。
(3) 放电诱发涂层内外层同时生长。在通道内的高温高压作用下,微区内形成的气体由通道逃逸出膜/液界面,通道内的反应产物或再熔融的涂层物质在压力作用下喷射并沉积于通道口附近,呈喷发的“火山囗”形态。火花熄灭后,熔融物质冷却凝固后在通道内壁沉积以封闭通道,形成致密的内层。并非所有的通道均完全封闭,一些未完全封闭的通道形成疏松的外层。涂层致密的内层与疏松外层同时生长(图2(c)~(f))。
(4) 放电诱发表面沉积物卷入涂层。取决于电解液体系,如Al(OH)4或H2SiO3等不溶凝胶在氧化过程中不断在涂层表面沉积(图2(h)),随后的放电过程中水合多聚物凝胶热解形成氧化物,并通过通道效应作为外层产物卷入涂层。表面沉积层产物不同,导致涂层的物相结构差异较大。
(5) 放电诱发反复熔凝过程。放电通道内形成的熔融产物在冷电解液及基体的瞬间快速冷却下,形成不同结构的纳米晶或非晶态组织。在放电通道周围也形成瞬时的温度梯度,从而在放电通道边缘生成柱状晶组织(如图4所示)。同时,邻近放电通道附近的涂层区域,在瞬时局部的加热与冷却循环作用下(因反复放电引起),发生熔融、冷淬及结晶过程(对应图2(c)~(f)阶段),此过程促进复杂氧化物形成及高温相变(如锐钛矿向金红石型氧化钛转变)。
(6)放电不会诱发基体组织改变。放电总是在膜基界面处或邻近界面区域产生,并形成贯穿涂层的通道,但放电产生的局部高能微区不会引起基体金属组织的任何改变。
对于微弧氧化涂层形成过程机理,研究者们正尝试通过高速摄影与发射光谱(OES)等先进的原位分析手段[23-26],试图对发生在微区的瞬间反应进行更深入的揭示,这将为功能化涂层(成分/结构/表界面)设计与制备方法调控提供了理论依据。
2 微弧氧化功能陶瓷涂层工艺控制策略明确微弧氧化工艺的影响因素是设计制备特殊功能化涂层的关键。微弧氧化系统的内在因素(电解液组成、pH及电导率)和外在因素(电源类型、电参数与溶液温度控制)影响微弧氧化涂层的生长与组织结构。其中,电解液的组成与配比、施加的电参数对微弧氧化过程起决定性作用,实际上通过以上参数设计可获得特殊物相组成与结构的涂层,以满足多种功能涂层的制备需求。另外,不同金属基体的性质(如电导率、组织及缺陷、第二相掺杂及含量等)对微弧氧化过程影响很大,有时必须采用微弧氧化前处理、特殊的溶液与电参数才能保证涂层生长。下面针对影响微弧氧化涂层生长的主要影响因素进行讨论。
2.1 电解液对涂层的影响电解液作为影响微弧氧化过程的内因,可以归纳为如下作用:①首先也是最重要的是,促进金属表面钝化形成薄的介质绝缘膜,它是在外电压下引起介质击穿导致放电的先决条件;②作为导电的溶液介质,传递发生在金属基体/电解液界面处进行氧化反应所需要的能量;③以含氧盐的形式提供氧化所需要的“O”源;④微弧放电过程可使电解液特定成分如离子、离子团、分子、纳米/微米金属(陶瓷或高分子)颗粒通过化学反应或电泳机制进入涂层中,这为进一步改性、改善或设计新颖的涂层提供途径。
为满足介质击穿的先决条件,能够促进强烈金属钝化的添加剂(如硅酸盐、铝酸盐和磷酸盐)被广泛用于电解液的基本组成。上述3类强钝化剂有如下优点:①微弧氧化放电前阶段,它们使火花电压很容易达到,因此节省时间;②电解液中存在的组分(如
实际上,已经证实即使在单一的NaOH碱性溶液(仅起钝化与导电作用)中可通过放电生长涂层,但是单一或简单的溶液体系导致涂层生长速率慢、能耗高,也不利于功能涂层的工艺设计制备与产业化生产。因此,依据功能化涂层的需求,复合电解液体系及配方的探索研究,对于学术与工程应用均产生极大吸引力。
2.2 电参数对涂层的影响微弧氧化陶瓷涂层一般分为内层致密层与外层多孔层,而较高的外层多孔层比例使涂层硬度等力学性能下降。限制或抑制多孔层生长,途径之一是优化电解液组成;另一途径是用特殊的电流制度(如双极脉冲电流配合优选的电参数)。双极脉冲电源可改善涂层的表面形貌,特别通过更短、更高能量的高频率放电作用以减小多孔层的厚度。
改变脉冲电参数实质上是改变单脉冲作用的能量,也就是微弧氧化过程中单脉冲能量的控制与作用。单脉冲作用的能量Ep可由式(5)定义:
式中,Up为脉冲电压(V),Ip为脉冲电流(A),tp为脉冲开放时间(s)。因此,改变脉冲参量可对表面放电特性进行调制,这直接影响涂层的生长、组织结构与相组成。
涂层生长速率与表面质量主要由单脉冲的放电能量决定。增大脉冲电压、减小频率、增大占空比或增大电流密度均使单脉冲的放电能量增大,使单脉冲的涂层生成量提高,因此涂层的生长速率增大。单脉冲放电能量增大时,热析出增大,等离子放电区的温度迅速升高,在放电区的金属基体和氧化涂层熔融量增大,等离子放电也更加强烈,这使得等离子放电通道形成物凝固后留下孔径更大的微孔,涂层表面变得更加粗糙。同时单脉冲放电能量的提高可使放电区域温度升高,这有利于涂层中特定氧化产物相结构的转变。
Kuskov等[27]在D16铝合金上的实验结果也表明提高电压与电流密度导致高的放电强度,使涂层的厚度增大,这是由于高的放电强度使涂层中高温高硬稳定相α-Al2O3含量增多。王亚明等[28]也提出电参数的阶段式氧化制度可以改善涂层的致密性。阶段递减的电流密度[29]比常电流密度极大地改善了涂层的结构,这与火花放电行为的改变有关,即放电后期自由衰减的电流密度导致前期已形成微孔孔隙的愈合。
总的来说,对于特定的电解液体系和金属基体,涂层的生长速率、物相组成与表面形貌可通过电参数进行优化设计。
2.3 基体成分与组织对涂层的影响用传统的阳极氧化工艺,高硅铸造铝合金或高铜铝合金很难进行氧化;而用微弧氧化工艺,尽管在高Si(或Cu)富集区有局部缺陷和稍高的能量消耗,但制备较厚涂层以改善抗磨与耐蚀性能是适用的。Si和Cu是引起微弧氧化涂层不均匀的主要元素[30],涂层表面不均匀性随合金中Si含量增加而增大,然而Si含量高达20%时仍然可形成较好质量的涂层,尽管由于孔隙率增加导致涂层硬度下降。
基体合金微观组织的不均匀特性仅对涂层的初期生长起作用。Duan等[31]发现镁合金组织中α相比β相表面有更高的生长速率,且β相微弧氧化涂层主要在α相形成的氧化膜的外围生长。这可能是由于α相有更高的反应活性,在高电场与Joule热的共同作用下可释放更多的Mg2+参与成膜反应[32]。
陶瓷颗粒或纤维增强的轻金属基复合材料由于具有低密度、高比强与比刚度等优异性能,在汽车、航空航天等领域有诱人应用潜力。但是,在服役环境(特别是腐蚀气氛)中它们比对应的轻合金更容易腐蚀,这是由于增强相与基体金属间的局部腐蚀或电流反应腐蚀、以及新化合物在界面的形成导致的选择腐蚀作用显著。传统的阳极氧化工艺已经用于在Al8090/SiC[33]和A6061/(Al2O3)p[34]表面制备薄的膜层,然而存在于界面的颗粒或晶须严重破坏膜层的完整性,因此腐蚀防护性能受限制。最近的研究[35-41](列于表1)和其它[42]研究结果表明微弧氧化有希望替代传统阳极化在铝或镁基复合材料表面制备高性能的防护涂层。不管怎样,金属基体中的增强相(如颗粒、晶须或纤维)不可避免影响微弧放电过程而导致低的涂层生长效率[34-35, 38],因为在微弧氧化火花放电前的初始阶段它们已经破坏了初始障碍层形成的完整性。薛文斌等[35]认为大部分的SiC增强相在火花放电通道内高温烧结作用下已经熔化形成氧化硅;但Arrabal等[38]研究表明SiC增强相没有改变的进入到涂层中。
Substrate | Metal-matrix
composite |
Reinforcement
phase size/μm |
Electrolyte
systems |
Coating phase
constituents |
Properties | Reference |
Al | 2024/15% SiCp | ~12.8 | Na2SiO3, 6−10 g/L
KOH, 1−2 g/L |
Mullite, α-Al2O3, γ-Al2O3,
amorphous SiO2 |
Anti-corrosion | [35-36] |
A356/20% SiCp | Na2SiO3, 15 g/L
NaAlO2, 3 g/L |
Mullite, α-Al2O3, γ-Al2O3 | Anti-corrosion
wear resistance |
[37] | ||
Mg | ZC71/12% SiCp | 2−20 | Na2SiO3, 0.05 mol/L
KOH, 0.1 mol/L |
MgO, Mg2SiO4 | Anti-corrosion | [38] |
AZ91/22% Al18B4O33w,
AZ91/22% SiCw |
Na2SiO3, 15 g/L
KF, 8 g/L, KOH, 8 g/L |
MgO, Mg2SiO4, MgF2 | Anti-corrosion | [39-41] |
膜基结合强度是各种微弧氧化功能涂层实现其使役性能的关键前提。尽管微弧氧化涂层因其冶金结合界面,一般认为膜基结合强度很高。但在不同的微弧氧化工艺条件下,膜基结合界面的特性会有较大的差异,因此探究涂层工艺对膜基结合强度的影响是非常必要的。不同金属基体表面微弧氧化涂层的结合强度如表2所示[43-53]。
(1) 火花放电强度对膜基结合强度影响
高电压、大电流导致微弧氧化火花放电强度增大,涂层生长速率高,涂层厚度增大。但发生在膜基界面的强烈火花放电,影响界面处涂层的致密性,进而降低涂层的膜基结合强度。通常情况下,涂层的厚度增加,致密度下降,导致膜基结合强度降低。Tang等[43]研究了占空比对AZ31B镁合金微弧氧化涂层结合强度的影响,随占空比增加,涂层厚度降低,剪切强度增加,占空比为40%时,最大结合强度为24.5 MPa。HAN J等[49]通过调控时间,在钛合金表面获得黑色和白色陶瓷涂层,高厚度涂层的结合强度明显降低。憨勇等[48]在Ta合金表面制备不同厚度微弧氧化涂层,也呈现出类似的结果。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Test method | Bonding strength | Reference |
Mg | AZ31B | Na2SiO3, KF, glycerol | MgO, MgSiO3 | Lap-shear strength | 24.5 MPa | [43] |
AZ31 | K3PO4, NaAlO2, MoS2 | MgO, MgAl2O4, MoS2 | Scratch test | Critical load>50 N | [44] | |
ZK60 | Na2HPO4·12H2O,
Na3PO4·H2O, (NaPO3)6 |
MgO, MgF2, ZnO, ZnF2, CaO, CaF2,
Ca3(PO4)2 |
Scratch test | (127.3±1.4) MPa | [45] | |
Al | 2024 Al | Na2SiO3, Na2P2O7 | α-Al2O3, γ-Al2O3 | Scratch test | Critical load: 100 N | [46] |
6063 Al | Na2SiO3+TiO2 | α-Al2O3, γ-Al2O3, TiO2 | Scratch test | Critical load: ~72 N | [47] | |
Ta | Pure Ta | (CH3COO)2Ca·H2O,
C3H7Na2O6P·5H2O |
Ta2O5, TaO, CaTa2O6 | Adhesion-tension
test |
(33.22±3.84) MPa | [48] |
Ti | Ti6Al4V | Na2SiO3·9H2O, NaOH | Rutile TiO2,
amorphous SiO2 |
Adhesion-tension
test |
(14.4±0.8) MPa and
(4.3±0.3) MPa |
[49] |
Ti6Al4V | NaAlO2, Na2CO3, | Al2TiO5 | Lap-shear strength | 110 MPa | [50] | |
Ti6Al4V | Na3PO4, FeSO4 | α-TiO2, γ-TiO2 | Adhesion-tension
test |
~37.5 MPa | [52] |
(2) 电解液成分对膜基结合强度影响
在不同体系的电解液中微弧氧化,电解液成分决定了涂层的物相与组织结构,因此影响膜基结合性能:在单一组分的基础电解液(如硅酸钠、磷酸钠、铝酸钠等)生长的涂层物相成分及组织简单,涂层结合强度高;在复合电解液体系,涂层生长效率高,但涂层结合强度会有所降低;在功能性无机盐(如FeSO4、NiSO4等)掺杂改性的复合电解液体系,涂层的膜基结合强度会有所下降;而纳米/微米粒子(石墨烯、碳纳米管、SiC、Al2O3等)混合改性的复合电解液体系,因主要以微弧沉积方式生长涂层,膜基结合强度亦会降低。王亚明等[50]研究了不同电解液体系涂层的结合强度,表明在NaAlO2/Na2CO3电解液体系中膜基界面强度最高,剪切强度达110 MPa。Yerokhin等[51]用划痕法测定了不同电解液中钛合金微弧氧化涂层的结合力,发现在KAlO2/Na3PO4电解液中制备的微弧氧化涂层,与基体具有最高的临界载荷LC2值(96 N)。Tang等[52-53]分别探索了电解液中改性无机盐(FeSO4和NiSO4)浓度对膜基界面强度的影响,随着浓度的增加,涂层的厚度增大,结合强度降低。Lou等[44]研究了MoS2粒子掺杂含量对镁合金表面微弧氧化涂层的膜基结合强度影响,随MoS2粒子的含量增加,涂层的结合强度降低,当纳米MoS2质量分数为2.5 g/L时,涂层具有致密的结构,因此结合性能最优异。
(3) 膜基结合强度测试方法对微弧氧化涂层适用性
微弧氧化涂层常用的膜基结合强度测试方式主要有:拉伸法[54]、剪切法[55]、划痕法[56]和压痕法[57]等。虽然划痕法和压痕法有操作简便、快速、高效等优点,但这两种方法测试误差较大,一般只做为粗略的比较。拉伸法和剪切法可定量测试微弧氧化涂层的膜基结合强度,是常用的表征方法;但因微弧氧化涂层外层多孔、内层致密,界面结合强度高的特点,决定其破坏的界面常发生在涂层内或者是粘结胶一侧,对测试数据准确性也产生一定的影响。
总体而言,微弧氧化制备的陶瓷涂层与金属基体为冶金结合,结合强度较高。为提高微弧氧化涂层的膜基结合性能,可通过调控微弧氧化电参数与电解液成分,以获得致密度高、结合性能优异的陶瓷涂层。
3 微弧氧化功能涂层设计制备与使役性能 3.1 抗磨减摩涂层设计与制备轻质高强金属(钛、铝、镁及其复合材料)替代高强钢等传统材料制作相对运动部件,在航空航天/舰船/汽车等轻量化减重增效中起到关键作用,但抗磨减摩性差限制其广泛替代使用。微弧氧化涂层相比于传统阳极氧化(酸性、碱性、化学转化)具有更厚、更致密的涂层结构、相比于硬质阳极化具有更强韧性、更好的抗磨性,且工艺简单环保,应国家绿色制造技术的需求,在一些场合替代上述工艺表现出极大潜力。铝合金表面微弧氧化涂层(以Al2O3为主相)具有高硬度(800~2 000 HV);镁合金表面微弧氧化涂层(以MgO为主相)的硬度在300~600 HV;钛合金表面微弧氧化(以TiO2为主)的硬度达到700 HV、以Al2O3、Al2TiO5或ZrO2为主的硬度达1 500 HV以上。对于同一基体,通过电解液成分与掺杂改性,或获得特定抗磨减摩强化的涂层物相组成与微观结构,进而设计出高性能/高精度的抗磨减摩涂层。
微弧氧化抗磨减摩功能化可通过对涂层的成分及结构设计来实现:①通过调控电参数(电压、电流密度、时间、频率等)及电解液成分(基础电解液体系、微纳米粒子改性体系),获得高硬度物相成分,以提高涂层的抗磨减摩性能;②通过对微弧氧化/其它工艺复合(喷涂、电沉积等)设计制备多层复合涂层。
表3总结了不同典型金属基体(Al、Mg、Ti)微弧氧化抗磨或减摩涂层体系与性能[58-82]。如铝合金微弧氧化涂层中主晶相为Al2O3,其中α-Al2O3相具有更高的硬度。可通过调节电参数及电解液成分获得高α-Al2O3相的陶瓷涂层[58]。Krishna等[59]研究表明在铝合金表面,随着涂层中α-Al2O3相增多,涂层的抗磨性能提高,且显著降低涂层的磨损率。通过在电解液中掺杂高硬度(金刚石、Si3N4、ZrO2、TiN等)或自润滑(石墨、PTFE、CNTs、MoS2等)粒子以提高涂层的抗磨或减摩性能。Yürektürk等[64]通过在电解液中添加CNTs,对比了涂层的磨损情况,表明掺杂后的微弧氧化涂层具有更优异的减摩效果。Chen等[70]探索了PTFE乳液含量对涂层的摩擦磨损性能影响,结果表明当PTFE质量分数20 g/L时,微弧氧化5 min,涂层具有超低且稳定的摩擦因数,仅为0.08。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Al | 6061 Al | Na2SiO3 | α-Al2O3, γ-Al2O3 | Friction coefficient: 0.5−0.6
(WC-6Co, 0.874 MPa) |
Wear resistance | [58-59] |
Wear rate decreases a factor of 12
(silica sand, 1 N) |
||||||
6061 Al | Na2SiO3, H3BO3,
diamond |
γ-Al2O3, diamond | Friction coefficient: 0.29
(Cr-steel, 2 N) |
wear resistance
and anti-corrosion |
[61] | |
ADC12 Al | Na2SiO3·5H2O,
NH4VO3, ZrO2 |
α-Al2O3, γ-Al2O3 m-ZrO2, t-ZrO2 |
Friction coefficient: 0.2
(Al2O3, 2 N) |
Wear resistance | [63] | |
6082 Al | Na2AlO3, CNT | α-Al2O3, γ-Al2O3 | Wear rate decreases 3 orders of
magnitude (Si3N4, 4 N) |
Wear resistance | [64] | |
Mg | ZK60 | Na3PO4 | MgO | Friction coefficient: 0.10
(52100 steel, 2 N) |
Wear resistance | [65] |
EV31A | Na5P3O10, Na2SiO3 graphite |
MgO, Mg2SiO4 Mg3(PO4)2 |
Wear rate decreases ~80%
(52100 steel, 2 N) |
Wear resistance and anti-corrosion | [68] | |
AZ91 | Na3PO4 PTFE |
MgO, Mg3(PO4)2 PTFE |
Friction coefficient: 0.08
(52100 steel, 5 N) |
Wear resistance | [70] | |
AZ31 | Na2SiO3·9H2O
(NaPO3)6, SiC |
MgO, Mg2SiO4 SiC |
Wear rate decreases 90%
(52100 steel, 10 N) |
Wear resistance and anti-corrosion | [72] | |
Ti | Pure Ti | Na2SiO3,
H3PO4 |
Rutile TiO2,
Anatase TiO2, Ti2O5 |
Friction coefficient: 0.4
(52100 steel, 5 N) |
Wear resistance | [74] |
Ti6Al4V | Na2SiO3·9H2O,
(NaPO3)6, NaAlO2, Na3MoO4 |
Rutile TiO2,
Anatase TiO2 |
Friction coefficient: 0.12−0.3
(52100 steel) |
Wear resistance and anti-corrosion | [75-77] | |
Ti6Al4V | Na3PO4,
MoS2 |
Rutile TiO2,
Anatase TiO2, MoS2 |
Friction coefficient: 0.15
(52100 steel, 2 N) |
Wear resistance | [81] | |
Ti6Al4V | MAO: NaAlO2,
Na2CO3+ Spraying graphite |
Al2TiO5, Al2O3, TiO2 | Friction coefficient: 0.12
(52100 steel, 1 N) |
Wear resistance and anti-corrosion | [82] |
微弧氧化层表面固有的多孔结构为构建高结合性的多层复合功能涂层提供很好的基质表面。目前喷涂、浸涂、化学转化、等离子体浸没离子注入等技术已与微弧氧化层复合,以制备低摩擦因数、低磨损率的复合涂层。
王亚明等[14-15]分别在钛合金与铝合金表面构建出纳米TiO2基/纳米Al2O3基减摩抗磨陶瓷涂层,突破了高减摩性致密纳米晶层的工艺控制、复杂型腔内壁涂层均匀生长关键技术难题,实现了高精度高腔深复杂壳体内壁减摩抗磨涂层的制备。纳米TiO2基减摩抗磨陶瓷涂层应用于XX大推力火箭伺服控制系统钛合金关键相对滑动密封壳体,该涂层工艺还拓展应用于XX钛合金活塞杆等运动部件;纳米Al2O3基减摩抗磨陶瓷涂层应用于XX飞行器伺服系统铝合金运动壳体。以上有效解决高精度钛合金或铝合金壳体内腔减摩抗磨油气密封问题,实现以轻质金属替代钢质壳体显著减重并增加有效载荷,提高控制精度可靠性与稳定性。
3.2 耐腐蚀涂层设计与制备轻量化金属合金(Al、Mg、Ti)应用于航空航天部件或者发动机等领域,当其服役于海洋环境中,海水、盐雾、潮湿、霉菌及烟尘等对轻合金构件或零件的腐蚀,特别是发动机燃气侵蚀和沾染沉积物部位更容易受到腐蚀,严重制约了金属构件的适用范围和寿命。微弧氧化陶瓷涂层及其复合涂层是解决这一问题的有效途径。微弧氧化涂层一般具有双层结构:外部多孔层和内部致密层[83]。由于腐蚀液容易渗透表面多孔层到达致密层,因此一般认为,内部致密层对涂层整体的抗腐蚀能力起到关键性的作用[84]。从电参数、电解液、粒子掺杂改性等角度入手,通过涂层结构的设计,提高内部致密层和外部多孔层的致密度可改善涂层的抗腐蚀性能;并且微弧氧化涂层表面的封孔工艺(如涂漆、硅烷化处理、电沉积、冷喷涂、溶胶凝胶)也是提高涂层的抗腐蚀性能的一种有效途径;此外改善涂层耐腐蚀性能手段还包括:金属基体表面前处理(纳米化)再微弧氧化制备兼具抗疲劳与抗腐蚀的复合涂层;在微弧氧化过程中引入超声辅助以提高微弧氧化涂层致密性,强化涂层的抗腐蚀性能。
表4总结了典型铝、镁、钛及其合金表面微弧氧化抗腐蚀涂层体系与性能结果[85-98]。微弧氧化处理可提高涂层的自腐蚀电位、降低自腐蚀电流。通过对电解液成分的调控(Al2O3、CeO2、氧化石墨烯等粒子掺杂)可进一步提高极化电阻2~3个数量级。Zhao等[89]在含有GO的电解液中制备的微弧氧化涂层可降低腐蚀电流达3个数量级,显著降低腐蚀速率。此外,对微弧氧化涂层采用功能化复合(超声辅助、旋涂、浸涂等)实现表面封孔,提高涂层的致密性,可有效改善涂层的耐腐蚀性能。Zhang等[92]制备的微弧氧化/化学转化涂层可提高极化电阻4个数量级;Gnedenkov等[93]将微弧氧化后的试样浸于PTFE溶液中,热处理后,试样表面接触角高于130°,且耐腐蚀性能显著增强。王亚明等[99-101]在微弧氧化前,采用表面机械研磨(SMAT)在LY12 Al合金表面制备纳米晶过渡层,并在纳米晶层上进行微弧氧化原位生长,形成的多层结构涂层在浸泡腐蚀过程中可起到自修复效果,有效改善了涂层的抗腐蚀性能,并且提高疲劳寿命10%~20%。
此外,应用到特殊工况下的金属(如核反应堆燃料包壳用锆合金)需要具备在严酷的高温高压下的抗腐蚀性能。在锆合金表面于不同体系电解液中构建了微弧氧化陶瓷涂层,研究了电解液体系对涂层的耐腐蚀性能的影响,结果表明于Na2SiO3溶液中制备的涂层在LiOH腐蚀液中具有优异耐腐蚀性能的涂层[102-104]。同时,学者们针对航空航天用铌合金、医学植入钽合金也进行了提高耐腐蚀性能的探索[105-106]。
铝、镁与钛合金微弧氧化涂层在提高抗腐蚀性能及抗接触电偶腐蚀方面已经取得了一些应用进展。ZL114铝合金海水过滤壳体表面制备的陶瓷涂层(厚20 μm),涂层可有效改善抗腐蚀性能,盐雾腐蚀1 500 h后,涂层表面无明显腐蚀痕迹[14]。用于海洋平台铝合金功率器件保护盒,涂层后使用一年以上,顶盖与底座间未发生锈蚀,解决了此前顶盖与底座之间发生严重接触腐蚀,导致顶盖打不开的问题。此外,镁合金微弧氧化作为底层再施加三防漆复合处理,用于某飞行器吊舱,抗盐雾腐蚀2 000 h以上,解决了镁合金抗腐蚀性能差的问题。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Al | 7075 Al | K2ZrF6, NaH2PO4 | t-ZrO2, α-A12O3 | icorr: decreases 2 orders of magnitude | Anti-corrosion and wear resistance | [85] |
6063 Al | Na2SiO3, Na2B4O7 | α-A12O3, γ-A12O3, Al9Si | icorr: decreases 2 orders of magnitude | Anti-corrosion | [86] | |
7075 Al | Na2SiO4, CeO2 | γ-A12O3, CeO2 | Impedance value improves 5 orders of magnitude | Anti-corrosion | [87] | |
Mg | AZ31 | Na2HPO4, NaF, sodium citrate, GO | MgO | icorr: decreases 2 orders of magnitude | Anti-corrosion | [89] |
ZK60 | Na2SiO3·9H2O, NaFspinning GO | MgO, MgSiO3, Mg2SiO4 | icorr: decreases 2 orders of magnitude | Anti-corrosion | [91] | |
AZ31 | NaAlO2 + chemical conversion | MgO, Al2O3, LDH | Polarization resistance improves 4 orders of magnitude | Anti-corrosion | [92] | |
MA8 | Na4SiO4, NaF+dipping PTFE | MgO, Mg2SiO4, PTFE | Contact angle >130°
Polarization resistance improves 3 orders of magnitude |
Anti-corrosion and wear resistance | [93] | |
AZ91 | Na2SiO3, NaAlO2+cold spray | MgO, MgAl2O4, MgSiO4,
Mg3Al2(SiO4)3 |
icorr: decreases 3 orders of magnitude | Anti-corrosion | [94] | |
Ti | Pure Ti | Na2SiO3, Na3PO4, NaAlO2+ Al2O3/CeO2 | Rutile TiO2, Anatase TiO2,
Al2O3, CeO2 |
Polarization resistance improves 1−3 orders of magnitude | Anti-corrosion | [95-97] |
Ti-6Al-4V | Na3PO4+PFOTS post treatment | Anatase TiO2 | Contact angle >150° Polarization resistance improves 1 orders of magnitude | Anti-corrosion | [98] |
轻质金属合金(钛、铝、镁、钛铝合金、铌合金等)及其复合材料在高速飞行器舱体、发动机热端部件、枪弹弹壳等轻量化装备中使用日益增多,但在高温气流冲刷使用环境下,传统的阳极氧化膜已达不到使用要求。在金属表面生长更厚、高结合强度的微弧氧化陶瓷涂层,兼具低热导率隔热、高抗热震性、优异的抗氧化性能,具有广泛应用潜力。表5总结了典型微弧氧化热防护涂层体系与性能结果[107-118]。Curran等[107]研究表明微弧氧化陶瓷涂层热导率仅为1 W/(m·K)左右,并且可通过涂层的成分设计(调控化学与物相组成)制备出更低热导率的涂层。如在Na2SiO3电解液中于铝合金表面制备出富含低热导率的莫来石相,涂层的热导率值可降至0.5 W/(m·K)[108];Apelfeld等[113]在Zr合金表面制备的微弧氧化涂层热导率仅为0.2 W/(m·K),为热障涂层的设计与制备提供了新思路。
优异的抗热震性能是决定金属(合金)在高温环境下服役寿命的关键因素。研究表明,Ti6Al4V微弧氧化涂层在经过500 ℃和700 ℃热震循环25次[111]及纯Al微弧氧化涂层试样在450 ℃循环40次后[112],涂层中仅产生了一些微观裂纹,宏观形貌基本没有发生变化。
尽管微弧氧化涂层表面多孔,但涂层内部致密,可有效抑制高温下氧向金属基体的扩散行为。王亚明[111]在Ti6Al4V表面制备微弧氧化涂层,700 ℃下循环氧化80 h,增重为0.98 mg/cm2远低于基体的增重(20 mg/cm2);此外,在Ti2AlNb合金表面制备的陶瓷涂层在800 ℃温度下,150 h后涂层氧化增重速率仅为0.003 1 mg2/(cm4·h),是金属基体的24.97%[114]。通过结构设计,制备复合涂层可进一步提高涂层的抗氧化性能。Du 等[116]在TC11合金表面原位制备TiO2涂层后,磁控溅射铝层,通过真空热处理扩散/反应过程,得到高界面结合强度的含α-Al2O3及TiAl金属间化合物的复合结构涂层。该涂层在700 ℃下的氧化增重速率低至0.03 mg2/(cm4·h),仅为基体的1/10。王亚明等[118]采用包埋渗与微弧氧化技术在Nb合金表面探索了新型NbSi2/Nb5Si3/Nb2O5-SiO2-Al2O3多层结构涂层,该涂层在高温氧化(1 250 ℃)时能够推迟其15 h的氧化进程,且氧化速率低至0.06 mg2/(cm4·h),仅为单一包埋渗层的1/12,显著改善了铌合金的抗高温氧化性能。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Al | 6082 Al | Na2SiO4 | α-Al2O3, γ-Al2O3 mullite |
Thermal conductivity
0.5 W·m−1·K−1 |
Thermal protection | [107] |
NSA | Hot dipping Al +
Na4P2O7, Zr(CO3)2(OH)2 |
m-ZrO2, t-ZrO2 c-ZrO2 |
Thermal conductivity
1 W·m−1·K−1 |
Thermal protection | [108] | |
Zr | Zr-1% Nb
alloy |
Na2SiO3·5H2O
Na(PH2O2) |
m-ZrO2, t-ZrO2, c-ZrO2 | Thermal conductivity
0.2 W·m−1·K−1 (1 327 ℃) |
Thermal protection | [113] |
Ti | Ti2AlNb | NaAlO2, Na2CrO4 | Rutile TiO2, α-Al2O3,
γ-Al2O3(Al0.928Cr0.052)2O3, Cr3O |
Parabolic rate constant
0.04 mg2/(cm4·h) (800 ℃) |
Oxidation resistance | [115] |
TC11 | Na2SiO3, Na3PO4+
magnetron sputtering |
TiO2, AlTi
α-Al2O3, Al3Ti, |
Parabolic rate constant 0.030 mg2/(cm4·h) (700 ℃) | Oxidation resistance | [116] | |
Ti-6Al-4V | Na2SiO3, Na2CrO4 | Rutile TiO2,
Anatase TiO2 |
Mass gain decreases a factor of 3, 0.0373 mg/cm2 (500 ℃) | Oxidation resistance | [117] | |
Nb | Nb-Hf | Pack cementation +NaAlO2 | Nb2O5-SiO2-Al2O3/NbSi2/
Nb5Si3 |
Parabolic rate constant
0.06 mg2/(cm4·h)(1250 ℃) |
Oxidation resistance | [118] |
在短时高温气流冲刷的极端使用环境下,传统的阳极氧化膜已达不到使用要求,高结合强度/低热导率的微弧氧化陶瓷涂层显示出优势,王亚明等测试表明,经氧乙炔焰烧蚀相同时间后,铝合金严重变形,而微弧氧化涂层耐冲刷、抗烧蚀性能优异,基体无变形。某型号铝合金子母弹靶试时出现的弹底脱落问题,采用阳极氧化膜不耐火药气体高温烧蚀,而采用微弧氧化陶瓷涂层,具有优异的耐冲刷、抗烧蚀性能,涂层耐2 000 ℃的高温气流冲击,20 s不脱落[119]。
3.4 热控涂层设计与制备随着航天技术飞速发展,热控涂层在调控航天器/高速飞行器表面温度方面至关重要。热控涂层是通过调节物体表面的太阳吸收率(αs)和发射率(ε)来控制物体的温度。可通过设计涂层的组成与表面结构来调控吸收率和发射率值。通过微弧氧化电解液组成与浓度、电参数调控,可在铝、镁、钛及铌合金表面制备出一系列热控涂层。其中,电解液成分及浓度对涂层的吸收率(αs)和发射率(ε)的比值起决定性作用,例如不同组分的添加物(KMnO4,FeSO4,Cr2O3和NH4VO3等)可获得不同光谱范围内可调控的涂层。不同金属基体表面微弧氧化热控涂层的典型研究结果列于表6[120-134]。在铝合金表面通过微弧氧化法制备的热控涂层在0.2~2.5 μm范围内吸收率>0.9,发射率>0.77[124]。AZ91D合金在含有Na3VO4电解液中制备的微弧氧化涂层吸收率和发射率分别为0.918和0.80[127]。在电解液中添加ZnSO4,制得的含有MgO和ZnO的涂层具有低吸收率和高发射率,可有效调节表面温度[129]。
除热控涂层外,耐高温/高发射率涂层在辐射热防护及辐射散热应用中也非常关键。王亚明等[120, 126, 130]在Al、Mg、Ti、Nb合金表面,较早开展了高发射率陶瓷涂层的制备与性能调控研究,通过设计电解液成分或混合掺杂(如CuO、CNT、SiC、Fe2O3等粒子)强化涂层的热辐射能力,进一步提高涂层表面发射率。通过特定成分掺杂与工艺调控,王亚明等[131, 134]在不同金属基体上已构建出耐高温/高结合的高发射率微弧氧化涂层,并在XX高速飞行器大面积钛合金壳体的辐射热防护系统上成功应用。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Al | 2024 Al | Na2SiO3 (NaPO3)6 |
γ-Al2O3 | Emissivity 0.85 (500 ℃)
(3−20 μm) |
Thermal radiation | [120] |
6061 Al | Na2SiO3 | α-Al2O3, γ-Al2O3,
SiO2, Al2SiO5 |
Emissivity 0.87 (70 ℃)
(8−16 μm) |
Thermal radiation | [121] | |
Pure Al | Na2SiO3 | CuO, Cu2O, Al2O3 | Emissivity 0.892 (100 ℃)
(5−20 μm) |
Thermal radiation | [122] | |
7075 Al | Na2SiO3·9H2O
(NH4)6Mo7O24·4H2O K2TiF6 |
γ-Al2O3 | Absorptance (0.9), emissivity
0.79 (0.2−1.4 μm) |
Thermal control | [123] | |
2024 Al | NaAlO2 | α-Al2O3, γ-Al2O3 | Absorptance >90, emissivity
>0.77 (300 K) (0.2−2.5 μm) |
Thermal control | [124] | |
7075 Al | Na2SiO3CNT | Al-O-SiCNT | Emissivity 0.9 (80 ℃)
(5−20 μm) |
Thermal radiation | [125] | |
Mg | AZ91D | Na2SiO3 | MgO, Mg2SiO4 | Emissivity 0.8 (350 ℃)
(8−20 μm) |
Thermal radiation | [126] |
AZ91D | Na2SiO3, Na3VO4 | Mg2SiO4, MgO, MgAl2O4 | Absorptance 0.918, emissivity
0.80 (25 ℃) (0.2−2.5 μm) |
Thermal control | [127] | |
Mg-Li | Na2SiO3 | Mg2SiO4, MgO | Absorptance 0.35, emissivity 0.82(25 ℃) (0.25−2.5 μm) | Thermal control | [128] | |
AZ31 Mg | Na5P3O10ZnSO4 | MgO, ZnO | Absorptance 0.35 (200−2 500 nm),
emissivity 0.88 (2−16 μm) |
Thermal control | [129] | |
Ti | Ti2AlNb | Na2SiO3, SiC | Rutile TiO2, anatase TiO2,
TiO2, Al2SiO5 |
Emissivity≥0.8 (600 ℃)
(3−20 μm) |
Thermal radiation | [130] |
Pure Ti | Na2SiO3, NaAlO2 | Rutile TiO2, anatase TiO2, | Emissivity 0.9 (700 ℃)
(8−14 μm) |
Thermal radiation | [131] | |
Ti6Al4V | Na3PO4, Fe2O3 | Rutile TiO2, anatase TiO2,
Fe2O3 |
Emissivity 0.87 (700 ℃)
(3−20 μm) |
Thermal radiation | [132] | |
TC4 | Na2SiO3, (NaPO3)6K2ZrF6 | Rutile TiO2, anatase TiO2,
brookite TiO2 |
Emissivity 0.89 (50 ℃)
(8−20 μm) |
Thermal radiation | [133] | |
Nb | Pure Nb | Na2SiO3,
Na2SiO3, NaAlO2, SiC |
Nb2O5, SiC | Emissivity 0.8 (600 ℃)
(0.25−2.5 μm) |
Thermal radiation | [134] |
高电绝缘性、低介电常数、低介电损耗材料是电子封装中用于承载电子元器件及其连接线路不可或缺的性能[135];此外,现代航空航天正朝着“电气化飞行器(MEA)”的概念迈进,保证电气设备在高温高压下长期运行,涂层材料应具有足够的绝缘强度[136]。在轻合金表面形成的陶瓷涂层(如Al2O3,SiO2,ZrO2等)具有较高的绝缘电阻和击穿强度,在绝缘涂层的应用方面表现出巨大的潜力。微弧氧化涂层在富含硅酸盐的电解液中形成的涂层以更高的速率生长并具有更大的厚度,特别是将含有SiO2、SiC等纳米粒子掺杂到涂层中,会大幅度增加介电强度。针对合金本身性质和涂层要求进行设计,绝缘涂层的耐压性可达千伏以上。例如,Al合金表面制备耐压强度较高的Al2O3陶瓷涂层,并通过改变电解液成分和控制微弧氧化参数调节涂层微观结构和物相组成,以提高其电绝缘性能。
微弧氧化介电绝缘功能化研究进展如表7所示[137-147]。Zhang等[137]通过不同浓度的ZrO2溶胶涂覆蚀刻Al箔的表面,在不同电压下进行微弧氧化,研究了ZrO2的浓度对涂层微观组织和电性能的影响,结果表明复合涂层的耐压性可保持700 V以上,并优化ZrO2溶胶的浓度从而提高了涂层的比电容。Hussein等[138]提到微弧氧化电绝缘涂层具有高的介电强度,并可在电子电气设备中广泛应用。同时,Walsh等[139]也提出微弧氧化涂层致密度高、厚度大、强度较高,具有优异的电绝缘性能。He等[147]利用微弧氧化技术在铍金属表面制备耐腐蚀、高绝缘的陶瓷涂层,并对其进行电化学分析。其他研究(铝、钛等微弧氧化绝缘陶瓷涂层)也有类似的结果[140-142]。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Al | Pure Al | C2H602 boric acid | ZrO2, Al2O3 | Breakdown voltage >700 V | Dielectric,
insulation |
[137] |
6063Al | Na2SiO3 | α-Al2O3,
γ-Al2O3, 3Al2O3·2SiO2 |
Insulation resistance 12.3 GΩ;
Breakdown voltage 489 V |
Insulation | [141] | |
Ti | Ti-6Al-2Zr-1Mo-3Nb | Phosphate | Rutile TiO2, anatase TiO2 | Insulation resistance >200 MΩ | Insulation,
anti-corrosion |
[142] |
TiN-Si | Ba(OH)2, Sr(OH)2 | BaxSr1−xTiO3, TiN, TiO2 | ε= 28−248; tanδ=0.284−0.846 | Dielectric | [144] | |
Ti6Al4V | Ba(OH)2 | BaTiO3, BaTiO3-x, BaCO3 | Ps=0.12 μC/cm2; Pr=0.032 μC/cm2; Ec=0.085 MV/m | Ferroelectric,
biomedical |
[146] | |
Be | Be | Na2CO3 | BeO | Impedance value 1 030.36 kΩ∙cm2 | Insulation,
anti-corrosion |
[147] |
除了上述绝缘涂层体系外,钛酸钡因其具有优异的介电、铁电和压电等性能,是电子工业中重要的功能性材料。利用这些特性,Lu等[143 - 144]利用微弧氧化技术制备BaTiO3和 BaxSr1−xTiO3薄膜,获得具有纳米层状结构的烧结状多孔形貌,并提出了BST膜的反应途径和形成机理。Wang等[145]在工业钛板上成功制备出BaTiO3和SrTiO3铁电薄膜,由于具有更高的介电常数,可能在微电子和光电子等领域中有更大的应用前景。
在伺服控制机构中,通过金属表面微弧氧化绝缘屏蔽涂层,切断电动泵中频电机强电干扰源的传播路径,彻底消除中频电机强电对伺服控制弱电的干扰,可解决现有密封结构复杂、泄露风险大与绝缘性能差等关键问题。已开发的某空间机构微弧氧化绝缘涂层底座及部件,有效减小信号干扰,提高工作可靠性。该涂层在航天高绝缘屏蔽设备及电力、电子仪器等领域有广阔的应用前景。
3.6 催化涂层设计与制备轻金属表面制备催化活性高的纳米涂层用于光催化领域,能够高效地分解有机物、光催化抗菌和分解有害气体,并可用于空气净化和污水治理等方面。通过微弧氧化电解液组成与工艺调控设计制备催化功能涂层(包括:催化活性层、催化活性物质载体、光催化和电催化等),涂层的表面结构及比表面积对催化性能起主要决定作用。单一的TiO2薄膜存在禁带宽度过大,在可见光波段的吸收效率低,以及薄膜内光生电子-空穴的复合效率高的问题。
通过电参数及电解液成分(催化活性粒子Tb、W、Co、Mn、Cu、Zn等)设计,可制备具有高催化活性成分的涂层,如表8所示[148-156]。Friedemann等[148]在不同的电解液和不同微弧氧化条件下制备陶瓷涂层以优化TiO2的多晶型组合物来调控光催化活性。Stojadinović等[149-150]利用微弧氧化在含有Tb4O7粉末的电解液中,形成Tb掺杂的TiO2涂层;并通过调控微弧氧化时间和Tb3+离子的浓度有效抑制了电子/空穴复合,改善了光催化活性。另外,Qin等[151]通过引入ZnO纳米颗粒,在钛基体上制备ZnO/WO3/TiO2复合涂层。并发现W和Zn含量与W∶Zn比值对光催化活性有很大影响。
为获得更高催化活性的表层,可通过以微弧氧化为底层,与浸涂、溶胶-凝胶、热处理等方式复合,设计制备新颖结构复合涂层。Rudnev等[152]利用微弧氧化前处理,随后通过浸渍、萃取和溶胶-凝胶合成等方法沉积催化活性组分,使之形成纳米线、纳米球或刺猬状结构,从而有效地提高催化剂的效率。同样,Lukiyanchuk等[154]在硅酸钠溶液中进行微弧氧化后,通过在硝酸盐溶液中浸渍,退火改性,获得了不同表面形态结构的Co、Cu、Si、Ti的氧化物复合材料。Chu等[155]利用微弧氧化技术成功制备具有3D网络“纳米片状”表面的二氧化钛层。并进行热退火处理使无定形纳米片转变成锐钛矿纳米微晶,进一步提高组装的DSSC的光伏效率。Markov等[156]在硼硅酸盐电解液中于铝基体上进行微弧氧化以形成可用作催化剂载体的多孔陶瓷涂层,并浸渍在硝酸镍溶液中,热处理后形成氧化镍沉积到陶瓷涂层的孔中来提高催化活性。研究者们探索在不同金属基体(Ti、Al、Mg、Nb等)表面,可通过一步法(调控微弧氧化工艺及成分)或多步法(微弧氧化与溶胶凝胶、热处理等复合)构建表面具有高催化活性的成分和结构的涂层,在催化领域具有广阔的应用前景。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Ti | Pure Ti | Na3PO4·12H2O, Tb4O7 | Anatase TiO2 | Improve photocatalytic activity | Photocatalysis | [149] |
VT1-0 | Na3PO4, Na2B4O7,
Na2WO4, Ni(CH3COO)2, Cu(CH3COO)2 |
Rutile TiO2, Anatase TiO2,
NiO, CuO |
Enhance the conversion rate of CO | Photocatalysis | [154] | |
VT10-Ti | Na2SiO3 | Co3O4/SiO2, TiO2/Ti, CuO/SiO2,
TiO2/TiCo3O4, CuO/SiO2, TiO2/Ti |
Activation energy 42.6−141.8 kJ/mol | Photocatalysis | [155] | |
Al | Pure Al | Brate, silicate | Al2O3 | Improve catalytic activity | Photocatalysis | [156] |
钛金属(及合金)因其优异的强韧性、抗腐蚀能力、抗磨损和优良的疲劳性能,被广泛应用于人体硬组织替换[157]。然而,钛合金表面无生物活性,易在其表面产生纤维(软)组织而导致与骨组织结合强度低,且初期炎症使创口不易愈合。因此,可通过微弧氧化涂层调控钛合金表面成分及结构,改善表面生物活性与抑菌特性,减小与骨组织的力学失配程度。此外,通过水热处理、水汽处理、热处理等方法,可进一步调控表面微纳结构与成分,以改善生物活性及提高抑菌能力。
通过微弧氧化电解液组成设计,可以将生物活性元素(如:Ca、P、Si和Sr等)引入到涂层中(如表9)[158-166]。魏大庆[159]采用微弧氧化在Ti6Al4V表面制备了含Ca、P涂层,后续在不同温度下对涂层进行热处理,得到 TiO2/Ca3(PO4)2复合涂层,研究表明该复合涂层具有较高的润湿性。Li等[162]对比研究了四硼酸盐电解液与磷酸盐电解液涂层对细胞粘附/增殖与骨整合能力的影响,发现四硼酸盐中的涂层具有“Cortex-like”状复合结构表现出更优异的生物活性。Santos-Coquillat等[163]通过设计电解液成分,将Si、F元素引入到微弧氧化涂层中,表明含F元素最多的涂层具有最优异的抗菌性能。此外,Muhaffel等[165]在电解液中加入AgNO3,以引入Ag+离子,通过调控Ag+的含量控制其释放速率,已达到抗菌目的。
通过溶液/电参数设计或与热处理、水热处理及电沉积等技术复合,获得合适孔径的多级微纳结构,可有效调控表面细胞与组织的附着行为[167-172]。Liao等[169]通过采用阶梯式电流控制分段微弧氧化工艺,在钛表面制备了具有微米/亚微米级的双级微孔结构。魏大庆等[170]对Ti6Al4V表面制备的含Ca、P涂层进行了碱热处理,在微观多孔结构表面生成了大量纳米网状多孔结构,生成的微米/纳米复合结构涂层经过模拟体液浸泡3天即可在表面诱导生成磷灰石沉积层,表现出优异的生物活性等;此外,将微弧氧化表面处理的钛金属与纯钛植入新西兰大白兔胫骨12周,发现通过微弧氧化及微弧氧化/水汽复合处理的钛金属生物活性表面使种植体与周围组织结合紧密,12周后种植体挤出力由纯钛的8 N提高到200 N,骨整合性能明显提高[172]。
3.7.2 镁合金腐蚀降解调控微弧氧化涂层可降解镁合金是具有良好生物安全性的新一代医用植入金属材料,在短期植入器件如硬骨替换、临时承载骨固定(如接骨板或螺栓)及心血管、腔道介入治疗支架等领域具有诱人的应用前景。但制约其应用的一个关键问题是,在复杂生理系统环境中镁合金植入物腐蚀降解速度过快,在组织没有完全愈合实现其基本功能之前,就丧失机械力学完整性。镁合金的均匀腐蚀、定向腐蚀及降解速度调控是镁生物材料应用中存在的国际性关键难题。采用微弧放电生成陶瓷涂层可以更好地解决镁过快腐蚀降解的实际问题,也逐渐引起了国内外研究者的关注。目前常用的途径有两种:①调控微弧氧化电参数及电解液成分,在强化涂层耐腐蚀性能的同时,提高涂层的生物活性;②以微弧氧化镁合金为底层,设计制备微弧氧化复合结构涂层(如表9)[173-186]。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Properties | Application | Reference |
Ti | Ti6Al4V | Ca(CH3COO)2·H2O
Na2SiO3, HA |
Amorphous Si, Ca, Ti, Na,
O, TiO2 |
Biological activity,
biocompatibility |
Biomedical | [159] |
Pure Ti | Ca(CH3COO)2·H2O
Na2SiO3 + heat treatment |
Rutile TiO2, anatase TiO2,
CaTi4(PO4)6 |
Good apatite-inducing ability | Biomedical | [161] | |
Pure Ti | Na2B4O7, Li2B4O7,
Na2HPO4 |
Rutile TiO2, anatase TiO2 | Cytocompatibility, osseointegration | Biomedical | [162] | |
3D Ti6Al4V | Ca(CH3COO)2·H2O,
NaH2PO4, |
Rutile TiO2, anatase TiO2,
HA |
Osseointegration | Biomedical | [164] | |
Ti6Al4V | Calcium acetate hydrate,
NaH2PO4, AgNO3 |
Rutile TiO2, anatase TiO2,
HA, CaTiO3 |
Biological activity, antibacterial | Biomedical | [165] | |
Pure Ti | Calcium acetate,
Strontium acetate + hydrothermal treatment |
Rutile TiO2, anatase TiO2,
CaSrTiO3, Ca9Sr1(PO4)6(OH)2 |
Cytocompatibility | Biomedical | [167] | |
TC4 | C3H7Na2O6P·5H2O
Na2SiO3·9H2O + electrodeposition |
Rutile TiO2, anatase TiO2 | Cytocompatibility | Biomedical | [168] | |
Mg | Pure Mg | Chemical pretreatment
+MAO: phosphate, Sr(OH)2 |
MgO | icorr: decreases 1 order of magnitude | Anti-corrosion, biomedical | [173] |
AZ31 | NaOH, KF+sol-gel | HA | icorr: decreases 97% | Anti-corrosion | [176] | |
AZ31/Pure | Na2SiO3·9H2O +
hydrothermal treatment |
MgO, HA, Mg(OH)2 | Improve anti-corrosion | Anti-corrosion biomedical | [177-179] | |
AZ91 | Na2SiO3, Na3PO4,
calcium gluconate + electrodeposition |
MgO, HA | Tensile strength after
immersion 297 MPa |
Anti-corrosion,
mechanical |
[180] | |
AZ31 | Na2SiO3, Na2B4O7,
C6H8O7·H2O+ plastic injection molding |
MgO | Tensile strength
120−130 MPa |
Anti-corrosion, mechanical | [181] | |
AZ31 | Phytic acid+dipping | MgO, SiO2 | icorr: decreases 3 orders of magnitude | Anti-corrosion | [182] |
采用一步微弧氧化技术来提高涂层的生物活性和耐蚀性能,目前主要方式有自封孔微弧氧化涂层、耐腐蚀微弧氧化障碍层等[173]。调控电解液成分与电参数是提高微弧氧化涂层耐蚀性能的有效途径。Toorani等[174]通过向磷酸盐电解液中加入硝酸镧,结果表明掺杂硝酸镧的微弧氧化层显著改善了涂层的耐蚀性能。Gu等[175]在Mg-Ca合金表面制备微弧氧化涂层,对比研究了电压对涂层的耐腐蚀性能及细胞相容性的影响,结果显示360 V电压下的微弧氧化涂层具有最优的耐浸泡腐蚀性能,且具有良好的生物相容性。
采用复合涂层结构设计以提高涂层耐腐蚀性能。微弧氧化封孔复合涂层的制备方法主要有溶胶凝胶法[176],水热法[177-179],电化学沉积法[180]等,同时可以制备微弧氧化-磷酸钙、微弧氧化-聚乳酸、微弧氧化-壳聚糖等复合涂层。曾荣昌等[184]通过在微弧氧化表面浸渍聚乳酸制备复合结构涂层,该涂层表现出优异的耐腐蚀性能,并且利于细胞的增殖分化,具有高的生物活性。憨勇等[179]采用水热法在微弧氧化涂层表面原位纳米化改性,将微弧氧化涂层原位改性为纳米棒状羟基磷灰石结构(HA),通过对表面微孔的封堵提高了其耐蚀性能,同时进一步提高了涂层的生物活性。王亚明等[185]采用微弧氧化的镁合金作为骨折固定物植入新西兰大白兔胫骨骨折部位,并进行了为期8周的实验,结果表明微弧氧化可调控镁合金降解速率,有效辅助促进骨折部位的愈合。
2013年德国Syntellix AG公司开发的MAGNEZIX MgYREZr可降解镁合金压缩螺钉成为全世界第一个获得CE认证的骨科产品,用于小骨和骨碎片的固定;韩国U&I公司的Mg-Ca镁合金螺钉也通过了韩国MFDS批准,用于关节的骨折固定;国内可降解纯镁螺钉已进入临床审批阶段。而微弧氧化涂层用于延长和调控镁合金的降解时间,解决力学性能不足的制约,突破上述在临床上承力固定应用的局限性,这将预示着可降解镁合金作为骨科植入物具有广阔的发展前景。
3.8 彩色涂层设计与制备金属表面涂层的颜色种类不仅起到装饰功能,而且在不同的应用环境(医用区分、光学吸收与辐射、3C产品及光学设备)中尤为重要。微弧氧化涂层的颜色主要受金属基体成分及涂层的物相组成影响,可通过调控电参数、电解液的成分((NaPO3)6、Na2WO4、K2TiF6、(NH4)6Mo7O24)、Cr2O3、CuO等粒子)及染料乳剂复合,制备不同颜色的涂层。研究结果显示,目前可在金属表面设计制备出白色、黄色、红色、蓝色、绿色、灰色、黑色等微弧氧化涂层。表10总结了不同颜色微弧氧化涂层的设计制备工艺[187-196]。Hwang等[187]通过调控合金基体的成分获得不同颜色的陶瓷涂层,解释了合金元素对涂层颜色的影响规律。Yeh等[193]在电解液中加入不同颜色的染料乳剂,制备出了不同颜色的涂层,研究发现通过预制一层障碍层可获得颜色均匀的表面。
Substrate | Grade | Electrolyte systems | Coating phase constituents | Proterties | Application | Reference |
Ti | Ti alloy | Calcium acetate, glycerophosphoric acid disodium | Rutile TiO2, anatase TiO2 | Light gray, yellow, grayish violet | Medical Modification | [187] |
Ti6Al4V | Na2SiO3·9H2O | Rutile TiO2 | Black, white | Thermal control | [49] | |
Ti6Al4V | (NaPO3)6, NaAlO2+calcine | Rutile TiO2, anatase TiO2 | Yellow | Coloration | [188] | |
Al | Pure Al | (NaPO3)6, Na2SiO3, NH4VO3 | Al2O3, V2O3 | Black | Coloration | [189] |
7075Al | Na2SiO4·9H2O, K2TiF6 | γ-Al2O3 | Black | Thermal control | [123] | |
6061/pure Al
6061 Al |
NaAlO2 + Cr2O3 pigment | α-Al2O3, γ-Al2O3 Cr2O3 | Green | Coloration | [191-192] | |
6061 Al | NaAlO2+ pigment | γ-Al2O3 | Red, yellow, orange, blueblack, white | Coloration | [193] | |
Mg | AZ80 Mg | Na2SiO3·9H2O, K2TiF6 | MgO, MgF2 | Grey | Electronic products | [194] |
AZ80 Mg | Na2SnO3, Na2SiO3 | MgO, SnO2 | Yellow | Electronic products | [195] |
最近,微弧氧化技术正在探索应用于磁性材料[197]及锂离子电池材料[198-199]中。Rudneva等[197]在含有Fe2(C2O4)3的电解液中,于钛合金表面制备含有Fe元素的非晶层,结果表明,电解液组成对涂层的磁性能和磁阻率影响较大。Gibaek Lee等[198]提出用微弧氧化法制备SiO2/TiO2复合膜作为电池的负极,该膜层具有优异的容量和循环稳定性,表明该技术有望应用于锂离子电池中。随后Lo等[199]制备了CNTs掺杂改性TiO2涂层,通过CNTs优异的导电性能,明显增加了锂离子电池的容量。
4 面临的挑战与未来发展方向展望目前,微弧氧化技术在金属表面改性方面愈来愈受到重视。采用微弧氧化技术,通过涂层的成分与结构设计已制备出各种功能化陶瓷涂层(抗磨减摩、耐腐蚀、热防护、热控、介电绝缘、催化、生物等),涂层具有独特的优异性能与较长的使用寿命;此外,通过低能耗电源与高效长寿命电解液体系的开发使涂层制造成本进一步降低,使其呈现出极大的竞争力与应用潜力。当前,微弧氧化工艺正处在基础研究向产业化应用的过渡阶段,但应用主要集中在轻合金的抗腐蚀、抗磨减摩防护。值得注意的是,对于设计制备特殊、高附加值的功能化涂层,微弧氧化是独特且不可替代的,已展现出广阔的应用潜力。
尽管如此,微弧氧化涂层形成机理、高性能功能涂层设计制备方面仍面临诸多挑战:
4.1 微弧氧化机理仍需深入完善
深入了解微弧氧化过程机理,是设计制备功能化涂层和进一步提升使役性能的前提。但由于微弧氧化火花放电过程时间非常短,仅为微秒级别,很难捕捉瞬间的微区火花放电状态与物理化学过程;同时膜基界面微区的放电状态对涂层的组织结构(尤其是膜基界面结构)影响较大,进而影响涂层的功能化应用。采用高速摄影、光发射光谱(OES)、原子发射光谱(AES)、原子示踪及其它新技术原位捕捉火花放电状态及元素和温度分布,采用FIB、TEM(包括原位)、SEM(包括原位)等手段分析对应涂层组织结构的的演变过程,仍然是阐释微弧氧化机理的有效途径。
4.2 高性能微弧氧化功能涂层的按需剪裁设计与精细调控制备仍需探索
(1) 抗磨减摩功能化涂层
轻量化金属(钛、铝及镁合金)及复合材料替代钢、铜等用于相对运动部件,在应用于高载荷抗磨环境中,要求涂层大厚度、高硬度;应用于低载荷精密减摩环境中,要求涂层高致密、低粗糙度与低摩擦因数。未来挑战仍然是通过微弧氧化参数设计提高致密层所占比例、通过溶液离子掺杂或石墨、PTFE、MoS2等低摩擦因数物相的复合,辅以精密抛光等后续加工,以满足航天或高端制造中对高精度、抗磨减摩涂层设计要求。
(2) 耐腐蚀功能化涂层
微弧氧化涂层对于提高轻量化金属(钛、铝及镁合金、铝锂合金、镁锂合金)及其复合材料的抗腐蚀与异种金属间电偶腐蚀非常有效,但由微弧氧化火花放电的本质决定,涂层表面有微孔、裂纹等缺陷,这为侵蚀性离子的渗入提供通道。通过微弧氧化参数设计提高致密层所占比例、通过微弧能量递减实现表面致密化自封孔、通过溶液掺杂实现表面微孔低熔点相形成自封闭是增强抗腐蚀性能的重要途径;此外,工程应用中通过复合工艺(如硅烷化处理、化学转化、电沉积、电镀、喷涂等)实现表面封孔及涂装,也是提高抗腐蚀性能行之有效的途径。
(3) 热防护功能化涂层
采用微弧氧化在金属(钛、铝及镁合金、钛铝合金、铌合金、钽合金)及复合材料表面构建低热导率(<2 W/(m·K))的热障陶瓷涂层,已引起研究者的潜在兴趣;通过调节微弧氧化电解液成分及低热导物相掺杂(如ZrO2),可进一步降低涂层的热导率。但热障涂层常常需要很高的涂层厚度(如100~1 000 μm),而微弧氧化制备高膜基结合、大厚度(>100 μm)涂层难度较大,因此发展底层为微弧氧化涂层、外层为其它热障涂层的复合结构涂层是可能解决的途径。
(4) 催化功能化涂层
微弧氧化涂层在催化中作为催化载体及催化活性结构。为提高催化效率,仍然需要通过调控电参数、电解液组成,设计制备具有类珊瑚状、纳米棒、纳米片等高比表面积的涂层;或通过微弧氧化后处理(热处理、溶胶-凝胶等)多步复合技术,以构建具有高催化活性的微纳结构涂层。
(5) 生物功能化涂层
钛合金表面微弧氧化生物活性涂层,可显著提高牙根、骨植入体与周围组织之间的骨整合/骨结合能力。通过钛合金多级孔前处理、电解液成分的掺杂改性、微弧氧化后处理(碱热、水热、水汽、等离子体、紫外活化)等进一步调控涂层的生物活性成分与表面多级微纳结构,构建易于细胞粘附增殖的微纳尺度的微环境,是继续探索的方向;构建兼具生物活性与抑(杀)菌的双重功能涂层,以预防早期的炎症影响愈合并促进组织修复也是关注的重要方向。如何有效调控镁合金的腐蚀降解速率是镁生物材料应用中的关键问题,微弧氧化涂层被认为是最有应用潜力的腐蚀降解调控涂层。但如何通过涂层设计实现镁金属的均匀腐蚀/定向腐蚀降解是研究的重要挑战,这也决定镁合金血管支架能否应用于临床的关键;微弧氧化涂层镁合金用于腔道支架、骨固定螺栓、骨板及大节段骨缺损修复支架等已经获得诱人的进展,但仍需要通过涂层结构设计、封孔及复合工艺涂层等实现降解速率精确调控及抑(杀)菌等功能。
(6) 其他功能化涂层
微弧氧化涂层在介电绝缘、热控、着色等功能化涂层方向正引起研究者及工程界的兴趣,在磁性、锂电、隐身等新方向也值得探讨。通过调控微弧氧化涂层电解液、工艺参数及后续改性等设计制备新型功能涂层,仍然是该领域研究者努力探索的方向。
对微弧氧化技术的研究已有30多年,因其诸多功能特性及可实现性强使其备受关注。未来的发展重点主要包括以下几方面。
在基础理论研究方面,进一步探讨微弧氧化涂层的生长机理以加深科学理解,明确微弧氧化过程-涂层结构-性能关系。
在新功能涂层工艺方面,探索新的功能化陶瓷涂层与可控合成方法,以推动特种功能涂层在高技术领域的扩大应用。
在工程应用研究方面,从大规模生产角度出发,继续通过溶液与电参数调控或复合涂层工艺强化各种功能特性;精细工艺集成、智能化制造与装备的人机对话选控将为推动应用插上翅膀;不同功能化涂层的国家(企业)标准需尽快建立,为推动应用提供设计依据。
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