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0 前言
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海洋自然资源丰富，具有重要的经济价值和国防战略地位。随着海洋强国战略的提出，人们对涉海装备的可靠性和服役寿命提出更高要求。磨损和腐蚀是涉海零部件失效的重要因素，海洋环境磨损、腐蚀或磨损-腐蚀交互作用下，工件失效造成了严重经济损失，仅 2018 年中国海洋基础设施腐蚀造成直接经济损失超 7 000 亿元[1]。统计表明，通过适当的防护策略可避免 5%～35%的腐蚀损失[2]。其中，服役部件表面减磨耐蚀涂层防护是延长其使用寿命和提升性能稳定性的重要手段。
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海洋工程设施与装备的机械运动系统部件长期服役于海洋环境，部件表面由于受到化学 / 电化学引起的腐蚀以及力学因素引起的摩擦磨损的交互作用，加速了零部件的损伤失效。表面涂层技术通过大幅提升零部件的耐腐蚀、耐磨损、抗氧化等性能而延长其服役寿命，是机械设备及其关键表面防护的最重要和最基本手段。
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减磨耐蚀防护涂层可分为有机涂层和无机涂层。其中，有机涂层虽具有良好的耐腐蚀或低磨损性能，但强度较低，无法承受较大载荷，且稳定性较差，不能在重载和高温环境服役[3-4]。无机涂层强度和稳定性较高，经过成分设计、结构调控或后处理等可使其具有良好的耐磨耐蚀性能。研究表明，阳极转换[5]、原位钝化调控[6]、纳米相协同[7]和梯度元素分布[8]等均能有效改善无机涂层的磨损腐蚀性能。然而，传统涂层仍存在氧化失效、钝化膜破裂和脱落等问题，研究开发新的具有良好减磨耐蚀涂层体系有助于解决传统涂层存在的弊端。
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2004 年 YEH 等[9]和 CANTOR 等[10]各自独立提出高熵合金（High entropy alloy，HEA）和等原子比多主元合金（Multi-principal element alloy，MPEA） 概念，开启材料设计新纪元。高熵合金（亦被称作多主元合金）涂层较传统涂层具有多主元特征，便于成分设计和结构调控，并且高熵合金涂层的高熵效应、晶格畸变、迟滞扩散和“鸡尾酒”效应使其能够获得优异的耐磨损、耐腐蚀和热稳定性等综合性能[11-12]。图1 为 Web of Science 数据库中，高熵合金涂层年发表论文数量及其摩擦腐蚀研究的关键节点[13-16]。目前，FeCoCrNi-和 VAlTiCr-体系高熵合金涂层耐磨损和耐腐蚀性能较被防护的 304 不锈钢、Q235 碳钢、镍基合金等基体呈数量级提升，并且涂层与基体之间具有良好结合力[17-20]。成分设计、结构调控、后处理和工艺优化等有望使高熵合金涂层实现在涉海环境耐磨防腐产业中的应用。
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本文针对海洋磨损、腐蚀和磨蚀工况，总结了 2019 年以来高熵合金涂层磨损腐蚀防护研究的最新进展，列举了高熵合金涂层常用制备方法的优缺点，对比了磁控溅射、激光熔覆和喷涂技术制备高熵合金涂层的构性关系和耐磨耐蚀机理。最后，提出高熵合金涂层减磨耐蚀防护研究仍需解决的问题及研究展望。
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1 高熵合金涂层制备方法
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目前，高熵合金涂层的制备方法主要包括磁控溅射、激光熔覆、热喷涂等，具有急热和急冷的特点，保留了高温状态的物相结构。根据吉布斯自由能公式 G＝H-TS，若系统的焓 H 不变，增加系统的温度 T 和混合熵 S，可以降低系统的自由能 G，从而提高系统的稳定性。因此，通过急冷工艺制备高熵合金涂层，有利于降低元素偏析、抑制中间相的成核和生长，形成固溶体相或非晶相，从而提高涂层的耐摩擦磨损、耐腐蚀等性能。
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磁控溅射技术是在真空状态下将一定能量的高能粒子以较高的速度撞击靶材，使溅射粒子从靶材表面发射，在基底表面沉积成膜的过程。磁控溅射过程中，原子随机碰撞，形成光滑致密、成分均匀的固溶体涂层。由于无需熔化靶材，磁控溅射可用于制备难熔高熵合金体系涂层，若溅射过程中通入反应气体（如 N2、O2、CH4 等），则可获得高熵化合物涂层。目前，高熵合金涂层靶材分为多主元复合靶、多主元拼接靶和单质多主元共沉积靶，可以灵活调控合金成分、种类及含量。根据电源的不同，该技术可以分为直流磁控溅射法和射频磁控溅射法。其中，直流溅射法一般只能用于金属靶材。
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激光熔覆技术是利用高能激光束熔化预制粉末和基体近表面，使得涂层与基体间冶金结合，具有高冷却速率（103～106℃ / s），被广泛应用于材料表面改性和零部件的表面修复。熔覆涂层表面粗糙度较高，需要抛光处理，且需注意基体元素稀释进涂层对结构和性能的影响。传统激光熔覆技术在基材表面形成熔池，热影响区大（500～1000 μm），涂层稀释率高（大于 10%）。近年来，超高速激光熔覆技术通过将光粉耦合位置移至基体表面上方位置 （0.2～3 mm），不仅大幅提高了沉积效率（10 倍以上）和粉体利用率（高达 90%），而且降低了涂层的稀释率、厚度以及表面粗造度，因此得到了广泛关注[21]。
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热喷涂技术是将涂层材料预制粉末经热源（如激光、火焰、等离子和电弧等）加热，借助高速气流雾化并加速喷射于基体表面，形成晶粒尺寸细小的合金或化合物涂层。热喷涂技术可用来喷涂几乎所有能形成熔融态粒子或类熔融态粒子的材料，而且基体材料不受限制。另外，喷涂过程中升温小，不产生应力和变形，具有适应性强和经济效益好等优点，在工业领域得到广泛应用。系的选择。
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表1 对比了磁控溅射、激光熔覆和热喷涂三种制备技术的优缺点。对于磁控溅射技术，若靶材是磁性材料，由于磁力线被靶材屏蔽，将大幅降低靶面上方的磁控作用，限制了其在强磁高熵合金涂层中的应用。激光熔覆和热喷涂技术均要求预制粉末熔点相近且熔点不能太高，限制了高熵合金涂层体系的选择。
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2 高熵合金涂层摩擦磨损性能
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摩擦磨损性能包含了减摩性能和耐磨性能。一般而言，金属材料通过添加润滑剂和硬质相来分别降低摩擦因数和减少磨损。与传统摩擦磨损防护涂层相似，高熵合金涂层磨损机理主要包括磨粒磨损、黏着磨损、疲劳磨损和氧化磨损，而提高涂层力学性能有利于改善受力作用为主的磨粒磨损、黏着磨损和疲劳磨损。摩擦过程中多种主元原位反应产生的纳米晶体-非晶氧化物、纳米梯度氧化物和复杂氧化物等也均能提升涂层的耐磨损性能。因此，润滑相和硬质耐磨相的设计和调控是高熵合金涂层实现低摩擦和耐磨损的关键。高熵合金的多主元设计理念拓宽了物相结构设计的丰富性，不仅有利于润滑元素 / 相的添加或生成， 而且通过调控多相的精细结构可以提高涂层强度和韧性，从而能够实现高熵合金涂层低摩擦耐磨损一体化。如表 2 所示，现有高熵合金摩擦磨损防护涂层主要为单相体心立方（BCC）、面心立方 （FCC）和非晶结构，室温平均摩擦因数波动范围较大（0.09～0.72），磨损率普遍达到 10−5 mm 3 /（N·m）量级，且含 N 元素的高熵合金涂层通常具有更低的磨损率。总体而言，高熵合金涂层耐磨损性能与传统耐磨损涂层性能相当，部分含 N 元素的高熵合金涂层耐磨损性能优于传统硬质涂层，在室温高载荷条件下磨损率低至 7.40× 10−7 mm 3 /（N·m）[22-23]。摩擦磨损影响因素众多，主要包括外界环境和材料本征特性，目前主要针对高熵合金涂层成分、结构、后处理和服役环境温度对其摩擦磨损性能及机理开展研究。
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2.1 成分 / 结构对摩擦磨损性能的影响
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元素种类或含量变化影响涂层的晶体结构，进而改变涂层力学性能和摩擦学性能[36]。多项研究表明，由 Al 元素和第四周期过渡金属元素组成的高熵合金涂层，其体系中的混合焓和晶格畸变随 Al 元素含量增加而增加，促使涂层由 FCC 向 BCC 结构转变，提高了涂层硬度[37]；涂层中引入小原子半径元素（Si、C、N 等）后，元素之间大的原子半径差（δ＞12%）导致晶格坍塌，促使涂层向硬质非晶态转变[25，38]；BCC 或非晶涂层中亲 N 元素（Al、 Cr、Mo、Nb、Ti、V、Zr 等）与 N 元素形成 Me-N 键，大量 Me-N 键将增加体系混合焓，提升涂层硬度并促使涂层向 FCC 结构转变[12]。可见，成分调控可以改变涂层的相结构，从而改善涂层的硬度和耐磨损性能[39]。另外，成分和制备工艺协同调控涂层结构也是改善涂层摩擦学性能的重要研究方向。涂层结构方面，复合结构涂层主要通过添加外加颗粒和原位相转变实现涂层强化和润滑协同，而多层结构涂层利用层间 / 层内共生多相实现耐磨和自润滑的协同控制[40-42]。因此，可基于涂层摩擦学性能需求改变涂层元素种类 / 含量、复合 / 多层结构，设计制备所需性能的高熵合金涂层。
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外加硬质颗粒，如 NbC[43]、TiC[44]和 CeO2 [45] 等具有抑制晶粒生长、增强晶界作用，用于提升涂层致密度和力学性能。外加颗粒周围产生的应力应变场或颗粒与新相间形成的低能表面均能增加相形核速率，进而优化涂层相结构、晶粒尺寸。然而，若外加颗粒发生团聚，将加剧涂层磨擦变形过程中的应力集中，导致早期损伤和破坏。LI 等[43]采用激光熔覆制备了添加 NbC 硬质纳米颗粒的 AlCoCrFeNi 高熵合金复合涂层。研究发现，少量的 NbC 硬质相固溶于涂层基体，提高了涂层体系 BCC 结构在高温条件下的稳定性，涂层基体由 FCC 向 BCC 结构转变；随着 NbC 硬质相增多，过量的 NbC纳米相在晶界析出而钉扎晶界，使晶粒得到明显细化，因此涂层硬度大幅提高，磨损量降低了 37%，平均摩擦因数降低了 19%。除引入颗粒种类和含量，颗粒尺寸和分布均匀性也是需要考虑的因素。LI 等[45]的研究结果表明，外加颗粒尺寸并非越小越好，相比纳米尺寸 CeO2颗粒，亚微米 CeO2 颗粒间由于相对较小的库仑力和范德华力，能够均匀分布于涂层中，降低了涂层应力，使用寿命更长。CeO2 的粒径主要显著影响涂层的黏着磨损和疲劳磨损。随 CeO2 的粒径逐渐增加，涂层的疲劳磨损先减小后加剧，而粘着磨损先增大后减小。因此，复合结构涂层需要根据摩擦磨损的性能需求，选择适配的外加颗粒。
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与外加颗粒增强相比，涂层体相或表面原位生长的颗粒 / 相分布更加均匀[44]。引入第三元素，如 N、O、C 元素或 Ag、Al、Mo、Ti 元素等可有效调控涂层晶体结构及相组成，从而实现原位相调控[46-49]。通常，引入非金属元素主要是对涂层起间隙固溶强化作用，含量较高时促进涂层发生析出或相转变；引入金属元素主要促使涂层相转变、细化晶粒和在摩擦接触表面形成强化氧化膜。并且，涂层体相或表面原位生长获得的纳米晶粒或梯度纳米晶粒在磨损时发生结构演变，将促进均匀的塑性变形，限制滑动引起的开裂和局部脆性断裂，涂层耐磨性提高。研究表明，高熵合金涂层制备过程中引入 N 元素后，体心立方晶粒以及 N 元素增强的晶界能够克服强度和延展性之间的不相容性，从而降低涂层磨损[50]。在耐磨损涂层中引入低剪切强度的 Ag 元素，摩擦过程中原位形成高强度和均匀变形的复合纳米氧化物表面也被证明有利于实现低摩擦耐磨损性能。西安交通大学 LIU 等[27]在 TiNbZr 涂层中引入 Ag 元素，提出“磨损反应防护”来实现涂层优异的耐磨损性能。其中，易剪切 Ag 元素保障涂层具有较低的摩擦因数（0.09），而涂层在干滑动时形成的强塑性复合纳米非晶-晶态氧化表面层使涂层磨损率较未形成该氧化层时降低了一个数量级。摩擦过程中，机械作用导致涂层表面产生高密度位错并形成亚晶界，使涂层表面晶粒细化，为氧进入涂层内部提供了丰富的路径，并且涂层晶界富集的 Ag 单质作为催化剂也加速氧的扩散，由此在涂层表面形成非晶包裹～10 nm Ag 单质的复合纳米非晶晶态氧化层。固溶体 Ag 纳米晶嵌入非晶基体中限制了非晶相剪切带的形成，Ag 纳米晶形成的孪晶界阻碍位错运动，并作为额外的独立剪切载体，从而使这种双相结构的晶体-非晶纳米复合氧化物表现出高强度和均匀的塑性变形，涂层裂纹和脆性破坏得到抑制。
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复合涂层经外加颗粒或原位相调控能够获得良好的耐磨损性能，然而涂层中颗粒分布或相尺寸控制难度较高，由此发展出多层涂层，并且可根据服役工况结合多种不同性能复合涂层，制备出多功能多层涂层。多层涂层共格应变强化能够抑制位错滑移，降低涂层磨损率。由于模板效应，亚稳态相能够稳定存在于层间，并且通过模板层可优化原子排列 / 择优生长取向，再结合层内或层间减摩相，能够实现多层涂层低摩擦-高耐磨一体化。河南工程学院 REN 等[29]在反应射频磁控溅射过程中调控通入 N2 含量及时长，获得不同调制周期（200）择优取向 FCC 固溶结构的 AlCrMoZrTi /（AlCrMoZrTi）N 多层涂层。较小的单层厚度、较强的模板效应和较高的层间约束力有效阻止了微裂纹的产生和扩展，从而降低了摩擦过程中的磨损。如图2 所示，西北工业大学王海丰团队利用磁控溅射技术在 NbMoWTa 涂层中引入 Ag 单质层，通过改变多层涂层调制周期，实现 Ag-FCC / NbMoWTa-BCC 到 Ag-BCC / NbMoWTa-BCC 相转变，最终既降低了涂层的摩擦因数，又保持了涂层的硬度，实现了基于相变和共格界面可控调节摩擦因数和磨损率，并提出制备这种多层涂层需要满足的三个条件：①选择力学性能优良的高熵合金；②引入界面处晶格错配度适中（＜ 5%）的固体润滑剂；③为获得最佳的磨损性能，应调整固体润滑剂的层厚[51]。
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图2 NbMoWTa / Ag 多层涂层制备与摩擦性能[51]（PVD 为物理气相沉积的缩写，h 为单层涂层厚度）
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Fig.2 Preparation and tribological properties of NbMoWTa / Ag multilayer coatings[51] (PVD is abbreviation of physical vapor deposition, h is individual layer thickness)
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2.2 工艺 / 后处理对摩擦磨损性能的影响
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工艺参数或后处理主要影响涂层的相组成和微观结构。通常，硬度越高的涂层由于相同应力下的接触面积减小，磨损率越低[52]。因此，可基于工艺参数或后处理调控涂层中硬质氧化物、非晶、细晶或第二相的含量和分布，提升涂层的力学性能[53]。研究表明，磁控溅射制备高熵合金涂层过程中，基体温度较低时，阴影效应占主导，并导致涂层边界处出现孔洞；基体温度较高时，表面扩散占主导，使得涂层形成致密的晶界，提高基体温度，有助于消除非晶引起的孔洞边界和促进晶化[54]。通过适当提高基体温度、减小基体表面非晶层厚度和涂层晶粒尺寸，能够提高涂层的力学性能[24]。另外，不同制备方法使涂层成分 / 结构产生差异，如图3 所示，火焰喷涂相较冷喷涂制备的 AlCoCrFeMo 高熵合金涂层会形成更高含量的 AB2O4（A=Fe / Mo，B=Al / Cr）氧化物，涂层硬度提高[34]。而利用等离子氮化处理降低涂层晶格系数或形成新的氮化物相，均能提升涂层的硬度，从而降低涂层的磨损率[25]。
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通常，热处理促使涂层晶粒尺寸增加或析出第二相，以此降低热应力或增加沉淀强化，能够有效提升涂层的力学性能，降低涂层磨损率[33，56]。特别地，根据涂层成分选择合适的热处理温度和升降温速率，在热处理过程中使涂层塑性应变引起再结晶，能够促使涂层晶粒细化，从而由细晶强化提升涂层的耐磨损性能[57]。ALVI 等[28]对磁控溅射制备的 CuMoTaWV 涂层 300℃退火处理后，涂层在基体上附着力提高，表面粗糙度和晶粒尺寸降低，摩擦学性能稳定性提高，而制备态涂层磨痕内呈现大的塑性变形且摩擦因数在整个摩擦过程中一直增加。值得注意的是，部分高熵合金涂层经不同时间（0、2、4 和 8 h）热处理后，其硬度和摩擦学性能并不呈线性关系，如 2 h 热处理后的（TiVCrAlMo）N 涂层硬度最低，而由于热应力释放和磨损颗粒氧化物 MoO3 具有润滑作用，涂层最终呈现最低的磨损率和摩擦因数[58]。等离子喷涂涂层通常孔隙率较高，其磨损机理主要为疲劳磨损、磨粒磨损和氧化磨损。高能量激光热处理常用于喷涂法制备的高熵合金涂层表面强化。区别于传统的固态热处理技术，该后处理方法使涂层表面经历快速熔化-凝固过程，能够细化晶粒和改善气孔、裂纹等缺陷，并且可实现局部强化处理。通过激光重熔等技术细化晶粒和加强固溶强化后，涂层磨损机理变为磨粒磨损和氧化磨损，平均摩擦因数和磨损率均优于等离子喷涂高熵合金涂层[55]。在 TiN 强化等离子喷涂 AlCoCrFeNi 高熵合金涂层中，激光重熔促使涂层发生铝热反应，形成 Al2O3，Al2O3 作为熔化态 TiN 非均相成核的核心，最终形成双相 TiN-Al2O3 陶瓷颗粒，在晶粒中和沿晶界分布，涂层耐磨性提高[59]。
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2.3 温度对摩擦磨损性能的影响
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海洋装备动力系统在服役过程中面临 300～800℃的高温，涂层热稳定性和氧化产物成为其成分设计的重要参考。高温环境下，涂层发生相变、软化或摩擦化学反应，将改变涂层磨损机理，而外加润滑相或原位自生润滑相的稳定性和分布状态决定涂层摩擦因数稳定性及对耐磨损性能的破坏程度。表3 为不同温度下高熵合金涂层的摩擦学性能对比情况。从中可知，制备态为 BCC、FCC 或非晶的高熵合金涂层在高温下均能获得良好的摩擦学性能，不同高熵合金涂层在高温下摩擦因数波动较大，磨损率普遍低于 10−4 mm 3 /（N·m）量级。
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温度较低时，涂层磨损率的高低主要取决于涂层表面硬度；而温度较高时，涂层磨损率主要取决于摩擦产物[63，71]。摩擦过程中，温度与力协同作用下，摩擦化学反应诱导涂层原位形成的耐磨 / 润滑相、相种类及分布状况影响涂层的摩擦性能[72-73]。如图4 所示，纳米晶氧化物不仅能够提升涂层表面硬度，还能充当固体润滑相来降低界面黏附作用[63]。摩擦过程中形成的非晶-纳米晶复合层及梯度纳米结构使涂层表面具有高强度和均匀变形性，限制了滑动引起的涂层开裂和局部脆性断裂，从而使涂层具有优异的耐磨损性能[26]。此外，由磨损产物压实形成的致密氧化物层也被证明在高温下能够抑制磨损[74]。
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为揭示高熵合金涂层晶体和非晶结构对其性能和摩擦机理的影响，宁波材料所蒲吉斌团队通过磁控溅射制备了两种 VAlTiCr-基高熵合金晶体涂层，并阐明了多主元晶体涂层热-力耦合高温摩擦机理[64-65，75]。结果表明，高温热效应主导了元素扩散氧化与固相反应，而摩擦力的机械混合效应促进了高温固相反应，最终形成 AlV3O9 和 Al2（MoO4）3 两种大晶面间距复杂氧化物高温润滑相。VAlTiCrW 涂层在 800℃摩擦磨损过程中，磨痕内原位形成 AlV3O9 复杂纳米氧化物，摩擦因数低至 0.15，磨损率降低至 10−5 mm 3 /（N·m）量级。而在 VAlTiCrMo 涂层中，Mo 元素促进了 Al 元素和 V 元素的扩散作用，在力作用下形成 Al2（MoO4）3，并且涂层表面形成分层纳米氧化物层，使得涂层在 700℃具有较低的摩擦因数和磨损率。随后，该团队又通过相图模拟和试验验证揭示了 700℃下 VAlTiCrSi 高熵非晶涂层从非晶态向由 FCC、Al8Cr5、（V，Ti，Cr）5Si3 和非晶相组成的混合物相结构转变是摩擦学性能提升的主要原因[75]。
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为获得具有自润滑性能的单相高温摩擦防护涂层，宁波材料所常可可和兰州化学物理研究所杨军等通过放电等离子烧结制备（HfMoNbTaTi）C 涂层，在高温摩擦诱导作用下，C 原子扩散，涂层在滑动界面处分解形成无定形碳膜，实现了对磨球与涂层易剪切接触，使得涂层最低摩擦因数仅为 0.1，磨损率降至 10−7 mm 3 /（N·m）量级[70]。此外，涂层高温下强韧相协同被证明能使涂层具有良好的摩擦性能[76]。原位生成硬质 SiO2与韧性双金属氧化物结合使涂层在 800℃下的平均摩擦因数低至 0.2，同时维持 10−5 mm 3 /（N·m）量级磨损率[68]。而外加润滑相能够改变摩擦氧化产物，抑制高温下氧化层的剥落，从而在降低涂层摩擦因数的同时提升其耐磨性[77]。
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3 高熵合金涂层腐蚀性能
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根据腐蚀形态，金属的腐蚀可分为均匀腐蚀和局部腐蚀。通常均匀腐蚀容易观测和预防，而局部腐蚀的发生位置具有不确定性，腐蚀速率难以检测，危害性和隐患性更大。局部腐蚀主要包括点蚀、电偶腐蚀和晶间腐蚀等。晶态涂层主要发生点蚀、晶间腐蚀和电偶腐蚀，而非晶涂层主要为点蚀，如图5 所示。从热力学角度，高熵效应促使高熵合金涂层倾向于形成单一的简单固溶体结构或非晶结构；从动力学角度，急冷工艺可以降低高熵合金涂层凝固过程中元素偏析，并促使纳米晶或非晶结构等亚稳结构的形成。相结构的简化有利于避免复杂相之间电位差引起的电偶腐蚀、晶间析出引起的晶间腐蚀，而非晶态涂层没有晶界、位错等晶体缺陷，不存在晶间腐蚀和电偶腐蚀。因此，高熵合金涂层往往表现出比传统耐蚀合金涂层更加优异的耐腐蚀性能。表4 列举了部分高熵合金涂层在室温条件下的耐 Cl− 腐蚀性能。从表中可知，大部分高熵合金涂层呈现单一简单的固溶体结构或非晶结构，耐腐蚀性能优于 304 不锈钢，腐蚀电流密度低至 nA / cm2 量级。部分高熵合金具有双相结构，依然展现出优异的耐腐蚀性能。从材料本征属性到服役环境，高熵合金涂层的腐蚀行为受诸多因素影响，当前研究的重点主要包括合金成分、组织结构、后处理及腐蚀环境温度等。
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图5 晶态涂层和非晶涂层主要腐蚀机制对比[78] （MM 为金属空位）
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Fig.5 Comparison of the main corrosion mechanisms of crystalline coatings and amorphous coatings[78] (MM is vacancy of the metal)
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Where Ecorr is corrosion potential, icorr is corrosion current density.
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3.1 成分 / 结构对腐蚀性能的影响
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涂层耐腐蚀性能受钝化膜的稳定性和致密度影响，而钝化膜的形成与合金成分及结构息息相关。 Al、Cr、Co、Cu、Fe、Mn、Mo、Nb、Ni、Ta、Ti、 V、Zr、C、N、Si 等元素常被用于耐腐蚀高熵合金涂层。其中，Al、Cr、Co、Ni 和 Ti 属于易钝化元素，Cr、Mo、Nb、N 元素能提高材料的耐点蚀能力， Ta、Nb、Cr、Ni、C、Si 等元素能提高材料的力学性能，Ti 元素能扩展材料钝化区，C 元素能提高钝化膜稳定性，Mn 元素会抑制钝化过程和降低钝化膜稳定性，Ni 元素能提高耐应力腐蚀[81，90-91]。例如， ALIYU 等[92]制备的 AlCrNiCoFeCu 涂层中，Cr 元素促进涂层表面金属钝化膜的形成，减少 Cl− 离子扩散。在 CoCrFeMnNi 合金体系中引入 Si 元素，形成非晶氧化物，能够降低 Cr 元素偏聚，而引入 Al 元素利于钝化膜变得稳定并具有更强的自修复能力[93-94]。高熵合金涂层中引入微量纳米稀土氧化物 CeO2 分布于晶界，细化了晶粒，并为 Ti、Cr、Mo、 Fe 元素的稳定态氧化物提供更多形核位点，促进形成稳定致密的钝化膜[95]。
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成分均匀、结构稳定有助于提升高熵合金涂层的耐腐蚀性能，通过元素设计和含量调控能够降低涂层成分偏聚和优化结构。其中，基于大原子半径差元素制备的高熵非晶涂层属于长程无序结构，具有优异的耐腐蚀性能[96]。如图6 所示，宁波材料所蒲吉斌团队采用磁控溅射制备的 VAlTiCrSi 非晶涂层在人工海水中的耐腐蚀性能较 304 不锈钢提高一个数量级[19，80]。VAlTiCrSi 非晶涂层钝化膜形成机制为瞬间形核，当 Cl− 穿透钝化膜后，主要与 Cr2O3 反应生成的氯化物在涂层 / 钝化膜界面堆积，同时钝化膜表面溶解减薄，最终导致涂层钝化膜破裂和点蚀萌生。在 VAlTiCrSi 非晶涂层中，钝化膜中未氧化金属起到生成钝化膜的补充剂效果，减缓了钝化膜损伤，并在致密化钝化膜后抑制了 Cl− 向内扩散，从而使涂层具有良好的耐腐蚀性能。而在 VAlTiCrMo 涂层中引入 N 元素后，涂层晶粒细化，其晶体结构由 BCC 转变为 FCC 相结构，涂层由非密排结构转变为密排结构，钝化区由 580 mV 提升至 990 mV[97]。
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另外，基于组分调控原位形成增强相也是提高高熵合金涂层性能的有效方法。增强相通常以第二相的形式存在于高熵合金中，通过调整阳极面积（基体相）与阴极面积（第二相）的比例来减少阳极溶解，从而提高耐腐蚀性。ZHANG 等[98]基于 C 元素与不同金属元素间混合焓的差异，调控 C / Al0.1CrNbSi0.1TaTiV 合金涂层中 C 元素含量，实现合金涂层中原位生成强化相 TiC。结果表明，适量小尺寸 TiC 相与基体相之间桥接形成的致密钝化膜层在一定程度上抑制了基体相的溶解，而过量的 TiC 相会加速电偶腐蚀过程，增加钝化膜的缺陷。在结构设计方面，利用多层结构打断磁控溅射柱状生长、优化涂层稳定性和降低生长缺陷，能够有效提升涂层的腐蚀性能。西安交通大学宋忠孝团队设计的非晶 TaNbHfZr＋FCC 相 CoCrNi 多层涂层的耐腐蚀性能大致为 CoCrNi 单层涂层的两倍[81]。
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3.2 工艺 / 后处理对腐蚀性能的影响
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降低金属元素的溶解性或降低钝化膜缺陷密度能够有效提升涂层耐蚀性，其中细化晶粒促进涂层形成稳定、致密的钝化膜便是一种常用方法[99]，并且晶粒细化还能抑制阴极相与基体之间的电荷转移过程[100]。如图7 所示，哈尔滨工业大学范成磊团队在气体钨极电弧熔覆 AlCoCrFeNi 涂层的工艺中引入超声波处理，空化效应和声流效应促使枝晶破碎和晶粒细化，晶粒尺寸从 285 μm 细化至 78 μm，Cr 元素在晶界偏聚，涂层从晶间腐蚀变为均匀腐蚀[101]。江苏理工学院丛孟启等研究表明，超声波的搅拌效应能够抑制 Cr 元素和 Mn 元素在涂层中偏聚，细化晶粒和均匀的微观结构提高了涂层的耐腐蚀性能[102]。另外，基于调整制备工艺参数，调控涂层纳米结构能够有效提升耐腐蚀性能。上海交通大学李伟课题组通过两步控温法（第一步在 473 K 下沉积 1.5 h，第二步在 373 K 下沉积 2 h）获得外层纳米薄片层和内层等轴晶层的梯度 Fe25.2Co25.2Ni27.4Cr22.2涂层，证明了多级结构涂层可获得优于铸态高熵块体合金的力学性能和与之相当的耐腐蚀性能[103]。类似地，冷喷涂辅助高速激光熔覆诱导原位双向扩散的梯度 CoCrFe0.5Ni1.5Mo0.1Nb0.68 共晶高熵合金涂层，在 3.5wt.% NaCl 溶液中的耐腐蚀性能较 Mg-Li 合金提高三个数量级[104]。
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图7 超声辅助电弧熔覆涂层形貌及腐蚀机理[101]
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Fig.7 Morphology and corrosion mechanism of the ultrasonic assisted arc cladding coatings [101]
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热处理通常能够改变涂层晶粒尺寸、晶体织构和物相组成，提高涂层元素分布均匀性或相稳定性，从而改善涂层耐腐蚀性能。XIE 等[105]对放电等离子烧结 AlxCoCrFeNi 涂层进行热处理。结果表明，热处理加速元素扩散，促进富 Ni 元素和 Al 元素的 B2 相向富 Cr 元素的 FCC 相转变，有助于形成高致密度钝化膜（富 Cr2O3 钝化膜致密度高于富 Al2O3 钝化膜），从而有效抑制涂层点蚀。另外，超声波技术具有点接触、高应变率和高能量密度输入特点，允许在更短的时间内形成厚的梯度冲击变形层，以及降低表面粗糙度，减小晶粒尺寸，诱发压缩残余应力，提高材料硬度和耐腐蚀性能。因此，超声波技术也开始被推广到高熵合金涂层的表面强化。中国石油大学（华东）ZHANG 等[106]对 Al1.5CoCrFeMnNi 制备态涂层进行超声波冲击强化处理，通过剧烈的塑性变形，产生和累积大量的位错，形成梯度结构，提升涂层的力学性能。而冲击过程中破碎表面的杂质相降低点蚀形核倾向，表面细小的树枝晶为钝化膜的形成提供更多的离子通道，使得涂层点蚀坑数量和大小降低，腐蚀电流密度降低近一个数量级。
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3.3 温度对腐蚀性能的影响
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轮船燃气轮机、海上风机和海上钻井平台等都一定程度上面临着热腐蚀。温度变化影响涂层腐蚀过程和钝化膜元素分布，并且，Cl−-O2 协同加速高熵合金涂层腐蚀破坏。目前，不同温度盐溶液环境腐蚀性能研究对象主要是高熵合金块体[107-110]，高熵合金涂层在不同温度溶液环境下的腐蚀性能研究不足[111]，主要被用于研究高温熔盐环境下的腐蚀性能。高温下，高熵合金涂层表面氧化生成 Al2O3 和 Cr2O3 等氧化物，而 NaCl 与 Al2O3 和 Cr2O3 等强保护性氧化物反应生成 NaAlO2 和 Na2CrO4 等弱保护性熔融氧化物，同时产生的 Cl2 通过涂层间隙加速涂层内部腐蚀[112]。如图8 所示，高熵合金涂层高温盐腐蚀后形成表面薄氧化层和内扩散层，由于产生 Cl2（2CrCl3＋3 / 2O2→Cr2O3＋3Cl2），单一种类盐腐蚀涂层的内扩散层孔洞比混合盐腐蚀产生的孔洞更大，并且随着温度升高，Cl− 加速攻击表面薄氧化层，涂层表面只剩下疏松内扩散层[113]。另外，高温促进涂层晶粒长大，表面氧化层与涂层应力增加使得高熵合金涂层产生裂纹或表面氧化物剥落，添加钇（Y） 元素和铪（Hf）元素被证明能够抑制高温环境下晶粒长大和降低高熵合金涂层表面氧化物剥落[114]。
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图8 激光熔化沉积涂层热腐蚀后截面形貌[113]
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Fig.8 Cross-sectional morphology of LMD (laser metal deposition) -fabricated coatings after hot corrosion [113]
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4 高熵合金涂层磨蚀性能
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磨蚀包含摩擦和腐蚀，由化学-电化学-力耦合作用加速材料失效，材料磨蚀损失体积包含磨损损失体积、腐蚀损失体积和磨损与腐蚀交互作用造成的损失体积[18]。摩擦运动对材料的腐蚀具有不可忽视的加速作用，而腐蚀的增大反过来又导致材料磨损的加剧，从而形成腐蚀介质特有的磨损与腐蚀交互作用，这成为海洋装备运动部件所面临的一个重大的科学和技术难题。耐磨蚀涂层要求其同时具有良好的耐摩擦磨损性能和耐腐蚀性能。传统 CoCrMo-合金、钛合金和 NiCrAlY-涂层是磨蚀研究的三大材料体系，而高熵合金涂层应用于磨蚀的研究起步较晚，2019 年前后才开始有少量的研究报道，并以 VAlTiCr-体系高熵合金涂层为主。磨蚀过程中，涂层或钝化膜被机械破坏，产生的裂纹将为 Cl− 提供更多向内快速扩散的通道，并且若摩擦破坏钝化膜，暴露的涂层新鲜表面也将加速涂层腐蚀，此时涂层的力学性能和再钝化能力强弱决定摩擦诱导的腐蚀比重。另外，腐蚀产生的氧化产物种类、含量和分布决定腐蚀诱导的涂层磨损比重，润滑和连续分布氧化物有利于降低腐蚀对磨损的影响。目前，高熵合金涂层磨蚀性能评价指标主要包括开路电位、动电位曲线、电化学阻抗、摩擦因数和磨损率，合金成分、结构、工艺或后处理对高熵合金涂层磨蚀性能的影响是人们目前研究的主要关注点，环境温度变化等影响因素研究鲜见报道。
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4.1 成分 / 结构对磨蚀性能的影响
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成分和结构设计可优化涂层本征特性及诱导原位摩擦-电化学反应，从而能够实现涂层良好的磨蚀性能调控。一般地，高熵合金耐磨蚀涂层包含易钝化元素（如 Al、Ti、Cr）和易形成硬质相元素（如 Al、Ti、N）。宁波材料所蒲吉斌团队设计并研究了 VAlTiCrCu 涂层在 3.5wt.% NaCl、0.5 mol / L H2SO4 和 1 mol / L NaOH 溶液中的磨蚀性能。在三种溶液中，涂层均表现为钝化膜被机械磨损破坏，开路电位降低，氧化物含量的动态变化引起涂层再钝化，当再钝化作用和磨损破坏达到平衡时，开路电位保持稳定[16]。由此可知，改善涂层力学性能和耐腐蚀性能是提升涂层磨蚀性能的关键。为进一步提升涂层耐磨蚀性能，在 VAlTiCrCu 涂层中引入 WC 硬质层，VAlTiCrCu / WC 涂层多层异质结构降低了涂层在 3.5wt.% NaCl 溶液中开路电位的磨损敏感性[115]。
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另外，成分调控和外加硬质相可改善涂层的力学性能和腐蚀性能，对提升涂层磨蚀性能具有显著效果。中国科学院兰州化学物理研究所隋旭东团队向（VAlTiCrNb）Nx涂层中引入 Mo 元素，随 Mo 元素含量增加，涂层表面粒径增大，较高的涂层硬度和摩擦过程中涂层表面生成的 Cr2O3 和 MoO3 氧化物降低了腐蚀-磨损协同损伤作用，涂层的腐蚀磨损率降低[116]。除改变高熵合金涂层中金属元素含量，调控非金属元素 / 非金属相种类及含量也能改善涂层的磨蚀性能。如图9 所示，S1～S4 分别表示 C 元素含量为 22at.%、 30at.%、 35at.% 和 40at.% 的（CrNbTiAlV）CxNy高熵合金涂层。C 元素的掺杂不仅使涂层结构致密而提升涂层的力学性能，还能形成非晶碳或碳化物而降低的涂层摩擦因数，使涂层具有良好的耐磨蚀性能。因此，引入高含量的 C 元素能够减弱摩擦-腐蚀协同破坏作用[53，117-118]。然而，部分高熵合金碳化物涂层的腐蚀加速磨损速率占总腐蚀磨损率的比值达到 80%[119]。高熵合金碳化物磨蚀机理还有待进一步研究。外加硬质颗粒常被用于提升涂层硬度，但因引入相界加速相间腐蚀，因此需要平衡腐蚀-磨损关系[120]。JIANG 等[121]通过在 CoCrNiMo 中引入 B4C 原位，形成共格陶瓷相（Cr，Mo）23（B，C）6，临界含量为 2at.%～3at.% B4C 即可实现最佳的磨损-腐蚀平衡。
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图9 高熵合金涂层磨蚀响应[118] （W 是材料在没有腐蚀的情况下，由于纯摩擦而产生的体积损失; ΔW 为材料在试验过程中由摩擦而产生的损耗; Wc是纯磨损损失的材料体积与试验过程中摩擦产生的体积之间的变化量; ΔC为由纯腐蚀造成的材料损失; Cw是材料在摩擦腐蚀过程中，由纯腐蚀引起的体积损失与由腐蚀引起的体积损失之间的变化量）
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Fig.9 Tribo-corrosion response of high-entropy alloy coating[118] (W is the volume loss of the material due to pure friction; ΔW is the loss of material due to friction in the absence of corrosion; Wc is the volume of change between the volume of material lost by pure wear and the volume produced by friction; ΔC is the volume loss due to corrosion; Cw is the change between the volume loss caused by pure corrosion and the volume loss due to corrosion of the material during tribo-corrosion.)
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4.2 工艺 / 后处理对磨蚀性能的影响
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机械作用会导致涂层表面及钝化膜破裂，从而引起腐蚀活性粒子向内扩散或钝化膜剥落[122]。因此，制备良好力学性能的致密纳米结构本征涂层或钝化膜涂层是获得良好磨蚀性能的关键。隋旭东团队通过 N 元素改变 VAlTiCrNb 涂层结构来调控磨蚀性能[123-124]。研究发现，随 N 元素含量增加，涂层从非晶转变为致密纳米晶，提高了涂层力学性能和表面化学惰性，开路电位受磨损影响敏感性降低，致密光滑且耐磨表面有效抑制了 Cl− 等活性离子的内渗破坏；通过调控该涂层溅射基体偏压，涂层择优取向由（200）变为（111），综合性能得到进一步改善。另外，海水环境中金属氧化物和氢氧化物等摩擦化学反应产物覆盖于磨痕上也能隔绝外界对涂层的直接损伤，从而提升涂层耐磨蚀性能[125]。鉴于钝化膜的主要成分为氧化物，可以通过氧化处理在涂层表面预先生成致密的氧化膜，一方面提高涂层表面硬度来降低磨损，另一方面缩减形成致密钝化膜的时间来抑制腐蚀介质的侵蚀。CHEN 等[69]发现 VAlTiCrNi 非晶涂层经 600℃氧化处理后，析出的 TiO2、Al2O3等细小硬质氧化物颗粒镶嵌在非晶基体中，起弥散强化作用，并形成致密氧化膜，最终实现在 3.5wt.% NaCl 溶液中涂层磨蚀电位差 0.015 V，摩擦因数 0.01，磨损率 2.36×10−6 mm 3 /（N·m）。
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5 总结与展望
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高熵合金以其“质剂不分”的独特设计理念而受到广泛关注。得益于热力学上的高熵效应和动力学上的极冷技术，高熵合金涂层具有成分设计自由度丰富、物相结构简单、组织成分均匀等特点，在干摩擦磨损、腐蚀和磨蚀材料领域具有潜在的应用价值。尽管目前关于高熵合金涂层的研究取得了一定的成果，但仍存在一些科学问题尚待解决，例如：
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（1）高熵合金涂层成分种类繁多，涂层构性关系不明确，缺少结构或性能设计广谱化的指导理论。
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（2）高熵合金涂层磨蚀性能及机理研究起步较晚，磨损、腐蚀和磨损-腐蚀交互作用对磨蚀影响机制及过程参照数据匮乏。
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（3）高熵合金涂层表面氧化膜 / 钝化膜的研究集中于试验后的检测分析，缺乏原位成分 / 结构表征测试，涂层摩擦化学、腐蚀化学或摩擦-腐蚀交互化学反应过程不清晰。并且，高熵合金涂层表面氧化膜 / 钝化膜成分及结构尚停留于由自然生长决定，缺乏类似本体涂层的成分和结构的可控制备。
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（4）随着海洋强国战略的不断推进，深海探测、资源开发及水下通讯等对深海高压耐磨蚀防护涂层的需求日益突出，然而高熵合金涂层腐蚀、磨损和磨蚀研究集中于常压溶液环境，欠缺对深海力-化学-电化学多场耦合极端环境下的腐蚀、摩擦和磨蚀研究，如图11 所示。
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为进一步促使高性能高熵合金涂层的高效研发和工程化应用，未来可以在以下方面开展深入研究：
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（1）结合大数据机器学习、第一性原理计算和高通量试验，加快高熵合金涂层设计、性能优化和机理分析，揭示元素及元素相互作用对高熵合金涂层构性关系的影响机制，形成高熵合金涂层成分-结构-性能预测大模型。
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（2）结合分步试验、理论计算和精细原位表征，揭示高熵合金涂层服役环境中磨损、腐蚀及磨蚀机制，特别是在涂层与介质作用过程中产生的钝化膜 / 氧化膜的演变规律、作用过程中元素 / 离子的迁移扩散等的原位表征。
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（3）结合材料特性、服役环境和制备工艺特点，构筑多层或复合涂层结构，揭示涂层制备-后处理强化间结构与性能的映射关系。例如，通过磁控溅射制备高熵合金 / 陶瓷纳米多层复合涂层、热喷涂高熵合金的激光 / 电子束表面改性等。
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（4）极端复杂环境中高性能高熵合金涂层的研发。“鸡尾酒”效应赋予高熵合金与生俱来的多功能性，在极端复杂服役环境中具有独特的优势。例如，在深海高压环境服役的运动结构件，受深海水压、温度、溶解氧、pH 等多重耦合因素作用，对涂层的强韧性、耐磨损和耐腐蚀等综合性能提出了更加苛刻的要求。丰富的成分设计和结构调控为强韧与多功能一体化高熵合金涂层的研发提供了广阔的发展空间。
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摘要
机械运动系统部件的磨损、腐蚀或磨损-腐蚀耦合损伤（磨蚀）是海洋工程设施与装备失效的最主要因素。涂层表面技术是目前广泛应用于提高部件服役性能与寿命的关键技术之一。其中，高熵合金涂层具有优异的强韧、耐磨、耐蚀等综合性能，对高熵合金涂层的开发和研究将拓展海洋装备零部件磨损、腐蚀防护涂层体系的选择范围和提升零部件表面综合服役性能。综述国内外高熵合金涂层在磨损、腐蚀和磨蚀方面的最新研究成果，从高熵合金涂层的主要制备方法出发，对比不同制备方法的优缺点，总结高熵合金涂层的摩擦磨损、腐蚀和磨蚀行为，探讨成分、结构、制备 / 后处理工艺以及服役温度对高熵合金涂层磨损、腐蚀和磨蚀性能的影响及其作用机理，重点介绍元素调控和第二相强化在优化高熵合金涂层耐腐蚀、耐磨损方面的研究进展。最后，指出当前高熵合金涂层磨损腐蚀研究中仍需解决的问题，并对其未来发展方向作出展望，这有助于推动高熵合金涂层在苛刻环境中的研究与应用。
Abstract
Wear, corrosion, and wear-corrosion synergism, namely, tribocorrosion acting on mechanical motion components are the most important factors in the failure of marine engineering facilities and equipment. Therefore, wear, corrosion, and tribocorrosion pose potential threats to marine economic development and operational safety. One-third to one-fourth of the energy of the world is wasted owing to friction, and the loss caused by the corrosion of marine infrastructure in China was more than 700 billion RMB in 2018. Because metals are the most used engineering materials, their protection has long been the pursuit of material studies; however, it is challenging to achieve. Presently, application of protective coatings, such as nitride, oxide, carbide, metal, and organic coatings on the surfaces of machine parts is considered an effective and economical way to reduce wear, corrosion, and tribocorrosion. Among various protective coatings, high-entropy alloy coatings have attracted research interest owing to their excellent comprehensive performance and unique advantages in wear, corrosion, and tribocorrosion resistance. In recent years, equiatomic or near-equiatomic high-entropy alloy coatings with BCC, FCC, and amorphous structures have been prepared as single-phase or multiphase coatings. These coatings have yielded abundant results in wear resistance and anticorrosion, providing a new approach for surface protection. To benefit from the on-demand design of wear-resistant and anticorrosion high-entropy alloy coatings, this article introduces the main preparation technologies for such coatings and reviews their latest results for corrosion, wear, and tribocorrosion from 2019 to the present. It elaborates the corrosion, wear, and tribocorrosion mechanisms of high-entropy alloy coatings from the perspectives of component design, structural regulation, doped elements, and postprocessing, thereby revealing the materials science tetrahedra of high-entropy alloy coatings. Moreover, the latest research results on the wear and corrosion properties of high-entropy alloy coatings are compared via tabulation. Some of the listed high-entropy alloy coatings show superior friction or corrosion properties to traditional protective coatings and have potential application prospects. Regarding the wear of high-entropy alloy coatings, synergistically improving the lubrication performance and surface mechanical properties contributes to their excellent tribological properties. When the temperature is low, typically lower than 300 ℃, the wear rate of a coating mainly depends on the hardness of the coating, and when the temperature is higher than 300 ℃, it mainly depends on the friction product. External particles, such as NbC, TiC, and CeO2, and in situ phase structures, such as nano-oxide AlV3O9 and coherent Ag-BCC / NbMoWTa-BCC, are two common materials for enhancing the tribological properties of high-entropy alloy coatings. Regarding the corrosion resistance of these coatings, using compact coatings and dense passive films has the advantage of a low corrosion current density. The corrosion current density of amorphous coatings, such as VAlTiCrSi, is generally lower than that of 304 stainless steels, and a compact high-entropy nitride coating with an FCC structure shows an expanding passivation zone. Similar to the method for reducing the wear of a coating, in situ formation of a reinforcing phase, such as TiC, promotes corrosion resistance. Interestingly, Si-related amorphous oxides uniformly distribute Cr in passive films, and Al improves the stability of a passive film to reduce corrosion. Moreover, the corrosion mechanism of a coating with a finer grain size transforms from intergranular corrosion to uniform corrosion through Cr segregation into the grain boundary. For tribocorrosion resistance, coatings should have excellent corrosion resistance and antiwear properties simultaneously. Therefore, the tribocorrosion resistance of high-entropy alloy coatings is generally derived from their compact structure, chemical inertness to active ions, and good mechanical performance. However, research on the tribocorrosion properties of high-entropy alloy coatings is limited, although VAlTiCr-based high-entropy alloy coatings have been extensively studied. According to these studies, wear destroys the chemical state of a coating surface and reduces its corrosion resistance. A multilayer coating with a hard layer or a composite coating with a nano-oxide forms an antiwear and corrosion-resistant passive film. In particular, the preoxidation process has been proven to have a positive effect on the tribocorrosion resistance of VAlTiCrNi amorphous coatings. From the aforementioned results, in the fields of wear, corrosion, and tribocorrosion resistance, the composition and structure of the oxide or passive film on the surface of a high-entropy alloy coating affect its friction, wear, and corrosion performance. The wear, corrosion, and tribocorrosion mechanisms of high-entropy alloy coatings tested in an atmospheric-pressure environment are discussed from the perspectives of composition, structure, and pre / post-treatment. Finally, the problems and perspectives regarding the corrosion, wear, and tribocorrosion of high-entropy alloy coatings are outlined to guide future studies. In this article, the in situ characterization of the oxide film / passive film originating from wear, corrosion, and tribocorrosion is discussed. The application of high-entropy alloy coatings on moving parts designed for deep-sea environments may be a key area for exploration in future. Additionally, a prediction model to improve the material design and study efficiency is expected.
Keywords
high-entropy alloy coating ； wear ； corrosion ； tribo-corrosion
