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作者简介:

张盼盼,女,1991年出生,博士,助理研究员,硕士研究生导师。主要研究方向为热防护涂层的表面改性。E-mail:panpanzhang166@163.com

中图分类号:TG156;TB114

DOI:10.11933/j.issn.1007−9289.20221209001

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目录contents

    摘要

    传统的等离子喷涂热障涂层在高温环境下服役易受熔融腐蚀盐渗透而过早剥落失效,研究激光合金化掺杂自愈合材料 TiC 对热障涂层热腐蚀行为的影响具有重要意义。采用大气等离子喷涂技术(Atmospheric plasma spray,APS)在 Inconel 718 镍基高温合金表面制备 NiCrAlY 粘结层,采用大气等离子喷涂技术在 NiCrAlY 粘结层上制备 8 wt.%氧化钇部分稳定的氧化锆(8 wt.% yttria partially stabilized zirconia,8YSZ)陶瓷层,构建典型双层结构热障涂层体系。采用 1 kW 光纤耦合激光器将自愈合材料 TiC 熔于 8YSZ 热障涂表层,并考察其在 900 ℃下 25%NaCl+75%Na2SO4混合熔盐中保温 4 h 的热腐蚀行为。结果表明,与等离子喷涂涂层相比,激光合金化改性热障涂层表面更加光滑,分布有网状裂纹,且结构致密。等离子喷涂涂层的热腐蚀产物主要是针状颗粒 Y2(SO4)3 和 m-ZrO2,但仅有较少的热腐蚀盐渗透至激光合金化改性热障涂层内部,其热腐蚀产物为 Y2(SO4)3 和少量的 TiO2。激光合金化改性热障涂层的抗热腐蚀性能较等离子喷涂态热障涂层提升 55.5%,一方面激光合金化改性层组织致密,可阻止热腐蚀盐渗透至涂层内部,另一方面,激光合金化改性热障涂层表面粗糙度更低,能减少与热腐蚀盐的接触面积。此外,自愈合材料 TiC 在高温下发生氧化反应引起体积膨胀,实现裂纹的部分自愈合效应,进一步阻止了热腐蚀反应的发生。采用激光表面改性技术将自愈合材料 TiC 引入热障涂层,激光合金化改性热障涂层不仅具有光滑的表面形貌,还具有致密的微观组织结构;同时自愈合材料 TiC 在高温环境下的裂纹自愈合效应有助于抑制热腐蚀盐的渗透,最终提高热障涂层的抗热腐蚀性能。

    Abstract

    Double-layered thermal barrier coatings (TBCs), which are composed of a top ceramic coating and a bonding coating, are widely used in the industry to reduce the surface working temperature of hot components. The primary materials used for the ceramic coating have been 6-8 wt.% Y2O3 partially stabilized ZrO2 (6-8YSZ), which has excellent performance of thermal insulation, high temperature resistance, low thermal conductivity, and effective thermal protection effect on metal substrates. However, traditional plasma-sprayed TBCs contain numerous pores and microcracks, and they are susceptible to corrosive salt penetration at high temperatures, leading to premature peeling failure. Doping with self-healing materials and laser post-treatment methods can effectively improve the hot corrosion resistance of TBCs. Therefore, this study aims to examine the effect of laser alloying on the hot corrosion behavior of plasma-sprayed TBCs. First, a NiCrAlY bonding coating is prepared on the surface of an Inconel 718 nickel-based superalloy via atmospheric plasma spray (APS) technology. An 8YSZ ceramic coating is then applied on the NiCrAlY bonding coating. Finally, self-healing TiC is melted on the plasma-sprayed 8YSZ coating by using a 1 kW fiber-coupled laser. The hot corrosion behaviors of the plasma-sprayed and laser-alloyed TBCs are investigated by immersion in 25% NaCl + 75% Na2SO4 mixed salt at 900 ℃ for 4 h. The weight losses of the plasma-sprayed and laser-alloyed TBCs following hot corrosion are examined. The microstructures of the plasma-sprayed and laser-alloyed TBCs before and after hot corrosion are studied using scanning electron microscopy. X-ray diffraction is used to characterize the phase composition of each coating, and energy-dispersive spectroscopy is used to analyze the elemental compositions. A high-precision electronic balance is used to measure the weights of the plasma-sprayed and laser-alloyed TBCs before and after hot corrosion, and the weight loss due to hot corrosion is determined. The results shows that the surface of the laser-alloyed TBCs is smoother. A few segmented microcracks are distributed on the laser-alloyed TBCs, which exhibits dense microstructure. The main corrosion products of the plasma-sprayed TBCs are needle-shaped Y2(SO4)3 particles and m-ZrO2. Meanwhile, only a small amount of corrosive salt penetrates the interior of the laser-alloyed TBCs, and its corrosion products are Y2(SO4)3 and a small amount of TiO2. After hot corrosion, the volume fraction of m-ZrO2 in the plasma-sprayed TBCs is 18.2%, whereas that in the laser-alloyed TBCs is only 8.1%. It is advisable to avoid the formation of m-ZrO2 during the preparation of 8YSZTBCs. If a detrimental phase transformation of t-ZrO2 to m-ZrO2 occurs, the original pores and cracks will serve as the starting points for stress relief, further exacerbating crack propagation and providing a pathway for the infiltration of corrosive salts, ultimately leading to coating delamination. On the one hand, the microstructure of the laser-alloyed TBCs is denser; this can prevent the penetration of corrosive salts into the interior of the coating. On the other hand, the surface roughness of laser-alloyed TBCs is lower, leading to lowered contact area with corrosive salts. Additionally, the self-healing material TiC undergoes an oxidation reaction during the hot corrosion test, resulting in partial self-healing of the pores and microcracks through volume expansion, further reducing the occurrence of hot corrosion reactions and the formation of harmful m-ZrO2 phases. Compared with plasma-sprayed TBCs, the laser-alloyed coatings shows an improvement in hot corrosion resistance by 55.5%.

  • 0 前言

  • 随着现代工业技术的发展,燃气轮机内部热端部件的工作环境不断向高温、高压和高推重比发展[1],其实际工作温度远超过高温合金承受温度的极限,从而严重影响燃气轮机的服役寿命和可靠性。工业上广泛应用双层结构热障涂层来降低热端部件的表面温度。双层结构热障涂层由顶层陶瓷层和底层金属粘接层组成,陶瓷顶层主要使用的材料为 6-8 wt.% Y2O3 部分稳定的 ZrO2(6-8 wt.% yttria partially stabilized zirconia,6-8YSZ),6-8YSZ 陶瓷具有强隔热、耐高温和热导率低的优异性能[2],对金属基体具有有效的热防护作用。

  • 航空发动机、燃气轮机叶片等高温部件在工作过程会遭受发动机燃料(如 Na、S 等元素)高温腐蚀、海水腐蚀和 CMAS(CaO-MgO-Al2O3-SiO2)腐蚀等,这些腐蚀物质常以 Na2SO4、NaCl 或 CMAS 形式存在于热障涂层表面,高温情况下熔融腐蚀盐渗透进入热障涂层内部,与稳定剂 Y2O3发生热腐蚀反应,导致 ZrO2 发生高温相变,从而使涂层因相变应力而剥落失效[3]

  • 通常热腐蚀机制是:① 腐蚀杂质附着在热障涂层表面,沿着涂层表面的裂纹、孔隙等通道渗入涂层内部;② 高温情况下粘结层会持续被氧化而使热生长氧化物的厚度增加且更加疏松,而使涂层内部裂纹宽度增加,提供更多熔盐渗透途径;③ 熔盐与 Y2O3 发生反应生成柱状或针状腐蚀物质,消耗了 Y 的含量,最终导致 ZrO2失稳相变[4]。腐蚀杂质附着在热障涂层表面会对叶片表面形状、发动机工作效率等有较大的影响。

  • 一些学者采用掺杂自愈合材料的方式以提高涂层的抗热腐蚀性能。自愈合材料是利用材料在应用过程中出现缺陷等问题时,无须附加作用便可以实现自我修复的特殊材料[5]。DERELIOGLU 等[6]在热障涂层(Thermal barrier coatings,TBCs)中掺入球形 MoSi2(B)修复颗粒,其高温下氧化分解形成非晶态 SiO2,流入裂纹并与裂纹面接触,裂纹表面润湿后,与涂层中的 ZrO2 发生化学反应形成固体 ZrSiO4。这种化学反应在愈合剂和基体材料之间产生了牢固的结合,并导致裂纹完全填充和密封,但是会面临 MoSi2 过早氧化的问题。OUYANG 等[7] 将 TiC 和 SiC 作为自愈合材料引入热障涂层,采用热喷涂技术制备出掺杂 TiC 和 SiC 的自愈合热障涂层,研究表明高温下涂层发生填充密封效应会导致涂层孔隙率下降,使熔化的热腐蚀盐无法穿透愈合的涂层,使用寿命约为传统 TBC 的 180%。

  • 一些学者采用激光后处理手段提升热障涂层的抗腐蚀性。虞礼嘉等[8]研究了等离子喷涂制备的热障涂层和激光重熔热障涂层在 75% Na2SO4+25% NaCl (质量分数)熔盐中的热腐蚀性能,结果表明喷涂态涂层在热腐蚀过程中出现了涂层剥落的现象,激光重熔热障涂层无剥落现象发生,改善了热障涂层的抗热腐蚀性能。GU 等[9]采用激光熔覆技术在 316L 不锈钢基体上制备 YSZ-Al2O3-TiC 和 YSZ-Al2O3-SiC 自愈合复合涂层,获得了柱状结构且生成的 TiO2 相实现了裂缝的填补。GUO 等[10]对大气等离子喷涂 YSZ TBCs 进行激光上釉改性,并研究了两种涂层在 700℃和 1 000℃下 V2O5熔盐中保温 4 h 的热腐蚀行为,结果表明激光上釉改性处理有利于改善热障涂层的抗热腐蚀性能,上釉层保持了结构完整性。然而上釉层中的垂直裂纹为熔盐渗透提供了路径,加速了未改性涂层的腐蚀。孙磊等[11]研究了 TiAl3 自愈合剂掺杂的激光合金化改性热障涂层的热腐蚀性能,结果表明 TiAl3 在高温下发生氧化反应生成了 Al2O3 和少量 TiO2 从而填补了裂纹,有助于抑制高温腐蚀盐的渗透,但尚未量化涂层的抗热腐蚀性能指标。综上,激光表面处理技术是一种制备自愈合涂层的可行方法。然而,采用激光合金化技术将 TiC 掺杂于 8YSZ 热障涂层的研究报道较少。本文采用激光合金化技术将自愈合材料 TiC 引入等离子喷涂 8YSZ 热障涂层,研究了激光合金化掺杂 TiC 对等离子喷涂 8YSZ 热障涂层热腐蚀行为的影响,最终揭示了激光合金化改性热障涂层的抗热腐蚀性能强化机制。

  • 1 试验

  • 1.1 材料

  • 本试验所选用的基体材料为Inconel718镍基高温合金。试验选用的粘结层材料为 NiCrAlY 粉末,其粉末粒径为 15~60 μm,其化学成分(质量分数) 如表1 所示。陶瓷层材料为 8 wt.% Y2O3-ZrO2 (8YSZ),其粉末粒径为 15~45 μm。本试验采用的自愈合粉末为 TiC,粉末粒径为 15~45 μm。为了避免自愈合粉末的热膨胀系数与热障涂层不匹配,将 10 wt.% TiC 与 CYSZ(CeO2、Y2O3 共同部分稳定的 ZrO2)粉末以球磨方式混合,利用行星球磨机 (QM-3SP4)进行球磨混粉制备而得,球磨转速 150 r / min,球磨时间 60 min,球料比为 3∶1。有研究表明,利用 CYSZ 粉末制备的涂层在高温下表现出更好的相稳定性和抗热腐蚀性能。此外,向涂层中掺入 CeO2可有效降低 ZrO2 材料的热导率,从而提高涂层的隔热性能、热膨胀系数和抗热震性能[12-13]

  • 表1 NiCrAlY 粉末的化学成分(wt.%)

  • Table1 Chemical composition of NiCrAlY powder (wt.%)

  • 1.2 涂层制备工艺

  • 利用电火花线切割机将 718 镍基高温合金切割成 20 mm×20 mm×7 mm 的尺寸,利用数控超声波清洗机进行超声清洗以除去表面油污、杂质等,然后对基体进行喷砂粗化处理,以提高基体表面粗糙度。粘结层与陶瓷层均采用大气等离子喷涂制备,粘结层厚度在 90~110 μm,陶瓷层厚度在 260~300 μm。

  • 1.3 激光合金化改性工艺

  • 在热障涂层表面铺上厚度 0.1 mm 的 10 wt.% TiC+90 wt.% CYSZ 混合粉末,采用 1 kW 光纤耦合激光器在涂层表面进行合金化处理。激光功率 140 W,激光扫描速度 5 mm / s,光斑直径 2 mm。

  • 1.4 热腐蚀试验

  • 在高温箱式炉(型号:SG-XL1400)中分别腐蚀喷涂态热障涂层和激光合金化热障涂层。具体步骤为称取一定量腐蚀盐,然后将 25% NaCl+75% Na2SO4 的混合熔盐粉末以 20 mg / cm2 的沉积量置于喷涂态热障涂层和激光合金化热障涂层表面,以避免边缘效应,铺腐蚀盐面积为 14 mm×14 mm; 以 10℃ / min 的升温速度升温至 900℃,将试样置于高温箱式炉中保温 4 h,允许试样在炉内冷却;将试样取出置于 98%无水乙醇中超声波清洗 10~15 min,以除去表面残留腐蚀盐和残留水分。

  • 1.5 表征方法

  • 利用金相切割机(型号 QG-50)对激光合金化改性后的热障涂层进行纵向切割,以观察截面形貌。采用德国 Zeiss EVO18 型扫描电子显微镜(SEM)观察涂层表面形貌、截面组织、涂层与基体结合区等微观特征;采用形状测量激光显微系统进行涂层表面粗糙度的表征,分别对喷涂态和激光合金化改性热障涂层的表面粗糙度进行测量。采用 Zeiss EVO18 型扫描电镜自带的 Bruker Xflash 6130 能量色谱仪(EDS)对涂层表面及截面元素含量和分布进行分析。采用 X 射线衍射仪(型号 X´Pert PRO) 检测粉末及涂层的物相组成。

  • 2 结果与讨论

  • 2.1 合金化形貌与组织结构分析

  • 图1 是等离子喷涂和 TiC 激光合金化热障涂层表面形貌图。观察图1a~1c,喷涂态形貌为扁平颗粒团聚层层堆叠,表面凹凸不平,表面粗糙度大约为 5.895 μm,同时存在熔化、半熔、未熔颗粒。观察图1d,合金化后激光作用区明显比周边涂层光滑,出现网状裂纹。图1e 中,涂层出现致密排布的柱状晶结构,有利于抑制腐蚀盐向涂层内渗透。激光合金化改性热障涂层表面粗糙度大约为 2.408 μm,明显小于喷涂态热障涂层的表面粗糙度。这是由于激光热源同时加热陶瓷层及铺在上方的 10 wt.% TiC 与 CYSZ 粉末使其温度瞬间上升形成熔池,当激光光斑移开后,熔池快速冷却凝固从而形成光滑的激光合金化改性区。图1b 中半熔、未熔颗粒形成的粗糙区域经过加热熔化凝固后,未观测到残留的半熔、未熔颗粒,光滑程度明显优于原本喷涂态颗粒的堆叠。激光合金化改性热障涂层表面更光滑,使其与腐蚀盐的接触表面积减少,可进一步减少热腐蚀反应的发生,从而改善热障涂层的抗热腐蚀性能。

  • 图1 等离子喷涂和 TiC 激光合金化热障涂层表面形貌

  • Fig.1 Surface morphology of plasma sprayed and laser-alloyed thermal barrier coatings

  • 图2 是等离子喷涂和激光合金化改性热障涂层的横截面形貌。图2a 为喷涂态热障涂层的横截面形貌,陶瓷层厚度为 290 μm,粘结层厚度为 110 μm。由图2b 可知,喷涂态陶瓷层存在较多的孔隙,孔隙形成的原因主要分为两种[14]:一方面,熔融粉末在飞行过程中快速冷却,导致在基体表面快速凝固收缩,而部分包裹在等离子体焰流中的环境气体和喷涂气体未能及时溢出,这些气体遗留在层状结构中,冷却后形成孔隙;另一方面,在等离子焰边上或尾部进入的粉末颗粒,未能被等离子焰完全熔化,导致粉末在基体上附着时是不均匀和不充分的变形,其他熔化粉末变形后与其形成不良的堆叠,产生孔隙。这些孔隙虽不利于涂层的抗热腐蚀和抗热震性能,但却能提高涂层的隔热性。图2c 为激光合金化改性热障涂层的横截面形貌,可以发现激光合金化改性热障涂层由深度为 130 μm 的改性层、110 μm 未改性的陶瓷层和 100 μm 粘结层组成,各层之间结合良好。激光合金化改性层结构致密且分布着少量的垂直裂纹,未改性区仍保持疏松多孔结构。进一步观察激光合金化改性层的放大形貌,改性区为致密的柱状晶结构,如图2d 所示。由于采用的激光束能量分布为高斯分布,激光光斑中心区域能量密度高周围低,能量密度分布不均匀,呈高斯分布。当激光移动时,光斑中心和四周区域冷却凝固速度不同,使得该区域产生较大的残余应力,当应力累积叠加到一定程度时,在改性层中就会产生垂直裂纹[15]。已有研究表明,引入垂直裂纹可以提高陶瓷层的应变容限,释放热循环过程中陶瓷顶层与粘接层不匹配产生的热失配应力,从而延长涂层寿命的有效途径之一[16-17]。激光合金化改性区的柱状晶结构主要取决于激光处理过程中温度梯度与凝固速度的比值,对应最高热梯度的方向。

  • 图2 等离子喷涂和激光合金化改性热障涂层的横截面形貌

  • Fig.2 Cross section morphology of plasma sprayed and laser-alloyed thermal barrier coatings

  • 图3a 是 8YSZ 粉末的 XRD 图谱,8YSZ 粉末中同时存在四方相 t、立方相 c 和单斜相 m 三种晶型的 ZrO2。图3b 是等离子喷涂热障涂层的 XRD 图谱,与图3a 对比发现,有害相 m-ZrO2消失,出现 t'-ZrO2。m-ZrO2 在 1 170℃会转变成 t-ZrO2,而 t-ZrO2 在 2 370℃会转变为 c-ZrO2。8YSZ 粉末经过高温喷涂后,m-ZrO2 发生马氏体相变形成 t'-ZrO2,这主要是喷涂过程中温度高冷却快,极冷极热的过程所导致,但是由于涂层中存在稳定剂 Y2O3,抑制了 t'-ZrO2 向 m-ZrO2 的相变。图3c 是激光合金化改性热障涂层的 XRD 图谱,发现经过改性后,涂层物相并未发生变化,主要为 t'-ZrO2和 c-ZrO2,表明激光合金化处理手段并不会促使有害相 m-ZrO2 生成,影响涂层质量。YSZ 热障涂层制备过程中应避免 m-ZrO2 生成,因为在 t'-ZrO2 向 m-ZrO2 转变过程中,同时伴随着 3%~5%体积的膨胀,在涂层内部产生一个较大的压应力。若有害相变发生将使原来的孔隙和裂纹作为应力释放的起点,进一步加剧裂纹的扩散,为腐蚀盐与氧的渗透提供路径,最终导致涂层剥落。此外,激光合金化改性涂层中没有检测到自愈合剂 TiC 及其氧化产物 TiO2 的衍射峰(图3c),可能是由于自愈合剂 TiC 的添加量较少,X 射线衍射方法的局限性使其无法检测到含量较低的物相。

  • 图3 粉末和各涂层的 XRD 图谱

  • Fig.3 XRD patterns of powders and coatings

  • 2.2 热腐蚀后物相分析

  • 图4 是热腐蚀后喷涂态和激光合金化改性热障涂层的 XRD 图谱。图4a 中发现涂层中有 t'-ZrO2、 m-ZrO2 和 Y2(SO43 生成,表明喷涂态涂层热腐蚀后生成了腐蚀产物发生物相转变,热腐蚀产物 Y2(SO43和m-ZrO2的生成会引起应力以及相变导致裂纹扩展涂层失稳剥落,最终导致热障涂层提前失效。而图4b 中可以看出,激光合金化改性涂层在热腐蚀后的主要物相为t'-ZrO2、c-ZrO2、少量Y2(SO43、 TiO2和 m-ZrO2,表明激光合金化热障涂层的热腐蚀产物为 TiO2、Y2(SO43和 m-ZrO2,可能是由于稳定剂被消耗较少,所生成的 m-ZrO2较少,被腐蚀盐所覆盖,所以 XRD 图谱只检测到少量 m-ZrO2,表明涂层内部未发生有害相的转变。

  • 图4 等离子喷涂热障涂层和激光合金化改性热障涂层热腐蚀后的 XRD 图谱

  • Fig.4 XRD patterns of plasma sprayed and laser-alloyed thermal barrier coatings after hot corrosion

  • 2.3 热腐蚀后表面形貌

  • 图5a、5b 是喷涂态热障涂层在 900℃下 25% NaCl+75% Na2SO4混合熔盐中热腐蚀 4 h 后的表面形貌,与图3a、3b 相比,热腐蚀后喷涂态涂层表面出现了空洞和点蚀,部分颗粒或扁平小块出现剥落,涂层疏松,且存在大量细小针状晶和块状的腐蚀盐颗粒。根据表2 的元素分析,区域 A 主要存在 Zr 和 O 元素,原子比约为 1∶2,表明该区域主要是 ZrO2;区域 B 中的不仅存在较多的 Zr、O 元素,还有较多的 Na 和 S 元素,根据原子比推断区域 B 为 ZrO2 和 Na2SO4 的混合物;区域 C 存在较多的 Y、S 和 O 元素,结合图4 的 XRD 图谱推断该区域是热腐蚀产物,即腐蚀盐与稳定剂 Y2O3 反应生成的 Y2(SO43

  • 图5 热腐蚀后等离子喷涂和激光合金化改性热障涂层表面形貌

  • Fig.5 Surface morphology of APS and laser-alloyed thermal barrier coatings after hot corrosion

  • 表2 热腐蚀后表面不同区域的化学成分(at.%)

  • Table2 Chemical composition of different areas on the face after hot corrosion (at.%)

  • 图5c、5d 是激光合金化改性热障涂层在 900℃ 下 25% NaCl+75% Na2SO4 混合熔盐中热腐蚀 4 h 后的表面形貌。经过热腐蚀试验后,激光合金化热障涂层表面仍保持光滑致密,网状裂纹分布更加明显,涂层表面有较小的腐蚀坑形成,同时存在有较大的块状晶和较小的针状颗粒,且分布分散,部分网状裂纹处有腐蚀盐聚集,但未渗透至涂层内部。结合图4 的 XRD 图谱与表2 的元素分析,区域 D 主要存在 Na、O、S、Zr 元素,且 Na、O、S 原子比约为 2∶1∶4,Zr 含量较小,所以区域 D 推断为 Na2SO4 与 ZrO2的混合物;区域 E 主要包含 O、Y、S 元素,根据其原子比例,可确定区域 E 物质为 Y2(SO43;区域 F 主要含有 Zr、O 元素,Zr 与 O 原子比约 1∶2.3,接近于 1∶2,可确认该区域主要为 ZrO2

  • 激光合金化热障涂层表面的未反应腐蚀盐成分比喷涂态多,有较少的腐蚀产物生成,有较好的抗热腐蚀性能。相比之下,合金化涂层的腐蚀坑更少,腐蚀产物 Y2(SO43 更少,等离子喷涂涂层腐蚀盐附着的密度远大于合金化涂层,腐蚀盐与稳定剂大量反应会导致涂层失稳相变,产生裂纹孔隙等又会加重腐蚀盐侵蚀程度。

  • 通常用 m-ZrO2 的含量来定量比较热腐蚀过程中涂层失稳的标准[18]。图6 是热腐蚀后喷涂态热障涂层和激光合金化改性热障涂层在衍射角 2θ 为 27°~33°时的 XRD 图谱。

  • 图6 热腐蚀后各涂层在 2θ 为 27°~33°的 XRD 图谱

  • Fig.6 XRD patterns of thermal barrier coatings when 2θ is 27°~33°

  • 根据式(1)来计算 m-ZrO2的体积分数,以表征两种涂层的抗热腐蚀性能。

  • m vol. %=Im(1¯11)+Im(111)Im(1¯11)+Im(111)+It(111)×100%
    (1)
  • 式中,Im(−111)和 Im(111)分别为单斜相(-111) 和(111)晶面的衍射强度;It(111)表示亚稳四方相 t'-ZrO2 晶面的衍射强度,m vol.%为 m-ZrO2 的体积分数。

  • 通过计算后得出结果如表3 所示,热腐蚀后喷涂态热障涂层中 m-ZrO2 体积分数为 18.2%,而激光合金化改性热障涂层中m-ZrO2体积分数仅为8.1%,激光合金化改性热障涂层相比较于喷涂态热障涂层提升了 55.5%,因此激光合金化改性热障涂层具备较优的抗热腐蚀性能。

  • 表3 热腐蚀后喷涂态和激光合金化改性热障涂层中的 m-ZrO2 含量

  • Table3 M-ZrO2 content of plasma-sprayed thermal barrier coatings and laser-alloyed thermal barrier coatings after hot corrosion

  • 2.4 热腐蚀后截面形貌

  • 图7 是热腐蚀后喷涂态和激光合金化改性热障涂层的横截面形貌及其 EDS 元素分析。从图7a 中可以看出,喷涂态涂层热腐蚀后截面出现明显孔洞。由图7b 的 EDS 元素分析,发现大孔洞处有大量 Na、 S、O 元素聚集,表明大量热腐蚀盐通过表面的裂纹与孔洞渗透到涂层内部,与 Y2O3稳定剂发生热腐蚀反应,破坏原本的涂层结构。

  • 图7c、7d 为激光合金化改性热障涂层的横截面形貌及其 EDS 元素分析,发现 Na、S 等腐蚀熔盐元素基本没有在激光合金化改性层内富集,裂纹与孔洞处有少量 Na、O、S 元素聚集,仅有少量腐蚀盐渗透至涂层内部,致密的激光改性层能够显著抑制腐蚀盐的渗透,避免了内部涂层与腐蚀盐的接触。 TiC 的自愈合作用也是涂层抗热腐蚀性增强的原因,其原理如下:TiC 氧化温度为 600℃[19],而热腐蚀试验温度远高于其氧化温度,部分 TiC 在热腐蚀过程中发生氧化反应,生成 TiO2和 CO2,氧化反应表达式如式(2)[20];TiC 在氧化反应前后发生一定的体积变化,TiC 与 TiO2 的摩尔比(物质的量比) 为 1∶1,TiC 的密度大致为 4.93 g / cm3[21],而 TiO2 的密度为 4.26 g / cm3[19],氧化反应前后的体积变化可以用 VTiO2 / VTiC=(MTiO2×ρTiC)/(MTiC×ρTiO2)来表示,通过计算可知,氧化反应前后体积增加了约 15.7%[22]

  • 图7 热腐蚀后等离子喷涂和激光合金化改性热障涂层截面形貌

  • Fig.7 Cross section morphology and EDS analysis of plasma sprayed and laser-alloyed thermal barrier coatings after hot corrosion

  • TiC+O2TiO2+CO2
    (2)
  • TiC 在氧化过程中生成 TiO2,伴随着一定的体积膨胀,随着氧化反应的进行,氧化物逐渐累积并填充孔隙、裂纹等缺陷。利用 TiC 在高温下发生氧化反应,生成的 TiO2 体积增生,使得涂层内部的孔隙、裂纹等缺陷逐渐自愈合,减少了腐蚀盐渗透的通道,从而提升热障涂层的抗热腐蚀性能。

  • 另外还可以通过热腐蚀失重量来表征热障涂层的抗热腐蚀性能,如式(3)[23]

  • R=W1-W0
    (3)
  • 式中,R 为热腐蚀单位面积失重量,mg / cm2;W0 为热腐蚀前热障涂层单位面积重量,mg / cm2;W1 为热腐蚀后热障涂层单位面积重量,mg / cm2

  • 通过计算 R 值来判断热障涂层的抗热腐蚀性能,R 值越小,热障涂层的抗热腐蚀性能越优异。结果表明,热腐蚀后等离子喷涂热障涂层失重 2.532 mg / cm2,而激光合金化改性热障涂层增重 0.143 8 mg / cm2。因此,激光合金化改性热障涂层的抗热腐蚀性能远优于等离子喷涂热障涂层。

  • 3 结论

  • (1)激光合金化改性热障涂层表面更加光滑,结构致密,分布有网状裂纹,激光合金化改性技术明显降低了涂层的表面粗糙度,减少与腐蚀盐的接触面积,提高耐热腐蚀性。激光合金化改性热障涂层的物相是 t'-ZrO2 和 c-ZrO2,未检测到有害相 m-ZrO2

  • (2)喷涂态热障涂层的热腐蚀产物主要是 Y2(SO43和 m-ZrO2,激光合金化改性热障涂层的热腐蚀产物主要是针状颗粒Y2(SO43、TiO2和m-ZrO2,较少的热腐蚀盐渗透至涂层内部,TiC 的氧化产生体积增生而对裂纹实现愈合,从而提升热障涂层的抗熔盐腐蚀性能。

  • (3)热腐蚀后喷涂态热障涂层中 m-ZrO2 体积分数为 18.2%,而激光合金化改性热障涂层中 m-ZrO2 体积分数仅为 8.1%,激光合金化改性热障涂层相较于喷涂态热障涂层提升了 55.5%。

  • (4)揭示了激光合金化改性热障涂层的抗热腐蚀强化机制:一方面利用自愈合颗粒 TiC 发生氧化反应生成氧化物填补裂纹,实现裂纹的部分自愈合,另一方面利用激光合金化改性层的致密结构,显著抑制腐蚀盐向涂层内的渗透,减少热腐蚀反应的发生。

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    • [5] PARK J H,BRAUN P V.Coaxial electrospinning of self-healing coatings[J].Advanced Materials,2010,22(4):496-499.

    • [6] DERELIOGLU Z,CARABAT A L,SONG G M,et al.On the use of B-alloyed MoSi2 particles as crack healing agents in yttria stabilized zirconia thermal barrier coatings[J].Journal of the European Ceramic Society,2015,35(16):4507-4511.

    • [7] OUYANG Taoyuan,SUO Jinping.TiC-self-healing thermal barrier coating structures and oxidation resistance[J].Surface & Coatings Technology,2021,412(7):127065.

    • [8] 虞礼嘉,梁文萍,林浩,等.激光重熔YSZ热障涂层 950 ℃的热腐蚀行为[J].中国腐蚀与防护学报,2019,39(1):77-82.YU Lijia,LIANG Wenping,LIN Hao,et al.Evaluation of hot corrosion behavior of laser as-remelted YSZ thermal barrier coatings at 950 ℃[J].Journal of Chinese Society for Corrosion and Protection,2019,39(1):77-82.(in Chinese)

    • [9] GU J J,JOSHI S S,HO Y S,et al.Oxidation-induced healing in laser-processed thermal barrier coatings[J].Thin Solid Films,2019,688:137481.

    • [10] GUO Lie,XIN Hui,ZHANG Zhao,et al.Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance[J].Journal of Advanced Ceramics,2020,9(2):232-242.

    • [11] 孙磊,郭雨嘉,张盼盼,等.激光合金化改性8YSZ热障涂层的热腐蚀性能研究[J].中国激光,2023,50(4):0402002.SUN Lei,GUO Yujia,ZHANG Panpan,et al.Hot corrosion resistance of 8YSZ thermal barrier coatings modified by laser alloying[J].Chinese Journal of Lasers,2023,50(4):0402002.(in Chinese)

    • [12] LEE C H,KIM H K,CHOI H S,et al.Phase transformationand bond oxidation behavior of phase-sprayed zirconia thermal barrier coatings[J].Surface & Coatings Technology,2000,124(1):1-12.

    • [13] 邓世均.热障陶瓷涂层的最新发展[J].材料保护,2003,36(3):5-7.DENG Shijun.Evolution of thermal barrier ceramic coatings[J].Materials Protection,2003,36(3):5-7.(in Chinese)

    • [14] 吴锦杨.自愈合热障涂层的制备和研究[D].武汉:华中科技大学,2012.WU Jinyang.Fabrication and research of self-healing thermal barrier coatings[D].Wuhan:Huazhong University of Science & Technology,2012.(in Chinese)

    • [15] 郭磊,辛会,张馨木,等.激光表面改性对熔盐环境下热障涂层相稳定性和微观结构的影响[J].表面技术,2020,49(1):41-48.GUO Lei,XIN Hui,ZHANG Xinmu,et al.Effects of laser surface modification on phase stability and microstructures of thermal barrier coatings in V2O5 molten salt[J].Surface Technolohy,2020,49(1):41-48.(in Chinese)

    • [16] 俞泽新,于景业,吴良敏,等.无冷却喷涂工艺对其制备的热障涂层的裂纹系统和寿命的影响[J].焊接学报,2016,37(10):55-58,132.YU Zexin,YU Jingye,WU Liangmin,et al.Effect of thermal spraying parameters on cracking and thermal cycling life of thermal barrier coatings[J].Transactions of the China Welding Institution,2016,37(10):55-58,132.(in Chinese)

    • [17] CHENG Bo,YANG Ning,ZHANG Qiang,et al.Sintering induced the failure behavior of dense vertically crack and lamellar structured TBCs with equivalent thermal insulation performance[J].Ceramics International,2017,43(17):15459-15465.

    • [18] ZHONG X H,WANG Y M,XU Z H,et al.Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate-sulfate salts[J].Journalof the European Ceramic Society,2010,30(6):1401-1408.

    • [19] 欧阳韬源.自愈合抗氧化热障涂层的制备、性能及其应用研究[D].武汉:华中科技大学,2016.OUYANG Taoyuan.Preparation,properties and application of the self-healing oxidation resisting TBC[D].Wuhan:Huazhong University of Science and Technology,2016.(in Chinese)

    • [20] BASHA G M T,SRIKANTH A,VENKATESHWARLU B.A critical review on nano structured coatings for alumina-titania(Al2O3-TiO2)deposited by air plasma spraying process(APS)[J].Materials Today:Proceedings,2020,22(4):1554-1562.

    • [21] 刘耀龙.TiC 陶瓷颗粒增强Mo基复合材料的制备工艺及性能研究[D].哈尔滨:哈尔滨工程大学,2020.LIU Yaolong.Study on preparation technology and properties of TiC ceramic particles reinforced molybdenum composites[D].Harbin:Harbin Engineering University,2020.(in Chinese)

    • [22] 陈宏飞,张弛,杨光,等.YSZ-Ti3SiC2热障涂层及其高温自愈合机制[J].航空制造技术,2019,62(18):90-96.CHEN Hongfei,ZHANG Chi,YANG Guang,et al.YSZ-Ti3SiC2 Thermal barrier coating and its self-healing mechanism under high temperatures[J].Aeronautical Manufacturing Technology,2019,62(18):90-96.(in Chinese)

    • [23] 丁柳柳,廖文俊,张艳梅,等.熔融盐对蓄热系统部件材料腐蚀行为的研究[J].装备机械,2015(1):41-46.DING Liuliu,LIAO Wenjun,ZHANG Yanmei,et al.An examination on the corrosion behavior of molten salt to the component materials in heat storage system[J].The Magazine on Equipment Machinery,2015(1):41-46.(in Chinese)

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    • [5] PARK J H,BRAUN P V.Coaxial electrospinning of self-healing coatings[J].Advanced Materials,2010,22(4):496-499.

    • [6] DERELIOGLU Z,CARABAT A L,SONG G M,et al.On the use of B-alloyed MoSi2 particles as crack healing agents in yttria stabilized zirconia thermal barrier coatings[J].Journal of the European Ceramic Society,2015,35(16):4507-4511.

    • [7] OUYANG Taoyuan,SUO Jinping.TiC-self-healing thermal barrier coating structures and oxidation resistance[J].Surface & Coatings Technology,2021,412(7):127065.

    • [8] 虞礼嘉,梁文萍,林浩,等.激光重熔YSZ热障涂层 950 ℃的热腐蚀行为[J].中国腐蚀与防护学报,2019,39(1):77-82.YU Lijia,LIANG Wenping,LIN Hao,et al.Evaluation of hot corrosion behavior of laser as-remelted YSZ thermal barrier coatings at 950 ℃[J].Journal of Chinese Society for Corrosion and Protection,2019,39(1):77-82.(in Chinese)

    • [9] GU J J,JOSHI S S,HO Y S,et al.Oxidation-induced healing in laser-processed thermal barrier coatings[J].Thin Solid Films,2019,688:137481.

    • [10] GUO Lie,XIN Hui,ZHANG Zhao,et al.Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance[J].Journal of Advanced Ceramics,2020,9(2):232-242.

    • [11] 孙磊,郭雨嘉,张盼盼,等.激光合金化改性8YSZ热障涂层的热腐蚀性能研究[J].中国激光,2023,50(4):0402002.SUN Lei,GUO Yujia,ZHANG Panpan,et al.Hot corrosion resistance of 8YSZ thermal barrier coatings modified by laser alloying[J].Chinese Journal of Lasers,2023,50(4):0402002.(in Chinese)

    • [12] LEE C H,KIM H K,CHOI H S,et al.Phase transformationand bond oxidation behavior of phase-sprayed zirconia thermal barrier coatings[J].Surface & Coatings Technology,2000,124(1):1-12.

    • [13] 邓世均.热障陶瓷涂层的最新发展[J].材料保护,2003,36(3):5-7.DENG Shijun.Evolution of thermal barrier ceramic coatings[J].Materials Protection,2003,36(3):5-7.(in Chinese)

    • [14] 吴锦杨.自愈合热障涂层的制备和研究[D].武汉:华中科技大学,2012.WU Jinyang.Fabrication and research of self-healing thermal barrier coatings[D].Wuhan:Huazhong University of Science & Technology,2012.(in Chinese)

    • [15] 郭磊,辛会,张馨木,等.激光表面改性对熔盐环境下热障涂层相稳定性和微观结构的影响[J].表面技术,2020,49(1):41-48.GUO Lei,XIN Hui,ZHANG Xinmu,et al.Effects of laser surface modification on phase stability and microstructures of thermal barrier coatings in V2O5 molten salt[J].Surface Technolohy,2020,49(1):41-48.(in Chinese)

    • [16] 俞泽新,于景业,吴良敏,等.无冷却喷涂工艺对其制备的热障涂层的裂纹系统和寿命的影响[J].焊接学报,2016,37(10):55-58,132.YU Zexin,YU Jingye,WU Liangmin,et al.Effect of thermal spraying parameters on cracking and thermal cycling life of thermal barrier coatings[J].Transactions of the China Welding Institution,2016,37(10):55-58,132.(in Chinese)

    • [17] CHENG Bo,YANG Ning,ZHANG Qiang,et al.Sintering induced the failure behavior of dense vertically crack and lamellar structured TBCs with equivalent thermal insulation performance[J].Ceramics International,2017,43(17):15459-15465.

    • [18] ZHONG X H,WANG Y M,XU Z H,et al.Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate-sulfate salts[J].Journalof the European Ceramic Society,2010,30(6):1401-1408.

    • [19] 欧阳韬源.自愈合抗氧化热障涂层的制备、性能及其应用研究[D].武汉:华中科技大学,2016.OUYANG Taoyuan.Preparation,properties and application of the self-healing oxidation resisting TBC[D].Wuhan:Huazhong University of Science and Technology,2016.(in Chinese)

    • [20] BASHA G M T,SRIKANTH A,VENKATESHWARLU B.A critical review on nano structured coatings for alumina-titania(Al2O3-TiO2)deposited by air plasma spraying process(APS)[J].Materials Today:Proceedings,2020,22(4):1554-1562.

    • [21] 刘耀龙.TiC 陶瓷颗粒增强Mo基复合材料的制备工艺及性能研究[D].哈尔滨:哈尔滨工程大学,2020.LIU Yaolong.Study on preparation technology and properties of TiC ceramic particles reinforced molybdenum composites[D].Harbin:Harbin Engineering University,2020.(in Chinese)

    • [22] 陈宏飞,张弛,杨光,等.YSZ-Ti3SiC2热障涂层及其高温自愈合机制[J].航空制造技术,2019,62(18):90-96.CHEN Hongfei,ZHANG Chi,YANG Guang,et al.YSZ-Ti3SiC2 Thermal barrier coating and its self-healing mechanism under high temperatures[J].Aeronautical Manufacturing Technology,2019,62(18):90-96.(in Chinese)

    • [23] 丁柳柳,廖文俊,张艳梅,等.熔融盐对蓄热系统部件材料腐蚀行为的研究[J].装备机械,2015(1):41-46.DING Liuliu,LIAO Wenjun,ZHANG Yanmei,et al.An examination on the corrosion behavior of molten salt to the component materials in heat storage system[J].The Magazine on Equipment Machinery,2015(1):41-46.(in Chinese)

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