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作者简介:

王蔚言,女,1991年出生,博士研究生。主要研究方向为碳/碳复合材料及其涂层表面缺陷预制、修复。E-mail:wwy1025@mail.nwpu.edu.cn;

付前刚(通信作者),男,1979年出生,博士,教授,博士研究生导师。主要研究方向为碳/碳复合材料防氧化及烧蚀。E-mail:fuqiangang@nwpu.edu.cn

中图分类号:TB332

DOI:10.11933/j.issn.1007−9289.20220218003

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目录contents

    摘要

    碳 / 碳(C / C)复合材料表面涂层在制备与服役过程中易出现裂纹、凹坑和孔洞等缺陷,使涂层失去完整性而极易导致防护失效,目前常用的解决方法为更换整体涂层,成本高、工艺复杂、耗时长,因此快速高效的涂层轻微缺陷修复技术是解决这一难题的有效途径。通过大气等离子焰流在 C / C 复合材料表面 SiC-ZrB2 (SZ)涂层表面预先构造缺陷,采用异丙醇以及高温下性能稳定的含硼聚氮硅烷胶粘剂作为修复剂,以 SiC-ZrB2粉末作为改性填料,Al2O3作为烧结助剂,对 SZ 涂层缺陷进行修复,研究修复前后涂层的微观结构演变与烧蚀防护性能。结果表明:经等离子焰流烧蚀后,未修复的 SZ 涂层试样中心出现圆形凹坑缺陷,裸露出 C / C 复合材料基底;而对于修复后的涂层试样,修复剂热解生成的 SiBCN 陶瓷和改性陶瓷填料均匀覆盖于缺陷处,使涂层保持较高完整性,且在氧乙炔烧蚀下生成致密的 SiO2玻璃膜可有效阻挡氧扩散,保护 C / C 复合材料免受机械冲蚀;修复后的涂层试样在氧乙炔焰流下烧蚀 60 s 后线烧蚀率与质量烧蚀率分别为 0.65 μm / s 和−0.28 mg / s,相比于未修复涂层试样分别降低了 83.54%和 129.47%,修复后涂层的抗烧蚀性能得到显著提升。

    Abstract

    Defects such as cracks, pits and holes, appear in the coatings of carbon / carbon (C / C) composite materials during the preparation and service process, which make coatings lose their integrity and lead to protection failure. Replacing the integral coating as a commonly used solution is expensive, complicated and time-consuming. Therefore, a fast and efficient repairing technology for minor defects on coatings is an effective way to solve this problem. Defects are pre-fabricated on the surface of SiC-ZrB2 (SZ) coated C / C composites by supersonic atmospheric plasma flame. To repair the defects of the SZ coating, isopropanol and boron-containing polynitrosilane adhesive with stable performance at high temperatures are used as repairing agents. SiC-ZrB2 and Al2O3 are used as modified filler and sintering aid, respectively. The microstructure and ablation resistance of the coating before and after repair are studied. The results show that circular pit defects appear in the center of the coating sample after plasma flame ablation, exposing the C / C composite substrate. For the coating sample after repair, SiBCN ceramics and modified ceramic fillers generated by the pyrolysis of the repair agent cover the defects evenly, and thus the coating sample maintains high integrity. A dense SiO2 glass film can be formed under oxyacetylene ablation, which effectively blocks oxygen diffusion and protect C / C composites from mechanical erosion. The repaired coating sample shows a better ablation resistance under oxyacetylene flame flow. After ablated for 60 s, its line ablation rate and mass ablation rate are 0.65 μm / s and -0.28 mg / s, respectively, which decrease by 83.54% and 129.47% compared with the unrepaired coating samples, suggesting that the ablation resistance of the repaired coating is significantly improved.

  • 0 前言

  • 碳 / 碳(C / C)复合材料是一种以热解碳或石墨为基体、碳纤维为增强体的先进高温结构材料,具有低密度(<2.0 g / cm3)、高强度、低热膨胀系数和高温下力学性能优异等特点[1-3],被认为是最有发展前途的高温结构材料之一,在航空航天等领域具有广泛的应用前景[4-7。然而,该材料在高于 400℃ 的有氧环境中会发生氧化,导致其性能大幅衰减。在 C / C 复合材料表面制备高温防氧化抗烧蚀涂层是最有效的解决方法[8-9]。硅基陶瓷涂层具有优异的耐高温和抗氧化性能,是目前研究最为深入的涂层体系[10-12],其中 SiC-ZrB2(简称 SZ)涂层由于其较好的高温抗烧蚀性能,以及可在 900℃以上与氧反应生成致密的 SiO2 玻璃保护层,进而阻碍氧气的扩散并愈合部分微小缺陷等优势,成为理想的 C / C 复合材料防氧化抗烧蚀涂层材料[13-16]。然而,SZ 陶瓷涂层自身易脆断,且与 C / C 复合材料热膨胀系数差异较大,导致在制备及服役过程中极易出现开裂和脱落等现象,从而引起涂层防护失效[17]。因此,寻找高效便捷的涂层缺陷修复方法,成为延长涂层防护寿命,拓宽 C / C 复合材料应用领域以及降低其应用成本的有效途径。

  • WANG 等[18]通过激光熔覆法对 C / C 复合材料表面 SiC 涂层的损伤位置进行修复。研究表明,经历 900℃和室温热循环 20 次后,修复后涂层比原始涂层的内部缺陷更少,且在 1 500℃静态空气中氧化 10 h 后,修复后涂层试样和无缺陷涂层试样的重量变化接近,比未修复试样减少 64.8%。DENG 等[19] 使用 B4C 掺杂聚硅氮烷(PSN)对 C / C 复合材料表面的硼硅玻璃涂层进行修复。研究表明,B4C 氧化生成的 B2O3 可溶解到硼硅酸盐玻璃中,填充孔隙,形成保护膜,显著提升了涂层的自愈合性能。目前,已有报道主要针对 C / C 复合材料抗氧化涂层的修复,却鲜有针对 2 000℃以上超高温抗烧蚀涂层缺陷的修复研究。

  • 含硼聚氮硅烷(PSNB)是一种主链含有硅氮键的有机树脂,在高温下可裂解生成 SiBCN 陶瓷,具有较好的抗氧化耐烧蚀性能,常被用作陶瓷及 C / C 复合材料的胶粘剂[20-21]。因此,本文采用 PSNB 胶粘剂作为修复剂基体,以 SiC-ZrB2 粉末作为改性填料,Al2O3 作为烧结助剂,利用 PSNB 高温裂解反应产物及其高粘度和流动性等特性,对 SZ 涂层中的缺陷进行修复,并对修复前后的涂层微观结构演变和抗烧蚀性能进行对比研究,旨在为提升涂层超高温烧蚀防护性能提供有效途径。

  • 1 试验

  • 1.1 C / C 复合材料表面 SZ 涂层的制备

  • 本文所用 C / C 复合材料以 2D 针刺碳毡为预制体,CH4 为碳源,采用热梯度化学气相渗积(TCVI) 工艺在碳纤维预制体内沉积热解碳基体所得,其密度约为 1.7~1.8 g / cm3,试样尺寸为 ϕ20 mm × 5 mm。以 Si 粉、C 粉及 Al2O3 粉为原料,通过包埋法在 C / C 复合材料表面制备 SiC 内涂层,以缓解因 SiC-ZrB2 涂层与 C / C 复合材料热膨胀系数差异过大而引起的涂层开裂或脱落现象。通过超音速大气等离子喷涂法(Supersonic atmospheric plasma spraying,SAPS),采用 HEPJ-Ⅱ型高效能 SAPS 设备,在 SiC 内涂层表面制备 SZ 涂层,首先将 60~80 wt.% SiC 粉末与 20~40 wt.% ZrB2粉末混合,再将混合粉末与聚乙烯醇(PVA)溶液、无水乙醇及去离子水以质量比 4∶4∶1∶1 混合均匀,通过离心喷雾干燥设备进行造粒,以提高陶瓷粉末流动性,保证喷涂过程中能够连续并稳定地输出粉料。随后使用 SAPS 设备将造粒后粉末均匀喷涂在 C / C 基体表面,喷涂参数如表1 所示,最终得到 SiC-ZrB2 / SiC 涂层 C / C 复合材料。

  • 表1 采用 SAPS 制备 SZ 涂层所用试验参数

  • Table1 Experimental parameters used to prepare SZ coating by SAPS

  • 1.2 涂层表面缺陷的预制

  • 本试验采用大气等离子焰流烧蚀法(Atmospheric plasma ablation,SAPA),以模拟涂层所经历的超高温热流机械冲刷及烧蚀环境,在 SiC-ZrB2 涂层表面预制出圆形坑洞缺陷。喷嘴与样品表面距离为 40 mm,等离子焰流烧蚀时间为 15 s,功率为 30 kW。图1 为涂层表面缺陷预制过程示意图。

  • 图1 缺陷预制过程示意图

  • Fig.1 Schematic diagram of the defect-prefabrication process

  • 1.3 涂层缺陷的修复

  • 采用 PSNB 和异丙醇 (IPA)作为修复剂基体, SiC-ZrB2陶瓷粉末作为填充剂,以缓解 PSNB 在热处理过程中出现的体积收缩现象,并保证修复后涂层高温下仍具有良好的抗烧蚀性能。加入少量 Al2O3 粉末作为陶瓷烧结助剂,经前期大量试验总结,选取各组元质量比为 PSNB∶IPA∶SiC-ZrB2∶Al2O3 =(1.5~3)∶(0.1~0.3)∶(0.8~1.2)∶(0.3~0.7)。将混合均匀的修复剂均匀涂覆在样品表面缺陷处,在烘箱中 200℃下放置 4 h,使 PSNB 充分交联固化,最后将试样放入高温热处理炉内,在Ar 气氛中 1 400~1 600℃ 下保温热处理 1~2 h,得到修复后的涂层试样。

  • 1.4 微观结构分析

  • 采用 X 射线衍射仪(XRD,X’Pert PRO)对涂层试样的物相组成进行表征分析;采用扫描电子显微镜(SEM,JSM-6460)观察涂层试样预制缺陷后、修复前后以及氧乙炔烧蚀前后的微观形貌;采用能谱分析仪(EDS)对涂层试样的元素组成及分布进行分析。

  • 1.5 氧乙炔火焰烧蚀测试

  • 采用氧乙炔火焰测试涂层试样的抗烧蚀性能,测试参数如表2 所示。通过式(1)和(2)分别计算涂层试样的线烧蚀率(Rl,μm / s)和质量烧蚀率 (Rm,mg / s),以表征其抗烧蚀性能。

  • R1=Δdt=d1-d2t
    (1)
  • Rm=Δmt=m1-m2t
    (2)
  • 式中,d1d2 为试样中心区烧蚀前后的厚度(μm); t 为烧蚀时间(s);m1m2 为试样烧蚀前后质量 (mg)。

  • 表2 氧乙炔火焰烧蚀时所用试验参数

  • Table2 Experimental parameters used in oxyacetylene flame ablation

  • 2 结果与讨论

  • 2.1 SZ 涂层的微观形貌与物相分析

  • 图2 为 SZ 涂层试样的表面形貌及截面形貌照片,由图2a 可知,喷涂涂层表面较为平整致密,相分布均匀,无明显孔洞及裂纹。图2b 为涂层试样表面放大的 SEM 照片。从图中可以观察到凝固的扁平化液滴以及少量未完全熔融的球形陶瓷颗粒,这是等离子喷涂涂层的典型形貌[22]。等离子体焰流中心区温度高达 10 000℃以上[23],当陶瓷颗粒与焰流接触时,颗粒最外层开始熔化,同时以高达 400 m / s 以上的速度喷射在 C / C 基体表面[24-26]。此时,完全熔化的陶瓷颗粒会在基体表面形成扁平化液滴。若陶瓷颗粒直径较大,颗粒内部在到达基体表面时并未完全熔化,无法在基体表面完全铺展,因此可观察到少量仍为球形的半熔融态陶瓷颗粒。从涂层试样截面形貌照片(图2c)可知,C / C 复合材料基体、SiC 内涂层以及 SZ 外涂层界面间结合紧密,无明显裂纹。SZ 涂层较为致密,厚度为 60~70 μm。

  • 图2 SZ 涂层试样的微观形貌照片

  • Fig.2 Micromorphology photos of SZ coating samples

  • 图3 为采用 SAPS 法制备的 SZ 涂层试样的 XRD 图谱。从图中可以看出,SZ 涂层试样中除了含有 SiC 和 ZrB2外,还存在少量 ZrO2和 SiO2。这是由于喷涂过程中温度较高,当陶瓷粉末在喷枪内部时有 H2对其进行保护,但从喷枪喷出至到达 C / C 复合材料表面之间仍有极短时间暴露在大气中,导致少量 SiC 和 ZrB2 粉末发生氧化。喷涂过程可能发生的反应如下:

  • ZrB2(s)+5/2O2(g)ZrO2(s)+B2O3(g)
    (3)
  • SiC(s)+1/2O2(g)SiO(g)+C(s)
    (4)
  • 2SiC(s)+3O2(g)2SiO2(l)+2CO(g)
    (5)
  • SiC(s)+4O2(g)SiO2(l)+CO2(g)
    (6)
  • 图3 采用 SAPS 法制备的 SZ 涂层试样的表面 XRD 图谱

  • Fig.3 XRD patterns of the surface of the SZ coating samples prepared by SAPS

  • 2.2 预制缺陷后涂层的微观形貌

  • 图4 为 SAPA 法预制缺陷后涂层试样的表面形貌照片。由图4a 可知,涂层试样中心有明显圆形凹坑,C / C 复合材料基体出现部分裸露,这是由等离子焰流高速冲击涂层表面所形成的。涂层试样中的 SiC 在高温大气环境下发生氧化生成玻璃态 SiO2,在等离子焰流高速冲击下向外飞溅,并在焰流区外迅速冷却凝固,因而在凹坑周围均匀分布着溅射状玻璃态 SiO2。从表面放大像可以看出,在大气等离子焰流冲蚀环境下,裸露出的碳纤维被快速氧化,在 C / C 复合材料中留下大量孔洞(图4b)。

  • 图4 预制缺陷后涂层中心缺陷表面形貌

  • Fig.4 Surface morphology of the defect on the coating

  • 图5 为预制缺陷后涂层试样截面的微观形貌照片。图5a 为预制缺陷后的涂层试样中心缺陷处的截面形貌照片,可以看到涂层中心区在等离子焰流冲击及氧化气氛下已完全消失,高温下部分液态 SiO2 从 C / C 复合材料表面的孔隙渗入其内部[27]。图5b 为预制缺陷后的涂层试样缺陷边缘区截面形貌照片。与烧蚀前的 SZ 涂层试样相比,涂层试样的厚度明显减小。图5c 为图5b 中矩形区域的局部放大图。等离子烧蚀后的涂层试样中出现贯穿性裂纹,并且涂层试样最上层的白色相表面覆盖了一层浅灰色相。通过 EDS 分析(图5d)可知,浅灰色相为 SiO2。图5c 中白色相为原 SZ 涂层,说明表面涂层被部分氧化,氧化后的玻璃相 SiO2 在焰流高速冲击作用下向外飞溅,覆盖在涂层试样表面的缺陷边缘区。由于焰流温度极高,冲击时间较短,冷却速度快,从而产生了较大残余应力,且因涂层与 C / C 复合材料的热膨胀系数存在差异,在升温和冷却过程中两者体积收缩不一致,导致裂纹出现。

  • 图5 预制缺陷后涂层试样的微观形貌

  • Fig.5 Micromorphology of the defect coating

  • 图6 为预制缺陷后涂层试样的 XRD 图谱。从图中可以看出,在预制缺陷后的涂层试样中检测到 C 的衍射峰,这与 C / C 复合材料基体出现部分裸露有关。此外,由于涂层被氧化,故与原始涂层相比 ZrO2 及 SiO2含量增多。

  • 图6 预制缺陷后涂层试样的 XRD 图谱

  • Fig.6 XRD patterns of the defect coating

  • 2.3 修复后涂层试样的微观形貌与烧蚀性能

  • 图7 为修复后涂层试样的表面形貌照片。由图7a 可知,涂层试样缺陷处裸露出的 C / C 复合材料已被完全覆盖,修复剂经热处理后表面各相分布均匀,PSNB 在高温下可裂解生成 SiBCN 陶瓷[28],其具有较高的力学性能、热稳定性、抗氧化以及抗蠕变性能,有利于修复后涂层在烧蚀过程中表现出更好的高温稳定性。此外,PSNB 在裂解过程中具有较高的化学活性[29],可作为高熔点陶瓷的烧结助剂,使加入的 SZ 填料粉末在热处理过程更易烧结,从而可得到较为均匀的修复层(图7b)。修复后能观察到裂纹,这是由于 PSNB 高温下裂解收缩导致,在烧蚀环境下,涂层经历高温后会发生体积膨胀,少量裂纹存在有利于缓解残余应力,降低涂层与基体开裂的可能性。如何缓解修复剂高温热处理过程因收缩导致的开裂趋势,还须做进一步研究。

  • 图7 涂层中心修复处表面形貌

  • Fig.7 Surface of the repaired coating

  • 修复后涂层的 XRD 图谱如图8 所示。与原始 SZ 涂层试样相比,修复后的涂层中出现了少量 Si3N4,这是 PSNB 裂解的产物,其具有高温抗氧化、耐磨损,以及抗冷热冲击等优异性能,对材料的抗烧蚀性能有改善作用。

  • 图8 修复后涂层试样的 XRD 图谱

  • Fig.8 XRD patterns of the repaired coating

  • 图9 为修复后与未修复的涂层试样经过氧乙炔烧蚀 60 s 后的表面形貌照片。从图9a 可以看出,修复后的涂层试样经过烧蚀后表面较为完整,出现了一些新的裂纹,但仍可有效保护 C / C 复合材料基体。而未经修复的试样烧蚀中心区出现涂层脱落现象,裸露出更多 C / C 复合材料基体。由图9c 可知,修复后涂层试样的修复区出现少量微孔,这是由于烧蚀过程涂层受到高温氧化腐蚀,有气态氧化产物生成,同时受到氧乙炔焰流的机械冲蚀作用,从而在涂层试样表面留下微孔。从图9c 中 EDS 图谱可以看出,涂层经烧蚀后表面白色相产物为 ZrO2。由图9d 可知,未经修复的试样表面裸露出的 C / C 复合材料在烧蚀环境下被明显氧化,且受到了冲蚀,表面粗糙,并出现较多坑洞。将修复区表层涂层揭下后,露出内层,形貌如图9e 所示,形成了均匀致密的玻璃态 SiO2,说明外涂层及内涂层的 SiC 被氧化,形成的玻璃膜很好地保护了内部的 C / C 复合材料基体。

  • 图9 氧乙炔烧蚀后试样表面形貌照片

  • Fig.9 Surface morphology photo of the sample after oxyacetylene ablation

  • 对两种试样的线烧蚀率及质量烧蚀率进行计算,结果如表3 所示。修复后涂层试样的抗烧蚀性能远远优于未修复的涂层试样,其在氧乙炔环境下烧蚀后的线烧蚀率和质量烧蚀率分别为 0.65 μm / s 和−0.28 mg / s,比未修复试样分别降低了 83.54%和129.47%。修复后涂层试样的质量烧蚀率为负值,这是由于在烧蚀过程中,涂层发生的氧化反应为增重过程,C / C 复合材料发生的氧化反应为失重过程。缺陷修复后,试样内的 C / C 复合材料未被氧化,故涂层增重,质量烧蚀率为负值。上述结果进一步说明缺陷修复后涂层保持较高完整性,可有效阻挡氧的扩散,保护 C / C 复合材料在氧乙炔烧蚀环境下不被氧化且免受机械冲蚀。图10 为烧蚀过程中试样表面中心区温度变化曲线,可以观察到,修复后试样初始表面温度低于未修复试样,且未修复试样表面温度在短暂烧蚀后温度略有下降,出现此现象的原因为表面涂层有部分脱落,消耗部分热量,引起温度降低。在烧蚀 25 s 后,两种试样温度变化均趋于平缓,,烧蚀 60 s 后,两试样表面温度均超过 2 000℃。

  • 表3 涂层试样氧乙炔烧蚀 60 s 后的线烧蚀与质量烧蚀率

  • Table3 Line ablation rates and mass ablation rates of the coated samples after oxyacetylene ablation for 60 s

  • 图10 烧蚀中心区表面温度变化曲线

  • Fig.10 Surface temperature change curve of the ablation central area

  • 图11a 为修复后涂层试样经氧乙炔烧蚀后中心区域截面形貌照片。图11b~11d 分别为图11a 中 A、 B 和 C 区域的放大图。由图11a 可以看出,涂层试样表面有气体挥发后留下的微孔,但总体仍较为完整。烧蚀后涂层试样中浅灰色玻璃相含量增多,因其具有较好的流动性,可愈合涂层氧化产生的部分微孔和微裂纹,使涂层仍保持较完整的形貌,阻挡了高温下氧气的扩散以及氧乙炔焰流的机械冲蚀,能够实现对基体的有效保护。图11c 为烧蚀后涂层修复区与 C / C 复合材料的界面处微观形貌照片。从图中可以看出,涂层与基体结合紧密,无明显裂纹。图11d 为烧蚀边缘区的微观形貌照片。在高温氧化及机械冲蚀双重作用下,涂层表面略显疏松,但靠近基体处仍较为致密。图11e 和 11f 为未修复试样氧乙炔烧蚀后的截面形貌照片。由图可知,裸露的 C / C 复合材料在烧蚀后,进一步被氧化与冲蚀,导致凹坑直径及深度均略增大。图11e 中右上角的插图为黄色矩形区域的放大图,可见烧蚀边缘区涂层开裂,开裂处的 SiC 内涂层几乎被完全氧化。图11f 为烧蚀边缘区形貌,与修复后试样相比,由于未修复涂层中 SiC 含量较少,涂层无法产生足够玻璃相,裂纹不能及时被玻璃相填充愈合,且涂层自身完整性遭到破坏,高温环境下残余应力更大,导致涂层在烧蚀后出现了贯穿性裂纹。

  • 图11 氧乙炔烧蚀后试样的截面形貌照片

  • Fig.11 Images of the cross-sectional morphology of the samples after oxyacetylene ablation

  • 综上所述,修复后的涂层试样仍保持自身完整性,在氧乙炔烧蚀环境下,涂层可对 C / C 复合材料基体进行有效保护,阻挡氧气与 C / C 复合材料直接接触,并保护 C / C 复合材料不受氧乙炔焰流的机械冲蚀损伤,提升了涂层试样的抗烧蚀性能。

  • 3 结论

  • SZ 涂层在等离子焰流高速冲击与超高温氧化下产生圆形凹坑缺陷,裸露出 C / C 复合材料基底; PSNB 在高温下裂解生成的 SiBCN 陶瓷,与 SZ 改性粉末烧结后得到较为均匀致密的修复层,修复后涂层在耐高温陶瓷的保护与玻璃相 SiO2 的自愈合作用下能保持自身完整性,阻挡了氧气向 C / C 复合材料扩散,且缓解了氧乙炔焰流的机械冲蚀,有效保护了 C / C 复合材料。该研究可为延长高温涂层防护寿命,提升其服役性能提供有效途径。

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