Effect of low-temperature plasma surface modification on the adhesive performance of CFRP
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摘要: 采用低温等离子体处理技术对碳纤维增强树脂复合材料(CFRP)表面进行处理，以氩气、氮气和氧气作为等离子体激发气体，通过接触角测量仪、原子力显微镜(AFM)、扫描电子显微镜(SEM)和光电子能谱仪(XPS)等测试分析手段，对CFRP表面的润湿性、粗糙度、微观形貌和化学组分进行表征，并结合拉伸剪切试验测试和失效形貌分析，研究了等离子体气体类型、放电功率和处理时间对CFRP表面理化特性和胶接性能的影响。结果表明，氩气、氮气和氧气等离子体处理可以显著改善CFRP胶接性能，当放电功率P=800W，处理时间t=20s时，与未处理相比，CFRP胶接强度分别提高了138%、172%和253%。表面测试分析可知，氩气等离子体处理后，CFRP胶接强度的增加主要是通过提高表面清洁度和增大界面粘接表面积，试样失效模式由界面失效转变为内聚失效为主的混合失效模式。与氩气相比，氮气等离子体处理后，CFRP表面生成了较多—NH2极性基团，表面活性增加，进一步提高了CFRP和胶粘剂之间界面的结合力。与以上两种气体相比，氧气等离子体刻蚀CFRP表面更为剧烈，并对表层基团进行重组，形成了极性较强—COOH官能团，试样胶接强度提高效果最佳，试样失效模式由界面失效转变为基板失效。此外，当活性粒子的密度和能量过高时，较大的等离子体刻蚀孔隙，在一定程度上会降低胶接性能。Abstract: Argon, nitrogen and oxygen were used as low-temperature plasma excitation gases to treat the surface of carbon fiber reinforced plastics (CFRP). The effects of plasma gas, discharge power and treatment time on the physicochemical properties, including wettability, roughness, microscopic morphology and chemical components of CFRP surface, were characterized by contact angle measurement, atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectrometer (XPS). The adhesive joint property was investigated through tensile shear experiment and failure morphology analysis. Compared with untreated, the tensile shear strength of CFRP adhesive joints after argon, nitrogen and oxygen plasma treatment can significantly improve the bonding performance of CFRP, and when the plasma discharge power is 800W and treatment time is 20s, the adhesive joint strength increases by 138%, 172% and 253%, respectively. The surface test analysis shows that the improvement of CFRP adhesive strength after argon plasma treatment is mainly induced by improving the surface cleanliness and increasing the surface area for interfacial adhesive, and the failure modes of samples changes from interfacial failure to mixed failure mode with cohesive failure as the main mode. Compared with argon, a greater number of polar chemical groups(—NH2)are generated on the CFRP surface after nitrogen plasma treatment, which increase the surface activity and further improve the interfacial adhesive strength between CFRP and adhesive. Compared with the above two gases, oxygen plasma etch the CFRP surface more vigorously, as well as reorganize the surface chemical groups, forming a more polar—COOH functional group, so that the specimen adhesive strength is improved most effectively, and the specimen failure mode changes from interface failure to substrate failure. In addition, under the excessively high density and energy of the active particles, the adhesive performance will be reduced to some extent with the expansion of the pores by plasma etching.
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Key words:
- CFRP /
- low temperature plasma /
- adhesive property /
- surface modification /
- wettability
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图 2 低压等离子体处理原理示意图
Figure 2. Schematic diagram of low pressure plasma processing principle
图 6 不同等离子体处理条件下CFRP试样的单搭接剪切强度
Figure 6. Lap-shear strength of CFRP samples under different plasma treatment conditions
图 7 (a) 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样的拉伸断裂形貌；(b) 不同放电功率和处理时间下，氧气等离子体处理后CFRP试样的拉伸断裂形貌
Figure 7. (a) Effect of plasma treatment with different gases on the tensile fracture morphology of the CFRP samples (P=800 W, t=20 s); (b) Tensile fracture morphologies of the CFRP samples under oxygen plasma treatment with different discharge powers and processing time
图 8 不同等离子体处理条件下CFRP试样的水接触角
Figure 8. Water contact angle of CFRP samples under different plasma treatment conditions
图 9 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样的水接触角：(a)未处理；(b) Ar；(c) N2；(d) O2
Figure 9. Effect of plasma treatment with different gases on the surface water contact angle of CFRP samples (P=800 W, t=20 s): (a) Untreated; (b) Ar; (c) N2; (d) O2
图 10 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样表面的2 D-3 D AFM图像：(a)未处理；(b) Ar；(c) N2；(d) O2
Figure 10. 2 D-3 D AFM images of CFRP samples surface treated under plasma treatment with different gases (P=800 W, t=20 s): (a) Untreated; (b) Ar; (c) N2; (d) O2
图 11 不同放电功率和处理时间下氧气等离子体处理后CFRP样品表面的SEM图像：(a)未处理；(b) P=200 W, t=30 s；(c) P=800 W, t=20 s；(d) P=800 W, t=30 s
Figure 11. SEM images of CFRP samples surface treated under oxygen plasma treatment with different discharge powers and processing time: (a) Untreated; (b) P=200 W, t=30 s; (c) P=800 W, t=20 s; (d) P=800 W, t=30 s
图 12 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样表面XPS图谱
Fig. 12 Effect of plasma treatment with different gases on the surface chemical elements of CFRP samples presented through XPS spectra (P=800 W, t=20 s)
图 13 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样表面XPS光谱：(a) C1s; (b) O1s; (c) N1s; (d) Si2p
Figure 13. Narrow-scan XPS spectra of CFRP samples surface treated under plasma treatment with different gases (P=800 W, t=20 s): (a) C1s; (b) O1s; (c) N1s; (d) Si2p
表 1 CFRP复合材料主要性能参数
Table 1. Main performance parameters of CFRP composites
Mechanical property Value Tensile modulus E11/GPa 121 Tensile modulus E22/GPa 8.6 Tensile modulus E33/GPa 8.6 Tensile modulus G12/MPa 3450 Tensile modulus G13/MPa 3450 Tensile modulus G23/MPa 2800 Poisson's ratio 0.3 Density/(kg·m−3) 1467 
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表 2 Araldite 2015主要力学性能参数
Table 2. Main mechanical property parameters of the Araldite 2015
Araldite 2015 Value Tensile strength/MPa 21.63 Shear strength/MPa 17.9 Elongation/% 0.33
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表 3 低压等离子体处理工艺参数
Table 3. Process parameters of low pressure plasma treatment
Item Nominal parameter value Plasma frequency/MHz 13.56 Power input/W 0-1000 Process pressure/Pa 100 Speed/(mL·min−1) 150
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表 4 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样表面化学元素组成及其所占比例
Table 4. Surface chemical composition and proportion of CFRP samples under plasma treatment with different gases (P=800 W, t=20 s)
Surface treatment Chemical composition in terms of atomic ratio/at% C O N Si O/C N/C Untreated 74.52 17.85 3.87 3.76 23.95 5.19 Ar 72.89 21.42 3.46 2.23 29.39 4.75 N2 66.54 19.61 11.48 2.37 29.47 17.25 O2 62.47 30.89 3.17 3.47 49.45 5.07
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表 5 放电功率P=800 W，处理时间t=20 s时，不同气体等离子体处理后CFRP试样表面XPS的C1s、O1s、N1s和Si2p分峰拟合数据
Table 5. C1s、O1s、N1s and Si2p peak-differentiating and imitating data for XPS of CFRP samples surface under plasma treatment with different gases (P=800 W, t=20 s)
Relative content of surface group/at% Surface treatment Untreated Ar N2 O2 C 1s C—C/C—H 71.92 70.95 68.01 60.36 C—O/C—N 23.71 25.49 27.96 33.37 C=O 4.37 3.56 4.03 6.27 O 1s C—O 79.57 86.13 81.28 61.76 C=O 20.43 13.87 18.72 25.96 O=C—O 0 0 0 12.28 N 1s C—NH2 100 100 100 100 Si 2p Si—C 69.21 66.32 67.17 45.47 Si—O 30.79 33.68 32.83 54.53
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