Carbon fiber-reinforced polyetheretherketone (CF/PEEK) composites exhibit characteristics such as light weight, high strength, fatigue resistance, high-temperature resistance, corrosion resistance, and recyclability, holding broad application prospects in aerospace, defense, robotics, and other fields. However, the significant modulus mismatch between carbon fiber (200-600 GPa) and the resin matrix (2-5 GPa) severely reduces stress transfer efficiency under low-velocity impact (LVI) loading. The team of Liu Huichao/Zhu Caizhen/Xu Jian from Shenzhen University addressed this interfacial modulus mismatch problem in CF/PEEK composites by proposing a novel interface regulation strategy. By constructing a gradient modulus interphase on the carbon fiber surface, they achieved simultaneous improvement in the in-plane mechanical performance and impact resistance of the composite.

On January 5, 2026, the related findings were published in the internationally renowned journal Chemical Engineering Journal (Chinese Academy of Sciences Zone 1, Top, IF: 13.2) under the title "Simultaneous improvement in-plane mechanical performance and impact resistance of CF/PEEK composites via constructing a gradient modulus interphase". The first author of the paper is Cui Jinze (Master's graduate from Shenzhen University, currently a Ph.D. student at the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences), and the co-first author is Luo Yutai (Master's graduate from Shenzhen University). Shenzhen University is the first corresponding affiliation for this article, and Assistant Professor Liu Huichao is the corresponding author.

Figure 1 Schematic diagram of constructing the gradient modulus interphase in CF/PEEK composites.

The research team utilized a PEEK derivative (PFEEKs) synthesized in their previous work—which is heat-resistant, easily impregnated, and possesses excellent compatibility—as an interfacial adhesive for CF/PEEK composites (Huichao Liu* et al. Composite Structures 2025, 362, 119107.). Using an ultra-thin fiber spreading technique, they spread and thinned 12K carbon fiber tows, preparing ultra-thin thermoplastic carbon fiber tapes with a width of 15 mm and a thickness as low as 30 μm (related work: Huichao Liu* et al. Composites Part B 2024, 284, 111718; Huichao Liu* et al. Polymer Composites 2025, 46, 13073). By employing a continuous modification process, they introduced varying contents of carboxylated carbon nanotubes (CNTs) into the carbon fiber tape, successfully constructing a microscale gradient modulus transition layer on the carbon fiber surface. Furthermore, through a layered structural design combining ultra-thin carbon fiber tapes and PEEK films, they fabricated CF/PEEK composites with both high in-plane mechanical performance and high impact resistance (see Figure 1).

Figure 2 Comparison of the flexural properties and ILSS of the CF/PEEK composites in this work with those reported in published literature.

The significant enhancement in in-plane mechanical performance intuitively verified the reinforcing effect of the gradient modulus interphase: Experimental results in Figure 2 show that the optimized CF/PEEK-1.2 composite achieved a flexural strength of 1073.0 MPa and a flexural modulus of 67.1 GPa, representing substantial increases of 52.6% and 87.4%, respectively, compared to the unmodified group. The achieved interlaminar shear strength (ILSS: 109.9 MPa) and flexural strength are superior to most reported results for similar composites in the literature.

Figure 3 Modulus distribution map and thickness of the interphase layer in CF/PEEK composites.

AFM DMT modulus mapping and microstructural characterization deeply revealed the intrinsic mechanism behind the simultaneous performance improvement: The core of the performance leap lies in the successful construction of the "gradient modulus interphase". Compared to the "cliff-like" modulus discontinuity at the interface of the unmodified composite, the introduction of 1.2 wt.% CNTs regulated the interphase thickness to approximately 361.4 nm, forming a smoothly transitioning gradient modulus layer (see Figure 3), thereby significantly alleviating stress concentration between the high-modulus fiber and the low-modulus matrix.

Figure 4 Front and back surface damage photographs and ultrasonic C-scan images of CF/PEEK composites after drop-weight impact.

The impact resistance of CF/PEEK composites is shown in Figure 4a1-a2. The CF/PEEK composite without CNTs exhibited a deep dent on the front surface after impact, while cracks on the back surface propagated in a zigzag pattern (Fig. 4a2). After adding CNTs, the shape and propagation direction of the cracks changed. With the incorporation of 0.6 wt.% CNTs, cracks propagated along the direction of the three-pronged star pattern on the back surface (Fig. 4b2). The CF/PEEK-1.2 composite showed the shortest crack length, as depicted in Fig. 4c2. Cracks in the CF/PEEK-1.8 composite propagated in a straight line, with a longer propagation length than that in the CF/PEEK-1.2 composite (Fig. 4d2).

To further quantify the impact damage, ultrasonic C-scan amplitude images were binarized, and the projected damage area on the laminate plane was calculated for all specimens (Fig. 4a3-d3). C-scan results indicated that the primary damage mode was delamination, and the calculated projected damage area was defined as the delamination damage projected area (DDPA). For the composite without CNTs, the DDPA was about 44.8 mm². After adding CNTs, both crack propagation length and damage area decreased (from 44.8 mm² to 44.2 mm²). The CF/PEEK-1.2 composite had a DDPA of 39.5 mm², a reduction of approximately 12%. The reduction in damage may be attributed to: In the absence of CNTs, the interphase could not effectively transfer stress to the carbon fibers under impact loading, leading to a larger matrix damage zone. Conversely, adding CNTs improved fiber-matrix interfacial bonding and mitigated the interfacial modulus gradient, enabling more effective stress transfer from the matrix to the fibers. Consequently, the matrix damage zone was constrained, crack propagation was inhibited, and impact damage was confined to the central region.

Figure 5 Schematic diagram of the gradient modulus interphase and impact resistance mechanism in CF/PEEK composites.

By studying the low-velocity impact behavior of CF/PEEK composites, the research team further elucidated the underlying logic of how the gradient modulus interphase reshapes the material's impact resistance. Without CNTs, weak interfacial bonding allowed cracks to propagate rapidly with little hindrance under impact loading, causing severe interface debonding, delamination, and fiber fracture. By introducing 1.2 wt.% CNTs, a crucial "gradient modulus transition interphase" was successfully constructed within the material, playing a core buffering role in stress transfer. Simultaneously, the introduced CNTs forced the crack propagation path to become more tortuous and elongated, significantly improving energy dissipation efficiency. Combined with the microscopic energy dissipation process during CNT pull-out from the matrix, this effectively curbed the lateral spread of delamination damage, thereby confining the damage to the central region to achieve higher post-impact compression strength (see Figure 5).

This study successfully addressed the interfacial modulus mismatch problem in CF/PEEK composites through a simple, industrially viable modification method. This gradient interphase design not only provides a theoretical basis for developing high-performance CF/PEEK materials but also opens a new pathway for future aerospace structural components to achieve higher degrees of weight reduction and impact resistance. This work was supported by the Guangdong Natural Science Foundation, Shenzhen Natural Science Foundation, and others.

Paper Information: Jinze Cui#, Yutai Luo#, Lingcong Kong, Changqiang Jia, Feng Bao, Jiali Yu, Caizhen Zhu, Jian Xu, Huichao Liu*. Simultaneous improvement in-plane mechanical performance and impact resistance of CF/PEEK composites via constructing a gradient modulus interphase. Chemical Engineering Journal 2026, 172690.

Link: https://doi.org/10.1016/j.cej.2026.172690

Furthermore, addressing the key scientific issue that the high melt viscosity of thermoplastic resins makes it difficult to impregnate large-tow carbon fibers or the interior of carbon fiber fabrics, easily leading to defects like voids which severely degrade the mechanical performance of carbon fiber-reinforced thermoplastic composites, the author's team employed a continuous mechanical fiber spreading technique to widen and thin large-tow carbon fibers. They also chopped the continuous carbon fiber tape into carbon fiber sheets (Chopped ultra-thin CF tape) to shorten the impregnation path for the melt, reduce void formation, and construct a gradient modulus interphase to improve the interfacial properties and reduce the porosity of CF/PA6 composites. Related work was published in Composites Science and Technology under the title "Continuous construction of gradient modulus interphase in CF/PA6 composites with enhanced interfacial properties and reduced porosity".

Paper Information: Guang Yang#, Jinze Cui#, Kewen Zeng, Yutai Luo, Feng Bao, Jiali Yu, Caizhen Zhu, Jian Xu, Huichao Liu*. Continuous construction of gradient modulus interphase in CF/PA6 composites with enhanced interfacial properties and reduced porosity. Composites Science and Technology 2025, 272, 111392.

Link: https://doi.org/10.1016/j.compscitech.2025.111392

Corresponding Author Profile:

Liu Huichao, Associate Researcher, Assistant Professor, Master's Supervisor at Shenzhen University, Shenzhen "Pengcheng Kongque" High-Level Talent. Main research areas: 1) Structure and properties of PAN-based carbon fibers; 2) Interface design and structure-function integration of carbon fiber-reinforced composites. In recent years, he has published multiple high-impact papers in top composite journals such as Composites Part B, Composites Science and Technology, and Chemical Engineering Journal.