In-situ thermal Raman mapping and stress analysis of CNT/CF/epoxy interfaces

Abstract

A study of the interfacial behavior and internal thermal stress distribution in fiber-reinforced composites is essential to assess their performance and reliability. CNT/carbon fiber (CF) hybrid fibers were constructed using electrophoretic deposition. The interfacial properties of CF/epoxy and CNT/CF/epoxy composites were statistically investigated and compared using in-situ thermal Raman mapping by dispersing CNTs as a Raman sensing medium (CNTR) in a resin. The associated local thermal stress changes can be simulated by capturing the G' band position distribution of CNTR in the epoxy at different temperatures. It was found that the G' band shifted to lower positions with increasing temperature, reaching a maximum difference of 2.43 cm−1 at 100 °C. The interfacial bonding between CNT/CF and the matrix and the stress distribution and changes during heat treatment (20–100 °C) were investigated in detail. This work is important for studying thermal stress in fiber-reinforced composites by in-situ thermal Raman mapping technology.

1. Introduction

Raman spectroscopy, as a newly-developed technique, is non-contact, effective, powerful and precise in micro-scale spatial solution[1], which has been widely applied to study both doping and thermal effects on microstructures of carbon nanotube (CNT) or graphene samples[2–5]. Furthermore, CNTs have been used as wireless sensing media and reinforcing materials in CNT/polymer composites for monitoring load transfer, residual stress, and thermal or mechanical strain[6] due to their perfect structure, and electrical, thermal and mechanical properties[7,8]. Studies have shown that the Raman signal of CNTs strongly depends on external stress or strain[9]. In detail, the Raman G' band (about 2 660 cm−1) of CNTs is very sensitive to deformation, which shifts significantly from its original position to a lower wavenumber position when CNTs are under tension and to a higher wavenumber position when under compression.

Carbon fiber (CF) reinforced polymer composites with high strength, stiffness and excellent thermal properties have great potential applications at room temperature and hyperthermal temperatures. However, the properties of interfacial bonding between carbon fiber and polymer matrix need improving with the fast development of industries. In particular, measuring and analyzing interfacial bonding properties is an important procedure for these composites. CNT-CF hybridized fiber (or called “fuzzy” fiber[10]) is a new generation composite, which was proposed in recent years for resolving the poor adhesion at the interface of CFs and polymeric matrix[11–13]. CNT-CF hybridized fibers are mainly prepared by chemical vapor deposition (CVD), chemical grafting, and electrophoretic deposition (EPD). The CVD method requires a rather high temperature of more than 1 000 °C for a long time and a complex carbon-hydroxide-oxygen atmosphere is detrimental to the strength and rigidity of CFs. The metal catalysts introduced on the CF surface are difficult to remove and will also affect the interfacial properties of CF based composites. Chemical grafting in solution requires chemical treatment of both CFs and CNTs to make them bind to each other. Still, their free contact and grafting characteristics of CFs in solution usually lead to uneven loading of CNTs. EPD is a method that utilizes an applied electric field to induce the directional movement of charged particles. When a neutral solution is used as a medium, the surface etching effect on CFs is minimized, and the performance deterioration is minimized. In addition, the deposition uniformity is mainly affected by the dispersion of CNTs in the suspension medium, and CNTs can be uniformly loaded on the surface of CFs by the effect of charge neutralization. EPD is a simple, fast and effective method for well-dispersed CNTs suspension systems.

The interface plays an important role in the application of composites, especially in certain conditions such as high-temperature environments, which makes local stress and temperature at the interface a study interest. Zhang et al. have successively reported a series of interfacial welding techniques using CNT and graphene networks, demonstrating their outstanding ability to improve phonon and electron transport at composite interfaces and verifying the feasibility of CNTs as thermal stress conductors[14–17]. Raman mapping technique has been used to map the internal stress distributions in electrical devices at dif...