Long Fibre Reinforced Ceramic Matrix Woven Composites (LFRCMWC), are promising candidates for advanced fields such as aerospace because of their excellent properties. These composites present a complex microstructure made of a matrix reinforced by woven fibres. The matrix easily breaks during loading process due to its brittle nature, initials cracking and poor fibres-matrix connection. This is the reason why fibres are seen as the main factor contributing to the strength of LFRCMWC. The usual way to improve the strength is to enhance the fibres arrangement. However, the matrix/fibres relationship is rarely studied.

Researchers from the Key Laboratory of Advanced Ceramics and Machining Technology of the  Tianjin University and from the Key Laboratory of Advanced Functional Composites of Beijing investigated the fibres and matrix contributions to bending strength of LFRCMWC [Wang et al., Ceramics International 45 (2019) 1143-1149, doi.org/10.1016/j.ceramint.2018.09.295]. The authors were particularly interested in the influence of matrix quality and of fibres loading direction on the bending strength.

They thus studied the bending strength of SiO2/SiO2 LFRCMWC in three different fibres directions (figure 1) and with different matrix quality. The matrix quality was characterised by the porosity (initial cracks or micro-pores) and by the distribution homogeneity of cracks and micro-pores. This distribution was measured by the Evaporation Characteristic Time (ECT) by determining the drying time of a wet sample. A small ECT is characteristic of a short drying time and so of large and few pores.

Figure 1: composite microstructure
Figure 1: composite microstructure

The authors observed a substantial influence of the matrix quality on the bending strength. They concluded that an increase, even a slight one, of the matrix quality can significantly make the composite stronger. Thus, a stronger matrix may reduce the number of internal pores and cracks which directly improves the bending strength. Furthermore, a denser matrix can provide a better matrix/fibres connection which leads to a better stress transfer from the matrix to the fibres and make, thereby, the fibres more efficient in the reinforcement role.

The authors also observed that the fibres loading direction have an influence on the bending strength. S shape fibres, fibres direction C on the figure 1, can endure more tensile stresses and deformations and thus, present a more efficient reinforcement role than tensed fibres of direction A and B.  In this way, S shape fibres present a higher tensing loading capacity which improves the bending strength. 

Figure 2: variation of bending stress with the porosity and ECT of composite.
Figure 2: variation of bending stress with the porosity and ECT of composite.

Finally, acoustic emission analysis gave valuable information on the contribution of fibres and matrix on the bending strength. The authors identified a fibre fracture peak frequency of 4.5 kHz, a fibre debonding peak frequency between 15–22 kHz and matrix crack of 30 kHz.

In this way, this paper provides a better understanding of the bending behaviour of LFRCMWC and more precisely of the contribution of the fibres and the matrix on the bending strength.