Ceramic Matrix Composites (CMCs) are the subject of considerable developments in the research field of aerospace industry as a promising candidate for high temperature structural applications. These composites present a complex microstructure as they are made of a SiC matrix reinforced by SiC woven fibres.

Diagram of the drilling of the composite with two different fibres orientation: (a), parallel to the cutting speed (0°) and (b), perpendicular to the cutting speed
Diagram of the drilling of the composite with two different fibres orientation: (a), parallel to the cutting speed (0°) and (b), perpendicular to the cutting speed

Because of the hard, brittle, heterogeneous and orthotropic nature of the composite, machining process can affect the properties of the composite in service and so reduce its service life. Researchers from the University of Nottingham and Rolls Royce investigate the mechanisms, and the influence on the microstructure of the machined surface, that occurs during material removal (cutting) process [Gavalda Diaz et al, Materials Science and Engineering A (2019), doi.org/10.1016/j.msea.2018.11.037].

Their study focusses on what occurs within the composite microstructure during the cutting process in two different areas of the composite: the fibres area and the matrix area. During the cutting process, severe mechanical and thermal loads are created within the composite surface which induced plastic strain.

Concerning the fibres area, the study reveals that plastic strain generated tensile residual stresses. Investigations on the composite microstructure shows that fibres undergo a brittle fracture-dominated behaviour during the cutting process. These two observations lead to the following conclusion: in the fibres area, a heat stress gradient is responsible of the compressive residual stresses.

The authors observed damage after machining such as radial cracks in the fibres coating. These cracks are induced by mechanical loads applied during the process. Furthermore, according to the authors, the columnar preferential grain growth of the coating facilitates the development of radial cracks, which follow the grain direction.

Concerning the matrix area, the study reveals that plastic strain generates compressive residual stresses and the matrix undergoes a plastic-dominated mechanism. It can be concluded, from these two observations, that plastic strain is induced by mechanical loads.  The authors also observed that some SiC particles have been pulled out during the cutting process as well as a redeposition of melted amorphous Si on the composite surface during machining.

 

 

Effect of cutting on different SiC-based constituents of the CMC (a) fibre, (b) polycrystalline 6 H-SiC particles and (c) CVI coating with a columnar structure
Effect of cutting on different SiC-based constituents of the CMC (a) fibre, (b) polycrystalline 6 H-SiC particles and (c) CVI coating with a columnar structure

The challenging point of this study was to investigate the effect of cutting process on different SiC-based constituents (fibres, coating and particles) found in the CMC but that present a different microstructure, which generates different removal mechanisms. In this way, this paper provides a better understanding of the mechanisms that occurs during the cutting process of a SiC/SiC CMC, which is essential as it might have a non-negligible influence on the microstructure and the mechanical behaviour of the composite.