1
Definition, Function, and Design of Interface in Ceramic-Matrix Composites

1.1 Introduction

To realize the advantages of operating systems under high-temperature conditions, it is necessary to master the properties of a large number of high-temperature materials and components. For example, a significant increase in the gas temperature will significantly increase the gas turbine efficiency. The introduction of new materials and new technology has gradually improved the high-temperature performance of gas turbine engine for more than 70 years, but the development of cooling methods and solutions has contributed more than 75% to the performance improvement (Li 2018).

Although component cooling methods and engine material properties have improved significantly, most high-temperature alloys currently operate at temperatures above 90% of their original melting point. Higher operating temperatures are required for more efficient engines, which will require higher component temperatures. As the operating temperature continues to increase, new materials with higher thermo-mechanical and thermo-chemical properties are required to meet high-temperature structural applications. Ceramic-matrix composites (CMCs) are considered to have the potential to provide high strength, high toughness, creep resistance, low notch sensitivity, and environmental stability to meet the needs of future high-performance turbine engines (Li 2019).

Figure 1.1 shows the tensile stress-strain curve of monolithic ceramic and fiber-reinforced CMCs. For a monolithic ceramic material, when it is subjected to tensile stress, it appears as elastic deformation at low stress level; as the stress increases, cracks occur in the defect region of the material, and the cracks rapidly expand, causing the material to undergo brittle fracture. When the CMC material is subjected to tensile stress, it is elastically deformed before matrix cracks; as the tensile stress increases, the matrix begins to crack, and the fibers begin to debond and play a role of crack bridging; as the tensile stress increases further, the cracks become saturated, and the bridging fibers begin to pull out; as the tensile stress continues to increase, the fibers begin to break until the material reaches the highest strength. The fracture modes of monolithic ceramics and CMCs are different, mainly because the interface plays a role in the fracture process of CMCs. The interface is a special domain between the matrix and the reinforcement. It is the link between the fiber and the matrix, and also a bridge for load transfer. The structure and properties of interphase directly affect the strength and toughness of CMCs.

Figure 1.1 The tensile stress-strain curves of monolithic ceramics and fiber-reinforced CMCs.

1.2 The Definition of Interface in Ceramic-Matrix Composites

CMCs possess good damage tolerance, mainly due to frictional sliding at the fiber-matrix interface, and the interphase enables frictional sliding to occur between the fiber and the matrix. The damage tolerance is manifested in CMC with 1% ductility, and the sensitivity to notches is comparable with that of aluminum alloys. CMCs also exhibit good room-temperature fatigue resistance. The fatigue strength (i.e. fatigue failure does not occur when the stress is less than the fatigue strength) is about 90% of the ultimate tensile strength, but the fatigue damage increases with temperature. The tensile strength of CMCs is usually volume-stable as the damage tolerance ability of CMCs decreases the scaling effect that often occurs in the monolithic ceramic materials due to the weakest link. Due to crack deflection and crack tip resistance, CMCs can have cracks that can cause catastrophic failure of monolithic ceramics without fracture. This thermomechanical performance is particularly suitable for producing large, static load, and high-temperature structural components.

CMCs can be divided into two types, i.e. oxide materials and non-oxide materials. Oxide composites include oxide fibers (e.g. Al2O3), interphase, and matrix. If any of the aforementioned three components contains non-oxide material (for example, SiC), the composite is classified as non-oxide CMC.

A lot of research and development work is conducted on non-oxide CMCs, especially SiC fiber-reinforced SiC matrix composite (SiC/SiC CMC) using pyrolytic carbon (PyC) or boron nitride (BN) as the fiber interface layer. Non-oxide CMCs have good high-temperature properties, such as creep resistance and microstructure stability. They also have high thermal conductivity and low thermal expansion, which show a good resistance to thermal stress. SiC/SiC composites are very suitable as thermal load components, such as chamber throat, flap, blade, and heat exchanger. Oxide CMCs (for example, oxide fiber-reinforced porous oxide-matrix composite without interphase) have excellent oxidation resistance, resistance to alkaline corrosion, low dielectric constant, and low price.

Both oxide and non-oxide CMCs exhibit some disadvantages. Non-oxide CMCs (for example, SiC/SiC) exhibit brittleness at intermediate temperatures (about 700 °C). Brittleness is more severe under cyclic loading conditions as oxygen enters from the cracks in the matrix and reacts with the interphase and fibers forming oxidation products, leading to the propagation of the matrix cracks. These oxidation reaction products limit the friction sliding mechanism between the fiber and the matrix inside the material, which can improve the toughness of CMCs. Although this oxidation effect does not occur when the stress is below the proportional limit stress, design and operation experience has shown that it is necessary to consider in advance that the overload stress exceeds the proportional limit stress. Therefore, the local brittleness has become the main limitation of non-oxide CMCs.

Compared with SiC/SiC, oxide CMCs do not undergo oxidation embrittlement, but have a temperature limit (about 1000 °C), which is related to creep and sintering. Moreover, the interface technology of oxide CMCs is less mature than that of non-oxide CMCs. The performance data of most oxide CMCs are obtained in systems without interphase, and the damage tolerance is based on a porous matrix.

1.2.1 Non-oxide CMCs

SiC/SiC CMCs maintain the advantages of SiC matrix such as high-temperature resistance, high strength, low density, and oxidation resistance, realize the strengthening and toughening effect of SiC fiber, and effectively overcome the fatal disadvantages of monolithic ceramics that are brittle, crack sensitivity, and low reliability. Compared with superalloys, SiC/SiC CMCs have lower density (usually 2.0-3.0 g/cm3, only 1/3-1/4 of superalloys) and higher temperature resistance (>1200 °C under non-cooling conditions) (Liu et al. 2018).

After decades of research, a variety of CMC preparation processes have been developed. Representative processes include chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP) process, and melt infiltration (MI). The main difference between the three processes is the densification of the SiC matrix.

1. (1) CVI process uses a gaseous precursor (for example, trichloromethylsilane) to pyrolysis and deposit on the surface of the SiC fiber to obtain an SiC matrix.
2. (2) PIP process usually infiltrates the fiber preform in liquid precursors (such as polycarbosilane), the precursors are ceramicized by high-temperature pyrolysis, and the infiltration and pyrolysis process is repeated to obtain a dense SiC matrix.
3. (3) Reactive melt infiltration (RMI) is the infiltration of molten silicon into a porous carbon preform, and the reaction of carbon and silicon generates an SiC matrix.
4. (4) Non-reactive melt infiltration infiltrated the molten silicon into the pores of the matrix, which mainly plays a filling role, and no reaction between silicon and carbon occurs.

The fundamental physical and mechanical properties of SiC/SiC composites prepared by different processes are shown in Table 1.1.

Table 1.1 Fundamental physical and mechanical properties of SiC/SiC composites prepared by different processes.