C/C Composites | Nikolai Zhivotenko

⚡️ Introduction

Carbon-carbon (C/C) composites are materials based on a carbon matrix and a braided frame of a 3-, 4- or 5-dimensional carbon fiber structure (Fig. 1). Designed for use in products of high-temperature, nuclear, and space-rocket technology. They can also be used as friction carbon ceramics in the braking systems of airplanes and trains.

The main value of C/C composites and C/SiC ceramics based on silicon carbide is that they allow the manufacture of structural elements of special equipment operating under conditions of exposure to extreme temperatures, high-speed gas flows, liquid metals, aggressive environments, mechanical and erosive wear. The most common examples are the manufacture of uncooled liquid rocket engine nozzles, low-thrust liquid fuel combustion chambers, parts of aviation gas turbine engines and industrial gas turbine units (Fig. 2).

This composite was developed for the reentry vehicles of intercontinental ballistic missiles, and is most widely known as the material for the nose cone and wing leading edges of the Space Shuttle orbiter and Buran spaceplane, however, the C/C Buran heat tiles have an antioxidant molybdenum disilicide coating (Fig. 3).

Body-reinforced C-C composite. NII ''Grafit'', Rosatom. https://strana-rosatom.ru/2020/09/28/uglerod-vezdesushhij-komu-i-kak-niigra/ — Fig. 1. Body-reinforced C-C composite.
NII ''Grafit'', Rosatom

Combustion chamber of liquid rocket engine made of C-SiC ceramic composite. JSC ''Kompozit'', Roscosmos. https://www.kompozit-mv.ru/en/nonmetallic-materials/112-oxidation-resistant-c-sic-composite.html — Fig. 2. Combustion chamber of liquid rocket engine made of C-SiC ceramic composite.
JSC ''Kompozit'', Roscosmos

Buran, http://www.buran.ru/ — Fig. 3. Buran spacecraft

⚡️ Problems

The prediction of the characteristics of composites can be carried out in two ways (Chunguang et al., 2020):

direct experiment with a set of a large number of statistical data;
computer modeling of the structure with further analysis by methods of mathematical physics and computational algorithms.

As a result of the complex spatial structure of the material (Fig. 4) and a wide range of operating temperatures, the prediction of various characteristics by direct experimental methods can be difficult or impossible. Therefore, computational methods such as the Finite Element Method are often used. And the main tool for predicting effective characteristics is the method of constructing a Representative Volume Element (Magnitsky, 2020). That is why microscopic data (Fig. 5) are the basis for predicting the properties of materials and qualitative comparison of macro-characteristics.

In order to study the microstructure characteristics of a carbon-carbon composite with volumetric reinforcement, comprehensive observations and studies of mesoscopic and microstructural characteristics are carried out. Mechanical and thermal tests are carried out in a wide temperature range - from cryogenic (120 K) to thousands of degrees (3000 K). Scanning optical and electron microscopy, micro-CT are also used to obtain the characteristics of the microstructure and the rules for the distribution of C/C composite material. In the future, the effective characteristics of the composite are determined, followed by validation with field experiments.

Fig. 4. Complex spatial structure of 3D, 4D, and 5D C/C composites

Fig. 6. Examples of workpiece and parts made of C/C composites

⚡️ Results

In the course of the project, were created:

physical model of the material;
geometric model of a representative unit (RVE unit).

A also reliably determined the characteristics of this type of material with deviations of no more than 5%. The characteristics of the microstructure show that a C/C composite with volumetric reinforcement is a heterogeneous material with cracks and pores of various sizes, which is a 3-dimensional and 4-directional body of carbon fibers as a reinforcing phase and pitch carbon as a binding matrix (Fig. 6).

⚡️ See also

In order to study the microstructure characteristics of an axially braided Carbon/Carbon (C/C) composite, a comprehensive observation and study of the mesoscopic and microstructure characteristics of an axially braided C/C composite is conducted. Scanning electron microscopy and Micro-CT were used to obtain the microstructure characteristics and distribution rules of the axially braided C/C composite material. The physical model of the material and the geometric model of the representative unit were established. At the same time, the characteristics of this kind of material are also obtained. The microstructure characteristics show that the axially braided C/C composite is a polymer with cracks and pores of different sizes, which is a three-dimensional and four-directions carbon fiber braided body as the reinforcing phase and pitch carbon as the reinforcing matrix. The microcosmic data obtained in this chapter is the basis for carrying out material property prediction and qualitative comparison of macro performance.

This paper defines the structural strength criterion for 4DL-reinforced carbon-carbon materials. For this scheme, fiber reinforcement consists of four groups of reinforcing elements, three of them are located in parallel planes with the angles of 120° between them and the fourth one is normal to them. The paper addresses the first failure of the material corresponding to its yield stress, in this point, one of the material components deviates from linear elastic behavior. A composite material is considered to be non-uniform structurally and consists of a matrix and reinforcing elements, rods. Those rods, in their turn, represent a unidirectional composite. To analyze the stress-strain state of individual components of the material, a three-level elastic model is built that uses the analytic approach at the micro level, while at higher levels it uses the finite element method. For numerical calculations, a structural cell of the material is taken. The boundary conditions provide small to negligible influence of the edge effects, thus simulating the behavior of the infinite volume of the material. For the material components, local strength criteria are introduced, where the fields of the criterion quantities are averaged over the volume of the structural cell. The strength surface of the material that corresponds to its first failure is obtained, and the conclusion is made that the suggested criterion provides a reasonable agreement with the available data on the typical carbon-carbon composite characteristics. Based on the calculated dependencies of the material’s yield stress on the load direction, a procedure is suggested to identify the model parameters based on the material failure behavior analysis using standard tensile and compressive tests. Estimated discrepancies between the results calculated using the suggested criterion and those obtained using the limiting stress criterion for biaxial stress states are given. It is shown that the discrepancy may reach tens of percent and in some cases the material strength increases in comparison with that in the uniaxial stress state. The results are subject to verification tests in order to verify the model for advanced spatially reinforced carbon-carbon composite materials.