# Study on Microstructure Characteristics of Axially Braided Carbon/Carbon Composites Based on SEM and Micro-CT

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

#### 2.1. Axially Braided C/C composite Material

#### 2.2. Sample Preparation and Instrument

#### 2.2.1. The Experimental Materials

#### 2.2.2. Low Damage Sample Preparation Method

#### 2.2.3. Experimental Equipment and Operative Conditions

^{3}thin sections, and the surface was polished and polished. With alcohol as solvent, ultrasonic cleaning equipment is used to clean the surface dirt and powder formed during processing. The ambient temperature of the test is 20 °C. In order to ensure the accuracy of the measurement results, it is carried out on a dust-free test platform.

#### 2.3. Dimensions and Representative Units of Axially Braided C/C Composites

## 3. Results

#### 3.1. Surface Structure Characteristics of Axially Braided C/C Composites

#### 3.1.1. Surface Microstructure Characteristics of the Reinforcing Phase

#### 3.1.2. Surface Microstructure Characteristics of the Matrix Phase

#### 3.1.3. Surface Microstructure Characteristics of the Interface

#### 3.2. Internal Mesoscopic/Microstructure Characteristics

## 4. Discussion

- The fiber rods in the Z-direction of axially braided C/C composites are Φ1.2 mm. The rods are arranged in a regular triangle. The center-to-center distance between the rods is about 3.2 mm. The fiber bundles are tiled in clearance of fiber rod, and the fiber bundles form 60° with each other, and a period of three layers of fibers with a distance of about 2.7 mm. The other parts of the composite material are filled with matrix charcoal to ensure the overall stability of the composite material. The carbon fiber filaments diameter obeys the log-normal distribution. The average axial fiber filaments diameter is 6.46 μm, and the variance is 0.04. The radial average is 6.84 μm, and the variance is 0.038.
- The interface between fiber rods (fiber bundles) and matrix charcoal in C/C composites is a weak bond, there are obvious pores between fiber rods (bundles) and matrix charcoal, and they are basically penetrating. The matrix charcoal Existing between carbon fiber rods, between layers of fibers, and in XY-direction fiber bundles. The pores and cracks are randomly distributed in the matrix charcoal.
- The axially braided C/C composite material is a porous material, and the pores are mainly distributed in the matrix charcoal, the interface, and the fiber rod (fiber bundles). The overall porosity of the material is between 0.045 and 0.055. The pore shape of the fiber rod is nearly triangular, and the porosity is about 0.043–0.0569. The interface porosity is between 0.254 and 0.3, the normal mean is 0.27, and the variance is 0.017. The porosity in the matrix is between 0.0327 and 0.04, which is normal mean of 0.036 and a variance of 0.015.
- The pore size distribution of axially braided C/C composites is concentrated in three areas: macro pore areas larger than 10μm, mesoscopic pore areas 0.1–10 μm, and micro pore areas smaller than 0.1 μm. The largest proportion of macro pores greater than 90 μm is the largest, above 0.7. The mesoscopic pores between 10 and 90 μm account for approximately 0.2–0.25, small pores between 0.1 and 10μm account for 0.03–0.05, and the micro pores little than 0.1 μm account for the smallest proportion, under 0.01. The macro pores and mesoscopic pores mainly exist in the matrix charcoal phase and interface phase of the material, while the small and micro pores mainly exist in the fiber rods and fiber bundles of the material. The pores in the fiber rod can be equivalent to a spherical shape, and the pore diameter obeys a normal distribution. The average value is 1.96 μm, and the variance is 0.04. The pores in the matrix and the interface can be regarded as ellipsoids with different long and short axes, and their sizes obey the log-normal distribution. The average sizes of the long semi axes corresponding to the pores in the matrix and interface are $94.2\text{}\mathsf{\mu}\mathrm{m}$ and $138.8\text{}\mathsf{\mu}\mathrm{m}$. The variances are 0.55 and 0.51. The average sizes of the short semi axes corresponding to the pores in the matrix and interface are $85.2\text{}\mathsf{\mu}\mathrm{m}$ and $50.6\text{}\mathsf{\mu}\mathrm{m}$. The variances are 0.47 and 0.42.

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**The braided form and manufacturing procedure of axially braided C/C composite material (

**a**) the braided form of axially braided C/C composite material.The intersecting directions of the fiber rod and fiber bundle are shown in the figure; (

**b**) the manufacturing procedure of axially braided C/C composite material.

**Figure 3.**Composite structure, representative unit and equivalent structure (The diagram illustrates the relationship between the three). (

**a**) Composite structure; (

**b**) The local element; (

**c**) Representative volume element; (

**d**) The equivalent structure after homogenization.

**Figure 4.**Representative volume element (RVE) unit of axially braided C/C composite (This is data obtained from the observation of 100 samples.)

**Figure 5.**Surface microstructure of the reinforcing phase of axially braided C/C composites. (

**a**) Axial micrograph of axially braided C/C composites by SEM; (

**b**) Radial micrograph of axially braided C/C composites by SEM; (

**c**) Micrograph of the fiber rod end surface by SEM; (

**d**) Micrograph of the fiber bundle end surface by SEM; (

**e**) Fiber morphology in the reinforcement phase by SEM; (

**f**) Interface morphology between fibers and matrix by SEM; (

**g**) Cross-section morphology of fiber rod by SEM; and (

**h**) Cross-section morphology of fiber bundle by SEM.

**Figure 6.**Fiber diameter distribution of axially braided C/C composites in different directions (more than 1000 fibers).

**Figure 7.**Surface microstructure characteristics of the matrix phase of axially braided C/C composite.

**(a)**Micrograph of the carbon matrix phase by SEM and

**(b)**morphology of carbon matrix pores by SEM.

**Figure 10.**Surface microstructure characteristics of the interface of axially braided C/C composites. (

**a**) Microstructure of interface layer of fiber rod by SEM; (

**b**) Microstructure of interface layer of fiber bundle by SEM.

**Figure 11.**Sectional view of the physical model of the axially braided C/C composites microstructure feature by Micro-CT.

**Figure 12.**Three-dimensional spatial structure of the axially braided C/C composites interface by Micro-CT.

**Figure 13.**Three-dimensional structure of axially braided C/C composite matrix charcoal by Micro-CT.

Name of Material | Dimensions | Forms | Research Carried Out |
---|---|---|---|

Direct braided and puncture C/C composites | 4D | The microstructure was analyzed by SEM, and the parameters of fiber rod diameter and interface defects were studied [17]. | |

Wound braided C/C composites | 2D/3D | A study on the correlation between weaving technology and performance was conducted by Micro-CT [18]. | |

Three-dimensional four-way braided C/C composites | 4D | Progressive damage analysis under bending load was carried out and crack propagation was observed by SEM [19]. | |

Multidimensional braided C/C composites | nD | A microscopic model was established to analyze the elastic properties and damage properties of the material [20]. | |

Needled carbon/carbon composites | 2.5D | The effects of acupuncture techniques and inclusions on mechanical properties were studied. Numerical prediction of performance based on SEM [21,22,23] |

**Table 2.**Axially braided C/C composite material braided structure parameters (The data obtained from the observation of 100 samples).

Fiber Rod Diameter | Fiber Bundle Section Size | Fiber Rod Braid Spacing | Fiber Bundle Monolayer Height | Interfacial Height |
---|---|---|---|---|

1.2 ± 0.1 mm | 1.19 × 0.95 mm | 3.2 ± 0.1 mm | 0.95 ± 0.05 mm | 0.02 ± 0.01 mm |

**Table 3.**Volume content of fiber reinforced phase in axially braided C/C composites (The data obtained from the observation of 100 samples).

Item | Area (mm^{2}) | Total Fiber area (mm^{2}) | Volume Content (%) |
---|---|---|---|

Fiber rod | 1.155 | 0.7884 | 68.26 |

Fiber bundle | 1.060 | 0.664 | 62.64 |

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**MDPI and ACS Style**

Wang, C.; Tang, M.; Liu, W.; Zhu, T.
Study on Microstructure Characteristics of Axially Braided Carbon/Carbon Composites Based on SEM and Micro-CT. *Materials* **2020**, *13*, 1414.
https://doi.org/10.3390/ma13061414

**AMA Style**

Wang C, Tang M, Liu W, Zhu T.
Study on Microstructure Characteristics of Axially Braided Carbon/Carbon Composites Based on SEM and Micro-CT. *Materials*. 2020; 13(6):1414.
https://doi.org/10.3390/ma13061414

**Chicago/Turabian Style**

Wang, Chunguang, Min Tang, Weikai Liu, and Tao Zhu.
2020. "Study on Microstructure Characteristics of Axially Braided Carbon/Carbon Composites Based on SEM and Micro-CT" *Materials* 13, no. 6: 1414.
https://doi.org/10.3390/ma13061414