A method for evaluating the CVD densification effect of carbon-carbon composites

Abstract

This invention discloses a method for evaluating the CVD densification effect of carbon-carbon composite materials. First, a long block with a height equal to the thickness of the carbon-carbon composite material is cut along its span. Then, this long block is segmented along both the original span and thickness directions to obtain several block samples. All block samples are prepared according to the testing requirements of an optical microscope to obtain test samples. The test samples are placed on an optical microscope, with the cut surface of the original carbon-carbon composite material as the observation surface. Measurements are taken of the surface and center of the test samples. During the measurement process, a perfectly circular carbon fiber is selected as the observation unit, and the thickness of the deposited carbon on the pyrolytic carbon is measured. The CVD densification effect of the carbon-carbon composite material is then evaluated. This testing method is relatively simple and highly operable.

Description

Technical Field

[0001] This invention belongs to the field of material preparation effect testing, specifically relating to a method for evaluating the CVD densification effect of carbon-carbon composite materials. Background Technology

[0002] Carbon-carbon composite materials not only possess excellent structural properties such as high specific strength and high specific modulus, but also exhibit superior thermophysical properties, effectively meeting the comprehensive performance requirements of the aerospace industry under high temperature and high speed conditions. They have already been applied to next-generation aerospace brake discs. Carbon-carbon composite materials are composites with carbon fiber as the skeleton and matrix carbon denser on the carbon fibers. The entire system is composed entirely of carbon elements, and its fiber structure is a multiphase heterogeneous mixture. The mechanical, thermophysical, and tribological properties of this material are closely related to its composition.

[0003] The matrix carbon in carbon-carbon composites is mainly formed during the CVD process. Isothermal CVD (ICVI) is a simple process, not limited by product geometry, has good process repeatability, and can be mass-produced. Furthermore, the deposited carbon can produce a uniform, rough pyrolytic carbon layer, resulting in good product quality and performance, with minimal decrease in the coefficient of friction under high energy loads. It is an ideal method for matrix carbon deposition. However, because the deposition process is greatly affected by gas diffusion efficiency, the permeability of the material gradually decreases with the deposition of matrix carbon in ICVI, thus significantly affecting the ICVI deposition rate and leading to inconsistent deposition results.

[0004] Although ICVI technology is relatively mature both domestically and internationally, key technical differences still exist between different manufacturers (raw materials, gas flow rate, deposition time, deposition temperature, etc.). Different process routes lead to variations in matrix carbon deposition effects, and due to differences in product thickness, the deposition results also vary significantly. Some carbon-carbon composites have a higher carbon deposition on the surface, resulting in a higher surface density, while the central area has a lower carbon content, leading to a lower density in the middle. Other carbon-carbon composites, due to their larger size, exhibit significant differences in deposition effects and densities between the internal, central, and external areas. Therefore, evaluating the overall matrix carbon deposition effect of a carbon-carbon composite material becomes an important method for evaluating ICVI production efficiency and even product performance.

[0005] Current methods for evaluating the deposition effect of matrix carbon mainly rely on observing cross-sectional metallographic images. The anisotropy of the material is measured by observing the reflection of polarized light by pyrolytic carbon and measuring the extinction angle. However, due to significant identification errors in measuring the extinction angle, the reliability of this evaluation method is low. It can only qualitatively assess whether the product is a rough layer, smooth layer, etc., and cannot quantitatively or semi-quantitatively compare products. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a simple, highly operable, and semi-quantitative method for evaluating the CVD densification effect of carbon-carbon composite materials.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention discloses a method for evaluating the CVD densification effect of carbon-carbon composite materials. First, a long block with a height equal to the thickness of the carbon-carbon composite material is cut along its span. Then, this long block is segmented along both the original span and thickness directions to obtain several block samples. All block samples are prepared according to the testing requirements of an optical microscope to obtain test samples. The test samples are placed under an optical microscope, using the cut surface of the original carbon-carbon composite material as the observation surface. Measurements are taken at selected observation points on the surface and middle of the test samples. At these observation points, perfectly circular carbon fibers are selected as the measurement units to measure the thickness of pyrolytic carbon deposited on the carbon fiber surface. Finally, based on the measurement results, the CVD densification effect of the carbon-carbon composite material is evaluated. When the average surface pyrolytic carbon thickness is ≥18 μm, and the ratio of the average middle pyrolytic carbon thickness to the average surface pyrolytic carbon thickness is >0.8, it is classified as Grade I. When the average surface pyrolytic carbon thickness is ≥18 μm, and the ratio of the middle pyrolytic carbon thickness to the average surface pyrolytic carbon thickness is 0.6–0.8, it is classified as Grade II. When the average surface pyrolytic carbon thickness is ≥18 μm, and the average intermediate pyrolytic carbon thickness is <0.6 μm, it is classified as Grade III; when the average surface pyrolytic carbon thickness is 16–18 μm, and the average intermediate pyrolytic carbon thickness is >0.8 μm, it is classified as Grade II; when the average surface pyrolytic carbon thickness is 16–18 μm, and the average intermediate pyrolytic carbon thickness is 0.6–0.8 μm, it is classified as Grade III. The thickness is 16-18 μm, and the average thickness of the intermediate pyrolytic carbon is <0.6 μm. It is classified as Grade IV. When the average thickness of the surface pyrolytic carbon is 12-16 μm, and the average thickness of the intermediate pyrolytic carbon is >0.8 μm, it is classified as Grade III. When the average thickness of the surface pyrolytic carbon is 12-16 μm, and the average thickness of the intermediate pyrolytic carbon is ≤0.8 μm, it is classified as Grade IV. When the average thickness of the surface pyrolytic carbon is ≤12 μm, it is classified as Grade IV.

[0009] This invention provides a method for evaluating the CVD densification effect of carbon-carbon composite materials. By selecting perfectly circular carbon fibers as the observation unit and measuring the thickness of the deposited carbon on pyrolytic carbon, the CVD densification effect of carbon-carbon composite materials can be evaluated semi-quantitatively. The inventors found that selecting perfectly circular carbon fibers as the measurement unit at the observation point is necessary to accurately evaluate the results. If the carbon fibers are not perfectly circular, the test results will be affected because the cross-section is not perpendicular.

[0010] The method for evaluating the CVD densification effect of carbon-carbon composite materials of the present invention evaluates the CVD densification effect by quantitatively obtaining the carbon deposition thickness of a single carbon fiber deposited in the carbon-carbon composite material and calculating the ratio of the average pyrolytic carbon thickness at the center of the cut surface of the carbon-carbon composite material to the average pyrolytic carbon thickness on the surface. In this invention, Grade I is superior to Grade II, which is superior to Grade III, which is superior to Grade IV. That is, the thicker the average pyrolytic carbon thickness on the surface and the larger the ratio of the average pyrolytic carbon thickness at the center to the average pyrolytic carbon thickness on the surface, the better. A larger ratio of the average pyrolytic carbon thickness at the center to the average pyrolytic carbon thickness on the surface indicates a smaller difference between the center and the surface, and better uniformity.

[0011] In a preferred embodiment, a long block with a width of 18-22 mm and a height equal to the thickness of the carbon-carbon composite material is cut along the span of the carbon-carbon composite material.

[0012] After the above-mentioned block is made into a test sample, the cut cross-section is the surface to be observed, and it is necessary to ensure that the surface to be observed is smooth and flat.

[0013] In a preferred embodiment, the length of the block sample is 20-30 mm and the height is 5-10 mm. In this invention, the length of the block sample is the distance measured along the span direction of the original carbon-carbon composite material, and the height is the distance measured along the thickness direction of the carbon-carbon composite material. When cutting, the edges are neat and the surface is flat. After the sample is segmented, it is marked sequentially from the inside out and from left to right.

[0014] In a preferred embodiment, the sample preparation process is as follows: a block sample is placed in a mold, then liquid epoxy resin is injected, cured, and then polished to obtain the sample. The liquid epoxy resin is composed of epoxy resin, butylene phthalate, and ethylenediamine.

[0015] In this invention, the sample preparation process for optical microscope testing is as follows: a block sample is placed in a cylindrical plastic mold coated with a release agent (Vaseline), and then the mold is placed in a vacuum device to create a vacuum of 9.1–9.5 × 10⁻⁶. 4At a pressure of Pa, turn off the vacuum pump and air inlet valve, and inject liquid epoxy resin from the top of the device to cover the sample by about 3 mm. Remove the sample and allow it to cure in air for about 1 hour. Before injecting the resin, add 1 / 4 of the resin weight of butylene phthalate as a solvent to dissolve the resin into a liquid state; then add 1 / 12 of the resin weight of ethylenediamine as a curing agent to harden the resin. After polishing the sample, perform fine polishing with 2.5 μm and 1 μm alumina suspensions. After fine polishing, the sample should be smooth and free of marks, and under a 100× microscope, no fine scratches should be visible, there should be no trailing, and the pores should be fully exposed, reflecting the true appearance.

[0016] In a preferred embodiment, the optical microscope is equipped with a polarizer and a rotatable analyzer.

[0017] In a preferred embodiment, the carbon fiber is an independent carbon fiber. The inventors have found that the carbon fiber should be relatively independent, and clustered carbon fibers should not be selected, otherwise it will affect the results.

[0018] In a preferred embodiment, the selection of observation points is still based on the positioning of the long block, and the distance between adjacent observation points on the surface is controlled to be between 20 and 30 mm. The observation points in the middle area should be within 10 mm above and below the thickness center line, preferably within 5 mm, and the distance between adjacent observation points in the middle area should be between 20 and 30 mm.

[0019] In the preferred approach, at least three perfectly circular carbon fibers are selected as the measurement unit at any given observation point.

[0020] Beneficial effects

[0021] The method for evaluating the CVD densification effect of carbon-carbon composite materials of the present invention selects perfectly circular carbon fibers as the observation unit and measures the thickness of the deposited carbon on the pyrolytic carbon, thereby semi-quantitatively evaluating the CVD densification effect of carbon-carbon composite materials. This test method is relatively simple and highly operable. Attached Figure Description

[0022] Figure 1 Example 1 Sampling diagram;

[0023] Figure 2 A schematic diagram showing the positional relationship of the segments in Example 1;

[0024] Figure 3 A schematic diagram of the observation point locations based on the long block in Example 1;

[0025] Figure 4 A schematic diagram of the observation point locations based on the long block in Example 1;

[0026] Figure 5Metallographic structure diagrams of the surface and intermediate observation points in Example 1.

[0027] Figure 6 Metallographic structure diagrams of the surface and intermediate observation points in Example 2.

[0028] Figure 7 Metallographic structure diagrams of the surface and intermediate observation points in Example 3.

[0029] Figure 8 Metallographic structure images of measurement points of carbon fibers with different morphologies in Comparative Example 1, among which Figure 8 A is the metallographic structure diagram of the circular carbon fiber measurement point. Figure 8 B is the metallographic structure diagram of the non-circular carbon fiber measurement point. Detailed Implementation

[0030] Example 1

[0031] 1. Sampling

[0032] Using a cutting machine, cut a rectangular strip with a span of 75mm, a width of 20mm, and a height equal to the thickness of the carbon disk (24mm). A sampling diagram is shown below. Figure 1 As shown, the cross-section of the sample along the radial direction is the surface to be observed, and this surface should be smooth and flat. The number of segments to be divided into is determined based on the span of the carbon disk, and a segmentation diagram is shown below. Figure 2 As shown, each sample segment is 25mm long and 8mm high, with neat edges and a flat surface. A total of 6 surface and middle block samples were obtained. After the samples were segmented, they were marked sequentially from the inner diameter to the outer diameter.

[0033] The sample was placed in a cylindrical plastic mold coated with a release agent (Vaseline), and then the mold was placed in a vacuum device to create a vacuum of 9.1–9.5 × 10⁻⁶. 4 At a pressure of Pa, turn off the vacuum pump and air inlet valve, and inject liquid epoxy resin from the top of the device to cover the sample by about 3 mm. Remove the sample and allow it to cure in air for about 1 hour. Before injecting the resin, add 1 / 4 of the resin weight of butylene phthalate as a solvent to dissolve the resin into a liquid state; then add 1 / 12 of the resin weight of ethylenediamine as a curing agent to harden the resin. After polishing the sample, perform fine polishing with 2.5 μm and 1 μm alumina suspensions. After fine polishing, the sample should be smooth and free of marks, and under a 100× microscope, no fine scratches should be visible, there should be no trailing, and the pores should be fully exposed, reflecting the true appearance.

[0034] 2. Measurement

[0035] All measurements were performed on an optical microscope equipped with a polarizer and a rotatable analyzer. Measurement positions were as follows: Figure 3 , Figure 4 As shown. Measurement locations should be evaluated on the surface and center of the carbon disk. Surface observation points should be between 20 and 30 mm away, while observation points in the central area should be located in the exact center of the sample, with adjacent observation points between 20 and 30 mm away.

[0036] At the observation points, perfectly circular carbon fibers (those with non-circular cross-sections that are not perpendicular) were selected as the measurement units, and the thickness of the deposited carbon on the pyrolytic carbon was measured. The selected carbon fibers should be relatively independent; clustered carbon fibers should be avoided as they may affect the results. The observation points should encompass representative areas from the inner diameter to the outer diameter and from the surface to the center, as shown in Table 2.

[0037] 3. Collect and analyze statistical data.

[0038] Then, based on the test results, the quality of the carbon deposition on the product matrix—the thickness of the deposited carbon—was evaluated. The metallographic image and results of the pyrolytic carbon from the carbon disk in Example 1 are shown. Figure 5 As shown in Table 2, the average thickness of the surface pyrolytic carbon is 18.53 μm, and the thickness of the intermediate pyrolytic carbon is 0.81 μm. Based on the evaluation results in Table 1, this product is classified as Grade I.

[0039] Table 1. Criteria for Judging the Carbon Deposition Effect of Pyrolysis Carbon Deposition Products

[0040]

[0041]

[0042] Table 2. Carbon deposition thickness at various locations in Example 1.

[0043]

[0044] Example 2

[0045] Due to production issues, a company produced a batch of defective product B. The sampling method was the same as in Example 1. Random samples were obtained from the surface and intermediate sections using this technique. Statistical data was collected and analyzed to evaluate the quality of the carbon deposition in the product matrix—the thickness of the deposited carbon. The metallographic image and results of the pyrolytic carbon from the carbon disk in Example 1 are also provided. Figure 6 As shown in Table 3, the average thickness of the pyrolytic carbon on the surface of Product B is <12μm. Based on the evaluation criteria, this product is classified as Grade IV.

[0046] Table 3 Carbon deposition thickness of different parts of product B

[0047]

[0048] Example 3

[0049] The CVD densification effect of carbon disk C produced by a certain company was evaluated. The sampling method was the same as in Example 1. Statistical data was collected and analyzed to evaluate the carbon deposition quality of the product matrix—the thickness of the deposited carbon. The metallographic image and results of the pyrolytic carbon of product C in Example 3 are also provided. Figure 7 As shown in Table 4, the average thickness of the surface pyrolytic carbon is 18.52 μm, and the ratio of surface pyrolytic carbon thickness to intermediate pyrolytic carbon thickness is 0.78. Based on the evaluation results in Table 1, this product is classified as Grade II.

[0050] Table 4. Carbon deposition thickness at various locations of a sample.

[0051]

[0052] Comparative Example 1:

[0053] In the observation unit, selecting perfectly circular carbon fibers as the measurement unit is essential for accurate evaluation of the results. If the carbon fibers are not perfectly circular, the non-perpendicular cross-section will affect the test results. For example... Figure 8 As shown, A represents a perfectly circular carbon fiber, and the metallographic thickness of the resulting products is relatively uniform (e.g., Figure 8 A), the metallographic thickness is approximately 13.83 μm. If a non-circular B is chosen as the measurement unit, the measurement results will differ significantly (e.g., Figure 8 B) The actual measurement results ranged from 12.88μm to 16.09μm, leading to inaccurate product performance.

[0054] Furthermore, during the actual exploration process, the inventors discovered that the carbon fibers should be relatively independent and not clustered, as this would also affect the results.

Claims

1. A method for evaluating the CVD densification effect of carbon-carbon composite materials, characterized in that: First, a long block with a height equal to the thickness of the carbon-carbon composite material is cut along its span. Then, this long block is segmented along both the original span and thickness directions to obtain several block samples. All block samples are prepared according to the testing requirements of an optical microscope to obtain test samples. The test samples are placed under an optical microscope, with the cut surface of the original carbon-carbon composite material as the observation surface. Measurements are taken at selected observation points on the surface and middle of the test samples. At these observation points, perfectly circular carbon fibers are selected as the measurement units to measure the thickness of pyrolytic carbon deposited on the carbon fiber surface. Finally, based on the measurement results, the CVD densification effect of the carbon-carbon composite material is evaluated. When the average surface pyrolytic carbon thickness is ≥18 μm, and the average middle pyrolytic carbon thickness is >0.8 μm, it is classified as Grade I. When the average surface pyrolytic carbon thickness is ≥18 μm, and the average middle pyrolytic carbon thickness is 0.6–0.8 μm, it is classified as Grade II. ≥18μm, with an average intermediate pyrolytic carbon thickness of <0.6μm at the surface, it is classified as Grade III; when the average surface pyrolytic carbon thickness is 16-18μm, with an average intermediate pyrolytic carbon thickness of >0.8μm at the surface, it is classified as Grade II; when the average surface pyrolytic carbon thickness is 16-18μm, with an average intermediate pyrolytic carbon thickness of 0.6-0.8μm at the surface, it is classified as Grade III; when the average surface pyrolytic carbon thickness is 16-18μm, with an average intermediate pyrolytic carbon thickness of 0.6-0.8μm at the surface, it is classified as Grade III. The average thickness of pyrolytic carbon is 8 μm. At the same time, the average thickness of pyrolytic carbon on the surface is <0.6 μm, which is classified as Grade IV. When the average thickness of pyrolytic carbon on the surface is 12-16 μm and the average thickness of pyrolytic carbon on the surface is >0.8 μm, it is classified as Grade III. When the average thickness of pyrolytic carbon on the surface is 12-16 μm and the average thickness of pyrolytic carbon on the surface is ≤0.8 μm, it is classified as Grade IV. When the average thickness of pyrolytic carbon on the surface is ≤12 μm, it is classified as Grade IV.

2. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: A long block with a width of 18-22 mm and a height equal to the thickness of the carbon-carbon composite material is cut along the span of the carbon-carbon composite material.

3. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: The length of the block sample is 20-30 mm and the height is 5-10 mm.

4. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: The sample preparation process is as follows: place the block sample in a mold, then inject liquid epoxy resin, cure it, and then polish it to obtain the sample. The liquid epoxy resin is composed of epoxy resin, butylene phthalate, and ethylenediamine.

5. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: The optical microscope is equipped with a polarizer and a rotatable analyzer.

6. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: The carbon fiber is an independent carbon fiber.

7. The method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1, characterized in that: The selection of observation points is still based on the positioning of the long block, and the distance between adjacent observation points on the surface is controlled to be between 20 and 30 mm. The observation points in the middle area should be within 10 mm above and below the thickness center line, and the distance between adjacent observation points in the middle area should be between 20 and 30 mm.

8. A method for evaluating the CVD densification effect of carbon-carbon composite materials according to claim 1 or 7, characterized in that: At any given observation point, select at least three perfectly circular carbon fibers as the measurement unit.

Images