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Original Article
DOI: 10.1016/j.jmrt.2019.08.036
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Available online 11 September 2019
Friction and wear behaviors of SiCNF modified carbon/carbon sealing materials
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Jie Chen
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chenjiecsu@163.com

Corresponding author.
, Rutie Liu, Xiang Xiong
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
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Received 14 December 2018. Accepted 22 August 2019
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Table 1. Thermal and physical properties of two composites.
Abstract

Silicon carbide nanofibers grew on the surface of carbon fibers of a unidirectional carbon preform by CCVD and then chemical vapor infiltration was used to densify the preform to obtain SiCNF-C/C composites. The effects of Silicon carbide nanofibers on friction and wear properties of SiCNF-C/C composites were deeply analyzed. Results show that the friction and wear properties of SiCNF-C/C composites are well improved due to the melioration of microstructure after modified by SiCNFs. The friction coefficient of SiCNF-C/C composites is more stable along with friction speed than that of C/C composites, it maintains high even at high friction speed. The wear of SiCNF-C/C composites is much lower and more stable than that of C/C composites with the increase of friction speed, especially in the vertical direction.

Keywords:
Silicon carbide nanofibers
Carbon/carbon composite
Interface
Friction and wear properties
Full Text
1Introduction

The sealing parts of aircraft engine have significant influence on the working performance and reliability of the whole engine. The shaft sealing materials used in aero-engine rotate against the sealed runway with high speed. Sealing is achieved by limiting the leakage of sealed fluid though a small gap between sealing materials and the runway during rotating [1,2]. Therefore high-performance sealing materials occupy an important position in the field of sealing technology. It often restricts the performance limit of engines and the implementation of some advance structures [3].

Carbon/carbon (C/C composite, carbon fiber reinforced carbon matrix composite) composite is a kind of high-performance composites and widely used in aerospace industry and automobile industry for the excellent properties, such as light quality, high module, high specific strength, low thermal expansion, high temperature resistance and outstanding corrosion resistance [4,5]. With the development of aero-engine sealing materials with high thrust/mass ratio, the operating conditions for inter-axial sealing devices are more and more harsh. The shaft seal ring frequently operates under the oxidizing condition with high temperature and high speed rotation. As for conventional C/C sealing materials, the stability of friction coefficient is relatively low especially in high temperature and the wear resistance is poor under these circumstances. Therefore, there is an urgent need to find a modification method to improve the properties of conventional C/C composites to adapt the harsh operating conditions.

Silicon carbide nanofibers (SiCNFs) are the products of extreme anisotropic growth of silicon carbide crystals. SiCNF is the single crystalline component with less structural defects and the aspect ratio is about 10 [6,7]. It has quasi-one-dimensional structure, so SiCNFs have high tensile strength, high corrosion resistance and excellent oxidation resistance. In addition, SiCNFs have excellent wear resistance and physical stability due to their special growth mechanism [8]. In recent years, many studies have reported that SiC fibers/particles added into C/C composites can change the microstructure of carbon matrix and improve the property of composites. Introducing SiC whiskers into the SiC fiber preform by chemical vapor deposition (CVD) method can obtain a composite with a higher density and a uniform densification [9]. When silicon carbide particles were dispersed in the matrix, the friction coefficient of C/C composites increased [10]. We have investigated the microstructure, mechanical and thermal properties of SiCNFs modified C/C composites in our previous research paper, literature [11]. But the friction and wear properties of SiCNF-C/C composites have not been discussed in our previous works.

To make full use of their well comprehensive performance, SiCNFs have grown on the fiber surface in this paper. SiCNFs were prepared on the surface of carbon fibers by catalytic chemical vapor deposition (CCVD). The carbon preform with SiCNFs was then densified by chemical vapor infiltration (CVI) to obtain SiCNF reinforced carbon/carbon (SiCNF-C/C) composites. On one hand, SiCNFs, as nanofibers with special surface structure, could modify the fiber surface resulting in the improvement of carbon microstructure and ultimately adjusted the tribological property. On the other hand, SiCNFs with excellent wear and corrosion resistance could directly improve the friction and wear properties of C/C composites.

2Experimental procedure2.1Preparation of composites

Carbon preforms were made from T700 polyacrylonitrile-based carbon fibers (Yixing Tianniao Co. Ltd., China), in which carbon fibers arranged unidirectional. Nickel particles (Hongxin Shiji Co. Ltd., China) were electroplated by 14wt% nickel sulfate aselectrolyte, which was used as catalysts covering around fiber surface.

After the preparation of catalysts, SiCNFs grow in situ on the fiber surface by CCVD. The CCVD was performed in the HTV-1 CVD furnace. During CCVD, the temperature was 980℃ and the pressure was 400Pa. The deposition time was 2.5h. The source of silicon carbide was methyltrichorosilane (MTS) and the carrier was hydrogen. Next, the SiCNFs-coated carbon clothes were stacked one by one along the same direction to get the carbon preform with fiber content of 35%. CVI was then used to densify the preform to obtain the SiCNFs modified composite (SiCNFs-C/C composite). The CVI was performed in the HR-2 CVI furnace. For comparison, a C/C composite with the same preform structure was prepared. Finally, the two composites were heated at 2300℃ under vacuum in GSS-2 graphitization furnace. The structure of carbon fiber preform and CVI process is similar to that of literature [11].

2.2Characterization

The bulk density of the composites was measured by the Archimedes water immersion method analyzer at room temperature. The microstructure of SiCNFs and surface morphology of carbon fibers were observed by Jeol JSM-5600LV scanning electron microscope (SEM). The microstructures of Pyrolytic carbon (PyC) and fiber/matrix interface were investigated by POLYVAR-MET polarized light microscopy (PLM).

The degree of graphitization g was calculated by the expression:

g=(0.3440d002)/(0.3440−0.3354) (1)
where g is graphitization degree (%), d002 is interlayer distance of (002) plane (nm), 0.3440nm is the d002 of turbostratic carbon, 0.3354nm is the d002 of ideal graphite. d002 was determined using Rigaku-3014 X-ray diffraction analyzer.

Thermal diffusivity of composites was calculated by laser flash method on JR-1 type synthetical thermal tester. Flexural testing was performed by three-point bend configuration on CSS-44100 universal testing machine. The test method is the same with that of literature [11]. The friction and wear properties of composites were examined on UMT-3 friction testing machine. The pair of parts is a chromium (Cr) steel ball. The friction load was 30N and the test time was 30min. The friction speed was 200r/min, 400r/min, 600r/min and 800r/min respectively. Specimens for property testing were machined in two dimensions, parallel direction and vertical direction, as show in Fig. 1.

Fig. 1.

Sketch map of property test (a) vertical direction; (b) parallel direction.

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3Results and discussion3.1Microstructure of composites

Fig. 2 shows the images of in-situ grown SiCNFs and carbon fibers covered by SiCNFs. Fig. 3 shows the energy spectrum result of in-situ grown SiCNFs located at the area of point A in Fig. 2(a). It can be seem from Fig. 3 that the obtained nanofibers by CCVD on carbon fibers are SiCNFs in this experiment condition. After modification, the in-situ grown SiCNFs have uniformly distributed on the surface of carbon fibers.

Fig. 2.

SEM images of in-situ grown SiCNFs (with selected area for Energy spectrum) and carbon fiber covering with SiCNFs (a) in-situ grown SiCNFs; (b) carbon fiber covering with SiCNFs.

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Fig. 3.

Energy spectrum result of SiCNFs located at point A in Fig. 2(a).

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The diameter of SiCNFs is 60–80nm and the length is 20–40μm. The carbon fibers are completely encapsulated by nanofibers. Fig. 4 shows the polarized-light microscopy of SiCNFs-C/C composite. PyC around carbon fibers of SiCNFs-C/C composite shows clear-cut distinction with strong optical reflectivity, belongs to rough laminar (RL) PyC [12,13]. The fiber/PyC interface is tight and jagged with many tiny granular. Further observation in Fig. 5 shows that this layer of tiny particles is the growing SiCNFs covered by PyC. It indicates that the graphite crystallite of the granular layer interface in SiCNF-C/C composites is more integrated.

Fig. 4.

Optical photographs (under PLM) of SiCNFs-C/C composite.

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Fig. 5.

SEM images of fiber surface of SiCNFs-C/C composite.

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As analyzed in literature [11], the peculiar quasi-one-directional structure of SiCNF has single crystalline component and less structural defects. It has great surface activity due to high lamellar orientation and big aspect ratio. Therefore, SiCNFs become the preferred nucleation centers. The hydrocarbon gases are easy to deposit orderly around their surface during CVI. So the layer of granular interface has high texture structure in the SiCNF-C/C composites. In summary, the in situ grown SiCNFs on carbon fibers have interposed the deposition of PyC and introduced high textured interface structure around the fiber surface.

3.2Effect of in situ grown SiCNFs on the friction property of C/C composites

Table 1 shows the thermal and physical properties of two composites. The graphitization degree, TC and flexural properties of SiCNF-C/C composites are much better than that of C/C composites. Fig. 6 shows the XRD patterns of the two composites. According to the analysis in literature [11], high textured PyC around carbon fibers have been induced by the in situ grown SiCNFs. This layer of high textured PyC is easier to graphitization. Therefore the graphitization degree of SiCNF-C/C composites is obviously higher than that of C/C composite after final graphitization due to the adjustment and improvement of microstructure, as in Fig. 6. On the other hand, fiber/matrix interface is the transmission bridge for heat and force [14]. The fiber/matrix interface bonding in SiCNF-C/C composites is well with no interface cracks, seen from Fig. 4. Therefore SiCNF-C/C composites with high textured carbon matrix and well interface bonding have more excellent thermal and mechanical properties by contrast with C/C composite.

Table 1.

Thermal and physical properties of two composites.

  C/C compositeSiCNF-C/C composite
Densification (g/cm31.631.64
Graphitization degree (%)  5468
Direction  ⊥  //  ⊥  // 
TC (W·m−1·K−112.4  110.6  21.6  156.4 
σ/MPa  159.5  11.7  241.7  25.8 
Fig. 6.

XRD patterns of SiCNFs-C/C composite and C/C composite.

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The friction coefficient and wear of two composites at different friction speed are shown in Fig. 6. It can be seen from Fig. 7(a) and (c) that the friction coefficient of two composites increases firstly and then decreases with the raising of the friction speed both in vertical and parallel direction. In addition, the friction coefficient of C/C composites changes greatly with the increase of friction speed and it is greatly reduced at high friction speed. However the friction coefficient of SiCNF-C/C composites is more stable along with friction speed than that of C/C composites and it remains high at high friction speed. As for wear, the wear of SiCNF-C/C composites is obviously lower than that of C/C composites at each friction speed whether in vertical direction or parallel direction, as in Fig. 7(b) and (d). And the wear of SiCNF-C/C composites has higher stability compared with C/C composites. Simultaneously, compared with parallel direction, SiCNFs have a greater influence on the wear property of C/C composite in the vertical direction.

Fig. 7.

Friction coefficient and wear of two composites along with the friction speed.

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According to molecular mechanics theory widely used in tribology [15], the friction coefficient is defined as

μ=β+KA (2)
Where β is the contribution of molecular adhesion caused by van der Waals force. K is the influence of mechanical deformation resistance. A is the accrual contact area between two friction surfaces.

With the raising of friction speed, friction heat on surface of materials enhances. Due to the enhancement of friction heat, the contacting asperities between two friction surfaces soften progressively. So the actual contact area A between friction surfaces is gradually increased. That is why the friction coefficient of two composites increases firstly. When friction speed increases to a certain value, oxidation of C/C composites occurs. Lots of oxides are repeatedly rolled in the friction process and then the friction layer between two friction surfaces is formed. When the friction layer is formed between friction surfaces, the influence of mechanical deformation resistance K is decreased, so the friction coefficient of two composites decreases finally. At the same time, it can be seen that the friction coefficient of SiCNF-C/C composites is higher than that of C/C composites at high friction speed. Although the formation of friction surface leads to the decrease of the mechanical deformation resistance K at high friction speed. But the modification of SiCNFs with excellent mechanical properties compensates for this loss to some extent, resulting in the higher friction coefficient of SiCNF-C/C composites. On the other hand, the graphitization degree of SiCNF-C/C composites is much higher than that of C/C composites, due to the modification of SiCNFs. The carbon matrix with high graphitization degree has more complete carbon layer. The complete layered microstructure can keep the smooth of friction surface even at high speed. So the friction coefficient of SiCNF-C/C composites is more stable along with friction speed than that of C/C composites.

The wear of SiCNF-C/C composite is obviously lower than that of C/C composite at each friction speed. Fig. 8 shows the morphologies of the worn surface of two composites. The worn surface of C/C composites is rough and uneven in two directions. However, the worn surface of SiCNF-C/C composite is smooth and glossy.

Fig. 8.

Morphologies of worn surface of two composites (a) C/C composite, ∥, (b) SiCNF-C/C composite, ∥ (b) C/C composite, ⊥, (d) SiCNF-C/C composite, ⊥.

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According to the Achard equation of adhesive wear [16]:

W=KL/3H(3)

The wear of materials is proportional to the load K and the slip distance L, and is inversely proportional to the hardness of the materials H. After modification by SiCNFs, the mechanical property of C/C composites is enhanced. The flexural strength of SiCNF-C/C composites is much higher than that of C/C composites, as in Table 1. Therefore, the hardness of SiCNF-C/C composites is much higher than that of C/C composites as well. The improvement of mechanical property is attributed to the high textured PyC matrix and well fiber/matrix interface bonding. It increases the resistance to the crack propagation. Simultaneously, the SiCNFs are high hard phases, which also attribute to the enhancement of the mechanical property. On the other hand, the high textured PyC matrix has good layered carbon microstructure, which brings lubrication effect during friction. Therefore the adhesive wear of SiCNF-C/C composites is lower than that of C/C composites.

In addition to adhesive wear, oxidation wear occurs for carbon composites during friction [17]. So thermal conductivity of materials could influence the oxidation wear as well. Friction heat can be conducted away from friction surface if the material has high TC. According to Table 1, the TC of SiCNF-C/C composites are higher than that of C/C composites. So the friction heat on contact surfaces of SiCNF-C/C composites can be conducted away timely and then the oxidation wear can be reduced relatively. Under this circumstance, the thickness of oxidation layer is maintained moderate. The lubrication layer can be seen in Fig. 9(a). Appropriate oxidation film between two friction surfaces induce the formation of lubrication layer, which guarantees the stability and low wear. In addition, the addition of SiCNFs with high wear resistance can obviously reduce the wear value of the composite. As for C/C-composites, friction heat cannot be conducted away timely due to the low TC. So the oxidation wear is more serious and lots of oxidations remain on the friction surface. In the process of repeated friction, the thick oxidations are crushed and broken becoming abrasive grains. In the subsequent repeated friction, the crushed oxidations become hard abrasives and cause serious furrow wear on the surface. So there are lots of scratches and debris on the friction surface of C/C-composites, as shown in Fig. 9(b). Therefore the friction surface of C/C-composites is rough with high wear at each friction speed. In summary, the wear value of C/C-composites is higher and not stable at different friction speed, compared with SiCNF-C/C composites.

Fig. 9.

SEM of worn surface of two composites (a) SiCNF-C/C composite (b) C/C composite.

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For unidirectional C/C composites, the properties in the vertical direction are mainly depended on the matrix and fiber/matrix interface, while the properties in the parallel direction are mainly depended on carbon fibers. According to the analysis above, the in situ grown SiCNFs on carbon fibers have introduced high textured interface structure around the fiber surface. The bonding between carbon fibers and the matrix is significantly enhanced. So the performance improved in the vertical is more obvious than that in parallel direction. Therefore the wear of SiCNF-C/C composites is obviously lower than that of C/C composites in the vertical direction.

4Conclusions

SiCNFs growing on the surface of carbon fibers induce the ordered deposition of PyC around carbon fibers during CVI. The bonding of fiber/matrix interface is improved by high textured PyC around fiber surface. After modification by SiCNFs, the friction and wear properties of SiCNF-C/C composites are enhanced obviously, compared with C/C composites due to the improvement of carbon microstructure. The friction coefficient of SiCNF-C/C composites is more stable along with friction speed than that of C/C composites. The friction coefficient of SiCNF-C/C composites maintains high even at high friction speed. The oxidation wear has been greatly reduced after modified by SiCNFs. The wear value of SiCNF-C/C composites is much lower and more stable than that of C/C composites with the increase of friction speed, especially in the vertical direction.

Acknowledgements

This research work is supported by the National Natural Science Foundation of China No. 51505503 and by International S&T Cooperation Program of China No. 2015DFR50580

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Journal of Materials Research and Technology

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