ARTICLE IN PRESS Tribology International 42 (2009) 823–827
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Mechanical and tribological properties of short-ﬁber-reinforced SiC composites Hanling Tang a, Xierong Zeng b,c,, Xinbo Xiong b,c, Long Li b,c, Jizhao Zou b,c a b c
College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, China College of Materials Science and Engineering, Shenzhen University, Shenzhen, China Shenzhen Key Laboratory of Special Functional Materials, Shenzhen, China
a r t i c l e in fo
Article history: Received 9 January 2008 Received in revised form 24 October 2008 Accepted 31 October 2008 Available online 21 November 2008
The short-carbon-ﬁber-reinforced SiC (Csf /SiC) composites were prepared by hot-pressing sintering with Si, Al and B as sintering additives. The effects of ﬁber volume fraction on the mechanical and tribological properties of the Csf /SiC composites were investigated. The results show that the bending strength values of the composites containing a certain content of the short carbon ﬁbers are higher than that of the monolithic SiC. The friction coefﬁcients of the composites decrease with increasing short carbon ﬁbers content. Except of the composite containing 53 vol% short carbon ﬁbers, the wear rates of the composites decrease with increasing short carbon ﬁbers content, and are lower than that of the monolithic SiC drastically. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Short-carbon-ﬁber-reinforced SiC composite Friction coefﬁcient Wear rate
1. Introduction SiC is a very promising material for wear components such as mechanical face seal rings, journal bearings, valves, nozzles, rotors etc. because of its excellent high temperature strength, low density, good oxidation resistance and low wear [1,2]. However, like other ceramics, it is very sensitive to defects, such as pores, cracks and large grains and generally exhibits rather high friction coefﬁcients typically in the range of 0.5–0.8, except for those sliding in water or under other lubricated conditions. Particulate, whisker and ﬁber-reinforced composites can overcome the unacceptable drawbacks. Some timely examples include composites with carbon ﬁbers [3–5]. In comparison with continuous ﬁber-reinforced composites, the short-carbon-ﬁber reinforced SiC composites have easy adaptability to conventional manufacturing techniques and low cost of fabrication, and therefore are increasingly studied [6–8]. It is very important for short-ﬁber-reinforced SiC composites to understand their properties, which depend strongly on ﬁber orientation and fraction [9,10]. However, few studies have been reported. Based on the short-carbon-ﬁber-reinforced SiC (Csf /SiC) composites with different ﬁber contents fabricated by hot pressing method, the objective of the work described in this paper was to
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(X. Zeng). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.10.017
study the microstructure, mechanical and tribological properties of the Csf /SiC composites with different volume fractions of the short carbon ﬁbers, and to investigate the friction and wear behavior of the short-carbon-ﬁber-reinforced SiC composites.
2. Experimental investigation 2.1. Sample preparation The commercial short carbon ﬁbers with an average diameter of 6–8 mm were used as the reinforcement of SiC matrix. Fine a-SiC powder (Kongwei Co. Ltd., Shenzhen, China) with an average particle size of 10 mm was used as the starting powder. In order to obtain well-densiﬁed composites, Si, Al and B, which had the same average particle size of 30 mm, were selected as sintering additives. The short carbon ﬁbers were dispersed in ethanol with OP-10 as dispersing agents by magnetic stirrer for 2 h and then added to composite powders with a-SiC and 1% additives, which were homogeneously mixed by planetary ball-mill for 12 h to form the slurry. The as-resulted slurry was dried and then pressed to the discs sheet with 50 mm in diameter and 10 mm in height. At last, the discs sheet was sintered in graphite mold by hot-press technology at 1800 1C under 30 MPa in Ar for 1 h. The short carbon ﬁber volume fractions of the sintered composite specimens were about 15, 30, 42 and 53 vol%, respectively.
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2.2. Measurements Bulk densities of the specimens were measured using Archimedes method. The bending strength was measured by a three point bending test with 30 mm span at a crosshead speed of 0.5 mm per min at room temperature. The dimension of the bending strength test bars was 4 mm 3 mm 40 mm. Fracture toughness was determined by Indentation Method. All test bars were machined from the sintered body and polished on the tensile surface perpendicular to the hot-pressing direction. Friction behavior was evaluated by unlubricated ball-on-disk experiments, carried out against a polished commercial SiC ball with diameter of 6 mm. Wear tests were carried out under dry condition in air in order to eliminate any lubrication effect from other sources. Sliding conditions were set at the 8 N normal load, 180 r/min sliding speed and 3 h sliding time. All friction plates were polished perpendicular to the hot-pressing direction. Wear volumes of the worn specimens were evaluated using a roughness tester. For the worn plate specimen, the cross sectional area of the worn track was taken as the average of that measured at four separate locations. The worn volume V and the worn rate W v were calculated according to the following equations: V ¼ ððS1 þ S2 þ S3 þ S4 Þ=4ÞLðmm3 Þ Wv ¼
V ðmm3 =N mÞ PL
From these micrographs, it was clear that the carbon ﬁbers homogeneously distributed in the matrix and mostly aligned within planes perpendicular to the hot-pressing direction. The strength of the longitudinal direction was expected to be higher
Fig. 2. X-ray diffraction patterns of the specimens containing the different ﬁber content: (a) 15 vol%, (b) 30 vol%, (c) 42 vol%, (d) 53 vol%, and (e) the original carbon ﬁber.
where S; L and P are the cross sectional area of the worn track, the sliding distance and the normal load, respectively. In all cases, the values reported were the average of ﬁve different tests. The constitution phases of the composites were determined by X-ray diffractometry (XRD). The distribution of the ﬁbers in the composites, fracture surfaces of the bending test and the worn tracks of the friction test were observed by scanning electron microscopy (SEM).
3. Results and discussion 3.1. Microstructures Sintered specimens with carbon ﬁbers content less than 42 vol% achieved nearly full density (499% theoretical density). For the 42 vol% and the 53 vol% carbon ﬁber additions, the relative densities of the specimens were 96% and 92%, respectively. Fig. 1 represents typical SEM images for the polished surfaces parallel and perpendicular to the direction of loading during hot pressing for a specimen containing 42 vol% short carbon ﬁbers.
Fig. 3. The inﬂuence of carbon ﬁber content on bending strength.
Fig. 1. SEM micrographs of the composite surfaces illustrating ﬁber orientation within the specimen containing 42 vol% carbon ﬁber (a) perpendicular and (b) parallel, to the direction of loading.
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than that of the transverse direction in the unidirectional shortﬁber-reinforced composites, as mentioned by Kragness et al. . So the following test plates were obtained from the direction perpendicular to the hot-pressing force. XRD analysis shows that the intensity of the ð0 0 2Þ graphite peak increased with increasing carbon ﬁbers content (Fig. 2). It was noted that the peak width of carbon ﬁbers after sintering was
narrower than that of the original carbon ﬁbers, which could result from the graphitization of the carbon ﬁbers spurred by the high temperature and the high pressure simultaneously .
3.2. Mechanical properties
Fig. 3 shows the dependence of carbon ﬁbers content on the bending strength of the composites. In the case of the composites containing 15 and 30 vol% ﬁbers, the bending strength values were higher than that of the monolithic SiC, which was caused by adding the carbon ﬁbers improving bending strength of the composites. However, the addition of the carbon ﬁbers more than 30 vol% led to the bending strength values of the composites
800 1200 Sliding time/s
Fig. 4. Fracture toughness of the composites.
Fig. 7. The variation of friction coefﬁcients with sliding distance for the composite and the monolithic SiC.
Fig. 5. SEM micrograph of the fracture surface of the composite.
Fig. 8. The relationship between carbon ﬁber volume fraction and friction coefﬁcients of the composites.
Fig. 6. SEM images of indented crack propagation path of the Csf /SiC composite.
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Wear rate/10-6 mm3 N-1 m-1
180 150 120 90 60 30
10 20 30 40 Content of carbon fiber/vol.%
Fig. 9. The relationship between carbon ﬁber volume fraction and wear rate of the composites.
decreased, and the bending strength values of the composite with more than 42 vol% carbon ﬁbers were lower than that of the monolithic SiC. When adding short ﬁbers was up to a value, the ﬁbers bridged to skeleton construction and the matrix material could not wrap up the ﬁbers and ﬁll voids, which resulted in the poor load transfer, weakening the ﬁber-reinforced and the strength of the material [13,14]. The ﬁber bridge function and fewer matrixes prevented densiﬁcation, which was considered to be the main reasons for the bending strength values of the composites decreased. Therefore, only adding the moderate amount of the short carbon ﬁbers can improve the strength of the composites. The fracture toughness of the composites was measured by indentation method. Fig. 4 compares the fracture toughness values of the composites with the different carbon ﬁbers contents. The fracture toughness of the composites was higher than that of the monolithic SiC signiﬁcantly and increased with increasing carbon ﬁbers content. Utilization of the short carbon ﬁbers increased the fracture toughness of the composites up to 7:1 MPa m1=2 in the case of 53 vol% carbon ﬁbers content with
Fig. 10. SEM images of the worn surfaces of the monolithic SiC and the composite specimens with different amount of carbon ﬁber volume fraction: (a, d) monolithic SiC; (b, e) 15 vol% carbon ﬁber addition; (c, f) 42 vol% carbon ﬁber addition; (a–c) low magniﬁcation images; and (d–f) higher magniﬁcation images.
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this value being 3 times higher compared to that of the monolithic SiC. A quantity of ﬁbers pullout absorbed energy from bridge joint in the process of fracture, which was responsible for the toughening effect of the Csf /SiC composite, as seen in Fig. 5. From the indenter microstructure of the composite (Fig. 6), the short carbon ﬁbers deﬂected, augmented crack growth path and even held up crack growing, which also contributed to improving the fracture toughness of the composites.
3.3. Tribological properties The variations of friction coefﬁcient with sliding distance for the composite and the monolithic SiC are shown in Fig. 7. In the case of monolithic SiC, the initial friction coefﬁcient indicated higher values, which were maintained in the steady state. The friction coefﬁcients of composite had smaller ﬂuctuation during the sliding time. The composite disc gave more reproducible friction coefﬁcient from test to test. Fig. 8 illustrates the relationship between carbon ﬁbers content and average friction coefﬁcients of the composites during sliding, neglecting the initial running in period. The friction coefﬁcients decreased with increasing ﬁber contents. Fig. 9 shows the wear rates of the composites with the different carbon ﬁber contents. For the composites with less than 42 vol% carbon ﬁbers addition, the wear rates values decreased with increasing ﬁber content, and were lower than that of the monolithic SiC drastically. The good solid lubrication effect of the carbon ﬁbers restricted the increasing of Hertz stress contact during sliding conditions and decreased the friction coefﬁcient and the wear rate of the composites . On the other hand, the enhancement of the short carbon ﬁber was hindrance the generation and expansion of the cracks of the composite and raised the toughness of the composite, and then reduced the scaling chips and rose the resistance wears performance of the composites. The worn tracks of the monolithic SiC and the composites are shown in Fig. 10. The worn track of the monolithic SiC indicated a rough surface with cracks and crushed debris, which resulted in the high friction coefﬁcient and wear rate. The wear mechanism of the monolithic SiC was considered to be abrasive wear because that the intrinsic brittle nature of SiC led to grain pullout, or grain fracture, grain comminution or plough of the worn surface during sliding condition due to high Hertz stresses. The worn tracks of the composites indicated the smooth appearances and covered with adhesive debris forming a ﬁlm, which consisted mainly of carbon. According to EDS analysis, the carbon content of the debris was higher than that of the origin surface of the composite. The carbon ﬁlm with adhesion on the surface acted as a solid lubricant that restricted the stress concentration and augmented the real contact area during sliding conditions, and then the wear mode of the composite was not changed to abrasive wear, thus resulting in smooth wear surfaces. As a result, the fabricated composites with less than 42 vol% carbon ﬁbers had higher wear resistance compared with the monolithic SiC. The wear rate of the composite with 53 vol% carbon ﬁbers addition is very high, even more than that of the monolithic SiC (Fig. 9). The worn surface of the composite with 53 vol% carbon ﬁbers occurs a lot of dropped grain and exposed ﬁbers (Fig. 11), which demolished the composite worn surface and led to wear rate increasing drastically. It was considered to be due to the high ﬁbers content and the weak grain boundary strength. The toughening effect of carbon ﬁbers decreased the generation and propagation of cracks of the composites, in addition to the carbon lubrication effect, which resulted in the good wear resistance of the fabricated composites, except of the composite with 53 vol% short carbon ﬁbers addition.
Fig. 11. SEM image of the worn surface of the Csf /SiC composite with 53 vol% carbon ﬁbers addition.
4. Conclusion Short-carbon-ﬁber-reinforced SiC composites were fabricated by hot pressing in Ar at 1800 1C under a pressure of 30 MPa. The obtained composites have the high fracture toughness and tribological performance. The friction coefﬁcient and wear rate of the composites are lower than that of the monolithic SiC drastically, except of with 53 vol% carbon ﬁber content. Adding the moderate amount of the short carbon ﬁbers can improve the properties of the composites. In this study, the composites with 15–42 vol% carbon ﬁber content have a good integrative capability. References  Knoch H, Fundus M. Sintered silicon carbide with deﬁned porosity for sliding applications. In: Ceramic technology international. Ohio: The American Ceramic Society; 1995. p. 59.  Sang KZ, Jin ZH. Unlubricated wear of Si/SiC and its composite with nickel Si/ SiC-Ni. Tribol Int 2001;34:315–9.  Zhou XG, You Y, Zhang CR, Huang BY, Liu XY. Effect of carbon ﬁber pre-heattreatment on the microstructure and properties of Cf/SiC composites. Mater Sci Eng A 2006;433:104–7.  He XB, Yang H. Densiﬁcation mechanism of Cf /SiC composites prepared by precursor pyrolysis and hot pressing. Mater Sci Eng 2002;20:358–60 (in Chinese).  Zhou CC, Zhang CR, Hu HF, Zhang YD, Wang ZY. Preparation of 3D-Cf /SiC composites at low temperatures. Mater Sci Eng A 2007.  Lee JS, Yoshida K, Yano TJ. Inﬂuence of ﬁber volume fraction on the mechanical and thermal properties of unidirectional aligned short-ﬁberreinforced SiC composites. J Ceram Soc Jpn 2002;110:985–9.  Walter K. Carbon ﬁber reinforced CMC for high-performance structures. Int J Appl Ceram Tech 2004;1:188–200.  Ju CP, Wang CK. Process and wear behavior of monolithic SiC and short carbon ﬁber–SiC matrix composite. J Mater Sci 2000;35:4477–84.  He XL, Zhou Y. Effect of sintering additives on microstructures and mechanical properties of short-carbon-ﬁber-reinforced SiC composites prepared by precursor pyrolysis-hot pressing. Ceram Int 2006;32:929–34.  Ding YS, Dong SM, Huang ZR, Jiang DL. Fabrication of short C ﬁber-reinforced SiC composites by spark plasma sintering. Ceram Int 2007;33:101–5.  Kragness ED, Amateau MF, Messing GL. Processing and characterization of laminated SiC whisker reinforced Al2 O3 . J Compos Mater 1991;25:416–32.  Chen Q, Li HJ. Inﬂuence of graphitization on oxidation resistance of carbon/ carbon composites prepared by high pressure impregnation and carbonization. Chin J Ceram Soc 2004;32:1515–9 (in Chinese).  Sambell RAJ. Carbon ﬁber composites with ceramic and glass matrices. J Mater Sci 1972;7:663–75.  Yang DA, Xu TX. Study on preparation process and mechanical properties of BMI composite material reinforced by short carbon ﬁber. Thermosetting Resin 1999;4:71–3 (Chinese).  Hideki H, Kiyoshi H, Hideki K. Inﬂuence of carbon ﬁber additions on friction properties and running-in behavior of silicon nitride-based composites under water lubrication. J Am Ceram Soc 88:3474–77.