Synergistically Toughening Effect of SiC Whiskers and Nanoparticles in Al 2 O 3 -based Composite Ceramic Cutting Tool Material

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1 CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 29,aNo. 5,a DOI: /CJME , available online at Synergistically Toughening Effect of SiC Whiskers and Nanoparticles LIU Xuefei 1, 2, LIU Hanlian 1, 3, *, HUANG Chuanzhen 1, 3, WANG Limei 1, 3, ZOU Bin 1, 3, and ZHAO Bin 1, 3 1 Centre for Advanced Jet Engineering Technologies, School of Mechanical Engineering, Shandong University, Jinan , China 2 School of Mechanical and Electrical Engineering, Qilu Institute of Technolegy, Jinan , China 3 Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, Shandong University, Jinan , China Received July 22, 2015; revised March 10, 2016; accepted April 11, 2016 Abstract: In recent decades, many additives with different characteristics have been applied to strengthen and toughen Al 2 O 3 -based ceramic cutting tool materials. Among them, SiC whiskers and SiC nanoparticles showed excellent performance in improving the material properties. While no attempts have been made to add SiC whiskers and SiC nanoparticles together into the ceramic matrix and the synergistically toughening effects of them have not been studied. An Al 2 O 3 -SiC w -SiC np advanced ceramic cutting tool material is fabricated by adding both one-dimensional SiC whiskers and zero-dimensional SiC nanoparticles into the Al 2 O 3 matrix with an effective dispersing and mixing process. The composites with 25 vol% SiC whiskers and 25 vol% SiC nanoparticles alone are also investegated for comparison purposes. Results show that the Al 2 O 3 -SiC w -SiC np composite with both 20 vol% SiC whiskers and 5 vol% SiC nanoparticles additives have much improved mechanical properties. The flexural strength of Al 2 O 3 -SiC w -SiC np is 730±95 MPa and fracture toughness is 5.6±0.6 MPa m 1/2. The toughening and strengthening mechanisms of SiC whiskers and nanoparticles are studied when they are added either individually or in combination. It is indicated that when SiC whiskers and nanoparticles are added together, the grains are further refined and homogenized, so that the microstructure and fracture mode ratio is modified. The SiC nanoparticles are found helpful to enhance the toughening effects of the SiC whiskers. The proposed research helps to enrich the types of ceramic cutting tool and is benefit to expand the application range of ceramic cutting tool. Keywords: Al 2 O 3 -based ceramic cutting tool materials, SiC whiskers, SiC nanoparticles, mechanical properties, toughening and strengthening mechanisms 1 Introduction It has become apparent that traditional high-speed-steel and cemented carbide tools are unable to meet the needs of industry, especially high speed machining. Ceramic cutting tools have proven to be an effective alternative to meet the needs of industry in high speed machining. Among the ceramic cutting tool materials, notably Al 2 O 3 -based ceramics and Si 3 N 4 -based ceramics, Al 2 O 3 -based ceramics are most commonly used because of the unique properties of alumina, such as good wear resistance, high hot hardness, great corrosive resistance and low cost. In recent years, the research on ceramic tool materials shows a trend of diversification in several aspects. The reseach of cermet [1 5] enriched the types of the ceramic matrix and improved the performance of the ceramic * Corresponding author. lhl70@sdu.edu.cn Supported by National Natural Science Foundation of China(Grant No ) Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2016 materials. The form of additives also changed from particles to whiskers and elongated grains [6] and the types of whiskers have developed from SiC whikser to TaC whisker [7], ZrC whisker [8] and others. The grain size range of the additives enlarged gradually. In the studies by TENG [9], LIU [10] and DONG [11], et al, different addititives of various grain sizes were applied to the matrix with different ratios to improve the performance of the ceramic materials. Moreover, the assessment of the mechanical properties of ceramic materials is not limited to static conditions in room temperature, but high temperature mechanical properties [12] and dynamic fatigue behavior [13] that are more relevant to practice. Many studies have shown that the mechanical properties of ceramic materials can be improved greatly when SiC whiskers or nanoparticles were added. SiC whisker is a monocrystalline fiber with high elasticity modulus and tensile strength. It contributes greatly to the flexural strength and fracture toughness of Al 2 O 3 -based ceramics when whiskers are dispersed uniformly and the composites are densified [14]. DENG [15]

2 978 LIU Xuefei, et al: Synergistically Toughening Effect of SiC Whiskers and Nanoparticles and GARNIER [16], et al, fabricated Al 2 O 3 -SiC w composites with different sintering methods and the results showed that the optimum flexural strength and fracture toughness of Al 2 O 3 -SiC w composites were obtained when the content of SiC whiskers was vol%. NIIHARA [17 18] is the first to add SiC nanoparticles into Al 2 O 3 and the flexural strength and fracture toughness of the composite fabricated were increased significantly when compared with monolithic Al 2 O 3, from 350 MPa to 1000 MPa and from 3.5 MPa m 1/2 to 4.7 MPa m 1/2 respectively. The subsequent studies of MESCHKE [19], WANG [20] and ZHANG [21], et al showed that the best flexural strength and fracture toughness of Al 2 O 3 -SiC np composites were obtained when vol% SiC nanoparticles were added. HUANG, et al [22], added both SiC whiskers and microparticles into Al 2 O 3 and fabricated Al 2 O 3 -SiC w -SiC p composites with excellent flexural strength and fracture toughness. Compared with monolithic Al 2 O 3, Al 2 O 3 -SiC p and Al 2 O 3 -SiC w, Al 2 O 3 -SiC w -SiC p composites showed obvious synergism of toughening between SiC whiskers and microparticles. In this study, SiC whiskers and nanoparticles are added into Al 2 O 3 matrix through a dispersing and mixing method and a new Al 2 O 3 -SiC w -SiC np composite is fabricated using hot-pressing sintering method. The synergism of toughening between SiC whiskers and nanoparticles is analyzed when comparing with the composites by adding SiC whiskers or SiC nanoparticles individually. mechanical stirring. Generally, the ball-milling time for ceramic composite powders is 48 h. However, it causes mechanical damage to SiC whiskers when the ball-milling time is too long [24]. Therefore, the ball-milling time should be shortened if the mixing uniformity is guaranteed. In the study, the ball-milling time was chosen as 15 h. The dispersion and mixing procedure is shown as Fig. 1. Five different composites powders with different components (shown as Table 2) were prepared to test the dispersion effect. It can be seen from Fig. 2 that SiC whiskers were well dispersed and no whiskers reunion were observed. As the content of SiC whiskers increased from 5 to 25 vol %, the amount of whiskers could be seen from Figs. 2(a) (e) increased correspondingly, which demonstrated the distribution of whiskers was uniform from one aspect. 2 Experimental Procedures 2.1 Raw materials. The Al 2 O 3 powders were bought from Zibo Dongchangye Co. Ltd., the average grain size was 0.5 μm and the purity was 99.99%. The β-sic w and β-sic np powders were bought from Changsha Sinet Advanced Materials Co. Ltd. and Shanghai Shuitian Material Technology Co. Ltd., respectively. The purity and density of β-sic w and β-sic np were 99.9 wt% and 3.21 g/cm 3. The particle size of β-sic np was 50nm. The physical properties of the β-sic w particles are shown in Table 1. Table 1. Physical parameters of β-sic w particles Materials Content φ / vol % Diameter Ф/ nm Length l / μm β-sic w Preparation procedure. Because of the surface force and static electricity of SiC whiskers, they intend to reunite and intertwine, which affects the uniformity of additive distribution and the density of the materials [23]. Meanwhile, SiC nanoparticles are easy to reunite because of their fine grain size and the large surface energy. Therefore, it is necessary to disperse SiC whiskers and SiC nanoparticles before adding them into the composites. In the study, SiC whiskers and SiC nanoparticles were dispersed in the solution of ethanol and polyethylene glycol through ultrasonic vibration and Fig. 1. Preparation procedure of the composite powders Table 2. Component of AW5P5, AW10P5, AW15P5, AW20P5 and AW25P5 (vol %) Composite Al 2 O 3 SiC w SiC np Y 2 O 3 MgO AW5P AW10P AW15P AW20P AW25P The composite powders were sintered through vacuum hot-pressing method and the sintered composites were cut, ground and polished into small specimens with a dimension of 34 mm 3 mm 4 mm. Flexural strength was measured at a span of 20 mm and a crosshead of 0.5 mm/min by the three-point bending test method on WD-10 electron universal tester. Hardness and fracture toughness was measured on the polished surface using a diamond pyramid

3 CHINESE JOURNAL OF MECHANICAL ENGINEERING 979 indenter with a load of 196 N and a loading duration of 15 s. The same sample was tested six times. Scanning electron microscope(sem, Supra-55 Zeiss Germany) attached with energy dispersive spectrometer(eds, PV9900 Philips Netherlands) was used to observe the microstructure of fractured surfaces. Fig. 2. Microstructure of different composite powders after dispersing and mixing 2.3 Materials components In order to study the toughening and strengthening effect of SiC w and SiC np either added together or individually, different morphologies of SiC with the same content (25 vol%) were used to fabricate composites with 25 vol% SiC w, with 25 vol% SiC np and with both 20 vol% SiC w + 5 vol% SiC np using the method described above. The materials components are shown as Table 3. 3 Results and Discussion 3.1 Mechanical properties The mechanical properties of AW25, AP25 and AW20P5 are shown in Fig. 3. It can be seen that there was not much difference in the flexural strength between AW25 and AP25; however, the flexural strength and fracture toughness of AW20P5 were improved remarkedly; also, the individual SiC w whisker showed more significant toughening effect than individual SiC np nanoparticle. Table 3. Component of AW25, AP25 and AW20P5 (vol %) Composite Al 2 O 3 SiC w SiC np Y 2 O 3 MgO AW AP AW20P

4 980 LIU Xuefei, et al: Synergistically Toughening Effect of SiC Whiskers and Nanoparticles AW25 and it is a result of the effect of SiC nanoparticles. On the one hand, SiC nanoparticles distributing on the grain boundary had a pinning effect on the boundary and improved the interfacial strength. On the other hand, SiC nanoparticles distributing in the matrix grain induced residual tension stress in the matrix grain because of the mismatch of thermal expansion coefficient and therefore induced more trans-granular fracture (shown as the block in Fig. 5). The fracture toughness of grain boundary is lower compared with the fracture toughness of the grains [25] and trans-granular fracture consumes more energy than inter-granular fracture. As a result, AW20P5 with both SiC w and SiC np additives showed higher flexural strength and fracture toughness compared with AW25. Fig. 3. Mechanical properties of AW25, AP25 and AW20P5 3.2 Toughening and strengthening mechanisms Effect of SiC w and SiC np additives on the composite hardness As shown in Fig. 3(b), the hardness of AP25 is the highest of the three composites and it is because of the finer grain and denser microstructure(fig. 4(b)). AW25 with 25 vol% SiC w alone has a lower hardness because of its coarse grains and lower density. When 5 vol% SiC w was displaced by 5 vol% SiC np, the hardness of AW20P5 was improved to be better than AW25. It can be seen from Fig. 4(a) that the grain size of AW25 is not identical and was larger than that of AP25 with 25 vol% SiC np alone(fig. 4(b)). When SiC np was added together with SiC w, it can be seen from Fig. 4(c) that the grain size of AW20P5 is more uniform and large grains(as shown by the circles in Fig. 4(a)) disappeared, demonstrating that adding 5 vol% SiC np helped to refine and homogenize grains and, therefore, improved the hardness of AW20P Effect of SiC w and SiC np on the composite fracture mode It can be seen from Figs. 4 (a) and (c) that the amount of trans-granular fracture in AW20P5 was more than that of Fig. 4. Microstructure of AW25, AP25 and AW20P5

5 CHINESE JOURNAL OF MECHANICAL ENGINEERING 981 (2) The improvements are more remarkable than the composite with SiC whiskers or SiC nanoparticles additives alone. This is mainly due to the composite grains which were refined further and the increased amount of trans-granular fracture mode, as well as the synergistic effect between SiC whiskers and nanoparticles in strengthening and toughening the composite whereby the residual strength brought in by SiC nanoparticles increased the toughening effect of SiC whiskers. Fig. 5. Microstructure of the fracture surface of AW20P5 It can be seen that the surfaces of SiC whiskers inside the AW25 composite(in the ellipse of Fig. 6) with SiC whiskers as the additive are smooth and clean; however, the surfaces of SiC whiskers in AW20P5 (in the ellipse of Fig. 5) are adhered with other materials and, as a result, the bonding area of SiC whiskers and the matrix is larger. Fig. 7. Microstructure of the composite powders of AW20P5 Fig. 6. Microstructure of the fracture surface of AW25 Because the grains of SiC nanoparticles were very fine, they tended to adhere to the surfaces of SiC whiskers, which can be seen from the arrows in Fig. 7, the SiC whiskers were covered by SiC nanoparticles. After sintering, the SiC nanoparticles were caught in the middle of SiC whiskers and the matrix. Because of the mismatch of thermal expansion coefficients, residual stresses were induced and the interfacial bonding strength of SiC whiskers and the matrix was improved. As a result, a suitable content of SiC nanoparticles contributed to the toughening effect of SiC whiskers and improved the flexural strength and fracture toughness of AW20P5. 4 Conclusions (1) When both 20 vol% SiC whiskers and 5 vol% SiC nanoparticles are added into the Al 2 O 3 -matrix, the flexural strength and fracture toughness of the fabricated composite will be increased to 730 MPa and 5.6 MPa m 1/2 respectively. References [1] ZOU B, HUANG C Z, SONG J P, et al. Effects of sintering processes on mechanical properties and microstructure of TiB 2 TiC+8 wt% nano-ni composite ceramic cutting tool material [J]. Materials Science and Engineering A, 2012, 540: [2] ZOU B, HUANG C Z, JI W B, et al. Effects of Al 2 O 3 and NbC additives on the microstructure and mechanical properties of TiB 2 TiC composite ceramic cutting tool materials[j]. Ceramics International, 2014, 40: [3] ZHAO G L, HUANG C Z, LIU H L, et al. Microstructure and mechanical properties of hot pressed TiB 2 SiC composite ceramic tool materials at room and elevated temperatures[j]. Materials Science & Engineering A, 2014, 606: [4] WANG L M, LIU H L, HUANG C Z, et al. Effects of sintering processes on mechanical properties and microstructure of Ti(C,N) TiB 2 Ni composite ceramic cutting tool material[j]. Ceramics International, 2014, 40: [5] ZOU B, ZHOU H J, XU K T, et al. Study of a hot-pressed sintering preparation of Ti(C 7 N 3 )-based composite cermets materials and their performance as cutting tools[j]. Journal of Alloys and Compounds, 2014, 611: [6] XU L, HUANG C Z, LIU H L, et al. In situ synthesis of ZrB 2 ZrC x ceramic tool materials toughened by elongated ZrB 2 grains[j]. Materials and Design, 2013, 49: [7] ZHAO G L, HUANG C Z, LIU H L, et al. Preparation of in-situ growth TaC whiskers toughening Al2O 3 ceramic matrix composite[j]. Int. Journal of Refractory Metals and Hard Materials, 2013, 36: [8] XU L, HUANG C Z, LIU H L, et al. Study on the synthesis and growth mechanisms of the refractory ZrC whiskers[j]. Int. Journal of Refractory Metals and Hard Materials, 2014, 42: [9] TENG X Y, LIU H L, HUANG C Z. Effect of Al 2 O 3 particle size on the mechanical properties of alumina-based ceramics[j]. Materials Science and Engineering A, 2007, : [10] LIU H L, HUANG C Z, TENG X Y, et al. Effect of special microstructure on the mechanical properties of nanocomposite[j]. Materials Science and Engineering A, 2008, 87:

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