- Email: [email protected]
Preparation and mechanical properties of in situ growth TiC whiskers toughening Al2O3 ceramic matrix composite
Materials Science and Engineering A 460–461 (2007) 146–148
Preparation and mechanical properties of in situ growth TiC whiskers toughening Al2O3 ceramic matrix composite Bingqiang Liu, Chuanzhen Huang ∗ , Meilin Gu, Hongtao Zhu, Hanlian Liu Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, Jinan 250061, China Received 23 September 2006; received in revised form 8 January 2007; accepted 8 January 2007
Abstract An in situ growth TiC whiskers toughening Al2 O3 ceramic matrix composite has been prepared based on a carbothermal reduction process by two separate steps such as in situ synthesizing of the TiC whiskers in alumina matrix powder and hot pressed sintering of the composite, respectively. The TiC content in the composite is 40% in terms of volume. The experimental results show that the composite is with a flexural strength of 780.9 MPa and a fracture toughness of 7.27 MPa m1/2 and such a preparation and fabrication process for in situ growth whiskers toughened ceramics can greatly improve the mechanical property of Al2 O3 matrix composite. © 2007 Elsevier B.V. All rights reserved. Keywords: TiC whisker; Carbothermal reduction; In situ growth; Al2 O3 matrix; Composite
1. Introduction As one of the important ceramic whiskers with high elastic modulus, TiC whisker is more appropriate to be a strengthening and toughening additive for majority of ceramic matrix composites than commonly used SiC whisker because of the higher thermal expansion coefficient. TiC whiskers is commonly produced by chemical vapour deposit (CVD) methods and added into matrixes by mechanical mixing procedures [1]. As a result, the application of TiC whiskers is limited by such disadvantages as high cost, health hazard and difficult dispersion. After the recently suggestions of a carbothermal reduction process for growth of TiC whiskers [2] and an in situ growth idea for synthesizing high performance composites [3], the combination of them will certainly become a promising way to produce in situ growth TiC whiskers toughening ceramic matrix composites with higher mechanical properties, much lower cost and less environmental pollution, etc. In the present work, an Al2 O3 ceramic matrix composite toughened by in situ growth TiC whiskers is prepared. The TiC whiskers are firstly synthesized by the carbothermal reduction
∗
Corresponding author. Tel.: +86 531 88392539; fax: +86 531 88392328. E-mail address: [email protected] (C. Huang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.009
process in the Al2 O3 matrix powder and subsequently the composite is sintered. The morphology of the synthesized powder and the microstructure, mechanical properties of the composite are studied. 2. Experiments Commercial Al2 O3 with an average grain size of 0.5 m is used as the matrix powder. Commercial TiO2 , amorphous carbon, synthesis pure NaCl are used for the synthesis of TiC whiskers and commercial Ni with an average grain size of 2.3 m is used as the catalyst. All of the raw materials mentioned above are proportionally added to produce a composite powder with about 40% TiC in terms of volume. The mixture is ball-milled for 24 h in ethanol medium. After dried and sieved, the homogeneous mixture is put into a graphite reactor for the synthesis of the composite powder, firstly heated to 1460–1480 ◦ C in 10–12 min in a flowing argon-gas atmosphere and held for 60 min duration, subsequently is heated to 1690–1710 ◦ C and held for 30 min duration. Such a synthesis process can either elevate the compound fraction of C in TiC whiskers or increase the chemical stability of the whiskers during the sintering procedure because that the powder will subsequently be sintered at 1700 ◦ C hereabout. The synthesized composite powder is put into a graphite die after dry ball-milled
B. Liu et al. / Materials Science and Engineering A 460–461 (2007) 146–148
Fig. 1. Morphology of the synthesized composite powder.
for 4.5–5 h, heated to 1690–1710 ◦ C and held for 30 min duration at a pressure of 25 MPa in a flowing argon-gas atmosphere. The flexural strength is measured using 3 mm × 4 mm × 30 mm polished test bars under three-point bending test on WD-10 electron universal tester with a span of 20 mm and a loading rate of 0.5 mm min−1 , considering an average of six specimens. The fracture toughness is evaluated according to the crack lengths of indentation method. The indenter is the Vickers DPH type and the applied static load is 196 N for 15 s. Crack lengths for toughness assessment are measured using optical microscope. The values of fracture toughness (KIC ) are calculated by the equation reported by Fukuhara et al. [4]: c −3/2 (1) KIC = 0.203HV a1/2 a
Fig. 2. Morphology of the polished surface and the propagation of the crack.
than 90 vol% with varies sizes TiC grains as byproducts [2]. However, a synergism between particle dispersion and whisker toughening [5], as well as a multi-scale effect [6,7] can be expected. From the SEM photographs on the polished surface of the composite shown in Fig. 2, three kinds of materials with different brightness can be observed. Besides the brightest phase which may be thought as the metal Ni because of its lowest volume
where 2a and 2c are the diagonal length of indentation and total length of cracks, respectively, and HV is Vickers hardness. At least 16 specimens were tested for the fracture toughness. The morphology of the synthesized powder and the microstructure of the sintered composite are investigated by a scanning electron microscope (Hitachi S-570 SEM). 3. Results and discussion Experimental results show that the mechanical properties of the composite are greatly improved comparing to pure Al2 O3 ceramics. The average flexural strength and fracture toughness of the composite are respectively 780.9 MPa and 7.27 MPa m1/2 . TiC whiskers are successfully synthesized distributing among the Al2 O3 matrix grains shown in Fig. 1. The TiC whisker yield in the carbothermal reduction process is commonly less
147
Fig. 3. Morphology of the fracture section.
148
B. Liu et al. / Materials Science and Engineering A 460–461 (2007) 146–148
the fracture toughness of the composite by crack deflection and bridging grains. Therefore either flexural strength or fracture toughness of the composite is greatly increased. Pullout of TiC whiskers can be well observed from the fracture surface shown in Fig. 3 on which some of the pullout holes are labeled by arrows. Obviously, toughening by TiC whiskers can be expected to be more effective than that by SiC whiskers because of the better physical compatibility between TiC whisker and alumina matrix. 4. Conclusion A kind of TiC whiskers toughening alumina ceramic matrix composite with flexural strength and fracture toughness of 780.9 MPa and 7.27 MPa m1/2 , respectively, has been prepared by in situ growth technology based on a carbothermal reduction process. The improvement on the mechanical properties can be attributed to the formation of the network microstructure, the change on the fracture mode and the presence of TiC whiskers. Acknowledgements Fig. 4. Fracture mode.
fraction, the dimmest phase was suggested to be Al2 O3 while the third phase was TiC [8]. It can be seen from Fig. 2 that the network microstructure is formed among with the TiC grains and the Al2 O3 matrix grains. Such a kind of microstructure is profitable to the improvement on the mechanical properties of the composite [8], which can be attributed to the presence of the whiskers and multi-scale grains where large TiC grains are joined by TiC whiskers and small TiC grains. The fracture mode with mainly intergranular fracture accompanied by partial transgranular fracture can be observed from the SEM photographs on the fracture surface shown in Figs. 3 and 4. The transgranular fracture mode will improve the flexural strength of the composite because of the higher interface fracture energy, whereas the intergranular fracture mode will improve
This project was supported by Excellent Young Teacher Support Program of Ministry of Education (No. 2003-79) and Special Foundation for University Doctor Discipline of Ministry of Education (20030422012), China. References [1] L. Sun, J.S. Pan, Mater. Lett. 51 (2001) 270–274. [2] N. Ahlen, M. Johnsson, M. Nygren, J. Am. Ceram. Soc. 79 (1996) 2803–2808. [3] Y.Q. Wu, Y.F. Zhang, J.K. Guo, Mater. Rev. 14 (2000) 20–22 (in Chinese). [4] M. Fukuhara, K. Fukazawa, A. Fukawa, Wear 102 (1985) 195– 210. [5] C.Z. Huang, X. Ai, China Ceram. 34 (1998) 20–22 (in Chinese). [6] Y.Y. Li, J.Z. Cui, Compos. Sci. Technol. 65 (2005) 1447–1458. [7] H.L. Liu, C.Z. Huang, J. Wang, B.Q. Liu, Key Eng. Mater. 315–316 (2006) 118–122. [8] F.Z. Li, Q. Ao, Y.C. Liu, Y.X. Liu, X.J. Tian, Mater. Mech. Eng. 20 (1996) 27–29 (in Chinese).
Preparation and mechanical properties of in situ growth TiC whiskers toughening Al2O3 ceramic matrix composite Bingqiang Liu, Chuanzhen Huang ∗ , Meilin Gu, Hongtao Zhu, Hanlian Liu Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, Jinan 250061, China Received 23 September 2006; received in revised form 8 January 2007; accepted 8 January 2007
Abstract An in situ growth TiC whiskers toughening Al2 O3 ceramic matrix composite has been prepared based on a carbothermal reduction process by two separate steps such as in situ synthesizing of the TiC whiskers in alumina matrix powder and hot pressed sintering of the composite, respectively. The TiC content in the composite is 40% in terms of volume. The experimental results show that the composite is with a flexural strength of 780.9 MPa and a fracture toughness of 7.27 MPa m1/2 and such a preparation and fabrication process for in situ growth whiskers toughened ceramics can greatly improve the mechanical property of Al2 O3 matrix composite. © 2007 Elsevier B.V. All rights reserved. Keywords: TiC whisker; Carbothermal reduction; In situ growth; Al2 O3 matrix; Composite
1. Introduction As one of the important ceramic whiskers with high elastic modulus, TiC whisker is more appropriate to be a strengthening and toughening additive for majority of ceramic matrix composites than commonly used SiC whisker because of the higher thermal expansion coefficient. TiC whiskers is commonly produced by chemical vapour deposit (CVD) methods and added into matrixes by mechanical mixing procedures [1]. As a result, the application of TiC whiskers is limited by such disadvantages as high cost, health hazard and difficult dispersion. After the recently suggestions of a carbothermal reduction process for growth of TiC whiskers [2] and an in situ growth idea for synthesizing high performance composites [3], the combination of them will certainly become a promising way to produce in situ growth TiC whiskers toughening ceramic matrix composites with higher mechanical properties, much lower cost and less environmental pollution, etc. In the present work, an Al2 O3 ceramic matrix composite toughened by in situ growth TiC whiskers is prepared. The TiC whiskers are firstly synthesized by the carbothermal reduction
∗
Corresponding author. Tel.: +86 531 88392539; fax: +86 531 88392328. E-mail address: [email protected] (C. Huang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.009
process in the Al2 O3 matrix powder and subsequently the composite is sintered. The morphology of the synthesized powder and the microstructure, mechanical properties of the composite are studied. 2. Experiments Commercial Al2 O3 with an average grain size of 0.5 m is used as the matrix powder. Commercial TiO2 , amorphous carbon, synthesis pure NaCl are used for the synthesis of TiC whiskers and commercial Ni with an average grain size of 2.3 m is used as the catalyst. All of the raw materials mentioned above are proportionally added to produce a composite powder with about 40% TiC in terms of volume. The mixture is ball-milled for 24 h in ethanol medium. After dried and sieved, the homogeneous mixture is put into a graphite reactor for the synthesis of the composite powder, firstly heated to 1460–1480 ◦ C in 10–12 min in a flowing argon-gas atmosphere and held for 60 min duration, subsequently is heated to 1690–1710 ◦ C and held for 30 min duration. Such a synthesis process can either elevate the compound fraction of C in TiC whiskers or increase the chemical stability of the whiskers during the sintering procedure because that the powder will subsequently be sintered at 1700 ◦ C hereabout. The synthesized composite powder is put into a graphite die after dry ball-milled
B. Liu et al. / Materials Science and Engineering A 460–461 (2007) 146–148
Fig. 1. Morphology of the synthesized composite powder.
for 4.5–5 h, heated to 1690–1710 ◦ C and held for 30 min duration at a pressure of 25 MPa in a flowing argon-gas atmosphere. The flexural strength is measured using 3 mm × 4 mm × 30 mm polished test bars under three-point bending test on WD-10 electron universal tester with a span of 20 mm and a loading rate of 0.5 mm min−1 , considering an average of six specimens. The fracture toughness is evaluated according to the crack lengths of indentation method. The indenter is the Vickers DPH type and the applied static load is 196 N for 15 s. Crack lengths for toughness assessment are measured using optical microscope. The values of fracture toughness (KIC ) are calculated by the equation reported by Fukuhara et al. [4]: c −3/2 (1) KIC = 0.203HV a1/2 a
Fig. 2. Morphology of the polished surface and the propagation of the crack.
than 90 vol% with varies sizes TiC grains as byproducts [2]. However, a synergism between particle dispersion and whisker toughening [5], as well as a multi-scale effect [6,7] can be expected. From the SEM photographs on the polished surface of the composite shown in Fig. 2, three kinds of materials with different brightness can be observed. Besides the brightest phase which may be thought as the metal Ni because of its lowest volume
where 2a and 2c are the diagonal length of indentation and total length of cracks, respectively, and HV is Vickers hardness. At least 16 specimens were tested for the fracture toughness. The morphology of the synthesized powder and the microstructure of the sintered composite are investigated by a scanning electron microscope (Hitachi S-570 SEM). 3. Results and discussion Experimental results show that the mechanical properties of the composite are greatly improved comparing to pure Al2 O3 ceramics. The average flexural strength and fracture toughness of the composite are respectively 780.9 MPa and 7.27 MPa m1/2 . TiC whiskers are successfully synthesized distributing among the Al2 O3 matrix grains shown in Fig. 1. The TiC whisker yield in the carbothermal reduction process is commonly less
147
Fig. 3. Morphology of the fracture section.
148
B. Liu et al. / Materials Science and Engineering A 460–461 (2007) 146–148
the fracture toughness of the composite by crack deflection and bridging grains. Therefore either flexural strength or fracture toughness of the composite is greatly increased. Pullout of TiC whiskers can be well observed from the fracture surface shown in Fig. 3 on which some of the pullout holes are labeled by arrows. Obviously, toughening by TiC whiskers can be expected to be more effective than that by SiC whiskers because of the better physical compatibility between TiC whisker and alumina matrix. 4. Conclusion A kind of TiC whiskers toughening alumina ceramic matrix composite with flexural strength and fracture toughness of 780.9 MPa and 7.27 MPa m1/2 , respectively, has been prepared by in situ growth technology based on a carbothermal reduction process. The improvement on the mechanical properties can be attributed to the formation of the network microstructure, the change on the fracture mode and the presence of TiC whiskers. Acknowledgements Fig. 4. Fracture mode.
fraction, the dimmest phase was suggested to be Al2 O3 while the third phase was TiC [8]. It can be seen from Fig. 2 that the network microstructure is formed among with the TiC grains and the Al2 O3 matrix grains. Such a kind of microstructure is profitable to the improvement on the mechanical properties of the composite [8], which can be attributed to the presence of the whiskers and multi-scale grains where large TiC grains are joined by TiC whiskers and small TiC grains. The fracture mode with mainly intergranular fracture accompanied by partial transgranular fracture can be observed from the SEM photographs on the fracture surface shown in Figs. 3 and 4. The transgranular fracture mode will improve the flexural strength of the composite because of the higher interface fracture energy, whereas the intergranular fracture mode will improve
This project was supported by Excellent Young Teacher Support Program of Ministry of Education (No. 2003-79) and Special Foundation for University Doctor Discipline of Ministry of Education (20030422012), China. References [1] L. Sun, J.S. Pan, Mater. Lett. 51 (2001) 270–274. [2] N. Ahlen, M. Johnsson, M. Nygren, J. Am. Ceram. Soc. 79 (1996) 2803–2808. [3] Y.Q. Wu, Y.F. Zhang, J.K. Guo, Mater. Rev. 14 (2000) 20–22 (in Chinese). [4] M. Fukuhara, K. Fukazawa, A. Fukawa, Wear 102 (1985) 195– 210. [5] C.Z. Huang, X. Ai, China Ceram. 34 (1998) 20–22 (in Chinese). [6] Y.Y. Li, J.Z. Cui, Compos. Sci. Technol. 65 (2005) 1447–1458. [7] H.L. Liu, C.Z. Huang, J. Wang, B.Q. Liu, Key Eng. Mater. 315–316 (2006) 118–122. [8] F.Z. Li, Q. Ao, Y.C. Liu, Y.X. Liu, X.J. Tian, Mater. Mech. Eng. 20 (1996) 27–29 (in Chinese).