- Email: [email protected]
Effect of TiC addition on the mechanical behaviour of Al2O3–SiC whiskers composites obtained by SPS
Journal of the European Ceramic Society 36 (2016) 2149–2152
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Effect of TiC addition on the mechanical behaviour of Al2 O3 –SiC whiskers composites obtained by SPS C.F. Gutiérrez-González a,b,∗ , M. Suarez a,b , S. Pozhidaev c , S. Rivera a , P. Peretyagin c , W. Solís c , L.A. Díaz b , A. Fernandez b , R. Torrecillas b,c a
Nanoker Research, Pol. Ind. Olloniego, Parcela 22A, Nave 5, 33660 Oviedo, Spain Nanomaterials and Nanotechnology Research Centre (CINN), CSIC-Universidad de Oviedo, 33940 El Entrego, Principado de Asturias, Spain c Moscow State University of Technology “STANKIN”, Vadkovskiy per. 1, Moscow 127994, Russian Federation b
a r t i c l e
i n f o
Article history: Received 25 January 2016 Accepted 30 January 2016 Available online 23 February 2016 Keywords: SPS Cutting tools Mechanical properties Composites
a b s t r a c t Large Al2 O3 –SiCw and Al2 O3 –SiCw –TiC composites disks (150 mm diameter) were prepared in this work by using the spark plasma sintering technique. The main physical and mechanical properties of these composites were measured in different disk zones in order to test out the homogeneity of the sintering process. It has been found that the incorporation of TiC as reinforcing phase to the Al2 O3 –SiCw composite, improved densification, mechanical strength and hardness. This result was analyzed in terms of SEM microstructure observations showing that the presence of TiC inhibited the alumina grain growth. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays the development of chemically stable cutting tool materials with improved mechanical properties is becoming critical. There is also a special effort on developing cutting tools capable of operating at high cutting speeds for longer times that the tools industrialized so far. Large production of these kind of tools is difficult because three main reasons: (1) Materials need to fulfil the required mechanical properties in order to ensure higher durability. (2) Traditional sintering methods are very limited not only in the amount of material processed but also in the time needed to complete a sintering cycle, resulting in an increase of final product price. (3) Typical materials used for these tools are based on nonconductive ceramics, which causes technological difficulties in the machining process and a further increment on the manufacturing costs. Because of its hardness, one of the most commonly used materials for fabricating cutting tools is alumina. However alumina is a brittle material and needs to be reinforced due to its low flexural strength and toughness values. One of the most common methods for this purpose is the incorporation of a secondary phase. Silicon carbide appears as a candidate in order to improve the fracture
∗ Corresponding author at: Nanoker Research, Pol. Ind. Olloniego, Parcela 22A, Nave 5, 33660 Oviedo, Spain. E-mail address: [email protected] (C.F. Gutiérrez-González). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.01.050 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
toughness and other mechanical properties [1–3]. For example, the fracture strength and creep resistance of Al2 O3 were improved by 3–5 times and by 3–4 orders by incorporating only 5% nano-sized SiC particles, respectively. However the strength of this composite is still lower than Si3 N4 . Moreover, industrial scale production of such tools is very costly not only because of the limited size of the blanks for machining the tools but also because of the machining costs. Furthermore, during the manufacturing of such tools, fast sintering is required to prevent grain growth and deleterious intercomponent reactions which, for example, occur above 1500 ◦ C in the Al2 O3 –TiC system [4]. In this respect, the sintering hybrid technique by plasma and induction, can rapidly manufacture large blank pieces (up to 400 mm diameter) from which it is possible to extract a higher number of tools per sintering cycle when compared to traditional methods as Hot Press. Regarding the improvement of the electrical conductivity of these composites it has been shown that the incorporation of TiC as secondary reinforcing phase not only improves this property but also the mechanical properties [5]. Taking into consideration all these facts, we are going to develop a new approach to the manufacturing of cutting tools in the Al2 O3 , SiCw and TiC system. For the first time, large size disks (150 mm diameter) will be sintered using the SPS technology. This will bring the opportunity of decreasing the fabrication costs for these kind of tools and, because of the TiC incorporation, the machining costs will be also reduced. The evaluation of the sintering process, in terms
2150
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
of density measurements, and the mechanical properties, in terms of flexural strength and hardness, will be also evaluated in order to prove that the sintering process has been homogeneous in all the disks regions. 2. Materials and characterization 2.1. Raw materials The following commercially available powders have been used as raw materials: (1) high-purity ␣-alumina (99.99%) (TM-DAR, Taimei Chemicals Co., Ltd., Japan) with an average particle size of d50 = 0.1 m, a BET specific surface area of 14.5 m2 /g, and the following chemical analysis (ppm): Si (10), Fe (8), Na (8), K (3), Ca (3), Mg (2), Cu (1), Cr (<1), Mn (<1), U (<0.004), Th (<0.005). (2) Silicon Carbide whiskers SC-9 M (Advanced Composite Materials LLC, Greer, SC, USA), 0.65 m diameter and an average length d50 = 11 m, with a silica content between 0.35 and 0.75 wt. % and free carbon between 0.05 and 0.3 wt. %. (3) Titanium Carbide STD 120 (H.C. Stark GmbH, Munich, Germany), with a total carbon content of min 19.6 wt. %, maximum oxygen content of 1.3 wt. % and an average particle size, d50 , between 1.0 and 1.5 m. 2.2. Powder processing and sintering Powders were attrition milled using isopropanol and alumina balls as milling media for 2 h, then dried at 100 ◦ C for 48 h and finally sieved through 180 m. The proportion of the TiC was adjusted to 22 vol. %, in order to obtain an electro-machinable material. The densification of the powders was performed by Spark Plasma Sintering in a HHPD400 equipment (FCT Systeme GmbH, Effelder-Rauenstein, Germany) using a 150 mm diameter tool. The sintering cycle was performed at 1780 ◦ C for 15 min and 30 MPa, under vacuum, with a heating rate of 25 ◦ C/min and free cooling. The cycle was controlled by a pyrometer situated at the top of the machine and pointing at the center of the blank (3 mm over the top surface). As a result, disks with 150 mm diameter and 9 mm thickness were obtained. For comparison purposes an Al2 O3 –SiCw disk with the same size and sintering cycle was also prepared.
Fig. 1. X-ray diffractogram for the starting powders and the sintered materials.
MA). The specimens were loaded to failure with a cross-head speed of 1 mm/min and a span of 12.5 mm according to ISO 6872:2008. Reported strengths represented the mean and standard deviation of at least 5 specimens, and were calculated according to Eq. (1): f =
3QL 2
2lh
(1)
where Q is the failure load, L is the span, l is the width and h is the height. The microstructural characterization of the polished surfaces with diamond to1 m roughness and thermally etched (1350 ◦ C, 5 min, vacuum atmosphere) was performed by scanning electron microscopy (FEI Quanta 650 FEG ESEM).
2.3. Characterization 3. Results and discussion In order to perform the characterization of the sintered disks and detect possible inhomogeneities on the measured properties, three different regions were distinguished in the sintered disks: Center (CE), center of the radius (CR) and the edge of the disk (ED). X-ray of the sintered materials were performed using Cu K␣ radiation (XRD Bruker AXS D8 ADVANCE, with a SolX energydispersive detector) in order to identify the different phases after the sintering process. The density was measured according to the Archimedes method using water as immersion media. The theoretical density was calculated by the rule of mixtures assuming a density of 3.96 g cm−3 for the Al2 O3 [6], 3.25 g cm−3 for the SiC and 4.9 g cm−3 for the TiC. The hardness was measured by performing 20 indentations on each area with a Vickers diamond microindenter (Leco 100A Microindentation Hardness Testing System, USA) on surfaces of cross sections polished down to 1 , with an applied load of 1 kgf for 10 s. The hardness was determined according to the equation HV = 1.853P/d2 , where P stands for the applied load (in N) and d stands for the diagonal length of the indentation (in mm). The bending strength, f , was determined by the three point bending test using prismatic bars with 4 mm width, 20 mm length and 3 mm thickness. The tensile surface of the bars was polished down to 1 m. The tests were performed at room temperature using a universal testing machine (Instron Model E10000, Boston,
Fig. 1 shows the X-ray diffractogram for the starting powders and the sintered materials. As it can be observed, the only present phases are Al2 O3 and SiC in the Al2 O3 –SiCw sample and Al2 O3 , SiC and TiC for the Al2 O3 –SiCw –TiC composite. No chemical reactions between components are observed. Fig. 2 shows representative microstructures of the sintered materials. Fig. 2A corresponds to the Al2 O3 –SiCw and Fig. 2B corresponds to the Al2 O3 –SiCw –TiC. In Fig. 2B three different phases can be observed. The darkest one corresponds to silicon carbide whiskers with an aspect ratio about 6–10, the medium gray phase is the TiC with a particle size of about 1.5 m and the lighter phase corresponds to the alumina matrix with a crystal size ranging from 0.5 to 1.5 m. As it can be observed all the components are homogeneously distributed, interfaces are well bonded and no microcracks or pores are observed. However the Al2 O3 –SiCw shows porosity and an inhomogeneous distribution of the SiC phase as it can be seen by the presence of some isolated alumina areas. Density measurements are presented in Table 1. As it can be observed the density values obtained in each material are very homogeneous and no significant differences between the center and the edge of each disc is observed. As mean values, the Al2 O3 –SiCw reached 95.35% and the Al2 O3 –SiCw –TiC 99.70%. This shows the important role that the TiC plays, improving the densifi-
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
2151
Fig. 2. Representative microstructures of the sintered materials. Fig. 2A corresponds to the Al2 O3 –SiCw and Fig. 2B corresponds to the Al2 O3 –SiCw –TiC. Table 1 Properties of the two different composites evaluated in three different areas of the disks. Zone
CE CR ED
Al2 O3 –SiCw
Al2 O3 –SiCw –TiC
Density (g/cm3 )
% TD
Bending strength, f (MPa)
Vickers hardness, Hv (GPa)
Density (g/cm3 )
3.54 ± 0.03 3.55 ± 0.02 3.54 ± 0.01
95.26 ± 0.70 95.53 ± 0.50 95.26 ± 0.27
498 ± 45 525 ± 27 537 ± 42
15.43 ± 2.02 15.85 ± 1.38 14.99 ± 3.01
3.97 ± 0.02 3.97 ± 0.02 3.96 ± 0.01
cation of the Al2 O3 –SiCw material. This densification behavior has been also previously observed in pure SiC materials [7]. The results for the bending strength test are summarized in Table 1. As it can be observed, in the case of the Al2 O3 –SiCw material the results are very similar in the entire sample with a small tendency to increase this value as we approach the edge of the sample. This trend is more pronounced in the case of the Al2 O3 –SiCw –TiC material with an increment of about 9%. However, the most obvious fact is the increased strength of the material when incorporating titanium carbide. This improvement varies between 23 and 38% depending on the area of the disc and is associated with the pinning effect provided by the TiC. It is well known that the mechanical strength of the alumina can be improved by obtaining microstructures with smaller grain sizes [8]. However this is very difficult to achieve when it comes to pure alumina sintering since it has a strong tendency to grain growth during sintering [9,10]. Clearly, the incorporation of silicon carbide fibers improves the mechanical strength of the alumina due to the pinning effect provided by them [11]. These silicon carbide fibers are located between the grain boundaries of the alumina acting as barriers preventing grain growth and thus obtaining a finer microstructure. Although this phenomenon can be observed in the Al2 O3 –SiCw composite, there are areas in which the alumina grains have grown because of the inhomogeneous SiC distribution. This fact can be observed in Fig. 3 where a SEM micrograph of the Al2 O3 –SiCw composite at higher magnification is shown. In this figure it can be seen that in the central area, where there is no SiC, alumina grains have grown much more than areas where SiC is distributed evenly. In the case of the Al2 O3 –SiCw –TiC composite, the incorporation of an additional third phase contributes to increase the pinning effect thus obtaining an even finer microstructure. Moreover, no isolated alumina areas with abnormal grain growth are observed in this composite. We can assume then, that the incorporation of titanium carbide is the responsible for the increased mechanical strength observed as the pinning effect is more effective. The results for the Vickers hardness are summarized in Table 1. In this case the addition of TiC to the Al2 O3 -SiCw matrix signif-
% TD
99.75 ± 0.37 99.74 ± 0.49 99.60 ± 0.27
Bending strength, f (MPa)
Vickers hardness, Hv (GPa)
684 ± 56 648 ± 37 744 ± 53
22.01 ± 1.45 21.60 ± 0.83 21.06 ± 1.29
Fig. 3. SEM micrograph of the Al2 O3 –SiCw composite at higher magnification. In the central area, where there is no SiC, alumina grains have grown much more than in the rest of the sample.
icantly improves the hardness values in all sample regions. This increment ranges from 36% to 42% depending on the sample zone. These results are to be expected if we consider that the TiC, as explained before, has improved the densification of the material eliminating residual pores. It is also important to notice that, as with the rest of measured properties, hardness is also homogenous in all the zones of the disk. 4. Conclusions Based on the study developed in this work, the following conclusions can be made:
2152
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
1. Large Al2 O3 –SiCw and Al2 O3 –SiCw –TiC composites disks up to 150 mm diameter with a homogeneous properties distribution have been sintered by SPS. 2. The incorporation of TiC as reinforcing phase to the Al2 O3 –SiCw composite, improved densification during sintering reaching values close to 100% of the theoretical density for this material. 3. The incorporation of TiC also improved the mechanical strength and hardness of the Al2 O3 –SiCw composite. The average improvement for these properties was found to be 31% and 39% respectively. 4. Analysis of the results showed that this improvement is due to the superior grain growth inhibition effect of the alumina grains provided by the presence of TiC. Acknowledgment Authors would like to thank The Ministry of the Russian Federation supported this work in the frame of Governmental Regulation of the Russian Federation No. 220, 9 April 2010 by contract 14.B25.31.0012, 26 June 2013.
References [1] Y.L. Dong, F.M. Xu, X.L. Shi, C. Zhang, Z.J. Zhang, J.M. Yang, et al., Fabrication and mechanical properties of nano-/micro-sized Al2 O3 /SiC composites, Mat. Sci. Eng. A Struct. 504 (2009) 49–54. [2] S.K. Sharma, B. Venkata Manoj Kumar, K.Y. Lim, Y.W. Kim, S.K. Nath, Erosion behavior of SiC–WC composites, Ceram. Int. 40 (2014) 6829–6839. [3] K. Takahashi, M. Yokouchi, S.K. Lee, K. Ando, Crack-healing behavior of Al2 O3 toughened by SiC whiskers, J. Am. Ceram. Soc. 86 (2003) 2143–2147. [4] M.P. Borom, M. Lee, Effect of heating rate on densification of alumina–titanium carbide composition, Adv. Ceram. Mater. 1 (1986) 335–340. [5] M. Kanamaru, T. Tatsuno, T. Kusaka, Hot-pressed Al2 O3 /SiC whisker/TiC nano-composites, J. Ceram. Soc. Jpn. 100 (1992) 408–412. [6] in: http://www.taimei-chem.co.jp/product/english/01.html#6. [7] H. Endo, M. Ueki, H. Kubo, Hot pressing of SiC–TiC composites, J. Mater. Sci. 25 (1990) 2503–2506. [8] D.M. Vernon, J. Rankin, C. Caragianis-Broadbridge, B.L. Laube, Novel processing route to control grain growth in submicrometer alumina compacts, J. Am. Ceram. Soc. 82 (1999) 2969–2977. [9] T. Sakuma, Mater. Sci. Forum (1996) 109–120. [10] A. Gupta, S. Sharma, M.R. Joshi, P. Agarwal, K. Balani, Mater. Sci. Forum (2010) 87–130. [11] L.C. Stearns, M.P. Harmer, Particle-inhibited grain growth in Al2 O3 –SiC: I experimental results, J. Am. Ceram. Soc. 79 (1996) 3013–3019.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Effect of TiC addition on the mechanical behaviour of Al2 O3 –SiC whiskers composites obtained by SPS C.F. Gutiérrez-González a,b,∗ , M. Suarez a,b , S. Pozhidaev c , S. Rivera a , P. Peretyagin c , W. Solís c , L.A. Díaz b , A. Fernandez b , R. Torrecillas b,c a
Nanoker Research, Pol. Ind. Olloniego, Parcela 22A, Nave 5, 33660 Oviedo, Spain Nanomaterials and Nanotechnology Research Centre (CINN), CSIC-Universidad de Oviedo, 33940 El Entrego, Principado de Asturias, Spain c Moscow State University of Technology “STANKIN”, Vadkovskiy per. 1, Moscow 127994, Russian Federation b
a r t i c l e
i n f o
Article history: Received 25 January 2016 Accepted 30 January 2016 Available online 23 February 2016 Keywords: SPS Cutting tools Mechanical properties Composites
a b s t r a c t Large Al2 O3 –SiCw and Al2 O3 –SiCw –TiC composites disks (150 mm diameter) were prepared in this work by using the spark plasma sintering technique. The main physical and mechanical properties of these composites were measured in different disk zones in order to test out the homogeneity of the sintering process. It has been found that the incorporation of TiC as reinforcing phase to the Al2 O3 –SiCw composite, improved densification, mechanical strength and hardness. This result was analyzed in terms of SEM microstructure observations showing that the presence of TiC inhibited the alumina grain growth. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays the development of chemically stable cutting tool materials with improved mechanical properties is becoming critical. There is also a special effort on developing cutting tools capable of operating at high cutting speeds for longer times that the tools industrialized so far. Large production of these kind of tools is difficult because three main reasons: (1) Materials need to fulfil the required mechanical properties in order to ensure higher durability. (2) Traditional sintering methods are very limited not only in the amount of material processed but also in the time needed to complete a sintering cycle, resulting in an increase of final product price. (3) Typical materials used for these tools are based on nonconductive ceramics, which causes technological difficulties in the machining process and a further increment on the manufacturing costs. Because of its hardness, one of the most commonly used materials for fabricating cutting tools is alumina. However alumina is a brittle material and needs to be reinforced due to its low flexural strength and toughness values. One of the most common methods for this purpose is the incorporation of a secondary phase. Silicon carbide appears as a candidate in order to improve the fracture
∗ Corresponding author at: Nanoker Research, Pol. Ind. Olloniego, Parcela 22A, Nave 5, 33660 Oviedo, Spain. E-mail address: [email protected] (C.F. Gutiérrez-González). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.01.050 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
toughness and other mechanical properties [1–3]. For example, the fracture strength and creep resistance of Al2 O3 were improved by 3–5 times and by 3–4 orders by incorporating only 5% nano-sized SiC particles, respectively. However the strength of this composite is still lower than Si3 N4 . Moreover, industrial scale production of such tools is very costly not only because of the limited size of the blanks for machining the tools but also because of the machining costs. Furthermore, during the manufacturing of such tools, fast sintering is required to prevent grain growth and deleterious intercomponent reactions which, for example, occur above 1500 ◦ C in the Al2 O3 –TiC system [4]. In this respect, the sintering hybrid technique by plasma and induction, can rapidly manufacture large blank pieces (up to 400 mm diameter) from which it is possible to extract a higher number of tools per sintering cycle when compared to traditional methods as Hot Press. Regarding the improvement of the electrical conductivity of these composites it has been shown that the incorporation of TiC as secondary reinforcing phase not only improves this property but also the mechanical properties [5]. Taking into consideration all these facts, we are going to develop a new approach to the manufacturing of cutting tools in the Al2 O3 , SiCw and TiC system. For the first time, large size disks (150 mm diameter) will be sintered using the SPS technology. This will bring the opportunity of decreasing the fabrication costs for these kind of tools and, because of the TiC incorporation, the machining costs will be also reduced. The evaluation of the sintering process, in terms
2150
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
of density measurements, and the mechanical properties, in terms of flexural strength and hardness, will be also evaluated in order to prove that the sintering process has been homogeneous in all the disks regions. 2. Materials and characterization 2.1. Raw materials The following commercially available powders have been used as raw materials: (1) high-purity ␣-alumina (99.99%) (TM-DAR, Taimei Chemicals Co., Ltd., Japan) with an average particle size of d50 = 0.1 m, a BET specific surface area of 14.5 m2 /g, and the following chemical analysis (ppm): Si (10), Fe (8), Na (8), K (3), Ca (3), Mg (2), Cu (1), Cr (<1), Mn (<1), U (<0.004), Th (<0.005). (2) Silicon Carbide whiskers SC-9 M (Advanced Composite Materials LLC, Greer, SC, USA), 0.65 m diameter and an average length d50 = 11 m, with a silica content between 0.35 and 0.75 wt. % and free carbon between 0.05 and 0.3 wt. %. (3) Titanium Carbide STD 120 (H.C. Stark GmbH, Munich, Germany), with a total carbon content of min 19.6 wt. %, maximum oxygen content of 1.3 wt. % and an average particle size, d50 , between 1.0 and 1.5 m. 2.2. Powder processing and sintering Powders were attrition milled using isopropanol and alumina balls as milling media for 2 h, then dried at 100 ◦ C for 48 h and finally sieved through 180 m. The proportion of the TiC was adjusted to 22 vol. %, in order to obtain an electro-machinable material. The densification of the powders was performed by Spark Plasma Sintering in a HHPD400 equipment (FCT Systeme GmbH, Effelder-Rauenstein, Germany) using a 150 mm diameter tool. The sintering cycle was performed at 1780 ◦ C for 15 min and 30 MPa, under vacuum, with a heating rate of 25 ◦ C/min and free cooling. The cycle was controlled by a pyrometer situated at the top of the machine and pointing at the center of the blank (3 mm over the top surface). As a result, disks with 150 mm diameter and 9 mm thickness were obtained. For comparison purposes an Al2 O3 –SiCw disk with the same size and sintering cycle was also prepared.
Fig. 1. X-ray diffractogram for the starting powders and the sintered materials.
MA). The specimens were loaded to failure with a cross-head speed of 1 mm/min and a span of 12.5 mm according to ISO 6872:2008. Reported strengths represented the mean and standard deviation of at least 5 specimens, and were calculated according to Eq. (1): f =
3QL 2
2lh
(1)
where Q is the failure load, L is the span, l is the width and h is the height. The microstructural characterization of the polished surfaces with diamond to1 m roughness and thermally etched (1350 ◦ C, 5 min, vacuum atmosphere) was performed by scanning electron microscopy (FEI Quanta 650 FEG ESEM).
2.3. Characterization 3. Results and discussion In order to perform the characterization of the sintered disks and detect possible inhomogeneities on the measured properties, three different regions were distinguished in the sintered disks: Center (CE), center of the radius (CR) and the edge of the disk (ED). X-ray of the sintered materials were performed using Cu K␣ radiation (XRD Bruker AXS D8 ADVANCE, with a SolX energydispersive detector) in order to identify the different phases after the sintering process. The density was measured according to the Archimedes method using water as immersion media. The theoretical density was calculated by the rule of mixtures assuming a density of 3.96 g cm−3 for the Al2 O3 [6], 3.25 g cm−3 for the SiC and 4.9 g cm−3 for the TiC. The hardness was measured by performing 20 indentations on each area with a Vickers diamond microindenter (Leco 100A Microindentation Hardness Testing System, USA) on surfaces of cross sections polished down to 1 , with an applied load of 1 kgf for 10 s. The hardness was determined according to the equation HV = 1.853P/d2 , where P stands for the applied load (in N) and d stands for the diagonal length of the indentation (in mm). The bending strength, f , was determined by the three point bending test using prismatic bars with 4 mm width, 20 mm length and 3 mm thickness. The tensile surface of the bars was polished down to 1 m. The tests were performed at room temperature using a universal testing machine (Instron Model E10000, Boston,
Fig. 1 shows the X-ray diffractogram for the starting powders and the sintered materials. As it can be observed, the only present phases are Al2 O3 and SiC in the Al2 O3 –SiCw sample and Al2 O3 , SiC and TiC for the Al2 O3 –SiCw –TiC composite. No chemical reactions between components are observed. Fig. 2 shows representative microstructures of the sintered materials. Fig. 2A corresponds to the Al2 O3 –SiCw and Fig. 2B corresponds to the Al2 O3 –SiCw –TiC. In Fig. 2B three different phases can be observed. The darkest one corresponds to silicon carbide whiskers with an aspect ratio about 6–10, the medium gray phase is the TiC with a particle size of about 1.5 m and the lighter phase corresponds to the alumina matrix with a crystal size ranging from 0.5 to 1.5 m. As it can be observed all the components are homogeneously distributed, interfaces are well bonded and no microcracks or pores are observed. However the Al2 O3 –SiCw shows porosity and an inhomogeneous distribution of the SiC phase as it can be seen by the presence of some isolated alumina areas. Density measurements are presented in Table 1. As it can be observed the density values obtained in each material are very homogeneous and no significant differences between the center and the edge of each disc is observed. As mean values, the Al2 O3 –SiCw reached 95.35% and the Al2 O3 –SiCw –TiC 99.70%. This shows the important role that the TiC plays, improving the densifi-
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
2151
Fig. 2. Representative microstructures of the sintered materials. Fig. 2A corresponds to the Al2 O3 –SiCw and Fig. 2B corresponds to the Al2 O3 –SiCw –TiC. Table 1 Properties of the two different composites evaluated in three different areas of the disks. Zone
CE CR ED
Al2 O3 –SiCw
Al2 O3 –SiCw –TiC
Density (g/cm3 )
% TD
Bending strength, f (MPa)
Vickers hardness, Hv (GPa)
Density (g/cm3 )
3.54 ± 0.03 3.55 ± 0.02 3.54 ± 0.01
95.26 ± 0.70 95.53 ± 0.50 95.26 ± 0.27
498 ± 45 525 ± 27 537 ± 42
15.43 ± 2.02 15.85 ± 1.38 14.99 ± 3.01
3.97 ± 0.02 3.97 ± 0.02 3.96 ± 0.01
cation of the Al2 O3 –SiCw material. This densification behavior has been also previously observed in pure SiC materials [7]. The results for the bending strength test are summarized in Table 1. As it can be observed, in the case of the Al2 O3 –SiCw material the results are very similar in the entire sample with a small tendency to increase this value as we approach the edge of the sample. This trend is more pronounced in the case of the Al2 O3 –SiCw –TiC material with an increment of about 9%. However, the most obvious fact is the increased strength of the material when incorporating titanium carbide. This improvement varies between 23 and 38% depending on the area of the disc and is associated with the pinning effect provided by the TiC. It is well known that the mechanical strength of the alumina can be improved by obtaining microstructures with smaller grain sizes [8]. However this is very difficult to achieve when it comes to pure alumina sintering since it has a strong tendency to grain growth during sintering [9,10]. Clearly, the incorporation of silicon carbide fibers improves the mechanical strength of the alumina due to the pinning effect provided by them [11]. These silicon carbide fibers are located between the grain boundaries of the alumina acting as barriers preventing grain growth and thus obtaining a finer microstructure. Although this phenomenon can be observed in the Al2 O3 –SiCw composite, there are areas in which the alumina grains have grown because of the inhomogeneous SiC distribution. This fact can be observed in Fig. 3 where a SEM micrograph of the Al2 O3 –SiCw composite at higher magnification is shown. In this figure it can be seen that in the central area, where there is no SiC, alumina grains have grown much more than areas where SiC is distributed evenly. In the case of the Al2 O3 –SiCw –TiC composite, the incorporation of an additional third phase contributes to increase the pinning effect thus obtaining an even finer microstructure. Moreover, no isolated alumina areas with abnormal grain growth are observed in this composite. We can assume then, that the incorporation of titanium carbide is the responsible for the increased mechanical strength observed as the pinning effect is more effective. The results for the Vickers hardness are summarized in Table 1. In this case the addition of TiC to the Al2 O3 -SiCw matrix signif-
% TD
99.75 ± 0.37 99.74 ± 0.49 99.60 ± 0.27
Bending strength, f (MPa)
Vickers hardness, Hv (GPa)
684 ± 56 648 ± 37 744 ± 53
22.01 ± 1.45 21.60 ± 0.83 21.06 ± 1.29
Fig. 3. SEM micrograph of the Al2 O3 –SiCw composite at higher magnification. In the central area, where there is no SiC, alumina grains have grown much more than in the rest of the sample.
icantly improves the hardness values in all sample regions. This increment ranges from 36% to 42% depending on the sample zone. These results are to be expected if we consider that the TiC, as explained before, has improved the densification of the material eliminating residual pores. It is also important to notice that, as with the rest of measured properties, hardness is also homogenous in all the zones of the disk. 4. Conclusions Based on the study developed in this work, the following conclusions can be made:
2152
C.F. Gutiérrez-González et al. / Journal of the European Ceramic Society 36 (2016) 2149–2152
1. Large Al2 O3 –SiCw and Al2 O3 –SiCw –TiC composites disks up to 150 mm diameter with a homogeneous properties distribution have been sintered by SPS. 2. The incorporation of TiC as reinforcing phase to the Al2 O3 –SiCw composite, improved densification during sintering reaching values close to 100% of the theoretical density for this material. 3. The incorporation of TiC also improved the mechanical strength and hardness of the Al2 O3 –SiCw composite. The average improvement for these properties was found to be 31% and 39% respectively. 4. Analysis of the results showed that this improvement is due to the superior grain growth inhibition effect of the alumina grains provided by the presence of TiC. Acknowledgment Authors would like to thank The Ministry of the Russian Federation supported this work in the frame of Governmental Regulation of the Russian Federation No. 220, 9 April 2010 by contract 14.B25.31.0012, 26 June 2013.
References [1] Y.L. Dong, F.M. Xu, X.L. Shi, C. Zhang, Z.J. Zhang, J.M. Yang, et al., Fabrication and mechanical properties of nano-/micro-sized Al2 O3 /SiC composites, Mat. Sci. Eng. A Struct. 504 (2009) 49–54. [2] S.K. Sharma, B. Venkata Manoj Kumar, K.Y. Lim, Y.W. Kim, S.K. Nath, Erosion behavior of SiC–WC composites, Ceram. Int. 40 (2014) 6829–6839. [3] K. Takahashi, M. Yokouchi, S.K. Lee, K. Ando, Crack-healing behavior of Al2 O3 toughened by SiC whiskers, J. Am. Ceram. Soc. 86 (2003) 2143–2147. [4] M.P. Borom, M. Lee, Effect of heating rate on densification of alumina–titanium carbide composition, Adv. Ceram. Mater. 1 (1986) 335–340. [5] M. Kanamaru, T. Tatsuno, T. Kusaka, Hot-pressed Al2 O3 /SiC whisker/TiC nano-composites, J. Ceram. Soc. Jpn. 100 (1992) 408–412. [6] in: http://www.taimei-chem.co.jp/product/english/01.html#6. [7] H. Endo, M. Ueki, H. Kubo, Hot pressing of SiC–TiC composites, J. Mater. Sci. 25 (1990) 2503–2506. [8] D.M. Vernon, J. Rankin, C. Caragianis-Broadbridge, B.L. Laube, Novel processing route to control grain growth in submicrometer alumina compacts, J. Am. Ceram. Soc. 82 (1999) 2969–2977. [9] T. Sakuma, Mater. Sci. Forum (1996) 109–120. [10] A. Gupta, S. Sharma, M.R. Joshi, P. Agarwal, K. Balani, Mater. Sci. Forum (2010) 87–130. [11] L.C. Stearns, M.P. Harmer, Particle-inhibited grain growth in Al2 O3 –SiC: I experimental results, J. Am. Ceram. Soc. 79 (1996) 3013–3019.