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carbon nanofiber composite
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Scripta Materialia 58 (2008) 520–523 www.elsevier.com/locate/scriptamat
Zirconia/carbon nanofiber composite Annama´ria Duszova´,a Ja´n Dusza,b,* Karel Toma´sˇek,a Jerzy Morgiel,c Gurdial Blugand and Jakob Kueblerd a
Technical University of Kosˇice, Faculty of Metallurgy, Letna´ 9, 042 00 Kosˇice, Slovak Republic Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04353 Kosˇice, Slovak Republic c Institute of Metallurgy and Materials Science of Polish Academy of Sciences, Reymonta 25, 30 059 Krakow, Poland d Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for High Performance Ceramics, 8600 Duebendorf, Switzerland b
Received 28 September 2007; revised 2 November 2007; accepted 4 November 2007 Available online 3 December 2007
The effect of the addition of carbon nanofibers (CNFs) on the microstructure, fracture/mechanical and electrical properties of the CNF/zirconia composite has been investigated. The microstructure of both ZrO2 and ZrO2–CNF composites consists of a very low grain sized matrix (approximately 160 nm) with relatively well dispersed carbon nanofibers in the composite. The mechanical properties slightly decreased after the addition of CNFs to the ZrO2 but the electrical resistivity decreased significantly, exhibiting approximately 0.1 X cm. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 3Y-TZP; Carbon nanofiber; Microstructure; Fracture; Electrical resistivity
During the last few years new ceramic/carbon nanotube composites have been developed with two aims: to improve the mechanical properties of the ceramic materials by reinforcing with carbon nanotubes (CNTs) and to develop functionalized ceramics with improved magnetic and electrical properties [1–3]. It seems that singlewall CNTs (SWCNTs) or multi-wall CNTs (MWCNTs) should be ideal reinforcing/functionalizing elements for composites due to their small size, low density, high aspect ratio, exceptional high tensile strength, and high electrical and thermal conductivity. A number of authors have reported improved mechanical and functional properties in the case of ceramic/CNT composites compared with the monolithic material [4–6]. According to the results of Zhan et al. [3], CNT/ceramic nanocomposites exhibit thermoelastic properties which give them the potential for use as promising thermoelastic materials. The investigations to date have focused mainly on alumina-based ceramic composites, with only limited work having been done on other systems, e.g. those based on silicon nitride or zirconia [7–10]. Zirconia (3Y-TZP) is a material extensively used for many structural applications due to its good mechanical
* Corresponding author. E-mail: [email protected]
properties. Zirconia and/or zirconia-based composites are interesting multifunctional materials that have been used for many applications, such as solid-oxide fuel cells, oxygen sensors and ceramic membranes, due to their good high-temperature stability, high breakdown electrical field or large energy bandgap [11]. As well as CNTs, there are similar carbon-based filamentous nanomaterials, e.g. hollow carbon fibers or carbon nanocoils, with similar mechanical and electrical properties but with slightly different sizes and shapes, which can also be utilized as reinforcing elements in ceramic-based composites [12–14]. The aim of the present contribution is to investigate the influence of carbon nanofiber additions on the microstructure, mechanical, fracture and electrical properties of zirconia. The starting materials were ZrO2 powder (TZ-3Y, Tosoh, Japan) and carbon nanofiber (1.07 wt.%). Carbon nanofiber (HTF150FF, Electrovac, Austria) with an average diameter of 80–150 nm, specific surface area in the range of 20–100 m2 g 1, Young’s modulus 500 GPa, tensile strength 7 GPa and electrical resistivity of 10 3–10 4 X cm was used. A water-based mixture was prepared with dispersant (DODECYLE MARANIL), carbon nanofibers (CNFs), ZrO2 and a polyvinylbutryl binder (MOWITAL B30T). The
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.11.002
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
dispersion was primarily done using ultrasonics and a magnetic stirrer. After the dispersion, the mixture was subsequently spray dried. Specimens in the form of discs with diameter of 10 and 20 mm were die pressed and then hot-pressed at 1300 °C and 40 MPa in argon. For comparison, monolithic ZrO2 was prepared under similar conditions (but without any binder). The CNFs have been characterized by scanning and transmission electron microscopy (SEM, TEM; Jeol Ltd). The density of the experimental materials has been measured using the Archimedes method. X-ray diffraction (XRD) analysis (Phillips, X-pert) was used for the determination of the phase composition of the experimental materials. Specimens for microstructure examination were prepared by diamond cutting, grinding, polishing and thermal etching. The microhardness (Leco instruments) and hardness were studied by the Vickers indentation method at loads from 0.25 to 150 N. Indentation toughness testing was carried out for estimation of the toughness of the materials at 150 N using a Vickers indenter and the Shetty equation [15]. The microstructure and the fracture surface of the materials were studied using SEM. The specimens were thermally etched at a temperature of 1250 °C and microfractography was used to analyse the fracture lines and surfaces of the specimens. The electrical resistivity was measured at room temperature using a four-point method. Specimens of dimensions of 1.5 mm 1.5 mm 10 mm were cut from the center of the hot-pressed samples for these measurements. The SEM and TEM analysis revealed that the CNFs used are in two forms: pipe-shaped hollow nanofibers, with an outside diameter of approximately 150–200 nm and an inside diameter of approximately 80 nm; and nanofibers with a ‘‘bamboo-like” structure consisting of several lateral units (Fig. 1a and b). The hot-pressed monolithic zirconia exhibits full density but the density of the ZrO2–CNF composite was lower due to the porosity in the composite attributable mainly to the clusters of CNFs present in its microstructure. XRD analysis revealed that the experimental materials consist of mainly t-ZrO2; however, a small amount of monoclinic zirconia and yttrium oxide carbide has also been found. In Figure 2 the microstructure of the bulk zirconia and the composite is illustrated. The monolithic zirconia consists of very small, submicron/nanometer-sized grains with randomly occurring defects in the form of
521
Figure 2. Microstructure of monolithic zirconia (a) and the CNF/ zirconia (b) composite, as seen by thermal etching/SEM.
very small pores, approximately 100–200 nm in diameter (Fig. 2a). The microstructure of the composite consists of similar sized or an even smaller grained matrix with relatively well dispersed CNFs (pores with their shape/ orientation indicating the locations of the burned out CNFs during the thermal etching) in the matrix (Fig. 2b). The smaller grain size of the zirconia in the composite compared with the monolithic material is evidence that the CNFs may hinder the grain growth in the composite during the sintering. A relatively high number of clusters of CNFs were found on the polished surface and the fracture surface of the samples, indicating that the mixing procedure needs to be improved (Fig. 3). The size of the clusters varies from a few microns up to approximately 20 lm, and porosity was always connected with these clusters. The presence of such clusters in the composite connected with porosity is the main reason for the lower density compared with the theoretical one (approximately 5.8 g cm 3). In Table 1 the basic properties such as the hardness and indentation toughness is presented together with the values of the electrical resistivity for the monolithic zirconia and for the composite. The hardness and indentation toughness decreased after addition of CNFs to the zirconia. On the other hand, the electrical resistivity decreased significantly, from a very high value of approximately 1012 (the exact value was not possible to measure because the resistivity was so high it was beyond the limitations of our measuring equipment) to a value of 0.105 X cm.
Figure 3. Cluster of CNFs accompanished with porosity on the fracture surface, as seen by SEM. Table 1. Properties of the investigated materials
Figure 1. Characteristic shapes of the carbon nanotubes used, as determined by SEM and TEM.
Material/ properties
Density (g/cm3)
HV1 (km/mm2)
KI-indentation (MPa m0.5)
Electrical resistance (X cm)
ZrO2 ZrO2–CNF
6.05 5.22
1395 ± 26 830 ± 28
6.24 ± 0.1 5.60 ± 0.15
ca. 1.0 1012 0.105 ± 0.05
522
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
The fracture mechanisms in the bulk zirconia material are mainly intergranular, with very little roughness of the fracture lines/surface, only apparent at the nanometer scale. No toughening mechanisms were revealed on the fracture lines/surfaces in this system. The composite reinforced by CNFs exhibits a slightly different behavior, with rougher fracture line/surfaces and crack deflection at the larger singular CNFs (Fig. 4a). The crack deflection is similar to that seen in whisker-reinforced ceramics, and represents one of the toughening mechanisms in similar systems which can improve the fracture toughness of composites. Besides the crack deflection, crack bridging and CNF pull-outs were often detected on the fracture surface of the failed composite; however, the pull-out length was usually short (Fig. 4b). Comparing the results of the present investigation with the results of a similar study, there are two main differences in the processing step. In our investigation we used CNFs with different geometry, with a larger diameter and a lower aspect ratio, compared with the CNTs in the similar investigations of ZrO2–CNT systems. The second difference was in the way the mixture was sintered: we used hot pressing, while similar investigations used spark plasma sintering (SPS) or pressureless sintering + post-hot isostatic pressing (HIP) treatment. The thermoelectric properties of 10 vol.% SWCNT/ 3Y-TZP composite produced by SPS have been studied by Zhan et al. [3]. According to these results, the electrical resistivity increased from approximately 0.02–0.5 X cm when the temperature increased from temperature to 545 K. Further increasing the testing temperature resulted in the slight decrease in electrical resistivity. Shi and Liang studied the effect of the MWCNT addition on the electrical and dielectric properties of MWCNT/3Y-TZP composite prepared by the SPS method [16]. They found a low percolation threshold of 1.7 wt.% of MWNTS for the composite and found the system attractive for some electrical applications. MWCNT/3Y-TZP composite was prepared by pressureless sintering + HIP by Ukai et al. [6]. For the materials with CNTs from 0.25 to 1.0 wt.% the electrical resistivity was found to be in the interval from 21 to 1.7 X cm, in very good agreement with our results. The electrical resistivity in our case was even lower than the results of Ukai et al., indicating the potential of the whisker-like nanotubes for improving the functionality of the ceramics. There are only a limited number of publications dealing with the mechanical properties of CNT/ZrO2 composites [5,6].
Sun et al. [5] studied the mechanical and fracture behavior of MWCNT/3Y-TZP composites containing 0.1–1.0 wt.% MWCNTs and SWCNTs prepared by SPS. They found that the addition of CNTs had a negative influence on the hardness of the composites and no influence (at 0.5 wt.%) or a negative influence on the fracture toughness. Comparing the influence of the addition of SWCNTs and MWCNTs on the fracture toughness, they found that the MWCNT are more effective in the toughening. However, they used the indentation technique for the toughness measurements, which is useful only for comparison purposes, cannot be considered a true material property and also tends to overestimate the KIc value as described by Quinn et al. [17]. Similar results were found by Sun et al. [6], who studied the failure properties of MWCNT- and SWCNT-reinforced zirconia prepared by the SPS process. According to them, the CNTs often agglomerate at the ZrO2 boundaries, and the weak bonding between the CNTs and zirconia are the reason why the reinforcing effect of the CNTs is limited. The results of the present investigation are in a very good agreement with these results and with the results of Ukai et al. [6] as regards the influence of the CNTs addition on the mechanical properties of zirconia (Fig. 5). As regards the mechanical properties, our results show that even the use of relatively coarse whisker-like CNFs is not effective in toughening the zirconia ceramic matrix at the current amounts used. On the other hand, we have to note that on the fracture surface/line we frequently found different toughening mechanisms, mainly in the form of crack deflection at the CNFs. The reason for the relatively low indentation fracture toughness is probably the poor dispersion and therefore the limited toughening effect of the CNFs. A more reliable technique (e.g. SEVNB ref.) has to be used for the measurement of the fracture toughness on fully dense composites to obtain information concerning the true effect of the CNFs on the fracture toughness. However, the indentation test can be used for characterization of different mechanical/damage properties of CNT- and CNF-reinforced composites. Wang et al. [18] investigated the contact-damage resistance of dense Al2O3/ SWNT composites prepared by the SPS method. Vickers and Hertzian indentation tests revealed that these composites are highly contact-damage resistant due to the shear-deformable SWNTs heterogeneities in the composites which help redistribute the stress field under
Figure 4. Fracture surface in the CNF/zirconia composite illustrating the crack deflections at CNFs.
Figure 5. Influence of the CNT addition on the Vickers hardness and indentation toughness of the CNT/3Y-TZP composites [5,6].
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
indentation, imparting the composites with contactdamage resistance. The lesser hardness of the composite compared with the monolithic material is dependent on the residual porosity which remains in the material after sintering, similar to that observed in other investigations [5]. Together with the porosity, the clusters of the CNFs/CNTs are characteristic processing defects present in our material and presented in all contributions dealing with similar composites. This indicates that there are still difficulties in the preparation of defect-free CNT- or CNF-reinforced ceramic composites, but that there is potential for the improvement of their functional and mechanical properties. Dense ZrO2/carbon nanofiber composites have been developed which exhibit slightly lower mechanical properties but significantly higher electrical conductivity compared with monolithic zirconia. The results point out the possibilities of improving the mechanical and functional properties of the ZrO2/carbon nanofiber composites by further optimizing the processing route of the composite. A future goal is to produce fully dense composites with improved dispersion of the CNFs. A.D. acknowledges the financial support of Empa during her stay there with the aim to prepare her diploma thesis. Further, the authors would like to thank Izaˇ urisˇinova´ and Maria´n Mihalik bel Silveira, Katarina D for their help with the dispersion of the CNT, XRD measurements and electrical resistivity measurements, respectively. The work was supported by NANOSMART, Centre of Excellence, SAS, by the Slovak Grant Agency for Science, grant No. 2/7914/27 and by the KMM-NoE EU 6FP Project.
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[1] S. Ijima, Nature 354 (1991) 56. [2] Ch. Laurent, A. Peigney, O. Dumortier, A. Rousset, J. Eur. Ceram. Soc. 18 (1998) 2005. [3] G.D. Zhan, A.K. Mukherjee, J. Appl. Ceram. Technol. 1 (2) (2004) 161. [4] G.D. Zhan, J.D. Kuntz, J.E. Garay, A.K. Mukherjee, P. Zhu, K. Koumoto, Scripta Mater. 54 (2006) 77–82. [5] J. Sun, L. Gao, M. Iwasa, T. Nakayama, K. Niihara, Ceram. Int. 31 (2005) 1131–1134. [6] T. Ukai, T. Sekino, A. Hirvonen, N. Tanaka, T. Kusunose, T. Nakayama, K. Niihara, Key Eng. Mat. 317–318 (2006) 661–664. [7] A. Peigney, Ch. Laurent, E. Flahaut, A. Rousset, Ceram. Int. 26 (2000) 677–683. [8] G.D. Zhan, J.D. Kuntz, J. Wan, A.K. Mukherjee, Nat. Mater. 61 (2003) 1899. [9] Cs. Balazsi, Z. Ko´nya, F. We´ber, L.P. Biro´, P. Arato´, Mat. Sci. Eng. C23 (2003) 1133–1137. [10] J. Tatami, T. Katashima, K. Komeya, T. Meguro, T. Wakihara, J. Am. Ceram. Soc. 88 (2005) 2889. [11] S.Y. Lee, H. Kim, P.C. McIntyre, K.C. Saraswat, J.S. Byun, Appl. Phys. Lett. 823 (2003) 2874–2876. [12] S-P. Chai, S. Hussein, S. Zein, A.R. Mohamed, Diam. Relat. Mater. 16 (2007) 1656–1664. [13] R. Vieira, M-J. Ledoux, C. Pham-Huu, Appl. Catal. A: General 274 (2004) 1–8. [14] X. Chen, S. Yang, K. Takeuchi, T. Hashishin, H. Iwanaga, S. Motojiima, Diam. Relat. Mater. 12 (2003) 1836–1840. [15] D.K. Shetty, I.G. Wright, P.N. Mincer, A.H. Clauser, J. Mat. Sci. 20 (1985) 1873–1882. [16] S.-L. Shi, J. Liang, J. Am. Ceram. Soc. 89 (11) (2006) 3533–3535. [17] G.D. Quinn, R.C. Brandt, J. Am. Ceram. Soc. 90 (2007) 673–680. [18] X. Wang, N.P. Padture, H. Tanaka, Nat. Mater. 3 (2004) 539–544.
Scripta Materialia 58 (2008) 520–523 www.elsevier.com/locate/scriptamat
Zirconia/carbon nanofiber composite Annama´ria Duszova´,a Ja´n Dusza,b,* Karel Toma´sˇek,a Jerzy Morgiel,c Gurdial Blugand and Jakob Kueblerd a
Technical University of Kosˇice, Faculty of Metallurgy, Letna´ 9, 042 00 Kosˇice, Slovak Republic Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04353 Kosˇice, Slovak Republic c Institute of Metallurgy and Materials Science of Polish Academy of Sciences, Reymonta 25, 30 059 Krakow, Poland d Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for High Performance Ceramics, 8600 Duebendorf, Switzerland b
Received 28 September 2007; revised 2 November 2007; accepted 4 November 2007 Available online 3 December 2007
The effect of the addition of carbon nanofibers (CNFs) on the microstructure, fracture/mechanical and electrical properties of the CNF/zirconia composite has been investigated. The microstructure of both ZrO2 and ZrO2–CNF composites consists of a very low grain sized matrix (approximately 160 nm) with relatively well dispersed carbon nanofibers in the composite. The mechanical properties slightly decreased after the addition of CNFs to the ZrO2 but the electrical resistivity decreased significantly, exhibiting approximately 0.1 X cm. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 3Y-TZP; Carbon nanofiber; Microstructure; Fracture; Electrical resistivity
During the last few years new ceramic/carbon nanotube composites have been developed with two aims: to improve the mechanical properties of the ceramic materials by reinforcing with carbon nanotubes (CNTs) and to develop functionalized ceramics with improved magnetic and electrical properties [1–3]. It seems that singlewall CNTs (SWCNTs) or multi-wall CNTs (MWCNTs) should be ideal reinforcing/functionalizing elements for composites due to their small size, low density, high aspect ratio, exceptional high tensile strength, and high electrical and thermal conductivity. A number of authors have reported improved mechanical and functional properties in the case of ceramic/CNT composites compared with the monolithic material [4–6]. According to the results of Zhan et al. [3], CNT/ceramic nanocomposites exhibit thermoelastic properties which give them the potential for use as promising thermoelastic materials. The investigations to date have focused mainly on alumina-based ceramic composites, with only limited work having been done on other systems, e.g. those based on silicon nitride or zirconia [7–10]. Zirconia (3Y-TZP) is a material extensively used for many structural applications due to its good mechanical
* Corresponding author. E-mail: [email protected]
properties. Zirconia and/or zirconia-based composites are interesting multifunctional materials that have been used for many applications, such as solid-oxide fuel cells, oxygen sensors and ceramic membranes, due to their good high-temperature stability, high breakdown electrical field or large energy bandgap [11]. As well as CNTs, there are similar carbon-based filamentous nanomaterials, e.g. hollow carbon fibers or carbon nanocoils, with similar mechanical and electrical properties but with slightly different sizes and shapes, which can also be utilized as reinforcing elements in ceramic-based composites [12–14]. The aim of the present contribution is to investigate the influence of carbon nanofiber additions on the microstructure, mechanical, fracture and electrical properties of zirconia. The starting materials were ZrO2 powder (TZ-3Y, Tosoh, Japan) and carbon nanofiber (1.07 wt.%). Carbon nanofiber (HTF150FF, Electrovac, Austria) with an average diameter of 80–150 nm, specific surface area in the range of 20–100 m2 g 1, Young’s modulus 500 GPa, tensile strength 7 GPa and electrical resistivity of 10 3–10 4 X cm was used. A water-based mixture was prepared with dispersant (DODECYLE MARANIL), carbon nanofibers (CNFs), ZrO2 and a polyvinylbutryl binder (MOWITAL B30T). The
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.11.002
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
dispersion was primarily done using ultrasonics and a magnetic stirrer. After the dispersion, the mixture was subsequently spray dried. Specimens in the form of discs with diameter of 10 and 20 mm were die pressed and then hot-pressed at 1300 °C and 40 MPa in argon. For comparison, monolithic ZrO2 was prepared under similar conditions (but without any binder). The CNFs have been characterized by scanning and transmission electron microscopy (SEM, TEM; Jeol Ltd). The density of the experimental materials has been measured using the Archimedes method. X-ray diffraction (XRD) analysis (Phillips, X-pert) was used for the determination of the phase composition of the experimental materials. Specimens for microstructure examination were prepared by diamond cutting, grinding, polishing and thermal etching. The microhardness (Leco instruments) and hardness were studied by the Vickers indentation method at loads from 0.25 to 150 N. Indentation toughness testing was carried out for estimation of the toughness of the materials at 150 N using a Vickers indenter and the Shetty equation [15]. The microstructure and the fracture surface of the materials were studied using SEM. The specimens were thermally etched at a temperature of 1250 °C and microfractography was used to analyse the fracture lines and surfaces of the specimens. The electrical resistivity was measured at room temperature using a four-point method. Specimens of dimensions of 1.5 mm 1.5 mm 10 mm were cut from the center of the hot-pressed samples for these measurements. The SEM and TEM analysis revealed that the CNFs used are in two forms: pipe-shaped hollow nanofibers, with an outside diameter of approximately 150–200 nm and an inside diameter of approximately 80 nm; and nanofibers with a ‘‘bamboo-like” structure consisting of several lateral units (Fig. 1a and b). The hot-pressed monolithic zirconia exhibits full density but the density of the ZrO2–CNF composite was lower due to the porosity in the composite attributable mainly to the clusters of CNFs present in its microstructure. XRD analysis revealed that the experimental materials consist of mainly t-ZrO2; however, a small amount of monoclinic zirconia and yttrium oxide carbide has also been found. In Figure 2 the microstructure of the bulk zirconia and the composite is illustrated. The monolithic zirconia consists of very small, submicron/nanometer-sized grains with randomly occurring defects in the form of
521
Figure 2. Microstructure of monolithic zirconia (a) and the CNF/ zirconia (b) composite, as seen by thermal etching/SEM.
very small pores, approximately 100–200 nm in diameter (Fig. 2a). The microstructure of the composite consists of similar sized or an even smaller grained matrix with relatively well dispersed CNFs (pores with their shape/ orientation indicating the locations of the burned out CNFs during the thermal etching) in the matrix (Fig. 2b). The smaller grain size of the zirconia in the composite compared with the monolithic material is evidence that the CNFs may hinder the grain growth in the composite during the sintering. A relatively high number of clusters of CNFs were found on the polished surface and the fracture surface of the samples, indicating that the mixing procedure needs to be improved (Fig. 3). The size of the clusters varies from a few microns up to approximately 20 lm, and porosity was always connected with these clusters. The presence of such clusters in the composite connected with porosity is the main reason for the lower density compared with the theoretical one (approximately 5.8 g cm 3). In Table 1 the basic properties such as the hardness and indentation toughness is presented together with the values of the electrical resistivity for the monolithic zirconia and for the composite. The hardness and indentation toughness decreased after addition of CNFs to the zirconia. On the other hand, the electrical resistivity decreased significantly, from a very high value of approximately 1012 (the exact value was not possible to measure because the resistivity was so high it was beyond the limitations of our measuring equipment) to a value of 0.105 X cm.
Figure 3. Cluster of CNFs accompanished with porosity on the fracture surface, as seen by SEM. Table 1. Properties of the investigated materials
Figure 1. Characteristic shapes of the carbon nanotubes used, as determined by SEM and TEM.
Material/ properties
Density (g/cm3)
HV1 (km/mm2)
KI-indentation (MPa m0.5)
Electrical resistance (X cm)
ZrO2 ZrO2–CNF
6.05 5.22
1395 ± 26 830 ± 28
6.24 ± 0.1 5.60 ± 0.15
ca. 1.0 1012 0.105 ± 0.05
522
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
The fracture mechanisms in the bulk zirconia material are mainly intergranular, with very little roughness of the fracture lines/surface, only apparent at the nanometer scale. No toughening mechanisms were revealed on the fracture lines/surfaces in this system. The composite reinforced by CNFs exhibits a slightly different behavior, with rougher fracture line/surfaces and crack deflection at the larger singular CNFs (Fig. 4a). The crack deflection is similar to that seen in whisker-reinforced ceramics, and represents one of the toughening mechanisms in similar systems which can improve the fracture toughness of composites. Besides the crack deflection, crack bridging and CNF pull-outs were often detected on the fracture surface of the failed composite; however, the pull-out length was usually short (Fig. 4b). Comparing the results of the present investigation with the results of a similar study, there are two main differences in the processing step. In our investigation we used CNFs with different geometry, with a larger diameter and a lower aspect ratio, compared with the CNTs in the similar investigations of ZrO2–CNT systems. The second difference was in the way the mixture was sintered: we used hot pressing, while similar investigations used spark plasma sintering (SPS) or pressureless sintering + post-hot isostatic pressing (HIP) treatment. The thermoelectric properties of 10 vol.% SWCNT/ 3Y-TZP composite produced by SPS have been studied by Zhan et al. [3]. According to these results, the electrical resistivity increased from approximately 0.02–0.5 X cm when the temperature increased from temperature to 545 K. Further increasing the testing temperature resulted in the slight decrease in electrical resistivity. Shi and Liang studied the effect of the MWCNT addition on the electrical and dielectric properties of MWCNT/3Y-TZP composite prepared by the SPS method [16]. They found a low percolation threshold of 1.7 wt.% of MWNTS for the composite and found the system attractive for some electrical applications. MWCNT/3Y-TZP composite was prepared by pressureless sintering + HIP by Ukai et al. [6]. For the materials with CNTs from 0.25 to 1.0 wt.% the electrical resistivity was found to be in the interval from 21 to 1.7 X cm, in very good agreement with our results. The electrical resistivity in our case was even lower than the results of Ukai et al., indicating the potential of the whisker-like nanotubes for improving the functionality of the ceramics. There are only a limited number of publications dealing with the mechanical properties of CNT/ZrO2 composites [5,6].
Sun et al. [5] studied the mechanical and fracture behavior of MWCNT/3Y-TZP composites containing 0.1–1.0 wt.% MWCNTs and SWCNTs prepared by SPS. They found that the addition of CNTs had a negative influence on the hardness of the composites and no influence (at 0.5 wt.%) or a negative influence on the fracture toughness. Comparing the influence of the addition of SWCNTs and MWCNTs on the fracture toughness, they found that the MWCNT are more effective in the toughening. However, they used the indentation technique for the toughness measurements, which is useful only for comparison purposes, cannot be considered a true material property and also tends to overestimate the KIc value as described by Quinn et al. [17]. Similar results were found by Sun et al. [6], who studied the failure properties of MWCNT- and SWCNT-reinforced zirconia prepared by the SPS process. According to them, the CNTs often agglomerate at the ZrO2 boundaries, and the weak bonding between the CNTs and zirconia are the reason why the reinforcing effect of the CNTs is limited. The results of the present investigation are in a very good agreement with these results and with the results of Ukai et al. [6] as regards the influence of the CNTs addition on the mechanical properties of zirconia (Fig. 5). As regards the mechanical properties, our results show that even the use of relatively coarse whisker-like CNFs is not effective in toughening the zirconia ceramic matrix at the current amounts used. On the other hand, we have to note that on the fracture surface/line we frequently found different toughening mechanisms, mainly in the form of crack deflection at the CNFs. The reason for the relatively low indentation fracture toughness is probably the poor dispersion and therefore the limited toughening effect of the CNFs. A more reliable technique (e.g. SEVNB ref.) has to be used for the measurement of the fracture toughness on fully dense composites to obtain information concerning the true effect of the CNFs on the fracture toughness. However, the indentation test can be used for characterization of different mechanical/damage properties of CNT- and CNF-reinforced composites. Wang et al. [18] investigated the contact-damage resistance of dense Al2O3/ SWNT composites prepared by the SPS method. Vickers and Hertzian indentation tests revealed that these composites are highly contact-damage resistant due to the shear-deformable SWNTs heterogeneities in the composites which help redistribute the stress field under
Figure 4. Fracture surface in the CNF/zirconia composite illustrating the crack deflections at CNFs.
Figure 5. Influence of the CNT addition on the Vickers hardness and indentation toughness of the CNT/3Y-TZP composites [5,6].
A. Duszova´ et al. / Scripta Materialia 58 (2008) 520–523
indentation, imparting the composites with contactdamage resistance. The lesser hardness of the composite compared with the monolithic material is dependent on the residual porosity which remains in the material after sintering, similar to that observed in other investigations [5]. Together with the porosity, the clusters of the CNFs/CNTs are characteristic processing defects present in our material and presented in all contributions dealing with similar composites. This indicates that there are still difficulties in the preparation of defect-free CNT- or CNF-reinforced ceramic composites, but that there is potential for the improvement of their functional and mechanical properties. Dense ZrO2/carbon nanofiber composites have been developed which exhibit slightly lower mechanical properties but significantly higher electrical conductivity compared with monolithic zirconia. The results point out the possibilities of improving the mechanical and functional properties of the ZrO2/carbon nanofiber composites by further optimizing the processing route of the composite. A future goal is to produce fully dense composites with improved dispersion of the CNFs. A.D. acknowledges the financial support of Empa during her stay there with the aim to prepare her diploma thesis. Further, the authors would like to thank Izaˇ urisˇinova´ and Maria´n Mihalik bel Silveira, Katarina D for their help with the dispersion of the CNT, XRD measurements and electrical resistivity measurements, respectively. The work was supported by NANOSMART, Centre of Excellence, SAS, by the Slovak Grant Agency for Science, grant No. 2/7914/27 and by the KMM-NoE EU 6FP Project.
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