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Al2O3 composites
Materials Science and Engineering A279 (2000) 81 – 86 www.elsevier.com/locate/msea
Crack growth resistance of Cr3C2/Al2O3 composites Jow-Lay Huang a,*, Ho-Don Lin a, Ching-An Jeng a, Ding-Fwu Lii b a
Department of Material Science and Engineering, National Cheng-Kung Uni6ersity, Tainan 701, Taiwan, ROC b Department of Electrical Engineering, Chinese Na6al Academy, Kaohsiung 813, Taiwan, ROC Received 22 February 1999; received in revised form 17 September 1999
Abstract The crack resistance of Cr3C2/Al2O3 composites was investigated. The indentation/strength technique was used to determine the R-curve behaviors. The small (2 mm) Cr3C2 particles were found to have a better toughening effect than that of large (7 mm) Cr3C2 particles in Cr3C2/Al2O3 composites. The R-curve behaviors in Cr3C2/Al2O3 composites could be attributed to the mechanisms of crack-deflection and crack-branching. Materials exhibiting R-curve behavior also had narrower strength distribution and larger Weibull modulus than those of Griffith materials. © 2000 Elsevier Science S.A. All rights reserved. Keywords: R-curve; Crack growth resistance; Indentation/strength technique; Weibull modulus
1. Introduction Alumina has been broadly used because of its excellent mechanical properties, good chemical stability and high temperature characteristics. Its intrinsic brittleness and relatively poor reliability however, made the toughening of alumina ceramics an important and challenging area. The incorporation of secondary phases (e.g. particulates, fibers or platelets) has been proven to be an easy, safe and economical toughening technique for alumina ceramics [1 – 4]. Chromium carbide has been successfully incorporated into Al2O3 [1] for toughening purposes owing to its high Young’s modulus and high-temperature erosion resistance. Quite promising mechanical properties and high temperature oxidation resistance of Al2O3/Cr3C2 composites were previously reported in the literature [1,5–7]. The R-curve behavior, illustrated by the fracture resistance as a function of crack length, has been observed in many materials, such as large-grained polycrystalline alumina [8 – 10], transformation-toughened zirconia [11] and whisker/fiber reinforced composites * Corresponding author. Tel.: +886-6-2383866; fax: + 886-62346290. E-mail addresses: [email protected] (J.-L. Huang), [email protected] (D.-F. Lii)
[12]. In these composites, additional energy is required to overcome the traction of aggregate interlock and pullout or the restraining forces of ligaments in the wake of the crack. Indentation/strength technique [13,14] has been successfully used to determine the R-curves. The modulus of Weibull distribution [15], as a statistical indicator of the variability of the strength has been successfully correlated to the slope of the R-curve [16]. A material exhibiting R-curve behavior also revealed higher strength and smaller scattering of strength. The phase stability, mechanical properties and fracture behaviors of pressurelessly sintered Al2O3/Cr3C2 composites have been previously investigated [17,18]. In this study, the crack resistance in Al2O3/Cr3C2 composites was studied by the indentation/strength technique and results were correlated with the strength distribution and Weibull Modulus.
2. Experimental procedures
2.1. Material preparation Alumina powder (A16-SG, Alcoa, USA, 0.5 mm in medium particle size) was ball milled with two different sizes of Cr3C2 powders (2 and 7 mm, grade 160, H.C. Stark, Goslar, Germany, 10% of total volume). The
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mixing was performed in a polyurethane bottle containing high purity alumina balls and ethanol for 24 h. The ratio of balls to charge to vehicle was 6:1:5 by mass. The slurry thus obtained was dried in a rotating vacuum condensor for 20 min. Dried agglomerates were then ground with alumina mortar and pestle, and further screened through a 100 mesh screen for pulverizing aggregates. In this paper, monolithic A12O3 is designated as ALO, A12O3 composites with Cr3C2 of 2 and 7 mm in size are designated as CA2 and CA7, respectively.
2.2. Kneading Powders of 400 g were mixed in a Sigma mixer (Irie Shokai Co., Japan, PN-1H) for 40 min. The binders were subsequently added and mixed for another 40 min. Temperature of the mixer was maintained at 190°C by a circulating hot oil. Binder system includes polypropylene (PP), paraffin wax (PW) and steric acid (SA) at a ratio of 30:65:5 by weight.
2.3. Injection molding Green compacts with the dimensions of 120× 12× 4 mm were molded using a reciprocating screw injection molding machine (Chen Hsong machinery Taiwan Co., Ltd., SM-50.) under a pressure of 700 Kgf/mm2 with a holding time of 8 s and injecting velocity of 40 cc s − 1. The barrel temperatures used for injection were 150, 155, 160, and 165°C from feeder to nozzle, respectively.
2.4. Debinding and sintering Samples were cut from the compact, soaked in heptane at 60°C for 8 h and dried in ambient air for 4 h. They were then thermally debinded, cold isostatically pressed at 100 MPa, and finally sintered in a graphite furnace at 1550°C for 2 h.
2.5. Mechanical properties Testing samples were machined into bars with dimensions of 2× 4×45 mm and 45° edge chamfers. The tensile surfaces were polished to 1 mm using diamond paste before test. Precracks were generated by a Vickers hardness indentor (Vickers, Hv, Akashi AVK-A). Flexural strength was measured by fourpoint bending on a universal testing instrument (series 8562, Instron Corporation, Canton, MA) at a displacement of 0.5 mm min − 1. The outer and inner spans were 30 and 10 mm, respectively. Each data point was the average of seven tests.
2.6. Microstructural analysis Microstructure and fracture surfaces were examined by optical microscope, scanning electron microscope (SEM, Hitachi S-2500) and scanning transmission electron microscope (STEM, JEOL 3010 AEM).
3. Results and discussion The relationship between the bending strength (s) and indentation load (P) can be described by the following equation [13,14]: log s= log a+ b log P
(1)
where a= [A/Y][(3+ 2n)/4] ×[4(x/A)/(1–2n)](2n – 1)(2n + 3)
(2)
b= (2n –1)(2n + 3),
(3)
Y is a dimensionless configuration coefficient with a value of 1.128 for semi-circular cracks. A and n are constants. The dimensionless constant, x (a characteristic of the material and indentor geometry), is related to the initiation crack length (ci) and the indentation load P [13,14], ci = [Px/A](2/3 + 2n)
(4)
According to Krause’s analysis [13], an analytical function of crack extension (Dc) to the fractional power; (n) is used to represent the fracture resistance (KR) as follows: KR = A(Dc)n
(5)
The crack extension, Dc = c−cn, applies to the tractioned crack surfaces, the term cn represent a preexisting, traction-free notch. The exponent (n) measures the susceptibility to R-curve behavior. When n= 0, KR becomes an invariant with the crack extension [13]. Varying flaw sizes, controlled by the Vickers indentation loading (P), were placed on the prospective tensile surfaces of specimens. The length of surface cracks (c) was measured using optical microscopy. Parameters n and A in Eq. (5) could be determined from Eqs. (1)–(4) based on the relationship between the crack length, corresponding fracture load, and the applied indentation load [13,14]. Fig. 1 shows the logarithmic plots of failure stress versus indentation load for monolithic alumina. The slope (b) of the line and the n value were calculated as −0.338 and −0.0058 from Eqs. (1) and (3). The calculated values of b and n were close to previous report of Griffith material (b= − 1/3 and n=0) [11]. It indicated that the ALO sample exhibited no evident R-curve behaviors due to the absence of toughening mechanism.
J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
83
Plots of failure stress versus indentation load for CA2 and CA7 are shown in Figs. 2 and 3 separately. The calculated n values (0.171 and 0.064 for CA2 and CA7) substantially deviated from what reported for a brittle material [11]. Therefore the Cr3C2/Al2O3 composites are essentially not Griffith materials, and could have R-curve behaviors and enhanced toughness. The crack growth resistance of monolithic alumina and Cr3C2/Al2O3 composites are expressed in KR − Dc
Fig. 3. Bending strength versus indentation load of Cr3C2 (7 mm)/ Al2O3 composites (CA7).
Fig. 1. Bending strength versus indentation load of A12O3 samples (ALO).
Fig. 4. Plots of toughness versus crack length determined by indentation/strength technique.
Fig. 2. Bending strength versus indentation load of Cr3C2 (2 mm)/ Al2O3 composites (CA2).
curves as shown in Fig. 4. The crack resistance of monolithic alumina appeared to be independent of crack extension. However, the concave rising R-curves were observed in the Cr3C2/Al2O3 composites. The calculated KR values varied from 5.26 to 7.33 MPa m1/2 for CA2 composites, and from 4.88 to 5.32 MPa m1/2 for the CA7 as the cracks extended from 100 to 700 mm. In addition, the small Cr3C2 particles consistently exhibit greater toughening effects than the larger ones. That the size of toughening particles played an essential role in toughening effects was previously discussed
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Fig. 5. The interaction between Cr3C2 particles and microcrackings; (a) crack branching; (b) crack deflection.
[19–21]. The reason for small particles exhibited greater toughening effects was attributed to the better densification and more particles contained in a unit volume [17,18]. Typical micrographs shown in Fig. 5 illustrate an interaction between the Cr3C2 particles and microcracks induced by Vickers indentor. Observations indicated that the toughening effects could be attributed to the crack deflection around the Cr3C2 particles, and crack branching [22]. Examination of the fractured surfaces by SEM revealed the cracks deflected by Cr3C2 and pores left by the pull out of Cr3C2 particles. The Al2O3 matrix grains were fractured in a transgranular mode. The crack extensions were plotted as a function of applied indentation load as shown in Fig. 6. The samples having smaller Cr3C2 particles consistently exhibited shorter crack extension under the same load. The observations, in agreement the results of Fig. 4, also suggested a better resistance for samples containing small Cr3C2 particles. Typical SEM micrographs showing the fractured surfaces of CA2 samples are shown in Fig. 7. The observations revealed an orthogonal crack growth upon an indentation load of 294N (Fig. 7a). The fractured surface appeared flat, and transgranular fracture was observed near the indentation region (Fig. 7b). However, rough surfaces with intergranular fracture mode were observed in the interior bulk (Fig. 7c). A Weibull approach [23], based on the assumption that a given volume of ceramic fails at the most severe flaw under uniform stress, was used to investigate the reliability of samples. Fig. 8 shows the failure probability of samples after bend test. More than 30 samples were tested for each plot. The Weibull Modulus (m) determined from the slopes in Fig. 8 were 9.85, 11.8 and 13.54 for ALO, CA7 and CA2, respectively. The trend of increase in the Weibull Modulus by adding Cr3C2 was similar to the trend of increase in the slopes of KR − Dc plots shown in Fig. 4 [16]. The relatively high ‘m’ values of CA2 and CA7 samples indicate that the reliability of alumina was enhanced with the incorporation of Cr3C2 particulates.
4. Summary and conclusions
Fig. 6. Crack extension after applying an indentation load.
(1) The crack growth resistance of Cr3C2/Al2O3 composites was consistently higher than that of monolithic alumina. A concave rising R-curve behavior was observed for Cr3C2/Al2O3 composites due to the toughening mechanisms (i.e. crack deflection and branching) by Cr3C2 particles. Small (2 mm) Cr3C2
J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
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Fig. 7. SEM micrographs showing the fractured surface of Cr3C2 (2 mm)/Al2O3 (a) an orthogonal crack after applying an indentation load of 294N; (b) transgranular fracture near the indentation region A in (a,c) intergranular fracture in the interior bulk region B in (a).
particulates had better toughening effect than that of large (7 mm) ones. (2) The Weibull modulus of Cr3C2/Al2O3 composites was higher than that of monolithic alumina. The greater ‘m’ value of the samples containing 2 mm Cr3C2 suggested it is more defect-tolerable and has small strength scattering. The trend of increase in the Weibull Modulus by adding Cr3C2 was
similar to the trend of increase in the slopes of KR − Dc plots Acknowledgements The authors would like to thank the National Science Council of the R.O.C. for its financial support under the contract no. NSC8522 1 6-E-006-034.
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J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
Fig. 8. Failure probability as a function of flexural strength.
References [1] C.T. Fu, J.M. Wu, A.K. Li, J. Mater. Sci. 29 (1994) 2671 – 2677. [2] S. Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J. Am. Ceram. Soc. 72 (10) (1989) 1880–1884. [3] W.J. Tseng, P.D. Funkenbusch, J. Am. Ceram. Soc. 75 (5) (1992) 1171 – 1175. [4] Y.S. Chou, D.J. Green, J. Am. Ceram. Soc. 75 (12) (1992) 3346 – 3352.
.
[5] C.T. Fu, A.K. Li, J.M. Wu, J. Mater. Sci. 28 (1993) (1993) 6285 – 6294. [6] K.M. Shu, C.T. Fu, D.M. Liu, J. Mater. Sci. Lett. 13 (1994) 1146 – 1148. [7] C.T. Fu, J.M. Wu, Bri. Ceram. Trans. 93 (5) (1994) 178–182. [8] Y.W. Mai, B.R. Lawn, Ann. Rev. Mater. Sci. 16 (1986) 415. [9] R. Knehans, R. Steinbrech, J. Mater. Sci. Lett. 1 (1982) 327– 329. [10] M.W. Swains, J. Mater. Sci. Lett. 5 (1986) 1313. [11] D.B. Marshall, J. Am. Ceram. Soc. 69 (3) (1986) 173–180. [12] K.W. White, M.J. Jenkins, A. Gosh, A.S. Kobayashi, R.C. Bradt, The R-curve behavior of SiC whisker polycrystalline alumina matrix composites to 1400°C, in: R.A. Bradley et al. (Eds.), Whisker- and Fiber-Toughened Ceramics-Proceedings of an International Conference, 1988, Oak Ridge, TN, pp. 281– 287. [13] R.F. Krause Jr., J. Am. Ceram. Soc. 71 (5) (1988) 338–343. [14] C.W. Li, J. Yamanis, Ceram. Eng. Sci. Proc. 10 (7/8) (1989) 632 – 645. [15] B. Bergman, J. Mater. Sci. Lett 4 (1985) 1143 – 1146. [16] R.F. Cook, D.R. Clarke, Acta Metall. 36 (3) (1988) 555–562. [17] D.F. Lii, J.L. Huang, K.C. Twu, A.K. Li, J. Ceram. Soc. Japan. 104 (1996) 9. [18] J.L. Huang, K.C. Twu, D.F. Lii, A.K. Li, Mater. Chem. Phys. 51 (1997) 211 – 215. [19] K.T. Faber, A.G. Evans, Acta Metall. 31 (4) (1983) 565–576. [20] K.T. Faber, A.G. Evans, Acta Metall. 31 (4) (1983) 577–584. [21] M. Taya, S. Hayashi, A.S. Kobayashi, H.S. Yoon, J. Am. Ceram. Soc. 73 (5) (1990) 1382 – 1391. [22] R.W. Rice, Ceram. Eng. Sci. Proc. 11 (7/8) (1990) 667–694. [23] D.W. Richerson, Modern Ceramic Engineering: properties, processing and use in design, Manufacturing Engineering and Materials Processing/8, Marcel Dekker, New York, 1992, pp. 326 – 322.
Crack growth resistance of Cr3C2/Al2O3 composites Jow-Lay Huang a,*, Ho-Don Lin a, Ching-An Jeng a, Ding-Fwu Lii b a
Department of Material Science and Engineering, National Cheng-Kung Uni6ersity, Tainan 701, Taiwan, ROC b Department of Electrical Engineering, Chinese Na6al Academy, Kaohsiung 813, Taiwan, ROC Received 22 February 1999; received in revised form 17 September 1999
Abstract The crack resistance of Cr3C2/Al2O3 composites was investigated. The indentation/strength technique was used to determine the R-curve behaviors. The small (2 mm) Cr3C2 particles were found to have a better toughening effect than that of large (7 mm) Cr3C2 particles in Cr3C2/Al2O3 composites. The R-curve behaviors in Cr3C2/Al2O3 composites could be attributed to the mechanisms of crack-deflection and crack-branching. Materials exhibiting R-curve behavior also had narrower strength distribution and larger Weibull modulus than those of Griffith materials. © 2000 Elsevier Science S.A. All rights reserved. Keywords: R-curve; Crack growth resistance; Indentation/strength technique; Weibull modulus
1. Introduction Alumina has been broadly used because of its excellent mechanical properties, good chemical stability and high temperature characteristics. Its intrinsic brittleness and relatively poor reliability however, made the toughening of alumina ceramics an important and challenging area. The incorporation of secondary phases (e.g. particulates, fibers or platelets) has been proven to be an easy, safe and economical toughening technique for alumina ceramics [1 – 4]. Chromium carbide has been successfully incorporated into Al2O3 [1] for toughening purposes owing to its high Young’s modulus and high-temperature erosion resistance. Quite promising mechanical properties and high temperature oxidation resistance of Al2O3/Cr3C2 composites were previously reported in the literature [1,5–7]. The R-curve behavior, illustrated by the fracture resistance as a function of crack length, has been observed in many materials, such as large-grained polycrystalline alumina [8 – 10], transformation-toughened zirconia [11] and whisker/fiber reinforced composites * Corresponding author. Tel.: +886-6-2383866; fax: + 886-62346290. E-mail addresses: [email protected] (J.-L. Huang), [email protected] (D.-F. Lii)
[12]. In these composites, additional energy is required to overcome the traction of aggregate interlock and pullout or the restraining forces of ligaments in the wake of the crack. Indentation/strength technique [13,14] has been successfully used to determine the R-curves. The modulus of Weibull distribution [15], as a statistical indicator of the variability of the strength has been successfully correlated to the slope of the R-curve [16]. A material exhibiting R-curve behavior also revealed higher strength and smaller scattering of strength. The phase stability, mechanical properties and fracture behaviors of pressurelessly sintered Al2O3/Cr3C2 composites have been previously investigated [17,18]. In this study, the crack resistance in Al2O3/Cr3C2 composites was studied by the indentation/strength technique and results were correlated with the strength distribution and Weibull Modulus.
2. Experimental procedures
2.1. Material preparation Alumina powder (A16-SG, Alcoa, USA, 0.5 mm in medium particle size) was ball milled with two different sizes of Cr3C2 powders (2 and 7 mm, grade 160, H.C. Stark, Goslar, Germany, 10% of total volume). The
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 6 4 1 - 3
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J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
mixing was performed in a polyurethane bottle containing high purity alumina balls and ethanol for 24 h. The ratio of balls to charge to vehicle was 6:1:5 by mass. The slurry thus obtained was dried in a rotating vacuum condensor for 20 min. Dried agglomerates were then ground with alumina mortar and pestle, and further screened through a 100 mesh screen for pulverizing aggregates. In this paper, monolithic A12O3 is designated as ALO, A12O3 composites with Cr3C2 of 2 and 7 mm in size are designated as CA2 and CA7, respectively.
2.2. Kneading Powders of 400 g were mixed in a Sigma mixer (Irie Shokai Co., Japan, PN-1H) for 40 min. The binders were subsequently added and mixed for another 40 min. Temperature of the mixer was maintained at 190°C by a circulating hot oil. Binder system includes polypropylene (PP), paraffin wax (PW) and steric acid (SA) at a ratio of 30:65:5 by weight.
2.3. Injection molding Green compacts with the dimensions of 120× 12× 4 mm were molded using a reciprocating screw injection molding machine (Chen Hsong machinery Taiwan Co., Ltd., SM-50.) under a pressure of 700 Kgf/mm2 with a holding time of 8 s and injecting velocity of 40 cc s − 1. The barrel temperatures used for injection were 150, 155, 160, and 165°C from feeder to nozzle, respectively.
2.4. Debinding and sintering Samples were cut from the compact, soaked in heptane at 60°C for 8 h and dried in ambient air for 4 h. They were then thermally debinded, cold isostatically pressed at 100 MPa, and finally sintered in a graphite furnace at 1550°C for 2 h.
2.5. Mechanical properties Testing samples were machined into bars with dimensions of 2× 4×45 mm and 45° edge chamfers. The tensile surfaces were polished to 1 mm using diamond paste before test. Precracks were generated by a Vickers hardness indentor (Vickers, Hv, Akashi AVK-A). Flexural strength was measured by fourpoint bending on a universal testing instrument (series 8562, Instron Corporation, Canton, MA) at a displacement of 0.5 mm min − 1. The outer and inner spans were 30 and 10 mm, respectively. Each data point was the average of seven tests.
2.6. Microstructural analysis Microstructure and fracture surfaces were examined by optical microscope, scanning electron microscope (SEM, Hitachi S-2500) and scanning transmission electron microscope (STEM, JEOL 3010 AEM).
3. Results and discussion The relationship between the bending strength (s) and indentation load (P) can be described by the following equation [13,14]: log s= log a+ b log P
(1)
where a= [A/Y][(3+ 2n)/4] ×[4(x/A)/(1–2n)](2n – 1)(2n + 3)
(2)
b= (2n –1)(2n + 3),
(3)
Y is a dimensionless configuration coefficient with a value of 1.128 for semi-circular cracks. A and n are constants. The dimensionless constant, x (a characteristic of the material and indentor geometry), is related to the initiation crack length (ci) and the indentation load P [13,14], ci = [Px/A](2/3 + 2n)
(4)
According to Krause’s analysis [13], an analytical function of crack extension (Dc) to the fractional power; (n) is used to represent the fracture resistance (KR) as follows: KR = A(Dc)n
(5)
The crack extension, Dc = c−cn, applies to the tractioned crack surfaces, the term cn represent a preexisting, traction-free notch. The exponent (n) measures the susceptibility to R-curve behavior. When n= 0, KR becomes an invariant with the crack extension [13]. Varying flaw sizes, controlled by the Vickers indentation loading (P), were placed on the prospective tensile surfaces of specimens. The length of surface cracks (c) was measured using optical microscopy. Parameters n and A in Eq. (5) could be determined from Eqs. (1)–(4) based on the relationship between the crack length, corresponding fracture load, and the applied indentation load [13,14]. Fig. 1 shows the logarithmic plots of failure stress versus indentation load for monolithic alumina. The slope (b) of the line and the n value were calculated as −0.338 and −0.0058 from Eqs. (1) and (3). The calculated values of b and n were close to previous report of Griffith material (b= − 1/3 and n=0) [11]. It indicated that the ALO sample exhibited no evident R-curve behaviors due to the absence of toughening mechanism.
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83
Plots of failure stress versus indentation load for CA2 and CA7 are shown in Figs. 2 and 3 separately. The calculated n values (0.171 and 0.064 for CA2 and CA7) substantially deviated from what reported for a brittle material [11]. Therefore the Cr3C2/Al2O3 composites are essentially not Griffith materials, and could have R-curve behaviors and enhanced toughness. The crack growth resistance of monolithic alumina and Cr3C2/Al2O3 composites are expressed in KR − Dc
Fig. 3. Bending strength versus indentation load of Cr3C2 (7 mm)/ Al2O3 composites (CA7).
Fig. 1. Bending strength versus indentation load of A12O3 samples (ALO).
Fig. 4. Plots of toughness versus crack length determined by indentation/strength technique.
Fig. 2. Bending strength versus indentation load of Cr3C2 (2 mm)/ Al2O3 composites (CA2).
curves as shown in Fig. 4. The crack resistance of monolithic alumina appeared to be independent of crack extension. However, the concave rising R-curves were observed in the Cr3C2/Al2O3 composites. The calculated KR values varied from 5.26 to 7.33 MPa m1/2 for CA2 composites, and from 4.88 to 5.32 MPa m1/2 for the CA7 as the cracks extended from 100 to 700 mm. In addition, the small Cr3C2 particles consistently exhibit greater toughening effects than the larger ones. That the size of toughening particles played an essential role in toughening effects was previously discussed
84
J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
Fig. 5. The interaction between Cr3C2 particles and microcrackings; (a) crack branching; (b) crack deflection.
[19–21]. The reason for small particles exhibited greater toughening effects was attributed to the better densification and more particles contained in a unit volume [17,18]. Typical micrographs shown in Fig. 5 illustrate an interaction between the Cr3C2 particles and microcracks induced by Vickers indentor. Observations indicated that the toughening effects could be attributed to the crack deflection around the Cr3C2 particles, and crack branching [22]. Examination of the fractured surfaces by SEM revealed the cracks deflected by Cr3C2 and pores left by the pull out of Cr3C2 particles. The Al2O3 matrix grains were fractured in a transgranular mode. The crack extensions were plotted as a function of applied indentation load as shown in Fig. 6. The samples having smaller Cr3C2 particles consistently exhibited shorter crack extension under the same load. The observations, in agreement the results of Fig. 4, also suggested a better resistance for samples containing small Cr3C2 particles. Typical SEM micrographs showing the fractured surfaces of CA2 samples are shown in Fig. 7. The observations revealed an orthogonal crack growth upon an indentation load of 294N (Fig. 7a). The fractured surface appeared flat, and transgranular fracture was observed near the indentation region (Fig. 7b). However, rough surfaces with intergranular fracture mode were observed in the interior bulk (Fig. 7c). A Weibull approach [23], based on the assumption that a given volume of ceramic fails at the most severe flaw under uniform stress, was used to investigate the reliability of samples. Fig. 8 shows the failure probability of samples after bend test. More than 30 samples were tested for each plot. The Weibull Modulus (m) determined from the slopes in Fig. 8 were 9.85, 11.8 and 13.54 for ALO, CA7 and CA2, respectively. The trend of increase in the Weibull Modulus by adding Cr3C2 was similar to the trend of increase in the slopes of KR − Dc plots shown in Fig. 4 [16]. The relatively high ‘m’ values of CA2 and CA7 samples indicate that the reliability of alumina was enhanced with the incorporation of Cr3C2 particulates.
4. Summary and conclusions
Fig. 6. Crack extension after applying an indentation load.
(1) The crack growth resistance of Cr3C2/Al2O3 composites was consistently higher than that of monolithic alumina. A concave rising R-curve behavior was observed for Cr3C2/Al2O3 composites due to the toughening mechanisms (i.e. crack deflection and branching) by Cr3C2 particles. Small (2 mm) Cr3C2
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Fig. 7. SEM micrographs showing the fractured surface of Cr3C2 (2 mm)/Al2O3 (a) an orthogonal crack after applying an indentation load of 294N; (b) transgranular fracture near the indentation region A in (a,c) intergranular fracture in the interior bulk region B in (a).
particulates had better toughening effect than that of large (7 mm) ones. (2) The Weibull modulus of Cr3C2/Al2O3 composites was higher than that of monolithic alumina. The greater ‘m’ value of the samples containing 2 mm Cr3C2 suggested it is more defect-tolerable and has small strength scattering. The trend of increase in the Weibull Modulus by adding Cr3C2 was
similar to the trend of increase in the slopes of KR − Dc plots Acknowledgements The authors would like to thank the National Science Council of the R.O.C. for its financial support under the contract no. NSC8522 1 6-E-006-034.
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J.-L. Huang et al. / Materials Science and Engineering A279 (2000) 81–86
Fig. 8. Failure probability as a function of flexural strength.
References [1] C.T. Fu, J.M. Wu, A.K. Li, J. Mater. Sci. 29 (1994) 2671 – 2677. [2] S. Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J. Am. Ceram. Soc. 72 (10) (1989) 1880–1884. [3] W.J. Tseng, P.D. Funkenbusch, J. Am. Ceram. Soc. 75 (5) (1992) 1171 – 1175. [4] Y.S. Chou, D.J. Green, J. Am. Ceram. Soc. 75 (12) (1992) 3346 – 3352.
.
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