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Effect of TiC particle size on the toughness characteristics of Al2O3–TiC composites
June 2001
Materials Letters 49 Ž2001. 235–238 www.elsevier.comrlocatermatlet
Effect of TiC particle size on the toughness characteristics of Al 2 O 3 –TiC composites Jianghong Gong ) , Hezhuo Miao, Zhe Zhao, Zhenduo Guan Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China Received 26 November 1999; received in revised form 26 October 2000; accepted 31 October 2000
Abstract The effect of TiC particle size on the toughness of Al 2 O 3 –30 wt.% TiC composites was examined by quantitatively analyzing the R-curves measured with direct indentation method for four samples having different TiC particle sizes. It was shown that the toughening effect in the composite is strongly dependent on the size of the TiC particles. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Alumina; Titanium carbide; Toughness; Indentation; Crack resistance
1. Introduction Adding amounts of titanium carbide ŽTiC. particles in the ceramic matrix has been recognized to be an effective way to make the alumina-based ceramics more suitable to be used as cutting tool materials w1x. During the past years, numerous studies have been carried out to examine the effect of adding TiC on the microstructures, the properties and the cutting performance of the resultant composites w2–5x. In these studies, much attention has been paid to the effect of the TiC particle content, however, little effort has been devoted to the effect of TiC particle size. In fact, recent studies on the toughening mechanism of ceramic composites w6,7x have shown that
)
Corresponding author. Fax: q86-10-627-71160. E-mail address: [email protected] ŽJ. Gong..
the toughening effect is strongly dependent on the size of the toughening particles, whiskers andror fibers. Considering that the fracture toughness is a key parameter for the characterization of the cutting performance for a given cutting tool material, the present study was performed to examine the effect of the TiC particle size on the toughness characteristics of Al 2 O 3 –TiC composites.
2. Experimental Four grades of Al 2 O 3 –TiC composites were prepared by hot-pressing the cold isostatically pressed green bodies of Al 2 O 3 powder Ž99.99% pure. mixed with 30 wt.% TiC particles Ž99.0% pure.. Hot pressing of each sample was carried out at 1650 y 17008C and 25 MPa for 30 min. The average sizes of the TiC particles used for preparing different samples are different with each other. Table 1 gives the average
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 7 6 - 1
J. Gong et al.r Materials Letters 49 (2001) 235–238
236
Table 1 Physical and mechanical properties of the test materials Property
spectively. The parameter ErH was then calculated from Ref. w9x
Sample AC01
Average size of TiC particles Žmm. Er H R-curve parameter K 0 ŽMPa m1r 2 . K` ŽMPa m1r2 . l Žmm.
b AC03
AC04
0.5
3.8
5.6
8.2
0.0370
0.0357
0.0366
0.0361
3.77 3.77 –
3.94 5.38 107.63
4.01 5.73 110.62
a
4.12 6.01 121.54
E
P
H c
3r2
ž /
7.11
y 0.45
H
ž / E
Ž 2.
The calculated ErH-values for all the four samples are listed in Table 1.
3. Results and discussion
size of the TiC particles measured from scanning electronic microscopy ŽSEM. micrographs for each samples tested. Specimens used for R-curve measurements were cut directly from the hot-pressed products, mounted in Bakelite, ground flat using a diamond grinding wheel and then polished carefully with successively finer diamond pastes to yield a scratch-free, mirrorlike surface finish suitable for indentation. Vickers indentation tests were conducted on each specimen at room temperature in the indentation load range from 49 to 294 N and at a constant dwell time of 15 s. After indentation, the lengths of the indentation-induced cracks were immediately measured by optical microscopy. A total of 10 perfect indentations, which exhibit clearly symmetrical indentation impressions and symmetrical crack patterns, were made at each load level. For each indentation, the crack resistance, K R , was calculated with the equation proposed by Anstis et al. w8x K R s 0.016
1 s
AC02
In Fig. 1, the measured crack resistance, K R , was plotted as a function of indentation crack length for each sample considered. In plotting this figure, the crack length and the crack resistance were respectively taken as the averages of the 10 measurements carried out at the same indentation load level. However, for the sake of conciseness, error bar corresponding to each datum point is presented only for samples AC01 and AC04, respectively. It can be seen that, although there are somewhat large scatters in the measured data, increasing tendencies in crack resistance with increasing crack length were observed for samples AC02, AC03 and AC04, while for sample AC01, the crack resistance keeps nearly constant independent of crack length. Thus, one can conclude directly that the significant toughening effect resulting from the mismatch between the mechanical orrand the thermal properties of TiC parti-
Ž 1.
where P is the indentation load, c is the average crack half-length, and E and H are elastic modulus and hardness of the test material, respectively. The Knoop indentation method proposed by Marshall et al. w9x was employed to determine the parameter ErH in the present study. For each sample, five Knoop indentations were made on the polished surface at an indentation load of 33.77 N and the long diagonal length, b, and the short diagonal length, a, of each indentation impression were measured, re-
Fig. 1. Crack resistance as a function of crack size for all the test samples. Solid lines are the best-fits of the experimental data to Eq. Ž3..
J. Gong et al.r Materials Letters 49 (2001) 235–238
237
cles and the Al 2 O 3 grains can only be obtained in Al 2 O 3 –TiC composites by using larger TiC particles. Ramachandran and Shetty w10x have proposed an empirical equation to describe the R-curve measured for toughened ceramics, c
ž l/
K R s K` y Ž K` y K 0 . exp y
Ž 3.
where K 0 , K` and l are adjustable parameters. The solid lines in Fig. 1 represent Abest-fitsB of Eq. Ž3. to the measured R-curve data. The corresponding best-fit values of the parameters K 0 , K` and l, obtained by iterative regression minimizing total variance, are listed in Table 1. It is clear that Eq. Ž3. gives good fits to the experimentally measured R-curves of samples AC02, AC03 and AC04. For comparison, one can reasonably assume K 0 s K` for sample AC01 and, correspondingly, K` for sample AC01 was determined as the average of the crack resistances measured at all the six load levels examined. As discussed by Ramachandran and Shetty w10x, for ceramic composites, K 0 is an estimate of the fracture toughness of the matrix. Note that, in general, there exists a systematic error of about 30% in the toughness value estimated with Eq. Ž1. w8x. There is reason to believe that the K 0-values listed in Table 1 may be reasonable estimations of the fracture toughness of the Al 2 O 3 matrix, whose toughness was frequently reported to be around 3–4 MPa m1r2 w11x. As can be seen in Table 1, the fracture toughness of the Al 2 O 3 matrix, i.e., K 0 , increases slightly with the TiC particle size. Such a slightly increasing tendency may be attributed to the effect of the residual stresses resulting from the mismatch between the TiC particles and the matrix grains w12x. From Eq. Ž3., K` is the maximum crack resistance or the fracture toughness of the composite. On the other hand, the parameter l in Eq. Ž3. may be related directly to the range of crack extension over which toughening effects should develop and saturate. In other words, l may be considered to be a measure of the so-called process zone formed behind the crack tip during crack extension. Based on the theoretical analysis of the toughening mechanisms in ceramic composites w6,13x, one can expect that larger TiC particles would result in more significant effects
Fig. 2. Variation of Ž K` y K 0 ., a measure of toughening effect, with the average size of the TiC particles existing in the composite.
of crack-bridging andror crack deflection in Al 2 O 3 – TiC composite, thereby a large process zone formed behind the crack tip, when compared with smaller TiC particles. This seems a reasonable explanation for the increasing tendencies in K` and l with the TiC particle size. In general, a simple way to evaluate the toughening effect is to compare the fracture toughness of the composite with that of the matrix. Fig. 2 shows the variation of the difference between K` and K 0 with the average size of the TiC particles existing in the composite. An increasing tendency was also obtained, giving a further support for the above analysis. In conclusion, TiC particle size has a significant effect on the toughness characteristics of Al 2 O 3 –TiC composites. Detailed study should be conducted in future to examine this effect and gain a basic understanding for the toughening mechanism in these composites.
References w1x M. Furukawa, J. Jpn. Soc. Powder Powder Metall. 38 Ž1991. 824. w2x Y.-W. Kim, J.G. Lee, J. Am. Ceram. Soc. 72 Ž1989. 1333. w3x E. Pippel, J. Woltersdorf, Phys. Status Solidi A 116 Ž1989. 165. w4x R. Matsuki, H. Ueda, T. Takenouchi, A. Nakahira, K. Niihara, J. Jpn. Soc. Powder Powder Metall. 38 Ž1991. 365.
238
J. Gong et al.r Materials Letters 49 (2001) 235–238
w5x X.S. Li, I.M. Low, J. Mater. Sci. 29 Ž1994. 3121. w6x P.F. Becher, J. Am. Ceram. Soc. 74 Ž1991. 255. w7x B.R. Lawn, N.P. Padture, L.M. Braun, S.J. Bennison, J. Am. Ceram. Soc. 76 Ž1993. 2235. w8x G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 Ž1981. 533. w9x D.B. Marshall, T. Noma, A.G. Evans, J. Am. Ceram. Soc. 65 Ž1982. C175. w10x N. Ramachandran, D.K. Shetty, J. Am. Ceram. Soc. 74 Ž1991. 2634.
w11x R. Morrell, Handbook of Properties of Technical and Engineering Ceramics: Part 1. An Introduction for the Engineer and Designer, Her Majesty’s Stationary Office, London, 1985. w12x B.R. Lawn, Fracture of Brittle Solids, Cambridge Univ. Press, New York, 1993. w13x A.G. Evans, J. Am. Ceram. Soc. 73 Ž1990. 187.
Materials Letters 49 Ž2001. 235–238 www.elsevier.comrlocatermatlet
Effect of TiC particle size on the toughness characteristics of Al 2 O 3 –TiC composites Jianghong Gong ) , Hezhuo Miao, Zhe Zhao, Zhenduo Guan Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China Received 26 November 1999; received in revised form 26 October 2000; accepted 31 October 2000
Abstract The effect of TiC particle size on the toughness of Al 2 O 3 –30 wt.% TiC composites was examined by quantitatively analyzing the R-curves measured with direct indentation method for four samples having different TiC particle sizes. It was shown that the toughening effect in the composite is strongly dependent on the size of the TiC particles. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Alumina; Titanium carbide; Toughness; Indentation; Crack resistance
1. Introduction Adding amounts of titanium carbide ŽTiC. particles in the ceramic matrix has been recognized to be an effective way to make the alumina-based ceramics more suitable to be used as cutting tool materials w1x. During the past years, numerous studies have been carried out to examine the effect of adding TiC on the microstructures, the properties and the cutting performance of the resultant composites w2–5x. In these studies, much attention has been paid to the effect of the TiC particle content, however, little effort has been devoted to the effect of TiC particle size. In fact, recent studies on the toughening mechanism of ceramic composites w6,7x have shown that
)
Corresponding author. Fax: q86-10-627-71160. E-mail address: [email protected] ŽJ. Gong..
the toughening effect is strongly dependent on the size of the toughening particles, whiskers andror fibers. Considering that the fracture toughness is a key parameter for the characterization of the cutting performance for a given cutting tool material, the present study was performed to examine the effect of the TiC particle size on the toughness characteristics of Al 2 O 3 –TiC composites.
2. Experimental Four grades of Al 2 O 3 –TiC composites were prepared by hot-pressing the cold isostatically pressed green bodies of Al 2 O 3 powder Ž99.99% pure. mixed with 30 wt.% TiC particles Ž99.0% pure.. Hot pressing of each sample was carried out at 1650 y 17008C and 25 MPa for 30 min. The average sizes of the TiC particles used for preparing different samples are different with each other. Table 1 gives the average
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 7 6 - 1
J. Gong et al.r Materials Letters 49 (2001) 235–238
236
Table 1 Physical and mechanical properties of the test materials Property
spectively. The parameter ErH was then calculated from Ref. w9x
Sample AC01
Average size of TiC particles Žmm. Er H R-curve parameter K 0 ŽMPa m1r 2 . K` ŽMPa m1r2 . l Žmm.
b AC03
AC04
0.5
3.8
5.6
8.2
0.0370
0.0357
0.0366
0.0361
3.77 3.77 –
3.94 5.38 107.63
4.01 5.73 110.62
a
4.12 6.01 121.54
E
P
H c
3r2
ž /
7.11
y 0.45
H
ž / E
Ž 2.
The calculated ErH-values for all the four samples are listed in Table 1.
3. Results and discussion
size of the TiC particles measured from scanning electronic microscopy ŽSEM. micrographs for each samples tested. Specimens used for R-curve measurements were cut directly from the hot-pressed products, mounted in Bakelite, ground flat using a diamond grinding wheel and then polished carefully with successively finer diamond pastes to yield a scratch-free, mirrorlike surface finish suitable for indentation. Vickers indentation tests were conducted on each specimen at room temperature in the indentation load range from 49 to 294 N and at a constant dwell time of 15 s. After indentation, the lengths of the indentation-induced cracks were immediately measured by optical microscopy. A total of 10 perfect indentations, which exhibit clearly symmetrical indentation impressions and symmetrical crack patterns, were made at each load level. For each indentation, the crack resistance, K R , was calculated with the equation proposed by Anstis et al. w8x K R s 0.016
1 s
AC02
In Fig. 1, the measured crack resistance, K R , was plotted as a function of indentation crack length for each sample considered. In plotting this figure, the crack length and the crack resistance were respectively taken as the averages of the 10 measurements carried out at the same indentation load level. However, for the sake of conciseness, error bar corresponding to each datum point is presented only for samples AC01 and AC04, respectively. It can be seen that, although there are somewhat large scatters in the measured data, increasing tendencies in crack resistance with increasing crack length were observed for samples AC02, AC03 and AC04, while for sample AC01, the crack resistance keeps nearly constant independent of crack length. Thus, one can conclude directly that the significant toughening effect resulting from the mismatch between the mechanical orrand the thermal properties of TiC parti-
Ž 1.
where P is the indentation load, c is the average crack half-length, and E and H are elastic modulus and hardness of the test material, respectively. The Knoop indentation method proposed by Marshall et al. w9x was employed to determine the parameter ErH in the present study. For each sample, five Knoop indentations were made on the polished surface at an indentation load of 33.77 N and the long diagonal length, b, and the short diagonal length, a, of each indentation impression were measured, re-
Fig. 1. Crack resistance as a function of crack size for all the test samples. Solid lines are the best-fits of the experimental data to Eq. Ž3..
J. Gong et al.r Materials Letters 49 (2001) 235–238
237
cles and the Al 2 O 3 grains can only be obtained in Al 2 O 3 –TiC composites by using larger TiC particles. Ramachandran and Shetty w10x have proposed an empirical equation to describe the R-curve measured for toughened ceramics, c
ž l/
K R s K` y Ž K` y K 0 . exp y
Ž 3.
where K 0 , K` and l are adjustable parameters. The solid lines in Fig. 1 represent Abest-fitsB of Eq. Ž3. to the measured R-curve data. The corresponding best-fit values of the parameters K 0 , K` and l, obtained by iterative regression minimizing total variance, are listed in Table 1. It is clear that Eq. Ž3. gives good fits to the experimentally measured R-curves of samples AC02, AC03 and AC04. For comparison, one can reasonably assume K 0 s K` for sample AC01 and, correspondingly, K` for sample AC01 was determined as the average of the crack resistances measured at all the six load levels examined. As discussed by Ramachandran and Shetty w10x, for ceramic composites, K 0 is an estimate of the fracture toughness of the matrix. Note that, in general, there exists a systematic error of about 30% in the toughness value estimated with Eq. Ž1. w8x. There is reason to believe that the K 0-values listed in Table 1 may be reasonable estimations of the fracture toughness of the Al 2 O 3 matrix, whose toughness was frequently reported to be around 3–4 MPa m1r2 w11x. As can be seen in Table 1, the fracture toughness of the Al 2 O 3 matrix, i.e., K 0 , increases slightly with the TiC particle size. Such a slightly increasing tendency may be attributed to the effect of the residual stresses resulting from the mismatch between the TiC particles and the matrix grains w12x. From Eq. Ž3., K` is the maximum crack resistance or the fracture toughness of the composite. On the other hand, the parameter l in Eq. Ž3. may be related directly to the range of crack extension over which toughening effects should develop and saturate. In other words, l may be considered to be a measure of the so-called process zone formed behind the crack tip during crack extension. Based on the theoretical analysis of the toughening mechanisms in ceramic composites w6,13x, one can expect that larger TiC particles would result in more significant effects
Fig. 2. Variation of Ž K` y K 0 ., a measure of toughening effect, with the average size of the TiC particles existing in the composite.
of crack-bridging andror crack deflection in Al 2 O 3 – TiC composite, thereby a large process zone formed behind the crack tip, when compared with smaller TiC particles. This seems a reasonable explanation for the increasing tendencies in K` and l with the TiC particle size. In general, a simple way to evaluate the toughening effect is to compare the fracture toughness of the composite with that of the matrix. Fig. 2 shows the variation of the difference between K` and K 0 with the average size of the TiC particles existing in the composite. An increasing tendency was also obtained, giving a further support for the above analysis. In conclusion, TiC particle size has a significant effect on the toughness characteristics of Al 2 O 3 –TiC composites. Detailed study should be conducted in future to examine this effect and gain a basic understanding for the toughening mechanism in these composites.
References w1x M. Furukawa, J. Jpn. Soc. Powder Powder Metall. 38 Ž1991. 824. w2x Y.-W. Kim, J.G. Lee, J. Am. Ceram. Soc. 72 Ž1989. 1333. w3x E. Pippel, J. Woltersdorf, Phys. Status Solidi A 116 Ž1989. 165. w4x R. Matsuki, H. Ueda, T. Takenouchi, A. Nakahira, K. Niihara, J. Jpn. Soc. Powder Powder Metall. 38 Ž1991. 365.
238
J. Gong et al.r Materials Letters 49 (2001) 235–238
w5x X.S. Li, I.M. Low, J. Mater. Sci. 29 Ž1994. 3121. w6x P.F. Becher, J. Am. Ceram. Soc. 74 Ž1991. 255. w7x B.R. Lawn, N.P. Padture, L.M. Braun, S.J. Bennison, J. Am. Ceram. Soc. 76 Ž1993. 2235. w8x G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 Ž1981. 533. w9x D.B. Marshall, T. Noma, A.G. Evans, J. Am. Ceram. Soc. 65 Ž1982. C175. w10x N. Ramachandran, D.K. Shetty, J. Am. Ceram. Soc. 74 Ž1991. 2634.
w11x R. Morrell, Handbook of Properties of Technical and Engineering Ceramics: Part 1. An Introduction for the Engineer and Designer, Her Majesty’s Stationary Office, London, 1985. w12x B.R. Lawn, Fracture of Brittle Solids, Cambridge Univ. Press, New York, 1993. w13x A.G. Evans, J. Am. Ceram. Soc. 73 Ž1990. 187.