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Investigation of strength degradation and strength recovery via short time heating for ground alumina ceramics with different grain size
Journal of Materials Processing Technology 145 (2004) 276–280
Investigation of strength degradation and strength recovery via short time heating for ground alumina ceramics with different grain size Meng Liu a,∗ , Jun-ichiro Takagi a , Akira Tsukuda b a
Department of Mechanical Engineering and Materials Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan b Kagawa Prefectural Industrial Technology Center, 587-1 Goto-Cho, Takamatsu 761-8031, Japan Received 27 September 2002; received in revised form 27 September 2002; accepted 17 February 2003
Abstract The strength of ground ceramics tends to be deteriorated due to the flaws induced by grinding process. And this strength degradation can be recovered by a short time heating method. Since the strength of ceramics is greatly related to the grain size, it can be considered that the strength degradation and the strength recovery of ground ceramics are strongly related to the grain size of ceramics. Therefore, in this study the relation between grain size of alumina ceramics and the strength degradation as well as the strength recovery was investigated experimentally. It was confirmed in this study that the strength degradation and the strength recovery are strongly related to the grain size. The experimental results indicate that the strength degradation and the strength recovery are more remarkable in the ceramics with small grain size than in the ceramics with large grain size. There exist a fine correlation between the strength degradation due to grinding process and the strength recovery via short time heating for the alumina ceramics with different grain size. © 2003 Elsevier B.V. All rights reserved. Keywords: Ground ceramics; Strength degradation; Strength recovery; Short time heating; Grain size
1. Introduction As for ceramic materials, the strength of ceramics increases as the grain size decreases generally [1,2]. This phenomenon is not caused by the grain size itself, but by the dimension of the initial flaws which are in proportion to the grain size. On the other hand, the strength of ground ceramics is greatly influenced by the flaw layer with micro-cracks and flaws induced by grinding process [3]. And the grinding mechanism and the formed flaw layer are greatly related to the grain size [4,5]. Therefore, it can be considered that there exists a correlation between the strength degradation of ground ceramics and the grain size of ceramics. In prior study [6], a method recovering the strength of ground ceramics by heating in minutes (short time heating) was attempted. Specimens of alumina ceramics and silicon nitride ceramics ground under various grinding conditions were heated under various heating conditions. The experimental results indicated that the strength of ground ceramics could be improved significantly by this short time heating. And the mechanism of this strength recovery was explained that the crack closure was attributed to relax partial stress
∗ Corresponding author. Fax: +81-45-3316593. E-mail address: [email protected] (M. Liu).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00465-5
concentration on grinding cracks, especially on the crack tip. It can be considered that this strength recovery may be affected by the grain size due to the difference of the grinding flaws. To apply this strength recovery technique, it is necessary to clarify the influence of the grain size on the strength recovery. Therefore, in this study, seven types of alumina ceramics with different grain size were produced using two types of ceramic powder. Using these seven types of alumina ceramics, the relation between the grain size and the strength degradation after grinding process as well as the strength recovery via short time heating for ground ceramics were investigated experimentally and discussed.
2. Experimental procedures 2.1. Ceramics Two types of high purity alumina powder (Type1: TAIMICRON TM-DA, purity 99.0%, average radius 0.2 m, TAIMEI Chemicals Co., Ltd., Japan; Type2: AL160SG3, purity 99.8%, average radius 0.6 m, SHOWA DENKO Co., Ltd., Japan) were used to produce ceramic specimens. The ceramic plates with size of 40 mm × 40 mm × 5 mm were formed by mold press (TM-DA, press under 30 MPa
M. Liu et al. / Journal of Materials Processing Technology 145 (2004) 276–280
277
Table 1 Mechanical properties of Al2 O3 ceramics Properties
Materials TM-DA powder
Sintering temperature (◦ C) Fracture toughness (MPa m1/2 ) Flexural strength (MPa) Young’s modular (GPa) Vickers hardness (Hv) Relative density (%) Average gram size (m)
AL-160 powder
TM-1
TM-2
TM-3
TM-4
TM-5
AL-1
AL-2
1300 4.0 461 395 2020 98.5
1340 3.92 480 408 1960 99.2
1400 3.56 524 414 1890 99.6
1500 5.0 352 402 1778 99.2
1600 4.44 344 408 1700 99.2
1550 4.3 580 376 1660 96.6
1650 4.9 330 391 1650 98.2
1.1
1.18
2.04
3.5
7.27
2.0
9.7
and AL160SG, press under 20 MPa), and then by CIP forming under pressure of 300 MPa. Then ceramic plates were sintered under the normal pressure for 1.0 h. The sintering temperature was varied as a variable to produce different grain size. For TM-DA materials, five kinds of sintering temperature of 1300, 1340, 1400, 1500, 1600 ◦ C were adopted. And for AL160SG materials, two kinds of sintering temperature of 1550, 1650 ◦ C were adopted. Then the ceramic plates were sliced and ground to prepare the ceramic specimens with dimensions of 3 mm × 4 mm × 40 mm (based on Japanese Industrial Standards JIS R1601). The pre-machining of all ceramic specimens were carried out with surface grinding at longitudinal direction of specimens 16–1.0 m of surface roughness Ry . The ceramic specimens were used for measuring the properties, grinding tests and heating tests. The properties of ceramic specimens are listed in Table 1. For the measurement of grain size, the specimens were polished to mirror surface and etched under the temperature lower 50 ◦ C than the sintering temperature for 10 min, then observed using SEM. Fig. 1 shows the representative microphotographs of etched ceramic surfaces.
Fig. 2. Schematic view of three-point bending test.
were ground with plunge grinding method in the direction perpendicular to longitudinal direction of specimens with up-cut grinding condition. The ground surface was induced as shown in Fig. 2. In order to keep the similar surface of the diamond wheels, the diamond wheels were trued with rotary truer to control the sharpness of diamond wheel before each grinding test [7], and then dressed with WA sticks. The experimental conditions are listed in Table 2 in detail.
2.2. Experimental procedures The grinding tests were carried out on a horizontal spindle surface grinding machine. The ceramic specimens
Table 2 Experimental conditions Diamond wheel specification Resin-bonded diamond wheel Truing condition Setup Truer Speed-ratio Dressing condition
Fig. 1. Microphotographs of etched ceramic surfaces.
Grinding condition Workpiece Wheel speed Feed rate Depth of cut Cumulative depth of cut Coolant condition
JIS-SDC140N100BS4, 180Dia × 10T Norton AXE-1416 Metal-bonded wheel #60/80 90Dia × 1.6T S = 0.8 (S = Vs (wheel)/Vt (truer) [7]) Grinding a alumina stick WA220G, WA400G Vw = 300 mm3 /mm Al2 O3 ceramics Vs = 16.67 m/s Vw = 3.0 m/min a = 5 m × 4 Pass ∆ = 20 m Wet
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Therefore, the grinding flaws will be formed in a large size [4,5].
4. Strength recovery by short time heating
Fig. 3. Bending strength for various grain size (before and after grinding).
Heating tests were conducted in air environment using a tube electric furnace. After the interior of the tube electric furnace reaching the desired temperature, put the specimen in the tube and keep the specimen for the desired heating time. And then the heated specimens were cooled down in air environment. Heating time and heating temperature were varied as experimental variables in this experiment. The fracture strength of all ceramic specimens was evaluated by using the average value of bending strength. The bending strength was measured at room temperature in three-point bending test according to the JIS R1601 standard (as shown in Fig. 2). At least five specimens were tested for each experimental condition.
3. Strength degradation of ground ceramics Fig. 3 shows the relation between the grain size and the strength before and after grinding process for seven types of ceramics. From the figure, the strength before grinding process shows a strong correlation with the grain size. The strength of ceramics increases greatly as the grain size decreases [2]. The strength degradation can be observed in all ceramic specimens with various grain size after grinding process. There exists no remarkable difference in the strength after grinding process under the grinding condition of this study among seven types of the ceramics. Observing the strength degradation due to the flaws induced by grinding process, two kinds of the grain size can be divided in this study, small grain size which the grain size is smaller than 2.04 m, and large grain size which the grain size is larger than 3.5 m. The ceramics with small grain size show a remarkable tendency of the strength degradation. But for the ceramics with large grain size, the strength changes slightly before and after grinding process, namely the strength degradation is not remarkable. This can be explained that there exists the large size of initial flaws in the ceramics with large grain size before grinding, these flaws affects the strength of ceramics mainly [2]. Another reasons can be considered that grinding mechanism about the ceramics with large grain size. As grinding the ceramics with large grain size, the brittle fracture between the grains will be produced generally.
Figs. 4–8 show the results of the strength of ground ceramics after heating tests for TM-1–TM-5 ceramics. The heating tests were carried out in the conditions with different heating temperature and different heating time. Heating temperatures were selected as four kinds of 600, 800, 1000, 1200 ◦ C. Two dotted lines show the strength of specimens before grinding and before the heating tests (after grinding). For the ground ceramics with small grain size, as shown in Figs. 4–6, in the condition of 600 ◦ C, there exists no remarkable improvement of strength after heating tests. As
Fig. 4. Variation of bending strength after heating (TM-1, 1.1 m).
Fig. 5. Variation of bending strength after heating (TM-2, 1.18 m).
Fig. 6. Variation of bending strength after heating (TM-3, 2.04 m).
M. Liu et al. / Journal of Materials Processing Technology 145 (2004) 276–280
Fig. 7. Variation of bending strength after heating (TM-4, 3.5 m).
the heating temperature increases, the strength recovery increases greatly. And the maximum strength recovery was obtained in the condition of 1200 ◦ C, 5 min for three types of the ceramics. For heating time beyond 5 min, there exists a tendency of the decrease of the strength when heat treatment was carried out for a long time to 30 min in all condition of the heating temperature [6]. On the other hand, for the ceramics with large grain size (Figs. 7 and 8), there exists no remarkable change in the strength in all heating conditions. In other words, the strength recovery cannot be obtained in the ceramics with large grain size via short time heating. For AL-1 and AL-2 ceramics, there also exists remarkable difference in the strength recovery between AL-1 and AL-2 ceramics. For the AL-1 ceramics with the grain size of 2.0 m, as the heating temperature increases, the strength recovery increases. The maximum strength recovery can be obtained in the condition of 1200 ◦ C, 5 min [6]. For the ceramics AL-2 with the grain size of 9.7 m, there exist almost no changes of the strength after heating tests in all heating conditions. Namely, the strength recovery cannot be obtained in AL-2 ceramics via short time heating.
279
Fig. 9. Variation of bending strength after heating (AL-1, 2.0 m).
two types of TM ceramics and AL ceramics, the similar phenomena can be concluded from above-mentioned experimental results. Namely, the remarkable strength recovery can be obtained in the ceramics with small grain size, but the strength recovery cannot be obtained in the ceramics with large grain size. In order to discuss the variation of the strength recovery with the grain size, the data of Figs. 4–10 were plotted to investigate the relation between the variation of strength after grinding and after heating tests for seven types of the ceramics. Fig. 11 shows the relation between the grain size and the strength degradation as well as the maximum recovery of strength. From the figure, the strength degradation and the strength recovery change significantly from grain size 2.0 to 3.0 m. For the alumina ceramics with the grain size smaller than about 2.0 m, the strength
5. Influence of grain size on strength recovery Seven types of alumina ceramics with different grain size were heated after grinding in various heating condition. For
Fig. 10. Variation of bending strength after heating (AL-2, 9.7 m).
Fig. 8. Variation of bending strength after heating (TM-5, 7.3 m).
Fig. 11. Variation of strength versus grain size.
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deteriorates remarkably after grinding process and the effect of the strength recovery via short time heating is remarkable. It is caused by the reason that the strength of the ceramics is very sensitive to grinding cracks induced during grinding process [5,6]. On the other hand, for the ceramics with large grain size larger than about 3.0 m, the strength degradation and strength recovery are not remarkable and the flaws induced by grinding affects the strength slightly. There exists a fine correlation between the strength degradation and the strength recovery via short time heating can be observed form the figure. Since the grinding cracks cause the partial stress concentration, the strength of ground ceramics is generally sensitive to the shape and size of grinding cracks. On the other hand, the strength recovery by heat treatment including heating for a long time has generally been attributed to the extra bonding force exerted by the glass phase on the crack wake, either through transport of the pre-existing glass phase across the crack plane [8–10] or due to the reaction of the crack surface with the environment [11–14]. For the strength recovery of alumina ceramics via short time heating, the changes in grinding cracks were caused by the diffusion of pre-existing glass phase, and crack closure occurred with a small amount of re-bonding near the crack tip. The crack closure is attributed to relax partial stress concentration due to grinding cracks, especially on the crack tip [6]. For above-mentioned mechanism of the strength recovery, it can be understood that the short time heating can recover the strength of ceramics, which the strength degradation was caused by the grinding flaws such as micro-cracks and defects etc. But for the ground ceramics which the strength is determined by the initial flaws induced before grinding process, not by the flaws induced by the grinding process, namely the strength changes slightly after grinding process, the short time heating has ineffective in recovering the strength of ground ceramics. Therefore, when applying the short time heating in recovering the strength of ground ceramics, the causes of the strength degradation must be considerable.
6. Conclusions In order to investigate the relation between the grain size and the strength degradation as well as the strength recovery via short time heating for ground alumina ceramics, seven
types of alumina ceramics were produced and tested under various heating conditions. It was confirmed in this study that the strength degradation and the strength recovery are strongly related to the grain size. The experimental results indicate that the strength degradation and the strength recovery are more remarkable in the ceramics with small grain size than in the ceramics with large grain size. There exist a fine correlation between the strength degradation due to grinding process and the strength recovery via short time heating for the alumina ceramics with different grain size.
References [1] M. Iwasa, K. Ando, N. Ogura, Process zone size fracture criterion for ceramics, J. JSME (A) 56 (531) (1990) 2353–2358 (in Japanese). [2] N. Miyahara, K. Yamaishi, Y. Muto, K. Uematsu, M. Inome, Effects of grain size on strength and fracture toughness in alumina, J. JSME (A) 58 (556) (1992) 2299–2306 (in Japanese). [3] K. Mori, H. Yoshinari, S. Nenno, T. Yokobori, Effect of surface grinding on flexural strength of alumina, J. Soc. Mater. Sci. Jpn. 41 (467) (1992) 1268–1272. [4] J.E. Mayer, G.P. Fang, Effect of grit depth of cut on strength of ground ceramics, Ann. CIRP 43 (1) (1994) 309–312. [5] A. Tsukuda, Y. Kondo, J. Takagi, M. Nishida, K. Uematsu, Grinding mechanism and residual stress of alumina ceramics with different grain size, J. Soc. Grinding Eng. 42 (6) (1998) 249–254 (in Japanese). [6] M. Liu, J. Takagi, A. Tsukuda, Strength recovery of ground ceramics via electric furnace heating, J. Mater. Process. Technol. 127 (2002) 107–114. [7] J. Takagi, T. Fukuda, K. Subramaniam, Sharpness control of vitrified-bonded CBN wheel face by precision rotary truer, in: Proceedings of the First International Conference on New Manufacturing Technology, Chiba, 1990, p. 269. [8] M. Hurukawa, K. Ogawa, T. Sugita, The strength of sintered alumina ceramics (1st report), J. Jpn. Soc. Prec. Eng. 35 (10) (1969) 634 (in Japanese). [9] M. Hurukawa, K. Ogawa, T. Sugita, The strength of sintered alumina ceramics (2nd report), J. Jpn. Soc. Prec. Eng. 36 (2) (1970) 101 (in Japanese). [10] S.R. Choi, V. Tikare, Crack healing of alumina with a residual glassy phase, strength, fracture toughness and fatigue, Mater. Sci. Eng. A 171 (1993) 77–83. [11] F.F. Lange, T.K. Gupta, Crack healing by heat treatment, J. Am. Ceram. Soc. 53 (1) (1970) 54–55. [12] F.F. Lange, Healing of surface cracks in SiC by oxidation, J. Am. Ceram. Soc. 53 (1970) 290. [13] J.E. Ritter, K. Jakus, L.A. Strazdis, W.P. Rogers, Effect of high temperature exposure in air on strength of hot-pressed silicon nitride, J. Am. Ceram. Soc. 66 (3) (1983) C53–C55. [14] Y.H. Zhang, L. Edwards, W.J. Plumbridge, Crack healing in a silicon nitride ceramic, J. Am. Ceram. Soc. 81 (7) (1998) 1861–1868.
Investigation of strength degradation and strength recovery via short time heating for ground alumina ceramics with different grain size Meng Liu a,∗ , Jun-ichiro Takagi a , Akira Tsukuda b a
Department of Mechanical Engineering and Materials Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan b Kagawa Prefectural Industrial Technology Center, 587-1 Goto-Cho, Takamatsu 761-8031, Japan Received 27 September 2002; received in revised form 27 September 2002; accepted 17 February 2003
Abstract The strength of ground ceramics tends to be deteriorated due to the flaws induced by grinding process. And this strength degradation can be recovered by a short time heating method. Since the strength of ceramics is greatly related to the grain size, it can be considered that the strength degradation and the strength recovery of ground ceramics are strongly related to the grain size of ceramics. Therefore, in this study the relation between grain size of alumina ceramics and the strength degradation as well as the strength recovery was investigated experimentally. It was confirmed in this study that the strength degradation and the strength recovery are strongly related to the grain size. The experimental results indicate that the strength degradation and the strength recovery are more remarkable in the ceramics with small grain size than in the ceramics with large grain size. There exist a fine correlation between the strength degradation due to grinding process and the strength recovery via short time heating for the alumina ceramics with different grain size. © 2003 Elsevier B.V. All rights reserved. Keywords: Ground ceramics; Strength degradation; Strength recovery; Short time heating; Grain size
1. Introduction As for ceramic materials, the strength of ceramics increases as the grain size decreases generally [1,2]. This phenomenon is not caused by the grain size itself, but by the dimension of the initial flaws which are in proportion to the grain size. On the other hand, the strength of ground ceramics is greatly influenced by the flaw layer with micro-cracks and flaws induced by grinding process [3]. And the grinding mechanism and the formed flaw layer are greatly related to the grain size [4,5]. Therefore, it can be considered that there exists a correlation between the strength degradation of ground ceramics and the grain size of ceramics. In prior study [6], a method recovering the strength of ground ceramics by heating in minutes (short time heating) was attempted. Specimens of alumina ceramics and silicon nitride ceramics ground under various grinding conditions were heated under various heating conditions. The experimental results indicated that the strength of ground ceramics could be improved significantly by this short time heating. And the mechanism of this strength recovery was explained that the crack closure was attributed to relax partial stress
∗ Corresponding author. Fax: +81-45-3316593. E-mail address: [email protected] (M. Liu).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00465-5
concentration on grinding cracks, especially on the crack tip. It can be considered that this strength recovery may be affected by the grain size due to the difference of the grinding flaws. To apply this strength recovery technique, it is necessary to clarify the influence of the grain size on the strength recovery. Therefore, in this study, seven types of alumina ceramics with different grain size were produced using two types of ceramic powder. Using these seven types of alumina ceramics, the relation between the grain size and the strength degradation after grinding process as well as the strength recovery via short time heating for ground ceramics were investigated experimentally and discussed.
2. Experimental procedures 2.1. Ceramics Two types of high purity alumina powder (Type1: TAIMICRON TM-DA, purity 99.0%, average radius 0.2 m, TAIMEI Chemicals Co., Ltd., Japan; Type2: AL160SG3, purity 99.8%, average radius 0.6 m, SHOWA DENKO Co., Ltd., Japan) were used to produce ceramic specimens. The ceramic plates with size of 40 mm × 40 mm × 5 mm were formed by mold press (TM-DA, press under 30 MPa
M. Liu et al. / Journal of Materials Processing Technology 145 (2004) 276–280
277
Table 1 Mechanical properties of Al2 O3 ceramics Properties
Materials TM-DA powder
Sintering temperature (◦ C) Fracture toughness (MPa m1/2 ) Flexural strength (MPa) Young’s modular (GPa) Vickers hardness (Hv) Relative density (%) Average gram size (m)
AL-160 powder
TM-1
TM-2
TM-3
TM-4
TM-5
AL-1
AL-2
1300 4.0 461 395 2020 98.5
1340 3.92 480 408 1960 99.2
1400 3.56 524 414 1890 99.6
1500 5.0 352 402 1778 99.2
1600 4.44 344 408 1700 99.2
1550 4.3 580 376 1660 96.6
1650 4.9 330 391 1650 98.2
1.1
1.18
2.04
3.5
7.27
2.0
9.7
and AL160SG, press under 20 MPa), and then by CIP forming under pressure of 300 MPa. Then ceramic plates were sintered under the normal pressure for 1.0 h. The sintering temperature was varied as a variable to produce different grain size. For TM-DA materials, five kinds of sintering temperature of 1300, 1340, 1400, 1500, 1600 ◦ C were adopted. And for AL160SG materials, two kinds of sintering temperature of 1550, 1650 ◦ C were adopted. Then the ceramic plates were sliced and ground to prepare the ceramic specimens with dimensions of 3 mm × 4 mm × 40 mm (based on Japanese Industrial Standards JIS R1601). The pre-machining of all ceramic specimens were carried out with surface grinding at longitudinal direction of specimens 16–1.0 m of surface roughness Ry . The ceramic specimens were used for measuring the properties, grinding tests and heating tests. The properties of ceramic specimens are listed in Table 1. For the measurement of grain size, the specimens were polished to mirror surface and etched under the temperature lower 50 ◦ C than the sintering temperature for 10 min, then observed using SEM. Fig. 1 shows the representative microphotographs of etched ceramic surfaces.
Fig. 2. Schematic view of three-point bending test.
were ground with plunge grinding method in the direction perpendicular to longitudinal direction of specimens with up-cut grinding condition. The ground surface was induced as shown in Fig. 2. In order to keep the similar surface of the diamond wheels, the diamond wheels were trued with rotary truer to control the sharpness of diamond wheel before each grinding test [7], and then dressed with WA sticks. The experimental conditions are listed in Table 2 in detail.
2.2. Experimental procedures The grinding tests were carried out on a horizontal spindle surface grinding machine. The ceramic specimens
Table 2 Experimental conditions Diamond wheel specification Resin-bonded diamond wheel Truing condition Setup Truer Speed-ratio Dressing condition
Fig. 1. Microphotographs of etched ceramic surfaces.
Grinding condition Workpiece Wheel speed Feed rate Depth of cut Cumulative depth of cut Coolant condition
JIS-SDC140N100BS4, 180Dia × 10T Norton AXE-1416 Metal-bonded wheel #60/80 90Dia × 1.6T S = 0.8 (S = Vs (wheel)/Vt (truer) [7]) Grinding a alumina stick WA220G, WA400G Vw = 300 mm3 /mm Al2 O3 ceramics Vs = 16.67 m/s Vw = 3.0 m/min a = 5 m × 4 Pass ∆ = 20 m Wet
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M. Liu et al. / Journal of Materials Processing Technology 145 (2004) 276–280
Therefore, the grinding flaws will be formed in a large size [4,5].
4. Strength recovery by short time heating
Fig. 3. Bending strength for various grain size (before and after grinding).
Heating tests were conducted in air environment using a tube electric furnace. After the interior of the tube electric furnace reaching the desired temperature, put the specimen in the tube and keep the specimen for the desired heating time. And then the heated specimens were cooled down in air environment. Heating time and heating temperature were varied as experimental variables in this experiment. The fracture strength of all ceramic specimens was evaluated by using the average value of bending strength. The bending strength was measured at room temperature in three-point bending test according to the JIS R1601 standard (as shown in Fig. 2). At least five specimens were tested for each experimental condition.
3. Strength degradation of ground ceramics Fig. 3 shows the relation between the grain size and the strength before and after grinding process for seven types of ceramics. From the figure, the strength before grinding process shows a strong correlation with the grain size. The strength of ceramics increases greatly as the grain size decreases [2]. The strength degradation can be observed in all ceramic specimens with various grain size after grinding process. There exists no remarkable difference in the strength after grinding process under the grinding condition of this study among seven types of the ceramics. Observing the strength degradation due to the flaws induced by grinding process, two kinds of the grain size can be divided in this study, small grain size which the grain size is smaller than 2.04 m, and large grain size which the grain size is larger than 3.5 m. The ceramics with small grain size show a remarkable tendency of the strength degradation. But for the ceramics with large grain size, the strength changes slightly before and after grinding process, namely the strength degradation is not remarkable. This can be explained that there exists the large size of initial flaws in the ceramics with large grain size before grinding, these flaws affects the strength of ceramics mainly [2]. Another reasons can be considered that grinding mechanism about the ceramics with large grain size. As grinding the ceramics with large grain size, the brittle fracture between the grains will be produced generally.
Figs. 4–8 show the results of the strength of ground ceramics after heating tests for TM-1–TM-5 ceramics. The heating tests were carried out in the conditions with different heating temperature and different heating time. Heating temperatures were selected as four kinds of 600, 800, 1000, 1200 ◦ C. Two dotted lines show the strength of specimens before grinding and before the heating tests (after grinding). For the ground ceramics with small grain size, as shown in Figs. 4–6, in the condition of 600 ◦ C, there exists no remarkable improvement of strength after heating tests. As
Fig. 4. Variation of bending strength after heating (TM-1, 1.1 m).
Fig. 5. Variation of bending strength after heating (TM-2, 1.18 m).
Fig. 6. Variation of bending strength after heating (TM-3, 2.04 m).
M. Liu et al. / Journal of Materials Processing Technology 145 (2004) 276–280
Fig. 7. Variation of bending strength after heating (TM-4, 3.5 m).
the heating temperature increases, the strength recovery increases greatly. And the maximum strength recovery was obtained in the condition of 1200 ◦ C, 5 min for three types of the ceramics. For heating time beyond 5 min, there exists a tendency of the decrease of the strength when heat treatment was carried out for a long time to 30 min in all condition of the heating temperature [6]. On the other hand, for the ceramics with large grain size (Figs. 7 and 8), there exists no remarkable change in the strength in all heating conditions. In other words, the strength recovery cannot be obtained in the ceramics with large grain size via short time heating. For AL-1 and AL-2 ceramics, there also exists remarkable difference in the strength recovery between AL-1 and AL-2 ceramics. For the AL-1 ceramics with the grain size of 2.0 m, as the heating temperature increases, the strength recovery increases. The maximum strength recovery can be obtained in the condition of 1200 ◦ C, 5 min [6]. For the ceramics AL-2 with the grain size of 9.7 m, there exist almost no changes of the strength after heating tests in all heating conditions. Namely, the strength recovery cannot be obtained in AL-2 ceramics via short time heating.
279
Fig. 9. Variation of bending strength after heating (AL-1, 2.0 m).
two types of TM ceramics and AL ceramics, the similar phenomena can be concluded from above-mentioned experimental results. Namely, the remarkable strength recovery can be obtained in the ceramics with small grain size, but the strength recovery cannot be obtained in the ceramics with large grain size. In order to discuss the variation of the strength recovery with the grain size, the data of Figs. 4–10 were plotted to investigate the relation between the variation of strength after grinding and after heating tests for seven types of the ceramics. Fig. 11 shows the relation between the grain size and the strength degradation as well as the maximum recovery of strength. From the figure, the strength degradation and the strength recovery change significantly from grain size 2.0 to 3.0 m. For the alumina ceramics with the grain size smaller than about 2.0 m, the strength
5. Influence of grain size on strength recovery Seven types of alumina ceramics with different grain size were heated after grinding in various heating condition. For
Fig. 10. Variation of bending strength after heating (AL-2, 9.7 m).
Fig. 8. Variation of bending strength after heating (TM-5, 7.3 m).
Fig. 11. Variation of strength versus grain size.
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deteriorates remarkably after grinding process and the effect of the strength recovery via short time heating is remarkable. It is caused by the reason that the strength of the ceramics is very sensitive to grinding cracks induced during grinding process [5,6]. On the other hand, for the ceramics with large grain size larger than about 3.0 m, the strength degradation and strength recovery are not remarkable and the flaws induced by grinding affects the strength slightly. There exists a fine correlation between the strength degradation and the strength recovery via short time heating can be observed form the figure. Since the grinding cracks cause the partial stress concentration, the strength of ground ceramics is generally sensitive to the shape and size of grinding cracks. On the other hand, the strength recovery by heat treatment including heating for a long time has generally been attributed to the extra bonding force exerted by the glass phase on the crack wake, either through transport of the pre-existing glass phase across the crack plane [8–10] or due to the reaction of the crack surface with the environment [11–14]. For the strength recovery of alumina ceramics via short time heating, the changes in grinding cracks were caused by the diffusion of pre-existing glass phase, and crack closure occurred with a small amount of re-bonding near the crack tip. The crack closure is attributed to relax partial stress concentration due to grinding cracks, especially on the crack tip [6]. For above-mentioned mechanism of the strength recovery, it can be understood that the short time heating can recover the strength of ceramics, which the strength degradation was caused by the grinding flaws such as micro-cracks and defects etc. But for the ground ceramics which the strength is determined by the initial flaws induced before grinding process, not by the flaws induced by the grinding process, namely the strength changes slightly after grinding process, the short time heating has ineffective in recovering the strength of ground ceramics. Therefore, when applying the short time heating in recovering the strength of ground ceramics, the causes of the strength degradation must be considerable.
6. Conclusions In order to investigate the relation between the grain size and the strength degradation as well as the strength recovery via short time heating for ground alumina ceramics, seven
types of alumina ceramics were produced and tested under various heating conditions. It was confirmed in this study that the strength degradation and the strength recovery are strongly related to the grain size. The experimental results indicate that the strength degradation and the strength recovery are more remarkable in the ceramics with small grain size than in the ceramics with large grain size. There exist a fine correlation between the strength degradation due to grinding process and the strength recovery via short time heating for the alumina ceramics with different grain size.
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