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Synthesis, microstructures and properties of γ-aluminum oxynitride
Materials Science and Engineering A342 (2003) 245 /250 www.elsevier.com/locate/msea
Synthesis, microstructures and properties of g-aluminum oxynitride Wang Xidong *, Wang Fuming, Li Wenchao Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Received 8 October 2001; received in revised form 22 April 2002; accepted 3 May 2002
Abstract This paper deals with the synthesis, microstructures and properties of g-aluminum oxynitride (AlON). The thermodynamic properties of AlON were analyzed and the Gibbs energy of AlON with different compositions and temperatures were evaluated. Based on thermodynamic studies, AlON has been synthesized. The microstructures, mechanical properties and oxidation resistance of the synthetic AlON have been examined and discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Synthesis; Mechanical property; Microstructure; g-Aluminum oxynitride
1. Introduction AlON is a solid solution of Al2O3 and AlN [1]. It has excellent chemical and mechanical properties and has potential applications as high-performance structural ceramics. Much attention has been focused on this material over the past 40 years. In addition, it has been processed into fully dense transparent material and shows promising mechanical and optical properties suitable for use in infrared and visible window applications [2]. However, the synthesis parameters and some properties of AlON have not been reported. In order to determine the parameters of AlON synthesis process, it is important to chart out a phase stability diagram. This, in turn, requires the Gibbs energy of AlON. But, thermodynamic properties of AlON are not consistent with each other by literature [3 /10]. Therefore, the thermodynamic properties of AlON were evaluated with both parabolic rule and sub-regular solution model in the present paper. Based on thermodynamic analysis, AlON was prepared by hot-press sintering. The microstructures and mechanical properties of the synthetic AlON sample were also examined and discussed. In view of the case the reported oxidation properties of AlON
* Corresponding author. Fax: /86-10-6232-7283 E-mail address: [email protected] (W. Xidong).
are not in accordance [11 /14], oxidation experiments of AlON oxidation have also been carried out.
2. Thermodynamic analysis of AlON The production of single phase AlON requires an understanding of the phase relationship in the AlN / Al2O3 system. Several experimental and calculated phase diagrams of this system have been reported [3 / 9]. However, the results are not consistent with each other. According to the computations of Kaufman [4], AlON could be thermodynamically stable from room temperature up to its melting point. This is not in concurrence with other investigations, which claim instability of the compound below some certain temperature. McCauley [5] reported the most completed experimental phase diagram. But, in this diagram, the ‘‘three-phase’’ line of AlON, Al2O3 and AlN (T / 1973K), with varying AlON composition is in violation of Gibbs’ phase rule. Based on the calculation and reassessment by Hillert [6] as well as Dumitrescu [7], Qiu [8] reported the latest phase diagram. The stability region of AlON was investigated by Willems et al. [9]. The result is shown in Fig. 1, which is based on the experimentally measured relationship between the composition and lattice parameter of AlON in the stability region of AlON.
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determining the experimental parameters for the synthesis of AlON. It is very difficult to measure the Gibbs energy of AlON at lines EA and EB directly. Therefore, estimation of the Gibbs energy values of AlON was attempted in the present work. Two methods of estimation have been carried out. 2.1. Gibbs energy of AlON evaluated with the thermodynamic geometric rule In the system of AlN and Al2O3, Gibbs energy of Al1.8O2.4N0.2, which is on the EB line in Fig. 1 can be estimated as shown in Fig. 2 and have X15 Df G (Al1:8 O2:4 N0:2 )5X2 Fig. 1. AlON stability region in N2 gas.
Kaufman [4] and Dorner et al. [10] have reported the Gibbs energy of formation of Al7O9N from the elements. Their results are shown as follows: DG f (Al7 O9 N) 5 357 1021052:215T (J mol 1 ) [4] DG f (Al7 O9 N) 5 315 213:21054:520T (J mol 1 ) [10]
(1) (2)
(6)
where X1 is the Gibbs energy of B (Al1.8O2.4N0.2) while it is in equilibrium with A (Al7O9N) and Al2O3 and X2 is the Gibbs energy of B while it is in equilibrium with A and AlN. This method is called the thermodynamic quasi-parabolic rule. Thus the Gibbs energy of AlON with any composition can be estimated by the geometric rule. Thermodynamic data are all chosen from JANAF thermodynamic compilations except the data analyzed in the present paper. Some of the results (with the compositions located at line EA and EB as well as in the region of AlON in Fig. 1) are listed in Table 1.
For the following chemical reaction (3)
2.2. Gibbs energy of AlON evaluated with sub-regular model
DG r (3) 8755 4:403T (J) (by Kaufman)
(4)
DG r (3) 32382:3 17:29T (J) (by Dorner)
(5)
As can be seen from Fig. 1, it is very easy to get the activity coefficient of AlN in AlON along line EA and the activity coefficient of Al2O3 in AlON along line EB. Based on sub-regular model, activity coefficient of AlN in AlON (gAlN) and the activity coefficient of Al2O3 in AlON (Al2O3) at fixed compositions can be calculated with the following relation:
AlN3Al2 O3 Al7 O9 N The Gibbs free energy change can be calculated as:
It can be seen from Eqs. (1) and (2), that the difference in Df G (Al7 O9 N) between the equations by Kaufman and Dorner et al. is less than 1% from room temperature to its melting point. However, the Dr G (3) in Eqs. (4) and (5) deduced from Eqs. (1) and (2) are totally different. According to Kaufman, Dr G (3) is always less than zero, and thus, Al7O9N can be thermodynamically stable at all temperatures. On the other hand, according to Dorner et al., Al7O9N can only be thermodynamically stable above 1873 K. The experimental results [9] show that AlON is thermodynamically stable only at temperatures higher than a certain temperature between 1873 and 1923 K. Thus, the Gibbs energy of Al7O9N by Dorner et al. [10] appear to be in agreement with the experimental results. It can be seen from Fig. 1, lines EA and EB represent the compositions of AlON co-existed with AlN and Al2O3, respectively, at different temperatures. Calculation of the thermodynamic properties, such as standard Gibbs energy of formation, of AlON having the compositions on lines EA and EB is important for
T1 ln(g1 ) T2 ln(g2 )
(7)
where T1 and T2 are temperatures, g1 and g2 are the activity coefficient of element at T1 and T2, respectively.
Fig. 2. Schematic diagram of quasi-parabola rule.
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Table 1 The evaluated Gibbs energy of AlON
Al1.8O2.4N0.2 Al1.67O2.01N0.33 Al1.72O2.16N0.28 Al1.762O2.286N0.238 Al1.69O2.07N0.31 Al1.72O2.16N0.28
T (K)
DfG 8 (J mol 1) (by geometric law)
DfG 8 (J mol 1) (by regular model)
2073 2073 2073 1973 1973 1973
828 7009400 708 7009700 754 7009300 819 9009100 752 1009400 780 4009200
826 400 708 300 752 500 818 400 751 000 778 900
According to Eq. (7), activity coefficient gAlN or Al2O3 can be calculated in the stability region of AlON at different temperatures. Then, based on Gibbs /Duhem equation, the following equation can be deduced: 2 XAl
2 XAlN
2 O3
g
1 XAl 2 O3
d(ln gAl2 O3 )
g (x
AlN =xAl2 O3 )
d(ln gAlN )
(8)
1 XAlN
Integration of Eq. (8), activity coefficient of both AlN and Al2O3 in AlON at different temperature and compositions can be calculated, and Gibbs energy of AlON with different compositions and temperatures can be evaluated. Parts of the results are shown in Table 1. As can be seen from Table 1, the results evaluated with thermodynamic quasi-parabolic law and that from regular model are somewhat similar, the differences are much less than 1%. Based on the value evaluated above, phase stability diagrams were calculated under different conditions. The results are shown in Fig. 3.
3. Synthesis of AlON It can be seen from the phase stability diagrams of Fig. 3 that the oxygen partial pressure in equilibrium with AlON is very low. This indicates that oxygen partial pressure of about 10 9 Pa has to be maintained for the synthesis of AlON at 101325 Pa of N2 at 2073 K. The commercial nitrogen gas has higher impurity oxygen content. Hence, in order to synthesize AlON, a reduction agent is usually required to purify the gas atmosphere. In the present work, Al2O3 and AlN were used as raw materials. A small amount of Al fine powder was added to the mixture as reducing agent. The preparation steps are as follows: 1) Appropriate amounts of the fine powders of Al2O3, AlN and Al were mixed by ball milling in ethanol medium for 24 h. The molar ratios of Al2O3, AlN and Al are 0.75, 0.24, 0.01, respectively. Al2O3, AlN and Al are all analytically pure with average size of 0.1, 0.56 and 0.95 mm, respectively.
Fig. 3. Phase stability diagram of Al/O/N system.
2) The mixture thus obtained, was placed in a graphite mould covered with boron nitride and was sintered at 2073 K, at 25 MPa, in N2 gas flow for 3 h. 3) The properties, crystalline characteristics and microstructures of the synthetic AlON sample were determined. X-ray diffraction (XRD) analysis of the sample after sintering was carried out and the results are shown in Fig. 4. All obvious peaks were indexed to the cubic cell ˚ ). The with the lattice parameter of AlON (7.940 A
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Fig. 4. X-ray diffraction pattern of AlON.
results match the JCPDS data of AlON and are quite consistent with the literature [9].
4. Microstructures and mechanical properties of AlON The AlON samples were analyzed with transmission electron microscopy (TEM) and the results are shown in Fig. 5. The two sets of diffraction pattern (from [411], by rotating at a certain angle, to [211]) are all indexed to be a face centered cubic cell with the parameter of AlON. High resolution electron microscope (HREM) was also employed to analyze the microstructure of the synthetic AlON. The results are shown in Fig. 6. All the phases are identified as AlON. The parameters indexed by HREM, XRD and TEM are all consistent with each other. As can be seen from Fig. 4, no phase other than AlON could be detected by powder XRD analysis. This confirms the fact that a small amount of Al powder added to the raw materials can ensure the required atmosphere of AlON synthesis. As also can be seen in Fig. 6, HREM analysis indicates that there is no glass phase produced at the interface of AlON grains. This indicates that the synthetic AlON grains were bonded directly. The density of AlON (sintered) specimen was determined by the Archimedes’ method and found to be 3.63 gcm 3, about 97.8% of its theoretical density. The theoretical density of AlON was reported to be 3.71 gcm 3. The synthetic AlON specimen was shaped into 4 /6 /40 mm cuboid. The fracture toughness were measured using the single-edge notched-beam (SENB) test with a notch depth of /3 mm and a notch width of 0.25 mm. A span of 30 mm and a loading speed of 0.5 mm min 1 of three-point bending test were used for strength measurement. Seven specimens were used for
Fig. 5. TEM results of AlON.
Fig. 6. HREM analysis results.
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each strength or toughness point. The toughness of the sintered AlON was found to be 3.96 MPa m 1/2. AlON obtained was also shaped into bars of 3 /4 / 40 mm3 and three-point bending strengths were measured at room temperature as well as at high temperatures up to 1723 K. The results are shown in Fig. 7. It can be seen that the three-point bending strength of AlON is significantly high in the entire temperature range. The strength increases slightly as temperature increases from room temperature to 1273 K. The main reason may be due to the reduction of residual stress of the material with rise in temperature. However, the strength decreases as temperature exceeds 1273 K. The fracture sections of AlON were analyzed with SEM. Parts of the results are shown in Fig. 8. As can be seen in Fig. 8, The fractures were found to be a mixed intercrystalline and cleavage fracture. It can also be seen (Fig. 8b) that the fracture origin is some glass phase containing elemental Fe and Si which might come from the impurities of raw material.
5. Oxidation resistance of AlON In view of the importance of the oxidation resistance of AlON, several papers have examined the oxidation behavior of AlON [11 /14]. However, the results were not consistent with each other and the oxidation mechanism is still far from being completely understood. Corbin and McCauley [11] found that the oxidation of AlON produces a protective oxide layer up to 1200 8C. But other authors have not reported this protective layer. Goursat [12] found that the oxidation starts at about 650 8C and reaches a maximum weight gain at 1150 8C. Heating beyond this temperature produces a weight loss. Lefret [13] found that the oxidation could not be observed below 1200 8C and it reaches a maximum weight gain at about 1550 8C. Goursat [14] studied the kinetics of the oxidation of AlON powder in the temperature range 900/1100 8C.
Fig. 7. Three-point bending strength of AlON.
Fig. 8. SEM photos of the fracture sections of AlON.
They found the reaction product to be alumina with extra nitrogen. In order to improve an understanding of the reaction mechanism, isothermal and non-isothermal oxidations of AlON were carried out on a SETARAM TGA92 (SETARAM; Calure cedex, France BP34-69641) system. AlON obtained was shaped into plates of 1 /6 / 20 mm3 and introduced to oxidation experiment at air in the temperatures range of 1273/1773 K. The experiments show that AlON can only be oxidized at a temperature higher than 1273 K in air. However, the oxidation product forms a protective layer which offers resistance to further oxidation below 1473 K. The product formed at lower temperatures was identified as metastable g-Al2O3 which is likely to be converted to a-Al2O3 structure by slow solid-state transformation. But, at a temperature higher than 1643 K, the product was clearly identified to be a-Al2O3. Due to the differences in the molar volumes between a-Al2O3 and AlON, cracks are likely to be formed in the product layer promoting further oxidation. This result was in agreement with literature [11].
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6. Conclusion In the present work, thermodynamic properties of AlON were examined. Gibbs energy of AlON with different compositions and temperatures were estimated. The results are shown in Table 1. Phase stability diagrams of Al /O /N at different conditions have been calculated. Under the condition given by these diagrams, AlON was prepared by sintering at 25 MPa in N2 flow at 2073 K for 3 h. XRD and TEM analyses show that no crystalline phase other than AlON could be observed in the synthesized material. HREM analysis showed that no glass phase could be observed at the crystalline interface. The density of AlON (sintered) is 3.63 g cm 3, about 97.8% of its theoretical density. The toughness was measured to be 3.96 MPa m 1/2. Three point-bending strengths of AlON were measured to be significantly high at room temperature as well as at high temperatures up to 1723 K. The strength increases slightly as temperature increases from room temperature to 1273 K. However, the strength decreases as temperature exceeds 1273 K. The fractures of AlON were found to be a mixed intercrystalline and cleavage fracture. The fracture origin is some glass phase containing elemental Fe and Si which might come from the impurities of raw material. The oxidation product of AlON forms a protective layer which offers resistance to further oxidation below 1473 K. The product initially formed at lower tempera-
tures was identified as metastable g-Al2O3 which is likely to be converted to a-Al2O3 by slow solid-state transformation. But, at a temperature higher than 1643 K, the product was clearly identified to be a-Al2O3. Due to the differences in the molar volumes between a-Al2O3 and AlON, cracks are likely to be formed in the product layer which promotes further oxidation.
References [1] N.D. Corbin, J. Eur. Ceram. Soc. 5 (1989) 143 /154. [2] J.W. McCauley, N.D. Corbin, J. Am. Ceram. Soc. 62 (1979) 476 / 479. [3] A.M. Lejus, Rev. Hautes Tempr. Refrac. 1 (1964) 53 /95. [4] L. Kaufman, CALPHAD 3 (1979) 275 /291. [5] J.W. McCauley, N.D. Corbin, NATO ASI Sci. 65 (1983) 111 / 118. [6] M. Hillert, S. Jonsson, Z. Metallkd. 83 (1992) 714 /719. [7] L. Dumitrescu, B. Sundman, J. Am. Ceram. Soc. 75 (1995) 239 / 247. [8] C. Qiu, R. Metselaar, J. Am. Ceram. Soc. 80 (1997) 2013 /2020. [9] H.X. Willems, M.M.R.M. Hendrix, G.D. With, R. Metselaar, J. Eur. Ceram. Soc. 10 (1992) 339 /346. [10] P. Dorner, L.J. Gauckler, H. Krieg, H.L. Lukas, G. Petzow, J. Weiss, CALPHAD 3 (1979) 241 /257. [11] N.D. Corbin, J.W. McCauley, SPIE Vol. 197, Emerging Optical Materials, 1981, pp. 19 /23 [12] P. Goursat, P. Goeuriot, M. Milly, Mater. Chem. 1 (1976) 131 / 149. [13] P. Lefort, G. Ado, M. Billy, J. Phys. 47 (1986) 521 /525. [14] P. Goursat, P. Goeuriot, M. Milly, Mater. Chem. 6 (1981) 81 /94.
Synthesis, microstructures and properties of g-aluminum oxynitride Wang Xidong *, Wang Fuming, Li Wenchao Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Received 8 October 2001; received in revised form 22 April 2002; accepted 3 May 2002
Abstract This paper deals with the synthesis, microstructures and properties of g-aluminum oxynitride (AlON). The thermodynamic properties of AlON were analyzed and the Gibbs energy of AlON with different compositions and temperatures were evaluated. Based on thermodynamic studies, AlON has been synthesized. The microstructures, mechanical properties and oxidation resistance of the synthetic AlON have been examined and discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Synthesis; Mechanical property; Microstructure; g-Aluminum oxynitride
1. Introduction AlON is a solid solution of Al2O3 and AlN [1]. It has excellent chemical and mechanical properties and has potential applications as high-performance structural ceramics. Much attention has been focused on this material over the past 40 years. In addition, it has been processed into fully dense transparent material and shows promising mechanical and optical properties suitable for use in infrared and visible window applications [2]. However, the synthesis parameters and some properties of AlON have not been reported. In order to determine the parameters of AlON synthesis process, it is important to chart out a phase stability diagram. This, in turn, requires the Gibbs energy of AlON. But, thermodynamic properties of AlON are not consistent with each other by literature [3 /10]. Therefore, the thermodynamic properties of AlON were evaluated with both parabolic rule and sub-regular solution model in the present paper. Based on thermodynamic analysis, AlON was prepared by hot-press sintering. The microstructures and mechanical properties of the synthetic AlON sample were also examined and discussed. In view of the case the reported oxidation properties of AlON
* Corresponding author. Fax: /86-10-6232-7283 E-mail address: [email protected] (W. Xidong).
are not in accordance [11 /14], oxidation experiments of AlON oxidation have also been carried out.
2. Thermodynamic analysis of AlON The production of single phase AlON requires an understanding of the phase relationship in the AlN / Al2O3 system. Several experimental and calculated phase diagrams of this system have been reported [3 / 9]. However, the results are not consistent with each other. According to the computations of Kaufman [4], AlON could be thermodynamically stable from room temperature up to its melting point. This is not in concurrence with other investigations, which claim instability of the compound below some certain temperature. McCauley [5] reported the most completed experimental phase diagram. But, in this diagram, the ‘‘three-phase’’ line of AlON, Al2O3 and AlN (T / 1973K), with varying AlON composition is in violation of Gibbs’ phase rule. Based on the calculation and reassessment by Hillert [6] as well as Dumitrescu [7], Qiu [8] reported the latest phase diagram. The stability region of AlON was investigated by Willems et al. [9]. The result is shown in Fig. 1, which is based on the experimentally measured relationship between the composition and lattice parameter of AlON in the stability region of AlON.
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 2 8 2 - 4
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determining the experimental parameters for the synthesis of AlON. It is very difficult to measure the Gibbs energy of AlON at lines EA and EB directly. Therefore, estimation of the Gibbs energy values of AlON was attempted in the present work. Two methods of estimation have been carried out. 2.1. Gibbs energy of AlON evaluated with the thermodynamic geometric rule In the system of AlN and Al2O3, Gibbs energy of Al1.8O2.4N0.2, which is on the EB line in Fig. 1 can be estimated as shown in Fig. 2 and have X15 Df G (Al1:8 O2:4 N0:2 )5X2 Fig. 1. AlON stability region in N2 gas.
Kaufman [4] and Dorner et al. [10] have reported the Gibbs energy of formation of Al7O9N from the elements. Their results are shown as follows: DG f (Al7 O9 N) 5 357 1021052:215T (J mol 1 ) [4] DG f (Al7 O9 N) 5 315 213:21054:520T (J mol 1 ) [10]
(1) (2)
(6)
where X1 is the Gibbs energy of B (Al1.8O2.4N0.2) while it is in equilibrium with A (Al7O9N) and Al2O3 and X2 is the Gibbs energy of B while it is in equilibrium with A and AlN. This method is called the thermodynamic quasi-parabolic rule. Thus the Gibbs energy of AlON with any composition can be estimated by the geometric rule. Thermodynamic data are all chosen from JANAF thermodynamic compilations except the data analyzed in the present paper. Some of the results (with the compositions located at line EA and EB as well as in the region of AlON in Fig. 1) are listed in Table 1.
For the following chemical reaction (3)
2.2. Gibbs energy of AlON evaluated with sub-regular model
DG r (3) 8755 4:403T (J) (by Kaufman)
(4)
DG r (3) 32382:3 17:29T (J) (by Dorner)
(5)
As can be seen from Fig. 1, it is very easy to get the activity coefficient of AlN in AlON along line EA and the activity coefficient of Al2O3 in AlON along line EB. Based on sub-regular model, activity coefficient of AlN in AlON (gAlN) and the activity coefficient of Al2O3 in AlON (Al2O3) at fixed compositions can be calculated with the following relation:
AlN3Al2 O3 Al7 O9 N The Gibbs free energy change can be calculated as:
It can be seen from Eqs. (1) and (2), that the difference in Df G (Al7 O9 N) between the equations by Kaufman and Dorner et al. is less than 1% from room temperature to its melting point. However, the Dr G (3) in Eqs. (4) and (5) deduced from Eqs. (1) and (2) are totally different. According to Kaufman, Dr G (3) is always less than zero, and thus, Al7O9N can be thermodynamically stable at all temperatures. On the other hand, according to Dorner et al., Al7O9N can only be thermodynamically stable above 1873 K. The experimental results [9] show that AlON is thermodynamically stable only at temperatures higher than a certain temperature between 1873 and 1923 K. Thus, the Gibbs energy of Al7O9N by Dorner et al. [10] appear to be in agreement with the experimental results. It can be seen from Fig. 1, lines EA and EB represent the compositions of AlON co-existed with AlN and Al2O3, respectively, at different temperatures. Calculation of the thermodynamic properties, such as standard Gibbs energy of formation, of AlON having the compositions on lines EA and EB is important for
T1 ln(g1 ) T2 ln(g2 )
(7)
where T1 and T2 are temperatures, g1 and g2 are the activity coefficient of element at T1 and T2, respectively.
Fig. 2. Schematic diagram of quasi-parabola rule.
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Table 1 The evaluated Gibbs energy of AlON
Al1.8O2.4N0.2 Al1.67O2.01N0.33 Al1.72O2.16N0.28 Al1.762O2.286N0.238 Al1.69O2.07N0.31 Al1.72O2.16N0.28
T (K)
DfG 8 (J mol 1) (by geometric law)
DfG 8 (J mol 1) (by regular model)
2073 2073 2073 1973 1973 1973
828 7009400 708 7009700 754 7009300 819 9009100 752 1009400 780 4009200
826 400 708 300 752 500 818 400 751 000 778 900
According to Eq. (7), activity coefficient gAlN or Al2O3 can be calculated in the stability region of AlON at different temperatures. Then, based on Gibbs /Duhem equation, the following equation can be deduced: 2 XAl
2 XAlN
2 O3
g
1 XAl 2 O3
d(ln gAl2 O3 )
g (x
AlN =xAl2 O3 )
d(ln gAlN )
(8)
1 XAlN
Integration of Eq. (8), activity coefficient of both AlN and Al2O3 in AlON at different temperature and compositions can be calculated, and Gibbs energy of AlON with different compositions and temperatures can be evaluated. Parts of the results are shown in Table 1. As can be seen from Table 1, the results evaluated with thermodynamic quasi-parabolic law and that from regular model are somewhat similar, the differences are much less than 1%. Based on the value evaluated above, phase stability diagrams were calculated under different conditions. The results are shown in Fig. 3.
3. Synthesis of AlON It can be seen from the phase stability diagrams of Fig. 3 that the oxygen partial pressure in equilibrium with AlON is very low. This indicates that oxygen partial pressure of about 10 9 Pa has to be maintained for the synthesis of AlON at 101325 Pa of N2 at 2073 K. The commercial nitrogen gas has higher impurity oxygen content. Hence, in order to synthesize AlON, a reduction agent is usually required to purify the gas atmosphere. In the present work, Al2O3 and AlN were used as raw materials. A small amount of Al fine powder was added to the mixture as reducing agent. The preparation steps are as follows: 1) Appropriate amounts of the fine powders of Al2O3, AlN and Al were mixed by ball milling in ethanol medium for 24 h. The molar ratios of Al2O3, AlN and Al are 0.75, 0.24, 0.01, respectively. Al2O3, AlN and Al are all analytically pure with average size of 0.1, 0.56 and 0.95 mm, respectively.
Fig. 3. Phase stability diagram of Al/O/N system.
2) The mixture thus obtained, was placed in a graphite mould covered with boron nitride and was sintered at 2073 K, at 25 MPa, in N2 gas flow for 3 h. 3) The properties, crystalline characteristics and microstructures of the synthetic AlON sample were determined. X-ray diffraction (XRD) analysis of the sample after sintering was carried out and the results are shown in Fig. 4. All obvious peaks were indexed to the cubic cell ˚ ). The with the lattice parameter of AlON (7.940 A
248
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Fig. 4. X-ray diffraction pattern of AlON.
results match the JCPDS data of AlON and are quite consistent with the literature [9].
4. Microstructures and mechanical properties of AlON The AlON samples were analyzed with transmission electron microscopy (TEM) and the results are shown in Fig. 5. The two sets of diffraction pattern (from [411], by rotating at a certain angle, to [211]) are all indexed to be a face centered cubic cell with the parameter of AlON. High resolution electron microscope (HREM) was also employed to analyze the microstructure of the synthetic AlON. The results are shown in Fig. 6. All the phases are identified as AlON. The parameters indexed by HREM, XRD and TEM are all consistent with each other. As can be seen from Fig. 4, no phase other than AlON could be detected by powder XRD analysis. This confirms the fact that a small amount of Al powder added to the raw materials can ensure the required atmosphere of AlON synthesis. As also can be seen in Fig. 6, HREM analysis indicates that there is no glass phase produced at the interface of AlON grains. This indicates that the synthetic AlON grains were bonded directly. The density of AlON (sintered) specimen was determined by the Archimedes’ method and found to be 3.63 gcm 3, about 97.8% of its theoretical density. The theoretical density of AlON was reported to be 3.71 gcm 3. The synthetic AlON specimen was shaped into 4 /6 /40 mm cuboid. The fracture toughness were measured using the single-edge notched-beam (SENB) test with a notch depth of /3 mm and a notch width of 0.25 mm. A span of 30 mm and a loading speed of 0.5 mm min 1 of three-point bending test were used for strength measurement. Seven specimens were used for
Fig. 5. TEM results of AlON.
Fig. 6. HREM analysis results.
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each strength or toughness point. The toughness of the sintered AlON was found to be 3.96 MPa m 1/2. AlON obtained was also shaped into bars of 3 /4 / 40 mm3 and three-point bending strengths were measured at room temperature as well as at high temperatures up to 1723 K. The results are shown in Fig. 7. It can be seen that the three-point bending strength of AlON is significantly high in the entire temperature range. The strength increases slightly as temperature increases from room temperature to 1273 K. The main reason may be due to the reduction of residual stress of the material with rise in temperature. However, the strength decreases as temperature exceeds 1273 K. The fracture sections of AlON were analyzed with SEM. Parts of the results are shown in Fig. 8. As can be seen in Fig. 8, The fractures were found to be a mixed intercrystalline and cleavage fracture. It can also be seen (Fig. 8b) that the fracture origin is some glass phase containing elemental Fe and Si which might come from the impurities of raw material.
5. Oxidation resistance of AlON In view of the importance of the oxidation resistance of AlON, several papers have examined the oxidation behavior of AlON [11 /14]. However, the results were not consistent with each other and the oxidation mechanism is still far from being completely understood. Corbin and McCauley [11] found that the oxidation of AlON produces a protective oxide layer up to 1200 8C. But other authors have not reported this protective layer. Goursat [12] found that the oxidation starts at about 650 8C and reaches a maximum weight gain at 1150 8C. Heating beyond this temperature produces a weight loss. Lefret [13] found that the oxidation could not be observed below 1200 8C and it reaches a maximum weight gain at about 1550 8C. Goursat [14] studied the kinetics of the oxidation of AlON powder in the temperature range 900/1100 8C.
Fig. 7. Three-point bending strength of AlON.
Fig. 8. SEM photos of the fracture sections of AlON.
They found the reaction product to be alumina with extra nitrogen. In order to improve an understanding of the reaction mechanism, isothermal and non-isothermal oxidations of AlON were carried out on a SETARAM TGA92 (SETARAM; Calure cedex, France BP34-69641) system. AlON obtained was shaped into plates of 1 /6 / 20 mm3 and introduced to oxidation experiment at air in the temperatures range of 1273/1773 K. The experiments show that AlON can only be oxidized at a temperature higher than 1273 K in air. However, the oxidation product forms a protective layer which offers resistance to further oxidation below 1473 K. The product formed at lower temperatures was identified as metastable g-Al2O3 which is likely to be converted to a-Al2O3 structure by slow solid-state transformation. But, at a temperature higher than 1643 K, the product was clearly identified to be a-Al2O3. Due to the differences in the molar volumes between a-Al2O3 and AlON, cracks are likely to be formed in the product layer promoting further oxidation. This result was in agreement with literature [11].
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6. Conclusion In the present work, thermodynamic properties of AlON were examined. Gibbs energy of AlON with different compositions and temperatures were estimated. The results are shown in Table 1. Phase stability diagrams of Al /O /N at different conditions have been calculated. Under the condition given by these diagrams, AlON was prepared by sintering at 25 MPa in N2 flow at 2073 K for 3 h. XRD and TEM analyses show that no crystalline phase other than AlON could be observed in the synthesized material. HREM analysis showed that no glass phase could be observed at the crystalline interface. The density of AlON (sintered) is 3.63 g cm 3, about 97.8% of its theoretical density. The toughness was measured to be 3.96 MPa m 1/2. Three point-bending strengths of AlON were measured to be significantly high at room temperature as well as at high temperatures up to 1723 K. The strength increases slightly as temperature increases from room temperature to 1273 K. However, the strength decreases as temperature exceeds 1273 K. The fractures of AlON were found to be a mixed intercrystalline and cleavage fracture. The fracture origin is some glass phase containing elemental Fe and Si which might come from the impurities of raw material. The oxidation product of AlON forms a protective layer which offers resistance to further oxidation below 1473 K. The product initially formed at lower tempera-
tures was identified as metastable g-Al2O3 which is likely to be converted to a-Al2O3 by slow solid-state transformation. But, at a temperature higher than 1643 K, the product was clearly identified to be a-Al2O3. Due to the differences in the molar volumes between a-Al2O3 and AlON, cracks are likely to be formed in the product layer which promotes further oxidation.
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