- Email: [email protected]
Mechanical properties of Sialon
ELSEVIER
Materials Science and Engineering A209 (1996) 175 179
Mechanical properties of Sialon Cosme Roberto Moreira da Silva, Francisco Cristovao Lourenc;o de Melo, Oliverio Moreira de Macedo Silva Centro Tecnico Aeroespacia!, fAEAitfR CEP-J2228-904, sao Jose dos Campos, SP, Bred!
Abstract Silicon nitride based ceramics have several important applications in the fabrication of hard materials, such as ceramic cutting tools, In this work, aluminum nitride, yttrium oxide and cerium oxide were used as sintering aids on pressureless sintered Si N , The relationships between the type and quantities of these additives with micro hardness (HV), fracture toughness and for:ned crystalline phases, such as Sialon, have been evaluated, The correlation between microstructure and the mechanical properties has been studied, using the scanning electron microscopy, Keywords: Mechanical properties; Scanning electron microscopy; Sialon nitride - - - - - - - - - - - - _ . _ - --
--- ------ --- - --------
1. Introduction
2. Experimental procedure
High temperature properties in silicon nitride are extremely affected by the existing intergranular glassy phase, between grains of fJ-Si 3 N 4 . Glass forming regions of several different systems have been studied, and the reaction products formed during sintering have been analysed [I]. Liquid phase sintering of silicon nitride shows better results using rare earth oxide as additives [2]. Using AI 20 3 as additive, for example, there will be changes in fJ -Si 3 N 4 , where AI and substitute for Si and Ni, respectively. There will be in this case, a solid solution formation, refered to as fJ -Sialon, with the general formula Si,Al,O, + 1.5\N s _ x [3]. .. Different types of Sialon have fJ -Si 3 N 4 structure, stabilized by the dissolution of some rare earth metals into interstitial sites, with the general formula M,(SiAI)12(0,N)16' where M is a metal and 0 < x ~ 2
2.1. Compositions
°
[4]. There is a general acceptance that the most relevant mechanism in Sialons sintering is dissolution/re-precipitation, and reaction kinetics pathways may depend on equilibrium phase relationship and physical/chemical characteristics of the used powders. 0921-5093/96/$15.00 © 1996 .- Elsevier Science S.A. All rights reserved SSDf 0921-5093(95)10133-0
In this work, the fJ -silicon nitride powder used was synthesized by UBE, using the ammonolysis of silicon tetrachloride (SiCI4) [5]. In this process, silicon diimide (Si(NH 2)) plus NH 4 are produced, and the pyrolisis between 1200 and 1450 °C produces a-Si,N 4 powder. The UBE SN E 10 has been chosen for this work, with average particle size of 0.55 micron. The Si,N 4 powder data is given in Table I. The sintering experiments has been performed using cerium oxide (Ce0 2) aluminum nitride (AIN) and yttrium oxide (Y"0,) as sintering aids. The compositions of materials evaluated in this work are given in Table 2. Each composition was ball-milled, using high purity alumina milling balls and dry ethanol, for 6 h. The slurry was dried and each sample die pressed at 25 MPa, followed by cold isostatic pressing at 300 MPa. Pressureless sintering was performed in a graphite resistance furnace in nitrogen atmosphere, at normal pressure, using the following heating schedule: 20°C min - 1 from room temperature to 1000 °C, followed by 15°C min - I from 1000 to 1750 °C, with holding time of 30
C.R.M. da Silva / Materials Science and Engineering A209 (/996) 175-179
176
Table I Typical powder data of UBE-Si 3 N4-SN E 10 Powder type
SN-IO
Chemical composition (wt.'X,)
x-Si 3 N4 (wt.';!o)
97
N-38.0
0-
~
Specific surface area (m 2 ) ~
Mean particle
sIze
II
0.55
1.3
erative for silicon nitride ceramics, has a resulting solution, diffusion-re-precipitation mechanisms, where the rate controlling step is more effective for yttrium containing materials, for the sintering conditions used in this work.
4. Micro hardness
C-~O.I
CI-0.005 Fe-0.002 AI-O.002 Ca-O.OOI
Table 2 Sample compositions Sample
Si 3 N 4 (wt.%)
AIN (wt.°lr,)
I 2 3 4 5 6
92.5 90.0 85.0 92.5 90.0 85.0
5.0 5.0 5.0 5.0 5.0 5.0
(Ce 2.5) (Ce 5.0) (Ce 10.0) (Y 2.5) (Y 5.0) (Y 10.0)
Y2 0 3 (wt.%)
Ce0 2 (wt.%) 2.5 5.0 10.0
2.5 5.0 10.0
min at this last temperature. Characterization has been performed by X-ray diffraction, to identify formed phases. Microhardness values have been determined at room temperature, using Vickers indenter with a 29.4 N load, with the average of ten indentations in each sample. The fracture toughness was determined by the indentation hardness technique. Scanning electron microscopy was used to evaluate microstructural aspects of etched samples in the chosen compositions.
3. Results and discussion The achieved relative densities for the studied compositions are presented in Table 3. The yttrium containing materials present higher densities, probably owing to a different dissolution behaviour of the nitride powder in the oxide plus aluminum nitride melt. The liquid phase sintering, opTable 3 Relative densities Relative density (%)
I
85 92 90 98 98 98
3 4 5 6
5. X-Ray diffraction The identified phases for all compositions are presented in Figs. 3-8. The number of crystalline phases increases with higher additive percentage. For the 10% ceria containing materials, the following phases were identified: f3 Silicon nitride; CeSi0 2Ncerium silicon nitride oxide; Ce sSi 30 12 N -cerium nitride silicate; Ce 4,67(Si04hO-cerium oxide silicate. Materials with 10% yttria additions form three main phases: f3 silicon nitride; Y2Si3N403-yttrium silicon oxide nitride; Al 6Si6N s0 9 -aluminum silicon nitride oxide, Sialon.
6. Fractures toughness
Composition
2
The micro hardness measurements for the cerium and yttrium containing materials are presented in Figs. I and 2, respectively. In the cerium containing materials, the hardness values are almost the same on the surface of the samples for all studied compositions, but decreases substantially when move toward the nucleus. This reduction is more effective for compositions with lower additive content. Such hardness reductions, as confirmed by SEM using EDS, are related to the diffusion of cerium to the surface of the samples during sintering, via grain boundaries, depleting the nucleus and enriching the surface with phases of higher hardness. Fig. 2 shows that for yttrium containing compositions, hardness values are higher, and remains almost constant from surface to the nucleus of the samples. There is a slight decrease for materials with 5% yttria additions. Scanning Electron Microscopy, using dispersive X-rays analyser energy, showed that there was no depletion of yttrium from the sample nucleus, for all studied compositions, with yttria additions.
Table 4 presents fracture toughness measurements, showing a composition dependence, which can be related to the f3 -grain aspect ratio (length to diameter ratio) and intergranular refractory phases. Higher aspect ratio will increase the fracture toughness, as occurring with the compositions with yttria additions. Tailoring strain state in the grain boundaries is one important factor for the improvement of fracture
C.R.M. da Silva
Materials Science and Engineering A209 (/996) 175179
-.
..
15
-..
.
-. -.
177
.
GPa
10 ~Ce02-2,5%
____ Ce02-5% --'-Ce02 -10%
5
0+---+--+---+--1---+---11---+----+--+---1
o
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
DISTANCE FROM SURFJlCE (mm) Fig. 1. Microhardness variation from surface to nucleus in ceria containing materials.
20,5 20 19,5 19 18,5 GPa
18 17,5
-+- Y203 ·2,5%
17
--A-Y203 -10'.
____ Y203 - 5%
16,5 16 0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
DISTANCE FROM SUFACE (mm) Fig. 2. Microhardness variation from surface to nucleus in yttria containing materials.
toughness for silicon nitride with low metal oxide additions. In this work, the better fracture toughness results obtained for yttria containing materials are related to higher aspect ratio of fJ -grain and the existing refractory crystalline phases, acting as reinforcement. Thermal expansion or elastic modulus mismatch can generate a stress field along the refractory phase/fJ grain matrix region, causing a crack deflection or crack branching mechanism.
microscopy, demonstrated differences for dissimilar compositions. (Figs. 9 and 10). The aspect ratio of (i -grains are very similar between nucleus and surface of the samples, for compositions with yttria additions. For cerium containing materials, the aspect ratio is clearly different in such regions, mainly for the sample with 2.5% cerium oxide. These results are in agreement with the differences obtained from hardness measurements, in such areas.
7. Scanning electron microscopy
8. Conclusions
The observed microstructures, obtained by scanning
Yttria containing materials, for the processing condi-
178
C.R.M. da Silva
I Materials Science and Engineering A209 (1996) 175-179
Y - 2,5 Ce - 2,5
28
28
1-f3-SN
1-f3-SN
Fig. 6. Crystalline phases for 2.5% yttria additions.
Fig. 3. Crystalline phases for 2.5% ceria additions.
Ce-5,0
1,3
45°
Y - 5,
35°
40°
°
50°
55°
28 1-f3-SN 2- CeSi0 2 N
28
3-Ce s Si 3 0 12 N 4- Ce4,67(Si04)30
1-f3-SN
Fig. 7. Crystalline phases for 5.0'1"0 yttria additions.
Fig. 4. Crystalline phases for 5.0'/;, ceria additions.
Ce-IO,O
Y - 10,0 1,3
2,3,4
55°
50°
35°
30°
20°
25°
28 1-f3-SN 2- CeSi 02N
3 -Ce S Si 3 0
12
N
4 - Ce4,67 (Si 04)3
°
Fig. 5. Crystalline phases for 10% ceria additions.
55°
50°
35°
28 I-
f3 -
SN
Fig. 8. Crystalline phases for 10% yttria additions.
C.R.M. da Silva
Materials Science and Engineerinli A209 (1996) /75 /79
179
I
Table 4 Fracture toughness (indentation method) Composition
K{C (MPa m' 2)
I (Ce 2.5)
6.55 ± 0.11 7.88±0.12 7.51 ±O.IO 7.68 ± 0.09 7.24 ± 0.04 7.68 ± 0.17
2 (Ce 5.0) 3 (Ce 10.0) 4 (Y 2.5)
5 (Y 5.0) 6 (Y 10.0)
tions in this work, gave higher values of micro hardness, density and fracture toughness. Such results are related to a more effective controlling step on the solution re-precipitation mechanism during liquid phase
Y203 5.0%
·i
Ce02 2.5%
._----'
Ce02 5.0%
,,'I •
,t'
Fig. 10. Etched samples of yttria containing materials, showing aspect ratio of II-grains.
sintering. The formation of highly refractory intergranular phases and higher aspect ratio for fJ -grains are also important. A mechanism based on crack deflection or crack branching will be operative, increasing the fracture toughness of yttrium based materials.
.~ V·I
.-""""". ;
Ce02 10.0%
Fig. 9. Etched samples of ceria containing materials, showing aspect ratio of fJ -grains.
References [1] M. Mitomo. J. Mater. Sci.• /0 (1975) 1169. [2] H.F. Priest. G.L. Priest and G.E. Gazza, J. Am. Ceram. Soc.. 60 (1977) 81. [3] A. Arias, J. Mater. Sci., 16 (1981) 787. [4] L.J. Gauekler, L.J. Lucas and G.J. PetlOw, J. Am. Ceram. Soc" 58 (1975) 346. [5] S. Hampshite. H.K. Park. D.B. Thompson and K.H. Jack, Natllr(', 274 (1975) 880.
Materials Science and Engineering A209 (1996) 175 179
Mechanical properties of Sialon Cosme Roberto Moreira da Silva, Francisco Cristovao Lourenc;o de Melo, Oliverio Moreira de Macedo Silva Centro Tecnico Aeroespacia!, fAEAitfR CEP-J2228-904, sao Jose dos Campos, SP, Bred!
Abstract Silicon nitride based ceramics have several important applications in the fabrication of hard materials, such as ceramic cutting tools, In this work, aluminum nitride, yttrium oxide and cerium oxide were used as sintering aids on pressureless sintered Si N , The relationships between the type and quantities of these additives with micro hardness (HV), fracture toughness and for:ned crystalline phases, such as Sialon, have been evaluated, The correlation between microstructure and the mechanical properties has been studied, using the scanning electron microscopy, Keywords: Mechanical properties; Scanning electron microscopy; Sialon nitride - - - - - - - - - - - - _ . _ - --
--- ------ --- - --------
1. Introduction
2. Experimental procedure
High temperature properties in silicon nitride are extremely affected by the existing intergranular glassy phase, between grains of fJ-Si 3 N 4 . Glass forming regions of several different systems have been studied, and the reaction products formed during sintering have been analysed [I]. Liquid phase sintering of silicon nitride shows better results using rare earth oxide as additives [2]. Using AI 20 3 as additive, for example, there will be changes in fJ -Si 3 N 4 , where AI and substitute for Si and Ni, respectively. There will be in this case, a solid solution formation, refered to as fJ -Sialon, with the general formula Si,Al,O, + 1.5\N s _ x [3]. .. Different types of Sialon have fJ -Si 3 N 4 structure, stabilized by the dissolution of some rare earth metals into interstitial sites, with the general formula M,(SiAI)12(0,N)16' where M is a metal and 0 < x ~ 2
2.1. Compositions
°
[4]. There is a general acceptance that the most relevant mechanism in Sialons sintering is dissolution/re-precipitation, and reaction kinetics pathways may depend on equilibrium phase relationship and physical/chemical characteristics of the used powders. 0921-5093/96/$15.00 © 1996 .- Elsevier Science S.A. All rights reserved SSDf 0921-5093(95)10133-0
In this work, the fJ -silicon nitride powder used was synthesized by UBE, using the ammonolysis of silicon tetrachloride (SiCI4) [5]. In this process, silicon diimide (Si(NH 2)) plus NH 4 are produced, and the pyrolisis between 1200 and 1450 °C produces a-Si,N 4 powder. The UBE SN E 10 has been chosen for this work, with average particle size of 0.55 micron. The Si,N 4 powder data is given in Table I. The sintering experiments has been performed using cerium oxide (Ce0 2) aluminum nitride (AIN) and yttrium oxide (Y"0,) as sintering aids. The compositions of materials evaluated in this work are given in Table 2. Each composition was ball-milled, using high purity alumina milling balls and dry ethanol, for 6 h. The slurry was dried and each sample die pressed at 25 MPa, followed by cold isostatic pressing at 300 MPa. Pressureless sintering was performed in a graphite resistance furnace in nitrogen atmosphere, at normal pressure, using the following heating schedule: 20°C min - 1 from room temperature to 1000 °C, followed by 15°C min - I from 1000 to 1750 °C, with holding time of 30
C.R.M. da Silva / Materials Science and Engineering A209 (/996) 175-179
176
Table I Typical powder data of UBE-Si 3 N4-SN E 10 Powder type
SN-IO
Chemical composition (wt.'X,)
x-Si 3 N4 (wt.';!o)
97
N-38.0
0-
~
Specific surface area (m 2 ) ~
Mean particle
sIze
II
0.55
1.3
erative for silicon nitride ceramics, has a resulting solution, diffusion-re-precipitation mechanisms, where the rate controlling step is more effective for yttrium containing materials, for the sintering conditions used in this work.
4. Micro hardness
C-~O.I
CI-0.005 Fe-0.002 AI-O.002 Ca-O.OOI
Table 2 Sample compositions Sample
Si 3 N 4 (wt.%)
AIN (wt.°lr,)
I 2 3 4 5 6
92.5 90.0 85.0 92.5 90.0 85.0
5.0 5.0 5.0 5.0 5.0 5.0
(Ce 2.5) (Ce 5.0) (Ce 10.0) (Y 2.5) (Y 5.0) (Y 10.0)
Y2 0 3 (wt.%)
Ce0 2 (wt.%) 2.5 5.0 10.0
2.5 5.0 10.0
min at this last temperature. Characterization has been performed by X-ray diffraction, to identify formed phases. Microhardness values have been determined at room temperature, using Vickers indenter with a 29.4 N load, with the average of ten indentations in each sample. The fracture toughness was determined by the indentation hardness technique. Scanning electron microscopy was used to evaluate microstructural aspects of etched samples in the chosen compositions.
3. Results and discussion The achieved relative densities for the studied compositions are presented in Table 3. The yttrium containing materials present higher densities, probably owing to a different dissolution behaviour of the nitride powder in the oxide plus aluminum nitride melt. The liquid phase sintering, opTable 3 Relative densities Relative density (%)
I
85 92 90 98 98 98
3 4 5 6
5. X-Ray diffraction The identified phases for all compositions are presented in Figs. 3-8. The number of crystalline phases increases with higher additive percentage. For the 10% ceria containing materials, the following phases were identified: f3 Silicon nitride; CeSi0 2Ncerium silicon nitride oxide; Ce sSi 30 12 N -cerium nitride silicate; Ce 4,67(Si04hO-cerium oxide silicate. Materials with 10% yttria additions form three main phases: f3 silicon nitride; Y2Si3N403-yttrium silicon oxide nitride; Al 6Si6N s0 9 -aluminum silicon nitride oxide, Sialon.
6. Fractures toughness
Composition
2
The micro hardness measurements for the cerium and yttrium containing materials are presented in Figs. I and 2, respectively. In the cerium containing materials, the hardness values are almost the same on the surface of the samples for all studied compositions, but decreases substantially when move toward the nucleus. This reduction is more effective for compositions with lower additive content. Such hardness reductions, as confirmed by SEM using EDS, are related to the diffusion of cerium to the surface of the samples during sintering, via grain boundaries, depleting the nucleus and enriching the surface with phases of higher hardness. Fig. 2 shows that for yttrium containing compositions, hardness values are higher, and remains almost constant from surface to the nucleus of the samples. There is a slight decrease for materials with 5% yttria additions. Scanning Electron Microscopy, using dispersive X-rays analyser energy, showed that there was no depletion of yttrium from the sample nucleus, for all studied compositions, with yttria additions.
Table 4 presents fracture toughness measurements, showing a composition dependence, which can be related to the f3 -grain aspect ratio (length to diameter ratio) and intergranular refractory phases. Higher aspect ratio will increase the fracture toughness, as occurring with the compositions with yttria additions. Tailoring strain state in the grain boundaries is one important factor for the improvement of fracture
C.R.M. da Silva
Materials Science and Engineering A209 (/996) 175179
-.
..
15
-..
.
-. -.
177
.
GPa
10 ~Ce02-2,5%
____ Ce02-5% --'-Ce02 -10%
5
0+---+--+---+--1---+---11---+----+--+---1
o
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
DISTANCE FROM SURFJlCE (mm) Fig. 1. Microhardness variation from surface to nucleus in ceria containing materials.
20,5 20 19,5 19 18,5 GPa
18 17,5
-+- Y203 ·2,5%
17
--A-Y203 -10'.
____ Y203 - 5%
16,5 16 0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
DISTANCE FROM SUFACE (mm) Fig. 2. Microhardness variation from surface to nucleus in yttria containing materials.
toughness for silicon nitride with low metal oxide additions. In this work, the better fracture toughness results obtained for yttria containing materials are related to higher aspect ratio of fJ -grain and the existing refractory crystalline phases, acting as reinforcement. Thermal expansion or elastic modulus mismatch can generate a stress field along the refractory phase/fJ grain matrix region, causing a crack deflection or crack branching mechanism.
microscopy, demonstrated differences for dissimilar compositions. (Figs. 9 and 10). The aspect ratio of (i -grains are very similar between nucleus and surface of the samples, for compositions with yttria additions. For cerium containing materials, the aspect ratio is clearly different in such regions, mainly for the sample with 2.5% cerium oxide. These results are in agreement with the differences obtained from hardness measurements, in such areas.
7. Scanning electron microscopy
8. Conclusions
The observed microstructures, obtained by scanning
Yttria containing materials, for the processing condi-
178
C.R.M. da Silva
I Materials Science and Engineering A209 (1996) 175-179
Y - 2,5 Ce - 2,5
28
28
1-f3-SN
1-f3-SN
Fig. 6. Crystalline phases for 2.5% yttria additions.
Fig. 3. Crystalline phases for 2.5% ceria additions.
Ce-5,0
1,3
45°
Y - 5,
35°
40°
°
50°
55°
28 1-f3-SN 2- CeSi0 2 N
28
3-Ce s Si 3 0 12 N 4- Ce4,67(Si04)30
1-f3-SN
Fig. 7. Crystalline phases for 5.0'1"0 yttria additions.
Fig. 4. Crystalline phases for 5.0'/;, ceria additions.
Ce-IO,O
Y - 10,0 1,3
2,3,4
55°
50°
35°
30°
20°
25°
28 1-f3-SN 2- CeSi 02N
3 -Ce S Si 3 0
12
N
4 - Ce4,67 (Si 04)3
°
Fig. 5. Crystalline phases for 10% ceria additions.
55°
50°
35°
28 I-
f3 -
SN
Fig. 8. Crystalline phases for 10% yttria additions.
C.R.M. da Silva
Materials Science and Engineerinli A209 (1996) /75 /79
179
I
Table 4 Fracture toughness (indentation method) Composition
K{C (MPa m' 2)
I (Ce 2.5)
6.55 ± 0.11 7.88±0.12 7.51 ±O.IO 7.68 ± 0.09 7.24 ± 0.04 7.68 ± 0.17
2 (Ce 5.0) 3 (Ce 10.0) 4 (Y 2.5)
5 (Y 5.0) 6 (Y 10.0)
tions in this work, gave higher values of micro hardness, density and fracture toughness. Such results are related to a more effective controlling step on the solution re-precipitation mechanism during liquid phase
Y203 5.0%
·i
Ce02 2.5%
._----'
Ce02 5.0%
,,'I •
,t'
Fig. 10. Etched samples of yttria containing materials, showing aspect ratio of II-grains.
sintering. The formation of highly refractory intergranular phases and higher aspect ratio for fJ -grains are also important. A mechanism based on crack deflection or crack branching will be operative, increasing the fracture toughness of yttrium based materials.
.~ V·I
.-""""". ;
Ce02 10.0%
Fig. 9. Etched samples of ceria containing materials, showing aspect ratio of fJ -grains.
References [1] M. Mitomo. J. Mater. Sci.• /0 (1975) 1169. [2] H.F. Priest. G.L. Priest and G.E. Gazza, J. Am. Ceram. Soc.. 60 (1977) 81. [3] A. Arias, J. Mater. Sci., 16 (1981) 787. [4] L.J. Gauekler, L.J. Lucas and G.J. PetlOw, J. Am. Ceram. Soc" 58 (1975) 346. [5] S. Hampshite. H.K. Park. D.B. Thompson and K.H. Jack, Natllr(', 274 (1975) 880.