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Effect of γ-aluminum oxynitride dispersion on some alumina properties
Ceramics International 15 (1989) 207-212
Effect of 7-Aluminum Oxynitride Dispersion on Some Alumina Properties* D. Goeuriot-Launay, P. Goeuriot, F. Thevenot Laboratoire des C~ramiques Sp~iales, ENSME, 158, cours Fauriei--42023 Saint-Etienne C~dex 2, France
G. Orange, G. Fantozzi GEMPPM, INSA, Bat. 502-20, Avenue A. Einstein, 69621 Villeurbanne C~dex, France
R. T r a b e l s i & D. T r e h e u x Laboratoire Diffusion dans les solides, Ecole Centrale de Lyon, 36 route de Dardilly, BP 163, 69131 Ecully C6dex, France Centre de Recherches Rh6ne-Alpes des C6ramiques Sp6ciales (Received 14 April 1988; accepted 6 June 1988)
Abstract: A T-aluminum oxynitride (),-AION) dispersion in an alumina matrix can lead to a composite ceramic with equivalent properties to those of a single phase alumina at room temperature. The AI203-TAION is obtained by pressureless sintering or hot-pressing of intimate mixtures of = alumina and aluminum nitride powders. The best treatment temperature is about 1700°C. The composite presents a high wear resistance; its flexural properties at high temperature are higher than those of alumina.
1 INTRODUCTION
2 FABRICATION
OF
THE
AI=O3-TAION
COMPOSITE Alumina is a widely used ceramic material, and many researches tend to reinforce it, with whiskers (SIC1), or dispersed grains, particularly zirconia. 2"3 The present work deals with the effect of),-aluminum oxynitride (TALON) dispersed in an alumina matrix. Our purpose is to describe at first the elaboration of the alumina-~,-aluminum oxynitride composite, and then its properties; wear resistance, mechanical properties at high temperature. * This work is a part of the doctoral thesis of D. GoeuriotLaunay, University of Lyon 1 (France), l I June 1987.
Fine powder of 7-AION is not yet available. So the alumina-~-aluminum oxynitride composite is prepared by sintering and reacting of alumina and aluminum nitride (specific area 6m2g -1) powder mixtures, using two starting alumina powders: - - E X A L alumina, in which the impurity content is less than 200 ppm. - - B A Y E R alumina containing about 0.2 wt % of impurities. Several oxynitrides exist in the AI2Oa-A1N
207 Ceramics International 0272-8842/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
208
D. Goeuriot-Launay et al.
system:4,s the y-aluminum oxynitride, with the composition limits at 1700°C, A I 2 . s I O 3 . s 6 N o . 4 4 t o A I 2 . 9 1 0 3 . 2 7 N o . 7 3 ; a tetragonal phase 6, with a higher alumina content. Similar phenomena occur during both pressureless sintering and hot-pressing, which is described below. 6 Initial mixtures containing 0 to l l m o l % AIN have been milled by attrition in anhydrous alcohol in order to obtain, after reaction hot-pressing, composites with 0 to 30vo1% A1ON. Hot-pressing experiments (1550 < T < 1800°C, 15 < t < 30min, P = 4 0 M P a , under nitrogen atmosphere) were performed using an uniaxial press (DEGUSSA). During pressure sintering, it is possible to observe the sample shrinkage. X-ray diffraction analyses were performed on a face which is perpendicular to the pressing axis. Metallographic observations (SEM) were also performed. X-ray diffraction analyses indicate that: --the impurities in the BAYER alumina promote the AI2Oa,AIN reaction that starts at 1550°C instead of 1600°C in the case of EXAL alumina. The y-spinel phase is the only oxynitride formed up to 1750°C. --above 1750°C for both aluminas other diffraction peaks appear. Some of them correspond to the tetragonal (6) aluminum oxynitride; others are near those of monoclinic (0) alumina. This tends to prove that an oxynitride of this structure can exist in the A1N-A12Oa system. Two ratios were defined to follow the oxynitrides evolution:
25.
R'
=
Is(400) 0' + 6)-AlaN Is(113) tz-Al20 3
x
f .4-......
20'
!5.
¥
.."" i
/
.""
i
/
.10-
i"
I
5 1500
lg00
14oo
I~O0'T ('c)
1750°C, the y-spinel oxynitride transforms into the tetragonal form while the alumina content decreases. X-ray diffraction observations can be related to the dilatometric curves (Fig. 3). Pure alumina shrinkage during pressure sintering slows down at about 1500°C and finishes at 1700°C. In the case of A12Oa-A1ON mixtures a maximum shrinkage occurs at about 1600 and 1650°C for respectively BAYER and EXAL aluminas. In the range of temperature 1650-1900°C (EXAL alumina) an expansion stage takes place that is accelerated above 1800°C. Fusion occurs at about 1900°C. With BAYER alumina, these phenomena are shifted about 50°C downwards. soaking time 30 rain
powder
} AI203 BAYER 30%AION ....... + ........ i axis ---x~-J. axis | D.o~non textured ~ A1'203 EXAL 30%AION
R' 120.
100
which depends on both ?- and 6-AlaN. The R' evolutions are similar if the X-ray patterns are performed on a face perpendicular to the pressing axis, on powder, or with a system that removes textural effects from the measurements (DOSOPHATEX system). Both R and R' increase up to 1700°C (Figs 1 and 2): y-spinel A l a N is formed at 1550°C, and its content increases until the aluminum nitride has completely reacted. For temperatures above 1700°C, R is almost constant while R' increases. This means that, above
'
Fig. 1. R ratio for the different hot-pressing conditions of the AI20 3 7 0 - A l a N 30 vol% composite. Soaking t i m e s : - - O - - , 15min and . . . + ..., 30rain for B A Y E R ; - - Q - - , 15min and - . - x - . - , 30rain for EXAL.
R = Is(220) y-AlaN x 100 Is(113) ~t-Al20 3 which depends on the y-spinel amount,
/j
.,A"P
/ .....
P
100 80 60-
-~..." .°.
20-
1500 Fig. 2.
jig..." ""
'
16'00
'
I"]00
I~k)0 T('C)
R' ratio for the different hot-pressing conditions of the AI20 3 7 0 - A l a N 30 vol% composite.
Effect of a y-aluminum oxynitride dispersion on alumina
209
At203 EXAL . . . At203 EXAL 10%AtON ...... At203 EXAL 30%AI.ON _-- At203 BAYER30%AI.ON
-At
=
.,...
-"-.=.2 . . . . . . . . ii,I
.,
i
".,, ',~;
(a)
k/
T (*C)
1500 1600I~ 180019'00 Fig. 3. Dilatometric curves during hot-pressing of AI2Oa-AIN mixtures: Al =f(T°C).
Finally the following interpretation of the dilatometric curves is proposed: the v-AION formation gives rise to a volume increase. In the range of temperature 1550-1700°C (BAYER) there is a competition between the alumina shrinkage and the V-oxynitride formation; this explains the shrinkage and then the dilatation observed on the dilatometric curves. At about 1750°C, the maximum amount of Vspinel is formed; so the dilatation is almost stopped. At 1780°C the y-phase dissolves alumina and then changes to the tetragonal 6- (and monoclinic 0-) oxynitrides: this phenomenon corresponds to the dilatation acceleration on the dilatometric curve. The viscosity of these phases decreases, a liquid phase appears that spreads out of the samples at 1900°C and which explains the beginning of shrinkage. The interval between the dilatometric curves corresponding to the two types of alumina powders (BAYER and EXAL) is explained by the impurities that promote the reactions that have been described• Metallographic observations (SEM) are performed on polished and thermally etched surfaces (the AION phase appears with a light grey colour on the photos). The alumina grain size is relatively fine (2-3/~m) in spite of the hot-pressing temperature (1700°C, Fig. 4a). The dispersion limits grain growth. Alumina grain coarsening occurs only at 1800°C (Fig. 4b): the viscosity of each oxynitride phase, present at triple points, decreases.
(b) Fig. 4. Microstructures (SEM) of a ~-AI203 80-y-AION 20vo1% hot-pressed under 40MPa. (a) T = 1700°C; (b) T = 1800°C.
3 M E C H A N I C A L PROPERTIES AT R O O M T E M PERATU R E
Table 1 presents composite mechanical properties at room temperature, these depend on both the type of sintering (with or without pressure) and the starting alumina. The presence of large defects due to the processing, especially for the pressureless sintered samples, leads to difficulties in the interpretation. However, some results can be pointed out: --high volume contents of y-AION increase the composite brittleness: 20v01% v-A1ON is the most interesting composition --hot-pressing leads to a control of the defect size. At room temperature the flexural properties of the AI203-AION composite are similar to those of single phase alumina. The beneficial effect of the Valuminum oxynitride will be now shown on other mechanical properties.
D. Goeuriot-Launay e t a ] .
210
Table 1.
Characterization of the AI=O3-AION composites: Mechanical properties at room temperature % AION
Density (gcm =)
Hv 10N (GPa)
Strength (MPa)
Klc (MPa ~/m)
E (GPa)
G1C (Jm -=)
10 20 30
3"92 3"91 3"88
20"9+0"9 20"6 + 0"9 21 "9 + 0'8
515+15 460 __+40 400 + 100
4"3 3"6 3"8
335 315 362
52 37 36
10 20 30
3"94 3-91 3"89
20"7 + 1 '1 21 "8 + 1 '2 21 '8 + 0"8
330 + 40 510 __+80 715 + 65
3"8 3"8 3"9
355 380 385
38 35 36
10 20 30
3"93 3'91 3'89
19'6 + 0"8 20"7 + 0"6 21 "3 + 0"7
335 + 25 500 + 50 430 __+100
4"4 4"2 4"0
----
----
10 20 30
3"94 3"89 3"84
20"5 + 0"8 20"8 + 0"7 20"5 + 1 "0
390 + 25 530 + 45 550 + 60
4"3 3"5 3"9
290 -325
58 -43
EXAL alumina Pressureless Sintering Hot pressing
Bayer alumina Pressureless Sintering
Hot-pressing
4 TRIBOLOGICAL AION COMPOSITE
BEHAVIOUR v'8
OF AI=O=-
The frictional samples (12 x 12 x 5 mm) are lapped then polished (diamond 3 pm). The friction experiments are carried out in water on a tribometer consisting of a rotary cylinder (52100 steel, 64HRC) against a fixed ceramic plane. The sliding speed is about 0.36 m s- 1. A load of 700 N has to be applied to obtain significant wear. The friction coefficient and the wear volume are respectively calculated from the tangential force and the width of the wear trace. Figure 5 represents the friction coefficient evolution for hot-pressed materials. A periodic evolution of the friction coefficient of pure EXAL At2 03 --.-- At20:3 - 10 % ALON . . . . AL203- 20 % ALON w . . At203- 30 % ALON At0N
O8 0.7 06 Q5
alumina is observed. This is due to the accumulation of wear debris in the contact area with involves an increase of the tangential force and its removal leads to the decrease. The Al2Oa-A1ON composite does not present a heavy changing of the friction coefficient. This can be explained by the trapping of wear debris (third body) into the contact. Consequently, as shown in Fig. 6, pure EXAL alumina presents a wear volume more important than those of the A12Oa-A1ON composite and pure A1ON. A minimum wear is observed in the case of Al2Oa-10 vol% AION, mainly after 2 h. The type of sintering (with or without pressure) and the type of alumina (BAYER or EXAL) have only a small effect on the tribological properties of the composites. The best results are obtained for hot-pressed samples with an EXAL alumina matrix. V mm3 05
-.----
AL2 03 At2 O3 - 10% AtON AL203-20% ALON
/
"
- -
t
~
I=
04 Q3
3'0
6"o
9"0
,9
120 Time(rain)
Fig. 5. Evolution of the friction coefficients versus time.
30
60
120 Time(rain)
Fig. 6. Evolution of the wear volumes versus time: AION content influence.
Effect of a ~-aluminum oxynitride dispersion on alumina
211
The presence of 7-A1ON enhances the alumina wear resistance against bearing steel (52100). The weak composite degradation occurs following an intergranular mechanism :crack formation along the grain boundaries, and then pull out of some alumina grains. The presence of the 7-A1ON phase impedes the propagation of intergranular cracks and modifies favorably the properties of the third body present in the contact, and the metal-ceramic transfer formation. These two phenomena implicate the decrease of the composite degradation.
'
BEHAVIOUR AT HIGH 5 FRACTURE T E M PERATUR E OF THE AI=O3-7-AION C O M POSITE 9
The composites whose properties are described in Table 1 have been tested at high temperature, in a nitrogen atmosphere. Figure 7 shows that the presence of ~-A1ON in the composite containing EXAL alumina remarkably delays the alumina strength decrease above 1000°C. The composite has elastic behaviour, even up to 1400°C. The critical stress intensity factor values are maintained at high temperature (Fig. 8) and even increase for high 7AION contents. This strengthening effect on mechanical properties at high temperature occurs for ),AION contents larger than 20 vol%, for both types of sintering. On the other hand the composites fabricated from BAYER alumina have poor mechanical properties (af and K~c) above 1000°C (Figs 7 and 8). of (MPa) 600
I,
K1C
(HPo .~/'m )
•r O
ot
600
Pressureless sintering x AI203 EXAL 10%AION + AI203 EXAL 30*/.AION - - B - - AL203 BAYER 30% AION Hot pressing -,,V-- A1203 BAYER 30 % AION • At203 EXAL 30% AION
800
% 10bO 12~10 1/,b0T(~C)
Fig. 8. Toughness versus temperature of the Ai203-7-AION composites. P.S. = pressureless sintered, H.P. = hot pressed.
This behavior can be explained by the particular microstructure of the composites observed on the high temperature (1400°C) fracture face (Fig. 9). The ),-AION crystals have two forms: small grains at triple point junctions and along alumina grain boundaries. In some boundaries we also observe a continuous intergranular phase. The ~-AION spherical particles, that are dispersed along grain boundaries, prevent a high percentage of alumina grains from sliding. This induces the absence of macroscopic plasticity and the high level of strength at high temperature. The good toughness values between 1000°C and 1400°C can be attributed to the presence of a grain boundary phase which leads to microplasticity, localized along grain boundaries, that permits a plastic relaxation in the plastic zone ahead of the main crack.
400,
200
x T (*C) + at203 x At 2133 • AL203 OA1203 • At203 • AI,2 03 Fig. 7.
0=3 wn • = 35 i~m EXAL 10% ALON(¢=S-15w-n) EXAL 30% ALON(¢=2_3Fm) EXAL 30% ALON (¢=5-10 p.m) BAYER 30% AI.ON(¢=2-3 [,u'n)
Flexural strength versus temperature of the hot pressed AI2Oa-7-AION composites.
l ~ J . ~ Fig. 9.
~ _;J. ¸
mmam~,
Fracture face at 1400°C of a AI203 80-7-AION 20 vol% hot-pressed composite.
212 In the case o f B A Y E R alumina, the mechanical properties are not maintained at high temperature. The impurities (Ca, Na, Mg etc.) lead to fusible phases at grain boundaries. The flexural strength decrease observed is due to the low viscosity of these phases. The alumina grain sliding is more important than the plastic relaxation phenomenon, so the fracture energy decreases; this explains the rapid decrease o f toughness above 1000°C. In this case a macroscopic plasticity is observed.
6 CONCLUSIONS TM Alumina-~,-aluminum oxynitride composites can be sintered with or without pressure from Al2Oa-A1N powder mixtures. These composites have mechanical properties at r o o m temperature similar to those o f alumina. The ~-A1ON dispersion has a beneficial effect on the mechanical properties (~rr, K~c) at high temperature and on wear resistance of an alumina ceramic. Wear applications are n o w being exploited in industry.
REFERENCES 1. CLAUSSEN, N. & PETZOW, G., Whisker-reinforced oxide ceramics. J. Physique, Colloque C1, Supplement 2, 47 (1986) C1-693-702.
D. Goeuriot-Launay et al.
2. CLAUSSEN, N., Fracture toughness of A1203 with an unstabilized ZrO2 dispersed phase. J. Am. Ceram. Soc., 59(1-2) (1976) 49-51. 3. ORANGE, G., FANTOZZI, G., HOMERIN, P., THEVENOT, F., LERICHE, A. & CAMBIER, F., Thermomechanical properties of zirconia toughened materials: effect of microstructure and temperature on toughening mechanisms. Science and Technology of Zirconia lII, Tokyo, 9-11 September 1986. 4. LEJUS, A. M., Th6se sur la formation, ~ haute temp6rature de spindles non stoechiomStriques et de phases d~riv~es dans plusieurs syst6mesd'oxydes ~ base d'alumine et dans le syst6me alumine-nitrure d'aluminium. Rev. Int. Htes Temp. Refract., 1(1) (1964) 53-95. 5. McCAULEY, J. W. & CORBIN, N. D., High temperature reactions and microstructures in the A12Oa-AIN system, 'Nato' Advanced Ceramic Meeting, University of Sussex England (1981). Progress in Nitrogen Ceramics, F. L. Riley, ed. Martinus-Nijhoff, Amsterdam, 1983, pp. 111-18. 6. GOEURIOT-LAUNAY, D., GOEURIOT, P. & THEVENOT, F., Hot-pressing synthesis of an Al2Oa-AION composite. Science of Ceramics, 14, Canterbury 1987 (in press). 7. TRABELSI, R., TREHEUX, D., GOEURIOT-LAUNAY, D., GOEURIOT, P., THEVENOT, F., ORANGE, G. & FANTOZZI, G., Friction, wear resistance and mechanical properties of an alumina-~-aluminum oxynitride composite (ALUMINALON), 6th CIMTEC, Communication, Milan, Italy, June 1986. 8. TRABELSI, R., Thesis Ecole Centrale de Lyon, January 1988. 9. LAUNAY, D., ORANGE, G., GOEURIOT, P., THEVENOT, F. & FANTOZZI, G., Reaction sintering of an AI2Oa-AION composite. Determination of mechanical properties. J. Mater. Sci. Letters, 3 (1984) 890-2. 10. GOEURIOT-LAUNAY, D., Thesis, Lyon, June 1987.
Effect of 7-Aluminum Oxynitride Dispersion on Some Alumina Properties* D. Goeuriot-Launay, P. Goeuriot, F. Thevenot Laboratoire des C~ramiques Sp~iales, ENSME, 158, cours Fauriei--42023 Saint-Etienne C~dex 2, France
G. Orange, G. Fantozzi GEMPPM, INSA, Bat. 502-20, Avenue A. Einstein, 69621 Villeurbanne C~dex, France
R. T r a b e l s i & D. T r e h e u x Laboratoire Diffusion dans les solides, Ecole Centrale de Lyon, 36 route de Dardilly, BP 163, 69131 Ecully C6dex, France Centre de Recherches Rh6ne-Alpes des C6ramiques Sp6ciales (Received 14 April 1988; accepted 6 June 1988)
Abstract: A T-aluminum oxynitride (),-AION) dispersion in an alumina matrix can lead to a composite ceramic with equivalent properties to those of a single phase alumina at room temperature. The AI203-TAION is obtained by pressureless sintering or hot-pressing of intimate mixtures of = alumina and aluminum nitride powders. The best treatment temperature is about 1700°C. The composite presents a high wear resistance; its flexural properties at high temperature are higher than those of alumina.
1 INTRODUCTION
2 FABRICATION
OF
THE
AI=O3-TAION
COMPOSITE Alumina is a widely used ceramic material, and many researches tend to reinforce it, with whiskers (SIC1), or dispersed grains, particularly zirconia. 2"3 The present work deals with the effect of),-aluminum oxynitride (TALON) dispersed in an alumina matrix. Our purpose is to describe at first the elaboration of the alumina-~,-aluminum oxynitride composite, and then its properties; wear resistance, mechanical properties at high temperature. * This work is a part of the doctoral thesis of D. GoeuriotLaunay, University of Lyon 1 (France), l I June 1987.
Fine powder of 7-AION is not yet available. So the alumina-~-aluminum oxynitride composite is prepared by sintering and reacting of alumina and aluminum nitride (specific area 6m2g -1) powder mixtures, using two starting alumina powders: - - E X A L alumina, in which the impurity content is less than 200 ppm. - - B A Y E R alumina containing about 0.2 wt % of impurities. Several oxynitrides exist in the AI2Oa-A1N
207 Ceramics International 0272-8842/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
208
D. Goeuriot-Launay et al.
system:4,s the y-aluminum oxynitride, with the composition limits at 1700°C, A I 2 . s I O 3 . s 6 N o . 4 4 t o A I 2 . 9 1 0 3 . 2 7 N o . 7 3 ; a tetragonal phase 6, with a higher alumina content. Similar phenomena occur during both pressureless sintering and hot-pressing, which is described below. 6 Initial mixtures containing 0 to l l m o l % AIN have been milled by attrition in anhydrous alcohol in order to obtain, after reaction hot-pressing, composites with 0 to 30vo1% A1ON. Hot-pressing experiments (1550 < T < 1800°C, 15 < t < 30min, P = 4 0 M P a , under nitrogen atmosphere) were performed using an uniaxial press (DEGUSSA). During pressure sintering, it is possible to observe the sample shrinkage. X-ray diffraction analyses were performed on a face which is perpendicular to the pressing axis. Metallographic observations (SEM) were also performed. X-ray diffraction analyses indicate that: --the impurities in the BAYER alumina promote the AI2Oa,AIN reaction that starts at 1550°C instead of 1600°C in the case of EXAL alumina. The y-spinel phase is the only oxynitride formed up to 1750°C. --above 1750°C for both aluminas other diffraction peaks appear. Some of them correspond to the tetragonal (6) aluminum oxynitride; others are near those of monoclinic (0) alumina. This tends to prove that an oxynitride of this structure can exist in the A1N-A12Oa system. Two ratios were defined to follow the oxynitrides evolution:
25.
R'
=
Is(400) 0' + 6)-AlaN Is(113) tz-Al20 3
x
f .4-......
20'
!5.
¥
.."" i
/
.""
i
/
.10-
i"
I
5 1500
lg00
14oo
I~O0'T ('c)
1750°C, the y-spinel oxynitride transforms into the tetragonal form while the alumina content decreases. X-ray diffraction observations can be related to the dilatometric curves (Fig. 3). Pure alumina shrinkage during pressure sintering slows down at about 1500°C and finishes at 1700°C. In the case of A12Oa-A1ON mixtures a maximum shrinkage occurs at about 1600 and 1650°C for respectively BAYER and EXAL aluminas. In the range of temperature 1650-1900°C (EXAL alumina) an expansion stage takes place that is accelerated above 1800°C. Fusion occurs at about 1900°C. With BAYER alumina, these phenomena are shifted about 50°C downwards. soaking time 30 rain
powder
} AI203 BAYER 30%AION ....... + ........ i axis ---x~-J. axis | D.o~non textured ~ A1'203 EXAL 30%AION
R' 120.
100
which depends on both ?- and 6-AlaN. The R' evolutions are similar if the X-ray patterns are performed on a face perpendicular to the pressing axis, on powder, or with a system that removes textural effects from the measurements (DOSOPHATEX system). Both R and R' increase up to 1700°C (Figs 1 and 2): y-spinel A l a N is formed at 1550°C, and its content increases until the aluminum nitride has completely reacted. For temperatures above 1700°C, R is almost constant while R' increases. This means that, above
'
Fig. 1. R ratio for the different hot-pressing conditions of the AI20 3 7 0 - A l a N 30 vol% composite. Soaking t i m e s : - - O - - , 15min and . . . + ..., 30rain for B A Y E R ; - - Q - - , 15min and - . - x - . - , 30rain for EXAL.
R = Is(220) y-AlaN x 100 Is(113) ~t-Al20 3 which depends on the y-spinel amount,
/j
.,A"P
/ .....
P
100 80 60-
-~..." .°.
20-
1500 Fig. 2.
jig..." ""
'
16'00
'
I"]00
I~k)0 T('C)
R' ratio for the different hot-pressing conditions of the AI20 3 7 0 - A l a N 30 vol% composite.
Effect of a y-aluminum oxynitride dispersion on alumina
209
At203 EXAL . . . At203 EXAL 10%AtON ...... At203 EXAL 30%AI.ON _-- At203 BAYER30%AI.ON
-At
=
.,...
-"-.=.2 . . . . . . . . ii,I
.,
i
".,, ',~;
(a)
k/
T (*C)
1500 1600I~ 180019'00 Fig. 3. Dilatometric curves during hot-pressing of AI2Oa-AIN mixtures: Al =f(T°C).
Finally the following interpretation of the dilatometric curves is proposed: the v-AION formation gives rise to a volume increase. In the range of temperature 1550-1700°C (BAYER) there is a competition between the alumina shrinkage and the V-oxynitride formation; this explains the shrinkage and then the dilatation observed on the dilatometric curves. At about 1750°C, the maximum amount of Vspinel is formed; so the dilatation is almost stopped. At 1780°C the y-phase dissolves alumina and then changes to the tetragonal 6- (and monoclinic 0-) oxynitrides: this phenomenon corresponds to the dilatation acceleration on the dilatometric curve. The viscosity of these phases decreases, a liquid phase appears that spreads out of the samples at 1900°C and which explains the beginning of shrinkage. The interval between the dilatometric curves corresponding to the two types of alumina powders (BAYER and EXAL) is explained by the impurities that promote the reactions that have been described• Metallographic observations (SEM) are performed on polished and thermally etched surfaces (the AION phase appears with a light grey colour on the photos). The alumina grain size is relatively fine (2-3/~m) in spite of the hot-pressing temperature (1700°C, Fig. 4a). The dispersion limits grain growth. Alumina grain coarsening occurs only at 1800°C (Fig. 4b): the viscosity of each oxynitride phase, present at triple points, decreases.
(b) Fig. 4. Microstructures (SEM) of a ~-AI203 80-y-AION 20vo1% hot-pressed under 40MPa. (a) T = 1700°C; (b) T = 1800°C.
3 M E C H A N I C A L PROPERTIES AT R O O M T E M PERATU R E
Table 1 presents composite mechanical properties at room temperature, these depend on both the type of sintering (with or without pressure) and the starting alumina. The presence of large defects due to the processing, especially for the pressureless sintered samples, leads to difficulties in the interpretation. However, some results can be pointed out: --high volume contents of y-AION increase the composite brittleness: 20v01% v-A1ON is the most interesting composition --hot-pressing leads to a control of the defect size. At room temperature the flexural properties of the AI203-AION composite are similar to those of single phase alumina. The beneficial effect of the Valuminum oxynitride will be now shown on other mechanical properties.
D. Goeuriot-Launay e t a ] .
210
Table 1.
Characterization of the AI=O3-AION composites: Mechanical properties at room temperature % AION
Density (gcm =)
Hv 10N (GPa)
Strength (MPa)
Klc (MPa ~/m)
E (GPa)
G1C (Jm -=)
10 20 30
3"92 3"91 3"88
20"9+0"9 20"6 + 0"9 21 "9 + 0'8
515+15 460 __+40 400 + 100
4"3 3"6 3"8
335 315 362
52 37 36
10 20 30
3"94 3-91 3"89
20"7 + 1 '1 21 "8 + 1 '2 21 '8 + 0"8
330 + 40 510 __+80 715 + 65
3"8 3"8 3"9
355 380 385
38 35 36
10 20 30
3"93 3'91 3'89
19'6 + 0"8 20"7 + 0"6 21 "3 + 0"7
335 + 25 500 + 50 430 __+100
4"4 4"2 4"0
----
----
10 20 30
3"94 3"89 3"84
20"5 + 0"8 20"8 + 0"7 20"5 + 1 "0
390 + 25 530 + 45 550 + 60
4"3 3"5 3"9
290 -325
58 -43
EXAL alumina Pressureless Sintering Hot pressing
Bayer alumina Pressureless Sintering
Hot-pressing
4 TRIBOLOGICAL AION COMPOSITE
BEHAVIOUR v'8
OF AI=O=-
The frictional samples (12 x 12 x 5 mm) are lapped then polished (diamond 3 pm). The friction experiments are carried out in water on a tribometer consisting of a rotary cylinder (52100 steel, 64HRC) against a fixed ceramic plane. The sliding speed is about 0.36 m s- 1. A load of 700 N has to be applied to obtain significant wear. The friction coefficient and the wear volume are respectively calculated from the tangential force and the width of the wear trace. Figure 5 represents the friction coefficient evolution for hot-pressed materials. A periodic evolution of the friction coefficient of pure EXAL At2 03 --.-- At20:3 - 10 % ALON . . . . AL203- 20 % ALON w . . At203- 30 % ALON At0N
O8 0.7 06 Q5
alumina is observed. This is due to the accumulation of wear debris in the contact area with involves an increase of the tangential force and its removal leads to the decrease. The Al2Oa-A1ON composite does not present a heavy changing of the friction coefficient. This can be explained by the trapping of wear debris (third body) into the contact. Consequently, as shown in Fig. 6, pure EXAL alumina presents a wear volume more important than those of the A12Oa-A1ON composite and pure A1ON. A minimum wear is observed in the case of Al2Oa-10 vol% AION, mainly after 2 h. The type of sintering (with or without pressure) and the type of alumina (BAYER or EXAL) have only a small effect on the tribological properties of the composites. The best results are obtained for hot-pressed samples with an EXAL alumina matrix. V mm3 05
-.----
AL2 03 At2 O3 - 10% AtON AL203-20% ALON
/
"
- -
t
~
I=
04 Q3
3'0
6"o
9"0
,9
120 Time(rain)
Fig. 5. Evolution of the friction coefficients versus time.
30
60
120 Time(rain)
Fig. 6. Evolution of the wear volumes versus time: AION content influence.
Effect of a ~-aluminum oxynitride dispersion on alumina
211
The presence of 7-A1ON enhances the alumina wear resistance against bearing steel (52100). The weak composite degradation occurs following an intergranular mechanism :crack formation along the grain boundaries, and then pull out of some alumina grains. The presence of the 7-A1ON phase impedes the propagation of intergranular cracks and modifies favorably the properties of the third body present in the contact, and the metal-ceramic transfer formation. These two phenomena implicate the decrease of the composite degradation.
'
BEHAVIOUR AT HIGH 5 FRACTURE T E M PERATUR E OF THE AI=O3-7-AION C O M POSITE 9
The composites whose properties are described in Table 1 have been tested at high temperature, in a nitrogen atmosphere. Figure 7 shows that the presence of ~-A1ON in the composite containing EXAL alumina remarkably delays the alumina strength decrease above 1000°C. The composite has elastic behaviour, even up to 1400°C. The critical stress intensity factor values are maintained at high temperature (Fig. 8) and even increase for high 7AION contents. This strengthening effect on mechanical properties at high temperature occurs for ),AION contents larger than 20 vol%, for both types of sintering. On the other hand the composites fabricated from BAYER alumina have poor mechanical properties (af and K~c) above 1000°C (Figs 7 and 8). of (MPa) 600
I,
K1C
(HPo .~/'m )
•r O
ot
600
Pressureless sintering x AI203 EXAL 10%AION + AI203 EXAL 30*/.AION - - B - - AL203 BAYER 30% AION Hot pressing -,,V-- A1203 BAYER 30 % AION • At203 EXAL 30% AION
800
% 10bO 12~10 1/,b0T(~C)
Fig. 8. Toughness versus temperature of the Ai203-7-AION composites. P.S. = pressureless sintered, H.P. = hot pressed.
This behavior can be explained by the particular microstructure of the composites observed on the high temperature (1400°C) fracture face (Fig. 9). The ),-AION crystals have two forms: small grains at triple point junctions and along alumina grain boundaries. In some boundaries we also observe a continuous intergranular phase. The ~-AION spherical particles, that are dispersed along grain boundaries, prevent a high percentage of alumina grains from sliding. This induces the absence of macroscopic plasticity and the high level of strength at high temperature. The good toughness values between 1000°C and 1400°C can be attributed to the presence of a grain boundary phase which leads to microplasticity, localized along grain boundaries, that permits a plastic relaxation in the plastic zone ahead of the main crack.
400,
200
x T (*C) + at203 x At 2133 • AL203 OA1203 • At203 • AI,2 03 Fig. 7.
0=3 wn • = 35 i~m EXAL 10% ALON(¢=S-15w-n) EXAL 30% ALON(¢=2_3Fm) EXAL 30% ALON (¢=5-10 p.m) BAYER 30% AI.ON(¢=2-3 [,u'n)
Flexural strength versus temperature of the hot pressed AI2Oa-7-AION composites.
l ~ J . ~ Fig. 9.
~ _;J. ¸
mmam~,
Fracture face at 1400°C of a AI203 80-7-AION 20 vol% hot-pressed composite.
212 In the case o f B A Y E R alumina, the mechanical properties are not maintained at high temperature. The impurities (Ca, Na, Mg etc.) lead to fusible phases at grain boundaries. The flexural strength decrease observed is due to the low viscosity of these phases. The alumina grain sliding is more important than the plastic relaxation phenomenon, so the fracture energy decreases; this explains the rapid decrease o f toughness above 1000°C. In this case a macroscopic plasticity is observed.
6 CONCLUSIONS TM Alumina-~,-aluminum oxynitride composites can be sintered with or without pressure from Al2Oa-A1N powder mixtures. These composites have mechanical properties at r o o m temperature similar to those o f alumina. The ~-A1ON dispersion has a beneficial effect on the mechanical properties (~rr, K~c) at high temperature and on wear resistance of an alumina ceramic. Wear applications are n o w being exploited in industry.
REFERENCES 1. CLAUSSEN, N. & PETZOW, G., Whisker-reinforced oxide ceramics. J. Physique, Colloque C1, Supplement 2, 47 (1986) C1-693-702.
D. Goeuriot-Launay et al.
2. CLAUSSEN, N., Fracture toughness of A1203 with an unstabilized ZrO2 dispersed phase. J. Am. Ceram. Soc., 59(1-2) (1976) 49-51. 3. ORANGE, G., FANTOZZI, G., HOMERIN, P., THEVENOT, F., LERICHE, A. & CAMBIER, F., Thermomechanical properties of zirconia toughened materials: effect of microstructure and temperature on toughening mechanisms. Science and Technology of Zirconia lII, Tokyo, 9-11 September 1986. 4. LEJUS, A. M., Th6se sur la formation, ~ haute temp6rature de spindles non stoechiomStriques et de phases d~riv~es dans plusieurs syst6mesd'oxydes ~ base d'alumine et dans le syst6me alumine-nitrure d'aluminium. Rev. Int. Htes Temp. Refract., 1(1) (1964) 53-95. 5. McCAULEY, J. W. & CORBIN, N. D., High temperature reactions and microstructures in the A12Oa-AIN system, 'Nato' Advanced Ceramic Meeting, University of Sussex England (1981). Progress in Nitrogen Ceramics, F. L. Riley, ed. Martinus-Nijhoff, Amsterdam, 1983, pp. 111-18. 6. GOEURIOT-LAUNAY, D., GOEURIOT, P. & THEVENOT, F., Hot-pressing synthesis of an Al2Oa-AION composite. Science of Ceramics, 14, Canterbury 1987 (in press). 7. TRABELSI, R., TREHEUX, D., GOEURIOT-LAUNAY, D., GOEURIOT, P., THEVENOT, F., ORANGE, G. & FANTOZZI, G., Friction, wear resistance and mechanical properties of an alumina-~-aluminum oxynitride composite (ALUMINALON), 6th CIMTEC, Communication, Milan, Italy, June 1986. 8. TRABELSI, R., Thesis Ecole Centrale de Lyon, January 1988. 9. LAUNAY, D., ORANGE, G., GOEURIOT, P., THEVENOT, F. & FANTOZZI, G., Reaction sintering of an AI2Oa-AION composite. Determination of mechanical properties. J. Mater. Sci. Letters, 3 (1984) 890-2. 10. GOEURIOT-LAUNAY, D., Thesis, Lyon, June 1987.