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LOW TEMPERATURE PHASE TRANSFORMATION STUDY IN THE Al2O3-Y2O3-Nb2O5 SYSTEM
E. S. Lima 1, a; A. P. O. Santos 2, b; R. F. Cabral 2, c; J. I. N. Fortini 3, d; J. B. Campos 4, e
1 Instituto Militar de Engenharia – IME – Praça general Tibúrcio, 80 – Urca, Rio de Janeiro – RJ - CEP 22290-270 – Brasil
2 Centro Universitário de Volta Redonda – UniFOA –Av. Paulo Erlei Alves Abrantes, 1325 – Três Poços, Volta Redonda – RJ – CEP 27240-560 – Brasil
3 Arsenal de Guerra General Câmara – AGGC – Rua General Daniel H. Balbão, s/n – General Câmara – RS - CEP 95820-000 – Brasil
4 Departamento de Engenharia Mecânica – UERJ/ - Rua São Francisco Xavier, 524 - Maracanã –Rio de Janeiro – RJ – CEP 20550-013 – Brasil
[email protected]; [email protected]; [email protected]; [email protected]; [email protected]
Keywords: Composite Al2O3-YAG, Nb2O5, sintering
Abstract. The mechanical properties and creep resistance of the Al2O3 have been improved with
the use of other oxide ceramics, among these the Y3Al5O12 (YAG - "yttrium aluminum garnet"), to
obtain the Al2O3-YAG composite. The aim of this work is to study the effect of addition of Nb2O5
at low sintering temperatures of the composite, starting from the eutectic composition Al2O3-Y2O3.
In this work, the compositions were produced by powder mixtures of Al2O3-Y2O3 and Al2O3-Y2O3-
Nb2O5 using high energy ball milling. The green bodies were pressed at 70 MPa and sintered at
1000, 1100, 1200 and 1300°C for 2 h. The materials were characterized by shrinkage and X-Ray
Diffraction (XRD) using the Rietveld method. The presence of Nb2O5 does not show any improve
neither for the YAG phase formation nor for the shrinkage of the sintered samples in the
temperature range studied.
Introduction
The Y3Al5O12 (YAG - "yttrium aluminum garnet"), is one of the higher creep resistance oxides.
In addition, this material is chemically stable in contact with Al2O3, whose thermal expansion
coefficient is close and with which forms a eutectic [1,2,3]. Thus, the YAG has characteristics that
allow its use in conjunction with Al2O3, resulting in the composite Al2O3-YAG. Several researches
have indicated superior mechanical properties of this material at temperatures above 1500°C
[2,3,4,5].
The phase diagram of Al2O3-Y2O3 [3] includes the phases YAG, YAP (YAlO3 - "Yttrium
Aluminum Perovskite"), and YAM (Y4Al2O9 - "Yttrium Aluminum Monoclinic"). The eutectic
Al2O3-YAG stable composite forms at 1826°C and 18.5 mols% of Y2O3.
The aim of this work is to perform a preliminary study of the Nb2O5 addition effect in the
production of Al2O3-YAG at low temperatures, starting from the Al2O3-Y2O3 eutectic composition
[6]. As usual additive of Al2O3 [6,7,8], this report will verify its behavior in the system Al2O3-Y2O3.
Materials and Methods
The starting powders were composed of Al2O3 (Type A GS-1000, Alcoa Aluminum SA, with
purity of 99.8%), Y2O3 (Type REO, Alfa-Aesar, purity of 99.9%) and Nb2O5 (Type HP 311,
CBMM - Companhia Brasileira de Metalurgia e Mineração, with purity of 98.5%).
Two mixtures were produced, both according to the eutectic composite Al2O3-YAG
stoichiometry [3], but one of them with Nb2O5, as shown in Fig. 1.
Materials Science Forum Vols. 798-799 (2014) pp 90-94Online available since 2014/Jun/30 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.798-799.90
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.194.20.173, Monash University Library, Clayton, Australia-05/12/14,17:12:37)
Fig. 1 Weight ratio mixtures of Al2O3-Y2O3 and Al2O3-Y2O3-Nb2O5
The mixtures were milled for 180 min. in the planetary ball mill Retsch, model PM-400, in order
to reduce the particle size, to increase the surface area and homogeneity of the mixture and to
improve the chemical affinity between the components. Subsequently, the powders were dried in an
oven Quimis, Q314 model M at 120°C for 48 h. Then the mixtures were deagglomerated in an
Al2O3 mortar and pestle and sieved. The specimens were uniaxially compacted in a cylindrical
matrix at a pressure of 70 MPa for 1 min. The samples were sintered air for 2 h with a heating and
cooling rate of 10°C/min at 1000, 1100, 1200 and 1300°C in an oven NETZSCH model 417/1.
The XRD analyzes were performed on a diffractometer PANalytical X'Pert Pro model, using
CuK radiation with a tube voltage of 40 kV and 40 mA and scanning 2 between 20 and 80°. The
collection time was 5s with step of 0.05°. The XRD were refined by the Rietveld method, in order
to quantify the observed phases [9,10], with a TOPAS software academic version.
In order to obtain the linear shrinkage, the green bodies and the sintered samples diameter
dimensions were measured with a precision caliper of 2.0 x 10-2
mm.
Results and Discussion
The Fig. 2 (a) shows the quantification phases coming from the Rietveld method of the Al2O3-
Y2O3 mixture sintered at 1000, 1100, 1200 and 1300°C. In this figure is observed the YAM, YAP
and YAG phase formation.
At 1000°C there was the formation of 30.77 wt% YAM, with the consumption of Al2O3 and
Y2O3. Both phases were reduced from 63.65 and 36.35 wt%, respectively, as shown in Fig. 1, to
61.60 and 7.63 wt%.
At 1100°C, YAM decreased to 5.32 wt% and there was the formation of 40.60 wt% of YAP.
Al2O3 and Y2O3 phases showed a continuous reduction, to 51.60 and 2.48 wt%, respectively.
At 1200°C was observed the reduction of YAM and YAP to 1.05 and 26.09 wt%, respectively,
and the formation of 29.03 wt% of YAG. Al2O3 and Y2O3 phases continued to show a reduction, to
43.50 and 0.33 wt%, respectively.
Materials Science Forum Vols. 798-799 91
Fig. 2: Phase quantification by Rietveld method of the mixtures a) Al2O3-Y2O3 and b) Al2O3-
Y2O3-Nb2O5
These results are in agreement with previous studies of Won et al [11], Neiman et al [12] and
Wen et al [13], wherein the heating of the mixture showed the formation of intermediate phases
YAM at 1000°C and YAP at 1100°C. The formation of YAG for the Al2O3-Y2O3 system is around
1300°C [11,14,15,16]. Its presence at 1200°C indicates that the processing conditions were
extremely favorable. The researches of Cabral et al [7,8,15] and Lima et al [14] also showed the
presence of YAG in similar temperatures.
However, in the sample treated at 1300°C, there was an increase formation of YAM and YAP,
with 11.95 and 31.84 wt% respectively, bucking the trend of its reduction. There was also a
reduction of the YAG formation, to 0.03 wt%, instead of its increase [13,16]. Probably, there was a
problem in heat treatment conditions of this sample as a malfunctioning furnace or a power outage.
The Fig. 2 (b) shows the evolution of phase quantitation with sintering temperature using the
Rietveld method calculations of the mixture Y2O3-Al2O3-Nb2O5 heat treated at 1000, 1100, 1200
and 1300°C. There was, besides the formation of YAM, YAP and YAG phases, as previously
observed in Fig. 2 (a), the YNbO4 phase formation.
In the composition treated at 1000°C, there was the formation of 13.42 wt% of YAM and 17.02
wt% of YNbO4. This phase formed preferably to the AlNbO4 [12,13], which forms when mixtures
of Al2O3 and Nb2O5 are heated up to 1200°C [7]. This was probably due yttrium (Y) being most
avid for aluminum (Al) than for niobium (Nb) [7,8,15]. The amount of Nb2O5 decreased to 0.76%
and was not plotted. There was the consumption of Y2O3, which were reduced from 34.90 wt%, as
shown in Fig. 1, to 7.63 wt%.
At 1100°C, YAM and YNbO4 decreased to 2.47 and 12.46 wt%, respectively. On the other hand,
there was the formation of YAP, with 27.41 wt%. The amount of Nb2O5 decreased to 0.72 wt% and
was not indicated on the graph. Al2O3 and Y2O3 phases showed a continuous reduction, to 54.02
and 2.92 wt%, respectively.
At 1200°C it was observed the YAM reduction and YAP increasing to 1.99 and 33.84 wt%,
respectively, with the formation of 10.79 wt% of YAG. These phases existences are once again in
accordance with previous researches [11,12,13]. It seems that the niobate formation contributed for
a minor presence YAG in relation of the former mixture in this same temperature. Al2O3 and Y2O3
phases were markedly reduced, to 46.22 and 2.07 wt%, respectively. From this temperature there
was no longer observed the presence of Nb2O5.
The YAG emerged as a major phase at a temperature of 1200°C for both mixtures, nearly 100°C
lower than would be expected. This result showed that the powders used have extremely favorable
characteristics, such as high homogeneity and excellent sinterability for forming the composite.
0 1000 1100 1200 1300
0
10
20
30
40
50
60
70
P
ha
ses
(wt%
)
Temperature (0C)
Al2O
3
Y2O
3
YNbO4
YAM
YAP
YAG
0 1000 1100 1200 1300
0
10
20
30
40
50
60
70
Ph
ase
s (w
t%)
Temperature (0C)
Al2O
3
Y2O
3
YAM
YAP
YAG
(a) (b)
92 Brazilian Ceramic Conference 57
Once again, the formation of YAG at 1200°C indicates that the processing conditions were
extremely favorable [11,14,15,16].
However, in the sample treated at 1300°C, there was a majority formation of YAM and YAP,
with 11.95 and 22.46 wt%, respectively, and a YAG reduction to 0.03 wt%, instead of its increase
[13,16]. This result was the same as for the Al2O3-Y2O3 mixture and is not consistent at this
temperature [11,12,13]. Once again, this fact indicates some experimental problem.
The graph in Fig. 3 shows the linear shrinkage of the diameter of the sintered Al2O3-Y2O3 and
Al2O3-Y2O3-Nb2O5 samples. There was a small upward trend of linear shrinkage with increasing
temperature up to 1200ºC in the two compositions, mainly on that without additive.
The shrinkage values, although low, is consistent for temperatures studied, since such mixtures
full sintering occurs only at temperatures higher than 1600°C [11,12,13].
Fig. 3: Shrinkage of the samples Al2O3-Y2O3 and Al2O3-Y2O3-Nb2O5
Considering both mixtures, the maximum shrinkage was observed below 2.5%. According
German [17] models for the initial stage of sintering at low temperatures do not allow a complete
sintering, but only the beginning, even where there is no grain formation, only the formation of
particles. It is likely that sintering temperatures used are located at a range of temperatures
corresponding to this stage, for this material [16,17].
At 1300°C there is a reduction of the shrinkage, which can be explained by experimental problems
reported by analysis of Fig. 2 (a) and (b) in that condition.
Conclusions
This research showed the phase transformation at low temperatures of the system Y2O3-Al2O3-
Nb2O5, as well of the Al2O3-Y2O3 system, by the Rietveld method. The phases formed in the system
Al2O3-Y2O3 were those provided by the literature, YAM, YAP and YAG.
The Nb2O5 was added to the mixture in order to carry out a preliminary study of its effect on the
production of the Al2O3-YAG biphasic composite at low temperatures. It was verified the presence
of phase yttrium niobate (YNbO4) in the lowest studied temperature, 1000°C. This occurred at the
expense of aluminum niobate (AlNbO4), present when there is only Al2O3 and Nb2O5. It seems that
the presence of Nb2O5 contributed for a minor formation of YAG in comparison with the Al2O3-
Y2O3 mixture.
Experimental problems did not allow performing phase transformation study in the temperature
of 1300°C, although it was possible to extract clear information from the other.
1000 1100 1200 1300
0,5
1,0
1,5
2,0
2,5
3,0
Al2O
3-Y
2O
3
Al2O
3-Y
2O
3-Nb
2O
5
Sh
rin
kag
e (
%)
Temperature (oC)
Materials Science Forum Vols. 798-799 93
The linear shrinkage was consistent with the temperatures used, since the sintering of Al2O3-
Y2O3 composite occurs only at temperatures above 1600°C. The Al2O3-Y2O3 mixture presented a
small higher shrinkage.
Further studies are in progress and will indicate the evolution of the system Al2O3-Y2O3-Nb2O5
at higher temperatures, as well as if the produced powders, besides the addition of Nb2O5, would
provide favorable conditions for the sintering of Al2O3-YAG biphasic composite. At this stage, is
clear that the presence of Nb2O5 does not show any improve neither for the YAG phase formation
nor for the shrinkage of the sintered samples in the temperature range studied.
Acknowledgement:
Dr. Campos would like to thanks CBPF (Centro Brasileiro de Pesquisas Físicas) for XRD
facilities.
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