Use of High Energy Ball Milling on the Sintering Optimization of Alumina Ceramics

Seventh International Latin American Conference on Powder Technology, November 08-10, Atibaia, SP, Brazil

Use of high energy ball milling on the sintering optimization of alumina ceramics

Helio R. Simoni1,a, Eduardo Saito1,b, Claudinei dos Santos1,2,c, Felipe Antunes Santos1,d, Alfeu Saraiva Ramos3,e, Olivério Moreira M.Silva4,f

1 USP-EEL - Universidade de São Paulo, Escola de Engenharia de Lorena, Polo Urbo Industrial, Gleba AI6, s/n, Mondesir, Lorena-SP, Brazil, CEP12600-000 2UNIFOA - Centro Universitário de Volta Redonda, Av. Paulo Erlei Alves Abrantes, 1325, Volta Redonda-RJ, Brazil, CEP 27240-560 3 UNESP-FEG - Unersidade Estadual Paulista, Faculdade de Engenharia de Guaratinguetá, Av. Dr. Ariberto Pereirada Cunha, 333, Guaratinguetá-SP, Brazil, CEP 12516-410 4 CTA-IAE/AMR - Centro Técnico Aeroespacial, Praça Marechal do Ar Eduardo Gomes, 50, S.J. Campos-SP, Brazil, CEP 12228-904

[email protected], [email protected], [email protected], [email protected], [email protected]

Keywords: Al2O3, Al2O3-Y2O3, high-energy ball milling, sintering, characterization

Abstract: In this work, the effect of the milling time on the densification of the alumina ceramics with or without 5wt.%Y2O3, is evaluated, using high-energy ball milling. The milling was performed with different times of 0, 2, 5 or 10 hours. All powders, milled at different times, were characterized by X-Ray Diffraction presenting a reduction of the crystalline degree and crystallite size as function of the milling time increasing. The powders were compacted by cold uniaxial pressing and sintered at 1550°C-60min. Green density of the compacts presented an increasing as function of the milling time and sintered samples presented evolution on the densification as function of the reduction of the crystallite size of the milled powders.

Introduction

Sintering of Al2O3-based ceramics is a critical process. To obtain materials with low porosity and high density, the use of high sintering temperatures and the choice of the starting-powders processing route are necessaries which can be made in different ways [1-2]. Alumina ceramics, Al2O3, are usually obtained by solid-state sintering at temperatures near 1600 to 1650°C [1], and in this case the undesirable grain growth may cause a reduction of the mechanical properties. An effective way to achieve a fine and uniform dispersion of nano-particles in ceramic systems is to obtain a supersaturated solid solution at room temperature by high-energy ball milling or mechanical alloying (MA) followed by precipitation at high temperatures [3]. However, the sintering temperature may be reduced due to the increased bulk and

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surface energies introduced during the milling process. Mechanical alloying (MA) is a method based on repeated cold-welding and fracture mechanisms of powder particles during high-energy ball milling [4-5], and has been extensively employed to obtain extended solid solutions, metastable phases and amorphous structures. Recent studies on MA are reported for ceramic systems including the formation of cubic- and tetragonal- stabilized zirconia from the mixture of a stabilizer oxide and pure zirconia powder [6-7]. In all cases, the final materials appeared to be suitable for structural applications and the sintered products showed increased mechanical properties compared to the conventionally prepared materials. In this work, the use of the high-energy ball milling, by mechanical alloying, is proposed for to improve the Al2O3 densification. Furthermore, comparatively, Al2O3-5%Y2O3 powder mixture is studied. The effect of the milling time and Y2O3 addition on the crystallite size of the Al2O3 -based ceramic is investigated.

Experimental Procedure Powder preparation As starting powders, Al2O3 (SG-1000, Almatis, USA) and Y2O3 (H.C.Starck, Germany) were used. Monolithic powder and powder mixture composed of 95wt.%Al2O3 and 5wt.%-Y2O3 was prepared by attrition milling in ethanol for 4h, 1000 rpm and ZrO2 balls (2mm diameter) as milling medium. After milling, the powders were dried at 90oC for 24 hours and deagglomerated. Processing The powder mixture was processed in a planetary ball mill (Fritsch P-5) under argon atmosphere, using a rotary speed of 200rpm, a ball-to-powder weight ratio of 10:1, and Si3N4 ceramic vial (225 mL) and balls (10 mm diameter). Samples were collected after different milling times: 0, 1, 2, 5 or 10h. Cylindrical specimen of 5mm height and 12mm diameter were obtained by cold uniaxially pressing under a pressure of 100MPa. These specimens were characterized by their green relative density and sintered at 1550°C for 60min, with heating and cooling rates of 10°C/min.

Characterization In order to study the phase changes of the powder mixture during MA, the crystalline phases were analyzed by X-ray diffraction (XRD) using Ni-filtered Cu-kα radiation, with a 2θ range of 20 to 80°, a step width of 0.05° and an exposure time of 2s. The full width at half maximum (FWHM) values were calculated by the integral method and the crystallite sizes were estimated by the Debye-Scherrer equation, Eq (1) and based on 5 measurements of the diffraction peak broadening [8-10]. Here, the corrected FWHM values without instrumental broadening are adopted. The standard peaks were obtained by using a silicon standard to correct instrumental broadening, which includes additional broadening because of slit widths, the sample size, penetration into the sample, and imperfect focusing [8-10].

D = (0,9.λ) / (β.cosθ) (1) where D –crystallite size, λ – wave length, β- full width at half maximum (FWHM) (rad) and θ peak position (in rad).

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The relative density of the green compacts was determined, associating the density of compacted specimen with the theoretical density of the Al2O3 or Al2O3-5%Y2O3 mixture, using the rule of mixtures. Crystalline phases formed in the sintered samples were determined by XRD analysis, as described previously. The microstructure of the sintered samples was examined in a LEO1450VP SEM of thermally etched (1400°C-15min) samples. The bulk density was measured using the Archimedes method in distilled water, and the relative density was determined correlating the bulk density with the theoretical density of the mixture.

Results and Discussion

Powder Characterization Figure 1 present XRD patterns of the Al2O3 powders in different milling times and with presence of the Y2O3 as dopant.

PowderCell 2.2

4223 PowderCell 2.2 AL2O3 4015 Alum ina pura.x_y AL2O3 Al2O 3 10h_b.x_y 113 116

2112 2008 110 300 024 012 012 300 116 104 214 113 104 214 110 024 10.10 10.10 122 122 211 211 125 202 202 006 208 125 006 0 208 0 20 25 30 35 40 45 50 55 60 65 70 75 80 20 25 30 35 40 45 50 55 60 65 70 75 80 (A) (B) 12000 12000 1 - Al O 2 3 1 1 - Al O 2 - Y O λ = 1,1276 A 2 3 λ = 1,1276 A 2 3 1 1 E = 10,9952 eV E = 10,9952 eV 9000 9000 Al2O3 - 5 Y2O3 - 10h Al2O3 - 5 Y2O3 - 1h

1 6000 1 6000 1 1 1 1 2 1 2 Intensity (counts) Intensity 3000 3000 1

Intensidade (contagens) 1 1 1 2 1 1 1 1 1 1 1 1 1 1 2 0 0 10 20 30 40 50 60 10 20 30 40 50 60

o o 2θ ( ) 2θ ( ) (C) (D) Figure 1- XRD patterns of Al2O3 and Al2O3-Y2O3 powder mixtures milled at different times: 0h and 10h.

It can be observed that monolithic alumina powders, Figure 1A and 1B, present a little reduction of the crystalline degree as function of the milling time, after 10h. Crystalline α-Al2O3 is observed in both, 0h and 10h-milling time. This phase is present in Y2O3-doped alumina ceramics (Figures 1C and 1D). On the other hand, Y2O3-phase presented sensibility to the amorphization after 10h milling time, Figure 1D. Figure 2 present results of the crystallite size of the alumina powders as function of the milling time and Y2O3 doping.

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28 Al2O3 Al O -Y O 26 2 3 2 3

Crystallite Size (nm) Size Crystallite 20

14 012345678910 Milling Time (h) Figure 2- Effect of the milling time on the crystallite size of the alumina powders.

It can be observed a reduction of the crystallite size as function of the milling time increasing, in both, Al2O3 and Al2O3-Y2O3 powders. The presence of the Y2O3 powder reduces the effect of the amorphization and fragmentation of the Al2O3-matrix probably because of this phase present high amorphization degree and consequently absorbed a considerable content of the energy generated during milling process. Different crystallite sizes promote effect on the compaction, as shown in Figure 3. A direct effect of the reduction on the particle size leads to the little increasing of the relative density of the green bodies.

0.580

Al2O3

0.575 Al2O3-Y2O3

0.570

0.565

0.560

0.555 Green Relative DensityT.D.) (% 0.550

012345678910 Milling Time (h) Figure 3 - Green density as function of milling time of the Al2O3 powder.

Sintered Samples Characterization Figure 4 present XRD patterns of the sintered samples, started to the powders submitted at different milling times.

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Pow derCell 2.2 3882 25000 AL2O3 alu10hsint.x_y 116 104 λ = 1,1276 A A A Al2O3 - 5 Y2O3 - 10 h + E = 10,9952 eV 1500oC / 60 min / ar 20000 A 012 A - Al2O3 Y Y - Y3Al5O12

15000 A

A 1941 300 110 Y A 10000

024 A A Y

Y 214

5000 Y Y Y Y A Y Y Y Y

10.10 Y A A A A A Y Y A A 119 A Y Y Y Y A Y A A Y Y A Y Y YA Y Y 220 A 122 223 202 211 0 125 006 217 036 208 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 10 20 30 40 50 60 (A) (B) 0 Figure 4 – XRD patterns of the samples: (A) Al2O3, (B) Al2O3-Y2O3, sintered at 1550 C- 60min.

Figure 4A present only α-Al2O3 as crystalline phase. On the other hand, Figure 4B indicates the presence of the both α-Al2O3 and Y3Al5O12 (Yttrium Aluminum Garnet) as crystalline phases, indicating that some alumina content was absorbed in the formation of this new phase by combination with amorphous Y2O3-starting powder. The formation of this new phase, Y3Al5O12, improved the sinterability of the alumina ceramic, as show in Figure 5.

100.0 Al2O3 99.5 Al2O3-Y3Al5O12

99.0

98.5

98.0

97.5

Relative Density (%.T.D.) 97.0

96.5

0246810 Milling Time (h) Figure 5 – Effect of milling time and Y2O3 addition on the relative density of the sintered samples

Previous works [1-2,11] relates the difficult to obtain full densification in Al2O3-based ceramics. This information is observation in Figure 5, which indicates that, the reduction of the starting powder with consequent increasing of the green relative density, lead to increasing to the final relative density. On the other hand, densification is not higher to 98.5% of theoretical density (3.98g/cm3), which can be lead to the reduction of the mechanical properties [1,11,12] reducing the reliability of these ceramics in structural applications. The presence of Y3Al5O12 as crystalline phase formed after solid-state reaction between Al2O3 and Y2O3, improves sinterability of the alumina powders reducing considerably the porosity on the sintered bulks (relative density superior to 99% of T.D.), in samples milled for 5h and 10h. Representative SEM micrographs of alumina ceramics sintered at 15500C, are show in Figure 6. It can observed a reduction of the

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porosity between Al2O3 grains in sintered samples, obtained with different particle-size powders, as received and milled for 10h.

0h 10h Figure 6 – SEM micrographs of the samples sintered at 15500C-60minutes, starting of the powders milled at different times.

Conclusions The high-energy milling improves the sinterability of the alumina ceramics by the reduction of the particles size promoting increasing the specific surface area of the powders, driving force of the sintering process. On the other hand, low particle sizes are not sufficient to the full densification of the Al2O3 ceramic. This behavior only is obtained by combination of the particle size reduction and use of the Y2O3powder, which forms a limited content of Y3Al5O12 crystalline phase during sintering, contributing with densification of these ceramics.

Acknowledgment The authors thank to FAPESP, CNPq and CAPES for financial support.

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