Directed Laser Processing of Compacted Powder Mixtures ... - Core

Science of Sintering, 45 (2013) 247-259 ________________________________________________________________________

doi: 10.2298/SOS1303247V UDK 621.375.826;621.315.612

Directed Laser Processing of Compacted Powder Mixtures Al2O3–TiO2-Y2O3 M. Vlasova1*), M. Kakazey1, P. A. Márquez Aguilar1, E.A. JuarezArellano2, R. Guardian Tapia1, V. Stetsenko3, A. Bykov3, S. Lakiza3, A. Ragulya3 1

Center of Investigation in Engineering and Applied Sciences of the Autonomous University of the State of Morelos (CIICAp-UAEM), Av. Universidad, 1001, Cuernavaca, Mexico 2 Instituto de Química Aplicada, Universidad del Papaloapan, Circuito Central 200, Parque Industrial, 68301 Tuxtepec, Oaxaca, Mexico 3 Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3, Krzhyzhanovsky St.,Kiev, 252680, Ukraine

Abstract: The phase formation, microstructure and surface texture of laser treated ternary powder mixtures of Al2O3–TiO2–Y2O3 had been studied. Rapid high temperature heating and subsequent rapid cooling due to the directed movement of the laser beam forms concave ceramic tracks. Phase composition and microstructure of the tracks depends on the Al2O3 content and the TiO2/Y2O3 ratio of the initial mixtures. The main phases observed are Y3Al5O12, Y2Ti2O7, Al2O3 and Al2TiO5. Due to the temperature gradient in the heating zone, complex layered structures are formed.The tracks consist of three main layers: a thin surface layer, a layer of crystallization products of eutectic alloys, and a lower sintered layer. The thickness of the crystallization layer and the shrinkage of the irradiation zone depend on the amount of Y3Al5O12 and Al2O3 crystallized from the melt. Keywords: Al2O3, TiO2, Y2O3 powders, Laser synthesis, Ceramics

1. Introduction The synthesis of composite ceramic materials based on binary powder mixtures of Al2O3-Y2O3, Al2O3-TiO2 and TiO2-Y2O3 have been of interest for many years due to their use as structural or functional materials [1-13]. This means that the physical properties required from the material depends of the application. Thus, physical and mechanical properties such as strength, hardness, fracture toughness, wear resistance, heat resistance and corrosion resistance are very important for structural ceramics. While for functional ceramics are important physical properties such as electrical, magnetic, thermal, optical, piezoelectric together with some of the properties mentioned above. Based on these ceramics requirements, a continuous development has been necessary [14-26]. Significant progress has been made to _____________________________

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Corresponding author: [email protected]

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___________________________________________________________________________ improve the properties of ceramics through the transition to the nanotechnology, which implies obtaining simple nano-powders and complex oxides by different methods [27-36]. For example, obtaining nano-powders eased the technology of synthesis of optically transparent ceramics, which are used as optical windows with a wide range of transparency, as optical and luminescent elements, as magneto elements, or as active elements of solid-state lasers [33, 34, 36-39]. Until recently, the main interest was focused on the development of technologies for the production of powders and ceramics based on binary eutectics as: Y3Al5O12, Al2O3/ Y3Al5O12, Al2O3/ Er3Al5O12, Al2O3/GdAlO3 etc. [40 - 43]. However, no less interest represents the synthesis of ceramics based on ternary eutectics: Al2O3/Y3Al5O12/ZrO2, Al2O3/Er3Al5O12/ZrO2 or Al2O3/GdAlO3/ZrO2 [44-47]. The aim of this work is to investigate the phase formation, the microstructure, and the surface texture on the laser synthesis zone of ternary powder mixtures of Al2O3–TiO2–Y2O3. This ternary system is very interesting, however its phase diagram has not been studied; especially, in conditions of rapid high temperature heating and subsequent rapid cooling due to the directed movement of the laser beam.This study provides insights into the features phase formation at metasTab. conditions and identifies differences in the self-organization microstructures of ternary powder mixtures.

2. Experimental Powder mixtures of x mol.% Al2O3 – y mol.% TiO2 – z mol.% Y2O3 were prepared using analytically pure Al2O3, TiO2, and Y2O3 powders (produced by REASOL). The particles size was ~1 μm. The compositions of the specimens prepared are presented in Tab. I. The compositions of the ternary mixtures were calculated so that, for increasing the Al2O3 content in the mixture, the molar ratio R = TiO2/Y2O3 remained constant. The selected values of R were: 0.25, 1 and 2.34. Tab. I. Composition of powder mixtures laser treated in this study. R = TiO2/Y2O3 Al2O3, 0.25 1 2.34 mol.% Y2O3, TiO2, Y2O3, TiO2, Y2O3, mol.% mol.% mol.% mol.% mol.% 50 55 60 65 70 75 80

40 36 32 28 24 20 16

10 9 8 7 6 5 4

25 22.5 20 17.5 15 12.5 10

25 22.5 20 17.5 15 12.5 10

15 13.5 12 10.5 9 7.5 6

TiO2, mol.% 35 31.5 28 24.5 21 17.5 14

After homogenizing the powder mixtures in a ball mill for 4 hours, the mixtures were compacted in pellets of 18 mm of diameter and 2 mm in thickness using a pressure of 300 MPa. Laser treatment was performed in an LTN-103 unit (continuous-action laser with λ = 1064 nm). The power of radiation used was 120 W, the diameter of the beam spot was 1.5 mm, and the linear traversing speed of the beam was 0.15 mm/s. After irradiation, concave channels (glassy tracks) were formed on the surfaces of the pellets. These glassy tracks were easily removed from the compacted specimens and were investigated [48]. Shrinkage of the pellets (Δh) in the central part of the channels was defined by the formula: ),

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___________________________________________________________________________ where l0 is the initial thickness of the pellets and l1 is the thickness of the samples after irradiation. The synthesis products were investigated by X-ray diffraction (XRD) and electron microscopy. XRD was performed using Cu Kα radiation in a DRON-3M diffractometer (Russia). Electron microscopy and X-ray microanalysis (EDS) were performed using a scanning electron microscope Superprobe 733 (JEOL, Japan) and a LEO VP 1450 (England). The microanalysis was carried out in the central part of the channels.

3. Results and discussion 3.1. X-ray diffraction data According to the XRD data, the laser treatment of the compacted mixtures promoted the formation of several composite ceramics. The main phases observed are: Y3Al5O12, Y2Ti2O7, α-Al2O3 and Al2TiO5 (Fig. 1, Tab. II).

Fig. 1. X-ray diffraction patterns of tracks formed after laser treatment. (a) powder mixtures with R = 0.25; (b) R = 1; (c) R = 2.34; (d) for Y3Al5O12 (card N 82-0575); (e) for Al2O3 (card N 83-2080); (f) for Y2Ti2O7 (card N 89-2065); and (g) for β-Al2TiO5 (card N 70-1435) [52]. Y3Al5O12 (o), Al2O3(▲),Y2Ti2O7 (●), β-Al2TiO5 (x).

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___________________________________________________________________________ Tab. II. Phase compositions obtained by X-ray diffraction of the tracks generated after laser treat compacted mixtures of Al2O3-Y2O3-TiO2. Al2O3, R = TiO2/Y2O3 mol.% 0.25 1 2.34 50 Y3Al5O12, Y2Ti2O7, Y2Ti2O7, Y3Al5O12, Al2O3, little Y2Ti2O7 traces Y2TiO5 Al2O3 little Al2TiO5, traces Al2Ti7O15 55 Y3Al5O12, Y2Ti2O7, Y2Ti2O7, Y3Al5O12, Al2O3, little Y2Ti2O7 Al2O3 little Al2TiO5 traces Al2Ti7O15 60 Y3Al5O12, Y3Al5O12, Y2Ti2O7, Al2O3, little Y2Ti2O7 Al2O3, Y2Ti2O7 little Al2TiO5, traces Y3Al5O12, traces Al2Ti7O15 65 Y3Al5O12, Y3Al5O12, Y2Ti2O7, Al2O3, little Al2O3, Al2O3, little Y2Ti2O7 Y2Ti2O7 little Al2TiO5, little Y3Al5O12 70 Y3Al5O12, Y3Al5O12, Al2O3, Y2Ti2O7, Al2O3, Al2O3, little Y2Ti2O7 little Al2TiO5 Y2Ti2O7 75 Y3Al5O12, O , Al2 3 Al2O3, Y3Al5O12, Y2Ti2O7, Al2O3, little Y2Ti2O7 little Al2TiO5 Y2Ti2O7 80 Y3Al5O12, Al2O3, Al2O3, Y3Al5O12, Y2Ti2O7, Al2O3, little Y2Ti2O7 little Al2TiO5 Y2Ti2O7 Note: The phases are listed in decreasing order of their content. Analyzing the change in the phase composition of the tracks (Tab. II), it can be seen that Y3Al5O12 is the most sTab. phase when the Y2O3 content is higher than the TiO2 content in the mixtures (R = 0.25), independently of the Al2O3 content. Always a small amount of Y2Ti2O7 is present at those conditions. However, if this ratio changes (R = 1 or 2.34) the phase stability also changes. At R = 1, the Y2Ti2O7, Y3Al5O12 and α-Al2O3 phases coexist although their content changes as the α-Al2O3 content increases. At R = 2.34, several phases are formed although the most sTab. is Y2Ti2O7. As expected, increasing the Al2O3 content in the mixtures increases the corundum content in the products. Corundum content gradually increases with α-Al2O3 ≥ 70 mol.% and it is the most sTab. phase at R = 1 or 2.34 when αAl2O3 ≥ 75 mol.%.

3.2. SEM data The macro-texture observed on the tracks surface is in form of arches (ridges) (Figs. 2 a, c, d). Their formation can be explained by the generation of a bath of melt and by its movement under the action of the laser beam (Fig. 2 a) [49, 50]. The concave form of the tracks (Fig. 2 d) is caused by the uneven heating of the sample´s surface due to defocus of the laser beam (Fig. 2 b). The surface of the cooled melt is severely deformed; it takes an ellipsoidal shape. Traces of these ellipsoids as well as the direction of the laser beam


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___________________________________________________________________________ movement can be observed on the walls of the tracks (Fig. 2 a, c). In addition, radial structures can be seen along the arches formed on the surface of the tracks (Fig. 2 c). The cross-sectional tracks (Fig. 2 d) showed that the tracks are essentially composed of several different layers, which are produced in various processes such as melting-crystallization (upper layer) or sintering (bottom layer).

Fig. 2. SEM micrographs of a track surface (a, c), its cross-section (d), and a schema of the surface heated by a defocused laser beam (b). The texture of the surface of tracks essentially depends on composition of initial mixtures. For example, at keeping the molar ratio R = 0.25 , but at a changing the Al2O3 content, the texture of tracks surface changes from a set of lamellae structures domains to an almost dendritic structure (Fig. 3). That change can be observed for tracks between 65% and 75% mol. of Al2O3, which correspond to Fig. 3 d, d´ and Fig.3 e, e´, respectively. According to Tab. II, above 70% mol. of Al2O3 the second dominant phase in the tracks is α-Al2O3. Therefore, α-Al2O3 can be the responsible for stabilizing the dendrite texture. Semi-quantitative EDS results are summarized in Tab. III, while the regions where the spectra were taken can be seen in Fig. 4. These data show the presence of mainly yttrium, aluminum and oxygen on the surface of the tracks. Only in a few regions titanium was observed (Fig. 4). It is also observed in secondary electrons that at 70% mol. of Al2O3 (Fig. 4 d) the microstructure changes from lamellae to semi-dendritic. Typically, dark regions are concentrated with aluminum, while light regions are concentrated with aluminum and yttrium (Fig. 4). Titanium was observed only in few light regions. Therefore this can indicate that in tracks with R = 0.25 the Y2Ti2O7 phase is not presented on the surface of the tracks. Based on data from EDS, the estimated phase composition obtained from the local places analyzed can be seen in Tab. III. Even with such rough composition estimation, it can

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___________________________________________________________________________ be concluded that increasing the Al2O3 content in the initial mixturesthe Y3Al5O12 content gradual decrease on the surface of the tracks. This is consistent with the XRD data.

Fig. 3. SEM micrographs of the tracks urface formed from mixtures with R = 0.25 and different Al2O3 (mol.%) content: 50 (a, a´); 55 (b, b´); 60 (c, c´); 65 (d, d´); 75(e, e´); 80 (f, f´). As discussed above, the studies of the cross-sectional tracks (Fig. 2d) showed that the tracks are composed of several layers, which are formed due to the peculiarities of laser heating the material. The process of heat propagation in a local zone is described by the follow equation [52]: where ρ is the density of the solid body, С is the heat capacity of the material, χ is the thermal conductivity coefficient, Q is the density of heat sources in a solid body. The local action on


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___________________________________________________________________________ the solid body is characterized by the appearance of a temperature gradient and local nonequilibrium processes.

Fig. 4. Secondary electrons SEM micrographs of subsurface layers of the tracks formed from mixtures with R = 0.25 and different Al2O3 (mol.%) content: 50 (a, a´); 60 (b, b´); 65 (c, c´); 70 (d, d´); 75 (e, e´); 80 (f, f´). The places were EDS were taken are marked with numbers on the microphotographs. A rough estimation of the thickness of the melting-crystallization zone (h) and the shrinkage (Δh) of the tracks is shown in Fig. 5. The largest melting-crystallization zone size of all the pellets studied was observed at R = 0.25 and 60 mol.% of Al2O3 (Fig. 5 a), while the maximum shrinkage (Δh) was also observed at R = 0.25 and 65 mol.% of Al2O3 (Fig. 5 b). Increasing the TiO2 content (R = 1), the largest melting-crystallization zone size and the maximum shrinkage values shifted to lower Al2O3 content (50 mol.%). At R = 2.34, almost any change can be observed in those parameters (Fig. 5). Comparing these results with XRD (Tab. II) and micro-analysis (Tab. III) data, it can be concluded that changes in the meltingcrystallization zone and the shrinkage of the tracks are affected significantly by mainly the presence of three phases: Y3Al5O12, Y2Ti2O7 and Al2O3. The higher the amount of Y3Al5O12

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___________________________________________________________________________ the largest is the crystallization zone and the bigger is the shrinkage. Increasing the TiO2 content (R = 1 and 2.34), the formation of Y2Ti2O7 dominates the phase equilibrium, which together with α-Al2O3, forms skeletons during rapid cooling that prevents changes in the melting-crystallization zone and in the shrinkage of the tracks (Figs. 6 c', 7d''-f''). The explanation of this effect is the following. During the cooling of the melt, the first phase to crystallize on the surface is corundum. This can be observed in some longitudinal section micrographs of the tracks (Fig. 6 b, a', c', a'') or in some cross section micrographs of the tracks (Fig. 7 a', b'). As soon as the surface of the tracks crystallizes, the heat transfer is slowed down changing the crystallization speed of Y3Al5O12. The Y3Al5O12 phase forms broadened layers of gray color between the Al2O3 crystallites in the direction to the sintering zone (Figs. 6 c', 7 f'). In turn, the crystallization of Y3Al5O12 + Al2O3 in the lower layers prolongs the movement of Y3Al5O12 and slow down further the heat transfer. As a result, the zone of crystallization increases and the shrinkage increases.

Fig. 5. Changein the thickness of the crystallization layer (a) and shrinkage of the irradiation zone (b) in the tracks formed during laser treatment depending on the Al2O3 content and different R values: (1) R = 0.25; (2) R = 1; (3) R = 2.34. It is clearly seen that at low content of corundum is generated directional solidification (crystallization) longitudinally (Fig. 6 a-c, a'-c '). The movement of the melt on the surface of the track partially flexes crystallites of layers underneath (Fig. 6 a). However, increasing the corundum content, the directed crystallization of Y3Al5O12 and Y2Ti2O7 will be locked by effect of the rapid crystallization of corundum (Fig. 6 a'' - c''). Note that, as a rule, at the boundary of the melting and sintering zones several pores are formed (Fig. 2 d). This indicates a combination of high temperature and high speed of the laser beam movement. Formation of bubbles is characteristic in processes of horizontal growth of Al2O3 single crystals [51] and requires the careful selection of melting-cooling speed mode to avoid them.


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Fig. 6. Secondary electrons SEM micrographs of the longitudinal section of tracks formed from mixtures with different R (0.25; 1 and 2.34) and different Al2O3 mol.% content: 50 (a,b,c); 60 (a´, b´, c´); 70 (a´´, b´´, c´´). Tab. III Element compositions obtained by EDS in localplacesof the tracks generated after laser treat compacted mixtures of Al2O3-Y2O3-TiO2 using R = 0.25and the assumedphase composition. Assumed phase Contentofelements, wt.% Al2O3, Nplace composition mol.% O Al Y Ti 50 1 38.96 20.47 40.57 Y3Al5O12 2 49.82 8.34 37.02 4.82 Y3Al5O12+Y2Ti2O7 60 1 39.99 19.7 40.32 Y3Al5O12 2 34.26 21.05 44.69 Y3Al5O12 3 44.75 28.04 25.42 1.79 Y3Al5O12+Y2Ti2O7 65 1 39.57 43.97 12.85 3.60 Y3Al5O12+Y2Ti2O7 2 32.42 42.2 23.62 1.72 Y3Al5O12+Y2Ti2O7 3 49.8 50.2 Al2O3 70 1 35.81 19.37 44.8 Y3Al5O12 2 45.77 54.23 Al2O3 3 36.64 21.74 41.62 Y3Al5O12 75 1 42.74 39.61 17.65 Al2O3+ Y3Al5O12 2 38.14 22.90 38.97 Y3Al5O12 80 1 48.11 36.83 15.06 Al2O3+ Y3Al5O12 2 43.99 35.97 20.05 Y3Al5O12 + Al2O3 Al2O3 47.95 53.96 Y3Al5O12 33.33 23.44 43.23 Y2Ti2O7 29.95 44.38 25.67

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___________________________________________________________________________ The evolution of the cross section microstructure of ceramic tracks obtained from mixtures with R = 0.25 (Fig. 7 a-f) is determined by the corundum content. Increasing Al2O3 content, the microstructure gradually changes from Y3Al5O12 crystals with Y2Ti2O7 layers (Fig. 7 a-b), to Y3Al5O12 crystals with thin layers of degenerated two- and three-phases eutectics (Y3Al5O12+ Y2Ti2O7; Y3Al5O12 + Al2O3; Al2O3 + Y2Ti2O7 + Y3Al5O12), Fig. 7 c-d. When the corundum content increase to 70 mol.%, the microstructure consist of large areas of binary eutectic (Al2O3+ Y3Al5O12), small amount of Al2O3 crystals and small degenerated areas of three-phase eutectic (Al2O3+Y2Ti2O7+ Y3Al5O12), Fig. 7 e- f.

Fig.7. Secondary electrons SEM micrograph of the cross section of tracks formed from mixtures with R (0.25; 1 and 2.34) and different Al2O3 mol.% content: 50 (a, a´, a´´); 55 (b, b´, b´´); 65 (c, c´, c´´); 70 (d, d´, d´´); 75 (e, e´, e´´); 80 (f, f´, f´´). The microstructure of the specimens obtained from mixtures with R = 1 can be seen in Fig. 7 a'-f'. The microstructure undergoes a gradual transition from Y2Ti2O7 crystals emerged from the three-phase eutectic (Al2O3 + Y2Ti2O7 + Y3Al5O12), Fig. 7 a'-c'; to a composition consisting of corundum crystals emerged from the three-phase eutectic matrix.


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___________________________________________________________________________ With further increase of alumina content in the initial mixtures, corundum crystals initially grow as individual dendrites and then as faceted crystals (Fig. 7 d'-f '). The microstructure of the specimens obtained from mixtures with R = 2.34 can be observed in Fig. 7 a''-f''. The microstructure at 50-55 mol.% of Al2O3 is represented by Al2O3 crystals emerged from the binary eutectic (Y2Ti2O7 + Al2TiO5), Fig. 7 a''-b''. The Y2Ti2O7 has gray color and the Al2TiO5 has white color.With further increase of alumina content, it is increased not only the amount of corundum crystals but also their size (Fig. 7 c''-f''). The secondary phase is a two-phase eutectic (Y2Ti2O7 + Al2TiO5). This investigation shows that the processes of melting, cooling, crystallization and sintering are generated simultaneously in compacted powder mixtures while a laser beam is moving due to the formation of temperature gradients in three directions (two on the surface and one in the vertical direction).

4. Conclusions The phase formation in the zone of laser irradiation proceeds within the framework of binary mixtures: Al2O3–Y2O3, Y2O3–TiO2 and Al2O3–TiO2. It is accompanied by the formation of composite ceramics. The main phases observed in this study are: Y3Al5O12, Y2Ti2O7, Al2O3 and Al2TiO5. The quantitative phase composition as well as the miсrostructure of the tracks depends on the Al2O3content and the ratio R = TiO2/Y2O3 in the initial mixtures. Due to the temperature gradient in the heating zone, complex layered structures are formed: thin textured surface layer, melting-crystallization layer of the multicomponent eutectic alloy, and a lower sintered layer. The thickness of the crystallization layer and the shrinkage of the irradiation zone depend on the amount of Y3Al5O12 and Al2O3 crystallized from the melt.

Acknowledgements The authors wish to thank CONACYT for financial support (Project 155731).

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Садржај: Проучавано је формирање фаза, микроструктура и површинска текстура смеше прахова Al2O3–TiO2–Y2O3 која је третирана ласером. Изузетно високо температурно загревање, праћено брзим хлађењем услед померања ласерског снопа, формира конкавне керамичке пруге. Фазни састав и микроструктура пруга зависе од садржаја Al2O3 и односа TiO2/Y2O3. У почетној смеши уочене су фазе Y3Al5O12, Y2Ti2O7, Al2O3 и Al2TiO5. Услед температурног градијента у зони загревања, долази до формирања слојевите комплексне структуре. Пруге се састоје од три главна слоја: слој танке површина, слој продуката кристализације еутектичких легура и нижи синтеровани слој. Дебљина кристализационог слоја и скупљање озрачене зоне зависе од количине Y3Al5O12 и Al2O3 који кристалишу. Кључне речи: прахови Al2O3, TiO2, Y2O3, синтеза ласером, керамика