Reactive coating on alumina substrates. Calcium and barium hexa aluminates

Pergamon

ScriptaMetallurgicae t Materialia, Vol. 31, No. 8, pp. 1049-1054, 1994 Copyright© 1994 Elsevier ScienceLtd Printed in the USA. All rights reserved 0956-716XJ94 $6.00 + 00 CONFERENCE SET No. 2

REACTIVE COATING ON ALUMINA SUBSTRATES, CALCIUM A N D B A R I U M H E X A ALUMINATES

J.S. Moya, A.H. de Aza, H.P. Steier, J. Requena and P. Pena Instituto de Ceramica y Vidrio CSIC, Arganda del Rey, Madrid, Spain

(Received May 6, 1994) (Revised May 27, 1994) Introduction Modifying the surface of ceramics by coating with a substance different from that of the bulk is currently an important technological issue in structural as well as in functional applications (microelectric, environment, sensors, thermal and chemical barriers, catalysis, etc). About 80% of all advanced ceramics are based on alumina [1]. In the present work we introduce reactive coating as a feasible route to designe an appropriate surface layer in dense alumina compacts, i.e. CaO'6A1203 (CAr) and BaO-6A1203 (BAr). CA6 and BA 6 are two compounds with the magnetoplumbite structure [2,3] exhibiting very promising technological properties, such as: (a) a wide range of solid solutions with iron oxides containing slags [4,5]; (b) high stability in reducing atmospheres [6]; (c) a coefficient of thermal expansion close to alumina (7.8x106 K~)[7] and (d) large primary crystallization fields, which mean low solubility in several multicomponent systems [6-8]. Recently CA6 and BAr based materials have found new applications fields in nuclear waste storage, optics, catalysis [9-11], as a support material for high temperature catalytic combustion [12] as well as a possible substrate material for thin, monocrystalline hexaferrite layers, serving as bubble storage or microwave devices [13]. Further applications of both compounds could be found in the field of solid state ionic conductors due to their g-alumina defect structure [3]. Several investigations have been undertaken with the aim of studying the mechanism involved in calcium and barium aluminate formation using diffusion couples made ' from oxide powders [14], single crystals [15], powder mixtures [16,17] or alkoxide-derived samples [17]. In the present work doloma (CaO-MgO), calcium carbonate (CaCO3) and barium carbonate (BaCO3) reactive coatings on A1203 substrates have been studied; the results have been discussed on the base of the high-temperature reactions predicted by the corresponding phase equilibrium diagrams. Experhnenta| procedure Dense alumina ~ (98% theoretical, >99.6 purity) was used as starting material. Cylindrical plates of 6ram in diameter and 2ram thick were diamond machined from the alumina blocks and the surfaces were subsequently polished to 1 ~m. These plates were coated by screen printing with suspensions of: doloma (CaO.MgO) z, CaCO32, and BaCO33 (these samples were labeled CM/A, C/A and B/A respectively). The suspensions were prepared by mixing ethylenglycol (55wt%) and the corresponding powder (45wt %) with 1%

~AlcoaCT 300 SG, Pittsburgh, Pa, USA. 25865 Merck Magnesiumoxide;2066 Merck Calciumcarbonate,Germany. 3Pro-BysBariumcarbonate,Spain. 1049

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defloculant 4. The screen printed substrates were dried at 80°C for 48h and heat treated using the following schedules: CM/A and C/A at 1650°C-4h, B/A at 1650°C-2h.

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Fig. 1 (A)Cross-section's SEM of CM/A fired specimen. (B)Top surface's SEM of the top layer. (C)Bottom surface's XRD pattern of the top layer (TL). (D)Top surface's XRD thin film of the botton layer. (E)Botton layer's XRD pattern. After firing the polished cross sections of all the studied specimens were observed by reflected-light optical microscopy (RLOM) and scanning electron microscopy (SEM-EDS) s'6.

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The phase present at different depths were identified in situ by XRD' analysis. Results and Discussion Fig. 1 shows the general view of the cross section of CM/A speciment with a characteristic microstructural detail as well as XRD patterns of the different interfaces. From this figure the following can be stated: (i) A crack along the interface (Fig. 1A) physically separates the glassy phase containing layer at the top from the glassy phase free layer formed mainly by textured CA~ crystals (Fig. 1E). (ii) The top surface of the top layer (Fig. 1B) is formed by large ( < 20#m) primary crystals of spinel and smaller secondary crystals of calcium aluminates (Ci2AT) formed probably during cooling by a desvitrification process. (iii) The bottom surface of the top layer is formed mainly by textured CA2 crystals (Fig. 1C). The thin film XRD of the surface of the bottom layer clearly shows the presence of CA2 textured crystals (Fig. 1D). (iv) From Fig. 2A it is clear that the CA6 textured crystals grow from alumina grains in the absence of any glassy phase. (The EDS average microanalysis quoted in 5 diferent crystals was as follows : 51.155%wt O, 43.042%wt AI, 5.803%wt Ca; Fig. 2B). 1.67pm c

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Fig. 2 BEDS spectrum quoted on the textured crystals.

The mechanism of formation of the CA6 layer can be explained considering the isothermal section at 1650°C of the MgO-A1203-CaO system (Fig. 3). At 1650 ° a liquid phase appears at the alumina interface. This liquid changes in composition along the doloma-A1203 line until point A is reached. At this point the liquid is saturated in alumina and Fig. 2 A SEM micrograph showing the interface MgA1204 (Spinel) precipitates. Subsequently the liquid between the textured CA6 crystals and the composition moves along the boundary L,-L2, at this point CA 2 nucleates probably on the surface of the alumina substrate alumina substrate. forming a textured layer as reported by Kohatsu and Brindley [15]. This CA2 layer separates the primary spinel plus liquid layer from alumina substrates. In a second step the CA2 layer reacts in solid state with the aluminiun substrate giving the stable CA6 compound. As the thermal expansion coefficient of CA2 is approximately half that of CA~ and Al~O3 [7], a crack can be formed during cooling as observed in Fig. 1A. CA2 and CA6 crystals grow along [100] and [110] directions respectively (Fig. 1C-D) in agreement with Kohatsu and Brindley [15].

~DiffraktometerD500, Siemens, Germany.

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REACTIVECOATING

In the case of C/A sample a similar phenomenon takes place. As observed in Fig. 4A at 1650°C a liquid is formed at the interface. This liquid is dissolving alumina until point A is reached. At this point CA2 precipitates. Subsequently, in a similar manner as in the previous case, CA2 reacts with the alumina substrate to form CA~ until all the liquid is consumed (Fig. 4B).

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O

Fig. 5A shows a general view of the B/A DOLOMA Moo+t l q * speciments after firing. As observed in the figure the coating consists of a top layer of - 3 0 tzm thickness and a bottom layer of - 8 0 t~m ...... / ,.¢//\\ thickness. The Vickers indentation print (1130 N load) shown in this figure, led to large cracks parallel to the interface. This fact means that the I CaO coating is subjected to compressive residual stress. CA, CA, A I 2 0 3 A similar phenomena was observed in the case of CA6 coated alumina substrates. The XRD pattern corresponding to the coating (Fig. 5B) shows the Fig. 3 Isothermal section at 1650°C of the system: MgOpresence of (a) BA oriented in the (hk0) directions CaO-AI~O3 [19].

Fig. 4 (A) CaO-AI~O3equilibrium diagram [18]. (B)SEM mierograph of the crosssection of C/A fired specimen.

and (b) BA6.6 and BA4.6 without any preferecial orientation. The Fig. 6 shows a close up view of the coating with the EDS analisis quoted in different areas. In the Fig. 7 the BaO-AI20~ equilibrium diagram is shown. Taking into account the high temperature phase relationship of this diagram it is possible to explain the nature and phases of the coating. At 1650 *C a liquid is formed at the alumina interface, this liquid penetrates into the substrate through the grain boundaries dissolving alumina and moving its composition along the 1650 °C isotherm until the point A is reached. At this moment BA crystals precipitate. Subsequently BA reacts with the alumina substrate to give BA~.6 and BA4.6 until all the remaining liquid with A-composition is consumed. The EDS spectra shown in Fig. 6 are consistent with the previous statements. The top layer of the coating is formed by remanent BA textured crystals and the bottom layer by BA6.6 + BA4.6. The remaining liquid phase has the higher Ba content composition as expected, considering the equilibrium diagram.

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REACTIVE COATING

,(

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x ! 2t]wta

~ :

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Fig. 5 (A) SEM micrograph of the cross-section of B/A fired specimen. (B) XRD pattern of the coating layer.

Fig. 6 SEM micrograph of the cross-section of B/A fired and thermally etched (1300 °C-lh) speciment and EDS spectra of different areas. GP: glassy phase.

Conclusions

The following conclusions can be drawn: 1. Reactive coating is a suitable route to design mechanically stable coatings on alumina substrates, such as CA6 and BA~. 2. The mechanisms of formation of the CA~ and BA6 coatings can be explained by means of the corresponding equilibrium diagrams.

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Tlme]

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L

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o AI203

10

t 20

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Fig. 7 A1203-BaO equilibrium diagram (from Ref. 13)

A¢knowled2ements This work was supported by CAICYT (Spain) project nb. MAT. 91-0878. One of the authors (H.P. Steier) was supported by COMETT. REFERENCES [1] E. D6rre and H. Hiibner, "Alumina", Springer-Verlag, 1984. [2] R.W.G.Wyckoff, Crystal Structures, Vol.3. Interscience Publishers, p52. [3] J.van Hcek, Ph.D. Thesis, Tech. University Eindhoven, Netherlands. [4] R.R.Dayal and F.P.Glasser.,"Science of Ceramics" Vol.3,191-24. Academic Press, London 1967. [5] J.A.Imlaeh and F.P.Glasser, Trans Brit. Ceram. Soc., 66, 287-93. (1967). [6] J.B.Task, D.J.Young, Ceram. Bull., 61,(7), (1982). [7] E.Criado, S.de Aza, 13ol. Soc. Esp. Ceram. Vidr. 15, 5,319-21, (1976). • [8] P.Pena, S.de Aza, J. Am. Ceram. Soc. 67, (1), C-3-5, (1984). [9] A.B.Harker, J.F.Flimtoff, Adv.Ceram. 8, 222, (1984). [10] J.D.Hodge, J.Electrochem. Soc. 133, (4), 833, (1986). [11] T.Okutani, R.Ryusaburo, Nippon Kagaku Kaishi, 9, 1485-90, (1975). [12] H.Arai and M.Machida, Catalysis Today, Vol. 10, 81-95,(1991) [13] F.Haberey, G.Oehlschlegel and K. Sahl, Ber. Dt. Keram. Ges.,54, (1977), Nr. 11 [14] I.Kohatsu and G.W.Bridleey. Zeitschrift for Physikaiische Chemie Neue Folge, Bd.60,S.79-89 (1968). [15] H.Tagai and I.Takayoshi, Yogyo Kyokai,77 (890),341-7 (1969). [16] V.Singh, M.M: All, U. K. Mandai, J. Am. Ceram. Sot., 73 (4), 872-76 (1990). [17] M.Machida, K.Eguchi and H.Arai, J. Am. Ceram. Soc., 71 [12], 1142-47, (1988) [18] B.Hallstedt, J. Am. Ceram. Soc., 73 [1], 15-23, (1990). [19] A.H.de Aza, P.Pena, S.de Aza, Unpublished research.