TEM STUDY OF ZrO2/Al INTERFACE

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TEM STUDY OF ZrO2/Al INTERFACE

G. Tremouilles, R. Portier

To cite this version:

G. Tremouilles, R. Portier. TEM STUDY OF ZrO2/Al INTERFACE. Journal de Physique Colloques,

1988, 49 (C5), pp.C5-299-C5-304. �10.1051/jphyscol:1988536�. �jpa-00228033�

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Colloque C5, supplément au n"10, Tome 49, octobre 1988 C5-299

TEM STUDY OF Zr02/ftl INTERFACE

G. TREMOUILLES and R. PORTIER*

ETCA, 16bls, Av. Prieur-de-la-Côte-d'Or, F-94414 Arcuell Cedex, France

*CNRS-VA 302, CECM, 15, rue Georges Urbain, F-94499 Vitry, France

RESUME : La zircone partiellement stabilisée à l'oxyde d'Yttrium est couplée à de l'Aluminium pur monocristallin par un processus de thermocompression. Une bonne adhésion est obtenue; des micro- cristaux d'Alumine sont observés à l'interface. Une diffusion d'O- xygène de la Zircone vers l'Aluminium a permis cette croissance.

Des relations d'orientations préférentielles sont observées à l'interface.

ABSTRACT : Yttria partially stabilized Zirconia is coupled to mo- nocrystalline pure Aluminium by thermal-compressive process. A good adhesion is obtained, and Alumina microcrystals are observed at the interface. The Oxygen diffusion from Zirconia allows Alumi- na's growth. Preferential orientations relationships exist at this interface.

INTRODUCTION

Yttria partially stabilized Zirconia and Aluminium can easily be bonded when they are kept in contact under a compressive stress at an appropriate combination of temperature and time. At once, the bonded interface between Zirconia (Zr02) and Aluminium is observed by transmission electron microscopy in order to investigate the bonding mechanism between the two materials.

I) EXPERIMENTAL PROCEDURE 1.1 Material

We use a sintered Yttria partially stabilized Zirconia (8% by weight) obtained from Ste Desmarquet, France. The Zirconia is cut to obtain square sheets of 0.3mm thick and 10 mm sides which are optically polished and 48 hours heat treated at 1400°C for recrystallization. Then the sheets are cleaned in hot sulfuric acid.Aluminium metal 99.999% purity single crystal is cut to obtain square sheets which are optically polished on the (111)planes and (HO)planes.

Sandwiches are done with the Zirconia and Aluminium sheets. These sandwiches are heated at 590°C (Al T/Tm=0.9) under compressive stress (60MPa), for 4 hours, under Argon atmosphere.

1.2 Thin foils

The thin foils are obtained by cutting, polishing to about 40pm thick and finally ion thinning (gatan dual ion mill, model 600). The thin foils are covered by a thin Carbon coat (l-2nm) which avoids electrical charging of specimen before investigation by transmission electron microscopy (T.E.M), using a JEM 1200EX electron microscope operating at 120KV.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988536

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C5-300 JOURNAL DE PHYSIQUE

1.3 Chemical analysis

Chemical analysis are performed across the ZrOz/Al interface in order to examine the change of chemical composition, or possible diffusion phenomena in both materials.

- Electron Probe MicroAnalysis (EPMA), using a Camebax SX50 electron microprobe, with a beam diameter of about 1 Dm

-

Energy Dispersive Spectroscopy (EDS) microanalysis in the microscope (ORTEC)

.

1.4 Mechanical test

The Vickers hardness is used a load of 20N; it is carried out at the specimen interface to form cracks just between Zirconia and Aluminium.

11) EXPERIMENTAL RESULTS 11.1 Elec.tron microscopy 11.1.1 Zi:rconia microstructure

The Zirconia have fine equiaxial grains less than lum F i g . The Tetragonal (t) and Cubic ( c ) phases appear; they are generally embedded. The t phase is in majority, more than 90%

(Fig.2).

Fig. 1 Fine grain Fig.2 Bright field Dark field g=220 Z r 0 ~ equiaxe Zirconia Fine layer of c phase superposed on the t matrix

product of electron beam focalization.

11.1.2 Aluminium microstructure

Aft.er the thermal-compressive process; no recrystallization of A1 is observed at the center of the specimen, A1 stays single crystal even at the interface.

At the borders and corners of the specimen, A1 is recovered by polygonizi~tion mechanism. The Al/Al grain boundaries 3re low angle; we can observed grain boundary dislocation network (Fig.3

a&b) and intragranular dislocations close to the interface (in Al, Fig. 3a).

11.1.3 Interface microstructure

Zirconia grains are curved at the interface, because of the sulfuric acid cleaning which works preferentially at grain boun-

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LOW angle grain boundaries ( B ) and Grain boundary dislocation network ( B j intragranular dislocation in A1 (D) and polygonisation (C)

Fig.4 Bright field Dark field g=lil zro2+022 yA120g Alumina grains epitaxial with Zirconia, the Loundary plane is curved.

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daries (Fig.l).There is Gamma fine grained (C100nm) Alumina layer about 100nm thick between Zirconia and Aluminium (Fig.5).

The orientation relationships study and diffraction patterns (Fig.6 a&b) show that there are preferential relationships between t ZrOz and \( Alz 03

.

The orientation relationships between Zr02 and A h 0 3 are :

and most often :

Fig.5 Bright field Dark field + g=004 yA1203 Alumina layer between Zirconia and Aluminium.

Fig.6 Diffraction pattern of ZrO tetragona?

2

z

= 011

interface corresponding to Fig.4

yA1203 A

z

= 011

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5 O ) ; they follow the Zirconia grain curve. The orientation relationships beetween Alumina and Aluminium is not preferential. We don't observe any dislocation at the interface in the interphase boundaries, but it is no exhaustive.

One specimen is realised with a process time of 30mn. In that con- dition the adhesion between Zirconia and Aluminium is very bad and we don't find any Alumina phase between them.

11.2 Microanalyses

The Alumina layer is about 100nm thick which is very small beside the electron beam diameter of EPMA (1000nm). So the linear traverse spectrum is difficult to read. We can read an Oxygen diffusion of about 100nm corresponding to the Alumina layer, but it could be a steep effect at the interface. We think that in the present case, we must not use the EPMA spectrum because of its too small spatial resolution.

EDS microanalyses by STEM near the interface give 3%at of A1 in Zirconia grain boundaries and less than l%at of A1 in Zirconia matrix. In Aluminium the analysis gives about l%at of Zr. These amounts are very small compared to the Oxygen diffusion necessary to Alumina's growth.

11.3 Vickers indentation test

The Vickers test with 2ON load forms cracks (Fig.7) Zirconia side. Those cracks go to the interface, are deflected by it and are stopped. If the test is done just at the interface, desadhe- sion cracks are formed between Zirconia and Aluminium (Fig.8); we observe slips band in Aluminium which are not deflected by grain boundaries, then is stays single crystal. The Vickers indenter test could give us information upon adhesion energy by measure- ment of the cracks length. The Vickers cracks are in thermody- namical equilibrium and the propagation kinetic is controlled.

Fig.7 Vickers test Zirconia side Fig.8 Vickers test at the interface

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111) DISCUSSION

As mentioned above, an Alumina layer is formed by solid phase reaction between Zirconia and Aluminium if the process time is not too short (4 hours). We can compare to other systems : in the case of aAlzO3/Nb,S.Morozumi and Al. (I), find an intermediate layer of NbOx, this result is in contrast with the study of the same system by M-Turwit and A1.(2&3) who don't find any intermediate layer with similar conditions of high temperature compressive stress.

The aAlzO~/Cu system can be bonded by high temperature compressive stress, or by gaz/metal eutectic reaction (Cu-0). In both cases it was found a CuA102 layer(4).

In our case, the Oxygen diffusion at 59U°C allows the following reaction:

The process temperature is lower than the reaction equilibrium temperature which is 900°C, the Aluminium reduces Zirconia and forms the Gamma Alumina which is face centered cubic, a=0.79nm. In this case the Alumina is Gamma because of the process temperature, it is a metastable form which becomes Alpha at 1000°C.

The preferential orientation relationships between t Zr02 and A1203 are epitaxial type, the Alumina layer grows from the Zirco- nia surface, depending on its surface orientation. This epitaxial growth minimizes the parameter misfit, and also the grain boundary energy. This energy decreasing might be favourable to a good bonding. The epitaxial growth of Alumina on Zirconia carries away no preferential orientation relationships between Alumina and Alumi- nium.

In the case of the specimens made with a process time of 30mn, we don't obtained a good adhesion, and we don't observe any Alumina phase. It seems that a chemical reaction must occur between Zirco- nia and Alumina to obtain a good bonding.

(1) S. Morozumi, M. Kikuchi, T. Nishino; "Bonding mechanism between Alumina and Nobium".

Jo.urna1 of Material Science 16 (1981) 2137-2144 (2) M. Turwitt, G. Elssner and G. Petzow: "Manufacturing

and properties of interfaces between Sapphire and No:biumW

.

Journal de Physique, Colloque C4, supplement au n04, tome 46, Avril 1985.

(3) M. Florjancic, W. Mader, M. Ruhle and M. Turwitt; "HREM and Diffraction study of an Alz03/Nb interface".

Journal de Physique, Colloque C4, supplement au n04, to:me 46, Avril 1985.

(4) C. Beraud; "Contribution par MET A l'btude de la liai- son Cuivre/AlumineW

.

Th~ese (1986). INSA, Lyon, France.