Orientation Imaging Microscopy Applied to Zirconia Ceramics

Marek Faryna1;�, Jerzy Jura2, and Krzysztof Sztwiertnia2

1 Jagiellonian University, Regional Laboratory of Physicochemical Analyses and Structure Research, Ingardena 3, 30-060Krak�ow, Poland; Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Reymonta 25, 30-059 Krak�ow, Poland

2 Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Reymonta 25, 300-59 Krak�ow, Poland

Abstract. Measurements of crystallographic orienta-

tions along with microscopic observations are the basis

of quantitative investigations of the microstructure of

crystalline materials. The technique that applies auto-

matic orientation measurements in the transmission or

scanning electron microscope is known as orientation

imaging microscopy (OIM) [1]. In this paper the

measurements and analyses of sets of single orienta-

tions gained from electron backscattered diffraction

(EBSD) registered in a scanning electron microscope

are presented. A quantitative description of microstruc-

ture of two polymorphs of zirconia, based on measure-

ments of single orientations, is also given.

Key words: Tetragonal zirconia polycrystals (TZP); orientationimaging microscopy (OIM); orientation distribution function(ODF).

The discovery that the stress-induced martensitic

tetragonal (t) to monoclinic (m) phase transformation

in ®ne ZrO2 precipitates, particles or grains can occur

in the vicinity of a propagating crack has lead to a new

class of strong and tough ceramic materials. This

transformation greatly enhances the mechanical

properties of ZrO2 ceramics [2]. Additionally, the

incorporation of hard carbide inclusions into the

polycrystalline tetragonal zirconia matrix (TZP)

proved to be an effective way of further improving

its mechanical properties [3 ± 5]. In the authors'

opinion the nature of such a toughening may be

better understood by establishing the crystallographic

relationships between several adjacent grains. Such

measurements are possible in the transmission

electron microscope (TEM). However, in the case

of TEM several limitations still exist making the

experiment dif®cult and time-consuming. Only very

thin ®lms transparent to electrons with high energies

can be analysed. Moreover, the observed area is

limited to several grains, which may lead to false

conclusions. In such a case orientation imaging

microscopy (OIM) in a scanning electron microscope

(SEM) can be very helpful as automatic measure-

ments of single orientations of thousands of grains

together with microscopic observations are realised

during one experiment.

Orientation imaging microscopy in the SEM is a

microtexture facility for obtaining electron diffraction

data from bulk samples. This technique known also as

electron backscattered diffraction (EBSD) provides a

wide range of information about the orientation of

crystals with a spatial resolution in the range of few

tens of micrometers, depending on the acceleration

voltage as well as the density of specimen. Although

the technique is known for at least two decades (®rstly

used by Venables and Harland in 1973 [6]), the

complexity of data and equipment has prevented

EBSD from becoming a standard technique. However,

recent developments in camera technology, together

with the availability of PC-driven scanning electron

microscopes, have made possible to apply this

technique to microtexture analysis of specimens

containing small grains and subgrains.

The principle of the EBSD technique is that a

phosphorus screen collects backscattered electrons

diffracted in a systematic manner and satisfying the

Bragg law. To produce an optimum EBSD signal in

Mikrochim. Acta 132, 517±520 (2000)

� To whom correspondence should be addressed

the SEM, the specimen has to be highly tilted towards

the electron beam. Information from EBSD appears as

bright bands, geometrically arranged on the phos-

phorus screen. The arrangements of these bands are

unique to a certain crystallographic orientation and

crystal symmetry. Each crystal of similar composition

will thus show a different pattern when oriented

differently.

Experimental

In this report two polymorphs of zirconia stabilised by Y2O3 werestudied using the EBSD technique. The cubic polymorph of ZrO2

stabilised by 8 mol.% Y2O3 is isostructural with mineral ¯uorite(CaF2), and has the m3m Laue group symmetry (three-fold axiswith two mirror plane) with lattice parameter a� 5.1437 AÊ . Thetetragonal form of ZrO2 stabilised by 2.9 mol.% Y2O3 has the4=mmm Laue group symmetry (four-fold axis with threeperpendicular mirror planes) and lattice parameters a� 5.1023 AÊ

and c� 5.1817 AÊ .Two discs of 30 mm diameter and 10 mm thickness, made from

both cubic and tetragonal zirconia were pressureless sintered in airin a furnace with Superkanthal heating elements at 1500 �C for 2 h.A detailed description of the sample preparation can be foundelsewhere [6]. The sintered samples were subsequently polished inthe conventional way. As the Bragg diffraction of electrons at allatomic planes in the crystal lattice occurs in a very thin layer(< 50 nm) in the subsurface of the specimen, it has to be free ofany mechanical damage produced by grinding and polishing. Toremove stresses introduced during polishing the samples wereadditionally heated at 1350 �C for 30 min (thermal etching). Basedon the assumption that the high tilt of the specimen surface withrespect to the incident electron beam reduces the amount ofcharging to a tolerable level [7], the specimens were normallyviewed without any coating. Nevertheless, several attempts weremade to collect electron backscattered diffraction images from thesamples covered with a thin carbon film to eliminate completely

the charging effect. However, it was found that due to the coating,the quality of Kikuchi pattern formed on the phosphorus screendeteriorated markedly making indexing almost impossible.

The measurements were carried out in a PHILIPS XL30 SEM atan acceleration voltage of 20 kV, tilt angle 72� and workingdistance 22 mm. The backscattered images were analysed andindexed with a NORDIF EBSD1 hardware and CHAN-NEL�EBSD software for fully automated pattern indexing. Aschematic diagram representing the NORDIF EBSD1 hardwareconfiguration is shown in Fig. 1.

Orientation measurements were performed over an area in theplan of the sample surface with grid nodes: X-direction� 200,Y-direction� 150 for cubic zirconia and X-direction� 150,Y-direction� 150 for tetragonal zirconia. The grid step was fixedat 1 mm for both cases, which corresponds to the mean graindiameter in the sample with the smaller grain size, i.e., tetragonalZrO2.

Results and Discussion

The measured sets of single orientations for both

polymorphs of zirconia have the form of orientation

maps describing the orientation topography, that is

topographical distribution of local crystallographic

orientations in the de®ned sample section. Examples

of such maps, one for cubic and one for tetragonal

zirconia, are shown in Figs. 2 and 3, respectively. The

correspondence between the colours seen in these

maps and the texture components is given in Tables 1

and 2.

The orientation topographies show that the mean

grain diameter in the cubic sample is about 3 ± 4 mm,

being much higher than in the tetragonal sample,

where the diameter does not exceed 1 mm. In both

samples a rather wide spectrum of orientations is

Fig. 1. Orientation Imaging Micros-copy NORDIF EBSD1 hardwarecon®guration [9]

518 M. Faryna et al.

observed, which are randomly distributed in the

sample space.

The manufacturing process may induce its speci®c,

statistical symmetry to the texture. In such a case,

each texture component is represented by its process

symmetry related variants. In order to check whether

these variants are present, the statistical triclinic

symmetry for the above process was assumed.

Using the orientation data from the sets of single

orientations the orientation distribution functions

(ODFs) were calculated by the method of Fourier

series expansion, assuming cubic and tetragonal

crystal symmetry for zirconia polymorphs. The ODFs

are shown in the Euler space ('1, �,'2) in cross-

section '2� constant.

The calculated ODFs are presented in the Figs. 4

and 5. The detailed analysis of these functions

con®rms the assumption of the lack of statistically

meaningful symmetry in both samples. There are only

few low local maxima observed on background of

wide scattered orientations. It is stressed that due to

the fact that analysed samples were discs the choice of

the X direction in the sample plane is chosen

arbitrarily. As a result, the '1 angle (the angle of

rotation around the sample normal) is also a free

parameter in this case. The '1 angles of all

orientations are changed by the same value with the

rotation around the normal. As a consequence, the

directions huvwi (parallel to the X direction) are

modi®ed into the other ones, also lying in the distinct

crystallographic planes (hkl).

Fig. 2. Orientation maps for the sample of cubic ZrO2, areascanned 200� 150 mm in the sample plane. (a) Orientationtopography, (b) distribution of the orientations a, b, c from theTable 1

Fig. 3. Orientation maps for the sample of tetragonal ZrO2, areascanned 150� 150 mm in the sample plane. (a) Orientationtopography, (b) distribution of the orientations a, b, c � � � h fromthe Table 2

Orientation Imaging Microscopy Applied to Zirconia Ceramics 519

Conclusions

Electron Backscattered Diffraction technique has been

successfully applied to obtain electron backscattered

patterns from zirconia ceramics, both from cubic and

tetragonal polymorphs. Due to high tilt of the

specimen surface with respect to the incident

electrons, the samples could be examined in the

scanning electron microscope without carbon coating.

In both zirconia polymorphs only certain orienta-

tions are present. Several crystallographic planes lying

parallel to the sample plane can be identi®ed in both

cubic and tetragonal samples.

At the present stage of investigations it is not

possible to judge whether these orientations are related

in some way to the sintering process.

Acknowledgements. Mr. Krzysztof Pawlik is gratefully acknowl-edged for the assistance with dedicated software for plotting theOrientation Distribution Function and Mr. Tomasz Rostek for com-piling orientation data. The experiments were performed on theequipment generously sponsored by the Alexander von Humboldt-Foundation, Bonn-Bad Godesberg, Federal Republic of Germany.

This work was ®nancially supported by the Polish Committee ofScienti®c Research under Grant 7 T08D 003 17.

References

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[2] A. H. Heuer, J. Amer. Ceram. Soc. 1987, 70, 689.[3] Zh. Dingh, R. Oberacker, F. Thummler, J. Europ. Ceram. Soc.

1993, 12, 377.[4] K. Haberko, Z. PeÎdzich, G. R�og, M. M. Bu�cko, M. Faryna,

Europ. J. Solid State Inorg. Chem. 1995, 32, 593.[5] M. Faryna, L. Lity�nska, K. Haberko, Z. PeÎdzich, J. Babiarz,

Mikrochim. Acta Suppl. 1998, 15, 83.[6] J. A. Venables, C. J. Harland, Phil. Mag. 1973, 27, 1193.[7] Z. PeÎdzich, K. Haberko, Ceramics International 1994, 20, 85.[8] D. J. Dingley, K. Baba-Kishi, Micro. Analys. 1990, 5, 29.[9] HKL Technology, Channel 4, Revealing Microstructure

(Manuals), 1999.

Table 1. The texture components for cubic zirconia

'1 � '2 hkl uvw Coloursattributed toorientations

a 260� 30� 10� � (159) [095] redb 170� 20 25� � (261) [3112] greenc 210� 20� 45� � (227) [3102] blue

Table 2. The texture components for tetragonal zirconia

'1 � '2 hkl uvw Coloursattributed toorientations

a 7.5� 75� 0� � (041) [100] redb 80� 60� 10� � (164) [023] yellowc 10� 85� 20� � (130) [310] greend 50� 65� 35� � (221) [122] bluee 60� 85� 35� � (681) [112] violetf 85 85 35� � (230) [001] blackg 45� 55� 60� � (211) [011] whiteh 30� 80� 75� � (721) [153] grey

Fig. 4. The ODF calculated from 30000 single orientationsmeasured on the cubic ZrO2

Fig. 5. The ODF calculated from 22500 single orientationsmeasured on the tetragonal ZrO2

520 Orientation Imaging Microscopy Applied to Zirconia Ceramics