HAL Id: jpa-00249226
https://hal.archives-ouvertes.fr/jpa-00249226
Submitted on 1 Jan 1994
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Benefits of HREM for the study of metal-ceramic interfaces
T. Epicier, C. Esnouf
To cite this version:
T. Epicier, C. Esnouf. Benefits of HREM for the study of metal-ceramic interfaces. Journal de Physique III, EDP Sciences, 1994, 4 (10), pp.1811-1831. �10.1051/jp3:1994242�. �jpa-00249226�
J. Phys. III France 4 (1994) 18 II-1831 OCTOBER 1994, PAGE 18 II
Classification
Physic-s Abstiacts
61.16D 61.50J 68.48
Benefits of HREM for the study of metal-ceramic interfaces
T. Ejicier and C. Esnouf
Groupe de Mdtallurgie Physique et de Physique des Matdriaux, URA CNRS 341, INSA de Lyon, Bit. 502, 69621 villeurbanne Cedex, France
(Receit,ed 28 Jai» 1994, accepted 2 August 1994)
R4sunt4, Cet article est consacrd h la discussion de l'intdrdt prdwntd par la Microscopie Electronique en Transmission h Haute Rdsolution (METHR) dans l'Etude des interfaces mdtal- cdramique, Une revue des mdthodes d'obtention d'interfaces adaptdes h l'observation au microscope dlectronique h transmission, ainsi que )es rdsultats d'Etudes rdcente~ sur le sujet, seront
d'abord prdsentds, Une sdlection d'exemples ont ensuite dtd choisis afin d'illustrer [es principaux points que la METHR peut aborder, h savoir : ddtection d'dventuels composts intermddiaires h
l'dchelle du nanombtre (cas des systbmes Nilmgo, NiO/Pt, MgO/Nb), accommodation des
marches prdsentes en surface de la cdramique (systbmes Fe/AI~O~, Au/MgO, Cu/MnO),
caractdrisation des dislocations de cohdrence (exemple du couple Cu/MgO), influence de la force
image sur la position des dislocations, dilatation du rdseau du mdtal causde par le ddsaccord
paramdtrique (exemple du couple Pd/MgO), d6termination de la nature chimique des plans atomiques terminaux (Pd/A[O~), Une discussion des problbmes spdcifiques de l'interprdtation des
images de METHR (principalement, l'analyse numdrique des images et [es simulations) sera mende et illustrde par le cas des interfaces incohdrentes dans le systbme Fe/AI~O~.
Abstract, The aim of the present paper is to discuss the advantages of High Resolution Electron Microscopy (HREM) studies of metal-ceramic interfaces. A review of methods available for the production of interfaces suitable for observation in a transmission electron microscopy, together with a litterature survey of recent relevant studies will be presented first. A selection of examples
will serve to illustrate the main points that HREM allows to be tackled : detection of nanometer-
size intermediate compounds (Nilmgo, NiO/Pt, MgO/Nb systems), accomodation of steps at the ceramic surface (Fe/Al~oj, Au/MgO, Cu/MnO systems), characterization of misfit dislocations
(e,g,, Cu/MgO), effect of the image force on the exact position of the dislocation cores, lattice
expansion due to elastic mismatch (e,g., Pd/MgO), determination of the chemical nature of the terminating atomic planes (e.g., Pd/AI~O~). An introductive discussion of specific problems in the
interpretation of HREM images (I,e., numerical image analysis and computer simulations) will be conducted and illustrated with the case of incoherent Fe/AI~O~ interfaces.
1, Introduction,
1-1 GENERALITIES. The understanding of the behaviour of heterophase boundaries, I,e.
interfaces between compounds with dissimilar structures, is one of the keys ofthe optimization
1812 JOURNAL DE PHYSIQUE III N° lo
of performances of modern materials, simply because these materials are more and more
composite systems, owing to the need of specific properties for their use in specific working
conditions,
Metal-ceramic interfaces constitute a class of such boundaries which is of the greatest interest for a lot of applications (electronic packaging systems, thin film technology, catalysis
and thermo-mechanical use e,g., high temperature engine components), The present paper
addresses the question of the structural characterization of the few atomic planes or « layers »
concerned by the joining of the metal and the ceramic. It presents the « state-of-art
» of what
can be done with High Resolution Electron Microscopy, which remains one of the most
powerful tool for undertaking a structural investigation at such a local scale.
Section explains why HREM is needed (Sect, 1.2), and describes the various methods that
can be used in order to produce metal-ceramic interfaces suitable for a TEM or HREM study (Sect.1.3); a review of significant TEM studies performed on metal-ceramic systems, excluding works which do not focu~ on the interface itself, will be given.
Section ? will present some application~ of the HREM technique, which illustrate advanced results obtained on specific problems according to an arbitrary classification introduced in i ?.
Section 3 is devoted to the « atomistic
» description of interfaces, as made possible with the help of computer simulation and treatment of HREM micrographs.
1.2 ABOUT THE NEED AND INTEREST OF HREM STUDIES OF METAL-CERAMIC INTERFACES.
As for any kind of structural defects in crystalline materials, Transmission Electron
Microscopy and HREM are unsurpa~sed techniques for a local microstructural characteri- zation, In HREM, one intends to observe the atomic structure of the object, which, under
optimal conditions, is imaged in terms of atomic columns in a convenient viewing direction.
For interfaces, the success of an HREM approach is depending upon the ability to obtain a reasonable combination of geometrical control of the interface orientation and attainable resolution. The electron microscopy technology has made significant progress during the last decade, and instruments with large tilt capabilities, together with a good experimental
resolution are now available for example, the JEOL 4000EX microscope jdouble-tilting up to
± 20° [1] and point-to-point resolution and information limit respectively equal to 0,17 and
0, l~ nm [2]j and the Atomic Resolution Microscopes JEOL ARM 1000 (double-tilting ± 45°
and routinely accessible resolution of 0,17 nm [3]) or JEOL ARM 1?50 (double-tilting ± 40°
and point-to-point resolution of about 0,105 nm [4]1 are operating machines which are frequently used for studies of interfaces.
Hence, it is interesting to state upon the type of information HREM can provide on the structure of interface~,
Figure i~ a diagram showing which kind of atomic defects can occur at or near an
heterophase interface. Some feature~ are more appropriately within the scoop of HREM
imaging (for example, point ill compared to (6)), and they will be illustrated through
dedicated examples from the literature in section ?. Discussions concerning main aspects of HREM ~tudies of general interfaces can be found in recent review papers, e-g- [5-1Il.
1.3 METHODS FOR PREPARING HREM SAMPLES OF INTERFACES. The variou~ methods that
can be undertaken in order to prepare thin foils adequate for a detailed atomistic characteri- zation of interfaces by HREM are listed below (abreviations reported are that used in table I, which reviews ~ignificant works on metal-ceramic interfacesj
(I) iii .iitu Chemical Reactions (CR) (see review [55] this method has widely been used in the case of oxide systems, where either the oxide precipitates in a metallic matrix (internal
oxidation), or the metal precipitates in an oxide matrix (internal reduction).
N° 10 HREM FOR THE STUDY OF METAL-CERAMIC INTERFACES 1813
. . . . . . . . . . . . ~ ~ # . .
. . . . . . . . . . . . ~ ~ . . .
. . . . . . . . . . . . . .'. .
. . .~. . . . . . . . . 'l'°. .
. . . .... $ . . ~~ .
. . . . . . . , o o°o o o7ffb
Ii .~. .~.l Iii "b
~i(~o
o~o
o
~°o$mro~~lo o~jif~~i~o o~o o~o o~o o~
~o o~o o~o o~o o~ o~o o~o o~o o~o o~
~o o~o o~~o~o o~o o~o o~o o~o o~o o~
~o o~o o~o
oo
o~o o~~o~o o~o o~o o~
Fig, I. Schematic representation of a heterophase interface for example, a metal jtop)/ceramic (bottom) interface and structural features : I) ceramic terminating layer at the interface 2) new interfacial compound; 3) step and 4) associated atomic relaxations 5) misfit dislocation ; 6) point defects (impurities) 7) segregation of impurities at the interface.
(it) Solid-State Bonding jSSB): a « ceramic-metal-ceramic
» sandwich is hot-pressed,
which allows planar interfaces to be produced.
(iii) Deposition Techniques (DT) one of the constituants (generally, the metal is deposited (evaporation, MBE methods) on a substrate (generally, the ceramic) under controlled
atmosphere or ultra-high vacuum conditions,
(iv) Real Materials (RM): metal-matrix composites, reinforced with ceramic fibers,
particles or whiskers offer a great choice of interfaces that can be studied.
Methods (I) are easy to develop, and allow a fast thin foil preparation~ with a large density of observable interfaces. However, they do not alloy a correlation to be established with the interfacial mechanical properties, contrarily to other methods, such as (it), and to a less extend (iii). Method (iv) is probably the less efficient methods for obtaining good HREM results,
owing to the difficulty of preparing adequate thin foils, and the undesirable effect of impurities
that may precipitate or segregate at the interfaces (this latter drawback can also be encountered in method (it), but can be avoided in methods (I) and (iii)).
Because of the inherent differences between the metal and the ceramic physical properties,
the ion-beam thinning method is required in all cases for preparing the TEM samples. For cases (it) and (iii), cross-sectional samples have to be made, and specific approaches have thus been
developed in order to make this delicate preparation step more efficient [56, 57].
2. Selected examples.
2.I FORMATION OF AN INTERMEDIATE COMPOUND AT THE INTERFACE. When formatiorJ Of
an intermediate compound (such as a binary-oxide in metal/oxide-based ceramic ~ystems)
occurs at a large scale as a consequence of thermodynamical equilibrium conditions, usual
diffraction techniques (XRD, Conventional TEM) can efficiently be undertaken in order to
identify the structure of the new phase (see, for example, AIO~ formation at the
Pt/AI~O~ interface [58]) ; thus, reasonnabie assumptions can be made on the reaction
mechanism leading to its formation, and significant information can be obtained on the
physical and mechanical behavior of the bonding.
1814 JOURNAL DE PHYSIQUE iii N° lo
Table I. Summarized results about metallceiamic investigations conducted by TEM
technique.
SYSTEM METHODOI ORIENTATION RELEVANT Rcf
PREPARAT~ON RELAT~ONSHIP TEMand REMARKS
Thcrmal ox>dafion of to mctalloxidc >s
(till and (l10) Aim but many areas HREM [31(
mr orby anodizauon crystallized under the
toofcu-Al alloy Cubc/cube with HREM
(Gliccpl boundary planes. tmage Simul. '~Y" ~~ ~h~ 122'
~~~
[00011A12°3~/liiilfd
«EM (magnetite) parttclcs
(po~ csumatcd lower and HREM within the iron interlayer [231
than IO''9Pa) ~~~~A1203II(I1°)Fd
controlled
highorccntrollcd CTEM by the Threshold [38]
vacuum" [00llNbII[2i101A1203 ~'~'~~~ [271
OR imposed bystarting geometry.
to of Nb-Al alloy «EM [33j
~~ ~~~~ ~~ ~~ ~'~~3 m~g~~~nul. ~~~
MBE growthof Nb (0001)A1~o~ Misfit dislocations with a j13j
films distance (avenge distance 0 34 nm). j18j
at
IO of Pd-Al alloy "~~ ~~~~~
i#
state, but Al-termi~~ting layer
~~~~
tmage simul annealingm an Al vaprur.
SSB(prr-annealedPt ~~~ lntermetallic reaction zone of
foils m vacuum or m and
~~~ ~sition. [4r]
mr at 1200°C-2h) II(011)pt
DT onto (112) involving Al,
~$~~$rn~d~r~c~/ i~(~ ~~~~~' ~~~~~ ~~
MB E onto (cooI) )VII(coo1) A1~03 j49j
sapp~urr Al rrrrrunating planeatthe interface
CTEM
MBE onto I1021 102)A1203 ~~~~ Mtsfit dtslocattons b= a/2< I10> [30]
sapp~urr ltfi~8C S'tfiU'
IOof Ag-Cd alloy m with I I HREM (hexagonal network is expected [4II
w Image simul.
DT Onto 001 M ~ ~~~~~ ~~~
HREM
~~d DSC theory
MBE on heated «EM network of edge,type misfit
]001]Mgc HREM with Burgers [19]
a/2<100>
network of mtsfit dislocauons )I'
DT onto MgO smoke CBED a of j39]
HREM the [4o]
Image simul. small parfidcs.
IOof Cu,Mg alloy III Studyof dislocaUon [12]
CTEM with CSL/DSC lattice model. [NJ
HREM plane of MgO is [29]
JR of(Mg-Cu)O (obtained atom" p robe
single crystals joojlmgcII[I12]c~ ttUCt°S*PY). ~~'
N° 10 HREM FOR THE STUDY OF METAL-CERAMIC INTERFACES 1815
DT on Nacl (iran dtrstrrsarrembcrded
~~d CTEM [37]
'"'~~ ~'8~~
(001)MgOII(001)~ CBE°
property
dustcrsarr~ HREM ongmate partly from an [161
appears
SSB onto(ml)MgO (ill)Nill[lmlmgo mEM region with good fit. lnterdiffusion j~~j
HREM advanced m order to explain
lot Ii MgOIIIIIiiNb
~~fi~~ dislocations not located at II 3]
00)MgO
II l00lNb ~~~~~ ~'~~' IO ofPd,Mg alloy m
air. with I type inUdacc HREM
and twinned ortentatton.
UV [36]
trucn<ubts prrparcd m Cube,cube ortentauon [30]
~l' I )cuII II I)MrO m order tc
Cu,MnO IO of Cu,Mn single b~a) + HREM the ii ii
crysUls with rrspect to a[I II Image simul ~~°l
MnO
Cube,cube dtslocation
A g,Ni O EO ofAg,Ni films with strongly HREM I1 [33]
orientation prsmon
Eo of Au,Ni films strongly faceted HREM is about 3 or 3 3 [1iii Au [331
~~'~~~ ~~~~' ~ " °~ [14]
RHEED
~~~ ~~~~ alo~~ [421
films IIli0lNiII(mllNiO AES ~
~~~ ~~,~g p~i~ dislocations j~s~
Cry~tallme or (oolj ~ CTEM
j26]
is j~gj
HREM HREM Ill
[43]
h~t
werrimrnergcdmto #E~
1341
annealingat 7m°C. ~~~~' ~~~~
([[[)Tic CTEM Coherency strains found inside Al
HREM scrnc interfaces. 21
bcarn '~fIU~~C~
[43]
Cu implantation into HREM are elongated
normally to the common plane(Iii)
Notations :
CTEM : Conventional Transmission Electron Microscopy.
CBED : Convergent Beam Electron Diffraction.
HREM : High Resolution Electron Microscopy.
RHEED Reflection High Energy Electron Diffraction.
IO : Internal Oxidation. EO : External Oxidation. IR
: Internal Reduction.
MBE : Molecular Beam Epitaxy. DT : Deposition Technique. SSB Solid State Bonding.
MMC : Metallic Matrix Composite. XPS : X Photoemission Spectroscopy.
1816 JOURNAL DE PHYSIQUE Ill N° 10
HREM will be required to ascertain that there is no such an intermediate compound at the
atomic scale, I-e- a few atomic layers thick phase. Examples have been reported in the
literature of such « thin
» intermediate phases unknown structure at the Nilmgo interface
[3?], NiPt at the NiO/Pt interface [26], probably MgO micro-domains at the MgO/Nb interface [13]. However, the coupling of HREM image~ with nano-probe analysis has not been
performed in any of these case~, and the crystal structure of these intermediate compounds has
not been unambiguously solved yet this combined approach is certainly a challenge for a near future, which has now become po~sible with the achievement of Field Emission Gun
microscopes.
2.2 AccomoDATioN OF STEPS. Atomic step~ are most generally due to deviation of the
« macro~copic » surface plane of the ceramic material from an exact, frequently preset, crystallographic and low-index plane. The pre~ence of such atomic steps may induce features of significant importance for the metal-ceramic bonding.
In the case of steps of large height (I,e, facets), different orientation relationships (OR) may develop, owing to crystallographic anisotropy of the interfacial energy.
When a high density of steps is present, the consecutive misorientation can be accomodated
by distorsions in the metallic part: Fig. 2a) is a typical example showing a slight
misorientation from the exact jI10j~~fl(1120)~j~~~ OR, owing to the presence of regularly- spaced ~teps (arrows) pre-existing at the ceramic surface. This finding clearly evidences the
« strength » of the preferred OR which takes place in this case, despite the non-favorable geometry imposed by the misoriented ceramic ~urface.
It may also occur that steps are required for the realization of the interface during in situ
chemical reactions, in order to minimize the lattice mismatch at the interface (for example, Au/MgO [28], Cu/MnO [?0]) in such case;, the steps are correlated to misfit dislocations, as
schematically drawn in Fig. ?b).
2.3 INTERFACIAL DISLOCATIONS.
2.3.I Misfit dislocation.<. A large number of HREM works on metal-ceramic interfaces have been devoted to the analysis of mi~fit dislocations jMDj. These defects accomodate mismatches between the metal and ceramic lattices, Periodic arrays of MD may form, which
are separated by a distance D given by the relation (see Fig. 3a) D
=
'j' 11)
where )b) is the modulus of the Burgers vector of the dislocations, and 6 the lattice misfit
along the direction perpendicular to the dislocation lines (see Fig. 3b) :
2 jij i~'j
~
~ Iii + i~) ~~~
with
I, d~
= (3)
(I, projection of the lattice distance
d~ on the interface plane d, d-spacings for compound ii = 1, 2), corresponding to the most close-packed planes joining at the interface),
Generally speaking, the dislocation structures can be characterized by Conventional TEM, However, the lattice mismatch can easily be a~ large as 10 % in many metal-ceramic systems,
N° lo HREM FOR THE STUDY OF METAL-CERAMIC INTERFACES 1817
a)
A1203 [0001
S
. -~
F j j
j
/ j
/ j
j step /
/
j~ - - .
/
' (Burgers
' circuit)
b) j
~-
Fig. 2. Steps at metal/ceramic interfaces. a) a-Fe/Aljoj interface : the high density of ~teps (arrows) lead to a « bulk
» misorientation of the metallic crystal ~ee enlarged detail (HREM. Jeol 4000EX).
b) Diagram showing a mi~fit di~location associated with a step.
1818 JOURNAL DE PHYSIQUE III N° lo
a) ~ b)
D "1(~
b
~-
[iioj
b = lfi Sol
[110] 2
[l12j
b = ~
b
=
)(l10J
. .
C) b ~
Jill 0J
- O-lattice cell
-
h
e)
Fig.
b) attice at the c) Square
grid of MD in the case of two
cube >> orientation ; j001) plane. d) exagonalnetwork of MD (same case as c),
but interfacial plane). ) Position of the core of a in the crystalline region of
match,
thus leading to very closely-spaced dislocations (if any,,,) ; then, observations at high resolution may be demanded for identifying unambiguously the presence of such defects, In the case of the joining of fcc metals le, g. Ag, Al, Au, Cu, Pd, Pt) to ceramics having a Na-Cl-
N° lo HREM FOR THE STUDY OF METAL-CERAMIC INTERFACES 1819
type structure (CdO, MgO, NiO), it has been shown by many authors (see corresponding
studies in Tab. I) that a « cube-cube » OR occurs preferentially. If the boundary plane is of the
(001)-type, the network is a square grid (as shown in Fig. 3c), and the misfit dislocations can
adequatly seen edge-on along a (100) viewing direction parallel to the (001) interface plane.
For ill I )-type boundary planes, an hexagonal network forms, with dislocations lying along (112) directions (see Fig. 3d) this latter case is more complex, since inclined dislocations
segments will be seen in any possible azimuth parallel to the interface plane: thus, superimposition effects will blur the usual « dot » contrast of atomic columns, and only periodic features in the experimental images will guide the investigator to conclude to the
presence of dislocation arrays (see, for example, Fig. 4).
Fig. 4. High-resolution electron micrograph showing the Cu/MgO li j interface as viewed along
the common [110] azimuth (courtesy F. Cosandey see II 2]). Note the characteristic « white-black »
contrast oscillation along the [l12] direction, due to misfit didocations.
In fact, the clear identification of misfit dislocations along semi-coherent interfaces is
directly depending upon the magnitude of the mismatch parameter 6 (Eq. (i )). According to the O-lattice theory, interfacial regions of good atomic match, centered on the O-lattice points,
are separated by regions of poor atomic match, where well-localized dislocations are expected
to lie in systems for which the lattice mismatch is small (see Fig. 3e). When 6 becomes large (nominally, m lo %), interfaces are expected to be incoherent, owing to the prohibitive
increase in interfacial energy that would be produced by an increase of the number of misfit dislocations. Table II lists relevant information about the misfit parameter 6 and the coherency
state for a selection of metal-ceramic interfaces.
