Methods for characterization of coating microstructures

Thin Solid Films, 73 (1980) 331-345 © ElsevierSequoiaS.A.,Lausanne--Printedin the Netherlands

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METHODS FOR CHARACTERIZATION OF COATING MICROSTRUCTURES* BIRGITE. JACOBSON Department of Physics and Measurement Technology, Link~ping University, S-581 83 Link~ping (Sweden)

(ReceivedApril21, 1980;acceptedApril25, 1980)

The use of deposition methods to produce both overlay coatings and freestanding sheets or pieces of metals offers the possibility of producing materials with unique properties not previously obtained by conventional materials processing. These properties are closely related to the internal microstructural features of the deposited film and can be varied over wide ranges by the selection of appropriate deposition parameters and film composition. Coatings with extremely fine grain size or amorphous structures can thus be produced as well as coatings with unusual distributions of voids and cavities or with dispersions of particles of non-equilibrium phase compositions. It is thus important in the evaluation of the coating properties to take these structural features into consideration in an effort to optimize the result of the deposition process. A variety of analysing tools exist for these structural investigations, each exhibiting advantages and limitations. The purpose of the present paper is to discuss some of the methods that have proved to be useful in the course of the development of coatings.

1. PROCESS--STRUCTURE--PROPERTIES--MODIFIEDPROCESS A close coupling of synthesis and characterization efforts with the investigation of mechanical, physical and chemical properties is highly desirable in the field of coating technology. The ideas which flow around this closed loop of integrated research, involving characterization of structure, investigation of properties and modification of the deposition process, generate a stimulating environment for creative development of tailor-made surface coatings for specific applications (Fig. 1). Until now, more research has been done along these lines in thin film technology than in thick film technology, and the most obvious lack of knowledge includes the understanding of the coating microstructure and its relation to both process parameters and properties. Thick deposited films exhibit characteristic structureproperty relationships both in the transition zone between the substrate and the deposit and in the deposit itself; the former are closely related to the substrate structure and composition, whereas the latter could be considered as the bulk properties of the coating itself. *Paper presentedat the InternationalConferenceon MetallurgicalCoatings,San Diego,California, U.S.A., April21-25, 1980.

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MODIFICATION OF PROCESS

CHARACTERIZATION OF MICROSTRUCTURE

OF PROPERTIES

Fig. 1. The closed loop of integrated research in coating technology.

Microstructures of both transition zones and deposited overlayers are in many ways different from those of conventionally processed materials, and they can be varied over much wider ranges of composition and defect distributions than in conventional materials. The three probably most important features are as follows: the possibility of producing fine grain sizes or amorphous structures; the presence of porosities and voids in a variety of size distributions and configurations; the possibility of producing non-equilibrium phases and non-conventional phase combinations. These unique microstructural features hold the promise of the development of surface coatings with unique and optimized properties if properly understood; more research is needed to this end. It is the object of this paper to discuss some important methods which are nowadays readily available and which allow the detailed characterization required for these kinds of materials. The methods are discussed with regard to selected aspects of the characterization process and are illustrated by means of various examples of coating structures and applications. 2. MICROSTRUCTURE, CRYSTALLOGRAPHY AND ELEMENTAL ANALYSIS The structural characterization of deposited films should include the following: macroscopic and microscopic investigations of grain morphology and defect distributions; crystallographic examination of phase compositions and grain orientation relationships; internal stress distributions; chemical analysis of elemental overall composition and elemental segregations, on both a long-range scale (over the thickness of the coating) and a short-range scale (within the individual grains) (Table I). TABLE I METHODS USED FOR THE CHARACTERIZATION OF DEPOSITED FILMS

Microstructure

Crystallography

Elemental analysis

Optical microscopy SEM TEM

X-ray diffraction Electron diffraction

Optical spectroscopy X-ray spectroscopy Electron spectroscopy

Deposited films are in most cases studied by conventional metallographic methods (optical microscopy) and scanning electron microscopy (SEM) to reveal

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their surface topography and grain morphology. However, many important features of the microstructure can only be observed at higher spatial resolution, for which transmission electron microscopy (TEM) is used. The most powerful features of TEM are its high spatial resolution capability (at the atomic level) and the variety of operational modes that can be used for structural analysis. Both of these features are particularly crucial in the case of fine crystalline structures with high imperfection densities, as in most deposited materials. With the various modes of imaging, contributions to the contrast from each individual object can be isolated and studied in detail. X-ray diffraction measurements of lattice d spacing for crystallographic phase characterization is more accurate than electron diffraction and is performed over considerably larger areas and volumes of the specimen than electron diffraction analysis 1. The two techniques are therefore complementary and should be combined for a more complete structural characterization. X-ray diffraction should also be used for a complete quantitative texture analysis. The sample is then placed in a texture goniometer and the result is projected on a stereogram which shows the preferred orientations of a selected set of planes (normally the close-packed planes) in relation to the film surface and a specific direction along the surface, e.g. a direction that is related to the geometry of the deposition process set-up. Chemical analysis can be performed using a variety of methods. An averaging of the composition over large volumes is obtained by wet chemical analysis, optical spectroscopy or X-ray fluorescence analysis. Analysis of smaller volumes, in the micron size range, can be obtained by electron microprobe analysis, using wavelength-dispersive X-ray analysis, or by SEM, using energy-dispersive X-ray analysis. Chemical analysis over large specimen surface areas (of the order of a few square millimetres) and extremely thin depth (in the range of some hngstr6ms up to tens of hngstr6ms) is obtained in cases where excited electrons are used for the elemental analysis, as in electron spectroscopy for chemical analysis (ESCA) and UV photoelectron spectroscopy. This is also the case for scanning Auger microscopy which exhibits the same depth resolution and a greatly improved spatial resolution (in the micrometre range) in surface area analysed. Both Auger analysis and ESCA are of great interest when oxides, carbides and nitrides are important components of the coating, because oxygen, carbon and nitrogen, which are present in various chemical environments, can be distinguished by these methods. Chemical analysis using transmission electron microscopy-scanning transmission electron microscopy (TEM-STEM) (as will be discussed below) yields results that cannot be achieved with any other technique. If we wish to determine the variations of the average composition in one direction only, e.g. the profile over a substrate-film interface, similar results could be obtained by sputter-etch depth profiling and simultaneous Auger analysis. However, the sputter-etching procedure can produce a change in composition by preferentially etching multiphase structures. This subject has been reviewed previously 2 and is not discussed further here. The general trend in the development of analysing tools for structural characterization is the combination of several methods in one and the same instrument. The goal for the development of the TEM-STEM instrument is a good example (Table II). Besides the capability of real-space image formation, modern TEM-STEM instruments possess the additional capability of electron diffraction

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pattern formation and chemical X-ray analysis from small selected volumes (30-50 A probe diameter) whereas other methods give only average compositional information. This focused electron probe can also be scanned over the surface to obtain the elemental line profile distribution or elemental surface mapping, i.e. microprobe analysis at an extreme spatial resolution level; the thin TEM specimen foil limits the lateral spread of the analysed X-ray volume. Light elements are analysed by an electron energy loss detector. Furthermore, a secondary electron detector located above the specimen can be used for surface topography studies; i.e. allowing studies to be done in the SEM mode with resolutions superior to those obtainable in conventional SEM instruments. Furthermore, all these analysing tools can be combined with dynamic experiments; in particular, heating and cooling experiments are useful in the study of phase transformations in deposited materials. Another important feature of the TEM-STEM instrument is, of course, the possibility of direct correlation between composition, crystallography and structure. This is especially important in the case of deposited materials, in which complex fine-grained polyphase structures are typical. TABLE II ANALYSING MODES AVAILABLEWITH A TEM -STEM INSTRUMENT

Microstructure

Diffraction

Bright field

Selected area

Dark field

Focused aperture

Out-of-focus

Focused probe

Chemical analysis " Energydispersive X-ray analysis Electron energy loss spectroscopy

Surface topography

Dynamic experiments

Secondary electron detector (SEM mode)

Heating, cooling, tensile testing

The following examples are therefore selected mainly from the TEM-STEM characterization method to encourage more work to be done with this instrument. The first example is taken from a study of electron-beam-evaporated Fe-Cr-A1-Y coatings for high temperature applications. The following examples are all from electron-beam-evaporated films of A15 compounds for superconducting applications. For more detailed information about the analytical methods discussed, the reader is referred to ref. 3. 3. SEM--TEM CHARACTERIZATION OF

Fe-Cr-A1-Y

COATINGS

For a specific application, Fe-25Cr-6Al-Y coatings were electron beam evaporated onto Udimet 500 turbine blade substrates; they were tested in fluidized bed environments and showed good results. It was therefore of interest to characterize their microsctructure in the as-deposited and tested conditions to learn more about the mechanisms involved in the coating performance during testing. This work is in progress and will be reported elsewhere. However, there are two approaches made in this work that are within the scope of the present paper. The first is the sample preparation method (ion beam milling) and the second is the direct

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correlation between SEM and TEM analyses which is made possible by the preparation method. Ion beam milling has proved to be a useful method for TEM thin foil preparation of complex structures of metals, ceramics, polymers and composites, where conventional electropolishing methods fail. An ionized argon beam bombards the central area of a rotating disc sample of diameter 3 mm from each side. The incident argon beam angle, voltage and current are determined by the sample material. Typical values for harder materials are 15-30 °, 4-6 kV and 25-100 I~A respectively. The higher values are used initially, when high rates are important; the lower values are used after penetration, when the radiation-damaged surface is to be removed as much as possible. The thinning rate is slow: typically 1 week per sample is needed. However, this slow rate allows smooth damage-free sample surfaces to be prepared 4 (Fig. 2(b)) and also permits detailed tailoring of the sample. The thin area can be located at the top surface, in the middle of the film or at the film-substrate interface. Alternatively, a cross section of the film can be prepared and studied. The latter method was used for the Fe-Cr-A1-Y study (Figs. 3-6).

3mm

Q Fig. 2. (a) Various geometrical configurations of T E M thin foils prepared by ion beam milling. The shaded areas indicate the substrate material and the unshaded areas the deposited material. (b) T E M micrograph of an evenly thinned foil of an Nb3Sn deposit 3 ~tm thick on a Hastelloy substrate.

An SEM study of a sample that was purposely slightly ion beam etched (Fig. 3(a)) reveals nicely the grain morphology and porosity structure of the coating. The layered structure (10 ~tm in thickness), which was obtained by rotation of the turbine blade during deposition, is clearly visible as well as each individual grain exhibiting the same diameter of 10 ~tm. It is interesting to note that the porosities are columnar and extend over several rotational layers of the film. The transition zone of 20 ~tm between the substrate and the film exhibits a fine-grained and less-ordered structure; the substrate surface was apparently mechanically cleaned before deposition and

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therefore also has a fine-grained structure next to the interface. A c h r o m i u m line profile a l o n g the straight line i n d i c a t e d (Fig. 3(b)) illustrates the high sensitivity of the detected X - r a y signal t o the t o p o g r a p h y of the s u r f a c e - - t h e crystal wavelengthdispersive d e t e c t o r is l o c a t e d to the left of the i m a g e - - a n d shows the care t h a t has to be t a k e n when q u a n t i t a t i v e chemical surface analysis is c o m b i n e d with ion b e a m etching a n d d e p t h profiling; the surface is in this case c o m p a r a t i v e l y s m o o t h for this k i n d of surface treatment.

Fig. 3. SEM micrographs of a cross section of an Fe-25Cr-6A1 Y deposit 120 gm thick on a US00 substrate. The surface is ion beam etched to reveal the grain and pore structure. (a) The top surface of the deposit is labelled A and the transition zone to the substrate C. The substrate is seen on the far right. (b) Etching is faster in the substrate than in the deposit and causes penetration along the interface. A chromium line "'composition" profile along the straight line indicated shows the surface sensitivity of the X-ray intensity rather than compositional variations. The X-ray crystal detector is located at the lower left. The c h a r a c t e r i z a t i o n of the F e - C r - A 1 - Y c o a t i n g m i c r o s t r u c t u r e was n o t c o m p l e t e d , however, with these studies; a further investigation was n e e d e d at a higher r e s o l u t i o n to reveal the internal structure of the grains. Thus, the same s a m p l e as discussed a b o v e was also investigated b y T E M . F i g u r e s 4 - 6 illustrate the typical structure of, respectively, the t o p layer, the i n t e r m e d i a t e b u l k of the coating, a n d the

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transition zone, as indicated by A, B and C in Fig. 3(a). A direct correlation between specific grains in the SEM study and the TEM study could in fact be made (not shown here). Conventional bright field (BF), dark field (DF) and selected area diffraction (SAD) imaging were used to study second-phase particles exhibiting a weak BF contrast but a strong DF contrast because of the close orientation relationship between the particles and matrix. All particles have the same orientation within one matrix grain and therefore contribute to the same DF image. This indicates that the particles had nucleated and grown in the solid state after condensation of the film. The SAD patterns are difficult to evaluate in detail because of the close relationship between the superimposed patterns and because of the possibilities of double diffraction with such a large sample volume contributing to the SAD pattern. An analytical TEM-STEM instrument which allows microdiffraction analysis is needed for definitive analysis.

Fig. 4. (a) TEM micrograph of the top surface structure of the Fe-Cr-AI-Y coating, labelled A in Fig. 3(a). (b) The corresponding SAD pattern.

The top layer of the Fe-Cr-A1-Y coating consists of ferritic 0c-Fe with a grain diameter of about 10 ~tm interspaced with a homogeneous distribution of particles of tetragonal cr-Fe-Cr 700-800 • in diameter and a relatively high density of dislocations tangled around the particles; the dislocations were probably generated during the peening process after deposition as they are not observed in the lower part of the coating (Fig. 4). The bulk of the coating is also ferritic and contains particles with three characteristic diameter distributions, namely 800 A, 200-300 A and about 50 A (Fig. 5). The larger particles area-Fe-Cr nucleated on the mediumsize particles; the BF contrast (Fig. 5(a)) reveals a second phase as black dots at the centre of the o phase particles. The transition zone is austenitic and exhibits a dense distribution of large ~ phase particles 2000-3000 A in diameter with a fine distribution of particles about 100 A in diameter in between (Fig. 6). The results are consistent: the austenite is stabilized in the transition zone by the presence of nickel diffused into the film from the U500 substrate, with the remaining ferrite probably

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Fig. 5. TEM micrographs of the bulk structure of the F~Cr A1-Ycoating labelled B in Fig. 3(a): (a) BF image; (b) the corresponding SAD pattern with the indexed spots generated from ferrite and the circled spots from the a phase; (c) DF image obtained from one of the circled spots in the SAD pattern.

transformed into the o phase; the bulk of the coating is ferritic because of the absence of nickel. The deposit was prepared in a temperature range where the a phase is likely to form, i.e. at 850--875 °C. The a phase is normally undesirable because of its hardness and brittleness. However, a phase particles are considerably m o r e coarse grained in conventional materials than in this case. This fine distribution of particles will p r o b a b l y contribute to the strength of the F e - C r - A 1 - Y coatings by dispersion strengthening yet still allow some ductility in the matrix phase. This stkucture is without d o u b t w o r t h y of more detailed analysis by T E M - S T E M .

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Fig. 6. TEM micrograph of the transition zone structure labelled C in Fig. 3(a): (a) D F image obtained from the circled spot in the SAD pattern shown in (b).

4. MICROANALYSIS BY TEM--STEM The TEM image is superior to the STEM image in quality. This is also true of the penetration when a tungsten filament is used and the material is crystalline and not amorphous. Thus, the STEM image as such is mainly considered to be an auxiliary mode for the direction of the electron probe onto a small volume of the structure selected for diffraction and chemical analysis. The usefulness of this technique will be demonstrated in the following. 4.1. Diffraction There are several ways to perform electron diffraction analysis in a TEMSTEM instrument, three of which are most useful and should be combiJaed to give complete information about the crystallography of the coating, i.e. to identify crystallographic phases present and their relative orientations. Before the introduction of the TEM-STEM microscope, the lens configuration was such that spatial aberration of the objective lens prevented diffraction patterns of an area less than 5000 A from being obtained. A selected-area aperture was therefore introduced to limit the area of the sample illuminated by the incident beam and thus contributing to the diffraction pattern. For many coatings the grain size of the matrix and second phases is much finer than this analysed volume, and the SAD pattern obtained is a ring pattern of superimposed reflections from all included crystals, the relative intensity and rotational symmetry of the rings:indicating preferred orientations in the structure, i.e. texture (Figs. 7(b), 1l(b) and 1l(d)). The possibility of obtaining microdiffraction from smaller areas is therefore extremely useful. This can be done in two ways: with techniques using a focused,condenser aperture or a focused probe. The first method is a quick and effective way to produce microdiffraction patterns from areas small enough to give single-crystal patterns from most deposited structures, i.e. it uses a beam that can be continuously varied

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Fig. 7. Microdiffraction analysis by the focused-aperture method. The beam size can be varied continuously down to a diameter of 300 A. The condenser aperture was 40 lam in this case. (a) BF image showing the actual beam sizeand thus the specimen volume contributing to the diffraction pattern shown in (b). (c) DF image obtained from one of the single-crystal diffraction pattern reflections shown in (d). d o w n to a diameter of 300 A, as illustrated in Fig. 7. The second condenser aperture is imaged on the specimen plane by the upper objective lens and thus only a small portion of the sample is illuminated and contributes to the pattern. Figs. 7(d) and 7(c) show a single-crystal pattern and the corresponding dark field image of the diffracting grain. The focused-probe technique (micro-microdiffraction) is used when a still higher resolution is needed (down to 50 A). F o r this purpose the S T E M unit is needed. The two condenser lenses and the upper objective lens act as a triple condenser system to produce a highly convergent probe that illuminates a small portion of the sample only. This probe is scanned across the sample by deflection coils and a fraction of the transmitted electrons is collected to form an image (the

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STEM image) on a cathode ray tube while the diffraction pattern itself is formed as before on the objective focal plane and can be recorded with an ordinary plate camera. The selected volume for so-called micro-microdiffraction analysis has to be chosen on the cathode ray tube screen by manual deflection of the probe to the actual spot, and this can be tedious because of the poor quality of the STEM image if a specific particle less than 100 A in size is of interest. Figure 8 illustrates a nice example of this method where individual twins in a small grain are identified in the micro-microdiffraction pattern.

J Fig. 8. Micro-microdiffraction analysis by the focused-probe method. The composition is NbSi and the twinned grain shown in (a) was crystallized during in situ annealing experiments of the amorphous structure shown in Fig. 9 prior to annealing. (b) The corresponding diffraction pattern.

4.2. Elemental microanalysis Ultrahigh resolution TEM analysis is greatly enhanced by the capacity to obtain direct chemical composition information at a comparable spatial resolution and thereby to place this chemical information in the context of the microstructure of the specimen. The same probing methods can be used as for generating electron diffraction patterns. The desired elemental information is carried either in the secondary emission of X-rays, which are analysed by an energy-dispersive (ED) detector located above the specimen, or in the transmitted electron energy loss spectrum (EELS), which reflects the primary excitations and which is analysed by an energy loss detector placed below the specimen. The ED detector can only be used in the analysis of elements of atomic number Z > 11 and the EELS detector is used for light element analysis.5 An example of this capability is given in Fig. 9 which shows an electron-beamevaporated superconducting NbSi film doped with oxygen to stabilize the amorphous structure. Only the amorphous phase is detectable with X-ray diffraction analysis whereas TEM analysis reveals a crystalline phase (black and white dots) of grain diameter 30-50 A dispersed in the amorphous phase (greyish background). The dark field image (Fig. 9(b)) demonstrates the crystalline character

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of the particles. The overall composition was measured without the STEM unit (Fig. 9(c)). The segregation of silicon into the 50 A crystalline particles was revealed by X-ray microanalysis of both phases (Fig. 9(d)). The electron probe size was in this case 40 A.

Nb

AMORPHOUS I I STRUCTURE t t

~ CRYSTALLITE / ~ 50 k DIAMETER

Fig. 9. X-raychemicalmicroanalysisof crystallitesof size 50 A (black and white) in an amorphous NbSi structure (grey background). (a) BF image. (b) DF image, revealing the crystallites as white dots. (c) Overall composition spectrum obtained without the STEM unit. (d) Focused-probe analysis of one selected crystalliteand the adjacent amorphous structure (beam size,40 A).

4.3. Characterization oj microcavities by throughoJocus imaging Deposited films often exhibit porosity (i.e. cavities or voids distributed in a variety of ways). These defects are extremely important for the properties of the films and must be included in the structural characterization. Large pores are easily observed by SEM (Fig. 3(a)). Small voids are sometimes hard to observe even by T E M and, if observed, are hard to distinguish from the contrast of second-phase particles present. For this purpose through-focus-imaging techniques are of great value. Small voids and cavities do not contribute significantly to the contrast when the image is in focus; in the under- and over-focused conditions, however, they change in contrast and, in the first case, appear as white dots with black rims and, in the second case, as black dots with white rims (Fig. 10); second-phase particles do

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not change in contrast under these conditions. The voids shown in this example are defects in a fully amorphous structure of NbSi with a size and distribution density, however, very similar to those of the crystaUites in Fig. 9(a) and 9(b). The distinction between these two structures is important to the superconducting properties of the NbSi film and was possible only by the use of the through-focus-imaging technique. Similar voids are often observed in deposited structures, both crystalline and amorphous, when the substrate temperature has been kept low or moderate.

I

4~

I

~ z _~-.~ i

.

.

.

;

.

.

.

.

.

.

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Fig. 10. Through-focus imaging of microvoids in a fully amorphous NbSi structure. (a) Contrast obtained in overfocused condition. (b) Corresponding contrast in underfocused condition and the diffuse halo obtained in the diffraction mode (RT indicates room temperature observation).

4.4. Dynamic studies by T E M - S T E M The TEM instrument allows dynamic experiments to be performed in situ in the microscope with the structural changes 6 resulting from heating, cooling or tensile deformation of the specimen being simultaneously observed and recorded. The temperature may be varied over the whole range from liquid-nitrogen temperature up to 1000 °C. These annealing experiments have a limited value for coinventional coarse-grained materials because of the presence of the thin foil surfaces preventing grain growth and enhancing dislocation annihilation. This surface influence is less dominant, however, when the grain size is only a small fraction of the foil thickness, as is often the case in deposited materials. The crystallization behaviour of amorphous structures could, for instance, be studied at the elevated temperature during transformation with all the analysing tools discussed above. An example is given in Fig. 11, which shows the annealing of the structure in Fig. 10; the sample was heat treated for 1 h at 700 °C and then for 1 h at 750 °C. First the voids are annealed out and this is followed by the nucleation and growth of the crystalline phase. 5. SUMMARIZING REMARKS AND RECOMMENDATIONS

A close coupling of synthesis, microstructure characterization and investigation of properties is highly desirable in the field of coating technology. More research is needed on microstructural features in relationship to both deposition

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process parameters and the coating properties, especially because deposited structures are in m a n y ways unique and possess properties that have not previously been obtainable by conventional materials processing.

Fig. 11. A dynamical annealing experiment performed by TEM on the same structure as shown in Fig. 10. (a) BF image exposed at an elevated temperature showing the disappearance of the voids after annealing for 1 h at 700 °C. (b) The correspondingSAD pattern indicating the beginningof ordering of the structure. (c) The same area after additional annealing for 1h at 750 °C. (d) The corresponding SAD pattern.

Characterization of coating microstructures should include investigation of grain and defect structure, crystallographic examination of phase compositions, chemical analysis of element distributions and characterization of internal stress distributions. A variety of analysing tools is readily available for these studies, and these should be employed in combination to give a systematic macroscopic and microscopic view of the coating structure. The modern analytical T E M - S T E M instrument as well as ESCA and Auger analysis should be used more frequently. Ion beam milling is a versatile method of sample preparation, both for preparing thin T E M foils of complex materials and for etching surfaces for SEM investigation of macrostructure. However, caution should be exercised when using the method for depth profiling in combination with surface-sensitive analysing methods for quantitative analysis.

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ACKNOWLEDGMENTS The a u t h o r w o u l d like to t h a n k Drs. R. D. F e l d m a n a n d R. H. H a m m o n d at H a n s e n L a b o r a t o r i e s , S t a n f o r d University, U.S.A., a n d Dr. S. A. Jansson, StalL a v a l T u r b i n e Co., Sweden, for s u p p l y i n g c o a t i n g materials. The following T E M facilities were k i n d l y m a d e available for the s t r u c t u r a l c h a r a c t e r i z a t i o n s : Xerox P a l o A l t o Research Center, U.S.A., Center for M a t e r i a l s Research at Stanford University, U.S.A., Jeol A p p l i c a t i o n Centre, L o n d o n , Gt. Britain, a n d H u d d i n g e H o s p i t a l , S t o c k h o l m , Sweden. REFERENCES 1 T.W. Barbee, Jr., B. E. Jacobson and D. L. Keith, Thin Solid Films, 63 (1979) 143. 2 J.W. Coburn, Thin Solid Films, 64 (1979) 371. 3 J.J. Goldstein, J. J. Hren and D. C. Joy (eds.), Introduction to Analytical Electron Microscopy, Plenum, New York, 1979. J. W. Edington, Practical Electron Microscopy in Materials Science, Van Nostrand-Reinhold, New York, 1976. J. A. Chandler, X-ray Microanalysis in the Electron Microscope, North-Holland, New York, 1976. 4 B.E. Jacobson, R. H. Hammond, T. H. Geballe and J. R. Salem, Thin Solid Films, 54 (1978) 243. 5 R. Sinclair and B. E. Jacobson, in 9th Int. Congr. on Electron Microscopy, August 1-9, 1978, Toronto, Canada, Microscopical Society of Canada, Toronto, 1978, pp. 510-511. 6 B.E. Jacobson, R. F. Bunshah and R. Nimmagadda, Thin Solid Films, 63 (1979) 357.