ELECTRON MICROSCOPY

3.5.1 Scanning Electron Microscopy (SEM)

Scanning electron microscopy was undertaken with a Cambridge Stereoscan 250 and a Jeol 6100 microscope. These were used to investigate the phase distributions within ceramic composites and for the observation of topographical features. Some chemical analysis was carried out using the X-ray detection facilities housed within the Cambridge microscope.

(i) SEM Specimen Preparation

Ceramic specimens were prepared for SEM examination by grinding one surface flat, using a coarse (70 pm) diamond-impregnated pad, then carefully polishing it to remove all scratches and evidence of grinding damage which might detract from

observations of the features under investigation. The specimens were often first

embedded in a conducting bakelite mould to ease handling. Since the materials

fabricated during this research were of particularly high wear resistance, the usual silicon carbide grinding pads were not used for any stage of the grinding or polishing operations, but successively finer grades of diamond-containing slurries (oil or water based) were used with a cast iron lapping plate that was covered with a nylon cloth for

use with the finer grades (from 3 to V4 pm). Particulate specimens, such as loose silicon

carbide platelets, were cemented onto an aluminium stub with a conducting, carbon paste. Finally, the specimens were cleaned in acetone and then coated with a thin film of either carbon or gold (by evaporative deposition) to provide a conducting path on the specimen surface.

(ii) SEM Operational Procedures

Topographical features were imaged by the detection of secondary electrons ejected from the specimen surface, whilst compositional information was obtained from

3.5 ELECTRON MICROSCOPY

3.5.1 Scanning Electron Microscopy (SEM)

Scanning electron microscopy was undertaken with a Cambridge Stereoscan 250 and a Jeol 6100 microscope. These were used to investigate the phase distributions within ceramic composites and for the observation of topographical features. Some chemical analysis was carried out using the X-ray detection facilities housed within the Cambridge microscope.

(i) SEM Specimen Preparation

Ceramic specimens were prepared for SEM examination by grinding one surface flat, using a coarse (70 pm) diamond-impregnated pad, then carefully polishing it to remove all scratches and evidence of grinding damage which might detract from

observations of the features under investigation. The specimens were often first

embedded in a conducting bakelite mould to ease handling. Since the materials

fabricated during this research were of particularly high wear resistance, the usual silicon carbide grinding pads were not used for any stage of the grinding or polishing operations, but successively finer grades of diamond-containing slurries (oil or water based) were used with a cast iron lapping plate that was covered with a nylon cloth for

use with the finer grades (from 3 to V4 pm). Particulate specimens, such as loose silicon

carbide platelets, were cemented onto an aluminium stub with a conducting, carbon paste. Finally, the specimens were cleaned in acetone and then coated with a thin film of either carbon or gold (by evaporative deposition) to provide a conducting path on the specimen surface.

(ii) SEM Operational Procedures

Topographical features were imaged by the detection of secondary electrons ejected from the specimen surface, whilst compositional information was obtained from

electrons back-scattered from the specimen, which depend strongly on the atomic density, allowing different ceramic phases to be distinguished by different contrast

levels in the resulting image. However, the images obtained from back-scattered

electrons are more limited in resolution than those due to secondary electrons which are

associated with a smaller interaction volume. T o differentiate between the Si3N4 and

SiC phases (similar atomic number contrast), it was necessary to reduce the distance

between the specimen and back-scattered detector to about 1 2 mm (for increased

detection efficiency) and use an electron accelerating voltage of at least 12 kV (to increase the strength of the back-scattered signal). The back-scattered detector was a solid state device and was divided into four quadrants, each receiving independent

signals which were combined for maximum atomic contrast. Topographical

information could also be obtained by reversing the polarities of signals received from opposite sides of the detector. This had the advantage of suppressing other sources of contrast and was found to be a useful way of imaging surface impressions, revealing more clearly defined impression boundaries than the secondary electron image.

Energy-dispersive X-ray analysis (EDAX) was performed on the X-rays ejected from the specimen by the electron beam. The specimens were brought within a 'working distance' of 15 mm (from the lens aperture) in order to allow efficient X-ray detection. Incoming X-rays produced a current in the detector that was amplified and stored in a multichannel analyser for rapid analysis using 'LINK' analytical software that stored the values of characteristic X-ray energies for the elements. Detection of the

'light' elements (those with atomic numbers below 1 1) required the removal of the

beryllium window protecting the detector from contamination. This was necessary to prevent the absorption of the softer X-rays emitted from the 'light' elements.

A digital X-ray mapping programme was also used, in order to acquire X-rays from selected elements simultaneously. As the beam was slowly scanned over the specimen, the data acquired was displayed on a grey level scale. However, the images obtained by this technique had lower resolution than the electron images.

3.5.2 Transmission Electron Microscopy (TEM)

A 200 kV, JEOL 2000FX microscope was used for higher resolution electron microscopy studies, enabling investigation of the individual ceramic grains and interfaces together with electron diffraction and elemental analysis, taking advantage of the better spatial resolution over that obtained in the SEM.

(i) TEM Specimen Preparation

Preparation of TEM specimens was more complicated than for SEM specimen

preparation. A thin ceramic slice (a few hundred pm) was diamond-cut from the bulk

material and ground to a thickness of about 100 Mm with a coarse (70 pm) diamond-

impregnated wheel and then polished on one side (using diamond slurries with

successively finer diamond particles, down to '/4 pm) to a much finer surface finish.

The ceramic slice was mounted for 'dimpling' (using a South Bay Technology

'dimpler'), to further reduce the thickness of a small, central area to about 40 pm, using a

brass wheel covered in a diamond impregnated (6 pm grains) paste. The dimpled area

was finally polished with a V4 pm diamond paste and finally, a brass supporting ring

(3mm diameter) was affixed around this area with araldite resin. The specimen was eventually reduced to electron transparency by bombardment with argon ions (accelerated at 5 kV) whilst being rotated to ensure uniform thinning. The 'ion beam thinning' continued until a visible hole had developed in the specimen centre, around

which could be found a region (< 1 pm in thickness) of electron transparency.

(ii) TEM Operational Procedures

High resolution images of the ceramic grains were obtained using the 'bright field' imaging mode, in which the image is formed from transmission of the direct electron beam (strongly scattered electrons are obstructed by the objective aperture) and consists of diffraction! phase and thickness contrast.

orientations within a selected area which was defined by inserting an aperture in the

electron beam. To minimise diffraction contributions from regions outside that

selected, the objective lens was focussed as accurately as possible on the image. The specimen could be tilted, in two perpendicular planes, to vary its orientation relative to the electron beam. Crystal lattice spacings were measured from the negatives for the diffraction patterns, using the relationship:

d= X L /r (3.1)

where d = the crystal lattice spacing, (XL) = the 'camera constant' (X = the electron wavelength), and r = the radial distance of a diffraction spot from the directly transmitted beam (measured). The camera constant was determined (at preset camera length (L) conditions) from the objective lens current, to accommodate fine adjustments in the focussing position.

The dark field imaging technique (in which the image is formed solely from a selected diffraction beam) was carried out by tilting the (directly transmitted) electron beam to allow the selected beam to travel along the optic (microscope) axis. The smallest objective aperture was then centred around the optic axis, preventing the transmisión of all off-axis diffraction beams.

Some elemental analysis was also undertaken in the TEM. For this, the

electron beam was focussed into a fine probe positioned on the area of interest and the presence of particular elements was identified using an EDAX spectrometer and associated software, already described in connection with the SEM. One disadvantage of analysis in the TEM was the proximity of the specimen holder and microscope column, both of which are metallic, to the specimen. Hence, metallic elements (such as Cu and Ni) were able to contribute to the X-ray spectrum. Electron energy loss spectroscopy (EELS) was also carried out and used to identify the cubic BN phase from the fine structure of the spectra obtained. This was carried out using a spectrometer (located beneath the viewing screen) that scanned across the energy spectrum arising from the inelastic scattering of electrons by the specimen.