Ceramic Technology and Processing, King
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Entertain one more example of how the SEM and EDX can be used to unravel ceramic puzzles using the following four figures. Firstly, examine Figure 11.56.
Figure 11.56: Wear Surface on an Al2O3/ZrO2 Abrasive Grain after Machining Titanium. SEM photograph. Surface shows a reticulated pattern of attached metal. Scale bar 10 μm.
This is the wear surface of an Al2O3-ZrO2 abrasive grain after it was used to machine Ti metal. It is evident that the Ti was molten because the material has formed rounded shapes indicating a whopping 1660 °C on the surface.
Figure 11.57 is the Ti distribution map, showing its position on the same field of view.
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Figure 11.57: Titanium Distribution on the Same Wear Surface. SEM photograph with titanium radiation. Titanium follows the reticulated pattern. Scale bar 100 μm.
As would be expected, the location of the Ti and that of the previously-molten metal are coincident. On the same field, Figure 11.58 shows the distribution of Zr.
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Figure 11.58: Zirconium Distribution on the Same Wear Surface. SEM photograph with Zr radiation. Zirconium follows the same pattern as titanium. Scale bar 100 μm.
Figure 11.59 is of the Al distribution on the same field.
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Figure 11.59: Aluminum Distribution on the Same Wear Surface. SEM photograph with aluminum radiation. Aluminum is not wet with the titanium. Scale bar 100 μm.
Here are the following conclusions.
•Ti was molten on the surface as it was machined.
•The distribution of the Ti was localized.
•Zr distribution follows the same pattern as Ti.
•Al distribution does not follow the same pattern.
•Ti wets the ZrO2 phase on the abrasive surface.
•Wear probably occurs by a reaction between the Ti and
ZrO2 in the presence of O2, where the mechanism is potentially viscous flow of the liquid titanium.
Energy dispersive analyses are fast and easy to obtain, depending on the precision needed and the concentration of the element being analyzed.
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When dealing with low concentrations, a peak is selected and a count made on that location. Then, as with bulk X-ray analyses, the adjacent background is counted and subtracted.
Wavelength Dispersive Analysis. The same fluorescent spectra taken from the sample surface can be separated by X-ray diffraction. To do so, the X- rays are collimated and impinge on a diffracting crystal selected for its lattice spacing. To cover the range of elements, it is necessary to include three crystals with different lattice spacings. The crystal diffracts radiation and a goniometer scans the spectrum just as in X-ray diffraction. The scan takes time to complete and thus is a slower process than energy-dispersive analysis. When the amount of a single element is sought, the angle is set on the peak and a count taken. The intensity is proportional to the amount present and is subject to the same restrictions as for other fluorescent analyses. While a slower process that requires more work, wavelengthdispersive analysis is capable of greater precision and sensitivity than energy-dispersive analysis.
Transmission Electron Microscopy (TEM)
TEM is a method where the electron beam is transmitted through the sample rather than just impinging upon its surface. The range of magnification is very broad, from a few hundred up to 1,000,000X with some instruments. Lattice planes of heavy elements can be resolved as can individual dislocations. The TEM can also produce an X-ray diffraction pattern that yields crystallographic information. In ceramic work, the TEM is used mostly for viewing powders, surface replicas, and thin sections. Figure 11.60 shows a TEM.
The instrument is essentially a column with an electron source, electromagnetic lenses for focusing the beam, a high potential for accelerating the electrons, and a stage for holding and manipulating the sample. Below the sample, there are sensors for collecting the image. The entire column is evacuated to a very low pressure.
Limiting is that the electron beam is not penetrative, so the sample must be very thin (<1 μm) or have low absorption and also be thin. There are
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techniques for looking at materials: dispersed powders that are viewed in silhouette on a carbon film, a carbon film replica that is shadowed with a heavier element to show surface detail, or a sample that has been thinned down to where it is transparent using specialized techniques.
Figure 11.60: TEM Instrument. Several good instruments are available with many attachments. (Courtesy of Philips)
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Dispersed Powders. A carbon film supported on a wire mesh is coated with a dispersion of the powder. The electron beam can penetrate the carbon, but not the particles that are seen only in silhouette. Figure 11.61 shows SiC particles on a carbon substrate.
Figure 11.61: Silicon Carbide Particles viewed with the TEM. A dispersion of particles on a transparent substrate can be directly viewed with a TEM. Scale bar 1.0 μm.
When the particles are fine enough, they are transparent to the electron beam and additional information can be obtained on their structure. Depending on the absorption of the particular material, the limit on size for transparency is about 1/2 μm.
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Surface Replicas. In this technique, a surface is coated with a stripping film that is pealed off after it dries. It is then placed on a mesh for support and vacuum-coated at an angle with a heavy element, such as Au or Pd. Figure 11.62 is a surface replica of an alumina-cutting tool wear surface, in a stereo pair.4
Figure 11.62: Wear Surface on an Alumina Cutting Tool. TEM photograph of a replica film shown as a stereo pair. Stereo gives a three dimensional view of surfaces. Scale bar 100 μm.
The wear surface is striated and contains ellipsoidal bumps on the surface (when using replicas, the vertical dimension is reversed and protrusions appear as depressions). Wear occurs by plastic deformation of the Al2O3-Fe2O3 phase on the surface by reaction of the alumina tool with steel in the presence of oxygen. This phase is a spinel that has cubic symmetry and can deform in bulk by plastic deformation when the temperature is high and the material is restrained by pressure from fracturing.
This techinque can be taken to the field and applied to large structures. Compared with a direct view as with the SEM, the TEM replica
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technique provides much more surface detail. Stereo pairs are useful when the surface has relief.
Another replica technique involves coating the sample with carbon and subsequently dissolving it. This leaves just the replica film that is then vacuum-coated and observed in the TEM, as seen in Figure 11.63.
Figure 11.63: Zirconia Particles, Replica viewed with a TEM. Surface detail is exquisite with TEM. Scale bar 1.0 μm.
The subject is monoclinic zirconia, which was produced by cracking zircon with lime at a high temperature. Surface detail is exquisite and far better than obtainable by other methods.
TEM can also be used to look at microstructure, as seen in Figure
11.64.
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Figure 11.64: Fused Cast Al2O3/ZrO2 Fracture Surface. The large flat area is an alumina crystal. The arrow points to a zirconia film encasing the crystal. Matrix is solidified alumina/zirconia eutectic. Scale bar 1.0 μm.
The sample is an arc-fused mixture of alumina and zirconia. This view is a fracture surface from which the sequence of crystallization can be deduced. There are three distinct regions in the structure. The large, flat surface on the lower left is an alumina crystal that was the first phase to crystallize out of the melt. As the melt composition became more concentrated with zirconia, it became supersaturated and plated out on the surface of the alumina crystal as a thin film about 1/2 μm thick. The alumina crystal is then isolated from the melt, which then crystallizes as an eutectic mix of both alumina and zirconia. It is this eutectic structure that imparts toughness to the material.