Ceramic Technology and Processing, King

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Current, well-designed microscopes have a built-in light source, automatic photographic capability, and, most importantly, a flat, clear field of view. Discussed below are different illumination techniques.

Vertical Illumination. In this configuration, the microscope is similar to a metallograph where polished sections of the samples are prepared. The light source passes through a vertical illuminator, through the objective lens, reflects off the polished surface, reenters the objective, and travels into the ocular lenses. Options include either one or two oculars; eye strain is decreased with two oculars.

The vertical illuminator contains the polarizer, another polarizer called an analyzer, and two iris diaphragms. One controls the light intensity, and the other controls the field of view. This diaphragm is set to where the field of view is just a little larger than that which is observed. This cuts down light scatter which makes for a foggy field. To proceed, close the diaphragm, center the spot with the levers provided, and then just open it up to the edges of the field. When viewing is really foggy, the field can be closed down to where just part of it is viewed. Another way to reduce light scatter is to sputter-on/vacuum evaporate a thin reflective metal film onto the polished surface. The film is far too thin to obscure any detail. Figure 11.28 is of a polished section with the surface coated with a gold film in one case and plain in the other.

The difference in obtaining a clear field of view is evident from the two photos. This is also a useful procedure for measuring microhardness indentations. The ends of the indents are much sharper. Now visible, the indent is always longer and the microhardness softer, but really more accurate.

Many instruments have Nomarsky interference contrast that imparts color to the viewed surface depending upon the relief. Relief is the difference in altitude of the surface. Hard phases stand out of the surface while softer phases are in slight depressions. This relief is easier to see if they show different colors. Nomarsky also sharpens etched grain boundaries and other features, as it imparts color to the field of view.

As an example, the slip-cast ceramic contains an alumina grog that is relatively coarse, and a finer bonding phase made up of alumina and glass. Figure 11.29 is a polished section of this material taken near the top of the casting observed with Nomarsky illumination.

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Figure 11.29: Slip Cast Coarse Grained Alumina Ceramic, near top. Polished section. Polarized light, Nomarski interference contrast. Concentration of fines at the top of the casting. Scale bar 200 μm.

Color can be adjusted depending on how much of the Nomarsky is added. In this case, blue brought out the distinction between the coarse grog and the finer-grained matrix, a matter of choice. Measure grain size by the grain intercept method. The concentration of large particles, small particles, and porosity can also be measured on this surface and is quantitative.

Another section taken near the base of the casting is seen in Figure

11.30.

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Figure 11.30: Slip cast Coarse-Grained Alumina Ceramic, near base. Polished section. Polarized light. Nomarski interference contrast. Large particles settled out during slip casting. Scale bar 200 μm.

The microstructure is entirely different. Coarse particles have settled out as the piece was cast, leaving the top too fine and the bottom too coarse. Firing shrinkage will be different as the fine top will shrink more than the coarse base. Such circumstances induce distortion and cracking.

Before leaving this example, it is useful to discuss what can be done to bring this serious segregation of particles under better control. Take a look at Stokes' law,

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where Vt is the terminal velocity of a spherical particle, g is the acceleration due to gravity, d1 is the density of the sphere, d2 is the density of the liquid, r is the spherical radius, and n is the viscosity of the liquid. In order to slow down segregation, Vt should be minimized.

Reducing "g" casting would require an orbit. That is not practical. "d1 an d2" are fixed by the systems composition.

"r" can be reduced if other properties of the ceramic are not seriously compromised. This has a large effect as the radius is to the power of 2.

"n" can be increased with thickeners such as long chain polymers or mineral thixotropic agents such as Hectorite. Polymers sometimes can interfere with the casting process or the plaster, but they are used. Hectorite is effective in very small (1% or less) amounts to increase viscosity.

"Vt" Any velocity is a function of both distance and time. Time can be cut substantially by pressure casting. That solution may be best as it does not interfere with the composition or surface chemistry.

See what can be learned from a polished section!

Measuring the grain intercept was described earlier. Random lines across the field are used to locate the grains or pores that are measured, obtaining the mean intercept and the standard deviation. When more than one phase is present, the volume percent of each phase is determined quantitatively. When only a rough idea of the grain intercept is necessary, one can visually scan the field and pick and measure one grain that appears to be average.

If the sample were comprised of spheres, there is a mathematical way to convert grain intercept to grain size. Since the ceramic sample does not have spherical grains, estimates to make the correction do not add to the information but rather subtract from it.

Oblique Illumination. This viewing method is where the illuminator shines directly upon the top of the specimen and does not go through the vertical illuminator. The field of view is "natural" such as that of a magnifying glass or stereo-binocular microscope. A flexible fiber optic illuminator is especially useful as it can be moved near the end of the objective close to the

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sample surface. The technique is good to about 100X, above which it is difficult to illuminate the surface as the clearance is too tight. Figure 11.31 is a view by oblique illumination of the surface of vacuum-fired porcelain enamel.

Figure 11.31: Vacuum-Fired Porcelain Enamel Defect. Oblique illumination is useful for viewing surfaces. Scale bar 400 μm.

An interest in vacuum firing derives from pin holes that sometimes penetrate porcelain enamel. By vacuum firing, the porosity can be eliminated. Ordinarily, porcelain enamel has a bubble structure that is essential to its structure. The figure shows that the bubbles have indeed been eliminated by the vacuum firing. This makes the coating extremely brittle. In the figure, the blue background is due to cobalt in the ground coat. Specks are mineral remnants.

The large central figure is where a large bubble emerged through the enamel coating. These bubbles derive from the decomposition of water on the steel surface.

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Hydrogen ions can then penetrate the steel, pass through an iron-rich layer at the interface, and then bubble up through the porcelain layer. Liquid glass is carried along with the bubble and an iron-rich phase precipitates out during cooling, creating the central feature in the figure.

Porcelain false teeth are vacuum-fired to remove porosity; this doesn't work well for porcelain enamel coatings.

Gases sometimes penetrate the steel substrate and form bubbles that then rise through the glass, carrying with it the iron-rich layer next to the steel. In the trade, these are known as copper heads. This is a view using oblique illumination. The structure and its implications would not have been evident by other viewing techniques.

Transmitted Illumination. In transmitted illumination, light is reflected up through the substage, through the slide containing the sample, into the objective lens, through the upper polarizer, and then into the ocular lenses. The substage contains a diaphragm controlling the light intensity, a second diaphragm controlling the field of view, a polarizer, and centering screws.

To center the field, close down the diaphragm and center using the screws. The objective lens also has centering screws that are used as the stage is rotated to make the substage coaxial with the objective. The oculars are focused in the same manner as described in the discussion on reflected light.

Since light is passed through the ceramic, there is much more information available than when it is simply reflected off its surface. Optical petrography is a fairly complex subject, much of which is not usually germane to the study of ceramics. With ceramics, the phases present are usually known and have a much more limited chemical constitution than the extensive array of minerals found in nature. After all, ceramics usually are constituted from only a few light elements. Other methods of analysis have largely supplanted optical microscopy for the identification of the phases present. The combination of X-ray diffraction and chemical analysis by a spectrographic method usually pinpoints the identity of the phases present. But, X-ray diffraction and an analysis do not reveal much about microstructure, size, or shape. This is where optical microscopy is useful.

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Transmitted light microscopy is applicable for examining films, powders, and thin sections. Films are usually polymers and not very applicable for ceramics. Powders are routinely examined by optical microscopy whenever the particle size is above about 2-3 μm. A slide is made by placing a small amount of powder on the slide and then adding a liquid, which is usually an index of refraction liquid. Mix the particles to disperse them and then put on a cover glass. Always use a coverglass to create a flat surface and to protect the objective lens. A rubber eraser is then used to move the cover glass around to distribute the powder suspension evenly and to squash the soft agglomerates.

When the particle size distribution is large, the sample segregates on the slide with the coarse particles moving toward the edge. Watch for this and make a new slide if it occurs. If water is used as the liquid, the small particles exhibit Brownian motion due to the low liquid viscosity. In order to see the particles, contrast is needed between the field and the transparent particles. This contrast is controlled by the difference in the index of refraction between the samples particles and that of the liquid. A big difference yields the most contrast.

Slides of particles are useful for measuring particle size and shape. Eyepieces are often fitted with a reticule or, even better, a movable scale called a filar. The filar is calibrated with a stage micrometer that is a precisely ruled slide. This yields an eyepiece calibration where a movable cross hair can be moved from one side of a particle to the other, the difference being the particle size. A revolving stage is recommended where the slide can be rotated, say 90 degrees, and the other dimension measured, resulting in some information on particle shape.

The microscope can also be fitted with an accessory called a mechanical stage. A mechanical stage is attached to the revolving stage and has rack and pinion movements where X/Y locations can be recorded. It is useful to be able to return to a previous location on the slide, especially when working at high magnifications, in order to place the cross hairs at exact locations. It is difficult to move the slide by hand with this precision. A mechanical stage is recommended.

Thin sections are slices cut from a sample and ground thin enough to become transparent without overlapping grains. Some considerations for making sections are addressed later. Obviously, thin sections are used with transparent ceramics. The birefringent color in polarized light depends on