Ceramic Technology and Processing, King
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Effects of Processing on Properties 373
Monoclinic%/MOE/Thermal Expansion
By continuing to decrease the amount of MgO added, the monoclinic content increases as seen in Figure 10.26.
Figure 10.26: MOE/Thermal Expansion, with Percent Monoclinic ZrO2. Both the MOE and thermal expansion decrease with increasing monoclinic phase.
As shown in the figure, both the modulus of elasticity and thermal expansion greatly decrease as the monoclinic content increases. As both these parameters decrease, the resistance to thermal shock increases. MgO-
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stabilized zirconia ceramics towards the right side of the composition are used in foundries where molten metal is poured over the cold lip of the crucible. Using such a technique, the crucible withstands a number of cycles before breaking.
Monoclinic%/MOR
This data is taken from a less dense body than that of Figure 10.23, and the monoclinic content is extended to much larger amounts. Figure 10.27 shows the MOR of the as-fired body and that after cycling between 260 °C and 1370 °C.
Figure 10.27: MOR-Thermal Cycling with Percent Monoclinic ZrO2. Strength decreases as the amount of the monoclinic phase increases.
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The two curves are parallel, with the MOR decreasing substantially with 20 cycles. Cycling over the transformation temperature loosens intergranular bonding due to the volume change in the monoclinic/tetragonal phase. The foundry crucibles discussed in the last section eventually fail due to decreased strength.
Monoclinic%/Porosity-Cycles/Expansion
Figure 10.28 provides further evidence of the body's disintegration after cycling.
Figure 10.28: Expansion/Porosity-Cycling with Percent Monoclinic ZrO2. Porosity increases with thermal cycling as the amount of the monoclinic phase increases.
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The body does not sinter as easily with a two-phase composition, and porosity results. This effect is visible in the figure where porosity increases from 0% to 0.4% at 61% monoclinic. The results are dramatic after the ceramic is thermally cycled, increasing from 0 to 8% after 20 cycles. There is an attendant expansion of the part's dimensions after cycling, from about 0.4% to about 1.6%. This provides evidence of the body coming apart due to the repeated transformation.
Check List, Microstructure/Properties
•Almost every case is special as there are so many variables.
•Porosity decreases strength, becoming very small at about 40v/o porosity.
•Porosity decreases the elastic modulus that becomes difficult to measure at about 50v/o porosity.
•Fracture toughness increases somewhat as the grain size becomes larger. The coefficient of variation is essentially flat.
•Hardness is flat as the grain size increases up to a point, and then drops significantly.
•Grain size (intercept) is flat as the sintering temperature increases up to a point and then increases steeply.
•The phase diagram of the ZrO2-MgO system charts the predicted behavior of the ceramic.
•MOR in the ZrO2-MgO system shows two maxima, with the larger one at about 3.5% MgO.
•MOR in this system is very sensitive to the amount of MgO avail able for stabilization.
•Properties in the ZrO2-MgO system are very dependant on the amount of the monoclinic phase in the ceramic. MOE drops linearly from 27.5% M to essentially 0 at 61% M. Thermal expansion drops linearly with monoclinic content, the curve at high monoclinic content has two humps that lower the overall expansion.
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•Thermal cycling of a 61% monoclinic body over the transformation temperature lowers the MOR to almost 0 after 20 cycles.
•Thermal cycling increases the porosity in this system at high monoclinic contents.
•The dimensions of a high monoclinic content body become greater after cycling.
•The ZrO2-MgO systems properties are sensitive to impurities that compete for MgO.
REFERENCES
1.Fred F. Lange, "Powder Processing Science and Technology for Increased Reliability," J. Amer. Ceram. Soc. 72[1]3-15 (1989).
2.Ronald S. Gordon, Personal Communication.
3.Morris Berg, Personal Communication.
4.O. Kubaschewsky and E.L. Evans, Metallurgical Thermochemistry, Pergamon Press.
5.The Industrial Graphite Engineering Handbook, Union Carbide Corporation.
6.Yohtaro Matsuo and Shiushichi Kimura, "Statistical Evaluation and
Strength Data of Engineering Ceramics," Jour. Ceram. Soc. Japan, 95 pp C400-409.
7.W. D. Kingery, Introduction to Ceramics, John Wiley & Sons, p622.
8.Ibid, p598.
9.Soo W. Lee, Stephen M. Hsu, and Ming C. Shen, "Ceramic Wear maps: Zirconia." J. Amer. Ceram. Soc. 76[8] pp1937-47 (1993).
10.V.S. Stubican and J.R. Hellmann, "Phase Equilibria in Some Zirconia Systems," Science and Technology of Zirconia, Edited by A.H. Heuer and L.W. Hobbs (Amer. Ceram. Soc. 1981).
11
Ceramic Property
Measurements
1.0 INTRODUCTION
This chapter covers analytical, slip properties, microscopy, and physical properties with emphasis placed on using these methods to characterize materials and densified ceramic articles. Properties provide clues to the history of the ceramic including how it was made, processes that occurred during its manufacture, and its environmental interactions during use. Also discussed in this chapter are the crafts used in these methods and in sample preparation.
2.0 ANALYTICAL
This section describes common analytical methods used to characterize ceramic materials. These methods include chemical composition, structure, surface area, thermal, particle size, and surface analytical methods. Since a comprehensive description of these methods is beyond the scope of this chapter, this discussion instead focuses on the function and application of the methods, craftsmanship in preparing samples, and making measurements.
Analytical measurements pose the serious problem of obtaining standards. Ideally, standards should derive from the material being analyzed.
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Ceramic Property Measurements 379
For example, when measuring silica in a ZrO2 powder, the standards should have chemistry over the same range and have the same phase composition as the sample. This is easily accomplished when the standard can be batched, which is sometimes the only option available. A hazard that must often be accepted is that anything done to the sample can change its chemistry.
Spectrographic
The spectrographic methods used here describe instances where some kind of spectrum is utilized for the measurement. Emission spectra are more widely used for analysis of ceramics than absorption spectra. While atomic absorption was an important absorption method for chemical analyses, it has largely been superseded by induction-coupled plasma (ICP), an emission technique. Other absorption spectra methods such as infrared are not commonly used with ceramics.
Arc Emission
The sample can be a wire, chunk, shaving, or can be pulverized, often in a capsule containing a metal ball. Contamination from the capsule and ball occurs as the assembly vigorously shakes in a machine. To circumvent this problem, the capsule and ball composition should differ from that of the sample materials analyzed. This is easily done for ceramics that are mostly comprised of light elements while the capsule is often WC/Co or steel. While a plastic capsule can be used, such a ball is not massive enough to crush most ceramics. A small B4C mortar and pestle can often be used. Larger pieces can be broken up in a Plattner mortar and pestle that is impacted with a hammer. However, pieces of the ceramic become embedded in the base and cause contamination in the next sample. This contamination can be reduced by first crushing a piece of the sample and then throwing it away. The next piece will have less contamination when crushed. Safety concerns also exist as there is the potential to break one's finger or thumb with the hammer should one's attention be momentarily diverted. While there are many other lab-sized crushers available, they are
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designed for materials much softer than most ceramics. Furthermore, these crushers all contaminate the sample.
With the sample now as a powder or lump, it can be packed or placed in a recessed graphite electrode and placed in an arc chamber. The arc vaporizes the sample at a temperature high enough to ionize the elements. Each element has a unique spectra by which it can be distinguished from most others. Because some interferences exist, not all elements can be analyzed by this method. The spectrographer simply indicates what can and cannot be done. Light from the arc is broken up into spectra with an Echelle optical design and read with a charge injection device (CID). The latter device has superseded photographic plates. An Echelle optical system consists of a shutter, mirror, prism, the Echelle grating, a mirror, and the CID detector. Since the light path folds back on itself, the instrument is smaller than the older spectrographs and can be bench-mounted. The spectrographer's care largely governs the method's accuracy. Accuracy is reported in ranges from major/minor/trace, to decades such as 1%-0.1%, to single digit values such as 2%. While single digit numbers often suffice, most spectrographic labs lack adequate standards for the specific material being analyzed and thus cannot achieve these numbers. Though standards are most important, other factors such as vibration, lab temperature, stability of the arc, and spattering of the melted sample also contribute. Here, meticulousness in technique is an asset.
An additional advantage of the arc emission spectrograph is that one can determine the rough composition of a very small sample, thereby following the motto that, "If it can be seen, it can be analyzed." An SEM with analytical capability proves even more useful. An emission spectrograph is shown in Figure 11.1.
Induction Coupled Plasma (ICP)
In this method, plasma is used to dissolve and vaporize the sample. The plasma is heated to a very high temperature with an induction coil. Since the coil is not in the plasma, it is non-contaminating. The high temperatures ionize the elements dissolved in the solution. This technique produces an emission spectra that can be dispersed with Echelle optics and sensed with a CID, similar to the arc emission spectrograph previously described. An ICP instrument is shown in Figure 11.2.
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Figure 11.1: Emission Spectrograph. Generally useful for analyzing many elements down to trace quantities. (Courtesy of Baird)
Figure 11.2: ICP Instrument. Widely used for chemical analysis. (Courtesy of Thermo Jarrell Ash)
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When trace elements are important, the chemical purity of the flux is of concern as it empties into the plasma along with the sample. Many ceramics are fused in a flux since they are insoluble in acid. When this interference exists, the flux impurity concentration must be analyzed in the same way and then subtracted from the sample results. Although this method decreases the precision of the analysis, it is the best option.
It is problematic that in dissolution of ZrO2 for a silica determination, silica can escape as SiF4, which has a high vapor pressure. One technique involves dissolving the zirconia in a Teflon-lined bomb with HF at an elevated temperature and subsequently chilling the bomb in ice water before it is opened.
X-Ray Fluorescent Analysis
The sample is either a compacted powder or a solid, flat specimen irradiated with X-rays. In the example, the tube has a Rh target, but other targets are available. Atoms in the sample become excited and fluoresce with their individual spectra that is analyzed with a diffractometer. The intensity of the spectral lines is proportional to the amount of the element in the sample. This intensity is reduced by adsorption of other elements in the sample, rendering a lower concentration reading than actually exists. As with all analytical techniques, interferences in the spectra do exist. Despite these interferences and adsorption errors, this method is otherwise nonproblematic and is generally useful. That the analyst is aware of the presence of both adsorption and interference makes these errors predictable, which is preferable to other methods where potential problems remain unknown.
Calibration standards are not as problematic because the amount of the element, rather than the phase, comes into play. Since weight analysis is very precise, the sole problem is to uniformly distribute the element in the sample, which becomes a mixing problem. To test the mixing technique, mix a small amount of red iron oxide pigment into a white powder of similar particle size. Take a small amount of the mixture and smear it onto a piece of white paper with your finger. Red streaks indicate that the pigment is still agglomerated and that it requires more shear to mix the material uniformly.