Ceramic Technology and Processing, King
.pdf
Effects of Processing on Properties 363
6.0 EFFECTS OF MICROSTRUCTURE ON PROPERTIES
Ceramics, as a technology rather than a science, are highly unpredictable. In fact, the generalization that, "Every case is special," often holds true in the realm of ceramics. This section presents some examples to illustrate this often-capricious behavior. Since these examples are not intended to be read quantitatively, only some values are given in the graphs to bring the illustration to scale.
Porosity/Properties
Strength
As expected, strength decreases as the volume fraction of pores increases.7 This is shown in Figure 10.17.
Figure 10.17: Strength with Volume Fraction Porosity. Strength drops rapidly as the porosity increases.
364 Ceramic Technology and Processing
The curve applies to a variety of materials. For most ceramics, the part will not hold together when the volume fraction of pores exceeds about 40-50% for a normal microstructure.
Elastic modulus
The elastic modulus follows a similar relationship as strength, with the modulus decreasing as the pore volume increases.8 This is shown in Figure 10.18.
Figure 10.18: MOE with Volume Fraction Porosity. The modulus of elasticity also drops off rapidly as porosity increases.
Effects of Processing on Properties 365
Thermal shock resistance increases with an increase in percentage pore volume due to the lower modulus.
Grain Size/Properties
Grain size (measured as the intercept on a polished surface) affects the physical properties of strength, fracture toughness, and hardness. The information shown in the next two figures is from an alumina ceramic made from a sub-micron powder which was sinterable at fairly low temperatures.
Strength
Over-firing a ceramic substantially decreases the MOR. A 100 °C increase in the soak temperature decreases the strength from 632 MPa to 337 MPa while increasing the grain intercept from 1.4 μm to 4.8 μm.
Fracture toughness
Figure 10.19 provides statistics on both the grain intercept measurements and fracture toughness. Regarding the measurements, CV is the coefficient of variation and SD is the standard deviation. Standard deviation increases as grain size increases. While this is expected since the measurements have larger dimensions, it is somewhat misleading. Dividing the standard deviation by the mean grain intercept normalizes the results such that there is even a slight decrease at the coarsest size. This result is consistent with observation, as the smallest grains have disappeared in the microstructure. Fracture toughness increases as the grains grow larger. Even though the increase is small, it is significant.
366 Ceramic Technology and Processing
Figure 10.19: Fracture Toughness with Grain Intercept. KIC-fracture toughness, SDstandard deviation, CVcoefficient of variation.
Hardness
Figure 10.20 shows the relation between Vickers hardness and grain intercept.
Hardness is flat until the microstructure begins to show exaggerated grain growth. The drop is fairly large. Figure 10.21 depicts the relationship between firing temperature and grain intercept.
Effects of Processing on Properties 367
Figure 10.20: Vickers Hardness with Grain Intercept. Hardness drops off as the grain size increases.
Figure 10.21: Effect of Temperature on Grain Intercept. Grains in this material start to grow rapidly above 1625 °C.
368 Ceramic Technology and Processing
Grain growth is flat up to 1550 °C and then starts to increase. The temperature rise of 50 °C to 1650 °C results in substantial growth. For most applications, this ceramic is over-fired. The microstructures of the 1550 and 1650 °C materials are shown in Figure 10.22.
Note that there is a five fold increase in magnification between the two photographs in Figure 10.22. Both photos are essentially fully dense. Measurements on grain intercept were made on these photos and two others at the other sintering temperature. Evidence of exaggerated grain growth is seen in the 1650 °C firing temperature photo.
Wear Resistance
Wear of TZP using a pin and disc configuration is shown in Figure
10.23.9
Wear is strongly influenced by fracture toughness, showing as great as a fourth power dependence as discovered by the authors. Tough TZP is quite wear resistant, especially when sliding against itself. This is a special case where wear rate correlates with a physical property. There exist additional cases where chemical reactions between two different materials control the wear mechanism. Because wear is a rather capricious property, there is no good substitute for a test.
Effects of Processing on Properties 369
Figure 10.22: Microstructure Alumina Fired at 1550-1650 °C. Note the change in magnification between the two figures. Scale bar 10 μm on both.
370 Ceramic Technology and Processing
Figure 10.23: Wear Rate with Fracture Toughness,TZP. Wear decreases very rapidly as the fracture toughness is increased.
Phase Composition
MgO-stabilized zirconia is used as an example in the discussions below. Consider the phase diagram of the ZrO2-MgO system.10 This is shown in Figure 10.24.
For a structural ceramic, sintering is in the cubic region of the phase diagram, close to 1800 °C and resulting in a 40 μm grain intercept. During aging in the tetragonal/cubic region, the amount of tetragonal phase varies and depends on the amount of MgO. These conditions have a pronounced effect on microstructure properties. Figures 10.23 through 10.26 are samples from the same experiment and can be compared with each other.
Effects of Processing on Properties 371
Figure 10.24: ZrO2-MgO Phase Diagram. C-SS cubic solid solution, T-SS tetragonal solid solution, M-SS monoclinic solid solution.
MgO %/Strength
Figure 10.25 shows the strength (MOR) behavior and thermal expansion to 1300 °C with varying MgO content for a fully dense structural ceramic with a 40 μm grain intercept.
372 Ceramic Technology and Processing
Figure 10.25: MOR with MgO Concentration, ZrO2-MgO system. The strength has two maxima.
The curve has two maxima at 2.6 and 3.5 w/o MgO. The first maximum at 2.6 w/o MgO is due to the elimination of free-standing, tetragonal grains that transform to monoclinic at lower temperatures. The volume change during this transformation weakens the structure. Further increasing the MgO content shifts the composition to the right. Optimal strength occurs at 3.5% MgO due to the best ratio of tetragonal inclusions in the matrix of the cubic grains. Further additions of MgO decrease the tetragonal content such that toughening is eliminated and the ceramic becomes embrittled. The 3.5% MgO maximum has a steep left side; another species competing for MgO (such as SiO2) shifts the composition off the crest and to the left. Due to this sensitivity, small amounts of impurities greatly affect MOR. Because strength derives greatly from composition, manufacture must be carefully monitored and controlled to obtain the highest strength. The 80,000 psi maximum shown is for a sample under optimal conditions.