Thermal Barrier Coatings for Low Emission, High Efficiency Diesel Engine Applications Page: 4 of 7
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O5 -.0404 _ 003 -4.002 -.0001 p
-40
-100
--1 40
_120
.. .-160fi)
Figure 2. Calcium titanate TTBC ceramic tested at 600
C showing the strain hysteresis developed
during compressive loading.-0.oa
5 -F.O14 -0.003 -dj.C02 "4.061 0
.-
-100
-120-1 4
Figure 3. Calcium titanate TTBC ceramic tested at 600
C but held at the maximum compressive load
for 10 hours before unloading.
The inelastic behavior of the TTBC ceramic material also
is dependent on the test temperature and loading rate.
The effect of loading rate is dramatically demonstrated by
holding the material at a load for a period of time. Figure
3 shows this effect for the same calcium titanate material.
A similar loading rate was used as for the sample in Fig-
ure 2, but the load was held at the maximum compres-
sive stress for 10 hours prior to unloading. This behavior
is now thought to be due to the unique microstructural
features of the plasma sprayed material.
The microstructure of the ceramic TTBC material reflects
the spraying method used to produce the material.
Ceramic powder is fed into a plasma flame, accelerated,
melted and deposited onto the substrate in a successive
manner similar to painting until the desired thickness of
material is built up. The resulting microstructure contains
pores, microcracks and areas of poor adherencebetween the individual "splats" produced by the molten
particles impacting the surface.
The pores in the microstructure of the ceramic TTBC
material, as shown in Figure 4, contribute to the low elas-
tic modulus when compared to the bulk ceramic modulus
(10 to 30% of bulk material elastic modulus). Microcrack-
ing from solidification stresses and poor adherence
between the "splat" structure of the TTBC material further
reduces the elastic modulus by allowing sliding between
the individual "splats", Figure 5. Friction prevents com-
plete recovery of this sliding motion and contributes to
the inelastic hysteresis. The presence of small amounts
of impurities such as silicon oxide creates glassy phases
that have been found in the microcracks and between
"splats", Figure 6. The calcium titanate material con-
tained 0.43 wt % of silicon oxide. This glassy phase con-
tributes to the creep when the TTBC material is held
under load at elevated temperature. This inelastic creep
at elevated temperature was also found to a lesser extent
in relatively high purity zirconia TTBC materials as shown
in Figure 7. This 8% yttria stabilized zirconia contained
less than 0.01 wt % silicon oxide.
, -f-_ -. -
-,...
' .mj
Figure 4. Typical microstructure (500X) of TTBC
ceramic material showing large pores (5-20
mm) created by faulty platelet (splat) stacking
during processing.2
+.
.. - - . _
f.i
. ..
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Beardsley, M. B.; Happoldt, P. G.; Kelley, K. C.; Rejda, E. F. & Socie, D. F. Thermal Barrier Coatings for Low Emission, High Efficiency Diesel Engine Applications, article, April 26, 1999; Warrendale, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc723040/m1/4/: accessed April 20, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.