I am looking for the experimental data in literature who could show the influence of porosity on coefficient of thermal expansion (CTE) and creep strain rate for ceramics such as YSZ. So far, no success. Any help would be greatly appreciated.
Theoretically，the uniformly distributed pores in the ceramic matrix should not affect the CTE of the sample. You can imagine there is YSZ embedded in the pore, it would not change the overall CTE . The pores might change the CTE only if they are aligned, eg, splitting the materials, which could affect the expansion of the sample. Hope this will help.
Thank you very much for your interesting answer.
Regarding the effect of porosity on creep rate, I found this article interesting
H. Echsler, et . al, Mechanical behaviour of as sprayed and sintered air plasma sprayed partially stabilised zirconia, 2004
where man could see the insignificant effect of sintering (much less porosity) on the creep rate (Fig. 11)
More comments are appreciated.
Hi Vahid,
There is probably no influence on CTE. A priori I would suggest that the creep rate is strongly affected by porosity for two reasons. The specific stress is higher in case of porous material as the porosity does not carry any load, on the other hand (an indirect effect) a porous PSZ will be more fine grained and thus there are more grain boundaries, the fine microstructure will show more creep according to the accepted theory.
During sintering and subsequent heating, pores are emerging as product of vacancies sinks mostly at triple junction positions in the grain boundary network. Also they can evolve on the eventual gas evolvement as reaction product, where they can deteriorate whole specimen. Porosity generally is determined with nitrogen adsorption and refers to pores present on the surface, while pores inside the polycrystalline specimen are called closed pores and they can be detected with SEM on the breakage and also quantified with Archimedes density determination compared with the XRD theoretical density. (Grain boundary difference in the subtraction is about 1 to 2% and is not considered). Closed pores usually unpin with prolonged heating from the grain boundary junction due to grain growth and became more rounded, even circular, in shape, but now inside the grain. All this events are continuous in their kinetics nature, since they are guided by diffusion processes, and are not recognizable on the dilatometric analysis during heating. CTE can be determined, thus, on the expansion diagram on the temperature intervals where l/l0 as a funciton of temperature difference T is linearly dependent. So if there are not some changes present as sintering, or chemical reaction with gas evolvement, where the slope of the diagram is changed by the overlaping of the shrinkage due to pore vanishing from the compacted powder, or swelling of the specimen, or phase transition where on the polycrystalline specimen CTE is changed due to crystallographic group substitution what can be registered as a slope change, than you can be sure that you are indicating lattice expansion of the specimen. This all means that there is no influence of the porosity. Further more you can compare dimensional change during heating of already sintered polycrystalline specimen with cooling slope on the first run (during sintering) as well as with second run cooling slope to see if there is difference in the slope representing CTE. That differences are indicating pore influence and their rearrangement on CTE. Best regards…
Thank you all for your time and good comments!!!
Due to the following works, one can see the increase in thermal conductivity over sintering time, which due to the first work is equivalent to reduction in expansivity.
Sung R. Choi and D. Zhu, Effect of Sintering on Mechanical and Physical Properties of Plasma-Sprayed Thermal Barrier Coatings, 2004
and
D. Zhu and R.A. Miller
Thermal Conductivity and Elastic Modulus Evolution of Thermal Barrier Coatings Under High Heat Flux Conditions, 1999
D. Zhu and R.A. Miller
Thermal Barrier Coatings for Advanced Gas Turbine and Diesel Engines, 2000
Thank you Mr. Kern for your comment about creep strain rate.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Ceramics