I have used the thermal etching method (vacuum atmosphere) for detection of grain boundaries. I would like to know which mechanisms or phenomena can lead to the appearance of grain boundaries? Do you have any reference?
by polishing you are cutting into the grains which then do no have their thermodynamically stable shape any more, the freshly created surface has higher surface energy than the surface of a as-fired part. By thermal activation grains tend to recrystallize forming a surface with less free energy. If you do this carefully enough you just see the onset of this process which makes the grain boundaries visible, actually by creating some different topographical levels beween the boundary and the adjacent grains.
by polishing you are cutting into the grains which then do no have their thermodynamically stable shape any more, the freshly created surface has higher surface energy than the surface of a as-fired part. By thermal activation grains tend to recrystallize forming a surface with less free energy. If you do this carefully enough you just see the onset of this process which makes the grain boundaries visible, actually by creating some different topographical levels beween the boundary and the adjacent grains.
To my mind the thermal etching method is not effective for visualization and detection of the grain boundaries in ceramics. The chemical treatment with further heating in vacuum are more preferable. If your ceramic has electric conductivity, the best methods may be EBIC or LBIC.
I totally agree with Dr. Kern.
In my experience, phosphoric acid is very effective for visualization and detection of grain boundaries.
S.K. Brantov is right as far as non-oxides such as SiC or Si3N4 are concerned, in fact here ion or plasma etching lead to better results, for the oxides thermal etching works very well. It has to be taken care that the etching is compatible with the material and also with the SEM used. Over-etching has to be avoided. If possible small acceleration voltages should be used in order to study what is on the surface and not to "look into" the material. I have attached some sample images to demonstrate the feasibility. it can be seen that the etching in air works well with the fine grained zirconia and Alumina zirconia materials but that in case of the oxide/non-oxide composite the hydrogen strongly attacks the titanium carbide. Images were taken with a Zeiss Gemini in-lens SEM in SE mode.
I think the difference between vapor pressure of grain and grain boundary at thermal etching temperature couses different topography. The grain boundary surface will be lower than the grain surface after etching because of higher vapor pressure of former. So, in SEM, you can observe grain and grain boundary regoin r
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