Dear Dr. Asl:

I just read your question with proper indexing-topics. I think that the question is further related with another topic I believe that this topic be Materials Science.

The mechanism or phenomenon responsible by emerging the grain boundary contrast after electron-beam scanning is thermodynamic in essence being ascribed to chemical potential and its correlation with optimization of surface area. Also, vice-versa it’s depending on the idea sequence to be developed.

As sintered, all internal interfaces are in thermodynamic equilibrium. This event is or tend to be true at center of sample, Thus, after cut, the central region can be investigated, as being representing of a genuine interface solid/solid, at equilibrium, as previously mentioed by Dr. Kern. At reach such sample region; in fact two another regions are created, as previously commented in another answer. Here, two finds are very important. The brain-boundary region is a continuous region along grain/grain-boundary/grain, after a professional polishing, mirror-like level, is very difficult the grain-boundaries visualization; despite of non-impossible typically, an systematic investigation of interface is not carried out.

As above mentioned, “the unity microstructural” called grain/grain-boundary/grain is continuous being in general its observation from beans of light or electrons. However, after cutting and polishing the interface grain-grain can be revealed since exhibits an excess of chemical potential, based on the chemical and structural distinct nature of grain boundary. See, at grain-boundary it is possible expect a higher number defects and defect species that at grain; as an example punctual as vacancies, substitution and interstitial, also defect clusters, also in line defect and variable range of lattice mismatch. This list of “defect” shows clearly that physical-chemistry nature of grain-boundary is a function of majority character of the chemical bond. In this sense, make sense; there is a ranking of sensibility or response at thermal etching of ceramics, also mentioned by Dr. Kern.

The chemical etching actuates under both grain ad grain-boundary, but due the excess of chemical potential further amount of mass is removed of grain-boundary region. In this sense, electroceramics, electronic ceramics and ceramics based on core-shell structures necessitates of an specific protocol of preparation, since strong alkaline or acid can dissolve the grain-boundary and further more. Sometimes, eventual segregation and precipitates at surface can be removed, that might give some favour of erroneous interpretation of events. In this sense, pH of solution for chemical-etching should be more closed to the neutral possible; more time of preparation more confidence.

In a broad sense, the thermal etching actuates re-distributing an amount of mass positioned at around the grain-grain interface region of grain-boundary being minimum, a priori it effect on the grain surface is very small. Thus, if chemical potential is great the response is great and more increased is the mass rearrangement, deep and broad grain boundary. Therefore, considering ionic ceramics a more intense effect of thermal etching can be expected. Otherwise, covalent ceramics gives an expected minor results.

However,it is possible that the core of concept of thermal etching seems positioned at around of the concept of thermodynamic equilibrium, obviously without discussed the exact nature and sense of concept equilibrium that all people have something to say about. The clear concept of thermodynamic equilibrium is that help to determine the macroscopic parameters of thermal treatment, as follow: temperature, time and sometimes atmosphere (rare events). Thus, in a broad sense, the temperature of thermal etching should be close to the sintering temperature, if the Lab has not a proper protocol, a general idea is start at temperatures far from of sintering-temperature. As example 100 oC and decreasing in a gradual way, 75, 50, 25 oC below of the sintering temperature. See, if the sintering temperature be very high, as a example slight below of the fusion of ceramic, the temperature of thermal etching can be only a bit below of the sintering-temperature, 2.5 oC!

Also, a long time of sintering affect the temperature of thermal-etching. As an hypothetical example, a ceramic sintered a 1280 oC during 1 hour have grain-boundary fully visualized after etch at 1230 oC. If the same green compact is sintered at 1280 oC during 24 hours, a similar etch level is possible, at the same time, only at etch at 1250 oC. See, as a whole, all mentioned events are possible due the “new surfaces” generated by cutting and polishing procedure. In this sense, some minutes are sufficient. Here, above also, it is important have in mind that a modification of the natural aspect is being engineered, a priori under absolute control, but excessive etching can easily modify the contact angle between grain, as an example.

Please, consider further reading:

i) PHYSICAL CERAMICS-Principles for Ceramic Science and Engineering.W. D. Kingery and co-authors. (WILEY. MIT). John Wiley & Sons, Inc., 1997. Chapter 02 and Chapter 03.

ii) Introduction to Ceramics. W. D. Kingery, H. K. Bowen., D. R. Uhlmann, Wiley-Interscience, John Wiley & Sons, Second Edition, 1976. Chapter 05 (at least)

iii) A Guide to Polishing and Etching of technical and Engineering Ceramics. D. J. Clinton. Editor: R. FREER. THE INSTITUTE OF CERAMICS. TEXT BOOK SERIES. 1987

 Kind regards

Marcos Nobre

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

Dear Dr. Asl:

I just read your question with proper indexing-topics. I think that the question is further related with another topic I believe that this topic be Materials Science.

The mechanism or phenomenon responsible by emerging the grain boundary contrast after electron-beam scanning is thermodynamic in essence being ascribed to chemical potential and its correlation with optimization of surface area. Also, vice-versa it’s depending on the idea sequence to be developed.

As sintered, all internal interfaces are in thermodynamic equilibrium. This event is or tend to be true at center of sample, Thus, after cut, the central region can be investigated, as being representing of a genuine interface solid/solid, at equilibrium, as previously mentioed by Dr. Kern. At reach such sample region; in fact two another regions are created, as previously commented in another answer. Here, two finds are very important. The brain-boundary region is a continuous region along grain/grain-boundary/grain, after a professional polishing, mirror-like level, is very difficult the grain-boundaries visualization; despite of non-impossible typically, an systematic investigation of interface is not carried out.

As above mentioned, “the unity microstructural” called grain/grain-boundary/grain is continuous being in general its observation from beans of light or electrons. However, after cutting and polishing the interface grain-grain can be revealed since exhibits an excess of chemical potential, based on the chemical and structural distinct nature of grain boundary. See, at grain-boundary it is possible expect a higher number defects and defect species that at grain; as an example punctual as vacancies, substitution and interstitial, also defect clusters, also in line defect and variable range of lattice mismatch. This list of “defect” shows clearly that physical-chemistry nature of grain-boundary is a function of majority character of the chemical bond. In this sense, make sense; there is a ranking of sensibility or response at thermal etching of ceramics, also mentioned by Dr. Kern.

The chemical etching actuates under both grain ad grain-boundary, but due the excess of chemical potential further amount of mass is removed of grain-boundary region. In this sense, electroceramics, electronic ceramics and ceramics based on core-shell structures necessitates of an specific protocol of preparation, since strong alkaline or acid can dissolve the grain-boundary and further more. Sometimes, eventual segregation and precipitates at surface can be removed, that might give some favour of erroneous interpretation of events. In this sense, pH of solution for chemical-etching should be more closed to the neutral possible; more time of preparation more confidence.

In a broad sense, the thermal etching actuates re-distributing an amount of mass positioned at around the grain-grain interface region of grain-boundary being minimum, a priori it effect on the grain surface is very small. Thus, if chemical potential is great the response is great and more increased is the mass rearrangement, deep and broad grain boundary. Therefore, considering ionic ceramics a more intense effect of thermal etching can be expected. Otherwise, covalent ceramics gives an expected minor results.

However,it is possible that the core of concept of thermal etching seems positioned at around of the concept of thermodynamic equilibrium, obviously without discussed the exact nature and sense of concept equilibrium that all people have something to say about. The clear concept of thermodynamic equilibrium is that help to determine the macroscopic parameters of thermal treatment, as follow: temperature, time and sometimes atmosphere (rare events). Thus, in a broad sense, the temperature of thermal etching should be close to the sintering temperature, if the Lab has not a proper protocol, a general idea is start at temperatures far from of sintering-temperature. As example 100 oC and decreasing in a gradual way, 75, 50, 25 oC below of the sintering temperature. See, if the sintering temperature be very high, as a example slight below of the fusion of ceramic, the temperature of thermal etching can be only a bit below of the sintering-temperature, 2.5 oC!

Also, a long time of sintering affect the temperature of thermal-etching. As an hypothetical example, a ceramic sintered a 1280 oC during 1 hour have grain-boundary fully visualized after etch at 1230 oC. If the same green compact is sintered at 1280 oC during 24 hours, a similar etch level is possible, at the same time, only at etch at 1250 oC. See, as a whole, all mentioned events are possible due the “new surfaces” generated by cutting and polishing procedure. In this sense, some minutes are sufficient. Here, above also, it is important have in mind that a modification of the natural aspect is being engineered, a priori under absolute control, but excessive etching can easily modify the contact angle between grain, as an example.

Please, consider further reading:

i) PHYSICAL CERAMICS-Principles for Ceramic Science and Engineering.W. D. Kingery and co-authors. (WILEY. MIT). John Wiley & Sons, Inc., 1997. Chapter 02 and Chapter 03.

ii) Introduction to Ceramics. W. D. Kingery, H. K. Bowen., D. R. Uhlmann, Wiley-Interscience, John Wiley & Sons, Second Edition, 1976. Chapter 05 (at least)

iii) A Guide to Polishing and Etching of technical and Engineering Ceramics. D. J. Clinton. Editor: R. FREER. THE INSTITUTE OF CERAMICS. TEXT BOOK SERIES. 1987

 Kind regards

Marcos Nobre