Please could someone suggest what are the possible reasons that the samples (Alumina and zirconia) which have been processed in exactly the same way ( Ceramics Processing and also the same recipe in SPS) obtain different colour and different density after sintering by SPS/FAST.
We have also seen such dark grey to white colors in SPS-ed Alumina, Spinel and Zirconia samples. The reason is not clear yet.
People have mentioned incorporation of Carbon into Alumina as a reason for its darkening. But also the occurrence on non-stoichiometry of the ceramic compounds has been mentioned.
We have not seen any influence in XRD or SEM-EDAX analysis.
Also the mechanical properties (Moduli, hardness, toughness) do not seem to be influenced by this color effect.
It might be due to reduction of cations. I would try to re-oxidise the samples in air/oxygen at intermediate temperature. If you heat them up to just below the sintering temperature, you´ll get nice thermally etched grain boundaries as well that looks very nice by SEM.
You should always polish some µm of the surface to obtain a more homogeneous material
It might be effected by lubricant u used during compaction. Secondly, incomplete reaction of atom at the interface, hence create nonstoichiometry materials. The presence of different charges in the sample would changed the physical properties, i.e. color
Dear Sami, there are several possible sources for absorption or color in these oxide ceramics. Three common sources are carbon contamination (from organics used in processing or the graphite dies or foil), color centers caused by reduction (in turn caused by low pO2 or carbon, especially for zirconia), and formation of aluminum oxycarbide (with a smaller bandgap, for alumina). As only ppm quantities are required to impart color, minute changes in concentration are all that is required to produce a visible effect. Re-oxidation should remove the color, but it doesn't tell you whether it's due to carbon contamination, color center formation, or aluminum oxycarbides. Also, carbon is inherently difficult to detect as it is present everywhere.
We briefly discuss coloration of magnesium aluminate in "Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel, Journal American Ceramic Society, 2013".
The difference in density is more difficult to explain and could be due to differences in density in the initial compacts (due to cold-pressing differences) or local variation in organic binders or lubricants (causing swelling).
Being applied for non-conducting powders, SPS-method is extremely sensitive to the physical state of initial treated bodies. So, all the output properties of SPS-produced ceramics in this case have always higher dispersions. I believe the different colour you have observed is mainly connected with high (and different!) gradients of pressure/composition inherent to the gas atmosphere inside your SPS-device.
I concur strongly with Marc Dumerac response. It could be any one of the three reasons he cited, oxygen vacancies, color center formation, carbon contamination and ppm of impurities. We see grey color in some air sintered Spinel samples due to carbon. Hot pressed spinel can experience carbon contamination from carbon die environment similar to SPS, but this is not always the case.
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