I had sintered many samples prepared using 3Y-YSZ nano size powder by PAS machine. SEM results for all samples showed an unusual microstructure. Also EDS detected that Cl is one of the compositions of the starting material. Details are available in attached document. Does anyone guess what is/are the problem/s?
it is an unusual phenomena. but keep try to do that under vacuum circumstances. which company you purchased this Nano powder? you synthesized that? or commercially bought?
Hello Abbas Fazel ANVARI-Yazdi
Thank you for your answer. I bought the powder.
Dear Dr Tayeb:
Seems that 3Y-PSZ mean 3% mol yttria partially stabilized ceramic, as in:
IOP Conf. Series:
Materials Science and Engineering 47 (2013) 012005 doi:10.1088/1757-899X/47/1/012005
In a general way, this type of material 3Y-PSZ can be classified as a refractory material, despite of dental ceramics application and another...
Seems that powder used is a solid solution, which was engineered at nanometric scales. That is different of a solid solution formed in situ during a solid state sintering, where a specific amount de Yttria powder is dispersed in the Zirconia.
In the paper above after sintering at high temperature for 2h, the sintered ceramic exhibits two phases, both allotropic forms of Zr2O4, and both solid solutions, being one of them the structure with tetragonal symmetry and the second the variety with monoclinic symmetry.
The idea of coexistence of tetragonal (t) and monoclinic (m) phases is that the toughness of solid solution can be enhanced by consuming a part of the energy of a crack in the transformation “t” to “m” and in the creation of a kind of compressive-stress field at around initial region of propagation of crack.
I would like to comment something that is very important about microstructures. An image of surface does not represent a or the microstructure of a material. In fact, the surface of a material is submitted to a series of particular events that might and in general distort the real microstructure. Thus, typically a transversal section of sample is taken and its center is investigated every far from boundaries of sample. Unfortunately, the mechanical cut of the sample can induce some kind of phase transformation, also changing the original physico-chemistry aspect of microstructure as sintered. See, with a set careful proceeding can be possible to access information about several microstructural parameters.
From image, seems that a surface was monitored. In this sense, it is possible to identify a great number of cracks, pink rectangles and red circles), in all its magnitude of occurrences such as: in line, zigzag, triangular and its further combinations. Typically, at this complex picture, both types of fracture can be further considered, as follow intergranular and intragranular. Seems that some grains be projected of the surface, that confer to this grains that exert a superior brightness to grain that turn a rapid identification of t-Zr2O4 more difficult.
Well, if the micrograph is of surface, some comments are above. In addition, I would like of comment a bit about the classical sintering procedure. In the past, when conventional Solid State Sintering of a 3Y-PSZ was carried out, inner cracks and surface cracks were observed during rapid cooling of sample. In part, some problems emerge due the refractory character of this class of material that generated a temperature gradient between surface and more inner region of sample during cooling down. This impossibility of attain a fast thermal equilibrium between surface and central region have strong influence on the mechanical behaviour of microstructure, specifically speaking when the temperatures are close to the temperature of phase transition. Here, the strategy is set up the cooling rate at very small values at around any phase-transition temperature; far from these temperatures (phase transition) the cooling rate can be a bit very fast.
I think that is possible in advanced Sintering methods, as an example Plasma Actived Sintering (PAS) that can be ocurring a very fast cooling rate at around temperature of phase transition "m" to "t" during the cooling step. Please, consider thermal analysis of the powder and ceramic via Differential Thermal Analysis (DTA) to find the temperature of phase transition of "t" to "m" during heating and of "m" to "t" during cooling stage. Also, further microstructural analysis should be considered if an inner portion analysis be feasible.
Kind regards
Marcos Nobre
Dear Khalid Tayeb,
could you please give some more information on the preparation of the SEM sample (cross section or fracture surface)?
The darker colour of the non-crystalline phase suggests a low atomic number material, maybe organics like embedding resin. The cracks may result from a thermal treatment with partial decomposition of the organic material.
To investigate the microstructure of YSZ samples often cross sections are prepared by embedding in epoxy resin with subsequent grinding and polishing. To reveal the grain boundaries a thermal treatment step called "thermal etching" at about 100°C below sintering temperature is performed (after removing the resin - which is difficult in case of open porosity).
The traces of Cl you detected in the starting material probably result from the preparation of the ZrO2; one production route uses chlorine to extract the material from ZrSiO4 sands.
Kind regards
Marcus Mueller
Checking the microstructure, it seems that some gases come out from inside during the PAC sintering. One possible reason is that nano crystalline ZrO2 was prepared from Zirconium oxychloride precusor, Cl was not completely washed out during in the as-prepared powders.
1) I suggest you to conduct a TGA analysis to see if there is weight loss and then check the composition again to see if there is still Cl in the powders.
2) Rewash and calcine the powders before PAC sintering.
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