After uniaxial pressing and pressureless sintering of a composite glass ceramic with Zr, the samples look bloated and as i am polishing, the porosites are opening up, i am unable to get the best refelctive ssurface. Can someone explain and give any suggestion?
can you see the porosities in SEM ? Method of characterization ?
actually u sinter the sample too high and also probably come from the large different of thermal expansion of each material to another. this case normally found in composite where u mix metal n ceramic togther
I think that it is expected, since the surface is the region that are "more sintered". You should try to get trough the corser abrasives as fast as possible, untill you reach the abrasives that doesn´t remove matrial, i.e. untill the wear mechanisms change from fragile to ductile mechanisms.
Put diamond paste in a velvet cloth and polish gently. So that you can avoid pores after sintering.
You should measure the density of your material so you can calculate the relative density compared to the theoretical one.
If your material exhibits poor relative density (i.e. a certain amount of porosity), you should modify the preparation process.
If you get a dense material then it means that you have grain pull-out as hypothesized by Roman Kubrin. In this case you should modify your polishing procedure reducing its severity.
Goffredo De Portu is quite right. If the true density is too low, try sintering at a higher temp. If the true density is adequate, the problem is grain pluck-out. Pluck-out can be mitigated but not necessarily eliminated by using one brief grinding (fixed abrasive) step for ~30 s, say 240-grit diamond, followed by ~3 longer polishing (free abrasive) steps, say 9, 6 & 1-um diamond for 5 min each. In a nonoxide ceramic, e.g. AlN, try sawing, polishing, grinding and washing without water -- water forms Al2O3 nanoparticles that weaken AlN grain boundaries significantly. If you are still getting massive pluck-outs, it probably means you have very weak grain boundaries due to a nanoprecipitate. The way to identify the bad actor is to examine a fresh fracture surface by SEM-WDS. Sintering additives that should be dissolving, or furnace contaminants, could be precipitating on your grain boundaries.
Ref: R.E. Chinn (yes, it is I!), Ceramography, ASM Intl. and ACerS, 2002, p 35-44, ISBN 0-87170-770-5.
It is important to know your glass composition as it maybe causing the type of pore. Further, any presence of water in the glass powder and presence of certain elements may or can cause the bloating.
If you are trying to keep the propensities in the matrix then try varying your particle distribution to get more homogeneous composition that will help obtain uniform composition through out the composite and produce uniform strength to the glass. You may want to check if there is variability in the glass composition, hope this helps.
Could you estimate first the value of real porosity in your samples and then compare it with your observations on the cross-sections of samples?!
Could you estimate first the value of real porosity in your samples and then compare it with your observations on the cross-sections of samples?!
Would you please describe the ceramographic procedure? I have some identical problems with SiC when there is a minimum porosity.
porosity is depends on compaction pressure and relative density. For getting best sample surface, the secondary operations such as hot extrusion, rolling can be done.
porosity is depends on compaction pressure and relative density. For getting best sample surface, the secondary operations such as hot extrusion, rolling can be done.
The question is why is it bloating, this is an indication of porosities, next is to increase the temperature and time of sintering, if this fails look at the composition of the initial glass ceramic and what was the starting materials used in making the glass ceramic. Zr does not bloat unless it has a phase change (volume change) causing weakness at grain boundaries, causing it to be pulled out.
If the Zr is present as crystalline ZrO2, be aware that ZrO2 changes phases during heating and cooling with significant expansion and shrinkage that would affect the microstructure. The bad actor might be ZrO2 changing from monoclinic to tetragonal when heated above ~1000°C, and the reverse upon cooling. This is why non-stabilized or partially stabilized zirconia ceramics should not be thermally etched -- they shatter. If the zirconia phase change is the source of the problem, try stabilizing its cubic phase with Y2O3 or MgO.
Ref: R.E. Chinn (him again!), Ceramography, ASM Intl. and ACerS, 2002, p 50, ISBN 0-87170-770-5.
As discussed earlier, the cause of bloating needs to be addressed. This could be due to starting material releasing gas, the powder particles not melting to increase density, and the sintering temperature is causing crystallisation around the gas bubbles, or pores created in sintering, trapping pores. you could add an additive to the composite that wont be detrimental to its final properties, to give a liquid/ glass phase along the grain boundary for pores or gas bubbles to travel to the surface at sintering temperature and time. Note the particle size of the glass ceramic powder is critical in the formation of bloating, which is related to the starting composition of the glass ceramic. Fine particles adsorb moisture and Co2 onto certain glass powders having specific starting materials.
Well again, its may be due to sintering or you may want to alter the time, temperature and manipulating variables also. You always have to try to get the best results.
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