Dear Sir/Madam,
The Lanthanum Magnesium hexaluminate or (LaMgAl11O19) was produced in situ inside ZTA ceramics during conventional sintering. How these elongated (LaMgAl11O19) grains can cause fracture toughness (KIC) improvement through R-curve behaviour?
it is well known that elongated hexaluminate grains can enhance the fracture toughness through crack deflection and crack bridging mechanism.
Thank you very much
Dear Zhwan,
from all the experiments I made to date I must say that I have never seen any relevant crack deflection at hexaaluminates in a dense alumina matrix (including ZTA). The cracks always go right through unless the platelet is hit under a very flat angle. This canbe understod quite easily looking at the basic theory on fiber reinforced composites (He and Hutchison "kinking of a crack out of an interface" !!!). The cracks are deflected when going from low modulus to high modulus material and when the interface is weak enough. From high modulus to low modulus (alumina matrix to MgLA6 )the cracks go through the "reinforcement". Confirmation:If you have a high fraction of platelets (>20%) the cracks try to "stay in the platelets" and do not come out again.
That is why the situation is completely different in e.g. Ce-TZP/MgLA6 and ZTA/MgLA6 systems. in the zirconia matrix composite the matrix has lower modulus (~200 GPA) and we can expect crack deflection in the ZTA system the matrix is high modulus (~400 GPa) no crack defelction. The Platelets have something like 270-300 GPa depending on composition.
Still there can be improved toughness, from my point of view this has something today with a reduced constraint and higher transformability of the zirconia and with a different state of residual stress between the three phases.
If your matrix is not dense the modulus maybe so low that you can observe some crack deflection, the same is true for materials containing a lot of glassy phase (kaolin addition).
best regards
Frank
Dear Professor @Frank Kern
Thank you very much for your detailed, scientific, well organised answer and explanation.
I used the conventional sintering (atmospheric pressure and room temperature) to ensure in situ formation of LaMgAll11O19 (LMA) inside ZTA composites (80 wt.% Al2O3 and 20 wt.% YSZ), or the ratio between Al2O3/YSZ is kept at 4.
The LMA formation was followed the stoichiometric ratio of its constitutes MgO, La2O3 and Al2O3. The mixed powders were mixed with the ZTA and then the normal processing, sintering, characterisation were followed.
Based on these facts, it is expected that the ZTA has a lower modulus than that of LMA due to low density. I have attached the crack micrographs (from (a) pure ZTA 0.0 wt.% LMA to (e) 9.0 wt.% of LMA inside ZTA). The crack bridging and crack deflection were found in the system. Additionally, for ZTA contains 9.0 wt.% of LMA (Fig (e)) the crack also going through the LMA elongated grains as you mentioned it.
Thank you very much again for your kind and full explained response.
Yours Sincerely,
Zhwan
The answer is in your question
" it is well known that elongated hexaluminate grains can enhance the fracture toughness through crack deflection and crack bridging mechanism. "
Dear Dr. Ori Yeheskel
Thank you very much for your kind response.
It is known that crack deflection, crack bridging, crack branching, pull out, ...are common contributors from elongated hexaluminate grains such as (CaAl12O19, SrAl12O19, LaAl11O18, LaMgAl12O19, ....etc) for toughening ceramics. However, many researchers discussed R curve behaviour associates with formation of elongated hexaluminate grains such as a study done by An and Chan (1996) who proved elongated CaAl12O19 causes R curve behaviour in Al2O3 matrix. Article R-Curve Behavior of In-Situ-Toughened Al2O3:CaAl12O19 Cerami...
I am working with formation of LaMgAl12O19 inside ZTA, I am suspicious if the same R curve associated with in situ formation of LaMgAl12O19
Thank you for your contribution
Zhwan
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