Abstract:
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Zirconia stabilized with 3% molar of yttria is a commonly used material in
biomedical field for prosthetic implants thanks to its high mechanical properties, in
particular a remarkable fracture strength (~1000 MPa) and an acceptable fracture
toughness (~ 5 MPA*m^0,5).
Zirconia high fracture toughness derives from a mechanism called “transformation
toughening” consisting in change of crystal structure around the path of a propagating
crack, change that causes an increase of volume of 5% able to reduce further crack
penetration.
After being successfully used for hip prosthesis and dental implants, this material was
found to suffer of deterioration when exposed for long periods to humid environment.
This process, called Low Temperature Degradation, triggers the phase transformation
of the material leading to an expansion of volume and progressive surface roughening
together with loss of mechanical contact properties and biocompatibility.
Further stabilization of 3Y-TZP with Cerium reduced the transformability of the
material preventing LTD but worsening its fracture toughness.
Alloying Cerium only to superficial grains appeared to be a successful solution to
prevent surface degradation without impairing mechanical properties, although this
procedure noticeably increased production costs.
The aim of this project was to develop a process that could obtain similar results
being at the same time feasible and suitable to industrial production.
In order to do so, infiltration of presintered material was chosen for introducing both
Cerium and alumina, known to improve respectively LTD resistance and mechanical
properties. Extensive studies of Cerium infiltration kinetic were performed in order to
limit its introduction to the superficial grains of the material.
While alumina introduction actually improved hardness, fracture toughness and even
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LTD, it also introduced major flaws in the material consistently lowering the ceramic
strength making it unsuitable for a structural prosthetic use.
Moreover, Cerium superficial introduction was accomplished calibrating the time of
infiltration, the viscosity of the solution and the conditions of the preform. The
material so obtained showed higher resistance to LTD and unchanged mechanical
properties.
Finally, an extensive mechanical characterization of samples covered with Cerium
through Magnetron Sputtering techni que was performed in order to compare the
results obtained through the infiltration one. Granted that Magnetron Sputtering is
known to be successful for superficial coating, it’s also quite expensive and difficult
to apply on prosthetic components. The two materials actually showed comparable
mechanical properties, confirming the validity of the infiltration technique.
In conclusion, improving the infiltration process for alumina introduction could be a
challenge for further studies, while in the case of Cerium doping the obtained results
were comparable to those obtained through more advanced and expensive techniques. |