Aging resistance and mechanical properties of novel zirconia based materials

Abstract

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 8 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.