Title : Zirconia based Bio-ceramics - new additives
Abstract:
Zirconia ceramic shows the best mechanical properties in terms of fracture toughness as it undergoes transformation toughening that occurs during the propagation of a crack inside the matrix. To exhibit transformation toughening, higher temperature phases (i.e., cubic or tetragonal) need to be stabilized at room temperature. For this purpose, additives like CaO, MgO, Y2O3, etc., are normally used as stabilizers. Among these additives, yttria is the most popular for stabilization purposes but it suffers from low-temperature degradation (LTD).
To overcome LTD issue, ceria stabilized zirconia (CSZ) was tried first as Ceria, having isomolecular formula to that of zirconia, does not create any vacancies upon substitution. Hardness up to 945 HV10 with an average grain size of 4.3 µm was found for CSZ systems. It is noteworthy that the rate of phase transformation (during ageing) is significantly reduced for ceria-containing specimens.
However, ceria-stabilized tetragonal zirconia (CSZ) possess poor mechanical properties like hardness and strength compared to yttria-stabilized tetragonal zirconia (3YSZ). The poor mechanical properties of CSZ are due to its large grain size. The grain growth of CSZ is due to the need for high temperatures and longer times for sintering and lack of proper grain growth restriction mechanism (segregation) like in 3YSZ. To overcome this issue, co-doping of Nb2O5 and Sm2O3 was attempted in recent days.
It is observed that Nb2O5 helped in decreasing the sintering temperature of CSZ, and Sm2O3 suppressed the grain growth by segregating at the grain boundaries of CSZ. The two-step sintering process refined the microstructure in both systems. The improved grain size of 1.57 µm, along with a hardness of 1175 HV10 and optimum fracture toughness of 6.2 MPa m1/2 achieved for 1 mol % niobia doped ceria stabilized zirconia. The fine grain size of 0.64 µm with a high hardness of 1288 HV10 along with an optimum fracture toughness of 5.37 MPa m1/2 is observed for Samaria doped ceria stabilized zirconia.
Audience Take Away Notes:
- Zirconia was highlighted as a ceramic biomaterial for load bearing orthopedics and dental applications
- In-vivo failure issues with yttria stabilized zirconia were discussed
- CeO2, Nb2O5 and Sm2O3 as an alternate of Y2O3, as a stabilizer to zirconia were investigated and results are highlighted
