Zirconia is one of the most prominent ceramics for numerous engineering products. All these products use yttria as stabilizer and it is also the prime reason behind the high cost of the end product. On the property aspects, YSZ (yttria-stabilised zirconia) also faces issues related to low temperature degradation, hydrothermal stability, moderate strength, fracture toughness etc. To mitigate all these issues, a specific research direction on stabilizing ZrO2 without Y2O3 with comparable/better properties (of YSZ) have been investigated with the help of two dopants: CeO2 and CaO.
A ‘burst nucleation’ approach yielded 20 mol.% ceria-doped zirconia nanopowders with high surface area, near-spherical morphology and high temperature tetragonal phase stability. Sinterability of the system also improved and a dense (~97%) sintered ceramic (at 1250 ?) with improved hardness (~14.5 GPa) could be obtained [1]. The long-standing issue of consolidation of a CeO2-doped ZrO2 ceramics, free from monoclinic phase using field-assisted sintering routes was also addressed. A successful suppression of the Ce+4 → Ce+3 conversion led to the formation of a dense ceramic sintered at low temperature (at 1175 ?) with an appreciably high hardness (>11 GPa) and toughness values in-between of tetragonal and cubic CeO2-ZrO2 ceramics [2].
A new factor, termed as, ‘shape strain’ was found to be responsible in stabilizing the CaO-doped tetragonal ZrO2 ceramics at nanoscale [3]. The ‘shape strain’ phenomenon had a strong analogy to size-dependent phase transformation in zirconia system as suggested by Garvie [4,5]. The synthesis [6] and processing [7] of the ceramic ensured a phase-pure, nanograined tetragonal zirconia ceramic stabilized with minimum (3-4 mol.%) CaO doping [8]. Phase-pure tetragonal structure along with uniform nanograins (90 nm) of the ceramic ensured the evolution of no monoclinic phase even after vigorous low-temperature degradation experiments (both thermal and hydrothermal aging for 80-100 h). The sintered ceramic recorded a high hardness (~15 GPa); the indentation toughness value was also comparable to a 3 mol% yttria-stabilized zirconia system. The use of ultrafine (~ 3 nm) nanoparticles of 10 mol.% CaO-doped ZrO2 together with a minimum dwelling time (15 min.) during pressureless sintering yielded high (> 99%)-density ceramic with a unique grain-locking microstructure comprising mostly cubic-phase (~ 83%) and minor tetragonal content (~ 17%) [9]. The CaO-doped ZrO2 nanopowders also showed promising dye-adsorption capacity, thereby, opening a new arena of potential application; industrial grade zirconia depicted little/no activity on this front [10]. The remarkable structure–property correlations in the CaO-stabilized ZrO2 ceramic suggests that the same may be worth examining for suitable future applications (e.g., in dental ceramics).
 
References:
 
[1]      J. Alloys Compd. 802 (2019) 318–325.
[2]      J. Eur. Ceram. Soc. (2023).  
[3]      Scr. Mater. 190 (2021) 52–56.  
[4]      Phys. Chem. 82 (1978) 218–224.  
[5]      J. Phys. Chem. 69 (1965) 1238–1243.  
[6]      Granted Indian Patent No. 201931036371, 2019.
[7]      J. Alloys Compd. 923 (2022) 166309
[8]      J. Am. Ceram. Soc. 104 (2021) 3497–3507
[9]      J. Mater. Res. 37 (2022) 4255–4267
[10]    Appl. Surf. Sci. 596 (2022) 153651