Sintering of Nano Particles of Zirconia
3.7.5. Sintering of Nano Particles of Zirconia
For the sintering of nano particles of zirconia, the procedures for the preparation of the powder materials into a compct form, in terms of mixing with green binders, compaction at high pressure in a isostatic presss, drying and then sintering in a high temperature furnace, are almost the same as detailed in the section - 2.7.1 for nano particles of SiC and for nano particles of alumina in the section - 3.4.3. Hence, they are not repeated here. But, there is one exception in that the pressure required for the compaction of nano particles of zirconia is quite high, as already mentioned in the section - 3.7.4 above. However, some results obtained by various workers in this field on microwave sintering of nano zirconia, which is quite important like in the case of sintering of nano particles of alumina as shown above, need to be described here.
The zirconia and alumina samples were sintered to over 99.5% of theoretical density using a 1.1 KWatt microwave generator in less than 45 minutes, or 1.5 hours 'cold to cold'. The density and vickers hardness were comparable to dense conventionally sintered materials, which were fired for a longer time of 12 to 16 hours. The container volume and susceptor position were found to have an impact on the heating rate and uniformity. The relationship between the applied power, the heating rate, tempera- ture and properties of the material were worked out, along with the scale-up for microwave sintering of zirconia for larger amount of materials preparation [70].
For the purpose of cost saving, the time of sintering is also important. The savings in processing time (up to 90%) and energy (20-80%) were expected in microwave sintering of ceramics, as this tech- nology breaks through into industrial firing processes, i.e. a hybrid microwave system. Typically, silicon carbide susceptors are used to initiate heating from room temperature, where many ceramics have 'low dielectric losses'. The loss increases with temperature, and at some “kick in” transition temperature, the ceramic load heats preferentially over the susceptors. In this work, the effect of dopant type and crystal structure of zirconia on the so-called ‘kick in’ temperature of transition was explored by using silicon carbide susceptors [71].
The zirconia toughened alumina (ZTA) is a ‘composite system’, which offers significant im- provements in toughness over alumina, with superior hardness compared to zirconia. There are numer- ous applications that would benefit from the use of ZTA. Unfortunately, the commercially available materials often suffer from inadequate transformation toughening and processing flaws associated with the spray drying method of preparation of the materials. In this work, the feasibility of using a simple ‘scalable process’ which avoids the spray drying step was explored. The ZTA bars with optimized toughness from this process were broken in four-point-bending test. The fracture surfaces for 40 vol.% zirconia ZTA were studied. The fracture analysis indicated that the strength was limited by cracks asso- ciated with slightly alumina rich agglomerates. The Weibull modulus was affected by the source of alumina powder, but the type of ‘critical flaw’ was not affected, which is an important observation [72].
As explained in the section 3.7.3 above, the stress-induced transformation of zirconia from te- tragonal to monoclinic phase offers an important contribution to toughening of ceramic composites.
NANO MATERIALS
The stabilization of the tetragonal phase is affected by the zirconia content and its grain size, and by stabilizing agents such as yttria. The zirconia phases in zirconia toughened alumina (ZTA) with several volume levels of zirconia (5 to 40 vol%) and yttira additions (0 to 3 mol%) were analyzed by quantita- tive X-Ray diffraction. The combined effect of zirconia and ytrria on hardness, toughness, and retention of the tetragonal phase was investigated. The amount of yttria required to stabilize the ‘optimum mix- ture’ of tetragonal and monoclinic zirconia was determined for 5 vol% zirconia [73-74].