Brief Introduction of Zirconia
Zirconia is a metal oxide having a high melting point, extremely stable chemical properties, wear resistance, high temperature resistance, and corrosion resistance. Zirconia can be used extensively in various fields as a new type of structural and functional ceramic material thanks to its strong mechanical properties, which include superior thermal shock resistance, high refractive index, good thermal stability, etc.
At low temperatures, ZrO2 has a monoclinic crystal system; at high temperatures, it has a tetragonal crystal system; and at higher temperatures, it has a cubic crystal system. The three crystal forms have the following transformation relationships:
(a picture)
Pure zirconia materials cannot be used in situations where there is a significant temperature shift because along with the crystal transformation, the reversible transition from the monoclinic phase to the tetragonal phase will be followed by a volume change of 7% to 9%, which causes the zirconia materials to crack easily during firing.
Additionally, ZrO2 has some unique characteristics in some specific industrial fields, such as excellent electrical conductivity, good high temperature degradation resistance, and so forth. As a result, metal ion oxides with a radius similar to Zr4+ are frequently added to ZrO2 in practical applications as stabilizers. The zirconia solid solution with high-temperature crystal forms maintained at room temperatures can obtain after high-temperature treatment, so that the zirconia material not only prevents the volume effect induced by phase transition during heating or cooling, but it also improves its performance and expands its applications.
Commonly-Used Stabilizers for Zirconia Ceramics
ZrO2 has a fluorite-like structure, with an ionic radius of 0.082nm for Zr4+. It is commonly accepted that cations having an ionic radius close to that of Zr4+ should be used for doping in order to generate a stable ZrO2 crystal structure at room temperature. At the moment, rare earth metal oxides, represented by Y2O3, and alkaline earth metal oxides, represented by CaO, are the two types of additives that have been investigated the most.
The following figure displays typical doping cations:
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Unary Doping
(1) Y2O3
When yttrium is employed as the doping element, the doping amounts of Y2O3 are typically 3mol%, 5mol%, and 8mol%; as the Y2O3 content rises, the stabilized zirconia ceramics can change from the tetragonal phase (partially stable) to the cubic phase (fully stable), which can be used in various fields. There are certain differences in the mechanical characteristics of stabilized zirconia with various Y2O3 contents. Y2O3 has a high density, superior sintering performance, and low sintering temperature when compared to other stabilizers. Excellent mechanical properties, chemical stability, and strong ion conductivity are all characteristics of yttrium stabilized zirconia.
Below is a comparison of the mechanical characteristics of zirconia ceramics with different yttrium contents:
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(2) CaO
In addition to having inexpensive raw material costs, CaO-stabilized zirconia materials also offer low firing temperatures, little change in sample grain size with an increase in CaO doping percentage, and a readily available cubic phase, all of which contribute to increasing electrical conductivity.
(3) Al2O3
Research have demonstrated that the doping of Al2O3 on YSZ (yttria-stabilized zirconia) is advantageous for grain refinement, suppressing phase transition, boosting densification, and improving mechanical characteristics. Adding Al2O3 to modify is one of the research hotspots.
(4) MgO
Magnesium-stabilized zirconia offers excellent mechanical properties at medium and room temperatures, as well as good wear resistance and low temperature degradation resistance.
Multiple Doping
The relative density, grain size, and mechanical characteristics of zirconia ceramics with varying quantities of element doping are depicted in the figure below:
(a picture)
Zirconia Powder Preparation Methods
The performance of ceramics is largely dependent on the characteristics of the powder. The method used to prepare the powder, the calcination temperature, grinding time, etc., determines the powder's crystal structure, particle size, specific surface area, and other properties, all of which have an impact on the performance of ceramics. There are numerous methods for producing stabilized zirconia powder. Taking yttrium-stabilized zirconia as an example, the commonly-used methods include the hydrothermal method, co-precipitation method, and sol-gel method, etc. Various factors will influence each powder preparation method. For instance, when manufacturing stable zirconia powder by hydrothermal method, the doping quantity is the key influencing factor for the crystal form, and the temperature of hydrothermal treatment, pH value, the mineralizer concentration, and the dopant concentration can affect powder performance.
Zirconia Powder Treatment
Before molding, the powder should be processed according to the features of each molding method and the properties of the molding powder. Particle size distribution and post-treatment have a significant impact on the sintering rate and densification of YSZ powders. Typically, the manufactured powder will agglomerate, and the degree of dispersion of the powder can be enhanced during the experiment by adding organic additives. Powder granulation is frequently necessary for dry pressing molding in order to assist pressing; prior to wet molding, a slurry with a high solid content and low viscosity should be made.
Forming
The forming method has a significant impact on the density and grain size of ceramics since it determines the contact area between particles and their packing density. Commonly-used molding methods for stabilized zirconia ceramics include dry pressing, hot die casting, isostatic pressing, tape casting, injection molding, gel injection molding, and additive manufacturing, etc. Each of these methods is influenced by a variety of factors. For instance, powder, molding pressure, pressure holding time, pressurization method, additive type and concentration, and pressurization speed are the factors influencing dry pressing, while powder, pH value, dispersant, plasticizer, and binder type and dosage are the factors influencing tape casting.
Sintering
Sintering Technology
Ceramic sintering methods include conventional sintering and rapid sintering. Rapid sintering technologies include microwave sintering, spark plasma sintering, self-propagating high temperature sintering, flash firing, cold sintering, and oscillating pressure sintering. Conventional sintering technologies include atmospheric pressure sintering, hot pressing sintering, etc. The most popular sintering method currently in use is atmospheric pressure sintering, which has the benefits of being inexpensive and simple to conduct but also has drawbacks like low density, uneven microstructure, and a lengthy production cycle.
Sintering System
Researchers investigated the effects of different sintering systems, such as sintering temperature and secondary sintering or two-step sintering, on ceramic preparation in order to produce stable zirconia ceramic materials with high density. Because of the high heating rate, the temperature inside the zirconia becomes uneven, resulting in the coexistence of larger and smaller grains, which readily leads to the appearance of cracks and the formation of pores. Hence, a lower heating rate should be used throughout the sintering process to achieve optimal performance.
The mechanical characteristics of Y-TZP ceramics are impacted by various sintering temperatures. The greatest fracture toughness appears between 1400°C and 1500°C, and the maximum elastic modulus is at 1400°C. Flexural strength, fracture toughness, and elastic modulus all first increase and subsequently decline with temperature. When utilizing microwave sintering method to produce 8YSZ, the density increases with the increasing holding time of 5, 10, and 15 minutes, as do the corresponding elastic modulus and hardness.
Multiple sintering and two-step sintering are also incorporated in pressureless conventional sintering. Two-step sintering involves heating the sample to a higher temperature to achieve a specific density, then rapidly cooling it to a lower temperature and holding it for an extended period of time. While grain boundary migration is restricted at this temperature, grain boundary diffusion can still occur. It is discovered that the hardness of two-step sintered samples rises as sintering time increases and it falls after 10 hours. There is a significant difference between the hardness levels of conventionally sintered samples and two-step sintered ones with residence times of 5h and 20h. Nevertheless, the value of flexural strength did not vary significantly with the sintering circumstances.