Zirconium is plentiful in the earth's crust, surpassing copper, zinc, and other metals. Zirconium oxide is known as ZrO2. Pure ZrO2 is a white, odorless crystal, while impurities turn it yellow-gray. ZrO2 is a novel ceramic with superior physical and chemical properties. It is an essential raw material for ceramic materials and functional materials, and is a hotspot for scientific research. Ceramics can have their corrosion resistance, hardness, and thermal expansion coefficient increased thanks to the addition of ZrO2, which has excellent heat resistance and corrosion resistance.
Effects of ZrO2 on Ceramic Properties
ZrO2 exists in three different crystal forms(as shown below), each of which changes depending on the temperature.
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One crucial factor to consider when evaluating a material's performance is its ability to withstand fracture. The greater the fracture toughness, the greater the resistance to crack propagation. The toughening effect of ZrO2 ceramic primarily makes use of the phase transition properties of ZrO2 to raise the flexural strength and fracture toughness of ceramic materials. At room temperature, ZrO2 is a monoclinic crystal, but as the temperature rises, it changes into a tetragonal crystal (a metastable state). Under the influence of crack tip stress, the metastable tetragonal ZrO2 goes through a phase transition and restructures into a monoclinic crystal. Volume expansion takes place during this process, which generates pressure to the crack, prevents further crack growth, and achieves the goal of toughening.
During the crystal transformation, there will be approximately 8% volume change. Cations must be added to the ceramic mud to aid in stabilizing the mud in order for the reaction to go smoothly and prevent damage to the ceramics (excessive volume expansion will cause the material to crack). The difference in radius between the added cation and the zirconium ion is less than 10%.
The fracture toughness of zirconia ceramics decreases when the grain size exceeds the critical phase transition size, causing the tetragonal crystal phase to transform into the monoclinic crystal phase. Conversely, when the grain size is less than the critical phase transition size, the metastable tetragonal crystal phase undergoes a phase transition, enhancing the material's fracture toughness. When ZrO2 is added to raw mud of daily-use ceramics, the effect that it has on increasing toughness is not ideal because the particles are in a restrained state. Therefore, the critical phase transition size of ZrO2 should be as large as possible to maximize the toughening effect.
Production Method of Zirconia-Enhanced Ceramics
Ceramic mud (natural silicate mineral mud) is used to make ceramics. The toughness of ceramics can be increased by adding a certain amount of ZrO2, and by simultaneously supplementing with Al2O3(adjust the melting temperature of the ceramic body, strengthen the bonding level between the materials, and increase the density of the ceramic body.) and MgO(increase the density of the ceramic body and its corrosion resistance.), the purpose of toughening ceramics can be achieved, as well as enhancing the corrosion resistance and thermal shock resistance of ceramics.
Following are the general steps:
(1) Weigh the necessary additives precisely, add them to the ceramic matrix mud, and combine thoroughly;
(2) Grind, refine, and mix using a ball mill;
(3) Add the proper quantity of distilled water to create ceramic slurry;
(4) Inject the prepared mud into the mold to shape and dry;
(5) Dry the trimmed block body in a warm, dry place (not in direct sunlight) until it turns white.
(6) Firing.
Set Firing Curve of Ceramics
The process of densifying the ceramic body is called ceramic sintering. The particle size and compactness are directly impacted by the sintering temperature. The ceramic sintering effect is significantly influenced by the holding period as well. If the holding period is too brief, the pores cannot completely discharge and instead remain between the grains, lowering the relative density. This is because there is insufficient time for diffusion between the particles. If the holding period is too long, the particles will grow abnormally, causing the accumulation of adjacent pores and a reduction in relative density.
The following is the sintering temperature curve:
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Fracture Toughness of Zirconia-Enhanced Ceramics
By using the indentation method, the fracture toughness of ceramic samples can be evaluated. The indentation method(IM method), is a technique that is used to determine the KIC. It is obtained through the measured crack length, elastic modulus E, and Vickers hardness H value that are generated by the indentation and its four corners. The disadvantage is that the measured KIC value will have some error due to the material's different properties.
According to the test results, the fracture toughness is positively correlated with the addition of ZrO2. The addition of alumina is also closely related to the fracture toughness, and the contents of zirconia and alumina affect each other, while the effect of magnesium oxide is not so obvious.
Hardness of Zirconia-Enhanced Ceramics
The Vickers hardness method can be used to gauge the hardness of ceramics. In order to accurately measure the diagonal length of the tiny indentation and observe the shape of the indentation during the Vickers hardness test, the surface that is going to be tested needs to be as smooth as possible. Due to the brittle nature of ceramics, a load of less than 98 N should be used during the test to prevent material fracture. According to the test results, the material's fracture toughness is negatively correlated with its hardness, i.e., the higher the ZrO2 content in the ceramic blank, the lower the hardness.
Corrosion Resistance of Zirconia-Enhanced Ceramics
Ceramics are typically not corroded and have good corrosion resistance. They can only be chemically corroded when they are submerged in liquids. ZrO2 is a weakly acidic oxide that is resistant to corrosion from acidic or neutral solutions, but is corroded by alkaline solutions. ZrO2 undergoes a phase change during the ceramic's firing process, generating phase transition toughening, which increases the density and corrosion resistance of ceramics.
Corrosion to acidic solution
The acid corrosion resistance of the sample improves as the ZrO2 content increases. There is a positive correlation between ceramics' ZrO2 content and their level of acid corrosion resistance.
Corrosion to alkaline solution
The alkali corrosion resistance of the sample improves as the Al2O3 content increases and the ZrO2 content decreases, so the level of the alkali corrosion resistance of the sample is positively correlated with the content of Al2O3, and negatively correlated with the content of ZrO2.
The following are the test results:
(a picture)
Conclusion
Using ordinary ceramic mud (natural silicate mineral) as the matrix, ZrO2, Al2O3, and MgO as additives, then mixing them to prepare the ceramic blank, and finally obtained the ceramic samples. For maximum fracture toughness and corrosion resistance, ceramic samples should have an additive ratio of 5:3:3 (5% ZrO2, 3% Al2O3, 3% MgO).
The surface of the ceramic sample produced in above test is not very smooth, and the degree of fusion between the additive and the matrix is still deficient, indicating that the degree of refinement and mixing of the ceramic blank is not high; the ceramic sample's density is not very high, which may be due to the presence of defects in formula or firing temperature curve. In the future, research on the toughening performance of ceramics should focus on improving the formula and the firing temperature curve, and further improving the operation method of experiment. It is anticipated that the research presented in this article will be applied to daily-use ceramics and artistic ceramics in order to improve the fracture toughness and corrosion resistance of ceramics, as well as to reduce the collision loss of ceramics during transportation and the corrosion of ceramics exposed to the external environment.