Transformation temperature Coefficient of thermal expansion Lattice constant Crystalline monoclinic tetragonal equiaxed ZK) 2 Monoclinic and tetragonal crystallographic transitions accompanied by volume changes, see. When heated, the monoclinic crystals transform into tetragonal crystals with a volume shrinkage of 3.25%; upon cooling, the tetragonal crystals transform into monoclinic crystals with 5% volume expansion.
ZrO2 monoclinic and tetragonal phase transitions have the following characteristics: 1 phase change with volume change; 2 phase transition is a non-thermal process, that is, the process is not at a specific temperature but within a certain temperature range; 3 phase change temperature Hysteresis, the positive transformation began at 1170, reverse transformation began at 1 000 ~ 850t:; 4 phase transformation to approximate the acoustic wave propagation speed in the solid phase, it is 2 to 3 times larger than the crack propagation speed; 5 phase transition No diffusion process is required. Since the monoclinic crystals and tetragonal atoms are arranged similarly, the phase transitions need not be accompanied by atomic diffusion, ie, the phase transition does not require a diffusion process; 6 phase transitions do not require atomic thermal activation. Modern studies have shown that the conversion of the fine tetragonal Zi02 to monoclinic crystals can be carried out at very low temperatures and even close to absolute zero. It can thus be seen that phase transitions do not require atomic activation, ie the phase transition activation energy is zero.
Due to the above characteristics of the ZH2 monoclinic and tetragonal transformations, ZrO2 has been widely used in the toughening of ceramic materials, and has now achieved significant results. See Table 2. Zrt>2 thermal expansion curve ZK) 2 monoclinic crystal directions Crystal transformation ceramic matrix Zr2-ceramic matrix composite material fracture toughness/MPam172 bending strength/MPa fracture toughness/MPam1/2 bending strength/MPa cubic Zr2 mullite spinel cordierite Table 2 The mechanical properties of ceramic materials can be seen , Zr02 multiphase ceramic material can increase the value of 1, so as to enhance the effect of strengthening, and recently improve the material's thermal shock stability.
2 Effect of oxygen vacancies on the phase transition of ZrO 2 Forming a cubic in the C-ZrO 2 lattice; forming in the t-ZrO 2 lattice, but the Zr-0 distance is different; in the m-ZK) 2 lattice, the pure ZrO 2 tetragonal phase is formed. In the case of an oblique phase transformation, the zirconium-oxygen coordination changes from the directional and the oxygen ions must be displaced. At room temperature, pure ZrO 2 cannot maintain the cubic and tetragonal structure and can only exist as monoclinic crystals.
For non-stoichiometric zirconia, introducing a certain amount of low-valence cations (eg, Y3+, etc.) in zirconia can allow the high temperature phase of Zi to remain at room temperature.
Experimental studies have shown that when the Y203 content is more than 7.5 mol%, ie, more than 7.5 mol%, the cubic phase is completely at room temperature; when the Y203 content is between 15 and 7.5% by mol, and the gl is 1.5 to 7.5 mol%, the tetragonal phase can be obtained. Retained to room temperature; When Y203 content is less than 1. 5mol%, B卩 less than 1.5mol%, completely monoclinic phase at room temperature. It can be seen that the high oxygen vacancy concentration allows the high temperature phase to remain at room temperature. At the same time, as the oxygen vacancy concentration increases, the phase transition point decreases.
Lu Xun et al.'s further study showed that the presence and changes of oxygen vacancies directly affect the phase transition process of ZrO2. For metastable tetragonal zirconia containing a certain amount of oxygen vacancies, the increase and decrease of oxygen vacancy concentration will further reduce its structure. Stability, making it more easy to monoclinic phase transition 3 Influence of zirconium dioxide toughened silicon nitride ceramics and promote the analysis of multiphase ceramic sintering factors are also very complicated, and the relevant issues are analyzed and discussed.
-Si3N4 system is easy to generate ZrN in the sintering process, and ZrN has great damage to zirconium dioxide deuterated silicon nitride ceramics, which is one of the main reasons for Zr02 toughening Si3N4 ceramics, so it should try to avoid its formation . When the pressureless firing temperature is higher than 1500T and hot-press firing is higher than 16001, the following reaction occurs: The pressureless sintering is an open system, the balance will be shifted to the right when heated and sintered, which is beneficial to the formation of ZrN; the closed air hole is closed during hot-press sintering. In the high gas pressure, the equilibrium will move to the left, can inhibit the formation of ZrN, but ZrN-Si02 is more unstable, in the 1400 Gauckler et al. that add liquid phase to form the material, in the sintering of the liquid phase, can achieve fast and dense To avoid gas reactions. The study shows that the best additive for Si3N4-Zr02 system is A1203 -A1N, and the material properties are improved by forming a Zi02-p-Sialcm in the transition liquid phase. In addition, the use of Y203-A1203 as a sintering aid also has a significant effect on inhibiting the formation of ZrN.
3.3 Firing temperature The increase of firing temperature has a greater contribution to the mobility of particles and can effectively promote sintering. Therefore, ceramic materials are often used to increase the firing temperature to increase the density of products. However, it should also be noted that when the temperature increases, it will lead to secondary recrystallization of the solid phase sintering, so that very few large particles grow abnormally, leading to deterioration of the material properties.
From the viewpoint of ZrN formation, too high a firing temperature is not suitable. Experiments show that at temperatures below 1 590 and nitrogen atmosphere, no ZrN is generated regardless of whether or not there is powdered protection. The sintering temperature is 1 640-1650T, no powder protection, no matter if it is argon atmosphere. In the nitrogen atmosphere, ZrO 2 is formed on the surface of ZrO 2, but only trace amounts of ZrN are generated under the condition of using powder and nitrogen atmosphere, but the firing temperature rises to 1700, even if powder protection is adopted. ZrN generation cannot be avoided. It can be seen that the firing temperature is an important factor in the formation of ZrN. If it is intended to increase the density of products by increasing the firing temperature, corresponding technological measures should be taken.
The size of ZrO2 has a direct influence on the phase transformation of ZrO2. It is very important to control the particle size of Zi02 reasonably for obtaining good microcrack toughening. When the particle size D of ZrO 2 is larger than the critical particle size DH of phase transition temperature, ZrO 2 particles have undergone phase transition at high temperature, and the phase transition is abrupt, and the crack size is large. These cracks can cause the main crack to propagate. The contribution of ceramic matrix toughening is not significant; when the particle size of ZrO2 is between DH and 011 (011 is the critical particle size at room temperature), microcracking will be induced at room temperature. At this time, the toughness of the ceramic will be significantly improved. However, the strength decreases due to the presence of microcracks. When the ZrO2 particles are further reduced to D less than DR, there is no phase change-induced microcracks in the ambient temperature ceramic matrix, but rather the phase-change elastic compressive strain energy is stored, at this time, when the matrix When subjected to an external force, the barrier effect of the elastic compressive strain energy on the main crack propagation is overcome, and t-Zr02 is transformed into m-Zi02 to induce a very fine crack. The toughness of the material can be greatly improved, and the strength is also improved at the same time. Get improved.
Regarding the ZrO2 particle size, except considering the above factors, when the ZrO2 particle size is too small, inevitably, the powder will agglomerate, the particle size will be small, the surface area will increase, the surface force will increase, and the agglomeration will occur. At the same time, the Zr2 particle size will be fine and solid phase sintering will occur. When the secondary recrystallization can not be ignored. These conditions will have a detrimental effect on Zr2 deuteration.
The ZrO2 particle size also has a direct effect on ZrN formation. Studies have shown that the finer the particle size of ZrO2, the more likely it is that ZrN is formed and the ZrN is extended from the surface to the inside, and the sample is easily oxidized, causing crazing cracking.
This shows that ZrO: Toughened Si3N4 ceramics need to consider Zr02 particle size to find a suitable particle size value.
3.5 Atmosphere During the later stage of sintering, the isolated pores in the green body gradually shrink, and the pressure increases, gradually cancelling out the surface tension as a driving force for sintering, and the sintering tends to be slow, making it difficult to achieve complete sintering under normal conditions. In addition to the diffusion of excess vacancies on the surface of the pores, densification plays an important role in the diffusion and dissolution of gases in the closed pores. Due to the large radius of the nitrogen atoms, it is difficult to diffuse and hinder the sintering.
According to the reaction (丨), the reaction is a reversible reaction. When the N2 pressure is increased, the equilibrium shifts to the left, which effectively controls the formation of ZrN, improves the toughening effect, and promotes the sintering to perform a visible nitrogen atmosphere. It has a dual role and should be comprehensively considered; +1/202(g), so adding appropriate amount of Si02 to the ground powder can increase the SiO partial pressure. According to the principle of physical chemistry, the refinement of Si02 particles is conducive to the production of SiO gas, and it will also help to suppress the formation of ZrN and prevent the product from cracking.
3.6 Pressure According to reaction (1), the reaction generates gas, and the increase in pressure will help the reaction to proceed to the left. Therefore, hot press sintering is advantageous in suppressing the formation of ZrN due to an increase in sintering pressure. In addition, hot pressing can provide additional impetus to compensate for the effect of surface tension being counteracted, allowing sintering to continue and accelerate. At the same time, under hot press conditions, the solid powder may exhibit some non-Newtonian fluid properties. When the shear stress exceeds its yield point, the flow will occur, the mass transfer rate will increase, and the closed pores will pass the viscous or plastic flow of the material. Elimination can be achieved, so the use of hot-press sintering can ensure that a high density sintered body is produced at a lower temperature and in a shorter time.
Hot-press sintering can reduce the temperature to achieve densification, so it can effectively avoid the rapid movement of grain boundaries during solid-phase sintering, prevent secondary recrystallization, and promote sintering.