Silicon nitride (Si3N4) ceramics are a beacon of advanced materials innovation. It has high hardness, high strength, and high toughness. These ceramics are wear-resistant and have excellent chemical and thermal stability. It is a structural ceramic with excellent comprehensive properties. They are used in a wide range of applications including mechanical, automotive, aerospace, and electronics. Examples include cutting tools, ceramic bearings, turbine rotors, and heat sink substrates. The sintering densification of silicon nitride ceramics plays a crucial role in its development and application. This article provides an in-depth look at the key factors influencing the densification of these ceramics.
1. Sintering Methods
Silicon nitride ceramics use a variety of sintering methods. These include hot pressing (HP), gas pressure sintering (GPS), and hot isostatic pressing (HIP).
1.1 Air pressure sintering
This method is used to inhibit the decomposition of silicon nitride powder at high temperatures. The purpose of introducing nitrogen during the sintering process is to achieve higher density. But, its contribution to densification is limited.
1.2 Hot isostatic pressing (HIP)
While HIP prepares high-performance silicon nitride ceramics, it also has high equipment requirements. At the same time, the cost is also higher. So there are challenges in large-scale application.
1.3 Hot-Pressing Method
This popular technique applies constant mechanical pressure at high temperatures. Compared with traditional sintering, hot pressing reduces the sintering temperature by 100-200°C. The sintering time is shortened. The final density reaches over 99%. But, external pressure has its inherent limitations. The strength of graphite molds limits higher pressure loads. If a pressure-assisted sintering method with constant pressure is used, there are limitations in promoting particle rearrangement and eliminating pores in the later stages of sintering.
1.4 oscillating pressure sintering
Researchers aim to achieve efficient densification of materials. It achieves to get ceramic materials with high density, high strength, and high reliability. Han Yao and his team proposed adding dynamic oscillating pressure to replace the existing static constant pressure during the sintering process of ceramic powder. That is, based on a large constant force, couple an oscillating pressure with adjustable frequency and pressure size. Thus, a bidirectional oscillating pressure with a larger value and controllable amplitude and frequency is applied to the sintered powder. Thereby it improves the sintering driving force.
The results show that silicon nitride ceramics achieve a complete phase change from α phase to β phase under the oscillating pressure sintering (OPS) process. The relative density reaches 99.82%. Compared with the hot press sintering process, the grain size of silicon nitride ceramics increases under the action of oscillating pressure. The average particle aspect ratio increased from 3.79 to 4.86. The bending strength, hardness, and fracture toughness are increased to 1333MPa, 16.2GPa, and 12.1MPa·m1/2. The fracture surface energy is also improved. Obvious deformation stripes and dislocation movement areas were observed on the grain surface of the OPS sample. Due to the introduction of oscillatory pressure, the densification rate and the driving force for grain growth increase. It can also promote the plastic deformation of silicon nitride during the densification process. Thereby accelerating the sintering densification process.
The team has applied this new oscillating pressure sintering method to a variety of high-performance structural ceramics such as Al2O3, ZrO2, and Si3N4/SiCw.
2. Sintering Temperature and Holding Time
The increase in sintering temperature helps mass transfer processes such as dissolution and diffusion. It can reduce the viscosity of the system and improve the fluidity. This in turn promotes densification. But, too high a temperature not only wastes energy but also causes too much liquid phase and too low viscosity. It also deforms the product, deteriorates its performance, and reduces its density. So, controlling the appropriate sintering temperature and holding time is an issue that must be considered in most studies.
Roger et al. studied the effect of sintering temperature on the densification of Si3N4 ceramics. They used MgSi2 as a sintering aid and controlled the temperature at 1300 to 1500°C for plasma activation sintering. They found that when the temperature was lower than 1350°C, the relative density of the sample was lower than 70%. When the temperature reaches 1400°C, the relative density of the sample is 99.6%. When the temperature is higher than 1400°C, the density of the sample almost no longer changes. Research shows that after the temperature reaches 1400°C, the rapid dissolution of α-Si3N4 in the liquid phase is promoted. β-Si3N4 is precipitated by precipitation. This causes the Si3N4 ceramic to further shrink, thereby increasing the degree of densification.
Liu Wenyong et al. used Al2O3-Y2O3 as a sintering aid. Si3N4 ceramics were prepared using the pressureless liquid phase sintering method. Incubate at 1650°C for 2 hours. Then the temperature is raised to 1800°C and the sintering system is maintained for 2 hours. In the obtained Si3N4 ceramic, the larger long columnar β-Si3N4 grains and the fine β-Si3N4 grains are distributed. The ceramic has a density of 98.4% and has excellent properties. Its hardness (HV10) is 15.7±0.5GPa, flexural strength and fracture toughness are 1037.3±48.9MPa and 5.8±0.2MPa.m1/2.
3. Sintering Aids and Aid Content
Silicon nitride will undergo an α→β phase transition during the sintering process. This phase change belongs to the structural reconstruction type. There must be breaking and forming of chemical bonds. For silicon nitride materials, high-energy covalent bonds are a disadvantage during the sintering process. The existence of Si-N covalent bonds results in a low atomic diffusion coefficient. So, it is difficult to prepare dense silicon nitride ceramic products by solid-phase sintering of pure silicon nitride powder. Metal oxides and rare earth oxides are often added as sintering aids. A eutectic liquid phase then forms to promote diffusion and bonding between particles and achieve the purpose of densification. This operation enhances the corrosion resistance and high-temperature mechanical properties of silicon nitride ceramics.
Currently used metal oxides and rare earth oxides include Al2O3, MgO, ZrO2, SiO2, RE2O3 (RE=La, Nd, Gd, Y, Yb, Sc), etc.
Besides, the research on sintering aids has developed from a single sintering aid to two or more composite sintering aids. Research has found that the use of a variety of composite sintering aids can improve liquid viscosity. It can also improve the high-temperature performance and thermal properties of Si3N4 ceramics.
Besides, the additional amount of sintering aids will also have a certain impact on the sintering density of silicon nitride. Liu Jian et al. used MgO and Y2O3 as sintering aids. Mix Si3N4, MgO, and Y2O3 in a certain molar ratio according to the formula design. After forming, the ceramic blank is placed in a boron nitride crucible for air-pressure sintering. The sintering temperature is 1890°C and the temperature is maintained for 2 hours. The entire sintering process is protected by a nitrogen atmosphere with a pressure of 2MPa.
The results show that when the addition amount of sintering aid is 5%MgO+4%Y2O3. The relative density of the sample reaches a maximum of 99.47%. That is, Si3N4 ceramics have been densified.
4. Conclusion
In summary, silicon nitride ceramics are multifunctional materials with excellent properties and great potential. Improving sintering densification meets the needs of modern industry for enhanced material performance. Despite significant progress in sintering aids, sintering temperatures and methods, challenges remain.
Although silicon nitride ceramic sintering densification technology already exists at home and abroad. But there is still a lot of room for development cost control and production efficiency.
