Three kinds of silicon nitride ceramics sintering additives are briefly described

As the most excellent comprehensive properties of structural ceramics material, silicon nitride in the high temperature, high speed, strong corrosive medium such as extreme environment with special application value, is considered to be one of the most development prospect of application of structure ceramic materials, in mechanical engineering, metallurgy, chemical industry, aerospace, biomedical, semiconductor and other industries are widely used.

The α→β phase transition occurs in the sintering process of silicon nitride, which belongs to the structural reconstruction type and inevitably has the fracture and formation of chemical bonds.  For silicon nitride materials, high-energy covalent bond is an unfavorable factor in the sintering process, and the existence of Si-N covalent bond leads to low atomic diffusion coefficient.  Therefore, liquid phase sintering is usually used for si3N4 sintering, which is easier to densify and optimize the properties in all aspects.  Liquid phase sintering requires the addition of sintering additives. In order to improve the performance of silicon nitride, it is very important to add appropriate sintering additives to adjust the composition and content of liquid phase.  

Oxide sintering assistant

Oxide sintering auxiliaries are most studied in Si3N4 ceramics sintering.  In the earliest sintering aid research, only a single metal oxide sintering aid was added for sintering, which played a certain role in promoting the sintering of Si3N4, but often the density is not enough, or the glass phase formed by low melting point, poor mechanical properties at high temperature, can not meet the application.  Subsequently, the researchers used rare earth oxides instead of metal oxides as sintering additives, and found the crystalline phase formed by rare earth elements and silicon, oxygen and nitrogen in the grain boundary to prepare Si3N4 ceramic materials with better performance.  

At present, the commonly used metal oxides and rare earth oxides are Al2O3, MgO, ZrO2, SiO2, RE2O3 (RE=La, Nd, Gd, Y, Yb, Sc) and so on.  

In addition, the research on sintering auxiliaries has developed from single sintering auxiliaries to two or more than two composite sintering auxiliaries.  It is found that the liquid viscosity and thermal properties of Si3N4 ceramics can be significantly improved by using a variety of composite sintering additives. Generally, metal oxides and rare earth oxides are used as multi-component composites.  Such as Y2O3/Al2O3, Y2O3/MgO, Y2O3/Al2O3/MgO, Y2O3/Al2O3/La2O3, Y2O3/La2O3 and so on.  

Non oxide sintering agent

With the further study of Si3N4 ceramics, it is found that reducing the content of lattice oxygen in the sintering process can effectively improve the thermal conductivity and other properties of Si3N4 ceramics.  

Non-oxide sintering auxiliaries such as borides (LaB6, YB2C2), silicides (ZrSi2), fluoride (YF3, YbF3, MgF2, LiF), nitride (MgSiN2, Y2Si4N6C), and hydrides (YH2, GdH2, ZrH2) are used instead of corresponding oxide sintering auxiliaries.  It is proved to be an economical and effective way to improve the thermal conductivity of silicon nitride.  The use of non-oxide sintering agent can effectively reduce the oxygen content in the liquid phase, and then hinder the formation of lattice oxygen, and the amorphous glass phase content decreases after the sintering body cooling, and the thermal conductivity is improved.  

Oxide - non oxide compound sintering agent

Wuchao Xing took Li2O+Y2O3 and LiF+Y2O3 sintering assistant systems as comparative tests to study their effects on the densification process, microstructure, mechanical properties and thermal conductivity of silicon nitride ceramics with high thermal conductivity.  

The results show:  

(1) The addition of LiF and Li2O can achieve good densification effect at low temperature by using Li+Y sintering assistant system, especially the final relative density of si3N4 ceramics with LiF-Y2O3 as additives can reach more than 99%;  

(2) Compared with the addition of Li2O, the liquid phase containing F promotes the α→β phase transition, so that Si and O are constantly removed from the liquid phase to improve the content of O in the liquid phase and reduce the content of O in the system. The thermal conductivity of hot-pressed silicon nitride ceramics obtained by using LiF-Y2O3 composite additives is higher. After 8 hours of hot-pressed sintering at 1750℃,  The thermal conductivity of silicon nitride ceramics can reach 59W/(m·K).

Wang Weide et al. prepared silicon nitride ceramics with high strength and high thermal conductivity using YBH2-MGO (YBHM) system as sintering agent, studied the effects of the system on the densification behavior, phase composition, microstructure, thermal conductivity and bending strength of silicon nitride, and clarified the action mechanism of YbH2 by comparing it with yB2o3-MGO (YBOM) system.  

The results show that:  

(1) In the pretreatment process, YbH2 can reduce SiO2 and generate Yb2O3 in situ;  

(2) The reduction of SiO2 is beneficial to the formation of "anoxic-nitrogen-rich" liquid phase, which is not conducive to densification, but conducive to the formation of lattice oxygen;  

(3) The grain size of si3N4 doped with YbH2 is larger, intergranular phase content is less, and thermal conductivity is improved;  

(4) With the increase of sintering temperature and the extension of holding time, the thermal conductivity of si3N4 sample increases gradually, but the bending strength decreases.

Ding Honghui explored the effects of yBF3-YB2O3 and YBF3-MgF2 on the phase composition, microstructure and mechanical properties of silicon nitride ceramics.  

The results show:  

(1) Silicon nitride ceramics were prepared with yBF3-YB2O3 binary composite sintering agent. YbF3 can obviously promote the α-β phase transformation, and obtain rod-like crystal with aspect ratio greater than 8 and good mechanical properties of silicon nitride ceramics.  When the composition of sintering agent is 5wt%YbF3 and 2.5wt%Yb2O3, the conversion rate of α-β phase is 77%, the slenderness ratio is 8.36 and the bending strength is 269MPa.  

(2) When yBF3-MgF2 was used as binary composite sintering agent, the α-β phase transformation rate of all samples was close to 100%.  MgF2 has a significant positive effect on promoting the growth of β -si3N4 along the C-axis, so that the high aspect ratio of β -Si3N4 can obtain a good lap network structure and excellent mechanical properties.  When MgF2 content increased from 2.5wt% to 7.5wt%, the sample aspect ratio increased from 8.60 to 11.11.  

(3) Compared with the YbF3 system, the mechanical properties of the yBF3-MGF2 system are significantly higher. The bending strength and porosity of the samples added with 2.5wt%MgF2 and 7.5wt%YbF3 reach 298MPa and 37%, which increase by 49% and 15% respectively compared with the corresponding samples containing 10wt%YbF3.  At this point, the sample reached the minimum structure factor of 3.23.