Introduction of 10 Sintering Processes for Alumina Ceramics(1-4)

06-03-2020

Introduction of 10 Sintering Processes for Alumina Ceramics(1-4)

1. Sintering at Normal Pressure 

Normal pressure sintering means that the material is sintered at atmospheric pressure without pressure, which is currently the most common sintering method. It includes atmospheric sintering under the condition of air and atmospheric sintering under the condition of certain special gas atmosphere. The method has higher sintering temperature, higher requirements for the furnace and greater waste of energy. 

Al2O3 has a high melting point, so the preparation of Al2O3 ceramics often needs to add sintering additives and compact by liquid phase sintering. This method can generally promote the sintering of Al2O3 ceramics. Liquid phase sintering of Al2O3 ceramics generates liquid phase, facilitated diffusion and viscous flow through chemical reaction to achieve particle rearrangement and mass transfer process, reduce sintering temperature of Al2O3 ceramics and accelerate effective sintering.

Since atmospheric sintering has no external driving force, it is very difficult to eliminate all pores inside the ceramic to reach the theoretical density. The special sintering process refers to adding sintering driving force during the sintering process of alumina ceramics to promote the densification of ceramics. At present, the common special sintering processes mainly include hot pressing sintering, hot isostatic pressing sintering, microwave heating sintering, microwave plasma sintering, discharge plasma sintering, etc.


2. Hot pressing sintering 

Hot pressing sintering is to apply one-way pressure to the sample at high temperature to promote the ceramic to achieve full densification. Compared with conventional sintering, sintering at a pressure of 15MPa reduces the sintering temperature of ceramics by 200℃ while increasing the density by 2%, and this trend increases with the increase of pressure. For pure alumina ceramics, conventional sintering requires a temperature above 1800℃. However, only 1500℃ is required for 20MPa hot pressing sintering. 

The pressure provided by hot pressing sintering promotes the flow of atoms in the particles, and the pressure and surface energy together act as driving forces to strengthen the diffusion effect. Because hot pressing sintering can sinter at a lower temperature, grain growth is inhibited, and the obtained sample is compact and uniform, small in grain size and high in strength. However, it is not suitable to produce products that are too high, too thick and complicated in shape, with small production scale and high cost.


3. Hot isostatic pressing sintering

Hot isostatic pressing sintering is the sintering that applies pressure to all directions of the ceramic body at the same time to reduce the sintering temperature of the ceramic, and the ceramic obtained by sintering has uniform structure and good performance. Although HIP sintering can successfully reduce the sintering temperature of ceramics and can obtain articles with complex shapes, HIP sintering requires encapsulating or presintering the green body in advance, and the pressure conditions are also relatively harsh.


4. Ultra-high pressure sintering

Ultra-high pressure sintering is sintering under higher pressure. due to higher pressure, atomic diffusion is inhibited and nucleation barrier is relatively small. therefore, alumina ceramics with high density (> 98%) and high purity can be prepared at lower temperature. In the ultra-high pressure sintering process, the existence of pressure increases the vacancy and atomic diffusion rate in the particles. The pressure and surface energy together act as the sintering driving force, which increases the diffusion effect. Ultra-high pressure sintering usually only needs to be carried out at a relatively low temperature to inhibit the abnormal growth of crystal grains, thus obtaining high-purity alumina ceramics with high densification degree, fine crystal grain size and uniform distribution.





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