In sintering, wettability has a great influence on densification, microstructure and properties of the alloy, and the wettability of liquid and solid particles is the wetting angle. Only with full or partial wetting conditions can the liquid phase penetrate into the micropores, cracks and even grain between the particles. There are many influence factors affecting the wetting angle, such as sintering temperature, contact time, additives, sintering atmosphere and so on.

 

 

1, Influence of Temperature and Time

Elevating temperature or prolonging the liquid-solid contact time can reduce the angle, which is limited. According to the wetting and interfacial chemical reaction based on thermodynamic theory, high temperature is beneficial to interfacial reaction, thus improving the wettability. The increase of temperature lowering of the system free energy advantage, so γsl decreased. However, the effect of temperature on γs and γsl is little. Wettability is a dynamic equilibrium process. And extending the time can establish the balance by interfacial reaction.

 

2, Influence of Surfactant

In the TiC-Ni hard alloy, adding a small amount of nickel molybdenum can make the wetting angle of nickel on TiC from 30° to 0°, and the dihedral angle 45° to 0°. The wettability can be improved only by reduce the γsl. Physical additives can promote the dissolution of solid phases and liquid phases and mild chemical reaction.

 

3, Influence of Powder Surface State

The surface of the powder gas, adsorption of impurities or oxide, oil, will reduce the liquid wettability of powder. As the adsorption itself reduces the surface free energy, the surface energy γs of solid particles adsorbed other material is always lower than the surface tension of the vacuum γ0. Considering the influence of adsorption film on solid surface, the wetting angle formula is

cosθ=[(γ0-π)-γsl]/γl （1）

 

4, Influence of Atmosphere

The atmosphere will affect the size of theta, of which the reason may be that the surface state of powder varies in different atmosphere. In most cases, oxide powder, hydrogen and vacuum are beneficial to eliminate the oxide film, thus improving the wettability. However, the vacuum is not necessarily better than the inert atmosphere on the wettability is more favorable without film.