Wetting and Joining of Structural Ceramic Components

Wetting

Wetting is the ability of a liquid drop to keep contact with a solid surface. The degree of wetting is qualified by a contact angle (θ_C), at which the liquid-vapor interface meets the solid-liquid interface, as shown in Figure 1. The contact angle is governed by the balance between adhesive and cohesive forces and it provides an inverse measure of wettability.

Figure 1.

A contact angle of a liquid drop on a solid substrate

If we denote the solid-vapor interfacial energy as γ_SG (or surface energy), the solid-liquid interfacial energy as γ_SL and the liquid-vapor interfacial energy as simply γ_LG (surface tension), we can use the following equation to describe the equilibrium (known as the Young Equation) (Young 1805):

Generally, a low contact angle (θ_C < 90°) means that wetting of the solid surface is very favorable, and hence the liquid will spread over a large area of the surface. A high contact angle (θ_C > 90°) usually indicates that wetting of the solid surface is unfavorable, and thus the liquid will minimize the contact area with the surface and form a compact drop.

It is important to study the wetting behavior of metal or glass liquids to a ceramic material. For instance, the joining of ceramics to metals by active metal brazing depends on the wetting of the ceramic by the braze filler. In the electronics industry, the packaging of microelectronic devices puts metals and ceramics in direct contact and the reliability of the device often relies upon the integrity of the metal/ceramic interface. Sintering in the presence of a liquid phase also primarily relies on the wetting of the ceramic compact by the liquid phase.