What is the term for the boundary between two different materials in contact?

The term that appropriately describes the boundary between two different materials that are in contact is "interface." This concept is fundamental in fields like materials science and engineering, as it refers to the specific surface where two distinct materials meet. The interface can significantly affect properties such as adhesion, strength, and corrosion resistance.

In various applications, understanding the characteristics of the interface is crucial. For example, in welding, coatings, or the analysis of composite materials, the properties at the interface can dictate the overall performance of the assembly or component. Factors like surface roughness, contamination, and the extent of bonding all play pivotal roles at the interface level.

While other terms refer to different phenomena or aspects related to materials, they do not specifically indicate the boundary itself between two materials. Thus, "interface" is the most accurate and widely accepted terminology for this context.