underlayer

An underlayer is a layer located beneath another layer in a multi-layered system. It functions as a base, foundation, or preparatory coating that affects adhesion, stability, insulation, or barrier properties of the layers above it.

In geology and soil science, the underlayer refers to deeper strata underlying the surface layer, such as bedrock or parent material. The properties of underlayers influence soil formation, drainage, groundwater movement, and stability of overlying soils or structures.

In construction and flooring, an underlayer (often called a sublayer) provides a smooth, level base for finish materials, adds thermal or moisture protection, and distributes loads to improve durability of floors or pavements.

In coatings and printing, an underlayer is a preparatory coating applied before the topcoat to improve adhesion, prevent color bleed, and enhance durability. In electronics and thin-film fabrication, underlayers provide adhesion to the substrate and can act as diffusion barriers or improve crystalline growth.

The term is often used interchangeably with sublayer, primer, or interlayer depending on the field. Understanding the underlayer is important for predicting performance and longevity of the final assembly.