Boundary lubrication occurs when solid surfaces are very close together, allowing contact between surface asperities. Friction and wear in this regime are determined by interactions between the solids and liquid lubricant at the asperity contacts. The bulk properties of the liquid play little role. Characteristics include liquid-solid interactions, asperity contact leading to increased friction and wear, and thin lubricant films less than 100nm thick consisting of only a few molecules. Examples include piston rings and cylinders in engines at top and bottom dead center positions. Protective boundary films can form via chemical interactions or reactions to absorb to surfaces and prevent direct contact between bodies, reducing friction.