Changes in friction of zinc flake coated threaded fasteners due to humidity, temperature and storage duration

Abstract

The friction coefficient of a specific material combination is often assumed to be invariant, even in manufacturing processes, such as during the tightening of threaded fastener joints. This paper considers the impact of storage conditions on threaded fastener friction. Fasteners were stored in hot humid and sub-zero temperature conditions to study friction in the thread and underhead contacts. Four Zn-flake coatings, commonly used in the automotive industry were considered. Consequent tightening of these fasteners at room temperature revealed that storage history had a significant impact on their friction coefficients, halving under-head friction in some cases. This varied behaviour was considered to be a response to coating nano-hardness and structure and differences in adsorption/absorption of water and zinc-oxide formation during storage.