An Experimental Study on Factors Affecting the Friction Coefficients in Electroplated Bolts

Abstract

Electroplated bolts have been widely used in the automotive industry for many decades. In the present work, for the first time, the role of each layer of the bolt coating (i.e., electroplating, passivation layer, and topcoat layer) on the coefficient of friction of a bolted joint were experimentally and systematically investigated. The coefficient of total friction (μ tot), coefficient of friction between threads (μ th), and coefficient of friction between bearing surfaces (μb ) were calculated by a torque–tension testing system and compared under different experimental conditions. It was found that the μ th, μb , and μ tot on pure Zn plating was slightly different from that on Zn-Ni alloy plating; the black passivation layer had lower μ th, μb , and μ tot than the clear passivate when topcoats were applied; and the topcoat played a dominant role in reducing the μ tot. Other factors including washer material, nut plating, and heat treatment were also studied. Results showed that the aluminum washer gave a dramatically higher μb than the steel washer. The tested plating layers of Zn, Ni, and Zn-Ni alloy on the nut did not significantly affect the μ th. Tested bolts could withstand heat up to 182 °C for 1 h. Higher temperature or longer heating time increased μ th, μb , and μ tot. These findings will increase the understanding of the factors that affect the friction coefficients in electroplated bolts and will help to better design bolted joints.