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.

中文翻译：

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影响电镀螺栓摩擦系数因素的试验研究

摘要 电镀螺栓已在汽车工业中广泛使用了几十年。在目前的工作中，第一次通过实验和系统地研究了螺栓涂层的每一层（即电镀层、钝化层和面漆层）对螺栓接头摩擦系数的作用。总摩擦系数（μ tot）、螺纹之间的摩擦系数（μ th）和轴承面之间的摩擦系数（μb）通过扭矩-张力测试系统计算，并在不同的实验条件下进行比较。发现纯Zn镀层的μ th 、μb 和μ tot 与Zn-Ni合金镀层的μ th 、μb 和μ tot 略有不同；涂上面漆时，黑色钝化层的 μ th 、 μb 和 μ tot 低于透明钝化层；而面漆在降低μ tot 方面起到了主导作用。还研究了其他因素，包括垫圈材料、螺母电镀和热处理。结果表明，铝垫圈的 μb 显着高于钢垫圈。螺母上测试的Zn、Ni和Zn-Ni合金镀层对μ th 没有显着影响。经测试的螺栓可承受高达 182 °C 的高温达 1 小时。更高的温度或更长的加热时间会增加 μ th 、 μb 和 μ tot 。这些发现将增加对影响电镀螺栓摩擦系数的因素的了解，并将有助于更好地设计螺栓连接。结果表明，铝垫圈的 μb 显着高于钢垫圈。螺母上测试的Zn、Ni和Zn-Ni合金镀层对μ th 没有显着影响。经测试的螺栓可承受高达 182 °C 的高温达 1 小时。更高的温度或更长的加热时间会增加 μ th 、 μb 和 μ tot 。这些发现将增加对影响电镀螺栓摩擦系数的因素的了解，并将有助于更好地设计螺栓连接。结果表明，铝垫圈的 μb 显着高于钢垫圈。螺母上测试的Zn、Ni和Zn-Ni合金镀层对μ th 没有显着影响。经测试的螺栓可承受高达 182 °C 的高温达 1 小时。更高的温度或更长的加热时间会增加 μ th 、 μb 和 μ tot 。这些发现将增加对影响电镀螺栓摩擦系数的因素的了解，并将有助于更好地设计螺栓连接。