Influence of Structure, Roughness, Microhardness, and Friction Coefficient of Zinc Coating on the Tightening Torque of a Threaded Connection

Samples of fasteners were studied: bolts, nuts, and washers with a zinc coating applied in various ways, namely galvanic, thermal diffusion, gas thermal, hot galvanization in molten zinc and in melt galfan (Zn + 5% Al), as well as with zinc lamellar coating. Data on roughness, microhardness, and friction coefficient of zinc coatings were obtained. It has been established that the thermal diffusion coating has the greatest hardness, and the coating obtained by hot-dip galvanizing in molten zinc has the least hardness. Maximum roughness R_a is observed in gas-thermal and thermal-diffusion coatings. The lowest roughness is found in coatings obtained by hot-dip galvanizing and galvanic deposition. The friction coefficients of coatings were studied in finger-disk geometry for all types of coatings. It has been established that zinc-lamella and thermal diffusion coatings have the highest coefficient of friction, and the lowest, gas-thermal coating and those obtained by hot-dip galvanizing. The nominal tightening torques for M3, M10, and M16 bolts are calculated. It has been established that the tightening torque of coatings applied by hot-dip galvanizing in molten zinc and gas-thermal spraying complies with the standards of RD 37.001.131–89. For other coatings, the friction coefficient requires adjustment through the use of lubricants or the application of additional coatings. The results can be used when choosing a zinc coating for fasteners.