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Threaded Joints and Fasteners (Bolts, Screws, Nuts and Pins)

Threaded joints are the most common type of detachable joints of the elements of machines and mechanisms, where fasteners - bolts, screws, nuts and pins, screwed to a certain torque are used.

Fasteners are constantly under stress under the influence of atmospheric conditions, they can be used at high and low temperatures, vibration, radiation, in a vacuum, aggressive gas and liquid media, and by the passage of electric current.

Under cyclic loads and micromovements, the use of materials with the same or similar chemical composition that are involved in the thread joint, fretting corrosion and diffusion adhesion of the surfaces occur. Due to increased roughness, defects in the microstructure, residual tensile stresses, stressing and gouging of the surface layer, the fatigue strength of fasteners is decreased. This leads to difficulties in disassembly, increase of duration, labor intensity and production costs during preventive maintenance, loosening of tension, unscrewing and breakage of fasteners. The problems are aggravated by the fact that 60-90% of the effort applied when tightening the fasteners is spent on overcoming the friction forces in the threads and on the end surfaces. Thus, to create a highly reliable threaded connection, one needs to use fasteners with increased strength, tribological, corrosion and conductive properties, which cannot be provided by one monolithic material.

To improve reliability of threaded joints and efficiency of fasteners, functional coatings such as zinc plating, cadmium plating, nickel plating, phosphating, oxidation, and also anti-seize lubricants are used. However, all these coatings have disadvantages - high friction coefficient, low mechanical and adhesion strength, surface hardening, temperature limitations and degradation.

To improve reliability of threaded joints and solve the problems with adhesion it is proposed to apply TriboPateks antifriction coating by plasma deposition (PACVD cold atmospheric pressure plasma) with a thickness of 2-3 µm on the fasteners. This coating has the properties of a solid lubricant, forms a barrier layer preventing the contact of two metal surfaces, resists significant mechanical stress and radiation exposure, high temperature and chemical inertness, and provides reduced friction and wear, in particular in a vacuum. The process is carried out at atmospheric pressure, the temperature of heating of parts is below 150°C, without changing the parameters of the initial roughness.