Fastener Finishings and Coatings
Fastener buyers are familiar with seeing black steel fasteners, Socket head cap screws being a popular fastener in the uncoated black condition. The 'as forged' black surface treatment does not alter the part dimensionally, and retains oil on the surface of the fastener, preventing corrosion. Also used for corrosion prevention is a range of metallic plating processes; zinc being the most popular. There are real dangers in the electroplating of fasteners above class 8.8, USA grade 5; Hydrogen embrittlement can lead to sudden failure of fasteners, and care needs to be taken to avoid this.
Titanium is famous for its tendency to gall at low levels of applied surface stress, even under very low sliding velocities and most especially against other titanium components. It is common therefore to find titanium fasteners that have their threads coated. Coatings involving lubricious compounds such as molybdenum or tungsten disulphide are often used and can be combined with other surface treatments such as anodising. Silver coated fasteners for motorsport is common they are used in high-temperature applications such as exhausts or turbochargers. Silver has a low shear strength, making it an excellent choice as a solid surface lubricant, and it also is very resistant to oxidation. High-strength silver-plated steel nuts are commercially available in both metric and imperial thread forms.
Materials from which commercially available threaded fasteners are produced range through aluminium and titanium to high-strength steels and superalloys and on to various polymers. Beyond the choice of materials, there are a large number of fastener coatings; these can range from those that are designed to prevent or delay the onset of corrosion, through to those that seek to change the behaviour of the material or to combat demanding environments.
This page contains fastener technical information for coating, finishes and plating. Barrel, electrolytic and other methods are used to coat fasteners.