This is the type of component that has provided Armoloy with some of its greatest successes and also some of its most embarrassing post-mortems.
Materials are typically 15-5PH, AMS5659 or 13-8PH, AMS5629, Corrosion Resisting Steel for aerospace or, on nuclear or sub-sea applications, 300 series stainless Steel or one of the Inconel series of Nickel based alloys.
What our experience has taught us is that you cannot generalise and say that a coated thread will outperform an uncoated one. In some instances, the very reverse of that statement can be much more accurate. We have learned from experience ( sometimes bitter ) that the quality and finish on the thread form is ultra important as is the post machining handling when some of the softer alloys have been used.
When asked, we have made much of our coating's superior anti-galling properties which can be of substantial benefit when a requirement exists for repeated assembly and disassembly of threaded fasteners in some of the more exotic materials. However, with these enhanced properties comes the accompanying problem of difficulty in machining.
Threads in these materials, unless cutting tips are maintained at their peak performance, show an alarming tendency to tear and leave an imperfect ragged edge.
Add a hard coating to this and you have the means to produce an instant razor blade - just waiting to inflict the maximum damage to that adjacent surface, already struggling to cope with micro-welding, ill-matched dimensions and surface geometry irregularities.
To keep it simple, uncoated threaded components can be often be tested using light, hand assembly. If the unit, in this condition, shows a tendency to tighten up and then ease off again, then coating one or both components will only worsen the situation. A mismatch is present and it must be cured if the coating is to have any chance of succeeding in its desired purpose.
What we have learned, over a lengthy period of practical experience, is that a coating of Armoloy NTDC on good threads is most definitely beneficial for in-service operations and does reduce significantly the risk of galling. The thickness of this coating may vary depending on the specification selected and the application.
One final cautionary statement - A good coating will not cure a bad thread.