Process optimisation of non-cyanide Ag–PTFE metal matrix composite electroplating for threaded connections

Abstract

Silver–polytetrafluoroethylene (PTFE) metal matrix composite coatings were deposited onto ANSI 304 stainless steel to assess the potential of using a novel nitrate–succinimide non-cyanide electroplating process to produce a corrosion resistant, self-lubricating composite coating. During this study, 32 different bath parameter sets derived from five variables were used to develop the non-cyanide electroplating process without the use of a surfactant or strike deposit. SEM/EDS as well as XPS microscopy were used to analyse the samples. In addition, the functional characteristics and adhesion strength of the coating were assessed using a bi-directional tribometer (to measure the coefficient of friction) and a pull-off test conducted on an Instron tensile test machine. The experimental results revealed that a friction coefficient of 0.23 was achievable (62% improvement over pure silver coating) where adhesion strength is not a primary consideration. For dry lubrication applications requiring high-coating adhesion strength, the maximum adhesion strength achieved for the Ag–PTFE composite coating was 2.5 MPa (44% reduction over pure silver coating) with a friction coefficient of 0.32 (47% improvement over pure silver coating).

Keywords:

• Non-cyanide silver electroplating
• Composite coating
• Adhesion
• Tribology
• PTFE

Acknowledgement

The authors wish to acknowledge the support of the technicians at the engineering and chemistry department at Plymouth University for their assistance in this research. Special acknowledgements are owing to the EPSRC scheme and Newcastle XPS team for supplying XPS facilities.