An investigation into the tribological performance of Physical Vapour Deposition (PVD) coatings on high thermal conductivity Cu-alloy substrates and the effect of an intermediate electroless Ni–P layer prior to PVD treatment

Abstract

The use of high thermal conductivity copper alloys in plastic injection moulds provides the benefit of rapid moulding cycles through effective heat transfer. However, copper alloys are relatively soft and wear rapidly so manufacturers are now developing copper alloys with increased hardness and wear resistance. Their wear resistance can be further improved by the deposition of hard coatings such as electroplated chromium, electroless nickel and Physical Vapour Deposition (PVD) coatings. In this paper, the tribological performance of three proprietary high-strength Cu alloys (Ampcoloy® 940, Ampcoloy® 944 and Ampcoloy® 83) coated with PVD CrN and CrAlN coatings has been evaluated. A medium phosphorous content electroless Ni–P (ENi–P) plated layer was also deposited as a pre-treatment to PVD CrN and CrAlN coatings to increase the load support. The effect of this intermediate ENi–P layer was also evaluated. Surface roughness and instrumented hardness measurements were used to characterise all coated systems in both plated (i.e. with the intermediate ENi–P coating) and standard (i.e. unplated) conditions. Scratch tests were also performed to evaluate the effect of the ENi–P on PVD coating adhesion to Cu alloy substrates. The tribological behaviour of PVD-coated Cu alloy systems was evaluated by pin-on-disc wear tests and ball-on-plate impact tests. Results demonstrate that the ENi–P layer improves the load support for PVD coatings on Cu alloys, thereby improving their tribological performance. However, for PVD-coated Cu alloys in the standard condition, the Cu alloy substrate type plays an important role in the tribological performance of PVD coatings. For instance, PVD CrN coatings were more suited to a certain Cu alloy type whilst CrAlN to the other two types.