Corrosion Resistance of High-Phosphorus Electroless Nickel with a Lower Coefficient of Friction, Nanoparticle Codeposition Electroless Nickel Layer
by Nicole Micyus, MacDermid Inc.
Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2013 in Rosemont, Ill., on June 11, 2013. A printable PDF version is available by clicking HERE.
The corrosion resistance of high phosphorus electroless nickel (EN) is well-known. But how is the corrosion resistance affected by an overlayer of a lower coefficient of friction electroless nickel? The two codeposited nanoparticles under study in the upper layer are polytetrafluoroethylene (PTFE) and boron nitride (BN), separately. EN/PTFE and EN/BN deposits of various wt% concentrations are plated over high phosphorus electroless nickel and the neutral salt spray (NSS) results are compared to those for a single layer of high phosphorus EN.
Keywords: Electroless nickel, high phosphorus electroless nickel, nanoparticle codeposition, PTFE, boron nitride, corrosion resistance, nanocomposite overlayers
The corrosion resistance of high phosphorus electroless nickel (EN) is well-known. Typically, high phosphorus deposits of 1 mil (25 µm) thickness or more can pass 1,000 hrs in neutral salt spray (NSS) testing. It has been shown that duplex EN coatings using high phosphorus as a base deposit can yield neutral salt spray (NSS) results well above 1,000 hrs with low/mid- and mid-phosphorus EN deposits.1 A high-phosphorus electroless nickel is the lower layer, which provides corrosion resistance, while the upper layer endows a particular property needed for a specific application. In this paper, the upper layer consists of a codeposition EN deposit with particles such as polytetrafluoroethylene (PTFE) and boron nitride (BN).
Both EN/PTFE and EN/BN deposits provide a lower coefficient of friction than that of high-phosphorus EN. They are utilized primarily in the automotive, machinery, engineering, and mold and die industries. The lower coefficient of friction deposits provide the hardness of electroless nickel and the lubricity of PTFE or boron nitride. The incorporation of PTFE particles provides dry lubrication and improved release properties. Some applications include plating molds and dies, self-lubrication for electronics, and other sensitive equipment, and low friction wear against mated surfaces. The incorporation of boron nitride particles improves the sliding friction characteristics of electroless nickel. EN/BN deposits can withstand higher abrasion forces and higher temperatures compared to EN/PTFE.
EN/PTFE and EN/BN are both porous deposits, which provide poor corrosion resistance. This study investigates whether or not the codeposition overlayer improves, decreases or has any effect on the corrosion resistance of the high phosphorus base layer when compared to a single layer of high phosphorus electroless nickel. Varying amounts of codeposited particles and varying layer thicknesses are explored.
Electroless Nickel Systems
The base layer for the duplex coatings consisted of a RoHS-compliant high phosphorus deposit of approximately 10.1-10.3 wt% P. The high phosphorus plated at 0.37-0.43 mil/hr (9-11 µm/hr). The EN/PTFE layer was plated to various PTFE compositions: 2-3, 6-7 and 10-11 wt%. All EN/PTFE baths had a plating rate of approximately 0.3 mil/hr (7.5 µm/hr). The high phosphorus matrix for the EN/PTFE was the same system used for the base layer. The EN/BN deposit was generated from a low/mid-phosphorus RoHS-complaint electroless nickel bath with a hexagonal boron nitride dispersion with low-to-moderate mechanical agitation to keep the particles suspended in solution and in constant motion. The EN/BN deposits plated at a rate of 0.6-0.7 mil/hr (15-18 µm/hr).
The surface topography of the high phosphorus EN used as a base layer for all testing is shown in Fig. 1. It is a smooth, pit-free deposit. The low porosity helps to establish a good base for the subsequent layers.