Hardness and Corrosion Resistance of Ni/NiB Bi-Layer and Ni/NiB/NiB-PTFE Tri-Layer Coatings Prepared by Electrodeposition and Dynamic Chemical Plating (DCP) Techniques

Abstract

Multilayer coatings having a thickness of 18–22 μm consisting of different Ni, Ni-B alloys and Ni-B-PTFE composite layers were produced on steel by combining electroplating with electroless dynamic chemical plating (DCP). The individual Ni layers and the multilayer coatings (Ni/Ni-B and Ni/Ni-B/Ni-B-PTFE) were characterized by scanning electron microscopy (SEM), electrochemical polarization measurements and salt spray tests. The Ni coatings presenting high corrosion resistance (6000 h, ASTM-B117) were obtained by electrodeposition from a sulfamate bath containing a samarium salt, whereas Ni-B layers of high hardness (700 to 800 HV) were produced by both methods (electroplating and DCP). Composite Ni-B-PTFE coatings (≈14% volume of PTFE) of high hardness (≈600 HV) and low friction coefficient (0.4 to 0.5) were obtained by the DCP technique. A duplex Ni(10.6 μm)/Ni-B (12μm) coating produced by electrodeposition exhibited a total hardness of 618 ± 9 HV and corrosion resistance higher than 1369 h in the salt-spray test. Combining electrodeposition with DCP produced a three-layer Ni (8 μm)/Ni-B (2.5 μm)/Ni-B-PTFE (7.8 μm) with high corrosion resistance and proper hardness as well as a low coefficient of friction.