Abstract
A multilayer coating on AISI 4140 steel was obtained by combining nickel electroplating and pack boriding methods. Boriding treatment was carried out in Ekabor® II powders consisting of SiC on the steel precovered with nickel electroplating at the temperatures of 900 and 1000 °C for 3 and 6 h. X-ray diffraction analysis showed that on the sample borided at 900 °C, Ni₃₁Si₁₂, Ni₂Si, Ni₃B and Fe₂B phases formed, whereas on the samples borided at 1000 °C Fe2B, Ni₅Si₂, Ni₇₄Si₂₆, Ni₂B and Ni₃B phases are present. It is probable that the formation of the nickel silicide layers was due to silicon in the boriding powder. The hardness of multilayer coatings was measured by using a Vickers indenter with a load of 100 g. Hardness measurements revealed that the hardness of multilayer coating was much higher than that of base steels. Hardness of nickel and silicon rich zones ranged between 700 and 900 HV, whereas hardness of iron, nickel and boron rich zones varied between 1400-1600 HV. Depending on boriding time and temperature the thickness of coating layers ranged from 51 to 165 µm. The best wear properties were obtained for the sample duplex treated at 1000 °C for 6 h. Corrosion tests showed that boriding at 900 and 1000 °C for 3 h followed by Ni electroplating improved the corrosion resistance.

Key words
nickel electroplating, pack boriding, silicides, wear, corrosion

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