A review of producing hard coatings by means of duplex treatments using an electroplated coating–thermochemical treatment combination

Abstract

Duplex treatments have been developed to overcome the disadvantages presented by simple treatments to surfaces of different materials and have, in a combined and complementary way, the properties that each of these methods supplies individually. The difference between thermal expansion coefficients for Fe and Cr in hard chrome plating leads to crack formation in the deposited coat, through which corrosive agents migrate and reduce the system’s integrity. Direct deposition by physical vapour deposition (PVD), used for obtaining chromium nitride films on steel substrates, is limited by high production costs, the low thickness obtained and low resistance to corrosion due to the presence of micro pores. Some studies have combined an electroplated chromium with thermochemical treatments made in a controlled atmosphere or vacuum furnaces or by plasma. This kind of duplex treatment allows compounds such as CrxN, CrxCyN and CrxCy to be obtained from chemical and micro structural transformation of chromium with nitrogen and/or carbon, the sealing of cracks in the coating and increasing the magnitude of properties like hardness and density, improving wear and abrasion and corrosion resistance.

Keywords

• Duplex treatment
• electroplated coating
• thermochemical treatment
• nitriding
• carburising
• nitrocarburising
• plasma