27th IFHTSE CONGRESS & EUROPEAN CONFERENCE ON HEAT TREATMENT 2022

Hydraulic cylinders are one of the main products in the fluid power industry. During their service life, piston rods are exposed to corrosive environments and wear stresses. Thus, a suitable surface treatment for enhancement of corrosion- and wear resistance is of high importance to guarantee a long lifetime of the parts. The main requirements to piston rod coatings include high surface hardness as well as a low and evenly distributed surface roughness to minimize wear of sealing and coating.
During the gas- or plasma nitriding process, atomic nitrogen diffuses into steels and leads to hardening of the surface and edge zone, where formation of nitrides with alloying elements such as Cr, Al and V represents the biggest contribution to hardness increase. Due to its ceramic character, the compound layer consisting of iron-nitrides, leads to an increased corrosion- and wear resistance. To reach a further increase in corrosion resistance, an oxidation step can be added after nitriding in the same process. Moreover, the slightly softer oxide layer can serve as a run-in layer to reduce sealing wear and the small pores act as a lubricant reservoir.
The PACVD technology can be used to deposit Si-doped amorphous carbon coatings (a-C:H:Si, “Diamond Like Carbon” - DLC). The Duplex DLC Xtended® process is a combination of plasma nitriding with subsequent DLC coating, enabling edge-zone hardening and coating in a single process.
Treatment of steels with RÜBIG technologies GASOX® or PLASOX® meet the requirements of 24-120h in the neutral salt spray test (NSS, ISO 9227). The Duplex DLC Xtended® thick coatings withstand more than 500h in the NSS test.
A comprehensive characterization of RÜBIG technologies in comparison with commercially available Cr- and Ni-Cr coatings including corrosion- and tribological testing with sealings as counterbody showed the applicability of of GASOX® and PLASOX® as alternatives for one layer Cr- coatings. For higher corrosion-requirements, Duplex DLC Xtended® coatings may be used.