Alternative Cr+6-free coatings sliding against NBR elastomer

Hexavalent chromium compounds result attractive primarily for industrial activity because they provide manufactured products with enhanced hardness, shininess, durability, color, corrosion resistance, heat resistance, decay resistance and tribological properties. 

On the other hand, it poses far more health hazards than trivalent chromium. It is a hazardous substance that increases the risk of developing lung cancer if itis inhaled. Ingestion or even simple skin exposure of chromic acid could increase the risk of cancer formation. In this situation, hexavalent chromium is classified by the International Agency for Research on Cancer (IARC) as a known human carcinogen (Group 1) (Working Group on the Evaluation of Carcinogenic Risks to Humans, 1987), where workers have the highest risk of adverse health effects from hexavalent chromium exposure.

Hexavalent chromium has been deeply used in tribological applications being friction and wear reduction also one of the main objectives in sliding mechanical parts for minimizing loss of energy and improving systems performance (Flitney, 2007) (Monaghan, 2008). In the last years, in fact, attention in maintenance costs saving also grow up, therefore a key question is to achieve low levels of friction as well as high wear resistance. In the field of elastomeric materials in 1978 A. N. Gent et al. (Gent, 1978) studied wear of metal by rubber attributing those phenomena at the direct attack upon metals of free radical species generated by mechanical rupture of elastomer molecules during abrasion. It suggested that such studies might lead to new metal texturing processes and surface treatment that can have the double effect of improving the tribological performances and protecting from external agents. Furthermore, coating technology is gaining ground thanks to new available technologies and focusing in particular to the need of using new alternative non toxic surface treatment with equivalent functionality of Cr+6.

The availability of new coating technologies like High Velocity Oxy-Fuel (HVOF) permits to have a wide range of hard coatings, but a deep study of their mechanical and tribological characteristics is needed due to the strong influence of their roughness, hardness, finishing and resistance to wear and corrosion.

HVOF thermal spray technique allows depositing variety of materials (alloys and ceramics). The powdered feedstock of deposition material is heated and accelerated to high velocities in oxygen fuel. The material hits and solidifies as high density well adherent coating material on the sample/ component. HVOF coatings are also strong and show low residual tensile stress or in some cases compressive stress, which enable very much thicker coatings to be applied than previously possible with the other processes.

An investigation is herein proposed considering NBR (Nitrile butadiene rubber) material sliding against HVOF coated steel rod in order to clarify the influence of the surface characteristics (hardness, roughness and texture) on the tribological measurements. Many times, in addition, the metallic parts need to have good corrosion resistance for protecting them from external hostile atmospheres. A study of the corrosion resistance of the HVOF coatings is then presented, in comparison with the reference Hard Chromium Plating (HCP) treatment.

In this situation, the main objective of this work was to investigate and compare the tribological and corrosion behavior of a reference tribopair NBR/ HCP versus some alternatives based on NBR/ HVOF coatings. These materials combinations simulates contact occurring in sealing systems, where polymer and metallic parts are rubbed each other (Conte, 2006).