Friction and wear characteristics of hard coatings

The friction and wear behavior of four hard coatings (Metco, Diamalloy, Stellite, and ZnSiC) was determined using a pin-on-disk machine. The coatings were thermal sprayed on cast iron disks. The coating compositions were Ni17Cr2.5Fe2.5Si2.5B0.15C (Metco), Fe30Mo2C (Diamalloy), Co30Cr12W2.4C (Stellite), and Zn50SiC (ZnSiC). Sliding was performed between cylindrical pins machined from non-asbestos organic (NAO) brake lining and the coated and uncoated disks. The lining, consisting of resin, aramid pulp, zirconia, graphite, calcium fluoride, rubber and barium sulfate, was developed as a material for automotive brake pads. The coatings were characterized by measuring their hardness, porosity, and corrosion resistance. The Metco and Stellite coatings had a uniform morphology in all directions. The Diamalloy coating had a lamellar microstructure whereas the ZnSiC coating was very porous. The corrosion resistance of the coatings was tested with exposure to 5% NaCl for 168 h. The Stellite coating had the best corrosion resistance. The friction and wear tests were conducted at contact pressures of 1.72, 3.45 and 6.89 MPa and sliding speeds of 1 and 3 m/s. The wear of the lining material was lowest when it slid against the Stellite coated disks and the highest coefficient of friction was observed for the Metco coated disks.