Rolling contact fatigue tests of reactively sputtered nitride coatings of Ti, Zr, Hf, Cr, Mo, Ti-Al, Ti-Zr and Ti-Al-V on 440C stainless steel substrates

Abstract

Eight different nitride coatings-TiN, ZrN, HfN, CrN, Mo 2 N, Ti 0.5 Al 0.5 N, Ti 0.5 Zr 0.5 N and (Ti-Al-V)N (from the aircraft alloy T i -6 wt .%A l -4 wt .%V-were deposited by reactive, unbalanced magnetron sputtering onto hardened (RC 60) 440C stainless steel specimens for rolling contact fatigue (RCF) tests. Four different coating thicknesses, i.e. 0.25, 0.50, 0.75 and 1.0 μm, of each type of coating were deposited on 440C test rods 9.5 mm in diameter and 75 mm long. The RCF tests were carried out under atmospheric conditions in a lubricated three-ball-on-one-rod test machine at two different stresses of 4.0 and 5.4 GPa. Six of the eight coatings gave significant improvements in RCF life, whereas only CrN and Ti 0.5 Al 0.5 N showed little or no improvement over that of an uncoated specimen. The best improvement was for the HfN coating 0.5 μm thick at both the hertzian stresses of 4.0 and 5.4 GPa, which gave an increase in RCF life of 13 and 12 times over that of an uncoated specimen respectively. The coating thickness is critical in improving the RCF life and, in general for these coatings, the optimum thickness is in the range 0.5−0.75 μm.