Ni-P and Mo: an excellent fretting wear resistant combination

The fretting wear behaviour of a number of hard coating combinations, on a bearing steel substrate, was studied without external lubrication. The coat ings used were Ni-P, Mo, 88% WC-12% Co, 60% Al2O3-40% TiO2 and Cr2O3. The N i-P coating, thickness 11 mu m, was deposited by electroless plating. The o ther coatings, thickness 60 to 80 mu m, were deposited by plasma spraying. The 'crossed-cylinders' test geometry was employed. A preliminary series of tests at strokes of 10 and 56 mu m, involving like-on-like and mixed coati ng combinations, was carried out in laboratory air, over 200,000 cycles, to identify the most promising coating combination. Mo exhibited a low coeffi cient of friction (approximate to 0.1 to 0.3) against itself and all the ot her coatings tested. Its coefficient of friction against Ni-P was particula rly low, at approximate to 0.1. All the other combinations of coatings exhi bited higher values of coefficient of friction, in some cases, in excess of 1.0. Mo against Ni-P achieved the lowest degree of wear under all test con ditions. A more exhaustive investigation of Mo against Ni-P, for fretting s trokes up to 200 mu m, showed that its performance was dependent on test co nditions. Both coefficient of friction and wear increased with stroke, for strokes in excess of 50 mu m, and with decreasing relative humidity, for va lues less than 35%. XPS revealed the presence of MoO3 and Na(OH)(2), both o f which exhibit a layered structure, on the corresponding unworn coatings a nd in the wear scars. Mo against Ni-P offers the prospect of a fretting res istant coating system, able to function in the problematic low stroke regim e, where lubricants have difficulty penetrating into the fretting interface . (C) 1999 Published by Elsevier Science S.A. All rights reserved.