Friction and wear reduction with tribological coatings

The basic requirements for a good tribological surface are (1) low sliding friction, (2) good resistance to scuffing, wear and abrasion, (3) long contact fatigue life and (4) adequate subsurface strength to provide dimensional stability. Coatings have inherent deficiencies. The major problem is failure at the interface between the coating and the substrate, which results in flaking, peeling or spalling of the coating under the repetitively applied contact stresses.Three types of coatings which employ different mechanisms to improve the tribological properties and to maintain coating integrity are described in this paper. Nitrocarburizing represents a class of coatings in which the elements are allowed to diffuse into the surface of the structural material to form an alloy with the substrate. Diffusion provides compositional gradients which result in hard wear-resistant surface and which at low shear strengths avoid the interfaces that frequently exist between coatings and substrates. Chemically vapor-deposited chromium and titanium carbides represent a class of coatings in which a chemically distinct layer is grown on top of the substrate and is bonded to the substrate by diffusion. In the third type of coating, hard particles are suspended in a soft matrix. The hard particles provide the wear and abrasion resistance and the soft matrix both bonds the particles together and provides the low friction. Although the bond strength of this coating to the substrate is lower than that provided by diffusion in the other coatings, the soft matrix will yield without flaking under the shear stresses which are developed at the interface.