PISTON-ring scuffing occurs most frequently during the break-in period and has been a problem to both the automobile and ring producers, for some time. Ring coatings have been under development for several years and their general adoption by nearly all automobile companies indicates both the need for them and their effectiveness. The coatings fall into two general classes, chemical and metallic. The chemical are: Ferrox, an iron oxide; Granoseal, an iron-manganese phosphate; Graphitox and Grafotox, a zinc-iron phosphate with colloidal graphite; and Surfide, ferrous sulphide. The metallic coating is of electrolytically deposited tin.
Careful tests under accelerated wear or scuffing conditions on newly finished surfaces showed that untreated rings produced twice the wear that occurred on the coated rings when only the compression rings were coated. The difference in wear was even greater when both compression and oil rings were coated properly. Coating the cylinder block gave even further aid, but improper coating was found to be detrimental, causing excessive wear.
Coated piston rings have definitely proved worth while and, with the development work now being carried on in the laboratories of both the piston-ring companies and automobile companies, further improvements are sure to come.