Wear Resistance

Phosphating is a widely used method of reducing wear on machine elements. The ability of phosphate coating to reduce wear depends on uniformity of the phosphate coating, penetration of the coating into metal, and affinity of the coating for oil. A phosphate coating permits new parts to be broken in rapidly by permitting retention of an adequate film of oil on surfaces at that critical time. In addition, the phosphate coating itself functions as a lubricant during the high stress of break-in.

Heavy manganese phosphate coatings (10.8 to 43.0 g/m2, or 3.5 x 10-2 to 0.14 oz/ft2), supplemented with proper lubrication, are used for wear-resistance applications. Parts that are manganese phosphate coated for wear resistance are listed in Table 5.

Table 5 Parts immersion coated with manganese phosphate for wear resistance

Part(a)

Material

Coating time, min

Supplementary coatings

Components for small arms, threaded fasteners®

Cast iron or steel; forged steel

15-30

Oils, waxes

Bearing races

High-alloy steel forgings or bar stock

7-15

Oils, colloidal graphite

Valve tappets, camshafts

Low-alloy steel forgings or bar stock

7-15

Oils, colloidal graphite

Piston rings

Forged steel, cast iron

15-30

Oils

Gears(c)

Forged steel, cast iron

15-30

Oils

(a) Coating weights range from 10.8 to 43.0 g/m2 (3.5 * 10-2 to 0.14 oz/ft2).

(a) Coating weights range from 10.8 to 43.0 g/m2 (3.5 * 10-2 to 0.14 oz/ft2).

(b) Coating may be applied by barrel tumbling.

(c) Coating weights range from 5.4 to 43.0 g/m2 (1.8 * 10-2 to 0.14 oz/ft2).

When two parts, manganese phosphated to reduce friction by providing lubricity, are put into service in contact with each other, the manganese coating is smeared between the parts. The coating acts as a buffer to prevent galling or, on heavily loaded gears, welding. The phosphate coating need not stand up for an extended length of time, because it is in initial movements that parts can be damaged and require lubricity. For example, scoring of the mating surfaces of gears usually takes place in the first few revolutions. During this time, the phosphate coating prevents close contact of the faces. As the coating is broken down in operation, some of it is packed into pits or small cavities formed in gear surfaces by the etching action of the acid during phosphating.

Long after break-in, the material packed into the pits or coating that was originally formed in the pits prevents direct contact of mating surfaces of gear teeth. In addition, it acts as a minute reservoir for oil, providing continuing lubrication. As work hardening of the gear surfaces takes place, the coating and the etched area may disappear completely, but by this time scoring is unlikely to occur.

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