Page 152 - The Tribology Handbook

A27 Bearing surface treatments and coatings

Table 27.4 Diffusion of materials into surfaces

PTOCeSS Mechanism Surface eflects De@ aspects
Carburising of steels Carbon is diffused into the Hard surfaces at depths up to Suitable, for example, for gears
surface at temperatures several mm depending on to increase surface fatigue
around 900°C followed by the diffusion time. strength or pump parts to
quenching and tempering Dimensional changes of increase abrasive wear
+. 0.1 % may occur resistance

Carbonitriding of steels Similar to carburising but with Slightly harder surfaces are Can be oil quenched with
addition of nitrogen as well obtainable some reduction in distortion
as carbon of the component
Boronising of steels Diffusion of boron into the Surface hardiiesses up to 1200 The core material needs to be
surface to form iron boride Hv relatively hard to support
the surface layer

Vanadium, niobium or Salt bath treatment at 1020°C Layer hardness up to 3500 Hv Used to reduce wear of the
chromium diffusion into to produce thin surface at up to 15 pm thickness surfaces of metal forming
steels layers of the metallic carbide tools
Chromising, aluminising and High temperature pack Increased surface hardness Improves high temperature
siliconising of steels processes for diffusing these corrosion resistance and
materials into the surface reduces fretting damage

Nitriding of steels which Treatment at about 550°C in Hardness up to 850 Hv with Forms a brittle white surface
contain chromium or cracked ammonia gas or surface layers up to 0.3 mm layer which needs to be
aluminium cyanide salt bath to produce deep removed before use in
chromium or aluminium tribological applications
nitrides in the surface layers
Nitrocarburising of steels Salt bath, gas or plasma Hard surface layer about 20 Reduces fretting damage. The
including plain carbon steels treatments are available pn thick with hardness of surface layer can be oxidised
(tufftriding) about 700 Hv and impregnated with
lubricant to produce a low
friction corrosion resistant
surface
Ion implantation of steel Surface bombardment by a Surface effects to a depth of Can improve resistance to
high energy nitrogen ion 0.1 Frn abrasion and fatigue
beam at 150°C produces up
to 30% implanted material
at the surface
Sulfinuz treatment of steel Salt bath treatment at about Surface layer contains Can give improved scuffing
600°C to add carbon, sulphides which act as a resistance with little risk of
nitrogen and sulphur solid lubricant in the surface component distortion during
treatment

Phosphating of steel Phosphate layer produced by Surface layer which is porous Assists running in and reduces
chemical or electrochemical and helps to retain lubricant risk of scuffing
action
Electrolytic deposition and Copper, aluminium and Creates surface layers at Gives improved scuffing and
diffusion of material into titanium can be treated at selected positions wear resistance
non-ferrous metals temperatures in the range
200-400"c
Hard anodising of aluminium The component is made the The hard layer is usually The coating is porous and can
anode for electrolytic 25-75 pm thick be overlaid with PTFE to
treatment in sulphuric acid. to give a low friction surface
The oxygen generated at the
surface produces a hard
porous oxide layer

A27.3