Page 153 - The Tribology Handbook

Bearing surface treatments and coatings A27

Table 27.5 The coating of surfaces

Process Mechanism Surface effects Design nspects
Electroplating with hard Plating is carried out at less Surface hardness of up to 900 A good surface for cylinder
chromium than 100°C so there are no Hv and thicknesses of up to Gners but needs to be
distortion problems. Current 1 mm. Also used in thin, as cleaned up by Si C honing.
density can be adjusted to deposited layers about 50 A good piston ring coating
produce tensile stresses p.m thick for use with cast iron liners
which, with diamond honing
produce regular distributed
cracks for good oil retention
Electroplating with tin, lead or Can be plated on to any Soft surface layers with Gives good resistance to fretting
silver metals thicknesses typically up to and improves the bedding in
50 wn and running properties of
harder bearing materials
Electroless nickel plating A low temperature process The hardness is about 550 Hv Good for cylinder liners and
with good throwing power as deposited hut can be all component surfaces
to follow component shapes. increased to 1000 Hv by requiring improved wear
Can also be used with hard heat treatment resistance. Inclusion of
wear-resistant particles or PTFE gives lower friction
PTFE particles dispersed in
the coating
Physical vapour deposition of The coating is transferred to T1.N coatings have a hardness Ideal for cutting tools and for
coatings such as Titanium the component via a glow of about 3000 Hv and are components subject to
nitride discharge in a vacuum usually 2-4 pm thick adhesive wear or low stress
chamber. Deposition abrasion such as hard
temperature is 250450°C particles sliding over the
surface. it needs an
undercoat of electroless
nickel if full corrosion
resistance is required
Chemical vapour deposition of The coating is produced by Coatings of Ti.N Tic. A1,0,. Very hard wear resistant
metals and ceramics decomposition of a reactive CrN WC and CrC in coatings can be created by
gas at the component thicknesses of the order of this process
surface. The operation 5-10 pm
occurs within a reaction
chamber at 800°C minimum
temperature
Spray coating using a gas The coating materid is Coatings can be of hardnesses Suitable for covering large
flame or electric arc supplied to the process as a up to 900 Hv in thicknesses areas of component surfaces
powder or as a wire or in the range of 0.05-1.0 mm
wires. The powder coating
may then be subsequently
fused to the surface
Plasma spray coating Uses an ionised inert gas to Typical coating thickness 0.1 Particularly suitable for
produce very high mm improving the abrasive wear
temperatures which enable resistance of components
oxides and other ceramics to
be coated onto metals.
Component heating is
minimal
Plasma arc spraying Carried out in a partially Up to 10 mm thicknesses of Suitable for high temperature
evacuated chamber or under very hard ceramic materials gas turbine components to
an inert gas shield. The can be deposited give improved oxidation and
component gets hot and fretting resistance. Also
good bonding is achieved. abrasion resistant
Higher temperatures can be components for mining and
achieved by having an agricultural machinery
electric current flow between
the arc and the surface.
The performance of the
materials can be further
improved by hot isostatic
pressing at temperatures of
the order of 1OOO"C and
above
A27.4