1. Electro-chemical Treatments
Electro-chemical coatings are produced by electrolysis of an aqueous
solution of a salt of the coating metal, the component to be coated being the
cathode. For good wear resistance, chromium is the most widely used coating
metal as it combines high hardness (about 1000 Hv) with corrosion resistance
and a low coefficient of friction against steel if the load is not excessive.
Chromium coatings are limited in thickness to 0.5 mm because of internal
stresses. Thicker coatings may be obtained using nickel, but the deposit
is relatively soft (about 250 Hv) so it is not normally used for wear resistance.
Hard particles of carbides (eg SiC, Cr2C3) and oxides, (eg Al2O3) can be incorporated in electro-chemical coatings of nickel and cobalt during plating to give a hardness of 600 Hv.
2. Chemical Treatments
Chemical coatings are produced by the immersion of the component in a solution
of a salt of the coating metal, no current being used.
Coatings of nickel,
phosphorus and nickel boron alloys are produced by the reduction of a nickel salt
by sodium hypophosphite or sodium borohydride respectively.
After heat treatment coatings have a hardness of up to 1000 Hv and good adhesive
wear resistance.
Electroless nickel coatings are more expensive than electro-chemical chromium coatings but they have superior corrosion resistance and uniform thickness can be obtained on complex shaped components. Hard particles, eg of SiC, can also be incorporated in the nickel during deposition to give a composite coating with a hardness of about 1300 Hv.
Thin (0.01 mm) coatings of metal phosphates are also formed chemically on steel components to provide a low friction surface and resistance to adhesive wear, usually to aid running in.
3. Chemical Vapour Deposition
Chemical vapour deposition (CVD) is a process whereby compounds are reacted in
the gas phase to form a dense layer on a heated substrate. The most widely used
wear resistant materials deposited by CVD are titanium carbide and titanium nitride
and with these materials the coating thicknesses are limited to about 10 micro m by
interfacial stresses.
Temperatures in the range 800oC to 1000oC are required for
deposition and at these temperatures thermal distortion and chemical reaction
between coating and substrate limit the choice of substrate.
In wear resistance applications, only cemented carbides and some tool steels
are used and in these cases the reaction that takes place results in very high
bond strengths.
Compared to PVD processes, CVD has much better 'throwing power' ie the ability to coat complex shaped components with a coating of uniform thickness.
4. Physical Vapour deposition
Physical vapour deposition (PVD) processes are performed at sub-atmospheric
pressures, the coating atmosphere being generated by thermal evaporation or
electrical sputtering of a suitable source material, possibly with the addition
of a reactive gas.
For titanium nitride, the most widely used wear resistant material deposited by
PVD, the coating rate is limited to few microns per hour with a thickness range
of 1 - 10 micro m.
Coatings are dense and with appropriate techniques, good adhesion to the
substrate is achieved. Substrate temperatures do not exceed 500oC.
5. Spraying Processes
A number of processes have been developed in which particles of the coating
material are heated to a molten or plastic state and projected at the substrate
which is relatively cold (less than 200oC). Coating density and
strength of bond with the substrate increase with projection velocity. This
may be about 100 m/s in a simple combustion gun; 500 m/s in a plasma gun in which
gases, usually argon and hydrogen, are heated in an electric arc to 15,000oC;
and 800 m/s in a gun in which metered quantities of acetylene and air are detonated.
Most metals and ceramics can be sprayed onto a wide range of metallic substrates.
All sprayed coatings are porous to some extent, the porosity varying from about 20% for application by combustion gun to less than 1% for detonation gun coatings.
The bond between the coating is mainly mechanical and is usually much less strong than those obtained by other coating processes.
6. Welding Processes
All of the various welding methods can be used to deposit wear resistant coatings
(hard facing). Coating materials range from low alloy steels to tungsten carbide
composites.
Rates of deposition are high and good bond strengths are obtained because of
alloying at the coating - substrate interface. There is no easily defined upper
limit to coating thickness although there is a tendency for some of the harder
materials to crack if deposited too thickly. These cracks are not usually harmful
since they are perpendicular to the surface and do not usually reach the interface.
It is however, impracticable to produce coatings less than 2 or 3 mm thick.
Most hard facing materials are based on:
Microstructurally most hardfacing alloys consist of hard phase precipitates
such as borides, carbides or intermetallics bound in a softer iron, nickel or cobalt
base alloy matrix.
Carbides are the predominant hard phases in iron and cobalt base hard facing alloys.
Borides and carbides are the main hard phases in nickel base hard facing alloys.
The matrix alloys in most cobalt, nickel and high alloy iron base hard facing
alloys generally contain up to 35% Cr, up to 30% Mo and up to 13% W, with smaller
amounts of silicon and manganese.
Return to module introduction
David Plane and David Grieve, 25th March 2003.