Adding a Layer of Material
to the Surface
There are numerous processes which involve coating with a layer of material,
not necessarily metallic, to meet the requirements of specific service
environments.
i) Weld or roll cladding usually
involves relatively thick layers (1mm to several cm). Weld cladding can
be used to good effect where abrasive wear is a problem, such as coating
digger teeth, tank tracks and mineral handling equipment. Roll cladding
is usually associated with corrosive or mild erosive wear problems, typically
those encountered in the chemical, wood pulp, paper and food process industries.
ii) Laser Alloying. In addition to
laser glazing and laser transformation, the power of the laser can be
used to alloy a mixture of metal or cermet powders on a component surface.
The process is normally concurrent, with the laser spot following the
spray nozzle, so that the coating is fused into an alloy and mixed with
the outer regions of the substrate material.
[Note the distinction between Lazer Alloying and Lazer Hardening
discussed earlier]
iii) Thermal Spraying involves heating
metal, ceramic or mixtures of metal and ceramic powders to a semi-molten
state and depositing them at high velocities on to components. These 'line
of sight' processes can be divided into flame, electric arc, plasma
arc and detonation gun techniques. In spray
fusing, the coating is heated after deposition (usually by a torch)
to fuse the material into a dense alloyed structure and produce a diffusion
bond to the substrate.
The thermal spray process is very versatile and the coating material
and application method can be tailored to produce specific surface properties.
These can range from extreme abrasion resistance with cermets (e.g. WC/Co)
and ceramics (e.g. chromium oxide), adhesive wear and corrosion resistance
(e.g. Ni/Cr with carbide additions), anti-scuffing (e.g. molybdenum),
abradables (e.g. ceramic/graphite coatings for gas turbine stators), thermal
barriers (e.g. zirconia) and corrosion resistant coatings (e.g. zinc).
The process can be automated and accurately controlled, with robot manipulation
of the gun, rotation of the component being sprayed, and computer control
of the spray parameters. For high integrity coatings the application of
hot isostatic pressing (HIPing) after coating has been found
to seal the porosity and further improve the bond to substrate quality.
Thermochemically formed coatings can be considered under
this category. they comprise a slurry of ceramic particles in an aqueous
chromium-based chemical. Through a sequence of applications (spraying,
painting or dipping) and heat curing cycles, the composite (which is free
form porosity) can be built to a thickness over 100 microns. The coatings
are hard (so effective against low stress abrasion) but tend to be brittle
under high loading.
iv) Electroplating Over 30 metals
can readily be deposited from aqueous solutions. There is a tendency to
think that electrolytic deposits are mainly for corrosion resistance,
decorative (e.g. gold, rhodium and platinum
or electronic/electrical usage, but there are many engineering and tribological
applications for electroplates. Hard or soft deposits are used, depending
on the particular function required.
Hard chromium plates (typically 1000Hv and up to 1mm thick)
are ideal for resisting abrasive wear, pick-up and corrosion/abrasion.
Porous or intentionally cracked chromium deposits are used for oil retention
as in automotive cylinder liners, precision bearing sleeves and piston
rings. Softer (600Hv), crack-free versions of Cr plate (maximum 30 microns)
are also available.
Nickel and copper deposits are applied mainly
as corrosion barriers, often as an undercoat for hard chrome, so that
the combination provides both wear and corrosion protection. Nickel deposits
are now available with the addition of a dispersion of file ceramic particles;
such layers provide excellent oil retention and wear properties for cylinder
liners in high revving engines.
Cadmium and zinc (usually around 10 microns
thick) are used to provide sacrificial corrosion protection. Because of
their position relative to iron in the galvanic scale, such coatings will
continue to protect the substrate even if they are scratched or worn.
In the case of cadmium, the environmental pressure is towards its replacement,
with zinc/nickel coatings currently providing some of the best
alternatives.
Soft deposits, such as tin, are used to facilitate 'running
in', prevent fretting and confer corrosion resistance, whereas silver
is used for anti-fretting.
Cobalt is used for high temperature oxidation resistance
and electrolytically deposited cobalt incorporating chromium carbide has
been successfully used in both dry and lubricated conditions at 800°C.
v) Electroless plating The autocatalytic
deposition of nickel/phosphorous and nickel/boron has many useful corrosion
and tribo/corrosion applications. Unlike the electrolytic processes, they
produce a deposit with completely uniform coverage. In the case of Ni
P, deposits around 25 to 50 microns thick with a hardness of about 500Hv
is obtained, but thermal ageing at temperatures around 400°C can develop
hardness values in excess of 1000Hv.
Composite Electroless Plated Deposits involve the production
of plated metals into which micron sized dispersions of non-metallic particles
are incorporated via co-deposition. Composite coatings of
electroless nickel containing silicon carbide exhibits superior abrasive
wear resistance to hard chromium plate in some applications. Incorporation
of 1 to 5 micron sized particles of PTFE as a solid lubricant in nickel
coatings produces low friction, self-lubricating surfaces. Other composite
electroless nickel coatings incorporate a polymer into and on to the surface
to provide a combination of low friction, non-stick and wear prevention.
vi) Galvanising and bath aluminising
are widely used for sacrificial corrosion protection of steels, for instance
in the construction industry and automotive exhausts. They are both based
on submersion in liquid metal (zinc, in the case of galvanising), usually
with a strip steel product being continuously fed through the bath.
vii) Chemical Vapour Deposition (CVD)
involves the dissociation of metal compound vapours at temperatures in
excess of 850°C to produce thin, diffusion-bonded, adherent coatings of
metal carbides, nitrides, carbo nitrides and oxides; typically TiN, TiC,
Ti(CN) and Al2O3. CVD coatings
are used on carbide tool tips (indexable inserts) and on selected tribological
items.
Only selected ferrous items can be treated, e.g. carbides with cobalt
binders or high speed steel items of simple shapes (the latter permitting
them to be re heat treated after deposition). However, techniques for
plasma assisted chemical vapour deposition (PACVD) have
developed which permit coatings to be deposited at temperatures well below
the tempering temperatures for high speed steel, i.e. <550°C. In particular,
this technique allows the deposition of ultra-hard carbon based coatings,
called Diamond-Like Carbon which confers unique properties
of low friction, wear resistance and 'kindness' to the sliding counterface.
viii) Physical Vapour Deposition (PVD)
is becoming increasingly important for small engineering components. PVD
embraces evaporative deposition, sputtering and ion plating in reactive
or inert environments. Process temperatures are relatively low, up to
400°C, thus minimising distortion and preserving the heat-treated state
of the substrate.
Reactive plating takes place in an inert gas. A partial
pressure of reactive gas supplies the carbon or nitrogen, and the metallic
species is added to the system by resistance heating, arc
or electron beam evaporation, or sputtering
from a solid target. Nitrides of titanium, zirconium, hafnium or chromium
and other metals have been deposited onto metallic components to provide
thin (3 5µm), hard (>3000Hv) layers of inert, low friction coefficient
compounds. These ceramic layers enhance the performance of cutting tools
and have considerable potential for many other small components.
Sputter ion plating techniques are also used to deposit solid
lubricants like MoS2, PTFE and lead onto bearing
surfaces, for instance for service in vacuum and space satellites. MoS2,
is particularly attractive, with the resulting layers producing the lowest
dry sliding friction coefficients so far obtained with any coating, even
under normal atmospheric conditions.
Corrosion protection layers and coating compounds for high temperature
tribological service in gas turbines are also effectively deposited by
PVD techniques.
ix) Painting to protect a substrate against
corrosion and improve its aesthetic appearance is probably the best known
surface modification process, with coatings based on acrylics, polyester,
polyurethane, etc. There have been considerable advances in paints, application
techniques and pre-coating/painting treatments. Surface preparation and
corrosion protection methods such as phosphating have brought painting
into the range of engineering coatings. Organic coatings deposited on
metal parts by spraying, brush application or dipping are replacing electroplated
deposits on some automotive parts.
Paints have two principal components, one liquid and the other solid.
The liquid component acts as a vehicle for the solid filler, providing
a uniform coverage. The liquid gives good application characteristics
and confers additional propeties to the finish coating, including elasticity
and impermeability. The solid phase functions by bloking corrosion processes,
building up an impermeable layer and providing physical strength.
Painting, dipping or spraying with organic resins and polymeric materials,
to which metallic, ceramic or solid lubricant compounds are added is providing
for both the corrosion and tribological requirements. One process consists
of zinc flakes bonded with zinc chromate and a proprietary organic material.
This process provides excellent surface protection and is widely used
in the automotive industry for fasteners, springs, clips, sintered parts
and items for steering gears.
Powder coating techniques are now increasingly used for
application of organics and polymers. The process of air-spraying and
electro-forritic deposition without the need for solvents or carriers
provides obvious environmental benefits. It is a rapidly growing area
of surface engineering and is used to provide coatings with non-stick
and low friction properties as well as corrosion protection.
Some current applications for polymeric resins are:
- Organic coatings deposited on metal parts are replacing electroplated
deposits on some automotive parts.
- Corrosion protection; a polymeric or composite coat is applied on
inner and outer tank surfaces, vessels, pipe-lines, etc, that are working
with chemical products or being buried.
- Zinc flakes bonded with zinc chromate and a propietary organic material
provides an excellent surface protection against corrosion.
- For mechanical characteristics; resin coating improves surface flexibility,
impact resistance, etc.
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