Surface Engineering Processes - A Review of Their Suitability for Space
Vacuum Applications
Introduction
There are many ways of treating metal surfaces to enhance the tribological
properties. These may be grouped into three broad categories:
Modifying the surface without altering the substrate's chemical constitution
There are two alternative methods for modifying the surface:
- By heating
When dealing with transformation hardenable alloys, in particular
carbon steels, low alloy steels and cast irons, there is an option
to harden (for improved wear properties) using flame, induction,
laser or electron beam techniques. In this case, instead of heating
the whole component (as in through hardening), only the surface is
affected, so that the bulk properties, specifically the toughness,
remain unaffected, and component distortion is minimised.
Such processes produce parts which must be post ground to size and
to provide a good surface finish. They produce no features that would
be detrimental in a space-vacuum environment, but the processes are
not applicable to light-weight alloys of titanium or aluminium and
are unlikely to find many space-related uses.
- By mechanical working
Cold working the surface by peening, shot blasting
or other specialised machining processes to produce deformed layers
increases the stored energy and compressive stress, thereby increasing
the hardness, fatigue and stress corrosion resistance.
Of these processes, only shot-peening is recommended for space applications.
The process is highly controlled and leaves no residues or inclusions
in the surface (as will blasting with an abrasive grit). Shot peening
can be applied to light-weight alloys to provide controlled surface
texture (influencing friction or prior to the deposition of a solid
lubricant coating).
Changing the surface layers by altering the alloy chemistry
There are a range of options:
- Thermochemical diffusion treatments introduce interstitial
elements, such as carbon, nitrogen and boron, or combinations of carbon
and nitrogen, into a ferrous metal surface at elevated temperatures.
However, the processes are not confined to interstitial diffusion; metallic
substitutional elements or metalloids are used in processes such as
chromising, aluminising and siliconising.
Interstitial element diffusion into steels falls into two categories:
- those carried out at low temperatures, i.e. within the ferritic
range, or
- high temperature treatments in the austenitic range.
Ferritic processes include gas nitriding (typically
525°C), plasma nitriding (400 to 600°C) and nitrocarburising
processes (approx 500°C). They are used to increase the hardness and
wear resistance of a steel surface and, for space components, those
processes using a gas or a plasma are recommended (salt bath processes
possibly leaving the risk of contaminating residues). Nitriding of
low alloy steels for gears is ideal, but if a light weight material
is required, only the technique of plasma nitriding can be applied
to titanium. It uses a temperature of near 900oC and post-finishing
would be required.
The austenitic treatments broadly include carburising
employing solid (pack), liquid (salt bath) or gaseous media, carbo-
nitriding and boronising. They are performed at temperatures
near 900°C and produce much greater case depths (up to several mm)
than the ferritic treatments. The process is applicable only to steel
and, if it is used to harden a space component, a gas or plasma-based
process would be recommended
- Oxide coatings on the surface of components can produce significant
tribological advantages. However, they are used primarily for terrestrial
applications, preventing scuffing, adhesive wear and metal transfer.
They are unlikely to be useful in space, with inherent porosity which
might produce out-gassing problems
- Anodising treatments for aluminium alloys produce oxide layers
which reduce adhesive wear and are significantly harder than the substrate
(up to 500Hv). In this case, the process of hard anodising is
carried out in an oxidising acid at around 0°C, so that a layer of oxide
up to 500 microns thick is produced. Surface growth is exactly half
of that layer thickness. It is ideally suited for improving the wear
properties of aluminium parts (eg gears) for space applications, with
an ability for precise control with no post-finishing. It cannot be
used on steel and, on titanium, provides only a sub-micron thick decorative
layer
Anodising may be followed by treatments to seal the surface and incorporate
solid lubricants into the surface to lower friction and reduce wear
rates. In this respect, the cellular structure of the layer readily
lends itself as a key and a reservoir for low friction polymers, particularly
PTFE
- Ion Implantation. In this process, atoms of gaseous or metallic
elements are ionised and pass to a high vacuum chamber, where they are
accelerated through a mass separator. Selected ions are then further
accelerated and implanted into the target component. The implanted species
occupy interstitial sites and distort the lattice. It is a low temperature
process, typically 150°C for small items and less for larger components.
The depth of effect is very shallow, 0.2 microns, but the surface properties
such as wear resistance, friction and oxidation/corrosion resistance
can be enhanced. It is a precise, clean process and might find applications
in space for improving wear resistance or tailoring the chemistry of
a surface to improve its lubricity.
Adding layers of material to the surface
There are many options for applying surface coatings:
- Welding and Surface Alloying
These industrial processes are not relevant to space applications;
they are used to produce massive deposits for extreme wear resistance.
- Thermal Spraying
This 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 such as wear resistance or a thermal barrier. For space
applications, it is recommended that those processes providing minimum
porosity (ie high energy plasma, detonation processes and high velocity
oxy-fuel processes) be employed so as to minimise the possibility
of outgassing.
- Electroplating Over 30 metals can readily be deposited from
aqueous solutions.
Those relevant to Space applications are:
- Hard chromium plates (typically 1000Hv and up to 1mm thick)
are ideal for resisting wear and galling. The coatings are space-
compatible, but the plating is micro-cracked and will require thorough
post cleaning to remove any acid residues that might cause out-gassing.
Post finish grinding will generally be required and specialists
pre-treatment procedures are required when plating aluminium or
titanium. Softer (600Hv), crack-free versions of Cr plate (maximum
30 microns) are also available.
- Nickel plating is used mainly for corrosion protection
in terrestrial applications, but may find uses in Space. It is soft,
bright and reflective.
- Copper deposits are ideal for providing good electrical
conductivity and low contact resistance (eg slip rings). An acid
based process (rather than cyanide) is recommended for space applications.
- 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. They will find applications on launch vehicles where long
periods open to corrosion can be anticipated
- Soft deposits, such as tin, are used to facilitate 'running
in', prevent fretting and can be used in Space. Equally, silver
is also used for anti-fretting, but more so for slip rings.
- Electroless plating The autocatalytic deposition of
nickel/phosphorous has many useful corrosion and tribo/corrosion applications.
Unlike the electrolytic processes, the chemical dip process produces
a deposit with completely uniform coverage. In the case of Ni P, a deposit
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.
The coatings are dense and precise and so provide a useful way of
improving wear resistance on Space components. Special pre-treatment
steps are required when plating aluminium or titanium.
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 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.
- Galvanising and bath aluminising are widely used for
sacrificial corrosion protection of steels. They will find Space related
applications only on launch structures and launch vehicles
- 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. The
process might find some applications in Space, but thin, hard coatings
like TiN are more likely to be applied by PVD (see below).
However, techniques for plasma assisted chemical vapour deposition
(PACVD) have developed which permit coatings to be deposited at temperatures
well below the tempering temperatures of materials like ball bearing
steels. In particular, this technique allows the deposition of ultra-hard
carbon based coatings (about 2 microns thick), called Diamond-Like
Carbon which confers unique properties of low friction, wear resistance
and 'kindness' to the sliding counterface. It provides low friction
in vacuum and is a strong candidate for precision parts in Space.
- 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. Being
a vacuum process, it is ideal for handling precision, clean parts that
are to be used in Space and where extreme wear resistance is required.
Coatings like TiN do not normally give low friction.
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 (TiN, the most common coating), zirconium, hafnium or
chromium and other metals have been deposited onto metallic components
to provide thin (3 to 5µm), hard (>3000Hv) layers of inert, low
friction coefficient compounds.
Sputter ion plating techniques are also used to deposit solid
lubricants like MoS2, PTFE and lead onto bearing surfaces,
particularly for service in vacuum and space satellites (eg ball bearings
and gears). 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.
- Powder coating techniques are used to deposit fluorinated polymer
coatings for low friction and non-stick. Care is needed in vacuum since
there may be unwanted outgassing. In general, the role of a compliant,
low friction coating for Space applications is best achieved with the
electroless nickel/PTFE system (see above).
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