Important: The notes below are not intended as detailed information they are simply to provide introductions to the processes. I have tried to provide links to sources of information which should be used for quality information
The modern trend is to use surface engineering techniques to provide high
performance surfaces on substrate materials providing vastly improved component
and tool performance, and life, at reasonable costs. The expansion of surface
engineering has largely taken place over the last thirty years. The present situation is
that most parts high performance engines have their surfaces treated to withstand
temperature, wear, and corrosion. The majority of the cutting tools used in machining
have improved surface hardness, wear resistance etc resulting from use of
surface engineering processes.
Surface Heat Treatments
Surface heat treatments are most generally used to obtain high surface hardness while maintaining the core toughness. Surface heat treatment processes include, flame hardening, case hardening (carburising), nitriding and induction hardening. Information on these processes can be found on webpage. Hardening.
Information on painting is found on webpage Painting
These coatings are bright smooth, hard, and heat resistant. The are most often used for
kitchen, bathroom and toilet surfaces, pots and pans. The resulting surface is attractive durable and
is easy to clean. The surface protection method is also used in all branches of engineering where surface
resistance to chemical attack, wear, or high temperatures are required.
Cladding is when a thin layer of one metal is laminated onto the surface of another e.g.
a thin sheet of stainless steel is laminated onto a thick mild steel plated to provide comparatively
low cost corrosion resistant plate. Most cladding is done be rolling sheets together in mills.
A specialised application of the cladding process is by using explosive charges. This metalworking technique
uses controlled detonations to force dissimilar metals into a high-quality,
metallurgically bonded joint. The transition joint has high mechanical strength, is ultra-high
vacuum tight and can withstand drastic thermal excursions.
Powder coating is an method of applying a decorative and protective finish to a wide
range of materials and products used in homes, construction and in engineering.
Zinc Coating (Galvanising)
Information on galvanising is found on webpage Painting
Tin plate is sheet steel covered with a layer of tin. The primary use of tinplate now is the
manufacture of tin cans. Unlike zinc, tin does not protect steel electrolytically. Tin will
only protect the iron if the tin-surface remains unbroken. It is therefore not safe to eat food
from a rusty tin can.
This process involves coating an object with a thin layer of metal by electro-deposition. Metals
that are most commonly deposited by electroplating are copper, nickel, chromium, tin , zinc, brass, gold and silver.
The anode surface area should be at least 1,5 times the cathode area and the current , for this application should be about 50 amperes per m2. The direct current voltage varies from about 0,5 to 2V.
Most cadmium produced is electroplated onto steel, iron, copper, brass, and other alloys to
protect them from corrosion. Cadmium plating is especially resistant to attack by alkali. Some is
used as the anode material in rechargeable batteries in which the oxide of nickel or silver is the cathode.
convenience of application,
This process is and electochemical process generally applied to Aluminium although the process is also used
for magnesium, titanium, zinc and magnesium. Anodising is an electrolytic passivation process
used to increase the thickness and density of the natural oxide layer on the surface of metal parts. Anodising
provides corrosion resistance and surface hardness,and is electrically insulating. The anodised finishes are
often porous. The porous surfaces can be dyed any colour which makes the process useful as a
Thermal Spray Coatings
The Plasma Spray Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating. This plasma spray process carried out correctly is called a "cold process" as the substrate temperature can be kept low during processing avoiding metallurgical changes and distortion to the substrate material.
The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled.
Pressurised plasma gas (argon, nitrogen, hydrogen, helium) flows through the anode and passes the anode. The cathode is
in the form of a constricting nozzle. The plasma is initiated by a high voltage discharge which
causes localised ionisation . This results in conductive path for a DC arc to form between cathode and anode.
The resultant heating from the arc causes the gas to reach extreme temperatures, dissociate and ionise to
form a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which
does not carry electric current). When the plasma is stabilised ready for spraying the electric
arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This
stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water
cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its
temperature and velocity. Powder is fed into the plasma flame most commonly via an external
powder port mounted near the anode nozzle exit. The powder is so rapidly heated
and accelerated and the spray distances can be in the order of 25 to 150 mm.
Tungsten Carbide-Cobalt Composites
Applications for this process include
Jet engine compressor blades
Notes below relate specifically to plasma nitriding as a typical process example
Plasma nitriding (also known as glow discharge or Ion nitriding) is a low temperature,
low distortion surface engineering process. A glow discharge plasma is used to transfer
nitrogen to the surface of the components undergoing treatment. In the case of plasma
nitro-carburising nitrogen and carbon are transferred to the surface.
The component to be surface treated are placed in special chambers.(furnaces). The components are made the cathode of an electrical circuit and the chamber is made the cathode the anode. Application of a voltage of about 400V between the anode and the Cathode electrodes at a low pressure ( ≤ 10 mbar) results in a current intensive glow discharge. This glow discharge covers all of the cathode supplying heat to the surface of the parts and a supply of nitrogen. Nitrogen diffuses into the surface combining with nitride forming elements such as chromium, and aluminium forming alloy nitrides, which significantly strengthen the surface.
The pack cementation, or pack diffusion, process diffuses the coating material
into the substrate, generally to impart oxidation and high temperature corrosion resistance
to the component substrate. Generally the coating material is a powder of aluminum,
chromium cobalt or alloys of these materials.
Ion implantation is an advanced materials engineering process by
which ions of a material can be implanted into the surface of another solid.
This can result in changing the physical properties of the solid.
Ion implantation is used in semiconductor device fabrication , in metal finishing,
and as a tool in materials science research.
The ions can introduce chemical/physical changes in the target surface,and
structural changes as the crystal structure of the target can be altered and even destroyed.
Ceramic and Cermet materials.
Ceramic and ceramic-metallic (cermet) surface coating materials have applications in
areas as diverse as aerospace parts and biomedical implants.
Chemical Vapour Deposition..
Chemical vapour deposition( CVD ) is a generic name for a group of processes
involving depositing a solid material from a gaseous phase onto the surfaces of components being treated..
Precursor gases (often diluted in carrier gases) are delivered into
the reaction chamber at approximately ambient temperatures. As they pass over or come
into contact with a heated component substrate, they react or decompose forming a solid phase which and are
deposited onto the component substrate. The component substrate temperature is important and can
influence what reactions will take place.
Physical Vapour Deposition (PDV) ..
Physical vapor deposition (PVD) is a general term used to describe any
of a variety of methods to deposit thin films by the condensation of a vaporized form
of the material onto various surfaces (e.g., onto semiconductor wafers).
The coating method involves purely physical processes such as high temperature vacuum evaporation or
plasma sputter bombardment rather than using a chemical reaction at the surface e.g.
chemical vapor deposition as described above.