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.
Painting
Paint is a liquid which dries to form a surface.
Vitreous Enameling
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.
Porcelain and ceramic materials are first mixed with water and clay then applied by brushing or spraying to the surfaces to be protected. The coated surfaces are then dried and the object is fired in a kiln at tempereratures high enough to fuse the particles together.
Cladding ..
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.
More mundane cladding process include simple plastic lining /sleeving of pipe bores.
Powder coating
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.
Powder coatings can be applied to metals, plastics, ceramics, composites, glass and wood.
The powder used for the process is a mixture of finely ground particles of pigment and resin, which is sprayed onto a surface to be coated. The charged powder particles adhere to the electrically grounded surfaces until heated and fused into a smooth coating in a curing oven. The result is a uniform, durable, high-quality, and attractive finish. Powder coating is one of the fastest-growing surface ooating technologies .
Zinc Coating (Galvanising)
Galvanizing is a method of protecting a steel surface from corrosion by providing a surface coating of Zinc. Galvanised coatings corrode preferentially to steel, providing sacrificial or cathodic protection to small areas of steel exposed through damage to the coating. Unlike organic coatings no touch up is needed.
Tin Plate
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.
Preparation of sheet steel includes cleaning, pickling, and polishing. The application of the tin coating is generally by use of an electrolytic process although sometimes a simple immersion processis used . Following tin plating the tinplate is passed through a induction coil heating process to melt the tin and add lustre. The finished thickness of tin is about 0.0008 millimetres.
Electroplating
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.
Taking copper plating as an example.refer to sketch below. A direct current is passed from a pure copper plate ( the anode) to the workpiece ( the cathode) through a copper sulphate solution (the electolyte ). A rather complicated electro-chemical process results in copper going into solution from the anode and being deposited from the solution at the cathode. The thickness of the resulting plated finish on the cathode depends on the plating time and the current strength.
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.
Cadmium Plating
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.
Cadmium plated fasteners are used widely in in aircraft manufacture because of cadmiums diverse range of properties including
convenience of application,
corrosion resistance associated with aluminium,
freedom from gummy or bulky corrosion products,
excellent lubricity and freedom from stick-slip for consistent torquing,
softness and malleability,
non-galling,
the ability to accept chromate conversion post-treatments
low levels hydrogen embrittlement.
Anodising
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 decorative finish.
The anodising process is an electolytic process in which the workpiece forms the anode and from the process name is derived from this feature.
Anodic films are more adherent and robust than painted surfaces.
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.
Typical spray materials are included below
Tungsten Carbide-Cobalt Composites
Nickel-Aluminium Composites
Aluminium Oxide (ceramic)
Aluminium-Bronze Composites
Nickel-Chrome- Ceramic Composite
Applications for this process include
Jet engine compressor blades
Lathe centres
Pump seals,
piston rods
Piston rings
Surface recovery
Plasma Nitriding/Carburising/Boronising.-
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.
Operating at temperatures between 400�C and 750�C these plasma processes can produce high surface hardnesses and hardened depths up to 0.8mm.
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.
Pack Cementation
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.
The parts to be coated are placed on a sled in a mixture of the coating material and an inert powder, such as aluminum oxide, along with a halide salt. The sled is then placed in a protective atmosphere (hydrogen or argon) furnace and brought up to the coating temperature. The salt vaporises and combines with the coating to generate the transporting vapor species. The sled is placed in a furnace and brought to a temperature at which the coating material will react with the salt to form a metallic halide vapor, which comes in contact with the surface of the parts to form the coating.
By controlling on the coating material, the coating material concentration in the pack, the time and temperature of the diffusion cycle, the desired surface coating results.
Ion Implantation
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.
Originally developed at the UKAEA's Harwell Laboratory, ion beam implantation is now available as an industrial scale process that allows toolmakers to increase hardness, fatigue life, corrosion resistance and accuracy of form. This extends tool lifetimes, by reducing wear and friction.
Ion implantation is a surface modification process in which ions are injected into the near-surface region of a substrate. Essentially, the technique involves accelerating ions to speeds of around 103m.s-1 through an electric field. The high-energy ions, typically 10�200 kiloelectron volts (KeV) in energy, are produced in an accelerator and directed as a beam onto the surface of the substrate. The ions impact on the substrate with kinetic energies 4�5 orders of magnitude greater than the binding energy of the solid substrate and form an alloy with the surface upon impact. Virtually any element can be injected into the near-surface region of any solid substrate. Commonly implanted substrates include metals, ceramics, and polymers. The most commonly implanted metals include steels, titanium alloys, and some refractory metals.
A significant advantage of ion implantation is that the treated surface is an integral part of the workpiece and does not suffer from possible adhesion problems associated with coatings. The moderate heating associated with the process virtually eliminates any risks of distortion or oxidation effects. Ion implantation produces no dimensional changes in the workpiece.
Ceramic and Cermet materials.
Ceramic and ceramic-metallic (cermet) surface coating materials have applications in areas as diverse as aerospace parts and biomedical implants.
Ceramics and cermet materials have unique thermal, mechanical, chemical and electrical properties, but their high fabrication cost, brittleness and size and shape limitations as monolithic components restrict many potential applications. There are advantages in using ceramics as coatings on metallic substrates. Thin ceramic coatings deposited by vapour deposition techniques (PVD and CVD) are widely used as wear and corrosion resistant coatings. These coatings include diamond, TiC, TiN, ZrN, Al2O3 etc. Since these deposition processes operate at atomic or molecular level well developed, dense structures having properties of dense monolithic materials can be produced. However, the thickness of these coatings is typically kept less than 10μm, to ensure the maximum benefit from the process.
Several applications require thicker ceramic coatings, e.g. for increased wear or corrosion resistance. A method of creating of thicker ceramic coatings has been the use of thermal spray technologies. In thermal spraying ceramic feedstock powder is fed to high temperature plasma or flame, melted to droplets, which are accelerated to high speed and solidified on the substrate forming a lamellar structure.
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.
The CVD process relies on the production of a species by the reaction of the element that is to be deposited with another element that results in the significant increase in the depositing elements vapour pressure. This volatile species is then passed over or allowed to come into contact with the substrate being coated. This substrate is held at an elevated temperature, typically from 800 - 1150�C. Finally, the deposition reaction then usually occurs in the presence of a reducing atmosphere, such as Hydrogen.
The procedure is used in the semi-conductor industry and to protect components from wear, corrosion or abrasion in engineering environments. These industrial fields are diverse and range including gas turbines blades, gas cooker parts, coinage and important nuclear power plant components.
CVD coatings can contain metals such as chromium, aluminium, boron, silicon, titanium, and manganese. The substrate materials include steel and non-ferrous alloys.
This is a high temperature process with the associated disadvantage of distortion of the components being treated.
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.
A typical PVD process is vacuum metallising (metalizing) involving depositing very thin coatings on metal or plastic parts. e.g Aluminium is most commonly used. The aluminium is vaporised electrically in a vacuum chamber containing the parts the vapour then condenses evenly on the parts providing very thin bright coatings.
Variations of this process include evaporative deposition, electron beam physical vaport deposition, Sputter deposition, Cathodic Arc Deposition and Pulsed laser deposition . These processes are used to form coatings which alter the mechanical, electrical, thermal, optical, corrosion resistance, and wear properties of the substrates.
