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  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 Laser Alloying and Laser 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 properties to the finish coating, including elasticity and impermeability. The solid phase functions by blocking 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:

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