Surface engineering modifies the surface of a component to provide properties which may be different from those required of the material beneath it. The objective may be to improve wear resistance, corrosion resistance, surface hardness, friction behaviour, temperature resistance, electrical properties or appearance while retaining the required bulk properties of the component.
This page provides an engineering introduction to the principal surface engineering processes. Some subjects, including painting, galvanizing and surface hardening, are covered in greater detail on separate RoyMech pages.
The correct treatment should be selected from the requirements of the component rather than from the coating process alone. Important factors include:
Several different surface treatments can sometimes provide a technically acceptable solution. Selection therefore involves both the required surface properties and the practical consequences of applying the process to the actual component.
| Process family | Examples | Typical engineering purpose |
|---|---|---|
| Surface hardening and diffusion | Flame and induction hardening, nitriding and related diffusion treatments | Hard, wear-resistant surface while retaining useful properties in the core. |
| Organic and polymer coatings | Paint, powder coating | Corrosion protection, appearance, identification and functional protection. |
| Metallic coatings | Zinc, tin, electroplating, cadmium plating | Corrosion protection, appearance, electrical properties, wear or other surface functions. |
| Conversion / oxide treatments | Anodising | Modification of the existing surface to produce a protective or functional surface layer. |
| Cladding | Roll-bonded and explosion-bonded metallic cladding | Combines a relatively thick functional surface material with a different substrate material. |
| Thermal spray | Plasma and other thermal spray processes | Deposited coatings for wear, corrosion, thermal protection and component restoration. |
| Diffusion coatings | Pack cementation | Changes near-surface chemistry to improve properties such as oxidation and high-temperature corrosion resistance. |
| Ion processes | Ion implantation | Modification of a shallow surface region without applying a conventional coating layer. |
| Vapour deposition | CVD, PVD | Thin engineered coatings providing properties such as wear resistance, hardness, corrosion resistance or specialised electrical and optical behaviour. |
| Ceramic and cermet coatings | Ceramic and cermet materials | Hardness, wear resistance, corrosion resistance, thermal protection and other specialised surface properties. |
Surface heat treatment is used where a hard or wear-resistant surface is required while retaining suitable properties in the underlying material. The treatment may change the microstructure of the existing surface or change its composition by introducing additional elements.
Processes include:
Flame and induction hardening principally alter the microstructure of a suitable steel surface by controlled heating and transformation. Processes such as carburising and nitriding modify the chemistry of the surface region through diffusion.
More detailed information is available on the RoyMech Hardening page.
Painting is one of the most widely used methods of protecting engineering surfaces. A correctly selected paint system can provide corrosion protection, colour, identification and other functional properties.
Successful painting depends strongly on surface preparation, coating compatibility, application conditions, film thickness and the environment in which the component will operate.
See the RoyMech Painting and Hot-Dip Galvanizing page for more detailed information.
Vitreous enamel is an inorganic glass-like coating fused to a substrate at elevated temperature. It can provide a hard, smooth and readily cleaned surface with useful resistance to heat, corrosion and chemical attack.
Enamel frit and other constituents are prepared and applied to the component, after which the coating is dried where necessary and fired so that the material fuses and bonds to the substrate.
Applications extend beyond decorative domestic products and include engineering equipment requiring chemically resistant, heat-resistant or easy-to-clean surfaces.
Cladding combines a layer of one material with a substrate of another material so that the component benefits from properties of both.
A common example is corrosion-resistant alloy cladding on a lower-cost structural steel substrate. The substrate provides the required bulk mechanical properties while the clad surface provides resistance to the service environment.
Metallic cladding can be produced by several processes including rolling, weld overlay and explosion bonding.
In roll bonding, prepared metal surfaces are brought together and subjected to sufficient deformation and pressure to produce bonding between the materials. The process is widely applicable to plate and sheet products.
Explosion bonding uses the controlled energy of an explosive to accelerate one metal plate against another at high velocity. Under suitable conditions the impact produces a metallurgical bond between materials which may be difficult to join by conventional fusion welding.
The process is particularly useful for producing clad plate and transition components containing dissimilar metals.
Powder coating applies a dry coating material to a prepared component and subsequently forms the required coating, commonly by heating and curing.
In a common electrostatic process, finely divided powder is electrically charged and sprayed towards an electrically grounded workpiece. The powder is attracted to the surface and the coated component is subsequently heated to form the finished coating.
Powder coatings can provide decorative and protective finishes and are widely used on engineering products, fabricated metalwork, appliances and other manufactured components.
The coating material, pretreatment, curing conditions and substrate must be compatible with the required application.
Zinc coatings are extensively used to protect steel from corrosion. Zinc can provide both barrier protection and sacrificial protection because it is anodic relative to steel.
Zinc may be applied by several processes, including hot-dip galvanizing and electrodeposition. These processes produce different coating structures and should not be treated as interchangeable merely because the deposited metal is zinc.
See the RoyMech Painting and Hot-Dip Galvanizing page for information on the hot-dip galvanizing process.
Tinplate consists of sheet steel coated with tin and has traditionally been used extensively for containers and other products requiring a thin, formable and corrosion-resistant surface.
Tin coatings on steel behave differently from zinc coatings. Zinc can provide sacrificial protection to exposed steel, whereas the protective effect of tin relies much more strongly on maintaining the integrity of the coating.
Tin can be applied to steel by electrodeposition and other coating processes. The required coating thickness and subsequent treatment depend on the particular product and specification.
Electroplating deposits a metallic coating onto a conductive workpiece by an electrochemical process.
In a simplified metal-plating system the workpiece forms the cathode in an electrolytic cell. Direct current drives the electrochemical reactions which result in metal being deposited onto the workpiece surface.
Depending on the process, the source of deposited metal may involve a soluble anode, metal ions already present in the electrolyte, or a combination of electrochemical reactions controlled by the plating bath.
Electroplated metals can include nickel, copper, chromium, zinc, tin, cadmium, silver, gold and other metals or alloys. The appropriate coating depends on the required corrosion, wear, electrical, decorative or other surface properties.
Coating thickness is influenced by electrical charge passed through the cell together with the efficiency and operating conditions of the particular plating process.
A deposited coating is not necessarily uniform over a complex component. Geometry affects current distribution, and edges, recesses, holes and shielded surfaces can plate differently.
Engineering considerations can include:
Cadmium plating has historically been used on steel components where a combination of corrosion protection, lubricity, predictable fastener behaviour and compatibility with particular service environments was required.
It became particularly important in aerospace and other demanding engineering applications, including some fastener systems.
Cadmium, however, is toxic and its use is subject to significant health, environmental and regulatory controls. Modern engineering should therefore not specify cadmium plating merely because it was traditionally used for a particular component.
For applications where cadmium remains technically necessary and legally permitted, processing must be carried out under the applicable controlled specification.
Anodising is an electrochemical process which deliberately develops an oxide layer on the surface of a metal. It is most strongly associated with aluminium and aluminium alloys, although anodic treatments can also be applied to some other metals.
The component being treated forms the anode in the electrolytic process, which gives anodising its name.
Depending on the anodising process, the resulting surface can provide:
Anodic coatings differ fundamentally from paint and electroplated metallic coatings because the oxide layer is formed from and into the substrate surface rather than simply being deposited as a separate layer.
Thermal spraying is a family of processes in which a coating material is heated and propelled towards a prepared substrate. The particles impact the surface and build up to form a coating.
Different thermal spray processes use different heat sources and particle acceleration methods. Examples include flame spraying, arc spraying, plasma spraying and high-velocity processes.
Thermal spray coatings can be selected to provide:
Plasma spraying uses an electric arc and process gas to produce a high-energy plasma jet. Coating material, commonly supplied as powder, is introduced into the jet where it is heated and accelerated towards the substrate.
The heated particles strike the prepared surface, deform and rapidly solidify. Repeated particle impacts build the required coating.
A plasma spray gun commonly contains water-cooled electrodes. Gas passing through the arc region is heated and ionised, producing the high-temperature plasma jet used to heat and accelerate the coating material.
Materials used in thermal spray systems include metals, alloys, ceramics and composite materials. Examples include:
Applications include turbine and compressor components, shafts, bearing and seal surfaces, piston and pump components, wear surfaces and restoration of worn engineering parts.
Plasma nitriding, also known as ion nitriding or glow-discharge nitriding, is a thermochemical surface treatment in which nitrogen is introduced into the surface of a suitable material using a low-pressure plasma.
The components are placed in a treatment chamber and normally form the cathode. Under suitable low-pressure gas conditions an electrical potential creates a glow discharge around the workpiece. The plasma assists the surface reactions and transfer of nitrogen into the material.
Nitrogen diffuses into the surface and interacts with the substrate and alloying elements. In suitable steels this can produce a hard, wear-resistant surface region while allowing useful core properties to be retained.
Related plasma-assisted processes can introduce carbon as well as nitrogen, depending on the required treatment.
Treatment temperature, case depth, hardness and processing conditions depend on material, required properties and the particular nitriding process. The old page's broad numerical operating ranges have therefore not been retained as universal design data.
Pack cementation is a diffusion coating process used to alter the chemical composition of the surface region of a component. It is particularly useful where improved oxidation or high-temperature corrosion resistance is required.
The component is surrounded by a pack containing a source of the coating element together with other materials required by the process. At elevated temperature, chemical transport reactions deliver the coating species to the component surface, after which diffusion produces the modified surface region.
Processes may introduce elements such as aluminium, chromium or silicon, depending on the substrate and required surface properties.
Aluminising, for example, is used in high-temperature applications where formation of a protective aluminium-rich surface is beneficial.
Ion implantation modifies the near-surface region of a material by accelerating selected ions towards the component and embedding them beneath its surface.
Unlike conventional coating processes, ion implantation does not normally build a distinct coating layer on top of the component. Instead, the composition and structure of a shallow region of the existing surface are modified.
Potential engineering applications include modification of wear, friction, hardness, corrosion or other surface characteristics. Ion implantation is also extensively used in semiconductor manufacture and materials research.
Because the treatment affects only a shallow surface region and does not require a conventional coating/substrate interface, it can be useful in specialised applications where dimensional change must be very small.
Ceramics and cermets can provide combinations of hardness, wear resistance, temperature capability, chemical stability and other properties which are difficult to obtain from conventional engineering metals alone.
Using these materials as coatings allows their surface properties to be combined with the toughness, strength, manufacturability or lower cost of a metallic substrate.
Examples of coating materials encountered in engineering include:
Thin ceramic coatings can be produced by vapour-deposition processes, while substantially thicker coatings can be produced by suitable thermal spray techniques.
Applications include cutting tools, wear components, aerospace components, thermal protection systems, biomedical components and corrosion-resistant surfaces.
Chemical Vapour Deposition (CVD) is a family of processes in which gaseous precursors react or decompose at or near a substrate surface to form a solid deposited coating.
The precursor gases are introduced into a controlled reaction environment. Conditions at the substrate cause the required chemical reactions to occur, forming the coating while gaseous reaction products are removed.
CVD can be used to deposit metals, carbides, nitrides, oxides and other materials. Engineering applications include wear-resistant cutting-tool coatings, high-temperature coatings, corrosion-resistant surfaces and specialised coatings for electronic and semiconductor manufacture.
Conventional CVD can require an elevated substrate temperature, which can limit its suitability for components sensitive to heat treatment, microstructural change or distortion. Other CVD variants operate under different conditions.
Physical Vapour Deposition (PVD) is a family of vacuum-deposition processes in which material is transferred from a source into the vapour phase and condensed onto the component surface to form a thin film.
Unlike CVD, the principal transfer mechanism is physical rather than relying on a chemical precursor reaction at the substrate surface, although reactive gases can be used in some PVD processes to form compound coatings.
PVD techniques include:
PVD coatings can be used to modify:
Hard PVD coatings are widely associated with cutting tools and other tribological applications, while thin metallic and functional films are used across many other engineering industries.
| Feature | CVD | PVD |
|---|---|---|
| Basic mechanism | Solid coating formed through chemical reaction or decomposition of gaseous precursors at the substrate. | Material is physically transferred into the vapour phase and deposited onto the substrate; reactive variants can also form compounds. |
| Process family | Chemical vapour deposition. | Physical vapour deposition. |
| Coating materials | Metals, carbides, nitrides, oxides and many other engineered materials. | Metals and numerous compound coatings including hard wear-resistant films. |
| Temperature | Strongly process-dependent; conventional processes can involve substantial substrate heating. | Strongly process-dependent and often selected where lower substrate temperature is advantageous. |
| Applications | Cutting tools, wear and corrosion protection, high-temperature coatings, electronics and specialised functional surfaces. | Cutting tools, wear surfaces, decorative coatings, optical/electrical films and specialised functional surfaces. |
Surface engineering processes operate over very different dimensional scales. Some modify only a very shallow region of the existing material, while others deposit a measurable coating and cladding processes can add a comparatively thick layer.
This distinction matters when tolerances, fits, threads, bearing surfaces, sealing surfaces or tool geometry are involved.
Many coating failures originate at the interface between the coating and substrate rather than in the coating material itself.
Depending on the process, preparation may include:
The correct preparation is specific to the substrate and surface treatment. A preparation method suitable for painting is not automatically suitable for electroplating, thermal spraying, PVD or another process.
A surface treatment should not be assumed to improve every mechanical property of a component simply because it improves wear or corrosion performance.
Surface condition is important in fatigue behaviour. A treatment may change surface roughness, residual stress, hardness, microstructure or introduce processing defects. Depending on the material and process these changes can be beneficial, neutral or detrimental to fatigue performance.
Fatigue-critical components therefore require the surface treatment to be considered as part of the complete material and manufacturing specification.
Some cleaning, pickling, electroplating and related surface-treatment processes can introduce hydrogen into susceptible materials. High-strength steels can be particularly sensitive to hydrogen-assisted cracking or embrittlement.
Where this is relevant, material strength, cleaning and plating processes, post-treatment requirements and the applicable engineering specification must all be considered.
Wear is not a single material property. Adhesive wear, abrasive wear, erosion, fretting and other mechanisms can require different surface solutions.
A very hard coating is therefore not automatically the best treatment for every wear problem. Coating toughness, substrate support, friction, lubrication, counterface material, contact pressure, temperature and the actual wear mechanism should all be considered.
Surface coatings are frequently selected to improve corrosion resistance, but the protection mechanism varies considerably between processes.
A coating may act principally as a barrier, provide sacrificial protection, form a chemically resistant surface, or alter the surface chemistry of the substrate itself.
Coating damage can therefore have very different consequences depending on the coating/substrate system. The behaviour of damaged paint, tin, zinc, ceramic coating and anodised aluminium should not be assumed to be the same.
When choosing a surface treatment, first define the engineering problem. For example:
The following resources provide additional information on surface engineering. RoyMech's original external resources are retained where appropriate for subsequent link checking.