Introduction:
The performance, durability and reliability of manufactured components depend not only on their internal structure but critically on their surface properties. In modern manufacturing, surface treatments and cleaning processes are essential steps that determine how well a part will perform in service. From the fatigue resistance of aircraft turbine blades to the corrosion protection of automotive fasteners, surface engineering technologies play a fundamental role in product quality and longevity. We will explore mechanical surface treatments that improve fatigue life through compressive residual stresses, thermal and mechanical coating processes that protect against wear and corrosion, and the essential cleaning operations that prepare surfaces for subsequent processing. Each technique offers specific advantages for particular applications and understanding these options enables manufacturers to select the optimal approach for their requirements.Mechanical surface treatments including shot peening, roller burnishing and explosive hardening provide critical improvements in fatigue life and wear resistance through compressive residual stresses and surface hardening. Thermal spraying and cladding processes enable application of protective coatings that extend component life in demanding environments. Chemical and mechanical cleaning methods ensure surfaces are properly prepared for subsequent processing operations. Each technique offers specific advantages for particular applications and proper selection depends on numerous factors including the material being processed, the required surface properties, production volumes and cost considerations. The integration of these processes into manufacturing sequences, from initial cleaning through final surface treatment enables the production of components with enhanced durability, reliability and functionality. Understanding the principles, capabilities and limitations of each method is essential for engineers and manufacturers seeking to optimise product quality and performance.
Mechanical Surface Treatments:
Mechanical surface treatments modify the surface properties of manufactured components through physical means inducing beneficial changes in surface structure and residual stress distribution. These treatments are widely applied to improve fatigue resistance, hardness and surface finish across numerous industrial sectors.Shot Peening:
Shot peening is one of the most common mechanical surface treatments, involving repeated impact of small shot particles on the workpiece surface. Cast steel, glass or ceramic shot ranging from 0.125 to 5 millimetres in diameter strike the surface, creating overlapping indentations that cause plastic deformation extending to depths of up to 1.25 millimetres. This deformation induces compressive residual stresses on the surface which significantly improves the component's fatigue life by delaying the initiation of fatigue cracks. The effectiveness of shot peening depends heavily on proper process control. If parameters are not managed correctly, the plastic deformation can become so severe that it damages the surface. Gravity peening offers a means of reducing deformation extent by using larger shot sizes with fewer impacts on the workpiece. This variation maintains beneficial effects while minimising potential surface damage. Shot peening finds extensive application on shafts, gears, springs, oil well drilling equipment and jet engine components including turbine and compressor blades. However a critical limitation exists for high temperature applications. When parts subjected to elevated temperatures undergo shot peening, the residual stresses begin to relax through thermal relaxation, greatly diminishing their beneficial effects. Gas turbine blades operating at high temperatures exemplify this limitation where the compressive stresses may be lost during service.Laser Shot Peening:
Laser shot peening also known as laser shock peening represents a more recent development first conceptualised in the mid 1960s but not commercialised until considerably later. This process subjects the workpiece surface to planar laser shocks from high power lasers, producing compressive residual stress layers typically 1 millimetre deep with less than 1 per cent cold working of the surface. The technique has been successfully and reliably applied to jet engine fan blades and materials including titanium, nickel alloys and steels, delivering improved fatigue resistance and some corrosion resistance. Laser intensities necessary for the process range from 100 to 300 joules per square centimetre, with pulse durations of approximately 30 nanoseconds. Currently, the primary limitation for industrial, cost effective applications is the high cost of the high power lasers, which must operate at energy levels of 100 joules per pulse and can reach up to 1 kilowatt in output.Water Jet Peening:
Water jet peening is a more recently developed process where a water jet at pressures as high as 400 megapascals impinges on the workpiece surface. This induces compressive residual stresses and surface and subsurface hardening at levels comparable to conventional shot peening. The process has been successfully applied to steels and aluminium alloys. Control of process variables including jet pressure, jet velocity, nozzle design and the distance from the nozzle to the surface is essential to prevent excessive surface roughness and surface damage. Proper parameter management ensures the desired beneficial effects are achieved without compromising surface quality.Ultrasonic Peening:
Ultrasonic peening employs a hand tool based on a piezoelectric transducer operating at a frequency of 22 kilohertz. The tool can accommodate various heads for different applications, offering flexibility for diverse surface treatment requirements. This method provides a convenient, localised approach to inducing compressive residual stresses on specific component areas.Roller Burnishing:
Roller burnishing also called surface rolling, cold works the component surface through contact with a hard and highly polished roller or set of rollers. The process is applicable to various flat, cylindrical or conical surfaces and delivers multiple benefits: improved surface finish through removal of scratches, tool marks and pits along with beneficial compressive surface residual stresses. Corrosion resistance is consequently improved since corrosive products and residues cannot become entrapped on the burnished surface. A variation known as low plasticity burnishing involves the roller travelling only once over the surface inducing residual stresses with minimal plastic deformation. Internal cylindrical surfaces can be treated through a similar process called ballizing or ball burnishing where a smooth ball slightly larger than the bore diameter is pushed through the length of the hole. Roller burnishing improves both mechanical properties and surface finish, and can be used independently or in combination with other finishing processes such as grinding, honing and lapping. Equipment can be mounted on various CNC machine tools for improved productivity and performance consistency. The process works on all types of metals, whether soft or hard and is typically used on hydraulic system components, seals, valves, spindles and fillets on shafts.Explosive Hardening:
Explosive hardening subjects surfaces to high transient pressures through placement and detonation of an explosive sheet layer directly on the workpiece surface. Contact pressures that develop can reach as high as 35 gigapascals and last approximately 2 to 3 microseconds. This method achieves major increases in surface hardness with very little change in component shape, typically less than 5 per cent. Railroad rail surfaces, for example, are treated through explosive hardening to improve wear resistance.Mechanical Plating and Cladding:
Mechanical Plating:
Mechanical plating also known as mechanical coating, impact plating or peen plating involves compacting fine metal particles over workpiece surfaces using glass, ceramic or porcelain beads propelled by rotary means such as tumbling. The process is essentially one of cold welding particles onto a surface and is typically used for hardened steel parts in automotive applications. Plating thickness usually remains less than 25 micrometres.Cladding:
Cladding also called clad bonding, bonds metals with a thin layer of corrosion-resistant metal through application of pressure by rolls or other means. A typical application is the cladding of aluminium where a corrosion resistant layer of aluminium alloy in sheet or tubular form is clad over an aluminium alloy body or core. The cladding layer is anodic to the core and usually has a thickness less than 10 per cent of the total thickness. Examples include 2024 aluminium clad with 1230 aluminium, and 3003, 6061, and 7178 aluminium clad with 7072 aluminium.Other applications include steels clad with stainless steel or nickel alloys. Cladding material may also be applied with dies, as in cladding steel wire with copper or through explosives. Multiple layer cladding is utilised in special applications. Laser cladding involves fusion of a different material over the substrate and has been successfully applied to metals and ceramics particularly for enhanced friction and good wear behaviour of components.
Case Hardening and Hard Facing:
Case Hardening:
Case hardening encompasses traditional methods including carburising, carbonitriding, cyaniding, nitriding, flame hardening and induction hardening. Beyond conventional heat sources such as gas and electricity, electron beams or laser beams can serve as heat sources in surface hardening of both metals and ceramics. The formation of martensite during case hardening generates compressive residual stresses on surfaces. These stresses are desirable because they improve component fatigue life by delaying initiation of fatigue cracks.Hard Facing:
Hard facing deposits a relatively thick layer, edge or point of wear resistant hard metal on the workpiece surface through fusion welding techniques. Numerous layers, known as weld overlay can be deposited to repair worn parts. This process enhances wear resistance of materials making them suitable for manufacture of tools, dies and various industrial components. Worn parts can also be hard faced for extended use.Spark Hardening:
Spark hardening also called electric spark hardening or electrospark deposition, deposits hard coatings of tungsten, chromium, or molybdenum carbides through an electric arc. The deposited layer typically measures 250 micrometres thick. Hard facing alloys can be used as electrodes, rods, wires or powder in spark hardening. Typical applications include valve seats, oil well drilling tools and dies for hot metalworking.
Thermal Spraying:
Thermal spraying encompasses a series of processes in which coatings of various metals, alloys, carbides, ceramics and polymers are applied to metal surfaces through a spray gun with a stream heated by an oxyfuel flame, electric arc or plasma arc. The earliest applications of thermal spraying date to the 1910s and involved metals with the term metallizing also used. These processes continue to undergo refinement. Surfaces to be sprayed are first cleaned of oil and dirt then roughened through methods such as grit blasting to improve bond strength. Coating material can be supplied in wire, rod or powder form. When droplets or particles impact the workpiece they solidify and bond to the surface. Particle velocities typically range from a low of approximately 150 to 1000 metres per second though they can be higher for special applications. Temperatures range from 3000 to 8000 degrees Celsius.The resulting coating is hard and wear resistant with a layered structure of deposited material. However coatings can have porosity as high as 20 per cent due to entrapped air and oxide particles from the high temperatures involved. Bond strength depends on the particular process and techniques used and is mostly mechanical in nature hence the importance of roughening the surface prior to spraying. Bond strength generally ranges from 7 to 80 megapascals depending on the specific process employed. Typical applications include aircraft engine components used in rebuilding worn parts, structures, storage tanks, tank cars, rocket motor nozzles and components requiring resistance to wear and corrosion. In automobiles, thermal spraying can be applied to crankshafts, valves, fuel injection nozzles, piston rings and engine blocks. The process also serves the gas and petrochemical industries for repair of worn parts and restoration of dimensional accuracy to parts not machined or formed properly. Energy sources in thermal spraying processes fall into two categories: combustion and electrical.
Combustion Spraying:
Thermal wire spraying uses an oxyfuel flame to melt the wire and deposit it on the surface. The bond is of medium strength and the process is relatively inexpensive. Thermal metal powder spraying is similar to flame wire spraying but uses metal powder instead of wire. Detonation gun spraying involves controlled and repeated explosions through an oxyfuel gas mixture delivering performance similar to plasma spraying. High velocity oxyfuel gas spraying or HVOF produces high performance comparable to detonation gun technology but at lower cost.
Electrical Spraying:
Twin wire arc spraying forms an arc between two consumable wire electrodes. The resulting bond has good strength, and the process is the least expensive among thermal spraying methods. Plasma spraying, whether conventional, high-energy or vacuum produces temperatures on the order of 8300 degrees Celsius and results in good bond strength with very low oxide content. Low pressure plasma spray and vacuum plasma spray both produce coatings with high bond strength and very low levels of porosity and surface oxides.Cold Spraying:
Cold spraying represents a more recent development where particles to be sprayed remain at lower temperature and are not melted, thus minimising oxidation. The spray jet is narrow and highly focused with very high impact velocities improving the bond strength of particles on the surface.Industrial Cleaning Processes:
Most work parts require cleaning one or more times during their manufacturing sequence. Chemical and mechanical processes accomplish this cleaning each serving specific purposes and offering distinct advantages.
Chemical Cleaning:
A typical surface is covered with various films, oils, dirt and other contaminants. Although some substances may operate beneficially such as the oxide film on aluminium, it is usually desirable to remove contaminants from the surface. Several important reasons justify why manufactured parts and products must be cleaned: preparing the surface for subsequent industrial processing such as coating application or adhesive bonding, improving hygiene conditions for workers and customers, removing contaminants that might chemically react with the surface and enhancing appearance and product performance.
General Considerations in Cleaning:
No single cleaning method can address all cleaning tasks. Just as various soaps and detergents are required for different household jobs, various cleaning methods are needed to solve different industrial cleaning problems. Important factors in selecting a cleaning method include: the contaminant to be removed, degree of cleanliness required, substrate material to be cleaned, purpose of the cleaning, environmental and safety factors, size and geometry of the part and production and cost requirements.Various kinds of contaminants build up on part surfaces, either from previous processing or the factory environment. Surface contaminants typically divide into categories: oil and grease including lubricants used in metalworking, solid particles such as metal chips, abrasive grits, shop dirt and dust, buffing and polishing compounds, and oxide films, rust, and scale.
Degree of cleanliness refers to the amount of contaminant remaining after a given cleaning operation. Parts being prepared to accept a coating such as paint or metallic film, or adhesive, must be very clean; otherwise, adhesion is jeopardised. In other cases, it may be desirable for the cleaning operation to leave a residue on the part surface for corrosion protection during storage, effectively replacing one contaminant with another that is beneficial. Degree of cleanliness is often difficult to measure quantitatively. A simple wiping test involves wiping the surface with a clean white cloth and observing the amount of soil absorbed.
The substrate material must be considered in selecting a cleaning method to avoid damaging reactions from cleaning chemicals. Aluminium is dissolved by most acids and alkalis, magnesium is attacked by many acids, copper is attacked by oxidizing acids such as nitric acid, and steels are resistant to alkalis but react with virtually all acids. Some cleaning methods are appropriate for preparing surfaces for painting, while others are better for plating. Environmental protection and worker safety are increasingly important and cleaning methods and associated chemicals should be selected to avoid pollution and health hazards.
Chemical Cleaning Processes:
Chemical cleaning uses various chemicals to remove contaminants from surfaces. Major chemical cleaning methods include alkaline cleaning, emulsion cleaning, solvent cleaning, acid cleaning and ultrasonic cleaning. In some cases, chemical action is augmented by other energy forms such as high frequency mechanical vibrations combined with chemical cleaning. Alkaline cleaning is the most widely used industrial cleaning method, employing an alkali to remove oils, grease, wax and various particles including metal chips, silica, carbon and light scale from metallic surfaces. Alkaline cleaning solutions consist of low cost, water soluble salts such as sodium and potassium hydroxide, sodium carbonate, borax, phosphates and silicates of sodium and potassium combined with dispersants and surfactants in water. The cleaning method is commonly accomplished by immersion or spraying usually at temperatures of 50 to 95 degrees Celsius. Following application of the alkaline solution, a water rinse removes the alkali residue. Metal surfaces cleaned by alkaline solutions are typically electroplated or conversion coated.Electrolytic cleaning, also called electrocleaning, applies a 3 to 12 volt direct current to an alkaline cleaning solution. The electrolytic action generates gas bubbles at the part surface, causing a scrubbing action that aids removal of tenacious dirt films. Emulsion cleaning uses organic solvents dispersed in an aqueous solution. Suitable emulsifiers result in a two phase cleaning fluid that functions by dissolving or emulsifying soils on the part surface. The process can be used on metal or nonmetallic parts and must be followed by alkaline cleaning to eliminate all organic solvent residues prior to plating.
In solvent cleaning, organic soils such as oil and grease are removed from metallic surfaces by chemicals that dissolve the soils. Common application techniques include hand wiping, immersion, spraying and vapour degreasing. Vapour degreasing uses hot vapours of solvents to dissolve and remove oil and grease on part surfaces. Common solvents include trichlorethylene, methylene chloride and perchlorethylene, all of which have relatively low boiling points. The process consists of heating liquid solvent to its boiling point in a container to produce hot vapours. Parts are introduced into the vapour which condenses on the relatively cold part surfaces dissolving contaminants and dripping to the container bottom. Condensing coils near the container top prevent vapours from escaping into the surrounding atmosphere. This is important because these solvents are classified as hazardous air pollutants under the 1992 Clean Air Act.
Acid cleaning removes oils and light oxides from metal surfaces by soaking, spraying or manual brushing or wiping. The process is carried out at ambient or elevated temperatures. Common cleaning fluids are acid solutions combined with water miscible solvents, wetting and emulsifying agents. Cleaning acids include hydrochloric, nitric, phosphoric and sulfuric acids, selection depending on the base metal and cleaning purpose. Phosphoric acid produces a light phosphate film on the metallic surface, which can be useful preparation for painting. Acid pickling involves more severe treatment to remove thicker oxides, rusts and scales generally resulting in some etching of the metallic surface that serves to improve organic paint adhesion.
Ultrasonic cleaning combines chemical cleaning and mechanical agitation of the cleaning fluid, providing a highly effective method for removing surface contaminants. The cleaning fluid is generally an aqueous solution containing alkaline detergents. Mechanical agitation is produced by high frequency vibrations of sufficient amplitude to cause cavitation, the formation of low pressure vapour bubbles or cavities. As the vibration wave passes a given point in the liquid, the low pressure region is followed by a high pressure front that implodes the cavity producing a shock wave capable of penetrating contaminant particles adhering to the work surface. This rapid cycle of cavitation and implosion occurs throughout the liquid medium making ultrasonic cleaning effective even on complex and intricate internal shapes. The cleaning process is performed at frequencies between 20 and 45 kilohertz and the cleaning solution is usually at an elevated temperature typically 65 to 85 degrees Celsius.
Mechanical Cleaning and Surface Treatments:
Mechanical cleaning involves physical removal of soils, scales, or films from the work surface by means of abrasives or similar mechanical action. These processes often serve additional functions beyond cleaning, such as deburring and improving surface finish.Blast Finishing and Shot Peening:
Blast finishing uses high velocity impact of particulate media to clean and finish a surface. Sand blasting using grits of sand as the blasting media is the best known method. Various other media are also used including hard abrasives such as aluminium oxide and silicon carbide and soft media such as nylon beads and crushed nut shells. The media is propelled at the target surface by pressurised air or centrifugal force. In some applications, the process is performed wet with fine particles in a water slurry directed under hydraulic pressure at the surface. Shot peening directs a high velocity stream of small cast steel pellets at a metallic surface, cold working the surface and inducing compressive stresses into the surface layers. Shot peening is used primarily to improve fatigue strength of metal parts with surface cleaning accomplished as a by product of the operation.Tumbling and Other Mass Finishing:
Tumbling, vibratory finishing and similar operations comprise mass finishing methods, involving finishing of parts in bulk by a mixing action inside a container usually in the presence of abrasive media. The mixing causes parts to rub against the media and each other to achieve the desired finishing action. Mass finishing methods are used for deburring, descaling, deflashing, polishing, radiusing, burnishing and cleaning. Parts include stampings, castings, forgings, extrusions and machined parts. Even plastic and ceramic parts are sometimes subjected to these operations to achieve desired results. Parts processed by these methods are usually small and are therefore uneconomical to finish individually.Tumbling, also called barrel finishing and tumbling barrel finishing, uses a horizontally oriented barrel of hexagonal or octagonal cross section in which parts are mixed by rotating the barrel at speeds of 10 to 50 revolutions per minute. Finishing is performed by a landslide action of the media and parts as the barrel revolves. The contents rise in the barrel due to rotation followed by tumbling down of the top layer due to gravity. This cycle of rising and tumbling occurs continuously subjecting all parts to the same desired finishing action over time. However because only the top layer of parts is being finished at any moment, barrel finishing is relatively slow compared to other mass finishing methods often requiring several hours of tumbling to complete processing. Other drawbacks include high noise levels and large floor space requirements. Vibratory finishing, introduced in the late 1950s as an alternative to tumbling uses a vibrating vessel that subjects all parts to agitation with the abrasive media as opposed to only the top layer. Consequently processing times are significantly reduced. The open tubs used permit inspection of parts during processing, and noise is reduced.
Most media in these operations are abrasive though some perform nonabrasive finishing operations such as burnishing and surface hardening. Media may be natural or synthetic materials. Natural media include corundum, granite, limestone and even hardwood, though these are generally softer and wear more rapidly and are nonuniform in size sometimes clogging in work parts. Synthetic media can be made with greater consistency in both size and hardness. These materials include aluminium oxide and silicon carbide, compacted into desired shapes and sizes using a bonding material such as polyester resin. Shapes include spheres, cones, angle cut cylinders and other regular geometric forms. Steel is also used as a mass finishing medium in shapes for burnishing, surface hardening, and light deburring operations. Selection of media is based on part size and shape as well as finishing requirements. In most mass finishing processes, a compound is used with the media. The mass finishing compound is a combination of chemicals for specific functions such as cleaning, cooling, rust inhibiting of steel parts and steel media and enhancing brightness and colour of parts especially in burnishing.
Thermal and Mechanical Coating Processes:
These processes apply discrete coatings that are generally thicker than coatings deposited by other methods based on either thermal or mechanical energy.Thermal Surfacing Processes:
Thermal methods use thermal energy in various forms to apply coatings whose function is to provide resistance to corrosion, erosion, wear and high temperature oxidation. Processes include thermal spraying, hard facing and the flexible overlay process. In thermal spraying, molten and semimolten coating materials are sprayed onto a substrate where they solidify and adhere to the surface. A wide variety of coating materials can be applied, including pure metals and metal alloys, ceramics including oxides, carbides and certain glasses, metallic compounds such as sulphides and silicides, cermet composites, and certain plastics including epoxy, nylon and Teflon. Substrates include metals, ceramics, glass, some plastics, wood and paper, though not all coatings can be applied to all substrates. When applying metallic coatings, the terms metallizing or metal spraying are used. Technologies used to heat the coating material include oxyfuel flame, electric arc and plasma arc. Starting coating material is supplied in wire, rod or powder form. When wire or rod is used, the heating source melts the leading end separating it from the solid stock. The molten material is then atomised by a high velocity gas stream, and droplets are spattered against the work surface. When powder stock is used, a powder feeder dispenses fine particles into a gas stream, transporting them into the flame where they are melted. Expanding gases in the flame propel molten or semimolten powders against the workpiece. Coating thickness in thermal spraying is generally greater than in other deposition processes typically ranging from 0.05 to 2.5 millimetres. The first applications of thermal spray coating were to rebuild worn areas on used machinery components and salvage work parts machined undersize. Success has led to use in manufacturing as a coating process for corrosion resistance, high-temperature protection, wear resistance, electrical conductivity, electrical resistance, electromagnetic interference shielding, and other functions.Hard facing applies alloys as welded deposits to substrate metals. What distinguishes hard facing is that fusion occurs between the coating and the substrate whereas the bond in thermal spraying is typically mechanical interlocking that does not stand up as well to abrasive wear. Thus hard facing is especially suited to components requiring good wear resistance. Applications include coating new parts and repairing used part surfaces that are heavily worn, eroded or corroded. Hard facing is readily accomplished outside relatively controlled factory environments by common welding processes such as oxyacetylene gas welding and arc welding. Common surfacing materials include steel and iron alloys, cobalt based alloys and nickel based alloys. Coating thickness is usually 0.75 to 2.5 millimetres although thicknesses as great as 9 millimetres are possible. The flexible overlay process deposits very hard coating material such as tungsten carbide onto substrate surfaces. This important advantage permits coating hardness up to about 70 Rockwell C and allows application only to selected regions of a work part. A cloth impregnated with hard ceramic or metal powders and another cloth impregnated with brazing alloy are laid onto a substrate and heated to fuse the powders to the surface. Thickness of overlay coatings is usually 0.25 to 2.5 millimetres. In addition to tungsten carbide and tungsten carbide cobalt coatings, cobalt based and nickel based alloys are also applied. Applications include chain saw teeth, rock drill bits, oil drill collars, extrusion dies and similar parts requiring good wear resistance.
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