The ASM Handbook  defines surface engineering as: “treatment of the surface and near-surface regions of a material to allow the surface to perform functions that are distinct from those functions demanded from the bulk of material.” The surface properties can be enhanced or changed to improve lubricity for forming, surface hardness for wear resistance, oxidation and corrosion resistance, mechanical properties such as fatigue or toughness, electrical properties, and aesthetic appearance (see Figure 1).
The surface-engineered surface is not independent of the substrate. They must be compatible to prevent spalling or separation of the coating from the bulk material. Often, thermal or mechanical treatments are performed that can change the substrate. A good example is carburizing, where additional carbon is diffused into the material, and the surface is changed to martensite after quenching. The interior or core material may be changed to another microstructure depending on the thermal process and alloy.
There are three primary categories of surface engineering: heat treatment, mechanical treatment, and surface substrate addition. Each of these are further divided into subcategories, shown in Figure 2.
Within the primary category of heat treatment, there are two subcategories: “change in surface chemistry” and “no change in surface chemistry.” No change in surface chemistry includes processes such as induction hardening or flame hardening, where a hard surface of martensite is created by the application of an induced eddy current or direct flame. This hard martensitic surface has compressive residual stresses for high fatigue strength and a hard surface for improved wear. The repeatability and level of automation available reduces the cost of materials, allowing lower hardenability steel to be used. Other than the hard surface of the surface-hardened part, the bulk material is unchanged. The change in the surface hardness can be accomplished by flame, induction, laser, and increasingly, by electron beam.
Where a change in surface chemistry is used, it is further subdivided by temperature or phase where the change in surface chemistry occurs. This division typically occurs either above or below the austenization temperature or the phases ferrite or austenite. Typical processes occurring in the austenite phase (generally above 725°C) include the diffusion of carbon in the familiar process of carburizing or the diffusion of nitrogen and carbon in carbonitriding. When quenched and tempered, the diffused layer of carbon and nitrogen produces a hardened layer of tempered martensite. This layer produces compressive residual stresses for improved fatigue life and a high hardness for improved wear. Typical applications are gears or other parts requiring excellent fatigue or wear properties.
Lower temperature surface-engineered processes, which change the surface chemistry of steel, are nitriding and nitrocarburizing. In this process, nitrogen is diffused into the substrate. The nitrogen can come from ammonia (gas nitriding), cyanide or cyanate salts in salt bath nitriding, or direct implantation of nitrogen as in ion or plasma ion nitriding. The nitrided part shows excellent wear properties due to high hardness, as well as good corrosion protection. Typical applications include gears, dies, fasteners, camshafts, and valve components. A typical microstructure of a nitrided part, including the “white layer,” is shown in Figure 3.
The mechanical treatment of steel to modify the surface is mostly limited to inducing strains to the surface of the material to produce compressive residual stresses. Typical processes include burnishing or shot peening the part surface. Burnishing is a small-scale forming operation that improves fatigue strength by producing a compressive residual stress. Surface compressive stresses can also reduce the chance for stress-corrosion cracking. Shot peening achieves the same goals of creating a layer of compressive residual stress by impacting small, hardened steel balls on the surface of the part. This can be localized to specific areas or the entire part surface. This type of process is applied to a large variety of parts, including turbine blades, landing gear, springs, and gears.
Surface Substrate Addition
Surface substrate addition is perhaps the widest field of surface engineering. In this process, a new material or alloy is added to the surface of the part. This is accomplished by one of three subcategories: plating, coating, or hardfacing.
Plating has been used for creating protective surfaces, or for decorative purposes, for decades. It is done to decrease wear, improve painting adhesion, and improve corrosion resistance. In this process, a thin layer of a different metal such as chromium, zinc, nickel, or copper is deposited on the surface of the steel by an electrochemical process.
In this process, an electric current is used to dissolve metal ions from the anode to be plated into a solution. This same electric voltage supplies electrons to the workpiece to be plated (cathode), attracting the negatively charged cathode. This reduces the oxidation state of the dissolved metal ions, and they plate onto the workpiece. The thickness and rate at which plating occurs is through control of voltage and current. Often, additives are used in the aqueous solution to improve the uniformity and adherence of the plating onto the workpiece. This is illustrated in Figure 4.
Electroless plating uses only one electrode (the workpiece) and no external power supply. Electroless nickel is a common form of electroless plating. A strong reducer is necessary in the solution to achieve effective plating.
Coatings for steel can run the gamut of organic coatings for corrosion protection and appearance or deposited coatings for electrical conductivity or corrosion protection. The organic coatings can be as simple as painting, or they can be complex coatings such as fluorocarbon polymers to improve lubricity. These organic coatings, such as molybdenum disulfide or fluorocarbons, reduce friction. They can be used on the final part or as an intermediate step in manufacture. Other polymer coatings such as epoxy coatings or ceramic epoxy coatings provide abrasion resistance. Coatings can also be used to provide antimicrobial properties, such as the addition of biocides, peptides , or nanoparticles.
Conversion coatings are those where the surface of the bulk material is changed through a chemical process. Examples of this are black oxide for steel and anodizing for aluminum. They are often used as a substrate for painting adherence and to provide corrosion resistance.
Chemical vapor deposition (CVD) is used to produce high-performance thin films on a metal substrate . In this process, a precursor gas is introduced into a vacuum chamber where objects to be coated are heated. A chemical reaction occurs on the surface of the part, resulting in the deposition of a thin film on the surface of the part. Reaction products are exhausted out of the chamber. Part temperatures can range from 200°C to 1600°C, depending on the substrate and the film to be deposited. CVD’s advantage is that the films produced are uniform in thickness and properties. The films are deposited with high purity and at a rapid rate. Unfortunately, the precursors used for CVD can be corrosive or hazardous. The reaction products can also be hazardous or corrosive. High temperatures are often used, which can limit applicability and substrate.
Plasma vapor deposition (PVD) can produce a wide variety of thin films. In this process, a target composed of the desired thin film material is stuck by a beam of electrons. The target material is turned into a plasma, which then condenses on the part. Common PVD coatings are aluminum and titanium nitride. PVD coatings are often corrosion-resistant and can be applied to different types of substrates — organic and inorganic. These thin films are used to provide corrosion resistance, wear resistance, and other properties. One disadvantage to PVD is that the coating may not be uniform in thickness, and the vacuum equipment used is expensive and maintenance-prone. A comparison of CVD and PVD is shown in Figure 5.
Hardfacing is a metalworking process where a hard or tougher metal is applied to the base metal. The primary purpose of hardfacing is to change the wear resistance of the part. However, depending on the hardfacing alloy used, it can be also used to increase resistance to heat, corrosion, or abrasion, and it can be used to minimize metal-to-metal adhesion. This process can also be used to build up a worn part. Hardfacing can be accomplished as part of the manufacturing process, repair process, or as preventative maintenance. It is often used to increase the life of a component . This is illustrated in Figure 6.
There are multiple processes used in hardfacing alloys, each with its own advantages and disadvantages. There are typically three types of processes used with hardfacing:
- Shielded metal arc welding
- Flux cored arc welding
- Submerged arc welding
Shielded metal arc welding (SMAW), or covered electrode welding, has the advantages of a wide alloy availability, with most hardfacing alloys available as covered electrodes. Generally, there is no material-thickness limitation with SMAW, as most thicknesses and alloys can be welded using SMAW. SMAW is versatile, with electrodes available for outside and out-of-position work. Disadvantages of SMAW are that typically at least three layers are needed to provide maximum wear properties. SMAW is a low-efficiency deposition process and is limited to about 0.5 to 3 kg/hr deposition rates.
With flux cored arc welding (FCAW), many alloys are available, almost as many with SMAW. Very high deposition rates, up to 1.8 to 12 kg/hr, are achievable with FCAW. FCAW is easy to operate and does not require extensive operator training. However, FCAW is generally used for flat positions and is not designed for out-of-position work. Like SMAW, it has a problem with dilution, with three layers required for adequate wear resistance.
Submerged arc welding (SMA) is easily automated and has high deposition rates. Training required is minimal, and the weld deposit produced is smooth and sound. The weld arc is also buried within the flux, so eye protection is minimized. SMA has limited alloy availability and is limited to flat positions. It has extremely high dilution so that many layers are required to achieve the optimum wear or corrosion protection. SMA has a high heat input, so it will tend to warp parts unless special precautions are made.
Thermal spray is another method of hardfacing or applying a specialized coating on a part. As this field has expanded, specialized societies have been formed to promote and educate regarding the process, such as the ASM Thermal Spray Society in the U.S.
Thermal spray is a method where a coating consists of a heat source and a coating material in the form of a wire or powder that is melted into tiny droplets and sprayed onto a surface at high velocity [5, 6]. The coating thicknesses range from 0.05 to 0.6 mm thick. Most metals can be used for the substrates, as well as many plastics. The coating materials have a wide material choice, including stainless steels, tungsten carbide, nickel-chrome carbides, and pure metals (gold, silver, copper, zinc, aluminum, etc.). There are four main methods used for thermal spray:
- Electric arc
- Flame spray
- Plasma spray
- HVOF (High velocity oxy-fuel)
Electric arc thermal spraying uses a similar method to wire arc welding. The coating material in wire form is electrically charged and melted. The molten droplets of metal are then sprayed onto the substrate using a high-velocity air stream to atomize and propel the material. This method is usually used to spray pure metals and metal alloys.
Flame spray is the original thermal spray technique used 100 years ago. A welding torch is used with the addition of a high-velocity air stream to deposit molten metal particles onto the substrate. The coating material is typically a wire or powder. Flame spray coatings are often fused after applying to enhance the coating density and coating adhesion.
Plasma spray uses an inert gas flowing past an electrode inducing a plasma. When the gases exit the gun, the gases return to a gas and high heat is produced. The coating wire or powder is introduced into the plasma flame and deposited onto the substrate.
High velocity oxy-fuel (HVOF) produces higher temperatures and has higher spray velocities than other methods of thermal spray. These properties mean that higher melting materials can be used for coatings such as tungsten carbides and nickel-chrome carbides. HVOF produces superior bond strength and coating density to other thermal spray processes.
Surface engineering is both an old but new field for materials science and engineering. The methods used vary widely but have the primary focus of improving parts by enhancing surface properties.
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