Friction and wear are common phenomena in nature. Friction is the resistance to relative motion between two mating surfaces, caused by atomic or molecular interactions arising from microscopic contact points. Wear, on the other hand, refers to the continuous loss of material from the surfaces of two mating parts due to their relative motion. As long as there is relative motion between surfaces, friction is inevitable; and where there is friction, wear is bound to occur. Working conditions that can lead to wear include sliding, micro-vibration, impact, scratching, and erosion. However, given the complexity of wear mechanisms and the uncertainty surrounding different types of wear, it is essential to thoroughly analyze the operating environment of components when selecting wear-resistant coatings. Thermal spraying technology can enhance the wear resistance of soft substrates or substrates that have already experienced wear.
In general, thermal-spray coatings exhibit higher wear resistance compared to cast or forged structures made from similar materials. This is because, during the thermal-spray process, the particles undergo rapid quenching at high speeds, resulting in a unique microstructure characterized by a certain degree of porosity. In metallic coatings, a small amount of oxide forms around the deformed particles. The microporous structure of the coating not only helps maintain a lubricating film on the part’s surface but also provides space to accommodate debris generated by wear, thereby significantly enhancing the wear resistance of the part’s surface.
According to the wear process and failure mechanisms of friction surfaces, wear can be classified into abrasive wear, adhesive wear, corrosive wear, fatigue wear, fretting wear, erosive wear, and high-temperature wear. High-temperature wear is essentially a combination of adhesive wear and abrasive wear. The characteristics of each type of wear and their corresponding performance requirements for coating materials are shown in the table.

Types and Characteristics of Wear, and Their Requirements for Coating Performance

1 Wear-resistant coating performance requirements
The requirements for wear-resistant coatings depend on the mechanical and chemical compatibility between the coating and the substrate material, the direction and magnitude of the applied load, and the performance characteristics of the coating itself. Depending on the specific application of the wear-resistant coating, factors such as coating hardness, chemical stability, coating yield strength, and the ability to resist crack nucleation and propagation all influence the coating's wear resistance.
(1) Coating Adhesion Strength Requirements The primary requirement for wear-resistant coatings is to ensure sufficient bonding strength between the coating and the substrate. To achieve this, the selection and design of both the substrate material and the coating material must be based on the premise of guaranteeing a strong bond between the coating and the substrate.
1) The substrate should be free of deformation. When a wear-resistant coating is used under high-load conditions, the substrate must have sufficient hardness and yield strength to prevent deformation of the coating.
2) Matching of elastic moduli between the coating and the substrate material: Under elastic strain conditions, if the elastic moduli of the coating and the substrate do not match, a sharp stress gradient will develop at the interface between the coating and the substrate when a load is applied. If the coating is stiffer than the substrate, the stress within the coating will increase. As the load increases and the difference in elastic moduli between the coating and the substrate grows, the stress will also increase.
The table lists the elastic moduli of high-speed steel and carbides.

Elastic Modulus of High-Speed Steel and Carbides

3) Rigid matching between the coating and the substrate material. To ensure a long service life for hard, wear-resistant coatings, there should be a reasonable match in rigidity between the coating and the substrate material. If a high-rigidity coating is deposited onto a substrate with low rigidity, the mismatch in rigidity will lead to increased tensile stress within the coating, causing cracks to form in the coating and propagate into the substrate, thereby resulting in premature failure of the coating.
4) Matching of thermal expansion coefficients. If the thermal expansion coefficient of the coating does not match that of the substrate material, stresses will develop due to volume changes. Typically, coatings are much thinner than their substrates; therefore, the substrate’s thermal expansion is largely unaffected by the coating’s thermal expansion, whereas the coating’s thermal expansion is strongly influenced by the substrate’s thermal expansion. The thermal stresses generated by the mismatch in thermal expansion between the coating and the substrate are essentially concentrated within the coating itself. The greater the difference in thermal expansion coefficients, the higher the stress within the coating—and the greater the likelihood of crack formation or even delamination. This is one of the primary reasons why many wear-resistant coatings fail prematurely through delamination even before experiencing significant wear.
The table lists the coefficients of thermal expansion for several carbide and nitride coating materials and steel.

Thermal Expansion Coefficients of Carbide and Nitride Coating Materials Compared to Steel

When the thermal expansion coefficient of the substrate is greater than that of the coating, the stress generated upon temperature increase is tensile; conversely, if the thermal expansion coefficient of the coating is greater than that of the substrate, the stress is compressive. As shown in the table, the thermal expansion coefficients of most carbide, nitride, and metal-ceramic coatings are lower than that of steel. Only TiN, NbN, and Cr3C2 have thermal expansion coefficients that are relatively close to those of high-speed steel, though the differences remain significant.
5) The affinity between the coating and the substrate material. The affinity—or chemical bonding capability—between the coating and the substrate directly affects the bonding strength between them. Generally, when the solid solubility of one compound in another is low, the bonding strength between them will also be weak. Only when the coating and the substrate exhibit maximum chemical affinity without forming a brittle interfacial phase can the bonding strength between them reach its peak, thereby fully realizing the wear-resistant performance of the coating.
(2) Coating wear resistance requirements Under the aforementioned condition that sufficient adhesion between the coating and the substrate must be ensured, we can then further specify requirements for the coating’s wear resistance, which primarily include the following aspects.
1) Coating hardness: Increasing the coating hardness helps to enhance the coating’s yield strength and prevent deformation. As the coating hardness increases, its resistance to abrasive wear improves, and the rate of abrasive wear is inversely proportional to the coating’s hardness. If the coating hardness exceeds the hardness of the abrasive particles, the abrasive wear rate drops sharply. Therefore, under conditions of abrasive wear, the coating hardness should be as high as possible. In contrast, under sliding wear conditions, it is advisable to use tough, single-phase soft coatings that do not contain second-phase hard particles; otherwise, severe abrasive wear may occur.
2) High-temperature wear resistance. When hard coatings are used as high-temperature wear-resistant coatings, they must not only exhibit excellent red hardness at high temperatures—that is, maintain high hardness at elevated temperatures—but also have low chemical solubility between the coating and the counter-friction material.
3) Corrosion and wear resistance. The wear resistance of wear-resistant coatings in corrosive media also depends on the coating’s corrosion resistance in chemical environments. Many hard coatings exhibit excellent corrosion resistance; in particular, ceramic coatings such as oxides and carbides are outstanding materials for corrosion- and wear-resistant coatings.
4) The bonding strength between coating particles is high. Hard-coating particles should exhibit strong interparticle bonding. For example, WC-Co metal-ceramic coatings are well-known wear-resistant coating materials. Cobalt has excellent wettability toward hard particles such as tungsten carbide, enabling the tungsten carbide particles to bond firmly together without peeling off. Only under these conditions can the high wear resistance of the tungsten carbide hard phase be fully exploited.
Obviously, the successful application of wear-resistant coatings depends not only on the coatings’ own friction and wear resistance properties but also on the appropriate matching of performance between the coating and the substrate.
2 Selection of Wear-Resistant Coating Spraying Materials  
In certain cases, coatings are required to exhibit both excellent wear resistance and outstanding corrosion resistance—for instance, components operating in environments such as petroleum, chemical industries, and marine atmospheres. If the wear resistance and corrosion resistance of a coating were evaluated qualitatively on a scale of ten levels, the results would be as shown in the table below. Here, Level 1 indicates the worst performance in either wear resistance or corrosion resistance, while Level 10 represents the best performance in these two aspects.

Wear-Resistant Coating Spray Material Selection Table

Depending on the differences in the operating environment of wear-resistant coatings, different spraying materials and spraying processes should be selected, for example:
(1) Coating for soft supports These coatings allow abrasive particles to become embedded and also permit deformation to adapt to the bearing surface. The coating materials are typically non-ferrous metals, such as aluminum bronze, phosphor bronze, Babbitt alloy, and tin coatings. Specific application parts include Babbitt bearings, hydraulic press bushings, thrust bearing pads, and compressor crossheads, among others.
(2) Coating for rigid supports Hard-supported surfaces typically operate under conditions of high load and low speed. Such supports are generally used in areas where embeddability and automatic adjustability are not critical, as well as in locations with limited lubrication. Spray-coating materials can include nickel-based and iron-based self-fluxing alloys, oxide and carbide ceramics (such as Al). ₂ O ₃-TiO₂ such as Co-WC, refractory metals like Mo, and Mo-based self-fluxing alloys. Specific application parts include: shock absorbers for presses, crankshafts, anti-abrasion bushings, rudder bearings, turbine shafts, necks of drive gears, and rotor components for fuel pumps with piston rings, among others.
(3) Abrasion-resistant coating When the operating temperature is below 540℃, the coating must be able to withstand the cutting and grooving effects of abrasive particles. Therefore, the coating hardness should exceed that of the abrasive particles. Suitable coating materials include self-fluxing alloys combined with Mo or Ni/Al mixed powders, high-chromium stainless steel, Ni/Al wires, T8 steel, and self-fluxing alloys combined with Co/WC mixed powders. Specific application parts include: mud pump piston rods, polishing rod bushings, screw conveyors for concrete mixers, tobacco grinding hammers, mandrels, and polishing/jetting fixtures, among others.
When the wear-resistant coating for abrasive particle wear is used at temperatures between 538 and 843°C, the coating must exhibit a hardness higher than that of the abrasive particles at high temperatures and also possess excellent oxidation resistance. Suitable spray-coating materials include iron-based, nickel-based, and cobalt-based coatings (such as cobalt-based Cr, Ni, and W alloy powders, Ni/Al wires, austenitic low-carbon stainless steels, and nickel- and cobalt-based self-fluxing alloys), as well as Cr. ₃ C ₂ Metal-ceramic powder; when subjected to impact or vibrational loads, a self-fluxing alloy is preferable if the temperature is below 760℃. However, when erosion is severe, it is best to use Cr. ₃ C ₂ If the primary application is antioxidant protection, iron-, nickel-, or cobalt-based coatings can be used.
(4) Hardface Wear-Resistant Coating When the operating temperature is below 538°C, wear occurs because, as the harder surface slides over a softer surface, the hard protrusions on the harder surface create grooves in the softer surface, thereby scraping off debris. This debris behaves similarly to abrasive particles. In such cases, the coating must be harder than the mating surface. Suitable coating materials include certain iron-based, nickel-based, and cobalt-based spray coatings, self-fluxing alloys, non-ferrous metals (such as iron-aluminum bronze), oxide ceramics, tungsten carbide, and certain refractory metal coatings. Specific application parts include wire-drawing winches, brake sleeves, forks, plug gauges, tube-sizing and piercing dies for rolling mills, extrusion dies, guide rods, pulp knives, rollers, blade crushers, fiber-guiding devices, forming tools, and pump seals, among others.
When the operating temperature of wear-resistant hardface coatings ranges from 540 to 815°C, although the general principles remain the same as described above, wear tends to accelerate significantly at higher temperatures. Therefore, it is necessary to use drill-based self-fluxing alloys, Ni/Al, and chromium carbide coating materials. When the temperature is below 760°C and there is an impact load, self-fluxing alloys are recommended; at higher temperatures, Cr-based coatings are more suitable. ₃ C ₂ Coatings—when the primary focus is on antioxidant properties, materials such as Ni/Al are preferred. Specific application parts include: forging tools, hot-break rolls, and hot-forming dies, among others.
(5) Micro-vibration-resistant wear coating Since wear is typically caused by unpredictable micro-vibrations, when the operating temperature is below 540℃, coatings with better toughness should be selected—such as self-fluxing alloys, oxides, carbide metal ceramics, certain Ni-, Fe-, and Co-based spray-coating materials, and non-ferrous metals. Specific application parts include servo motor pivots, cam followers, rocker arms, cylinder liners, anti-gas rings, guide vanes, propeller reinforcing rods, and the like.
When the operating temperature of wear-resistant micro-vibration coatings ranges from 538 to 843°C, specific iron-based, nickel-based, cobalt-based materials, as well as chromium carbide ceramic materials, can be used due to the higher operating temperatures. Specific application components include turbine seals, sealing rings, sealing gaskets, and turbine blades for jet engines.
(6) Cavitation-resistant coating Since the coating needs to withstand gas impacts in liquid flows, it must exhibit excellent toughness, high wear resistance, resistance to fluid corrosion, and no brittleness. Suitable materials include Ni-based self-fluxing alloys, copper alloys containing 9.5% Al and 1% Fe, copper alloys containing 38% Ni, self-fluxing alloys mixed with Ni/Al powders, Type 316 stainless steel, and ultrafine Al. ₂ O ₃ and pure Cr ₂ O ₃ Moreover, all coatings should undergo sealing treatment. Specific application parts include turbine blades, wear-resistant rings, nozzles, and diesel engine cylinder liners, among others.
( 7) Erosion- and wear-resistant coating These coatings must be able to withstand wear caused by sharp, hard particles. Several options are available, including Ni-based self-fluxing alloy powders, self-fluxing alloy powders mixed with fine copper powder, high-Cr stainless steel powders, and ultrafine Al. ₂ O ₃ Powder, pure Cr ₂ O ₃ Powder, Al ₂ O ₃ 87%+TiO ₂ L3% composite powder and Co/WC composite powder. Specific application parts include: exhaust fans, hydroelectric valves, and cyclone dust collectors, among others.

3 Wear-resistant coating spray parameter settings
When applying wear-resistant coatings using the JP5000 supersonic flame spraying equipment manufactured by Praxair of the United States, the parameter settings are shown in the table below.

JP5000 Spray Coating Parameter Selection Table for Various Wear-Resistant Coatings

Since supersonic flame spraying has its own unique characteristics, the following two points should be carefully considered during the actual spraying process: ① Because supersonic flame spraying delivers a relatively large amount of heat to the substrate, it is essential to strictly control cooling measures to prevent overheating and deformation of the substrate, which could otherwise lead to the scrapping of the workpiece; ② Special attention must be paid to shielding and protecting areas that are not to be sprayed. In the supersonic flame spraying process, the sprayed particles are not heated to a fully molten state; instead, they retain a certain degree of rigidity and fly at much higher velocities—far exceeding the particle velocities used in conventional plasma spraying. Consequently, the conventional methods of using anti-stick coatings and masking tapes can no longer meet the shielding requirements, and it is necessary to adopt the method of wrapping and shielding with thin sheets of iron or copper.
4 Post-processing of wear-resistant coatings
For wear-resistant coatings, grinding is the only practical and feasible finishing method. Since the bonding between coating particles relies primarily on mechanical interlocking and the coating contains a certain degree of porosity, heat transfer occurs relatively slowly from a grinding perspective. Consequently, conventional grinding techniques used for dense materials are not suitable for grinding coated surfaces. If the grinding pressure is too high or the grinding speed is too fast, it may cause particles on the coating surface to shift or displace, leading to the detachment of internal particles within the coating and even resulting in the entire coating peeling off from the substrate. Factors that determine the choice of grinding process include: the type of coating, the shape of the workpiece, the required surface finish, and the specified tolerances.
Generally speaking, the finer the spray powder, the lower the porosity of the coating, and the more uniform the coating, the better the surface finish after grinding.
When selecting a grinding wheel, factors such as coating type, hardness, workpiece size and shape, grinding allowance, surface finish requirements, and grinding machine type should be taken into account. Generally, the following principles should be followed:
(1) Use the sharpest grinding wheel possible. This type of grinding wheel features high cutting speed and is less prone to overheating. The sharpness of the grinding wheel depends on the type and size of abrasive grains used in its manufacture. For grinding wheels designed to remove wear-resistant coatings, commonly used abrasive grains include silicon carbide and diamond. This is because silicon carbide grains, when fractured during grinding, expose fresh, sharp cutting edges; while diamond grains offer excellent durability and can cleanly grind a wide variety of hard, wear-resistant coatings. When the abrasive grain size is smaller, not only does the surface area decrease, but the cutting edges also become sharper. Such fine grains are more likely to penetrate deeply into the coating material, resulting in higher surface finish quality. Generally speaking, for rough grinding of wear-resistant coatings, abrasive grain sizes range from 125 to 150 mesh, whereas for fine grinding, the grain size typically falls between 380 and 400 mesh.
(2) The internal structure and hardness grade of the selected grinding wheel must be able to deliver free grinding performance. The internal structure of a grinding wheel refers to the spacing between individual abrasive grains within the wheel. Grinding wheels with a porous structure offer superior grinding performance, as the gaps between grains provide greater chip-storage capacity. The hardness of the grinding wheel also affects the performance of wear-resistant coatings during grinding: harder wheels have a longer service life than softer wheels. When grinding stresses are relatively low, the contact area is large, and the grinding speed is high, it is recommended to use a softer grinding wheel. Conversely, when grinding stresses are high, surface finish requirements are stringent, the contact area is small, and the grinding wheel is narrow, it is advisable to opt for a harder grinding wheel.
(3 ) Select the most suitable grinding wheel bond type. There are two commonly used types of grinding wheel binders: ceramic binders and resin binders. Grinding wheels made with ceramic binders can withstand higher grinding speeds and tighter dimensional tolerances, and are unaffected by water, acids, oils, or temperature fluctuations. However, they require that the grinding machine’s spindle speed be kept below the wheel’s safe operating speed—typically less than 33 meters per second. On the other hand, grinding wheels using resin binders can operate at even higher grinding speeds and achieve superior surface finish.
For WC-based wear-resistant coatings applied to JP5000, the recommended grinding process is as follows:
1) Use precision, high-quality grinding equipment;
2) Use a water-soluble cooling medium for rinsing;
3) Use resin-bonded diamond grinding wheels: When the workpiece outer diameter is less than 50 mm, select a φ500 grinding wheel; when the workpiece outer diameter is greater than 50 mm, select a φ762 grinding wheel.
4) The grinding process is carried out using a two-step method. In the first step, rough grinding is performed with the following parameters: abrasive grain size of 125–150 mesh; wheel speed of 25–30 m/s; workpiece speed of 0.3 m/s; grinding depth less than 0.01 mm; feed rate of 0.2–0.3 m/min, with a longitudinal feed rate of 0.025–0.05 mm. In the second step, fine grinding is performed with the following parameters: abrasive grain size of 380–400 mesh; wheel speed of 25–30 m/s; workpiece speed of 0.5 m/s; grinding depth less than 0.005 mm; feed rate of 0.05–0.1 m/s, with a longitudinal feed rate of 0.025–0.05 mm.
During the grinding process, the condition of the grinding wheel’s cutting surface changes. After a period of use, either the abrasive grains on the wheel’s surface become worn down to the point where their height equals that of the bonding matrix, or the wheel’s surface becomes filled with material from the workpiece being ground. Both of these conditions can weaken the wheel’s grinding capability, leading to increased friction (burnishing) relative to actual grinding. In such cases, the grinding wheel should be dressed or replaced with a new one. When using diamond tools to dress the grinding wheel, the cross-feed speed of the tool across the wheel’s surface significantly affects the wheel’s final cutting performance. A rapid cross-feed can open up the wheel’s surface, re-sharpening the abrasive grains and thereby enhancing the wheel’s grinding capacity. Conversely, a slower cross-feed can cause the wheel’s surface to become clogged, dulling the abrasive grains and making the wheel harder. For thermal spray wear-resistant coatings, the slow-dressing method is not recommended. Therefore, when using diamond grinding wheels to grind wear-resistant coatings, it is crucial to keep the wheel sharp, as this helps achieve a higher surface finish.
For grinding wear-resistant coatings, wet grinding is generally recommended. However, dry grinding can also be used if appropriate protective measures are taken. Nevertheless, the advantages of wet grinding far outweigh those of dry grinding. During wet grinding, harder grinding wheels can be employed without increasing the likelihood of chipping or thermal cracking. This minimizes the removal of surface particles and results in a superior surface finish. Moreover, the grinding wheel remains less clogged, reducing the frequency of dressing required. In addition, wet grinding helps flush away grinding debris and residues. The filtration and proper concentration of the grinding fluid also significantly influence the surface finish.
In short, as long as the grinding process is carried out carefully, it is possible to obtain a wear-resistant coating surface with excellent surface finish. Below are some factors that need to be considered when determining the grinding process for wear-resistant coatings.
1) Using a softer, freely grinding abrasive wheel can significantly reduce the chances of burnishing and abrasive grain shedding.
2) Keep the grinding wheel surface clean and sharp;
3) Use the correct grinding wheel dressing technique;
4) When performing rough grinding, it’s best to use a coarse-grit grinding wheel; when performing fine grinding, a fine-grit grinding wheel should be used. If you attempt to achieve a good surface finish using a coarse-grit wheel, it could lead to abrasive particle shedding, contamination, or burning.
5) Use light grinding. Wear-resistant coatings are typically thin; excessive grinding pressure may cause delamination of the coating surface or detachment of surface particles.
6) When performing the final grinding operation, non-sparking grinding should be used; otherwise, the grinding wheel surface may become dull or glazed.
7) Always maintain pressure on the coating; only by spraying with the nozzle angled downward toward the substrate can delamination and particle detachment be minimized.
8) Optimization of grinding process parameters. Variations in grinding parameters significantly affect grinding speed and surface finish; the surface finish of wear-resistant coatings largely depends on the selected grinding process. When issues arise with a given grinding wheel, adjustments should be made to the wheel speed, feed rate, workpiece speed, and dressing process.

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