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Product Description
[Coating Preparation] Wear-resistant Coating
Wear is a common phenomenon in nature. Friction is the resistance to relative motion between two mating surfaces, caused by the interaction—on an atomic or molecular level—resulting from microscopic contact points between these surfaces. 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 will inevitably occur; and where there is friction, wear is bound to follow. Working conditions that can lead to wear include sliding, micro-vibration, scratching, and erosion. However, given the uncertainty surrounding the causes of wear, it is essential to thoroughly analyze the operating environment of components when selecting wear-resistant coatings. The thermal spraying technique 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 the corresponding performance requirements for coating materials are shown in the table below.
1. Performance requirements for wear-resistant coatings
The requirements for wear-resistant coatings depend on the mechanical compatibility and chemical compatibility between the wear-resistant coating and the substrate material, the direction and magnitude of the applied load, as well as the performance characteristics of the coating itself.
(1) Coating adhesion strength requirements.
The primary requirement for wear-resistant coatings is to ensure that the coating has sufficient bonding strength with the substrate. To this end, the selection and design of both the substrate material and the coating material should 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 should have sufficient hardness and yield strength to prevent deformation of the coating.
2) Matching of the elastic modulus between the coating and the substrate material,
Under elastic strain conditions, if the elastic modulus of the coating does not match that of the substrate, a sharp stress gradient will develop at the interface between the coating and the substrate when a load is applied. If the coating’s stiffness exceeds that of the substrate, the stress within the coating will increase. As the load and the difference in elastic moduli between the coating and the substrate grow larger, the stress will also increase.
The table lists the elastic moduli of high-speed steel and carbides.
3) Rigidity matching between the coating and the substrate material.
To ensure a long service life for hard, wear-resistant coatings, the stiffness of the coating should be reasonably matched with that of the substrate material. If a highly rigid coating is deposited onto a substrate material with low stiffness, the mismatch in stiffness 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, stress 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 resulting from the mismatch in thermal expansion between the coating and the substrate are concentrated primarily within the coating itself. The greater the difference in thermal expansion coefficients, the higher the stress in the coating—and the greater the likelihood of cracking or even delamination. This is one of the primary reasons why many wear-resistant coatings fail prematurely by delaminating before they have even begun to show significant wear.
The table lists the coefficients of thermal expansion for several carbide and nitride coating materials and 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 that 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 strong chemical affinity without forming brittle interfacial phases can their bonding strength be high enough to fully exploit the wear-resistant properties 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 enhance the coating’s yield strength and prevent deformation. As 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 will 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 superior corrosion resistance—for example, components operating in environments such as oil, chemical industries, and marine atmospheres. If the wear resistance and corrosion resistance of a coating are 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 signifies the best performance in these two aspects.
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 adjustment are not critical, as well as in locations with limited lubrication. Coating materials that can be employed include nickel-based and iron-based self-fluxing alloys, oxide and carbide ceramics (such as Al2O3-TiO2 and Co-WC), refractory metals like molybdenum, and molybdenum combined with self-fluxing alloys. Specific application components include: crankshafts for press dampers, anti-abrasion bushings, rudder bearings, turbine shafts, necks of drive gears, and rotor assemblies for fuel pumps with piston rings.
(3) Wear-resistant particle-abrasion 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’s 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 materials include iron-based, nickel-based, and cobalt-based coatings (such as cobalt-based Cr-Ni-W alloy powders, Ni/Al wires, austenitic low-carbon stainless steels, nickel- and cobalt-based self-fluxing alloys, etc.), as well as Cr3C2 metal-ceramic powders. Under conditions involving impact or vibration loads, if the temperature is below 760°C, self-fluxing alloys are preferred; whereas when erosion is severe, Cr3C2 is recommended. If the primary requirement is oxidation resistance, coatings based on iron, nickel, or cobalt can be employed.
(4) Hard-face 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, shift 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 sealing rings, among others.
When the operating temperature of hardface wear-resistant 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 employ drill-based self-fluxing alloys, Ni/Al coatings, and chromium carbide coatings. When the temperature is below 760°C and impact loads are present, self-fluxing alloys are recommended; at higher temperatures, Cr3C2 coatings are more suitable. If oxidation resistance is the primary consideration, coatings such as Ni/Al should be selected. Specific application components include forging tools, hot-break rolls, and hot-forming dies.
(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 others.
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 parts include turbine seal rings, sealing rings, sealing gaskets, and turbine blades for jet engines.
(6) Cavitation-resistant coating.
Since the coating must withstand gas impacts in liquid flows, it is required to possess 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, ultrafine Al2O3, and pure Cr2O3. All these coatings should undergo sealing treatment. Specific application components include turbine blades, wear-resistant rings, spray nozzles, and cylinder liners for diesel engines.
(7) Erosion- and wear-resistant coating.
These coatings must be able to withstand wear caused by sharp, hard particles. Several types of materials can be used, including Ni-based self-fluxing alloy powders, self-fluxing alloy powders mixed with fine copper, high-Cr stainless steel powders, ultrafine Al2O3 powders, pure Cr2O3 powders, composite powders consisting of 87% Al2O3 and 13% TiO2, and Co/WC composite powders. Specific application components include fans, hydroelectric valves, and cyclone dust collectors.
3. 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 during grinding occurs relatively slowly. 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 sharp grinding wheels whenever 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 used to remove wear-resistant coatings, silicon carbide and diamond are commonly employed as abrasive grains. 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 coarse 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 deliver 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 use a harder grinding wheel.
(3) Select an appropriate grinding wheel bond type. There are two commonly used grinding wheel bonds: ceramic bond and resin bond.
Grinding wheels using ceramic binders can withstand higher grinding speeds and precise fit tolerances, and are unaffected by water, acids, oils, or temperature fluctuations. However, they require that the grinding machine’s spindle speed be lower than the wheel’s safe operating speed—typically below 33 meters per second. In contrast, grinding wheels with resin binders can be used at even higher grinding speeds and achieve superior surface finish.
For WC-based wear-resistant coatings applied to JP8000, 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 working 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 more friction (burnishing) than 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 spalling or thermal cracking, minimizing the removal of surface particles and resulting in superior surface finish. Moreover, wet grinding prevents rapid clogging of the grinding wheel, thus 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 play a significant role in achieving a high-quality 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 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 likelihood of burnishing and abrasive particle 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 may result in abrasive grain 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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Product Description
[Coating Preparation] Wear-resistant Coating
Wear is a common phenomenon in nature. Friction is the resistance to relative motion between two mating surfaces, caused by the interaction—on an atomic or molecular level—resulting from microscopic contact points between these surfaces. 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 will inevitably occur; and where there is friction, wear is bound to follow. Working conditions that can lead to wear include sliding, micro-vibration, scratching, and erosion. However, given the uncertainty surrounding the causes of wear, it is essential to thoroughly analyze the operating environment of components when selecting wear-resistant coatings. The thermal spraying technique 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 the corresponding performance requirements for coating materials are shown in the table below.
1. Performance requirements for wear-resistant coatings
The requirements for wear-resistant coatings depend on the mechanical compatibility and chemical compatibility between the wear-resistant coating and the substrate material, the direction and magnitude of the applied load, as well as the performance characteristics of the coating itself.
(1) Coating adhesion strength requirements.
The primary requirement for wear-resistant coatings is to ensure that the coating has sufficient bonding strength with the substrate. To this end, the selection and design of both the substrate material and the coating material should 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 should have sufficient hardness and yield strength to prevent deformation of the coating.
2) Matching of the elastic modulus between the coating and the substrate material,
Under elastic strain conditions, if the elastic modulus of the coating does not match that of the substrate, a sharp stress gradient will develop at the interface between the coating and the substrate when a load is applied. If the coating’s stiffness exceeds that of the substrate, the stress within the coating will increase. As the load and the difference in elastic moduli between the coating and the substrate grow larger, the stress will also increase.
The table lists the elastic moduli of high-speed steel and carbides.
3) Rigidity matching between the coating and the substrate material.
To ensure a long service life for hard, wear-resistant coatings, the stiffness of the coating should be reasonably matched with that of the substrate material. If a highly rigid coating is deposited onto a substrate material with low stiffness, the mismatch in stiffness 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, stress 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 resulting from the mismatch in thermal expansion between the coating and the substrate are concentrated primarily within the coating itself. The greater the difference in thermal expansion coefficients, the higher the stress in the coating—and the greater the likelihood of cracking or even delamination. This is one of the primary reasons why many wear-resistant coatings fail prematurely by delaminating before they have even begun to show significant wear.
The table lists the coefficients of thermal expansion for several carbide and nitride coating materials and 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 that 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 strong chemical affinity without forming brittle interfacial phases can their bonding strength be high enough to fully exploit the wear-resistant properties 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 enhance the coating’s yield strength and prevent deformation. As 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 will 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 superior corrosion resistance—for example, components operating in environments such as oil, chemical industries, and marine atmospheres. If the wear resistance and corrosion resistance of a coating are 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 signifies the best performance in these two aspects.
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 adjustment are not critical, as well as in locations with limited lubrication. Coating materials that can be employed include nickel-based and iron-based self-fluxing alloys, oxide and carbide ceramics (such as Al2O3-TiO2 and Co-WC), refractory metals like molybdenum, and molybdenum combined with self-fluxing alloys. Specific application components include: crankshafts for press dampers, anti-abrasion bushings, rudder bearings, turbine shafts, necks of drive gears, and rotor assemblies for fuel pumps with piston rings.
(3) Wear-resistant particle-abrasion 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’s 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 materials include iron-based, nickel-based, and cobalt-based coatings (such as cobalt-based Cr-Ni-W alloy powders, Ni/Al wires, austenitic low-carbon stainless steels, nickel- and cobalt-based self-fluxing alloys, etc.), as well as Cr3C2 metal-ceramic powders. Under conditions involving impact or vibration loads, if the temperature is below 760°C, self-fluxing alloys are preferred; whereas when erosion is severe, Cr3C2 is recommended. If the primary requirement is oxidation resistance, coatings based on iron, nickel, or cobalt can be employed.
(4) Hard-face 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, shift 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 sealing rings, among others.
When the operating temperature of hardface wear-resistant 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 employ drill-based self-fluxing alloys, Ni/Al coatings, and chromium carbide coatings. When the temperature is below 760°C and impact loads are present, self-fluxing alloys are recommended; at higher temperatures, Cr3C2 coatings are more suitable. If oxidation resistance is the primary consideration, coatings such as Ni/Al should be selected. Specific application components include forging tools, hot-break rolls, and hot-forming dies.
(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 others.
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 parts include turbine seal rings, sealing rings, sealing gaskets, and turbine blades for jet engines.
(6) Cavitation-resistant coating.
Since the coating must withstand gas impacts in liquid flows, it is required to possess 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, ultrafine Al2O3, and pure Cr2O3. All these coatings should undergo sealing treatment. Specific application components include turbine blades, wear-resistant rings, spray nozzles, and cylinder liners for diesel engines.
(7) Erosion- and wear-resistant coating.
These coatings must be able to withstand wear caused by sharp, hard particles. Several types of materials can be used, including Ni-based self-fluxing alloy powders, self-fluxing alloy powders mixed with fine copper, high-Cr stainless steel powders, ultrafine Al2O3 powders, pure Cr2O3 powders, composite powders consisting of 87% Al2O3 and 13% TiO2, and Co/WC composite powders. Specific application components include fans, hydroelectric valves, and cyclone dust collectors.
3. 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 during grinding occurs relatively slowly. 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 sharp grinding wheels whenever 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 used to remove wear-resistant coatings, silicon carbide and diamond are commonly employed as abrasive grains. 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 coarse 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 deliver 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 use a harder grinding wheel.
(3) Select an appropriate grinding wheel bond type. There are two commonly used grinding wheel bonds: ceramic bond and resin bond.
Grinding wheels using ceramic binders can withstand higher grinding speeds and precise fit tolerances, and are unaffected by water, acids, oils, or temperature fluctuations. However, they require that the grinding machine’s spindle speed be lower than the wheel’s safe operating speed—typically below 33 meters per second. In contrast, grinding wheels with resin binders can be used at even higher grinding speeds and achieve superior surface finish.
For WC-based wear-resistant coatings applied to JP8000, 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 working 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 more friction (burnishing) than 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 spalling or thermal cracking, minimizing the removal of surface particles and resulting in superior surface finish. Moreover, wet grinding prevents rapid clogging of the grinding wheel, thus 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 play a significant role in achieving a high-quality 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 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 likelihood of burnishing and abrasive particle 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 may result in abrasive grain 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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