The present invention generally relates to a low-friction coating system and a method of forming a low-friction coating on a metal substrate.
Vehicle systems often include rotatable components. For example, a vehicle steering system may include a wheel bearing rotatable with respect to a constant velocity (CV) joint, and/or a rotor rotatable with respect to a wheel. Such components may be joined by a washer, e.g., a torque washer, to distribute a load between the two components and/or to prevent one component from spinning freely.
Rotation of the components with respect to each other generates friction and heat, and therefore may also produce audible noise, and/or induce wear and corrosion on mating surfaces of the components.
A method of forming a low-friction coating on a metal substrate includes ferritic nitrocarburizing the metal substrate to form a surface of the metal substrate, wherein the surface includes a compound zone and a diffusion zone disposed subjacent to the compound zone. After ferritic nitrocarburizing, the method includes oxidizing the compound zone to form a porous portion defining a plurality of pores, and, after oxidizing, coating the porous portion with polytetrafluoroethylene. Further, the method includes curing the polytetrafluoroethylene to thereby form the low-friction coating.
A low-friction coating system includes the metal substrate having the surface including the compound zone and the diffusion zone disposed subjacent the compound zone, wherein the compound zone includes the porous portion defining the plurality of pores. The low-friction coating system also includes a cured film formed from polytetrafluoroethylene disposed sufficiently on the porous portion so as to at least partially fill at least one of the plurality of pores.
In one variation, the low-friction coating system is configured for minimizing audible noise from friction during component rotation. The low-friction coating system includes a first component and a second component. The second component is disposed in contact with the first component, and is rotatable with respect to the first component. The low-friction coating system further includes a torque washer disposed in contact with each of the first component and the second component. The torque washer has the surface including the compound zone and the diffusion zone disposed subjacent to the compound zone, wherein the compound zone includes the porous portion defining the plurality of pores. The low-friction coating system also includes the cured film formed from polytetrafluoroethylene disposed sufficiently on the porous portion so as to at least partially fill at least one of the plurality of pores. The torque washer minimizes audible noise from friction during rotation of the second component with respect to the first component.
The methods and systems minimize audible noise between two components during component rotation. In particular, the methods and systems minimize a stick slip condition between rotatable components. Further, the torque washer of the low-friction coating system exhibits a low coefficient of friction, excellent strength, stiffness, and corrosion-, wear-, and heat-resistance. Additionally, the torque washer maintains excellent clamp load between components during component rotation while simultaneously exhibiting excellent wear-resistance and a low coefficient of friction. Moreover, the methods and systems are cost-effective.
The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.
Referring to the Figures, wherein like reference numerals refer to like elements, a method of forming a low-friction coating on a metal substrate is disclosed herein. The method may be useful for forming a low-friction coating system, shown generally at 10 in
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The torque washer 12 may have a coefficient of friction of about 0.09 when disposed in contact with the first component. Therefore, because of the low-friction coating, i.e., the cured film 26 (
Additionally, the torque washer 12 of the low-friction coating system 10 maintains excellent clamp load between the first component 16 and second component 28 during rotation, while exhibiting excellent wear-resistance, as provided by the diffusion zone 20, and a low coefficient of friction, as provided by the cured film 26 formed from polytetrafluoroethylene disposed on the porous portion 22 of the compound zone 18. That is, the excellent wear-resistance and low coefficient of friction of the torque washer 12 minimize fretting wear on surfaces of the rotating components 16, 28. Since clamp load may deteriorate as surfaces wear, the torque washer 12 therefore provides excellent clamp load for joints. Excellent clamp load is particularly important for applications including the wheel bearing and CV joint since consistent clamp load maintains excellent wheel bearing performance, e.g., sealing and stiffness.
The method of forming the low-friction coating on the metal substrate 12 is described with general reference to
The metal substrate 12 may be ferritic nitrocarburized by any suitable method, e.g., solid-, liquid-, and/or gaseous-ferritic nitrocarburizing. Ferritic nitrocarburizing produces the surface S, which may be known as a case hardened surface S, including the compound zone 18 and the diffusion zone 20.
More specifically, gaseous ferritic nitrocarburizing may expose the metal substrate 12 to a nitrogen-containing gas, e.g., ammonia, and a carbon-containing gas, e.g., a hydrocarbon gas such as methane or propane, at a temperature of from about 550° C. to about 590° C. For example, the metal substrate 12 may be exposed to a blended gas including ammonia, methane, and oxygen at a temperature of about 570° C. Exposure to the blended gas may induce cracked nascent ammonia gas to dissociate at the surface S of the metal substrate 12 and react with the hydrocarbon gas according to the following reactions.
In particular, ammonia dissociates on the surface S of the metal substrate 12 according to reaction (1).
NH3→N+3/2H2 (1)
And, carbon dioxide is generated according to the water-gas reaction (2).
CO2+H2H2+CO (2)
Further, when the metal substrate 12 is exposed to a gaseous atmosphere including ammonia and an endothermic gas mixture including carbon monoxide, a dominant carburizing reaction (3) occurs at a temperature of about 570° C. As used herein, carburizing refers to diffusion of carbon into the surface S of the metal substrate 12.
CO+H2C+H2O (3)
In particular, carburizing occurs according to a relationship expressed by equation (4).
wherein
ac=carbon activity,
K3=equilibrium constant,
ρCO=partial pressure of carbon monoxide,
ρH2=partial pressure of hydrogen,
ρH2O=partial pressure of water vapor.
Likewise, nitriding activity occurs according to a relationship expressed by equation (5). As used herein, nitriding refers to introduction of nitrogen into the surface S of the metal substrate 12.
wherein
a′N=nitriding activity,
K1=equilibrium constant,
ρNH3=partial pressure of ammonium,
ρH23/2=partial pressure of hydrogen.
Ammonia addition at constant pressure to the gaseous atmosphere surrounding the metal substrate 12 results in a drop in the partial pressure of hydrogen and an increase in the nitriding activity according to a relationship expressed by reaction (6).
NH3+COHCN+H2O (6)
And, hydrogen cyanide present in the gaseous atmosphere as a result of ammonia interaction with carbon monoxide supplies nitrogen in parallel to nitrogen present according to dissociation reaction (1). Therefore, mass transfer of nitrogen to the compound zone 18, i.e., build-up of nitrogen in the compound zone 18, occurs according to reaction (7), and nitriding activity of the compound zone 18 occurs according to equation (8).
HCN→C+N+½H2 (7)
wherein
a′N=nitriding activity,
K2=equilibrium constant,
ρHCN=partial pressure of hydrogen cyanide,
ac=carbon activity,
ρH21/2=partial pressure of hydrogen.
In another variation of ferritic nitrocarburizing, solid ferritic nitrocarburizing may expose the metal substrate 12 to a nitrogen- and carbon-containing salt bath at a temperature of from about 550° C. to about 590° C. For example, the metal substrate 12 may be exposed to a cyanide salt bath at a temperature of about 570° C. for from about 1 to about 2 hours. Exposure to the cyanide salt bath may induce cyanate ions to react at the surface S of the metal substrate 12 according to reactions (9)-(12).
4KOCN→K2CO3+CO+2N* (9)
2CO→CO2+C** (10)
KCN+CO2→KOCN+CO (11)
2KCN+O2→2KOCN (12)
And, nitrogen and carbon react with iron of the ferrous metal substrate 12 according to reactions (13) and (14).
*N=3Fe→Fe3N (13)
**C+3Fe→Fe3C (14)
Therefore, ferritic nitrocarburizing forms the surface S of the metal substrate 12 including the compound zone 18 and the diffusion zone 20.
The method may also include preparing the metal substrate 12 for ferritic nitrocarburizing. For example, the method may include descaling and/or heating the metal substrate 12 prior to ferritic nitrocarburizing. That is, the metal substrate 12 may be exposed to an acid, e.g., muriatic acid, sulfuric acid, and/or phosphoric acid, to remove scale, i.e., iron oxide, from the metal substrate 12 prior to ferritic nitrocarburizing. Likewise, the metal substrate 12 may be heated, e.g., to about 400° C. in a convection furnace. Heating the metal substrate 12 prior to ferritic nitrocarburizing minimizes moisture in the metal substrate 12, which may react with the nitrogen-containing gas, the carbon-containing gas, and/or contents of the nitrogen- and carbon-containing salt bath.
With continued reference to
For applications including solid ferritic nitrocarburizing via the cyanide salt bath set forth above, oxidizing may expose the compound zone 18 to an oxidizing salt bath at a temperature of from about 425° C. to about 430° C. for from about 10 minutes to about 30 minutes to form the porous portion 22. For example, the metal substrate 12 may be immersed in the oxidizing salt bath at a temperature of about 427° C. for about 20 minutes.
The oxidizing salt bath may be an alkali hydroxide/nitrate mixture that oxidizes the compound zone 18 of the metal substrate 12 to form an oxide/nitride mixture in the compound zone 18. The formation of the oxide/nitride mixture provides the metal substrate 12 with excellent corrosion-resistance. As such, the oxidizing salt bath may include from about 2 parts by weight to about 20 parts by weight, e.g., from about 10 parts by weight to about 15 parts by weight, nitrate ions based on 100 parts by weight of the oxidizing salt bath. Suitable nitrate ions may include, but are not limited to, sodium nitrate, potassium nitrate, and combinations thereof. Likewise, the oxidizing salt bath may include from about 25 parts by weight to about 40 parts by weight carbonate ions based on 100 parts by weight of the oxidizing salt bath. Suitable carbonate ions may include, but are not limited to, sodium carbonate, potassium carbonate, and combinations thereof. Moreover, the oxidizing salt bath may include from about 40 to about 73 parts by weight hydroxide ions based on 100 parts by weight of the oxidizing salt bath. Suitable hydroxide ions may include, but are not limited to, sodium hydroxide, potassium hydroxide, and combinations thereof.
In particular, oxidizing may occur according to reactions (15)-(17).
CN−1+3OH−1+NO3−1→CO3−2+NO2−1+NH3+O−2 (15)
CNO−1+3OH−1→CO3−2+NH3+O−2 (16)
[Fe(CN)6]−4+6NO3−1→FeO+5CO3−2+5CO3−2+6N2+CO2 (17)
Therefore, oxidizing the compound zone 18 forms the porous portion 22 defining the plurality of pores 24.
With continued reference to
The method may further include pre-treating the metal substrate 12 after oxidizing and prior to coating the porous portion 22 with polytetrafluoroethylene. For example, the metal substrate 12 including the compound zone 18 and the diffusion zone 20 may be degreased in a solvent solution or a water-based caustic such as sodium hydroxide. Then, after degreasing, the metal substrate 12 may be iron- or zinc-phosphated to passivate the surface S of the metal substrate 12 and provide further improved adhesion of the cured film 26 formed from polytetrafluoroethylene to the porous portion 22.
Additionally, the method may further include cooling the metal substrate 12 after oxidizing and prior to coating the porous portion 22 with polytetrafluoroethylene. That is, the metal substrate 12 may be cooled to room temperature by air cooling and/or thermal quenching with water.
After coating, the method further includes curing the polytetrafluoroethylene to thereby form the low-friction coating, i.e., the cured film 26. The polytetrafluoroethylene may be cured at a temperature of from about 220° C. to about 345° C. for from about 5 to about 20 minutes to coat the porous portion 22 of the compound zone 18. Therefore, curing the polytetrafluoroethylene on the porous portion 22 of the compound zone 18 forms the cured film 26, i.e., the low-friction coating, on the metal substrate 12, and thereby provides the metal substrate 12 with a low coefficient of friction.
The method provides the metal substrate 12, e.g., the torque washer, with the aforementioned low coefficient of friction and excellent strength and corrosion-, wear-, and heat-resistance. The method also provides the torque washer 12 with unexpected stiffness. That is, the cured film 26 disposed on the porous portion 22 of the compound zone 18 of the torque washer 12 imparts a stiffness to the torque washer 12. As such, the tabs 14 (
The methods and systems 10 minimize audible noise between two components 16, 28 during component rotation. In particular, the methods and systems 10 minimize a stick slip condition between rotatable components 16, 28. Further, the torque washer 12 of the low-friction coating system 10 exhibits a low coefficient of friction, and excellent strength, stiffness, and corrosion-, wear-, and heat-resistance. Additionally, the torque washer 12 maintains excellent clamp load between components 16, 28 during component rotation while simultaneously exhibiting excellent wear-resistance and a low coefficient of friction. Moreover, the methods and systems 10 are cost-effective.
While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.