Multifunctional alumina-based coating on cast iron, steel, copper or copper alloy

Abstract

This invention involves a coating that is ceramic-based and has an inner and outer layered structure. The inner layer is made of inorganic compounds generated by electrochemical reaction between an electrolyte and a metallic component surface, and the outer layer is an alumina-based ceramic generated by a simultaneous process of electrochemical reaction and plasma sintering in the electrolyte. The coating can be deposited on cast iron, steel, copper and copper alloys for enhanced anti-wear, anti-corrosion, electric insulation, thermal barrier, and antifouling properties for applications on brakes, motor parts, bearings, gears, shafts, washers, bushes, pistons, food processing devices, marine structures, or aquaculture cages.

Description

Multifunctional alumina-based coating on cast iron, steel, copper or copper alloy TECHNICAL FIELD

[0001] This invention involves a coating on cast iron, steel, copper and copper alloy for enhanced anti-wear, anti-corrosion, electric insulation, thermal barrier, or antifouling properties for applications on brakes, bearings, laminates, wires, windings, gears, shafts, washers, bushes, pistons, or in marine and aquaculture environment. BACKGROUND OF THE INVENTION

[0002] Considering the increasing use of electric vehicles (EVs) to avoid tailpipe exhaust emissions, the relative contribution of non-exhaust emission (NEE) will become increasingly more important for total traffic-related emissions. NEE are predominantly from brake wear, tire wear, road surface wear and resuspended road dust, where brake discs (rotors) generate a large portion of brake wear dust. A common idea for EV s is that regenerative braking will remove the brake wear or the need for brakes altogether. Unfortunately, regenerative braking may not be able to bring a vehicle to a stop all on its own and the friction brake systems are always in play as emergency stops. The combination of regenerative braking and friction braking would remain necessary in the future for safety reasons. Although the lack of tailpipes indeed improves the air quality for urban cities, brakes still play a significant role in NEE from EV s. At the same time, corrosion-induced materials loss of the brake discs in EVs should be valued more due to the reduced or less-frequent usages of the friction brakes and the lower disc temperature, which prevents the disc from drying and increases the exposure time to wetness and highly corrosive de-icing salts especially in the winter season. When friction brake systems on vehicles with regenerative braking system (RBS) are slow to reach bedded conditions, the braking operation could lead to increased brake wear emissions. If brake discs and pads are degraded due to the reduced use because of RBS, rusted surfaces could lead to poor bedding conditions and higher brake wear.

[0003] EVs use electric motors which involve the electrical power and magnetic field. The bearings, gears and shafts in the drivetrain may need to have electric insulation to avoid electric discharginginduced erosion. Washers and bushes may also need such an electrical insulation to avoid the mentioned electric erosion and to reduce galvanic corrosion. Some pistons in brake calipers and even in conventional combustion engines need to have thermal barrier coatings for thermal management. An antifouling coating is also needed for marine and aquaculture applications. Therefore, there is a great demand for numerous components to have a multifunctional coating I Date Re9ue / Date Received 2022-04-01 surface which has the enhanced anti-wear, anti-corrosion, electric insulation, thermal barrier, and antifouling properties.

[0004] For examples, surface treatments including chrome plating, thermal spraying and laser cladding processes have been explored to combat problems of wear and corrosion for cast iron brake disc applications. Besides a thermal spraying coating proposed by an automaker stated in a US patent application 20110278116Al, an auto supplier [US patent application 20200217382Al] also adopted the hard tungsten carbide coating on the cast iron brake disc using a thermal spraying process and claims that it has a 90% reduction in brake dust than a conventional brake disc. However, such a kind of brake disc can cost 5-10 times more than a conventional cast iron disc as shown in its application in a sports car. After the thermal spraying process, a portion of coating materials needs to be machined or ground off to obtain the required surface roughness on brake friction ring areas. Furthermore, chemicals used in thermal spraying processes likely contain heavy metals which may have adverse effects on environment and human health.

[0005] Another auto supplier also introduced a new brake disc which features a hard layer of coating applied to its ring, using High-Velocity-Oxy-Fuel (HVOF) technology that reduces environmental impact through reducing particular emissions [US patent US9,879,740]. Recently, a carbidic brake rotor surface coating applied by high-performance laser-cladding was tested by a U.S. automaker in Europe [EuroBrake 2020-MDS-020]. Very low rotor wear rates and reduced pad wear show high potential for reduction of brake particular emissions, especially for European low metallic brake pad materials. However, both thermal spraying (including HVOF) and laser-cladding technologies are designed most likely to meet the demands of premium and luxury cars only again due to the high cost. Furthermore, as stated in US patent US I 0,895,295, all the carbide-based coatings have a preparatory metallic alloy of support layer containing chromium, nickel, and / or tungsten heavy metals. Even oxide-based coatings (including aluminium oxide-based coatings) prepared by the thermal spraying (including plasma thermal spraying and HVOF) have a similar bonding layer containing chrome and nickel. Those metals may be harmful for environment, water and human health when they are worn and emitted as brake dust (e.g., PM2.5 and PMI0).

[0006] Relatively low-cost technology called Ferritic Nitrocarburizing (FNC) for cast iron discs was developed by another U.S. automaker to improve corrosion resistance mainly for North America market. However, PNC-treated rear wheels in used EV s still need frequent braking in order to keep the rust from affecting the braking performance. 2 Date Re9ue / Date Received 2022-04-01

[0007] In order to increase corrosion resistance and lightweighting of brake discs, aluminum have already been deployed to make such as aluminum metal-matrix composite (Al-MMC) discs. Plasmaelectrolytic oxidation (PEO) process is an environmentally safe coating process which offers aluminium alloy discs with protection against corrosion and wear. The terminology of PEO was firstly defined and used in one of the inventors' Ph.D. dissertation [Nie, Ph.D. Dissertation, Hull, UK, 2000; Nie, Yerokhin, Matthews, et al, Surf Coat Technol, 122 (1999) 73-93]. The PEO process ( or micro-arc oxidation) is then widely proposed to apply for valve metals including aluminum (Al), magnesium (Mg) and titanium (Ti) and their alloys [Canadian patent CA 2,479,032]. The PEO process utilizes a high electric voltage to induce the dielectric breakdown of a self-passive film on a light metal surface. Consequently, a ceramic oxide coating forms on the surface. In terms of the coating growth mechanism, the PEO is actually an oxidation of a light metal itself being treated. Therefore, the treated metal decides chemical composition of the oxide. In other words, the oxide is aluminium oxide if the treated material is aluminium, or magnesium oxide if the treated material is magnesium, or titanium oxide if the treated material is titanium. The ceramic oxide film can be tailored to provide desirable (thermo-) mechanical properties for engineering ( or even for biomedical) applications. However, for a car brake application, the aluminum base alloys with relatively low melting temperature can be soften at high braking temperatures and undergo plastic deformation at extreme panic braking situations [EuroBrake 2020-EB5-032]. Still, the PEO coating on Al-based brake discs may find applications in EV brake systems in future when a new brake test standard is proposed or regulators exempt EV from some current brake test standards by considering aluminium lightweighting and also contribution of E-motor regenerative braking power.

[0008] A novel cost-effective coating technology, called plasma electrolytic aluminating (PEA), has been developing by inventors of this patent for improving corrosion and wear resistance of cast iron materials at a small sample level (i.e., on sample coupons) [Nie et al., ACS Sustainable Chemistry and Engineering. 7 (2019) 5524-5531; 8 (2020) 893-899]. In brief, the PEA process is a new coating method which combines electrochemical reaction, plasma discharging in a liquid environment, and plasma sintering of in-situ formed ceramic coating materials in the liquid environment. The PEA process is significantly different from known ceramic and coating fabrication methods. When someone speaks about ceramic sintering, the sintering is often operated in a furnace to densify a ceramic bulk material, not in a form of a coating [US patent US 20 l 7 / 00884 71 Al]. The sintering is performed by high temperature heating, not involved in plasma discharging. A traditional ceramic coating can be prepared in liquid environment through electrochemical [US patent US 4882014] or 3 Date Re9ue / Date Received 2022-04-01 sol-gel methods where again no plasma discharging is involved but sometime furnace sintering as a post-heat treatment is applied [patent US20060147699Al]. When a plasma discharging is relied on for a ceramic coating deposition, it is often that the coating operation is carried out in a vacuum system, e.g., physical vapour deposition or chemical vapour deposition [Canada patent number CA 2850270], or in air environments, e.g., thermal spraying [Canada patent number CA 2883157]. A plasma discharging in a liquid environment for a ceramic coating preparation can be found in plasma electrolytic oxidation (PEO) processes [Surf Coat Technol, 122 (1999) 73-93]. However, the conventional PEO process can be applied only onto engineering materials made of aluminium, titanium and magnesium and their alloys, since the PEO process is a coating conversion process associated with its substrate. A US patent [patent US 9701 l 77-B2] disclosed a coating method, called plasma electrochemical deposition, which is actually similar to the PEO process that can be applied again only to aluminium, titanium and magnesium. The PEA process used in this invention is applied on components that are not made of aluminum, titanium or magnesium. Therefore, the proposed PEA process in this invention is significantly different from the PEO and totally different from vacuum plasma and thermal spraying coating process. The PEA process is applied on components that are brake discs ( or called brake rotors in North America) made of cast iron; bearings or laminates made of steels; or windings made of copper. The components can be gears, shafts, washers, bushes, pistons, food processing devices, marine structures or aquaculture cages. The disclosed coating process is to provide metallic surfaces (i.e., cast iron, steel, Cu and Cu alloy) with multi-functionality which includes anti-wear, anticorrosion, electric insulation, thermal barrier, and antifouling properties. As a few examples, the coating process disclosed in this invention is to make an alumina-based coating for decreasing NEE and corrosion of cast iron disc and drum brakes, for increasing electrical insulation of steel bearings and laminates, or for increasing thermal durability and corrosion resistance of copper wires and windings. SUMMARY OF THE INVENTION

[0009] The invention hereby involves an alumina-based coating which is synthesized using a process similar to the PEA (plasma electrolytic aluminating). The PEA process reported so far is an immersion treatment process where a small cast iron sample is immersed into the electrolyte. The coating process in this invention for a large component is that the component is located preferentially outside the electrolyte tank and the aqueous electrolyte is sprayed onto the localized component surface where the coating is needed. The coating has a double layered structure with surface pores. The inner layer is formed through electrochemical reaction between the electrolyte 4 Date Re9ue / Date Received 2022-04-01 and the metallic surface being treated. The formed inner layer is made of environmental-friendly inorganic compounds (i.e., iron aluminate, copper aluminate, or aluminum hydroxide depended on the electrolyte used), which is totally different from the metal-based supportive layer (e.g., chromium and nickel) involved in the previously-mentioned thermal spraying and laser-cladding technologies. The outer layer is an alumina-based ceramic comprising alumina (i.e., AbO3 in chemical formula), aluminium-silicon-oxide, aluminium-titanium-based oxide, or titanium-based oxide, depended on the electrolyte used but independent from the component' s material being treated. Thus, the coating contains no heavy metal in this invention. This coating has multifunctionality which includes anti-wear, anti-corrosion, electric insulation, thermal barrier and antifouling properties. Obviously, the coating pores can be partially or fully sealed by a commercial coating sealing method if needed.

[0010] The said electrolyte is the deionized or distilled water dissolved with 4-40 grams / litre at least one of sodium aluminate, potassium aluminate, sodium silicate, potassium silicate, sodium phosphate, potassium phosphate, or sodium titanium oxalate.

[0011] The said metallic component surface is made of cast iron (including grey, compact graphite, and ductile cast iron), steel, copper, or copper alloy.

[0012] The said voltage during the coating process is 60-600 Volts of a DC or pulsed DC power with a current density of 0.05-5 A / cm2 .

[0013] The said coating has a double layers, and the inner layer thickness is 0.5-5 microns, the outside layer thickness is 5-100 microns.

[0014] The said coating has anti-wear and anti-corrosion resistances 5-10 times higher than the substrate material without the coating. The coating has a thermal conductivity of 0.5-10 Walt per meter per Kelvin (W / m•K) and electric resistance of 1-100 Giga-ohms (GO).

[0015] The said coating on Cu and Cu alloys also has anti-fouling property.

[0016] The coated components (or parts) can be used for applications in brake discs (rotors), bearings, gears, shafts, washers, bushes, pistons, marine structures, or aquaculture cages. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic illustration of a coating synthesized using a coating process similar to the PEA process. DETAILED DESCRIPTION OF THE INVENTION 5 Date Re9ue / Date Received 2022-04-01

[0018] Referring to the schematic illustration in FIG. 1, a metallic component 1 is sprayed with an electrolyte. The surface of the component 1 firstly reacts with an electrolyte under a positive bias voltage of 60V-600V to form a thin inorganic compound layer 2. The compound layer would dielectrically breakdown when the bias voltage is increased to a certain voltage (preferably beyond 200V up to 600V). Under the increased bias voltage, precursors (ionic compounds) in the electrolyte stick onto the top of the inorganic compound layer surface 2 where a simultaneous process of electrochemical reaction and sintering (by plasma discharging) occur and form a hard ceramic outer layer 3. Therefore, the coating has a two-layered structure. The plasma electric discharging under the high bias voltages also generates pores 4 with microns or submicron in sizes. The inner layer thickness can be in a range of 0.5-5 microns and the outer layer thickness can be in a range of 5-100 microns, depended on the treatment time, current density used and the final ending voltage. Longer treatment time, high current density used and high ending voltage generate a thicker coating.

[0019] In accordance with embodiments of this invention, a metallic component made of cast iron or steel is treated using a process similar to the PEA (plasma electrolytic aluminating) process. The coated surface has double layers. The inner layer results from the reaction of the component material with the electrolyte which warrants interface adhesion strength between the top layer and the component surface. The inner layer is made of iron aluminate (AhFeO4). The top layer is a hard ceramic (i.e., alumina) which provides wear and corrosion resistance. Exampled applications of this alumina-based coating are for brake discs (rotors) of automotive vehicles or wind turbine electric power generators. The alumina-based coating deposited on friction ring areas of automotive brake discs (rotors) is to reduce brake dust and corrosion in a cost effective way.

[0020] In accordance with embodiments of this invention, a metallic component made of cast iron, steel, copper or copper alloy is treated using a coating process similar to the PEA (plasma electrolytic aluminating) process. The coated surface has double layers. The inner layer results from the reaction of the component material with the electrolyte which warrants interface adhesion strength between the top layer and the component surface. The inner layer is made of iron aluminate, copper aluminate, or aluminum hydroxide, depended on the electrolyte used. The top layer is an alumina-based ceramic which provides wear and corrosion resistance as well as electric insulation. Exampled applications of this coating are for bearing, gears, shafts, washers and bushes in electric motors and in situations where those multifunctional properties are necessary. The electric insulating property of the coating is applied onto inner diameter (ID) or outer diameter (OD) surfaces of bearings, gears and shafts especially for avoiding electric erosion problems that may 6 Date Re9ue / Date Received 2022-04-01 otherwise occur on the bearing rollers or gear teeth during the service involving electric current and voltage.

[0021] In accordance with embodiments of this invention, a metallic component made of cast iron or steel is treated using a coating process similar to the PEA (plasma electrolytic aluminating) process. The coated surface has double layers. The inner layer results from the reaction of the component material with the electrolyte which warrants interface adhesion strength between the top layer and the component surface. The inner layer is made of iron aluminate. The top layer is an alumina ceramic which has a thermal insulating property and also be able to withstands high pressure and temperature. Exampled applications of this coating are for pistons in brake calipers and combustion engmes.

[0022] In accordance with embodiments of this invention, a metallic component made of copper or copper alloy is treated using a coating process similar to the PEA (plasma electrolytic aluminating) process. The coated surface has double layers. The inner layer results from the reaction of the component material with the electrolyte which warrants interface adhesion strength between the top layer and the component surface. The inner layer is made of copper aluminate or aluminum hydroxide. The top layer is an alumina ceramic which provides corrosion and wear resistance; however, the inner layer contains copper oxide which can be released to environment for antifouling applications. Exampled applications of this coating are for marine sector and aquaculture for anticorrosion and antifouling.

[0023] In accordance with embodiments of this invention, a metallic component made of cast iron, steel, copper or copper alloy is treated using a coating process similar to the PEA (plasma electrolytic aluminating) process. The coated surface has double layers. The inner layer is formed through electrochemical reaction between the electrolyte and the metallic component surface being treated. The formed inner layer is made of environmental-friendly inorganic compounds of iron aluminate, copper aluminate, or aluminum hydroxide, depended on the electrolyte used. The outer layer is an alumina-based ceramic made of alumina, aluminiumsilicon-oxide, aluminium-titaniumbased oxide, or titanium-based oxide, depended on the electrolyte used. The coating has multifunctionality including anti-wear, anti-corrosion, electric insulation, thermal barrier and antifouling properties. The coated components and parts are brake discs (rotors), bearings, gears, shafts, washers, bushes, pistons, food processing devices, marine structures or aquaculture cages. To be specific for but not limited to examples hereby, the coating process disclosed in this invention is to make an alumina-based coating for decreasing NEE and corrosion of cast iron disc and drum brakes, 7 Date Re9ue / Date Received 2022-04-01 for increasing electrical insulation of steel bearings and laminates, or for increasing thermal durability and corrosion resistance of copper wires and windings. 8 Date Re9ue / Date Received 2022-04-01

Claims

Multifunctional alumina-based coating on cast iron, steel, copper or copper alloy The invention claimed is:

1. A coating process, comprising: spraying an aqueous electrolyte onto a surface of a metallic component; performing electrochemical reaction and simultaneously plasma sintering on said surface; and forming a ceramic coating on said surface, wherein said ceramic coating has an inner and outer layered structure in which said inner layer is made of an inorganic compound and said outer layer is made of an alumina-based ceramic.

2. The coating as claimed in claim 1, wherein said inorganic compound in the inner layer is iron aluminate, copper aluminate, or aluminum hydroxide.

3. The coating as claimed in claim 1, wherein said alumina-based ceramic in the outer layer is aluminium oxide, aluminium-silicon oxide, aluminium-titanium oxide, or titanium oxide.

4. The coating as claimed in claim 1, wherein said coating has an inner layer thickness of 0.5-5 microns and an outer layer thickness of 5-100 microns.

5. The coating as claimed in claim 1, wherein said metallic component as the coating substrate is made of cast iron, steel, copper or copper alloy.

6. The coating as claimed in claims 1 and 5, wherein said metallic component is a cast iron brake disc, a cast iron brake drum, a steel bearing, a steel laminate, a copper wire, or a copper winding. 9 Date Re9ue / Date Received 2022-05-13