Patent Application
20100080982
The present disclosure is directed to a thermal spray coating and, more particularly, to surface preparation for bond enhancement of a thermal spray coating.
Several well known high temperature thermal spray methods for coating a substrate exist in the industry today such as, for example, high-velocity oxygen fuel (HVOF) spraying. HVOF is a combustion process in which oxygen is mixed with a fuel gas and ignited, forming an exhaust gas. The exhaust gas is accelerated toward a substrate via a spray torch as metal and/or ceramic material is injected into the gas stream. The injected material becomes molten and is propelled at a high velocity toward the substrate to be coated. One shortcoming of thermal spray methods such as HVOF may be that the bond strength that is achieved between a coating and a substrate may be limited.
U.S. Pat. No. 5,688,564 (the '564 patent) issued to Coddet et al. discloses a process for the preparation of a substrate surface to increase bond strength. The '564 patent discloses irradiating a substrate surface via a pulse laser beam prior to applying a thermal spray coating. The pulse laser beam imparts a large amount of energy into the substrate surface in a very brief amount of time. The pulse laser may improve bond strength of the coating by creating a plasma of vaporized material that expands to cause a shockwave. The shockwave may have a cleaning and roughening effect on the substrate surface that may improve bond strength between the coating and the substrate surface.
Although the process of the '564 patent may provide a method for affecting a shockwave effect to roughen a substrate surface, it may fail to improve the coating bond for metallurgically joining the coating and the substrate. The process described in the '564 patent does not provide a significant increase in thermal energy available at a contact surface between the substrate and the thermal spray particles.
The present disclosure is directed to overcoming one or more of the shortcomings set forth above and/or other deficiencies in the art.
In accordance with one aspect, the present disclosure is directed toward a method for coating an element. The method includes irradiating a surface of the element with a continuous laser to heat the element. The method also includes coating the surface of the element with a thermal spray coating after irradiating.
According to another aspect, the present disclosure is directed toward a coating. The coating includes a substrate material and a thermal spray layer. The coating also includes an interface layer bonding the substrate material to the thermal spray layer, the interface layer being greater than about 75% contaminant-free.
As illustrated in
Depositing device 14 may be any suitable thermal spraying device for depositing a coating material 16 onto substrate 12. Coating material 16 may be deposited onto substrate 12 via any suitable method known in the art such as, for example, combustion wire spraying, combustion powder spraying, twin wire arc spraying, plasma transfer wire arc spraying, wire or powder high-velocity oxygen fuel (HVOF) spraying, or combustion flame spraying. HVOF is a combustion process where oxygen may be mixed with a fuel gas and ignited, forming an exhaust gas stream. The exhaust gas stream may be accelerated toward a substrate at high velocities such as, for example, velocities in excess of about 1400 meter/second (m/s). Coating material 16 may include powder metals or ceramic cermets that are injected generally axially or radially into the exhaust gas stream and become molten as they are propelled toward substrate 12. High velocities of coating material 16 may contribute to mechanical bond strength between coating material 16 and substrate 12. Depositing device 14 may be any suitable application device such as, for example, an HVOF spray gun, a wire arc spray gun, or a plasma arc spray gun. Coating material 16 may be in any suitable form such as, for example, powder, liquid, or wire, and may be introduced into a plasma jet produced by depositing device 14. Depositing device 14 may deposit coating material 16 via any suitable technique such as, for example, a raster motion on flat surfaces or a spiral pattern on rotating elements. Depositing device 14 forms a thermal spray layer 24 on substrate 12.
Laser 18 may be a continuous laser suitable for preparing a surface for a coating such as, for example, a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser, a carbon dioxide laser, or a high power diode laser (HPDL). Laser 18 may be a continuous wave (CW) laser and may operate at a suitable power level for coating such as, for example, of between about 100 and about 2000 W/mm2. For example, laser 18 may operate at a power level of between about 500 and about 1500 W/mm2. Laser 18 may also operate at a power level of about 400 W/mm2. Power level may be determined based on laser spot, which may be a surface area irradiated by laser beam 22. Laser spot may be measured based on the full width at half maximum (FWHM) of the laser power distribution across laser beam 22.
Laser 18 may be mounted on a same fixture as depositing device 14, or alternatively on a different fixture that precedes depositing device 14 in a direction of motion 25 of coating. Laser 18 may be moved in direction of motion 25 at a suitable rate for coating such as, for example, of between about 200 and about 3000 millimeter/second (mm/s) relative to substrate 12. Alternatively, substrate 12 may be moved at a rate of between about 200 and about 3000 mm/s relative to laser 18 and depositing device 14. For example, laser 18 may be moved at a rate of between about 500 and about 1500 mm/s. Depositing device 14 may follow closely behind laser 18 in direction of motion 25, with coating material 16 contacting a surface location at an interval such as, for example, between about 1 and about 20 milliseconds after laser 18 irradiates the surface location.
Laser 18 may emit a laser beam 22 that contacts a surface 20 of substrate 12 and/or a previously applied layer 24. As illustrated in
Depth 27 and the temperature gradient of laser-affected zone 26 may affect bond strength between coating 15 and substrate 12. Although heating substrate 12 may improve bond strength, bond strength may be weakened by too much heat, i.e., by laser-affected zone 26 being too large and/or temperatures being too high. Bond strength may also be weakened by laser-affected zone 26 being too small and/or temperatures being too low. Decreasing a rate of movement of laser 18 may increase the amount of time that laser beam 22 imparts heat into a given location of substrate 12, thereby imparting more heat into substrate 12 than when laser 18 moves at a faster rate. Therefore, controlling the rate of movement of laser 18 may control the amount of heat imparted to substrate 12, and may produce a desired laser-affected zone 26 of an appropriate size and temperature for optimizing bond strength for a given coating material and substrate material. Laser-affected zone 26 may be controlled via laser 18 to avoid melting of substrate 12. Melting may be undesirable because it may significantly reduce a hardness of substrate 12.
As illustrated in
Laser beam 22 may affect at least one previously applied layer 24 and a portion of substrate 12 to combine together to form a single interface layer 28 within laser-affected zone 26. After a suitable amount of passes of laser 18 and depositing device 14 such as, for example, about twenty to thirty passes, interface layer 28 may have a thickness of up to about 150 μm. For example, interface layer 28 may be between about 1 and 100 μm thick, or between about 1 and 50 μm thick. Interface layer 28 may have a hardness that is greater than a hardness of substrate 12. Hardness may be measured by a suitable micro-hardness test that measures hardness of a small volume of material such as, for example, a Vickers or Knoop hardness test.
Coating system 10 may include an application of flux to clean surface 20 of substrate 12, and/or surfaces of previously applied layers 24, before irradiation by laser 18. The flux may be any suitable flux known in the art for preventing oxidation such as, for example, fluoride-containing or calcium-containing flux. Oxidation occurs when oxygen molecules interact with molecules of a surface, causing an oxide film to form that may decrease bond strength. Oxidation may occur nearly instantaneously such as, for example, when oxygen molecules contact surface 20. Any suitable method known in the art for applying a thin film of material may be used to apply the flux over an area of surface to be coated such as, for example, via a dispensing device that sprays a thin layer of flux onto a surface. The flux may be inert at relatively low temperatures such as, for example, an ambient outdoor temperature. When subjected to relatively high temperatures such as, for example, laser beam 22, the flux may react with any oxide film that has formed on surface 20 and/or surfaces of layer 24 due to oxidation, to vaporize both the flux and the oxide film. The removal of oxides prior to coating may improve a bond strength between coating 15 and substrate 12.
Coating system 10 may be used in any coating application. For example, coating system 10 may be used in any manufacturing and remanufacturing applications requiring a thermal spray coating. Laser 18 may improve bond strength by producing a desired laser-affected zone 26 via laser beam 22.
Coating system 10 may be used for new manufacturing of an article, remanufacturing of an article, sealing of an article, and wear resistance applications on an article. Coating system 10 may be used on engine components such as, for example, cylinder heads, cylinder blocks, pistons, piston rings, and crankshafts. Coating system 10 may also be used on track assembly undercarriage components such as, for example, rotatable bushings, idlers, track rollers, track pins, track shoes, track faces, and track links of a chain assembly. For example, coating system 10 may be used for sealing an end face of a bushing. Coating system 10 may be used on implement tool components such as, for example, ripper tips, buckets and bucket edges, blades, ground engaging tools, implement cutting edges, and linkage pins. Coating system 10 may also be used on hydraulic cylinder components such as, for example, rods, rod eyes, rod seals, piston seals, and piston tubes. Coating system 10 may also be used on suspension cylinder components such as, for example, shock-absorbing truck struts.
As illustrated in
Coating system 10 may improve the bond strength between coating 15 and substrate 12. Controlling a rate of movement of laser 18 may produce a desired laser-affected zone 26 of substrate 12, which may improve bond strength. A desired laser-affected zone 26 may be selected, based on material properties of substrate 12 and coating 15, to achieve a desired bond strength. Controlling laser-affected zone 26 may thereby achieve a desired, uniform bond strength. Coating system 10 may also provide a relatively small interface layer 28 having metallurgical properties that may improve bonding between coating 15 and substrate 12. Metallurgical properties of interface layer 28 may also reduce the probability of feathering (i.e., removing coating and exposing uncoated substrate material) during machining after applying coating 15. Laser 18 may also clean a surface to be coated, which may improve bond strength of coating 15 and may eliminate the need for grit-blasting the surface.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed coating system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed method and apparatus. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
