Cold spraying

Abstract

A method comprising: cold-spraying a surface of a substrate with a bond material to form a bond coating; and cold-spraying a surface of the bond coating with a coating material to form a top coating. The bond material is different from the coating material and harder than the surface of the substrate.

Claims

1. A method comprising the steps of: cold-spraying a surface of a substrate with a bond material to form a bond coating; cold-spraying a surface of the bond coating with a coating material to form a top coating; and heating the coated substrate after forming the top coating; wherein the substrate comprises a material comprising a non-metallic, intermetallic, ceramic or oxide phase; the bond material comprises cobalt, a cobalt-based alloy, titanium, or a titanium-based alloy; the coating material comprises nickel or a nickel-based alloy; the bond material is different from the coating material and harder than both of the top coating and the surface of the substrate; and wherein a difference between a Vickers hardness of the bond material and a Vickers hardness of the surface of the substrate is at least 100 HV when measured under the same conditions.

2. The method of claim 1, wherein the coating material is a nickel-based superalloy.

3. The method of claim 1, wherein heating the coated substrate comprises heating the coated substrate for at least 30 minutes.

4. The method of claim 1, wherein heating the coated substrate comprises holding the coated substrate at a temperature from about 200° C. to about 1000° C.

5. The method of claim 1, wherein the method further comprises mechanically preparing the surface of the substrate prior to forming the bond coating.

6. The method of claim 1, wherein the substrate is a structural component.

7. The method of claim 1, wherein the bond coating is from about 0.1 millimeter (mm) to about 2 mm thick.

8. The method of claim 1, wherein the top coating is from about 0.5 millimeter (mm) to about 1 mm thick.

9. The method of claim 3, wherein heating the coated substrate comprises heating the coated substrate for at least 2 hours.

10. The method of claim 9, wherein heating the coated substrate comprises heating the coated substrate for at least 4 hours.

11. The method of claim 1, further comprising mechanically preparing the surface of the substrate by milling or grinding prior to forming the bond coating.

12. The method of claim 1, wherein the substrate is an engine block.

13. The method of claim 9, wherein heating the coated substrate comprises heating the coated substrate at a temperature of about 500° C.

Description

DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIGS. 1 (a) to (d) illustrate schematically, in sectional side views, a process of repairing a damaged surface of an engine block by cold-spraying a coating including a bond coating and a top coating;

(3) FIG. 2 is a flowchart illustrating a cold-spraying method;

(4) FIG. 3 is an optical micrograph of a ground, polished and etched metallurgical sample of an interface between a cast iron substrate and a cold-sprayed coating of nickel-based superalloy;

(5) FIG. 4 is an optical micrograph of a ground, polished and etched metallurgical sample of an interface between a cast iron substrate and a cold-sprayed coating of nickel-based superalloy;

(6) FIG. 5 is an optical micrograph of a ground, polished and etched metallurgical sample through a cast iron substrate coated with a cold-sprayed bond coating of cobalt-chromium-tungsten alloy and a top coating of nickel-based superalloy;

(7) FIG. 6 is an optical micrograph of a ground, polished and etched metallurgical sample of an interface between a cast iron substrate coated and a cold-sprayed bond coating of cobalt-chromium-tungsten alloy;

(8) FIG. 7 is an optical micrograph of a ground, polished and etched metallurgical sample of an interface between a cold-sprayed bond coating of cobalt-chromium-tungsten alloy and a top coating of nickel-based superalloy; and

(9) FIG. 8 is a bar chart showing interfacial bond strength (in MPa), measured by a glue failure method, of cold-sprayed samples A, B, C, D and E.

DETAILED DESCRIPTION

(10) A method of repairing a diesel engine block 1 is illustrated schematically by way of FIGS. 1 (a) to (d).

(11) The engine block 1 includes an engine block body 2 formed predominantly from grey cast iron. As shown in FIG. 1 (a), a surface portion 3 of the engine block 1 has been damaged through use, for example by cavitation erosion and wear. Repair of the engine block 1 to remove the damaged surface portion 3, and subsequently to achieve dimensional restoration, is necessary.

(12) The damaged surface portion 3 of the engine block 1 may be removed by any suitable methods known in the art. For example, the damaged surface portion 3 may be removed using milling, grinding, sand blasting and/or polishing processes. Removal of the damaged surface portion 3 results in the formation of a new surface 4 of the engine block body 2, as can be seen in FIG. 1 (b).

(13) Following removal of the damaged surface portion 3, dimensional restoration of the engine block 1 is achieved by cold-spray coating the engine block body 2.

(14) In a first cold-spraying step, as illustrated in FIG. 1 (c), a bond coating 5 is formed on the surface 4 by cold-spraying a bond material onto the surface 4. In the present example, the bond material is a cobalt-chromium-tungsten (Co—Cr—W) alloy. The bond coating 5 is from about 0.5 mm to about 1 mm thick (i.e. measured in a direction locally perpendicular to the surface 4 of the engine block body) and has an external surface 6.

(15) In a second cold-spraying step, as illustrated in FIG. 1 (d), a top coating 7 is formed on the surface 6 of the bond coating by cold-spraying a coating material onto the surface 6. In the present example, the coating material is a nickel-based superalloy (e.g. an Inconel® alloy). The top coating 7 is from about 2 mm to about 3 mm thick (i.e. measured in a direction locally perpendicular to the surface 4 of the engine block body).

(16) Following the second cold-spraying step, a heat treatment is performed in which the engine block is held at a temperature of about 500° C. for about 4 hours.

(17) As discussed in more detail below under Examples, the inventors have found that cold-spraying the bond material to form the bond coating on the engine block body, prior to cold-spraying the coating material to form the top coating, results in improved adhesion of the top coating to the engine block body in comparison to cold-spraying the coating material directly onto the engine block body (e.g. directly onto surface 4 formed by removal of the damaged portion 3). The inventors have also found that heat-treating the coated engine block leads to a further improvement in coating adhesion.

(18) Although the example shown in FIG. 1 relates to repair of an engine block, similar methods may be used to repair other types of component (such as other types of vehicle or engine component). More generally, similar methods may be used to form coatings on substrates of any type. In each case, however, the method includes (as illustrated schematically in FIG. 2): first, cold-spraying a bond material to form a bond coating (block 100 in FIG. 2); and, second, cold-spraying a coating material to form a top coating on the bond coating (block 101 in FIG. 2). The method may further comprise carrying out an optional heat treatment (block 102 in FIG. 3).

(19) The substrate (e.g. the component) which is to be repaired or coated may be formed from any type of material. However, the inventors have found that the use of a cold-sprayed bond coating is particularly effective in improving adhesion of a cold-sprayed top coating when the substrate comprises non-metallic, intermetallic, ceramic or oxide phases. Such phases may be present in substrates formed from metals or metal alloys, for example as metal oxide surface coatings or as non-metallic, intermetallic, ceramic or oxides phases in an alloy microstructure also including predominantly metallic phases. For example, ferrous alloys, and in particular cast irons, may include phases such as graphite (e.g. in grey cast iron) or cementite (e.g. in white cast iron) which may be characterised as non-metallic, intermetallic or ceramic.

(20) It will be appreciated that different bond materials may be selected for different applications. However, the inventors have found that the bond material should be harder than the material from which the substrate is formed, in order to achieve good adhesion between the bond coating and the substrate. In particular, the Vickers hardness of the bond material should be about 100 HV, for example about 150 HV, higher than the Vickers hardness of the surface of the substrate to be cold-sprayed. Suitable bond materials include metals or metal alloys (such as Co- or Ti-based alloys) or ceramics (such as alumina).

(21) It will also be appreciated that different coating materials may be selected for different applications. In many applications, however, the coating material will be a metal or a metal alloy. The inventors have found that the method is particularly suitable for coating substrates with superalloys such as nickel-based superalloys (e.g. an Inconel® alloy).

(22) It will also be appreciated that the cold-spraying conditions (for example, cold-spray apparatus parameters) may be varied dependant on the materials to be deposited and the thickness of the coatings to be obtained. Exemplary cold-spray parameters are provided below under Examples.

(23) It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

EXAMPLES

Example 1

(24) A grey cast iron engine block was repaired by machining away a damaged portion of a surface of the block and subsequently cold-spraying the machined surface of the block with a layer of Inconel® (IN718) nickel-based superalloy.

(25) The microstructure of the engine block and the cold-sprayed layer, at the interface between the block and the layer, was investigated by imaging a metallurgical sample cut in cross-section perpendicular to the interface. The sample was ground, polished and etched according under standard metallurgical sampling conditions and was imaged in an optical microscope. FIGS. 3 and 4 are optical micrographs of the interface.

(26) In both FIGS. 3 and 4, a region of grey cast iron is indicated generally at C and a region of Inconel® nickel-based superalloy is indicated generally at I. As can be seen in the micrographs, the cast iron includes a ferrite matrix, labelled α, and flakes of graphite, G. As can be seen in FIG. 3, the Inconel® nickel-based superalloy appears to bond well to the ferrite matrix of the cast iron. However, as can be seen in FIG. 4, the Inconel® nickel-based superalloy does not bond well to graphite flakes and, indeed, delamination (labelled D) of the Inconel® nickel-based superalloy layer adjacent interfacial graphite flakes is observed.

(27) The strength of the bond between the layer of nickel-based superalloy and the grey cast iron substrate, as tested by a glue failure method, was found to be poor.

Example 2

(28) A sample was prepared by cold-spraying a substrate with a bond material to form a bond coating and subsequently cold-spraying the bond coating with a coating material to form a top coating.

(29) The substrate was formed from a grey cast iron (GJL 250).

(30) The bond material was a Co—Cr—W alloy (Co452). The bond material was cold-sprayed using the following cold-spraying parameters:

(31) Propellant Gas: N.sub.2

(32) Gas Temperature: 1000° C.

(33) Gas Pressure: 45 bar

(34) Particle Speeds: 700-800 m/second

(35) Gas Flow: 80 m.sup.3/hour

(36) Gun Scan Speed: 500 mm/second

(37) Step Size: 1 mm

(38) The coating material was an Inconel® (IN718) nickel-based superalloy. The coating material was cold-sprayed using the following cold-spraying parameters:

(39) Propellant Gas: N.sub.2

(40) Gas Temperature: 800° C.

(41) Gas Pressure: 40 bar

(42) Particle Speeds: 600-700 m/second

(43) Gas Flow: 80 m.sup.3/h

(44) Gun Scan Speed: 500 mm/second

(45) Step Size: 3 mm

(46) In both cases, a standoff distance between the cold-spray gun nozzle and the substrate was 30 mm and a SIC de Laval nozzle having an inlet diameter of 13 mm, a throat diameter of 2.52 mm, an outlet diameter of 6 mm, an expansion ratio of 5.6, and a convergent length of 15 mm, was used.

(47) The cast iron substrate was preheated to 300° C. for 5 minutes prior to cold spraying the bond material. The substrate was not preheated prior to cold spraying the coating material.

(48) FIG. 5 shows an optical micrograph of a ground, polished and etched cross-section through the sample perpendicular to the interfaces between the substrate, the bond coat and the top coat. As can be seen in the micrographs, the Co—Cr—W alloy bond coating, B, is well-adhered to the cast iron substrate, S, and the nickel-based superalloy top coating, T, is well-adhered to the bond coating, B. The substrate-bond coating (I.sub.SB) and bond coating-top coating (I.sub.BT) interfaces are shown in more detail in FIGS. 6 and 7, respectively. No continuous crack is observed along the substrate-bond coating interface or along the bond coating-top coating interface.

(49) The strength of the bond between the coating (comprising the bond coating and the top coating) and the cast iron substrate, as tested by a glue failure method, was found to be improved in comparison to the sample in Example 1.

Example 3

(50) Five different samples were prepared as follows.

(51) Samples A, B and C were prepared by cold-spraying a nickel-based superalloy (Inconel® 625) onto a cast iron substrate. In sample A, the substrate was formed from a ductile cast iron and was sandblasted prior to cold-spraying. In sample B, the substrate was formed from a grey cast iron and was polished prior to cold-spraying. In sample C, the substrate was formed from a grey cast iron and was ground prior to cold-spraying.

(52) Samples D and E were prepared by, first, cold-spraying a cast iron substrate with a bond material to form a bond coating and, second, cold-spraying the bond coating with a coating material to form a top coating. In sample D, the substrate was formed from grey cast iron, the substrate was polished prior to cold-spraying, the bond material was a Co—Cr—W alloy (Co452), and the coating material was a nickel-based superalloy (Inconel® 625). In sample E, the substrate was formed from grey cast iron and was polished prior to cold-spraying, the bond material was a Co—Cr—W alloy (Co452), the coating material was a nickel-based superalloy (Inconel® 625), and the sample was heat-treated by holding at 500° C. for 4 hours.

(53) The interfacial bond strength for each sample was measured using the adhesion strength test (also known as the glue failure test) following the ASTM C633 standard. The samples were wire-cut into circular buttons each having a diameter of 25 mm. The buttons were ground flat. Top and bottom button surfaces and fixtures were sand-blasted with P80 alumina particles, cleaned with ethanol, and assembled together with adhesive glue. The assembled sets were then placed in an oven in which the sets were cured at 150° C. for 60 minutes and left to cool to room temperature (about 23° C.). The sets were then tested using a tensile tester with a load cell of 50 kN in tensile mode with an extension rate of 0.8 mm/minute until the sets failed. The results of the adhesion strength testing are shown in FIG. 8. As can be seen in FIG. 8, samples D and E (which include a bond coating between the layer of nickel-based superalloy and the cast iron substrate) exhibit improved interfacial bond strengths in comparison to samples A, B and C (in which nickel-based superalloy was cold-sprayed directly onto the cast iron substrate). In addition, it can be seen that the interfacial bond strength of sample E (which was subjected to a heat treatment after cold spraying) is twice that of sample D (which was not heat treated).