EP4448186A1

EP4448186A1 - Thermal spray system and coating

Info

Publication number: EP4448186A1
Application number: EP22829823.8A
Authority: EP
Inventors: Tanvir Hussain; Federico VENTURI; Sunil CHADHA; Kah LENG; Acacio Rincón ROMERO
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2021-12-16
Filing date: 2022-12-13
Publication date: 2024-10-23
Also published as: WO2023111539A1

Abstract

A thermal spray system (50), a thermal spray nozzle (200) and a method of thermal spray coating are disclosed. The thermal spray system (50) comprises a combustion chamber (101) configured to produce a thermal spray flame (106) from the reaction product of a fuel fluid and an oxidiser fluid; a first feedstock injection port (107) configured to entrain a first feedstock material in the thermal spray flame (106); an inert gas injection port (202) configured to shroud the thermal spray flame (106) with an inert gas; and a second feedstock injection port (207) configured to entrain a second feedstock material in the thermal spray flame (106). The first feedstock injection port (107) is spaced apart from the second feedstock injection port (207) along a longitudinal axis of the thermal spray flame (106).

Description

THERMAL SPRAY SYSTEM AND COATING

FIELD OF INVENTION

The present invention relates to method and apparatus for thermal spray coating, and to coatings produced by thermal spray deposition.

BACKGROUND

Thermal spray coating is a process that allows the deposition of a coating through high- temperature and high-velocity spray of a feedstock. The feedstock normally consists of ceramics, cermets or metallic materials. The feedstock can be either in the form of powder, suspension or solution precursor.

Thermal spray apparatuses consist of a combustion chamber where fuel and oxidiser react, and an expansion nozzle where the exhaust is ejected at high temperature and high velocity. The feedstock can be injected in the combustion chamber or in the expansion nozzle.

Depending on the combustion power, the feedstock can experience a different temperature, velocity and interaction time, leading to melting, semi -melting or no melting. Each of these states can be optimal for different class of materials.

Once injected, the feedstock follows the exhaust path, undergoes acceleration and heating, and is projected onto a substrate to form a coating.

High velocity oxy-fuel (HVOF) thermal spray is a particular thermal spray method. In HVOF thermal spraying, a mixture of gaseous or liquid fuel and oxygen is combusted to generate a combustion flame. The feedstock is fed into the flame, which heats the feedstock and accelerates it towards the surface to be coated. Flame temperatures are typically in the range 2500-3100° C. The flame accelerates the feedstock to very high speeds (at least supersonic), for example approaching 800 m/s. HVOF spraying typically achieves coatings that have low porosity and high bond strength. Low porosity is important for maintaining the structural integrity of the coating.

Broadly, all the HVOF thermal spray torches can be classified as either gas fuelled or liquid fuelled. Typically, the feedstock is injected directly into the combustion chamber for a gas-fuelled torch, and in the liquid fuel torches, the feedstock is injected downstream. Typically, process parameters are adjusted for a particular torch in order to achieve an appropriate coating.

In traditional HVOF spraying, the feedstock is a dry powder. However, it is also known to employ suspension high velocity oxy-fuel (SHVOF) thermal spray to form wearresistant coatings incorporating emerging 2D nanomaterials like graphene nanoplatelets (see WO2018/224830). In SHVOF thermal spray, particles of material are dispersed in a suspension (i.e. in a fluid such as a liquid). The suspension is used as the feedstock and is fed into the combustion flame, similar to how dry powder is fed into the flame in conventional HVOF thermal spray. Suspension sprayed coatings tend to have a finer grain size and pore size, less anisotropy, and lower surface roughness than conventional, dry, HVOF thermal sprayed coatings. Suspension sprayed coatings can be thinner than conventional thermal spray coatings, bridging the gap between thermal -sprayed coatings and vapour deposition coatings.

Excessive heat and an oxidising environment can degrade some sensitive feedstock materials, such as polymers, nanomaterials, carbon-based materials, carbides, nitrides and metals.

Although some progress has been made in addressing these issues in the prior art, a thermal spraying apparatus and method that can accommodate a wider range of feedstock materials is desirable.

SUMMARY

According to a first aspect of the invention, there is provided a thermal spray system, comprising: a combustion chamber configured to produce a thermal spray flame from the reaction product of a fuel fluid and an oxidiser fluid; a first feedstock injection port configured to entrain a first feedstock material in the thermal spray flame; an inert gas injection port configured to shroud the thermal spray flame with an inert gas; and a second feedstock injection port configured to entrain a second feedstock material in the thermal spray flame; wherein the first feedstock injection port is spaced apart from the second feedstock injection port along a longitudinal axis of the thermal spray flame.

The first feedstock injection port may be configured to introduce the first feedstock material to the combustion chamber, or to the thermal spray flame, at a distance of no more than 30 mm from the combustion chamber. The second feedstock injection port may be at least 30 mm away from combustion chamber along the longitudinal axis of the thermal spray flame.

The second feedstock injection port may be at least 30 mm (or at least 20 mm) downstream of the first feedstock injection port.

The thermal spray system may comprise a thermal spray gun and a nozzle, wherein the thermal spray gun comprises the combustion chamber and the first feedstock injection port, and wherein the nozzle is configured to detachably couple to the thermal spray gun, and the nozzle comprises the inert gas injection port and the second feedstock injection port.

According to a second aspect, there is provided a nozzle configured to detachably couple to a thermal spray gun comprising a combustion chamber configured to produce a thermal spray flame and a first feedstock injection port, the nozzle comprising: an inert gas injection port, for shrouding the thermal spray flame with an inert gas; a second feedstock injection port configured to entrain a second feedstock material in the thermal spray flame within the nozzle.

The following features are applicable to the first or second aspect.

The nozzle may be configured to surround the thermal spray flame for a distance of at least 50mm (or at least 80mm) from an outlet of the thermal spray gun.

The nozzle may comprise an exit nozzle portion by which the thermal spray exits the nozzle.

The exit nozzle portion may have a length of 50mm to 100mm. The exit nozzle portion may be diverging, with a half-cone angle of 1 to 10 degrees

The exit nozzle portion may have an inlet internal diameter of between 10mm and 20mm.

The second feedstock injection port may open into the exit nozzle portion at a distance of between 5 mm and 30 mm from an inlet end of the exit nozzle portion.

The nozzle may comprise a proximal end for coupling to the thermal spray gun, and a distal end remote from the proximal end. The second feedstock injection port may be nearer to the proximal end than the distal end

The second feedstock injection port may be at an angle of up to 30 degrees from a radial direction. The radial direction is perpendicular to the thermal spray flame axis (i.e. centreline).

There may at least two nozzle feedstock injection ports (e.g. at the same axial position, for example distributed around a circumference).

The inert gas port may be at a distance of between 5mm and 25 mm from an outlet end of the nozzle (e.g. at a distance of between 5mm and 25mm from an outlet end of the diverging nozzle portion).

The nozzle may comprise a holder module for adapting the nozzle to couple to the thermal spray gun, the holder module detachable from the rest of the nozzle

The holder module may comprise a converging portion configured to communicate the thermal flame from the tip of the thermal spray gun to a downstream portion of the nozzle.

According to a third aspect, there is provided a method of using the nozzle of the second aspect, the method comprising: attaching the nozzle to the thermal spray gun; producing a thermal spray flame with the thermal spray gun; adding a feedstock material to the thermal spray flame via the first and/or second feedstock injection port; shrouding the thermal spray flame with an inert gas via the inert gas injection port; and depositing a coating on a substrate.

According to a fourth aspect, there is provided a method of thermal spray coating, comprising: reacting an oxidiser fluid with a fuel fluid in a combustion chamber to form a thermal spray flame flowing out of the combustion chamber; injecting a first feedstock material into the thermal spray flame at a first position; injecting a second feedstock material into the thermal spray flame at a second position; depositing a coating comprising the first and second feedstock material on a substrate; wherein the second position is spaced apart from the first position along a longitudinal axis of the thermal spray flame.

The following features may be applicable to the third or fourth aspect.

The second position may be at least 20 mm (or at least 30 mm) from the combustion chamber (any reference to a distance from the combustion chamber may be measured from an outlet of the combustion chamber).

The second feedstock material may comprise a suspension or solution based feedstock. The feedstock material may comprise a suspension or solution based feedstock.

The first feedstock material may be different from the second feedstock material, and the coating may comprise a composite material.

The first feedstock material may comprise a metal material, and the second material may comprise a ceramic material. The first feedstock material may comprise a ceramic material, and the second feedstock material may comprise a suspension of nanoparticles.

The feedstock material may comprise a suspension comprising silicon carbide particles and yttrium aluminium garnet particles.

The feedstock material may comprise a silicon carbide particles and a solution based yttrium aluminium garnet precursor solution. Depositing the coating may comprise synthesising yttrium aluminium garnet by chemical reactions in the thermal spray flame.

The feedstock material may comprise a suspension of alumina and Inconel particles.

The first feedstock material may comprise a calcium sodium phosphosilicate glass and the second feedstock material may comprises a gallium oxide suspension.

According to a fifth aspect, there is provided a method of depositing a composite layer comprising a first material and a different second material by thermal flame spray, comprising: reacting an oxidiser fluid with a fuel fluid in a combustion chamber to form a thermal spray flame flowing out of the combustion chamber; injecting a first feedstock comprising particles of the first material into the thermal spray flame; injecting a second feedstock comprising a solution of precursors into the thermal spray flame; synthesising the second material in the thermal spray flame; depositing the composite coating comprising the first material and the second material on a substrate.

The first material may be or comprise silicon carbide (SiC). The second material may be or comprise yttrium aluminium garnet (YAG).

The first feedstock may be carried by a gas (e.g. nitrogen). The first feedstock may comprise a liquid suspension of particles (e.g. SiC particles).

The precursors may comprise nitrate salt precursors of the second material. The precursors of the second feedstock may comprise aluminium trioxonitrate V nonahydrate and yttrium trioxonitrate V hexahydrate (i.e. A1(NC>3)3.9H2O and Y(NO3)3.6H₂O).

The first feedstock and the second feedstock may be combined into a combination feedstock - e.g. a YAG precursor solution carrying SiC particles in suspension.

The first feedstock and the second feedstock may be injected into the thermal spray flame at different positions along the thermal spray flame. The different positions may be separated by at least 20mm or at least 30mm along the axis of the thermal spray flame.

The method may comprise using a thermal spray system according to the first aspect or the second aspect (including any of the optional features thereof) . The first feedstock may be provided to the first feedstock injection port and the second feedstock provided to the second feedstock injection port. Alternatively the first feedstock may be provided to the second injection port and the second feedstock may be provided to the first injection port. In some embodiments a combination feedstock may be provided to either the first feedstock injection port or the second feedstock injection port.

According to a sixth aspect, there is provided a coating produced according to the method of any of the third, fourth or fifth aspects, including any optional features thereof.

The features (including optional features) of each aspect may be combined with the features (including optional features) of any other aspect. Nozzles and flame spray systems according to embodiments may be used in the methods according to certain embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure l is a schematic diagram of a thermal spray system according to an embodiment, comprising a first axial feedstock injection port and a second downstream radial feedstock injection port; Figure 2 is a schematic diagram of a thermal spray system according to an alternative embodiment, comprising a first radial feedstock injection port and a further downstream second radial feedstock injection port;

Figure 3 is a sectioned 3D view of a nozzle according to an example embodiment;

Figure 4 is a 2D section of the nozzle of Figure 3;

Figure 5 is a set of views of a first holder module;

Figure 6 is a set of views of a second holder module;

Figure 7 is a set of views of a feedstock injection module;

Figure 8 is a set of views of a gas injection module;

Figure 9 is a set of views of an inner module;

Figure 10 shows the effect of varying expansion ratios (i.e. nozzle outlet diameters) on the centreline gas velocity and centreline gas temperature;

Figure 11 shows the effect of varying nozzle outlet diameter on oxygen mass fraction within a nozzle according to an embodiment

Figure 12 shows the effect of changing inert gas flow rates on the oxygen mass fraction within a nozzle according to an embodiment;

Figure 13 shows the effect of different shrouding gas flows on (a) centreline gas velocity and the (b) centreline gas temperature;

Figure 14 shows the effect on (a) average particle velocity; and (b) average particle temperature of adding a nozzle according to an embodiment to an existing flame spray gun; Figure 15 (a) shows a cross section of a composite alumina/graphene coating produced according to an embodiment, and Figure 15 (b) and (c) show wear rates and coefficients of friction for the same coating;

Figure 16 (a) shows a cross section of a composite Inconel/alumina coating produced according to an embodiment, and Figure 16 (b) shows the measured thermal spray flame temperature and velocity at a stand-off distance of 150mm;

Figure 17 (a) shows a cross section of a hybrid 45 S5 bioglass/ Ga2Os coating produced according to an embodiment; and Figures 17 (b) and (c) compare delamination from a similar coating produced by a prior art flame spray gun with the coating according to an embodiment; and

Figure 18 shows a schematic of a nozzle used in an example method;

Figure 19 shows cross sections of the example SiC/YAG coating, with Figure 19(a) showing low magnification and Figure 19(b) showing high magnification;

Figure 20 shows X-ray diffraction results obtained from a SiC/YAG coating according to an example;

Figure 21 shows wear results obtained from example SiC/YAG coatings;

Figure 22 shows SEM images of wear tracks obtained from testing example SiC/YAG coatings; and

Figure 23(a) and 23(b) show a cross section of a silicon carbide coating produced according to an embodiment; and Figure 23(c) shows x-ray diffraction results obtained from the feedstock material and the silicon carbide coating according to an embodiment.

DETAILED DESCRIPTION

Figure 1 is a schematic diagram of a thermal spray system 50 according to an embodiment. The flame spray system 50 comprises a flame spray gun 100 and a nozzle 200, coupled to the flame spray gun 100. The flame spray gun 100 comprises a combustion chamber 101, oxidiser inlet 103, fuel inlet 104 and first feedstock injection port 107. The oxidizer inlet 103 is configured to provide an oxidiser fluid (such as oxygen gas) to the combustion chamber 101. The fuel inlet 104 is configured to provide a fuel fluid to the combustion chamber 101 (e.g. a gas phase fuel). The fuel and oxidiser mix in the combustion chamber 101 and react to form a thermal spray flame 106, which expands out of the combustion chamber 101 at high temperature and high velocity.

The first feedstock injection port 107 is configured to inject a first feedstock material into the thermal spray flame 106, so that the first feedstock material 108 can be carried by the flame 106 to a substrate 300 and be deposited at the substrate 300 as a coating 301. The feedstock material 108 introduced at the first feedstock injection port 107 may, for example, comprise a solid phase particulate material carried by a fluid stream (e.g. a liquid suspension of particle, or particles entrained in a gas flow).

The flame spray gun 100 may be any prior art flame spray gun 100. In this embodiment the first feedstock injection port 107 is depicted as introducing feedstock material into the combustion chamber 101 (which is typically used in flame spray guns employing gas phase fuel), but this is not essential. The direction in which the first feedstock injection port 107 injects feedstock into the combustion chamber 101 is parallel to the direction of propagation of the thermal spray flame 106 from the flame spray gun 100 (the first feedstock port 107 may therefore be referred to as an “axial” feedstock port).

The nozzle 200 is configured to couple to a distal tip of the flame spray gun 100, and comprises a through channel for the thermal spray flame 106 to pass through on its way towards the substrate 300. The nozzle 200 may be removably attached to the flame spray gun 100, for example by a clamp, thread, fastener or any other suitable fastening means. In some embodiments the nozzle 200 may be permanently attached to the flame spray gun (e.g. by welding it in place).

The nozzle 200 comprises a second feedstock injection port 207. The second feedstock injection port 207 is downstream from the first injection port 107, and therefore provide feedstock to a different position in the thermal spray flame 106. The environment experienced by a second feedstock material injected at the second feedstock injection port 207 may therefore differ (potentially significantly) from the environment experienced by the first feedstock material injected at the first feedstock injection port 107. For example, the first feedstock material may experience very high temperatures associated with the combustion chamber 101. The thermal spray flame 106 may be significantly cooler at the second feedstock injection port 207. The provision of more than one feedstock injection port 107, 207, at different positions along the flame spraying direction increases the flexibility of the flame spray system 50. This can be particularly significant where more than one feedstock material is to be combined in the thermal spray flame 106. For example, a second feedstock material may be more thermally sensitive than a first feedstock material (indeed, the first feedstock material may have a high melting point and require high temperatures for flame spraying). In the example of Figure 1, the more thermally sensitive material can be introduced at the second feedstock injection port 207, and the less thermally sensitive material may be introduced at the first feedstock injection port 107.

The nozzle 200 further comprises an inert gas injection port 202. The inert gas injection port 202 is configured to introduce an inert gas to shroud the thermal spray flame 106 within the nozzle 200 (and potentially after the nozzle 200), thereby reducing the potential for less stable feedstock materials to degrade, for example, as a result of oxidation as a result of contact with ambient air. The introduction of the inert gas may also reduce the temperature of the thermal spray flame 106.

The provision of both a second feedstock injection port 207 and the inert gas injection port 202 enables improved quality coatings that were not possible with prior art flame spray systems, particularly where the feedstock material is sensitive to thermal or chemical degradation during spraying.

Although an embodiment has been depicted in which a nozzle 200 is provided, for example as a removable item that can be decoupled from an existing flame spray gun 100 using hand tools, in some embodiments a flame spray system 50 can be designed from scratch with similar functionality. In certain embodiments the nozzle 200 need not be a separate part from the spray gun 100. The specifics of the flame spray gun 100 can vary - what is shown in Figure 1 is merely an exemplary schematic of a flame spray gun 100.

Figure 2 shows a thermal spray system 50 with similar features to that shown in Figure 1. In this embodiment, however, the thermal spray gun 100 comprises concentric fuel and oxidiser inlets 104, 103, and the first feedstock injection port 107 is radial, and introduces the first feedstock material to the thermal spray flame 106 downstream of the combustion chamber 101. The first feedstock injection port 107 may introduce the first feedstock material within 30 mm of the combustion chamber (or within 20 mm or within 10 mm). The second feedstock injection port 207 is downstream of the first feedstock injection port (for example, by at least 20mm).

Figures 3 to 9 show a nozzle 200 according to an example embodiment (suitable for use as shown in Figures 1 and 2), comprising a holder module 210, feedstock injection module 220, gas injection module 230 and inner module 240.

The nozzle 200 is generally cylindrical, with an axis corresponding with the thermal spray flame longitudinal axis or centreline (when the nozzle is coupled to a spray gun 100). Each of the modules 210, 220, 230, 240 is similarly generally cylindrical.

A modular construction may be advantageous in facilitating straightforward manufacturing of the nozzle 200, and improving the ease of reconfiguring the nozzle (e.g. for a different flame spray gun), but is not essential. Any of the modules described herein may be combined. A monolithic combination of modules may be formed, for example using additive manufacturing methods. The monolithic combination of modules may, for example, combine the functions of all or some of the modules. For example, the functions of the feedstock injection module 220, gas injection module 230 and inner module 240 may be combined in a single part, leaving the holder module 210 as a replaceable adapter for coupling the nozzle 200 to different spray guns 100. Different holder modules 210 may be provided that are adapted to fit different spray guns (e.g. as a kit of parts along with the other nozzle modules, integrated or otherwise).

Referring to Figure 5, the holder module 210 comprises a gun recess 21 1 for receiving a distal tip of the thermal spray gun 100. The gun recess 211 in this example comprises an internal cylindrical surface and a gun shoulder 214. The gun shoulder 214 locates the gun 100 axially (along the thermal spray flame axis) relative to the nozzle 200 when the distal tip of the gun 100 abuts the gun shoulder 214. The gun recess 211 may be at least 10mm deep (in the axial direction).

Downstream of the gun recess 211, the holder module 210 further comprises a converging portion 212. The converging portion 212 may form, with the inner module 240, a divergent nozzle. Although the example nozzle is depicted with a diverging inner surface, this is not essential. In some embodiments the inner surface of the nozzle may have parallel sides, or comprise a converging exit portion.

A distance between the gun shoulder 214 and the end of the holder module 210 (or the start of the inner module 240) may be in the range 30mm to 70mm (for example ~ 50mm). A taper angle of the converging portion may be in the range 5 to 10 degrees (e.g. -between 7 and 8 degrees).

The holder module 210 further comprises an external thread 213, configured to engage with a corresponding internal thread 223 on the feedstock injection module 220.

Figure 6 shows an alternative holder module 210a, which is similar to the holder module 210, except that the gun recess 211 is modified to accommodate a different gun tip. In the example of Figure 6, the holder module 210a comprises an internal thread, by which the nozzle may be coupled to a thermal spray gun 100. Holder module 210a comprises a slightly longer converging portion, with a shallower taper angle.

Referring to back to Figures 3, 4 and 7, the feedstock injection module 220 comprises a second feedstock injection port 207, coolant circulation ports 224, a coolant gallery 225, internal thread 223 for coupling to the holder module 210, an external thread 226 for coupling to the gas injection module 230, and an inner surface 221 for engagement with the inner module 240.

There are two nozzle feedstock injection ports 207 spaced apart on an external circumference of the feedstock injection module 220. In some embodiments, there may be a single nozzle feedstock injection port 207, or more than two. The feedstock injection port 207 is substantially radial, but has a slight inclination in the direction that the flame propagates through the nozzle 200. The angle of the injection port may be important in achieving efficient penetration of the feedstock into the flame The injection angle 246 (between the radial direction and an axis of the nozzle feedstock injection port 207) may be between 0 and 15 degrees.

The coolant circulation ports 224 are configured to enable a cooling fluid (e.g. water) to be circulated in contact with the nozzle 200, so as to keep the nozzle 200 cool. A cooling plenum or gallery 225 is provided between an outer wall of the feedstock injection module 220 and the inner module 240, so that the cooling fluid is in direct contact with the inner module 240 and the feedstock injection module 220. The other modules 210, 230 are cooled indirectly, by thermal conduction with the directly cooled module 220, 240.

The feedstock injection module 220 further comprises a tapering internal wall 221 that is arranged to conform with a corresponding external wall 241 of the inner module 240. The tapering internal wall 221 locates the inner module 240 in the nozzle 200, when the inner module is pushed axially (towards the rightward direction in Figure 7) as is clear from Figure 4.

The gas injection module 230 comprises an internal thread 236 that is configured to engage with the external thread 226 of the feedstock injection module. The gas injection module 230 comprises an inert gas injection port 202, through which an inert gas (e.g. nitrogen) can be introduced so as to shroud the flame 106, and reduce oxidation of any sensitive feedstock within the flame 106. The gas injection module 230 further comprises a locating shoulder 231 that abuts an end of the inner module 240, maintaining an external tapering wall 241 of the inner module 240 in engagement with the tapering inner face 221 of the feedstock injection module 220.

The gas injection module 230 and inner module 240 together define a plenum 233 therebetween, by which the inert gas is communicated from the inert gas injection port 202 to inner inert gas injection ports 242 through the inner module 240. In this example, there are more inner gas injection ports 242 than gas injection ports 202 in the inert gas injection module 230. Specifically, in this example there are at least 20 inner gas injection ports 242 and only two inert gas injection ports 202 in the inert gas injection module 230. There may be more or fewer inert gas injection ports 202 in the inert gas injection module 230, or a different number of inner has injection ports 242.

The gas injection module 230 is provided with a gas seal groove 235, for receiving a seal element (e.g. an o-ring) for preventing the inert gas escaping the nozzle 200.

The inner module 240 defines the inner surface of an exit portion of the nozzle. In this example the inner module 240 provides a diverging exit portion, but this is not essential to the invention.

The inner module 240 in the example is substantially frusto-conical (i.e. a tapering cylinder). The length of the inner module 240 may be in the range 50 mm to 110 mm (e.g. ~80 mm). A taper angle 244 of the inner module 240 be less than 15 degrees (e.g.~ 5 degrees).

The taper angle 244 may be defined based on a difference between the inlet diameter 248 at one end of the diverging region of the inner module 240 and the larger outlet diameter 249 at the opposite end of the diverging region of the inner module 240. The inlet diameter may be in the range 10 mm to 18 mm (e.g. ~ 14 mm). The outlet diameter may be in the range 24 mm to 32 mm (e.g. ~ 28 mm).

The external tapering wall 241 of the inner module 240 is provided with coolant seal grooves 243, for receiving a sealing element (such as an o-ring) for maintaining a seal to retain the coolant fluid in the cooling gallery 225.

Figure 10 shows the results of simulations investigating the effect of different expansion ratios in a diverging exit portion of a nozzle 200. The nozzle 200 in these simulations comprises a converging portion from the gun shoulder to the start of the diverging portion. The converging portion has an inlet diameter of 49 mm, an outlet dimeter of 36 mm and a length of 50 mm. The inlet diameter 248 of the diverging portion is 14 mm and the length of the diverging portion is 80 mm. The inner inert gas injection port 242 is at a distance of 11.5mm from the outlet of the nozzle 200.

Figure 10 shows results for outlet (larger) diameters of 24 mm, 28 mm, and 32 mm for the diverging portion. Curves for gas velocity (a) and gas temperature (b) are shown with X position = 0 corresponding with the outlet edge of the spray torch 212 In this simulation the total gas flow rate (oxygen as oxidizer and hydrogen as fuel) was 5.9 x 10’³ kg/s at 300 K, with an equivalence ratio of 1 and a nitrogen shrouding gas flow of 2 g/s. A curve is depicted in which there is no gas injected through port 202.

High gas velocities are desirable, and low oxygen entrainment into the nozzle. A large outlet diameter results in greater velocities, but increases oxygen entrainment into the nozzle (see Figure 11) which may lead to oxidation.

Figure 11 shows simulation results obtained for the same range of simulation parameters shown in Figure 10 (with outlet diameters of 24 mm, 28 mm and 32 mm), illustrating the oxygen mass fraction in the diverging nozzle portion and near the outlet of the nozzle 200. Decreasing outlet diameter results in lower mass fractions of oxygen within the nozzle, which will inhibit oxidation of sensitive feedstock in the flame within the nozzle.

Figure 12 shows the effect of changing inert gas flow rates on the oxygen mass fraction, for the same nozzle parameters that are described with reference to Figure 10, with a nozzle outlet diameter of 28mm. The gas mass flow rates are (a) 0 g/s, (b) Ig/s, (c) 2g/s and (d) 3 g/s. Gas flow rates greater than 2 g/s are sufficient to substantially eliminate oxygen from within the nozzle.

Figure 13 shows the effect of different shrouding gas flows on (a) centreline gas velocity and the (b) centreline gas temperature, for a nozzle outlet diameter of 28mm and other parameters as described with reference to Figure 10. The different shrouding gas flow rates do not make a significant difference in the centreline gas velocity, and make only a very small difference to the centreline gas temperature.

Figure 14 shows the effect on (a) average particle velocity; and (b) average particle temperature of adding a nozzle similar to that shown in Figures 3 to 9 to an existing flame spray gun (with x position = 0 corresponding with the exit of the flame spray gun). The “no shroud” results are those with no nozzle, and the “shroud” results are obtained with a nozzle according to an embodiment. At distances of greater than 200mm from the exit of the flame spray gun the average particle velocity is 20-30% lower with the nozzle than without. At distances over 200mm from the exit of the flame spray gun the average particle is significantly higher, typically by at least 20%.

Figure 15 shows results obtained from a coating deposited according to an embodiment. The arrangement used for deposition was similar to that shown in Figure 1, in which a nozzle like that shown in Figure 3 was attached to a thermal spray gun comprising an axial feedstock injection port. The first feedstock was an aqueous suspension of alumina (aluminium (III) oxide), injected axially at the first (gun) feedstock injection port. The second feedstock was an aqueous suspension of graphene nanoplatelets (with an average diameter in the range 1-7 microns, which may be defined as the average lateral size, measured by SEM), which was injected radially at the second (nozzle) feedstock injection port (downstream of the first injection port) .

An inert shrouding gas was injected in the inert gas injection port of the nozzle at a rate of ~2 g/s. The nozzle parameters were as described with reference to Figure 9 (and shown in Figures 3 and 4), with a nozzle outlet diameter of 28 mm.

Figure 15 (a) shows a cross section of the resulting composite alumina / graphene coating. The top layer is cold mounted resin from sample preparation, middle layer is the coating and the bottom layer is the metallic substrate.

Figure 15 (b) and (c) show wear rates and coefficients of friction at different loads, obtained from a pin on disc wear test. The graph shows very low specific wear rate and very low coefficient of friction up to 40 N load — the wear rate increases at 60 N load.

Figure 16 shows results obtained from a coating deposited according to an embodiment. The arrangement used for deposition was similar to that shown in Figure 1, in which a nozzle like that shown in Figure 3 was attached to a thermal spray gun that comprised an axial feedstock injection port. The first feedstock was Inconel 625 (Al 0.4, C 0.03, Cr 21-23, Co 1, Fe 0.4, Mn, 0.5, Mo 8-10, Ni Bal, Nb 3.2-3.8, P 0.01, Si 0.4, S 0.01, Ta 3.2, Ti 0.4 by mass) powder entrained in a gas, injected axially at the first (gun) feedstock injection port. The second feedstock was an aqueous suspension of alumina, which was injected radially at the second (nozzle) feedstock injection port (downstream of the first injection port). An inert shrouding gas was injected in the inert gas injection port of the nozzle at a rate of ~2 g/s. The nozzle parameters were as described with reference to Figure 9 (and shown in Figures 3 and 4), with a nozzle outlet diameter of 28mm.

Figure 16 (a) shows a cross section of the resulting composite Inconel / alumina coating.

Figure 16 (b) shows the measured thermal spray flame temperature (2011 K) and velocity (454 m/s) at a stand-off distance of 150 mm.

Figure 17 shows results obtained from a coating deposited according to an embodiment. The arrangement used for deposition was similar to that shown in Figure 1, in which a nozzle like that shown in Figure 3 was attached to a thermal spray gun that comprised an axial feedstock injection port. The first feedstock was an aqueous suspension of 45 S5 bioglass (calcium sodium phosphosilicate), injected axially at the first (gun) feedstock injection port. The second feedstock was an aqueous suspension of Gallium (III) Oxide (Ga2O3), injected at the second (nozzle) feedstock port. Gallium (III) Oxide has antimicrobial properties.

Figure 17 (a) shows a cross section of the coating obtained according to an embodiment.

Figure 17 (b) shows the results of attempting to coat the same feedstock by mixing the first and second feedstock and providing both to the axial (gun) feedstock port. It is clear that delamination of the coating is a significant problem.

Figure 17 (c) shows the coating obtained according to an embodiment, which has absolutely no issues with delamination. The delamination is likely due to improper mixing of two completely different phases injected with one injection point. The embodiment solves the improper mixing issue by multiple injection points and only allowing mixing to take place in the flame in appropriate points. Example SiC/YAG coating from liquid suspension feedstock

Certain embodiments comprise coating SiC/YAG coatings from a liquid suspension. A specific example will be described in detail below. The example is merely illustrative - other coatings are also possible in accordance with embodiments.

Silicon carbide (SiC) is a ceramic widely used for its superior mechanical properties, making it an interesting, exciting material with applications, e.g., machining as a cutting tool, automotive as brake discs, and structural material for ballistic vests. The use of SiC as a coating is extensive, especially for its tribological properties of low friction and low wear, which make it ideal for wear resistant applications. Applications in coating form include safety critical parts such as offshore wind turbine bearings. SiC can be generally produced with high purity silica and carbon black at high temperature by the Lely process or by spark plasma sintering.

The situation becomes more challenging when depositing SiC coatings, due to the tendency of SiC to decarburise at high temperature, bond with oxygen and form SiCU, which is ultimately detrimental to the overall quality of the coating. To prevent SiC particles from oxidation when depositing coatings, a binder material is often added, which ultimately yields composite coatings with varying SiC content. The binder, as the name suggests, also has the role of keeping together the SiC particles.

Recent advances in Suspension High Velocity Oxy-Fuel (SHVOF) thermal spray have enabled the deposition of SiC/YAG coatings with no or minimal oxidation¹ using a GTV Top Gun SHVOF torch (GTV, Germany). The suspension route is an interesting approach; however not all HVOF torches are designed to allow suspension injection, such as the Metallisation Met Jet torch (Metallisaion, Dudley, UK). This torch is a kerosene fuelled HVOF (referred to as HVOLF), which provides greater velocity and lower temperature than its hydrogen-based counterparts, potentially proving improved performance for oxidation sensitive materials.

A combination of physical shrouding of the thermal spray flame, gas shrouding (by injection of an inert gas) and radial injection has been found to limit or eliminate

¹ A. Rincon, Z. Paia, T. Hussain, A suspension high velocity oxy-fuel thermal spray manufacturing route for silicon carbide - YAG composite coatings, Materials Letters 281 (2020) 128601, decarburation and thermal degradation. Radial injection alone (i.e. of all the feedstock) may be feasible in hydrogen-fuelled HVOF thermal spray torches, which normally reach around 100 kW flame power, but a desirable flame penetration for entirely radially injected feedstock may challenging for liquid-fuelled HVOF torches capable of reaching 250 kW flame power.

The provision of a physical shroud (e.g. in the form a nozzle 200) and radial injection can enable suitable flame penetration since the feedstock injection happens within the physical shroud, with no need to overtake the flame boundaries.

An embodiment provides a novel setup for HVOLF that includes the use of a solid shroud, a shrouding inert gas, radial feedstock injection, and the use of suspension feedstock of SiC/YAG composite particles. This embodiment enables a Metallisation Met Jet HVOLF torch (or similar product) the capability of handling liquid feedstock such as a liquid suspension of particles or a solution precursor (which may react to form particles in the flame).

Coatings with no SiC oxidation detectable through X-Ray diffraction were obtained according to embodiments. The mechanical and tribological properties of the coating were analysed. Other heat and oxidation sensitive materials may be deposited using a similar approach.

In an embodiment, the feedstock material may be a commercially obtained SiC/YAG powder in which particles comprise SiC grains in a YAG binder.

High velocity oxy-fuel thermal spray was carried out with a liquid fuelled system (MetJet IV from Metallisation Ltd, United Kingdom). The kerosene flow was 500 ml/min, and the oxygen flow 900 1/min, yielding a stoichiometry of 91% and a total mass flow of 28 g/s. This stoichiometry was selected to leave a small fraction of the fuel unburnt in order to ensure that no free oxygen was available for oxidising the feedstock material. In other embodiments, different fuel and oxygen flow rates may be used. Preferably, the stoichiometry is selected to provide an equivalence ratio greater than 1. The substrates coated in the example were 60 x 25 x 2 mm coupons of stainless steel AISI304 (nominal composition: 18% Cr, 8% Ni, 2% Mn, 0.08% C, 0.045% P, 0.03% S, 0.75% Si, 0.1% N, all in wt. %, and Fe balance), polished to a 1 pm finish (grit size P240, P400, P800 and P1200 and diamond pad polishing at 6 pm and 1 pm). The substrates were placed at a stand-off distance of 203 mm (6 inches). The substrates were grit blasted with SiC grit at 6 bar and ultrasonicated in deionised water and industrial methylated spirit mixture, and placed at a stand-off distance of 203 mm (6 inches). The coating was sprayed in a raster path with 4 mm overlap, a traverse speed of 1 m/s and for a total of 20 passes. The specifics of the substrate are not essential, and are described here for completeness.

A nozzle 200, as described hereinbefore, was used with the thermal spray gun 100, as illustrated in Figure 20. The suspension feedstock 98 was injected radially into the flame 106 via the second feedstock injection port 207. A shrouding gas 92 (nitrogen) was injected via an inert gas injection port of the nozzle 200, downstream of the second feedstock injection port 207.

The suspension feedstock was injected at 50 ml/min using a feedback loop-controlled pressurised vessel. In this setup, no feedstock is injected axially in the original thermal spray torch; however, nitrogen carrier gas was fluxed at 9 ml/min to prevent the hot jet from coming back into the feeder tubes.

The following paragraphs describe characterisation methods that were employed to evaluate a coating according to an embodiment. Other characterisation techniques may be used, and the techniques are described below for completeness.

The porosity of a coating deposited as described above was measured with ImageJ (NIH, USA) using the thresholding technique on three 500 pm² areas on the polished crosssection and average values with associated standard error were reported. The microhardness of the coating was measured using a Microhardness tester (Buehler, UK) with 10 indents at the centre of the cross section, with a 25 gf load. The associated error is the standard error. The Scanning Electron Microscopy (SEM) images were acquired using a 6490LV SEM (JEOL, Japan) and an XL30 SEM (FEI, The Netherlands) at 20 kV accelerating voltage. The X-ray diffractograms were acquired using a D8 Advance Da Vinci (Bruker, Germany) with Cu cathode (K_a = 1.5406 A) in the range 10° < 20 < 68° at a 0.02° step size. Two configurations were chosen: Bragg -Brentano geometry with 0.15 s dwell time for the powder, and 2° glancing angle configuration with 20 s dwell time for the coating to remove the signal contribution from the substrate.

Wear tests were carried out using a ball on flat rotary tribometer (Ducom, The Netherlands) with a 6 mm alumina ball (Dejay ltd, United Kingdom) as countersurface. The tests were carried out on the top surface of the coating, ground with SiC grinding discs (grit size P240, P400, P800 and P1200) and polished with 6 pm and 1 pm diamond pads. The loads applied were 10, 20, 30 and 40 N, and the tests ran for 30 minutes at 60 RPM along a 12 mm diameter circular wear track for a total of 1800 cycles or 68 m. Each of the test was repeated twice and the standard error from the two was associated with the specific wear rate values.

Profilometry was carried out using an Alicona 5G XL (Bruker, Germany) with a 10X objective, yielding a lateral resolution of 2 pm and a vertical resolution of 50 nm. The cross-sectional profile of the wear track was measured in 4 positions along the wear track. The wear volume loss of the coating was calculated by multiplying the average cross-sectional area by the wear track length. The wear volume loss of the ball was calculated assuming the removal of a spherical cap equivalent. The specific wear rate was then calculated by dividing by the load and distance.

Figure 19 shows a cross section of the example SiC/YAG coating, with Figure 21a showing a low magnification image and Figure 21b showing a high magnification image. A ~20 micron thick coating was obtained, showing a good coating/substrate interface. The SEM BSE image in Figure 21b helps to identify the different phases present. According to their contrast level, it is possible to find YAG rich areas (bright areas - circle), SiC rich areas (grey areas - square) and porosity (dark areas - rhombus).

Mechanical properties of the coating were evaluated by measuring porosity and microhardness. The coating porosity was measured to be (9.2 ± 0.5) %. This porosity value is lower than that reported for similar prior art coatings for tribology applications. This may be due to the lower momentum and heat transfer between the flame and the feedstock from radial injection compared to axial injection. Also, the liquid suspension medium (water) needs to be accelerated and vaporised, absorbing momentum and heat, respectively. These factors contribute to hinder the decarburisation of the SiC, but at the same time favour an increase of the porosity of the coating. A lower porosity value is normally desirable for wear applications as it yields higher microhardness and, in turn, better wear resistance. The coating microhardness was (300 ± 19) HV0.025. This value is similar to other prior art SiC/YAG coatings. The difference between the bulk hardness and the hardness of this coating can be explained by the presence of the YAG binder and the porosity, which is known to yield lower microhardness values.

Figure 20 shows X-ray diffractometry results obtained from coatings produced according to the example. The diffractogram obtained from the powder feedstock shows the presence of SiC, both in Moissanite-4 (PDF #00-073-1664) and Moissanite-6 (PDF #00-074-1302) form, and YAG (PDF #00-079-1891). Moissanite 4 and 6 are two polytypes of SiC which contain 8 and 12 atoms in the unit cell, and are therefore indicated byhP8 and hP12 Pearson symbol, respectively. The diffractogram from the coating confirms the presence of these same materials . In addition, elemental Si (PDF #00-027-1402) appears as a new phase in the coating. According to the Reference Intensity Ratio (RIR) method (I/I_c), the relative concentration of the various phases was calculated and is listed below in Table 1. Regarding SiC, an initial equal relative quantity of Moissanite -4 and 6 is observed in the feedstock, which then changes into having mainly Moissanite -6 in the coating, hinting that a transformation occurs during the spray. In addition, a -20% decrease in YAG relative content is observed, along with the appearance of a 4 wt. % of Si. The high -temperature flame causes some of the SiC in the feedstock to decarburise, but the oxygen -depleted environment provided by the nozzle (thanks to the longer expansion nozzle and the inert gas shroud) prevents oxidation, leaving elemental Si. The partial transformation of SiC into Si through decarburisation, along with the lower deposition efficiency of YAG compared to SiC explain the relative decrease of YAG concentration of YAG in the coating compared to the feedstock. measured by XRD and calculated by the RIR method. The wear performance of the coatings was tested against alumina in order to provide a ceramic-on-ceramic wear couple. The results are shown in Figure 21. The volumetric specific wear rate is shown in Figure 21a. The wear loss of the coating generally appears higher than that of the counterbody. Also, for the coating, an overall increase of the wear loss occurs as the load increases, whereas for the counterbody, the values remain very similar at all loads. The wear loss increase of the coating appears to be steeper between 20 N and 30 N loads, and also the variability as shown by the error bars increases at the same loads, suggesting the inception of cracking wear mechanism of the ceramics. The coefficient of friction at the various loads as a function of wear distance is shown in Figure 21b. The bedding in period appears very different between the 10 N test and all the other tests, with the former starting at a low value of 0.2 and gradually increasing up to a stable value along with the whole wear distance, and the latter increasing steeply up to values above 0.5 and stabilising as early as after 20 m. The highest steady-state coefficient of friction value is shown by the 20 N load test. In this case, the highest load before large-scale cracking mechanisms occur to release the stress of the coating. The higher loads of 30 and 40 N cause relatively more cracking and debris that helps keep the coefficient of friction lower. The 10 N load test shows a different behaviour because it is so mild that the plastic deformation of the coating top surface takes the whole test length to occur. Overall, the coating shows a remarkably low coefficient of friction at low load (< 10 N), suggesting its best suitability for low- load applications. The coefficient of friction values are similar to those reported for prior art SiC/30%YAG coatings against ceramic (SiC) counterbody in unlubricated conditions.

The specific wear rate is comparatively higher than prior art SiC/30%YAG coatings against stainless steel counterbody, with a value of 2.5 • 10’⁵ mm’/Nm here compared to 1.9 • 10’⁶ mm’/Nm in the prior art, but in the prior art case the load was also lower (6 N), for which a lower specific wear rate has to be expected. The amount of binder material (YAG) in the example coating according to an embodiment was around 11 wt. %, which may explain the wear performance since a higher amount of YAG is correlated with a lower coefficient of friction and lower wear rate.

Further insight into the wear mechanisms at the various loads is given by the SEM images in Figure 22, which shows the full -width wear tracks (left) along with high- resolution details (right) from within the wear tracks. The main features emerging from the low magnification SEM images are the increase of wear track width as the load increases, and especially between the 10 N load test and all the others. Cracking is evident at all loads but is more pronounced at 40 N. The high magnification images add more details to this picture. At 10 N, a mixture of flat, plastically deformed areas coexists with rougher areas full of debris. Even if the coefficient of friction bedding in period had already taken place, some parts of the coating top surface were still to be flattened by the counterbody, and the debris tended to be stored in those areas. At the higher load of 20 N, the wear track appears more uniformly flattened, with a smooth, slightly cracked surface, and little debris. The higher coefficient of friction observed at 20 N can be explained by the larger contact surface given by the smoother finish and lack of debris. At 30 N, the cracking increases slightly, and much more debris is forming. The 40 N test is the harshest scenario, with large-scale and micro cracking, debris, ploughing and grain pull out. The micro cracking appears as small wrinkles, the ploughing as long horizontal trails across the image, and the grain pull out appears along the crack at the bottom left of the image, where the right crack border appears brighter due to the weaker electrical contact from the pull out.

In the example coating method, a liquid-fuelled HVOF thermal spray torch is used. Liquid fuelled HVOF may yield a particularly powerful flame in HVOF thermal spray (e.g. >150kW, or >200kW, e.g. -250 kW) and tends to be characterised by high-velocity and moderate temperature. The nozzle placed at the exit of the thermal spray gun creates a constrained expansion towards the environment, modifying the thermodynamics of the flame. The use of radial injection in the nozzle allows exploiting a fraction of this power by letting the feedstock interact with the thermal spray flame within the nozzle expansion section, easily injecting the feedstock inside the flame and allowing mixing and the heat and momentum transfer. The choice of liquid feedstock - suspension in this case - makes it easier to handle fine powder that would not be suitable otherwise. The presence of water in the suspension means it has to be accelerated and vaporised by the thermal spray flame, reducing the flame power. The milder feedstock -flame interaction in accordance with an embodiment, along with the shrouding gas protection at the nozzle end, results in the feedstock reaching lower temperatures, limiting melting, heat degradation and decarburisation. This may be accompanied by a lower momentum and accordingly a lower deposition efficiency - but a higher purity of the deposited coating. In the example coating, this is demonstrated by the lack of SiC>2 and the presence of elemental Si, meaning that when decarburisation occurs, the oxygen - depleted environment resulting from the nozzle, the reducing gas ratio of the HVOLF and the YAG shell, prevent the Si from bonding with oxygen and lets it deposit in elemental form.

This example demonstrates that HVOLF with liquid feedstock may be used to coat heat and oxidation sensitive materials, enabling coatings that were previously not possible . The flexibility of flame spray systems according to embodiments allow for the deposition of composites with mixing in situ by injecting ordinary powder feedstock within the flame spray torch and, simultaneously, liquid feedstock through the second injection port of the nozzle.

Example SiC/YAG coating from liquid solution based precursors

The example above discloses deposition of SiC/YAG from particles that comprise both SiC and YAG. In other embodiments, a liquid solution can be used, comprising precursors that react in the thermal spray flame to form particles for deposition by the flame. This approach is applicable to SiC/YAG and to any other type of coating comprising components that can be formed from precursors by reaction in the thermal spray flame.

For example, a SiC/YAG coating may be formed by HVOF thermal spray using a feedstock comprising a liquid (e.g. water) in which SiC particles are suspended and in which the YAG phase is included in the form of precursors (e.g. in solution). During spraying, the precursors in the liquid feedstock are transformed into YAG by chemical reactions in the flame and subsequently melt during spraying. This synthesis of YAG during spraying contributes to enhanced protection of SiC from oxidising conditions, helps avoid SiC degradation during the spray process and supports the development of dense coatings.

An example of a suitable precursor solution for synthesis of YAG in the thermal spray flame is a stoichiometric solution of aluminium trioxonitrate V nonahydrate and yttrium trioxonitrate V hexahydrate represented as A1(NC>3)3.9H2O and Y(NC>3)3.6H2O, respectively. The reaction of these precursors to form YAG has been investigated by thermogravimetric analysis and differential scanning calorimetry, which identifies three stages in the calcination reaction. The first stage, in which adsorbed water is driven off and gases (nitrous oxides) are evolved from the decomposition of the nitrates, occurs at around 450 degrees C. No crystallisation occurs during this first stage. In the second stage, at a temperature of around 750 degrees C, crystalline YAG phases begin to form due to aluminium (Al³⁺) coordination site rearrangement and yttrium ion (Y³⁺) substitution. The third and final stage, at around 900 degrees C yields crystalline (cubic) YAG. The calcination reaction is endothermic, which takes thermal power from the thermal spray flame.

The solution based precursor YAG feedstock may be introduced axially (into the combustion chamber), or radially a short distance from the combustion chamber outlet (e.g. within 30 mm, for example at the first feedstock injection port of Figure 2). The solution based precursor YAG feedstock may be introduced radially at a second feedstock injection port 207 (e.g. of a nozzle), more than 30 mm downstream of the combustion chamber outlet (or more than 50 mm downstream). The solution based precursor YAG feedstock may be combined with a suspension of SiC particles, so that the SiC particles and YAG precursor are co-injected at the same position (e.g. more than 30 mm downstream of the combustion chamber outlet, at the second feedstock injection port, or (alternatively) less than 30mm downstream of the combustion chamber e.g. at the first feedstock injection port as shown in Figures 1 or 2) . In other embodiments the SiC particles and the YAG precursor solution may be injected in different positions. For example, the YAG precursor solution may be injected at the first injection port and the SiC particles (in liquid or gas suspension) injected at the second injection port. Alternatively the YAG precursor solution may be injected at the second injection port and the SiC particles (in liquid or gas suspension) injected at the first injection port.

An example coating has been produced by the inventors and analysed by X-ray diffraction analysis, as shown in Figure 23. The relationships between processing and microstructure were studied in terms of porosity phase distribution and mechanical properties. The wear behaviour of the produced coatings at room and at high temperature (600 °C) was studied, demonstrating that the solution based precursor approach could be applied to obtain coatings with similar performance to the example coating produced from suspended SiC/YAG feedstock (described above). Figure 23 shows results obtained from a coating deposited according to an embodiment from a solution based precursor. The arrangement used for deposition was similar to that shown in Figure 1, in which a nozzle like that shown in Figure 3 was attached to a thermal spray gun that comprised an axial feedstock injection port. Silicon carbide is highly reactive to oxygen, and cannot be flame sprayed without decarburising in a conventional spray. An aqueous suspension comprising silicon carbide (SiC) and yttrium aluminium garnet (YAG) precursors in the form of nitrates, was injected radially at the second (nozzle) injection port. YAG particles were synthesised in the thermal spray flame and co-deposited with SiC.

Figure 23 (a) shows a cross section of the coating obtained using 75 kW spray conditions according to an embodiment.

Figure 23 (b) shows a cross section of the coating obtained using 100 kW spray conditions according to an embodiment.

Figure 23 (c) compares x-ray diffraction (XRD) results obtained for the coating after spraying at different conditions. XRD data show that SiC do not decompose and degradation or oxidation into SiC>2 of the SiC was not observed

The method of SiC/YAG coating described herein, in which SiC particles are codeposited in a thermal spray flame (e.g. HVOF) with YAG formed from solution based precursors (which may be termed S-YAG). In certain embodiments, the SiC/S-YAG may be deposited using a physically shrouded flame spraying system (e.g. employing a nozzle as described herein or an equivalent), but this is not essential. At least some components of the feedstock for forming the SiC/S-YAG may be introduced at a downstream radial injection port, at a distance of at least 30 mm from the combustion chamber, but this is also not essential.

Although example embodiments have been described in which a specific nozzle has been used, it is anticipated that similar results can be obtained using variations of the nozzle, or with a gun that integrates features from the nozzle. The example nozzle described herein is modular, and comprises a number of separable parts that are attached to one another using threaded portions, but (as already mentioned) this is not essential. Methods according to an embodiment may use either of the feedstock injection ports (without adding feedstock via the other of the feedstock injection ports), or both at the same time.

The scope of the present invention is not intended to be limited by the example embodiments, but should be determined with reference to the accompanying claims.

Claims

1. A thermal spray system, comprising: a combustion chamber configured to produce a thermal spray flame from the reaction product of a fuel fluid and an oxidiser fluid; a first feedstock injection port configured to entrain a first feedstock material in the thermal spray flame; an inert gas injection port configured to shroud the thermal spray flame with an inert gas; and a second feedstock injection port configured to entrain a second feedstock material in the thermal spray flame; wherein the first feedstock injection port is spaced apart from the second feedstock injection port along a longitudinal axis of the thermal spray flame.

2. The thermal spray system of claim 1, wherein the first feedstock injection port is configured to introduce the first feedstock material to the combustion chamber, or to the thermal spray flame, at a distance of no more than 30 mm from the combustion chamber, and the second feedstock injection port is at least 30 mm away from combustion chamber along the longitudinal axis of the thermal spray flame.

3. The thermal spray system of claim 1 or 2, comprising a thermal spray gun and a nozzle, wherein the thermal spray gun comprises the combustion chamber and the first feedstock injection port, and wherein the nozzle is configured to detachably couple to the thermal spray gun, and the nozzle comprises the inert gas injection port and the second feedstock injection port.

4. A nozzle configured to detachably couple to a thermal spray gun, the thermal spray gun comprising a combustion chamber configured to produce a thermal spray flame and a first feedstock injection port, and the nozzle comprising: an inert gas injection port, for shrouding the thermal spray flame with an inert gas; a second feedstock injection port configured to entrain a second feedstock material in the thermal spray flame within the nozzle.

5. The thermal spray system of any of claims 1 to 3 or the nozzle of claim 4, wherein the nozzle comprises an exit nozzle portion by which the thermal spray exits the nozzle.

6. The thermal spray system or nozzle of claim 5, wherein the exit nozzle portion has an inlet internal diameter of between 10 mm and 20 mm.

7. The thermal spray system or nozzle of claim 5, wherein the exit nozzle portion has a length of 50 mm to 100 mm.

8. The thermal spray system or nozzle of claim 5 or 6, wherein the exit nozzle portion is diverging and has a half-cone angle of 1 to 10 degrees

9. The thermal spray system or nozzle of any of claims 5 to 8, wherein the second feedstock injection port opens into the exit nozzle portion at a distance of between 10 mm and 30 mm from an inlet end of the exit nozzle portion.

10. The thermal spray system or nozzle of any preceding nozzle claim, wherein the nozzle comprises a proximal end for coupling to the thermal spray gun, and a distal end remote from the proximal end, wherein the second feedstock injection port is nearer to the proximal end than the distal end.

11. The thermal spray system or nozzle of any preceding claim, wherein the second feedstock injection port is at an angle of up to 30 degrees from a radial direction.

12. The thermal spray system or nozzle of any preceding claim, wherein the inert gas port is at a distance of between 5mm and 25 mm from an outlet end of the nozzle.

13. The nozzle of any preceding nozzle claim, wherein the nozzle comprises a holder module for adapting the nozzle to couple to the thermal spray gun, the holder module detachable from the rest of the nozzle

14. The nozzle of claim 13, wherein the holder module comprises a converging portion configured to communicate the thermal flame from the tip of the thermal spray gun to a downstream portion of the nozzle.

15. A method of using the nozzle of any preceding nozzle claim, comprising: attaching the nozzle to the thermal spray gun; producing a thermal spray flame with the thermal spray gun; adding a feedstock material to the thermal spray flame via the first and/or second feedstock injection port; shrouding the thermal spray flame with an inert gas via the inert gas injection port; and depositing a coating on a substrate.

16. A method of thermal spray coating, comprising: reacting an oxidiser fluid with a fuel fluid in a combustion chamber to form a thermal spray flame flowing out of the combustion chamber; injecting a first feedstock material into the thermal spray flame at a first position; injecting a second feedstock material into the thermal spray flame at a second position; depositing a coating comprising the first and second feedstock material on a substrate; wherein the second position is spaced apart from the first position along a longitudinal axis of the thermal spray flame.

17. The method of claim 16, wherein the second position is at least 20 mm from the combustion chamber.

18. The method of claim 16 or 17, wherein the second feedstock material comprises a suspension or solution based feedstock; or the method of claim 15, wherein the feedstock material comprises a suspension or solution based feedstock.

19. The method of any of claims 16 to 18, including the subject matter of claim 16, wherein the first feedstock material is different from the second feedstock material, and the coating comprises a composite material.

20. The method of claim 19, wherein: i) the first feedstock material comprises a metal material, the second material comprises a ceramic material; ii) the first feedstock material comprises a ceramic material, and the second feedstock material comprises a suspension of nanoparticles; or iii) the first feedstock material comprises a calcium sodium phosphosilicate glass and the second feedstock material comprises a gallium oxide suspension.

21. The method of claim 15, wherein the feedstock material comprises a suspension comprising silicon carbide particles and yttrium aluminium garnet particles.

22. A method of depositing a composite layer comprising a first material and a different second material by thermal flame spray, comprising: reacting an oxidiser fluid with a fuel fluid in a combustion chamber to form a thermal spray flame flowing out of the combustion chamber; injecting a first feedstock comprising particles of the first material into the thermal spray flame; injecting a second feedstock comprising a solution of precursors into the thermal spray flame; synthesising the second material in the thermal spray flame ; depositing the composite coating comprising the first material and the second material on a substrate.

23. The method of claim 22, wherein the precursors comprise aluminium trioxonitrate V nonahydrate and yttrium trioxonitrate V hexahydrate .

24. The method of claim 22 or 23, wherein the method steps are performed using a thermal spray system according to any preceding thermal spray system claim.

25. The method of any of claims 22 to 24, wherein: i) the first feedstock and the second feedstock are injected into the thermal spray flame at the same position along the thermal spray flame; ii) the first feedstock and the second feedstock are injected into the thermal spray flame at different positions along the thermal spray flame.