Deposition of coatings on substrates

Abstract

A process and apparatus are disclosed for the deposition of a layer of a first material onto a substrate of a second material. Powder particles of the first material are entrained into a carrier gas flow to form a powder beam directed to impinge on the substrate. This defines a powder beam footprint region at the substrate. The powder beam and the substrate are moved relative to each other to move the powder beam footprint relative to the substrate, thereby to deposit the layer of the first material. A laser is operated to cause direct, local heating of at least one of a forward substrate region and a powder beam footprint region. The laser beam direction is defined with reference to a plane coincident with or tangential to a surface of the substrate at the center of the laser beam footprint in terms of an elevation angle from the plane to the laser beam direction and in terms of an acute azimuthal angle from the movement direction to the laser beam direction. The elevation angle is 80 or less and the azimuthal angle is 60 or less. In the apparatus, there are provided at least three laser sources arrayed around the powder beam footprint, the angular spacing between the laser sources being 120 or less.

Claims

1. A coating process for the deposition of a layer of a first material onto a substrate of a second material, the second material optionally being different from the first material, the process including the steps: entraining powder particles of the first material into a carrier gas flow to form a powder beam directed to impinge on the substrate, thereby defining a powder beam footprint region at the substrate; causing relative movement of the powder beam and the substrate in a movement direction to move the powder beam footprint relative to the substrate to deposit the layer of the first material; and operating a laser source to direct a laser beam along a laser beam principal axis direction to provide a laser beam footprint to cause direct, local heating of at least one of the forward substrate region and the powder beam footprint region, without macroscopic melting of the first material, the laser beam principal axis direction being defined with reference to a plane coincident with or tangential to a surface of the substrate at the centre of the laser beam footprint in terms of an elevation angle from the plane to the laser beam principal axis direction and in terms of an acute azimuthal angle from the movement direction to the laser beam principal axis direction, wherein the elevation angle is 80 or less and the azimuthal angle is 60 or less.

2. The coating process according to claim 1, wherein the elevation angle is 30 or more.

3. The coating process according to claim 1, wherein the movement direction is variable, in order to provide variation in the shape of the deposited layer.

4. The coating process according to claim 1, wherein there are provided two or more laser sources and the azimuthal angle for at least one of the laser sources is non-zero.

5. The coating process according to claim 1, wherein a single layer of the first material is deposited on a substrate of a second material, the first material having a different composition to the second material.

6. The coating process according to claim 1, wherein multiple layers are applied sequentially, each previously-deposited layer acting as the substrate for the layer being applied.

7. The coating process according to claim 1, wherein the first material is an anti-corrosion coating or a wear coating.

8. The coating process according to claim 1, wherein the laser is operated directly to heat the forward substrate region but not the powder beam footprint region.

9. The coating process according to claim 1, wherein the laser source is operated directly to heat the forward substrate region using a first intensity profile and to heat at least part of the powder beam footprint region using a second intensity profile, wherein the average intensity of the first intensity profile is greater than the average intensity of the second intensity profile.

10. The coating process according to claim 1, wherein the carrier gas is selected from: nitrogen and air.

11. The coating process according to claim 1, wherein the carrier gas is not heated.

12. The coating process according to claim 1, wherein the powder particles in the powder beam have average kinetic energy Ek, Ek optionally varying with position across the powder beam, and Ek is selected so that without direct heating of the forward substrate region and/or the powder footprint region, the powder particles do not adhere to the substrate, the process including the step of selectively deactivating the laser in order to prevent the powder particles from adhering to the substrate.

13. The coating process according to claim 1, wherein the deposited layer is at least 0.1 mm thick.

14. The coating process according to claim 1, the process further including the steps: controlling the laser and the relative movement of the powder beam and the substrate to provide a spatial temperature distribution at the powder footprint region of the substrate in which the local temperature of the substrate is in the range 0.5 Ts to less than Ts in a volume from the surface of the substrate at least up to a depth of 0.2 mm from the surface of the substrate and not more than 0.25 Ts at a depth of 1 mm from the surface of the substrate, wherein Ts is the solidus temperature (in K) of the second material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic side view of a laser-assisted cold spray deposition process in which the laser beam (Gaussian intensity profile) and the powder beam are coaxial.

(2) FIG. 1A shows a schematic plan view of the arrangement of FIG. 1.

(3) FIG. 2 shows a plot of the measured temperature profile for the surface of the substrate for the arrangement of FIGS. 1 and 1A along the x-axis through the centre of the laser beam.

(4) FIG. 3 shows a schematic side view of a laser-assisted cold spray deposition process in which the axis of the laser beam (Gaussian intensity profile) is displaced forwardly of the axis of the powder beam.

(5) FIG. 4 shows a schematic plan view of the arrangement of FIG. 3.

(6) FIG. 5 shows a plot of the measured temperature profile for the surface of the substrate for the arrangement of FIG. 3, measured in a similar manner as FIG. 2.

(7) FIG. 6 shows a schematic side view of a laser-assisted cold spray deposition process in which the laser beam has a tailored intensity profile.

(8) FIG. 7 shows a schematic plan view of the arrangement of FIG. 6.

(9) FIG. 8 shows a plot of the measured temperature profile for the surface of the substrate for the arrangement of FIG. 6.

(10) FIG. 9 shows a schematic side view of a laser-assisted cold spray deposition process in which the laser beam is scanned at the substrate in order to provide a required intensity profile.

(11) FIG. 10 shows a schematic plan view of the arrangement of FIG. 9.

(12) FIG. 11 shows plots of measured temperature profiles for the surface of the substrate for the arrangement of FIG. 9.

(13) FIG. 12 shows a schematic view of an arrangement for modelling temperature profiles.

(14) FIGS. 13-15 show the temperature profile results of modelling of a prior art disclosure.

(15) FIGS. 16-18 show the temperature profile results of modelling of an embodiment of the invention.

(16) FIG. 19 shows a cross sectional view of a cold spray deposition process according to an embodiment of the invention.

(17) FIG. 20 shows a plan view of the process of FIG. 19.

(18) FIG. 21 shows a schematic cross sectional view of the deposition of a titanium track layer on a steel substrate, the view being taken in a direction across the width of the track.

(19) FIG. 22 shows an optical micrograph of the cross section of the track illustrated in FIG. 21.

(20) FIG. 23 shows a cross sectional view of a cold spray deposition process used in another embodiment of the invention.

(21) FIG. 24 shows a schematic plan view of the arrangement of FIG. 23.

(22) FIG. 25 shows one mode of operation of the embodiment of FIG. 23.

(23) FIG. 26 shows a schematic plan view of the arrangement of FIG. 25.

(24) FIG. 27 shows the particle size distribution of a Sn powder.

(25) FIG. 28 shows a schematic plan view of a rectangular track deposited on a substrate to illustrate the effect of azimuthal angle.

(26) FIG. 29 shows a cross sectional micrographic view of the track taken for the track deposited in direction D1 in FIG. 28.

(27) FIG. 30 shows a cross sectional micrographic view of the track taken for the track deposited in direction D2 in FIG. 28.

(28) FIG. 31 shows a cross sectional micrographic view of the track taken for the track deposited in direction D3 in FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND OPTIONAL ADDITIONAL FEATURES OF THE INVENTION

(29) Preferred embodiments of the invention will now be described, with reference to the drawings. Additionally, some arrangements are described and illustrated which are outside the scope of protection, but they are described and illustrated in order to provide a fuller understanding of the invention.

(30) Cold spraying (CS) is a process in which high velocity particles impact and bond onto a substrate when their velocities are above a critical value. Achieving this critical value typically involves the use of a high mach number gas (e.g. helium) and gas heating. In the preferred embodiments of the present invention, localised heating of the substrate facilitates a reduction of the critical velocity allowing for the use of lower cost carrier gas (e.g. nitrogen) and/or a reduction in the amount of gas heating required. Both of these significantly reduce the potential cost of the process implementation.

(31) FIGS. 1 and 2 illustrate an arrangement which is presented for reference in order to assist in an understanding of the preferred embodiments of the invention.

(32) In FIG. 1, a substrate 10 is provided. In many of the embodiments of the invention, substrate 10 is formed of steel. Steel is present here as being used in an embodiment of the invention but it will be understood that this invention is not necessarily limited to the use of steel substrates. However, other substrate materials are contemplated and can be used. A layer 12 is deposited on the substrate 10 by the process described in more detail below.

(33) The substrate is moved in direction A. A powder beam 14 is formed in a known manner by entraining powder particles of typical diameter 5-50 m into a high speed flow of an inert carrier gas such as nitrogen and ejecting the powder beam from a suitably-located nozzle (not shown). A laser source directs high intensity laser light in the form of a laser beam 16 to be coaxial with the powder beam. The temperature of the surface of the substrate is measured using a pyrometer.

(34) Typically, the kinetic energy Ek of the powder particles in the powder beam 14 is not uniform across the diameter of the powder beam. Instead, it is typical for the powder particles towards the centre of the powder beam to have higher energies than those towards the outside of the powder beam. The distribution of Ek across the diameter of the powder beam may, for example, be Gaussian or near-Gaussian.

(35) Similarly, in typical arrangements, the distribution of intensity across conventional laser beams is not uniform but instead is also Gaussian or near-Gaussian, being greater towards the centre of the beam.

(36) FIG. 2 shows plots of measured temperature profiles for the surface of the substrate for the arrangement of FIG. 1. FIG. 2 is aligned with FIG. 1, so that the zero on the x-axis is aligned with the centre of the laser beam 16. The temperature distribution at the substrate along the x-axis is shown in FIG. 2. This demonstrates that the highest temperature is found behind the powder beam. This is wasteful and inefficient. Also, there is insufficient heating forward of the powder beam to allow the substrate to be conditioned to achieve good adherence and compaction of the incoming particles. Furthermore, the deposited layer typically has a different (and typically higher) absorption property for the laser light. This can result in the deposited layer being subjected to a temperature which is too high, leading to unwanted melting or even removal of the deposited layer.

(37) FIG. 3 shows a schematic side view of a laser-assisted cold spray deposition process in which the axis of the laser beam is displaced forwardly of the axis of the powder beam. This is a modification of the arrangement of FIG. 1. The laser beam once more has a Gaussian intensity profile. FIG. 4 shows a schematic plan view of the arrangement of FIG. 3. FIG. 5 shows plots of measured temperature profiles for the surface of the substrate for the arrangement of FIG. 3. Again, FIG. 5 is aligned with FIG. 3, so that the zero on the x-axis is aligned with the centre of the laser beam 16. The temperature distribution at the substrate along the x-axis is shown in FIG. 5. This demonstrates that the highest temperature is found at the centre of the powder beam. This assists in the generation of a suitable temperature profile at the substrate to ensure good adherence of the incoming powder particles onto the substrate without waste of the laser energy. Furthermore, displacing the axis of the laser beam forwardly of the axis of the powder beam means that less laser energy is directed to the already-formed (and typically high absorbent) layer 12, meaning that there is a reduced risk of melting or burning away of the deposited layer 12.

(38) FIG. 6 shows a schematic side view of a laser-assisted cold spray deposition process which is a modification of the arrangement of FIG. 3 in that the laser beam 18 has a spatial intensity profile that is tailored to further improve the deposition characteristics in the process. As shown in FIG. 7 (plan view), laser beam 18 has a forward portion 18a which has a relatively high intensity and a rearwards portion 18b which has a relatively low intensity. Forward portion 18a is forwards of the powder beam, meaning that the forward portion does not overlap with the powder beam footprint. The effect of this is that the heating provided by forward portion 18a is only dependent on the absorption of the laser light by the surface of the substrate 10. Rearwards portion 18b of the laser beam overlaps with the powder beam footprint and with part of the deposited layer that is rearwards of the powder beam footprint. In view of the higher absorption at the deposited layer, the intensity of the rearwards portion of the laser beam is correspondingly lower, in order to avoid melting or removal of the deposited layer. Heating of the deposited layer to a limited extent can assist with compaction and/or stress relief of the deposited layer.

(39) Control of the spatial intensity of the laser beam 18 is provided by an optical element (not shown) that is typically a refractive optical element. However, a diffractive or reflective optical element may be used. The control provided by such an optical element is typically fixed controli.e. the resultant spatial intensity profile of the laser beam is typically fixed for a particular optical element. However, it is possible to use different optical elements (providing different spatial intensity profiles) for different coating conditions, e.g. different substrates, different coating materials, etc. Swapping suitable optical elements in and out of the system may be done in an automated manner, e.g. robotically (for safety and reproducibility reasons).

(40) FIG. 9 shows a schematic side view of a laser-assisted cold spray deposition process which is a modification of the arrangement of FIG. 6 in that the required laser intensity profile is provided by suitable control of scanning of the laser beam 20. As shown in FIG. 10 (plan view), the laser beam footprint has a forward portion 20a which has a relatively high intensity and a rearwards portion 20b which has a relatively low intensity. In a similar manner to the arrangement of FIGS. 6 and 7, forward portion 20a is forwards of the powder beam, meaning that the forward portion does not overlap with the powder beam footprint and so can have a high intensity to ensure that a sufficient amount of laser energy is absorbed by the substrate. Rearwards portion 20b of the laser beam footprint has a lower intensity because it overlaps with the powder beam footprint and with part of the deposited layer that is rearwards of the powder beam footprint.

(41) Suitable scanning optics will be well known to the skilled person in order to provide suitable control of the spatial intensity of the laser beam footprint. The advantage of this arrangement in comparison to the arrangement of FIGS. 6-8 is that the relative intensity of the different parts of the laser beam footprint can be controlled in real time and can be adjusted according to the circumstances. This allows the laser assisted cold spray deposition process to be extremely flexible in terms of the materials that can be deposited and the substrates that such materials can be deposited on.

(42) Accordingly, in these preferred embodiments of the invention, the laser is fired in the vicinity of the powder beam footprint in order to increase the local temperature reducing the yield stress of the material of the substrate (i.e. the original substrate and/or any previously deposited particles). Localised spatial control of the heat input improves (and in the most preferred embodiments, maximises) the process efficiency. This is particularly apparent in the deposition of the first layer, were a significant amount of energy is typically needed ahead of the powder beam to enable adhesion, whereas overheating of the trailing edge can result in coating damage due to higher absorption levels.

(43) This technology provides a very significant improvement in the application of cold spraying in view of the cost reductions and increases in coating speed that are possible. The technology has application, for example, in the production of anti-corrosion and/or anti-wear coatings.

(44) The process has additional controllability when compared to more conventional cold spraying. For example, because the energy of the laser beam heats the substrate to a temperature at which particle adherence is possible, if the laser beam is turned off, the particles will not adhere to the substrate. This allows very precise patterning of the deposited layer because the laser beam can be controlled exceptionally tightly both in terms of spatial location and in terms of switching on and off. Furthermore, due to the control of the spatial temperature profile of the substrate using the laser, it is possible to heat only a very narrow track on the substrate, meaning that particles will adhere only at the narrow track. In this way, it is possible to deposit layers with a width that is smaller than the width of the powder beam, corresponding instead to the width of the track of the substrate heated to a suitable temperature.

(45) The present inventors have considered the disclosure of Kulmala and Vuoristo (2008) in detail. They have assessed the disclosure of Kulmala and Vuoristo (2008) based on computer modelling, in order to determine the temperature profile of the substrate during processing.

(46) Kulmala and Vuoristo (2008) discloses a low pressure cold spray process. Ceramic (alumina) powder particles are entrained in the powder beam in order to assist compaction. The substrates used are low carbon mild steel substrates.

(47) The relevant parameters in Kulmala and Vuoristo (2008) are as follows: Temperatures 650 to 1000 C. At 650 C. laser powers 2.0 kW to 1.8 kW. At 1000 C. laser powers 2.9 to 2.7 kW for multiple layers. 6 kW laser power available. Processing speed 40 mm.Math.s.sup.1. Laser spot 5.823.5 mm. Uniform intensity across laser spot. Powder nozzle diameter 5 mm. Powder beam diameter 5.8 mm. Power absorption approximately 35%.

(48) The surface temperature of a substrate can be measured using a pyrometer. When the substrate is heated using a laser, a suitable frequency filter can be used at the pyrometer to filter out the wavelength of the laser light. Power absorption can be determined by first measuring the reflectivity of the substrate, i.e. by measuring the intensity of light reflected from the substrate compared with the intensity of light incident at the substrate. The power absorption is then (1-reflectivity)100, to express as a percentage. The temperature distribution in the substrate can then be determined in a known manner based on the thermal materials properties of the substrate, described below.

(49) In FIGS. 13-18, temperature profiles are shown. The contours are 100K apart. In each of the figures, the lowest temperature contour is 373K. In FIGS. 13 and 15 the highest temperature contour is 973K. In FIG. 14 the highest temperature contour is 773K. In FIGS. 16-18 the highest temperature contour is 1273K.

(50) It should be noted that the modelling work reported here is material dependent. Here, the substrate is steel. The material to be deposited in the modelling is Ti.

(51) FIG. 12 shows a schematic view of an arrangement for modelling temperature profiles. A stationary laser source (not shown) provides a laser beam 50 that is passed through suitable beam optics 52 to form a laser footprint 54 of a specific shape on a substrate 56. The substrate 56 is moved in direction A order to provide relative movement between the substrate 56 and the laser footprint 54. Axes X, Y and Z are shown in FIG. 12. The relative movement between the substrate and the laser footprint is along axis X. The thickness direction of the substrate is along axis Z. The centre of the laser footprint is at X=Y=Z=0. At the beam footprint, the laser power is delivered to the substrate. The beam footprint width is Sw. The beam footprint length is SI. The substrate thickness is Mt.

(52) Based on the disclosure of Kulmala and Vuoristo (2008), the laser intensity is uniform across the laser footprint.

(53) The low carbon mild steel substrate used in Kulmala and Vuoristo (2008) is assumed to have the following properties: Thermal conductivity 25.6 W.Math.m.sup.1.Math.K.sup.1 Specific heat 925 J.Math.kg.sup.1.Math.K.sup.1 Density 7640 kg.Math.m.sup.3 Melting temperature 1765K Boiling temperature 3000K

(54) Ambient temperature is assumed to be 20 C.

(55) The resultant isotherms are shown in FIGS. 13, 14 and 15. FIG. 13 shows the temperature profile of the surface of the substrate in plan view. FIG. 14 shows the temperature profile of the substrate in a cross section view of the y-z plane (at x=0). FIG. 15 shows the temperature profile of the substrate in a cross section view of the x-z plane (at y=0). Based on the disclosure of Kulmala and Vuoristo (2008), the centre of the powder beam would be at about x=+9 mm in FIG. 15.

(56) However, the present inventors consider that the arrangement in Kulmala and Vuoristo (2008) is inefficient. Much of the heat delivered to the substrate is wasted. Heating of the substrate is useful if it affects the adherence of the incoming powder particles. However, in Kulmala and Vuoristo (2008), much of the energy is used to heat deeper regions of the substrate (e.g. deeper than about 0.5 mm).

(57) The temperature at the surface of the substrate at the powder beam footprint in Kulmala and Vuoristo (2008) is about 700 C. (973K). The temperature at about 0.5 mm from the surface below the powder beam footprint is about 600 C. (873K). The temperature at about 1 mm from the surface below the powder beam footprint is about 400 C. (673K). The temperature at about 2 mm from the surface below the powder beam footprint is about 200 C. (473K).

(58) In an embodiment of the present invention, a titanium substrate is used. The dimension of the substrate are identical to those in Kulmala and Vuoristo (2008). The relevant parameters of this embodiment are as follows: Maximum temperature 900 C. 4 kW laser power available. Processing speed 500 mm.Math.s.sup.1. Laser spot diameter 6 mm. Gaussian intensity profile across laser spot. Power absorption approximately 40%.

(59) The titanium substrate used in this embodiment has the following properties: Thermal conductivity 6.8 W.Math.m.sup.1.Math.K.sup.1 Specific heat 564 J.Math.kg.sup.1.Math.K.sup.1 Density 4428 kg.Math.m.sup.3 Melting temperature 1941K Boiling temperature 3560K

(60) The resultant isotherms are shown in FIGS. 16, 17 and 18. FIG. 16 shows the temperature profile of the surface of the substrate in plan view. FIG. 17 shows the temperature profile of the substrate in a cross section view of the y-z plane (at x=0). FIG. 18 shows the temperature profile of the substrate in a cross section view of the x-z plane (at y=0). The preferred location for the centre of a powder beam would be at about x=+3 mm in FIG. 18.

(61) As can be seen from these results, this embodiment of the invention provides suitably deep heating of the substrate in order to promote good adhesion and density of the deposited layer but avoids the wastage of heat into the deeper parts of the substrate below the powder beam footprint.

(62) With reference to FIGS. 19 and 20, there is shown a substrate 100 moving in movement direction A. Onto the substrate is directed a powder beam 102 along a powder beam direction 104 and a laser beam 106 along a laser beam direction 108. In this embodiment, the laser beam heats the substrate at the powder beam footprint in order to deposit a layer 110 formed of the particulate material in the powder beam. The angle e subtended in FIG. 19 between the plane of the substrate 100 and the laser beam direction is the elevation angle. As will be clear, if the elevation angle was 90, the laser beam direction would be parallel to the powder beam direction 102. However, in this embodiment, the elevation angle e is about 45. Where the substrate is non-planar, the elevation angle e is defined with reference to a plane tangential to surface of the substrate at the centre of the laser beam footprint.

(63) As shown in FIG. 20, angle a is the azimuthal angle of the laser beam direction 108. This is the acute angle subtended between the movement direction A and a projection of the laser beam direction onto the surface of the substrate 100.

(64) Azimuthal angle a is 60 or less from the movement direction. As will be understood, this places a restriction on the direction in which the substrate can be moved relative to the laser beam direction. If it is wanted to form a deposited layer in any direction on the surface of the substrate, it will be necessary either to provide a laser source that is moveable to provide the required laser beam direction, or it will be necessary to provide more than one laser beam source, to be switched in and out of operation depending on the shape of the track and hence the direction of movement of the substrate. Given the limitation of 60 for the range of suitable laser beam directions for providing adequate shape for the deposited layer, it will be understood that preferably an array of laser sources is provided, preferably at least 3 laser sources, angularly arranged around the powder beam footprint with an angular spacing of 120 or less.

(65) The term laser source here is intended to mean a device which functions to provide a laser beam along the required direction. Therefore where more than one laser source is provided, and thus more than one laser beam is provided, it is possible for the laser beams to be derived from the same laser. This can be done by a suitable arranged of optical elements such as fibre optics.

(66) As discussed above, when the laser is applied in a non coaxial manner the direction which the laser enters the powder beam is significant. It has been found that if the azimuthal angle is greater than 60 then one or both of the deposition efficiency and track shape becomes impaired. When the laser is shaded by deposited powder the deposition efficiency of the process is altered. This manifests as a distortion of the track when the laser is primarily coming into the side of the deposit. When incident from the rear of the track there is a uniform drop in deposition efficiency.

(67) FIG. 21 illustrates the situation. Here there is shown a schematic cross sectional view of the deposition of a titanium track layer 150 on a steel substrate (not shown), the view being taken in a direction across the width of the track. The mass distribution 152 across the powder jet P is shown as a symmetrical, near-Gaussian distribution. However, if the laser beam L arrives at a non-zero azimuthal angle (here 30), then the shape of the track 150 becomes asymmetrical (as seen by X1 and X2 being unequal), due to the differential heating at the powder footprint. In FIG. 21, the track height is Th, the powder beam footprint width is 8 mm, the laser footprint is 4.5 mm and the laser power distribution is 154.

(68) FIG. 22 shows an optical micrograph of the cross section of the track described above.

(69) FIGS. 23-26 illustrate another embodiment of the invention, adapted to control the track shape and to allow the track direction to be varied at will. Similar features already described with reference to FIGS. 19 and 20 are given the same reference numbers and are not necessarily described again.

(70) In FIGS. 23 and 24, 6 incoming laser beams 206A-206F are provided along respective directions 208A-208F. Azimuthal angle A is about 30, and azimuthal angle B is about 30. As will be understood, the measurement of these azimuthal angles depends on the direction of movement of the substrate as shown in FIGS. 19 and 20. Azimuthal angles for beams 206C, D, E and F are not shown but can be measured according to the explanation above. As shown in FIG. 23, elevation angle e of the laser beams 206A and 206E is the same and is about 45.

(71) As shown in FIGS. 25 and 26, when the substrate is moved in direction A, trailing and side-facing laser beams 206C, D, E and F can be switched out of operation. As will be understood, the substrate direction can be changed and one or more of the laser beams 206C, D, E and F can be switched into operation in order to provide a suitable heating profile for the substrate, the powder footprint region and/or the deposited layer. This arrangement therefore allows the substrate to move in any direction desirable to form a deposition track, by suitable switching in and out of operation the various laser sources as needed.

(72) FIG. 27 shows the particle size distribution of a Sn powder, showing a typical particle size distribution measured using a Malvern Mastersizer 2000 instrument. The shape of the distribution is typical of powders suitable for use with embodiments of the invention, but it is noted here that the average particle size shown in FIG. 27 is slightly too low for optimum suitability with the preferred embodiments of the invention. Alternative powders that can be used are, for example, Ti powder or stellite powder. Stellite is a CoCr alloy. Typically, the average particle size required for use in the invention depends to some extent on the density of the material to be sprayed. A more dense material (e.g. stellite) typically requires a finer particle size because particles that are too large will not accelerate well enough in the gas jet. A suitable average particle size for stellite is about 40 m. A less dense material (e.g. Ti) has a coarser particle size for use in the embodiments of the invention. A suitable average particle size for Ti is about 55 m.

(73) Experimental work to show the advantage of the preferred embodiment of the invention has been carried out, as illustrated in FIG. 28 which shows a schematic plan view of a rectangular track 302 deposited on a substrate 300 to illustrate the effect of azimuthal angle.

(74) A powder beam was formed to impinge perpendicularly on a substrate as described above. The laser beam had an elevation angle of about 60. The direction of movement of the substrate with respect to the powder beam and laser beam was varied so as to vary only the azimuthal angle. The specific variation used in this experiment was to form a deposited layer along the outline of a rectangular track, starting at position S. The track was deposited first along direction D1, along one side of the rectangle, with the azimuthal angle at 0. Then the direction of movement of the substrate was changed and the track was then deposited along the next side of the rectangle along direction D2, with the azimuthal angle at 90. Next, the direction of movement of the substrate was changed again and the track was deposited along the next side of the rectangle along direction D3, with the azimuthal angle at 180. Finally, the direction of movement of the substrate was changed again and the track was deposited along the next side of the rectangle along direction D4, with the azimuthal angle at 270, to return to position S.

(75) The relative deposition efficiency for each side of the rectangle was determined, on the fair assumption that the particle beam was constant and the laser power was constant during the full spraying treatment. The results showed that the deposition efficiency for azimuthal angle being 0 was significantly better than the deposition efficiency for the other azimuthal angles tested in the experiment.

(76) FIG. 29 shows a cross sectional micrographic view of the track taken for the track deposited in direction D1 in FIG. 28. FIG. 30 shows a cross sectional micrographic view of the track taken for the track deposited in direction D2 in FIG. 28. FIG. 31 shows a cross sectional micrographic view of the track taken for the track deposited in direction D3 in FIG. 28. Each micrographic image is of an etched microstructure. As can be seen, track D1 is relatively symmetrical and uniform in microstructure. However, where the azimuthal angle is 90, 180 or 270 (90), the deposited tracks are asymmetrical and/or molten and partially oxidised.

(77) The present inventors have also demonstrated the control of the deposition of Ti powder particles when the powder beam is maintained on but the laser is turned on and off to control the position of the formation of the deposited layer along the track. This has been demonstrated for a constant traverse speed along the substrate of 500 mm/min, the laser being turned on for 2 seconds at a time to deposit individual islands of the coating layer.

(78) The invention is considered at present to have considerable merit for the deposition of coatings on tubes. Typically the coating is deposited on the external surface of the tube (e.g. as an anti-corrosion coating). In this application, typically the tube is rotated and the relative axial position between the powder beam footprint and the tube is controlled to provide a continuous coating.

(79) The invention is also considered at present to have considerable merit for the deposition of coatings on relatively small localised area(s) of a substrate. This is of interest in particular for the repair of surface defects on high value components such as turbine blades. In this application, control over the azimuthal angle as defined above is considered to be particularly important, to allow the coating to be applied in a desired pattern to cover the localised area as required.

(80) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(81) All references referred to above are hereby incorporated by reference.