Thermal spray method integrating selected removal of particulates

Abstract

A thermal spray system and method includes a hot gas generator with nozzle accelerating heated gas towards a substrate in the form of a gas column projecting onto the substrate surface as a spot. One or more feedstock injectors proximate the nozzle exit, directed towards the gas column, are connected to a feedstock source. The hot gas stream transfers heat and momentum to the feedstock, causing the feedstock particles to impact onto a substrate to form a coating. The system further comprises one or more liquid injectors proximate the nozzle exit, directed towards the axis, and connected to a source of liquid. The system controls the flow and velocity with which the liquid is injected, permitting control of the depth of penetration of the liquid into the gas column. The method selectively prevents suboptimal feedstock particulates from adhering to the substrate and provides for the in-situ removal of suboptimal deposits.

Claims

1. An integrated method of providing controlled in-situ removal of surface debris comprising weakly-adhered feedstock and grit particles during a thermal spray coating process used to form a coating on a substrate surface, comprising: providing a source of heated gas and a nozzle for shaping heated gas into a gas stream column coaxial with the nozzle, the gas stream column extending in a downstream direction away from the nozzle and projecting onto the substrate surface; providing a feedstock injector that is configured to inject feedstock into a first side of the gas stream column; providing a liquid injector that is configured to inject liquid water that does not contain feedstock into the first side of the gas stream column transverse to the gas stream column; providing feedstock having a particle size distribution; establishing a feedstock profile corresponding to the particle size distribution; determining a portion of the feedstock profile as optimal for forming lamellar structure on a substrate and the balance of the feedstock profile as suboptimal to permit at least some of the suboptimal feedstock to clean abrasively when the suboptimal feedstock is cooled by liquid water injected transverse to the gas stream column; determining two volumetric regions within the gas stream column, including a first region wrapped around the axis of the column and a second region surrounding the first region and coaxial with it, the first region projecting into a spot on the substrate surface and the second region projecting into an annular ring on the substrate surface, the annular ring coaxial with the spot and surrounding it, wherein the first region is hotter and faster than the second region and extends continuously from the nozzle to the substrate surface and heats the substrate surface; moving the gas stream column relative to the substrate surface whereby the spot and the annular ring move relative to the substrate surface; injecting feedstock into the gas stream column from the feedstock injector and adjusting injection parameters of the feedstock injected by the feedstock injector to control a depth of feedstock penetration into the gas stream column so that the optimal feedstock is entrained within the hotter and faster first region of the stream while the suboptimal feedstock is entrained within the second region of the stream; injecting liquid water that does not contain feedstock into the gas stream column from the liquid injector downstream of the feedstock injector in a direction that is transverse to the gas stream column and adjusting injection parameters of the water from the liquid injector to control, separately from the adjustment of the feedstock injection parameters, a depth of water penetration into the gas stream column transverse to the gas stream column so that the water is entrained substantially within the second region of the stream without penetrating the hotter and faster first region, the water reducing the temperature of the suboptimal portion of the feedstock entrained within the second region of the stream, and the temperature reduction being sufficient to prevent adherence of at least some suboptimal feedstock entrained within the second region of the stream on the substrate surface, wherein at least some of the suboptimal feedstock impacts the substrate surface and acts as abrasive media and removes weakly-adhered feedstock and grit particles of surface debris to prevent the weakly-adhered feedstock and grit particles of surface debris from being entrapped in a coating formed by the optimal feedstock to provide a cleaned substrate surface ahead of the spot on the substrate surface; wherein the injection parameters of the liquid water are also adjusted separately from the adjustment of the feedstock parameters so that liquid water impacts the substrate removing debris on the substrate; and forming a coating on the substrate surface by depositing feedstock substantially from within the spot projected on the surface by the hotter and faster first region of the gas stream column, wherein the hotter and faster first region extends to the substrate surface such that the optimal feedstock has optimal temperature and velocity and attains a molten status and speed sufficient to deform into a lamellar structure when the optimal feedstock impinges on the cleaned substrate surface, the coating, thus, consisting substantially of optimal feedstock deposited with optimal temperature and velocity conditions and forming lamellar structures.

2. The method as described in claim 1 further comprising; stopping the feedstock flow; adjusting pressure and velocity of the liquid water such that the liquid water penetrates the first region of the gas stream column; and: moving the column over one or both of the coating and surfaces adjacent to the coating for the purpose of removing debris.

3. The method as described in claim 1 wherein: the source of heated gas is a combustion chamber.

4. The method as described in claim 3 wherein: the feedstock is in the form of a slurry comprising a liquid containing suspended fine particles of coating material.

5. The method as described in claim 1 wherein: the source of heated gas is a plasmatron.

6. The method as described in claim 5 wherein: the feedstock is in the form of a slurry comprising a liquid containing suspended particles of coating material.

7. The method as described in claim 1 wherein: the feedstock is a slurry comprising a liquid containing suspended particles of coating material.

8. The method as described in claim 1 wherein: the water contains suspended abrasive particulates and the conditions are adjusted such as not to cause the adherence of the abrasive particulates.

9. The method as described in claim 1 wherein: the liquid injector is directly adjacent the feedstock injector.

10. The method as described in claim 1, wherein: the liquid injector and the feedstock injector inject liquid and feedstock, respectively, in directions that are parallel to one another.

11. The method as described in claim 1, wherein: the nozzle includes an end face and an aperture in the end face forming an exit, wherein the gas stream column exits the nozzle at the exit; and wherein: the feedstock injector is spaced apart from the exit and end face in a downstream direction to form a gap.

12. The method as described in claim 1, wherein: the feedstock profile is Gaussian and the optimal feedstock profile comprises a range of particle sizes having upper and lower bounds, wherein the suboptimal feedstock profile comprises particle sizes above and below the upper and lower bounds, respectively; the liquid injector comprises a first liquid injector; and including: providing a second liquid injector that is located on an opposite side of the gas stream column relative the first liquid injector; adjusting the size of the first liquid injector and the speed of liquid injected from the first liquid injector such that liquid from the first liquid injector is entrained substantially in a first portion of the second region on the first side of the gas stream column without reaching the first region; adjusting the size of the second liquid injector and the speed of liquid injected from the second liquid injector and the speed of liquid injected from the second liquid injector such that liquid from the second liquid injector is entrained substantially in a second portion of the second region that is on a second side of the gas stream column opposite the first side; adjusting feedstock injection such that the suboptimal feedstock particles having sizes below the lower bound are entrained in the first portion of the second region on the first side of the gas stream column and abrasively clean the substrate surface ahead of the spot on the substrate surface, and the suboptimal particles having sizes above the upper bound pass through the first portion of the second region on the first side of the gas stream column and through the gas stream column, and wherein the suboptimal particles having sizes above the upper bound are entrained in the second portion of the second region on the second side of the gas stream column and act as abrasive media trailing the spot and remove surface debris from the coating formed within the spot.

13. The method as described in claim 1, wherein: the feedstock is injected in a direction that is transverse to the gas stream column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-1a are side and end views showing a general presentation of the conventional thermal spray process providing a hot gas column extending from a nozzle to a substrate surface, with the coating being deposited onto the substrate surface within the spot projected by the gas column onto the substrate surface.

(2) FIGS. 2-2a are side and end views showing a step in a preferred embodiment of the thermal spray method wherein two volumetric concentric regions are defined within the gas column, a hotter and faster first region 15 surrounding the axis 5 of the gas column, and a cooler and a slower second region 16 wrapped around region 15.

(3) FIG. 2b is a graph showing particulate size versus count.

(4) FIGS. 3-3a are side and end views showing one step in a preferred embodiment of the thermal spray system and method wherein feedstock is injected via injector 19, with the optimal feedstock particles being entrained within region 15 and the suboptimal particles being entrained within the upper portion of region 16. Also shown is liquid injector 21, which is used to inject liquid to become entrained substantially within the upper portion of the second region 16.

(5) FIGS. 4-4a are side and end views showing another step in a preferred embodiment of the thermal spray system and method wherein feedstock is injected via injector 19, with the optimal particles being entrained within region 15 and the suboptimal particles being entrained within the upper and lower portions of region 16. Two opposed liquid injectors 21 and 31 are also shown; the injectors are used to inject liquid to become entrained substantially within the upper and lower portions of region 16, respectively.

(6) FIGS. 5-5a are side and end views showing another step in a preferred embodiment of the thermal spray system and method wherein feedstock is injected via opposed injectors 19 and 25, with the optimal particles being entrained within region 15 and the suboptimal particles being entrained within the upper and lower portions of region 16. Two opposed liquid injectors, 21 and 31 are also shown; these injectors are used to inject liquid to become entrained substantially within the upper and lower portions of region 16.

(7) FIG. 6 shows a schematic front view of the nozzle 3 with a plurality of feedstock injectors 19 and 25 and a plurality of liquid injectors 21 and 31 arranged about axis 5.

(8) FIGS. 7-7a are side and end views showing a preferred embodiment of the method wherein a coating is deposited and the substrate surface is cleaned by alternate steps of the method described in the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

(9) A thermal spray apparatus/system and a method are provided for the continuous in-flight reduction of suboptimal feedstock deposition and the in-situ removal of debris, such as less adherent feedstock and surface preparation grit particulates, from the substrate and coating. The apparatus (FIGS. 2-2a) includes a hot gas generator 2 and nozzle 3, which are used to generate a high temperature gas column 4 that projects into a spot onto the substrate surface 6. In an illustrative embodiment of the invention, the hot gas column properties, coating performance requirements, and feedstock characteristics combine to define an optimal feedstock size range; thus, any particle sizes outside this range would be classified as suboptimal, or undesirable. As mentioned above, a feedstock size distribution consisting only of particles within the optimal size is impractical. In practice, the most efficient scenario is to center the feedstock particle size distribution within the optimal size range, as shown schematically in FIG. 2b. Accordingly, within the gas column 4, the locations of the feedstock particle from each category define two volumetric regions: region 15 and region 16.

(10) Region 15 surrounds axis 5 and projects onto the substrate surface 6 in a central spot 17. This region is characterized by the location of the optimal feedstock particles, meaning the particle temperature and velocity conditions generated in region 15 produce an optimal coating on the surface 6.

(11) Region 16 surrounds region 15 and projects onto the substrate surface 6 in an annular region 18 that surrounds the central spot 17. Region 16 is characterized by the location of suboptimal feedstock particles; thus, the particle temperature and velocity conditions generated in region 16 are insufficient to produce an optimal coating on the surface 6. Consequently, region 18 is formed by the deposition of suboptimal particles.

(12) FIGS. 3-3a show an embodiment wherein the system comprises a first injector 19 to inject feedstock 20 into the gas column and a second injector 21 for injecting a liquid 22 into the gas column, with the second injector shown positioned downstream and adjacent to the first injector. For this embodiment, the feedstock particle size distribution is skewed, consisting only of particles in the optimal size range and smaller. Resultantly, the size of injector 19 and the speed of feedstock injection produce the penetration of the optimal feedstock particles 23 into region 15, while the suboptimal feedstock particles 25 are confined to the upper portion of region 16. The optimal feedstock particles 23 entrained in region 15 are transferred sufficient heat and momentum from the hot gas stream to impact substrate surface 6 and form an optimal quality coating 24, which is confined to the spot 17. The suboptimal feedstock particles 25 entrained in the upper portion of region 16 are cooled by liquid 22, which is primarily entrained into the upper portion of region 16 by adjusting the size of injector 21 and the speed of liquid injection. As shown in FIG. 3, the cooling produced by liquid 22 can reduce the degree of suboptimal feedstock particle melting to the point that splat formation is prevented, causing cooled suboptimal feedstock particles 27 to hit surface 6 and bounce off without adhering and forming a coating. Thus, liquid 22 and cooled suboptimal feedstock particles 27 can impact surface 6 and act as abrasive media, removing the weakly-adhered feedstock and grit particles represented by surface debris 26 ahead of the movement of spot 17 and the formation of coating 24. Furthermore, liquid 22 and cooled suboptimal feedstock particles 27 acting as abrasive media on surface 6 may dislodge embedded surface debris such as grit particles 28, removing them from the surface and preventing them from being entrapped in the coating. Moreover, it is possible that heating by the hot gas stream and cooling by the impinging liquid may cause the expansion and contraction of surface 6 and weakly-adhered/embedded debris particles 26 and 28, respectively, in a way that aids the removal of these debris particles from the surface. If an enhanced abrasive process is required, the liquid 22 may contain a suspension of fine abrasive particulates, such as silicon or aluminum oxides. The fine particulates would be entrained in the upper portion of region 16 where they would be accelerated towards surface 6 without achieving the velocity or degree of melting necessary to adhere to surface 6 upon impact. These fine particulates would therefore enhance the removal of debris 26 and 28.

(13) FIGS. 4-4a depict an embodiment where the feedstock particle size distribution is Gaussian and contains particles below and above the optimal size range. In this case, larger than optimal particles 29 injected with the feedstock stream 20 would penetrate through region 15 and become entrained in the lower portion of region 16. Because these particles 29 do not receive sufficient heat and momentum in region 16, they form a suboptimal deposit, represented by surface debris 30, which trails the movement of spot 17 and the formation of coating 24. As discussed here above with reference to FIG. 3, the smaller than optimal feedstock particulates 25 do not have enough momentum to penetrate into region 15. As a result, suboptimal feedstock particles 25 entrain in region 16 where they do not receive enough heat and momentum to form optimal coating 24 upon impacting surface 6, so instead suboptimal feedstock particles 25 add to surface debris 26. The negative situations associated with surface debris 26 and 30 are resolved by incorporating opposing liquid injectors 21 and 31, as shown in the preferred embodiment of FIG. 4. The size of injector 31 and the speed of injection are adjusted so that the entrainment of liquid 32 occurs substantially within the lower portion of region 16. Some particles 29 are then cooled by liquid 32 to impact the substrate with a degree of melting that is insufficient to adhere to the substrate; these cooled suboptimal feedstock particles 33 hit surface 6 and bounce off without adhering and forming a coating. Thus, liquid 32 and suboptimal feedstock particles 33 can impact surface 6 and act as abrasive media, removing weakly-adhered surface debris 30 in the portion of region 18 trailing the motion of the spot 17 and the formation of coating 24. This cleaning mechanism may also remove from surface 6, embedded debris such as grit particle 34. Moreover, it is possible that heating by the hot gas stream and cooling by the impinging liquid may cause the expansion and contraction of surface 6 and weakly-adhered/embedded debris particles 30 and 34, respectively, in a way that aids the removal of these debris particles from the surface.

(14) With regards to the upper portion of region 16, the mechanism of action is the same as described here above with reference to FIG. 3. The cooling of suboptimal feedstock particles 25 in region 16 by liquid 22 reduces adherence to surface 6 upon impact. As shown in FIG. 4, some cooled suboptimal feedstock particles 27 hit surface 6 and bounce off without adhering at all. Thus, liquid 22 and suboptimal feedstock particles 27 can impact surface 6 and act as abrasive media, removing the weakly-adhered feedstock and grit particles represented by surface debris 26 ahead of the movement of spot 17 and the formation of coating 24. Furthermore, liquid 22 and suboptimal feedstock particles 27 acting as abrasive media on surface 6 may dislodge embedded surface debris, such as grit particle 28, removing them from the surface and preventing them from being entrapped into the coating. Moreover, it is possible that heating by the hot gas stream and cooling by the impinging liquid may cause the expansion and contraction of surface 6 and weakly-adhered/embedded debris particles 26 and 28, respectively, in a way that aids the removal of these debris particles from the surface.

(15) When increased output requires larger volumes of feedstock to be injected, multiple feedstock injectors can be distributed about axis 5 of the gas stream. FIGS. 5 and 5a of the drawings presents another preferred embodiment of the system shown in FIG. 4 with an additional feedstock injector 35 being located opposite to feedstock injector 19. The mechanism of injection and removal of suboptimal particulates and surface debris is a mirror of the mechanisms described for the embodiments shown in FIG. 3 and FIG. 4.

(16) In another embodiment of the present invention, FIG. 6 of the drawings shows a schematic front view of nozzle 3 with a plurality of feedstock injectors 19 and 35 and a plurality of liquid injectors 21 and 31 arranged about axis 5.

(17) Another preferred embodiment of the thermal spray system incorporating the invention is shown schematically in FIGS. 7 and 7a of the drawings. The gas stream column is shown extending from nozzle 3 to substrate surface 6, the column having a defined core region 15 surrounding axis 5. Feedstock injector 19 is shown having flow control valve 37. Similarly, liquid injector 21 is shown having flow control valve 38. One of each injector is shown in FIG. 7; however, only one injector connected to both control valves 37 and 38 could be incorporated, or a plurality of injectors arranged about axis 5 may be employed as previously described with reference to FIG. 6. For the embodiment shown in FIG. 7, in a first step, the thermal spray system moves relative to surface 6 parallel to arrow 8 to deposit one or multiple layers of coating 11 or 24 in a manner described here above with reference to FIG. 1, 3, 4, or 5. In a second step, feedstock flow is stopped with valve 37, and the liquid velocity is adjusted with valve 38 so that the liquid is entrained substantially within region 15 of the gas stream. In a third step the thermal spray system moves relative to surface 6 in the direction(s) of arrow 8 and/or arrow 39 to clean debris particles 26 and 28 from surface 6 and coating 11 or 24 according to the method described here above with reference to FIG. 3, 4, or 5. In a fourth step, control valve 37 is opened and the feedstock and liquid flows are adjusted to deposit one or multiple layers of coating 11 or 24 in a manner described here above with reference to FIG. 1, 3, 4, or 5.

(18) It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.