GB2268430A

GB2268430A - Method of coating materials

Info

Publication number: GB2268430A
Application number: GB9214374A
Authority: GB
Inventors: G M Bedford
Original assignee: Frictec Ltd
Current assignee: Frictec Ltd
Priority date: 1992-07-07
Filing date: 1992-07-07
Publication date: 1994-01-12
Anticipated expiration: 2012-07-07
Also published as: GB9214374D0; GB2268430B; WO1994001238A1

Abstract

In the rotary friction surfacing of a first material (11) with a second material (22) by bringing a rotating body (11) into contact with the surface of the second material (22) by movement in one plane only, a containment ring (26) prevents spreading of the first and second material at the region where the deposit is formed. The first material is cooled by a spray of water from one or more nozzles (35).

Description

METHOD OF COATING MATERIALS This.invention relates to a method of forming a coating of a first material on a second material by rotary friction surfacing.

A coating of a relatively hard material can be formed at a useful coating depth i.e. greater than 0.2 mm along an edge of a relatively soft substrate on relative movement between the coating material and the substrate as disclosed in patent specification W087/04957. A useful depth of a relatively hard material can also be coated on a relatively soft substrate under circumstances where there is no relative translational movement therebetween if the substrate is positively cooled to bring about movement of the shear layer at which heat is being generated in a direction away from the substrate, as disclosed in EP-A-0377562. It is with a process of the latter type that this invention is particularly concerned.

In one aspect the invention provides a method of forming a surface of a first material on a second material by bringing a rotating body of the first material into contact with the surface of the second material by movement in one plane only, the first material being relatively hard and the second material being relatively soft, characterised by the step of confining the first and second materials adjacent to the zone of contact therebetween to reduce or prevent radial spreading of the deposit of the first material and the second material.

It was an unexpected finding that close contact with an external body could confine the spreading of the second or substrate material in the region of a contact zone with the relatively hard first or "mechtrode" material, without undesirable adhesion taking place, and that the shape of the deposited relatively hard or mechtrode material could follow the shape of the body which brings about that confinement. It has also been found that where the containment ring is conductive and especially where it is a relatively massive body and the deposits are shallow, it can bring about some interface movement because it can act as a heat sink and cool the material being deposited on the substrate. However, the provision of additional cooling of the substrate and/or the containment ring is preferred.

In this invention the first material will generally be in the form of a rod, though the use of other shapes e.g. tubular is not excluded. The second material may be in the form of a plate, but is also preferably in the shape of a rod. The body in which they are confined may be a one piece annular body or may be annular body defined by a multiplicity of jaws which are urged together. The body may have a lower thermal conductivity than the material of the substrate, in which case it is preferably a ceramic material, for example alumina or magnesium oxide.

Less preferably it has a similar thermal conductivity to that of the substrate, in which case the confinement body may be of high speed steel, tool steel, stellite, or an iron, nickel, cobalt or aluminium-based metal matrix composite material. Also the first and second material may be confined, adjacent to the region where they are in contact, within a body having a thermal conductivity greater than that of the second material e.g. of copper. The ratio between the diameter of the body of the first material and the body of second material can be from 0.8 to 1.04, preferably from 0.9 to 1.02, and preferably the contact zone of the first and second materials is contained within a containment ring whose diameter relative to the diameter of the body of first material is from 1.04 to 1.25.

The containment ring may be at a fixed position relative to the first and second members and may be of height at least 3 mm. Contact between the first and second materials may be maintained until the deposit produced has a depth from 10 to 100% of the depth of the containment ring within which the contact zone between the first and second members is contained.

Alternatively, for the production of deposits of semi-infinite depth, the containment ring may be relatively movable in a direction parallel to the axis of rotation of the first material so as to follow a progressive deposition of the first material on the second material.

The deposit of the first material on the second material may have a substantially smooth cylindrical surface or may be stepped to provide a region of increased or decreased diameter.

Contact between the first and second materials may take place within a containment ring which has a wear resistant surface on an internal cylindrical surface thereof. The wear resistant surface may be of a ceramic thin film material deposited by CVD or PVD.

The second member may protrude into containment ring a distance from 10 to 100% or more of its diameter.

The first member may be a nickel-based, iron-based, cobalt-based or aluminum-based material including alloys and metal matrix composites thereof.

The second member may conveniently be plain carbon steel, alloy steel or stainless steel or another metal e.g. nickel or cobalt.

In the formation of the coating, the first and second materials may initially be contacted at a given rate of rotation and applied load and the rate of rotation and/or the load may be varied during the contact period which may conveniently last from 4 to 100 seconds. The speed of rotation of the first material and the pressure applied at the interface between the first and second materials should be such that a shear layer at which heat is being generated moves away from the surface of the second material in the direction of the axis of rotation of the first material so that on removing the first material from contact with the second material, the second material is found to be surfaced with a layer at least 0.2 mm deep of the first material. The second member is preferably cooled by a spray of water or another atomised liquid.

In a further aspect of the invention there is provided a method of forming a surface of a first material on a second material by bringing a rotating body of the first material into contact with the surface of the second material by movement in one direction only, wherein the second material adjacent to the surface in contact with the first material is cooled by means of a spray of water or another atomised liquid.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which Figure 1 is a diagramatic side view of a substrate and hard material coating rod ("mechtrode') at the region of the interface between them, with rotary friction surfacing having been carried out in the absence of any confinement at the interface; Figure 2 is a diagramatic section of a rotary friction surfacing apparatus according to the invention; Figure 3A shows a substrate and a deposit of relatively hard material formed according to the invention, but using confinement means of relatively high thermal conductivity; and Figure 3B shows the same substrate and deposit of relatively hard material after grinding to reduce the diameter thereof; Figure 4 is a diagramatic view of a second form of the deposition apparatus according to the invention; Figure 5 is a diagram illustrating the formation of a deposit under conditions where the substrate has entered insufficiently into the confinement region; Figure 6 is a diagramatic perspective view of the apparatus of Figure 5 with part of the confinement structure removed, and illustrating the pattern of heat flow.

Figure 7 is a diagramatic section of a second embodiment of a confinement for a material of low thermal conductivity; and Figures 8A to 8D show further forms of the invention in which the containment ring is one-piece and is made of steel; Figure 9 shows a yet further form of the invention in which cooling is via a water bath; Figures 10 and 11 show diagramatically various coating rods and substrates used in experimental tests according to the invention.

Figure 1 shows a substrate 10 which is in the form of a rod and has a deposit thereon of relatively hard material 12 which has been deposited by the process disclosed in our European patent specification EP-A-0377562. In that specification there is disclosed a method of forming the surface deposit 12 of a relatively hard material on the substrate 10 which is relatively soft by bringing a rotating body of the relatively hard material 12 into contact with the surface of the relatively soft material 10. The substrate 10 is postively cooled in the region adjacent to the interface 14 between the materials 10, 12 by contact with jaws of relatively conductive material, or by supply of or immersion in a cooling liquid.There is no relative movement between the rotary body of relatively hard material which forms the deposit 12 and the substrate 10 except in a direction parallel to the axis of the rotating body.

By virtue of the cooling, the speed of rotation of the body which forms the deposit 12 and the pressure applied at the interface 14, a shear layer at which heat is being generated moves away from the surface 14 along the rotating body so that on removing the rotating body from the hot region of relatively hard material, that relatively hard material is found to be formed as a surface deposit on the relatively soft material. During the deposition process, the heat of friction between the relatively hard and relatively soft substrate generates a hot plasticised layer at the end of the rod 10 which gives rise to a deposit 12 of extremely fine and homogeneous microstructure with full density and with a narrow localised heat affected zone in the substrate 10.The coating is a solid phase process and during the coating cycle the applied layer 12 reaches a temperature of about 40 degrees C below its melting point. Thus the coating 12 is a product of a vigorous hot forging action rather than a casting action as in fusion welding. However, in the process described in the above European patent, part of the substrate 10 collapses during the deposition process to form a radially upset region 16 of relatively large diameter which is covered by a similar relatively enlarged region 18 which typically has an uneven side surface as shown. Both of the substrate 10 and coating 12 have to be machined down to a diameter 20 at which both are defect-free following post-coating heat treatment of alloys where necessary.This machining operation includes extensive grinding of the deposit 18 to reduce its diameter, and because of the undesired radial enlargement of the diameter and the unevenness of the readily enlarged portion 18 of the mechtrode deposit, lengthy grinding is necessary to convert the article to its finished state.

The present invention enables the efficiency of the deposition process as disclosed in EP-A-4377562 to be improved by reducing undesired spreading of the substrate and the deposit in the region where the deposit is formed, and thereby to enable valve stem tips and similar articles to be produced with reduced finishing cost and at a higher production rate.

In Figure 2 there is shown a substrate 10 on which a deposit is to be formed of relatively hard material by rotary friction surfacing using a sacrificial rod 22 of the relatively hard material. The end 24 of the substrate is held within a close fitting anular container 26 which in this instance may be of a material such as copper which has a higher thermal conductivity than that of the substrate 10 which is typically mild steel or which may be of another iron based material which has a generally similar thermal conductivity to that of the substrate 10. With this arrangement, the container 26 limits the spreading of the substrate 12 during deposition, and the deposit of material from the mechtrode 22 forms within the container 26 and it is also thereby limited in the extent of its radial spread.With this arrangement, samples have been produced in which the upset diameter of the substrate and the corresponding diameter of the deposit are such that there is only a small increase in diameter in the depositional zone.

However, with the apparatus shown in Figure 2 which uses a one piece container of a material having a greater or similar conductivity to that of the substrate, it has been found that undercuts 30 are prone to develop around the edge of the deposit 12 where it is joined to the substrate 10, and although the article can be ground to a reduced diameter where these undercuts 30 are not present as in Figure 3B, again unnecessary machining cost is involved.

In order to allow the end 24 of the substrate to be present within the container 26, but still be adequately cooled, a modification was made to the cooling system disclosed in EP-A-0377562. In that specification cooling is by contact with copper jaws, or by contact with a body of cooling water. It has now been found that satisfactory cooling of the substrate 10 can be achieved by spraying water from one or more spray nozzles 35 towards the end of the substrate 10. Where more than one spray nozzle is used, as is preferred, they are distributed equi-angularly about the substate 10. The use of spray nozzles enables the deposition process to be carried out with the substrate 10 directed vertically or horizontally, or in any other desired direction, whereas when cooling is by a body of water as in the earlier European patent, the substrate 10 has to be directed vertically.A suitable spray unit for cooling a rod-like substrate is sold by Wymark Technical Products Limited of Cheltenham, England under the trade name VAPORCOOL 100. It will be noted that in the present process the deposited material does not, or does not noticeably, metallurgically bond to the inner face of the containment ring and can be removed relatively easily. It is believed that this is the result of cooling in the deposition region which maintains the substrate and the containment ring relatively cold. The containment ring can therefore be re-used many times.

The problem of undercut can be reduced in the apparatus shown in Figures 4 and 6 in which the end of the substrate 10 fits within a containment ring 40 of low thermal conductivity ceramic material which may be for example óf a alumina or magnesium oxide. The containment ring 40 is supported within a cup 42 formed in its base wall with a circular opening through which the substrate 10 protrudes. A stepped diameter plug 44 (fig 4) fits within a vertical side wall of the cup 42 to hold the containment ring 40 firmly therebetween as shown. Various dimensions are marked in Figure 4 for the part of the containment ring assembly shown, and these will be discussed individually below.

The base wall of the cup 42 has a thickness indicated by the dimension L1, and this dimension should be made as small as possible. As L1 is reduced the effectiveness of the cooling action of the water spray from nozzle 35 is increased. For a substrate of diameter 10 to 25 mm the thickness L1 is desirably in the range of 2 to 10 mm preferably about 3 mm.

Before the deposition process starts, the end of the substrate 10 protrudes a vertical distance L5 into the containment ring 40. The distance L5 must be sufficient that the deposit of hard material forms on the substrate 10 at a clearance from the lower wall of the cup 42. If L5 is zero or is an insufficient distance, the hard material deposit becomes welded to the base of the cup 42 as shown in Figure 5. For a substrate of diameter 10 to 25 mm the value of L5 is preferably 1 to 4 mm The height of the ceramic containment ring is shown as L2. If it is required to produce a deposit of hard material of depth H, then L2 should obey the relationship: L2 > H + L5 The depth of the deposit of hard material on the end of the substrate should normally be greater than 0.2 mm and is typically 3 mm or more, and may be 5 mm or more or even 8 mm or more.The inventors have not found at present any practical upper limit on the depth of the deposit which can be produced, which is set only by the machine time required to produce that depth of deposit, and deposits of depth greater than 40 mm have been produced using the method of this invention thereby enabling blanks for milling cutters, drills and the small tooling to be made. Contact between the mechtrode and the substrate may be maintained until the deposited material has reached a depth of from 10 to 100% of the depth of the containment ring 40 within which a contact zone between the mechtrode material and the substrate is contained.Alternatively, for the production of deposits of semi-infinite depth, the containment ring 40 may be made axially movable in a direction parallel to the axis of rotation of the mechtrode 10 so as to follow a progressive deposit of the mechtrode material on the substrate.

L2 + L3 + L4 - L5 determines the distance which the mechtrode has to protrude into the containment ring assembly to bring about formation of the required deposit. Ideally L3 and L4 are minimised.

The diameter + 1 of the mechrode 22 in Figure 5 can be greater than the diameter 2 of the substrate 10, and the mechtrode 22 is typically from 0.05 to 2 mm greater than the diameter of the substrate 10, preferably 0.2 to 0.4 mm greater than the diameter of the substrate 10. There are some exceptions to this rule where some materials can have a larger substrate than the mechtrode e.g. when using certain metal matrix composite materials. The aperture in the base of the cup 42 is preferably substantially the same diameter of that of the substrate 10 so that the top of the substrate 10 should be dry during the initial phase or "touchdown" of the coating operation.

Q 3 represents the internal diameter of the containment ring 40, and this diameter is greater than or # 2. The diameter of the overcoating 12 produced on the substrate 10 depends directly upon the diameter &commat; 3 because with a ceramic bush it has been found that the diameter of the overcoating is almost precisely the same as the internal diameter 3 of the ceramic bush. The ceramic containment rings 40 must have an external diameter t 4 sufficient to enable it to withstand the pressures during the deposition operation.The external diameter 5 of the cup should be sufficient to provide adequate structural strength and typically 5 = 4:4 + 10 mm In a typical example, for a substrate of diameter 10 mm and a mechtrode of diameter 10.4 mm and a deposit thickness of 2 mm, L1 is about 3 mm, L2 and L3 are about 5 mm, L4 is 2 mm, L5 is 1 mm, t3 is 10.4 mm, 4 is 18 mm and 105 is 43 mm.

As is apparent in Figure 6, the reason why the deposit 12 of relatively hard material forms on the substrate is that the main heat flow from the area of contact between the mechtrode 22 and the substrate 10 is via the substrate itself. The mechtrode 22 may be of high speed steel which has a thermal conductivity of 25 W/MK the ceramic retaining ring 40 typically has a thermal conductivity in the range of 1 to 10 W/MK.

The substrate which may be of mild steel typically has a thermal conductivity of about 50 W/MK and provides the predominant heat path from the contact zone as is shown by the arrows, some heat also entering the base wall of the cup 42 as shown and the surrounding container.

An alternative structure for the containment ring assembly is shown in Figure 7 where the ceramic containment ring 40 fits flush into the cup 42 and is held in position by means of a lid 50 having a downwardly facing flange which fits onto the external side wall of the cup 42 as shown. With this arrangement the length of the mechtrode 22 that has to enter the containment ring assembly is reduced.

Figures 8A - 8D show various geometries used in tool steel containment rings 26. In figure 8A the containment ring is a constant diamater and is slightly larger than the substrate. Figures 8B to 8D are similar but show different containment ring geometries that give rise to different deposit shapes which may be near the net shape desired.

Figure 9 shows a substrate 10 held between parts 90, 92 of a c-clamp by means of an hydraulic jack 94.

The substrate is in a container 96 containing a body 98 of water in which the containment ring 40 is immersed. The containment ring 40 is held in place by a pressure plate 100 during the deposition period.

The level of the water in the container 96 is kept just below the rubbing interface to bring about cooling and hence interface movement. Alternatively, if the required growth is relatively small e.g. about 3 mm, cooling may be effected by the circulating water which is kept at a constant level.

If the upper portion of the containment ring 40 is stepped to defined a region of relatively large diameter, then it is possible to produce a coating on the substrate having an enlarged free end. Figure 10 shows a mechtrode and a plain or slightly tapered coating of relatively hard material on a substrate and Figure 11 shows a stepped material coating on a substrate.

The accompanying table shows for the dimensions indicated in Figures 10 and 11 various coatings that have been produced and the axial load, rate of relative rotation and coating period necessary to give rise to the indicated coating.

TYPE COATING SUBSTRATE dS dC1 dC2 dM L1 L2 L3 LOAD RPM TIME KN SEC I MMC 708 M40 BS970 28.5 24.7 - 24.7 109 45 - 20 300 45 WITH 3165 ALUMINA PARTICLES I MMC AS ABOVE AS ABOVE 28.5 24.7 - 24.7 109 17 - 35 300 30 RESEARCH I MMC AS ABOVE 9.97 10.3 - 8.17 92 7.3 - 4.5 650 7 MATERIAL 7.5 I AS ABOVE AS ABOVE 9.97 10.3 - 8.17 92 12 - 4.5 650 12 7.5 IV AS ABOVE AS ABOVE 9.4 9.5 11.2 8.17 92 13 6.5 4.5 650 12 7.5 III BM2 AS ABOVE 9.97 10.6 - 10.2 97.8 7 - 4.5 750 5 10

Claims

1. A method of forming a surface of a first material on a second material by bringing a rotating body of the first material into contact with the surface of the second material by movement in one plane only, the first material being relatively hard and the second material being relatively soft characterised by the step of confining the first and second materials adjacent the region of contact therebetween to reduce or prevent radial spreading of the deposit of the first material on the second material.

2. A method according to claim 1, wherein the first and second materials are confined within a body having a lower thermal conductivity than the material of the substrate.

3. A method according to claim 2, wherein the first and second materials are confined within a body of a ceramic material.

4. A method according to claim 3, wherein the first and second materials are confined within a body which is of alumina or magnesium oxide.

5. A method according to claim 1, wherein the first and second materials are confined at the region where they are in contact within a body having a similar thermal conductivity to that of the substrate.

6. A method according to claim 5, wherein the confinement body is of high speed steel, tool steel, stellite or iron, nickel, cobalt or aluminum-based metal matrix composite material.

7. A method according to claim 1, wherein the first and second materials are confined, adjacent to the region where they are in contact, within a body having a thermal conductivity greater than that of the second material.

8. A method according to claim 7, wherein said body is of copper.

9. A method according to any preceding claim, wherein the first and second materials are both generally cylindrical bodies, and the ratio of the diameter of the body of first material to the body of second material is from 0.8 to 1.04.

10. A method according to claim 9, wherein the ratio of the diameter of the body of the first material to the diameter of the body of second material is from 0.9 to 1.02.

11. A method according to any preceding claim, wherein the contact zone of the first and second materials is contained within a containment ring whose internal diameter relative to the diameter of the body of first material is from 1.05 to 1.25.

12. A method according to claim 11, wherein the containment ring is of fixed position relative to the first and second members and is of height at least 3 mm.

13. A method according to claim 12, wherein the containment ring is of height above 5mm.

14. A method according to claim 12, wherein the containment ring is of height above 40mm.

15. A method according to any of claims 1 to 12, wherein the containment ring is relatively movable in a direction parallel to the axis of rotation of the first material so as to follow a progressive deposit of the first material on the second material.

16. A method according to any preceding claim, wherein the contact between the first and second materials is maintained until the deposit has a depth from 10 to 100% of the depth of a containment ring within which the contact zone between the first and second members is contained.

17. A method according to claim 16, wherein the process is continued until the deposit of the first material has approximately 100% of the depth of the containment ring.

18. A method according to any preceding claim, wherein the diameter of the rotating member is from 0.05 to 2 mm different from the diameter of the second material.

19. A method according to claim 18, wherein the body of first material is of diameter 0.2 to 0.4mm different from the diameter of the body of second material.

20. A method according to any preceding claim, wherein the deposit of the first material on the second material has a substantially smooth cylindrical or tapered surface.

21. A method according to any preceding claim, wherein the contact between the first and second material is within a containment ring having a wear resistant surface on an internal cylindrical surface thereof.

22. A method according to claim 21, wherein the wear resistant surface is of a ceramic thin film deposited by CVD or PVD.

23. A method according to any preceding claim, wherein the contact between the first and second materials is within a containment ring supported by a support member for resisting axial and/or radial loads.

24. A method according to any preceding claim, wherein the second member protrudes into the containment ring a distance from 10 to 100% of its diameter.

25. A method according to claim 24, wherein the second member protrudes a height of 10 from to 100% of its diameter into the containment ring.

26. A method according to any preceding claim, wherein the coating material is a nickel-based, iron-based cobalt-based or aluminum-based material, including alloys and metal matrix composite materials.

27. A method according to any preceding claim, wherein the second member is of a nickel-based, iron-based cobalt-based or aluminum-based material, including alloys and metal matrix composite materials.

28. A method according to any preceding claim, wherein the first and second materials are initially contacted at a given high rate of rotation and applied load, and the rate of rotation and/or the load is varied in a predetermined pattern or cycle during the contact period.

29. A method according to any preceding claim, wherein the contact period is from 4 to 100 seconds.

30. A method according to any preceding claims, wherein the speed of rotation of the first material and the pressure applied at the interface between the first and the second materials are such that a shear layer at which heat is being generated moves away from the surface of the second material in the direction of the axis of rotation of the first material, so that on removing the first material from contact with the second material the second material is found to be surfaced with a layer at least 0.2mm deep of the first material.

31. A method according to any preceding claim, wherein the second material is positively cooled adjacent the surface where it contacts the first material.

32. A method according to claim 31, wherein the second member is cooled by a spray of atomised liquid.

33. A method according to claim 32, wherein the atomised liquid is water.

34. A method of forming a surface of a first material on a second material by bringing a rotating body of the first material into contact with the surface of the second material by movement in one plane only, wherein the second material adjacent to the surface in contact with the first material is cooled by means of a spray of atomised liquid.