WO1994018354A1

WO1994018354A1 - Coatings

Info

Publication number: WO1994018354A1
Application number: PCT/GB1994/000223
Authority: WO
Inventors: David Mark Williams
Original assignee: Mcpherson's Limited
Priority date: 1993-02-06
Filing date: 1994-02-07
Publication date: 1994-08-18
Also published as: AU6003694A; GB9302384D0

Abstract

The invention relates to the provision of a hard surface coating on the cutting edge of a knife or other cutting implement and has for its objective to improve upon the process disclosed in co-pending EP Patent Application 93303062.9 which objective is met by a process for the production of a carbo-nitride coating comprising grinding one face of a V-shaped cutting edge of a knife blade or other cutting implement, locating the part-ground blank in a chamber provided with a means able to produce ions of a metal carbo-nitride, and subsequent to the coating process final grinding a second face of a V-shaped cutting edge, characterised by the blank being oriented such that the face to be coated is directed towards the cathode evacuating the chamber, impressing a current on a metallic element of the required metal to induce ion release, and to provide a required potential difference between the metallic element and the blank to cause ion bombardment of the blank at least over the area of the said ground face, said ion bombardment being controlled by the potential difference to induce heat in the blank to a level below the tempering temperature of the metal of the blank, releasing nitrogen and carbon-carrying gas, such as methane, into the chamber to apply a carbo-nitride coating to the face to be coated by physical vapour deposition at a temperature to cause the growth of columnar crystals, removing the blank from the chamber and grinding the second face of the V-shaped cutting edge to expose the carbo-nitride coating at the cutting tip of the cutting edge.

Description

COATINGS

This invention relates to coatings and is particularly concerned with the provision of a hard surface coating on at least the cutting edge of a cutting implement such as a knife blade, or other cutting implement. It has long been known that the surface hardness and wear resistant properties of metal objects can be enhanced by the provision of a hard surface, and in co-pending EP Patent Application No. 93303062.9 there is described and claimed a knife blade comprising a V-shaped cutting edge formed on a blank and such that the cutting tip lies substantially centrally of the width of the blank, one side face of the V- shaped cutting edge being provided with a coating of a carbo- nitride, the actual cutting edge being formed wholly of the carbo-nitride material, and the carbo-nitride material displaying a columnar crystal structure that extends away from the surface of the blank and to the outer face of the carbo- nitride coating.

It is the object of the present invention to provide an improved process for the production of such a carbo-nitride coating.

According to the present invention, a process for the production of a carbo-nitride coating comprises grinding one face of a V-shaped cutting edge of a knife blade or other cutting implement, locating the part-ground blank in a chamber provided with a means able to produce ions of a metal carbo- nitride, the blank being oriented such that the face to be coated is directed towards the cathode evacuating the chamber, impressing a current on a metallic element of the required metal to induce ion release, and to provide a required potential difference between the metallic element and the blank to cause ion bombardment of the blank at least over the area of the said ground face, said ion bombardment being controlled by the potential difference to induce heat in the blank to a level below the tempering temperature of the metal of the blank, releasing nitrogen and carbon-carrying gas, such as methane, into the chamber to apply a carbo-nitride coating to the face to be coated by physical vapour deposition at a temperature to cause the growth of columnar crystals, removing the blank from the chamber and grinding the second face of the V-shaped cutting edge to expose the carbo-nitride coating at the cutting tip of the cutting edge. The metallic element of the required metal may be a strategically located cathode, the impressed current causing the release of ions, and there may be an associated magnet system to provide a force field to control the random dispersal of ions and increase the degree to which the blank is bombarded. Equally, the metallic element of the required metal may be a block of metal in a strategically located crucible, with an attendant electron beam means to evaporate the metal in the crucible and cause the release of ions, there being an impressed current on the crucible and the metal therein to provide a required potential difference with the blank. Here, the ions react with the gas to create a plasma directed at the blank to a degree dictated by the potential difference with again control over the temperature to which the blank is raised, to be below the tempering temperature of the metal of the blank and to ensure the growth of columnar crystals in the applied carbo-nitride coating.

Ordinarily, physical vapour deposition is effected such as to avoid the presence of columnar crystals in the coating, it having been long known that a fine equiaxed and dense structure is to be strived for to maintain the integrity of the coating at its surface. In contrast with this, the deliberate creation of a columnar crystal coating at the cutting tip of a knife or other cutting implement has a most significant effect on the cutting characteristics and on edge retention of the knife or other cutting implement.

The blank may be located in the chamber in fixed and parallel relationship to the metallic element with the first ground face of the cutting edge facing the metallic element or may be located in fixed perpendicular relationship to the metallic element with the ground end of the blank directed towards the cathode. Desirably however, the blank is oriented between a parallel and a perpendicular position, such as at 45° and with the first ground face of the blank facing the cathode. Equally, the blank may be mounted for rotational or orbital movement in relation to the metallic element.

In the circumstance where the material of the blank is a martensitic stainless steel, the potential difference between the blank and the metallic element and the resultant ion bombardment, for the cleaning of at least the face to be coated and subsequently and in conjunction with introduced nitrogen and methane gas, the application of a coating of carbo-nitride material by physical vapour deposition, should be such as to ensure that the temperature of the blank is not raised to greater than 550°C but more than 200°C. Preferably, the temperature of the blank is raised to between 300°C and 400°C and in particular to 350°C.

. he duration of the physical vapour deposition process should be for a period of time to provide a coating of a thickness of between 8 and 12 microns.

As a consequence of the production of the coating of carbo-nitride material by physical vapour deposition the build-up of the coating thickness is by the growth of columnar crystals extending from the surface of the face to be coated to the outer surface of the coating.

Whilst the full surface of a blank may be coated, it is preferred to limit the application of the coating to the first ground face of the V-shaped cutting edge. Thus, the side faces of the blank may be masked to prevent the application of a coating. To enable batch production a number of blanks may be held together in abutting side-by-side relationship and staggered such that a first blank masks the side face of an adjacent blank but leaves its first ground face exposed and so on, over the number of blanks to be simultaneously coated. The first such blank can be provided with an appropriate masking plate or of itself serve the purpose of a mask and used as the first blank of a next batch.

Thus, the blanks may be positioned on an appropriate jig for location within a chamber in which the metallic element, cathode or crucible, is strategically located, the jig being either fixed in relation to the metallic element, or rotatably mounted. In a relatively small chamber, a single jig can be provided and mounted for rotation about its own axis. In larger chambers, a number of jigs can be provided and mounted not only for rotation about their own axes but also for rotation around the chamber.

In the circumstance where the jig can be rotated and/or orbited, each jig may be formed by a number of jig plates on a common support, each jig plate being correspondingly multi-sided and each side correspondingly profiled to provide for the positive support of a number of overlapping blanks bridging the respective sides of the jig plate, and there being a clamp to lock the blades to the jig plates, with, when provided, a masking plate overlying the first, exposed, blank. Dependent on the blade size being treated, such as, for example, triangular, square or hexagonal jig plates can be employed, increasing considerably the number of blanks that can simultaneously be treated, the rotation/orbiting of the jigs ensuring that the blades are repeatedly oriented as required in relation to the metallic element during the process consequent on the continuous rotation/orbiting of the jigs.

Titanium may be employed as the cathode or the metal in the crucible, and whereby to form a coating of titanium carbo-nitride. To provide an enhanced appearance at the cutting edge of a blade or the like, following the efflux of time needed to produce the required thickness of titanium carbo-nitride coating, the supply of methane to the chamber can be discontinued to leave physical vapour deposition in the presence of nitrogen to overlay the coating with a further coating of titanium nitride. Thus, physical vapour deposition in the presence of nitrogen alone can be, for example, for 10 minutes to provide an overlayer of titanium nitride 0.1 to 1.0 micron thick. This then masks the relatively dull colour of titanium carbo-nitride with the gold colour of titanium nitride. Equally, a chromium may be employed as the cathode or the metal in the crucible, to provide a coating of chromium carbo-nitride.

The invention will now be described in relation to the accompanying drawings, in which:

Figure 1 is a schematic cross-sectional side elevation of a treatment chamber for effecting the process of the invention;

Figure 2 is a plan view of Figure 1;

Figure 3 is a schematic plan view of a second process chamber for effecting the process of the invention; and

Figures 4 to 6 are graphical representations of comparative testing as between identical blades, one of which was coated in accordance with the invention, and the other uncoated.

In Figures 1 and 2 a treatment chamber 1 has located therein a jig array 2 the jig array being located on and supported by a drive shaft 3. Located within the chamber are four titanium cathodes 4, in equal spaced relationship around the chamber, and mounted on the chamber, are four magnet/electromagnet arrays 5. Each jig member of the jig array is formed by a number of support plates 6 which, and as is shown more particularly by Figure 2, have a number of sides (five illustrated) each of stepped configuration, to accept a number of blades 7 in overlapping relationship and with direct contact between adjacent blades, the blades being held in place by a clamp 8. The first blade may serve as a mask but preferably the first blade is overlaid by a masking plate 9. The disposition of the masking plate and the blades on a particular face of the support plates is such that only the edge of the blade to be coated is exposed.

An electrical connection is made as between the cathodes 4 and the jig array, such as to ensure a required potential difference as between the cathodes and the jig array.

During operation of the process the jig array is caused to rotate by the shaft 3 at a required rate, and additionally each jig caused to rotate intermittently about its own axis by providing a gear means 10 attached to the jig shaft to contact a rack means 11 located in the casing and such that during each rotation of the jig array, each jig is caused to self-rotate by, for example, one-quarter turn. With a current impressed on the cathodes there is the release of ions, and with a potential difference as between the cathodes and the blades ions are directed at the blades to bombard the exposed edges of the blades to be coated. Simultaneously there is the release of nitrogen and a carbon-carrying gas such as methane, with the resultant applicatiron to the exposed surface of the blades of a carbo-nitride coating by physical vapour deposition.

The presence of the magnet/electromagnets 5 causes the provision of a force field to combat random dispersion of ions from the cathode and to ensure that a greater proportion of the ions are directed towards the blades on the jig array.

The control over the potential difference as between the cathodes 4 and the blades 7 on the jig array is such that the coonsequential heating of the blades is held to below the tempering temperature of the blades, i.e. to approximately 350°C and which ensures that the carbo-nitride deposition on the exposed surfaces of the blades is by the growth of columnar crystals.

As is illustrated schematically in Figure 3, here it is the case of the provision of a chamber 12 in the bottom of which is provided a crucible 12A to hold titanium metal. Here, jigs of essentially similar character to those of Figures 1 and 2 are located on overhead conveyor means 13 to hang down in the chamber. To prevent any unwanted swinging of a jig, a lowermost track means may be provided to guide the jigs. In common with the construction of Figures 1 and 2, a rack and gear means 14 may be provided in similar manner to the rack and gear means 10/11 of Figure 1, to cause each jig to rotate about its own axis intermittently. In common with what is described in relation to Figure 1, an impressed current is provided on the crucible 13 and each jig array and hence the blades on each jig array to create a required potential difference. In accordance with known practice, an attendant electron beam means is activated to evaporate the metal in the crucible with the simultaneous release of a carbon-carrying gas such as methane, and nitrogen with the result that the ions released from the metal react with the gas to create a plasma that is directed at the blades on the jig arrays to generate a carbo-nitride coating,, the potential difference between the crucible and the jig arrays/blades being controlled such that the temperature induced in the blades is held below the tempering temperature i.e. to approximately 350°, and to guarantee that the applied coating is in columnar crystal form.

In both processes, the duration is controlled to provide a required thickness of coating. Desirably the thickness of the coating is held to between 8 and 12 microns with a process duration of approximately three to eight hours.

It will be understood that prior to location in the jigs, each blade had been subjected to grinding of one face of a V-shaped cutting edge. Subsequent to the coating process, the second face of the edge is ground to result in a cutting tip formed wholly of the coated material.

With titanium as the cathode and the metal in the crucible, with a resultant titanium carbo-nitride coating applied to the blade edge, its colouration may not be aesthetically acceptable. Consequently, and before the termination of the process, the supply of a carbon-containing gas such as methane to the chamber, is suspended and nitrogen continued to be supplied and such as to apply a final surface coating of titanium nitride of a colour more aesthetically acceptable. in the circumstance where another metal is employed, such as, for example, chromium, a resultant coating of chromium carbo-nitride is of a colouration substantially identical to that of the blade and hence virtually impossible to detect by the naked eye. Consequently, it may be desirable again to suspend the delivery of a carbon-containing gas before the end of the process but with the continued supply of nitrogen, to provide a final surface coating of chromium nitride of an acceptable colouration but more to signal to the user that it is a hard coated and exceedingly sharp knife that is being employed.

Blades made in accordance with the process above defined were subjected to test in accordance with the proposals in the draft European Standard Test Procedure NO.CEN/TC194WG4. Each test, the results of which are portrayed in Figures 4 to 6, compared respective identical blades with an edge configuration in accordance with European Patent No.0220362. Each comparison blade was left uncoated, and the blades of the invention coated to the serrated side of their edge, the plain side of the edge being the final grinding stage after coating.

It will immediately be seen from the graphical presentations in Figures 4 to 6, that the blades of the invention have cutting characteristics that are a major improvement over the comparison blades and when it is borne in mind that the comparison blades made in accordance with European Patent No. 0220362 constituted a major advance over conventional blades, at its date, the significance of the improvements provided by the invention will be appreciated.

As will also be immediately apparent from the graphical presentations of the test results, the testing of the comparison blades continued until such time as the edge was blunt by the cutting of 230, 200 and 400 cards, whereas the testing of the blades of the invention were curtailed considerably prior to any indication that the blade edge was degrading, with the tests of Figures 4 and 5 suspended after the cutting of 1000 cards and the test of Figure 6 suspended after the cutting of 450 cards. The clear absence of any significant degrading of the cutting blades of the invention establishes that the cutting edges of the blades was being subjected to an effective renewal during cutting by micro- fragmentation of the exposed tip of the coating material encouraged by the effective flaw lines at the junction of adjacent columnar crystals and with attendant micro-wear of the ground face of the edge of the blade immediately behind the exposed coating material, but which micro-wear did not detract from the support provided by the blade material perpendicularly behind the cutting tip.

The tests as illustrated in Figures 4 to 6 were conducted employing titanium as the metal of the cathodes or in the crucible, and such that the coating was of titanium carbo-nitride. It will be fully understood that other metals can be employed, such as, for example, chromium and vanadium.

Claims

1. A process for the production of a carbo-nitride coating comprising grinding one face of a V-shaped cutting edge of a knife blade or other cutting implement, locating the part-ground blank in a chamber provided with a means able to produce ions of a metal carbo-nitride, and subsequent to the coating process final grinding a second face of a V-shaped cutting edge, characterised by the blank being oriented such that the face to be coated is directed towards the cathode evacuating the chamber, impressing a current on a metallic element of the required metal to induce ion release, and to provide a required potential difference between the metallic element and the blank to cause ion bombardment of the blank at least over the area of the said ground face, said ion bombardment being controlled by the potential difference to induce heat in the blank to a level below the tempering temperature of the metal of the blank, releasing nitrogen and carbon-carrying gas, such as methane, into the chamber to apply a carbo-nitride coating to the face to be coated by physical vapour deposition at a temperature to cause the growth of columnar crystals, removing the blank from the chamber and grinding the second face of the V-shaped cutting edge to expose the carbo-nitride coating at the cutting tip of the cutting edge.

2. A process for the production of a carbo-nitride coating as in Claim 1, characterised in that the metallic element of the required metal is a strategically located cathode in the chamber there being an impressed current on the cathode causing the release of ions and an impressed current on the knife blank to provide a potential difference as between the cathode and the knife blank.

3. A process for the production of a carbo-nitride coating as in Claim 2, characerised in that a magnet system is associated with the chamber to provide a force field to control the random dispersal of ions and increase the degree to which the blank is bombarded.

4. A process for the production of a carbo-nitride coating as in Claim 1, characterised in that the metallic element of a required metal is contained in a crucible strategically located in the chamber, with an attendant electron beam means to evaporate the metal in the crucible to cause the release of ions there being an impressed current on the crucible and on the blank to provide a required potential difference and to cause a plasma created by the reaction of the ions and the gas to be directed at the blank to a degree dictated by the potential difference.

5. A process for the production of a carbo-nitride coating as in any of Claims 1 to 4, characterised in that the blank is located in the chamber in fixed and parallel relationship to the location of the metallic element.

6. A process for the production of a carbo-nitride coating as in any of Claims 1 to 4, characterised in that the blank is located in the chamber in fixed perpendicular relationship to the location of the metallic element.

7. A process for the production of a carbo-nitride coating as in any of Claims 1 to 4, characterised in that the blank is located in the chamber in fixed angular relationship to the location of the metallic element.

8. A process for the production of a carbo-nitride coating as in any of Claims 1 to 4, characterised in that the blank is mounted for rotational and/or orbital movement in relation to the metallic element.

9. A process for the production of a carbo-nitride coating as in Claim 8, characterised in that the blank is mounted on jig means for location in the chamber, the jig means being capable of rotational and/or orbital movement.

10. A process for the production of a carbo-nitride coating as in Claim 9, characterised in that the jig means has a support carrying a number of jig plates having a multiplicity of edges, each edge being correspondingly profiled to enable a number of blanks to be located on each side in overlapping and abutting relationship and whereby each blank serves as a mass for the blank immediately behind to leave the edges of the number of blanks exposed, and there being clamp means to hold the blanks to the jig plates.

11. A process for the production of a carbo-nitride coating as in Claim 10, characterised in that a blanking plate is provided to overlay the first blank on each edge of each jig plate to leave the edge of the first blank exposed.

12. A process for the production of a carbo-nitride coating as in any of Claims 8 to 11, characterised in that each jig is mounted for rotation about the axis of the support.

13. A process for the production of a carbo-nitride coating as in any of Claims 8 to 11, characterised in that each jig is mounted for orbital movement around the chamber.

14. A process for the production of a carbo-nitride coating as in any of Claims 1 to 13, characterised in that towards the end of the process, there is the cessation of the supply of carbon containing gas and the continued supply of nitrogen to the chamber and whereby to provide a final coating of the nitride of the required metal on the coating of carbo- nitride.

15. A process for the production of a carbo-nitride coating as in any of Claims 1 to 13, characterised in that the temperature of the blank during the process is held to between 200°C and 550°C.

16. A process for the production of a carbo-nitride coating as in Claim 15, characterised in that te temperature of the blank during the process is held to 350°C.

17. A process for the production of a carbo-nitride coating as in any of Claims 1 to 16, characterised in that the duration of the process is between 3 and 8 hours to provide a coating thickness of between 8 and 12 microns .