GB2061325A - Chemical vapour deposition coating of razor blades - Google Patents

Abstract

A process for the formation of metal and metal compound coatings on cutting edges by chemical vapour deposition, in which a static electric field is established between the cutting edge and a counter electrode positioned in the deposition chamber, the field potential and gas pressure being such that glow discharge does not take place and the field potential additionally being such that a dense adherent coating is obtained having an aspect ratio of more than 1. Carbides of Cr, Ti and Mo may be deposited on stainless steel from the carbonyls.

Description

SPECIFICATION Formation of coatings on cutting edges and coated cutting edges This invention is concerned with the formation of metal and metal compound coatings on cutting edges, particularly those of razor blades, by chemical vapour deposition and with coated cutting edges.

Chemical vapour deposition (CVD) is a known technique for forming coatings of metals and metal compounds on a variety of substrates; it has been used, for example, in the production of tantalum and tungsten coatings on steels, silica coating on silicon, and titanium carbide and nitride coatings on cemented carbide tools.The technique essentially comprises contacting the substrate to be coated with the vapour of a compound of the coating metal and, where a metal compound coating is to be formed which cannot be obtained by decomposition of the vaporised coating metal compound, a vaporised or gaseous compound which contains the additional element(s) required for the desired metal compound coating and where it is required to reduce the coating metal compound and/or the additional element(s) containing compound, hydrogen, while maintaining the substrate at a temperature at which decomposition or reaction of the compound(s) takes place with formation of the desired coating.The use of CVD to form metal and metal compound coatings on razor blade cutting edges is described in British Specification 1 41 6 887; this specification gives examples of a number of different coatings and the compounds which can be used to form them.

CVD is carried out in a suitable deposition chamber at an elevated temperature and usually at a reduced pressure (as required or desirable in order to vaporise the compound(s) used). The deposition chamber may form part of a closed system with the means for supplying the vaporised metal compound or a flow system may be used in which fresh vaporised metal compound is supplied to the deposition chamber and decomposition or reaction products are removed therefrom. The latter type of system is usually preferred because it is more versatile, easier to operate and permits a greater degree of control to be maintained over the deposition parameters.

In the following description reference will be made to the accompanying drawings, in which: Figures 1 a and 1 b are diagrammatic sections of the tip of a razor blade cutting edge which has been coated by conventional CVD (1 a) and illustrating the re-sharpening thereof (1 b), Figures 2a and 2b are similar sections of the tip of a razor blade cutting edge which has been coated by the process of the present invention (2a) and illustating the re-sharpening thereof (2b), Figures 3 and 4 are photomicrographs at magnifications of xl 000 and x2000 respectively of tree growths as described below, and Figure 5 is a curve obtained by plotting minimum voltage required to initiate glow discharge against operating pressure.

We have found that when conventional CVD conditions are used to form coatings on razor blade cutting edges which are of sufficient thickness (that is more than about 0.1 micrometers) to contribute significantly to the strength of the edge, the tip radius increases in proportion to the coating thickness and there is a significant loss of sharpness which is such as to necessitate re-sharpening of the blade.

This is illustrated diagrammatically in Figures la and 1 b.

These Figures show the original sharpened cutting edge 10 and the CVD coating 11 thereon. In Figure 1 b, the hatched area 1 2 represents the portion of the coating 11 which has to be removed in order to re-sharpen the edge.

We have found specifically that CVD carried out under conventional conditions produces approximately equal thickness of coating on the tip, T, and on the facets, F, which may be.expressed in the form that the aspect ratio, T/F, of the coating is 1.

Since the primary interest in the CVD coating of cutting edges is the formation of very hard coatings, such as chromium carbide, molybdenum carbide or titanium carbide coatings which have hardnesses ranging from about 2000 to 3600 Hv, it will be appreciated that the re-sharpening of such coated cutting edges is difficult and time-consuming.

We have further found that the aspect ratio of a CVD coating on a cutting edge can be increased to a significant extent, for example to 2, by carrying out CVD with a static electric field established between the cutting edge and a counter electrode positioned in the deposition chamber, the field potential and gas pressure being such that glow discharge does not take place and the field potential additionally being such that a dense adherent coating is obtained. The effect of obtaining coating aspect ratios having values greater than 1 is to decrease the amount of coating material that has to be removed in resharpening the edge.This is illustrated in Figures 2a and 2b wherein the reference numerals have the same significance as in Figures 1 a and 1 b, it will immediately be seen that far less material has to be removed in order to resharpen the edge than is the case when the aspect ratio of the coating is 1 as shown in Figures lea and 1 b.

According to the present invention, therefore, we provide a process for the formation of metal and metal compound coatings on cutting edges by chemical vapour deposition, in which a static electric field is estabilished between the cutting edge and a counter electrode positioned in the deposition chamber, the field potential and gas pressure being such that glow discharge does not take place and the field potential additionally being such that a dense adherent coating is obtained which has an aspect ratio of more than 1 and, preferably, about 2.

The present invention further comprises a razor blade having a cutting edge which has a metal or metal compound coating thereon, the thickness of the coating on the tip of the cutting edge being greater than that on the facets of the cutting edge. The aspect ratio of the coating is preferably about 2.

The counter electrode may take any convenient form. One arrangement we have found convenient is to introduce the vaporised compound(s) and any inert gas used in the CVD process into the deposition chamber through one or more metal inlet pipes which are electrically insulated from the deposition chamber and are connected as the counter electrode. In a preferred arrangement of this kind, each gas inlet pipe is arranged parallel to the longitudinal axis of a stack of razor blades to be treated and has a row of holes along its length through which the vapour/gas passes out of the pipe, the row being substantially coextensive with the length of the razor blade stack. One or more exhaust gas outlet pipes are also provided which have similar rows of holes and are also arranged parallel to the longitudinal axis of the razor blade stack.In a system for treating double edge blades, the inlet holes preferably face the cutting edges and the exhaust holes preferably face the ends of the blades. In this way, the vapour/gas flows through the deposition chamber in a direction parallel to the blade edges and concentration gradients along the length of the blade stack, which would produce poor coating uniformity, and thereby prevented.

The field potential may be either positive or negative with respect to the cutting edge being treated. It is generally convenient to connect the cutting edge to earth- and to make the counter electrode either anodic or cathodic relative to the cutting edge.

As indicated above, the field potential and the pressure in the deposition chamber should be such that glow discharge does not take place. For any particular operating pressure, it is found that there is a minimum voltage which will initiate-flow discharge and a field voltage of less than this minimum must be used. A curve showing the relationship between operating pressure and the minimum voltage to initiate glow discharge for a particular set of CVD conditions is referred to below, by way of example; those skilled in the art will have no difficulty in determining what field voltages can be used for other sets of operating conditions.

There is a further limitation upon the magnitude of the field voltage. If it is too high, it is found that a dense adherent coating is not formed at the tip of the cutting edge, but rather a relatively thin coating which is characterised by the formation of nodular "trees" extending several millimetres from the cutting edge tip. Figures 3 and 4 of the accompanying drawings are photomicrographs at magnifications of x 1000 and x2000 respectively of typical tree growths of this kind. The tip of the cutting edge can be discerned in Figure 3 as the line running substantially diagonally across the photograph to which, at one point, the bese of the "tree" is attached.

The voltage at which this phenomenon occurs depends on the particular CVD conditions used, but can be readily determined by routine trial by those skilled in the art. Higher positive voltages can be used than negative voltages, that is the phenomenon of tree growth occurs at smaller negative voltages than positive voltages. Suitable blade voltages will usually be up to +2kV and up to -0.5kV.

In order that the invention may be more fully understood, the following example is given by way of illustration only.

EXAMPLE CVD was carried out on a stack of sharpened stainless steel razor blades in a glass deposition chamber which could be evacuated to a controlled pressure using Edwards pumping system and pressure controller. The blade stack was held on an electrically heated knife, the temperature of which was controlled by a Eurotherm proportional controller via a thermocouple welded to one of the blade edges. The vaporised metal compounds (Cr(CO)e and Mo(CO)6, see below) and an inert carrier gas (nitrogen) were introduced into the chamber via two inlet pipes, each parallel to the stack and having a row of holes facing th-e blade edges. Gaseous reaction products were removed from the chamber through two outlet pipes, also parallel to the stack and having a row of holes facing the blade ends and positioned on the other side of the stack from the inlet pipes.

The two inlet pipes passed through and were electrically insulated from the base plate of the chamber and were connected to a high voltage d.c. supply via a vacuum connector in the base plate.

The inlet pipes could be made either anodic or cathodic relative to the blade stack which was held at earth potential.

CVD of chromium/molybdenum carbide is typically carried out under the following conditions L substrate temperature 3750C vapour/gas supply: nitrigen saturated with Cr(CO)6 and Mo(CO)e at 300C flow rate: 100 mls/min chamber pressure: 0.4 torr deposition time: 30 min.

This produces an alloy carbide coating 2 jum thick with a typical composition, by weight, of 80% Cr2C and 20% Mo2C.

A series of runs was first carried out with the apparatus and system described above and using the typical chromium/molybdenum carbide CVD conditions just mentioned, except that the chamber pressure was varied. The runs were carried out at various voltages and chamber pressures to determine the minimum voltages required to initiate glow discharge over a range of pressures from 1 to 35 torr. The results obtained are shown graphically in Figure 5 which is a curve obtained by plotting the minimum voltages, in kV, against the pressure, in torr. It will be understood that all combinations of voltage and pressure above and to the left of the curve will give rise to glow discharge, while all combinations below and to the right of the curve will not.

At the higher pressures used, that is at 30 torr and above, the rate of deposition was reduced and to obtain a coating 2 Mm thick, it is necessary to increase the deposition time to 2 hours.

A further series of runs was then carried out using voltages ranging from 0 to +4kV and from 0 to -4kV and in each case a chamber pressure such that glow discharge did not take place. Following completion of deposition, sample blades from each run were sectioned perpendicularly to the cutting edge by a standard metallographic technique and the sections were examined with a scanning electron microscope and the aspect ratios, T/F, of the coatings were measured.

The results obtained are summarised in the following Table.

Run Blade No. Voltage T/F 1 +4kV - Tree growth 2 +3kV 22 r, 3 +2kV 1.88 Normal deposit 4 +1.5kV 1.91 5 +1kV 1.81 6 +0.5kV 2.00 ,, SI 7 0 1.20 SI IS 8 4.5kV 2.00 9 -1kV - Tree growth 10 -2kV - ..

11 3kV 12 1kV - .. .. S,

Claims (8)

1. A process for the formation of metal and metal compound coatings on cutting edges by chemical vapour deposition, in which a static electric field is established between the cutting edge and a counter electrode positioned in the deposition chamber, the field potential and gas pressure being such that glow discharge does not take place and the field potential additionally being such that a dense adherent coating is obtained, which has an aspect ratio of more than 1.

2. A process according to claim 1, in which the coating has an aspect ratio of about 2.

3. A process according to claim 1 or 2, in which the vaporised compound(s) and any inert gas used in the chemical vapour deposition process is/are introduced into the deposition chamber through one or more metal inlet pipes which are electrically insulated from the deposition chamber and are connected to the counter electrode.

4. A process according to claim 3 for the coating of razor blade cutting edges, in which each gas inlet pipe is arranged parallel to the longitudinal axis of a stack of razor blades to be treated and has a row of holes along its length through which the vapour/gas passes out of the pipe, the row being substantially coextensive with the length of the razor blade stack, and one or more exhaust gas outlet pipes are also provided which have similar rows of holes and are also arranged parallel to the longitudinal axis of the razor blade stack.

5. A process according to any of claims 1 to 4, in which the cuttng edge voltage is up to e2kV or up to -0.5kV.

6. A process for the formation of metal compound coatings in razor blade cutting edges according to claim 1, substantially as herein described in the Example.

7. A razor blade having a cutting edge which has a metal or metal compound coating thereon, the thickness of the coating on the tip of the cutting edge being greater than that on the facets of the cutting edge.

8. A razor blade according to claim 7, in which the coating has an aspect ratio of about 2.