GB2099806A

GB2099806A - Diamond-like material

Info

Publication number: GB2099806A
Application number: GB8214642A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-05-20
Filing date: 1982-05-19
Publication date: 1982-12-15
Also published as: GB2099806B

Abstract

A grinding and polishing material showing diamond-like characteristics is formed by depositing a layer of hard carbon onto a substrate, such as glass or stainless steel, in a glow discharge system. The substrate is chosen to give a weak bond to the carbon layer and may be flexible to assist in removing the carbon layer. As the layer thickness increases on a glass substrate the layer spontaneously breaks into small platelets on exposure to air. These platelets can be used as formed or crushed to form a powder. Hard carbon grown in a glow discharge system.

Description

SPECIFICATION Grinding and polishing material The invention concerns a grinding and polishing material.

One material frequently used in grinding and polishing is diamond in the form of small and carefully graded particles. This can be used as a free abrasive, generally in the form of a slurry, on a variety of surfaces, designated laps or polishers and appiied with friction to the surface to be worked -- either by hand or by machine.

Diamond can also be used as a fixed abrasive by embedding it in a suitable matrix. In both cases the surface finish obtained is dependent on the diamond particle size which must be uniform.

Diamonds are obtained by mining or can be synthesised by very high pressure techniques and particles are obtained by crushing and grading.

Other materials may be used for grinding and polishing, e.g. alumina, silicon carbide, boron nitride, but their hardness is below that of diamond and for certain materials their polishing efficiency is less. In general terms the complexity and cost of their production increases as they approach the properties of diamond.

The present invention provides a hard diamond like material, for use in grinding and polishing, without the need to use conventional very high pressure equipment.

According to this invention a grinding and polishing material is formed by growing a layer of hard carbon onto a substrate in a glow discharge system. The substrate is chosen to have a weak bond to the carbon layer so that growth induced stress in the layer cause the carbon to detach from the substrate in small platelets of a uniform thickness.

Suitable substrates are glass, metals, such as stainless steel, etc. and may be made sufficiently thin as to be deformable to assist in detaching the carbon layer.

A substrate is placed on a cathode plate in a vacuum chamber into which a hydrocarbon gas is admitted at low pressure, while a D.C. or A.C. (of suitable frequency) glow discharge is formed between the cathode and an anode electrode mounted nearby. lonisation of the gas causes carbon ions to strike the substrate and form a layer whose thickness depends on time, applied voltage and hence current, and gas pressure.

Layers of more than a certain thickness spontaneously break into small platelets which become detached on exposure to air. These hard carbon platelets although having a different crystalline form a physical and chemical properties similar to those of diamond.

An alternative method of producing hard carbon films which can be removed in platelet form is to mount the substrate on the anode electrode which is maintained at a temperature of 250 to 3000C. The glow discharge is run as before, using a carbon or aluminium covered cathode for example. The anode grown layers are virtually stress free and may need to be removed mechanically or by etching the substrate away.

Growth of hard carbon onto an anode is described in U.K. Patent Application No.8001383.

As a second alternative the cathode and anode deposition techniques may be combined and run simultaneously.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a glow discharge apparatus for growing hard carbon onto a cathode; Figure 2 shows a glow discharge apparatus for growing hard carbon onto an anode.

A glass substrate 1 is placed on a metal cathode 8 in an air tight metal container 2 which is first evacuated by a vacuum pump 3 to about 10-5 Torr. A hydro carbon gas such as butane, methane or propane is then continuously bled into the container 2 via an inlet pipe 4 and a control valve 9 and the pressure maintained at about 10-2 Torr by throttling the pump 3. The container 2 is connected to earth whilst the cathode 8 is connected via an insulated connector 5 to a 1,000 Volts RF supply 6 via a series capacitor 10.

As a result ionisation of the gas takes place with consequential dissociation of carbon and hydrogen ions and the cathode attains a negative potential of around 1,000 Volts. Carbon ions impinge on the substrate 1 to form a carbon layer 7 having a time dependent thickness. For example a 1 ,um thick layer 7 is formed in about 60 minutes, with an RF power of about 100 watts and a cathode of 120 mm diameter.

Stresses in the carbon layer 7 cause it to fracture into small platelets on opening the container which may then be removed.

Alternatively the container 2 may have a hopper (not shown) and two interlocking vacuum tight doors so that growth of hard carbon may be continuous using a plurality of substrates rotated sequentially into a growth position then onto discharge position to tip the platelets into the hopper.

The platelets are all of similar thickness and may be used as formed. They may alternatively be crushed into finer powder and graded if desired.

As an alternative to RF glow discharge DC glow discharge may be used. In this technique the RF supply is replaced by a DC supply at 2000 Volts with a series current limiting resistor.

Operating conditions are the same as for RF operation. A grid may be mounted above the substrate to collect secondary electrons to minimise charging of the insulating substrate.

Figure 2 shows an alternative method of producing the hard carbon layer. The substrate 1 is fixed on an anode 11 spaced typically 50 mm from the cathode which is carbon or aluminium covered. The anode 11 is heated by a heater 1 2 to avoid trapping hydrogen in the grown layer.

Growth of the layer 7 on the anode is from carbon ions in the ionised gas that strike the cathode and bounce off onto the substrate 1.

Such a growth technique is described in U.K.

Patent Application No. 80 01383.

Claims

1. A method of producing a grinding and polishing material comprising the steps of growing a layer of hard carbon on a substrate in a glow discharge system and removing the carbon layer to form small pieces of hard carbon.

2. The method of claim 1 wherein the substrate is grown until growth induces stresses cause the layer to break into small platelets whilst growing or on subsequent exposure to air.

3. The method of claim 1 wherein the substrate is flexible so that the carbon layer may be detached and broken into platelets by flexing of the substrate.

4. The method of claims 2 or 3 wherein the substrate is a glass material.

5. The method of claims 2 or 3 wherein the substrate is stainless steel.

6. The method of any one of claims 1 to 5 wherein the platelets are crushed into a finer powder.

7. A grinding and polishing material of hard carbon platelets formed by the method of any one of claims 1 to 5.

8. A grinding and polishing material of hard carbon powder formed by the method of claim 6.

9. A material according to claim 7 formed substantially as hereinbefore described with reference to the accompanying drawings.