A method for coating steel components with daimond by chemical vapour deposition (CVD) and steel components obtained thereby
In order to increase the hardness of articles steel can be coated with diamond. Diamond coated steel components find applications in cutting or grinding tools, abrasive wheels, end mills or load bearing elements such as, ball-bearings or types of inserts. Steel components are coated with diamond by the chemical vapour deposition technique comprising of heating the components at high temperatures up to 500-900 degrees celsius in a vacuum chamber under flow of reactant gases (hydrocarbon, hydrogen and carbon dioxide) at low pressure of 20-100 Torr within the chamber. CVD may be carried with a heating filament of 400-1000 Watts. After cooling the surface is ready.
This process does not make the quality of steel hardness as good. This is because carbon is highly soluble in iron at high temperatures used in CVD of diamond on bare steel, the diamond precursors diffuse into the steel. Diffused carbon atoms, due to the catalytic effect of iron in steel, form graphite clusters which are deposited on the steel surface. Diamond is deposited further on the graphite phase instead of the steel. The adhesion of diamond on graphite is poor, making the deposition of diamond on the steel surface of less than desired hardness. To circumvent this problem method of pre- coating the steel layer prior to CVD with titanium, nickel, chromium nitride and molybdenum have been investigated but they have caused other problems and have not helped much. The main object to improve hardness would be to improve the adhesion of diamond on steel.
They have devised a method to coat steel that improves the quality of diamond coat on steel components thus enhancing adhesion and resultant hardness.
First the steel surface is cleaned by washing components with detergent, followed by deionized water. Next the components are washed with an aqueous alkali solution and then deionized water. Lastly to remove oxides on the outer layer of steel electro-cleaning or anodic is done using a sulphuric acid bath. The clean component is further electroplated with a strike layer of nickel 0.05 to 1.2 microns thickness. It is then further electroplated with a nickel- diamond composite layer of 15 to 30 microns thickness. On this processed layer a chromium layer of 3-8 microns thickness is added also by electroplating. The next step is to use chemical vapour deposition coating of diamond as known.
The three pre-layers are thermally stable. The nickel layer enhances the second nickel-diamond coating, chromium helps adhering of diamond very well. These layers have helped to solve the problem of poor adhesion due to formation of graphite cluster on bare steel formed during CVD. Carbon does not diffuse into steel because the three layers of chromium, nickel-diamond and nickel form a barrier. Hence, adhesion on steel components has been drastically improved. The adhesion was verified using scanning electron microscopy, Raman spectroscopy, and micro-hardness gauging methods showing good adhesion thus greater hardness.
Industrial application: Tools industry
1. Grinding /cutting tools
2. Abrasive wheels
3. End mills
4. Ball bearings(medical and dental instruments also)
- Devi Shanker Misra, Arun Kumar Sikder