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
Industrial application: Tools industry
1. Grinding /cutting tools
2. Abrasive wheels
3. End mills
4. Ball bearings(medical and dental instruments also)
Indian patent application no. 377/MUM/2000 Patent grant no. 200842
Inventors: Devi Shanker Misra and Arun Kumar Sikder