Introduction
Cubic boron nitride (cBN) is an attractive material for electronic and mechanical
applications under extreme circumstances because of its large band gap energy, high
room temperature thermal conductivity, high chemical inertness, good transmittance over
a large spectral range, and exceptional strength. Because of these highly desirable
properties there has been an extensive worldwide effort to synthesize thin films of cBN.
Boron nitride, like carbon, has two main phases: hexagonal boron nitride (hBN), a
graphite-like material of low hardness, and cubic boron nitride which has the zinc blende
or diamond structure and is second only to diamond in hardness. It has the advantage
over diamond of low solubility in iron enabling it to be used for the machining of ferrous
materials.
The cubic phase of boron nitride has significant technological potential for thin film
applications. Due to its high Vickers hardness of about 5000 kg/mm2 it is highly suitable
for hard, protective coatings. The fact that cBN does not react readily with ferrous metals,
can be deposited in thin film form at low temperatures and has a high resistance to
oxidation makes it even more attractive for tooling applications.
Trends in the metal cutting industry are driven by the manufacturers need to continually
improve performance and reduce costs. The fundamental material characteristics that
enable cBN to enhance metal cutting operation are its extreme hardness, good toughness,
chemical inertness and high thermal conductivity. Today there is a significantly greater
and wider use of cBN tooling such as ball-nosed type cutters for the machining of dies
and moulds, machining of hard, medium hard and cast iron moulds.
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Deposition Techniques
Recently, new processes for the deposition of cubic boron nitride have been investigated,
wherein BN films have been deposited from the vapor phase at low temperatures. There
are many methods for preparing the c-BN thin films that can be classified mainly into
two areas: the physical vapor deposition (PVD) methods and Chemical Vapor Deposition
(CVD) methods.
Physical Vapor Deposition:
Physical vapor deposition is a thin film deposition process in the gas phase in which
source material is physically transferred in the vacuum to the substrate without involving
any chemical reactions. Electron beam evaporation, sputtering, arc vapor evaporation are
the physical vapor deposition techniques that are commonly used to deposit metals.
(a) Sputtering: This physical vapor deposition technique is commonly used to
deposit metals and oxides. It is the process of bombarding the target by highenergy chemically inert ions extracted from plasma that cause ejection of atoms
from the target, these ejected atoms from the target are then redeposited on the
surface of the substrate, which is located in the vicinity of the target. The target
here is the metal or the oxide that is to be deposited and often Argon is used as
inert gas ion.
(b) Electron Beam Evaporation: An electron beam evaporator designed for use in
ultra high vacuum environment. It provides the capability to evaporate materials
in a controlled manner at rates less than one monolayer/min to over one
micron/min. In this method, the target material is evaporated as a result of
localized heating by bombardment with high-energy electrons generated in the
electron gun, which then is directed towards the surface of the source material.
The evaporated material is very pure. The bombardment of metal with electrons is
accompanied by generation of low intensity X-rays that may create defects in the
oxide present on the surface of the substrate. To annihilate those defects the
substrate material can be annealed.
(c) Arc Vapor Deposition: Arc vapor deposition uses a high current, low-voltage arc
to vaporize a cathodic electrode or anodic electrode and then deposit the
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vaporized material on a substrate. The vaporized material is highly ionized and
usually the substrate is baised so as to accelerate the ions to the substrate surface.
This method has wide range of applications in electrical contact engineering, arc
melting of alloys.
Chemical Vapor Deposition
In chemical vapor deposition, thin film deposition occurs by initiating a chemical
reaction inside the chamber filled with reagents vaporized in an inert carrier gas. As this
carrier gas flows over the target substrate, energy supplied by the surroundings causes
diffused reagents to react thus forming the desired material thin film across the target
surface. This process is widely used to fabricate semiconductor devices. Few important
types of chemical vapor deposition methods are: Plasma enhanced chemical vapor
deposition, Rapid thermal chemical vapor deposition.
(a) Plasma Enhanced CVD: Some applications require depositing films at very low
substrate temperatures. In order for the film deposition to occur at low
temperatures, alternative source must be used to supply the activation energy and
one way to do this is to introduce plasma into the reaction chamber. Plasma
Enhanced CVD is a process of chemical vapor deposition in which species to be
deposited are generated in plasma, as a result deposition using the same source
gases is taking place at lower wafer temperature then conventional CVD where
high wafer temperature is required to break the bonds and to release desired
species from the input gases.
(b) Thermal CVD: Thermal chemical vapor deposition is the deposition of atoms or
molecules by the high temperature reduction or decomposition of a chemical
vapor precursor species, which contains the material to be deposited. Reduction is
normally accomplished by hydrogen at an elevated temperature and
decomposition is accomplished by thermal activation. The deposited material
gives compounds when they react with other gaseous species in the system.
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Experimental Procedure
Material Used
Boron Nitride is a synthetic material discovered in 19th century. Boron and Nitrogen have
the same number of outer shell electrons and are neighbors of carbon. Hence boron
nitride exhibits same crystal structure as that of carbon and has both hexagonal and cubic
forms. Hexagonal boron nitride (hBN) is a graphite-like material of low hardness, and
cubic boron nitride, which has the zinc blende or diamond structure and is second only to
diamond in hardness.
(a) Properties: Cubic boron nitride exhibits variety of highly desirable mechanical,
thermal, electrical and optical properties. Thus it has significant technological
potential for thin film applications. It is also one of the most perspective materials
for hard coatings because of its unique chemical and physical properties. Some
important physical and chemical properties of cBN are shown in table 1.
(b) Application in tooling Industry: Cubic boron nitride when deposited on a Silicon
substrate can be used in variety of electrical and thermal applications and at the
same time cBN deposited on Tungsten Carbide substrate has wide applications in
tooling industry. Its high hardness combined with low chemical reactivity with
the iron group metals and its high thermal stability makes it an exceptionally
interesting material for protective and wear-resistant coatings for cutting tools.
Among all the applications of cBN cutting tools, precision hard turning attracts
great interest since it provides an alternative to conventional grinding in
machining high precision, high hardness components in small volume production.
So the area of interest is to prepare cBN cutting tools and this can be achieved by
depositing them on tungsten carbide substrate using electron beam evaporation.
Electron Beam Evaporation of Cubic Boron Nitride
As discussed earlier cubic boron nitride can be deposited using different physical and
chemical vapor deposition techniques. Out of all the processes the most prominent one is
e-beam evaporation. The operating principle of the electron beam evaporator with the
experimental setup and different parameters to deposit cubic boron nitride are discussed
in the later sections.
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(a) Operating Principle: An electron beam evaporator is designed to use in an ultra
high vacuum environment. It provides capability to evaporate materials in a
controlled manner at evaporation rates between less than one monolayer/min to
over one micron/min. The principle behind e-beam evaporation is the use of
electron-beam induced heating of the target material to the temperature at which
the desired evaporation rate is reached. During this process, some ionization of
target vapor occurs and it is measured as ion current. This positive ion current is
used as an indicator of the evaporation rate.
(b) Experimental Setup: An electron beam evaporator has four separate pockets,
each one of which can be loaded with material either in the form of rods or can be
placed as pellets inside the crucible. For the present purpose, boron nitride pellets
are loaded into the crucible and placed in one of the pockets of the evaporator.
Each pocket has its own filament, which enables to heat the crucible to the desired
temperature. Crucibles such as tungsten that have high melting point than boron
nitride are chosen. The crucible is then held at a high potential, upto 2 kV.
Electrons emitted from the filament, at earth potential, are accelerated onto the tip
of the rod by the high field gradient between the tip and the filament. Since the
total power available is 200 W the emitted electron beam current can go upto
100mA.
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