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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.
Sputtering: This physical vapor deposition technique is commonly used to deposit metals
and oxides. It is the process of bombarding the target by high-energy 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.
Electron beam evaporation: 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.
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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 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.
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.
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
chemical vapor deposition.
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.
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.
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.
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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
ion beam assisted deposition.
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. A
schematic electron beam evaporator gun is shown in fig (1). 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.
Fig 1: Schematic of an electron beam evaporator gun
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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. The typical operating conditions of an e-beam evaporator are shown in table 1.
Table 1: Operating conditions of an Electron beam evaporator
Item
Conditions
Chamber pressure
5X10-7 Torr
Substrate temperature
4000C
Filament temperature
20000C
Electron beam energy
2kV
Source to target distance
35 cm
Results and Discussion
Cubic boron nitride deposition using an electron beam evaporator occurred at an emission
current of 35 mA and it was maintained at the same emission current for about 5 mins to
obtain thin films of cubic boron nitride having a thickness of about four monolayers at a
rate of 0.2 nm/sec. Fourier Transform Infrared Spectroscopy (FTIR) was performed to
observe the composition of the film. The FTIR spectra (fig 2) shows the sharp peaks or
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bands at 750 and 1090 cm-1. The band at 750 cm-1 is attributed to B-O stretching
vibration and the peak at 1090 cm-1 is due to cubic boron nitride (Ratna Phani et al).
Fig 2: FTIR spectra of a cubic boron nitride thin film
Cubic boron nitride is much more suitable as a cutting tool for all kinds of steels than
diamond as diamond is not stable in the presence of Fe, N, and O at high temperatures.
So to further investigate the applicability of cBN in tooling hardness tests were
performed using nano indenter and it was found that the cBN films had microhardness
values between 53 and 60 GPa on tungsten carbide substrates (M.Keunecke et al., 2001).
Conclusions
From the results it can be concluded that the cubic boron nitride films can be deposited
by electron beam evaporation of solid boron nitride. Characterization of films using FTIR
reveals that there is a formation of cBN on tungsten carbide substrate at an evaporation
temperature of nearly 20000C and nano indentation tests confirmed that cBN films are
hard and their hardness is next only to diamond thus proving its high potential in
applications such as wear resistant tool coatings, and tool inserts.
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References
A. R. Phani, G. N. Chaudhari, S. Manorama, Journal of Solid State
Chemistry, 118 (1995) 99.
[2] K. Sell, H. Holleck, H. Leiste, M. Stuber, S. Ulrich, J. Ye,
Diamond Relat. Mater., 11 (2002) 1272.
G. Schwarz, F. Friess, G. K. Wolf, Surf. Coat. Technol., 125
(2000) 106.
[4] J. Ullmann, D. Heyden, G. Schwarz, G. K. Wolf, D. Baba, R.
Hatada, Surf. Coat. Technol., 97 (1997) 281.