Chemistry Reference
In-Depth Information
for milling and where sharp edges are needed, e.g., finishing-cut operations. The
grain microstructures are finer and the microhardness is higher. Coatings deposited
with PEPVD are not limited to equilibrium compositions. Innovations in the coating
system process, e.g., incorporating of pulsed dc power, have evidently solved the
PEPVD process difficulty to deposit insulating coating materials [398]. Isolating
coatings with appropriate properties can be deposited by magnetron sputtering in a
stable regime in pulsed mode only [399].
Because of the lower pressure in case of PEPVD, the deposition of three-
dimensional substrates is more difficult than in the case of CVD. PEPVD processes
are highly suitable for multilayer hard coatings. Unfortunately, multilayer designs
are still uncommon.
8.2.4.2.4.3 Plasma-Enhanced Deposition of TiN and TiC
One of the all-purpose deposition materials for wear protection is TiN. TiN belongs
to the covalent bond type, but crystallizes in the rock-salt (NaCl) structure type. How-
ever, its properties are dominated mainly by the defect structure. Lattice distortions
and microcrystallinity enhance the hardness of TiN layers.
Titanium nitride can be formed by conversion of titanium chloride in plasma in
the presence of hydrogen and nitrogen, or ammonia (NH
3
). The formation of titanium
nitride from interacting with hydrogen and nitrogen is described by
1
2
(
(
TiCl
4
)
gas
+
2
(
H
2
)
gas
+
N
2
)
→
(
TiN
)
solid
+
4
(
HCl
)
gas
.
(8.75)
By the application of NH
3
, TiN can be deposited according to
1
2
H
2
→
(
(
TiCl
4
)
gas
+
(
NH
3
)
gas
+
TiN
)
solid
+
4
(
HCl
)
gas
.
(8.76)
If the deposition is supported by a plasma, the deposition temperature can be
reduced to about 500
◦
C [400] in comparison to 900
◦
C in normal CVD. A
deposition temperature of 300
◦
C is sufficient if a metalorganic precursor like
tetrakis-(dimethylamido)titanium is used in PECVD.
For PEPVD of TiN, different methods like low-voltage electron beam, cathodic
arc, hollow cathode arc, and reactive magnetron sputtering can be applied. Especially
magnetron sputtering has been a well-approved coating technique. The deposition of
appropriate TiN coatings requires a plasma density that cannot be attained with con-
ventional DC magnetrons. For hard TiN coatings, bias currents of about 10 mA/cm
2
are essential. Higher ion current density at the substrate improves the hardness of
reactive-sputtered TiN layers. An increase of the ion current density on substrates
in magnetron sputtering can be achieved by additional discharges, such as the usage
of unbalanced magnetrons [401] or pulsed magnetron sputtering [399]. In case of
unbalanced magnetrons, deposition rates of 500 nm/min were reported. Normally, an
external heating of the substrate is not necessary. Nevertheless, the substrate will be
heated by the plasma itself. In case of a 5 μm layer, the substrate reaches temperatures
between 250
◦
C and 300
◦
C [402]. In case of pulsed magnetron sputtering, frequencies
up to 100 kHz are used. This technique allows the deposition of TiN coatings with
remarkable improved properties at 450
◦
C.
