Chemistry Reference
In-Depth Information
The environment-friendly plasma activation allows low-temperature processing, but
like vacuum technology, it is expensive.
Conventional plasma nitriding uses ion energies
200 eV [176]. Usually low-
pressure plasmas in N
2
/H
2
mixtures, sometimes also with Ar admixture, are applied
for this process. The metallic work piece is used as cathode. N
+
ions are acceler-
ated to the cathode and impinge into the cathode surface. They form there nitrides
in the compound layer (thickness of some tens μm); others diffuse into the bulk
material (diffusion zone). The hardening of the work piece results firstly from
the hard metal nitride in the compound layer and secondly from the stress gen-
erated by the nitrogen particle in the metal lattice in the diffusion zone. Besides
the energetic N
+
ions according to charge transfer reactions, fast N atoms collide
with the cathode surface. The Ar and H
2
admixture ensures an effective cleaning
of the surface. Plasma nitriding operates in a temperature range between
<
∼
∼
300
◦
C
and
600
◦
C.
Various materials are suitable for nitriding: Ti, Al, and stainless steel, as well as
alloy steels with elements that form nitrides such as Al, Cr, Mo, and Ti.
PIII is used also for implantation of other elements like Cu into Ti [177]. An
RF discharge 13.56 MHz operates between the grounded vessel and two disk-shaped
cupper electrodes in an H
2
O vapor atmosphere (2 Pa). Pulsed voltage (5 kV, 50 Hz)
was applied to Ti samples. Cu was determined by XPS in the sample and antimicrobial
properties could be demonstrated, what is important for application of this material
for endoprosthesis.
∼
8.2.3.2.3 Plasma Oxidation
Oxidation in oxygen plasmas occurs with metals that have floating potential (plasma
oxidation) or metals that are biased on a potential above floating potential (plasma
anodization)[90].Plasmaanodizationleadstohigherratesandthickeroxidefilm.The
development of Cu oxides by plasma oxidation in an oxygen discharge is studied in an
inductive coupled O
2
discharge (13.56 MHz, 300 W, 200 Pa, O
2
flow 0.25 dm
3
min
−
1
)
[178]. After 5 min treatment time, Cu
x
O is formed, then a mixture of Cu
2
O/Cu
3
O
(10 min), and later CuO (20 min) is observed. The layer thickness is about 0.3 μm.
8.2.3.2.4 Plasma Electrolysis
Plasma electrolysis is an interesting technique of surface engineering [179]. This
method is used for plasma electrolytic, e.g., oxidation, nitriding, carburizing. The
work piece is immersed in a bath of liquid electrolyte and different potentials are
applied between work piece and counter-electrode. A pulsed voltage higher than
200 V leads to an electrical breakdown between the metal and the electrolyte through
the dielectric surface film (e.g., oxide film) or gas bubbles on the surface. The
developed microplasma modifies the deposited layer or the surface, e.g., by collisions
of energetic ions. Plasma electrolysis uses different electrolyte compositions for the
various applications (see Table 8.9).
Thick wear resistant oxide layers on Al with maximum hardness 18-23 GPa
and thickness about 500 μm have an excellent adhesion. Nitriding, carburizing, and
