Medical Design Briefs - January 2022 - 18

Implantable Metals
L
mm. With less wall mass, the furnace
requires less energy and time to heat,
while still protecting workers that may accidentally
touch the outside of the chamber.
With better overall control, the
PulsPlasma nitriding furnaces offer multiple
heating and cooling zones with each
controlled by its own thermocouple. This
will create a very uniform temperature distribution
within plus or minus 5 °C from
the bottom to the top of the furnace.
Uniformity of temperature within a
chamber pays a dividend beyond the
consistency of nitriding results. With an
even temperature throughout the chamber,
the entire space is available for loading
components, which effectively in -
creases the chamber's capacity.
Stainless Steel
Plasma nitriding has a broad appeal to surface treat many metallic materials used in medical applications,
such as titanium alloys, as well as ferritic and austenitic stainless steel. (Credit: PVA TePla)
become a focal point for additional innovations,
and broader application in the
medical device field, particularly for
manufacturers who seek a more environmental
and safe solution.
In pulse plasma nitriding, parts are
processed by loading components into a
vacuum vessel. Components are first positioned
on a support structure or grate.
Spacing is maintained for treatment uniformity.
Mechanical masking can also be
used to isolate sections from treatment.
After parts are positioned, and thermo
couples are placed, a bell housing is lowered
to enclose the load. The chamber is
evacuated to below 10 Pascals prior to
heating and a pulsating DC voltage of several
hundred volts is applied between the
charge (cathode) and the chamber wall
(anode). A low-flow process gas is introduced,
and the oscillating electrical pulse
ionizes the gas. For plasma nitriding, a
mixture of nitrogen and hydrogen gases
are used, to which carbon containing gas,
such as methane, may be added.
Depending on treatment time and temperature,
nitrogen atoms diffuse into the
outer zone of components and form a diffusion
zone. This can be atomic nitrogen,
dissolved in the iron lattice, as well as in
the form of included nitrate (metallic
nitride or special nitrides) deposition.
Adding further precision, innovators
in pulse plasma nitriding have discovered
methods to optimize the process
through better control of the pulses. In
the PulsPlasma® process developed by
18
Cov
PVA TePla AG Industrial Vacuum
Systems, for example, a precision regulated
gas mixture of nitrogen, hydrogen,
and carbon-based methane is used. A
pulsating DC voltage signal of several
hundred volts is delivered in less than 10
microseconds per pulse to ionize the
gas. This serves to maximize the time
between pulses for superior temperature
control throughout the chamber.
If there is a temperature variance of
±10° within a batch, it produces completely
different treatment results. However, by
controlling the pulse current by means of
an exact pulse on and off time management,
the overall temperature can be precisely
managed with a uniform distribution,
from top to bottom, throughout the
hot wall chamber.
A unique feature with this approach is
that the system can be switched on to a
stable glow discharge at room temperature.
Most systems cannot do this because
the generators do not supply stable plasma.
To compensate, those systems must
first be heated to 300-3500 °C before
plasma can be applied, adding time to
the process. With PulsPlasma, the heat-up
time to temperature is used to prepare
the surface by giving it a fine cleaning.
Even the materials of construction used
to manufacture the nitriding systems furnace
itself have been optimized. In all systems,
PlaTeG uses insulative materials
developed in the aerospace industry to
create a furnace wall as thin as 40 mm,
compared to the industry standard of 150
www.medicaldesignbriefs.com
ToC
For medical device manufacturing,
one of the key advantages of pulse plasma
nitriding is that it is more suited to
heat treating of high alloy materials such
as stainless steel.
When working with steels that have a
higher chromium content, liquid nitriding
and gas nitriding can react with the
elemental content and " rob " the metal
of some of its corrosion resistance.
Stainless steel has a natural passivation
layer of chromium oxide that inhibits
corrosion. To bring nitrogen into the
material, the chromium oxide layer
must first be removed. With gas nitriding,
removal
of the passivation layer
requires the application of a special gas
chemistry. Stainless steels can also be
nitrided in salt baths, but it comes with a
tradeoff: Corrosion resistance is sacrificed
at the surface.
With PulsPlasma nitriding, the treatment
is applied directly through controlled
ionic bombardment of the surface.
By choosing a nitriding temperature
below 450 °C, and with exact control
of the gas mixture, the material surface
can be treated without reducing the
corrosion resistance of the material.
Increased Production Throughput
Nitriding is a batch process. Innovation
in furnace design, through an optimized
mechanical operation, can increase efficiency
and increase production capacity.
While the actual time for nitriding does
not change, efficient loading and
unloading scenarios plays an important
part. The PlaTeG plant design can use
any one of a Mono, Shuttle, or Tandem
footprint, to manage throughput,
resources, and operations costs.
Medical Design Briefs, January 2022

http://www.medicaldesignbriefs.com http://info.hotims.com/82317-803

Medical Design Briefs - January 2022

Table of Contents for the Digital Edition of Medical Design Briefs - January 2022