The Plasma Source Ion Implantation [11] technique was used to
implant Ti-6Al-4V flats with nitrogen ions at relatively low
temperatures. In PSII, targets are placed directly in a plasma source
chamber and are then pulse-biased to high negative voltages. During
this pulse, electrons are repelled from the target on the time scale
of the electron plasma frequency which is short relative to the ion
motion, leaving behind an ion matrix sheath. This sheath then expands
and the ions uncovered by the sheath accelerate into the target with
roughly normal incidence on all sides. This minimizes the sputtering
of the target, thus maximizing the retained dose. The implants
conducted for this paper were performed under typical PSII conditions.
The base pressure was torr and the operating neutral
pressure was
torr. The bias pulses had a duration
of 10
s and a frequency of 100 Hz. The bias voltage was 50 kV and
the delivered ion dose was
atoms/cm
. The
retained dose was measured to be on the order of
.
Details of the retained dose measurements can be found
elsewhere [12].
The targets implanted for this study were square flats (2.54 cm on a
side, and 0.318 cm thick). They were cut from 15.24 cm bar and sheet
stock, then prepared for implantation by mechanically grinding and
polishing to a 0.06 m surface roughness (peak-to-valley). The
original microstructure was
plus intergranular
,
resulting from simple mill annealing.
The nitrogen concentration depth profiles produced by the ion implantation were measured using a scanning auger microprobe (SAM) equipped with an argon ion gun at an energy of 3 keV. Details of this analysis have been published prevly [12]. Of particular importance is the fact that an internal calibration technique was used to overcome the problem associated with the overlap of nitrogen and titanium peaks at an energy of 385 eV [13].
Both hardness and wear tests were conducted on implanted and
unimplanted samples. The wear tests were conducted on a pin-on-disk
wear tester, using a 3 mm diameter ruby ball as a stylus and Hank's
solution
as a
lubricant. The rotational speed of the disk in these tests was 40
RPM, providing linear speeds of 25.1 and 31.4 mm/s, at track radii of
12 and 15 mm, respectively. Curves representing the wear depth as a
function of the number of wear cycles were generated by repeatedly
interrupting the wear test to measure the depth of the wear track
using a profilometer. The pin-on-disk test rig was modified somewhat
to assure that the location of the stylus was not altered by
these interruptions.
The hardness tests were conducted using a nanoindenter [14],
which is a depth-sensing instrument with a depth resolution of
approximately 0.4 nm and a force resolution of approximately 0.3
N. The nanoindenter used for this study was built by
Microscience, Inc. (Norwell, MA). The indenter used on this machine
is a diamond Berkovitch indenter and calibration is used to account for
the inevitable blunting of the indenter tip. The indents in the
implanted sample were made in a 4
5 array at a spacing of 20
m in one direction and 30
m in the other direction. Some
data was lost, though, due to insufficient storage space on the
testing machine, so only fifteen of the 20 indentations yielded data.
The indents in the unimplanted sample were made in a 3
3 array
at a spacing of 30
m in both directions. For each of these
indents, hardness values were recorded at several depths, yielding
several data points for each specimen. These results were compared to
finite element results in order to estimate the yield stress changes
induced by the ion implantation.