106
D. W. Eastman et al.
Fig. 6 Schematic of laser
machining path for
microtensile sample
demonstrating trepanning
method. Similar to the
method presented with the
FIB, the sample geometry is
cut closer to the final
geometry with each pass. For
this final shaping of the
sample, the laser energy
remains the same for each
pass and is on the order of
20 μJ for machining René
88DT
energy that is above the ablation threshold of the material, but is not high enough to
cause significant damage in the subsurface layer of the material [83–86]. The stage
speed and number of passes can also be varied to affect the effective number of
laser pulses that hit the sample during machining. A slower speed with fewer passes
removes more total material from the sample, but a higher speed with more passes
provides a higher quality sample because less redeposited material builds up on it.
Another challenge of laser machining is the taper of the machined sample edges.
There are a few techniques that can be used to minimize the effect of taper. One is
to use a two-step process, as has been described for FIB and wire EDM machining.
An outer series of passes of the sample geometry at higher energy is first used to
remove most of the material that needs to be cut away. Then, a series of subsequent
passes with less energy is used in a trepanning method in order to achieve the final
sample geometry. A diagram showing this trepanning method is shown in Fig. 6.
In the current study, each sample was cut using 60 total passes with a beam energy
of 50 μJ. Every 10 passes the beam was moved 5 μm closer to the final sample
geometry. The first 30 passes were performed using a stage speed of 0.5 mm/s, and
the final 30 passes were performed at a faster speed of 2.0 mm/s. This methodology
reduces the taper and creates less damage in the final specimen. The taper in a
sample can also be reduced by tilting the objective lens that focuses the laser beam
on the sample. The objective lens is mounted on a rotator that can be manually
adjusted to tilt the incoming beam by about 1 degree, allowing the edge of the beam
to cut parallel to the sample edge, rather than at the tapered angle. A drawback of
this technique is that changing the cut direction requires changing the tilt of the
objective lens on the fly.
Laser machining, and femtosecond laser machining in particular, is a promising
technique in sample fabrication with numerous benefits. A main reason the tech-
D. W. Eastman et al.
Fig. 6 Schematic of laser
machining path for
microtensile sample
demonstrating trepanning
method. Similar to the
method presented with the
FIB, the sample geometry is
cut closer to the final
geometry with each pass. For
this final shaping of the
sample, the laser energy
remains the same for each
pass and is on the order of
20 μJ for machining René
88DT
energy that is above the ablation threshold of the material, but is not high enough to
cause significant damage in the subsurface layer of the material [83–86]. The stage
speed and number of passes can also be varied to affect the effective number of
laser pulses that hit the sample during machining. A slower speed with fewer passes
removes more total material from the sample, but a higher speed with more passes
provides a higher quality sample because less redeposited material builds up on it.
Another challenge of laser machining is the taper of the machined sample edges.
There are a few techniques that can be used to minimize the effect of taper. One is
to use a two-step process, as has been described for FIB and wire EDM machining.
An outer series of passes of the sample geometry at higher energy is first used to
remove most of the material that needs to be cut away. Then, a series of subsequent
passes with less energy is used in a trepanning method in order to achieve the final
sample geometry. A diagram showing this trepanning method is shown in Fig. 6.
In the current study, each sample was cut using 60 total passes with a beam energy
of 50 μJ. Every 10 passes the beam was moved 5 μm closer to the final sample
geometry. The first 30 passes were performed using a stage speed of 0.5 mm/s, and
the final 30 passes were performed at a faster speed of 2.0 mm/s. This methodology
reduces the taper and creates less damage in the final specimen. The taper in a
sample can also be reduced by tilting the objective lens that focuses the laser beam
on the sample. The objective lens is mounted on a rotator that can be manually
adjusted to tilt the incoming beam by about 1 degree, allowing the edge of the beam
to cut parallel to the sample edge, rather than at the tapered angle. A drawback of
this technique is that changing the cut direction requires changing the tilt of the
objective lens on the fly.
Laser machining, and femtosecond laser machining in particular, is a promising
technique in sample fabrication with numerous benefits. A main reason the tech-