|
1. Mechanism of Ductile Cutting
Fig. 1.1
shows the schematic diagram of chip formation in ductile cutting of silicon
wafer with a tool having a large negative
rake angle and an arc cutting edge, where DE is the tool rake face,
BD is the arc cutting edge, BC is the tool flank face, K
is a point on the arc cutting edge BD, O is the center of the
arc cutting edge BD, AB is a curved shear plane, r is the cutting edge radius,
g is the tool rake angle, and ao
is the depth of cut. The cutting forces in
the cutting region are the resultant tool force Fr, the
cutting force Fc, the thrust force Ft,
the shear force on the shear plane, Fs, the normal force
on shear plane, Fns, the frictional force on tool face,
Ff, and the normal force on tool face, Fn.
Fig. 1.1 Schematic diagram of chip formation in ductile
cutting of a brittle material.
In the cutting of
brittle materials, with the undeformed chip thickness being sufficiently
small and the ratio of the radius of tool cutting edge to undeformed chip
thickness being larger than 1, the chip formation will be in a ductile mode.
This is because the chip formation will be dominated by dislocation rather
than fracture due to the following three effects. First, under the cutting
geometry formed by extremely small undeformed chip thickness, and with the
ratio of tool cutting edge radius to undeformed chip thickness being larger
than 1, the workpiece material in the cutting region undertakes extremely
large compressive stress and shear stress, with the compressive stress being
much larger than the shear stress. This stress status produces a largely
reduced stress intensity factor KI, and activates
dislocation emission in the material. Second, at the mesoscale of chip
formation, the dislocation hardening largely strengthens the normal flow
stress of the workpiece material, which increases the fracture toughness of
the material, KC. Third, at the mesoscale of chip
formation, the strain gradient also largely strengthens the normal flow
stress of the workpiece material, which also increases the fracture
toughness of the material, KC. As a result, crack
propagation due to workpiece material pre-existing flaws are blocked, and
dislocations dominate the chip formation process. It should be noted that in
the ductile chip formation mechanism of a brittle material the key issue is
that the value of fracture toughness KC is larger than the
value of stress intensity factor KI. This can be achieved
by reducing the undeformed chip thickness in the cutting region. Since the
KC is a material property that varies with the workpiece
material, the value of undeformed chip thickness for ductile chip formation
will also vary with the workpiece material.
Back to Top |
|
3. Nanometric Cutting
Fig. 3.1 shows the schematic diagram of the maximum undeformed chip
thickness in the ultra-precision face turning experiments. Here, O1
and O2 are the centers of two adjacent arc cutting edges,
and the distance between O1 and O2 is
the feed rate used in the experiments. The nanometer scale values for
undeformed chip thickness were achieved by arranging combinations of the
radius of tool corner R, depth of cut ao and feed
rate f, as shown in Fig. 3.1. The maximum undeformed chip thickness
dmax can be simplified using the equation when
 as
shown in Fig. 3.1 (a):
(3.1)
The maximum undeformed chip thickness dmax can be
determined using the equation when
 as
shown in Fig. 3.1 (b):
(3.2)
(a)
(b)
Fig. 3.1 Schematic diagrams of maximum undeformed chip thickness.
Back to Top |
|
4. Chip
Formation
SEM micrographs
of chips obtained in cutting of silicon wafers using the diamond tool with
the cutting edge radius of 45 nm under cutting speed of 4 m/s
(1000 rpm) and feed rate of 5
mm/rev
(5 mm/min) are shown in Fig. 4.1. It was found that when the
undeformed chip thickness was smaller than 40 nm, the chips obtained
were in the form of ribbons and layers, i.e. the chips formed were
continuous.
The
appearance of such chips was similar to the chip generated when machining
ductile metals. On the other hand, as the undeformed chip thickness
increased beyond 40 nm,
discontinuous and
fractured chips were obtained. Hence, the cutting at the undeformed chip
thickness larger than 40 nm indicated that the chip formation was in
a brittle mode.
|
.jpg) |
 |
|
(a)
Undeformed chip thickness: 20 nm |
(b)
Undeformed chip thickness: 50 nm |
|
Fig. 4.1
Chips formed at different undeformed chip thickness in cutting of Si-wafers.
|
Cutting of
silicon wafers
was made using the diamond tool
with the cutting
edge radius of 647 nm under cutting speed of 4 m/s (1000
rpm) and feed rate of 5
mm/rev
(5 mm/min).
SEM
micrographs of chips obtained are shown in Fig. 4.2. The micrographs
show that the chips formed in cutting with values of the undeformed chip
thickness in between 50 nm to 596 nm were in the form of long
ribbon, which is similar to the chip generated in machining of ductile
metals as shown in Fig. 4.2 (a). The chips are continuous with fine ripples
on the surfaces, indicating that the chip formation was in a ductile mode.
On the other hand, the chips in cutting with values of the undeformed chip
thickness larger than 655 nm were fragmented and discontinuous with
non-uniform length as shown in Fig. 4.2 (b). These chips are fractured
particles and blocks with sharp ends. Such appearance indicates that in the
cutting process, the chip formation was dominated by crack propagation and
brittle fracture.
|
 |
 |
|
(a)
Undeformed chip thickness: 200 nm |
(b)
Undeformed chip thickness: 690 nm |
|
Fig. 4.2
Chips formed at different undeformed chip thickness in cutting of Si-wafers.
|
Back to Top |
|
5. Ductile Cutting Surface
SEM photographs of the machined workpiece surfaces achieved in cutting of
silicon wafers with the cutting
edge radius of 335 nm under cutting speed of 4 m/s
(1000 rpm) and feed rate of 5
mm/rev
(5 mm/min)
are shown in Fig. 5.1. SEM observations on the machined wafer surfaces
indicated that larger undeformed chip thickness would cause more percentage
of fracture surfaces. It should be noted that feed marks were clearly
displayed on the machined workpiece surfaces in cutting of silicon wafers.
Fracture surfaces were achieved in the pre-trimming of silicon wafers due to
cutting in a brittle mode. Maybe some of them were left and remained in
cutting of silicon wafers with smaller undeformed chip thickness.
 |
 |
|
(a) Undeformed chip thickness: 242 nm |
(b) Undeformed chip thickness: 337 nm |
|
Fig. 5.1
SEM photographs of the machined workpiece surfaces of Si-wafers. |
The machined workpiece surface of a silicon wafer achieved in ductile mode
with the cutting
edge radius of 335 nm under cutting speed of 4 m/s
(1000 rpm) and feed rate of 5
mm/rev
(5 mm/min)
was examined using an optical inspection measurement system (OIMS), as well
as the original polished surface of a silicon wafer. The OIMS photos are
shown in Fig. 5.2. Compared the machined wafer surface with the polished
wafer surface, a good workpiece surface for silicon wafers can be achieved
by using the ductile cutting technology, as shown in Fig. 5.2.
|
 |
 |
|
(a) Ductile cutting surface |
(b) Original polished Surface |
|
Fig. 5.2 OIMS photos of ductile cutting and
original polished surfaces of Si-wafers. |
AFM photographs of the machined surfaces achieved in cutting of
silicon wafers with the cutting edge radius of 335 nm under cutting speed of 4 m/s
(1000 rpm) and feed rate of 5
mm/rev
(5 mm/min)
are shown in Fig. 5.3. AFM observations of the surface topography indicated
that smoother surfaces were obtained in cutting of silicon wafers with
smaller undeformed chip thickness. The larger the undeformed chip thickness,
the rougher the machined workpiece surfaces, and the larger the machined
workpiece surface roughness. Experimental
results achieved in cutting of silicon wafers indicated that
the cutting modes changed from ductile to brittle when the undeformed chip
thickness increased within an extremely small range, whatever from the SEM
observations of chip formation, and the AFM and SEM examinations of the
machined workpiece surfaces. All the experimental results indicated that
silicon wafers could be machined in a ductile cutting mode with certain
cutting conditions and certain cutting tool geometry.
 |
 |
|
(a) Undeformed chip thickness: 166 nm |
(a) Undeformed chip thickness: 348 nm |
|
Fig. 5.3 AFM photographs of the machined workpiece surfaces of Si-wafers. |
Back to Top |
