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Original article
Carbon fiber-reinforced polymer
composite drilling via aluminum
chromium nitride-coated tools: Hole
quality and tool wear assessment
Journal of Reinforced Plastics
and Composites
2017, Vol. 36(19) 1403–1420
! The Author(s) 2017
Reprints and permissions:
DOI: 10.1177/0731684417709359
Muhammad Harris1, Muhammad Asif Mahmood Qureshi2,
Muhammad Qaiser Saleem3, Sarmad Ali Khan3 and
Muhammad Mahmood Aslam Bhutta2
Drilling into carbon fiber-reinforced polymer composites presents a stiff challenge in terms of ensuring hole quality
aspects such as delamination, roughness, roundness, etc. while maintaining an adequate tool life. The hardened carbon
fibers in laid up form generally give rise to chips in powdered form that contribute significantly to tool wear problem. To
overcome the aforementioned issues, the use of high performance tools has been reported. This paper reports on the
usage of yet another novel high performing but much economical aluminum chromium nitride-coated tools that are
typically employed for metallic parts. The effect of aluminum chromium nitride tools at three spindle speeds with
constant feed rate was evaluated on hole quality, tool wear, and chip formation pattern of carbon fiber-reinforced
polymer composite plates. The results were compared with the performance of conventional high speed steel tools
taken as baseline. It was found that the coated tools help significantly improve hole quality and provide appreciable tool
life at specific machining parameters.
Hole quality, tool wear, drilling, carbon fiber-reinforced polymer (CFRP) composite, aluminum chromium nitride (AlCrN)
The demand of carbon fiber-reinforced polymer
(CFRP) composites has seen a consistent increase in
advanced engineering applications such as aerospace
and automotive industries as well as structural rehabilitation, etc. This is primarily due to their high strengthto-weight ratio and formability at affordable cost.1–5
Owing to their aforementioned positive traits, they
are dominant among the materials replacing the conventional metals. The CFRP composites are being
introduced in the fabrication of major structural members of aircrafts with floor beams, frame panels and
significant portion of the tail sections being the specific
examples.6 CFRP members in these applications are
typically joined through mechanical fastening, i.e. riveting and bolting, requiring as much as tens of
thousands of drilled holes for the complete assembly.
The machining quality of drilled holes thus plays a vital
role in reliability and safety of the product. Hole
quality is measured in terms of hole surface roughness,
Department of Industrial & Manufacturing Engineering, Rachna College
of Engineering & Technology, Gujranwala, Pakistan
Department of Mechanical Engineering, University Engineering &
Technology, Lahore, Pakistan
Department of Industrial and Manufacturing Engineering, University
Engineering & Technology, Lahore, Pakistan
Corresponding author:
Muhammad Qaiser Saleem, Department of Industrial and Manufacturing
Engineering, University Engineering & Technology (UET), Lahore,
Email: [email protected]
roundness, cylindricity, delamination, and fraying
damage. Owing to inhomogeneous, fibrous and anisotropic nature of CFRP composite, despite the many
advantages it offers, the challenges encountered in
machining of this class of materials are different from
those of metals.5,7
The low cost conventional high speed steel (HSS)
tools with high fracture toughness, the property considered as most important specifically for drilling operations,8 invite natural inclination towards it as first
choice for such applications. Although its use has
been reported for drilling of CFRP composites,9–11
however the advancement in tooling materials and
coating technology has resulted in reportage of the
use of certain other advanced tools namely SiAlON
ceramics, carbides, Tin-coated carbides, TiAlN-coated
tools, and chemical vapor deposition (CVD) diamondcoated tungsten carbides (WC).6,12–15 Aluminum chromium nitride (AlCrN) coating over cobalt substrate is a
relatively new class of coated tools that has not been
found to be investigated for drilling purposes of CFRP
composites. Considering the distinct benefits it offers as
cutting tool such as high wear resistance, excellent hot
hardness, good thermal shock stability, and ability to
be utilized at higher cutting speeds,16 a pertinent investigation is necessitated for its use for assessment of hole
quality and tool wear aspects while drilling into CFRP
composites, in the context of which, the use of this
advanced class of tools has not been found to be available in open literature.
This research paper reports the results of a comprehensive investigation of AlCrN-coated drilling tools on
CFRP composite materials. The 8-ply CFRP composite
material test pieces were prepared in the laboratory,
while cutting tools were provided by a standard supplier. The performance of AlCrN-coated tools was evaluated in terms of hole quality and tool wear and
compared with that of HSS tools with logical
Literature review
The relevant aspects of research on CFRP composite
drilling, carried out by different researchers, is presented herein.
Li et al.6 investigated the machining characteristics
of CVD diamond-coated tungsten carbide tools with
optimized double point geometry while drilling into
CFRP composite plates having 36–40 quadriaxial
plies (t & 10 mm, Vf & 60%). They employed two feed
rates, i.e. 0.2 mm/rev and 0.4 mm/rev, reducing each to
0.01 mm/rev in last 0.5 mm of plate thickness to prevent
delamination at exit. They took the termination criteria
to be either VBmax ¼ 0.3 mm or 384 holes whichever
occurred first. The primary tool wear mode was
Journal of Reinforced Plastics and Composites 36(19)
found to be abrasion and chipping/peeling up to
123 mm. The hole quality was measured in terms of
2D and 3D surface roughness parameters (Ra, Sa, St,
in mm). They observed that plies with 135 fiber orientation showed poorer surface quality along with fiber
pullout phenomenon. The fibers were found to have
fractured or peeled-off in general. The reason of this
specific orientation contributing more to the pullout
phenomenon was not comprehensively explained.
Also, roughness was reported to be significantly less
at the exit as compared to that at the entrance in general. This was attributed to the polishing effect of the
tool and reduced feed in last half mm thickness of the
sheet. It was also seen that the superior hole quality
generated by new tool degraded as machining
Shyha et al.17 investigated the machining behavior of
uncoated tungsten carbide twin lipped stepped drills
( ¼ 1 mm, 1.5 mm) on 12 ply quadriaxial CFRP
plates of 3 mm thickness, consisting of woven and unidirectional fibers in three types of resins. A constant
spindle speed of 9500 r/min and feed rates of 0.2 mm/
rev and 0.4 mm/rev were taken for experimentation
with termination criterion of VBmax ¼ 0.1 mm. The outcome of the findings point towards the role of fiber
orientation to be influencing the results wherein 3750
holes were drilled on a woven fabric plate in comparison to only 82 holes in plate with unidirectional fibers
with all other things being constant. A more comprehensive reasoning of the physical phenomenon
involved, however, is still needed.
Heisel and Pfeifroth18 investigated the influence of
different point angles of cemented carbide tools on hole
quality of biaxial CFRP plates (Vf ¼ 55%, t ¼ 9 mm,
t ¼ 800 MPa and E ¼ 67 GPa) in terms of delamination, fraying damage and bur height. It was reported
that the feed force is proportional to the point angle.
The tools with point angles greater than 180 gave least
delamination at the entrance of the hole but considerable delamination took place at the exit. The vice versa
was observed with tool angles less than 180 . For the
results pertaining to fraying damage, higher point angle
was found to be better at both entrance and exit.
Hocheng and Tsao10 experimented with HSS tools
of different geometries, namely, saw drill, candle stick
drill, and step drill to conclude that the variable drill
geometry and feed rate influenced the thrust force.
They also got a decrease in tangential force as the
point angle increased.
Madhavan and Prabu19 used solid carbide (K20),
HSS, and poly crystalline diamond insert drills to
study the effect of feed rates, cutting speeds and geometry of drill bit on delamination, surface roughness, and
chip formation. They observed that carbide drills produced continuous ribbon like chips whereas HSS and
Harris et al.
Figure 1. (a) Schematic of drilled workpiece, (b) drill tools, (c) laying up, and (d) machine setup.
Table 1. Experiment plan.
Test #
Tool description
Speed (r/min)
Feed (mm/rev)
Evaluation parameters
AlCrN coating on cobalt
steel substrate
Machined surface quality:
1. Delamination at entrance and exit
2. Fraying damage
3. Hole roughness
4. Hole roundness
5. Chip formation
Tool wear:
1. Tool flank wear
2. Rake face wear
3. Chisel edge wear
4. Corner edge(heel) wear
Uncoated HSS
polycrystalline diamond (PCD) tools produced discontinuous particle like chips. They argued that discontinuous particle like chips contributed to crater wear due to
promotion of micro-ploughing action by free abrading
material in the form of these chips. They also observed
delamination to be minimum at high cutting speed irrespective of the tool type employed.
Ramirez et al.20 monitored the wear of K20 carbide
drill of diameter 12 mm and tool tip angle 140 while
machining 30 mm thick CFRP composite plates
(Vf ¼ 60%). They embedded the thermocouples in the
resin to measure the temperature from tip to corner of
the cutting edge in an effort to map the temperature
profile to see its relevance with machining performance.
The cutting forces and torque were measured at a feed
rate of 0.05 mm/rev and cutting speed of 100 m/min
whereas the wear at flank and rake faces were measured
at regular machining intervals at the radii of 1.5, 3, 4.5,
Journal of Reinforced Plastics and Composites 36(19)
Figure 3. Roundness error measurement.
Figure 2. Schematic of variables used for Fd.
and 5 mm. They observed gradual degradation of the
tool flank that ultimately led to chipping due to abrasion. They also concluded that chipping phenomena
contributed to increase in localized temperature.
Research work has also been reported on drilling of
hybrid metal-CFRP composite materials using different
types of twist drills.21,22 It was found that drilling of metal
(titanium) influenced the flank while that of CFRP composites affected the cutting edge. The defects in hole quality (delamination, diametral errors and surface roughness)
were more prominent in hybrid Ti-CFRP workpieces as
compared to simple CFRP composites due to excessive
heat generation. Also, presence of Ti chips caused adhesive action on the drill bit resulting in more unstable
machining. Among the types of drill bits employed,
PCD drill tools showed better hole quality at all given
conditions as compared to tungsten carbide (WC) tools.
Drilling of composite materials reinforced by other
types of fibers has also been investigated by various
As evident from the presented literature review,
though various tool materials and aspects have been
reported however the use of novel AlCrN-coated (on
cobalt steel substrate) drills for machining of CFRP
composites with comprehensive reportage on factors
like hole quality and tool wear has not been found.
These aspects are focused in the current research work.
Workpiece material
A 200 g/m2, 3 K carbon fiber woven in 0 /90 orientation was used in epoxy matrix to make an eight layered
4 mm thick CFRP composite specimen of fiber architecture [0 /90 /+45 /45 ]2 using hand lay-up method.
Figure 4. Flank wear and rake face wear.
Machining details
For machining purposes, following two types of drills
(round shank, two flute, right handed, diameter 12 mm
and point angle 130 ) were used (i) AlCrN coating on
cobalt steel substrate (HSS-E) with hardness of substrate HV 987, (ii) conventional uncoated HSS drills
with hardness of HV 940. Nineteen holes were drilled
in each plate. The drilling scheme, the drills used, actual
layup and the machine setup used for experimentation
are shown in Figure 1. The experiments were performed
at constant feed of 0.05 mm/rev with three spindle
speeds of 1300, 2000, and 2700 r/min on CNC machining center (First MCV-600 machining center by Long
Chang Machinery Co Ltd. having three axis, 15HP and
8000 Top RPM with ATC). Since the underlying objective of the work was to obtain good quality delamination
free holes so the machining conditions were carefully
selected to help achieve the objective. The choice of
low feed rate was influenced by literature13,25 suggesting
good quality holes and delamination free machining to
be possible only at smaller feed rates. The specific value
of 0.05 mm/rev was selected herein for it being reported
Harris et al.
Figure 5. (a) AlCrN-coated new tool and (b) corner edge wear after 50 holes at 1300 r/min.
Figure 6. (a) Delamination at entrance and (b) average of delamination at entrance of drilled holes.
Journal of Reinforced Plastics and Composites 36(19)
Figure 7. Delamination at entrance.
to be close to industrial cutting conditions.26 So keeping
this value of feed rate constant, the effects of spindle
speed were evaluated. As for the spindle speed, though
the range in general was based on the typical values
available in the literature however, one value (2700 r/
min) in specific was purposely taken to be on a higher
side. The reason was to also evaluate the performance of
these tools at a representative high speed in light of the
earlier reported literature15,27 suggesting reduction in
delamination due to smooth machining made possible
by material matrix softening as a result of higher temperature generated at high speed.
Six experiments were performed in total. For each
experiment, drilling was done on separate CFRP composite plates as per the experiment plan given in Table 1.
Machining quality
A number of methods are available in established literature to measure delamination.5,9,28 Keeping in view
the experimental constraints, delamination factor (Fd )
as given by equation (1)5,9 was used for quantifying the
degree of hole delamination
Fd ¼
where Dmax was measured at entrance and exit with the
help of Coordinate Measurement Machine (CMM)
(CE-450 DV Coordinate Measurement Machine by
Chen Wei Precise Technology Co Ltd.) and Dnom was
taken to be equal to diameter of the drill bit. Figure 2
gives the schematic of measurement.
Fraying damage was qualitatively assessed via
images taken at 10X magnification on CMM. Hole
roughness was measured with surface texture meter
(Surtronic 25 by Taylor Hobson Precision Ltd having
gauge range 300 mm, resolution 0.01 mm, traverse length
0.25–25 mm with diamond stylus) with cutoff length
and evaluation length of 0.8 mm and 2.4 mm, respectively. Readings were taken at six random locations
(three at holes’ entrance and three at exit) and the
mean value of roughness is reported herein.
Roundness error was measured using least square
circle method with the help of roundness testing machine
(Talyrond 130 by Taylor Hobson Precision Co Ltd.). In
this method, the sum of radial difference of the minimum
Harris et al.
Figure 8. (a) Delamination at exit and (b) average of delamination at exit of drilled holes.
circumscribed circle (MCC) to the reference circle (least
square circle) and the maximum inscribed circle (MIC)
to the reference circle (least square circle) were
accounted for. The schematic of roundness error measurement is shown in Figure 3.
Chips were examined visually for better comprehension of the phenomena involved.
Tool wear
CMM was used for measuring flank wear, corner edge
wear, wear on rake face, and chisel edge. The flank
wear was measured as the largest wear on the flank of
each tool after every five holes. The test was terminated
upon reaching either 50 holes or flank wear VBmax criterion of 0.3 mm29 whichever occurred first.
Additionally, wear on rake face was measured only
for the drills employed for machining at high speed of
2000 and 2700 r/min after every 10 holes. Here again
CMM was employed for the purpose. Figure 4 shows
the schematic of measurement of flank wear and wear
on rake face.
Wear at corner edge was also evaluated at high speed
of 2000 and 2700 r/min after every 10 holes using equation (2). Figure 5 shows the schematic of the measurement. Here, C1 refers to the distance from a reference
point O to the unworn edge, whereas C2 shows the
measurement up to worn edge
¼ C1 C2
Journal of Reinforced Plastics and Composites 36(19)
Figure 9. Delamination at exit.
Wear at chisel edge was visually examined also at
high speed of 2000 and 2700 r/min after every 10 holes.
Results and discussions
Machining quality
Delamination. The delamination caused by the two types
of tools at three spindle speed levels is shown in
Figure 6, wherein (a) represents the values at the
entrance of the hole measured for every fifth hole
drilled and (b) represents the average values for all 50
holes. In general, AlCrN-coated tool performed better
than HSS tool at 2000 and 2700 r/min. At low speed of
1300 r/min, however, the delamination caused by
coated tool is seen to be comparable to that of HSS
tool. Figure 7 is a pictorial comparison of delamination
observed at entrance of the drilled holes for first and
last (50th) hole for all speeds. Here, too, it can be seen
that though the two types of tools give comparable
results at 1300 r/min, AlCrN-coated tools result into
much better performance at high speeds of 2000 and
2700 r/min in terms of delamination. It can be attributed to lower coefficient of friction associated with
AlCrN-coated tools. Coated tools would result into
smoother chip flow and thus reduced thrust force and
delamination in comparison to their uncoated counterparts. The effectiveness is seen to be more prominent at
higher operating speeds (2000 & 2700 r/min) where the
need to overcome this friction becomes more significant
and it is there that the role of coating becomes more
evident. In comparison, uncoated tools in absence of
such an attribute yield inferior quality holes at these
conditions. It is also observed that as the number of
holes drilled increase, though the overall trend of
delamination seems to be logically increasing however
the behavior is somewhat uneven during the course of
experimentation for all the speeds and tool types in
general (see Figure 6(a)). This is most probably due
to complex interaction of tool wear with fiber orientation during the experimentation.
The values of delamination at exit of the hole are
shown in Figure 8. Here too, similar trends are
observed as were observed to those at the entrance.
At 2700 r/min, there is a difference of 2.1% in the average delamination values caused by the two tool types.
Figure 9 gives the pictorial representation of the hole
quality at exit, showing the superior hole quality of the
AlCrN-coated tool after 50 holes. The results obtained
herein are found to be in the vicinity of those reported
for other tools when operated at comparable operating
conditions while drilling into CFRP composites. The
value of 1.046 obtained herein for exit delamination
at 2700 r/min is comparable to 1.043 reported for
coated TiAlN drills when operated at similar rpm of
2800 at same feed rate of 0.05 mm/rev.13 As for comparison with tungsten carbide (uncoated & coated)
tools, the average maximum value obtained for exit
Harris et al.
Figure 10. (a) Roughness values and (b) average of roughness values.
delamination here is 3.4% and 5.2% less in comparison to those reported for uncoated & TiN-coated
WC tools, respectively, at comparable spindle speed of
3000 r/min and feed rate of 0.04 mm/rev.15
The phenomenon of delamination can be explained
as follows. Upon initiation of the drilling process, the
indentation by drill tip is followed by shearing off the
material as drill rotates. During drilling, fibers stretch
in one orientation and pull out in another. Due to the
torsion effect of rotating drill, and subsequent shearing
of the fibers, some of the fibers are pulled out whereas
some are pushed in after being cut. This wraps the
fibers around the drill flank. The uncoated HSS tool
surface that is more irregular in nature results into
entanglement of fibers with cutting tool and thus
promotes the phenomenon. Since coating generates a
smoother surface and lower friction on top of cutting
edge in comparison to surface of uncoated drill bit, it
would essentially help to overcome the entanglement
issue thereby resulting into better delamination results.
However, beyond 25th hole, coating starts to degrade
(as would be evident in tool wear section wherein the
tool evaluation results at the end of experimentation
have been included) that could be credited with no
marked difference for delamination result obtained
for the two tool types of tools in the specified range
on overall basis.
Fraying damage. The pattern for fraying damage was
observed to be in conformity with the one observed
Journal of Reinforced Plastics and Composites 36(19)
Figure 11. (a) Roundness error and (b) average values of roundness error.
for delamination. It was maximum at the exit of hole
and minimum at the entrance. Similar pattern was
observed by others with PCD drills, coated and
uncoated carbide drills.27,30–32 As was observed for
delamination, here too, coated tools resulted into
much less fraying damage in comparison to baseline
HSS as can be seen in Figures 7 and 9.
Hole roughness. Figure 10 shows the results for hole surface roughness under all machining conditions. As can
be seen by results, AlCrN-coated tools outperformed the
HSS counterparts at all the machining speeds. At 2700 r/
min, the highest surface roughness value was observed to
be 23.4 mm for HSS tool whereas for its coated counterpart the maximum value observed was 9.6 mm (59%
less), indicating the extent of benefit obtainable by the
coated drill. Also the overall variation observed for all
the speed regimes for HSS tool was 15.8% on average
compared to only 7% obtained by AlCrN-coated tool.
This indicates more uniform performance in terms of
surface quality demonstrated by coated tools on overall
basis. Although it is not possible to have a direct comparison of these results with previously cited work on
CFRP due to differences in machining conditions however for reference purposes some work can be cited25
wherein Ra has been reported to be 4 mm over the
course of experimentation when uncoated carbide twist
drill was employed at constant spindle speed of 2000 r/
min and 0.02 mm/rev feed rate.
Hole roundness. From the results obtained for hole
roundness, at maximum speed of 2700 r/min, the maximum error in roundness value was observed to be
33.72 mm whereas the corresponding value for HSS
Harris et al.
tool being 257.69 mm, indicating 87% reduction for
the case of coated drill. The smaller deviation observed
for the roundness values in case of coated tools can be
attributed to the fact that they resulted into less fiber
pull out (as explained before) and less drill wander.
Less drill wander would be made possible by coated
tool retaining its cutting edges and corner integrity
for a longer period in comparison to its uncoated counterpart as will be discussed in tool wear section. Raj and
Karunamoorthy25 also state this retained cutting edge
and corner integrity to be responsible for reduced
roundness error. Figure 11 presents the dominance of
coated tool at all speeds whereas Figure 12 gives the
schematic of the actual measurement.
Chip formation. The reasons of better machining performance by AlCrN tools can also be explained by visually observing the types of chips produced by the two
types of tools. Chips produced by AlCrN-coated tool
were continuous whereas those produced by HSS tools
were observed to be in powdered form, as shown in
Figure 13. The lengths of AlCrN chips were very
long, apparently hard to break. They had a tendency
to wrap themselves over the tool at all speeds investigated herein instead of being clogged; here, entanglement was more visible phenomena as opposed to
burning and adherence typically observed in clogged
scenarios of metallic machining.
Considering that unclogged continuous long chip
always represents favorable machining conditions,
verify why coated tool had higher surface finish as compared to HSS. Contrarily, the relatively rough edges of
HSS would cause intermittent cutting points and excessive torsion and shearing at micro level as already mentioned. It would be resulting into more quashing of the
composite and pulled-out fibers, thereby resulting into
powdered chips. Smoother AlCrN tools on the other
hand would drag the pulled out fiber with more length
in comparison.
Tool wear
Figure 12. Screen shot of roundness error measurement.
Figure 13. (a) AlCrN-coated tool chip and (b) HSS tool chips.
Flank wear. The graph between hole number and flank
wear is shown in Figure 14. At 1300 r/min, AlCrNcoated tool showed negligible tool wear up to 15th
hole as compared to a near-uniform flank wear of
HSS tool. Beyond 15th hole, the increase in flank
Journal of Reinforced Plastics and Composites 36(19)
Figure 14. (a) Tool flank wear and (b) average of tool flank wear.
wear started becoming visible and continued till the end
of experimentation. This explains why the coated tool
effectiveness was degraded for hole quality, specifically
delamination, where at higher hole counts the results
started becoming comparable for two tool types on
average (at entrance), as was shown in Figures 6 and 7.
An interesting outcome is for the difference in type
of wear for two types of tools. For HSS the wear is
more in the form of abrasion and for AlCrN-coated
tool, it is rather chipping of ceramic coating at
micron level as shown in Figure 15.
The reasons for these differences could be that high
mechanical stresses induced by carbon fiber might have
caused the weakening of grain binder resulting in
damage in coated layer and thus chipping and fracture
of cutting edges.33,34 For the HSS, on the other hand,
the wear mechanism is more of abrasion due to uniform
composition of the tool. Similar observance was made
by others,6,26 who reported work on CFRP composite
drilling using uncoated and coated carbide drills. For
uncoated carbide drills, abrasive wear was reported to
be the tool wear mechanism26 whereas for coated carbide drills chipping phenomenon was observed to be
dominating the abrasive wear.6
Wear at rake face. At 2000 r/min, the AlCrN-coated tool
underwent considerably less wear than the HSS tool,
i.e. a difference of 59%, as can be seen in Figure 16.
However, the wear at rake face was comparable for
both types of tools at 2700 r/min, i.e. just 5.7% difference of the average values. The nature of wear for HSS
shows that maximum rake face wear occurred near the
tip of the tool (chisel edge) and fades along the length.
However, in case of AlCrN-coated tool; it was distributed over the whole length of the cutting edge of the
rake face. As a rule of thumb, the rake angle for a
metallic tool is selected in such a way that the impact
of cutting forces exerted by the chips is in the vicinity of
the cutting edge. This is due to the fact that a metallic
tool can withstand such impacts and can be resharpened periodically. The ceramic tools cannot be
re-sharpened; hence they are designed with a rake
Harris et al.
Figure 15. Tool flank wear.
angle such that the impact of cutting forces is on the
rake face rather than on the edge. Since AlCrN-coated
tool belongs to ceramic class of tools so its different
rake angle is most likely the cause of wear pattern
observed herein. The reasoning gains strength by looking at the chips produced for HSS tool which were
powdered in nature and thus more concentrated near
the tip resulting into more wear near tip as shown in
Figure 17. Contrarily, in case of AlCrN tool, the long
continuous chips flowed over the rake face along the
cutting edge causing more uniformly distributed wear
up to the corner edge. This wear pattern however was
not spread over the whole rake face but was observed
to be very close to the edge only. This might be because
of less feed rate preventing the flow of chips from
inflicting the whole rake face.
Wear at corner edge (heel). Here, too, the AlCrN-coated
tools outperformed the HSS tools at all speeds under
investigation, as can be seen in Figure 18. Not only the
magnitude of wear was many times less in case of
AlCrN tools, but also it was almost unaffected by the
speed of the cutting tool. Despite the fact that maximum cutting speed is experienced at the outer periphery (cutting edge) of the drill, this coating still appears
to act as an efficient thermal barrier for substrate over
the range of cutting speeds employed herein. Its superior hot hardness most likely contributes to high wear
resistance resulting into much lesser values of corner
edge wear in comparison to its uncoated counterparts.
The value of 50 mm (.05 mm) at 2000 r/min after 50
holes obtained with AlCrN-coated tool herein is an
order of magnitude lesser than the value of 500 mm
Journal of Reinforced Plastics and Composites 36(19)
Figure 16. (a) Wear at rake face and (b) average of wear at rake face.
Figure 17. Wear at rake face (a) AlCrN coated at 2700 r/min after 50 holes and (b) HSS at 2700 r/min after 50 holes.
Harris et al.
Figure 18. (a) Wear at corner edge, (b) average of wear at corner edge.
Figure 19. Schematic of fiber fraying during machining.
Journal of Reinforced Plastics and Composites 36(19)
Figure 20. Wear at chisel edge (a) HSS at 2000 r/min after 50 holes, (b) AlCrN at 2000 r/min after 50 holes.
reported by Xia et al.32 with uncoated carbide tools
after drilling 50 holes at spindle speed of 2000 r/min
at same feed rate of 0.05 mm/rev while drilling of
quasi-isotropic CFRP composites.
Another reason of the observed trends can be sought
in the fraying of fibers, as visualized in Figure 19. Since
HSS caused more fraying, i.e. uncut elongated fibers,
they are bound to rub against the drill and propagate
the wear up to the corner. This phenomenon was not
that severe in case for coated tools, where corner wear
was observed to be less in general.
Wear at chisel edge. The wear at chisel edge was found to
have followed a specific pattern wherein the wear at
ends of the edge was relatively more than that at the
center. As the center of the chisel edge coincides with
the axis of rotation, torque due to cutting forces is zero
here, whereas at the ends, a higher torque would cause
more wear thereby resulting into the edge losing its
sharpness more in relation to the center. In case of
AlCrN-coated tool, wear band is observed to be relatively thin compared to uncoated tool (see Figure 20)
which is attributed to its greater wear resistance.
The overall outcome of this work is that AlCrN-coated
drills outperform HSS drills while machining CFRP
composites. More specifically, following conclusions
can be drawn from the presented work.
1. The use of AlCrN-coated tools is seen to favor the
machining of CFRP composite plates specifically at
higher speeds. Machining is carried out with generation
of preferred long and continuous chips with lesser tendency to clog thereby resulting in better machining performance in comparison to HSS counterparts.
2. AlCrN-coated tools resulted in less delamination in
comparison to HSS tools. A trend with respect to
speed was observed wherein more speed is seen to
result into less delamination in general. The best
results were obtained at 2700 r/min for delamination
both, at entrance and exit. Also, coated tools exhibited negligible fraying damage as compared to HSS
tools. However, in both types of tools, it was found
to be higher at exit than at entrance.
3. For the case of surface roughness, at 2700 r/min,
59% reduction in the maximum of the observed
values was obtained for AlCrN-coated tool in comparison to HSS tool. Similarly AlCrN-coated tools
resulted into significantly less roundness error.
Again, machining at 2700 r/min by AlCrN-coated
tools resulted in 87% less roundness error in maximum of the values observed herein in comparison to
HSS counterpart.
4. The flank wear of the coated tool is almost negligible
up till 15th hole at lowest speed of 1300 r/min but
after that it degrades significantly till the end of
experimentation and becomes comparable to HSS.
5. Different types of wear patterns were obtained for
the two types of tools. Though no significant wear at
rake face was found for both, however, for coated
tools it was more distributed over the whole length
of the cutting edge of the rake face whereas for HSS
tool it was more concentrated near the tip of the
6. Wear at corner edge of the drill (heel wear) was recorded more for HSS than for AlCrN-coated tool
that is attributed to the high percentage of uncut
fibers (fraying damage). For the case of wear at
chisel edge, a specific pattern with more worn out
edges in comparison to the center is observed
herein; the pattern being more dominant for HSS
Harris et al.
Authors’ note
This work is based on MSc Thesis titled ‘‘Evaluation of Hole
Quality and Tool Wear of CFRP Composites Using Different
Tool Bits’’ submitted to University of Engineering &
Technology, Lahore, Pakistan.35
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
The author(s) received no financial support for the research,
authorship, and/or publication of this article.
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Appendix 1
distance from a reference point O to the
unworn edge, mm
measurement taken from a reference point O
after the corner edge is worn out, mm
maximum delaminated diameter, mm
nominal diameter of the drilled hole, mm
Young modulus, GPa
delamination factor, Fd ¼ D
Vickers hardness number, kgf/mm2
surface roughness, mm
3D surface roughness parameter, mm
3D surface roughness parameter, mm
thickness, mm
fiber volume content, % (percentage)
maximum flank wear, mm
tensile strength, MPa
drill diameter, mm
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