553
Wear mechanisms of TiN-coated CBN tool during
finish hard turning of hot tool die steel
J S Dureja1*, V K Gupta1, V S Sharma2, and M Dogra3
1
Department of Mechanical Engineering, Punjabi University, Patiala, India
2
Department of Industrial and Production Engineering, Ambedkar National Institute of Technology, Jalandhar, India
3
SSG Panjab University Regional Centre Hoshiarpur, Punjab, India
The manuscript was received on 23 May 2009 and was accepted after revision for publication on 16 September 2009.
DOI: 10.1243/09544054JEM1664
Abstract: The aim of the present investigation was to identify the wear mechanisms of TiN-
coated CBN tools prevalent under different machining conditions during hard turning of hot
tool die steel. The different wear mechanisms observed were abrasion wear at low cutting
speed, low feed rate, and high workpiece hardness; formation of a transferred layer resulting
from tribochemical reactions between constituents of the tool and workpiece material at high
speed; and the formation of built-up edges at moderate cutting speed. Hard carbide particles of
the work material at higher feed rate severely abraded the tool flank land, resulting in shallow
grooves due to the detachment of CBN grains. At greater depth of cut, the built-up edges and
transferred layer reduced friction and tool wear. Excessive adhesion of workpiece material
followed by plastic deformation and notching were clearly visible at low workpiece hardness
(47 HRC). The influence of cutting speed, feed rate, depth of cut, and workpiece hardness on the
progressive tool flank wear, i.e. flank wear rate (VBr, mm/mm) in the steady wear region, was
also analysed. The flank wear rate was observed to decrease with increase in cutting speed,
depth of cut, and workpiece hardness, but after an initial decrease it increased with increase in
feed rate.
Keywords: TiN-coated CBN tool, hot tool die steel, scanning electron microscopy
1 INTRODUCTION
turning is preferred over grinding [1]. CBN tools are
especially suitable for hard machining of steels
Finish hard turning, an emerging machining process,
because of their chemical inertness to steel. PCBN
enables manufacturers to machine tool steels and
has excellent mechanical properties such as high-
other steel parts in their hardened state with a single-
temperature strength, high thermal conductivity for
point polycrystalline cubic boron nitride (PCBN tool;
dissipating excessive heat generated during dry cut-
commonly known as CBN) or ceramic cutting tool
ting, high abrasive wear resistance accompanied with
without application of cutting fluid. Traditionally,
good chemical and thermal stability, and hardness
hardened steels are finish machined by the time-
next to diamond [2 5].
consuming grinding process. The shape of the
PCBN is broadly categorized as high CBN content
grinding wheel limits its application to regular
(CBN-H) or low CBN content (CBN-L) material.
workpiece geometries. The ability of machining
CBN-H grades contain approximately 80 95 per cent
complex workpiece geometries in a single setup,
CBN with a metallic-type binder. CBN-L grades
increased metal removal rate, and reduced setup
contain 40 70 per cent CBN and, of these, the
time are the key features due to which finish hard
majority have ceramic-based binder systems such as
TiC and TiN. The work conducted so far has revealed
*Corresponding author: Department of Mechanical Engineering,
that CBN-L materials perform better than CBN-H
University College of Engineering, Punjabi University, Patiala, materials during hard turning in terms of tool life and
Punjab 147002, India. surface finish [6]. Some authors have explained this
email: [e-mail] phenomenon to be the result of greater bonding
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
554 J S Dureja, V K Gupta, V S Sharma, and M Dogra
strength of CBN-L tools, while others have proposed 17 19]. The binder in PCBN cutting tools is more
that welded layers on the tool flank wear land of liable to this form of wear and some of the phases in
CBN-L tools act as protective layers and reduce wear the tool react sharply with the workpiece material,
of these tools [7 9]. Some researchers have attributed resulting in structural changes [18]. The structural
the poor wear performance of CBN-H tools to greater changes can make the binder less wear-resistant,
plastic deformation and higher defect density in thereby leading to an increase in abrasive wear.
these tools [10, 11]. The hardness and fracture Similar to diffusion, high temperature at the cutting
toughness of CBN tools decrease with reduced CBN edge promotes chemical reactions in this region.
content [12]. Owing to the ceramic binder phase, Many researchers have reported the presence of a
CBN-L tools have a lower thermal conductivity, layer on the surface of the PCBN tool, outside the
which causes increasing temperature of the cutting contact area of contact between the tool and work-
edge during hard turning, leading to softening of piece, known as the built-up layer [15, 17, 18, 20 23].
workpiece material in the cutting zone, reducing It is formed due to chemical reactions occurring
cutting forces, and tool wear. Matssumoto et al. [13] between the tool and the workpiece in the area of
reported that CBN-L tools are more suitable for finish contact and is facilitated by high temperature.
turning of hardened steel. At low cutting speeds, the Because of relatively large cutting forces, these reac-
tool life of CBN-L tools is superior to that of CBN-H tion products move to the surrounding area and get
tools, whereas at higher cutting speeds the reverse is deposited on the non-contact tool surfaces. During
true; also the surface roughness is less favourable investigation of deposited layers, Klimenko et al. [21]
when using CBN-H tools [14]. found that these consist of elements from the tool
The extreme cutting conditions during dry turning and the workpiece (B, C, N, Si, Al, Cr, Fe) and their
of hardened parts results in high temperatures at the reaction products with atmospheric oxygen. It is also
tool chip interface and tool work interface, which observed that the built-up layer protects the tool
accelerates tool wear and leads to rapid deterioration from wear. Luo et al. [15] found that the life of PCBN
of surface finish and dimensional and shape accu- tools increases with increasing cutting speed until a
racy. It is therefore important to understand tool critical value is reached, beyond which tool life
wear mechanisms and the influence of cutting con- decreases. It is suggested that the adhered layer pro-
ditions on the performance of these tools under dif- tects the tool until a temperature is reached at which
ferent machining applications. The prominent wear the layer becomes soft and gets removed, leading to
mechanisms that govern CBN tool wear in hard increased tool wear.
turning are abrasion [1, 2, 15 17], adhesion resulting Ohtani and Yokogawa [24] revealed that the main
from tribochemical interactions [2, 17, 18], diffusion wear mechanism of CBN and ceramic tools during
[2, 17 19], and chemical wear [15, 17, 20 23]. The machining of cold work tool SKD11 (18 60 HRC) is
occurrence of each mechanism depends on abrasion by hard alloy carbide particles contained in
mechanical and thermal loading during machining, the workpiece material. Narutaki and Yamane [2]
CBN content of the tool, type of binder phase, com- found that tool wear is primarily abrasion during
position of the workpiece material, and the cutting machining of work materials containing ultra-hard
conditions. The different wear mechanisms coexist carbides in their study. The tool wear resistance
under different cutting regimes. Even for the same increases with increase in CBN content. During hard
tool workpiece combination the wear mechanisms turning of AISI 4340 alloy steel with a PCBN tool
are different, as different cutting conditions bring the having a ceramic-based binder (TiC þ Al2O3), Luo
tool into significantly different cutting regimes. et al. [15] suggested that the main wear mechanism
Abrasion is caused by the presence of hard carbide for PCBN tools is abrasion of the binder phase caused
particles in the workpiece material and CBN grains by hard particles present in the workpiece. Davies
present in the cutting tool [1, 2, 15 17]. As the binder et al. [16] found for a CBN-H metal-matrix tool that
material gets abraded by hard carbide particles from interactions between the binder and the work mate-
the workpiece, CBN grains are more easily detached rial produce highly adherent layers and that bond
and contribute to further abrasion [15]. Adhesion failure between CBN grains and the matrix leads to
occurs when material from the workpiece or chip the plucking of CBN grains, which causes significant
melts due to high temperature and prevailing stress abrasive wear and grooves on the tool flank. During
conditions at the cutting edge, and adheres to the hard turning of AISI 52100 steel, Poulachon et al. [25]
non-contact surface of the tool [15, 17, 18, 21 23]. observed abrasion by hard alloy carbide particles
The area and thickness of the deposited layer depend present in the workpiece as the main wear mechan-
on the cutting conditions and tool wear rate, as these ism for the CBN tool. The wear mechanism depends
factors determine the temperature in the cutting on the nature of the carbides, their distribution, and
zone. Diffusion is facilitated by the high temperature size. Klimenko et al. [26] observed a coating resulting
reached during the metal cutting process [1, 2, from the tool wear process at the tool rake and flank
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
Wear mechanisms of TiN-coated CBN tool 555
faces, which consists of elements from the tool and Eda et al. [32] revealed the superiority of a TiN
workpiece material as well as products of their coating on CBN tools for machining both hard and
interaction with oxygen. During turning of hardened soft materials as it retards crater wear (diffusion
AISI 52100 steel with a CBN tool, Chou [27] noticed wear) and also reduces the friction force on the rake
that adhesive wear associated with the tribochemical face. Although previous works [2, 33] established that
process is the dominant wear mechanism. The cobalt CBN grains are quite stable and do not react with
binder in the BZN6000 tool (CBN-H) tends to form iron, recent research [18, 19] indicates that a tribo-
strong bonds with the piled-up layer, resulting in logical reaction occurs between the elements B, N,
more severe adhesive wear. However, the titanium and the chip (FeO) which is responsible for the delay
nitride binder present in BZN8100 (CBN-L) results in in diffusion wear in the presence of a TiN layer. The
relatively weaker bonds with the piled-up layer, thus high cutting speed accompanied by high cutting
increasing adhesive wear. temperature and the presence of abrasive particles
In their investigation of the performance and wear rapidly remove the TiN layer and the coated insert
behaviour of CBN tools during finish hard turning of behaves like an uncoated insert. However, at mod-
AISI 52100 steel, Chou et al. [20] revealed that adhe- erate cutting speed the coated insert exhibits the best
sion is the primary wear mechanism under normal results in terms of tool life. More et al. [34] investi-
cutting conditions. However, chemical diffusion gated the performance of CBN TiN composite-
dominates for extended periods of machining time coated and PCBN tipped inserts during hard turning
under aggressive cutting conditions. Dearnley and applications. The crater wear of the CBN TiN com-
Grearson [28] found that the CBN tool is prone posite-coated inserts was found to be less than that of
to diffusion/dissolution in the high-cutting-speed the PCBN inserts, which may be attributed to lubri-
range. Suh [29] observed that a diffusion/dissolution city of the TiN capping layer.
type of mechanism occurs when a hard cemented Although different wear patterns of CBN tools have
tungsten carbide or CBN tool is used to cut steel. In been reported, researchers normally regard flank
the machining of steel and superalloys by CBN tools, wear land width, VB, as the tool life criterion owing to
Kramer [30] mentioned that the cutting temperature its significant influence on surface finish and
may exceed 1200 C; thus the wear rate becomes dimensional accuracy of the machined part [35 37].
diffusion-dominated and both chemical stability and Sometimes the crater depth, KT, is also used as tool
diffusivity control the wear rate. Narutaki and life criterion especially during machining of titanium
Yamane [2] in their investigation of chemical inter- alloy [38]. Crater wear has a strong influence on
actions between CBN tool and pure iron workpiece process reliability as it can lead to instantaneous
revealed that there is no diffusion wear. However, failure due to chipping or fracture of the tool edge.
during tests with ceramic-binder PCBN tool and Hard turning with CBN cutting tools requires pru-
carbon steel workpiece, boron diffuses into the steel dent design of the tool geometry. CBN cutting tools
and the concentration of boron in the PCBN tool gets have lower toughness than other common tool
depleted up to a depth of 50 mm. In case of metal- materials and are more prone to chipping [38].
binder PCBN material, cobalt was observed to diffuse Therefore, a nose radius and proper edge preparation
from the tool but only up to a depth of 10 mm or less. are essential in order to increase the strength of the
The rate of diffusion was found to increase with cutting edge and attain favourable surface char-
increasing temperature but the authors concluded acteristics on the finished metal components [39].
that since the cutting temperature with PCBN is CBN cutting tools designed for hard turning possess
relatively low (<900 C), significant diffusion wear is negative rake and are provided with chamfer or hone,
noticed under extremely severe cutting conditions. or even both.
Eda et al. [31] suggested that CBN-H material has Due to the change in properties with increase in
higher thermal conductivity and therefore a lower hardness of workpiece material, the basic shearing
cutting temperature. Because of higher thermal con- process and formation of chips differ in hard turning
ductivity, CBN-H materials dissipate heat more [39]. Prior research showed that workpiece hardness
rapidly. Heat generated at the cutting edge facilitates has a profound effect on the performance of CBN
plastic deformation of the workpiece in the shear tools [2, 3, 20], cutting forces, residual stresses, and
zone. The CBN-L material, due to its lower thermal also integrity of the finish machined surfaces [40, 41].
conductivity, retains more heat at the cutting tip in Nakayama et al. [33] indicated that cutting forces in
the shear zone, which softens the workpiece and the machining of hard materials are not necessarily
promotes deformation and shearing. The higher high compared with those of soft materials. A high
temperature generated in the CBN-L material allows shear angle and the formation of saw-toothed chips
the formation of a protective layer on the tool. The due to poor ductility reduce the forces despite
study also revealed that TiC is more resistant to the high strength of hard materials. Ohtani and
chemical wear than CBN. Yokagawa [24] revealed that the lifespan of carbide
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
556 J S Dureja, V K Gupta, V S Sharma, and M Dogra
tools decreases as workpiece hardness increases, required for chip formation with the increase in
while the lifespan of CBN and ceramic tools shows material hardness.
the opposite trend. Enomoto et al. [42] indicated that The literature consulted so far reveals that only a
during turning of Cr Mo steels having various hard- limited number of studies pertaining to hard turning
ness levels, CBN tools have the shortest life when the
of hot tool die steel, particularly AISI H11 steel, have
steel is rather soft (35 HRC), whereas carbide tools
been carried out. The objectives of the present work
showed shorter life with the increase of work material
were to investigate the different wear mechanisms of
hardness. In their investigation of the wear behaviour
a TiN-coated CBN tool, having constant tool geo-
and cutting performance of CBN and ceramic tools
metry, prevalent under different machining condi-
during dry cutting of AISI 4340 alloy steels having
tions and to analyse the effect of cutting parameters,
different hardness (35, 45, 50, and 55 HRC), Luo et al.
i.e. speed, feed rate, depth of cut, and workpiece
[15] revealed that the wear of ceramic and CBN tools
hardness, on the progressive flank wear and surface
decreases with an increase in workpiece hardness,
roughness during hard turning of AISI H11 steel. The
but at about 50 HRC the wear starts to increase. This
influence of machining parameters on flank wear
behaviour is consistent with the variation of cutting
rate, corresponding to the steady-state region of the
force, which in turn is inversely related to the
flank wear versus cutting length curve, has also been
cutting temperature. A similar phenomenon has also
evaluated. For the purpose of machining, a CBN-L
been reported by Narutaki and Yamane [2], and
tool with a chamfer and hone was selected in order to
Matsumoto et al. [13]. During hard turning of AISI
attain best possible quality of machined surface.
52100 steel having three different hardness levels
(38, 52, and 60 HRC) with CBN tools, Poulachon et al.
[25] defined 50 HRC as a limiting value of hardness,
2 EXPERIMENTAL DETAILS
below which the cutting forces decrease with work-
piece hardness. Derakhshan and Akbari [43] in their
2.1 Work material
investigation of the effect of workpiece hardness (45,
50, 55, and 60 HRC) and cutting speed on surface
In this study, the chosen workpiece material was hot
roughness during hard turning with CBN tools found
tool die steel (AISI H11) in the form of round bars
that average surface roughness Ra increases with
having 35 mm diameter and 150 mm length. The
increase in work hardness, but was observed to be
workpiece was through-hardened followed by tem-
lowest at 65 HRC using CB7020 tools. However, with
pering to achieve three hardness levels of 42, 47, and
CB50 tools surface roughness decreases with increase
52 HRC. The structure obtained was martensitic with
in work hardness and was lowest at 60 HRC. When
cementite and a small amount of other metal car-
investigating the effect of process parameters during
bides. The chemical composition of the workpiece
hard turning of AISI H13 steel at two hardness levels
material is given in Table 1. The steel investigated
(51.3 and 54.7 HRC) with CBN tools, Ozel et al. [44]
here is widely employed for the production of forging
revealed that the effects of workpiece hardness, cut-
dies, jigs and fixtures, back-up and support tools, etc.
ting edge geometry, feed rate, and cutting speed on
surface roughness are statistically significant. The
effects of two-factor interactions  of the edge geo-
2.2 Cutting tool
metry and the workpiece hardness, the edge geo-
The tool insert selected had low CBN content (50 per
metry and the feed rate, and the cutting speed and
cent CBN) with fine, medium-grained titanium
feed rate  also appeared to be important. Especially
binder phase, CBN tipped and coated with TiN
honed edge geometry and lower workpiece surface
and was in accordance with ISO specification
hardness resulted in better surface roughness.
WNGA080408S01030A. The CBN-L tool was selected
Tamizharasan et al. [45] analysed tool wear and sur-
in this study due to its superior performance and tool
face finish during hard turning of crank pins at three
life compared with CBN-H tools during continuous
hardness levels using different grades of PCBN cut-
turning, as stated in section 1. The edge preparation
ting tool and described various characteristics in
provided on the insert was 30 · 0.1 mm chamfer with
terms of component quality, tool life, tool wear,
light honing. During the conduct of each test, a fresh
effects of individual parameters on tool life and
cutting edge was used.
material removal, and economics of operation. Dur-
ing their investigation on the effect of cutting force
during turning of hardened tool steels at different
Table 1 Chemical composition of AISI H11 steel (wt%)
hardness levels (44, 48, 52, 58 HRC) with CBN inserts,
CSiMn Cr Mo V
Qian and Hossan [46] observed that cutting force
increases slightly with the increase in workpiece
0.33 0.95 0.27 5.32 1.22 0.36
hardness. This is because more deformation energy is
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
Wear mechanisms of TiN-coated CBN tool 557
2.3 Experimental procedure feed rate, depth of cut, and workpiece hardness on
progressive flank wear of the tool, the experiments
The cutting inserts were clamped to a right-hand
were performed at three different levels of these
tool holder with ISO designation MWLNR 2525 M08.
individual parameters as given in Table 2. The range
The clamping of the insert on the tool holder resulted
in 6 rake angle, 6 clearance angle, and 95 of machining parameters selected is in accord-
ance with the range specified by the manufacturer
approach angle.
(Sandvik Coromant) of the inserts [47]. The lower
Before actual testing, the oxidized layer present on
limit of workpiece hardness, i.e. 42 HRC, is the
the workpiece surface was removed. For this purpose,
minimum recommended workpiece hardness for
the outermost 1 mm thick layer of material was
hard turning operations [48], whereas the high limit
removed from all test specimens by turning with
of workpiece hardness, i.e. 52 HRC, is the maximum
coated carbide inserts. Subsequently, longitudinal
recommended hardness for AISI H11 tool steel used
turning tests were conducted under dry conditions
in this study [49]. The intermediate levels of the
on a computer numeric control lathe (model MSC-
machining parameters and workpiece hardness were
ZL25MC-187; Mori Seiki, Japan) having the following
chosen in between the higher and lower limit speci-
specifications: maximum power ź 56 kW (75 hp),
fied. The range of machining parameters selected
maximum diameter of work ź 300 mm, maximum
herein is also well supported by some earlier inves-
cutting length ź 600 mm, and maximum spindle
speed ź 3500 r/min. The experiments were inter- tigations [1, 22, 50 55].
rupted at regular intervals, i.e. after every 2, 4, 6, 10,
14, and 20 passes, to record the tool flank wear and
average surface roughness of the machined surface.
3 RESULTS AND DISCUSSION
Each single pass consisted of axial cutting length of
125 mm. The flank wear, defined as the average width
3.1 Effect of machining parameters on flank wear
of flank wear land VBavg of worn-out inserts, was
Figures 1(a) to (d) show the progression of flank wear
measured with the help of an optical microscope
with helical cutting length for different combinations
(Leica, Germany) equipped with an integral digital
camera and computer interface having magnification of machining parameters. The flank wear decreases
in the range of 100 · to 1000 · and least count equal to with increase in cutting speed (Fig. 1(a)). Figure 1(b)
0.01 mm. The tool wear was characterized through depicts that the flank wear initially decreases with
scanning electron microscopy (SEM) equipped with
feed rate and then increases with increasing feed rate.
energy-dispersive X-ray analysis (EDAX) (model
It is important to mention that at the highest feed
Quanta F-200; FEI, Netherlands) having a maximum
rate of 0.15 mm/r, the experiment was terminated
magnification of 106 times, resolution of 2 nm, and
after ten passes due to excessive chattering. The
operating voltage in the range of 200 V to 30 kV. The
excessive chattering resulted in poor surface finish
surface roughness, Ra (mm), of the machined samples
(Ra > 1.6 mm) and so the test was stopped. The
was measured with a surface analyser (model SurfT-
increase in depth of cut reduces the flank wear as
est SJ-401; Mitutoyo, Japan) with a cut-off length of
shown in Fig. 1(c). The test corresponding to the
0.8 mm over three sampling lengths, which were
minimum depth of cut (0.05 mm) was stopped after
distributed circumferentially at an angle of 120 . The
ten passes, as surface roughness was excessive
average of these three values of surface roughness Ra (Ra > 1.6 mm). Figure 1(d) depicts the influence of
was used to quantify the roughness achieved on the
workpiece hardness on flank wear. The flank wear
machined surfaces.
exhibits a decreasing trend with increasing workpiece
The experiment was terminated when either of the
hardness. The tests corresponding to workpiece
following two conditions was met:
hardness of 42 and 47 HRC were stopped after 14
passes as the flank wear was excessive (VB > 200 mm).
(a) condition 1, VB > 200 mm;
(b) condition 2, Ra > 1.6 mm.
The limiting value of average flank wear (i.e. Table 2 Machining parameters and their levels*
200 mm) was selected as the tool life criterion
Parameter Range
according to JIS B4011-1971 standard, whereas for
hard turning to replace grinding, the surface rough-
Cutting speed, V (m/min) 100, 140, 180
Feed rate, f (mm/r) 0.05, 0.1, 0.15
ness (Ra) should be less than 1.6 mm.
Depth of cut, a (mm) 0.05, 0.10, 0.20
Workpiece hardness, WH (HRC) 42, 47, 52
2.4 Plan of experiments
* While one parameter was being varied, the remaining parameters
In order to identify different wear mechanisms of the
were kept constant during the tests at the following levels:
CBN tool and to analyse the effect of cutting speed, V ź 140 m/min, f ź 0.05 mm/r, a ź 0.1 mm, and WH ź 52 HRC.
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
558 J S Dureja, V K Gupta, V S Sharma, and M Dogra
Effect of Speed on VB
Effect of Feed on VB
90
100
80 90
80
70
70
60
60
50
50
40
40
30
30
Feed=0.05mm/rev.
20 Speed=100m/min.
20
Feed=0.1mm/rev.
Speed=140m/min.
10
10
Speed=180m/min. Feed=0.15mm/rev.
0 0
0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000
Cutting length (mm)
Cutting Length (mm)
(b)
(a)
Effect of Depth of cut on VB Effect of Work Hardness on VB
90 250
80
200
70
60
150
50
40
100
30
W.Hard.=42
50
20 D.O.C.=0.05mm
W.Hard.=47
D.O.C.=0.1mm
10
W.Hard.=52
D.O.C.=0.2mm 0
0
0 500 1000 1500 2000 2500 3000
0 500 1000 1500 2000 2500 3000
Cutting Length (mm) Cutting Length (mm)
(c) (d)
Fig. 1 Effect of machining parameters on progressive tool flank wear
In order to clearly understand and compare the cles of the work material on the flank face of the
influence of machining parameters on flank wear, inserts [44]. At low cutting speed of 100 m/min, the
the flank wear rate (VBr ź DVB/D cutting length) was binder present in the cutting tool gets easily abraded
obtained from the slope of the flank wear versus axial from the substrate due to the high cutting force,
cutting length curves in Figs 1(a) to (d). For this resulting in less softening of the work material as a
purpose, the data corresponding to each test run in result of low cutting temperature. Owing to high
Figs 1(a) to (d) were linearly fitted by excluding the cutting forces, CBN grains are detached from the
data in the initial wear-in/rapid wear period and the substrate, thereby causing abrasive wear [56, 57]. At
data falling in the accelerating/tertiary region. The locations from where the binding material has been
steady wear rate, thus obtained, is plotted in Figs 2(a) abraded by the rubbing of carbide particles present in
to (d). Figures 3(a) to (l) show optical micrographs of workpiece material, the freshly exposed CBN grains
worn-out inserts, taken at 200 · magnification, for are more easily removed and contribute to further
varying machining conditions and Figs 4(a) to (l) abrasion [18]. As the cutting speed increases from
show the corresponding SEM micrographs of these 100 m/min to 140 m/min, the wear rate decreases
worn-out inserts. slightly. A transferred/protective layer resulting from
The steady-state flank wear rate VBr decreases with diffusion of the bonding material (TiC) of the tool
increasing cutting speed as shown in Fig. 2(a). Similar forms at the tool chip interface. This transferred
results have also been reported by Chou et al. [20] layer acts as a diffusion barrier against chip material;
and Ozel et al. [44]. At all cutting speeds, abrasive hence the tool wear rate is decreased. The transferred
wear marks are observed on the flank wear land of layer can be seen in Figs 3(b) and 4(b). In order to
the inserts as revealed from the optical micrographs confirm the composition of transferred layer, EDAX
in Figs 3(a) to (c) and the SEM micrographs in analysis of the worn-out insert used at 140 m/min
Figs 4(a) to (c). The wear pattern at the lowest cutting was performed at two different locations as indicated
speed (i.e. 100 m/min) is abrasive wear, accompanied by crosshair mark in the SEM images given in Figs 5
by adhesion of work material (transferred layer), as (a) and (b). The EDAX pattern shows that besides
clearly observed in Figs 3(a) and 4(a). The scratch oxygen, the transferred layer consists of very high
marks result from the rubbing of hard carbide parti- concentrations of Fe and other elements such as Mn,
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
VB(µm)
VB(µm)
VB(µm)
VB(µm)
Wear mechanisms of TiN-coated CBN tool 559
Flank wear rate Vs Speed
Flank wear rate Vs Feed
18
40
16
35
14
30
12
10
25
8
20
6
15
4
10
2
5
0
0
90 140 190
Speed (m/min) 0 0.05 0.1 0.15 0.2
feed (mm/rev.)
(a)
(b)
Flank wear rate Vs W. Hardness
Flank wear rate Vs Depth of cut
90
25
80
20
70
60
15
50
40
10
30
20
5
10
0
0
0 0.1 0.2 0.3
38 43 48 53
DOC (mm)
W.Hard. (HRC)
(d)
(c)
Fig. 2 Flank wear rate versus machining conditions
Cr, and Si, which are the constituents of workpiece feed rate as shown in Fig. 2(b). At the lower feed rate
material. The EDAX pattern also reveals the presence of 0.05 mm/r, abrasive marks are observed on the
of elements like Ti, N, and Al, which are present in flank wear land as evident from the optical micro-
the binder phase of the CBN tool. The transferred graph in Fig. 3(d) and SEM micrograph in Fig. 4(d). At
layer is formed at higher cutting speeds due to the high higher feed rate, higher cutting temperature expe-
temperature generated. The transferred layer acts as a dites the oxidation of TiN binder, exposing some of
diffusion barrier against chip material and limits fur- the CBN particles on the flank wear land, which are
ther increase in tool wear. Hence, tool wear rate is severely gouged by the hard particles of workpiece
reduced. Luo et al. [15] revealed that wear of PCBN material, thus enhancing the tool wear. The high
tools decreases with increasing cutting speed until a temperature generated at higher feed rate leads to
critical value is reached after which tool life decreases. removal of transferred layer from the surface of the
A further drop in the flank wear rate is observed with insert, thereby accelerating the tool wear, as evident
increasing speed to 180 m/min. Some shallow pockets from Figs 3(f) and 4(f). The experiment conducted at
can be noticed in the SEM micrograph (Fig. 4(c)) the highest feed rate (0.02 mm/r) was discontinued
because of detachment of a portion of transferred after ten passes, due to excessive tool chattering, as
layer, confirming adhesion wear at high speed [58]. the surface roughness exceeded the limiting value
The flank wear rate decreases marginally with (Ra > 1.6 mm).
increasing feed rate from 0.05 mm/r to 0.1 mm/r; At low depth of cut, metal becomes too thin to be
however, it increases rapidly with further increase in cut off. In such a situation, the squeezing action on
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
VBr (µm/mm)
VBr (µm/mm)
VBr (µm/mm)
VBr (µm/mm)
560 J S Dureja, V K Gupta, V S Sharma, and M Dogra
Abrasion marks
Grooves
Transferred layer
(a) (Speed=100 m/min) (b) (Speed=140 m/min) (c) (Speed=180 m/min)
Varying cutting speed
Grooves
(d) (Feed=0.05 mm/r) (e) (Feed=0.1 mm/r) (f) (Feed=0.15 mm/r)
Varying feed rate
Transferred layer
(g) (DOC=0.05 mm) (h) (DOC=0.1 mm) (i) (DOC=0.2 mm)
Varying depth of cut
Transferred
Adhered material
material
on tool nose
Adhesion followed by
Abrasion marks
plastic deformation
25 kV Mag 400X 10 µm
(j) - (Work Hard. = 42HRC) (k) - (Work Hard. = 47HRC) (l) - (Work Hard. = 52HRC)
Adhesion followed by
Varying workpiece hardness
Fig. 3 Optical micrographs of worn-out inserts obtained at: varying cutting speed, (a) to (c); varying feed
rate, (d) to (f); varying depth of cut, (g) to (i); varying workpiece hardness, (j) to (l)
the workpiece by the round portion of the cutting 0.05 mm to 0.1 mm, as evident from Fig. 3(h). With
edge can be intensified, thus increasing the friction, the increase in depth of cut to 0.1 mm, a built-up
which leads to increased tool wear as evident in edge and transferred layer are formed on the tool
optical and SEM micrographs, Figs 3(g) and 4(g). The face, which decreases friction at the tool chip inter-
experiment conducted at the lowest depth of cut face, thereby decreasing the tool wear as revealed in
(0.05 mm) was stopped after ten passes as the surface Figs 3(h) and 4(h). As the depth of cut is increased
irregularities enhance tool chattering, resulting in further to 0.2 mm, the tool works against relatively
poor surface roughness which exceeds the limiting softer material as the hardness of the cylindrical
value of Ra > 1.6 mm. The flank wear rate is observed workpiece decreases upon moving radially inward,
to decrease with increase in depth of cut from owing to differences in the quenching rate from the
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
Wear mechanisms of TiN-coated CBN tool 561
Fig. 4 SEM micrographs of worn-out inserts obtained at: varying cutting speed, (a) to (c); varying feed
rate, (d) to (f); varying depth of cut, (g) to (i); varying workpiece hardness, (k) to (l)
outer surface towards the core during the hardening and Si, which are the constituents of workpiece
process. As a result, the tool wear decreases as is material. The EDAX pattern also reveals the presence
evident in Figs 3(i) and 4(i). In order to confirm the of elements like Ti, N, and Al, which are present in the
composition of the built-up layer, EDAX analysis of binder phase of the CBN tool.
the worn-out insert used at 0.2 mm depth of cut was The flank wear rate decreases with increase in
carried out at the position indicated by the crosshair workpiece hardness, Fig. 2(d). This phenomenon was
mark in the SEM image shown in Fig. 6. The EDAX explained by Luo et al. [15]. They observed during
pattern shows that besides oxygen, the built-up layer turning work of hardness below 50 HRC by ceramic
consists of Fe and other elements such as V, Cr, Mn, and CBN tools that the cutting temperature is
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
562 J S Dureja, V K Gupta, V S Sharma, and M Dogra
Fig. 5 EDAX pattern of the worn-out insert (V ź 140 m/min, f ź 0.05 mm/r, a ź 0.1 mm, WH ź 52 HRC)
Fig. 6 EDAX pattern of the worn-out insert (V ź 140 m/min, f ź 0.05 mm/r, a ź 0.2 mm, WH ź 52 HRC)
increased with the increase of the work material and the cutting temperature starts to decrease. As a
hardness. This causes the workpiece to become result, the degree of softening of the workpiece
softer, so that the cutting forces produced are remains smaller and there is less amount of adhered
decreased. Furthermore, the adhesive force of the layer on the tool face. Moreover, high shear stress and
chip tool interface at higher temperature would strain on the saw-tooth chip are produced. Hence,
increase, which can cause the adhered layer on the the cutting force starts to increase, and tool wear also
tool face to increase, thus protecting the cutting becomes larger. When workpiece hardness is below
edges and reducing the tool wear. However, in the 47 HRC, excessive adhesion of workpiece material
case of turning work material having hardness above is observed as shown in Figs 3(j) and (k) and Figs 4(j)
50 HRC, the saw-tooth chip is gradually produced and (k). The composition of adhered material on the
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
Wear mechanisms of TiN-coated CBN tool 563
Fig. 7 EDAX pattern of the worn-out insert (V ź 140 m/min, f ź 0.05 mm/r, a ź 0.1 mm, WH ź 47 HRC)
tool edge, corresponding to workpiece hardness of 47 expected, the surface roughness increases with
HRC, was confirmed by EDAX analysis (Fig. 7) at two increasing cutting length because the blunt/worn-out
different locations as indicated by crosshair marks in
tool results in chatter marks on the machined surface
the SEM micrographs. The EDAX pattern reveals high
and decreases the surface finish. The best surface fin-
concentrations of Fe, Mn, and Cr, which are con- ish is achieved corresponding to the combination of
stituents of workpiece material, adhered to the tool
lowest feed and highest speed, Fig. 8(a) and (b). The
and undergoes plastic flow, as evident in Figs 3(k)
experiment conducted at the highest feed rate, i.e.
and 4(k). However, as this material gets detached, it
0.15 mm/r, was stopped after ten passes as surface
carries away a huge portion of the tool edge, resulting
roughness exceeded the limiting value (Ra > 1.6 mm).
in notch wear as observed in Figs 3(j) and 4(j). The
The experiment performed at the lowest depth of cut
experiments corresponding to workpiece hardness of
(0.05 mm) was also stopped after ten passes as surface
42 HRC and 47 HRC were stopped after 14 passes as
roughness exceeded the limiting value. The increase in
flank wear exceeded the limiting value (VB > 200 mm).
depth of cut results in decreased surface roughness as
The excessive deformation of the tool below work-
evident from Fig. 8(c). As pointed out in section 3.1, at
piece hardness below 47 HRC suggests non-suit-
low depth of cut metal becomes too thin to be cut off;
ability of this tool material below this workpiece
therefore the squeezing action on the workpiece
hardness. As the workpiece hardness increases to 52
by the round portion of the cutting edge gets intensi-
HRC, the tool wear mechanism changes to abrasive
fied, thus increasing the friction and deformation
mode as observed in Figs 3(l) and 4(l).
which subsequently leads to poor roughness of the
machined surface. In general, the surface roughness
3.2 Effect of machining parameters on
increases marginally with increased workpiece hard-
surface roughness
ness (Fig. 8(d)). This is because the effort required to
plough the work material (i.e. cutting forces) increases
Surface roughness is an indicator of the surface quality
of machined surfaces and also acts as a yardstick for with the increase in workpiece hardness, leading to
predicting tool failure. Figures 8(a) to (d) show the chip segmentation that results in rapid tool wear and
variation of surface roughness with cutting length. As poor surface finish.
JEM1664 Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture
564 J S Dureja, V K Gupta, V S Sharma, and M Dogra
Effect of cutting speed on Ra Effect of Feed on Ra
1.4 1.6
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4 0.4 Feed=0.05mm/rev.
Speed=100m/min.
Feed=0.1mm/rev.
0.2
Speed=140m/min.
0.2
Feed=0.15mm/rev.
Speed=180m/min. 0
0
0 500 1000 1500 2000 2500 3000
0 500 1000 1500 2000 2500 3000
Cutting Length (mm)
Cutting length (mm)
(a) (b)
Effect of Depth of cut on Ra Effect of Work Hardness on Ra
1.4
1.8
1.6
1.2
1.4
1
1.2
0.8
1
0.8 0.6
0.6
0.4
W.Hard=42HRC
D.O.C.=0.05mm
0.4
W.Hard=47HRC
D.O.C.=0.1mm 0.2
0.2
W.Hard=52HRC
D.O.C.=0.2mm
0
0
0 500 1000 1500 2000 2500 3000
0 500 1000 1500 2000 2500 3000
Cutting Length (mm)
Cutting Length (mm)
(c) (d)
Fig. 8 Surface roughness versus cutting length for different machining conditions
4 CONCLUSIONS 6. For workpiece hardness below 47 HRC, excessive
adhesion of workpiece material followed by plas-
The following conclusions can be drawn from the tic flow is observed. However, abrasive wear pre-
present study. vails at higher workpiece hardness.
1. The steady-state wear rate decreases with Thus, the present investigation helps to identify the
increase in cutting speed, depth of cut, and main wear mechanisms involved with TiN-coated
workpiece hardness. CBN tools under various machining conditions over
2. The flank wear rate initially decreases slightly with the range of parameters investigated. This will assist
increase in feed rate, followed by rapid increase manufacturers in selecting proper machining para-
with further increase in feed rate. meters for hard machining of tool steels. The study
3. At low cutting speed, low feed rate, and higher has revealed that PCBN is not economical for
work hardness, abrasion dominates the tool wear. machining tool steel having hardness below 47 HRC
The transferred layer (adhesion wear) and built- as it leads to excessive adhesion of work material on
up edge formation reduce the tool wear rate the tool face, thereby causing significant flank wear.
with increase in cutting speed at relatively higher
feed rate.
4. Higher feed rate increases the cutting tempera- ACKNOWLEDGEMENTS
ture resulting in oxidation of TiN binder, exposing
some of the CBN particles on flank wear land The authors wish to thank Sandvik Coromant for
which are severely abraded by the hard carbide supplying tool inserts to conduct the experiments;
particles of workpiece material, and thereby the Institute of Auto Parts and Hand Tools, Ludhiana,
accelerating the tool wear. for their support in execution of the tests; and the
5. At larger depth of cut the built-up edge and Indian Institute of Technology, Roorkee, for provid-
transferred layer formed on the tool face decrease ing characterization facilities.
friction at the tool chip interface, thereby
decreasing tool wear. Ó Authors 2010
Proc. IMechE Vol. 224 Part B: J. Engineering Manufacture JEM1664
Ra(µm)
Ra(µm)
Ra(µm)
Ra(µm)
Wear mechanisms of TiN-coated CBN tool 565
nitride cutting tools using Auger electron spectroscopy
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