Cutting tools need to be wear resistant, hard and chemically inert to prevent
chemical interaction between the cutting tool, workpiece material and cutting
environment during machining process. It was found that the high hardness surface
of the cutting tool increases the wear resistance of cutting tool (Kumar
et al., 2007; Jianxin et al., 2011).
To accomplish these objectives, some of the cutting tool is coated with single
or multiple materials to increase the wear resistance and improve the performance
of the cutting tool. In North America and Western Europe, it was estimated that
more than half of the metal-cutting tools are coated by either Chemical Vapour
Deposition (CVD) or Physical Vapour Deposition (PVD) (North,
Coating basically can increases the wear resistance, prolonged the tool life
and reduces the cutting forces and cutting temperature (Jindal
et al., 1999). It is well known that thin and/or hard coatings can
reduces tool wear and improves tool life and hence the productivity (Chung-Chen
and Hong, 2002; Yigit et al., 2008). Hard
coatings such as TiN provides markedly improvements in many manufacturing processes
and offer particular productivity advantages when applied to the tools used
in metal-cutting operations. For examples, Tang et al.
(2000) found a great improvement in tool life with diamond-cobalt boride
Similarly, Shao et al. (2007) found that coating
could effectively reduce the formation of adhesive layers on the tool and thus
prevent the formation of BUE. Liew and Ding (2008) had
reported that coating increased the cracking, fracture and abrasion wear resistance
in milling stainless steel at low speed. Therefore, coated tool had longer tool
life than uncoated tool. Coating was also found to enhance the lubricity of
the tool, improve the mechanical (such as mechanical cracking and fracture)
and chemical wear resistance (such as oxidation and diffusion wear) and reduces
cutting temperature and temperature variation of the tool, rendering it less
susceptible to crack (Nordin et al., 2000; Arndt
and Kacsich, 2003; Ghani et al., 2004; Liew
and Ding, 2008).
The machining condition and wear mechanism are dependent on the cutting environment,
cutting parameters, cutting tool and workpiece (Ema and
Davies, 1989; Chung-Chen and Hong, 2002; Richetti
et al., 2004; Korkut and Donertas, 2007; Tzeng,
2007). Hence, research is essential to determine the optimum cutting conditions.
Recommendations from manufacturers should only be used as a guide because better
cutting conditions for a specific situation can be only found through research
(Richetti et al., 2004).
Ti-BASE HARD COATING, TiAlN AND TiCN
Titanium alloy are widely used owing to their low density, high specific strength,
exceptional corrosion resistance and can stand high temperature. However, titanium
alloys suffer from the serious disadvantages of poor tribological properties
such as high friction coefficient, difficulty to lubricate and low adhesive
and fretting wear resistance which prevent the applications of it as engineering
tribological components (Yang et al., 2007).
Various surface modification techniques have been proposed for the enhancement
of its friction and impact resistant. The most recognized method consists of
the formation of a thin layer of a hard ceramic phase, such as TiN and TIC,
at the surface of cutting tool (Yang et al., 2007).
The advantages of hard coating such as high hardness, lower friction coefficient
and chemically stable had attracted the eyesight of the industry and the uses
of hard coating was highly increased in the past 30 years (Chung-Chen
and Hong, 2002). Ti-base coatings were widely applied to improve the lifetime
and performance of a wide variety of tools.
TiN, TiC, TiCN and TiAlN and are the common used Ti-base hard coating. TiN
possesses some very useful properties, despite its relatively lower hardness.
It is chemically more stable and provides excellent resistance to the formation
of BUE (Narasimhan et al., 1995). Despite of
improving the tool wear resistance, the adhesive of TiN on the metal is poor
due to the surface contaminants and residual tensions at the layer-metal interface
(Yang et al., 2007).
Conventionally, a layer of TiN coating is often used to increase the surface
hardness, reducing the coefficient of friction and to provide a layer of heat
barrier to the cutting tool. The TiC coating has relatively higher affinity
to the cemented carbide substrate compared to the TiN coating. TiC coating has
greater propensity to develop the brittle eta phase at the interface and it
has been suggested by many researchers that the formation of a small amount
of eta phase is beneficial whereby it provides a diffusion interface to reduce
the transfer of heat into the tool and leading to an improvement in adhesion
(Narasimhan et al., 1995).
Titanium Carbonitride (TiCN) is the solidification of TiN and TiC. The inclusion
of carbon atoms in the TiN lattice results in a substantial increase of the
film hardness and in a lower friction coefficient. For these reasons Ti (C,
N) coatings are often used in cutting tools and cold forming applications, replacing
conventional TiN coatings (Bemporad et al., 2001).
TiCN owns the excellence of both TiC and TiN. This has arisen from the fact
that TiCN possesses high hardness and excellent wear resistance (Yang
et al., 2007). Narasimhan et al. (1995)
had found that TiN coatings had a larger crystallite size than the TiCN coatings
and the adhesion property of TiN is better than TiCN. However, the microhardness
of TiCN is higher than TiN, the coefficient of friction of TiCN is constantly
lower than TiN and TiCN coatings can significantly reduces the cutting force
compare to TiN coatings. Hence, TiCN coated cutting tool had better wear resistance
and longer tool life than TiN coated cutting tool.
TiCN had chemical stability and superior mechanical properties such as low
friction, high hardness (HV 2500-3000), high toughness, high melting point (3050°C),
good electrical conductivity and excellent wear resistance (Narasimhan
et al., 1995; Cheng et al., 2010;
Yang et al., 2010). TiCN had better anti-wear
capabilities compare to TiN and TiC (Narasimhan et al.,
1995; Yang et al., 2010).
THE EFFECTS OF COATING DEPOSITION METHOD ON COATING PERFORMANCE
Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD) are the
most common and conventional methods use to produce coating for cutting tool
applications. The first cutting tool coating was produced by CVD in late 1960s
and early '70s (Destefani, 2002). Generally, in CVD
process, the tools are heated in a sealed reactor to about 1000°C (1830°F).
Gaseous and volatile compounds supply the metallic and non-metallic constituents
of the coating materials and inert gas is sometimes introduced into the reactor,
depending on the needs. Thickness of CVD coatings can range from 5-20 μm.
PVD emerged in the 1980s as a viable process for applying hard coatings to
cemented carbide cutting tools (Destefani, 2002) and
is generally performed in vacuum chamber (Destefani, 2002;
Suresha et al., 2006). The metallic sources of
the coating, obtained via arc evaporation or sputtering, react with gaseous
that provides non-metallic sources and subsequently produces thin films that
deposited onto the substrate (Destefani, 2002; Suresha
et al., 2006). Due to low pressure, the coating atoms and molecules
undergo relatively few collisions on their way to the substrate. PVD is therefore
a line-of-sight process that requires moving fixtures to ensure uniform coating
deposition (Destefani, 2002).
The chief difference between PVD and CVD is the processing temperature. PVD
process is carries out at a temperature much lower than CVD process, below 500°C,
while CVD process is performs at a higher temperature of above 700°C (Hintermann,
1984; Nordin et al., 2000; Kopac
et al., 2001; Destefani, 2002). The high
temperature of CVD may changes or destroys the tribological properties of the
substrate, increase the potential of catastrophic tool failure (Hintermann,
1984; Nordin et al., 2000; Kopac
et al., 2001; Destefani, 2002; Yang
et al., 2010) but enhances the adhesion of coating to the substrate
(Hintermann, 1984; Destefani, 2002).
For instance, Transverse Rupture Strength (TRS) of substrates decreased after
CVD process or high temperature coating process but remained relatively unchanged
after PVD process (Quinto et al., 1988). High
temperature is needed to reach sufficient adhesion strength between coating
and substrate (Yang et al., 2010).
The differences in deposition process condition resulted intrinsic microstructural
differences between PVD and CVD coatings (Quinto et al.,
1988). The grain and crystalline structure of PVD coating is very fine,
resulted a very smooth and low coefficient of friction bright coating (Kopac
et al., 2001; Destefani, 2002). The fine
grain and crystalline structure of PVD coating enhances the diffusion wear resistance
while CVD coating does not barrier to the diffusion wear (Kopac
et al., 2001). However, CVD has the advantages of more uniform and
homogeneous coating deposition (Hintermann, 1984). Smoother
coating may adhere better to substrate (Destefani, 2002).
One relatively new approach to dealing with coating roughness is post-coat mechanical
polishing. The polished coating resulted smoother coating that has increase
in lubricity and may adhere better to the substrate than untreated coating which
is more susceptible to flaking (Destefani, 2002).
Among PVD, the sputtering PVD produced smoother and finer coating, while cathodic
arc evaporation produced denser coating due to the higher proportion of ionized
metal vapour. During arc evaporation, the bombardment of energetic metal ions
may penetrated into substrate at angstrom levels and knock out some atoms, promoting
the adhesivity of the coating to substrate (Suresha et
al., 2006). Hence, the density and adhesivity of cathodic arc evaporation
PVD coating are better than sputtering PVD coating (Suresha
et al., 2006). Cathodic arc evaporation PVD process is also known
as cathodic arc PVD (CAPVD).
PVD coating is essentially free of the thermal crack which is common in conventional
CVD coating (Santhanam et al., 1996; Destefani,
2002). CVD coating is usually in residual tension stress at room temperature
(Quinto et al., 1988; Santhanam
et al., 1996; Selinder et al., 1998;
Destefani, 2002), because coating material is generally
have higher coefficient of thermal expansion than substrate (Destefani,
2002). The high residual tension stress may relieved by transverse cracks
that don't affect coating adhesion but may initiate cracking and tool fracture
(Destefani, 2002). High compressive stress that helps
to resist crack initiation and propagation is developed in PVD coating (Quinto
et al., 1988; Santhanam et al., 1996;
Selinder et al., 1998; Destefani,
2002). Minimizing crack formation and propagation help in improving tool
edge security and edge-chipping resistance, thus preventing premature tool failure
(Quinto et al., 1988; Santhanam
et al., 1996; Selinder et al., 1998;
Destefani, 2002). Excessive stresses, however, causing
poor adhesion and brittle behaviour of the coating (Selinder
et al., 1998). Compressive stress can changes to tensile stress by
annealing heat treatment (Quinto et al., 1988;
Santhanam et al., 1996).
Other than thermal crack, high temperature CVD also results in formation of
eta phase in substrate-coating interface, while eta phase formation is eliminated
during PVD process (Santhanam et al., 1996; Destefani,
2002). Coupled with the formation of eta phase and high tendency of grown-in
thermal crack, CVD-coated tool has lower edge fracture strength than PVD-coated
tool, increase the tendency of edge chipping, particularly in interrupted machining
when cyclic thermal and mechanical stresses were more influenced (Santhanam
et al., 1996; Jindal et al., 1999;
Destefani, 2002). This allowed PVD to coat on sharp
edges and complex chip-breaker geometries tool, while in contrast, CVD-coated
tools require an edge-honing or chamfer to strengthens its edge (Santhanam
et al., 1996; Destefani, 2002). Therefore,
comparison of the performance among coating shall takes the effects of tool
geometries into account (Santhanam et al., 1996).
Nevertheless, Narasimhan et al. (1995) has proved
that their novel conventional CVD technique in TiCN coating production could
controlled or even eliminated the formation of the brittle eta phase, despite
it is often took place in conventional CVD process.
Although, it is recognised that the formation of brittle eta phases during
coating deposition is harmful but some researchers has been suggested that the
formation of a very small amount of eta phase during the formation of coating
is beneficial, because it provides a diffusion interface that leading to an
improvement of adhesion properties (Narasimhan et al.,
CVD coatings are generally thicker than PVD coatings, on the order of microns
thick versus as thin as a few nanometers. PVD is therefore often uses to produced
multilayer coating (Destefani, 2002). Moreover, thinness
PVD coating also enhance the ability to coat sharp edges (Destefani,
2002). The deposition of multilayer coating by PVD is more complex than
single layer coating production which is usually involving the moving of the
substrate or movable shutter to modulate the fluxes. Movable shutters involving
complex engineering structures and thus are expensive. In some cases, interruption
of the deposition by shutters may also lead to asymmetric deposition rate. Furthermore,
modulation of fluxes using movable shutters involves the moving of shutters
synchronously with evaporation from the sources. As result, it was difficult
to obtain precise control of the coating properties as expected, especially
when the layers are very thin (Suresha et al., 2006).
It is more practical to produced superlattice coating by controlling the deposition
fluxes, rotation and evaporating time. The substrates are placed between two
targets and the revolving substrate is alternately exposed to each target, resulting
in a layered thin film with periodicities that can be controlled by varying
the rotation speed (Suresha et al., 2006).
Despite these multiple advantages brought by PVD process, CVD remains the dominant
cutting tool coating process, especially in the United States. Some applications
may require higher adhesion that produced by the chemical bond during CVD process.
While PVD can deposit a wider range of coating materials on various substrates,
there are some coatings and thin layers that can't be deposited using PVD process.
For example, CVD is currently the sole method uses to produce diamond coating
Several researchers had reported that tribological properties of coating which
includes composition and microstructure, are sensitive to deposition conditions
and parameters (Navinsek et al., 1997; Kim
et al., 2000; Bemporad et al., 2001;
Destefani, 2002; Suresha et al.,
2006; Yigit et al., 2008; Cheng
et al., 2010). For instances, possibility of inter-layer slip can
be resolved by varying deposition parameters (Bemporad et
al., 2001), while adjusting process parameters in PVD allows modification
from a columnar to an equiaxed microstructure (Destefani,
2002). Cheng et al. (2010) had reported that
the growth orientation of the TiCN coating is strongly dependent on the deposition
During coating deposition by PVD that involves rotation, the composition of
coating was significantly changes with small excursions in distance due to the
planetary rotation. Moreover, the variances in the vapour pressure among coating
components may causing composition inhomogeneity. Thus, coating with rich variety
of composition and properties could be produced with this technique (Suresha
et al., 2006).
In the production of thin crystalline layers by CVD, the probability of the
adhesion is increased with temperature and thus leading to epitaxial growth.
However, growth rate for CVD are too low to allow boule production, usually
tens of micrometers per hour. The growth rate is increases with the temperature
but this led to difficulty of controlling crystalline growth and problem of
homogeneous nucleation in the gas phase may occur (Yigit
et al., 2008).
Low deposition temperature not exceed 250°C should be used to PVD-coat
CrN on alloyed tool steels and other heat sensitive substrate, because higher
temperature will ruins the substrate (Navinsek et al.,
1997). On the other hand, Kim et al. (2000)
found that small changes in evaporation temperature had significantly affected
the coating deposition. In the deposition of TiCN on aluminum alloy by metal-organic
plasma-assisted CVD, the substrate was just partially coated at evaporator temperature
below of 72°C and coating was spalled above 82°C. Good coating layers
were obtained at temperature range of 74-78°C (Kim et
The concentration of gases and precursors used during coating deposition had
significant effects to the tribological properties of coating and also the ease
of process. Cheng et al. (2010) reported that
concentration of mixture CH4 and N2 gases affected the
composition, microstructure and subsequently properties of coating, in the production
of TiCN coating by large area filtered arc deposition.
On the other hand, Kim et al. (2000) had found
that chlorine from titanium precursor, TiCl4, caused the deterioration
of mechanical properties and increased the stresses induced in coating, when
they produced TiCN coating by plasma assisted chemical vapour deposition process
at temperature of approximately 450°C. They also had found that the ratio
of hydrogen to nitrogen in chamber affected the hardness of the coating produced,
where hardness was reduced with the increasing of nitrogen. Increased nitrogen
content led to a bad dissociation of the precursor and thus more undissociated
precursor molecules were incorporated in the layer and resulted low hardness
(Kim et al., 2000). Mean while, Narasimhan
et al. (1995) reported that in the CVD of TiCN coating, using organometallic
precursor could reduces the deposition temperature to 800°C, as compared
with 1000-1100°C in conventional CVD process and the formation of eta phases
was reduced significantly (Narasimhan et al., 1995).
It can be derives that the deposition methods, coupled with the deposition
conditions and parameters, had immense influences to the tribological properties
and microstructure of the coating and also had non-negligible effects to the
substrate and thus coated tool. Therefore, conditions and parameters during
coating deposition process must be carefully controlled. Alternative deposition
techniques which have high efficient, good consistency, more controllable, lower
deposition temperature that prevents the damaging of the properties of substrate,
are developed. These methods, for examples, are magnetron sputtering, laser
technology and laser assisted deposition, cathodic arc evaporation and plasma
assisted deposition. Unfortunately, all these alternative deposition methods
also have its limitations and will not discuss in this study.
FACTORS AFFECTING THE COATING PROPERTIES
The properties of the coating can affect it performance which is strongly depend
on the composition, stoichiometry, impurities, microstructure, imperfections
and the preferred orientation (texture). The properties of the coating can be
controlled during the process of the deposition of coating (Bunshah,
Microhardness of a coating plays an important role in the wear mechanism and
higher microhardness will lead to higher wear resistance of the coating (Yang
et al., 2010). In general, wear rate and wear mechanism strongly
dependent on the microstructure and mechanical properties of the coatings and
the characteristics of the impacting particles (Yang et
al., 2010). In multilayer, microhardness was dependent on the bilayer
period (Balaceanu et al., 2005).
The increasing of nitrogen contents will reduced the hardness and residual
stress is increased with increasing of C/Ti ratio in the TiCN coatings (Kim
et al., 2000; Cheng et al., 2010).
The hardness is reduced because of the increasing number of undissociated precursor
molecules which incorporated in the coating layer and resulting the reduction
of hardness (Kim et al., 2000). Similarly, Narasimhan
et al. (1995) had reported that the coarseness of the TiCN coating
increased with the increasing of the contents of nitrogen in the film, mean
while (Bemporad et al., 2001) had found that
the hardness and friction properties of Ti (C, N) coating changes with the content
of carbon (Bemporad et al., 2001).
Generally, TiCN lattice parameter values are located between the lattice parameter
values of TiN and TiC. TiCN coatings is a solid solution of C atoms in TiN crystal
lattice, the variation in the carbon content in the coatings causes significant
changes in their crystalline and bonding structure. Internal stress and lattice
parameter increases with increasing of C content in the TiCN coatings (Cheng
et al., 2010). Since TiC and TiN are isomorphous, a TiCN coating
can have a wide range of compositions, from carbon rich to nitrogen rich. Thus,
TiCN lends itself eminently to the tailoring of composition and properties during
deposition. The deposition of TiCN can be carried out on a wide range of substrates
without greatly sacrificing the adhesion of the coating. A gradient of hardness
can be achieved by controlling the C:N ratio in the TiCN coating layers. Properties
and behaviour of the TiCN coating is a strong function of composition (Narasimhan
et al., 1995; Cheng et al., 2010).
The sequences of the layer of coating had significantly effects to the mechanical
and tribological properties of the coating. Surface properties such as wear
resistance, hardness and coefficient of friction depend strongly on the properties
of the outer layer of the film 20 (Narasimhan et al.,
1995; Bemporad et al., 2001). Narasimhan
et al. (1995) had improved the surface properties of the TiCN coating
by providing a carbon-rich top layer to increase the hardness, wear resistance
and surface smoothness of the coating (Narasimhan et
al., 1995). Similarly, depositing TiCN coating as the surface layer
in multilayer coating can improve the wear resistance significantly, this is
due to the lower friction coefficient of TiCN compare to other Ti-base hard
coating such as TiN and TiAlN (Hsieh et al., 1998;
Chung-Chen and Hong, 2002).
Bemporad et al. (2001) had investigated the
properties of multilayer TiN/TiCN coating. They found no noticeable interdiffusion
is present but interlayer-slip has occurred, leading to the hypothesis that
better bond may obtained when TiN is deposited on TiCN and not the contrary.
To eliminate the interlayer-slipping with abrupt transition, they deposited
a thin layer (some nm) between each TiN and TiCN film to obtain a graduated
transition from the nitride to the carbon nitride and vice versa (Bemporad
et al., 2001).
In this study, the most significant factor that affected the properties and
performance of coating is unable to conclude but composition, thickness of coating,
sequences of layers and deposition method are the major factors that influencing
the properties and performance of coating and multilayer film.
PERFORMANCE OF COATING ON THE MACHINING PROCESS
TiAlN had the characteristics of high hardness, better wear resistance, chemical
and thermal stability (Hsieh et al., 1998; Nordin
et al., 2000; Ghani et al., 2004).
When cutting with the TiAlN coated tool, a dense and highly adhesive protective
Al2O3 surface film was found to form and prevent diffusion
of oxygen into the tool. It was also found that considerably more heat was dissipated
via chip removal. This allowed machining to be carried out at higher speeds
without causing excessive thermal stresses on the substrate. However, the performance
of TiAlN coating was less superior than TiN coating in low speed machining or
interrupted cutting process due to its brittleness and high friction coefficient
despite having better thermal stability (Ghani et al.,
2004; Sokovic et al., 2004). Wear mechanism
such as abrasion, attrition, chipping, plastic deformation and Built up Edge
(BUE) were predominant when cutting cast iron at cutting speed 120-220 m min-1
using TiN coated tools (Siow et al., 2011).
Jaharah et al. (2009) also found that good machined
Surface was produced when end milling hardened steel using P10 TiN coated carbide
tools. Wear mechanism and performance of PVD coated cemented carbides when milling
of Ti-6Al-4V under dry condition was studied by Ahmad Yasir
et al. (2008). They found attrition wear on flank and rake were the
main cause of tool failure. Rapid tool wear was observed when the coating layer
had been delaminated which explains that the chemical wear also contributes
to the tool failure.
Yigit et al. (2008) had carried out an investigation
on the performance of cemented carbide cutting tool coated with multilayer of
TiCN+TiC+TiCN+Al2O3+TiN in dry turning of nodular cast
iron. They had found that multilayer of TiCN+TiC+TiCN+Al2O3+TiN
coated-cutting tool with 10.5 μm thickness of coating exhibited lower tool
wear, had higher wear resistance, produced better surface finish and the cutting
forces is lower compared to the multilayer of TiCN+TiC+TiCN+Al2O3+TiN
coated-cutting tool with 7.5 μm thickness of coating. Their results shown
that thickness of the coating had marginally effects on the performance of the
cutting tool. Similarly, several researchers had found that the coating thickness
of the multilayered coatings had significant effect on the wear resistance.
Sahin and Sur, (2004) studied the effect of Al2O3,
TiN and Ti (C, N) based CVD coatings on tool wear in machining metal matrix
composites. Flank wear increased with increased cutting speeds for all three
tools. Kok (2010) studied the wear of cutting tools
in the machining of 2024Al alloy composites reinforced with Al2O3
particles using varying sizes and volume fractions of particles up to 23.3 vol%
by a turning process using TiN coated carbide tools and Ti+Ti(C, N)+TiN) coated
carbide tools at different cutting speeds. The results show that the TiN coated
tool sustains the least flank wear due to the extreme hardness and therefore
high wear resistance of this material. Ti+Ti(C, N)+TiN) coated tools is found
to be very unsatisfactory and sustains the most severe flank wear (Kok,
Jindal et al. (1999) had investigated the properties
and performance of TiAlN, TiCN and TiN PVD-coated tungsten carbide tool. They
had found that TiAlN coating had better adhesion properties than TiCN, where
TiCN possessed higher residual stress that caused slips of the coating. Hardness
evaluation showed that TiCN had the highest hardness among the coatings in room
temperature but TiAlN had the highest hardness when the temperature exceeds
750°C. Both TiCN and TiAlN are harder than TiN in both range of temperature.
In turning inconel, medium carbon steel (SAE 1045) and ductile cast iron under
flood lubrication using these cutting tools, they discovered that TiAlN coated
tool had the best wear resistance (crater wear and abrasive wear) and longer
tool life, followed by TiCN and TiN coated tools and both TiAlN and TiCN coated
tools were performed significantly better than TiN coated tool in all the tests.
The better properties and performance of TiCN and TiAlN coated tool compared
to TiN coated tool was partly attributed to the solid solution effect of either
carbon or aluminum in the TiN lattice that strengthening the coating and increases
the hardness of the coating. The formation of a stable Al2O3
layer on TiAlN coating endowed the TiAlN coated tool with higher resistance
to abrasive wear and crater wear and makes TiAlN coated tools preformed better
than TiCN coated tool and had longer tool life (Jindal et
TiAlN had many advantages properties, i.e. high hardness at elevated temperature,
low thermal conductivity and high thermal and chemical stability. Furthermore,
TiAlN possessed high oxidation stability owing to the formation of a stable
and protective Al2O3 film which in turn causes an improvement
in the wear resistance of TiAlN coated cutting tool during machining process.
The films also prevented intensive interaction between tool-workpiece and depressed
the tendency of adherence of the workpiece material to tool surface (Fox-Rabinovich
et al., 2004). Fox-Rabinovich et al. (2004)
had report that grain size refinement by using Filtered Arc Deposition (FAD)
of TiAlN could accelerate the formation of alumina protective layer, leading
to the improvement in oxidation wear resistance. This is a significant improvement
especially in high speed machining when the oxidation wear is dominant. Additionally,
finer grain size led to better surface finish that will further lowering the
propensity of the adherence of workpiece material to tool surface and improved
the chip flow (Fox-Rabinovich et al., 2004).
TiAlN coating is better than TiN and TiCN coatings in high speed machining
but suffers greater damage than TiN and TiCN in more mechanically influenced
processes such as interrupted cutting or slow speed cutting. TiN tends to oxidize
in temperature above 500°C and the formation of rutile phase TiO2
which is brittle and poor in adherence ruined the protectability of the TiN
coating and the formation of TiO2 increases with increasing temperature.
The oxidation of TiAlN led to the formation of Al2O3 which
is a protective oxide layer that has high chemical stability that prevents diffusion
wear, make TiAlN coating more superior in high speed machining. However, TiAlN
coating is more brittle and had higher friction coefficient than TiN coating,
this caused poorer performance of TiAlN coating in low speed and interrupted
machining compare to TiN coating (Hsieh et al., 1998).
Kim et al. (2000) had found that the hardness
of TiAlN coating was higher than TiN coating.
||Comparison among TiAlN, TiCN and TiN for tool life in high
speed machining (HSM) and surface microhardness
In sliding tests with alumina and steel balls, they had discovered that TiN
coating had lower friction coefficient at lower sliding speed but higher than
TiAlN coating when increasing the sliding speed to 0.5 m sec-1. This
suggested that TiAlN is more superior than TiN in high speed machining (Kim
et al., 2000). An index comparison had been plot (Fig.
1) to compare the tool life in High Speed Machining (HSM) and surface microhardness
of TiAlN, TiCN and TiN.
Chung-Chen and Hong (2002) reported that TiCN and TiAlCN
hard coating can improve tool life in the end milling of quenched AISI 1045
carbon steel. They found that TiCN coated tool had better tool life compared
to TiAlCN coated tool. Chang et al. (2006) reported
that multi component of TiAlCrN coatings synthesized by PVD exhibited better
mechanical performance due to its advanced tribological properties and high
temperature oxidation resistance. The incorporation of aluminum in the Face
Centered Cubic (FCC) of TiN structure enhanced the coating thermal stability
and hardness effectively. Hence, the oxidation resistance of the TiAlN coating
can be further improved by the incorporation of chromium to form Cr based nitrides
which are good in anti-corrosion. This could be the reason for the excellent
performance shown by the AlTiCrN coated tool compared to the TiAlN coated tools
The advantageous of the Ti-base hard coating are strongly dependent in the
cutting condition and some researchers are not satisfied with the performance
of single/mono Ti-base hard coating, they thus combine two to three type of
Ti-base hard coating become a Ti-base multilayer coating. For example, multilayer
of TiCN/TiN coating can tremendously improve the wear resistance and properties
of single TiCN and TiN coating. The duplex coating showed a marked reduction
in the wear rate and cutting forces as compared with the single coating in the
machining of 4150 steel, 1045 steel and nodular cast iron (Narasimhan
et al., 1995).
Camuscu and Aslan (2005) had investigated different
types of cutting tool performance in end milling of AISI D3 tool steel. They
had found that TiCN/TiAlN coated carbide tool and TiAlN coated cermet tools
exhibited similar performances in terms of tool life and surface finish, while
TiCN coated carbide tool exhibited the worst performance. This also shows that
TiAlN is a better coating material than TiCN for the machining applications
of hardened tool steels. Ghani et al. (2004)
found TiN coating perform better than uncoated tool when machining hardened
steel at high cutting speed.
Yigit et al. (2008) had found that the TiN in
multilayer of TiCN+TiC+TiCN+Al2O3+TiN coated cemented
carbide tool made a consideration contribution to the tool wear resistance in
dry turning of nodular cast iron. However, TiN in that multilayer coating had
adverse effects on the quality of machined surface.
A thin film is deposited onto the surface of a cutting tool to improve the
performance of the cutting tool. Ti-based hard coating is currently the most
popular hard coating that applied in cutting tool application and is proved
that can tremendously improve the performance and tool life of cutting tool,
especially when machine difficult-to-cut materials. TiAlN coating is indeed
performed better than TiN and TiCN coatings in high-speed machining but suffers
greater damage than TiN and TiCN coatings in more mechanical influenced processes
such as interrupted cutting and low-speed machining.
The performance of first and second generation of Ti-based hard coating is
limited in certain cutting conditions. A new trend in developing Ti-based hard
coating is toward tertiary Ti-based coating or high complexity Ti-based coatings
like TiAlNbN and TiAlCN. On the other hand, some researchers combined two to
three types of Ti-base hard coatings to form a new Ti-based multilayer coating,
such as TiN/TiCN and TiCN/TiAlN. All these developments and researchers are
aimed to improve performance of Ti-based hard coating.
It was found that the composition of the coating, thickness of the coating,
sequences of the layer in multilayer coating and the coating deposition methods
can affect the properties of the coating and hence the performance.
There are researchers reported that Ti in Ti-based coating was oxidised during
high temperature machining. Thus, other elements are searched to replace Ti.
These elements, for examples, are Cr and Zr that forming CrN and ZrN, aiming
to further improve the performance of coating. However, the superiority of CrN
and ZrN to TiN is yet to conclude.