Materials Science and Engineering, A 163 ( 1993) 141-148
141
Titanium carbonitride-based cermets: processes and properties
ShanyongZhang
School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 2263 (Singapore)
(Received October 23, 1992)
Abstract
Cermet is a structural material in which approximately equiaxed fine grains of ceramic hard phase embed in a matrix of
metal or alloy binder. Titanium carbonitride-based cermets were first invented in the 1930s, but the boom of the cutting
grades really started in the early 1970s when titanium carbide-based cermets were established. However, because of their
superior properties, the Ti(C,N)-based cermets are now in a process of replacing the TiC-based cermets for cutting tool
applications. In traditional titanium carbonitride-based cermets, molybdenum is regarded as the indispensable ingredient
for wettability and sinterability, at the expense of grinding machinability. With the recent invention of the pre-sintering
solid-solution treatment of the ceramic hard phase, the materials development of titanium carbonitride cermets has come
to a new stage where molybdenum is no longer indispensable. For cermets of very high nitrogen content, however, the
combination of solid-solution treatment and a moderate molybdenum addition is predicted to be the way out.
I. Introduction
Cermet, or "ceramic metal", is a structural material
composed of a ceramic hard phase and a metal binding
phase, in which approximately equiaxed fine grains of
the ceramics, which constitute approximately 15-85%
by volume, are embedded in a matrix of metal or alloy
binder. The combination of metal and nonmetal in
cermets occurs on a microscale. Cermets incorporate
the desirable qualities and suppress the undesirable
properties of both metal and ceramic. Cermet cutting
tools are light in weight (compared with conventional
hard metals with densities ranging from 12 to 15 g
c m -3, c e r m e t s have densities of only around 6-7.5 g
cm 3, even lighter than steel), and are high in mechanical strength, toughness and heat conductivity. A high
heat conductivity leads to a low temperature gradient
resulting in less thermal stresses and cracks and is thus
greatly preferred for high speed cutting tool materials.
In cutting performance, cermet cutting tools produce
better control of geometry accuracy of the work pieces,
better chip and tolerance control, longer tool life,
improved surface finish, increased feeding rate and
consistent maintenance of critical dimensions. Yet,
cermet inserts cost about the same as coated tungsten
carbide but can machine at significantly higher speeds.
Cermets are now competing, in prices and properties,
with conventional hard metals, even with coated hard
metals, particularly at high cutting speeds and in finishing and milling operations [1-3].
Commercially there are two kinds of cermets: titanium carbide-based cermets and titanium carbonitridebased cermets. TiC-based cermets were on the market
in the mid-1960s [4]. Though Ti(C, N)-cermet was first
invented in 1931 [5] the boom of research and production really started after the systematic investigation
conducted by Kieffer and co-workers around
1968-1970 [6] which corrected the prejudice against
nitrogen inclusions. Since then many grades have been
developed [7-10]. Table 1 provides a glance of the
history of the development.
As compared with titanium carbide-based cermets
(Table 2), the titanium carbonitride-based cermets have
higher hot hardness, much higher transverse rupture
strength (TRS), much better oxidation resistance (less
weight gain in oxidation tests) and much higher thermal
conductivity [9, 11-13]. The hard phase of titanium
carbonitride-based cermets is of much finer grains,
consequently the high temperature creep resistance is
greatly improved. In cutting performance, the relatively
high enthalpy of formation of titanium carbonitride
increases its resistance to built-up edges, scaling and
crater formation; favorable flank wear when cutting a
tough steel at a relatively high cutting speed prolongs
tool life and increases total chip removal between tool
changes. As a result, titanium carbonitride cermet
cutting tools are used for the high speed milling, roughing and semi-finishing of carbon alloy and stainless
steels. Excellent surface finishes and close tolerances
are obtained, even on superalloys and other difficultElsevier Sequoia
S. Zhang
142
/
Titanium carbon#ride-based cermets
TABLE 1. Development of titanium carbonitride-based cermets [7-10]
Year of establishment
Hard phase
1931
1970
1974
1980-1983
1988
1988
1991
Binder phase
Ti(C, N)
Ti(C, N)
(Ti, Mo)(C, N)
(Ti, Mo, W)(C, N)
(Ti, Ta, Nb, V, Mo, W)(C, N)
(Ti, Ta, Nb, V, W)(C, N)
(Ti, Ta, Nb, V, W, Mo, etc)(C, N)
Ni(Co, Fe)
Ni-Mo
Ni-Mo
Ni-Mo-A1
(Ni, Co)-Ti2A1N
Ni-Co
Ni-Cr
TABLE 2. Comparison of high-temperature properties of a TiC-cermet and a Ti(C, N)-cermet [14]
Cermets
1000 °C
Microhardness
(kg mm- 2)
900 °C
Strength (TRS)
(MPa)
1000 °C
Weight gain
(mg cm- 2 h- 1)
1000 °C
Therm. cond.
(W m- 1 deg- 1)
TiC-cermeta
Ti(C, N)-cermet b
500
600
1050
1360
11.8
1.6
24.7
42.3
aTiC- 16.5Ni-9Mo; bTiC-20TiN- 15WC- 10TaC-5.5Ni- 11Co-9Mo.
to-machine materials for which TiC-based cermets
cannot be used [7]. Thus for tool applications carbonitride-based cermets are the primary cermets at
present. Titanium carbide-based cermets are in the
process of being replaced by titanium carbonitridebased cermets.
T h e purpose of this paper is to survey the structure,
properties and production of titanium carbonitride
hard phase and that of the corresponding cermets with
emphasis on process and property and finally tap the
trend of the materials development. For other detailed
machining tests and performances, the readers are
referred elsewhere: turning and milling operations [14],
abrasive wear test [15], cutting test [ 16 and 17].
2. Titanium carbonitride
Titanium carbide (TIC) and titanium nitride (TIN)
are the basis for titanium carbonitride (Ti(C, N)). Both
TiC and T i N have a sodium chloride structure, where
the corner of the face-centered-cubic (f.c.c.) lattice
formed by C atoms (or N atoms in the case of TiN)
locates at the point (1/2, 0, 0) of the f.c.c, superlattice
formed by Ti atoms (Fig. 1). T h e lattice parameter of
TiN is slightly smaller than that of TiC. T i N has better
thermal conductivity than TiC, which makes the Ti(C,
N)-based cermets more thermally conductive, thus
more thermal-shock resistant than the TiC-based
counterpart. Other basic data for TiC and T i N crystals
[18-20] are listed in Table 3.
Since TiC and TiN are isomorphous, the carbon
atoms on the TiC superlattice can be replaced by
CorN
Q
I
I
TiN: 0.4240 nm
TiC: 0.4320 nm
Ti
I
t
Fig. 1. Illustration of lattice structures of TiN or TiC crystals.
The corner of the f.c.c, lattice formed by C or N atoms is located
at the point (1/2, 0, 0) of the f.c.c, lattice formed by Ti atoms, a
typical NaC1 structure. The C and N atoms in TiC and TiN are
interchangeable in any proportion, thus a continuous solid
solution of titanium carbonitride (or Ti(C1 _xNx) where 0 ~<x ~<1
can be made.
TABLE 3. Basic data for TiC and TiN
Melting
temp.
(°C)
TiC 3140
TiN 2930
Ref. [18]
Micro
Density Lattice
Lattice
hardness
(g cm- 2) parameter parameter
(kgf mm -2 )
(A)
(A)
3200
2000
[18]
4.92
5.22
[18]
4.322
4.242
[19]
4.320
4.240
[20]
nitrogen atoms in any proportion (c.f., Fig. 1 ), therefore
a continuous series of solid solutions can be prepared:
Ti(C1_ xNx) where 0 ~<x ~< 1. One of the most c o m m o n
ways of making titanium carbonitride is to hot-press a
S. Zhang
/
Titanium carbonitride-based cermets
thoroughly blended mixture of TiC and TiN powders
in vacuum, for instance, at 1800 °C and 27.6 MPa for
5 h [21] or at 1600 °C and 3000 MPa for 1 h [20].
Other methods explored by Pastor [22] include:
( 1 ) high temperature diffusion of TiC and TiN:
4.34
4.32
4.30
4.28
TiC + TiN --~ Ti(C, N) at 1700 °C x 2 h in nitrogen
(2) high temperature nitridation of titanium plus
titanium carbide:
=,
,.a
4.26
4.24
T i C + T i + N : --~ Ti(C, N) at 1700 ° C x 2 h
4.22
0.0
I
0.2
I
0.4
(3) carbonitridation of titanium dioxide:
TiO2 + C + N 2 flow ~
kinetics study)
I
0.6
I
0.8
1.0
x in Ti(Cl.xNx)
CO + Ti(C, N) (see [23] for the
(4) thermal decomposition of titanium
chloride-amine (or nitrile) complexes:
143
tetra-
Fig. 2. Lattice p a r a m e t e r o f titanium c a r b o n i t r i d e as a function
of nitrogen c o n t e n t [cf, 21]. Since lattice p a r a m e t e r o f T i N is
smaller than that of TiC, the lattice p a r a m e t e r o f Ti(C~_~N~)
decreases linearly with increasing x.
3000
TiC14 + C2HsN 2 + CCl4(solvent )
2600
---,- Complex
Ar
+ CH3 -- N H 2
600-900 °C
---- Ti(CN) + C
(5) carbonitridation of titanium powder by a
methylamine-Ar gaseous mixture gas at 800-1400 °C.
Ti(C~_,N~) can be identified by X-ray diffraction
with, for instance, Ni-filtered Cu K~ radiation. The
lattice parameter can be measured by high angle X-ray
diffraction by extrapolating to 2 0 = 180 ° [21]. The
physical and mechanical properties of the carbonitride
vary with increasing amount of nitrogen. As x increases
in Ti(C~-xN,), the lattice parameter decreases linearly
as shown in Fig. 2. This linearity was also demonstrated by Neumann et al. [19], Shimada et al. [20],
Lengauer and Ettmayer [24], Suzuki and Tanaka [25]
and others. Pastor's [22] expression for the lattice
parameter variation is a ( A ) = 4 . 3 0 5 - 0 . 0 7 0 x , with
reliability r = 0.95. Titanium carbonitride is a different
material from titanium carbide and titanium nitride, so
the material properties such as hardness and toughness
or the sinterability are also different [26]. Because TiN
has lower microhardness and higher thermal conductivity than TiC (see Table 2), in titanium carbonitride,
the microhardness decreases (Fig. 3) and the thermal
conductivity increases (Fig. 4) with increasing amount
of nitrogen.
In making Ti(C, N) or sintering of Ti(C, N)-based
cermets, stability of Ti(C, N) is very important. It is
known that the stability of titanium carbonitrides
Ti(C t ~Nx)varies with the nitrogen content x, nitrogen
partial pressure and temperature. According to
Pastor's [22] thermodynamic calculations, it is impossible to get titanium carbonitride at 1527°C in
nitrogen flow (1 atm) when x~<0.2 because in this
%
2200
1800
\
g~
1400
0.00
I
!
0.20
0.40
I
0.60
I
0.80
1.00
x in Ti(Cl.xNx)
Fig. 3. Microhardness of titanium carbonitride as a function of
nitrogen content [cf 21]. Since the TiN is softer than TiC, the
microhardnessof Ti(C~-xN~)decreases with increasingx.
range TiN plus free carbon are the stable phases. When
x >0.65, Ti(C~ _xN~) is stable as a single phase. When
0.2 ~ x ~<0.625, Ti(C~ _xNx) partially decomposes and
produces some free carbon. This decarbonization
tendency increases with increasing nitrogen partial
pressure and decreases with increasing temperature as
shown in Table 4. Kieffer et al. [27] studied the stability
of transition metal carbides against N 2. Their results
concerning the stability of Ti(C, N) are also included in
the table for easy comparison.
3. Titanium carbonitride-based cermets
As outlined before, titanium carbonitride has a high
melting point, high hardness and oxidation resistance
and is used as the material for cutting tools or wearresistant machine parts. However, a sintered body of
pure titanium carbonitride is rarely used for these
144
S. Zhang /
Titanium carbonitride-based cermets
0
0
30
TiN
O
O O
O
0
25
0
O
0
0
TIC03N 0.7
0
0
O •
0
•
Q
[]
•
[]
13
I"I []
rl
•
•
|
1:1
[] TiCo.4N 0.6
ii
[]
•
TiC 0.6N 0.4
2o
•
OOID
[] []
i
D
•
•
•
[n
a
15
a
10
0
TiC0.TN 0.3
n
a n n&,,
&&
anna
TiC
&&L
n
i
I
t
200
I
4~
n
t
I
6~
I
I
8~
Temperature (°C)
Fig. 4. Thermal conductivity [20]. The thermal conductivity of
Ti(C~_xNx) increases with increasing temperature and nitrogen
content x.
TABLE 4. Composition of titanium carbonitride as a function
of temperature at 1 atm of N2
Temp.
(°c)
1400
1527
1800
2027
Stablephases
Ref.
TiN + C
Ti(C~_xNx) + C
Ti(C1_~Nx)
no data
x<0.2
no data
does not exist
no data
0.2< x<0.625
no data
x<0.16
x > 0.60
x>0.65
x > 0.35
x > 0.16
[27]
[22]
[27]
[22]
purposes because of its brittleness and low breaking
strength. To overcome these shortcomings, metal or
alloy is used as the binder or matrix to form what are
called cermets.
In production of titanium carbonitride cermets, the
most commonly used production routes are hot pressing, hot isostatic pressing (HIP'ing), sintering or a
combination of HIP'ing and sintering, in a vacuum,
nitrogen or argon atmosphere.
For the hard phase, the starting materials are either
TiC and TiN powders [21, 25, 28, 29] or Ti(C, N)
powders [9, 10, 12, 22, 30-32]. Other ingredients used
in some compositions include Mo2C, VC, WC, NbC,
TaC, ZrC, HfC, Cr3Cz, etc. These elements form
carbonitride solid solution (TIN, W, Nb, Ta, Zr, etc.)
(C, N) to strengthen the hard phase through a solidsolution strengthening mechanism.
Nickel is the basic binder. Cobalt is often added in
the binder to increase the machinabitity and to lower
the solubility of Ti in Ni [33] to stabilize the carbonitride. Molybdenum is a strong solid-solution
strengthening element in nickel (atomic radius of Mo is
1.39 A while that of Ni is 1.24 A), so it is added in
some cermets to strengthen the binder phase. When
both Ni and Co are added as the binder phase, the
weight ratio of Ni/(Ni+Co) is preferably within
0.3-0.8 considering the miscibility or affinity with a
mixed carbonitride of the hard phase [9]. Recently a
large amount of Cr (5-30 wt.%) was also added in the
binder phase in place of Co for wettabitity, toughness,
high temperature strength and oxidation resistance.
Too tittle Cr will not bring out the effect, yet too much
of it will result in excessive precipitation of chromium
carbide that reduces toughness. In this case, the weight
ratio of Ni/(Ni + Cr) is preferably within 0.6 to 0.98 or
Cr/(Ni + Cr)= 0.02-0.4 [10].
A strong metal-ceramic bond is essential for a satisfactory cermet product. The sinterability of the material and the properties of the sintered body depend
largely on the metal-ceramic bonding between the
hard phase and the binder phase. Though good wetting
does not mean good bonding, good bonding requires
good wetting. The contact angle (wetting angle)
between the ceramic and the liquid metal is a measure
of the wettability. Non-wetting occurs when the contact
angle is greater than 90 °, and complete wetting occurs
when the contact angle is 0 °, as Ni wets WC in vacuum
at 1500 °C [39] where liquid Ni spreads over the
surface of the solid WC particles, or in microstructure,
liquid Ni spreads between the boundaries of WC
grains. Good bonding usually occurs when the hard
phase has a small amount of solubility in the binder
phase during liquid phase sintering. Improvement in
wetting can occur when there is preferential absorption
of atoms at an interface thus lowering interfacial energy
or when there is a diffusion gradient established across
solid-liquid interfaces thus reducing the contact angle
[40]. The wettability of a carbide is also manifested in
the stability of the carbide: the lower the negative heats
of formation of a carbide, the smaller the wetting angle
between the carbide and liquid metal. In carbides of
groups IVb, Vb and VI b (American notation) of the
periodic table of the elements, MozC has the lowest
negative heats of formation (Table 5), thus it has the
lowest wetting angle or best wettability with liquid
metals. Therefore, to improve the wetting and thus the
sinterability Mo2C is added as an indispensable ingredient in the conventional cermets.
Addition of Mo2C also increases toughness, decreases particle size, suppresses graphite formation and
contributes to high temperature strength [7, 10].
Molybdenum carbide exists in an intermediate phase
S. Zhang /
TABLE 5. Heats of formation at 298 K ( k J mol ~)of carbides ( I g b ,
TiC
- 183.7
ZrC
- 199.2
HfC
-209.2
145
Titaniumcarbonitride-based cermets
VC
- 126.4
between a hard phase and binder phase, separating the
hard phase from the liquid during sintering thus preventing the grain growth of the hard phase owing to
dissolution and reprecipitation. In a typical microstructure of titanium carbonitride cermet containing
molybdenum carbide, the Ti(C,N) core is surrounded
by a rim enriched in molybdenum carbide. The
Ti(C, N) core is called a' phase, which contains nearly
all the nitrogen from the starting mixture and the rim is
called a" phase, which contains most of the molybdenum. The combination of the a' and a" phases is
then embedded in the metal binder matrix [3, 8, 13].
The two isostructural phases a' and a " were decomposed spontaneously from (Ti, Mo)(C, N) (spinodal
decomposition). The a" phase has an inherently better
wettability with respect to the binder both in the liquid
and in the solid states and hence greatly improved
strength of the cermet.
Grain size (/~) of the hard phase is shown as a power
(r) function of sintering time (t) [30]:/z oc t ~. The melting
temperature of nickel is 1455 °C and that of cobalt is
1495 °C [18]. To ensure a liquid phase sintering, the
sintering temperatures of titanium carbonitride
cermets usually range from 1400 to 1600 °C depending on the kinetics and resultant microstructures
desired. The sintering time is kept short to prevent
grain coarsening that damages mechanical properties.
For example, the flexural strength and hardness of a
cermet sintered at 1550°C both dropped with
increased holding time from 1 to 3 h [22]. For a
molybdenum carbide toughened titanium carbonitride
cermet the typical Vickers microhardness is 1650 kg
mm -2, the modulus of rupture or MOR is 1500 MPa
and the critical stress intensity of mode I or K~c is 8.5
MPa m 1/2. The tantalum/niobium carbide toughened
titanium carbonitride cermets have a little lower hardness but higher strength as compared in Table 6. MOR
measures the bending strength, or Mc/I where M is the
bending moment, c is the distance from the neutral axis
to the tensile surface and I is the moment of inertia. Kic
is a measure of the fracture toughness. K I= (raY C 1/2
where o~ = applied stress, c is half length of the crack
and Y is a dimensionless term determined by the crack
configuration and loading geometry.
Because titanium carbonitride constitutes the
majority in composition, various properties of the
titanium carbonitride-based cermets follow the suit of
that of the carbonitride as discussed in the previous
section. Nitrogen additions to the hard phase resulted
Vb
and V I b groups) [40]
NbC
- 142.3
TaC
- 161.1
Cr3C2
-89.7
Mo2C
- 17.6
WC
-35.1
TABLE 6. Typical properties of titanium carbonitride-based
cermets [14]
Ti(C,N)
cermets
Hardness
MOR Ktc
Young's modulus
(kgmm 2) (MPa) (MPam ~/2) (GPa)
Type A"
TypeBh
1650
1500
1500
1800
8.5
10
450
410
~'Type A: (Ti, Mo/W)(C,N)-based cermets that are referred to
as molybdenum carbide toughened cermets. The ISO grades
P01, P15, K01 and K10 are in this group, bType B: (Ti, Mo/W,
Nb, Ta)(C, N)-based cermets that are referred to as tantalum/
niobium carbide toughened cermets. The ISO grades PI0, P20,
K05 and K20 are in this group.
x = TiN/(TiC+TiN)
Work p ece: 4340 sta nless stee
.125
.100
E
"~
._o
x = 0.15
x=O
x=o l
.OTS
E
x=O.13
.050
/,/,
.025
0
100
200
300
380
Cutting Speed (m/rain)
Fig. 5. Effect of nitrogen addition [21]. At the same Ni binder
and nose deformation of the cutting edge, nitrogen addition
makes cutting at higher speed possible.
in higher wear resistance and improved plastic deformation (Fig. 5) of the cutting edge thus permitting
machining at high cutting speed [1]. Titanium carbonitride is also used to coat titanium carbonitride-based
cermets of low nitrogen content to increase the wear
resistance [34]. On the TiC reach side, the grain size of
hard phase in the titanium carbonitride cermet decreases with increasing nitrogen content [21, 28, 30],
resulting in hardness and strength increase [11, 12] up
to the level where the grains are around the size of 1
~ m [12], leading to a longer tool life [35] (Fig. 6). With
increasing nitrogen, the thermal conductivity of the
cermet is increased, resulting in improved thermal
shock resistance [11, 35]. Fracture toughness increases
with increasing nitrogen content up to x = N/(C + N ) =
146
S. Zhang
/
Titanium carbonitride-based cermets
0.5, further increase of the TiN results in a decrease in
fracture toughness [25, 36] because of increased grain
growth [30]. With increasing nitrogen content, transverse rupture strength (TRS), Young's modulus of elasticity, oxidation resistance [25, 37] and wear resistance
[15] increase while the steady-state creep rate
decreases [12, 38] (Fig. 7).
4. Trends in the development of Ti(C, N)-based
cermet
In the Ti(C,N)-based cermets, carbonitrides tend to
decompose during the process of production, thus
nitrogen is released through bubbling owing to the
reaction between the carbonitrides and the binding
metal [31]. This is called denitrification. Denitrification
/
/
J
• J•
f
I
I
0.2
I
I
0.4
I
I
0.6
x in Ti(Cl_xNx)
Fig. 6. Cutting life of a titanium carbonitride cermet tool as a
function of nitrogen content [35]. Turning of Cr-Mo steel JIS,
cutting speed 150 m min-1, depth of cut: 1.0 mm, feed
0.31 mm rev- 1.
~_
~
Ti(Cl_x Nx)-I4%Mo2C- 16%Ni
Ix=0
3-Point Bending: 400 MPa
..~
x=0.
F
0
I
I
20
I
I
I
40
I
60
I
I
80
Loading Time (min)
Fig. 7. Curves of Ti(C~_xNx)-14%Mo2C-16%Ni cermets
deformed at 1000 °C and 400 MPa. The steady-state creep rate
decreases with increasingnitrogen content [12].
takes place during the formation of Ti(C,N) solid
solution from TiC and TiN [28], even during rising
temperature to the sintering temperature when TiN
particles are denitrified and carburized [29] and during
formation of carbonitride of Ti, Ta, Nb, Mo, W, etc., in
particular, when WC is dissolved in Ti(C,N)[9]. Denitrification depletes nitrogen, thus the actual nitrogen
content in the material is reduced from the predetermined level; denitrification makes the cermet inhomogeneous to warp the surfaces and sides; the nitrogen
gas generated creates micropores in the sintered body
that serve as the fracture origin [12, 29]; denitrification
also raises the liquidus of the material, thus less liquid
is available at fixed sintering temperature for an effective liquid-phase sintering [9, 29]. The total effect is
the lowering of sinterability and deterioration of
strength. Adding molybdenum offsets these effects
through improvement in wetting and sinterability at the
expense of machinability. However, with increasing
nitrogen content in the titanium carbonitride cermets,
denitrification becomes increasingly vigorous. To
maintain the sinterability, a large amount of Mo is
therefore indispensable and the price is the loss of
machinability.
The pre-sintering solid-solution or PSSS treatment,
devised by Tobioka et al. [9], is an effective way of
reducing the amount of molybdenum while keeping the
nitrogen content at a high level. The PSSS treatment
aims to form a solid-solution of the hard phase composed of carbonitride of Ti, W, Ta, etc., at a temperature higher (at least not lower) than the sintering
temperature. Such treated hard phase is then pulverized and mixed with Ni and Co metal binders before
sintering. Because the liquid phase of Ni or Co has a
solubility of about 10 at.% for C (while in solid state at
1318 °C the solubility dropped to 0.55% [33]), without
the PSSS treatment, carbides of Ta, W, Yi, etc., tend to
dissolve in the liquid phase during sintering and then
precipitate on the hard grains during cooling, thus
resulting in grain growth. After solid-solution treatment, these metals are bonded in the carbonitride solid
solution (Ta, W, Ti, etc.)(C,N). Since the liquid phase
of Ni or Co has little solubility for N, accordingly the
dissolution and precipitation processes of the hard
phase are suppressed, thus no grain growth takes place.
This eliminated the need for adding molybdenum.
Therefore the new cermets enjoy the advantage of high
nitrogen content but do not have the negative effects
brought along with molybdenum. This leads to high
strength, high toughness, great wear resistance and
good machinability at the same time. Tobioka et al. [9]
produced cermets containing essentially no molybdenum. Table 7 lists properties of four titanium carbonitride cermets of similar composition with or without
PSSS treatment. The effect of the PSSS treatment with-
S. Zhang
/
147
Titanium carbonitride-based cermets
TABLE 7. Comparison of properties for cermets with and without the pre-sintering solid-solution (PSSS) treatment [9]
Cermet
Hard
phase
(%)
PSSS
treat
with
5%
Mo2C
Hardness
Hv
(kgmm -2)
Fracture
toughness
(MPam 1/2)
Strength
TRS
(kgmm 2)
Crater
depth
(mm)
Flank
wear
(ram)
A
C
B
D
85
85
80
80
yes
no
yes
no
no
no
yes
yes
1600
1300
1630
1500
8.5
9.0
5.5
5.5
200
180
180
180
0.05
too worn
0.23
0.25
0.08
too worn
0.08
0.12
out the addition of 5% m o l y b d e n u m can be seen by
comparing cermet A and cermet C: beside the higher
strength and hardness, the PSSS treated cermet is much
i m p r o v e d in crater depth and flank wear resistance-the cermet without the PSSS treatment is too worn to
measure. T h e effect of the PSSS treatment with addition of 5% m o l y b d e n u m can be seen by comparing
cermet B and cermet D: the PSSS treated cermet has
better crater depth and flank wear resistance. T h e
method of PSSS treatment of the hard phase can bring
the nitrogen content to about N/(C + N) = 0.6 in atomic
ratio without causing denitrification. W h e n the ratio
goes above 0.6, however, denitrification still occurs,
thus the sintering property and the wear resistance
deteriorate.
5. Outlook of the materials development
Titanium carbonitride cermets of high nitrogen
content still maintaining good sintering properties and
grinding machinability are the future of titanium carbonitride cermet materials. T h e key to the success m a y be
the combination of the following: using carbonitrides as
starting powders, for example, Ti(C,N) and W(C,N)
instead of TiC, T i N and WC (probably it is also true
for other transition metal carbides, such as TaC, NbC,
VC, etc.); and adding a certain a m o u n t of m o l y b d e n u m
and performing solid solution treatment of the hard
phase before sintering with the binder.
Acknowledgments
T h e author gratefully acknowledges Professor H.
Pastor for his kind help. T h a n k s are also due to Drs. K.
Xia, D. Z h u and Q. Li for critical discussions.
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