CERAMICS TOOL MATERIALS WITH
ALUMINA MATRIX
Katarina GERIC
Abstract: Ceramics cutting tool are being more widely
used, as they can satisfy the high criteria of industry.
Main trends in research of ceramic materials are shifting
from high purity single phase like Al203 to multiphase
composite ceramics, and nowadays extensive research is
done to identify all the combinations which would satisfy
the requirements: high hardness, high toughness, high
temperature resistance and inertness toward machining
part. In this paper, we analysed the composition,
strengthening and toughening mechanism, as well as the
application of cutting tool materials with alumina matrix.
its good resistance to oxidation, relatively high hardness
(compared with that of cemented carbide cutting tool
materials) and low friction coefficient with metals. Main
trends in research ceramic materials are shifting from
single phase of high purity to multiphase composite
ceramics.
Advances in ceramic processing technology have resulted
in a new generation of high performance ceramic cutting
tools exhibiting improved properties. Considerable
improvements have been achieved in tool properties such
as flexural strength, fracture toughness, thermal shock
resistance, hardness, and wear resistance by incorporating
one or more other components into the base material to
form ceramic–matrix composite tool materials. The
reinforcing component is often in the shape of particles or
whiskers. Ceramic tool materials with oxide matrices,
particularly alumina, Al203, are of increasing interest, as
they exhibit high hot-hardness and very high abrasion
resistance, thus making it suitable for high speed
machining.
O ther
7%
C e ra m ic
5%
TS
45%
C e rm e t
8%
Key words: cutting tool material, alumina composite.
1. INTRODUCTION
There is an axiom in metal working that urges the use of
the right tool for the job. Selecting the correct cutting tool
material for a specific machining operation is the first step
in creating the most effective process plan for
manufacturing a part. The cutting tool material is
dependent on the work material to be machined and the
operation to be performed. Often, there are several
possible choices of tool materials that will successfully
(but not cost-effectively) produce parts. Additional factors
then must be considered and these include:
machine tool horsepower, speed range, rigidity,
productivity demands,
tooling budget limitations
machine tool burden rate.
In an increasing number of applications, the right tools for
the jobs are ceramic cutters. According to some data,
application of ceramic cutting tools is about 5% of all the
other cutting tool materials, Fig 1. While available as
solid body tools, the most popular form of ceramic cutters
is the indexable ceramic insert.
With the development of high speed machining, the
requirements for cutting tool materials have become
higher and higher. In recent years, a new kind of material
has been widely used as cutting tool material because of
HSS
35%
Fig. 1. Application of cutting tool materials
Addition of hard particles or whiskers to the Al203 matrix
may enhance its mechanical properties considerably.
Some of these tool materials, such as Al203/TiC,
Al203/TiB2, Al203/ZrO2, Al203/Ti(CN), Al203/WC, and
Al203/SiC, have been used in various machining
applications and offer advantages with respect to friction
and wear behaviors. A large variety of strengthening and
toughening mechanisms, including particle dispersion
toughening,
transformation
toughening,
whisker
reinforcing as well as micro-crack toughening, have been
developed to improve toughness, strength and thermal
shock resistance of ceramic tool materials.
2. CUTTING TOOL OF PURE ALUMINA
Cutting tools from alumina, Al203, are used since the
beginning of twentieth century for cutting low hardness
steel, but are usually used for grey cast iron. Cutting tools
of pure Al203 are usually used in mass production, as they
are cheap. Material for cutting tool is fine grain at least
99% pure Al203 (less than 5 µm), with high relative
density, i.e. containing less than 2% porosity [1].
367
Hardness of alumina at room temperature is at the same
level as that of cemented carbides (1600HV, 16GPa
Knoop).
When a few of volume percent of carbon is added, a
fracture toughness approximately 20% higher than that of
the corresponding ceramic material containing no carbon
additive is achieved, while the hardness and the flexural
strength do not decrease much [3]. Strip-like graphite
(carbon) with a width of about 100 nm, is distributed at
the boundary area of Al203 and TiC and Al203 grains. It is
found that microcracking, crack deflection, crack
branching and crack bridging may contribute dominantly
to the increase in the fracture toughness of the carbon
containing Al203/TiC ceramic tool material. Therefore, it
can get higher fracture resistance while retaining good
wear resistance in continuous and interrupted machining
of hardened carbon steel.
This composite has been widely used for high speed
cutting of hard steel, superalloys, or cast iron.
Fig. 2. Cutting tool materials comparisons
Hot hardness and compression strength are larger than
that for cemented carbides. However, fracture toughness
is low, with the value KIC for alumina tools being 1.75 to
4.3 MPa m1/2, much lower than those for cemented
carbide, fig. 2 [2].
One way to address the toughness issue of pure alumina,
was by mixing (ceramic + carbide). Ceramic composite
cutting tool material contains Al203 and 30% or more
carbide of refractory metals. TiC or (Ta,Ti)C are widely
used since 1960. Nowadays, WC and NbC are added.
Al2O3-based ceramic composites reinforced with
refractory carbides have been developed as an alternative
to cemented carbide.
Table 1 summarizes some mechanical properties of WC
as compared to Al203 and other reinforcing carbides.
Although all carbides show similar hardness, WC features
higher fracture strength and elastic modulus, compared to
NbC and TiC.
Table 1. Mechanical properties of Al203 and carbides
Al203
NbC
TiC
WC
Flexural strength
(MPa)
200-400
200-400
200-400
560
Elastic mod.
(GPa)
310-410
340
310-410
520-700
Hardness
(GPa)
20
24
18-32
18-24
3. ALUMINA BASED COMPOSITE
Al203 + TiC composite
The Al203/TiC composites consist of small titanium
carbide grains dispersed in an alumina matrix. The
mixture is hot pressed and is dark gray in color Fig. 3.
Microstructure is fine grain Al203 with dispersed carbide
grains of diameter equal to few microns. These cutting
tools are mainly used for working on gray cast iron with
high speed, and it is expected to be used more widely than
pure Al203, as its toughness is increased by addition of
TiC, 4,92 MPam1/2
368
(a)
(b)
Fig. 3. Pure Al203 (a) and Al203 composite (b)
Al203–TiC–Co composite
Incorporation of a ductile phase into a brittle ceramic
matrix has been proved to be an effective mechanism to
toughen the ceramic because the existence of ductile
phase can dissipate the energy of crack initiation and
propagation through plastic deformation, thus making a
larger contribution to the increment of toughness of brittle
ceramic materials. Thin film of metal cobalt was cladded
on the surface of Al203 and TiC powder by a chemical
deposition method before the mixture of powder was hotpressed into an Al203–TiC–Co (about 70 wt.% Al203–Co
and 30 wt.% TiC–Co) [4]. Due to the presence of cobalt
film, ceramic exhibits a higher fracture strength and
fracture toughness than Al203–TiC (about 70 wt. % Al203
and 30 wt. % TiC).
Al2O3/TiCN composite
The problem with these tools is obtaining them, i.e.
sintering. By using new technique of sintering, i.e.
repetitious-hot-pressing technique Al203/Ti(C0.3N0.7),
cutting tool material was fabricated successfully. TiCN is
very attractive as a cutting tool due to its lower friction
coefficient in comparison to TiC. The obtained material
contains 45% of Al203, 55% of Ti(C0.3N0.7), with Nickel +
Molybdenum (4,5%) being added as binders. A small
amount of MgO (0,5%) was used to limit the abnormal
growth of Al203 grains during sintering [5]. This
composite has good mechanical properties (Flexural
strength 820 MPa, Fracture toughness 7.4MPam1/2,
Vickers Hardness 20.4 GPa), especially high fracture
toughness compared to other Al203-based ceramics. The
reason of the toughness increment of this composite is the
formation of a spatial net structure during sintering.
Ti(C0.3N0.7) grains surround almost each grain of Al203
and vice versa. This can be attributed to the molybdenum
existence between hard phases and bonding phase,
separating the hard phase from the liquid during sintering
thus preventing the grain growth of the hard phase owing
to dissolution and reprecipitation, so its addition can fine
Ti(C0.3N0.7) grains. The combined effects of transgranular
fracture and grain bridging mechanisms lead to the higher
toughness of the composite, compared with Al203-based
ceramics. Inserts of this cutting tool material
Al203/Ti(C0.3N0.7), is suitable for continuously cutting of
cast iron, hardened steel, especially intermittent cutting
hardened steel.
Similar composite, Al203/TiCN, have been fabricated by
gas pressure sintering [6], which overcomes the
limitations of hot pressing. Up to now, most of the Al203/
TiC composites have been fabricated by hot pressing
which have many limitations for mass production. The
densification behavior and mechanical properties of the
Al203 gas pressure sintered with 30 wt%. TiCN achieved a
relative density of 99.5%, a bending strength of 772 MPa,
a hardness of 19.6 GPa, and a fracture toughness of 5.82
MPam1/2. Gas-pressure sintering Al203/TiC composites
consist of small titanium carbide grains dispersed in an
alumina matrix and have been used as excellent cutting
tools for a long time. Gas pressure sintering is now widely
used for manufacturing high-performance ceramics
because it combines the advantages of hot pressing,
pressureless sintering and hot isostatic pressing. TiCN is
very attractive for cutting tool due to its lower friction
coefficient than TiC.
Mechanical properties will be achieved for the Al203/
Ti(C,N)/SiC ceramic material when the volume fraction
of both SiC and Ti(C,N) is nearly 15%. The flexural
strength, the fracture toughness and the hardness of ATS
ceramic material is measured to be 721MPa, 5.4MPam1/2
and 19.0 GPa, respectively, which is higher than that of
the pure alumina ceramic. It is shown that when
machining hardened tool steel at low speed, wear mode
of the kind of ceramic tool material is mainly flank wear
with slight crater wear. The adhesion between tool and
work pieces is relatively weak [7]. With the increase of
cutting speed, cutting temperature increases consequently.
As a result, the adhesion is intensified both in the crater
area and flank face.
Al203 -TiC whiskers composite
This material, with in situ growth of whiskers in a matrix
material, has advantages in comparison to other methods,
as the other methods of application of whiskers are still
limited by such disadvantages as healthy hazard, difficult
dispersion, a high cost, etc. In situ growth of TiC
whiskers, strengthening and toughening of Al203 matrix
composite tool materials can greatly reduce the cost and
environment pollution by such advantages as the direct
synthesizing of whiskers in a matrix material and
avoiding the complicated mixing procedures. The
majority of the synthesized whiskers display an ideal
aspect ratio of 10–30 with a diameter of 1– 3 mm [8].
Whiskers have high elastic modulus as an additive can
greatly improve the flexural strength and fracture
toughness of ceramic materials.
Al203 /TiCN/ Y2O3 composite
Recently, rare earths are used as effective additives in a
lot of ceramics Al203 /TiCN-O.2% Y2O3 composites were
fabricated by hot pressing sintering. The bending strength,
fracture toughness and Vickers hardness was respectively
1015 MPa, 6.89 MPam1/2, and 20.82 MPa till 1650 °C [9].
This kind of composite tool material has very good wear
resistance and it is suitable for continuously cutting
quenched steel, and especially intermittently cutting
quenched steel and gray cast iron. The addition of Y203
formed YAG, special compound that precipitated at grain
boundaries of Al203 during sintering and inhibited crystal
growth. The formation of YAG increased bend strength,
fracture toughness, cracks-initiation resistance and cracksextension resistance. The cracks propagated into Al203
grains and were often deflected by TiCN grains. Crack
bridging by TiCN grains and/or Al203 /TiCN bonded
grains might occur. It is considered that this crack
deflection and crack bridging became more effective by
this chain-like structure by rare earth. Al203 /TiCN-0. 2%
Y203 tool gave excellent overall cutting performance, i.e.
longer tool life, lower cutting force, less tool-chip friction
and higher machining quality.
Al2O3– ZrO2 composite
Composite of Al2O3– ZrO2 are used for continuus
shallow cuts, (semifinishing or finishing operation).
Typical applications include turning carbon steels, alloy
steels, tool steels (with lower hardness) and gray, nodular
or malleable cast iron.
The TiC and ZrO2 particles with a diameter of
approximately 20 nm were distributed randomly in the
Al203 matrix [10]. The addition of ZrO2 nanoparticles
reduced the grain size and improved the distribution of
different phases. The mechanical properties increase with
the increasing amount of ZrO2 up to 10 wt%. The Al2O3–
TiC–10 wt% ZrO2 nanocomposites showed the best
mechanical properties with flexural strength of 706MPa
and fracture toughness of 6.3MPam1/2, which was
approximately 20% higher than that of Al203–TiC
composite. Fracture mode of Al203–TiC– ZrO2 being a
mixture of intergranular and transgranular fracture
compared to a mainly intergranular fracture of Al203–TiC.
The residual stresses are generated by the thermal
expansion coefficient mismatch between different phases,
which leads to the generation of dislocations and
microcracks around the nanoparticles. The effects of
nanoparticles on the deflected propagation, nailing and
blocking of the dislocation and microcracks are believed
to contribute to the improvement of the strength and
toughness of Al203–TiC– ZrO2.
Al203-TiB composite
Al203/TiB2 ceramic cutting tools material was produced
by hot pressing [11]. When machining with this tool,
wear rates and the friction coefficient at the tool–chip
interface of Al203 /TiB2 ceramic cutting tools in dry highspeed machining of hardened steel were reduced
comparing to that of low-speed machining. The
mechanisms responsible were determined to be the
formation of a self-lubricating oxide film on the tool–chip
interface owing to the tribological–chemical reaction by
the elevated cutting temperature. The appearance of this
self-lubricating oxide film contributed to the improvement
in wear resistance and the decrease of the friction
369
coefficient. Cutting speed was found to have a profound
effect on the self-lubricating behavior. In dry low-speed
machining of hardened steel, the Al203 /TiB2 tools showed
mainly adhesive and abrasive wear. While in dry highspeed machining, oxidation wear of the ceramic tools was
the dominant mechanism due to the very high cutting
temperature.
Moreover, it offers cost reduction in machining. Dry
machining is becoming increasingly popular due to
concern regarding the safety of the environment.
Recently, consumption of cutting fluids has been reduced
considerably by using mist lubrication. However, mist in
the industrial environment can have serious respiratory
effects on the operator. In dry machining, there will be
more friction and adhesion between the tool and the
workpiece, since they will be subjected to higher
temperatures. This will result in increased tool wear and
hence reduction in tool life. In high-speed machining, the
maximum cutting temperature of the insert involved can
reach more than 1000 ◦C. Conversely, the limit of cutting
speed is a function of the cutting tools used.
Al203/ TiB2/SiCw composite
Al203/TiB2/SiCw ceramic cutting tools with different
volume fraction of TiB2 particles and SiC whiskers were
produced by hot pressing [12]. Results showed that the
fracture toughness and hardness of the composite tool
materials continuously increased with increasing SiC
whisker content up to 30 vol%. The relative density
decreased with increasing SiC whisker content, the trend
of the flexural strength being the same as that of the
relative density. Cutting speeds were found to have a
profound effect on the wear behaviors of these ceramic
tools. The ceramic tools exhibited relative small flank and
crater wear at cutting speed lower than 100 m/min, within
further increasing of the cutting speed the flank and
crater wear increased greatly.
The composite tool materials with higher SiC whisker
content showed more wear resistance. Abrasive wear was
found to be the predominant flank wear mechanism.
While the mechanisms responsible for the crater wear
were determined to be adhesion and diffusion due to the
high cutting temperature.
Al203-NbC composite
The attempt was made to add Al203 and NbC. Fully dense
samples, both with and without NbC, showed hardness
values around 18 GPa [13], which is in accordance with
values reported for similar systems densified using other
additives The presence of a residual second phase did not
reveal any deleterious effect on the hardness of the
composite. No significant toughening has been obtained
by increasing NbC contents. Additions of small amounts
of TiO2 and MnO to plain Al203 have been proven
effective in attaining sintered bodies with densities of
98%
Al203-WC composite
Pressureless sintered and hot-pressed Al203–WC
composites with use of Y2O3 as a sintering additive have
also been evaluated. Additions of up to 30 wt% WC
resulted in limited grain boundary pinning and
corresponding high densification. Although the addition
370
of Y2O3 improved sintering, the presence of a residual
grain boundary phase (YAG) was harmful to the fracture
toughness of the composites, as it affected the
effectiveness of the crack deflection mechanism that takes
place at the interfaces between Al203 and WC grains [14].
Hot-pressing resulted in hardness 17.5 GPa and fracture
toughness 7 MPam1/2 which is an improvement compared
to alumina reinforced by other refractory carbides. The
resulting materials should withstand elevated tempe
ratures and high erosion rates for high-speed and wearresistant cutting tools. Although these materials do not
have the same thermal stability of ceramic-ceramic
composites, they benefit from crack deflection and crack
bridging mechanisms at WC grain boundaries improved
toughness
Al203-YSZ composite
Ceramic cutting inserts produced from Al203/yttria
stabilized zirconia (YSZ) system compositions were
varied from 0 wt% to 100 wt%.
Each Al203 and YSZ composition was mixed and
uniaxially pressed. The results show that 20 wt% of YSZ
produced the minimum wear area [15]. When the amount
of YSZ was increased, the wear area also increased.
However, the Vickers hardness of the inserts decreased
with the increase of YSZ, while the fracture toughness of
the cutting inserts shows a continuous increase up to 60
wt% YSZ. Above 60 wt% of YSZ, the microstructure of
the polished samples started to show microcracks and
formed larger grain sizes of YSZ, thus hindering the
transformation toughening mechanism from functioning
effectively.
The increase of wear resistance of ZTA can be explained
as a result of transformation toughening mechanism that
originated from YSZ when surrounded by alumina matrix.
Al203 - SiC composite
The SiC can be added as powder, as whiskers or as nano
composite. It increases the hardness and the toughness.
Al2O3–SiC composites containing up to 30 wt% of
dispersed SiC particles (Φ280 nm) were fabricated by
hot-pressing and machined as cutting tools [16]. The
Al203–SiC particulate composites exhibit higher hardness
than their unreinforced matrix because of the small grain
structure, i.e. inhibited grain growth by adding SiC and
the presence of hard secondary phase (SiC). For
machining composite, the tool showed the longest tool
life, seven times longer than a commercial tool made of
Al203–TiC composite. The fracture toughness of the
composites remains relatively constant. The large SiC
particles on the grain boundary is believed to contribute to
the increment of the toughness at high (≥20 wt.%) SiC
loadings. The reduced grain size and the transformation of
the fracture mode from intergranular to intragranular of
the composites may lead to the reduction of the fracture
toughness whereas crack deflection by SiC particles is
expected to contribute the increase in toughness. Thus,
these two competing effects seemed to result in the small
change of the fracture toughness in the composites.
As the main drawback of ceramic inserts is their
britleness, small crystals “whiskers” of SiC are added in a
Al203 base [17]. The powder mixture with more 25% SiC
is hot pressed in order to eliminate porousness. Whiskers
are around 1-2 µm in diameter, and around 20 µm long,
very hard and tough, have no imperfections and are
randomly distributed in the Al203 base, fig 4. During the
production process they are pressed insert blanks which
are cut in a wanted shape.
Fig. 4. SiC crystals [17]
In the Greenleaf's insert, the ratio is around 50/50 alumina
to silicon carbide whiskers. These cristals are like fibers
and act as strengtheners for brittle ceramics. They act in a
similar way as fiberglass fibers when strenghtening
rubber. Fracture resistance is an important advantage of
such a reinforced ceramic insert cutter. Their random
distribution in ceramic base helps the base to have
predictable wear patterns to these inserts. This means
mainly higher resistance to crack growth due to deflection
crack on interfaces ,relief of stress at the crack tip when
SiC whiskers pull/out from their sockets in the Al203.
This ceramics has a special area of application and in
general, it is used for machining hard ferrous steel and
nickel alloys. Work piece, often chemically react with
cutting tools, but are inert towards titan.
Alumina-based composites with nanosized SiC and C
inclusions and with alumina matrix grain size varying
from submicrometer to approximately 4 µm. [18].
Materials with submicrometer alumina matrix grains
wear predominantly by plastic deformation and grooving.
Coarse-grained materials wear by mixed wear mechanism
involving crack initiation and interlinking accompanied
by grain pull-out, plastic deformation and grooving.
The wear rate of composites increases with increasing
volume fraction of SiC. Coarse-grained materials wear
much faster then those with submicron microstructure. In
all cases the wear resistance of composites was higher
than that of pure alumina of comparable grain sizes used
as reference materials.
The nanocomposites of the coarse-grained materials wear
by inter- and intra-granular fracture combined with plastic
deformation and grooving. The low wear rate is observed
in composites with lower volume fraction of mainly
intragranularly located SiC, and is correlated with higher
extent of plastic grooving. Nanocomposites with higher
volume fractions of SiC, and larger number of grain
boundary wedging SiC particles, wear comparatively
faster.
Mechanical properties of cutting tool materials with
Al203 matrix was given in Tab. 2
Table 2. Mechanic properties of cutting tool materials
H
Composite
FT
1/2
BS
(MPa)
Ref.
GPa
MPam
Al203/TiC
19.1
4.9
817
[3]
Al203/TiC/C
18.9
5.9
782
[3]
Al203–TiC–Co
19,5
7.9
714
[4]
Al203/TiCN
20.4
7.4
820
[5]
Al203/TiCN
19.6
5.8
772
[6]
Al203/
Ti(C,N)/SiC
19.0
5.4
721
[7]
Al203
/TiCN/Y2O3
20.8
6.9
1015
[9]
Al203/TiC/ZrO2
94.4
HRA
6.3
706
[10]
Al203 /TiB2
20,8
5.2
785
[11]
Al203/ TiB2/SiCw
22.0
8.4
670
[12]
Al203-NbC
18.0
Al203-WC
17.5
7.0
[14]
Al203-YSZ
1700
HV
4.0
[15]
Al203 - SiC
23.0
3.7
[16]
Al203 - SiC
22.2
5.2
[16]
Al203 - SiC
21.1
4.8
[18]
[13]
H-hardness, FT- fracture toughness, BS- bending strength
4. CONCLUSIONS
It can be expected that with the use of various new
ceramic cutting tools, the development of high speed
cutting technology will be promoted. Ceramic cutting
tools have better hardness than other materials, but the
toughness presents a problem. However, cutting tools
brittleness decreases once we add carbide, oxide and
nitride in alumina matrix as they will, with some
additional additives, increase toughness. Current research
is focused on the different combinations of material
composite, using known strengthening and toughening
mechanisms, in order to obtain best possible properties of
cutting tools.
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CORRESPONDENCE
Katarina GERIC, Professor
University of Novi Sad
Faculty of Technical Sciences
Trg Dositeja Obradovica 6
21000 Novi Sad, Serbia
gerick@uns.ac.rs