Study on the Microstructure and the Machining Performance of

Solid State Phenomena Submitted: 2017-11-14
ISSN: 1662-9779, Vol. 281, pp 426-431 Revised: 2018-03-27
doi:10.4028/www.scientific.net/SSP.281.426 Accepted: 2018-04-03
? 2018 Trans Tech Publications, Switzerland Online: 2018-08-31
Study on the Microstructure and the Machining Performance of
Ti SiC -TiB -TiC Composite Ceramic
3 2 2
1,a 1,b 1,c 1,d
Xiangjun Tang , Junshou Li , Fang Zhao , Qing Li
1
Army Engineering University, Advanced Material Institute, Shijiazhuang 050003, Hebei, China
a b c d
[e-mail], [e-mail], [e-mail], [e-mail]
Keywords: Ti SiC ; TiB ; composite ceramic; Machining Performance
3 2 2
Abstract. The Ti SiC -TiB -TiC three-phase ceramics are prepared by Spark Plasma Sintering (SPS)
3 2 2
method with Self-propagating High-temperature Synthesis (SHS) using Ti, Si, C andB C powders.
4
The characterization of sintering product’s image and structure is analyzed by XRD and SEM. Most
of TiB ’s images are angular cuboid or short bar-shaped and most of TiC phase’s images are irregular
2
spherical particles which are evenly embedded in Ti SiC substrate and have a good combination
3 2
interface with Ti SiC . In the composite ceramic SPS sintering process, sinter sample’s displacement
3 2
along Z-axis goes through three stages of falling, balance and rising along with the change of heating
temperature, which reflects the sample’s change rule between heated expansion force and pressure.
Finally its machining performance is analyzed by wire cutting method and machining method. The
Ti SiC -TiC-TiB block composite ceramic proves to have a good machining performance.
3 2 2
Introduction
Machinable ceramic is a kind of ceramic materials which can be machined by using the traditional
machining methods and tools at room temperature, such as turning, drilling, milling, planing,
grinding and tapping, and the machining precision can be up to 10μm or so[1]. Ti SiC is a typical
3 2
representative of the newly layered machinable ceramic materials. Also it refers to the ternary carbide
material whose general expression is M A X sometimes. M refers to the transition metal, A refers to
3 1 2
Si, Al, Ge and so on, X refers to the carbon element. As early as 1967, the ternary carbide was
synthesized at 2000 °C with TiH , Si and C by Jeischko et al, and its crystal structure and theoretical
2
density were also determined[2]. But it was not until 1997 that the single phase Ti SiC which has
3 2
good machining performance was prepared by Barsuom et al using hot pressing method[3]. The
domestic study of machinable ceramics such as Ti SiC gradually began in 2000[4-10]. Due to the
3 2
higher requirements of raw materials, equipment and technological process for the pure Ti SiC ’s
3 2
preparation, the cost of product is bound to increase. Therefore exploring a kind of composite
material with advantages of low cost and excellent performance has become an inevitable
requirement. TiB and TiC are conductive ceramics, both of which have the high hardness, high
2
temperature resistance and a good compatibility with Ti SiC ’s physical and chemical properties[11].
3 2
All of these provide the prerequisite for the material composition.
Experiment
With Ti, Si, C and B C as the raw materials, the three-phase composite ceramic powders
4
composed of Ti SiC , TiB and TiC are prepared by SHS. In the two groups of samples, the relative
3 2 2
content of Ti SiC , TiB and TiC is 45.7%, 28.0%, 26.3% (No.A1) and 42.7%, 29.5%, 27.8%
3 2 2
(No.A2) [12] respectively.
Samples A1 and A2 are fully grinded first, and then are put in the graphite mould to form a whole.
Finally, they are burned in spark plasma sintering system (As is shown in figure 1) into wafers of bulk
(φ=20mm) materials.
Specific sintering parameters are shown in Table 1.
In the sintering process, the parameters on the direction of Z axis, such as displacement,
temperature and pressure should be recorded. The number of the samples which have already been
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Solid State Phenomena Vol. 281 427
sintered is A1 and A2 respectively. The surface morphology is observed by SEM and the phase
composition is analyzed by EDS. The morphology changes between the bulk materials and powder
materials are contrasted, with the related mechanical properties and electrical properties measured
and the machining performance analyzed.

Fig.1 Spark plasma sintering system

Table 1 Sintering parameters
Heating rate Sintering
Pressure Holding Protective
Cooling way
(MPa) ( °C /min) temperature ( °C) time (min) atmosphere
Cooled to below
30 100 1250 10 Vacuum
600°C in 3 minutes


(a) (b)
Fig.2 Displacement changing curves along Z-axis direction as the temperature rises during the
heating
428 High-Performance Ceramics X
Results and Analysis
The analysis of the SPS sintering process. Figures 2 (a) and (b) are respectively the
displacement changing curves along Z-axis direction as the temperature rises during the SPS heating
of sample A1 and A2. When 650°C ≤ T ≤ 900°C, the height reading of the sample on the Z axis
direction gradually reduces. It is caused by the heat expansion. At this point, the radial pressure is less
than the expansion force, so the sample is shown to be longer. When 950°C ≤ T ≤ 1050°C, the curve
tends to be a straight line. At this point, the radial pressure and expansion force are balanced. When T
> 1050°C, the reading on the Z axis direction increases gradually. It shows that the sample has been
gradually compressed during the process of temperature rising until it becomes dense. The changing
trend of sample A2 in displacement-temperature changing curve is roughly the same as that of the
sample A1. Only the speed of displacement change is slightly different within their respective
temperature range.
Figures 3(a) and (b) reflect respectively the displacement changing curves along Z-axis direction
of composite ceramic sample A1 and A2 in the process of the SPS heating preservation with time. It
can be seen from the figure that the changing tendencies of the two samples are roughly the same in
the process of heat preservation. The changes of compression ratios of the samples in the whole
process are not big. After ten minutes of heat preservation, the displacements on the Z axis of the two
samples still have the tendency of increasing. That is to say, the densification degrees of the samples
don’t reach the highest this time. Therefore, in the follow-up experiments, the heat preservation time
should be increased to achieve the higher degree of density.


(a) (b)
Fig.3 Displacement changing curves along the Z-axis direction as time during the heating
preservation


(a) (b)
Fig.4 SEM images of Ti SiC -TiB -TiC composite ceramics
3 2 2

SEM image analysis. Figure 4 are the SEM images of Ti SiC -TiB -TiC composite ceramics after
3 2 2
being magnified 5000 times. In figure 4(a), the two groups of samples are made up of three phases
Solid State Phenomena Vol. 281 429
which have different colors and images, as is shown at 1, 2 and 3. After the preliminary judgment, the
black phase is TiB whose image is angular cuboid or short bar-shaped. The width of cuboid TiB
2 2
crystal is about 500nm and the length of it is 2μm mostly, and a small number of them are 1μm. The
shape of another part is irregular, but the diameter is all within 500nm. The gray phase is TiC which is
in the form of irregular particles and its diameter is between 250nm and 500nm. Both of them are
inlaid in the light color matrix of Ti SiC , and have a closely combined interface with the Ti SiC
3 2 3 2
crystal. Figure 4(b) is the SEM image of Ti SiC -TiB -TiC composite ceramics after being magnified
3 2 2
9000 times. In the figure, the crystal image is more clearly with each phase being more obvious and
the interfaces of each phase combined well.
The machining performance of the electrical discharge. Electrical discharge machining is also
called electro discharge machining. The material is cut by electric erosion phenomenon when the
pulse discharges between the tool electrode and workpiece electrode. According to different
purposes, the electrical discharge machining can be divided into EDM wire cutting, EDM processing
and EDM grinding, etc. Among them, the wire cutting is one of the commonly used method of the
conductive ceramic processing. Therefore, the EDM performance of ceramic material depends not
only on the melting point, specific heat and conductive coefficient, but also on the carrier
concentration, carrier mobility, resistivity and other electrical properties.
The sintered samples processed by the wire cutting equipment are cut into small wafers with the
diameter of 20mm and thickness of 1mm. After being grinded, polished, cleaned and dried, four
electrodes are welded on the sample to make conductive performance test sample. The hall effect
device is used to measure the carrier concentration, carrier mobility and resistivity. The test results
are listed in table 2.
As can be seen from the table, the carrier concentrations of the two samples are similar. Though
the differences between carrier mobility and resistivity are larger, the data still belongs to the same
order of magnitude. It is analyzed that its density is the main cause leading to the differences.

Table 2 Resistivity of Ti SiC -TiB -TiC composite ceramic
3 2 2
Ti SiC -TiB -TiC carrier concentration carrier mobility resistivity
3 2 2
-3
(cm ) (cm/Vs) (Ω·cm)
15 5 -3
A1 3.86×10 6.42×10 2.52×10
15 5 -3
A2 3.65×10 1.40×10 12.26×10

Table 3 Comparison of resistivity among different materials
material
Resistivity (ρ/Ω·cm)
-6
Cu 1.678×10
elemental
-6
Fe 9.71×10
-6
Al brass (70Cu-30Sn) 2.6×10
-6
alloy
Stainless steel (18-8) 5.0×10
-6
Fe-42Ni alloy 7.2×10
-5
TiB 1.44×10
2
-5
TiN 2.5×10
non oxide
-5
ZrC 4.9×10
ceramic
-5
TiC 6.1×10
-5
Ti SiC 2.22×10
3 2

Table 3 shows the resistivity of some common materials[12]. It can be seen that the resistivity of
all the non-oxide ceramics is roughly one order of magnitude higher than that of the elemental metal
and alloy. The result in this paper shows that the resistivity of the two composite ceramic samples is
two orders of magnitude higher than that of the each phase. The resistivity of sample A2 is 4.87 times
of sample A1. This is because there are a certain number of pores in the sample and pores cannot
conduct electricity. The density of sample A1 and A2 is 83.8% and 80.7% respectively, which are
430 High-Performance Ceramics X
measured by the Archimedes method. Thus the density is the leading cause that influences the
different resistivity of composite ceramic.
The material is processed by wire cutting and the machining performance of Ti SiC -TiB -TiC
3 2 2
composite ceramic is evaluated by measuring the surface roughness and observing the processing
surface smoothness after the cutting. Table 4 lists the parameters set during the wire cutting.

Table 4 Wire cutting parameters
Wire electrode (mm) Pulse width (μs) Peak current (A) Pulse interval (μs)
0.18 18 3 8
Feed speed (step/s) Power amplifier tube (piece) Voltage (V) Working solution
0.001 2 90 NH-A/Z special emulsion

The surface roughness of TiB -Ti SiC -TiC composite ceramic is measured by type SJ-201P
2 3 2
surface roughness tester after wire cutting. The roughness of the two samples is 6.52μm and 6.68μm
respectively, which is inversely proportional to the density of the composite ceramic. Therefore, the
greater the density is, the smoother the surface is, the smaller the roughness is after wire cutting, the
better the machining performance is.
Figure 5 is a photo of TiB -Ti SiC -TiC composite ceramic cutting sample. The composite
2 3 2
ceramic sample is cut into a slice with thickness of 1mm. Although its surface has obvious wire
cutting stripes, its shape is complete and the machining defects such as cracks and pits cannot be
observed with naked eyes. Studies have shown that its excellent mechanical performance is mainly
caused by the weak combination of Ti-Si bonds. Raman spectra also shows a fairly small shear
SiC crystal has the similar
modulus value between Si-Si planes[13]. This is mainly because the Ti
3 2
structure with graphite which results in the self lubrication.


Fig.5 Slice figure of the Ti SiC -TiB -TiC composite ceramic
3 2 2


(a)drilling sample (b)wear of drill
Fig.6 Machining behaviors of Ti SiC -TiB -TiC composite ceramic
3 2 2

Machining performance. The samples are drilled by the twist drill of high-speed tool steel
(GB/T6135.3) which is rolled by the tico international Co., LTD. The drill type is HSS-R-W43 and
the specification is 3.0mm. Processing result is shown in figure 6(a). Although the drilling is
successful and the wall of the hole is smooth, we can see from the outside that one end of the hole is
Solid State Phenomena Vol. 281 431
neat and there is some breakage on the edge of the other side. The drill is badly worn, as is shown in
figure 6(b). The roughness of the inner hole on the sample is 9.05μm, which is measured by the type
SJ-201P surface roughness tester. Thus the TiB -Ti SiC -TiC composite ceramic sintered material
2 3 2
has good machining performance.
Conclusions
(1) With Ti SiC , TiB , TiC three-phase composite powders prepared by SHS as the raw materials,
3 2 2
the relatively dense Ti SiC -TiC-TiB block composite ceramics can be obtained by SPS. Most of the
3 2 2
images of TiB are angular cuboid or short bar-shaped, and most of the images of TiC are irregular
2
particles. Both of them are embedded in the Ti SiC matrix, having a closely combined interface with
3 2
Ti SiC crystal.
3 2
(2) In the SPS sintering process of the Ti SiC -TiC-TiB block composite ceramic, sinter sample’s
3 2 2
displacement along the Z axis goes through three stages of falling, balancing and rising along with
change of heating temperature, which reflects the sample’s changing rule between the heated
expansion force and the pressure.
(3) The Ti SiC -TiC-TiB block composite ceramic has a good performance of wire cutting and
3 2 2
machining prepared by SPS. After the wire cutting and the drilling process, the average surface
roughness of the samples is 6.6μm and 9.05μm respectively.
Acknowledgement
This paper is supported by the National Natural Science Foundation of China (No.51172281).
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