MOLYBDENUM SILICIDES AS A BONDING PHASE IN DIAMOND COMPOSITES
Jaworska L.a, Morgiel J.b, Stobierski L.c, Lis J.c, Krolicka B.a, Maziarz W.b
a
Institute of Metal Cutting, Materials Engineering Department, 37a Wroclawska St., 30-011 Krakow,
Poland,
b
Institute of Metallurgy and Materials Science, Polish Academy of Science, 25 Reymonta St.,
30-059 Krakow, Poland.
c
AGH University of Science and Technology, Faculty of Materials Science and Ceramics,
30 Mickiewicza St., 30-59 Krakow, Poland,
ABSTRACT
Molybdenum silicides were obtained by the Self Propagating Synthesis (SHS) method. Diamond
composites containing 30 wt.% of MoSi2 or Mo5Si3 bonding phase had been prepared using HT-HP
Bridgman type apparatus. Sintering of the diamond composites were carried out at 2073±50K and
8±0.2 GPa. The interactions in diamond-silicide composites were studied by means of X-ray
diffraction and transmission electron microscope (TEM). Results of both mechanical properties and
thermal resistance measurements are reported. Hardness HV1 for the composite with
the predominantly MoSi2 bonding phase was found to be around 42GPA. After heat treatment
in 1200°C for 30 min. in vacuum HV1 decreases to 29GPa. In the second composite containing
the Mo5Si3 bonding phase the HV1 of 28.0GPa after thermal exposure changes to 21.0GPa.
Keywords: polycrystalline diamond, HT-HP sintering, silicides, binder material, properties.
1. INTRODUCTION
The application of polycrystalline diamond (PCD) with the cobalt-bonding phase
is in some cases limited by its low thermal resistance above 700°C. One of the possibilities
to increase the thermal resistance of PCD materials is through preparation of composites with nonmetallic bonding phase. The most promising seems to be the MoSi2, which
is characterized by high melting point, good thermal conductivity, relatively modest thermal
expansion coefficient and excellent oxidation resistance up to 1700°C (Table 1). There are two
allotropic phases of MoSi2. Low temperature phase has tetragonal structure. High temperature phase
has hexagonal crystal structure. The allotropic conversion is in 1850-1900°C. Only the creep
resistance of MoSi2is poor, but it might be improved by alloying with other silicides like Mo5Si3
[2, 3], having even higher high melting temperature. Information on the crystal structure, lattice
parameter, density, melting point, linear thermal expansion and microhardness of discussed silicides
were gathered in Table 1.
Table 1. Selected properties of silicides from Mo-Si system [4, 5].
Symbol
Crystal
structure
Lattice
parameters,
nm
MoSi2
Tetragonal
0.32/0.786
Density, Melting
g/cm3 point, °C
6.3
2050
Linear thermal
expansion α,
10-6K-1
Microhardness
HV1, GPa
αa 8.2
αc 9.4
12.9
αa 5.2
11.7
αc 11.5
The intermetallic powders intended as bonding phase may be prepared using relatively low cost
Self-propagating High Temperature Synthesis (SHS) technique. The SHS is especially useful for
Mo5Si3
Tetragonal
0.964/0.49
8.19
2180
silicides production due to low temperature of ignition starting this process. The high porosity of
SHS synthesized silicides makes them easily milled and allows obtaining fine powders to be mixed
with diamond crystallites. A certain drawback of SHS technique is that the combustion products except the main phase - usually contain also other minor phases.
The results of X-ray phase composition analysis and transmission electron microscopy
microstructure observations of PCD were confronted with some physical and mechanical properties
of obtained compacts. The thermal resistance of composites was evaluated by comparing properties
and phases composition before and after thermal exposure.
2. EXPERIMENTAL PROCEDURE
The intermetallic compounds MoSi2 and Mo5Si3 were produced from stoichiometric
mixtures of powders. The purity and grain size of element powders are shown in Table 2.
Table 2. Powder characteristics
Element
Purity, %
Grain size, µm
Silicon*
99.8
0-60
Molybdenum (Goodfellow)
99.9
2 (average particle)
* Obtained by an autogenic milling of silicon waste chips produced by ZA Tarnow, Poland.
The homogeneity of these mixtures were ascertain through extensive mixing of their components
for 12 hours in a vibratory rotary mill with Teflon balls suspended in anhydrous isopropyl alcohol.
Drying was carried out during mixing. Next, powders were formed into discs by pressing in steel
matrix and synthesized using SHS technique. The reaction was started at 1415°C using graphite
crucible with the graphite foils lining in an argon-filled chamber. After ignition, i.e. rising part of
material to high temperature, the front of reaction propagated across the reaction crucible resulting
in full transformation of the loaded material. The products of SHS reaction were crushed in Abbich
mortar to powder with grain size to 0.5mm and next they were milled in rotary-vibratory mill with
WC grinding media in anhydrous isopropyl alcohol to power with specific surface area to 10m2/g.
Diamond powders of 3-6µm (MDA, De Beers) were mechanically mixed with 30wt.%
of silicides powders obtained using described above procedure. It is confirmed that the amount of
bonding phase has a significant influence on compressive strength and hardness
of composites; using of 30wt.% bonding phase is especially advantageous [6].
The resulting mixture was formed into discs (φ =15mm, h=5mm) by pressing in a steel
matrix under pressure of 200MPa. Samples were heated using an assembly provided with
an internal graphite heater. Compacts were sintered at pressure of 8.0±0.2GPa and temperature
1800±50°C in the high pressure Bridgman type apparatus.
Diamond composites were analyzed qualitatively by X-ray diffraction (XRD) using Philips
PW 1710 diffractometer using Co Kα radiation filtered graphite monochromator single crystal.
Vickers indentation tests were performed on compacts using FM-7 micro hardness tester.
The applied load was 10N for 10s. Pieces of sintered compacts intended for hardness measurements
were subjected to lapping on the Strues plate with diamond abrasive. The microstructure
observations were performed using Philips CM20 transmission electron microscopy (TEM) with
EDAX Phoenix energy depressive spectroscopy (EDS) microanalysis attachment. Ultrasonic
drilling, mechanical polishing and ion milling with Gatan DuoMill 600 prepared thin foils.
The density was measured using hydrostatic method.
For the thermal resistance studies diamond composites with 30wt.% of the molybdenum silicides
bonding phase were annealed for 30 min at 1200°C in 0.8 Pa vacuum using
the GERO HTK 8/22G Furnace. The heating rate was 10°/min.
3. RESULTS AND DISCUSION
The X-ray analysis of the SHS product of 33,3% molybdenum and 66,6% silicon indicated
presence of both MoSi2 and Mo5Si3. The presence of the latter amounts up to ~6% (the phase
content was estimated using the direct intensity comparison method of B.L.Averbach
and M.Cohen). On the other hand, the next product from SHS of containing 62.5% molybdenum
and 37.5% silicon was composed mostly of Mo5Si3, but again with several percent of MoSi2
and trace presence of Mo3Si.
Transmission electron microscopy observations of microstructure of PCDs produced with
characterized above SHS reaction products proved their very low porosity (Fig. 1a).
a)
b)
Fig. 1. TEM microstructure of PCD with binding material containing predominantly:
a) MoSi2, b) Mo5Si3
The compacts consisted of large blocky diamond crystallites separated by thin channels
filled with fine crystalline material. As the observations were performed in thicker areas (to avoid
areas with perforation) the latter usually showed much darker contrast due to its stronger inelastic
scattering electrons by metal atoms than carbon in diamond, what is well documented in Fig. 1a
and Fig.1b. The cracks present in this microstructure might have originated during sample
preparation or handling of thin foil but it still indicates that the weakest link in this PCD is diamond
- bonding material link. The center of bonding material was usually filled with crystallites
of slightly larger size than those being in contact with diamond. The analysis electron diffraction
patterns obtain from fine crystallites proved them to be a mixture of molybdenum silicides
and molybdenum silicon carbides, while the larger crystallites in the center were either
molybdenum silicides. X-ray diffraction pattern of the composite with the 30wt.% predominantly
MoSi2 bonding phase (Fig. 2-as sintered) confirm that this material is composed of tetragonal
MoSi2, Mo24Si15C3 and diamond. In the case of composite with 30wt.% of predominantly Mo5Si3,
this compact is composed of hexagonal form of MoSi2, Mo24Si15C3 and diamond.
Table 3. Selected physical properties of PCD with 30 wt.% of the silicides bonding phase.
Bonding material
Hardness HV1, GPa
App. Density, g/cm3
Young modulus a, GPa
MoSi2 (>Mo5Si3)
40.0±3.0
3.77
348
Mo5Si3 (>MoSi2)
27.5±1.0
3.92
327
a) Young’s modulus measured with the ultrasonic method.
The hardness measurements of the investigated compacts indicated, that while those with
MoSi2 were around 40 GPa, though the PCD with Mo5Si3 were characterized by distinctly lower
hardness, Table 3. On the other hand, the Young’s modulus was highest for the PCD with MoSi2
based binding material at the level of ~350 GPa while the second compact has hardness below 330
GPa.
The use of polycrystalline materials on the base of diamond with cobalt bonding phase was
in some cases limited by low temperature using conditions because these composites was observed
the tendency to the graphitization process. A suitable criterion for thermal resistance was
the evaluation of PCD phase composition and properties before and after thermal exposure,
particularly hardness and density changes. The phase’s changes during thermal treating were
observed on X-ray pattern, Fig. 2 and Fig.3.
After the thermal treatment of both compacts practically no more molybdenum silicides were
present. Intensities of peaks connected with Mo24Si15C3 increased, so one may conclude that
amount of this compound raised. Lattice parameters of hexagonal Mo24Si15C3 were a=0.7286 nm,
c=0.5046 nm, c/a=0.692, vol.of cell=232 Å3 [7]. For these compacts after heat-treating atoms
distance in Mo24Si15C3 are changing and are close to theoretical values (Table 4).
Table 4. Changes of lattice parameters in Mo24Si15C3 cells for diamond composites with 30wt.%
of molybdenum silicides bonding phases, before and after heat treatment.
Type of bonding
a, nm
b, nm
c, nm
vol., nm3
Phase
MoSi2 (>Mo5Si3)
0.7304
0.5093
0.697
235.3
MoSi2 (>Mo5Si3)T*
0.7294
0.5069
0.695
233.5
Mo5Si3 (>MoSi2)
0.7297
0.5087
0.697
234.6
Mo5Si3 (>MoSi2)T*
0.7291
0.5053
0.693
232.6
*) T-after heat treatment in 1200°C for 30 min.
M oSi2_tet
Intensity [a.u.]
M o24Si15C3
C_D
1200°C
as sintered
20
30
40
50
60
2Theta [degrees]
Fig. 2. X-ray pattern of diamond composite with 30wt.% of predominantly MoSi2
before and after heat treatment.
M o24Si15C3
C_D
Intensity [a.u.]
M oSi2_hex
1200°C
as sintered
20
30
40
50
60
2Theta [degrees]
Fig. 3. X-ray pattern of diamond composite with 30wt.% of predominantly Mo5Si3
before and after heat treatment.
Table 5. Hardness and density of diamond composites with 30wt.% of molybdenum silicides before
and after thermal exposure in 1200°C for 30min in vacuum
Apparent density
Apparent density
Hardness
Hardness
Type and amount of
before treat,
after treat,
HV1*,
HV1**,
bonding phase
3
3
g/cm
g/cm
GPa
GPa
MoSi2 (>Mo5Si3)
3.800
3.627
42.0
29.0
Mo5Si3 (>MoSi2)
3.965
3.788
28.0
21.0
*) Hardness before heat treatment
**) Hardness after heat treatment
For the composite with predominantly the MoSi2 bonding phase hardness changes are about
13 GPa, for the composite with Mo5Si3 about 7.0 GPa, Table 5. In both cases, graphite is not
detected.
4. CONCLUSIONS
The performed investigations indicate that mixing of diamond powder with silicides from
Mo-Si systems allow to produce compacts characterised by good bonding of filler material
and diamond crystallites, i.e. the SHS produced silicides has high chemical activity at high pressure
and high temperature at which this compacts are prepared.
The X-ray phase analyses indicate that the bonding material has a multiphase composition
including carbides and silicides. Properties of compacts were found related to the type
of dominating silicides in the bonding phase, i.e. compacts based on the MoSi2 strongly differ
in properties from that based on Mo5Si3.
ACKNOWLEDGMENT
The State Committee for Scientific Research supports project (grant T 08D 024 21).
The authors would like to thank S.Kotas for X-ray diffraction.
REFERENCES
[1] E.L.Courtright, Materials Science and Engineering A261(1999), pp. 53-63.
[2] J.R.Jokisaari, S.Bhaduri, S.B.Bhaduri, Mat. Sci. and Engineering A323 (2002) pp. 478-483.
[3] F.Chu. D.J.Thoma, K.J.McClellan, P.Peralta, Mat. Sci. and Engineering A261 (1999)
pp. 44-52.
[4] G.V.Samsonov, L.A.Dropina, B.M. Rul’, “Silicidy”, Metallurgia, Moskwa, 1979.
[5] R.Riedel, “ Handbook of Ceramic Hard Materials”, WILEY-VCH, vol.II, 2000.
[6] L.Jaworska, T.Gibas, B.Krolicka, J.Morgiel, S.J.Skrzypek, High Pressure Research 18 (2000)
pp.271-277.
[7] E.Parhe, W.Jeitschenko, Acta Cryst. 19 (1965) pp. 1031.