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Processing and properties of nanostructured WC-Co
Nan~TRUCTURED MATERIALS MOL, 1, PP. 119-124, 1992 COPYRIGHT~1992 PERGAMONPRESSLtd. ALL'RIGHTS RESERVED
0965-9773/92 $5.00 + .00 PRINTEDIN THE USA
PROCESSING AND PROPERTIES OF NANOSTRUCTURED WC-Co
L.E. McCandllsh, B.H. Kear, and B.K. Kim Department of Mechanics and Materials Science Rutgers University Piscataway, NJ 08855-0909
Introduction The traditional method of making WC-Co cemented carbides is by crushing, grinding, blending and consolidation of the constituent powders [I]. Taking this approach, the microstructural scale can be no smaller than the size of the milled powders, typically i-I0 microns in diameter. With great effort, the microstructural scale can be reduced to about 0.5 micron (500 nm) in premium WC-Co grades. Meeting the challenge of obtaining improved properties by further reduction in grain size, Figure i, requires a new approach. Our approach is to start with precursor compounds, in which W and Co are mixed at the molecular level, and then to transform these compounds into nanostructured WC-Co powder by thermochemical treatment [2-9]. Table I contrasts the conventional powder metallurgical method with the new chemical processing method for making WC-Co powders.
Table I Old vs. New Method of Making WC-Co Powder A.
Conventional Powder Metallurgy Method I. 2. 3. 4. 5.
B.
Reduce APT powder to W powder React W powder and C powder at 1600°C to make WC powder Ball mill WC powder with high purity Co powder Add lubricant and press to shape by cold compaction Liquid phase sinter at 1425°C
New Thermochemical Synthesis Method i. 2. 3. 4.
Precipitate molecular precursor powder at 100°C thereby mixing W and Co at the molecular level React precursor powder at controlled carbon and oxygen activity at 600-i000°C to produce nanostructured WC-Co powder Add lubricant and press to shape by cold compaction HIP-sinter at 1200°C
The thermochemlcal synthesis method provides a more direct route for making WC-Co powder. Processing conditions are milder and reaction times are shorter. These conditions favor the production of nanostructured materials.
Consolidated WC-Co powders are used for metal cutting or rock drilling applications, and for wear parts. Hardfacings are produced by thermal spraying methods, such as plasma spraying, plasma transferred arc deposition, and hypersonic jet spraying. Cutting tools and wear parts are made by cold compaction followed by liquid phase sintering [i]. Recently, there has been interest in net-shape processing by powder injection molding [i0]. In principle, all of these techniques may be used to consolidate nanostructured WC-Co powders, provided that the thermal transient in the liquid state is so brief that coarsening of the phases is minimized. In this paper, we note that nanostructured WC-Co powders may also be sintered rapidly in the solid state, which is the preferred method for obtaining the finest microstructures.
119
120
LE MCCANDLISH,BH KEARAND BK KIM
Powder Processin~ A prerequisite for the chemical processing of nanostructured WC-Co powder is a homogeneous precursor powder, in which W and Co are intimately mixed at the molecular level, such as in the compound trls(ethylenediamlne) cobalt tungstate, Co(en)3W04, which after reduction and carburization yields WC-23 wt% Co [2]. Other compositions (i.e. W/Co ratios) can be obtained from aqueous solution mixtures, such as Co(en)3WO 4 + H2WO 4 or AMT + CoCla (where AMT (NH4)s(H2WI2040).4HzO) [5]. These solutions, upon atomization and rapid drying, precipitate homogeneous powders with amorphous or microcrystalline structures, which are suitable precursors for subsequent thermochemical processing to the desired nanostructured we-co powder. Spray dried powders have been used to produce WC-Co powders with from 3-30 wt% Co. Figure 2 shows a TGA trace of the reduction of Co(en)3WO 4 powder in i:i H2:Ar gas, as the powder is heated from room temperature to 700°C at a rate of 250C/mln [2]. The ethylenedlamine llgands are cleanly removed at between 150-250°C and the reduction is complete at 650°C. The result of this reduction step is a nanostructured powder, whose x-ray diffraction pattern is shown in Figure 3. The powder does not have the equilibrium composition CoTW6 + W expected from the Co-W binary phase diagram, but rather consists of a homogeneous nanostructured mixture of W and Co. This intermediate powder is nanoporous, with a surface area of 40 m2/g. It is extremely reactive, combusting spontaneously on exposure to air at room temperature. In practice, this is not a problem, because the reactive powder is not normally removed from the reactor before carburization. The nanoporosity of the mixed metal powder means that it can be carburized easily by reaction with a buffered gas mixture, such as C0-C02, that is capable of supplying carbon. This gas mixture can supply carbon via the reaction: 2 CO - CO 2 + C
(i)
Thus, the carbon activity of the CO-CO d mixture is given by a c - (Pco2/P~2) exp(AGO/RT)
(2)
At carbon activities suitable for the formation of WC-Co, the small oxygen pressure due to the equilibrium 2 COz - 2 CO + 02 (3) is not high enough to form WO2, the most stable oxide at the carburization temperature. In fact, the precise measurement of Po2, with a ZrO 2 oxygen sensor, allows the calculation of the ratio of Pco2/Pco. This ratio plus knowledge of the total pressure, Pco + Pco2, allows calculation of a c by equation 2. In this way an oxygen sensor can be used to program and control the carbon activity of the gas phase inside the reactor. An isothermal section through the quaternary phase diagram for W, Co, C and O is a tetrahedron. Two sides of the IIO0°C section are shown in Figure 4 [ii]. Starting with a s p e c i f i e d W / C o ratio, the tie line connecting this point to the carbon vertex defines the accessible equilibrium compositions, provided that the oxygen partial pressure is below that required to form any metal oxide phase. The latter requirement is easily satisfied for WC-Co, since the carbon activity in the two phase field is relatively h i g h a n d the oxygen pressure in the gas phase is relatively low. Application of the Phase Rule indicates one degree of freedom across the WC-Co phase field. Thus, at about IIO0°C, the carbon activity can be varied between about 0.2 - 0.99 without precipitation of Co3W3C or C, and without formation of oxide(s). From a practical viewpoint, the range of W/Co atom ratios extends from about 0.5 - 0.95. Figure 5 shows a schematic of a commercial-scale unit for chemical processing of nanostructured WC-Co powder. AMT and a water soluble cobalt salt are dissolved in the solution mixing tanks. The aqueous solution is pumped through a filter to a gas-fired spray dryer, where it is atomized and dried by forced hot air to form a precursor powder. The powder product, which is entrained in the air stream, is separated by a cyclone and deposited in a temporary holding vessel. The precursor powder is then transferred to a fluid bed reactor for thermochemical conversion (reducton and carburization) to WC-Co powder. Figure 6a shows the characteristic dimpled structure of the nanostructured WC-Co powder particles. Figure 6b shows a high resolution SEM back scattered electron image of a typical powder particle; the fine scale of the interconnected porosity (black contrast), the network of WC grains (white contrast) and the associated network of Co grains (gray contrast) are clearly visible.
NANOSTRUCTURED WC-Co
~,~ Compresm~ Strength
121
600C
2500 ~
*•EI !I,°°°
5C0C %
subm*crmWC ~
5000
i x
'E
~
1500 " ' - " " ~ - , . 1000
~
3ooc
e~,,o
200( o=
z
500 10
15 ~ WEIGHT XCOS~T
30o0
~
2ooo
lOOO
5
25
Temperature ( ' C } 100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0
7s0p
(
g 2O
N
3O
60
1.198
I 0
I 4
I 8
I 12
I 16
Time(rain.)
-4H20
J
[ 20
28
I 24
20
Coi~
w
w
70
60 1.541
1.343
c tJ
2.252
Fig. 3 X-ray diffraction pattern of nanophase Co-W powder [2].
WC
c~%
kl
40 Two Thetald-spacing 50 1.668
c
p-c=
=
c°Gwf'"V°Tw6
I
Fig. 2 Reductive decomposition of Co(en)3WO 4 to form the reactive intermediate [2].
Co
25
. . . . . .
0oo
At:H2=1:1
UmulThermal r Ramp c ~
15
Fig. l(b) Transverse rupture strength and fracture toughness of WC-Co hardmetals as a function of composition and WC grain size [12].
60'/
CO(en)GWO4 -2Ira
I0
WEIGHT ZCOBALT
Figure l(a) Hardness and compressive strength of WC-Co hardmetals as a function of composition and WC grain size [12].
10
15mnlc~SwUbmcronl eno,tlWC na•l hardmetz~110
$
1000 5
w
20
F r a c t ~ ~
W
1
Fig. 4 Isothermal sections for the ternary systems (a) W-Co-O, I100-1300°C; (b) W-Co-C, I127°C.
W
122
LE MCCANDLISH,B H
KEAR AND BK KIM
$OLUTIOIV MIX TANK
RF.,4CT,4AIT ~SES
PRODL~T
F i g . 5 Commercial s c a l e u n i t f o r c h e m i c a l p r o c e s s i n g o f WC-Co powder.
Fig. 6 SEMmlcrographs of thermochemically processed WC-10 wt. Co powder (a) low magnification (x2000), . . . . . . . . . gnlfication (x35,000).
2000 19oo o .~
!
~"""~
NanodyneWC-Co S~dvk WC-Co
18oo
1700 1600 15oo
2;0
" ,;o
" ,;o
,;o
,ooo
Grain Size (nm)
Fig. 7 High magnification (x120,O00) TEM micrograph of sintered WC-10 wt.% Co, showing faceted 200 nm grains of WC in Co binder.
Fig. 8 Dependence of hardness on WC grain size in WC-10 wt% Co material
NANOSTRUCTUREDWC-Co
123
Powder Consolidation An accepted method of consolidating conventional WC-Co powders is by cold compaction, followed by liquid phase sintering at about 1400°C in vacuum, i.e. just above the WC-Co pseudo-binary eutectic temperature, 1320°C. In general, sintering times measured in hours are needed to achieve theoretical densities in high WC volume fraction alloys. Cold compaction and liquid phase sintering is also an attractive option for consolidating nanostructured WC-Co powders. However, it is clearly essential to minimize the time spent at the sintering temperature in order to minimize particle coarsening, which can be quite rapid in the presence of liquid Co. Tests have shown that dense structures in WC-10 wt% Co can be achieved in 30 seconds at 1400°C, which gives WC grain size of about 200 nm. An additional 30 seconds sintering time increases the WC grain size to 2.0 microns. Such rapid grain growth is characteristic of ultrapure WC-Co. A small amount of uncombined C, or an addition of Cr, markedly inhibits grain growth during liquid phase sintering. On the other hand, ultrapure WC-Co powders can be consolidated by solid state sintering, where grain growth is much slower. Figure 7 shows 200 nm WC grain size in solid state slntered WC-10 wt% Co. Hardness Hardness values of WC-10 wt% Co with WC grain size 200 nm, 400 nm, and 800 nm, are respectively 1950, 1700 and 1550 Kg/mm 2. Figure 8 compares the hardness of 200 nm material prepared from chemically processed powder with Sandvik's fine and ultrafine grades of WC-Co. Clearly, the 200 nmmaterial offers an advantage because the high hardness is achieved in a high Co volume fraction alloy, where the ductility and fracture toughness is greater.
Summary A novel chemical processing method is described for making nanostructured WC-Co powders. Critical to the success of the process is the control of thermodynamics and kinetics of gas-solid reactions in a fluid bed reactor. Of particular importance is the precise control of carbon and oxygen activities in the fluidizing gas stream. The as-synthesized powders have a high surface area and each powder particle is composite in nature. In contrast to conventional WC-Co powders, the extremely good mixing of the ceramic and metal phases in the powder particles, and the interconnected nanoperosity enables these powders to be consolidated by solid state sintering at relatively low temperatures. A low sintering temperature and a short sintering time ensure retention of a nanostructure in the consolidated material. Preliminary evidence indicates that as the scale of the nanostructure is reduced, the mechanical properties, such as hardness, are enhanced, which is in keeping with the wellestablished trend in properties of conventional WC-Co material. References i. 2. 3.
4.
5.
6.
F.V. Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, Princeton, NJ (1980) 383. L.E. McCandlish, R.S. Pollzzotti, Control of Composition and Microstructure in the Co-W-C system Using Chemical Synthetic Techniques, Sol. State lonics, 32/33 (1989) 795. L.E. McCandlish, B.H. Kear, B.K. Kim, L.W. Wu, Metastable Nanocrystalline Carbides in Chemically Synthesized W-Co-C Ternary Alloys, in Multicomvonent Ultrafne Microstructures, L.E. McCandlish, D.E. Polk, R.W. Siegel, B.H. Kear, Ed., MRS Symposium Proceedings, Vol. 132 (1989) 67. L.E. McCandlish, B.H. Kear, B.K. Kim, L.W. Wu, Low Pressure Plasma-Sprayed Coatings of Nanophase WC-Co, in Protective Coatings: Processin~ and Characterization, R.M. Yacizi, Ed., The Metallurgical Society, Warrendale, PA (1990). L.E. McCandlish and B.H. Kear, Chemical Processing of Nanophase WC-Co Composite Powders, Proceedings of the Cottrell Conference on Advanced Materials, Special Issue of Mat. Sci. Tech., Institute of Metals, Ed. J.l. Harris, London, in press (1990). L.E. McCandlish, B.K. Kim and B.H. Kear, Chemical Synthesis of Nanophase Metal/Ceramlc Composites, in High Performance Comoosites for the 1990's, to be published by the Metallurgical Society (1991).
124
7.
8. 9. I0. II. 12.
LE McCANDLISH,BH KEARAND BK KIM
B.H. Kear, B.K. Kim, and L.E. McCandllsh, Chemically Processed Nanophase WC-Co Composites, in Frontiers of Chemlstrv: Materials by Desizn, American Chemical Society's Chemical Abstracts Service, columbus, Ohio (1990) 65. L.E. McCandlish and R.S. Polizzotti, Multiphase Composite Particle, US Pat. No. 4,851,041 (1990). L.E. McCandlish, B.H. Kear and J. Bhatia, Spray Conversion Process for the Production of Nanophase Composite Powders (U.S. Pat. App. S.N. 433,742). R.M. German, D. Lee, C. Chung and R.W. Messier, Near Net Shape Manufacturing, P.W. Lee and B.L. Furguson (eds.), ASM International, Metals Park, Ohio (1988) 187. T.D. Halllday, F.H. Hayes and F.R. Sale, The Industrial Use of Thermochemical Data, T.I. Barry (ed.), Chemical Society, London (1980) 291. P. Ettmayer, Hardmetals and Cermets, Annu. Rev. Mat. Sci. 1989, 19:145:64.
0965-9773/92 $5.00 + .00 PRINTEDIN THE USA
PROCESSING AND PROPERTIES OF NANOSTRUCTURED WC-Co
L.E. McCandllsh, B.H. Kear, and B.K. Kim Department of Mechanics and Materials Science Rutgers University Piscataway, NJ 08855-0909
Introduction The traditional method of making WC-Co cemented carbides is by crushing, grinding, blending and consolidation of the constituent powders [I]. Taking this approach, the microstructural scale can be no smaller than the size of the milled powders, typically i-I0 microns in diameter. With great effort, the microstructural scale can be reduced to about 0.5 micron (500 nm) in premium WC-Co grades. Meeting the challenge of obtaining improved properties by further reduction in grain size, Figure i, requires a new approach. Our approach is to start with precursor compounds, in which W and Co are mixed at the molecular level, and then to transform these compounds into nanostructured WC-Co powder by thermochemical treatment [2-9]. Table I contrasts the conventional powder metallurgical method with the new chemical processing method for making WC-Co powders.
Table I Old vs. New Method of Making WC-Co Powder A.
Conventional Powder Metallurgy Method I. 2. 3. 4. 5.
B.
Reduce APT powder to W powder React W powder and C powder at 1600°C to make WC powder Ball mill WC powder with high purity Co powder Add lubricant and press to shape by cold compaction Liquid phase sinter at 1425°C
New Thermochemical Synthesis Method i. 2. 3. 4.
Precipitate molecular precursor powder at 100°C thereby mixing W and Co at the molecular level React precursor powder at controlled carbon and oxygen activity at 600-i000°C to produce nanostructured WC-Co powder Add lubricant and press to shape by cold compaction HIP-sinter at 1200°C
The thermochemlcal synthesis method provides a more direct route for making WC-Co powder. Processing conditions are milder and reaction times are shorter. These conditions favor the production of nanostructured materials.
Consolidated WC-Co powders are used for metal cutting or rock drilling applications, and for wear parts. Hardfacings are produced by thermal spraying methods, such as plasma spraying, plasma transferred arc deposition, and hypersonic jet spraying. Cutting tools and wear parts are made by cold compaction followed by liquid phase sintering [i]. Recently, there has been interest in net-shape processing by powder injection molding [i0]. In principle, all of these techniques may be used to consolidate nanostructured WC-Co powders, provided that the thermal transient in the liquid state is so brief that coarsening of the phases is minimized. In this paper, we note that nanostructured WC-Co powders may also be sintered rapidly in the solid state, which is the preferred method for obtaining the finest microstructures.
119
120
LE MCCANDLISH,BH KEARAND BK KIM
Powder Processin~ A prerequisite for the chemical processing of nanostructured WC-Co powder is a homogeneous precursor powder, in which W and Co are intimately mixed at the molecular level, such as in the compound trls(ethylenediamlne) cobalt tungstate, Co(en)3W04, which after reduction and carburization yields WC-23 wt% Co [2]. Other compositions (i.e. W/Co ratios) can be obtained from aqueous solution mixtures, such as Co(en)3WO 4 + H2WO 4 or AMT + CoCla (where AMT (NH4)s(H2WI2040).4HzO) [5]. These solutions, upon atomization and rapid drying, precipitate homogeneous powders with amorphous or microcrystalline structures, which are suitable precursors for subsequent thermochemical processing to the desired nanostructured we-co powder. Spray dried powders have been used to produce WC-Co powders with from 3-30 wt% Co. Figure 2 shows a TGA trace of the reduction of Co(en)3WO 4 powder in i:i H2:Ar gas, as the powder is heated from room temperature to 700°C at a rate of 250C/mln [2]. The ethylenedlamine llgands are cleanly removed at between 150-250°C and the reduction is complete at 650°C. The result of this reduction step is a nanostructured powder, whose x-ray diffraction pattern is shown in Figure 3. The powder does not have the equilibrium composition CoTW6 + W expected from the Co-W binary phase diagram, but rather consists of a homogeneous nanostructured mixture of W and Co. This intermediate powder is nanoporous, with a surface area of 40 m2/g. It is extremely reactive, combusting spontaneously on exposure to air at room temperature. In practice, this is not a problem, because the reactive powder is not normally removed from the reactor before carburization. The nanoporosity of the mixed metal powder means that it can be carburized easily by reaction with a buffered gas mixture, such as C0-C02, that is capable of supplying carbon. This gas mixture can supply carbon via the reaction: 2 CO - CO 2 + C
(i)
Thus, the carbon activity of the CO-CO d mixture is given by a c - (Pco2/P~2) exp(AGO/RT)
(2)
At carbon activities suitable for the formation of WC-Co, the small oxygen pressure due to the equilibrium 2 COz - 2 CO + 02 (3) is not high enough to form WO2, the most stable oxide at the carburization temperature. In fact, the precise measurement of Po2, with a ZrO 2 oxygen sensor, allows the calculation of the ratio of Pco2/Pco. This ratio plus knowledge of the total pressure, Pco + Pco2, allows calculation of a c by equation 2. In this way an oxygen sensor can be used to program and control the carbon activity of the gas phase inside the reactor. An isothermal section through the quaternary phase diagram for W, Co, C and O is a tetrahedron. Two sides of the IIO0°C section are shown in Figure 4 [ii]. Starting with a s p e c i f i e d W / C o ratio, the tie line connecting this point to the carbon vertex defines the accessible equilibrium compositions, provided that the oxygen partial pressure is below that required to form any metal oxide phase. The latter requirement is easily satisfied for WC-Co, since the carbon activity in the two phase field is relatively h i g h a n d the oxygen pressure in the gas phase is relatively low. Application of the Phase Rule indicates one degree of freedom across the WC-Co phase field. Thus, at about IIO0°C, the carbon activity can be varied between about 0.2 - 0.99 without precipitation of Co3W3C or C, and without formation of oxide(s). From a practical viewpoint, the range of W/Co atom ratios extends from about 0.5 - 0.95. Figure 5 shows a schematic of a commercial-scale unit for chemical processing of nanostructured WC-Co powder. AMT and a water soluble cobalt salt are dissolved in the solution mixing tanks. The aqueous solution is pumped through a filter to a gas-fired spray dryer, where it is atomized and dried by forced hot air to form a precursor powder. The powder product, which is entrained in the air stream, is separated by a cyclone and deposited in a temporary holding vessel. The precursor powder is then transferred to a fluid bed reactor for thermochemical conversion (reducton and carburization) to WC-Co powder. Figure 6a shows the characteristic dimpled structure of the nanostructured WC-Co powder particles. Figure 6b shows a high resolution SEM back scattered electron image of a typical powder particle; the fine scale of the interconnected porosity (black contrast), the network of WC grains (white contrast) and the associated network of Co grains (gray contrast) are clearly visible.
NANOSTRUCTURED WC-Co
~,~ Compresm~ Strength
121
600C
2500 ~
*•EI !I,°°°
5C0C %
subm*crmWC ~
5000
i x
'E
~
1500 " ' - " " ~ - , . 1000
~
3ooc
e~,,o
200( o=
z
500 10
15 ~ WEIGHT XCOS~T
30o0
~
2ooo
lOOO
5
25
Temperature ( ' C } 100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0
7s0p
(
g 2O
N
3O
60
1.198
I 0
I 4
I 8
I 12
I 16
Time(rain.)
-4H20
J
[ 20
28
I 24
20
Coi~
w
w
70
60 1.541
1.343
c tJ
2.252
Fig. 3 X-ray diffraction pattern of nanophase Co-W powder [2].
WC
c~%
kl
40 Two Thetald-spacing 50 1.668
c
p-c=
=
c°Gwf'"V°Tw6
I
Fig. 2 Reductive decomposition of Co(en)3WO 4 to form the reactive intermediate [2].
Co
25
. . . . . .
0oo
At:H2=1:1
UmulThermal r Ramp c ~
15
Fig. l(b) Transverse rupture strength and fracture toughness of WC-Co hardmetals as a function of composition and WC grain size [12].
60'/
CO(en)GWO4 -2Ira
I0
WEIGHT ZCOBALT
Figure l(a) Hardness and compressive strength of WC-Co hardmetals as a function of composition and WC grain size [12].
10
15mnlc~SwUbmcronl eno,tlWC na•l hardmetz~110
$
1000 5
w
20
F r a c t ~ ~
W
1
Fig. 4 Isothermal sections for the ternary systems (a) W-Co-O, I100-1300°C; (b) W-Co-C, I127°C.
W
122
LE MCCANDLISH,B H
KEAR AND BK KIM
$OLUTIOIV MIX TANK
RF.,4CT,4AIT ~SES
PRODL~T
F i g . 5 Commercial s c a l e u n i t f o r c h e m i c a l p r o c e s s i n g o f WC-Co powder.
Fig. 6 SEMmlcrographs of thermochemically processed WC-10 wt. Co powder (a) low magnification (x2000), . . . . . . . . . gnlfication (x35,000).
2000 19oo o .~
!
~"""~
NanodyneWC-Co S~dvk WC-Co
18oo
1700 1600 15oo
2;0
" ,;o
" ,;o
,;o
,ooo
Grain Size (nm)
Fig. 7 High magnification (x120,O00) TEM micrograph of sintered WC-10 wt.% Co, showing faceted 200 nm grains of WC in Co binder.
Fig. 8 Dependence of hardness on WC grain size in WC-10 wt% Co material
NANOSTRUCTUREDWC-Co
123
Powder Consolidation An accepted method of consolidating conventional WC-Co powders is by cold compaction, followed by liquid phase sintering at about 1400°C in vacuum, i.e. just above the WC-Co pseudo-binary eutectic temperature, 1320°C. In general, sintering times measured in hours are needed to achieve theoretical densities in high WC volume fraction alloys. Cold compaction and liquid phase sintering is also an attractive option for consolidating nanostructured WC-Co powders. However, it is clearly essential to minimize the time spent at the sintering temperature in order to minimize particle coarsening, which can be quite rapid in the presence of liquid Co. Tests have shown that dense structures in WC-10 wt% Co can be achieved in 30 seconds at 1400°C, which gives WC grain size of about 200 nm. An additional 30 seconds sintering time increases the WC grain size to 2.0 microns. Such rapid grain growth is characteristic of ultrapure WC-Co. A small amount of uncombined C, or an addition of Cr, markedly inhibits grain growth during liquid phase sintering. On the other hand, ultrapure WC-Co powders can be consolidated by solid state sintering, where grain growth is much slower. Figure 7 shows 200 nm WC grain size in solid state slntered WC-10 wt% Co. Hardness Hardness values of WC-10 wt% Co with WC grain size 200 nm, 400 nm, and 800 nm, are respectively 1950, 1700 and 1550 Kg/mm 2. Figure 8 compares the hardness of 200 nm material prepared from chemically processed powder with Sandvik's fine and ultrafine grades of WC-Co. Clearly, the 200 nmmaterial offers an advantage because the high hardness is achieved in a high Co volume fraction alloy, where the ductility and fracture toughness is greater.
Summary A novel chemical processing method is described for making nanostructured WC-Co powders. Critical to the success of the process is the control of thermodynamics and kinetics of gas-solid reactions in a fluid bed reactor. Of particular importance is the precise control of carbon and oxygen activities in the fluidizing gas stream. The as-synthesized powders have a high surface area and each powder particle is composite in nature. In contrast to conventional WC-Co powders, the extremely good mixing of the ceramic and metal phases in the powder particles, and the interconnected nanoperosity enables these powders to be consolidated by solid state sintering at relatively low temperatures. A low sintering temperature and a short sintering time ensure retention of a nanostructure in the consolidated material. Preliminary evidence indicates that as the scale of the nanostructure is reduced, the mechanical properties, such as hardness, are enhanced, which is in keeping with the wellestablished trend in properties of conventional WC-Co material. References i. 2. 3.
4.
5.
6.
F.V. Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, Princeton, NJ (1980) 383. L.E. McCandlish, R.S. Pollzzotti, Control of Composition and Microstructure in the Co-W-C system Using Chemical Synthetic Techniques, Sol. State lonics, 32/33 (1989) 795. L.E. McCandlish, B.H. Kear, B.K. Kim, L.W. Wu, Metastable Nanocrystalline Carbides in Chemically Synthesized W-Co-C Ternary Alloys, in Multicomvonent Ultrafne Microstructures, L.E. McCandlish, D.E. Polk, R.W. Siegel, B.H. Kear, Ed., MRS Symposium Proceedings, Vol. 132 (1989) 67. L.E. McCandlish, B.H. Kear, B.K. Kim, L.W. Wu, Low Pressure Plasma-Sprayed Coatings of Nanophase WC-Co, in Protective Coatings: Processin~ and Characterization, R.M. Yacizi, Ed., The Metallurgical Society, Warrendale, PA (1990). L.E. McCandlish and B.H. Kear, Chemical Processing of Nanophase WC-Co Composite Powders, Proceedings of the Cottrell Conference on Advanced Materials, Special Issue of Mat. Sci. Tech., Institute of Metals, Ed. J.l. Harris, London, in press (1990). L.E. McCandlish, B.K. Kim and B.H. Kear, Chemical Synthesis of Nanophase Metal/Ceramlc Composites, in High Performance Comoosites for the 1990's, to be published by the Metallurgical Society (1991).
124
7.
8. 9. I0. II. 12.
LE McCANDLISH,BH KEARAND BK KIM
B.H. Kear, B.K. Kim, and L.E. McCandllsh, Chemically Processed Nanophase WC-Co Composites, in Frontiers of Chemlstrv: Materials by Desizn, American Chemical Society's Chemical Abstracts Service, columbus, Ohio (1990) 65. L.E. McCandlish and R.S. Polizzotti, Multiphase Composite Particle, US Pat. No. 4,851,041 (1990). L.E. McCandlish, B.H. Kear and J. Bhatia, Spray Conversion Process for the Production of Nanophase Composite Powders (U.S. Pat. App. S.N. 433,742). R.M. German, D. Lee, C. Chung and R.W. Messier, Near Net Shape Manufacturing, P.W. Lee and B.L. Furguson (eds.), ASM International, Metals Park, Ohio (1988) 187. T.D. Halllday, F.H. Hayes and F.R. Sale, The Industrial Use of Thermochemical Data, T.I. Barry (ed.), Chemical Society, London (1980) 291. P. Ettmayer, Hardmetals and Cermets, Annu. Rev. Mat. Sci. 1989, 19:145:64.