Applications

Presently, industrial application of FAST sintering methods is in the production of both hard and soft magnetic materials and cutting tools in Japan (Ref 43, 44). For instance, manganese-zinc ferrites for high-frequency power supplies are PAS sintered using 60 s pulse current cycle followed by a heating current of 2000 A under 49 MPa pressure (Ref 43). Densities of >99% and grain size of 1 /''m are obtained. This fine grain size minimizes the magnetic high-frequency core losses from 1800 kW/m3 for a conventionally sintered 9 /'m sample to only 720 kW/nr\ Commercial Nd-Fe-Co-B magnets are densified by PAS.

Laboratory-scale experiments have succeeded in field-activation sintering of a large variety of materials from metals to intermetallic compounds to ceramics and composites. Examples of metal powders that have been consolidated include pure metals (nickel, tungsten), intermetallics (TiAl, NiAl, Nb3Al, FeSi2), and composites (Ref 23, 27, 28, 41, 42, and 45). The field-sintered ceramics were oxide (Al2O3, superconductors) and nonoxide-type (AlN, Si3N4) (Ref 25, 37, 38, 39, and 46). The process parameters, density, grain size, and hardness values achieved in PAS-consolidated materials are shown in Tables 2 and 3.

Table 2 Field-activated sintering parameters and properties of metal powders

Material

Temperature, K

Holding time, min

Pressure, MPa Density, g/cm3(%)

Grain size, nm

Hardness

Nickel

1270

2

30

8.57 (96.2)

NA

228 HV

Tungsten

2163

6

61

17.4 (90)

-■ 1500

NA

Fe-2%C(a)

973

3

47

7.22

52

41 HRC

Fe-2%Al(a)

973

3

47

7.34

38

43 HRC

Nb3Al(a)(b)

1423

3

30

7.57 (-■ 100)

-■ 200

1073 HV

NiAl

1616

1.5

30

5.88 (99.5)

NA

38 HRC

TiAl(c)

1602

<3

39

(100)

>100

NA

Fe-85vol%Fe3C

723

3

63

(98)

45

1050 HV

WC-10%Co

1673

1

45

14.45

NA

1756 HV

Fe-Ni-5vol%TiC

1473

10

33

7.84

NA

249 HV

FeSi2(d)

1123

5

59

(83.4)

NA

566 HV

performed in air with pulse cycle time of 30 s, and on (off) durations of 30 ms. NA, not applicable.

Mechanically alloyed powders.

Nb3Al compound synthesis occurred simultaneously with powder densification. Pulse duration 100 ms. Pulsing cycle time = 99 s

All sintering tests were performed in air with pulse cycle time of 30 s, and on (off) durations of 30 ms. NA, not applicable.

Mechanically alloyed powders.

Nb3Al compound synthesis occurred simultaneously with powder densification. Pulse duration 100 ms. Pulsing cycle time = 99 s

Table 3 Field-activated sintering parameters and properties of ceramic powders

Material

Temperature, K

Holding time, min

Pressure, MPa

Density, %

Grain size, /-'m

AlN

2005

5

50

99.3

0.8

i t -Al2O3(a)

1573

<10

40

98.2

0.7

-Al2O3(a)

1673

6

66

>99.5

1

J -Si3N4-5%Y2O3-2%MgO(b)

1823-1873

7-8

49

98.1-98.5

0.3

WC

2073

2

45

100

NA

TiN

1273

3

66

93.7

NA

BCCSO

1073

8.3

14

100

NA

YBCO

1173

15

15

100

NA

All sintering tests were

Multiple pulsing applied. Sintered in a vacuum

Multiple pulsing applied. Sintered in a vacuum

To illustrate some advantages of field application, a comparison of conventional and FAST sintering of aluminum nitride powders should be considered. Aluminum nitride was field sintered to near-full density in 5 min at 2000 K without any additives (Ref 46). The resulting densities ranged between 3.18 and 3.24 g/cm3 (97.5 and 99.3%, respectively). For comparison, undoped aluminum nitride can be sintered to 95% at 2200 K for 30 h. When dopants are used, the densities achieved are 97 to 98% by conventional sintering at 2070 to 2220 K for 3 to 4 h.

Simultaneous synthesis and densification of MoSi2 compound from elemental powders has been achieved by FAST (Ref 47). A density of 99.2% was obtained by using a 30 s pulse cycle at 60 MPa pressure followed by heating to 1700 °C in vacuum. This density is higher than that reported for conventional hot pressing of MoSi2 powders (95 to 97%). The hardness values of field-densified MoSi2 compare favorably with conventional specimens (>9.4 GPa).

Functionally graded materials combining refractory ceramics on a metal substrate, such as ZrO2 on NiCrAlY and TiAl, have been consolidated to high densities using FAST methods (Ref 48, 49). A temperature gradient was achieved by either using punches of different materials (i.e., one graphite and one tungsten) or using a stepped die. The temperature gradient was >100 K in the former case and >700 K for the latter.

Bonding capabilities of FAST process should be similar to those in spark sintering. However, only one attempt was reported on diffusion bonding of cubic boron nitride on metal substrates (Ref 50). The good adhesion obtained suggests that field sintering can be used for ceramic-to-metal joining.

Net-shape parts may be produced by FAST methods. One such example is a diamond-shaped cutting tool insert that was fabricated by PAS using a special punch-and-die set (Ref 51). The diamond-shaped cavity of the mold was formed by divided pieces of graphite to prevent mold destruction during sintering (Fig. 9).

Fig. 9 Die-and-punch configurations for net-shape sintering of cutting tool insert. Courtesy of K. Yamazaki (U.C. Davis)

The rapid rate of densification, no need for preliminary powder preparation steps such as cold compaction, additive use and debinding, and air sintering are some densification characteristics that make the FAST process economically competitive. This is especially true for materials that are difficult to cold press; oxygen-sensitive, metastable materials; or for critical bonding and purity requirements. Final-net-shape and diffusion bonding of different materials with minimal microstructural changes are high-potential capabilities of FAST consolidation that are worth future explorations.

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