Process Mechanisms

Because loose powders are directly loaded into the sintering machines, the use of binders to achieve the strength of the green part becomes unnecessary. Consequently, the debinding step commonly used to burn out the volatiles is rendered superfluous. This is not only helpful in eliminating processing steps and increasing productivity, but part purity also may be enhanced.

The accelerated densification by electric field application may be partly explained by the ability to remove the oxides and impurities from the particle surfaces. In general, the remaining oxides on powder particles are known to cause consolidation difficulties and low fracture toughness of the sintered part. If direct boundary contacts between grains can be achieved, this permits full advantage to be taken of the intrinsic strength of the material. Oxide removal may be attributed to phenomena ranging from electric breakdown of the oxide layer to a possible plasma generation among powder particles.

As in any sintering process, field-activated densification starts with a highly porous body in which pores form an interconnected network. The initial pressure application proceeds with neck formation. Because the pressure is at a low level, most of these necks are only interparticle point contacts. At this stage, the pulsed current is applied, and the goal is to achieve a uniform current path rather than local channels that may concentrate all passing current. Usually, the initial 1 to 2 pulse applications ensure a uniform current path, particularly for high resistance powders. Next, the current is forced to choose the path of the least resistance, that is, through the contact points to complete the current path through powders. The powder particles have an inherent oxide layer on their surface. This way, small capacitors are formed across any two particle contact points. Electrical discharges are generated across these capacitor gaps. The interfering surface oxide films are "broken" when a certain voltage level is achieved. This level, or electrical breakdown voltage, is dependent on the dielectric strength of the oxide layer. The mechanisms of electrical failure that take place at the breakdown voltage involve arcing across adjacent points on particles and electrical breakdown of dielectric film on the powder particle surface.

Alternatively, the electrical discharges across the sample may generate a plasma, that is, an ionized gas among powder particles. It appears that plasma generation occurs only in some conditions that are not yet well defined. If plasma is generated, surface oxide removal and elimination of surface adsorbates take place. The resultant "clean" surfaces provide a better intergranular cohesion, thus enhancing the neck formation. Consequently, enhanced sinterability is observed upon further densification. The dc current applied in the second stage actually promotes diffusion bonding of these clean interparticle contacts.

Presently, the type of surface effects caused by electric field activation has not been unambiguously established. However, experimental evidence suggests that particle surfaces are "cleaned," and direct grain-to-grain contact is observed in the field-sintered specimens. For instance, a transmission electron microscopy (TEM) micrograph of aluminum nitride sintered under an electrical field application shows clean grain boundaries with no secondary or amorphous boundary layers (Fig. 3). A high-resolution electron microscopy image of a typical grain boundary in this aluminum nitride confirmed this direct grain-to-grain bonding (Ref 35). Furthermore, measurements of the oxygen content in electron energy loss spectra did not show any oxygen segregation at the grain boundary.

Fig. 3 Transmission electron micrograph of aluminum nitride sintered by PAS. Courtesy of T.A. Guiton, Dow Chemical Company. 69,000x

This result is in contrast to the conventional resistance-sintered specimens that do not involve an activation step. In this case, the higher resistance of the surface oxide films gives rise to higher temperatures than in the interior of the particles themselves and therefore causes melting at particle boundaries. This liquid-sintering mechanism explains the higher densities and shorter times for densification. When molten films develop, secondary phases are observed at grain boundaries of the sintered part (Ref 36).

Another indication of some special surface effects have been noticed in PAS densification of superconductors. Resistivity/temperature curves of Y-Ba-Cu-O (YBCO) superconductors consolidated by PAS showed new transition onsets from 240 to 278 K (Ref 37). Such transitions have not been observed in non-PAS consolidated superconductors. They can be explained by new surface phenomena (i.e., surface accumulation of electronic charge) that may be induced by a plasma generation.

After the particles have been connected, the second stage starts when a direct current is applied. Diffusional processes and plastic flow are the main contributors to the densification in this stage. The highest heating occurs at the thin necks formed by discharge. Diffusion is activated by the passage of the current. The highest temperatures achieved in the necks provide the highest diffusion rates and thus enhance matter transport toward the neck area. This is the area in which most of the matter transport is required for sintering. Therefore, field application intensifies the sintering rate. In addition, plastic flow of metal particles is also enhanced by the combination of applied pressure and high temperatures. The plastic flow may occur through the entire particle or, possibly, only localized plastic flow at the necks. In both cases, plastic flow significantly contributes to pore closure and therefore enhances overall densification rate. As a result, high density may be achieved and in shorter times or at lower temperature than in the absence of the applied current. The final result is a dense compact with good intergranular bonding.

Similar to metals, in ceramics electric charge is built up in the individual powders, which can be considered capacitors. A correlation between the dielectric properties of ceramics and the PAS sinterability has been established (Ref 38, 39). Ceramics with high dielectric constant and low dielectric strength sinter well under electrical field. When the dielectric constant of the ceramic powder is high, the powder surface can hold higher electric charges. This is the case for alumina and aluminum nitride. Conversely, no FAST sintering has been possible for low dielectric constant and high dielectric strength ceramics such as boron nitride and silicon nitride unless oxide additives are used. The favorable effect of a high dielectric constant on sinterability may be also noticed in sintering conditions that increase the dielectric strength of the material (Ref 39). For instance, the dielectric constant of alumina increases by heating. When pulses are applied at higher temperatures, alumina sinterability is enhanced. This has been achieved by multiple pulsing of powders (Fig. 4). The dielectric constant increases when alumina is heated up at increasingly higher temperatures before next pulse is applied.

Fig. 4 Effect of multiple pulsing on the densification of O-AI203 and 7-AI203 powders. The single pulse cycle was applied at 293 K. The pulse cycle current was 750 A, pulse duration 60 ms for a total of 60 s. The multiple pulse cycle involved 4 cycles, one each at 293, 473, 773, and 1273 K. The pulsing parameters were the same as single pulse cycle and kept constant for all the cycles. The numbers in bracket denote holding time at sintering temperature for each run. The initial pressure was 40 MPa and increased to 66 MPa in the sintering stage. Courtesy of R.S. Misra (U.C. Davis)

Fig. 4 Effect of multiple pulsing on the densification of O-AI203 and 7-AI203 powders. The single pulse cycle was applied at 293 K. The pulse cycle current was 750 A, pulse duration 60 ms for a total of 60 s. The multiple pulse cycle involved 4 cycles, one each at 293, 473, 773, and 1273 K. The pulsing parameters were the same as single pulse cycle and kept constant for all the cycles. The numbers in bracket denote holding time at sintering temperature for each run. The initial pressure was 40 MPa and increased to 66 MPa in the sintering stage. Courtesy of R.S. Misra (U.C. Davis)

Field sintering of alumina indicated a dependence on the pulse duration. Sintering is enhanced when longer pulse duration (30 to 160 ms) is used. The longer discharge times allow the ions in the ceramics more time for polarization and therefore more electric energy to be built up before electrical breakdown.

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