SPECIAL REPORT: MATERIALS HANDLING/POWDER PROCESSING: Optimizing Ceramic Powder Specification

December 1, 2009
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Dry powder pressing techniques are the most cost-effective, flexible choice for the manufacture of a wide range of ceramic components. Identifying powder blends and granulates that are best suited to such applications is an essential element of process development, but it brings its own challenges. One critical factor is to select an appropriate method for characterizing ceramic powders, the importance of which is illustrated here using experimental data from a study of die filling.

A first step toward the successful manufacture of a ceramic component using a powder pressing technique is to develop an appropriate feed material. As ceramic powders are often supplied as submicron or nano-sized particles, it is usual practice to include an initial granulation step. The ideal outcome from granulating a well-specified mix of ceramic powder, binders, lubricants and dispersants to a suitable endpoint will be a feed of consistent composition that delivers high levels of productivity and meets product quality targets. Material must flow easily from a hopper, fill a die adequately, and compress to form a stable green body that can be successfully ejected from the mold before sintering and final machining. A good match between the feed and the rest of the plant will result in a robust and stable process that maximizes throughput and quality.

Developing granulates with properties that are suitable for a defined application requires careful manipulation of a variety of parameters, including particle size and distribution, degree of agglomeration, composition (e.g., binder content), and particle shape and texture. It is common, for example, to select relatively fine granules for the production of ceramics that have very demanding specifications, such as those used for medical applications. Finer particles tend to form more homogeneous green bodies and ultimately deliver a final product with few defects.

One of the major challenges implicit in this development process is how to characterize granulates in ways that relate to in- process performance and finished product quality. Without appropriate measurement tools, the performance of a potential feed material can be assessed only through plant trials on either a lab- or pilot-scale-a costly and time-consuming option. Characterizing powder properties effectively can both accelerate and optimize the development cycle.

Characterizing Powders for Ceramic Processing

Although ceramic processors have a wide selection of powder characterization techniques from which to choose, the applications within the industry exert some quite specific (though not unique) demands. Quantifying flowability is essential because the material needs to flow freely, in a controlled way, out of a hopper into the feed shoe and from there to the die. Understanding the response of the powder to air is also critical.

Air exerts a major influence on powder flow properties and is especially important in die filling via gravity.1,2,3 Here, the air content of the powder in the feed shoe, while often uncontrolled, affects flowability during discharge. Powder entrains air as it leaves the shoe, which lubricates the filling process and simultaneously reduces bulk density. If this air is not easily released as the powder settles in the die, then bulk density remains low, compromising fill weight. De-aeration behavior is therefore also highly pertinent. Sensitive differentiation in all of these areas is a vital attribute of a truly useful characterization tool.

Many conventional powder measurement methods are limited by their lack of reproducibility and questionable relevance to the manufacturing process. For example, zirconia has excellent flow characteristics when tested using the Hall flowmeter, but can flow poorly during die-filling operations.1 Furthermore, many techniques record just a single parameter for a given powder, failing to capture the behavioral complexity that ceramic processors need to assess. Powder characterization technology has, however, developed significantly over the past decade. The maturation of dynamic measurement techniques and the emergence of universal powder testers that offer multi-faceted characterization are both worthy of note.

A universal powder tester* combines conventional shear and bulk testing with more modern dynamic methodologies. Dynamic measurements of the powder in motion not only quantify flowability in an intuitively relevant way, but also permit investigation of the impact of air. Samples can be measured in a compacted, conditioned, aerated or even fluidized state. Closely defined test protocols and sample conditioning give such instruments the excellent reproducibility that makes them highly differentiating. They are therefore valuable to those charged with developing ceramic granulates that process well.

One example of a universal powder tester is the FT4 from Freeman Technology.

The Importance of Conditioning

Powder samples arrive in a laboratory with a processing history. They may have caked or compacted in storage, for example, or become aerated during pneumatic conveying. Conditioning a sample prior to measurement largely eliminates this history, setting a baseline state for analysis.

Gentle agitation of the powder bed, using a closely defined procedure, breaks up weak agglomerates, releases air from an overly aerated sample, or introduces it into a compacted one, leaving a homogeneous, loosely packed bed. Pre-measurement conditioning greatly improves analytical reproducibility.

Figure 1. Schematic drawing for the die-filling process.

Investigating Die-Filling Performance

To compare and contrast the in-process behavior of various materials, conditioned samples of tungsten, aluminum and glass beads of two sizes were processed in a lab-scale die-filling rig. The rig mimics a commercial operation and consists of a stationary die and a motor unit that drives a powder-filled shoe across the die surface at a controlled velocity of between 50 and 300 mm/s (see Figure 1). The cylindrical die has a volume of 10 ml and diameter of 25 mm.

The mass transferred into the die during a single pass of the shoe was measured for each powder to determine filling ratio (the ratio of actual mass of powder in the die to a mass calculated from die volume and conditioned bulk density). The experiment was repeated three times for each material, using a newly conditioned sample of powder.

Figure 2. Die-filling ratio as a function of shoe speed for four different materials.

Filling ratios for all four materials are shown in Figure 2. The larger glass bead sample, GL, clearly processes most successfully in this unit, while tungsten gives the worst performance. High filling ratios are achievable with tungsten when shoe speed is low, but this would translate to a productivity restriction in an industrial setting.

Figures 3 and 4. The die-filling properties under different powder packing conditions of aluminum (top) and tungsten powder (bottom).

To broaden the study, the impact of the packing state of the material in the shoe was assessed for two materials: tungsten and aluminum (see Figures 3 and 4). This was achieved by switching from a conditioned feed to an aerated one, and, in a separate experiment, to one that was slightly compacted. The compacted feed was produced by tapping the sample 20 times, while a lightly aerated state was reached by passing air through the material at a velocity of 20 mm/s (aluminum) and 10 mm/s (tungsten) immediately before loading the shoe.

Compacted powders are less likely than conditioned or aerated materials to flow freely under gravity because of their closer particle packing and the loss of entrained air. However, powders vary in this respect: cohesive powders can be greatly affected, while coarse-grained materials often are not. Here, the aluminum flows more freely into the die when aerated, improving the filling ratio. In contrast, when the feed is slightly compacted, filling ratio falls. With tungsten, a more dramatic reduction in filling performance is observed with the compacted material, but results for the aerated and conditioned feed were similar. In some cases with the tapped material, the powder bed formed a stable bridge that prevented flow from the shoe, prohibiting any further filling.

Figure 5. The size and morphology of tested powders by SEM: (a) GL glass beads, (b) GS glass beads, (c) granular aluminum powder, and (d) tungsten powder.

Rationalizing Process Performance

The particle size and shape of the four materials were characterized by laser diffraction and scanning electron microscopy, respectively, to provide some general information about the powders (see Table 1 and Figure 5). Measuring an array of bulk, shear and dynamic properties for all four materials produces a database of values that permits rationalization of the observed processing performance (see Table 2†).

†All results in Table 2 were generated using the FT4 from Freeman Technology; see References 4 and 5 for additional details.

Table 1. General powder properties for four different materials.

The flow energy parameters of specific energy and basic flow energy, which reflect flow behavior under unconfined and forced conditions, respectively, indicate that tungsten is considerably less free-flowing than the GS glass beads. Other indicators of flowability include consolidation index, shear stress, cohesion, unconfined yield strength and compressibility. Generally speaking, lower values of all of these parameters are associated with powders that flow more readily.

Figure 6. Plot showing the relationship between filling ratio (at shoe speed of 230 mm/s) and specific energy.

The “flow” variables suggest that the flowability ranking of the four materials is (from the most free-flowing to the least): GS glass, GL glass, aluminum, tungsten. While this ordering broadly correlates with die-filling performance, there is an obvious anomaly in that the small glass beads exhibit lower die-filling ratios than the larger ones (see Figure 6). In this case, analyzing flowability only partly rationalizes process behavior, failing to accurately predict the relative behavior of the two different samples of glass beads. The response of the powders to air is the missing link.

Figure 7. Graph showing flow energy as a function of aeration velocity.

With a powder rheometer, aeration behavior is studied directly by measuring flow energy as a function of air velocity through the sample. Figure 7 shows how flow energy varies with air velocity (the raw data from aeration testing), while Figure 8 is a normalized, derivative plot illustrating the rate of change of flow energy as a function of air velocity. The latter is an especially effective method of presenting the data in order to clarify differences between the samples.

Figure 8. Normalized graph showing the rate of change of flow energy as a function of air velocity.

The results show that the GS glass beads aerate to the point of fluidization extremely quickly (total energy of ~ 0 mJ). Even at relatively low flows, air permeates evenly through the entire powder bulk, separating virtually all of the particles and causing the bed to fluidize. The flow energy of the tungsten also reduces rapidly with air flow, but, importantly, it never becomes fluidized. Instead, it stabilizes at a total energy of around 300 mJ. The strong inter-particulate bonding of the fine tungsten particles gives the bed a cohesiveness that prevents fluidization; the air channels through the sample rather than lubricating each particle. Aluminum and the GL glass beads both fluidize, but at significantly higher air velocities than the GS glass beads. The GL beads, in particular, require a significantly larger air flow to reach the fluidized state.

Permeability data suggest why this might be. Permeability, which is a measure of how easily a material can transmit a fluid (in this case, air) through its bulk, is influenced by many physical properties such as particle size and distribution, cohesiveness, particle stiffness, shape, surface texture and bulk density. It is important in many process environments and matters for this study because it determines the discharge rate of entrained air during die filling.6

Table 2. Dynamic, shear and bulk properties of four different samples.

The large spherical GL beads give rise to a bed with significant voidage and relatively high permeability (low pressure drop, see Table 2), from which air is easily released. The relatively large, irregularly shaped aluminum granules produce a similar effect. In contrast, the GS beads are around six times less permeable than the larger bead and are therefore much more likely to retain air.

According to these data, the GL beads are more difficult to fluidize than the GS beads for two reasons. First, they have a higher mass-to-surface area ratio, which means that the upward force needed to support them, a pre-requisite of fluidization, is greater. Second, the fact that the GL bed is highly permeable means that it provides little resistance to air flow and easily releases any entrained air, so fluidization is more difficult.

In combination with specific energy measurements, aeration and permeability results rationalize the superior die filling ratios achieved with the GL bead sample. Good flowability is confirmed by a low value of specific energy, describing its excellent filling potential, and a high aeration ratio, quantifying low cohesive strength. Once in the die, it rapidly releases air as quantified by its high permeability.

In contrast, although it is more free-flowing (lower SE, higher AR), the GS sample holds on to air in the die, reducing bulk density and consequently the mass fill. This behavior was confirmed visually, though not quantified, by observing bed collapse rates for all materials.

With its high aeration ratio, the aluminum sample shows it has minimal cohesive strength, and, with its relatively high permeability, it is clear it would de-aerate efficiently in the die. However, due in part to its irregular particle morphology and lower bulk density, its filling performance is not as good as either of the glass beads, as shown by a higher specific energy. It is likely that a modification of the particle shape to a more spherical morphology would transform this material into one that has the highest filling ratio.

These findings suggest that optimal die filling performance requires a powder that is free-flowing with relatively high permeability. An ideal powder aerates enough to flow easily yet de-aerates sufficiently quickly to ensure complete die filling. Quantifying these characteristics provides a specification for feed materials that will perform well in this piece of equipment.

In Conclusion

Powder characterization that is both reliable and relevant to the process makes it easier to develop a blend or granulate suitable for ceramic powder processing applications. The study described here highlights correlations between dynamic, bulk and shear properties, and performance in die-filling operations.

Experimental data demonstrate that if powder flowability, aeration/de-aeration characteristics and permeability are quantified reliably, it is possible to predict die-filling performance. Modern universal powder testers sensitively and precisely determine these essential characteristics of powder behavior, supporting and accelerating development of the best possible feed materials.

For more information regarding powder characterization, contact Freeman Technology Ltd., Boulters Farm Centre, Castlemorton Common, Welland, Worcestershire, England, WR13 6LE; call (44) 01684-310860; fax (44) 01684-310236; e-mail tim.freeman@freemantech.co.uk; or visit www.freemantech.co.uk.

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