Plastic clay-like flow stress of saturated advanced ceramic powder compacts

Abstract

The flow stresses of saturated, consolidated alumina powder compacts and a commercial throwing clay were measured and compared. The flow stresses of certain saturated alumina powder compacts formed by consolidating salt coagulated slurries were found to be almost identical to the flow stress of the commercial throwing clay. The effect of volume fraction of particles, particle size, interparticle attraction, and consolidation pressure on the flow stress were systematically investigated. Plastic behavior and flow stresses, controlled by changing the forces between the particles, may be useful in developing low cost plastic forming processes for the production of advanced ceramic components.

Introduction

Colloidal processing of advanced ceramic powders is important in removing strength limiting flaws from high performance ceramic components, by passing the powder, formulated as a dispersed slurry through a filter prior to further processing.1, 2, 3 Since the diamond machining of ceramic components is expensive, it is economically advantageous to produce objects that do not require machining subsequent to firing. Forming techniques that produce green bodies that are very close to the desired final shape (and shrink uniformly during firing) are referred to as near-net-shape processes. These processes only require a minimal machining. Plastic forming processes that are compatible with the removal of flaws by filtration will allow the economical production of near-net-shape ceramic components of high strength and reliability. Traditional clay forming processes are well developed and inexpensive. If the plastic behavior of wet advanced ceramic powder compacts can be controlled to match that of clay, inexpensive plastic forming methods can to be used to produce high strength complex shaped ceramic components. Such a process has the potential to economically produce reliable, high performance ceramics.

Although the plastic properties of clay have been the subject of much research, there is still no general agreement to the mechanism responsible for plasticity.4, 5, 6 Interpenetrating strong and weak particle networks, hydration repulsion and relative densities far from the maximum packing density probably contribute to this phenomena. On the other hand, as reported below, the development of plasticity for advanced powders such as alumina, zirconia and silicon nitride, has stemmed from the basic understanding relating interparticle pair potentials to the rheological behavior of particle networks. This basic understanding has directly lead to clay-like rheology and new near-net-shape forming processes.

The production of a plastic powder compact that can be used for net-shape forming requires two processing steps. The first step is formulating a slurry (powder+liquid mixture). The slurry must first be formulated such that particles are highly repulsive to allow the strength degrading inclusions to be removed by filtration.¹ As detailed below, after the flaws are removed from the powder, the slurry needs to be reformulated to cause the particle to be weakly attractive. Weakly attractive particle networks are needed to develop plastic behavior. Repulsive and attractive particle networks produce a wide variety of rheological properties that are controlled by changing the surface chemistry of the particles through changes that include pH, salt and polymer concentration.3, 7, 8 Other parameters such as particle volume fraction, size and shape can also affect the rheology of slurries. It will be shown that the preferred interparticle pair potential required for a shape forming operation is one in which the particles form a weakly attractive network.3, 8 The weakly attractive network is created when the slurry is formulated to produce a short range repulsive potential. When combined with the pervasive van der Waals attractive potential, the short-range repulsive potential prevents attractive particles from coming into contact. Instead, the particle ‘sit’ in a potential well separated by an equilibrium distance. Particles in the weakly attractive network must be pulled apart to induce flow. The force needed to pull the particle apart and the number of particle per unit volume will control the flow stress.

The second processing step to form a plastic powder compact is called consolidation, namely, a process that increases the volume fraction of powder. Consolidation is generally performed by forcing the liquid within the slurry through a filter while retaining the particles on the filter in the form of a ‘cake’ that is saturated with the liquid. This method of consolidation is generally known as either pressure filtration or de-watering. During consolidation, the particle network supports the applied pressure. Particles that are separated by either a short- or a long-range repulsive potential can be pushed into touching contact during consolidation.⁹ The applied pressure is not equally shared by all particles, thus, a fraction of the particles within the network are the first to be pushed into touching contact during consolidation. After consolidation, the particles that have been pushed into contact form a much stronger network that interpenetrates a much weaker particle network that existed within the slurry. The much stronger particle network can act as a skeleton that supports the much weaker network. Higher consolidation pressures cause a greater fraction of particle to be pushed into contact. The strong particle network causes the consolidated body to initially exhibit an elastic stress/strain behaviour during mechanical testing. The elastic regime occurs until a peak stress is observed where the strong network is broken apart.⁹ The peak stress increases with the consolidation pressure, i.e. increases with the fraction of particles that are forced into contact during consolidation. For bodies consolidated below a critical pressure, further deformation causes flow at a flow stress. As detailed below, the flow stress depends on the nature of the interparticle pair potential that existed in the slurry, the particle size, and the volume fraction of powder within the saturated, powder compact. When the body is loaded a second time no peak stress is observed since the strong network is already broken apart. Bodies consolidated above a critical pressure are elastic and fracture before they flow, i.e. their yield stress exceeds their flow stress. In, the stress/stain data is generally reported for the second time the body is loaded. These data will exclude the peak stress because it is not observed once the strong network is broken apart during the first loading period.

This paper systematically explores the parameters which control the flow stress of plastic, saturated powder compacts. By way of reviewing the recent literature, a complete picture is presented of how each parameter effects the flow stress. These parameters include the volume fraction of the particles, the average size of the particles, the magnitude of attraction between the particles, and the consolidation pressure. It will be seen that saturated, consolidated powder compacts formed from advanced ceramic powders can be produced with a flow stress that is identical to that of a commercial throwing clay.