Densification of zirconia from submicron-sized to nano-sized powder particles

Densi®cation of green powder compacts is primarily a process of eliminating voids in the compacts through the use of elevated temperature. Mass diffusion is thermally activated between contacting particles as the temperature rises to some critical level. On heating, shrinkage and grain growth may concurrently take place to different extents depending on a number of factors such as the sintering temperature, sintered density and initial particle packing con®guration or ef®ciency. Exaggerated grain growth is in general most undesirable primarily because it decreases end-point density and degrades the ®nal properties of the products. This is particularly pronounced in the densi®cation of some nanocrystalline ceramic materials [1], e.g., Al2O3 and ZrO2. Therefore, the control of grain growth, in most cases, is one of the most important subjects on densi®cation. To overcome such undesirable grain growth behaviour, a number of techniques have been developed such as pressure-assisted or ®eld-assisted sintering [2, 3] and solute doping [4]. Most of these have attained satisfactory results. Unfortunately, these techniques have their own respective limitations such as high manufacturing costs, geometrical restriction of parts and enhanced high-temperature due to undesired grain-boundary segregation. Recently, the advancement of powder processing technology makes ceramic products with a high endpoint density and (ultra)®ne microstructure highly feasible. Before further discussion can proceed, one must keep in mind that a starting (ultra)®ne and pure powder particle is principally responsible for the resulting (ultra)®ne microstructure. Therefore, the use of (ultra)®ne pure ceramic powders has become one of the increasingly important considerations in the fabrication of advanced ceramics. Furthermore, the smaller the powder particle size used, the lower is the temperature required to achieve full density and this may ensure to some extent a ®ner microstructure. Recently, many studies have experimentally found that a full density ceramic part can be achieved at a sintering temperature lower by several hundred degrees Celsius for nano-sized powders (from a few nanometres to several tens of nanometres) than for coarse powders [5±7]. This opens up the possibility of making ceramics with ®ne or ultra®ne microstructure (termed nanostructure with a grain size generally well below 100 nm) at a relatively low temperature. In practice, low-temperature pressureless sintering is always the most desirable route to fabricate ceramic parts for industrial manufacturers and is also an important subject of considerable interest for ceramists. This methodology may in some aspects ensure an economic way to produce ceramics with a unique microstructure for technological applications. Low-temperature sintering may usually effectively reduce the tendency of grain growth to some signi®cant extent and greatly retains the ®nal microstructure with a scale close to that initially controlled in the green state as if additional doping is incorporated [6]. Recently, ceramic materials with such nanostructure have been reported to be greatly attractive because of the uniqueness of the resulting microstructure-sensitive properties [8]. However, in view of the literature, systematic investigations on the determination of the possibly attainable lowest temperature, Tmin, that can be utilized to densify fully a given ceramic powder compact consisting of particle sizes either on a submicrometre scale or on a nanometre scale are rarely found. It is believed that such classi®cation because of the difference in particle size may re ect a difference in Tmin determination and this is the focus of this study to be addressed. The resulting sintered microstructure will not be the focus to be discussed in this letter. The in uence of particle size on sintering has been well documented; a theoretical consideration derived by Hansen et al. [3] for the densi®cation rate, =d t, for all stages of isothermal sintering is given as