Dean-Mo Liu

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

Microstructure and high-temperature strength of pressureless-sintered silicon carbide

Silicon carbide (SIC) cermic has been recognized as a prime candidate material for structural applications such as heat engine components, heat exchangers etc., due to its good strength, excellent resistance to corrosion and oxidation, and excellent thermomechanical as well as thermophysical properties. The densification of SiC via pressureless sintering has been an important subject for years, and the first work was pioneered by Prochazka [1] who obtained a dense SiC with the addition of boron and carbon as sintering aids by means of a liquid-phase sintering mechanism. In spite of sufficient flexural strength, SiC ceramics in the SiC + B + C system showed no improvement in the resistance to fracture [2, 3]. A number of investigations indicate the importance of sintering aids, which play a crucial role in changing the mechanical as welt as high-temperature properties of the SiC ceramic [3-6]. Omori and Takei [7], who used alumina and yttria as sintering aids, obtained a dense SiC ceramic with an improved mechanical strength of greater than 6 5 0 MPa. More recently, Lee et al. [3] obtained a dense SiC ceramic with a fracture toughness as high as -8 .3 MPam 1/2, by allowing exaggerated growth of the SiC grains at 2000 °C to form a microstructure containing considerable amounts of plate-like a-SiC grains. Unfommately, the fracture strength of their SiC ceramics decreased to -450 MPa. A recent investigation by the present authors has demonstrated that dense SiC ceramics with a four-point flexural strength above 600 MPa and a fracture toughness as good as 6 . 0 M P a m 1/2 are achievable through the control of microstructure evolution in monolithic SiC ceramic, by starting with a mixture of ozand /3-SIC powders via a two-step pressureless sintering procedure [8]. The requirement of high-temperature strength is a prime consideration in the development of SiC ceramics, since one of their principal uses is for heat-engine applications where the high-tempera~tre properties are critically important. Although studies on the high-temperature strength of SiC ceramics are extensive, most of the investigations involving sintering aids were focused on boron and carbon systems. For employing A1203 and 57203 as sintering aids, the available data on the strength at elevated temperatures are not extensive. In a number of patent references [6,9] the data for high-temperature strength of SiC ceramics, in spite of their high room-temperature strength, showed a considerable reduction in strength by over 25% at 1200 °C.

Porosity development in ceramic injection mouldings via different burnout strategies

Abstracts are not published in this journal