The use of solid solution additives has been shown to be very effective for the formation, by conventional sintering, of ceramic materials with high density and with controlled grain size. However, the number of systems for which such additives have been successfully found remains quite small, and the role of the additives is fairly well understood in only two or three of these. This paper describes the initial part of a systematic study into the effects of solid solution additives on the sintering of ceramics. Cerium oxide was chosen as a model host powder for this work because it has appreciable solubility for many additives. A combination of kinetic data and microstructural observations indicate that the sintering and grain growth are influenced significantly by the additive size but less significantly by the additive charge. The density versus grain size relationship is almost independent of the additive below relative densities of = 0.90 but depends strongly on the additive above this density. The data are interpreted in terms of the effect of the additives on the densification to coarsening ratio.

Calcium compounds and yttrium compounds are frequently used as additives to densify AlN ceramics[l,2]. They react with oxygen impurities, such as A1203 in the raw AlN powder and in aluminate forms. A phase diagram is helpful in deciding the amount of additives to add, but the AlN sintering process doesn't come to equilibrium. We investigated the differences between sintering AlN with CaCO3 and with Y2O3.

With CaCO3 added, the calcium-aluminate liquid phase evaporates from the sintered AlN body at high sintering temperatures. The AlN thermal conductivity doesn't increase above a sintering temperature of 1700°C because residual oxygen substitutes into the AlN grains. With Y2O3 added, AlN sinters below 1850°C, as predicted by a phase diagram. With a large amount of Y2O3 added, 5 wt%, thermal conductivity doesn't depend on residual oxygen. This is because yttrium doesn't evaporate from the sintered AlN body and residual oxygen substitutes into AlN grains at an early stage of the AlN sintering.

Commercially available silicon nitride powders were coated with various metal oxides with the help of metal-organic surfactants in an aqueous medium. A surfactants could also act as a dispersant. The coatings were studied by XPS, TEM and accoustophoresis and the mechanism for formation of these coatings is proposed. The amount of sintering aid incorporated was determined by the surface area and surface chemistry of the nitride powder, the pH of the solution, the structure of the surfactant and the surface charge behavior of the powder and the coating. Multicomponent coatings were also possible by precise control of these parameters. The sintering behavior of the coated powders and the properties of the sintered material were promising.

Small-angle scattering techniques have been used in a number of studies to characterize pore evolution in ceramic compacts. Parameters characterizing the pore distribution, such as total pore surface area, pore size, and pore density, have been measured through both intermediate and final stage sintering. A review of these results indicates that pore sizes were generally found to remain constant during intermediate-stage sintering; supporting a topological decay model of sintering. Pore sizes generally increased and the size distribution broadened during final-stage sintering. The scattering results also suggest the presence of a unique pore surface area versus density curve.

Lithium niobate prepared by sol-gel techniques can crystallize at temperatures as low as 200°C, and achieve a high homogeneity. In this sutdy, nucleation, crystallization, and morphological development are examined by TEM for alkoxide derived LiNbO3. Preliminary studies show that the as-dried gel is completely amorphous and homogeneous. Between 150-200°C, crystallization begins with crystallites on the order of 10-20 nm appearing in the amorphous matrix. By 450°C, single crystal spheroidal particles (0.05 - 0.2 gm) form. By 600°C, facetted large crystallites on the order of 1 gtm develop. Grain size and porosity are strongly dependent on the amount of water of hydrolysis in thin films.

The sintering rate of a ceramic powder matrix can be reduced significantly by the presence of rigid inclusions. The factors that lead to this reduced sinterability have been the subject of much debate in the last 5–10 years. Recent work indicate that the reduction in the matrix sinterability is a processing-related problem. The main factors that control the sinterability of ceramic particulate composites include the packing of the matrix immediately surrounding the inclusions and interactions between the inclusions which constrain the matrix. A processing method which avoids the difficulties associated with these two factors is outlined. The method involves the synthesis of coated inclusion particles. Composites containing up to 40 volume percent inclusions can be freely sintered in the solid state to full density.

The sintering of glass powder packings with metal inclusions has been studied using (a) a loading dilatometer, and (b) an in-situ SEM hot-stage. The addition of a small volume fraction of metal particles unexpectedly increases the sintering rate of the composite. With a further increase in the volume fraction of inclusions the sintering rate decreases rapidly to zero at a well-defined threshold. The in-situ sintering experiments have been used to observe the sintering mechanisms and to determine the parameters that contribute to this phenomenon: to assess the distribution of inclusions, and the nature of inclusion-inclusion and matrix-inclusion interaction. Predictions of the effective sintering rate are computed using a truss model with randomly distributed inclusions. The computations capture accurately the decrease in sintering rate (beyond the initial increase) with increasing volume fraction of inclusions. The initial increase is attributed to a reduction in viscosity of the glass due to chemical interaction with the metal.

This study focuses on (1) the co-dispersion behaviors of dissimilar phases exhibiting different surface chemistry in ceramic suspensions and (2) the effect of local whisker distribution on matrix densification. Si3N4 whiskers and Al2O3 powders are mixed and slipcast in aqueous solution. Local whisker clusters are observed and remain stable even after prolonged sintering. High shear-rate agitation of the composite suspension does not eliminate the whisker agglomeration. Surface pre-treatment of whiskers, such as acid/base-washing, effectively reduces the clustering and sintered flaw size. From SEM observation it is found that the matrix powder densifies highly inhomogeneously because of the constraint imposed by the reinforcement. Matrix powder is inhibited from diffusing into “inter-whisker” voids.

One approach for improving mechanical properties of ceramics is to add reinforcing whiskers; however, addition of whiskers often inhibits densification. Formation of SiCcordierite composites by pressureless sintering was studied. The cordierite composites densified by viscous sintering; after densification the amorphous matrix then crystallized. It was shown that high green density composites obtained by using small particle size powder gave the highest sintered density composites. Reducing the aspect ratio of the SiC phase also improved sintered composite densities. Densities greater than 96% were obtained using small particle size cordierite powder formed by a sol-gel process and either classified SiC whiskers or SiC powder at SiC loadings of 20 vol%

Significantly improved reaction kinetics and properties have been realized for reaction bonded Si3N4 (RBSN) by using Si powders having unusual physical and chemical characteristics combined with clean, oxygen-free processing conditions. For instance, complete nitridation can be achieved in < 1 h at 1150°C or < 100 m at 1250°C without pre-nucleation, or < 1 h at 1100°C with pre-nucleation. These compare with normal schedules of ∼100 h at 1470-1500°C. Resulting strengths (up to 870 MPa) and oxidation resistances of the RBSN essentially equal those of fully dense Si3N4 even though RBSN includes 25% open porosity. The results show that every aspect of the process, ranging from powder synthesis to < ppm levels of oxygen in the nitriding atmosphere, have demonstrable effects on the reaction kinetics and the resulting properties of the parts. With careful processing, reaction formed ceramics can compete advantageously with fully dense counterparts.

Self-propagating high temperature synthesis (SHS) can be used to prepare near net refractory shapes which are difficult to produce by other methods. In SHS, green compacts capable of strong exothermal reaction are ignited at one end and transformed into the desired products by a self-propagating reaction. Physical processes involved in SHS include chemical kinetics, macroscopic transport phenomena and phase transformations. This paper describes a simplified mathematical model of the SHS process derived by combining the heat equation with the chemical reaction rate equation. The model equations are solved numerically to obtain representation of the SHS process. Parametric studies have been performed to investigate the relative importance of the physical parameters in the model.

The synthesis of Al2O3/ZrO2/ SiC composite powders by aluminothermic reduction of zircon in the presence of carbon is reported. The effect of the temperature on the progress of the reaction, as assessed by XRD analysis, is outlined. The morphological characteristics of the reaction products are described. Based on the experimental results, a reaction scheme is proposed. The possible propagation of the reaction is discussed in terms of the adiabatic temperature.

Solid state displacement reactions are being studied at Pacific Northwest Laboratory (PNL) as a method of synthesizing intermetallic and ceramic composites in situ. A high-strength in situ composite of MoSi2/SiC was synthesized using a solid state displacement reaction between Mo2C and Si. Diffusion couples between Mo2C and Si processed at 1200°C revealed the formation of aligned SiC platelets in a MoSi2 matrix. In situ composites were also synthesized by blending Mo2C and Si powders and vacuum hot-pressing the powders at 1350°C for 2 h followed by 1 h at 1700°C. The resulting microstructure consisted of 1-μm diameter SiC particles (30 vol%) uniformly dispersed in a fine grained MoSi2 matrix. Densities of 5.53 g/cm3 were obtained along with a microhardness of 14.2 GPa. Bend bars and chevron-notched bars revealed a strength of 475 MPa and a fracture toughness of 6.7 MPa√m at room temperature. Bend strengths increased to 515 MPa at 1000°C and then decreased to 112 MPa at 1200°C. Measured fracture toughness increased to 10.5 MPa√m at 1050°C.

The combustion synthesis of the B4C-2Al203-xAl ceramic-metal composite system has been studied using the propagating mode of ignition for the reaction 2B203 + C + (4 + x)Al = B4C + 2Al203 + xAl in which x was varied between 0 and 3. The volatile constituent(s), eg. B2O3, resulted in the formation of an expanded or foamed ceramic with a uniform porous structure. The porosity represents ≥65%, and an expansion of 300-500% occurred mainly in the vertical direction. The effect of green density and reaction stoichiometry on product density, pore morphology, propagation rate and overall activation energy is discussed.

The role of biomimetic approaches in materials engineering is reviewed. In the case of ceramics, close parallels in structure and function can be seen between synthetic and biological materials. Biological ceramics have resolved difficulties which are limiting the application of synthetic materials, particularly in the need for greater toughness. The enhanced toughness of materials such as shell and tooth enamel can be attributed to microstructural factors. Both natural materials contain a small amount of polymer. If this is essential, then any synthetic mimetic material would be limited by the temperature resistance of the polymer. In fact, much of the toughness is dependent on the elongated particle morphology and does not require polymer. Routes to forming ceramics with such elongated particles are discussed.

New generations of polymer-ceramic nanocomposites are being developed by intercalation of layered solids. Synthetic approaches include intercalative polymerization, direct polymer intercalation and molecular dispersion of the host layers in the polymer matrix. The unique physical and mechanical properties of the hybrids are attributed to the molecular dispersion and the synergism between the individual components.

Ion exchange was carried out on a lithium-doped fluorophlogopite machinable glassceramic with composition, in wt%, 52.2 SiO2, 14.0 Al2O3, 14.2 MgO, 12.5 MgF2, 1.4 Li2O, and 5.7 Na2O, to study the strengthening effect. The crystalline phases after annealings were Na-fluorophlogopite and betaspodumene, with a volume fraction of about 2:1. Ion exchange of Li by K in the glassceramic at 770° C for 4 h improved 2.1 to 2.3 times the three point bending strength (TPBS). Hence strong machinable ceramic with TPBS of greater than 300 MPa can be made. Ion exchange increases 100% electrical resistivity and improves 21% chemical stability, due to a surface layer of about 100 micrometer enriched with K.

To date, processes used to synthesize to silicon nitride and silicon carbide ceramics from polysilazane preceramic polymers have focused on techniques which seek fully dense monoliths or matrices. An alternative approach, phase separation of polysilazane solutions using appropriate gelation agents, yields infusible preceramic monoliths having controlled two-phase microstructures. Subsequent removal of the solvent phase by supercritical fluid exchange leads to aerogels with defined pore structures. Controlled pore structures were explored as a means of providing diffusional pathways for reactant and product gases during pyrolysis. Resulting changes in the gel microstructure were followed by scanning electron microscopy and surface analysis by nitrogen desorption methods. It was found that micropores present in the unpyrolyzed gels are absent after pyrolysis, but that larger pore structures are maintained.

The logical development of thermally stable polymers is based upon forming true molecular composites comprised of both polymer and ceramic components. This paper describes our initial studies on the synthesis of composite materials that are expected to produce films with thermal stability to 500°C.