The development of a synthetic bone substitute able to mimic both the chemical composition and morphology of bone's inorganic component is a priority for biomaterials research. Hydroxyapatite is a promising material for this application. The role of nanostructured apatites in the design and preparation of biomimetic and bioactive scaffolds is particularly crucial. These new materials characterised by nanodomain and modified chemical features show improved performance in terms of interaction with natural tissue, allowing and taking part in bone ingrowth. Thus interest has shifted from biocompatible to biomimetic and bioactive materials. The importance of non-stoichiometric/defective apatites similar to the mineral part of bone, in particular those containing HPO42-, CO32- and Mg2+, is explained. Attention is focused on innovative synthesis techniques that can yield powders with higher reactivity to build synthetic implants with a porous structure resembling that of spongy bone that can ensure complete penetration and consequent replacement of the bony tissue. The use of cellulosic sponges soaked with powder suspensions can give adequate porosity in terms of both size and distribution, and this appears to be a suitable method for the repair of both loadbearing and non-loadbearing osseous defects. Such sponges can also function as in situ drug delivery systems if they are charged with pharmacologically active compounds. So far the characteristics of artificial bone tissues have been shown to be very different from those of natural bone, mainly because of the absence of the self-organising interaction between apatites and the proteinic component that modifies the intrinsic features of each constituent. For that reason attention is currently focused on so called biologically inspired materials, in this case composites synthesised to exploit the ability of biological systems to store and process information at the molecular level. Following this new approach, nanosize bladelike crystals of hydroxyapatite have been nucleated in situ on self-assembling collagen fibres. The characteristics of the resulting composites are described.

This chapter contains sections titled: Introduction Experimental Procedures Results and Discussion

The effects of: - different Si3N4 powders, - different powder processing routes (mechanical-, ultrasonic-, attritor mixing of additive powders and chemical coprecipitation of metal hydroxides), - different sintering aid systems (La2O3+Y2O3, Al2O3+Y2O3, AlN+Y2O3) on microstructure and properties of hot pressed silicon nitride were evaluated and compared. All the properties: strength, hardness, toughness, Young's modulus, thermal expansion were related to the microstructural features (grain size and morphology, grain boundary phases, flaws) which are specific to a material and its process history.

The effects of different powder processing routes for the addition of sintering aids (La2O3+Y2O3 and Al2O3+Y2O3) in silicon nitride mixtures on sintering behaviour, microstructure and mechanical properties of hot pressed dense samples were evaluated and compared The investigated powder processing methods were the following: mechanical-, ultrasonic-, attritor-mixing of additive powder and chemical methods: coprecipitation of hydroxides, microencapsulation process, pyrolysis from tartrates. Improvements in performance were found to strongly depend on the powder processing as well as on the additive system Materials with a very low strength degradation (12-20%) from R.T. to 1400 degrees C were produced, with values ranging from similar to 850-1100 MPa at R.T. to similar to 550-800 MPa at 1400 degrees C. All the properties: strength, hardness, toughness are related to microstructural features: grain size and morphology, grain boundary phases, crack propagation and flaws, which are specific to a material and its process history.

BSCCO (2223) phase superconducting powder was synthesized by organic route: pyrolysis of citrates was used as alternative method to the solid state reaction. Powder with the nominal composition Bi1.84Pb0.35Sr1.9Ca2.1CU3OX was synthesized in very short time: 24h including calcination and firing at 858 degrees C with various intermediate grinding steps. Reactions among components were investigated by thermogravimetric and differential thermal analysis. The identification of the reaction intermediates which play a crucial role in the synthetic process was systematically performed. Hot pressing was carried out in order to prepare highly dense superconducting materials and the densification behaviour of the powders extensively studied

Two commercial Si3N4 powders and several sintering did systems (Y2O3-La2O3; AlN-Y2O3; Al2O3-Y2O3) were chosen to evaluate the influence of: powder characteristics, powder processing method (mechanical or chemical mixing) and additive system on the sintering behaviour, microstructure and mechanical properties of dense materials.

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics were sintered by the hot-pressing technique. The densification mechanism was investigated at 800°C and 850°C and a densification phenomenology has been proposed. Samples hot pressed at 850°C showed high density and T c ≈110 K.

The oxidation resistance of Si3N4-Al2O3-Y2O3 materials is strongly dependent on the secondary phases and processing conditions, as evidenced from the behaviour of two materials of the same starting composition but sintered in different conditions: I) when only Y-silicates are present as secondary phases (hot pressed material, 1710 C, 30 MPa, 1 hour) no modifications are detected at T < ~1100° C and the oxidation resistance is good up to~1400°C; parabolic oxidation kinetics and structure and composition of the oxide suggest the diffusion of Y and Al cations from the bulk to the reaction interface as the rate-governing step; II) when also Y-based oxynitrides form as a consequence of high temperature processing (gas-pressure sintered material, T = 1950°C, 1.5 MPa, 1.5 hours), the thermal instability at low temperatures (800–1000°C) of these phases results in linear kinetics, and at T > 1100°C parabolic kinetics are observed, but the oxidation rate constants are higher in comparison with (I).

The compounds under investigation were prepared with nominal composition MBa2 Cu3 O7-x, M being Y or Sm, through solid state reaction starting from Y2O3, Sm2O3, BaCO3 and CuO powders. Reactions were investigated by thermo-gravimetric and differential thermal analysis in the temperature range 900–980°C. Reaction kinetics of MBa2Cu3O7-x solid state synthesis were evaluated according to the mathematical relationships theoretically derived for phenomenological models. In the Y-based system the phase boundary model provides a reaction kinetics suitable to describe the process, while a three dimensional diffusion model fits the data in the Sm-based system. The influence of the mixing method on the reaction kinetics was evaluated. Microstructural features of superconducting samples were analyzed.

Oxidation of dense Si3N4-Al2O3-ZrO2 and Si3N4-Al2O3 compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<∼1623 K (ΔH= ∼100kJ mol−1)), whereas at T> ∼1623 K (ΔH = ∼800kJ mol−1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si3N4-Al2O3-ZrO2 composites compared to other ZrO2-Si3N4 materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.

The microhardness A Si3N4-based materials may be related to their phase and chemical compositions and to microstructural parameters such as porosity, grain size and secondary phases. By reducing the amount of intergranular phase, the microhardness may reach values up toHV500 = 3000 kg mm−2. Moreover, a comparison among different materials must include the microhardness tests in a wide range of values of the applied load. The relationd n =B1 +B2d2, obtained from those of Meyer and Kick, gives two constants:B1 may represent one class of materials andB2 is specific for each single material.

The behavior of hot-pressed Si{sub 3}N{sub 4} tools during the machining of cast iron was tested. Hot-pressed materials having different types and amounts of intergranular phase and different microstructures and levels of microhardness were considered. The performance of Si{sub 3}N{sub 4}-based materials depends on the characteristics of the intergranular phase, which condition the chemical resistance of the tool under attack of the chemical elements of the cast iron, and on the microhardness. By using alumina as a sintering aid and zirconia as the toughening phase, hot-pressed materials with different amounts of {beta}{prime}-sialon were obtained, which, through the optimization of the microstructure and phase composition, exhibit very promising behavior as cutting tools.

Development of microstructure, thermal expansion behaviour and room temperature MOR, Vickers microhardness and toughness have been studied for 83wt%Si3N4-5wt%Al2O3-12wt%ZrO2 composites hot pressed at 34.4MPa for various times and temperatures. The deleterious presence of Zr-O-N phases is avoided in these materials. Microcracking has been observed because of the mismatch between thermal expansion coefficients. A phenomenological model is suggested to account for the variation of cell parameters of the β'-phase with the progress of the α→β' conversion. No detectable improvement in hardness and toughness results from the presence of zirconia. The substantial MOR increase, from ~370 MPa for a Si3N4-Al2O3 reference material to~630 MPa for the ZrO2 composite has been ascribed more properly to a grain boundary phase effect, than to specific ZrO2-induced microcrack strengthening.

Heat treatments of hot-pressed Si3N4MgOY2O3ZrO2 composites in an oxidizing atmosphere result in serious material instability at ∼ 800 < T < ∼ 1200 K where an oxygen controlled auto-catalytic oxidation mechanism possibly associated with minor amounts of zirconium oxynitride secondary phases is suggested to cause the catastrophic oxidation and loss of structural integrity of the material. The irreversible increase in the dimensions of the samples subjected to thermal expansion tests possibly derives from cracks generated by the permanent volume increase associated with oxidation of zirconium oxynitride phases.

Hot pressing of two sodium beta-alumina powders of nominal composition Na2O · 6.9Al2O3 and Na2O · 8.5Al2O3 at 1100 – 1600°C and 150 – 350 kg/cm2 in inductively heated graphite dies under vacuum, proved an easy method for obtaining fully dense materials in very short pressing times. Densification rates and microstructures developed are strongly dependent on the characteristics of the starting powders. The microstructure of the materials obtained from the first powder consisted of approx. equiaxed grains of the β and β″ phases; the intermediate and final stages of densification (ϱ > ∼ 85%) obeyed first-order kinetics and showed stress exponents in the steady state creep equation (ε ∼ σn) from 0.74 to 1.76. The apparent volume diffusion coefficients at 90% relative density for which n values close to unity were found, calculated on the assumption of the Nabarro-Herring diffusional creep model were of the order of 10−9 cm2/sec. The apparent activation energy for densification was 55 kcal/mole. A duplex microstructure was observed in the materials hot-pressed from the second powder consisting of nearly equiaxed grains and strongly anisotropic elongated crystals grown perpendicularly to the hot-pressing direction. First-order densification kinetics were observed also for this powder but with much lower densification rates. The apparent activation energy for densification was 130 kcal/mole. A strong texture was observed in the materials hot-pressed from both powders with most of the c axes oriented parallel to the hot-pressing direction.