Preparation of zirconia-toughened bioactive glass-ceramics

Bioactive glass-ceramics toughened by tetragonal zirconia polycrystal (TZP) were prepared by hot-pressing mixed powders of the MgO-CaO-P₂O₅-SiO₂ glass and TZP containing 20 to 80% alumina. The bending strength and the fracture toughness of the composite materials were improved compared with those of the material without TZP. These composites showed high bending strengths (400 to 500MPa) and high fracture toughness ( 2.8MPa m^1/2). The existence of a crack deflection mechanism was observed by scanning electron microscopy. After soaking in simulated physiological solution at 100 °C, no phase transformation from tetragonal to monoclinic of TZP in the composites and no degradation in bending strength occurred.     

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with ¹¹B⁺ or ²⁸Si⁺ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 10¹⁷ B/cm² (half at 40 keV and half at 160 keV) or 1 × 10¹⁷ Si/cm² (80 keV). Sapphire implanted with 1 × 10¹⁸ B/cm² had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 10¹⁷ B/cm². Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 10¹⁷ B/cm², half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 10² N/m at 20° and 1.4 × 10² N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 m increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within 1–3% of that of unimplanted material at 3.3 m. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 m, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.     

Effect of annealing on mechanical properties of self-reinforced alpha-silicon carbide

Alpha-SIC powder containing 7.2 wt % Y₃Al₅O₁₂ (YAG, yttrium aluminum garnet) and 4.8 wt % SiO₂ as sintering aids were hot-pressed (SC0) at 1820°C for 1 h and subsequently annealed at 1920°C for 2 h (SC2), 4 h (SC4) and 8 h (SC8). When the annealing time was increased, the microstructure changed from equiaxed to elongated grains and resulted in self-reinforced microstructure consisted of large elongated grains and small equiaxed grains. Development of self-reinforced microstructure, consisted of mostly 6H phase, resulted in significant improvements in toughness. However, the improved toughness was offset by a significant reduction in strength as in the materials consisted of 4H originated from -SiC. The fracture toughness and strength of the 8-h annealed materials were 5.5MPa · m^1/2 and 490 MPa, respectively.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl₂O₄) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl₂O₄ fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (1.2 MPa m^1/2), (2) fractography of machining flaw fracture origins was the most effective source of K _IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).Retired.     

Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained 23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4m. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4m. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is 43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.     

Microsphere-filled lightweight calcium phosphate cements

The incorporation of inorganic and organic microsphere fillers into calcium phosphate cement (CPC) to produce lightweight cementitious materials that could be used under hydrothermal conditions at high temperatures between 200 and 1000 °C was investigated. An aluminosilicate based hollow microsphere, with a density of 0.67 gcm^–3 and a particle size of 75–200 m, was the most suitable having a low slurry density of 1.3 gcm^–3, and a compressive strength greater than 6.89 M Pa. This microsphere-filled lightweight CPC exhibited the following characteristics: 1. after autoclaving at 200 °C, amorphous ammonium calcium orthophosphate (AmCOP) salt and Al₂O₃·xH₂0 gel phases, formed by the reaction between calcium aluminate cement and an NH₄H₂P0₄ based fertilizer, were primarily responsible for the development of strength; 2. at a hydrothermal temperature of 300 °C, the microsphere shell moderately reacted with the CPC to form an intermediate reaction product, epistilbite (EP), while crystalline hydroxyapatite (HOAp) and boehmite (BO) were yielded by the phase transformations of AmCOP and Al₂O₃·xH₂O, respectively; 3. at an annealing temperature of 600 °C, the HOAp phase remained in the cement body, even though an EP anorthite (AN) phase transition occurred; 4. at 1000 °C, the phase conversion of HOAp into whitlockite was completed, while the AN phase was eliminated; and 5. the microsphere demonstrated excellent thermal stability up to temperatures of 1000 °C.This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016.     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Dense SiC ceramics were obtained by hot pressing of -SiC powders using Al₂O₃-Y₂O₃ and La₂O₃-Y₂O₃ additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to 750 MPa at RT and up to 550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m^1/2 and hardness from 21 to 23 GPa.     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Mechanical properties of precipitation-strengthened Ni-Al-Cr alloy based on an NiAl intermetallic compound

Mechanical properties of a ternary alloy Ni-30.3 at. % Al-6.6 at. % Cr have been studied in the temperature range 25 to 1100° C. The material was heat-treated to produce a stable dispersion of incoherent rod-shaped Ni₃Al precipitates, 1m in diameter and 20m long. The tensile properties were found to be temperature dependent. Below 750° C the material had high strength, low ductility and low strain-rate sensitivity, whilst above 750° C the strength fell, ductility increased and the material became strain-rate sensitive. The room temperature fracture toughness of the single-phase material was 6 MN m^–3/2 and increased to 50 MN m^–3/2 in the two-phase material. This can be attributed to the effect of Ni₃Al on crack nucleation and propagation.     

In situ synthesis of Mo(Si,Al)2-SiC composites

An in situ reaction was proposed and investigated to produce Mo(Si_{1 – x} Al _x )₂-SiC composites. The starting powders were MoSi₂, Al and C. A direct current hot pressing (DCHP) method was used to prepare these composites. When the mixed powder was hot pressed at temperatures lower than 1500°C, the phase composition was Mo(Si,Al)₂ and -SiC. When the hot pressing temperature was higher than 1600°C, however, Nowotny phase Mo₅Si₃C₁ appeared. The chemical stoichiometry of the proposed in situ reaction becomes difficult because of the formation of solid solution among these phases and the appearance of Mo₅Si₃C phase. The in situ formed SiC phase in the x = 0.3 sample was partly in whisker shape. However, the SiC phase in x = 0.15 sample was in particle shape. These in situ formed SiC particles and whiskers acted as crack deflection and bridging elements and improved the fracture toughness. The Vickers hardness and fracture toughness of the x = 0.3 sample hot pressed at 1700°C for 60 min in vacuum were 15.6 GPa and 5.39 MPa · m^1/2, respectively.     

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

Single phase-sialon ceramics, Si_6–z Al _z O _z N_8–z , have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into a-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y₂O₃ to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y₂O₃ addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of the-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phase-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1m at lowz-values to 5m at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y₂O₃ to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.     

Thermal stability in the Al-Al3Ti system

Directionally solidified samples of an Al-2 wt% Ti alloy were annealed at temperatures between 435° C and 660° C to investigate the thermal stability of phases formed during an incomplete peritectic transformation. The proportion of Al₃Ti present in the assolidified alloy is less than equilibrium up to about 480° C, and more than equilibrium at higher temperatures. Hence, Al₃Ti particles will be stable up to about 500° C and will tend to dissolve at higher temperatures. Diffusion due to non-equilibrium composition of the phase continues at all temperatures but is sluggish up to about 600° C. The diffusion coefficient of Ti in Al at 635° C is estimated to be 2×10^–11 cm² sec^–1.     

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti₃SiC₂/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti₃SiC₂/SiC composite was finer than that of monolithic Ti₃SiC₂, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti₃SiC₂ matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m^1/2 and 1080 kg mm^–2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti₃SiC₂ was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti₃SiC₂.     

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.     

Preparation of TiB2 sintered compacts by hot pressing

A sintered compact of titanium diboride (TiB₂) was prepared by hot pressing of the synthesized TiB₂ powder, which was obtained by a solid-state reaction between TiN and amorphous boron. Densification of the sintered compact occurred at 20 MPa and 1800° C for 5 to 60 min with the aid of a reaction sintering, including the TiB₂ formation reaction between excess 20 at % amorphous boron in the as-synthesized powder (TiB₂ + 0.2B) and intentionally added 10 at % titanium metal. A homogeneous sintered compact of a single phase of TiB₂, which was prepared by hot pressing for 30 min from the starting powder composition [(TiB₂ + 0.2B) + 0.1 Ti], had a fine-grained microstructure composed of TiB₂ grains with diameters of 2 to 3 m. The bulk density was 4.47 g cm^–3, i.e. 98% of the theoretical density. The microhardness, transverse rupture strength and fracture toughness of the TiB₂ sintered compact were 2850 kg mm^–2, 48 kg mm^–2 and 2.4 MN m^–3/2, respectively. The thermal expansion coefficient increased with increasing temperature up to 400° C and had a constant value of 8.8 x 10^–6 deg^–1 above 500° C.     

Silicon carbide-toughened zirconia ceramics

Tetragonal zirconia polycrystalline (TZP) ceramics containing SiC reinforcement in the form of fine particles (nano-scale), particles (micro-scale), whiskers and platelets were synthesized by hot-pressing. The effects of morphology and grain size of SiC reinforcement on the strength and fracture toughness at room temperature were investigated. The addition of SiC (in whatever form) caused decreases in strength and toughness at room temperature with the exception of whisker-reinforced materials. Toughness fell off with increasing temperature, but nevertheless retained about one-half of the room-temperature value for that particular SiC reinforcement. However, the whisker- and particle-reinforced materials had higher K _lc values at high temperature than fine particle- or platelet-reinforced materials, with values in excess of 7 MPa m^1/2 at 1000 °C. The microstructure was examined for SiC whisker-reinforced/TZP materials by TEM and HREM, to examine the nature of the whisker/zirconia interface.     

Microstructural modification to improve mechanical properties of a 70%Si3N4-30%BAS self-reinforced ceramic composite

Different microstructures of the 70%Si₃N₄-30%BAS self-reinforced composite, fine, coarse and bimodal, are obtained by pressureless sintering at 1920°C. Flexural strength, fracture toughness and crack-growth resistance (R-Curve) behavior of each microstructure are characterized by three-point bending, indentation and modified compact tension methods respectively, at room temperature. The crack deflection, whisker bridging and pullout are considered as major toughening mechanisms in the composite. It is found that coarsening -Si₃N₄ whiskers of this composite can improve the toughness/fracture resistance but deteriorate the strength. When limited large abnormally grown whiskers are introduced into the microstructure, the composite shows an improved toughness/fracture resistance behavior and concurrently sustains a high strength.     

Residual stresses in particle-reinforced ceramic composites using synchrotron radiation

Residual stresses were determined in particle-reinforced ceramic composites using synchrotron based x-ray diffraction. The baseline Si₃N₄ and the Si₃N₄-TiN composites were processed by turbomilling, pressure casting, and isopressing. They were then continuously sintered to full density, under a pressureless, flowing nitrogen atmosphere. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following quenching in water from 1000°C, 1100°C, and 1200°C. The residual stresses for both the baseline Si₃N₄ and the Si₃N₄-TiN composites were determined from the (441) and (531) reflections, by applying the 2-sin² method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the mechanical properties of each material. In both the baseline Si₃N₄ and Si₃N₄-TiN composites, after thermal shocking, the compressive residual stresses were developed in directions both parallel and perpendicular to the sample surface. The residual compressive stresses for the Si₃N₄-TiN composites were much higher than the baseline Si₃N₄. As a result, both fracture toughness and flexural strength of the Si₃N₄-TiN composites were improved. In addition, the addition of the TiN appears to improve both the strength and toughness of the baseline Si₃N₄.