Porous HA ceramic for bone replacement: Role of the pores and interconnections – experimental study in the rabbit

Hydroxyapatite (HA) porous ceramics are increasingly used in biomedical applications. Their physical characteristics, such as porous volume, require perfect control of the pore shape, as well as the number and the size of their interconnections.The aim of our study was to evaluate a new HA ceramic using polymethylmethacrylate microbeads (PMMA) as the porous agent. Four interconnection sizes (30, 60, 100 and 130 m) with a 175–260 m pore size and three pore sizes (175–260, 260–350 and 350–435 m) for a 130 m interconnection size were tested. Various HA implants were appraised by microscopic evaluation in a 4.6×10 mm rabbit femur cancellous bone defect 12 weeks after implantation. The best osteoconduction result was obtained in the center of the ceramic by means of a 130 m interconnection size and a 175–260 m mean pore size. Bone formation obtained within the pores was double that obtained in our previous study where naphtalen microbeads were used as the porous agents.© 2001 Kluwer Academic Publishers     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO₃)₂, (NH₄)₂HPO₄ and HNO₃; the concentrations of Ca²⁺ and PO₄ ^3– were in the ranges of 0.10 to 0.90 mol·dm^–3 and 0.06 to 0.54 mol·dm^–3, respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300–500°C) and the upper furnace for the pyrolysis of the precipitated metal salts (750–900°C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca²⁺ (0.50 mol·dm^–3) and PO₄ ^3– (0.30 mol·dm^–3) at the temperatures of 800°C (the upper furnaces) and 400°C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of 1 m or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200°C for 5 h attained maxima 95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was 3 m. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ · mol^–1 · K^–1.     

Laser surface melting of Ti-6Al-4V alloy

Surface hardening of Ti-6Al-4V alloy with laser surface melting (LSM) in a nitrogen atmosphere has been studied. In LSM, hardness increased almost three-fold in comparison to that of the substrate, the latter having a Vickers hardness of 350, by the formation of TiN in the range of 100m of melt depth. Hardness, then, decreased slowly and reached a minimum of 580 VHN at a maximum melt depth of 750m. -Ti was formed in the heat-affected zone (HAZ) with a VHN of 450. Ageing treatments were performed for all specimens at 450 °C and different ageing times (1–20h). Short ageing treatments increased the hardness in the melted zone as well as in the HAZ (1–3h). Long ageing treatments (7–20h) resulted in uniform hardness distribution in the melted zone.     

Infrared transmission of sintered 3 mol% Y2O3-ZrO2 gel

Translucent ZrO₂ film was successfully prepared by gelling hydrothermally produced nano-ZrO₂ powders. The film (300 m thick) was found to transmit light to 6.5 m (40% transmission) when sintered at 1200 °C, but transmission was totally lost after sintering at 1300 °C for 1 h. Residual organic material such as urea, which was used for preparing the powder, dominated the transmission of the film in the region between 1.3 and 4.5 m when sintered below 1000 °C. When sintered above 1000 °C, the microstructure controlled the transmission. Both organic residuals and the microstructure of the zirconia were found to determine the transmission in 4.5–6.5 m region.     

Microstructural investigation of low-density carbon-carbon composites

The microstructure of low-density (0.13–0.64 Mg m^–3) carbon-carbon composites was investigated using optical microscopy, scanning electron microscopy and image analysis. All samples initially contained varying proportions of rayon precursor carbon fibres, recycled fibrous material and phenolic resin precursor matrix, and were manufactured utilizing a vacuum moulding technique. Some of the composites were densified using the chemical vapour deposition (CVD) of pyrolytic carbon. All the composites were shown to have a two-dimensional planar random microstructure, with a distinct layering effect being seen on the microscopic (and sometimes macroscopic) level. The degree of layering in the composites was quantified utilizing image analysis and was found to be most pronounced in samples containing no recycled material, and least pronounced in samples containing all of its fibrous constituent as recycled material. The composites were found to be very porous, the pores consisting of mainly interconnecting open pores (typically 65–85% of the sample volume). In non-CVD samples the fibrous material was held together by thin (<5 m) discrete matrix bonds, with a few large (typically 100 m×200 m×800m) fibre bundles also existing within the structure. In the CVD-processed material the deposit coat on the fibres was of even thickness throughout the composite and joined together fibrous material that was not previously in contact.     

Characterization of a δ/γ duplex stainless steel

A duplex stainless steel was investigated in both as-received sheet and after annealing at temperatures ranging from 850 to 1100°C. The sheet presents a deformation texture in both phases, austenite and ferrite, induced by cold rolling. Microstructure in the as-received material consists of island-like austenitic grains in a ferrite matrix. These austenitic grains are elongated with an average size of 6, 20 and 40 m along the normal (ND), transversal (TD) and rolling direction (RD). Quantitative texture measurements demonstrated that texture components are distributed mainly along the -fiber (ND ‹100›) and -fiber (RD ‹110›) for the ferrite and the -fiber (ND ‹110›) for the austenite. After recrystallization, a decrease in the intensity of the mean fibers and an increase in the minor components was observed in both, ferrite and austenite. Therefore, a similar texture was reached in both phases after annealing at 1050°C. Microstructural characterization after annealing at temperatures above 850°C showed that the elongated austenitic grains transform in colonies of equiaxic grains of about 10–15 m in size. These colonies are surrounded by a ferritic matrix at annealing temperatures above 1000°C or by a laminar microstructure at temperatures below 950°C. This laminar microstructure includes sigma phase and austenite formed from delta ferrite, and untransformed delta ferrite.     

Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co2Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1–3 m across the hexagonal plane and 0.1–0.3 m thickness at 1000°C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200°C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material.     

Nucleation control of diamond synthesized by microwave plasma CVD on cemented carbide substrate

Diamond was coated on to cemented carbide substrate by microwave plasma CVD, in which nucleation control of diamond crystals was investigated under constant deposition conditions; total pressure 30 torr, CH₄ flow rate 1 ml min^–1, H₂ flow rate 199 ml min^–1 and microwave power 550 W. Nucleation tends to occur selectively on the edge part of WC grains of the cemented carbide substrate with coarse WC grain size of about 1 m, where the nucleation density was 9×10⁶ cm^–2. The density increased to about 5×10⁷ cm^–2 when using a finegrained substrate (WC grain size 0.5 m). A considerably enhanced nucleation was observed by introducing a number of fine microflaws on to the substrate surface. Microflawing treatment with diamond fine powder (grain size 0–1/4 m) suspended in an ultrasonic cleaner bath was effective for increasing the diamond nucleation density up to 5×10⁸ cm^–2. The grain size of grown diamond crystals decreased with increasing microflawing time.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

Development of the Porous Substrates of Ceramic Membrane Filters

Results of studying the processes of manufacture of the porous substrates of ceramic membrane filters have been presented. Specimens of ceramic porous substrates suitable for the manufacture of membrane filters have been obtained in the porosity interval 30–60% and the pore-size interval 0.4–100 m based on the investigations carried out.     

Effect of fiber content on the thermoelectric behavior of cement

The effect of discontinuous stainless steel fibers (diameter 60 m) as an admixture in cement paste on the thermoelectric behavior (the Seebeck effect) was systematically studied as a function of fiber volume fraction from 0 to 0.50 vol%. Without fibers, cement paste has an absolute thermoelectric power of +3 V/°C. A fiber content of up to 0.20 vol% makes the absolute thermoelectric power more negative (down to –63 V/°C), whereas a fiber content of 0.20–0.50 vol% makes the absolute thermoelectric power more positive (up to +31 V/°C)—even more positive than the positive value for the steel fiber by itself (+8 V/°C). The value is zero at a steel fiber content of 0.27 vol%. The effects are probably due to carrier scattering rather than conduction.     

A flexible route to high strength α-alumina and aluminate spheres

The formation of hollow alumina spheres is accomplished by coating polystyrene beads of 3 m and 50–80 m diameter with carboxylic acid functionalized alumina nanoparticles (alumoxanes) from aqueous solution 2–8 wt%. The resulting coated beads were heated to 220°C to calcine the alumoxane to porous amorphous alumina before washing with toluene to remove the polystyrene from inside the ceramic coating. The resulting hollow spheres were sintered at 1000°C to form -alumina. The -alumina spheres have been characterized, by SEM (scanning electron microscopy), BET, and hardness measurements, that show the hardness of the hollow alumina sphere (1900 ± 100 Kg.mm^–2) approaches that of corundum (ca. 2000 Kg.mm^–2). Multilayered bi-phasic spheres may be prepared by subsequent coating the -alumina spheres with a solution of a metal-doped alumoxane. After calcining, the mixed metal oxide phase (CaAl₁₂O₁₉, Er₆Al₁₀O₂₄, MgAl₂O₄, Al₂TiO₅, and Y₃Al₅O₁₂) forms outside of the alumina sphere resulting in a composite like ceramic bi-layer sphere. Pre-formed hollow alumina spheres were incorporated into a resin and ceramic thin film formed from a 1 wt% A-alumoxane aqueous solution. The hardness of the composites is compared to the matrix materials themselves.     

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO₂ laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s^–1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 m sized sub-grains with sub-micrometre and precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale precipitates were present in the as-solidified -Al matrix but there was no evidence for the , S and T₁ phases. The heat-affected zone was only 100 m wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.     

Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 m thick cladding layers of copper on a 125 m thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 m grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.     

Characterization of silver-palladium submicronic powders

Various properties of submicronic silver-palladium powders (70/30 and 75/25 Ag/Pd % wt/wt), recently obtained by a new process of precipitation in aqueous solution, are described. These powders are intended to be used in the multilayer ceramic capacitor industry. The morphology of the particles has been analysed through various measurements, such as SEM images, tap density, BET surface area, and the size distribution by a sedimentation technique. The powders were found to be made of spherical shaped particles of mean diameter close to 0.3 m with a narrow size distribution. The individual grains were agglomerated only to a small extent. After being conditioned as a paste by blending with an organic binder, screen-printed and fired, the particles sintered into a conductive layer of low resistivity (15–30 Wcm for 2 m thickness) with only a few remaining voids, as observed by SEM. Thermal measurements indicated no melting of a pure silver phase at 960 °C, thus indicating that alloying was achieved before this temperature.     

Low cost porous mullite-corundum ceramics by gelcasting

A gelcasting process has been successfully employed to fabricate porous mullite-corundum ceramic composites from natural clay and corundum powders. The specimens based on the mullite composition are found with an open porosity of 45–47.9%, mean pore size of 1.28–2.55 m, and nitrogen permeability of 965–5038 m³· m^–2· bar^–1· hr^–1by reactively sintering the gelled mixture of kaolinite and -alumina at 1100–1500°C. The open porosity (_o), mean pore size (d _p), pore size distribution and gas permeability can be controlled by adjusting raw material ratios and sintering temperatures. The gas permeability of the specimens is found to be more dependent on the pore size distribution as well as d _pthan on _o. In addition, the gas transportation mechanism in porous mullite-corundum ceramic composites is dominated by viscous flow.