The influence of sintering temperature on the properties of compacted bovine hydroxyapatite

On the sintering characteristic of hydroxyapatite (HA), the resulting microstructure and properties are influenced not only by the characteristic and impurities of materials but also are found to be dependent on the thermal history during the fabrication process. This research is concerned with the effect of sintering temperature on the relative density, hardness, and phase purity after sintering process. Bovine HA (BHA) powder obtained from heated local cortical bovine bone at 900 °C for 2 h was uniaxially pressed at 156 MPa into green bodies using a 20 mm cylindrical dies. The compacted green body was pressurelessly sintered in air atmosphere at temperatures ranging from 1000 to 1400 °C, at a furnace ramp rate of 5 °C/min and dwell time of 2 h. The BHA starting powder was characterized using XRD and FTIR. SEM was also used for observing the microstructures of the starting material. The sintered BHA specimens were analyzed using Archimedes method for measuring density; XRD for phase stability; and Vickers method for hardness measurement. The analysis results show that the starting BHA powder and the sintered BHA specimens contain HA. The intensity of the three main peaks of HA decreases with increasing sintering temperature which may be due to decomposition of HA at high temperature. The density and hardness of BHA increases with increasing sintering temperature based on the results obtained.     

Influence of temperature and aging time on HA synthesized by the hydrothermal method

The influence of temperature and aging time on the morphology and mechanical properties of nano-sized hydroxyapatite (HA) synthesized by a hydrothermal method is reported here. The pre-mixed reactants were poured into a stirred autoclave and reacted at temperatures between 25–250°C for 2–10 h. HA powders thus obtained were examined using X-ray diffraction (XRD), high-resolution field emission scanning electron microscopy (FESEM) and a particle size analyzer. It was found that the aspect ratio of the particles increased with the reaction temperature. The length of the HA particles increased with the reaction temperature below 170°C, but it decreased when the temperature was raised above 170°C. The agglomerates of HA particles were formed during synthesis, and their sizes were strongly dependent on reaction temperatures. As the reaction temperature increased, the agglomerate size decreased (p = 0.008). The density of the discs pressed from these samples reached 85–90% of the theoretical density after sintering at 1200°C for 1 h. No decomposition to other calcium phosphates was detected at this sintering temperature. A correlation existed (p = 0.05) between the agglomerate sizes of HA particles synthesized at various conditions and their sintered densities. With the increase of the agglomerate size, the sintered density of the HA compact decreased. It was found that both the sintered density and flexural strength increased with increasing aging time and reaction temperature. A maximum flexural strength of 78 MPa was observed for the samples synthesized at 170°C for 5 h with the predicted average at these conditions being 65 MPa. These samples attained an average sintered density of 88%.     

Development of controlled strength-loss resorbable beta-tricalcium phosphate bioceramic structures

Controlling the strength-loss rate during biodegradation is a bottleneck in developing viable resorbable ceramic implants. Resorbable beta-tricalcium phosphate (β-TCP) bioceramic is known for its excellent biocompatibility. However, it exhibits poor sinterability and poor flexural strength. Here, we improved sintering behavior and biaxial flexural strength of β-TCP bioceramic without altering its biocompatibility by introducing multi-oxide sintering additives, in small quantities. These additives could also tailor the rate of resorption and hardness deterioration of β-TCP. A range of additives were prepared and introduced into β-TCP powder. Resultant powders were uniaxially pressed and sintered at 1250 °C, in air. Considerable improvement in densification (up to 33%) and biaxial flexural strength (up to 43%) were achieved. X-ray powder diffraction (XRD) analysis confirmed that the additives didn't alter the phase purity. In vitro cytotoxicity and biocompatibility analyses were performed using a prostate cancer cell-line. Results showed that the doped and pure β-TCP structures were non-toxic and biocompatible.     

Ceramic composites: TiB2-TiC-SiC

The composition 20% TiB₂-55% TiC-25% SiC (mol%) was selected and the hot pressing parameters were optimized using optimal design. The optimized hot-pressed material had a bend strength > 1200 M Pa, a thermal shock resistance of 300 °C, and a bend strength at 1200 °C of 456 MPa. The pressureless sintering of the selected composite was optimized, with and without additives. Powders sintered without additives (2200 °C, 2 h) had a low density (92.2%) and strength (480 MPa). Carbon and boron additives led to higher densities (97.5%) but did not improve the strength.     

Synthesis and sintered properties evaluation of calcium phosphate ceramics

The present paper describes calcination of calcium phosphate ceramics (hydroxyapatite, HA) at 200, 400, 600, 800 and 1000 °C to observe the phase change using X-ray diffraction. HA phase was found to be stable up to 600 °C and later got dissociated into other non-stoichiometric phases like tricalcium phosphate (Ca₃(PO₄)₂ [TCP]), calcium pyrophosphate (Ca₂P₂O₇ [CPP]) and calcium hydrogen phosphate (CaHPO₄ [CHP]). TCP was found to be the major phase above 1000 °C. FTIR spectra showed the presence of various PO₄³⁻ and OH⁻ groups present in the powder. Powders compacted and sintered at 900, 1000, 1100 and 1200 °C showed an increase in density from 2.11 to 2.95 g/cm³ while biaxial flexural strength (BFS) was found to be higher (48.7 MPa) when the samples were sintered at 1100 °C and it decreased with further increase in sintering temperature.     

Tensile properties of iron-based P/M steels with ferrite + martensite microstructure

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction.     

Composites of bovine hydroxyapatite (BHA) and ZnO

Composites of calcinated bovine bone-derived hydroxyapatite (BHA), doped with 2.5, 5, and 10 wt.% ZnO were produced by sintering. Scanning electron microscopy (SEM) and X-ray diffraction analysis together with measurements of density, compressive strength, and Vickers microhardness were carried out in the sintered samples. The experimental results showed that the best mechanical properties were achieved in the samples with 5% addition of ZnO. The highest value of compression strength was achieved after sintering at 1200 °C (72 MPa) and of microhardness at 1300 °C (548 HV). Prolong heat treatment at 1300 °C results in vulnerable BHA–ZnO composites to over-firing effect.     

Effects of sintering temperature and particle size on the translucency of zirconium dioxide dental ceramic

The aim of this study was to evaluate the effects of sintering temperature and particle size on the translucency of yttrium stabilized tetragonal zirconia polycrystals (Y-TZP) dental ceramic. Eighty disc-shaped and cylindrical specimens were fabricated from zirconia powers of particle size 40 and 90 nm. These specimens were sintered densely at the final sintering temperature 1350, 1400, 1450 and 1500°C, respectively. The visible light transmittance, sintered density and microstructure of the sintered block were examined. The results showed that the sintered densities and transmittances increased with the temperature from 1,350 to 1,500°C. Y-TZP could gain nearly full density and about 17–18% transmittance at the final sintering temperature of 1,450–1,500°C. The 40-nm powders had higher sintered density and transmittance than the 90-nm. The translucency of Y-TZP dental ceramic could be improved by controlling the final sintering temperature and primary particle size.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Crystallization and microstructural evolution process from the mechanically alloyed amorphous SiBCN powder to the hot-pressed nano SiC/BN(C) ceramic

The mechanically alloyed amorphous SiBCN powders were hot pressed at 1500, 1600, 1700, 1800, and 1900 °C under a pressure of 80 MPa in the nitrogen atmosphere for 30 min. The crystallization, the microstructural evolution, and the properties of the prepared ceramics were carefully studied by XRD, TEM, HRTEM, and property testing. Results show that the crystallization of β-SiC, turbostratic BN(C), and α-SiC in the amorphous matrix starts at about 1500, 1600, and 1700 °C, respectively. When the powder is hot pressed at the temperatures higher than 1700 °C, the prepared ceramics always consist of nano β-SiC, α-SiC, turbostratic BN(C), and amorphous body. With the increase of the sintering temperature, the ceramic crystallinity becomes higher, the grains get larger, and the amorphous content becomes lower. At the temperatures lower than 1800 °C, the bulk density, the relative density, the flexural strength, the Young’s modulus, and the fracture toughness of the prepared ceramics show persistent but insignificant increase. However, when the ceramic is sintered at 1,900 °C, these properties are rapidly improving to 2.6 g/cm³, 91.8 %, 331.0 MPa, 139.4 GPa, and 2.8 MPa m^1/2.     

Fabrication and oxidation resistance of silicon nitride fiber reinforced silica matrix wave-transparent composites

Wave-transparent ceramic matrix composites for the high temperature use should possess excellent oxidation resistance. In this work, Si_3N_(4f)/SiO_2 composites with different fiber content were fabricated by filament winding and sol gel method. The oxidation resistance was investigated by tracking the response of flexural strength to the testing temperature. The results show that the flexural strength and toughness of the composites with fiber content of over 37% can reach high levels at around 175.0 MPa and 6.2 MPa m1/2, respectively. After 1 h oxidation at 1100?C, the flexural strength drops a lot but can still reach 114.4 MPa, which is high enough to ensure the safety of structures. However, when the oxidation temperature rises to 1200–1400?C, the flexural strengths continue to fall to a relatively low level at 50.0–66.4 MPa. The degradation at high temperatures is caused by the combination of over strong interfacial bonding, the damage of fiber and the crystallization of silica matrix.     

Mechanical,corrosion, and sliding wear behavior of intermetallics reinforced austenitic stainless steel composites

The present work investigates the effect of supersolidus sintering and intermetallics (Ni₃Al, Fe₃Al) additions on the densification, mechanical, tribological, and electrochemical behavior of sintered austenitic (316L) stainless steels. The performances of the supersolidus liquid phase sintered (SLPS) compacts are compared with the conventional solid-state sintered (SSS) compacts of similar compositions. Correspondingly, the sintering was carried out at two different temperatures 1200 °C (SSS) and 1400 °C (SLPS). Supersolidus sintering results in significant improvement in densification, wear resistance, corrosion resistance, strength, and ductility in both straight as well as aluminide added composites.     

低温烧结细晶Y-TZP

通过在Ｙ－ＴＺＰ中加入适量的硅酸盐玻璃添加剂,使其烧结温度明显降低,并且制备出具有细晶粒、高强度的四方相氧化锆增韧陶瓷材料．分析了添加剂含量及烧结温度与材料致密度、显微结构及力学性能的关系,发现在Ｙ－ＴＺＰ材料中加入１ｗｔ％的添加剂,可以使材料在１４００℃下烧结,氧化锆晶粒尺寸约为１００～２００ｎｍ;其抗折强度可达９５０ＭＰａ．     

Effect of bioglass additions on the sintering of Y-TZP bioceramics

The objective of this work was to evaluate the influence of bioglass additions on the sintering and mechanical properties of yttria-stabilized zirconia ceramics, Y-TZP. Samples containing different bioglass additions, varying between 0 and 30 wt.%, were cold uniaxial pressed at 80 MPa and sintered in air at 1200 °C or 1300 °C for 120 min. Sintered samples were characterized by X-ray Diffractometry and Scanning Electron Microscopy. Hardness and fracture toughness were determined using Vickers indentation method. As a preliminary biological evaluation, in vitro cytotoxicity tests by Neutral Red Uptake method (using mouse connective tissue cells, NCTC clone L929 from ATCC bank) were realized to determine the cytotoxicity level of ZrO₂-bioglass ceramics. The increasing of bioglass amount leads to the decreasing of relative density due to martensitic (tetragonal-monoclinic) transformation during cooling of the sintered samples. Y-TZP samples sintered at 1300 °C containing 5 wt.% of bioglass presented the best results, with high relative density, hardness and fracture toughness of 11.3 GPa and 6.1 MPa m^1/2, respectively. Furthermore, the un-cytotoxic behavior was observed in all sintering conditions and bioglass amounts used in this study.     

Oxidation resistance of dense carbon fiber/Si–O–C glass composite fabricated by pre-oxidation and spark plasma sintering

The carbon fiber/Si–O–C glass composite was prepared from the silicone and carbon fiber by pre-oxidation and spark plasma sintering (sintered composite). The mass loss of the sintered composite oxidized at 1200 °C for 90 min was 5%, which was lower than that of same dimension for similar composites, although the mass loss at 600 °C was still high. This indicated its excellent oxidation resistance at elevated temperature. No cracks and pores were found in the sintered composite, indicating that the combination of pre-oxidation and spark plasma sintering was better than the pyrolysis for manufacturing dense composites. Compared with the flexural strength of about 60 MPa for carbonaceous composites, the flexural strength of the sintered composite was obviously improved to 220 MPa. Moreover, microstructures of the specimen before and after sintering as well as after oxidation were investigated.     

Mechanical strength of extrusion freeformed calcium phosphate filaments

Hydroxyapatite–tricalcium phosphate mixtures of various compositions were extruded by a solid freeforming process to form lattice structures to serve as hard tissue scaffolds. The unwelded filaments, sintered at temperatures from 1100 to 1300°C, had radii from 115 to 135 μm and were tested in three point flexural loading using a purpose-built fixture. Flexural strength ranged from 20 to 100 MPa depending on composition and sintering temperature. Weibull moduli up to 13 were obtained. Compositions with 50% or more tri-calcium phosphate did not develop strengths much above 40 MPa and the strength of most compositions fell when the sintering temperature exceeded 1250°C. Multiple layer lattice structures were created and tested in compression.     

碳化硅泡沫陶瓷浆料成分与烧结性能

将用于制备碳化硅泡沫陶瓷的浆料烘干、制粉、干压、烧结,从而探讨浆料的成分与 烧结性能的关系.样品的抗弯强度与烧结助剂的含量之间存在最佳搭配关系,烧结对强度的贡 献主要来自于新相莫来石的生成和玻璃相对碳化硅颗粒的包覆、连接作用.同种成分的样品开 气孔率随着烧结温度的升高表现出单调下降的趋势,而抗弯强度与开气孔率的变化并没有表现 出完全相反趋势;不同成分样品的耐火度均保持在1730℃没有变化.采用最佳配方的浆料和 最佳烧结温度制备的碳化硅泡沫陶瓷抗弯强度可达到0.72MPa.     

In situ formation of sintered cordierite–mullite nano–micro composites by utilizing of waste silica fume

This study aims at in situ formation of sintered cordierite–mullite nano–macro composites having high technological properties using waste silica fume, calcined ball clay, calcined alumina, and magnesia as starting materials. The starting materials were mixed in different ratios to obtain different cordierite–mullite composite batches in which the cordierite contents ranged from 50 to 100 wt.%. The batches were uni-axially pressed at 100 MPa and sintered at 1350, 1400 and 1450 °C to select the optimum temperature required for cordierite–mullite nano–macro composites formation. The formed phases were identified by X-ray diffraction (XRD) pattern. The sintering parameters in terms of bulk density (BD) and apparent porosity (AP) were determined. The microstructure of composites has been investigated by scanning electron microscope (SEM). Cold crushing strength (CCS) of the sintered batches was evaluated. The result revealed that the cordierite–mullite nano–macro composites were in-situ formed at 1400 °C. The batch containing 70 wt.% cordierite showed good physical and mechanical properties.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Microstructural evolution and magnetic properties of pressureless-sintered nanosized iron prepared by a facile combustion-based route

High-activity iron nanopowders were prepared through a facile combustion-based route, combining solution combustion synthesis and hydrogen reduction. The as-synthesized iron nanopowders were densified by pressureless sintering. The densification behavior, microstructure, mechanical properties and magnetic properties of the iron bulk sintered at different temperatures were studied in detail. At a low sintering temperature of 700 °C, the relative density of the sintered iron reaches 97.3%, because the iron nanopowders exhibit a low sintering activation energy of 123 KJ/mol. The iron bulk sintered at 700 °C exhibits relatively regular equiaxed grains with an average grain size of 0.45 μm, and has a maximum tensile strength of 510.1 MPa, a high microhardness of 201.9 HV, and a saturation induction of 1.75 T. As the sintering temperature increases, the grain size and magnetic properties of the sintered iron enhance, while the mechanical strength decreases. For the sintered iron prepared at 1300 °C with the average grain size ~ 80.9 μm, the saturation induction value reaches 1.78 T and the tensile strength is 334.9 MPa.