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1.
Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina
K. Uematsu M. Sekiguchi J. -Y. Kim K. Saito Y. Mutoh M. Inoue Y. Fujino A. Miyamoto 《Journal of Materials Science》1993,28(7):1788-1792
Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions. 相似文献
2.
A number of polymeric precursors to silicon nitride were prepared and evaluated as binders in cold pressing/pressureless sintering operations. These polymers exhibited ceramic yields in excess of 75% by weight, and powder compacts made using them as binders displayed improved green handling properties. Compacts pyrolysed at 800 °C exhibited unusual microstructures, including the development of whiskers in situ. Based on microstructural observation, compacts sintered under pressureless conditions appeared to show enhanced densification relative to those processed without preceramic binders. Preceramic binders appeared to enhance the formation of -Si3N4 and may enhance densification of compacts sintered under pressureless conditions. 相似文献
3.
《Materials Science & Technology》2013,29(8):1241-1248
AbstractThe influence of process route on the microstructure and tensile behaviour of specimens prepared from hot isostatically pressed powders and extruded ingot of the burn resistant alloy, Ti–25V–15Cr–2Al–0·2C (wt-%), has been investigated. Samples based on gas atomised (GA) and plasma rotating electrode process (PREP) powders have been studied. Microstructural examination shows that many PREP powder particles are single crystals, whereas GA particles are polycrystalline. The mechanical properties of hot isostatically pressed specimens have been assessed using tensile testing monitored by acoustic emission, while microstructures have been characterised by synchrotron X-ray microtomography and optical and analytical scanning electron microscopy. Tomographic examination revealed a small fraction (<0·002 vol.-%) of pores in samples made from hot isostatically pressed GA powders, but no porosity was detected in samples made from hot isostatically pressed PREP powder. In view of their similar tensile behaviour, it is concluded therefore that the porosity does not contribute to the scatter and poor ductility in these hot isostatically pressed samples. These pores increased in size and volume fraction after heat treatment above the hot isostatic press temperature. The large scatter in tensile properties of both hot isostatically pressed GA and PREP samples was correlated with the presence of large (100–400 μm) circular crack initiation sites on the fracture surfaces, but the origin of these initiation sites has not been identified. 相似文献
4.
An improvement in processing of hydroxyapatite ceramics 总被引:17,自引:0,他引:17
M. G. S. Murray J. Wang C. B. Ponton P. M. Marquis 《Journal of Materials Science》1995,30(12):3061-3074
Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h. 相似文献
5.
Evolution of porosity and microstructure was observed during densification of lutetium oxide ceramics doped with europium (Lu2O3:Eu) fabricated via vacuum sintering and hot isostatic pressing (HIP’ing). Nano-scale starting powder was uniaxially pressed and sintered under high vacuum at temperatures between 1575 and 1850 °C to obtain densities ranging between 94% and 99%, respectively. Sintered compacts were then subjected to 200 MPa argon gas at 1850 °C to reach full density. Vacuum sintering above 1650 °C led to rapid grain growth prior to densification, rendering the pores immobile. Sintering between 1600 and 1650 °C resulted in closed porosity yet a fine grain size to allow the pores to remain mobile during the subsequent HIP’ing step, resulting in a fully-dense highly transparent ceramic without the need for subsequent air anneal. Light yield performance was measured and Lu2O3:Eu showed ∼4 times higher light yield than commercially used scintillating glass indicating that this material has the potential to improve the performance of high energy radiography devices. 相似文献
6.
Gibson IR Ke S Best SM Bonfield W 《Journal of materials science. Materials in medicine》2001,12(2):163-171
The effect of different sintering conditions on the sintered density and microstructure of two different hydroxyapatite (HA) powders was examined. The powder characteristics of a laboratory synthesized HA powder (Lab HA) were low crystallinity, a bimodal particle size distribution, a median particle size of 22 m and a high specific surface area (SSA) of 63 m2/g. By contrast, a commercial calcined HA (commercial HA) was crystalline and had a median particle size of 5 m and a low SSA of 16 m2/g. The different powder characteristics affected the compactability and the sinterability of the two HA powders. Lab HA did not compact as efficiently as commercial HA, resulting in a lower green density, but the onset of sintering of powder compacts of the former was approximately 150 °C lower than the later. The effect of compaction pressure, sintering temperature, time and heating rate on the sintered densities of the two materials was studied. Varying all these sintering conditions significantly affected the sintered density of commercial HA, whereas the sintered density of Lab HA was only affected significantly by increasing the sintering temperature. The Vickers hardness, Hv, of Lab HA was greater than commercial HA for low sintering temperatures, below 1200 °C, whereas for higher sintering temperatures the commercial HA produced ceramics with greater values of hardness. These trends can be related to the sinterability of the two materials. 相似文献
7.
Palanki Balakrishna B. Narasimha Murty K. P. Chakraborthy R. N. Jayaraj C. Ganguly 《Materials and Manufacturing Processes》2000,15(5):679-693
Nuclear ceramic grade UO2 powders are usually prepared by the wet chemical ammonium diuranate route. The powders are pressed and sintered before incorporation into nuclear fuel assemblies. The processing is complex at all stages and the specifications are stringent. In powder preparation, slow addition of the precipitating reagent at a low temperature is recommended. The conditions for the drying of the precipitate, calcination and reduction are chosen to result in an agglomerate free, fine and porous powder that does not require milling or binder addition and is capable of being compacted and sintered to desired density with a homogeneous microstructure. The pressing conditions are chosen to give compacts that are free from defects such as cracking, chipping and end-capping. Sintering conditions are such that desintering, bloating, weathering and nitriding are avoided. Some insights that have been gained in powder preparation, pressing and sintering are presented in this paper. The relationship between powder characteristics and pressing and sintering properties is described. 相似文献
8.
Solid state reaction, sintering and Pb removal activity of hydroxyapatite/zirconia layered composite
In order to form a layered hydroxyapatite/zirconia ceramic, the solid state reaction and sintering were examined by the three processes of powder mixture, dry-pressing compaction and tape cast. The solid state reaction between hydroxyapatite and zirconia occurred in the thin width of 10–50 m at interface in a layered composite body. In both sintered layer composites from dry compaction and tape cast, the significant deformation of composite bodies was observed, depending on sintering temperatures. By selecting a sintering temperature of 1200°C, we fabricated a layer ceramic composite of hydroxyapatite/zirconia exhibiting the flat film shape. The tape cast process was useful to form a porous sintered composite of hydroxyapatite and zirconia. The porous composite showed the removal performance of aqueous lead from wastewater. 相似文献
9.
Shock compaction features are experimentally investigated for some selected ceramic materials and ceramic composite systems, i.e. SiC, AIN, SiC/AIN, and AIN/Al2O3. A typical microstructure to support the skin model proposed in a previous paper is obtained by using SiC powder with a particle diameter of several micrometres. AIN transforms into an unknown phase. The transformation needs a small amount of oxygen. The amount of the unknown new phase decreases with the increase of shock temperature. Chemical reaction of AIN/Al2O3 mixture into ALON (cubic aluminium oxynitride spinel) occurs under shock loading and proceeds along with increasing shock temperature. Crystallite size and microstrain of the samples are determined by X-ray line broadening analysis. The microstrain for mixture sample and fine powder is larger than that for single-component sample and coarse powder, respectively. As suggested by the skin model, the requirement that the initial particle size is less than 1 m is essential for ceramic powder to be consolidated by a shock compaction technique in order to yield good compacts of optimum strength and to achieve chemical reaction accompanying mass transport. 相似文献
10.
W. Tillmann M. Gathen Prof. E. Vogli C. Ullrich 《Materialwissenschaft und Werkstofftechnik》2007,38(6):473-480
Novel encapsulation technique for diamond composites using PVD‐process For machining of mineral materials diamond tools consisting of a steel body combined with diamond impregnated segments are used. Frequently, these segments are hot pressed. Other process routes are pressureless sintering of green compacts partly combined with hot isostatic pressing and hot isostatic pressing of encapsulated powder mixtures. The compaction effect of hot isostatic pressing require a low porosity of sintered components realized by using ultra‐fine metal powder or an impermeable capsule made of metal or glass. The Institute of Materials Engineering pursues a novel process route by physical vapor deposition of a coating on pressureless sintered composites. The thin coating acts as a capsule and guarantees the pressure transfer in the following hot isostatic pressing process. Although bronze powders with particle sizes up to 90 μm are used, the manufacturing of diamond composites with low porosities is possible. In comparison to conventional encapsulation‐techniques the main advantages of this novel process route are the use of comparatively coarse metal powders and a larger geometric flexibility. 相似文献
11.
Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al2O3+22 wt% SiO2 (high-Al2O3 material) and 72 wt% Al2O3+28 wt% SiO2 (low-Al2O3 material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al2O3 materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al2O3 compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al2O3/SiO2 ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al2O3 were produced from high-Al2O3 powders calcined between 600 and 1100 C. Low-Al2O3 sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al2O3 materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al2O3 compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries. 相似文献
12.
Linear relationships between mean strength and nitrogen weight gain are established for isostatically pressed silicon compacts nitrided to weight gains of less than 60%. For a particular silicon powder the relationship depends upon the isostatic pressure used in compact fabrication, i.e. the green density. A linear relationship between mean strength and nitrided density is also demonstrated and this is independent of green density for the particular compacts studied. The implications of these relationships are discussed and their potential value for developing high strength reaction sintered silicon nitride explained. 相似文献
13.
Pure nanocrystalline -alumina powders were coated with different fractions (5, 10, and 15 vol%) of SiO2-SrO glass using the sol-gel technique. The isostatically cold pressed powders were pressureless sintered in air for 5 h in the temperature range of 1250°C to 1550°C. The relative densities were ranged between 60 to 90% of the theoretical and were composition dependent. The density was increased with the sintering temperature. In pure alumina, the to phase transformation went to completion by sintering at 1250°C. However, in the glass-coated samples, transition -alumina was mostly retained after sintering at the same temperature. Pure nanocrystalline alumina sintered at 1350°C exhibited vermicular structure with isolated pores. The microstructure of the low glass-containing samples exhibited nanocrystalline to submicron size grains arranged in platelet-shaped clusters. Samples with higher glass contents exhibited also micron-size needle-shape grains of strontium aluminate. 相似文献
14.
D. J. Curran T. J. Fleming G. Kawachi C. Ohtsuki M. R. Towler 《Journal of materials science. Materials in medicine》2009,20(11):2235-2241
Hydrothermal treatment is traditionally employed to improve the sinterability of powder compacts by reducing porosity and
increasing apparent density. The effect of hydrothermal treatment on green powder compacts has been assessed in order to better
understand how treatment may affect the sinterability of the bodies. Laboratory synthesised nano sized hydroxyapatite (HA)
and a commercial zirconia (ZrO2) powder have been ball milled together to create composite mixtures containing 0–5 wt% ZrO2 loadings. Disc shaped bodies have been formed using uniaxial and subsequent isostatic pressure. The resultant coherent samples
were subjected to hydrothermal treatment at either 120 or 250°C for 10 h in order to assess the effect of this processing
technique on the physical, mechanical and microstructural properties of the green composites. ZrO2 loadings up to 3 wt% increased apparent density from 90 to 92%, whereas increased loading to 5 wt% increased flexural strength,
from 6 to 9 MPa. Increasing the hydrothermal treatment temperature increased open porosity, from ~44 to ~48% and reduced biaxial
flexural strengths of the treated bodies compared to those of their room temperature isostatically pressed counterparts (~10
to ~6 MPa). 相似文献
15.
V. T. Dovgii A. I. Linnik S. Yu. Prilipko G. Ya. Akimov Yu. F. Revenko A. A. Novokhatskaya T. A. Linnik N. V. Davydeiko V. N. Spiridonov A. V. Zhebel’ 《Inorganic Materials》2011,47(11):1253-1257
Phase-pure La0.52Sr0.28Mn1.2O3 manganite nanopowders with average crystallite sizes of 30, 60, and 200 nm have been synthesized using coprecipitation and multiple cold isostatic pressing at 1 GPa. The crystallite size is shown to have a significant effect on the electrical and magnetic properties of the nanopowders: with decreasing particle size, their resistivity rises by several orders of magnitude, their Curie temperature decreases significantly, and the peak in their magnetic susceptibility broadens. The electrical and magnetic properties of powder compacts are compared to those of ceramic samples. The powder compacts show conventional magnetic hysteresis behavior, whereas the ceramics produced by sintering the compacts at 1270 K have an anomalous hysteresis. A mechanism is proposed that accounts for the anomalous hysteresis behavior. 相似文献
16.
Y. J. He A. J. A. Winnubst H. Verweij A. J. Burggraaf 《Journal of Materials Science》1994,29(24):6505-6512
Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 m and a zirconia grain size of 0.2 m. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the alumina phase stage shifted to lower temperature. During pressureless sintering the to alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature. 相似文献
17.
S. M. Wiederhorn B. J. Hockey D. C. Cranmer R. Yeckley 《Journal of Materials Science》1993,28(2):445-453
Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y2O3 as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined. 相似文献
18.
Preparation of Porous Ceramics from Nanocrystalline Zirconia and Their Microstructure 总被引:1,自引:0,他引:1
D. S. Nikitin V. A. Zhukov V. V. Perkov S. P. Buyakova S. N. Kul'kov 《Inorganic Materials》2004,40(7):760-763
Data are presented on the compaction behavior of nanocrystalline yttria partially stabilized zirconia powder and the effects of compaction pressure, sintering temperature, and sintering time on the microstructure of the resultant ceramics. It is shown that even relatively low (50 MPa) compaction pressures lead to the disintegration of powder particles and aggregates. The compaction behavior of the powder points to changes in the densification mechanism: from quasi-liquid sliding of powder particles at the beginning of the process to the breakdown of large microstructural constituents at the end. In the initial stages of sintering, a robust skeleton forms, which plays a key role in determining the pore structure of the ceramic. 相似文献
19.
The paper presents an investigation of the influence of silver particles on the microstructure and mechanical properties of
barium titanate. Barium titanate-silver composites have been prepared by ball milling precursor powder constituents; followed
by drying, sieving and calcination prior to powder compaction. After sintering the green compacts, microstructural analysis
was undertaken involving measurement of grain size, silver particle size, phase composition and phase content. Characterisation
of mechanical strength, toughness, hardness and stiffness was also undertaken. Reaction product phases between silver and
barium titanate could not be detected. The dispersed silver particles were shown to inhibit densification. Silver particles
below 1 μm in size were intragranular and attached to domains. The size of the intergranular silver particles increased with
silver content. An increase in silver content improved whereas strength, hardness and stiffness decreased, while toughness
was unchanged. 相似文献
20.
《Materials Letters》2004,58(1-2):115-118
Low-cost steel porous structure materials have been prepared from hollow sphere mill-scale powders by using a simple method. Hollow spheres of mill-scale material were fabricated by coating polystyrene spheres, using pelletizer disc. The coated powders were mixed with 1 wt.% inorganic binder diluted in 4 wt.% water, and then were uniaxially pressed at 0, 100, 200 and 400 MPa in a die to produce rectangular shape compacts (30×15×15 mm), calcined at 550 °C for 30 min (to burn off the organic materials). After calcination, the compacts were then sintered in hydrogen atmosphere for 40 min at 1150 °C.The resulted steel porous materials have different relative densities varied from 32.3% to 50.89%, depending upon the previous compaction pressing before sintering. It is noted that the relative density increased with the increase of compaction pressing, whereas the porosity content decreased. Increase of properties, such as hardness, transverse rupture strength, and compression yield stress, occurred due the increase of compaction pressing of the compacts before sintering. 相似文献