Effects of organic binders on the sintering of isostatically compacted zirconia powders

Certain processing-related flaws in cold isostatically pressed ceramic powder compacts may arise from the delayed burn-out of organic binders until the sintering temperature is approached, although the isostatic compaction technique usually gives a higher and much more uniform green density than the conventional die compaction technique. For the 3 mol% Y₂O₃-doped zirconia powder in which 3 wt% PEG 1500 was introduced, the sintered density and sintering shrinkage were found to decrease in a near linear manner with increasing isostatic compaction pressure. The processing-related defects were identified as intergranular pores (1–5 m). It is considered that these processing-related defects are a consequence of incomplete organic burn-out at low and intermediate temperatures in the heating-up period and the swelling of intergranular pores associated with the burn-out of residual organic binders at temperatures close to the sintering temperature. A higher calcination temperature and an extended calcination dwell time may be required to eliminate the organic residuals in the isostatically pressed ceramic powder compacts than in the conventional die-pressed samples.     

Filter cake forming and hot isostatic pressing for TZP-dispersed hydroxyapatite composite

The combination of a filter cake forming process and hot isostatic pressing was applied to prepare hydroxyapatite composites containing dispersed tetragonal zirconia polycrystal (TZP) with high strength and toughness. Fine TZP powder was dispersed into as-synthesized hydroxyapatite slurry, formed with the filter cake process and hot isostatically pressed at 800–1150 °C at 100 MPa for 2 h. The temperature needed for densification increased with increasing TZP content; 1100 °C was needed to fully densify the composite with 26.8 wt% TZP. No phase change was found in TZP nor in the hydroxyapatite phase up to the maximum temperature examined in hot isostatic pressing. Significant phase change was found in specimens annealed in air at 1200 °C. The strength and toughness achieved were respectively 190 MPa and 2.3 MPa m^1/2. These values were approximately 20% and 100% higher than the corresponding values for hydroxyapatite ceramics without TZP particle dispersion.     

Microstructure and properties of hot isostatically pressed powder and extruded Ti25V15Cr2Al0·2C

Abstract

The 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.     

Microstructural evolution in nanocrystalline alumina containing silicate glasses

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.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

Sintering of Ti(C,N)-Based Cermets: The Role of Compaction

Different compaction processes were employed in studying the effect of green state on the sintering of TiC-TiN-Ni-Mo₂C and Ti(C_0.3N_0.)-Ni-Mo₂C cermets. Specimens were uniaxially pressed and some being subsequently compacted by cold isostatic pressing. Liquid and solid phase sintering was carried out in nitrogen atmosphere. Green state porosity, pore size distributions, as sintered morphologies, micro-hardness and microstructural development were characterized. Mercury porosimetry measurements indicate that with increasing compacting pressure, large pores are eliminated and the pore size distribution curves shift towards smaller pores. Higher compaction load also leads to decrease in average pore size and a wider pore size distribution. It was found that the Ti(CN)-based cermets are superior than the (TiC-TiN)-based cermet. Cermets with carbowax additives are better sintered than those without the wax. The two-stage consolidation (diepressing + CIP) results in better sintering properties than die-pressing alone.     

微量SiO2添加对Pr:Lu3Al5O12陶瓷光学及闪烁性能的影响

在制备透明陶瓷时, 广泛采用烧结助剂来提升陶瓷的光学质量。但烧结助剂的添加可能会恶化陶瓷的发光性能。本研究采用真空预烧结合热等静压烧结制备了0.25at%Pr:LuAG闪烁陶瓷, 研究了微量SiO₂烧结助剂对陶瓷光学及闪烁性能的影响。结果表明, 添加少于200 ppm的微量SiO₂(1 ppm表示添加量为1×10^-6 g/g)能有效促进热等静压过程中气孔的排出, 有效提升了Pr:LuAG陶瓷的光学性能。150 ppm SiO₂添加的Pr:LuAG陶瓷在400 nm处的直线透过率约为77%。同时研究了预烧温度及时间对Pr:LuAG陶瓷光学性能的影响。在实现完全闭气孔结构时, 进一步升高预烧温度或延长保温时间会降低热等静压过程中的致密化速率, 不利于气孔的排出, 从而降低了Pr:LuAG陶瓷的光学质量。此外, 添加微量SiO₂对Pr:LuAG陶瓷闪烁性能的影响较小。添加微量SiO₂结合热等静压烧结是制备Pr掺杂石榴石闪烁陶瓷的有效途径。     

Hot isostatic pressing of Si3N4 with Y2O3 additions

The effect of Y₂O₃ additive on the properties of hot isostatically pressed silicon nitride was studied. The influence of small additions of Y₂O₃ on the densification of silicon nitride was investigated. The density and elastic moduli of the product increase with increasing of the Y₂O₃ additions. The hot isostatically pressed pure silicon nitride consists of -Si₃N₄, -Si₃N₄and Si₂N₂O; phase content of the hot isostatically pressed silicon nitride with 10 wt % Y₂O₃addition consists of -Si₃N₄, yttrium silicate and Y₂Si₃O₃N₄. The effect of the outgassing of the specimens prior to hot isostatical pressing on the properties of the final material is discussed.     

Thermoelectric properties of n-type SbI3-doped Bi2Te2.85Se0.15 compound fabricated by hot pressing and hot extrusion

n-type 0.1 wt% SbI₃-doped Bi₂Te_2.85Se_0.15 compounds were fabricated by hot pressing and hot extrusion. The hot pressed compounds were densified up to 99.2% of theoretical density. The grains were preferentially oriented and contained many dislocations due to the hot pressing. The figure of merit of the compounds hot pressed at 420°C was 2.35 × 10_-3/K. On the other hand, the grains of the extruded compounds were small, equiaxed, (1.0 m) and contained many dislocations due to the dynamic recrystallization during the extrusion. The fine grains significantly improved the bending strength and figure of merit. The grains were also preferentially oriented through the extrusion. The bending strength and figure of merit were increased with increasing extrusion temperature. The figure of merit of the compounds hot extruded at 440°C was 2.62 × 10_-3/K.     

Effect of fluorine content on mechanical properties of sintered fluoridated hydroxyapatite

The effect of fluorine content, preparing method, and sintering temperature on both the bulk density and biaxial flexural strength of sintered fluoridated hydroxyapatite (FHA) was studied. Both uniaxially pressed un-milled (UPU) and cold isostatically pressed milled (IPM) FHA discs were sintered at temperatures between 1200∼1400 °C at an interval of 100 °C. It was found that the fluorine content had significant impact on the sintering behavior, densification, and mechanical properties of FHA discs. At a low fluorine content, the specimens decomposed between 1200–1400 °C, which resulted in a relatively low sintered density and biaxial flexural strength of the specimens. In comparison, the specimens with high fluorine content did not decompose even at the sintering temperature of 1400 °C. The sintered density and biaxial flexural strength of these specimens increased with the sintering temperature. The maximum sintered density and biaxial flexural strength of the UPU specimens were only 85% of the theoretical density and 45 MPa, respectively. In contrast, much higher sintered density (∼95% of the theoretical density) and mechanical strength (∼100 MPa) were attained for the specimens subjected to the cold isostatic pressing and ball milling. A close correlation between the sintered density and biaxial flexural strength of the specimens was revealed, where the biaxial flexural strength increased exponentially with the sintered density.     

Interfacial precipitation in hot isostatically pressed diffusion bonds of 17-4 PH stainless steel

Abstract

The embrittlement of hot isostatically pressed (hipped) diffusion bonds manufactured from 17-4 PH stainless steel has been investigated by Auger electron spectroscopy (AES) of in situ fracture specimens. Depth profiling by AES has revealed copper precipitation at the interface of the diffusion bond. This precipitation, up to a few monolayers in thickness, occurs during the ramp up to temperature and pressure of the hot isostatic pressing (hipping) cycle and is not readily removed by subsequent heat treatment. This effect is explained in terms of the metallurgical characteristics of copper within the steel. Results suggest that the extent of the precipitation decreases with increasing process temperature. In the case of PH 13-8 Mo stainless steel, where the precipitation hardening phase is NiAl, the interface is weakened by sulphur segregation and the formation of oxide particles.     

Transparent Lu2O3:Eu ceramics by sinter and HIP optimization

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.     

The mechanical properties of pressed processed wheat material

Breakfast wheatflake materials, produced by two methods, were milled and different sieve fractions reconstituted by hot pressing into bar-shaped test pieces, to reduce the geometry and structure effects of flakes. The stiffness and fracture properties of these pressed bars of different particle size in ranges <0.5 mm, 0.5–1 mm, 1–1.4 mm, 1.4–2 mm and of different water content were compared. Dynamic mechanical thermal analysis showed that the bending modulus, E, superimposed as a function of temperature in the range -40 to 140 °C. The value of E at 20 °C decreased with increasing water content corresponding to depression of the glass transition temperature. Microscopy of the test pieces revealed that starch was the continuous phase. The stiffness properties were similar in many respects to data published for pressed starch specimens. However, the energy to break samples at 7% water content (wet weight basis) was greater when a range of particle sizes was used compared to the results of narrow particle size ranges. This is consistent with published results on fracture toughness of particulate compacts. The energy to break samples increased with increasing water content.     

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from -to -alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.     

Fracture mechanisms and mechanics of an 18-4-1 high speed steel

Cast, wrought, and directly sintered smooth and precracked beam specimens of BT1 steels were studied in three- and four-point bending at room temperature. Following austenitization at 1250° C and tempering between 500 and 560° C, brittle fracture strengths varied between 1.1 and 2.8 GN m^–2 and the fracture toughness of the materials was in the range 18 to 25 MN m^–3/2. Combining these data, the critical Griffith-Irwin flaw sizes were calculated to be typically of the order of 100 m. This is in reasonable agreement with the observed sizes of some failure-initiating sites, particularly pores in sintered material, but generally several times larger than the carbide and grain sizes. In wrought specimens, failure frequently originated from groups of carbides, apparently fracturing on contiguous planes. No evidence of sub-critical cracking of carbides was detected (as in BT42), in contrast to BM2, BT6 and sintered and hot isostatically pressed BT1. Only inter-powder particle parting occurred in this sintered material. Conventional fracture mechanics thus successfully interprets results on sintered specimens, but only on several of the wrought specimens. For the majority of the latter it appears necessary to invoke operation of propagation mechanisms involving short, 10 m, cracks under monotonic loading or to associate the brittle fracture stress with that for crack nucleation: e.g. cleavage of a carbide cluster.     

Microstructurally controlled mullite ceramics produced from monophasic and diphasic sol-derived pastes using extrusion

Mullite ceramics with controlled microstructure in terms of grain size/shape, pore and glassy phase content were produced from sol-derived pastes using extrusion. Particular attention has been given to the development of a continuous process which is suitable for the preparation of high-solids-loading mullite pastes from two different starting mullite precursors, namely, diphasic and molecular mixed mullite sols. A combined processing technique comprising vacuum filtering and pressure filtration was introduced in order to obtain extrudable mullite pastes from low solids-loading colloidal sols. It is shown that glassy phase free stoichiometric 3:2 mullite (3Al₂O₃·2SiO₂) with fine (0.94 m) equiaxed grain microstructure is achievable from monophasic precursors after pressureless sintering at 1400°C for 3 h using the developed technique which can control both the sol-derived paste microstructure and process parameters. It is also found that the room and high temperature (1300°C) flexural strength and toughness of extruded mullites are mainly controlled by the grain size, the presence and location of glassy phase, nano-inclusions and pores at the grain boundaries. Pressureless sintered mullite derived from the monophasic sol-derived pastes provides flexural strength values of 345 and 277 M Pa for room temperature and 1300°C, respectively.     

Sintering behaviour of gel-derived powders

85Al₂O₃-15ZrO₂ (wt%) powders were synthesized by gel precipitation starting from AlCl₃ · 6H₂O and ZrCl₄ solutions and dried by two different methods: (i) by spray-drying and (ii) via a sol-gel route by n-octanol in a pilot plant. The particles by process (i) were spherical granules of diameter 15 m, and those by process (ii) were microspheres of 25 m. The powders were characterized in terms of morphology, particle size distribution, surface area, weight loss and crystallization behaviour at different temperatures. Shrinkage, microstructure, density and pore size distribution were evaluated on compacts at different temperatures to study the sintering kinetics. Experimental observations suggest that (a) the use of controlled-geometry powders allows one to obtain high green densities; and (b) the sintering of gel-derived powders develops in two steps: during the first one, sintering mainly takes place inside each microsphere (or granule), and during the second step, mainly between the microspheres (or granules). At temperatures > 1100°C, sintering produces shrinkage of the microspheres (or granules) leading to pore formation between them, which prevents the achievement of high densities. Only by using hot pressing is it possible to obtain theoretical densities and high mechanical properties.     

Effect of green density and electric field direction on densification of YAG nano-powders by spark plasma sintering

Nanocrystalline YAG powders with 34-nm average particle size were cold isostatic pressed (CIP) between 10 and 240 MPa to form compacts with 43–52% green densities. Spark plasma sintering (SPS) was performed for 5 min at 1,400 °C and 90 MPa using clusters of three disc specimens packed in series with the electric field direction. The pore size and its volume fraction decreased with the increase in the CIP pressure. The final density varied between 86% and 99% and strongly depended on the specimen location in the series. Density, grain size, and pore size analyses showed that the specimens facing the electric current experienced higher SPS temperatures. Possible temperature gradient due to axial displacement of non-conducting oxide was too low to explain the observed effect. Effect of the green density on the final density was marginal.     

Strength characterization of yttria-partially stabilized zirconia/alumina composite

The flexural strength of yttria-partially stabilized zirconia/alumina composite in the sintered and hot isostatically pressed condition (Super PSZ) was evaluated as a function of temperature (20–1300°C in air environment), applied stress and time. Failure was essentially governed by the presence of processing defects such as zirconia or alumina agglomerates. The sudden decrease in fracture strength at relatively low temperatures (400–600°C) is believed to be due to the stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 300–1000°C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000°C at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Liquid phase sintering of bimodal size distributed alumina powder mixtures

Fine and coarse alumina powder mixtures (non-additive specimen) and those containing the additive formed liquid phase during firing (additive specimen) were compacted and fired at 1400–1600°C. Liquid phase sintering proceeded markedly at 1400–1500°C and additive specimens had much higher relative density than non-additive specimens at 1500°C. As the liquid phase sintering proceeded, the open pore volume decreased abruptly, but the open pore size changed depending on the packing structure. The open pore size decreased in the specimens where the fine particles formed matrix structure, while it increased in the specimens where the coarse particles formed skeletal structure. At 1600°C all additive specimens having different mixing ratios of fine and coarse powders had similar microstructure and the same relative density of 97%. However, spherical large pores were formed and remained in all additive specimens even at 1600°C. The bending strength of those specimens was about 400 MPa.