Effect of diopside addition on sintering and mechanical properties of alumina

In this paper, diopside was introduced in alumina as a sintering aid and fine structural alumina matrix ceramic materials were fabricated by pressureless sintering. The relative density, hardness, fracture toughness and bending strength of the new fabricated composites were measured. Tribological tests were carried out at a given rotation speed of 160 rpm and in a normal load ranged from 50 to 200 N. The experiment results show that the introduction of diopside can enhance densification rate, which may contribute to the improvement in mechanical properties and result in enhanced wear resistances. The effects of diopside on mechanical properties and microstructures of fine structural alumina matrix ceramic materials were analyzed and discussed.     

Fabrication of ultrafine-grained alumina ceramics by two different fast sintering methods

The preparation of ultrafine-grained alumina ceramics by the fast sintering technique Self-propagating High-temperature Synthesis plus Quick Pressing (SHS-QP) method and spark plasma sintering (SPS) technique was reported. The effects of different heating rates (SHS-QP-1600 °C/min, SPS-200 °C/min) on the preparation of ultrafine structure were compared. The densification and grain growth as a function of sintering time and temperature were discussed. Within a short sintering time (<3 min), the full-dense alumina with ultrafine-grained structure was obtained by SHS-QP at 1550 °C under 100 MPa. By SPS, the sintering temperature was lower (1200 °C) than that of SHS-QP. The differences in densification parameters were explained by analyzing the thermodynamics of sintering process.     

Enhanced properties of pure alumina ceramics by oscillatory pressure sintering

A novel oscillatory pressure sintering (OPS) process to consolidate high-quality pure alumina ceramics is reported. The microstructure of the ceramics prepared by OPS develops into a higher final density, a smaller and a narrower distribution of grain sizes compared with those prepared by conventional pressureless sintering (PS) and hot-pressing (HP) processes. Enhanced mechanical properties of alumina ceramics were investigated by OPS process. The bending strength, hardness and elastic modulus of the OPS specimen reached about 546 MPa, 19.1 GPa and 374 GPa, respectively, i.e values significantly higher than that of the specimens by PS and HP. XRD analysis indicates the strengthening of atomic bonds aided by oscillatory pressure. The results suggest OPS to be an effective technique for preparing high-quality pure alumina ceramics.     

Effect of carbon contamination on the sintering of alumina ceramics

The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.     

复合烧结剂对高铝质干式料烧结性能的影响

以粒度3～5mm,1～3mm,≤1mm,≤0.088mm的高铝矾土为主要原料,以粒度≤0.1mm的硼酸(H3BO3>99.6%)、粒度≤0.05mm的粘土、粒度≤0.1mm的钾长石和粒度≤0.1μm的硅灰(SiO2>90%)为复合烧结剂,按m(骨料)∶m(细粉)=65∶35的配比配料。将混合料在陶瓷模具中手工捣打成型,将成型好的试样分别在600℃、700℃、800℃、900℃和1000℃下均保温2h后脱模。测量热处理后各试样的耐压强度和显气孔率;采用XRD分析了试样的物相组成。结果表明:复合烧结剂中钾长石和硼酸在中温、低温下具有良好的烧结作用,在700～800℃热处理后,试样耐压强度和显气孔率明显增加。硼酸含量为2%的试样,在800～1000℃热处理后,显气孔率增幅较大。添加硅灰可以降低钾长石烧结温度;而复合烧结剂中的粘土在中温、低温下不利于干式料的烧结,低于800℃热处理后的试样,耐压强度和显气孔率没有随粘土含量增加而变化;900～1000℃热处理后的试样,耐压强度随粘土含量的增加而降低,显气孔率增加不大。     

Impact of microwave sintering on dielectric properties of screen printed Ba0.6Sr0.4TiO3 thick films

The influence of 30 GHz microwave sintering compared to conventional sintering has been investigated on polycrystalline Ba_0.6Sr_0.4TiO₃ (BST60) thick films with respect to an application as tunable dielectrics. The BST thick films were prepared as metal–insulator–metal (MIM) capacitors on alumina substrates. The average grain size (440 nm) and the porosity (approx. 30%) of the sintered films are only little affected by the sintering method. However, permittivity, dielectric loss and tunability have been influenced substantially. The dielectric improvement by microwave sintering is interpreted in terms of an increased crystal quality (ξ_S) and/or a decrease of defect concentrations. It is assumed that microwave sintering preferably heats up parts of the film where an increased defect density exists and therefore causes a selective heating process. This may heal up charged defects, inhomogeneities, and structural defects.     

Preparation of mullite from alumina/aluminum nitrate and kaolin clay through spark plasma sintering process

In the present study, the in-situ synthesized mullite has been prepared successfully by mixing kaolinite with alumina and aluminum nitrate nonahydrate (ANN) powders through high energy milling followed by spark plasma sintering (SPS). Using a high-energy ball-mill, the stoichiometric compositions of the starting powders, considering their final transformation to Al₂O₃ and SiO₂, have been mixed. The SPS process has been performed at 1400 and 1375?°C for the specimens containing Al₂O₃ and ANN, respectively. XRD patterns of the milled powders after 30?h showed the formation of quartz from kaolinite for both starting batches. The displacement-temperature-time (DTT) curves and the corresponded vacuum changes indicated the dehydration and phase transformation of ANN and kaolinite at different stages of the sintering process. The XRD patterns of the sintered samples revealed the formation of mullite alongside un-reacted Al₂O₃ and crystobalite for the batches containing Al₂O₃ and ANN, respectively. The results of the physical and mechanical properties tests showed higher amounts of bending strength (397?±?18?MPa), Vickers hardness (16.32?±?0.21?GPa) and fracture toughness (3.81?±?0.24?MPa?m^?1/2) alongside a lower porosity (0.070?±?0.02%) for the prepared sample containing Al₂O₃, than those of the sample containing ANN.     

Influence of processing atmosphere on the microstructural evolution of submicron alumina powder during sintering

Investigations into the sintering of submicron oxide powders have revealed interesting behavior, particularly insofar as it concerns their microstructural evolution in the early, low temperature transformations during heating. In this work, experiments were conducted on a submicron alumina powder, whose microstructural evolution and densification were characterized after sintering from 900 °C to 1400 °C in air, dry air and high vacuum (10⁻⁸ atm). The results indicated that the processing atmosphere strongly influences the particle size distribution at low temperatures before shrinkage occurs. Shrinkage began concomitantly with grain growth and the sintering atmosphere influenced the sintering kinetics. This factor, which is associated with previous narrowing of the particle size distribution, may affect grain growth and densification during the final stage of sintering.     

Effects of B4C addition on the microstructure and properties of porous alumina ceramics fabricated by direct selective laser sintering

Direct selective laser sintering (dSLS) is a promising method for the fabrication of complex-shaped ceramic parts. In this paper, boron carbide (B₄C) was used as an inorganic additive to improve the laser sintering behavior of alumina. The effects of B₄C addition on the microstructure and mechanical properties of porous alumina ceramics were investigated. Mixture of alumina powders and different amount of B₄C were directly sintered using different SLS parameters. Results indicated that the process window of alumina could be expanded by the addition of B₄C. Furthermore, the amount of B₄C played an important role in surface morphologies of alumina ceramics. It could be explained by the increase of mass transfer due to the addition of B₄C, which enhanced the densification process. The compressive strength of sintered samples increased with the increase of B₄C, which reached its maximum value when the content of B₄C was 7?wt% and the density of the samples after post treatment could reach 1.4?g/cm³. In addition, a size expansion phenomenon was observed. The size expansion could reach 5% after SLS, which could be attributed to the pin effects and oxidation behavior of B₄C particles.     

Effect of alumina addition on initial sintering of cubic ZrO2 (8YSZ)

The effect of 0–10 wt% alumina addition on the initial sintering of 8 mol% Y₂O₃ cubic ZrO₂ (8YSZ) was studied. Activation energy and initial stage of sintering mechanism were analyzed in order to understand the effect of the alumina in the sintering process. The analysis was carried out using the analytical method for constant rate heating (CRH). The activation energy decreased from 716 to 599 kJ/mol for undoped 8YSZ to 2.16 wt% of alumina–8YSZ, respectively. The mechanism for the initial stage of sintering for <2.16% Alumina–8YSZ changed from grain boundary diffusion (GBD) to volumetric diffusion (VD). With 10 wt% of alumina, the activation energy increased to 854 kJ/mol which was thought due to the change in the initial stage of sintering mechanism from VD to GBD.     

Permalloy/alumina soft magnetic composite compacts obtained by reaction of Al-permalloy with Fe2O3 nanoparticles upon spark plasma sintering

Composite sintered soft magnetic materials of permalloy/alumina type have been obtained by reactive spark plasma sintering. The composite compacts have been obtained by sintering of Ni71.25Fe23.75Al5 alloy with 3 and 5% (wt.) Fe₂O₃ nanoparticles. The Ni based alloy with large particles (up to hundreds of μm) have been covered by a thin layer of iron ferric oxide nanoparticles (20–40 nm). The as obtained composite particles have been subjected to sintering process using a homemade installation at 900 °C for 10 min. Upon sintering process several reactions between Ni-based alloy and iron oxide are induced, the main phase resulting from reaction is alumina-Al₂O₃ as it results by X-ray diffraction investigations. According to the scanning electron microscopy and energy dispersive X-ray spectroscopy investigations, alumina forms a matrix embedding the Ni-based particles. The alumina matrix is continuous, but the layer has large variation in width, and offers a high electrical resistivity. A mechanism of formation is proposed for the alumina matrix composite compacts when using Al-permalloy powder and iron oxide. The compacts have been tested in DC and AC for magnetic characteristics.     

A further report on finite element analysis of sintering deformation using densification data—Error estimation and constrained sintering

In a previous paper, Kiani et al. [Kiani, S., Pan, J., Yeomans, J. A., Barriere, M. B. and Blanchart, P., Finite element analysis of sintering deformation using densification data instead of a constitutive law. J. Eur. Ceram. Soc., 2007, 27, 2377–2383] proposed an empirical numerical method to calculate the sintering deformation of ceramic powder compacts without knowing the viscosities and sintering potential. The method was validated by free sintering experiments using specimens with non-uniform initial densities. Two new developments are reported in this paper: (a) a method of error estimation is developed which can be used to check if the empirical analysis is valid after the analysis; (b) a range of case studies are presented showing that the empirical solutions provide very good approximations to the solutions obtained using full constitutive laws not only for free sintering but also for highly constrained sintering of single- or multi-layered films.     

红柱石粒度对氧化铝纤维增强红柱石基复合材料烧结性能的影响

以南非红柱石和多晶氧化铝纤维为原料,在纤维加入量(w)分别为5%、10%、15%和20%,烧成温度分别为1350℃和1500℃的条件下,研究了红柱石原料粒度为0.2～101.5μm和0.1～34.7μm时对传统无压烧结工艺制备的氧化铝纤维增强红柱石基复合材料烧结性能的影响。结果表明:随着红柱石粒度的减小,基体材料的莫来石化温度和烧结温度明显降低,但纤维团聚现象加剧;由于纤维与基体界面结合力较强,纤维的增强作用以纤维的脱粘和断裂为主;在材料烧结后,红柱石粒度的变化对其常温耐压强度影响不大。     

Experimental based full process simulation of alumina selective laser processed parts densified by cold isostatic pressing and solid state sintering

The compound process of cold isostatic pressing (CIP) of alumina selective laser processed (SLP) parts and solid state sintering (SSS) and its full process simulation were realized in this paper, focusing on studying the overall deformation, relative density distribution, grain growth and sintering stress variation during the process. Especially, correlation was established between the macroscopic deformation and microscopic evolution. Model parameters for alumina are presented, which were optimized in accordance with the experimental results. CIPed part still exhibited density inhomogeneity, of which SSS tended to increase the overall density and homogenize density distribution. The sintering behavior was studied with the employment of dilatometer experiments. Furthermore, compared with conventional heating strategy, fast firing turned out to decrease sintering production time as well as drive the matter diffusion and densification process. The master sintering curve (MSC) moves upward a little under the condition of fast firing.     

Cyclic ablation of high-emissivity Sm-doped ZrB2/SiC coatings on alumina substrates

Samarium-doped ZrB₂/SiC (ZBS) coatings possess properties of high emissivity and excellent ablation performance suitable for hypersonic applications. Of interest in the current study is how cyclic ablation affects the scale development on alumina substrates. ZBS coatings with 3, 5 and 8 mol% of samarium (Sm) dopant were prepared via shrouded plasma spray onto alumina substrates and subjected to two 60-s ablation cycles with temperatures reaching up to 1700 °C. Blisters were observed on the Sm-doped coatings after the 1st cycle as a result of a local eutectic reaction between the ablation products and alumina substrate. A Sm-stabilized t-ZrO₂ phase was identified through X-ray diffraction after the ablation of the Sm-doped coatings. The ZBS with 5 mol% of Sm dopant produced a flower-like microstructure after the 2nd cycle due to the formation of convection cells.     

Effect of binder on the structure and mechanical properties of lightweight bubble alumina ceramic

The effect of binders such as ammonium aluminum sulphate, phosphoric acid and composite binder on the properties of lightweight bubble alumina ceramic was studied. The composite binder was composed of ammonium aluminum sulphate and phosphoric acid. Ammonium aluminum sulphate solution can improve compressive strength of alumina bubbles effectively but can not improve that of lightweight bubble alumina ceramic due to the fewer nano-alumina powders in situ decomposed of ammonium alumina sulphate. Trans-ball fractures occurred in thermal shock test. Phosphoric acid solution can improve compressive strength of alumina bubble ceramic because of promoting sintering properties of aluminum phosphate in situ produced by phosphoric acid and alumina component during sintering but decrease that of alumina bubbles. Along-ball fractures occurred in thermal shock test. The composite binder combined with the advantages of ammonium alumina sulphate and phosphoric acid and improved the compressive strength of both alumina bubbles and lightweight bubble alumina ceramic, and effectively reduce the amount of the binders and lower the product cost. At the sintering temperature of 1700 °C, with composite ammonium alumina sulphate and phosphoric acid as binder, the density of lightweight bubble alumina ceramic was between 1.20 and 1.60 g/cm³, and the compressive strength was 18-42 MPa.     

Improvement of Vickers hardness measurement on SWNT/Al2O3 composites consolidated by spark plasma sintering

Dense alumina composites with different carbon nanotube content were prepared by colloidal processing and consolidated by Spark Plasma Sintering (SPS). Single-wall carbon nanotubes (SWNTs) were distributed at grain boundaries and also into agglomerates homogeneously dispersed. Carrying out Vickers hardness tests on the cross-section surfaces instead of top (or bottom) surfaces has shown a noticeable increase in the reliability of the hardness measurements. This improvement has been mainly attributed to the different morphology of carbon nanotube agglomerates, which however does not seem to affect the Vickers hardness value. Composites with lower SWNT content maintain the Vickers hardness of monolithic alumina, whereas it significantly decreases for the rest of compositions. The decreasing trend with increasing SWNT content has been explained by the presence of higher SWNT quantities at grain boundaries. Based on the results obtained, a method for optimizing Vickers hardness tests performance on SWNT/Al₂O₃ composites sintered by SPS is proposed.