Dense Alumina Ceramics Research Articles

Investigation of mechanical properties based on grain growth and microstructure evolution of alumina ceramics during two step sintering process

Alumina ceramics having small grain size and high density yield good mechanical properties, which are required in most mechanical applications. Two Step Sintering (TSS) is used to develop dense alumina ceramics. In this research work the effect of sintering temperatures on microstructure and density of the alumina specimens developed by using TSS has been investigated. It has been observed that TSS is more efficient in controlling grain growth and increasing the density as compared to One Step Sintering (OSS) of alumina. Scanning electron micrographs of sintered alumina specimens have been compared. It has been observed that TSS proves to be a better technique for increasing density and controlling grain growth of alumina ceramics than OSS. More relative density, hardness, fracture toughness and small grain size was achieved by using TSS over OSS technique.

Transparent alumina ceramics densified by a combinational approach of spark plasma sintering and hot isostatic pressing

In order to increase the in-line transmission of fine transparent alumina in visible light the grain growth during sintering of alumina ceramics was supressed using a combined densification process. This process combines presintering of a green body by spark plasma sintering with final hot isostatic pressing. The presintering by spark plasma sintering provided bodies with a substantially smaller grain size than pressureless presintering. It is shown that the fine-grained presintered microstructure could be retained during final hot isostatic pressing and alumina ceramics doped with spinel and zirconia nanoparticles in particular could be sintered to full density with only minor grain growth during final hot isostatic pressing. The novel combined densification process enhanced by the unique nanoparticle doping approach provided fully dense alumina ceramics with an average grain size of 237nm and an in-line transmission of 76.2% at a wavelength of 632.8nm and a sample thickness of 0.8mm.

DLP 式光造形法によるアルミナ３次元積層造形緻密体の作製

DLP 式光造形法によるアルミナ３次元積層造形緻密体の作製

Additive Manufacturing of Dense Alumina Ceramics

In this study, a new process for additive manufacturing (AM) of dense and strong ceramic objects is described. The lithography‐based ceramic manufacturing (LCM) technique is based on the selective curing of a photosensitive slurry by a dynamic mask exposure process. The LCM technique is able to produce strong, dense and accurate alumina ceramics without virtually any geometrical limitations. With over 99.3% of a theoretical alumina density, four‐point bending strength of 427 MPa, and very smooth surfaces, the LCM process distinguishes itself from other AM techniques for ceramics and provides parts with very similar mechanical properties as conventionally formed alumina.

Direct coagulation casting of alumina using magnesium citrate as coagulating agent with glycerol diacetate as pH regulator

Direct coagulation casting of alumina suspension via high valence counter ions (DCC-HVCI) using magnesium citrate as coagulating agent with glycerol diacetate as pH regulator was investigated. Influence of concentrations of glycerol diacetate and magnesium citrate on pH and rheological properties of alumina suspension was investigated. High viscosity enough to coagulate the suspension is achieved by adding magnesium citrate and glycerol diacetate. Hydrolysis of glycerol diacetate shifts the suspension to lower pH which promotes the decomposition of magnesium citrate and contributes enough magnesium ions to coagulate the suspension. Coagulated samples with different shapes can be demolded without deformation by treating 55 vol% alumina suspension with 2 vol% glycerol diacetate and 0.5–1.2 wt% magnesium citrate at room temperature for 2.5–6 h. Dense alumina ceramics with relative density above 99.5 % can be prepared by DCC-HVCI using magnesium citrate as coagulating agent with glycerol diacetate as pH regulator sintered at 1550 °C for 2 h.

Colloidal Shaping of Alumina Ceramics by Thermally Induced Gelation of Methylcellulose

Dense alumina ceramics have been fabricated by the colloidal shaping of alumina slurry using methylcellulose as the networking additive. Concentration as low as 0.08 wt% of methylcellulose had been found to be sufficient enough to convert an alumina slurry of 74–75 wt% solid loading, to consolidated shapes by thermal gelation at 70°–80°C. Cast shapes could be sintered to dense alumina (99% TD) at temperatures in the range of 1500°–1525°C at a minimum heating rate of 180°C/h. The effect of methylcellulose concentration on the viscosity of suspensions was studied and green strengths of samples are investigated.

On the transparency of nanostructured alumina: Rayleigh-Gans model for anisotropic spheres

A light scattering model under the Rayleigh-Gans-Debye approximation has been developed for polycristalline alumina. The model states that transmittance of dense alumina ceramics basically depends not only on the maximum grain size but also on the preferential orientation of their c-axis, or texture. The effect of texture in transparency has been experimentally measured on several dense alumina samples with different grain size and compared to that obtained from x-ray Rietveld refinements with a very good agreement. The Rayleigh-Gans-Debye approximation also allows to represent optical data in a very simple way (logarithm of transmittance vs. the inverse of the wavelength square). Using these variables, a straight line is obtained for the Rayleigh-Gans-Debye approximation, its slope being proportional to the maximum grain size and textural parameter. Deviation from this law implies the presence of pores or grain of extremely large size.

Fabrication of submicron alumina ceramics by pulse electric current sintering using M 2+ (M = Mg, Ca, Ni)-doped alumina nanopowders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using M 2+(M: Mg, Ca, Ni)-doped alumina nanopowders at 1250 °C under a uniaxial pressure of 80 MPa. The M 2+-doped alumina nanopowders (0–0.10 mass%) were prepared through a new sol–gel route using high-purity polyhydroxoaluminum (PHA) and MCl 2 solutions as starting materials. The composite gels obtained were calcined at 900 °C and ground by planetary ball milling. The powders were re-calcined at 900 °C to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification and microstructural development depend on the M 2+ dopant species. Dense alumina ceramics (relative density ≥99.0%) thus obtained had a uniform microstructure composed of fine grains, where the average grain size developed for non-doped, Ni-doped, Mg-doped and Ca-doped samples was 0.67, 0.67, 0.47 and 0.30 μm, respectively, showing that Ca-doping is the most promising method for tailoring of nanocrystalline alumina ceramics.

The effect of powder forming method on the pull-out flaw populations observed on polished surfaces of alumina ceramics

Quantitative image analysis was used to study the effect of the powder forming method on the population of pull-out flaws for polished surfaces of dense alumina ceramics. By comparing the area fractions, the size distributions and the extreme values of the pull-out areas it was concluded that slip casting with dispersed slips resulted in significantly fewer and smaller pull-outs compared to slip casting with flocculated slips or powder pressing. The implications of these observations for the effect of the powder forming method on the strength of alumina ceramics are discussed.

Fabrication of Submicron Alumina Ceramics by Pulse Electric Current Sintering Using Mg2+-Doped Transition Alumina Powders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using Mg 2+ -doped transition alumina powders at 1200-1350°C under a uniaxial pressure of 40 or 80 MPa. The Mg 2+ -doped transition alumina powders (0-0.50 mass% MgO base) were prepared through a new sol-gel route using high-purity polyhydroxoaluminum (PHA) and MgCl 2 solutions as starting materials. The composite gels obtained were calcined at 900°C and ground by planetary ball-milling. Upon heating, the composite gels transformed into a single-phase γ-alumina or mixed phase of γ- and Χ-aluminas, depending on the MgO content. The resultant transition alumina powders were solid solutions, in which Mg 2+ cations were substituted into the crystal lattice. The powders were re-calcined to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification depended on the MgO content and loading pressure. The critical Mg 2+ -doping for suppressing grain growth was found to be 0.10 mass% MgO. Higher loading pressures led to full densification at lower temperatures, resulting in a more uniform and finer microstructure. Thus, dense alumina ceramics (relative density? 99.6%) with a uniform microstructure composed of fine grains with an average size of 0.47μm could be obtained by PECS at 1250°C under 80 MPa.

Solid freeform fabrication of alumina using laser-assisted ESAVD

Electrostatic spray assisted vapour deposition (ESAVD) has been used successfully to produce films and coatings. A combination of laser-assisted (LA) deposition and ESAVD has been developed for the direct fabrication of alumina ceramics. This novel laser-assisted ESAVD technique involves spraying a liquid precursor onto a substrate whilst a laser beam is simultaneously used to heat the precursor and cause the decomposition and chemical reactions of the precursor in order to produce a solid deposit. A desired three-dimensional (3-D) object can be formed through layer-by-layer deposition using a computer-controlled platform. Dense or porous alumina ceramics can be deposited on the substrate by optimizing the processing parameters. This paper reports the fabrication of alumina parts by the laser-assisted ESAVD method. The effect of processing parameters including laser scanning speed, laser beam size and laser power on the microstructure will be presented. Scanning electron microscope (SEM) and X-ray diffraction (XRD) have been used to characterise the phase and structure of the deposited alumina parts.

Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field

Highly crystalline-textured pure dense alumina ceramics were fabricated from spherical alumina powder without any seed particles and sintering additives by electrophoretic deposition (EPD) in a strong magnetic field of 10 T. The crystalline texture was confirmed by x-ray diffraction (XRD) for alumina ceramics deposited at 10 T followed by sintering at 1873 K. The angle between the directions of the magnetic and electric fields (φB-E) was altered to control the dominant crystal faces of the α-alumina monoliths. The average orientation angles estimated from the XRD diagram of the samples prepared at φB-E = 0°, 45°, and 90° were 16.52°, 45.15°, and 84.90°, respectively. Alumina/alumina laminar composites with different crystalline-oriented layers were also fabricated by alternately changing the φB-E layer by layer during EPD in a 10 T magnetic field. It was demonstrated that by using this technique, it is possible to control the crystalline orientation by changing the angle of E versus B during the EPD.

Microstructure and mechanical characteristics of laser-alloyed alumina ceramics

Commercially available alumina ceramic Al24 containing 5% porosity was alloyed by adding HfO 2 or mixtures of HfO 2 with 5 or 10 mol% SiO 2, CeO 2 or cordierite glass using infrared laser radiation. Thickness of the alloyed surfaces ranged between 500 and 800 μm depending on the composition of the alloyed powders and laser parameters used. The resulting microstructures and worn surfaces were analysed by electron microscopy. As a result of the alloying, the tendency of crack formation and grain growth was substantially reduced in the Al 2O 3 ceramic during laser treatment. The composite layers, showing no open porosity, contained 20–40 vol.% second phases, which were homogeneously distributed at the grain boundaries of the alumina matrix. Due to alloying, average grain size, hardness and Young's modulus of the composite layers were reduced, but resistance to cracking under the mechanical action of a sliding diamond (scratch test) abrasive wearing were substantially enhanced compared with the unmodified dense alumina ceramics Al23.

Alumina ceramics based on seeded boehmite and electrophoretic deposition

It is shown that seeding a commercial boehmite sol with crystallographically suitable seeds reduces both the crystallization temperature for the final α-A1 2O 3 phase and the sintering densification temperature. The seeding component was a tailored combination of δ- and α-alumina particles in nanometre and micrometer ranges, respectively, dispersed in water. Electrophoretic deposition (EPD) provided a quick, simple and cost-effective processing route to prepare dense monolithic alumina ceramics from the seeded boehmite suspensions. EPD-formed green bodies could be sintered and densified at temperatures as low as 1250 °C for 2 h.

Reaction-based Processing of Textured Alumina by Templated Grain Growth

Highly textured, dense alumina ceramics were fabricated by a new processing route which utilizes a mixture of Al metal powder, alumina powder, alumina platelet (template) particles and a liquid phase former. The process involves dry forming the powder mixture (e.g. uniaxial pressing, and roll compaction) to align the plate-like template particles. The addition of a calcium aluminosilicate glass reduces constrained densification by the template particles and allows attainment of high density at ∼1450°C. The degree of orientation (i.e. r is 1 for a random sample and 0 for a perfectly textured material) and volume fraction of textured material, f, were measured by X-ray-based rocking curve technique and SEM-based stereological analysis, respectively. It has been shown that texture quality (the r parameter) is controlled by initial strain during forming, sintering time and temperature. In addition, alumina ceramics with the volume fraction of textured material ranging from 1 to ∼100% can be obtained.

Alumina Ceramics Fabricated by Colloidal Processing

Alumina ceramics were fabricated using an inorganic particulate sol as a dispersing medium. High purity alumina was dispersed in boehmite sol and the interparticle potential in powder-sol medium was controlled by pH adjustment to obtain a well dispersed alumina suspension. The dispersion stability and suspension viscosity with varying pH in boehmite as well as water medium were, studied and presented. Alumina-boehmite slurry of more than 80 wt% solid content was achieved by controlled flocculation and green compacts were made through slurry compaction. Packing characteristics of alumina particles under slurry compaction and dry pressing were also studied and compared. Alumina slurry containing 14 wt% of boehmite has the maximum theoretical green density of about 63% at 14 MPa and the theoretical sintered density of about 96.9% at 1450°C. The microstructural features of sintered alumina show uniformly distributed grains of 2 μ size. Alumina after sintering has a reasonable hardness of 2500 kg.mm−2. Slurry compaction is found to be a novel method for producing high dense alumina ceramics.

Relationship between Grinding Mechanism and Residual Cracks in Single Point Grinding Test of Alumina Ceramics with Different Grain Sizes

In order to discuss the effect of grain size on material removal mechanism and crack type beneath the groove, the single point grinding tests of alumina ceramics with three kinds of grain sizes were carried out by use of conical diamond. The specimens of dense alumina ceramics were prepared by hot isostatic pressing (HIP) after presintering at various temperatures. The state of grooves were observed by SEM, and grinding force were measured by dynamometer. Main conclusions are obtained as follows: (1) Material removal mechanism changed from pseudo-plastic flow type to brittle fracture type with increase in depth of cut. The depth of cut, which varied from pseudo-plastic flow type to brittle fracture type, was increased with decrease in grain size. (2) The crack type varied from lateral and median type to microcrack with increase in grain size. (3) The normal grinding force was influenced by material removal mechanism of alumina ceramics.

The formation of NaMg2Al15O25 in an α-Al2O3 matrix and its effect on the mechanical properties of alumina

An in-situ sintering reaction was designed to produce lath-like \‴-alumina in an α-alumina matrix in order to make alumina ceramics stronger and tougher. The reaction sequence to produce \‴-alumina (NaMg2Al15O25) requires the formation of \-alumina (NaAl11O17) and spinel (MgAl2O4) at around 1100 dgC followed by a solid-state reaction of these two phases to give \‴-alumina at elevated temperatures; this reaction is complete at around 1600 °C. The in-situ sintering reaction produces near-theoretically dense alumina ceramics in which lath-like \‴-aluminas are homogeneously distributed. The bending strength and fracture toughness increase to 620 MPa and 5 MPam1/2, respectively; these increases are thought to be due to the suppression of grain growth as well as the crack deflection and bridging associated with lath-like \‴-alumina.