Rosa Maria da Rocha

Effect of TiO2 and TiB2 on Pressureless Sintering of B4C

This paper presents results of experiments on pressureless sintering of boron carbide (B4C) with addition of titânia (TiO2) and titanium diboride (TiB2). The TiB2 powder was added as a second phase and the TiO2 powder for reactive sintering and in-situ formation of TiB2. The final concentrations of TiB2 in the composites were 0 to 10 vol%. Sintering was performed at 2050 °C/30min in argon atmosphere. TiO2 was completely transformed into TiB2 with fine equiaxed grains distributed homogeneously. Composites obtained by in-situ reaction showed a densification increase with the concentration increase, while the composites with TiB2 powder mixture showed low densification in all compositions. Relative Density of the composite with 10 vol% of TiB2 obtained in-situ was 91% (TD) compared to 86 % for B4C only. Vickers hardness was about 29 GPa.

Sintering of B4C-SiC Powder Obtained In Situ by Carbothermal Reduction

Boron carbide (B4C) and silicon carbide (SiC) are materials with high hardness and low density what make them very useful in various applications, such armor plates. Hot press sintering of B4C-SiC composites was studied. The powder mixture was obtained in situ by carbothermal reduction using B2O3, SiO2 and carbon black as precursors. Seeds of commercial B4C and SiC were used to accelerate the synthesis reaction. Compositions were prepared to obtain powders after carbothermal reduction with 10, 30 and 50 wt% of SiC. Carbothermal reaction was conducted in argon atmosphere at temperatures up to 1700 °C. The synthesized powders were analyzed by DRX and SEM. Hot pressing at 1850 °C/30 min (20 MPa) in argon atmosphere was applied. Densities higher than 93% of theoretical density and microhardness of 34 GPa were achieved for hot pressed samples.

Composites Obtained from Alumina and Polymer Derived Ceramic

Alumina-mullite composites with low shrinkage can be made by reaction bond using mixtures of alumina, aluminum and silicon carbide. In this work, an alternative route is used to produce alumina composites with low shrinkage. Here alumina samples containing additions of 10 and 20 wt% of a preceramic polymer were warm-pressed and treated in the range of 900 -1500°C to produce alumina based composites. The obtained composites were analyzed by linear shrinkage and compared to pure alumina samples sintered at the same temperature range. It were also evaluated the density variation and crystalline phases formed during heat treatment of alumina composites. Results showed that alumina-silicon oxycarbide and alumina-mullite composites were obtained with lower shrinkage than pure alumina samples.

Effect of TiB2 on Pressureless Sintering and Hot Pressing of ZrB2

Zirconium diboride (ZrB2) is a material of particular interest because of the excellent and unique property combination of high melting point and high electrical and thermal conductivity. In this work, the effect of TiB2 addition on pressureless sintering and hot pressing sintering of ZrB2 was investigated. Four compositions were prepared with 0, 5, 10 and 20 wt% of TiB2. First, ZrB2 and TiB2 powders were milled by planetary mill with SiC spheres at for 4 h and then they were wet mixed. Compacted samples were pressureless sintered at 2150 oC/1h and hot pressed at 1850 °C/30min with 20 MPa, both in Ar atmosphere. The added TiB2 completely dissolved into the structure and formed a solid solution with ZrB2. Addition of TiB2 in ZrB2 ceramic improved densification and hardness for both sintering process, but hot pressed samples exhibited better results.

Processing of Aqueous Tape Casting of Glass-Ceramic in the SiO2-Al2O3-MgO-Li2O System with ZrO2 and B2O3 Addition

The aim of this work was to fabricate glass-ceramic substrates using aqueous tape casting technique of the parent glass powder in the SiO2-Al2O3-MgO-Li2O system with addition of ZrO2 and B2O3. The powder mixture was melted at 1650°C/1h and the obtained glass (frit) was milled and processed by tape casting in thin substrates with 700 μm thickness. Suspensions were prepared by aqueous medium and latex emulsion binder. Flexible green tapes with few defects were obtained using suspensions prepared with 7 and 10 wt% of binder in relation to glass powder. Substrates were submitted to burn out at 700 °C and thermal treatment at 950 oC/30min, 950 oC/4h and 1000 °C/1h. The green density of the substrates was around 1.55 g/cm3 and 2.10 g/cm3 after sintering/crystallization. Microstructure in all treatments showed high porosity. The major crystalline phases identified by XRD were Li0,6Al0,6Si2,4O6, ZrO2 and ZrSiO4.

Adhesive Selection for Lamellar Composites Alumina/Kevlar® for Impact Absorption

Alumina has been applied in systems of protection against high speed impact due mostly to the mechanical properties, low cost and ease of processing. A new approach to this application involves the manufacture of lamellar bioinspired composites of thin plates of alumina obtained by tape casting process, and fabrics of poly (p-aramid), Kevlar ®. The aim of this study is to investigate the interaction between the ceramic and polymeric fibers fabric observing the influence of different nature adhesives to select the most suitable for further preparation of bioinspired lamellar composite for impact protection applications. The strength of adhesion is evaluated by 90° Degree Peel Test and the interaction is investigated using techniques of optical microscopy, scanning electron microscopy (SEM) and spectroscopy with Fourier transform infrared (FT-IR) to characterize the surfaces of substrates after testing. Polyurethane aqueous based adhesive presented better interaction with both materials when compared to organic solvent based adhesive.

Hot Pressing Sintering of ZrB2 with β-SiC Addition

Zirconium diboride (ZrB2) is a material of particular interest because of the excellent and unique property combination of high melting point, high electrical and thermal conductivity. In this work, the effect of addition of beta-silicon carbide (β-SiC) on hot pressing sintering of ZrB2 was investigated. Four compositions were studied with 0, 10, 20 e 30 vol% of SiC. ZrB2 powder and mixtures were prepared by planetary milling with SiC spheres during 4 h. Samples were sintered at 1850 °C/1h with a pressure of 20 MPa in argon atmosphere. β-SiC has undergone phase transformation to α-SiC during sintering. The addition of SiC increased densification with increasing of SiC content. The total densification of sample was 96.8 % of theoretical density for sample with 30 vol% of SiC and Vickers hardness was 19.9 ± 0.3 GPa.

Pressureless Sintering of ZrB2 with β-SiC Addition

Zirconium diboride (ZrB2) is a covalent compound that leads the category of ultra high temperature ceramics materials owing to its unique properties. In this work, the effect of addition of beta-silicon carbide (β-SiC) in pressureless sintering of ZrB2 was investigated. Four compositions were prepared with 0, 10, 20 e 30 vol% of SiC. ZrB2 powder and mixtures were prepared in by planetary milling with SiC spheres at 4 h. Two sintering temperatures were used, one at 2050 oC/1h and other at 2150 °C/1h. The addition of SiC has promoted an increasing in densification with the increasing of SiC content. The total densification of sample sintered at 2050 oC was 90% of theoretical density for sample with 30 vol% of SiC, while the maximum densification for temperature of 2150 oC was 91,0 %TD.

Comparison of Different Alumina Powders for Aqueous Tape Casting Process

Tape casting process was used to produce Al2O3 substrates in an aqueous system with acrylic latex emulsion as binder. The present work studied the slurry formulations in aqueous medium of Al2O3 powders with different particle size distribution and made correlation to the green and sintered tapes. Two commercial alumina powders, one sub-micrometric and other micrometric were used. Compositions of Al2O3 slurries with 80 and 83 wt% of solids were prepared by dispersing the powders in water with a dispersant with subsequently additions of 7 and 10 wt% of binder. Sub-micrometric Al2O3 resulted in a high densification tapes regardless solid concentration and binder amount in the slurry, though green density was affected. For micrometric alumina, increasing the solid concentration resulted in a little higher final density.

Sintering of B4C-SiC Composites with ALN-Y2O3 Addition

One suitable material candidate to improve B4C mechanical properties is SiC. B4C-SiC ceramic composites are very promising armor materials because B4C and SiC are intrinsically very hard. In this work a pressureless sintering study of B4C-SiC ceramics was made. B4C-SiC mixtures were prepared with SiC concentration from 10 to 50 wt%. Without the external applied pressure during sintering it was necessary to add sintering aid. The additive system AlN-Y2O3 was investigated as sintering aid. Samples were densified by pressureless sintering at 2000 °C/30 min in an argon atmosphere. B4C-SiC composites were analyzed by XRD and SEM. Bulk density and total weight loss were also measured. Density higher than 93 % of the theoretical value was determined and microhardness of 30.3 GPa was achieved for composite with 10 wt% of SiC sintered with AlN-Y2O3 additive.