Rosa Maria Rocha

In this study, a hypersonic plasma setup was constructed based on a vortex plasma heater with prenozzle gas-dynamic insertion. The prenozzle allows the improvement of the characteristics of the vacuum system according to the necessities of the experiments. The plasma setup produces a hypersonic thermal flow, which is capable to test the thermal oxidation of ultrahigh temperature ceramics (UHTC) composites, such as zirconium diboride (ZrB2). Thereby, ZrB2 samples were prepared with a variation of 10, 20, and 30% of silicon carbide (SiC) in volume, in order to investigate the oxidation mechanisms and microstructural properties of the samples tested under hypersonic thermal flow. The results of the oxidation tests showed that the samples with 10 and 30% of SiC undergo to the active oxidation and forms an unstable and fragile ZrO2 oxide. The formed ZrO2 does not withstand the drag force and the thermal flux of the hypersonic plasma jet, partially volatilizing the oxide layer, causing an accentuated loss of mass. For the oxidation tests of the sample with 20% of SiC, the gain of mass was observed due to the formation of ZrSiO4 passivation layer, which is a stable oxide and promotes mechanical resistance, and low degradation rate. These results can be associated with the variation of SiC, which demonstrates an ideal proportion of 20% of SiC in ZrB2, which influences the oxidation mechanisms and produce a protective layer.

Zirconium diboride is an ultra high temperature ceramic material that leads this emerging class of materials because of its distinct combination of properties, including high melting temperature (> 3000 °C) and the lowest theoretical density (6.09 g·cm^-3) among the borides. This combination of properties makes ZrB₂ candidate for airframe leading edges on sharp-bodied reentry vehicles. In this work, the effect of particle size of ZrB₂ on the pressureless sintering of ZrB₂-SiC composites was studied, using ZrB₂ powder with average particle size of 2.6 and 14.2µm. Four different vol% concentration of ß-SiC (0, 10, 20 and 30 vol%) were added to as-received and planetary milled ZrB₂ powder. Samples were pressureless sintered at 2050 °C/1h in argon atmosphere. The reduction of initial ZrB₂ particle size led to composites with better results of densification, mechanical properties and oxidation resistance regardless ß-SiC addition, showing relative densities around 92.5 %Theoretical Density (Td) and flexural strength and microhardness around 260 MPa and 17.5 GPa, respectively. Composites processed with as-received ZrB₂ powder showed increasing in densification and flexural strength with the SiC content increasing. Relative density varied from 74.7 to 90.8 %TD and flexural strength from 102 to 241 MPa, for 0 and 30 vol% of SiC, respectively.

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.

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.

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.

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 B₂O_3. 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 Li_0,6Al_0,6Si_2,4O₆, ZrO₂ and ZrSiO₄.

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.

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.