Micro-Raman spectroscopy of indentation induced phase transformation in nanozirconia ceramics

Micro-Raman spectroscopy has been employed as an effective technique to determine the phase transformations in nanostructured yttria stabilised zirconia (YSZ) ceramics with different yttria content. Samples have been prepared with varying mean grain sizes by a slip casting route followed by a microwave assisted two-step sintering cycle starting with aqueous nanozirconia suspensions. Indents were generated using a Vickers pyramidal indenter at different loads and the resulting phase transformations were mapped using micro-Raman spectroscopy. The results were compared to those of a commercial submicron 3YSZ. The amount of transformation was found to be much lower for nanozirconia compared to the submicron zirconia with similar yttria content

Influence of laser beam brightness during surface treatment of a ZrO2 engineering ceramic

A comparative study between fibre and Nd:YAG (neodymium, yttrium, aluminium, garnet) laser surface treatment on a cold isostatic pressed (CIP) ZrO2 engineering ceramic was conducted to investigate the individual differences of laser brightness (radiance) produced by the two laser sources. The effects of brightness exhibited by the two lasers were investigated in respect to the change in the hardness, dimensional size of the laser radiated zones and the microstructure of the ZrO2 engineering ceramic. The results showed that the hardness of the ZrO2 engineering ceramic was reduced by 36% for the Nd:YAG laser in comparison to the as-received surface. However, only 4% reduction in the surface hardness was found from employing the fibre laser surface treatment which was not significant as much as the results of the Nd:YAG laser radiation. The change in hardness occurred due to softening of the laser radiated surface of the ZrO2 with a changed composition which was softer than the laser unaffected surface. The dimensional size of the fibre laser radiated track was also found to produce broader surface profiles in comparison to that of the Nd:YAG laser. The fibre laser radiated surface track was 32% larger in width and 51.5% longer in depth of penetration in comparison to that of the Nd:YAG laser. Change in microstructure of the ZrO2 engineering ceramic radiated by both lasers was found as opposed to the ground and polished untreated surface with the fibre laser affecting the grain morphology to a greater extent in comparison to that of the Nd:YAG laser radiation. The physical and micro-structural effects from applying the two laser types to the ZrO2 engineering ceramic differed as deep penetration and broader laser radiated track as well as larger grains were produced by the fibre laser, despite using identical laser processing parameters such as spot size, power density, traverse speed, gas flow rate, wavelength and the Gaussian beam profile. This occurred due to the high brightness exhibited by the fibre laser radiation which generated larger power per unit area which in turn induced into the ZrO2 engineering ceramic and resulted to producing high processing temperature, larger fibre laser-ceramic-interaction zone and melt-pool at the laser-ZrO2 interface in comparison to that of the Nd:YAG laser which intrinsically resulted to a change in physical attributes of the ceramic

Compositional effects in nanostructured yttria partially stabilised zirconia

There is a considerable current interest in learning how to process genuinely nanostructured ceramics as they offer the potential for significantly enhanced properties; however, it is often difficult to make large enough components to allow more than the most basic of property measurements. In this work, densified components measuring up to 50 mm in size have been produced and a number of very interesting properties have been measured and demonstrated

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB2 ultra-high temperature ceramic (UHTC) particulate filled Cf/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc-jet testing of the UHTC composites highlighted the excellent ultra-high temperature oxidation performance of these materials

UHTC composites for hypersonic applications

A dream for many scientists, engineers and sci-fi enthusiasts is of an aerospace vehicle that can take off from an airport, fly through the atmosphere and travel to the other side of the earth at hypersonic speeds, and then return through the atmosphere to the same or another airport. Thanks to programs like DARPA’s Falcon Hypersonic Technology Vehicle 2 program (Figure 1), the dream is taking form

Hydrothermal aging behaviour of nanocrystalline Y-TZP ceramics

The influence of grain size and yttria content on the hydrothermal aging behavior of yttria-stabilized zirconia ceramics has been investigated via the preparation of genuine, fully dense nanozirconia ceramics with <100 nm grain size by two-step sintering. The samples were aged at different temperatures for varying periods of time and XRD and micro-Raman spectroscopy have been used to monitor the aging process while the hardness of the samples was used as a measure of the deterioration in the physical properties. The results were compared with those of a commercial submicrometer 3YSZ zirconia sample subjected to the same aging treatment and characterization. Nano3YSZ with <100 nm grain size did not exhibit degradation, whereas the submicrometer 3YSZ samples underwent severe degradation

Dense nanostructured zirconia by two stage conventional/hybrid microwave sintering

The processing of 3 mol% yttria-partially stabilised zirconia nanopowder into components has been investigated via slip casting low viscosity but high solids content nanosuspensions and subsequent pressureless sintering via one and two stage sintering involving both pure conventional heating and hybrid conventional-microwave heating. Very homogeneous and uniform green bodies with densities up to ~54% of theoretical could be produced, the major limitation being cracking on drying when the highest solid content suspensions were used. This could be partially overcome via the use of humidity drying. The pressureless sintering of the bodies revealed that the two stage sintering process allows a much finer average grain size to be retained than conventional single stage firing, whilst the use of hybrid heating gave further improvements. Greater than 99% dense ceramics with average grain sizes of ~65 nm could be produced from a powder with an average particle size of ~16 nm

Heat flux mapping of oxyacetylene flames and their use to characterise Cf-HfB2 composites

Cost effective and fast ultra-high-temperature testing methods such as oxyacetylene torch testing are extremely useful for the rapid screening of ultra-high-temperature ceramic (UHTC) materials for hypersonic applications. There is no report in the literature, however, of an organised study to measure the heat flux and how it varies with gas flow rate, gas flow ratio and distance from the nozzle tip for an oxyacetylene flame. In this paper, the authors report for the first time the heat flux mapping of an oxidising, neutral and reducing flame. The measured heat flux was as high as ∼17 MW m-2 at a distance of 10 mm from the nozzle, which is much higher than that previously reported in the literature. Torch testing was carried out for Cf-HfB2 UHTC composites at this heat flux and the results are presented along with detailed microstructural characterisation

Nano-crystalline ultra high temperature HfB2 and HfC powders and coatings using a sol-gel approach

Nano-crystalline HfB2 and HfC powders have been synthesized through a simple sol-gel route by using inorganic precursors like hafnium (IV) chloride (HfCl4), boric acid (H3BO3) and phenolic resin as a source of hafnium, boron and carbon respectively. The resulting HfB2 powders had an average crystallite size of ~35 nm whilst the HfC powders were ~75 nm in diameter. The precursor gels of HfC and HfB2 were also used to dip coat SiC fibre bundles, on heat treatment, a continuous coating of HfC and HfB2 was obtained. The wettability of the gels was determined using contact angle measurements. The continuity of the coatings on the SiC fibre bundles were characterized using optical and scanning electron microscopy

Screw dislocation assisted spontaneous growth of HfB2 tubes and rods

The mechanism of anisotropic growth of HfB2 rods has been discussed in this study. HfB2 powder has been synthesized via a sol–gel-based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron respectively, though a small number of comparative experiments involved amorphous boron as the boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of anisotropic growth of HfB2 has been investigated. It is hypothesized that imperfect oriented attachment of finer HfB2 particles results in screw dislocations in the coarser particles. The screw dislocation facilitates dislocation-driven growth of particles into anisotropic HfB2 rods