Daniel Doni Jayaseelan

TEM investigation of hot pressed ‐10 vol.%SiC–ZrB2 composite

Abstract Abstract The polytypism of SiC, phase transformation of ZrB2 and the interfaces between SiC and ZrB2 were investigated using high resolution TEM in a hot pressed 10 vol.%SiC‐ZrB2 composite. In most cases, no grain boundary interphases between hexagonal ZrB2 and 6H‐SiC phases were observed with SiC being both inter‐ and intragranular. Occasionally, 6H‐SiC transformed into 3C and 15R and hexagonal ZrB2 transformed into cubic ZrB. High resolution TEM showed no grain boundary interphases in most regions. Energy dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy analyses showed the presence of oxygen throughout the sample. The phase transformation of SiC and ZrB2, and the interphase formation between SiC and ZrB2 grains are discussed.

Powder characteristics, sintering behavior and microstructure of sol-gel derived ZTA composites

Abstract A series of alumina/zirconia composites of varying compositions of zirconia were prepared through the sol–gel technique. Precursors were calcined at different temperatures ranging from 300 to 1400°C and sintered at 1530°C for 3 h. Compacts made from the powder calcined at 950°C yielded density up to >99% of theoretical density by pressureless sintering. Pore size distribution and the densification behavior were explained with respect to calcination temperature. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia grains in the alumina matrix. It is also observed that the faceted intergranular zirconia grains are at the grain junctions and the corners of the alumina matrix.

Influence of MgO on microstructure and properties of mullite–Mo composites fabricated by pulse electric current sintering

Abstract Mullite–Mo (10 vol.%) composites and monolithic mullite were fabricated using a pulse electric current sintering technique. Both monolith and composites of mullite were sintered up to theoretical density at 1500°C within few minutes. MgO of 0.25 wt.% was added as a sintering aid to both the mullite and composites. Addition of MgO significantly increased the bending strength of the monolithic mullite and mullite/10 vol.% Mo composites to 441 and 634 MPa respectively. The apparent increase in the bending strength of the composites was attributed to the combinational effect of Mo and MgO present in the composites. The fracture toughness of the composites also increased from 2 to 3.9 MPa.m 0.5 for the mullite/10 vol.% Mo composites, which was nearly twice that of the mullite. Crack-bridging and frontal process-zone elongation were expected to be the toughening mechanisms operated in these composites. The addition of Mo having high thermal diffusivity slightly increased the thermal diffusivity of the composites, because the 10 vol.% Mo particles were well dispersed and discontinuous in the matrix. Elongated mullite grains were observed for the composites without MgO, whereas the composites with MgO have a controlled microstructure.

Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system

TaC, HfC and their solid solutions are promising candidate materials for thermal protection structures in hypersonic vehicles because of their very high melting temperatures (>4000 K) among other properties. The melting temperatures of slightly hypostoichiometric TaC, HfC and three solid solution compositions (Ta1−xHfxC, with x = 0.8, 0.5 and 0.2) have long been identified as the highest known. In the current research, they were reassessed, for the first time in the last fifty years, using a laser heating technique. They were found to melt in the range of 4041–4232 K, with HfC having the highest and TaC the lowest. Spectral radiance of the hot samples was measured in situ, showing that the optical emissivity of these compounds plays a fundamental role in their heat balance. Independently, the results show that the melting point for HfC0.98, (4232 ± 84) K, is the highest recorded for any compound studied until now.

Comparison of Water Vapor Corrosion Behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBC's

The corrosion behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBCs were examined at 1500oC in water vapor environment. These oxides were coated on silicon nitride specimens by oxidation-bonded reaction sintering technique. Among Ln2Si2O7 system, though the thermal expansion coefficient of Yb2Si2O7 phase is closer to silicon nitride than that of Lu2Si2O7 phase, the corrosion resistance of silicon nitride with Lu2Si2O7 EBC was higher than that of Yb2Si2O7 EBC sample. In these EBC materials, boundary silica phase was easily corroded by water vapor. Among ASiO4 system, though the corrosion rate of HfSiO4 bulk was larger than that of ZrSiO4 and TiSiO4 phases, the oxidation of the silicon nitride substrate for HfSiO4 coated sample was smaller than that of other two. Many cracks were in ASiO4 EBC layer during the corrosion test. The introduced crack length in HfSiO4 EBC layer is shorter than in ZrSiO4 EBC layer due to the thermal expansion mismatch between EBC materials and the silicon nitride substrate.

Sintering and microstructure of mullite-Mo composites

Abstract Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m 1/2 , respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.

Carbon nanotube–SiO2 composites by colloidal processing

Abstract The surfactants assisted colloidal processing of 1˙0 wt-% carbon nanotube (CNT)–SiO2 composites using acid treated CNTs and colloidal silica has been investigated. By using different combinations of anionic and cationic surfactants five systems were developed with the aim of coating the CNTs with silica particles and creating a homogenously dispersed composite. Sintering of CNT–SiO2 composites was performed for 3 h at 1300°C in argon atmosphere. Pressureless sintering yielded composites with a density >95% of the theoretical. X-ray diffraction analysis showed the presence of cristobalite, but no other crystalline phases were detected in the sintered composites. Microstructural observation revealed isolated CNTs in the SiO2 matrix both in the as dried powder and in the sintered samples. It is concluded that the simple colloidal process developed here can be a convenient alternative to the sol–gel based methods used for fabrication of inorganic matrix composites containing CNTs.

Synthesis of ZrB2/SiC composite powders by single-step solution process from organic–inorganic hybrid precursor

A precursor of a zirconium diboride/silicon carbide (ZrB2/SiC) composite was synthesised via an organic–inorganic hybrid derived from gum karaya, tetraethyl orthosilicate, boric acid and zirconyl chloride starting materials. Fourier transform infrared spectroscopy of the as-synthesised dried hybrid revealed the formation of Si–O, Zr–O–C and B–O–B. X-ray diffraction revealed that the powder consists of only ZrB2 and β-SiC. Scanning electron microscopy and TEM of the composite powders showed that SiC and ZrB2 occurred in intimately mixed aggregates of spheroidal submicron sized particles for low (3M) boric acid concentration, while at high (5M) boric acid concentration, the two phases are larger with the ZrB2 adopting a blocky, angular morphology (∼10–30 μm long by 5 μm wide and thick), while the SiC remains spheroidal with ∼1 μm diameter particles in 10–20 μm diameter aggregates. Thermogravimetry–differential thermal analysis with the help of X-ray diffraction analysis revealed that the formation temperature was low at 1275°C for ZrB2 and 1350°C for the SiC with 40 wt-% yield.

Development of EBC for Silicon Nitride

Various coating methods of EBC layer for silicon nitride were discussed. High density EBC layer was successfully coated by different techniques such as sputtering, sol-gel and reaction sintering methods. Water vapor corrosion and recession mechanisms of Lu2Si2O7 which is a potential material for EBC were discussed. The problems in the development of EBC revealed by corrosion tests were summarized. The most important problem addressed here was the corrosion of silica at grain boundary. Due to corrosion of silica at the boundary, formation of porous surface is inevitable, then the silicon nitride substrate gets easily oxidized and/or corroded by water vapor. To resolve this issue, we propose a new EBC material without boundary silica and the corrosion mechanism of this improved EBC material is discussed.

Thermally stable high-strength porous alumina

A two-step heating schedule involving pulse electric current sintering, a kind of pressure-assisted vacuum sintering, and a subsequent postsintering in air was used to fabricate sintered porous alumina compacts. During pressure-assisted vacuum sintering, a dense microstructure of the A1 2 0 3 -C system was obtained and in the second stage (i.e., during postsintering in air at different temperatures ranging from 800 to 1300 °C for more than 10 h) carbon particles present in the A1 2 O 3 -C system burned out to form a highly porous A1 2 O 3 compact. In this work, the porosity (30%) was successfully controlled and did not change with the postsintering temperature. The intriguing aspect of this study is that porous alumina compacts are fabricated with high strength and remain stable against the postsintering temperature and extended soaking time. This behavior merits the material fabricated here as a potential porous compact, mechanically withstanding for high-temperature applications.