13 results found
Dayi EN, Al Nasiri N, 2019, Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications, Journal of the European Ceramic Society, Vol: 39, Pages: 4216-4222, ISSN: 0955-2219
The effect of exposure temperature and time on the diffusion rate of rare-earth oxides applied on silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) have been investigated. Knowledge on diffusion mechanism between the deposited rare-earth (RE) slurry and silica layer is necessary to understand the process governing EBCs formation and their properties. SEM/EDS analysis were used to study the effect of microstructure on diffusivity. The diffusion coefficient increases with increasing sintering temperature and time. The measured diffusion coefficients of the RE-coating into silica layer were in the order of 10−15-10-17 m2/s revealing an overall good adhesion on the SiC/SiC CMCs.
Al Nasiri N, Patra N, Pezoldt M, et al., 2019, Investigation of a single-layer EBC deposited on SiC/SiC CMCs: Processing and corrosion behaviour in high-temperature steam, Journal of the European Ceramic Society, Vol: 39, Pages: 2703-2711, ISSN: 0955-2219
Two rare earth monosilicates (Yb 2 SiO 5 , and Lu 2 SiO 5 ) were deposited using a low-cost coating application method to produce a single-layer coating. RE- oxides slurries were dip coated on oxidised CMC samples and subsequently heat treated at high temperature to ensure reaction between SiO 2 and RE-oxides to form the RE-monosilicate. A single, continuous, homogeneous thick coating of 25 μm was obtained. X-ray diffraction (XRD) confirmed formation of, RE-monosilicates in Yb and Lu silicate systems. Coated samples were exposed to 90% H 2 O static steam environment at 1350 °C for 25, 50, 100, and 150 h. Scanning Electron Microscopy (SEM) indicated that both coatings adhered strongly to the substrate. Coating thickness reduced from 22 μm to 11 μm for Yb-coating and 13 to 4 μm for Lu-coating with increasing corrosion time from 25 to 150 h, however there was no significant attack of the CMC for all steam exposure times.
Patra N, Al Nasiri N, Jayaseelan DD, et al., 2018, Thermal properties of C-f/HfC and C-f/HfC-SiC composites prepared by precursor infiltration and pyrolysis, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 38, Pages: 2297-2303, ISSN: 0955-2219
Al Nasiri N, Patra N, Jayaseelan D, et al., 2017, Water vapour corrosion of rare earth monosilicates for environmental barrier coatings application, Ceramics International, Vol: 43, Pages: 7393-7400, ISSN: 1873-3956
Water vapour corrosion resistance of five rare earth monosilicates Y2SiO5, Gd2SiO5, Er2SiO5, Yb2SiO5, and Lu2SiO5 was investigated during testing at 1350 °C for up to 166 h in static air with 90% water vapour. Four of the RE-silicates showed little weight gain (0.859 mg cm−2) after 166 h of exposure. Prior to testing the microstructure consists of equiaxed grains of 4- 7±0.4 µm. XRD analysis showed that after 50 h exposure to water vapour corrosion Y, Er, Yb and Lu-silicates had both mono and disilicates present on their surfaces as a result of the reaction between monosilicate and water vapour to form disilicate, while Gd-silicate has converted completely to G4.67Si3O13 making it less stable for environmental barrier coating application. The microstructures of corroded Y, Er, Yb and Lu-silicates contain ridges and cracks, while that of Gd-silicate contains rounded grains suggesting melting along with striped contract grains.
Al Nasiri N, Patra N, Ni N, et al., 2016, Oxidation behaviour of SiC/SiC ceramic matrix composites in air, Journal of the European Ceramic Society, Vol: 36, Pages: 3293-3302, ISSN: 0955-2219
Oxidation of silicon melt infiltrated SiC/SiC ceramic matrix composites (CMC) was studied in air at 1200–1400 °C for 1, 5, 24 and 48 h. Weight gain and oxide layer thickness measurements revealed the oxidation follows parabolic reaction kinetics with increase in temperature and time. XRD showed the extent of oxide layer (SiO2) formation was greatest after 48 h at 1400 °C: an observation confirmed by X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) analyses. Oxide layer thickness varied from 1 μm after 48 h at 1200 °C to 8 μm after 48 h at 1400 °C. Oxidation of SiC/SiC composites is both temperature and time dependent with an activation energy of 619 kJ mol−1. BN coatings around SiC fibres showed good resistance to oxidation even after 48 h at 1400 °C.
Nasiri NA, Saiz E, Giuliani F, et al., 2016, Grain bridging locations of monolithic silicon carbide by means of focused ion beam milling technique, Materials Letters, Vol: 173, Pages: 214-218, ISSN: 1873-4979
A slice and view approach using a focused ion beam (FIB) milling technique was employed to investigate grain bridging near the tip of cracks in four silicon carbide (SiC) based materials with different grain boundary chemistries and grain morphologies. Using traditional observations intergranular fracture behaviour and hence clear evidence of grain bridging was found for SiC based materials sintered with oxide additives. More surprisingly, in large grain materials, the FIB technique reveals evidence of grain bridging irrespective of the grain boundary chemistry, i.e. also in materials which macroscopically fail by transgranular failure. This helps to explain why the toughness of large grained materials is higher even if failure is transgranular.
Patra N, Al Nasiri N, Jayaseelan DD, et al., 2016, Synthesis, characterization and use of synthesized fine zirconium diboride as an additive for densification of commercial zirconium diboride powder, Ceramics International, Vol: 42, Pages: 9565-9570, ISSN: 1873-3956
Zirconium diboride (ZrB2) was synthesized by a solution-based technique using zirconyl chloride (ZrOCl2·8H2O, ZOO), boric acid (H3BO3, BA) and gum karaya (GK) as the sources of zirconium, boron and carbon, respectively. The initial formation temperature of ZrB2 was 1200 °C and complete conversion was achieved by 1400 °C. Preceramic precursors and as-synthesized ZrB2 powders were characterized by XRD, TG-DTA, SEM, TEM, EDX and compared with commercial ZrB2 powder made by carbothermic reduction. FT-IR of as-synthesized dried preceramic precursor revealed the formation of Zr–O–C and Zr–O–B whereas SEM showed agglomerated spherical particles with mean diameter of <1 µm. Commercial ZrB2 and as-synthesized fine ZrB2 powder were spark plasma sintered (SPS) at 1900 °C for 10 min. Addition of 10 wt% of synthesized fine powder improved the fired density from 87% to 93% of theoretical. A significant cost benefit arises for the utilization of cheap synthesized fine powder as an additive for the densification of the more expensive commercial powder.
Cedillos-Barraza O, Grasso S, Al Nasiri N, et al., 2016, Sintering behaviour, solid solution formation and characterisation of TaC, HfC and TaC-HfC fabricated by spark plasma sintering, Journal of the European Ceramic Society, Vol: 36, Pages: 1539-1548, ISSN: 1873-619X
Solid solution formation and sintering behaviour of TaC–HfC ceramics made from commercial TaC and HfC powders prepared using spark plasma sintering (SPS) at temperatures up to 2450 °C was investigated. Phase analysis and lattice parameter measurements using X-ray Diffraction (XRD) showed mutual diffusion of Hf and Ta with increasing sintering temperature. High Resolution Transmission Electron Microscopy (HRTEM) confirmed that solid solution formation and densification were achieved by a solid-state mechanism. Solid solutions were achieved for 4TaC–1HfC, 1TaC–1HfC and 1TaC–4HfC powders after sintering at temperatures of at least 2350 °C. Fracture toughness (KIC) values were in the range of 2.7–3.4 MPa m1/2 for all ceramics measured using Vickers indentation. Thermal conductivity of TaC was 55.8 W/m K at 1400 °C. Coefficients of thermal expansion (CTE) varied from 7.08–7.66 × 10−6/K (in the range of 25–2000 °C), with TaC at the lower end.
Al Nasiri N, Patra N, Horlait D, et al., 2016, Thermal properties of rare-earth monosilicates for EBC on Si-based ceramic composites, Journal of the American Ceramic Society, Vol: 99, Pages: 589-596, ISSN: 0002-7820
Rare-earth (RE) monosilicates are promising candidates as environmental barrier coating (EBC) materials for ceramic matrix composites for aerospace applications. Five rare-earth monosilicate materials have been investigated: Y2SiO5, Gd2SiO5, Er2SiO5, Yb2SiO5, and Lu2SiO5 produced from RE oxides and silica starting materials pressed and sintered at 1580°C under flowing air. Relative densities above 94% were obtained for all samples and ceramics were made containing 85–100 wt% of the RE monosilicate according to X-ray diffraction (XRD) with RE disilicates as the second phase in the Gd, Yb, and Lu silicate systems. Microstructures were characterized using scanning electron microscopy and XRD, and thermal properties measured including specific heat, thermal expansion, and thermal diffusivity. For the first time, specific heat capacity values are reported for the monosilicates [0.45–0.69 J·(g·K)−1]. Thermal expansion coefficients (TECs) of the dense samples ranged between 5.9 and 10.3 × 10−6 K−1 measured for 473 to 1473 K. All EBCs have low thermal conductivities [1.8 W·(m·K)−1 or less] making them excellent EBC insulators.
Horlait D, Grasso S, Al Nasiri N, et al., 2016, Synthesis and Oxidation Testing of MAX Phase Composites in the Cr-Ti-Al-C Quaternary System, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 99, Pages: 682-690, ISSN: 0002-7820
Patra N, Al Nasiri N, Jayaseelan DD, et al., 2015, Low-temperature solution synthesis of nanosized hafnium carbide using pectin, Ceramics International, Vol: 42, Pages: 1959-1963, ISSN: 0272-8842
Nano-sized hafnium carbide was synthesized from organic–inorganic hybrid polymer based on polycondensation and carbothermal reduction reaction from pectin and hafnium tetrachloride followed by thermal treatment. Orthorhombic and monoclinic hafnia formed on pyrolysis which above 1300 °C transformed to hafnium carbide. Conversion of amorphous to crystalline hafnia was initiated at ~800 °C while the endothermic carbothermal reduction reaction started at ~1275 °C. Total yield of hafnium carbide was ~62%. The resulting carbide powders were equiaxed with narrow size distribution of crystallite size ~50 nm. SEM and EDX mapping confirm the uniform distribution of Hf and C. The high ceramic yield, uniform size particles, and simplicity of the process make it a promising route for polymer infiltration pyrolysis of carbon fiber/ultra high temperature composites.
Al Nasiri N, Ni N, Saiz E, et al., 2015, Effect of microstructure and grain boundary chemistry on slow crack growth in silicon carbide at ambient conditions, Journal of the European Ceramic Society, Vol: 35, Pages: 2253-2260, ISSN: 0955-2219
Silicon carbide (SiC) is being used increasingly as a room temperature structural material in environments where moisture cannot always be excluded. Unfortunately, there have been almost no reports on slow crack growth (SCG) in SiC at room temperature. To address this gap, SCG in SiC was studied using constant stress rate and double torsion tests in water. SiC based materials were produced with a wide range of grain boundary chemistries and microstructures, which may affect their slow crack growth behaviour. To clarify the role of chemistry and microstructure respectively, solid state (SS) sintering with carbon and boron along with liquid phase (LP) sintering using oxides additives were used to produce materials with fine and coarse grains. The LP-SiC was three times more sensitive to SCG than SS-SiC materials. Moreover, the larger grained material with a higher toughness was less sensitive to SCG than the materials with fine grains.
Al Nasiri N, Saiz E, Giuliani F, et al., 2015, Effect of microstructure and slow crack growth on lifetime prediction of monolithic silicon carbide, Materials Science and Engineering A - Structural Materials Properties Microstructure and Processing, Vol: 627, Pages: 290-295, ISSN: 0921-5093
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