1,720,990 research outputs found
Spark plasma sintering and hot pressing of ZrB2–MoSi2 ultra-high-temperature ceramics
No full textZrB2 + 15 vol.% MoSi2 composites were densified by spark plasma sintering and conventional hot pressing at the same sintering conditions, i.e. maximum temperature: 1750 ◦C, applied pressure: 30MPa and heating rate: 100–150 K/min. The densification behaviour was investigated
through the analysis of the hot pressing and spark plasma sintering shrinkage curves. The microstructures were analyzed and compared in order to understand the influence of the two sintering techniques. The following mechanical properties were measured:Vickers hardness, fracture toughness, Young’s modulus and 4-pt bending strength. The main outcome of the present work is that when the ZrB2-based composite was densified either by spark plasma sintering or by hot pressing using the same sintering parameters, no differences were observed in terms of microstructural features and mechanical properties
Improvement offered by coprecipitation of sintering additives on ultra-fine SiC materials
No full textA commercial SiC powder is processed through either conventional ultrasonication or coprecipitation of sintering additives, to obtain dense materials. The coprecipitation process involves a change of the surface properties of SiC particles which concerns the formation of amorphous precipitates (see the figure) deriving from the sintering aids and partial dissolution of the surface silica. These factors lead to microstructural refinement and reduction of the intergranular phase amount in dense samples
Oxidation behaviour of a pressureless sintered HfB2–MoSi2 composite
No full textThe thermal stability of a 80-vol.% HfB2 + 20 vol.% MoSi2 Composite is tested under oxidizing environment. Oxidation tests are carried out in flowing synthetic air in a TG equipment from 1000 to 1400 ◦C with exposure time of 30 h. At temperatures ≥1200 ◦C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing hafnium diboride from fast degradation. Analysis of the kinetics is carried
out through fitting of the thermogravimetric curves. Between 1200 and 1400 ◦C, the kinetic curves deviate from a parabolic behaviour, being more close to a logarithmic–parabolic behaviou
Sinterizzazione di carburo di silicio in fase liquida
No full textMateriali densi a base di carburo di silicio sinterizzato in fase liquida sono stati ottenuti da due polveri (di tipo α-SiC e β-SiC) utilizzando come additivi (Y2O3+Al2O3) Sono state analizzate le densità e la microstruttura dei materiali, in relazione alle caratteristiche e trattamento delle polveri
Nanoindentation characterization of SiC-based ceramics
No full textDepth-sensing indentation tests were carried out on several SiC-based liquid-phase-sintered SiC ceramics characterized by different mean grain size spanning from 78 to 540 nm. The indentation tests were performed with peak loads ranging from 5 to 400mN in order to investigate the property variation with the variation of the peak load and material microstructure. The values of indentation hardness and Young’s modulus were calculated according to the models developed by Oliver and Pharr (O&P) and by Cheng and Cheng (C&C). According to the O&P model, the finest grained SiC ceramics did not show the indentation size effect (ISE) which was observed in the largest grained SiC ceramics. The grain-size dependence of the indentation hardness can be described by an inverse Hall–Petch relation. With the C&C model, the correlation of the indentation hardness with the peak load and the grain size was less evident. The indentation hardness calculated by the O&P model was lower than that calculated by the C&C model and in better agreement with the values of Vickers microhardness. The O&P indentation Young’s modulus was higher than the C&C indentation Young’s modulus but the latter was in very good agreement with the values measured by resonant frequency. For both models,
the indentation Young’s modulus was almost load-independent even if a dependence on the microstructure was observed
Sub-micro and nano-sized SiC ceramics: comparison and mechanical properties
No full textMaterials with grain sizes down to the nanometer range have recently attracted a lot of scientific interest, as an increase in hardness, mechanical strength, Weibull modulus and wear resistance is expected.
In this work, SiC-based materials having mean grain size ranging from 500 down to 70 nm are produced starting from diverse SiC powders, commercial and laboratory synthesized.
Different processing routes are carried out in order to optimise the final microstructure.
Dense specimens are obtained through hot pressing with the aid of Al2O3 and Y2O3 as sintering agents. Microstructural characteristics are analysed by XRD, SEM, EDX, TEM techniques. The following mechanical properties are measured: four-point bending strength (from room temperature up to 1300°C), fracture toughness, hardness and Young modulus.
Microstructural parameters and mechanical properties are discussed to point out if the decrease of mean grain size down to 100 nm or less results in a actual improvement of the performance of this specific syste
Short-term oxidation of a ternary composite in the system AlN–SiC–ZrB2
No full textThe present study investigates the oxidation behaviour in air of a structural ceramic composite with the following volumetric composition: 55% AlN–15% SiC–30% ZrB2. This kind of ternary composite is electroconductive (3×10−4 cm) and has significant strength (∼700MPa) and toughness (4MPam1/2) up to 1000 ◦C. Oxidation tests were carried out in a TG equipment from 700 to 1300 ◦C with exposition time of 30 h. Significant weight gain is observed at T > 1000 ◦C. In the range 700–900 ◦C, the process is dominated by the oxidation of ZrB2 into zirconia and boria and the kinetic is nearly parabolic. At temperatures in the range 1000–1100 ◦C, boria reacts with alumina forming aluminium borate and borosilicate glass and the kinetic largely deviates from parabolic behaviour. In the samples oxidized at temperatures in the range 1200–1300 ◦C, aluminium borate and mullite crystallize on the surface. The kinetics is para-linear in this temperature range but at 1300 ◦C, the rupture of the outer layer, results in accelerated damage of the sample. The composite is recommended for applications up to
1100 ◦C
Effects of powder processing on colloidal and microstructural characteristics of Beta-SiC powders
No full textThree different powder processing methods for introducing sintering aids (yttria and alumina) into a commercial silicon carbide powder are studied: mechanical mixing, heterocoagulation and coprecipitation. The surface characteristics of the rawpowders and powder mixtures are analysed
by acoustophoretic measurements and compared. The effectiveness of the coprecipitation process for additivation is evaluated by the comparison of ESA measurements and by TEM observations. The powder mixtures are pressureless sintered in argon atmosphere and the microstructure of the sintered materials is analysed. The coprecipitation process allows a higher final density to be obtained in a shorter sintering time, with a finer and more homogeneous microstructure
High strength and toughness electroconductive SiC-based composites
No full textThe development of silicon carbide (SiC) based composites using MoSi 2 as the secondary electroconductive phase for the matrix was investigated. A mixture of SiC and 10 vol % MoSi2 was hot pressed at 1900°C, without any addition of sintering aids. An improvement of the final; relative density from 60 to 86% was obtained with the addition of 2 wt% of Y2O3. In the reinforced composite containing 30 vol% of MoSi2, the fracture toughness increased of about 27 % in comparison with the reference SiC material. It was observed that the addition of MoSi 2 particles to the Sic matrix caused a gradual decrease of the electrical resistivity of the composites. The hardness of the composites decreased with increasing the MoSi2 content, while the Young's Modulus existed near the SiC reference value
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