C.A. Nannetti

Brazing of SiC and SiCf/SiC composites performed with 84Si-16Ti eutectic alloy: microstructure and strength

The microstructure and strength of brazed joints for monolithic SiC and SiCf/SiC composites are presented and discussed; the brazing technique is based on the use of the 84Si-16Ti (at%) eutectic alloy. The rather low melting point of the used alloy allows to avoid a degradation of the fibre/matrix-interfaces in the composite materials. All the joints did not show any discontinuities and defects at the interface and revealed a fine eutectic structure. Moreover, in the case of composites, the joint layer appeared well adherent both to the matrix and the fibre interphase, and the brazing alloy infiltration looked sufficiently controlled. High resolving electron microscopic investigations of the microstructure and of the nanochemistry (HREM, EELS, esp. ELNES) revealed atomically sharp interfaces without interdiffusion or phase formation at the interlayer leading to the conclusion that direct chemical bonds are responsible for the adhesion. The joints of SiCf/SiC composites showed 71 ± 10 MPa shear strength at RT and nearly the same values at 600°C.

SiC–SiCf CMC manufacturing by hybrid CVI–PIP techniques: process optimisation

SiC–SiCf ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation.

Development of 2D and 3D Hi-Nicalon fibres/SiC matrix composites manufactured by a combined CVI–PIP route

SiCf/SiC composites with true 3D textures were manufactured in order to investigate the effects of the texture on thermal and mechanical properties. Hi-NicalonTM fibre preforms having a total fibre volume content of about 40% with 25 and 50% relative fibre content through the thickness were infiltrated using a mixed chemical vapour infiltration–polymer infiltration and pyrolysis technique. The issues and the main characteristics of the obtained 3D composites are presented and compared with the ones of 2D composites prepared employing the same materials and process.

Joining of SiC based ceramics and composites with Si-16Ti and Si-18Cr eutectic alloys

Silicon carbide and SiCf/SiC ceramic matrix composites are attractive materials for energy application because of their chemical stability and mechanical properties at high temperature. For these materials advanced joining techniques are under development and, among them, brazing is one of the most promising. In this paper a brazing technique based on the use of the Si-16Ti (at.%) and the Si-18Cr (at.%) eutectic alloys (with 1330°C and 1305°C melting points, respectively) is presented and discussed. The eutectic alloys were prepared by several melting operations, reduced in powders and finally used for the joining experiments. The brazing trials led to joints without discontinuities and defects at the interface and with fine eutectic structures exhibiting a morphology comparable with that of the starting alloys. Microstructure and nanochemistry investigations showed neither interdiffusion nor phase formation at the interface leading to the conclusion that direct chemical bonds are responsible for the adhesion. Joint specimens showed high shear strength with failure occurring mainly in the base material.

Low activation brazing materials and techniques for SiCf/SiC composites

Abstract A low activation brazing technique for silicon carbide fiber reinforced silicon carbide matrix composites (SiCf/SiC) is presented; this technique is based on the use of the 78Si–22Ti (wt%) eutectic alloy. The joints obtained take advantage of a melting point able to avoid composite fibre-interface degradation. All the joints showed absence of discontinuities and defects at the interface and a fine eutectic structure. Moreover, the joint layer appeared well adherent both to the matrix and the fibre interphase and the brazing alloy infiltration looked sufficiently controlled. The joints of SiCf/SiC composites showed 71±10 MPa almost pure shear strength at RT and up to 70 MPa at 600 °C.

Thermal shock tests of β-sic pellets prepared from laser synthesized nanoscale sic powders

Abstract Nanoscale SiC powders, produced by laser synthesis from gaseous precursors, have been successfully used to prepare sintered pellets. All the sintered samples showed the low temperature β-SiC structures and presented an enhanced thermal conductivity (> 20%) with respect to materials prepared from commercial powders. Samples hardness and toughness, comparable with commercial products, confirmed the good samples quality. The thermal shock tests have been performed by irradiating the pellets with a Nd-YAG pulsed laser (pulse duration and energy: 0.25 μs and 0.18 J or 0.4 ms and 0.65 J). The laser fluence (power density) was increased by reducing the spot size, up to the appearance of a visible surface damage. The threshold values for the structural damage were quantified using a heat flux parameter φabs√tp. The measured threshold value rose from ca. 20 MW/m2 √s for the best materials prepared from commercial SiC powders to ca. 24 MW/m2√s for the newly developed β-SiC.

In-situ tritium release (CORELLI-2 experiment) and ex-reactor ionic conductivity of substoichiometric LiAlO2 breeder ceramics

LiAlO2 pellets with about 5% Li deficiency, prepared by a “wet” and a “dry” route were tested in situ for tritium release properties in nearly the same environmental conditions (CORELLI-2 experiment). Both the “wet” and “dry” route specimens were characterized by 80% of theoretical density (TD), almost fully open porosity and grain size ≤ 0.5 μm. The tritium removal rate evolution, following temperature or sweep gas changes during the irradiation, were observed to be nearly the same for both materials, in spite of their different preparation routes and impurities concentration. The ionic conductivities, as determined by impedance spectroscopy, were also similar. The presence of LiAl5O8 spinel phase in both samples apparently influenced the defect structure related transport properties of both lithium and tritium in these materials.

Effect of purge gas oxidizing potential on tritium release from Li-ceramics and on its permeation through 316L SS clads under irradiation (TRINE experiment)

Abstract The effect of red—ox potential of helium purge gas (variously doped with H2, H2O and O2) was examined on tritium release from Li-ceramics (LiAlO2 and Li2ZrO3 pellets) and on its permeation rate through the 316L stainless steel clads (bare and coated) held at 500°C. Decreasing the H2 content from 1000 vpm (reference ‘R’ gas mixture) to 100 vpm, and substituting H2O for H2, the tritium permeation rate (ca. 1.41010 atoms cm−2 s−1 in R-gas) increases. Tritium inventories in the Li ceramics were increased too. When a strong oxidizing purge (1000 vpm O2 added to He containing 100 vpm H2O) was used, a retention time (τ) of two days at 400°C was measured for Li2ZrO3. In this oxidizing environment the tritium permeation loss dropped by a factor five for the uncoated capsules while an aluminide coating became a very effective tritium barrier: tritium permeation flux at 550°C fell below the measurable limit.