Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

The effect of Al2O3-MgO additives on the microstructure of spark plasma sintered silicon nitride

It was shown that spark plasma sintered silicon nitride with a high content of Al₂O₃ and MgO consists of α and β silicon nitride, the main phase being α silicon nitride. The increase in the sintering temperature did not lead to significant changes in the phase composition as occurs in silicon nitride added with Al₂O₃-Y₂O₃. It was found that increasing in SPS temperature above 1650 °C leads to an insignificant increase in the density. A complex shaped equiaxed grain microstructure was shown in both cases. However, doping with aluminum and yttrium oxides allows obtaining an elongated grain microstructure. The Hall-Petch effect was observed for the microhardness of the investigated SPSed silicon nitride. The microhardness of the described ceramics was rather high and more than 1900 HV compared to the pressureless sintered at 1800 °C silicon nitride with the microhardness equal to 1511 HV.     

Low-temperature joining of SiC ceramics using NITE phase with Al2O3-Ho2O3 additive

In order to avoid the property degradation resulting from high-temperature joining process, nano-infiltrated transient eutectoid (NITE) phase with the Al₂O₃-Ho₂O₃ as the joining adhesives was adopted to join silicon carbide (SiC) ceramics with the attempts to lower down the joining temperature. The liquid-phase-sintered silicon carbide (LPS-SiC) specimens were joined at 1500-1800°C by spark plasma sintering (SPS) under the pressure of 20 MPa. The results of the shear test and microstructure observation showed that the joining process could be finished at a relatively lower temperature (1700°C) compared to other NITE-phase joining. In contrast to the shear strength of 186.4 MPa derived from the SiC substrate, the joint exhibited the shear strength of 157.8 MPa with the fully densified interlayer.     

Pressure-less joining of alumina ceramics by the reaction-bonded aluminum oxide (RBAO) method

The present work demonstrates a pressure-less and reliable joining technique for alumina ceramics through a reaction-bonded aluminum oxide (RBAO) method. Effective joining relies on the RBAO mechanism, in which Al particles are converted to alumina through oxidation and bond with alumina particles from the parts to be joined upon sintering. Alumina ceramics in a green state were successfully joined with the use of an Al/Al₂O₃ powder mixture as an interlayer. The oxidation behavior of the Al particles was confirmed by thermogravimetry and X-ray diffraction analyses. Joining was performed in ambient air at 1650 °C for 2 h without applying any external pressure. Microstructural observations at the joining interfaces indicated a compact joining. The joining strengths were assessed by determining the biaxial strengths at room temperature, and the joined samples exhibited no fractures at the joining interfaces. Moreover, the joints had a strength of almost 100 % when compared with those of the parent alumina ceramics.     

Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, T_g, is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al₂O₃ does not reveal an internal friction peak at T_g, i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y₂O₃, 6 wt.% Y₂O₃+2 wt.% Al₂O₃, or 2 wt.% Y₂O₃+4 wt.% Al₂O₃+2 wt.% TiN, do show an internal friction peak near T_g. This “T_g-peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y₂O₃+Al₂O₃+TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y₂O₃+Al₂O₃ or Y₂O₃ as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y₂O₃+Al₂O₃ sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y₂₀N₄Si₁₂O₄₈) and O-sialon (Al₀.₀₄Si₁.₉₆N₁.₉₆O₁.₀₄). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y₂O₃ sintering additive, however, is thermally stable. The presence of Ti⁴⁺ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.     

Pressureless joining of SiC ceramics with magnesia-alumina-silica glass ceramics

Liquid phase sintered SiC ceramics were joined using magnesia-alumina-silica (MAS) glass-ceramic fillers without applied pressure. Four different filler compositions with 9.3–25.2 wt.% MgO, 20.7–33.6 wt.% Al₂O₃, and 49.2–68.1 wt.% SiO₂ were studied. The effects of filler composition and joining temperature (1450–1600°C) on the joint strength were investigated. All compositions exhibited an optimum joining temperature at which the maximum joint strength was obtained. A low joining temperature resulted in poor wetting of the SiC substrate due to the high viscosity of the filler. Whereas a high joining temperature caused dewetting and large unfilled sections in the interlayer due to the deleterious interfacial reactions. The joint strength was inversely proportional to the interlayer thickness, which was a function of filler composition and joining temperature. The SiC ceramic joined at 1525°C with MgO-25 wt.% Al₂O₃-60 wt.% SiO₂ filler exhibited a four-point bending strength of 286 ± 40 MPa.     

Secondary phases,microstructures and properties of AlN ceramics sintered by adding nitrate sintering additives

AlN ceramics were sintered at a temperature range from 1650 to 1800°C through adding the Ca and Y nitrate sintering additives. Secondary phases, microstructures and properties of the AlN ceramics were studied. When the AlN ceramics are sintered at 1650 or 1675°C, CaO and Y₂O₃ from the sintering additives react with Al₂O₃ in the AlN powder to generate CaAl₄O₇ and Y₃Al₅O₁₂. Part of Y₃Al₅O₁₂ reacts with CaO and Al₂O₃ to form CaYAl₃O₇ at 1700°C. At 1800°C, CaYAl₃O₇ decomposes into CaAl₄O₇ and Y₃Al₅O₁₂. Finally, CaAl₄O₇ volatilises and only Y₃Al₅O₁₂ remains. As the sintering temperature increases, the AlN grains grow continuously and the bending strength and thermal conductivity of the AlN ceramics increase first and then decrease. The AlN ceramics sintered at 1700°C are fully dense and have the highest bending strength and thermal conductivity of 373·7 MPa and 136·7 W m^?1 K^?1 in this work.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Joining of SiC,alumina, and mullite by the Refractory Metal—Wrap pressure-less process

The aim of this work was to discuss the suitability of the joining process called “RM-Wrap” (RM = Refractory Metals, ie, Mo, Nb, Ta, Zr) as a pressure-less and tailorable technique to join several different ceramics such as SiC, alumina, and mullite (3Al₂O₃.2SiO₂). In the RM-Wrap joining technique the refractory metal foil is used as a wrap containing one or more silicon foils. It is performed at 1450°C, under flowing argon, and the resulting joining materials are in situ formed composites made of refractory metal disilicides (MoSi₂, NbSi₂, TaSi₂, or ZrSi₂) embedded in a silicon-rich matrix; their coefficient of thermal expansion has been calculated and the Laser Flash Method was used to measure the thermal diffusivity of one of them (MoSi₂/Si) in 25°C-1000°C range, then to calculate its thermal conductivity. All the obtained joints are uniform, continuous, and crack free. Some preliminary oxidation tests were carried out on all joints at 1100°C, 6 hours in air, giving unchanged morphology of the interface and the joining materials itself; the joint strength of RM-Wrap joined SiC was measured at room temperature using three different mechanical tests: (a) single lap (SL), (b) single lap off-set (SLO) and (c) torsion on hourglass-shaped samples (THG) (on Mo-wrap joined SiC).     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

Rare earth (RE: Nd,Dy, Ho,Y, Yb,and Sc) aluminosilicates for joining silicon carbide components

Six different types of glass 12.18 RE₂O₃‐22 Al₂O₃‐65.82 SiO₂ (mol %) where RE: Nd, Dy, Ho, Y, Yb, and Sc were tested for joining silicon carbide (SiC) components. The different types of glass vary in their thermal properties but they are similar in their behavior for the joining process when a laser‐based heating technology was used. The quality of the joints was characterized by microscopic analysis, mechanical tests, and measurements of tightness. Annealing experiments were conducted at temperatures in the range of the glass transition and crystallization allowing an assessment of the compositions for usability as glass and as glass‐ceramic interlayers. Five of the investigated compositions can be recommended for application up to temperatures of 900°C. The Y‐ and Yb‐based compositions guarantee a high joint quality at temperatures up to 1200°C. The high temperature assessment was based on tightness and microstructural analyses of the joints after the annealing procedures. The results can be transferred to joining processes with lower heating and cooling rates.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.     

Solid-state joining of SiC using a thin Ti3AlC2, TiC,or Ti filler

SiC monoliths containing 5 wt.% Al₂O₃-Y₂O₃ additive were joined using a thin Ti₃AlC_2, TiC, or Ti filler. After joining at 1900 °C for 5 h under 3.5 MPa, the joint properties were compared in terms of the microstructure, phase evolution, joining strength, and possible elimination of the joining layer. Although all samples showed a sound joint, the microstructure differed according to the filler. SiC joined with Ti₃AlC₂ filler showed an indistinguishable joining interface due to the filler decomposition followed by solid-state diffusion into the SiC base, whereas TiC filler remained at the interface without showing decomposition or diffusion. In contrast, the Ti filler showed a possible elimination of the joining layer because of the diffusion of Ti and the formation of TiC. The mean joining strengths for the Ti₃AlC₂, TiC, and Ti fillers were 300, 234, and 248 MPa, respectively, which were comparable to that of the base SiC material (250 MPa).     

Glass-ceramics for joining oxide-based ceramic matrix composites (Al2O3f/Al2O3-ZrO2) operating under direct flame exposure

This work focuses on the joining processes of oxide-based ceramic matrix composites (Al₂O_3f /Al₂O₃-ZrO₂), which are used as radiant tube furnace components in the steel industry. These components have to operate in harsh environments, and under high temperatures, and they therefore have to resist corrosion, humidity, and combustion. Two glass-ceramics systems, which have Y₂Ti₂O₇ as their main crystalline phase, as well as specific and optimized properties to withstand severe operating conditions, including temperatures of 900 °C, are here proposed as joining materials. The adhesion of the glass-ceramics to the composite was found to be excellent after mechanical and thermal tests in which they were in direct contact with a 900 °C flame and thermal cycling of between 400 °C and 900 °C.     

Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO‐RE₂O₃ and results were compared with a commercial Al₂O₃‐Y₂O₃‐doped material. Decreasing stress intensity range (ΔK) fatigue tests were conducted using compact‐tension specimens to measure steady‐state fatigue crack growth rates. Specimens doped with MgO‐RE₂O₃ additives showed a significantly higher resistance to crack growth than those with Al₂O₃‐Y₂O₃ additives and this difference was attributed to the much higher grain aspect ratio for the MgO‐RE₂O₃‐doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R‐curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO‐RE₂O₃‐doped silicon nitrides displayed similar steady‐state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.     

Conversion of Alumina to Aluminum Nitride Using Polymeric Carbon Nitride as a Nitridation Reagent

Polymeric carbon nitride, which was synthesized by polymerization of dicyandiamide at 500°C, was used as a nitridation reagent in the conversion of δ‐alumina (δ‐Al₂O₃) to aluminum nitride (AlN). The products obtained at various reaction temperatures were characterized by powder X‐ray diffraction, ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). δ‐Al₂O₃ began to convert to AlN at 900°C, which is the lowest temperature reported for the formation of AlN from Al₂O₃, and completely converted to AlN at 1400°C. The occurrence of reaction intermediates during nitridation was confirmed by ²⁷Al MAS NMR and XPS. The change in Raman spectra with reaction temperatures indicated that lattice defects in AlN were reduced by calcining at higher reaction temperatures.     

Enhanced thermal conductivity and flexural strength of sintered reaction-bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

Sintered reaction-bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y_0.96Eu_0.04)₂O₃ and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post-sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction-bonded silicon nitride (RBSN) doped with Y₂O₃ and MgO, the β-Si₃N₄ content and nitridation degree were 51.1% and 93.8%, respectively. However, the β-Si₃N₄ content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y_0.96Eu_0.04)₂O₃ and MgO. After post-sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y_0.96Eu_0.04)₂O₃ and MgO was increased by 16.5% compared to that of the samples doped with Y₂O₃ and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2-mol.% (Y_0.96Eu_0.04)₂O₃ and 5-mol.% MgO.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

The temperature dependence of the relative grain‐boundary energy of yttria‐doped alumina

Atomic force microscopy was used to measure the dimensions of grain‐boundary thermal grooves on the surfaces of Al₂O₃, 100 ppm Y‐doped Al₂O₃, and 500 ppm Y‐doped Al₂O₃ ceramics heated at temperatures between 1350°C and 1650°C. The measurements were used to estimate the relative grain‐boundary energies as a function of temperature. The relative grain‐boundary energies of Al₂O₃ decrease slightly with increased temperature. When the doped samples were heated, there was an overall increase in the grain‐boundary energy, attributed to a reduction in the grain boundary excess at higher temperature. The overall trend of increasing grain‐boundary energy was interrupted by abrupt reductions in grain‐boundary energy between 1450°C and 1550°C. In the same temperature range, there is an abrupt increase in the grain‐boundary mobility that is associated with a complexion transition. When the 100 ppm Y‐doped sample was cooled, there was a corresponding increase in the relative grain‐boundary energy at the same complexion transition temperature, indicating that the transition is reversible.     

cBN-Al2O3 composites in NaCl environment under high pressure and high temperature conditions

A series of experiments of cubic boron nitride (cBN)-Al₂O₃ composites was conducted in NaCl environment under a pressure of 5 GPa at 1200–1650 °C using a Chinese multi-anvil high-pressure apparatus. The oxidation resistance of cBN-Al₂O₃ composites reached 1300 °C, which was 200 °C higher than that of raw cBN powder. The porosity was estimated by the content of NaCl impurities in cBN-Al₂O₃ composites. The content of NaCl impurity increases with increasing temperature and decreases with increasing Al₂O₃ level under high pressure and high temperature conditions. cBN+30 vol% Al₂O₃ sintered at 5 GPa and 1200 °C shows no NaCl impurity, and the Vickers hardness of the sample is 21.6 GPa which is half of cBN+10 vol% Al.