Microstructure and mechanical properties of the Si3N4/42CrMo steel joints brazed with Ag–Cu–Ti + Mo composite filler

The Si₃N₄ ceramic was joined to 42CrMo steel using Ag–Cu–Ti + Mo composite filler. Effect of Mo particles content on the microstructure and mechanical properties of the joints were investigated. Defect-free joints were received when the Si₃N₄/42CrMo steel joints were brazed with Ag–Cu–Ti + Mo composite filler. The results show that a continuous reaction layer which is composed of TiN and Ti₅Si₃ was formed near the Si₃N₄ ceramic. A double reaction layer which consists of Fe₂Ti and FeTi was also formed adjacent to 42CrMo steel, with Fe₂Ti being located near the steel. The central part of the joint is composed of Ag based solid solution, Cu based solid solution, Mo particles and some Cu–Ti intermetallic compounds. The maximal bending strength reached 587.3 MPa with 10 vol.% Mo particles in the joint, at which the joint strength was 414.3% higher than the average strength for the case without Mo particles addition.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

Tailoring the microstructure of high porosity Si3N4 foams by direct foaming with mixed surfactants

The mixed surfactants were successfully applied to fabricate the highly porous Si₃N₄ ceramic foams by the direct foaming method. The oppositely charged surfactants mixed in slurries could combined into catanionic surfactant by the electrostatic attraction and facilitate the formation of ultra-stable foams. The microstructure of the Si₃N₄ ceramic foams, including pore structure, mean pore size, pore size distribution and porosity were tailored by varying the mixing ratio of surfactant, mixed surfactants concentration and solid content of the initial slurries. Si₃N₄ ceramic foams with porosity of 92%-97%, mean pore size of 140-240 µm and compressive strength of 0.85-5.38 MPa were obtained by adjusting mixed surfactants between 0.1 and 0.4 wt% and solid content between 22 and 30 vol%. The compressive strength of Si₃N₄ ceramic foams in current work was much higher than most reported results.     

Preparation of Si3N4 ceramic foams by simultaneously using egg white protein and fish collagen

Fish collagen, a kind of fibrous protein, and egg white protein were selected as foaming agent to prepare ceramic foams by protein foaming method. Ceramic foams with open porosity of 84.8–86.9%, average pore size of 216–266 μm and compressive strength of 8.7–13.7 MPa were fabricated. Studies of fish collagen addition on the influence of open porosity, pore size distribution and mechanical property of ceramic foams were investigated. In comparison with single addition of 8 wt% egg white protein, the combinational addition of 2 wt% fish collagen and 6 wt% egg white protein results in 23% increase in average pore size. In addition, the introduction of fish collagen decreases the count of small pores. Moreover, with the introduction of fish collagen, pores become regular in shape.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

In situ synthesis of Si2N2O/Si3N4 composite ceramics using polysilyloxycarbodiimide precursors

In situ synthesis of Si₂N₂O/Si₃N₄ composite ceramics was conducted via thermolysis of novel polysilyloxycarbodiimide ([SiOSi(NCN)₃]_n) precursors between 1000 and 1500 °C in nitrogen atmosphere. The relative structures of Si₂N₂O/Si₃N₄ composite ceramics were explained by the structural evolution observed by electron energy-loss spectroscopy but also by Fourier transform infrared and ²⁹Si-NMR spectrometry. An amorphous single-phase Si₂N₂O ceramic with porous structure with pore size of 10–20 μm in diameter was obtained via a pyrolyzed process at 1000 °C. After heat-treatment at 1400 °C, a composite ceramic was obtained composed of 53.2 wt.% Si₂N₂O and 46.8 wt.% Si₃N₄ phases. The amount of Si₂N₂O phase in the composite ceramic decreased further after heat-treatment at 1500 °C and a crystalline product containing 12.8 wt.% Si₂N₂O and 87.2 wt.% Si₃N₄ phases was obtained. In addition, it is interesting that residual carbon in the ceramic composite nearly disappeared and no SiC phase was observed in the final Si₂N₂O/Si₃N₄ composite.     

Wetting of AgCu-Ti filler on porous Si3N4 ceramic and brazing of the ceramic to TiAl alloy

The effect of Ti content on the wettability of AgCu-Ti filler on porous Si₃N₄ ceramic was studied by the sessile drop method. AgCu-2 wt% Ti filler alloy showed a minimum contact angle of 14.6° on porous Si₃N₄ ceramic during the isothermal wetting process. The mechanism of AgCu-Ti filler wetting on porous Si₃N₄ ceramic is clarified in this paper. Porous Si₃N₄ ceramic was brazed to TiAl alloy using AgCu-xTi (x = 0, 2 wt%, 4 wt%, 6 wt%, 8 wt%) filler alloy at 880 °C for 10 min. The effect of Ti content on the interfacial microstructure and mechanical properties of porous-Si₃N₄/AgCu-xTi/TiAl joints are studied. The typical interfacial microstructure of p-Si₃N₄/AgCu-Ti/TiAl joint is p-Si₃N₄/penetration layer (Ag(s,s)+Si₃N₄+TiN+Ti₅Si₃)/Ag(s,s)+Cu(s,s)+TiCu/AlCu₂Ti/TiAl. The maximum shearing strength of the brazed joint was 14.17 MPa and fracture that occurred during the shearing test propagated in the porous Si₃N₄ ceramic substrate for the formation of the penetration layer.     

Effect of different preparation methods on the microstructure and mechanical properties of Si3N4 ceramic composites

In this study, Si₃N₄ ceramic composites were fabricated by using ball-milling, titration preparation and urea preparation methods, respectively. The effect of different preparation methods on microstructure and mechanical properties of the Si₃N₄ ceramic composites was investigated. Obviously, the Si₃N₄ ceramic composite prepared by the urea preparation method (U-SN sample) showed better sintering behavior and higher mechanical properties than that prepared by the other two methods. Compared with the Si₃N₄ ceramic composite by the titration preparation method (T-SN sample), we could avoid the complex titration process or uncontrollable pH value during the preparation process of the U-SN sample. Meanwhile, the coated Y-Al precursor layer in thickness of nanometers was more homogeneous than that prepared by the traditional titration method. B-SN represented the Si₃N₄ ceramic composite prepared by the ball-milling method. These samples were all sintered from room temperature to 1750 °C via hot-pressing sintering. The U-SN specimen showed the optimal flexural strength and fracture toughness of being 817 MPa and 6.90 MPa/m², respectively, which could be attributed to its smallest grain size (0.46 µm) among these three samples.     

The role of MgSiN2 during the sintering process of silicon nitride ceramic

The reaction process between MgSiN₂ SiO₂ and Si₃N₄ was investigated by analyzing the composition change of the powder mixture of 61 wt% MgSiN₂, 34 wt% SiO₂ and 5 wt% α-Si₃N₄ after heat treatment at different temperatures. The phase and chemical compositions of the grain boundary phase in the silicon nitride ceramic was analyzed by x-ray diffraction, transmission electron microscope, and energy-dispersive x-ray spectroscopy. The results demonstrated that MgSiN₂ reacted with the surface silica and Si₃N₄ to form Mg–Si–O–N liquid phase, which promoted the consolidation densification of silicon nitride powders through liquid-phase sintering mechanism. The amount of Mg–Si–O–N glass boundary phase using MgSiN₂ as additives is much less than that using the same amount of MgO additive, owing to the lower oxygen concentration and higher nitrogen content.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

Microstructural and mechanical characterization of the Si3N4/Si3N4 joint brazed using Au–Ni–V filler alloys

In this study, two Au–Ni–V filler metals were used to braze Si₃N₄ ceramic in the form of foils. The effects of brazing temperature and V content in the filler alloy on microstructure and bonding strength of the joint were studied. The results reveal that a VN reaction layer with a thickness about 4 μm was formed at the interface between Si₃N₄ substrate and filler alloy. With increasing brazing temperature or V content the thickness of VN reaction layer increased. A maximum joint bending strength of 242 MPa was achieved when the joint was brazed at the temperature of 1423 K for 30 min using Au58.7Ni36.5V4.8 filler alloy. The bonding mechanism was discussed with reference to the discovered phases and brazing parameters.     

Effects of surfactant and particle size on the microstructure and strength of Si3N4 foams with high porosity

The highly porous Si₃N₄ ceramic foams were prepared by direct foaming with mixed surfactants. The surface tension and viscosity of slurries were tailored by different carbon chain length of surfactants and different mean Si₃N₄ particle size to achieve the pore size controlling. The nearly linear relationship between the pore size and the ratio of surface tension to viscosity was observed, which indicates that the pore size could be precisely tailored by the slurry properties. Si₃N₄ ceramic foams with porosity of nearly 94%, mean pore size of 110–230 µm, and compressive strength of 1.24–3.51 MPa were obtained.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Microstructure and mechanical properties of porous Si3N4/Invar joints brazed with Ag-Cu-Ti+Mo/Cu/Ag-Cu multi-layered composite filler

Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was successfully designed to braze porous Si₃N₄ and Invar alloy. To further reduce the CTE mismatch between the porous Si₃N₄ and brazing filler, Mo particles were introduced into Ag-Cu-Ti. The effects of the Mo addition on the microstructure and mechanical properties of the brazed joints were studied. The results showed that, the addition of Mo particles into Ag-Cu-Ti lowered the CTE mismatch and improved the joint strength to a certain degree. However, an excessive content was harmful. The Mo particles could absorb Ti at high temperature, causing Ti shortage in the reaction with the ceramic. When cooling down, the absorbed Ti was released. The released Ti could react with Cu to generate Cu-Ti phase. So, additional Ti was adopted in the brazing filler as a supplement. When the Ti content was 5 wt%, the reaction layer on the ceramic interface was too thin to transfer enough load. However, when it reached 15 wt%, the Cu interlayer dissolved completely and Fe-Ti and Ni-Ti phases appeared. The maximum joint shear strength (83 MPa) was obtained with 10 wt% Ti and 5 vol% Mo, which had exceeded 90% of the porous Si₃N₄ and was 56% higher than the joint brazed without Mo particles.     

Foaming of flat glass cullet using Si3N4 and MnO2 powders

The introduction of a compound capable of releasing oxygen, such as MnO₂, greatly improves the foaming ability of Si₃N₄ used as foaming agent in soda-lime glass powder, leading to expansion at a relatively low temperature (800–850 °C) and short processing time (7–30 min). The effect is based on the supply of oxygen, in addition to that in the furnace atmosphere. At the highest level of porosity, however, the strength of foams is negatively affected by a coarse microstructure, determined by cell coalescence. The reduction of firing temperature or, above all, the reduction of the processing time, was found to limit the coalescence and significantly improve the strength of the foams.     

Processing of highly porous TiO2 bone scaffolds with improved compressive strength

In the present study, the effect of sintering time and recoating procedures on the pore architectural parameters and compressive strength of highly porous ceramic TiO₂ foams were investigated. Long sintering times (>5 h) at 1500 °C led to an inward collapse of one wall of the triangular voids typically found in the strut interior of foams prepared using the replication method. This strut folding led to increased compressive strength, while the pore architectural features were not significantly affected. Furthermore, majority of the internal porosity of the foam struts was partially eliminated and became accessible for infiltration with TiO₂ slurry. Recoating procedures were found to markedly reduce the flaw size and number in the TiO₂ foam struts, which led to significant strengthening of the ceramic structure (0.7 → 3.4 MPa) by improved structural uniformity and slightly increased strut diameter.     

Structure–property relations of ferroelectric BaTiO3 ceramics containing nano-sized Si3N4 particulates

Barium titanate/silicon nitride (BaTiO₃/xSi₃N₄) powder (when x = 0, 0.1, 0.5, 1 and 3 wt%) were prepared by solid-state mixed-oxide method and sintered at 1400 °C for 2 h. X-ray diffraction result suggested that tetragonality (c/a) of the BaTiO₃/xSi₃N₄ ceramics increased with increasing content of Si₃N₄. Density and grain size of BaTiO₃/xSi₃N₄ ceramic were found to increase for small addition (i.e. 0.1 and 0.5 wt%) of Si₃N₄ mainly due to the presence of liquid phase during sintering. BaTiO₃ ceramics containing such amount of Si₃N₄ also showed improved dielectric and ferroelectric properties.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Processing of ceramic-metal interpenetrating composites

Interpenetrating composites consist of three-dimensionally interpenetrating matrices of two different phases; in the present work they were alumina and aluminium–magnesium alloys and were produced by infiltrating ceramic foams with molten alloys. The foams were made by mechanically agitating ceramic suspensions to entrain gases and then setting the structure via the in-situ polymerisation of organic monomers, a process known as gel casting. This resulted in the foams having a very open and interconnected structure that could be easily infiltrated by the molten metals. Previous composites have been produced at Loughborough University using squeeze casting; however, whilst infiltration was usually accomplished in a matter of seconds, the resulting size and shape of the composite was limited. Hence the present work has focused on investigating the potential for using gravity-fed infiltration. Whilst this was much slower, often taking several minutes, when optimised it is believed it will offer the potential for the production of large and complex-shaped pieces.The composites were produced at atmospheric pressure by infiltrating 2–10 wt.% magnesium content Al–Mg alloys into 20% dense Al₂O₃ foams with highly interconnected porosity. The processing parameters of temperature, ≥900 °C, and atmosphere, flowing N₂–Ar, were investigated to determine the processing window and infiltration kinetics. In-situ observation of the process shows that infiltration is faster at higher temperatures, Mg contents and N₂ partial pressures. Both optical and scanning electron microscopy (SEM) have been used to characterise the composites.