Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

Influence of Cu addition on the self-propagating high-temperature synthesis of Ti5Si3 in Cu–Ti–Si system

The self-propagating high-temperature synthesis (SHS) reactions can take place in Cu–Ti–Si systems with Cu additions of 10–50 wt.%, and the products only consist of Ti₅Si₃ and Cu phases, without any transient phase. In Ti–Si system, most of the Ti₅Si₃ grains synthesized exhibit the polygon-shaped coarse appearance with an obviously sintered morphology. When Cu content increases from 10 to 50 wt.%, however, the Ti₅Si₃ exhibits cobblestone-like shape with a relatively smooth surface, and its average size decreases significantly from 15 to 2 μm or less. The formation mechanism of Ti₅Si₃ in Cu–Ti–Si system is characterized by the solution, reaction and precipitation processes. Furthermore, the addition of Cu has a great influence on the volume change between green and reacted preforms. The volume change increases with Cu content increasing from 0 to 20 wt.%, and then decreases with the content further increasing from 20 to 50 wt.%. The addition of Cu to Ti–Si system significantly decreases the onset temperature of the reaction during differential scanning calorimetry process, which is even much lower than the α → β transition temperature of Ti (882 °C), suggesting that the reaction could be greatly facilitated by Cu addition. As a result, the role of Cu serves not only as a diluent but also as a reactant and participates in the self-propagating high-temperature synthesis reaction process.     

Effect of holding time on microstructure and mechanical properties of Si3N4/Si3N4 joints brazed with Au58.7Ni36.5V4.8 filler alloy

Si₃N₄ ceramics were brazed using Au–Ni–V metal foils at 1423 K for different holding times. Effect of holding time on microstructure and mechanical properties of the joints was investigated. The results indicate that a reaction layer of VN exists at the interface between Si₃N₄ ceramic and filler alloy. With increasing holding time from 0 to 90 min, thickness of the VN reaction layer increases from 0.4 to 2.8 μm, obeying a linear relation. Mechanism of the interfacial reaction was discussed by calculating the formation of free energy of VN. No specific orientation relationship exists between VN reaction layer and Si₃N₄ ceramic. In addition, Ni₃Si intermetallic compound appears in the joint when the holding time increases to 90 min, resulting in the deterioration of the joint strength.     

Effect of brazing temperature on microstructure and mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> joints brazed with Ag–Cu–Ti + Mo composite filler

Mo particles have been introduced into Ag–Cu–Ti brazing alloy for the joining of Si₃N₄ ceramic. Effect of brazing temperature on microstructure and mechanical properties of the joints were investigated. The result shows that a continuous reaction layer which is composed of TiN and Ti₅Si₃ was formed at the Si₃N₄/braze interface. The central part of the joint was composed of Ag-based solid solution, Cu-based solid solution, Mo particles, and Cu–Ti intermetallic compounds. By increasing the brazing temperature, both the thickness of the reaction layer and amount of Cu–Ti intermetallic compounds in the joint increased, being beneficial for the joint strength. Whereas, the reaction between Ti and Si₃N₄ ceramic proceeded excessively and more Cu–Ti intermetallic compounds were precipitated in the joint while elevating the brazing temperature to 950 °C, leading to deterioration of the bending strength. The maximal bending strength reached 429.4 MPa at 900 °C for 5 min when the Si₃N₄ ceramic was brazed with Ag–Cu–Ti + Mo composite filler.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Microstructure and brazing mechanism of porous Si3N4/Invar joint brazed with Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler

Porous Si_3N_4 was brazed to Invar alloy in this study, and Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was designed to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallic compounds. The effects of the brazing temperature and the thickness of Cu interlayer on the microstructure and mechanical properties of brazed joints were investigated. The typical microstructure of the joint brazed with multi-layered filler was porous Si_3N_4/TiN + Ti_5Si_3/Ag-Cu eutectic/Cu/Ag-Cu eutectic/Cu-rich layer + diffusion layer/Invar. When the brazing temperature increased, the reaction layer at the ceramic/filler interface grew thicker and the Cu interlayer turned thinner. As the thickness of Cu interlayer increased from 50 to 150μm, the joint strength first increased and then decreased. In this research, the maximum shear strength(73 MPa) was obtained when being brazed at 1173 K with a 100μm Cu interlayer applied in the filler, which was 55% higher than that brazed with single Ag-Cu-Ti brazing alloy and had reached 86% of the ceramic. The release of residual stress and the barrier effect of Cu interlayer to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallics played the major role in the improvement of joint strength.     

Joining of Si3N4 using Ag57Cu38Ti5 brazing filler metal

Hot-pressed Si₃N₄ was joined using Ag₅₇Cu₃₈Ti₅ brazing filler metal at 1103 to 1253 K for 5 min in a vacuum. The interface reactions between Si₃N₄ and the brazing filler metal during brazing are reported. An important event is sufficient interface reaction, characterized by the formation of a layer of TiN with an appropriate thickness at the ceramic-filler interface. The joining strength of the butt joint depends on the interface reaction, and a maximum joining strength of 490 MPa measured by the four-point-bend method is achieved for the Si₃N₄-Si₃N₄ joint brazed at 1153 K for 5 min. It is also discussed how to design the best brazing filler metal for joining ceramic to ceramic or ceramic to metal.     

A TEM analysis of the Si3N4/Si3N4 joint brazed with a Cu-Zn-Ti filler metal

Si₃N₄ ceramic was joined to itself using a filler alloy of (CuZn)85Ti15 at 1123–1323 K for 15 min. TEM observation showed that a reaction zone of TiN and/or Ti₂N exists at the interface between the ceramic and filler alloy, and the center of the joint is composed of Cu-Zn solid solution in which there are Cu₂TiZn and Ti₅Si₃ reaction phases. With increasing brazing temperature, both the thickness of the reaction zone and the amount and size of the Ti₅Si₃ phase increase, while the amount and size of the Cu₂TiZn phase decrease. When the brazing temperature reached 1323 K, the Cu₂TiZn phase disappeared. When the brazing temperature is lower than 1223 K, the interfacial reaction zone is mainly composed of Ti₂N, which has a cylindrical shape and orientates randomly in the zone. There is a crystal orientation relationship between the Ti₂N in the reaction zone and the Cu in the Cu-Zn solid solution, which is: {110}_Ti2N//{420}_Cu, 001_Ti2N//001_Cu. When the brazing temperature is higher than 1223 K, the interfacial zone is composed of TiN, which has a plate shape crossing each other.     

Thermal stability of nano-layered structure and hardness of TiAlN/Si3N4 nanoscale multilayered coating

Thermal stability of the TiAlN/Si₃N₄ nanoscale multilayered coating that was reported to show excellent hardness and toughness, has been investigated in terms of the nano-layered structure and hardness. TiAlN/Si₃N₄ nanoscale multilayered coatings with various thickness of Si₃N₄ layer were prepared by alternating deposition of TiAlN and Si₃N₄. In contrast to other nanoscale multilayered coating system such as AlN/CrN in which the intensity of the low angle XRD peaks decreases with increasing annealing temperature by interdiffusion between adjacent layers, the low angle XRD peak intensity of the nanoscale multilayered TiAlN/Si₃N₄ coatings increased after heat-treatment in an N₂ atmosphere up to 800 °C. Such a thermal stability of the nano-layered structure is believed to be due to spinodal type phase separation of TiAlN and Si₃N₄, which increased the hardness value of the TiAlN/Si₃N₄ coating at high temperatures.     

SPS制备氮化硅/锌铝基复合材料的组织和性能研究

通过放电等离子烧结工艺制备了氮化硅/锌铝基复合材料,重点探讨了氮化硅添加量对氮化硅/锌铝基复合材料致密度、硬度和摩擦性能的影响.采用扫描电子显微镜(SEM)及电子探针X射线显微分析仪(EPMA)对样品的微观组织进行了分析,并使用显微硬度仪、旋转摩擦试验仪对其性能进行了研究.结果表明:氮化硅在样品中分散均匀,且氮化硅的加...     

In situ synthesis of TiB whisker reinforcements in the joints of Al2O3/TC4 during brazing

Al₂O₃ ceramic has been successfully joined to Ti-6Al-4V alloy with Ag-Cu-Ti-B mixed powder. The TiB whiskers in the brazing layer were in situ synthesized during brazing. The effects of B content in reactant on the phase composition, microstructure and shear strength of the joints were investigated using SEM, EDS, and shear test. Results indicate that B content in the filler has a great impact upon the microstructure of the joints via exerting an influence on the volume fraction of in situ synthesized TiB whiskers. When the TiB content is 40 vol.%, the shear strength reaches the maximum value of 77.9 MPa. The higher content of TiB (≥40 vol.%) depresses the shear strength of the joints due to the interfacial thermal stress cannot be relaxed. Reaction phases (Ti₃Cu₂AlO, Ti₂Cu, Ti₂(Cu, Al), Ti(Cu, Al) and Ti₃Al) appear in the joint, moreover, as the volume fraction of TiB increase, Ag (s.s) and Ti(Cu, Al) distribute more uniform and fine in the brazing layer, as well as TiB whiskers mainly distribute in them. Eventually, Ti₃Cu₂AlO, TiB and TiB₂ firstly generate based on the thermodynamic analysis, and in excessive Ti circumstances, TiB whiskers remain in the brazing alloy.     

Enhanced mechanical response of hybrid alloy AZ31/AZ91 based on the addition of Si3N4 nanoparticles

The main aim of this study was to simultaneously increase tensile strength and ductility of AZ31/AZ91 hybrid magnesium alloy with Si₃N₄ nanoparticles. AZ31/AZ91 hybrid alloy nanocomposite containing Si₃N₄ nanoparticle reinforcement was fabricated using solidification processing followed by hot extrusion. The nanocomposite exhibited similar grain size to the monolithic hybrid alloy, reasonable Si₃N₄ nanoparticle distribution, non-dominant (0 0 0 2) texture in the longitudinal direction, and 13% higher hardness than the monolithic hybrid alloy. Compared to the monolithic hybrid alloy (in tension), the nanocomposite simultaneously exhibited higher yield strength, ultimate strength, failure strain and work of fracture (+12%, +5%, +64% and +71%, respectively). Compared to the monolithic hybrid alloy (in compression), the nanocomposite exhibited higher yield strength and ultimate strength, lower failure strain and higher work of fracture (+35%, +4%, −6% and +6%, respectively). The beneficial effects of Si₃N₄ nanoparticle addition on the enhancement of tensile and compressive properties of AZ31/AZ91 hybrid alloy are investigated in this paper.     

Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Ferrimagnetic and semiconducting CaCu3Fe2Sb2O12 by first principles

CaCu₃Fe₂Sb₂O₁₂ is mechanically stable, thermodynamically stable at pressures above 18 GPa. Both GGA and GGA + U methods predict that it is a ferrimagnetic semiconductor with Fe³⁺ in high spin state (S = 5/2). The coupling of Fe–Cu is antiferromagnetic, while that of Cu–Cu is ferromagnetic. The calculated total spin moment is 6.17 μ_B.     

Doping-induced microstructural, textural and optical properties of In2Ti1−xVxO5+δ semiconductors and their role in the photocatalytic splitting of water

The physicochemical properties of V-doped indium titanates (In₂Ti_1−xV_xO_5+δ, 0.0 ≤ x ≤ 0.2) were investigated by using XPS, powder XRD, UV–vis, SEM and luminescence spectroscopy techniques. The Rietveld refinement of XRD data revealed that even though the V-containing samples were isostructural with In₂TiO₅ (orthorhombic space group Pnma), a systematic x-dependent variation was noticeable in the Ti–O bond lengths in [TiO₆] octahedral units, cell parameters and in the value of δ. XPS results confirmed the coexistence of V⁵⁺ and V⁴⁺ states, leading thereby to an enhancement in oxygen non-stoichiometry in the doped samples. A loading-dependent progressive shift from 400 to 750 nm was also observed in the onset of the absorption edge, indicating a significant narrowing of the band gap. Furthermore, the samples with higher V-content were comprised of the grain clusters having larger size and an irregular shape. The UV–vis, photoluminescence and thermoluminescence studies indicate that the doping-induced lattice defects may give rise to certain closely spaced acceptor/donor energy levels in between the band gap of host matrix. The indium titanates are found to serve as stable photocatalysts for water splitting under visible light, where oxygen was the major reaction product. The role of microstructural and morphological properties in the photocatalytic activity is discussed.     

Microstructure and hardening mechanisms in a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanocrystalline Si₃N₄/TiN nanostructured multilayer films were fabricated by radio-frequency reactive magnetron sputtering. The microstructure and properties of these films were measured using an X-ray diffractometer, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and nanoindenter. The superhardness effect was found in Si₃N₄/TiN multilayers. The hardness of Si₃N₄/TiN multilayers is affected not only by modulation periods, but also by layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500 °C and a layer thickness ratio (l_Si3N4/l_TiN) of 3/1. The hardening mechanisms in this system are discussed in the light of our experimental results. Results of calculation of the theoretical stress distribution in the multilayers suggests that alternating stress fields caused by thermal mismatching between Si₃N₄ and TiN is one of the main reasons for the superhardness effect observed in Si₃N₄/TiN multilayers.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Phase stabilities of self-organized nc-TiN/a-Si3N4 nanocomposites and of Ti1 − xSixNy solid solutions studied by ab initio calculation and thermodynamic modeling

Bulk properties of stable binary fcc-TiN and hcp(β)-Si₃N₄, hypothetical fcc-SiN and hcp(β)-Ti₃N₄, and ternary Ti_1 − xSi_xN_y phases are calculated by ab initio method. The values of total energies are then used for thermodynamic calculations of the lattice instabilities of hypothetical binary phases of fcc-SiN and hcp-Ti₃N₄, and of the interaction parameters of ternary Ti_1 − xSi_xN_y phases. Based on these data, Gibbs free energy diagrams of the quasi-binary TiN_y-SiN_y system are constructed in order to study the relative phase stability of the metastable ternary fcc- and hcp-Ti_1 − xSi_xN_y phases over the entire range of compositions. The results are supported by the published data from chemical and physical vapor deposition experiments. The constructed Gibbs free energy diagram and phase selection diagram of quasi-binary TiN_y-SiN_y system in fcc structure show that metastable fcc-Ti_1 − xSi_xN coatings should undergo chemically spinodal decomposition into coherent fcc-TiN and fcc-SiN. Due to a high lattice mismatch between fcc-TiN and hcp-Si₃N₄, and to much higher lattice instability of fcc-SiN with respect to stable hcp-Si₃N₄, only about one monolayer of pseudomorphic SiN_y interfacial phase is stable.     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.