Microstructure and reaction phases in Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was self-jointed using a filler alloy of Cu–Pd–Ti, and the microstructure of the joint was analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15 (at.%), a high quality Si₃N₄/Si₃N₄ joint was obtained by brazing at 1100–1200 °C for 30 min under a pressure of 2 × 10⁻³ MPa. The microstructure of the Si₃N₄/Si₃N₄ joint which was observed by EPMA, XRD and TEM, and the results indicated that a reaction layer of TiN existed at the interface between Si₃N₄ ceramic and filler alloy. The center of the joint was Cu base solid solution containing Pd, and some reaction phases of TiN, PdTiSi and Pd₂Si found in the Cu [Pd] solid solution.     

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.     

Formation mechanism of elongated β–Si3N4 crystals in Fe–Si3N4 composite via flash combustion

Elongated β–Si₃N₄ crystals have a significant influence on the mechanical property of Fe–Si₃N₄ composite. In this paper, the formation mechanism of elongated β–Si₃N₄ crystals in Fe–Si₃N₄ composite was investigated. During the preparation process, β–Si₃N₄ crystals developed in a spiral and layer growth mechanism in the dense areas. They kept growing from the dense areas and formed radially distributed elongated crystals with hexagonal prismatic morphology as time went on. As for the formation mechanism, the (100) crystal plane of β–Si₃N₄ from Si-N-O melt is mainly the vicinal crystal planes growth with different angles from the (100) crystal plane. At the later stage, the crystallization and the diffusion forces in Si-N-O molten phase decreased. However, the short range diffusion remained active and resulted in the gradient distribution of N content near the boundary. With the temperature decreasing, the disappearance of the short range diffusion implied the end of the crystallization process of the elongated β–Si₃N₄ crystals.     

Microstructure and mechanical properties of Al2O3/Cu joints brazed with Ag–Cu–Ti+Zn composite fillers

In this study, Al₂O₃ ceramic and Cu bars were brazed with newly designed Ag–Cu–Ti(ABA)+Zn composite fillers. Systematic analysis of the microstructure of the brazed joints indicated that the volatilization of Zn atoms during the brazing process could promote the spreading of liquid brazing fillers on the surface of the Al₂O₃ ceramic, resulting in a uniform dendritic interfacial structure. The typical interfacial structure was an Al₂O₃/TiO/(Cu, Al)₃Ti₃O+Ag(s, s)/Cu interface. Notably, the tensile strength was improved to 20.89 MPa for Al₂O₃/Cu joint brazed with ABA+Zn composite fillers at 900 °C for 20 min, approximately 67.6% higher than the sample brazed without Zn foil. In this case, the fracture model was straight and sharp-angled inside the Al₂O₃ ceramic. In addition, the joint strength decreased with increased brazing temperatures from 900 to 940 °C.     

Microstructure and joining strength evaluation of SiC/SiC joints brazed with SiCp/Ag–Cu–Ti hybrid tapes

SiC particulates were mixed with Ag–Cu–Ti powders to fabricate SiC_P/Ag–Cu–Ti (SICACT) sheets by tape casting process, which were used to braze the sintered SiC ceramics with the structure of SiC/Ag–Cu–Ti foil/SICACT sheet/Ag–Cu–Ti foil/SiC. Microstructure and joining strength both at room temperature and at high temperature were characterized by electron probe X-ray microanalyzer, electron dispersive spectroscopy, transmission electron microscopy, and flexural strength test. The SiC particulates from the SICACT sheets were randomly distributed in the filler alloy matrix and reacted with Ti from the filler alloy. Reaction products TiC and Ti₅Si₃ were found in the interfacial reaction layer. With the increase in SiC particulates volume fraction, the joining strength at room temperature first increased, and then decreased, which was affected by both CTE mismatch and the thickness of the reaction layer. In addition, the joining strength of joints brazed using SICACT sheets at 600?°C can reach 197 MPa, which was obviously higher than that brazed using Ag–Cu–Ti filler alloy.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

Enhanced mechanical properties and thermal cycling stability of Al2O3-4J42 joints brazed using Ag–Cu–Ti/Cu/Ag–Cu composite filler

This study investigates the mechanical properties and microstructure evolution of Al₂O₃-4J42 joints brazed using Ag–Cu–Ti (ACT) and Ag–Cu–Ti/Cu/Ag–Cu (ACTCA) fillers during thermal cycling from 0 °C to 500 °C. The reaction products between Al₂O₃ and brazing filler of these two types of joints are mostly composed of Ti–O compounds, Ti₄Cu₂O and Al-based compounds. Brittle intermetallic compounds (IMCs) are observed in ACT joints, but not found in ACTCA joints. The reaction layer in ACT joints becomes thinner and discontinuous with thermal cycles, while that in the ACTCA joints hardly changes. Besides, the stress-induced cracks occur within the Al₂O₃ ceramic near the Al₂O₃/filler interface in the ACT joints, but no crack is found in the ACTCA joints. The mechanical tests show that the ACTCA joints maintain at least 217%, 154% and 144% higher shear strength than the ACT joints at 0, 10 and 20 thermal cycles, respectively. The Cu interlayer with low yield strength releases stress through plastic deformation, meanwhile acts as a barrier to prevent elements diffusion and the formation of the brittle IMCs, thus improving the mechanical properties and thermal cycling stability of the joints.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

In situ preparation of Si3N4–TiN composite by pyrolysis of polytitanosilazane (PTSZ)

Si₃N₄–TiN composite powders were obtained by in situ pyrolysis of polytitanosilazane. Dense Si₃N₄–TiN composites were prepared by hot-pressing at 1800 °C under 20 MPa for 2 h without sintering additive. Crystallization of amorphous PTSZ powders occurred between 1400 and 1500 °C with major phases, α-Si₃N₄, β-Si₃N₄, and small amount of phase TiN. Mechanical properties and microstructure of Si₃N₄–TiN composites were characterized. The results showed that the mechanical strength was 620 MPa, the fracture toughness was 7.8 MPa m^1/2 and the Vickers hardness was 8.5 GPa. SEM analysis indicated that Si₃N₄–TiN composite possessed excellent fracture toughness because TiN grains produced by in situ pyrolysis were well dispersed in Si₃N₄ matrix.     

Si3N4粉粒度对振荡压力烧结Si3N4陶瓷的影响

为了提高Si₃N₄陶瓷的烧结致密度,采用振荡压力烧结工艺分别在1 745和1 775℃制备了Si₃N₄陶瓷,主要研究了Si₃N₄粉的粒度（平均粒径分别为0.4、2.0、2.3μm）对Si₃N₄陶瓷的显微结构和性能的影响。结果显示：1)在两种温度的振荡压力烧结工艺下,由三种不同粒度的Si₃N₄粉制备的Si₃N₄陶瓷的相对密度都很大,为99.65%～99.86%,彼此相差很小。2)由平均粒径为0.2μm的Si₃N₄粉在1 745℃烧结制备的试样的微观结构最均匀,其β-Si₃N₄晶粒平均长径比、抗弯强度和维氏硬度均最大,分别达到5.0、（1 364±65） MPa和（15.72±0.8） GPa;由平均粒径为2.3μm的Si₃     

Sn/Si3N4复合镀层中纳米Si3N4含量测定方法的比较

采用电熔解法溶解Sn/Si3N4复合镀层,以紫外可见分光光度法测定了镀层中纳米Si3N4的含量,并与传统酸溶解及重量法对比。结果表明,电解法在10 min内使镀层完全溶解且在测定波长448 nm处纳米Si3N4溶液的吸光度值与其浓度在10～200 mg/L范围保持良好的线性关系,该方法简单,省时,误差小,回收率98.5%～100%。     

Simultaneous synthesis and consolidation of nanostructured TaSi2–Si3N4 composite by pulsed current activated combustion

Dense nanostructured 4TaSi₂–Si₃N₄ composite was synthesized by pulsed current activated combustion synthesis (PCACS) method within 3 min in one step from mechanically activated powders of TaN and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense 4TaSi₂–Si₃N₄ composite with relative density of up to 98% was produced under simultaneous application of a 60 MPa pressure and the pulsed current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

C_f–Si₃N₄ sandwich composites were prepared by gelcasting using α-Si₃N₄ powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of C_f–Si₃N₄ sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 °C. The investigation of microwave absorbing property reveals that, compared with the Si₃N₄ ceramics, both the real (ε?$\epsilon ?$) and imaginary (ε??$\epsilon ??$) permittivity of C_f–Si₃N₄ sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the C_f–Si₃N₄ sandwich composites are significantly enhanced compared with pure Si₃N₄ ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si₃N₄ ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of C_f–Si₃N₄ sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε?$\epsilon ?$ and ωε??$\omega \epsilon ??$ are inversely proportional to the frequency square ω²${\omega }^2$, and the predicted results agree quite well with the measured data.     

Elastic–plastic analysis of ultrafine-grained Si2N2O–Si3N4 composites by nanoindentation and finite element simulation

Ultrafine-grained Si₂N₂O–Si₃N₄ composites are fabricated by hot-press sintering of amorphous nano-sized silicon nitride powders at 1600, 1650, and 1700 °C with nano-sized Al₂O₃ and Y₂O₃ as additives. Sintered materials of increasing average grain sizes of 280, 360, and 480 nm were obtained with increasing sintering temperature. Hardness and elastic modulus are tested by nanoindentation. Finite element simulations of the nanoindentation are performed to study the elastic and plastic mechanical properties based on the elastic modulus and P–h curve that were obtained through the nanoindentation tests. A theoretical method is proposed for calculating the stress–strain relationship of brittle ceramic materials based on the experimental nanoindentation data. Several relevant coefficients in the theoretical calculation formula are determined by comparing the calculation and simulation results.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

低温热压制备高硬度、高韧性Si3N4–SiCw–ZrB2陶瓷

通常低温热压烧结的Si₃N₄陶瓷具有较高的硬度和较低的断裂韧性;而高温热压烧结的Si₃N₄陶瓷具有较低的硬度和较高的断裂韧性。为了获得高硬度、高韧性Si₃N₄陶瓷,添加20%SiC_w（SiC晶须,体积分数）和2.5%ZrB2,在1 500℃低温热压制备了Si₃N₄基陶瓷,开展其相组成、致密度、显微结构和力学性能研究,并与1 800℃高温热压烧结Si₃N₄进行了对比研究。结果表明：SiC_w的引入阻碍了Si₃N₄低温致密化,致密度从97.9%降低到92.9%,Vickers硬度从20.5 GPa降低到16.4 GPa,断裂韧性从2.9 MPa·m^1/2增加到3.4 MPa·m^1/2。同步引入SiC_w和ZrB₂     

Paper derived SiC–Si3N4 ceramics for high temperature applications

Biomorphic Si₃N₄–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N₂ or N₂/H₂), in the temperature range of 1300–1450 °C SiC–Si₃N₄ ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si₃N₄ and/or β-Si₃N₄ phases depending on the nitridation conditions. The SiC–Si₃N₄ ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.     

Si3N4粉体表面化学分析及表面改性

Si3N4陶瓷是一种重要的结构陶瓷,本文着重阐述了Si3N4粉体的表面分析方法,以及为了提高固相体积分数而进行的Si3N4粉体表面改性方面的进展.     

非晶态Si3N4结晶及α-Si3N4晶种的调控结晶行为

采用液相界面反应法制备高α相的Si₃N₄粉体。以SiCl₄为硅源,液氨为氮源,甲苯为溶剂,通过液相界面法合成硅亚胺,硅亚胺在1 000℃分解为非晶态Si₃N₄粉体,在1 550℃晶化为α-Si₃N₄粉体。研究了非晶态Si₃N₄的晶化过程,并讨论了添加α-Si₃N₄晶种对非晶态Si₃N₄结晶过程的影响。结果表明:晶核从具有短枝状结构的非晶态Si₃N₄团聚体的表面和内部形成,进而生长成为α-Si₃N₄晶粒;添加α-Si₃N₄晶种,能够增加形核数量,加快了非晶态Si₃N₄向α-Si₃N₄的结晶转...