Self-joining of Si3N4 using metal interlayers

This work focuses on various aspects of diffusion bonding of Ti-foil and Nb-foil interlayers during the self-joining of Si₃N₄. Joints were diffusion joined by hot-uniaxial pressing at temperatures ranging from 1200 °C to 1600 °C using different holding times. The microstructural characterization of the resulting interfaces was carried out by scanning electron microscopy, electron-probe microanalysis (EPMA), and X-ray diffraction. The results showed that Si₃N₄ could not be bonded to Ti at temperatures lower than 1400 °C; however joining was successful at higher temperatures. On the other hand, Si₃N₄ was solid-state bonded to Nb at temperatures ranging from 1200 °C to 1600 °C. Joining occurred by the formation of a reactive interface on the metal side of the joint. Ti₅Si₃, TiSi, and TiN were detected at the Si₃N₄/Ti interface, and Nb₅Si₃ and NbSi₂ at the Si₃N₄/Nb interface, resulting from high-temperature reaction between Ti or Nb and Si₃N₄. Four-point bending testing gave a maximum joint strength of 147 MPa for Si₃N₄/Ti/Si₃N₄ samples hot pressed at 1500 °C and 120 minutes. However, strong joints were obtained above 1450 °C (>100 MPa). These results indicated that there is a strong relationship between the thickness of the interface and the mechanical strength of the preceding joints. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Infrared brazing Cu and Ti using a 95Ag-5Al braze alloy

Dynamic wetting angle measurements, microstructural evolution, reaction kinetics, and shear strength of infrared brazing Cu and Ti using a 95Ag-5Al braze alloy are evaluated. The specimen infrared brazed at 900 °C consists mainly of Cu₂Ti and Cu₄Ti. Both CuTi and Cu₄Ti₃ are observed at the interface between the braze and Ti substrate. Microstructures of Ti/95Ag-5Al/Cu joints infrared brazed at 830 °C and 850 °C are very different from that of the joint infrared brazed at 900 °C, because the dissolution of both substrates significantly decreased as the brazing temperature decreased. Specimens infrared brazed at 830 °C and 850 °C are primarily comprised of Ag-Cu eutectic and the Cu-rich phase. Two interfacial reaction layers, including Ti₂Cu and AlCu₂Ti, are found in the experiment. The shear strengths of infrared brazed specimens at 830 °C and 850 °C are between 160 and 198.5 MPa, and are fractured along the interfacial reaction layers, AlCu₂Ti and Ti₂Cu, between the braze alloy and Ti substrate. The use of the infrared brazing provides an effective way to inhibit the growth of intermetallics at the interface between the braze alloy and substrate.     

Stress relaxation and bonding in Si3N4/MA6000 joints by reactive interlayers

Diffusion bonding of Si₃N₄ to the new generation of ODS-superalloys, such as MA6000, may yield strongly joined metal-ceramic systems for high temperature applications. Si₃N₄ has been diffusion bonded to MA6000 during HIP at 100 MPa at 1100-1300°C. Stresses caused by the large thermal mismatch were reduced by multiphase interlayers. To promote the chemical adhesion, reactive and adhesive interlayers were used at the metal-ceramic interface which in the absence of such layers fail at low stresses. It has been shown that during reactive bonding brittle phases are frequently formed at the interfaces which may lead to a failure of the joint. The reduce of thermal stresses by thin soft interlayers is very limited but can be obtained by a microcrack induced stress relaxation mechanism. During adhesive diffusion bonding the mechanical strength of the bond is limited by the stress state and the strength of the ceramic component.     

Brazing Inconel 625 Using Two Ni/(Fe)-Based Amorphous Filler Foils

For MBF-51 filler, the brazed joint consists of interfacial grain boundary borides, coarse Nb₆Ni₁₆Si₇, and Ni/Cr-rich matrix. In contrast, the VZ-2106 brazed joint is composed of interfacial Nb₆Ni₁₆Si₇ precipitates as well as grain boundary borides, coarse Nb₆Ni₁₆Si₇, and Ni/Cr/Fe-rich matrix. The maximum tensile strength of 443?MPa is obtained from the MBF-51 brazed specimen. The tensile strengths of VZ-2106 brazed joints are approximately 300?MPa. Both amorphous filler foils demonstrate potential in brazing IN-625 substrate.     

Fabrication and characteristics of AA6061/Si3N4p composite by the pressureless infiltration technique

The tensile properties and microstructures of AA6061/Si₃N₄ particle composites fabricated by pressureless infiltration under a nitrogen atmosphere were analyzed. In addition, the control AA6061 without Si₃N₄ particles fabricated by the same method was investigated to separate the effect of Si₃N₄ particle addition. It was found that AlN particle layers formed on the surface of Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the following reaction: Mg₃N₂ + 2Al → 2AlN + 3Mg. Thus, the spontaneous infiltration results from a great enhancement of wetting via the formation of Mg₃N₂ by the reaction of Mg vapor and nitrogen gas. The increased tensile strength and 0.2 pct offset yield strength in the control AA6061 were largely due to fine AlN particles formed by the aforementioned in situ reactions, as compared to commercial AA6061. In the composite reinforced with Si₃N₄ particles, of course, the AlN was also formed through the following additional reaction at the Si₃N₄ particle/Al melt interfaces: Si₃N₄ + 4Al → 4AlN + 3Si. However, this AlN may not contribute to the increase in strength because its formation is compensated by the consumption of Si₃N₄ particles. Consequently, the strength increase of the composite fabricated by the present method is attributed to the fine AlN particles formed in situ, as well as the fine reinforcing Si₃N₄ particles, as compared to commercial AA6061.     

Infrared Brazing Fe<Subscript>3</Subscript>Al Using Ag-Based Filler Metals

The microstructural evolution and bonding shear strength of infrared brazed Fe₃Al using Ag and BAg-8 (72Ag-28Cu in wt pct) braze alloys have been studied. The Ag-rich phase alloyed with Al dominates the entire Ag brazed joints, and the shear strength is independent of the brazing time. The BAg-8 brazed joint contains Ag-Cu eutectic for all brazing conditions, and its shear strength increases slightly with increasing brazing time. The highest shear strength of 181 MPa is acquired from the joint infrared brazed at 1073 K (800 °C) for 600 seconds. A thin layer of Fe₃Al is identified at the interface between the brazed zone and the substrate for both braze alloys. An Al depletion zone in the Fe₃Al substrate next to the interfacial Fe₃Al is identified as the α-Fe phase. The dissolution of Al from the Fe₃Al substrate into the molten braze causes the formation of α-Fe in the Fe₃Al substrate.     

Brazing of Ti-6Al-4V and niobium using three silver-base braze alloys

Evaluations of the (infrared)-brazed Ti-6Al-4V and niobium joints using three silver-base braze alloys have been extensively studied. According to the dynamic wetting angle measurement results, the niobium substrate cannot be effectively wetted by all three braze alloys. Because the dissolution of Ti-6Al-4V substrate causes transport of Ti into the molten braze, the molten braze dissolved with Ti can effectively wet the niobium substrate during brazing. For infrared-brazed Ti-6Al-4V/Ag/Nb joint, it is mainly comprised of the Ag-rich matrix. The TiAg reaction layer is observed at the interface between the braze and Ti-6Al-4V substrate. In contrast, Ti-rich, Ag-rich, and interfacial TiAg phases are found in the furnace-brazed specimen. The dominated Ti-rich phase in the joint is caused by enhanced dissolution between the molten braze and Ti-6Al-4V substrate. The infrared-brazed Ti-6Al-4V/72Ag-28Cu/Nb joint is mainly comprised of the Ag-rich matrix and Ag-Cu eutectic. With increasing the brazing temperature or time, the amount of Ag-Cu eutectic is decreased, and the interfacial Cu-Ti reaction layer(s) is increased. The infrared brazed joint has the highest average shear strength of 224.1 MPa. The averaged shear strength of the brazed joint is decreased with increasing brazing temperature or time, and its fracture location changes from the braze alloy into the interfacial reaction layer(s) due to excessive growth of the Cu-Ti intermetallics. The infrared-brazed Ti-6Al-4V/95Ag-5Al/Nb joint is composed of Ag-rich matrix and TiAl interfacial reaction layer. With increasing the brazing time, the amount of Ag-rich phase is greatly decreased, and the interfacial reaction layer becomes Ti₃Al due to enhanced dissolution of Ti-6Al-4V substrate into the molten braze. The average shear strength of the infrared-brazed joint is 172.8 MPa. Additionally, the existence of an interfacial Ti₃Al reaction layer significantly deteriorates the shear strength of the furnace-brazed specimen.     

High-Temperature Oxidation Resistance of Ceramic Matrix Composites Si3N4 – Cf

We have studied the behavior of ceramic matrix composites in the system Si₃N₄ – C_f, reinforced with different types of carbon fibers, at moderate (1100°C) and maximum (1500°C) service temperatures. We have studied the effect of the composition of ceramic matrix composites and the oxidation temperature conditions on their strength properties and microstructure.     

Brazing Inconel 625 Using the Copper Foil

Brazing Inconel 625 (IN-625) using the copper foil has been investigated in this research. The brazed joint is composed of nanosized CrNi₃ precipitates and Cr/Mo/Nb/Ni quaternary compound in the Cu/Ni-rich matrix. The copper filler 50 μm in thickness is enough for the joint filling. However, the application of Cu foil 100 μm in thickness has little effect on the shear strength of the brazed joint. The specimen brazed at 1433 K (1160 °C) for 1800 seconds demonstrates the best shear strength of 470 MPa, and its fractograph is dominated by ductile dimple fracture with sliding marks. Decreasing the brazing temperature slightly decreases the shear strength of the brazed joint due to the presence of a few isolated solidification shrinkage voids smaller than 15 μm. Increasing the brazing temperature, especially for the specimen brazed at 1473 K (1200 °C), significantly deteriorates the shear strength of the joint below 260 MPa because of coalescence of isothermal solidification shrinkage voids in the joint. The Cu foil demonstrates potential in brazing IN-625 for industrial application.     

Resistive Properties of the Sintered Ceramic Composite Si3N4 - SiC

A technology has been developed for activated sintering of resistive Si₃N₄ - SiC ceramic composite. The microstructure, electrophysical properties and strength of the materials obtained have beenstudied over a wide range of concentration.     

Microstructure and thermal stability of coated Si3N4 and SiC

Microstructure of coatings obtained by treating Si₃N₄ and SiC in Cr powders at 1273–1623 K has been studied employing XRD, SEM, AES and TEM. In accordance with thermodynamic calculations and kinetic consideration, the coatings have layered structures and contain metal-rich silicides and metal-rich nitrides or carbides. The microstructure of the coatings has been found to depend on the treatment conditions. The kinetics of the coatings growth obeys a parabolic growth law, the activation energies being close to the activation energies for self-diffusion of the corresponding metals. Thermal stability of the coated and uncoated Si₃N₄ and SiC in Fe-, Ni- and Co-based matrices has been studied and the coatings have been found to considerably improve the stability of Si₃N₄- and SiC-metal interfaces.     

Reactions in the systems MoSi3N4 and NiSi3N4

The reaction products, formed during annealing of porous powder mixtures of Si₃N₄ with non-nitride forming metals like Ni or Mo, will depend on the partial pressure of N₂ in the atmosphere. In a diffusion couple, however, nitrogen has to be released at the Si₃N₄-interface during the formation of a metal silicide. It cannot escape easily and builds up a higher pressure of nitrogen at this interface. Therefore, the reaction products are different from those in porous pellets. This has been verified for NiSi₃N₄ and MoSi₃N₄ couples. The role of traces of oxygen on these reactions will be discussed.     

钎焊温度和热处理对钨铜连接性能的影响

采用厚20 μm的非晶态Ti-Zr-Ni-Cu钎料,真空钎焊连接用于聚变堆面向等离子体部件的钨和铜铬锆合金,钎焊温度分别为860、880和900℃,对880℃下的钎焊样品进行热等静压(HIP)处理.采用SEM和EDS分析连接接头的形貌和成分,用静载剪切法测量焊接接头强度.测试结果表明在860～880℃下钎焊10 min能够获得较好的连接界面,经880℃钎焊后焊接接头的剪切强度为16.57 MPa,880℃钎焊后HIP处理的试样界面结合强度提高至142.73 MPa,说明真空钎焊后HIP处理可以显著改善接头的结合强度.     

Mechanical characteristics of electrotechnical materials of the system Si3N4-SiC

Conclusions With selected optimal technology of producing materials Si₃N₄,-SiC, their mechanical characteristics may change within fairly broad limits, and they are determined primarily by the composition of the initial charge. Material with optimal composition has a bending strength of 500 MPa and a critical stress intensity factor 6.8 MN/m^3/2.To obtain ceramics with high values of _b and K_1c, it is expedient to use finely disperse highly active silicon carbide (10–30 volume%), and also oxide-free-activators for hot pressing.Increasing the grain size of the conducting phase SiC to 120 m and the amount of activating additive leads to reduced _b of the materials, however, the overall level of strength remains fairly high (>200 MPa).Translated from Poroshkovaya Metallurgiya, No. 1(313), pp. 57–61, January, 1989.     

In-Situ Reactive Synthesis of Si2N2O Ceramics and Its Properties

Si₂N₂O is considered as a new great potential structural/functional material in place of Si₃N₄ for high-temperature applications. In the present work, Si₂N₂O ceramics were in-situ reactive synthesized by a nitridizing powder mixture of Si and SiO₂ using an optimized two-step sintering process according to thermodynamic analyses. The results showed that the purity of Si₂N₂O in the produced ceramics increased with an increase in final sintering temperature, while the shrinkage and Vickers hardness decreased. After final sintering above 1923?K (1650?°C), pure nanograined Si₂N₂O ceramics can be obtained. Flexural strength and fracture toughness both showed peak values at 1873?K (1600?°C). The reaction mechanism was proposed and then the difference of the produced ceramics was discussed.     

Synthesis and characterisation of Si3N4p/Cu nanocomposite powders by high energy ball milling

Abstract

The present work reported the preparation of Cu–25 wt-%Si₃N₄ nanocomposite powders via high energy ball milling (HEBM). The phases and morphologies of as-milled powders with various milling times were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that with increasing the milling time, the irregularly shaped Cu powder became flattened, and, subsequently, refined and near spherical. After 12 h milling, the particle size of Cu–Si₃N₄ composite powders was in the range of 200–300 nm, while the grain size of Si₃N₄ particulates, 10–25 nm, was well within a nanometre scale. A uniform distribution of the nanosized Si₃N₄ reinforcing phase throughout the Cu matrix was successfully obtained. A reasonable mechanism for the formation of Cu–Si₃N₄ nanocomposite powders during HEBM was also proposed.     

Improving the Strength of the ZrC-SiC and TC4 Brazed Joint Through Fabricating Graded Double-Layered Composite Structure on TC4 Surface

In order to improve the ZrC-SiC ceramic and TC4 brazed joint property, graded double-layered SiC particles (SiCp)-reinforced TC4-based composite structure (named as GLS for convenience) was designed to relieve the residual stress in the joint. The GLS was successfully fabricated on TC4 substrate by double-layered laser deposition technology before the brazing process. The investigation of the GLS shows that the volume fraction of SiCp in the two composite layers was graded (20 and 39 vol pct, respectively). Ti₅Si₃ and TiC phases formed in the GLS due to the reaction of SiCp and TC4. The laser power-II (the laser power for the second deposition layer) affected the microstructure of the GLS significantly. Increasing the laser power-II would promote the reaction between the SiCp and TC4. But the high laser power-II made the layer I remelt completely and the two layers became homogeneous rather than graded structure. In the ZrC-SiC and TC4 brazed joint, the CTE (coefficient of thermal expansion) was graded from the TC4 to the ZrC-SiC due to the GLS, and the strength of the joint with the GLS (91 MPa) was higher than that without the GLS (43 MPa).     

Diffusion bonding of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-TiN composite with nickel-based interlayers

The diffusion bonding of a Si₃N₄-TiN composite with Ni, INVAR (Fe-Ni alloy), and IN600 (Ni-Cr-Fe alloy) interlayers has been investigated between 1100 °C and 1350 °C, under argon or nitrogen atmosphere. For the chosen bonding conditions, the Si₃N₄ phase of the composite reacts with the interlayer phase, leading to the release of silicon and nitrogen, whereas the TiN phase remains stable. The bonding mechanisms with nickel and INVAR (Ni-Fe alloy) interlayers are rather similar. Released silicon diffuses into the reaction layer and into the interlayer, forming a solid solution, whereas the released nitrogen remains gaseous. The bonding rate depends then on the elimination rate of nitrogen from the reaction interface. The thermal stability of these joints is very high up to 1100 °C. However, the interfacial porosity and the internal stresses created by the high nitrogen pressure are pernicious for the mechanical strength. The bonding mechanism with IN600 (Ni-Fe-Cr alloy) interlayer is rather different. The released nitrogen can form nitrides with interlayer elements (Cr, Al). Released silicon diffuses into the reaction layer and forms silicides. The joint porosity is less significant for the IN600 interlayer, which suggests a good mechanical strength. However, the formation of silicide is pernicious, because of the brittleness of these phases.     

Infrared repair brazing of 403 stainless steel with a nickel-based braze alloy

Martensitic stainless steel (403SS) is extensively used for intermediate and low-pressure steam turbine blades in fossil-fuel power plants. The purpose of this investigation is to study the repair of shallow cracks on the surface of 403SS steam turbine blades by infrared repair brazing using rapid thermal cycles. A nickel-based braze alloy (NICROBRAZ LM) is used as filler metal. The braze alloy after brazing is primarily comprised of borides and an FeNi₃ matrix with different amounts of alloying elements, especially B and Si. As the brazing temperature increases, more Fe atoms are dissolved into the molten braze. Some boron atoms diffuse into the 403SS substrate primarily via grain boundary diffusion and form B-Cr-Fe intermetallic precipitates along the grain boundaries. The LM filler metal demonstrates better performance than 403SS in both microhardness and wear tests. It is also noted that specimens brazed in a vacuum have less porosity than those brazed in an Ar atmosphere. The shear strength of the joint is around 300 MPa except for specimens brazed in short time periods, e.g., 5 seconds in Ar flow and 30 seconds in vacuum. The fractographs mainly consist of brittle fractures and no ductile dimple fractures observed in the scanning electron microscope (SEM) examination.