Microstructure and mechanical properties of diffusion bonded SiC/steel joint using W/Ni interlayer

This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W₅Si₃ and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni₄W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.     

Joining of Si3N4/Si3N4 with CuNiTiB paste brazing filler metals and interfacial reactions of the joints

The joining of Si₃N₄/Si₃N₄ was carried out using CuNiTiB paste brazing filler metals. The maximum room-temperature three-point bend strength of the joints is 338.8 MPa. The cross-section microstructures of the joints and the element area distribution were examined by scanning electron microscope (SEM) equipped with wave dispersive X-ray spectroscopy (WDS). The phases appeared on the fracture surfaces of the joints were determined by means of X-ray diffraction analysis (XRDA) method. A model is established of the interfacial reactions between Si₃N₄ and the CuNiTiB brazing filler metals. With this model, the relationship between the joint strength and the interfacial reactions is discussed.     

Finite element method simulation of residual stresses in Al2O3–AISI 304 steel joints

Abstract

Thermal residual stresses are very detrimental to the mechanical resistance of metal–ceramic joints and thin metallic foils acting as stress relieving interlayers have been used to reduce their effect. The present work presents finite element method simulations of the residual stress field in Al₂O₃–AISI 304 steel joints using interlayers. Different interlayer materials (Ti, Ni, Mo, and Cu) were considered, either separately or in combination. Calculations show that among the different interlayer materials considered, Cu and Ti/Cu are most effective in reducing the thermal stresses and that this role is determined mainly by the ductility of the interlayer material. The calculated results were validated by shear tests performed on real joints obtained by diffusion bonding and it was concluded that residual stresses control the mechanical resistance of the joints.     

Kinetic and microstructural aspects of the reaction layer at ceramic/metal braze joints

The formation and stability of the reaction layer when brazing non-oxide ceramic materials were studied. Si₃N₄-Si₃N₄, SiC-SiC and Si₃N₄-stainless steel braze joints were produced and investigated. Several filler metals, most Cu- and Ag/Cu-based, containing different amounts of titanium were used to evaluate the effect of titanium on the formation and growth of the reaction layer. Some braze joints were processed using filler metals containing precious metals for high-temperature and oxidation-resistant applications. It was established that the matrix composition of titanium-bearing filler metals affects the ceramic wetting characteristics and the reaction layer kinetics. In the Si₃N₄ braze joints, the reaction layer consisted of TiN and titanium silicides. An activation energy corresponding to the diffusion of nitrogen in TiN was calculated for the growth of the reaction layer. During fabrication of the braze joints with precious-metal-containing filler metals at 1250°C, Si₃N₄ decomposed and a sound joint could not be processed. Premetallizing the Si₃N₄ with an AgCulnTi filler metal resulted in the formation of the reaction layer and permitted the fabrication of sound braze joints at 1250°C. Attempts to produce SiC braze joints with CuTi filler metals were unsuccessful owing to the decomposition of the SiC; a TiCreaction layer had developed, but this did not prevent the diffusion of copper into the ceramic substrate, nor did it slow down the decomposition of the SiC.Visiting Professor at Werkstoffwissenschaften, Aachen, Germany.     

Heat-resistant joints of Si3N4 ceramics with intermetallic compounds formed in situ

The possibility of the improvement on the heat resistance of Si₃N₄ ceramic joints with intermetallic compounds formed in situ was investigated. The Si₃N₄ ceramics were joined with Ti/Ni/Ti multi-interlayers between 1000 and 1150°C. The effects of various parameters, which include the thickness of Ti and Ni foils, the pressure imposed during bonding, the bonding temperature and the holding time, on the microstructures and the strength (both at room temperature and at high temperature) of the joints were studied. The results indicated that the sound joints with higher strength both at room temperature and at elevated temperature could be acquired with intermetallic compounds formed in situ under appropriate bonding parameters. The shear strength at 800°C could sustain about 88 MPa.     

Effects of temperature on interface microstructure and strength properties of titanium–niobium stainless steel diffusion bonded joints

Abstract

The effects of temperature on interface microstructure and strength properties of Ti/stainless diffusion bonded joint using Nb interlayer, processed in the temperature range 800–950°C for 1·5 h in vacuum were investigated. The stainless steel/Nb interface is free from intermetallic phase up to 900°C; however, Fe₂Nb+Fe₇Nb₆ phase mixture has been observed at 950°C processing temperature. The Nb/Ti interface is free from intermetallic for all processing temperatures. The maximum tensile strength of ～287 MPa (～90% of Ti) and shear strength ～222 MPa (～75% of Ti) along with 6·9% ductility have been achieved in the diffusion bonded joints, when processed at 900°C. The bonded samples failure takes place through the stainless steel/Nb interface for all processing temperatures during the loading.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.     

Transient liquid phase bonding of steel using an Fe–B interlayer

Transient liquid phase bonding processes have been performed to join two carbon steel tubes using Fe_96.2B_3.8 wt% amorphous ribbons as interlayers. Welding experiments were performed at the temperature T ≈ 1,250 °C for different durations and under pressures of 2, 3 and 4 MPa. From metallographic inspection it is concluded that the bonding process ends in 7.0 min if a pressure of 4 MPa is applied whereas the process results incomplete if less pressure is applied. A 1D model using a finite element method has been developed for the simulation of a transient liquid phase bonding process. This model was applied to the bonding of steel/Fe–B glassy metals/steel. The computed results allow us to understand the role played by the variables involved in the bonding process.     

Microstructure and mechanical properties of diffusion bonded W/steel joint using V/Ni composite interlayer

Diffusion bonding between W and steel using V/Ni composite interlayer was carried out in vacuum at 1050 °C and 10 MPa for 1 h. The microstructural examination and mechanical property evaluation of the joints show that the bonding of W to steel was successful. No intermetallic compound was observed at the steel/Ni and V/W interfaces for the joints bonded. The electron probe microanalysis and X-ray diffraction analysis revealed that Ni₃V, Ni₂V, Ni₂V₃ and NiV₃ were formed at the Ni/V interface. The tensile strength of about 362 MPa was obtained for as-bonded W/steel joint and the failure occurred at W near the V/W interface. The nano-indentation test across the joining interfaces demonstrated the effect of solid solution strengthening and intermetallic compound formation in the diffusion zone.     

Effect of a silicon sintering additive on solid state bonding of SiC to Nb

Pressureless-sintered (PLS) SiC was joined to Nb by solid state bonding in a vacuum. The joining strength of the PLS SiC/Nb joint increases to the saturated value of 108 MPa with increasing joining pressure at a joining condition of 1673 K and 7.2 ks. This saturated value of PLS SiC/Nb joint is higher than that of the reaction sintered (RS) SiC/Nb joint. The strength of SiC itself affects the strength of the SiC/Nb joint. The high stability of the intermediate phase Nb₅Si₃ in the interface at elevated temperature leads to the high heat resistance of the joint. The thickness of the intermediate phase Nb₅Si₃ in the PLS SiC/Nb system is lower than that of the RS SiC/Nb system at a constant joining time, although the activation energy, 452 kJ mol^–1, of growth for the phase in the PLS SiC/Nb system is almost the same as the RS SiC/Nb system, 456 kJ mol^–1. The rate constant of the growth for the phase in the PLS SiC/Nb system is lower than that in the RS SiC/Nb system. The excess silicon in RS SiC promotes the formation of the Nb₅Si₃ phase at the interface between SiC and Nb.     

BONDING OF CERAMICS TO METALS WITH ACTIVATED COATING LAYERS MADE BY PLASMA SPRAYING

Bonding of ceramics to metals was carried out by using thermal sprayed coatings as interlayer materials. This bonding method is referred as the thermal spray bonding. In the present study, Si₃N₄ and Al₂O₃ ceramics were bonded to SS41 mild steel using activated Ti-Cu multilayer coatings plasma-sprayed on the steel in a low pressure atmosphere. The bondability was estimated by SEM/EDX analysis and shear tests of the joints. The results were compared to those obtained using Ti-Cu multilayer foils as interlayers. When using Ti-Cu foils as interlayers, brittle Fe-Ti compound layers were formed in the joint area after long time heating at a bonding temperature of 900°C, which deteriorated the joint strength. In contrast with the activated interlayers made by plasma spraying, the Ti-Cu eutectic reaction took place uniformly at the joint immediately after heating to the bonding temperature. This improved the bondabilrty, and the resultant joints exhibited shear strengths of about 180 MPa.     

Pressure-assisted direct bonding of copper to silicon nitride for high thermal conductivity and strong interfacial bonding strength

To achieve superior thermal and mechanical properties of copper-bonded (Cu-bonded) Si₃N₄ substrate, a pressure-assisted direct bonded Cu (DBC) technique was applied to bond Cu foil with Si₃N₄ plate. The effects of oxide layer (SiO₂) thickness of Si₃N₄ plate on the microstructure, thermal and mechanical properties of the Si₃N₄-DBC samples were investigated. The successful bonding of Cu foil to Si₃N₄ plate was confirmed by the presence of the interfacial products of Cu₂MgSiO₄ and CuYO₂. Additionally, it was demonstrated that a thin SiO₂ layer can result in a discontinuous distribution of interfacial products while a thick one can lead to the formation of pores in SiO₂ layer. Notably, the sample prepared by Si₃N₄ plate with 5-μm-thickness SiO₂ layer and Cu foil with 5.9-μm-thickness oxide layer (Cu₂O) exhibited the optimally comprehensive properties with thermal conductivity of 92 W·m^?1·K^?1 and shearing strength of 102 MPa, which demonstrates significant promise for application in power electronic modules.

Graphical abstract

     

Silicon nitride-stainless steel braze joining with an active filler metal

Hot-pressed Si₃N₄ was brazed to 410-stainless steel using a Ag-Cu-Ti alloy foil in a vacuum. The occurrence of cracking due to processing was examined by systematically varying the brazing temperature and time between 840 and 900 °C and 6 and 60 min, respectively. Cracks were found in Si₃N₄ parallel to the bonding interface when the braze joints were processed at the lower temperatures (for all processing times at 840 °C and for times of 6 and 12 min at 860 °C). A reaction layer was observed to develop in the filler metal adjacent to Si₃N₄, rich in Ti and containing some Si. The thickness of this layer depended on brazing temperature and time. Microcracks were found in the reaction layer normal to the bonding interface in the joints processed at higher brazing temperatures (880 °C for 60 min and at 900 °C for 30 and 60 min). The low temperature cracks occurred, apparently, as a result of the incomplete relaxation of thermal stresses due to the presence of a hard continuous titanium strip in the filler metal; the high temperature microcracks seemed to be affected by the increase in thickness of the reaction layer and by the precipitation of intermetallic compounds. The compressive shear strength of the braze joints were evaluated and correlated with the cracking behaviour and microstructure changes in the joint. A strong braze joint was obtained when the reaction layer was relatively thin and no cracks were present in either the reaction layer or the Si₃N₄.     

Microstructure and bond strength of Ni-Cr steel/Si3 N4 joint brazed with Ag-Cu-Zr alloy

Abstract

The interfacial microstructure and bond strength were examined for a Ni-Cr steel/Si3 N4 joint brazed using an Ag-Cu eutectic alloy containing 5 wt-%Zr. The reaction of Si₃N₄ with the brazing alloy formed a very thin ZrN layer with a cubic structure (a = 4.577 Å) at the interface. No Zr silicide (Zr₅Si₃) was present at the interface even though its formation is thermodynamically possible. The reaction product did not contain any of the dilute ceramic phases or intermetallics which are commonly seen in other active metal brazing systems. This strongly implies that the elements of the Ni-Cr steel and Ag or Cu in the brazing alloy, did not participate in the interfacial reaction. The shear strength of the joint was strongly dependent on the thickness of the reaction layer and the morphology of CuZr₂ precipitates in the brazement. A joint with a reaction layer thickness of 0.5 μm, which was formed by brazing at 950°C for 30 min, showed the highest fracture shear strength (～ 202 MPa).     

Transient liquid phase bonding of magnesium alloy (Mg – 3Al – 1Zn) using copper interlayer

Abstract

The mechanism, microstructure and mechanical properties of transient liquid phase (TLP) bonded magnesium alloy (Mg – 3Al – 1Zn) joints using copper interlayers in an argon atmosphere have been investigated. The formation process of the TLP joint comprises a number of stages: plastic deformation and solid state diffusion, dissolution of the interlayer and base metal, isothermal solidification and homogenisation. The composition profiles and microstructures of the joint depend on the bonding time at a temperature of 530°C. With an increase in bonding time from 10 to 60 min, the concentration of copper and the amount of CuMg₂ compound in the joint decrease. For longer bonding times, the most pronounced features of the joint are composition homogenisation, grain coarsening and elimination of the bond line within the joint centre. The presence of brittle CuMg₂ and grain coarsening of the joint are the main reasons for impairing the joint shear strength. A joint shear strength of 70.2 MPa, which is 85.2% of the base metal strength (82.4 MPa), can be achieved by bonding at 530°C for 30 min.     

Solidification microstructures and mechanical properties of vertical centrifugal cast high speed steel

Abstract

This study was undertaken to investigate the influence of Nb and V alloying elements and manufacturing conditions on the microstructural behaviour and mechanical characteristics of HSS (high speed steel) roll manufactured by a VCC (vertical centrifugal casting) process. In the Fe - 2C - 6Cr-1.5W - 3Mo - 4V alloy, the amount of MC carbide was increased and the the amount of M₇C₃ carbide decreased with an increase in V and Nb content. In steel containing 3%Nb, primary NbC carbide was formed within the cell in the matrix. The hardness of steel containing 6.5%V but no Nb was increased a little but when 9%V was added, the hardness decreased in the specimen owing to the soft ferritic matrix. The hardness of the matrix in steel containing 1.5%Nb increased, but decreased for 3%Nb addition. In wear tests, wear loss decreased with increasing rotational wear speed.     

Vacuum diffusion bonding of TiB2 cermet to TiAl based alloys

Abstract

Vacuum diffusion bonding of TiB₂ cermet to TiAl based alloys was carried out at 1123 – 1323 K for 0.6 – 3.6 ks under 80 MPa. The microstructural analyses indicate that a compound Ti(Cu, Al)₂ is formed in the interface of the TiB₂ /TiAl joints, and the width and quantity of the Ti(Cu, Al)₂ compound increase with the increase of the bonding temperature and bonding time. The experimental results show that the shear strength of the diffusion bonded TiB₂ /TiAl joint is 103 MPa, when TiB₂ cermet is bonded to TiAl based alloy at 1223 K for 1.8 ks under 80 MPa.     

Solid-state bonding of alumina to austenitic stainless steel

A low temperature and low pressure bonding process for alumina and 316L austenitic stainless steel has been developed using a titanium/molybdenum laminated interlayer. The intermetallic compounds of Ti₃Al (or Ti₂/Al) and TiAl were formed at the alumina/titanium interface on bonding at above 1273 K. The activation energy of the layer growth was about 142 kJ mol^–1. The construction of Al₂O₃/Ti/Mo/steel gave the most stable joints. The highest tensile strength was above 60 MPa with a titanium 0.4 to 0.6mm thick/molybdenum 0.4 to 0.5 mm thick interlayer on bonding at 1273 K for 3 h under pressure of 12 MPa.     

Friction and wear behaviour of β-silicon nitride–steel couples under unlubricated conditions

Abstract

The friction and wear behaviour of colloidally processed and pressureless sintered β-silicon nitride (Si₃N₄) ceramics against steel DIN Ck45K under unlubricated condition were investigated using a pin on disk tribometer. β-silicon nitride (Si₃N₄) ceramics consolidated by slip casting from suspensions with different solid loading have been studied, aiming at increasing the use of β-Si₃N₄ as cutting tools in industrial applications. The morphology of the worn surfaces of β-silicon nitride ceramics was analysed by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Under the conditions used, it was found that β-Si₃N₄ ceramics exhibited a low wear rate (10^?6 mm³ N^?1 m^?1) and the frictional behaviour of β-Si₃N₄ ceramics–steel couples depended on a metallic layer transferred from the steel disk to the β-Si₃N₄ ceramic pin.     

Compressive creep behaviour of Yb4Si2O7N2 containing silicon nitride ceramic between 1400 and 1500°C

Abstract

A Yb₂O₃-SiO₂ doped silicon nitride ceramic, prepared such that the composition was placed directly on the Yb₄Si₂O₇N₂-Si₃N₄ tie line, was hot pressed sintered. The compressive creep behaviour of the sintered Yb₄Si₂O₇N₂-Si₃N₄ material was examined at 1400, 1450 and 1500°C under a stress range of 250-400 MPa in a nitorgen atmosphere. The sintered material exhibited high resistance to creep. The stress exponents were found to be ~1.9 at 1400°C, ~2.1 at 1450°C and ~2.1 at 1500°C. The activation energy obtained was 510 ± 25 kJ mol^-1. The values of the stress exponents and the activation energy suggest a cavitational process, accommodated by grain boundary sliding, viscous flow and solution-reprecipitation, as the most probable dominant creep mechanism.