A study of the wetting,microstructure and bond strength in brazing SiC by Cu-X(X = Ti,V,Nb,Cr) alloys

In the active brazing of SiC by copper-based alloys, the effects of various active elements such as titanium, vanadium, niobium and chromium on the wetting, microstructure and bond strength are investigated. In wetting, Cu-Cr alloys have the lowest wetting angles on SiC of 10°–20° depending on the chromium content. SiC is decomposed on contact with alloy melts during brazing. Carbon and silicon released from this decomposition of SiC react with active elements to produce their carbides and suicides at the interface. The reacted layers have different microstructures depending on the brazing alloys, but Cu-Ti and Cu-Cr alloys show similar microstructure, as do Cu-V and Cu-Nb alloys. In the four-point bend tests, the brazed joints of Cu-5 at % Ti, Cu-5 at % V and Cu-5 at % Nb alloys have similar bend strengths of 86.9, 80.3 and 92.4 MPa, respectively. The brazed joints of Cu-2 at % Nb alloys show a high bend strength of 154 MPa, although the wetting angle is a little higher, at about 60°. Niobium is found as a new active element of copper-based alloys to braze SiC. Cu-Nb alloys are promising for substitution for Cu-Ti alloys.     

Advances in brazing of ceramics

The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with primary emphasis on those influencing wetting of solid ceramics by liquid filler metals. In general, wetting of ceramics by conventional brazing alloys has been regarded as difficult. As a consequence, premetallization of the faying surfaces is frequently used to facilitate the brazing of ceramics. However, it is evident from the literature that recent developments in filler metals, based on active metal (e.g. Ti) additions (the amount depending on alloy composition and type of ceramic), have provided a basis for a substantial reduction of the contact angle. This favourable effect is caused by their reactivity, resulting in the formation of oxides when joining oxide ceramics (e.g. Al₂O₃), and nitrides or carbides and suicides in the case of nonoxide ceramics (e.g. Si₃N₄ or SiC). In addition to insufficient wetting, the mismatch in thermal expansion between the joint members may give rise to a low strength level due to the formation of high residual stresses on cooling. These stresses may limit the maximum allowable flaw size in ceramics to a few micrometres, i.e. of a similar size to that of pores.     

Microstructure of brazed joints between mechanically metallized Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and stainless steel

Brazing has been increasingly used to join metals to advanced ceramics. Brazing covalent materials requires either the use of active filler alloys or the previous metallization of the surface. To that end, a new and simple mechanical technique has been applied to metallize advanced ceramics, thus avoiding the use of costly Ti-based active filler alloys. The mechanical metallization of Si₃N₄ with Ti was employed as an alternative route to deposit active metallic films prior to brazing with stainless steel using 72% Ag--28% Cu or 82% Au—18% Ni eutectic alloys. The brazing temperatures were set to 40°C or 75°C above the eutectic temperature of each filler alloy. Ti-films of average thickness 4 μm produced adequate spreading of both filler alloys onto Si₃N₄ substrates, which were subsequently brazed to stainless steel. The interface of Si₃N₄/310 stainless steel basically consisted of a reaction layer, a precipitation zone and an eutectic microconstituent. Mechanically sound and vacuum-tight joints were obtained, especially upon brazing at relatively lower temperatures. Increasing the brazing temperature resulted in thermal cracking of the Si₃N₄, possibly due to increased thermal stress.     

Joining Joining of silicon nitride with Al-Cu alloys

The sessiie drop technique was used to evaluate the equilibrium contact angle and work of adhesion of molten Al-Cu alloys on Si₃N₄ at 1373 K under vacuum. The wettability of Al-Cu alloys on Si₃N₄ is improved by an addition of copper content up to 20 at%. The joining of Si₃N₄ to Si₃Ni₄ was also conducted using Al-Cu filler metal at a brazing condition of 1373 K for 3.8 ksec. The dependence of strength of the Si₃N₄ joint against the copper content in the filler corresponds to the copper content dependence of work of adhesion for molten Al-Cu alloy on Si₃N₄. The superior wettability and mechanical property of filler provide the superior strength of Si₃N₄ brazed with the filler. In particular, the Si₃N₄ joint brazed with Al-1.7 at% Cu filler exhibits the maximum fracture shear strength of 188.3 MPa at room temperature. This superior strength of Si₃N₄ brazed with Al-1.7 at% Cu filler is maintained at elevated temperatures up to 850 K.     

Some observations on the wetting and bonding of nitride ceramics

Several series of experiments have been conducted to gain information about the wettability of AlN, BN, Si₃N₄ and two sialon ceramics by potential braze materials. It was possible to achieve wetting of all five ceramics using aluminium, copper-titanium alloys, and a Ag-28Cu-2Ti alloy. Wetting by aluminium and the Ag-28Cu-2Ti alloy was usually good. Both wetting and non-wetting alloys containing titanium reacted to form TiN and it is argued that the achievement of wettability is associated with a certain degree of hypostoichiometry. While aluminium should also have reacted, no clear evidence was obtained. In supplementary experiments it was found that bonds formed by brazing with aluminium at 1000 °C could have shear strengths as great as 60MPa. Although the experimental work was preliminary in nature, it suggested that good brazing systems could be developed.     

Wetting of silicon nitride by alkaline-doped MgSiO3

The wetting behaviour of Si₃N₄ by alkaline-doped MgSiO₃ was investigated by the sessile drop method. It is shown that the alkaline oxide additions improve the wetting of MgSiO₃ on Si₃N₄. The hot-pressing of Si₃N₄ is controlled by a liquid phase sintering process where the dissolution of Si₃N₄ in silicate glass promotes good wetting and a well bonded interface by lowering the liquid-solid interfacial energy. Controlling total alkaline impurity level between 50 and 100 ppm is suggested for an optimal strength performance.     

The wetting,reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si₃N₄ by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si₃N₄ to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si₃N₄ was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si₃N₄(s) + 4Ti (1 ? sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si₃N₄, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si₃N₄ to Si₃N₄ was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.     

Structure of wetting front in the Ag-Cu-Ti/SiC reactive system

The nanostructure around the wetting triple line in the Ag-Cu-Ti/SiC reactive wetting system was studied. The reaction product consisted of an upper separated Ti₅Si₃ layer more than 20 nm thick, and a lower TiC layer less than 10 nm thick was observed exceeding the front line of Ag-Cu-Ti brazing metal. At the top of the reaction product, the lower TiC layer was observed to project beyond the front line of the upper Ti₅Si₃ layer. In Ag-Cu-Ti/Si₃N₄ system, structural change around the wetting triple line as reactive wetting progressed has been reported, however, in the Ag-Cu-Ti/SiC system, the structure around the triple line did not change. The difference between the two systems was explained from the viewpoint of a stable phase change with the decrease of Ti activity as reactive wetting progressed.     

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.     

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.     

Wetting of ceramics by molten silicon and silicon alloys: a review

The surface of silicon is very sensitive to interactions with oxygen present as impurity in furnace atmosphere. It is shown that three types of Si surfaces can be obtained depending on the oxygen’s partial pressure in the furnace and on temperature: oxidized, oxide-free but containing adsorbed oxygen and adsorption free. The influence of oxygen on the surface tension of molten Si is also discussed. Wetting by Si and Si alloys is then described and analysed for three types of ceramics: (i) ionocovalent oxides (Al₂O₃, SiO₂, MgO, etc.), with a particular emphasis on the Si/silica couple, (ii) the different types of carbons where wetting is assisted by the reaction between Si and carbon, and (iii) the predominantly covalent ceramics (SiC, Si₃N₄, AlN, BN, etc). The role of wetting in the processing of silicon or silicon-based multimaterials is also illustrated.     

Nitriding of Ferrosilicon Powders

The process of nitriding of high-silicon ferrosilicon alloys has been investigated. Formation of Si₃N₄ is found to proceed as a result of nitrogen diffusion into several eutectic melts of iron silicides and silicon. At T > 1673, the process of Si₃N₄ dissociation develops.     

Combining thermodynamics and DSC to characterize the melting and wetting behavior of a composite powder used for joining ceramics

The diffusion-assisted melting behavior of a Cu-coated Ti-powder was characterized as a function of the intermetallic formation and corresponding melting temperature using DSC. The endothermic and exothermic events were identified using the respective binary phase diagram. The results demonstrated that initial melting is independent of the heating rate; however complete melting occurred at 900°C when heated at 10°C/min as oppose to 915°C for a heating rate of 40°C/min. The wetting results of the brazing alloy on Si₃N₄ ceramic show that the brazing alloy maintains its initial shape until the complete melting is achieved. Equilibrium contact angle measurements obtained are similar to results previously published but the wetting kinetics observed are different.     

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.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

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.     

Microstructural characterisation of transient liquid phase zone of activated diffusion brazed nickel base superalloy

Abstract

Microstructural investigations of activated diffusion brazed (ADB) joints in IN738 plate were conducted. Joints of this type, which had wide gaps, were formed using the brazing alloys Nicrobraz 150, BRB, and DF4B. Tests on the wettability of the brazing alloys when associated with an IN738 surface showed that a slight degree of wetting was associated with a rapid onset of isothermal solidification and conversely a heavy degree of wetting was associated with a relatively low rate of isothermal solidification. The microstructure of the joint area was characterised using a scanning electron microscope and an electron probe X-ray microanalyser (EPMA). These showed that borides with a blocky morphology were present in joints associated with all three brazing alloys examined. A major difference in matrix phase chemistry was observed however for the three brazing alloys, e.g. an Ni–Ni₃ B eutectic phase was observed in Nicrobraz 150, but DF4B and BRB exhibited a coarsened gamma prime (γ′) phase and an absence of a Ni₃ B matrix phase.     

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.     

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.     

Microstructure characterization of in situ synthesized porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–Si<Subscript>2</Subscript>N<Subscript>2</Subscript>O composites using feldspar additive

Porous Si₃N₄–Si₂N₂O bodies fabricated by multi-pass extrusion process were investigated depending on the feldspar addition content (4–8 wt% Si) in the raw silicon powder. The diameter of the continuous pores was about 250 μm. The polycrystalline Si₂N₂O fibers observed in the continuous pores as well as in the matrix regions of the nitrided bodies can increase the filtration efficiency. In the 4 wt% feldspar addition, the diameter of the Si₂N₂O fibers in the continuous pores of the nitrided bodies was about 90–150 nm. A few number of rope typed Si₂N₂O fibers (∼4 μm) was found in the case of 8 wt% feldspar addition. However, in the 8 wt% feldspar addition, the matrix showed highly porous structure composed of large number of the Si₂N₂O fibers (∼60 nm). The relative densities of the Si₃N₄–Si₂N₂O bodies with 4 wt% and 8 wt% feldspar additions were about 65% and 61%, respectively.