Al2O3-5 wt% Al composites by ICP sintering of synthesized precursor

Microstructure developments during the milling of Al₂O₃-5wt% Al composite powder in an attritor and subsequent sintering of the precursor by inductively coupled argon plasma are presented. After 4 h of milling the precursor contained tubular ceramic-metal and uniform ceramic regions. With an increase in the milling period the ceramic-metal regions broke into smaller and almost globular regions, and the smaller regions became dispersed in the ceramic regions. After 8 h of milling the composite powder had a stable microstructure and contained 0.25–0.35 m clusters. The sintered composite was > 99.7% dense and its microstructure consisted of ceramic-metal regions which were dispersed in the matrix of a ceramic region. The sizes of ceramic grains in ceramic-metal regions and the ceramic regions were 0.3–2.2 and 0.8–1.8 m, respectively. Many ceramic grains in ceramic-metal regions were separated by 30–100 nm wide metal layers. The microstructure of the ceramic-metal region showed many features of interpenetrating phase composites. The Knoop and Vickers microhardnesses of the composites at 5–10 N loads were 410–450. Under 10 N loads in Knoop and Vickers microhardness tests the crack length was 11±3 and 3 ± 0.5 m, respectively. The crack propogation mechanisms in the indented areas are discussed.     

Morphology,strength and pre-coating effect in YSZ/Nl bonding

The quality of ceramic-metal bond is strongly influenced by the microstructure of the transition region between the ceramic and the metal. Sandwich-like ceramic-metal-ceramic specimens are fabricated by solid state bonding of 3 mol% yttria doped zirconia ceramic with nickel foils. Time dependence of the shear strength of the bonding assembly is evaluated at 900° C under a bonding pressure of 8.17 MPa. An optimum strength is obtained for the bonding time between 10 and 25 minutes. The shear strength is also measured as the function of the bonding pressure and bonding temperature. The dependence of the processing parameters on the shear strength of the bond assembly is investigated on the basis of the morphological development in the ceramic-metal interface. In addition, the effect of a pre-coated Ni film on the strength of YSZ/Ni bonding is discussed.     

Interface reactions and bonding strength in aluminium alloy (AA-7075)-alumina diffusion bonds

Diffusion bonds between alumina and high-strength aluminium alloy (AA-7075) have been produced and studied in the present work. Direct diffusion bonding in the solid state was tested as a possible joining method for both materials. The nature of the AA7075-Al₂O₃ interface was investigated paying special attention to the chemical interaction processes between the alloying elements and the ceramic material, as well as their influence on the joint strength. SEM images and energy-dispersive microanalysis were used to determine the formation of reaction layer between both parent materials. Shear strength was used as an optimum method to evaluate the bond strength and the influence of the bonding parameters on it. A maximum shear strength of 60 MPa was achieved using bonding temperatures and pressures of 360 °C and 6 MPa, respectively, during very prolonged bonding times (100h). Fractographic studies of the failure surface gave additional information on those aspects.     

Preparation and properties of a new porous ceramic material used in clean energy field

At high temperature, the oxide redox reactions of ceria can split H₂O and CO₂ to produce H₂ and CO, so porous ceria with high temperature resistance and high specific surface area has an important foreground in clean energy applications. In this work, a reticulated porous ceria ceramic material with interconnected porous structure was prepared by the impregnation technique using organic polyurethane sponges as template. The influences of pretreated sponge, dipping time length, pore size and sintering temperature on the porosity and strength of the porous ceria ceramics were systematically studied. With the increasing sintering temperature, the glass phase occurred and led to an increase in strength, but an decrease in porosity. Eventually, we analyzed the relationships between porosity and strength of these porous materials, aiming to provide theoretical and practical references for its application in clean-energy field.     

Diffusion bonding of a nickel (chromium) alloy to zirconia: Mechanical properties and interface microstructures

Diffusion bonding of a Ni(Cr) alloy with ZrO₂ has been studied. It was found that the processes were controlled by chemical reactions at the metal/ceramic interface forming compounds within the composition range NiO_1–x Cr₂O_3–y ZrO_2–z (0<x,y,z<1). Reaction products were characterized by examination of the fractured metal/ceramic halves and from cross-sections using optical, scanning and transmission electron microscopy and energy dispersive X-ray microanalysis. These metallographic studies indicate that the interface reactions are accompanied by local melting indicating a possible eutectic reaction following interdiffusion. Pre-oxidizing the metal foil enhances these reactions and lowers the reaction temperatures. Differential thermal analysis found endotherms at 1050 and 1110 °C with sintered NiCr/ZrO₂ powders, and at 980 and 1100 °C when pre-oxidized powders were used. These are shown to be associated with local melting at the metal/ceramic interface. This thus explains the existence of a critical bonding temperature for good-quality diffusion bonding, where a minimum temperature has to be satisfied for the eutectic reaction to occur.     

Thermo-compression bonding of alumina ceramics to metal

Alumina ceramics and Kovar with aluminum interlayer are pressed together under vacuum at temperatures around 600°C for joining. This process produces mechanically strong ceramic to metal bonds in one step in an economic manner. In order to arrive at the optimum conditions for solid-state bonding, effects of bonding temperature, pressure and time on the bond strength have been studied. Bonding kinetics is also elucidated. Irradiation of 99% Al₂O₃ ceramics by 4–5 MV X-rays has been found to increase the bond-strength sharply from 33 to 60 MPa with a dose of 15 k Rads for bonding temperatures around 540°C. The apparent activation energy for the bonding process (Q _B) depends strongly on the type of alumina ceramics. Irradiation of alumina ceramics (99%), prior to joining with Kovar, accelerates the solid-state bonding by reducing (Q _B) from 209 to 76 kJ/mole.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Polymer/filler derived NbC composite ceramics

NbC containing ceramic composites were manufactured from poly(siloxane)/Nb/NbC filler mixtures by a high temperature reaction bonding process. During heating in an inert atmosphere the Si—O—C ceramic residue of the polymer reacted with the metallic Nb filler to form Nb _x Si _y , NbO and NbC. Samples with a high Nb/NbC ratio showed reduced porosity and increased hardness after pyrolysis at 1200°C.     

Transient liquid phase bonding of HfC-based ceramics

Transient liquid phase (TLP) bonding enables joining at lower temperatures than traditional bonding techniques and preserves the potential for high-temperature applications, making it particularly attractive for joining ultra-high-temperature ceramics (UHTCs) such as carbides and borides. The feasibility of a TLP joint between “pure” carbides has been recently demonstrated. The present study examines the interactions that occur between undoped HfC or MoSi₂-doped HfC and a Ni/Nb/Ni multilayer interlayer during TLP bonding. Bonding is performed at 1400 °C for 30 min in a high-vacuum furnace. SEM–EDS characterization shows that the reaction layer formed at the interlayer/ceramic interface contains mixed carbides and depending upon the ceramic, Ni–Nb–Hf, or Ni–Nb–Hf–Si, or Ni–Nb–Si alloys. Nanoindentation tests traversing the reaction layer between the bulk ceramic and Nb foil midplane also show a clear transition zone across which the indentation modulus and hardness vary. Crack-free joints have been obtained with undoped HfC. The addition of 5 vol% MoSi₂ introduces small (<5 μm long) isolated cracks within the reaction layer, whereas with 15 vol% MoSi₂ added, cracking was pervasive within the reaction layer. When the reaction layer exceeds a critical thickness, as in the case of the bond obtained with HfC doped with 15 vol% MoSi₂, residual stresses become sufficiently large to cause extensive cracking and bond failure. The results suggest a need to characterize and balance the positive role of additives on sintering with the potentially deleterious role they may have on joining.     

A new method for preparation of direct bonding copper substrate on Al2O3

The oxygen content is usually difficult to control in direct bonding copper process. In this study a new method for preparation of direct bonding copper on alumina ceramic substrates was realized. 96% Al₂O₃ ceramic substrates were first oxidized by pasting a thin layer of Cu₂O and firing at 1150 °C in air. Then copper foil was bonded to the substrate by heating to 1070 °C in pure N₂ atmosphere. Microstructure and composition of the interface between the copper and Al₂O₃ ceramic were analyzed. The XRD and EDS results show that an interphase of CuAlO₂ was formed and a eutectic transformation between oxygen and copper took place at the interface of the copper and the ceramic substrate. The interface was much thicker than the traditionally bonded substrates, which resulted in a better bonding strength. The directly bonded copper alumina substrate samples showed no evidence of de-bonding after 50 thermal cycles comprising quenching from 220 °C to room temperature.     

The effect of interlayer metals on the strength of alumina ceramic and 1Cr18Ni9Ti stainless steel bonding

The effects of interlayers of molybdenum and copper on the strength of alumina ceramic and 1Cr18Ni9Ti stainless steel bonding with Ag₅₇Cu₃₈Ti₅ filler metal were investigated. The interfacial morphologies were observed and analysed by scanning electron microscopy and energy dispersive X-ray (EDX) analysis, respectively. The joint strength was examined by shear tests. When using a molybdenum interlayer, the joint strength could be greatly improved because molybdenum not only reduced the interfacial residual stress, but also did not affect the interfacial reaction between the ceramic and the filler metal, and the maximum value was obtained when it was about 0.1 mm thick. When using copper as an interlayer, the joint strength was not increased but decreased, because copper reduced the activity of titanium in the filler metal, resulting in an insufficient interfacial reaction between the ceramic and the filler metal and the formation of poor interfacial adhesion. Therefore, in selecting an interlayer metal to reduce or avoid interfacial residual stress in joining ceramics to metals, in which the interfacial reaction of ceramic and filler metal is important to the joints, the interaction of interlayer metal and filler metal must be considered.     

Diphasic xerogels: I. Ceramic-metal composites

The sol-gel process has been extended to the preparation of new diphasic xerogels leading to new hybrid ceramic-metal materials. The final component compositions were Al₂O₃, SiO₂, ZrO₂ as oxides and Cu, Pt, Sn, and Ni as metals.The xerogels as examined by XRD, SEM, TEM, consisted or a noncrystalline (in a few cases, microcrystalline) ceramic matrix with small metallic islands (5–50 nm) and micro and macro pores. The fine structure of these materials can be controlled by both preparation steps: the gelation parameters and subsequent thermal treatment.     

The influence of brazing conditions on joint strength in Al2O3/Al2O3 bonding

The brazing of alumina ceramic to itself was performed using Ag₅₇Cu₃₈Ti₅ filler alloy. The bonding was carried out in a vacuum of 7 × 10^?3 Pa, and the joining conditions were at 1073, 1123, 1173, 1223, 1273 and 1323 K for 1.8ks under a pressure of 0.01 MPa, at 1123 K with a pressure of 0.01 MPa for 0, 0.3, 0.9, 1.8, 2.7 and 3.6 ks, and at 1123 K for 1.8 ks with pressures of 0, 0.01, 0.05, 0.10, 0.15, 0.20 and 0.30 MPa, to determine the effects of joining temperature, pressure and holding time on the joint strength. The joint strength was measured by shear tests. The interface microstructures and fractured surfaces after testing were observed by scanning electron microscopy (SEM). It was shown that the shear strength of Al₂O₃/Al₂O₃ joints was largely affected by the joining conditions; it first increased and then decreased with increasing joining temperature, pressure and holding time and depended mainly on the strength of interfacial reaction layer itself and the interface bonding strength between the reaction layer and the ceramic. The maximum joint strength was obtained when the reaction occurred under a suitable temperature, pressure and time, and the reaction layer thickness was about 2 μm. SEM observations revealed that there were four types of fracture and each kind corresponded to a different strength.     

Chemical reaction assisted transient liquid phase bonding of alumina in combination with cold isostatic pressing

Abstract

A new method of transient liquid phase (TLP) bonding of alumina specimens has been developed using a mixture of aluminium powder and silica powder as insert materials. A chemical reaction of aluminium with silica occurs in the inter layer to produce alumina and silicon. Some of the specimens were subjected to cold isostatic pressing (cipping) before bonding to improve the bonding strength. Specimens with an interlayer of powder mixture were joined for Al/SiO₂ ratios of 1 : 0.84 and 1 : 0.42, but did not join for an interlayer with a theoretical ratio of 1 : 1.67. When specimens were subjected to cipping before bonding, bonds were far stronger than bonds without cipping in a temperature range from room temperature to elevated temperatures above the melting point of aluminium. In the mechanical test (bending test), fracture occurs at the boundary between the alumina matrix and the interlayer at room temperature, and in the interlayer at temperatures above the melting point of aluminium.     

Herstellung und Prüfung festkörpergeschweißter Keramik-Metall-Verbindungen

Preparation and Testing of Solid-State Bonded Ceramic-to-Metal-Joints Ceramic-to-metal joints were manufactured by solid-state bonding in a R. F.-high-vacuum apparatus. The influence of welding temperature, welding time and welding pressure on the bond strength was investigated for an alumina/niobium-combination consisting of a layered composite ceramic-metal foil-ceramic. As a measure of bond quality the fracture resistance K_ICV was chosen. K_ICV date were obtained by the use of 4-point bend tests and tensile tests. For comparison, data are presented concerning the fracture resistance K_IC of the bulk alumina and the conventional bond strength of the joints. In addition to the Nb/Al₂O₃-data, K_ICV factors are determined for other metal/alumina combinations and for a Zr/Si₃N₄ joint. The present solid-state bonding apparatus was used for the preparation of the specimens. A device for high-temperature bend testing in a high vacuum was constructed. Some data for the high-temperature bond strength of solid-state bonded joints are given.     

Preparation of ceria nanoparticles supported on carbon nanotubes

By chemical reaction of CeCl₃ and NaOH on carbon nanotube solution and subsequent heat treatment, ceria nanoparticles supported on carbon nanotubes were prepared. The processing parameters affecting the size of ceria particles were discussed. The particles were characterized by XRD and TEM. XRD patterns revealed that the particles exhibited CaF₂-type crystal structure. The TEM micrograph showed that the mean sizes of ceria particles were about 6 nm.     

Direct silver bonding – an alternative for substrates in power semiconductor packaging

Bonding between silver and ceramics like Al₂O₃, ZrO₂, MgO, AlN, sapphire or quartz glass is obtained by a liquid phase bonding process based on the pseudo-binary eutectic between Ag and CuO (at 1 mol %). It melts 15 K below the melting point of pure Ag in air. Excellent wetting between the eutectic liquid and the ceramic surfaces gives mechanically strong, reliable bonds. The bonding mechanism is similar to the well known direct copper bonding (DCB)-process. Our new process is simple and works at 1219±2 K in plain air. It therefore has the potential of massive cost reductions compared to the more complicated DCB-process.     

Studies of ceramic-liquid metal reaction interfaces

An investigation has been made of the nature and extent of chemical reactions between various liquid metals and a range of engineering-grade ceramics typically used as cutting tool inserts. Such possible reactions are relevant to chemical wear effects during metal cutting but also relate to liquid metal containment by ceramics and ceramic-metal joining. The experimental procedure has involved immersing pre-polished ceramic sections in liquid metals for controlled times with subsequent sectioning and examination of the reaction interface. The ceramics studied were two alumina-based materials and five silicon nitrides and sialons. The metals were pure iron, pure nickel and four iron-nickel alloys (a mild steel, a stainless steel and two nickel-based superalloys) and span a range of Fe-Ni compositions. The reaction rates of the alumina materials were found to be much lower than those of the silicon nitride-based materials and reflect the chemical stability of the Al-O bond array. Zirconia-toughened alumina showed little evidence of reaction with clean iron alloys but substantial attack by oxygen-containing iron-based materials was found resulting in the formation of iron-aluminium spinel reaction products. Al₂O₃-TiC/N exhibited preferential metal attack of the carbonitride phase with dissolution and/or replacement of the TiC/N dispersion. Within the silicon nitride-based group, ferrous alloys were found to be more damaging than mainly nickel alloys and silicon nitrides were more readily attacked than sialons. The difference in behaviour between the sialons and silicon nitrides is attributed to alumina additions in the former group of materials increasing resistance to attack by molten metals. A detailed mechanism of attack for these mixed-phase ceramics is proposed whereby a silicon concentration gradient is established from the crystalline ceramic phases, through the glassy binding phase, to the metal. The result is dissolution of the crystalline phase and an increase in volume fraction of the glassy binder at the metal-ceramic interface with concomitant progressive disruption of the ceramic microstructure.     

29Si MAS-NMR investigation of the conversion process of a polytitanocarbosilane into SiC-TiC ceramics

A polytitanocarbosilane has been prepared from polycarbosilane and titanium n-butoxide.²⁹Si MAS-NMR was used to characterize the various steps of the conversion process of the polymer into the final ceramic. The reaction of titanium butoxide with polycarbosilane introduces oxygen into the polymer that seems to play an important role in the pyrolysis process. In the first stage up to 1000 ° C, the study reveals the cleavage of Si-C bonds and the formation of SiC_4-xO_x units. In the second stage, above 1000 ° C, the number of Si-O bonds decreases, probably due to a carbothermal reduction process. At 1500 ° C, the product can be described as a mixture of crystalline SiC and TiC with no excess carbon.