Evaluation of Brazed Silicon Nitride Joints: Microstructure and Mechanical Properties

Sintered Si₃N₄ has been bonded to itself and to AISI 316 steel by the active-metal brazing route. A commercial Ag-35Cu-1.6Ti filler has been used with joining taking place during a 30 min hold at 850°C under vacuum. Si₃N₄/ Si₃N₄ joints have been produced with strength distribution (average bend strength = 773.5 MPa, Weibull modulus = 11.2) similar to that of the monolithic ceramic. Direct brazing of the Si₃N₄ to AISI 316 steel was unsuccessful. However, reliably strong (bend strength of 250–400 MPa) ceramic/ steel joints with 20 × 20 mm² cross sections were fabricated by using Cu, Mo, or Nb interlayers. The most potent inter-layer used in this work was Mo, whose coefficient of thermal expansion matches best that of the ceramic.     

Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Solid-State Bonding of Silicon Nitride Ceramics with Nickel-Chromium Alloy Interlayers

Si₃N₄ ceramics with Al₂O₃ and Y₂O₃ as additives were joined with an 80 wt% Ni-20 wt% Cr alloy sheet as an insert layer. Joining was performed by hot-pressing between 1000° and 1350°C in argon, and under uniaxial pressures in the range of 50 to 100 MPa. The average joint strength, evaluated by four-point bending, was large enough (>300 MPa) for some industrial applications. However, the scatter in strength was relatively large, because of the formation of interfacial pores, which were not distributed uniformly at the bond interface. The effects of joining pressure and N₂ gas partial pressure on the formation of the pores were confirmed microscopically. Cr coating on the Si₃N₄ ceramic before joining contributed to reduce the joint strength scatter. The major interfacial reaction products were Cr nitrides.     

Effects of Brazing Time and Temperature on the Microstructure and Mechanical Properties of Aluminum Air Brazed Joints

High-purity aluminum foil was used to join alumina substrates directly in air at temperatures ranging from 800° to 1200°C and soak times of 1–100 h. It was found that the bend strengths of the resulting Al₂O₃/Al/Al₂O₃ joints generally increase with increasing brazing temperature and time, with a maximum bend strength of 135 MPa on average achieved in samples joined at 1200°C for 100 h. Additionally it was determined that measurable ductility is retained in the joint even after exposure under extended high-temperature conditions. During joining, an Al₂O₃ scale forms along the interface between the aluminum and adjacent substrates. An increase in brazing temperature and/or time leads to intergrowth and sintering between this thermally grown oxide layer and the substrate surface, which appears to be the primary source of improved joint strength. Fracture analysis indicates that the Al₂O₃/Al/Al₂O₃ joints generally fail via one of three mechanisms, (1) by de-bonding along the foil/substrate interface in specimens that were joined at low temperature or held at temperature for an insufficient period of time; (2) by ductile rupture in specimens that were joined at conditions that promoted sintering between the oxidized foil and adjacent alumina faying surfaces, but left behind a continuous residual aluminum layer within the joint; or (3) by mixed-mode fracture in specimens joined at high temperature and long exposure times, in which the thermally grown alumina that forms between the two substrates is interrupted by dispersed pockets of residual aluminum metal.     

Joining of Carbon-Carbon Composites by Graphite Formation

Joining of carbon-carbon (C-C) composites by graphite formation, using manganese, magnesium, and aluminum interlayers, has been investigated. The process involved the formation of a metal carbide by chemical reaction between the metal interlayer and the composite, followed by the decomposition of the carbide and evaporation of the metal at elevated temperatures. The maximum bonding temperature in these experiments was 2200°C. Bonding of composite specimens occurred when manganese or a powder mixture of aluminum and graphite was used as interlayers. Attempts to join C-C pieces using a magnesium interlayer were unsuccessful. The double notch shear strengths of the joints produced using Mn interlayers were very low and ranged from 0.15 to 1.61 MPa at test temperatures of 1200° and 1400°C. The interlayer, after completion of the joining operation, consisted, in most cases, only of graphite. The joints produced with aluminum plus graphite interlayers were even weaker, with strength values of 0.11 MPa or less. The presence of aluminum could be detected in some of these joints, suggesting incomplete dissociation of Al₄C₃ at the maximum bonding temperature of 2150°C.     

Joining of Carbon—Carbon Composites Using Boron and Titanium Disilicide Interlayers

The feasibility of joining of 3-D carbon—carbon (C–C) composites by using B and TiSi₂ interlayers has been investigated. The optimum temperature for joining with a B interlayer was determined to be about 1995°C and that for joining with a TiSi₂ interlayer was about 1490°C. The shear strengths of the joints made at these optimum temperatures were found to increase with the shear testing temperature up to a point, followed by a decrease at higher temperatures. For C–C specimens bonded at 1995°C with a B interlayer, the maximum joint shear strength (average value 18.35 MPa) was observed at the test temperature of 1660°C. The shear strength of joints produced with a TiSi₂ interlayer showed a maximum at the test temperature of 1164°C, with an average value of 34.41 MPa. The B interlayers reacted with C–C composite pieces during joining, and the product of reaction was identified as B₄C. In specimens joined with TiSi₂ interlayers, the reaction between TiSi2 and C did not go to completion, and the bond interlayer contained TiC, SiC, and TiSi₂.     

Post-Hot-Pressing and High-Temperature Bending Strength of Reaction-Bonded Silicon Nitride-Molybdenum Disilicide and Silicon Nitride-Tungsten Silicide Composites

A hot-pressing technique was used for the further densification of reaction-bonded silicon nitride-molybdenum disilicide and silicon nitride-tungsten silicide (Si₃N₄-MoSi₂ and Si₃N₄-WSi₂, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon-molybdenum or silicon-tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha-Si₃N₄ that formed during the reaction-bonding process was transformed to β-Si₃N₄ and, moreover, a very small amount of Mo₅Si₃ (W₅Si₃) was formed in addition to MoSi₂ (WSi₂). Three- and four-point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si₃N₄-WSi₂ composite increased slightly from room temperature up to 1000°C, whereas the Si₃N₄-MoSi₂ composite showed a more-pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.     

High-Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites

SiC–Mo₅(Si,Al)₃C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three-point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.     

High-Temperature Strength and Creep Behavior of Melt-Infiltrated SiC-Mo≤5Si3C≤1 Composites

Molybdenum carbosilicide composites (SiC-Mo_≤5Si₃C_≤1) were fabricated via the melt-infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼10⁵ Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle-ductile transition of the intermetallic phase. The composites retained ∼90% of their room-temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°-1850°C and a stress range of 200–250 MPa. The creep rate of the SiC-Mo_≤5Si₃C_≤1 composites was approximately an order of magnitude higher than that of reaction-bonded SiC.     

Joining of Zirconia Ceramic to Stainless Steel and to Itself Using Ag57Cu38Ti5 Filler Metal

The brazing of ZrO₂ ceramic to 1Cr18Ni9Ti stainless steel and to itself was performed using Ag₅₇Cu₃₈Ti₅ filler metal under a vacuum of 7 × 10^-3 Pa. The effects of interlayer copper on the ceramic to stainless steel joint strength, and the brazing temperature (1073 to 1323 K) and holding time (0 to 60 min) on ceramic to ceramic joint strength were investigated. The joint strength was evaluated by shear testing. An interfacial reaction layer between the ceramic and the filler metal was found, and the reaction products were δ-TiO and γ-AgTi₃. The joint strength of ZrO₂ ceramic to stainless steel was improved by using a layer of copper of a suitable thickness. The brazing temperature and holding time had a strong influence on the joint strength of ceramic to ceramic, and the joint strength was mainly controlled by reaction layer thickness and the properties of the reaction products. The maximum shear strength was obtained for brazing at 1123 K for 30 min and an interfacial reaction layer thickness of ∼4.4 μm.     

Diffusion Bonding of Silicon Nitride Using a Superplastic β-SiAlON Interlayer

A superplastic β-SiAlON was used as an interlayer to diffusionally bond a hot-pressed silicon nitride to itself. The bonding was conducted in a graphite furnace under a constant uniaxial load of 5 MPa at temperatures varying from 1500° to 1650°C for 2 h, followed by annealing at temperatures in the range of 1600° to 1750^oC for 2 h. The bonds were evaluated using the four-point-bend method at both room temperature and high temperatures. The results indicate that strong, void-free joints can be produced with the superplastic β-SiAlON interlayer, with bond strengths ranging from 438 to 682 MPa, and that the Si₃N₄ joints are heat resistant, being able to retain their strength up to 1000°C (635 MPa), and therefore have potential for high-temperature applications.     

Reactive Hot Pressing of SiC/MoSi2 Nanocomposites

Dense SiC/MoSi₂ nanocomposites were fabricated by reactive hot pressing the mixed powders of Mo, Si, and nano-SiC particles coated homogeneously on the surface of Si powder by polymer processing. Phase composition and microstructure were determined by methods of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectrometry. The nanocomposites obtained consisted of MoSi₂, β-SiC, less Mo₅Si₃, and SiO₂. A uniform dispersion of nano-SiC particles was obtained in the MoSi₂ matrix. The relative densities of the monolithic material and nanocomposite were above 98%. The room-temperature flexural strength of 15 vol% SiC/MoSi₂ nanocomposite was 610 MPa, which increased 141% compared with that of the monolithic MoSi₂. The fracture toughness of the nanocomposite exceeded that of pure MoSi₂, and the 1200°C yield strength measured for the nanocomposite reached 720 MPa.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

Hydrothermal Solidification of Kaolinite–Quartz–Lime Mixtures

The strength development of hydrothermally solidified kaolinite–quartz–lime systems with kaolinite as the aluminum source was studied. The starting materials were mixed so that the Ca/(Si + Al) atomic ratio was in the range 0.23 to 0.25, and the Al/(Si + Al) ratio was between 0 to 0.50. Specimens were formed by uniaxial pressing and hydrothermal treatment under saturated steam pressure at 200°C for 2 to 20 h. For quartz-rich systems with Al/(Si + Al) = 0 and 0.05, strength development by the formation of calcium silicate hydrates, such as C–S–H and tobermorite (Ca₅(Si₆O₁₈H₂)·(4H₂O), was observed. On the other hand, in the case of kaolinite-rich systems with Al/(Si + Al) = 0.24 to 0.50, strength development by the formation of hydrogarnet (Ca₃Al₂(SiO₄)(OH)₈) was recognized, resulting in flexural strengths between 15 to 20 MPa. It is proposed that strength development is related to the formation of mesopores (∼0.04 μm) that accompanied formation of the hydrogarnet.     

Microstructure-Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia-Toughened Alumina

The rates of densification and the mechanical properties of pure Al₂O₃ and ZrO₂-toughened Al₂O₃ (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding-induced and fracture-induced monoclinic ZrO₂ formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four-point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m^1/2.     

Diffusion Bonding of Zirconia/Alumina Composites

ZrO₂/Al₂O₃ composites with from 0% to 100% Al₂O₃ content were diffusion bonded at 12.5 MPa for 30 min in the temperature range 1450° to 1500°C. Under appropriate bonding conditions, a bonding strength greater than 1000 MPa was achievable between dissimilar materials with different thermal expansion coefficients.     

Properties of Isostatically Hot-Pressed Silicon Nitride

Hot isostatic pressing was studied for densification of reaction-bonded Si₃N₄ containing various levels of Y₂O₃. Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y₂O₃. An Si₃N₄-5 wt% Y₂O₃ composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa .     

Formation, Powder Characterization and Sintering of MgCr2O4 by the Hydrazine Method

Picrochromite (MgCr₂O₄) crystallizes at 480° to 530°C from an amorphous material prepared by the hydrazine method. The MgCr₂O₄ powders were characterized for particle size and surface area. Individual particles tend toward a hexagonal morphology above 1000°C. Dense MgCr₂O₄ ceramics (99.5% of theoretical) with an average grain size of 2 μm have been fabricated by spark plasma sintering for 5 min at 1400°C and 30 MPa. Their fracture toughness and bending strength are 3.7 MPa·m^1/2 and 310 MPa, respectively.     

Microstructure of Alumina Brazed with a Silver-Copper-Titanium Alloy

Joints of high-purity Al₂O₃ were made with a 56Ag-36Cu-6Sn-2Ti (wt%) experimental alloy by vacuum brazing at 900°C for 20 min. The microstructure at the ceramic-metal interfaces was examined in cross-sectional specimens of the joints, and phases formed by reaction between the braze filler metal and the Al₂O₃ were analyzed for composition and crystal structure in a transmission electron microscope. A 0.1- to 0.2-μm-thick layer formed directly on the Al₂O₃ surface. This layer was Ti-rich, had the face-centered-cubic crystal structure with a lattice parameter of 0.423 nm, and was identified as oxygen-deficient γ-TiO. The second reaction layer was ∼3 μm thick, and it separated the TiO layer from the metallic phases of the braze filler metal. The structure of this layer, as determined by electron diffraction, was diamond cubic of the space group Fd3m , with a lattice parameter of 1.137 nm. The combination of microanalysis and diffraction data confirmed that this phase was Ti₃(Cu_0.76Al_0.18Sn_0.06)₃O and indicated that it can be classified as an n phase.     

Joining Si3N4-Based Ceramics with Oxidation-Formed Surface Layers

A simple processing technique has been developed for joining Si₃N₄-based ceramics. Thin (<5 μm thick), amorphous, or partially crystalline SiO₂-based surface layers were formed, via low-temperature oxidation (at 1200°C), on the faces to be joined. Joining of the surface-coated pieces could then be performed in an inert environment at typical sintering/joining temperatures (i.e., 1700°C), with or without applied gas pressure, via a transient viscous/liquid phase. This method was most effective for Si₃N₄ ceramics with single oxide sintering additives when a thin (∼1 μm thick), highly smooth (RMS roughness <60 nm) SiO₂ layer was formed, and essentially 'pore-free' joints could be formed. However, the method was less suitable for a multi-additive SiAlON material under current experimental conditions, as relatively high roughness (RMS roughness >400 nm) oxide scales formed, leaving residual porosity at the joint interface.