Interfacial strength and chemistry of additive-free silicon nitride ceramics brazed with aluminium

Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints. The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints.     

The nature of sialon joints between silicon nitride based bodies

Quite strong joints between silicon nitride based bodies have been made by incorporating a layer of aluminium and oxides between the bodies and heating in a nitriding atmosphere. The joints are resistant to thermal shock and maintain their strength at 1200° C. Microscopic, DTA and X-ray diffraction studies indicated that sialon phases are present in the joints, and that the bonding reaction involves the reduction of Si₃N₄ by aluminium and the subsequent renitriding of the resultant silicon, as well as the simultaneous nitriding of a portion of the aluminium. Transmission electron microscopy of a joint between hot pressed and reaction bonded silicon nitrides showed that 15R aluminium nitride polytype sialon was present on the reaction bonded side of the joint and ß-sialon on the hot pressed side.     

Thermal expansion coefficient of Y-α′-sialon

A sialon composite composed of Y-α′-sialon and β′-sialon has been fabricated by hot pressing mixtures of Si₃N₄, Y₂O₃ and AlN powders. Thermal expansion coefficients of the Y-α′-sialon and β′-sialon were determined by the high-temperature X-ray diffraction technique. The thermal expansion coefficient of Y-α′-sialon depended on the composition, being minimum at x=0.3 in the formula Y_x(Si_12-4.5x, Al_4.5x)(O_1.5x, N_16-1.5x). The coefficient of β′-sialon increased with increasing lattice constant, that is, the z value in the formula Si_6-zAl_zO_zN_8-z. The thermal expansion coefficient of sialon composites determined by a differential dilatometer increased with increasing amount of Y-α′-sialon. This revised version was published online in November 2006 with corrections to the Cover Date.     

New process for brazing ceramics utilizing squeeze casting

A new joining process for ceramics to ceramics and ceramics to metals, SQ brazing, has been developed. This process utilizes squeeze casting; a brazing material is squeezed into the interface channel to be brazed and is solidified under a high pressure. This new process has several advantages, low cost, mass producibility, high interface strength and high reliability, no severe reaction, etc. Alumina to alumina and silicon nitride to silicon nitride brazing with pure aluminium are shown as examples. Alumina containing silica as a sintering additive brazed by a conventional method severely reacted with aluminium braze so that the joint strength was low. After SQ brazing, reaction was moderate and the strength almost reached that of the parent alumina. Silicon nitride could be brazed by SQ brazing. Although the simple SQ brazing could not make a strong interface, pre-oxidization treatment of silicon nitride increased the joint strength beyond 400 M Pa.     

Preparation and properties of stable dysprosium-doped α-sialon ceramics

Dense ceramics with overall compositions DyxSi12-4.5xAl4.5xO1.5xN16-1.5x, where 0.2≤x≤1.0, along the Si3N4–Dy2O3·9AlN tie line were prepared by hot-pressing at 1800°C. The dysprosium-doped α-sialon phase formed in the composition range 0.3≤x≤0.7. Sintered materials of different compositions were post-heat-treated at temperatures in the range 1300–1750° C for different times and it was shown that the Dy-α-sialon phase is stable over a large temperature interval and during heat treatment times up to 30 days. Unlike corresponding neodymium- and samarium-doped α-sialons, dysprosium-doped α-sialon does not decompose into β-sialon and rare-earth-rich grain-boundary phase(s) at temperatures below 1550°C. The α-phase can coexist with a liquid phase at temperatures ≥1550°C and with the Dy-M′-phase (Dy2Si3-xAlxO3+xN4-x) at lower temperatures. When heat treated at 1450°C, any residual liquid grain-boundary phase reacted with minor amounts of the α-sialon phase and devitrified to Dy-M′-phase, yielding a glassy phase-free material. The Dy-M′-phase formed had the maximum aluminium substitution, i.e. x≈0.7. Dysprosium-doped α-sialon exhibited very high hardness (Hv10=22 GPa) and a fracture toughness of 4.5 MPa m1/2, and the hardness and toughness decreased only slightly after devitrification of the glassy phase. Some elongated α-sialon grains were formed at high x values in glassy phase-containing materials, but their presence did not affect the toughness significantly. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structure and strength of AlN/V bonding interfaces

AlN ceramics are bonded using vanadium metal foils at high temperatures in vacuum. Different bonding temperatures were used in the range 1373–1773 K with bonding times of 0.3–21.6 ks. The AlN/V interfaces of the bonded joints were investigated using SEM, electron probe microanalysis and X-ray diffraction. A bonding temperature of 1573 K was found to be suitable to activate both parts to initiate a phase reaction at the interface, because a thin V(Al) solid solution layer formed adjacent to the ceramic at 1573 K just after 0.9 ks, and a small flake-shaped V₂N reaction product formed inside the vanadium central layer. The formation of V(Al) and V₂N controls the interfacial joining of the AlN/V system at 1573 K up to 5.4 ks bonding time. The pure vanadium layer quickly changed to vanadium-containing V₂N. The diffusion path could be predicted for the AlN/V joints up to 0.9 ks at 1573 K following the sequence AlN/V(Al)/V₂N/V, while after 0.9 ks, the interface structure changed to AlN/V(Al)/V₂N + V by the growth Of V₂N into the vanadium. The AlN/V joints shovyed no ternary compounds at the interface. A maximum bond strength could be obtained for a joint bonded at 1573 K after 5.4 ks having a structure of AlN/V(Al)/V₂N + V. At 7.2 ks, nitrogen, resulting from AlN decomposition, escaped and the remaining aluminium reacted with V(Al) to form V₅Al₈ intermetallic, which is attributable to the decrease in bond strength.     

Fracture of silicon nitride joined with an aluminium braze

Mechanical Properties of silicon nitride joined with aluminium braze have been investigated using fracture mechanics. The highest bonding temperature, 1133 K, produced the highest four-point bend strength of 417 MPa, the strength depending strongly on stress rate. The fracture parameter,N, for slow crack growth in the joint was 29.7 which was near to that of the silicon nitride. Stress corrosion cracking is believed to be one of the serious problems associated with ceramic joining.     

Phase relationship between β′- and O′-sialon at 1623 K

-sialon whiskers and co-products of synthesis, such as -sialon powders and O-sialon powders, were annealed at 1623 K for 8 h in a closed graphite reaction tube under 1 atm nitrogen. Phase stabilities, Si/Al ratios, and crystallographic features were investigated. The O-sialon phase, which formed in the early stage of synthesis when oxygen partial pressure was relatively high, became less stable in the present annealing condition and decomposed. The majority of released aluminium and possibly oxygen from the decomposed O-powder was incorporated into -sialon whiskers with little change in its lattice parameters, when the -sialon whiskers were included in annealing. The aluminium contents were always lower in the -whiskers than in the powders even after increasing its aluminium content during 8 h annealing. The lattice parameters of both -whiskers and powders increased with increasing aluminium content and became closer after annealing. The lattice parameters of -whiskers remained the same before and after annealing despite the increased aluminium content, while the lattice parameters of -powders decreased despite its aluminium content remaining unchanged. The lattice parameters of O-sialon increased with increasing aluminium content, and the increase in thea direction is the largest when compared with other parameters.     

Electric field-assisted and field-depressed segregation of reactive metals to the bonding interface in braze alloy joining

Segregation of reactive metals at the bonding interface has been observed in various ceramic and/or metal joints bonded with reactive metal-bearing braze alloys. When a d.c. of 20 mAcm⁻² is applied to the ceramic/braze/ceramic system at a brazing temperature of, say, 1373 K, the electric field assists the segregation at the braze-ceramic interface on the cathode side and suppresses the segregation at the interface on the anode side. This may imply that reactive metal atoms in the braze can migrate as a cation. E.m.f. measurement on the ceramic (AIN or ZrB₂)-metal foil systems with increasing temperature shows that a negative e.m.f. to the ceramic pole appears from about 900 K for AIN and from 500 K for ZrB₂, as does the thermally stimulated current in polymers. These temperatures coincide well with those where the electrical conductivity of AIN and ZrB₂, respectively, begins to increase with increasing temperature. Therefore, it is considered that the polarization of the ceramics may take place and assist the migration, and consequently segregation, of reactive metals in braze alloys to the braze-ceramic interface during brazing.     

Transient liquid phase bonding of titanium to aluminium nitride

Titanium has been successfully joined to aluminium nitride AlN at a temperature as low as 795 °C, using Ag–Cu Cusil^® commercial braze alloy. While reactive wetting and spreading proceeds at the AlN/braze alloy interface, chemical interactions develop at the titanium side rendering possible isothermal solidification of the joint. The determining factor in the solidification process is the fast formation of TiCu₄ crystals by heterogeneous nucleation and growth in the liquid phase. As a consequence, the braze alloy is depleted in Cu and solid Ag precipitates. After annealing, the re-melting temperature of the resulting joint can be increased up to about 910 °C which is nearly 130 °C higher than the melting point of the starting braze alloy.     

Joining of Al-plasma-sprayed Si3N4ceramics

Aluminium was coated on silicon nitride ceramics by a low-pressure plasma spraying method, in order to form a tight bond between aluminium and the ceramics. Aluminium nitride formed as a interfacial reaction product between the aluminium coating layer and the ceramics. Two pieces of the aluminium-coated Si₃N₄ ceramics were then joined using the aluminium coating layers as filler metal in a vacuum of 1.3×10^–3 Pa at 973 K. The average bending strength and Weibull modulus of the joint are 340 MPa and 6.3 respectively, considerably higher than the 230 MPa and 0.9 of a Si₃N₄ ceramics joint brazed with an aluminum plate under the same condition.     

Joint of silicon nitride and molybdenum with vanadium foil,and its high-temperature strength

Joints of silicon nitride and molybdenum with a vanadium interlayer were fabricated using a vacuum hot-pressing facility. The optimum joining conditions for producing a joint with the highest shear strength were found to be as follows: a temperature of 1328 K, a mechanical pressure of 20 MPa and a bonding time of 5.4 ks. The effect of test temperature on shear strength was also examined. The strength level was initially 118 MPa at room temperature and this level gradually decreased as the test temperature rose. At 973 K, the strength level was still 70 MPa. Observations of the interface by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA) revealed that a layer of reaction product V₃Si formed at the silicon-nitride-vanadium interface.     

Interface fracture surface energy of sol–gel bonded silicon wafers by three-point bending

To probe the interface of silicon sol–gel bonded wafers we developed in–situ micromechanical bending test coupled with optical microscopy. The silicon wafers were bonded together at room temperature using sol–gel silica and dried at 60 °C and sintered at 600 °C. Beam specimens were cut from the bonded wafers, then notched and tested in three-point bending. During bending the crack opening from a notch and the deviation along the interface was observed with an optical microscope. To quantify the interfacial debonding from considering the experimental results, a simple energy balance allows an apparent interfacial fracture surface energy to be determined. Experiments and the determined interfacial surface energies show that the bonding of the silicon wafers depends on the silica sol–gel chemistry and on the temperature of the thermal treatment during the bonding process.     

Liquid phase bonding of siliconized silicon carbide

Aluminium was used as a braze to join siliconized silicon carbide to itself. Brazes were carried out in the 700–1100 °C temperature range, in vacuum. A thick reaction layer forms in the ceramic adjacent to the braze film, due to reaction between the metal braze and the free silicon present in the ceramic matrix. The silicon concentration of the braze film reaches values well above the maximum liquid solubility at the brazing temperature. A pseudotransient aluminium-silicon liquid phase promotes the formation of a 100% silicon braze film when either high temperatures, long holding times or very slow cooling rates are used. The dominant mechanism responsible for the formation of the braze microstructure is the preferential unrestrained solidification growth of Si plates on the braze plane, supported by fast liquid Si diffusion. Strong joints were produced and, when pure silicon brazes formed, four-point bend strengths over 200 MPa were obtained at testing temperatures as high as 700 °C. Fracture occurs either in the reaction layer-ceramic boundary or in the braze, the crack propagation plane changing from one side of the braze-ceramic interface to the other and through the braze itself.     

Interfacial reactions between SiC and aluminium during joining

Reactions between SiC and liquid aluminium were studied. Transmission electron microscopy (TEM) showed that aluminium carbide (Al₄C₃) phase was formed at the interface between pressureless sintered SiC and aluminium. In contrast, the Al₄C₃ phase was not detected at the reaction sintered SiC-Al interface. This difference in microstructures results in the change in bending strength of the joints. Mixtures of SiC and aluminium powders were heated to react in vacuum in the temperature range 973 to 1473 K and the reaction products were examined using X-ray powder diffraction. It was confirmed that Al₄C₃ and silicon were formed, and that the extent of reaction between SiC and aluminium was decreased by the addition of silicon into aluminium.     

Void-free strong bonding of surface activated silicon wafers from room temperature to annealing at 600 °C

In order to investigate the high temperature application of surface activated silicon/silicon wafer bonding, the wafers were bonded at room temperature and annealed up to 600 °C followed by optical, electrical, mechanical and nanostructure characterization of the interface. Void-free interface with high bonding strength was observed that was independent of the annealing temperature. The bonding strength was as high as 20 MPa. The normalized interfacial current density was increased with the increase in the annealing temperature. A thin interfacial amorphous layer with a thickness of 8.3 nm was found before annealing, which was diminished at 600 °C. A correlation between the current density and nanostructure of the interface was observed as a function of the annealing temperature. The high quality silicon/silicon bonding indicates its potential use not only in low temperature microelectronic applications, but also in high temperature harsh environments.     

Interfacial structure and reaction mechanism of AlN/Ti joints

Bonding of AlN to Ti was performed at high temperatures in vacuum. The bonding temperature ranged from 1323 to 1473 K, while the bonding time varied from 7.2 up to 72 ks. The reaction products were examined using elemental analysis and X-ray diffraction. TiN, Ti3AlN (τ1), and Ti3Al were observed at the AlN/Ti interface, having various thickness at different bonding conditions. The thickness of TiN and Ti3AlN layers grew slowly with bonding time. On the other hand, growth of the Ti3Al layer followed Fick’s law. The activation energy of its growth was found to be 146 kJ mol-1. When thinner Ti foil (20 μm) was joined to AlN at 1473 K for a long time (39.6 ks), the Ti central layer has completely consumed and another ternary compound Ti2AlN(τ2) started to form. A maximum bond strength was achieved for an AlN/Ti (20 μm) joint made at 1473 K for 28.8 ks, after which the bond strength of the joint deteriorated severely. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties and microstructure of reaction sintered β′-sialon ceramics prepared by a slip casting method

Reaction sintered β′-sialon ceramics Si_6-zAl_zO_zN_8-z, were prepared by slip casting from α-Si₃N₄, Al₂O₃, and AlN starting powders. The mechanical properties and microstructures of sintered bodies were investigated as a function of composition (varying the z value). The maximum value of the flexural strength, ∼ 600 MPa, and fracture toughness, ∼ 4.1 MPa m^1/2 were observed in the z range of 0.5–1. In the z value range of 2–4, the mechanical properties decreased drastically. This phenomenon is attributed to the variation of fracture energy, which is greatly affected by the sintered crystallite size. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of active metal coatings on the mechanical properties of silicon nitride-based ceramics

The effects of titanium, zirconium, hafnium and tantalum coatings on the mechanical properties of three silicon nitride ceramics were studied. The titanium coatings was found to cause a 50% decrease in the four-point bend strength of one of the silicon nitride ceramics while the effects of the zirconium, hafnium and tantalum coatings on all three silicon nitride ceramics were moderate. The reactions at a high temperature (940–980°C) between titanium and the grain-boundary glassy phase was the major cause for the degradation of the ceramic properties by the titanium coating. Residual tensile stress developed at the reaction interface replaced the glassy grain-boundary phase. Analytical electron microscopy showed the formation of a 180 nm thick Ti₅Si₃ layer and the crystallization of the amorphous grain-boundary phase. An indentation technique was used to measure qualitatively the residual stress developed at the reaction interface.     

Structure and properties of diffusion bonded transition joints between commercially pure titanium and type 304 stainless steel using a nickel interlayer

The solid-state diffusion bonding was carried out between commercially pure titanium and Type 304 stainless steel using nickel as an interlayer in the temperature range of 800–900 °C for 9 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in the optical and scanning electron microscopes. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis. TiNi₃, TiNi and Ti₂Ni are formed at the nickel–titanium (Ni–Ti) interface; however, the stainless steel–nickel (SS–Ni) diffusion interface is free from intermetallic compounds up to 850 °C temperature. At 900 °C, the Ni–Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti₂Ni and λ + χ + α-Fe, λ + FeTi and λ + FeTi + β-Ti phase mixtures occur at the SS–Ni interface. The occurrence of different intermetallics are confirmed by the x-ray diffraction technique. The maximum tensile strength of ∼276 MPa and shear strength of ∼209 MPa along with 7.3% elongation were obtained for the diffusion couple processed at 850 °C. At the 900 °C joining temperature, the formation of Fe–Ti base intermetallics reduces the bond strength. Evaluation of the fracture surfaces using scanning electron microscopy and energy dispersive spectroscopy demonstrates that failure takes place through Ni–Ti interface up to 850 °C and through the SS–Ni interface of the joint when processed at 900 °C.