Reaction chemistry at joined interfaces between silicon nitride and aluminium

Joined interfaces of HIPed additive-free silicon nitride ceramics/aluminium braze bonded at a low temperature of 1073 K for 18 ks or at a high temperature of 1473 K for 1.8 ks in vacuum of 1.3 mPa and of β silicon nitride powders/aluminium powders bonded at the low temperature for 1.8 ks or 18 ks in the same vacuum are identified by analytical transmission electron microscopy and X-ray diffraction method. Mullite, some small crystals and β′-sialon are detected at the interface of the ceramics/aluminium braze bonded at the low temperature and 15R AIN-polytype sialon, β′-sialon, aluminium nitride, mullite and silica-alumina noncrystalline are detected at that bonded at the high temperature. At the interface of the two kinds of powders, aluminium nitride and silicon are also detected besides β′-sialon and silica-alumina noncrystalline even though the bonding was conducted at the low temperature. The interfacial reactions of the joints are influenced not only by bonding temperature but also by the oxide formed at the interface before bonded.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

Fracture toughness of multilayer silicon nitride with crack deflection

The fracture resistance of a multilayer silicon nitride consisting of alternate dense and porous layers was investigated by a single-edge-V-notched beam (SEVNB) technique. Since silicon nitride whiskers were aligned parallel to the laminar direction in the porous layer, the crack deflected macroscopically along the whiskers, resulting in high apparent K_I values, 15–25 MPa m^1/2. The crack then propagated in mode I, and was arrested when K_I was reduced to the fracture resistance without the crack deflection effects. These fracture resistance behaviors were well-explained in terms of the notch-insensitivity and the shielding effects of pull-out of the aligned whiskers.     

Effect of oxide additive in silicon nitride on interfacial structure and strength of silicon nitride joints brazed with aluminium

Silicon nitride ceramics with Y₂O₃ and Al₂O₃ as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.     

Sintering of nano crystalline α silicon carbide doping with aluminium nitride

Sinterable silicon carbide powders were prepared by attrition milling and chemical processing of an acheson type α-SiC. Pressureless sintering of these powders was achieved by addition of aluminium nitride together with carbon. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid state sintering process.     

Fracture strength of low-pressure injection-moulded reaction-bonded silicon nitride

Reaction-bonded silicon nitride (RBSN) samples were fabricated via a low-pressure injection-moulding technique. Sample batches of 58, 68, and 70 vol % silicon solids loading were moulded using a multicomponent, nonaqueous binder. These samples were analysed in terms of their nitrided bulk density, flexural strength, and microstructure. Bulk densities of 2.9 g cm^–3 (91% theoretical density) and three-point moduli of rupture in excess of 304 MPa (44×10³ p.s.i.) were achieved. These results indicate a potential use of low-pressure injection moulding as a forming technique for the fabrication of RBSN components.     

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.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Fracture toughness of silicon nitride determined by cyclic-fatigue-cracked specimens

An experimental technique for determining fracture toughness has been developed. In this method, a fatigue precrack is introduced in single-edge notched beam specimens by cyclic fatiguing in four-point bend at an elevated temperature. The resulting fatigue precracks satisfy all conditions required by the ASTM Standard Test for plane-strain fracture toughness of metallic materials. The applicability of this technique to provide reproducible fracture toughness values is demonstrated by experimental results obtained for silicon nitride sintered in two different ways in comparison with those obtained by means of the indentation technique for the same materials.     

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)₂ · Al₂O₃ by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.     

Reduction of sodium-accelerated oxidation of silicon nitride ceramics by aluminium implantation

Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×10¹⁵cm^-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×10¹⁵cm^-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture of flash oxidized,yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after flash oxidation was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.     

Fracture behavior of silicon nitride ceramics subjected to hypervelocity impact

A new advanced ceramic thruster made of monolithic silicon nitride ceramics was developed for the planetary exploration spacecraft AKATSUKI (PLANET-C) at Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). To ensure its operation onboard the spacecraft, the reliability of the ceramic thruster against micrometeoroid hypervelocity impact has been investigated. Silicon nitride plates were impacted by spheres of stainless-steel and other materials with 0.2-0.8-mm diameters in the velocity range up to 8.0 km/s using a two-stage light-gas gun. Using crater depth data under various impact conditions, the penetration equation of silicon nitride was determined. The impacted samples showed fracture patterns of three types: cratering, cratering with spallation, and perforation. These fracture patterns were well categorized by the multiple forms of the penetration equation.     

Fracture phenomenology of a sintered silicon nitride containing oxide additives

Crack propagation mechanisms in a sintered silicon nitride containing various oxide additives (ceria, magnesia, zirconia and strontium oxide) were studied as a function of initial flaw size, temperature, applied stress and time. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw technique. At 20° C, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 35 were strength-controlling. The flexural strength was found to be independent of temperature from 20 to 800° C and the mode of crack propagation was primarily transgranular. At temperatures above 800° C the flexural strength decreased significantly, due to viscous flow of the glassy phase present in the material and resulting in sub-critical crack growth (SCG). The mode of crack propagation during SCG was essentially intergranular. Flexural stress-rupture evaluation in the temperature range 800 to 1000° C has identified the stress levels for time-dependent and time-independent failures.     

Growth of oriented aluminium nitride films on silicon by chemical vapour deposition

Aluminium nitride films were grown on silicon substrates using the chemical vapour deposition (CVD) method. The properties of the films were studied by scanning electron microscopy (SEM), atomic force microscope (AFM) measurements, X-ray diffraction and Raman scattering. The resulting films were strongly textured and had a preferential orientation with the c-axis normal to the surface, the Raman spectra showed two peaks at 607 and 653 cm^–1 and two large bands at 750 and 900 cm^–1 of smaller intensity. Both the macro- and micro-Raman spectra showed the same peaks.     

Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization

Anodization is an effective surface treatment for improving the corrosion resistance of aluminium-matrix composites. For SiC particle-filled aluminium, anodization was performed successfully in an acid electrolyte, as usual. However, for AlN particle-filled aluminium, anodization needed to be performed in an akaline (0.7 N NaOH) electrolyte instead of an acid electrolyte, because NaOH reduced the reaction between AlN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration of NaOH caused the dissolution of the anodizing product (Al2O3) by the NaOH, whereas too low a concentration of NaOH did not provide sufficient ions for the electrochemical process. The corrosion properties and anodization characteristic of pure aluminium, Al/AlN and Al/SiC were compared. Without anodization, pure aluminium had better corrosion resistance than the composites and Al/SiC had better corrosion resistance than Al/AlN. After anodization, the corrosion resistance of Al/AlN was better than Al/SiC and both composites were better than pure aluminium without anodization, but still not as good as the anodized pure aluminium.