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.     

Bond strength of vacuum brazed Mg-PSZ/steel joints

Mg-PSZ/steel joints vacuum brazed by the aid of amorphous Cu-30 w/o Ti foils were prepared in the temperature range 883 to 990 °C. A maximum bond strength of 176 MPa (flexural strength, four-point bend test) was obtained after brazing at 930 °C/5 min. Fracture energy and fracture resistance data of the interface region and the adjacent dark-coloured, oxygendeficient zone of Mg-PSZ were obtained from experiments using notched specimens. A drastic decrease of fracture energy of Mg-PSZ from 178 J/m² for the as-received material to 60.2 J/m² for the blackened zirconia adjacent to the braze was observed. This effect is assumed to influence the bond strength of the brazed joints.     

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.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

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.     

Microstructure and strength of high nitrogen steel joints brazed with Ni-Cr-B-Si filler

Brazing of high nitrogen austenitic stainless steels was carried out by using Ni-Cr-B-Si filler metal. The effects of brazing temperature (1020–1100°C) on the microstructure and shear strength of the joints were investigated. The results show that BN compounds with hexagonal structure are formed at the interface by the reaction of N from substrate and B from filler. The brittle Cr₅B₃ compounds with high microhardness are observed in the centre of brazing seam. The BN content increases and the Cr₅B₃ content decreases with the increase in brazing temperature. However, the content of BN compounds played a determinable role on the joint strength. The optimal shear strength of joints was 176.7?MPa when the joining temperature was 1020°C.     

Hardness and mechanical property relationships in directionally solidified aluminium-silicon eutectic alloys with different silicon morphologies

Hardness and tensile property measurements made on directionally solidified Al-Si eutectic alloys show that definite but different hardness-growth velocity and hardness-silicon interparticle spacing relationships exist for alloys with different silicon eutectic phase morphologies. Tensile property measurements show that hardness and 0.2% proof stress follow silicon interparticle spacing relationships of the same form. It is suggested that hardness can only be used to measure proof stress when the eutectic structure displays a single silicon eutectic phase morphology.     

Pressure-assisted direct bonding of copper to silicon nitride for high thermal conductivity and strong interfacial bonding strength

To achieve superior thermal and mechanical properties of copper-bonded (Cu-bonded) Si₃N₄ substrate, a pressure-assisted direct bonded Cu (DBC) technique was applied to bond Cu foil with Si₃N₄ plate. The effects of oxide layer (SiO₂) thickness of Si₃N₄ plate on the microstructure, thermal and mechanical properties of the Si₃N₄-DBC samples were investigated. The successful bonding of Cu foil to Si₃N₄ plate was confirmed by the presence of the interfacial products of Cu₂MgSiO₄ and CuYO₂. Additionally, it was demonstrated that a thin SiO₂ layer can result in a discontinuous distribution of interfacial products while a thick one can lead to the formation of pores in SiO₂ layer. Notably, the sample prepared by Si₃N₄ plate with 5-μm-thickness SiO₂ layer and Cu foil with 5.9-μm-thickness oxide layer (Cu₂O) exhibited the optimally comprehensive properties with thermal conductivity of 92 W·m^?1·K^?1 and shearing strength of 102 MPa, which demonstrates significant promise for application in power electronic modules.

Graphical abstract

     

The strength and oxidation of reaction-sintered silicon nitride

The structure of reaction-sintered silicon nitride is studied using scanning electron and optical microscopy at various stages during nitriding, for a range of nitriding and compacting conditions. The strength is then evaluated and interpreted in terms of the microstructure. It is found that fracture always occurs in a brittle manner by the extension of the largest pores. The effects of prolonged annealing in air above 1000† C on both the structure and strength are investigated. At 1400† C, cristobalite is formed. If the temperature is then maintained above 250† C, the strength is enhanced, but below this temperature the oxide layer cracks and reduces the strength.     

The wetting of silicon nitride by chromium-containing alloys

The wetting of ceramic materials by metallic melts is the most important characteristic of brazing alloys. The effect of chromium additions to copper-base alloys on the wetting of silicon nitride was investigated. Wetting experiments were carried out on Si₃N₄ using liquid Cu-Cr, Cu-Ni-Cr, Cu-Si-Cr and Cu-Ni-Si-Cr alloys. The addition of chromium to liquid copper up to its solubility limit promoted wetting on Si₃N₄. Improved wetting with a higher chromium content was achieved by the addition of nickel to the Cu-Cr alloys. The formation of an interfacial reaction layer, which is detrimental for brazing ceramics, was suppressed by the addition of silicon to the chromium-containing brazing alloys.     

Microstructure and strength of brazed joints of Ti3Al-base alloy with NiCrSiB

Brazing of Ti₃Al alloys with the filler metal NiCrSiB was carried out at 1273–1373 K for 60–1800 s. The relationship of brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1323–1373 K, brazing time is 250–300 s. The maximum shear strength of the joint is 240–250 MPa. Three kinds of reaction products were observed to have formed during the brazing of Ti₃Al alloys with the filler metal NiCrSiB, namely, TiAl₃ (TiB₂) intermetallic compounds formed close to the Ti₃Al alloy. TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds layer formed between TiAl₃ (TiB₂) intermetallic compounds and the filler metal and a Ni[s,s] solid solution formed in the middle of the joint. The interfacial structure of brazed Ti₃Al alloy joints with the filler metal NiCrSiB is Ti₃Al/TiAl₃ (TiB₂)/TiAl₃+AlNi₂Ti (TiB₂)/Ni[s,s] solid solution/TiAl₃+AlNi₂Ti (TiB₂)/TiAl₃ (TiB₂)/Ti₃Al, and this structure will not change with brazing time once it forms. The formation of over many intermetallic compounds TiAl₃+AlNi₂Ti (TiB₂) results in embrittlement of the joint and poor joint properties. The thickness of TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) in the brazed joints of Ti₃Al alloys with the filler metal NiCrSiB are 349 kJ/mol and 24.02 mm²/s, respectively, and the growth formula was y²=24.04exp(−41977.39/T)t. Careful control of the growth of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) can influence the final joint strength.     

Origin of strength anisotropy in hot-pressed silicon nitride

Three- and four-point room-temperature bend tests were carried out on specimens of a hot-pressed silicon nitride oriented parallel and perpendicular to the hot-pressing direction. Scanning electron microscopy was used to identify and measure the fracture-initating flaws, thus enabling the fracture toughness to be calculated. The strength anisotropy exhibited by this material was attributed to variation in the severity of the inherent flaws in the material with orientation, with the fracture toughness showing no significant anisotropy in this case.     

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.     

Improving the strength of brazed joints to alumina by adding carbon fibres

The addition of short, bare, carbon fibres to a silver-based active brazing alloy (63Ag-34Cu-2Ti-1Sn) resulted in up to 30% improvement in the shear/tensile joint strength of brazed joints between stainless steel and alumina. The optimum fibre volume fraction in the brazing material was 12%. This improvement is attributed to the thinning and microstructural simplification of the alumina/braze reaction product (titanium-rich) layer, the softening of the brazing alloy matrix, the strengthening of the braze and the reduction of the coefficient of thermal expansion. The depth of titanium diffusion into the alumina was decreased by the fibre addition. The first two effects are due to the absorption of titanium by the fibres. This absorption resulted in less titanium in the brazing alloy matrix, a braze/fibre particulate reaction product (titanium-rich) on the fibres and the diffusion of titanium into the fibres. In contrast, the use of an active brazing alloy with a lower titanium content but without carbon fibres gave much weaker joints. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Interfacial structure in silicon nitride sintered with lanthanide oxide

Three independent research groups present a comparison of their structural analyses of prismatic interfaces in silicon nitride densified with the aid of lanthanide oxide Ln₂O₃. All three groups obtained scanning transmission electron microscope images which clearly reveal the presence of well-defined Ln segregation sites at the interfaces, and, moreover, reveal that these segregation sites are element-specific. While some results differ across the three research groups, the vast majority exhibits good reproducibility.     

The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

Residual strength of ground hot isostatically pressed silicon nitride

Surface flaws are introduced during grinding of most high strength ceramics. These flaws reduce the strength and it is therefore important to choose grinding parameters such that surface damage is minimized. The assumption that it is the same mechanism that causes cracking beneath both an indenter and a diamond tool made it possible to propose a grinding model. According to this model high wheel speeds, low workpiece velocities and low depths of cut would reduce the grinding forces and thus be beneficial to the strength after grinding. Grinding experiments on hot isostatically pressed silicon nitride showed that this was the case. The experiments also showed that the grinding direction had the strongest influence on the strength, and if possible the direction ought to be parallel with the expected principal stress. Even what can be considered to be mild machine parameters introduce flaws and residual compressive stresses in the surface of the workpiece.