Interfacial microstructure and mechanical properties of porous-Si3N4 ceramic and TiAl alloy joints vacuum brazed with AgCu filler

Porous Si₃N₄ ceramic was firstly joined to TiAl alloy using an AgCu filler alloy. The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of porous-Si₃N₄/AgCu/TiAl joints were studied. The typical interfacial microstructure of joints brazed at 880 °C for 15 min was TiAl/AlCu₂Ti/Ag-Cu eutectic/penetration layer (Ti₅Si₃+TiN, Si₃N₄, Ag (s, s), Cu (s, s))/porous-Si₃N₄. The penetration layer was formed firstly in the brazing process. With increasing brazing temperature and time, the thickness of the penetration layer increased. A large amount of element Ti was consumed in the penetration layer which suppressed the formation and growth of other intermetallic compounds. The penetration layer led the fracture to propagate in the porous Si₃N₄ ceramic substrate. The maximum shear strength was ~13.56 MPa.     

Microstructure and mechanical properties of porous Si3N4/Invar joints brazed with Ag-Cu-Ti+Mo/Cu/Ag-Cu multi-layered composite filler

Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was successfully designed to braze porous Si₃N₄ and Invar alloy. To further reduce the CTE mismatch between the porous Si₃N₄ and brazing filler, Mo particles were introduced into Ag-Cu-Ti. The effects of the Mo addition on the microstructure and mechanical properties of the brazed joints were studied. The results showed that, the addition of Mo particles into Ag-Cu-Ti lowered the CTE mismatch and improved the joint strength to a certain degree. However, an excessive content was harmful. The Mo particles could absorb Ti at high temperature, causing Ti shortage in the reaction with the ceramic. When cooling down, the absorbed Ti was released. The released Ti could react with Cu to generate Cu-Ti phase. So, additional Ti was adopted in the brazing filler as a supplement. When the Ti content was 5 wt%, the reaction layer on the ceramic interface was too thin to transfer enough load. However, when it reached 15 wt%, the Cu interlayer dissolved completely and Fe-Ti and Ni-Ti phases appeared. The maximum joint shear strength (83 MPa) was obtained with 10 wt% Ti and 5 vol% Mo, which had exceeded 90% of the porous Si₃N₄ and was 56% higher than the joint brazed without Mo particles.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Microstructure and mechanical properties of BN-Si3N4 and AlON joints brazed with Ag-Cu-Ti filler alloy

AlON was successfully brazed to BN-Si₃N₄ using a Ag-Cu-Ti filler alloy. SEM, TEM and XRD studies revealed that a TiN + TiB₂ + Ti₅Si₃ reaction layer formed adjacent to the BN-Si₃N₄ while a (Cu,Al)₃Ti₃O layer formed adjacent to the AlON. In addition, Ag-Cu eutectic, Cu(s,s) and AlCu₂Ti were observed in the brazing filler. The effect of brazing temperature on the microstructure and mechanical properties of the joints was investigated. As the brazing temperature increased, the reaction layers became thicker, while the thickness of the brazing seam decreased. Meanwhile, the amount and the size of AlCu₂Ti intermetallic compounds decreased. The shear strength of the joints first increased and then dropped with increasing the brazing temperature. A joint with a maximum strength of 94 MPa was obtained when it was brazed at 850 °C for 15 min.     

Laser-induced metallization of porous Si3N4 ceramic and its brazing to TiAl alloy

The poor wettability of traditional brazing filler alloys on the surface of ceramics always lead to the formation of defects in the joints and weaken the bonding strength eventually, especially the porous ceramics. Metallization on ceramics is an effective way to improve the wettability. In this work, laser-induced cladding process was applied to metalize the surface of porous Si₃N₄ ceramic, and the traditional AgCu eutectic filler alloy can wet on the metalized surface completely. The metalized porous Si₃N₄ ceramic brazed to TiAl alloy successfully using AgCu filler alloy. The interfacial microstructure and mechanical property of the porous Si₃N₄/TiAl alloy brazed joint was significantly improved by the novel laser-induced metallization process.     

Effect of different preparation methods on the microstructure and mechanical properties of Si3N4 ceramic composites

In this study, Si₃N₄ ceramic composites were fabricated by using ball-milling, titration preparation and urea preparation methods, respectively. The effect of different preparation methods on microstructure and mechanical properties of the Si₃N₄ ceramic composites was investigated. Obviously, the Si₃N₄ ceramic composite prepared by the urea preparation method (U-SN sample) showed better sintering behavior and higher mechanical properties than that prepared by the other two methods. Compared with the Si₃N₄ ceramic composite by the titration preparation method (T-SN sample), we could avoid the complex titration process or uncontrollable pH value during the preparation process of the U-SN sample. Meanwhile, the coated Y-Al precursor layer in thickness of nanometers was more homogeneous than that prepared by the traditional titration method. B-SN represented the Si₃N₄ ceramic composite prepared by the ball-milling method. These samples were all sintered from room temperature to 1750 °C via hot-pressing sintering. The U-SN specimen showed the optimal flexural strength and fracture toughness of being 817 MPa and 6.90 MPa/m², respectively, which could be attributed to its smallest grain size (0.46 µm) among these three samples.     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

The interfacial and tribological properties of Ni–Cr alloy/ceramic tribo-couples under dry sliding contact

In this paper, the tribological behaviors of Ni–Cr alloy sliding against Si₃N₄ and WC–Co at 20 °C and 600 °C were investigated on a tribometer with a ball-on-disk configuration. The experimental results indicated that Ni–Cr alloy sliding against WC–Co exhibited higher wear resistance than that sliding against Si₃N₄. From the viewpoints of the interfacial interactions between metal and ceramic (chemical reaction, wetting, adhesion, transference), the wear mechanisms were elucidated. The tribological behaviors of Ni–Cr alloy/ceramic tribo-couples were well correlated with the interfacial characteristics, namely the reactive interface and the non-reactive interface. Ni–Cr alloy/Si₃N₄ tribo-couple showed severe adhesive wear as a result of the interfacial reaction between Ni and Si₃N₄, while the non-reactivity of Ni/WC interface is the most important factor corresponding to the moderate adhesive wear in Ni–Cr alloy sliding against WC–Co. Finally, the relations among the interfacial characteristics, wear behavior, and temperature were discussed. The results may provide some experimental evidences on the design and optimization of metal/ceramic tribo-couples.     

Microstructure and mechanical properties of the Si3N4/42CrMo steel joints brazed with Ag–Cu–Ti + Mo composite filler

The Si₃N₄ ceramic was joined to 42CrMo steel using Ag–Cu–Ti + Mo composite filler. Effect of Mo particles content on the microstructure and mechanical properties of the joints were investigated. Defect-free joints were received when the Si₃N₄/42CrMo steel joints were brazed with Ag–Cu–Ti + Mo composite filler. The results show that a continuous reaction layer which is composed of TiN and Ti₅Si₃ was formed near the Si₃N₄ ceramic. A double reaction layer which consists of Fe₂Ti and FeTi was also formed adjacent to 42CrMo steel, with Fe₂Ti being located near the steel. The central part of the joint is composed of Ag based solid solution, Cu based solid solution, Mo particles and some Cu–Ti intermetallic compounds. The maximal bending strength reached 587.3 MPa with 10 vol.% Mo particles in the joint, at which the joint strength was 414.3% higher than the average strength for the case without Mo particles addition.     

Joining of Cf/SiC composite to Ti-6Al-4V with (Ti-Zr-Cu-Ni)+Ti filler based on in-situ alloying concept

In this paper, a novel brazing process based on in-situ alloying concept was carried out to join C_f/SiC composite to TC4 alloy at 940 °C for 20 min. Mixed powders of Ti-Zr-Cu-Ni alloy and pure Ti metal were used as interlayer. In the process, Ti-Zr-Cu-Ni alloy melted and then dissolved pure Ti metal via liquid-solid reactions, achieving in-situ alloying of the interlayer. The interfacial microstructure and formation mechanism of the brazed joints were investigated. The effect of Ti powder content on the microstructure and the mechanical properties of joints were analyzed. The results showed that: the maximum lap-shear strength of the in-situ alloying brazed joints was 283±11 MPa when using (Ti-Zr-Cu-Ni)+40 vol% Ti composite filler, and this value was 79% higher than the mechanical strength when using Ti-Zr-Cu-Ni alone. A reaction layer of (Ti,Zr)C+Ti₅Si₃ formed near C_f/SiC composite side, while a diffusion layer of Ti₂Cu+Ti(s,s) formed near Ti-6Al-4V side. In the interlayer, lots of Ti(s,s) were distributed uniformly and few of Ti-Cu compounds were found, contributing to the plasticity of joints. Adding moderate Ti powder was beneficial for improving the interfacial reaction between C_f/SiC composite and filler material, which affected the lap-shear strength of joints.     

Improving the electrical and microwave absorbing properties of Si3N4 ceramics with carbon nanotube fibers

Carbon nanotube fibers (CNTFs) reinforced Si₃N₄ ceramics has been prepared by incorporating CNTF preforms into ceramic precursor and followed by the sintering process. A SiC interface layer is formed due to the chemical reaction between Si and CNTs, leading to a good bonding between CNTs and Si₃N₄ matrix. Due to the ceramic deposition, the oxidation resistance is increased of 200 °C. Furthermore, the CNTs have well orientation and high-volume distribution (7 wt%) in the hybrid composites. Obvious improvements of the electrical conductivity (up to 103 S/m) and the microwave absorbing performance (up to 6 dB at 15 GHz) are obtained for the composites containing CNTFs. Our work provides a meaningful way to fabricate multifunctional ceramics possessing high electrical and microwave absorbind properties.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Zirconia ceramic and Nb joints brazed with Mo-particle-reinforced Ag-Cu-Ti composite fillers: Interfacial microstructure and formation mechanism

Zirconia (ZrO₂) ceramic and Nb were successfully brazed using a Mo-particle -reinforced Ag-Cu-Ti composite filler. The effect of the Mo content of the composite filler on the interfacial microstructures and mechanical properties of ZrO₂/Nb-brazed joints was investigated. The calculated Ti activity initially increased and then decreased as the Mo content was increased from 1 to 40 wt%, and played a decisive role in the evolution of interfacial products formed adjacent to the ZrO₂ ceramic. When 40 wt% Mo particles were added to the composite filler, TiO+Ti₃Cu₃O reaction layers formed adjacent to the ceramic substrate. By decreasing the Mo content of the filler, the TiO layer became thinner or even vanished, whereas the thickness of the Ti₃Cu₃O reaction layer increased gradually with decreasing Mo content. Concurrently, a bulky TiCu compound grew near to the ZrO₂ ceramic, and further fine TiCu particles were observed in the brazing seam. This microstructure evolution, as well as the mechanism for the formation of joints brazed with composite fillers of differing Mo content, is discussed based on TEM analyses. The shear strength of the brazed joint is clearly improved when a suitable amount of Mo is added to the Ag-Cu-Ti filler. A maximum shear strength of 370 MPa was obtained when ZrO₂/Nb joints were brazed with Ag-Cu-Ti+5 wt% Mo composite filler.     

Microstructure evolution and mechanical property of ZrC-SiC/Ti6Al4V joints brazed using Ti-15Cu-15Ni filler

ZrC-SiC ceramic and TC4 alloy were successfully brazed using a self-prepared Ti-15Cu-15Ni filler. The microstructure and mechanical property of the joints obtained at different brazing temperatures were investigated. The results indicated that Ti from the Ti-15Cu-15Ni and the TC4 reacted with the ZrC-SiC to form TiC phase adjacent to the ZrC-SiC ceramic. In the brazing seam, Ti₂(Ni, Cu) intermetallic compounds zone (IMCs Zone), Hypoeutectic Zone and Hypereutectoid Zone formed. The brazing temperature affected the dissolution of TC4 into the braze filler significantly, and then determined the microstructure of the joint. The formation of α-Ti in the brazing seam could decrease the hardness and the brittleness of the brazing seam, which was beneficial to the property of the brazed joint. The joint strength reached a maximum value of 43 MPa when the joint was brazed at 970 °C and cracks propagated in the ZrC-SiC substrate near the brazing seam.     

Sintered silicon nitride/nano-silicon carbide materials based on preceramic polymers and ceramic powder

A flexible method is presented, which enables the fabrication of porous as well as dense Si₃N₄/nano-SiC components by using Si₃N₄ powder and a preceramic polymer (polycarbosilazane) as alternative ceramic forming binder. The SiCN polymer benefits consolidation as well as shaping of the green body and partially fills the interstices between the Si₃N₄ particles. Cross-linking of the precursor at 300 °C increases the mechanical stability of the green bodies and facilitates near net shape machining. At first, pyrolysis leads to porous ceramic bodies. Finally, subsequent gas pressure sintering results in dense Si₃N₄/nano-SiC ceramics. Due to the high ceramic yield of the polycarbosilazane binder, the shrinkage during sintering is significantly reduced from 20 to 15 lin.%. Investigations of the sintered ceramics reveal, that the microstructure of the Si₃N₄ ceramic contains approx. 6 vol.% nano-scaled SiC segregations, which are located both at the grain boundaries and as inclusions in the Si₃N₄ grains.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Uncovering the critical factor in determining the residual stresses level in Si3N4–GM filler alloy–42CrMo joints by FEM analysis and experiments

The influence of gradient materials (GM) filler alloy on the distribution of thermal stresses and on the bending strength of the brazed Si₃N₄–42CrMo steel joints was examined by using finite element modeling (FEM) computations in combination with experiments. In order to form a smooth thermal expansivity change across the whole joint, a novel GM filler alloy was fabricated by stacking each layer with different content of Mo particles (Ag–Cu–Ti+Mo) addition together. We examined the effect of GM compositions, layer numbers and thicknesses on the residual stresses in the brazed joint. In particular, the monolayer composite filler produced by incorporating 10 vol% Mo particles induced the minimum residual stresses in the joint, agreeing with the experimental results. The results indicated that the CTE mismatch between the joined materials and the ability of plastic deformation in the filler alloy were two factors that determine the residual stresses level in a brazed joint. The results reported here will provide us guidance to choose an appropriate filler alloy for improving the ceramic–metal joint performance.     

Revealing the strengthening mechanism in Si3N4 ceramic joint by atomic force microscopy coupled with nanoindentation techniques

The composite filler has been widely introduced for joining ceramic. However, the underlying formation and strengthening mechanisms in the joint remain uncertain. In this study, a commercial Ag–Cu–Ti brazing alloy with Mo particles reinforcement has been introduced for joining Si₃N₄ ceramic and the effect of Mo particles on the microstructure and flexural strength of the joint was investigated. Nanoindentation was employed to characterize the mechanical properties for reaction phases in the joints. The modulus and hardness values for Cu–Ti intermetallics and brazing alloy in the joint were first reported, providing a strong evidence to elucidate the strengthening mechanism. In addition, the strength was increased from 200 MPa, with Ag–Cu–Ti alone, to a maximum of 429 MPa while using Ag–Cu–Ti + 5 vol.% Mo composite filler. We are convinced that, for a well-bonded joint, the thermal expansion mismatch between the joined materials and the plastic deformation in the brazing alloy determined the joint strength.