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 Si3N4/42CrMo joints brazed with TiNp modified active filler

Si₃N₄ ceramic/42CrMo steel joints were obtained by employing TiNp modified Ag–Cu–Ti active filler and subsequently the effect of TiNp content on the microstructure and mechanical properties of the joints was investigated. Microstructural examination revealed that TiN+Ti₅Si₃ reaction layer was adjacent to the Si₃N₄ ceramic while a TiC reaction layer was close to the steel substrate. With the increase of TiNp content, more fine grains and less Ag–Cu eutectic appeared in the joint and the reaction layers near the two base materials became thinner. The flexural strength of the joint obtained by four-point bending test climbed about 100% with the optimum TiNp content of 5 vol%, comparing with the case without TiNp. Thermal stress distributions in the joint were analyzed using finite element modeling computations, which accorded well with the bending test results.     

Microstructure and mechanical properties of ZrB2-SiC/Nb joints brazed with CoFeNiCrCuTix high-entropy alloy filler

ZrB₂-SiC ceramics and Nb alloy were brazed at 1160°C for 60 min with CoFeNiCrCuTi_x high-entropy alloy filler. The influence of Ti content on the interface structure and mechanical properties of ZrB₂-SiC/Nb joint was systematically studied. It is found that the rich-Ti Laves phase was formed due to the addition of large atomic size Ti fill into the filler alloy or brazing joint, and its content increases with Ti content. The joint brazed by high-entropy alloys filler without Ti can be divided into a tooth-shaped Cr₂B reaction layer and a central area composed of a eutectic mixed structure of FCC phase and rich-Nb lamellar Laves phase. Ti and Nb are mutual solid solution elements. The increase of Ti content in the joint makes the FCC phase and the rich-Nb lamellar Laves phase to transform into a big bulk Ti-rich Laves phase and the quadrilateral (Ti, Nb)B phase. The tooth-shaped Cr₂B was disappeared. The residual stress generated in the joint during the brazing process tends to cause defects such as holes and microcracks in the bulk Ti-rich brittle Laves phase. Therefore, with the addition of Ti, the normal temperature performance of the joint decreases from 216 MPa to 52 MPa. However, with the increase of Ti, the high-temperature mechanical properties of the joint first decrease, and then increase. It was mainly due to the formation of rich-Ti Laves phase and quadrilateral (Ti, Nb)B with excellent high-temperature mechanical properties. When brazing with CoFeNiCrCuTi_1.5 filler, the high-temperature performance of the joint reached 92% of its room temperature performance.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.     

Microstructural and mechanical properties of porous Si3N4 carbon coated ceramic brazed with a cobalt-silicon filler

The Co_22.5Si_77.5 (at.%) braze was used to bond porous Si₃N₄ ceramics. The effects of brazing temperature on microstructure and the bonding strength of the joint were studied. The results reveal that no visible reaction layer was observed. The corresponding joint strength was low. In order to improve the joint strength a carbon coated modification of the porous Si₃N₄ substrate was suggested. The impact of this modification on the joint properties was examined. It was established that a SiC reaction layer with a thickness from ∼15 μm to ∼65 μm was formed at the interface and SiC nanowires were observed when the temperature increased from 1280 °C to 1340 °C. The maximum shear strength of the carbon coated and uncoated joints were 115 MPa and 44 MPa, respectively. The significant improvement of the joint strength was attributed to the SiC reaction layer and a strengthening by the presence of SiC nanowires. .     

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.     

Interfacial microstructure and mechanical properties of zirconia ceramic and niobium joints vacuum brazed with two Ag-based active filler metals

Reliable brazing of a zirconia ceramic and pure niobium was achieved by using two Ag-based active filler metals, Ag-Cu-Ti and Ag-Cu-Ti+Mo. The effects of brazing temperature, holding time, and Mo content on the interfacial microstructure and mechanical properties of ZrO₂/Nb joints were investigated. Double reaction layers of TiO and Ti₃Cu₃O formed adjacent to the ZrO₂ ceramic, whereas TiCu₄+Ti₂Cu₃+TiCu compounds appeared in the brazing interlayer. With increasing brazing temperature and time, the thickness of the Ti₃Cu₃O layer increased with consumption of the TiO layer, and the total thickness of the reaction layers increased slightly. Meanwhile, the blocky Ti-Cu compounds in the brazing interlayer tended to accumulate and grow. This microstructural evolution and its formation mechanism are discussed. The maximum shear strength was 157 MPa when the joints were brazed with Ag-Cu-Ti at 900 °C for 10 min. The microstructure and bonding properties of the brazed joints were significantly improved when Mo particles were added into the Ag-Cu-Ti. The shear strength reached 310 MPa for joints brazed with 8.0 wt% Mo additive, which was 97% higher than that of joints brazed with single Ag-Cu-Ti filler metal.     

The effect of a carbon layer on the microstructural and mechanical properties of porous BN/Si3N4 ceramic brazed with a titanium-silicon filler

The Ti₂₂Si₇₈ (wt.%) braze was used to bond porous BN/Si₃N₄ ceramic. The results revealed that the joint strength was low. In order to improve the joint strength, a carbon coated modification of the porous BN/Si₃N₄ substrate, achieved by the pyrolysis of phenol-formaldehyde resin was suggested. The thickness of carbon layer was controlled by the volume ratio of phenol-formaldehyde resin and methyl alcohol (R/M). The effect of the carbon layer on the microstructure and mechanical properties of the joint was examined. It was established that the maximum joint strength was achieved at the level of 80 MPa (as opposed to the uncoated joint with only 30 MPa) when R/M was 1:2, at the given brazing temperature. The improvement of the joint strength is attributed mainly to the formation of a SiC infiltration reaction layer, a significant population of SiC nanowires, as well as TiN_0.7C_0.3 and TiB₂ particles.     

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.     

Wetting of AgCu-Ti filler on porous Si3N4 ceramic and brazing of the ceramic to TiAl alloy

The effect of Ti content on the wettability of AgCu-Ti filler on porous Si₃N₄ ceramic was studied by the sessile drop method. AgCu-2 wt% Ti filler alloy showed a minimum contact angle of 14.6° on porous Si₃N₄ ceramic during the isothermal wetting process. The mechanism of AgCu-Ti filler wetting on porous Si₃N₄ ceramic is clarified in this paper. Porous Si₃N₄ ceramic was brazed to TiAl alloy using AgCu-xTi (x = 0, 2 wt%, 4 wt%, 6 wt%, 8 wt%) filler alloy at 880 °C for 10 min. The effect of Ti content on the interfacial microstructure and mechanical properties of porous-Si₃N₄/AgCu-xTi/TiAl joints are studied. The typical interfacial microstructure of p-Si₃N₄/AgCu-Ti/TiAl joint is p-Si₃N₄/penetration layer (Ag(s,s)+Si₃N₄+TiN+Ti₅Si₃)/Ag(s,s)+Cu(s,s)+TiCu/AlCu₂Ti/TiAl. The maximum shearing strength of the brazed joint was 14.17 MPa and fracture that occurred during the shearing test propagated in the porous Si₃N₄ ceramic substrate for the formation of the penetration layer.     

Interfacial microstructure and mechanical properties of ZTA/ZTA joints brazed with Ni-Ti filler metal

The reliable brazing of the ZTA ceramic joints was successfully obtained using Ni-Ti filler metal. The microstructure and mechanical properties of the joints brazed at different temperatures were investigated. During the process of brazing, both Al₂O₃ and ZrO₂ in the ZTA reacted with the Ni-Ti filler, resulting in the formation of the AlNi₂Ti + Ni₂Ti₄O reaction layer adjacent to the ZTA substrate when brazed at 1350 °C for 30 min. NiTi and Ni₃Ti compounds precipitated at the center of brazing seam. When the brazing temperature increased from 1320 °C to 1380 °C, the thickness of AlNi₂Ti + Ni₂Ti₄O layer increased gradually. As the brazing temperature varied from 1400 °C to 1450 °C, TiO was formed adjacent to the ZTA substrate, along with the reduction of Ni₂Ti₄O. AlNi₂Ti distributed at the interface and center of brazing seam. The maximum shear strength of 152 MPa was obtained when brazed at 1420 °C for 30 min.     

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.     

Brazing ZrO2 ceramic and TC4 alloy by novel WB reinforced Ag-Cu composite filler: Microstructure and properties

Residual stresses in ceramic-metal joints is the important factor for their reliable implementation in cutting-edge industries. Composite fillers is reported to be a promising approach to reduce the residual stresses. Until now, few experimental researches on the brazing of ZrO₂ ceramic and TC4 alloy using composite fillers have been reported. In this study, to release the residual stresses and improve the joints strength, novel WB reinforced Ag-Cu composite filler was fabricated to braze ZrO₂ ceramic and TC4 alloy. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were applied for the analysis of microstructure and phases structure in the joints. The TiB whiskers and W particles were in situ synthesized via the reaction between active Ti and WB particles, and randomly distributed in the brazing seam. The effect of brazing temperature and WB content on interfacial microstructure and mechanical strength in the brazed joints were investigated. When brazed at 870 °C for 10 min, favorable microstructure reinforced by TiB whiskers and W particles in the brazing seam was achieved with 7.5 wt% WB addition in composite filler. The maximum average shear strength of the joints was 83.2 MPa, which about 59.4% increase over the joints without WB addition.     

Microstructure and mechanical properties of C/C composite/Ti6Al4V joints with a Cu/TiCuZrNi composite brazing alloy

C/C composites and Ti₆Al₄V sheets were joined using a novel Cu/TiCuZrNi composite braze and the effect of Cu foil thickness on the microstructure and mechanical properties of the joints were investigated. A composite joining material, consisting of a TiC particle reinforced brazing alloy formed in situ during the brazing process was obtained. Before the joining process, the C/C composite surface was modified by solid state reaction with chromium to form a chromium carbide coating. This carbide reaction layer can react with the Ti-based brazing alloy and form evenly distributed TiC particles. The maximum apparent shear strength of as-received joint was 39±8.5 MPa obtained by using a 40 μm thick Cu foil together with the TiCuZrNi alloy; this value was 70% higher than the mechanical strength obtained for joints brazed with the TiCuZrNi alloy only.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Microstructure and mechanical properties of the C/C composite and Nb joints brazed with an active Ag-based filler metal

An active Ag-based filler metal, containing trace alloy elements of Al, Cu and Ti, was successfully applied to braze C/C composite and Nb. The microstructure and formation mechanism of the C/C composite and Nb brazed joint were investigated in this study. Moreover, the influence of brazing parameters on microstructural evolution and mechanical properties of brazed joints was evaluated. The typical interfacial microstructure of the joint obtained at 950 °C for 600 s was C/C/TiC + AlCu₂Ti/Ag(s, s) + AlCu₂Ti + particle Cu/AlCu₂Ti + AlCuTi + (Ti, Nb)₃Al + Nb(Ti)/Nb. The dispersive AlCu₂Ti phase was uniformly distributed in the Ag matrix, which was a beneficial structure for the brazed joint. The shear strength of the brazed joint was sensitive to the brazing temperature and holding time, which was closely related to the TiC layer bordering the C/C composite. The thickness of the TiC layer first increased as temperature increased to 950 °C, and then decreased when temperature reached 970 °C. The carbon fiber eroded by the filler at 970 °C entered to the brazing seam and reacted with Ti, resulting the reduction of the thickness of TiC, thus damaging the strength of the joint. With extension of holding time from 300 s to 1200 s, the interface reaction became more sufficient. Therefore, the thickness of the TiC layer increased. However, at 1200 s, the over-thick TiC broke the C/C substrate because of the mismatch of coefficient of thermal expansions. The maximum shear strength of the joint (950 °C/600 s) reached 55 MPa at room temperature and 35 MPa tested at 550 °C.     

Interfacial microstructure and mechanical properties of TC4/Ti3SiC2 contact-reactive brazed joints using a Cu interlayer

Reliable contact-reactive brazed joints of TC4 alloy and Ti₃SiC₂ ceramic were obtained using a Cu interlayer. The interfacial microstructure of a TC4/Ti₃SiC₂ joint brazed at 920?°C for 10?min was TC4/Ti₂Cu +?α-Ti +?β-Ti/Ti₂Cu +?AlCu₂Ti +?Ti₅Si₃/Ti₅Si₃ +?Ti₅Si₄/Ti₃SiC₂. The interfacial microstructure and mechanical properties of TC4/Ti₃SiC₂ joints brazed at different temperatures were investigated. With increasing temperature, the shear strength of the brazed joints first increased and then decreased. The maximum shear strength was 132?±?8?MPa, and the corresponding fracture occurred along the Ti–Si reaction layer and the Ti₃SiC₂ substrate adjacent to the Ti–Si reaction layer. The microhardness test also demonstrated that the Ti–Si reaction layer possessed the highest microhardness, 812?±?22 HV. The Ti-Si reaction layer was the weakest part of the brazed joints. To eliminate the Ti-Si reaction layer and improve the mechanical properties of TC4/Ti₃SiC₂ brazed joints, a 40-μm Ni layer was plated on the surface of the Ti₃SiC₂ ceramic before brazing. The results showed that the Ti–Si reaction layer that formed adjacent to the Ti₃SiC₂ ceramic was thin and intermittent. Moreover, the interface between the Ti₃SiC₂ ceramic and the TC4 alloy became jagged. The shear strength of the TC4/nickel-plated Ti₃SiC₂ brazed joints improved to 148?±?8?MPa; the corresponding fracture occurred mainly in the Ti₃SiC₂ ceramic and only a small portion of the fracture occurred in the brazing seam.     

Fabrication processing and mechanical properties of Si3N4 ceramic turbocharger wheel

A composite technique was chosen to fabricate Si₃N₄ ceramic turbine wheel based on modified investment casting (MIC), slip casting (SP) and mold constraint hot isostatic pressing (MCHIP) aided by a near-net dimension using the gypsum mold and multi-piece Y₂O₃ ceramic mold. The detailed fabrication processing of Si₃N₄ ceramic turbine wheel was described. And the flexural strength and fracture toughness after different work temperature and speed were discussed. The results showed that owing to occurrence of phase transformation and chemical reaction, excess temperature resulted in interface cracking and interface debonding, flexural strength and fracture toughness decrease. Thermal expansion and centrifugal force under excessive high speed brought many pores in the microstructure and resulted in crack initiation and crack propagation. The critical work temperature was 700?°C and critical work speed was 100,000 r/min, which were obtained from the test and simulation results.     

Characterization of carbon/carbon composite/Ti6Al4V joints brazed with graphene nanosheets strengthened AgCuTi filler

Carbon/carbon (C/C) composite and Ti6Al4V alloy (wt%) were successfully brazed with graphene nanosheets strengthened AgCuTi filler (AgCuTiG). Graphene nanosheets (GNSs) with low CTE and high strength were dispersed into AgCuTi filler by ball milling. The interfacial microstructure was systematically characterized by varieties of analytical means including transmission electron microscopy (TEM). Results show that typical interfacial microstructure of the joint brazed at 880 °C for 10 min is a layer structure consisting of (Ti6Al4V/diffusion layer/Ti2Cu + TiCu + Ti3Cu4 + TiCu4/GNSs + TiCu + TiC + Ag(s,s) + Cu(s,s)/TiC/C/C composite). The interfacial microstructure and mechanical properties of brazed joints changed significantly as temperature increased. High temperature promoted the growth of TiCu and TiC phases, which were attached to GNSs. Meanwhile, the diffusion layer and primary reaction layers thickened as temperature increased, while the thickness of brazing seam decreased. The maximum shear strength of 30.2 MPa was obtained for the joint brazed at 900 °C for 10 min. GNSs decreased the thickness of brittle reaction layers and promoted the formation of TiCu and TiC phases in brazing seam, which caused the strengthening effect and decreased the CTE mismatch of brazed joints. The fracture modes are also discussed in this paper.     

High temperature mechanical properties of porous Si3N4 prepared via SRBSN

The high temperature strength and fracture behavior of porous Si₃N₄ ceramics prepared via reaction bonded Si₃N₄ (RBSN) and sintered reaction bonded Si₃N₄ (SRBSN) were investigated at 800–1400?°C. The weight gain after oxidation for 15?min and the microstructure of the edge and center of the fracture surface clearly show that the internal oxidation of porous SRBSN is unavoidable with porosity of ~ 50% and mean pore size of 700?nm. The oxidation of Si₃N₄ and intergranular Y₂Si₃O₃N₄ phase may responsible for the high temperature strength degradation of SRBSN. Porous Si₃N₄ ceramics prepared with addition of 1?wt% C showed low strength degradation at temperature >?1200?°C.