Joining of ZrO2-4.5 wt% Y2O3 (Y-TZP) ceramics using nanocrystalline tape cast interlayers

Nanocrystalline Y-TZP tape casts were used as interlayers to join conventional Y-TZP ceramic pellets. The joining experiments were performed by hot pressing at 1000°C to 1300°C under constant pressure of 55 MPa. Two types of joints were obtained with and without a nanocrystalline interlayer. At 1100°C, the successful joints were enabled only with the interlayer; four point bending test results revealed an average joint strength of 206 ± 10 MPa. The joint strength increased with the joining temperature. The specimens joined at 1300°C with an interlayer exhibited a joint strength of 613 ± 40 MPa, which is 96% of the strength of the ceramic pellets. The interlayer at the joint exhibited homogeneous and crack free microstructure and preserved its nanocrystalline nature at all temperatures. The advantage of the nanocrystalline interlayer for joining is pronounced at lower joining temperatures and most probably for pellets with large grain size.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Joining of Si-Ti-C-O fibre-assembled ceramic composites with 72Ag-26Cu-2Ti filler metal

Si-Ti-C-O fibre-assembled ceramic composites were joined with 72Ag-26Cu-2Ti filler metal at 1123 K and 1223 K in vacuum. The composites consisted of Si-Ti-C-O fibres, which were assembled unidirectionally, and oxide material filling the spaces between the fibres. During the joining process, frothing occurred at the joining interfaces. Joining interfaces were observed by SEM and analysed by electron probe microanalysis and X-ray diffraction. The strength of the joints was evaluated by four-point bending tests. Most of Si-Ti-C-O fibre/filler metal interfaces and the oxide material/filler metal interfaces were firm without cracking and separation. At the fibre/metal interfaces, a high concentration of titanium was confirmed. Among the specimens joined at 1123 K, the average strength, measured by the bending test, was 96 MPa. It was inferred that the defects at the joining interfaces formed by frothing had decreased the strength of the joints. Metallizing of the surfaces to be joined with the same filler metal as a pretreatment before joining, was effective in preventing frothing during joining and improving the joining strength. The average strength of the joints with pretreatment was 211 MPa.     

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.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi₂ composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi₂ were investigated. There was good bonding between the SiC and MoSi₂ particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (2 nm) layer, however, was observed at the SiC/MoSi₂ interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is 20% loss in comparison to that of RB-SiC alone (containing 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.     

Joining of Silicon Carbide Using MgO-Al2O3-SiO2 Filler

Pressureless sintered SiC specimens were joined using MgO-Al2O3-SiO2 (MAS) filler. MAS filler showed excellent behaviour of wetting on SiC substrate above 1480 °C, and the wettability was much influenced by the joining atmosphere. The joining was carried out at 1500 and 1600 °C for 30 min in Ar atmosphere. The flexural strength of the joined specimen showed 342–380 MPa up to 800 °C. However, the flexural strength of the joined specimen decreased to about 80 MPa at 900 °C due to softening of the joint interlayer. The results of the XRD and WDS showed that the reaction between SiC and the MAS filler produced the oxycarbide glass.     

Fabrication and properties of novel composites in the system Al–Zr–C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 m long and 133 m thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 m diameter and a 1–2 m thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.     

C/C复合材料与LAS陶瓷连接的显微结构与性能

采用Y2O3-Al2O3-SiO2-TiO2(YAST)玻璃作为中间层,对SiC-MoSi2表面改性的C/C复合材料与Li2CO3-Al2O3-SiO2(LAS)陶瓷进行热压连接,所施压力为20MPa,保温时间为30min,连接温度分别为1150℃,1200℃,1250℃,1300℃。利用SEM,EDS和BEI(背散射电子像)对SiC-MoSi2涂层,连接界面的形貌和断口进行了分析,研究结果表明,SiC-MoSi2涂层与基体结合紧密,Si、C元素在界面处呈梯度状分布,形成厚度约为15μm的过渡层。YAST玻璃与基体润湿良好,接头的剪切强度可达26.21MPa。     

Mechanical and thermal properties of Si–C–N material from polyvinylsilazane

Commercial polyvinylsilazane was crosslinked and then crushed to powder. The powder was compacted by cold isostatic pressing at 630 MPa and pyrolysed at 1050 °C in flowing argon. Crack-free Si–C–N material was obtained. Bulk density of the material was 1.95 Mg m–3. Open porosity was 9.6%. The material was amorphous as a result of X-ray diffraction analysis. Elastic modulus measured by pulse–echo method was 105 GPa. Vicker's hardness calculated from indentation at 98 MPa was 6.1 GPa. Fracture toughness measured by indentation fracture method was 2.1 MPa m1/2. Average bending strength was 118 MPa. The material shrank 1.9% in length during heating up to 1400 °C in nitrogen. The thermal expansion coefficient of the material heat treated up to 1400 °C increase from 3.08 × 10–6 °C–1 at 100 °C to 3.96 × 10–6 °C–1 at 1200 °C.     

Employing reactive synthesis for metal to ceramic joining for high temperature applications

Joining of dissimilar materials allows the properties of both materials to be exploited in a device or structure. The main reasons for the incorporation of dissimilar materials are to achieve function, improve efficiency and to reduce cost.Silicon nitride is an engineering ceramic that has outstanding properties but has yet to find its full commercial potential. Silicon nitride is suitable for high temperature applications, however, its incorporation into devices or structures tends to be restricted due to a lack of suitable joining techniques.This paper presents the results of joining between the high temperature and corrosion resistant iron-chromium-aluminium alloy (Fecralloy) with silicon nitride by a nickel aluminide (NiAl) interlayer. The formation of NiAl from its constituent elements (Ni-Al compact was used) by reactive synthesis is highly exothermic and this was utilised to cause partial melting of the Fecralloy interface and reactive wetting of the silicon nitride interface.Joints with average shear strength of 94.3 MPa were fabricated under optimum processing conditions (900°C, 15 min, 45 MPa). Thermal cycling at 850°C in air showed that the joints could be used at this temperature.The primary focus of this work was on the effects of process conditions upon the microstructure and mechanical properties of the joint. The reactive synthesis of NiAl was studied using differential thermal analysis (DTA), where the effects of varied heating rate were investigated.     

Heat-resistant joints of Si3N4 ceramics with intermetallic compounds formed in situ

The possibility of the improvement on the heat resistance of Si₃N₄ ceramic joints with intermetallic compounds formed in situ was investigated. The Si₃N₄ ceramics were joined with Ti/Ni/Ti multi-interlayers between 1000 and 1150°C. The effects of various parameters, which include the thickness of Ti and Ni foils, the pressure imposed during bonding, the bonding temperature and the holding time, on the microstructures and the strength (both at room temperature and at high temperature) of the joints were studied. The results indicated that the sound joints with higher strength both at room temperature and at elevated temperature could be acquired with intermetallic compounds formed in situ under appropriate bonding parameters. The shear strength at 800°C could sustain about 88 MPa.     

Joining of silicon nitride to SUS304 stainless steel using a sputtering method

Silicon nitride with thin sputter-deposited titanium and nickel films was joined to SUS304 stainless steel (18% Cr-8% Ni) using metallic buffers in a series of silicon nitride/nickel/ molybdenum/nickel/SUS304, and the joining strength and microstructures were investigated. Four-point bending tests showed fracture strength of the joints up to 169 MPa. Cracks were formed at the interface between the silicon nitride and its adjacent nickel buffer, and frequently extended into the silicon nitride. Microstructural analyses revealed that the silicon nitride reacted with the sputter-deposited titanium producing titanium nitride and isolated silicon atoms, and that silicon and titanium diffused into the nickel buffer. Calculations using a finite-element method indicated a marked reduction in thermal stress induced in the joined silicon nitride with increasing thickness of the molybdenum buffer. The strong interfacial bond inducing the fracture of the joined silicon nitride was interpreted in terms of a good interfacial reaction, the interdiffusions and the reduction of thermal stress being due to the insertion of the molybdenum buffer.     

Tensile testing at high temperatures of ex-PCS Si-C-O and ex-PCSZ Si-C-N single filaments

A microtensile tester consisting mainly of an induction-heated furnace, a 0–2 N load cell, a 0.1/1 m sensitivity straining device and hot grips has been designed and used to test ceramic single ceramic filaments at 25–1600°C under vacuum (0.1 Pa) or in controlled atmospheres. Both failure strength and Young's modulus were measured with an isothermal gauge length of 30 mm. A system compliance correction was applied for each test temperature and material. The apparatus was used to characterize an ex-poly-carbosilane Si-C-O fibre (Nicalon NLM-202) and an ex-polycarbosilazane Si-C-N experimental single filament almost free of oxygen (-ray curing). Both materials exhibit a significant strength loss at 1200–1600°C when tested under vacuum, assigned to a decomposition process with an evolution of gaseous species (SiO/CO or N₂) and the formation of a mechanically weak decomposition surface layer. Conversely, the Si-C-N filament undergoes no strength loss when tested in an atmosphere of nitrogen (P=100 kPa) at 1200°C, the decomposition being impeded by the external nitrogen pressure. In all cases, no significant decrease in Young's modulus was observed.     

Joining of carbon/carbon composites for nuclear applications

Using hot pressing, carbon/carbon composites were joined using a Ti₃SiC₂/SiC interlayer which was in situ synthesized by the reaction of TiC and Si. Phase composition of the interlayer was characterized by X-ray diffraction. Morphologies of the joints before and after shear test were determined by scanning electron microscope and energy dispersive spectroscopy. The mechanical strength of the joints was assessed by shear strength test. Phase analysis reveals that the interlayer was mainly composed of ternary Ti₃SiC₂, SiC, and little TiC. The microstructure observation results show that the dense and uniform interlayer adheres firmly to the C/C composites. A composition gradient reaction layer was formed at the joining interface between C/C substrates and interlayer. The room temperature average shear strength of the joints is about 38.9 ± 3.6 MPa. The joining mechanism and failure behavior of the joints were also discussed.     

Sinterability of magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 m; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.     

The preparation and performance of a novel room-temperature-cured heat-resistant adhesive for ceramic bonding

A novel adhesive for joining ceramic materials was made using silicon-epoxy interpenetrating polymer networks (IPNs) as matrix (based on silicon resin (SR) and diglycidyl ether of bisphenol A (DGEBA) epoxy resin (EP)), γ-glycidoxypropyltrimethoxysilane (γ-GPS) as cross-linking agent, dibutyltin dilaurate (DBTDL) as catalyst, Al, low melting point glass (GP) and B₄C powders as inorganic fillers, low molecular polyamide (LMPA 650) as curing agent. The character and heat-resistance property of the IPNs and adhesive were tested by FT-IR, DSC and TG. The compressive shear strength of ceramic joints was investigated at different temperatures in atmosphere surroundings. The modification mechanism of inorganic fillers was studied using XRD. Results showed that the IPNs were a homogeneous morphology of inter-crosslinked network structure with single T_g. The adhesive could be cured at room temperature with good heat-resistance property due to the chemical bond of epoxy group and Si-O-Si. The optimum compressive shear strength (9.44 MPa at 1000 °C) occurred at SR/EP ratio: 9/1, content of KH560: 2%, Al/GP/B₄C ratio: 3.2/4/3, fillers/IPNs ratio: 6/4. The adhesive had good heat-resistance property with 10% weight loss at 435 °C. Failure mode of joint was mixing failure due to the high chemical bonding force.     

C/SiC复合材料有序多孔陶瓷接头的制备及其连接技术研究

采用stöber法制备出单分散氧化硅小球, 并以此为模板, 结合先驱体转化技术成功制备出C/SiC复合材料纳米有序多孔陶瓷接头, 并对该接头制备工艺条件作了优化。对制备出的C/SiC多孔陶瓷接头分别采用先连后浸法(SJM)和直接浸渍法(DSM)进行了连接。结果显示, 两种方法连接的连接件的抗弯强度分别达82.4和20.5 MPa, 表明C/SiC多孔陶瓷接头采用SJM连接较好。     

Evaluation of Si3N4 joints: bond strength and microstructure

Joining of pressurelessly sintered silicon nitride ceramics was carried out using adhesive slurries in the system Y-Si-Al-O-N in a nitriding atmosphere. The effects of bonding parameters, such as joining temperature (1450–1650°C), applied pressure (0– MPa) and holding time (10–60 min), on the bond strength of joint were evaluated. A typical microstructure of the joint bonded with the optimum adhesive was investigated. The three point bend testing of joined samples with 3 × 4 × 36 mm³ in dimension was employed to study the bond strength of joints. The results show that an optimum joining process was achieved by holding at 1600°C for 30 min under an external pressure of 5 MPa and the maximum bond strength was 550 MPa, compared to 700 MPa of unbonded Si₃N₄ ceramic, using the adhesive having the Si₃N₄/(Y₂O₃ + SiO₂ + Al₂O₃) ratio of 0.39. The good bond strength is attributed to the similarity in microstructure and chemical composition between joint zone and ceramic substrate. The fracture modes were classified into two types according to the values of bond strength. This revised version was published online in September 2006 with corrections to the Cover Date.     

Structure and properties of Si-Ti-C-O fibre-bonded ceramic material

Si-Ti-C-O fibre-bonded ceramic material was synthesized from pre-oxidized Si-Ti-C-O fibre with an oxide layer 400–600 nm thick, by hot-pressing at 2023 K under 50–70 MPa. The interstices in the Si-Ti-C-O fibre-bonded ceramic material were packed with an oxide material which existed on the surface of the pre-oxidized Si-Ti-C-O fibre, and the oxide material formed a small amount of the matrix phase (10 vol%). At the fibre-matrix interface, aligned turbostratic carbon, which was oriented around the fibre, was formed during hot-pressing. The existence of the interfacial carbon layer indicated the Si-Ti-C-O fibre-bonded ceramic material to have a fibrous fracture pattern with high fracture energy. The Si-Ti-C-O fibre-bonded ceramic material showed excellent durability even at 1773 K in air, because a protective oxide layer is formed on the surface at a high temperature (above 1273 K) in air. Moreover, the Si-Ti-C-O fibre-bonded ceramic material almost maintained its initial strength in the bending and tensile tests, even at 1773 K in air.