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.     

采用Ag-Cu-In-Ti焊料连接碳化硅陶瓷

采用四元Ag-Cu-In-Ti焊料成功地连接了常压烧结SiC陶瓷. 研究了钎焊温度和保温时间对碳化硅连接强度的影响, 同时通过EPMA和TEM分析连接界面的微观结构, 并且探讨了连接的原理. 试验结果表明, 在700～780℃试验温度范围内, 碳化硅的连接强度存在峰值, 最高四点弯曲强度达到了234MPa, 但是连接强度随着保温时间的增加呈现单调下降趋势. 接头微观结构由基体SiC、反应层和焊料三部分组成, 连续致密的反应层紧密连接基体和焊料, 反应层由带状层、TiC层和Ti5Si3层组成, 带状层宽度约20nm, 由Ag、In、Si和少量的Ti、Cu组成. 元素线扫描结果显示焊料中的活性元素Ti含量在反应层内形成峰值, 活性元素Ti与SiC发生反应生成新的反应层是连接的主要因素.     

The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints

Silica carbide modified carbon cloth laminated C–C composites have been successfully joined to lithium–aluminum–silicate (LAS) glass–ceramics using magnesium–aluminum–silicate (MAS) glass–ceramics as interlayer by vacuum hot-press technique. The microstructure, mechanical properties and fracture mechanism of C–C/LAS composite joints were investigated. SiC coating modified the wettability between C–C composites and LAS glass–ceramics. Three continuous and homogenous interfaces (i.e. C–C/SiC, SiC/MAS and MAS/LAS) were formed by element interdiffusions and chemical reactions, which lead to a smooth transition from C–C composites to LAS glass–ceramics. The C–C/LAS joints have superior flexural property with a quasi-ductile behavior. The average flexural strength of C–C/LAS joints can be up to 140.26 MPa and 160.02 MPa at 25 °C and 800 °C, respectively. The average shear strength of C–C/LAS joints achieves 21.01 MPa and the joints are apt to fracture along the SiC/MAS interface. The high retention of mechanical properties at 800 °C makes the joints to be potentially used in a broad temperature range as structural components.     

树脂基多孔碳孔结构对Cf/SiC复合材料连接性能的影响

连接技术是实现大尺寸以及复杂构型C_f/SiC复合材料制备及工程化应用的关键技术。本工作使用酚醛树脂作为碳源, 通过反应连接法实现了C_f/SiC复合材料的稳定连接, 研究了多孔碳坯的体积密度和孔径对接头连接性能和微观结构的影响, 讨论了惰性填料含量对接头连接性能和显微组织的影响。研究表明: 树脂基多孔碳素坯的体积密度和孔径分别选定在0.71~0.90 g·cm^-3和200~600 nm比较合适, 随着多孔碳素坯孔径增加, 游离硅尺寸逐渐增大; 当孔径为190 nm时, 连接件强度最大为(125±12) MPa。添加SiC惰性填料可以明显减小多孔碳素坯的体积收缩, 当SiC惰性填料质量分数为50%时, 连接件强度最高达到(216±44) MPa, 基本与基体材料强度相当。总体而言, 本研究为实现C_f/SiC复合材料稳定连接提供了理论指导, 对实现复杂形状或大型C_f/SiC复合材料的制备和工程应用具有重要意义。     

Brazing of SiC to a wrought nickel-based superalloy using CoFeNi(Si, B)CrTi filler metal

Newly-developed CoFeNi(Si, B)CrTi brazing filler metal was used for joining of SiC to a wrought nickel-based superalloy (GH3044). The brazing alloy was fabricated into brazing foils by a rapid solidifying technique, and the brazing temperature was fixed at 1150 °C. The SiC/GH3044 joints using single interlayer Ni or triple interlayers of Ni/W/Ni showed very low strength, and this was because the Ni severely interfered with the normal reactions between the SiC and the brazing alloy. When using triple interlayers of Kovar/W/Ni for the SiC/GH3044 joining, the joint strength was remarkably elevated to 62.5-64.6 MPa. Kovar has a low coefficient of thermal expansion. Moreover, when Kovar was used as an interlayer neighboured to the brazed SiC, it basically ensured the normal interfacial reactions between the brazed SiC and the used brazing alloy. These two factors should account for the improvement of the joint strength.     

High-resolution electron microscopy of a SiC/SiC joint brazed by a Ag-Cu-Ti alloy

A high-resolution electron microscope observation (HREM) was performed on the joined portion of a brazed polycrystalline or single crystal SiC to itself with (Ag-28wt% Cu) + 2wt% Ti alloy foil. The brazing was done under vacuum at temperatures of 800° C to 950° C with a holding period of up to 30 min. Reaction products formed at the joined interface were found to be mainly TiC. In the specimen brazed at 800° C with the holding time of 0 min, reaction product TiC formed itself into small crystallites with a diameter of less than 20 nm, and an amorphous like layer was found between SiC and TiC. On the other hand, TiC was formed as a layer along the joined interface for the specimen brazed at 950° C for the holding time of 30 min. Lattice matching of SiC to TiC crystals appeared to be good so the high bonding strength of the joint was attributed to the formation of this epitaxial interface between SiC and TiC.     

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.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

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.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.     

SiC matrix composites containing transition metal boride particulates internally synthesised by carbothermal reaction

SiC matrix composites reinforced with the various borides of the transition metals in group IV a-VI a, which were synthesized from the transition metal oxide, boron carbide and carbon mixed with SiC powder. Dense composites containing boride particulates of titanium, zirconium, niobium and chromium were prepared through reactive hot-pressing. The morphology of the internally synthesized boride particles reflected that of the starting oxide powders. SiC-NbB₂ composites with four-point flexural strength of 500 to 600 MPa and better oxidation resistance than SiC-TiB₂ were prepared even through pressureless sintering process. Pressureless-sintered and HIPed SiC-20 vol% NbB₂ exhibited the four-point flexural strength of 760 MPa at 20 °C and 820 MPa at 1400 °C.     

Bonding mechanism between silicon carbide and thin foils of reactive metals

Pressureless-sintered SiC pieces and SiC single crystals were joined with foils of reactive metals at 1500° C (1773 K) for titanium and zirconium foils or at 1000° C (1273 K) for Al/Ti/Al foils. Bend testing at various temperatures up to 1400° C (1673 K), optical and electron microscopy, and electron-probe X-ray microanalysis studies were carried out on the specimens. From the results, it was concluded that the fairly high bond strength of titanium-foil joined SiC specimens might be attributed to the formation of a Ti₃SiC₂ compound, since good lattice matching between SiC and Ti₃SiC₂ was obtained in the SiC single crystals. Also in the Al/Ti/Al-foil joined SiC, high bond strength was obtained, but it decreased steeply at 600° C (873 K) because of a retained aluminium phase. The bond strength in the zirconium-foil joined SiC was low.     

Joining of reaction-bonded silicon carbide using a preceramic polymer

Ceramic joints between reaction-bonded silicon carbide (RBSiC) were produced using a preceramic polymer (GE SR350 silicone resin) as joining material; samples were heat treated in an argon flux at temperatures ranging from 800–1200°C without applying any pressure. The strength of the joints was determined by four-point bending, shear and indentation tests. Microstructural and microchemical analyses were performed by optical microscopy, SEM, TEM and AEM. The room-temperature strength of the joints increased with the joining temperature. Maximum values as high as 220 MPa in bending and 39 MPa in shear tests were reached for samples joined at 1200°C. No detectable residual stresses were observed both in the joining material and the joined parts, and the fracture mechanism was nearly always cohesive. The joint thickness was shown to depend on the processing temperature, and ranged from about 2–7 m. The joining material was a silicon oxycarbide amorphous ceramic, with no oxygen diffusion occurring between this and the RBSiC joined parts. The lack of compositional gradients, precipitates or reaction layers indicate that the SiOC ceramic acted as an inorganic adhesive, and that the joining mechanism involved the direct formation of chemical bonds between the RBSiC parts and the joining material. © 1998 Chapman & Hall     

Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane

Three-dimensional braided carbon fiber reinforced silicon carbide composites (3D-B C_f/SiC) were prepared through eight cycles of vacuum infiltration of polycarbosilane (PCS) and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of C_f/SiC were discussed. It was found that the heating rate had great effect on the mechanical properties of C_f/SiC composites. With the increase of heating rate, the density of C_f/SiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of C_f/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 °C/min. The results showed that the flexural strength of the C_f/SiC composites fabricated at a heating rate of 15 °C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600 °C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300 °C in vacuum, the C_f/SiC showed higher flexural strength (680 MPa) than that (557 MPa) at room temperature.     

Joining ceramic to metal using a powder metallurgy method for high temperature applications

Fecralloy was successfully joined to calcia stabilised zirconia (CSZ) using a mixture of Fe, Cr and Al powders as a brazing filler and a screen printing and powder metallurgy method. The joining process was achieved at 1000 °C for 5 h in vacuum. During the joining process the filler wetted the surfaces of the CSZ and the Fercalloy foil, and formed a Fe(Cr, Al) alloy. The joint produced using the filler of (Fe-30Cr-5Al)-0.06 (Y₂O₃) (wt.%) showed good thermal stability and good thermal cycling oxidation resistance at temperatures up to 850 °C in air, even though the joint contained some porosity.     

Joining of SiC ceramic-based materials with ternary carbide Ti3SiC2

The joining of two pieces of SiC-based ceramic materials (SiC or C_f/SiC composite) was conducted using Ti₃SiC₂ as filler in vacuum in the joining temperatures range from 1200 °C to 1600 °C. The similar chemical reactions took place at the interface between Ti₃SiC₂ and SiC or C_f/SiC, and became more complete with joining temperature increases, and with the consequent increased joining strengths of the SiC and C_f/SiC joints. Based on the XRD and SEM analyses, it turns out that two reasons are most important for the high joining strengths of the SiC and C_f/SiC joints. One is the development of layered Ti₃SiC₂ ceramic, which has plasticity in nature and can contribute to thermal stress relaxation of the joints; the other is the chemical reactions between Ti₃SiC₂ and the base materials which result in good interface bonding.     

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti₃SiC₂/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti₃SiC₂/SiC composite was finer than that of monolithic Ti₃SiC₂, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti₃SiC₂ matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m^1/2 and 1080 kg mm^–2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti₃SiC₂ was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti₃SiC₂.     

Pressureless-sintered and HIPed SiC-TiB2 composites from SiC-TiO2-B4C-C powder compacts

Dense SiC-TiB₂ composites with prescribed compositions were obtained through pressureless sintering of SiC-TiO₂-B₄C-C powder compacts. During the process, TiO₂, B₄C and C reacted to form TiB₂, followed by the consolidation of SiC matrix with the aid of excess B₄C and C. The effects of the composition of the starting powders on the final density were investigated and the mechanical properties of the composite were evaluated. The sintered body with additional HIPing at 1900 °C exhibited the average four-point flexural strength of more than 700 MPa at both 20 and 1400 °C.     

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.     

反应烧结制备Si3N4/SiC复相陶瓷及其力学性能研究

以两种不同配比Y₂O₃/Al₂O₃ (A, 2:3; B, 3:1, 总量15 wt%)为烧结助剂, 通过添加不同质量分数的SiC粉体,反应烧结制备了高强度的氮化硅/碳化硅复相陶瓷。并对材料的相组成、相对密度、显微结构和力学性能进行了分析。结果表明: 在1700℃保温2 h情况下, 烧结助剂A 与B对应的样品中α-Si₃N₄相全部转化为β-Si₃N₄; 添加5wt% SiC, 烧结助剂A对应样品的相对密度达到最大值94.8%, 且抗弯强度为521.8 MPa, 相对于不添加SiC样品的抗弯强度(338.7 MPa)提高了约54.1%。SiC能有效改善氮化硅基陶瓷力学性能, 且Si₃N₄/SiC复相陶瓷断裂以沿晶断裂方式为主。