Various toughening mechanisms, summarizing the nanocomposite and ceramic grain boundary stress design to solve the problem of ceramic brittleness Alumina ceramics have a series of excellent properties such as high temperature resistance, high wear resistance, corrosion resistance and oxidation resistance, and can withstand metallic materials and polymers The harsh working environment in which the material is incompetent is one of the most widely used ceramic materials currently produced. However, the brittleness of ceramic materials greatly limits its application. Therefore, the toughening of ceramic materials is always a core issue in ceramic research. The main toughening methods of ceramics include phase change toughening, fiber (whiskers) toughening, and granules. Toughening and compound toughening.
Ceramic-based metal composites are multiphase composites consisting of one or more ceramic phases and metals or alloys. The ceramic component in the composite material gives the composite material the required hardness, thermal strength and wear resistance; the metal phase gives the product the necessary toughness. The properties of the ceramic-based metal composite material depend on the properties of the ceramic and the volume of the metal. Percentage of the combination of the two properties and the interface strength of the bond.
1 Toughening of metal particles Alumina ceramics A suitable amount of CrAlNiTiCuMo was added to ceramics to study the dispersion and toughening of alumina with metallic chromium powder. The relationship between the toughening results and the interfacial bonding strength was investigated. The fracture toughness value of the material was found to be related to Cr. Increase in volume fraction and increase. When the 15% (Cr particle 500 ° C oxidation 3h), the fracture toughness value of the material is 8. 38MPa.m1/2, is 2.4 times the fracture toughness value of AI2O3 matrix material so Cr toughening Al23 material is Cr main way Particles bridge the cracks of the matrix and plastically deform as the crack expands, thereby consuming energy at the crack tip. The bonding strength of the two phases plays a decisive role in the way of fracture and fracture of the bridged Cr grains. The interfacial bonding strength of Cr and Al23 matrix depends on the solid solution of Cl23 and Al23 on the surface of Ci particles. The higher the content of Cr23, the greater the interfacial bonding strength. Table 1 lists several metal particles added to Al2 3 ceramics to improve the mechanical properties of the ceramic Table 1 Alumina metal particles Mechanical properties of composite ceramics Addition of metal% relative density /% bending strength fracture toughness (KIc / K0) Toughening of intermetallic compounds Alumina metal particles Toughening and reinforcing ceramics also have some drawbacks. With the increase of metal content, the mechanical properties of ceramic materials are strengthened. However, due to the low melting point of metals and poor wetting of ceramics, it can be used as The amount of metal added to the ceramic matrix by the reinforcing agent is limited; and due to the poor oxidation resistance and corrosion resistance of the metal, the addition of the metal tends to reduce the chemical stability of the ceramic material.
In recent years, intermetallic compounds have been used as a toughening agent in ceramic materials due to their excellent properties. 17 Since the intermetallic compounds have the property of coexistence of metallic bonds and covalent bonds, the use temperature can be intermediate between the metallic superhard alloy and ceramics. Between (1 1400°C), the brittleness is higher than that of metallic materials, but it is lower than that of ceramic materials. The characteristics of using intermetallic compounds between metals and ceramics are designed to design an intermetallic compound when a ceramic intermetallic compound is compounded. The respective disadvantages of the ceramic materials can be compensated by the advantages of each other.
ChouWB et al.: When the NiAl particle content reaches 20%, the NiAl particles are connected together. At this time, the resistance suddenly drops. The strength of the Afcft composite material with NiAl is higher than that calculated using the mixing rule, and the strength of the composite material increases. It is the synergistic strengthening effect of thermal stress strengthening caused by matrix strengthening, reinforcement enhanced interface strengthening and thermal expansion mismatch, and the prediction is only predicted by matrix and reinforcement. In order to enhance the combination of metal and ceramic, it was found that adding 0.25% Fe in NiAl can increase the bonding strength between Al23 and NiAl, thereby increasing the fracture toughness of the Ab3-based composite ceramic.
3 etc. Metal FeAl particles were added to Afc3 to make a composite material containing Fe and Fe-Al compounds. Table 2 gives the raw material composition and the bending strength of the resulting composite material when the added Al content is high (C5), It is evident from the XRD experiments that FeAl is produced. The results show that when the amount of Al/Fe is less than a critical value, an Fe-containing Ab3 composite material is formed. When the Al/Fe is greater than a certain critical value, Fe-Al is formed. Compound Al23 matrix composites Table 2 Raw material composition and flexural strength of composite materials No. of raw material composition) Intermetallic compounds were made into composites by pressure infiltration into Al23, due to the former having good wetting of Al23, Therefore, the fracture toughness of this composite material in 3 alumina-based nanocomposite ceramics has been proven by many studies. Adding nano-metal particles to the ceramic can significantly improve its toughness. The mechanism of toughening a new nanoparticle in a new raw material boils down to: (1) Refinement. Inhibits grain growth and mitigates abnormal grain growth; (2) Residual modulus E and coefficient of thermal expansion a improve strength, toughness, etc.; (4) Intragranular nanoparticles form a secondary interface within the matrix particles, and The particle size and position of the doping-doping particles (intergranular and intragranular) that have pinning dislocations similar to those of the grain boundary nanophase have a large influence on the mechanical properties of the nanocomposites. In the material research, it was found that about 70% of the doped Ni was distributed at the grain boundary position. The addition of Ni prevented the growth of Al23 grains. The grain size of the Ni particle at the grain boundary was 140 nm, which was far greater than that of the intragranular Ni particles (70 nm). ). The strength and toughness of Al2O3-based nanocomposite ceramics are 1.35 and 1.2 times that of Al2O3 matrix, respectively. It is considered that the performance improvement is due to the grain refinement of the matrix, Tohru Sekino et al. used two different processes to prepare Afcft/Ni nanocomposite ceramics: One method is to mix pure Al23 and 50 mesh NiO with a ball mill (A process). Another method is to add Ni(N3)2°6B0 to Afcft for post-reaction. The brewing process will reduce the powder for sintering txnAi.net Tsinghua University of Science and Technology. The ceramic substrate has a crystal grain size of about 卟m. The average grain size of the B-process sample is 0.64 pm, while the grain size of the At3 ceramic substrate is 1.2 Mm. The smaller grain size in the composite material produced by the B process is due to the limitation of the uniformly dispersed Ni particles. The relationship between fracture strength and Ni content is also studied in this paper. The strength of the process can reach 1090 MPa when 9 (Ni) = 5%, and the strength decreases when 9 (Ni)> 10%. The addition of Ni does not improve the toughness of the ceramic. When 9(Ni)20% cKic increases with increasing Ni content. Previous reports on the Al23/Ni system mostly believed that the Ni plasticity improved the toughness of the Ab3, but Ni did not appear in the front of the crack. The authors of Ductile Fracture believe that stress-induced micro-cracks are a possible mechanism by which particles can diffuse and toughen ceramics.
TEM observations revealed that the bonding between Al23 and large Ni particles was not very strong, because cracks always pass through the interface of Ab3/Ni, so this weak interface cannot transfer stress to Ni particles. In order to obtain greater toughness, it must be Improving the Interface Structure of Ceramics and Metals Xudong Sun Table 3 Mechanical Properties of Several AI2O3 Metal Nanocomposites "Commodity 26 Composition Metal Source Strength Intensity Toughness Young's Modulus / GPa W, Powder Hydrogen Reduction, Hot Press Sintering 4 Ceramic Grain Boundary Stress Design The toughening stress design of ceramics is to design the grain boundary stress of ceramics, so that there is the ability to transfer, consume and absorb external energy in the ceramic in order to achieve the purpose of strengthening and toughening the ceramic material. The coefficient of expansion or the grain boundary of the different elastic moduli or the second phase particles, in the ceramic firing process, due to the mismatch in the thermal expansion coefficient or elastic modulus, there must be various stresses in the grain boundary phase.When the ceramic material is in When subjected to external stresses, these grain boundary stresses are encountered in the process of main crack propagation, and various interactions will occur, possibly resulting in The following conditions: (1) When the main crack reaches the grain boundary compressive stress with pressure, partial or total cancellation of the tensile stress of the main crack will cause the stress concentration at the tip of the main crack to slow down, or even stop the crack propagation; (2) When the main crack When the grain boundary stress with shear stress is reached, micro cracks may be formed along the grain boundary, so that the stress concentration at the main crack tip is dispersed, and the crack is thus deflected. Therefore, the effect of the grain boundary stress and the applied stress will result. The absorption and transfer of energy can be expected to achieve a physical mismatch in the design of the stress, artificially in the ceramic material, and to introduce appropriate stresses between the phase boundaries, which contributes to the strengthening and toughening of the ceramic material. One of its essential and even fatal weaknesses is that due to the mismatch between the elastic moduli and the thermal expansion coefficient of the phases, it causes thermal stress during the heating and cooling process. Such stresses seriously affect the performance of the composite material. , and even determine its fate. Therefore, the correct analysis and evaluation of the stress of multiple-phase materials has a rational use for the preparation and use of such materials. The effect of the thermal stress of the composite material is mainly based on the embedding of the reinforcing phase (particles, etc.) in the hollow matrix model, but it is only applicable to the case where the reinforcement volume fraction is not too large, and the reinforcing phase particles account for the multiple phase material. In the case of larger volume fractions, a double-layered nesting model is used, which embeds the reinforcing particle spheres and the matrix hollow spheres in the cavity of an equivalent composite medium to form a “double nested model”.
It has been recognized that the residual stress in multiphase ceramics has an important influence on its mechanical properties, and its toughness and strength are closely related to the number and distribution of microcracks caused by residual stress in porcelain. Therefore, the purpose of strengthening and toughening the two-phase particle complex ceramic material can be analyzed by finite element method. The results of the experiment are shown in Table a. The distribution of stresses within the ceramic model is shown in Fig. 23-NiAlCMMCs.KeyEngineering 28 Zheng Maosheng, Jin Zhihao, Wang Hongqi. Two-layer nested model for thermal stress analysis of particle reinforced composites. Chinese Science, 1994,24(12):13,134029 Yang Xuegui, Liu Feng, Hou Yaoyong. Finite Element Analysis and Internal Toughening Mechanism of Internal Stress in Composite Ceramics with Two Phase Particles
