Preparation, Microstructural Control And Properties Optimization Of The B₄C-SiC-?TiB₂? Composite Ceramics

As an important and promising structural material in engineering, boron carbide has been utilized in the field of national defense, aerospace, nuclear engineering and other spheres due to its outstanding physical and chemical properties. However, low fracture toughness and ultral-high sintering temperature severely restrict the further development of B4 C materials.In this doctoral dissertation, two practical and economical methods for the frabrication of B4 C composites explored to resolve the drawbacks of the B4 C. In addition, a comparative analysis for the two preparation technologies was executed to recognize their respective advantages and disadvantages. Theoretical and experimental bases for the industrial preparation of the high-performance B4 C ceramics were provided. 1. B4C-Si C composite ceramics fabricated through hot pressing assisted by mechanochemistry(MC-HP).Firstly, highly active B4C-Si C composite powders were synthesized by mechanochemical method with B4C-Si-graphite system as starting materials. During this process, starting reaction conditions, reaction thermodynamics and kinetics, and microstructure evolution mechanisms of the composite powders were studied. Relationships between mechanochemical process and the composite powder characteristics were obtained, thereby realizing the controllable preparation for the B4C-Si C composite powders. After that, obtained B4C-Si C composite powders were sintered by hot pressing to fabricate B4C-Si C composite ceramics. In order to obtain the B4C-Si C composite ceramics with optimum overall properties, the effects of Si C content, sintering temperature etc. on microstructure and mechanical properties of the B4C-Si C composite ceramics were studied to optimize the sintering process. Meanwhile, the densification mechanism, disorder-order transformation and sintering kinetics of the composite powders during the sintering are also studied. Then, through depth analysis on microstructure and detailed discussion on the toughening mechanism, the relationship between preparation, microstructure and mechanical properties of the B4C-Si C composite ceramics were revealed.The results show that B4C-Si C composite ceramics with high toughness can be fabricated by MC-HP in a relatively low temperature with B4C-Si-graphite system as starting powders. In addition, the following conclusions can also be obtained.(1) Highly disorder structured B4C-Si C composite powders can be synthesized successfully through MC with B4C-Si-graphite system as starting materials. The synthesized composite powders are composed of nanocrystalline B4 C and amorphous Si C, thereby owning highly sintering activity. During the milling, dilution effect of B4 C on reactants of C and Si hinders the self-propagating reaction, resulting in that the synthesis of composite powders is only controlled by the gradual diffusion reaction mechanism.(2) During the sintering, densification process of the samples is rapidly accomplished within the narrow temperature range(1700-1900 °C), resulting in that a part of nano-sized Si C are embedded into B4 C grains to form intergranular/intragranular composite structure. The intergranular/intragranular crystal structure plays an important role in reinforcing the toughness of the B4C-Si C composites.(3) Si C content and sintering temperature have obvious impacts on microstructure and mechanical properties of the B4C-Si C composite ceramics. The relative density, Vickers hardness, flexure strength and fracture toughness respectively reach 98.6±0.29%,34.3±0.5GPa?461±12MPa and 6.0±0.MPa?m1/2 for the B4C-20%Si C composites sintered at 1950°C/30 MPa. 2. B4C-Si C-Ti B2 composite ceramics fabricated by hot pressing combined with in situ reaction(IR-HP).High-performance B4C-Si C-Ti B2 composite ceramics were fabricated by IR-HP with the reaction system of B4C-Ti C-Si as raw materials. Homogeneous-dispersed additions of Ti B2 and Si C were introduced through in situ reaction during heating process, which can realize integration of powder preparation and sintering. Firstly, in order to realize the controllability of the in situ reaction, start conditions, duration time, thermodynamic and kinetic of the reaction, and the formation rule of the product powders were studied. Meanwhile microstructural evolution model and the reaction mechanism of the in situ reaction were revealed. Afterwards, during the sintering, the effects of sintering pressure, sintering temperature,(Si C-Ti B2) content, and the particle size of the raw powders on the microstructure and mechanical properties of the B4C-Si C-Ti B2 composite ceramics were studied. On this basis, through deep analysis on the microstructure and detailed discussion on the toughening mechanism, the relationship between preparation, microstructure and mechanical properties of the B4C-Si C-Ti B2 composite ceramics are revealed.The results show that B4C-Si C-Ti B2 with high toughness can be fabricated by IR-HP at a relatively low temperature with B4C-Ti C-Si system as starting powders. In addition, the following conclusions can also be obtained.(1) The reaction process of the B4C-Ti C-Si system is controlled by diffusion kinetic and is divided into two steps. Diffusion of B atoms and melt of Si are the important controlling factors for the reaction. The whole reaction is accomplished before 1400°C. Among the raw powders, the particle size of B4 C has no effects on the particle of product powders. However, the particle size of Si has a medium influence on the product powders. All the Si C particles stemming from Si are nano-sized. Especially, the particle size of Ti C has a markedly impact on the product powders because the particle size of the Ti B2 in product powders is controlled by the Ti C particles.(2) The sintering pressure, sintering temperature,(Si C-Ti B2) content, and the particle size of the raw powders have obvious impacts on the microstructure and mechanical properties of the B4C-Si C-Ti B2 composite ceramics. Under the conditions of 1950°C and 60 MPa, when the small-sized Ti C(40nm) and Si C(150nm) are adopted at the same time, the B4C-20%(Si C-Ti B2) composite ceramics can obtain minimum fracture toughness(5.11 MPa?m1/2) and maximum flexure strength(632 MPa). When the big-sized Ti C(3?m) and Si C(70?m) are adopted at the same time, the B4C-20%(Si C-Ti B2) composite ceramics can obtain maximum fracture toughness(6.38 MPa?m1/2) and minimum flexure strength(567MPa).(3) Additions in the B4C-Si C-Ti B2 composite ceramics not only include Si C and Ti B2, but also include Ti B2-Si C composite element which is constituted by interlocked Si C and Ti B2. The formation of the interlocked Ti B2-Si C composite element is affected by the particle size of raw powders Ti C and Si. The interlocked Ti B2-Si C composite element is the unique structure which only belongs to the B4C-Si C-Ti B2 composite ceramics fabricated by the IR-HP. This unique structure plays an important role in reinforcing the toughness of the B4C-Si C-Ti B2 composites. 3. Comparison of the two kinds of composite ceramics fabricated by two preparation technologies.Merit and demerit of the two kinds of composite ceramics and two preparation technologies were analyzed and compared according to the materials cost, experimental conditions, operation difficulty of the technologies and the properties of the products. Theoretical and experimental bases for the industrial preparation of the high-performance B4 C ceramics were provided.The results show that the MC stage for preparing B4C-Si C composite powders in MC-HP technology requires complex technological process, harsh environmental conditions and expensive equipment. But HP stage in MC-HP technology is simple. For IR-HP technology, the powder preparation and sintering are grouped in the integrated. Therefore, the sintering stage requires complex and accurate control. In addition, higher pressure of 60 MPa must be executed to obtain dense samples.In the same hot pressing conditions(1950°C, 30MPa), mechanical properties of the B4C-Si C composite ceramics fabricated by MC-HP are superior to that of the B4C-Si C-Ti B2 composite ceramics fabricated by IR-HP. However, if the relative densities of the two kinds of composite ceramics are similar, the mechanical properties of the B4C-Si C-Ti B2 composite ceramics fabricated by IR-HP have an advantage over that of the B4C-Si C composite ceramics fabricated by MC-HP. In addition, the workability of the B4C-Si C-Ti B2 composite ceramics is superior to that of B4C-Si C composite ceramics.