复相增强增韧碳化硼陶瓷的研究

碳化硼是一种非常坚硬的材料,硬度可达到5000MPa(即50GPa),在1000℃可以抵抗空气的腐蚀,具有较高的抗酸性和抗碱性,可以用作磨料、切削刀具、耐磨零件喷嘴、轴承、车轴等,利用它导热性好,热膨胀系数低、能吸收热中子的特性,可以制造高温热交换器,核反应堆的控制剂。利用它耐酸碱性好的特性,可以制作化学器皿,熔融金属坩埚等。通过B4C和TiC的化学反应、Si和C的化学反应,在碳化硼基体内原位反应生成TiB2和SiC晶相,生成的TiB2晶粒粒径:0.5~1μm,SiC晶粒粒径: 3μm以下,且两种晶相均匀分布在碳化硼基体中。当加入20wt%TiC粉末和 14wt%Si粉末时,复相碳化硼陶瓷的断裂韧性KIC值达到 6.08 MPa.M1/2,维氏硬度HV值达到32.5Gpa,分别比纯碳化硼性能提高19.8%、10.9%;当加入26wt%TiC粉末和18wt%Si粉末时,则复相碳化硼陶瓷断裂韧性KIC值达到6.48 MPa.M1/2和维氏硬度HV值达到 32.63 Gpa,分别比纯碳化硼性能提高33%、11.4%。在复相陶瓷中B4C、TiB2和SiC这3种晶相热膨胀系数的不匹配产生残余应力,从而产生应力偏转和微裂纹增韧,同时裂纹沿着B4C、TiB2和SiC的晶界扩展,避免出现穿晶出现,是陶瓷增韧增强的主要机理,同时产生的游离碳和游离硅反应生成SiC晶相,能进一步延长裂纹在晶界处扩展速率。

In-situ homogeneous synthesis of carbon nanotubes on aluminum matrix and properties of their composites

Using nickel catalyst supported on aluminum powders, carbon nanotubes （CNTs） were successfully synthesized in aluminum powders by in-situ chemical vapor deposition at 650 ℃. Structural characterization revealed that the as-grown CNTs possessed higher graphitization degree and straight graphite shell. By this approach, more homogeneous dispersion of CNTs in aluminum powders was achieved compared with the traditional mechanical mixture methods. Using the in-situ synthesized CNTs/Al composite powders and powder metallurgy process, CNTs/Al bulk composites were prepared. Performance testing showed that the mechanical properties and dimensional stability of the composites were improved obviously, which was attributed to the superior dispersion of CNTs in aluminum matrix and the strong interfacial bonding between CNTs and matrix.

Effects of SiC amount on phase compositions and properties of Ti_3SiC_2-based composites

The phase compositions and properties of Ti3SiC2-based composites with SiC addition of 5%-30% in mass fraction fabricated by in-situ reaction and hot pressing sintering were studied. SiC addition effectively prevented TiC synthesis but facilitated SiC synthesis. The Ti3SiC2/Ti C-SiC composite had better oxidation resistance when SiC added quantity reached 20% but poorer oxidation resistance with SiC addition under 15% than Ti3SiC2/TiC composite at higher temperatures. There were more than half of the original SiC and a few Ti3SiC2 remaining in Ti3SiC2/Ti C-SiC with 20% SiC addition, but all constituents in Ti3Si2/TiC composite were oxidized after 12 h in air at 1500 °C. The oxidation scale thickness of TS30, 1505.78 μm, was near a half of that of T,2715 μm, at 1500 °C for 20 h. Ti3SiC2/Ti C composite had a flexural strength of 474 MPa, which was surpassed by Ti3SiC2/TiC-SiC composites when SiC added amount reached 15%. The strength reached the peak of 518 MPa at 20% SiC added amount.

Improved oxidation resistance of chemical vapor reaction SiC coating modified with silica for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a SiC coating modified with SiO2 was prepared by a complex technology. The inner SiC coating with thickness varying from 150 to 300 μm was initially coated by chemical vapor reaction (CVR): a simple and cheap technique to prepare the SiC coating via siliconizing the substrate that was exposed to the mixed vapor (Si and SiO2) at high temperatures (1 923?2 273 K). Then the as-prepared coating was processed by a dipping and drying procedure with tetraethoxysilane as source materials to form SiO2 to fill the cracks and holes. Oxidation tests show that, after oxidation in air at 1 623 K for 10 h and thermal cycling between 1 623 K and room temperature 5 times, the mass loss of the CVR coated sample is up to 18.21%, while the sample coated with modified coating is only 5.96%, exhibiting an obvious improvement of oxidation and thermal shock resistance of the coating. The mass loss of the modified sample is mainly contributed to the reaction of C/C substrate with oxygen diffusing through the penetrating cracks formed in thermal shock tests.

Phase transformation of nano-grained W(Co, C) composite powder and its phase constitute

A new process of WC-Co cemented carbide was developed by using nano-grained W（Co, C） composite powders as raw materials processed by high-energy ball milling. X-ray diffraetion（XRD）, differential thermal analysis （DTA）, thermo-gravimetrie （TG） analysis and coercive forces of the sintered samples were adopted to analyze the phase transformation and constitution, and the microstructures of sintered samples were characterized by scanning electron microscopy（SEM）. The results show that the as-milled powders are transformed into transitional phases W2C and η （Co3W3C or Co6W6C） during sintering, and finally transformed into WC and Co phases completely at 1 250℃ for 30 min, and a large number of fibrous WC grains with about 1.2μm in length and 100 nm in radial dimension are formed in the sintered body at 1 300 ℃.

Effect of nanofibers at surface of carbon fibers on microstructure of carbon/carbon composites during chemical vapor infiltration

Before densification by chemical vapor infiltration,carbon or SiC nanofibers were grown on the surface of carbon fibers by catalytic chemical vapor deposition using electroplated Ni as catalyst.The modification and mechanism of nanofibers on the pyrocarbon deposition during chemical vapor infiltration were investigated.The results show that the nanofibers improve the surface activity of the carbon fibers and become active nucleation centers during chemical vapor infiltration.They can induce the ordered deposition of pyrocarbon and adjust the interface bonding between pyrocarbon and carbon fibers during the infiltration.

Microstructure and mechanical properties of liquid phase sintered silicon carbide composites

Silicon carbide (SiC) composites were prepared by hot-press sintering from α-SiC starting powders with BaAl2Si2O8 (BAS). The effects of additives on densification, microstructure, flexural strength, and fracture behavior of the liquid phase sintered (LPS) SiC composites were investigated. The results show that the served BAS effectively promotes the densification of SiC composites. The flexural strength and fracture toughness of the SiC composites can reach a maximum value of 454 MPa and 5.1 MPa·m1/2, respectively, for 40% (w/w) BAS/SiC composites. SiC grain pullout, crack deflection, and crack bridging were main toughening mechanisms for the sintered composites.