Effect of HfB2 on microstructure and mechanical properties of ZrB2–SiC-based composites

In this work, ZrB₂–SiC-based composites with different additives were sintered in different temperature (1600, 1700, 1800 and 1900), time (4, 8, 12 and 16) and pressure (10, 20, 30 and 40 MPa). The effect of HfB₂ (0, 5, 10, 15 vol.%) on ZrB₂-based ceramics was investigated. Microstructure evaluation and phases were done by SEM and EDAX. Effect of SPS conditions (temperature, time and pressure) on microstructure was investigated. The results showed that Hf and Zr diffuse in each other and (Zr, Hf)B₂ solid solution will be formed. In addition, it was concluded that by increasing temperature and time, solid solution formation increases. Pressure has little effect on diffusion. Also, MoSi₂ and ZrB₂ form (Zr, Mo)B₂ solid solution. Finally it is cleared that HfB₂ increases open porosity percent slightly. Finally, it was observed that HfB₂ has negative effect on densification while improves flexural strength due to (Zr, Hf)B₂ solid solution.     

Spark plasma sintering of ZrB2-based composites co-reinforced with SiC whiskers and pulverized carbon fibers

Spark plasma sintering (SPS) process was used to preparation of ZrB₂-based composites co-reinforced with SiC whiskers as well as various amounts of pulverized carbon fibers. The effects of C_F content on microstructure and mechanical characteristics of ZrB₂–SiC_w–C_F composites were scrutinized. Although all composites approached high densification, a fully-dense sample was fabricated by the addition of 2.5 wt% C_F. The growth of ZrB₂ grains was remarkably inhibited in C_F-reinforced composites. No in-situ formed phase was detected by XRD; however, trace of nano-ZrC clusters was observed in SEM fractographs of ZrB₂–SiC_w–C_F samples. The formation of such nano-sized ZrC refractory phase was also proved by thermodynamics. The hardness of composites slightly decreased from 21.9 to 19 GPa with increasing the C_F content. Reversely, the fracture toughness values enhanced from 4.7 to 6 MPa.m^½ with increasing the amount of pulverized carbon fibers.     

原位合成ZrB2增强铜基复合材料的组织及性能与制备温度的关系

以Cu-5B中间合金和海绵锆为原料,通过熔铸法在Cu熔体中原位合成了ZrB2颗粒增强Cu基复合材料,并利用光学显微镜、X射线衍射仪、扫描电镜(SEM)和能谱仪等研究了不同制备温度下Cu-ZrB2复合材料的组织及性能.结果表明:随着制备温度的升高,复合材料中的ZrB2颗粒趋向集聚,生成量减少,且ZrB2聚集团的平均尺寸快...     

Anti-oxidation properties of ZrB2 modified silicon-based multilayer coating for carbon/carbon composites at high temperatures

To improve the anti-oxidation ability of silicon-based coating for carbon/carbon (C/C) composites at high temperatures, a ZrB₂ modified silicon-based multilayer oxidation protective coating was prepared by pack cementation. The phase composition, microstructure and oxidation resistance at 1773, 1873 and 1953 K in air were investigated. The prepared coating exhibits dense structure and good oxidation protective ability. Due to the formation of stable ZrSiO₄–SiO₂ compound, the coating can effectively protect C/C composites from oxidation at 1773 K for more than 550 h. The anti-oxidation performance decreases with the increase of oxidation temperature. The mass loss of coated sample is 2.44% after oxidation at 1953 K for 50 h, which is attributed to the decomposition of ZrSiO₄ and the volatilization of SiO₂ protection layer.     

Mechanical properties and microstructure of TiB2 ceramic influenced by ZrB2 additive

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The effect of (ZrB2-SiC) addition on microstructure and mechanical properties of NbMo-matrix composites fabricated by hot-pressing

The effects of (0–60%) vol% (70 vol% ZrB₂ + 30 vol% SiC) additions on microstructure and properties of NbMo substrate fabricated by hot-pressing were studied at room temperature. Types of formed phase were decided by the amount of ZrB₂ and SiC additives. The effective eutectic phase was observed in 15 vol% (70 vol% ZrB₂ + 30 vol% SiC)-NbMo, which was attributed to the addition of SiC. 30 vol% (70 vol% ZrB₂ + 30 vol% SiC)-NbMo had the highest relative density of 98.69%. Compared with x ZrB₂-NbMo composites, the addition of SiC could further improve the hardness of NbMo_ss in x (70% ZrB₂ + 30% SiC)-NbMo when the value of x was same, and NbMo_ss in 60 vol% (70 vol% ZrB₂ + 30 vol% SiC)-NbMo had the highest hardness of 6.82 GPa. Only the 15 vol% (70 vol% ZrB₂ + 30 vol% SiC) addition could improve the compressive strength of NbMo matrix. The reasons for the low strength of 30, 45, 60 vol% (70% ZrB₂ + 30% SiC)-NbMo were the lack of ductile phase and the large amount of hard phase production.     

Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles

The NbMo-matrix composites reinforced with (0–60 vol%) ZrB₂ were fabricated by hot-pressing at 2400 °C for 10 min under a pressure of 50 MPa in dynamic vacuum in the induction heating furnace specially designed in our institute. The optimum ZrB₂ content in NbMo solid solution was determined to be 30 vol% for excellent comprehensive mechanical property. NbMo-30 vol% ZrB₂ has the highest density of 99.63%, the most uniform microstructure, high fracture toughness of 5.75 MPa m^1/2. The highest ZrB₂ concentration that reacts with NbMo solid solution is at the range of 30 to 45 vol%. The types of the formed niobium borides were decided by the original ratio of Nb to B. The distribution of Mo and Zr was mutually exclusive in low ZrB₂ content composites, however, there was Mo₂Zr in high ZrB₂ content composite. Except for NbMo-45 vol% ZrB₂, the compressive strength increased with ZrB₂ content (from 927.09 MPa to 1635.91 MPa). The Young's modulus values were directly proportional to ZrB₂ content. The fracture toughness (from 6.34 MPa m^1/2 to 3.99 MPa m^1/2) was inversely proportional to ZrB₂ content. The big residual ZrB₂ particles in high ZrB₂ content samples such as NbMo-45 vol% ZrB₂ and NbMo-60 vol% ZrB₂ was the main reason for nonhomogeneous microstructure, low density (94.09% and 94.83%, respectively) and low fracture toughness (4.58 MPa m^1/2 and 3.99 MPa m^1/2, respectively).     

Effect of ZrB2 and SiC addition on TiB2-based ceramic composites prepared by spark plasma sintering

TiB₂-based ceramic composites with different amounts of ZrB₂ and SiC were prepared by spark plasma sintering at 1700 °C with an initial pressure of 40 MPa and a holding time of 10 min. The (Ti_xZr_y)B₂ solid solution was found in the sintered TiB₂/ZrB₂/SiC composites by XRD. The microstructural and mechanical properties of the prepared samples were investigated. The composite with the addition of 30 vol.% ZrB₂ shows better comprehensive performances, and the bending strength and the fracture toughness of the composite are 780.5 MPa and 7.34 MPa m^1/2, respectively. The generation of the (Ti_xZr_y)B₂ solid solution makes the microstructures of the composites finer and more homogeneous, which has played a very important role in grain refinement and interface fusion.     

Hydrogen generation via hydrolysis of nanocrystalline MgH2 and MgH2-based composites

Nanocrystalline MgH2 and MgH2-based composites with 25% （mass fraction） of Al, Ca, or CaH2 as an individual additive respectively were prepared by ball milling. The crystallite size and morphology of the as-milled powders were characterized and their hydrolysis behaviours were investigated in comparison with commercial polycrystalline MgH2. The results show that the crystallite size of both MgH2 and MgH2-based composites is reduced to less than 13 nm after milling for 15 h. Due to its enhanced specific surface area and unique nanocrystalline structure, the as-milled MgH2 shows much better hydrolysis kinetics than the commercial polycrystalline MgH2, with the hydrolysed fraction upon hydrolysing for 70 min enhances from 7.5% to about 25%. As compared with the as-milled MgH2, the MgH2-based composites with either Call2 or Ca as an additive present further greatly improved hydrolysis kinetics, with the hydrolysed fraction for 80 min achieving about 76% and 62% respectively. However, the addition of Al doesn＇t show any positive effect on the improvement of the hydrolysis kinetics of Mg H2.     

Effect of short carbon fiber addition on pressureless densification and mechanical properties of ZrB2–SiC–Csf nanocomposite

In the present paper, ZrB₂–SiC–C_sf composites were produced by pressureless sintering method. Carbon fiber and SiC nanoparticles with different weight percentages were added to the milled ZrB₂ powder. The mixed powders were formed by hot pressing and cold isostatic press (CIP) and after the pyrolysis, were sintered at 2100 °C and 2150 °C. In order to compare the microstructure and mechanical properties of samples scanning electron microscopy (SEM) equipped with EDS spectroscopy, XRD analysis, hardness and toughness tests were used. The results show that with the increase in weight percentage of carbon fiber, the porosity increases but the hardness, fracture toughness and density decrease. On the other hand, with the increase in weight percentage of SiC nano-particles, the porosity decreases and fracture toughness, hardness and density increase. The results indicate that in an optimal percentage of both additives, the hardness and toughness increase. Additionally, with the increase in sintering temperature, the values of hardness and fracture toughness increase and porosity decreases.     

微波烧结工艺对Ti-Mg复合材料组织和性能的影响

为了确定制备Ti-Mg复合材料的最佳微波烧结工艺,采用微波烧结制备了Ti-15Mg复合材料。采用扫描电镜、差热分析、X射线衍射、光学显微镜、压缩试验以及耐腐蚀性测试等系统性地研究了烧结温度、保温时间对复合材料微观组织、力学性能和耐腐蚀性能的影响。结果表明,烧结温度为540~600 ℃,随着烧结温度的升高,复合材料的致密化程度提高,孔隙率降低,抗压强度增强,耐腐蚀性增强;烧结温度为600 ℃时,镁均匀地分布在钛基体中,复合材料的性能最佳,满足作为医用材料的性能要求;烧结温度继续升高则会导致复合材料中镁的大量挥发,孔隙率增加,复合材料的强度下降。微波烧结制备Ti-15Mg复合材料具有快速、稳定烧结的特点,因此保温时间对复合材料性能的影响不明显。     

熔渗法制备CrW/Cu复合材料的组织与性能研究

采用熔渗法制备了CrW/Cu复合材料,即首先把铬粉和钨粉的混合粉末烧结成骨架,然后把铜液渗入CrW骨架的孔隙中制备CrW/Cu复合材料;研究了气氛、熔渗温度、溶渗时间对CrW/Cu复合材料的显微组织和性能的影响.研究结果表明:真空环境下溶渗法制备的CrW/Cu复合材料组织最致密;当熔渗温度大于1 350℃时,尽管没有达到铬的熔点温度,但W-Cr骨架中的Cr颗粒开始逐步消失,随着熔渗温度的提高或熔渗时间的延长,Cu取代其位置;Cr通过Cu合金液迁移到W骨架的孔隙中,与W形成Cr-W固溶体;材料组织的变化导致材料的硬度明显升高,导电率明显降低.     

Wear resistance of ZrB2 based ceramic composites

The wear resistance and tribological characteristics of spark plasma sintered ZrB₂ + B₄C, ZrB₂ + SiC and ZrB₂ + ZrC composites were investigated under dry sliding conditions at applied loads of 5 N and 50 N in air. The microstructure, deformation and damage characteristics were studied using scanning electron microscopy, confocal electron microscopy, a focused ion beam and atomic force microscopy. The friction coefficient values were very similar for all composites with values ranging from 0.63 to 0.72 and with the lowest value recorded for the ZrB₂ + SiC composite at a 5 N applied load. The ZrB₂ + ZrC composite was the most wear resistant, with wear rates at a 5 N load of 6.15 × 10⁻⁶ mm³/(N·m) and at a 50 N of 7.3 × 10⁻⁶ mm³/(N·m). None, or a very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB₂ + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all composites, with similar chemical compositions but different sizes and thicknesses at the 50 N load.     

Microstructure dependent physical and mechanical properties of spark plasma sintered ZrB2-MoSi2–SiCw composites

A comparative evaluation has been carried out off the physical and mechanical properties of ZrB₂–20 vol% MoSi₂ composites reinforced with 5, 10 and 20 vol% SiC whiskers (SiC_w). The abovementioned hybrid composites have been densified by spark plasma sintering (SPS) at 1600 °C for 10 min. The present study focuses on the effect of the quantity of SiC_w on the densification, electrical conductivity, hardness, and fracture toughness ZrB₂–MoSi₂–SiC_w composites in detail. Microstructural characterization suggests that SiC_w restricts the grain growth of ZrB₂ during densification. The electrical conductivities of the ZrB₂–MoSi₂–SiC_w composites have been found to vary in the range ~2.4–3.5 × 10⁶ S/m, and it reduce with increasing SiC_w content. Results exhibit that the indentation fracture toughness of ZrB₂–20 vol% MoSi₂ improves ~70%, ~112% and ~130% with an addition of 5, 10 and 20 vol% SiC_w, respectively. An increase in SiC_w content leads to more amount of crack deflection and crack arrest that successively improves the fracture toughness of the composites.     

树脂对碳毡硅化处理后的显微组织与性能影响

研究了树脂对碳毡经处理后的显微组织与性能影响。结果表明,未浸渍树脂的碳毡硅化处理后的显微组织特点是反应生成的碳化硅颗细注且均匀分布在游离硅中;而浸渍树脂的碳毡硅化处理后的显微组织特点是反应生成的碳化硅颗粒粒径及分布均不均匀。大颗粒碳化硅的出现造成了试样断裂强度的降低,ＸＲＤ结果表明Ｓｉ－ＳｉＣ复相陶瓷的主晶相为α－ＳｉＣ,β－ＳｉＣ和游离Ｓｉ,不同晶型ＳｉＣ的出现与试样烧结过程中Ｓｉ／Ｃ反应的放热     

Oxidation and ablation resistance of ZrB2–SiC–Si/B-modified SiC coating for carbon/carbon composites

ZrB₂–SiC–Si/B-modified SiC coating was prepared on the surface of carbon/carbon (C/C) composites by two-step pack cementation. The coating could efficiently provide protection for C/C composites from oxidation and ablation. The improvement of oxidation resistance was attributed to the self-sealing property of the multilayer coating. A dense glassy oxide layer could afford the high temperature up to 2573 K and efficiently protect C/C composites from ablation.     

Effects of ply-orientation on microstructure and properties of super-aligned carbon nanotube reinforced copper laminar composites

The super-aligned carbon nanotube (SACNT) films reinforced copper (Cu) laminar composites with different orientations of CNT ply were fabricated by electrodeposition. The results show that the tensile strength and yield strength of cross-ply composite with 5.0% (volume fraction) of SACNT reach maximum of 336.3 MPa and 246.0 MPa respectively, increased by 74.0% and 124.5% compared with pure Cu prepared with the same method. Moreover, the electrical conductivities of all the prepared composites are over 75% IACS. The result of TEM analysis shows that the size of Cu grain and the thickness of twin lamellae can be reduced by adding SACNT, and the refining effect in cross-ply composites is more significant than that in unidirectional ply composites. The enhanced strength of the Cu/SACNT composites comes from not only the reinforcing effect of SACNT films but also the additional strengthening of the Cu grain refinement caused by CNT orientation.