Papers by Author: Zhi Hao Jin

Abstract: An improved chemical liquid vaporized infiltration process was developed to fast densify carbon/carbon (C/C) composites. A disc-shaped carbon felt was chosen as preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, and the precursor was heated by the third heat source separated. By this method, carbon felts (bulk density ~0.2 g/cm3) were densified to C/C composites with density of 1.29, 1.61 and 1.72 g/cm3 when prepared for 3h at 900°C, 1000°C and 1100°C, respectively. Scanning electron microscopy (SEM) reveals that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbon. The deposition process is analyzed by dividing the reactor into four regions associated with specific functions and the reasons for the rapid fabrication are proposed as the short convection and diffusion path for the precursor and the existing thermal gradients across the preform.

Abstract: The B4C/BN nanocomposites were fabricated by hot-pressing sintering of the B4C/BN nanocomposite powders at 1850oC for 1h under the pressure of 30MPa. The composite powders with the microstructure of B4C particles coated with nano-sized BN particles were prepared by the chemical reaction of H3BO3 and CO(NH2)2 on the surface of B4C particles at high temperature. The microstructure investigation of the nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed in the B4C matrix. With the increasing content of h-BN, the density of the B4C/BN nanocomposites decreased gradually; the fracture strength and fracture toughness of the B4C/BN nanocomposites decreased gradually, the strength and toughness of the B4C/BN nanocomposites with the h-BN content of 10wt% and 20wt% achieved high values. The Vickers hardness of the B4C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the machinability of the B4C/BN nanocomposites was significantly improved. The B4C/BN nanocomposites with the h-BN content more than 20wt% exhibited excellent machinability.

Abstract: AlN/BN laminated ceramic composites were fabricated using tape-casting and hot-pressing by optimizing the designs of the structure and geometry of AlN/BN laminated ceramic composites. The results showed that the fracture toughness and bending strength for AlN/BN laminated ceramics reached 9.1MPa.m1/2 and 378MPa respectively. The fracture toughness is two times higher than that of AlN monolithic ceramics. The excellent fracture toughness of AlN/BN laminated ceramics could be mainly attributed to crack deflection, delaminating, branching, parallel propagation and crack laminate pilling out at the AlN/BN weak interface.

Abstract: The machinable B4C/BN ceramics composites were fabricated by hot-pressing sintering process at 1850oC for 1h under the pressure of 30MPa. The mechanical property, thermal shock behavior and machinability of the B4C/BN ceramics composites were investigated in this article. The experimental results showed that the fracture strength and fracture toughness of the B4C/BN nanocomposites were significantly improved in comparison with the B4C/BN microcomposites. The Vickers hardness of the B4C/BN nanocomposites and B4C/BN microcomposites decreased gradually with the increasing content of h-BN, while the machinability of the B4C/BN nanocomposites and B4C/BN microcomposites were significantly improved. The B4C/BN ceramics composites with the h-BN content more than 20wt% exhibited excellent machinability. The thermal shock resistances of the B4C/BN ceramics composites were better than that of the B4C monolith, and the thermal shock resistance of the B4C/BN nanocomposites was much better than that of the B4C/BN microcomposites. The thermal shock temperature difference (-Tc) of B4C monolith was about 300oC, while the -Tc of the B4C/BN microcomposites was about 500oC, the -Tc of the B4C/BN nanocomposites was about 600oC.

Abstract: Porous Si3N4 ceramics by gel casting preparation has received considerable attention because of its excellent performance. In the paper, orthogonal experimental design L16(45) was used to investigate the preparation of porous Si3N4 ceramics by gel casting preparation. Three variables including solid loading, monomer content and the ratio of crosslinking agent to monomer were studied. Through range analysis and variance analysis, the results suggest that the ratio of crosslinking agent to monomer has significant influences on both the flexural strength and porosity of Si3N4 sintering body. For the flexural strength of Si3N4 sintering body, the order of significance levels was as follows: the ratio of crosslinking agent to monomer, solid loading and monomer content. For the porosity of Si3N4 sintering body, the order of significance levels was as follows: the ratio of crosslinking agent to monomer , monomer content and solid loading.

Abstract: High purity alumina/stainless steel joints were produced via activated molybdenummanganese (Mo-Mn) route using 72Ag-28Cu solder. Microstructures of the metallized ceramic and joint sections were observed by scanning electron microscopy. Joint strength was tested by shear-loading method. Some process factors were characterized and analyzed, which include temperature, holding time and heating and cooling rate in ceramic metallization process. The effects of Ni plating and succedent annealing were also investigated. Experimental results show that, migration of glassy phases is the main mechanism of the ceramic metallization. Glass migration direction is from metallizing layer to ceramic side. In the ranges of temperature and holding time of metallization, joint strength firstly increases and then falls with temperature raising and time extending. More fully sintered metallizing layer can be obtained while the temperature increases from 1200oC to 1500oC, and the time prolongs from 30min to 60min. Over-sintering of the metallizing layer will take place with metallizing temperature of 1600 oC and overlong holding time of 70min, which reduces the joint strength. The slower heating and cooling rate, and the annealing after Ni plating both help enhance the seal strength, due to relieving or eliminating interlayer residual thermal stress. However, too slow heating and cooling rate, such as 5 oC /min, is equivalent to overlong holding time and finally also decline the strength. A thin Ni coating helps solder wet metallizing surface, and stops solder erode metallizing layer.

Abstract: Machinable mica glass ceramics with more ZnO and B2O3 showed the phase separation by spinodal decomposition. the size of two phases formed by phase separation is in micron meter order. The nucleation and growth of crystal is performed through the diffusion of atoms. Another kind of materials with no ZnO and B2O3 addition behaves different mechanism of phase separation in terms of nucleation and growth. Many fine particles are obtained after the phase separation , the size of which is less than 100nm in diameter. Subsequently nucleation and growth in the crystallization is accomplished by aggregation and unification of the granular phase precipitation. The mechanism of the aggregation and unification nucleation-growth is different from one controlled by atom diffusion in nature. The different mechanisms in crystal nucleation and growth, caused by the composition changing.

Abstract: Carbon/carbon (C/C) composites with smooth laminar texture pyrocarbon (SL), rough laminar texture pyrocarbon (RL) and RL pyrocarbon adding particle resin carbon were prepared, respectively. The open porosities of the composites were measured by Archimedes principle, and their mechanical and thermophysical properties were tested. The friction properties of the braking disc prepared from the three types C/C composites were tested at stimulating airplane braking conditions. The friction coefficient of the SL C/C composites with is 0.26 under normal landing condition. For the RL C/C composites with open porosity of 13%, the friction coefficients are 0.35, 0.24, 0.29 under normal landing (NL), rejected take-off (RTO), damp landing (DL) conditions, respectively. The friction coefficients of C/C composites with RL pyrocarbon adding particle resin carbon (open porosity is 8%) are 0.36, 0.31, 0.36 under NL, RTO, DL conditions, respectively. The results show that the RL pyrocarbon increases the friction coefficients of the C/C. The addition of particle resin carbon in the C/C increases the braking efficiency of C/C composites under high energy landing. Low porosity C/C has better braking efficiency under DL condition.

Abstract: The oxidation behavior and electrical resistivity of reaction-bonded silicon carbide (RB-SiC) at high temperature (900 °C) had been studied in this paper. The results showed that the weight of RB-SiC would be increased when it was oxidized at 900
. The relationship between the weight-gain of RB-SiC and oxidation times followed the parabolic curve. The oxidation resistance of RB-SiC at 900
could increased by the increase of SiC particles sizes. But the electrical resistivity of RB-SiC had not affected by the oxidation at 900
. The oxidation mechanism of RB-SiC and the affecting factor on oxidation of RB-SiC were analyzed and discussed.

Abstract: Based on low-temperature hot-press sintering and rapid thermit reaction, heat-resistant metal matrix composites with nano-ceramic reinforcement were prepared via reactive hot pressing. According to XRD, the composites comprised predominantly of (fcc) Cr0.19Fe0.7Ni0.11, (fcc) Fe-Cr and alumina at 700°C through the highly-exothermic thermit reaction between the starting powders. Three-point bending strength, fracture toughness, Vickers hardness and relative density increased with the increase of hot-press sintering temperature and holding time. The improving mechanical properties may be explained by increasing of content of (fcc) Cr0.19Fe0.7Ni0.11. SEM analysis showed a microstructure consisting of equiaxial granules at 700°C for 1 h and a uniformly dispersed network of very fine grains at 700°C for 2 h. It is considered that, in the reactive hot-pressing process, Al atoms diffused into the metal matrix (Fe2O3, Cr, Ni) sites and formed Al2O3 and Fe-Cr-Ni matrix. Such a technique offers the possibility of synthesizing heat-resistant metal matrix composites with nano-ceramic reinforcement materials at considerably lower temperature.