Zheng Yi Fu

Composites fabricated by self-propagating high-temperature synthesis

Abstract Self-propagating high-temperature synthesis (SHS) is a promising method for the fabrication of composites. However, the products obtained directly from the SHS process are generally porous, which limits the application of SHS. SHS plus dynamic mechanical pressing, which combines synthesis and densification in one step was proposed and was named SHS/QP. In the process, immediately after the SHS reaction, a quick pressing force acts on the “hot and soft” sample for densification. In this paper, the fabrication of cermets by SHS/QP is reported. The optimum conditions of SHS/QP for cermets were studied, and the densification mechanism for cermets was analyzed.

Spark plasma sintering of aluminium nitride transparent ceramics

Abstract Aluminium nitride (AlN) transparent ceramic was fabricated by a spark plasma sintering (SPS) technique. Samples were processed at 1800°C, 25 MPa pressure, and 6 Pa vacuum without sintering additives, and at different heating rates and sintering durations ranging from 4 to 20 min. X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) investigation indicated that the ceramics prepared by SPS had good transparency, because of its high purity, fine particle size, and uniform microstructure. Dislocations and pores in crystals were observed by TEM, which adversely affect the transmittance of AlN transparent ceramics. Experiments showed that the SPS technique is an effective method for the fabrication of transparent ceramics.

Numerical Simulation of the Temperature Field in Sintering of BN by SPS

The temperature field during the whole process of SPS sintering TiB2-BN sample was numerically simulated based on the simplification of the temperature rising process. The result shows that the highest temperature is found in the punch and the heat flows from punch to sample and die in SPS sintering system. In the radial direction, the center temperature of the sample is much higher than the circumference temperature of the sample. In the axial direction, the center temperature of the sample is lower than the border temperature of the sample. The temperature difference in the sample is growing bigger in the sintering process and reaches the maximum at the end of the heating-up process.

Characterization of Nanocrystalline CoCrFeNiCuAl High-Entropy Alloy Powder Processed by Mechanical Alloying

The equiatomic multicomponent CoCrFeNiCuAl high-entropy alloy powder was synthesized by mechanical alloying. The effects of milling time and heat treatment on the structure and morphology of the ball milled alloy were investigated. Single BCC solid solution structure appears when the alloy is ball milled more than 30h. The 60h ball milled alloy powder shows a mean particle size of 3 μm, which is actually hard agglomerations of nanosized crystals with crystalline size less than 10nm. The 60h ball milled alloy exhibits good chemical homogeneity. The single BCC solid solution structure transforms to a BCC and a FCC phases when annealled at 600°C for 1h, which can be attributed to the supersaturatable solid solution formation during the mechanical alloying process.

Fabrication of translucent boron nitride dispersed polycrystalline silicon nitride ceramics

Optical transparency was achieved at infrared region and overall translucent silicon nitride was fabricated using hot press sintering (HPS). The increase in h-BN content decreased the optical transparency. Microstructral observations shows that the optical, mechanical and tribological properties of BN dispersed polycrystalline Si3N4 ceramics were affected by the density, α:β-phase ratio and content of h-BN in sintered ceramics. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN at 1850°C. The composite contained from 0.25 to 2 mass % BN powder with sintering aids (9% AlN + 3% MgO). Maximum transmittance of 57% was achieved for 0.25 mass % BN doped Si3N4 ceramics. Fracture toughness was increased and wear volume and friction coefficient were decreased with increase in BN content.

Annealing effects on structure and mechanical properties of CoCrFeNiTiAlx high-entropy alloys

Novel CoCrFeNiTiAlx(x:molar ratio, other elements are equimolar) high-entropy alloys were prepared by vacuum arc melting and these alloys were subsequently annealed at 1000 ?C for 2 h. The annealing effects on structure and mechanical properties were investigated. Compared with the as-cast alloys, there are many complex intermetallic phases precipitated from the solid solution matrix in the as-annealed alloys with Al content lower than Al1.0. Only simple BCC solid solution structure appears in the as-annealed Al1.5 and Al2.0 alloys. This kind of alloys exhibit high resistance to anneal softening. Most as-annealed alloys possess even higher Visker hardness than the as-cast ones. The as-annealed Al0.5 alloys shows the highest compressive strength while the Al0 alloy exhibits the best ductility, which is about 2.6 GPa and 13%, respectively. The CoCrFeNiTiAlx high-entropy alloys possess integrated high temperature mechanical property as well.

Effect of solid solution formation on densification of spark plasma sintered ZrC ceramics with TiC as sintering aid

The densification of ZrC ceramics doped with different contents of TiC prepared by spark plasma sintering at the temperatures between 1750 and 1850°C has been investigated. The microstructure and mechanical properties of the ceramics have been characterised. It was shown that TiC additions effectively promoted the densification process by forming (Zr,Ti)C solid solution. The relative densities and mechanical properties of ZrC samples increased with the increasing of TiC content or the sintering temperature. Ceramic with the content of TiC up to 10 vol.-% sintering at 1850°C showed an excellent combination of properties including a relative density of 98.7%, hardness of 20.8 GPa and flexural strength of 605 MPa.

Preparation of ZrC Powder by Self-Propagating High-Temperature Synthesis

ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) using exothermic reaction of ZrO2-C-Mg system. By theoretical calculating, the adiabatic temperature (Tad) for the system is about 2235K enough to react as SHS process. The Tad observed during experiment is 1850K. The results show that high pure ZrC powder is obtained with appropriate Mg contents. The scanning electron micrograph shows that the average size of ZrC particles is about 2μm.

Reinforcement of B4C Ceramics with Multi-Walled Carbon Nanotubes

Here we have prepared B4C/CNTs composites using the spark-plasma sintering (SPS) method. Mechanical property measurements reveal obvious enhancement confirming the fabrication of true B4C/CNTs composite materials with improved toughness properties.The addition of 1wt% CNTs in the B4C increased the fracture toughness by about 1.6 times from 2.5 to 4 MPa.m1/2 because the CNTs presented at the B4C grain boundaries, made the length of cracks shorten.

Preparation of Al2O3-ZrO2-Ni Nano-Composites by Spark Plasma Sintering

In the present study, a process to prepare Al2O3-ZrO2-Ni nano-composite is explored. The nano-sized nickel particles, around 1-10 nm, can be successfully prepared by using a solution and coating technique. These nano-sized Ni particles disperse uniformly onto the surface of micro-sized Al2O3 and ZrO2 particles after the coating treatment. The powder mixtures were then consolidated by employing a spark plasma sintering (SPS) technique at 1350C for 1 to 5 minutes. Dense composite is resulted after SPS, though a slight density variation within the specimen is also noted.