De Gui Zhu

Oxidation Behaviors of TiB2-TiCX and TiB2-TiCX/15SiC Ceramics

Fully dense samples of TiB2-TiCX and TiB2-TiCX/15SiC ceramic composites were fabricated by in-situ synthesis under hot isostatic pressing from TiH2, B4C and SiC powders. Their oxidized behaviors at different temperatures were tested. Optical micrograph studies and thermo-gravimetric analyses show that the highest effective temperature of oxidation resistance is 700°C for TiB2-TiCX, and 1100°C for TiB2-TiCX/15SiC. The weight gain of TiB2-TiCX/15SiC below 1100°C is quite low, and it rises up suddenly when the temperature reaches 1200°C. Thus, the highest effective temperature of oxidation resistance is 1100°C for TiB2-TiCX/15SiC. The oxidation dynamic curves of TiB2-TiCX/15SiC ceramics accord with the parabola’s law. The activation energy of TiB2-TiCx/15SiC (189.87kJ.mol-1) is higher than that of TiB2-TiCx (96.44kJ.mol-1). In the oxidation process of TiB2-TiCx/15SiC, TiB2 reacts with oxygen and generates TiO2 and B2O3 at first. A layer of whole homogeneous oxide film cannot be formed, in the mean time, the oxidation of TiC begins. When temperature goes up to 1000°C, TiC phase is totally oxidized. SiC is oxidized to SiO2 at about 900°C, Meanwhile, TiO2 forms denser film than B2O3, which grows and covers the surface of the material, and gives better property of oxidation resistance.

In Situ Synthesis of TiB2-TiC0.8-40vol%SiC by Hot Pressing

TiB2-TiC0.8-40vol%SiC multiphase ceramics were prepared by in-situ hotpressing sintering. The phase composition and microstructures of the materials were characterized by optical mic- oscope, X-ray diffraction and scanning electron microscopy. The effects of sintering temperature on the phases, microstructures and mechanical properties of the ceramics were investigated. The results show that density, bending strength and fracture toughness of the ceramics are increased with the elevation of sintering temperature (1800-1950°C). High densified TiB2-TiC0.8-40vol%SiC multipha- se ceramics and optimized microstructure is obtained by sintering at 1900°C, in which the uniform distribution of lath-shape TiB2 and bulk TiC0.8 grains can be observed obviously. Nano-SiC particles distributed dispersively in the TiB2 and TiC0.8 grains and at boundaries. The Vickers hardness, fract- ure toughness, flexural strength and electrical conductivity of the TiB2-TiC0.8-40vol%SiC multipha- se ceramics sintered at 1900°C are 24.055GPa, 8.27±1.0MPa∙m1/2, 516.69MPa and 2.2×106S∙m-1, respectively. However, up to 1950°C, TiB2 and TiC0.8 grains gradually grew up, the bending stren- gth of multiphase ceramics was decreased greatly. In addition, TiB2, TiC0.8 and SiC particles were incorporated together to improve the particulate strength and toughness of composite material by the synergistic mechanism effects among the crystal phases in the multiphase ceramics, such as crack deflection, grain’s pull-out and fine-grain toughening.

Microstructure and Properties of In Situ Fabricated Al-5wt.%Si-Al2O3 Composites

Alumina reinforced aluminum matrix composites (Al-5wt.%Si-Al2O3) fabricated by powder metallurgy through hot isotactic pressing were sintered in different processes, i.e. solid and liquid phase sintering. Optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), Energy Dispersive X-ray (EDX) techniques were used to characterize the sintered composites. The effects of solid phase and liquid phase sintering on density, microstructure, microhardness, compression and shear strength were investigated. It was found that in situ chemical reaction was completed in solid phase sintering, but the composites had lower microhardness, comprehension and shear strength due to low density and segregation of alumina and Si particles in microstructure. Segregation of reinforcement particles in solid phase sintering resulted from character of solid reaction and Si diffusion at high temperature over a long hold time.

Effect of Isothermal Forging on Microstructure and Mechanical Properties of TiAl-Based Alloys

The effect of isothermal forging on microstructure and mechanical properties of Ti-44Al-4Nb-4Zr-0.2Si-1B alloys were investigated by means of BSE, TEM, tensile and the high cycle fatigue test at room temperature. The results showed that the lamellar thickness and volume fraction of equiaxed γ phases and B2+ω phases decreased, the grain size and volume fraction of lamellar α2+γ colonies was raised after isothermal forging. The lamellar was bending. The tensile strength and yield strength was increased by 80MPa although the total elongation hardly changed. The fatigue limit was increased by 230MPa. The effect of boride, lamellar thickness and B2+ω phases on the mechanical properties were studied.

Effect of Tungsten Addition on Microstructure and Mechanical Properties of TiAl-Based Alloy

The effect of different tungsten addition on microstructure and mechanical property of TiAl-based alloy was investigated. The results indicate that the size and amount of β (B2) phase and equiaxed γ phase gradually increase with increasing content of tungsten. The grain size and lamellar spacing obey a parabolic law. The tungsten addition can increase the room temperature strength, high temperature strength but have a little effect on the ductility. The creep rupture life has inverse correlation relationship with the content of tungsten.

Sliding Wear Resistance of Tempered T15m High Speed Steel

The dry sliding wear tests were carried out on T15m high speed steel with the M-2000 type wear testing machine, and studied the influence of wear performance due to the different loads and speeds on T15m, SEM was employed to analysis the worn surface morphology to discuss the mechanism of wear and tear. The results demonstrated that the mechanisms as well as the wear rate of T15m high speed steel were under different loads and speeds were different. Under the load of 100N, it is oxidative wear, while abrasive wear is the main reason for 150N and 200N. However, under 250N it is converted to serious oxidative wear and fatigue wear.

Low-Temperature Preparation of the BaO-B2O3 Matrix Composite Ceramics

In this work, the energy-saving LTCC composite ceramics containing quartz, fused quartz and zirconia ceramic particles, respectively based on the binary system BaO-B2O3 were prepared by traditional solid-state preparation process at a sintering temperature of 900°C. Sintering mechanism and physical properties of the LTCC composite ceramics are investigated and discussed in detail in terms of their mineral phase composition. The results indicate that the introduction of α-alumina to the binary system BaO-B2O3 can improve the sintering behavior whereas the presence of the functional ceramic particles in the composite ceramics is important to achieve the peculiar physical characterlistics, which consequently supply more possibilities to regulate on the physical properties of the composite ceramics.

Preparation of the BaO-B2O3 Matrix Low-Temperature Sintering Ceramics via an Aqueous Suspension Route

Due to the low-meting characteristics of the BaO-B2O3 binary composition, the dense ceramics with peculiar properties, so called low-temperature sintering ceramics, were prepared by the introduction of other oxides, e.g., SiO2, Al2O3, ZnO, CaO to the BaO-B2O3 binary compositions at a sintering below 1000°C. The low-temperature preparation of the ceramics are achieved by milling the aqueous suspension mixtures of Ba (OH)2·8H2O, H3BO3 ,H2SiO3, Al (OH)3, ZnO. By the aqueous suspension process, some compounds with a low-melting point such as hydrated barium borate, and the precursor compounds of the functional mineral phases, e.g., BaSiO3, ZnSiO3, BaAl2Si2O8, etc. can be formed easily from a suspension solution, which consequently supply more possibilities to fabricate the low-temperature sintering ceramics with peculiar physical properties.

Study of Friction and Wear Properties of C-Cu Composites Reinforced with Carbon Fiber Cloth

C-Cu composites reinforced with carbon fiber cloth were prepared by hot pressing sintering technology. Wear test was conducted with copper ring under dry friction condition. The effects of load and sliding speed on friction and wear properties of the composites were studied respectively and meanwhile the wear mechanism was analyzed by combination with the morphologies of worn surfaces. The results show that carbon fiber cloth improves the wear resistance of composites and fiber grindings possess the ability of alleviating adhesive wear. These make abrasion loss small in the range of normal load and sliding velocity. With increasing of the load and sliding speed, friction coefficient and abrasion loss also increase. The main wear mechanism is transformed from slight adhesive wear into delamination wear and is accompanied by abrasive wear and slight oxidative wear at the same time.

Investigation on Recovery and Recrystallization of Al-Si-Al2O3 Composites

Al-Si-Al2O3 composites were prepared by powder metallurgy with in-situ synthesis technology. The recovery and recrystallization behavior of Al-Si-Al2O3 composites which underwent compression and then heat-treatment under different temperature were studied using micro-hardness tester, optical microscope (OM) and scanning electron microscopy (SEM) . The results showed that the hardness of composites increased dramatically after compression, and the sample containing 5wt% Si was increasing more evidently than the sample including 10wt%Si. Heat treatment gradually eliminated work hardening; meanwhile the fact that the hardness of composites trended to decline greatly when subjected to annealing suggested occurrence of recovery and recrystallization inside the composites. Recrystallization nucleation preferentially took place in the region near the particle, while the growth of recrystallized grains can also be hindered owning to the pining effect of particles. Depending on the analysis of microstructure and microhardness, it can be concluded that the recrystallization temperature of Al-wt.5%Si-Al2O3 composites was 500°C and the Al-wt.10%Si-Al2O3 composites was 525°C.