Densification effects on the fracture in fused silica under Vickers indentation

This study investigates the effects of densification on the deformation and fracture in fused silica under Vickers indentation by both the finite element analysis (FEA) and experimental tests. A refined elliptical constitutive model was used, which enables us to investigate the effects of the evolution of yield stress under pure shear and elastic properties with densification. The densification distribution was predicted and compared with experiments. The plastic deformation and indentation stress fields were used to analyze the initiation and morphology of various crack types. The formation mechanism of borderline cracks was revealed for the first time. This study reveals that the asymmetry of the densification distribution and elastic-plastic boundary significantly influences the cracking behavior. Under the Vickers indentation, conical cracks have the largest penetration depth. When these cracks emerge from a region far from the impression, they extend with constant radii to form circles on the sample surface. Otherwise, they tend to be initiated at the centers of the indenter-material contact edges before propagating towards the impression corners with increasing radii. Therefore, the borderline cracks consisting of successive partial conical cracks can form at a low load and makes them the first type of crack to appear.     

Facile synthesis of MgO–Mg2SiO4 composite ceramics with high strength and low thermal conductivity

The recycling of solid waste is a win-win solution for humans and nature. For this purpose, magnesite tailings and silicon kerf waste were employed to prepare MgO–Mg₂SiO₄ composite ceramics by solid-state reaction synthesis in the present work. Then, effects of sintering temperature and raw material ratio on as-prepared ceramics were systematically studied. As-prepared ceramics showed improvement in their relative density (from 47.55%–68.12% to 90.96%–95.25%) and cold compressive strength (from 7.34–118.66 MPa to 303.39–546.65 MPa) with the increase in sintering temperature from 1300 to 1600 °C. In addition, it was found that Si promoted synthesis process of Mg₂SiO₄ phase through transient liquid phase sintering and Fe₂O₃ accelerated sintering process through activation sintering. Consequently, the presence of Mg₂SiO₄ phase effectively improved the density and strength of MgO–Mg₂SiO₄ composite ceramic, while reducing its thermal conductivity. This work provides a potential reutilization strategy for magnesite tailings, and as-prepared products are expected to be applied in fields of construction, metallurgy, and chemical industry.     

Physical characteristics of AFEX-pretreated and densified switchgrass,prairie cord grass,and corn stover

The goal of the study was to evaluate and compare the physical properties of control, pretreated and densified corn stover, switchgrass, and prairie cord grass samples. Ammonia Fiber Expansion (AFEX) pretreated switchgrass, corn stover, and prairie cord grass samples were densified by using the comPAKco device developed by Federal Machine Company of Fargo, ND. The densified biomass were referred as “PAKs” in this study. All feedstocks were ground into three different grind size of 2, 4 and 8 mm prior to AFEX pretreatment and the impact of grinding on pellet properties was studied. The results showed that the physical properties of AFEX-PAKed material were not influenced by the initial grind size of the feedstocks. The bulk density of the AFEX-PAKed biomass increased by 1.2–6 fold as compared to untreated and AFEX-pretreated materials. The durability of the AFEX-PAKed materials were between 78.25 and 95.2%, indicating that the AFEX-PAKed biomass can be transported easily. To understand the effect of storage on the physical properties of these materials, samples were stored in the ambient condition (20 ± 2 °C and 70 ± 5% relative humidity) for six months. After storage, thermal properties of the biomass did not change but glass transition temperature decreased. The water absorption index and water solubility index of AFEX-treated and AFEX-PAKed biomass showed mixed trends after storage. Moisture content decreased and durability increased upon storage.     

Influence of ZrB2 hard ceramic reinforcement on mechanical and wear properties of aluminum

In this work, the effect of ZrB₂ (0, 5, 10 and 20?vol%) ceramic reinforcement on densification, structure, and properties of mechanically alloyed Al was investigated. The milling of Al-ZrB₂ powder compositions resulted in formation of agglomerates with varied size. In particular, the size of agglomerates was reduced considerably with increased addition of ZrB₂ to Al. Interestingly, the densification of hot pressed Al increased from 96.06% to 99.22% with ZrB₂ addition. The reduction of agglomerates size was attributed to the enhanced densification of Al-ZrB₂ composites. Pure Al showed relatively low hardness (0.94?GPa) and it was improved to 1.78?GPa with the addition of 20?vol% ZrB₂. The mechanical properties have significantly been improved for Al-ZrB₂ composites. Especially Al - 20?vol% ZrB₂ possessed a very high yield strength (529?MPa), compressive strength (630?MPa) and compressive strain of 19.25%. Realization of such a good combination of mechanical properties is the highest ever reported for Al composites so far in the literature. The coefficient of friction (COF) of Al-ZrB₂ varied narrowly between 0.33 and 0.40 after dry sliding wear against steel disc. The wear rate of Al-ZrB₂ composites was within mild wear regime and varied between 98.88?×?10^?6 and 34.66?×?10^?6 mm³/Nm. Among all the compositions, Al - 20?vol% ZrB₂ composite exhibited the lowest wear rate and high wear rate was noted for pure Al. Mild abrasion, tribo-oxidation, third body wear (wear debris) and delamination were the major material removal mechanisms for Al-ZrB₂ composites. Overall the hardness, strength and wear resistance of Al - 20?vol% ZrB₂ composite was improved by 84.3%, 84.3% and 64.2%, respectively when compared to pure Al.     

Densification mechanism during reactive hot pressing of B4C-ZrO2 mixtures

The densification behaviors of pure B₄C and B₄C-ZrO₂ mixtures were compared during hot pressing. The results showed that in-situ formed ZrB₂ effectively enhanced the densification process of B₄C-ZrO₂ mixtures, more significantly during the intermediate stage. Within the relative density ranging from 0.75 to 0.90, the B₄C-15?wt%ZrO₂ mixture (B15Z) achieved the maximum densification rate as twice much as that of pure B₄C. The stress exponent n>3 indicated plastic deformation was the dominant densification mechanism of B15Z. The viscosities of plastic flow were evaluated using Murray-Rodger-William equation and the viscosity of B15Z was only a quarter of that in pure B₄C. The sintering activation energy was calculated to be 305.9?kJ/mol for pure B₄C and 197?kJ/mol for B15Z, respectively. It was proposed that the lower viscosity of plastic flow and activation energy accelerated the sliding and propagating motions of plastic flow, by which underlain the enhanced densification behaviors of B₄C-ZrO₂ mixtures.     

烧结工艺对β-氮化硅陶瓷的影响

以β-Si3N4粉末为原料,以YAG（钇铝石榴石）为烧结助剂,通过气氛压力烧结（GPS）制备出致密的β-氮化硅陶瓷材料,形成大小均匀的柱状颗粒和小球状颗粒复合显微结构,研究了烧结助剂质量分数、烧结温度以及保温时间对β-氮化硅陶瓷致密化程度及力学性能的影响.     

Analytical and numerical study of the densification of carbon/carbon composites by a film-boiling chemical vapor infiltration process

The film-boiling chemical vapor infiltration (CVI) process is a fast process developed for composite material fabrication, and especially carbon/carbon composites. In order to help define optimal conditions, a local 1D model has been developed to study the densification front which establishes itself during the processing of a carbon/carbon fibrous preform. The model features heat conduction, precursor gas diffusion, densification reactions and structural evolution of the porous medium. The effects of total mass flux, Thiele modulus, porous medium geometry on front behavior parameters such as width, velocity and residual porosity are presented as semi-analytical correlations. An existence criterion is produced, which involves a minimal heat flux. Comparison between process-scale experiments and simulation is then possible by inserting the semi-analytical results achieved in the local study of the front into a light numerical model involving the entire preform. The model has been validated with respect to previous experimental and numerical data.     

纳米陶瓷的烧结研究

纳米陶瓷的制备过程中，烧结是关键一环。总结了现有的烧结理论，介绍了国内外流行的纳米烧结技术，并提出了今后的研究方向。     

粉末烧结体镦粗致密与成形的有限元分析

选择典型的镦粗工艺对粉末烧结坯的致密与成形行为和规律进行了研究，提出了多孔材料的致密模型并简要介绍了多孔材料的塑性理论。然后，采用有限元法模拟了不同镦粗压下量的烧结坯的致密与成形过程。密度与等效应变分布的一致性表明，烧结坯的致密强烈依赖于材料的塑性变形。     

Prediction model for surface temperature of roller and densification of iron powder during hot roll pressing

The hot roll pressing of iron powder with several rotating speeds was carried out on a pilot-plant scale. From these experiments, it was confirmed that the surface temperature of roller was increased with increasing its rotating speed. It was also known that the heat transfer coefficient between the iron powder and the roller surface is closely related with the rotating speed of roller. These results were quantitatively described by using a mathematical model which was derived based on the steady-state heat transfer during hot roll pressing. In addition, the densification behavior of iron powder during the hot roll pressing was simulated by a finite element model based on the arbitrary Lagrangian and Eulerian (ALE) method and a yield criterion for metal powder. From these models, the maximum critical rotating speed of roller during the hot roll pressing, in which the roller could hold out against the thermal fracture, could be determined.