Preparation of Poly(N-butyl methacrylate-co-glycidyl methacrylate) and Toughness Improvement for Powder Epoxy Resin E663

Copolymers of poly(n-butyl methacrylate-co-glycidyl methacrylate) were prepared by a free radical polymerization. The structures of the copolymers were characterized. The copolymers could be added to a powder epoxy resin (E663) to form modified epoxy resins. It was found that toughness of the cured modified epoxy resins were greatly improved, impact strength increased 3 times and fracture elongation increased 20% compared to the cured unmodified E663 resin. This is because that the copolymers had epoxy groups and flexible butyl groups, the former were involved in cross-linking reaction of the E663, and the latter made contribution to the toughness improvement.     

Data-driven optimization of hardness and toughness of high-entropy nitride coatings

High coating hardness and toughness are mutually contradicting properties and are challenging to be achieved simultaneously. Combining the vast component space of high entropy systems and the powerful high-dimensional data processing tools is expected to be the best solution to this problem. In this paper, high-entropy nitride coatings data for quinary and hexagonal systems were collected and machine learning prediction models were trained. Using a new material system combined with multi-objective optimization, high-entropy nitride coatings with the optimal hardness and elastic modulus combination were successfully obtained and verified by experiments. In addition, the partial dependence heatmaps were used to visualize how elemental content affects mechanical properties prediction in this system. This approach helped to better interpret the optimization results and discover the unknown mapping relationships between elemental content and the mechanical properties of high-entropy nitrides in machine learning models.     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.     

Improving the braking performance of the novalac-based CNFs/carbon composite by the wet chemical oxidation of CNFs

In this research, the braking performance of the CNFs-carbon composite pads was improved by the oxidation of CNFs using wet chemical oxidation. Oxidation of CNFs resulted in the formation of functional groups on the CNFs that improved adhesion and bonding of the CNFs to the matrix. Through this process, the toughness, tensile, compressive, and bending strength of the composite were strongly increased. Also, the hardness and wearing properties of the composite were improved by the oxidation of CNFs. For the composite reinforced by the oxidized CNFs, the low content of the nanofiber pull-out increased the COF. The main disadvantage of the oxidized CNFs-carbon composite was the sample decomposition at temperatures above 400 °C. This feature can deteriorate the braking performance of the composite when it is pushed too hard and overheated. To fix this problem, a SiC coating layer was applied to the composite by CVD method. Applying SiC postponed the thermal degradation of the composite up to 1000 °C. Weight losing of the SiC-coated sample was recorded at about 3.13 % at 1000°C.     

Fracture in CVD diamond

The fracture behaviour of thick, textured films of chemical-vapour-deposited diamond is discussed with particular emphasis on the influences asserted by the polycrystalline microstructure. Cracking is investigated on two different scales, firstly where it is large in that it traverses many grains and pertains to the fracture of the bulk. The second is where the fractures are localised, resulting from repeated small particle impacts and the mechanisms of material removal at the grain-size-scale are elucidated. The behaviour at this smaller level can be rationalised in terms intermediate to bulk fracture and that observed in single crystal diamond. The effects of grain size, grain boundaries, crystallographic orientation, twinning, internal stresses and pre-existing flaws are discussed. A new value for the fracture toughness is calculated and the gravimetric erosion rate for different surface orientations measured.     

Five decades of crack path research

The complete solution of a crack propagation problem includes determination of the crack path. At the present state of the art the factors controlling the path taken by a propagating crack are not completely understood. In the last five decades there have been substantial advances in the understanding and prediction of both macroscopic and microscopic aspects of crack paths. This paper is a retrospective covering research on several crack path topics with which the author has been involved over the past five decades. All have a long history, but are still of interest and have practical engineering applications.     

超高韧性水泥基复合材料基本力学性能

超高韧性水泥基复合材料(简称UHTCC)是一种中等纤维体积掺量的随机分布的短纤维增强高性能水泥基复合材料。本文通过单轴拉伸试验、四点弯曲试验、单轴抗压试验、三点弯曲缺口梁断裂试验研究了这种新型材料的抗拉、抗弯、抗压和断裂性能。试验结果表明，超高韧性水泥基复合材料在拉伸和弯曲荷载作用下具有假应变硬化和多缝开裂特性，以及高延性、高韧性和高能量吸收能力。极限荷载时的最大裂缝宽度在50μm左右，如此小的裂缝宽度可以有效地阻止侵蚀性物质的侵入，提高钢筋混凝土结构的耐久性。拉伸和弯曲试验测得的超高韧性水泥基复合材料的极限拉伸应变在3%以上，平均裂缝间距1mm左右。超高韧性水泥基复合材料的抗压强度类似于混凝土，抗压弹性模量较低，但受压变形能力比普通混凝土大很多。通过三点弯曲缺口梁试验证明，超高韧性水泥基复合材料的峰值荷载和峰值荷载对应变形都较基体有明显的提高。缺口拉伸试件和缺口梁试件均证明，超高韧性水泥基复合材料可以将单一裂缝细化成多条细裂缝，同时超高韧性水泥基复合材料具有对小缺口不敏感的特性。四种试验的结果证明超高韧性水泥基复合材料在各种破坏荷载作用下均能保持良好的整体性，不发生碎裂破坏。     

Thermo-Mechanical Properties of Semi-Degradable Poly(β-amino ester)-co-Methyl Methacrylate Networks under Simulated Physiological Conditions

Poly(β-amino ester) networks are being explored for biomedical applications, but they may lack the mechanical properties necessary for long term implantation. The objective of this study is to evaluate the effect of adding methyl methacrylate on networks’ mechanical properties under simulated physiological conditions. The networks were synthesized in two parts: (1) a biodegradable crosslinker was formed from a diacrylate and amine, (2) and then varying concentrations of methyl methacrylate were added prior to photopolymerizing the network. Degradation rate, mechanical properties, and glass transition temperature were studied as a function of methyl methacrylate composition. The crosslinking density played a limited role on mechanical properties for these networks, but increasing methyl methacrylate concentration improved the toughness by several orders of magnitude. Under simulated physiological conditions, networks showed increasing toughness or sustained toughness as degradation occurred. This work establishes a method of creating degradable networks with tailorable toughness while undergoing partial degradation.     

Surprising shape-memory effect of polylactide resulted from toughening by polyamide elastomer

Melt blending of polylactide (PLA) and biodegradable polyamide elastomer (PAE) has been performed in an effort to toughen the PLA. DMA tests showed good compatibility between PAE and PLA blends, and the PAE were dispersed in PLA matrix uniformly shown in SEM photos. Mechanical properties of blends with different PAE concentrations were observed. With the PAE contents increasing, the elongation at break of blends increased and the brittle break became ductile break. When the PAE content is 10%, the tensile strength of blend is similar to neat PLA, and the elongation increased to 194.6% significantly. Remarkably, the blends showed wonderful shape-memory effect. PAE domains act as stress concentrators in system with the stress release locally and lead to energy-dissipation process. These will prevent PLA matrix from breaking under high deformation, and lead to the PLA molecular orientation. Consequently, the blends occurred to deformation upon tensile load, and heating up the material will reform the shape back to the original shape.     

Fracture toughness of coated TiCN–WC–Co cermets with graded composition

Mixed TiCN–WC–Co cermets are developed to improve at the same time toughness and resistance to deformation of materials for cutting tool applications. Moreover, graded materials joining optimum properties according to the functional part of the tool are elaborated. To this end, TiCN–WC–Co cermets are interesting because they develop a WC–Co layer at the surface during the sintering. This tough layer at the surface limits the crack propagation that can lead to the rupture of the tool. Such materials show a good resistance to the deformation in the bulk and a good toughness at the surface, where the cracks are initiated upon machining. Cutting tools are often coated by CVD to improve the wear resistance. This paper proposes a method to measure the toughness K_IC at high temperature by using this CVD coating for initial crack formation. The coating thickness is the precrack length of traditional K_IC measurements. Samples are fractured by three point bend tests. The rupture stress is measured by Weibull statistics. This method is particularly interesting for graded structure materials where the influence of surface layers on toughness must be estimated. The comparison between cermets with and without WC–Co layer shows an improvement of 28% of the toughness when the layer is present. The possible bias of internal stresses on the results is discussed.