Toughening alumina with silver and zirconia inclusions

Both silver and zirconia inclusions are added into an alumina matrix, the strength and toughness of the composites are determined. The toughening agents prohibit the grain growth of the matrix, the strength of alumina is, therefore, enhanced. The addition of two toughening agents also enhances the toughness of alumina. The presence of Ag inclusions raises the transformation ability of ZrO₂; however, the toughness increase of the Al₂O₃–ZrO₂–Ag composites is slightly lower than the sum of the toughness increase of Al₂O₃–ZrO₂ and of Al₂O₃–Ag composites. The present study demonstrates that the toughening effects contributed by a transformation toughening agent and a ductile toughening agent can interact with each other; nevertheless, such interaction depends strongly on the microstructure of the composites.     

Toughening macroporous alumina membrane supports with YSZ powders

Macroporous alumina is an important support in membrane fields because of its stabilities to withstand exposure to high temperature, harsh chemical environment and high mechanical strength. However, the essence of brittleness can greatly shorten the life span and restrict the application fields. In this paper, YSZ (ZrO₂ stabilized by 3 mol% Y₂O₃) powders were added into alumina powders to improve the fracture toughness of macroporous Al₂O₃ supports sintered at 1400 °C and 1600 °C. The results show that the fracture toughness and the corresponding bending strength of supports are simultaneously greatly influenced by various YSZ contents. When YSZ content is 6 wt%, the maximum value of the fracture toughness is 3.0 MPa·m^1/2, and the bending strength is up to 90 MPa. By SEM and XRD analysis, the phase transformation of the uniform distribution t-ZrO₂ into m-ZrO₂ is the main cause which improves the fracture toughness of macroporous Al₂O₃ supports. Lowering of the sintering temperature by adding YSZ additives is also discovered here. The fracture toughness of the supports sintered at 1400 °C by adding YSZ powder is higher than that of the supports sintered at 1600 °C without adding any additives.     

Toughening structural adhesives via nano- and micro-phase inclusions

It has been clearly demonstrated that the addition of low concentrations of nanosilica particles to a typical rubber-toughened adhesive, based upon a two-part epoxy formulation, leads to very significant increases in the toughness of the adhesive and also to increases in the glass transition temperature and the single-lap shear strength. The nano-SiO 2 particles have an average particle diameter of 20 nm and are very well dispersed in the epoxy adhesive, and only a concentration of about 1% to 8% by mass of such nanoparticles are needed to achieve significant improvements in the mechanical and thermal performance of the rubber-toughened two-part epoxy adhesive.     

Crushing behavior of alumina inclusions with different porosity during hot compression

Alumina inclusions in commercial as-cast 2.25Cr1Mo0.25V aluminum deoxidized steel exhibited a feature of porous structure. In order to investigate the crushing characteristics of alumina inclusion during hot working, a series of alumina blocks with different porosity whose properties are similar to the alumina inclusions in ingots were prepared using spark plasma sintering. The crushing behavior of alumina blocks during hot compression with quasi-static load was studied. A prediction model of compressive strength of alumina inclusions considering apparent porosity was established on basis of hyperbolic sine Arrhenius equation. A novel crushing mode diagram for alumina inclusions characterized by Z parameter was proposed. The crushing mechanism of alumina inclusions under different deformation parameters was clarified by fracture characteristics. The results showed that the hot compression process of alumina presented a typical brittle fracture, the compressive strength was more sensitive to deformation conditions at lower apparent porosity as compared with the conditions of higher apparent porosity. With the increase of Z, the crushing mode of alumina inclusions gradually changed from intergranular fracture to transgranular fracture.     

Controlled stripping of aluminide coatings on nickel superalloys through electrolytic techniques

Aluminide diffusion coatings are widely employed to improve the oxidation and/or the corrosion resistance of highly added value turbine components operating in harsh environments at high temperatures. Refurbishment of such components requires appropriate removal of worn coatings and of the corrosion products layer—usually an oxide scale. Stripping is mostly carried out using hazardous chemical baths of limited reliability. In this work, an alternative stripping method based on electrochemical techniques has been carried out at laboratory scale for CVD Al diffusion coatings on a directionally solidified Ni base superalloy. Both the galvanostatic and the potentiostatic modes have been investigated. Prior to them, in situ gas bubbling induced by cathodic polarization seems to be an effective way to remove the superficial oxide scales. Measuring the open circuit potential during the experiments allows easy monitoring of the progress of the selective dissolution of the different layers. Complete removal of the aluminide coatings is indicated by potential values similar to those of the substrate. The correlation between the electrochemical features and the surface state after stripping has been carried out by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The electrochemical approach is a promising means to strip out surfaces in a selective and reliable manner.     

Toughening plastics by crack growth inhibition through unidirectionally deformed soft inclusions

We demonstrate that without changing the filler content mechanical properties of commodity plastics with immiscible soft inclusions can be decisively enhanced, simply by pressure induced flow processing in the solid state. As an example, we have chosen acrylonitrile-butadiene-styrene (ABS), where shape and orientation of the soft fillers were changed by processing, resulting in an array of aligned and oriented nanosize deformed rubber domains. These deformed domains effectively controlled the propagation of cracks inside the solid matrix and were responsible for a multifold increase of tensile and impact toughness. Thus, appropriate processing allows manufacturing plastics with high impact resistance in accordance with engineering needs.     

Toughening mechanisms in nacre-like alumina revealed by in-situ imaging of stress

Nacre-like aluminas are bio-inspired, ceramic-based composites that display high toughness and strength. Their toughness arises from their brick-and-mortar microstructure. We must understand the role of several microstructural features over their mechanical properties. Techniques that allow microstructural imaging during mechanical tests are therefore desired. Here, we use both in situ fluorescence spectroscopy to image the stress field around cracks propagating in samples, and in situ scanning electron microscopy to image the crack-microstructure interactions. Stress concentrates around locally disordered zones where the crack is pinned while crack propagation is delayed. In situ imaging shows that obstacles to crack propagation are either larger-than-average alumina platelets or bundles of alumina platelets misaligned with respect to the majority of the platelets. Such microstructural heterogeneities are therefore important to impede crack propagation in nacre-like alumina. The approach proposed here can be used to understand the structure/properties relationships of other types of materials.     

Activity and stability of alumina zeolite nickel catalysts

The catalytic properties of nickel catalysts (50 wt.% of alumina and 50 wt.% of Ni,H-ZSM-5) were investigated and related to the amount of NiO (0–8 wt.%) and the method of nickel incorporation (8 wt.% NiO). Consideration was also given to the method by which zeolite and alumina were combined. The cracking properties of the catalysts increased when the amount of NiO was raised up to 4 wt.%. To decrease the content of aromatic hydrocarbons in the products it is necessary to raise the amount of NiO to a higher level than 4 wt.%. The catalyst prepared by peptisation of the mixture of zeolite and aluminium hydrogel (with ageing process) displayed reduced activity and stability because of the low susceptibility of NiO to reduction.     

Toughening of zirconia/alumina composites by the addition of graphene platelets

A study on graphene platelet/zirconia-toughened alumina (GPL/ZTA) composites was carried out to evaluate the potential of the new structural materials. GPL–ZrO₂–Al₂O₃ powders were obtained by ball milling of graphene platelets and alumina powders using yttria stabilized ZrO₂ balls. Samples were sintered at different temperatures using spark plasma sintering. Fracture toughness was determined by the single-edge notched beam method. The results show that the GPLs are uniformly distributed in the ceramic matrix and have survived high temperature sintering processes. Several sintering experiments were carried out. It is found that at 1550 °C, GPL/ZTA composites were obtained with nearly full density, maximum hardness and fracture toughness. A 40% increase in fracture toughness in the ZTA composite has been achieved by adding graphene platelets. The toughening mechanisms, such as pull out, bridging and crack deflection, were observed and are discussed.     

Thermal shock resistance of refractory composites with Zirconia and Silicon-Carbide inclusions and alumina binder

With the goal of designing a castable refractory for an aerospace application with optimum resistance to thermal shock, three different particle-reinforced ceramic composites are designed and compared. Different volume fractions of Silicon Carbide (SiC) particles, Zirconia (ZrO₂) bubbles, and Zirconia solid particles dispersed in an alumina (Al₂O₃) matrix are used in the fabrication of these castables. Destructive and nondestructive testing procedures are implemented to evaluate their thermomechanical properties, both before and after a custom designed thermal shock cycle. After demonstrating the applicability of thermal shock indices, the variation of these indices due to thermal shock is measured experimentally and utilized as a design tool. Multiple micro-scale damage mechanisms were observed, all of which are various forms of structural deformation.     

Reduction of nickel oxide on alpha alumina: coalescence of nickel to metallic crystallites

Reduction of alpha-alumina-supported nickel oxide was studied with hydrogen consumption, magnetization and XRD measurements. Rupture of Ni-O bonds at 270–350 ° C is much faster than nucleation of metallic nickel and precedes growth into crystallites. Water vapor and low hydrogen flows retard both processes. At higher temperatures, growth is more rapid than Ni-O bond rupture.     

Carbonaceous deposition associated with the catalytic steam-reforming of hydrocarbons over nickel alumina catalysts

The reactions of n-pentane, n-hexane, n-heptane, 1-heptene, cyclohexane, benzene, toluene, ethylbenzene, and the xylenes with steam have been investigated using 75% w/w nickel/alumina catalysts at 475 °C, with particular reference to the formation of surface carbonaceous deposits. Using steam/hydrocarbon ratios of between 0 and 15, up to four distinct types of surface carbonaceous deposit have been identified. Extensive carbon filament formation is observed when no steam or low steam concentrations are used. A hydrocarbonaceous polymeric species, extractable from the used catalysts with tetrachloromethane, is formed with each hydrocarbon under all reaction conditions. The chemical identity of the polymer, established by infrared spectroscopy and mass spectrometry, is apparently independent of the reactant hydrocarbon. Evidence for a reactive and a nonreactive surface carbon has also been obtained. Admission of [¹⁴C]CO or [¹⁴C]CO₂ with the reactant hydrocarbon leads to scrambling of the [¹⁴C]label among the reaction products and the surface deposit. A mechanism for the formation of both the gaseous reaction products and the various surface residues is discussed in terms of the adsorbed intermediates. The use of carbon monoxide adsorption as a probe for exposed surface nickel shows that, although the exposure of the nickel surface is markedly reduced by the formation of surface deposits, the activity of the catalyst for the gasification of the hydrocarbons is virtually unaffected. The possibility of reaction occurring on a carbonaceous overlayer on the nickel surface is discussed.     

In-flight formation and characterization of nickel aluminide powders in a dc thermal plasma jet

The strength of some intermetallics increases with temperature instead of exhibiting a decrease: thus, they are ideally suited for high temperature applications. The formation of intermetallic compound during the spray process leads to metallurgical bonding due to the high exothermicity of the formation reaction. In this paper formation of nickel aluminide in a thermal plasma jet from ball-milled nickel–aluminium powders is reported. Commercially available nickel and aluminium powders were mixed in the appropriate amount and injected in to a thermal plasma jet produced by an atmospheric plasma torch. During in flight the formation of nickel aluminide takes place in plasma jet. Powders were collected at two different collection distances (40 and 80 cm) for three power levels (10, 15 and 20 kW), different plasma gas flow rates (15 and 20 lpm) and different powder feed rate (7 and 14 g/min). The powders are characterized using SEM, optical micrograph, TG/DTA and XRD analysis. The effect of plasma parameters on the formation have been studied and reported. The formation is better at higher power levels, low powder feed rate, low plasma gas flow rate and longer collection distances.     

Toughening enhanced at elevated temperatures in an alumina/zirconia dual-phase matrix composite reinforced with silicon carbide whiskers

A unique temperature dependence of toughening is observed in an alumina/zirconia dual-phase matrix composite reinforced with silicon carbide whiskers. The work of fracture (WOF) of the composite is maximized at 400 °C to 130 J/m², which is about 6.5 times larger than that of monolithic alumina at room temperature. The WOF decreases sharply with an increase in temperature above 400 °C. The enhanced toughening at elevated temperatures is described by the stress-induced transformation toughening of tetragonal zirconia, which is affected by the internal thermal stress owing to thermoelastic mismatch between the matrix and the whiskers. The maximum WOF is not given only by the stress-induced transformation but also by the crack-face bridging of the whiskers. The WOF was optimized at a specific zirconia volume fraction of 0.7 in the matrix, which was essentially due to the maximized tensile internal stress on zirconia in the dual-phase matrix.     

聚碳酸亚丙酯增韧聚乳酸研究

利用双螺杆挤出机研究了聚碳酸亚丙酯(PPC)对聚乳酸(PLA)的增韧效果.结果表明:PPC树脂对PLA有明显的增韧作用,但增韧的同时会引起PLA拉伸强度和维卡软化温度的降低,随PPC用量增加,冲击强度持续提高,而拉伸强度和维卡软化温度持续降低;PLA/PPC共混体系中加入二苯基甲烷二异氰酸酯(MDI)后,可提高两者的相容性,从而起到增韧的作用,随MDI用量增加,PLA/PPC共混物的冲击强度和拉伸强度呈现先增加后减小的趋势.     

Toughening epoxies with halloysite nanotubes

An experimental attempt was made to characterize the fracture behaviour of epoxies modified by halloysite nanotubes and to investigate toughening mechanisms with nanoparticles other than carbon nanotubes (CNTs) and montmorillonite particles (MMTs). Halloysite-epoxy nanocomposites were prepared by mixing epoxy resin with halloysite particles (5 wt% and 10 wt%, respectively). It was found that halloysite nanoparticles, mainly nanotubes, are effective additives in increasing the fracture toughness of epoxy resins without sacrificing other properties such as strength, modulus and glass transition temperature. Indeed, there were also noticeable enhancements in strength and modulus for halloysite-epoxy nanocomposites because of the reinforcing effect of the halloysite nanotubes due to their large aspect ratios. Fracture toughness of the halloysite particle modified epoxies was markedly increased with the greatest improvement up to 50% in K_IC and 127% in G_IC. Increases in fracture toughness are mainly due to mechanisms such as crack bridging, crack deflection and plastic deformation of the epoxy around the halloysite particle clusters. Halloysite particle clusters can interact with cracks at the crack front, resisting the advance of the crack and resulting in an increase in fracture toughness.     

Toughening epoxy resins with polyepichlorohydrin

Polyepichlorohydrin (PECH) rubbers were found to toughen epoxy resins based on the diglycidyl ether of bisphenol A (DGEBA) and cured with piperidine. The degree of toughening depends on the molecular weight of the PECH and on the curing temperature. Best toughening was achieved with PECH of the highest nominal molecular weight of 3400 (Hydrin 10 × 2). Hydrin 10 × 1 (nominal molecular weight 1700) did not toughen the epoxy resin unless bisphenol A was also added, whereas Hydrin 10 × 2 toughened it in the absence of bisphenol A. Curing resins containing bisphenol A and Hydrin 10 × 1 at 160°C resulted in a slightly more brittle resin than when cured at 120°C. The effect of PECH rubbers on the T_g, modulus, and hot/wet properties is similar to that of carboxy-terminated butadiene-acrylonitrile rubbers (CTBN). Dynamic mechanical thermal analysis (DMTA) and scanning electron micrographs (SEM) of fractured surfaces show that the PECH separates as a discrete phase during curing. © 1993 John Wiley & Sons, Inc.     

Toughening biodegradable polylactide with nanopores

The inherent brittleness of biodegradable polylactide (PLA) limits its large-scale applications. A common route for toughening PLA is to incorporate other components by blending. In achieving good toughness, however, other properties such as strength and transparency are usually sacrificed to a large extent. Here we presented a facile approach to toughen PLA with nanopores. By tailoring solution phase separation abundant nanopores with size of about 30 nm was achieved in the PLA films. The presence of nanopores significantly enhanced tensile toughness to about 67 MJ/m³, and good strength and high transparency were retained. Both multiple crazing and shear yielding were contributed to the remarkable toughness improvement. Our approach, advantageous over other routes, opens new opportunities to fabricate tough PLA with a better balance of comprehensive properties.     

Minimum amount of nano-sized nickel particles to enhance the strength of alumina

Previous studies demonstrated that the strength of alumina could be enhanced by incorporating >5 vol.% nano-sized nickel particles. In the present study, the possibility of using a much smaller amount, <0.5 vol.%, of nano-sized Ni particles to improve the mechanical properties of Al₂O₃ is explored. As the Ni content is low, the densification of Al₂O₃ is affected little and the Ni inclusions remain small after pressureless sintering at 1600 °C. The presence of Ni inclusions can refine the matrix grains; the strength of Al₂O₃ matrix is consequently enhanced.     

硅改性氧化铝及负载镍基催化剂的制备与表征

以拟薄水铝石为原料,经过胶溶、混捏和挤条成型制备硅质量分数为0.5%~7%的系列改性氧化铝,并采用等体积浸渍法制备负载镍基催化剂,研究硅改性对氧化铝载体和镍基催化剂性能的影响。采用NH3-TPD和H2-TPR对样品进行表征,结果表明,硅改性的氧化铝载体酸性随着硅含量的增加逐渐增强;负载镍后表面酸类型发生改变,弱酸及强酸中心消失,只有中强酸中心存在;镍基催化剂活性组分与载体之间相互作用也随着硅含量的增加逐渐增强。