Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Short SiC fiber/Ti3SiC2 MAX phase composites: Fabrication and creep evaluation

The compressive creep of silicon carbide fiber reinforced Ti₃SiC₂ MAX phase with both fine and coarse microstructure was investigated in the temperature range of 1000-1300°C. Comparison of only steady-state creep was done to understand the response of fabricated composite materials toward creep deformation. It was demonstrated that the fibers are more effective in reducing the creep rates for the coarse microstructure by an increase in activation energy compared to the variant with a finer microstructure, being partly a result of the enhanced creep rates for the microstructure with larger grain size. Grain boundary sliding along with fiber fracture appears to be the main creep mechanism for most of the tested temperature range. However, there are indications for a changed creep mechanism for the fine microstructure for the lowest testing temperature. Local pores are formed to accommodate differences in strain related to creeping matrix and predominantly elastically deformed fibers during creep. Microstructural analysis was done on the material before and after creep to understand the deformation mechanics.     

Mechanical and electrical properties of carbon nanofibers reinforced aluminum nitride composites prepared by plasma activated sintering

High density carbon nanofibers (CNFs) reinforced aluminum nitride (AlN) composites were successfully fabricated by plasma activated sintering (PAS) method. The effects of CNFs on the microstructure, mechanical and electrical properties of the AlN composites were investigated. The experimental results showed that the grain growth of AlN was significantly inhibited by the CNFs. With 2 wt.% CNFs added into the composites, the fracture toughness and flexural strength were increased, respectively to 5.03 MPa m^1/2 and 354 MPa, which were 20.9% and 13.4% higher than those of monolithic AlN. The main toughening mechanisms were CNFs pullout and bridging, and the main reason for the improvements in strength should be the fine-grain-size effect caused by the CNFs. The DC conductivity of the composites was effectively enhanced through the addition of CNFs, and showed a typical percolation behavior with a very low percolation threshold at the CNFs content of about 0.93 wt.% (1.51 vol.%).     

Fabrication,microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics

The fabrication, microstructure and high-temperature creep behavior of chemically compatible, three-phase alumina/erbium aluminum garnet (Er₃Al₅O₁₂, EAG)/erbia fully-stabilized cubic ZrO₂ (ESZ) particulate composites with the ternary eutectic composition is investigated. The composites were fabricated by a solid-state reaction route of α-Al₂O₃, Er₂O₃ and monoclinic ZrO₂ powders. The final phases α-Al₂O₃, EAG and ESZ were obtained after calcination of the powder mixtures at 1400 °C. High dense bulk composites were obtained after sintering at 1500 °C in air for 10 h, with a homogeneous microstructure formed by fine and equiaxed grains of the three phases with average sizes of 1 μm. The composites were tested in compression at temperatures between 1250 and 1450 °C in air at constant load and at constant strain rate. As the temperature increases, a gradual brittle-to-ductile transition was found. Extended steady states of deformation were attained without signs of creep damage in the ductile region, characterized by a stress exponent of nearly 2 and by the lack of dislocation activity and modifications in grain size and shape. The main deformation mechanism in steady state is grain boundary sliding, as found in superplastic metals and ceramics. In the semibrittle region, microcavities developed along grain boundaries; these flaws, however, did not grow and coalescence into macrocracks, resulting in a flaw-tolerant material. Alumina is the creep-controlling phase in the composite because of the grain boundary strengthening caused by the (unavoidable) Er³⁺- and Zr⁴⁺-doping provided by the other two phases.     

Surface modification of graphene oxide nanoplatelets and its influence on mechanical properties of alumina matrix composites

This paper discusses the effect of modified graphene oxide nanoplatelets (RGO-Al₂O₃) and unmodified graphene oxide nanoplatelets (GO) addition on the microstructure and mechanical properties of alumina matrix composites. The sinters were prepared by powder metallurgy processing using Spark Plasma Sintering to consolidate the powder mixtures. Moreover, the influence of applied reinforcing phase on the fracture mechanism was also investigated. Significant improvement of the fracture toughness (60%) for the composites with 0.5 wt.% RGO-Al₂O₃ compared to the reference sample was observed. Moreover, 20% higher K_IC was noticed for RGO-Al₂O₃ reinforced composites than for Al₂O₃-GO.     

Fabrication and creep properties of eutectic-composition Al2O3/YAG/YSZ sintered composites

Three-phase alumina/YAG/yttria-stabilized cubic zirconia (YSZ) composites were fabricated by a solid-state reaction route starting from commercial powders of Al₂O₃, Y₂O₃ and monoclinic ZrO₂. The final phases Al₂O₃, YAG and YSZ were obtained after calcination of the powder mixtures at 1400 °C. Dense bulk composites were obtained after sintering, with a homogeneous microstructure of fine and equiaxed grains with sizes of 1 μm. Compressive mechanical tests were performed at 1300–1450 °C in air at constant load and at constant initial strain rate. A brittle-to-ductile transition was found with increasing temperature. Grain boundary sliding is the main deformation mechanism in the ductile regime, characterized by a stress exponent of 2 and by the absence of dislocation activity and changes in grain morphology. Alumina seems to be the rate-controlling phase owing to the improvement in creep resistance by the presence of yttrium and zirconium of the other two phases.     

Abrasive wear of Al2O3–SiC and Al2O3–(SiC)–C composites with micrometer- and submicrometer-sized alumina matrix grains

The response of Al₂O₃, Al₂O₃–SiC–(C) and Al₂O₃–C nanocomposites to grinding was investigated in terms of changes of quality of ground surfaces and of the weight losses with time. The study used monolithic polycrystalline aluminas as references, and alumina-based composites with nanosized SiC and C inclusions and with alumina matrix grain size varying from submicrometer to approximately 4 μm. The studied materials can be roughly divided into two groups. Materials with submicrometer alumina matrix grains (Group 1) wear predominantly by plastic deformation and grooving. Coarse-grained materials (Group 2) wear by mixed wear mechanism involving crack initiation and interlinking accompanied by grain pull-out, plastic deformation and grooving. The wear rate of composites increases with increasing volume fraction of SiC. The Group 2 materials wear much faster then those with submicron microstructure. In all cases (with one exception) the wear resistance of composites was higher than that of pure aluminas of comparable grain sizes used as reference materials.     

不同分散剂对复掺GO/CNFs水泥基复合材料力学和导电性能的影响

新型碳纳米材料氧化石墨烯(GO)和纳米碳纤维(CNFs)在分散性良好的前提下可用于改善传统水泥基材料的性能.采用聚羧酸减水剂(PCs)、十二烷基硫酸钠(SDS)、十二烷基苯磺酸钠(SDBS)3种不同分散剂对复合GO和CNFs在水泥基材料中进行分散,研究分散剂种类对复掺GO/CNFs水泥基复合材料的力学及导电性能的影响,...     

Grain Size Effect on Creep Deformation of Alumina-Silicon Carbide Composites

A study of the flexural creep response of aluminas reinforced with 10 vol% SiC whiskers was conducted at 1200° and 1300°C at stresses from 50 to 230 MPa in air to evaluate the effect of matrix grain size. The average matrix grain size was varied from 1.2 to 8.0 μm by controlling the hot-pressing conditions. At 1200°C, the creep resistance of alumina composites increases with an increase in matrix grain size, and the creep rate (at constant applied stress) exhibits a grain size exponent of approximately 1. The stress exponent of the creep rate at 1200°C is approximately 2, consistent with a grain boundary sliding mechanism. On the other hand, the creep deformation rate of 1300°C was not sensitive to the alumina grain size. This was seen to be a result of enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increases. Observations also indicated that the prevalent site for nucleation and growth of creep cavities in coarsegrained materials is at two-grain junctions (grain faces), whereas in fine-grained materials cavities nucleate primarily at triple-grain junctions (grain edges). Electron microscopy studies revealed that the content of any amorphous phase present at whisker-alumina interfaces is independent of alumina grain size (and hot-pressing conditions). In addition, the alumina grain boundaries are quite devoid of amorphous phase(s). This variation in amorphous phase content does not appear to be a factor in the present creep results.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Enhanced dye photocatalysis and recycling abilities of semi-wrapped TiO2@carbon nanofibers formed via foaming agent driving

To enhance photocatalysis and recycling abilities of catalyst simultaneously, novel microstructure in which TiO₂ nanoparticles were semi-wrapped in carbon nanofibers (CNFs) was proposed and produced successfully. Betaine was employed as a foaming agent to drive the TiO₂ nanoparticles to migrate from inner space to the surface of CNFs gradually under the function of calcination. Various characterizations were used to research the surface and crystal evolution process of TiO₂ and CNFs, and the semi-wrapped microstructure of TiO₂@CNFs was well built up when the composites were carbonized at 800 °C. TiO₂ was immobilized stably on CNFs while exposed partly to air distinctively. This unique semi-wrapped structure endowed the composites with strong interfacial interaction between TiO₂ and CNFs, and the exposed section of TiO₂ provided sufficient reaction sites for organic dyes. Notably, photocatalytic degradation ratio of Rhodamine B in the first time reached 98.2% and even remained 95.4% after being recycled for 5 times under the irradiation of ultraviolet light, indicating that the photocatalysis and recycling abilities were obviously strengthened simultaneously compared with other counterparts reported in literature.     

A comparison of the effects of multi-wall and single-wall carbon nanotube additions on the properties of zirconia toughened alumina composites

The use of multi-wall carbon nanotubes (MWCNTs) or single-wall carbon nanotubes (SWCNTs) as filler in ceramic matrices could create composites with exceptional mechanical properties. We have prepared dense monolithic alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) composites with additions of 0.01 wt% of MWCNTs or 0.01 wt% of SWCNTs by conventional sintering and have demonstrated that the mechanical properties depend on (a) the distribution of CNTs in the matrix and (b) the interaction between the ceramic phases and CNTs. The fracture toughness of Al₂O₃ ceramics reinforced with SWCNTs was significantly better than those reinforced with MWCNTs. However, fracture toughness in MWCNT-reinforced ZTA increased 41% over ZTA free of the toughening agent and 44% over ZTA reinforced with SWCNTs. A well dispersed and small amount of MWCNTs was enough to produce an increase of fracture toughness in ZTA composites.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Effect of LaB6 additions on densification,microstructure, and creep with oxide scale formation in ZrB2-SiC composites sintered by spark plasma sintering

A comparative study has been carried out on densification, microstructure, and creep with oxide-scale formation in ZrB₂-20 vol.% SiC-(7, 10 or 14 vol.%) LaB₆ composite containing B₄C and C as additives, and prepared by spark plasma sintering at 1800 °C under 70 MPa ram pressure. Addition of LaB₆ has promoted densification of composites by scavenging oxygen impurity, thereby increasing their hardness. Constant load compressive creep tests at 1300 °C under 47 and 78 MPa stresses have shown the lowest creep rate in the 10 vol.% LaB₆ composite. The stress exponents obtained for composites having 10 vol.% LaB₆ (～1.3 ± 0.1) and 14 vol.% LaB₆ (～2.6 ± 0.2) suggest respectively, grain boundary diffusion with intergranular glassy phase formation and dislocation glide as operating mechanisms. Intergranular cracking caused by grain boundary sliding appears as the damage mechanism. Oxide scales formed during creep exhibit greater thickness and defect concentration than those by isothermal exposure at 1300 °C within similar duration.     

Effects of surface modification of carbon nanofibers on the mechanical properties of polyamide 1212 composites

In this study, the effects of carbon nanofiber (CNF) surface modification on mechanical properties of polyamide 1212 (PA1212)/CNFs composites were investigated. CNFs grafted with ethylenediamine (CNF‐g‐EDA), and CNFs grafted with polyethyleneimine (CNF‐g‐PEI) were prepared and characterized. The mechanical properties of the PA1212/CNFs composites were reinforced efficiently with addition of 0.3 wt % modified CNFs after drawing. The reinforcing effect of the drawn composites was investigated in terms of interfacial interaction, crystal orientation, crystallization properties and so on. After the surface modification of CNFs, the interfacial adhesion and dispersion of CNFs in PA1212 matrix were improved, especially for CNF‐g‐PEI. The improved interfacial adhesion and dispersion of CNFs in PA1212 matrix was beneficial to reinforcement of the composites. Compared with pure PA1212, improved degree of crystal orientation in the PA1212/CNF‐g‐PEI (CNF‐g‐EDA) composites was responsible for reinforcement of mechanical properties after drawing. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41424.     

Improvement of Vickers hardness measurement on SWNT/Al2O3 composites consolidated by spark plasma sintering

Dense alumina composites with different carbon nanotube content were prepared by colloidal processing and consolidated by Spark Plasma Sintering (SPS). Single-wall carbon nanotubes (SWNTs) were distributed at grain boundaries and also into agglomerates homogeneously dispersed. Carrying out Vickers hardness tests on the cross-section surfaces instead of top (or bottom) surfaces has shown a noticeable increase in the reliability of the hardness measurements. This improvement has been mainly attributed to the different morphology of carbon nanotube agglomerates, which however does not seem to affect the Vickers hardness value. Composites with lower SWNT content maintain the Vickers hardness of monolithic alumina, whereas it significantly decreases for the rest of compositions. The decreasing trend with increasing SWNT content has been explained by the presence of higher SWNT quantities at grain boundaries. Based on the results obtained, a method for optimizing Vickers hardness tests performance on SWNT/Al₂O₃ composites sintered by SPS is proposed.     

Mechanical properties of graphene oxide reinforced alumina matrix composites

Graphene oxide (GO) reinforced alumina matrix composites have been fabricated by using graphene oxide synthesized by a modified Hummer's method. Samples were prepared by powder metallurgy and consolidated by Spark Plasma Sintering (SPS). The influence of GO addition on the microstructure and mechanical properties of the composites was investigated. Results show a significant increase (almost 35%) of the fracture toughness for composites containing 0.5 wt% graphene oxide compared to sintered pure alumina. In order to find reasons for this improvement Scanning/Transmission Electron Microscopy (SEM/TEM) observations were carried out. They reveal a good interface between the reinforcement and the matrix as well as such mechanisms like branching, deflection and bridging of crack propagation.     

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al₂O₃ and Al₂O₃-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al₂O₃-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al₂O₃ (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.     

Deformation behaviour of iron-doped alumina

Compressive creep tests in air were carried out on 1 cat.% Fe-doped alumina at a temperature T=1400 °C. Iron doping affected the plastic deformation by different ways in relation with Fe²⁺ cations population. Fe²⁺ cations sped up the deformation rates. FeAl₂O₄ spinel precipitates were identified and they were found (i) to interact with alumina grain boundaries (ii) to limit the grain growth within a range of strain. The Fe²⁺ cations underwent oxidation and this resulted in the dissolution of the some precipitates and in the decrease of deformation rates. It was suggested that deformation sped up this evolution through mass transport and that time was not a dominating parameter.