Preparation and mechanical properties of alumina composites reinforced with nickel-coated graphene

This paper discusses the influence of nickel–phosphorus coated graphene (Gn–Ni–P) and uncoated graphene (Gn) addition to an alumina matrix and its impact on the mechanical properties of obtained composites. The composites are prepared via powder processing and consolidated using the Spark Plasma Sintering (SPS) method. The effects of the addition of coated graphene and coating thickness on mechanical properties were evaluated. Physical properties such as relative density, hardness and fracture toughness were analyzed. Significant improvement of the fracture toughness (60%) for the composites with 2 vol% Gn–Ni–P compared to reference sample was observed. Moreover, 35% higher K_IC was noticed for Gn–Ni–P reinforced composites than for Al₂O₃–Gn.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

SiC and ZrN nano-particulate reinforced AlON composites: Preparation,mechanical properties and toughening mechanisms

Due to excellent chemical stability, high rigidity, superior corrosion and wear resistance, aluminum oxynitride (AlON) has been considered as one of most promising candidate ceramic materials in high-performance structural, advanced abrasives and refractory fields. However, it usually exhibited relatively low flexural strength and poor fracture toughness. The study is aimed to develop silicon carbide (SiC) and zirconium nitride (ZrN) nano-particulate reinforced AlON composites with improved mechanical properties and fracture resistance via a hot-press sintering process. It was found that the addition of ZrO₂ nanoparticles would be transformed into ZrN during sintering. Due to the pinning effect of SiC and ZrN nano-particles positioned at grain boundaries of micro-sized AlON particles, the presence of SiC and ZrN nano-particles resulted in the reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in AlON to intragranular cracking in composites. With presence of 8 wt% SiC and 5.2 wt% ZrN nano-particles, the relative density, microhardness, Young’s modulus, flexural strength and fracture toughness increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in flexural strength and fracture toughness.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

The effects of phenolic resin-derived PyC interlayers on microstructure and mechanical properties of Cf/SiC composites

A novel, easy and cost-effective way, infiltration and pyrolysis of phenolic resin solution, was exploited to prepare pyrolytic carbon (PyC) interlayers for carbon fiber/silicon carbide (Cf/SiC) mini-composites. X-ray photoelectron spectroscopy, dynamic contact angle measurement and scanning electron microscope were carried out to characterize chemical structure of carbon fibers (CFs), wetting properties between CFs and phenolic resin solution and microstructure of CFs and their composites, respectively. Remarkably, SEM results showed regulation of uniformity and thicknesses of PyC interlayer could be achieved through controlling the concentration of phenolic resin solution and oxidation condition of CFs. When CFs were treated by 10?min' oxidation with 40?mg/L ozone followed by dip-coating with 4?wt% phenolic solution, uniform PyC interlayer with approximately 120?nm were prepared on CFs. The corresponding Cf/SiC specimens had the largest increase in tensile strength and work of fracture with the improvement of 26.2% and 71.6% from the PyC-free case.     

The effect of carbon nanotubes introduction on the mechanical properties of reaction bonded boron carbide ceramics

The influence of carbon nanotubes (CNTs) on the mechanical properties and structure formation during reactive sintering of B₄C materials with Si addition was studied. Upon infiltrating the B₄C structure with molten silicon, a non-porous composite was formed with a density of 2.45-2.55 g/cm³ and a hardness of 22-27 GPa. The formation of highly dispersed B-C-Si phases was observed in the interphase of adjacent B₄C particles due to the incorporation of Si into B₄C structures. These phases increase the bonding strength between B₄C particles. In spite of the fact that the addition of 1-5 wt% Multi-Walled Carbon Nanotubes decreases the green density of the compacts, the flexural strength of the infiltrated material significantly increased. The improvement of the strength of ceramics modified with MWCNTs was interpreted in terms of the formation of thin flattened SiC crystals at the interfaces between B₄C and B-C-Si particles, which strengthen the interfaces between ceramic particles.     

The effect of interfacial bonding of calcium phosphate cements containing bio-mineralized multi-walled carbon nanotube and bovine serum albumin on the mechanical properties of calcium phosphate cements

The aim of this study was to investigate the effect of adding bio-mineralized hydroxyl functionalized multi-walled carbon nanotubes (MWCNTs–OH) on the compressive strength of calcium phosphate cements (CPCs). Bovine serum albumin (BSA) was also incorporated as a protein which acts as promoter of hydroxyapatite (HA) crystal growth when bounded to CPC granules. The results show that the strong interfacial bonding of CPC/MWCNTs–OH is essential to improve the mechanical properties of CPC/bio-mineralized MWCNTs–OH/BSA composite.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

Sintering effect on mechanical properties of composites of natural hydroxyapatites and titanium

This study presents the fabrication and characterization of composite materials of hydroxyapatite and Ti. Hydroxyapatite (HA) powder was obtained from bovine bones (BHA) and human enamel (EHA) via calcination technique. Fine powders of HA were admixed with 5 and 10 wt.% fine powder of metallic Ti. Powder-compacts were sintered at different temperatures between 1000 and 1300 °C. Compression strength, Vickers microhardness and elastic modulus as well as density were measured. SEM and X-ray diffraction studies were also conducted. The experimental results showed that addition of Ti to EHA and BHA decreases the elastic modulus, comparing to samples of pure BHA. The best mechanical properties for BHA–Ti composites were obtained after sintering in the range of 1200–1300 °C and for EHA–Ti composites in the range of 1100–1300 °C.     

Structure and mechanical properties of hot-pressed B4C-NdB6 composites

High-performance B₄C-NdB₆ composites were fabricated by hot-pressing sintering at the temperature of 2050 °C for 20 min holding time and 20 MPa pressure with Nd₂O₃ (1～4 wt%) as the aiditive. The effects of Nd₂O₃ on the sintering process of the B₄C were studied. The reaction mechanisms of B₄C and Nd₂O₃ at different temperature were investigated. Based on the results of TG-DSC and thermodynamic calculation,. NdB₆ was formed via Nd₂O₃ react with B₄C in the sintering process, which greatly enhanced the densification of B₄C and promoted the sintering process. The flexural strength, fracture toughness and hardness of the B₄C-NdB₆ composites rose to 366.42 MPa, 5.27 MPa m^1/2 and 38 GPa by adding 3 wt% Nd₂O₃, respectively. The coexistence of transgranular and intergranular fracture is the major fracture mode. The phenomenon of pull-out contributed to improvement of the fracture toughness.     

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si₃N₄) ceramics were fabricated using a mixture of Si₃N₄ and silicon (Si) powders via conventional processing and sintering method. These Si₃N₄ ceramics with sintering additives of ZrO₂ + Gd₂O₃ + MgO were sintered at 1800 °C and 0.1 MPa in N₂ atmosphere for 2 h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si₃N₄ samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39 g/cm³ to 2.92 g/cm³ except for the sample without initial Si₃N₄ powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si₃N₄ ceramics with a good combination of mechanical and thermal properties.     

Technological properties of celsian reinforced glass matrix composites

Monoclinic celsian derived from an innovative route, i.e. cation exchanged zeolites heat-treated at low temperature, was added at different contents (10, 20, 30 wt%) to a glass matrix, in order to improve its mechanical and electrical performances. The effect of the celsian reinforcement was evaluated by testing several properties of the composite materials, such as the elastic modulus, abrasion resistance, flexural strength and electrical insulation. The results so far obtained suggest that the addition of the monoclinic celsian to the glass matrix may produce low-cost particulate composites with interesting technological properties.     

Structure and mechanical properties of aluminosilicate geopolymer composites with Portland cement and its constituent minerals

The compressive strengths and structures of composites of aluminosilicate geopolymer with the synthetic cement minerals C₃S, β-C₂S, C₃A and commercial OPC were investigated. All the composites showed lower strengths than the geopolymer and OPC paste alone. X-ray diffraction, ²⁹Si and ²⁷Al MAS NMR and SEM/EDS observations indicate that hydration of the cement minerals and OPC is hindered in the presence of geopolymer, even though sufficient water was present in the mix for hydration to occur. In the absence of SEM evidence for the formation of an impervious layer around the cement mineral grains, the poor strength development is suggested to be due to the retarded development of C-S-H because of the preferential removal from the system of available Si because geopolymer formation is more rapid than the hydration of the cement minerals. This possibility is supported by experiments in which the rate of geopolymer formation is retarded by the substitution of potassium for sodium, by the reduction of the alkali content of the geopolymer paste or by the addition of borate. In all these cases the strength of the OPC-geopolymer composite was increased, particularly by the combination of the borate additive with the potassium geopolymer, producing an OPC-geopolymer composite stronger than hydrated OPC paste alone.     

Effects of copper on microstructure and mechanical properties of Cf/ZrC composites fabricated by low-temperature liquid metal infiltration

Carbon fiber-reinforced zirconium carbide matrix (C_f/ZrC) composites were fabricated by a liquid metal infiltration process at 1200 °C, using low melting Zr₇Cu₁₀, ZrCu and Zr₂Cu alloys as infiltrators. The effects of Cu on microstructure and mechanical properties of the composites were investigated. The results indicated that the products were composed of either single- or polycrystalline ZrC, C and Cu. With increasing Cu content in the infiltrators, the yield of ZrC decreased from 43.7 vol% to 27.9 vol%. When ZrCu was used as an infiltrator, the obtained composites exhibited a better bending strength of 98.2±3.1 MPa. What is more, the use of Zr₂Cu could provide the highest fracture toughness of the composites with a moderate debonding.     

Effect of thickness of SiC films on compression and thermal properties of SiC/CF composites

Commonly, carbon foam derived from commercially available melamine foam showed brittle characteristics. In this paper, the carbon foam was prepared via the direct carbonization of the melamine foam, and chemical vapor deposition was employed to deposit ultra-thin SiC films on the CF skeleton. The evolution, microstructure, mechanical strength, and thermal properties of the as-prepared SiC/CF composites were investigated. Test results showed that a novel SiC skeleton with a three-dimensional interconnected network was prepared successfully. The thickness of the SiC filmes had a significant influence on the compression and thermal properties of the composites. The SiC/CF-II possessed a higher compression performance than that of SiC/CF-I, while the thermal insulation was relatively much poorer. This present work had some reference meaning to the correlation studies of the thermal insulation material for the potential applications while bearing live loads.     

The microstructural and mechanical properties of geopolymer composites containing glass microfibres

This paper presents the effects of microfibre contents on mechanical properties of fly ash-based geopolymer matrices containing glass microfibres at 0, 1, 2 and 3 mass%. The influence of glass microfibres on the fracture toughness, compressive strength, Young's modulus and hardness of geopolymer composites are reported, as are the microstructural properties investigated using scanning electron microscopy. Results show that the addition of 2 mass% glass microfibres was optimal, exhibiting the highest levels of fracture toughness, compressive strength, Young's modulus and hardness. The results of the microstructural analysis indicate that the glass microfibres act as a filler for voids within the matrix, making a dense geopolymer and improving the microstructure of the binder. This leads to favourable adhesion of the composites, and produces a geopolymer composite with good mechanical properties, comparable to pure geopolymer. The failure mechanisms in glass microfibre-reinforced geopolymer composites are discussed in terms of microstructure.     

Effects of carbonyl nickel powder addition on the mechanical properties,microstructure, and yttrium segregation of sintered 3YSZ composites

In this study, the effects of the addition of carbonyl nickel powder on the density, microstructure, and mechanical properties of sintered yttria-stabilized zirconia (3YSZ) were investigated. Sintering at 1300 °C resulted in the optimum comprehensive mechanical properties. The addition of 5 vol% carbonyl Ni increased the fracture toughness and flexural strength from 9.51 MPa m^1/2 to 14.5 MPa m^1/2 and from 747 MPa to 873 MPa, respectively. The addition of carbonyl nickel showed greater improvement than did the addition of spherical Ni powder. The dendritic morphology improved the interface bonding between the ceramic and the metal, enabling a bridging mechanism of the ductile phase. However, further Ni addition decreased the mechanical properties. X-ray diffraction results showed that the amounts of the monoclinic phase (M) and cubic phase (C) of 3YSZ increased, whereas the amount of the tetragonal phase (T) decreased. The Y segregation near the Ni particles, which was confirmed by an energy dispersive spectrometer (EDS), caused the phase changes. The segregation of Y occurred during the cooling stage, rather than the holding stage, of sintering. During the cooling stage, the heat mismatch between Ni and ZrO₂ resulted in strong elastic strain energy, which promoted Y segregation.     

Microstructure and properties of (SiC, TiB2)/B4C composites by reaction hot pressing

(SiC, TiB₂)/B₄C composites were fabricated by reactive hot-pressing B₄C, Si₃N₄, -SiC and TiC powders, with (Al₂O₃ + Y₂O₃) as sintering additives. According to the thermodynamics principles, the possible reaction equations and the reaction products for the system were determined. By means of XRD, SEM of surface thermally etched and TEM the phase composition was determined. It was shown that the phase composition of sintered body was B₄C, -SiC, BN and TiB₂, and the matrix was B₄C and -SiC. The typical values of hardness, bending strength, fracture toughness and the relative density of the composites can reach HRA 88.6, 554 MPa, 5.6 MPa m^1/2 and 95.6%, respectively. Furthermore, the microstructures of the composites were analyzed by TEM, SEM and energy spectrum methods. The results show the presence of laminated structure and a clubbed frame dispersion phase and bunchy dispersion phase among the matrix. Some intragranular structures were also found in the B₄C grains. Microstructural analysis indicates that the new formed phase, uniform and fine grains, and the layered and clubbed structure play an important role in improving the properties of the composites. Fractography and crack propagation suggest that crack deflection and crack bridging are the possible toughening mechanisms.     

O＇－Sialon－ZrO_2复合材料的显微结构与力学性能的研究

研究了Ｏ＇－Ｓｉａｌｏｎ－ＺｒＯ２复合材料的显微结构与力学性能的关系。结果表明，Ｏ＇－Ｓｉａｌｏｎ形成连续网络编织状结构。ＺｒＯ２加入量较少时充当填充结构骨架的作用；ＺｒＯ２加入量增多时（至４０％），会有更多的ＺｒＯ２形成聚集体。随着ＺｒＯ２引入量的增加，材料的常温抗折强度提高，但高温抗折强度下降。Ｏ＇－Ｓｉａｌｏｎ的编织状结构可能阻碍晶界滑移。这种复合材料的高温抗折强度在１４００℃为１１２～１７３ＭＰａ。     

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.%).