An indentation model for erosive wear in Al2O3/SiC nanocomposites

The erosive wear resistance of Al₂O₃ has been shown to be improved by the addition of 5 vol.% of sub-micron sized SiC particles to form a ‘nanocomposite’, in agreement with previous results. The erosive wear was measured directly, and also estimated by an indentation model consisting of closely spaced grids of indentations that mimic the effect of successive particle impacts; in the model, particle impacts cause sub-surface cracking but loss of material from the surface occurs only from an impact within a region damaged by a previous impact. The volume of material lost from within indentation grids was used to predict the wear rate. These predictions agreed well with the directly measured values.The commonly observed change in fracture mode from intergranular for Al₂O₃ to transgranular for nanocomposites was confirmed. Transgranular fracture can allow a smaller volume of material to be removed during an impact and hence increase erosion resistance.     

Correlation between microstructure and mechanical properties of Al2O3/ZrO2 nanocomposites

The aim of this work was to correlate the microstructure of alumina matrix nanocomposites containing 1, 3, and 5 vol% of monoclinic zirconia nanometric particles with the mechanical properties and wear resistance of these composites. The microstructural analysis showed the beneficial effect of the zirconia particles in the alumina matrix regarding grain growth and improvement of the properties: up to 8% for the microhardness, 11% for the flexural strength and 23% for the wear resistance for nanocomposites containing 5 vol% of particles when compared to inclusion-free alumina.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

Influence of C doping on the fracture mode and abrasive wear of Al2O3

Previous studies have shown that the addition of SiC nanoparticles to Al₂O₃ changes the fracture mode from intergranular to transgranular and in doing so improves the wear resistance. The reason for this is not clear but a change to the grain boundary chemistry caused by impurities such as C added with the SiC may be involved. The aim of the current study was to investigate the influence of small amounts of C doping on the fracture mode and wear properties of Al₂O₃. The microstructure and properties of Al₂O₃ doped with 0 and 0.012 wt% C were studied. Al₂O₃ showed mainly intergranular fracture. The addition of 0.012 wt% C to Al₂O₃ changed the fracture mode to mainly transgranular. The wear resistance improved and the percentage of surface grains pulled out was lower compared to pure Al₂O₃.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Microstructure and mechanical properties of hot pressed Al2O3/SiC nanocomposites

Al₂O₃/SiC micro/nano composites containing different volume fractions (5, 10, 15, and 20 vol.%) of SiC were prepared by mixing a sub-micron alumina powder with respective amounts of either micro- or nano-sized silicon carbide powders. The powder mixtures were hot pressed 1 h at 1740 °C and 30 MPa in the atmosphere of Ar. The effect of SiC addition on the microstructure and mechanical properties, i.e. hardness, fracture toughness, and room temperature flexural strength were investigated. The flexural strength increased with increasing volume fraction of silicon carbide particles. The maximum flexural strength (655 ± 90 MPa) was achieved for the composite containing 20 vol.% of coarse-grained SiC, which is more than twice as high as in the Al₂O₃ reference. Hardness and fracture toughness were also moderately improved. The observed improvement of mechanical properties is mainly attributed to alumina matrix grain refinement and grain boundary reinforcement.     

Mechanical properties and fracture behavior of fibrous Al2O3/SiC ceramics

Fibrous Al₂O₃ ceramics with a mixture of SiC and Al₂O₃ as the cell boundaries were fabricated by extrusion-molding and hot-pressing techniques. The effects of the cell boundary composition on the mechanical properties and fracture behavior are investigated. It is shown that a 65:35 mixture of SiC:Al₂O₃ can act as a suitable cell boundary for Al₂O₃ cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and apparent toughness are up to 1349 J/m² and 6.0 MPa m^1/2, respectively.     

Thermal microstress measurements in Al2O3/SiC nanocomposites by Cr3+ fluorescence microscopy

Alumina/SiC ‘nanocomposites’ show significant property improvements compared with pure alumina. The improvements are thought to stem at least in part from the microstresses caused by the thermal expansion mismatch between alumina and SiC. These microstresses have been measured previously by neutron and X-Ray diffraction. This paper reports stress measurements using Cr³⁺ fluorescence microscopy of the alumina matrix. The results show that although fluorescence microscopy is less powerful than the diffraction techniques in terms of the range of information provided, it does provide an alternative method of measuring subsurface microstresses in these materials which is quicker, cheaper and higher in spatial resolution.     

Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites

Dry ceramic block-on-steel ring wear tests were performed at high loads in several Al₂O₃/20 vol.% SiC composites as a function of the SiC grain size, which ranged from 0.2 to 4.5 μm in d₅₀. The wear resistance of the monolithic alumina was radically improved by the addition of the SiC particles, reducing down to one order of magnitude wear rate. Two different behaviours were identified according to the microstructural observations on the worm surfaces: intergranular fracture and grain pull-out in the monolithic Al₂O₃, and plastic deformation and surface polishing in the composites. The wear resistance of the Al₂O₃/SiC composites increased with the SiC grain size due to their fracture toughness enhancement.     

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.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

Indentation size effect and wear characteristics of spark plasma sintered,hard MWCNT/Al2O3 nanocomposites

Magnesia doped multiwalled carbon nanotube (CNT)/α-alumina nanocomposites have been fabricated by spark plasma sintering at 1500°C under 50 MPa in argon. Owing to combined grain refining effect of nanotube and magnesia, nanocomposites possessed smaller matrix grains and extensively lower matrix crystallites than pure alumina. Thermal expansion mismatch between matrix and filler rendered up to four times higher compressive lattice microstrain to the nanocomposites over pure alumina. Despite very low CNT loading (e.g. 0·13?wt-%), nanocomposites offered considerably higher hardness (as high as 24·42?GPa), negligible indentation size effect (Meyer exponent?=?1·906???1·941) and enhanced elastic response over pure alumina. Up to 0·27?wt-% nanotube loading, much higher wear resistance was observed for the nanocomposites over pure alumina. The presence of uniformly dispersed and structurally intact nanotubes coupled with lower matrix grains and crystallites having compressive lattice strain were the key factors behind achieving such improved mechanical properties of the present nanocomposites.     

Relationship between microstructure and abrasive wear resistance of Al2O3-FeAl2O4 nanocomposites produced via solid-state precipitation

The work investigates the correlation between the microstructure and wear behaviour of novel Al₂O₃-FeAl₂O₄ nanocomposites, developed by precipitation of FeAl₂O₄ particles through reduction aging of Al₂O₃-10 wt.% Fe₂O₃ solid solutions in N₂/4%H₂. Reduction aging at 1450 °C for 10 and 20 h resulted in considerable improvements in abrasive wear resistance. The nanocomposites developed from solid solutions doped additionally with ∼250 ppm of Y₂O₃ contained finer intergranular second phase particles (by a factor of ∼2) and showed further improvements in the wear resistance. Doped nanocomposites reduction aged for 20 h at 1450 °C exhibited the minimum wear rate (reduced by a factor of ∼2.5 with respect to monolithic Al₂O₃). The suppression of fracture-induced surface pullout in the presence of intragranular nanosized second phase particles was the major factor responsible for the improved wear resistance of the nanocomposites with respect to monolithic alumina; microstructures without these intragranular nanoparticles showed no improvement. Higher aging temperature led to the presence of coarse (>2 μm) intergranular FeAl₂O₄ particles which had a detrimental effect on the wear resistance.     

多孔Al2O3/SiC、MoSi2/SiC复合材料的制备及吸波性能

以Si、Al₂O₃、MoSi₂微粉和生物竹材为原料,采用包埋烧结法分别制备出SiC多孔材料、Al₂O₃/SiC、MoSi₂/SiC复合材料。采用XRD、SEM及波导法测试其物相组成、显微结构及吸波性能。结果表明：MoSi₂/SiC复合材料的厚度为2 mm时有明显的吸波特性,有效吸收带宽在X波段的9.65~12.4 GHz频率范围内达2.75 GHz,且最低反射损耗为-38.27 dB。Al₂O₃/SiC复合材料孔道内的Al₂O₃与SiC晶须交缠,形成大量电偶极矩,产生介电损耗;MoSi₂/SiC复合材料除介电损耗外还存在电阻损耗,使得复合材料电磁损耗增加,是较有前途的结构功能吸波材料。     

Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites

Alumina/polymethylmethacrylate (PMMA) nanocomposites were produced by incorporating alumina nanoparticles, synthesized using the forced gas condensation method, into methylmethacrylate. The particles were dispersed using sonication and the composites were polymerized using free radical polymerization. At an optimum weight percent, the resulting nanocomposites showed, on average, a 600% increase in the strain-to-failure and the appearance of a well-defined yield point when tested in uniaxial tension. Concurrently, the glass transition temperature (T_g) of the nanocomposites dropped by as much as 25°C, while the ultimate strength and the Young's modulus decreased by 20% and 15%, respectively. For comparison, composites containing micron size alumina particles were synthesized and displayed neither phenomenon. Solid-state deuterium NMR results showed enhanced chain mobility at room temperature in the nanocomposites and corroborate the observed T_g depression indicating considerable main chain motion at temperatures well below those observed in the neat polymer. A hypothesis is presented to relate the thermal and mechanical behavior observed in the composites to the higher chain mobility and T_g depression seen in recent ultrathin polymer film research.     

Effects of Y2O3 additives and powder purity on the densification and grain boundary composition of Al2O3/SiC nanocomposites

Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al₂O₃. However, these additions also restrict sintering.Al₂O₃ and Al₂O₃–5%SiC ‘nanocomposites’ were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y₂O₃. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al₂O₃–SiO₂ composites was also made, as it has been proposed that SiC partially oxidises during processing of Al₂O₃–SiC nanocomposites.The addition of 5 vol.% SiC to Al₂O₃ hindered densification, as did addition of 0.15 wt.% Y₂O₃ or 0.1 wt.% SiO₂. In contrast, the addition of 0.15 wt.% Y₂O₃ to Al₂O₃–5% SiC nanocomposites improved densification.The composition of Al₂O₃–Al₂O₃ grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO₂ caused segregation of Si, and Y₂O₃ addition caused segregation of Y. The segregation of each element was equivalent to <10% of a monolayer at the grain boundary. However, if SiC and Y₂O₃ were simultaneously added the segregation increased to 40% of a monolayer. The enhanced segregation was attributed to increased oxidation of SiC in the presence of Y₂O₃ allowing formation of a SiO₂–Al₂O₃–Y₂O₃ eutectic phase or a segregated layer which may explain the improvement in sintering rate when Y₂O₃ was added to nanocomposites.     

The relationship between microstructure and quality factor for (Al,Mg,Ta)O2 microwave dielectrics

The relationship between microstructures and quality factor (Q) of (1−x)(Al_1/2Ta_1/2)O₂–x(Mg_1/3Ta_2/3)O₂ ceramics was investigated. The extrinsic loss of microwave dielectrics depended on cations ordering, grain size, and porosity. (Al_1/2Ta_1/2)O₂ has a disordered structure and (Mg_1/3Ta_2/3)O₂ has an ordered trirutile structure. As (Mg_1/3Ta_2/3)O₂ content increased, (1−x)(Al_1/2Ta_1/2)O₂–x(Mg_1/3Ta_2/3)O₂ ceramics revealed an ordered phase and were of single phase for x>0.6. The increase of the ordered phase and grain size enhanced the Q. When ordering was completed at (Mg_1/3Ta_2/3)O₂ concentration over 60 mol%, the grain size was a major factor in the increase the Q value. In contrast the porosity degraded the Q value. Therefore, the Q value depended on order/disorder, the porosity, and the grain size in that order.     

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al₂O₃/SiC. Feedstock Al₂O₃/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al₂O₃ powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al₂O₃ transformed to metastable γ- and δ-Al₂O₃ phases during plasma spraying. Other minority phases in the sol-gel Al₂O₃/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.     

Liquid phase formation in the system SiC,Al2O3, Y2O3

The liquid phase formation in the system SiC–Al₂O₃–Y₂O₃ was investigated via differential thermal analysis (DTA) combined with thermogravimetry (TG). For this purpose mixtures of various alumina and yttria mol ratios and 10 and 20 mol% silicon carbide were densified and heat treated at different temperatures. It was shown that silicon carbide in the examined amounts has low influence on the melting temperature of the oxide phase. The compositions and microstructures formed were studied by SEM, EDX and XRD. The results were compared to thermodynamic calculations.