Simultaneous improvement of the strength and fracture toughness of MgO-Al2O3-SiO2 glass/Mo composites by the microdispersion of flaky Mo particles

The effect of shape and volume percent of Mo particles on theflexural strength and fracture toughness of MgO-Al₂O₃-SiO₂(MAS) glass/Mo composites was investigated. The flexural strengthand fracture toughness of composites depends heavily on Mo particleshapes, and there is greater improvement in composites reinforcedwith flaky rather than massive Mo particles. In the compositesreinforced with flaky Mo particles, fracture toughness increases withvolume percent of Mo and, at 50 vol% Mo, is 11.6 MPam,which is approximately 6.7 times higher than that of the matrix. Increases in fracture toughness of composites reinforced with flakyMo particles is greater than with SiC whiskers, SiC platelets, SiC particles or ZrO₂ particles. Fabricating composites reinforcedwith flaky Mo particles is an effective toughening technique capableof simultaneously improving the strength and toughness of brittlematerials, such as monolithic Al₂O₃ and MAS glass, by utilizing plastic deformation of ductile phase.     

Crack growth resistance (R-curve) behaviour and thermo-physical properties of Al2O3 particle-reinforced AlN/Al matrix composites

Crack growth resistance behaviour and thermo-physical properties of Al₂O₃ particle-reinforced AlN/Al matrix composites have been studied as a function of AlN volume fraction as well as Al₂O₃ particle size. The fracture toughness of the composites decreased with increase in vol% AlN and decrease in Al₂O₃ particle size. All the composites exhibited R-curve behaviour which has been attributed to crack bridging by the intact metal ligaments behind the crack tip. The Young’s modulus of the composites increased with the vol% of AlN whereas the thermal diffusivity and coefficient of thermal expansion followed a reverse trend. The composites exhibited hysteresis in thermal expansion as a function of temperature and the hysteresis decreased with decrease in metal content of the composite.     

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al₂O₃–SiO₂ (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO₂ particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

Addition effects of aluminum and in situ formation of alumina in MoSi2

Phase composition change and mechanical properties at room temperature of MoSi₂ materials with the addition of aluminum were investigated. An explanation was given to the appearance of Mo₅Si₃ when hot pressing MoSi₂ raw powder containing oxygen. The mechanical properties including Vickers hardness, bending strength and fracture toughness were improved with the addition of aluminum up to the limit content needed for absorbing the oxygen in MoSi₂ raw powder. More aluminum addition than the limit content (in this study it is 5 wt %) will result in the formation of Mo(Si ,Al)₂ and Si. The in situ formed Al₂O₃ could act as a crack pinning element. However, because the thermal expansion coefficients of Al₂O₃ and MoSi₂ are near and there is a strong bonding between them, the toughening effect by such in situ formed small Al₂O₃ particles (less than 2 m) is limited.     

The effect of particulate loading on the mechanical behaviour of Al2O3/Al metal-matrix composites

An investigation was made to determine the effect of particulate loading on the elastic, tensile, compressive and fracture properties of Al₂O₃/Al metal-matrix composites fabricated by a pressureless-liquid-metal-infiltration process. The elastic modulus was found to be strongly affected by the reinforcement content, falling within the Hashin-Shtrikman bounds. The Young's modulus of the most highly loaded composite was 170 GPa; compare with 65 GPa for the unreinforced alloy. The strength systematically increased with loading, and the rate of increase also increased with loading. The measured yield strengths were nominally the same in both tension and compression; however, the composites possessed far greater ultimate strengths and strains-to-failure in compression than in tension. At 52 vol % reinforcement, yield strengths in tension and compression of 491 and 440 MPa, respectively, were measured, whereas the associated ultimate strengths were 531 and 1035 MPa, respectively. In tension, the yield and ultimate strengths of the base alloy were found to be 170 and 268 MPa, respectively. The composites displayed a nearly constant fracture toughness for all particulate loadings, with values approaching 20 MPa m^1/2 compared to a value of 29 MPa m^1/2 for the base alloy. Using fractography, the tensile-failure mechanism was characterized as transgranular fracture of the Al₂O₃ particles followed by ductile rupture of the Al-alloy matrix, with no debonding at the matrix/reinforcement interfaces.     

Influence of cobalt phase on thermal shock resistance of Al2O3-TiC composites evaluated by indentation technique

Cobalt-coated Al₂O₃ and TiC powders were prepared using an electroless method to improve resistance to thermal shock. The mixture of cobalt-coated Al₂O₃ and TiC powders (about 70 wt.% Al₂O₃-Co + 30 wt.% TiC-Co) was hot-pressed into an Al₂O₃-TiC-Co composite. The thermal shock properties of the composite were evaluated by indentation technique and compared with the traditional Al₂O₃-TiC composite. The composites containing 3.96 vol.% cobalt exhibited better resistance to crack propagation, cyclic thermal shock and higher critical temperature difference (ΔT_c). The calculation of thermal shock resistance parameters (R parameters) shows that the incorporation of cobalt improves the resistance to thermal shock fracture and thermal shock damage. The thermal physic parameters are changed very little but the flexure strength and fracture toughness of the composites are improved greatly by introducing cobalt into Al₂O₃-TiC (AT) composites. The better thermal shock resistance of the composites should be attributed to the higher flexure strength and fracture toughness.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.     

Mechanical reinforcement of unsaturated polyester by AL2O3 nanoparticles

Nanometer-sized Al₂O₃ particles (15 nm average diameter) were used as reinforcements to enhance the fracture toughness of a highly crosslinked, nominally brittle, thermosetting-unsaturated polyester resin. It was observed that the addition of untreated, as-received Al₂O₃ particles does not result in enhanced fracture toughness. Instead, the fracture toughness decreases by 15% as the volume fraction of the particles is increased from 0% to 4.5%. Similar degradation in fracture toughness was observed for reinforcement by 1- and 35-μm Al₂O₃ particles. The lack of reinforcement was attributed to poor particle-matrix bonding, as observed from scanning electron micrographs of the fracture surfaces. However, considerable reinforcement was observed when the nanocomposites were fabricated using an organofunctional silane to enhance particle-matrix interface strength. For the case of a 4.5% volume fraction of well-bonded Al₂O₃ particles added to the unsaturated polyester, the fracture toughness was increased by almost 100%.     

Mechanical properties of hot-pressed SiC particulate-reinforced Al2O3 matrix

The effect of SiC particulate dispersoids on the fracture toughness and strength of hot-pressed Al₂O₃-based composites was evaluated. Addition of 20 vol % SiC particulates was found to increase both the fracture toughness and strength of Al₂O₃. The relationships between mechanical properties and SiC additions are discussed.     

Room temperature mechanical properties and high temperature oxidation behavior of MoSi2 matrix composite reinforced by adding La2O3 and Mo5Si3

MoSi₂ matrix composites containing 0.8 wt.%La₂O₃ and different volume fractions of Mo₅Si₃ were synthesized by self-propagating high temperature synthesis. The room temperature mechanical properties and high temperature oxidation behavior at 1200 °C were studied. Results showed that La₂O₃ and Mo₅Si₃ caused the grain size to decrease of the MoSi₂ matrix composite. The flexure strength and fracture toughness are improved compared with pure MoSi₂. The strengthening mechanism of La₂O₃–Mo₅Si₃/MoSi₂ is fine-grain strengthening, and its toughening mechanisms are fine-grain toughening, crack deflection, crack branching and crack bridging. With an increase in Mo₅Si₃ content, the oxidation resistance gradually decreased. This is attributed to the poor oxidation resistance of Mo₅Si₃, grain refinement and relative density decrease of the composites. In this experiment, a La₂O₃–Mo₅Si₃/MoSi₂ composite was found to have optimal mechanical properties and high temperature oxidation resistance after adding 0.8 wt.%La₂O₃ and 16.3 wt.% Mo₅Si₃.     

Effect of Al2O3 and Fe3Al on the phase formation and mechanical properties of β-sialon

The paper evaluated the mechanical properties of β-sialon composites prepared by hot-pressing sintering at 1600 °C in N₂ atmosphere using α-Si₃N₄, Al₂O₃, Y₂O₃ and Fe₃Al as raw materials. The influence of Al₂O₃ and Fe₃Al content on flexure strength, fracture toughness, hardness, and relative density was investigated. And phase formation and morphology of the composites were characterized by X-ray diffraction and electron microscopy. The experimental results indicate that the raw material Fe₃Al reacts with α-Si₃N₄ to form silicides at elevated temperature, and supplies more liquid phase to assist densification. Besides, the variation of flexure strength, fracture toughness and hardness is mainly consistent, and also in good agreement with the relative density measurements. The values all increase firstly, and then decrease when the Al₂O₃ content increases. Scanning electron microscopy illustrates that the metal particles act to inhibit the crack propagation.     

Microstructure and mechanical properties of chromium and chromium/nickel particulate reinforced alumina ceramics

A range of Al₂O₃-Cr and Al₂O₃-Cr/Ni composites have been made using either pressureless sintering in the presence of a graphite bed or hot pressing. Examination of the microstructures shows that they are fully dense (typically 98–99% of the theoretical density) and that the micrometre-scale metallic particles remain discrete and homogeneously dispersed in all composites. All of the hot pressed specimens had higher flexural strengths than the sintered materials. Within each processing route, the composites had slightly lower strength values than the equivalent monolithic alumina specimens. This was attributed to weak interfacial bonding. Fracture toughness behaviour was investigated using indentation and double cantilever beam methods. All of the composites were found to be tougher than the parent alumina and to show resistance-curve behaviour. For the composites, maximum fracture toughness values were 5–6 MPa m^1/2 (about double the value for alumina) for process zone sizes of a few millimetres, although steady state was not reached in the limited number of specimens tested. Examination of fracture surfaces and indentation cracks showed that the toughening potential of the metal particles was not exploited to any significant extent. This was mainly due to weak metal-Al₂O₃ interfaces, but also because of carbon embrittlement of the metallic particles in which chromium was the major constituent.     

Effect of the microstructure on the mechanical properties of a directionally solidified Y3Al5O12/Al2O3 eutectic fiber

The strength and fracture behaviors of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth (EFG) technique. The microstructure was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The room temperature tensile strength and Weibull's modulus of the eutectic fiber before and after heat treatment at 1460°C were measured. The fracture toughness and crack propagation behaviors were investigated using an indentation technique. Significant coarsening of the lamellar microstructure was observed after heat treatment at 1460°C in air. The degradation of the room temperature tensile strength in the Y₃Al₅O₁₂/Al₂O₃ eutectic fiber after heat treatment was attributed to the development of surface grooves at the surface of the fiber. Also, the Y₃Al₅O₁₂/Al₂O₃ eutectic fiber showed a radial (Palmqvist) crack type and exhibited an anisotropic crack propagation behavior during the indentation tests.     

Properties and microstructures of Lanxide® Al2O3-Al ceramic composite materials

A study has been made of four Al₂O₃-Al composite materials fabricated by the directed oxidation of molten aluminium alloys. Their microstructures are described and measurements of density, coefficient of thermal expansion, thermal conductivity, hardness, elastic constants, compressive strength, flexural strength, fracture toughness, work of fracture, and thermal shock resistance are reported. Compared to a typical dense sintered Al₂O₃, such as Durafrax^® 1542, which is somewhat harder, stiffer, and stronger in compression, the new composites can be stronger in flexure, particularly at high temperatures, far tougher, and considerably more resistant to thermal shock. Attempts are made to relate their differences in properties to microstructure.     

Interfacial phenomenology of SiC fibre reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

The microstructures of SiC fibre-reinforced Li₂O·Al₂O₃·6SiO₂ glass-ceramic composites with Ta₂O₅, Nb₂O₅, TiO₂ and ZrO₂ dopants were investigated. An amorphous carbon-rich layer, from 100–170 nm thick, was observed in the interfacial region between fibre and matrix. A second interfacial layer of TaC, NbC, or TiC precipitates, appeared adjacent to the C-rich layer. Low bond strength between these two interfacial layers resulted in low interfacial shear strength, and this in turn led to an increase in toughness of the composites containing 4 mol% Ta₂O₅ or Nb₂O₅ dopant. 2 mol% Ta₂O₅ dopant in this composite acted as a nucleating agent for the matrix but was not adequate to form an appreciable volume of TaC particles in the interfacial region, hence a flexural strength decrease was observed. The composite containing TiO₂ dopant exhibited low flexural strength and fracture toughness resulting from the formation of a TiC layer which had a larger coherent bond strength with the interfacial C-rich layer, and attacked the structural integrity of the fibres.     

Multilayer composites in Al2O3/MoSi2 system

A new multilayer composite with a super-plastic layer, a hard layer and a weak interface was proposed. The hard layer can provide the strength of the multilayer composites at high temperature, the plastic layer can deform plastically at high temperature and disperse the applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Such multilayer composites were prepared by tape casting in the Al₂O₃/TiC/MoSi₂–Mo₂B₅ system. It was found that such design is effective on the increase of fracture energy both at room temperature and at high temperature, and the strength at high temperature could be remained in a relatively high revel.     

Microstructure and mechanical properties of Cr3C2 particulate reinforced Al2O3 matrix composites

Al₂O₃ matrix with three grades of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) composites were fabricated by a hot-pressing technique. Fully dense compacts with Cr₃C₂ content up to 40 vol % can be acquired at 1400 °C under 30 MPa pressure for 1 h. The flexural strength increases from 595 to 785 Mpa for fine Cr₃C₂ particle (0.5 m) reinforced Al₂O₃ matrix composites. The fracture strength is significantly dependent on the fracture modes of matrix (intergranular or transgranular). The transgranular fracture with a compressive residual stress gives a high fracture strength of composites. At the same time, the fracture toughness increases from 5.2 MPa m^1/2 (10 vol % Cr₃C₂) to 8.0 MPa m^1/2 (30 vol % Cr₃C₂) for the coarse Cr₃C₂ particle (7.5 n) reinforced Al₂O₃ matrix composites. The toughening effects of incorporating Cr₃C₂ particles into Al₂O₃ matrix originate from crack bridging and deflection. The electrical conductivity and the possibility of electrical discharge machining of these composites were also investigated.     

Mechanical properties of Al2O3 particle-Y-TZP matrix composite and its toughening mechanism

Dense Al₂O₃ particle-Y-TZP matrix (Al₂O₃<40 vol%) composite was prepared by pressureless sintering at 1550°C. Composites with 10–30 vol% Al₂O₃ particles showed enhanced fracture toughness, bending strength and Vicker's hardness as compared to single-phase Y-TZP. The highest strength (1150 MPa) and highest toughness (12.4 MPa m^1/2) were obtained for the composite containing 10 vol% Al₂O₃. It was found that, in addition to the contribution by the crack-deflection effect, the enhanced phase transformation from tetragonal to monoclinic during fracture was the main toughening mechanism in operation in the composites.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.