Bulk WC-Al2O3 composites prepared by spark plasma sintering

WC and WC-Al₂O₃ materials without metallic binder addition were densified by spark plasma sintering in the range of 1800-1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC-Al₂O₃ composite were investigated. The addition of Al₂O₃ facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al₂O₃ additive helps to limit the formation of W₂C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV₁₀) of 25.71 GPa, fracture toughness of 4.54 MPa·m^1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al₂O₃ composites are 24.48 GPa, 6.01 MPa·m^1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al₂O₃ are thought to result from the reduction of W₂C phase, the crack-bridging by Al₂O₃ particles and the local change in fracture mode from intergranular to transgranular.     

Fractal analysis of crack paths in Al2O3-TiC-4%Co composites

1 Introduction Al2O3-TiC composite (denoted by AT) is an important material for structural components due to the high strength, hardness, as well as chemical stability and wear resistance. However, the low fracture toughness still cannot match the comman…     

The influence of powder characteristics on mechanical properties of Al2O3–TiC–Co ceramic materials prepared by Co‐coated Al2O3/TiC powders

Ultrafine Al₂O₃–TiC–Co (ATC) ceramic is prepared in order to improve the bending strength and fracture toughness of ceramic materials. The ultrafine Co‐coated Al₂O₃ and TiC powders have been synthesized by electroless plating at room temperature, and the composite powders were sintered by hot‐pressing to compact ATC samples. The average bending strength, average hardness and average fracture toughness values of ATC ceramic with different particle sizes and Co contents were investigated. The toughening mechanism of the ultrafine ATC ceramic was studied by transmission electron microscopy (TEM) and ceramic performance testing methods. The results show that the relative density, bending strength and fracture toughness values increase remarkably with the increase of Co content. The ultrafine grain of original powders is beneficial to improve the relative density, strength and toughness values of ATC ceramic. The Co phase hinders the growth of ATC ceramic grains during sintering. The Co phase forms a three‐dimensional mesh skeleton structure during sintering, improving the fracture toughness and strength of the composite ceramic.     

Strengthening and toughening forming of Al/Al2O3 composite in pseudo-semi-solid state

A new technology—thixo-die-forging of the composite in pseudo-semi-solid state was proposed based on the powder metallurgy technology combing with semi-solid metal process, and the cup shells with Al/Al₂O₃  composite was prepared successfully. The metallographic analysis and performance test show that the microstructure of parts is dense and mechanical properties are excellent with the volume fraction of Al is 37 %. The bend strength and fracture toughness of the composite are about 570-690 MPa and 8.5-16.8 MPa m^1/2, respectively. Comparing with reaction in situ and high temperature oxidation technologies the bending strength and fracture toughness are improved greatly. At the same time, it shows that the technology parameters have great influences on the properties. So it is feasible to prepare metal/ceramics composites by the proposed technology.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al₂O₃ composites were studied. The effect of TiC and Al₂O₃ starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δa). The mechanical properties and thermal fatigue resistance of TiC/Al₂O₃ composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al₂O₃ composites is due to a “true” cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

Dependency of fracture toughness of plasma sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on lamellar structure

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of critical strain energy release rate G _Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al₂O₃ coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G _Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al₂O₃ coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G _Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G _Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding.     

Effects of SiC interfacial coating on mechanical properties of carbon fiber needled felt reinforced sol-derived Al2O3 composites

3D carbon fiber needled felt and polycarbosilane-derived SiC coating were selected as reinforcement and interfacial coating, respectively, and the sol−impregnation−drying−heating (SIDH) route was used to fabricate C/Al₂O₃ composites. The effects of SiC interfacial coating on the mechanical properties, oxidation resistance and thermal shock resistance of C/Al₂O₃ composites were investigated. It is found that the fracture toughness of C/Al₂O₃ composites was remarkably superior to that of monolithic Al₂O₃ ceramics. The introduction of SiC interfacial coating obviously improved the strengths of C/Al₂O₃ composites although the fracture work diminished to some extent. Owing to the tight bonding between SiC coating and carbon fiber, the C/SiC/Al₂O₃ composites showed much better oxidation and thermal shock resistance over C/Al₂O₃ composites under static air.     

In-situ formed γ-Al2O3 nanocrystals repaired and toughened Al2O3 coating prepared by plasma spraying

During process of plasma spraying of alumina with the stable α phase in the as-received powder, metastable γ phase tends to form in the as-deposited coating. In this paper, plasma-sprayed Al₂O₃ coating was investigated using transmission electron microscopy. In situ formation of γ-Al₂O₃ nanocrystals was observed in the pre-existing cracks of the as-sprayed Al₂O₃ coating, which contributes to self-repairing and self-toughening of the Al₂O₃ coating. Except for the predominant intergranular fracture, the pre-formed γ-Al₂O₃ nanocrystals would effectively obstruct the crack development in the coating and then the coating fracture toughness is enhanced.     

BN/Al2O3 复合粉末及其聚合物基复合材料的制备与导热性能

采用固-液相共混法制备了多种BN/Al₂O₃复合粉末,通过冻融法和表面修饰法对BN进行了改性处理,改变表面修饰剂类型和摩尔比得到了前驱体和烧结态BN/Al₂O₃复合粉末,并利用机械混合法制备了聚合物基BN/Al₂O₃复合材料,并测试分析了其导热性能。结果表明,经冻融处理的BN分散性和界面相容性明显优于未经冻融处理的BN。多巴胺对BN的改性效果优于聚乙二醇。采用多巴胺作为表面修饰剂且BN与Al(NO₃)₃的摩尔比为1:1时,能够得到纳米Al₂O₃均匀包覆的微米BN粉末,即BN/Al₂O₃微纳复合粉末,其聚合物基复合材料的导热系数可达0.62 W·m^-1·K^-1,是纯聚合物导热系数的3倍,是采用纯微米BN粉末制备的聚合物基复合材料导热系数的1.5倍。在BN表面附着的Al₂O₃可以形成层状热传导通道,能够有效提高聚合物基BN/Al₂O₃复合材料的热导率。     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Ti3AlC2-Al2O3-TiAl3 composite fabricated by reactive melt infiltration

Porous preforms were fabricated by cold-pressing process using powder mixture of TiC, TiO₂ and dextrin. After pyrolysis and sintering, Al melt was infiltrated into the porous preforms, leading to the formation of Ti₃AlC₂-Al₂O₃-TiAl₃ composite. Effects of cold-pressing pressure of preforms on microstructures and mechanical properties of the composites were studied. Synthesis mechanism and toughening mechanism of composite were also analyzed. The results shows that TiO₂ is reduced into Ti₂O₃ by carbon, the decomposition product of dextrin, which causes the spontaneous infiltration of Al melt into TiC/Ti₂O₃ preform. Then, Ti₃AlC₂-Al₂O₃-TiAl₃ composite is in-situ formed from the simultaneous reaction of Al melt with TiC and Ti₂O₃. With the increase of cold-pressing pressure from 10 MPa to 40 MPa, the pore size distribution of the preforms becomes increasingly uniform after pre-sintering, which results in the reduction of defects, and the decrease of property discrepancy of composites. Nano-laminated Ti₃AlC₂ grains and Al₂O₃ particles make the fracture toughness of TiAl₃ increase remarkably by various toughening mechanisms including stress-induced microcrack, crack deflection and crack bridging.     

Fabrication of A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties

Aluminum/alumina composites are used in automotive and aerospace industries due to their low density and good mechanical strength. In this study, compocasting was used to fabricate aluminum-matrix composite reinforced with micro and nano-alumina particles. Different weight fractions of micro (3, 5 and 7.5 wt.%) and nano (1, 2, 3 and 4 wt.%) alumina particles were injected by argon gas into the semi-solid state A356 aluminum alloy and stirred by a mechanical stirrer with different speeds of 200, 300 and 450 rpm. The microstructure of the composite samples was investigated by Optical and Scanning Electron Microscopy. Also, density and hardness variation of micro and nano composites were measured. The microstructure study results revealed that application of compocasting process led to a transformation of a dendritic to a nondendritic structure of the matrix alloy. The SEM micrographs revealed that Al₂O₃ nano particles were surrounded by silicon eutectic and inclined to move toward inter-dendritic regions. They were dispersed uniformly in the matrix when 1, 2 and 3 wt.% nano Al₂O₃ or 3 and 5 wt.% micro Al₂O₃ was added, while, further increase in Al₂O₃ (4 wt.% nano Al₂O₃ and 7.5 wt.% micro Al₂O₃) led to agglomeration. The density measurements showed that the amount of porosity in the composites increased with increasing weight fraction and speed of stirring and decreasing particle size. The hardness results indicated that the hardness of the composites increased with decreasing size and increasing weight fraction of particles.     

Effect of the Al2O3 content on the mechanical properties and microstructure of 3Y-TZP ceramics for dental applications

Alumina-reinforced zirconia composites containing 0 to 30 vol% of alumina were fabricated by sintering at 1550 °C for 2 h in air. The effect of the Al₂O₃ content on the mechanical properties and microstructure of 3Y-TZP ceramics was investigated. Al₂O₃ acted as an inhibitor of the grain growth of 3Y-TZP. As the alumina content increased, the fracture mode changed gradually from the transgranular mode to the intergranular mode and the Young’s modulus and hardness increased. The biaxial flexural strength also showed a slight increase with an increase in Al₂O₃ content, due to the grain size refinement of the ZrO₂ matrix, while the fracture toughness, which was investigated by the SEVNB method, showed a contrary tendency. The decrement of the fracture toughness can be explained by the increase in the critical transformation stress, the decrease in the volume fraction of the transformable t-ZrO₂ and the increase in the tensile residual stress.     

CNTs含量对微弧等离子喷涂CNTs/AT40涂层力学性能的影响

采用微弧等离子喷涂成功制备了CNTs/Al_2O_3-40%TiO_2复合吸波涂层,借助扫描电镜、X射线衍射仪、拉伸试验机、显微硬度计等测试手段,对涂层的组织结构、结合强度、显微硬度和热震性能进行了测试分析。结果表明:涂层组织结构致密,孔隙率低;CNTs的含量对复合涂层的力学性能有较大影响,随CNTs含量的增加,CNTs/AT40复合涂层的结合强度、显微硬度和热震性能都先增大后减小。CNTs质量分数为3%时,平均结合强度最高,厚度0.5 mm时结合强度为32.08 MPa,比AT40涂层的结合强度24.09 MPa提高了33.2%,平均显微硬度为821.6 HV0.1,比AT40涂层提高了20.4%,厚度2.0 mm时热震次数为63次,比AT40涂层增加了16%。     

Mechanical properties improvement related to the isothermal holding time in Si3N4 ceramics sintered with an alternative additive

In the present work, the influence of the isothermal holding time on the physical (relative density and mass loss), chemical (α–β transformation and intergranular phase crystallization) and mechanical (hardness and fracture toughness) properties of Si₃N₄ ceramics with Al₂O₃ and CTR₂O₃ as additives has been studied. CTR₂O₃ is a natural rare earth oxide mixture, produced at DEMAR-FAENQUIL from the mineral xenotime, consisting mainly of Y₂O₃, Yb₂O₃, Er₂O₃ and Dy₂O₃. The increase in hardness and fracture toughness with increasing duration of isothermal sintering is discussed in regard of densification, α–β Si₃N₄ phase transformation and microstructure. The microstructural variations were decisive for the increase of fracture toughness, because larger grains (>4 μm) with higher aspect ratios (>6) developed during increased sinter periods, enhancing crack deflection and crack-bridging mechanism. In this way longer isothermal holding times contribute to the improvement of the physical and mechanical properties of silicon nitride based ceramics.     

Tensile and wear properties of rolled Al5Mg-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> or C particulate composites

Al5Mg alloy matrix composites reinforced with different percentages of Al₂O₃ (60 μm) or C (90 μm) particulates were prepared by the vortex method. The composites were then subjected to hot or cold rolling with different reduction ratios. The microstructures of the rolled composites revealed that the matrix grains moved around the particulate causing deformation. By continuing deformation, the particulates rearranged themselves in the matrix, leading to lensoid distortion. It was found that the addition of Al₂O₃ or C particulates increased the 0.2% proof stress and reduced both the tensile strength and ductility, compared with the monolithic alloy. Scanning electron microscopy (SEM) fractographic examinations showed that the composites reinforced with Al₂O₃ particulates failed through particulate fracture and matrix ligament rupture. However, the failure of the composites reinforced with C particulates was through particulate decohesion, followed by ductile failure of the matrix. Abrasive wear results showed that the wear rate of the Al5Mg alloy decreased with the addition of C particulates. However, increasing the volume fraction of C particulates did not have a prominent effect on the wear rate. The composites reinforced with Al₂O₃ particulates exhibited a higher wear rate than that of the unreinforced alloy. Furthermore, addition of both C and Al₂O₃ particulates into the Al5Mg matrix alloy did not significantly improve the wear resistance. For all composites studied in this work, hot or cold rolling had a marginal effect on the wear results.     

The Microstructures and Mechanical Properties of V2O5-Doped Al2O3/TiAl In-Situ Composites by Reactive Hot Pressing Process

Al₂O₃/TiAl composites are successfully fabricated by the in-situ hot pressing method from the elemental powders of Ti, Al, TiO₂, and V₂O₅. The effect of V₂O₅ addition on the microstructure and mechanical properties of the Al₂O₃/TiAl in-situ composites is investigated in detail. It is found that the as-synthesized composites mainly consist of V-dissolved γ-TiAl, α₂-Ti₃Al, and Al₂O₃ particles along with a small amount of V₃Al phase, and the in-situ-formed fine Al₂O₃ particles tend to disperse on the grain boundaries of TiAl matrix. With increasing V₂O₅ content, the density and Vickers hardness of the resulting composites gradually increase, whereas the fracture toughness and flexural strength first increase and then decrease with the increase of V₂O₅ content. The composite with 3.5 wt.% V₂O₅ has the maximum value of 9.35 MPa m^1/2 and 713.36 MPa for the fracture toughness and flexural strength, respectively. The toughening mechanism is also discussed in detail.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on the microstructure and sliding wear of 7075 Al alloy manufactured by squeeze casting method

Aluminum (Al) alloy 7075 reinforced with Al₂O₃ particles was prepared using the stir casting method. The microstructure of the cast composites showed some degree of porosity and sites of Al₂O₃ particle clustering, especially at high-volume fractions of Al₂O₃ particles. Different squeeze pressures (25 and 50 MPa) were applied to the cast composite during solidification to reduce porosity and particle clusters. Microstructure examinations of the squeeze cast composites showed remarkable grain refining compared with that of the matrix alloy. As the volume fraction of particles and applied squeeze pressure increased, the hardness linearly increased. This increase was related to the modified structure and the decrease in the porosity. The effect of particle volume fraction and squeeze pressure on the dry-sliding wear of the composites was studied. Experiments were performed at 10, 30, and 50 N with a sliding speed of 1 m/s using a pin-on-ring apparatus. Increasing the particle volume fraction and squeeze pressure improved the wear resistance of the composite compared with that of the monolithic alloy, because the Al₂O₃ particles acted as load-bearing constituents. Also, these results can be attributed to the fact that the application of squeeze pressure during solidification led to a reduction in the porosity, and an increase in the solidification rate, leading to a finer structure. Moreover, the application of squeeze pressure improved the interface strength between the matrix and Al₂O₃ particles by elimination of the porosity at the interface, thereby providing better mechanical locking.     

Investigation of residual stress effects in an alloy reinforced ceramic/metal composite

This study aims at investigating the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al₂O₃/A356 CMCs composites with an interpenetrating network structure and varying metal content, ranging from 10 to 40 vol.%, were produced using the pressure infiltration technique of Squeeze casting. Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. Residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic/metal interface of the Al₂O₃/A356 composites. The relationship of residual stresses and the contact area of the ceramic/metal interface are also discussed.     

Influences of annealing temperature on microstructure and mechanical properties of Mo-La2O3

Influences of annealing temperature on the microstructure and mechanical properties of Mo-La₂O₃ were investigated. Effects of annealing temperature on tensile properties, fracture toughness, and microhardness are discussed. Microstructure and fracture morphology of Mo alloys are observed by optical microscope, SEM, and TEM. The results indicate that grain size increased while tensile strength, fracture toughness, and microhardness decreased with increasing annealing temperature. Larger La₂O₃ particles are located at grain boundaries or sub-boundaries, while the majority of smaller La₂O₃ particles are located within the grain. The strengthening effect is quantitatively assessed, which yielded a predicted yield strength in good agreement with measurements.