High-temperature shear strength of lead-free Sn-Sb-Ag/Al2O3 composite solder

The lead-free Sn-1.7Sb-1.5Ag solder alloy and the same material reinforced with 5 vol.% of 0.3-μm Al₂O₃ particles were synthesized using the powder-metallurgy route of blending, compaction, sintering, and extrusion. The mechanical properties of both monolithic and composite solders were studied by shear punch testing (SPT) at temperatures in the range of 25-130 °C. Depending on the test temperature, the shear yield stress (SYS) increased by 4.8-8.8 MPa, and ultimate shear strength (USS) increased by 6.2-8.8 MPa in the composite material. The strength improvement was mostly due to the CTE mismatch between the matrix and the particles, and to a lesser extent to the Orowan strengthening mechanism of the submicro-sized Al₂O₃ particles in the composite solder. The contribution of each of these mechanisms was used in a modified shear lag model to predict the total composite-strengthening achieved.     

Influence of the surface constitution on the as-sintered strength of Si3N4 micro specimens

In order to study the significance of as-sintered surfaces for the strength of Si₃N₄ micro components, micro bending specimens with dimensions of 240 μm × 240 μm × 1400 μm were prepared from sintered reaction-bonded silicon nitride. The processing parameters sintering temperature and dwell time were varied, and two different types of powder bed were applied during sintering. The characteristic strength σ₀ - determined by micro-3-point-bending tests - varied from about 500 to 1200 MPa. Strength values of about 1000 MPa and higher were observed when powder beds were applied which were newly prepared and doped with the sintering additives Y₂O₃, Al₂O₃, and MgO. In that case, the mass loss during sintering was negligible or even a small mass gain occurred. Samples sintered in used and undoped powder beds by contrast showed mass loss and an enrichment of the secondary phase YN-melilite (Y₂Si₃O₃N₄) at the surface; for these samples σ₀ was found to be restricted to about 500-700 MPa.Contrary to expectations, it was observed that the residual porosity - ranging from less than 1% up to 11% - does not act as a predominant strength-determining factor. Instead, the more or less pronounced formation of surface-near defects, due to the decomposition and volatilization of Si₃N₄ during sintering, is considered to be decisive for the achievable strength. Mass loss and YN-melilite formation are indicators for defect creating reactions. Due to the increased surface-to-volume ratio of micro components, the affected surface zone in particular determines the overall mechanical properties.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Effect of quenching mode on the mechanical properties of a Zr64Al10Ni15Cu11 metallic glass

This paper studied the effect of quenching mode on the mechanical properties of a Zr₆₄Al₁₀Ni₁₅Cu₁₁ metallic glass. It has been shown that the specimen through annealing and water quenching treatments has a higher strength of 1698 MPa than the specimen without annealing treatment (with a strength of 1572 MPa) due to the presence of nanocrystals in the amorphous matrix, which act as obstacles to the shear bands movement during deformation and thus improve the strength. It has also been shown that the specimen through annealing and air quenching treatments has a lower strength of 1119 MPa than the specimen without annealing treatment (with a strength of 1572 MPa) due to the heterogeneous distribution of the internal stress, which induces the crack initiations and catastrophic failure at a rather low external applied stress.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

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.     

The effect of bond coat on mechanical properties of plasma-sprayed Al2O3 and Al2O3-13 wt% TiO2 coatings on AISI 316L stainless steel

In this study, Al₂O₃ and Al₂O₃-13 wt% TiO₂ were plasma sprayed onto AISI 316L stainless-steel substrate with and without Ni-5 wt% Al as bond coat layer. The coated specimens were characterized by optical microscopy, metallography and X-ray diffraction (XRD). Bonding strength of coatings were evaluated in accordance with the ASTM C-633 method. It was observed that the dominant phase was Al₂O₃ for both coatings. It was also found that the hardness of coating with bond coat was higher than that of coating without bond coat. Metallographic studies revealed that coating with bond coating has three different regions, which are the ceramic layer (Al₂O₃ or Al₂O₃-13 wt% TiO₂), the bond coating, and matrix, which is not affected by coating. The coating performed by plasma-spray process without bond coating has two zones, the gray one indicating the ceramic layer and the white one characterizing the matrix. No delamination or spalling was observed in coatings. However, there are some pinholes in coating layer, but they are very rare. The bonding strength of coatings with bond coat was higher than that of coating without bond coat. The strength of adhesion and cohesion was determined by means of a planemeter. It was seen that percentage of cohesion strength was higher than that of adhesion strength.     

Refinement of microstructure and improvement of mechanical properties of Al/Al2O3 cast composite by accumulative roll bonding process

Some important problems associated with cast metal matrix composites (MMCs) include non-uniformity of the reinforcement particles, high porosity content, and weak bonding between reinforcement and matrix, which collectively result in low mechanical properties. Accumulative roll bonding (ARB) process was used in this study as a very effective method for refinement of microstructure and improvement of mechanical properties of the cast Al/10 vol.% Al₂O₃ composite. The average particle size of the Al₂O₃ was 3 μm. The results revealed that the microstructure of the composite after eleven cycles of the ARB had an excellent distribution of alumina particles in the aluminum matrix without any noticeable porosity. The results also indicated that the tensile strength and elongation of the composites increased as the number of ARB cycles increased. After eleven ARB cycles tensile strength and elongation values reached 158.1 MPa and 7.8%, which were 2.54 and 2.36 times greater than those of the as-cast MMC, respectively.     

High temperature fatigue deformation behaviors of thermally sprayed steel measured with electronic speckle pattern interferometry method

High temperature fatigue (R=0) damage and deformation behaviors of SUS304 steel thermally sprayed with Al₂O₃/NiCr coating were investigated using an electronic speckle pattern interferometry (ESPI) method. Surface cracks and delamination occurred after 1×10⁵ cycles test when σ_max was 202 MPa at 873 K. The lengths and number of cracks and delamination largely decreased when σ_max or temperature decreased to 115 MPa or 573 K, respectively. Strain values along cracks measured with the ESPI method were much larger than other areas due to crack opening under the tensile load. The positions of strain concentration zones on strain distribution figures by ESPI method were well corresponded to those of cracks on sprayed coatings. Strain values decreased largely where local delamination occurred.     

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.     

Densification and properties of barium zirconate ceramics by addition of P2O5

The influences of P₂O₅ doping on the sintering behavior, phase formation and properties of barium zirconate ceramics were investigated. Unmodified BaZrO₃ was difficult to densify, even at 1600 °C. Only a porous microstructure could be obtained. However, doping BaZrO₃ with P₂O₅ markedly enhances its sinterability. 94.2% of theoretical density was achieved with the inclusion of 4 mol% P₂O₅ sintered at 1600 °C for 4 h. The bending strength of the samples sintered at 1600 °C for 4 h was improved by almost 8 times by the addition of 4 mol% P₂O₅. The average bending strength of 152.3 ± 16.7 MPa was obtained. The Vickers hardness of 4 mol% P₂O₅ modified BaZrO₃ reaches 8.8 ± 0.4 GPa.     

Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatomaceous earth (DE) and rice husk ash (RHA) with high starting SiO₂/Al₂O₃ ratios of 13.0-33.5 and Na₂O/Al₂O₃ ratios of 0.66-3.0 were studied. The effects of fineness and calcination temperature of DE, concentrations of NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties and microstructures of the geopolymer pastes were investigated. The results indicated that the optimum calcination temperature of DE was 800 °C. Increasing fineness of DE and starting Na₂O/Al₂O₃ ratio resulted in an increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave higher compressive strengths than those with KOH. The optimum curing temperature and time were 75 °C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm³ and compressive strength of 15 kg/cm² was obtained. Incorporation of 40% RHA to increase starting SiO₂/Al₂O₃ and Na₂O/Al₂O₃ ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to 24 kg/cm² with only a marginal increase of bulk density to 1.01 g/cm³. However, the geopolymer materials with high Na₂O/Al₂O₃ (>1.5) were not stable in water submersion.     

Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process

Core-shell structured HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites were fabricated using a multi extrusion process. The shell of Al₂O₃-(m-ZrO₂) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO₂ dispersed in the HAp matrix. The t-ZrO₂ and m-ZrO₂ particles (< 400 nm) were homogeneously dispersed in the HAp and Al₂O₃ phases, respectively. In the HAp-(t-ZrO₂) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively.     

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.     

Effect of metal volume fraction on the mechanical properties of alumina/aluminum composites

The influence of metal volume fraction on the mechanical properties of Al₂O₃/Al composites with constant diameter of metal ligaments was studied. Alumina/aluminum composites with interpenetrating networks and metal content between 12 and 34 vol.% were fabricated by gas-pressure infiltration technique. The fabricated composites exhibited good mechanical properties, e.g. the bending strength of 740 MPa for samples containing 12 vol.% of Al. The bending strength of the composites decreased with increasing volume fraction of metal phase. High strength of the fabricated composites was explained by strong interfacial bonding between alumina and aluminum. The fracture toughness of the composites increased, however, with increasing volume fraction of aluminum. The highest fracture toughness values of 6 MPa m were measured for the composites containing 25 vol.% of Al. Fractographic analysis of fractured surfaces showed deformed metal ligaments which demonstrated that crack bridging by plastic deformation of the metal phase is the main toughening mechanism in Al₂O₃/Al composites.     

Fracture toughness of alumina/lanthanum titanate laminate composites with a weak interface

Layers of lanthanum titanate (La₂Ti₂O₇) and α-alumina (α-Al₂O₃) were employed to form a layered composite in order to improve the fracture toughness of monolithic alumina. The composites were produced by two different processing methods. In the first, individually presintered pellets of α-Al₂O₃ and La₂Ti₂O₇ were stacked together and hot-forged. In the second, tape cast molten salt La₂Ti₂O₇ and dense α-Al₂O₃ were stacked together and hot-forged. The forged composite samples were investigated by optical microscopy, scanning electron microscopy (SEM), Vickers indentation and three-point bending. During the hot-forging process, an interphase, aluminum titanate (Al₂TiO₅) was found to form as a result of the reaction between α-Al₂O₃ and La₂Ti₂O₇. The flexural strength and the fracture toughness of the resulting laminate composites were found to be 320 MPa and 7.1 MPa m^1 / 2, respectively. Indentation experiments showed that the newly formed Al₂TiO₅ at the interface is sufficiently weak to promote crack deflection and hence increase the fracture energy and mechanical properties of the composite.     

Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Einfluß trockener Friktionsbeanspruchungen auf Schädigungsverhalten,Restfestigkeit und Oberflächeneigenspannungen von Al2O3

Influence of dry frictional loading on damage behaviour, bending strength and surface residual stresses of Al₂O₃ ceramics The present contribution gives results of dry friction tests of an Al₂O₃‐plate against pellets made of grey cast iron. The influence of the frictional loading on the damage and fracture behaviour as well as on the surface residual stresses within the ceramic plate are presented and discussed. Furthermore FEM simulations of the friction system are given. The damage of the Al₂O₃ is mainly determined by the friction induced heat flux dQ/dt. At dQ/dt values higher than 1.3 kJ/s small radial surface cracks within the friction area occur. The location of crack initiation as well as the direction of crack propagation are predicted correctly by the FEM analysis. Increasing dQ/dt lead to additional tangential cracks connecting the initial radial cracks and, consequently, leading to a fragmentation of the alumina plate. The thermally induced radial cracks show a transcrystalline, the secondary radial cracks a mainly intercrystalline crack path. The occurrence of fine radial cracks is connected with a decrease of the bending strength of the Al₂O₃ of about 20%. After one friction cycle a bimodal strength distribution is observed. In spite of the fact that cyclic frictional loading does not alter the macroscopic damage behaviour with respect to one cycle, it leads to a more unimodal distribution of the residual strength. The residual stress state changes from compressive surface stresses in the as‐received state to biaxial tensile residual stresses with strong gradients after frictional loading.     

Effect of liquid-forming additives on the sintering and mechanical properties of Al2O3/3Y-TZP (30 vol.%) composite

Al₂O₃/3Y-TZP (30 vol.%) composite was pressurelessly sintered with addition of TiO₂MnO₂ and/or CaOAl₂O₃SiO₂ glass. It was found that TiO₂MnO₂ addition greatly enhanced the densification of the composite by the formation of a low-viscosity liquid at sintering temperature. In contrast, the high-viscosity liquid formed by CaOAl₂O₃SiO₂ glass improved mechanical properties because of its repressing effect on grain growth. The composite could be obtained at a temperature as low as 1400°C by co-doping with TiO₂MnO₂ and CAS glass. Bending strength of 552±64 MPa and fracture toughness of 6.03±0.22 MPa m^1/2 were obtained with a doping level of 2 wt.% TiO₂MnO₂ and 2 wt.% CAS glass.