Densification and mechanical properties of mullite-SiC nanocomposites synthesized through sol-gel coated precursors

Mullite-SiC nanocomposites are synthesized by introducing surface modified sol-gel mullite coated SiC particles in the matrix and densification and associated microstructural features of such precursor are reported. Nanosize SiC (average size 180 nm) surface was first provided with a mullite precursor coating which was characterized by the X-ray analysis and TEM. An average coating thickness of 120 nm was obtained on the SiC particles. The green compacts obtained by cold isostatic pressing were sintered in the range 1500–1700°C under pressureless sintering in the N₂ atmosphere. The percentage of the theoretical sintered density decreases with increase in SiC content. A maximum sintered density of 97% was achieved for mullite-5 vol.% SiC. The fractograph of the sintered composite showed a highly dense, fine grained microstructure with the SiC particles uniformly distributed along the grains as well as at the grain boundaries inside the mullite. The Vicker’s microhardness of mullite-5 vol.% SiC composite was measured as 1320 kg/mm² under an applied indentation load of 500 g. This value gradually decreased with an increase in SiC content.     

Insight of the interface of electroless Ni–P/SiC composite coating on aluminium alloy,LM24

Electroless nickel composite coatings with silicon carbide, SiC, as reinforcing particles deposited with Ni–P onto aluminium alloy, LM24, having zincating as under layer were subjected to heat treatment using air furnace. The changes at the interface were investigated using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to probe the chemistry changes upon heat treatment. Microhardness tester with various loads using both Knoop and Vickers indenters was used to study the load effect clubbed with the influence of second phase particles on the coating at the vicinity of the interface. It was observed that zinc was absent at the interface after elevated temperature heat treatment at 400–500 °C. Precipitation of copper and nickel with a distinct demarcation (copper rich belt) along the coating interface was seen with irregular thickness of the order of 1 μm. Migration of copper from the bulk aluminium alloy could have been the factor. Brittleness of the coating was confirmed on heat treatment when indented with Vickers. However, in composite coating the propagation of the microcrack was stopped by the embedded particles but the microcracks continue in the matrix when not interrupted by second phase particles (SiC).     

Influence of SiC particles distribution on mechanical properties and fracture of DRA alloys

In the last 20 years a new class of metal matrix composite material (DRA – Discontinuously Reinforced Aluminum) with aluminum alloy matrix and SiC particles as secondary phase has been developed. The most important step during composite production is the homogenization process of metal and ceramic powder particles. Quantitative analysis of a SiC particles distribution in the aluminum alloy matrix (CW67) was used to determine the optimum homogenization parameters of different powders. It was found out that the level of mixture homogeneity largely depends on the amount of mixing dish filling, homogenization time and characteristics of reinforcing particles. By introducing the concept of homogeneity index, it was shown that the lowest values of the mentioned parameter correspond to the best uniformity of SiC particles in the CW67 matrix. Composite with the lowest value of homogeneity index was the one with 5 vol.% of SiC, homogenized during 60 min and the amount of mixing dish filling of 20 vol.%. This composite displayed the best values of mechanical properties and fracture resistance.     

Influence of high velocity oxygen-fuel spraying parameters on the wear resistance of Al–SiC composite coatings deposited on ZE41A magnesium alloy

High velocity oxygen-fuel (HVOF) has been used as fabrication technique to deposit aluminium coatings reinforced with silicon carbide particles on Mg–Zn substrates. The aim of the investigation is to improve the tribological performance of the ZE41A magnesium alloy. The parameters of the thermal spraying system have been optimized in order to maximize the SiC particles incorporation in the aluminium matrix of the coating and to minimize the mechanical deterioration of the light alloy substrate. Pin-on-disc tests were developed to characterize the tribological behavior of the different specimens. Composite coatings with thicknesses of 120 μm, reinforced with ∼10 wt% and with high adhesion to the substrate were achieved. The wear resistance of the substrates was increased and the wear rate decreased in two orders of magnitude respect to that of the bare Mg-alloy after the optimization of the spraying parameters.     

Hardness recovery of ceramic coated aluminium matrix composites using thermal-shock resistant sol-gel silica coatings

Sol-gel silica coatings increase wear and corrosion resistance of aluminium matrix composites, but a heat treatment stage at temperatures as high as 500 °C is usually required to consolidate the ceramic coating. Under these thermal conditions, sol-gel ceramic coatings tend to crack and the aluminium matrix hardness strongly reduces. Present paper describes how the combination of a certain sol-gel procedure accompanied with quenching treatment in silicone oil allows recovering most of the hardness of the substrate, while keeping undamaged the sol-gel silica coating deposited.     

Microstructure and abrasive wear behaviour of B<Subscript>4</Subscript>C particle reinforced 2014 Al matrix composites

In this study, the microstructure and abrasive wear properties of varying volume fraction of particles up to 12% B₄C particle reinforced 2014 aluminium alloy metal matrix composites produced by stircasting method was investigated. The density, porosity and hardness of composites were also examined. Wear behaviour of B₄C particle reinforced aluminium alloy composites was investigated by a block-on-disc abrasion test apparatus where the samples slid against the abrasive suspension mixture (contained 10 vol.% SiC particles and 90 vol.% oil) at room conditions. Wear tests performed under 92 N against the abrasive suspension mixture with a novel three body abrasive. For wear behaviour, the volume loss and specific rate of the samples have been measured and the effects of sliding time and the content of B₄C particles on the abrasive wear properties of the composites have been evaluated. The dominant wear mechanisms were identified using SEM. Microscopic observation of the microstructures revealed that dispersion of B₄C particles was generally uniform while increasing volume fraction led to agglomeration of the particles and porosity. The density of the composite decreased with increasing reinforcement volume fraction but the porosity and hardness increased with increasing particle content. Moreover, the specific wear rate of composite decreased with increasing particle volume fraction. The wear resistance of the composite was found to be considerably higher than that of the matrix alloy and increased with increasing particle content.     

Threshold stress during crack-healing treatment of structural ceramics having the crack-healing ability

Alumina/15 vol.% SiC particles composite and mullite/15 vol.% SiC particles composite, that have excellent crack-healing ability are subjected to crack-healing under elevated static and cyclic stresses at 1373 K or 1473 K. The bending strengths of the specimens crack-healed under stress were investigated at the crack-healing temperature. From the results, the threshold stresses during crack-healing were determined. Crack-healing was found to occur although the pre-crack is grown by the applied stress. It is found that crack-healing can eliminate the pre-crack under stress below 64% fracture stress of the cracked specimen, if the ceramic components have an adequate crack-healing ability.     

Joining of C/C composite to TC4 using SiC particle-reinforced brazing alloy

Silicon carbide particles were used as reinforcement in the Ag-26.7Cu-4.6Ti (wt.%) brazing alloy for joining C/C composite to TC4 (Ti-6Al-4V, wt.%). The mechanical properties of the brazed joints were measured by shear strength testing. The effects of the volume percentage of SiC particles on the microstructures of the brazed joints were investigated. It is shown that the maximum shear strength of the joints is 29 MPa using 15 vol.% SiC in the brazing alloy which is greater than that with Ag-26.7Cu-4.6Ti brazing alloy alone (22 MPa). Ti is reacted with SiC particles, forming Ti–Si–C compound in the particle-reinforced brazing alloy. Due to this, more SiC particles in the brazing alloy, the thickness of TiC/TiCu reaction layer near C/C composite decreases. Moreover, SiC particles added to the brazing alloy can reduce the CTE of the brazing alloy which results in lower residual stress in the C/C composite-to-metal joint. Both of the above reasons lead to the increasing of the shear strength of the brazed joints. But excessive SiC particles added to the brazing alloy lead to pores which results in poor strength of the brazed joint.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Aluminum matrix composites based on preceramic-polymer-bonded SiC preforms

Polymethylsiloxane (PMS) was used as a binder to make self-supporting SiC preforms for pressurized aluminum melt infiltration. The SiC particles were coated with preceramic polymer by spray drying; this ensured a fine and homogeneous distribution coupled with a high yield of the binder. The conditioned SiC powder mixtures were processed into preforms by warm pressing, curing and pyrolysis. A polymer content of 1.25 wt.% conferred sufficient stability to the preforms to enable composite processing. Using this procedure, SiC preforms with various SiC particle size distributions were prepared. The resulting Al/SiC composites with SiC contents of about 60 vol.% obtained by squeeze casting infiltration exhibit a 4-point bending strength of ∼500 MPa and Young’s moduli of ∼200 GPa. These values are comparable to those of compositionally identical, but binder-free composites. It is thus shown that the PMS-derived binder confers the desired strength to the SiC preforms without impairing the mechanical properties of the resulting Al/SiC composites.     

Effect of submicron size SiC particles on microstructure and mechanical properties of AZ31B magnesium matrix composites

The magnesium matrix composites reinforced with three volume fractions (3, 5 and 10 vol.%) of submicron-SiC particles (∼0.5 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. With increasing the volume fraction of the submicron SiC particles (SiCp), the grain size of matrix in the SiCp/AZ31B composites was gradually decreased. Most of the submicron SiC particles exhibited homogeneous distribution in the SiCp/AZ31B composites. The ultimate tensile strength and yield strength of the 10 vol.% SiCp/AZ31B composites were simultaneously improved. The study of interface between the submicron SiCp and the matrix in the SiCp/AZ31B composite suggested that submicron SiCp bonded well with the matrix without interfacial activity.     

Processing of Al/SiC composites in continuous solid–liquid co-existent state by SPS and their thermal properties

Silicon carbide (SiC)-particle-dispersed-aluminum (Al) matrix composites were fabricated in a unique fabrication method, where the powder mixture of SiC, pure Al and Al–5mass% Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction was detected at the interface between the SiC particle and the Al matrix. The relative packing density of the Al–matrix composite containing SiC was higher than 99% in a volume fraction range of SiC between 40% and 55%. Thermal conductivity of the composite increased with increasing the SiC content in the composite at a SiC fraction range between 40 vol.% and 50 vol.%. The highest thermal conductivity was obtained for Al–50 vol.% SiC composite and reached 252 W/mK. The coefficient of thermal expansion of the composites falls in the upper line of Kerner’s model, indicating strong bonding between the SiC particle and the Al matrix in the composite.     

SiCP混杂对C/Al浸渍成型复合材料性能的影响

碳纤维经混杂SiC_P后用压力浸渍成型方法制备成C/Al复合材料,分析混杂的SiC_P对C/Al复合材料力学性能的影响。测试了制备成的复合材料性能,并用SEM对复合材料断面组织与断口形态进行分析。结果表明,混杂的SiC_P可以分隔纤维,有利浸渍,使纤维分布均匀从而提高了复合材料的性能,而用sol-gel方法涂复SiC层并混杂SiC_P可获得最佳的性能。     

Evaluation of microstructure and mechanical properties of Al/Al2O3/SiC hybrid composite fabricated by accumulative roll bonding process

In this investigation, a new kind of metal matrix composites with a matrix of pure aluminum and hybrid reinforcement of Al₂O₃ and SiC particles was fabricated for the first time by anodizing followed by eight cycles accumulative roll bonding (ARB). The resulting microstructures and the corresponding mechanical properties of composites within different stages of ARB process were studied. It was found that with increasing the ARB cycles, alumina layers were fractured, resulting in homogenous distribution of Al₂O₃ particles in the aluminum matrix. Also, the distribution of SiC particles was improved and the porosity between particles and the matrix was decreased. It was observed that the tensile strength of composites improved by increasing the ARB passes, i.e. the tensile strength of the Al/1.6 vol.% Al₂O₃/1 vol.% SiC composite was measured to be about 3.1 times higher than as-received material. In addition, tensile strength of composites decreased by increasing volume fraction of SiC particles to more than 1 vol.%. Scanning electron microscopy (SEM) observation of fractured surfaces showed that the failure mechanism of broken hybrid composite was shear ductile rupture.     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

Influence of microstructure and texture on mechanical properties of aluminium alloy 2124 + 5%SiC particulate composites

Abstract

A study of texture, microstructure, mechanical properties, and crack propagation mechanisms was carried out on aluminium alloy 2124 reinforced with 5 vol.-%SiC particles (3 μm). Three fabrication techniques have been used to produce the composites. composite I wasfabricated by blending followed by hot isostatic pressing. composite II was fabricated by mechanical alloying followed by hot isostatic pressing. composite III wasfabricated by agglomeration of aluminium powder by mechanical alloying followed by blending with SiC and hot isostatic pressing. All three composites were hot rolled to nominally 12.5 mm thick plate. Similar textures were observed for all composites. A model of the observed texture is {001} (211), {111} (211), and {211} (111) for rolling, side, and transverse planes respectively. Composite I showed a homogeneous distribution of SiC particles. Transmission electron micrographs of composite I showed good interface bonding, stacking faults present in SiC particles, and segregation of aluminium, oxygen, copper, and magnesium to the interface. composites II and III showed an inhomogeneous distribution of SiC particles. The elastic modulus was slightly higher in the (211) direction than in the (111) direction. The fracture toughness of composite I was higher in the (211) crack direction whereas that for composite II and composite III was higher in the (111) crack direction. Secondary crack propagation modes follow the crystallographic orientations of {100} and {111} planes.     

Effect of surface modification on silicon carbide particles with nanocrystalline spinel LZO for enhanced damping and mechanical property

In our proof-of-concept studies, pure aluminium matrix composites reinforced with spinel lithium zirconate (Li₂ZrO₃, LZO) coated silicon carbide (SiC) particles were fabricated by stir casting. Initially, we presented the results of an investigation on spinel LZO coating layer on SiC particles of sub-micron size via facile isothermal hydrolyzing and chemical precipitation procedure under relatively low temperature. Composite materials combining high stiffness (storage modulus) and mechanical properties by reinforcing SiC@ZrO₂ and SiC@LZO obtained by different techniques, thoroughly investigated by various characterization techniques. The amount and coverage of zirconia and LZO crystals on SiC surface were studied through X-ray energy dispersion spectroscopy (EDX) analysis and elemental mapping observation. Further, thermal conductivity and zeta potential characteristics of the surface modified SiC particles were investigated for adequate dispersion and stability of SiC particles. The resulting aluminium metal matrix composite exhibits 90% higher stiffness and 3.8-fold increase in tensile strength, respectively as compared to the pure aluminium.     

Study on corrosion resistance of aluminium coating deposited on magnesium-zinc-yttrium-calcium alloy by cold spray

Pure aluminium coatings were prepared on magnesium-zinc-yttrium-calcium alloy substrate via cold spraying technology with different scanning speeds and working gas temperature. The correlation between the corrosion resistance of the coatings and the different spraying process was studied. While the working gas temperature is 600 °C and the scanning speed is 1 mm/s, aluminium coating has less porosity and the coating was well combined with the substrate. Higher temperature of working gas increases the plastic deformation of particles, which lead to a dense aluminium coating. The relationship of corrosion resistance on working gas temperature and scanning speed of aluminium coatings has also been investigated by immersion corrosion test and electrochemical impedance spectroscopic techniques. The results show that the cold sprayed aluminium coatings revealed a lower porosity and higher corrosion resistance with the decreasing scanning speed and the increasing temperature of working gas. The porosity and corrosion current densities were 0.938 vol.% and 2.427 ⋅ 10⁻⁶ A/cm². The experimental results show that the aluminium coating prepared by cold spraying has a good protective effect on magnesium alloy.     

Vacuum brazing of aluminium metal matrix composite (55 vol.% SiCp/A356) using aluminium-based filler alloy

Aluminium matrix composites with high volume fractions of SiC particles, as the reinforcements, are potentially suitable materials for electronic packaging. These composites, due to their poor weldability, however, have very limited applications. The microstructure and shear strengths of the bonds made in 55 vol.% SiC_p/A356 composite, using an aluminium based filler alloy containing Cu, Si, Mg and Ni, were investigated in this paper. The brazing temperature had a clear effect on the bond integrity, and the samples brazed at 560 °C demonstrated good bonding between the filler alloy and the SiC particles. The maximum shear strength achieved in this work was 102 MPa.     

Fluidity of aluminium alloy AlSi7Mg–SiC particulate composite melts

Abstract

A spiral test was used to evaluate the flowability of an AlSi7 Mg0·6 alloy reinforced via 10–30 vol.-%SiC particles having average diameters of 9, 13, and 23 μm. The results show that the composite melt has the same ability to flow into long thin sections as unreinforced aluminium, when the particle content is <20 vol.-%. The flowability of the particulate metal matrix composite is significantly reduced for particle volume fractions >20 vol.-% and approaching 30 vol.-% as a result of increased viscosity. At these higher levels of SiC, an increase in pouring temperature cannot compensate for the reduced flowability. The particle distribution in the solidified material depends on the length which the metal flows before solidification. After 150–200 mm of flow, dendritic grains growing in the composite melt push the particles so that the solidified microstructure consists of open areas without particles. The particle free areas increase in diameter with the flow distance.

MST/3005