Effects of copper addition on the sintering behavior and mechanical properties of powder processed Al/SiC<Subscript><Emphasis Type="Bold">p</Emphasis></Subscript> composites

The effect of copper addition on powder processed Al-10 vol% SiC composites was studied in regards to their sintering responses. Copper was mixed with aluminum powder either as elemental powders or as the coated layer on SiC particles. After sintering at 600°C for 1 h, Al-SiC composites with no copper addition showed little densification. It also demonstrated very low bend strengths of 49 and 60 MPa, indicating poor bonding between the powders in the sintered composite. The addition of 8% copper to the Al/SiC system effectively improved the sintering response, producing over 95% theoretical density, a bend strength of 231 MPa with the copper coated SiC, and a 90% density with over 200 MPa bend strength with the admixed copper.The as-sintered microstructures of the Al–SiC composites clearly revealed particle boundaries and sharp pores, indicating that only a limited neck growth occurred during sintering. In the case of Al–Cu–SiC composites, however, a liquid phase was formed and spread through particle boundaries filling the interfaces or voids between SiC particles and the matrix powders. The coated copper on SiC particles produced a somewhat better filling of the interface or voids, resulting in a little more densification and better sintered strength. Since the solubility of copper in aluminum is less than 2% at the sintering temperature, the alloying of copper in the aluminum matrix was limited. Most of the copper added was dissolved in the liquid phase during the sintering and precipitated as CuAl₂ phase upon cooling.     

Processing of heat-treated silicon carbide-reinforced aluminum alloy composites

The present study investigates the processing of heat-treated silicon carbide (SiC) particle-reinforced 6061 aluminum alloy (AA) composites. As-received SiC powders were heat treated at 1300ºC, 1400ºC, and 1500ºC in nitrogen atmosphere for 2 h, and the 6061 AA–SiC composites were developed by spark plasma sintering at 560ºC and 60 MPa for 5 min in argon atmosphere. The amorphous silicon nitride is found to form in SiC particles as a result of heating at 1400ºC. The microstructure of the composites exhibited uniform distribution of SiC or SiC/Si₃N₄ particles in 6061 AA matrix. Further, the heat-treated SiC-reinforced 6061AA composites exhibited improved mechanical properties. A typical combination of UTS of 240 MPa and elongation of 21% is obtained for the 6061 AA composites prepared using SiC powders heated at 1400ºC.     

Development of Al356/SiCp cast composites by injection of SiCp containing composite powders

One of the great challenges of producing cast metal matrix composites is the agglomeration tendency of the reinforcements. This would normally result in poor distribution of the particles, high porosity content, and low mechanical properties. In the present work, a new method for uniform distribution of very fine SiC particles with average size of less than 3 μm was employed. The key idea was to allow for gradual in situ release of properly wetted SiC particles in the liquid metal. For this purpose, SiC particles were injected into the melt in three different forms, i.e., untreated SiC_p, milled particulate Al–SiC_p composite powder, and milled particulate Al–SiC_p–Mg composite powder. The resultant composite slurries were then cast from either fully liquid (stir casting) or semisolid (compocasting) state. Consequently, the effects of the casting method and the type of the injected powder on the microstructural characteristics as well as the mechanical properties of the cast composites were investigated. The results showed that the distribution of SiC particles in the matrix and the porosity content of the composites were greatly improved by injecting milled composite powders instead of untreated-SiC particles into the melt. Casting from semisolid state instead of fully liquid state had similar effects. The average size of SiC particles incorporated into the matrix was also significantly reduced from about 8 to 3 μm by injecting milled composite powders. The ultimate tensile strength, yield strength and elongation of Al356/5 vol.%SiC_p composite manufactured by compocasting of the (Al–SiC_p–Mg)_cp injected melt were increased by 90%, 103% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiC_p injected melt.     

Wear and corrosion studies of graphite‐aluminum composite reinforced with micro/nano‐TiB2 via spark plasma sintering

The demand for lightweight materials in the automobile and aerospace industries has led to various researches on graphite and graphite‐aluminum composites. The aim of this study was to investigate the effect of the addition of micron/nano TiB₂ particles on the properties of graphite‐aluminum composite particularly the wear resistance. The powders were sintered at 550 °C and 50 MPa with more attention on the effect of the sintering temperature on densification, microhardness, coefficient of thermal expansion, wear and frictional force. The results show that the addition of nano TiB₂ reduces the densification while improving the hardness of Gr?Al composite with the lowest value being 96.0 % of relative density and the highest microhardness of 43.58 HV 0.1. The coefficient of thermal expansion and frictional force of the composite materials increases with increasing TiB₂ content and heating rate (100 °C/min–150 °C/min). TiB₂ particles enhance the wear resistance of graphite‐aluminum composite. The addition of micro/nanoparticles of TiB₂ to graphite‐aluminum composite increases its corrosion rate with improved passivation behavior in 3.5 wt.% NaCl solution. Nevertheless, 5 wt.% nano (100 °C/min) TiB₂ additions do not affect the overall corrosion rate. This work has shown that we can take advantage of some of the properties of TiB₂ to improve the performance of graphite‐aluminum composite.     

Effects of reinforcement distribution on low and high temperature tensile properties of Al356/SiCp cast composites produced by a novel reinforcement dispersion technique

A major challenge for full utilization of the potentials of SiC_p reinforced metal matrix composites is the uniform dispersion of very fine SiC particles in the matrix alloys. In this study, a novel method for gradual in situ release of properly wetted SiC particles with average size of less than 3 μm in the liquid metal was employed which greatly overcame this challenge. SiC particles were injected into the melt in three different forms, i.e., untreated SiC_p, milled particulate Al-SiC_p composite powder, and milled particulate Al-SiC_p-Mg composite powder. The resultant composite slurries were then cast in either a fully liquid state (stir casting) or semisolid state (compocasting). Subsequently, the effects of the type of the injected powder and the casting method on the microstructural and mechanical characteristics of the cast composites at room temperature and 300 °C were investigated. The results demonstrated that distribution of SiC particles in the matrix were greatly improved by injecting milled composite powders instead of untreated SiC particles into the melt. Also casting the composite slurries in a semisolid state instead of fully liquid state slightly improved the distribution. The ultimate tensile strength, yield strength and elongation at room temperature of Al356/5 vol.% SiC_p composite manufactured by compocasting of the (Al-SiC_p-Mg)_cp-injected melt were increased by 113%, 90% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiC_p injected melt. The improvements in these properties at 300 °C were about 100%, 103% and 129%, respectively. Almost all the composite samples retained more than 90% of their strengths at 300 °C, whereas the monolithic samples lost more than 25% of their strength at this temperature. The composites manufactured by compocasting of (Al-SiC_p-Mg)_cp-injected melts exhibited a typical ductile fracture surface with equiaxed dimples at both room temperature and 300 °C.     

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.     

Processing,microstructure and properties of in-situ reinforced SiC matrix composites

SiC matrix composites were fabricated by in-situ formation of transition metal boride and carbide particles from oxide powders by carbothermal reactions. Dense composites with various microstructures were produced by pressureless sintering and additional hot-isostatic pressing. The microstructures and mechanical properties of the composites were dependent upon the pressureless-sintering temperature. The use of submicron-sized TiO₂ lead to fine and equiaxial TiB₂ particulates. The composites exhibited high flexural strengths (>700 MPa). At higher sintering temperatures, the grain growth of SiC swept the boride into clusters with larger sizes and anisotropic shapes, which improved the fracture toughness of the composite at the expense of strength.     

Sinter-HIP of polymer-derived Al2O3–SiC composites with high SiC contents

Submicrometer Al₂O₃ composites with more than 20 vol.% of SiC particles were produced using a multiple infiltration of porous bodies with a liquid polymer SiC precursor. The fully dense composites were successfully densified using a sinter-HIP process. Parameters of sintering and HIP steps are discussed with respect to both densification and microstructure evolution of the composites. The initial pressure during the sintering step plays an important role for the preparation of fully dense composites with a submicrometer alumina matrix at 1750 °C. Optimized densification schedule of sinter-HIP represents a novel approach of densification at relatively mild conditions compared to previously reported or common densification methods of Al₂O₃–SiC composites with high SiC content, such as pressureless sintering, hot pressing and post-HIPing. The method expands the possibilities for preparation of alumina based composites with SiC volume fraction > 20 vol.%, filling the gap in available literature data.     

Mechanical properties of rolled A356 based composites reinforced by Cu-coated bimodal ceramic particles

Three kinds of A356 based composites reinforced with 3 wt.% Al₂O₃ (average particle size: 170 μm), 3 wt.% SiC (average particle size: 15 μm), and 3 wt.% of mixed Al₂O₃–SiC powders (a novel composite with equal weights of reinforcement) were fabricated in this study via a two-step approach. This first process step was semi-solid stir casting, which was followed by rolling as the second process step. Electroless deposition of a copper coating onto the reinforcement was used to improve the wettability of the ceramic particles by the molten A356 alloy. From microstructural characterization, it was found that coarse alumina particles were most effective as obstacles for grain growth during solidification. The rolling process broke the otherwise present fine silicon platelets, which were mostly present around the Al₂O₃ particles. The rolling process was also found to cause fracture of silicon particles, improve the distribution of fine SiC particles, and eliminate porosity remaining after the first casting process step. Examination of the mechanical properties of the obtained composites revealed that samples which contained a bimodal ceramic reinforecment of fine SiC and coarse Al₂O₃ particles had the highest strength and hardness.     

Nanosilicon carbide/hydroxyapatite nanocomposites: structural,mechanical and in vitro cellular properties

In this study, bioceramic nanocomposites were synthesized by sintering compacted bodies of hydroxyapatite (HA) mixed with 5 or 15 wt% nanosilicon carbide at 1100 or 1200°C in a reducing atmosphere. Pure hydroxyapatite was also prepared for comparison. Phase compositions, structural and physical properties of the composites were studied using appropriate techniques. Some in vitro biological properties of the composites were also investigated by using newrat calvaria osteoblastic cells. X-ray diffraction analysis indicated that tricalcium phosphate (TCP) comprising negligible α-TCP and considerable β-TCP were formed in composites during sintering meanwhile hydroxyapatite and silicon carbide (SiC) were also existed in the composition. Based on the results, that composite made of 5 wt% nanosilicon carbide exhibited higher bending strength, fracture toughness and bulk density than pure HA and composite with 15 wt% silicon carbide. The scanning electron microscopy coupled with energy dispersive X-ray analysis revealed that the addition of nanosilicon carbide suppressed the grain growth and yielded a feature of island-type clusters consisting of blistered calcium phosphate (HA and TCP) and SiC grains. Also, in this study, better proliferation rate and alkaline phosphatase activity were observed for the osteoblastic cells seeded on top of the composites compared to pure HA. Overall, the results indicated that the composite of 95 wt% hydroxyapatite and 5 wt% SiC exhibited better mechanical and biological properties than pure HA and further addition of SiC failed strength and toughness.     

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.     

Ultrasonic assisted fabrication of particle reinforced bonds joining aluminum metal matrix composites

This paper presents a method of producing uniform particle strengthened bonds between pieces of aluminum metal matrix composite (Al-MMCs), of strength equal to that of the substrate material. SiC particle reinforced Zn-based filler metals were fabricated by mechanical stir casting and ultrasonic treatment, and then used to join pieces of SiC_p/A356 composite with the aid of ultrasonic vibration. The filler metals made by mechanical stirring were porous and contained many particle clusters. Ultrasonic vibration was used to disperse the agglomerates and prevent further coagulation of SiC particles during joining, but the method failed to eliminate the porosity, resulting in a highly porous bond. The filler metal treated by ultrasonic vibration was free of defects and produced a non-porous bond strengthened with uniform particles between pieces of SiC_p/A356 composite. The presence of surface oxide films at the bonding interface significantly degraded the performance of SiC particle reinforced bond. Removal of this oxide film by at least 4 s of ultrasonic vibration significantly increased the bond strength, reaching a value equal to that of the substrate metal.     

Effect of SiC Nanoparticles Content and Mg Addition on the Characteristics of Al/SiC Composite Powders Produced via In Situ Powder Metallurgy Method

In the present study, the effect of the nanosized SiC particles loading and Mg addition on the characteristics of Al/SiC composite powders produced via a relatively new method called “in situ powder metallurgy” (IPM) was investigated. Specified amounts of SiC particles (within a size range of 250 to 600 µm) together with SiC nanoparticles (average size of 60 nm) were preheated and added to aluminum melt. This mixture was stirred via an impeller at a certain temperature for a predetermined time. The liquid droplets created by this process were then subsequently cooled in air and screened through 250 µm sieve to separate micron-sized SiC particles from solidified aluminium powder particles containing nanosized SiC particles. Results of SEM and TEM studies together with microhardness measurements revealed that the commercially pure (CP) Al could not embed as-received SiC particles. However, the nanosized particles were distributed uniformly in the Al-1 wt% Mg powders. The process yield and microhardness of the Al-1Mg composite powders increased with increasing the contributed amount of nanosized SiC particles.     

The sintering of combustion-synthesized titanium diboride

A comparative study of the sinterability of combustion-synthesized titanium diboride was conducted over the temperature range of 1800 to 2100° C. During the initia! sintering stage, the densification rate was slightly higher in the combustion-synthesized than in the commercially obtained titanium diboride. For sintering times of > 30 min, however, the shrinkage rates for both types of powders were the same. The activation energy for the late sintering stage was 774 ± 46 kJ mol^–1, consistent with 8 volume diffusion mechanism, end was the same for both combustion-synthesized and commercial powders. The microstructures of sintered specimens with initial particle size below 1O µm exhibited a grain size ranging from 5 to over 40 µm after 30 min of sintering. The addition of 5 wt% NbB₂ to the combustion-synthesized resulted in enhanced shrinkage during the initial sintering stage, but did not affect later stage kinetics. Various amounts of additives of CrB₂, NiB and TiC had no effect on early and late stage sintering kinetics, with the exception of 50 wt% TiC which appreciably inhibited densification.     

Effects of amorphous silica coatings on the sintering behaviours of SiC whisker-reinforced Al2O3 composites

In this study, pressureless sintering of silicon carbide whisker (SiC_w)-reinforced alumina composites was investigated. SiC whiskers or Al₂O₃ powders were coated with amorphous silica, and sintering behaviour was analysed according to the powder characteristics of the composite. It was found that amorphous silica coatings improved densification as compared with uncoated powders, because the viscous flow allows the release of any tensile stress due to differential shrinkage between the matrix and the silicon carbide whiskers. Mullite occurred when amorphous silica coatings reacted with alumina at 1500 °C, which resisted the viscous sintering of the amorphous silica coatings.     

Preparation of AlN-Cu composite powders by electroless plating of controlled oxidized nitride particles to prevent degradation by hydrolysis

The present work describes the preparation of AlN-Cu composite powders by electroless plating. Initially, the hydrolysis reaction of the ceramic particles in the electroless solution was studied as a reference element for the design of a protective surface barrier that enabled the coating process, with no ceramic phase degradation. The metal source of the electrolytic bath was copper sulfate, with formaldehyde as the reducing agent, under alkaline conditions of pH 12. The microstructural characterization indicated the formation and growth of aluminum hydroxides from AlN particles, inhibiting the coating of Cu by increasing the OH⁻ ions in the solution. As the exposure time increased, the ellipsoidal bayerite grew from AlN and transformed into prismatic particles of the thermodynamically more stable gibbsite phase. To prevent the degradation of AlN, a controlled oxidation stage was implemented to form a protective barrier of non-reactive alumina on the surface through thermal treatment in oxidizing atmospheres. An atmosphere of dry air was found to be more appropriate than pure oxygen for the formation of a continuous and dense layer of crack-free alumina on nitride surfaces, and a temperature of 1000 °C for 1 h enabled the formation of 3.9 by weight of α-Al₂O₃, capable of reducing the hydrolysis reaction of AlN. The process of autocatalytic deposition on the passivated particles, applied in three consecutive steps of metallization, led to AlN-Cu composite powders with 29 wt% Cu. Finally, the coated powders were treated in a hydrogen-reducing atmosphere at 400 °C to remove traces of the Al(OH)₃ phase encountered, as well as to improve the adhesion of the nanostructured deposit of the cauliflower-like structure to the AlN surfaces, obtaining AlN-Cu composite powders suitable for the preparation of metal/ceramic composites.     

Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si₃N₄ particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si₃N₄ powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si₃N₄ powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.     

A comparison study of the agglomeration mechanism of nano- and micrometer aluminum particles

The agglomeration mechanism of micro- and nanosize aluminum particles with a primary mean particle diameter of 4.5 μm and 75 nm, respectively, was comparatively investigated under an incident shock wave. The morphology, particle size, and agglomeration process of micro- and nanometer alumina particles were comprehensibly compared by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. Images of X-ray diffraction reveal that a varied of phases of alumina (γ-, δ-, ε-, and α-Al₂O₃) were simultaneously found in the nanosize alumina products, which may give some detail information of the wide variety of reacting temperature of aluminum nanoparticles, while Al₄C₃ was detected in micrometer alumina products, which also gives some dynamic information of aluminum to alumina, i.e., aluminas have actually reacted with the free active carbon atoms to produce their intermediates. The microstructure of aluminas induced by the incident shock waves was detected and analyzed by using transmission electron microscopy combined with X-ray photoelectron spectroscopy spectrum. These results are an additive evidence to support that the initial stage sintering of the alumina nanosize powders is dominated by grain boundary diffusion, while the volume diffusion is the main character for the initial stage sintering of the micrometer alumina powders.     

Effect of heat treatment on microstructure and interface of SiC particle reinforced 2124 Al matrix composite

The microstructure and interface between metal matrix and ceramic reinforcement of a composite play an important role in improving its properties. In the present investigation, the interface and intermetallic compound present in the samples were characterized to understand structural stability at an elevated temperature. Aluminum based 2124 alloy with 10 wt.% silicon carbide (SiC) particle reinforced composite was prepared through vortex method and the solid ingot was deformed by hot rolling for better particle distribution. Heat treatment of the composite was carried out at 575 °C with varying holding time from 1 to 48 h followed by water quenching. In this study, the microstructure and interface of the SiC particle reinforced Al based composites have been studied using optical microscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electron probe micro-analyzer (EPMA) associated with wavelength dispersive spectroscopy (WDS) and transmission electron microscopy (TEM) to identify the precipitate and intermetallic phases that are formed during heat treatment. The SiC particles are uniformly distributed in the aluminum matrix. The microstructure analyses of Al–SiC composite after heat treatment reveal that a wide range of dispersed phases are formed at grain boundary and surrounding the SiC particles. The energy dispersive X-ray spectroscopy and wavelength dispersive spectroscopy analyses confirm that finely dispersed phases are CuAl₂ and CuMgAl₂ intermetallic and large spherical phases are Fe₂SiAl₈ or Al₁₅(Fe,Mn)₃Si. It is also observed that a continuous layer enriched with Cu and Mg of thickness 50–80 nm is formed at the interface in between Al and SiC particles. EDS analysis also confirms that Cu and Mg are segregated at the interface of the composite while no carbide is identified at the interface.