SiC Preforms Research Articles

Effect of controlled interfacial reaction on the microstructure and properties of the SiCp/Al composites prepared by pressureless infiltration

High volume fraction SiCp/Al composite is prepared by pressureless infiltration. In this paper, the effect of both the addition of Si to aluminum and passive oxidation of SiC to control SiC/Al interfacial reaction on the microstructure, thermal conductivity and coefficient of thermal expansion of infiltrated composite is investigated. TEM and X-ray results showed that severe interfacial reaction occurred between SiC reinforcement and Al matrix by the infiltration of the unoxidized SiCp preform by Al–8 wt% Mg at 1000 °C. And the reaction can be suppressed by the use of aluminum with 12 wt% Si addition or the SiC preform oxidized in air at 1200 °C for 3 h. The method of addition of Si to aluminum is more favorable to prepare composites with higher thermal conductivity, lower coefficient of thermal expansion and porosity as compared to the method of passive oxidation of SiC. The effect mechanism of both methods on the microstructure and thermal–physical properties is discussed.

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.

Composites by aluminum infiltration of porous silicon carbide derived from wood precursors

Composites were fabricated by infiltration of molten Al–13Si–9Mg into porous SiC derived from wood. The composite microstructure retains the heterogeneous orthorhombic symmetry of the initial wood material. As compared to the uninfiltrated SiC preform, the composite shows much improved transverse compressive strength and stiffness, resulting in reduced anisotropy.

Effect of calcination process on the properties and microstructure of SiC preform and corresponding SiCp/Al composites synthesis by pressureless infiltration

The influence of calcination time and temperature on the property and microstructures of the SiC preform prepared by ceramic injection molding and corresponding SiCp/Al composites by pressureless infiltration was investigated. It was found that the commutative bonding of generated SiO 2 layer on the surface of the SiC was the source of obturated pores and strengthened the preform. The effect of the temperature on the open porosity ratio ( P OPR) and strength of the preform was more than that of calcination time because the diffusion of oxygen in the oxide layer was controlled by the temperature, not by the time. The aluminum could destroy the obstacle of SiO 2 during the infiltration by the interface reaction and infiltrated the obturated pores so that the porosity of the composite was lower than the obturated porosity of the corresponding preform. A better calcination condition was found to be at 1100 °C for 8 h in vacuum at which the compressive strength of the preform was about 0.4 MPa and the porosity of the corresponding composites was below 0.3% though the obturated porosity ratio of the preform was as high as 2.6%.

Al/SiC Carriers for Microwave Integrated Circuits by a New Technique of Pressureless Infiltration

Materials with high thermal conductivity and thermal expansion coefficient matching with that of Si or GaAs are being used for packaging high density microcircuits due to their ability of faster heat dissipation. Al/SiC is gaining wide acceptance as electronic packaging material due to the fact that its thermal expansion coefficient can be tailored to match with that of Si or GaAs by varying the Al:SiC ratio while maintaining the thermal conductivity more or less the same. In the present work, Al/SiC microwave integrated circuit (MIC) carriers have been fabricated by pressureless infiltration of Al-alloy into porous SiC preforms in air. This new technique provides a cheaper alternative to pressure infiltration or pressureless infiltration in nitrogen in producing Al/SiC composites for electronic packaging applications. Al-alloy/65vol% SiC composite exhibited a coefficient of thermal expansion of 7/spl times/10/sup -6/ K/sup -1/ (25/spl deg/C-100/spl deg/C) and a thermal conductivity of 147 Wm/sup -1/K/sup -1/ at 30/spl deg/C. The hysteresis observed in thermal expansion coefficient of the composite in the temperature range 100/spl deg/C-400/spl deg/C has been attributed to the presence of thermal residual stresses in the composite. Thermal diffusivity of the composite measured over the temperature range from 30/spl deg/C to 400/spl deg/C showed a 55% decrease in thermal diffusivity with temperature. Such a large decrease in thermal diffusivity with temperature could be due to the presence of micropores, microcracks, and decohesion of the Al/SiC interfaces in the microstructure (all formed during cooling from the processing temperature). The carrier showed satisfactory performance after integrating it into a MIC.

Fabrication of the Aluminum Matrix Composite by Ultrasonic Infiltration Technique

In order to infiltrate the molten aluminum alloy to the reinforcement preform by low pressure in casting process and acquire the high performance composites with high density, the effect of the ultrasonic vibration on the infiltration was investigated by model experiments using clear solution and glass or aluminum borate preform, which is correspond to a molten matrix and reinforcement, respectively. Ultrasonic vibration improves the wettability of liquid polyester resin on glass plate, dramatically. The final infiltration height and infiltration speed of liquid polyester resin in glass capillary were improved by the ultrasonic vibration. Furthermore, the infiltration speed of water to aluminum borate preform accelerated by ultrasonic vibration. This effect was more remarkable, when the infiltration height is lower or infiltration time is shorter. In actual, the molten aluminum alloy infiltrate to SiC preform using ultrasonic vibration easily and acquire the high dense composites without pores.

Processing of squeeze cast Al6061–30vol% SiC composites and their characterization

6061 aluminum matrix composites containing 30vol% SiC with mean mass particle size of 16 and 22 μm as the reinforcing phase were synthesized by squeeze casting route. The process consisted of fabricating of SiC preform followed by squeeze casting of 6061 aluminum alloy. The SiC preform was made by blending SiC particles with silica as a coal and sintering. The microstructural features, aging behavior, mechanical properties and wear characteristics of the composites were investigated. The results indicated that a homogenous distribution of SiC particles in the aluminum matrix, which is almost free of pores, can be obtained by squeeze casting method. Higher strength and lower wear rates were observed in the composite materials than the unreinforced aluminum alloy part. However, a marked decrease in ductility of the composites arising from adding of SiC particles was obtained. The aging behavior of the aluminum alloy was found to be influenced by the reinforced materials, i.e. the time to obtain the peak hardness in T6 treatment was lower for the composites than the unreinforced material. The fracture surfaces of the unreinforced and composite samples exhibited dimple surfaces. The wear resistance of the aluminum alloy was improved by reinforcing with SiC particles and it is comparable with unalloyed pearlitic cast iron. This article presents the effect of SiC particles on the aging behavior, mechanical properties and wear resistance of 6061 aluminum alloy made by squeeze casting method.

Infiltration of SiC preforms with iron silicide melts: microstructures and properties

Spontaneous infiltration of SiC preforms with Fe X Si Y (=Fe 3Si, Fe 5Si 3 and FeSi) melts produced SiC/Fe X Si Y composites with nearly full density (≥96.5% theoretical density). The phases, microstructures and mechanical properties have been studied with conventional materials characterization techniques including X-ray diffraction, optical microscopy, scanning electron microscopy etc. A key issue behind these studies is dissolution of SiC in Fe X Si Y melts during infiltration. The dissolution of SiC in Fe 3Si melt led to carbon precipitation and phase changes (Fe 3Si disappeared and Fe 5Si 3 and FeSi formed). However, Fe 5Si 3 and FeSi infiltration gave no carbon precipitation, but SiC sintering, particle coalesce and grain growth instead. Extra-large SiC grains and SiC single crystals were found in fine SiC (0.5 μm) infiltrated by Fe 5Si 3. Phase equilibrium calculations for Fe–Si–C system were performed at 1873 K using chemsage in order to study SiC solubility and the condition for the precipitation of SiC rather than C in Fe–Si melt. Mechanical properties including micro-hardness, bending strength and Weibull modulus of all infiltrated samples were tested and analyzed on the basis of phase and microstructure observations.

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

SiCp/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiCp/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiCp/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiCp/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiCp/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiCp/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.

Fabrication of Silicon Carbide‐Mullite Composite by Melt Infiltration

The melt infiltration method was used to fabricate a SiC‐mullite composite at high temperature. Mullite was successfully obtained from a SiO2 and Al2O3 powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO2 and leaving Al2O3 and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.

The production of Al-Mg alloy/SiC metal matrix composites by pressureless infiltration

Metal matrix composites have been produced by pressureless infiltration of Al-Mg alloys into SiC preforms at 900°C under N2 for different infiltration times. The wettability of the ceramic reinforcement by the Al-Mg alloy is crucial in determining whether an MMC can be produced by pressureless infiltration. Sessile drop results show that Al alloys with Mg contents greater than 8 wt% had a contact angle lower than 90°C after 5 minutes contact time. This was in agreement with the pressureless infiltration results as MMCs have been produced after 30 minutes with these alloys. Sessile drop experiments also show that SiC is similarly wetted by Al-Mg alloys under both N2 and Ar. It is concluded that the infiltration process does not involve the intermediate nitride phase suggested by other authors.

Al–SiC ELECTRONIC PACKAGES WITH CONTROLLED THERMAL EXPANSION COEFFICIENT BY A NEW METHOD OF PRESSURELESS INFILTRATION

A method is described whereby SiC preforms may be infiltrated in air by a molten aluminum alloy to yield a metal matrix composite with thermal expansion coefficients that are suitable for electronic packaging, such as in microwave integrated circuits. Crucible geometries and infiltration conditions may be designed to provide inexpensive and reusable containers and useful section thicknesses in ∼1 hr. Intricate geometries may be fabricated in the green state of the ceramic, prior to infiltration, thereby minimizing final machining of the composite.

Infiltration of Al-Mg Alloy into SiC Preform in the Production of Metal Matrix Composites by Pressureless Infiltration

ABSTRACTMolten Al-Mg alloys has been infiltrated into SiC to produce Al matrix composites at 900oC for different infiltration times under N2. The wettability of the ceramic reinforcement by the Al-Mg alloy is crucial in determining whether Al can be infiltrated into ceramic preform. Sessile drop results showed that Al alloys with Mg contents greater than 8wt-% had a contact angle lower than 90o, after 5 minutes contact time, this was associated with the infiltration of Al-Mg alloys into SiC preform occurred after 30 minutes. Sessile drop experiments also show that SiC is similarly wetted by Al-Mg alloys under N2. It is concluded that the infiltration process does not involve the intermediate nitride phase suggested by other authors. These materials were also charaterised both mechanical properties and microstructure.

Degradation at 1200°C of a SiC coated 2D-Nicalon/C/SiC composite processed by SICFILL® method

The thermal stability of a 2D-Nicalon/C/SiC composite was studied through the variation of both mechanical properties and microstructure occurring during heat treating. The composite was processed by infiltration of SiC preforms according to SICFILL® method. The material toughness was enhanced by a carbon interphase put between the fibers and the matrix. In order to improve the thermal stability a CVI layer was deposited on the carbon interphase and the specimen surfaces were CVD covered by an external SiC seal coating about 165 μm thick. The aging tests were carried out at 1200°C in air or in non oxidizing environment (vacuum). Other specimens were thermally cycled between 25 and 1150°C. Three point bending tests and Charpy impact measurements were performed before and after these treatments. The composite microstructure was investigated by scanning electron microscope (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), reflectance infrared spectroscopy (FTIR) and surface area BET measurements. The as-processed material showed a modulus of rupture (MOR) of 483 MPa and appreciable toughness. These characteristics were retained after aging (200 h at 1200°C) under vacuum. Air thermal treatments caused heavy loss of strength and increase of brittleness. Strong oxidation occurred during these last treatments at both the carbon interlayer and the matrix, while the SiC external sample coating was not oxidized. The oxygen needed for composite bulk oxidation flowed through the SiC coating due to the occasional presence of very few structural defects.

Role of Mg and Mg+Si as external dopants in production of pure Al–SiC metal matrix composites by pressureless infiltration

Metal matrix composites have been produced by pressureless infiltration of pure Al into Mg doped SiC preforms after 1 h at 900°C. Aluminium has been found to infiltrate preforms containing between 2 and 14 wt-%Mg, however Al did not infiltrate a preform containing 1 wt-%Mg. Preforms doped with 1 wt-%Mg and Si did result in infiltration. Increasing the Mg content or increasing the Si content in the preform resulted in more extensive infiltration. The effect of Mg and Mg mixed with Si on pressureless infiltration of pure Al, microstructure of MMC as well as mechanical properties are discussed. Although the dopant was uniformly distributed throughout the preform microstructural analysis and hardness measurements indicate that the resultant composite may not be uniform due to infiltration inwards from the edge to the centre of preform.

High-temperature fracture toughness of SiC-Mo5Si3C composite

Silicon carbide-based composites are candidate materials for high-temperature structural applications in heat engines and heat recovery systems [1, 2]. For these applications, the ceramic components are often needed to be manufactured by processes with near-net-shape capability to minimize the requirements to perform very expensive and difficult post-machining operations [3]. Among the various fabrication processes, the melt infiltration process, which has the advantages of near-netshape capability and short processing time, is proposed to be one of the potential processes for such applications [4]. However, SiC-based composites made by conventional melt infiltration processes showed poor high-temperature mechanical properties, which are primarily limited by the low melting-point residual phases as exhibited by the reaction-bonded SiC [5–7]. Much effort has thus been made to reduce or eliminate the low-melting-point residual phases in the composites. One way to reduce the residual phases is through alloy infiltration [8–10]. Better approaches have been demonstrated by Lim et al. [11] and Shobu et al. [12], where high-melting-point MoSi2 and Mo(Si,Al)2 were directly infiltrated into the SiC performs. Both the infiltrated SiC-MoSi2 and SiC-Mo(Si,Al)2 composites exhibited significant increases (more than a 40% increase at 1400 ◦C) in fracture strength between 1200 and 1400 ◦C. In a previous report [13], a novel SiCMo5Si3C composite was fabricated by directly infiltrating high-melting-point Mo5Si3C into the SiC preforms. The composite also showed significant increases in fracture strength between 1200 and 1500 ◦C, which was attributed to the toughening effects provided by the infiltrated Mo5Si3C phases at elevated temperatures. To clarify this point, the fracture toughness of the composite is studied in the present report. The fracture toughness for ceramics can be characterized by various techniques, such as the double torsion (DT) technique [14], the double cantilever beam (DCB) technique [15], the indentation fracture (IF) technique [16], the indentation strength (IS) technique [17], the single edge precracked beam (SEPB) technique [18], the chevron notched beam (CNB) technique [19], etc. The IS method was adopted in the present study because it has proved to be efficient and economical [20]. The preparation procedure for the composite as well as the infiltration characteristics have been reported elsewhere [13, 21], so they are only briefly introduced here. Namely, a SiC preform of dimensions 40× 16× 6 (mm) was prepared from the raw SiC powder (α, circa 3.2 μm, purity 99%, Showa Denko, Japan) through a cold isostatic pressing (CIP) process. An infiltrant of the Mo45Si30C5 composition was prepared from the raw powders of MoSi2 (circa 2.93 μm, Japan New Metals), SiC (the same as above) and Mo (circa 1.3 μm, purity 99.94%, Japan New Metals). The infiltration was performed at 2100 ◦C in an induction furnace in 1 atm argon. Typically, the temperature was ramped to 2100 ◦C in 5 min from 1000 ◦C, held for 20 min, followed by a 5-min cool down to 1000 ◦C, where the power was cut off. The powder mixture was found to melt above 2000 ◦C, and spontaneous infiltration could be achieved above 2050 ◦C. Although 20 min for infiltration was employed in the present study, several minutes were found to be sufficient for a full infiltration. The infiltrated composites are relatively dense, but the relative density could only reach 95% of the theoretical value at best. The remaining porosity is attributed to the uninfiltrated areas up to 20μm and closed pores in the SiC matrix that could not be reached during the infiltration process, as exhibited by the typical microstructure of the composite in Fig. 1. Also, the volumetric shrinkage during solidification would contribute to the porosity. Further improvement in product density is very difficult. The XRD and SEM/EDS investigations confirmed that the composite is only composed of SiC and Mo5Si3C. Beams for the IS testing were cut from the infiltrated samples using a diamond saw, and ground to the nominal dimensions of 3× 1.5× 12 (mm). The tensile surface was polished with 1 μm diamond paste to remove residual stress due to machining and to produce a finish for optical microscopic observations. The edges of the prospective tensile face were slightly chamfered.

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiCp/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO2 as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiCp/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10−6/K were obtained in 50–71 vol% SiCp/Al MMCs. The thermal conductivity of SiCp/Al composite decreased with increasing volume fraction of SiCp and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiCp/Al composite decreased with increasing volume fraction of SiCp, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.

Oxidation of MoSi2 and MoSi2-based materials

Oxidation experiments, at 500°C, of MoSi2 and MoSi2-based compounds such as Mo(Al,Si)2 and MoSi2+1 wt% C compacts have been carried out. These compacts were prepared byin situ synthesis and a compaction method, starting from the elemental powders. For comparison, commercial MoSi2 and Mo(Al,Si)2 infiltrated into SiC preform were also studied under similar conditions. It was found that the synthesized high density MoSi2 and Mo(Al,Si)2 infiltrated into SiC preform did not show any oxidation even after 100 h of heating in air. The colour of the polished surfaces of commercial MoSi2, Mo(Al,Si)2 and MoSi2+1 wt% C had changed. The SEM of Mo(Al,Si)2 showed open blisters with rods of MoO3 in them whereas MoSi2+1 wt% C surface had MoO3 rods but no blisters and the oxidation was superficial with no penetration into the compact. It is suggested that in compounds, the presence of small amounts of impurities is not as detrimental to pesting as presence of defects like open pores or cracks. Hence, high density of the compact is essential for the prevention of complete disintegration of the compact.

Microstructural evolution of silicon carbide/aluminum oxide composites processed by melt oxidation

The nucleation and growth mechanisms during high temperature oxidation of liquid Al-3 wt% Mg and Al-3 wt% Mg-7 wt% Si alloys were studied to provide a better understanding of the composite fabrication process, especially in the presence of SiC reinforcement. Al2O3-matrix composites with and without SiC particulates have been produced by directed oxidation of aluminum alloys. The microstructure consists of three interpenetrating phases: the SiC preform, a continuous α-Al2MO3 matrix, and a network of unoxidized metal. The volume fraction of metal within the oxidation product decreases with increasing processing temperature. The preform does not show any evidence of degradation by the molten alloy, but the growth front tends to climb up the particles, increasing the oxidation area and therefore enhancing the rate of composite growth. The amount of porosity was found to increase with the Mg content in the alloy, from 2.0 vol% for 0.5 wt% Mg to 5.8 vol% for 3 wt% Mg. The role of magnesium and silicon in the growth process are discussed.

Microwave densification of electrophoretically infiltrated silicon carbide composite

A new method to fabricate SiC composites by microwave heating SiC preforms is described. Preforms were produced by electrophoretically infiltrating SiC fibre (Nicalon) preforms with SiC powder. Samples were heated to 1600°C in a matter of minutes and held at temperature for 5 min to minimize fibre degradation. To achieve densification, heated preforms required the application of a uniform load. Bulk densities increased from ∼ 0.8 gcm-3 for the as-infiltrated preforms to over 1.9 gcm-3 for microwave-heated samples with a small applied load of ∼13 kPa. Microstructural analysis revealed the presence of some pores and cracks in the matrix; however, large areas of dense SiC matrix material are apparent.