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.     

Processing of diamond-particle-dispersed silver-matrix composites in solid–liquid co-existent state by SPS and their thermal conductivity

Diamond-particle-dispersed silver (Ag) matrix composite was fabricated in a unique fabrication method, where solid–liquid coexistent state of the powder mixture of diamond, pure Ag and pure Si was designed to create during spark plasma sintering (SPS) process. The composite is well consolidated in a temperature range between 1113 K and 1188 K and no reaction was detected by scanning electron microscopy at the interface between diamond particles and the Ag matrix. The relative packing density of the diamond–Ag composite fabricated was 95–97% in a volume fraction range of diamond between 40% and 50%. The thermal conductivity of the diamond–Ag composite containing 50 vol.% (v/o) diamond reached 717 W/mK, approximately 80% the theoretical thermal conductivity calculated by Maxwell–Eucken’s equation. This result suggests that the solid–liquid co-existent state during SPS consolidation is very effective not only for rapid densification of the composite but also for producing strong bonding between the diamond particles and the Ag matrix.     

Effect of metalloid silicon addition on densification,microstructure and thermal–physical properties of Al/diamond composites consolidated by spark plasma sintering

Aluminum matrix composites reinforced with diamond particles were consolidated by spark plasma sintering. Metalloid silicon was added (Al–Si/diamond composites) to investigate the effect. Silicon addition promotes the formation of molten metal during the sintering to facilitate the densification and enhance the interfacial bonding. Meanwhile, the alloying metal matrix precipitates the eutectic-Si on the diamond surfaces acting as the transitional part to protect the improved interface during the cooling stage. The improved interface and precipitating eutectic-Si phase are mutually responsible for the optimized properties of the composites. In this study, for the Al–Si/diamond composite with 55 vol.% diamonds of 75 μm diameter, the thermal conductivity increased from 200 to 412 Wm⁻¹ K⁻¹, and the coefficient of thermal expansion (CTE) decreased from 8.9 to 7.3 × 10⁻⁶ K⁻¹, compared to the Al/diamond composites. Accordingly, the residual plastic strain was 0.10 × 10⁻³ during the first cycle and rapidly became negligible during the second. Additionally, the measured CTE of the Al–Si/diamond composites was more conform to the Schapery’s model.     

Thermomechanical properties of copper–carbon nanofibre composites prepared by spark plasma sintering and hot pressing

Several types of carbon nanofibres (CNF) were coated with a uniform and dense copper layer by electroless copper deposition. The coated fibres were then sintered by two different methods, spark plasma sintering (SPS) and hot pressing (HP). The Cu coating thickness was varied so that different volume fraction of fibres was achieved in the produced composites. In some cases, the CNF were pre-coated with Cr for the improvement the Cu adhesion on CNF. The results show that the dispersion of the CNF into the Cu matrix is independent of the sintering method used. On the contrary, the dispersion is directly related to the efficiency of the Cu coating, which is tightly connected to the CNF type. Overall, strong variations of the thermal conductivity (TC) of the composites were observed (20–200 W/mK) as a function of CNF type, CNF volume fraction and Cr content, while the coefficient of thermal expansion (CTE) in all cases was found to be considerably lower than Cu (9.9–11.3 ppm/K). The results show a good potential for SPS to be used to process this type of materials, since the SPS samples show better properties than HP samples even though they have a higher porosity, in applications where moderate TC and low CTE are required.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 mm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macroand microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^' (Al₃Li) and S^' (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

Characterization of hydroxyapatite– and bioglass–316L fibre composites prepared by spark plasma sintering

To improve the mechanical properties of pure hydroxyapatite (HA) ceramics and pure 45S5 bioglasses, HA–316L fibre composites and bioglass 45S5–316L fibre composites were produced by spark plasma sintering (SPS) at 950 and 850 °C, respectively. While the HA phase in the HA–316L fibre composites did not decompose after the SPS process, microcracks were found around the 316L fibres in the composites. Consequently, the HA–316L fibre composites could not effectively improve the mechanical properties of the pure HA ceramics. In contrast, the bioglass 45S5–316L fibre composites showed no microcracks around the 316L fibres and thus exhibited bending strengths of up to 115 MPa.     

Microstructure and mechanical properties of a copper–stainless-steel dual-phase system prepared by spark plasma sintering

Spark plasma sintering (SPS) has been used to synthesise samples of a model dual-phase (DP) system, consisting of copper and AISI420 martensitic steel. Mixed powders were sintered at different temperatures in the range from 850°C to 1000°C using a loading pressure of 60?MPa. Tensile testing revealed dimpled fracture surfaces in samples sintered at higher temperatures, indicating ductile failure and good interfacial bonding between the phases, with the best results overall obtained for samples sintered from wet-mixed powders. The results show that by control of the mixing, sintering and post-sintering heat treatment conditions, the fabrication of dense samples covering a range of DP microstructures is possible using SPS, with independent control of the microstructure and properties of the two phases.     

Microstructures and properties of high-fraction Sip–6061Al composites fabricated by pressureless sintering

Pre-treated Si powder (Sip) and 6061Al powder were used to fabricate high-fraction Sip/6061Al composites via pressureless sintering, and the effects of the Sip content and the sintering temperature on the microstructures and properties of the composites were studied. The results show that in the composites, there exist MgAl₂O₄ nanocrystalline particles, and the Si phase varied from a discontinuous particulate state to a semi-continuous skeleton state as the Si content increased from 30 to 50?wt-%. Densities, bending strengths, hardness, and thermal conductivities of the composites all increased initially and then decreased with the sintering temperature. The 680°C sintered 30?wt-% Sip/6061Al composites and the 700°C sintered 50?wt-% Sip/6061Al composites have the optimal mechanical and thermophysical properties.     

Mechanical properties of carbon nanotube–alumina nanocomposites synthesized by chemical vapor deposition and spark plasma sintering

Carbon nanotubes–alumina (CNT–Al₂O₃) nanocomposites with variable CNT content were directly synthesized by chemical vapor deposition (CVD). The as-grown CNT–Al₂O₃ mixture was densified by spark plasma sintering (SPS) at 1150 and 1450 °C. Vickers hardness of 9.98 GPa and fracture toughness of 4.7 MPam^1/2 were obtained for 7.39 wt.% CNT–Al₂O₃ nanocomposite. The addition of CNTs gives rise to 8.4% increase in hardness and 21.1% increase in toughness over that of the pure Al₂O₃. The optimum amount of CNTs is considered to be able to significantly enhance the mechanical property of ceramics in composites.     

Al matrix composites reinforced by in situ synthesized graphene–Cu hybrid layers: interface control by spark plasma sintering conditions

Journal of Materials Science - Tremendous impacts are usually made by the synthesis method and consolidation technique on microstructure and interface of graphene/Al composites. In the present...     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Effect of Fe on the sintering and thermal properties of Mo–Cu composites

Effects of Fe on the sintering and thermal properties of Mo–Cu composites have been investigated. Mo–Cu–xFe composites are fabricated by powder metallurgy techniques with addition of various Fe contents ranging from 0.4 wt% to 2.2 wt%. The thermal properties and action mechanism of Fe to Mo–Cu composites are discussed. Results have indicated that the coefficient of thermal expansion (CTE) and thermal conductivity (TC) of Mo–Cu composites are greatly affected by the addition of Fe contents. It has also been observed that the fabricated composite powders with Fe additions exhibit high sinterability. Also, the inclusion of Fe can active the sintering course in shorter times and decline the sintering temperature thus also improving the physical properties of composites. Furthermore, it is also concluded that the utilization of steel kettle and steel balls for milling the Mo–Cu powders is also beneficial to improve the physical and thermal properties of Mo–Cu alloy.     

Fabrication and characterizations of Zn1?xCoxO bulk ceramics prepared by solid state reaction combined with spark plasma sintering

Zn_1?xCo_xO (x?=?0.01, 0.05 and 0.1) bulk ceramics were prepared through a two-step, solid state reaction method combined with spark plasma sintering technique. The single phase Zn_1?xCo_xO powders were synthesized using ZnO and Co₃O₄ at 935?°C in air for 3?h. The Zn_1?xCo_xO bulks were prepared at sintering temperature from 900 to 1,100?°C for 5?min by SPS. The relative density of Zn_1?xCo_xO bulk ceramics sintered at 1,100?°C is higher than 99% of the theoretical value. The Structure, composition analysis, optical absorption, Raman and XPS measurements revealed that the Co²⁺ substituted Zn²⁺ ions and was incorporated into the lattice of ZnO in both of the single phase Zn_1?xCo_xO powders and bulk ceramics. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior and that the paramagnetic Co amount is smaller than the nominal Co concentration for all of Zn_1?xCo_xO samples at 4?K. The paramagnetic magnetism of bulk ceramics is apparently larger than that of powders with the same composition. The electrical properties measurements reveal that the Co concentration has a slight influence on the electrical properties of Zn_1?xCo_xO bulk ceramics. The carriers concentration is about 1?×?10²⁰?cm^?3 and with the Co concentration increases the resistivity slightly increases from 3.56?×?10^?3 (x?=?0.01) to 5.58?×?10^?3 (x?=?0.1) Ωcm.     

Formation of pure $$tau$$ τ -phase in Mn–Al–C by fast annealing using spark plasma sintering

Journal of Materials Science - Mn–Al–C is intended to be one of the “gap magnets” with magnetic performance in-between ferrites and Nd-Fe-B. These magnets are based on the...     

Structure and mechanical properties of TiB2/TiC – Ni composites fabricated by pulse plasma sintering method

TiB₂/TiC – Ni composites were synthesized starting from the powders of Ti, B₄C and Ni, using Pulse Plasma Sintering (PPS) method. Typically used one-step (1100?°C–10?min.) and novel double-step sintering processes (900?°C–10?min. +1100?°C–5?min.) were applied and compared. XRD studies showed that the composite obtained by double-step sintering consisted of TiB₂, TiC and Ni phases. For one-step processing additionally undesired Ni₃B and graphite were detected. SEM observations revealed dark-grey grains of TiB₂, light-grey grains of TiC (both around 25?µm in size) and Ni areas surrounded by TiC. The composites synthesized in one- and double-step processes revealed the hardness and relative density of 2335 HV5 (±110) and 97.8% and 2470 HV5 (±70) and 99.8%, respectively. Novel double-step sintering process allowed to avoid undesired phases (graphite, Ni₃B) and only TiB₂, TiC and Ni were present in the structure. Additionally it was possible to decrease the temperature of the process comparing to other fabrication methods.     

Enhanced properties of alumina–aluminium titanate composites obtained by spark plasma reaction-sintering of slip cast green bodies

Full dense alumina + 40 vol.% aluminium titanate composites were obtained by colloidal filtration and fast reaction-sintering of alumina/titania green bodies by spark plasma sintering at low temperatures (1250–1400 °C). The composites obtained had near-to-theoretical density (>99%) with a bimodal grain size distribution. Phase development analysis demonstrated that aluminium titanate has already formed at 1300 °C. The mechanical properties such as Vickers hardness, flexural strength and fracture toughness of bulk composites are significantly higher than those reported elsewhere, e.g. the composite sintered at 1350 °C show values of about 24 GPa, 424 MPa and 5.4 MPa m^1/2, respectively. The improved mechanical properties of these composites are attributed to the enhanced densification and the finer and more uniform nanostructure achieved by non-conventional fast sintering of slip-cast dense green compacts.     

Enhanced thermal conductivity and dielectric properties of Al/β-SiCw/PVDF composites

Polymeric composites with high thermal conductivity, high dielectric permittivity but low dissipation factor have wide important applications in electronic and electrical industry. In this study, three phases composites consisting of poly(vinylidene fluoride) (PVDF), Al nanoparticles and β-silicon carbide whiskers (β-SiC_w) were prepared. The thermal conductivity, morphological and dielectric properties of the composites were investigated. The results indicate that the addition of 12 vol% β-SiC_w not only improves the thermal conductivity of Al/PVDF from 1.57 to 2.1 W/m K, but also remarkably increases the dielectric constant from 46 to 330 at 100 Hz, whereas the dielectric loss of the composites still remain at relatively low levels similar to that of Al/PVDF at a wider frequency range from 10⁻¹ Hz to 10⁷ Hz. With further increasing the β-SiC_w loading to 20 vol%, the thermal conductivity and dielectric constant of the composites continue to increase, whereas both the dielectric loss and conductivity also rise rapidly.     

Processing and superplastic properties of fine grained Si3N4/Al–Mg–Si composites

Abstract

Composites with an Al–Mg–Si alloy matrix containing 20 vol.-% of either Si₃N₄ whiskers or Si₃N₄ particulates were extruded at 773 K with a reduction ratio of 100: 1, and tensile experiments were performed under conditions of constant true strain rate. Recrystallisation and dynamic precipitation occurred during hot extrusion so that very small grain sizes of less than ~ 3 ;amp;#x03BC;m were produced. The extruded composites showed superplastic behaviour at high strain rates (above 10^?1 S^?1). The high strain rate superplasticity of the composites is attributed to the very small grain sizes. Internal cavities developed during straining and density studies revealed that the rate of increase of the extent of cavitation was lower at a temperature slightly above the partial melting temperature than at a temperature lower than the partial melting temperature. It is concluded that the presence of a liquid phase restricts the development of cavities because the liquid phase serves to relax the stress concentrations.

MST/3139     

Tribological behavior of Ti3SiC2/(WC–10Co) composites prepared by spark plasma sintering

The dry sliding friction and wear behavior of Ti₃SiC₂/(WC–10Co) composites (TWCs) against GCr15 steel pair at room temperature was investigated through the determination of friction coefficient and wear rate under different conditions and the analysis of the morphologies and compositions of wear debris, worn surfaces of TWC and GCr15 steel. The friction coefficients of TWC with 3 wt.% WC–10Co were in the range of 0.40–0.48, and the wear rate varied from 0.6 × 10⁻⁴ mm³ (N m)⁻¹ to 1 × 10⁻⁴ mm³ (N m)⁻¹. At the load of 10 N and sliding speed of 0.353 m/s, the glazes were formed on the worn surfaces of TWC. The wear mechanisms were complicated, including micro-cutting and abrasive wear of TWC, oxidation wear of GCr15 steel, as well as adhesive wear caused by the glaze flaking.     

Phase,microstructure and properties evolution of fine-grained W–Mo–Ni–Fe alloy during spark plasma sintering

Fine-grained tungsten (W) heavy alloy containing molybdenum (Mo) with W particle sizes of less than 5 μm were fabricated by spark plasma sintering (SPS) pre-milling W–2Mo–7Ni–3Fe powder at a lower temperature of 1000–1250 °C. Phase, microstructure and mechanical properties evolution of W–Mo–Ni–Fe alloy during spark plasma sintering were studied in detail. As increasing sintering temperature, the hardness of the alloy decreased rapidly. However, bending strength of the alloy demonstrated a fall–rise–fall trend, and the maximum strength was obtained at 1150 °C. The W–2Mo–7Ni–3Fe alloy microstructure was composed of white W-grain, gray W-rich structure, black γ-(Ni, Fe, W, Mo) binding phase, and deep-gray W-rich structure. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–2Mo–7Ni–3Fe alloy.