Effect of coating of graphite particles with polyaniline base on charge transport in epoxy-resin composites

Investigations of charge transport, in epoxy resin composites of graphite particles coated with a non-conducting polyaniline-base layer, showed that particle shape and surface structure may crucially affect the percolation behaviour of the systems. In contrast to the gradual increase in the DC conductivity of composites in the range 20–52 vol.% for pure graphite particles, due to their fragmentary nature, a steep rise of several orders of magnitude in conductivity, appeared in composites of graphite particles coated with 10 wt.% of polyaniline base at a particle concentration 50 vol.%. The frequency and temperature dependences indicate that, in both cases, at the maximum loading used (52 vol.%), the obtained material had ohmic conductivity. In contrast, the conductivity of epoxy composites of graphite particles, coated with 20 wt.% of polyaniline base, only slightly increased over the whole range of particle concentrations. These findings suggest that, in the case of 10 wt.% polyaniline coating, due to the irregular surface structure, a certain amount of uncoated material is present, which enables the formation of conducting contacts with ohmic conductivity in the percolation area. The 20 wt.% polyaniline coating forms a compact non-conducting layer on the surface of the graphite particles, thus preventing electrical contact.     

Influence of coating on short steel fiber reinforcements on corrosion behavior of aluminium base short steel fiber reinforced composites

Steel fiber reinforced aluminium composites are attractive materials of high specific strength but exhibit poor resistance against electrochemical corrosion. The study discusses the electrochemical corrosion behavior of uncoated, copper and nickel coated short steel fiber reinforced aluminium and Al–2Mg matrix composites in 1 (N) NaCl solution. Galvanic corrosion between the steel fiber and aluminium governs the corrosion behavior of these composites. It has been observed that open circuit potential (OCP) is shifted to more negative side with copper coating on the fibers and to the more positive side on coating the fibers with nickel. Compared to the uncoated fiber higher corrosion current density indicates corrosion rate was observed for the copper coated fiber reinforced composites where as a lower current density was noted for the nickel coated fiber reinforced composites was observed. Addition of 2 wt% magnesium to aluminium alloy matrix increased the corrosion current density. The corrosion mechanism in these composites is dominated by galvanic cell formation that is evident from the dissolution of Al matrix near the peripheral region of steel fibers.     

Preparation and properties of cast aluminium-ceramic particle composites

A casting technique for preparing aluminium-alumina, aluminium-illite and aluminium-silicon carbide particle composites has been developed. The method essentially consists of stirring uncoated but suitably heat-treated ceramic particles of sizes varying from 10 to 200 m in molten aluminium alloys (above their liquidus temperature) using the vortex method of dispersion of particles, followed by casting of the composite melts. Recoveries and microscopic distribution of variously pretreated ceramic particles in the castings have been reported. Mechanical properties and wear of these composites have been investigated. Ultimate tensile strength (UTS) and hardness of aluminium increased from 75.50 MN m^–2 and 27 Brinell hardness number (BHN) to 93.15 MN m^–2 and 37 BHN respectively due to additions of 3 wt % alumina particles of 100 m size. As a contrast, the tensile strength of aluminium-11.8 wt % Si alloy decreased from 156.89 MN m^–2 to 122.57 MN m^–2 due to the addition of 3 wt % alumina particles of the same size. Adhesive wear rates of aluminium, aluminium-11.8 wt % Si and aluminium-16 wt % Si alloys decreased from 3.62×10^–8, 1.75×10^–8 and 1.59×10^–8 cm³ cm^–1 to 2.0×10^–8, 0.87×10^–8 and 0.70×10^–8 cm³ cm^–1, respectively, due to the additions of 3 wt % alumina particles.Formerly with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India.     

Al/SiC composite coatings of steels by thermal spraying

Thermal spraying has been used to coat carbon steels (F112) and austenitic stainless steels (AISI 304) with aluminium matrix composites. Mixtures of aluminium powder and SiC particles were used as spraying material. A sol-gel silica coating was laid on SiC particles to reduce the porosity of the composite coatings and to inhibit the formation of aluminium carbide. The sol-gel silica coating acts as an active barrier enhancing the wettability of the reinforcement by molten aluminium. Coatings with a reinforcement volume fraction up to 30 vol.% were obtained with porosities of about 1.0 vol.%. The incorporation of sol-gel silica coated SiC particles reduces the coefficient of thermal expansion of the composite coating and enhances its adhesion to the substrates more than when uncoated SiC particles were used.     

Thermal conductivity of Cu–Zr/diamond composites produced by high temperature–high pressure method

Copper matrix composites reinforced with about 90 vol.% of diamond particles, with the addition of zirconium to copper matrix, were prepared by a high temperature–high pressure method. The Zr content was varied from 0 to 2.0 wt.% to investigate the effect on interfacial microstructure and thermal conductivity of the Cu–Zr/diamond composites. The highest thermal conductivity of 677 W m⁻¹ K⁻¹ was achieved for the composite with 1.0 wt.% Zr addition, which is 64% higher than that of the composite without Zr addition. This improvement is attributed to the formation of ZrC at the interface between copper and diamond. The variation of thermal conductivity of the composites was correlated to the evolution of interfacial microstructure with increasing Zr content.     

Thermal conductivity of diamond/Ag composites with chromium carbide coated diamonds for the building materials of high power modules

Abstract

This paper reports on a study of the preparation and characterisation of diamond/Ag composites for the building materials of high power modules. The Cr₇C₃ coated diamond particles are utilised to improve the interfacial bonding between the Ag and diamond and composites are prepared by hot pressing technique. The characteristics of Cr₇C₃ coating layers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the Cr₇C₃ coatings on the diamonds result in a strong interfacial bonding and a greatly enhanced thermal conductivity of the composites. A largely enhanced thermal conductivity of 768 W m^?1 K^?1 is obtained in Cr₇C₃ coated composites, which increases 168% relative to that of uncoated composites at 65% diamond volume fraction. The measured thermal conductivity agrees reasonably well with the predictions by a differential effective-medium (DEM) model.     

Laser cladding nickel-titanium carbide composite coating on a 45 carbon steel: Preparation,microstructure and wear behavior

Almost fully dense nickel-titanium carbide composite coatings with varied titanium carbide content were deposited on 45 carbon steel by laser cladding. High content of titanium carbide particles up to 50 wt.% with bimodal microstructure could be homogeneously distributed in the nickel based matrix. Due to the presence of the harder nickel-titanium carbide composite coating on the 45 carbon steel, the surface hardness and wear properties were significantly improved. The Vickers hardness (HV 3) increased from about 260 HV 3 for the 45 carbon steel to 300 HV 3 – 360 HV 3 for nickel based composite coating containing 30 wt.% titanium carbide and 550 HV 3 – 680 HV 3 for nickel based composite coating containing 50 wt.% titanium carbide composite coating, respectively. The coefficient of friction and volume wear rate was reduced down to 0.41×10⁻⁶ mm³ N⁻¹ m⁻¹ and 9.3×10⁻⁶ mm³ N⁻¹ ⋅ m⁻¹ when a nickel based composite coating containing 50 wt.% titanium carbide was coated on the 45 carbon steel, respectively. The enhanced wear performance of the composite coating was due to presence of harder nickel-titanium carbide composite coating and formation of varied soft and lubricant metal oxides consisting of mainly titanium oxides and minor iron and nickel oxides.     

Investigation on mechanical properties and tribological behaviour of stir cast LM13 aluminium alloy based particulate hybrid composites

LM13 aluminium alloy with boron carbide (0 wt.%–7.5 wt.%) and fly ash (2.5 wt.%) reinforced particulate hybrid composites were fabricated using liquid metallurgy route. Microstructure and mechanical properties viz., hardness, ultimate tensile strength and ductility were investigated. Wear behaviour of composites was tested by varying sliding distance and load. Fracture surface and worn surface of composites were examined using field emission scanning electron microscope. Microstructure of hybrid composites revealed uniform dispersion of particles in LM13 aluminium alloy. Hardness and tensile strength of composites increased with increasing wt.% of boron carbide and fly ash particles. Wear test results showed that addition of particles significantly decreased the weight loss and coefficient of friction. Also cumulative weight loss decreased up to 47.2 % for 10 wt.% of hybrid composites as compared to LM13 aluminium alloy. Fracture surface of composites showed dimples with particle cracking on the surface. Worn surface of LM13 aluminium alloy showed continuous grooves due to ploughing with delamination. However, worn surface of composites showed fine grooves due to the presence of hard reinforcements on the surface. Boron carbide and fly ash reinforced LM13 aluminium hybrid composites exhibited superior mechanical properties with excellent wear resistance as compared to LM13 aluminium alloy.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Modification of the dynamic damping behaviour of jute/vinylester composites with latex interlayer

This paper reports the tailoring of jute reinforced composites for damping by the use of a suitable interlayer. The interlayer helped to improve the damping capacity of the composites, with a lowering in the stiffness values. In this work, jute cloths were taken and roller-coated with SBR latex, dried and used as reinforcement in vinylester resin matrix composites to develop interlayer jute composite. Untreated composites were prepared with 35 wt.% of jute content and 65 wt.% of resin content. In the latex interlayer composites, 30% of the matrix vinylester resin was replaced by latex, while the jute content was same. Tensile properties of the latex interlayer composites were compared with the untreated jute reinforced composites. The tensile strength and tensile modulus were lowered by 66% and 93% respectively in case of the interlayer composites compared to the uncoated ones, but the energy absorption increased by 183%. The interlayer composites showed high loss modulus values at room temperature. The damping parameter of the interlayer composites were found to be much higher than the untreated composites, which indicated their suitability for high damping applications.     

An investigation on the fatigue behaviour of Al 7075-T6 coated with titanium nitride using physical vapour deposition process

This paper investigates the fatigue behaviour of aluminium alloy 7075-T6 coated with 3-μm-thick titanium nitride (TiN) coatings using a physical vapour deposition (PVD) process. Preliminary work demonstrated that the high operating temperature of the deposition process markedly reduced the tensile properties of the coating-substrate system by comparison to the uncoated Al 7075-T6. The low tensile properties of the coated material caused a considerable reduction in the load-carrying capacity under fatigue loading. Consequently, a substantial decrease of 90% was found in the fatigue life of the coated material. The lost tensile properties were then satisfactorily recovered by applying a post heat treatment to the coated alloy leading to significant fatigue life improvements to approach original properties of the uncoated Al 7075-T6. The evaluation of fatigue fracture surfaces at high magnifications indicated that the fatigue cracks initiated at the outer surface of the aluminium substrate under the coating thin layer.     

Facile electroless copper plating on diamond particles without conventional sensitization and activation

Conventional electroless plating of copper on diamond particles needs SnCl₂ sensitization and PdCl₂ activation pretreatments, which needs noble metal and consumes a large amount of reducing agent. In this paper, metallic tungsten coatings were first plated onto diamond particles by microwave-heating salt-bath plating (MHSBP) method, and then copper layer was directly plated onto the out surface of the tungsten layer by an electroless plating method with no need of SnCl₂ sensitization and PdCl₂ activation pretreatments. Composition and morphology of the coatings was analyzed by XRD, SEM, and EDS. The results show that the copper coating on the diamond surfaces can be adjusted by control the concentration of CuSO₄·5H₂O and plating temperature, and a full copper coating is achieved with content of CuSO₄·5H₂O of 19.6 g/L in the plating solution at 60 °C. The bending strength of the coated diamond/Cu composites is as high as 630 MPa, which increases 93.3% than the uncoated composites. This work presents an electroless plating of copper can directly on the surfaces of diamond particles with no need of conventional sensitization and activation, and a strong interface combination between coated diamond and copper.     

Preparation of high thermal conductivity copper–diamond composites using molybdenum carbide-coated diamond particles

Molybdenum carbide (Mo₂C) coatings on diamond particles were proposed to improve the interfacial bonding between diamond particles and copper. The Mo₂C-coated diamond particles were prepared by molten salts method and the copper–diamond composites were obtained by vacuum pressure infiltration of Mo₂C-coated diamond particles with pure copper. The structures of the coatings and composites were investigated using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated that the Mo₂C coatings effectively improved the wettability between diamond particles and copper matrix, and Mo₂C intermediate layers were proved to decrease the interfacial thermal resistance of composites. The thermal conductivity of the composite reached 608 Wm^?1 K^?1 with 65 vol.% Mo₂C-coated diamond, which was much higher than that with uncoated diamond. The greatly enhanced thermal conductivity is ascribed to the 1-μm-thick Mo₂C coatings. Mo₂C coatings on diamond particles are proved to be an effective way to enhance the thermal conductivities of copper–diamond composites.     

Microstructure and mechanical properties of Al–2Mg alloy base short steel fiber reinforced composites prepared by vortex method

Fabrication and characterization of cast Al–2Mg alloy matrix composites reinforced with short steel fibers are dealt with in the present study. Three types of steel fiber were used: uncoated, copper coated and nickel coated. All the composites were prepared by the liquid metal route using vortex methods. When tested in tension, all composites exhibited improvement in strength due to high relative strength of steel fibers. The ductility was lowered except for the composite with copper coated fibers. Copper coated fiber reinforced composites gave the highest strength. Higher strength accompanied with appreciable ductility demonstrated by composites with copper coated fibers is attributed to the solid solution and fiber strengthening as well as good bonding at the interface. Composites reinforced with uncoated and Ni coated steel fibers did not exhibit strengthening to the level exhibited with copper coated fibers because brittle intermetallic phases are formed at the interface. These phases promote initiation and facilitate propagation of cracks. The observed fracture mechanism of composites was dimple formation, fiber breakage and pullout of fibers. Fracture surface of uncoated and Ni coated composites showed extensive pull out of fibers as well as fiber breakage confirming the above inference. In case of the copper coated composites dimple formation and coalescence was more extensive. EDX analysis showed a build up Cu, Ni, and Fe at the interface.     

Aggregation of CaCO3 particles in PP composites: Effect of surface coating

The occurrence and effect of aggregation in PP composites containing seven different precipitated CaCO₃ fillers coated with stearic acid are described in this study. The particle size and specific surface area of the filler varied in a relatively wide range, the latter changed between 2 and 12 m²/g. The fillers were characterized by various methods including surface area, particle size and bulk density. PP composites were prepared in an internal mixer in the composition range of 0–0.3 volume fraction filler content and their structure was studied by optical microscopy. The tensile and rheological properties of the composites were related to their structure. The results prove that the unambiguous detection of the presence of aggregation is difficult in particulate filled composites. The coating of CaCO₃ fillers with a surfactant changes the behavior of the particles considerably, but does not eliminate aggregation completely. The association of filler particles depends on the relative magnitude of adhesion and separating forces. Although coating decreases the surface free energy of the filler significantly, gravitational forces are much smaller than adhesion between either uncoated or coated fillers thus powder particles always aggregate. Different forces act in suspensions used for the determination of the particle size distribution of the filler; the shape of the distribution may indicate the presence of aggregation. Coated fillers form much looser aggregates with more diffuse interphases, than uncoated particles. Composite stiffness is completely insensitive to changes in structure or interaction, but the direct evaluation of other tensile properties may also lead to erroneous conclusions. Model calculations, oscillatory viscometry, as well as the proper representation of the results allow the unambiguous detection of aggregation.     

Influence of coated SiC particulates on the mechanical and magnetic behaviour of Fe–Co alloy composites

Near-equiatomic Fe–Co alloy composites containing 0, 5 and 10 vol% of uncoated and coated SiC particles were prepared by applying a uniaxial pressure of 80 MPa at 900 °C for 5 min in a spark plasma sintering furnace. The SiC particles used in this study were coarse, with an average particle size of 20 μm and their surfaces were coated with four different types of coatings, namely Ni–P, Cu, Co and duplex Cu and Ni–P by an electroless plating method. Quasi D.C. magnetic, bending and hardness tests were performed on the composites. The influence of particulate coatings on the magnetic and mechanical behaviour of the composites was investigated by correlating their properties with their microstructures as observed using scanning electron microscopy and optical microscopy and crystallographic information as obtained using X-ray diffraction. The cobalt coated particles were found to exhibit the best wettability with the matrix without the formation of deleterious intermetallic compounds at the interface. Because of the better interfacial bonding in the composites with Co coated particles, there was an enhancement in flexural strength and permeability compared to the uncoated and other coated particulate composites studied. In addition, inclusion of cobalt coated SiC particulates produced an increase in hardness and a decrease in coercivity compared to the monolithic material.     

二水磷酸氢钙包覆镁粉的组织及其在NaCl溶液中的腐蚀行为

运用液相化学沉积技术制备了二水磷酸氢钙包覆镁粉粉末。利用扫描电镜(SEM)、能谱仪(EDS)、X射线衍射仪(XRD)及傅里叶变换红外光谱仪(FT-IR)分析包覆层特征,用比表面积和孔隙度分析仪测定包覆前后粉末的比表面积,并通过浸泡实验研究包覆粉末的腐蚀行为。结果表明,包覆处理后的镁粉颗粒仍保持球形,表面形成了一层厚度2~4μm细片状的二水磷酸氢钙(DCPD),与未包覆粉末相比,其比表面积增加约60倍,在3.5%(质量分数)NaCl溶液中的耐腐蚀性能提高四个数量级。     

Experimental investigation on mechanical properties and wear characterization of Al-Si12CulMg1 aluminium alloy reinforced with titanium diboride and boron carbide particulate hybrid composites using stir casting process

LM13 aluminium alloy (Al−Si12CulMg1) with titanium diboride (TiB₂) and boron carbide (B₄C) particulate hybrid composites have been prepared using stir casting process. Wt% of titanium diboride is varied from 0–10 and constant 5 wt% boron carbide particles have been used to reinforce LM13 aluminium alloy. Microstructure of the composites has been investigated and mechanical properties viz., hardness, the tensile strength of composites have been analyzed. Wear behavior of samples has been tested using a pin on disc apparatus under varying load (20 N–50 N) for a sliding distance of 2000 m. Fracture and wear on the surface of samples have been investigated. Microstructures of composites show uniform dispersion of particles in LM13 aluminium alloy. Hardness and tensile strength of composites increased with increasing wt % of reinforcements. Dry sliding wear test results reveal that weight loss of composites increased with increasing load and sliding distance. Fracture on the surface of composites reveals that the initiation of crack is at the interface of the matrix and reinforcement whereas dimples are observed for LM13 aluminium alloy. Worn surface of composites shows fine grooves and delamination is observed for the matrix.     

Metal-shell char particulate composites using copper-coated particles

Optimum conditions to obtain uniform and continuous coatings of copper on 125 m size coconut shell char particles (by using Fehling formaldehyde solution, activating char surfaces by copper tartarate complex followed by copper coating) are reported. 30 wt % of copper, in the form of coatings with almost theoretical density, could be depostied on shell char particles in the present investigation. Copper-coated shell char particles were dispersed in Al-11.8 wt % Si alloy melts using the vortex method and the melts were poured into moulds to produce cast Al-alloy shell char particulate composites, with higher recoveries than those obtained with uncoated shell char particles. Copper shell char composites using coated char particles were also successfully made by powder metallurgy techniques. As a result of 4 wt % dispersions of coconut shell char in the as-sintered copper shell char composites (a) hardness decreases from 45 BHN to 35.7 BHN (b) density decreases from 7.9 g cm^–3 to 7.06 g cm^–3, (c) electrical resistivity increases from 5 ohm cm to 10.05 ohm cm in close agreement with calculated value, and (d) wear rate and friction coefficient under dry conditions decrease from 6.635× 10^–11 cm² and 0.250, to 3.89×10^–11 cm² and 0.129, respectively, suggesting that these composites can be used in antifriction applications. Repressing and annealing increases the density and hardness, and decreases the electrical resistivity values of the as-sintered pieces.     

The UPAL process: a direct method of preparing cast aluminium alloy-graphite particle composites

A direct method of preparing cast aluminium alloy-graphite particle composites using uncoated graphite particles is reported. The method consists of introducing and dispersing uncoated but suitably pretreated graphite particles in aluminium alloy melts, and casting the resulting composite melts in suitable permanent moulds. The optical pretreatment required for the dispersion of the uncoated graphite particles in aluminium alloy melts consists of heating the graphite particles to 400° C in air for 1 h just prior to their dispersion in the melts. The effects of alloying elements such as Si, Cu and Mg on the dispersability of pretreated graphite in molten aluminium have also been reported. It was found that additions of about 0.5% Mg or 5% Si significantly improve the dispersability of graphite particles in aluminium alloy melts as indicated by the high recoveries of graphite in the castings of these composites. It was also possible to disperse upto 3% graphite in LM 13 alloy melts and retain the graphite particles in a well distributed fashion in the castings using the pre-heat-treated graphite particles. The observations in this study have been related to the information presently available on wetting between graphite and molten aluminium in the presence of different elements and our own thermogravimetric analysis studies on graphite particles. Physical and mechanical properties of LM 13-3% graphite composite made using pre-heat-treated graphite powder, were found to be adequate for many applications, including pistons which have been successfully used in internal combustion engines.