Fabrication and squeeze casting infiltration of graphite/alumina preforms

The manufacturing of a suitable rigid porous graphite/alumina preform has been investigated taking into account the influence of the binder type, the graphite/alumina content in the preform and the percentage of binder in water. The preforms showing an acceptable rigidity have been infiltrated with a CuSn12 bronze alloy by squeeze casting considering two different pouring temperatures. The composite quality is strongly influenced by both the graphite/alumina volume fractions and the binder type. The optimal quality has been obtained by infiltrating a Carsil 2000^TM binded preform containing 30 vol % graphite flakes and 70 vol % alumina fibres.     

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% SiC_p/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 SiC_p/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% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, 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.     

Studies on mechanical and wear properties of alloyed hypereutectic gray cast irons in the as-cast pearlitic and austempered conditions

The mechanical and wear behavior of a series of as-cast gray iron alloys were compared with properties obtained after austempering at 360 °C. The austempered alloys showed equivalent or moderately enhanced mechanical strength than the as-cast pearlitic gray irons. The specific wear rates of all the austempered alloys decreased significantly by 7–15 times and friction coefficient reduced by 30–50% compared to pearlitic alloyed gray irons. The dry sliding wear studies of as-cast alloys against high carbon 1%Cr through-thickness hardened steel shows that the specific wear rate ranged from 5.6 to 19.1 (×10⁻⁷) g/Nm with friction coefficient from 0.55 to 0.7. While, the austempered alloys showed wear rates from 0.5 to 2.6 (×10⁻⁷) g/Nm with friction coefficient ranging from 0.23 to 0.4. The improved wear resistance was attributed to the layer wise surface phase transformation associated with strain induced martensite formation of the stabilized austenite in the austempered matrix, lubrication of the interface by the flake graphite and better heat conduction from the rubbing interface by higher volume fraction of the graphite. Cast iron alloyed with Ni shows enhanced mechanical properties and wear resistance. The tensile strength shows decreasing trend with increase in carbon equivalent and graphite volume. The specific wear rate and friction coefficient shows decreasing trend with increase in hardness and graphite flake volume.     

Effect of sliding speed on friction and wear of uni-directional aramid fibre-phenolic resin composite

A previous study on the tribological performance of a compression-moulded aramid fibre-phenolic resin composite, containing 30% continuous fibre, showed that this composite provides a reasonable combination of the friction coefficient and wear rate to be used as a friction component, such as a brake shoe. In the present work, the effect of sliding speed on the friction and wear behaviour of this composite has been investigated. The sliding experiments were conducted in a speed range of 0.1–6 m s^–1 at two normal pressure levels of 1.0 and 4.9 MPa. The coefficient of friction was found to be stable over a wide range of sliding speeds and normal pressures. The wear of the composite was found to be insensitive to changes in the speed in the higher speed range. The results have been supplemented with scanning electron micrographs to help understand possible friction and wear mechanisms.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

Production of cast aluminium-graphite particle composites using a pellet method

Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm^–2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm^–2 and the optimum size and percentage of aluminium powder were 400 to 1000m and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.Formerly with Indian Institute of Science, Bangalore, India.     

Influence of copper and molybdenum on dry sliding wear behaviour of sintered plain carbon steel

Study of wear behaviour of sintered low alloy steels is required to ascertain their applications for wear resistance. In the present work the influence of copper and molybdenum on wear behaviour of plain carbon steel (Fe–0.5%C) using pin-on-disk arrangement has been addressed. Atomized iron (Fe), graphite (C), copper (Cu) and molybdenum (Mo) elemental powders were suitably weighed and thoroughly mixed in a pot mill to yield the alloy powders of Fe–0.5%C, Fe–0.5%C–2%Cu and Fe–0.5%C–2%Mo. Admixed alloy powders were then compacted and sintered for obtaining preforms of aspect ratio (height/diameter) 1.3 and diameter 25 mm. The sintered preforms were then hot extruded and subsequently machined to obtain wear test specimens of diameter 6 mm and height 50 mm. Using Design of Experiment software, the sliding wear experiments were planned and conducted on a pin-on-disk tribometer. It has been found that there is a substantial improvement in wear resistance of the P/M plain carbon steel by the addition Mo rather than Cu. However coefficient of friction is higher due to presence of hard microstructural phases. Delamination wear is found predominant for both the alloy steels. Empirical correlations for mass loss and coefficient of friction with respect to load/speed have been developed for the alloy steels.     

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.     

Microstructure and tribological performance of CNx–TiNx composite films prepared by pulsed laser deposition

CN_x–TiN_x composite films were prepared on high-speed steel (HSS) substrate by pulsed KrF excimer laser co-deposition process with graphite/Ti combined targets and a substrate temperature of 200 °C. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The adhesion and tribological performance of the films were investigated using a conventional scratch tester and a ball-on-disk tribometer, respectively. In the graphite/Ti range of 0.5–2.0 of the target, TiN_x, a-CN_x and metallic Ti phase were found in the composite films. The TiN_x disappeared in the films at a high graphite/Ti ratio of the target. With increasing the graphite/Ti ratio of the target, the adhesion to substrate of the composite films deteriorated from 46 N to 26 N, and the friction coefficient decreased from 0.23 to 0.17. The composite film deposited at the graphite/Ti ratio of 1.0 showed a low friction coefficient, good adhesion and wear rate of 3.2 × 10⁻⁷ mm³/Nm in humid air.     

Grain refinement and microstructural effects on mechanical and tribological behaviours of Ti and B modified aluminium bronze

Microstructural effects on mechanical and tribological behaviours have been studied for a series of aluminium bronzes with different microstructures. ASTM 1045 and 52100 steels were used as the counterparts in the friction and wear tests. Experimental data show that the coefficients of friction, wear rate and mechanical properties strongly depend on the volume fraction of -phase present in the alloy and, to a lesser extent, on the average -grain size. The minimum coefficients of friction and the wear rate correspond to a yield strength of 370 N mm^–2 and a bulk hardness of HB 168. Except for extreme low average -grain size (corresponding to low volume fraction), both the coefficients of friction and the wear rate show linear relations with the reciprocal of the yield strength of the alloy, but not to the reciprocal of the hardness as expected. Based on these results, a design principle for high strength wear-resistant aluminium bronze has been developed.Dr T. Leo Ngai is also a part-time Research Fellow in the Materials Research Center, City Polytechnic of Hong Kong, Hong Kong.     

Abrasive wear of thick silica films prepared by vacuum evaporation

We investigate the wear behaviour of thick silica films 2–3m thick evaporated on a polymethylmethacrylate substrate in vacuum. The wear test is performed with kaolin mineral powder suspended in water. Abrasive wear is not affected by silica film hardness varying from 100 to 550 kg mm^–2, which corresponds to film preparation pressures ranging from 5×10^–3 to 5×10^–5 torr. The wear characteristics and the appearance of the wear tracks are strong evidence that abrasive wear of the deposited silica films consists of the mechanical and molecular removal of a hydrolysed silica film surface by abrasive kaolin grains. This is very similar to the case of glass polishing.     

Wear behavior of carbide coated Co–Cr–Mo implant alloy

The wear behavior of a new type of metal carbide surface coating on Co–Cr–Mo implant alloy was studied. The coating was created using a microwave plasma-assisted reaction. Codeposition of impurity diamond film, diamond particles, and soot was prevented by controlling process conditions. Wear tests were carried out using a sapphire ball-on-Co–Cr–Mo disc unidirectional sliding configuration with harsh conditions of high contact stress and slow sliding speed in both no-lubrication, and deionized water lubrication environments. In the case of uncoated Co–Cr–Mo discs, the effect of deionized water lubrication was remarkable and reduced the wear factor by one order of magnitude compared to the no-lubrication tests. The wear factor of carbide coated Co–Cr–Mo discs was slightly smaller than that of uncoated Co–Cr–Mo discs with deionized water lubrication (2.7×10^–6 mm³N^–1m^–1 vs. 4.2×10^–6mm³N^–;1m^–1). The addition of deionized water lubrication did not greatly affect the wear factor of carbide coated Co–Cr–Mo discs. The influence of surface geometry resulting from the brain coral-like surface morphology of carbide layers on wear behavior was analyzed considering stress concentrations and effective contact area.     

Preparation of cast aluminium alloy-mica particle composites

The optimum conditions for producing cast aluminium alloy-mica particle composites, by stirring mica particles (40 to 120 m) in molten aluminium alloys (above their liquidus temperatures), followed by casting in permanent moulds, are described. Addition of magnesium either as pieces along with mica particles on the surface of the melts or as a previously added alloying element was found to be necessary to disperse appreciable quantities (1.5 to 2 wt.%) of mica particles in the melts and retain them as uniform dispersions in castings under the conditions of present investigation. These castings can be remelted and degassed with nitrogen at least once with the retention of about 80% mica particles in the castings. Electron probe micro-analysis of these cast composites showed that magnesium added to the surface of the melt along with mica has a tendency to segregate around the mica particles, apparently improving the dispersability for mica particles in liquid aluminium alloys.The mechanical properties of the aluminium alloy-mica particle composite decrease with an increase in mica content, however, even at 2.2% the composite has a tensile strength of 14.22 kg mm^–2 with 1.1% elongation, a compression strength of 42.61 kg mm^–2, and an impact strength of 0.30 kgm cm^–2. The properties are adequate for certain bearing applications, and the aluminium-mica composite bearings were found to run under boundary lubrication, semi-dry and dry friction conditions whereas the matrix alloy (without mica) bearings seized or showed stick slip under the same conditions.     

The influence of pressure on the electrical tribology of carbon nanotube–silver–graphite composite

The effect of pressure on the friction and wear properties of carbon nanotube–silver–graphite composite with 10 A/cm² and without electrical current has been investigated. The results show that the wear of composite increase with the increase of pressure under mechanical wear, but the wear of composite varies with the pressure in the shape of U under electrical wear. Pressure is a factor related to both the electrical heating, friction heating and abrasive wear. At a reasonable load without much increase in the frictional heating and mechanical wear, the electrical heating could be reduced which will result in lower total thermal effect, and the resultant wear rate could arrive at a minimum. The electrical wear is higher than mechanical wear by 6–20 times. The differences between the no-current and with-current wear is Joule heat released in the friction zone which leads to breakdown of the lubricating film, roughening of the brush surface, and intensification of the adhesive interaction at the contact spots. The wear of positive brush is higher than that of negative brush. The friction coefficient of composite with current is greater than that without current.     

Sliding wear of Al2O3P/6061 Al composite

The mild sliding wear behaviour of a 15 vol % Al₂O_3P/6061 Al composite has been investigated by using a pin-on-disc reciprocating sliding machine. The composite has been shown to exhibit an excellent wear resistance as compared to the unreinforced matrix alloy. The wear rate of the composite under dry wear conditions with a 12N load is approximately one tenth of that in the 6061 aluminium alloy. The wear rate of the composite under lubrication with 15W/40 gear oil under a 100N load is only one thousandth ofthat in the 6061 aluminium alloy.The dry wear resistance of an over-aged sample is shown here to be better than a peak aged or under-aged sample when the composite was aged at 160°C. The coefficient of friction of the composite was approximately 0.5–0.6 under dry conditions and 0.07 in lubricated wear experiments.In the initial stage, the worn surface of the composite under dry conditions is primarily composed of ploughed grooves and ductile tear. The composite makes a conducting contact with the steel pin. The worn surface is composed of compacted powder and the contact potential gradually increases when the period of the wear experiment goes beyond 2 h.     

Corrosion and tribological properties of a composite based on aluminum with graphite particles

We study the corrosion resistance and tribological characteristics of composite materials based on an aluminum-silicon alloy with dispersed metallized graphite particles. Graphite decreases the friction coefficient and makes the wear resistance of materials 3–5 times higher as compared with the matrix alloy without decreasing its corrosion resistance. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 119–121, November–December, 2007.     

Role of zirconia filler on friction and dry sliding wear behaviour of bismaleimide nanocomposites

This paper discusses the friction and dry sliding wear behaviour of nano-zirconia (nano-ZrO₂) filled bismleimide (BMI) composites. Nano-ZrO₂ filled BMI composites, containing 0.5, 1, 5 and 10 wt.% were prepared using high shear mixer. The influence of these particles on the microhardness, friction and dry sliding wear behaviour were measured with microhardness tester and pin-on-disc wear apparatus. The experimental results indicated that the frictional coefficient and specific wear rate of BMI can be reduced at rather low concentration of nano-ZrO₂. The lowest specific wear rate of 4 × 10⁻⁶ mm³/Nm was observed for 5 wt.% nano-ZrO₂ filled composite which is decreased by 78% as compared to the neat BMI. The incorporation of nano-ZrO₂ particles leads to an increased hardness of BMI and wear performance of the composites shows good correlation with the hardness up to 5 wt.% of filler loading. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms.     

Effect of coal tar pitch modified by sulfur as a binder on the mechanical and tribological properties of bronze-impregnated carbon-matrix composites

Bronze-impregnated carbon-matrix composites were prepared through compression molding, carbonization and impregnation. The mechanism of sulfuration was studied, and the effect of coal tar pitch modified by sulfur as a binder on the mechanical and tribological properties of composites was investigated by varying the content of sulfur. The results showed that the sulfur addition increased the softening point, carbon yield and C/H atomic ratio of coal tar pitch but decreased the toluene solubility and quinoline solubility due to the dehydrogenating polymerization of pitch molecules. The micro-hardness, bending strength and compressive strength of the composites were enhanced by increasing the mass percentage of sulfur and reached a maximum of 160 HV, 132.82 MPa and 293 MPa at 7 wt. % of sulfur, respectively. However, both the hardness and strength of the composites decreased as the content of sulfur increased beyond 7 wt. %. The friction coefficient value of composites increased monotonously, but the wear rate decreased with increasing sulfur content; subsequently, the wear rate reached a minimum of 3.045 × 10⁻⁷ mm³/Nm at 7 wt. % of sulfur and then ascended. The wear mechanisms of the composites were adhesive wear, abrasive wear and oxidative wear. However, adhesive wear and oxidative wear occurred slightly for the composites with the binder modified by sulfur.     

Friction and wear studies of bulk polyetherimide

Polyetherimide, ULTEM 1000, was studied as a neat resin for investigating its friction and wear behaviour while sliding against mild steel, under unlubricated conditions on a pin on disc machine. Experimental parameters selected were different loads, sliding distances and counterface roughness. It was observed that unlike other semicrystalline highly linear thermoplastics and some thermoplastic thermosetting polyimides, this polyimide does not transfer a film on the counterface under the conditions of the study. It seems that it wears mainly by fatigue mechanism but crack propagation could not be observed in SEM micrographs of pin surfaces. The frictional coefficient was low as compared with other polyimides and varied with contact pressure from 0.4 to 0.22, although the initial value was high. The cycles required to initiate wear particles were around 90 kcycles at 13 N cm^–2 contact pressure. Once the wear started, the specific wear rate was high (K ₀=10^–13m³N^–1 m^–1).     

Sliding friction and wear behaviour of vinylester and its composites under dry and water lubricated sliding conditions

In this paper, we studied and explored the tribological performance of pure vinylester (V), glass fiber reinforced (GFR), SiC filled glass fiber reinforced vinylester composite under dry and water lubricated sliding conditions. Friction and wear tests were carried out with configuration of a pin on a rotating disc under ambient conditions. Tests were conducted at normal load 10, 30 and 50 N and under sliding speed of 1.6 m/s, 2.8 m/s and 4 m/s. The results showed that the coefficient of friction decreases with the increase in applied normal load values both under dry and water lubricated conditions. On the other hand for pure vinylester specific wear rate increases with increase in applied normal load under dry sliding condition and decreases with increase in applied normal load under water lubricated conditions. However the specific wear rate for GFR vinylester composite and SiC filled GFR vinylester composite decreases with the increase in applied normal load both under dry and water lubricated conditions. Moreover, for the range of load and speeds used in this investigation the coefficient of friction and specific wear rates using water lubricant registered lower values than that of the dry condition. The specific wear rates for pure vinylester and vinylester + 50 wt.% GFR and SiC filled GFR vinylester composite under dry and water lubricated sliding condition were in the order of 10⁻⁷ mm³ N⁻¹ mm⁻¹.