Formation of composite Cu–graphite and Cu–PTFE coatings and their tribological characterization

The Dynamic Chemical Plating (DCP) technique allows production of 2-μm copper films containing particles of graphite or PTFE in 18 and 15 min, respectively, at ambient temperature. DCP yields composites with particle-incorporation fractions of 12% for graphite micro-particles and 22% for PTFE nano-particles. The composite films show excellent tribological properties, acting as self-lubricating coatings with friction coefficients as low as 0.18.     

Cross-property connections for copper–graphite composites

Copper–graphite composite materials in the range of 0–10 vol% of carbon phase were prepared from the mixture of copper and graphite powders by hot isostatic pressing. The microstructure, mechanical (tensile strength, elongation to fracture) and physical (electrical and thermal conductivity) properties of composite samples were investigated, and the cross-property connections were calculated. It was shown that electrical and thermal conductivity cross-property (Lorenz number) is almost constant and increases only slightly (no more than 10 % increase was observed). This implies that in the investigated composition range the Lorenz number of a copper–graphite composite system behaves according to Franz–Wiedemann law for pure metals at constant temperature. On the contrary, the conductivity to tensile strength cross-property connections showed significant linear increase (over 200 % in the investigated composition range) for both electrical conductivity and thermal conductivity of composite materials. The cross-property connections of conductivity to the elongation to fracture exhibit a nonlinear dependence on the volume fraction of graphite.     

Effect of addition of graphite particulates on the wear behaviour in aluminium–silicon carbide–graphite composites

Aluminium matrix composites with multiple reinforcements (hybrid AMCs) are finding increased applications because of improved mechanical and tribological properties and hence are better substitutes for single reinforced composites. Few investigations have been reported on the tribological behaviour of these composites with % reinforcement above 10%. The present study focuses on the influence of addition of graphite (Gr) particulates as a second reinforcement on the tribological behaviour of aluminium matrix composites reinforced with silicon carbide (SiC) particulates. Dry sliding wear tests have been performed to study the influence of Gr particulates, load, sliding speed and sliding distance on the wear of hybrid composite specimens with combined % reinforcement of 2.5%, 5%, 7.5% and 10% with equal weight % of SiC and Gr particulates. Experiments are also conducted on composites with % reinforcement of SiC similar to hybrid composites for the sake of comparison. Parametric studies based on design of experiments (DOE) techniques indicate that the wear of hybrid composites decreases from 0.0234 g to 0.0221 g as the % reinforcement increases from 3% to 7.5%. But the wear has a tendency to increase beyond % reinforcement of 7.5% as its value is 0.0225 g at.% reinforcement of 10%. This trend is absent in case of composites reinforced with SiC alone. The values of wear of these composites are 0.0323 g, 0.0252 g and 0.0223 g, respectively, at.% reinforcement of 3%, 7.5% and 10% clearly indicating that hybrid composites exhibit better wear characteristics compared to composites reinforced with SiC alone. Load and sliding distance show a positive influence on wear implying increase of wear with increase of either load or sliding distance or both. Whereas speed shows a negative influence on wear indicating decrease of wear with increase of speed. Interactions among load, sliding speed and sliding distance are noticed in hybrid composites and this may be attributed to the addition of Gr particulates. Such interactions are not present in composite reinforced with SiC alone. Mathematical models are formulated to predict the wear of the composites.     

Studies on the formability of powder metallurgical aluminum–copper composite

Formability is concerned with the extent of deformation that the materials undergo before failure; thus its investigation is critical for successful processing of materials during bulk deformation. The present investigation has been undertaken to generate the forming limit diagrams for powder metallurgical aluminium–copper composites for different initial relative densities and copper contents. Sintered aluminium–copper composite compacts of 2%, 4% and 6% copper content with different initial relative densities have been prepared by applying recommended powder compaction pressures. The material properties such as apparent strain hardening exponent and strength coefficient were determined using stage wise compression test to generate the formability limit diagram. Densification curves were plotted to investigate the effect of initial relative density and copper content on the pore closure phenomena during deformation. Theoretical and experimental investigations using standard ring compression test were carried out to determine friction factor between tool and work piece interfaces for different initial relative density and copper content. The critical transition densities vide the forming limit diagram were found to be 84%, 85.3%, 86% and 87.5% for pure sintered aluminium, Al–2%Cu, Al–4%Cu and Al–6%Cu composites respectively. The friction factor between tool and work piece interfaces has showed increasing pattern for all the cases with decrease in the initial relative density and increase in the copper content of the composite.     

Thermal conductivity of wood-derived graphite and copper–graphite composites produced via electrodeposition

The thermal conductivity of wood-derived graphite and graphite/copper composites was studied both experimentally and using finite element analysis. The unique, naturally-derived, anisotropic porosity inherent to wood-derived carbon makes standard porosity-based approximations for thermal conductivity poor estimators. For this reason, a finite element technique which uses sample microstructure as model input was utilized to determine the conductivity of the carbon phase independent of porosity. Similar modeling techniques were also applied to carbon/copper composite microstructures and predicted conductivities compared well to those determined via experiment.     

Effect of environment atmosphere on thermal shock resistance of the ZrB2–SiC–graphite composite

The thermal shock resistance of the ZrB₂–SiC–graphite composite was evaluated by measuring the retention of the flexural strength after the electrical resistance heating to the temperature ranging from 1000 °C up to 2500 °C. The experiment was operated in two different environment atmospheres (pure oxygen and low oxygen partial pressure which mixed O₂ and Ar with 1:9) at total pressure 2000 Pa. The residual strength for the specimen decreased gradually as the temperature increased up to 2200 °C, and it was slightly higher when heated in low oxygen partial pressure environment than in pure oxygen. In contrast to the specimen heated in low oxygen partial pressure environment, the residual strength for the specimen in pure oxygen increased steeply as the temperature increased from 1600 °C up to 1800 °C. The analysis of the SEM observations combined with EDS confirmed that the surface oxidation played a positive role in the thermal shock resistance of the ZrB₂–SiC–graphite composite with different environment atmospheres. The results here pointed out a potential method for charactering the effect of environment atmosphere on thermal shock resistance of the ZrB₂–SiC–graphite composite.     

Experimental equipment for the evaluation of the strength characteristics of carbon-carbon composite mateials within the temperature range 20–2200°C

We suggest procedures for testing and describe the design of a UKM-2200 specialized experimental installation for tensile and bending testing of specimens made of carbon-carbon composite materials in a vaccum or inert media within the temperature range 20–2200°C. We discuss the results of strength tests for specimens of carbon-carbon composite materials with multidirected spatial reinforcement of the structure and with felt-like structure in a vacuum for the same temperature range. Translated from Problemy Prochnosti, No. 3, pp. 130–138, May–June, 1999.     

Thermomechanical behaviour of a copper–chromium in situ composite

Abstract

The mechanical response of an in situ copper–chromium composite was investigated over a range of temperatures by means of tensile and isothermal creep tests. Scanning electron microscopy was used to characterise the extent, type, and distribution of damage. It was found that the failure mechanisms fell into distinct regimes. At cryogenic temperatures damage tended to occur in the form of reinforcement fracture. Around room temperature, very little damage was observed in the composite. At temperatures of about 400°C, extensive damage was again observed in the form of reinforcement failure and cavitation. Further increase in the test temperature resulted in a transition from a local to a global load sharing, with damage distribution becoming more homogeneous. These experimental observations were rationalised by considering the relative extent of deformation within the two phases as a function of temperature.     

Laboratory scale erosion testing of a wear resistant glass–ceramic

Abstract

The testing of erosion resistant materials is usually performed on laboratory scale test rigs under accelerated wear conditions. The validity of this is questionable because of the dependence of wear mechanisms on a variety of impact parameters. Data are presented that have been obtained from such tests on a glass–ceramic (Silceram) which has been developed as a low-cost erosion resistant lining material. Various impact conditions have been investigated, including impact angle, particle velocity, and impact frequency. The data concerning the effects of particle velocity show very good agreement with one particular erosion model, although there is an apparent dependence on other test variables.

MST/611     

Fabrication and oxidation behavior of a reaction derived graphite–ZrC composite for ultrahigh temperature applications

Graphite containing nominally 40 vol.% ZrC (graphite–ZrC) was prepared from commercially available ZrO₂ and graphite powders by hot pressing at 2000 °C in a vacuum. The oxidation behavior of the graphite–ZrC composite was carried out in dry stagnant air at the temperatures of 1200 and 2200 °C. Compared with the pure graphite, the graphite–ZrC composite exhibited good oxidation resistance because the mass loss of the composite powder was significantly lower than that of the pure graphite. The mass loss of graphite–ZrC at 2200 °C was lower than pure graphite at 1200 °C. Furthermore, the introduction of ZrC also improved the strength of the graphite–ZrC composite.     

Experimental correlations between wear rate and wear parameter of Al–Cu–Mg/bagasse ash particulate composite

The experimental correlations between wear rate and wear parameter of Al–Cu–Mg alloy composite reinforced with 10 wt.% bagasse ash particles produced by double stir casting method was developed in terms of applied load and sliding speed using the empirical linear regression and analysis of variance method. The wear behaviour of the specimen was investigated using pin-on-disc method. An empirical linear regression equation was used in predicting wear rate within a selected experimental domain. The predicted wear rate of the alloy and composite samples were found to lie close to that of the experimentally observed ones. The confirmation of experiments was conducted using analysis of variance (ANOVA) to verify the optimal testing parameters. The interactions of applied load and sliding speed of the composite had no significant effect, while the alloy had a significant effect on the wear rate.     

Mechanical and tribological properties of acrylonitrile–butadiene rubber filled with graphite and carbon black

In this work, acrylonitrile–butadiene rubber (NBR)/expanded graphite (EG)/carbon black (CB) micro- and nanocomposites were prepared by two different methods, and the resulting mechanical and tribological properties were compared with those of NBR/CB composites. Meanwhile, the effects of graphite dispersion and loading content, as well as the applied load and sliding velocity on the tribological behavior of the above composites under dry friction condition were also evaluated. The worn surfaces were analyzed by scanning electron microscopy (SEM) to disclose wear mechanism. As expected, the better the dispersion of graphite, the more remarkable enhancement on tensile and dynamic mechanical properties, and the greater reduction in the coefficient of friction (COF) and specific wear rate (W_s). It was found that a small amount of EG could effectively decrease COF and W_s of NBR/CB composites because of the formation of graphite lubricant films. The COF and W_s of NBR/CB/EG composites show a decreasing trend with a rise in applied load and sliding velocity. NBR/CB/EG nanocomposite always shows a stable wearing process with relatively low COF and W_s. It is thought that well-dispersed graphite nano-sheets were beneficial to the formation of a fine and durable lubricant film.     

The tribological behavior of W–S–C films in pin-on-disk testing at elevated temperature

W–S–C films were deposited by magnetron sputtering in an Ar atmosphere with a Ti interlayer. A carbon target with several pellets of WS₂ incrusted in the zone of the preferential erosion was used. The number of pellets was changed to modify the carbon content in the films, which varied from 29 up to 70 at%. Doping W–S films with carbon led to a substantial increase of the hardness in the range 4–10 GPa; the maximum of hardness was obtained for coatings with the carbon content of 40 at%. X-ray diffraction (XRD) patterns showed that there was a loss of crystallinity with the increase of the carbon content in the film.The coatings were tested by pin-on-disk from room temperature (RT) up to 400 °C. At RT, the friction coefficient was in the range 0.2–0.30. At temperatures higher than 100 °C, the friction is below 0.05 for all compositions. The tribological behavior of the coatings with increasing temperatures depended on the films carbon content. For low-carbon content up to 40%, the wear rate was almost independent of the temperature up to 300 °C, while it increased dramatically in the case of the coatings with high-carbon content. In general, the limiting temperature for W–S–C coatings is 400 °C.     

Microstructure and tribological characteristics of aged Co–28Cr–5Mo–0.3C alloy

In this study, an attempt to investigate the role of isothermal aging on the microstructure and tribological characteristics of Co–28Cr–5Mo–0.3C alloy was made. Regarding the results, isothermal aging at 850 °C and 950 °C for at least 16 h contributed to the formation of lamellar-type carbides at the grain boundary regions. Moreover, at higher aging times (over 16 h), the amount of lamellar-type carbides decreased. The wear properties of as-cast and heat treated samples were determined at 0.5 ms⁻¹ speed several under normal applied loads such as 50, 80, and 110 N. At the lowest applied load (50 N), the samples were isothermally aged at 850 °C for 8 and 16 h and also the ones were aged at 950 °C for 16 h had higher wear resistance probably due to more volume fraction of lamellar-type carbides when compared to as-cast and the other aged samples, but, at higher applied loads (80 and 110 N) due to the formation of adhesive oxide layer on the as-cast sample surface, the wear rate of as-cast samples is lower compared with all heat treated ones.     

On modeling the abrasive wear characteristics of in situ Al–12%Si/TiC composites

In this work the abrasive wear characteristics of in situ synthesized Al–12%Si/TiC composites were investigated based on the plan of full factorial design. During the experiment applied load, sliding distance and weight percentage of reinforcement considered as input variables and weight loss and coefficient of friction as responses. The experimental results revealed that the sliding distance and weight percentage of TiC had relatively higher influence on coefficient of friction. On the other hand, the applied load had relatively higher influence on the weight loss. The developed regression equations for predicting the weight loss and coefficient of friction were validated with a number of test cases and it has been observed that the percentage error for both responses is less than ± 10%. It indicates that there is a close agreement between the predicted and measured results. Multi-response optimization technique was also employed to optimize the control factors.     

Study on tribological behavior of electrodeposited Cu–Si3N4 composite coatings

Cu–Si₃N₄ composite coatings were prepared by electrolysis from a copper sulphate solution containing dispersed Si₃N₄ particles of 0.4 or 1 μm mean size. Wear behavior of Cu–Si₃N₄ composite and pure copper coatings were evaluated using a pin-on-disc test machine under dry condition sliding. Effects of current density and particle concentration on the incorporation percentage of Si₃N₄, the preferred orientation of copper crystallites, the microstructure, the microhardness and the wear resistance of the coatings were determined. Si₃N₄ particles in the copper matrix resulted in the production of composite deposits with smaller grain sizes and led to change the preferred orientation growth from [1 0 0] to [1 1 0]. It was proved that the presence of Si₃N₄ particles decreases the wear loss and the friction coefficient of the coating. According to the results, the friction coefficient decreased dramatically from 0.52 to 0.26 for pure copper coatings to 0.16–0.24 for Cu–Si₃N₄ composite coatings. In addition, fluctuation of friction coefficient values for Cu–Si₃N₄ composite coating was lower compared with the pure copper coating. The wear properties of Cu–Si₃N₄ composite coatings were shown to depend on the weight fraction, the size and the distribution of co-deposited particles.     

Combining Maxwell’s methodology with the BEM for evaluating the two-dimensional effective properties of composite and micro-cracked materials

Maxwell’s methodology is combined with the boundary element method (BEM) for evaluating the two-dimensional effective elastic properties of composite, porous, and microcracked isotropic materials with periodic or random structure. The approach is based on the idea that the effective properties of the material can be deduced from the effects that a cluster of fibers, pores, or cracks embedded in an infinite matrix has on the far-fields. The fibers, pores, or cracks can have arbitrary shapes, sizes, and elastic properties, provided that the overall behavior is isotropic, and their effects are assumed to be the same as those of an equivalent circular inhomogeneity. The key aspect of the approach is to precisely account for the interactions between all the constituents in the cluster that represent the material in question. This is done by using the complex-variables version of the BEM to solve the problem of a finite cluster of fibers, pores or cracks embedded an infinite isotropic, linearly elastic matrix. The effective properties of the material are evaluated by comparing the far-field solutions for the cluster with that of the equivalent inhomogeneity. It is shown that the model adequately captures the influence of the micro-structure of the material on its overall properties.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Effect of titanium on copper–titanium/carbon nanofibre composite materials

Copper/carbon nanofibre composites containing titanium varying from 0.3 wt.% to 5 wt.% were made, and their thermal conductivities measured using the laser flash technique. The measured thermal conductivities were much lower than predicted. The difference between measured and predicted values has often been attributed to limited heat flow across the interface. A study has been made of the composite microstructure using X-ray diffraction, transmission electron microscopy and Raman spectroscopy. It is shown in these materials, that the low composite thermal conductivity arises primarily because the highly graphitic carbon nanofibre structure transforms into amorphous carbon during the fabrication process.     

Fabrication and characterization of multi-filament copper matrix–polyethylene fibres composite wire

In this work, the process of co-drawing followed by bundling and restacking was used to fabricate composite wires with a metallic copper matrix and High Density Polyethylene (HDPE) polymer fibres. After processing, copper matrix composite wires with 3770 polymer domains of approximately 20 μm in diameter were obtained. Differential Scanning Calorimetry analysis, optical microscopy and microhardness measurements were performed on the polymer and the copper components of the composites. The volume fraction and the degree of crystallinity of the polymer remain constant during the drawing. This procedure leads to a lamellar reorganization and fragmentation of the crystalline HDPE phase.