This review summarises the research work carried out in the field of carbon nanotube (CNT) metal matrix composites (MMCs). Much research has been undertaken in utilising CNTs as reinforcement for composite material. However, CNT-reinforced MMCs have received the least attention. These composites are being projected for use in structural applications for their high specific strength as well as functional materials for their exciting thermal and electrical characteristics. The present review focuses on the critical issues of CNT-reinforced MMCs that include processing techniques, nanotube dispersion, interface, strengthening mechanisms and mechanical properties. Processing techniques used for synthesis of the composites have been critically reviewed with an objective to achieve homogeneous distribution of carbon nanotubes in the matrix. The mechanical property improvements achieved by addition of CNTs in various metal matrix systems are summarised. The factors determining strengthening achieved by CNT reinforcement are elucidated as are the structural and chemical stability of CNTs in different metal matrixes and the importance of the CNT/metal interface has been reviewed. The importance of CNT dispersion and its quantification is highlighted. Carbon nanotube reinforced MMCs as functional materials are summarised. Future work that needs attention is addressed.

Carbon nanotube reinforced metal matrix composites - a review

The purpose of this study was to investigate the influence of Cu-coating on the spreading kinetics and equilibrium contact angles of aluminum on ceramics using a sessile drop technique. Al2O3 and SiC plates were coated by electroless plating. The copper film overcomes the low wetting of the uncoated samples by dissolution in the drop at 800 °C in argon, showing an intrinsically favorable effect on the adhesion energy. Just after 2 min, the contact angle decreased to 12.6° and 26°for Al/Cu–Al2O3 and Al/Cu–SiC, respectively. However, a de-wetting behavior was observed, reaching equilibrium contact angles of 58.3° and 45.5° for the couples. The dissolution reaction rate at the triple junction was so high that the spreading process was controlled by local diffusion rather than chemical reaction kinetics.

Wettability and spreading kinetics of molten aluminum on copper-coated ceramics

Theoretical concepts of sintering were originally based upon ideas of the discrete nature of particulate media. However, the actual sintering kinetics of particulate bodies are determined not only by the properties of the particles themselves and the nature of their local interaction with each other, but also by macroscopic factors. Among them are externally applied forces, kinematic constraints (e.g. adhesion of the sample's end face and furnace surface), and inhomogeneity of properties in the volume under investigation (e.g. inhomogeneity of initial density distribution created during preliminary forming operations). Insufficient treatment of the questions enumerated above was one of the basic reasons hindering the use of sintering theory. A promising approach is connected with the use of continuum mechanics, which has been successfully applied to the analysis of compaction of porous bodies. This approach is based upon the theories of plastic and nonlinear-viscous deformation of porous bodies. Similar ideas have recently been embodied in a continuum theory of sintering. The main results of the application of this theory for the solution of certain technological problems of sintering are introduced including their thermo–mechanical aspects.

Theory of sintering: from discrete to continuum

Ionic liquids and their solid-state analogues, organic ionic plastic crystals, have recently emerged as important materials for renewable energy applications. This Review highlights recent advances in the synthesis of these materials and their application as electrolytes for batteries, capacitors, photovoltaics, fuel cells and CO2 reduction.

Ionic liquids and their solid-state analogues as materials for energy generation and storage

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites. The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of “in situ hybridising” and “in situ alloying” are described.

Reinforcement coatings and interfaces in aluminium metal matrix composites

Amorphous molybdenum nitride thin films were prepared by pulsed direct current (dc) reactive sputter deposition. The effect of the sputtering gas nitrogen content on the structure of as-deposited thin film was investigated. The nitrogen content in the sputtering gas affected the crystallinity of deposited thin films. When the sputtering gas contained from 10 to 30% nitrogen, crystalline γ-Mo 2 N was observed. A decrease in the γ-Mo 2 N peak intensity along with peak shifting and broadening was observed in X-ray diffraction (XRD) spectra as the deposition nitrogen content increased. An amorphous molybdenum nitride thin film was obtained as the nitrogen content in the sputtering gas increased to 50%. X-ray photoelectron spectroscopic analysis (XPS) of the as-deposited thin films showed that the binding energy of Mo 3d 3/2 , Mo 3d 5/2 and Mo 3p 3/2 peaks as well as the amount of nitrogen in thin film increased as the nitrogen content in sputtering gas increased. Thermal treatment of the amorphous thin film showed that the amorphous film had survived 700 °C thermal annealing with no evidence of crystallization and reaction. However, crystallization of amorphous film and reaction of the amorphous thin film with Si substrate were observed after thermal annealing at 800 °C.

Amorphous molybdenum nitride thin films prepared by reactive sputter deposition

Chromium and chromium-nitride coatings on steels have been deposited with a magnetron sputter-deposition system. The deposition power was 200 W pulsed DC with a frequency of 185 kHz and 96% deposition duty. The substrate temperature was maintained at 200°C. The sputtering gas was argon mixed with 0, 3, 5 or 7% nitrogen. Results show that the average deposition rate was 1.2 μm/h and was not affected by the nitrogen content in the sputtering gas. Using X-ray diffraction analysis, it was observed that the deposited Cr under pure argon condition, was nanocrystalline bcc chromium with a particle size in the range of 7–8 nm. With an increasing nitrogen content in the sputtering gas the amount of CrN increased. The measured microhardness of the chromium-coated steel increases with the increasing nitrogen content. With less than a 2-μm Cr coating, the steel hardness increases from 129 to 255 HV when the nitrogen content in the gas is 7%. The microhardness of Cr/CrN-coated steel prepared with sputter deposition is superior to that prepared with electroplating. The hardness of the coating layer calculated from these data is 1270 HV. Scratch tests were used to characterize coating adhesion. The critical load was determined to be the applied load under which an acoustic noise was found and cracks in the scratch track were first observed. The critical loads of deposited films with 0, 3, 5 and 7% N 2 in the gas, were 1.57, 5.68, 8.33 and 20.29 N, respectively.

Sputter-deposited nanocrystalline Cr and CrN coatings on steels

The effect of substrate temperature and sputtering gas composition on the structure and properties of chromium-chromium nitride films deposited on C-1040 steel using r.f. magnetron sputter deposition was investigated. X-ray diffraction analysis was used to determine the structure and the composition of the films. The average grain size of the films was determined using the full width at half-maximum method (or Scherrer's formula) from the X-ray diffraction pattern. Films were deposited at temperatures ranging from room temperature to 400 °C. At 200 °C all the films consisted of a single phase, while at 400 °C dual-phase films were observed at certain partial pressures of nitrogen. Microhardnesses of the films were evaluated from the composite hardness (substrate with films on it) values using a physical model. At 200 °C, the hardness of the films was seen to increase with increasing nitrogen content in the sputtering gas as a result of the hard CrN phase formation. At higher deposition temperatures, the residual stresses lead to spalling of film.

Reactive sputter deposition of chromium nitride coatings

A sintering model, taking into account the effect of particle size distribution and the effect of grain growth, was developed previously. Experimental data from the sintering of high-purity alumina were used to testify this model. The sintering was carried out at 1500 °C in air for various lengths of time. The results agreed with the prediction of the model. The powders with a narrower starting particle size distribution exhibited a lower sintering rate prior to the occurrence of grain growth, but a higher densification rate after the grain growth took place. The grain size/density trajectory was applied to reveal the effect of particle size distribution on sintering behaviour. It is suggested that powders with a narrower size distribution are preferable.

Effect of particle size distribution on sintering

A sintering model, taking into account the effect of particle size distribution and the effect of grain growth, was developed previously. Experimental data from the sintering of high-purity alumina were used to testify this model. The sintering was carried out at 1500°C in air for various lengths of time. The results agreed with the prediction of the model. The powders with a narrower starting particle size distribution exhibited a lower sintering rate prior to the occurrence of grain growth, but a higher densification rate after the grain growth took place. The grain size/density trajectory was applied to reveal the effect of particle size distribution on sintering behaviour. It is suggested that powders with a narrower size distribution are preferable.

Ray Y. Lin

Papers

Amorphous molybdenum nitride thin films prepared by reactive sputter deposition

Sputter-deposited nanocrystalline Cr and CrN coatings on steels

Reactive sputter deposition of chromium nitride coatings

Effect of particle size distribution on sintering

Effect of particle size distribution on sintering: Part II Sintering of alumina