Microstructural investigation on antifriction characteristics of self-lubricating copper hybrid composite

Abstract

Owing to good antifriction properties and high wear resistance, copper hybrid composites reinforced with hard ceramic particles and solid lubricant components are regarded as promising materials for applications in sliding electrical contacts. The present work investigates the antifriction mechanism of a (SiC+Gr)/Cu composite from a microstructural viewpoint, so as to assist the development and application of this material. A graphite rich tribolayer formed on the worn surface was responsible for good tribological properties of the composites. Testing results showed that nanoparticles of graphite were involved in a mechanically mixing process by adhering to both the other wear debris and the two contacting surfaces, thereby developing a solid lubricant tribolayer. The nanographite to nanographite contacting mode, formed between the composite and the counterface, significantly improved wear resistance and friction stability. The forming and failure process of the graphite rich tribolayer was studied. A mechanism has been developed based on the experimental results.     

Incorporation of new technique for processing of Al/SiCp composites based on evaporative pattern casting (EPC) method

Abstract

A336 Al matrix composites containing different volume fraction and mean mass particle size of SiC particles as the reinforcing phase were synthesised by evaporative pattern casting (EPC) route. The process consisted of fabricating of EPS/SiC_p composite pattern followed by EPC of A336 Al alloy. The EPS/SiC_p pattern was made by blending SiC particles with expandable polystyrene (EPS) beads and placing them in expanding mould heating with steam until EPS beads expand completely. Uniform distributed SiC particles around the EPS beads and locally movement of them during pouring and degradation leads to homogenous distribution of particles in final Al/SiC_p composite. Higher modulus, strength and hardness were observed in the composites than the unreinforced Al alloy part. The fracture surfaces of the composite samples exhibited dimple surfaces and fracture in SiC particles.     

Corrosion resistance of Ni/Al2O3 coatings in NaCl solution

Abstract

This paper presents results of a research on the corrosion resistance of composite Ni/Al₂O₃ coatings electrochemically deposited from Watts baths containing different amounts (0, 20, 100 g dm^?3) of Al₂O₃ particles. Potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 3% solution of NaCl. The coatings with about 6 wt-% of corundum, deposited from a bath with 100 g dm^?3 of a powder, showed the best protective properties. The rate of corrosion of such coatings after 7 days of exposure in the NaCl solution was over two times slower than that of coatings containing 2 wt-% of Al₂O₃ and six times slower than that of a standard nickel coating. Two equivalent circuits: one consisting of two RC circuits and the other one made up of three RC circuits were used for the analysis of the impedance spectra. Regardless of the presence and amount of the Al₂O₃ particles in the nickel coating, during first day of exposure in the NaCl solution a layer of nickel oxides and hydroxides forms on the surface of the coatings increasing their corrosion resistance. In the case of coatings with 6 wt-% of Al₂O₃, the passive layer is least vulnerable to the aggressive action of Cl^? ions.     

Long term corrosion characteristics of metallic materials in marine environments

Abstract

Long term corrosion test data in marine environments are essential data to guide the design and application of marine engineering. Corrosion test results, over a 16 year period from October 1986 to October 2002, using eight kinds of metallic materials are reported in this paper. Materials were exposed in the splash zone, tidal zone, and full immersion zone of four corrosion test stations located in Qingdao, Zhoushan, Xiamen and Yulin respectively, and therefore the corrosion behaviour of various ferrous and non-ferrous materials in these marine environments has been obtained. The corrosion of carbon steel in the seawater fits the following general relationship: D=A+K(t–1). The variation in the general corrosion rates for the different carbon steels in seawater is relatively small and consistent. However, the pitting corrosion rates for the different carbon steels are all significantly different. The marine splash zone is about 0–2˙4 m above the mean high water level of the seawater. The corrosion peak was located about 0˙6 to 1˙2 m above the mean high water level. The order of the corrosion potential of stainless steels with the corrosion resistance of stainless steels is consistent in seawater. The corrosion of copper was found to be serious in the full immersion zone of the Yulin sea area. The effective lifetime of the cathodic protection for a wrapped aluminising layer of the hard and ultrahard aluminium may be 16 years.     

Active screen plasma nitriding of materials

Abstract

The rapid development and the uptake of plasma nitriding technology into industrial surface engineering have slowed down in recent years. This is attributed to some of the inherent shortcomings of conventional dc plasma technology, for example, difficulties in maintaining a uniform chamber temperature, instability of the plasma and potential surface damage to parts caused by arcing. Efforts in overcoming these problems have led to the development of active screen plasma nitriding (ASPN) technology. This review demonstrates that with all its technological and environmental advantages, ASPN can be used to treat low alloy steels, tool steels, stainless steels and other steels which can conventionally be nitrided with dc plasma technology. In addition, ASPN can be used to treat non-conducting materials such as oxidised steels and polymeric materials which are not suitable for a dc plasma nitriding system. In the longer term, environmental friendly and technologically advanced plasma nitriding will outperform conventional salt bath and gaseous methods. Active screen plasma nitriding is a novel way of possibly achieving the full potential of plasma technology for thermochemical surface engineering.     

Titanium–hydroxyapatite composite biomaterial for dental implants

Abstract

The objective of the present study was to investigate high velocity compaction of titanium powder and to prepare a dense composite biomaterial of titanium and hydroxyapatite with the purpose of forming dental components with improved early healing properties. A high purity titanium powder was compacted using high velocity compaction to study the density distribution. Then, a titanium–hydroxyapatite composite was prepared by mixing titanium powders and hydroxyapatite grains. Dental implant components were formed from the high velocity compacted specimens, exposing the hydroxyapatite grains at the component surface. The green density reached more than 98·5% after more than one impact. The composite was heated to 500°C, enough to bind the titanium grains, but to avoid observable reactions. Compacted pure titanium could be sintered to full density. The heated composite material reached 99% density, no reaction was observed between titanium and hydroxyapatite, and the composite material could be formed into dental implants.     

Effect of Al2O3 and B4C particle additions on microstructure of PM copper based brake linings

Abstract

This paper highlights the effect of different ceramic particles on the structure of PM copper based brake linings. The copper based brake linings using a range of ceramic additives (1–6 wt-%) were prepared by powder metallurgy (PM). The optimum conditions for the production of brake linings were determined as compaction under 400 MPa and sintering at 805°C for 20 min in an argon atmosphere. The density of copper based brake linings decreased after sintering with the increase in ceramic powder contents for both Al₂O₃ and B₄C ceramic particles. The microstructural characterisation of produced samples showed that the lower boiling point elements in the as supplied powder vaporise during sintering from the structure and this leads to an increase in the porosity amount of the final component.     

Experimental study of coal based direct reduction in iron ore/coal composite pellets in a one layer bed under nonisothermal,asymmetric heating

Abstract

The reduction of hematite iron ore fines by different carbonaceous materials in composite pellets in a one-layer bed under nonisothermal asymmetric heating from above is studied. Temperature measurements at various points in the bed show that there are large gradients both inside and outside the pellets, with the reduction fastest at the top where the temperature is highest. The relative difference between the degree of metallisation between the top and the bottom of the pellet can be even greater during the reduction. The pellets undergo nonuniform swelling/shrinking towards the end of the reduction, and their height is less than their width. Reoxidation can start at the top of the pellet while the bottom has not yet been fully reduced. Those with a higher initial volatile content but with the same amount of fixed carbon as other pellets, gave a significantly higher degree of reduction without the involvement of fixed carbon gasification reactions.     

Testing of packages with LSA materials in very severe mechanical impact conditions with measurement of airborne release

Abstract

To assess the risks associated with transport accidents involving solid LSA-II and LSA III materials a comprehensive experimental programme was conducted to quantify and characterise airborne release of radioactive particulate matter in transport and handling accidents with mechanical impact of varying severities and to determine the dependency from influencing parameters such as LSA material and packaging properties and size. The experimental approach combined well-controlled and very reproducible impact experiments with small scale specimens and drop tests of larger scale specimens from different heights up to 27m. In both cases the associated airborne release of particulate matter is determined by measuring the amount and aerodynamic particle size characteristics of released dust. The small scale tests revealed fundamental results on airborne release and size distribution which helped to design the test matrix of the large scale experiments, especially with brittle material. In the large scale tests, volumes of specimens were varied systematically up to 200L and the LSA material was contained either within packaging or without protective packaging in order to determine the influence of the packaging on the airborne release and to be able to extrapolate other configurations of package sizes and impact severities. The LSA surrogate materials were either concrete, used to immobilize radioactive wastes as representative brittle material, or appropriately chosen powders representing dispersible materials. Based on the experimental results it can be concluded that the requirements of the current IAEA Transport Regulations sufficiently limit potential radiological consequences from transport accidents with mechanical impact involving packages with LSA-II or LSA-III materials.     

Ultrasupercritical power plant development and high temperature materials applications in China

Abstract

The rapid development of Chinese economy demands sustainable growth of power generation to meet industrial and domestic demand. The total installed capacity of electricity and annual overall electricity generation are now both the second highest in the world, close to those of the USA. Forecasts of China's electricity demand over the period 2010–20 are presented. Chinese power plants, like those worldwide, are facing demands to increase thermal efficiency and to decrease the emission of CO₂, SO_X and NO_X. In light of the national resource of coal and electricity market requirements in the next 15 years, power generation – especially ultrasupercritical (USC) power plants with the steam temperature over 600°C – will undergo rapid development. The first 1000 MW USC power unit, with steam parameters 600°C, 26·25 MPa, entered service in November 2006. It is estimated that more than 350 USC power units will be installed in China by 2020. USC power plant designs will adopt a variety of qualified high temperature materials for boiler and turbine manufacturing applications. Among these materials, the modified 9–12%Cr ferritic steels, Ni–Cr austenitic steels and certain nickel base superalloys have received special attention in the Chinese materials market.