Mechanical, fatigue and wear properties of sintered carbon nanotube-based functionally graded material

This PhD thesis is concerned with the development of nickel coated carbon nanotubes (CNTs) functionally graded metal matrix composites, based on an aluminium-silicon alloy matrix, obtained by powder metallurgy. Besides their development and production, this thesis focus on these composites characterization when regarding metallurgical, mechanical, fatigue and wear behaviour. The experimental and theoretical work undertaken on this thesis was performed through the following steps: Assessment of several mixing processes for mixing aluminium powders and the reinforcement; Assessment of the optimum reinforcement fraction for these composites; Study and explanation of the effective reinforcing mechanisms on these composites (tensile, fatigue and wear) and Development of an equipment to obtain components with selected graded composition and design of a graded composition (reinforcement distribution) for piston rings production. CNTs dispersion is a common issue, to which great efforts are being placed by the scientific community. This work also faced this topic, with CNT agglomerates being found in these composites produced with several mixing procedures. This thesis allowed to conclude that higher improvements on CNTs dispersion is possible by using a combination of different mixing processes. Several CNT-reinforced composites were produced, aiming to assess the optimum reinforcement fraction for several properties. The 2 wt.% CNT-reinforced composite revealed the highest tensile strength, showing that, for CNT's reinforcement wt.% higher than 2%, the detrimental effect of agglomerates is higher that the beneficial effect of well dispersed CNTs. In spite of the reduction on mechanical properties for higher CNT content, a wear resistance increase was verified. This work also showed that the fatigue behaviour was found to increase by adding CNTs until a certain percentage. Regarding wear behaviour the agglomerates may also contribute to improved wear performance. In this thesis, hybrid composites reinforced by CNTs and SiCp were also produced and characterized. It was possible to conclude that the addition of a second reinforcement brought a beneficial effect on tribological and mechanical behaviours, unachievable by using solely one of the reinforcements. The final outcome of this thesis was the development of an equipment to produce components with selected graded composition (functionally graded material - FGM) by powder metallurgy. This development also included the design of a graded composition (CNT distribution). This equipment was found adequate for the production of these graded composites. This study is very appealing for components design once it allows to develop controlled materials gradients suited for particular requirements. This work developed a possible solution for a piston ring, but a similar analysis/process can be applied to other components, where there is differential mechanical and wear requirements along a component. This study also showed, that the use of commercial aluminium alloys (at least Al-Si alloys) reinforced with Ni coated CNTs seems to be adequate for industrial applications.