Metal matrix composites reinforced with SiC long fibers and carbon nanomaterials produced by electrodeposition

The research work of this PhD thesis was done on the study, production and characterization of two types of metal matrix composites: 1) fiber reinforced metal matrix composites and, 2) carbon nanomaterials reinforced metal matrix composites. In fiber reinforced metal matrix composites, a metal or an alloy is reinforced with continuous or discontinuous fibers in order to improve the specific strength and stiffness at high temperatures. For example superalloys are the typical materials for the hot parts of aeronautic engines. They are very important in the aerospace field as they offer high temperature mechanical strength together with a good resistance to oxidation and corrosion. But due to high temperatures involved in the service conditions, buckling of the material may occur. In order to avoid this high temperature buckling phenomena, a reinforcement of the superalloy could be needed to maintain the mechanical properties. For this reason it was thought to investigate the possibility of realizing continuous fibers reinforced metal matrix composites over the superalloys that can further improve the high temperature properties. Due to its simplicity and viability, electrochemical deposition was chosen as the production technique to produce this kind of composite materials. The selected substrate for electrodepositing the nickel matrix composite was nickel based superalloy Inconel-718, and monofilament continuous silicon carbide fibers were chosen as reinforcement. First of all chemical compatibility was studied between the nickel matrix, superalloy and silicon carbide fibers, both in the uncoated form, and coated with carbon or carbon/titanium diboride. Both theoretical calculations and experiments were conducted, suggesting the use of a carbon coating over fibers and a buffer layer of nickel to increase the interface quality as well as to avoid unwanted reactions between substrate and silicon carbide fibers. After studying the chemical feasibility of all the related components, electrodeposition of the composites was performed in order to demonstrate the practical feasibility of the process. Then final composites were deposited on the dog-bone shaped specimens of Inconel-718. The produced composites were subjected to mechanical tests in order to evaluate the mechanical properties at room temperature and at high temperatures (400 °C and 600 °C). Evaluation of the results shows improvement in the yield strength of the produced composites with respect to the superalloy alone at room temperature. There is a decrease in the yield strength at high temperatures due to the failure of the interface between the superalloy and the composite layer but it is demonstrated that if a hot pressing treatment is done, then the interface strength is retained at high temperature and thus yield strength values will also increase. Fracture analysis and EDS analysis were also performed on the fractured surfaces of the samples with the help of field emission scanning electron microscope in order to study the fracture mechanisms involved and the composition of the interface after their testing at high temperatures. The fracture mechanisms in Ni/SiC composite layer was of debonding and pull out type which is typical characteristic of the fiber reinforced composites. The superalloy fractures with ductile behavior at room temperature. At temperatures of 400 °C and 600 °C, the superalloy fails with more ductile character after extensive amount of plastic deformation before its fracture. Carbon nanomaterials are widely being used to reinforce the metallic materials in order to improve their electrical, thermal, corrosion, wear and friction resistance for particular applications. In this part of the research work, graphene nanoplatelets (GNPs) and graphene oxide (GO) were chosen as nano reinforcements to produce nickel matrix nanocomposites with the intention to evaluate their wear and friction behavior. Graphene nanoplatelets and graphene oxide consist of few layers of graphene and graphene oxide respectively and their layered structure coupled with the small size of these materials can be helpful reducing the wear rate of the composites. Crystallite size, hardness and roughness of the coatings were also studied in order to understand the effect of nano phases on these properties. Again the production technique employed consists of electrodeposition of the composites over a conducting steel substrate. A lot of work has been done in producing metal matrix composites reinforced with carbon nano fibers, carbon nanotubes and graphene. But graphene nanoplatelets and graphene oxide are not much evaluated for reinforcing the metallic matrices especially with the electrodeposition technique. To produce these types of composites by electrodeposition, a uniform and stable dispersion of the carbon nanomaterials in the nickel deposition baths is necessary. So particular attention was given to this aspect and uniform and stable dispersions were obtained by using a suitable dispersant, chosen after a wide screening, namely poly sodium styrene sulphonate (PSS). The dispersing technique employed the ultrasonication of the deposition bath with the help of an ultrasonic probe. The obtained coatings were strong and well adherent to the steel substrate, and presented rather well dispersed graphene oxide or graphite nanoplatelets, even if some agglomerates were still present in samples obtained from highly concentrated suspensions. The nanocomposites were characterized in terms of microhardness, crystallite size, roughness and wear and friction behaviors. The composites with GO show very little effect on the microhardness whereas Ni/GNP composites show slight increase in the hardness. The effect on the crystallite size is not significant. Low concentration of the nano phase gives the composites a good smooth surface with less roughness whereas, by increasing the concentration of the carbon nanomaterials, the composites produced presents a rougher surface. Pin-on-disk tests were chosen to evaluate the wear behavior of the composites. The obtained results demonstrated a significant decrease in the wear rate, percent mass loss and volume loss of the composites as compared to the pure nickel one. The worn tracks observations suggest that the nanocomposites were worn by adhesive wear mechanism.