3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Influence of ruthenium and molybdenum ion implantation on the machining performance of WC-Co straight grade inserts(2019) Mkhaliphi, Thuli GoodnessThe aim of this study was to determine the effect of Ru and Mo ion implantation on the tool life of WC-Co straight grade cutting tool inserts during face milling of AISI H13 tool steel. The cutting tool inserts were implanted at 6 different ion implantation dosages. The acceleration voltage was kept constant for 5 ion implantation dosages at 100 keV and it was then increased to 120 keV for one of the ion implantation dosages. The face milling tests were conducted under unlubricated conditions according to ISO standard 8688-1: 1989. In order to ensure intermittent conditions the radial depth of cut was set at 30% of the cutter diameter. A 5 insert cutter was fully loaded during machining. The non-implanted inserts were used as the baseline for the volume of material to cut. The induced cutting forces of the ion implanted inserts were higher than that of non-implanted inserts due to increased shear area as a result of cutting edge chipping. The estimated heat generated when using ion implanted inserts had a scattered relationship with respect to the non-implanted inserts; this was attributed to the several factors involved in heat generation during machining. Several overlapping failure mechanisms were observed between the non-implanted and ion implanted inserts. The flank wear scar of the ion implanted inserts was smaller than that of non-implanted inserts indicating abrasion resistance from increased micro-hardness resistance. Even though the surface roughness of the machined components using ion implanted inserts was slightly higher than that of non-implanted inserts, it was still within allowable tolerances. Uncoated inserts failed prematurely due to workpiece adhesion that resulted to flaking and excessive chipping on the rake face. The ion implantation dosage of 8E+15 ions/cm2 at 100 keV and 2E+16 ions/cm2 at 120 keV were the best dosages with low wear rate, acceptable surface roughness and heat generation similar to that of non-implanted inserts.Item Using ruthenium to modify surface properties of austenitic stainless steel for improved corrosion resistance(2017) Moyo, FortunateChromium oxide provides an inexpensive and practical means of increasing the corrosion resistance of austenitic stainless steel in most environments. However, the oxide is prone to dissolve in reducing acids and in chloride containing solutions, which compromises the durability and effective operation of structures made of austenitic stainless steel. This research project explored the use of thin ruthenium surface alloys produced by ion implantation, RF sputtering and pulsed electrodeposition (PED) to improve the corrosion resistance of AISI 304L austenitic stainless steel in reducing acids and chloride solutions via a technique known as cathodic modification. The properties of the alloyed 304L stainless steel were evaluated using a number of tools including X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS). Preliminary tests in 1 M sulphuric acid showed that the ruthenium surface alloys sufficiently raised the corrosion potential of 304L stainless steel to ranges where the stability of chromium oxide is guaranteed. Surface alloys produced by RF sputtering and PED were associated with the best corrosion resistance, and protection efficiencies of at least 85%, but they spalled during corrosion exposure rendering them unsuitable for corrosion application. The corrosion of the ruthenium implanted surface alloys exhibited a strong dependence on the surface roughness of the stainless steel, with the least corrosion rates achieved on rough 304L stainless steel samples implanted with 1016 Ru/cm2 at 50 keV. Corrosion characterisation of these ruthenium implanted surface alloys was studied in various corrosive media including sulphuric acid, sodium chloride, magnesium chloride and simulated fuel cell solutions. Their corrosion rates in sulphuric acid decreased with increase in acid concentration, and exhibited non-Arrhenius behaviour in the acid solutions; corrosion rates were unaffected by increasing exposure temperature from 25 to 50°C. In 3.5 wt% sodium chloride, addition of ruthenium via ion implantation changed pit morphology from elongated to circular, indicating a diminished tendency for pits to initiate at manganese sulphide stringers. Corrosion rates of the ruthenium implanted stainless steels in the simulated fuel cell solutions were at least 69% lower than the target corrosion rate for use in polymer electrode membrane fuel cells (PEMFCs), thus presenting a possible practical application of ruthenium surface alloyed austenitic stainless steel.