https://www.um.edu.mt/library/oar/handle/123456789/76422 Wed, 08 Apr 2026 06:14:19 GMT 2026-04-08T06:14:19Z https://www.um.edu.mt/library/oar/handle/123456789/101532 Title: An investigation on laser surface modification of an A356 aluminium alloy Abstract: The laser surface modification of an A356 aluminium alloy was investigated as a means to improve the tribological behaviour of the surface of such alloy. Laser remelting and alloying were carried out using a CO2 laser, equipped with a co-axial powder delivery system. The parameters needed to create uniform and repeatable surface layers were identified and optimized. Although limited hardness improvement was achieved with laser remelting, considerable improvement was achieved with laser alloying with nickel. A hardness improvement of nearly 3 times that of the untreated material was reported for uniform crack-free surfaces, whilst a 7-fold improvement was reported for fairly uniform surfaces that experienced moderate cracking. Microstructural, chemical and phase analysis of such surfaces showed that dendrites of Al3Ni and Al3Ni2 intermetallics were formed in the microstructures; these being the cause of the high hardness achieved. The introduction of ceramics to this aluminium nickel structure proved to be successful. Alloying using a novel technique involving Ni-Ti-C resulted in a uniform distribution of fine TiC particles in an Al-Ni and Al-Si-Ti intermetallic structure. Using Ni-Ti-SiC on the other hand resulted in a similar structure but with additional SiC particles, less TiC particles and less uniform distribution of the particles. The hardness values obtained were related to the overall content of alloying elements in the alloyed surfaces, and were similar to those obtained when alloying with nickel. Due to the fact that surfaces alloyed with Ni-Ti-C resulted in better dispersion of the carbides compared to the surfaces alloyed with Ni-Ti-SiC, only the former were further investigated. The wear resistance of both surfaces alloyed with nickel and Ni-Ti-C was highly improved when using an overlap rate of 25% and powder flow rates of more than 0.04 g/s, reaching values of over 22 times less volume loss compared to the untreated material. The type of wear in fact changed from severe, for the untreated material, to mild for the surfaces treated with these parameters. Corrosion testing showed that the alloyed surfaces had reduced corrosion resistance compared to the untreated material, therefore when these surfaces would be exposed to a corrosive environment while resisting wear, processing parameters more suited to limit this degradation need to be chosen at the expense of wear resistance. Description: PH.D Wed, 01 Jan 2014 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/101532 2014-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/101215 Title: Tribological and mechanical characteristics of surface modified austempered ductile iron Abstract: Compared to other ferrous materials, austempered ductile iron (ADI) has marked economic advantages such as low melting temperature, low shrinkage, excellent castability, good machinability and high damping capacity. This makes it a potential candidate material for automotive components such as gears. Gears in service are continuously being subjected to cyclic loads and are therefore required to have a tough and ductile bulk structure, together with superior tribological properties at the surface. In this study, ADI alloyed with 1.6 wt% copper and 1.6 wt% nickel has been investigated as a material for gears. The high toughness and ductility required in the core of gears was obtained by austempering in the temperature range of 360-400°C. A high content of stable austenite in austempered ductile iron is known to yield an optimum combination of mechanical properties. In this study, ductile iron specimens were austempered using different heat treatment parameters with the purpose of obtaining microstructures with contrasting austenite volume fractions and austenite carbon contents. The stability of the austenite was assessed using X-ray diffraction patterns and also by subjecting the austempered samples to cryogenic temperatures. The austempering parameters yielding optimal results were then used throughout the rest of the study. In addition to optimising the bulk properties, shot peening and laser hardening were studied as potential surface treatments for ADI gears. Different peening intensities and different shot materials were used to determine the peening parameters which yield the desired properties at the surface. The optimum parameters (S330 shots and 0.38 mmA intensity) were then applied to shot peen ADI specimens, which were then used to evaluate the resistance to bending fatigue, dry and starved lubricated sliding wear, and rolling contact fatigue. The results showed that while shot peening was beneficial in increasing the bending fatigue life, it did not have a positive influence on all the tribological characteristics. Therefore, laser surface hardening was studied as a second surface engineering technique. In particular, spot matrix hardening was investigated, in which ADI surfaces were irradiated with an array of laser spots. Studies were carried out using single spots and different beam powers, beam durations and beam diameters, with the purpose of determining the parameters which yield a surface with maximum hardness, hardened depth, no cracking and minimal distortion. Following optimisation of the process parameters on single spots (beam duration of 300 ms, beam diameter of 1.75 mm and beam power of 600-660 W), analysis was carried out using different spot patterns. The tribological behaviour of such treated surfaces was studied by carrying out starved lubricated sliding wear tests and rolling contact fatigue tests. Results showed that laser hardening is a suitable surface treatment to improve the tribological characteristics of ADI. Therefore, for the first time, the results obtained on test specimens indicate that shot peening could be applied to the root of Cu-Ni ADI gears in order to increase the bending fatigue resistance. Subsequently, the face and flank of gear teeth can be spot laser hardened with the intent of improving the tribological characteristics of ADI gears. Description: PhD Wed, 01 Jan 2014 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/101215 2014-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/101192 Title: Inorganic and hybrid transparent protective coatings for clean and corroded steel surfaces : a conservation approach alignment Abstract: Corrosion degradation of metal artefacts is a major concern in museums where the indoor atmospheric conditions are uncontrolled. A common approach to mitigate corrosion on museum objects is to apply protective organic coatings. This thesis explores the possibilities of using inorganic and hybrid organic-inorganic coatings as candidates for the protection of historical metal surfaces: if successful, such coatings would represent a complete breakaway from traditional measures insofa1 as steel artefacts are concerned. Inorganic protective films would offer advantages over organic coatings in being less permeable to oxidative and other corrosive gases and electrolytes and far more resistant to photochemical degradation. Hence, less maintenance would be necessary than with organic coatings. The Palace Armoury collection (mid-16 to 17 century) situated in the Grandmaster's Palace in Valletta, Malta was chosen as case study for this work The collection was chosen on grounds of it being almost entirely composed of steel objects and which displayed visible signs of corrosion. A comprehensive study of the Palace armoury indoor atmosphere was undertaken as a first part of this work which included an 18- month-long measurement campaign for relative humidity, 03, S02 and N02• The results showed that the internal atmospheric environment is not conducive to the safe storage and optimal display of general metals collections, pointing to the need for more effective protection of the displayed and stored artefacts. In a second part of this study, an in-depth metallurgical examination of a representative set of authentic plate-armour objects from the same collection was carried out yielding significant information on both the manufacturing technology of these artefacts and their state of preservation. X-ray diffraction studies performed on the corrosion layers indicated that the iron oxy-hydroxide phases present ubiquitously on the armour surfaces, including the er-containing akageneite, were indeed very similar to what is observed on steels exposed to a marine coastal environment. In a next phase of the study, the potential application of Ti02 and Si02 films as protective measures was investigated. Transparent films of these oxides were deposited as coatings using a physical vapour deposition technique starting off with titanium (III/IV) oxide and fused silica sources respectively. The Ti02 films were found to be heavily tinted by interference colours and poorly protected the underlying steel substrate. Further work on the use of this material as a 'conservation-friendly' coating was eventually abandoned on grounds of poor aesthetical qualities. Slightly better results were achieved with Si02 films in that interference colours were subdued but costly alterations to the deposition chamber were required in order to deposit coatings with improved anti-corrosion properties at lower temperatures. Attention was thus turned to the novel use of four groups of organic-inorganic hybrid coatings. The first group was based on a 60:40 mol% methyltriethoxysilane (MTES): tetraethylorthosilicate (TEOS) precursor mixture. The latter was altered by additions of 3-methacrylopropyltrimethoxysilane (y-MPS) in exchange for TEOS to furnish 60:30:10 mol% and 60:20:20 mol% MTES:TEOS:MPS sols. The second group was based on a 50:50mol% mixture of y-MPS and TEOS. Other member within this group were 40:40:20 mol% TEOS:MPS:MTES and 40:40:20 mol% TEOS:MPS: octyltriethoxysilane (OCTES). In the latter two sols, alkyltriethoxysilane precursors were added in an attempt to improve the water repellence of the final coatings. The third and fourth group of coatings were based on binary mixtures of either MTESTEOS or phenyltriethoxysilane (PTES) and TEOS where the phenyl- or methyltriethoxysilane precursor was varied from 20 to 80mol%. All coating lacquers were prepared by an acid catalysed hydrolysis-condensation procedure and the neutralized sols deposited onto mild steel substrate and glass slides by a dip-coating technique. Prepared sols for groups 1 and 2 were characterized by viscosity measurements and particle sizing and their temporal stability during the coating process was ascertained. Coatings were deposited by a dip-coating process on glass slides and steel substrate which was either cleaned uncorroded or pre-corroded to simulate the surfaces of historical steel. [...] Description: PH.D Tue, 01 Jan 2013 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/101192 2013-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/100020 Title: The laser deposition of stainless steel-matrix composite surfaces Abstract: Austenitic stainless steels are alloys of iron and chromium designed to have excellent corrosion resistance. Their resistance to wear is however less remarkable, and improvements to this and to their mechanical properties are desirable. The aim of this research was to investigate the improvement of these properties by means of the laser deposition of stainless steel matrix composite layers or tracks. Tungsten carbide agglomerates, chromium carbide and alumina were selected as the reinforcing materials. A methodology was devised for improving deposition process investigation times and economy. This was then applied to the determination of deposition conditions which promoted particle survival and improved wear resistance without excessively diminishing corrosion resistance. The effectiveness of coating ceramic materials with metal in order to enhance their probability of surviving the process was evaluated using silver-coated alumina and tungsten carbide agglomerates. The outcome of the deposition process was investigated by means of characterization techniques and by wear, erosion and corrosion testing. Description: PH.D Mon, 01 Jan 2007 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/100020 2007-01-01T00:00:00Z