This licentiate thesis adds a new piece to the puzzle that describes how the microstructural characteristics influence the hardness behavior of a multilayer coating. It contains a presentation of the manufacturing and the subsequent characterization of multilayer thin films. These multilayers consist of alternating layers of crystalline titanium nitride (TiN) and amorphous silicon nitride (Si3N4), deposited with a physical vapor deposition technique referred to as reactive magnetron sputtering. The microstructure of as-deposited films was examined with cross-sectional transmission electron microscopy (XTEM) and x-ray diffraction (XRD). XRD studies revealed a transition in preferred orientation for TiN, from a pure 002 orientation to a mixed 111/002 orientation as the TiN layer thickness increased from 4.5 nm to 9.8 nm. XTEM studies showed a microstructure consisting of equiaxed or elongated TiN grains, depending on layer thickness, limited in size by the amorphous interlayers. Selected area diffraction verified the observed transition in preferred orientation in TiN. For small silicon nitride layer thicknesses (~0.3 nm) an epitaxial stabilization of Si3N4 to the crystalline TiN lattice was observed through high resolution electron microscopy studies. Instead of amorphous interlayers a cubic silicon nitride rich phase (SiNx) was observed. This is to the present knowledge of the author the first time this phenomenon has been observed within this material system. In order to explain the observed behavior a model based on the involved energies were developed. Nanoindentation was performed to evaluate the mechanical behavior of the coatings as the layer thicknesses varied. All multilayers were harder than the monolithic TiN film, which had a hardness of 18 GPa compared to 32 GPa for the hardest multilayer. An interesting observation was that the hardest multilayer corresponds to the presence of cubic silicon nitride. Curvature measurements were performed and showed that the residual stresses within the multilayers were compressive and relatively constant, 1.3±0.7 GPa. In addition to the XTEM studies of as-deposited samples, XTEM studies of deformed multilayers were also conducted. The 300 mN load produced plastic deformation in the substrate under the indent. Cracks within the multilayer normally propagated along TiN/Si3N4 interfaces, which suggest that a lower energy is needed for cracking along an interface compared to intralayer cracking. The observed hardness increase can be ascribed to the multilayered structure of these films. By the interruption of TiN growth with intermittent Si3N4 layers the produced microstructure consisted of small TiN grains, separated in the growth direction by amorphous or crystalline interlayers. Small grains are known to contribute to hardening, but the interlayers also contribute, acting as dislocation obstacles either due to the amorphous tissue or to coherency stresses. / Godkänd; 2004; 20060917 (cira)

Godkänd; 2008; 20081121 (ysko)

and the placement of the material. Hence, 3D printing can be an advantageous new method of constructing supercapacitors.In this thesis, the aim was to investigate how the different parameters of Electrohydrodynamic printing (EHD printing) will affect the spread of gold nanoparticles. The electrohydrodynamic printing method is a printing method that utilizes an electric field to cause droplet ejection from the nozzle. When the electric field exerts a force on the solution containing nanoparticles, it stretches the meniscus to a point where it becomes unstable and forms a droplet. EHD printing utilizes an electric field which gives the method a high spatial accuracy while being able to print droplets with within a separation distance of tens of nanometers.Different parameters were evaluated to achieve desired distribution of gold nanoparticles across a silicon wafer substrate. This thesis focuses on print speed, frequency, heat treatment and voltage, and how printing parameters affect the results. The results revealed a variation, while the printing patterns follow a trend. The best results achieved in this work came from a low nozzle-substrate voltage, high frequency, and high printing speed. The varying results could be brought on by variation in ink composition, the nozzle diameter, and the metal coating of the capillary, to name a few possible causes.Handledare:

Bibliography: pages 85-89. / A study has been made of the unlubricated sliding wear behaviour of austempered ductile irons under conditions of sliding velocity and load. The load was varied between 0.9 and 2.8 MPa, whilst the sliding velocity range was between 0.5 and 2.0 ms⁻¹. Two commercial grades of spheroidal graphite irons, SG42 and SG60 were austempered between 250⁰C and 400⁰C. A distinction in the wear behaviour was found with metallic type wear dominating at the lower sliding velocities and an oxidative type wear being evident at the higher sliding velocities. It was however found that an increase in the load resulted in an earlier onset of the oxidative type wear regime, for a specific sliding velocity. On austempering these spheroidal graphite irons the mechanical properties as well as the sliding wear resistance increased dramatically. Furthermore, the austempered irons' outperformed a series of steels of much higher hardness by factors between 2 and 28 times under the same conditions. At the lower velocity of testing the outstanding wear resistance is attributed to the austempered iron's unique microstructure of acicular ferrite and retained austenite and a partial transformation of austenite to martensite. However, at the higher sliding velocity the exceptional wear resistance is derived from a development of an tribologically protective oxide film together with the formation of a hardened white layer. The development of the work hardened layer is linked to the high carbon in the matrix of these irons. The work hardened layer leads to a similar wear rate prevailing for all irons austempered from a specific parent iron. The synergism of variation in load, sliding velocity and wear counterface together with the effect of initial microstructure has been explain in terms of simple wear models.

This thesis reports a study of the microstructural evolution of chromium containing duplex ferrite-martensite steels and examines the effects of the microstructure on the mechanical properties. Emphasis has been placed on determining the microstructural factors responsible for the persistent occurrence of anisotropy in a modified 12 wt% Cr steel designated 3CR12. in addition an investigation has been carried out in order to refine the grain structure of a ferritic steel containing 16-17 wt % Cr by inducing a duplex ferrite-martensite phase structure. The microstructural evolution of 3CR12 was studied during cooling from a solution heat treatment at 1380°C and the natures of the phase transformations evident were investigated. Energy dispersive X-ray spectroscopy (EDS), in association with a scanning electron microscope (SEM), was used to determine the composition of the phases arising from the solid state δ-ferrite to austenite transformation. It is shown that the high temperature δ-ferrite phase partially decomposes to austenite via a Widmanstatten growth mechanism and consequently a banded two phase structure is produced after hot rolling. The element partitioning which arises during the solid state δ-ferrite decomposition ieads to compositional banding with an indelible nature. A model is proposed for the events leading to the generation of the banded phase structure and the formation of an elongated ferritic microstructure in 3CR12 after sub-critical annealing. The type and distribution of non-metallic inclusions occurring in 3CR12 has also been assessed. Characteristic fracture modes developed during impact testing have been related to the grain morphology and the occurrence of non-metallic inclusions. It is shown that splits form parallel to the rolling plane when Charpy specimens are subjected to impact testing and that both impact energy and mode of fracture are dependent on the directional properties of the 3CR12 microstructure. Splitting is predominantly caused by the low energy crack path provided by long, undulating grain boundaries parallel to the rolling plane, and inclusions, particularly manganese sulphides (MnS), facilitate low energy modes of fracture associated with the splitting phenomenon. MnS inclusions are also found to affect the corrosion resistance of 3CR12 and careful control of the chemistry of the steel permits these inclusions to be restricted to levels at which acceptable impact and corrosion properties are maintained. Refinement of the grain structure of ferritic steels containing 16-17 wt % Cr was carried out by modifying the ratio of ferritising elements to austenitising elements in the steel chemistry. Suitable ruckel additions have been determined which provide alloys with sufficient austenitising ability to refine the high temperature δ-ferrite phase and consequently a duplex ferrite-martensite microstructure is produced. Tempering of these alloys at 700°C results in a lamellar ferrite-martensite structure which gives rise to an attractive combination of impact and tensile properties which may provide a stainless steel with superior cost effectiveness to austenitic grades.

Metastable stainless steels are promising engineering materials demonstrating good corrosion resistance and mechanical properties. Their mechanical properties are however significantly affected by the deformation induced martensitic transformation. Hence, in order to use these steels to their full potential it is vital to have profound knowledge on this martensitic phase transformation. The aim of this thesis was therefore to investigate the evolution of phase fractions, texture, microstrains and microstructure to improve the current understanding of the deformation induced martensitic transformation in AISI 301. To investigate the deformation behavior of AISI 301, in-situ high-energy x- ray diffraction during tensile loading has been performed on samples suffering different cold rolling reduction. Ex-situ transmission electron microscopy, electron back-scattered diffraction and optical microscopy were also used to characterize the microstructure at different deformation levels. The results show that parts of the austenite transforms to both ά- martensite and ε-martensite during deformation of AISI 301. The transformation behavior of ά-martensite is however completely different from the transformation behavior of ε-martensite. ε-martensite forms in a parabolic behavior, while the ά-martensite transformation can be divided in three characteristic stages. The third transformation stage of ά-martensite has previously not been reported and it is characterized by a series of rapid transformations, each of which is followed by a period of yielding without any transformation. Moreover, the lattice strain evolution in the austenite at high plastic strains was found to be oscillatory, which is correlated with the stepwise transformation of ά-martensite as well as changes in x-ray peak broadening. This behavior was also coupled with the evolution of microstructure, where a distinct banded structure consisting of slip bands and Ü-martensite was observed at low plastic strains. This banded structure was however broken at high plastic strains when the ά-martensite grew larger and formed a block- shaped morphology. These findings lead to the conclusion that the three stages of ά- martensite transformation is due to different stages of nucleation and growth. The ά-martensite will first form as small nucleus, mainly at dislocation pile-ups along slip bands. The nucleuses will grow moderately in size and the structure will become saturated with nucleuses. Hence, the only way more ά-martensite can form is by growth of the existing nucleuses. This growth is very localized and seen as bursts in the transformation curve. The oscillatory behavior observed for the lattice strains during martensite formation possibly originate when semicoherent boundaries between austenite and ά-martensite become incoherent as the ά-martensite grow large. / Godkänd; 2005; 20061213 (haneit)

Adhesive joints are used extensively in the automotive industry. There are many ongoing studies on the area of application of joining composite to other material using adhesive joints. In this study, an analysis of mechanical behaviour of composite single lap-joint (SLJ) for carbon fibre reinforced polymer (CFRP) assembled with steel is presented and the analyses are divided into three phases. The first phase consists of a parametric study on a SLJ using Volkersen analytical model (AM), which is the effect of adhesive thickness and overlap length of the SLJ under tensile load. It was found that with increasing the adhesive thickness the final peak load (strength of the joint) increases. The peak load also increases with increasing the overlap joint, but the limit value for the overlap length varies for different adhesive thickness. For example for the case of adhesive thickness of 0.5 mm, the curve reaches to its plateau with overlap length of 40 mm. It was also observed the increase of adhesive thickness leads to decrease of maximum shear stress at the edges of the single lap joint, but it increases as it approaches the middle of the overlap length. Phase two of this study consist of a shear stress comparison with the Volkersen AM with the finite element model (FEM) using ANSYS Parametric Design Language(APDL) software. The purpose of this comparison was to validate the AM. It was found that the AM has a good agreement with the numerical-model (NM). However, the shear stress from the AM at the edge is a little higher than the NM, this is because the analytical method only takes into account the shear stress in one direction but the NM also takes into account the normal shear stress in the other direction. Phase three of this study consists of an experimental analysis of SLJ mechanical behaviour due to the change in temperature of 180 degrees and change in adhesive thickness and also a comparison with the NM. Three adhesive thickness 1, 0.5 and 1.5 mm were tested. Different boundary conditions (BC), namely as with frame and fixed BC are tested for NM. The one with frame BC is to compare with experimental setup and the fixed BC is the equivalent to Volkersen’s geometry. Both experimental and numerical results, show that the relative deformation of the SLJ decreases with the increase of the adhesive thickness. Although the experimental values were much lower than the numerical one, they agree well with the numerical result in term of trend of relative deformation. In experimental analysis, it was found that increasing the adhesive thickness from 0.5 mm to 1.5 mm decrease the relative deformation from 7.8% to 5.3%. It was concluded that increasing the adhesive thickness decreases the stiffness of the joint and allows more thermal movement in the joint.

This investigation evaluates the abrasive-corrosive wear behaviour of aluminium alloys with the aim of establishing a data base of performance and guide lines for material optimisation. Wear test apparatus and standard tests developed by previous research programmes were utilised (Noel and Allen, 1981; Barker, 1988). Further tests were then devised for a more detailed characterisation of wear behaviour. Tests conducted showed that aluminium alloys have approximately a quarter to half the abrasion resistance of mild steel. Poor microfracture properties of Al-Si cast alloys were observed as a result of coarse and brittle silicon rich phases contained in the aluminium matrix. Non heat-treatable wrought alloys exhibit ductile micro-deformation characteristics whilst heat-treatable alloys, having the best abrasion resistance, possess better combinations of strength, hardness and toughness. Tests with combined corrosion and wear showed that most aluminium alloys are subject to pitting corrosion due to localised differences in electrode potentials at constituent sites. Higher series alloys with a large number of constituent particles exhibit higher pitting densities. Due to the high electrode potentials of silicon phases and copper and zinc solid solutions, the alloys LM6+Sr, 2014 and 7075 have poor corrosion resistance and are subject to localised and pitting attack. As a consequence the alloys 2014, 7075 and LM6+Sr show a decrease in wear performance under abrasive-corrosive conditions. In contrast the good corrosion resistance of the alloys 5083, 6261 and 7017 provide a significant improvement in wear performance under conditions of long corrosion periods with light abrasive intervals. This study concludes that the abrasion resistance of wrought alloys may be optimised by designing an alloy with a good combination of tensile strength, fracture toughness and hardness together with an intermediate microstructural size distribution of second phase particles in the aluminium matrix. Ageing of heat treatable alloys improves abrasion resistance significantly, peak hardness and strength conditions resulting in optimum abrasion properties.

Bibliography: pages 129-137. / This investigation was undertaken as part of an endeavour to design an ideal wear resistant material for particular applications. The research was aimed at the alleviation of wear in the gold mining industry. In order to achieve this objective it was necessary to examine the surfaces of worn materials in order to gain a better understanding of the different wear mechanisms and also to examine the extent and depth of deformation induced by abrasive wear. Numerous proprietary wear resistant materials and stainless steels presently used in the gold mining industry together with other materials were included in this investigation. The abrasion and corrosion- abrasion wear resistance of two particular proprietary wear resisting materials was determined to be superior to mild steel and attempts were made to explain the good performance of these materials in terms of micro- structural and mechanical properties. Various techniques were used to study the effects of low and high stress wear of materials which had been tested in both the laboratory and in-situ in the mines. These techniques include scanning and transmission electron microscopy, optical metallography and microhardness studies. It was found that as the nominal load on the abrasive increased, the mode of material became more severe, the depth of deformation increased and the surface hardness increased. Attempts were made to explain these phenomena in terms of microstructural considerations, work hardening capacity, phase transformations and recovery and recrys- tallization. This work has assisted in the specification of the composition and microstructure of steels which should provide improved performance in severe working conditions.

Godkänd; 1998; 20070404 (ysko)