Browsing by Author "Lawson, K. J."
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Item Open Access Carbon and titanium diboride (TiB2) multilayer coatings.(2004-11-01T00:00:00Z) Rao, Jeff; Cruz, R.; Lawson, K. J.; Nicholls, John R.Titanium Diboride, (TiB2) is a metal-based refractory ceramic material that has been investigated in industrial applications ranging from, cutting tools to wear parts and for use in the aerospace industry. The unique properties which make this material so fascinating are, its high hardness, high melting point and its corrosion resistance. TiB2 is prevented from wider mainstream use because of its inherent brittle nature. With a view to overcome this in coating form and with the aim of providing in addition inherent lubricity, in this study 50 layer TiB2/C multilayer stacks have been fabricated, with varying volume fractions of ceramic, whereby the interfaces of the layers limit crack propagation in the TiB2 ceramic. TiB2 has been multilayered with carbon, to make use of the unique and hybrid nature of the bonding in carbon coatings. DC magnetron sputtering with substrate bias was the preferred route for the fabrication of these coatings. AISI tool steel has been used as the substrate material. By varying the amount of TiB2 ceramic from 50% to 95%, the Hardness of the coating is seen to increase from 5 GPa to 17GPa. The Hardness is observed to decrease as a function of increasing carbon content, agreeing with other studies that the carbon layers are not load-bearing. The graphitic nature of the sp2 bond, however, acts as a lubricant layer.Item Open Access Low temperature ITO thin film deposition for solar cells(Cranfield University, 2008-09) Seraffon, Maud; Impey, Susan A.; Lawson, K. J.; Rao, JeffThe electrical and optical properties as well as the microstructure of indium tin oxide thin films deposited on glass and flexible substrates is demonstrated in this project report. This project is a part of an MSc in Advanced Materials at Cranfield University. The understanding and improvement of indium tin oxide coatings deposited by magnetron sputtering on both glass and polymer substrates has been an area of extensive research in the last decade. This technology appears to be very interesting in terms of money saving and efficiency in the solar cell domains where the coatings are used in thin film solar cells. Research into the thin film solar cells mechanism, the sputtering process and ITO coatings is reported, along with detailed consideration of the best results obtained in the past in terms of ITO films’ electrical and optical properties. ITO thin films were deposited on glass and different sputtering parameters were changed in order to investigate their influence on the coatings properties: film thickness, chamber pressure, rotation, oxygen amount and sputtering power. A decision was made to establish the best sputtering parameters. These parameters were set to deposit ITO on polyethylene terephthalate (flexible substrate). The samples were also annealed at 150°C and 370°C. An ITO thin film with a resistivity of 1x10-4 Ωcm and a 90% transmissivity was obtained. An Energy Dispersive Spectrometry analysis was finally made on samples showing a substoichiometric composition of the ITO films.Item Open Access Methods to reduce the thermal conductivity of EB-PVD TBCs.(Elsevier Science B.V., Amsterdam., 2002-03-01T00:00:00Z) Nicholls, John R.; Lawson, K. J.; Johnstone, A.; Rickerby, David S.This paper reviews the advantages and disadvantages of various thermal barrier coating (TBC) systems, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity. Methods of heat transfer within zirconia based ceramics are discussed, including the influence of coating microstructure and ceramic composition. It is shown the addition of dopant atoms (colouring) is effective in reducing â  phononâ  transport and that layered microstructures are effective in reducing â  photonâ  transport. Advanced processing, using EB-PVD coating methods has allowed both coloured and layered ceramic coatings to be produced. Measured thermal conductivities of 1.0 W mKâ  1 have been achieved using these methods, much lower than current commercial EB- PVD coatings at 1.5âÂItem Open Access Nickel titanium and nickel titanium hafnium shape memory alloy thin films(Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Rao, Jeff; Roberts, T,; Lawson, K. J.; Nicholls, John R.Shape memory alloy (SMA) coatings of NiTi and NiTiHf have been deposited onto Si substrates using pulse DC sputtering. Coatings of NiTi with compositions containing 45 to 65 at.% Ti have been fabricated by co-sputtering NiTi with Ti. NiTiHf coatings with Hf compositions ranging from 2 to 30 at.% Hf have been fabricated by co-sputtering NiTi with Hf. XRD results reveal the as-deposited coatings as amorphous. A high temperature, 1100 à °C anneal followed by a low temperature, 550 à °C anneal was employed to crystallise the coatings. The XRD then shows the coatings to be martensitic at room temperature. Two sets of samples were produced for characterisation; one set was used for indentation studies and the other set used to prepare freestanding films required for differential scanning calorimetry (DSC) studies. Using the DSC, a NiTi coating containing 52 at.% Ti shows an endothermic austenite peak phase transformation, (Ap) at around 105 à °C and an exothermic peak martensite phase transformation, (Mp) at 65 à °C, resulting in a hysteresis of 40 à °C. For a NiTi coating containing 65 at.% Ti the hysteresis remained unchanged at 40 à °C, but there was a decrease in the phase transformation enthalpies when compared with the coatings containing 52 at.% Ti. Calculated phase transformation enthalpies in the NiTi coatings ranged from 6 to 13 J/g for the austenite phase and â  8 to â  11 J/g for the martensite phase. The NiTiHf coating shows SMA behaviour for a film containing 30 at.% Hf. DSC reveals an â  Râ  phase transition in this film. It is understood that this phase is present in films that have high internal stresses and is understood to nucleate near Ti3Ni4 precipitates. Phase transformation temperatures occur at 98 à °C and 149 à °C during heating and occur at 99 à °C during cooling. Phase transformation enthalpies range between 2 and 3 J/g for the austenite phase and â  7 J/g for the martensite phase. A scratch tester equipped with a 5 mm spherical tip has been utilised with loads ranging from 1 to 5 N to determine the recovery properties of the films. The results in this study conclude that NiTi films containing 65 at.% Ti deform 3 times more than films containing 52 at.% Ti. For NiTiHf thin films, increasing the Hf composition from 2 at.% to 30 at.%, doubled the deformation measurItem Open Access Protein adsorption on heterogeneous surfaces(American Institute of Physics, 2009) Aggarwal, Nitesh; Lawson, K. J.; Kershaw, Matthew; Horvath, R.; Ramsden, Jeremy J.The adsorption of the protein bovine serum albumin from an aqueous solution onto substrata made from pure silica, pure zirconia, and a mixture of the two has revealed that the adsorption behavior of the protein onto the mixture very significantly diverges from the corresponding mean of the behaviors with the pure substrata. A tentative explanation in terms of matching substratum heterogeneity with protein surface heterogeneity is offered.Item Open Access TiB2 ceramic and DLC Multilayered PVD coatings(Cranfield University, 2007) Santos da Silva Cruz, Ricardo Manuel; Nicholls, J. R.; Lawson, K. J.Titanium diboride (TiB2) has been investigated as a potential candidate for several industrial applications, such as: cutting tools, electric devices, wear parts and many more fields of application. The main drawback of TiB2 is its brittle nature, which has limited its range of applications. Diamond-like Carbon (DLC) has been used in industrial applications, mainly for wear resistant parts. However, the application of DLC films has been limited by the level of internal stress accumulated during deposition. This thesis investigates the deposition of TiB2/DLC multilayer wear resistant coatings to overcome these limitations. The thesis focuses on the tribological effects caused by different deposition techniques on TiB2 ceramic used by the coatings industry today, in order to overcome the brittle nature of TiB2 and the lack of adhesion of DLC films. The multilayer coatings consisted of 25 bi-layers of TiB2/DLC. These TiB2/DLC coatings were fabricated, maintaining a constant composition wavelength (sum of two layers λ =200 nm) for an array of ceramic fractions ranging from 25% to 95% by volume using as substrates, tool steel AISI 1095 and powder metallurgy aluminium alloys, type 2618. The effect of the DLC content on the structure and performance (hardness, adhesion and wear) of the films was investigated. The bi-layer thickness influences the failure patterns observed in the scratch testing. These observations have been used to optimise the multilayered coating structure. Multilayer coatings have been manufactured to increase surface hardness and wear resistance as applied to a commercial powder metallurgy Al alloys (Al 2618) used in the automotive industry. Optimum coatings have found hardness values of 27.8 GPa, with a critical load of 20 N and a friction coefficient of 0.47. As a result of wear tests the multilayer with 10% of DLC was found to be best compromise between high hardness (23.8 GPa), good adhesion (critical load higher than 20 N), low friction and low wear rate.