The present thesis focuses on characterisation of microstructure and the resulting mechanical and tribological properties of CVD and PVD coatings used in metal cutting applications. These thin and hard coatings are designed to improve the tribological performance of cutting tools which in metal cutting operations may result in improved cutting performance, lower energy consumption, lower production costs and lower impact on the environment.  In order to increase the understanding of the tribological behaviour of the coating systems a number of friction and wear tests have been performed and evaluated by post-test microscopy and surface analysis. Much of the work has focused on coating cohesive and adhesive strength, surface fatigue resistance, abrasive wear resistance and friction and wear behaviour under sliding contact and metal cutting conditions. The results show that the CVD deposition of accurate crystallographic phases, e.g. α-Al2O3 rather than κ-Al2O3, textures and multilayer structures can increase the wear resistance of Al2O3. However, the characteristics of the interfaces, e.g. topography as well as interfacial porosity, have a strong impact on coating adhesion and consequently on the resulting properties.  Through the deposition of well designed bonding and template layer structures the above problems may be eliminated. Also, the presence of macro-particles in PVD coatings may have a significant impact on the interfacial adhesive strength, increasing the tendency to coating spalling and lowering the surface fatigue resistance, as well as increasing the friction in sliding contacts. Finally, the CVD-Al2O3 coating topography influences the contact conditions in sliding as well as in metal cutting. In summary, the work illuminates the importance of understanding the relationships between deposition process parameters, composition and microstructure, resulting properties and tribological performance of CVD and PVD coatings and how this knowledge can be used to develop the coating materials of tomorrow.

In this thesis a new design concept of tribologically active coatings aimed for low friction applications, have been explored. Materials modeled by ab initio DFT calculations were realized through deposition of carbide and nanocomposite coatings by DC-magnetron sputtering. The design concept employs destabilization of a carbide material by alloying with a weak carbide-forming element, which refines the structure into a nanocomposite. The destabilization creates a driving force for superficial ejection of carbon in a tribological contact, forming a lubricious graphitic carbon layer. The otherwise hard material limits the real contact area and the transformed layer accounts for low shear resistance. Hence, the ideal situation for low friction is provided by formation of an easily sheared thin surface layer on a hard material. TiAlC was chosen as a model system for the theoretical modeling as well as for the depositions. The elemental composition, microstructure and mechanical properties of the coatings were characterized to relate the inherent properties to the experimentally achieved tribological response. As predicted by theory, TiAlC coatings were shown to provide self-lubrication on the atomic scale by giving low friction through a tribologically induced surface restructuring. It was shown possible to reduce the friction coefficient from 0.35 for TiC to 0.05 by addition of Al. Alloying with Al also proved to be a potent method in tailoring residual stresses from high and often detrimental levels to acceptable levels, with no significant reduction in either hardness or Young’s modulus. The effect of adding Al into TiC on the oxidation resistance was also explored. The critical temperature for onset of oxidation proved to increase with the Al-content from about 350°C for TiC to about 450°C for TiAlC with about 7 at% Al. A further increase in Al content did not change the onset temperature further but reduced the oxidation rate.

The widespread use of low friction PVD coatings on machine elements is limited by the high costs associated with fulfilling the demands on the surface quality of both the supporting substrate and the counter surface. In this work, an attempt is made at lowering these demands, by adding a sacrificial carbon overcoat to a TaC/aC low friction coating. Both coatings were deposited by planar magnetron DC sputtering, as separate steps in a single PVD-process. Coatings were deposited on substrates of two different surface roughnesses, in order to test the ability of this coating system to function on rougher substrates. Reciprocating ball on disc tests was performed, using balls with two different surface roughnesses. The worn surfaces were investigated using 3-D profilometry and SEM. The ability of the different overcoats to initially reduce the roughness of both the coated surface and the counter surface and to produce stable, low-friction conditions was examined for the different initial roughnesses. The implications for design of efficient run-in coatings for various systems are discussed.

This thesis is to study the microstructure and its related surface properties of CrxNy¡BDLC and TiO2 films prepared by different plasma energy processes such as physical vapor deposition (PVD) and plasma immersion ion implantation (PIII) processes. In the first chapter of this thesis, the different Cr-based coatings (Cr, Cr2N and CrN) were prepared by PVD process with lower plasma energy condition (less than 100 eV). The surface energy properties of these Cr-based coating were studied and the relationship between the surface properties and adhesion forces with respect to epoxy molding compound (EMC) were also investigated. It is found that the PVD-Cr2N coating has the largest contact angle of water and the lowest polar components and surface energy. The low surface energy is attributed to the high density of surface reactive sites and the formation of N-H bonds and O-H bonds on the film surface that act as the effective adsorption sites for carbon. The second chapter focuses on the modification of the electrical and mechanical properties of DLC films with high plasma energy (5¡ã45 keV) , PIII post-treatment. It is found that using high ion energy and a short treating time can modify the sp3 content, the nano-hardness and the Young¡¦s modulus of the DLC films. The formation mechanisms of sp3 C-C bonds in the implanted DLC film involve the reaction among compressive stress field, bonds breakage and recombination process induced by hydrogen and carbon ion. The third chapter deals with the structure and photocatalytic properties of nitrogen-doped TiO2 film prepared by PVD process with lower plasma energy condition (less than 100 eV). The TiO2-xNx films are indirect transition type and optical band gap energy is various with increasing nitrogen flow rate. The TiO2-xNx film with high crystallinity and porous surface morphology shows the best degradation rate of methylene blue solution.

<p>This Thesis presents two different deposition techniques for the synthesis of Ti₂AlC coatings. First, I have fabricated Ti₂AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211^TMTi₂AlC powder of size ~38 µm in an H₂/O₂ gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti₃AlC₂, TiC, and Al_xTi_y alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211^TM Ti₂AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti₂AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti₂AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti₂AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti₂AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti₂AlC (101̅7) // Al₂O₃ (0001).</p> / Report code: LIU-TEK-LIC-2008:15.

This licentiate thesis has the main focus on evaluation of the wear of coated and uncoated polycrystalline cubic boron nitride cutting tool used in cutting operations against hardened steel. And to exam the surface finish and integrity of the work material used. Harder work material, higher cutting speed and cost reductions result in the development of harder and more wear resistance cutting tools. Although PCBN cutting tools have been used in over 30 years, little work have been done on PVD coated PCBN cutting tools. Therefore hard turning and hard milling experiments with PVD coated and uncoated cutting tools have been performed and evaluated. The coatings used in the present study are TiSiN and TiAlN. The wear scar and surface integrity have been examined with help of several different characterization techniques, for example scanning electron microscopy and Auger electron spectroscopy.   The results showed that the PCBN cutting tools used displayed crater wear, flank wear and edge micro chipping. While the influence of the coating on the crater and flank wear was very small and the coating showed a high tendency to spalling. Scratch testing of coated PCBN showed that, the TiAlN coating resulted in major adhesive fractures. This displays the importance of understanding the effect of different types of lapping/grinding processes in the pre-treatment of hard and super hard substrate materials and the amount and type of damage that they can create. For the cutting tools used in turning, patches of a adhered layer, mainly consisting of FexOy were shown at both the crater and flank. And for the cutting tools used in milling a tribofilm consisting of SixOy covered the crater. A combination of tribochemical reactions, adhesive wear and mild abrasive wear is believed to control the flank and crater wear of the PCBN cutting tools. On a microscopic scale the difference phases of the PCBN cutting tool used in turning showed different wear characteristics. The machined surface of the work material showed a smooth surface with a Ra-value in the range of 100-200 nm for the turned surface and 100-150 nm for the milled surface. With increasing crater and flank wear in combination with edge chipping the machined surface becomes rougher and showed a higher Ra-value. For the cutting tools used in milling the tendency to micro edge chipping was significant higher when milling the tools steels showing a higher hard phase content and a lower heat conductivity resulting in higher mechanical and thermal stresses at the cutting edge.

Zugl.: Aachen, Techn. Hochsch., Diss.

Techn. Hochsch., Diss.--Aachen, 2005.

Zugl.: Darmstadt, Techn. Univ., Diss., 2007

Zugl.: Aachen, Techn. Hochsch., Diss., 2008