Synthesis and Characterization of Transition Metal Diborides

Sammanfattning: This thesis is devoted towards physical vapor deposition (PVD) of thin films of transition-metal (TM) diborides, focused on the material system TiBx, Ti1-xAlxB2-y and CrBx. The metal diborides are a large family of compounds with both metallic and ceramic properties, due to its bonding nature being a mix of covalent and ionic bonds. Their characteristics include, e.g., good mechanical, electrical and thermal properties, while an improved oxidation and corrosion resistance are currently sought after. Furthermore, while the ideal composition of these diborides is TMB2, i.e. with a B to metal ratio of 2, the stoichiometry in the PVD deposited films typically diverges from this ratio. TiBx is often reported to be overstoichiometric, with x well above 2. One of the most known and commonly used member of the TM diboride family is TiBx, primarily used in hard-coating applications such as tools for machining Al. However, the material displays a fracture toughness and oxidation resistance that ideally needs to be improved.The films presented in this thesis were deposited by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Using both methods facilitates an improved control of both microstructure and composition, and hence the materials properties. With HiPIMS, understoichiometric TiBx films were grown and it was shown that these films can match and even exceed the overstoichiometric counterpart, deposited with DCMS, in terms of mechanical properties. The hardness and fracture toughness for TiB1.43 films were measured at 43.9±0.9 GPa and 4.2±0.1 MPa√m, compared to TiB2.70 films at 37.7±0.8 GPa and 3.1±0.1 MPa√m. Furthermore, the understoichiometric films significantly improve the oxidation resistance. Air annealing of TiB1.43, TiB2.20, and TiB2.70 films at 400 °C showed an average oxidation rate of 2.9±1.5, 7.1±1.0, and 20.0±5.0 nm/h, respectively, explained by the microstructural difference between over- and understoichiometric material. In TiBx films where x > 2, there is a B-rich tissue phase in the grain boundaries which is suggested to enhance oxidation. The hydroscopic nature of B2O3 causes more rapid oxidation and evaporation thus providing an easy oxidation pathway in B-rich regions.  However, understoichiometric films where x < 2 do not show any significant boundary phases. Instead, the B deficiency is presented as planar defects with Ti-rich stacking faults. Hence the absence of the B-rich tissue phase has strongly contributed to increasing the oxidation resistance.Oxidation resistance and mechanical properties were also investigated for understoichiometric Ti1-xAlxB2-y coatings with varying Ti:Al and B:M ratios, obtained from both HiPIMS and DCMS depositions. Al alloying of the TM diboride TiBx significantly enhances the oxidation resistance. However, incorporating too much Al is at the expense of the excellent hardness seen in the pure TiBx, going from 46.2±1.1 GPa to 22.6±1.1 GPa for Ti0.9Al0.1B1.3 and Ti0.3Al0.7B1.3, respectively. Hence, a reduction in the Al content is needed to retain the mechanical properties. The boundary phase in this material consists of a Ti1-xAlxB2-y tissue phase, rich in either Al or B depending on the x and y values. An improved oxidation resistance in Ti1-xAlxB2-y was seen with reduced Al and B content, proposed to be due to absence of tissue phase in the grain boundaries, in line with the observations for TiBx. The oxide scale thickness of Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.9 after air annealing at 600 °C for 10 h was measured to be 205 nm and 320 nm, respectively. Moreover, the trends indicate a reduced oxidation rate as the oxide scale grows thicker.A systematical study of DCMS deposited CrBx coatings, 1.90 ≤ x 2.08, was also performed, motivated by CrBx being a material of interest for providing potential corrosion resistance. All films, irrespectable of the deposition conditions, exhibited (001) texture, with epitaxial growth observed when increasing temperature from 500 °C to 900 °C. Higher density (5.2 g/cm3) and smoother surfaces was seen in films grown at lower pressure, 5 mTorr (0.67 Pa), compared to higher pressure, 20 mTorr (2.67 Pa), and was explained by less gas scattering leading to more energetic particles impinging on the surface. CrBx film composition show no apparent dependence on substrate temperature, and has a slight dependence on deposition pressure for the samples deposited at 900 °C, with reduced B content for increasing pressure. Overstoichiometric CrB2.08 films showed the presence of large B-rich inclusions, and B deficiency in CrB1.90 films presented as planar defects with Cr-rich stacking faults, similar to understoichiometric TiBx.The thorough investigations of all the systems in this thesis are aimed towards improving the understanding of the correlation between the thin film synthesis process and the resulting composition and microstructure, which in turn dictates the properties of thin films. A particular emphasis is put on control of composition.