Nanostructured TiN/ZrAlN and HfAlN Thin Films : Effect of Structure on Mechanical Properties

Sammanfattning: Transition metal nitrides are a remarkable group of ceramic materials that offer exceptional properties such as high hardness, low tribological wear, excellent thermal stability, and high oxidation resistance. Alloys such as TiN, CrN, VN, ZrN, and HfN have been identified as ideal candidates for protective coatings on cutting tool inserts in the metal processing industry. While TiAlN has been widely accepted, ZrAlN and HfAlN alloys have much unexplored potential. With a melting point of HfN at 3300 °C, approximately 400 °C higher than TiN, HfAlN shows great potential for age-hardening at even higher temperatures. These remarkable materials inspire us to push the limits of what is possible, and to continue to innovate materials science.The work performed in this thesis focuses on the development of hard coatings using ionassisted reactive magnetron sputtering. The coatings are based on group IV TM-Al-N, where TM is either Ti, Zr, or Hf. The aim is to enhance the performance of these ceramic coatings by simultaneously increasing their hardness and toughness. To achieve this, the growth mechanisms, structure, and mechanical properties of the films were studied in detail. The coatings were deposited onto single crystal Si(001) and MgO(001) substrates.The first study describes the development of a multilayer structure, consisting of alternating layers of TiN and Zr0.37Al0.63N1.09, with a bilayer period of 20 nm, with the aim of combining the unique properties of the constituent materials. Cubic rocksalt TiN is known for its high hardness and unfortunate brittleness. Hexagonal wurtzite Zr0.37Al0.63N1.09 is less hard, but also more ductile. The crystal structure of the multilayers varied depending on the substrate temperature during growth. At temperatures below ~350 °C, the ZrAlN layers grew near amorphous, while they were nanocrystalline between 500 °C and 800°C. At 900 °C, the ZrAlN segregated into a nanolabyrinthine structure consisting of w-AlN and c-ZrN. The hardness of the films increased significantly with increasing deposition temperature, from 24 GPa to 36 GPa. The films also showed superior fracture stress compared to the available literature, increasing from 6.1 to 7.7 GPa. The fracture toughness of the films was also improved compared to the binary constituents, up to 2.8 MPa√m. These findings illustrate the potential of combining diverse materials, to create new structures with enhanced properties and highlight the importance of optimizing the growth conditions to achieve the desired film functionality.In a second study, single-crystal Hf1-xAlxNy films were grown at high temperatures on MgO(001) substrates. Excess nitrogen in HfNy (y=1.22, 1.33) film created ordered nanosized domains of variations in the nitrogen composition, leading to the formation of a compositionally modulated superstructure. In Hf0.93Al0.07N1.15, the immiscibility of the constituents (c-HfN and c-AlN) causes the formation of a superstructure consisting of isostructural Al-rich and Hf-rich domains due to surface initiated spinodal decomposition. Micropillar compression tests reveal a ductile HfN1.22 and substantial strain hardening upon deformation. Hf0.93Al0.07N1.15 exhibited a brittle nature, although at a substantially increased yield stress in comparison, consistent with the improved hardness from 26 GPa to 40.5 GPa, measured by nanoindentation.