ICMCTF2013 Session E2-2: Mechanical Properties and Adhesion

Sputter-deposited NiTi thin films are known to exhibit unusual properties such as shape memory effect and superelasticity making them attractive materials for MEMS and biomedical applications. Furthermore, their thermoelastic martensitic transformation is accompanied by reversible changes in surface roughness, which is an important factor in tribology. Thus, it might be possible with these films to control surface roughness and/or surface features during the sliding process through external heating with consequent reduction of friction and wear. The application of these films as smart surfaces for tribology requires detailed understanding of microstructural phenomena involved during the martensitic transformation; moreover, low hardness and wear resistance limit the use of these films.

In this study, Ni-Ti-(Cu) coatings deposited by plasma-assisted magnetron sputtering have been investigated. To obtain transformation temperatures higher than room temperature and improved mechanical properties, Ti-rich NiTi films were deposited and co-sputtered with copper, respectively. The coatings with a thickness of 1.5 µm were isothermally annealed at 500°C for 1 h in an argon atmosphere, with the aim to produce coherent precipitates (GP-zones) in the matrix. A scanning electron microscope (SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to observe cross-section of the coatings and their chemical composition, respectively. Their structure was evaluated by X-ray diffraction (XRD) and the mechanical properties were measured by a depth sensing nanoindenation.

The Ti/Ni ratio in the deposited films was kept constant when a series of five Ni-Ti-Cu coatings was prepared with copper content increasing up to 15 at.%. After annealing of the as-deposited amorphous coatings, XRD analyses showed a dominant martensitic structure at room temperature for all investigated compositions and thus indicating prevalent shape memory behaviour. Doping with copper affected the microstructure, since different kinds and densities of precipitates (i.e. coherent plates and spherical semi-coherent precipitates with the matrix) were identified. Application of the Scherrer equation to the strong martensitic peaks suggested a decreasing crystalline size with increasing Cu content. Nanoindentation revealed the effect of Cu content on mechanical (hardness and Young’s modulus) and functional properties, as well as their variation with change in transformation path, and thus in the native phase from B19’ to B19, for Cu contents lower and higher than 10 at.%, respectively.

Nanotwinned metals have received increasing attention recently as high density nanotwins can lead to unique electric, thermal and mechanical behavior, which are largely different from nanocrystalline metals with high angle grain boundaries. Twin boundaries serve as barriers to the transmission of dislocations, and are effective sources and sinks for dislocations during plastic deformation. We will review recent studies on a variety of sputtered nanotwinned fcc coatings, including 330 stainless steel, Cu and Ag. Both coherent {111} and incoherent {112} twin boundaries are observed, and the average twin spacing, on the order of ~ 10 nm, can be achieved by varying deposition conditions. In situ nanoindentation studies reveal that twin boundaries are mobile during deformation. Numerous dislocation-twin boundary interaction mechanisms are discussed.

Multilayered Al-Cu-Fe thin films have been deposited by triple-target unbalanced high vacuum magnetron sputtering onto Si and Al₂O₃ substrates. Isothermal annealing was performed using both an in situ XRD furnace, where the phase evolution was monitored, and in a tube furnace at temperatures up to 700 °C and annealing times up to 100 h.

It was found that when using Al₂O₃ substrates the icosahedral quasicrystalline phase Al₆₂Cu_25.5Fe_12.5 was formed at about 500 °C which was improving in structural quality and orientation with increasing temperature, but with the addition of a secondary β-phase, Al₅₀(CuFe)₅₀. For a Si substrate, Si starts to diffuse into the thin film at temperatures above 300 °C, preventing the quasicrystalline phase from forming. Instead, the cubic Al_62.5-xSi_xCu₂₅Fe_12.5 approximant phase forms at 430 °C. With increasing annealing time at 600 °C the Si content increases from x=8.3 at.% after 4 h to x=12 at.% after 64 h.

In this work, ZrCuAlNi thin film metallic glasses (TFMGs) were grown on Si wafer and AISI420 steels by magnetron sputtering system. Different amount of nitrogen flow rates were controlled by plasma emission monitoring (PEM) during sputtering. And boron element was doped to evaluate the influences on the mechanical properties and microstructure of TFMGs. The supercooled liquid region of TFMG was determined by the differential scanning calorimetry (DSC) analysis. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phase structure was confirmed by using x-ray diffraction (XRD). The mechanical properties were measured by nanoindentation. The scratch tester and ball-on-disk wear tests were employed to evaluate the adhesion and tribological properties. The surface roughness was determined by atomic force microscopy (AFM). It was concluded that the glass-forming ability and hardness of TFMGs were strongly influenced by the nitrogen and boron contents. The proper nitrogen and boron concentrations for ZrCuAlNi TFMG were proposed in this work.

In this report, we studied the effect of vanadium concentration on W-V-N superhard thin films deposited on Si(100) substrates at 700°C using reactive magnetron sputtering. The concentration of vanadium was varied in the range 0-21 at%. The resulting films microstructure, surface morphology, hardness, young modulus, fracture toughness and adhesion strength were studied by X-ray diffraction, atomic force microscopy, and nanoindentation, respectively. The Results showed that the W-V-N solid solution formed with preferred orientations (111) and (200) for all the W-V-N films. Nanoindentation hardness and young modulus of W-V-N films initially increased and then decrease with increasing vanadium content. The maximum values of hardness (~ 44 GPa) and young modulus (~ 420 GPa) were found for V content in the range of 8- 15 at %. The fracture toughness of ~ 5.22 MPa m^1/2 and adhesion strength of ~ 30-45 N was found in W-V-N with silicon substrate. A significant improvement in structural and mechanical properties of W-V-N superhard thin films would make them very useful protective coatings for machining tools.