ICMCTF2017 Session E2-1: Mechanical Properties and Adhesion

Microelectromechanical systems (MEMS) relays have orders of magnitude higher figure of merit compared to semiconductor devices, along with lower power consumption and insertion loss. Unfortunately, MEMS switches have not penetrated high volume applications partly due to contact adhesion, contamination, high cycle switch life and microstructural evolution leading to performance drift.

Recent work has shown that it is possible to achieve extraordinarily stable nanocrystallinity in some binary metal alloys via solute segregation in the as-deposited state, even at high temperatures. One of these alloys, Pt-Au, may provide solutions to several challenges preventing greater adoption of MEMS switches. As-deposited nanocrystalline Pt grains are stabilized by the presence of Au at grain boundaries, presenting the possibility of reduced creep and relaxation in the electrical contacts and structural elements of a MEMS switch. Sputtered Pt-Au was used to construct MEMS switches, and initial results suggest performance improvements relative to baseline switches made from Au. MEMS switch performance and associated material evolution mechanisms will be discussed.

The amorphous nature of thin film metallic glasses (TFMGs) provides outstanding mechanical properties, including high strength, large elastic limits, and excellent corrosion and wear resistance. The grain boundary-free structure of TFMGs produces an exceptionally smooth surface and low surface free energy, resulting in high hydrophobicity and a low coefficient of friction.

The texture transformation has been attributed to a competition between strain energy and interface energy. In FCC materials, the (111) orientation has the lowest interface energy, while the (100) orientation has the lowest biaxial modulus. Thus, at a given strain, the interface energy per unit volume decreases as the inverse of the film thickness, while the strain energy per unit volume is constant with film thickness. However, recent studies have questioned the role of both stresses and interface energies in this texture transformation. Where stresses are known with certainty, they have been shown to be insufficient to produce the texture transition, and the transitions seem to occur in films of similar thickness regardless of the interface conditions. We simulated the driving forces using a first principle density functional theory for the orientation selection mechanisms and investigated the transformation by using a bulge test apparatus to induce different stresses in thin Ag films under identical annealing conditions. In situ synchrotron XRD measurements show the change in texture during annealing, and reveal that applied stresses have no effect on the transformation. Stress analysis shows that differences in driving forces for texture transformation due to applied bulge pressure were significant (≈200 kJ/m³), suggesting that a different, much larger driving force must be responsible. Reduction in defect energy has been proposed as an alternative. However, vacancy and dislocation densities must be exceptionally high to significantly exceed the strain energy and do not provide obvious orientation selection mechanisms. Nanotwins in reported densities are shown to provide greater driving force (≈ 1000 kJ/m³) and may account for orientation selection. The large difference between the calculated strain and defect energies and the driving force for grain growth (21,100 kJ/m³) casts doubt on the applicability of a thermodynamic model of texture transformation.

Among many ternary nitride protective coatings, the CrAlN coatings have been paid much attention to the cutting tool’s coating applications due to their excellent properties such as high hardness, low surface roughness, and excellent thermal stability. It was reported that the interlayer with the median hardness to elastic modulus ratio (H/E ratio) between the value of the coating and the substrate improved the wear resistance of the coating. In this work, various interlayers such as CrN, CrZrN, CrN/CrZrSiN were synthesized between the CrAlN coating and the tungsten carbide substrate to improve mechanical properties of the coatings. All the coatings were produced by an unbalanced magnetron sputtering system on the WC-6 wt.% Co substrate, and total thickness was controlled to be 3 μm. The microstructure, hardness and elastic modulus, and friction coefficient were evaluated by field-emission scanning electron microscopy (FE-SEM), nano-indentation, and ball-on-disc type wear tester, respectively. All the coatings were annealed at temperatures from 600 to 1000˚C in furnace for 30 min, and the hardness values were investigated using nano-indentation.

The hardness and elastic modulus of all the CrAlN coatings were not affected significantly by type of the interlayer, and they were measured to be in the ranges of 35.5 to 36.2 GPa and 424.3 to 429.2 GPa, respectively. However, wear test showed that the CrAlN coatings with the CrN and CrN/CrZrN interlayer exhibited improved friction coefficient of 0.34 compared to the CrAlN coating with the CrZrN interlayer (COF 0.41), and the wear rate and width of those coatings showed lower values. These improved wear properties could be attributed to the H/E ratio of the interlayer between the CrAlN coating and the WC substrate. In view of the coating structure, there exists a gradual decrease in the H/E ratio from the CrAlN coating (H/E, 0.089), to the CrZrSiN interlayer (H/E, 0.083) and CrN interlayer (H/E, 0.076), and the WC substrate (H/E, 0.040). The CrZrSiN and CrN interlayers induced a smooth transition of the stress effectively under loading conditions during the wear test, and this led improved wear resistance of the CrAlN coating. During the thermal stability tests, the hardness of the CrAlN coating with the CrN/CrZrSiN coating was maintained up to 1000˚C due to excellent oxidation resistance of the CrZrSiN layer consist of the amorphous SiN phase .