ICMCTF2012 Session E2-3: Mechanical Properties and Adhesion

Recently metallic glass thin films represent a class of promising engineering materials for structural applications. In this work, a series of Zr-based ZrNiAlSi metallic glass thin films were fabricated by sputtering process. Different amount of nitrogen gas was introduced during thin films sputtering. The amorphous structures of coatings were determined by a glancing angle X-ray diffractometer and transmission electron microscope (TEM), respectively. The surface and cross sectional morphologies of thin films were examined by a field emission scanning electron microscopy (FE-SEM). The surface roughness of thin films was explored by atomic force microscopy (AFM). A nanoindenter, a scratch tester and pin-on-disk wear tests were used to evaluate the hardness, adhesion and tribological properties of thin films, respectively. The influences of nitrogen concentration on the mechanical and tribological properties of metallic glass thin films were discussed. A proper nitrogen content of metallic glass thin films was proposed to achieve an amorphous structure with adequate mechanical properties in this work.

The parallel nano-scanning calorimeter (PnSC) is a novel silicon-based micromachined device for calorimetric measurement of nanoscale materials in a high-throughput methodology. The device contains an array of calorimetric sensors, each one of which consists of a silicon nitride membrane and a tungsten heating element that also serves as a temperature gauge. The small mass of the individual sensors enables measurements on samples as small as a few hundred nanograms at heating rates up to 10⁴ K/s. This device was used to study thin-film samples of Ni-Ti-Zr shape memory alloys to evaluate the effects of high-temperature (900°C) heat treatments and low-temperature (450°C) thermal cycling on the characteristics of the martensite transformation. The response of the samples to heat treatments depends on composition and is controlled by a precipitation mechanism. Two precipitate types, a Ti₂Ni base phase at low Zr concentration and a Ni₁₀Zr₇ base phase at high Zr concentration, affect the martensite transformation characteristics by altering the composition and the stress state of the shape memory phase. Thermal fatigue behavior, induced by thermal cycling, is improved compared to previous results. The most stable sample demonstrates a transformation temperature reduction of just 11°C for 100 cycles. The improved stability of the samples is attributed to the very small grain size of approximately 5-20 nm. The high heating and cooling rates characteristic of nanocalorimeters allowed this study to be performed in a high-throughput manner with efficiencies not previously achieved.

The Ti-Cr-B-N thin films with various boron contents were deposited by pulsed DC magnetron sputtering on silicon substrates and SUS420 stainless steel discs. Heat treatments were carried out in a vacuum furnace at 600, 800 and 1000°C for 1 hour, respectively. The crystalline structures and BN bonding nature of thin films before and after heat treating were characterized by grazing angle X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. The surface and cross sectional morphologies of heat treated thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A nanoindenter and nanoscratch tester were adopted to evaluate the mechanical properties of coatings before and after heat treating. It was found that a hardening effect occurred after heat treating. Evolution of microstructures and mechanical properties after heat treatment at different temperature was investigated. The possible hardening mechanism of Ti-Cr-B-N coatings was also proposed.

This presentation will include three case studies:-

1) Improvements to the Oliver and Pharr approach to take account of time dependency in nanoindentation, as applied to nanoindentation at 750C

2) The importance of high temperature nanoindentation of DLC in accurate modelling of sliding contact

3) High-speed reciprocating nano-wear of DLC and metallic samples

In addition it will be demonstrated how the such extracted generic material parameters can be used for computer aided optimizing of coating systems.

Extraction of mechanical properties of thin layers from nanoindentation tests needs determination of the projected contact area. Different formulas and procedures can be used to calculate this indentation surface from the displacement measurement. However, material properties are more difficult to obtain for small penetration depths, below 100 nm. A new method based on second harmonic detection for dynamic nanoindentation testing is proposed. This technique permits to determine the Young modulus and the hardness of materials at small depths. With this new method, the measurement of the normal displacement is not used, avoiding needing precise contact detection. Moreover, the tip defect and thermal drift influence on the measurements are reduced. Results show that the amplitude of second harmonic can be correctly measured at small depths. The Young modulus and the hardness of the tested materials can be obtained from this measurement with rather good accuracy. The mechanical properties determined with this new method are in good agreement with values obtained with classical nanoindentation tests and extend their domain. Influence of indentation size effect at small penetration depths and the limitation of the technique are discussed .

Investigating the mechanical behavior of thin films, coatings, and small-scale structures to understand the unpredicted behavior associated with scale-related mechanisms has lead to the development of specialized micro- and nano-scale techniques. Through this work and the application of these techniques, situations where property-changing effects must be considered have been defined. We will present a methodology where these techniques and insights were leveraged to explore the use of micro-scale methods to evaluate bulk materials. In the first case, a new approach to perform tests on tensile specimens with ~300 μm long gauge sections will be discussed. A micro-fabricated support frame was used for specimen gripping, and to create a link between the specimens and the displacement actuators and the load sensor. A digital image correlation technique was applied measure strains. In the second case, a fully integrated, on-chip, bending-fatigue MEMS device will be discussed. The system utilized an electrostatic scheme for actuation and sensing. Optical methods were used to measure fatigue-crack growth. In both cases EDM or chemical etching was utilized to section specimens from bulk material prior to integration into the MEMS testing platforms. Tensile and fatigue results of several stainless steels and aluminum will be presented. Early results show promise for use of this methodology to extract mechanical property and fracture data from small sections of materials, potentially opening the door for more thorough investigations of materials where conventional techniques are not applicable.

Mechanical property characterization with spatial resolution of a nanometer was recognized as a grand challenge for nanotechnology a decade ago. Because atomic force microscopes (AFM) interrogate the samples mechanically and provide resolution down to the level of atoms, they are a natural candidate for nanomechanical mapping. However, factors such as tip geometry characterization, load and displacement calibration and control, and the models used to compute material properties substantially complicate the measurements. For industrial applications, throughput is an additional challenge.

In the last decade, much progress has been made. A series of calibration methods for force and tip geometry have been developed. Various tip-sample interaction models were developed and validated by comparison with bulk measurements. During material property mapping, the time scale of tip-sample interaction now spans from microseconds to seconds, tip sample forces can be controlled from piconewtons to micronewtons, and spatial resolution can reach sub-nanometer. AFM has become a unique mechanical measurement tool having large dynamic range (1kPa to 100GPa in modulus) with the flexibility to integrate with other physical property characterization techniques in versatile environments. The methods of mechanical mapping have also evolved from slow force volume to multiple-frequency based dynamic measurements using TappingMode^TM and contact resonance.

Even more recently, high speed and real-time control of the peak force of the tip sample interaction has led to a fundamental change in AFM imaging, providing quantitative mapping of mechanical properties at unprecedented resolution. In addition, ease of use improvements and development of high speed AFM have led to faster, simpler, and more quantitative SEM like operation with the AFM. This presentation will review this recent progress, providing examples from a wide range of fields that demonstrate the dynamic range of the measurements and the speed and resolution with which they were obtained.