ICMCTF2015 Session TS2-1: Advanced Characterization of Coatings and Thin Films

Determination of reliable fracture properties in brittle thin films and coatings is challenging due to the restrictions in test specimen size and geometry vis a vis the material system. Direct extrapolation of bulk properties to smaller length scales without accounting for extrinsic and intrinsic size effects is not valid. Focused ion beam machining and in-situ SEM based micro-mechanical testing are popular tools today to characterize material properties directly at the smaller length scales. Three different material systems: single crystal Si wafer, Co based metallic glass thin films and PtNiAl based gradient bond coats, all of which are known to be nominally brittle, but with increasing degree of microstructural complexity, were probed using the above mentioned tools. The chosen systems vary considerably in structure and morphology and belong to extreme ends of the application spectrum ranging from MEMS devices to aerospace engine components but are all plagued by their low ductility and fracture toughness. Relevant fracture properties like fracture strength, initiation K_IC and R-curve behavior were extracted along with recording of crack trajectories to propose failure mechanisms. Different small-scale fracture toughness test geometries, including the single cantilever bending, clamped beam bending, double cantilever bending and pillar splitting were utilized for the same.

Distinct fracture features in these 3 materials systems will be illustrated. The single crystal Si (100) fractures similarly at the bulk and the micron-scale and was used as a calibration tool for the testing techniques. Both the metallic glass thin films and the PtNiAl bond coats exhibited size effects at the micron-length scale. The Co based metallic glass film exhibited fracture strengths close to the theoretical strength of the material. The PtNiAl bond coats showed a rising fracture toughness with crack growth within a few microns. In this context, specific aspects of fracture toughness testing at the small-scale will be highlighted. A description of the properties measured and their significance in the overall material design and development along with the advantages and limitations of the testing techniques will be discussed during the talk.

Advanced hard coatings providing multi-functional properties like wear and oxidation protection, self-hardening by spinodal decomposition of metastable phases and self-lubrication combined with high toughness require sophisticated materials selection and architecture design. For a knowledge-based development of such coatings, advanced characterization techniques to investigate coating microstructure and properties from the macro- via the micro- to the atomic scale are needed. Within this contribution, recent progress in coating characterization methods is highlighted. Examples included are three-dimensional atom probe tomography to study decomposition of metastable phases and interdiffusion in layered systems as well as cross-sectional nano-diffraction using focused X-ray synchrotron beams to illuminate microstructure evolution during coating growth. The acquired detailed knowledge about composition and microstructure enables to establish correlations to mechanical properties, where recently new approaches for determination of hot-hardness based on high-temperature nanoindentation as well as fracture strength and fracture toughness determined by micromechanical tests have been suggested. Combining such techniques with failure analysis of hard coatings during application or during micromechanical tests enables to understand coating degradation, thus providing the basis for further optimization of coating materials and architectures.

A versatile bulge test setup was used to perform mechanical tests on rectangular gold membranes with thicknesses ranging from 100 nm to 350 nm. It can be used in a conventional way to calculate stress-strain curves and determine parameters such as residual stress and plane-strain modulus. Alternatively, the setup in Erlangen can be inserted into an atomic force microscope (AFM) which allows in-situ imaging of the surface of the deforming membrane.

In order to determine the fracture toughness of thin films, narrow slits of 10 µm length were milled into the center of the membranes by focused ion beam (FIB). Subsequently, the membranes were loaded until rupture. The investigated samples comprised freestanding gold films and films supported by a 100 nm thick silicon nitride film.

The fracture toughness obtained for both kinds of gold films is far below the fracture toughness reported for bulk gold. In detail, the fracture toughness of freestanding films is slightly higher than the one of supported films. Besides, the fracture toughness of freestanding films stays constant within the studied film thickness range whereas the fracture toughness of supported films increases linearly with film thickness. The different behavior is also reflected in the results of in-situ bulge tests in the AFM. In the freestanding films, widespread out-of-plane motion of individual columnar grains was revealed. In contrast, only little plastic deformation could be observed directly in front of the crack tip of supported films.

The results from conventional and in-situ bulge tests are compared to each other and discussed in regard to existing models for the size dependence of fracture toughness.

Measuring the interface toughness between brittle coatings or thin films and their substrates is difficult due to the effects of substrate plasticity and complex specimen geometries. A novel method has been developed to probe the interface toughness of transparent thin films on opaque substrates. A femtosecond laser is used to ablate a small amount of the substrate with negligible local heating and damage. Due to the disparity of timescales for the ablation event and resonant frequency of the detached film, the film is analyzed as subjected to a pressure impulse resulting in an initial out-of-plane velocity. The subsequent behavior of the film, interface crack propagation and/or film cracking is used to determine the properties of the interface.

Femtosecond lasers are ideal tools for creating interface flaws below transparent films because they incorporate limited damage to the substrates in the low fluence ablation regime. With a pulse duration that is six orders of magnitude shorter than common nanosecond lasers, local heat effects are minimal. The power and focused size of the laser can be adjusted to change the initial upward velocity of the film and the size of the detachment. The local nature of this test allows for spatial or microstructural dependent variations in toughness to be rapidly assessed. Experiments and modeling of the response of the film to irradiation will be presented for FeCrAl and Si substrates.

In order to evaluate the performance of hard coating materials used in aggressive cutting operations such as high-performance milling or high speed machining, it is necessary to investigate the material properties at the high temperatures generated in service. However, the small length scale of the coatings limits the utility of conventional high temperature techniques. The recent extension of nanoscale techniques such as nanoindentation and nanoimpact testing to elevated temperatures has recently made this a possibility, while vacuum techniques for high temperature nanoindentation has also allowed measurement at temperatures where oxidation of the indenter or sample may be problematic. Micropillar compression at high temperature offers several attractive advantages over hardness testing: indentation is highly dependent on tip geometry for accurate measurements, and indenter geometry variation due to erosion by oxidation and abrasive wear or plastic deformation of the tip is a serious concern at high temperatures. The micropillar geometry additionally offers a uniaxial stress state in contrast to the tri-axial stress state of indentation providing a direct measurement of the yield stress, while in situ micro-pillar compression allows direct visualization of the deformation mechanism changes at elevated temperature so that buckling, fracture, or plastic deformation can be immediately observed. Similarly, ion-machining of notched-cantilever geometries, and in situ observation of fracture under a point-like load, can allow for the investigation of the fracture toughness at high temperature; an equally important parameter for application.

The elevated temperature performance of a wide range of Chromium Nitride-based PVD hard coatings was therefore evaluated using in situ micro-mechanical methodologies. This allows both the first direct measurement of the uniaxial high temperature yield strength, rather than the hardness, of such coatings, as well as the elevated temperature fracture behavior. The microstructure of the coatings was analyzed using X-ray diffraction, Raman spectroscopy and focused ion beam cross sectioning followed by electron microscopy. Micropillars were examined using electron microscopy before and after compression, while for cantilevers the effect of ion-machining was thoroughly scrutinized. Trends in deformation behavior, yield stress, and fracture with temperature are discussed in relation to coating microstructure.

Zirconium diboride (ZrB₂) is a ceramic that has been shown to exhibit high hardness and elastic modulus as well as good wear-erosion-corrosion resistance in the bulk form. These attributes, which are retained at high temperature, are potentially useful for numerous thin film applications. However, thin film growth of ZrB₂ films with well-defined properties seems complicated as the literature reports growth of nonstoichiometric films with B/Zr> 2 or films with high amount of contaminants, e.g. O up to 30 at.%. Recently, we demonstrated epitaxial growth of ZrB₂ films on 4H-SiC(0001) by direct current magnetron sputtering (DCMS) using a ZrB₂ compound target [1].

In this work, we investigate the nanomechanical properties of epitaxial ZrB₂ films deposited on 4H-SiC(0001) as well as 10-10 weakly textured ZrB₂ films grown on Si(100), using a deposition temperature of 900 ^oC. The film growth was carried out in an UHV system by DCMS, using a 99.5 % pure ZrB₂ target, a 425 W power-discharge in Ar plasma kept at 0.53 Pa, a substrate bias of ‑80 V, and with no substrate rotation. For comparison, we also investigated 0001 fiber textured ZrB₂ films deposited on Si(100) without external heating as well as bulk ZrB₂.

For mechanical testing performed at room temperature, the epitaxial films deposited on 4H-SiC(0001) present a super-hardness H = 47.3 GPa and a reduced elastic modulus E_r = 359 GPa. High temperature measurements show a hardness decrease to H = 32.8 GPa at 600 ^oC, while E_r does not change significantly. For the 10-10 textured film, we measured H = 30.8 GPa at room temperature, which decreases to H = 16.9 GPa at 600 ^oC, while E_r slightly decreases from 243 to 235 GPa. We find the mechanical properties for this textured film to be affected by a film/substrate reaction during growth, similar to that found for epitaxial ZrB₂ films deposited on Si(111) [1]. We also compare the properties of these films at room temperature with those of 0001 fiber textured ZrB₂ films deposited on Si(100) with a hardness of 25 GPa, E_r = 291 GPa, and elastic recovery R = 96%, as well as bulk ZrB₂ with H = 31 GPa, E_r = 340 GPa and R = 81%.

High resolution electron microscopy with selected area electron diffraction of the indented area in the 0001 fiber textured film reveals a retained continuous ZrB₂ film and no sign of phase transformation, despite massive deformation of the Si substrate. Thus, the former observation supports the measured high elastic recovery of 96% for this film, while the latter one shows that it is stable in high stress fields.

[1] L. Tengdelius, J. Birch, J. Lu, L. Hultman, U. Forsberg, E. Janzén, H. Högberg, Phys. Status Solidi A 211 (2014) 636.

Mechanical characterization of ultra-thin protective coatings is often difficult due to the influence of the interfaces and substrate effects, such that the default is often to use scratch or wear methodology for a pass-fail analysis of film properties. However, recent developments in ultrasensitive nanoindentation instrumentation can be coupled with advanced models, such as Li and Vlassak [J. Mater. Res. 24 3 2009], in order to quantitatively extract the elastic properties of the film via through thickness measurements.

Here, a MEMS based transducer with a noise floor similar to that of contact mode AFM is used. However, this system is actuatated linearly, so avoids the rocking, bending, or sliding modes typical of AFM indentation. Samples are anonymously grown diamond like carbon (DLC) on a glass platter with variation in thicknesses from 3 to 12nm, and native oxides of aluminum and chromium.

Indentation can then be coupled with through-tip electrical characterization, where a bias can be applied to the sample, and the resulting current is measured. Current-voltage information can be modeled using a simple trapezoidal tunneling model to extract local film parameters. These parameters include the barrier height, or local film thickness, and the barrier voltage, a parameter that includes chemical information. These experiments can also be ported into an electron microscope, coupling advanced imaging methodology and mechanical measurements.

The bulk metallic glass (BMG) materials have drawn lots of attention from researchers and industries due to their unique mechanical properties since 1980s. Meanwhile, the thin film metallic glass (TFMG) represents a class of promising amorphous engineering material for structural applications. Among several kinds of material systems of TFMG, the tungsten based TFMG has been seldom explored. In this work, ternary W-Zr-Si TFMGs with different W concentrations were fabricated on Si wafer and AISI 420 stainless steel disk substrates by a co-sputtering system. The glass transition temperature, T_g, and crystallization temperature, T_x, of TFMGs were determined by differential scanning calorimetry (DSC). The annealing treatment of TFMGs at temperature ranges below T_g, among T_g and T_x, and above T_x were carried out in Ar atmosphere at a heating rate of 40 K/min and a holding time of 60 s. Phases of as-deposited and annealed TFMGs was determined by the X-ray diffractometer (XRD). The microstructures of thin films were investigated by the field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The hardness and Young’s modulus of TFMGs were evaluated by a nanoindenter .The electrical properties of coatings were determined by a four-point probe tester. Effects of annealing treatment on the microstructure, electrical and mechanical property of W-Zr-Si TFMGs were discussed in this work.

Keywords: W-Zr-Si, thin film metal glass, nanoindentation, annealing, glass transition temperature

Fast sputter rate, large concentration dynamic range (ppm - 100%) and large depth dynamic (nm to more than 100 microns) are the most important features of this technique for Elemental Depth Profile Analysis.

Thanks to the pulsed Rf Source used, any type of materials can be sputtered (Conductive, poorly conductive, non conductive, fragile, organics, glass, Si..)

This technique is also a very good tool for defects analysis.

After a brief description of the technique, the presentation will be focused on applications in these specific fields:

- PV PhotoVoltaic (CIGS, CdTe...)

- Energy Storage and more precisely Li Ion Batteries (Cu anode with Organic coating with Li)

- Semi conductor (Doped wafer, Implantation in Si...)

- Hard Disk, DVD

- Thick Organic coating