ICMCTF 2021 Virtual Conference Session HP: Advanced Characterization Techniques for Coatings, Thin Films, and Small Volumes (Symposium H) Poster Session

Transmission EBSD mapping offers the ability to target site-specific grain or phase boundaries for Atom Probe Tomography (APT) analysis, and correlate boundary chemistries with grain misorientations. In this study we demonstrate that high-resolution transmission electron back scattering diffraction (tEBSD) maps can be acquired on needle-shaped APT specimens that consist of grains of size ranging from few hundred nanometers to few micrometers. The use of this correlative technique will be demonstrated with thermal barrier coatings (TBCs) used in turbine engines to operate at temperatures greater than the melting temperatures of engine components and consequently achieve better propulsive power performance and fuel efficiency. The general structure consists of three layers: a top is a coat made of yttrium-stabilized ZrO₂ (or YSZ), which has excellent thermal resistivity, a thermally grown oxide (TGO) scale, that consists of a-alumina grains, and a bond coat layer at the coating/substrate interface that improves adhesion of the ceramic layers on the superalloy substrate.

The nanoscale microstructural and residual stress gradients across hard coatings on cutting tools are of high scientific and industrial interest. Here, cross-sectional X-ray nanodiffraction with a beam size of 35×25 nm² was used to retrieve structural and mechanical gradients in the cutting edge area of a ~2 μm thick TiN coating deposited by cathodic arc evaporation on a WC-Co substrate. Scanning small-angle X-ray scattering microscopy (SAXSM) is presented and utilized for the first time to investigate the nanoscale defect density in the cutting edge area. At the cutting edge, interface-like planar domains of high scattered intensity were indicated, while a gradual increase of the SAXS intensity at the rake face was correlated with pores found by scanning electron microscopy. Furthermore, the coating’s <111> fibre texture axis orientation correlates with the substrate’s surface normal, showing abrupt orientation changes across the former mentioned interface-like structures. The planar regions next to the edge exhibit gradual and constant stress profiles with anisotropic defect build-ups on the flank and rake faces, respectively. Directly at the edge, nonlinear lateral and cross-sectional compressive residual stress gradients ranging from ~0 to −3 GPa were observed, which together with the interface-like planar domains may represent a reliability issue during operation.

Transition Metal Nitrides (TMN) are well known for their good mechanical stability, chemical inertness, as well as tribological properties. Hence, they successfully found application in the metal forming industry, and are in use as protective coatings in the automotive and aerospace industry. Besides TiN, CrN is one of the most used and best investigated hard coatings, preferably applied in conditions that require a low coefficient of friction. A decisive disadvantage of these hard coatings, however, is their low fracture tolerance. Premature failure of the coating due to crack initiation and propagation leads to economic disadvantages or completely excludes an application. In recent years, micromechanical testing methods have made it possible to measure and specifically improve precisely fracture toughness of thin film materials. There are various methods known for measuring K_IC obtaining all advantages and drawbacks, especially with respect to intrinsic material characteristics and accuracy.

In this contribution, we perform distinct micromechanical tests on cathodic arc evaporated CrN coatings. These coatings were deposited with different bias voltages and deposition temperatures in order to obtain a variation in both, microstructure (specifically crystallite size) and defect density. The importance of the microstructure on fracture characteristics has recently shown by Ast et al. for Ti-Al-N deposited by diverse PVD techniques [1]. However, a clear correlation between the column size and the density of column boundaries is still missing.

We found a significant influence of the residual stress state on the fracture properties of such hard coatings using the indentation fracture method. Furthermore, we used pillar splitting and cantilever bending tests to determine the intrinsic fracture toughness of our coatings with respect to the microstructure and defect density. These results were complemented by HR-TEM investigations together with x-ray diffraction studies, and nanoindentation tests.

References

[1] Ast J., et al., (2019). Fracture toughness determination of arc-PVD and HiPIMS hard coatings by micro-cantilever and pillar splitting tests.

Examination of materials cross sections often provides essential information about the crystal structure, layer or film thicknesses, existence of voids or cracks and other properties that might impact materials performance and reliability. Cross-sectional observation is especially essential in thin film technology, to examine layer thickness, deposition integrity (voids/adhesion), as well as film growth and crystallographic orientation. Currently various methods are used to prepare specimen cross sections for scanning electron microscope (SEM) observation. Mechanical methods of cutting and polishing are widely used, particularly for metallographic sample preparation. However, mechanical polishing presents several problems: a) in composite materials with different hardness values, the polished surface becomes uneven as the softer components are cut faster and more easily than the harder components; b) in soft materials, particles of hard abrasive can be buried in the material being polished; c) in materials with voids, the edges of the voids can stretch and deform; e) for metals, due to the strain caused by mechanical polishing on the polished surface, the information about the crystal structure by means of electron back-scatter diffraction (EBSD) becomes difficult or impossible to obtain; f) fine features like hairline cracks and small voids can get smeared shut and will not be recognized as such.

This paper presents utilization of broad ion beam instrument (JEOL CP polisher) for cross-sectional preparation of various thin film/substrate combinations. This table top instrument utilizes Ar ion beam to produce large area cross-sections of materials, with the ability to employ cryogenic (LN2) temperatures to address beam sensitive and eutectic metal systems. Moreover, the use of FE-SEM equipped with in-lens detectors and high sensitivity backscatter detecotr allows observation of the resulting samples to investigate nanoscale features, including voids, grain boundaries and layers. Additionally, EDS and EBSD can be utilized to provide additional characterization of the thin film specimens in terms of compositional variations and crystallographic orientation.

We will present examples of Zn thin films - depending on the film composition these may require cryogenic preparation to preserve film integrity. Additionally we would present examples of other materials - anodized films, evaporated metal thin films, solar films, etc.