Laves phases are intermetallic phases well known for their excellent strength at high temperatures but also for their pronounced brittleness at low temperatures. Especially in high-alloyed steels, Laves phases were long time regarded as detrimental phases as they were found to embrittle the material. Perusing the more recent literature, it seems the negative opinion about the Laves phases has changed during the last years. It is reported that, if the precipitation morphology is properly controlled, transition metal-based Laves phases can act as effective strengthening phases in heat resistant steels without causing embrittlement. For a targeted materials development, the mechanical properties of pure Laves phases should be known. However, the basic knowledge and understanding of the mechanical behavior of Laves phases is very limited. Here we present an overview of experimental results obtained by micromechanical testing of single-crystalline NbCo 2 Laves phase samples with varying crystal structure, orientation, and composition. For this purpose, diffusion layers with concentration gradients covering the complete homogeneity ranges of the hexagonal C14, cubic C15 and hexagonal C36 NbCo 2 Laves phases were grown by the diffusion couple technique. The hardness and Young's modulus of NbCo 2 were probed by nanoindentation scans along the concentration gradient. Single-phase and single crystalline microcantilevers and micropillars of the NbCo 2 Laves phase with different compositions were cut in the diffusion layers by focused ion beam milling. The fracture toughness and the critical resolved shear stress (CRSS) were measured by in-situ microcantilever bending tests and micropillar compression tests, respectively. The hardness, Young's modulus and CRSS are nearly constant within the extended composition range of the cubic C15 Laves phase, but clearly decrease when the composition approaches the boundaries of the homogeneity range where the C15 structure transforms to the off stoichiometric, hexagonal C36 and C14 structure on the Co-rich and Nb-rich, respectively. In contrast, microcantilever fracture tests do not show this effect but indicate that the fracture toughness is independent of crystal structure and chemical composition of the NbCo 2 Laves phase.

In order to guarantee their protective function, thermal sprayings must be free from cracks, which expose the substrate surface to e.g. corrosive media. Cracks in thermal sprayings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross-sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology as well as coating defects were inspected using light microscopy on metallographic cross-sections. Additionally, the resulting crack patterns were imaged in 3D by means of X-ray micro-tomography.

The wear resistance of thermal spray coatings mainly depends on coating properties such as the microstructure, hardness, and porosity, as well as on the residual stress in the coating. The residual stress is induced by a variety of influences e.g. temperature gradients, difference of the thermal expansion coefficient of the coating / substrate materials, and the geometry of the components. To investigate the residual stress, the Impulse Excitation Technique was employed to measure the Young’s and shear moduli. The residual stress was determined by using the hole-drilling method and X-ray diffraction. Pin-on-Disc and Pin-on-Tube tests were used to investigate the wear behavior. After the wear tests, the wear volume was measured by means of a 3D-profilometer. The results show that the value of the residual stress can be modified by varying the coating thickness and the substrate geometry. The compressive stress in the HVOF-sprayed WC-Co coatings has a significant positive influence on the wear resistance whereas the tensile stress has a negative effect.

A special pin-on-disc test setup designed for vacuum environments was used to conduct wear tests in a large chamber scanning electron microscope. Arc-sprayed NiCrBSi and HVOF-sprayed WC-12Co coatings were tested using a pin with an Al 2 O 3 ceramic ball as the wear counterpart. During testing, different wear mechanisms were identified and the processes were recorded in short video streams.

This study investigates the wear resistance of WC-FeCSiMn coatings deposited on 3D surfaces by two-wire arc spraying. Wear behavior was evaluated by means of pin-on-disc testing, pin-on-rotating tube testing, and a method in which a robot arm moves a pin over test specimens with arbitrary surface geometries. Residual stresses were determined by incremental hole drilling and were found to have a dependency on substrate geometry. After wear testing, a 3D profilometer determined wear volume and coating surfaces were examined by SEM. The results indicate that wear resistance is strongly influenced by the geometry of the substrate.

Thermally sprayed coatings are usually defined by their hardness, porosity, roughness and wear resistance. Even though the Young’s modulus is an essential property, which describes the mechanical behavior of the coated components during their use, only few efforts were made to determine this property. The most common measurement methods of the Young’s modulus of thermally sprayed coatings are tensile tests, bending tests, and nanoindentations. During the tensile and bending tests a sliding of the splats can occur due to the laminar structure of the thermally sprayed coatings, influencing the measurement value. When using the nanoindentation test, only the elastic behavior of a single splat can be determined because of a minimal measuring volume. However, the Young’s Modulus of thermally sprayed coatings can also be determined by means of a resonant method, called impulse excitation technique (IET). In this paper, the values of the Young’s moduli of thermally sprayed coatings, measured by several methods are compared with each other and correlated to the microstructure of the coatings, investigated by means of scanning electron microscopy.

The anti-wear coatings in this study are developed for the use in forming tools, which need a good resistance against different wear mechanisms such as surface fatigue, adhesion and abrasion. NiCrBSi is suitable for this purpose because of its high resistance against different types of wear at ambient and high temperatures. Due to the low thermal and kinetic energy during the spraying process, the arc-sprayed NiCrBSi coating shows a high interlaminar boundary porosity as well as numerous interface cracks between the coating and the substrate, which can reduce the wear resistance and the adhesion to the substrate. In this work, arc-sprayed NiCrBSi coating were remelted by a treatment with an oxyacetylene flame, induction heating, furnace and laser. The treatment parameters were investigated and optimized. The modified microstructure was characterized using an optical microscope and a scanning electron microscope as wells as an EDS analysis. The hardness of the coatings was evaluated before and after remelting. To examine various wear mechanisms, pin-on-disc tests were conducted with different wear counterpart. The wear and coefficients of the as-sprayed and remelted coating were determined. In order to show the change of the wear resistance as well as the wear mechanisms, wear tracks were characterized with a scanning electron microscope.