A novel micro-scale repetitive impact test has been developed to assess the fracture resistance of hard coatings under dynamic high strain rate loading. It is capable of significantly higher impact energies than in the nano-impact test. It retains the intrinsic depth-sensing capability of the nano-impact test enabling the progression of the damage process to be monitored throughout the test, combined with the opportunity to use indenters of less sharp geometry and still cause rapid coating failure. The micro-impact test has been used to study the resistance to impact fatigue of Al-rich PVD nitride coatings on cemented carbide. The impact fatigue mechanism has been investigated in nano-and micro-scale impact tests. Coating response was highly load-dependent. A Ti 0.25 Al 0.65 Cr 0.1 N coating with high H 3 /E 2 performed best in the nano-and micro-impact tests although it was not the hardest coating studied. The

The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterisation. Experimental results point out a strong strength decrease associated with localised corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.

The influence of the microstructure on the tolerance of WC-Co cemented carbides to corrosion damage was studied by using residual strength as the critical design parameter. In doing so, samples were immersed in synthetic mine water solution for different times, and changes induced by corrosion exposure were assessed. A detailed 3D FIB/FESEM tomography characterization of corrosion damage-microstructure interactions is included. Results reveal that corrosion damage may result in relevant strength degradation on the basis of stress rising effects associated with the formation of surface corrosion pits. Thus, as immersion time increases strength gradually decreases. Fractographic examination reveals the formation of semi-elliptical and sharp corrosion pits for studied medium-and ultrafine-sized cemented carbides, respectively. The latter has a much more pronounced stress rising effect, and consequently higher strength losses were determined for ultrafine grades. Corrosion process consists of a selective attack of the binder that is dissolved in the corrosive media. Initially, it is located at centres of binder pools and as exposure time in the media increases, corrosion evolves consuming the rest of the pools which finally leaves an unsupported WC grain skeleton at the surface.

A high temperature micro-impact test has been developed to assess the fracture resistance of hard coatings under repetitive dynamic high strain rate loading at elevated temperatures. The test was used to study the temperature dependence of the resistance to micro-scale impact fatigue of TiAlSiN coatings on cemented carbide at 25-600 C. Nanoindentation and microscratch tests were also performed over the same temperature range. The results of the microimpact tests were dependent on the impact load, coating microstructure, coating and substrate mechanical properties, and their temperature dependence. At higher temperatures there was a change in failure mechanism from fracture-dominated to plasticity-dominated behaviour under the cyclic loading conditions. This was attributed to coating and substrate softening.

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.