Cutting tool performance is mainly characterized by material substrate, cutting edge geometry and coating, and also by a good choice of the cutting parameters, mainly cutting speed, depth of cut and feed. In drilling a good choice of substrate/coating can reduce production costs per hole cut by 50%. Coatings evolution has gone from monolayer to nanostructured and/or nanometric-scale multilayer coatings. These are used because of their high hardness, and good corrosion and oxidation resistance and thermal stability. Cutting edge preparation on one hand and droplet elimination after the coating process in the other are important issues for reaching a good tool/coating performance, being a key issue.In this article a series of coatings for drilling low and medium carbon alloyed steels are presented, along with their performance. Validation tests were carried out on steel 42CrMo4, very used in the automotive sector. Seven coatings were tested, including AlCrSiN, μAlTiN, TiAlCrN, AlTiCrN, AlCrN, AlTiSiN and TiAlSiN. Flank wear, evolution of drilling thrust force and torque, damage on cutting edge faces on primary cutting edge and behaviour of drill bit secondary edges were studied. A final elimination Downloaded by [134.of droplets by drag grinding was performed in several cases. Process monitoring, SEM microscopy and EDX analysis were used, concluding that the best results were for μAlTiN, TiAlSiN and AlTiSiN. Reasons for the good behaviour are the good surface finishing after droplet elimination and the high thermal stability of these protective layers.

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The durability of two solar-selective aluminium titanium oxynitride multilayer coatings was studied under conditions simulating realistic operation of central receiver power plants. The coatings were deposited by cathodic vacuum arc applying an optimized design concept for complete solar-selective coating (SSC) stacks. Compositional, structural and optical characterization of initial and final stacks was performed by scanning electron microscopy, elastic recoil detection, UV-Vis-NIR-IR spectrophotometry and X-Ray diffraction. The design concept of the solar selective coatings was validated by an excellent agreement between simulated and initial experimental stacking order, composition and optical properties.Both SSC stacks were stable in single stage tests of 12 h at 650°C. At 800°C, they underwent a structural transformation by full oxidation and they lost their solar selectivity. During cyclic durability tests, multilayer 1, comprised of TiN, Al .64 Ti .36 N and an Al 1.37 Ti .54 O top layer, fulfilled the performance criterion (PC) ≤ 5% for 300 symmetric, 3 h long cycles at 600°C in air. Multilayer 2, which was constituted of four Al y Ti 1-y (O x N 1-x ) layers, met the performance criterion for 250 cycles (750 h), but was more sensitive to these harsh conditions. With regard to the degradation mechanisms, the coarser microstructure of multilayer 1 is more resistant against oxidation than multilayer 2 with its graded oxygen content. These results confirm that the designed SSCs based on Al y Ti 1-y (O x N 1-x ) materials withstand breakdown at 600°C in air. Therefore, they can be an exciting candidate material for concentrated solar power applications at high temperature.

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cermets present both normal and abnormal growth of faceted titanium diboride (TiB 2 ) grains during liquid-phase sintering. Abnormal grain growth (AGG) is preferentially found at high sintering temperatures in specimens processed from powder mixtures with a wide particle size distribution. The WC additions to the initial powder mixtures have proved efficient in reducing the number and size of these large TiB 2 grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB 2 AGG control is obtained by decreasing TiB 2 dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB 2 AGG but also the porosity levels obtained by previous powder processing routes. TiB 2 cermets produced by aggressive milling present a higher amount of alumina particles in the matrix after sintering, which, in addition, appear more homogeneously dispersed in the microstructure. The distortion produced by these particles on the facets of TiB 2 growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB 2 particles from the powder mixtures, which could act as seeds for TiB 2 uncontrolled growth. TiB 2 -Ni 3 (Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K IC of about 10 MPa , data clearly out of the range covered so far by other TiB 2 -based materials.1m