dc.description.abstract
Hard coatings like TiN, Ti1-xAlxN, CrN, and Cr1-xAlxN synthesized by physical vapour deposition processes like cathodic arc evaporation or sputtering are widely used to protect tools in numerous machining and forming applications during which they have to sustain high temperatures and tribological loads. Their high usage is based on their excellent properties, such as high hardness, high thermal stability, high wear and oxidation resistance. It is well known that the coating performance can be tailored, for example, by the deposition parameters used - like partial pressure, gas mixture, bias potential, or temperature - or by the coating architecture (e.g. multilayer or even superlattice arrangements, graded composition, and defect or stress design by applying alternating bias potentials). However, also the target material itself can significantly influence the coating structure and thus the coating properties. Therefore, we have studied the impact of specially developed targets - with a comparable Al/Cr ratio, powder metallurgically prepared from metallic Al and Cr, intermetallic Al8Cr5, or ceramic AlN and CrN powder - on structure, morphology, mechanical properties, and thermal stability of reactively and non-reactively sputtered AlxCr1-xN coatings. All reactively sputtered coatings exhibit a pronounced columnar structure with a preferred (111) orientation whereas the coating deposited non-reactively from the ceramic target is nano-crystalline with a preferred (200) orientation. Furthermore, the hardness is highest for this non-reactively sputtered nitride coating and the deposition rate is twice as high as for coatings reactively sputtered from metallic Al/Cr targets. Also the thermal stability, especially with respect to the onset of Cr-N dissociation, is shifted to higher temperatures for coatings prepared from ceramic targets. Further on, monolithically grown as well as multilayered AlxCr1-xN coatings were deposited by cathodic arc evaporation using metallic AlxCr1-x targets with compositions of x = 0.7, 0.75, 0.85, and 0.9. Monolithically grown Al0.7Cr0.3N and Al0.75Cr0.25N coatings exhibit a single phase cubic structure. A mixed cubic/hexagonal structure is observed for Al0.85Cr0.15N, and a single phase hexagonal structure for Al0.9Cr0.1N coatings. Multilayer variations, combining single phase cubic layers (using Al0.7Cr0.3 and Al0.75Cr0.25 targets) with the mixed cubic/hexagonal (using Al0.85Cr0.15 targets) or hexagonal layers (using Al0.9Cr0.1 targets) show also hexagonal phase fractions in addition to the cubic phases when applying low bias potentials of -40 V. However, increasing the bias potential supports the coherency strains to supress the growth of the less dense hexagonal phase. Therefore, even for a high overall Al/Cr ratio of 77/23 at% hardness values around 33 GPa are obtained. However, the thermal stability, with respect to phase decomposition and Cr-N dissociation to cubic Cr under N2-release, is reduced by increasing the applied bias potential. Due to the reduced thermal stability, the hardness gain by increasing the bias potential is already lost for an annealing temperature - 600 °C in (as deposited) single-phase cubic structured coatings. Therefore, the Al0.7Cr0.3N coatings prepared with -40 V outperform their -120 V bias counterparts in hardness already for an annealing temperature - 600 °C. This is different for the multilayer coatings comprising cubic and hexagonal wurtzite type phases. Especially for Al0.75Cr0.25N/Al0.9Cr0.1N multilayers the increased bias during deposition helps to prepare dense coatings and the arrangement with high Al-containing layers allows for an improved resistance against decomposition and Cr-N dissociation. As nitride thin coatings are exposed to very high temperatures during metalworking the diffusion of C, Cr, and Fe - which are common transfer - elements during machining - into such TiN, Ti0.5Al0.5N, CrN, and Cr0.3Al0.7N coatings is studied. Our results show that the mechanical properties of nitride thin coatings are not always negatively influenced by the inward-diffusion of such foreign elements. For example, the hardness of TiN is nearly unaffected by the diffusion of C, Cr, and Fe up to 1000 °C and 30 min. It was shown that especially C and Cr exhibit almost 15 and 10 times larger diffusion coefficients in Ti0.5Al0.5N than in TiN coatings. In CrN, C exhibits an even up to 35 times larger diffusion coefficient than in Cr0.3Al0.7N. Based on our results we can conclude that microstructural changes within coatings significantly determine diffusion processes. The diffusion of C, Cr, and Fe is more pronounced within Ti0.5Al0.5N than in TiN but less pronounced within Cr0.3Al0.7N than in CrN.
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