ICMCTF2012 Session B1-1: PVD Coatings and Technologies

The study reports on the effect of the addition of copper in the Al₂O₃ film on its mechanical and optical properties. The Al-Cu-O films were reactively co-sputtered using dc pulse dual magnetron in a mixture of Ar+O₂. One magnetron was equipped with a pure Al target and the second magnetron with a composed Al/Cu target. The amount of Al and Cu in the Al-Cu-O film was controlled by the length of pulse at Al/Cu target. The Al-Cu-O films with ≤ 16 at.% Cu were investigated in detail. The addition of Cu in Al₂O₃ film strongly influences its structure and mechanical properties. It is shown that (1) the structure of Al-Cu-O film gradually varies with increasing Cu content from γ-Al₂O₃ at 0 at.% Cu through (Al_8-2x,Cu_3x)O₁₂ nanocrystalline solid solution to CuAl₂O₄ spinel structure, (2) the Al-Cu-O films with ≥3 at.% Cu exhibit (i) relatively high hardness H increasing from ~15 GPa to ~20 GPa, (ii) enhanced elastic recovery W_e increasing from ~67 % to ~76% with increasing Cu content from ~5 to ~16 at.% Cu and (iii) low values of Young’s modulus satisfying the ratio H/E>0.1 at ≥5 at.% Cu, and (3) highly elastic Al-Cu-O films with H/E>0.1 exhibit enhanced resistance to cracking during indentation test under high load.

The arc-evaporation technique is one of the most commonly used deposition technique in industry, as it indicates high growth rates combined with superior coating properties. Nevertheless, reactive cathodic arc-evaporation processes show high influence of the reactive partial pressure used, especially when depositing Al-Cr oxides. Recent investigations pointed out that an increased oxygen partial pressure enhances the formation of crystalline Al-Cr oxides. Such high gas flow rates result in formation of oxide islands on the target surface, increasing the droplet density of the film formed as well as indicates a non-steady-state process. Therefore, investigations were directed to vary the Al to Cr composition in the target to overcome these drawbacks and assure high quality (Al,Cr)₂O₃ films and a controlled process. Here we show first results on arc-evaporated (Al,Cr)₂O₃ using ternary Al-Cr-Si targets and their positive effects on the formation of crystalline corundum structured oxides as well as decreased target poisoning due to oxide formation. Furthermore, theoretical investigations using density function theory are carried out to support the evaluated results.

Understanding failure mechanisms of cutting tools and the tool-work interface is necessary for further improvement of tools life. Therefore, in this study, wear and failure mechanisms of coated cutting tools were studied. TEM cross-sectional analysis was used to observe the cutting edges of tools after being used for machining tests, cutting temperature was evaluated by measuring the thermal electromotive force of tool-material, EPMA was used to analyze the coating surface and to study the effect of Si and Al contents on cutting performance.

The TEM cross-sectional analysis of the cutting edges shown that the coatings deformed plastically toward the axis with formation of micro cracks. The TEM-EDS analysis of the coatings after the cutting test have shown Co-diffusions into the coatings and phase transitions of the coatings in the actual cutting process. According to the measurement of the cutting temperature, the cutting speed found to be an effective factor rather than hardness of work-material.

EPMA analysis of tool-work interface showed that an oxide layer was formed on the coating surface during cutting, which was Mn-Si-O or Al-O. The oxide layer is called “belag”. Formation of the Mn-Si-O layer could help increasing the cutting speed compared to formation of an Al-O layer. The belag formation was related to the chemical composition of work materials, chemical composition of the coatings and cutting temperature.

Furthermore, cutting tests using coatings with various chemical composition showed that grain size and crystal structure of coatings has good agreement with the cutting performance. It was concluded that minimizing the grain size of a coating is one of the most important factors for developing cutting tool with higher tool life.

Face centered cubic (Al_1-xCr_x)₂O₃ solid solution films, with x in the range 0.60<x<0.70, have been deposited using dual reactive RF magnetron sputtering from Al and Cr targets in mixed Ar/O₂ discharge at a substrate temperature of 500 °C. The films have a strong <100> preferred orientation. The unit cell parameter is 4.04 Å determined by x-ray diffraction and high resolution transmission electron microscopy techniques. Nanoindentation shows that the films exhibit hardness values up to 26 GPa and reduced modulus of 220-235 GPa. In the present work, ex-situ annealing studies were performed on as-deposited samples for a series of temperatures up to 1000 °C and annealing time of 0-8 h. The fcc structure remains intact up to 700 °C. The onset of phase transformation from fcc to corundum is observed in the sample annealed at 900 °C for 2 h, where annealing for 2 h at 1000 °C results in complete transformation to α-(Al_1-xCr_x)₂O₃. There is no indication of any phase separation into α-Cr₂O₃ and Al₂O₃ prior and after the annealing, confirmed by the in-plane and out-of-plane line scans performed in EDX TEM and long-time small step size θ/2θ XRD patterns. The kinetics of phase transformation studied by the Jahnson-Avrami-Mehl model shows the apparent activation energy of the phase transformation process in the range of 380-480 kJ/mol. Ongoing work is focused on theoretical studies of the stability and composition of the structure of the as-deposited fcc-(Al_1-xCr_x)₂O₃ solid solution films, which are suggested to have a non-stoichiometric NaCl structure with 33% vacancy occupancy on Al/Cr sites.

Common targets used in physical vapour deposition techniques to synthesise thin films and coatings present a high level of purity in order to avoid the incorporation of impurities into the growing films. The removal or reduction of contaminations during target production, on the other hand, is cost intensive especially if multi-element targets are produced. Only very limited information is available on the effect of impurities in nowadays frequently used target materials regarding the deposition process and coating properties. For the present investigation AlCr targets produced by powder metallurgy were chosen and the influence of Fe contents of up to 2 at.% on structure, mechanical properties and oxidation behaviour of the deposited AlCr(Fe)N coatings was studied. All coatings were synthesised by cathodic arc-evaporation in an industrial-scale Oerlikon Balzers INNOVA deposition system. The analysis by scanning electron microscopy and X-ray diffraction revealed no significant changes in the microstructure and crystal structure. All coatings presented a face-centred cubic structure with grain sizes in the range from 20 to 30 nm. The hardness values measured by nanoindentation were generally ranging from 31 to 33 GPa without a pronounced correlation with the Fe concentration in the coating. The oxidation resistance, on the other hand, reduced with increasing Fe content. Analyses of the oxidised coatings by X-ray diffraction and Raman spectroscopy revealed the formation of oxides, in the present case Cr₂O₃, at a temperature of 800°C if Fe was present as compared to 900-1000°C for the reference AlCrN coating.