Effect of work function and cohesive energy of the constituent phases of Ti-50 at.% Al cathode during arc deposition of Ti-Al-N coatings
Introduction
(TixAl1−x)N coatings are frequently used as a high performance wear and corrosion resistive coating on cutting tools due to its age hardening properties, during which the coating decomposes into coherent cubic (c)-TiN and (c)-AlN rich domains [1], activated at elevated temperatures.
Industrially, such coatings are typically deposited by PVD techniques, in particular by cathodic arc deposition [2]. Cathodic arc transforms the cathode material into a dense, highly ionized plasma [3]. The transformation of the cathode material into the plasma phase occurs explosively in a temporally and spatially confined cathode spot on the cathode surface [4]. The cathode spot appears to be stationary on the cathode surface for a time period of a few ns [5,6]. The current density of the cathode spot is reported to be of the order of 1010–1012 A/cm2 [5,6]. This high current density leads to the Joule heating of the cathode surface [7,8]. The heat modifies the surface by forming a so-called converted layer of few microns [[7], [8], [9]]. Several research papers have discussed the association and dynamics of the cathode spot with surface roughness [10,11], surface contamination [12,13], surface temperature [14], inclusions [15], steering magnetic field [16], and material properties like work function [17] and cohesive energy [[18], [19], [20]]. The significance of material properties of the cathode, e.g., work function and cohesive energy become prominent in the dynamics of the cathode spot, especially if the cathode comprises more than one phase. In the case of Ti-Si cathodes, it has been shown that more cathode spot events occur on the phase with low work function than the phases with comparatively higher work function during arcing [17]. Consequently, the low work function phase experiences higher erosion. Similarly, it has been shown that the cohesive energy plays an important role in ion erosion [18,20]. The cohesive energy of the cathode material has been shown to be critical for the vacuum arc operation as it dictates how much energy is needed for the formation of cathode spots [20,21].
In this paper we attempt to tune the cathode material properties to influence the dynamics of the cathode spot, which in turn influences the process conditions and the coating properties. We show that by altering the work function and cohesive energy of Ti-50 at.% Al cathodes the reactive deposition conditions and the properties of the resulting Ti-Al-N coatings are influenced.
Section snippets
Experimental methods
Classified by microstructure, two Ti-50 at.% Al grades were prepared. The first grade was prepared by pressing followed by forging Ti and Al powders of equal atomic percentage. The temperature during the forging was 380 °C, which is below the melting temperature of Al but above its recrystallization temperature. This treatment ensures that the produced grade is an elemental mixture of pure α-Ti and Al (for short E.M. grade). The reported work functions of the two phases present in the E.M.
Results
The dual-phase microstructures of the virgin states of E.M. and I.M. grades are distinctly different, as can be seen in Fig. 1 and Fig. 2. The E.M. grade has a microstructure with α-Ti particles (lighter contrast) embedded in an Al matrix (darker contrast) (Fig. 1 (a)), whereas the I.M. microstructure consists of γ (TiAl) and α2 (Ti3Al) as can be seen in Fig. 2 (b). Fig. 1 (b) shows the coexistence of the equiaxed γ and lamellar γ/α2 grains. A grain size of 50 μm (measured by the intercept
Discussion
The arcing process supplies electrons and ions to the plasma that are needed to maintain its existence. The constituent phases of the cathode surface that require the least energy for ejection of electrons and ions are the phases with the lowest work function and cohesive energy [17,42]. During the single trigger event experiments, the cathode spot is active on the virgin material. In the case of the E.M. grade, the disparity among the virgin parent phases in terms of work functions (W.F.) and
Conclusion
Parities in the work functions and cohesive energies of the constituent phases of the Ti-50 at.% Al cathodes are achieved by transforming the α-Ti and Al phases to γ and α2 phases through reactive hot isostatic pressing of Ti and Al compacts. Such changes of the cathode phases promote uniform erosion of cathodes and potentially higher deposition rates as in the case of Ti-Al-N coatings. Data suggests that the thermal conductivity of Ti-50 at.% Al cathode is the dominant factor in macro-particle
Acknowledgements
The authors acknowledge the financial support from VINN Excelence Excellence Center in Research and Innovation on Functional Nanoscale Materials (FunMat-II; grant number 2016-05156) by the Swedish Governmental Agency for Innovation Systems. The authors would like to thank G. Greczynski for his assistance with the UPS measurements and W. Wan for his assistance with transmission EBSD measurements.
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