Morphology and microstructure evolution of Ti-50 at.% Al cathodes during cathodic arc deposition of Ti-Al-N coatings
Abstract: Today's research on the cathodic arc deposition technique and coatings therefrom primarily focuses on the effects of, e.g., nitrogen partial pressure, growth temperature, and substrate bias. Detailed studies on the morphology and structure of the starting material—the cathode—during film growth and its influence on coating properties at different process conditions are rare. This work aims to study the evolution of the converted layer, its morphology, and microstructure, as a function of the cathode material grain size during deposition of Ti-Al-N coatings. The coatings were reactively grown in pure N2 discharges from powder metallurgically manufactured Ti-50 at.% Al cathodes with grain size distribution averages close to 1800, 100, 50, and 10 μm, respectively, and characterized with respect to microstructure, composition, and mechanical properties. The results indicate that for the cathode of 1800 μm grain size the disparity in the work function among parent phases plays a dominant role in the pronounced...
Summary (2 min read)
- Morphology and microstructure evolution of Ti-50 at.%.
- This work aims to study the evolution of the converted layer, its morphology, and microstructure, as a function of the cathode material grain size during deposition of Ti-Al-N coatings.
- Detailed understanding and abundant research material on Ti-Al-N,4 ZrAl-N,5 Ti-Si-N,6 etc., are readily available.
- In-depth knowledge of such mechanisms is needed for a comprehensive understanding of the arc deposition process and perhaps a mean to control the microstructure and composition of arc deposited coatings.
- In addition, the effect of cathode material grain size on coating microstructure, composition, and mechanical properties is discussed.
II. EXPERIMENTAL DETAILS
- Classified by different average grain sizes distributions, four grades of titanium–aluminum with equal atomic percentage were used in this study.
- This ensures that the arc traverses the surface of the piece only once (single trigger event).
- To capture the surface state after multiple arcing events (steady state arcing) and to analyze differences in the resulting coatings, full sized circular cathodes having a diameter of 63 mm were also made from all four grades.
- The macro-particle areal density of the coatings was measured by the box-counting method, i.e., by dividing SEM micrograph with an area of 2015 lm2 into 60 boxes and then counting the macro-particles in each box.
- The load of 25 mN and taper polishing were used to mitigate the influences from the substrate and surface roughness on the recorded hardness.
- Figure 2 shows SEM micrographs of polished virgin materials prepared from four grades.
- These diffractograms show that during the initial stage of arcing the 1800 lm-grade retains its virgin composition of Ti and Al, the 100 lm-grade also shows the existence of just virgin phases, the 50 lm-grade shows the additional trace of f (Al5Ti2), and the 10 lm-grade additionally contains the intermetallic phase g (Al2Ti).
- The converted layer on the 1800 lm-grade hosts craters with diameters ranging from sub-micron to 3100 lm.
- Cross-sections of the converted layer on the four grades are shown in Fig.
- The coatings grown by the 100 lm-grade, as well as 50 lm-grade, have only spherical macro-particles, and the coating grown by the 100 lm-grade shows the fewest macro-particles among the four coatings.
A. Formation and evolution of the converted layers
- The composite manufactured material of all the four grades of Ti-50 at.%.
- A third factor yielding a more pronounced erosion of Al than Ti is related to the fact that metals with low melting temperatures emanate more macro-particles.
- It anchors the arc at this position and brings the liquid Al to boil.
- The grain size is too small for arc anchoring in the other three grades, which explains why macro-bubbles are not observed for these grades.
- On the other hand, the large interface area and poor thermal conductivities in the cases of 50 and 10 lm-grades generate more favorable conditions for the formation of intermetallic phases.
B. Steady-state condition of the converted layers and the effect on coatings
- The multiple instances of the cathode spot even out the macro-bubbles observed during the initial stage of the converted layer on the 1800 lm-grade [see Fig. 7(i)].
- Al has a lower melting temperature than Ti, it is expected that these flattened macro-particles are Al rich and they require a lower temperature or a longer time to solidify; as a result, these macro-particles reach the substrate in the liquid state.
- The relatively thick (50 lm) converted layer on the surface of 1800 lm-grade is a result of preferential erosion and the anchoring of the cathode spot.
- The temperature is expected to be quite high due to the residual heat generated from the self-sustaining reaction, and the ignition of the cathode spot further increases the temperature of the region surrounding this site.
- The high macro-particle density of the coatings grown by 50 lm-grades compared to 100 lm-grade is related to the self-sustaining exothermic reaction, which consumes the Al phase.
- Al powder metallurgical cathodes, as a function of their grain sizes (1800, 100, 50 and 10 lm), during the cathodic arc deposition of Ti-Al-N coatings.
- These two factors contribute to the high Al content in the deposited coatings.
- The reduction in grain size to 100 lm enhances the intermixing of the parent phases, which mitigates the anchoring phenomenon and results in the coatings having equal content of Ti and Al.
- Further reduction in the grain size creates favorable conditions for the initiation of the self-sustaining reaction to form the c phase.
- For the 10 and 50 lm grain size cathodes the self-sustaining reaction is the dominant factor affecting the evolution of the morphology and microstructure of the cathodes during arcing.
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Q1. What contributions have the authors mentioned in the paper "Morphology and microstructure evolution of ti-50 at.% al cathodes during cathodic arc deposition of ti-al-n coatings" ?
In this paper, the evolution of the morphology and microstructure of Ti-50 at. % Al powder metallurgical cathodes, as a function of their grain sizes ( 1800, 100, 50 and 10 lm ), during the cathodic arc deposition of TiAl-N coatings was reported.