Abstract
Herein, we investigate the influence of powder metallurgical manufactured Ti0.5Al0.5 cathode grain size (45-150 μm) on the properties of a DC arc discharge, for N2 pressures in the range 10-5 Torr (base pressure) up to 3x10-2 Torr. Intermetallic TiAl cathodes are also studied. The arc plasma is characterized with respect to ion composition, ion charge state, and ion energy, and is found to change with pressure, independent on choice of cathode. Scanning electron microscopy, X-ray diffraction, and Energy-dispersive X-ray spectroscopy of the cathode surfaces and the concurrently deposited films are used for exploring the correlation between cathode-, plasma-, and film composition. The plasma has a dominating Al ion content at elevated pressures, while the film composition is consistent with the cathode composition, independent on cathode grain size. Cross-sections of the used cathodes are studied, and presence of a converted layer, up to 10 μm, is shown, with an improved intermixing of the elements on the cathode surface. This layer is primarily explained by condensation of cathode material from the melting and splashes accompanying the arc spot movement, as well as generated plasma ions being redeposited upon returning to the cathode. The overall lack of dependence on grain size is likely due to similar physical properties of Ti, Al and TiAl grains, as well as the formation of a converted layer. The presented findings are of importance for large scale manufacturing and usage of Ti-Al cathodes in industrial processes.
Highlights
Vacuum arc plasma from Ti0.5Al0.5 compound cathodes with different grain sizes has been characterized with respect to ion composition, ion charge state, and ion energy in vacuum and reactive N2 atmosphere
Concurrent deposition of thin films has been done for evaluation of primarily macroparticle generation and film composition
A converted layer up to 10 μm was found at the cathode surface, with an improved intermixing of the elements, possibly contributing to the lack of dependence of cathode grain size on the arcing process
Summary
Arc evaporated (Ti,Al)N thin films are widely used as hard and wear-resistant coatings due to their superior mechanical properties and oxidation resistance. Industrial PVD synthesis of (Ti,Al)N by DC arc with plasma generation from a Ti-Al compound cathode in a nitrogen atmosphere is an established technique. Extensive studies have been carried out on the effect of the properties of the arc generated material flux on the resulting coatings, including the influence of cathode composition, nitrogen pressure, electrical potential (bias) and temperature of the substrate. to date, little is known about the influence of microscale properties and grain size of Ti-Al cathodes on the plasma generation process and subsequent film formation.With a particular focus on a compound cathodes used in the arc deposition process, input parameters such as phase composition, arc current, applied magnetic field, and operation temperature, are acknowledged to have significant impact on the arc behavior, generated plasma characteristics, and coating properties, see e.g. Refs. 16–20 and 15. Industrial PVD synthesis of (Ti,Al)N by DC arc with plasma generation from a Ti-Al compound cathode in a nitrogen atmosphere is an established technique.. Extensive studies have been carried out on the effect of the properties of the arc generated material flux on the resulting coatings, including the influence of cathode composition, nitrogen pressure, electrical potential (bias) and temperature of the substrate.. With a particular focus on a compound cathodes used in the arc deposition process, input parameters such as phase composition, arc current, applied magnetic field, and operation temperature, are acknowledged to have significant impact on the arc behavior, generated plasma characteristics, and coating properties, see e.g. Refs. Movement of the arc spot on the surface of a compound cathode, where areas composed of different phases may exist, can obviously lead to a change in discharge properties. A local variation of the work function, the melting temperature, and electrical and thermal conductivity, have a strong influence on the arcing process, exemplified by, e.g., a change in macroparticle
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