Investigations of the Cathode Spot Dynamics in a Vacuum Arc Coating Process

Abstract

Influence of cathode materials (Ti, Al, Cu, TiN), ambient gases (Ar, Nz, p = 0.1-1 Pa) and the arc current itself on the motion and the velocity of cathode spots in an arc coating process have been investigated with the help of a new high speed framing camera. It was found, that the cathode material causes different spot currents but in general the spot arrangement and the motion on the surface are similar. Surface contaminations due to ambient gases affect this dynamics in several ways. Insulating layers like AlN can drastically increase the instantaneous spot velocity, for example from <5 m/s on A1 up to 170 m/s on AIN contaminated areas. TIN layers with a high conductivity increase the spot mobility at first. But at nearly completely contaminated surfaces (simulated by a TIN cathode), the mobility is strongly decreased. The values change from an average velocity of 6.3 m/s with a diffusion constant of 54 cm2/s (Ti, 0.01 Pa) to 2 m/s and 6.4 cm2/s at TiN.The course of the instantaneous spot velocity during the spot splitting phase was investigated too. The instantaneous spot velocity of each of the two new spots originated from the starting spot is relatively high (30-40 m/s) within the first 50 p. The cathode material and the ambient gases are of slight influence in this phase. The movement is directed. In the further development the instantaneous spot velocity is decreasing to values under 5-10 m/s. The motion is now more and more random. Additionally it could be proved, that the lower stability limit for a stable discharge is strongly connected with the spot current, which depends on discharge conditions.

Temporal development of the composition of Zr and Cr cathodic arc plasma streams in a N2 environment

“…On a contaminated surface the dielectric layers cause additional electron emission mechanisms, which favor the formation of new spots. As a result, type 1 spots on contaminated surfaces tend to show random movement over larger distances than type 2 spots on clean surfaces [2,9,14,15].…”

“…The spot velocity is also affected by contaminants. Ellrodt et al [14] reported the influence of cathode material and ambient gas on the motion and velocity of the cathode spots. They found that in general the spot arrangement and motion were similar on various metals as the surface was contaminated, and the velocity of the spots increased.…”

Machining with AlCr-oxinitride PVD coated cutting tools

“…The evaporators were operated with DC current in steered arc mode in order to maintain low macroparticle emission, which is well known to affect both the surface texture and morphology of cathodic arc PVD coatings [32][33][34][35][36]. The coating architecture adopted consisted of 3 layers, an AlCrN bonding and functional layer of 1.5-2 µm thickness, a 100 nm transition layer with a linear variation of non-metal content from nitride to oxinitride with a composition controlled by the reactive gas volumetric flow rates.…”

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

“…Despite these advantages, the main drawback is the emission of liquid droplets, also called macroparticles (MPs) contamination [1,2], MPs deposited on the films deteriorate the surface roughness, structure quality and many other particular property. It is widely accepted that apparent cathode spot motion is actually a series of arc events where a new cathode spot ignites after the old spot extinguishes during a short duration [3][4][5]. Cathode spots motion randomly on the target surface cut down the utilization factor of cathode material and the lower motion velocity increase the number of droplets emitted from liquid pool [6].…”