Model calculation of the charge composition of a plasma in a vacuum-arc discharge with a composite cathode

Abstract

Composite cathodes manufactured by mixing (sin tering or melting) various components in the desired proportion are widely used to obtain various func tional coatings by electric arc spraying. In particular, titanium-aluminum composite cathodes are used to deposit wear resistant and heat resistant coatings on cutting tools [1]. During spraying of a composite cath ode in a vacuum arc source material for multicompo nent ion deposition (implantation), the main task is to form a near surface layer with a certain stoichiometric composition on the processed substrate (target). For this reason, it is an important to know how the charge composition of ions of the initial components entering the vacuum discharge plasma changes as compared to the relative yield of these components from the cath odes of corresponding pure materials.The present Letter proposes a simple model for cal culating changes in the charge composition of a vac uum arc plasma in discharge with a composite cath ode. The results of model calculations are compared to recent experimental data [2,3]. Zhirkov et al.[2] pre sented the results of an experimental study of the mass and charge composition in a vacuum arc discharge plasma with a Ti 1 -x Al x (x = 0.16-0.7) composite cathode operating in a continuous mode. In [3], the mass and charge composition in a vacuum arc dis charge plasma had a cathode with a similar composi tion (Ti 0.67 Al 0.33 ), but operating in a pulsed discharge regime.The proposed model is based on the ecton mecha nism of vacuum arc cathode spot operation [4]. According to this mechanism, a vacuum arc plasma is formed as a result of the operation of separate cells (explosive emission centers) in the cathode spot of the vacuum arc. The arc current buildup is accompanied by an increase in the number of simultaneously oper ating cells of the cathode spot. For this reason, the ion flux parameters (the distribution of ion velocities and charges, ion erosion rate, ion current fraction) are independent or weakly depend on the arc current up to the kiloampere level [4][5][6][7].It was shown [8] that the elemental composition of the vacuum arc plasma almost coincides with the ele mental composition of the composite cathode. For this reason, the proposed model assumes that compo nents in the cathode spot of the composite electrode (in this case, Al and Ti) erode simultaneously and the relative rate of their erosion corresponds to their rela tive content and does not change with time. In addi tion, the results of Zhirkov et al. [2,9] show that ions of various sputtered components have almost identical velocities. The results of our recent model calculations [10] for a three component plasma showed that the drift velocities of ions with even significantly different masses (zirconium and deuterium) exhibit leveling as a result of strong ion-ion interaction in a dense near cathode plasma. For this reason, the proposed model assumes that ions of different components have the same velocity and, hence, the plasma can be described using the two temperat...

Cathode and plasma phenomena in vacuum-arc sources of hydrogen isotope ions. II. Ionization processes in the arc plasma

“…In a number of studies (see, e.g. [5,6,[14][15][16][17][18]), it was shown that simulation models based on the equations of magnetohydrodynamics are quite applicable to describe the evolution of the cathode jet plasma in vacuum arcs with pure metal cathodes and give predictions consistent with the experimental data. In particular, it was shown that the plasma expansion near a cathode spot cell is almost spherically symmetric [14].…”

On plasma jet formation in vacuum arc with composite cathode

“…In this case the ions of different mass will have the same average velocity due to very strong ion-ion collision in dense cathode spot plasma [10]. In the framework of the model of [11] the calculated average velocity of Zn-Pb plasma was 0.61*10 6 cm/s for all ion types. It is necessary to clarify that the ions originated from the cathode spots have quite a wide distribution of kinetic energy [12].…”

“…At different stages of the cycle the ions emitted from the spot have different drift velocities, but at any stage the ions with different charges have the same drift velocity [14]. From this viewpoint the velocity obtained within the model of [11] can be treated as a velocity averaged over the ecton cycle [1,13]. The ion flux angular distributions calculated for this case are shown in Figure 8.…”