The Mechanism of Electrode Erosion in Electrical Discharges PHYSICAL BASIS OF THE LOW EROSION RATE OF THE PLATINUM METALS By Professor F. Llewellyn-Jones, M.A., D.Phil., D.Sc., F.1nst.P. Department of Physics, University College of Swansea instance when the latter effect is undesirable This paper outlines the nature of the in practice is that of the well-known sparking various classes of electrical discharges plug of internal combustion engines, and an -the high voltage capacitative spark article referring to this effect has recently discharge, the normal arc discharge, and appeared in this journal (I). Electrical the low voltage arc that occurs at contacts of various types represent other electrical contacts-and discusses the examples in which the deleterious effects of physical processes that determine the electrical discharges lead to the erosion of dependence of the rote of electrode the electrode metals (z), and Want (3) in an erosion on the physical properties of article in this journal has discussed the the metal. It is concluded that the most design of light duty contacts. Consequently, important properties conducive to low an understanding of processes that result in erosion rates are high boiling point, the erosion of electrode materials can be of high thermal conductivity and high considerable technical as well as fundamental density, so providing the physical reason physical interest. for the low erosion rates observed for the platinum metals when used as electrodes Types of Erosion and electrical contacts. There are various processes by which metal electrodes can erode or wear away. Most of the technical applications of First, there is the obvious one of chemical electrical discharges through gases are those attack such as oxidation or corrosion. When which employ electrodes to establish the the products of chemical action are volatile required electric field, and in such cases the erosion can be rapid at high temperatures. the electrodes themselves then play an im- Another process is that of the actual dis- portant role in the discharge processes. The integration of the structure under atomic cathode suffers positive ion bombardment or ionic bombardment; and, thirdly, there is and sputtering often results; also when the process of electrical erosion by the action arcing occurs, hot spots are formed and the of an electrical discharge. It is well known very high temperatures that result can pro- that at local regions of an electrode in a duce evaporation. Such evaporation has at discharge tube hot spots at very high tem- least two effects which are sometimes highly peratures can be produced. The anode undesirable, namely, the production of the undergoes bombardment by electrons while vapour of the electrode metal thus changing the cathode suffers bombardment by positive the ambient gas atmosphere, and the erosion ions leading to sputtering and evaporation. and wearing away of the metal itself. An High temperatures can also be produced in Platinum Metals Rev., 1963, 7 , (2),58-65 58 some gases by gas-atomic association which anode; the energy released there (eV) by this may involve the liberation of a great deal of electron bombardment is clearly dependent heat; this produces more evaporation, but upon the actual value of the contact voltage V the release of energy of the electrical dis- which, on account of the presence of local charge by electronic or ionic bombardment self-inductance, can be greater than the static of the electrodes is a primary cause of the contact voltage. This bombardment can production of hot spots and resulting erosion. remove metal from the anode. It is to a consideration of this latter Secondly, as the gap widens and the con- phenomenon that this paper is devoted, centration in it of metal vapour increases, this with particular reference to platinum and the vapour becomes ionised by the electron platinum group metals. It has long been stream, so producing positive ions. These realised that these metals appear to have a are attracted towards and bombard the peculiar resistance to erosion by electrical cathode thus leading to loss of material from discharges, and the physics of the phenomena the cathode. Because of the presence of of electrical erosion will now be discussed electrons, and ions as well as gas atoms, a in an attempt to elucidate the process. plasma or positive column is formed, direct In any practical device involving electrical high energy electron bombardment of the discharges, properties other than that of anode is reduced and, as the gap lengthens, a simple resistance to electrical erosion have normal arc discharge is set up. The energy to be taken into account. Some of these, released at the cathode is then of the order hardness for example, naturally concern the eV,, where V, is the cathode fall of potential suitability of the metal for the required between the electrode and the plasma column. practical fabrication process. Many of the In this stage the release of energy at the required properties of a contact material and cathode can exceed that at the anode. assembly are closely interrelated, and in a Clearly, for the fully developed arc and practical device it is often not possible to when a high current is passed, considerable change one characteristic without altering bombardment of both electrodes can occur. others. However, this paper will be concerned In such conditions considerable energy is only with the question of erosion due to an also dissipated in the positive column itself electrical discharge. where very high temperatures can be attained. However, in the present discussion attention Electrode Processes in Contact is mainly directed to the simple states of the Discharges contact discharge during the earliest stages A discussion of the various mechanisms of its development and when the gap separa- of electrode bombardment occurring at an tion is short. This separation d is approxi- opening contact has been given previously by mately that of the largest molten metal bridge the present author (4). Briefly, there are and is of the order of IO-~cm. Two types of different phases of operation of the arc short arc have been classified and the term discharge depending upon the state of “anode arc” is given to the case of predomi- development of the arc itself and therefore nant anode Ioss of material, and “cathode upon the gap separation and power dissipated. arc” to the case when the loss is confined First, the arc is formed when the micro- to the cathode (5). scopic molten metal bridge blows up and the So far the post-contact arc has been metal contact is replaced by a gaseous and considered, but it is also known that a pre- vapour region between the electrodes (2). contact arc can occur with cold electrodes (6). Since the cathode is at a high temperature Consideration of the electron emission ac- (2 melting point of the metal) it thermally tivity (7) of the cathode surface shows that emits electrons which are attracted to the this can be initiated when the gap separation Platinum Metals Rev., 1963, 7 , (21, 59 Photomicrograph ( X 420) of a molten metal bridge between pure platinum contacts operat- ing in air, showing the stable nodoid form. The temperature of the glowing metal ranges from about 2000°K at the ends to ubout 4500°K near the middle of the bridge (Phoragraeh b. F. Lleaellyn-3ones und Michael Price) is of the order of IO-* cm and must be long period, any rise in electrode temperature, initiated by an electron avalanche (n electrons and therefore consequent evaporation, would each of charge e) striking the anode and be negligible. Now, the initial stage of the releasing energy in some cases as much as setting up of the discharge takes place in times n eV. of the order of the electron and ion transit Again, as in the case of a spark discharge times t, and is therefore -d/W+, where W- is passed between two cold electrodes of self the ionic drift speed this is 105, giving t, of capacity C, the energy of the spark released the order of IO-~ sec. Clearly then, the by particle bombardment of the electrodes practical consequences of an extremely rapid must be some fraction of the total energy release of discharge energy W (- kinetic (-CV2/z) of the gap which is discharged energy of ions and electrons plus the potential by the spark current. energy of the ions and less the work required Thus in all the cases considered, whether a in the secondary emission of electrons) is a spark, or the pre-contact or post-contact rapid rise in temperature over the “hot spot” micro-arc, electrical discharges have this area and throughout a small volume of elec- basic feature in common in that a fraction of trode underneath. the total energy available in the charged spark gap is released at either or both of the Electrode Hot Spot and Energy electrodes. Balance Thus, from elementary considerations of Thus it seems reasonable to regard the main the discharge mechanism, it can be seen that consequences of the electrical discharge particle bombardment, whether electronic or (neglecting chemical effects) as far as the ionic, of electrodes can occur, and it is now electrode is concerned as being due to the necessary to consider the consequences to the sudden release of energy over a certain small electrodes of this release of discharge energy. area of either electrode, and clearly the mag- The first important factor to appreciate is nitude of that release is a matter of physical the great rapidity with which a significant significance. amount of energy can be released at an The considerations put forward above of electrode surface.