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. If the total energy W of a the basic processes in the establishment of discharge were released at a slow rate over a an arc discharge show that the area of the

Platinum Metals Rev., 1963, 7 , (21, 60 electrode significant in the bombardment metal to these processes and therefore to processes is the area over which either the physical properties of the metal. This is electron avalanche or the positive ion ava- v= aV2- PT’-yA(T- 0) (1) lanche will strike the appropriate electrode. p {(T - 0)s +Z I T/A) Theoretical estimates of these areas have been volume of electrode metal eroded, or made by the present author (8) for certain boiled off, per elementary discharge cases of practical interest, such as that of the P= density of metal sparking plug of an internal combustion T= boiling point of the metal engine, and it is found that very high current x= thermal conductivity densities -107 amp cm-2 can occur at A= atomic weight s= specific heat “hot spots” about IO-~cm2 in area. V= effective discharge potential dif- The energy balance following this sudden ference release of energy at areas on the electrode of a=f(C) =function of the local capacity C and this order must now be considered. A certain discharge conditions which determine the fraction of the total energy (microscopic)volume of metal lying under this released at the electrodes and avail- area will be suddenly heated, and part of this able heat must be dissipated by thermal conduc- P= a constant which includes Stefan’s constant and the area of the hot spot tion through the bulk of the metal; the rate Y= a geometrical factor related to the at which this can occur has been given pre- rate of thermal conduction of energy viously (8). Heat will also be lost by thermal through the metal the steady temperature (in “K) at radiation from the hot spot surface of area A points remote from the hot spot. and temperature ToK at a rate proportional to AT4. The erosion equation can also be written Considerations such as these lead to in a simplified form by noting that T> >O the conclusion that the mass of metal be- and a is a constant in any given circuit when neath the hot spot will play a vanishingly electrical parameters are given; further, LYV small part in the energy balance there which can be replaced by a constant LY’ when the determines the processes of disposal of the gap voltage is given and remains constant for energy released, and part of which may even different electrode materials, so that

involve boiling of some metal. Thus, the v =a’- PT’-ylT (2) boiling of the electrode material involves p(s+21/A)T the energy of the discharge current and not the current directly considered simply as a trans- The Erosion Equation port of charge. It follows that these processes Before discussing experimental data that of disposal must include the processes of the may be used to test the validity of this thermal conduction through the metal itself equation, it is of interest to consider what away from the surface of the hot spot; the guidance this relation gives concerning the process of the heating up of a small volume behaviour of metals with different physical of metal underneath the hot spot, first to the properties when exposed to an electrical melting point, followed by melting; if suf- discharge. ficient energy is available, there occurs the Inspection at once brings out the impor- further heating of the volume of molten metal tance of the boiling point, the density and the up to the boiling point; and, finally, evapora- thermal conductivity, high values of which tion or boiling can occur when there is should lead to low values of electrode erosion; further excess available, this leading to a high boiling point is clearly of the greatest removal of metal from the electrode. significance in considering those physical The author has previously obtained an properties of a metal for which low electrical expression (8) relating the rate of boiling of erosion is desired. Another factor of impor-

Platinum Metals Rev., 1963, 7 , (21, 61 tance is the first term, Vaf(C) or cc’, represent- interpretation, on the basis of fundamental ing the amount of electrical energy released physical processes, of experimental results at the electrodes. When this is comparatively must be carried out with some care when small, the amount of resulting erosion, being considering the dependence of erosion directly given by the difference of two terms, is not on current or total charge passed. On the necessarily proportional to the energy re- other hand, in the case of comparatively leased; but when the energy term a’ greatly high-voltage, short-time or capacitative spark exceeds the other terms and the erosion is discharges, the energy released could depend great, the volume rate of erosion li is then on the square of the initial spark gap voltage very approximately proportional to CIV. V,, and this then becomes an important Further, in cases of comparatively prolonged parameter. This condition applies to the case arcs as distinct from short, almost instan- of the ignition spark discharge of an internal taneous, discharges of the gap and local combustion engine, in which turbulence and circuit capacity, the gap or arc potential V, other factors lead to rapid arc quenching and can be almost constant, and then the arc the discharge itself takes the form of a SUC- energy dissipated V, x I, x t has become cession of high voltage sparks (8). roughly proportional to the arc current I, and also to I,t, i.e. to Q, the total charge passed. Applications and Test of Erosion Clearly, the highest possible values of these Equation quantities lead to the lowest possible rate of The expression derived above for the rate matter erosion; further, it would appear that of volume erosion in terms of the physical the most important property, for any given properties of the electrode has been applied amount of energy released, is that of high in a number of different cases for which boiling point. The physical reason for this is, experimental data on spark erosion are of course, that high values of this property available. Since these cases were concerned lead to high dissipation of energy, so reducing with widely different experimental and physi- the amount of excess energy available for cal conditions, it may be of interest to describe actual boiling of metal. them. The three distinct sets of discharge Now these properties are precisely those conditions were : associated with metals such as platinum and (a) the high voltage “capacitative” elec- iridium, which have very high boiling points trical spark as employed at sparking as well as high densities compared with those plugs of internal combustion engines, of other metals. Thus the theory of electrical (b) the erosion of electrical contacts by erosion due to electrical discharges briefly the normal arc discharge at break, and outlined above indicates the physical reasons (c) the erosion of electrical contacts by for the peculiar property of platinum and the low-voltage (V-4 volts) arc at iridium in resisting erosion by an electrical break and occurring immediately after discharge and therefore for their general suita- the rupture of the molten metal con- bility as contact or spark gap electrodes, as tact bridge. far as that particular property is concerned. It can also be seen that approximate (a) If the constants CI,p and -( in any given measurements in certain special conditions, contact circuit were known, it would be for example with constant arc voltage V,, can possible to calculate the mass or volume lead to the result that erosion appears to be eroded per spark for any particular metal proportional to the total charge passed, as when sustained low-voltage arcing and chemi- would be the case in “ionic transfer’’ of cal reaction are negligible. However, such matter from one electrode to the other (9, 10). detailed calculations are not possible at This line of thought indicates that the physical present since neither the precise time during

Platinum Metals Rev., 1963, 7 , (21, 62 which the hot spots were maintained in the materials. In their experiments, the arc development of the spark discharge nor the current I, lay in the range 1-5 amp and the precise areas of the hot spots are known. gap arc voltage V, remained constant at about The procedure in such cases is then to 50 V in a resistive and inductive circuit, and find the constants cc, p and y empirically by consequently these authors considered that applying the expression to the particular the discharge obtained on opening the con- experimental data for three materials of tacts to have been a normal arc discharge. known, preferably differing, physical proper- They applied the general erosion equation in ties. This procedure has in fact been fol- its simplified form (2) where o is the matter lowed for the metals platinum, nickel and transfer coefficient expressed in cm3 per austenitic steel, which have been employed coulomb. The thermal conductivity 1 is in experimental sparking plugs and used in expressed in joules per second - cm per degree aero engines (8). The three simultaneous centigrade; the boiling point T in "C, the equations were then solved, and the values density p in g per cm3j specific heat s in joules obtained were per g"C. Ittner and Ulsh used the following a=O.gg X IO-'' F, values of the constants for their specific p=2.00 x IO-~O cal per (o'K)~, experimental conditions : Y=O.pj x IO-~ cm sec. cc'=2.40 x IO-~ Inspection then showed that these values 9 =2.04 X I0-I' for the particular conditions were physically y =2.52 x 10-6 reasonable. On the basis of these values it and with these used the expression (2) to could be deduced, for instance, that for these calculate the transfer coefficient (or cathode sparking plug discharges only per cent of 11 loss as it was in their experiments). the total spark energy was actually released Some of their results for metals of the at the two electrodes, the remainder being platinum group are given in the table, from dissipated in the gas. which they concluded that expression (2) is (b) Consider next the second example of useful in giving a first approximation to the practical application. Ittner and Ulsh (I I), expected cathode arc transfer of most at the IBM laboratories, New York, measured materials and so permits evaluation of normal the erosion of a large number of contact arc transfer coefficients which can then be

~

Data on Physical Properties and Erosion

Transfer coefficient Thermal Specific (cathode loss) Metal Boiling Density point :ond u ctivity heat, Atomic in cma x IO-"C jouleslsec joules/ weight g/cm3 "C cmi*C g'C 1 - - 0.33 0.34 Ag 10.5 i950 4.2 0.23 I08 Au 0.34 1.1 19.3 2600 2.9 0.13 I97 Ir 0.25 22.4 4800 0.58 0.13 I93 Pd 1.51 I.59 12.2 2200 0.70 0.29 I07 Pt 21.4 4300 0.70 0.13 I95 0.43 0.4I 10% Ir-Pt 0.63 0.8 21.6 4400 0.3 I 0.13 I94 I 20% Ir-Pt 0.60 0.67 21.7 4500 0. I8 0.13 I94 35% Ir-Pt 0.54 0.54 21.8 4600 0.20 0.13 I94 Rh 0.42 0.5 12.4 3880 0.88 0.24 -I03

Platinum Metals Rev., 1963, 7 , (21, 63 used as a guide for the choice of contact some results at a recent conference in materials. The table gives, for comparison, London (12). These are given in the graph the data for more common metals in addition and were explained on the following lines. to those for the platinum metals. With low series circuit inductance < 10-7H Ittner and Ulsh also drew attention to the there was no region over which the transfer fact that the simplified erosion equation gives a was constant and independent of inductance, qualitative explanation of the behaviour of the so that the transfer was not solely due to iridium-platinum alloys included in the table. bridge transfer; in fact, there must have The observed effect of alloying indium with been a short duration, short length post- platinum was first to increase matter transfer bridge rupture arc. In that case, it is reason- due to arcing, but this eventually decreased able to assume that expressions (I) or (2) as the percentage of indium is increased. might be applicable. Indeed, the same They attributed this increase in transfer with argument as used for these materials by small amounts of iridium to the decrease in Ittner and Ulsh, who had shown that the arc thermal conductivity in the alloy as compared transfer was greater for the alloys than for with pure platinum. pure platinum, might be expected to apply to these materials for these discharges also. (c) The third application of the erosion equation to the platinum metals also lies in Conclusion the field of electrical contacts, but this time The experimental results described above to a different class of electrical discharge. show that the rate of matter erosion due to The contact voltage was 4V, the parallel electrical discharges is, to a first approxima- quenching capacitance was 4pfd, and the tion, given by the expressions (I) or (2). series inductance was controllable in the Further, the experiments involved a number range of 10-8 to 10-5 Henry. The erosion and of different metals, including the metals of net transfer produced at electrodes of plati- the platinum group, which covered a wide num and iridium-platinum alloys have been range of physical properties. Consequently, investigated at Swansea, using a radio-active it is reasonable to conclude that this agree- tracer technique, and C. H. Jones reported ment is experimental evidence in support of

Platinum Metals Rev., 1963, 7 , (21, 64 the basic physical assumptions regarding the help to form the basis of a fuller under- mechanism of the erosion of metal electrodes standing, not only of the behaviour of plat- by electrical discharges. This mechanism, inum and of other metals, but also of the in cases which exclude chemical activity, erosion process itself due to electrical dis- corrosion and sputtering, envisages actual charges. boiling of metal at the electrode hot spots References by the energy available in excess of that I G. W. Shoobert, Platinum Metals Rev., 1962, dissipated by radiation and thermal conduc- 6,9294 tion from the hot spot. Further, the agree- 2 F. Llewellyn-Jones, The Physics of Electrical ment between theoretically predicted and Contacts, Oxford, Clarendon Press, 1957 3 J. 6. Want, Platinirm Metals Rev., 1961, 5, experimentally observed erosion rates may 42-50 also be regarded as evidence in support of the 4 F. Llewellyn-Jones, Proc. Inst. Elec. Engrs., analysis of the different types of electrical Part I, 1949, 96, (Nov.), 305-312 discharges given above in the section or 5 L. H. Germer and W. S. Boyle, Paper D3, 8th Annual Gaseous Electronics Con- electrode processes. ference, Schenectady, N.Y., 20th October, From these conclusions, then, it follows 195s 6 L. H. Germer and I. L. Haworth, Phys. Rev., that the dependence of the discharge erosion 1948,739 (9X IlZf rate of the electrode on its physical properties 7 F. Llewellyn-Jones, Proc. Inst. Elec. Engrs., is mainly on the lines indicated by the Part I, 19.53, Inn, (Jul.), 169-173 relation (I), and so substantiates the general 8 F. Llewellyn-Jones, Nature, 1946, 157, (Mar. 9th), 289-299; Brit. 3. Appl. Phys., observations given in the section on the 19.50, 1, (31, 60-65 erosion equation. This is of particular in- 9 R. Holm, Electrical Contacts, Stockholm, terest as far as the platinum metals are Hugo Gebers, 1946 I0 W. B. Ittner, J. Appl. Phys., 1956, 27, (4), concerned, and it supplies the physical basis 382 of the observed low erosion rates of metals I1 W. B. Ittner and H. B. Ulsh, Proc. Inst, Elec. of the platinum group: this also explains Engrs., 1957, Part B, 104, (Jan.), 63-68 the low erosion rates of metals like tungsten 12 C. H. Jones, Paper read at Symposium on Electrical Contacts, organised by the and molybdenum in chemically inactive Institute of Physics and Physical Society, ambient atmospheres. These experimental London, 1961; see also M. R. Hopkins and C. H. Jones, Brit. 3. Appl. Phys., in applications of the erosion equation, therefore, the press. Electro-organic Synthesis at Controlled Potentials MERCURY CATHODE AND PLATINUM GAUZE ANODE Although catalytic hydrogenation and other ated calomel reference electrode. A high chemical reduction processes are employed in current capacity potentiostat, developed re- the synthesis of many organic chemicals, their cently, maintained potential control. The application is sometimes limited by lack of various proportions of p-aminophenol, aniline, selectivity. An alternative method of reducing azoxybenzene and p-phenetidine produced organic compounds-controlled potential were dependent on the electrolysis time and electrolysis-has been investigated recently on the potential used. by W. H. Harwood, of Continental Oil The results of this laboratory-scale investi- Company, and R. M. Hurd and W. H. gation indicate that potential control is Jordan, of Tracor Inc., using nitrobenzene as effective in directing the course of electro- the starting material (Ind. Eng. Chem., Process chemical reduction reactions and in producing Design & Development, 1963, 2, (I), 72-77). higher yields of some reduction products than Electrolyses were carried out over the are obtainable with chemical reducing agents. potential range -0.4 to -0.9 volt in sulphuric On an industrial scale, this technique may acid solutions, in a cell with a platinum gauze have many applications in the manufacture of anode, a mercury pool cathode and a satur- organic compounds.

Platinum Metals Rev., 1963, 7 , (21,65565 65