Wear, 65 (1981) 399 - 401 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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Letter to the Editor

Reply to comments on “The unlubricated wear of cast irons”

Dr. Stobo’s comments [l] on our paper [2] are most interesting. We also admit that a metallographic examination is necessary to investigate the wear of metals and alloys since phase transformations induced by the fric- tional heating cycle, frictional force and applied load are expected on the sliding surface.

The phase tr~sfo~ation mechanism of severe wear [3] is an attractive proposal. We agree with the statement, “Clearly, when the experiment is stopped to allow metallographic examination of the surface, martensite or retained austenite will form, but this only proves that the surface tempera- ture during rubbing was above A,, . It does not prove,. . that martensite was the surface phase during rubbing”. This implies that martensite and/or retained austenite should form on the surface after sliding if a significant fraction of the contacting asperities during sliding are above A,, and the subsurface tem- perature is below M,. At the maximum wear rate of cast irons-where the mean temperature of the apparent sliding surface is considerably lower than M,, neither martensite nor retained austenite was detected on the worn surface by optical microscopic observation and X-ray diffraction. Only the base material phases present before sliding were found in wear debris, which must be cooled at a higher rate than the surface after sliding [4, 51. If the phase transformation plays an important role in severe wear around a wear maxi- mum, the phase transformation occurs on a grain size scale [3]. In this case martensite on the surface ought to be detected by X-ray diffraction or micro- scopic observation when sliding is stopped prior to me~lo~aphic analysis. Therefore we cannot say that at 300 - 400 “C, the flash temperature coincid- ing with the maximum wear rate, a significant fraction of the contacting asperities are above 800 “C. We do not disagree with the assumption that the temperature of a small fraction of the asperities may reach above A,,. Mar- tensite, retained austenite and oxides are clearly detected on the worn surface and in the wear debris by X-ray diffraction in the mild wear range at higher sliding speeds than that of the maximum wear rate [ 4,5] .

In so-called adhesive wear, wear occurs through the transfer of a frag- ment to the mating surface and/or the peeling-off of a fragment from the surface. These actions are induced by the force applied on the surface. This force is considered to be produced not only by the mechanical interlocking of asperities but also by the formation of adhesive junctions. Since adhesion is a solid phase welding between two solids which come into contact under load or relative motion the surface temperature must be the most important factor controlling the formation of an adhesive junction between the sliding

Temoerature of real area of contact

Fig. 1. Schematic illustration of wear, strength of adhesive junctions and frequency of formation of adhesive junctions as a function of the temperature of the real area of contact.

surfaces if the pair of materials and the atmosphere in which sliding occurs are defined. It is reasonable to assume that this temperature is not the mean temperature of the apparent sliding surface but the temperature of the real area of contact. An increase in the temperature of the real area of contact must facilitate the formation of an adhesive junction but must weaken the adhesive strength, as shown in Fig. 1. The adhesive force is considered to be expressed by the product of the area, i.e. the frequency of formation, of an adhesive junction and its strength, so that the wear rate varies, and has a maximum, with the temperature of the real area of contact. Regarding the flash temperature in our definition, when this is 300 - 400 “C the wear rate of cast irons reaches a maximum. We do not believe that the greater part of real areas of contact is above A,, during sliding at the maximum wear rate.

If relative motion occurs only at the interface of the junction as in perfect fluid lubrication, friction appears but wear does not occur. Wear debris is derived from shearing in the subsurface layer. Thus the temperature gradient from the surface to the inside must be an important factor in wear and is considered to be one of the principal factors controlling where shearing occurs. As it is not possible to measure precisely or to calculate this temperature gradient we estimated the mean temperature of the apparent sliding surface. Similar temperatures have been adopted in other works [6, 71. Since a rise in the mean temperature results in a decrease in the strength of the sub- surface, we consider that wear increases with increase in the mean tempera- ture when the flash temperature is maintained constant. Such a suggestion is confirmed by the wear of 17Cr stainless steel, 4/6 brass and aluminium alloys, although the degree of increase varies with the flash temperature [ 8 - lo]. The wear of cast irons and of carbon steels is similar if the mean temperature is above 400 - 500 “C for cast irons and above 200 - 250 ‘C for carbon steels. Wear of these materials depends only slightly on the mean temperature be-

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low such values; the reason for this is probably that the strength of the mate- rials does not decrease much up to such temperatures.

We suggested that mild wear occurs when the flash temperature is too low for adhesive junctions to take place frequently or when the flash temper- ature and the mean temperature are high and low respectively, i.e. when only the interface of the contact becomes very soft or viscous and the subsurface is kept at a considerable strength. For sliding in air the sliding surface oxidizes when the wear loss is small. The surface is bright and apparently free from oxide during severe wear of annealed or normalized steels and of pearlitic or ferritic cast irons. This is probably because the wear rate is larger than the oxidation rate of the surface. The wear rates of a quenched steel and a white cast iron vary with sliding speed with a maximum. At maximum wear rates the wear of these materials is small compared with the wear of annealed steels and pearlitic cast irons. Oxides visibly form on the surface [8, 111. Hardness is related not only to the shear strength but also to the real area of contact on which the flash temperature depends.

We cannot agree with the postulation that mild wear at high loads or high speeds can be attributed to the formation of surface martensite by local heating, because the hard phase forms after sliding is stopped. The phase transformation mechanism might appear to describe dry wear phenomena consistently [ 31. This mechanism does not account for the wear of low carbon ferritic or austenitic stainless steels, since these materials rarely pre- sent or do not present martensitic transformation induced by heating and quenching but exhibit a similar variation in wear with sliding speed to that of carbon steels. We have some doubts about the phase transformation mech- anism of dry wear.

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J. J. Stobo, Wear, 65 (1980) 131. M. Kawamoto and K. Okabayashi, Wear, 58 (1980) 59. J. J. Stobo, J. Aust. Inst. Met., 18 (1973) 146. K. Okabayashi, M. Kawamoto and H. Notani, J. Jpn. Foundrymen’s Sot., 38 (1966) 501. M. Kawamoto and K. Okabayaahi, Wear, 17 (1971) 123. W. Hirst and J. K. Lancaster, hoc. R. Sot. London, Ser.A, 259 (1960) 228. S. W. E. Earles and M. G. Hayler, Wear, 20 (1972) 51. M. Kawamoto, S. Shintani, T. Sone and K. Okabayashi, J. Jpn. Inst. Met., 37 (1973) 1236. K. Okabayashi, M. Kawamoto and T. Kajimoto, J. Jpn. Sot. Lubr. Eng., 21 (1976) 695. K. Okabayashi, M. Kawamoto and T. Kajimoto, J. Jpn. Light Met. Inst., 26 (1976) 124. K. Okabayashi and M. Kawamoto, J. Jpn. Foundrymen’s Sot., 48 (1976) 520.

(Received October 3, 1980; in final form October 21, 1980)

M. KAWAMOTO and K. OKABAYASHI

College of Engineering, University of Osaka Prefecture, 4-804, Mozu-Umemachi, Sakai, Japan