Subsurface effects during sliding wear

Subsurface effects during sliding wear

63 Wear, 35 (1975) 63 - 67 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands SUBSURFACE EFFECTS DURING SLIDING WEAR J. A. KIRK and T...

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Wear, 35 (1975) 63 - 67 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

SUBSURFACE

EFFECTS DURING SLIDING WEAR

J. A. KIRK and T. D. SWANSON Department of Mechanical M~ry~nd 20742 (U.S.A.)

Engineering,

University

of Maryland,

College Park,

(Received April 8, 197 5)

Summary Carefully sectioned copper wear surfaces were examined in the scanning electron microscope and evaluated in a microhardness tester. The microscopy results showed the presence of subsurface cracks under the wear track and the microhardness results showed the presence of a zone of low microhardness very near the wear surface. Both results support the assumptions of delamination wear as put forth by Suh. Introduction The adhesive theory of wear, as described by Archard [I], postulates the formation of wear particles at the sliding interface of two contacting members. When wear particle formation occurs it is assumed to take place at the contacting asperities, resulting in wear particles which are hemispherical in shape. Rabinowicz [ 21 has amplified Archard’s work and presents an equation relating the volume of transferred wear particles directly to the normal load and distance slid and inversely to the hardness of the wearing material. Although this equation is consistent with most experimental results, it does not account for the basic metallurgy of the sliding members and makes several arbitrary assumptions. Furthermore, Seifert and Westcott [3] have demonstrated that wear particles appear more like thin sheets than either hemispherical or semielliptical fragments. Suh et al. [4 - S] have recently propdsed a del~ination theory to explain wear particle formation at low sliding speeds. This model takes into account the physics and metallurgy of the wear process and is based on dislocation mechanics and the plastic deformation and fracture behavior of metals near a free surface. Suh postulates that load induced dislocations concentrate in a layer just below the free surface. Eventually the dislocations coalesce into voids which then form cracks which can propagate to the free surface. Suh has demonstrated that many of the wear particles are disc-like, as opposed to the hemispherical shape assumed by the adhesion theory. He has further shown that bulk material hardness per se is not the controlling

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factor in wear 171. A major assumption of the delamination theory is the formation of a zone of low dislocation density near the sliding interface. Indications of the existence of this zone would provide additional support for the acceptance of the delamination theory. The purpose of the paper is to verify the formation of subsurface cracks (as predicted by the delamination theory) and demonstrate the existence of a zone of low dislocation density under the wear track. Experimental Frocedure Numerous direct and indirect methods are potentially applicable to verify the existence of a low dislocation density zone. To observe a zone directly techniques such as X-ray diffraction [9], tran~ission electron microscope [ 10, ll] and use of etchants [lo] (to reveal dislocation pits) might be attempted. The X-ray diffraction and TEM approaches require special techniques and considerable sample preparation. Etchants may reveal individual dislocations, but to do so effectively requires a nearly perfect crystal and observation on specific crystallographic planes. On the other hand an indirect approach, such as a microh~dness test, may be used to infer the dislocation density information from the surface hardness. Since it has been firmly established that the generation, motion, and interaction of dislocations determine a material’s strength [lo], it can be assumed that the greater the relative hardness of a specific material the greater the dislocation density. This method was chosen for this paper. A pm on disc wear test apparatus, as described by Rabinowicz [2], was used to generate the wear surfaces for both scanning electron microscopy observation and microhardness testing. An annealed highly pure (99.9%+) sample of copper was used as the disc due to its expected thick zane of low dislocation density [4]. A hard steeI ball bearing (AISI type 52100 hardened to R, = 61) was employed as the pin. The disc was rotated at 26.2 r.p.m. for a contact velocity of 5.90 cm/s. The applied load was 682 g and the test was conducted for approximately 5 min in air (relative humidity 70%) at room temperature. The copper disc was then sectioned normal to the wear surface and samples were taken in directions both parallel and perpendicular to the wear track (directly underneath the center of the wear track). To avoid destroying the subsurface variations in dislocation density an Agietron spark cutter (electron discharge machine) was used to section the copper disc. After the disc was sectioned with the spark cutter it was then lightly polished to a 0.3 micron metallographic finish and examined in the SEM and/or tested in a Rockwell MO Tukon microhardness tester. All microh~dness tests employed a Knoop indenter with a 5 g load. The long diagonal of the indenter was oriented parallel to the wear track and a series of penetrations were made parallel to and progressively away from the bottom of the wear track. A-schematic diagram of the sectioned sample is

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shown in Fig. 1 and a scanning electron micrograph of the sectioned copper surface is shown in Fig. 2. The length marks shown on the micrograph are micron markers. WEAR

TRACK

$REE

SURFACE COPPER

SAMPLE

Fig. 1. Schematic diagram of sectioned sample.

Results and Discussion Figure 3 shows the results of Knoop penetration hardness number (KHN) versus distance from the bottom, of the wear track. Note that there are several hardness numbers at approximately the same depth below the wear track. These indentations were obtained at different horizontal locations underneath the wear track (as shown in Fig. 2). Although the data points are scattered, they do indicate a decrease in hardness in the proximity of the wear track. At a distance of 10 microns under the track, the average hardness is 125% of a reference hardness value taken 250 microns below the track. This is opposed to a hardness of 160% the reference value at 80 microns depth. It is interesting to note that the hardness of the soft zone is not much greater than the hardness of the undisturbed annealed sample. It is believed that the soft zone is the result of a relatively low dislocation density and that the harder zone beneath it is a result of work hardening (i.e., dislocation pile-up). In general the accuracy of the Knoop hardness number is limited by edge effects when indentations approach the free surface [12]. For readings taken for this paper all indentations had symmetrical short diagonals which were less than 3 microns in length, and indentations were taken at least 8 microns from the free surface. By exercising care in placing indentations near the surface the softening indicated in Fig. 2 is not considered an edge effect. It is also important to note that the Knoop hardness numbers are

Fig. 2. Micrograph of sectioned copper specimen showing indentation marks.

l

800 I

l :. . /. . . ”

. IO

LOAD=5

GM.

REF: KHN OF 705 0 0=250 20

30

60

70

80

Fig. 3. Penetration hardness us. distance under wear track.

significantly higher than those normally expected for annealed copper. This is a normal result of microhardness testing at very small loads [12] . Figure 4 shows a scanning electron micrograph of a section of the copper sample perpendicular to the wear track (the slider would move over this section into the plane of the micrograph). Note there is definite subsurface cracking approximately 50 microns beneath the free surface. Other micrographs taken on sections parallel to the wear track showed similar subsurface cracks. Conclusions Under conditions of light load and low sliding velocity a thin zone of low microhardness (low dislocation density) appears to form very near the wear surface of a copper sample. Microhardness readings taken from 80 microns beneath the wear track up to 10 microns from the surface of the track, show a continually decreasing microhardness as the bottom of the wear track is approached. These results are in agreement with the delamination theory of wear as discussed by Suh.

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Fig. 4. Subsurface cracks in the copper s~pie-viewed (slider motion out of the plane of the micrograph).

perpendicular to the slide direction

Scanning electron micrographs of sections perpendicular and parallel to the wear track (of the copper specimen) have shown the presence of subsurface cracks and voids, also as predicted by the delamination theory of wear. Acknowledgements The authors would like to thank the University of Maryland Center of Materials Research for providing scanning electron microscopy time for this project. References

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7 8 9 10 11 12

J. F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys., 24 (1953) 981 - 988. E. Rabinowicz, Friction and Wear of Materials, Wiley, London, 1966. W. W. Seifert and V. C. Westcott, A method for the study of wear particles in lubricating oil, Wear, 21(1972) 27 - 42. N. P. Suh, The delamination theory of wear, Wear, 25 (1973) 111 - 124. N. P. Suh, S. Jahanmir, E. P. Abrahamson, II and A. P. L. Turner, Further Investigation of the delamination theory of wear, J. Lubric. Technol., 96 F (1974) 631- 637. N. P. Suh, S. Jahanmir, D. A. Colling and E. P. Abrahamson II, The delamination theory for wear of metals sliding at low speeds, Proc. 2nd N. Amer. Me~lworking Research Conf., 1974, pp. 117 - 127. N. P, Suh, S. Jahanmir and E. P. Abrahamson II, Microscopic observations of the wear sheet formation by delamination, Wear, 28 (1974) 235 - 249. N. P. Suh, E. P. Abrahamson II, S. Jahanmir and D. A. Colling, Failure by delamination during wear, Proc. Scanning Electron Microscopy Conf., 1974, 889 - 894. R. W. Armstrong and C. Wu, X-Ray diffraction microscopy, Tools and Techniques for Microstructural Analysis, Plenum Press, New York, 1973. J. P. Hirth and J. Loth, Theory of Dislocations, McGraw-Hill, New York, 1968. P. R. Swann, C. J. Humphreys and M. J. Goringe (eds), High Voltage Electron Microscopy (Proc. 3rd International Conf.), Academic Press, New York, 1974. V. E. Lysaght and A. DeBellis, Hardness Testing Handbook, American Chain and Cable Co., 1969.