332 



D. SCOTT AND H. M. SCOTT 



Fig. I. Initiation of Wear Damage on Soft Steel. An area 

 of running track near the start of a test run. Tiie material 

 of the cylinder is E.N. 8 steel of hardness aproximately 

 200 V.P.N. Plastic deformation of this fairly ductile material 

 is evident, also small areas of surface damage produced 

 by the displacement of material. Magnification 7500. 



Fig. 2. Initiation of Wear Damage on Hard Steel. Initial 

 damage to the surface of a case hardened E. N.36 steel 

 cylinder of hardness over 800 V.P.N. There is little plastic 

 deformation of this hard material of low ductility and 

 initial damage appears to be by displacement of material 

 and cracking following overstressing of the surface. 

 Magnification 6000. 



Visual damage by optical microscopic examina- 

 tion after only one revolution of the wear machine 

 appeared to be confined to circumferential scratches, 

 but from the electron micrograph it appeared that, 

 within each individual scratch, plastic flow had oc- 

 curred over a narrow track and a step-wise structure 

 had been formed suggesting a stick-slip method of 

 formation. 



It is not to be expected that the complex mecha- 

 nism of wear should be easily elucidated for the very 

 process itself eliminates its own initial stages as 

 cumulative action develops towards catastrophic 

 failure. For the study of factors initiating such a 

 failure it is desirable that a single traverse be made 

 of a surface so that the development of surface 

 failure may be followed. For this purpose a special 

 crossed-cylinder machine (1) has been developed at 

 the Mechanical Engineering Research Laboratory 

 in which one cylinder is rotated and a mating cylin- 

 der, at right angles, so traversed that the zone of 

 contact moves along the surface of both cylinders 

 and fresh contact surfaces are continually being 

 presented. The helical track round the cylinder 

 reveals how the damage to the surface builds up 

 with increase of load. Using this machine and 

 cylinders with a fine surface finish of average rough- 

 ness 1 // inch C.L.A. some interesting electron micro- 

 graphs have been obtained of the initial stages of 

 the wear process when using a conventional lubri- 

 cant. The electron micrographs 1-3 are reversed prints 

 of shadowed single-stage formvar replicas. 



Another important wear problem is fretting corro- 

 sion, the term given to the surface damage that 



results when there is slight relative oscillatory motion 

 between solid surfaces in contact. The damage mani- 

 fests itself by severe pitting of the surfaces and the 

 generation of considerable amounts of oxidised deb- 

 ris. In conjunction with the research on fretting 

 (8) the electron microscope has been used to study 

 the initial stages of surface damage (fig. 4). 



Formvar replicas have been taken from taper 

 sections (7) prepared for metallographic investiga- 

 tion of rubbing surfaces. E.M. examination of these 

 shadowed replicas has helped considerably in eluci- 

 dating the metallographic changes in the immediate 

 sub-surface areas of contact. Interesting electron 

 micrographs have also been obtained of the deforma- 

 tion of crystals in the sub-surface area of contacting 

 points of rubbing surfaces. 



The electron microscope has been used for the 

 examination of debris from fretting corrosion and 

 wear tests, since test conditions giving reasonably 

 low rates of wear require the high magnification of 

 the electron microscope for satisfactory examina- 

 tion. 



When debris particles are available in the dry state 

 they can be brushed directly on to a prepared formvar 

 film supported on a copper microscope grid and 

 examined in the microscope. Shadowing improves 

 definition and enables a better interpretation to be 

 made of the shape of the particles. 



When a lubricant is used in a test it is necessary 

 to separate the debris froin the lubricant. This is 

 done by first centrifuging the sample with a solvent 

 for the lubricant. The solvent is replaced during 





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Fig. 3. Effect of E.P. Lubricant on the Initiation of Wear 

 Damage. The build up of a surface film when using extreme 

 pressure additives in the lubricant which seem to form a 

 contaminated surface film in the area of contact. This 

 film may be expected to be of low shear strength and 

 may prevent metallic contact and seizure. The material of 

 the cylinder is again case-hardened E.N. 36 steel. 

 Magnification 15,000. 



Fig. 4. Initial Stages of Fretting Corrosion. The surface 

 of a highly polished tool steel specimen after 15 seconds 

 fretting (750 oscillations under a load of 10 kg and amplitude 

 of slip 10-3 inches). Overstressing of the surface has mani- 

 fested itself by plastic deformation and displacement of surface 

 material; the mechanism being similar to that observed 

 during the initial stages of wear of hard steel cylinders. 

 Magnification 7000. 



