Original Report
Some Observations of Metals under Repeated Sliding
HiroshiFURUICHI HajimeYOSHIDA ToshiharuNAITO
（Received August 31，1976） Abstract The structual changes of metals under unidirectionally repeated sliding（repeated friction） was studied． It was found that the failure in this case is quite the same neither as an ordinary wear nor as a fatigue failure including an ordinary fretting fatigue． 1． Introduction It is often observed that many machines fracture because of a repeated sliding． The mechanism of it is， however， not so well known． The main purpose of this paper is to present a special type of a failure of metals under a repeated sliding， which， as for the initiation of a crack， has a resemblance with a fatigue failure， is different not only from a wear but also from a fatigue failure as a whole， comprising a fretting fatigue． 2． Experimental The specimens were electropolished 65／35 brass
plate（Table 1），24×7×3mm3， which had been
kept at 600°C for l hour and then cooled slowly， in carbon powders． These specimens were given repeated sliding in the longitudinal direction with a rounded brass shoe of radius 2 mm， which was treated in the same way as the specimens． As far as the observations through an optical microscope were made， all the crystals were in a Table l Chemical composition（wt％）of the specimen and the shoe state ofα一brass． ／ The sliding speed was 42 mm／sec， the cycle was 15rpm， the sliding distance was 18mm and the load per unit length to the transverse direction was 24 kg／mm． The scheme of the repeated sliding machine is shown in Fig．1． 3． Results and I）iscu88三〇ns Figure 2 shows the side of a specimen before the repeated sliding． Figure 3 shows the distribu・ tion of dislocation etch pits on the side of a speci− men， after 25， OOO cycles of the sliding． It is seen that， near the slid surface， there is aregion where grain boundaries have disappeared
and much deeply electropolished and etched in the process of revealing the pits． The thickness of this①
＼ ／Cu
65．1Fe
0．003Pb
0．0014Ni
0．0002Zn
Bal． ざ ▽”い 2 3 ＊post graduate student1：Shoe 2：Specimen 3：Spring
Fig．1 Scheme of the repeated slid・ ing machine一14一
Some Observations of Metals under Repeated Sliding
Table 2 Hardness
Specimen
Annealed
Cold hammering
Cold rolling Continuous frictlon Repeated sliding＊ （25，000cycles）Hardness（VHN）
62．9土0．6 188±3．7 166±2．6 133±0．7 283±8．9 Fig．2 Brass specimen before the repeated sliding Fig．3 Distribution of dislocation etch pits on the side of a brass specimen， after 25，000cycles of the repeated sliding region is often more than 50μm． The hardness of this region is abnormally high，compared with that of an ordinary continuous
friction of the same load per unit length and the same speed of the sliding as the repeated sliding and that of a severe cold hammering or of a severe cold rolling（Table 2）． Judging from the result of an electron probe micro analysis， no difference of the chemical composition exists between the hard region and the other region of a specimen． There’ fore， it can be said that the cause of the high hardness is not the difference of the chemical com・ ． position． This region is different from the hard “white layer”， which appears in the case of an ordinary continuous friction and is etched less deeply than the other region of a specimen． This is different from the authors’hard layer1）． Figure 4 shows the back reflection micro−focus ＊Near the slid surface Fig．4 X−ray diffraction pattern of the slid surface of a brass specimen pin hole photograph of this region；the diameter of the slit was 100μm and the camera length was 10mm． Nothing can be seen except highly diffused ． rlngS・ Therefore， it is concluded that this region is non・crystalline or composed of very fine grains．＊＊ The same phenomenon was observed in a specimenwhich was repeatedly slid in the argone with
5％hydrogen atmosphere．
This hard region may act as an obstacle to the growth of cracks， because of the dithculity of the plastic deformation． On the other hand， this hard region can be utilized for surface harden’ng． As shown in the enlarged view of this region， no crack could be observed （Fig．5）． It is seen in Fig．3， below this region， grains were observed and these grains become coarser and less distored ＊＊In cases of non・crystalline metals， the hardness is abnormally high and only very diffused rings can be seen in the diffraction photographs2）．一15一
December 1976
Report of the Faculty of Engineering， Yamanashi UniversityNo．27
Fig．5Micro・
structure of harden・ ed region near the slid surface （25，000cycles） Fig．6 0ptical micrograph showing slip lines and cracks after the repeated sliding（25，000 cycles） with the distance from the slid surface， till they reach the same structure as that before sliding． In some grains， cracks could be seen（Fig．6）． Consequently， cracks form not at the slid surface but below it． It is said that， in the case of a fretting fatigue， cracks originate at the slid surface， independent of the way of the stressing3）． It is generally recognized that pitting occurs in metals which received an ordinary fretting fatigue3）． In the present experiment， however， no pitting was observed． It is confirmed that new grains appeared even below the recrystallization temperature， in the region where slip lines were observed4）． Figure 7 shows one of the examples of slip lines after the repeated sliding． Some of them are ex・ truded or intruded． This phenomenon is very often Fig．7 Scanning electron micrograph showing slip lines （25，000 cycles） Fig．8 Scanning electron micrograph showing extruded or intruded triangles（50，000 cycles） ① Slip lines ／ Cross−slip mechanism Fig． g One of the examples of the mecha− nism of the formation of the extruded or the intruded triangles by cross・slipPing 1：Extruded triangle 2：Intruded triangle一16一
Some Observations of Metals under Repeated Sliding observed in the case of a fatigue failure and is known to be one of the causes of the initiation of afatigue crack5）． Extruded or intruded triangles are shown in Fig．8， this phenomenon can be ex・ plained by modifying the mechanism of the extrusion or the intrusion of a fatigue failure6）（Fig．9）． From the facts mentioned above， it may be said that， the mechanism of the fracture of the repeated sliding is quite the same neither as a wear nor as a fatigue failure inclu（玉ing a fretting fatigue， though as for the initiation of a crack， the mecha・ nism is the same as that of a fatigue failure． 4． Conclusion The structual changes during a repeated sliding of a brass specimen was studied． The specimen was a plate and the shoe was rounded with the radius of 2 mm， to make line contact with the エ speclmen・ The speed of the sliding was 42 mm／sec， the sliding cycle was 15 rpm， the sliding distance was
