WEAR AND LUBRICATION 



(a) Normal illumination. 



Fig. 1. Surface of a worn gear tooth. (After Scott') 50X 



(b) Interference micrograph showing rippled na- 

 ture of the surface. 



microscope. A particular surface feature may 

 be emphasized by a specific form of illumina- 

 tion. Some surface irregularities may be re- 

 vealed more easily by dark field or oblique 

 illumination, the irregularities which diffract 

 the Hght appearing light on a dark back- 

 ground. Phase-contrast microscopes may be 

 used to advantage to examine wear damage 

 and surface deformation as phase contrast is 

 very sensitive to changes in surface levels 

 and can reveal minute slip lines and fine 

 surface irregularities hardly visible or quite 

 invisible under normal illiunination (Fig. 2). 

 The use of polarized light can supplement 

 information obtained with phase-contrast 

 and interference techniques. 



In order to examine microscopically se- 

 lected surface areas of large, unwieldy speci- 

 mens or parts of machines without disman- 

 tling the machine or seriously interrupting 

 its use, replica techniques may be used to 

 advantage (1). Replicas may be rendered 

 more highly reflecting by coating in vacuo 

 with a metal such as aluminum. 



To study both surface and sub-surface 

 changes brought about by rubbing, sliding, 

 rolling or corrosive action it is usual to cut a 



cross-section for microscopic examination ; if 

 the specimen is sectioned at an acute angle 

 to the plane of the surface, a further mag- 

 nification factor is obtained of dimensions 

 perpendicular to the original surface. Taper- 

 sectioning (Moore (3)), carried out by grind- 

 ing at a predetermined small angle to the 

 plane of the surface following suitable plating 

 and mounting of the specimen, is useful in 

 studying changes of surface profile and sub- 

 surface metallographic changes (1, 4, 5). 



To those engaged in a detailed study of 

 wear and the changes caused by rubbing con- 

 tact, knowledge of the physical properties of 

 individual crystals, aggregates and surface 

 films, etc., is essential, and micro-hardness 

 testing technique (Taylor (6)) may be used 

 in conjunction with microscopic and metal- 

 lographic examination. It is often important 

 to determine the extent of work hardening on 

 a surface, the depth and hardness of surface 

 films, as well as to elucidate metallographic 

 changes produced by the wear process (Fig. 



3). 



The success of microscopic and metal- 

 lographic investigations of surface and sub- 

 surface changes depends largely on the 



309 



