A　Rockwell　C　120　degrees　diamond　indenter　with　a　spherical　tip　radius　of　100　mu　m　was　used　to　measure　the　coefficient　of　friction　by　microscratch　test　under　different　normal　loads.　The　measured　friction　coefficient　was　found　to　increase　with　normal　load,　which　was　rationalised　by　a　geometrical　intersection　model.　Although　plastic　deformation　increases　with　normal　load,　its　contribution　into　the　total　deformation　becomes　smaller　with　the　increase　in　normal　load.　Elastic　deformation　predominates　in　the　total　deformation　under　large　normal　loads.　It　is　the　adhesion　shear　stress　over　the　contact　area　that　causes　plastic　deformation.　Lateral　force　was　found　to　be　proportional　to　penetration　depth,　especially　under　large　normal　loads　when　elastic　deformation　predominated　the　deformation,　with　the　proportionality　representing　deformation　or　shearing　resistant　toughness.