This paper researched the microstructure and contact fatigue strength of Ni-P and Ni-Co alloy coating on the tooth surface of gears made from 45 carbon steel. With the help of Electron Microscope （EM） and Optical Microscope （OM）

several studies on the alloying coating

such as surface shape

subsuper ficical metellographic structure

interface structure of longitudinal section

surface roughness

binding strength between alloy coating and matrix metal

element analysis of coating

microhardness and shape characteristic of fatigue pitting hole were carried out. The experimental results are analysed and conclusion is as follows: the contact fatigue life and strength of the test piece made from 45 carbon steel and treated with alloy coating were increased remarkablely than that of the test piece which have no alloy coating

their contact fatigue strength can compair with that of the test piece made from 40Cr steel with hardening and tempering.