Study On Preparation Processes And Properties Of Gradient Coatings On 40Cr Surface

Gear,as an important mechanical transmission part,in addition to high strength,must have excellent surface properties,such as wear resistance,corrosion resistance,fatigue resistance and so on.Our previous studies have shown that chromizing is an effective method to improve the surface properties of gear materials.However,after chromizing,the existence of carbon-poor layer（CP-layer）between the coating and substrate is inevitable,which can cause the spalling of chromizing coating in the work.In this paper,it is hoped to obtain excellent coating,and solve the problem of carbon-poor layer at the same time.Thus,boron-chromium alloy coating and silicon-chromium alloy coating were prepared on the AISI 5140 steels.The main works are as follows:（1）In this paper,surface modification of gear materials is reviewed,the early study results of the research group were summarized,a comprehensive research on the experiment and theory of thermo-diffusion chromizing is conducted,and how to solve the problem of CP-layer caused by chromizing is systematically analyzed;（2）A two-step pack cementation process（pre-boronizing and then chromizing）was employed to prepare the B-Cr duplex-alloyed coating on the steel.After the first step of pre-boronizing（PB sample）,box-type furnace chromizing（BC-1 sample）and induction heating chromizing（BC-2 sample）were carried out,respectively.;（3）For the B-Cr duplex-alloyed coating,the evolution of microstructure and properties and the growth kinetics of the pre-boronizing+induction heating chromizing coating with the increase of the second chromizing time were also investigated;（4）Two types of Si-Cr coatings（SiCr-1 and SiCr-2 samples）were fabricated on AISI 5140 steels by pack-cementation via different Si feedstocks.Through studying,the primary conclusions are as follows:For the B-Cr coating,the heating mode of the second step of chromizing has a significant effect on the phase composition and microstructure of the B-Cr coating.The efficiency of induction heating is higher than that of the box furnace heating,resulting in a thicker,denser,flatter surface,and B-Cr coating with fully reacted B and Cr elements.The wear and corrosion resistance of the steel is found to be significantly improved by the formation of effective B-Cr coating.The formation mechanisms and properties of the two duplex-alloyed coatings are investigated and discussed.As the second-step chromizing time increases,the net-like Fe₂B and rod-like CrFeB phases in the coating gradually disappear,and finally completely transform into the block-like Cr₂B and Cr_xC_y(Cr₇C₃ and Cr₂₃C₆)phases.The growth kinetics analysis shows that interface reaction dominates the coating growth during the early stage of chromizing,while atomic diffusion gradually controls the coating growth at the later stage.The evolution mechanism of the B-Cr duplex-alloyed coating was also discussed.For the Si-Cr coatings,both coatings mainly consist ofα-Fe（with Si and Cr in concentration）,Fe₃Si,CrFe₈Si,and CrC.However,there are apparent differences in the coating morphology,element content,microhardness,and wear resistance,which is owing to the difference in the coating growth mechanism.The coating growth of SiCr-1 sample using Si powders as Si feedstock is controlled by the interdiffusion mechanism of the substrate and coating elements,while the coating growth of SiCr-2sample using ferrosilicon powders as Si feedstock is dominated by the reaction deposition mechanism.For both samples,the presence of micropores in the coatings is observed and their formation is mostly due to the non-equilibrium diffusion induced Kirkendall effect.A new gradient growth mode of micropores based on the Kirkendall effect was proposed.