| The mold is one of the core equipment in the industry,and a large number of components are produced by the mold.However,the average life of the mold manufactured in China is significantly lower than that in the developed countries.In particular,the hot-work dies are usually used in harsh environments and there are several kinds of damages,such as wear,corrosion and fatigue,which greatly shortening their service life.Since the failure of hot-work dies is mostly caused by the surface damages,using the advanced surface engineering technology to modify the surface of the molds becomes the key to improve their service life.In this thesis,the Cr3C2-NiCr and Cr3C2-CoNiCrAl Y coatings were prepared respectively by activated combustion high velocity air fuel(AC-HVAF)spraying on the surface of H13 steel and the spraying parameters were optimized through the orthogonal experiment.The microstructures of coatings were investigated by SEM,EDS and XRD.The basic properties of the coatings such as porosity,bonding strength and microhardness were observed.Moreover,the wear resistance,high temperature oxidation resistance and thermal shock resistance performances of the coatings were studied and compared emphatically.The main conclusions of the thesis are as follows:(1)Orthogonal testing was carried out to optimize the AC-HVAF spraying process of Cr3C2-CoNiCrAlY coating.Results show that the effects of spraying distance,fuel?pressure and powder feed rate on the microhardness of the coating are ranked as follows: fuel? pressure > spraying distance > powder feed rate.And the optimal process parameters appear when the fuel? pressure,the spraying distance and the powder feed rate are 103 psi,32 mm and 20 %,respectively.(2)High quality Cr3C2-NiCr and Cr3C2-CoNiCrAlY coatings with good bonding strength,compact and uniform structure can be fabricated via AC-HVAF technique,which avoids the oxidation and decarburization.Some properties of two coatings were listed as follows: porosities are 0.87 % and 1.93 %,microhardness are 1002.44 HV0.3 and 998.02 HV0.3,fracture toughness are 2.75 MPa·m1/2 and 2.87 MPa·m1/2,respectively.The bonding strengths of both coatings are higher than 70 MPa.(3)The friction and wear testing results show that: the wear rates of H13 steel,Cr3C2-NiCr and Cr3C2-CoNiCrAlY coatings are 16.67×10-6,7.51×10-6,8.01×10-6 mm3/N·m respectively at room temperature and 83.53×10-6,23.78×10-6,19.83×10-6 mm3/N·m respectively at 700 ?.Theses two types of coating show excellent tribological properties at both room temperature and 700 ?.Furthermore,the wear rate of Cr3C2-NiCr coating is slightly lower than that of Cr3C2-CoNiCrAlY coating at room temperature,but higher than that of Cr3C2-CoNiCrAlY coating at 700 ?.Besides,the friction and wear mechanisms of Cr3C2-NiCr and Cr3C2-CoNiCrAlY coatings are fatigue wear at room temperature,while the friction and wear mechanism is abrasive wear and oxidation wear at 700 ?.(4)The oxidation weight gains of Cr3C2-NiCr and Cr3C2-CoNiCrAlY coatings are only 0.87 mg·cm-2 and 0.80 mg·cm-2 after cyclic oxidation at 800 ? for 100 h,which are far less than H13 steel(78.38 mg·cm-2).Both coatings exhibite outstanding high temperature oxidation resistance and the Cr3C2-CoNiCrAlY coating is superior to the former.(5)In the thermal shock testing from room temperature to 700 ?,the number of cycles for Cr3C2-NiCr is significantly lower than that of Cr3C2-CoNiCrAlY coating,therefore,the latter series provide superior thermal shock resistance compared with the former. |
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