Abstract
Magnesium alloy has been widely used due to its excellent properties. How to improve its corrosion resistance, hardness, wear resistance and thermal shock resistance is very important. These problems can be solved by plasma sprayed ceramic coating. The plasma sprayed 80wt% Al2O3+20wt% ZrO2(AZ20), 50 wt % Al2O3+50 wt % ZrO2(AZ50),20 wt% Al2O3+80 wt% ZrO2(AZ80) composite coatings and NiCoCrAlY-AZ50 gradient coating were deposited on AZ91D magnesium alloy in this paper. NiCoCrAlY alloy was used as bond coating. The microstructure and properties of these coatings were investigated by means of SEM、EDS、XRD and so on. The best process parameters of AZ50 composite coating were optimized by orthogonal experimental design.
The results show that the key parameters on bonding strength of AZ50 composite coating are ranked as the sequence: power, powder feeding rate, primary gas flow rate and spray distance. The optimized process parameters of bonding strength were: power 40kw,powder feeding rate 13g/min, spray distance 70mm, primary gas flow rate 55psi, the corresponding bonding strength was as high as 32.56MPa.The key parameters on deposition efficiency of AZ50 composite coating are ranked as the sequence: power, spray distance, powder feeding rate and primary gas flow rate. The optimized process parameters of deposition efficiency were: power 40kw, powder feeding rate 13g/min, spray distance 70mm, primary gas flow rate 50psi. The bonding strength of Al2O3-ZrO2(AZ) composite coatings decreased with the increase of coating thickness and the content of ZrO2. The tensile fracture of AZ composite coatings occurred at the interface between ceramic coating and bond coating.
The AZ composite coatings (double layer structure) were characterized by a lamellar structure with defects of few pores, microcracks and semi-molten particles. The mechanical combination was the primary bond form of magnesium alloy and coating. The ceramic coatings consisted of α-Al2O3, γ-Al2O3, t-ZrO2, t’-ZrO2 and c-ZrO2.
The microcracks, pores and loose tissue of AZ composite coatings increased and the diffraction intensity ofα-Al2O3，γ-Al2O3 became weaker with the increase of ZrO2 content.
The microhardness of AZ composite coating (double layer structure) showed a scatter distribution and significantly conformed to Normal and Weibull distribution. The friction coefficient of AZ composite coatings increased with increase of linear speed, while decreased with the increase of load. The wear mechanism of AZ composite coatings was adhesive wear. The thermal shock failure of AZ composite coatings includes three stages:crack forming, crack extension and coating spallation. The spallation of coating was related with thermal stress and oxidation of bond coating. The bonding strength, hardness, wear resistance and corrosion resistance decreased while the thermal shock resistance increased with the increase of ZrO2 content. The plasma sprayed AZ composite coatings could
effectively improve the hardness, wear resistance and corrosion resistance of AZ91D magnesium alloy. The corrosion resistance of coating was better after sealing treatment.Compared to AZ50 with double layer structure, the corrosion resistance, bonding strength and thermal shock resistance of NiCoCrAlY-AZ50 gradient coating were better.
Keywords: magnesium alloy; plasma spraying; Al2O3; ZrO2; gradient coating; process parameters
本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://en.sunspraying.com/
|
