Nano Al2O3 +TiO2 technology of magnesium surface plasma spraying

97% Al2O3+3%TiO2 nano-ceramic layer was sprayed on the surface of AZ91D magnesium alloy by plasma spraying. The ceramic particle size was 150nm and 120nm, and the coating thickness was 50μm ~ 100μm. The thermal efficiency ηQ was about 65%.  The surface of the sample is preheated to 100℃ before spraying, the spraying distance is 60mm ~ 80mm, the plasma flame flow axis and the workpiece surface are 45° ~ 60° Angle, the flow rate of N2 of the powder feeding gas is 0.5m3/h, and the powder feeding rate is 1kg/h ~ 1.5kg /h.  After coating, hardness test, SEM (scanning electron microscopy) and XRD (X-ray diffraction) analysis of coating layer, bonding strength and corrosion resistance test of coating and substrate were carried out. 

Microstructure analysis

The distribution of powdered particles of nano-ceramics is quite uniform, but there are still agglomerated particles.  97% of the powder was composed of α-Al2O3 and 3% rutile - like TiO2.  The coating is a lamellar layer, due to the adhesion of high temperature particles and solidified particles, so that the coating is composed of many particles, the surface is quite rough.  

Due to the low melting point of AZ91D alloy matrix, part of the matrix surface will melt when impacted by high-temperature plasma ceramic particles.  XRD and SEM analysis showed that there are columnar γ-Al2O3 and equiaxed nano-α-Al2O3 particles in the coating.  Because γ-Al2O3 grows along the direction of heat flow during the formation process, the unmelted α-Al2O3 particles are caused by cooling during the plasma thermal spraying process.  This structure is beneficial to the improvement of the mechanical properties of the sprayed nanometer layer. There is no TiO2 in the XRD spectrum, because the contact area of alumina and titanium oxide is large and completely solid dissolved during thermal spraying. Due to their mutual solid solution, the columnar interlayer bonding force is enhanced, which is conducive to the lifting of the coating strength. 

It is found that a small amount of MgO exists in XRD spectra, which is formed by the interface reaction of magnesium alloy surface impacted by high temperature ceramic particles during thermal spraying.  The formation of MgO can effectively restrain the growth of Al2O3 grains, and the refined Al2O3 is beneficial to the improvement of coating strength. Meanwhile, it can also strengthen the lubrication between ceramic and magnesium alloy matrix, and adjust the distribution of stress in the interface.  

The coating performance

Hardness:

After plasma spraying 97% Al2O3+3%TiO2 nano-ceramic on AZ91D alloy, the hardness of ceramic layer is HV950 ~ HV 980, which is higher than that of conventional ceramic layer HV750 ~ HV800.  Because there are a lot of equiaxed nanometer α-Al2O3 in the coating.  

Binding strength:

 According to ASTM C633, the bonding strength of the coating is 19MPa ~ 22.5MPa, while that of the conventional plasma sprayed ceramic coating is only 16MPa. The former is 18.75% ~ 40.63% higher than that of the latter.  

Corrosion resistance of coatings:

According to the standard of GB6458-1986, the scientists sprayed the nano-ceramic coating on the surface by plasma, and the sealed samples were subjected to the neutral salt spray test for 72h. The surface was intact without rust spots.