Amorphous AlPO4 coating formed on titanium alloy for high temperature oxidation protection: Oxidation kinetics and microstructure

• Amorphous AlPO4 based coatings on titanium alloy were prepared by dipping method.
• The coatings used amorphous aluminum phosphate as binder.
• The coatings improved oxidation resistance of titanium alloy at 600 and 800 °C.
• The infrared emissivity value (> 0.8) of coatings was much higher than titanium alloy.

Amorphous AlPO4 based ceramic coatings with 13 μm thick were prepared on Ti–6Al–2Zr–1Mo–1V titanium alloy surface using amorphous aluminum phosphate as a binder and Al2O3/Cr2O3 based mixing particles as the fillers. The phase composition and microstructure of the coated samples before and after exposing oxidation test were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). After exposing oxidation at 600 °C for 100 h, no oxide scales were found either at coating/substrate interface or on coating surface, exhibiting a significantly lower weight gain (0.07 mg/cm2) compared with the bare titanium (0.54 mg/cm2). After exposing oxidation at 800 °C, the TiO2 dominated oxide scales began to form on coating surface after oxidation for 5 h and at coating/substrate interface after oxidation for 60 h, finally got weight gain of 1.4 mg/cm2 after oxidation for 100 h. The coating had a higher infrared emissivity value (> 0.8) than titanium substrate (0.15–0.3) in the wide wavelength range of 3–20 μm, which was attributed to the uniform dispersion of high emissivity Al2O3 and Cr2O3 particles in the amorphous AlPO4 binder matrix.