In the present work, chromia and alumina coatings were produced by atmospheric plasma spray process. Phase composition, microstructure and mechanical properties of chromia and alumina composite coatings on carbon steel substrates were studied and compared to individual Cr2O3 and Al2O3 coatings. Morphology and microstructure of coatings were characterized using scanning electron microscopy equipped by EDS. The porosity of the plasma-sprayed coatings was measured by the Archimedes water immersion technique and image analysis method. X-ray diffraction was employed to identify phase composition of coatings. Also some mechanical properties such as bonding strength, Vickers microhardness and resistance to crack propagation of coatings were measured. The results showed different phases for Al2O3 in different coatings while Cr2O3 remained in eskolaite phase in all of the coatings. Addition of 75 wt% Cr2O3 prevented formation of metastable and undesirable γ-Al2O3 phase. Alumina and rich-alumina coatings showed denser microstructure and higher flattening degree of splats as well as lower surface roughness. Bonding strength of individual Cr2O3 and Al2O3 coatings was higher than composite one. Vickers microhardness increased with addition of Cr2O3. Alumina improved spray ability and crack propagation resistant of chromia.

Microstructure, phase composition and mechanical properties of plasma sprayed Al2O3, Cr2O3 and Cr2O3-Al2O3 composite coatings

Microstructure and mechanical properties of plasma sprayed Al 2 O 3 -YSZ composite coatings

Al 2 O 3 non-gradient ceramic coating (marked as 1# coating) and NiCrAlY/Al 2 O 3 gradient ceramic coating (marked as 2# coating) were prepared with the plasma spraying and slurry spraying methods, respectively. The coating specimens were characterized by scanning electron microscopy and X-ray diffractometry. The high temperature oxidation kinetics curves of the coatings were examined; the bonding strength between the coatings and the metal substrates was studied, and analyses were conducted on the thermal shock resistance and the failure mechanism of the coatings. The results show that spraying of Al 2 O 3 ceramic slurries can effectively fill the cracks generated during the process of plasma spraying of NiCrAlY coatings, and metallurgical bonds form between the coatings and the metal substrates. α-Al 2 O 3 and rutile TiO 2 are the main crystalline phase structures of the temperature resistant ceramic coatings. The oxidation rate constant of 2# coating was 1.323 × 10 − 4  mg 2  cm − 4  h − 1 , which shows the best resistance to high temperature oxidation. The effect of high-temperature melting of the Al metal in the bonding layer was found to be beneficial to the bonding strength. Under the same temperature conditions, 2# coating had a higher bonding strength than 1# coating, and when the oxidation time was 96 h, its bonding strength reached up to 26.9 MPa. The thermal expansion coefficient of the NiCrAlY bonding coating fell between those of the metal substrate and ceramic top coating, thus reducing the thermal expansion coefficient difference between the two and decreasing the rate of propagation of the cracks to improve the thermal shock resistance of the coating. An integrated study indicated that 2# coating had a better heat protective performance for the metal substrates.

S. Sathish

Papers

Comparative study on corrosion behavior of plasma sprayed Al2O3, ZrO2, Al2O3/ZrO2 and ZrO2/Al2O3 coatings