Transformation of plasma-sprayed zirconia coating from monoclinic to cubic structure by liquid manganese penetration

Abstract

Plasma sprayed ceramic coatings have been used to give the surfaces of metallic products resistance to high temperature oxidation, corrosion and wear. The coatings, however, contain defects such as voids which are not filled with the sprayed powder particles, microcracks formed owing to the thermal shrinkage of each of the flattened particles composing the coating and non-bonded portions between the flattened particles. These defects are often connected to each other to form connected pores through which corrosive gases and liquids in the environment can reach the metallic substrate, leading to deterioration of the coating functions. Ohmori et al. [1] have shown that connected pores in plasma sprayed alumina coatings could be eliminated by a manganese penetration treatment at a high temperature. In the present work, this penetration treatment is applied to a plasma sprayed zirconia coating and the effect of the manganese on the microstructure and the crystal structure of the coating is examined. Zirconia powder with particle sizes from 10 μm to 44 μm and 99.8% purity was low pressure plasma sprayed onto a substrate to a thickness of about 100 μm. The substrate was a SS400 medium carbon steel plate with a thickness of 3 mm. The plasma spraying conditions are listed in Table I. A manganese plate of 99.99% purity and weighing about 3–5 g was put on the surface of the zirconia coating and this assembly was held at 1573 K for 10.8 ks in a vacuum of 1.33 3 10 Pa. To minimize the loss of manganese owing to its evaporation at this temperature, a zirconia cover was placed over the manganese as shown in Fig. 1. Fig. 2a shows a scanning electron micrograph of the cross-section of the as-sprayed coating and Fig. 2b shows that of the coating heat-treated without manganese at 1573 K for 10.3 ks in a vacuum of 1.33 3 10 Pa. The as-sprayed coating contained many pores and cracks. This structural feature was not altered significantly by the heat treatment at 1573 K. When the temperature of the coating in contact with the manganese plate was raised to 1573 K, the manganese melted and readily penetrated the coating through the connected pores. The manganese penetration drastically changed the structure of the coating, as shown in Fig. 2c. Almost all of the large pores and cracks disappeared and the zirconia phase granulated, which indicated that liquid phase sintering occurred in the coating. As can be seen in Fig. 2d, which is the characteristic X-ray image of manganese taken from the same position as that in Fig. 2c, the manganese penetrated throughout the coating and manganese-rich precipitates formed in the substrate. The easy penetration implies that sufficient wetting of the zirconia occurred with the liquid manganese. The manganese concentration was high in the large interstices among the zirconia granules. Manganese was also detected within each of the zirconia granules. The spray powder was exclusively composed of monoclinic zirconia as shown in Fig. 3a. During spraying, the powder particles had been rapidly heated in the plasma flame to extremely high temperatures, then rapidly cooled on impinging onto the substrate or onto the already-formed deposit. Despite this thermal history of the zirconia particles, the constituent of the as-sprayed coating was the same as that of the spray powder, as shown in Fig. 3b. No change occurred in the crystal structure of zirconia after holding the as-sprayed coating at