The XRD results indicated that in both alumina coatings, α-Al2O3, γ-Al2O3 and amorphous phase were the predominant phases(Fig. 1a–b, Table 3). In the electro-melted Al2O3 reference refractory used as reference, α-Al2O3 was the main phase (Fig. 1c, Table 3). In the uncoated silico-aluminous refractory used as substrate, mullite and silica phases were prevalent (Fig. 1d, Table 3). It is important to notice that after the spraying process for both Al2O3 oatings, the α-Al2O3 from the powder feedstock became mainly in γ-Al2O3 in the as-sprayed coatings. This behavior is due to the initial features of feedstock powders (Table 2) and the spraying parameters (Table 1) as it was previously mentioned by other authors . From statistical p-values it was established that in Al2O3 coatings the quantity of amorphous phase, α-Al2O3and γ-Al2O3 has a significant change in samples tribologically tested at 1000 °C (p-values < 0.05). For its part, the mullite and silica phases percentage in the uncoated silico-aluminous refractory change significantly in the specimens tested from 750 °C. Being of all these, more relevant the increasing of α-Al2O3, from the decreasing ofγ-Al2O3and amorphous phases produced in alumina coatings tested at 1000 °C.
Table 1 Plasma spraying parameters
Table 2 Main physical-chemical features of the feedstock powders
Table 3 Crystallographic phases detected before and after the wear tests for all materials evaluated.
Fig. 4. XRD patterns at 25, 500, 750 and 1000 °C of: a) Al2O3 coating from the agglomerated powder, b) Al2O3coating from the fused & crushed powder, c) electro-melted Al2O3 refractory used as reference and d) pressed & sintered silico-aluminous refractory used as substrate.
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