The role of bulk nucleation in the formation of crystalline cordierite coatings produced by air plasma spraying

Near stoichiometric cordierite (2MgO–2Al2O3–5SiO2) glasses doped with 6.3, 8.3 and 10.1 weight percent (wt.%) TiO2 were plasma sprayed onto SiO2-based refractory concrete substrates. Substrate temperatures were controlled such that the maximum surface temperature exceeded the crystallization temperature during plasma spraying. Only the cordierite powders containing 8.3 and 10.1 wt.% TiO2 produced crystalline coatings. For compositions with less than 8 wt.% TiO2 the crystallization is believed to be controlled by surface nucleation at imperfections. Surface nucleation became more difficult with increasing preheat temperature and is related to improved wetting between coating splats. Bulk nucleation-controlled crystallization was observed at compositions above 8 wt.% TiO2 and the presence of Al2TiO5 was detected in these compositions.