Mullite Plasma Spraying for In Situ Repair of Cracks in Mullite Refractories: Simultaneous Optimization of Porosity and
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A. Schrijnemakers, B. G. Francq, R. Cloots, B. Vertruyen, and F. Boschini (Submitted November 26, 2012; in revised form May 21, 2013) We report a laboratory-scale study about the suitability of the plasma spraying process for ‘‘in situ’’ repair of cracks in mullite refractories of industrial furnaces. The ‘‘design of experiments’’ approach is used to investigate how the coating porosity and thickness are influenced by six experimental parameters. Arc current, secondary gas (H2) flow rate, and stand-off distance are the most significant parameters for both responses. Several interaction terms also affect significantly the thickness response. The validity of the model equations is discussed both from a statistical point of view and regarding the physical credibility of the main model terms. Additional experiments confirm that the measured properties lie into the prediction intervals provided by the model. Using a set of parameters optimized for minimal porosity and high thickness (relevant for the crack repair application), coatings with 6% porosity and 1070 lm thickness can be prepared reproducibly.
Keywords
design of experiments, furnace repair, mullite, plasma spraying
1. Introduction Thermal spraying techniques are well-known production routes for thermally and chemically resistant coatings (Ref 1-4). Compared to other coating processes, thermal spraying techniques allow the deposition of very thick coatings (up to 1 mm) of oxide and non-oxide ceramics in relatively short production times. During the plasma spraying process (Ref 5), solid particles are injected into a plasma jet created either by a DC arc or by a RF field. The feedstock material is melted in the high temperature region of the plasma and the molten particles are accelerated until impact on the substrate. Then, rapid solidification of impacted droplets and deposit buildup occur. In molten glass processing furnaces, molten glass infiltrates the pores of the sintered ceramics and may dissolve the refractories until failure occurs. ‘‘In situ’’ repair of A. Schrijnemakers, R. Cloots, B. Vertruyen, and F. Boschini, LCIS/ GreenMat, Department of Chemistry B6, University of Lie`ge, 4000 Lie`ge, Belgium; B.G. Francq, ISBA, Institute of Statistics, Biostatistics and Actuarial Sciences, University of Louvainla-Neuve, 1348 Louvain, Belgium; and F. Boschini, APTIS, Physic Institute B5, University of Lie`ge, 4000 Lie`ge, Belgium. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
damaged refractories by plasma spraying would offer the possibility to avoid or delay expensive and time-consuming furnace dismantling operations. Early-stage damages often consist in cracks. It is therefore necessary to optimize the plasma spraying process for localized deposits. The coating quality depends on various process parameters (Ref 6) and statistical methods can be useful for the optimization of coatings properties (Ref 7, 8). Statistical design of experiments is a mathematical method to establish the relationships between the properties of t
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