Process Design and Monitoring for Plasma Sprayed Abradable Coatings
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Abradable coatings in compressor and high-pressure stages of gas turbines must provide specific hardness and porosity values to achieve an optimal cut-in of the blade tips. A fractional factorial experimental plan was designed to investigate the influence of the plasma spraying parameters argon flow rate, current, spraying distance and powder feed rate on these properties of magnesia spinel. Based on the results, magnesia spinel coatings with low (~400 HV0.5), medium (~600 HV0.5) and high hardness (~800 HV0.5) could be reliably manufactured. Further incursion rig tests confirmed the dependence of the rub-in behavior and abradability on the coating characteristics and process parameters, respectively. Process monitoring was also applied during plasma spraying of magnesia spinel abradables on batches of turbine components. The recorded particle characteristics and coating properties showed a good reproducibility of the spraying process.