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Influence of critical plasma spray parameters on microstructure and properties to alumina

Time:2013-07-09 09:28:34  From:CNKI  Author:Ma Donglin

  Abstract
  Plasma spraying technology is one of novel surface modification process which has been developed rapidly in recent years. The metal or ceramic feed stock powder melt or semi-melt in plasma flame. And then eject onto compounds surface to forming high performance coatings which protect apparatus against wreck environment. The alumina-titania coatings deposited by plasma spraying are applied in various areas such as space, machinery, power,light industry, textile, petrochemical and etc., due to its excellent thermal barrier resistance,insulation property, wear resistance, antifriction property and corrosion resistance property.
  In this research program, the NiCrAl bond coating was deposited on the Q235 steel at first, then two different alumina-titania coatings (nanostructured or conventional) were respectively deposited on it by plasma spraying with APS2000 plasma spraying apparatus (prima gas is Ar, second gas is H2). The optimal plasma spraying parameters was selected at 60V, 600A, 35L/min by orthogonal experiment, and then six CPSP parameters (672、733、825、900、1021、1114 A•V/L•min-1) were designed according to the optimal CPSP parameters.
  The microstructure and phase constitution of coatings fabricated in different CPSP conditions were measured by SEM and XRD. And other mechanical properties such as micro-hardness,thermal shock resistance and abrasion resistance were evaluated also. Additionally, the relation between CPSP and these characteristic, thermal shock failure mechanism and abrasion failure mechanism of coatings were studied.
  The results showed that, the microstructure of conventional and nanostructured coatings surface and cross section were bio-module structure, which divided into partial melting region and full melted region. There were also some pores and cracks existed in coatings surface and cross section. Phase constitution of conventional and nanostructured coating were alpha Al2O3,gamma Al2O3 and rutile TiO2. Coating crystallite size was in nanometer range which confirmed by Scherrer methods from its XRD results. With the increasing of CPSP, the relative contents of alpha Al2O3 decrease and gamma Al2O3 increased. And the relative contend of PM region, porosity and cracks in coatings decreased while the CPSP increased.
  On the contrary, micro-hardness increased as increasing of CPSP.With the increase of CPSP, the thermal shock cyclic times of conventional and nanostructured coatings increased at first and then decrease. And the cyclic times change trend of nanostructured coating was more sensitive than conventional ones with CPSP variation. Coating, deposited in moderated CPSP condition (conventional ones at 825 A•V/L•min-1, nanostructured ones at 900 A•V/L•min-1), possessed the best of thermal shock resistance of all. More over, thermal shock resistance of nanostructured coatings sprayed in optimum CPSP condition was better than conventional ones. Such as stress field induced by heating-cooling circulation and phase transformation during thermal shock experiment,oxygen elements permeated inside NiCrAl bond coting subsequently produced TGO layer,cracks formation around defects were main reason of coatings failure at thermal shock test.
  While spherical pores, partial melted region and nano-size crystalline in coatings can enhance the thermal shock resistance of it.Wear resistance of nanostructured coating was better than conventional one, wear resistance of nanostructured coatings increased as CPSP increasing. As the test temperature elevated, the wear resistance of coatings receded. Pull-out of coating lamina and plastic deformation of coating materials were main failure mechanism of conventional and nanostructured coatings. The wear resistance property of coatings could be improved by partial melted region and nanostructured grain in coatings.
  Key words: Al2O3+13wt.%TiO2 coating; CPSP; Microstructure; Thermal shock resistance;Wear resistance 
    
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