Cr <sub> 2 </ Sub> O <sub> 3 </ Sub> Preparation Of Nanostructured Thermal Spray Powder And Wear-resistant Coating Research

Nanostructured materials have unique properties in the aspects of hardness, strength, ductility and toughness. Nanostructured ceramics materials have low brittleness and good toughness, compared to microstructured ceramics. Nanocrystalline Cr2O3 powder purchased from market has low apparent density and poor flow ability so that it can not be directly used for plasma spraying. By using steps of ball milling, slurry forming, spray drying and heat treatment, the nanocrystalline powder of Cr2O3 can be reprocessed into nano-particles agglomerates for plasma spraying. This study focuses on the influence of organic binder concentration, solid loading, spray drying condition, heat treatment temperature on the physical properties of the agglomerated powder such as free holding density, tap density and flow ability. Scanning electron microscopy and X-ray diffraction were used to characterize the original nanocrystalline powder and the agglomerated powder. A geometry model was established for explaining the various particle shapes under different heating condition during spray drying. In addition, a geometry model was established for describing the change of particle shapes during heat treatment. Experimental results show that the particles of the agglomerated powder are almost spheric and have the size in the range of 10 ~100 m. After heat treatment at a temperature higher than 1000 C, the organic binder inside agglomerates was removed and weak connection among nano-particles of large agglomerates was formed. The powder has good flow ability and apparent density, which is suitable for thermal spray.D.C. plasma was used to spray the agglomerated ceramic powder, forming Cr2O3 wear-resistant coating on stainless steel substrate. Free surface and cross section of the coating were examined by scanning electron microscopy and optical microscopy. It is revealed that a part of the coating is microstructured and another part is nanostructured. Unlike conventional plasma-sprayed coatings, no obvious laminar layer can be found in nanostructured coating. X-ray diffraction result does not show any change of phase constituent in the coating when compared to thepowder. Nanostructured coating has lower microhardness than conventional microstructured coating and thus the toughness of the nanostructured ceramic coating is improved.