An amorphous nanoparticulate aluminosilicate 3/2-mullite precursor has been synthesized and carefully characterized. The sol contained 2-nm particles of Q3(3Al) silica species together with six-coordinated alumina, which suggested an allophane-like structure of the nanoparticles. The sol remained stable for years, and formed an easily redispersible physical gel upon solvent evaporation. The gel crystallized to mullite at temperatures below 1000 °C, without going through any intermediate spinel phase. Thus, the nanoparticulate precursor is regarded as a homogeneous high-purity mullite precursor with a high Si–O–Al bond density, which is useful in the preparation of various nanostructured Al-rich aluminosilicate materials. The sols and gels were characterized by small-angle x-ray scattering, dynamic light scattering, x-ray diffraction, 27Al and 29Si magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy, and differential thermal analysis.
The potential of the high-velocity oxy-fuel (HVOF) thermal spray process for reduced porosity in coatings compared to those produced by other ambient thermal spray processes is well known. The ability to produce high-density ceramic coatings offers potential in high-performance applications in the field of wear, corrosion resistance, and dielectric coatings. However, due to operational limit of the HVOF process to effectively melt the ceramic particles, the process-structure relationship must be well optimized. It has been also demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. One strategy to improve melting of ceramic particles in relative low-flame temperatures of HVOF process is to modify particle crystal structure and composition. In this paper the effect of the powder manufacturing method and the composition on deposition efficiency of spray process as well as on the mechanical properties of the HVOF sprayed are studied. Effect of fuel gas, hydrogen vs. propane, was also demonstrated. Studied materials were alumina-, chromia-, and titania-based agglomerated powders. Coating properties such as microstructure, hardness, abrasive wear resistance, and relative fracture toughness were compared to the coating manufactured by using conventional fused and crushed powders. It can be concluded that powder size distribution and microstructure should be optimized to fulfill process requirements very carefully to produce coatings with high deposition efficiency, dense structure, improved fracture toughness, and adhesion.
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