Synthesis of nanostructured Al[subscript 2]O[subscript 3]-Ni(Cr) particles using high-energy mechanical alloying process for high velocity oxy-fuel coatings development

Abstract

The research study presented in this thesis focuses on the synthesis and characterization of nanostructured cermet Al[superscript 2]O[superscript 3]-Ni(Cr) particles using a high-energy mechanical alloying (HE-MA) powder processing technology. The feedstock is used to develop protective coatings using a high velocity oxy-fuel (HVOF) thermal spray methodology. Two independent composite design-of-experiments (DoE’s) were developed to process the materials using the HE-MA process and four distinct milling parameters were targeted to optimize the process condition: milling time, milling type, ball to powder weight ratio and weight composition of starting materials. The underscoring particle design objectives were twofold: (1) reduce the ceramic oxide grain size below 20 nm, and more precisely; (2) embed nano-scaled grains inside metal/alloy binder particles resulting in nanostructured cermet particles. Results from the first phase of the study (DoE-1) showed that the yield was low against these objectives due to the low rpms (150 rpm) provided by an attritor mill which took 36 hours of synthesis time to achieve a ceramic (Al[superscript 2]O[superscript 3]) grain size reduction to 141 nm. In DoE-2, the synthesis was exercised in a shaker mill (1200 rpm speed), and the milling time and material design composition varied. Extended results obtained from both experimental and analytical characterization techniques showed that it was possible to meet the above objectives following a high-impact milling protocol. The results were nanocrystalline Al[superscript 2]O[superscript 3] grains (within a 20 nm size range) and the embedment of near-uniform Al[superscript 2]O[superscript 3] grains in the Ni(Cr) binder matrix at 12 hours milling. Keywords: Cermet; HVOF; high-energy mechanical alloying; nanostructured particle