Novel processing route for design and manufacturing of metal toughened nanoceramics: Al–Al2O3 nanocermets

Abstract

Monolithic alumina is a highly brittle ceramic with a very low fracture toughness, making it an unreliable material for mechanical and structural applications. Ceramic-metal composites (cermets) are the subject of intensive interest to obtain materials which can address the intrinsic brittleness of ceramics without compromising their high strength and light weight. In the present investigation, aluminium-alumina (Al–Al 2 O 3 ) nanocermets were developed. The nanocermets possessed the toughness of Al phase and the strength of reinforced nano Al 2 O 3 particles. The nanocermets green bodies were prepared such that upon sintering each adjacent Al 2 O 3 grain was conformally coated with a continuous Al film. Mechanical milling via a combination of cryo-milling and ball-milling was used to prepare Al–Al 2 O 3 nano-powder mixture. Spark plasma sintering was used to consolidate the green nanocermets. Results show that specimens prepared from 35 h milled powders exhibited superior mechanical properties as compared to other nanocermets, with an improvement in microhardness, elastic modulus, compressive strength, and flexural strength by 88%, 78%, 69% and 40% with respect to sintered pure Al. The indentation toughness of 35 h milled nanocermet showed an increment of 12.5% with respect to sintered pure Al 2 O 3 . Characterization techniques such as XRD, SEM, HR-TEM, X-ray tomography were employed to investigate the phase evolution and structural morphology, while FEM simulations were performed for understanding the crack propagation mechanisms in nanocermets. Figure Description: A novel processing route devised from the combination of cryo-milling and ball-milling of elemental Al and Al 2 O 3 powders, followed by spark plasma sintering (SPS) for the development of toughened and strengthened Al–Al 2 O 3 nanocermets (metal-ceramic nano-systems). An attempt has been made to utilize the favourable physical and mechanical attributes of ductile metal phase and brittle ceramic phase to fabricate nanocermets with enhanced mechanical properties. These nanocermets will find use in structural applications requiring components with high strength to weight ratio, such as automotive parts, ballistic armours and jet propulsion systems. • Al 0.2 -(Al 2 O 3 ) 0.8 nanocermets possessing the toughness from ductile Al and strength from brittle Al 2 O 3 are developed. • A combination of cryo-milling and ball-milling is used for the preparation of nanosized Al-Al 2 O 3 powder mixtures. • An optimized cryo-milling of 10 h transforms micron-sized Al particles into nanometric thin sheets. • Enhanced wettability of Al and Al 2 O 3 at the grain boundary interface is achieved for metal-ceramic nano-systems. • Toughening of brittle Al 2 O 3 by metallic Al phase transforms the inter-granular fracture into trans-granular mode.