Review on developments of bulk functionally graded composite materials

Monolithic components are often insufficient when a combination of conflicting properties, or a variation in properties at different regions within a material is required. Functionally graded composite materials (FGCM) are a relatively new class of materials with a systematic variation of the composition, microstructure, and properties along one direction. The graded structure offers tailorable property combinations not achievable with monolithic composites and significant recent research has been carried out on their development. An effort has been made in this article to review the latest developments on bulk FGCMs since 2015. The different processing techniques used for their fabrication have been discussed, highlighting their advantages, limitations, and recent advancements pertaining to FGCM fabrication. Various physical, mechanical, and thermal properties of FGCMs have been treated separately, and wherever possible, summary plots combining data from various research articles have been developed. Finally, potential areas for directing future FGCM research have been identified.Abbreviations: AM: Additive manufacturing; ANOVA: Analysis of variance; ATZ: Alumina toughened zirconia; BCP: Biphasic calcium phosphate; BEI or BSE: Backscattered electron images; BG: Bioactive glass; CAD: Computer aided design; CDHAp: Calcium-deficient hydroxyapatite; CNT: Carbon nanotube; CoCGO: Cobalt and gadolinium doped ceria; CTE: Coefficient of thermal expansion; DED: Direct energy deposition; DIC: Digital image correlation; ELDS: Electro discharge sintering; EDX: Energy dispersive x-ray analyser; FDM: Fused deposition modelling; FGCC: Functionally graded cemented carbides; FGCM: Functionally graded composite materials; FGHM: Functionally graded hard metals; FSP: Friction stir processing; FSW: Friction stir welding; GDI: Graded density impactor; HA: Hydroxyapatite; HIP: Hot isostatic pressing; LCM: Lithography based ceramic manufacturing; LDMD: Laser direct metal deposition; LMD: Laser metal deposition; LSCF: Lanthanum cobalt ferrite; MLG: Multilayered graphene; MMC: Metal matrix composites; MWCNT: Multi-walled carbon nanotubes; PAS: Plasma activated sintering; PFC: Plasma facing components; PM: Powder metallurgy; RHA: Rice husk ash; RVE: Representative volume element; SEM: Scanning electron microscope; SHAp: Stoichiometric hydroxyapatite; SLA: Stereolithography; SLM: Selective laser melting; SLS: Selective laser sintering; SOFC: Solid oxide fuel cells; SPS: Spark plasma sintering; TEM: Transmission electron microscope; UHTC: Ultra-high temperature ceramics; WAAM: Wire arc additive manufacturing; YSZ: Yttria stabilized zirconia