The mechanical and thermal response of materials with spatial gradients in composition and microstructure is of considerable interest in numerous technological areas such as tribology, optoelectronics, biomechanics, nanotechnology and high temperature technology. The graded transition in composition across an interface of two materials (for instance, metal and ceramics or polymer) can essentially reduce the thermal stresses and stress concentration at intersection with free surfaces. Similarly the stress intensity factor at the crack tip can be altered by varying the gradient properties across the interface. The ceramic-metal FGMs
exhibit higher fracture resistance parameters resulting in higher toughness due to crack bridging in a graded volume fraction. Varying thermal expansion in graded layers induces residual stress and affects the crack growth mode. In fact, the interface bonding is much improved by providing smooth composition variation when traversing the interface. The interest in graded materials focused primarily on the control of thermal stresses in elements exposed to high temperatures (to 1600C), for instance in gas turbine blades, aerospace structures, solid-oxide fuel cells, energy conversion systems using thermoelectric or thermionic materials (thermal barrier coatings, TBC). Subsequent applications include fusion and fast-breeder reactors as a first-wall composite material, piezoelectric and thermoelectric devices, high density magnetic recording media, in optical applications as graded refractive index materials in audio-video discs, in bioengineering as dental and orthopaedic implants, in structures as fire retardant doors and penetration resistant materials for armour plates and bullet-proof vests.
The concept of thin surface layers
is closely related to functionally graded materials. In fact, all surface treatments and coatings aimed to increase wear and fracture resistance induce gradual or stepwise transition to bulk properties. Well established techniques such as shot-peening, laser treatment, ion implantation, have been developed and extended to generate nanocrystalline surface coatings with grain sizes of order of few tens of nanometres. New techniques include thermal spray, electrodeposition, electrophoretic deposition, chemical (CVD) and physical vapour deposition (PVD), ion beam assisted deposition (IBAD), etc. The surface layer can be created with grain sizes varying smoothly from the surface to the bulk. Also the gradients in porosity and density can be controlled to increase the damage resistance and reduce stress intensity factors at crack tips. Recent experimental and theoretical works demonstrated that controlling gradients in thermal and mechanical properties provides a new potential for design of surfaces and interfaces with higher resistance to cracking and wear subjected to mechanical surface loading and thermal gradients. The diverse applications include load-bearing engineering structures, protective coatings, bioimplants, magnetic storage media.
There are several types of FGMs
that exhibit exceptional multifunctional properties and multisectoral applications as shown below:
|Ceramic/metal bulk FGMs||Thermal stress relaxation; high heat resistance and wear resistance, high mechanical strength||Spark plasma sintering process||High efficiency engine components|
|Titanium (alloys) with graded density or porosity||Combination of good mechanical properties and light weight||Additive, layer-wise process: direct metal laser sintering (DMLS) of powders||Light weight structures for aircraft and space industry, implants|
|Toolsteels with C, V, Cr gradients; steels or Ni superalloys with ceramic (oxide, carbide) particle gradient||Combination of toughness and hardness or wear resistance||Additive, layer-wise process: 3D-printing with local material composition control (generating a green part of powdered material and sintering or infiltration)||Tools, medical instruments, implants, aircraft and space industry.|
|Functionally graded cemented carbide: titanium based surface ceramic layer, tough cemented carbide core and intermediate layer with graded composition||Wear resistance, breakage resistance, thermal crack resistance||sintering||Cutting insert|
|Ferritic-austenitic combinations like 316L and 17-4PH; steel-ceramic combinations||Magnetic and non-magnetic; ductile and stiff and other||Co-injection moulding and co-sintering (building a graded interface)||Automotive industry, sensors; medical instruments|
|Precious metals like Pt, Ag (catalysis) and metal oxides like SnO2 (sensors) with graded porosity from bulk to nanometre scale||High specific surface and strong gas-metal interaction; graded porosity combines optimised contact on substrates (bulk side) and high functionality (nano-structured side)||PVD based on sputter techniques and inert gas evaporation and condensation, with in-situ design of the deposed structures by controlling the process parameters||Gas sensors and catalytic active layers, low-temperature bonding for electronic connections|