The inception of high-entropy alloy promises to push the boundaries for new alloy design with unprecedented properties. This work reports entropy stabilisation of an octonary refractory, HfMoNbTaTiVWZr, high-entropy thin film metallic glass, and derived nitride films. The thin film metallic glass exhibited exceptional ductility of ≈60% strain without fracture and compression strength of 3 GPa in micro-compression, due to the presence of high density and strength of bonds. The thin film metallic glass shows thermal stability up to 750 °C and resistance to Ar-ion irradiation. Nitriding during film deposition of HfMoNbTaTiVWZr thin film of strong nitride forming refractory elements results in deposition of nanocrystalline nitride films with compressive strength, hardness, and thermal stability of up to 10 GPa, 18.7 GPa, and 950 °C, respectively. The high amount of lattice distortion in the nitride films leads to its insulating behaviour with electrical conductivity as low as 200 S cm−1 in the as-deposited film. The design and exceptional properties of the thin film metallic glass and derived nitride films may open up new avenues of development of bulk metallic glasses and the application of refractory-based high entropy thin films in structural and functional applications.

Electron beam powder bed fusion (EBPBF) is a beneficial processing route to fabricate Ti6Al4V alloy for aerospace applications due to its relatively low lead time and the possibility of topology optimization. The dry sliding wear behavior of EBPBF-Ti6Al4V against steel and alumina counterballs from room temperature (RT) to 500 °C was investigated to evaluate the influence of EBPBF processing and microstructure on the wear properties for broadening the application criteria of this lightweight alloy. The wear tests revealed that the wear rate decreased with increasing temperature due to formation of stable oxide glaze layer. This study reveals elevated temperature sliding wear behavior, wear mechanisms and microstructural changes below the wear track of EBPBF Ti6Al4V alloy against steel and alumina counterbodies.

The phase stability, compressive strength, and tribology of tungsten alloy containing low activation elements, W0.5(TaTiVCr)0.5, at elevated temperature up to 1400 °C were investigated. The spark plasma sintered W0.5(TaTiVCr)0.5 alloy showed body centered cubic (BCC) structure, which was stable up to 1400 °C using in-situ high temperature XRD analysis and did not show formation of secondary phases. The W0.5(TaTiVCr)0.5 alloy showed exceptionally high compressive yield strength of 1136 ± 40 MPa, 830 ± 60 MPa and 425 ± 15 MPa at 1000 °C, 1200 °C and 1400 °C, respectively. The high temperature tribology at 400 °C showed an average coefficient of friction (COF) and low wear rate of 0.55 and 1.37 × 10−5 mm3/Nm, respectively. The superior compressive strength and wear resistance properties were attributed to the solid solution strengthening of the alloy. The low activation composition, high phase stability, superior high temperature strength, and good wear resistance at 400 °C of W0.5(TaTiVCr)0.5 suggest its potential utilization in extreme applications such as plasma facing materials, rocket nozzles and industrial tooling.

High temperature wear behaviour of selective laser melted (SLM) 316L stainless steel (SS) was studied to elucidate the influence of characteristic microstructure of SLM 316L SS on the wear properties. The wear tests were conducted from room temperature (RT) to 600 °C using ball-on-disc setup with alumina counter ball. The effect of temperature on the wear rate and the underlying mechanisms were evaluated and compared with conventional 316 SS. The RT coefficient of friction (COF) and wear rate of SLM 316L SS and conventional 316 SS were 0.5 and 4.6 ± 0.4 x 10−4 mm3/Nm and 0.7 and 4.5 ± 0.1 x 10−4 mm3/Nm, respectively. The wear rate of conventional 316 SS slightly decreased with increasing temperature from 4.5 ± 0.1 x 10−4 mm3/Nm at RT to 3.2 ± 0.1 x 10−4 mm3/Nm at 300 °C, followed by increasing to 4.9 ± 0.4 x 10−4 mm3/Nm at 400 °C, while the wear rate of SLM 316L SS was twofold lower with 2.3 ± 0.6 x 10−4 mm3/Nm at 300 °C and 2.7 ± 0.3 x 10−4 mm3/Nm at 400 °C. The wear rate at 600 °C was found to be comparable between SLM 316L SS and conventional 316 SS with a wear rate of 6.4 ± 0.7 x 10−4 mm3/Nm and 6.6 ± 0.6 x 10−4 mm3/Nm, respectively. The lower wear rate in SLM 316L SS at higher temperatures of 300 °C and 400 °C was due to its stable hierarchical microstructure, cellular subgrains, formation of stable oxide glaze and higher hardness. Moreover, the cross-sectional microscopy of wear track after 600 °C wear tests showed that the deformation zone below the wear track in SLM 316L SS was 10–15 μm compared to 30–40 μm for conventional 316 SS. The two folds low wear rate of the SLM 316L SS at 300 °C and 400 °C compared to conventional 316 SS could potentially render it for usage in applications where high temperature wear resistant SS are needed.

High-entropy ceramics is an emerging class of high-entropy materials with properties superior to conventional ceramics. Recent research has been focused on the development of new high-entropy ceramic compositions. High-entropy oxides, carbides, borides, silicides, and boron carbides had been reported with superior mechanical, oxidation, corrosion, and wear properties. The research work on the processing and characterization of bulk high-entropy ceramics and coating systems has been summarized in this chapter. The composition design, structure, chemistry, composite processing of bulk high-entropy ceramics, and evolution of microstructure and properties are reported. The literature on the deposition of high-entropy ceramic coating and the influence of coating parameters have been discussed to produce high-entropy ceramic coatings with superior mechanical, oxidation, and wear properties. 

High-entropy alloy (HEA) is a multi-component alloy constituting five or more principal elements in equi- or near equi-atomic percentages. The high configurational entropy in a HEA composition, in contrast to conventional alloys, leads to the stabilisation of the alloying elements in stable solid solutions of face-centred-cubic (FCC), body-centred-cubic (BCC) and/or amorphous structures. The characteristic properties of HEAs are mainly governed by lattice distortion, sluggish diffusion and entropy- and cocktail-effects. Refractory high-entropy alloys (RHEAs), consisting of refractory elements, are considered as a paradigm shift in developing materials for high temperature applications.

The current PhD project investigates four different aspects of RHEAs. First, it involves developing CuMoTaWV RHEA by spark plasma sintering (SPS) and utilising the cocktail effect of HEAs for high temperature tribological application. The use of the cocktail effect, defined as selecting favourable compositions for particular applications, is utilised for RHEA compositions in order to yield adaptive tribological behaviour at changing temperatures or environments. The sintered CuMoTaWV showed formation of BCC solid solution and a composite microstructure. The high temperature tribological investigations showed an adaptive behaviour at different temperatures. At lower temperatures Cu lowered the wear rate through formation of CuO, and at higher temperatures V enhanced the tribological resistance through formation of lubricating V₂O₅ phases.

The second aspect involves studying the effect of lattice distortion on mechanical properties of magnetron sputtered thin film after adding Cu to the refractory elements of Mo, Ta, W and V. A target of CuMoTaWV was developed through partial sintering and used to deposit thin film on different substrates. The deposited film showed formation of BCC solid solution, which was verified through DFT calculations. The lattice distortion in CuMoTaWV film showed high hardness and nano-pillar compressive strength. Furthermore, the tribological properties were enhanced at temperatures up to 400^oC due to the addition of Cu.

The third aspect involves studying the effect of configurational entropy on the formation and high temperature stability of refractory high-entropy thin film metallic glass and its nitrides, by increasing the number of principal elements. A partially sintered target of TiVZrNbMoHfTaW was used to deposit thin films of metallic glass and nitrides through magnetron sputtering. The metallic glass thin films and its nitrides were found to have high hardness of 7.3 GPa and 19–43 GPa, respectively. Furthermore, the metallic glass thin films showed a high nano-pillar compressive strength of up to 3 GPa, almost twice as high as conventional metallic glass films. The phase stability of metallic glass and its nitride thin films were found to be stable at temperatures up to 750^oC and 950^oC, respectively. The exceptionally superior mechanical properties and high temperature stability has been attributed to the presence of high configurational entropy.

The last part of this PhD thesis consists of studying high-entropy-based W-rich alloys for high temperature applications. A W-based alloy of composition W_0.5(TaTiVCr)_0.5 was consolidated using SPS. The resulting alloy revealed a BCC solid solution structure. The microstructure of W-rich alloys consist of a combination of W-rich, high-entropy and TiC phases. The BCC solid solution structure in W-rich alloys was found to be stable with exceptionally high compressional strength up to 1,400^oC. A high compressive yield strength of 1136 ± 40 MPa, 830 ± 60 MPa and 425 ± 15 MPa was found at test temperatures of 1,000^oC, 1,200^oC and 1,400^oC, respectively. The resulting high strength has been related to the formation of high-entropy phases, which in return induces sluggish diffusion at higher temperatures. The high temperature tribology at 400^oC showed an average COF and low wear rate of 0.5 and 1.37 x 10^-5 mm³/Nm, respectively. The high temperature wear resistance at 400^oC was enhanced due to the presence of HEA and TiC phases.

The studies carried out in this thesis suggest the possibility of utilising the full potential of the cocktail effect, lattice distortion and configurational entropy in designing new high-entropy compositions for applications requiring adaptive tribological behaviour, superior mechanical properties and high temperature phase stability.

Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700–880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10–6 mm3/N m (RT) and 2.5 × 10–5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.

Atomic layer deposition (ALD) was used to deposit ZnO/Al₂O₃/V₂O₅ nanolaminate coatings to demonstrate a coating system with temperature adaptive frictional behaviour. The nanolaminate coating exhibited excellent conformity and crack-free coating of thickness 110 nm over Inconel 718 substrate. The ALD trilayer coating showed a hardness and elastic modulus of 12 GPa and 193 GPa, respectively. High-temperature tribology of the nanolaminate trilayer was tested against steel ball in dry sliding condition at 25 °C (room temperature, RT), 200 °C, 300 °C, and 400 °C. It was found that the nanolaminate coating showed a low coefficient of friction (COF) and wear rate at RT and 300 °C. The trilayer coating was found intact and stable at all temperatures during the friction tests. The adaptability of nanolaminate coating with the temperature was verified by performing the cyclic friction test at 300 °C and RT. The low COF and wear rate had been attributed to the (100) and (002) basal plane sliding of ZnO top layer, and the interlayer sliding of weakly bonded planes parallel to (001) plane in V₂O₅ bottom layer. Furthermore, even after the removal of ZnO coating during the tribotest, the bottom V₂O₅ layer coating stabilized the COF and wear rate at RT and 300 °C.

Duplex stainless steel, 71 wt.% austenite, 13 wt.% ferrite and 16 wt.% sigma, was made upon heat treating of fully ferritic as-built selective laser melted (SLM) 2507 stainless steel at 1200 °C. Formation of sigma phase in the heat treated SLM 2507 was investigated using optical microscopy and scanning electron microscopy (SEM). The heat treated SLM 2507 demonstrated a yield strength of 686 MPa, ultimate tensile strength of 920 MPa and an elongation of 1.8% at room temperature with a brittle fracture morphology. Precipitation of sigma phase during heat treatment and slow cooling improved the mechanical and wear properties at high temperatures (1200 °C and 800 °C, respectively). The tensile strength and elongation of the heat treated SLM 2507 was measured 400 MPa and 20% as compared to casted duplex steel with 19 MPa and 30% elongation at 1200 °C. The 20 times higher mechanical strength as compared to casted duplex steel was attributed to sigma precipitates. Tribological behaviour of heat treated duplex SLM 2507 containing sigma at 800 °C showed very low wear rate of 4.5 × 10⁻⁵ mm³/mN compared to casted duplex steel with 1.6 × 10⁻⁴ mm³/mN.

An equiatomic high entropy alloy (HEA) CuMoTaWV was designed for room temperature to high temperature wear applications using spark plasma sintering of elemental powder mixture at 1400 °C. The sintered solid solution showed uniform distribution of elements in a BCC high entropy alloy phase along with V rich solid solution phase with an average hardness of 600 Hv and 900 Hv, respectively. Room temperature (RT) dry sliding wear tests, against alloy steel (700–880 Hv) for 200 m sliding distance at 5 N normal load, showed negligible wear of 5 × 10⁻⁷ mm/N m and a coefficient of friction (COF) of 0.5. Sliding wear characterization of sintered CuMoTaWV alloy against Si₃N₄ (1550 Hv) counter body from RT to 600 °C showed an increasing average COF of 0.45–0.67 from RT to 400 °C and then reducing to 0.54 at 600 °C. The wear rate was found to be lower at RT (4 × 10⁻³ mm³/N m) and 400 °C (5 × 10⁻³ mm³/N m), and slightly higher at 200 °C (2.3 × 10⁻² mm³/N m) and 600 °C (4.5 × 10⁻² mm³/N m). The CuMoTaWV alloy showed wear mechanisms specific to the test temperatures. The wear of CuMoTaWV alloy was governed by adhesive wear at RT and 200 °C and oxidative wear at 400 °C and 600 °C. The analyses of wear surfaces showed that the low wear rate at RT was due to the high hardness of the HEA, presence of V rich zones and formation of W and Ta tribofilm. At 400 °C, the formation of CuO tribolayer reduced the wear and hindered oxidation of wear track. At 600 °C, the wear rate increased due to oxidation of Cu, Ta and W. Moreover, the formation of lubricating elongated magneli phase V₂O₅ in V rich regions of CuMoTaWV alloy reduced the COF to 0.54.