Synthesis and mechanical and elevated temperature tribological properties of a novel high-entropy (TiVNbMoW)C _4.375 with carbon stoichiometry deviation

Abstract

High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability, high hardness (H), high strength, and superior creep resistance that they display. Due to high melting points and low lattice diffusion coefficients, however, the high-entropy carbides are usually difficult to consolidate to a nearly full density. To cope with this challenge, herein, binary carbides including TiC, V 8 C 7 , NbC, Mo 2 C, and WC with different carbon stoichiometry were used to prepare dense high-entropy (TiVNbMoW)C 4.375 , and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy (TiVNbMoW)C 4.375 were investigated. Intriguingly, although the starting binary carbides have different crystal structures and carbon stoichiometry, the as-prepared high-entropy material showed a rock-salt structure with a relatively high density (98.1%) and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness (K IC ) of 4.02 MPa•m 1/2 . More importantly, the high-entropy (TiVNbMoW)C 4.375 exhibited low coefficient of friction (COF) at room temperature (RT) and 800 ℃. Wear rate (W) gradually increased with the temperature rising, which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear. At 800 ℃, lubricating films formed from sufficient oxidation products of V 2 O 5 and MoO 3 effectively improved tribological behavior of the high-entropy (TiVNbMoW)C 4.375 . Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions. These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics (HECs) for sliding parts under high-temperature serving conditions.

High‐entropy (Ti0.2V0.2Nb0.2Mo0.2W0.2)Si2 with excellent high‐temperature wear resistance

“…Compared with HE‐MSi 2 , Mo‐Si‐based ceramics (such as MoSi 2 , Mo 5 Si 3 , and Mo 5 SiB 2 ), high‐entropy carbides (such as (MoTaWVTi)C, (HfMoNbTaTi)C, and (TiVNbMoW) 4.375 ), and traditional single‐phase ceramics (such as Sialon, Si 3 N 4 , Al 2 O 3 , SiC, and ZrO 2 ), 61–69 the high‐entropy carbides demonstrate the best wear resistance at RT with wear rates as low as 10 −7 mm 3 /N·m (Figure 5). 67,68 HE‐MSi 2 is slightly less wear‐resistant than Mo‐Si‐based ceramics at RT.…”

“…It can be concluded from XPS (Figure 6) that the sublimation point of V 2 O 5 is much higher. MoO 3 and V 2 O 5 are solid lubricants with an easily sheared and slipped lamellar structure that can form a lubricating film during sliding (Figure 9C,C’), therefore HE‐MSi 2 exhibits the lowest friction coefficient and wear rate at 600°C 69 . When the temperature rises to 900°C, a large amount of MoO 3 sublimates, resulting in no lubricant film forming on the worn surface (Figure 9D,D’), which weakens the friction‐reducing and wear‐resistant effects.…”

“…The lowest wear rate is attained at 600 • C, which amounts to (1.88 ± 0.15)×10 −6 mm 3 /N⋅m for HE-MSi 2 and 1.94×10 −6 mm 3 /N⋅m for the whole wear systems. [61][62][63][64][65][66][67][68][69] the high-entropy carbides demonstrate the best wear resistance at RT with wear rates as low as 10 −7 mm 3 /N⋅m (Figure 5). 67,68 HE-MSi 2 is slightly less wear-resistant than Mo-Si-based ceramics at RT.…”

“…[61][62][63][64][65][66][67][68][69] the high-entropy carbides demonstrate the best wear resistance at RT with wear rates as low as 10 −7 mm 3 /N⋅m (Figure 5). 67,68 HE-MSi 2 is slightly less wear-resistant than Mo-Si-based ceramics at RT. Nonetheless, HE-MSi 2 shows excellent high-temperature wear resistance (≥600 • C) beyond Mo-Si-based ceramics, high-entropy carbides, and traditional single-phase ceramics.…”

“…Additionally, the compacted debris layer is thicker and can withstand a higher load, thus protecting the worn surface from further damage during sliding. This results in the lowest friction coefficient and wear rate at 600 • C. 69 When the temperature rises to 900 • C, a large amount of MoO 3 sublimates, resulting in no lubricant film forming on the worn surface (Figure 9D,D'), which weakens the friction-reducing and wear-resistant effects. However, since the high melting points of TiO 2 , WO 3 , and Nb 2 O 5 on the worn surface provide good support, and the SiO 2 on the worn surface protects the interior from further oxidation, the wear rate of HE-MSi 2 at 900 • C is still significantly lower than that of Mo-Si-based ceramics (such as MoSi 2 , Mo 5 Si 3 , Mo 5 SiB 2 ), high-entropy carbides (such as (MoTaWVTi)C, (HfMoNbTaTi)C, (TiVNbMoW)C 4.375 ), and traditional single-phase ceramics (such as Sialon, Si 3 N 4 , Al 2 O 3 , and SiC, ZrO 2 ) (Figure 5).…”