Abstract
Refractory transition-metal diborides (TMB2) are candidates for extreme environments due to melting points above 3000 °C, excellent hardness, good chemical stability, and thermal and electrical conductivity. However, they typically suffer from rapid high-temperature oxidation. Here, we study the effect of Al addition on the oxidation properties of sputter-deposited TiB2-rich Ti1-xAlxBy thin films and demonstrate that alloying the films with Al significantly increases the oxidation resistance with a slight decrease in hardness. TiB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a TiB2 target, while Ti1-xAlxBy alloy films are grown by hybrid high-power impulse and dc magnetron co-sputtering (Al-HiPIMS/TiB2-DCMS). All as-deposited films exhibit columnar structure. The column boundaries of TiB2.4 are B-rich, while Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by a Ti1-xAlxBy tissue phase which is predominantly Al rich. Air-annealing TiB2.4 at temperatures above 500 °C leads to the formation of oxide scales that do not contain B and mostly consist of a rutile-TiO2 (s) phase. The resulting oxidation products are highly porous due to the evaporation of B2O3 (g) phase as well as the coarsening of TiO2 crystallites. This poor oxidation resistance is significantly improved by alloying with Al. While air-annealing at 800 °C for 0.5 h results in the formation of an ~1900-nm oxide scale on TiB2.4, the thickness of the scale formed on the Ti0.68Al0.32B1.35 alloys is ~470 nm. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale that considerably decreases the oxygen diffusion rate by suppressing the oxide-crystallites coarsening.
Highlights
Refractory transition-metal diborides (TMB2), classified as ultrahigh temperature ceramics, are promising materials for extreme thermal and chemical environments which include, individually or in combination, temperatures above 2000 °C, drastic chemical reactivity, hydrostatic pressure, mechanical stress, wear, and very high levels of radiation and heat gradients [1, 2]
The column boundaries of TiB2.4 are B-rich, while the Ti0.68Al0.32B1.35 alloy films have Ti-rich columns surrounded by an Al-rich Ti1-xAlxBy tissue phase which is B deficient
The column boundaries of TiB2.4 films grown by dc magnetron sputtering (DCMS) are B-rich, while the Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by an Al-rich Ti1-xAlxBy tissue phase which is B deficient
Summary
Refractory transition-metal diborides (TMB2), classified as ultrahigh temperature ceramics, are promising materials for extreme thermal and chemical environments which include, individually or in combination, temperatures above 2000 °C, drastic chemical reactivity, hydrostatic pressure, mechanical stress, wear, and very high levels of radiation and heat gradients [1, 2]. Their high strength at elevated temperatures together with thermal conductivity, which provides a high thermal-shock resistance in severe heat fluxes, make TMB2 ceramics suitable for aerospace applications such as rocket components, atmospheric reentries, jet engine turbines, propulsion systems, and sharp leading edges in hypersonic vehicles, with speeds exceeding Mach 5 [1,2,3,4,5,6,7,8,9,10,11,12,13]. This unique combination of ceramic and metallic properties makes TM diboride thin films promising candidates for many applications
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