TY - THES

T1 - Thermal stability and oxidation behavior of magnetron sputtered Mo-Si-B Thin films

AU - Vieweg, Annika

N1 - embargoed until 26-05-2019

PY - 2014

Y1 - 2014

N2 - Refractory elements such as Mo, Ta, or W have the highest melting points of all elements, which makes them a very interesting field of studying for high temperature applications. For those applications the bulk material is as important as a protective coating to withstand high thermal loads. One system on the merge are molybdenum alloys. While the bulk material shows excellent creep behavior, molybdenum-based thin films have outstanding thermal protective properties. The main problem with molybdenum is its drastic oxidation behavior when exposed to oxygen above temperatures of 400°C. This so called “pesting” phenomenon is based on the formation of volatile molybdenum oxide (MoO3) resulting in an intense mass loss. The addition of silicon to molybdenum leads to the formation of a dense protective SiO2-based scale increasing the stability of molybdenum based materials at high temperatures. To also improve the oxidation resistance at lower temperatures boron is added to the system, forming a glassy B2O3 layer during oxidation. The oxidation resistance is highly affected by the pre-existing phase composition. Within the ternary system Mo(1-x-y)Si(x)B(y) several binary phases (Mo5Si3(D8m, tl32, W5Si3-prototype), Mo3Si(A15, cP8, Cr3Si-prototype), MoSi2(C11, tl6, MoSi2-prototype)), as well as the so called T2 phase (Mo5SiB2 (T2, l4/mcm, Cr5B3-prototype)) exhibit outstanding properties. The formation of this phase is difficult and mostly requires post-annealing treatments after deposition. In this study the mechanical and thermal properties of different chemical compositions within the ternary system Mo(1-x-y)-Si(x)-B(y) were analyzed. Different compositions were achieved through magnetron sputtering with Mo-Si compositional targets and an elementary boron target or three elementary targets (Mo, Si, and B). The targeted compositions of the films were all within the molybdenum rich region of the ternary phase diagram. XRD analysis in the as deposited state showed no T2 phase but mostly Mo5Si3(D8m)and Mo3Si(A15) structures or an XRD amorphous state, which is more pronounced with increasing boron content. The hardness level of the coatings rises up to a constant level of 20 GPa, when containing more than 5 at% Si and B. Vacuum annealing at 900, 1100, and 1300 °C was conducted to investigate the thermal stability of the phases, as well as their transformations. All coatings exhibit a fully crystalline structure after annealing at 900°C and also the T2 phase could be obtained. The hardness of all coatings increased due to the annealing process (e.g., up to 26 GPa for Mo0.53Si0.37B0.10), which is correlated to the formation of the T2 phase. Oxidation treatments in ambient air for 1h at 900, 1100, and 1300 °C revealed that the Mo0.59Si0.33B0.19, Mo0.53Si0.37B0.10, and Mo0.58Si0.28B0.14 coatings exhibit superior oxidation resistance. For these coatings, the T2 phase develops during the oxidation process. Detailed investigations (e.g., by cross sectional EDX line scans) show that the Mo0.58Si0.28B0.14 coating, having the highest B/Si ratio investigated, exhibits the lowest consumed film thickness, and hence highest oxidation resistance also due to the formation of the T2 phase. This thesis experimentally proofs the predicted influence of the B/Si ratio on the oxidation behavior and thermal stability of Mo-Si-B alloys. For distinct chemical compositions the T2 phase develops during oxidation, resulting in an excellent oxidation resistance even without pre-annealing treatments in vacuum.

AB - Refractory elements such as Mo, Ta, or W have the highest melting points of all elements, which makes them a very interesting field of studying for high temperature applications. For those applications the bulk material is as important as a protective coating to withstand high thermal loads. One system on the merge are molybdenum alloys. While the bulk material shows excellent creep behavior, molybdenum-based thin films have outstanding thermal protective properties. The main problem with molybdenum is its drastic oxidation behavior when exposed to oxygen above temperatures of 400°C. This so called “pesting” phenomenon is based on the formation of volatile molybdenum oxide (MoO3) resulting in an intense mass loss. The addition of silicon to molybdenum leads to the formation of a dense protective SiO2-based scale increasing the stability of molybdenum based materials at high temperatures. To also improve the oxidation resistance at lower temperatures boron is added to the system, forming a glassy B2O3 layer during oxidation. The oxidation resistance is highly affected by the pre-existing phase composition. Within the ternary system Mo(1-x-y)Si(x)B(y) several binary phases (Mo5Si3(D8m, tl32, W5Si3-prototype), Mo3Si(A15, cP8, Cr3Si-prototype), MoSi2(C11, tl6, MoSi2-prototype)), as well as the so called T2 phase (Mo5SiB2 (T2, l4/mcm, Cr5B3-prototype)) exhibit outstanding properties. The formation of this phase is difficult and mostly requires post-annealing treatments after deposition. In this study the mechanical and thermal properties of different chemical compositions within the ternary system Mo(1-x-y)-Si(x)-B(y) were analyzed. Different compositions were achieved through magnetron sputtering with Mo-Si compositional targets and an elementary boron target or three elementary targets (Mo, Si, and B). The targeted compositions of the films were all within the molybdenum rich region of the ternary phase diagram. XRD analysis in the as deposited state showed no T2 phase but mostly Mo5Si3(D8m)and Mo3Si(A15) structures or an XRD amorphous state, which is more pronounced with increasing boron content. The hardness level of the coatings rises up to a constant level of 20 GPa, when containing more than 5 at% Si and B. Vacuum annealing at 900, 1100, and 1300 °C was conducted to investigate the thermal stability of the phases, as well as their transformations. All coatings exhibit a fully crystalline structure after annealing at 900°C and also the T2 phase could be obtained. The hardness of all coatings increased due to the annealing process (e.g., up to 26 GPa for Mo0.53Si0.37B0.10), which is correlated to the formation of the T2 phase. Oxidation treatments in ambient air for 1h at 900, 1100, and 1300 °C revealed that the Mo0.59Si0.33B0.19, Mo0.53Si0.37B0.10, and Mo0.58Si0.28B0.14 coatings exhibit superior oxidation resistance. For these coatings, the T2 phase develops during the oxidation process. Detailed investigations (e.g., by cross sectional EDX line scans) show that the Mo0.58Si0.28B0.14 coating, having the highest B/Si ratio investigated, exhibits the lowest consumed film thickness, and hence highest oxidation resistance also due to the formation of the T2 phase. This thesis experimentally proofs the predicted influence of the B/Si ratio on the oxidation behavior and thermal stability of Mo-Si-B alloys. For distinct chemical compositions the T2 phase develops during oxidation, resulting in an excellent oxidation resistance even without pre-annealing treatments in vacuum.

KW - Mo-Si-B

KW - Hochtemperatur Oxidation

KW - Oxidationsbeständigkeit

KW - T2-Phase

KW - thermische Stabilität

KW - Mo-Si-B

KW - high temperature oxidation

KW - oxidation resistance

KW - T2-phase

KW - thermal stability

M3 - Diploma Thesis

ER -