TY - THES

T1 - Phase Stability and Mechanical Properties of Yttrium alloyed Ti-Al-N Thin Films

AU - Riedl, Helmut

N1 - embargoed until 23-05-2017

PY - 2012

Y1 - 2012

N2 - Protective coating systems for cutting and milling tools are an indispensable application to enhance properties of the bulk material and simultaneously extend the lifetime. These thin films have to combine excellent thermal and mechanical properties. Ti(1-x)Al(x)N is a well-established material system meeting such requirements. In order to fulfil the increasing demands during cutting and milling operations further advancements have to be achieved. Alloying Ti(1-x)Al(x)N with another element such as Yttrium (Y), to form a quaternary alloy Ti(1-x-y)Al(x)Y(y)N, is a promising approach to increase hardness, Young's modulus and oxidation resistance in high-temperature environments. Yttrium is a transition metal and typically has a high melting point (Tm=1526°C) and at least one 4d electron. The latter enables Y to form a stable cubic nitride, which is crucial for the formation of a supersaturated single phase cubic solid solution of Ti(1-x-y)Al(x)Y(y)N. The high affinity of Y towards oxygen (reactive-element effect) combined with these attributes makes Ti(1-x-y)Al(x)Y(y)N coatings extremely encouraging to attain the desired properties. In this study, all deposited Ti(1-x-y)Al(x)Y(y)N coatings were performed in a plasma-assisted reactive magnetron sputtering process. To achieve various Ti- and Y-contents four different targets with a constant Ti/Al ratio of 1:1 and Y contents of 0,2,4,8 at.% were used. Additionally, a specific number (0,4,8,12,16) of Ti-pellets were utilised at the targets to increase the variety of compositions. The intent to use such a large number of compositions was to clearly identify the borderline for the cubic/wurtzite transition and hence to show the influence of the obtained structure on the thermal stability. In a first step, all these coatings were deposited onto single crystalline Si substrates and analysed regarding their structural and mechanical properties. Here, we show that by decreasing Yttrium and increasing Titanium mole fractions the cubic phase is stabilised. The supersaturated solid solution of c-AlN, c-TiN and c-YN can be maintained up to an Y-content of ~5%, respecting different Al-contents below ~60% of the metal sublattice. In a next step, certain compositions were selected for vacuum annealing treatments. The hardness, Young's modulus, sheet resistivity, structural changes and oxidation behaviour were investigated as a function of temperature for those thin films. However, all coatings offer more or less an age-hardening effect and show a slight increase from ≈ 30GPa to ≈ 33GPa at 1100°C before a deterioration can be observed. Wurtzite structured AlN can be detected after annealing at Ta≈1200°C. The behaviour of the sheet resistivity also correlates with the thermally induced structural changes during annealing. The coating with the composition Ti(0.51)Al(0.47)Y(0.02)N even achieves a hardness of ≈ 32GPa up to 1200°C and furthermore H ≈ 28GPa at 1400°C. This coating also performs best in the oxidation tests with only ≈ 20% of the coating thickness (~3.5µm) being transformed into the typically Al2O3 and Ti-rich layered oxide scale. This study significantly supports and extends the scientific understanding about the influence of 4d transition metals, like Yttrium, on the properties of Ti(1-x-y)Al(x)Y(y)N coatings and also points out which compositions actually are highly attractive for applications.

AB - Protective coating systems for cutting and milling tools are an indispensable application to enhance properties of the bulk material and simultaneously extend the lifetime. These thin films have to combine excellent thermal and mechanical properties. Ti(1-x)Al(x)N is a well-established material system meeting such requirements. In order to fulfil the increasing demands during cutting and milling operations further advancements have to be achieved. Alloying Ti(1-x)Al(x)N with another element such as Yttrium (Y), to form a quaternary alloy Ti(1-x-y)Al(x)Y(y)N, is a promising approach to increase hardness, Young's modulus and oxidation resistance in high-temperature environments. Yttrium is a transition metal and typically has a high melting point (Tm=1526°C) and at least one 4d electron. The latter enables Y to form a stable cubic nitride, which is crucial for the formation of a supersaturated single phase cubic solid solution of Ti(1-x-y)Al(x)Y(y)N. The high affinity of Y towards oxygen (reactive-element effect) combined with these attributes makes Ti(1-x-y)Al(x)Y(y)N coatings extremely encouraging to attain the desired properties. In this study, all deposited Ti(1-x-y)Al(x)Y(y)N coatings were performed in a plasma-assisted reactive magnetron sputtering process. To achieve various Ti- and Y-contents four different targets with a constant Ti/Al ratio of 1:1 and Y contents of 0,2,4,8 at.% were used. Additionally, a specific number (0,4,8,12,16) of Ti-pellets were utilised at the targets to increase the variety of compositions. The intent to use such a large number of compositions was to clearly identify the borderline for the cubic/wurtzite transition and hence to show the influence of the obtained structure on the thermal stability. In a first step, all these coatings were deposited onto single crystalline Si substrates and analysed regarding their structural and mechanical properties. Here, we show that by decreasing Yttrium and increasing Titanium mole fractions the cubic phase is stabilised. The supersaturated solid solution of c-AlN, c-TiN and c-YN can be maintained up to an Y-content of ~5%, respecting different Al-contents below ~60% of the metal sublattice. In a next step, certain compositions were selected for vacuum annealing treatments. The hardness, Young's modulus, sheet resistivity, structural changes and oxidation behaviour were investigated as a function of temperature for those thin films. However, all coatings offer more or less an age-hardening effect and show a slight increase from ≈ 30GPa to ≈ 33GPa at 1100°C before a deterioration can be observed. Wurtzite structured AlN can be detected after annealing at Ta≈1200°C. The behaviour of the sheet resistivity also correlates with the thermally induced structural changes during annealing. The coating with the composition Ti(0.51)Al(0.47)Y(0.02)N even achieves a hardness of ≈ 32GPa up to 1200°C and furthermore H ≈ 28GPa at 1400°C. This coating also performs best in the oxidation tests with only ≈ 20% of the coating thickness (~3.5µm) being transformed into the typically Al2O3 and Ti-rich layered oxide scale. This study significantly supports and extends the scientific understanding about the influence of 4d transition metals, like Yttrium, on the properties of Ti(1-x-y)Al(x)Y(y)N coatings and also points out which compositions actually are highly attractive for applications.

KW - Thermal stability

KW - Oxidation resistance

KW - Quaternary system Ti-Al-Y-N

KW - Age-Hardening

KW - Thermische Beständigkeit

KW - Oxidationsbeständigkeit

KW - Quaternäres System Ti-Al-Y-N

KW - Age-Hardening

M3 - Diploma Thesis

ER -