TY - THES

T1 - Impact of Al on structure and mechanical properties of NbN and TaN

AU - Zhang, Zehua

N1 - embargoed until null

PY - 2011

Y1 - 2011

N2 - Hard coatings, such as transition metal nitrides (TMN) are widely used for wear-resistance in machining, casting or hot-forming applications. However, TMN oxidise rapidly and often these oxides have a porous morphology which enables a rapid scale growth. Therefore, ternary compounds (TM,Al)N are researched in order to improve the oxidation resistance but also mechanical properties of TMN coatings. Important here is, that the superior oxidation stability and mechanical properties are obtained for the face centred cubic (fcc) structure, which requires investigations of the phase stability ranges of (TM,Al)N coatings. In this work, a series of NbNy and TaNy coatings were deposited on Si- and austenitic stainless steel substrates with an increasing N2 partial pressure by a reactive unbalanced magnetron sputtering technique. These studies were necessary to find the deposition parameters to grow single-phase fcc NbN and TaN layers. Furthermore, the fcc-NbN and fcc-TaN phases are alloyed with Al to form a ternary Nb1-xAlxN and Ta1-xAlxN compound. With increasing N2-partial pressure during sputtering of a Nb-target the obtained coatings crystallize to form hex-Nb2N, hex-Nb2N+fcc-NbN, hex-NbN+fcc-NbN and finally almost single-phase fcc-NbN (measured by X-ray diffraction, XRD). In contrast, the TaNy films crystallize to form hex-Ta2N, hex-Ta2N+fcc-TaN, and finally fcc-TaN+hex-TaN. The changed structures cause a significant change of the mechanical properties measured by nanoindentation. The hardness value of fcc-NbN is approximately 28 GPa. The single phase fcc-TaN could not be obtained. But the hardness of the fcc+hex-TaN is similar to the hardness of fcc-NbN. These binary nitrides were alloyed with an increasing Al-content, x=Al/(Nb,Ta+Al), of x=0.11 to 0.70. A single phase fcc-Nb1-xAlxN exists in the range of x=0.11 to 0.44, and the lattice parameter decreases with increasing Al-content at the same time the hardness increases until it reaches ~32 GPa. The transition zone to form a dual phase of fcc-Nb1-xAlxN+wur-Nb1-xAlxN is in the range of x=0.44 to 0.61 with decreasing hardness. A single phase wur-Nb1-xAlxN exists for x>0.61 with hardness values of ~22-23 GPa. In contrast, the transition zone for Ta1-xAlxN is already in the range of x=0.36 to 0.65. The highest hardness value of fcc-Ta1-xAlxN is ~34 GPa. The hardness values of single phase wur-Ta1-xAlxN are around ~23-24 GPa. In conclusion, the results show that the NbNy and TaNy films present a high complexity due to the variety of crystallographic phases that can be formed. Through the incorporation of Al atoms into the crystal lattice the fcc structure is stabilised for x below 0.44 (Nb1-xAlxN) and 0.36 (Ta1-xAlxN). Higher Al contents promote the formation of the wurtzite phase.

AB - Hard coatings, such as transition metal nitrides (TMN) are widely used for wear-resistance in machining, casting or hot-forming applications. However, TMN oxidise rapidly and often these oxides have a porous morphology which enables a rapid scale growth. Therefore, ternary compounds (TM,Al)N are researched in order to improve the oxidation resistance but also mechanical properties of TMN coatings. Important here is, that the superior oxidation stability and mechanical properties are obtained for the face centred cubic (fcc) structure, which requires investigations of the phase stability ranges of (TM,Al)N coatings. In this work, a series of NbNy and TaNy coatings were deposited on Si- and austenitic stainless steel substrates with an increasing N2 partial pressure by a reactive unbalanced magnetron sputtering technique. These studies were necessary to find the deposition parameters to grow single-phase fcc NbN and TaN layers. Furthermore, the fcc-NbN and fcc-TaN phases are alloyed with Al to form a ternary Nb1-xAlxN and Ta1-xAlxN compound. With increasing N2-partial pressure during sputtering of a Nb-target the obtained coatings crystallize to form hex-Nb2N, hex-Nb2N+fcc-NbN, hex-NbN+fcc-NbN and finally almost single-phase fcc-NbN (measured by X-ray diffraction, XRD). In contrast, the TaNy films crystallize to form hex-Ta2N, hex-Ta2N+fcc-TaN, and finally fcc-TaN+hex-TaN. The changed structures cause a significant change of the mechanical properties measured by nanoindentation. The hardness value of fcc-NbN is approximately 28 GPa. The single phase fcc-TaN could not be obtained. But the hardness of the fcc+hex-TaN is similar to the hardness of fcc-NbN. These binary nitrides were alloyed with an increasing Al-content, x=Al/(Nb,Ta+Al), of x=0.11 to 0.70. A single phase fcc-Nb1-xAlxN exists in the range of x=0.11 to 0.44, and the lattice parameter decreases with increasing Al-content at the same time the hardness increases until it reaches ~32 GPa. The transition zone to form a dual phase of fcc-Nb1-xAlxN+wur-Nb1-xAlxN is in the range of x=0.44 to 0.61 with decreasing hardness. A single phase wur-Nb1-xAlxN exists for x>0.61 with hardness values of ~22-23 GPa. In contrast, the transition zone for Ta1-xAlxN is already in the range of x=0.36 to 0.65. The highest hardness value of fcc-Ta1-xAlxN is ~34 GPa. The hardness values of single phase wur-Ta1-xAlxN are around ~23-24 GPa. In conclusion, the results show that the NbNy and TaNy films present a high complexity due to the variety of crystallographic phases that can be formed. Through the incorporation of Al atoms into the crystal lattice the fcc structure is stabilised for x below 0.44 (Nb1-xAlxN) and 0.36 (Ta1-xAlxN). Higher Al contents promote the formation of the wurtzite phase.

KW - Nb-Al-N

KW - Ta-Al-N

KW - phase stability

KW - deposition

KW - hardness

KW - Nb-Al-N

KW - Ta-Al-N

KW - Phasensatabilität

KW - Beschichtung

KW - Härte

M3 - Diploma Thesis

ER -