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Hollow cathode gas flow sputtering of NixAly coatings on Ti-6Al-2Sn-4Zr-6Mo: Mechanical properties and microstructures

: Kohns, A.; Ortner, K.; Jung, T.


Advanced engineering materials 11 (2009), Nr.1-2, S.67-70
ISSN: 1438-1656
ISSN: 1615-7508
Fraunhofer IST ()

This paper reports on the deposition of Ni(x)Al(y) coatings on Ti-6Al- 2Sn-4Zr-6Mo by hollow cathode GFS (gas flow sputtering). It is possible to deposit Ni(x)Al(y) coatings with a large variety of mechanical properties, compositions and microstructures. The latter can be influenced significantly by variations of the bias voltage. Hydroxides or oxides can develop within the porosities due to multi-stage chemical processes during or after sampling the coated specimen out of the vacuum chamber. With regard to the nickel content in the coating, a significant enrichment can be observed for bias voltages of 50 V and more. With regard to the mechanical properties, it occurred that plastic hardness and elasticity correlate for variations of the substrate bias and the process pressure. The reduction of porosities for bias voltages of 25 and 50 V goes along with higher levels of hardness and elasticity. Peak values for both factors are found at U(ind B) = 50V, while the elastic indentation modulus reaches a minimum there. At U(ind B) = 150V, variations of the oxygen content do not occur for different process pressures. However, for high bias voltages an increasing process pressure can lead to a significant temperature increase at the substrate, which influences the mobility of the atoms during deposition. This effect can have a measurable impact on the mechanical properties and might explain the variations regarding hardness and elasticity for different process pressures. At U(ind B) = 50 V, it appears that these effects compensate as no significant variations can be observed. High compressive stresses were detected in the coating deposited without substrate bias. Due to the high porosity of the coating, this effect is attributed to the creation of oxides/hydroxides in the hollow spaces and the related increase in density. The variation of residual stresses between compressive and tensile state for bias voltages between 25 and 150 V is discussed. The results of this study can be used as a basis to diminish the fatigue strength reduction of Ti-6Al-2Sn-4Zr-6Mo components coated with an erosion resistant multilayer system. In contrast to the standard deposition parameter (U(ind B) = 25 V, p(ind Ch) = 0.49 mbar), a substrate bias of 50 V, for example produces a coating with a microcrystalline structure, higher hardness and elasticity values and a lower stiffness. Coatings like these might have the potential to diminish the propagation of cracks into the substrate.