C. Reinhard

Influence of the bias voltage on the structure and mechanical performance of nanoscale multilayer CrAlYN∕CrN physical vapor deposition coatings

The effects of bias voltage on the microstructure and the related tribological properties of CrAlYN∕CrN nanoscale multilayer superlattice coatings were investigated. The coatings were deposited at 450°C substrate temperature by combined high power impulse magnetron sputtering (HIPIMS) and unbalanced magnetron sputtering techniques. The substrates were 304 stainless steel, M2 high speed steel for structural analysis and mechanical testing, as well as cemented carbide substrates end mills for dry high speed milling applications. Substrates were pretreated by HIPIMS etching. The bias voltage Ub was varied between −75 and −150V. The chemical composition was determined by neutral mass spectroscopy. The microstructure was characterized by x-ray diffraction and cross sectional transmission electron microscopy. All coatings had a single phase B1 fcc structure. The chemical composition was not affected by the bias voltage. Local epitaxial or axiotaxial growth attributed to the HIPIMS etching pretreatment was obser...

Enhanced sulphidation/oxidation resistance of Ti–45Al–8Nb alloy by multilayered coatings at 850°C for up to 675 h

Abstract The high temperature sulphidation/oxidation behaviour of three multilayered coatings CrAlYN/CrN etched by Y+, CrAlYN/CrN etched by Cr+ and CrAlYN/CrN etched CrAl+ and the uncoated γ-TiAl (Ti–45Al–8Nb (at-%) used as reference sample was studied at 850°C for 675 h. Sulphidation/oxidation test was performed in the environment of H2/H2S/H2O, yielding pS2 = 10−1 Pa and pO2 = 10−18 Pa. Kinetic data obtained by discontinuous gravimetric method showed that the multilayered coatings effectively enhanced the sulphidation/oxidation resistance of γ-TiAl alloy. The corrosion resistance decreasing in order: CrAlYN/CrN etched by Y+>CrAlYN/CrN etched by CrAl+>CrAlYN/CrN etched by Cr+>γ-TiAl. Scale development studies using SEM, EDX and X-ray diffraction confirmed two regions of the coated materials: ‘affected,’ where coating cracked and developed non-protective TiO2 scale, and ‘unaffected,’ where protective (Al,Cr)2O3 scale formed. The uncoated γ-TiAl material, after exposure, showed a typical multilayered structure consisting of layers of TiO2 and Al2O3.

Oxidation performance of nano-scale multilayer coatings on γ-TiAl

There is a major drive to introduce γ-TiAl into gas turbine engines in order to reduce weight. However, this will require the development of coatings that protect against oxidation at high temperature, but do not adversely affect the mechanical properties. This work reports the high temperature degradation mechanisms of a nanoscale CrAlYN/CrN multilayer coating deposited on γ-TiAl(8Nb) by a combined high power impulse magnetron sputtering / unbalanced magnetron sputtering. Detailed TEM/STEM of FIB prepared specimens from isothermal static oxidation tests at 850°C for up to 1030 hours is presented. The evolution of the complex oxide structure and the implications for future coating development is discussed.