Investigation On Erosion-corrosion Behaviors And Cavitation Erosion-Corrosion Behaviors Of Al Alloying β-Ta₅Si₃ Nanocrystaline Coating

In this work,to obtain the optimized Al addition,the structural stability,mechanical properties and electronic structure ofβ-Ta₅Si₃ with different Al additions are systematically investigated by first-principles density functional calculations.The binary and ternaryβ-Ta₅Si₃ are thermodynamically and mechanically stable,while Al addition reduces their thermodynamic stability slightly.The elastic constants,bulk modulus（B）,shear modulus（G）and Young’s modulus（E）ofβ-Ta₅(Si_1-xAl_x)₃ are calculated.The results for theβ-Ta₅(Si_1-xAl_x)₃ show that the values of bulk modulus（B）,shear modulus（G）and Young’s modulus（E）decrease with increasing Al content,suggesting that the Al alloying weakens the hadness and shear deformation resistance ofβ-Ta₅Si₃.Meanwhile,the toughness ofβ-Ta₅Si₃ after Al addition has been evaluated according to the bulk/shear modulus（ratio B/G ratio）and Poisson’s ratio criteria,indicating Al alloying improves the toughness ofβ-Ta₅Si₃.In general,the D8m-structured ternaryβ-Ta₅Si₃,where two Si^4a atoms are replaced by two Al atoms,shows the better ductility compared to other Al alloyedβ-Ta₅Si₃.To further explore the toughening mechanism,the density of states forβ-Ta₅(Si_1-xAl_x)₃ have been calculated,demonstrating that the form of Al-Ta metallic bonds and the reduction of covalent property for theβ-Ta₅Si₃ should be accounted for the improvement of ductility.Depending on on the optimal amount of Al addition,theβ-Ta₅Si₃ andβ-Ta₅(Si_0.83Al_0.17)₃nanocrystalline coatings are deposited onto mirror-polished 316L stainless steel substrates by a double cathode glow discharge plasma method.The phase identification,microstructural characteristic and composition of the coatings are analyzed using X-ray diffraction（XRD）,scanning electron microscopy（SEM）incorporating an X-ray energy dispersive spectroscopy（EDS）analyser attachment and transmission electron microscopy（TEM）.The XRD result shows that both coatings are consisted of only tetragonalβ-Ta₅Si₃ phase.The cross-sectional SEM morphology indicates that the two coatings with a thickness of²0μm exhibit a dense and homogeneous microstructure along the thickness and well-attached coating/substrate interface.The as-depositedβ-Ta₅(Si_0.83Al_0.17)₃ coating is composed of nearly rounded D8_m-β-Ta₅Si₃ grains with an average size of⁴ nm and exhibited the intense（002）and（400）preferred orientation.After Al alloying,the hardness and the residual stress of theβ-Ta₅Si₃decreased by nanoindentation tests,but the contact damage resistance and adhesion strength are increased.The electrochemical tests in 3.5 wt.%NaCl solution show that the Al alloying improves the corrosion resistance of theβ-Ta₅Si₃.The behavior of erosion-corrosion and cavitation erosion-corrosion for theβ-Ta₅Si₃ and Al-alloyedβ-Ta₅(Si_0.83Al_0.17)₃ coatings are investigated with the assistance of electrochemical measurements in laboratory.Based on the test results and the analysis of the materials failure,β-Ta₅(Si_0.83Al_0.17)₃ coating is found to possess the best resistance against erosion-corrosion and cavitation erosion-corrosion in 3.5 wt.%NaCl solution among the tested materials.