| Due to their low density and high specific strength,titanium alloys are serving as components of gas turbine engines.However,the titanium alloys are commonly subject to severe oxidation at elevated temperatures above 500?.In addition,when it was used in marine environments,NaCl from the atmosphere accumulated on the surface,and corrosion involving deposited salt and water vapor occurred.The presence of salt and water vapor led to the increase of oxidation rate and the-formation of thick brittle a case at relatively low temperature which undermines the mechanical properties of these alloys.Surface modification such as coating is one favorable solution for improving corrosion and oxidation resistance of titanium alloys.In this thesis,more than 10 ?m thick Ti(Al)N single layer coatings and Ti/Ti(Al)N(composite)multilayer coatings with different Ti layer thicknesses were designed and deposited on Ti-6Al-4V alloy by arc ion plaiting(AIP)and TiAl coatings were also prepared by AIP under high vacuum level.The influences of these coatings on high temperature corrosion resistance of Ti-6A1-4V alloy were investigated.The oxidation kinetics of the bare and coated alloys at 450?-750?were studied using thermo-balance in the presence of solid NaCl,H2O,and O2 and cyclic oxidation setup in air.The morphology,chemical and phase composition,microstructure of the coated alloys before and after corrosion were investigated using X-ray diffraction(XRD),scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS),electron probe micro-analyzer(EPMA),transmission electron microscopy(TEM).The residual stress of the coatings was measured by XRD and the variation of internal stress in the coatings with temperature during the thermal cycles was calculated by finite element method(FEM).The main results are as follows:In the Ti/Ti(Al)N(composite)multilayer coatings,Ti(Al)N layer exhibited columnar structure,while the grains of Ti layer were equiaxed.The introduction of Ti layer into Ti(Al)N coatings disrupted the continuity of columnar grains of the TiAlN layer.With the increasing of Ti layer thickness,more and larger droplets were incorporated in Ti/Ti(Al)N(composite)multilayer coatings and the roughness value of the coatings became higher.The corrosion resistance of Ti-6Al-4V in the presence of solid NaCl,H2O and O2 was significantly improved at 450? by the deposition of TiN single layer coating and Ti/TiN multilayer coating with Ti layer thickness of 0.2 ?m and at 600? by the deposition of TiAlN single layer coating and Ti/TiAlN(composite)multilayer coating with Ti layer thickness of 0.15 ?m.These was due to that the accelerated corrosion of titanium alloy induced by Cl-was suppressed by Ti(Al)N layer.With the increasing of Ti sublayer thickness in the Ti/Ti(Al)N(composite)multilayer coating,the corrosion resistance of coatings decreased.The reason was that the inward diffusion of Cl-,H2O and O2 along the through-thickness defects of the Ti(Al)N layer such as pinholes offered the opportunities of their reaction with Ti sublayer.The resultant volume expansion from the oxidation process led to upward bending and cracking of the coating,which facilitated the inward diffusion of Cl-,H2O and O2.In addition,with the increasing of Ti layer thickness,more and larger droplets were incorporated in Ti/Ti(Al)N(composite)multilayer coatings,which exerted an adverse effect on corrosion resistance of coatings.Ti(Al)N single layer coating,Ti/TiN multilayer coatings and Ti/TiAlN composite multilayer coatings showed excellent oxidation resistance after cyclic oxidation at 600?.The oxidation resistance of the TiAlN single layer coating and Ti/TiAlN composite multilayer coatings was much better than that of the TiN single layer coating and Ti/TiN multilayer coatings.The thickness of the Ti sublayer did not play an important role on the oxidation resistance of the(composite)multilayer coatings.Ti/TiN multilayer coatings was unstable at this temperature,N element from TiN layer diffused into Ti layer,reacted with it and led to the formation of Ti2N and the total consumption of Ti layers.By contrast,N element in TiAlN layers did not diffuse to Ti layers and Ti/TiAlN composite multilayer coatings was stable at 600?.After cyclic oxidation at 750?,thin oxide scale formed on the surface of the thick TiAlN single layer coating and Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.15 ?m and 0.3 ?m.However,many cracks formed in the thick TiAlN single layer coating and Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.15 ?m after the cyclic oxidation.The Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.3 ?m did not crack and showed excellent cyclic oxidation resistance.The coatings were under high compressive stress in the temperature decreasing stage of the thermal cycle.The local Ti-6Al-4V substrates beneath the thick TiAlN single layer coating and Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.15 ?m deformed when suffered so large compressive stress,which led to the origination and propagation of cracks in the coatings.The triangle cracks with narrow top and wide bottom finally formed under the cyclic thermal stress.It was the 0.3 ?m thick Ti sublayer in the Ti/TiAlN composite multilayer coating that accommodated and released the stress by plastic deformation,and decreased the brittleness of the coating,so that no crack formed in the coating during thermal cycle at 750?.Bi-phased TiAl coatings which were composed of ?-TiAl+?2-Ti3Al were deposited on Ti-6Al-4V titanium alloys by high vacuum arc ion plating.Due to-different levels of ion bombardment under the substrate bias ranging from 0 V to-350 V,the relative decrease of Al contents was about 16%from 49 at.%to 41 at.%,while a dramatic 86%decrease of the relative porosity of coating was obtained from 3.12%to 0.44%.Al2O3-rich oxide layers were formed on the TiAl coatings during oxidation at 700?,showing good protectiveness of the TiAl coatings for Ti-6Al-4V alloys.Meanwhile,only thin IDZ layer composed of ?2-Ti3Al was observed after oxidation for 100 h.More compact coatings deposited under higher bias eliminated the internal oxides as appeared in the lower bias-deposited ones after oxidation,indicating that higher substrate bias is more favorable for improving the oxidation resistance of the TiAl coatings. |
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