Preparation And Properties Of High-temperature Oxidation Resistant Coating On Titanium Alloy Based On Double Glow Plasma Surface Metallurgy Technology

The titanium alloy，which is regarded as an important aeronautical material because of its highstrength weight ratio, is being helpful to reduce the weight of airplanes and aero engines. It is widelyconsidered the titanium alloy is positive to improve the performance of aero engines since the fiftiesof the20th century. However the weakness of the titanium alloy in oxidation resistance at hightemperature cannot be ignored. At the work condition of500℃, the oxide film of titanium alloy isvulnerable and prone to further oxidized, and then the whole component fails.The even more seriousis the fact that“titanium fire” causes the plane crashunder certain conditions. Further applications oftitanium alloy are restricted by the disadvantages mentioned above.Therefore, in the21st century, theoperating temperatures of the titanium alloys are required to be raised to adapt to the development ofhigh performance aero-engine. The key problems in the practical using of titanium alloy are its poorwear resistance at high-temperature above500℃.At present, high-temperaturesurface oxidation resistance of titanium alloy`has been a hot area ofresearch. The double glow plasma surface metallurgy technique, which offerssome unique properties,provides a new technical method to improve the antioxidant performance of titanium alloy. In thisstudy, the chemical composition and structure of high temperature oxidation resistant coatingwasfirstly designed, and Ti-Cr and Ti-CrNi coating was obtained on the surface of TC4alloy by thetechnique. Then the component, organization and the structure of coatings were studied in-depth.Effects of the processing on microstructures and properties were also analyzed, and the hightemperature oxidation behavior and thetribological behavior of coatings were researchedsystematically.Based on the oxidation mechanisim of titanium alloy, the influence of alloying elements ontitanium alloy and the principle of double glow plasma surface metallurgical technology, thedissertation given a design criteria about high temperature oxidation resistant coating. Firstly, themelting point of alloying elements must be higher than the processing temperature of double glowplasma surface metallurgical technology. Secondly, the alloying elements must be able to inform solidsolutions and intermetallics with Ti, which would be translated into compact, continuous andprotective oxidation film. Thirdly, the high temperature oxidation resistant coating must include acomplete structure of settled layer, mutual diffusion layer and sputtering-affected zone. The settledlayer should be compact and continuous, which would prevent the reaction of Ti with O. The thickness of mutual diffusion layer should be controlled in a certain range to realize the balance ofoxygen blocking and metallurgical bonding. On the premise of guaranteeingcontent and construction,the thickness and phase change of sputtering-affected zone should be strictly controlled. Based on theabove principle, the dissertation designed the contentsand constructions of the Ti-Cr and Ti-CrNicoating.The dissertation studied the best preparation conditions of the Ti-Cr and Ti-CrNi coating on thesurface of TC4alloy by the technology. In general, the thickness of coating was about25～35μm,which had a good bond with TC4alloy. And there was no holes detected in the coating. With theincreasing of Ni, the surface hardness of coating gradually decreased. However, the hardness ofmutual diffusion layer and sputtering-affected zonedid not changed with the amount of Ni. With theincreasing of Ni, the elastic resilience of settled layer decreased, and the elastic resilience of mutualdiffusion layer was constant. Ni increased the toughness and adhesion of settled layer, lowered themodulus of elasticity.The dissertation discussed the impact of the preparation processing on the mechanical propertiesof the substrate. The amount of α phase increased in TC4substrate after the processing, the coarsevermicular widmanstatten organizations and basket network organizations were also appeared. Thepreparation process had a little effect on the tensile properties of the TC4substrate. The effect ofpreparation processing on fatigue performance was relevant to the stress level. The cycle lives of TC4alloy was decreased by20to40%in the high stress level （≧200MPa）, the widmanstattenorganizations and basket network organizations were the main reasons to lead the decline of thetensile and fatigue properties of TC4alloy.This dissertation systematically studied the high-temperature oxidation behavior of the Ti-Cr andTi-CrNi coating at different temperatures and the oxidation mechanism was in-depth analysis. Thehigh-temperature oxidation mechanism of the Ti-Cr coating was: the outer diffusion grew of Cr and Ti,Al resulted the oxide films were formed; the intrusion diffusion of oxygen was limited to theinterdiffusion layer. However, the great brittleness of Ti （Cr, Al）2precipitated phase layer weakenedthe bonding strength of the oxide film and substrate. The accumulation of cyclic alternating stress wasthe main reason of the spalling of oxide film. When the Ti-CrNi coating was oxidized, Ni distributedin the oxide film in two ways. The one was NiO oxide mixed with Cr₂O₃film, the other was theformation of an independent continuous NiO film. The distribution form of NiO was not only relatedto the content of Ni in the coatings, but also to the influence of the oxidation temperature. The Niaddition of a certain amount was able to enhance the isothermal oxidation resistance performance of coatings. However, an excessively high content was to reduce the isothermal oxidation resistance ofcoatings. The basic principle of Ni addition wasNi should not interfer the formation of densecontinuous Cr₂O₃film at high-temperature oxidation. A certain amount of Ni could form a continuousNiO film beneath the Cr₂O₃film, it could not only enhance the capacity of oxygen barrier of the oxidefilm, then to enhance the high-temperature protective property; but also enhance the toughness andadhesion of coatings. The NiO film could achieve a transition between the oxide film and theprecipitation of Ti-CrNi phase layer, it bridge the comprehensive stress differenceof thermalexpansion coefficient of coatings. When the content of Ni was40at.%, the coating had the best hightemperature oxidation resistance.A comparative study on the behaviors of friction was conducted at room temperature and hightemperature. At room temperature, the settled layer of the coating mainly was adhesive wear, and theinterdiffusion layer was abrasive wear. The coating significantly improved the wear resistance of TC4alloy at room temperature. At high temperature, the high temperature and pressure exacerbated theoxidation and deformation of the alloy layer, the friction and wear of the coating exacerbated. Thewear volume and specific wear rate of the coating were rising rapidly with the load and sliding speedincreased, and the wear resistance decreased rapidly at high temperature. In the following experimentconditions:500℃, load730g, sliding speed10m/min, Si₃N₄as the friction pair, the coating wasinvalid for wear after60min.