Erosion Resistance And Failure Mechanism Of Ti/TiAlN Multilayer Coating

Erosion is caused by the impact of solid particles against the surface of an object.The impacting particles gradually remove material from the surface through repeated deformations and cutting actions.Erosion phenomena widely exist in many industries,for instance,mechanics,metallurgy,energy,building materials,aeronautics and astronautics.Depositing coatings with good erosion resistance on the surface of materials or parts can enhance their erosion resistance,improve their properties and prolong their service life.The crack resistance and toughness of traditional hard coatings like Ti N and TiAlN are generally poor,which cannot satisfy many harsh application conditions like turbine blades.In this thesis,Ti/TiAlN multilayer coatings were fabricated on Ti-6Al-4V titanium alloy by a home-made hybrid multisource cathodic arc ion deposition system aiming to achieve the high performance of coatings combined with high hardness,toughness,and good erosion resistance,and reveal the role of metal layers and interfaces of multilayer coatings in improving the crack resistance and erosion resistance of coatings.The thickness,modulation period and modulation ratio of the multilayer coatings are important parameters that affect their strength and toughness.Firstly,the effects of coatings’ thickness(6 μm,11 μm,15 μm),modulation period(6,12,24)and modulation ratio(1:2,1:3,1:5)on their surface roughness,phase composition,hardness and the adhesion strength were studied by orthogonal analysis.The results showed that the architecture of coatings had a limited effect on the surface roughness and phase composition of the coatings.The order of importance of the factors affecting the hardness and erosion rate of Ti/TiAlN multilayer coatings was modulation ratio,modulation period and coatings’ thickness;the order of importance of the factors affecting the adhesion strength was modulation ratio,coatings’ thickness,modulation period.The hardness of the coatings increased with the decrease of modulation ratio and decreased with the increase of modulation period,whereas the change was not obvious with the change of coatings’ thickness.With the decrease of modulation ratio or increase of the coatings’ thickness,the adhesion strength increased.With the increase of the modulation period,the adhesion strength increased at first but decreased then.With the decrease of modulation ratio,the erosion rate of the coatings’ decreased.With the increase of the modulation period,the erosion rate of the coatings’ increased but decreased with the increase of coatings’ thickness.Crack resistance of the coatings can evaluate their erosion resistance indirectly.Furthermore,the Ti/TiAlN multilayer coating with good comprehensive mechanical properties selected through the orthogonal experiment was used to comparatively study the crack resistance of the Ti/TiAlN multilayer coating and the TiAlN monolithic coating by scratch test,and the crack resistance and failure mechanism of the coating under scratch test conditions were investigated in terms of structural evolution.The results showed that compared with the Ti/TiAlN multilayer coating,the TiAlN monolithic coating had higher hardness and elastic modulus,but poor plasticity and bearing capacity.Ductile Ti layers had three functions of bonding,energy absorption,and coordinated deformation,improving the toughness and bearing capacity of the coating.Ti layers also induced a large number of strong interfaces in the multilayer coating,hindering the propagation of the cracks.The cohesion and adhesion strength between different layers and between the coating and the substrate was enhanced.A home-made erosion test rig has been built,which can simulate various erosion environments such as different erosion media,different impact angles and different erosion speeds.The erosion resistance of the coating was evaluated by the erosion tester,and the effects of type of impacting particles and impacting angles on the erosion resistance and failure mechanism of the coating were discussed.The results showed that under all test conditions,the Ti/TiAlN multilayer coating had a good protective performance on the Ti-6Al-4V substrate.The Ti/TiAlN multilayer coating had a better protective effect on the substrate eroded by the angular silica,showing the nature of“layer by layer” damage.Eroded by a low angle impact accelerated the failure of the coating,but had little impact on the erosion failure mechanism.Under the erosion of the normal impact of angular silica,Ti/TiAlN multilayer coating showed the feature of“area” damage.Eroded with a low angle impact accelerated the failure of coating,showing the mechanism of "area" damage and “layer by layer” damage.The inducing of the Ti layers increased the number of interfaces within the coating,and the propagation of radial cracks was deflected at the interfaces,which enhanced the erosion resistance of the coating.