Research On Crack Behaviour With Texturization Of Bonding Layer In Thermal Barrier Coating Based On Digital Image Correlation

The thermal barrier coating is a multi-layer and composite structure.The coating interface is the location of stress concentration,which is also the important factor causing of coating peeling.Rough morphology of coating interface can improve the bonding strength of the coating,but it causes stress concentration of interface.Therefore,this working mainly focuses on problem,which is balancing the interface micro-morphology and stress concentration.The purpose of this paper is to explore the influence of interface micro-morphology on stress and strain distribution and crack evolution behavior.Thermal barrier coatings are prepared by supersonic plasma spraying technology in this working.Uniform groove morphologies of different depths on the surface of the bonding layer are prepared by laser.The surface strain characteristics of the coating are obtained by using of the stretching-digital image correlation,and the influence of the interface texture on the surface strain of the coating is explored.Combining the optical microscope and the plane strain model,the damage mechanism of interface texturing thermal barrier coating is analyzed.Based on the surface strain of coating,a simple method to judge cracks is proposed.The main works in this paper are as follows:(1)The stretch-digital image correlation experiment is designing to explore the surface strain characteristics of the coating.It is found that there are four stages of surface strain during the stretching process: the strain concentration area is generated at one end of the effective gauge length of the coating,and the strain decreases with the increase of coating thickness;the strain fringes appear on the surface of the coating,and the distribution of strain fringes is distributed with equal width;when the coating reaches the strain limit,cracks occur firstly in the strain concentrated area;the number of cracks increases continuously until the cracks reach a saturated state.(2)The laser is used to prepare groove morphologies of different depths on the surface of the bonding layer.When the strain of the substrate is 2%,the surface strain of the textured thermal barrier coating is larger than the non-textured coating.Before the crack occurred,the surface strain of textured coating is lower than non-textured coating,and the surface strain decreased as the groove depth increased.Small matrix strain can produce cracks.However,the surface of the bonding layer formed uniform groove morphology through the interface texture treatment.The strain concentration areas at the tip and valley bottom alternate with each other,so that the surface strain curve of the coating presents a relatively uniform wave shape,reducing the influence of strain concentration at the interface between the ceramic layer and the bonding layer.(3)There are two inflection points in the strain curve of characteristic points on the thermal barrier coating surface.The first inflection point indicates to the occurrence of surface cracks.The second inflection point indicates to the propagation of interface cracks.The surface texture of the bonding layer has stress concentration in the tip and valley bottom area.As the groove depth increases,the stress value between the tip and the bottom of the valley increases,making the time earlier of surface crack generation.But the time of interface crack propagation is delayed,which is of great significance to resist tensile damage.(4)Combining optical microscopy and plane strain finite element model,the crack propagation behavior of the interface-textured thermal barrier coating is proposed.The cracks were first initiated in the TC layer and vertically extended to the peak of the ceramic layer.Then,cracks extended to the valley of the BC layer along the junction of the peak and valley.When the value of Rc at the top ceramic and bonding coating interface was less than 22 ?m,the cracks expanded along the top ceramic and bonding coating interface,and adjacent cracks were connected,leading to spalling in the top ceramic layer.On the contrary,the cracks expand along the interface between the bonding layer and the ceramic layer,and adjacent cracks are connected to cause the ceramic layer to peel off.