Review of Numerical Simulation of TGO Growth in Thermal Barrier Coatings

Thermally grown oxide(TGO)is a critical factor for the service life of thermal barrier coatings(TBC).Numerical simulations of the growth process of TGO have become an effective means of comprehensively understanding the progressive damage of the TBC system.At present,technologies of numerical simulation to TGO growth include two categories:coupled chemical-mechanical methods and mechanical equivalent methods.The former is based on the diffusion analysis of oxidizing elements,which can describe the influence of bond coat(BC)consumption and phase transformation in the growth process of TGO on the mechanical behavior of each layer of TBC,and has high accuracy for the thickness evolution of TGO,but they cannot describe the lateral growth of TGO and the rumpling phenomenon induced.The latter focuses on describing the final stress and strain state after the growth of a specific TGO rather than the complete growth processes of TGO.Based on the measured TGO thickness growth curve,simulations of thickening and lateral growth can be achieved by directly applying anisotropic volumetric strain to oxidized elements and switching elements properties from the BC to the TGO.

Effect of Supersonic Fine Particles Bombarding on the Service Life of Thermal Barrier Coating

Thermally Grown Oxide（TGO） is a dominating component in controlling the effectiveness of thermal barrier coating.During the growth of TGO,whether we could homogeneously distribute Al atom on the TGO and the intermediate metal layer will be the key factor in forming TGO with continuous,uniform and single-ingredient（Al2O3）.In this experiment,we bombarded particles on to the metallic bound layer.We studied the influence of supersonic particle bombardment on the diffusion of Al.We hope to control the growth of TGO by monitoring the diffusion of Al.Thermal barrier coating（TBC）,which consists of a NiCoCrAlY bond coat and a ZrO2-8Y2O3（wt.%） topcoat（TC）,is fabricated on the nickel-base superalloy by air plasma spray（APS）.NiCoCrAlY bond coat is treated by supersonic fine particles bombarding（SFPB）.The morphology,oxidation behavior of TBC and phase are characterized by scanning electron microscope（SEM） equipped with an energy dispersive spectromrter（EDS） and X-ray diffractometer（XRD）.The influence of supersonic fine particles bombarding technique on the service life of thermal barrier coating is studied.The results show that SFPB technique improves the flaw of excessive surface undulation in the as-sprayed bond coat.A continuous,uniform and single-ingredient（Al2O3） TGO can quickly form in the SFPB TBC during high temperature oxidation process.The thickening of TGO is relatively slow.These will effectively suppress the formation of other non-protective oxides.Therefore,SFPB technique reduces the growth stress level generated by the continuous growth of TGO,and also avoids the stress concentration induced by formation of the large particle spinal oxide.Thermal barrier coating still remains well after 350 thermal cycles.The service life of TBC is improved.The proposed research provides theoretical basis and technical references to further improve and enhance the SFPB technique.

Influence of the Thermal Barrier Coatings Design on the Oxidation Behavior

The properties of two different types of thermal barrier coatings （TBCs） were compared to improve the surface characteristics on high temperature components. These TBCs consisted of a duplex TBC and a five-layered functionally graded TBC. NiCrAIY bond coats were deposited on a number of Inconel-738LC specimens using high velocity oxy-fuel spraying （HVOF） technique. For duplex coating, a group of these specimens were coated with yttria stabilized zirconia （YSZ） using plasma spray technique. Functionally graded NiCrAIY/YSZ coatings were fabricated by plasma spray using co-injection of the two different powders in a single plasma torch. The amount of zirconia in functionally graded coatings were gradually increased from 30 to 100 vol. pct. Microstructural changes, thermally grown oxide （TGO） layer growth and damage initiation of the coatings were investigated as a function of isothermal oxidation test at 970℃. As a complementary test, the performance of the fabricated coatings by the optimum processing conditions was evaluated as a function of intense thermal cycling test at 1100℃. Also the strength of the adhesive coatings of the substrate was also measured. Microstructural characterization was analyzed by scanning electron microscopy （SEM） and optical microscopy whereas phase analysis and chemical composition changes of the coatings and oxides formed during the tests were studied by XRD （X-ray diffraction） and EDS （energy dispersive spectrometer）. The results showed that microstructure and compositions gradually varied in the functionally graded coatings. By comparison of duplex and functionally graded TBCs oxidation behavior （duplex failure after 1700 h and funcitionally graded TECs failure after 2000 h）, thermal shock test and adhesion strength of the coatings, the functionally graded TBC had better performance and more durability.

Spallation Process of Thermally Grown Oxides by In-Situ CCD Monitoring Technique

In cooling process of Fe-Cr-Al alloy oxidized at 1 300 ℃, the effect of cooling speed and exposure time on oxide spalled area fraction and successive variety of the spalled region were studied by investigating evolvement of the thermally grown oxide using in-situ CCD monitoring technique. The results showed that oxide spallation can be restrained by controlling cooling speed and the critical temperature drop of spallation initiation which is closely related to the oxide thickness or exposure time, and the spallation process of a little region may be described in more detail as two routes： from the oxide/substrate interface micro-decohesion, micro-buckles, buckle spreading, buckle crack to spallation and from the interface micro-decohesion, micro-buckles, buckle crack and spallation to the residual oxide decohesion and spallation.

Time of flight improved thermally grown oxide thickness measurement with terahertz spectroscopy

As a nondestructive testing technique,terahertz time-domain spectroscopy technology is commonly used to measure the thickness of ceramic coat in thermal barrier coatings(TBCs).However,the invisibility of ceramic/thermally grown oxide(TGO)reflective wave leads to the measurement failure of natural growth TGO whose thickness is below 10μm in TBCs.To detect and monitor TGO in the emergence stage,a time of flight(TOF)improved TGO thickness measurement method is proposed.A simulative investigation on propagation characteristics of terahertz shows the linear relationship between TGO thickness and phase shift of feature wave.The accurate TOF increment could be acquired from wavelet soft threshold and cross-correlation function with negative effect reduction of environmental noise and system oscillation.Thus,the TGO thickness could be obtained efficiently from the TOF increment of the monitor area with different heating times.The averaged error of 1.61μm in experimental results demonstrates the highly accurate and robust measurement of the proposed method,making it attractive for condition monitoring and life prediction of TBCs.

Substrate Effects on the High-Temperature Oxidation Behavior of Thermal Barrier Coatings

The high-temperature oxidation behaviors of the NiCrAIYSi/P-YSZ thermal barrier coatings （TBCs） produced by electron beam-physical vapor deposition （EB-PVD） on directionally solidified （DS） and single crystalline （SC） Ni-based superalloy substrates were investigated. The cross-sectional microstructure investigation, isothermal and cyclic oxidation tests were conducted for the comparison of oxidation behaviors of TBCs on different substrates. Although TBC on DS substrate has a relatively higher oxidation rate, it has a longer thermal cycling lifetime than that on SC substrate. The primary factor for TBC spallation is the mismatch of thermal expansion coefficient （TEC） of the bond coat and substrate. The morphological feature of thermally grown oxide （TGO） has a strong influence on the TBC performance. By optimizing the elemental interdiffusion between bond coat and substrate, a high quality TGO layer is formed on the DS substrate, and therefore the TBC oxidation behavior is improved.

Effect of TGO on the tensile failure behavior of thermal barrier coatings

Thermally grown oxide(TGO)may be generated in thermal barrier coatings(TBCs)after high-temperature oxidation.TGO increases the internal stress of the coatings,leading to the spalling of the coatings.Scanning electron microscopy and energy-dispersive spectroscopy were used to investigate the growth characteristics,microstructure,and composition of TGO after high-temperature oxidation for 0,10,30,and 50 h,and the results were systematically compared.Acoustic emission(AE)signals and the strain on the coating surface under static load were measured with AE technology and digital image correlation.Results showed that TGO gradually grew and thickened with the increase in oxidation time.The thickened TGO had preferential multi-cracks at the interface of TGO and the bond layer and delayed the strain on the surface of the coating under tensile load.TGO growth resulted in the generation of pores at the interface between the TGO and bond layer.The pores produced by TGO under tensile load delayed the generation of surface cracks and thus prolonged the failure time of TBCs.