|Alternative Title||Research on Strengthening and Failure Mechanisms of Coating/Substrate Systems|
|Place of Conferral||北京|
|Keyword||涂层/基底体系 强化 内聚力模型 损伤 失效|
Coating/substrate systems have been widely used in aerospace, automobile and ship industry due to the coating’s excellent properties of thermal insulation, wear resistance, corrosion resistance and so on. For example, thermal barrier coatings (TBCs) are used in turbine blades of aircraft engines to reduce the temperature of superalloy substrate, which can protect turbine blades and improve thermal efficiency of engines.
Since coating/substrate systems are always subjected to mechanical loads inevitably in practical applications, coatings not only play a role in important functions like thermal insulation, but also strengthen the system. It has been found that the strengthening effects are actually remarkable, which involves various complex micro mechanisms. However, little concern is paid on the macro strengthening behaviors, for example, influences of coating properties including the coating bonding area and the coating modulus as well as the interface properties on strengthening effects. Furthermore, the failure behaviors involving coating cracking and interface delamination after the strengthening may lead to the loss of strengthening effects and other functions such as thermal insulation. Therefore, it is of great practical significance to investigate the strengthening and failure mechanisms of coating/substrate systems. In this thesis, by means of theoretical modelling and finite element simulations using cohesive zone model, strengthening and failure mechanisms of coating/substrate systems were studied systematically. The main work and results could be stated as follows:
(1) Based on the beam theory and some assumptions about interfacial stresses, a theoretical model to reflect the strengthening effects for systems under three-point bending was developed. The explicit expression of the system stiffness was given. Influences of coating properties and interface properties on the system stiffness and interfacial stresses were revealed. It is found that with the increase of coating bonding length, the system stiffness increases and tends to saturation eventually, while singularities of interfacial stresses at the coating end become weaker. The critical coating bonding length corresponding to the saturation of system stiffness increases monotonously with the increase of coating thickness and coating modulus, as well as the decrease of interfacial stiffness. Thus, it is most efficient and economical to take the critical bonding length as the coating length, but the singularities of interfacial stresses at the coating end should be taken into consideration carefully.
(2) The overall mechanical responses of systems with cohesive zone model were derived theoretically from a series model at first, which serves as a theoretical basis for modelling of failure behaviors of coating/substrate systems. When cohesive elements are introduced into single-material media, in order to ensure that the introduction of cohesive elements should not affect the original mechanical properties, a selection criterion of stiffness of finite-thickness cohesive elements was proposed. As for bi-material systems, for example, coating/substrate systems, the thickness-dependent fracture characteristic on the macroscopic scale was revealed by introducing cohesive elements to the interface: as the system thickness such as coating thickness increases, the fracture of the system tends to be closer to the catastrophic failure. This is in accordance with literature results of molecular dynamics simulations on the microscopic scale.
(3) Through finite element method (FEM) using cohesive element model, the failure behaviors of coating/substrate system under three-point bending in different failure modes were modelled, and influences of coating thickness, the strength and fracture toughness of coatings and interface were discussed. For systems with very strong interface, the failure mode is mainly coating cracking. Thus, only transverse cohesive elements were inserted into the coating, and values of cohesive stiffness were chosen according to selection criterion proposed in this thesis. The results indicate that with the increase of coating strength, transverse crack length increases firstly and then decreases. And with the increase of coating fracture toughness, transverse crack length decreases monotonously. For systems with very weak interface, the failure mode is mainly interface delamination. Thus, only cohesive elements were inserted to the interface of coating and substrate. The results imply that with the increase of interfacial strength, interface delamination is easier at first and then becomes harder, while the increase of interfacial fracture toughness always makes the delamination harder to occur. Generally, for systems in mixed failure mode, the dominated failure mode is closely related to coating thickness: with the increase of coating thickness, the dominated failure mode transitions from coating cracking to interface delamination, which is consistent with experimental results. The damage defined by dimensionless crack length with dimensionless loading displacement obeys the power-law damage and failure model, and the damage rate shows the singularity of 0.5 near the catastrophic failure point. The damage of thick coating systems whose failure are dominated by interface delamination is faster than thin coating systems whose failure are dominated by coating cracking. It can be found that with the increase of interfacial strength, the damage of thick coating systems is slower, which can be used to account for the slower damage of nanostructured ceramic coatings with higher interfacial strength.
|龙浩. 涂层/基底体系的强化及失效机制研究[D]. 北京. 中国科学院大学,2018.|
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