Papers by Author: J. Yan

Abstract: Pressure-pushing method of evaluation for adhesion strength of high performance coatings material and substrate is presented. This method, based on the pressure-testing device of high performance bond strength test sample patent, is one that the sample is cracking along the interface by putting the static load. According to the sample’s stress boundary conditions, the formula of the adhesion strength is given. The surface coating/substrate interface stress distribution is analyzed by using the finite element method, which provides the theory basis for the correct evaluation of coating/substrate strength. The experiments are conducted on the strength of common coating substrate materials as well. All the results show that pushed method can test the adhesion strength of high performance coatings and substrate, whose data is smaller than the international standard and A STMC633-79. And the experimental data of the pushed method are free from the influence of random factors.

Abstract: Strategies for time-economic lifetime assessment of thermal barrier coatings (TBC) in service are described and discussed on the basis of experimental results, achieved on material systems with coatings applied by electron beam physical vapour deposition. Service cycles for gas turbine blades have been simulated on specimens in thermo-mechanical fatigue tests, accelerating the fatigue processes by an increase of load frequency. Time dependent changes in the material system were imposed by a separate ageing, where the samples were pre-oxidized prior to the fatigue test. Results of thermo-mechanical fatigue tests on pre-aged and as-coated specimens gave evidence of interaction between fatigue and ageing processes. An alternative approach is used, which is focused on the evolution of a failure relevant damage parameter in the TBC system. The interfacial fracture toughness was selected as a damage parameter, since one important failure mode of TBCs is the spallation near the interface between the metal and the ceramic. Fracture mechanical experiments based on indentation methods have been evaluated for monitoring the evolution of the interfacial fracture toughness as a function of ageing time. It was found that the test results were influenced by both changes of the interface (which is critical in service) and changes in the surrounding material.