山崎 泰広

The effect of the bond-coating on the adhesion strength after the thermal fatigue test was investigated in the air plasma sprayed thermal barrier coatings (APSed TBCs). In this work, two kinds of the CoNiCrAlY powder with the different particle size were used for spraying the bond coating material. In addition, the bond coat was sprayed by either the low-pressure plasma spray (LPPS) method or the high velocity oxy-fuel (HVOF) one. The adhesion strength and the residual stress were evaluated as a function of the thermal fatigue cycles. The experimental results indicated that the adhesion strength after thermal cycle fatigue was varied with the bond-coating. The effects of the residual stress and the thermal fatigue damage, i.e., micro-cracks, on the adhesion strength were discussed.

産業用発電システムで運用される先進高効率ガスタービンでは高温部の主要部材であるタービン動静翼に対する遮熱コーティングが不可欠となっている.本研究では,大気プラズマ溶射遮熱コーティングを対象として,熱サイクル数と保持時間を変数とした熱サイクルを与えた試験片の残留応力を評価した.また,熱サイクル損傷量の代表値として界面近傍のトップコーティング内における微小き裂のき裂長さを測定するとともに,残留応力,熱サイクル損傷と熱サイクル波形の関連性について検討した.

The tensile and the 4-point bending tests of the free-standing air plasma sprayed thermal barrier coating (APSed TBC) were carried out in order to evaluate the mechanical properties. The effects of the process variables in the APS and the isothermal exposure on the mechanical properties were investigated. The experimental results indicated that the mechanical properties of the TBC ceramic top coating were significantly changed by the process variables and the isothermal exposure. The elastic modulus evaluated from the unloading curve was much higher than that from the initial loading curve. The microstructural and analytical investigations were also carried out and the relationship between the mechanical properties and the splat structure was discussed. It was cleared that the mechanical properties of APSed TBC was controlled with the change of the splat structure by the initial stage of the sintering.

The correlation between the mechanical properties and the splat microstructure of an APSed TBC was studied by using the simple sintering model and the numerical simulation, i.e., the distinct element method. It was cleared from results of the numerical simulation that the mechanical properties of an APSed TBC were controlled with the change of the splat structure by the initial stage of the sintering. The change of mechanical properties in the APSed TBCs by the thermal exposure is predictable by using the proposed method in this work.

Thermal barrier coatings (TBCs), that reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of industrial gas turbines. Recently, in order to take full advantage of the potential of the TBC systems, experimental and analytical investigations in TBC systems have been performed. However there is a little information on the deformation behavior of the top coating. In addition, the effects of the thermal exposure and the process parameters on the mechanical properties of the top coating have never been clarified. From these backgrounds, the effects of the process variables in APS and the thermal exposure on the mechanical properties were investigated in order to optimize the APS process of top coatings. The experimental results indicated that the mechanical properties of the APS-TBC, i.e. the tensile strength and the elastic modulus, were significantly changed by the process variables and the long term thermal exposure. The microstructural investigation was also carried out and the relationship between the mechanical properties and the porosity was discussed.

The delamination resistance in a plasma-sprayed thermal barrier coating (TBC) system was evaluated by means of a modified four-point bending test. In this work, in order to study the effect of the interface roughness between the bond coating (BC) and the top coating (TC), two kinds of the powder with the different particle size were used for spraying the BC material. In addition, the influence of the isothermal aging on the delamination resistance was also investigated. The experimental results indicate that the effect of the TC/BC interface roughness wasn't significant. The energy release rate Gc, which was estimated by the modified four-point bending tests, increased with increasing aging time at 1000 °C, on the other hand, the scatter of it decreased with increasing aging time. The crack mainly propagated in the top coating for as-sprayed condition and at the top-coating/bond-coating interface after thermal aging for 2000 h. The energy release rate Gc was correlated with the fracture strength of the weakest parts of the TBC specimen. The energy release rate Gc increased with thermal aging until a critical time due to the sintering of the top-coating expired. It can be expected that Gc will decrease with the thermal aging after the critical aging time because the top-coating/bond-coating interfacial strength decreases by the TGO growth. The critical aging time in this work is approximately 2000 h. © 2005 Elsevier B.V. All rights reserved.

The thermal cycle tests of the air plasma sprayed thermal barrier coatings were carried out by means of the electric furnace or the high frequency induction heating system. In this study, the in-situ damage behavior was measured by using the acoustic emission system. The experimental results indicated that the thermal cycle damage occurred in cooling stage mainly. The effects of the heating method and the thermal cycle wave on the damage behavior were discussed.

The cyclic indentation test of the air plasma sprayed thermal barrier coating was carried out. The experimental results indicated that the local delamination area and the peak indentation depth were increased with the increasing of the number of cycles. The acoustic emission signals occurred in the early stage of cycles and it was related with local delamination damage. However after a few cycles, the acoustic emission signals were lost.

The effects of the porosity on the process variables in the APS and the high temperature thermal exposure were studied. The experimental results indicated that the porosity of the APS-TBC was significantly changed due to the APS process. In addition, it was decreased by the thermal exposure at 1000℃. The relationship between the porosity and mechanical properties in APS-TBC was discussed. The Young's modulus and the tensile strength of the APS-TBC were increased with the decreasing of the porosity.

Behavior of thermo-mechanical fatigue (TMF) failure of a new generation titanium alloy matrix composite, SP700/SCS-6, was studied, based on a series of investigations on ; (i) mechanical properties under monotonic loading of the SP-700/SCS-6 composite and the matrix alloy; (ii) fiber push-out tests to represent the fiber/matrix interfacial strength at elevated temperatures ; (iii) isothermal low cycle fatigue (ILCF) tests, (iv) interfacial reaction zone to characterize the microscopic damage evolution under the TMF test and the thermal cycle test without external load. A special focus was paid to understand both the mechanisms of TMF failure, and the dissimilarities and the similarities between the TMF and ILCF failures. From the results of these tests, it was shown that the differences of damage characteristics may be concerned with the degradation of the fiber/matrix interface by the crack propagation and reaction layer due to thermal cycles. These systematic investigations showed that the lives and the failure mechanisms of the TMF should be clearly distinguished from those of the isothermal ILCF, whereas there were a few similarities between them. It was also suggested the difference should be primarily originated from the cracking by the repeat of the thermal cycle and the resultant change in fiber/matrix adhesion strength.

The isothermal low cycle fatigue (LCF) fracture behavior of a new generation titanium alloy matrix composite, SP-700/SCS-6, were investigated compared with the monolithic matrix alloy. To study the effects of the test temperature and the loading frequency on the LCF fracture behavior in SP-700/SCS-6, the LCF tests were performed under strain-controlled condition at room temperature and 450°C. In the LCF tests two strain wave shapes were applied : the fast-fast wave in which the loading frequency was 0.5 Hz, and the slow-slow wave in which it was 1/360 Hz, respectively. The conclusions obtained from the experimental results are summarized as follows : (1) The LCF lives of the composite were almost comparable to those of the matrix alloy, on the basis of strain range. In other words, the performance of the present composite material can be absolutely improved, since the stiffness and the deformation resistance can significantly increased by the composition. (2) The fatigue cracks were initiated from the matrix part at the specimen corner. The fiber pull-out was not so significant on the fracture surface. (3) The LCF lives of the composite at 450°C were longer than those at room temperature. This trend was in contrast to the monolithic matrix alloy which exhibits a normal temperature dependence in the LCF lives. (4) The LCF lives of the composite under the slow-slow wave was noticeably shorter than those under the fast-fast wave at high temperature. This trend was familiar to the conventional monolithic materials.

The 4-point bending test of the freestanding air plasma sprayed thermal barrier coating (APS-TBC) was carried out in order to evaluate the mechanical properties of the TBC. The effects of the process variables in APS and the isothermal aging on the mechanical properties were investigated. The experimental results indicated that the mechanical properties of the TBC were significantly changed by the process variables and the long term thermal exposure. The peculiar stress-strain response was observed in the loading and unloading bending test; the initial loading is nearly linear, however the non-linear behavior was seen in unloading and the significant irreversible strain occurred at zero stress. The elastic modulus evaluated from the unloading curve was much higher than that from the initial loading curve.

Effects of the process variables in air plasma spray (APS) and the long term thermal exposure on the mechanical properties of the thermal barrier coating (TBC) film were, studied. The experimental results indicated that the mechanical properties of TBC film, i.e. the tensile strength and the elastic modulus, were significantly changed by the process variables in APS and the long term thermal exposure. The microstructural investigation of the TBC film was also carried out and the relationship between the change of the mechanical properties due to the thermal exposure and the porosity were discussed.

Titanium alloy matrix composites (TMCs) have received considerable interest as structural materials for aeronautical applications, because of their higher specific strength and stiffness. When applying TMCs at elevated temperatures, high temperature isothermal low-cycle fatigue (LCF) failure is one of critical issues to be concerned. A unidirectionally reinforced SCS-6/SP-700 composite is a tentative target in this work, where the matrix alloy, SP-700 is a new generation high strength Titanium alloy developed by NKK Inc., and the SCS-6 is a beta-SiC fiber developed by Textron Specially Materials, respectively. A merit to employ the SP-700 is that this alloy enables to reduce a fabrication temperature, because of its capability for superplasticity at relatively lower temperatures. The 7-plies composite specimen was produced by hot isostatic pressing (HIP) at 800°C for 0.5 hrs. in vacuum, alternating layers of thin-foils of the SP-700 and the green tapes of the SCS-6 fibers, so that the fibers were uniformly distributed as a hexagonal array in the matrix. The volume fraction of the fibers in the composite is about 28%. In this work, the following articles in a unidirectionally reinforced SCS-6/SP-700 composite have been studied and evaluated: (i) mechanical properties of the SCS-6/SP-700 composite and the matrix alloy at temperatures ranged between room temperature and 450°C; (ii) LCF lives and the failure modes of the composite and the matrix alloy at room temperature and 450°C; (iii) fiber push-out tests at elevated temperatures ranged between room temperature and 600°C, to represent the fiber/matrix interfacial strength; and (iv) observation and the characterization of the interfacial reaction zone by means of a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDS). Based on these experimental results, the effects of temperature and the loading frequency on LCF failure of the SCS-6/SP-700 composite were discussed.

Thermo-mechanical fatigue (TMF) tests of a unidirectionally reinforced SP700/SCS-6 composite were carried out, where the matrix alloy, SP700 is a new generation high strength titanium alloy. The damage evolution during the TMF was explored, through the investigations of the following articles, compared with that in the monolithic matrix alloy: (i) mechanical properties of both the SP700/SCS-6 composite and the matrix alloy at elevated temperatures; (ii) TMF and isothermal low cycle fatigue (LCF) failure lives of the composite and the matrix alloy, (iii) fiber push-out tests at elevated temperatures to represent the fiber/matrix inter facial strength; (iv) observation and the characterization of the interface by means of a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDS); (v) crack initiation and the propagation during the thermal cycle test under no external loading; and (vi) changes of the interface during the TMF, LCF and thermal cycles. These systematic investigations showed that the lives and the failure mechanisms of the TMF should be clearly distinguished from those of the LCF, whereas there were a few similarities between them.