For a certain period of high temperature exposures, TBC experienced degradations such as sintering, hot corrosion, erosion, wear, foreign object damage (FOD) etc. Under real gas turbine operation, TBC will experience the combination of any of these, which will not only affect the thermal insulation performance of TBC, but also the efficiency of the cooling system 
. In worst situations, premature failure of TBC will occur prior to the actual design life.
The available test rigs used in various TBC studies can be divided into two main groups, based on the actual gas turbine operational modes. Terms used by researchers: isothermal oxidation and thermal cyclic test, are determined by the TBC conditions in certain gas turbine operating modes and conditions.
4.1. Thermal Cyclic Test Rig
Most TBC studies have been conducted under thermal cyclic test condition. Selected studies have been briefly described to present the available thermal cyclic test rig in simulating TBC. The selections are limited to thermal cyclic tests with test temperatures beyond 1000 °C to represent the advanced gas turbine operating temperatures.
Vaben et al. used custom-made burner rig to simulate TBC in thermal cyclic condition at 1500 °C, which was equipped with 1050 °C cooling air to determine the effects of transient cooling 
. Transient cooling during the high temperature exposure of TBC will accelerate the failure. In the study, it was found that the cooling rate will bring a significant effect to the lifetime of TBC. At the cooling air rate of 100 K/s, the TBC lifetime will reduce by a factor of two to three as compared to the TBC with the cooling air rate of 10 K/s in thermal cyclic condition. This study shows the importance of cooling air effects on the performance of TBC.
Mahade et al. used a custom-made thermal shock burner rig to conduct the test at 1400 °C and the substrate surface remained at 1050 °C for the multi-layered TBC with the addition of a rare-earth element, gadolinium (Gd) 
. Multi-layered TBC and the addition of rare-earth elements in typical TBC system has been widely discussed in recent studies, aiming to improve the properties of TBC and enhance the capability in higher exposed temperatures. Back to the thermal cyclic test conducted, the burner was removed from the TBC top coat surface after 5 min of heating, and the compressed air was applied for cooling mode. However, the temperature of the compressed air is not mentioned by the authors. Through this study, the performance of advanced TBC, which was featured with a multi-layer and additional rare-earth elements, was evaluated at a higher temperature of 1400 °C equipped with the cooling effect. Mahalingam et al. in their study also used a burner flame of 1400 °C for thermal cycle test to determine the thermal stability of TBC systems with different types of bond coats, where one was applied with air plasma spray (APS) and the other one was applied with high velocity oxygen fuel (HVOF) 
. However, the evaluations in all these studies are limited to the TBC behaviors in cyclic condition, not in the prolonged, steady-state condition.
Wang et al. used a custom-made burner rig associated with calcium–magnesium–alumino-silicate (CMAS) feature to conduct the test at 1350 °C with 1150 °C cooling air 
. This study has used the burner rig to determine the TBC performance under cyclic conditions. CMAS liquid was suspended on the TBC surface prior to the thermal cyclic test. Three TBC systems, distinguished by the weightage percentage (wt.%) of yttrium (Y) element, have been tested and ranked, and were compared by their ability in CMAS resistance. The burner rig used is shown to be capable to produce the ‘aged’ TBC under the designed experimental test parameters. Cooling air effect also has been taken into consideration, which it believed to simulate the working condition of TBC. However, the burner rig was operated under cyclic condition, not in the prolonged, steady-state condition.
Zhang et al. in their study conducted both isothermal oxidation and thermal cyclic tests on TBC system 
. For isothermal oxidation test, TBC specimens were exposed to 1100 °C for a maximum of 100 h using muffle furnace. Thermal cyclic was conducted using a custom-made burner rig at 1225 °C. No cooling system was used in this study. Through this study, some presumptions were made. During the isothermal oxidation test using a commercial furnace, the whole perimeter surface of the TBC specimen was exposed to the test temperature of 1100 °C. This means, not only the TBC top coat surface, but the substrate surface were also exposed to the test temperature. The average melting temperature for Ni-based superalloys that are typically consumed in gas turbine and have been used as substrate material is approximately 1250 °C. Failure analysis that was conducted by Kolagar et al. found that Ni-based superalloy will experience overheating when exposed to temperatures beyond 1000 °C for a prolonged duration 
. This finding can be related to the reason why 1100 °C has been selected by the authors for the isothermal oxidation test. The selected isothermal oxidation test temperature of 1100 °C is not as high as the temperature used in thermal cyclic test, 1225 °C. With no cooling air or other cooling system applied, the TBC specimens are expected to experience overheating and fail.
Ma et al. also conducted both isothermal oxidation tests for prolonged temperature exposure, and thermal shock test to simulate the thermal cyclic working condition of TBC 
. The isothermal oxidation test was conducted using commercial air furnace at 1300 °C for a total exposure of 95 h. The thermal cyclic simulation was conducted also using the air furnace at 1200 °C. The authors conducted an isothermal oxidation test at temperatures beyond 1000 °C, which differs to the test conducted by Zhang et al. 
, as discussed above. There are possible reasons for the higher test temperature used. Ma et al. placed the TBC specimens in the crucibles made of alumina where the crucibles are capable of protecting the substrate surface from direct contact with furnace hot air 
. The other thing is, the isothermal oxidation test was conducted for 20–25 h, and only then the specimens were taken out from the furnace and ambient air cooled prior to TGO measurement. Then the TBC specimens were placed back in the furnace to continue the isothermal exposure for another two to three cycles with similar methods until reaching the total of 95 h. Being protected by the crucibles and relatively short exposure time are the possible reasons for the exposures of the whole TBC specimens to temperatures beyond 1000 °C. The selected test temperatures beyond 1200 °C are believed to simulate TBC in the advanced gas turbine operating temperatures. The simulation for TBC in advanced gas turbine operating conditions seemed not to have been completed where no cooling feature was applied to attribute to the cooling effect.
Other than the custom-made burner rig, there was a facility developed, and this was used in the study by Yingsang et al. 
. They used a high heat flux rig to simulate TBC in gas turbine working condition at the exposed temperature of 1150 °C with 950 °C cooling air. The laser power was used to provide the test temperature of 1150 °C and the coverage of heating was only at the middle surface of specimens. It was mentioned that the heating did not reach the edge of the specimens. In TBC evaluations, the condition of TBC edges should be considered as edge delamination is prone to occur on the coating system with the edge surface. There are many studies that have reported that edge delamination is one of the main damage mechanisms for TBC. For example, Jiang et al. reported that the interfacial cracks within TBC were initiated from the edge area and propagated through the interface during the cooling or shutdown period 
. A study by Abedini, Dong and Davis used finite element method (FEM) on the TBC delamination by the edges 
. The software simulation was conducted to optimize the interface conditions to minimize the occurrence of TBC edge delamination. Even the developed facility was equipped with the compressed air as the cooling device; the use of a laser should have improved heating on the whole TBC top coat surface, so that TBC could be simulated fully to establish the optimum TBC behavior including the resistance to edge delamination.
Liu et al. in their study conducted both isothermal oxidation and thermal cyclic tests to evaluate the performance of TBC for gas turbine blades application 
. Thermal cyclic test was conducted using a custom-made furnace with automation specimen lifting, to simulate the TBC under cyclic conditions at 1100 °C. No cooling air or other cooling system was equipped in this custom-made furnace. While for isothermal oxidation test, commercial muffle furnace was used with the same test temperature of 1100 °C. As discussed in the previous section, hot gas path components including the blade are equipped with not only the TBC system, but also the cooling system such as air-cooled, for advanced gas turbine application. In this study, both isothermal oxidation test for prolonged exposure and thermal cyclic test for peak and/or part peak condition cannot represent the TBC simulation in advanced gas turbine, where no cooling system has been used in both custom-made and commercial test facilities.
TBC was tested under cyclic conditions either by using a custom-made test rig that was equipped with the cooling air system 
, that was not equipped with the cooling air system 
, or a commercial test rig that was not equipped with the cooling air system 
. These showed that there are a number of studies have developed test rigs that are capable of simulating TBC-assisted cooling air system. However, the above-mentioned articles are limited to the TBC simulation under cyclic conditions.
For certain studies without a cooling system, a number of limitations have occurred, such as limited materials can be applied especially for those substrates where typically the melting temperatures of the materials are below 1250 °C. Also, the test temperature is limited to the materials used in the TBC system. These two factors may limit the full potential to explore the performance of TBC assisted with cooling air in simulative advanced gas turbine conditions. For example, Xiao et al. and Negami et al. have used commercial furnace and custom-made hydrogen burner rig, respectively, to conduct TBC thermal cyclic tests at similar test temperature of 1080 °C 
. A number of studies were conducted using commercial furnaces such as by Doleker et al. at a test temperature of 1150 °C 
; Goral et al., Shamsipoor et al., Fritscher et al. and Barhanko et al. all used a test temperature of 1100 °C 
; and Adam et al. and Taleghani et al. used test temperatures of 1050 °C and 1000 °C, respectively 
; and Yang et al. and Jing et al. both used commercial muffle furnace with a test temperature of 1050 °C 
. However, without cooling air or other cooling system equipped, the tests are limited to certain temperatures that do not represent the actual exposure of advanced gas turbine specifically under cyclic conditions.
For isothermal oxidation test, most researchers have used commercial furnace or equipment to simulate TBC in steady-state operating condition. Similar to thermal cyclic test, the selected studies that will be briefly described later are having TBC temperature exposures beyond 1000 °C only. The selections are of relevance with the high temperature of advanced gas turbines.
4.2. Isothermal Oxidation Test Rig
Similar to TBC that has been tested under cyclic conditions, TBC under steady-state condition also shows that a small number of studies have been conducted using a custom-made test rig. For example, Fan et al. conducted the isothermal oxidation test using high voltage heating equipment to simulate the TBC at the maximum temperature of 1800 °C for 40 h 
. The voltage was applied on the TBC top coat surface until it reached the intended temperature. Under this study, an extremely high temperature has been tested for advanced TBC application, but this does not represent the steady-state condition of TBC in advanced gas turbine working condition when no cooling system is equipped. The use of high voltage to provide a high temperature TBC top coat is a brilliant idea where the temperature can be achieved efficiently. However, there is still a need to include the cooling system during the TBC test rig simulation to get complete performance and behavior for advanced gas turbine application. The other important thing that needs to be considered thoroughly is the safety aspect during the test.
Nau et al. used a high pressure test rig to achieve the operating temperature of 1526.85 °C 
. The purpose of their study is to determine the feasibility of the device in measuring the combustor wall in a high temperature and high pressure environment. A probe was used to represent TBC-coated combustor wall. Even the main idea of the study regards the measurement technique and device, but the concept that they have been introduced to exposes the probe, which is a TBC specimen, to a high temperature beyond 1500 °C. shows the test rig facility used in the study. Referring to the figure, TBC top coat surface can be exposed to high temperature but no cooling system is applied to completely simulate the TBC in advanced gas turbine application. Some modifications can be made to achieve this working condition.
Schematic of the test rig used by. Adapted with permission from ref. 
. Copyright 2019 ASME.
Naraparaju et al. used a mini turbine to simulate TBC 
. The mini turbine, as shown in , has a single stage compressor section, a combustor chamber, a vane section, and a rotating bladed disk. Two TBC systems were applied on the blades. The test was conducted at 1250 °C for 1 h. From the configuration of the mini turbine, it can be ascertained that the developed mini turbine is suitable to be used for former types of gas turbine. However, if some modifications can be made with the installation of suitable cooling system at the combustor chamber and the blade sections, the use of this mini turbine can be extended not only to study the performance of TBC but any aspect in advanced gas turbine application.
Mini turbine unit used by Naraparaju et al. in their study. Adapted with permission from ref. 
. Copyright 2018 Elsevier.
Other than as discussed above, almost all available test rigs to simulate TBC under steady-state condition are of commercial furnace. The highest test temperature applied in the isothermal oxidation test using the commercial furnace was reported by Bobzin et al.; 1300 °C and 1200 °C for a prolonged exposure of 50 h 
. In the study, a multilayer TBC system, which consists of typical 7YSZ and the use of a rare-earth element of lanthanum zirconate, La2
, has been evaluated. As discussed in the previous section, the addition of rare-earth elements in TBC are widely studied, which adds a benefits by protecting the underlying components to higher temperature in advanced gas turbine. The use of a rare-earth element in the multilayer TBC conveys the author’s effort to evaluate the advanced TBC system, which is preferably applied in the advanced gas turbine. However, the use of commercial furnace does not completely represent the advanced gas turbine simulative operating condition.
Another example to be discussed is Karaoglanli et al. who used commercial furnace to simulate TBC at 1000 °C, 1100 °C, and 1200 °C for maximum prolonged exposures of 100 h 
. A comparative study has been conducted where TBC specimens were deposited using two deposition techniques, APS and HVOF, has been evaluated. A limited test temperature of 1200 °C when commercial furnace is used for prolonged exposure is expected. To study the performance of TBC, a number of parameters need to be evaluated. The parameters included the addition of a rare-earth element, the thickness of each TBC layer, the test condition, which will represent the actual operating condition of the particular TBC, and the deposition technique used to produce the TBC system. Different deposition techniques, deposition parameters, and surface preparations all will affect the performance of TBC. The authors have successfully evaluated the TBC system produced with different deposition techniques. We can get some overviews on the behaviors of these TBC systems and this might help to predict which technique is suitable to our application. In power generation gas turbine fleet, each OEM may use different TBC deposition techniques on their gas turbine components. However, without the assistance of any cooling system, this study is not suitable to be applied in simulating the advanced gas turbine.
Other examples have used commercial furnace in their studies such as Essa et al. and Chen et al. where they simulated TBC in the maximum exposed temperature of 1150 °C for a prolonged exposure of 1970 h and 312 h, respectively 
; and Yang et al. conducted the TBC prolonged isothermal oxidation test at 1150 °C for a maximum of 200 h using commercial muffle furnace 
A number of studies have been reported to conduct the TBC isothermal oxidation at 1100 °C. For example, Jonnalagadda et al. conducted the test with a maximum exposure of 3000 h using commercial furnace 
; Goral et al. and Izadinia et al. conducted the test for 1000 h also using the commercial furnace 
; Paraschiv et al. conducted the test for maximum 600 h 
; and Vorkotter et al. used commercial thermogravimetric analyzer (TGA) for a maximum test exposure of 70 h 
. The temperature of 1100 °C is the lowest maximum material temperature of gas turbine components that are made of superalloys and coated with TBC 
. For the higher material temperature beyond 1200 °C, not only TBC has been applied on the superalloy components, but it also assisted with the cooling system. All these facts have explained why most of the studies that conducted isothermal oxidation tests on TBC-coated specimens using a commercial furnace, which is not equipped with any cooling system, chose 1100 °C as the test temperature. Xiao et al. in their study chose a slightly lower test temperature of 1080 °C to conduct a TBC isothermal oxidation test for 1300 h using a commercial furnace 
Not only 1100 °C, but 1000 °C also has become a popular choice as the test temperature for TBC isothermal oxidation. Chang et al. in their study used real gas turbine material that was cut from the after-service blades from thermal power plants in Korea 
. The scrapped specimens were then TBC coated and heat treated prior to a prolonged 8000 h exposure using a furnace. Three test temperatures have been selected, which are 850 °C, 950 °C, and 1000 °C. The behavior of TBC-coated specimens that were exposed to these temperatures under steady-state condition was evaluated. Many studies have also been reported to choose 1000 °C in isothermal oxidation test. For example, Taleghani et al. conducted the test using a commercial furnace for a maximum exposure period of 200 h 
; Zulkifli et al. used a commercial furnace to isothermally expose the TBC specimens for 120 h 
; Karaoglanli et al., Shi et al., Zakeri et al. and Doleker et al. also used a commercial furnace for 100-h isothermal oxidation test 
; whereas Shi et al. and Doleker et al. used a commercial muffle furnace with a similar test duration 
The above-mentioned articles 
do not consider the addition of cooling systems in their work. Studies that have been conducted using typical commercial furnace 
have also proven to not be capable of simulating the cooling air effect on TBC in advanced gas turbine operating condition. The developed custom-made test rigs are also limited to short test duration under cyclic condition and no cooling air effect under prolonged steady-state operating conditions. TBC in advanced gas turbine operating condition should be equipped with the cooling air effect to get the optimum degraded TBC to be studied.