The materials and coated members of equipments exposed to high temperatures, such as air-craft engines, gas turbines, reciprocating engines and power switchgears, are subject to temperature gradient at all times. As popularly known, the temperature gradient determines the macro/micro stress distributions in the materials and coatings, and that it influences thermal cycle damage from the repetition of start-up and shut-down operations of the equipment. Therefore, development for such materials and coatings require a standard testing method for heat resistance under temperature gradient. So far, the uniform heating test and the thermal shock test with water-quenching have been applied to evaluate heat resistance [1,2]. However, these methods cannot realize a heat resistance test with simulation of an actual temperature gradient, because the inner temperature distribution of the test pieces is automatically determined by their shape, material and test environment only.
On the other hand, the heat-resistance tests involving heat treatment on the surface of the test piece, such as the burner rig test, has so far been applied as a simulative test using a small test piece. These methods are capable of realizing necessary temperature gradient by controlling both the surface and rear face temperatures of the test piece. In addition, combining these heating tests with an acoustic emission method enables an in-situ monitoring of the damage progress during the cyclic heating test. In this manner, the surface heating tests have been demonstrated to be effective for evaluation of heat resistance under temperature gradient. However, no industrial standard regarding such a surface heating test has been established worldwide. Therefore, an exploratory committee for the standardization was organized in 1998, and consequently new Japanese Industrial Standard named “Testing Method for Heat Resistance under Temperature Gradient” was established by the Minister of Economy, Trade and Industry in 2005, after deliberations by the Japanese Industrial Standards Committee, in accordance with the Industrial Standardization Law[3].
This article firstly describes the outline of this newly established Japanese Industrial Standard. Subsequently, we introduce a practical example of the cyclic heating test for thermal barrier coatings using a burner heating apparatus, according to this standard.
Testing Method for Heat Resistance under Temperature Gradient:
The new standard specified the testing method for heat resistance of materials under temperature gradient and coated members of equipment exposed to high temperatures, such as aircraft engines, gas turbines, reciprocating engines, accelerators, power switchgears and so on.
Principle:
The principle of this new standard is indicated in Fig. 1. The test piece mounted on the jig is subjected to temperature gradient by heating its surface and cooling the rear surface of the jig. The heating source shall be chosen from among burner heating, arc heating, plasma heating and beam heating by lamp, laser or electron beam. The cooling source, such as water and gas, should have the performance enough to obtain the required temperature gradient.
Experimental Procedure:
Burner Heating Test Apparatus
Figure 1: shows the burner heating test apparatus. The test piece was mounted to the supporting holder with the water cooling functioning to cool the back face of the jig. The
Figure 2: Schematic of the test piece. The test piece shall be (a) soldered or mechanically bonded to the jig with the base material, (b) coating provided to the jig, or (c) a part of test piece functioning as jig. (d) The typical shape and dimension of the test piece.
Figure 3: Schematic of the burner apparatus for heating test.
Figure 4: Optical micrograph of cross-section for thermal barrier coating test piece.
temperature of the water was kept constant by a chiller unit. Combustion of oxygen/hydrogen (O2/H2) was applied for the burner flame, and the flow rate of the gases controls the burner output. As described above, the temperatures on the surface and in the jig were monitored at all times by the radiation thermometer and thermocouples, respectively. The acquired temperature data were converted into heat flux, temperature difference and effective thermal conductivity in real time by a computer. The shutter has a function to interrupt the flame, and the repeating ON/Off of the shutter realizes the heat cycle. Additionally, the acoustic emission (AE) sensor is mounted on the jig. This sensor is also capable of detecting the initiation and propagation of cracks in real time.
Thermal Barrier Coating Test Piece The duplex coating test pieces consisting of partially stabilized zirconia (PSZ) and NiCrAlY alloy layers were prepared by the air pressure plasma spraying. The thickness of the PSZ layer and NiCrAlY layers were approximately 320 and 80 μm, respectively. Type 304 stainless steel was chosen as the material for jig.
Figure 5: Relationships between the burner output, the temperatures of front and rear faces and temperature gap.