JP5384540B2 - Thermal barrier coating inspection method - Google Patents

Description

  The present invention relates to a thermal barrier coating inspection method suitable for gas turbine members and the like.

  In recent years, in gas turbines, the combustion gas temperature has been increased to achieve higher efficiency. At present, the combustion gas temperature has already exceeded the melting point of heat-resistant alloy members such as turbine blades and stationary blades, and in addition to various cooling technologies, the thermal barrier coating (Thermal coating on the surfaces of turbine blades and stationary blades). The construction of Barrier Coating (TBC) has become widespread.

  The TBC is composed of a top coat for heat insulation and a bond coat for ensuring oxidation resistance and corrosion resistance, and reduces the heat flux from the combustion gas to the heat-resistant alloy substrate, The effect of reducing the surface temperature of the alloy substrate is obtained.

  For the top coat, yttria partially stabilized zirconia (YSZ), which is an oxide having low thermal conductivity, is widely used. YSZ has a crystal structure stabilized by adding yttria. For the bond coat, MCrAlY alloy (M includes one or more metal elements of Ni, Co and Fe) and aluminides such as Ni-Al and Ni-Al-Pt are used. Thermal bond oxide (TGO) is formed on the surface of the bond coat when exposed to high temperature, and it is resistant to oxidation and corrosion by blocking oxidizing gas and corrosive gas. Is secured.

  In the case of recent TBC, it is said that there is a heat shielding effect of about 150 ° C. However, in a gas turbine, in addition to being constantly exposed to high temperatures, the change in thermal stress accompanying heating / cooling is large when starting and stopping, and the top coat peels off and falls off. When the top coat peels off, the bond coat is directly exposed to the high-temperature combustion gas, and the temperature locally rises, causing damage to heat-resistant alloy members such as turbine blades and stationary blades. Therefore, in order to maintain the soundness of the thermal barrier coating and improve the reliability of the gas turbine, an inspection method for predicting the deterioration of TBC, particularly the topcoat peeling is desired.

  In order to solve such a problem, the damaged part formed by colliding a steel ball or the like with the top coat is heated, the temperature distribution of the damaged part is detected by an infrared imaging device, and the peeling length is calculated to cause deterioration. A method for inspecting deterioration of a ceramic coating material to be judged has been developed (Patent Document 1). In addition, mechanical load load is repeatedly applied to the TBC test piece at room temperature, the amount of strain and the amount of peeling are measured, the peeling parameter is set as a function of the amount of strain and the number of repetitions for the amount of peeling, and the amount of peeling is determined by the peeling parameter. A coating peeling evaluation method to be evaluated has been developed (Patent Document 2).

  Patent Document 3 and Patent Document 4 describe the formation of interfacial oxide in the bond coat layer as a life management method of a thermal barrier coating that estimates the deterioration state of a high temperature component equipped with TBC and manages the life of the high temperature component. Focusing on the average metal content, a technique for estimating the lifetime of the thermal barrier coating from the correlation of measured values is disclosed.

JP 2000-206072 A JP 2006-47074 A JP 2008-14747 A JP 2008-14748 A

  Conventionally, as an inspection after thermal barrier coating (TBC) construction in industrial gas turbine members, a method that can be carried out in a short time is desired from the viewpoint of productivity, and appearance of cracks, peeling, discoloration, etc. on the surface of TBC is desired. Although inspection is the main and external defects that occur during construction can be found, it is difficult to predict the internal state of the TBC and deterioration such as peeling off of the top coat during use.

  If the technique described in Patent Document 1 or 2 is used, there is a certain effect in diagnosing and predicting the deterioration of TBC. However, since the degree of deterioration after damage is measured, the test requires a lot of time and cost, and it is difficult to inspect various TBCs in a short time. Moreover, it cannot be said that it is possible to predict how much aged TBC immediately after construction which is not damaged shows deterioration over time during use.

  Regarding the technique described in Patent Document 3 or 4, the state of high-temperature parts used in an actual machine is measured, and it is difficult to estimate the life before use. In the technique described in Patent Document 3 or 4, the average content of the interface oxide forming metal in the bond coat layer, that is, the “composition” of the interface oxide forming metal is measured. It does not focus on the “structure” of metal.

  An object of the present invention is to provide an inspection method that can easily and quickly determine the performance of TBC in a state before use.

  In the thermal barrier coating inspection method of the present invention, the release life of the thermal barrier coating composed of a bond coat applied to the surface of the base material of the high-temperature member for gas turbine and a top coat applied to the surface of the bond coat When estimating the crystallization area ratio of a plurality of types of bond coats and their peeling life, the crystallization area ratio of the test piece is measured, and the crystallization area ratio of the test piece is peeled off from the correlation. Estimate life.

  According to the present invention, it is possible to easily predict the topcoat peeling of the TBC in a short time and determine the durability of the TBC and the quality of construction.

  Further, according to the present invention, it is possible to determine whether or not a part is available before assembling or shipping the part.

It is a flowchart which shows the procedure of the test | inspection method of thermal barrier coating. It is a graph which shows the frequency | count of heating and cooling until it reaches topcoat peeling in the thermal cycle test of TBC. It is an EBSD image which shows the structure of the bond coat A after TBC construction. It is an EBSD image which shows the composition of bond coat B after TBC construction. It is an EBSD image which shows the structure of the bond coat C after TBC construction. It is a notional graph which shows the correlation with the crystallization area ratio of a bond coat, and the frequency | count of heating and cooling until it reaches topcoat peeling.

  The present invention relates to a method for inspecting a bond coat of a thermal barrier coating suitable for a gas turbine member, for example.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by using the correlation between the topcoat peeling of the TBC and the crystal structure of the bond coat after the construction, and the present invention has been completed. I let you.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “this embodiment” as appropriate) will be described in detail. However, the present embodiment is not limited to the following contents and does not depart from the gist thereof. Any change can be made within the range.

  Hereinafter, a thermal barrier coating inspection method according to an embodiment of the present invention will be described.

  The thermal barrier coating inspection method estimates the peeling life of a thermal barrier coating composed of a bond coat applied to the surface of a base material of a gas turbine high temperature member and a top coat applied to the surface of the bond coat. It is an inspection method, a step of obtaining a correlation between a crystallization area ratio of a plurality of types of bond coats and a peeling life thereof, a step of measuring a crystallization area ratio of a test piece, a crystallization area ratio of a test piece, and the above And a step of estimating the peeling life from the correlation of

  In the method for inspecting the thermal barrier coating, when calculating the crystallization area ratio, the cross section of the part used for measuring the crystallization area is polished.

  In the thermal barrier coating inspection method, the crystallization area of the bond coat is measured by electron backscatter diffraction.

  The thermal barrier coating inspection method is performed mainly through the following procedure as shown in FIG.

  (1) Various TBCs are applied to the base material of the high-temperature member for gas turbine (TBC application process).

  (2) Cross-sectional observation of various TBCs is performed, and the crystal state of each bond coat is measured (crystal state measurement step).

  (3) The area ratio (crystallization area ratio) of the crystalline part of the bond coat described in (2) above is calculated (crystallization area ratio calculation step).

  (4) A correlation between the ratio of the crystallized area and the number of heating / cooling times until the top coat is peeled (a value corresponding to the peel life of the top coat) is obtained (correlation calculation step).

  (5) Using the correlation described in (4) above, the peel life of the top coat of the test piece before being incorporated into an actual machine is estimated (peel life estimation step).

  (6) The durability of the TBC of the test piece according to the above (5) and the quality of construction are determined (determination step).

  The above test piece may be used by cutting out a part of the high-temperature member for gas turbine (preferably a portion not incorporated in the actual machine), or separately prepared when TBC is applied to the base material of the high-temperature member for gas turbine. It may be applied simultaneously to the base material of the test piece.

  Moreover, various TBC used in order to obtain | require the correlation as described in said (4) may be applied to the base material of the actual high-temperature member for gas turbines, or it may be applied to the base material of said test piece. Good.

  Hereinafter, each process will be described.

(TBC construction process)
In the present embodiment, first, a test piece is prepared by sequentially laminating a bond coat and a top coat on a heat-resistant alloy base material and constructing a TBC to be evaluated.

  As the bond coat, for example, MCrAlY that exhibits excellent corrosion resistance and oxidation resistance is used. Although the thickness is not particularly limited, it is usually about 100 to 200 μm. Specific film forming methods include, for example, a low pressure plasma spray (LPPS), a high speed flame spray (High Velocity Oxy). -Fuel Frame-spraying (HVOF) etc. can be used.

For the top coat, a material having low thermal conductivity, for example, yttria-stabilized zirconia (YSZ: ZrO ₂ -6 to 8Y ₂ O ₃ ) is used, which is usually about 300 to 500 μm. For the deposition of the top coat, for example, an atmospheric plasma spray (APS) under atmospheric pressure is usually used.

  In this way, the TBC test piece to be evaluated is prepared. In this embodiment, since the inspection is performed by observing the cross section of the TBC, the test piece is in a state where it is applied to the gas turbine member or the test. For example, various shapes applied to a base material having a simple shape such as a flat plate or a disk shape can be used.

  Thus, the point which can test | inspect without being influenced by the shape of a test piece and a measurement site | part is an advantage of this embodiment. In film formation by thermal spraying, the film quality may be different between the flat surface and curved surface due to its properties, but it can be used for local inspection and evaluation and examination of construction conditions, and is highly reliable Contributes to TBC construction.

  On the other hand, in the case of the method of colliding the steel balls shown in Patent Document 1, the degree of damage varies depending on the angle of collision of the steel balls, and when the shape of the test piece is complicated such as a blade shape, the deterioration is accurately estimated. It is difficult. In addition, in the case of the evaluation method for applying a mechanical load described in Patent Document 2, the shape of the test piece is limited to a simple shape such as a flat plate, a cylinder, or a round bar in order to measure the strain amount.

(Crystal state measurement process)
After preparing the TBC test piece, cross-sectional observation is performed and the crystal state of the bond coat is measured.

  For the adjustment of the cross section, as in a general method for observing a metal structure, an arbitrary evaluation portion is sampled, embedded in a resin, and the cross section to be observed is polished.

  The crystal state of the cross section after polishing is measured. The crystal state can be measured using, for example, electron back-scattering diffraction (EBSD). This EBSD is a method of measuring a crystal orientation at an arbitrary point by analyzing a pattern of Kikuchi lines in reflection electron diffraction of a scanning electron microscope (SEM). By using EBSD, the structure of the alloy (crystal state distribution) can be observed.

  Recently, a fully automatic analysis system has been developed, and a large number (tens of thousands to hundreds of thousands) of crystal orientations can be measured at intervals of several μm, and information on the crystal orientation of the observation plane can be easily obtained as image information. It is like that.

  As described above, in the present embodiment, the time required from the preparation of the test piece to the measurement of the crystal state is shorter than the evaluation method for performing the test described in Patent Documents 1 and 2. It is.

(Crystallized area ratio calculation process)
Using the image information obtained in the crystallization state measurement step, the area ratio (crystallization area ratio) of the crystalline portion in the bond coat is measured.

  In the EBSD measurement, it is difficult to measure the crystal orientation at a portion where the crystal lattice is distorted such as a crystal grain boundary or an amorphous portion, and the measurement accuracy is lowered. In general, a reliability index (CI value) is used as an index relating to measurement accuracy, and a portion where the CI value is a certain value or more is determined as crystalline. In image display, it is possible to arbitrarily set a CI value threshold, and it is possible to distinguish between a crystalline portion and an amorphous or highly strained portion according to the level of the CI value.

  In this step, from the image information obtained by measuring the crystal state, the area of the crystalline portion where the CI value occupying the unit area of the bond coat is a certain value or more is measured.

(Correlation calculation process)
The correlation between the crystalline area ratio obtained in the above process and the topcoat peeling lifetime is determined.

  The top coat peeling life is defined by the number of heating and cooling times (number of cycles) until the top coat is peeled. The number of cycles is determined by conducting a thermal cycle test. Conditions such as the temperature and holding time of the heat cycle test can be arbitrarily set. In the case of a high-temperature member for gas turbine, the step of holding at a high temperature of 900 to 1200 ° C. for a certain period of time and then cooling and holding to room temperature It is usual to carry out as one cycle. The holding time is arbitrarily set according to the operating conditions of the actual machine.

  Prepare several TBCs with different crystallized areas in advance, measure the crystallized area, conduct several thermal cycle tests, measure the topcoat peel life of each, determine the crystallized area and topcoat A master curve (correlation graph) is created using the peel life as a parameter.

(Peeling life estimation process)
The topcoat life of the test piece is estimated by comparing the correlation calculated in the correlation calculation step with the crystallization area ratio obtained from the test piece. That is, the crystallization area ratio obtained from the test piece is made to correspond to the master curve obtained in the correlation calculation step.

(Judgment process)
Based on the result of the estimation of the topcoat peeling life depending on the crystallization state, TBC sound (TBC good) or TBC unhealthy (TBC bad) is determined. If the TBC is healthy (good), the product is shipped, and if the TBC is defective, the product is not shipped, and re-coating, review of construction conditions, etc. are made, and the quality is controlled.

  Hereinafter, description will be made using examples.

  A Ni-based alloy IN738 suitable for a gas turbine member was cast into a rod shape and machined to a diameter of 1 inch and a thickness of 3 mm to obtain a base material.

As a pretreatment for thermal spraying on the surface of the substrate, alumina particles having a particle size of 24 were used, and blasting was performed under a pressure of 5 kgf / cm ² .

  Although it is obvious that various methods can be used as the method for forming the bond coat, in this embodiment, high-speed flame spraying (HVOF) or low-pressure plasma spraying (LPPS) is used to form the bond coat on the substrate. The film was formed with a thickness of about 150 μm.

  Bond coats A and D described later are Ni-rich alloys, and bond coats B and C are Co-rich alloys. Bond coats A and B use high-speed flame spraying (HVOF), and bond coats C and D use low-pressure plasma spraying (LPPS). Here, the Ni-rich alloy powder is NiCoCrAlY (composition formula: Ni-23Co-17Cr-12.5Al-0.5Y (mass%)), and the Co-rich alloy powder is CoNiCrAlY (composition formula: Co-32Ni-21Cr-8Al-0.5Y (mass%)).

  For the topcoat, commercially available yttria stabilized zirconia (YSZ) was applied at a thickness of about 300 μm using atmospheric pressure plasma spraying (APS).

  In order to confirm the effectiveness of the inspection method of the present embodiment, a thermal cycle test is performed on the TBC produced by the above method at 1093 ° C. for 10 hours, cooled to room temperature, and held for 15 minutes for one cycle. did.

  FIG. 2 is a graph showing the number of peeling cycles in the thermal cycle test.

  The horizontal axis indicates the type of bond coat, and the vertical axis indicates the number of heating / cooling cycles (number of cycles: number of cycles (also referred to as the number of peeling cycles)) until the top coat is peeled off.

  From this figure, it can be seen that the bond coat A has a small cycle number of 30 or less, and the bond coats B and C have a large cycle number of 60 or more.

  3, FIG. 4 and FIG. 5 are EBSD images showing the crystal state of the cross section of the test piece on which the TBC subjected to the thermal cycle test shown in FIG. 2 was applied.

  The test pieces shown in these drawings are obtained by sequentially applying a bond coat 2 and a top coat 3 on a substrate 1.

  In these figures, the above-mentioned portion having a CI value of 0.1 or less was determined as an amorphous or high strain portion. In the figure, the darkest gray portion, that is, the black portion is an amorphous or highly strained portion.

  In the bond coat A shown in FIG. 3, most of the bond coat is amorphous or highly strained, and in the bond coat C shown in FIG. 5, almost the whole is crystalline.

  As a result of measuring the area (crystallized area ratio) of the crystallized portion in the unit area of the bond coat by image analysis, the bond coat A is 5%, the bond coat B is 22%, and the bond coat C is 100. %. That is, it can be seen that the crystallinity increases in the order of bond coat A, bond coat B, and bond coat C.

  Moreover, although the EBSD image is not shown, in the case of the bond coat D, the crystallization area ratio was 100%.

  From the magnitude relationship of the crystallization area ratio and the number of peel cycles in the thermal cycle test shown in FIG. 2, it was found that the peel life tends to increase in proportion to the crystallized portion. Therefore, the relationship was organized using the area of the crystallized portion and the number of peeling cycles as parameters.

  FIG. 6 is a conceptual graph showing the correlation between the crystallization area ratio (Fraction of Crystallized Area) and the number of times of heating and cooling (Number of Cycles) until the top coat is peeled off.

  From this figure, it is possible to estimate the peeling life from the crystallization ratio. In this example, when the crystallization area ratio is about 10%, the top coat peeling is shown as early as about 30 cycles. However, when the crystallization area ratio is about 20% or more, about 70 cycles and a good peeling life are obtained. (Long peeling life). Therefore, the soundness of the TBC can be determined by defining the threshold value of the crystallization area ratio showing a good peeling life as 20% or more.

  Table 1 summarizes the evaluation results of bond coats A to D.

  In the table, the numerical value in parentheses indicates the crystallization area ratio (%), and ◯ and x indicate the quality of the peeling life (the peeling life is sufficiently long and the peeling life is short), respectively. Yes.

  As described above, according to the inspection method of the bond coat thermal barrier coating of the present invention, the top coat peeling life can be estimated from the crystallization ratio of the bond coat of TBC, the soundness of TBC is evaluated, the quality Management can be performed efficiently.

  1: base material, 2: bond coat, 3: top coat.

Claims (3)

  An inspection method for estimating the peeling life of a thermal barrier coating composed of a bond coat applied to the surface of a base material for a high-temperature member for a gas turbine and a top coat applied to the surface of the bond coat. The step of obtaining the correlation between the crystallized area ratio of the bond coat and the peel life thereof, the step of measuring the crystallized area ratio of the test piece, and the peel life of the test piece from the crystallized area ratio and the correlation A method for inspecting a thermal barrier coating comprising the step of estimating the thermal barrier coating.   The thermal barrier coating inspection method according to claim 1, wherein when calculating the crystallization area ratio, a cross section of a portion used for measurement of the crystallization area is polished.   The thermal barrier coating inspection method according to claim 1, wherein the crystallization area of the bond coat is measured by electron backscatter diffraction.

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