EP0236520A1

EP0236520A1 - Ceramic-coated, heat-resisting member and process for preparing the same

Info

Publication number: EP0236520A1
Application number: EP86103159A
Authority: EP
Inventors: Nobuyuki Iizuka; Fumiyuki Hirose; Naotatsu Asahi; Yositaka Kojima
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1984-09-10
Filing date: 1986-03-10
Publication date: 1987-09-16
Also published as: JPH0563555B2; JPS61174385A

Abstract

A ceramic-coated member comprising a substrate having a high mechanical strength, a bonding layer of corrosion-resistant alloy containing Al, and having higher resistances to oxidation and corrosion in a higher temperature atmosphere than those of the substrate formed by plasma melt injection in an atmosphere of low oxygen content, and a ceramic coat layer formed on the bonding layer, wherein an Al-based oxide layer is formed at the boundary between the bonding layer and the ceramic coat layer by high temperature treatment of the member in the atmosphere.

Description

BACKGROUND OF THE INVENTION

This invention relates to a heat-resisting member for use at a high temperature or in a corrosive atmosphere at a high temperature, and also to a process for preparing the same.
With a view to the improvement of power generation efficiency of gas turbine plants for power generation, techniques for a higher temperature application of gas turbines have been studied. With the high temperature application, it has been desired to elevate the heat-resisting temperature of gas turbine members. With developments of Ni-based or Co-based alloys, the heat-resisting temperature of these heat-resisting alloys has been elevated, but has been now saturated at about 850°C.
Ceramic materials are better in the heat resistance than metallic materials, but have problems in toughness, etc. when used as materials of construction. Thus, in order to cope with the higher temperature application of such members, extensive studies have been so far made to prevent the members from exposure to a higher temperature. For example, various methods for cooling the members have been studied. Another method so far studied is coating of the surfaces of metallic members with a ceramic having a low thermal conductvity. The coating is called "thermal barrier coating", which will be hereinafter referred to as "TBC". TBC can have a better effect, when combined with various cooling methods. For example, it has been reported that the temperatures of TBC-applied metallic members as substrates can be lowered by 50° to 100°C than those of TBC-unapplied metallic members. By use of these methods, the reliability of the constituent members of high temperature gas turbines, etc. can be increased.
The technical tasks of TBC to be solved are problems of bonding mechanism between a substrate and a ceramic coat layer and its reliability, because TBC is based on a combination of a heat-resisting alloy as a substrate and a ceramic coat layer having different physical properties than those of the substrate. Particularly in gas turbines, etc, damages such as peeling, falling-off, etc. of the ceramic coat layer occur due to the thermal cycle of the start-up, shut-down, etc. Various measures have been so far taken to solve these problems. According to one measure, a bonding layer composed of a metal alloy is provided between the ceramic coat layer and the substrate [Japanese Patent Application Kokai (Laid-open) No. 55-112804], where the bonding layer serves to lessen the differences in the physical properties between the ceramic coat layer and the substrate. However, in that case, the bonding mechanism between the ceramic coat layer and the bonding layer is based only on a mechanical bonding with a bonding strength of 2 to 5 kg/mm².
Furthermore, in addition to the bonding layer it has been proposed to provide an intermediate layer composed of a mixture of the alloy material as the member for the bonding layer and the ceramic material as the member for the ceramic coat layer or a plurality of the intermediate layers having varied mixing ratios of the alloy material to the ceramic material, or an intermediate layer whose mixing ratio of the alloy material to the ceramic material is continuously changed from the single alloy material to the single ceramic material, between the bonding layer and the ceramic coat layer. These measures are directed to lessening of the differences in the physical properties between the ceramic coat layer and the bonding layer and are all based only on the mechanical bonding of the ceramic material to the alloy material. Thus, damages such as peeling, falling-off, etc. of the ceramic coat layer, bonding layer and intermediate layer start to take place at positions of weak bonding strength, when a large thermal stress is applied to TBC due to the thermal cycle, etc.
The ceramic coat layer, bonding layer and intermediate layer for TBC are formed mainly by plasma melt injection, because the layer-forming speed is high and the layer formation is economical, and also the melt injected layers can have a porous structure, which is preferable particularly in case of the ceramic coat layer. That is, pores or fine cracks in the porous structure can be utilized for lessening the thermal stress. The ceramic melt injected layer, formed by plasma melt injection, is superior to the dense ceramic coat layer, formed by sputtering, etc. in the resistance to a thermal shock due to a thermal cycle, etc. on one hand, but TBC in the ceramic coat layer of such a porous structure has problems of high temperature oxidation or high temperature corrosion of the alloy material as the member for the bonding layer or the intermediate layer on the other hand, because TBC is used at a high temperature and under a high temperature corrosive condition by impurities, etc. contained in fuel. The alloy material is a member of distinguished resistances to oxidation and corrosion at a high temperature, but the expected distinguished resistances of the proper alloy material to oxidation and corrosion at a high temperature cannot be always attained and seem to depend upon the methods for forming the alloy layers.
The present inventors have conducted durability tests of the conventional TBS at a high temperature and have found that the bonding between the substrate and the bonding layer is relatively stable, whereas the bonding between the bonding layer and the ceramic coat layer is deteriorated within a short time, and cracks develop in the ceramic coat layer or the ceramic coat layer is peeled off.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the reliability of TBC and provide TBC with a stable bonding strength between the ceramic material and the substrate for a long time and with less occurrence of cracking or peeling.
Another object of the present invention is to provide a process for preparing TBC with less occurrence of cracking or peeling.
The present invention provides a heat-resisting member which comprises a substrate of metallic material, a bonding layer of alloy having higher resistances to oxidation and corrosion at a high temperature than those of the substrate, formed on the substrate, and a ceramic coat layer formed on the bonding layer of alloy, wherein an oxide layer composed of Al as the main component is formed at the boundary between the bonding layer of alloy and the ceramic coat layer.
According to the present invention, the oxide layer composed of Al as the main component is stable even in a high temperature atmosphere and can prevent progress of oxidation of the bonding layer of alloy and also can prevent occurrence of cracking or peeling of the ceramic coat layer, even if used for a long time, owing to a high bonding strength between the oxide layer and the ceramic coat layer.
The present invention further provides a process for preparing a ceramic-coated, heat-resisting member which comprises a step of forming a bonding layer of alloy containing at least one of Ni and Co, and further containing Cr and Al, and having higher resistances to oxidation and corrosion at a high temperature than those of a substrate containing at least one of Ni, Co and Fe as the main component on the surface of the substrate, a step of forming an oxide layer containing Al as the main component on the surface of the bonding layer of alloy, and a step of forming a ceramic coat layer on the surface of the oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a picture of cross-sectional structure of TBC according to the present invention.
Fig. 2 shows the appearance of a gas turbine combustor with TBC.
Fig. 3 is a cross-sectional view along the line X-X of Fig. 2.
Fig. 4 shows the appearance of a combustor of another type with TBC.
Fig. 5 shows the appearance of a moving vane of gas turbine.
Figs. 6 and 7 are pictures of cross-sectional structures of the conventional TBC after the oxidation at a high temperature for the purpose of comparison.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Situations in which the present invention has been attained will be outlined below, before going into description of the embodiments of the present invention.
As a result of exposure of the conventional TBC to an atmosphere of oxidation or corrosion at a high temperature and successive thermal cycle test, the present inventors have found that the durability of TBC is considerably lowered. In that case, it seems that, in addition to the fact that the bonding between the ceramic material and the alloy material is low in strength because of the mere mechanical bonding, the surface of alloy material at the boundary is oxidized or corroded to further lower the bonding strength. Thus, in the conventional TBC, it seems that the bonding strength between the ceramic material and the alloy material is low and is further lowered by surface changes of the alloy material due to oxidation, corrosion, etc. at a high temperature. These problems will considerably lower the reliability of TBC.
In the plasma melt injection, on the other hand, melt injection into the atmosphere and also into an atmosphere under reduced pressure are carried out. In the latter case, the atmosphere around the plasma arc is controlled and furthermore the pressure in the atmosphere is controlled. By the melt injection into an atmosphere in reduced pressure the melt injection particles are not contaminated with oxygen, etc. during the melt injection, and thus a very good bonding layer of metallic alloy can be formed. The thus formed bonding layer of metallic alloy is utilized as a coat layer for preventing oxidation and corrosion at a high temperature of gas turbine members exposed to a high temperature.
Taking the foregoing facts into consideration and with a view to an increase in the reliability of TBC, the present inventors have made extensive studies to improve the bonding mechanism between the ceramic material and the alloy material. That is, the present inventors have studied TBCs of various materials so far employed. For example, the present inventors conducted a high temperature oxidation test of TBC comprising a ZrO₂-based ceramic coat layer and a bonding layer of metallic alloy material, where application of TBC to gas turbine parts at a high temperature or to gas turbine parts locally exposed to a high temperature was taken into account. As a result, it was found that considerable oxidation took place at the boundary between the ZrO₂-based coat layer and the bonding layer of the conventional TBC. As a result of measuring the bonding strength of TBC before and after the test, it was found that the bonding strength at the boundary between the ZrO₂-based coated layer and the bonding layer was decreased to 1/2 - 1/4 of the original strength in every cases after the oxidation test at 1,000°C for 500 hours. The decrease in the bonding strength was remarkable in every cases, though there were slight differences, depending upon the thickness and porosity of the ZrO₂-based coat layer and further upon the species and amount of an additive to ZrO₂, and also upon the species of the alloy material components of the bonding layer. The decrease in the bonding strength at the boundary was more pronounced at a higher temperature in the oxidation test or with a longer test time. In a test at 1,100°C for 100 hours, peeling damages occurred locally at the boundary. In the oxidation test of TBC using an intermediate layer composed of a mixture of metallic alloy material and ZrO₂-based material, the decrease in the bonding strength was more remarkable. These test results were in good accordance with the results of high temperature thermal cycle tests conducted by the present inventors. That is, in a test of repeating maintenance at temperatures of 970°C, 1,020°C, 1,070°C or 1,120°C for 30 minutes, and air cooling down to 150°C, the number of repetitions till occurrence of a damage on TBC was considerably lowered at a higher test temperature. These problems inherent to the conventional TBC have been a serious obstacle to application of TBC of high reliability coping with a higher temperature application of gas turbines. That is, in application of TBC to prevent a high substrate temperature of gas turbine parts and lower the substrate temperature, it has been difficult to throughly lower the substrate temperature of gas turbine parts because the parts provided with the conventional TBC has a short durability of TBC at a high temperature.
Thus, the present inventors have studied gas turbine members provided with TBC of long durability at a high temperature, capable of lowering the substrate temperature of gas turbine parts even under high temperature operating conditions in place of gas turbine parts provided with the conventional TBC. That is, as a result of extensive studies to obtain TBC of high reliability in applying gas turbines to a higher temperature in the foregoing circumstances, the present inventors have invented gas turbine members provided with TBC of distinguished durability.
The present invention will be described in detail below.
At first, the present inventors studied the problems of conventional TBC in detail and investigated the causes for the problems. Cross-sectional structures of TBC subjected to various oxidation tests were inspected. Examples of the results are shown in Figs. 6 and 7, where the picture show the cross-sectional structures of bonding layer region with a magnification of 100. Fig. 6 shows that a defect occurs at the boundary between the ZrO₂-based coat layer and the bonding layer, and Fig. 7 shows the result of TBC having an intermediate layer composed of a mixture of the alloy material and the ZrO₂-based material, formed between the bonding layer of alloy material and the ZrO₂-based coat layer, where the alloy material in the intermediate layer is considerably oxidized. These phenomena are also observable in the high temperature thermal cycle test. That is, TBC has a new problem of oxidation of the bonding layer or intermediate layer through the ZrO₂-based coat layer having a porous or fine crack structure capable of lessening the thermal stress. Such an oxidation considerably lowers the bonding strength at the boundary, causing a peeling damage to TBC at the boundary owing to the thermal stress, etc. One of important causes for the oxidation at the boundary is that the ZrO₂-based material turns to a semi-conductor at high temperature, facilitating an oxygen migration and increasing an oxygen partial pressure at the boundary. Such oxidation seems to be accelerated due to an increase in the boundary area, for example, when an intermediate layer is provided.
As a result of analysis of the boundary state of the conventional TBC, it was found that oxides containing Cr as the main component were formed at the boundary. Such Cr-based oxides were unstable at a high temperature, and damages occurred at the positions where such oxides were formed. Thus, it was necessary in the TBC for gas turbines in a high temperature application to take the oxidation at the boundary into due consideration.
As a result of extensive studies of various measures, the present inventors have found that it is promising to form an oxide film of dense structure containing Al as the main component at the boundary. Al-based oxides are stable at a high temperature and do not turn to semi-conductors even at a high temperature in contrast to the ZrO₂-based material. Thus, the Al-based oxide layer is effective as a barrier for preventing an internal oxidation. When the Al-based oxide layer has too large a thickness, the Al-based oxide layer can serve as an additional intermediate layer having the physical properties of the Al-based oxide. As a result, damages due to a thermal stress, etc. start to occur at the Al-based oxide layer. When the Al-based oxide layer has too small a thickness on the other hand, the Al-based oxide layer fails to act as a barrier capable of thoroughly preventing the internal oxidation. Thus, it is desirable that the Al-based oxide layer has a thickness of 0.1 to 20 µm. The Al-based oxide layer having a thickness in such a range as above can serve as a barrier capable of thoroughly preventing the internal oxidation of the bonding layer.
The present inventors have further found that the Al-based oxide layer has another important function, i.e. a function of improving the bonding strength between the ZrO₂-based ceramic coat layer and the bonding layer. That is, the conventional TBC is based on the mechanical bonding between the ZrO₂-based ceramic material and the metallic alloy of the bonding layer, whereas the bonding mechanism between the ZrO₂-based ceramic coat layer and the bonding layer through the Al-based oxide layer found by the present inventors is based on bonding at the boundary of oxides, i.e. Al-based oxide and ZrO₂-based ceramic material, and also the Al-based oxide originating from the Al component in the metallic alloy of the bonding layer, and thus is very strong. For example, oxidation tests of TBC containing such an Al-based oxide layer at 1,000°C for 500 hours revealed that the bonding strength between the bonding layer and the ZrO₂-based ceramic coat layer was not substantially lowered and had 7 kg/mm² or higher.
Fig. 1 shows an example of cross-sectional structure of TBC after the high temperature oxidation test with a magnification of 100, where the substrate was Hastelloy X, Ni-based alloy (22 wt.% Cr - 1.5 wt.% Co - 9 wt.% Mo - 19 wt.% Fe - 0.1 wt.% C - the balance being Ni); the bonding layer was a Co-based alloy (10 wt.% Ni - 25 wt.% Cr - 7 wt.% Al - 0.6 wt.% Y - 5 wt.% Ta - the balance being Co): the ceramic coat layer was 92 wt.% ZrO₂ - 8 wt.% Y₂O₂); an Al oxide layer having a thickness of a few µm was formed between the bonding layer and the ceramic coat layer, though not clearly shown in the structural picture of Fig. 1. A process for forming the Al oxide layer will be described later.
Fig. 1 shows that there are no defects at the boundary between the ZrO₂-based ceramic coat layer and the bonding layer. Oxidation tests at 1,100°C for 100 hours also revealed that neither lowering of the bonding strength nor development of defects at the boundary was observed. Results of thermal cycle tests of repeating maintenance at 1,030°C, 1,070°C, 1,120°C, or 1,170°C for 30 minutes and air cooling to 150°C are shown in Table 1.
In Table 1, test pieces Nos. 201-204 have the conventional TBCs, and test pieces Nos. 205-208 have TBCs having an Al-based oxide layer according to the present invention. It is seen from Table 1 that the present TBCs having an Al-based oxide layer have about 3 to 7-fold numbers of repetitions until TBCs have been damaged as compared with those of the conventional TBCs. The higher the test temperature, the more pronounced the effect. The TBC having Al-based oxide film found by the present inventors has a remarkable effect particularly at a higher temperature. Heat-resisting parts with such a TBC will be stable even at a higher temperature. Furthermore, TBC having a ZrO₂-based coat layer bonded through the Al-based oxide layer has a bonding strength of the ZrO₂-based coat layer of 7 kg/mm² or higher, which is much higher than that of the ZrO₂-based coat layer of the conventional TBC, i.e. 3-5 kg/mm². Thus, damages of TBC at gas turbine combustor parts, etc. due to combustion-vibration or at high speed revolution parts, such as moving vanes of gas turbine, etc. can be prevented.
The present inventors have thus investigated the effect of providing the TBC. By providing substrate parts of gas turbine member exposed to a high temperature such as combustor parts, particularly parts exposed to a high temperature combustion gas, with the said TBC of high durability at a high temperature, the substrate temperature can be stably reduced. For example, durability of cylindrical combustor parts for low NOx emission provided with the TBC having the said Al-based oxide layer on the inside surface of the parts exposed to a high temperature combustion gas until the TBC has been damaged has been found to be about 3 times as long as that of the parts provided with the conventional TBC. This is because the TBC having an Al-based oxide layer has a distinguished durability, particularly, at a high temperature. Thus, the effect on the reduction of the substrate temperature of combustor parts by providing the present TBC can be stably maintained, whereas in case of the combustor parts provided with the conventional TBC, the TBC will be damaged within a short time, and particularly the TBC at the parts at a high substrate temperature will be considerably damaged. As a result, the effect on the reduction of the substrate temperature by the TBC will be lost, and consequently the substrate temperature is elevated, resulting in damaging of the parts.
At the combustor parts whose cooling by compressed air, etc. cannot be thoroughly carried out due to the strength of the combustor parts or the structural restriction such as fixing, etc. of the combustor parts, the substrate temperature is liable to increase. At such parts, TBC plays a particularly important role, and TBC having a ceramic coat layer of low thermal conductivity can prevent a local increase in the substrate temperature besides the reduction of the substrate temperature by the thermal barrier effect of TBC, and thus has a function to equalize the substrate temperature. Consequently, TBC plays a very important role in preventing a local increase in the temperature of parts due to the structural restriction or combustion conditions and preventing deformation or damages of the parts due to the local increase in the substrate temperature. However, the conventional TBC has a problem in the durability, particularly, at a high temperature, and the TBC at the parts whose substrate temperature is locally elevated will be damaged within a short time. In case of the combustor parts, the substrate vibrates by combustion-vibration, and thus the TBC, the bonding strength of whose ceramic coat layer has been lowered due to the exposure to a high temperature, will be more damaged. That is, the TBC will not be thoroughly effective at the parts for which the effect of TBC is most required. The substrate temperature at the parts with the damaged TBC can be rather high than that at the other parts with undamaged TBC. For example, in case of the combustor parts in contact with flames, the substrate temperature at parts with the damaged TBC might be higher in some cases than that at parts provided with no TBC at all due to the radiation effect of the ceramic coat layer in the TBC. The thermal barrier effect by the conventional TBC has been known, but has not be practically used at the gas turbine parts requiring a high reliability, such as combustor parts, etc. In the past, TBC was experimentally used at such parts, but the reliability of the parts exposed to a high temperature was lost to the contrary.
On the other hand, at the gas turbine parts provided with the TBC having an Al-based oxide layer, for example, combustor parts, even if the substrate temperature is locally elevated at such parts due to the structural restriction or combustion conditions, the TBC is hardly damaged even at a high substrate temperature, because the present TBC has a distinguished durability, particularly, at a high temperature. Thus, the thermal barrier effect by the present TBC can be thoroughly maintained, and also the function to lessen the local increase in the temperature by TBC can be attained. As a result, the gas turbine parts provided with the present TBC have a high reliability. It is effective to provide only parts, at which the substrate temperature is locally elevated, with TBC having an Al-based oxide layer, because the local increase in temperature can be prevented by the thermal barrier effect of the TBC. Furthermore, a heat input to the substrate at the parts provided with TBC can be decreased by virtue of the radiation effect of the ceramic coat layer in the TBC, when there is no TBC at other parts, and can be balanced with the heat input at the other parts without TBC, thereby preventing the local increase in temperature of the substrate, and thus it can be expected that the temperature can be equalized throughout the substrate.
By providing the entire surface or portions of the gas turbine parts to be exposed to a high temperature with TBS having an Al-based oxide layer, the effect of TBC can be fully obtained in every cases. In the foregoing, the local increase in the substrate temperature has been described, referring to the combustor parts as an example, but this also occurs at other gas turbine parts. For example, in case of stator vanes, moving vanes, etc., it is difficult to equalize the temperature of the vane substrates owing to the restriction of vane cooling structure. Furthermore, with application of gas turbines to a higher temperature, such difference in temperature distribution tends to be larger. Thus, the gas turbine parts provided with TBC of high durability having an Al-based oxide layer can assure application of gas turbines to a higher temperature with a high reliability.
The present invention will be described in detail below, referring to Examples.

Example 1

The surface of Ni-based alloy, Hastelloy-X (22 wt.% Cr - 1.5 wt.% Co - 9 wt.% Mo - 19 wt.% Fe - 0.1 wt.% C - the balance being Ni) as a substrate was washed and defatted, and then subjected to blasting with steel grits and then to plasma melt injection to form a coat layer of alloy material (10 wt.% Ni - 25 wt.% Cr - 7 wt.% Al - 0.6 wt.% Y - 5 wt.% Ta - the balance being Co). The plasma melt injection was carried out in an Ar atmosphere under a pressure of 200 Torr, and the oxygen partial pressure in the plasma melt injection atmosphere was found to be not more than 10⁻³ atmosphere by an oxygen sensor. The plasma power was 40 kW. Under these conditions, the coat layer of Co - Ni - Cr - Al - Y alloy having a thickness of 0.01 mm was formed as a bonding layer of TBC. Then, a coat layer of 92 wt.% ZrO₂ - 8 wt.% Y₂O₃ having a thickness of 0.3 mm was immediately formed on the said bonding layer by plasma melt injection in the atmosphere with a plasma power of 50 kW. Then, the substrate with the coat layers was subjected to heat treatment in vacuum at 1,060°C for 10 hours to conduct a diffusion treatment between the substrate and the bonding layer.
For comparison, TBCs consisting of coat layers having the same thickness as in the present invention were formed from the same TBC materials as those of the present invention according to the conventional process. That is, the said alloy material was melt injected in the atmosphere using an Ar gas according to the conventional process, and then ceramic material of 92 wt.% - 8% Y₂O₃ was coated thereon in the same manner as above.
To make sure of the effect of TBC according to the present invention, the following various tests were carried out. At first, oxidation tests were carried out at various temperature, and after the tests, appearance inspection, cross-sectional structure inspection and bonding strength tests were carried out. Table 2 shows the results of appearance inspection and bonding strength tests.
In Table 2, test pieces Nos. 1-6 relate to the conventional TBCs, and test pieces Nos. 7-11 relate to the present TBCs prepared according to the present process of Example 1. That is, in case of the conventional TBCs, the ceramic coat layer of 92 wt.% - 8 wt.% Y₂O₃ peeled off at a temperature of 1,070°C or higher for 100 hours, and the TBCs were damaged, whereas the present test pieces Nos. 7-11 had no damage on the appearance of TBCs. Results of bonding strength tests of TBCs after the oxidation tests revealed that the conventional TBCs of test pieces Nos. 1-6 with no damages on TBCs has a bonding strength of 2-5 kg/mm², and the bonding strength was lowered with increasing oxidation test temperature. The break site at the bonding strength tests was at the boundary between the bonding layer and the ceramic coat layer of 92 wt.% ZrO₂ - 8 wt.% Y₂O₃.
On the other hand, in all the cases of the present TBCs of test pieces Nos. 7-11, no decrease in the bonding strength of TBC was observed under any of the oxidation test conditions, and the bonding strength had more than 7 kg/mm², i.e. the limit value according to the bonding strength test using an adhesive having a bonding strength of 7 kg/mm². Thus, the break sites after the bonding strength tests were all in the adhesive region.
Then, thermal cycle tests of test pieces were carried out after the oxidation tests by repeating cycles each consisting of maintenance at 750°C for 15 minutes and maintenance in water at 20°-25° for 15 seconds. The results are shown in Table 3.
Test pieces shown in Table 3 were test pieces after the oxidation test. In case of the conventional TBCs of test pieces Nos. 1-3 in Table 3, the ceramic coat layer of 92 wt.% - 8 wt.% Y₂O₃ peeled off after the thermal cycle tests of 200 to 500 repetitions, and TBCs were damaged. On the other hand, the present TBCs of test pieces Nos. 7-11 in Table 3 had no damages even after the thermal cycle tests of 1,400 - 1,700 repetitions, and damages on TBC was observed in the thermal cycle test of maximum 1,700 repetitions. Thus, the present TBCs had a higher resistance to oxidation at a high temperature, a higher resistance to thermal shock and thus a higher resistance than those of the conventional TBCs.

Example 2

TBC was formed from the same materials under the same melt injection conditions as in Example 1. Then, the substrate with the TBC was heated in vacuum at 1,060°C for 3 hours to conduct a diffusion treatment between the Co-Ni-Cr-Al-Y coat layer as the bonding layer and the substrate, and then subjected to a heating treatment in the atmosphere at 1,000°C for 15 hours. The thus prepared TBC of the present invention had a boundary layer having a thickness of about 5 µm substantially uniformly formed at the boundary between the ceramic coat layer of 92 wt.% - 8 wt.% Y₂O₃ and the bonding layer of Co-Ni-Cr-Al-Y. It was found by EPMA analysis or X-ray diffraction that the boundary layer contained Al-based oxides as the main component.
For comparison, TBC was formed from the same materials as those for the present TBC in the conventional manner as in Example 1, and the substrate with the conventional TBC was subjected to the same diffusion treatment in vacuum and the heat treatment in the atmosphere as those for the present TBC.
In Table 3, results of thermal cycle tests of the present TBC, test piece No. 102, and the conventional TBC, test piece No. 101, conducted in the same manner as in Example 1 are shown. In the case of the conventional TBC, test piece No. 101, the ceramic coat layer of 92 wt.% ZrO₂ - 8 wt.% Y₂O₃ peeled off after about 500 repetitions, whereas the present TBC, test piece No. 102 was damaged after about 1,500 repetitions, as shown in Table 3. Thus, the present TBC had a durability about 3 times as long as that of the conventional TBC.
One embodiment of applying the present TBC to a gas turbine liner is shown in Fig. 2.
TBC was applied to the inside surface of the cylindrical part of combustor liner 1 in Fig. 2. At the downstream side of the combustor liner 1 were provided openings for cooling air (which will be hereinafter referred to as louvers 2), and the metal temperature was much elevated there. Thus, TBC was applied to the region indicated by "A" in Fig. 2. The substrate material of combustor liner 1 was Hastelloy-X (22 wt.% Cr - 1.5 wt.% Co - 9 wt.% Mo - 19 wt.% Fe - 0.1 wt.% C - the balance being Ni). The TBC having an Al-based oxide layer was formed by plasma melt injection as follows. At first, the liner was washed and defatted, and then blasted with Al₂O₃ grits. After formation of fresh surface on the substrate in this manner, an alloy material of 10 wt.% Ni - 25 wt.% Cr - 7 wt.% Al - 0.6 wt.% Y - 5 wt.% Ta - the balance being Co was deposited on the fresh surface by plasma melt injection to form a bonding layer. The desirable conditions for forming the bonding layer were as high a plasma power as possible and control of the atmosphere around the plasma jets during the melt injection. Particularly the factor for controlling the atmosphere was to lower the oxygen partial pressure, preferably, to 10⁻³ atmospheres or less. Other factor for controlling the atmosphere was to conduct the plasma melt injection under reduced pressure. By controlling the atmosphere, a preferable bonding layer for the present invention could be formed.
In the present embodiment, the plasma melt injection was carried out in an Ar atmosphere whose oxygen partial pressure was controlled to 10⁻³ atmospheres or less, and whose pressure was controlled to 200 Torr. The substrate temperature was maintained preferably at 500° to 1,000°C during the melt injection, but maintained in a range of 600° to 700°C in the present embodiment. Under these conditions, a bonding layer having a thickness of about 0.1 mm was formed. Then, a coat layer of ceramic material composed of 94 wt.% ZrO₂ - 6 wt.% Y₂O₃ was formed on the bonding layer. The coat layer having a thickness of about 0.3 mm was formed by plasma melt injection with a high plasma power, i.e. 55 kW. After the formation of TBC in this manner, the part with the TBC was heated in vacuum to conduct a diffusion treatment between the bonding layer and substrate in vacuum of about 10⁻⁵ Torr at 1,060°C for 5 hours, and then subjected to heat treatment in the atmosphere at 900°C for 20 hours. The conditions for the diffusion treatment and the heat treatment were not particularly limited, but it was preferable to conduct the diffusion treatment at a temperature ranging from the melt injection temperature for the substrate to 800°C for 3 to 100 hours, whereas it was preferable to conduct the heat treatment at a temperature ranging from 600° to 1,200°C for 1 to 200 hours. The combustor liner coated with the TBC having an Al-based oxide layer was formed in this manner.
The combustor liner 1 was in a structure having cooling louver 2, as shown in Fig. 3. To fully attain the cooling effect of such louver 2, it was necessary to keep the dimensional precision of the louver within the predetermined range. That is, when the thickness of TBC was extremely large at the louver site, the cooling effect was considerably lowered at the site, resulting in an increase in the substrate temperature. Furthermore, when the thickness of TBC was locally large, the durability of TBC at that site was considerably shortened. Thus, in the present embodiment, plasma melt injection was carried out onto the inside surface 3 within an angular range "B" shown in Fig. 3. By forming a bonding layer or a ceramic coat layer of 94 wt.% ZrO₂ - 6 wt.% Y₂O₃ under the said conditions, TBC was formed at the louver part to the thickness of TBC not larger than the necessary one. The TBC thus formed on the combustor liner had the cross-sectional structure similar to that of Fig. 1, where the boundary layer of Al-based oxides having a thickness of about 3 µm was formed between the bonding layer and the ceramic coat layer of 94 wt.% - 6 wt.% Y₂O₃. Thermal cycle test of repeating maintenance at 1,000°C for 30 minutes and maintenance in water at 20°-25°C for minutes was carried out with the combustor liner having such TBC.
For comparison, a combustor liner with the conventional TBC having no Al-based oxide layer was prepared in the same manner as in the present combustor liner and subjected to the same thermal cycle test. It was found that the present combustor liner had no damages on TBC even after 500 repetitions, whereas the combustor liner with the conventional TBC had damages on TBC only after about 90 repetitions.
The present combustor liner thus prepared and the conventional combustor liner prepared for comparison were subjected to combustion tests under the same conditions. In the tests for about 1,500 hours, the conventional TBC was damaged at the parts without the cooling louvers shown in the region "A" in Fig. 2, whereas no damages were observed on the present TBC throughout the entire region "A" of the present combustor liner. Furthermore, the part in the region "A" of the combustor liner shown in Fig. 2 was cut to inspect the state of TBC, and it was found by cross-sectional structure inspection that no damages were found at all at the TBC.
Dimensional change in the liner diameter at the part of the present combustor liner in the region "A" shown in Fig. 2 was less than about 3%, whereas the dimensional change in the liner diameter of the conventional combustor liner with the damaged TBC was as large as about 5%. As described above, the effect of the present TBC on the combustor liner can be maintained for a long time and thus problem such as deformation of combustor liner, etc. can be effectively prevented in the present invention.
TBC was provided on a combustor liner of another structure as shown in Fig. 4. Increase in the substrate temperature in the region "C" of the combustor liner of such a structure shown in Fig. 4 was remarkable. Thus, a combustor liner provided with the TBC at the inside surface in the region "C" to be exposed to the combustion gas shown in Fig. 4, was prepared, using the same materials, for the coat layers under the same conditions as in case of Fig. 2. For comparison, a combustor liner provided with the TBC having no Al-based oxide layer at the inside surface in the region "C" was prepared. These combustor liners were tested under the same combustion conditions, and it was found that no damages were observed on the TBC of the present combustor liner after the test for about 2,000 hours, and no deformation of combustor liner such as a change in the liner diameter, etc. occurred, whereas the conventional TBC on the combustor liner was considerably damaged after the test for about 2,000 hours, and the change in the liner diameter in the region "C" was large, and the combustor liner was deformed. Thus, the present combustor liner provided with the present TBC only at the part whose substrate temperature was elevated had a distinguished durability, that is, a high reliability. When TBC was provided on the entire inside surface of the combustor liner shown in Fig. 4 in the same manner as in case of Fig. 2, the thermal barrier effect was more improved with less deformation of the liner.
A moving vane for gas turbine having such a structure as shown in Fig. 5 was provided with the present TBC having an Al-based oxide layer at all the vane surface 6 and shroud 7. The moving vane 5 was made of Inconel-738. The materials of TBC were the same materials as in case of Fig. 2, and conditions, etc. were also the same as in case of Fig. 2. For comparison, a moving vane provided with the conventional TBC was prepared. The thus prepared vanes were subjected to the same thermal cycle test as in case Fig. 2, and it was found that the moving vane according to the present invention had 2 to 4-fold number of repetition until the TBC had been damaged, as compared with the moving vane provided with the conventional TBC.
In the foregoing embodiments, the combustor liner and the moving vane have been described, but the present invention is also effective for other gas turbine parts to be exposed to a high temperature. Furthermore, any known alloy material can be used for the bonding layer in TBC, so far as it contains Al as a component, preferably 5 to 30% by weight of Al. Any material can be used, so far as it contains ZrO₂ as the main component, and can contain any of CaO, MgO, Y₂O₃, etc. as a stabilizer. The thickness of the coat layers is not particularly limited, but it is preferable from the viewpoint of the thermal carrier effect and durability of TBC that the bonding layer has a thickness of 0.03 to 0.5 mm and the ZrO₂-based coat layer has a thickness of 0.05 to 0.8 mm.

Claims

1. A heat-resisting member which comprises a substrate (1) containing at least one of Ni, Co, and Fe as the main component, a bonding layer of corrosion-resistant alloy containing at least one of Ni and Co, and Cr and Al and having a higher resistance to oxidation at a high temperature than that of the substrate (1), formed on the substrate (1) and a ceramic coat layer containing ZRO₂ as the main component, formed on the bonding layer, wherein an Al-based oxide layer is formed at the boundary between the bonding layer and the ceramic coat layer.

2. A heat-resisting member according to Claim 1, wherein the substrate (1) is a Ni-based alloy containing 35 to 61% by weight of Ni, 1.0 to 3.0% by weight of Co, and 14 to 27% by weight of Fe.

3. A heat-resisting member according to Claim 1, wherein the ceramic coat layer is composed of a material containing ZrO₂ as the main component, and at least one of CaO, MgO, and Y₂O₃.

4. A heat-resisting member according to Claim 1, wherein the bonding layer of corrosion-resistant alloy is composed of a material containing Cr and A, and at least one of Hf, Ta, Y, Si, and Zr.

5. A heat-resisting member according to Claim 1, wherein the Al-based oxide layer is formed by oxidation of the Al component in the bonding layer by maintaining the substrate (1) having the bonding layer and the ceramic coat layer in the atmosphere at 600° to 1,200°C for at least one hour.

6. A heat-resisting member according to Claim 1, wherein the Al-based oxide layer has a thickness of 0.1 to 20 µm.

7. A heat-resisting member according to Claim 6, wherein the bonding layer has a thickness of 0.03 to 0.5 mm and the ceramic coat layer has a thickness of 0.05 to 0.8 mm.

8. A process for preparing a ceramic-coated, heat-resisting member, which comprises a step of forming a bonding layer of corrosion resistant alloy containing at least one of Ni and Co, and having higher resistances to oxidation and corrosion at a high temperature than those of a substrate (1) containing at least one of Ni, Co, and Fe as the main component on the surface of the substrate (1), a step of forming a coat layer of ceramic containing ZrO₂ as the main component on the surface of the bonding layer, and a step of forming an oxide layer containing Al as the main component at the boundary between the bonding layer and the ceramic coat layer.

9. A process according to Claim 8, wherein the step of forming the Al-based oxide layer comprises maintaining a substrate (1) having a bonding layer and a ceramic coat layer formed on the bonding layer in an oxygen atmosphere at 600° to 1,200°C for at least one hour, thereby oxidizing the Al component in the bonding layer.

10. A process according to Claim 8, wherein the step of forming the bonding layer of the corrosion-resistant alloy comprises forming the bonding layer by plasma melt injection in an atmosphere having an oxygen partial pressure of not more than 10⁻³ atmospheres.

11. A process according to Claim 10, wherein the step of forming the Al-based oxide layer comprises heating a substrate (1) having a bonding layer and a ceramic coat layer formed on the bonding layer in the atmosphere at a temperature of 600° to 1,200°C for 1 to 200 hours.