LOW COEFFICIENT OF EXPANSION ALLOYS HAVING A THERMAL BARRIER COATING

Abstract: A low coefficient of expansion alloy is coated with a thermal barrier coating having a low coefficient of expansion compatible with the alloy. The alloy is preferably a chromium-free iron-base alloy and is fist coated with an oxidation reistant intermediate bond coating and then with a zirconia containing thermal barrier. An article of manufacture made from this material could be utilized in heat engines where close tolerances are required.

Description: The instant invention relates to a thermal barrier coating system in general, and more particularly, to a thermal barrier coating system for alloys having a low coefficient of expansion. In order to increase the efficiency of heat engines, such as gas turbines and reciprocating engines, there usually must be a concomitant increase in the operating temperatures and pressures of these devices. Unfortunately, most current materials systems ultimately fail at elevated conditions thereby causing a practical limit on operating parameters. Over the years various materials have been proposed and introduced to boost the operating temperatures and pressures of these engines. One common system includes the application of a thermal barrier coating ("TBC") including zirconia to a superalloy substrate. An intermediate oxidation resistant bond coating of MCrAlY is disposed between the TBC and the substrate. Such systems are disclosed, for example, in US-A-4 485 151. The thermal expansion mismatch between conventional superalloys and their ceramic TBC's is partially accommodated by deliberately making the ceramic coating 10% porous. This is a half step at best. Under the circumstances, zirconia has been the material of choice since its coefficient of expansion is somewhat similar to those of the available nickel-base and cobalt-base superalloys now in production. In addition ZrO₂ has the lowest thermal conductivity of the common refractory materials. MgO and Al₂O₃ are not very suitable because their thermal conductivities are much greater than ZrO₂. A difficulty with the available systems is that the superalloys have a moderate coefficient of expansion that must be taken into account when the internal components of the engines are fabricated. In jet aircraft engines, for example, turbines may reach temperatures of 1093°C (2000°F) and more. Although the refractory coating enables the superalloy to operate within such an environment serving as both a thermal barrier as well as an adjunct to the corrosion resistant properties of the alloy, the expansion of the superalloy substrate material may introduce certain inherent design inefficiencies in the engine. Close operating tolerances of the critical components are absolutely critical in turbine design. As a result of the extreme conditions encountered in such power plants, low coefficient of expansion alloys have not been generally used in the more critical areas. Although possessing wonderfully low coefficient of expansion values which would allow increased engine component tolerances, these alloys generally do not exhibit the requisite high temperature and corrosion resistant characteristics as do the nickel-base and cobalt-base superalloys. Low expansion cast and wrought alloys such as the 900 series of chromium-free iron-nickel base alloys are used for shafts, seals and shrouds in gas turbine engines where they are limited to components operating at 649°C (1200°F) or lower. This is because of the reduced oxidation resistance at this temperature and above. The problem is further compounded by the fact that at temperatures above about 649°C (1200°F) the alloys undergo phase changes that embrittle them. However, as discussed above, to improve engine efficiency through tighter sealing, gas turbine manufacturers would welcome the opportunity to extend the use of chromium-free low expansion alloys to higher operating temperatures and pressures, but currently are stymied in view of the perceived shortcomings of the alloys. There are numerous coating systems in the literature. U.S. Patents 4,055,705; 4,248,940; 4,255,495; 4,485,151; 4,535,037; 4,375,190 are associated with refractories deposited on superalloy substrates. In contrast to these systems, the present invention employs a chromium-free low expansion alloy having an oxidation resistant, thermal barrier duplex coating. More particularly, it provides a coated article of manufacture for use at temperatures up to 871°C having a thermal barrier coating system comprising a substrate, an oxidation resistant intermediate bond coating thereon and a thermal barrier coating over the intermediate bond coating exhibiting a coefficient of expansion at 649°C similar to that of the substrate, wherein the substrate is an alloy having the nominal composition of 38% nickel, 13-15% cobalt, 3-4.7% niobium, 1.4-1.5% titanium, and optionally silicon (up to 0.4%) and aluminium (up to 0.9%), the balance, apart from impurities, being iron. The thermal barrier coating may include yttria and may be a partially stabilized zirconia-yttria thermal barrier coating. The resultant system exhibits the necessary low expansion characteristics and superalloy properties while simultaneously providing satisfactory oxidation resistance. Articles of manufacture may withstand temperatures of about 871°C (1600°F). Higher temperatures may be withstood provided that air cooling is employed. The thermal barrier coating has a very low thermal conductivity and a coefficient of expansion acceptably compatible to the cast and wrought low coefficient of expansion alloys in the expected temperature ranges. Although the anticipated internal operating temperatures many be higher than the results herein, the insulating characteristics of the thermal barrier will reduce the temperature of the substrate to acceptable levels. The invention will now be described in more detail with reference to the accompanying drawing, which shows graphically the linear thermal expansion (in percent) of various materials versus temperature. The ultimate thrust of the instant invention is the increased utilization of low coefficient of expansion chromium-free iron-nickel base alloys, more particularly the 900 series of low-expansion alloys, in heat engines. By applying the refractory thermal barrier and the intermediate bond layer to the alloy, the alloy's low expansion characteristics may be used to great advantage. The thermal barrier, preferably a partially stabilized zirconia ("PSZ" - 8% Y₂O₃-ZrO₂) fairly matches the coefficient of expansion of the substrate 900 series of alloys (903, 907 and 909) and is compatible therewith. By insulating the substrate from the high internal temperature ravages of the engine, the components made from the instant invention may permit closer manufacturing tolerances thereby resulting in greater operating efficiencies. The 900 series of low-expansion alloys nominally contain about 38% nickel, 13-15% cobalt, 3-4.5% or 4.7% niobium, 1.4-1.5% titanium, optional silicon (up to about 0.4%) and aluminum (up to about 0.9%), small quantities of other materials depending on the formulation and the remainder essentially iron (42%). The nominal compositions of the individual alloys of the 900 series (available as INCOLOY® alloys 903, 907 and 909 [trademark of the applicant company]) are as follows:- Alloy Ni % Co % Ti % Al % Si % Nb % Fe(bal) % 903 38 15 1.4 0.7(0.9) - 3 41.5 907 37.4(38) 13 1.5 0.1(0.03) 0.1(0.15) 4.7 42 909 37.4(38) 13 1.5 0.1(0.03) 0.4 4.7 42.7 These alloys are the subject respectively of US-A-3 157 495 (Alloy 903), US-A-4 200 459 (Alloy 907) and US-A-4 487 743 (Alloy 909), in which further details of their compositions are given. To achieve the low coefficient of expansion ("COE"), the chemistry is restricted. In particular, chromium is substantially absent and aluminum must be limited to levels normally less than contained in other classes of superalloys. Normally this will cause oxidation at elevated temperatures as well as a rise in the COE. However, by coating these materials, oxidation resistance is maintained and temperature stresses within the substrate are kept to acceptable levels. In particular, the 900 series of iron-base alloys were developed to take advantage of their low COE. For example, the COE of alloy 909 is about 10.3 µm/m/°C (5.7 x 10⁻⁶ in/in/°F) at about 649°C (1200°F) whereas the nickel-base superalloys INCONEL® alloy 718 (trademark of the applicant company), RENE' 41® (trademark of Teledyne Allvac) and WASPALOY (trademark of United Technologies Corp.) have a combined average COE of about 15.3 µm/m/°C (8.5 x 10⁻⁶ in/in/°F) at 649°C (1200°F) (about 48% higher than 909). The coefficient of expansion of the alloys used according to the invention is at least 25% lower than that of such superalloys. Iron-base superalloy A-286 has a COE of about 17.6 µm/m/°C (9.8 x 10⁻⁶ in/in/°F) at 649°C (1200°F) (about 71% higher than 909). The Figure provides a comparison between various materials. It should be noted that the low expansion 900 series of alloys (particularly 907 and 909) are closer to zirconia than the typical nickel-base (and iron- and cobalt-base) superalloys. For example, the COEs for alloys 907 and 909 at -18 to 93°C (0-200°F) are about 8.0 µm/m/°C (4.46 x 10⁻⁶ in/in/°F). The COE of PSZ at this same temperature range is about 11.0 µm/m/°C (6.1 x 10⁻⁶ in/in/°F). The COE's of the PSZ at the temperatures of interest are similar since they do not appreciably change with temperature. The coatings may be applied to the substrate by techniques known and available to those in the art. Plasma spraying was utilized in obtaining the following data. A METCO® 9MB plasma spray unit was employed. It should be appreciated, however, that the other suitable methods of applying the thermal barrier and intermediate bond coating are appropriate as well. MCrAlY (M = Ni, Fe, Co, NiFe, NiCo or mixtures thereof) was preferentially selected for the bond coating since it is highly effective for essential oxidation resistance. MCrAl and MAl may also be utilized if the conditions are less demanding. Test specimens were prepared. In most instances low COE INCOLOY® alloy 909 was the substrate alloy. For the purposes of comparison, INCONEL® alloy 718 and INCOLOY® alloy 800 were used for five substrates. The powders used for the intermediate bond coating were procured from commercial sources. Table I lists the compositions for the substrates and the intermediate bond coating. COMPOSITION OF SUBSTRATES AND INTERMEDIATE LAYERS Alloy Substrates Ni Co Fe Cr Cb Ti Al Si Other 909 38.2 13.0 Bal. -- 4.7 1.5 0.03 0.4 -- 718 52.0 1.0 max. Bal. 19.0 5.2 0.8 0.5 0.35 max. -- 800 32.0 -- Bal. 21.0 -- 0.4 0.4 1.0 max. -- Intermediate Layers Ni 211 Bal. -- -- 22.0 -- -- 10.0 -- 1Y Fe 124 -- -- Bal. 24 -- -- 8 -- .5Y Ni 963 60.0 -- Bal. 22.5 -- -- 6.8 -- -- Tables II and III list the particulars of the specimens. Table II relates to the entire system, whereas Table III details three different thermal barrier coating compositions. COMPOSITION OF TEST PIN SERIES Sample No. 8% Y₂O₃ - ZrO₂ microns (mils) Type of PSZ Powder Intermediate Layer microns (mils) Type of Powder 0 500 (20) Sintered 100 (4) Ni 211 1 500 (20) Fused 100 (4) Fe 124 2 500 (20) Fused 100 (4) Fe 124 3 500 (20) Fused 100 (4) Fe 124 4 500 (20) Sintered 100 (4) Fe 124 5 500 (20) Sintered 100 (4) Fe 124 6 500 (20) Sintered 100 (4) Fe 124 7 500 (20) Fused 100 (4) Ni 211 8 500 (20) Fused 100 (4) Ni 211 9 500 (20) Fused 100 (4) Ni 211 10 500 (20) Sintered 100 (4) Ni 211 11 500 (20) Sintered 100 (4) Ni 211 12 500 (20) Sintered 100 (4) Ni 211 16 500 (20) Fused 10 (0.4) Ni 963 19 500 (20) Fused 100 (4) Ni 211 20 500 (20) Fused 100 (4) Ni 211 21 500 (20) Sintered 100 (4) Ni 211 22 1000 (40) Sintered 100 (4) Ni 211 23 2000 (80) Sintered 100 (4) Ni 211 30 500 (20) Sintered 200 (8) Ni 211 31 1000 (40) Sintered 200 (8) Ni 211 32 2000 (80) Sintered 200 (8) Ni 211 37 1000 (40) Sintered 100 (4) Ni 211 39 1000 (40) Sintered 200 (8) Ni 211 42 1000 (40) Sintered 100 (4) Ni 211 All domed pin substrates are INCOLOY® alloy 909 (about 12.7 mm [0.5 in] diameter x 76.2 mm [3.0 in] long) except 19, 20, 37 and 38 which were INCONEL® alloy 718 and 42 which was INCOLOY® alloy 800. COMPOSITION OF VARIOUS TYPES OF PARTIALLY STABILIZED ZIRCONIA MONOLITHS Monolith No. Composition Type M- 2 8% Y₂O₃ - ZrO₂ Fused and crushed M- 5 8% Y₂O₃ - ZrO₂ Sintered and crushed M- 7 8% Y₂O₃ - ZrO₂ Spherodized M-10 12% Y₂O₃ - ZrO₂ Fused and crushed M-11 12% Y₂O₃ - ZrO₂ Fused and crushed M-13 20% Y₂O₃ - ZrO₂ Fused and crushed The specimens were evaluated in a cyclic oxidation rig under a variety of conditions aimed at evaluating: (i) the comparative resistance to thermal fatigue (coating crack or spalling) in order to find the preferred coating system; (ii) the operating temperature below which failure does not occur and (iii) the temperature gradient across the thermal barrier coating. The monoliths of Table III were made by spraying the various PSZ powders on a 12.5 mm (.5 in) diameter x 76,2 mm (3 in) length copper substrate followed by acid dissolution of the copper. The thermal cycling data from the specimens are presented by test run in Table IV. Each test run varied in thermal conditions and duration, consequently the data are reported by Test Run number (TR). The monoliths (second series of specimens) are described in Table V along with the distribution of phases in the as-received condition. Data from Table IV are presented in Table VI to show the effect of furnace temperature on the thermal cycle resistance of both types of PSZ (fused and sintered) coatings of FeCrAlY and NiCrAlY intermediate layers using an INCOLOY® alloy 909 substrate. Table VII summarizes the sustainable temperature gradient as measured at different operating temperatures and the thermal cycle history of air cooled (0.2 m³/hr [8 ft³/hr]) hollow pin 12. A more detailed sustainable temperature gradient versus environmental temperature is given in Table VIII for pin 12. These temperature values are not rounded. Table IX compares the thermal cycle resistance of sintered PSZ coating on Ni 211 intermediate layer on INCOLOY® alloy 909 versus INCONEL® alloy 718 substrates at two furnace temperatures. Table X shows the effect of sintered PSZ thickness on two thicknesses of Ni 211 intermediate layer of INCOLOY® alloy 909 at a furnace temperature of 1000°C (1830°F). Table XI presents the thermal cycle resistance of INCOLOY® alloy 909 substrate versus INCONEL® alloy 718 and INCOLOY® alloy 800 substrates at equivalent sintered PSZ coating and Ni 211 intermediate layer thicknesses at 1000°C (1830°F). PHASE IDENTIFICATION OF PLASMA SPRAYED MONOLITHS Monolith No. Type PSZ Phase Distribution Tetragonal FCC Monoclinic M- 2 8% Y₂O₃ PSZ-Fused 100% -- -- M- 5 8% Y₂O₃ PSZ-Sintered 100% -- -- M- 7 8% Y₂O₃ PSZ-Spheroidized 100% -- -- M-10 12% Y₂O₃ PSZ-Fused 57% 43% -- M-11 12% Y₂O₃ PSZ-Fused 41% 59% -- M-13 20% Y₂O₃ PSZ-Fused -- 100% -- EFFECT OF FURNACE TEMPERATURE ON THERMAL CYCLE RESISTANCE OF FUSED AND SINTERED PSZ COATINGS ON FeCrAlY and NiCrAlY INTERMEDIATE LAYERS USING AN INCOLOY® ALLOY 909 SUBSTRATE TBC Type Intermediate Layer Cycles @ 650°C(1220°F) Cycles @ 910°C(1670°F) Cycles @ 1015°C(1860°F) Fused Fe 124 >1877 1052 72 Sintered Fe 124 >1877 1186 208 Fused Ni 211 >1877 1160 72 Sintered Ni 211 >1877 5076 216 HISTORY OF AIR COOLED (0.2 m³/hr [8 ft³/hr]) PIN #12 TR Furnace, °C (°F) Core, °C (°F) Difference, °C (°F) # Cycles 3 880 (1610) 740 (1365) 140 (245) 1044 6 1015 (1860) 910 (1670) 105 (190) 144 7 650 (1200) 505 ( 940) 145 (260) 595 SUSTAINABLE TEMPERATURE GRADIENT EXPERIENCED BY PIN #12 VERSUS ENVIRONMENTAL TEMPERATURE USING INTERNAL AIR FLOW OF 0.2 m³/hr (8 ft³/hr) Furnace Temperature, °C (°F) Core Temperature, °C (°F) Difference, °C (°F) 1019 (1866) 916 (1680) 103 (186) 890 (1634) 742 (1368) 148 (266) 809 (1488) 650 (1202) 159 (286) 719 (1325) 556 (1032) 163 (293) 604 (1120) 440 ( 824) 164 (296) THERMAL CYCLE RESISTANCE OF SINTERED PSZ COATING ON NiCrAlY (Ni 211) INTERMEDIATE LAYER ON INCOLOY® ALLOY 900 VERSUS INCONEL® ALLOY 718 SUBSTRATES AT 900°C (1650°F) AND 1015°C (1860°F) Furnace Temperature, °C (°F) Cycles to Failure Alloy 909 Alloy 718 900 (1650) 5076 360 1015 (1860) 216 72 EFFECT OF SINTERED PSZ THICKNESS ON TWO THICKNESSES OF NiCrAlY (Ni 211) INTERMEDIATE LAYER ON INCOLOY® ALLOY 909 AT FURNACE TEMPERATURE OF 1000°C (1830°F) Pin No. Ni 211 Thickness microns (mils) Sintered PSZ thickness, microns (mils) Cycles to Failure 21 100 (4) 500 (20) 700 22 100 (4) 1000 (40) 190 (minor crack on dome) 23 100 (4) 2000 (80) 25 30 200 (8) 500 (20) 700 31 200 (8) 1000 (40) 40 32 200 (8) 2000 (80) 25 The results demonstrate: (i) the compatibility of duplex PSZ coating with low COE alloys, particularly 909 and (ii) that duplex PSZ coated INCOLOY® alloy 909 can be used in higher temperature environments than uncoated INCOLOY® alloy 909. It has been additionally shown that thermal barrier coated INCOLOY® alloy 909 as a composite system has greater thermal cycle resistance than does thermal barrier coated INCONEL® alloy 718 or thermal barrier coated INCOLOY® alloy 800. The instant system has been shown to tolerate at least up to 871°C (1600°F) with an air flow of only 0.2 m³/hr (8 ft³/hr) for at least 5000 cycles. It outperformed an identically tested alloy 718 NiCrAlY bond coat/PSZ composite which failed in less than 400 cycles. Table IV presents the thermal cycle resistance of the composite specimens tested under a variety of thermal cycle conditions. The tests were run in order to select a preferred type of 8% Y₂O₃-ZrO₂ (fused or sintered) and a preferred choice for MCrAlY intermediate layer (M = Ni or Fe). Additionally, four duplicate coated pins with INCONEL® alloy 718 substrates were tested for comparative purposes with INCOLOY® alloy 909. One plasma vapor deposited ("PVD") intermediate layer on INCOLOY® alloy 909 was also evaluated. The results (Table VI) suggest that at temperatures near 910°C (1670°F) and above sintered PSZ is preferably to fused PSZ. The reason for the preferred performance is not yet known. Similarly as shown also in Table VI, the performance of the two intermediate layers (NiCrAlY and FeCrAlY) are equivalent at 650°C (1220°F). However, at 910°C (1670°F) and above, the NiCrAlY intermediate layer (Ni 211) is preferred over the FeCrAlY intermediate layer (Fe 124). Again, a reason for this result has yet to be established. Table X clearly shows the marked improvement in thermal cycle resistance that is achieved when INCOLOY® alloy 909 is used as the substrate for a sintered PSZ coating (0.5 mm [0.02 in]) on a NiCrAlY (Ni 211) (0.1 mm [0.004 in]) intermediate layer over that of the duplicate coating on INCO alloy 718 (see Table XI for additional comparison with INCONEL® alloy 718 and INCOLOY® alloy 800). The PVD coating of Ni 963 was not roughened prior to either PVD or plasma spraying (of PSZ coating), consequently, the composite failed rapidly (Pin 16 in TR 4 and 5, data presented in Table IV). The effect of PSZ coating thickness on thermal cycle resistance is shown in Table X using the pins described in Table II. The 0.5 mm (20 mils) coating is preferred over the thicker coating thicknesses. These same Tables (X and II) present data to suggest that there is essentially no difference in performance between 0.1 mm (4 mils) and 0.2 m (8 mils) NiCrAlY intermediate layer thicknesses. Zirconia undergoes a drastic phase change near 950°C upon cooling which results in a 3.5% volume expansion. Additions of Y₂O₃ (6-8%) initially stabilize the high temperature tetragonal phase to much lower temperatures. Compositions near 12% Y₂O₃-ZrO₂ are 50% tetragonal - 50% FCC, while those near 20% Y₂O₃-ZrO₂ are totally FCC. This is confirmed in Table V for the monoliths of Table III. Note that the type of 8% Y₂O₃-ZrO₂ does not alter the phase distribution. Pin #12 from Table II was machined with a 6.3 mm (0.25 in) drill to make a hole to the dome of the test pin and using a small diameter tube and laboratory compressed air at 0.2 m³/hr (8 ft³/hr), the pin was air cooled while being tested in TR's 3, 6 and 7. The sustained temperature gradient and number of thermal cycles at each test temperature are given in Table VIII and a profile of the sustainable temperature gradients from 604°C (1120°F) to 1019°C (1866°F) are given in Table IX. Note that the sustainable temperature gradient gradually decreases as the temperature increases for the 0.5 mm (0.02 in) PSZ coating. Nonetheless, air cooled engine components made from the materials disclosed herein appear to be acceptable and overcome the difficulties envisioned with conventional 900 series alloys.

Claims: A coated article of manufacture having a thermal barrier coating system for use at temperatures up to 871°C comprising a substrate, an oxidation resistant intermediate bond coating thereon and a thermal barrier coating over the intermediate bond coating exhibiting a coefficient of expansion at 649°C similar to that of the substrate, wherein the substrate is an alloy having the nominal composition 38% nickel, 13-15% cobalt, 3-4.7% niobium, 1.4-1.5% titanium, and optionally silicon (up to 0.4%) and aluminum (up to 0.9%), the balance, apart from impurities, being iron. An article according to claim 1 wherein the substrate is alloy 903, alloy 907 or alloy 909. An article according to claim 1 or claim 2 wherein the intermediate bond coating comprises MCrAlY, MCrAl or MCr, M being selected from the group consisting of Ni, Fe, Co and mixtures thereof. An article according to any preceding claim wherein the thermal barrier coating comprises zirconia. An article according to claim 4 wherein the thermal barrier coating also comprises yttria. An article according to claim 5 wherein the thermal barrier coating comprises sintered partially stabilised zirconia. An article according to claim 6 wherein the thermal barrier coating comprises 8% Y₂O₃-ZrO₂. An article according to claim 1 including a NiCrAlY intermediate bond coating and a sintered partially stabilised zirconia thermal barrier. An article according to claim 7 or claim 8 wherein the intermediate coating has a thickness of about 0.1 mm and the thermal barrier coating has a thickness of about 0.5 mm. An article according to any preceding claim disposed in a heat engine. An article according to any preceding claim including an air passage disposed therein for internal cooling of the article. An article according to any preceding claim wherein the intermediate bond coating and the thermal barrier coating have been successively applied to the substrate by plasma spraying.

Inventors: BELL, JAMES ALEXANDER EVERT, DEBARBADILLO, JOHN JOSEPH, SMITH, GAYLORD DARRELL
Applicants: INCO LIMITED
Application year: 1989
Main classification: C23C4
IPC codes: C22C19, C23C4, F02B75, C22C30, C23C28, C22C38