Low Coefficient of Expansion Alloys Having a Thermal Barrier

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.

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 ZrO.sub.2 has the lowest thermal conductivity of the common refractory materials. MgO and Al.sub.2 O.sub.3 are not very suitable because their thermal conductivities are much greater than ZrO.sub.2.

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.degree. C. (2000.degree. 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 iron-base alloys are used for shafts, seals and shrouds in gas turbine engines where they are limited to components operating at 649.degree. C. (1200.degree. 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.degree. C. (1200.degree. 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 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. Pat. Nos. 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.

Accordingly, there is provided a low expansion iron-base alloy having an oxidation resistant, thermal barrier duplex coating. A 900 series low expansion alloy includes a partially stabilized zirconia-yittria thermal barrier coating and an intermediate oxidation resistant 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.degree. C. (1600.degree. 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 may be higher than the results herein, the insulating characteristics of the thermal barrier will reduce the temperature of the substrate to acceptable levels.

The FIGURE is a graphical depiction of 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 iron-base 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% Y.sub.2 O.sub.3 -ZrO.sub.2) 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 alloys nominally contain about 38% nickel, 13-15% cobalt, 3-4.5% niobium, 1.5% titanium, optional silicon and aluminum, small quantities of other materials depending on the formulation and the remainder essentially iron (42%). To achieve the low coefficient of expansion ("COE"), the chemistry is restricted. In particular, chromium 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 (available as INCOLOY.RTM. alloys 903, 907 and 909 [trademark of assignee]) were developed to take advantage of their low COE. For example, the COE of alloy 909 is about 10.3 .mu.m/m/.degree. C. (5.7.times.10.sup.-6 in/in/.degree. F.) at about 649.degree. C. (1200.degree. F.) whereas the nickel-base superalloys INCONEL.RTM. alloy 718 (trademark of assignee), RENE'.RTM. 41 (trademark of Teledyne Allvac) and WASPALOY.RTM. (trademark of United Technologies Corp.) have a combined average COE of about 15.3 .mu.m/m/.degree. C. (8.5.times.10.sup.-6 in/in/.degree. F.) at 649.degree. C. (1200.degree. F.) (about 48% higher than 909). Iron-base superalloy A-286 has a COE of about 17.6 .mu.m/m/.degree. C. (9.8.times.10.sup.-6 in/in/.degree. F.) at 649.degree. C. (1200.degree. 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 COE's for alloys 907 and 909 at -18.degree. to 93.degree. C. (0.degree.-200.degree. F.) are about 8.0 .mu.m/m/.degree. C. (4.46.times.10.sup.-6 in/in/.degree. F.). The COE of PSZ at this same temperature range is about 11.0 .mu.m/m/.degree. C. (6.1.times.10.sup.-6 in/in/.degree. F.). The COE's of the PSZ at the temperature of interest are similar since they do not appreciably change.

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.RTM. 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.

For the purposes of this specification a low COE is meant to be a value at least 25% lower than a corresponding nickel, iron or cobalt-base superalloy or an article of manufacture made therefrom at a given temperature.

Test specimens were prepared. In most instances low COE INCOLOY.RTM. alloy 909 was the substrate alloy. For the purposes of comparison, INCONEL.RTM. alloy 718 and INCOLOY.RTM. 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.

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.

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 (0.5 in) diameter .times.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 on FeCrAlY and NiCrAlY intermediate layers using an INCOLOY.RTM. 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 m.sup.3 /hr [8 ft.sup.3 /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.RTM. alloy 909 versus INCONEL.RTM. 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.RTM. alloy 909 at a furnace temperature of 1000.degree. C. (1830.degree. F.). Table XI presents the thermal cycle resistance of INCOLOY.RTM. alloy 909 substrate versus INCONEL.RTM. alloy 718 and INCOLOY.RTM. alloy 800 substrates at equivalent sintered PSZ coating and Ni 211 intermediate layer thicknesses at 1000.degree. C. (1830.degree. F.).

The results demonstrate: (i) the compatibility of duplex PSZ coating with low COE alloys, particularly 909 and (ii) that duplex PSZ coated INCOLOY.RTM. alloy 909 can be used in higher temperature environments than uncoated INCOLOY.RTM. alloy 909. It has been additionally shown that thermal barrier coated INCOLOY.RTM. alloy 909 as a composite system has greater thermal cycle resistance than does thermal barrier coated INCONEL.RTM. alloy 718 or thermal barrier coated INCOLOY.RTM. alloy 800. The instant system has been shown to tolerate at least up to 871.degree. C. (1600.degree. F.) with an air flow of only 0.2 m.sup.3 /hr (8 ft.sup.3 /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% Y.sub.2 O.sub.3 -ZrO.sub.2 (fused or sintered) and a preferred choice for MCrAlY intermediate layer (M=Ni or Fe). Additionally, four duplicate coated pins with INCONEL.RTM. alloy 718 substrates were tested for comparative purposes with INCOLOY.RTM. alloy 909. One plasma vapor deposited ("PVD") intermediate layer on INCOLOY.RTM. alloy 909 was also evaluated. The results (Table VI) suggest that at temperatures near 910.degree. C. (1670.degree. F.) and above sintered PSZ is preferable 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.degree. C. (1220.degree. F.). However, at 910.degree. C. (1670.degree. 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.RTM. 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.RTM. alloy 718 and INCOLOY.RTM. 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 mm (8 mils) NiCrAlY intermediate layer thicknesses.

Zirconia undergoes a drastic phase change near 950.degree. C. upon cooling which results in a 3.5% volume expansion. Additions of Y.sub.2 O.sub.3 (6-8%) initially stabilize the high temperature tetragonal phase to much lower temperatures. Compositions near 12% Y.sub.2 O.sub.3 -ZrO.sub.2 are 50% tetragonal - 50% FCC, while those near 20% Y.sub.2 O.sub.3 -ZrO.sub.2 are totally FCC. This is confirmed in Table V for the monoliths of Table III. Note that the type of 8% Y.sub.2 O.sub.3 -ZrO.sub.2 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 m.sup.3 /hr (8 ft.sup.3 /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.degree. C. (1120.degree. F.) to 1019.degree. C. (1866.degree. 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.

While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.