Thermal and sputtered aluminum oxide coatings for high temperature electricalinsulationKenneth G. Kreider and Stephen Semancik Citation: Journal of Vacuum Science & Technology A 3, 2582 (1985); doi: 10.1116/1.572838 View online: http://dx.doi.org/10.1116/1.572838 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in High Temperature Thermal Stability and Oxidation Resistance of Magnetronsputtered Homogeneous CrAlONCoatings on 430 Steel AIP Conf. Proc. 1099, 303 (2009); 10.1063/1.3120037 Electrical Properties of High Temperature Insulation Coatings under various Pressures at 298 K and 77 K AIP Conf. Proc. 899, 431 (2007); 10.1063/1.2733221 Control of the structure and properties of aluminum oxide coatings deposited by pulsed magnetron sputtering J. Vac. Sci. Technol. A 17, 945 (1999); 10.1116/1.581669 Insulator-coated sputter gun for growing superconducting oxide films J. Vac. Sci. Technol. A 15, 2605 (1997); 10.1116/1.580779 Reactive direct current magnetron sputtering of aluminum oxide coatings J. Vac. Sci. Technol. A 13, 1188 (1995); 10.1116/1.579859

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Thermal and sputtered aluminum oxide coatings for high temperature electrical insulation

Kenneth G. Kreider and Stephen Semancik Chemical Process Metrology Division, National Bureau o/Standards, Gaithersburg, Maryland 20899

(Received 3 May 1985; accepted 16 May 1985)

Aluminum oxide coatings have been investigated as electrically insulating layers for mounting thin film Pt-PtlRh thermocouples on gas turbine blade and vane alloys (MAR M200 + Hf and MAR M509). Thermal oxides were grown directly onto NiCoCrAIY and FeCrAIY coatings on these alloys at temperature between 1300 and 1400 K in oxygen partial pressures 10- 7 to 2 X 104

Pa. Although these thermal oxides exhibited good adherence, analytical characterizations using electron and optical microscopy, as well as x-ray photoelectron spectroscopy (XPS) showed that they had defects and impurities which limited their insulating ability. The insulating quality of the coating was greatly improved however by reactively sputtering an aluminum oxide film over the thermal oxide. Results are presented on the electrical performance of the 2-5 J-lm thick composite layers for temperatures up to 1300 K.

I. INTRODUCTION

High temperature thin film sensor technology is being devel­oped at the National Bureau of Standards, I NASA, and ma­jor turbine engine manufacturers. 2

,3 These thin film sensors promise to provide temperature measurements of critical areas of the turbine engine blades and vanes with better accu­racy than present thermocouple technology, Presently used wire thermocouples are imbedded in the blade in machined grooves with their insulation and are cumbersome and bulky by comparison, They lead to distortions in reproducing ex­act heat transfer conditions of the blade or vane and distur­bances in the flow of the gas stream. A thin 2-5 J-lm film thermocouple mounted directly on the critical component has the advantage of excellent heat transfer characteristics to define precise component metal temperatures without dis­turbing the gas stream,

The thin film thermocouple assembly is constructed on top of the turbine engine blade or vane coating as depicted in Fig. 1, Virtually all first stage turbine engine blades and vanes have MCrAIY coatings, where M stands for nickel, cobalt, iron, or a combination of these metals. This coating is used to protect the blade from oxidation and sulfidation and extensive research in the industry has been used to optimize the composition and fabrication processing of this coat­ing.4-14 All of these coatings feature the growth of primarily A1 20 3 from the aluminum in the coating (typically 10%-12%) which forms an excellent barrier against oxidation of the base metal. Fortunately, the A1 20 3 is also an excellent electrical insulator even at 1300 K. In fact, among the alloy­ing ingredients in superalloys such as Ni, Fe, Cr, Co, Ti, Zr, Ta, Nb, W, and Mo the oxide of aluminum stands along as a good electrical insulator at that temperature. Therefore, the construction of a thin film thermocouple or a gas turbine engine blade or vane starts with careful growth of aluminum oxide on the coating. Unfortunately, this native oxide on the coating does not provide a fully adequate insulation.

The MCrAIY oxide has impurities and imperfections which can degrade its electrical properties, For example, the growth of grains of NiO, Fe20 3 or Cr20 3 which are 105 and

2582 J. Vac. Sci. Technol. A 3 (6), Nov/Dec 1985

107 times as conductive as A1 20/5 would lead to shorts. Other defects related to the growth habit of A1 20 3 cause a lack of adherence, or spalling during thermal cycling, The technology developed by Dils2 and Grant et al. 3 has included the coating of sputtered aluminum oxide between the ther­mal oxide and the metal thermocouple legs. Sputtered type S thermocouples, platinum and platinum plus 10% rhodium, have been used for their outstanding oxidation resistance and durability as the melting points of the superalloys.

Several problems remain with this thin film thermocouple system. Practically, they involve electrical shorts through the insulation, delamination in service, and open circuits de­veloped on thermal cycling. The central problems are the adhesion to the oxide by the precious metal of the thermo­couple and the oxide's insulating integrity. Our research fo­cused on studying the thermal and sputtered oxides with an objective of relating their processing, structure, composi­tion, and performance.

II. EXPERIMENTAL MATERIALS AND METHODS

Discussions with NASA and the major engine manufac­turers led to the choice of MAR M200 + Hf as a typical blade alloy and MAR M509 as the vane alloy. Commercial

~ 'f'i j~i1$I~.3 .1"'.rel~.:I ... I .•• 1 .•.. r~~:~~::u~:: ::~Oxide Layer

- MCrAIY Coating T 4~~~~~~r4-o<~ (sputtered or evaporated)

A -Original Metal Surface

-Baae Metal

FIG. 1. Schematic cross section of thin film sensor.

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2583 K. G. Kreider and S. Semancik: Thermal and sputtered aluminum oxide coatings 2583

alloys with both electron beam and sputter deposited MCrAIY coatings were obtained from a major engine manufacturer. The coatings include NiCoCrAIY (Bal Ni,0.23Co,0. 18Cr,0. 12AI,0.005Y) and FeCrAIY (Bal Fe, 0.18Cr,0.IIA 1,0.OO7Y). These material systems included the most successful combinations to date for blade and vane thin film thermocouples.

Thermal oxides on the MCrAIY alloys were grown in oxi­dizing atomspheres including air, dry ultrahigh purity (UHP) argon « 2 ppm impurities), argon plus room tem­perature dew point H20, and argon plus 4% H2 • This en­abled a wide range of oxygen partial pressures (104 to 10-7

Pa) for oxidation The lower levels should suppress the for­mation of all but A1 20 3 and Y20 3 and are recommended by Grant and Przybyszewski3 in the initial stages of oxidation to promote a dense, hard, adherent layer of A1 20 3 • Prior to oxidation, the surfaces were ground and polished to either a - 600 mesh or 1 !-lm finish and cleaned ultrasonically in

acetone followed by a methyl alcohol and deionized water rinse.

In-house sputtering included the reactive sputtering of A1 20 3 and rf magnetron sputtering of platinum and plati­num lO%-rhodium thermocouple alloys. The sputtering was accomplished using a 60 cm diam vacuum sputtering chamber and a 3000 W rf generator at 13.56 MHz, an imped­ance matching box, and a 5 cm diam planar magnetron. A quartz oscillator thickness gauge was used to monitor the deposition during sputtering and a mass spectrometer was used for residual gas analysis. Bottled 99.999% pure argon and 99.99% pure oxygen were used in reactive sputtering. Typical parameters for Pt-Pt/Rh sputtering were: target to substrate distance 15-25 cm; initial vacuum 4-15 X 10-5 Pa; argon sputtering pressure 0.14 Pa; 250 W power; deposition rates 0.4-0.8 nm per s metal; and rf impedance 50 n. The reflected power was less than 1 %. Typical parameters for reactive sputtering of A1 20 3 were: 99.99% aluminum tar­get; target to substrate distance - 15 cm, initial vacuum 4-14X 10-5 Pa; argon pressure 0.14 Pa; oxygen pressure 0.014 Pa; 400 W power; deposition rates 0.05-0.1 nm per s; and rf impedance 50 n. The reflected power was less than 1 %. Typical residual gas analysis measured with the mass spectrometer indicated the following pressures: H 20 = 10-4

, Ar = 10-6, O2 = 10-5, CO-N2 = 3x 10-5,

CH4 = 5 X 10-6 Pa. The thickness of deposition was checked by weight gain on the sample and monitored on a glass slide. Visual inspection of the glass slide also served as a convenient way to insure the stoichiometry of the A1 20 3

since any deviations affected the transparency of the deposit. Thicknesses of the A1 20 3 deposits were also checked using interferometry with thallium illumination which indicated fringes at step heights of 0.27 !-lm.

Analytical techniques were used during this study to char­acterize the layers, surfaces and interfaces present in the thin film thermocouple devices. The overall objective was to identify the most desirable procedures for each of the fabri­cation steps. Specific analyses, described below, were done to document the quality of the insulating aluminum oxide lay­er, the degree of metal diffusion into the aluminum oxide, and the amount and type of near-surface impurities.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

The techniques used included scanning electron micros­copy (SEM), energy dispersive x-ray analysis (EDX), and x­ray photo emission spectroscopy (XPS). These analysis meth­ods were chosen since they can provide complementary information relating to the structure, composition and chemistry of interfacial regions. For example, SEM mea­surements give a direct picture of surface (or cross sectional) microstructure, while EDX can identify the composition of features seen within the SEM micrographs. While EDX has a relatively high lateral spatial resolution, its sampling depth is about 1 !-lm; XPS, on the other hand, has a nominal sam­pling depth of 5 nm, and can therfore provide key informa­tion about the composition at the surface of films used in the thermocouple devices. In addition, XPS can detect elements oflow atomic number (like oxygen), which EDX cannot, and XPS also indicates the chemical environment in which the detected atoms exist.

SEM and EDX measurements were done using commer­cial instrumentation equipped with an electron microscope and integrated EDX capabilities. Specimens were carbon coated to reduce charging effects produced by the incident SEM electron beam. Several commercial spectrometers were used during the course of this work to perform the x­ray photoemission measurements. Samples were placed "as is" into the ultrahigh vacuum enclosures of the photoelec­tron spectrometers and lightly sputter etched to obtain "clean" surfaces. XPS spectra from the outermost layers of these samples provided information on surface impurities, the presence of Co, Ni, Fe, and Cr in the oxide layers, and oxide stoichiometries.

Mechanical and electrical tests were used to evaluate the adhesion, stability, electrical performance, and overall suit­ability of the aluminum oxide coatings and Pt-Pt/Rh ther­mocouple films. The adhesion test employed a commercial adhesion tester which operates by pulling an epoxy bonded tab from the bonded film. These tests are reported directly as a shear strength of the film bond or strength up to 70 MPa (10 000 psi). The electrical insulation and dielectric test was performed using a test jig in an air atmosphere furnance. The 1.8 X 1.8 cm test coupon was held with platinum and plati­num-lO% rhodium connections to the thin film thermocou­ple electrodes and heated to approximately 1200 K. The out­put voltage and resistance with a 1 V potential were monitored between the Pt-Pt/Rh thin film and between the thin film and substrate (ground) while heating the test cou­pon. This test detects any premature dielectric breakdown of the insulator and any instability of the thin film sensor. Al­though the test cannot insure the stability of the thermocou­ple electromotive force (emf), since the entire thin film ther­mocouple is at virtually one temperature, it does detect increases in resistance or deterioration mechanisms such as oxidation of rhodium during high temperature exposure.

Thin film thermocouples were produced with 10 cm legs on long bars to measure output voltages between the hot and cold junctions. These test bars were placed through the wall of the furnace with the hot junction and the cold extension wire connections monitored with type S reference grade thermocouples. Extension connections were made with cop­per thermocouple extension wire.

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2584 K. G. Kreider and S. Semancik: Thermal and sputtered aluminum oxide coatings 2584

FIG. 2. Scanning electron micrograph of D4 (1530X). Oxides are penetrat­ing inward from surface heat treated at 1090 'C for 50 h.

III. RESULTS AND DISCUSSION

Thermal oxidation of the MCrAlY coatings is a critical step in the creation of a durable, adherent, insulating layer between the turbine engine hardware and the thin film ther­mocouple sensor. Forty-eight 1.S X 1.S cm test coupons were oxidized at temperatures ranging from 1300 to 1400 K for up to SO h in the four atmospheres (air, Ar + 3% H20, Ar, and Ar + 4%H2). The four coating-substrate combinations in­cluded FeCrAIY on both MAR MS09(A) and MAR M200 + Hf (B) and both electron beam deposited (C) and sput­

tered (D) NiCoCrAIY on MAR M200 + Hf. All samples were examined for color, quality, and perfection of the oxide at SOO X and were checked for the effectiveness of the insula­tion by measuring the resistance between the base metal and a tap water wetted 1 em spot on the insulation. The tap water effectively finds pores and gaps in the oxide which should have more than 2 Mil resistance. It was found that tempera­tures above 1360 K (1090 0C) lead to uneven, coarse oxides with both the FeCrAIY and NiCoCrAIY coatings. Other problems with the thermal oxide included spalling on the

FIG. 3. AI map of D4 (1530 X). A 1,03 penetration is evident from diffusion pipes.

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

FIG. 4. Thermal oxide of FeCrAIY on B1O. Dark grains are related to the uneven growth pattern (500x).

thicker thermal oxides such as sample A3S (FeCrAIY on MAR MS09) where 10 pm diam chips spalled off with a void and faceted surface beneath. Similar problems with the oxide on the NiCoCrAIY (e.g., sample D4) seemed to relate to oxide "pipes" which were identified with SEM and EDX (Figs. 2 and 3). This void formation has been noted on MCrAIY alloys by Sprague et al.9 and by Tien and Pettit. 13

Other problems relate to the growth of 1 pm dark red grains (probably Fe20 3 ) in the FeCrAIY (Fig. 4) or green-blue grains (probably NiO) on the NiCrAIY when the oxide is grown in air. The use ofthe dry high purity argon for the first 4 h of the oxidation as recommended by Grant et al. 3 appears to have helped suppress these inferior oxide growths. It was determined that temperatures between 1300 and 1350 K could be used for SO h starting with a very low p02 (pure argon) for 4-10 h to obtain a useful thermal oxide. A typical good thermal oxide is shown in Fig. 5. No advantages were obtained using argon with H20 or H2 and those treatments are not recommended. The adhesion tests were performed on thermal oxides, sputtered oxides and on the platinum­platinum plus 10% rhodium thin films. The NiCoCrAlY thermal oxidation had less of a problem with spalling and its concomitant void formation than the FeCrAIY. The oxide on the NiCoCrAIY tended to have a needlelike morphology

FIG. 5. Thermal oxide of electron beam deposited NiCoCrAIY on C21. Small fibrous habit of the oxide is apparent in addition to the film growth (500x).

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2585 K. G. Kreider and S. Semancik: Thermal and sputtered aluminum oxide coatings 2585

FIG. 6. Sputtered oxide on C19 (NiCoCrAIY) which is similar to that on FeCrAIY (500X).

superimposed on the film growth similar to that reported by Smeggil and Bornstein. 16 They attributed the problem to the presence of 0.61 ppm NaCl in the gas. Strieff et a/.17 have attributed the needlelike growth to high aluminum levels in the /3 NiAI phase.

Thermal oxidation was followed by the deposition of a reactive sputtered Al20 3 from an aluminum target as was described above. The weight gain was followed and used to control the thickness to be 1-4 f1m for the sputtered oxide film. A typical sputtered oxide is shown in Fig. 6 which looks like a clear lacquer coating on the thermal oxide at 500 X . The room temperature "water spot" resistance test was used to confirm the integrity of the sputtered oxide. Adhesion tests on both thermal and sputtered oxides indicated interla­minar shear strengths of 70 MPa (10 000 psi) were obtaina­ble. Low adhesion of the oxide films was observed only for two circumstances: very thin « 0.1 f1m) thermal oxide which was grown in the Ar + 4% H2 atmosphere and thick ( > 3 f1m) sputtered oxide films.

Analytical investigations were carried out on thermal ox­ide layers produced in different growth atmospheres and at different oxidizing temperatures. Various surface impurities were observed in the XPS spectra of many of these samples. Although carbon levels were sometimes quite high, it is be­lieved that much of the carbon collected on the surfaces dur­ing postoxidation handling. Other impurities, like Si and As, at levels of approximately 2.5 and 0.5 at. %, were also ob­served. Si is an impurity that has been identified by the sub­strate/coating vendor. IX The high surface sensitivity ofXPS demonstrated that Ni, Co, Cr, and Fe did diffuse to within 5 nm of the oxide surface. Sample A42, a FeCrAIY-coated specimen air oxidized at 1075 °C, had Fe and Cr levels near 0.1 at. % in its oxide layer, while sample C12, a NiCoCrAIY specimen Ar + H20 oxidized at 1090 °C had Ni and Cr (and presumably Co, which was not recorded) in its oxide at about 0.5 at. %. The C12 sample also appeared to have a surface enrichment of yttrium, apparently due to the diffusion of Y to the surface where it reacted with oxygen. The yttrium was noted in other thermally treated samples as well.

X-ray photoemission was used not only to estimate the level of Fe, Cr, Co, and Ni, but also to identify their chemical state. The spectrum in Fig. 7 shows the Ni 2P3/2 region for

J. Vac. ScI. Technol. A, Vol. 3, No.6, Nov/Dec 1985

sample D1 (a NiCoCrAlY-coated specimen thermally oxi­dized at 1050°C in Ar and H20). The two photoemission features are very similar to those observed in the XPS spec­trum of nickel oxide. 19

Despite some presence of nickel, chromium (and cobalt and yttrium) oxides in almost all the thermally grown layers, the insulating oxide was predominantly (typically >95%) aluminum oxide. Quantitative evaluations of elemental con­centrations were obtained using XPS peak areas and ac­counting for core level dependent photoemission cross sec­tions and instrument transmission functions. The aluminum to oxygen ratio was generally close to 2:3 when corrected, indicating nearly stoichiometric Al20 3 at the surface, al­though some samples seemed to be somewhat oxygen rich at the surface.

Since the surfaces of the studied specimens were primarily aluminum oxide, core level chemical shifts of Al and oxygen features for the various samples would be expected to be small. However, shifts in the core levels were observed, and these are attributable to photoemission-induced surface charging. The charging that occurs on insulating films pro­duces a uniform shift to higher binding energy for all XPS features, and gives a useful measure of the insulating quality of the oxide. For example, it was found that oxides grown in air provided better insulation (higher XPS charging) than those grown in Ar + H 20. This result may be directly corre­lated to the higher levels of Ni oxide and Cr oxide found in Ar + H 20-grown oxides, since the presence of these species could be expected to reduce the resistivity of an alumina layer.

An extreme case of surface charging is exhibited by the comparison of the two samples in Fig. 8. A charging shift is measured on the surface of sample Cl4 which is 4 eV larger than that measured for the Ar + H20 thermal oxide on C 12. This result illustrates the general finding that combination (sputtered over thermal) oxide films provide insulating lay­ers which are far superior to thermal oxide layers alone. The

o ~ 1.50 U w fI)

II: W Q.

fI) I­Z

5 0.75 U

• o ~

o

XPS Sample D1

(Thermal oxide grown

in Ar+ H20)

875 870 865 860 855 850 845 840

BINDING ENERGY (eV)

FIG. 7. XPS nickel2Pm core level region indicating the presence of oxidized nickel in the surface aluminum oxide of sample D 1.

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2586 K. G. Kreider and S. Semancik: Thermal and sputtered aluminum oxide coatings 2586

o ~ 1.6 u w C/)

a: 1.2 w 11.

C/)

!Z 0.8 :::> o u

0.4 .. o

OXYGEN 10

536.3 eV j

532.3 eV

I C14

XPS Mg Ko

C12 / ... Sputtered ./ \y- Thermal

oxide surface! \ oxide I \ surface

/ \ (Ar+ H20) / \

I \ t' \

-' \ '-

O'_-~ '­'-

540 538 536 534 532 530

BINDING ENERGY (eV)

FIG. 8. Oxygen Is XPS core levels from samples C12 and C14. The better insulating properties of the C 14 sputtered oxide surface (over a thermal oxide) lead to an upward shift in binding energy which positions the 0 Is level 4 eV higher than 0 Is level from the Ar + H20 thermal oxide at the surface of C12. Similar shifts were observed in comparisons of other fea­tures (e.g., C Is) derived from these samples.

poorer insulating quality of thermally grown oxides is prob­ably connected not only to differences in the total oxide thickness, but also to the presence of pores and pits that have been optically observed in such layers and the presence ofNi and Cr in the oxide. Sputtered aluminum oxide overlayers probably fill these pores, thereby leading to the observation oflarger charging shifts indicative of better insulating layers.

The surfaces of sputtered aluminum oxides would also be expected to have higher resistivities because they showed no signs of Ni, Cr, or Co oxides. Figure 9 shows an example

c z o () w en a: w Q.

en I­z ::J o ()

'" o

XPS Mg Ka

Expected location

Cr 2P3/2 j

Sputtered oxide surface

C12 Thermal oxide surface (Ar+ H 20)

x2.5

Cr 2P3/2

592 590 588 586 584 582 580 578 576 574

BINDING ENERGY (eV)

FIG. 9. XPS spectra for samples CI2 and CI4 in the binding energy region near the Cr 2P3/2 level. The features in the CI2 spectrum indicate some presence of chromium for CI2 which is absent in the CI4 case. (Charging shifts have been applied to locate the positions of the Cr 2r3 / 2 levels for these samples.)

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

2000 g w () z < I-en en w 1000 a: en ~ J: 0

SPUTTERED OXIDE FILM RESISTANCE

• o

400 600

TEST TEMPERATURE (OC)

800 1000

FIG. 10. Sputtered oxide film resistance. A40 is FeCrAIY on Mar MS09 BI6isFeCrAIY on Mar M200 + Hf, C24isEB NiCoCrAIYon MarM200 + Hf, D2 is sputtered NiCoCrAIY on Mar M200 + Hf.

comparison of the Cr 2P3/2 regions for a thermal oxide (C12) and the sputter deposited surface of sample C 14. Chromium features are only observed at the Cl2 surface.

Platinum and platinum plus 10% rhodium thin films were sputtered 1 mm wide with a 5 mm central overlap on the 1.8 cm square coupons using steel shim stock masks. This per­mitted high temperature electrical testing. Thicknesses were controlled to be between 1.5 and 2.5 f.1m and adhesion testing indicated that 70 MPa interlaminar shear strength could be attained. The conditions required for good adhesion ap­peared to be optical surface roughness in the 1-3 f.1m range (a - 600 mesh polish), cleanliness and freedom from carbon

bearing films, thermal oxide of at least 1 f.1m thickness, and sputtered oxide films less than 3 f.1m thick. No chemical etch­ing was required for platinum adhesion.

The high temperature insulation test was used to evaluate the oxide resistance and dielectric strength as a function of temperature up to 1325 K. Typical results of testing A, B, C, and 0 type sputtered oxide films are presented in Fig. 10. The oxide films on NiCoCrAIY (type C and 0) had resis­tance values greater than 2 MD. below about 1175 K but the resistance fell rapidly to several hundred kilohms at tem­peratures above 1325 K. The steeply decreasing slope in ox­ide resistance with increasing temperature is comparable to those given in Samsanov. 15 The oxide films on FeCrAIY were apparently growing above 975 K as indicated by the increase in resistance measured at the 1075 K region (Fig. 10). Generally, the resistance of these films was not as great as those formed on NiCoCrAIY. The test fixture also permit­ted evaluation of dielectric breakdown. A 1 V potential was placed over the oxide film at each temperature up to the maximum temperature but no dielectric breakdown was ob­served on any test sample. The high dielectric strength of Al20 3 is no doubt adequate for thermocouple voltages and even for thin film strain gages with similar dimensions.

Tests on the Pt-Ph/Rh alloy films were conducted with the same equipment. In addition to measuring the resistance to ground, a continuous measurement of the metallic thin

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2587 K. G. Kreider and S. Semancik: Thermal and sputtered aluminum oxide coatings 2587

~~-------------------------------------,

=:: 1Il ~ :::J Cl LJ Cl ::0;:

8S 2IIl :r: I-

::0;: -' L: :z: :r: I-

100

4 SLOPE = O. 8309

100 200 300 400 500 600 TEMP. DIFFERENCE - REFERENCE THERMOCOUPLES K

FIG. II. Thin film thermocouple test.

film resistance was made. Samples of each of the four materi­al systems were fabricated with thicker oxide layers than A40, B16, C24, and D2 to improve the high temperature performance. In fact, all four test coupons had greater than 2 MO resistance between ground and the platinum at test tem­peratures up to 1400 K. The C type (EB NiCoCrAlY on MAR M200 + Hi) resistance fell to 350 kO at 1440 K. The platinum alloy bimetallic strip increased in resistance at high temperatures as expected.

Long (10 cm) test bars were fabricated to test the thin film thermocouple output versus temperature. Some further problems were encountered in maintaining good adhesion and freedom from insulation defects on these larger speci­mens which may relate to contamination or unevenness of the sputtered layer. The results of testing one of these 10 cm bars through the furnace wall are presented in Fig. 11. The figure is used to compare the emf output of the thin film thermocouple on FeCrAlY to the difference between two reference grade type S thermocouples stationed at the hot junction and the extension wire connections outside the fur­nace. One problem related to this comparison is that the reference thermocouples do not measure the exact metal temperature of the bar and they are affected by their radi­ation environment more than the bonded thin film thermo­couple. A second important problem relates to the exact composition of the sputtered thin film thermocouple which may be altered during sputtering. The test is considered a demonstration of the feasibility of the approach rather than an exact calibration. Nevertheless, the comparison in Fig. 11 indicates rather good agreement and the linear slope of the data is within 20% of the reference thermocouples.

IV. SUMMARY

Thermally grown aluminum oxide films on FeCrAlY and NiCoCrAlY and sputtered aluminum oxide films were in-

J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985

vestigated for mounting thin film Pt-PtiRh thermocouples. The thermal oxide films were found to have defects and im­purities which reduced their insulating properties. These de­fects were characterized using electron and optical micros­copy as well as XPS. The combination of the thermal oxide plus sputtered aluminum oxide was found to have over 1 MO resistance at 1300 K on the NiCoCrAlY and approximately 100 kO at 1300 K on the FeCrAlY. Type S (Pt-PtlRh) sput­tered thin film thermocouples on the oxide insulator were found to perform comparably to wire thermocouples on test bar samples.

ACKNOWLEDGMENTS

The authors wish to acknowledge sponsorship for this re­search by the Turbine Engine Hot Section Technology (HOST) program of the National Aeronautics and Space Agency and the Energy Conversion and Utilization Tech­nology (ECUT) program of the Conservation and Renew­able Resources, DOE. We would like to thank Ray Hollanda of NASA, Joseph Carpenter of ORNL, and James Eber­hardt of DOE for their helpful guidance in this project. We would also like to thank James Mullaly of Pratt and Whit­ney, Florida for his help in materials and Craig Olson of NBS for his help with electron microscopy.

'K. G. Kreider, S. Semancik, and C. Olson, National Bureau of Standards Internal Report No. 84-2949 (National Bureau of Standards, U. S. GPO, Washington, D. C. 1984).

2R. R. Dils and P. S. Follansbee, Superalloys: Metallurgy and Manufacture, edited by B. H. Kear, D. R. Muzyka, J. K. Tien, and S. T. Wlodek (Claitor, Baton Rouge, LA, 1976), pp. 37-44.

3H. P. Grant etal., NASA Report No. CR-165476 (11 Jan. 1982). 4F. S. Pettit, Trans. Metall. Soc. AIME 239, 1296 (1967). 5G. W. Goward, D. H. Boone, and C. S. Giggins, Trans. of ASM 60, 228 (1967).

"T. K. Redden, Trans. Metall. Soc. AI ME 242,1695 (1968). 7c. S. Giggins and F. S. Pettit, Trans. Metall. Soc. AIME 245,2495 (1969). "G. W. Goward, J. Met. 22, 31 (1970). 9J. A. Sprague, G. K. Johnston, F. A. Smidt, G. H. Meier, S. Y. Huang, and F. S. Pettitte, ONR Contract No. NOOI4-K-0355 (1981).

lOW. C. Hagel, Corros. Eng. 21,317 (1965). liD. P. Whittle, D. H. Boone, and I. M. Allam, Thin Solid Films 73, 359

(1980). 12E. A. Gulbransen and K. F. Andrew, J. Electrochem. Soc. 106,294 (1959). 13J. K. Tien and F. S. Pettite, Metall. Trans. 3, 1587 (1972). 14W. E. Boggs, J. Electrochem. Soc. 118,906 (1971). "G. S. Samsanov, The Oxide Handbook (Plenum, New York, 1973). 16J.G. Smeggi1 and N. Borstein, J. Electrochem. Soc. 125,1283 (1978). 17R. Strieff, J. M. N'Gandu Muamba, and D. H. Boone, Thin Solid Films

119,291 (1984). 18James Mullaly (private communication). 19Physical Electronics Handbook of X-ray Photoelectron Spectroscopy (Per­

kin-Elmer, Eden Prairie, MN, 1979), p. 80.

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