PUBLICATIONS A 1 1 1 0 4 A 5 7 5

NBSIR 84-2949

Advanced Thin Film Thermocouples

Kenneth G. Kreider*, Stephen Sernancik*, and Craig Olson**

*U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Chemical Engineering

Chemical Process Metrology Division

Gaithersburg, MD 20899

**U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 20899

October 1 984

Contract: C-54715-D

Prepared for National Aeronautics and Space Administration

Lewis Research Center

Cleveland, OH 44135

NATIONAL BUREAU OF STANDARDS

LIBRARY NBSIR 84-2949

ADVANCED THIN FILM THERMOCOUPLES

Kenneth G Kreider*, Stephen Semancik*, and Craig Olson**

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*U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Chemical Engineering

Chemical Process Metrology Division

Gaithersburg, MD 20899

**U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 20899

October 1 984

Contract: C-54715-D

Prepared for National Aeronautics and Space Administration

Lewis Research Center

Cleveland, OH 44135

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

The research performed on this contract is intended to provide information

in order to improve the technology of thin film thermocouples for gas turbine

engine blades and vanes. Two principal target areas were selected for emphasis.

They included the coating-substrate compatibility problem and the properties of

the insulating oxide on which the thin film thermocouple is bonded. The insulating oxide must not only provide the electrical insulation for the thermocouple

but also must structurally bond the thermocouple to the engine part being measured. Therefore, both insulating and adhesion properties were studied.

The approach included using commercial materials technology up to the fabrication of the insulator and thermocouple followed by mechanical , thermal, and

electrical testing of properties. This effort was combined with analytical

techniques including optical microscopy. X-ray diffraction, optical interferometry, scanning electron microscopy (SEM), energy dispersive X-ray analysis

(EDX), and X-ray photoemission spectroscopy (XPS). We would like to thank James

Allen for his help in the fabrication and testing of the test devices. We would

like to express appreciation for technical advice to James Mulally and Ralph

Hecht of the coatings group at Pratt and Whitney-Florida, William Stowell of the

Instrumentation Engineering at General Electric-Cincinnati , and Prof. Fred

Pettit of the University of Pittsburgh. We would also like to thank Ray

Holanda, the NASA Project Manager for his excellent guidance and technical

advice in accomplishing this project.
Table of Contents

Section Page

Summary

I Introduction ................... 1

II Experimental Materials and Methods 5

A. Material Selection and Procurement .... 5

B. Test Coupon Design . . ........ 6

C. Cleaning and Oxidation Procedures .... 8

D. Sputter Coating 10

E. Thermal Mechanical and Electrical Testing ........ 12

F. Analytical Characterization Techniques 13

1. SEM and EDX . 14

2. X-ray Photoemission 15

III Results

A. Substrate-Coating Stability ..... 16

B. Dielectric Strength and Adhesion 21

1. Thermal Oxidation ......... 21

2. Sputtered Thin Films 27

3. Sputtered Coating Adhesion and

Thermal -Mechanical Performance ...... 31

4. Electrical Performance ................ 33

IV References . 37

List of Tables 40

i i

List of Illustrations 41
Summary

Four substrate-coating combinations were investigated as a materials system base for thin film platinum-platinum 10% rhodium thermocouple sensors. The

cobalt-base vane alloy MAR M509 was combined with a sputtered FeCrAlY coating

and the nickel-base blade alloy MAR M200+Hf was combined with coating alloys of

FeCrAlY and both electron beam and sputter deposited NiCoCrAlY. All combinations were successfully thermally oxidized and sputter coated with AI 2 O 3 to

form a good insulating foundation for the Pt-PtlORh sputter coated thin film

thermocouple. Thermal oxidation was performed in air, wet argon, dry argon, and

argon with 4% hydrogen to control the rate and morphology of the oxides on the

MCrAlY. The best oxides were formed by starting the oxidation in dry argon followed by air atmospheres at 1050-1090°C. Oxides on the NiCoCrAlY were generally

smoother and more uniform than those on FeCrAlY. These were characteri zed using

optical micrography, visual inspection, XPS and EDX for composition, and electrical measurements. The thickness, 1. 5-2.0 urn, measured by weight gain, was

considered optimum.

Aluminum oxide coatings were deposited by reactively sputtering from an

aluminum target in Ar+10% O 2 using an RF planar magnetron. These coatings

were evaluated microscopical ly, by interferometry for thickness, by EDX and XPS

for composition, for adhesion, and by high temperature electrical testing. XPS

studies indicated considerable improvement in composition and insulating properties of the sputtered oxide compared to the thermal oxide. This pure A1 2 O 3 should be 1-2 pm thick to avoid shorts and minimize coating stresses that would

lead to delamination.

To prevent substrate degradation from interdiffusion with the coating,

AI 2 O 3 barrier layers were investigated. Sputtered A1 2 O 3 in 0.6 and

2.0 urn layers were heat treated and compared with samples containing no barrier

i i i
layer using EDX to determine interdiffusion of Al, Fe, Ni , Co, and Cr. It was

apparent that bridging of the barrier 1 ayer by the FeCrAlY coating nullified the

effectiveness of the barrier layer.

The platinum-platinum 10% rhodium thin film thermocouples were sputter

deposited to a thickness of approxmately 1.5 vm. These were evaluated for

adherence, high temperature stability against shorting and material loss, and

for thermocouple e.m.f. Thin films of less than 1 ym tended to last less than

20 hrs at 1050°C. Some problems of platinum film adherence in high temperature

cycling were encountered and may need further attention. These problems may be

alleviated using an improved treatment or a reactive metal bond enhancer in the

system.

IV
I. INTRODUCTION

Conventional surface measurement techniques on turbine airfoils perturb the

boundary 1 ayer on the surface of an airfoil or the heat flows within the airfoil

to the extent that detailed analyses of data are quite difficult. This problem

is particularly acute in cooled turbine airfoils, where increases in the turbine

inlet temperatures have been accompanied by complex air and water cooling designs and attendant reductions of the component cross section. To meet this

need, a new technology for constructing arrays of thin, insulated, sensing films

directly on turbine airfoil surfaces must be developed.

While the use of thin film sensors for low temperature surface measurements

has been common for some time (1,2,3), practical thin film devices for use in

harsh environments must satisfy several design constraints which preclude the

use of conventional methods and materials [and severely limit the choice of

alternate materials and techniques of construction].

There is a new method of construction of high temperature thin films which

has provided the first thin film turbine airfoil devices in large scale aircraft

gas turbines (4,5). A brief review of this technology indicates the potential

and the problems of high temperature thin film devices in gas turbines. The

existing method of high temperature thin film construction uses a thermally

grown metal oxide to electrically insulate the sensing element from the substrate (6-9). In this technique, a turbine component is coated with an alloy

capable of growing an adherent, high dielectric strength metal oxide. The coatj ed component is then oxidized and arrays of sensors and leads are sputtered on

the oxidized surfaces. Wire leads are attached to the sensors and the assembly

is reoxidized. The development of practical surface sensors for use in harsh

environments is primarily dependent on the development of a coating which is

compatible with the substrate material and capable of growing useful insulates
surface oxide layers- For turbine alloys, the nickel base alloy NiCoCrAlY and

the iron base alloy, Fe 18Cr 11A1 0.7Y, have been selected from the group of

MCrAlY high temperature alloys which form adherent aluminum oxides during oxidation. An extensive investigation of the electrical properties of the aluminum

oxides grown on these alloys led to the initial selection of FeCrAlY by Dils

(10). The dielectric strengths of oxides grown on this alloy (>5 x 10^V/ cm

at 1093°C) are high and the conductivities of the oxides 10"8fi-l at

1093°C) are low. In addition, the alloy has desirable mechanical properties

throughout the operating range of the sensors and devices using this alloy are

unusually durable. Similar properties of the native oxide on NiCoCrAlY and its

commercial use in coating materials for turbine engine hardware has led to its

selection.

Each element of the sensor is chosen for its specific electrical, chemical,

or mechanical properties and, as a result, it is necessary to combine materials

with widely varying types of chemical bonding. The metal oxide is an insulator

with strong ionic bonds, the coating is a high temperature metal alloy and the

sensing element is usually a noble metal alloy. Therefore, when the temperature

of the assembly is changed, thermal mechanical strains develop within the assembly. In a gas turbine, the assembly experiences thermal mechanical strains during normal operation. When the engine is decelerated from takeoff to idle, high

compressive strains are developed in the uniform oxide layer (11,12). At imperfections in the oxide, shear tractions are applied to the metal/metal oxide

interface and cause a concentration of inplane stresses at the interface. In

addition, the combustor exhaust gases induce small, high frequency temperature

oscillations at the metal surface; in the typical gas turbine, the oxide and

metal coating near the surface osci 11 ate +_5°C several times a second(12). These

temperature oscillations induce only a _+60 um/m strain (+21 MN/m? stress)

2
in the oxide and a +120 um/m strain (+_ 12 MN/m? stress) in the coating but the

latter strain is adequate to cause plastic deformation of coatings which are

sufficiently weak and ductile to accommodate the strains associated with the

large infrequent temperature cycles- The deformation of the substrate displaces

the surface material into the boundary layer where the oxide and metal are

apparently subjected to additional large heat flows (13).

The resultant thermal mechanical strains within the assembly can be accommodated by a limited number of techniques. A scanning electron microscope study

of devices after extensive use in laboratory combustor exhaust gases has indicated that, in general, the thermal strain in useful (< 1-2 ym) thin oxides is

stored as elastic strain energy in the oxide, while strain relief in usable

thick (2-5 ym) oxides occurs by buckling and attendant plastic deformation of

the substrate (11). It has been concluded that, in order for the sensor assembly to remain intact, it is necessary to select a system in which: 1) the compressive shear stress of the oxide is greater than the buckling stress of the

oxide when it is attached to the coating, 2) the metal oxide/metal interface

yield and shear strengths are greater than the coating yield strength, 3) the

coating yield strength is small and 4) the coating is ductile over the entire

operating range of the sensors. In successful devices, the coating is sufficiently uniform that the oxide layer remains attached to the coating and continues to insulate the sensing film from the coatings. NiCoCrAlY and FeCrAlY satisfy these requirements; their oxides are adherent and the material is sufficiently ductile throughout the operating range that thermal strain relief occurs

primarily by oxide bucklinq and regeneration by deformation of the MCrAlY.

The thermal mechanical strains across the noble metal/oxide interface are

much smaller since the thermal expansion coefficients of both materials are

similar and the yield stress of the noble metal is low. However, it was found

3
that high velocity combustor exhaust gas flow over the sensors does cause delamination of the sensing elements deposited on smooth oxide surfaces. There is

little chemical bonding between aluminum oxide and useful noble metal alloys

and, therefore, the coating is abraded before oxidation in order to cause the

growth of an irregular oxide. The films are then sputtered into the irregular

surface and interlocked with the oxide.

However, several problems remain in order to achieve a practical technology

that can be used in a routine manner in the gas turbine industry (6). This

investigation addresses the critical problem identified by the above mentioned

reference (7-9). These are areas of immediate concern which have resulted in

insufficient film adhesion, metal oxide insulation failures, and degradation of

the substrate alloy properties.

Initial studies have indicated that those coatings which form the highest

dielectric strength oxide may be unstable on nickel base alloys. Interdiffusion

between a coating and an alloy can cause a loss of adhesion of the oxide film,

and more importantly, may weaken the thin substrate alloy. It is essential that

the sensor technology employ a coating that will not reduce the life of the turbine component. The properties of the oxide layer are also critical. Specifically, the interrel ationshps between the dielectric properties of the oxide layer, the adhesion of the oxide to the coating, and the adhesion of the noble

metal films to the oxide must be considered. As described above, there are

relatively few coatings which accommodate the thermal mechanical strains within

the sensor. The properties of the coating also effect the electrical breakdown

of the oxide. The morphology of the oxide affects the oxide adhesion, electrical breakdown of the oxide and the adherence of the noble metal thin film. A

successful sensor technology will balance the effects of the coating, surface

4
morphology oxidation cycles and sputtered aluminum oxide films on the oxide

properties.

II. EXPERIMENTAL MATERIALS AND METHODS

A. Material Selection and Procurement

The most important planned application for the thin film sensors is in the

hot section of the gas turbine engine. Both blades and vanes have significant

needs for accurate surface temperature measurements in commerical and military

engines. Discussions with General Electric (14) and Pratt and Whitney (14) engineers were used to select the candidate materials. Both GE and P&WA selected

MAR M509 as the vane alloy; P&WA selected MAR M200 + Hf as the blade alloy and

GE selected Re 80H. Subsequent negotiations to obtain the materials from the

manufacturers favored the MAR M200 + Hf and the MAR M509. The standard coating

techniques include the pack cementation Co-Dep process, sputtered MCrAlY and

electron beam (EB) coating. Since the latter two coating techniques are used on

the selected substrate alloys, EB and sputtering were chosen for the coatings.

The EB coating of NiCoCrAlY could be commercially prepared and purchased to

specification on the MAR M200 + Hf and it was selected as one of the coatingsubstrate combinations. It was procured from Chromalloy and our samples were

designated with a "C". The sputtered coatings could also be procured from Pratt

and Whitney: both FeCrAlY and NiCoCrAlY on both the MAR M200 + Hf and MAR M509.

This approach has several advantages: it ensures that the material systems

represent the state of the art in advanced turbine engine material technology,

and it simplifies the transfer of gained technology to commercial practice.

The selection of coating alloys was based on two premises: first, the

NiCoCrAlY should be investigated because it was the most successful coating for

5
the MAR M200 + Hf and we could study two deposition techniques and compositions

second FeCrAlY had been proposed by Oils with support from P&WA as a possible

improvement on the system with the hope that the oxide dielectric capabilities

and adhesion would be superior. In summary. Table 1 is a list of the alloys in

vestigated with their nominal compositions and Table 2 is a list of the combina

tions investigated with their designations.

For all studies of dielectric strength, adhesion, and thermocouple properties, the above materials were procured from P&WA and Chromalloy without any

problems. For the substrate-coating stability tests, an intermediate layer of

AI 2 O3 was sputtered between the MCrAlY and substrate alloy. These samples

were also procured but considerable problems were encountered in obtaining coat

ings with adequate adherence to the AI 2 O 3

. The standard procedure for

obtaining good adherence to the substrate is to abrade the surface with 50 urn

glass shot peening thus giving the coating more purchase. This technique can

lead to "leaders" or imperfections which might shunt the AI 2 O 3 diffusion

barrier and therefore an optical polish was used to insure a high quality

AI 2 O 3 coating. Unfortunately, the adherence problem marred the benefits of

this approach.

B. Test Coupon Design

The test coupon design for this investigation is intended to simulate the

applications of thin film thermocouples on present and near future turbine

engine first stage turbine blades and vanes. The materials of construction of

that hardware was discussed above in 1 1 -A but other considerations are also

important in the design of the test coupon. A schematic cross section of the

thin film sensor is presented in Figure 1 with the thickness of each layer of

the device. These thickness relationships must be preserved in the test coupon

6
design to preserve the thermal expansion relationships and simulate the stress

and strain environment of the actual application. Actual first stage blades in

engines are typically 4-5 cm long and 0.3 to 1.0 cm in thickness with internal

cooling passages. These dimensions are so large compared to the layer thicknesses of the gage that it was felt test coupons 2 cm x 2 cm x 0.5 cm would properly relate the blade and vane dimensions. Essentially, the substrate material

MAR M509 or MAR M200 + Hf determines the thermal expansion of the structure in

both the engine hardware and test coupon. All the test coupons used in the

investigation were prepared and coated by commercial processes to insure direct

correspondences of metallurgical structural aspects.

The test coupon design also had the following requirements. It must be

suitable for thermal oxidation; surface grinding, polishing, and cleaning; sputtering of uniform layers; electrical performance evaluation up to 1100°C; optical and electron microscopic analysis; analysis by energy dispersive X-ray analysis and X-ray photoemission spectroscopy; and evaluation as a thermocouple. To

efficiently and economically satisfy the above requirements, a flat test coupon

approximately 2 x 2 x 0.5 cm was employed which was suitable for all but the

last requirements . Only the determination of diffusion profiles for the barrier

layer investigation of the coating-substrate stability tests required a destructive (sectioning) analysis of the test coupon. The requirement of evaluating

thermocouple performance, however, dictates that the legs of the thermocouple

pass through the thermal gradient from the connections to the measured temperature at the junction. This thermocouple evaluation requires a long test coupon

so that the dimensions of 10 x 0.5 x 0.5 cm were chosen for this test. Both

test coupons could be processed in a virtually identical manner throughout the

fabrication procedure. A metal masking technique was chosen for obtaining the

thermocouple pattern. Steel shimstock 0.12 mm thick was chosen as the masking

7
material and it could be cut accurately to approximately 0.3 mm tolerances. The

thermocouple pattern for the test bars included 1.0 mm wide legs spaced 1.0 mm

apart. A diamond saw was used to cut the masking slot. Although placement of

the masks on the samples was done by hand (no precision fixtures were employed)

alignment was adequate for two legged thermocouples on a 0.5 cm wide bar. Estimated resolution of this technique is approximate! y 0.1 mm but this could be

improved with fixtures.

C. Cleaning and Oxidation Procedures

The MCrAlY coating test samples were received with a nodular surface which

was ground and polished to prepare the surface for oxidation. Electron beam deposited NiCoCrAlY samples have a "matte" finish and required only 600 grit preparation. These "C" samples were all ground with 600 grit paper and some

received a polish with 1 ym diamonds followed by a 50 ym shot peening with glass

spheres. Since no advantage was perceived comparing the shot peening to the 600

grit polish, the shot peening was discontinued. Following the 600 grit polish,

the cleaning procedure included an alconox detergent followed by water rinse,

ultrasonic cleaning in acetone, methyl alcohol rinse and de-ionized water rinse.

Samples were handled only with tongs and rubber gloves when clean. The sputtered coatings required more grinding (400 mesh) to obtain a flat, smooth surface and FeCrAlY coatings in some cases required considerable surface removal

(10-20 ym). In fact, pits and defects in the FeCrAlY were a problem in surface

preparation and caused the rejection of some samples.

The recommended oxidation treatment for the NiCoCrAlY coatings by Grant and

Przybyszewski? includes 4 hours at 1350K in dry hydrogen followed by 50 hours

at 1300K in air. They recommend that p02 be kept below 10*5 Pa in the

early stages of oxidation to suppress the formation of chromium and cobalt

8
oxides during the initial AI 2 O 3 formation and to start the migration of

aluminum toward the surface. This approach promotes the gradual growth of a

dense, hard, and adherent layer of AI 2 O 3

. Both argon and dry hydrogen have

been suggested to achieve this selective oxidation and it was decided to investigate the relative advantages of various partial pressures of oxygen and water

vapor. Oxide formation on MCrAlY alloys has been studied in depth and reported

in the literature(10>H»15-25) . in addition, the authors consulted with GE

and P&WA engineers and Prof. Fred Pettit(15,17) (y. Pittsburgh) to determine the most useful approach in achieving the optimum thermal oxide on the specific FeCrAlY and NiCoCrAlY alloys used in this investigation. The desired properties of the oxide are that it be durable and self healing, have excellent

insulating and dielectric properties, and provide for good adhesion of the

lamellae of the thin film sensor. Experiments were designed to analyze and test

the oxide for its morphology, uniformity, electrical properties, thermal cycling

stability, and adhesion. These experiments will be described in subsequent

sections

.

The elements in the MCrAlY coatings have various rates of oxidation depending on the temperature, partial pressure of oxygen, concentration of the metal

species at the reacting surface, and the free energy of formation of the oxide

formed. Selective oxidation of one or more species can be most effectively controlled by controlling the partial pressure of oxygen. Thus, by maintaining

PO 2 to below 10-7 p a (io-12 atm) at 1000°C only the aluminum and

yttrium will form oxides. Above this range, chromium oxidizes and above 0.1 Pa

(10"6 atm) nickel, cobalt and iron oxides form. The method used in this investigation included oxidation with argon plus 4% hydrogen to suppress the formation of chromium oxides (p02 < 10"7 Pa); and argon with both dry (p 02 <

1 Pa), and wet (p02 = 1 Pa, PH 2 O = 3000 Pa) argon to suDpress cobalt.

9
nickel, and iron oxides; and air for the final oxide growth. Once AI 2 O 3 is

formed, further oxide growth is primarily AI 2 O 3 because of the diffusion and

kinetics of the oxide formation process. In fact, the MCrAlY alloys were designed to form the AI 2 O 3 as a protective barrier for turbine engine hardware. Its stability and extremely low diffusion coefficients make the AI 2 O 3

superior for oxidation protection as well as being useful as an electrical insulating layer for the thin film sensors. For use as an insulating layer in a

thin film sensor, however, the purity and continuity requirements are most

stringent. Therefore, suppression of formation of any oxide such as a nickel

oxide which would have inferior electrical properties is important. The detection of iron, nickel, and cobalt oxides is simple since these oxides have characteristic colors and are apparent in visual inspection. Photomicrography at

500X is also useful to detect inclusions, second phase oxides, and other perturbing irregularities in the alumina film which are detrimental to the electrical properties of the films.

The thermal oxidation was performed using the four different atmospheres

described above at temperatures between 975 and 1120°C for periods up to 80

hours on both the FeCrAlY and NiCoCrAlY alloys. Thickness and quality of the

oxide was monitored by recording weight gain and microscopic evaluation.

D. Sputter Coating

The FeCrAlY and NiCoCrAlY sputtered coatings were procured from Pratt and

Whitney Aircraft. NiCoCrAlY coatings were approximately 0.1 mm thick and had an

as deposited composition of 0.573 Ni , 0.152 Co, 0.167 Cr, 0.108 Al, and 0.0013Y

weight fractions as determined from electron microprobe measurements. A photomicrograph of the coating after heat treatment of 1075°C for 4 hours in argon is

given in Fig. 2. The two phase structure of the y - y' is apparent. Fig. 3

10
is a .yttrium X-ray map indicating that the yttrium was not lost and still has

broad distribution in the coating thereby implying that its use in improving

bonding is preserved through the sputtering operation- The FeCrAlY coatings

were measured to have a composition of Fe plus 0.17 Cr, 0.11 A1 , 0.003 Si, and

0.001Y (weight fractions). A 1000X photomicrograph is shown in Fig- 4 which

indicates the faceted growth and the need for significant material removal to

obtain a smooth surface.

In-house sputtering included the reactive sputtering of AI 2 O 3 and RF

magnetron sputtering of platinum and platinum 10%-rhodium thermocouple alloys.

The sputtering was accomplished using a Varian (14) 3118 vacuum sputtering chamber and Plasmatherm (14) HFS-3000D RF generator (3000 watt) at 13.56 MHz, Plasmatherm (14) 3000 AMSE impedance matching box, and a 5 cm magnetron of the type

sold by U.S. Guns (14). An Inficon (14) IC 6000 quartz oscillator thickness

gauge was used to monitor the deposition during sputtering and a mass spectrometer was used for residual gas analysis. Bottled UPC 99.999% Argon and HP

99.99% oxygen were used as the sputtering gases. Typical parameters for platinum and platinum + 10 % rhodium sputtering were: target to substrate distance

15-25 cm; initial vacuum 4-15x10-5 Pa; argon sputtering pressure 0.14 Pa;

250 watts power; deposition rates 0.4-0 .

8

nm per sec metal; and RF impedance 50

ohms. The reflected power was less than 1%. Typical parameters for reactive

sputtering of AI 2 O 3 were: aluminum target, target to substrate distance, 15

cm, initial vacuum 4x10*5 to 1.4x10*4 Pa; argon pressure 0.14 Pa oxygen

pressure 0.014 Pa, 300 watts power, deposition rates 0.05-0.1 nm per second, and

RF impedance 50 ohms. Typical residual gas analysis from the mass spectrometer

indicated the following pressures: H 20= 10 * 4

, Ar= 10*5, 02 = 10

* 5

,

C 0 -N 2 = 3 xl 0

“ 5

, CH4

= 5 x 10*5 Pa. The thickness of deposition was

checked by weight gain on the sample and monitored on a glass slide. The glass

11
slide is also a convenient way to insure the stoichiometry of the AI 2 O 3

since any problems affect the transparency of the deposit. Thickness of the

AI 2 O 3 deposits was also checked using interferometers with thallium illumination which indicated fringes at step heights of 0.27 urn.

E. Thermal, Mechanical, and Electrical Testing

Several thermal, mechanical and electrical tests were used to evaluate the

adhesion, stability, electrical performance, and overall suitability of the

aluminum oxide coatings and platinum-platinum rhodium thermocouple films. They

included a mechanical adhesion test, electrical insulation and dielectric performance at temperature test, a thermal cycling stability test, and a thermocou

pie performance test. The adhesion test employed a commercial Sebastian (14)

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 fiber bond

or strength up to 10,000 psi.

The electrical insulation and dielectric test is performed using the test

jig pictured in Fig. 5. The 2 cm x 2 cm test coupon is held with platinum and

platinum- 10% rhodium connections to the thin film thermocouple electrodes

described in Section I I B and heated to 1075°C in an air furnace. The output

voltage and resistance with a one volt potential are monitored between the Pt

and PtRh 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 emf since the entire thin film

thermocouple is at virtually one temperature, it does detect increases in resis

tance or deterioration mechanisms such as oxidation of rhodium during high temperature exposure.

12
The thermal cycling test is also performed on the 2 cm square test coupon

by cycling the coupon to 1000°C. The heating cycle is generated by a propane

air torch in 60-90 seconds and cooling to below 100°C is also accomplished in

less than 120 seconds. Temperatures were measured with a carefully placed

thermocouple and by optical pyrometry. Both techniques for measurement underestimate the maximum temperature of the thin film itself which tends to develop

hot spots as defects are generated. This test detects adhesion problems as well

as overall film stability under harsh environmental conditions.

The thermocouple performance test was performed on the C type test coupon

described in section I IB - The 10 cm long bar is inserted into a propane air

torch flame to immerse the thermocouple junction while the thin film connections

are made remote from the high temperature junction. A reference grade Pt-PtRh

thermocouple is joined to the junction area of the bar to monitor the temperature for comparison.

F. Analytical Characteri zation Techniques

Analytical techniques have been applied to characterize various aspects of

the layers, surfaces and interfaces present in the thin film thermocouple

devices. The overall objective was to identify the most desirable methods for

each fabrication step. Characteri zations were carried out 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. The

role of AI 2 O 3 diffusion barriers was also investigated. Comparative studies

were conducted on samples from all four classes (A-D) of substrates; these samples had been treated in different atmospheres at different temperatures , and

had received different types of oxide coatings (thermal or rf sputtered) in an

attempt to develop the best fabrication procedures.

13
The techniques used included scanning electron microscopy (SEM), energy

dispersive x-ray analysis (EDX) and x-ray photoemission 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 urn; XPS, on the other hand, has a

nominal sampling depth of 5 nm, and can therefore provide key information about

the composition at the surface of films used in the thermocouple devices. In

addition, XPS can detect elements of low atomic number (like oxygen), which EDX

cannot, and XPS also indicates the chemical environment in which the detected

atoms exist.

1. SEM and EDX

SEM and EDX measurements were done using two separate commercial instruments each equipped with an electron microscope and integrated EDX capabilities.

Samples were potted in epoxy either for viewing a face, or for cross-sectional

cutting, polishing and viewing. Specimens were carbon- coated to reduce charging effects produced by the incident SEM electron beam. SEM was used to observe

various structural features, including phase separations, and to position samples for EDX analysis. "Spot mode" EDX permitted the composition of different

phases to be determined, and in the "integrated area mode" EDX was applied to

obtain composition profiles across the oxide/coating and coating/substrate

interfaces. In particular, a window of analysis was chosen for the EDX, that is

an area at a specific magnification, so that analysis covered small changes in

chemical composition. This was done for all heat treated materials and compared

to a 'reference piece' of non-heat treated material. The results of the

14
analysis were all computerized using a standard- less semi-quantitative program

of the EDX unit. Results compared favorable to the compositional values given

for the 'reference' sample.

All SEM/EDX procedures were kept constant, i.e., KV, tilt, working distance, time of analysis, so that accurate comparisons could be made.

2. X-Ray Photoemission

Surface effects can play a critial role in the adherence and durability of

thin overlayers. Therefore, x-ray photoemission spectroscopy was used to study

the outermost layers of certain samples to evaluate the fabrication methods.

Several brands of commercial spectrometers were utilized [Leybold-Heraeus (14),

Physical Electronics (14) and Vacuum Generators (14)] during the course of this

project. Samples to be investigated were placed "as is" into the ultrahigh

vacuum enclosure of the photoelectron spectrometer, and lightly sputter-etched

to obtain a "clean" surface.

XPS measurements allowed correlations to be made between fabrication conditions and the composition and chemical state of species produced at the surface

of a variety of samples. The chemical composition of the surface region obtained from XPS data was used to identify impurities that might degrade adherence. XPS results were also used to detect oxide-stabilizing additives (like Y)

as well as the presence of metal atoms which had segregated from the coatinq

into the surface region of the insulating (thermal or sputtered) oxide layer.

Since all elements except hydrogen can be detected in an XPS spectrum, the relative amounts of the different elements observed could be estimated and used to

find stoichiometries of compounds at the specimen surface (e.g. x and y in

Al x 0y) . In doing so, however, differing photoemission cross sections for

the energy levels of the elements had to be accounted for.

15
Chemical shifts of the core electron levels were also used to obtain information on the bonding environment of a given type of atom in the surface

region. In addition, the insulating quality of a poorly conducting layer could

be inferred by the uniform charging-induced shift of all core levels from a sample which was losing photoelectrons without being completely neutralized.

III. RESULTS

A Substrate-Coating Stability

A series of the nickel blade alloy (MAR M200 + Hf) was coated with sputtered NiCoCrAlY. As mentioned above, considerable difficulty was encountered in

obtaining test coupons with the barrier layer sputtered AI 2 O 3 plus the

NiCoCrAlY, but, three coupons were in the appropriate condition to study the

effectiveness of the barrier coating in this system. Since the primary diffusion problem in this system is aluminum diffusion into the structural alloy MAR

M200 + Hf, aluminum was the primary focus of the investigation. The samples

included a comparison sample (D24) which had vendor sputtered AI 2 O 3 approximately one ym in thickness and the standard vendor sputtered NiCoCrAlY (nominally 23% Co, 18% Cr, 12% A1 and 0.5% Y). A second coupon (D4) was heat treated at

1090°C for fifty hours without the barrier 1 ayer also for comparison and the

third coupon (D14) was heat treated at 1080°C for fifty hours with the 1.0 ym

barrier layer to determine if the barrier layer would deter interdiffusion.

An electron scan of test coupon D24 is given in Fig. 6 in which the A1 signal is highlighted on the electron dispersive X-ray analysis display. It is

evident that the aluminum oxide is approximately 2 ym thick. This oxide is composed of thermal oxide generated by the vendor and sputtered oxide. The thermal

16
oxide presence is established by the aluminum depleted layer in the substrate

immediately below the oxide. Data on the composition profile are presented in

Table 3 and the comparisons to nominal composition indicate good agreement

although the substrate evidently has considerable silicon. The low yttrium in

the analysis may be inaccurate in our measurement, but was not considered important to the study. It can be inferred that the spot should be at least 3 m

from the oxide to obtain unbiased readings.

Test coupon D4 was heat treated at 1090°C for 50 hrs without the barrier

coating and a scanning electron micrograph is presented in Fig. 7. The white

areas related to insulator charging and are not considered important. However,

it is clear that oxides are not only forming on the surface but are penetrating

into the coating. This is confirmed in the A1 map of the same area of the coating (Fig. 8). Pettit has reported similar oxide growth and this may result from

diffusion pipes or actual pores in the sputtered coating. This columnar structure of the coating is difficult to suppress at high sputtering deposition

rates.

The diffusion profile of test coupon D4 is presented in Table 4 and Fig. 9.

Because the Ni, Co, and Cr concentrations are similar in coating and the substrate, it is difficult to deduce any precise information. This problem is complicated by the two phase structure of both the coating (Fig. 2) and substrate

alloys which partition Ni, Co, and Cr in slightly different amounts. That partitioning is evident in the Ni, Co, and Cr maps of both the coating and substrate and was a factor in the choice of spot size and location for the EDX

analysis. The diffusion profiles of Ni, Co, and Cr indicate that Ni is entering

the coating to make up approximate! y half of the difference between coating and

substrate compositions. The penetration depth over which the Ni concentration

increases significantly is probably 50-60 ym. Si and Ti appear to have little,

17
if any, mobility and that relates to the fact that they are undoubtedly tied up

as oxides. The aluminum concentration is the primary species of interest. The

expectations are that aluminum will be depleted near an oxidizing surface and

that aluminum would move into the substrate from the higher aluminum containing

coating. These tendencies are masked by inhomogeni eties primarily caused by the

formation of AI 2 O 3 in the bulk phases.

The depletion of aluminum near the surface is evident in Fig. 9. Little

aluminum penetration into the substrate is evident and that is probably a result

of the vendor's heat treatment before coating which depletes the aluminum from

the edge of the substrate (Fig. 6 ).

The test coupon D14 was obtained with a 1 pm AI 2 O 3 barrier coating

between the NiCoCrAlY and MAR M200 and heat treated 50 hrs at 1080°C. Figures

10, 11, 12 and 13 show the scanning electron micrograph, the A1 map, the Co map,

and the Cr map at 580x. The data from one of the diffusion profiles are presented in Table 5 and Fig. 14. The concentration 10 ym in from the edge indicates

primarily the A1 depletion due to oxidation. This is followed by a gradual

build up of Ni which has come from the substrate and a Ni depleted area in the

substrate near the AI 2 O 3 barrier. This latter effect is related to the presence of Hf, Zr, Nb, Si, and Ti in that area as precipitates which results in

depression of the Ni concentration. It may be that the EDX analysis is underreporting the Ni which reaches a level of 64% rather than 70% in the bulk at 200

pm distance from the edge. Co and Cr have diffused from the coating into the

substrate primarily in the zone between 80 and 100 pm from the edge. This zone

near the interface between the coating and substrate is a region of considerable

phase transformations and because of uneven diffusion may even have voids.

In this investigation with several samples, it almost appeared that the interface had moved as will be noted below. The Co and Cr concentrations show the

18
general movement out of the coating and into the substrate but the problem mentioned above in discussing D4 is also present. Aluminum concentration values

again indicate the depletion near the surface and very little penetration into

the substrate.

In summary, the NiCoCrAlY coating on the MAR M200 + Hf appears to be a well

matched combination which, through consumption of aluminum by oxidation, prevents softening of the substrate from aluminum diffusion. The investigation of

the barrier coating indicated little difference between the two coupons with and

without the barrier coating. This difference is lessened by the similar compositions of coating and substrate and by the complexities of the two microstructures. It should be observed that the commercial application is planned without

a barrier layer.

The barrier layer is much more important in the systems employing FeCrAlY

as the coating material. Both aluminum and iron can diffuse into the substrate

and lower the creep strength. For this investigation, the combination of

FeCrAlY plus MAR M509 was chosen because the contrast in compositions would more

clearly delineate the diffusion profiles and the high temperatures of the vane

alloy in service. The aluminum oxide barrier 1 ayer was sputtered on by the vendor prior to the sputtered FeCrAlY. Two temperatures , 1125°C and 1075°C, were

used to heat treat both 0.6 ym and 2 ym thickness barrier layers. Delamination

of the coating at the barrier layer was a serious problem and the principal data

was obtained from samples A62 which was not heat treated, A40 which had no barrier layer and was heat treated 50 hours at 1075°C, and A19 which had 2 m of

AI 2 O 3 and was heat treated 86 hrs at 1075°C. The same procedures were

followed as with the "D" coupons which were described above.

Test coupon A62, the comparison test coupon, was used to confirm the technique and explore any unusual conditions that might complicate the procedure.

19
The AI2O3 appeared to be uneven in the SEM images and the AI2O3 was damaged in the sample preparation so that it was difficult to tell if the layer had

perfect continuity. The EDX results are reported in Table 6. The small spot

size lxl ym tends to pick out inhomogenieties particularly in the two phase

structures. No quantitative analysis was recorded for the tungsten, tantalum,

titanium, or zirconium in the substrate alloy since the principal interest in

this study was to follow iron and aluminum penetration into the cobalt alloy.

The low value of aluminum in the coating may have resulted from losses in the

sputtering process or the lower yield of aluminum compared to that of iron. The

complexity of the cobalt alloy is evident by the variability of the Co and Cr

contents

.

Test coupon A40 was heat treated at 1075°C for 50 hours without the barrier

coating to determine the diffusion profiles. The results of the EDX investigation are presented in Table 7. Figure 15 and 16 are typical diagrams of counts

versus energy from which the data are generated. Fig. 15 is at point 10 in the

substrate material 113 ym from the edge of the sample. The diffusion profiles

of Fe, Al, Cr, and Co are plotted in Fig. 17. It is evident that Co diffuses

rapidly through the 80 ym of the b.c.c. FeCrAlY coating and reaches a 25% weight

fraction all the way to the edge. The aluminum concentration is well below 1%

only 30 -ym from the interface between the coating and substrate indicating that

even without the barrier aluminum diffusion is not a serious problem. These

results correspond with the previously reported results (9) of the Ni CoCrAlY/MAR

M200 + Hf system. Iron penetration is not very severe after 50 hours at 1075°C

reaching less than one quarter of its concentration 10-12 ym from the barrier

and showing vanishingly small concentrations at 30 ym (Fig. 17).

Test coupon A19 was heat treated at 1075°C for 86 hours with a 2 ym

AI2O3 barrier layer. The longer heat treatment yields a "go-no go" test;

20
that is, if the iron and aluminum penetration are not significantly less the

barrier coating is not working. A scanning electron micrograph is presented in

Fig. 18. The photograph at 1000X indicates some damage to the FeCrAlY coating

at the interface, but perhaps more significantly it appears that metal bridges

are formed through the A1 2 O 3 barrier layer. The diffusion data is presented

in Table 8 and a second profile in Table 9. The data from Table 8 are also presented in Fig. 19. Again, the cobalt is rapidly diffusing through the FeCrAlY

reaching approximately 25% by weight in the b.c.c. phase. The iron concentration is penetrating at approximately 40 ym to guarter of peak height. This is a

greater amount than the diffusion in the A40 coupon which had no barrier. Aluminum penetration to quarter height is also approximately 40 ym. It is clear

that the AI 2 O 3 barrier is not preventing (or slowing down) diffusion of iron

or aluminum into the substrate and it is not slowing down cobalt diffusion into

the coating. Chromium is probably not a problem in these alloys. The indications are that the coating is being breached by cracks or holes and the impermeability of the AI 2 O 3 for Al, Fe, and Co is not being utilized. Perhaps an

improvement of the AI 2 O 3 coating technique (for both bonding and continuity)

coupled with a rigorous inspection technique could yield a useful barrier

1 ayer

.

8 . Dielectric Strength and Adhesion

1. Thermal Oxidation

Thermal oxidation of the MCrAlY coatings is a critical step in the creation

of a durable, adherent, insulating 1 ayer between the turbine engine hardware an'

the thin film thermocouple sensor. As was discussed in Section II, Experimental

21
Materials and Methods, considerable research has lead to the technology of the

MCrAlY coatings and the protective oxide. In order to optimize the oxide for

the insulating layer, considerable care must be taken to obtain a uniform, continuous AI 2 O 3 base for the sputtered AI 2 O 3 film. Selective oxidation

has been used to suppress the formation of nickel, cobalt, and iron oxides which

have poorer electrical properties and may lead to spinels and other second

phases which would disturb the continuity of the oxide film and weaken its

resistance to cracking during thermal cycling. This investigation studies the

effects of oxidation under various oxygen partial pressures and temperatures on

various FeCrAlY and NiCoCrAlY coatings.

The FeCrAlY - MAR M509 system had been recommended because it forms "forgiving" oxide; however, in order to obtain good adherence of. the sputtered

layers, a certain degree of surface roughness was recommended. It was found

that too rough a surface would lead to uneven oxide growth. Test coupon A4

(Fig. 20) had a 400 mesh final surface preparation and yielded oxide channeling

as depicted in the figure; therefore, all subsequent samples were given a 600

mesh finish. The need for surface removal is made clear in Fig. 4 where the

uneven FeCrAlY surface is clear. The dark area in the figure is FeCrAlY with

the metal substrate below and a nickel plating to protect the faceted FeCrAlY

coating. The columnar nature of the sputtered coating is evident in the photograph. The FeCrAlY coating also had a tendency to form pits below the oxide

film especially when the oxide was grown in air. Fig. 21 illustrates this problem; the oxide film has cracked and spalled revealing the pitting surface underneath. The pitting is probably a result of excessive aluminum loss from specific locations. The sample A35 in Fig. 21 was oxidized in dry argon (H 2 OMO

ppm) for 13 hours at 1070°C followed by 49 hours in air at 1070°C. Samples

without the low oxygen partial pressures in the early stages were generally

22
worse and samples oxidized at temperatures , i.e., 1120°C were severely spalled

and contained black oxides. A complete oxide film could be obtained under the

conditions used for sample A42A, (1050°C for 80 hours in wet argon) and for sample A43A (1080°C for 50 hrs in dry argon). A42A had an oxide thickness calculated from its weight gain of 1.6 ym and A43A had a thickness of 1.9 ym. A photomicrograph of sample A43A is shown with the sputtered coating in the next

section

.

FeCrAlY coatings on MAR M200 + Hf had similar problems and thermal oxide

thicknesses are presented in Table 11. Fig. 22 illustrates a second problem on

sample BIO which was exposed at 1090°C for 66 hours in an argon plus 4% H 2

atmosphere. Here the AI 2 O 3 growth habit is not only in a film but generates

"leaders" of more rapidly growing oxide in mounds. The average thickness of

this oxide was calculated to be 1.6 ym. Adequate film growth was obtained under

the conditions used for sample B41 shown in Fig. 23 (13 hours in dry argon plus

49 hours in air, both at 1070° C )- This oxide was 2.1 ym thick.

Fewer problems were encountered with MAR M200 + Hf plus the electron beam

deposited NiCoCrAlY system. A cross section of the sputtered NiCoCrAlY coating

was given in Fig. 2. The two phase coating has a smoother surface (top dark

line). Typically, the oxide appeared as in Fig. 24 with a fairly smooth scale

plus small AI 2 O 3 needles. This coupon is sample C21 heat treated at 1075°C

for 16 hours in argon and 33 hours in air, resulting in approximately 2 ym of

oxide scale. Poor results were obtained at 1120°C where green oxide seemed to

break through the scale and disrupt the continuity. Similarly, the sputtered

NiCoCrAlY on MAR M 200 + Hf generated a good oxide. Sample 08 (Fig. 25) was

heat treated under the same conditions as C21 (1075°C for 13 hrs in Ar, 33 hrs

in air) and also displayed the AI 2 O 3 needles. Smeggil and Bornstein(25)

have related the needle habit of AI 2 O 3 on NiAl to the presence of 0.61 ppm

NaCl

.

23
For thermal oxide layers, different growth atmospheres and oxidizing temperatures were investigated analytically. In general, the substrate was not

found to have a significant effect on the oxide obtained, but some differences

were observed for the oxide grown on the FeCrAlY and NiCoCrAlY coatings. EDX

measurements on both FeCrAlY-based oxides (like C26 [Ar+3%H20 oxidized,

1050°C]) indicated that metals from the coating diffused into the thermal oxide

during heating. The nominal 1 m depth of the EDX technique meant that significant amounts of metals like Fe and Ni, Cr and Co had diffused to within 1 m of

the surface, but no conclusive statement could be made about the important

region just below the oxide surface.

XPS measurements, however, with their higher surface sensitivity, showed

that these metals did diffuse to within 5 nm of the oxide surface. The FeCrAlYcoated sample A42 [air oxidized, 1075°C] had Fe and Cr levels near 0.1 atomic

percent in its oxide layer, while NiCoCrAlY-coated C12 [Ar+H20 oxidized at

1090°C] had Ni and Cr (and presumably Co, which was not recorded) in its oxide

at about 0.5 atomic percent. The C12 sample also appeared to have a surface

“enrichment of yttrium, apparently due to the diffusion of Y to surfaces where it

reacted with oxygen. The yttrium effect was noted in other thermally treated

samples as well.

Various impurities were also observed in the XPS spectra of many of the

samples analyzed. Although carbon levels were sometimes quite high, it is

believed that much of the carbon collected on the surfaces during post-oxidation

handling. Other impurities, like Si and As, at levels of approximatel y 2.5 and

0.5 atomic percent, were also observed. Si is an impurity that has been identified by the substrate/coating vendor.

The key role to be played by the oxide layer on these samples is as electrical insulation. This property could be monitored using resistivity

24
measurements, but the core level shifts of the XPS peaks were also found to provide a useful diagnostic of the insulating quality of these oxide layers. Core

level shifts can occur either due to chemical factors, or because of surface

charging. The latter effect produces a uniform shift to higher binding energy

for all recorded XPS features. Insulating qualities determined from observed

charging shifts were generally in agreement with available resistivity

measurements

.

Charging shifts for thermally oxidized samples were always smaller than

those observed from sputtered 1 ayers of aluminum oxide over a thermal oxide.

For example, Figure 26 illustrates an approximatel y 6 eV shift (mainly from

charging since both surfaces are predomi nantly aluminum oxide) between the oxygen Is electrons emitted from the sputtered oxide on C14 (2.8 vm sputtered over

2.7 urn thermal) and the Ar+b^O-grown oxide on C12 (1.1 urn). In this case, the

difference in the thicknesses may explain at least part of the larger shift for

C14. Charging shifts for air-oxidized samples like A42 [1075°C], B23 [1077°C]

and 02 [1025°C] were usually intermediate between the sputtered and Ar+H20

grown oxides (1.5 eV or more above the Ar+b^O oxides, and about 1.5 eV below

the sputtered overlayer). The charging shift in Figure 27 shows that the airoxidized D2 (2.7 urn) has a better insulating layer than D1 (Ar+b^O, 2.6 urn)

even though these layers have essentially the same thickness. Notice that there

is some difference in the 01s position for the Ar+b^O oxidized samples C12

[1090°C] and D1 [1050°C] in Figures 26 and 27. This difference, as well as the

generally poorer insulating quality of the thermally grown oxides may be connected, not only to differences in oxide thickness, but also to the presence of

pores and pits that have been optically observed in such layers. It is possible

that the sputtered layers fill these pores, thereby leading to the 1 arger charging shifts that are indicative of better insulating layers.

25
It is interesting to note also that the Ar+h^O grown oxides may have a

somewhat higher Ni and Cr content than their air-grown analogues. Figure 28

shows what is probably the strongest Ni core level signals observed during this

study from an aluminum oxide, in this case a Ar+^O-grown layer on sample Dl.

A feature attributable to nickel oxide is also observed several electron volts

higher in binding energy. The metallic Ni peak would indicate that the Ni oxidation reaction did not go to completion in the Ar+b^O atmosphere.

Despite some presence of nickel, chromium and cobalt oxides and yttrium

oxide in almost all the thermally grown layers, the insulating oxide was predominantly (typical ly _> 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 (see

Figure 29), indicating stoichiometric AI 2 O3 at the surface (e.g. A63 [air,

1025°C], B23 [air, 1075°C]); however, certain samples seemed to be somewhat

oxygen rich at the surface (e.g. A42 [air, 1075°C], B38 [Ar+b^O, 1065°C]).

The principal tests for thermal oxide on both the FeCrAlY and NiCoCrAlY

coatings were appearance (no color) and electrical impedance. The latter test

was conducted by containing the sample in two drops of moisture with two probes

to insure at least one megaohm d.c. resistance. The oxide was also scratched

and observed in a light microscope. Indications were that no preference could

be ascertained related to the use of argon with 4% H 2 which suppresses the

formation of chromium oxides. The NiCoCrAlY coatings seemed to generate good

thermal oxides using both argon followed by air oxidation and using air oxidation. FeCrAlY (18:11:1) was more troublesome with the generation of the pits

and oxide spalling but adequate oxides could be obtained by starting with an

26
argon atmosphere. Spalling was a more serious problem with thicker 2 m oxide

scales

.

2. Sputtered Thin Films

The thin film thermocouples are composed of various layers approximatel

y

1-2 ym thick. Their electrical performance requires an insulator of high dielectric performance to prevent breakdown and high resistance (> 10 kfi) over distances of approximately 2 ym. Therefore, the insulator must be pore free and

free of 2 ym diameter defects that would affect the electrical properties. As

was mentioned in the Introduction, aluminum oxide is an excellent choice for the

insulator and reactive vacuum sputtering has been found to be a useful way of

achieving these properties. Similarly, the platinum and platinum-rhodium alloy

films must have excellent electrical properties. Thin film deposition of

platinum group metals can be achieved by numerous means such as electroplating,

electron beam coating, slurry and sinter techniques, el ectrophoresi s , electroless deposition, etc., but the sputtering technique has been found by the electronics industry to be preferred. The metal films of the thermocouple must be

uniform, strong, ductile, adherent, stable, and durable. The sputtering technique has been used in the aircraft engine industry and can be commercialized

with confidence because of its use by many vendors in the electronics industry.

Reactive sputtering of aluminum oxide has been described by Berry26 5 Thornton27,28 s Schwartz29 ? Maissel30 s Waits^l, Urbanek^

and Nowicki33. The development of RF sputtering enabled the production of

non-metallic films leading to unique capabilities of this technique. The

AI 2 O 3 sputtered film can be produced using an aluminum target or an alumina

target since the RF field does not require a conducting target. Since the bombarding species is almost completely of atomic form both targets are similar in

27
practice- To achieve AI 2 O 3 in the substrate film oxygen must be present in

the sputtering atmosphere which reacts with the deposited film as it is built

up. In fact, with oxygen present in the sputtering atmosphere, an aluminum target is rapidly oxidized so that both aluminum and alumina targets have similar

surfaces- Aluminum targets are to be preferred because of the ease of fabrication and superior heat transfer characteristics.

The technique for deposition of the AI 2 O 3 insulating film was planar

magetron DC sputtering. This technique was pioneered by Nowicki33 at Honeywell for MOS devices and is superior to CVD because of the better control of

residual stresses. The magnetron has the added advantage of high deposition

rates and therefore greater promise as a commercial technique. In this investigation, 1100 aluminum was used in preference to the 6061 alloy used by Nowicki

to avoid contamination by silicon and magnesium. The presence of magnesium,

however, may promote better adhesion (34,35).

Experimental investigations of sputtering rates, atmospheres, target to

substrate distances, and power levels were used to optimize coating purity,

smoothness, and adhesion. Purity is verified by optical clarity and absences of

color. Smoothness is verified with the optical microscope, and adhesion was

measured with a Sebastian adhesion tester. The best quality films were produced

using 250-300 watts on a 5 cm diameter target of 1100 aluminum at 15 cm target

to substrate distance. The pressure of the sputtering atmosphere of 90% argon,

10% oxygen, was 2x10^ Pa to optimize the film. Typical sputtering rates were

0 . 2

- 0 . 3 nm per second so that a 1 pm film required three hours to deposit.

Typical good sputtered films are shown in Figs. 30 and 31. Test coupon A43A in

Fig. 30 has 2.6 ym of sputtered AI 2 O 3 and C19 has 1.9 ym. At 500X, the

coating is colorless, clear, and appears to have 1-2 ym hills which are a reproduction of the thermal oxide topography. Each coating is made on a glass slide

28
as well as the test coupon. The coatings on the glass slide were used to evaluate the clarity, thickness, and smoothness.

Thickness of the coating is an important parameter and three methods were

used for its measurement. During the coating, the rate and accumulation are

measured using a calibrated quartz oscillating crystal. Weight gain was measured on each sample and glass slide. The weight gain was calculated using

estimates of the covered area and some allowance had to be made for wrap around

coating on unmasked areas. The weight gain estimate of thickness is probably

within 10% of the true value. The third method employed a Leitz (14) optical

interferometer. Typical interferometric measurements are shown in Fig. 32

indicating 1.6 and 1.9 urn, respecti vely. The technique used thallium radiation

with a 0.27 ym wavelength which is appropriate for films 1-2 ym in thickness.

The photographs indicate the step height from masked to unmasked areas of the

glass slide. Assumptions of thickness uniformity are based primarily on manufacturers specifications relating the target to substrate geometries. The best

interferometric examination of AI 2 O 3 films confirmed the absence of any

major deviations on areas 10x2.5 cm of the glass slide. Although the thickness

measurements are given with two significant figures throughout the report, the

best estimate of accuracy would be +10%. Some thickness measurements are

reported with one significant figure and the experimental accuracy for these

measurements is not estimated to be within the +10%. The adhesion properties of

the AI 2 O 3 and metal films are reported in the next section.

Analytical studies were also made of sputtered oxide layers. Even though

thermal oxidation did not produce high quality insulating layers, it was found

that the thermal oxide served as a good base for a reactively sputtered oxide

overlayer. All sputtered aluminum oxides examined by XPS were deposited on a

thermally grown oxide layer, and the XPS charging shifts indicated that such

29
sputtered oxide surfaces were consistently less conductive than thermal oxide

surfaces as discussed above in reference to Figure 26.

Sample C14 is representative of specimens with a highly insulating layer

consisting of both thermal and sputtered oxides. Although carbon was observed

at the surface of this sample, the only other elements observed were aluminum

and oxygen; other possible components, like Ni , Co, Cr or Si (see above) were

not present at levels that could be observed, if they were present at all. Figures 33 and 34 compare spectra of the Ni and Cr 2p regions of the C14 sputtered

oxide surface with those from an Ar+^O grown oxide surface (C12). The lower

surface conductivity of C14 could have been due in part to the absence of any

Ni , Cr or Co oxides in such specimens. These species are much less likely to

diffuse up into the sputtered oxide layer at the relatively low temperatures the

samples reach during sputter deposition.

Platinum and platinum-10% rhodium films for the thermocouples were also

deposited using planar magnetron vacuum sputtering. The best films were made

using a target to substrate distance of 15-20 cm, 200-250 watts, 0.14 Pa argon

sputtering gas and electrically grounded substrates. The target materials were

case pure 10 cm discs approximately 0.3 cm in thickness. The deposition rates

were approximately 0.6-0. 8 nm per second and no significant difficulties were

encountered in obtaining good quality coatings which were general ly mirror

smooth on the glass slide and reproduced the surface topography of the test coupon. Adhesion and durability of these coatings is discussed below.

The masking for the thermocouple pattern was accomplished using 0.12 mm

thick steel shim stock. This material is easy to cut and provided the necessary

resolution for the thermocouple patterns required. Simple spring clamps could

be used to provide masking for thermocouple legs of 1 mm width. The small test

coupon with a sputtered thermocouple pattern is shown in Fig. 35 and the long

30
thermocouple test coupon is shown in Fig. 36. Typical thicknesses deposited for

the platinum and platinum-rhodium were 1-2 ym.

3. Sputtered Coating Adhesion and Thermal-Mechanical Performance

One of the critical properties of the thin film thermocouple relates to its

adhesion and stability on the substrate. As was described in earlier sections

the tests used to verify this integrity include thermal cycling and adherence

testing of the sputtered oxide and platinum alloy films. The results of these

tests are presented in Tables 10, 11, 12 and 13 for each of the material

systems.

Table 10 is a summary of the heat treatments, thicknesses and adhesion

strength of the thin film sensor samples fabricated on the MAR M509 plus

FeCrAlY. The system heat treatments were made at temperatures ranging from 1025

to 1120°C but the highest temperature used successfully was 1090°C. In order to

obtain 1.5 ym thickness of thermal oxide in approximate! y 50 hrs, temperatures

of over 1075°C were required with dry Argon (less than 10 ppm oxygen); even

1090°C was not adequate for 66 hours in argon plus hydrogen. Air or argon plus

3% H

2 O atmospheres lead to adequate (1.5 ym) oxides at 1050°C and above after

50 hours. Adhesion of the sputtered oxide was not a serious problem in any of

the tests (A4, A15, A43, and A43A) . The adherence of the platinum film was a

problem on A4 which had the rough thermal oxide (Fig. 20). The dark sputtered

oxide on samples A62 and A63 probably relates to metallic contamination of the

coating and these samples were note tested.

Table 11 is a summary of the heat treatments, oxide thicknesses, and adhesion strengths of the thin films of the MAR M200+Hf plus FeCrAlY system (B).

Results on oxide thickness similar to those observed on the "A" samples would

lead to recommendations of temperatures of 1050-1090°C for 50 hours in air or

31
argon plus H 2 O at the dew point (~3% H 2 O) . Again, the sputtered oxide

adherence was high (B16, B27A, B41, B45, B47, B49). The platinum adherence was

also good on B16 and B47. Sample B49 had poor platinum adherence and also had a

rough thermal oxide. Photo-micrographs of B49 indicate some spalling of the

thermal oxide which may have related to contamination of the surface.

Table 12 summarizes the heat treatments, oxide thicknesses, and adhesion

strengths of the thin films on the MAR M200+Hf plus NiCoCrAlY (electron beam

deposited) samples. Again, it was difficult to achieve adequate thickness of

thermal oxide in dry argon or argon plus 4% hydrogen at temperatures below

1100°C. Generally, the thermal oxides were thicker and smoother than those on

the FeCrAlY. Adherence of sputtered oxide was good on all samples tested (C12,

C13, C14, C15, C19, C20, C23» C24, C28) except C22 which had a thin oxide grown

in dry argon. Adherence of the platinum films was good on C19, C20 and C24.

These samples had thermal oxides matured in air. C12 had poor platinum film

adherence.

Table 13 summarizes the heat treatments, oxide thickness and sputtered film

adherence for the "D" samples of MAR M200+Hf plus NiCoCrAlY (sputtered). The

thermal oxide was generally darker than those oxides deposited by the electron

beam method. Deposited NiCoCrAlY samples and some "D" samples had a blue -oxide

contamination eminating from the uncoated edges. Samples with non-chromatic

oxides had good sputter film adherence.

In order to evaluate the adherence of the platinum-platinum 10% rhodium

films, thermal cycling tests were used. The test coupons weigh approximate! y 10

grams and are approximately 0.8 cm thick. They have 70-100 urn of NiCoCrAlY or

FeCrAlY plus 1-2 ym thermal oxide and 1-2 urn sputtered aluminum oxide under the

2 urn thermocouple film. Each coupon was heated with a torch to 1000°C in 60-90

sec and cooled in air to 500°C, then water quenched to below 100°C and recycled.

32
The tests included eight cycles for each sample. Temperatures were measured

with a type S thermocouple in an AI 2 O 3 tube between the base of the specimen

and a refractory brick. The surface temperature of the sample was also monitored with a L&N disappearing filament pyrometer. The peak temperature recorded

is conservative since emissivity corrections were not made and the thermocouple

is not directly exposed to the flame. In fact, certain portions of the film

were obviously hotter. The thermal cycling did produce defects in the platinum

and platinum-rhodium films which were apparent under the microscope and lead to

microscopic rupture in one sample (B16). The defects appeared in photo-micrographs as small blisters 10-100 urn in diameter which are expanded probably due

to excessive overheating and plastic deformation at peak temperatures and lack

of good adhesion or thermal contact. Both B16 (FeCrAlY on MAR M200) and C24

(NiCoCrAlY on MAR M200) demonstrated this behavior. A40 (FeCrAlY on MAR M509)

did not have the blisters, but the oxide on this specimen was rough.

4. Electrical Performance

The electrical performance of all four types of test coupons (A, B, C, and

D) were evaluated. In fact, all sputtered oxide and thermal oxide films were

checked at room temperature by direct contact with an ohmmeter probe capable of

measuring up to two megahoms. The area of the film under the ohmic probe was

wet with tap water (200-500 K ohms at 1 cm) to connect any pores with the probe.

Thermal oxide films such as Oil, A38, and B23 which were overheated did not pass

this test and were not sputter coated. The high temperature electrical test was

made on the fixture described in Section IIE (Fig. 5) and permitted monitorinq

electrical resistance between the platinum thin film and the substrate, along

approximate! y 1.5 cm of length of the bimetallic film, or between two points on

the oxide 1.5 cm apart. Typical results of testing A, B, C, and D type

33
sputtered oxide films are presented in Fig. 37. The oxide films on NiCoCrAlY

(type C and D) had resistance values greater than two megohms below about 900°

C

but the resistance fell rapidly to several* hundred kiloohms at temperatures

above 1Q50

0 C. The steep decreasing slope in oxide resistance with increasing

temperature is comparable to those given in Samsanov36,37 . The data indicate decreases from 4 x 10^ ohms at 600°C to 3.5x10^ at 800°C to 5xl04 at

1000°C and lxlO^ at 1100°C for sintered aluminum oxide and similar decreases

occur for single crystals. This very steep slope of one to two decades per

100°C in the critical range strongly indicates the need for high temperature

testing. The oxide films on FeCrAlY were apparently actively growing above

700°C because of the increase in resistance measured at 800°C region (Fig. 37).

Generally, the resistance of these films was not as great as those formed on

NiCoCrAlY. The test fixture also permitted evaluation of dielectric breakdown.

A one volt potential was placed over the 2-4 ym thick 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 AI 2 O 3 is no doubt adequate

for thermocouple voltages and even for thin film strain gages with similar

dimensions.

Tests on the platinum-platinum rhodium alloy films were conducted with the

same equipment. In addition to measuring the resistance to ground, a continuous

measurement of the metallic thin film resistance was made. Samples A43A, C19,

D4 and D5 also 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 two megohms resistance between ground and the platinum at the

following test temperatures : 970°C for A43A, 960°C for D4, 1080°C for D5, and

1120°C for C19. The C19 test coupon was taken to 1169°C where the resistance

34
fell to 350 K ohms. The platinum alloy bimetallic strip increased in resistance

at high temperatures as expected.

Full size thin film thermocouple test specimens shown in Fig. 36, were

tested for thermocouple output. This specimen which is over 10 cm long permitted testing in a propane torch on one end while maintaining the cold end near

room temperature. Type S reference grade thermocouples (250 ym diameter) were

fixed to each end of the 10 cm bar during heating to monitor metal temperatures

.

Although this test was not sufficiently precise to be considered a calibration,

it was used to determine that the thermocouple was indeed working properly.

Sample C31 thin film thermocouple test coupon followed the output of the

attached thermocouple. Some difficulties were encountered in maintaining the

attached thermocouple in intimate contact with the test bar above 900°C in the

flame so that stabi 1 ity measurements were not possible with this arrangement.

The same thin film thermocouple bar, C31, was inserted through the wall of

a silicon carbide rod heated furnace to obtain long time stability information.

The refractory wall of the furnace was 9 cm thick leaving approximatley 0.5 cm

extension of the junction into the furnace and 0.5 cm for connections outside

outside the furnace. Both ends were monitored' with Pt-Pt 10% Rh thermocouples

touching the MAR M200 bar. The heat transfer along the bar lead to intermediate

values of thermocouple output voltages on the thin film thermocouple compared to

the thermocouples used for monitoring. For example, the monitors read 0.99 mv

(165°C) and 9.13 mv (975°C) on the outboard and inboard points respectively when

the thin film thermocouple was reading only 6.36 (725°C).

This severe temperature gradient along the test bar may have contributed to

a failure in the form of an open circuit at approximately 10 hours. The platinum leg had debonded from the test bar which terminated the test. The results

of these thin film thermocouple tests indicated the general validity of the

35
fabrication method but are not considered a good simulation of true engine test

conditions. These tests should be conducted by engine manuf acturers where the

complex geometries and application conditions can be used.

36
REFERENCES

1. Burger, H.C. and Van Citter, P.H., "Preparation of Bismuth-Antimony Vacuum

Thermal Elements by Vaporization", Z. Phys., Vol . 66, 1930, p. 210.

2. Harris, L., and Johnson, E.A., "Technique of Sputtering Sensitive Thermocouples", Rev. Sci . Inst. 5, 1934, p. 153.

3. Chopra, K.L., Thin Film Phenomena , McGraw Hill, New York, 1969.

4. Dils, R.R., "Process of Coating a Gas Turbine Engine Alloy Substrate", U.S.

Patent 3, 890, 456, 17 June 1975.

5. Dils, R.R. and Follansbee, P.S., "Superalloy Sensor", Superal Toys : Metal - lurgy and Manufacture , Kear, B.H., Muzyka, D.R., Tien, J.K., and Wlodek,

S.T., Eds., Claitor, Baton Rouge, La., 1976, pp. 37-44.

6. Grant, H.P. and Przybyszewski , J.S., "Heat Transfer Coefficients Around

Cylinders in Crossflow in Combustor Exhaust Gases", JOURNAL OF ENGINEERING

FOR POWER, TRANS, ASME, 99, No. 4, p. 497, 1977.

7. Grant, H.P., and Przybyszewski, J.S., "Thin Film Temperature Sensor", NASA

CR-159782, Feb. 15, 1980.

8. Grant, H.P., Przybyszewski , J.S., and Claing, R.G., "Turbine Blade Temperature Measurements Using Thin Film Temperature Sensors", NASA CR- 165201

,

March 17, 1981.

9. Grant, H.P., et al . , "Thin Film Temperature Sensors Phase III", NASA

CR-165476 , Jan. 11, 1982.

10. Dils, R.R. and Follansbee, P.S., "Electronic and Ionic Conduction in Aluminum Oxides Grown on High Temperature Metal Alloys", presented at Symposi urn on High Temperature Electron and Ion Transport in Solids , Electrochemical

Society Meeting, San Francisco, May 12-17, 1974.

11. Wells, C.H., Follansbee, P.S. and Dils, R.R., "Mechanisms of Dynamics

Degradation of Surface Oxides", Stress Effects and Oxidation of Metals,

J.V. Cathcart, Ed., AIME, New York', T975

12. Dils, R.R. and Follansbee, P.S., "Dynamic Oxidation and Corrosion in Power

Generating Units", CORROSION, _33, No. 11, p. 385, 1977.

13. Dils, R.R., "Dynamic Gas Temperature Measurements in a Gas Turbine Transition Duct Exit", Journal of Engineering for Power, Trans. ASME, Vol. 95,

Series A, No. 3, July 1973, p. 265.

14. Certain commercial equipment, instruments, and materials are identified in

this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the material or equ foment is necessarily the best available for the purpose.

37
15- T.K. Redden, "Ni-Al Coating-Base Metal Interactions in Several Nickel-Base

Alloys", Trans. Metalurgical Society of AIME, Vol . 242, p. 1695, August

1968.

16. G.W. Goward, D.H. Boone and C.S. Giggins, "Formation and Degradation

Mechanisms of Aluminide Coatings on Nickel-Base Superal loys", Tran, of ASM,

Vol. 60, p. 228, 1967.

17. F.S. Pettit, "Oxidation Mechanisms for Nickel -A1 uminum Alloys at

Temperatures Between 900° and 1300°C", Trans. Metallurgical Society of

AIME, Vol. 239, p. 1296, September 1967.

18. E.A. Gulbransen and K.F. Andrew, "Oxidation Studies on the

Iron-Chromium-Al uminum Heater Alloys", J. Electrochemical Society, p. 294,

April 1959.

19. G.W. Goward, "Current Research on the Surface Protection of Superalloys for

Gas Turbine Engines", J. of Metals, p. 31, October 1970.

20. W.C. Hagel , "The Oxidation of Iron, Nickel and Cobalt-Base Alloys

Containing Aluminum", Corrosion - Nat'l Assoc, of Corrosion, Engineers,

Vol. 21, p. 317, October 1965.

21. C.S. Giggins and F.S. Pettit, "Oxidation of Ni-Cr Alloys Between 800° and

1200°C", Trans. Metallurgical Society of AIME, Vol. 245, p. 2495, December

1969.

22. W.E. Boggs, "The Oxidation of Iron-Aluminum Alloys from 450° to 900°C", J.

Electrochem, Soc.: Solid State Science, p. 906, June 1971.

23. R.E. Grace and A.U. Seybolt, "Selective Oxidation of A1 from an Al-Fe

Alloy", J. Electrochemical Society, p. 582, October 1958.

24. J. Stringer, P.S. Corkish and D.P. Whittle, "Oxidation of Two-Phase

Alloys", in Stress Effects and the Oxidation of Metals, J.V. Cathcart,

Editor, Proc. Symp. 1974 TMS-AIME Fall Meeting, Oct. 21-24, 1974, Materials

Science Symposium, publication of the Metallurgical Society of AIME, 1975.

25. Smeggil, N. Borstein, "The Effect of NaCICg on Oxidation of NiAl", J.

Electrochem. Soc., Solid State Science and Technology, p. 1283, Aug. 1978.

26. Berry, R.W., P.M. Hall, and M.T. Harris, Thin Film Technology , Chapter 4,

Van Nostrand, New York, 1968.

27. Thornton, J.A. and A.S. Penford from Thin Film Processes , J.L. Vossen and

W. Kern, Academic Press, New York, 1978, p. 76-115.

28. Thornton, J.A. and V.L. Hedcoth, J. Vac. Sci., Tech. 13, 117 (1976).

29. Schwartz, N. Trans. 10th Nat. Vac Symposium, 1963, MacMillan, New York, p.

325 (1964).

"

30. Maissel, L.I., Physics of Thin Films. Vol. Ill, Academic Press, New York

(1966).

38
31. R.K. Waits, Thin Film Processes , J.L. Vossen and W. Kern, Academic Press

New York, pp - 131-175 (1978).

32. Urbanek, K., "Solid State Tech", 20(4), 87 (1977).

33. Nowicki, R.S., J. Vac. Sc i - Tech., Vol . 14, No. 1, p. 127 (Jan/Feb 1977)

34. Breinan, E.M., and K.G. Kreider, "Braze Bonding and Joining of Aluminum

Boron Composities", Metal Engineering Quarterly, ASM, p. 5 (Nov. 1969).

35. Kreider, K.G., and K.M. Prewo, "Boron Aluminum Composites" in Metallic

Matrix Composites , K.G. Kreider, Ed. Academic Press, New York ( 1974)

.

36. Samsonov, G.S., The Oxide Handbook, Trans. CN Turton, NY IFI/Plenum

(1973).

37. Pentecost, J.L. Ceramic Age 71:26 (1958).

39
List of Tables

1. Alloy Compositions in Weight %

2. Materials Systems and Designations

3= D24 Composition Profile - No Heat Treatment

4. . D4 Composition Profiles - No Barrier

5. D14 Composition Profile with Barrier and Heat Treatment

6. A62 Composition Profile - No Heat Treatment

/. A40 Composition Profile - Without Barrier

8. A19 Composition Profile - With Barrier and Heat Treatment

9. A19(B) Composition Profile - With Barrier and Heat Treatment

10. Adhesion Strength and Heat Treatments Cobalt Alloy

MAR M509 plusFeCrAlY

11. Adhesion Strength and Heat Treatments Nickel Alloy

MAR M200+Hf plus FeCrAlY

12. Adhesion Strength and Heat Treatments Nickel Alloy

MAR M200+Hf plus NiCoCrAlY (EB)

13. Adhesion Strength and Heat Treatments Nickel Alloy

MAR M200+Hf plus NiCoCrAlY (S)

40
List of Illustrations

1. Schematic cross section of thin film sensor.

2. NiCoCrAlY coating on Ni alloy.

3. Yttrium X-ray Map of NiCoCrAlY coating as deposited with Ni backing.

4. Sputtered FeCrAlY coating on Ni alloy Ni backing on top.

5. High temperature electrical test fixture.

6. A1 map of D24.

7. Scanning electron micrograph of D4. Oxides are penetrating inward from

surface.

8. A1 Map of D4.

9. Diffusion profiles of D4.

10. Scanning electron photograph of D14.

11. A1 map of D14.

12. Co map of D14.

13. Cr map of D14.

14. Diffusion profiles of D14.

15. EDX intensity vs. energy spectrum from coating of A40.

16. EDX intensity vs. energy spectrum of substrate material in A40.

17. Diffusion profiles of A40 at region of interface; without barrier layer;

after heating to 1075°C, 50 hours.

18. Scanning Electron Micrograph of A19.

19. Diffusion profiles of A19.

20. Thermal oxide on A4.

21. Thermal oxide spalling of FeCrAlY sample A35.

22. Thermal oxide of FeCrAlY on FeCrAlY on BIO.

23. Thermal oxide of FeCrAlY on B41.

24. Thermal oxide of electron beam deposited NiCoCrAlY on C21.

25. Thermal oxide of sputtered NiCoCrAlY on D8.

41
List of Illustrations (Cont.)

26. Oxygen Is XPS core levels from samples C12 and C14.

27. Comparison of the 01s levels for samples D1 and D2.

28. XPS nickel 2 P 3/2 core level region indicating the presence of metallic

and oxidized nickel in the surface aluminum oxide of sample Dl.

29. Wide energy range XPS spectrum obtained from the air-oxidized surface of

sample B23.

30. Sputtered oxide on A43A.

31. Sputtered oxide on C19.

32. Interferometric micrographs of sputtered aluminum oxide.

33. XPS spectra for samples C12 and C14 in the binding energy region near the

Ni 2 P 3/2 level.

34. XPS spectra for samples C12 and C14 in the binding energy region near the

Cr 2 p 3/2 level

.

35. Small test coupon for thin film Pt and Pt 10% Rh.

36. Thin film thermocouple test coupon.

37. Sputtered oxide film resistance.

42
Table 1 Alloy Compositions

in weight %

Alloy

MAR M200+Hf Ni lOCo 9Cr 5A1 2T i 2Hf lCb 0.15B 0-14C

MAR M509 Co 24Cr ION i 7W 3.5Ta

0.2Ti 0.5Zr 0.6C

FeCrAlY Fe 18Cr 11A1 0.7Y

NiCoCrAlY Ni 23Co 18Cr 12A1 0 - 5 Y

Table 2

Material Systems and Designations

Substrate Coating Process Designation

MAR M509 FeCrAlY DC Sputtering A

MAR M200+Hf FeCrAlY DC Sputtering B

MAR M200+Hf NiCoCrAlY Electron Beam C

MAR M200+Hf NiCoCrAlY DC Sputtering D
Table 3

D24 Composition Profile - No Heat Treatment

(in weight percent)

Nominal

Coating -5

Distance from

-3 -1

barrier

1

(in pm)

3 5 Nominal

Substrate

Ni 54 55 55 48 56 63 64 71

Co 17 16 17 16 11 14 13 10

Cr 19 18 16 18 11 10 11 9

A1 12 11 11 18 14 5.3 5.3 5

Si 0 0.6 0.7 - 5.3 5.9 5.5 -

Ti - - - - 1.8 2.1 2.1 2

D4 l Table 4

Composition Profiles - No Barrier

(in weight percent)

Distance from edge (in ym)

0 10 20 30 40 70 80 90 100 120 140 150 200

Ni ' 51 56 62 56 65 64 66 66 68 66 68 66

A1 100 4 6.4 9.6 5.6 4.0 3.5 3.5 3.5 2.9 3.4 3.5 3.5

Si 0 0.9 0 0 0.8 3.7 5.8 5.7 5.7 5.2 4.5 4.2 5.4

Cr 0 23 19 13 18 10 12 10 11 10 12 10 11

Co 0 22 19 16 20 15 13 12 13 12 13 12 12

Ti 0 1.6 2.0 2.0 1.5 1.7 2.0 2.1 2.3
Table 5

D14 Composition Profile with Barrier

(in weight percent)

10 20 30 40

Ni 50 53 54 57 56

A1 CO• 6.9 5.7 9.6 13

Si 4.8 0.7 0.9 0.5 0

Ti 0.3 0.3 0.4 0.4 0

Cr 20 21 21 18 16

Co 17 18 18 16 15

Y - - - - 0.7

Zr - - - - -

Hf —, «>

distance from edge (in pm)

50 60 70 80 90 100

59 58 63 59 55 53

9.8 8.8 12 7.8 4.9 3.5

0 0.7 0.6 1.1 1.7 2.0

0.5 0.4 0 0.5 0.5 1.9

16 16 11 16 20 19

15 16 13 16 18 17

- - 0.4 - - -

- - - - - 1.1

CB a, at, <=> 0.2

no 120 130 140 150 200

64 64 62 62 64 63

5.5 5.5 4.9 4.4 4.7 4.2

10 7.8 5.6 6.7 6.6 6

1.9 2.3 1.9 1.9 1.9 1.6

6.5 8.1 12 12 10 11

11 12 14 14 13 14

0.9
Table 6

A62 Composition Profile - No Heat Treatment

(in weight percent)

Nomi nal

Coating 21 12

Distance

5

from

3

barrier

0

(in

1 ym)

6 150

Nomi nal

Substrate

Fe 70 75 76 77 75 10 - - - -

Cr 18 21 21 18 18 18 26 28 24 24

A1 11 4<,6 2.5 5.1 6.3 45 5.0 _ - -

Co «> «> dX> 24 55 59 60 55

10 11 11 10
Table 7 A40 Composition Profile - Without Barrier

(in weight percent)

Nomi nal

Coating 3 19

Fe 70 53 52

Cr 18 17 17

Si 0 1.0 1.0

A1 11 4.8 2.8

Co 0 25 24

Ni 0 0 4.3

Distance from Surface (i

31 40 56* 79

51 50 50 44

17 17 16 14

1.1 1.2 1.0 1.1

4.7 5.1 3.8 7.6

23 23 25 27

4.1 4.4 4.6 5.8

ym)

92 113*

Nomi nal

Substrate

12 0 0

21 24 24

3.6 3.8 0

2.6 0 0

52 61 55

10 11 10
Table 8

A19 Composition Profile - with Barrier and Heat Treatment

(in weight percent)

Distance from Barrier (in urn)

Coating Substrate

Edge 4 20 10 6 4 2 0 1 3 8 12 18 25 30 36 42 58 68

Fe 35 48 49 49 48 47 46 8.0 48 47 30 32 27 26 32 15 0 0 0

Cr 13 17 18 17 17 16 15 3.6 16 16 47 34 50 43 17 19 25 24 25

Si 1.2 1.2 1.1 1.0 1.0 - 1.2 - 1.0 1.2 5.0 3.9 5.8 4.1 2.1 3.3 4.2 4.0 3.7

A1 32 cn CO 6.0 6.6 6.0 5.7 7.2 83 8.4 6.3 3.0 4.3 2.2 4.0 3.1 3.2 1.3 0 0

Co 19 25 26 26 25 27 27 5.5 27 26 15 20 15 19 41 53 61 61 63

Ni 0 3.5 0 0 3.5 5.4 3.9 0 3.7 0 5.0 0 4.3 5.3 6.3 8.5 12 9.0
Table 9

A19(B) Composition Profile - with Barrier and Heat Treatment

(in weight percent)

Distance from Edge (in yin)

Nominal

Substrate

32 68 96 115 126 130 146 242

Fe 56 55 47 42 38 31 3.6 0.5 0

Cr 18 17 21 9.4 14 27 23 31 24

Si 0.7 1.0 1.5 1.0 1.4 2.7 3.1 3.4 0

A1 4.0 4.2 4.2 7.4 7.3 4.5 1.7 0.2 0

Co 19 20 23 33 33 29 58 55 55

Ni 2.6 2.9 3.5 7.0 7.0 5.9 10 9.8 10
Table 10

Adhesion Strength and Heat Treatments

Cobalt Alloy MAR M509 plus FeCrAlY

Sample °C Hrs Atmosphere

Thermal

Oxide

urn

Sputtered

Ox i de

ym

Adhesion

Oxide

MPa

Adhesion

Pt

MPa

A4 1075 40 air 0.9 - 70 1.8

A15 1050 66 Ar 0.9 2.8 60 65.0

A19 1050 66 Ar 0.9 - - -

A28 1070 50 air 1.1 - - -

A35 1070 60 Ar, air 1.5 - - -

A38 1120 25 Ar, air heavy,

mixed

- - -

A40 1065 47 air 1.8 2.3 - -

A42A 1050 80 Ar+H 2 0 1.6 - - -

A43 1065 46 Ar <1 2.0 62 -

A43A 1090 50 Ar 1.9 2.6 39 17.8

A52 1090 66 Ar+H 2 1.0 dark - -

A63 1025 48 air (3)* dark - -

*heavy oxide on uncoated edge
°c

1090

1060

1060

1077

1025

1090

1050

1065

1070

1120

1075

1050

1075

Table 11

Adhesion Strength and Heat Treatments

Nickel Alloy MAR M200+Hf plus FeCrAlY

Hrs Atmosphere

Thermal

Oxide

urn

Sputtered

Oxide

urn

Adhesion

Oxide

MPa

Adhesion

Pt

MPa

66 air 1.6 = - -

50 air 3.0 = - -

47 air 2.0 0.8 >70 32

50 air - - - -

48 air 1.2 - - -

50 Ar+H20 1.0 2.5 >70 -

80 Ar+H20 - - - -

46 Ar 0.4 <1 - -

62 Ar , ai

r

2.0 - - -

25 air - - - -

50 air 2.5 3.0 67 -

66 Ar+H2 0 2.0 1.1 68 62

40 air 2.0 rough 66 low
Table 12

Adhesion Strength and Heat Treatments

Nickel Alloy MAR M200+Hf plus NiCoCrAlY (EB)

Sample °C Hrs Atmosphere

Thermal

Oxide

ym

Sputtered

Oxide

ym

Adhesion

Oxide

MPa

Adhesion

Pt

MPa

Cll 1120 25 Ar,air Green,

Defective

- - -

C12 1090 25 Ar+H2 0 1.1 2.8 64 45

C13 1075 50 air 2.8 2.6 >70 -

C14 1075 50 air 2.7 2.8 69 -

C15 1075 40 Ar s air 5.5 66 low

C18 1025 48 air 3.3 - - -

C19 1025 48 air 2.1 1.9 >70 55

C20 1125 63 air - 0.6 69 >70

C21 1075 49 Ar,air 2.2 - - -

C22 1065 46 Ar 0.1 1 3.6 3.4

C23 1075 49 Ar,air 2.0 1.5 >70 -

C24 1065 47 air 2.4 0.8 55 25.5

C25 1090 66 Ar+H 2 1.3 Damaged - -

C26 1050 80 Ar+H 2 0 1.0 1.2 - -

C28 1050 66 Ar 0.4 3.0 35
Table 13

Adhesion Strength and Heat Treatments

Nickel Alloy MAR M200+Hf plus NiCoCrAlY (S)

Sample °C Hrs Atmosphere

Thermal

Oxide

ym

Sputtered

Oxide

ym

Adhesion

Oxide

MPa

Adhesion

Pt

MPa

D1 1050 80 Ar+H 2 0 2.6 - - -

D2 1025 50 air 2.7 1.9 15.6 -

D3 1025 50 air 2.0 - 67.0 -

D4 1090 50 Ar+H 2 0 3.2 1.5 - -

D5 1125 63 air 3.5 - 62.0 65

D6 1075 50 air 2.9 - - -

D7 1075 49 Ar,air 3.6* - - -

D8 1075 49 Ar ,air 2.9* - - -

* Blue contamination

Sputtered Film

Thermocouple

Oxide Layer

MCrAlY Coating

(sputtered or evaporated)

Original Metal

Surface

Base Metal

Figure 1. Schematic cross section of thin film sensor

Figure 2. NiCoCrAlY coating on Mi alloy

Nickel backing is on top. Test sample heat treated 4 hours in argon ( SOOx

)
Figure 3. Yttrium X-ray Map of NiCoCrAlY coating

as deposited with Ni backing (860x)

Figure 4. Sputtered FeCrAlY coating on Ni alloy

Ni backing on top. Notice faceted growth (lOOOx)
~ i gure 5. High temperature electrical test fixture

-igure 6 . Electron scan of D24 iljjx,

AI highlighted displaying AI 2 O 3 phase
Figure 7. Scanning electron micrograph of D4 ( 1 5 30 x

)

Oxides are penetrating inward from surface

Heat treated at 1090°C for 50 hours

Figure 8 . A1 Map of D4 (15 30x

)

AI 2 O 3 penetration evident from diffusion pipes

mmmmm
Figure 9. Diffusion profiles of D4

Heat treated at 1090 for 50 hrs without barrier
Figure 10. Scanning electron photograph of D14

AI 2 O 3 is dark 1 ym line 580x

Figure 11. A1 map of D14 (580x)
Figure 12. Co map of D14 (580x)

Figure 13. Cr map of D14 (580x)
50

40

30

20-

i

10

Figure 14. Diffusion profiles of D14.

1 yr.i barrier layer 1080°C, 50 hrs
ENERGY (key)

Figure 15. EDX intensity vs. energy of A40 in coatinq

LT= 60 SECS A-40, 17 JAN 84, 10

Figure 16

ENERGY (keV)

EDX intensity vs. energy of A40 in suDstrate material
Figure 17. Diffusion profiles of A40

No barrier layer, 1075°C, 50 hrs
Figure 18. Scanning Electron Micrograph of A19

Note bridging of AI 2 O 3 barrier layer with metal

Figure 19. Diffusion profiles of A19

2 ym barrier, 1075°C, 86 hrs
(Vtr p. ?

~

, o

7 Ol

*'

... ' r i

.

. f#

' ' # * v<f*

. » r C

^

v'>

€

' * *V ^ 1* «*

% ^ v ,

. V

"’y' ® * * ,

' fT* > V * *

* ** . . #•>,

_ i * > > T

<L -m +< /Vi

4 “V -^ r

#*

Figure 20. Thermal oxide on A4 uneven oxide due to surface grooves from

polishing (500x)

Figure 21. Thermal oxide spalling of FeCrAlY sample A35

Note holes under spalled areas (lOOx)
Figure 22. Thermal oxide of FeCrAlY on FeCrAlY on BIO

Dark grains are related to uneven growth pattern (500x)

s0

•'o

'""‘"ilnm,

40

Figure 23. Thermal oxide of FeCrAlY on B41 oxide is 2.0 um Thick (250x)
Figure 24. Thermal oxide of electron beam deposited NiCoCrAlY on C21

Small fibrous habit of oxide apparent in addition to film growth (500x)

Figure 25. Thermal oxide of sputtered NiCoCrAlY on D8
10 4

COUNTS

PER

SECOND

Figure 26. Oxygen Is XPS core levels from samples C12 and C14. The better insulating properties of the C14 sputtered oxide surface lead to an upward shift in binding energy which positions the 01s level 4eV

higher than 01s level from the Ar+H20 thermal oxide at the

surface of C12. Similar shifts were observed in comparisons

of other features (e.g., Cls) derived from these samples.
BINDING ENERGY (eV)

Figure 27. Comparison of the 01s levels for samples D1 and D2. The D2

oxide grown in air has a larger charging shift than the D1 oxide grown

in Ar and H 2 O. Similar shifts occur between the Cls and other core levels for these samples.
1

40

COUNTS

PER

SECOND

875 870 865 860 855 850 845 840

BINDING ENERGY (eV)

Figure 28 . XPS nickel 2P3/2 core level region indicating the presence

of metallic and oxidized nickel in the surface

aluminum oxide of sample Dl.
1

50

COUNTS

PER

SECOND

2.4

1.8

1.2

0.6

0

700 600 500 400 200 100 ' 0

BINDING ENERGY (eV)

Figure 29. Wide energy range XPS spectrum obtained from the air-oxidized

surface of sample B23. The 01s and Al features indicate

an AI 2 O 3 stoichiometry at the surface.
Figure 30. Sputtered oxide on A43A

Note smooth pebbly film (500x)

Figure 31. Sputtered oxide on C 19

Similar to A43A (500x)
Figure 32. Interferometric micrographs of sputtered aluminum oxide

Above indicates 1.6 ym thick; below indicates 1.9 urn thick
10

2

COUNTS

PER

SECOND

BINDING ENERGY (eV)

Figure 33. XPS spectra for samples C12 and C14 in the binding energy

region near the N i 2

p

3 /

2

level. The thermally oxidized C12

surface has Ni present at an estimated level near 0.5%,

while essentially no Ni is observed in the sputtered

aluminum oxide on C14.
10*

COUNTS

PER

SECOND

BINDING ENERGY (eV)

Figure 34. XPS spectra for samples C12 and C14 in the binding energy

region near the Cr2p3/2 level. The feature in the C12 spectrum

indicates a Cr concentration in the thermal oxide near 0.5%,

while Cr appears to be completely absent from the sputtered

oxide surface of C14.
Figure 35. Small test coupon for thin film Pt and Pt 10% Rh

Figure 36. Thin film thermocouple test coupon
OHMS

RESISTANCE

(K)

SPUTTERED OXIDE FILM RESISTANCE

TEST TEMPERATURE (°C)

Figure 37. Sputtered oxide film resistance
NBS-114A 'REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NB SIR-84-2 9 49

2. Performing Organ. Report No. 3. Publ ication Date October 1984

4. TITLE AND SUBTITLE

Advanced Thin Film Thermocouples

5. AUTHOR(S)

Kenneth G. Kreider, Stephen Semancik, and Craig Olson

6. PERFORMING ORGANIZATION (If joint or other tnan NBS, see instructions) 7. Con tract/Grant No.

NATIONAL 3UREAU OF STANDARDS C- 5471

5

DEPARTMENT OF COMMERCE 8. Type of Report 8. Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

NASA Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FiPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary or

bi bl iography or literature survey, mention it here) rost significant information, f document includes a si;

Abstract

The fabrication, materials characteri zation, and performance of thin film

platinum-platinum rhodium thermocouples on gas turbine engine alloys has been

investigated. The materials chosen for the study were the turbine blade alloy

systems MAR M200+Hf with NiCoCrAlY and FeCrAlY coatings; and vane alloy systems

MAR M509 with FeCrAlY. Research was focussed on making improvements in the

problem areas of coating-substrate stability, adhesion, and insulation reliability and durability. Diffusion profiles between the substrate and coating with

and without barrier coatings of A1 2 O 3 are reported. The relationships

between fabrication parameters of thermal oxidation and sputtering of the insulator and its characteri zation and performance are described. The best thin

film thermocouples were fabricated with the NiCoCrAlY coatings which were thermally oxidized and sputter coated with A1 2 O 3

-

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; ana separate key woras Oy ter- r- high temperature sensors; reactive sputtering; thermocouples; thin film thermocouples,

turbine alloy coatings; turbine engine sensors

-

13. AVAILABILITY

L^Unlimited

i | For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[Xj Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGE'

83

15. Pr