207100

PROJECT CLOSING REPORT

/ _"C/7-

o6 7//p"

"PLASMA-SPRAYED REFRACTORY OXIDE COATINGS ON

SILICON-BASE CERAMICS"

%

(NCC-3,285)

(Project Period:..3/01/93-11/15/96)

Submitted to:

NASA-Lewis Research Center

Cleveland, OH 44135

by:

Prof'. Surendra Tewari,

Department of Chemical Engineering Deprtment,

Cleveland State University,

Cleveland, OH 44115

Sept 18, 1997

1
PLASMA-SPRAYED REFRACTORY OXIDE COATINGS ON

SILICON-BASE CERAMICS

INTRODUCTION

Silicon-base ceramics are promising candidate materials for high temperature structural

applications such as heat exchangers, gas turbines and advanced internal combustion engines.

Composites based on these materials are leading candidates for combustor materials for HSCT

gas turbine engines. These materials possess a combination of excellent physical and mechanical

propertie_ at high temperatures, for example, high strength, high toughness, high thermal shock

resistance, high thermal conductivity, light weight and excellent oxidation resistance.

However, environmental durability can be significantly reduced in certain conditions such

as when molten salts, H2 or water vapor are present. The oxidation resistance of silicon-base

materials is provided by SiO2 protective layer. Molten salt reacts with SiO2 and forms a mixture

of SiO2 and liquid silicate at temperatures above 800C. Oxygen diffuses more easily through the

chemically altered layer, resulting in a catastrophic degradation of the substrate. SiC and Si3N4

are not stable in pure I-t2 and decompose to silicon and gaseous species such as CH4, Sill, Sill4,

N2, and NH3. Water vapor is known to slightly increase the oxidation rate of SiC and Si3N4.

Refractory oxides such as alumina, yitria-stabilized zirconia, ytria and mullite

(3A1203.2SIO2) possess excellent environmental durability in harsh conditions mentioned above.

Therefore, refractory oxide coatings on silicon-base ceramics can substantially improve the

environmental durability of these materials by acting as a chemical reaction barrier. These oxide

coatings can also serve as a thermal barrier.

The purpose of this research program has been to develop refractory oxide

chemical/thermal barrier coatings on silicon-base ceramics to provide extended temperature range

and lifetime to these materials in harsh environments.

PROPOSED RESEARCH AND TECHNICAL BACKGROUND

Research at Solar Turbines, Inc. and Oak Ridge National Lab showed that refractory oxide
coatings on SiC tend to crack on thermal cycling. It is not surprising that alumina or

ytria-stabilized zirconia coatings crack on thermal cycling considering the high thermal expansion

mismatch between these oxides (>>9x10-6/oC for alumina and ))llxl0-6/oC for ytria-stabilized

zirconia) and silicon-base ceramics ())3-5x10-6/oC), which is likely to produce a high thermal

stress. However, thermal expansion coefficient (CTE) of mullite 0)5.4x10-6/oC) is fairly close to

that of silicon-base ceramics that CTE mismatch may not be the major factor for the observed

cracking.

When molten mullite is rapidly quenched, crystallization is suppressed and metastable

amorphous mullite is formed. Crystallization of amorphous mullite occurs at ))1000C which is

%

accompanied by a shrinkage of volume, resulting in a high residual stress. In a previous study by

Lee et.al., phase transformation of amorphous mullite during thermal cycling was identified as the

key factor for the observed poor thermal shock resistance of plasma-sprayed mullite. Solid

solution range of mullite is dependent on the processing route and is very difficult to control.

Plasma spray of mullite tends to precipitate second phase A1203 and SiO2 which may also cause

a high residual stress and thus contribute to the cracking. During previous research at NASA it

was seen that deposition of metastable amorphous mullite was feasible, and that this process could

yield significant improvement of thermal shock resistance.

Since oxide ceramics with less amount of SiO2 possess better environmental resistance in •

hydrogen or molten salts, it is desirable to apply alumina, ytria-stabilized zirconia or ytria

overlayer coatings on mullite base coating for further enhancement of the environmental durability

of silicon-base ceramics. CTE mismatch between mullite and these oxides are so substantial that a

high thermal stress is likely to develop at the interface on thermal cycling. However, CTE of

materials can be tailored by mixing two or more materials with different CTEs. Therefore, CTE

mismatch between mullite base coating and alumina, ytria- stabilized zirconia or ytria overlayer

coatings can be minimized by applying compositionally gradient coatings of mullite and these

oxides.

In the following section we will list the major accomplishments during this research

project.
KEY ACCOMPLISHMENTS

A. Development of Environmental/Thermal Barrier Coatings for Si-Based Ceramics.: Siliconbased ceramics are the leading candidate materials for high temperature structural applications.

However, an environmental protection scheme is necessary to realize the full potential of these

materials. Key environmental issues include molten salt corrosion and evaporation of silica in

reducing gases or water vapor, leading to a rapid recession of material. This task was aimed at

developing protective refractory oxide coating systems for Si-based ceramics to provide

protection from these severe environments.

A. 1. Characterization of Long-Term Durability of gWullite Coating for SiC Ceramics: A new

%

fully-crystalline mullite coating was developed in our laboratory. New mullite(5-10 mil)-coated

sintered SiC and SiC/SiC composites were tested for long-term environmental durability and

chemical compatibility by cycling the sample between room temperature and 1200/i300°C with 1

or 2 hr cycle. No macroscopic cracking or spallation of coating occurred after over 500 cycles.

After 600 cycles at 1200°C, mullite showed excellent thermal shock resistance and chemical

compatibility (only limited oxidation at the interface). After 250 cycles at 13000C, mullite still

showed excellent thermal shock resistance; however, substantial oxidation of SiCoccurred,

leading to the formation of a large number of pores at the coating/SiC interface. Continued

formation of pores may cause the delamination of coating in long term exposures at 1300°C.

Thermally-cycled mullite-coated SiC (600 1hr cycles at 1200°C) was tested for molten salt

corrosion. It exhibited excellent resistance to molten salt corrosion in a burner rig (Jet A fuel,

2ppm Na, 4 Atm, 50 hr). This confirmed the potential of mullite as a barrier coating against

molten salt corrosion.

Mullite-coated SiC/SiC composite exposed to a rich burn gas in a high pressure burner rig

at 1260°C revealed a selective evaporation of silica from the surface. This will lead to a rapid

recession of mullite coating. Therefore, an overlay coating such as yttria-stabilized zirconia

(YSZ) is necessary to protect the recession of SiC ceramic from reducing gases or water vapor.

A. 2. Development and Characterization of Mullite/YSZ Dual Layer Coatings: YSZ is more

stable than mullite in reducing gases or water vapor because it has much lower vapor pressure

than silica and it is not known to form a hydroxide. Mullite is used as a bond coat because of its

4
excellentthermalshockresistanceandchemicalcompatibility.YSZ wasappliedonto new mullite

bond coat by conventionalplasmaspraying.Mullite(5 mil)/YSZ(5 mil)-coatedSiC andSiC/SiC

compositeswere cycled between room temperatureand 1200/1300°Cwith 2hr cycle. No

macroscopiccrackingor spallationof coatingoccurredatler over 350 cycles. After 250 cycles at

1200°C, mullite/YSZ coating showed excellent thermal shock resistance and chemical

compatibility (similar to the mullite coating/SiC system). Only limited reaction was observed at

the mullite/YSZ interface after 250 cycles at 1300°C. However, substantial cracking occurred

around the mullite/YSZ interface after 250 cycles at 1300"C. This is likely due to the large CTE

mismatch between mullite and YSZ (5.6 x 10 "_ °C "_ for mullite vs 10-12 x 10 -6 °C'_ for YSZ) and

%

the resultant thermal stress. Phase transformation of YSZ from tetragonal (as-sprayed) to cubic

and monoclinic (13000C in long exposure) may be another contributor to the cracking. YSZ stays

as tetragonal at 1200°C even in long exposure. Cracking becomes more severe with a thicker

YSZ---5 rail mullite/15 mil YSZ coating showed a massive cracking only after 20 hr at 1260°C in

a high pressure burner rig.

SiC or SiC/SiC composites exhibit paralinear oxidation when exposed to reducing gases

or water vapor due to the evaporation of silica by SiO(g) or Si(OI-I)4, respectively. Similar

behavior is expected for mullite-coated SiC. When mullite(5 mil)/YSZ(5 mil) coating was applied

on SiC/SiC composites, the weight loss portion from the paralinear oxidation was eliminated in"

50% I-I20-50°/o 02 at 1300°C. This demonstrated that mullite/YSZ coating is a promising system

to protect SiC from evaporation in reducing gases or water vapor.

A. 3. Development of Mullite amt Mullite-Based Coatings for Si_N_ Ceramics: Because of

industries' current need to protect Si3N4 ceramics from molten salt environments, muUite coating

technique developed for SiC ceramics has been applied to Si3N4 ceramics. Two major technical

barriers have been identified. First, there is a significant CTE mismatch between Mullite and Si3N4

ceramics (5.6 x 10"6 °C "l for mullite vs 3-4 x 10"6 *C"_ for Si3N4 ceramics). Second, it is very

difficult to roughen the surface of Si3N4 ceramics, which is essential to achieve a good mechanical

bonding at the coating/substrate interface.

B. Characterization of Cyclic. Oxidation Bchavi0r of Superalloys and Superalloy-Matrix

Composites for EPM Nozzle Backstructure
Cyclic oxidation study of MA956 and sapphire/MA956 composites has been completed.

Distinctively different cracking behavior was observed in monolithic MA956 and composite.

Oxidation lifetime was projected based on the oxidation kinetics data. Stress analysis of the

system using finite element analysis explained the different cracking behavior.

(_. Characterization ofTherrna, l Protection Systems for EPM .Nozzle Thermal Blanket_

Environmental durability and thermal properties of candidate thermal protection systems

for ELM nozzle thermal blankets have been tested. Test includes thermal aging at high

temperatures in furnace and subsequent exposure to burner rig.

%

PUBLICATIONS

1. K,N. Lee, R.A. Miller, and N.S. Jacobsen, "Development of Thermal Shock Resistant Mullite

Coatings on Silicon Carbide," Ceramic Transactions Vol 38, 565-575 (1994).

2. K.N. Lee and 1LA. Miller, "Long-Term Durability of Mullite-Coated Silicon-Based

Ceramics," Ceramic Engineering & Science Proceedings Vol.A, 547-554, July-August, 1994.

3. K.N. Lee, N.S. Jacobsen, and R.A. Miller, "Refractory Oxide Coatings on SiC Ceramics,"

MRS Bulletin, 14110] 35-38 (1994).

4. K.N. Lee, N.S. Jacobsen, and R.A. Miller, "Refractory Oxide Coatings on SiC Ceramics,"

NASA TM 106677, 1994.

5. K.N. Lee and W.L. Worrell, "High-Temperature Oxidation Behavior of Iridium-Based

Alumina-Forming Ternary Intermetallics," Oxid. Met. Vol 41, 37-63 (1994).

6. K.N. Lee and R.A. Miller, "Refractory Oxide Coatings on Silicon-Based Ceramics," HITEMP

Review 1994, 62/1-62/9 (1994).

7. K.N. Lee, V.K. Arya, G.R. Halford, and C.A. Barrett, "Kinetics of Cyclic Oxidation and

Cracking and Finite Element Analysis of MA956 and Sapphire/MA956 Composite System

(PART I-EXPERIMENTAL)," HITEMP Review 1994, 51/1-51/10 (1994).

8. K.N. Lee, V.K. Arya, G.R. Halford, and C.A. Barrett, "Kinetics of Cyclic Oxidation and

Cracking and Finite Element Analysis of MA956 and Sapphire/MA956 Composite System

(PART II-ANALYTICAL)," HITEMP Review 1994, 52/1-52/12 (1994).

9. K.N. Lee, R.A. Miller, and N.S. Jacobsen, "New Generation of Plasma-Sprayed Mullite

Coatings on SIC," (J. Am. Ceram. See.)

10. K.N. Lee and N.S. Jacobsen, "Chemical Stability of the Fiber Coating/Matrix Interface in

Silicon-Based Ceramic Matrix Composites," (J. Am. Ceram. See.)

11. K.N. Lee, V.K. Arya, G.R. Halford, and C.A. Barrett, "Kinetics of Cyclic Oxidation and

Cracking and Finite Element Analysis of MA956 and Sapphire/MA956 Composite System,".

12. K.N. Lee and R.A. Miller, "Durability of Mullite Coatings On SiC And SiC/SiC Composites

under Thermal Cycling Between Room Temperature and 1200-1400°C," (J. of Mat. Sci.)

13. K.N. Lee, R.A. Miller, N. S. Jacobsen, and E.J. Opila, "Environmental Durability of Mullite

Coating/SiC and Mullite-YSZ Coating/SiC Systems," (1995 Cocoa Beach Meeting)

6
14.N.S. Jacobson,K.N. Lee,andT. Yoshio,"Corrosion of Mullite by Molten Salts."

PRESENTATIONS

1. 18th Annual Conference of Composites and Advanced Ceramics, January 9-12, 1994, Cocoa

Beach, FL.

2. Aeromat'94 Conference, June 6-9, 1994, Anaheim, CA.

3. Hitemp Review 1994, October 24-26, 1994, Westalke, OH.

PATENT DISCLOSURES PROPOSED

1. CVD SiC/Plasma-Sprayed MulliteDual Layer Coating Systems for Silicon-Nitride Ceramics.

2. Mullite/Glass Ceramic/Mullite/Yttria-Stabilized Zirconia Multilayer Coating System for

Silicon-Based Ceramics.

NASA GRANTEE SUBCONTRACTOR

NEW TECHNOLOGY SUMMARY REPORT

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Report is submitted immediately following the grant's technical

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Subcontractor Name _ • N " '7"E t,,J 7q,'Q I

and Address C t.._- w E LP_ _ i'>

"l

bFpr

Report Submitted by:

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S it-tCO/v' .. _ tqSE- C E tLrq/3"_j6 _

cN_LL_. , 3. 2,5 ,_-

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Today's Date: # / 17- /. g 7-

New technology may be either patentable or non-patentable.

Large business subconlracmrs mus! disclose PATENTABLE and NON-PATENTABLE items O.e., any invention or discovery

conceived or first reduced to practice during the performance of the or subcontract) as they are discovered.

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o,,v

PLEASE COMPLETE THE REVERSE SIDE OF THIS FORM AND MAIL TO THE FOLLOWING ADDRESS:

NASA Lewis Research Center

Attn: Kathy Kerrigan

Commercial Technology Office; Mail Stop 7-3

Cleveland, Ohio 44135

NASA C-3044 (3-97)
RESEARCH

CORPORATION

TECHNOLOGIES

RECEIVED

OFFICE OF RESEARCH SERVICES

JUL 1 1994

LCLEVELAND STATE UNIVERSITY

Joseph G. Stumpf

Diter.lor. PhysiCal Sc*ence$ Group

101 North _Vdmot Road. Suite bOO

Tucson. A[izona 85711-3335

1602) 748-.4400

Fax (6021 748-0025

June 28,1994

Mr. Charles Urbancic

Interim Director

Office of Research Services

Cleveland State University

1983 East 24th Street

Cleveland, OH 44115

Re: RCT Disclosure No. 198A-B323-94

Plasma-Sprayed Mullite/Glass Ceramic Dual Layer Coating

for Silicon-Based Ceramics; K. N. Lee & R. A. Miller

RCT Disclosure No. 198A-B324-94

Plasma-Sprayed Mullite/Yttria-Stabilized Dual Layer Coating

for Silicon-Based Ceramics; K. N. Lee & R. A. Miller

Dear Mr. Urbancic:

This will confirm our recent telephone conversation.

We note that one of the co-inventors of the referenced disclosures is a US government

employee. This being the case, the US government has ownership rights to these inventions.

These rights could have a negative impact on Research Corporation Technologies' (RCT) ability

to license these technologies.

As a result, we are declining to proceed further with an evaluation.

If the US government is willing to relinquish its dghts or to assign its rights to Cleveland

State University, RCT would be pleased to proceed with a technical and commercial evaluation.

Thank you for your interest in RCT. Please contact me if you have any questions.

c: B. Weinberg

JGS:dm

Internet: rctecl-_cc_vx I .ccit.arizona.edu
National Aeronautics and

S pace Ad ministration

Lewis Research Center

Cleveland, Ohio

44135

ReDly to Attn of

July 7, 1994

Attn: Mr. Charls Urbancic

Research Services

Cleveland State University

Cleveland, OH 44115

Dear Mr. Urbancic:

"Enclosed is my invention disclosure entitled, "Mullite/Glass

Ceramic/Mullite/Yttria-Stabilized Zirconia Multilayer Coating

System for Silicon-Based Ceramics."

I strongly recommend that Cleveland State University assigns

the right of the invention to NASA-Lewis Research Center because

NASA-Lewis has a great interest in this invention and also has the

facility and expertise to fully develop it.

Considering the great technological implication of this

invention, immediate processing of the patent application will be

very much appreciated.

Sincerely,

Kang N. Lee

RECEIVED

OFFICEOFRESEARCHSERVICES

JUL - 8 1994
RESEARCH CORPORATION TECHNOLOGIES

Invention Disclosure Form

I. Description

Please provide a title for your invention and a brief description.Inventionsinclude new processes, products,apparatus, compos_ons of

matter, living organisms--OR improvements to (or new uses for) things that already exist. Use additionalsheets and attach descriptive

materials to expand answers to questions.(Sketches, drawings,photos,reportsand manuscripts will be helpful.)

Mullite/Glass Ceramic/Mullite/Yttria-Stabilized Zirconia

InventionTale Mult_]ayer Coat/n_ System for Silicon-Based Ceramics

B. Description A New multilayer coatinK system of mullite/glass ceramic/mullite/yttria-stabilized

zirconia on silicon-based ceramics by plasma spraying, which provides excellent

thermal as well as environmental protection in severe envirorments containing

%

molten salts 2 water vapor and/or reducing atmosphere.

C. What aretheimmediateand/orh._ureapp_cationsoftheinvention?

Potential i,mediate applications include thermal/chemical barrier coatings for combustors

for advanced gas turbine engines and protective coatings for heat exchangers and advanced inter

combustion engines.

D. Why is the inventionbetter--more advantageous--than presenttechnology?What are its novel and unusualfeatures?What problems

does it solve?

Provide a complete protection of silicon-based ceramics in molten salts, water vapor, and/or

reducin_ atmospheres. C0mbin_ the benefits of mullite, glass ceramic and zircon/a coatings.

E. Is work on the inventioncontinuing?Acethere limitationsto be overcome orothertasks to be done priortopracticalapplication?Acethere

any test data?

No limitations to be overcome prior to practical application.

Test data available to confirm its durability and adherence.

Work will continue to further optimize the process.

F. Have prcx:lucts,apparatusor compositions,etc. actually been madeand tested? .....

The coating system was processed and tested (Figs la and ib).

II. Publications, Public Use and Sale

Note: valid patent protectiondepends on accurate answere to the followingitems.

A. Has invention been disclosedin an absb'act,paper, talk, news storyor a thesis?

Type of disclosure

(Please encdosea copy)

DisclosureDate
._-m ro

.'-.__

•_.,_ +

l- e.. _

_ m

m ID _

_ _,-_

_,.___

e_m
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NEW TECHNOLOGY .REPORT

NASA requireseach researchgrantee,researchcontractorand resemrchsubcontractorto reportnew technology to the

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Titleof Re_x)rt

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Timetable

The granteesubcontractordiscloseseach discovery of new teclmology

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SubcontractorName

and Address:

Cleveland State University

1960 E. 24th .%t., Cleve±ana, un 44i15

Report Submitted by:.

Telephone Number:.

NASA Grant "l_fle:

Kang N. Lee

NASA Grant Number:.

NASA Grant Monitor:.

NCC 3. 285

Subcontract Completion Date: I /

Today's Date: _.....7_I...6._I....1..q94

%

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A arab.led lnvezULIonu Is amy hlvesdou or discover, ,0v_|¢ln is or may be m reusable, dtat was contrived or lust .cmdly _luc,ut m practl= dnn-Xns ,ue

pcr_ormau_ of tl_ eOUalCI ca" n11_.OUlmlCt, SmaU busht,-,_ coutraclors and sub¢oulz'lcloxs mLSt, at • miLLmum, dbr.Jo_ subje¢l h, vudoas as they m'e

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ATTN: KAREN GRASSE.

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CLEVELAND, 0HI0 44135
I General Information

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Internal Docket Patent Patentable

e . " Aool. Filed . Item

Mullite_ss Ceramlc/_/Yt tria-S _aDl£1 zed

I. Zirconia Multilayer Coating Syst_______ for Silicon-Based Ce_cs ( )

2. () () () -

3. _ () () ()

4. _ () () ()

Nonpatentable

I,cm

tit Second and Third Tier Subcontractors

Pleasecompletethefollowingscaionlistingallresearchsubcontractorsparticipating to date.Includeeachsubcontractor's

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Use of the forms identified or_the reverse side of this page is optional; however, an alternate must at

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New Technology Report Form ApproY~d 

O»'.B. No. 2700·0009 

INS'7RUCTIONS 

This form mlY be used wh~n reporting inventions. discoy· 

eries. improvements or innovations to NASA. Use of this 

repOrt form is option.l; provided, however, that whitever 

repOrt form It is used cont.in the essenti.1 informltion requested 

nerein. 

In completing elch section. use ~tever detail deemed 

appropriate for a "full and complete disclosure," as required by 

tnt New Technology or Property RightS in Inventions Cl.use. For 

further guid.nce IS to whit constitutes I satisfactory reDOrt. 

pi use refer to NHB 2170.3. Documentltion Guidelines for 

New Technology Reporting. 

Available additional documentation which provides a full, de· 

tailed description should be attached. as well as any additional 

expl.natory sheets where neceu.ry. 

1. TITLE . C . S Mullite/Glass Ceramic/Mullite/Yttria-Stabilized Zirconia Multilayer oat1ng ystem 

for Silicon-Based Ceramics 

2. INNO"ATO"ISI (1V~~,"dSDc;"S«u'irylVo.l· 

Kang N. 'Lee*; ' 

Robert· A. Miller*-IL 

~. EM" LOVE" (Orpfliutio" ."d tli.,;uo"J 

"* Dept. of Chern. Eng., CSU 

** Materials Division, NASA-Lewis 

4. A:lO"ESS {Plat:. of hrlo"".nc.J *, **. NASA-Lewis, 21000 Brook Park Rd. 

Cleveland, OH 44135 

SECTION 1- DESCRIPTION OF THE PROBLEM THAT MOTIVATED THE TECHNOLOGY DEVELOPMENT (£",.,A-G __ ,.I 

o.,c,jprion of !+obI.", ObiKri".: 6.-1( ... 0' UrJiqu.!'robI.", Ot.rx,.,inc: c.~r H4fOfY,p,io, TKhfliQu.: D.-Limiuriolll.' /'';0' TcII"itlllftl 

AlB. ~nrraI Drscripdon orproblrm ObjectivclCharactrristic:s 

Silicon·hued ceramics ICC leading candidate materials Cor high tcrnper2IUrc SIIU~ra1 app1ications such as heat exc:hangcn, gas turbine engines and 

advanced intcmal combustion engines. They have excellent oxidation resistance in clean oxidizing enviroMlents due to the formation oCprotcctive silica 

scale (SiOJ. Howeva-, dur.lbility in high temperalUrc enviroMlents containing molten salts, high level oCwaler vapor, and/or a reducing aImOSphcre can 

limit cheir applications. Molten salts react wich silica to Conn liquid silicaICS which no longer ICC protective. High levels oCwater vapor and a reducing 

atmosphere lead to che evaporation of silica scale via Si(OH).(g) and Sio(g), rc:sp«tively. One approach to overcome these potential environmental 

limitations is to apply barrier ClOII1ings which are environmenlally stable in chese severe ea\"iroMlents. These CG;ltinp can serve as thmnal barrier coatings 

as well This document discloses a concept of and a process Cor a new multilayer <:eating system ofmullitelglass c:cramiclmullitclyaria-stabillz.ed zirconia 00 silicon-based c:er.unics by plasma spraying. The new coating system provides silicon-based ceramics wich excellent environmental resistance in molten 

salts, water vapor. andlor reducing atmosphere. 

C. Prior Art ". 

Previously, we developed a new plasma-spraying process to apply a fully-gystalline mullite coating on silicon-based ceramics (U.S. paIent applicatioo 

#08/031,444). This new fully-crystalline mullite coating exhibited dramatically improved adI!en:nce and thermal shock resUtance compaRd to 

tlOOwntionally plasma-spn.yed mullitc coatings. The aew mullitc coatioc provida some proccctioa against molten salt corTOIioa. ~ in acidic: aaItI. 

We also disclosed inventions oC mullitclgJass cmmic (12·20-93) and mulljtelzjrsonia (2-8-94) cIuallayer coatings Cor silic:on-based CCPIIlic:s. The glaa 

c:cnmic in the mJUitclglass c:eramic dual layer coating stops the propaptioo oCthrougb-thidtncss c:nda in mullite, preventing the pendnlioo oC molten 

salts. Zirconia in the mullitclzirconia cIuallayer coating provides proc.ec;tioo against vaporization in high water vapor and reducing atmosphere because 

it has very low volatility in those environments. The zirconia layer also deflects some througb-thidtncss c:nda originating from the mullile layer. 

D. Umitations or Prior Art 

The prior art does DOt provide CICI11'Idc proledion in cnviroaments c:oataiDing molten salts, IUgh water vapor. and/or reducing aImosphere. ~ coatings 

~Iop through-thic:kness aadcs on repealed c:ycling. Mohcn salts em penetrate throup the c:racb and altadc the silic:on-bucd substnle in Ioag tam 

exposures. M!illi!! abo bas a fairly high silica &CIivity of about 0.4. Tbcn{ore tbe wporizatioo of silica ia waIcr" wpor or reducing umo.pbcra ia IliIl 

sipUfic:m enoup to limit its usefulness Cor Joac tam applicatioas. The rim cmmjc GUIer laya-in tbe muJlitc/gius c:cramic dual layer -'ina bu too 

high of a silica aaivityto ~ tbe silica nraporatiaa in hip water VIIpOC' IDd reducinc a!mOSpbere. The ~ layer in tbe IIIIIDiteIzircoaia dual layer 

coating does not resolve tbe issue oClbc througb-thickDaa c:ndcs in tbclllUUite laya-. wbida provides a path for molten salt peuetratiaa. 

• Supplying this information is voluntary, in .c.cordance with Public Uw 83·579 (Privacy Act of 19741. Howw.r. it is reQuired for 

eligibilitY in the establishment of Space Act monetary aWlrek.. . . 

NASA 'DRM iliA A~" IS ~"IVIOuS IOITION IS OISOLITI. 
SECTION II -- TECHNICALLY COMPLETE AND EASILY UNDERSTANDABLE DESCRIPTION OF NEW TECHNOLOGY THAI

WAS DEVE LOPED TO SOLVE THE PROBLEM OR MEET THE OBJECTIVE (Lr-rsrsssot-'oo,'*_ _L-.r_*cil/c _r_t,on olitwn;

_. --Stare Of d(rv_o_,_r- _ --4_qt3lrioll 88 • un#t; O. --FuttctioJl4_ oOefution/ E. --_pOoftiv# _orf ; F. --En_n4_t_l_g gJ_qC;f_r.Bt_ottJr; G. --_#_O_

muiomenr..H.-Orswi_. 9",Dhl. t_:.- I.--_r= ot ir_Wrfd;fn=,li•_-8nd J..-4_;,r_4i,_ce. _;sl_'lint. Mfer_ tmcronl

Identification of Steps

Since niullite coatings do not provide protection against vaporization in high water vapor and reducing

atmospheres, refractory oxides with low silica activity or refractory oxides containing no silica are needed in those

environments. Yttria-stabilized zirconia has a much lower vapor pressure than silica or mullite, and is not known

to form any hydroxide in water vapor. In addition, _rttria-stabilhed zirconia has lower thermal conductivity than

muUite, making it a more effective thermal barrier coating. However, yttria-stab;l;Ted zlrconia sprayed directly onto

silicon-based ceramics debonds on thermal cycling, presumably due to chemical incompatHibity between zirconla

and the silica scale which eventually forms on sUicon-based ceramics. The detailed nature of the chemical

incompatibility is not known yet, although sintering ofzirconia by silica and reaction between yttria and silica have

been suggested. Therefore, a chemically compatible intermediate layer such as mullite is needed. We have found

that a mullite/yttria-stahilized zlrconia dual layer coating exhibits excellent thermal shock resistance and chemical

compatibility.

Another issue with mulfite coatings is the penetration of salt via through-thickness cracks in long term exposures.

Yttria-stabillzed zirconia overlay coating on mullite does not resolve this issue. We have discovered that glass

ceramic overlay coatings on mullite are very effective in stopping the propagation of through-thickness cracks in

mullite layer. Like mullite, however, the glass ceramic does not provide protection against vaporization in high

water vapor and reducing atmosphere.

Therefore, the combination of glass ceramic and yttria-stabilized zirconia overlay coatings can potentially resolve

all the environmental durability issues mentioned above. In this coating concept, the mullite layer provides strong

bonding and chemical compatibili_', the glass ceramic layer provides crack arrest, and the yttria-stabUized

zirconia provides protection against vaporization. When the glass ceramic has a relatively high silica activity, such

as cordierite used in thls study, another mullite layer is needed between the glass ceramic and the yttria-stabUized

zlrconla to prevent chemical reaction bet, m'een the two layers. Combination of all these concepts results in

mullite/glass eeramic/mullite/vttrla-stabilized zirconia multUayer coatings. The second mullite layer is not necessary

if the glass ceramic is chemically compatible with the .vttria-stabilized zirconia.

Mullite, glass ceramic, and yttria-stabilized zirconla powders purchased from commercial vendors were plasmasprayed onto SiC x (2.54cm • 0.45cm • 0.2cm) and SiC/SIC composite 2 (2.54cm • lcm • 0.3cm). Mullite and glass

ceramic were sprayed with our new technique (U.S. patent application #08/031,444 and our invention disclosure

of mulllte/glass ceramic dual layer coating filed on 12-20-93) to obtain fully-crystalline phase. The principle of the

new technique is heating of the substrate before and during the spraying process to maintain the substrate

temperature above the crystallization temperature ofmulllte and glass ceramic (about 1000*C). It is very critical

to maintain a uniform substrate temperature because local cold spots below the crystallization temperature or local

hot spots can cause the cracking of substrate. Yttria-st_biliTed zlrconia was sprayed with conventional techniques.

Powder particle size was between 100 and 200 mesh; plasma gun power was 35-45kw; plasma are gas was Ar-40%

He; and powder carrier gas was argon with flow rates between 2 and 6 SLPM (standard liter per minute). SiC

coupons were etched in molten sodium carbonate at 860-900"C for 8-10 hours to produce a surface sdltable for good

adherence (roughness Ra = 150-250 microinch). SiC/SiC composite coupons were used as received, for

they already have appropriate surface roughness.

Coated specimens were cycled between 1300"C and room temperature with two-hour cycle to test the adherence,

crack resistance, and chemical compatibility. For each cycle, the coated coupon was inserted into a preheated

furnace (1300"C) In air, held in the furnace for two hours and then ah--quenched to room temperature. After fifty

cycles, metaHographic examination revealed that the entire coating was still well bonded without any evidence of

chemical reaction between each layer, and glass ceramic layer stopped the propagation of cracks (Figs.Is and lb).

*Hexoloy TM, Carborundum,

2Dupont, Newark, DE.

°

Niagara Falls, NY.
The above results clearly demonstrate that the mulllte/glass ceramic/mulllte/yttria-stablllTed zirconla multHayer

coating system can effectively serve as an excellent environmental/thermal barrier coating for silicon-based

ceramics in severe environments containing molten salt, water vapor, and/or reducing atmosphere.

Alternate Embodiments

l_he substrate may be any other s$con-based ceramlc/metal such as SigN4or MoSi2, composites based on SiC, SI_N4

or MoSi_, or any other ceramics, metals, or metal alloys. Additional intermediate layers such as microcomposites

ofmullite/yttria-stabilized zirconla or a composttionally gradient coating b envisioned to reduce the CTE mismatch

between each layer. Spray parameters other than those described in this disclosure may be used.

SECTION III -- UNIQUE OR NOVEL FEATURES OF THE TECHNOLOGY AND THE RESULTS (OR BENEFITS) OF ITS APPLICATION (Enmr_ a_mOrimreA.-No,,elorun;quetar_m- R.--De_l_n_ _ co_m_ pmbi_. ___m_ ¢__. mf _m,.

D.--,An41_r;sof cal_b,7;r/_:E.-Sotteceof #trot;RndF.-AdvJnw_/thormomin_]

Advantage Over Prior Arts

Prior arts have drawbacks in the following areas: Mullite coating does not provide protection against vaporization

in high water vapor and reducing atmosphere, and through-thickness cracks in mullite coating causes penetration

by molten salt_; MuHite/glass ceramic dual layer coating does not provide protection against vaporization in high

water vapor and reducing atmosphere; Mullite/_tria-stabiUzed zireonia dual layer coating does not resolve the

issue of through-thickness cracks. In comparison, however, the new Mullite/glass ceramic/muilite/yttria-stabillzed

zirconia multilayer coating system provides complete protection in severe environments containing molten salts,

high water vapor, and/or reducing atmosphere.

Unique Features of the Technology

The concept of multilayer mullite/glass ceramic/mullite/yttria-stabilized ztrconia coating to combine the merits of

each layer:,strong bonding and chemical compatibility from.mullite, crack arrest from glass ceramic, and resistance

to vaporization in water vapor and reducing atmosphere from yttria-stabilized zirconla.

Benefits of Application "

Substantially extend lifetime of silicon-based ceramics In high temperature structural applications by providing

excellent thermal as weU as environmental protection in severe environments containing molten salts, water vapor,

and/or reducing atmosphere.
SECTION III ICon.)

SECTION IV -- ADDITIONAL DOCUMENTATION i'lnc/uov or I_f _low _!, pe_nenr 6c_.,men==rio,, w_h ,im in ;n, u,d'e'rmndiW or ,_lic

I_(m cf me new mehnotclw. IF NOT TOO IULICY OR DIFFICULT TO REPRODUCE. INCLUDE COPIES WITH THIS REPORT. For m_ eel•fences

Or _it_c_ml d_'um_nmbon #_ilmble but NOT incluclecf in _ _ [¢PuetO mar _ing _na_ent_/ m • _ unC_nmnding @f =_ new _=_noic_t, _L

• mP.h nwr lie cewtty m nm_ee st tw_e) ¢emwrre lwm A. below)

1. PAP|RS. ARTICLllS , , X 4. ASSEM|LV/MFG. ORAWINQS _" I?. TEE _T DATA

A. AVAILAELE 2. CONTRACTOR REPORTS S. PARTSON INGRE0 LIST |. ASSEMEL.Y/MPG. PIIOCED.

OOCUMENTS X 3. ENGINEERINO SPECS. • OPERATING MANUAI._ •. COMPUTER TAPES/CARDS l#_,k_k *--J *

tom/IS•Ill') =10. OTHER _8_¢/P/'J

Noteboo'k oF_ 1_n_ R. Lee and George LeissleJ: (l:X]asma Ted'•d clan)

• . INDICATE THE DATES OR THE APPROXIMA'rE "riME PERIOD DURING WHICH THIS TECHNOI.OGY WAS OEYEt.OPED [Le. ¢¢mc_'m_.

¢¢,_eeucred. ramrod, eT_J

Conce±vo:l: 12-9-93

Con_let±on of f±rst moil1:4-5-94

First successful operational test: 5-16-94

C. LIST THE FIRST PU|t'_C'A'I"ION OR PuE;.IC DISCLOSURE OF THE NEW TECHNOLOGY. AND DATES

O. t.IST THE DATES AND ANY PARTICUI.ARI.Y PERTINENT PAGE NUMBERS OF OTHER PU|FDIGATIONS WHICH ARE AVAIL.A|t.E |UT N_

AT'rACHED

E. DEG RE E OF TECHNOI,.OGICAL SIGNIF ICIANCE /'_41¢k in yOKIr _fst _l(t_'l'sent Cite mfll'fft_t w_iclt _ •J¢_t'lf¢8_ me (I(_rlNI @t _nolo_..81

,ifn;r_c_nce of m;, tllchnoloIwJ

'. MOO,P,_LT,GN "rE ,X_T,N_ "rECHNO_mOGY rl_ =. ,UlrrAN'T,A_ ADVANCE r_ =L MJk.JO. ,REACT.ROUgH

I I L.J IN THE ART I...J

COMMENTS

0.,, "7"- - q "/

v

RESEARCH CORPORATION TECHNOLOGIES

Invention Disclosure Form

I. Description

Please provide a title for your invention and a brief description. Inventions include new processes, products, apparatus, compositions of

tnatter, living organisms--OR improvements to (or new uses for) things that already exist. Use additional sheets and attach descriptive

materials to expand answers to questions. (Sketches, drawings, photos, reports and manuscripts will be helpful.)

CVD SiC/Plasma-Sprayed Mullite Dual Layer Coating System for Silicon Nitride

Invention Title Ceramics

3. Description A new multilayer coatin_ system of CVD SiC/mullite on silicon nitride ceramics,

which will provide thermal as well as environmental protection in severe

environments containing molten salts, water vapor, and/or reducing gases.

• Whataretheimmediate and_rfutureapplications oftheinvention?

Potential applications include thermalJenvironmental barrier coatings for advanced gas

turbine engines, advanced internal combustion enRines, or heat exchangers.

Whyistheinvention be_er--moreadvantageous--thanpresenttechnology? Whatareitsnovelandunusualfeatures?Whatproblems

:,esitsolve?

i:_v'DSiC bond coat provides substantially improved bonding of mullite coating compared to

i:he mullite coating alone on silicon nitride ceramics. It also eliminates the extensive

i:oughenin_ process on silicon nitiride ceralnics prior to the mullite application.

Is wo_ontheinventioncon_nuing? _etherelimitationstobeovercomeorother_s_tobedonepdortopra_icalapplication?Are there

_'testdata?

i_o limitations to be overcome prior to the practical applications

est data available to confirm its suprior adherence and resistance to thermal cycling

iork will continue to further optimize the process

t_aveprcxlu_s, appa_tus or composi_ons, etc. actually been made andtes_d? ....

_e coating system was processed and tested (Fig. i)

II. Publications, Public Use and Sale

_.:valid patent protection depends on accurate answers to the following items.

_as invention been disclosed in an abstract, paper, talk, news story or a thesis?

of disclosure

(Please enclose a copy)

Disclosure Date
II. (Publications,PublicUseandSale--Continued)

B. isa publicationorotherdisclosureplannedinthenextsixmonths?

Typeofdisclosure

(Enclosedrafts,abstracts, prepdnts)

C. Hastherebeenanypublicuseorsaleofproductsembodying, theinvention?

Describe,givingdates

Date

D. Areyouawareofrelateddevelopments byothers?If"yes," please give citations. Copies of any relevant patents or publications would

be appreciated.

IlL Sponsorship

If the research that led to the invention was sponsored, please fillinthe details and attach a copy of the contract or agreement if possible.

A. Government agency _IASA-Lewis Research Center contract/grant no. NCC-3-285

B. Name of industry, university, foundation or other sponsor:

C. Has the invention been disclosed to industry representatives? If "yes," please provide details, including the names of companies and

their representatives.

No

IV. For Our Records

A. Names and titles of inventors (please print; sign where indicated) i<___ p_)

1. Kang N. Lee* Signature Date

2. Robert A. Miller** Signature Date

3. Lisa C. Veitch *** Sign_atbrem, Date

%

216

B. Contact for more data Kang N. Lee Tel. ( )

* 1835 Coe's Post Run, Westlake, OH 44145

C. Mailing address for inventor(s)

............. ** 9224 Highland Dr. Bred_svillev OH 4414-I

*** 18 Stony Brook Ln, Wakeman, OH 44889

D. Name and title of institutional representative (please sign where indicated)

433-5634

Signature Date

Department Tel. ( )

Mailing address

RESEARCH CORPORATION TECHNOLOGIES

6840 East Broadway Boulevard

Tucson, Arizona 85710-2815

Telephone 602/296-6400
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Nabonal Aeronautics and

Space Administration

Lewis Research Center

Cleveland, Ohio

44135

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March 27, 1995

Reply to Attn of

Attn: Mr Charls Urbancic

Research Services

Cleveland State University

Cleveland, OH 44115

Dear Mr. Urbancic:

Enclosed is an invention disclosure entitled, "CVD SiC/PlasmaSprayed Mullite Dual Layer Coating System for Silicon Nitride

Ceramics".

I recommend that Cleveland State University assigns the right

of the invention to NASA-Lewis Research Center based on the fact

that this invention is of great interest to NASA-Lewis and NASALewis has the facility and expertise to optimize the process,

fabricate model parts, and perform critical tests, leading to the

practical application.

Considering the technological implication of this invention

and the need by NASA-Lewis, immediate attention on this matter will

be greatly appreciated.

Sincerel_l

Kang N. Lee

tl- ...... z_..

f,_ _I c, ,, .- '/5