afwal-tr-83-4039 rain erosion mechanisms … · ment operation, the united states ... and sem...

AFWAL-TR-83-4039

RAIN EROSION MECHANISMS OFFLUOROELASTOMER AND POLYURETHANECOATINGS ON SELECTED COMPOSITESUBSTRATES

Charles J. HurleyUniversity of Dayton Research Institute

Joseph ZahaviIsrael Institute of Metals, Technion

George F. SchmittAir Force Wright Aeronautical Laboratories

An APRIL 1983

S Special Interim Report for the Period July 1980 - February 1983

TI

Approved for public release; distribution unlimited

MATERIALS LABORATORYSAIR FORCE WRIGHT AERONAUTICAL LABORATORIES DTIQ

AIR FORCE SYSTEMS COMMANDWRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433 E L T

A

NOTICE

When Government drawings, specifications, or other data are used for any

purpose other than in connection with a definitely related Government procure-ment operation, the United States Government thereby incurs no responsibilitynor any obligation whatsoever; and the fact that the Government may have

formulated, furnished, or in any way supplied the said drawings, specifica-tions, or other data, is not to be regarded by implication or otherwise as inany manner licensing the holder or any other person or corporation, or convey-ing any rights or permission to manufacture, use, or sell any patented inven-tion that may in any way be related thereto.

This report has been reviewed by the Office of Public Affairs (ASD/PA)and is releasable to the National Technical Information Service (NTIS). AtNTIS, it will be available to the general public, including foreign nations.

This technical report has been reviewed and is approved for publication.

G. F. SCHMITT W. C. KESSLER, ChiefProject Engineer Coatings & Thermal Protective

& Materials Branch

FOR THE COMMANDER

f. D. CHERRY, ChefNonmetallic Materials Division

"If your address has changed, if you wish to be removed from our mailing list,or if the addressee is no longer employed by your organization, please notifyAFWAL/MLBE, WPAFB, OH 45433 to help us maintain a current mailing list."

Copies of this report should not be returned unlecs return is required bysecurity consiCerations, contractual obligations, or notice on a specificdocument.

UNCIASSIFIED%rjCURITY CLASSIFICATION OF THIS PAGE (*7mVi, Datgnfered),

PAGE. READ INSTRUCTIONSREPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM

1. A'loolk NUM .... Z. OOVT ACCESSION NO. S. RECIPIENT'S CATALOG NUMBER

AFWAL-TR-83-4039 _Vý.,4/ý F 'v '714, TITLI (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

RAIN EROSION MECHANISMS OF FLUOROELASTOMER AND Special Interim Report

POLYURETHANE COATINGS ON SELECTED COMPOSITE July 1980 - February 1983SUBSTRATES 6. PERFORMING O1G. REPORT NUMBER

1. AUTHOR(.) S. CONTRACT OR GRANT NUMBER(*)

Charles J. Hurley, Joseph Zahavi, and George F33615-81-C-5030F. Schmitt F33615-82-C-5039

F33615-79-C-5129'. PERFORMING ORGANI.ATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK

PERFRMIN ORGNIZAION AMEAREA & WORK UNIT NUMBERS

University of Dayton Research Institute

300 College Park Avenue 24220101Dayton, OH 45469

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

Materials Laboratory (AFWAL/MLBE) April 1983Air Force Systems Command 13. NUMBER OF PAGES

Wright-Patterson AFB, OH 45433 76

14. MONITORING AGENCY NAME & ADDRESS(If dillerent from Controlling Office) 15. SECURITY CLASS. (of this report)

Unclassified

ISa. DECLASSIFICATION/DOWNGRADINGSCHEDULE

I&, DISTRIBUTION STATEMENT (of this Report)

Approved for public release; distribution unlimited

17. DISTRIBUTION STATEMENT (of the abstract entered In Block 20, If dlfferent from Report)

16. SUPPLEMENTARY NOTES

Prepared in cooperation with:Israel Institute of Metals, Technion, Haifa (Israel) andMaterials Laboratory (AFWAL/MLBE), Wright-Patterson AFB, OH 45433.

19. KEY WORDS (Continue on reverse side If necoescry and Identify by block number)

Fluoroelastomer Quartz PolyimidePolyurethane Graphite EpoxyCoated Composites Rain Erosion Test and EvaluationGlass Epoxy

20. ABSTRACT (Contlnue on reverse side It necessary end Identify by block number)

-This report documents a test program which was conducted to evaluate therain erosion resistance of selected coated composite materials at 30# and 90O

impact angles in a 1 inch/hour rainfall at 500 mph. Tests included: weightloss, percent coating removal, and SEM examinations.q--

DD I FJo2 1473 EDITION oF I '4oV 6 Is OBSOLETE UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE (When Date ,'ntered)

PREFACE

The effort documented in this report was conducted as ajoint enterprise by the following persons: Charles J. Hurley,University of Dayton Research Institute, under Contract Nos.F33615-81-C-5030 and F33615-82-C-5039; Dr. Joseph Zahavi,Visiting Scientist, under Contract No. F33615-79-C-5129 withUniversal Technology Corporation; and George F. Schmitt, Air

\t-Force Wright Aeronautical Laboratories, Materials Laboratory,under Project No. 2422, Task No. 242201. The work describedherein was conducted during the period from July 1980 toFebruary 1983.

The authors wish to acknowledge the contributions ofTimothy Courney and James Gannon of the University of DaytonResearch Institute for performing the rain exposure tests.Sincere appreciation is extended to Robert Brodecki of SystemsResearch Laboratories for the scanning electron microscopyoperations and photography.

"IN

TABLE OF CONTENTS

Section Page

INTRODUCTION 1

1. BACKGROUND 1

2. OBJECTIVE

II EXPERIMENTAL PROCEDURE 2

1. MATERIALS 2

2. TEST SPECIMEN FABRICATION 2

3. TEST SPECIMEN PRETEST EXAMINATION 2

4. MACH 1.2 RAIN EROSION TEST APPARATUS 2

5. POST-TEST OBSERVATION AND EXAMINATION

PROCEDURES 5

a. Visual Observation 5

b. Target Mass Loss 5

c. Scanning Electron MicroscopyExamination

III TEST RESULTS 8

1. RAIN EROSION TEST RESULTS - VISUAL

EXAMINATION 8

a. Fluoroelastomer Coating - Glass Epoxy

Substrate - 90 0 Impact Angle 8

b. Fluoroelastomer Coating - Graphite

Epoxy Substrate - 900 Impact Angle 8

c. Fluoroelastomer Coating - Quartz

Polyimide Substrate - 900 Impact

Angle 8

d. Fluoroelastomer Coating - Glass Epoxy

Substrate - 300 Impact Angle 10

e. Fluoroelastomer Coating - Graphite


f. Fluoroelastomer Coating - Quartz

Polyimide Substrate - 300 Impact

Angle 10

g. Polyurethane Coating - Glass Epoxy


h. Polyurethane Coating - Graphite Epoxy


V

PRECEDING PAGE BLANK

TABLE OF CONTENTS (Continued)

Section Page_

III TEST RESULTS (Continued)

i. Polyurethane Coating - Quartz Poly-

imide Substrate - 900 Impact Angle 12

j. Polyurethane Coating - Glass EpoxySubstrate - 300 Impact Angle 12

k. Polyurethane Coating - Graphite


1. Polyurethane Coating - Quartz Poly-

imide Substrate - 30° Impact Angle 14

2. RAIN EROSION TEST RESULTS - TARGET MASS

LOSS 14

a. Fluoroelastomer Coating - Glass Epoxy


b. Fluoroelastomer Coating - Graphite


c. Fluoroelastomer Coating - Quartz

Polyimide Substrate - 90* Impact

Angle 16

d. Fluoroelastomer Coating - Glass Epoxy


e. Fluoroelastomer Coating - Graphite


f. Fluoroelastomer Coating - Quartz

Polyimide Substrate - 300 impactAngle is

g. Polyurethane Coating - Glass Epoxy


h. Polyurethane Coating - Graphite


i. Polyurethane Coating - Quartz Poly-


j. Polyurethane Coating - Glass Epoxy


k. Polyurethane Coating - Graphite


1. Polyurethane Coating - Quartz Poly-


vi

TABLE OF CONTENTS (continued)

Section Page

III TEST RESULTS (continued)

3. RAIN EROSION TEST RESULTS - SEMOBSERVATIONS 23

a. Fluoroelastomer Coating - Glass EpoxySubstrate - 900 Impact Angle 23

b. Fluoroelastomer Coating - GraphiteEpoxy Substrate - 900 Impact Angle 28

c. Fluoroelastomer Coating - QuartzPolyimide Substrate - 901 ImpactAngle 28

d. Fluoroelastomer Coating - Glass EpoxySubstrate - 300 Impact Angle 34

e. Fluoroelastomer Coating - GraphiteEpoxy Substrate - 300 Impact Angle 34

f. Fluoroelastomer Coating - QuartzPolyimide Substrate - 300 ImpactAngle 39

g. Polyurethane Coating - Glass EpoxySubstrate - 900 Impact Angle 39

h. Polyurethane Coating - GraphiteEpoxy Substrate - 90° Impact Angle 39

i. Polyurethane Coating - Quartz Poly-imide Substrate - 900 Impact Angle 44

j. Polyurethane Coating - Glass EpoxySubstrate - 300 Impact Angle 44

k. Polyurethane Coating - GraphiteEpoxy Substrate - 301 Impact Angle 50

1. Polyurethane Coating - Quartz Poly-imide Substrate - 300 Impact Angle 50

IV DISCUSSION 57

1. RAIN EROSION BEHAVIOR-FLUOROELASTOMER 57

a. Kinetics 57

b. Surface Characterization 57

c. Effect of Impact Angle 57

d. Effect of Exposure Time 58e. Effect of Composite Material 59

vii

TABLE OF CONTENTS (continued)

section Page

IV DISCUSSION (continued)

2. RAIN EROSION BEHAVIOR-POLYURETHANE 59

a. Kinetics 59

b. Surface Characterization 60

C. Effect of Impact Angle 60

d. Effect of Exposure Time 60

e. Effect of Composite Material 61

V CONCLUSIONS 63

VI REFERENCES 65

viii

LIST OF ILLUSTRATIONS

Figure Page

1 Rain Erosion Test Specimen for 90° Impact Angle 3

2 Rain Erosion Test Specimen for 300 Impact Angle 4

3 Mach 1.2 Rain Erosion Test Apparatus 6

4 Fluoroelastomer Coating - % Coating Removal vs.Exposure Time, 900 9

5 Fluoroelastomer Coating % Coating Removal vs.Exposure Time, 300 11

S6 Polyurethane Coating - % Coating Removal vs.Exposure Time, 90° 13

7 Polyurethane Coating - % Coating Removal vs.Exposure Time, 300 15

8 Target Mass Loss of Fluoroelastomer CoatedComposites Exposed at a 900 Impact Angle,500 mph, 1 inch/hr Rainfall 17

9 Target Mass Loss of Fluoroelastomer CoatedComposites Exposed at a 300 Impact Angle,500 mph, 1 inch/hr Rainfall 19

10 Target Mass Loss of Polyurethane CoatedComposites Exposed at a 900 Impact Angle,500 mph, 1 inch/hr Rainfall 21

11 Target Mass Loss of Polyurethane CoatedComposites Exposed at a 300 Impact Angle,500 mph, 1 inch/hr Rainfall 22

12 Fluoroelastomer Coated Glass Epoxy Composite 24

13 Fluoroelastomer Coated Glass Epoxy Composite,900 Impact Angle 25


15 Fluoroelastomer Coated Glass Epoxy Composite,

900 Impact Angle 27

16 Fluoroelastomer Coated Graphite Epoxy Composite 29

17 Fluoroelastomer Coated Graphite Epoxy Composite,900 Impact Angle 30

ix

LIST OF ILLUSTRATIONS (Continued)

Figure Page

18 Fluoroelastomer Coated Quartz PolyimideComposite 31

19 Fluoroelastomer Coated Quartz PolyimideComposite, 90° Impact Angle 32

20 Fluoroelastomer Coated Quartz PolyimideComposite, 90 0 Impact Angle 33

21 Fluoroelastomer Coated Quartz PolyimideComposite, 900 Impact Angle 35


23 Fluoroelastomer Coated Graphite EpoxyComposite, 300 Impact Angle 38

24 Fluoroelastomer Coated Quartz PolyimideComposite, 300 Impact Angle 40

25 Polyurethane Coated Glass Epoxy Composite 41

26 Polyurethane Coated Glass Epoxy Composite,900 Impact Angle 42

27 Polyurethane Coated Graphite Epoxy Composite,900 Impact Angle 45


29 Polyurethane Coated Quartz Polyimide Composite47

30 Polyurethane Coated Quartz Polyimide Composite,900 Impact Angle 48


32 Polyurethane Coated Glass Epoxy Composite,300 Impact Angle 51



x

LIST OF ILLUSTRATIONS (Concluded)

Figure Page



X.

SECTION IINTRODUCTION

1. BACKGROUND

For many years, rain droplet impingement erosion of polymeric

coatings and composite materials used in exterior aircraft struc-

tures have been a problem. (1'2,3'4) Aircraft and missile systems

operating in adverse weather environments at moderate to high

velocities were subject to progressive surface erosion or struc-

tural damage. Current fluoroelastomer and polyurethane coatings

for leading edges, radomes, antenna covers, and helicopter rotor

blades havt been relatively successful in protecting reinforced

plastic components from erosion by rain. Developments in improved

rain erosion resistance elastomeric coatings were characterized or

evaluated by rotating arm test apparatus and actual flight tests.

These tests provided a relative ranking of materials performance.

2. OBJECTIVE

The object of this program was to investigate the erosion

damage mechanisms involved with polymeric coatings on different

types of reinforced plastic composites. The polymeric coated

composites were to be evaluated for visible erosion damage, weight

loss after rainfield exposure and in-depth analysis of erosion

mechanisms utilizing electron microscopy techniques. The per-

formance of fluoroelastomer and polyurethane coatings on similar

reinforced plastic composites was to be assessed. One military

specification fluoroelastomer and polyurethane coating was to be

evaluated on glass epoxy, graphite epoxy, and quartz polyimide

substrates.

!1

SECTION II

EXPERIMENTAL PROCEDURE

1. MATERIALS

The materials for this investigation were supplied by the

Air Force Wright Aeronautical Laboratories. Materials were

furnished in flat sheet form and processed to be representative

of the coated substrates used in aircraft components. The MIL-C-83445A white polyurethane coating was spray-applied 14 mils thick

to 100 mil glass epoxy, 95 mil quartz polyimide, and 85 mil

graphite epoxy composite sheets. The AF-C-VBW-15-15 white fluoro-

elastomer coating was spray-applied 13 mils thick to similar com-

posite sheets.

2. TEST SPECIMEN FABRICATION

All test specimens were fabricated from the parent material

sheets. Great care was taken to insure that the coated surfaces

were not damaged or adversely affected by fabrication. The test

specimen configurations used for rain erosion evaluations are

shown in Figures 1 and 2.

3. TEST SPECIMEN PRETEST EXAMINATION

The as-received coated composite test specimens were num-

bered and logged according to the standard procedure required forthe rain erosion test apparatus. All specimens were visually

examined for defects or damage. Test specimens were dried over-

night in a forced air oven at 125 0 F, cooled to ambient tempera-

tures in a dessicator and then weighed. Pretest scanning elec-

tron micrographs were prepared on selected specimens from each

coated substrate group.

4. MACH 1.2 RAIN EROSION TEST APPARATUS

The rotating arm apparatus consists of an eight-foot diam-

eter double arm blade. It is designed to produce high tip

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Figure 1. Rain Ercsion T•est Specimen for 900 Impact Angle.

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Figure 2. Rain Erosion Test Specimen for 300 Impact Angie.

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velocities with negative lift and a low drag coefficient. Fated

test specimens are mounted at the leading edge tip sections of

the double rotating arm. The test specimens can be subjected to

variable speeds of 0 to 900 mph. The double arm is mounted hori-

zontally on a vertical drive shaft as shown in Figure 3. Simu-

lated rainfall is produced by four curved manifold quadrants.

Each manifold quadrant has 24 equally-spaced capillaries. Rain-

drop size and drop rate are controlled by the capillary orifice

diameter and the head pressure of the water supply. The manifold

quadrants are mounted above the tips of Lhe double rotating arm.

Raindrops from the simulation apparatus impact the test specimens

throughout their entire annular path. Rain droplets are 2.0 mm

diameter and generated at the rate of one inch/hour of simulated

rainfall. This apparatus is fully described in AFML-TR-70-240. (5)

For the purposes of this study, matched pairs of specimens

were inserted into the specimen holders at a 300 or 900 angle of

incidence to the rain droplet impact. All rain erosion testing

wF• conducted at 500 mph. Duration of the rainfield exposure

tests was variable depending upon the observed surface conditions

afte.r 5 and 10 minute intervals of rainfield exposure.

5. POST-TEST OBSERVATION &ND EXAMINATION PROCEDURES

a. Visual Observatirn

All specimens were examined visually after rainfield

exposure with a lighted magnifier and the surface condition was 2

recorded. Recorded comments included scratches, pittinAg, per-

centage of coating removal, coating adhesion loss, and composite

damage.

b. rarget Mass Loss

All test specimens were forced air oven dried at 125%F

overnight and cooled to ambient temperature in a dessicator after

rainfield exposure. Target mass loss was recorded after each

exposure interval. Mass loss was recorded to within 0.001 g.

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c. Scanninq Electron Microscopy Examination

The ETEC Autoscan High Resolution Scanning ElectronMicroscope is a second generation instrument. It incorporates

such features as electron optics, specimen chamber, electrondetection and display systems. This instrument bridges the

specimen gap between light and transmission electron microscopes.

Specimens are vapor shadowed with a heavy metal or carbon to

provide contrast. Resolution is of the order of 2009 and useful

magnification up to 50,OOOX.

7

SECTION III

TEST RESULTS

1. RAIN EROSION TEST RESULTS - VISUAL EXAMINATION

a. Fluoroelastomer Coating - Glaas Epoxy Substrate -

906 Impact Angle

Visual examination, under a lighted magnifier, of the

fluoroelastomer coated glass epoxy substrates exhibited initia-

tion of pitting on the exposed edges of coated specimens during

the first five-minute interval of rainfield exposure. At theten-minute interval of rainfield exposure, surface pitting and

several arear of very localized adhesion loss were observed at

the exposed edges of the coated specimens. Continued rainfield

exposure resulted in increased pitting with 10% coating removal

and increasing adhesion loss at the 15-minute interval. Thirty

minutes of rainfield exposure resulted in 50% coating removalwith additional adhesion loss and erosion of the glass epoxysubstrate. Percent coating loss as a function of rainfield

exposure time is shown in Figure 4.

b. Fluoroelastomer Coating - Graphite Epoxy Substrate -9Q0 Impact Angle

The fluoroelastomer coated graphite epoxy specimensexhibited no visible surface damage after five minutes of rain-

field exposure. At the 10-minute interval of rainfield exposure,

however, coating losses averaged 5% of the coated surface areaand up to 50% adhesion loss of the fluoroelastomer from the

graphite epoxy substrate. Percent coating loss versus rainfield

exposure time is shown in Figure 4.

c. Fluoroelastomer Coating - Quartz Polyimide Substrate -90O Impact An__e

Five minuter cf rainfield exposure of the fluoroelas-

tomer coated quartz polyimide substrate resulted in the initia-

tion ot pitting and 1% coating removal. Ten-miiute exposures

exhibited increased pitting, approximately 2% coating removal,

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and partial adhesion loss of the fluoroelastomer coating to the

substrate. Continued pitting, 10% coating loss and partial adhe-sion loss occurred at the 15-minute interval of rainfield exposure.

Thirty-minute exposures exhibited 35% coating loss, partial coat-

ing adhesion loss, and severe erosion of the exposed quartz poly-

imide substrate. Results are shown in Figure 4.

d. Fluoroelastomer Coating - Glass Epoxy Substrate -

M0- Impact Angle

Examination of the fluoroelastomer coated glass epoxysubstrates exposed to a five-minute interval of rainfield expo-

sure resulted in no visible damage. Thirty-itinute rainfield

exposures visually indicated pitting initiation and no measurablecoating loss. Increased pitting, partial coating adhesion loss

and composite substrate damage occurred after 165 minutes of

rainfield exposure at 500 mp~h. Results are shown in Figure 5.

e. Fluoroelastomer Coatin - Graphite Epoxy Substrate -T0" Impact Angle

Five-minute rainfield exposure of the fluoroelastomercoated graphite epoxy substrate resulted in no visible damage.

Thirty-minute exposure resulted in partial coating adhesion loss

and composite damage. Exposure to 180 minutes of rainfield

exposure showed partial adhesion loss and increased composite

substrate damage. The results are shown in Figure 5.

f. Fluoroelastomer Coating - Quartz Polyimide Substrate -3Q0 Impact Angle

Visual examination of the fluoroelastomer coated quartz

polyimide substrate after five minutes of rainfield exposure

indicated no visible damage. Exposure to thirty minutes of the

rainfield environment also resulted in no visible damage. Fur-

ther exposure to 180 minutes of the simulated rainfall produced

pitting, partial coating edge delamination, and composite damage.

The results are shown in Figure 5.

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g. Polyurethane Coatn - G1ass Epoxy Substrate -

Visual examination, under a lighted magnifier, of the

polyurethane coated ([Lass epoxy substrate revealed no visible

damage afteL five minutes of rainfield exposure. At the 10-

minute interval of rainfield exposure, 2% of the coating was

removed with partial coating adhesion loss at the edges of the

specimens. Continued rainfie]d exposure resulted in slight

pitting, no increase in coating removal, and increased adhesion

To0s at the coating edges at the 20-minute exposure interval.

Forty minutes of rainfield exposure exhibited 10% coating removal

and increased coating adhesion loss. Results are shown in Fig-

ure 6.

h. Polyurethane Ccating - Graphite Epoxy Substrate_-9 o oImpac

The polyurethane coated graphite epoxy specimens exhib-

ited no visible surface damage after five minutes of rainfield

exposure. ALh the 10-minute interval, coating removal losses

averaged 1.0% off the coated surface area and up to 60% coating

adhnsion loss of the polyurethane from the graphite epoxy sub-

strates. Percent coating loss versus rainfield exposure time is

shown in Figure 6.

i. Polyurethane Coating - Quartz Polyimide Substrate_-Im actA

Five minutes of: rainfield exposure of the polyurethane

coated quartz polyimide substrates resulted in partial coating

adhesion loss at the specimen edges. Ten-minute exposures exhib-

ited increased coating adhesion loss of the polyurethane from

the quartz polyimide specimens and 2% coating removal. Further

rainfield exposures totaling 20 minutes produced 1.0% coating

loss and increasing coating adhesion losses. Results are shown

in Figure 6.

j. Folyurethane Coating - Glass Epox Substrate3 0 Impact Angle

Five-minute and 30-minute rainfield exposures of the

polyurethane coated glass epoxy substrates revealed no visible

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damage. Exposures to 180 minutes of the rainfield environment

resulted in 3% coating removal and partial coacinci delamination

from the glass epoxy sh,bstrates. Percent coating removal versus

rainfieJd exposure tim,- is shown in Figurc 7.

k. Polyurethane Coating - Graphite Epoxy Subs.trate -30"• Impact Angle

Exposures of five minutes and 30 minutes to rainfleldexposure conditions resulted in no visible damage of the poly-urethane coated graphite epoxy specimens. One hundred and eighty

minutes of exposure produced partial coating adhesion losses and

substantial damage to the graphite epoxy composites. Results Eire

shown in Figure 7.

1. Polyurethane Coating - Quartz Polyijide Substrate -

No visible damage was observed after five minutes and

30 minutes exposure to rainfield conditions for the polyurethane

co~ted quartz polyimide specimens. One hundred and eighty min-utes of exposure produced slight coating adhesion loss and very

slight damage to the quartz polyimide substrates. Results are

shown in Figure 7.

2. RAIN EROSION TEST RESULTS - TARGET MASS LOSS

a. Fluoroelastomer Coating - Glass Epoxy Substrate -Th-•Impac Angle

Fluoroelastomer coated glass epoxy substrates exposed

at a 900 impact angle resulted in an average of 0.0035 g weight

loss after five minutes of rainfield exposure. Tfen minutes of

rainfield exposure produced an average of 0.0060 g of specimen

weight loss. Fifteen minutes produced a magnitude increase inweight loss of 0.0188 g. Rainfield exposures of 30 minutes dura-

tion exhibited another magnitude increase in mass loss nf 0.1313 g.

b. Fluoroelastomer Coating - Graphite Epoxy Substrate -

TV-Impact Angle

Rainfield exposure of five minute duraticn with fluoro-

elastomer coated graiphite epoxy specimens resulted in an average

14

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raass loss of 0.0076 q. An order of mnagnitude increase in target

mass loss occurred at 10 minutes of rainfield exposure. Mass losn

at this interval resulted in a 0.039 g weiqht loss.

C. Fluoroelastomer Coating - Quartz Polyimide Substrate -90* Impact Angle

Five-minute rainfield exposures of fluoroelastomer

coated quartz polyimide specimens resulted in an average target

mass loss of 0.0078 g. An order of magnitude mass loss occurred

at the 10-minute interval of rainfield e.posure. This resulted

in an average weight loss of 0.0117 g. Fifteen minutes of expo-

sure exhibited a mass loss average of 0.0296 g. Rainfield expo-

sures of 30 minutes resulted ii, another order of magnitude increase

in target mass loss. These specimens averaged a 0.2114 g weight

loss.

Figure 8 shows a cumparison of target mass loss for the

fluoroelastomer/cornposite combinations exposed at a 900 impact

angle arid a velocity of 500 miles per hour.

d. Fluoroelastomer Coating - Glass Epoxy Substrate -

30* impact An2ie

Fliaoroelastomer coated glass epoxy substrates exposed

to a 300 impact angle resultcd in an average weight loss of

0.0030 g after five minutes of rairifield exposure. Thirty min-

utes of rainfield exposure produced an average target mass loss of

0.0130 g. This was an order of magnitulde increase in mass loss.

A minimal weight loss increase occurred after 165 Minutes of

rainfield exposure. Target mass loss t.~taled 0.0199 g.

e. Fluoroelastomer Coating- Graphite Epoxy Substrate -

30T Impact Angle

Five-minute rainfiell exposure of the fluoroelastomer

ccated graphite epxy substrates resulted in an average mass loss

of 0.0097 g. An order of magnitude change in target mass loss

occurred aL the 30-minute exposure interval. The average weight

loss was 0.0616 g. A minimal increase in mass loss occurred

after 180 minutes of rainfield exposure. Average target maos

loss totaled 0.0960 g.

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f. Fluoroelastomer Coating Quartz Polyimide Substrate -

30 Impact Ang le

Five-minute rainfield exposure of the fluoroelastomer

coated quartz polyimide substrates resulted in an average weight

loss of 0.0055 g. An average mass loss of 0.0218 g occurred after.

30 minutes of rainfield exposure. One hundred and eighty minutes

of exposure produced an average total mass loss of 0.0454 g.

Figure 9 shows a comparison of target mass loss for the

fluoroelastomer/composite combinations at a 300 impact angle and

a velocity of 500 miles per hour.

g.Polyurethane Coatin - Glass Epoxy Substrate

Polyurethane coated glass epoxy substrates exposed to

a 900 impact angle exhibited an average target mass loss of

0.00265 g after five minutes of rainfield exposure. Ten-minute

rainfield exposure produced an average mass loss of 0.0077 g. A

mass loss of 0,0075 g occurred at the 20-minute interval. A total

average target mass loss of 0.0172 g was measured after 40 minutes

of rainfield exposure.

h. Polyurethane Coating -_Graphite Epoxy Substrate

900 Impact An le

Rainfield expost *e of five minute duration with poly-

urethane coated graphite epoxy specimens resulted in an average

mass loss of 0.0080 q. Exposure to 10 minutes of rainfield con-

ditions produced an order of magnitude increase in target mass

loss for these materials. Total average mass loss at 10 minutes

was 0.0331 g.

1. Po1 urethane Coating - Quartz Polyimide Substrate -

90° Imact AngleFive-minute rainfield exposure of the polyurethane

coated quartz polyimide substrates resulted in an average mass

loss of 0.0052 g. An average mass loss of 0.0088 g occurred

after 10 minutes of exposure for this ccating/substrate combi-

nation. Twenty-minute rainfield exposure produced an average

total target mass loss of 0.0188 g.

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0g~04

HH

0 0H

L 1 )

C) 0004 r-I e-I

x H)0

0o 0 04f

00

0 01

,(~ H N Ln r-4-0~*00 04 0 0ý0

too 01 mý4UlI

(5) E90) SSP 4-!

16N

Figure 10 shows a comparison of target mass loss for the

polyurethane/composite combina ions exposed at a 900 impact anglearid a velocity of 500 miles per hour.

j. Polyurethane Coating - Glass Epoxy Substrate -

300 Impact Angle

!,oiyurethane coated glass epoxy substrate exposed at a30* impact angle resulted in an average target mats loss of

0.0070 g after five minutes of rainfield exposure. Thirty minutes

of rainfield exposure produced an order of magnitude increase in

mass loss averaging 0.0170 g. A minimal weight loss increase

occurred after 180 minutes of rainfield exposure. Target mass

losses averaged 0.0285 g.

ka Polyurethane Coating Graphite Epoxy Substrate -

Target mass loss for the polyurethane coated graphiteepoxy substrates exhibited an. order of magnitude increase for

each exposure interval. At five minutes of rainfield exposure,an average mass loss of 0.0077 g was recorded. Thirty minutesexposure resulted in an average target mass loss of 0.0183 g.

Weight loss averaging 0.1183 g was recorded after the 180-minute

rainfield exposure.

1. Polyurethane Coating - Quartz Polyimide Substrate -300 Impact Angle

Five-minute rainfiel( exposure of the polyurethane

coated quartz polyimide substrates resulted in an averaqe mass

loss of 0.0074 g. An average mass loss of 0.0244 g occurred after

30 minutes of rainfield exposure. One hundred and eighty minutesof exposure produced an average total mass loss of 0.0718 g.

Figure 11 compares the target mass loss of the poly-

urethane/composite combinations at a 30' impact angle arnd a

velocity of 500 miles per hour.

20

S•l......ii.. .. . -I ......... - . ..... T"

0

3.4

04

2 .~4.*-4

Of4

V 04- P4

(t 0

m 0D

vi wý 41

- C- __j 001.ID (D 0* .J-I

o0a0

ssol sew abj0

2-I

aiDo 4 o

0

04-

0 0

1-'1

0 04

X C4 H0( 0

OD 40.0

HL 9- or ssW 4bUV

02

3. RAIN EROSION TEST RESULTS - SEM OBSERVATIONS

Selected test specimens which were exposed for varying time

intervals to a 1 inch/hour simulated rainfall at 500 miles per

hour were examined by scanning electron microscopy (SEM) for

characterization of the mode of erosion damage and erosive pro-

cesses. The surface morphology of the glass epoxy, graphite

epoxy and quartz polyimide coated with the fluoroelastomer or

polyurethane coatings was observed before and after exposure to

rainfield conditions. The results obtained for these materials,supplied by the U.S. Air Fgrce, are described herein.

a. Fluoroelastomer Coating -_Glass Epoxy Substrate -

90• Impact Angle

The (AF-C-VBW-15-15) white fluoroelastomer coating on a

glass epoxy substrate before exposure to rainfield conditions is

shown in Figure 12. The surface of the fluoroelastomer coating

exhibited a uniform morphology with homogeneous distribution of

titanium dioxide pigment as shown in Figure 12A. However, the

coating also contained elongated striations of the type shown in

Figure 12B.

Exposure of the fluoroelastomer coated glass epoxy com-

posite substrates t, a rainfield environment at a 901 impact

angle for timed durations up to 30 minutes resulted in localized

surface erosion damage as well as crack formation, coating removal,

and subsequent erosion damage of the glass epoxy composite as shown

in Figures 13, 14, and 15.

The formation of isolated eroded areas as shown in Fig-

ure 13 are clearly observed after rainfield exposure durations

of five minutes (Figure i3A), 10 minutes (Figure 13B) and 30 min-

utes (Figure 13C). These localized erosion zones were in the

order of 10 microns in size and tended to combine or agglomerate

together as shown in Figure 13B. The erosion damage observed on

the exposed surfaces was characterized by localized erosion

events (Figur, 13) or agglomeration of events (Figure 14C). Other

damage mechanisms were also observcd, such as coating buckling

23

S-" 4" - . i~ . . .. .

'PI

pA No Painfield Exposure4 00 OX

B No Pa"int Hild E~xposure400OX

Figure 12. PinoronLastom('r- Coated(-( clajss Epoxy Composite.

24

IIAN

A 5.0 mil Ralnflfc d xpsue 10.0 mill RairLfi~lc E xposureo1000<BC O

... -. .7

C. 30.0 si H n I d l~pasi1A re300GM

lhicur T.IItIIJ)!a lioit't Coo Loul Vpn; rnxy Compo~sit

2%

A 15.0 mill Rainfield Exposure B 15.0 mm~ Rainfield Exposure170OX 400OX

C 15.0 min Rainfield Exposure D 30.0 mil Rainfiold Exposure10 OX 50COX

PiqUreO 1-4. F.Ituorocilastomer Coaited Glass Epoxy Comp~osite,90' Imipact- A~ncie.(

26

¾A 30.~0 min Rainf iold Exposure

¾ G)OX

B3 30.0 nuin Rmain -t I dEpm

Figure 15 .F1 u ornon 1 n sLeninr (,Ii Lc C as po)-y Comjpos IJto90" U ImpactI Am (Ii v

'27

..........

(Figures 14A and 14B), microcracks (Figure 14D) leading to coating

delamination and removal (Figure 15A) and subsequently to sub-

strate erosion damage as shown in Figure 15B. It should be noted

that these processes were associated with the presence of elon-

gated striations found in the fluoroelastomer coating beforeexposure to rainfield conditions as shown in Figure 12B.

b. FluoroeA.astomer Coating - Graphite Epoxy Substrate -

The fluoroelastomer coated graphite epoxy composite sub-

strate exposure to rainfield conditions is shown in Figure 16.

The coating contains a uniform surface morphology with homogeneous

distribution of titanium dioxide pigment in the size range of 0.5

microns. This description is exhibited in Figure 16A. It also

contained elongated striation defects as shown in Figure 16B.

Erosion damage was introduced into the fluoroelastomercoating during exposure to rainfield conditions at an impact angle

of 90g. rhe surface damage after five and 10 minutes of rainfieldexposure is shown in Figure 17. The erosion damage was charac-terized by l.ocal removal of coating materials, resulting in single

crater formations in the 10 micron size range (Figure 17A) or

agglomeration of such craters (Figure 17B). Furthermore, crater

formation was associated with microcracks in the coating as

revealed in Figure J7C. These microcracks were several microns

in length and une to tw6 microns in width.

c. Fluoroelastomer Coating -- Quartz Polyimide Substrate -

TC7impact Anýl

A uniform fluoroelastomer coating was obtained onquiArtz polyimide substrates Ls shown in Figure 18A. The bright

zones indicated a homogeneous distribution of titanium dioxide

pigment. However, the coating contained elonqated striations

probably resulting from the coating application procedure as

shown in Figure 18B. The elongated striations were approximately

one micror in width and several dozen microns in length.

Figures 19 and 20 exhibit the erosion damage at a 900

impact angle on the fluoroelastomer coating surface after

28

- Iij

1XIi

t ~ ~ I )Y

1?ti11~ h ICC I i CC C I I I I )~Y (~ C1C ).)i. Vt

*A 5.0 niih nfcii B 1.0 .0 mill Rai afield Exposure500ox 3000x

I W il

K 10 .0CtM 2P min R i t'icli ET),x oUra'2000 X

17 L01. 1 1 7. i ~i* ) &:Lfu o e C"rinjlibi t. Epoxy ComposdiLa,90' {ilnpac I An~ltic

_ _ _ _ __ .. '. . . .

4 4

~ ~¶1A

A No.Raif~el IxFosur

400041

4 q-j

1A No R~iirfic tixli : ,osure400OX

I~~ igure 18. PI ~~.6ýoo Ii ae ole Qat oymie C m oie

~771

tv

A 5-0 min Railfield Exposure B mi ran Rainfjeld Exposure300OX 5OQOX

Ni !4,-

3,11

C 1.0.0 min ~Iinlfild Extposurce D) 10.0 min- Rainfi-eld Exposure700 OX 63000X

Fi~qure .1.9 F1. LAOT"Ce Istoiner C oait-od Qu~at z. PoliV 1 fllA1 Compos1. toq- r Lflpacft Ancil~ct.

32j

A 15.0 min Rainfield Exposure B 15.0 min Rainfield Exposure

50OX 3 OQOX

90 mp.t tnqie.

33A

exposure to rainfield conditions up to 30 minutes in duration.The damage illustrated was characterized by formation of local-ized erosion sites in the 10 micron size range after five minutes(Figure 19A and 19B) and 10 minutes (Figure 19C and 19D) of rain-field exposure. Rainfall exposure intervals of 15 minutes to 30

minutes resulted in the formation of clusters of single erosionsites as shown in Figure 20. Furthermoze, the erosion damage

after 30 minutes of rainfield exposure at a 900 impact angleresulted in coating cracking and delam":.-.ition (Figure 21A) fol-lowed by severe erosion damage incurred in the quartz polyimidesubstrate (Figure 21B and 21C). Erosion damage in the quartz

polyimide substrate material was characterized by broken quartzfibers (Figure 21B) as well as by polyimide resin removal (Fig-

ure 21C).

d. Fluoroelastomer Coating - Glass Epoxy Substrate -360 Impact Angle•

The fluoroelastomer coated glass epoxy substrates

exposed to rainfield conditions at a 300 impact angle for time

intervals up to 165 minutes exhibited a surface erosion damage

mode shown in Figure 22. Isolated erosion sites of localizedcoating removal in the range of 10 microns were characterized

and were observed in Figures 223 and 22C. A rainfall exposureinterval of 165 minutes resulted in localized erosion sites aswell as evidence of microcracks associated with. the isolated

erosion sites.

e. Fluorc "astomer Coating-. Graphite` Epoxy Substrate -O'w-r--i• ,-c-f-A-ngle

The graphite epoxy substrates coated with the fluoro-elastomer coating and exposed to rainfield conditions at a 300

impact angle for time intervals of five minutes to 180 minutesduration resulted in surface erosion damage as shown in Figure 23.The mode of damage was characterized by local coating material

removal followed by the formation of isolated craters severa3

microns in size.

34

.N.

A 30.0 min Rainfield Exposure B 30.0 min Rainfield Exposure

900X 20 OX

C 30.0 min Ralmnficild EFxposure

Figure 21. Fluoroolastornor Coa~ted Quairtz Pol-yirnide, Composite,90' ImpacL Anqie.

A 5.0 min Rainfield Exposure B3 30.0 min fainfield ExposuieiGOox 3. 500X

c 10.0 min Rairifield Exposuire1000 X

Piqcjur e 22 . P'1 v nro iciTI tI01Wr coiitied GII SF, Epocxy Conipos iLe,30' ~imrict 7\zi1 eo

D~ 165.0 minh Rainfbicld Expur150(O X

()Iq!

E1.05 .0 roi~n Rzain filed Exposure!5 n l r) X

F4.cjure 22 (Con ' d) I !ý'Itiiioroe 1 d*I~iumu llr Coat ed Gla&ss E~poxy Compon i.he,30' [ni-xp' t Anq 1 e.

-17

A 5.0 min Rainfield Exposure B 30.0 min Rainfield Exposure2000X 500OX

0I,

iti

F i IIPo22. 1 I no1 rnO (I Sr t~i Gi~t' Cooa Led t ruj)i .1 Le' i )pOXy C po tit(10 r, v) wp ic Am; 1 c,

f. Fluoroelastomer Coatj.nxi - Quartz Polyimide Substrate -

TV 0iM act Angle

Scanning electron microscopy observations of fluoro-elastomer coated quartz polyimide substrate materials exposed at

a 30* impact angle to rainfield conditions for five minutes to

180 minutes revealed surface erosion damage as shown in Figure 24.

The erosion damage was characterized by localized coating removal

at erosion sites resulting in crater formation. These local

events up to 10 microns in size were either isolated as shown in

Figure 24A or agglomerated as depicted in Figures 24B and 24C.L4;

4 g. Polyurethane Coatin4 - Glass Epoxy Substrate -

Ol' The MIL-C-83445A white polyurethane coating on qlas6

epoxy composite substrate is shown in Figure 25 prior to rainfield

exposure. The coating exhibits a uniform homogeneous distribution

of titanium dioxide pigment as depicted in Figure 25A up to 1

micron i.• dirmneter. Unexpectedly, however, low magnification

siF;ting of "grain boundaries" having a hexagonal ring shape.

These structures range in size up to 100 microns in diameter a are

clearly observed in Figure 25B.

Surface etructure and morphology of the polyurethane

coated g)lass epoxy composite substrates after exposure to rain-

field coa:ditions at a 900 impact angle for inteTvals of five

minitres to 40 minutes are shown in Figure 26. The 'rosion darloge

was asnociated with localized material removal from the coatinq

surface resulting in single craters or clusters of craters -s

shown in Figure 26C and 26E. The presence of the cjrain-type.

strut-tdres as well as the titanium dioxide pigmentk were not

directly associated with local erosion processes as clearly shown

in Figures 26A, 26D, and 26B, respecttvely.

h. Polyurethanc= Coatijii-. Gr•,hite Eipox Subs.itrat-

9_ tipact Angle

The polyurethanr coatLed graphite epoxy composite sub-

strates exh;bited the stim, uniform homogenaJty of pigment

39

A * mm ~ tfivI d 11xposlire 13 3.'mR iels-o14 Expost)ro800 OX 8000:c

........... . .....

I H G lli I P~il r LI I '14) r

xL

'1q

I . 4t i

A '~ ?:3i1~)~;t;~ X;~A1v RLJ1 lidEpsr

44 1 t

W` I'

'v' ' wf i p y

NMI,

Alliit4!i~~

No Ra li %fld

/JtI W''

Fi~cjUrc 25. 1't) Iytll(it Ane (¼a ( 11Mfl C I -ui 'Oxy ('0inpOsi t'

[BOAt lOQO

I, 20. 11 ' 1-i e4 I' i I

I I 2 1 0 IcIIIi I h I't I (! o~ Ve I 1 1) y lf I I

42

77..

....... .. ......

1 0 0 () \

lr L' 2e 26 (C 11t '(I) aIy IAI~(A.n h Via -(l d (11,,ý Fpo ~xy Cmo1

901 I'mp ic I All' I

41

dispersion as previously described and is again depicted in Fig-ure 27. Th,. same hexagonal grain structure ring shape was

observed in tPigý_res /.7A an(' 2'B and previously described.

Erosion darnace was introduced into the poLyurethane

coating surface on thie grarphite epoxy composite materials during

rainfield exposure at a 900 impact angle. Five-minute expo~sures

to rainfield conditions resul~ted in single craters or clusters of

craterG as illtistrated in Figures 28A and 28B. An exposure to

raintield conditions of 10 minutes duration exhibited extensive

micrc~cracking in the polyurethane coating surface and is shown

i* . Ilxkurethaiie Coatin~ - Quartz POliid Substr-ate-

900 Impact Angle

As expected, the polyurethane coated quartz polyimide

composite suhbstrates exhibited the familiar unifoý-m pigmentdispersion characteristics. Hexagonal ring struQLures výp to

100 microns in diameter again -,ppeared in the coating surface.

These observations are illustrated in Figures 29A and 29B.

Figures 30 and 31 exhibit the erosion dampre at a 900

impact angle on the polyurethane coated quartz pollyimide compos

substrates after exposure to rainfield coiditions up to 20 minutes

in duration. Localized surface erosion in the form of craters

and microc~ acking of the polyurethane coating was exhibited after

five minutes and 10 minutes of rainfield expostire as shown i1n

Figures 30A, 30~3, 30C, and 30D, Some minor coating tCearing alongthe path zýf the micrccracks %..as noted as in Figure 30B. Surfacehcharacterization after 20 minuteE duration indicated continued

localized c-atur production and increased microcracking. The

Inicrocracks observed did not app.ar to be associated with crater

forma.tion as shown in k'igure 31.

j. -Pol urethano-, Coatin - Glass Epoxy Substrate-30 Impact Angl

Trhe pciyurethane coated glass epoxy substrates exposed

to rainfield coiidi~tions at a 300 impact angle for intervals of

44

FiI

ANo R.1 Ill i(, (I xposure

1 000\X

Figjure 27. 1)ovue 11t' 7t1rt111 ( (1 ri hI i)i t e Vtpoxy Conipos toc

9 0 1 Ill

W4

A 5.0 nuin Rairifieid Expaosure B3 5.0 min Rainfield Exposure1000YI 3000X

1 0. 0 min Rat'nf teld EJK-pý '.xIre2'0 00 X

F i.ciir'' 28~ 1-) ýI yurcMh-:ine Coated Cr -phi ti !,poyy Comnpos~ite9J0 Tlmlpact /\nil(:.1e

I C;

A No 0 Rainfiftcld E.xpo~sure10 00 X.

1 N"- Rciinfin.cid Exposure'300X

Figure 29. Polyuroi:1iarw' Coated Quairtz p'OlytmIidC Compos ite,90' fpa L AII(1 n

4-7

t~ir

A r-( milln Rainfield Exposure B 5.0 min Rainfield Exposure200OX 2000X

II~

c .1.0.0 uu~n ýa..infi io1 di Fxposure 1) 10 .0 min Ra-timi tiC 1 [,:xpusut'r2000 X 4(100 X

Fi quro '30 . PC) iyurv thane Coctedl Qumat Iz P0 ly iniLde Comnlpom;i tei

9 0 t, Aia I. Ai I e

4 8

A 20.0 Thin Rainfield Exposqure B 20.0 min Rainfield Exposure2000X 2000X

c 20.0 mnRoinfield Exposure4000OX

1Fi riuro 31. POYWHI W ½ ti (CaLLed Quo ri.z Poiyirnidc' Composji o,,900 1ll~lt An ilo.

4 9

__________ '> 'I.I~f~r'.-rt~mtrtr~r~~ . .. fl~k%.lIflr ~ rr...?..flc..

five minutes to 180 minutes resulted in surface erosion as shown

in Figure 32. The erosion damage was characterized by the forma-

tion of isolated craters in the coating surface after 30 minutes(Figure 32C) and 180 minutes (Figure 32D) of rainfield exposure.Furthermore, microcracking of the polyurethane coating was

observed after 30 minutes exposure (Figure 32B) and 180 minutes

exposure (Figure 32E), respectively. These microcracks are probablyassociated with the "hexagonal shaped grain boundary structure"

previously observed in the polyurethane coating as shown in Figure32B. The observed microcracks in the polyurethane coating were

a surface phenomena and did not penetrate to the glass epoxy inter.-

face (Figure 32E).k. Polyurethane Coating - Graphite Epoxy Substrate -

impact Angle

Figures 33 and 34 illustrate the mode of erosion damageintroduced into the polyurethane coating on graphite epoxy con,-

posite substrates, exposed at an impact angle of 300, after rain-field exposure conditions for five minutes to 180 minutes. Micro-

cracks developed in the coating; however, they did not penetrateto the coating/substrate interface as is clearly shown in Figures

33D and 33D. In addition to microcrack formation, isolatedcraters were observed in Figures 33C and 34B. These

localized erosion zones did not penetrate to the coating/substrate

interface even after 180 minutes of rainfield exposure.

1. Polyurethane Coating - Quartz Polyimide Substrate -

300 Impact Angle

Exposure to a 300 impact angle in the rainfield of the

polyurethane coated quartz polyimide substrates for 30 minutesto 180 minutes resulted in localized erosion damage as well as

in coating crack formation. These events are shown in Figures 35and 36. The cracks were several microns in length and 1 micron

in width as illustrated in Figures 35C and 36C, respectively. In

some cases, the cracks were associated with localized crater for-

mation and ire depicted in Figure 36A. The cracks did not pene-

trate through to the coating substrate interface and, therefore,

no coating delamination was observed.

50

2

A 5.0 min Rainfield Exposure B 30.0 min Rainfield E,,xposure

200 OX 200X

c 30.0 nil n Rainfield 1;xpo4u~re ')180 .0 mm Pin Ra 1 v'd P'Xpc)Surr('

2000 X 1000 X

FiqjurC 32. 1polyurc lhance Coated (GIas ES poxy Colinpositcu

300 ' mpac t Anq-1e.

E 180.0 IIIil Railifie-ld Exposure200 OX

Fi.qure '32 (Cont' d) . Polyurethane. CoaLedl Glass Epoxy Comporgit.e,300 Impact Ancile.

.. .. .. . . ... ...

IV

c 30 .0 'Ii TA Roi-I- Fiu Id Exposu re D 30 .0 min 1ainf icd pour3000 X 300 DX

FA i (IUI-C 3.3 .Poiyur o~ianc, Coated (Iraphi. L E~poxy Comiposi Lu,3f0' Impact. Angle.

5:3

ANII

A 180.0 min Mai~rifiolcl I,:xposul.rro 13 LOC) 0 min Rimnfiold E'.xposure20OX 400 OX

H' I ). 0 Il iI Rlnhi.IIFi o 1A. Y! Expc) qI r C200 OX

1'~ur 3. 0 4. 1u ri i i~ Coa te.d (1 -Iphi tj.1 I> Jox y Coimmpe ' ibCi

. . ... ... .. .. .

A 30 .0 mmrain f Rýifild 3xad ra1 0 . 0 mi n R a in fi a1ci B.X 1) 03U I (5 0O O0x 4000X

f If

C' mC3ii n Rainri-iol V-,xposiu~r-

1Filqurc ' Pu .Yi t 1111 (lu C')C 'Ld QvIdr1m : I ol y i mi duc! lp~ .t0(

5

A 180 .0 mill ]vic LaW V1;posura, 13 18F10.0 Invi. n Rmi iii I i IdXxporurc,o1 oI. o x.C )O

I u ý !I. A w '

SECTION IV

DISCUSSION

iL RAIN EROSION BEI1AVIOR- FLUCROELASTOMER

a. Kinetics

The kinetic behavior of the fluoroelastomer coated glass

epoxy, graphite epoxy and quartz polyimide composite materialsunder rain erosion conditions were evaluated in terms of specimenmass Toss and percent coating removal. At the 900 impact angle,coating removal increased gradually with rainfield exposure time

up to the ten minute interval as shown in Figure 4. This correlateswith tho gradual increase in target mass loss shown in Figure 8.Further exposure of the coated glass epoxy and quartz polyimide

to rainfield conditions at a 900 impact angle resulted in sharpincreases in coating removal and target mass loss. The materiallosses were associated with, excessive coating removal after a

rainfield exposure interval of f.i, "'en minutes followed by severe

erosion damage of the composite L ILrate material.

At an impact angle of 300 under rainfield exposureconditions, the amount of visible coating removal was practically

nil and independent of exposure time as shown in Figure 5. However,the target mass loss increased gradually with rainfield exposureup to the thirty minute interval and thereafter no further signi-

ficant mass loss was observed as shown in Pigure 9. This behaviorindicated material removal from the coaLing surface without visiblyapparent coating removal and substrate erosion.

b. Surface Characterization

Coating surface morphology was characterized by scanninq

electron microscopy observations. The effects of rainfleld

exposure conditions such as impact angle, exposure time and thetype of composite substrate material were evaluated.

c. Effect of Impact Angle

Exposure of the fluoroelastomer coated composite sub-

strate at a 90' impact anole resulted in the formation of single

5,7

craters or cratering agglonteration in the coating surfaýw' as shown

in Figure 13. Furthermore, microcracks were formed resulting

in coating delaminr tion as depicted in Figure 14. Thiu 1-d to

subsequent damage of the composite substrate material shown in

Figure 15.

Rainiield ý-xposure conditions at a 3s* inr.pact angle

resulted primarily in the formation of isolated craters in the

coaLing surface as shown in Figure 22C. Some microcracking was

evic!;nt in the coatinq after an exposure duration of one hundred

and sixty five minutes for the flunrocarbon coated glass epoxy

composite as shown in Figure 22D. Furthermore at the 300 irpact

cngle, no visibly substantial coating removal was observed. Those

observations correlate with the kinetic data shown in Figure 5.

do ffect of Exposure Time

The exposure duration of the various fluoroelastomer

coated composites under rain erosion conditions resulted in the

* formation of craters in the coating surface after five minutes

of rainfleld exposure at both 900 and 300 impact angles. After

five minutes exposure at a 900 impact angle, xwicrocracks were

also formed. This resulted in partial coating delamination and

separation at the fifteen minute rainfield exposure interval.

Subsequently, damage was introduced into the composite substrate

material during the fifteen to thirty minute exposure interval.

It should be noted that the coating removal and sabstrate damage

incu;,e:d during this interval :'s in correlation with the kinetic

curves E1.own in Figure 4 and 8. High increases in target mass

loss were observed after the fifteen and thirty minute exposure

intervals.

W-hen the test specimens were exposed to rainf~icld

exposurL conditions at a 300 impact angle, crack formation was

observed only after one hundred and six.y five minutes as shown

in Figure 22D. This coirelates well with the amount of coating

r-eo•a. versus iainfield exposure time shown in Figure 5.

58

e. Effect of Compo:ite Material

The effect of compositcn construction on the rain erosionperformance of the fluoroelastomer coating is discussed herein. Thefluoroelastomer coated glass epoxy composite specimens exposed ata 900 impact angle to rainfie].d conditions exhibited the least

amount of overall visible damage and mass loss. FluoroelastomerAcoated quartz pclyimidc followed the same general trend as tiecoated glass epoxy, but exhibited severe erosion of the compositesubstrate after coating removal. The graphite epoxy coated withfluoroelastomer exhibited the most significant degree of visibledamage and target mass at the 900 impact angle. At the 300 impact.angle, the fluoroelastomer coated composites exhibited essentiallyno visible damage until thirty minutes of rainfield exposure. In

terms of target mass loss, the coated composites exhibited anorder of magnitude increase in mass loss during the first thirtyminutes of rainfield exposure conditions. From thirty to onehundred and sixty five minutes of rain exposure, the coatedcomposites exhibited no substantial increase in mass loss. Thefluorocarbon coated graphite epoxy showed the largest increasein material loss during the first thirty minutes of exposure.

2. RAIN EROSION BEHAVIOR.-POLYURETHANE

a. Kinetics

The kinetic behavior of the polyurethane coated glassepoxy, graphite epoxy and quartz polyimide composite materia)runder rainfield exposure conditions were eval.uated in terms ofspecimen mass loss and percent coating removal. At a 900 impactangle, coating removal increased gradually with rainfield exposuretime up to the ten minute interval as depicted in Figure 6. Thiscorreiates well with the target mass loss curves shown in Figure10. Further exnosire of the coated glass epoxy and quartz polyi-mide to rainfield exposure conditiui-s at a 900 impact angle

resulted in slight increases in coating removal and target massloss. The material losses were associated with moderate coatingremoval after a rainfield exposure interval of fifteen minutes.

59

- • i I - - I I_ _ I I I aI I

At the 300 impaci an',le unde rainf-uld exposuie

conditions, .he amount of ,toating removal was isentially nil

and independcnt of expo3ure:-. tirre a• shou:. in Pigure 7. Targetmass loss increased gradiuaily with rainfi-ld exposure tinie up to

thirty minutes and increased slightly thereafter, except for thepolyurethane coated graphite epoxy specimens. Target mass loss

versus rainfield exposure is shown in Figure 11. Their behavior

indicated material removal from the coating surface withoutvisibly apparent coating removal and substrate erosion.

b. Surface Characterization

Coating surface morphology was characterized by

scanning electron microscopy observations. The effects of rain-

field exposure conditions such as impact angle, exposure time and

type of composite substrate materia2 were evaluated.

c. Effect of Impact Angle

Exposure of the polyurethane coated composite substrates

at a 900 impact angle resulted in the formation of single craters

or cratering agglomeration in the coating surface as shown in

Figure 26. Furthermore, microaracks were formed resulting in the

initiation of coating delamination as depicted in Figure 28.

Rainfield exposure conditions at a 300 impact angle

resulted primarily in the foimation of isolated craters in the

coating surface. Microcracking was evident in the polyurethanecoating on all three substrates. The microcracking process

occurred between five minutes and one hundred and eighty minuteso0 rainfield exposure depending upon the type of substrate. Inno case did the cracks penetrate to the substrate, therefore, nodelamination of the polyurethane coating was observed. These

observations correlate with the kinetic data showzn in Figure 7.

d. Effect of Exposure Time

The exposure duration of the various polyurethane coated

composite specimens evaluated under rainfield exposure conditionsresulted in the formation of craters in the coating surface after

60

I

* five minutes for polyurethane coated graphite epoxy and quartz

polyimide specimens and twenty minutes for the polyurethane coated

glass epoxy at the 900 impact anqle. After ten minutes exposure,* microcracks were formed on the coated graphite epoxy and quartz

polyimide specimens. No microcracking was observed in the poly-

urethane coated glass epoxy after forty minutes of rainfield ex-posure ýt a 900 impact angle. This correlates well with the

kinetic curves shown in Figures 6 and 10.

Rainfield exposure at a 300 impact angle resulted inisolated craters at the thirty minute interval for all the poly-

urethane coated composites. Crack formation was observed afterthirty minutes of rainfield exposure in the polyurethane coated

graphite epoxy and quartz polyimide composite specimens arid afterone hundred and eighty minutes as shiown in Figure 32 for the poly-

urethane coated glass epoxy. This correlates well with the amountof coating removal versus rainfield exposure time shown in Figure 7.

e. Effect of Composite Material

The effect of composite construction on the rain erosion

performance of the polyurethane coating is discussed herein. Thepolyurethane coated glass epoxy composite specimens exposed at a

900 impact angle to rainfield conditions exhibited the least amountof overall visible damage and mass loss. Polyurethane coated

cuartz polyimide exhibited the same amount of coating removal asthe coated glass epoxy, but in only half the exposure time as thecoated glass epoxy. Mass loss versus exposure duration of the

coated quart% Oolyimide followed the same general curve shape,but slightly higher. The graphite epoxy coated with polyurethaneexhibited the most significant degree of visible damage and target

nass loss at the 90* impact anqi ,. At th.c 300 impact angle, the

polyurethane coated composites exhibited little visible damage up

to the thirty minute interval of rainfield exposure. In terms of

target mass loss, the coated composites exhibited an order ofmagnitude increase in LOass loss during the first thirty minutes ofrainfield exposuiLe. From thirty to one hindred and eighty minutes

of rain exposure, the coated composites (the coated nlaL~s epoxy

61

- - - - - - A . .. * .. :... *i

specimens) exhibited no substantial increase in mass loss. The

coated graphite epoxy specimens exhibited a continuing increase inmass loss. The coated quartz polyimide specimens exhibited a

continuing mass loss with a lessening of the rate of mass loss at

the one hundred and eighty minute interval.

62

4.--

SECTION V

CONCLUSIONS

The polyurethane coated composites exhibited greater observable

visual damage after exposure to a 90' angle of droplet impingement

versus the 300 angle of exposure. The fluoroelastomer coated

composites also revealed greater observable visual damage at a

900 droplet impact angle versus a 300 angle of exposure. The poly-

urethane coatings exhibited a significantly lower rate of mass loss

than the fluoroelastomer coatings at 900, except in the case of the

graphite coated composites (see Figures 2,3). Polyurethane and

fluoroelastomer coatings on graphite epoxy composites followed the

same general trend of mass loss rate at 900 impingement angles.

Mass loss rates for both coatings at 301 impingement angles were

generally similar (see Figures 4, 5).

Polyurethane coated glass epoxy exhibited the least amount of

visible damage and mass loss after exposure at 900. Polyurethane

coated quartz-polyimide closely followed the results of the glass-

epoxy materials. Polyurethane coated graphite epoxy exhibited the

greatest degree of visible surface damage at the 900 angle of drop-

let impact. Fluoroelastomer coated compositev exposed at 900

followed the same general trend of damage distribution among the

composite substrates but exhibited greater degrees of damage in

shorter exposure periods, No substantial difference between the

polyurethane and fluoroelastomer on the three different composite

substrates at 300 angle of rainfield exposure occurred.

Polyurethane coated composites exhibited less visible damage

and less mass loss than the fluorocarbon coated composites at a900 angle of exposure for time periods of up to 40 minutes,

Polyurethane and fluorocarbon coated composite structures of glass

epoxy, glass polyimide, and graphite epoxy exposed at a 30' angle

of droplet impact exhibited essentially minor differences in the

amount of target mass loss after 180 minutes of rainfield exposure.

63

hiiiiniln ipllii~il~lr~iil nilillliI llll I~i m il eann ...... I

Selected specimens were examined using scanning electron

microscopy techniques. Cratering initiation and crack propagation

in the polymeric coating was observed. The degree of coating

surface damage was greater for the fluoroelastomer coated compositesas a function of rainfield exposure time regardless of dropletimpact angle. In terms of composite construction, the graphite

epoxy substrate was the most significant in its influence on the

degree of damage to the coating surfaces. Glass epoxy substrates

exhibited the least amount of damage to the surface coatings.

Of the two erosion resistant coatings, the polyurethane

coating provided greater protection of the composite substrates

than the fluorocarbon coating. The glass-epoxy substrates ex-hibited increased performance for the polyurethane and fluoro-

elastomer coatings. Quartz-polyimide substrates closely followed

the performance of the coated glass epoxy materials. Polyure-thane and fluorocarbon-coated graphite-epoxy substrates exhibited

the greatest degree of coating surface damage. Surface erosion

mechanisms and characteristics of coated materials can be identi-

fied and analyzed by SEM techniques.

The degree of protection which these two classes of coatings

provide should be kept in perspective since they are by far themost rain erosion resistant polymeric coatings available. Other

polymeric type coatings fail very rapidly under similar rainexposure conditions and only with these elastomeric systems is

true rain protect ion fcr extended exposure periods possible.

64

SECTION VI

REFERENCES

1. G. F. Schmitt, Jr. "The Erosion Behavior of Polymeric Coatingsand Composites at Subsonic Velocities", Procee n f theThird Intl. Conference on Rain Erosion and Associated PhenomenaFarnborough, 1970, Royal Aircraft Establishment, Farnborough,England, 1970, pp. 107-128.

2. E. Van Rensen "The Behavior of Fiber Materials Under Erosionand Cavitation Stresses", Proceedinqs of Fourth Intl.Conference on Rain Erosion and Assoc. Phenomena, A. A. Fyalland R. B. King-e-ds), Royal Aircra ft Establishment, Farnborough,England, 1974 pp. 485-512.

3. G. F. Schmitt, Jr. "In-Service Performance of Polyurethaneand Fluorocarbon Rain Erosion Resistant Radome Coatings",Materials & Processes - In-Service Performance Science ofAdvanced Materials and Process Engineering Sernes, Vol 9,SAMPE, 1977, pp. 377-391.

4. G. F. Schmitt., Jr. "Materials Parameters that Govern theErosion Behavior of Polymeric Composites in Rain Environments",Conýoite Materials: Testing and Design (Third Conference)LASTM-STP-546, American Society for Testing and Materials,1974, pp. 303.-323.

5. C. J. Hurley and G. F. Schmitt, Jr. Development and Cali-bration of a Mach 1.2 Rain Erosion Test Apparatus, AFML-TR-70-240, October 1970, AD No. A715-909.

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