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AD-769 286

DEVELOPMENT AND EVALUATION OF PG/SiC CODEPOSITED COATINGS FOR ROCKET NOZZLE INSERTS. VOLUME II

Kenneth E, Undercoffer

Atlantic Research Corporation

Prepared for:

Air Force Rocket Propulsion Laboratory

1973

DISTRIBUTED BY:

KJTn National Technical Information Service U. S. DEPARTMENT OF COMMERCE 5285 Port Royal Road, Springfield Va. 22151

Unclassified secuniTv CLASSIFICATION or THIS PAGE onitn o«* Snort«

Ad-TZf^sf REPORT DOCUMENTATION PAGE

1 HEPORT NUMBER

AFRPL-TR-73-70 Vol. II 2. OOVT ACCESSION NO

4 TiTLF (md subim.) Development and Evaluation of PG/SiC Codeposited Coatings for Rocket Nozzle Inserts. Volume II. Vapor Deposition of Pyrolytic Graphite/Silicon Carbide Codeposited Coatings

7 AutMORft;

Kenneth E. Undercoffer

9 PERFORMING ORGANIZATION NAME AND ADDRESS

Atlantic Research Corporation 5390 Cherokee Avenue Alexandria, Va. 22314 CO.NTJJOLLINQ OFFICE NAME AIJD ADDRESS "iir Force Rocket Propulsion Laboratory Director of Laboratones Edwards, CA 93523 Air Force System Command. United States Air Fnrce

Ti MONITORING AGENCY NAME 1 AOORESS(7l dlltmrml f.-om Controlling Olllce)

Commander, DCASD Baltimore Building 22, Fort Holabird Baltimore. Maryland 21219

READ INSTRUCTIONS BEFORE COMPLETING FORK

S. RECIPIENT'S CATALOG NUMBER

S. TVPE OF REPORT » PERIOD COVERED

Special -Jan. 1972 to July 1973

6 PERFORMING ORG, REPORT NUMBER 46-5544.11

i. CONTRACT OR GRANT NUMBERf«J

FO4611-72-C-0047

10. PROGRAM ELEMENT, PROJECT, TASK AREA » WORK UNIT NUMBERS

Project No. 3148 BPSN: 623148

12. REPORT DATE

1973 13. NUMBER OF PAGES

91 IS. SECURITY CLASS, (at Ihlt raporlj

Unclassified

IS«. DECLASSIFICATION DOWNGRADING SCHEDULE N^A

16. DISTRIBUTION STATEMENT (ol Ihli R.porl)

17, DISTRIBUTION STATEMENT (ol th» •bmtact enterttd In Block 20, It dlttorrtnt Irom Rtporl)

18. SUPPLEMENTARY NOTES

Rtprodu^cd by

NATIONAL TECHNICAL INFORMATION SERVICE

U S Deparlmanl of Commtrc» 5pring(i»IH VA 35 Ml

19. KEY WORDS fConflnu» on rrvtr«« »Id» II n»c»»»mrY mnd ld»nllly by block numb»r)

Pyrolytic Graphite Silicon Carbide Rocket Nozzles Vapor Deposition

Coatings

Maj. ot illustration, in ^iSnt,nayl* Wt«r

20. ABSTRACT (Contlnuo on r»v»r»» »Id» It n»c»»»mry mnd ld»ntlly by block numbmr)

Phase I - Task I. Parameters were established to vapor deposit PG/SiC coatings on nozzle components with throat diameters up to 1.7 inches. The coatings were deposited from dilute methane and methyltrichlorosilane in rapidly flowing nitrogen at atmospheric pressure. The coatings were characterized with respect to morphology and SiC content. Coated throat liner components for I.O-irch throat diameter subscale test nozzles and for 1.7-inch throat diameter HIPPO test nozzles vere fabricated.

DD I J°NM73 1473 EDITION OF 1 NOV 85 IS OBSOLETE

SECURITY CLASSIFICATION OF THIS PAGE (Wbtr. Dmlm Bnl»t»d) 4: /

■Ill I I I IHM I II IW tttmmim. ""- " j..-- ■..[ 11 iirmil

l|iL ■"mmummmwimm.

TABLE OF CONTENTS

P»ge

1 INTRODUCTION 1

II OBJECTIVE I

III SCOPE 2

IV CONCLUSION 2

V GENERAL PROCEDURES 3

1. COATING PROCEDURE 3 2. COATING EVALUATION Pi^OCbDURES 7

VI COATING DEVELOPMENT 10

1. FABRICATION TECHNIQUE DEVELOPMENT 10 2. l.OIN^H ENTRANCE AND EXIT SECTIONS 28 3. 1.72-INCH HIPPO NOZZLE THROAT INSERTS 33 4. 1.72-INCH HIPPO ENTRANCE APPROACH SECTIONS 52

VII REPRODUCIBILITY STUDY 64

1. SiC CONTENT/DENSITY RELATIONSHIP 64

VIII SiCl4 AS SILICON SOURCE MATERIAL 74

IX QMS STUDIES 80

X DICUSSSION 83

Preceding page blank

in

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r LIST OF ILLUSTRATIONS

Figure Page

1 Water-Cooled Copper Injectcr 4

2 MTS Bubbler System 5

3 Typical Coating Microstructures 11

4 Standard 1.0-Inch Nozzle Throat Insert Substrate 13

5 1.0-Inch Wraparound Nozzle Throat Insert Substrate (large radius) 14

6 1.0-Inch Wraparound Nozzle Throat Insert Substrate (small radius) IS

7 1.0-Inch Subscale Throat Insert Deposition Assembly 1 f)

8 1.0-Inch Throat Insert Fabricated Using Variable Deposition Conditions. All Views are from Exit End of 03XX-19 at 800X 29

9 1.0-Inch Entrance Approach Section Substrate 30

10 1.0-Inch Exit Section Substrate 31

11 1.0-Inch Entrance Approach and Exit Section Deposition Assembly 32

12 1.72-Inch Standard Nozzle Throat Insert Substrate 38

13 1.72-Inch Wraparound Nozzle Throat Insert Substrate 39

14 1.72-Inch HIPPO Throat Insert Deposition Assembly 40

15 1.72-Inch HIPPO Entrance Approach Section Substrate 53

16 1.72-Inch HIPPO Entrance Approach Section Deposition Assembly 54

17 Bubbler N, Rate Versus CHjSiClj Rate (for 5-gallon bubbler operating at 50 psig and 70oF) 59

18 HIPPO 1.72-Inch Throat Insert Sectioning Plan 65

19 HIPPO 1.72-Inch Wraparound Throat Insert Sectioning Plan 66

20 HIPPO 1.72-Inch Entrance Approach Sectioning Plan 67

iv

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LIST OF ILLUSTRATIONS (continued)

Figure

21 Relationship Between SiC Content and Density of PG/SiC Co-Deposit

22 Microstructure of PG/SiC Made with SiCI4-Run No. 4 (289-63) Magn. 200X

Page

72

78

i 1 1 i

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Table

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

XIII

XIV

XV

XVI

XVII

XVIII

XIX

XX

XXI

UST OF TABLES

h|e

Coating Index System 6

Description of Various PG-SiC Coating Microstructural Code Types 8

Coated Surface Condition Code 9

1.0-Inch Throat Insert Coating Process Conditions 17

1.0-Inch Throat Insert Coating Evaluation Data 20

1.0-Inch Throat Insert Data Summary (fired nozzles onl-') 22

1.0-Inch Insert Substrate Dimensional Data 23

1.0-Inch Throat Insert 27

1.0-Inch Entrarce Approach and Exit Section Coating Process Conditions 34

1.0-Inch Entrance Approach and Exit Section Coating Evaluation Data 35

1.0-Inch Entrance Approach and Exit Section Data Summary (fired nozzles only) 36

1.0-Inch Enlrancc Approach and Exit Section Substrate Dimensional Data 37

1.72-lndi Throat Insert Coating Process Conditions '!

1.72-Inch HIPPO Throat Insert Coating Evaluation Data 44

1.72-Inch HIPPO Throat Insert Data Summary (fired nozzles only* 46

1.7:-lnch HIPPO Throat Insert Substrate Dimensional Data 48

1.72-Inch HIPPO F:ilrance Approach Section Coating Process Conditions 55

1.72-lncli HIPPO hntrancc Approach Section Coaling Evaluation Data 57

1.72-Inch HIPPO Entrance Approach Section Data Summary (fired nozzles only) 58

1.72-Inch HIPPO Entrance Approach Section Substrate Dimensional Data 61

HIPPO Throat Insert Reproducfliility Part Shape 2324 68

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LIST OF TABLES (continued)

Table Page

XXII HIPPO Throat Insert Reproducibiiity Part Shape 2424 (wraparound) 69

XXIII HIPPO Entrance Approach Section Reproducibiiity Part Shape 2722 70

XXIV HIPPO Throat Insert Reproducibiiity Part Shape 2324 71

XXV Coating Process Variables Using SiCl^ 75

XXVI Appearance of Coating at 2-Inch Increments rrom Injector Tip 76

XXVII QMS Summary 81

XXVIII CoatingRun Variables Summary (for coatings that were test flred only) 84

XXIX Phase 1 - Task I Coating Runs 86

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SEaiONI

INTRODUCTION

The basic technique for codepuiiting pyrolytic graphite silicon carbide (PG/SiC) coatings was developed by Atlantic Research prior to the onset of this program. Work performed for the AFRPL under Contract FO4611-71-C-0014 provided process refinement and nozzle Performance data which, when added to other prior work, formed the technical basis for the process studies performed on this program.

Task I of Phase I of this program was undertaken to study the specific process parameters of interest in applying coatings to nozzle throat component of 1.7-inch diameter. Uniformity and coating reproducibility were central concerns in this effort.

SECTION II

OBJECTIVE

The program objective is to develop a technique for fabricating reproducible PG/SiC coatings for nozzles for tactical missiles.

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SEaiON III

SCOPE

Coaling lechnimivs were developed lor the fabrication of I.O-inch and 1.7-inch throal diameter, lluoat •nserts and entrance approach sections using, primarily, ATJ graphite as substrate material. Coating thicknesses were nominally 100 to ISO mils Successful test firings were completed under Phase I, Task I, and Phase II of the parent report. A lew wraparound throat inserts were fabricated in both sizes. Six pieces of 1.7-inch coated hardware were sectioned and examined to check reproducibility and homogeneity of the coatings. The accuracy of the SiC content Jcteriiiination was checked and its relationship to codeposit densi'> was determined. The potential of the Quantitative Metallurgical System, QMS, in accurately measuring and defining the si/e, shape and degree ol dispersion of the SiC particles was examined in a preliminary way. Two sources of methyl trichlorosilane (MTS) and SICI^ were checked as veactant materials for SiC formation.

SECTION IV

CONCLUSION

Conditions were established for coaling 1.0- and 1.7-inch throat diameter, throat inserts and entrance approach sections with SiC concentrations of approximately 20 percent, coating thicknesses up to 150 mils, and with a good dispersion of line SiC particles. Good reproducibility was demonstrated. The established coalini! parameters (n the test fired components are contained in the respective tables.

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SECTION V

GENERAL PROCEDURES

1. COATING PROCEDURE

The graphite component to be coated is placed inside a graphite canister with graphite jigging in such a way that only tae surface to be coated is in contact with the reaction gases. The canister is cylindrical in shape and has an inlet and an outlet graphite tube leading in and out through its front and back ends, respectively. This canister is placed inside of a horizontal carbon tube resistance furnace. The reactant gases arc highly diluted with nitrogen and are maintained in the furnace under one atmosphere pressure. The reactant gases are introduced through a one-hole, water cooled, copper injector which is sketched in Figure 1. The injector spacing is defined as the distance

in inches from the tip of the injector to the throat or to the exit end if an extrance approach section. The liquid methvltrichlorosilane is cent? med in a stainless steel upright cylindrical container with a Viton gasketed cover as shown in Figure 2. The container is immersed in a water bath and held at 90oF, while nitrogen gas is bubbled through it. Saturation, or near saturation, of the nitrogen with MTS is assumed. This gas is then further diluted with N2 and methane is added. This reactant g,as mixture is introduced through the injector. A small amount of nitrogen is also introduced through the annulus between the entrance graphite tube and the water cooled copper injector to prevent backflow in that area. Temperature is monitored with a Leeds & Northrup Model 8632F optical pyrometer, sighted through a hole in the graphite heater tube onto the CD. of the canister. The calibration of the 4-inch Pereny furnace has shown a maximum difference of 40oF between this site point and the coating surface under coating

conditions.

A coating index system was devised to define the type of part fabricated and the major coating characteristics. This is shown in Table I. For cataloging purposes, the part shapss were given sequential numbers based on the fust 2 digits only. A second series of numbers denoted the deposition log book run number. An example of the system as used in this report is:

2322-7

1.7 inch

Standard Insert •

15%SiC-

100 mils thick-

Sequence

033-38

Page 38

Log Book No. 033

mm^

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Figure 2. MTS Bubbler System.

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Description of Part

Nominal Size

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1.5

1.72

2.0

Shape

Standard Insert

Wraparound - Large Radius

Wraparound - Small Radius

Entrance Approach

Exit Section

Nominal Composition (percent SIC)

0 (Pyrolytic Graphite)

8

15

25

Graded Coating

Metal Carbide (Hf, Zr)

Ternaries (SIC + metal carbide)

Nominal Thickness

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100

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250 and beyond

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2. COATING EVALUATION PROCEDURES

The insert coatings were evaluated on the basis of thickness, composition, density, and microstructurc The density and composition were determined at each end on rings made available during the insert trimming operation. Microstructurc was determined by polishing each end of the insert. The end coating thicknesses were also determined on the polished ends by using a filar microscope and allowing for the angle between the polished surface and the coating surface. The coating thickness at the throat was determined by measurement of the throat diameter, using calipers, before and after coating.

Density was determined on small specimens by taking a short segment of each ring and using a sink-float method with density iluids 0.05 g/cc apart at the start of the program and 0.025 g/cc apart later in the program. Composition was determined by ashing at l,900oF in air and weighing the codeposit sample and the final SiO-, poduct. During the program, it was discovered that weight equilibrium was not being achieved in this gravimetric analytical process and the ashing method was abandoned in favor of a density/SiC content relationship as described in Section 13.0.

The poliilied inserts were examined on a Zeiss inverted standard metal microscope with magnifications of 40X, 125X, 400X and I000X. A light polarizer was used to give contrast between the PC matrix and the SiC crystals. In some cases where maximum contrast is needed, such is QMS analysis, the sample was electrolytically etched in a 5 percent KOH solution to preferentially oxidize the PG phase thereby darkening the matrix and enabling better observation of the SiC. Determination of PG cone angles and SiC crystal diameters were made by the use of angular and linear reticules placed in the microscope optics which superimposes these scales on the observed material.

A code system was devised for identification of the various types of microstructures encountered in the program. This code system, which is shovn in Table II, is based primarily on SiC crystal size and degree of dispersion. The ends of the inserts were made ready for microstructural observation by standard metallographic grinding and polishing techniques. Specimens up to 4 inches CD. were subjected to grinding on a series of silicon carbide abrasive paper. Grits No. 240, 320,400 and 600 are normally used. The inserts were ground on a !5-micron diamond disc which is adhesive backed and mounted on an 8-inch diameter bronze polishing wheel. Water was used to carry away removed material and to lubricate the grinding surface. After grinding the insert was ready for polishing and the insert was polished on the rotating wheel. Oil and/or water was used to lubricate during the polishing process.

A 6-micron diamond polish was usually sufficient to provide clear microscopic observation. This polish could be slightly improved by use of a 1 -micron diamond paste which yields a lower material removal rate.

A code system was also devised to categorize the degree of surfaced roughness of the coatings, as shown in Table III. The system is based on: (a) the average cone (surface module) diameter, (b) the size range of these cone diameters, and (c) the numbers of very large modules, or scabs present in the insert coating.

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Table II. Dtscrlptlon of Variout PG-SIC Coating Microitructural Code Types.

Group 10

Group 11

Group 20

Group 21

Group 30

Group 40

Group 60

Group 70

SIC phase fine, evenly dispersed, acicular, needles less than 0.035-mil diameter with an L/D ratio greater than 3/1.

Similar to Group 10 with SiC phase showing A tendency to cluster in the PG cone boundaries.

SIC phase medium sized, evenly dispersed, acicular, needles 0.035-mil to 0.105-mil diameter with an L/D ratio greater than 3/1.

Similar to Group 20 with SiC phase showing a tendency to cluster in the PG cone boundaries.

SiC phase coarse, moderately well dispersed, acicular, needles greater than 0.105-mil diameter with an L/D ratio greater than 3/1. PG growth cones usually contain occasional, intraconlcal, delaminations and strain lines.

SiC phase fine, well dispersed, granules less than 0.035- mil diameter.

SiC phase coarse, poorly dispersed, acicular, needles greater than 0.105-mil diameter with an L/D ratio of greater than 3/1. PG cones contain many delaminations and strain lines.

SiC phase has no definite structure. Usually consists of coarse string-like growths surrounding unalloyed PG cones. PG cones contain many delaminations and strain lines.

——--

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Table III. Coated Surface Condition Code.

Digit

Ist 2nd 3rd

Average Surface Cone Diameter

< 10 mils 0

10/20 mils 1

20/40 mils 2

40/80 mils 3

80/160 mils 4

> 160 mils 5

Cone Diameter Range

0.8X* to 1.25X maximum 0

0.M to 2X maximum 1

0.2X to 5X maximum 2

< 0.2X to > 5X 3

Presence of Nodules

None 0

Few (< 5) 1

Moderate (5 to 25) 2

Prolific (> 25) 3

*X = average cone diameter.

SECTION VI

COATING DEVELOPMENT

A total of lll runs were made in order to fabricate sufficient throat inserts and entrance approach sections to satisfy the 1.01 and 1.0 mch ubscaJe test firings at Atlantic Research and the 1.7·inch HIPPO test firing at RPL. Figure 3 show the typical coating microstructures achieved. Considerable difficulties resulted from inact:urate SiC content determinations. It was 1'!amed during the run series that the ashlng method gave low SiC contents. Consequently, some runs which wen originally thought to be too low in the Sit content were eventually found to be satisfactory . This problem was corrected and all SiC contents in th1s report are considered to be accurate.

1. FABRICATION TECHNIQUE DEVELOPMENT

a. 1.0 Inch NozzJe Throat Insert

Twenty~ight tandard (see Figure 4) and three wraparound (see Figures 5 and 6) I.O·inch nozzle throat iu rt depo ifon runs were conducted. The insert substrate dimension , shown, are before coating and prior to tritr.mmg. A Pereny 4·inch carbon tube resistance furnace was utilized as the heat source. The deposition assembly configuration was similar for all 31 runs and is hown in Figure 7.

Coating process conditions are shown in Table IV and coating characterization data are showr. in Table . The nucrostructures were all coded recently in accordan e with Table II. The process and coating evaluation data

for the seven inserts that were test fired are ummarized in Table VI.

All sub trates, with a few e ccptmn a indicated in Table JV, were fabricated from ATJ graphite. In all ca the throat axis was nom~al to the grain direction of the graphite material u ed.

Sub trate dimensions were recorded before and after coating to detennine if any significant mi match exi ted between th CTE of the coating and that of the substrate. The data are shown in Table VII. An average decrease of 0.02 perc nt in the substrate outside diameter wa recorded indi a ling that the thermal expansion of the ATJ graphite in the With Grain, WG , dir" ·tion i slightly lower than that of the compo ite coating, as expected. The substrate length showed a decrease of 0.05 percent whi ·h was compl!!tely unexpected since the measured CTE of the coating, in the ab plane, i less than that of A TJ graphite 1n the acros• grain, AG, direction . A possible explanation may be that substrate warpage occurred because of the biaxrnl stress fields set up by the complicated coati.-1g/sub:;trate interface. Note that max unum stre is at the interface but the position of strain mea urement is at the .ubstrate periphery. On the other hand, the very poor conSJ.st~:ncy as indicated by the high standard deviation rPnci ~r the data inconclusive.

The first five 1.0-inch inserts were fabricated under identical conditions. The injector spacing, identified the distan e between the tip of the injector and the in rt throat, was 3-1/4 inches. The first of these 0332-1 was

utilized for the first firing of the program, CM·l . The secona 0332·2 was sectioned axiaUy and photomicrographs

I The graded coatings and 20-inch coating are not included in these l I I runs.

10

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Group II Microstructure, 1260X Exit End View

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Figure 3. Typical Coating Microstructures.

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were taken of the entrance, throat and exit areas. Although good even SiC dispersion was obtained and good axial microstructutal uniformity was achieved, the needles were shorter than expected. The third insert, 0332-3, was sectioned, circumierentially, at the entrance, throat and exit planes. These sections were then cut into four equal size quadrants and the density and SiC concentration of the coating determined for each. The results are given in Table VIII. These first three inserts, which wue fabricated using an injector spacing of 3-1/4 inches, yielded coatings with an average coating thickness ratio of 0.89/1 between the entrance end and the throat. After the third run a new heater tube was installed in the furnace. The next two runs, 03224 and 0332-3, were conducted utilizing the same conditions as the first three runs. However, the entrance/throat coating thickness ratio dropped to 0.60/1. Apparently the installation of a new furnace tube affected the deposition rate profile in the deposition chamber. These changes are probably caused by changes in the longitudinal thermal gradient within the furnace tube which altered the thermal profile of the deposition chamber. To increase the entrance end coating thickness the Injector spacing was increased from 3-1/4 to 3-3/4 inches. Other conditions remained the same. This increased the entrance/throat coating thickness ratio loan average of 0.75/1 through Run No.0332-15, excluding No. 0322-11.

Run No. 0322-11 was conducted to determine if reducing the process nitrogen rale and Increasing the deposition temperature would increase the SiC crystal size. The resultant coating was In the group 70 category Indicating that these conditions were unsatisfactory.

The Injector spacing was held at 3-3/4 during fabrication of all of the remaining standard throat inserts. An average entrance/throat coating thickness ratio of 0.75/1 was obtained on all of these inserts. However, considerable fluctuation in this ratio occurred throughout the program. Ratios as low as 0.63/1 and as high as 0.97/1 were encountered. No definite explanation of these fluctuations can be given. It is likely that they arc related to longitudinal thermal gradients which always exist within the deposition chamber. These gradients do change with furnace use as changes In the heater tube thermJ gradient occur. However, such changes usually take place over an extended period of time as the heater tube slowly degenerates and would not explain abrupt changes between runs. These are more likely caused whenever the deposition chamber is not returned to the same position in the furnace after each run. Since thermal gradients obviousl; ^o exist within the furnace tube, any change In the location of the deposition chamber would result In changes of its temperature profile.

After 3-3/4 inches was established as the standard injector spacing a second insert, 0332-9, was sectioned In the same manner as 0332-3. These data are also shown in Table Vlll.

Prior to Run No. 0322-15 bubbler weights were not recorded before and after deposition. After these weight recordings were begun It became apparent that the reproducibility of the CH^SICl^ rate, from run to run, was less than desirable. This lack of reproducibility was found to be caused by leakage to the atmosphere, from the nitrogen Inlet lines leading into the bubbler and between the bubbler and the injector. All of the lines and fittings from the nitrogen supply, through the bubbler, and Into the injector were replaced. After Run No. 0322-18 these lines were leak tested prior to each deposition mn.

At about this same time it was considered desirable to check the effect of temperature and gas velocity on the coating microstructure. To accomplish this purpose a deposition run, 03xx-19, was conducted In which the coating was deposited in five distinct layers, each separated by a thin PG band. The first four layers were deposited at 2,900, 3,000, 3,100 and 3,200oF, respectively. The fifth layer was deposited at 3,200oF with a 50 percent

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increase in the process nitrogen flow rate to determine the effect of flow rate and turbulence on the coating microstructure. The results of this special deposition run are shown in Figure 8. The SIC particle size increases with increasing deposition temperature. The extreme coarseness of the fourth layer is not understood and more runs of this type should be made.

The next three runs, 0322-20, 0312-21 and 0312-22, were conducted at a reduced CHjSiCl^ rale to determine the compatibility of a PG/8'/f SiC coating with ATJ graphite. Only one of these runs, 0312-21, yielded a coating with a SiC concentration near that desired. The SiC phase was extremely fine and granular. No flaws were visible in either the coating or the substrate. Insert No. 0312-22 contained 12 percent SiC and was of similar microstructure. However, it contained a delamination within the coating near the substrate. This was believed to have been caused by variations in the CH^SiCI^ How rate during deposition which, in turn, caused variations in the SiC concentration and induced excessive stresses within the coating.

The next deposition run, 0336-23, was conducted to fabricate a 200-mil-thick coating. Post deposition examination showed the coating, near the substrate, to be of a coarse microstructure with a poor SiC dispersion. This band was 4 to 6 mils thick and contained a fine delamination. This situation had been encountered in the past and is believed to be caused by a lack of nucleation sites for the SiC during the initial phase of deposition. To provide these sites the CHjSlCU is introduced, into the furnace, 30 to 60 seconds before starting the CH,. flow. Another cause of this type of anomaly is an insufficient soak time at deposition temperature before coating is started. At least 15 minutes of soak time are required to ensure that the deposition chamber has reached equilibrium before deposition Is started.

The remaining five standard throat insert deposition runs, 0332-24 through 0334-28, were all conducted to fabricate inserts for motor test firings. Of these, three yielded coatings of acceptable quality.

Seven standard design 1.0-inch nozzle throat inserts were produced for test firing. Deposition conditions for these inserts were nearly identical as shown in Table VIII. The calculated linear flow rale of the reactant gases at the throat is 52 feet per second at the end of coaling. Deposition rates were fairly consistent, around 15 mils per hour. The SiC content at the throat is higher than aimed (20 percent) because incomplete ashing during the SiC assay gave low answers. These SiC data are taken from the density/%SiC relationship and average 27 percent at the Ihroat.

The wraparound design run results served to demonstrate that the fabrication of these coated shapes is feasible. No difficulties were encountered.

2. 1.0 INCH ENTRANCE AND EXIT SECTIONS

Fileven 1.0-inch entrance approach sections, Figure '), and one I.O-inch exit section. Figure 10, deposition runs were conducted. The first five, 0734-1 through 0712-5, were conducted using a 6-incli carbon tube furnace heated by a 100 kW, 10 Kc, Ajax motor generator. The remaining parts were fabricated in the 4-inch Pereny carbon lube resistance furnace. The deposition configuration for all twelve runs was similar and is shown in Figure

II.

28

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1st Layer- 140SCFH Process N2.

2900oF deposition temperature.

Group 10-20 microstructure.

Dispersion Is very good and SiC is acicular.

2nd Layer - 140 SCFH Process N2.

3000oF deposition temperature.

Group 10-40 microstructure.

Crystals appear to have been cut across axis. This may explain the apparently small L/D ratio.

Note: Crystal length in lower right hand corner of picture.

3rd Layer - 140 SCFH Process N2.

3100oF deposition temperature.

Group 11 microstructure.

Note: Poorer dispersion with SiC starting to concentrate in cone boundaries.

4th Layer - 140 SCFH Process Nj.

3200°F deposition temperature.

Group 30 microstructure.

SiC phase shows definite tendency to concentrate in cone boundaries.

5th Layer - 210 SCFH Process N2.

3200° deposition temperature.

Group 30 microstructure.

Similar to 4th layer. Somewhat better dispersion may be the result of reduced deposition temperature which would be caused by cooling effect of increased N2 flow rate.

Figure 8. 1.0-Inch Throat Insert Fabricated Using Variable Deposition Conditions. All Views are from Exit End of 03XX-19 at 800X.

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Coating process conditions are shown in Table IX and coating characterization data are shown in Tadle

Additional process and coating data for the seven parts that were Tired are summarized in Table XI.

All substrates were fabricated from ATJ graphite with the throat axis normal to the grain direction.

Substrate dimensions were recorded before and after coating. The data are shown in Table XII. There appeared to be a slight decrease in the average substrate outside diameter and a slight increase in the length after coating. All permanent substrate outside diameter deformations recorded were between -0.1 and +0.04 percent. However, the data are fragmentary, and because of this, it is difficult to reach any definite conclusions.

In the first five runs, which were conducted in the 6-inch induction furnace, the SiC concentration, at the exit end, decreased steadily from a high of 30 percent in the first run, 0734-1, to a low of 12 percent in the fifth run, 0712-5. This was undoubtedly related to the continuous degeneration of the furnace tube as described in Section VII. After the loss of the 6-inch furnace tube the process was changed to the 4-inch Pereny furnace. At this time the deposition temperature was reduced from 3.350oF to 3,200oF and the total process gas flow ate was increased by 22 percent by increasing the process nitrogen flow rate. The remaining seven runs were conducted with no serious problems.

One exit section, 0934-1, was fabricated using the same parameters as the latter entrance approach sections.

It may be noted that the coating rate decreased and the SiC content at the exit end increased when the switch was made to the Pereny furnace. The rate decrease and part of the SiC increase may be attributed to 'he decrease in CH4 content of the reactant gases. The main cause of the increase in SiC content is probably due to the decrease in temperature. On the other hand, the SiC needles coarsened when the process was switched to the Pereny furnace. This cannot be explained at this time. However, the latter Pereny run microstructures arc consistent with the 1.0-ir.ch throat insert microstructures.

3. 1.72-INCH HIPPO NOZZLE THROAT INSERTS

Forty-four standard (see Figure 12) and three wraparound (see Figure 13) 1.72-mch HIPPO nozzle throat insert deposition runs were conducted. The deposition assembly configuration is shown in Figure 14. All except the first four and the last two runs utilized a Pereny 4-inch carbon tube resistance furnace as the heat source.

Coating process conditions are shown in Table XIII and coating characterization data are shown in Table XIV. Additional process and coating data for the eight fired throat inserts are summarized in Table XV.

The coating mictostructure of all the 1.72-inch inserts, fabricated in the 4-inch Pereny furnace, were similar and were mostly code 20.

Most of the substrates were fabricated of ATJ graphite and machined with the axis normal to the grain direction. A few HLM substrates were coated for sectioning as rcproducibility specimens. Two annealed ATJ substrates were also coated. The substrate material used, if other than ATJ, is listed below the deposition run number in Table XIII.

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Substrate dimensions were recorded before and after coating tu check for potential coating/substrate CTE mismatch. The data are shown in Table XVI. No significant change is apparent.

The first four 1.72-inch deposition runs were conducted in the 6-inch, induction heated, carbon tube furnace. Only one of these, 2322-2, yielded an acceptable coating. This part was sectioned axially to determine the SiC concentration in the throat area which was found to be acceptable at 18 percent.

After the fourth 1.72 inch fabrication run, the deposition process was shifted to the Pereny 4-inch carbon tube resistance furnace. At this time the deposition temperature was reduced to 3,200CF which was considered to be nominal in the 4-inch furnace. Also, the process nitrogen gas flow rate was changed to and fixed at 150SCFH.

The first two 1.72-incli deposition runs, in the 4-inch furnace, were conducted using identical deposition parameters. The first of these, 2332-5, utilized Dow CH^SiCl-j as the SiC source and 'he second, 2322-6, Union Carbide CH-jSiClj. Both parts were sectioned axially and examined. The coating at the entrance end of 2332-5 was 6 percent higher in SiC concentration and less thick than 2322-6.

Indications were that Union Carbide CH^SiCl-j was equal to and possibly superior to the Dow material when depositing on this configuration. At this time Union Carbide CH^SiClj was selected as the SiC source material and was utilized for all subsequent deposition runs in the program.

The next four deposition runs, 2322-7 through 2322-10, were all conducted utilizing similar deposition conditions except that the injector to throat spacing was reduced to 5 inches. The entrance/throat coating thickness ratio as well as tl e cntrance/exit%SiC ratio in these four runs was inconsistent. This thickness ratio inconsistency continued through to Run 2324-26, although the composition became consistent. At the same time, for some inexplicable reason, the SiC needle size rose to code 3U. At Run 2324-26 the injector spacing was changed to 6 inches. After this the entrance/throat ratio never fell below 0.75/1 and averaged 0.90/1.

For deposition run, 2334-27, the process nitrogen flow rate was increased from 150 to 230 SCFH to improve the coating surface texture. This corresponded to a calculated linear flow rate of reactants, at the throat, of 20 and 29 feet per second, respectively. The coating surface texture was improved. However, it was difficult to maintain this rate using the existing flowmeter, and to alleviate this problem, the next six runs, 2324-29 through 2324-33, were conducted using 210 SCFH. Starting with Run 2324-34 a new flowmeter tube was installed and a process nitrogen rate of 230 SCFH was utilized for all subsequent 1.72-incf üoposition runs.

The increase in process nitrogen rate did not immediately solve the surface roughness problem. Much of this turned out to be the result of poor technique in cleaning and assembling the deposition hardware. To assure the best possible coating texture requires that the following procedure must be followed scrupulously;

a. All mating diameters in the deposition chamber should be concentric within 0.010 inch.

b. All of the deposition fixturing should be wiped with a dry, lint-free cloth and blown with dry nitrogen before assembly.

c. A^er assembly the depositioi. .ixturing should be blown out, again to remove dust generated during assembly.

47

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d. Thoroughly clean and blow off injector before insertion into depoiition assembly.

e. After deposition temperature is reached, blow out deposition chamber at maximum rate possible through injector.

f. Check exhaust tube periodically and clean if necessary.

Whenever this procedure was followed no serious problems were encountered with surface texture of the coating using the established deposition conditions.

No significant problems were encountered in the deposition process from Run 2324-35 to 2324-39. During this time inserts for Firings 0005 and 0006 and one reproducibility specimen, 2324-39, were fabricated.

During Run 2324-40, the throat deposition rate decreased from 19.5 mils/hr in previous runs to 17.8 mils/hr. This was apparently caused by a malfunction in the furnace which changed the temperature profile in the deposition chamber. This reduction in the deposition rate required an increase in the deposition time. Because of the increased deposition time more CHjSiClj had to be added to the bubbler. This filled the bubbler to capacity which caused droplets of CH^SiCU to become entrained in the gas stream during the initial stage of deposition. This caused large momentary increases in the CFUSiCI^ rate which formed thin bands of increased SiC concentration in the coating. These bands were so prominent in the coatings on 2324-40 and 233441 that they were considered to be unsuitable for firing. Run 233442 was acceptable for Firing 0007 and was the last 1.72-inch insert coated in the 4-inch Pereny furnace.

The last two standard 1.72-inch inserts were fabricated in the 6-inch Pereny furnace because the 4-inch furnace was inoperable. A definite change in the coating characteristics took place whenever furnaces were changed. The SiC phase was much finer and the surface condition was considerably less rough. This is probably related to differences in the longitudinal temperature profile of the deposition chambers.

The first run, 232243, conducted in the 6-inch furnace utilized exactly the same deposition parameters as the previous run which was conducted in the 4-inch furnace. The throat coating deposition rate dropped from 17.5 to 13.7 mils/hr and the SiC concentration increased from an average of 21 percent to an average of 30 percent. The entrance/throat coating thickness ratio fell from 0.87/1 to 0.79/1.

The next deposition run, 232444, was conducted with the injector spacing changed from 6 to 6-1/2 inches. All other parameters were the same as those of the previous run. Post deposition examination showed the throat coating deposition rate to have increased to 16.7 mils/hr and the SiC concentration to have fallen to 19 percent. The entrance/throat coating thickness ratio was 0.96/1. This part was accepted for the last HIPPO motor firing of the program.

Three 1.72-inch wraparound nozzle throat inserts were fabricated to determine the uniformity of the coating using this configuration. AU three coatings were applied to PC precoated HLM substrates to facilitate removal of the coating from the substrate. These runs were conducted between standard 1.72-inch deposition Runs 2324-35 and 2324-38.

51

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The first wraparound 1.72-inch insert, 2424-1, was fabricated using the same deposition conditions as the previous standard 1.72-inch insert deposition run. Post deposition examination showed the entrance/throat coating thickness ratio to be 0 57/1. This was somewhat less than desired.

The next two wraparound nozzle insert fabrication runs, 2442-2 and 2424-3, were conducted in the 6-inch Pereny furnace with the Injector spacing increased to 6-3/4 inches. This increased the entrance/throat coating thickness ratio to 0.74/1 and 0.89/1, respectively. These two coatings were used in the coating reproduclbllity studies.

4. 1.72INCH HIPPO ENTRANCE APPROACH SECTIONS

Twenty-seven 1.72-inch entrance (see Figure 15) approach section fabrication runs were conducted. The deposition assembly configuration is shown in Figure 16. The first 16 of these utilized the 6-inch Induction heated furnace as the heat source. The remainder were conducted in the Pereny 6-inch resistance furnace.

Coating process conditions arc shown in Table XVII and coating characterization data are shown in Table XVIII. Additional process and coating data, for the eight fired parts, are summarized in Table XIX.

A higher CHjSiCli flow rate was required to fabricate the 1.72-inch entrance approach sections than was required for the smaller parts labriculcd. This necessitated the use of a much larger bubbler. The basic design of this bubbler was similar to that of the one used for smaller parts. Maximum capacity of this bubbler was five gallons. A broad range of CH-jSiCU rales were used during this series of runs. A graph showing the CHjSiCU rate versus bubbler nitrogen flow rate is shown in Figure 17. This graph was used to determine the CHiSiCU rate during the first twelve runs when bubbler weights had not been recorded.

Three distinct microstructural variations were encountered during the fabrication of these parts.

The first microstructure group, Group A, was produced during the first sixteen runs when depositions

were being conducted in the 6-inch induction furnace. Included in this first group are the entrance approach sections fired in HIPPO's 0001,0002,0003 and 0004. The coatings fabricated during this phase, contained moderate to large acicular SiC and were generally classified as groups 20, 21,30 and occasionally as groups 60 and 70.

When the deposition proccess was shifted to the 6-inch resistance furnace the coating microstructure changed drastically. This resulted in the formation of Group B which are Runs 17 to 25 inclusive. The SiC phase was extremely fine and although frequenly classified in groups 10 and 11 it was much finer than that which is normally associated with these groups. On two occasions the coatings fell into the group 40 class. The deposition rate also showed a marked decline. Attempts were made to coarsen the SiC needles throughout the Group B series. However, this goal was never achieved until the temperature was raised in Group C. Two of the entrance approacl. sections fabricated during the Croup B phase were utilized for HIPPO firings 0005 and 0006.

The third microstructure variation Group C, occurred during the last two runs, when the deposition temperature was increased to 3,500oF. The two parts were utilized in HIPPO firings 0007 and 0008. The microstructure of these parts was definitely in the group II to group 21 range; i.e., they contained coarser SiC needles than previously encountered with the 6-inch Pereny. The deposition rate increased markedly over that of the previous series and was even slightly above that of the first series of runs conducted in the 6-inch induction furnace.

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Tablt XVIII. 1.72-Inch HIPPO Entranct Approach Section Coating Evaluation Data.

Idant. 1 No. Hun Ho.

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Coat int) TTiicknaHi (in,)

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Covvrud witii •oft qroWttl«

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100 ! i70U-3 ll)'J-2 Q.Ö19 0.02U 2.00 2.15/2.21 u 1 aiDu|> 70

Not Vllltil*

at i.OOX

100 Group 70

i ms-i 10'J- J I) U'4 0,092 2.20 2.20 ü j b 00(1 Hoi viaiPlv at toox - 000 1

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at I0OOX

40 - '.'. 211 siL- phaat'

not viBlbla «t 100UX

40 - 55 211 1

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jJTeiiant around |iur inittt<r . Not

micropatiahDd at this und.

L'oatinij rough and acallopcd. Hot 1 micropol lahud. . l!

1 i KM-7 U.U4h o.oyt. 2.27V

.'. 10" 2.125 11 24 Thin unalluyud fO band viaiblv at tntaifaeu at both «ndH. ! |

CJmup io j 55 ^oo oroui- 21 i5 200

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K CoÄtinq GOWrod wiU. aoduiua resulting Irutn »eltin^ of the exit uleanuit 1 tulia onto ttt« suliutratu. Coating und» not nucropol inlied. 1 j

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2. JV2.40 22 JO (.rouj' 10 210 Croup 2] JO ■ 210 |

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21

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[ 17U- i JÜ4-24 0.1 J4 O.ldl 2. J'. 2.45 27 17

14

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[ (0üU4 iU9-27 0.1U o.Ki. 2.12V

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121 Croup 20 40- .0

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121 [

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2-1 14 üraup 11 GO 100 Croup 10

Coating BllOWS Ldndir.ij

CO loo 1

JÜ4-40 9,108 Ü.152 2.2S J{ Group 20 75 100 Group 10 55 000 I 2714- 4 10005

JO'J-42 0.120 o.UO 2.25 2.10/2 15 15 2 Group 10 50 000 Group 10 50 000 i

2724- Q iü'i-ib 0.120 o.im 2,25 2.275/ 2. 100

15 19 Croup 40 55 100 Croup 40 55 000 1

27J4-21 JU"*-«? 0.107 0.142 2.225/ 2.2V,

2.325/ 2. 150

U 25 Group 10 ,0 010 Croup 10 CO 010 1

1 2712-22 KW-S2 Ü.10Ü 0.118 2.250/ 2.100/ 2.325

lb ,'2 ' 110 up 40 5U 010 Group 40 50 010 j

2714-2J 109-54 0.120 0.140 2.200/ 2.250

2.125/ 2. 150

12 25 Group 11

Coating con ainti 10, f 100

id to und, axi Croup 11

1 cracks.

H5 000 J

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2. t5o 2.400

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27,'4-37 (000 7

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It appears that these variations in microstructure occur because of differences in the lateral thermal profile of the two deposition furnaces. It would at first appear that the deposition temperature of the surface being coated varies. However, since the temperature is read at the backside of the deposition canister and both canisters were of the same size, it is highly unlikely thai variations in the order of 300oF could occur. Furthermore, periodic optical pyrometer calibrations revealed no large anomalies.

From the first deposition run, 2700-1, to the fourteenth, 2734-14, Dow C^SiC^ was used. All subscquem runs utilized Union Carbide CHjSiCI^. The CHjSiClj flow rate during the early runs was considerably higher than that which was later established as nominal. These high rates were though to be necessary to achieve a nominal IS percent SiC containing coating. However, the density fluids, being used to determine the SiC concentration, were found to be in error and, as a result, the indicated SiC concentrations were too low. When this error was discovered the f HjSiCK rate was reduced to bring the SiC concentration nearer to the nominal IS percent desired. It is interesting to note that coatings with SiC concentrations as high as 52 percent were fabricated without cooldown cracks. Flawed coatings were encountered, later in the program (Run 2734-23) at much lower concentrations, indicating that SiC concentration is not the only factor influencing the CTE of the coating.

Starting with Run 2734-1S Union Carbide C^SiClj was utilized as the SiC source.

All of the substrates coated were fabricated from ATJ graphite with the axis normal to the grain direction.

Substrate dimensions were recorded before and after coating to check for potential coating/substrate CTE mismatch. The data are recorded in Table XX. A reduction in the substrate outside diameter occurred in nearly all of the parts coated. The data were also grouped according to SiC concentration to see if any difference in deformation occurs between high and low concentration of SiC. No significant difference was apparent.

The reduction in substrate diameter, which occurred, reinforces the characterization data which indicate that the CTE of the coating is higher than that of the ATJ substrate. Because of this mismatch cooldown stresses were, in some cases, high enough to cause coating failure as occurred in 2734-23 and 2724-25.

An average increase in the substrate length was indicated, as expected.

The summarized information on the eight coating runs that were used in the test firings is divided into the three groups, A, B and C in Table XIX. Again, there is no obvious reason why the products of Group A and Group B are so different except that a relationship may exist between the SiC crystal size and the MTS concentration in the reactant gas. It may also be caused by differing thermal gradients in different furnaces. It should be emphasized that the control of codeposit properties as a function of deposition parameters is still not well understood.

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SECTION vn

REPRODUCIBILITY STUDY

A study was undertaken to determine whether the coating characteristics, density, percent SiC, microstructure, and thickness were reasonably consistent in both the axial and the circumferential directions along the inserts. Two standard HIPPO throat inserts, two wraparound design HIPPO throat inserts and two HIPPO entrance approach sections were each sectioned at four axial locations in planes perpendicular to the axU. The specific axial locations are shown in Figures l;i. 19, and 20, The results of the study are contained in TablesXXI, XXII, and XXII1.

One of the standard throat inserts was examined at four eq'iispaced, 90 degree, circumferential locations at each axial location, as shown in Table XXIV. No unexpected anomalies occurred, except: (1) some inconsistencies in density around the periphery (T?Me XXIV) and (2) some inconsistencies in the relationship between SiC content and density, particularly in the case of Run 2732-6 in Table XXIII.

The SiC contents were determined by LUV AC, Incorporated, by a wet chemical method. The weighed sample was pulverized in a mortar and pestle so as to completely pass through a 200 mesh screen. The powder was fused with Na-,CO, in a platinum crucible and leached with dilute HC1. Perchloric acid was added and the silicon was precipitated as hydrated silicic acid by evaporation of the volatile constituents from the liquor. The silicic acid was converted to SK^ by ignition with a Bunsen burner. This product was weighed. HF was then added and the material was again evaporated to drive off the silicon as SiF^,. The residue was then weighed. The difference between the two weighings gave the SiO-t weight and allowed calculation of the SiC content in accordance with the formula:

%SiC = Weight of Si02 X 66.7

Weight of Sample

1. SiC CONTENT/DENSITY RELATIONSHIP

Using the SiC content data by wet chemistry from Li:vac and the Atlantic Research density data for forty-one specimens from the reproducibility study containing between 15 and 25 percent SiC, the relationship between density and SiC content was established using a least squares equivalent technique and using the formula:

1

^composite

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321 lOOdr

In this way, the density of the PG phase was determined to be 2.14 gm/cc, and the coefficient of variation was found to be 13 percent of the SiC content. The density of SiC was assumed to be 3.21 gm/cc. The standard deviation with respect to the PG phase density was found to be ±0.02 percent. The new plot of percent SiC versus density, based on the above formula is shown in Figure 21.

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The raw data for the plot is contained in Section 8.0, Tables XXI through XXIII. The data are less reliable below IS percent and above 25 percent. However, above 25 percent, the density relationship is considered to be more reliable than the ashing technique at Atlantic Research. The ashing technique tended to yield lower than true SIC contents, particularly at the higher SIC contents. This was undoubtedly caused by incomplete conversion of the SiC to S10-) which was probably due to inaccurate temperature control during ashing. This could probably be easily remedied. However, other proven techniques are now available. The density/%SiC relationship is recommended as adequate with an occasional spot check assay.

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SECTION V11I

SiCI4 AS SILICON SOURCE MATERIAL

A small effort was made to investigate the use of SiC(4 as the silicon containing source material. It was felt that SiC could be deposited, from the SiCI4, at higher temperatures than are attainable utilizing CHjSiClj. The achievement of higher deposition temperatures would, hopefully, permit the formation of a higher density pyrolytic graphite matrix than is obtained when depositing fiom CHjSiCl-j, at 3,200oF. A higher matrix density would be expected to increase the erosion resistance of the composite coating.

From the data gathered during this initial series of experiments, it was indicated that the use of SiCl4 as the source will yield coatings at a significantly higher deposition temperature than can MTS, The possible advantages of this higher deposition temperature may, however, be outweighed by the poorer dispersion of the SiC phase within the PC matrix.

Eight deposition runs were conducted in the 4-inch Pereny furnace utilizing 2-inch I.D. tubular specimens, 10 inches in length. After deposition the coated tubes were sectioned transversely into five segments at 2-inch increments from the injector. These segments were then polished and examined microscopically. The coating thickness was measured microscopically and the SiC concentration was estimated visually.

Coating process data are shown in Table XXV and coating data are shown in Table XXVI.

The first deposition run was conducted at 3200°F using deposition gas flow rates established during prior codeposition studies. The first 3-1/2 inches of the substrate tube were covered with a soft deposit. The next portion of the coating was hard and gray in color. Where present, the SiC phase existed as long, stringlike crystalline spikes normal to the substrate surface. The SiC crystals were more massive than those previously grown from MTS. The dispersion was also poorer than that found when using MTS as the silicon source.

The second deposition run was conducted for 2 hours at 3200CF. During the first hour no hydrogen was used. During the second hour, hydrogen was added at twice the rate of Run No. 1. The hydrogen addition increased the SiC concentration but significantly decreased the coating deposition rate. From this observation it appears likely that the hydrogen addition inhibits the deposition of PG rather than increasing that of the SiC. The SiC phase in both portions of the coating was in the form of thick crystalline spikes with a definite tendency to cluster at the PG cone boundaries. The PG cones contained significant amounts of intraconical delaminations.

Deposition Runs 3, 4, 5 and 6 were conducted at 3600oF using various deposition gas flow rates. With the exception of deposition Run No. 4, this series yielded coatings of inferior quality, and the SiC phase showed poor dispersion where present. Run No. 4 contained moderately well dispersed SiC for the first 6 inches. The crystalline form was thick and spike like. Although the majority of the crystals were oriented 90 degrees to the substrate surface, a significant amount could be seen growing at angles up to 45 degrees from the normal plane. Visible within the coating were many pure PG growth cones which contained fine strain lines and some intraconical delaminations. Figure 22 shows the coating microstrucutre achieved in Run No. 4 at the 4-inch position.

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The seventh deposition was conducted at 3800oF using the deposition gas flow rates of Run No. 4. No SiC phase was visible in any area of the coating.

The eighth and last deposition run was conducted at 3600°F using a S-/S0 MTS and SiCI4 mixture as the silicon source. No SiC phase was visible in any area of the coating.

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SECTION IX

QMS STUDIES

The quantitative metallurgical system, QMS, is capable of automatically scanning a photomicrograph of a two-phase system such as pyrographite/silicon carbide codeposit and measuring the total area, total length and breadth along two perpendicular axes, and total number of particles of one of the phases. From this, it automatically calculates the average particle area, the average length, L, and width, D, of the particles, and the elongation ratio, L/D, and the mean free path. It also is capable of automatically yielding the particle size distribution of the second phase. The microphotograph may be taken by either light optics or by electron optics. In any case, good color contrast between the phases and good edge definition of the particles must be obtained.

Three PG/SiC specimens were submitted to Micron, Inc. under this program. The following data were requested: (1) total area of field, (2) total area occupied by SiC particles, (3) total number of SiC particles in field, (4) average area of SiC particles, (5) average length of SiC particles in longitudinal direction, (6) average length of SiC particles in transverse direction, (7) average elongation ratio, (8) mean free path, and (9) SiC particle size distribution. The results of the QMS study at Micron, as well as of the pertinent prior examination at Atlantic Research, are summarized in Table XXVII. The complete QMS study is contained in the Micron reports which are attached as Appendices A and B. It was attempted to check the QMS results by comparison with; (1) approximation of the average particle area by light optics at Atlantic Research, and (2) percent SiC area calculated from the SiC concentration obtained at Atlantic Research from density measurements.

This first specimen submitted to Micron, No. 162-74, was the product from a test firing of a 1-inch subscale nozzle, CM4. The particular PG/SiC codeposit area examined was near the throat and near the substrate. The specimen was electropolished at Atlantic Research using KOH electrolyte to improve the definition of the SiC particle outline and to improve the color contrast between the SiC and the graphite matrix. The resulting microstructure is contained in Appendix A. Color contrast and phase edge sharpness are quite good. The particular area examined appears considerably coarser, at least in some parts, than the Code 21 that was assigned to this material. This explains, at least in part, why the QMS gave an average particle size area of 19.4 square microns compared with the expected area, from the code, of 4.5 square microns. The SiC area comparison in the table also suggests that the particular area examined by QMS was lower in SiC content that the entire specimen.

The second specimen, 001-52, was a portion of the coating from a 7-inch throat insert from the Scale-up Program. Micron's QMS examination is described in Appendix B and the results are compared with Atlantic Research light microscopy and composition in Table XXVII. Atlantic Research prepared and photographed the specimen by light microscopy. After diamond polishing, the specimen was etched electrolytically in 3 percent KOH at 15 volts and 5 amperes for 60 seconds. Kodak Tri-X film was given a 30-second exposure and the positives were printed on Kedalith high contrast paper. Micron repolished and then photographed the same specimen using electron imaging techniques. As seen in Figures 1 and 2 of Appendix B, the resolution with the electron imaging was quite poor. The average particle area agreement with Atlantic Research was best in the case of the light optics, as seen in Table XXVII. On the other hand, the percent SiC area showed best correlation in the case of the electron microprobe. Micron did not conduct the particle size distribution analysis because of the poor resolution obtained. It should be noted that Micron had confused Specimen No. 4, previously submitted and studied in Appendix A, with Run 001-52.

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The third specimen studied at Micron was PG/SiC graded codeposit from a 1-inch throat insert, 033-16. The QMS results with this material are also reported in Appendix B and compared with Atlantic Research results in Table XXVII. This was a graded coating and the QMS work was done on the PG/SiC area near the substrate. Consequently, the Atlantic Research data reported in Table XXVII are an average of the data from the 1-inch throat inserts that were fired under the Air Launch Program. This particular specimen was examined by the scanning electron microscope, SEM, only, in the unpolished condition; i.e., in the as-coated condition. Fair checks with Atlantic Research data were obtained. The SEM photographs of this specimen, which are shown in Appendix B, are excellent (see Figure 4 of Appendix B) and show some gradation in SiC crystal population from the substrate (bottom of Figure 4) outward. However, it is doubtful whether edge definition is adequate with SEM since the flat surface is not photographed. The lower than expected elongation ratios obtained in this study may be explained by the fact that the long needles appear dominant to an observer of a microstructure whereas the small nonacicular particles are generally not too noticeable.

This brief study should be regarded as preliminary. It does demonstrate the QMS technique may have potential. It's use would appear to be more feasible after improved reproducibility and homogeneity are achieved. At least until that time, ten or twelve areas should be examined and the results averaged.

One area in which the current QMS technique appears to be lacking is that of defining the degree of dispersion of the particles. The mean free path is only an average of the distance between the particles. An attempt should be made to define the degree of distribution of the free path about this mean. SiC particles often segregate in ar.u/or have a higher elongation ratio in the PG grain boundaries.

A number of specimens have been submitted to AFML and Micron, Inc., under the SCALE-UP Program. Twelve areas of each specimen are being examined by electron optics. Some tentative results have been received orally from Lieutenant Hollenberg at AFML. This indicates good agreement between the SiC by QMS and Atlantic Research density determinations, provided the SiC crystals are not finer than Code 20. AFML also find that the best edge definition and phase contrast can be obtained with the electron microprobe. They are also attempting to measure the degree of dispersion of the particles.

A specification on the QMS, as applied to PG/SiC codeposit, will be issued at a later date as a separate document.

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SECTION X

DISCUSSION

A summary of the major coatinv run variables is listed in Table XXVIII. The most common microstructure found with these test fired codings was code 20 but some codes 10, II, 21, and 30 were also considered to be acceptable. No clear patterr. for the microstructural variation emerges although it is thought to be associated mainly with deposition temperat jre, thermal gradient and gas flow rate. Composition and deposition rate were never considered to be under good .ontrol, although they were known to be functions of temperature, MTS content of the reactant gas, and probably lesidence time and temperature gradient. The molar ratio of CH^/MTS in the reactant gases is approximately 7. The Ueposition reactions are;

and CH4 = C + 2H2

CH3SiCI3 SIC + 3HCI

Assuming stoichiometry, the PG/SiC ratio in the product should be 7. However, it is approximately 14. This indicates that pyrolysis, of the MTS, is not complete. This effect is even more pronounced at higher temperatures.

Although the effect of deposition temperature was not determined systematically, in this series, it has been and is known that an increase in temperature coarsens the SiC needles and decreases the SiC content. In this work, every attempt was made to hold the temperature at 3200oF. Frequent checks of the optical pyrometers using primary temperature standards as well as checks against other recently calibrated instruments showed the pyrometers to always be in calibration to within 50oF.

The distance between the injector lip and the substrate was determined to be extremely important, particularly in controlling the coating thickness profile. The 1.0-inch throat insert entrance to throat thickness ratio was quite inconsistant until the spacing was increased by 0.5 inch. It then remained at 0.75±2 for the remainder of the 1.0 inch series. Similarly, the same thickness ratio for the 1.7 inch HIPPO inserts became consistent only after run 26 when the spacing was increased by 1 inch.

Another coating variable that may be important is the thermal gradient along the furnace axis. This may explain why coatings change in character when a new furnace element is installed.

The nitrogen flow rate is also considered to be important in that the PG cones are smaller and the coating surface is smoother at higher flow rates.

A few special points on good coating technique wc-re discovered in this investigation. At least a IS minute soak time at temperature is required and, further, the MTS must be turned on prior to the methane presumably in order to provide sites for growth of the SiC needles. Otherwise, the SiC crystals may be coarse and the dispersion poor early in the deposition cycle. Another point of good technique is that the substrate must be clean of all loose particles and debris prior to coating.

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The dimension«! changes on the outside envelope of the components change only very little during coating. It appears, from this work, that the change Is detectable only on sufficiently large components; In this case the 1.7-inch HIPPO entrance approach sections.

The number of coating runs made In each type of furnace and for each type of coated component are listed In Table XXIX. The Phase I Task I goal was achieved in that sufficient pieces for the 1.0-inch subscale (7) and the 1.7-inch HIPPO tests firings (8) were fabricated. However, it was not possible to control the coating parameters so as to accurately engineer the effect of a change in substrate geometry or size. Preliminary coating experiments must still be conducted with each new geometry. A fundamental coating study to assess the effect of all coating parameters Is recommended.

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4-Inch Pereny

Resistance Furnace

6-inch Pereny

Resistance Furnace

6-inch Induction Furnace Total

1.0-inch Throat Inserts 28 0 0 28

1.0-Inch Entrance Approach Sections

7 5 0 12

1.7-inch Throat Inserts 38 2 4 44

1.7-inch Entrance Approach Sections

0 11 16 27

Totals 73 18 20 111

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