M. Ivosevic, R. Knight, S.R. Kalidindi, and G.R. Palmese

Drexel University, Philadelphia, Pennsylvania

J.K. Sutter

Glenn Research Center, Cleveland, Ohio

Microstructure and Properties of Thermally

Sprayed Functionally Graded Coatings for

Polymeric Substrates

NASA/TM—2003-212119

March 2003

https://ntrs.nasa.gov/search.jsp?R=20030032481 2018-05-15T15:57:11+00:00Z
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M. Ivosevic, R. Knight, S.R. Kalidindi, and G.R. Palmese

Drexel University, Philadelphia, Pennsylvania

J.K. Sutter

Glenn Research Center, Cleveland, Ohio

Microstructure and Properties of Thermally

Sprayed Functionally Graded Coatings for

Polymeric Substrates

NASA/TM—2003-212119

March 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

International Thermal Spray Conference and Exposition

cosponsored by the ASM Thermal Spray Society (TSS), German Welding Society (DVS),

and the International Institute of Welding (IIW)

Orlando, Florida, May 5–8, 2003
Acknowledgments

The authors gratefully acknowledge financial support from the Propulsion and Power program at NASA Glenn

Research Center, Cleveland, Ohio (NCC3–825). The authors also greatly appreciate the assistance of Mr. Dustin

Doss during the HVOF spraying of the coatings and Mr. David Von Rohr during the microscopy and analysis.

Available from

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Available electronically at http://gltrs.grc.nasa.gov

The Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.
NASA/TMó2003-212119 1 

Microstructure and Properties of Thermally Sprayed Functionally Graded 

Coatings for Polymeric Substrates 

M. Ivosevic, R. Knight, S.R. Kalidindi, and G.R. Palmese 

Drexel University 

Philadelphia, Pennsylvania 19104 

J.K. Sutter 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Abstract 

The use of polymer matrix composites (PMC's) in the gas flow 

path of advanced turbine engines offers significant benefits for 

aircraft engine performance but their useful lifetime is limited 

by their poor erosion resistance. High velocity oxy-fuel 

(HVOF) sprayed polymer/cermet functionally graded (FGM) 

coatings are being investigated as a method to address this 

technology gap by providing erosion and oxidation protection 

to polymer matrix composites. The FGM coating structures 

are based on a polyimide matrix filled with varying volume 

fractions of WC-Co. The graded coating architecture was 

produced using a combination of internal and external 

feedstock injection, via two computer-controlled powder 

feeders and controlled substrate preheating. Porosity, coating 

thickness and volume fraction of the WC-Co filler retained in 

the coatings were determined using standard metallographic 

techniques and computer image analysis. 

The pull-off strength (often refered to as the adhesive strength) 

of the coatings was evaluated according to the ASTM D 4541 

standard test method, which measured the greatest normal 

tensile force that the coating could withstand. 

Adhesive/cohesive strengths were determined for three 

different types of coating structures and compared based on 

the maximum indicated load and the surface area loaded. The 

nature and locus of the fractures were characterized according 

to the percent of adhesive and/or cohesive failure, and the 

tested interfaces and layers involved were analyzed by 

Scanning Electron Microscopy. 

Introduction 

For the successful integration of high temperature polymer 

matrix composites (PMCís) into jet engines, key long-term 

performance characteristics such as erosion resistance and 

thermo-mechanical fatigue (TMF) behavior must be 

addressed. Even with significant benefits such as weight 

savings, improved strength, reduced part counts and reduced 

manufacturing costs, the successful application of PMCís into 

the gas flow path of advanced turbine engines is still limited 

by their poor erosion resistance. Yet, there is little published 

information describing solutions for this erosion problem on 

polymer composites [1]. Erosion-resistant coatings are needed 

to protect the composite materials, at least through the first 

overhaul interval, and preferably for the full life of the 

component. 

In addition, components used in engine gas flow paths must 

exhibit an acceptable surface finish to ensure good 

aerodynamic performance. The goal of this work was to 

develop thermally sprayed erosion/oxidation resistant 

functionally graded (FGM) coatings for PMC substrates. The 

goal was to grade the coating composition from pure 

polyimide, similar to the matrix of the PMC substrate on one 

side, to 100% WC-Co on the other [2]. Both step-wise and 

continuous gradations of the WC-Co loading in these coatings 

are being investigated. A 100% WC-Co outer layer should 

provide improved wear and oxidation protection to the 

substrate. The thermosetting polyimide coating matrix should 

help manage the differences in coefficient of thermal 

expansion (CTE) between pure WC-Co and the PMC substrate 

material, and improve the TMF properties of the 

coating/substrate system. Secondary goals included improved 

surface finish relative to coatings developed during a previous 

study [3] and evaluation of the thermo-mechanical fatigue 

(TMF) properties of the coating/substrate system. Results 

reported here focus on coating microstructure and aspects of 

the adhesive and/or cohesive properties of the FGM coatings. 

Background 

The limited work reported on the thermal spray deposition of 

erosion-resistant coatings on fiber-reinforced polymer 
NASA/TMó2003-212119 2 

composites [4ñ6] almost exclusively used metallic (zinc, 

nickel or aluminum) bondcoats or polymeric (polyamide, 

polyimide or PEEK) layers to enhance the adhesion of the 

wear-resistant topcoats (typically carbides, borides or nitrides) 

to PMC substrates. However, for applications subject to 

thermal fatigue or thermo-mechanical fatigue this may be a 

limitation owing to the large differences in CTE within the 

overall coating/substrate system. Work on the plasma spraying 

of epoxy-based materials [7, 8] reported some difficulty in 

obtaining satisfactory build-up and properties of thermosetting 

polymeric coatings. It was reported that external substrate 

preheating might enhance the deposition behavior and curing 

reaction during the thermal spraying of thermosetting 

polyimide materials. Preliminary proof-of-concept evaluations 

resulting in the successful deposition and build-up of pure 

polyimide and polyimide/WC-Co powders onto electrically 

preheated PMR-15 substrates were previously reported [2, 7]. 

Improved substrate preheating set-ups were subsequently 

developed during this work. 

The potential benefits of using polymer matrix coatings filled 

with commercial or nano-sized ceramic (e.g. silica or alumina) 

reinforcements or fillers has been reported by a number of 

authors [9, 10]. The high velocities and non-uniform velocity 

distributions of thermal spray jets, in combination with 

variations in particle size, density and morphology can, 

however, result in significant segregation when dissimilar 

materials are co-sprayed. This represents a serious challenge 

when the desired feedstock materials have significant 

differences in density, as was the case for the two materials 

used in this work, polyimide and WC-Co, which have 

densities of 1.39 and 12.5 gm/cm3

, respectively. 

Consequently, spraying techniques that minimized material 

segregation were a key requirement. One approach considered 

for minimizing material segregation was dry ball-milling to 

produce a ìcompositeî feedstock, with the ceramic phase 

embedded into the polymer component. Ball-milling of the 

polyimide and WC-Co powders was investigated here, 

however, as reported previously [2], little or no embedding of 

the harder WC-Co particles into the softer, yet somewhat 

brittle, thermosetting polyimide particles was obtained. 

An alternative approach investigated to minimize material 

segregation was a powder feeding configuration that allowed 

simultaneous internal and external feeding of the two 

materials (Fig. 1). External feeding of the much denser WCCo component and internal feeding of the polyimide enabled a 

balance between the differences in momentum and kinetic 

energy of the two feedstock materials to be established. 

Moreover, this configuration also helped to minimize the 

differences in heating of the polymer and cermet feedstocks. 

Internal feeding of the polyimide material, with a much lower 

thermal conductivity (~0.2 W/mK), afforded longer residence 

times within the HVOF jet than those experienced by the 

externally fed, higher thermal conductivity (~100 W/mK) 

WC-Co. 

Computer 

Control

Powder 

Feeder I

WC-Co

HVOF 

Gun

Powder 

Feeder II

POLYIMIDE

Computer 

Control

Powder 

Feeder I

WC-Co

HVOF 

Gun

Powder 

Feeder II

POLYIMIDE

Figure 1: Simultaneous internal/external powder feeding of 

the polyimide and WC-Co feedstocks. 

Experimental Approach 

Thermosetting polyimide (end-capped) of an oligomeric form 

was selected as the matrix material for the proposed FGM 

coatings. Molding polyimide material produced by Maverick 

Corp. was mechanically crushed and cryo-ground (Shamrock) 

to produce a powder that flowed well, with a particle size 

distribution in the range (ñ100, +20 μm), suitable for HVOF 

spraying. A previous study [3] utilized a conventional 88/12 

WC-Co powder (Sulzer Metco 71VF-NS) for the functional 

surface. A similar WC-Co powder - Amperit 515.0 (H.C. 

Stark, Inc.), with a particle size range of (ñ22.5, + 5.6 μm) was 

selected as the reinforcing/filler material. Substrates were 25 x 

75 x 3 mm (1 x 3 x 1/8 in.) coupons of carbon-fiber reinforced 

polymer matrix (PMR-15) composite material. Prior to 

spraying, the PMR-15 substrates were grit blasted using 

250 μm (60 mesh) Al2O3 grit. 

Pure thermosetting polyimide coatings and composite 

polyimide/WC-Co composite coatings were sprayed using a 

Stellite Coatings, Inc. Jet Kote II HVOF combustion spray 

system, using hydrogen as fuel gas. All the coatings were 

sprayed using the spray parameters summarized in Table 1. 

Table 1: HVOF spray parameters for depositing pure 

polyimide and various ratios of polyimide/WC-Co coatings on 

graphite fiber reinforced PMR-15 composite substrates. 

HVOF Spray Parameter Value

Spray distance (m) 0.15 

Polyimide feed rate (gm/min) 2 - 9 

WC-Co feed rate (gm/min) 20 - 50 

Carrier gas Ar 

Carrier gas flow rate (m3/s) 0.5 x 10ñ4

H2:O2 ratio 0.4 - 0.5 

H2 and O2 flow rates (m3/s) 3 x 10ñ3 / 6-7 x 10ñ3

Surface speed (m/s) 0.11 

Substrate temperature (°C) 230 - 340 

Coating thicknesses obtained were in the range 300ñ600 μm 

(12ñ24 mils). A 100% WC-Co outer layer, or topcoat, was 

sprayed using a UTP UNI-Spray-Jetô flame-powder spray 
NASA/TMó2003-212119 3 

system, again using hydrogen as the fuel gas. The flame spray 

parameters are summarized in Table 2. 

Table 2: Typical flame spray parameters used for spraying 

pure WC-Co topcoats onto polyimide/WC-Co coatings. 

Flame Spray Parameter Value

Spray distance (m) 0.08 

WC-Co feed rate (gm/min) 70 

Carrier gas Ar 

Gas pressure - H2 (MPa) 2900 

Gas pressure - O2 (MPa) 5075 

Gas pressure - Cooling air (MPa) 1450 

Surface speed (m/s) < 0.11 

Substrate temperature (°C) 15 

Spraying of the graded coating structures required reliable, 

repeatable simultaneous feeding of two materials, WC-Co and 

polyimide. A LabVIEWÆ -based computer-controlled system 

was developed and used for simultaneously controlling two 

identical Praxair Model 1207 volumetric powder feeders 

(Fig. 1). Electric strip heaters (Omega type PT 502/120) were 

used as external heat sources in contact with the rear of the 

substrate, together with a hot air gun directed at the front face 

[11]. Preheating was only used while spraying the pure 

polyimide and polyimide/WC-Co layers, not during the flame 

spraying of the WC-Co topcoat. 

Porosity, coating thickness and volume fraction of the WC-Co 

filler retained in the coatings were determined using standard 

metallographic techniques - sectioning, mounting and 

polishing - and computer image analysis using Scion Image 

software. The pull-off strength, often referred to as adhesion, 

of the coatings was determined according to the ASTM D4541 standard test method by measuring the greatest normal 

tensile force that the coatings could withstand. A schematic of 

the pull-off test method used is shown in Figure 2. 

Pull Stub 

[ÿ 13 mm]

Bearing 

Ring

Epoxy

Coating

Substrate

Pull Stub 

[ÿ 13 mm]

Bearing 

Ring

Epoxy

Coating

Substrate

Figure 2: Schematic of the ASTM D 4541 pull-off test method.

The test conditions used were as follows: 

! Self-aligning sample fixtures adapted for laboratory use 

on an Instron 5800R mechanical testing machine (Model 

58R1127 in conjunction with Instron Merlin software). 

! Temperature and relative humidity of 20 °C and 65%, 

respectively. 

! 12.7 mm (0.5 in.) aluminum pull stubs (type PS-25, M. E. 

Taylor Engineering, Inc.) prepared in accordance with the 

ASTM D 2651 guide for the preparation of metal 

surfaces for adhesive bonding. 

! 3M type DP-460 off-white adhesive, with a curing time 

of 60 minutes at a temperature of 60 °C. 

The bearing ring was located concentrically around the 

loading fixture on the coating surface (Fig. 2).The 

adhesive/cohesive strengths of three different types [Table 3] 

of coating were measured and compared based on the 

maximum indicated load and the original surface area stressed. 

Six samples of each coating type were tested and the mean 

value of the results reported. 

Table 3: Types of coating evaluated by tensile adhesion 

testing. 

Coating Designation Composition 

Pure Polyimide Coating A + B 

2-Layer Coating A + B + C 

3-Layer Coating A + B + C + D 

Where: 

A ñ Carbon-fiber reinforced PMR-15 substrate. 

B ñ Pure polyimide coating layer. 

C ñ WC-Co/polyimide composite coating layer. 

D ñ Pure WC-Co topcoat. 

The nature and locus of the failures were characterized 

according to the percent of adhesive and cohesive failures. 

The surfaces of the as-tested samples were subsequently 

analyzed by scanning electron microscopy using an Amray 

Model 1830 SEM. 

Results and Discussion 

An important first step of this work was the development of 

HVOF spray parameters for depositing the pure polyimide 

matrix material onto PMC substrates, owing to the reported 

difficulty in obtaining satisfactory initial coating build-up. An 

adherent pure polyimide coating (region B, Fig. 3) was 

obtained after extensive parameter development and 

understanding of the substrate preheating requirements. These 

coatings exhibited good adhesion and clean coating/substrate 

interfaces with little or no apparent debonding. 

A significant level of porosity ñ the large black areas within 

region B of Fig. 3 - was observed in the polyimide coating. 

The high void level (~26%) may have been due to gas 

evolution during the crosslinking reaction (the onset of 

polymerization of the oligomer end-groups) that began above 

~280 °C. Out-gassing of the PMC substrate material may also 
NASA/TMó2003-212119 4 

have contributed to the excessive porosity observed, even 

though the substrates were vacuum dried at ~120 °C for ~2 

days and stored in a desiccator prior to spraying. Since gas 

evolution during crosslinking is a function of the polyimide 

chemistry and imidization kinetics, this problem may be 

addressed in the future by further studying these relations. 

Figure 3: Pure polyimide coating (B) HVOF sprayed onto a 

carbon-fiber reinforced PMC substrate (A). 

Two- and three-layer polyimide/WC-Co coating microstructures are shown in Figs. 4 and 5. 

Figure 4: Two-layer HVOF sprayed FGM coating comprising 

a WC-Co/polyimide outer layer (C) and a pure polyimide 

layer (B) on a PMC substrate (A). 

Image analysis of the WC-Co/polyimide layers (region C) 

shown in Figs. 4, 5, 8 and 13, indicated that the composite 

layer had the following typical composition: 

! 46% polyimide matrix. 

! 26% WC-Co filler. 

! 28% voids/porosity. 

Figure 5: Three-layer FGM coating comprised of a pure WCCo topcoat (D), a WC-Co/polyimide layer (C) and a pure 

polyimide layer (B), on a PMC substrate (A).

Neglecting voids, this indicated that the relative proportions of 

the polyimide matrix and WC-Co filler were 58% and 42%, 

respectively. 

Results of the tensile adhesion tests are shown in Fig. 6. 

Coatings similar to those shown in Figs. 4 and 5 both 

exhibited lower pull-off strengths (5ñ6.2 MPa) than the pure 

polyimide coatings (~8.4 MPa).The tensile strength of the 

PMC substrate was determined using the same pull-off test to 

establish a reference value for the substrate adhesive/cohesive 

characteristics. Uncoated substrates failed due to delamination 

in the direction perpendicular to thickness at a tensile strength 

~17.6 MPa. The locus of failure of the samples tested was 

characterized according to the location and percent of 

adhesive and cohesive failures, together with SEM 

characterization of the interfaces and coating layers. 

8.53

5.32

6.67

0

1

2

3

4

5

6

7

8

9

10

Pure Polyimide 2-Layer 3-Layer

Type of Coating

Adhesive/Cohesive Strength [ MPa]

Figure 6: Results of the pull-off tensile adhesion tests for three 

types of coating (ASTM D-4541). 
NASA/TMó2003-212119 5 

Most of the samples failed due to delamination within the 

substrate (Fig. 7). These results were surprising given that the 

tensile strength of the reference (uncoated) substrate 

(~17.6 MPa) was much higher than the tensile strength of any 

of the coatings tested. This may have been due to the damage 

caused by the grit blasting used to roughen the substrate 

surfaces prior to spraying. The uncoated substrates tested were 

not grit blasted prior to testing. 

Pure Polyimide Coating

A, 95%

B, 5%

2-Layer Coating

A, 70%

C, 30%

3-Layer Coating

A, 80%

C, 20%

Figure 7: Estimated percentages and locations of adhesive 

and cohesive failures. Pure polyimide coating ñ failure mainly 

within the substrate (A); two- and three-layer coatings ñ 

failure within both the substrate (A) and composite layers (C). 

Grit blasting damaged the surface of the substrate, likely 

reducing its tensile strength, by breaking carbon fibers, as 

shown in Fig. 8. 

Figure 8: Three-layer composite coating showing the 

substrate surface fiber damage [W] caused by grit blasting. 

SEM micrographs showing the failed surface of a pure 

polyimide coating are shown in Figs. 9 and 10. The mode of 

fracture in the thermosetting polyimide was brittle, associated 

with the formation of cracks in regions between the gas voids 

where localized stress concentrations likely occurred. At a 

higher magnification of a region between gas voids (Fig. 11), 

parallel flat plateaus were observed, indicating brittle crack 

propagation along multiple parallel planes. 

Figure 9: Fracture surface of a failed pure polyimide coating. 

Cohesive failure within the polyimide coating (B) combined 

with cohesive failure in the substrate (A). 

The two- and three-layer coatings both failed by the same 

mechanism, a combination of the cohesive failure within the 

composite layer (region C) and cohesive failure within the 

substrate (region A), as shown in Fig. 7. 

Figure 10: Detail of the fracture surface of a pure polyimide 

coating (B). 

SEM micrographs showing the failed surfaces of tested twoand three-layer coatings are shown in Figs. 12, 13 and 14. 

W 
NASA/TMó2003-212119 6 

Cracks appeared to have propagated partially along the 

interfaces between the WC-Co filler and the polyimide matrix, 

as shown at the left side of Fig. 14. This may have contributed 

to the lower pull-off strengths measured for the two- and 

three-layer coatings relative to the pure polyimide coating. 

Figure 11: High magnification (500X) image of the inter-pore 

region of a tested pure polyimide coating. The parallel plateau 

and ìriverî patterns indicated brittle crack propagation along 

multiple parallel planes in the polyimide coating (B). 

Figure 12: Typical image of the fractured surface of two- and 

three-layer coatings. Cohesive failure within the composite 

layer (C) was combined with delamination within the 

substrate (A). 

Figure 13: Fracture surface of a two-layer coating within the 

WC-Co/polyimide composite layer (C). 

Most of the tensile failures within the WC-Co/polyimide layer 

(region C) occurred due to crack propagation between WC-Co 

particles (Fig. 14) which was believed to be due to low 

elongation prior to yielding and the high modulus of rigidity 

of the thermosetting polyimide matrix. 

Figure 14:Brittle cracks propagating from a pore within the 

composite layer (C) of a three-layer coating. 

Summary and Conclusions 

The feasibility of depositing FGM coatings consisting of 

layers of pure polyimide, polyimide + WC-Co and pure WCCo has been demonstrated using a combination of internal and 

external feeding of the two feedstock materials. Pure 

polyimide and polyimide + WC-Co were sprayed by the 

HVOF process and the WC-Co topcoat was flame sprayed. 
NASA/TMó2003-212119 7 

The porosity and volume fraction of the WC-Co filler retained 

in the sprayed coatings were determined by standard 

metallographic techniques and image analysis. The relative 

proportions (on a pore-free basis) of polyimide matrix and 

WC-Co filler were 58% and 42%, respectively. The porosity 

of the pure polyimide coating was determined to be ~26%, 

likely due to gas evolution during crosslinking above 280 °C. 

The tensile adhesion behavior of three types of coatings 

sprayed onto carbon-fiber reinforced PMC substrates has been 

evaluated using a standard pull-off tensile adhesion test 

(ASTM D 4541). The adhesive/cohesive strengths of the 

different coating types were measured and compared to that of 

an uncoated substrate. The two- and three-layer composite 

coatings both exibited lower pull-off strengths (5ñ6.2 MPa) 

than pure polyimide coatings (~8.4 MPa), and in all cases 

these values were lower than the tensile strength (~17.6 MPa), 

of the uncoated PMC substrate used as a ìreferenceî for 

assessing coating adhesion/cohesion. The nature and locus of 

the failed surfaces following tensile testing were characterized 

according to the percent adhesive and cohesive failures. The 

majority of the failures were due to delamination within the 

substrate, which occurred at lower strengths than those 

exhibited by uncoated substrates. The difference was likely 

due to substrate damage caused during grit blasting prior to 

coating. 

The mode of failure in the thermosetting polyimide was brittle 

fracture associated with the formation of cracks at inter-pore 

regions where localized stress concentrations would occur. 

The two- and three-layer FGM coatings both failed by the 

same mechanism - a combination of cohesive failure in the 

composite layer and delamination within the substrate. 

Additional work is being carried out, including continued 

spray parameter development and optimization, assessment of 

the repeatability of the results and detailed characterization of 

the coatings, including evaluation of the thermo-mechanical 

fatigue properties of the coating/substrate system, and their 

erosion resistance. Polyimide chemistry optimization will also 

be investigated in order to reduce gas evolution during 

crosslinking reactions. 

References 

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High Temperature Polymer Composites,î Proc. 44th

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Technical Memorandum

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National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

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13

Microstructure and Properties of Thermally Sprayed Functionally Graded

Coatings for Polymeric Substrates

M. Ivosevic, R. Knight, S.R. Kalidindi, G.R. Palmese, and J.K. Sutter

Coatings; Composites; Polyimide

Unclassified -Unlimited

Subject Category: 27 Distribution: Nonstandard

Prepared for the International Thermal Spray Conference and Exposition cosponsored by the ASM Thermal Spray Society

(TSS), German Welding Society (DVS), and the International Institute of Welding (IIW), Orlando, Florida, May 5–8,

2003. M. Ivosevic, R. Knight, S.R. Kalidindi, and G.R. Palmese, Drexel University, Philadelphia, Pennsylvania 19104;

J.K. Sutter, NASA Glenn Research Center. Responsible person, J.K. Sutter, organization code 5150, 216–433–3226.

The use of polymer matrix composites (PMC’s) in the gas flow path of advanced turbine engines offers significant

benefits for aircraft engine performance but their useful lifetime is limited by their poor erosion resistance. High velocity

oxy-fuel (HVOF) sprayed polymer/cermet functionally graded (FGM) coatings are being investigated as a method to

address this technology gap by providing erosion and oxidation protection to polymer matrix composites. The FGM

coating structures are based on a polyimide matrix filled with varying volume fractions of WC-Co. The graded coating

architecture was produced using a combination of internal and external feedstock injection, via two computer-controlled

powder feeders and controlled substrate preheating. Porosity, coating thickness and volume fraction of the WC-Co filler

retained in the coatings were determined using standard metallographic techniques and computer image analysis. The

pull-off strength (often referred to as the adhesive strength) of the coatings was evaluated according to the ASTM D 4541

standard test method, which measured the greatest normal tensile force that the coating could withstand. Adhesive/

cohesive strengths were determined for three different types of coating structures and compared based on the maximum

indicated load and the surface area loaded. The nature and locus of the fractures were characterized according to the

percent of adhesive and/or cohesive failure, and the tested interfaces and layers involved were analyzed by Scanning

Electron Microscopy.