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

Drexel University, Philadelphia, Pennsylvania

A. Tsurikov

Naval Surface Warfare Center, West Bethesda, Maryland

J.K. Sutter

Glenn Research Center, Cleveland, Ohio

Optimal Substrate Preheating Model for

Thermal Spray Deposition of Thermosets

Onto Polymer Matrix Composites

NASA/TM—2003-212120

March 2003

https://ntrs.nasa.gov/search.jsp?R=20030032486 2018-05-15T15:56:55+00:00Z
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M. Ivosevic, R. Knight, S.R. Kalidindi, and G.R. Palmese

Drexel University, Philadelphia, Pennsylvania

A. Tsurikov

Naval Surface Warfare Center, West Bethesda, Maryland

J.K. Sutter

Glenn Research Center, Cleveland, Ohio

Optimal Substrate Preheating Model for

Thermal Spray Deposition of Thermosets

Onto Polymer Matrix Composites

NASA/TM—2003-212120

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

 Dr. Antonios Zavaliangos during the model formulation, Mr. Dustin Doss during the HVOF spraying of the

coatings, and Mr. David Von Rohr during the microscopy and analysis.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

The Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

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

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/TMó2003-212120 1 

Optimal Substrate Preheating Model for Thermal Spray Deposition of 

Thermosets onto Polymer Matrix Composites 

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

Drexel University 

Philadelphia, Pennsylvania 19104 

A. Tsurikov 

Naval Surface Warfare Center 

West Bethesda, Maryland 20817 

J.K. Sutter 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Abstract 

High velocity oxy-fuel (HVOF) sprayed, functionally graded 

polyimide/WC-Co composite coatings on polymer matrix 

composites (PMC's) are being investigated for applications in 

turbine engine technologies. This requires that the polyimide, 

used as the matrix material, be fully crosslinked during 

deposition in order to maximize its engineering properties. 

The rapid heating and cooling nature of the HVOF spray 

process and the high heat flux through the coating into the 

substrate typically do not allow sufficient time at temperature 

for curing of the thermoset. It was hypothesized that external 

substrate preheating might enhance the deposition behavior 

and curing reaction during the thermal spraying of polyimide 

thermosets. An additional difficulty arises from the low 

thermal conductivity and low specific heat capacity of the 

PMC substrate, which prevent effective substrate preheating 

by the HVOF jet as in the case of metallic substrates. 

A simple analytical process model for the deposition of 

thermosetting polyimide onto polymer matrix composites by 

HVOF thermal spray technology has been developed. The 

model incorporates various heat transfer mechanisms and 

enables surface temperature profiles of the coating to be 

simulated, primarily as a function of substrate preheating 

temperature. Four cases were modeled: (i) no substrate 

preheating; (ii) substrates electrically preheated from the rear; 

(iii) substrates preheated by hot air from the front face; and 

(iv) substrates electrically preheated from the rear and by hot 

air from the front. 

Thermal properties of the polyimide needed for the 

simulations were determined by Differential Scanning 

Calorimetry (DSC) and Thermo-Gravimetric Analysis (TGA). 

Microstructural characterization of the coatings and the 

morphology of polyimide splats sprayed both with and 

without substrate preheating were analyzed using standard 

metallographic techniques. 

Coating temperature in cases (iii) and (iv) never dropped 

below the crosslinking temperature of the polyimide 

feedstock. This was the critical condition required for the 

curing reaction and successful deposition of thermosets by 

HVOF thermal spraying. 

Introduction and Motivation 

In response to the need for processable high temperature 

resistant polymer matrix materials, NASA developed a new 

class of thermosetting polymers known as PMR 

(Polymerization of Monomeric Reactants) polyimides [1]. 

Polyimide based matrix composites reinforced with carbon 

fibers are widely used in the aerospace industry due to their 

excellent high-temperature mechanical properties, thermooxidative stability and easy processability. PMR-15, for 

example, the first commercial generation of PMR resin, is 

being used to fabricate a range of aircraft engine components 

ranging from small, compression molded, bearings to large, 

structural autoclave-molded engine ducts used on the F404 

engine for the US Navy FA-18 Hornet. A cost/benefit study 

indicated that significant cost and weight savings [2] could be 

achieved by replacing the titanium duct with a composite one. 

New applications of PMC's in turbine engines, however, 

require improvements in compositesí environmental 

durability, which may be achieved by the development of 

functionally graded material (FGM) coatings based on a PMR 

polyimide matrix, similar to the substrate matrix material, 

filled with varying volume fractions of ceramic particles. This 

requires that the polyimide matrix be fully crosslinked during 

deposition in order to maximize its engineering properties. 

Thermal spray technology is being studied as an efficient tool 

for the deposition of polyimide based FGM coatings onto 

PMC substrates. Thermoset deposition by thermal spray 

technology is a challenge in itself due to the time sensitive 

nature of the in situ curing reaction of these polymers. 
NASA/TMó2003-212120 2 

The limited work published on the HVOF spray deposition of 

polymers reports almost exclusively on the spraying of 

thermoplastics - Nylon, polypropylene, polycarbonate, PEEK, 

PAEK, PPS, PE, ETFE etc. Previous work on the plasma 

spraying of epoxy-based materials [3, 4] reported some 

difficulty in obtaining satisfactory build-up and properties of 

thermosetting polymeric coatings. This was attributed to the 

rapid heating and cooling nature of the thermal spray process 

and the high heat flux from the coating to the substrate, which 

did not allow sufficient time at temperature for curing of the 

thermoset to occur. The results indicated that external 

substrate preheating might enhance the deposition behavior 

and curing reaction during the thermal spraying of polyimide 

thermoset materials. 

Preliminary proof-of-concept studies resulted in the successful 

spraying of pure polyimide and polyimide/WC-Co blends onto 

electrically preheated PMR-15 substrates [5, 6]. The substrate 

preheating setups were developed further during this work. 

Problem Statement 

The goal of this work was to determine temperature profiles of 

the thermosetting polyimide splats during HVOF spray 

deposition onto a PMC substrate. A temperature profile model 

of the coating was generated, primarily as a function of 

preheating temperature, by allowing for different heat transfer 

mechanisms. This model will help understand the process 

conditions necessary to produce highly crosslinked polyimide 

molecular architectures by the HVOF thermal spray process. 

Background 

The unit process in the deposition of thermally sprayed 

coatings is the impact of an individual droplet onto the 

substrate surface to form a splat. Assuming that a spherical 

droplet of diameter d is deformed into a disc or a cylinder of 

diameter D and thickness h under constant volume conditions, 

the thickness of an individual splat can be calculated using 

experimental values for the D/d ratio according to equation 1. 

πd3/6 = π D2 h/4 (1) 

For an average 50 μm diameter droplet, the calculated splat 

thickness would be ~8 μm. During cooling, the splat may 

undergo a change of state from liquid to crosslinked solid with 

the resulting enthalpy change acting as a heat source. Coating 

change of state can be incorporated into the heat transfer 

equations through the mean specific heat of the splat, 

calculated according to equation 2 [3]. 

 Cp = [(Cpc(Tx - ε) + Cpc(Tx +ε))/2] + [∆Hx/(Tx+ε - Tx-ε)] (2) 

Where ∆Hx is the enthalpy of crosslinking, which relates to the 

area of the valley above the baseline on the DSC graph (Fig. 1, 

b-c). Cpc(Tx-ε) and Cpc(Tx+ε) are the specific heats of the lower 

and upper limits of the liquid range, respectively. 2ε is the 

magnitude (in Kelvin) of the crosslinking temperature range of 

the polymer. The mean specific heat calculated in this way 

was 1,300 J/kg K. 

Differential Scanning Calorimetry (DSC) and ThermoGravimetric Analysis (TGA) scans for the polyimide material 

are shown on Figs. 1 and 2, respectively. Numerical values 

obtained from these plots and reported thermal properties 

[1, 7] of the PMR polyimide thermoset material are listed in 

Table 1. 

Figure 1: DSC (in air) scan result for the polyimide powder. 

Figure 2: TGA (in Air) profile of the polyimide powder. 

Table 1: Thermal properties of the PMR polyimide powder. 

Relaxation/Fusion Temp. (°C) 170 - 250 

Crosslinking Temperature (°C) 300 - 400 

Degradation Temperature (°C) ≥ 500 

Specific Heat (J/kg K) 1,300 

Thermal Conductivity (W/m K) 0.3 

Density (kg/m3) 1,400 

Gel Time @ 300 °C (s) ~15 

(a) 

(b) 

(c) 

[a] 

Heat Flow (mW)
NASA/TMó2003-212120 3 

The thermal conductivity of composite materials can vary 

widely based upon both fiber orientation and volume fraction. 

Investigations by several researchers [7] indicated that the 

thermal conductivity of graphite epoxy composites can be as 

low as 1.0 W/m K. A thermal conductivity of 1.0 W/mK and 

specific heat of 900 J/kgK were assumed for the graphite 

fiber-reinforced polyimide composite used in this work. 

The maximum allowed substrate preheating temperature of 

320 °C was specified as a temperature at least 28 °C below the 

glass transition temperature (Tg) of the fully cured substrate 

matrix polyimide (Tg = 350 °C), as recommended [8]. The 

maximum temperature of the HVOF combustion spray jet is 

~3,500 °C and thus the outermost layers of thermoset particles 

may be continuously decomposing during their flight to the 

substrate. The mean temperature of the particle immediately 

prior to impact will thus be between its softening and 

decomposition temperatures, and calculations indicated that it 

was closer to the latter. For the purpose of these calculations, 

the temperature of the droplet immediately before impact was 

equated with the decomposition temperature of the polymer 

(500 °C), since this provided a definite value (Fig. 2). 

Experimental Approach 

In order to validate the theoretical model, a pure polyimide 

coating was sprayed onto a PMC substrate, preheated 

according to theoretical model, using a Stellite Coatings, Inc. 

Jet Kote II HVOF combustion spray system. The coating was 

sprayed using the spray parameters summarized in Table 2. 

Table 2: Typical HVOF spray parameters for spraying pure 

polyimide onto graphite fiber reinforced PMR-15 substrates. 

HVOF Process 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) 280 - 340 

A molding grade of thermosetting polyimide (produced by 

Maverick Corp.) in an oligomeric form was mechanically 

crushed and cryo-ground (Shamrock) to produce a flowable 

(ñ100, +20 μm) particle size range powder suitable for HVOF 

spraying. The substrates used were 25 x 75 x 3 mm (1 x 3 x 

1/8 in.) coupons of carbon-fiber reinforced PMR-15 polymer 

matrix composite material. Glass slides were used to observe 

individual polyimide splats. Coating microstructures and splat 

morphologies of polyimide sprayed both with, and without, 

substrate preheating were analyzed using standard 

metallographic techniques. 

Omega type PT 502/120 electric strip heaters were used as an 

external heat source from the rear, together with hot air 

preheating from the front face of the substrate, as described 

later in Case 4. 

Simulation Details 

To a first order approximation, the heat flow in the radial 

direction of the coating may be neglected and the cooling of a 

splat can be simplified to a one-dimensional non-steady-state 

heat flow through the splat thickness to the substrate. This 

enables the conductive heat flow to be described by the 

principal equation: 

 (k/ρCp) (∂2

T/∂x2) = (∂T/∂t) (3) 

where T is the absolute temperature within the splat at a 

distance x from the splat/substrate interface, t is the time and 

k, ρ and Cp are the thermal conductivity, density and specific 

heat of the splat. 

A finite-difference method was used to solve the above 

equation, which in explicit form can be written as: 

 ks(Ti+1, j-2Ti, j+Ti-1, j)/∆x =(ρsCps∆x/2)(Ti, j+1-Ti, j)/∆t (4) 

Since temperature T(x, t) is a function of both time and 

distance, in order to solve equation 4, discretization must be 

conducted along two axes ñ t and x, (Fig. 3). 

If the temperatures Ti+1, j, Ti, j and Ti-1, j are known then 

temperature Ti, j+1 can be determined, as shown in Fig. 3. 

Ti-1,j

Ti,j

Ti+1,j

Ti,j+1

IC

BC

BC

Time t [s]

Coating Thickness x [mm]

0 1 Ö i Ö n

δt

δx

0 1 Ö j Ö m

Ti-1,j

Ti,j

Ti+1,j

Ti,j+1

IC

BC

BC

Time t [s]

Coating Thickness x [mm]

0 1 Ö i Ö n

δt

δx

0 1 Ö j Ö m

Figure 3: Schematic of the discretization model along two 

axes ñ x and t. BC = boundary conditions, IC = initial 

conditions. 
NASA/TMó2003-212120 4 

The finite difference equations for the five boundary 

conditions (BC) shown in Fig. 4 were: 

! At the substrate/environment interface: 

 ks(Tb-Ta)/∆y + hE(TE-Ta) = (ρsCps∆y/2)(Tía-Ta)/∆t (5) 

! Within the substrate: 

 ks(Th-2Tg + Tf)/∆y = (ρsCps∆y/2)(Tíg-Tg)/∆t (6) 

! At the substrate/coating interface: 

kc(Tl-Tk)/∆x + ks(Tj-Tk)/∆y 

 = (ρsCps∆y/2)(ρcCpc∆x/2)(Tík-Tk)/∆t (7) 

! Within the coating: 

 kc(Tr-2Tq+Tp)/∆x = (ρcCpc∆x/2)(Tíq-Tq)/∆t (8) 

! At the coating/environment interface: 

 kc(Tu-Tv)/∆x + hE(TE-Tv) = (ρcCpc∆x/2)(Tív-Tv)/∆t (9) 

a b f g h j k

l p q r v u

Substrate

Splat

δy δx

Conduction

Convection Convection 

Conduction

Environment E [Flame, Air, Hot Air]

Electro Resistant Heater or Environment [Air]

a b f g h j k

l p q r v u

Substrate

Splat

δy δx

Conduction

Convection Convection 

Conduction

Environment E [Flame, Air, Hot Air]

Electro Resistant Heater or Environment [Air]

Figure 4: Schematic of differential elements used for the heat 

transfer analysis. 

The required initial (IC) condition (equation 10) is of the 

form: 

 T(x,0) = Tin (10) 

The following four cases were modeled, assuming different 

initial substrate temperatures: 

! Case 1: No substrate preheating (Fig. 5). 

HVOF Gun

PMC 

Substrate

T Ta = 25 °C a = 25 °C

Figure 5: Schematic of the substrate/gun system with no 

substrate preheating. 

! Case 2: Substrate electrically preheated from the rear 

(Fig. 6). 

HVOF Gun

PMC 

Substrate

Tk = 204 °C Ta = 320 °C

~

Figure 6: Schematic of the substrate/gun system with the 

substrate preheated from the rear only. 

! Case 3: Substrate preheated by hot air from the front face 

(Fig. 7). 

HVOF Gun

PMC 

Substrate

Tk = 320 °C

Ta = 204 °C

Hot Air

Figure 7: Schematic of the substrate/gun arrangement with 

the substrate preheated from the front face only. 

! Case 4: Substrate preheated by the electrical resistance 

heater from the rear and by hot air from the front side 

(Fig. 8). 

HVOF Gun

PMC 

Substrate

T Tk = 320 °C a = 320 °C

~

Hot Air

Figure 8: Schematic of the substrate/gun system with the 

substrate preheated from the rear and front faces. 
NASA/TMó2003-212120 5 

In all cases the initial linear temperature gradient along the 

substrate cross-section was calculated using equation 11: 

 ks = (Ta - Tk)/L = -q (11) 

where ks is the thermal conductivity of the substrate, Ta and 

Tk the temperatures of both surfaces of the substrate, L the 

thickness of the substrate and q the heat flux generated by the 

heaters. The time interval ∆t was governed by the stability 

condition (equation 12) using a non steady-state finite 

difference method. 

 (k/ρCp)(∆t/∆x2) < 1/2 (12) 

Which gave ∆t = 2 x 10ñ6 s. The substrate thickness (3 mm) 

was discretized using 5 equi-spaced points; and each 8 μm 

thick splat was divided into 3 equi-spaced points. The 

temperature within each layer (splat) was calculated until a 

steady-state temperature condition was achieved. Once this 

equilibrium was reached the deposition of the next layer 

(splat) was simulated. 

Results and Discussion 

CASE 1: No substrate preheating.

0

100

200

300

400

500

600

0 2 4 6 8 10

Distance from Interface [μm]

Temperature [C]

10

20

30

60

69

10

Tx

Time From Impact [μs]

2

(a) 

0

100

200

300

400

500

600

0 20 40 60 80 100

Distance from Interface [μm]

Temperature [C]

2

20

60

10000

10

Tx

Time From Impact [μs]

20

80

(b) 

Figure 9: Temperature profiles in polyimide coatings at 

various times after impact with the substrate: (a) single 8 μm 

splat; (b) 80 μm thick coating after deposition of the final 

(10th) splat. 

CASE 2: Substrate electrically preheated from the rear. 

0

100

200

300

400

500

600

0 2 4 6 8 10

Distance from Interface [μm]

Temperature [C]

10

20

30

60

69

10 Tx

Time From Impact [μs]

2

(a) 

0

100

200

300

400

500

600

0 20 40 60 80 100

Distance from Interface [μm]

Temperature [C]

2

20

60

10000

100

Tx

Time From Impact [μs]

200

800

(b) 

Figure 10: Temperature profiles in polyimide coatings at 

various times after impact with substrate, (a) single 8 μm 

splat, (b) 80 μm thick coating after deposition of the final 

(10th] splat. 

CASE 3: Substrate preheated by hot air on the front face only. 

0

100

200

300

400

500

600

0 2 4 6 8 10

Distance from Interface [μm]

Temperature [C]

10

20

30

60

690

100

Tx

Time From Impact [μs]

2

(a) 

Time From Impact (μs) 

Time From Impact (μs) 

Time From Impact (μs) 

Time From Impact (μs) 

Time From Impact (μs) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Temperature (°C) Temperature (°C) 

Temperature (°C) Temperature (°C) Temperature (°C) 
NASA/TMó2003-212120 6 

0

100

200

300

400

500

600

0 20 40 60 80 100

Distance from Interface [μm]

Temperature [C]

2

20

60

10000

100

Tx

Time From Impact [μs]

200

800

(b) 

Figure 11. Temperature profiles in polyimide coatings at 

various times after impact with the substrate: (a) single 8 μm 

splat: (b) 80 μm thick coating after deposition of the final 

(10th) splat. 

CASE 4: Substrate preheated by electrical resistance heater 

from the rear and by hot air from the front face. 

0

100

200

300

400

500

600

0 2 4 6 8 10

Distance from Interface [μm]

Temperature [C]

10

20

30

60

690

100

Tx

Time From Impact [μs]

2

(a) 

0

100

200

300

400

500

600

0 20 40 60 80 100

Distance from Interface [μm]

Temperature [C]

2

20

60

10000

100

Tx

Time From Impact [μs]

200

800

(b) 

Figure 12: Temperature profiles in polyimide coatings at 

various times after impact with the substrate: (a) 8 μm single 

splat: (b) 80 μm thick coating after deposition of the final 

(10th) splat. 

A theoretical model was developed to calculate the 

temperature profiles of 8 μm thick individual splats and 80 μm 

thick coatings built up from ten overlapping splats. In 

addition, temperature profiles were also calculated for the 

entire coating during the total time taken for the deposition of 

10 individual layers. All calculations were done for four cases 

of substrate preheating. The temperature within each layer 

(splat) was calculated until a steady-state temperature was 

achieved. Once this equilibrium was reached the deposition of 

the next layer (splat) was simulated. Thermosets generally 

cure over a range of temperature, but for the purposes of this 

investigation, the crosslinking temperature (the horizontal 

dashed line shown on each graph at Tx = 280 °C) was taken as 

the lower limit of the range. 

Case 1: 

After a single droplet was deposited, its temperature decayed 

rapidly with time and all parts of the splat cooled from the 

initial 500 °C below the crosslinking temperature Tx = 280 °C 

in only 80 μs (Fig. 9a). The gel time of the polyimide was 

~15 s @ 280 °C and hence the process model predicted that 

curing of an isolated 8 μm thick splat was not possible in cases 

where the substrate was not preheated. As the coating 

thickness increased with the deposition of successive layers, 

the time required to achieve the steady-state temperature 

increased to 500 μs for the 10th layer (Figs. 9b and 13) which 

was still several orders of magnitude shorter than the gel time. 

Case 2: 

The substrate was preheated to a maximum permitted 

temperature of 320 °C from the rear, but due to the low 

thermal conductivity of the PMC substrate, the temperature on 

the front side where the coating was being sprayed only 

reached 204 °C. This situation was slightly better than Case 1, 

since the temperature decayed rapidly below the crosslinking 

temperature in 300 μs. for one splat (Fig. 10a). It is interesting 

to note that after deposition of the 10th layer, the equilibrium 

coating temperature finally reached the polyimide crosslinking 

temperature (Figs. 10b and 14), which indicated that 

succeeding layers would remain above the crosslinking 

temperature. 

Cases 3 and 4:

The coating temperature in cases 3 and 4 never dropped below 

the substrate/coating interface temperature (the baseline on 

Figs. 11a, b, 15, 12a, b and 16) thus in these cases the 

deposited material was always above the crosslinking 

temperature. This was the critical condition necessary for 

successful deposition of thermosets by HVOF combustion 

spraying. Further work will be carried out using Dynamic 

Mechanical Analysis (DMA) of coatings sprayed according to 

theoretical prediction in order to determine the crosslinking 

density of the polyimide coatings. 

Time From Impact (μs) 

Time From Impact (μs) 

Time From Impact (μs) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Distance From Interface (μm) 

Temperature (°C) Temperature (°C) Temperature (°C) 
NASA/TMó2003-212120 7 

Figure 13: Temperature profiles for a ten-layer coating during deposition (~80 μm thick coating) ñ Case 1. 

Figure 14: Temperature profiles for a ten-layer coating during deposition (~80 μm thick coating) ñ Case 2. 

Figure 15: Temperature profiles for a ten-layer coating during deposition (~80 μm thick coating) ñ Case 3. 

Temperature at Coating/Substrate interface 

Individual Splat 

Temperature 

Profiles 

Temperature at Coating/Substrate Interface 

Individual Splat 

Temperature 

Profiles 

Individual Splat 

Temperature 

Profiles 

Temperature at Coating/Substrate Interface 

Tx
NASA/TMó2003-212120 8 

Figure 16: Temperature profiles for a ten-layer coating during deposition (~80 μm thick coating) ñ Case 4. 

Polyimide splats sprayed onto glass slides with, and without, 

substrate preheating, are shown in Figs. 17 and 18, 

respectively. 

Figure 17: Partially melted polyimide splat on a glass slide. 

Figure 18: Well melted polyimide splat on a glass slide. 

Splat tests indicated that external substrate preheating should 

enhance the deposition behavior during the thermal spraying 

of polyimide thermoset materials. Polyimide sprayed onto a 

preheated substrate resulted in well-melted splats (Fig. 18). 

After the above tests, thermosetting polyimide coatings were 

successfully sprayed onto carbon-fiber reinforced PMC 

substrates preheated according to the theoretical model 

described in Case 4. Adherent polyimide coatings were 

obtained with little or no debonding at the coating/substrate 

interface (Fig. 19). 

Figure 19: Pure polyimide coating sprayed onto a PMC 

substrate. 

Individual Splat 

Temperature 

Profiles 

Temperature at Coating/Substrate Interface 
NASA/TMó2003-212120 9 

Conclusions 

! A finite difference method has been demonstrated to be a 

viable tool for modeling the temperature profiles of 

thermosetting polyimide coatings deposited by the HVOF 

process. 

! The process model (Case 1) with no substrate preheating 

predicted that curing of isolated splats or a 10-layer 

coating could not occur. 

! The theoretical model indicated that deposition of 

thermosetting polyimide coatings could be successfully 

achieved using the substrate preheating conditions 

described in Cases 3 and 4. 

! Substrate preheating from the front using hot air (Case 3) 

was the recommended method since it was simpler than 

the dual preheating model (Case 4) and still maintained 

the required temperature profile within the coating. 

! The finite difference method used was found to be very 

sensitive to the stability conditions. Modeling of the heat 

transfer through cross-sections of 8 μm thick splats 

required that the time interval be at the microsecond level 

(∆t = 2 x 10-6 s) in order to achieve stable results. 

References 

1. NASA Tech Briefs LEW-15399, ìHigh-Flow, HighMolecular Weight, Addition-Curing Polymers,î NASAGlenn Research Center, Cleveland, OH, (1999). 

2. Stotler, C.L., ìThe 1980ís ñ Payoff Decade for Advanced 

Materials,î Proc. SAMPE, (1980), 176ñ183. 

3. Bao, Y. and Gawne, D. T., ìProcess Modelling of 

Thermal Spray Deposition of Thermosets,î Surface 

Engineering, 11 (3), (1995), 215ñ222. 

4. Bao, Y. and Gawne, D.T., ìEffect of Processing 

Parameters on the Wear Resistance of Thermally Sprayed 

Epoxy Coatings,î Practical Solutions for Engineering 

Problems, (Proc. 9th National Thermal Spay Conf., 

Cincinnati, OH), C. C. Berndt, Ed., ASM InternationalÆ, 

Materials Park, OH, (1996), 227ñ230. 

5. Ivosevic, M., Knight, R., Twardowski, T.E., Kalidindi, 

S.R., Sutter, J.K. and Kim, D.Y., ìDevelopment of 

Thermally Sprayed FGM Erosion/Oxidation Resistant 

Coatings for Polymeric Substrates,î Proc. ITSC-2002 

Intíl. Thermal Spay Conf., Essen, Germany, E. 

Lugscheider, Ed., ASM InternationalÆ, Materials Park, 

OH, (2002), 705ñ709. 

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

and Sutter, J.K., ìMicrostructure and Properties of 

Thermally Sprayed Functionally Graded Coatings for 

Polymeric Substrates,î (Proc. ITSC-2003 Intíl. Thermal 

Spay Conf., Orlando, FL), C. Moreau, Ed., ASM 

InternationalÆ, Materials Park, OH, (2003). 

7. Engineered Materials Handbook, Desk Edition, 

ìThermoset Polyimide Resin and Fiber-Resin 

Composite,î Michelle, M. Gauthier, Ed., ASM 

InternationalÆ, Materials Park, OH, (1995). 

8. Naik, S., Macri, D. and Sutter, J.K., ìErosion Coatings for 

High Temperature Polymer Composites,î Proc. 44th

International SAMPE Symposium, 44 (1), (1999), 68ñ81. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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

High velocity oxy-fuel (HVOF) sprayed, functionally graded polyimide/WC-Co composite coatings on polymer matrix

composites (PMC's) are being investigated for applications in turbine engine technologies. This requires that the

polyimide, used as the matrix material, be fully crosslinked during deposition in order to maximize its engineering

properties. The rapid heating and cooling nature of the HVOF spray process and the high heat flux through the coating

into the substrate typically do not allow sufficient time at temperature for curing of the thermoset. It was hypothesized

that external substrate preheating might enhance the deposition behavior and curing reaction during the thermal spraying

of polyimide thermosets. A simple analytical process model for the deposition of thermosetting polyimide onto polymer

matrix composites by HVOF thermal spray technology has been developed. The model incorporates various heat transfer

mechanisms and enables surface temperature profiles of the coating to be simulated, primarily as a function of substrate

preheating temperature. Four cases were modeled: (i) no substrate preheating; (ii) substrates electrically preheated from

the rear; (iii) substrates preheated by hot air from the front face; and (iv) substrates electrically preheated from the rear

and by hot air from the front.

NASA TM—2003-212120

E–13772

WBS–22–708–31–14

15

Optimal Substrate Preheating Model for Thermal Spray Deposition of Thermosets

Onto Polymer Matrix Composites

M. Ivosevic, R. Knight, S.R. Kalidindi, G.R. Palmese, A. Tsurikov,

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;

A.Tsurikov, Naval Surface Warfare Center, West Bethesda, Maryland 20817; and J.K. Sutter, NASA Glenn Research

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

March 2003