Influence of Substrate Temperature on the Structure and Cohesive/Adhesive Strength of Fe Co Si B Nb Metallic Glass Coating Films Produced by Thermal Spraying

In

fl

uence of Substrate Temperature on the Structure

and Cohesive

/

Adhesive Strength of Fe

­

Co

­

Si

­

B

­

Nb Metallic

Glass Coating Films Produced by Thermal Spraying

Masahiro Komaki

, Tsunehiro Mimura

, Saori Tsuji

, Kenji Amiya

,

Yasunori Saotome

and Tohru Yamasaki

1_{Nakayama Steel Works, Ltd., R&D Division, Osaka 551-8551, Japan}

2_{Institute for Materials Research, Kansai Center, Tohoku University, Himeji 671-2280, Japan} 3_{University of Hyogo, Himeji 671-2280, Japan}

The influence of the substrate temperature on the structure, pore distribution and cohesive/adhesive strength of Fe­Co-based metallic glass coatingfilms has been examined. The metallic glass coatingfilms have been produced by a thermal spraying technique using our developed cylindrical nozzle on SS400 substrates. The splat morphology of the sprayed particles changed from an irregular splash to a disk shape at a transition temperature of about 300°C. When the substrate temperature increased to the transition temperature region (300­323°C), the porosity in the boundaries between the sprayed coatingfilms and the substrates decreased. This can be produced by the strong increase in the wettability of the sprayed particles which is accompanied with a morphological change from splashed to disk-shaped particles. At temperatures ranging from 375 to 400°C, the porosity in both the boundary and inside regions decreased, and the volume fraction of the amorphous phase increased with temperature, resulting in a increase in the cohesive/adhesive strength up to about 27 MPa. [doi:10.2320/matertrans.M2012282]

(Received August 20, 2012; Accepted September 26, 2012; Published November 25, 2012)

Keywords: iron­cobalt­silicon­boron­niobium, metallic glass, sprayed coating film, thermal spraying method, substrate temperature,

cohesive/adhesive strength

1. Introduction

In our previous study, the influence of the substrate

temperature on the structure, pore distribution and cohesive/

adhesive strength of Fe­Cr-based amorphous coating films

was examined by a thermal spraying technique using our

developed cylindrical nozzle on SS400 substrates.1)

The volume fraction of amorphous phase in the sprayed

coatingfilms was found to strongly dependent on the cooling

rate of the sprayed particles. This volume fraction increased at temperatures ranging from 325 to 400°C. In this temper-ature region, the heat transfer may be substantially increased with substrate temperature because of the lower porosity in the boundary region, resulting in a higher cooling rate of the sprayed particles. Therefore, the optimum substrate temper-ature providing the maximum cooling rate of the sprayed particles may be determined.

In the present study, the influence of the substrate

temperature on the structure, pore distribution and

cohe-sive/adhesive strength of Fe­Co-based metallic glass coating

films have been examined with the aim to collect the basic

data required for spray-coating soft magnetic materials onto various magnetic sensors.

2. Experimental Procedures

The thermal spraying equipment used in this study was

composed of a gas flame spraying gun and a cylindrical

nozzle. The thermal spraying material was a gas-atomized

Fe36Co36Si4B20Nb4 (Fe­Co, numbers indicate at%) powder

(grain size: 63­88 µm) with a melting temperature of 1103°C.

SS400 steel plates (120 mm©120 mm©6 mm) were used as

substrates. The surface finishing of the substrates was done

by shot blasting with alumina powders, while the surface

roughness was adjusted to about Ra=4 µm. The

micro-vickers hardness of the thermal sprayed coatingfilms on the

substrates was measured by using a 0.2 kg load and a loading time of 10 s at the center of the inside region in cross section.

Structural analysis of the sprayed coating films was carried

out by X-ray diffraction (XRD, Cu­K¡ radiation with a

graphite monochromator, 40 kV, 200 mA) and scanning electron microscope (SEM, 15 kV, Hitachi SU1500). The volume fraction of the amorphous phase in the sprayed

coatingfilms separated from the substrate was estimated by

measuring the heat evolution during crystallization in differ-ential scanning calorimetry (DSC, SII 6220) with a heating

rate of 40°C/min under a high-purity argon gas atmosphere.

The weight of the samples was set to about 20 mg.

Figure 1 shows a schematic illustration of the cylindrical

nozzle-type thermal spraying gun and the flat substrate.

Table 1 summarizes the thermal spraying conditions used in the present study. In order to prevent the formation of oxides

in the sprayed coatingfilms, theflame was kept in a reducing

atmosphere by using an acetylene-rich gas fuel mixture while Table 1 Thermal spraying condition for Fe­Co­Si­B­Nb metallic glass

coatingfilms.

Acetyleneflow rate 37.8 dm3_/min

Oxygenflow rate 28.3 dm3_/min

External cooling gas (N2)flow rate 400 dm3/min

Cooling air pressure from the back side 0­0.1 MPa

Gun scan rate 300 mm/s

Spraying pitch 5 mm

Thickness of the sprayed coating 400 µm

introducing nitrogen gas into the cylindrical nozzle.2) With the aim to control the substrate temperature, the cooling air pressure from the rear side was controlled to be between 0 and 0.1 MPa with an intermittent operation of thermal spraying, as described later in Fig. 5.

Figure 2 shows schematic illustrations of the tensile test

specimen used for measuring the cohesive/adhesive strength

between the sprayed coating film and the substrate. In

particular, Fig. 2(a) shows the tensile test specimen. The

substrate was first bonded to an aluminum dolly (effective

area: 314 mm2, diameter: 20 mm) using the 3M Company

adhesive agent AF163-2, and then annealed at 120°C for 2 h to harden the adhesive. Figure 2(b) shows the assembly diagrams of the tensile test jig and the tensile test specimen. The aluminum dolly and the substrate were installed in upper and lower test jigs, respectively. The cross-head speed of the

tensile test was set to 1 mm/min.

Figure 3 shows the shape of the substrate designed to

measure the cohesive/adhesive strengths of the tensile test

specimens with an effective area 20 mm in diameter. As a pretest, substrate surface temperatures at the center points of

the four central specimens (6, 7, 10, 11) and the center point of the substrate were measured by the spot-welded K-thermocouples, respectively. In the result, the temperature

difference of five points changed less than «5°C. For this

reason, the substrate surface temperature was measured at the center point by a spot-welded K-thermocouple as a typical temperature. After the thermal spraying treatment, the substrate was cut along the dotted lines by wire cutting equipment. Finally, four specimens around the

K-thermo-couple were selected for subsequent cohesive/adhesive

strength measurements. After bonding the coated substrate

and the aluminum dolly, the remaining sprayed coatingfilm

was removed from the effective area by shot blasting. With this purpose, a ring groove with a 2 mm depth around the 20 mm diameter area was used for precise removal.

Figure 4 shows the thermal gradients of the spraying

flame, as measured by the thermocouple, along the distance

from the cylindrical nozzle head to the substrate. In this diagram, the 0 mm curve indicates the thermal gradient in the

center of the flame, while the 5 and 10 mm curves refer to

the thermal gradient at the corresponding distances from the

Flat substrate

(SS400: 120 mm × 120 mm × 6 mm)

Flame

Spraying film (Fe-Co: 400 µm)

External cooling gas (N2) Spray powder material

(Carrier gas: N2) _{Cylindrical nozzle}

(Twofold cylinder)

Front end of thermal spray-gun nozzle Acetylene/Oxygen

Substrate cooling gas (Air)

Air nozzle

K-thermocouple (Φ0.5)

Fig. 1 Schematic illustration of the cylindrical nozzle-type thermal spraying gun and theflat substrate.

Upper test jig Tensile direction

Tensile test piece Aluminum dolly (Φ20)

Sprayed coating film (Φ20) Fe-Co

(b)

Ring groove with the 2 mm depth

Substrate SS400

Tensile direction Lower test jig Adhesive agent (Φ20)

3M AF163-2

30 mm × 30 mm × 6 mm (a)

center of theflame. Although theflame hits the substrate at a temperature of approximately 1000°C at its center, this temperature decreased sharply to about 300 and 500°C at 10 and 5 mm, respectively, from this position.

Figure 5 shows the thermal spraying scanning patterns on the SS400 substrates. The spraying gun was mounted on a

robot arm and scanned at a feed rate of 300 mm/s. In afirst

step, the thermal spraying was started at point A, and subsequently scanned back and forth along the solid lines with a 10 mm pitch. Once the spraying reached point B, the

spraying gun was returned to point A and held in standby while the substrate temperature decreased to the preset value. In the second step, the scanning pattern was shifted about 5 mm downward. In the third step, the scanning pattern returned to that of the 1st step. After that, these steps were repeated until reaching the preset thickness value of the

sprayingfilms.

Metal: SS400

120 6

Φ20 2

1 2 4

5 6 7 8

9 10 11

Φ26 30

Tensile test side

Cross-section observation and XRD area

Wire cut line

Spot-welded

Selected specimens to measure the cohesive/adhesive strength

K-thermocouple (Φ0.5) Ring groove with

the 2 mm depth

K-thermocouple

(Sprayed coating film: Fe-Co)

120

30

12 3

Fig. 3 Shape of the substrate designed to measure the cohesive/adhesive strengths of the tensile test specimens with an effective area 20 mm in diameter.

0 200 400 600 800 1000 1200 1400 1600

0

Sprayed flame temperature,

T

/

°

C

Distance from the cylindrical nozzle head, l/mm Cylindrical nozzle

5 mm

10 mm

Cross section of the flame.

0 mm position

Lower temperature area of about 400 °C High temperature area of about 1000 °C Temperature distribution of the flame.

0 mm

5 mm

10 mm

Position of the substrate

120 100

80 60

40 20

Fig. 4 Thermal gradients of the spraying flame, as measured by the thermocouple, along the distance from the cylindrical nozzle head to the substrate.

Waiting position of the cylindrical-nozzle type thermal spraying gun.

Flat substrate

(SS400: 120 mm × 120 mm × 6 mm) First scanning line (Feed rate: 300 mm/s)

Second scanning line (Feed rate: 300 mm/s)

Spot-welded K-thermocouple

A

B

10

5

Figure 6 shows a representative example of the substrate temperature over 50 consecutive steps for a given target temperature of 300°C. The substrate temperature was measured by the spot-welded K-thermocouple at the center

point. Figure 7 shows a magnified picture of the dotted

circle area represented in Fig. 6. After preheating the substrate above the preset value (305°C), the gun was held in standby at the starting point while the substrate cooled

down to 305°C. Once this point was reached, the first

spraying step was started. In this case, the bottom

temperature of the substrate (Tmin) was obtained when the

thermal spraying was started, while the local maximum

temperature (Tmax) was reached when the gun was scanned

over the spot-welded point of the K-thermocouple. After

scanning the first step, the gun was held at standby at the

starting point while the substrate temperature decreased to the preset value. Once the substrate temperature had decreased below 305°C, the second spraying step was

started. In this study, 400-µm-thick films were prepared

over 50 steps. Hereafter, the average value of Tmin for all

the steps will be referred to substrate temperature (Ts). In

this case, Ts was 300°C. The difference between Tmax and

Tmin remained within 50°C, and Tmin ranged within «6°C,

as shown in Fig. 8.

3. Results

3.1 Structure of the sprayed coatingfilms

Figure 9 shows cross-sectional SEM micrographs of the

Fe­Co-based metallic glass coating films on SS400

sub-strates. The pores in the cross sections are shown as white

spots. A high porosity (²4.0%) was observed in the

boundary region framed by the dotted line for Ts¯300°C

(Figs. 9(a)­9(c)). The porosity in the boundary region

decreased to about 3.0% at Ts=323°C, and a further

increase in Ts to 401°C led to a drop in the porosity to a

minimum of about 2.6%. Porosity in the boundary region

gradually increased to about 3.4% when Ts increased to

451°C. In the inside region of the sprayed coating film,

framed by the solid line, the observed porosity reached only

1.0% or above at Ts values ranging from 250 to 323°C, as

shown in Figs. 9(a)­9(d). AtTs²350°C, the porosity in the

inside region remained below 1.0%, reaching a minimum

value of about 0.2% at 401°C. A further increase in Ts

(451°C) led to a gradual increase in the inside region porosity

to a maximum of about 0.6%. Figure 10 shows the porosity

of the sprayed coatingfilms as a function ofTsas summarized

the results of Fig. 9.

Figure 11 shows the hardness of the sprayed coatingfilms

as a function of Ts. The hardness was found to initially

increase withTs(³HV1080 at 300°C) and dropped thereafter

(³HV1060 at 325°C). The hardness reached a maximum

value of about HV1100 for Ts=450°C.

Figure 12 shows the XRD patterns of the as-sprayed Fe­

Co coatingfilms at variousTsvalues. Broad diffraction peaks

(2ª=40­50°) were obtained at Ts¯425°C, indicating that

the structure of the sprayed coating films was practically

amorphous. When Ts was increased to 451°C, some sharp

peaks attributable to ¡-Fe, Fe3B and Fe2B phases appeared

superimposed on the broad pattern of the amorphous phase, thereby indicating the partial crystallization of the amorphous phase.

Figure 13 shows the DSC curves for the Fe­Co sprayed

coatingfilms at variousTsvalues. In the case of the rapidly

quenched ribbon sample, a large exothermic peak was

100 150 200 250 300 350 400 450 500 550 600 0 Substrate temperature, T / ° C

Thermal spraying time, t/min Preheating Thermal spraying

Crystallization temperature (586 °C)

Average bottom temperature of substrate Cooling

Fig.7

Substrate temperature: Ts=300 °C 50 steps 20 18 16 14 12 10 8 6 4 2

Fig. 6 Representative example of the substrate temperature over 50 consecutive steps for a given target temperature of 300°C.

250 300 350 400

0.5

Preheating Thermal spraying

Waiting Spot-welded K-thermocouple point

Thermal spraying Substrate temperature, T / ° C

Thermal spraying time, t/min 1 step

2 step Waiting

3 step Tmax-Tmin= 10~50 °C

Preset temperature

(305 °C) Ts=300 °C

3.5 3.0 2.5 2.0 1.5 1.0

Fig. 7 Magnified picture of the dotted circle area represented in Fig. 6.

200 250 300 350 400 450 500 0

Substrate minimum temperature,

T

/

°

C

Thermal spraying cycle number

Ts= 323 °C Ts = 350 °C Ts = 376 °C Ts = 401 °C Ts = 425 °C Ts = 451 °C

Ts= 250 °C Ts = 274 °C Ts = 300 °C

65 55 45 35 25

15 20 30 40 50 60

10 5

observed with a peak temperature of 600°C. For powder

(grain size: 38­63 µm), peak temperature shifted to higher

value as compared with ribbon sample. This may be attributed to a compositional shift due to the difference in

a production method. For sprayed coating films, the peak

temperatures did not change with an increase in Ts as

compared with powder.

Figure 14 shows the volume fractions of the amorphous

phase for the Fe­Co sprayed coatingfilms as a function ofTs.

These values were estimated from the sizes of the exothermic peaks based on that of the rapidly quenched ribbon sample. The volume fraction of the amorphous phase decreased with

Tsto a minimum value of about 91% at 275°C. This value

increased to a maximum value of about 97%at aTsvalue of

about 400°C, and decreased thereafter.

Boundary region: 2.6% Inside region: 0.2% (g)

Inside region: 1.0%

Boundary region: 2.9% (d)

(b)

Boundary region: 4.9%

(c)

Inside region: 1.1%

(i)

Boundary region: 3.1% Inside region: 0.7% (h)

Inside region: 0.8%

Boundary region: 3.1% (e)

Inside region: 0.7%

Boundary region: 2.9% (f)

(a)

Inside region: 1.4%

Boundary region: 4.6%

Boundary region: 3.4% Inside region: 1.8%

Inside region: 0.6% Boundary region: 4.3%

Fig. 9 Cross-sectional SEM micrographs of the Fe­Co-based metallic glass coating films on SS400 substrates: (a) Ts=250°C; (b)Ts=274°C; (c)Ts=300°C; (d)Ts=323°C; (e)Ts=350°C; (f )Ts=376°C; (g)Ts=401°C; (h)Ts=425°C; (i)Ts=451°C.

0 1 2 3 4 5 6 7 8 9 10

225

Boundary region

Inside region

Substrate temperature, Ts/°C

Porosity (%)

Transition temperature

475 450 425 400 375 350 325 300 275 250

Fig. 10 Porosity of the sprayed coating films as a function of Ts as summarized the results of Fig. 9.

900 950 1000 1050 1100 1150 1200

Substrate temperature, Ts /°C

Hardness of the coatings (HV)

225 250 275 300 325 350 375 400 425 450 475

3.2 Tensile tests

Figure 15 shows the fracture surfaces of the four speci-mens around the K-thermocouple region after tensile tests at

each Ts value. Small fractured areas with dark colored

cohesive/adhesive agent were observed at Ts=350, 401,

425 and 451°C, thereby indicating that the fracture was

caused by the sprayed coating film peeled from the dolly.

However, almost entire fractured area having rough metallic luster surface was created by the fracture inside the sprayed

coatingfilm.

Figure 16 shows the average cohesive/adhesive strength

of the four specimens around the K-thermocouple region (as

previously shown in Fig. 3) as a function ofTs. An increase

in Ts between 250 and 450°C led to increased cohesive/

adhesive strengths (²19 MPa, minimum value at 300°C,

maximum of 27 MPa at 400°C). This maximum can result from the observed decrease in porosity in both the boundary and inside regions, as previously shown in Fig. 10. A further

increase in Ts (²400°C) led to lower cohesive/adhesive

strengths, which may be due to partial crystallization and embrittlement accompanying crystallization of the Fe-based

amorphous phase, as previously shown in Fig. 12.3,4)

4. Discussion

In order to obtain large volume fractions of the amorphous

phase and high cohesive/adhesive strengths between the

sprayed coatingfilms and the substrates, control overTsmay

be an important factor.5,6)

The splat morphology of the sprayed particles has been reported to be a useful parameter to estimate optimum

thermal spraying conditions.5,6)_{The splat morphology of the}

sprayed particles changed from irregular splash to a disk

shape at the transition temperature.5)_{It is therefore suggested}

that the cohesive/adhesive strength is strongly influenced

byTs.

Figure 17 shows the morphological changes of a Fe­Co

thermal spray droplet on a flat SS400 substrate (90 mm©

90 mm©6 mm) at variousTsvalues. In this case, the thermal

spraying gun was scanned only one time. Figure 18 shows schematic descriptions of the shapes and cross-sectional optical micrographs of the thermal spray droplets represented

in Fig. 17. The droplet splashed irregularly at Ts¯249°C,

as shown in Figs. 17(a) and 17(b). The splat morphology

changed to a disk shape at Ts=300°C, the transition point.

At this temperature, the disk-shaped droplets showed very smooth surfaces with metallic luster, with some small spots that may be small crystallites (indicated by arrows in

Figs. 17(c) and 17(d)). AtTs=400°C, the droplet was also

disk-shaped, although many small branches were observed, as shown in Fig. 17(e). It has been reported that these branches can be formed when the relative wettability between

the droplet and the substrate increases.7)Small spots on the

surface of the droplet were rarely observed at Ts=400°C.

This temperature yielded the maximum volume fraction of the amorphous phase, as previously shown in Fig. 14. When

Tswas increased to 450°C, some spots were observed on the

surface of the droplet and many long branches which shrank by the surface tension effect were observed around it, as shown in Fig. 17(f ). This may be attributed to the decrease of

Intensity /arbitrary unit

2θ/degree

20 30 40 50 60 70 80

Ts= 451°C

Ts= 425°C

T_s= 376°C

T_s= 350°C Ts= 401°C

Ts= 323°C

T_s= 300°C

T_s= 274°C

Ts= 250°C Fe3B Fe2B

α-Fe

Fig. 12 XRD patterns of the as-sprayed Fe­Co coatingfilms at variousTs values (Cu­K¡radiation with a graphite monochrometer, 40 kV, 20 mA).

450

Powder

Temperature, T_s/°C

Exothermic /arbitrary unit

Peak

TX

ΔH= 49.1mJ/mg

ΔH= 47.6mJ/mg

ΔH= 46.9mJ/mg

ΔH= 47.3mJ/mg

ΔH= 48.0mJ/mg

ΔH= 46.5mJ/mg

ΔH= 46.6mJ/mg

ΔH= 44.5mJ/mg

ΔH= 44.6mJ/mg

ΔH= 44.9mJ/mg

T_s =451°C Ts =425°C T_s =350°C

Ts =401°C Ts =376°C T_s=274°C Ts=250°C

Ts=300°C

Ts=323°C

Ribbon ΔH= 48.6mJ/mg

650 600

550 500

Fig. 13 DSC curves for the Fe­Co sprayed coatingfilms at various Ts values.

50 55 60 65 70 75 80 85 90 95 100

225

V

o

lume fraction of the amorphous phase (%)

Substrate temperature, Ts/°C

475 450 425 400 375 350 325 300 275 250

the relative wettability between the droplet and the substrate by oxide generation on the surface of the SS400 substrate and the sprayed particles, resulting in partial crystallization and embrittlement accompanying crystallization of the Fe-based amorphous phase. In this temperature region, the adhesion

failure by this droplet may be occurred in the sprayed coating

film, this resulting in the gradual increase in the porosity. On

the basis of the results shown in Figs. 14 and 17, it can be concluded that 300°C is the transition temperature for the morphological change of the sprayed droplet, while the optimum temperature leading to a maximum volume fraction of the amorphous phase is about 400°C.

Figure 19 shows the relationships between the porosity (in

both the boundary and inside regions), cohesive/adhesive

strength, and volume fraction of the amorphous phase in the

Fe­Co sprayed coatingfilms as a function ofTs.

At Ts¯300°C, the volume fractions of the amorphous

phase in the sprayed coating films were high (²90%).

However, a high porosity (>4.3%) in the boundary region

between the sprayed coating film and the substrate was

observed. This may be the result of the low wettability of

the sprayed particles showing irregular splash shape.5,6) In

this temperature region, both the volume fraction of the

amorphous phase and the cohesive/adhesive strength

de-creased with increasing Ts.

AtTsvalues ranging from 300 to 375°C, the porosity in the

boundary region decreased to about 3%, and this can be

attributed to the increase in the wettability of the sprayed

(b) (c)

(d) (e) (f)

(a)

(g) (h) (i)

Fig. 15 Fracture surfaces of the four specimens around the K-thermocouple region after tensile tests at eachTsvalue: (a)Ts=250°C; (b)Ts=274°C; (c)Ts=300°C; (d)Ts=323°C; (e)Ts=350°C; (f )Ts=376°C; (g)Ts=401°C; (h)Ts=425°C; (i)Ts=451°C.

0 5 10 15 20 25 30 35 40

225

Cohesi

v

e/adhesi

v

e strength,

A

/

MP

a

Substrate temperature, Ts/°C

σ

475 450 425 400 375 350 325 300 275 250

particles as a result of the morphological change from splash

to disk shape.5,6)

At Ts values ranging from 375 to 400°C, the porosity in

both the boundary and inside regions dropped with decrease in viscosity of the sprayed particles, whereas the volume

fraction of the amorphous phase increased with Ts, this

resulting in an enhancement of the cohesive/adhesive

strength up to 27 MPa. In general, the volume fraction of the amorphous phase was found to be strongly dependent on

the cooling rate of the sprayed particles; whenTsis increased,

the cooling rate of the sprayed particles should decrease. In this temperature region, the amount of heat transfer may be substantially increased because of the lower porosity in both the boundary and inside regions, resulting in an improved

(e) (f)

(c)

(a) (b)

(d)

Fig. 17 Morphological changes of a Fe­Co thermal spray droplet on a flat SS400 substrate at various Ts values: (a) Ts=200°C; (b)Ts=249°C; (c)Ts=300°C; (d)Ts=349°C; (e)Ts=400°C; (f )Ts=450°C.

(a)

Shrink Splash

Substrate Thermal spray droplet Thermal spray particle

Thermal spray droplet

Oxide (d)

(c) (b)

cooling rate of the sprayed particles.8) Therefore, the

optimum Ts value leading to a maximum cooling rate of

the sprayed particles was determined to be about 400°C.

At Ts²400°C, the increase in the porosity was

accom-panied with a drop in the volume fraction of the amorphous

phase with Ts, resulting in a decrease in the cohesive/

adhesive strength to about 20 MPa. In this temperature region, the heat transfer may be negatively affected by a lower wettability as a result of oxide generation on the surface of the SS400 substrate and the sprayed particles, this resulting in a partial crystallization and embrittlement accompanying crystallization of the Fe-based amorphous

phase.3,4)

5. Summary

Control of Ts is the most important point to achieve

Fe36Co36Si4B20Nb4metallic glass coatingfilms with superior

properties such as high hardness, high corrosion resistance, and soft magnetic properties.

The volume fraction of the amorphous phase of the

sprayed coatingfilms was found to be strongly dependent on

the cooling rate of the sprayed particles. The volume fraction

of the amorphous phase was found to increase forTsranging

from 375 to 400°C. In this temperature region, the heat

transfer may be substantially improved withTsas a result of

the lower porosity in both the boundary and inside regions, resulting in a faster cooling of the sprayed particles.

Therefore, the optimum Ts leading to a maximum cooling

rate of the sprayed particles was determined to be around

400°C at which cohesive/adhesive strength was also

maximum.

REFERENCES

1) M. Komaki, T. Mimura, R. Kurahashi, H. Odahara, K. Amiya, Y. Saotome and T. Yamasaki:Mater. Trans.53(2012) 681.

2) M. Fukumoto, C. Yokoi, M. Yamada, T. Yasui, M. Sugiyama, M. Ohara and T. Igarashi:Quarterly J. Japan Weld. Soc.25(2007) 323.

3) M. Douyama and R. Yamamoto:Amorphous Materials, (University of Tokyo Press, Tokyo, 1983) p. 42.

4) A. Inoue:New Functional Materials, Fundamentals of Metallic Glasses

and their Applications to Industry, (Technosystem, Tokyo, 2009) p. 79.

5) M. Fukumoto: Yosha39(2002) 172.

6) M. Fukumoto, Y. Okuwa, M. Yamada, T. Yasui, Y. Motoe, K. Nakashima and T. Igarashi:Quarterly J. Japan Weld. Soc.26(2008) 74.

7) K. Murakami, H. Nakajima, N. Fijita, Y. Hanatate and S. Adachi: J. Thermal Spray Tech.21(2001) 63.

8) K. Murakami: Yosha40(2003) 18.

0 5 10 15 20 25 30 35 40

50 55 60 65 70 75 80 85 90 95 100

225

0 1 2 3 4 5 6 7 8 9 10

Porosity in the boundary region Transition

temperature

Disk and

Splash Disk

Disk and crystallization

Volume fluctuation of the amorphous phase

Porosity in the inside region

Porosity (%)

Substrate temperature, Ts/°C

V

olume fraction of the amorphous phase (%)

Cohesive/adhesive strength

Cohesi

v

e/adhesi

v

e

strength,

A

/

MP

a

475 450 425 400 375 350 325 300 275 250

σ