EFFECT OF SUBSTRATE SURFACE TREATMENT ON DURABILITY OF THERMALLY SPRAYED WC CERMET COATING IN ROLLING/SLIDING CONTACT A. NAKAJIMA, T. MAWATARI Faculty of Science and Engineering, Saga University, 1, Honjo-machi, Saga-shi, Saga 840-8502, JAPAN; e-mail: nakajima@me.saga-u.ac.jp SUMMARY Using a two-roller testing machine, the authors have examined the surface durability of thermally sprayed WC-Cr-Ni cermet coating in lubricated rolling or rolling with sliding contact conditions. In the present investigation, the coating of 20 to 200μm in thickness was formed onto the blasted or ground roller specimens made of a thermally refined carbon steel, an induction hardened carbon steel and a carburized alloy steel by means of the high energy type flame spraying (Hi-HVOF) as well as the conventional high velocity oxygen-fuel flame spraying (HVOF). The WC cermet coated roller finished to a mirror-like condition was mated with the carburized steel roller without coating and a maximum Hertzian stress of PH=1.2 or 1.4GPa was applied in line contact. As a result, it was found that the coating on the blasted substrate roller generally shows a higher durability compared with that on the ground substrate roller and this difference appears more distinctly as the coating thickness decreases. In addition, it was also confirmed that the life to flaking of coated roller is prolonged as the hardness of substrate is raised and the coating thickness increases.

Keywords: Thermally Sprayed Coating, WC Cermet, HVOF, Surface Durability, Substrate Treatment

1 INTRODUCTION Thermal spraying has been applied steadily in various fields of industry as one of the most important material processing or surface modification technologies [1]. Especially, the hard coatings with a metallic carbide like WC cermet generally offer excellent tribological proper-ties such as wear resistance and sliding performance [2]. Therefore, a wide variety of applications to the contact surfaces of machine elements are anticipated. However, so far only a few basic researches have been done on the surface durability or the tribological properties of sprayed coatings in rolling/sliding contact conditions[3], although some simple evaluation tests such as the inden-tation hardness test, scratch test, and wear resistance test under sliding conditions have been widely practiced.

With these points as background, using a two-roller testing machine the authors have examined the surface durability of thermally sprayed WC-Cr-Ni cermet coat-ing in lubricated rolling or rolling with sliding contact conditions. In the previous paper [4][5], mating the carburized steel roller with the WC-Cr-Ni cermet coated roller formed by the conventional type high velocity oxy-fuel flame spraying (HVOF) or by the high energy type spraying (Hi-HVOF), it was found that some fac-tors such as substrate material, coating thickness and tangential force have significant effects on the occur-rence of flaking of coated roller. In the present investi-gation, the effect of substrate surface treatment on the durability of WC cermet coated roller was examined. 2 EXPERIMENTAL 2.1 Testing machine and test rollers Experiments were carried out using a two-roller testing machine having a centre distance of 60 mm. The main part of this machine is shown in Figure 1.

Figure 1: Two-roller testing machine

Figure 2 shows the shapes and dimensions of test rollers. Cylindrical specimens F roller without coating and D roller with WC cermet coating were mated. The outside diameter of both rollers is 60mm and the effective track width is 10mm. In nearly pure rolling conditions, D roller was driven by mating F roller with the aid of roll-ing friction (slip ratio s≈0%). In rolling with sliding conditions, a pair of rollers F/D were forcibly driven by gears with the gear ratios of 27/31 (slip ratio given for the coated D roller side s = -14.8%), 26/31 (s =-19.2 %) and 25/32 (s = -28.0 %).

The material of F roller was a carburized and hardened chrominum molybdenum steel (SCM415 according to JIS G4105, the surface hardness ≈760 HV). The contact surface of F roller was finished to a roughness of Ry = 0.1~0.4 μm (Ry: maximum height of the profile accord-ing to ISO 4287-1997 or JIS B 0601-1994, sampling length 0.25 mm) by precision grinding. As the substrate material of D roller, a carburized alloy steel, an induction hardened carbon steel (S45C according to JIS G 4051, ≈590 HV) and a thermally refined carbon steel (S45C, ≈280 HV) were used. Prior to spraying, the outer surface of D roller was finished by grinding or roughened by

shot-blasting. The roughnesses of ground substrate and blasted substrate surfaces were Ra ≈ 0.15 μm and Ra ≈ 4.0 μm respectively (Ra: arithmetic average roughness according to ISO 4287-1997 or JIS B 0601-1994).

As shown in Figure 3, the WC(Bal.)-Cr(20 mass%)- Ni(7 mass%) cermet coating was formed onto the ground or blasted D rollers not only by the conventional high velocity oxygen-fuel spraying (HVOF) but also by the high energy type flame spraying (Hi-HVOF). The spraying conditions in both processes are shown in Ta-ble 1. The coatings of about 20 μm, 50 μm, 100 μm and 200 μm in thickness were prepared. The contact surface of D roller after spraying was finished smooth to a mir-ror-like condition with a roughness of 0.1 ~ 0.2 μm Ry by grinding and subsequent polishing. The micro-Vickers hardness of the coating formed by HVOF was ≈960 HV (test load: 2.94 N), while the hardness of the coating formed by Hi-HVOF was ≈1130 HV.

2.2 Experimental conditions and procedures A summary of the present experiments is given in Table 2. In tests AB, BB and CB, blasted substrate rollers were used. While in tests AG, BG and CG, ground substrate rollers were used. The rotational speed of the driving side roller was 3583±10 rpm and the normal load which gives a maximum Hertzian stress of PH = 1.2 or 1.4 GPa in line contact was applied using a coil spring. In tests AB-1 and AG-1, where the HVOF sprayed D roller was used, both rollers were rotated under pure rolling condi-tions (s ≈ 0 %).

Figure 2: Test rollers

(a) Blasted (b) Ground

Figure 3: Cross sections of sprayed coating

Coating method HVOF Hi-HVOF Pressure, Oxygen 0.7 1.0 MPa Fuela 0.5 0.9 Flow rates, Oxygen 28.8 53.6 m3/h Fuela 4.2 0.02 Spray distance, mm 150 380 Velocity of particles, m/s 700 1080 Velocity of gas, m/s 1620 2160

N.B. aFuel: Propylene (HVOF), Kerosene (Hi-HVOF)

Table1: Spraying conditions

Roughness Rye, μm Test

No. Hardness Hvc

Thick- ness d, μm Before After

Hertzf stress PH, GPa

Slip ratio s, %

Coeffi. of friction

μ

hming, μm (318K)

Number of cycles N × 104

Weight loss, g

Surface damage

AB-1 F Da

823 925 / 720 26 0.2

0.2 0.2 0.3 1.2 ~0.0 ~0.001 0.87

(1.17) 2000 0.00 0.00

F:No D:No

AG-1 F Da

792 989 / 722 20 0.4

0.2 0.6 0.4 1.2 ~0.0 ~0.001 0.87

(1.17) 45 0.01 0.02

F:No D:Flaking

BB-1 F Db

770 1211 / 276 201 0.3

0.2 0.2 0.2 1.2 -14.8 0.033

~0.032 0.40

(1.11) 2000 0.00 0.00

F:No D:No

BB-2 F Db

767 1203 / 303 202 0.1

0.2 0.1 0.3 1.4 -14.8 0.032

~0.032 0.31

(1.07) 128 0.00 0.22

F:No D:Flaking

BG-1 F Db

740 1108 / 278 201 0.1

0.2 0.2 0.1 1.2 -14.8 0.036

~0.031 0.48

(1.11) 2000 0.00 0.01

F:No D:No

BG-2 F Db

758 1122 / 276 199 0.2

0.1 2.8 1.6 1.4 -14.8 0.033

~0.033 0.38

(1.07) 31 0.06 0.53

F:No D:Flaking

CB-1 F Db

771 1068 / 586 48 0.1

0.1 0.1 0.2 1.4 -14.8 0.036

~0.034 0.40

(1.07) 2000 0.00 0.02

F:No D:No

CB-2 F Db

765 1115 / 584 98 0.2

0.2 0.1 0.3 1.4 -19.2 0.035

~0.030 0.25

(1.06) 2000 0.00 0.00

F:No D:No

CB-3 F Db

764 1114 / 590 100 0.1

0.2 0.2 0.5 1.4 -28.0 0.032

~0.030 0.21

(1.03) 2000 0.00 0.00

F:No D:No

CG-1 F Db

743 1102 / 574 200 0.1

0.2 0.1 0.2 1.4 -14.8 0.035

~0.032 0.35

(1.07) 2000 0.00 0.00

F:No D:No

CG-2 F Db

747 1153 / 576 202 0.1

0.2 3.8 0.5 1.4 -28.0 0.035

~0.030 0.32

(1.03) 1698 0.00 0.16

F:No D:Flaking

N.B. aWC/CrNi cermet coated roller by HVOF. bWC/CrNi cermet coated roller by Hi-HVOF. cSurface hardness of F roller and coating hardness/substrate hardness of coated D roller. dThickness of coating. eRy: maximum height of the profile according to ISO 4287-1997 or JIS B 0601-1994 (sampling length 0.25mm). fPH: maximum Hertzian stress. ghmin: EHL oil film thickness calculated using the oil viscosity at roller surface temperature. ( ) is hmin calculated for the inlet oil temperature 318 K.

Table 2: Summary of experiments and main results

Except for above two tests, the Hi-HVOF sprayed D roller and the carburized F roller without coating were mated under rolling/sliding conditions, and the coated D roller was placed on the slower (driven) side (sm =-14.8 %, -19.2 % and -28.0 %). As lubricant, a paraffinic mineral oil without EP additives (kinetic viscosity ν: 62.9 mm2/s at 313 K, 8.5 mm2/s at 373 K, pressure-viscosity coefficient α: 13.3 GPa-1 at 313 K, specific gravity 288/277K: 0.878) was supplied to the inlet side of rotating rollers at a flow rate of 15 cm3/s and at a constant oil temperature of 318 K. The theoretical EHL oil film thickness hmin was calculated using the oil viscosity at the actual temperature of the roller surfaces which was measured successively by trailing thermo-couples. For reference, the oil film thickness for the inlet oil temperature of 318 K was also calculated. As shown in Table 2, the minimum oil film thickness hmin for the actual surface temperature was 0.87 μm in the nearly pure rolling condition, while the hmin was reduced significantly to 0.21 ~ 0.48 μm in the rolling with sliding condition. The state of oil film formation between two rollers was continuously monitored by means of an electric resistance method [7]. The friction force be-tween rollers was measured using strain gauges stuck on the driving shaft (via slip rings). 3 RESULTS AND DISCUSSION

3.1 Comparison of surface durability The testing machine is equipped with an automatic stop-ping device which works in response to the abnormal vibration induced by the occurrence of flaking or any other surface damages. In the present investigation, the life to flaking or the rolling contact fatigue life N is defined as the total number of revolutions of the flaked roller side. When the testing machine continued to run without any serious damage, the running was discontin-ued at N =2 ×107 cycles. The main results such as num-ber of cycles N in each test, whether flaking occurred or not, weight loss due to flaking and/or wear of each roller, etc. are summarized in Table 2. Figure 4 shows several test results. With the blasted substrate roller, the occurrence of flaking was apt to be restrained compared with the ground substrate roller. This tendency remarkably appeared in the carburized steel substrate roller with coating of about 20 μm in thickness. Although the thermally refined steel substrate roller generally showed a lower durability than the car-burized steel or the induction hardened steel substrate roller, the effect of the substrate surface treatment on the life to flaking became relatively small as the coating thickness became thick like 200 μm.

Figure 5 shows the relation between the life to flaking and the substrate material when the ground substrate D roller with coating of about 200 μm in thickness was used. As apparent from the figure, the life to flaking of the ground substrate roller was prolonged extremely as the substrate hardness was raised, and these results showed the same tendency as the previous results ob-tained using the blasted substrate roller [4][5]. Figure 6 shows the ratio of the depth of flaking to the coating thickness.

N=4.5 ~105

N=2.0 ~107

1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Number of cycles N

Ground

Blasted

Subs

trat

e su

rfac

e

Carburized steelPH=1.2GPa, s à0%Thickness à20μm

(a) SCM415 carburized steel

1.2GPa1.4GPa

Ground

Blasted

N=2 ~107

N=1.28 ~106

N=2 ~107

N=3.1 ~105

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

Num

ber o

f cyc

les

NPH

Substratesurface

Thermally refinedThickness à200μms=-14.8%

(b) S45C thermally refined steel

Figure 4: Effect of substrate surface treatment on life to flaking

N=3.1 ~105

N=1.7 ~107N=2 ~107

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

Thermallyrefined

s=-14.8%

Inductionhardeneds=-14.8%

Inductionhardeneds=-28.0%

Num

ber o

f cyc

les

N

PH=1.4GPaHi-HVOFThickness à200μmGround substrate roller

Figure 5: Effect of substrate hardness on life to flaking

Hi-HVOFs=-14.8%(202μm)

HVOFs=-14.8%(86μm)

Hi-HVOFs=-14.8%(199μm) Hi-HVOF

s=-28.0%(202μm)

0.0

0.5

1.0

1.5

2.0

Thermallyrefined,Blasted

Inductionhardened,

Blasted

Thermallyrefined,Ground

Inductionhardend,Ground

Dep

th o

f fla

king

/coa

ting

thic

knes

s

PH=1.4GPa ( ) : Coating thickness

Figure 6: Ratios of depth of flaking to coating thickness

The result of the HVOF sprayed coating of 86 μm in thickness [4] is also plotted. When a thermally refined steel was used as substrate, flaking was apt to occur along the interface between the coating layer and the substrate, and often developed deep beyond the interface independent of the substrate surface treatment. When the induction hardened steel substrate roller blasted was used, flaking occurred at a shallower depth than the coating thickness and the damages were confined within the coating layer.

104 105 106 1070

5

10

15

Number of cycles N

Volta

ge E

ab, m

V

slip ratio Thickness�BB-2, -14.8%, 202 µm�BG-2, -14.8%, 199 µm�CB-3, -28.0%, 100µm�CG-2, -28.0%, 202 µm

2× 107

0 0.002 0.004 0.006

0

2

4

6

8

Dept

h/b

Equivalent plastic strain εεεεpav

PH=1.4GPaµ=-0.13PassS45C thermally refines steel

:Thickness=0.15b:Thickness=1.5b

Coating(1.5b)

Coating(0.15b)

Figure 7: Oil film formation between rollers Figure 8: Distributions of equivalent plastic strain εpav (Eab=0mV: contact, 15mV: separation) in the subsurface layer

While, in the case of the ground substrate roller, flaking occurred near the coating/substrate interface, and the depth of flaking was slightly deeper than the coating thickness.

3.2 States of oil film formation and friction Figure 7 shows some results of the measurement of the state of oil film formation between rollers. When the oil film is developed fully, the voltage Eab recorded on a chart reaches 15 mV. As shown in the figure, the proc-esses of oil film formation were slightly different ac-cording to the rolling/sliding conditions. However, it could be considered that the state of oil film formation is hardly affected by the substrate surface treatment.

The results of friction measurement in each test are given in Table 1 as the coefficient of friction (initial value ~ steady value). Although some differences in the variation of friction force were observed depending on the operating conditions such as Hertzian stress, slip ratio, etc., it seemed that the substrate surface treatment has no effect on the friction.

3.3 Elastic-plastic analysis In view of the fact that the life to flaking was affected by the coating thickness and the substrate treatment, the elastic-plastic behavior of the subsurface layer under repeated rolling with sliding contact loads was analysed by a finite element method (FEM). In the analysis, the coating thickness was fixed at 0.15b or 1.5b. Where, b is the half-width of Hertzian contact. Incidentally, under the maximum Hertzian stress PH=1.4GPa, 0.15b and 1.5b are equal to 56μm and 557μm respectively. As regards other analytical conditions, the same way as the previous investigation [4][5] was adopted. Figure 11 shows typical examples of the distributions of equivalent plastic strain εpav both in the coated layer and in the thermally refined steel substrate. In the present analysis, since the equivalent plastic strain hardly varied after 3 times pass of loading, the calculation was discontinued up to 3 times pass. It was found that the plastic strain immediately beneath the coated layer extremely in-creased as the coating thickness decreased. 4 CONCLUSIONS Using a two-roller testing machine, the effect of the substrate surface treatment on the durability of thermally sprayed WC-Cr-Ni cermet coating formed by the HVOF

or the Hi-HVOF process was investigated in lubricated rolling/sliding contact conditions. The results are sum-marized as follows: (1) It was found that the coating on the blasted substrate

roller generally shows a higher durability compared with that on the ground substrate roller.

(2) This tendency appeared distinctly with thin coating on the hard substrate, and the blasted substrate was effective.

(3) As the coating thickness increased, the life to flaking of coated roller was prolonged especially with ther-mally refined steel substrate, and the effect of sub-strate surface treatment on the durability of coating became small.

(4) Such a distinctive feature was also confirmed by the elastic-plastic analysis of the subsurface layer.

5 REFERENCES [1] Harada, Y.: Recent Development of Thermal Spray-ing Technology and it’s Applications. Bull. Jpn. Inst. Metals, 31 (1992) 5, 413 - 421 [2] Knapp, J. K.: Nitta, H.: Fine-Particle Slurry Wear Resistance of Selected Tungsten Carbide Thermal Spray Coatings. Tribology Int., 30 (1997) 3, 225-234 [3] Ahmed, R.: Hadfield, M.: Rolling Contact Fatigue Performance of Detonation Gun Coated Elements. Tri-bology Int., 30 (1997) 2, 129-137 [4] Nakajima, A.: Mawatari, T.: Yoshida, M.: K. Tani, K.: Nakahira, A.: Effects of Coating Thickness and Slip Ratio on Durability of Thermally Sprayed WC Cermet Coating in Rolling/Sliding Contact. Wear, 241 (2000) 166-173 [5] Nakajima, A.: Mawatari, T.: Yoshida, M.: K. Tani, K.: Nakahira, A.: Surface Durability of WC Cermet Coating in Rolling/Sliding Contact - Effects of Substrate Material and Coating Thickness -. ITC NAGASAKI 2000, Oct. 29 – Nov. 2, 2000, Nagasaki [6] Tani, K.: Nakahira, H.: Miyajima, K.: Harada, Y.: Thermal and elastic anisotropy of thermally sprayed coatings. Mater. Trans. JIM, 33 (1992) 6, 618-626 [7] Ichimaru, K.: Nakajima, A.: Hirano, F.: Effect of Asperity Interaction on Pitting in Rollers and Gears. Trans. ASME, J. Mech. Des., 103 (1981) 2, 482-491