Relationship between crack growth and wear mechanism of brittle materials during sliding contact

Tribology Research: From Model Experiment to Industrial Problem G. Dalmaz et al. (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

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Relationship between Crack Growth and Wear Mechanism of Brittle Materials during Sliding Contact Hatsuhiko USAMI Junji SUGISHITA, Hiroshi MURASE* and Kei-ichi INADA* Materials science and engineering department, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, 468-8502 Japan *Graduate student of the school of mechanical engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, 468-8502 Japan

ABSTRACT Effects of crack growth on wear mechanisms of brittle materials such as ceramics were evaluated using a testing apparatus having a reciprocal movement at a constant frequency with various normal loads in air. Soda lime glass bars and alumina spheres were mainly used for specimens. The friction and wear behavior showed that the wear of the glass depended on the crack growth near the wear scar and that the coefficient of friction was greater corresponding to the wear rate. Critical stresses for crack growth in sliding contact were determined from indentation damage technique. The crack growth below the wear scar was estimated from inert strength measurements after the friction experiment. The maximum stress at the trailing edge in sliding contact was calculated from elastic contact theory. It was confirmed that the tangential traction at the interface had great influence on the stress at the trailing edge and that crack growth occurred during sliding contact when the stress exceeded the critical value for crack growth. 1.

INTRODUCTION

Hard materials such as ceramics are candidate materials for anti wear systems because of their hardness and chemical stability (1, 2). The friction and wear behavior of ceramics has been studied for the past decade (3). It was found that the basic wear mechanisms of ceramics are similar to those of metals and it is well recognized that the wear rate is usually smaller. However, when crack growth occurs around the interface, the wear volume becomes considerably greater (4). Damage development due to friction is usually characterized by the change of the surface profile and removed volume of the surface, and the wear property of the material is evaluated by the wear amount of the surface. As shown in tracks of ball bearings and indentation damage, crack growth occurs around the surface (5) and results in large wear loss despite of the small change of the surface profile. Therefore, it is important for evaluating the wear properties, to not only measure the wear amount but also characterize the subsurface layer.

Herein, cracking is one of the most important factors for brittle materials. The present study is focused on the effect of crack growth on the wear mechanisms of brittle materials during sliding friction. Soda lime glass bars and alumina spheres were used as model materials. The indentation damage technique was used to determine the critical stress for the crack growth, and inert strength measurements to evaluate the crack growth in sliding friction. The influence of crack growth on the wear mechanism of the glass bars is discussed.

2.

EXPERIMENTAL

2.1 Materials Mostly, soda lime glass bars and alumina spheres were used for specimens. Table 1 shows the mechanical properties of the specimens. The glass bars had rectangular shape (3 X 4 X 40 mm) in conformity with Japanese Industrial Standards, JIS R 1601 (6). The diameter of the alumina sphere was

830

9.5 mm. The specimens had mirror finished surfaces and the comers of the glass bar were beveled to eliminate stress concentrations in strength measurements. Indentation damage and inert strength measurements were carried out using the same specimen after the friction experiment. Table 1 Mechanical properties of specimens

Materials

Young's modulus

Fracture toughness

Poisson's ratio

Soda lime glass

71 GPa

0.8 MPa m o.s

0.22

Alumina

400 GPa

4.0 MPa m 0.s

0.25

2.2. Testing apparatus A testing apparatus having a reciprocal movement was used for friction experiments. Figure l shows a schematic of the testing apparatus. The alumina sphere was fixed into the holder, then was attached to the octagonal-shaped dynamometer that consisted of two curved beams and 8 strain gages. The glass bar was fixed to the guide way driven by a DC motor with a connecting rod. The normal load was applied by mounting dead weights on the upper end of the dynamometer. The experiments were carried out in air. The glass bar was made to contact the alumina sphere, the normal load was applied and then the glass bar was driven. The testing conditions are shown in Table 2.

Dyamo-meter 1 / Normal load

/

Holder Alumina

Glass bar Guide way

DC motor Rod

Fig. 1 Schematic of testing apparatus Tests were interrupted after specific numbers

of cycles to measure the weight loss of the specimens and to observe the surface. Commercial grade oil was supplied to reduce the tangential traction at the interface. The oil was spread just before the test and was not replenished during the test. Since the oil was consumed by the friction cycles, the contact condition at the interface evidently changes with the increase in the number of friction cycles. Table 2 Testing condition

Applied load

Stroke amplitude

Maximum speed

Number of cycles

15,40,100 N

2 mm

2.1 m m/s

1000

3. RESULTS 3.1. Coefficient of friction The friction force was measured continuously during the test and was recorded in terms of coefficient of friction. The coefficient of friction as a function of number of cycles is shown in Fig. 2. In dry conditions, the coefficient of friction was initially greater with a maximum value at approximately 50 cycles, then decreased and reached steady state after 200 cycles. The values at maximum and at steady state were 1.0 and 0.6 to 0.7, respectively. In the lubricated condition, the coefficient of friction was approximately 0.1 and was almost constant during the test. The influence of the applied normal load on the coefficient of friction was small in dry and lubricated conditions. 3.2. Wear behavior The wear amount of the specimen was measured from the weight loss of the specimen and was indicated as its value divided by the applied load (wear rate). It was difficult to evaluate the wear of the alumina sphere and of the glass bar in the lubricated condition since the weight change was too small to measure. Figure 3 shows the value of the wear rate of the glass bar as a function of the number of cycles. The wear rate after 1000 cycles increased with the increase in the applied load. The wear rate was greater at initial stage and reduced with the increase in the number of cycles. After 200 cycles, it seemed that the specific wear rate reached to the steady state

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of the cycles where the transition of the wear rate occurs.

region. .Applied load, N.

1.0

0 25

s=" 0.8

oA 1 04o 1 0

3.3. W e a r scar and w e a r debris

O

===

o

c9

1,1.,

0.6

o

c 0.4 o r

in=

0 tO

0.2

0

200

400 600 800 Number of Cycles, n

1000

Fig. 2 Variations of coefficient of friction (Filled points indicate the results in

the

lubricated condition.) Z

Optical micrographs of the surface after the test are shown in Fig. 4. Crack growth near the wear scar was seen in the dry condition (a, b). The spacing of the cracks was about 100 gin. Such crack growth was observed in all cases in the dry condition beginning in the initial stage, and the crack length was larger with the increase in the applied load. In the lubricated condition (c, d), cracks were not observed at the initial stage and the lower applied load (c). However, semi circular cracks were seen after 1000 cycles and more than 40 N (d). Therefore, it is concluded that not only the applied normal load but also the tangential traction, corresponding to the coefficient of friction, influence the critical value for crack growth.

15

E E

b

12

qp=

o

9

"o

=.

0-

6

o=

Applied load, N

[ olO 4o1

"

25

A 100

E

o_ 9

0

!

!

I

200 400 600 Number of Cycles, n

I 8 0 (}

1000

Fig. 3 W e a r v o l u m e as a function of n u m b e r of Cycles (Filled points indicate the results in the lubricated condition.)

Fig. 4 Optical micrographs of w e a r scars The change of the wear rate in the dry condition from the higher to the lower occurred at 50 cycles at 15 N and 200 cycles at 100 N. Slopes in the Figure correspond to the specific wear rate. The average values of the specific wear rate at less than 50 cycles and at steady state were approximately 108 mm2/N and 10-1~ respectively. The difference in the values of the specific wear rates against the variation of applied load was small. As a result, it is clear that the wear volume after 1000 cycles depended on the number

Wear debris was collected from the surface near the wear scar of glass bars tested in the dry condition (Fig. 5). It was difficult to collect the debris in the lubricated condition because of the spread-out oil. The size and the shape of the wear debris depended on the number of cycles. At the initial stage, where the specific wear rate was larger, the shape of the debris was of flakes whose size was approximately 100 gm (Fig. 5a). Comparing with the intervals between cracks on the surface shown in

832

Fig. 4, it seemed that interaction among the cracks during growth resulted in the large wear debris. At steady state (Fig. 5b), the wear debris became much smaller. Considering the wear behavior, the wear volume depends on the size of the wear debris.

applied after the first AE signal was detected, the crack growth resulted in a cone crack extending from the ring below the surface. It was confirmed that the radius of the ring crack was about l0 to 20 % larger than that of the contact area predicted from Hertzian contact theory (7). The maximum load was approximately 300 N, which was larger than that of the maximum applied load in the friction experiments.

Piston

. oui0e

Way

Indentor (Alumina) Specimen (Glass)

4. Discussion 4.1. Critical stress for crack growth

Friction and wear tested in the dry condition was larger in the initial stage and smaller at steady state region and when tested in the lubricated condition. Crack growth occurred on the surface both in the dry condition and in the lubricated condition at higher applied loads. The size of the wear debris collected in the initial stage in the dry condition was larger and corresponded to the higher specific wear rate. In order to determine the critical stress for crack growth at the glass surface, indentation damage tests were carried out. Figure 6 shows the testing procedure, consisting of an electromechanical testing apparatus (AG-10B, Shimazu. Co. Ltd. Japan) and an acoustic emission (AE) measurement system. Herein the glass bar was mounted on the silicon nitride plate. The alumina sphere was used as indentor and was pushed by the piston with a constant cross head speed of 0.05 mm/min. Two sets of AE sensors were attached to the silicon nitride plate near the glass bar. The AE signal was measured continuously during the test. The critical load for crack growth was determined from the AE signal, then the load was removed. After the experiment, the testing surface of the glass bars was observed by optical microscopy to measure the radius of the crack. No plastic behavior was observed on the loaddisplacement curve. All the start of cracking, crack growth occurred near the contact area and formed a ring crack. When additional load was

AE sensor

9 Base Plate (Silicon nitride)

Fig. 6 Schematic of indentation damage test

Assuming that the contact between the alumina sphere and the glass bars was elastic, the critical stress for the crack growth under applied normal load is found as, (l- 2v) p fin = 2n:r 2 Where, P, r, v were maximum load, radius of the ring crack and Poison's ratio, respectively. Four point bending tests (outer/inner span of 30/10 mm) at room temperature were also carried out and the bending strength was calculated using the equation.

where, P, L, I, t and b are maximum load, outer span (=30 mm ), inner span (-10 mm), specimen thickness and width, respectively. Results of these measurements are shown in Fig. 7. The average value of the strength was 66 MPa in bending and 290 MPa in the indentation damage tests. The Weibull modulus, m, was 6.6 in the indentation damage and 7.5 in the bending tests. The stress of indentation damage was found to decrease with the

833

increase in the ring crack radius. In order to estimate the effect of the tangential traction and applied load on crack growth, the maximum stress at the trailing edge was calculated from the following equations (based on elastic contact theory by Mindlin (8))

Oma x =On+ o l

traction results in the decrease in the normal loads for crack growth. Crack growth in sliding contact was observed (Fig. 9). After 0.5 cycles (a), i.e. uni-directional sliding, the cracks have semi circular shape on the surface. Atter l cycle (b), semi circular cracks formed on the reverse direction and the region beside the wear scar had risen up from the surface. Therefore, it is concluded that the stress around the interface influences the wear properties of the glass surface and that the interaction of crack growth and wear occurs around the interface and results in the large wear debris.

[ 1,.]

104 where, a is the radius of the contact area and Po is the maximum normal stress. Herein the value of the maximum stress is given at the trailing edge of the interface i.e. x=a(=r), because the contribution of ot on Omax is greater than that of o, and decreases with the increase in x at x>a.

Applied load, N 40 N 100N

ii

m a.

Lubricated Condition

10 3 :-" ;<

(p=O.1

E

Nominal Strength

99.99 o~ 99.9

.>' -9

99

90

~.

70

=e =

50 30

1:., o

40

(p=o)

J

Bending m=7.5 n=18

Indentation m=6.6 ,1~

n=

.

.

.

1020.01

.

.

.

.

.

.

.

.

I

I

I

I

0.1 Coefficient of Friction, p

I Ill=

Fig. 8 Maximum stress at the trailing edge

.

,

:

. .

60

80 100 200 300 400 Fracture Strength, MPa Fig. 7 Results of the strength measurements

The maximum stress at the trailing edge derived from Fig. 7 is shown in Fig. 8. The value of the coefficient of friction p=l and p=0.1 correspond to the dry and the lubricated condition at the initial stage of the friction experiment, respectively. Under the indentation damage (i.e. p=0), the deduced stress for the crack growth is 290 MPa. The maximum stress increases with the increase in the coefficient of friction and exceeds the deduced stress (290 MPa) even at a load of 15 N in dry condition. This suggests that the tangential

Fig. 9 Crack growth in sliding contact 4.2.Crack growth during friction Measurements of the fracture strength of the bar specimens were carried out after the friction experiments in conformity with the 4 point bending tests (6) to evaluate crack growth below the wear scar. The position of the wear scar was located on the lower surface and was subjected to tensile stress. The cross head speed was 0.01 mm/min. The observed inert strength as a function of the number of friction cycles is shown in Fig. 10. As seen, the strength degradation was larger at greater normal

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load and at the initial stage of dry sliding. In the lubricated condition, the degradation was relatively small compared with the results in the dry condition. Particularly, at 40 N The strength degradation after 20 friction cycles, where no cracks were observed on the surface. Since cracks were observed after 1000 cycles, it seems that the residual strength degradation depends on the crack growth below the wear scar.

80

7 1 7 Nominal m

a. 60

strengt~

i o~ 50

~

U

C 1_

or}

-~ 40

r 15N El 40N O100 N

"0

o.,.

1

II 1

O

El

8SS

II

O

0 aN

II

nm 5. CONCLUSION

e D

88o

1 10 100 Number of Cycles, n

fracture origin was the cone cracks formed below the wear scar. Crack growth occurred with increasing number of cycles (a, b). In the lubricated condition at lower normal load, crack growth was small (c). At higher normal load (d), the crack became larger and had a complicated shape. Considering the strength degradation (Fig. 9), the crack growth mainly occurs in the initial stage and then has a complicated path. Consequently, the crack growth occurs below the wear scar in sliding contact by the tangential traction even at the lower applied load predicted from the indentation damage tests.

1000

Fig. I 0 Inert strength after the friction test (Filled points indicate the results in the lubricated condition.)

The effect of crack growth on the friction and wear of a ceramic material was evaluated for the case of the soda lime glass bars as the model material. Indentation damage tests for the determination of the critical stress for crack growth and inert strength for estimating crack growth below the wear scar were also were carried out. The followings is the summary of the obtained results. 1)

2)

3)

In the glass bars, interaction among cracks results in the formation of larger wear debris and a greater wear volume. Interaction of the cracks occurs when the stress at the tailing edge exceeds a critical value for crack growth. The decisive crack growth occurs below the wear scar during the friction cycle.

ACKNOWLEDG EMENTS This work was generously supported by ICHIHARA international research and scholarship foundation, Nagoya JAPAN.

Fig. I I Fracture surface after residual strength measurements Figure

11 shows fracture surfaces.

The

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Fractural Aspects of Microtribology" Japanese Journal of Tribology 3 7,11 (1992) 895-90 l 5) A. Yoshida et al, "Fundermental study on applying fine ceramics to rolling bearings under water lubrication "International Tribology Conference Yokohama (1995) Vol. 1 485-490

6) JIS R 1601 (1993) 7) B. R. Lawn, "Indentation of Ceramics with Spheres:A Century after Hertz", J. Am. Ceram. Soc., 8, (1998)19771994 8) R. D. Mindlin, "Compliance of Elastic Bodies in Contact". J. Applied Mechanics Sept. (1944) 259-268