Unlubricated sliding wear of ceramics against graphitized cast irons

Unlubricated sliding wear of ceramics against graphitized cast irons

129 Wear, I71 (1994) 129-134 Unlubricated sliding wear of ceramics against graphitized cast irons Liang Fang, Yimin Gao, Lin Zhou and Peng Li Dep...

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129

Wear, I71 (1994) 129-134

Unlubricated

sliding wear of ceramics against graphitized

cast irons

Liang Fang, Yimin Gao, Lin Zhou and Peng Li Department of Mechanical Engineering, Xim Jiaotong University, Xian 710049 (China) (Received

April 16, 1993; accepted

August

19, 1993)

Abstract sliding wear tests of A&Oa, S&N4 and ZrOz against ductile iron and grey cast iron were carried out on a ring-block wear tester at room temperature. It was found that the wear volume of ceramics increased in the order A1203, ZrOz, SiaN,. The wear volume of grey iron was greater than that of ductile iron. Energydispersive X-ray analyses of worn surfaces of the ceramics indicated that a large amount of cast iron formed a transfer layer on A1203, making the wear volume of A&O3 the smallest among the three ceramics. The graphite in the cast irons can decrease the wear of the ceramic-cast iron pairs. -

Unlubricated

1. Introduction

Ceramics have found increasingly wider applications in industry as engineering tribomaterials. For example, if some wear components in engines are made of ceramics instead of metals, the engines’ efficiency can be improved considerably [l, 21. Ceramics cutting tools can machine extremely hard materials with increased cutting speed. Tribologists are paying more attention to the study of ceramic tribology. Ceramics are very hard and generally more wear resistant than metals. On contrary, when ceramics are sliding against graphitized cast iron, the graphite in cast iron is a wellknown solid lubricant. In wear processes the graphite could probably lower the wear of ceramic-cast iron pairs. Therefore, it can be expected that ceramics and cast irons could form a promising wear couple. As a result, the study of friction and wear behaviour of ceramicest iron is very valuable. Nakamura and Hirayama f33 have made rolling wear tests with ring-on-ring contact under a normal load 490 N and indicated that in the case of ceramics-grey cast irons the wear of ceramics was much smaller than that in the case of ceramics-ceramics. The wear of ceramic components against grey cast iron increased in the order Sic, S&N,, ZrOz. The wear of grey cast iron was smallest when coupled with Sic, but was largest when coupled with ZrO,. They considered that the thermal conductivity of ceramics determines the wear behaviour of ceramic-grey cast iron pairs. Within the framework of the Versailles Project on Advanced Materials and Standards (VAMAS), Czichos et al. [4, 51 compared the tribological behaviour of a-

~3-1~8/~/$07.~ 0 1994 Efsevier Sequoia. All rights reserved SSDZ 0043-1648(93)06343-3

A1203, S&N, and AISI 52100 steel combinations in a ball-on-disc tester under conditions of dry sliding with a normal load of 10 N and sliding velocity of 0.1 m s-l. They concluded that the wear predominantly occurs at the Si,N, com~nent for S&N,-steel sliding pairs. However, the wear was hardly observed to occur at the (Y-A&O, specimens for a-Al,O,-steel sliding pairs. They found material transfer occurring from steel to a-Al,03. For the Si3N4--steel sliding system, in addition microfracture, the abrasive action of S&N, wear debris embedded in the steel counterpartner contributes to the high wear of S&N, ~mponents. Clearly, from reports by Czichos et al. [4,5] the wear behaviour of ceramic-steel pairs does not predominantly depend on the thermal conductivity of ceramics, and is rather different from the situation for ceramic-grey cast iron pairs reported by Nakamura and Hirayama [3]. As mentioned above, the graphite in cast iron is able to be considered as a solid lubricant during sliding of ceramic-cast iron pairs. If the graphite forms a lubricating film between ceramic and cast iron specimens, the tribological behaviour of ceramics will be different from that when they are coupled with steels. The friction and wear of ceramic-cast iron pairs may also be obviously decreased. However, the tribology of ceramic-graphitized cast iron is less reported in literature than that of ceramics-ceramics and ceramics-steels. So, through friction and wear tests of ceramics against graphitized cast irons, it is possible to find the beneficial contributions of graphite to the tribological behaviour of ceramics. On the basis of this consideration three kinds of cast irons (grey cast iron, ductile iron and steel) and the

three kinds of ceramics (AlzOli, ZrO, and Si,N,) were used to form wear pairs to be tested. The test conditions were unlubricated sliding wear at room temperature. These tests arc used to study the wear behaviour of the ceramic-cast iron pairs.

2. Experimental details Wear tests were carried out on a ring-block wear tester (type M-200 produced by the Jinan Plant of Material Testing Machines). The load applied was 30 N. The lower specimen was a cast iron ring and the sliding velocity relative to the upper specimen was 0.4 m s-‘. The upper specimen was a ceramic block. It was in contact with the ring specimen and reciprocated along the axis of the ring specimen with a frequency of 16 min-’ and an amplitude of 3.2 mm. The test principle and the size of the specimens are shown in Fig. 1. The temperature during wear tests was room temperature, around 20 “C. The humidity was the natural humidity in the laboratory. The relative humidity was in the range 30%-40%. As well as the lower specimen of cast iron, a 0.8% C steel specimen was also used to research the influence of graphite during sliding wear. The weight loss was measured on a balance with a sensitivity of 0.1 mgf. The weight loss was converted into volume loss by using the cast iron or steel density. Before weighing, the specimen was cleaned with alcohol and acetone, then weighed after being dried. The amaunt of wear of the upper specimen of ceramics was very small and cannot be obtained by weighing on the balance. The volume loss of the ceramic specimen can be expressed from geometric ~nsiderations by

-(4R2-b2)“2 I

where I_ is the length of the wear groove (equal to the length of the ceramic specimen, ix. 4 mm), U, il are the widths of the wear groove at the two sides of the ceramic specimen and R is the radius of the ring specimen, i.e. 20 mm. The sizes of a and b were measured under a microscope with a magnification of 50~ The ceramic specimens were sintered AI,O,, ZrO, and S&N, under normal pressure. Their basic properties are shown in Table 1. The cast iron specimens were melted in a medium frequency induction furnace. Then, they were treated to give ductile iron and grey iron. Their chemical compositions are shown in Table 2. The cast iron was normalized or quenched and tempered to produce pearlitic or martensitic matrices respectively. Table 3 shows the hardness of the cast iron specimen after heat treatment. The hardness of 0.8% C steel is about equal to that of normalized grey cast iron. Before testing, the surface roughness R, of the ground cast iron and steel ring specimens was 0.8 Km. The density of the metal specimens was determined by a buoyancy method. The density of the grey cast iron and ductile iron specimen were 7.1445 g cm-’ and 7.0744 g cm-’ respectively, while that of the 0.8% C steel was 7.6000 g cmm7. By using the density the weight loss was converted into vofume loss. In order to examine the worn surface details, the observation and analyses of the worn surface were TABLE

1. Basic

properties

of test ceramics -

Ceramic

[email protected],

zro,

S&N,,

Density (g cm-“) Bending strength (MPa) Fracture toughness (MN Hardness

3.25 330 3.4 1500 HV

5.53 608 10.1 1150 W

3.23 682 9.55 92 HRA

TABLE

2. Chemical

m-“)

compositions

of test irons

(weight

per cent)

c

Si

Mn

S

P

Re

Mg

3.34 3.76

2.18 2.66

0.36 0.47

0.04 0.024

0.037 0.030

0.057

0.049

of test

irons

30N Grey cast iron Ductile iron

TABLE

3. Hardness

Ductile

Hardness Fig.

1. Principle

of wear

test

and

specimen

sizes.

(HRC)

Grey

iron

cast

iron

Normalized

Quenched

Normalized

Quenched

27.2

54

22

40

L. Fang et al. / Ceramic+raphitized

conducted by using scanning electron microscopy and electron spectroscopy for chemical analysis (ESCA).

3. Results and discussion Figure 2 shows the wear of ceramics when coupled with normalized cast irons after wear tests. These curves indicated that the wear of S&N, is much larger than that of the other two ceramics. When coupled with normalized cast irons, the wear of ceramics increased in the order Al,O,, ZrO, Si,N,. It can also be seen that the wear of ceramics when coupled with ductile irons is slightly greater than that when coupled with grey cast irons. Figure 3 shows the wear of ceramics when coupled with quenched cast irons after wear tests. These curves indicated that the wear of ceramics increased in the same order: Alu203, ZrOz, S&N,. The wear of ceramics when coupled with ductile irons is also slightly greater than that when coupled with grey cast irons. As compared with Fig. 2 the two sets of

SisNdDI o Si&/GI

GI-grey icon DI-duetile iron

b

“x 60

j

40

5 *

20

0

1

2

3

4

5

6

sliding distance (km)

Fig. 2. The wear of ceramics against normalized and ductile iron vs. sliding distance.

grey cast iron

Fig. 3. The wear of ceramics against quenched and ductile iron vs. sliding distance.

gey

cast iron

cast iron wear

131

curves have similar features. The slope of the ZrO, curves in Fig. 3 is larger than that in Fig. 2. Figure 4 shows the morphology of the worn surfaces of ceramics after wear tests against ductile irons. Figure 4(a) is the morphology of the worn surface of S&N, when coupled with ductile iron. It is clear that the worn surface is very smooth. Energy-dispersive analysis of X-rays (EDAX) of the surface revealed that the composition is (weight per cent) 32.8 Si, 41.15 Y, 18.48 Zr and 7.79 Fe. This composition implies that there is no adhesive film of cast iron on the worn surface of S&N,. However, the situation is different on the worn surface of A&O,. Figure 4(b) is the morphology of the worn surface of A&O, after a wear test against quenched ductile iron. The surface is quite rough. There are many overlapped lamellae on the ceramic when observed under higher magnification (Fig. 4(c)). Meanwhile, plastic flow can clearly be seen on the worn surface of Al,O,. EDAX of the surface of Fig. 4(b) shows the composition (weight per cent) to be 83.31 Fe, 16.69 Al. It is thought that the cast iron has adhered on the ceramic surface in the form of overlapped lamellae. The adhered lamellae could protect the AlzO, surface against wear, decreasing the wear of A&O, and increasing the wear of cast iron. The upper left part of Fig. 4(d) is the original surface of ZrO,, and the lower right part is the worn surface after a wear test against quenched ductile iron. EDAX results for the original and worn surfaces are shown in Table 4. It is seen that there is enrichment of iron on the worn surface of ZrO,. Some dark areas on the worn surface shown in Fig. 4(d) are really caused by the transfer of cast iron debris. It can be seen from Fig. 4 that the worn surface of ceramics containing predominantly ionic bonds, such as A&O, and ZrO,, can easily form cast iron transfer layers when coupled with graphitized cast irons. The amount of cast iron transferred on the worn surface of Al,O, is greater than that on ZrO,. The existence of the graphite in the cast irons seems to have no influence on the material transfer between the ceramic and cast iron during wear. From Figs. 5 and 6 it is seen that the wear of cast iron coupled with A&O, is always greater than that of cast iron coupled with other ceramics. The reason is that the cast iron is consumed by the formation of the overlapped lamellae. This phenomenon is coincident with the above discussion of wear of Al,O,. In Figs. 5 and 6 the wear of ductile iron is always smaller than that of grey iron. However, in Fig. 6 one point is abnormal, i.e. for DI-ZrO, at a sliding distance of 5 km. Figure 7 shows the wear of ceramics and cast irons after a wear test with a test travel of 5 km, when normalized grey iron, ductile iron and 0.8% C steel were tested against the ceramics A&O, and S&N,. It

132

L. Furry EI ul. / CeramicTaphiiized

Cd)

(cl Fig. 4. The morphology ZrOz.

cast iron wear

of the worn surfaces

of ceramics

after tests against quenched

ductile iron: (a) S&N,; (b), (c) A&O,; (d)

TABLE 4. Energy-dispersive analysis of X-rays results for worn surface of ZrO, (weight per cent)

Worn surface Original surface

Fe

Ce

Zr

31.57 1.61

3.44 5.90

64.99 92.49

41

I

Fig. 6. The wear of quenched grey cast iron and ductile iron against ceramics vx. sliding distance.

0

1

2

3

sltdingdistancc

4

5

6

(*m)

Fig. 5. The wear of normalized grey cast iron and ductile iron against ceramics vs. sliding distance.

can be seen that the wear of ceramics-cast irons is much smaller than that of ceramics-steel. The matrices in both cast irons and steel are pearlitic. The matrices have almost the same hardness. The only difference is that in the cast irons there is graphite, flake graphite in grey iron and spheroidal in ductile iron. Therefore, from Fig. 7 it can be inferred that the graphite is able to reduce the wear of ceramicxast iron pairs.

L. Fang et al. / Ceramic-graphitized cast iron wear

133

>1:

j

i2

20

ac

1

0.8%C

steel

Fig. 9. The surface morphology against A120,.

of ductile iron after a wear test

Fig. 10. The surface morphology against ZrOz.

of ductile iron after a wear test

Fig. 8, ESCA results for C Is peak of [email protected]

Figure 8 shows the ESCA results for the original and worn surfaces of 210~. ZrO, is chemically quite stable. If the binding energy (182.1 eV) of Zr in ZrOz is taken as the calibration basis, a comparison of the peak location of the binding energy of carbon before and after wear test can be made as shown in Fig. 8. It can be seen that the binding energy peak of carbon on the original surface of ZrO, has moved from the peak of o~~ally adhered carbon (due to ~llution in air) to the location of the graphite peak after the wear test. This means that there is graphite on the worn surface of ZrOz after the wear test. Concerning Al,O, and Si,N,, because of tribological chemical reactions taking place in the wear process [6, 71, it is difficult to determine the peak location of the binding energy of carbon in the same way as mentioned above. The transfer film can only be found metallo~aphically as shown in Figs. 4(b) and 4(c) for Alz03. Figures 9 and 10 show the morphology of the worn surfaces of quenched and tempered ductile iron after wear tests against Al,O, and ZrOz respectively. It can be seen in these pictures that the graphite nodules

have spalled, leaving craters on the deformed worn surfaces. The spalled graphite could have served as a solid lubricant, resulting in the decrease in the wear of ceramic-cast iron pairs. According to the above considerations it could be supposed that in the wear process of ceramic-cast iron pairs considerable plastic deformation takes place at the subsurface beneath the worn surface. Graphite particles then spa11 from the metallic matrices. The spalled graphite particles form a lubricating fihu under the successively repeated wear actions of ceramic-cast iron pairs, and thus reduce the wear of the pairs. The whole process can be shown schematically as in Fig. 11. Under the present test conditions the hardness of ceramics has no direct effect on the wear of the ceramics themselves. In the previous work of the present authors [8] it has been found that the wear of ceramics does not depend on the hardness but on the wear mechanisms of the ceramic-cast iron pairs. For example, a transfer film of cast iron forms on the surface of A&O, as shown in Fig. 4(c). The transfer film can protect Al,4

%

Fig. 7. [email protected]&N ef nomz&zed grey iron, ductile iron and 0.8% C steel coupled v&h AttO, and Si,N, at a sliding distance of 5 km.

286

284

282

Bindim auw,eV

r----

l-

--------

-1

ceramic

(2) The wear of ductile iron when coupled with ceramics is smaller than that of grey cast iron. (3) In the case of Al,O,-cast iron, the worn surface of Al,O, is adhered with cast iron lamellae, decreasing the wear of Al,O, and increasing the wear of the cast iron.

(b)

(4) wear (5) wear

The graphite in the cast irons can reduce the of ceramic-cast iron pairs. Hardness is not a unique factor influencing the of ceramic-cast iron pairs.

Acknowledgments Cc)

Cd)

Fig. 11. Sketch showing the formation of the graphite lubricating film during the wear of ceramic-cast iron pairs: (a) before wear; (b) subsurface deformation; (c) graphite spalling; (d) formation of graphite film.

and reduce its wear. This wear does not relate to the hardness. As one more example, the hardness difference of Si,N, and Al,O, is not very large (see Table l), but the wear of S&N, may be 10 times of A&O,. Comparing Fig. 5 and Fig. 6, it can be seen that the wear of normalized iron and of quenched and tempered iron is not very different although their hardness difference is quite large (Table 3). Therefore, hardness is not a unique factor affecting the wear of ceramic-cast iron pairs.

4. Conclusions (1) Under the conditions of unlubricated wear at room temperature, the wear of ceramics increases always in the order A1203, ZrO,, Si,N,, in spite of pearlitic or martensitic matrix and ductile iron or grey cast iron. Furthermore, the wear of ceramics coupled with ductile iron is always greater than that of ceramics coupled with grey cast iron.

This work was supported by the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Academia Sinica. The authors give thanks to Lining Cai and Wen Shu for their help in the tests.

References A.V. Levy and N. Jee, Unlubricated sliding wear of ceramic materials, Wear, 121 (1988) 363-380. S. Jahanmir, Tribological application for advanced ceramics, Mater. Res. Sot. Symp. Proc., 140 (1989) 285-291. Y. Nakamura and S. Hirayama, Wear tests of grey cast iron against ceramics, Wear, 132 (1989) 337-34.5. H. Czichos, S. Becker and J. Lexow, Multilaboratory tribotesting: results from Versailles Advanced Materials and Standards Programme on wear test methods, Wear, 114 (1987) 109-130. H. Czichos, S. Becker and J. Lexow, International multilaboratory sliding wear tests with ceramics and steel, Wear, 13.5 (1989) 171-191. M.G. Gee, The formation of aluminium hydroxide in the sliding wear of alumina, Wear, 153 (1992) 201-227. Y. Tsunai and Y. Enomoto, Tribochemical wear of silicon nitride in wear, n-alcohols and their mixtures. In KC. Ludema (ed.), Proc. Int. Conf. on Wear of Materials, ASME, New York, 1989, pp. 369-374. L. Zhou, L. Fang and J. Zhou, A wear study of ceramics against high chromium cast irons with different matrices, 2nd Nail. Conf. on Heat Treatment, (in Chinese).

October

1246,

1992, pp. 3541