sliding wear mechanism of fine cerar tics A&Cl3 and high chromium cast irorP

@eceived September 6, E493; accepted February XS, 2994)

Abstract

The unlubri~ted sliding wear behaviaur of hot-pressed ceramics AL& and ZrOz E pinst high chromiuxn CBS~~ iron with varks matrices was studied. The wear rn~~r~~rn~~han~srn of A&,0, and ZrO was discussed. The results showed the f~l~~ug: (1) The wear of Al& depends on the f#rmaticm af transfer f3m coming from the cast iron; the austenite in the matrix of the cast iron promotes the formation of tram ‘er film which can reduce s~m~tane~u~~ the wear of bath A&& and cast iron; (2) the direct factor csontroitin ; the ftxmation of transfer film on ceramics is the adhesive energy; (3) the wear couple ~~U~austenjte cast iron 5 a promising ~m~inat~~n~ because it possesses minimum wear of both sides; (4) the wear of ZrOa depends I n transfer film and phase tra~sf~~ati~n; (5) the EDAX analysis can be used to deter~ne ruugh&~ the relatk : thickness of the trausfer film; f6$ the tardier af marten&k cast Irons does not affect the wear i3f ceramb

1. IntMuction

Fine ceramics are new ~~~i~~~~~~ materials, Corn- pared with meta& materials, they have high hardness, high stiffness, small co&cients of thermal expansion, high heat resistance, high corrosion resistance and Iow specific gravity, etc. As ~us~~ti~u~ materials, ce- ramics are good candidates. However, the app~~t~o~ of ceramics is stilI knited because of their tow toughness and tensile strength. In view of the properties mentioned above, the mast promising fields of application are as follows: (1) high temperature resistance; (2) severe corrosion resistance; (3) wear resistance.

There have been a number of papers ~o~ce~~ng the ap~~~cat~a~ of fine ceramics as wear~re~istant materials, FiDe ceramics have been su~eessf~i~~ used as ding bearings in machine tools, sliding bearings in water pumps and rocker-arm pads in engines [I]. The service life of ceramic bearings in ~on~e~t~ng rods of automob~e engines has exceeded SOa Ooo km &I]. ft has become a common practice to adopt Al&based ceramic ma- terials in cutting tools [3,43. However, most of the triboiogical investigations on ceramics are carried out macroscopically, Less reports can be found about the microscopic research on wear of ceramics. At the present stage, this work is not enough to provide an effective

instruction for the applk ation of ceramics in the wear field, As a matter of fact, the wear research lags behind its application. Therefore , research on the wear mech- anism of ceramics is ne ided.

Sliding wear is a COXT xxm wear type. Under u&t- bricated sliding, ceramics ,ehave di~ere~t~ from metab. The wear resistance uf o ramics does sot decrease and the coefficient of f&t& n does not increase due tu adhesion as they do wi h metals. This a~ntage of ceramics makes them rn( re promising as vacuum corn- ponents and components in precision equipment where the working condition is unlubricated sliding and high wear resistance and pre dsion are required.

Therefore, the aim of the present paper is to study the wear behaviour and wear m~cro”me~hanism of ce- ramic materials co&de *ed under un~~b~~ated con- ditions.

The preparation of fh~ : ceramics is implicated. The ceramics are difficult tc machine. Fine ceramics are very expensive. The appli ation of ~er~~~~rarn~~ cotl- pies would be limited. The couple, ceramic against highly wear-resistant me1 i1Xic materials may be of great potential, Therefore, a : nxdy on the wear of ceramic/ metal couples is valuabl :.

High chromium cast i on has been used recently as a highly wear-resistant naterial. Owing to the large amount of annoying efem :nQ, the carbide phase in the iron transfom from the less hard type &SC (93%1000 HV) to the very hard &UC3 ~~3~lg~

40 L. Z&U et al. I Un~b~cated sliding wear mechanism of fine ceramics

E-IV). At the same time, the carbide mo~holo~ is also improved. As a result the toughness and wear resistance of high chromium cast iron are superior to ordinary white cast iron. High chromium cast iron is easier to prepare and cheaper. Therefore, fine cerami~~igh chromium cast iron is chosen as a friction couple to be investigated. In the present paper, the unlubri~ted sliding wear behaviour of fine ceramics Al&.& and ZrO, against high chromium cast iron with various matrices is studied and the wear micro-mechanism is discussed.

2. Test materials

The test ceramics A&,0, and ZrOz were provided by Shanghai Institute of Ceramics, Chinese Academy of Science. The ceramics were hot pressed. The specimen size was 3 mm X 4 mm X 12 mm with two ends ground. Their conventional mechanical properties are listed in Table 1.

2.2. Higi2 chromium cast iron The chemical composition of the high chromium cast

iron is listed in Table 2. The iron of this composition was heat treated to obtain various matrices, i.e. mar- tensitic, austenitic and pearlitic. When the quenching temperature is low, the austenite contains less carbon, transforms to less hard martensite. With increasing quenching temperature, the austenite contains more carbon and the transformed martensite becomes harder, ~ternatively, with increasing quenching temperature, more carbon and alloys dissolve in the austenite. The austenite becomes more stable, because the martensite transformation temperature M, is lowered by these elements. The stable austenite makes the iron contain more retained austenite which will decrease the matrix hardness of the iron.

These two con~adicto~ phenomena produce a peak on the curve of matrix hardness versus quenching tem- perature as shown in Fig. 1.

Maximum matrix hardness appeared at 1000 “C, while the hardness of the martensitic matrix reached a peak of 1020 HV. When the quenching temperature was as high as 1160 “C, the matrix was almost completely

TABLE 1. Mechanical properties of ceramics

austenitic, Using the relation shown in Fig. 1, seven regimes of heat treatment have been chosen to obtain the following: (1) four martensitic matrices of different hardness; (2) two matrices containing different amounts of austenite; (3) annealed pearlitic matrix. The cryogenic treatment was adopted for irons M, to M4 in order to eliminate retained austenite. The irons become simple martensitic containing di~erent carbon content and exhibiting different matrix hardness. These seven irons were used for research purposes. Their heat treatment and properties are listed in Tables 3 and 4. Their typical micros~ucture is shown in Fig. 2. From Table 3 it is seen that the carbide volume fraction is in a narrow range. The carbide volume fraction can roughly be considered as the same. So, a single variable (ma- trices) research becomes possible.

3. Experiment

3J. Wear test The wear test was carried out on a test machine

type M-200. The ambient temperature was around 25 “C and the relative humidi~ was 60% + 5%. The ring- block test con~guration was used for the present re- search (Fig. 3). The upper ceramic block specimen was loaded on the lower iron ring specimen using a load of 30 N. The rotational speed of the lower specimen was 200 rev min- I. Running-in was taken as 1000 m for each specimen to e~minate the error caused by the initial contact condition. The volume loss of the upper and lower specimens (block and ring) was mea- sured after a test travel of 4000 m.

The volume loss of the ceramic specimen was cal- culated according to the following expression:

-b arcsin - (P-o’)‘n)

+ 3Rr_6) [{(R2-ti2)3)1/2 -((Rz- b2j3]‘“]

Ceramics Density”

(g cm?

Hardness”

WV)

Bendinga strength

(MFa)

Elastic [5] modulus (GN ml’)

Fractureb toughness KIc (MN m-“)

Additivr ,”

-

N203 3.95 202.5 430 410 ZrOz 6.06 1724 1300 190

*Provided by the Shanghai Institute of ceramics, Chinese Academy of Sciences. bDetermined by Vickers indent method.

5.75 MgO 5.69 Y&3

2.94 0.22 0.29 0.032 0.023 18.6 3.02 0.98

V, vohme kxs d ceramic specimen; L, length of the test surface of Geramk §p~~e~~ -R, radius of iron SpeG~e~ (SB mmf; #, f;7, half ads of the wo%x greove at both ends on the test smface, and ar >b. L, a and b cao be directly measured on the ceramic specimen after the test as schematically shown in Fig. 4.

The Wright loss of the iron s~c~mens was dete~in~d on a balance with a sensitize of 0.1 mg. Then the weight loss was converted into volume loss by using

the iron ~~~s~~ (7.50 g cm-J]. 1x3 order t0 obtain a high accuracy of weight d~t~~i~~t~on~ each tested ~~~c~~n was cleaned in ethanol for 10 min and then in acctone fox 10 min by u~traso~~e vibration. The specimen was dried in a 100 “C dryer for 15 min. Then it was cooled to room temperature in a desiccator and weighed,

The accura~ of vofume kxs d~t~~oat~o~ of the ceramic and iron s~ec~meus were checked as O.oOl mm’ and Q.02 mm3 res~ct~~*

In order tu study the ~c~~rn$~ha~~srn of wear, some worn surfaces of the tested couches were firstly cleaned by ultrasonic rinsing and then observed under a scanning microanalyzer type JAX-$40. The surface morphology was studied a.ad EDAX was also done on these surfaces. The ceramic specimens were m~talI~ed by gold for scalping electron microscopy analysis.

The unlubricated sliding wear test was done on c~~~rnics Al@, and ZrOa against seven irons with various matrices. The test results are listed in Tables 5 and 6.

4.2.Z. ~~~~~~~ of rn~~~~ on c~rni~ ~~~~~ There have been a number of reports ~~~~~~ng

the mass trausfer phenomena on ceramic surfaces* Reference 5 ~u~es~gated t&e transfer phenomena in cer~mi~~crarn~c friction ~u~~~s, and has found a the- oretical basis for the fo~a~~~ of transfer film through an estimation of Van der Waals fm-ces, Some qualitative e~~~~at~ons of transfer ~h~norn~~ were made in some ceramic/metal couples using the mechanical properties of metals, such as elastic modulus, shear mudulus and

where V,, Ys %trt: the and x3 respectively.

~~1ecnIar volume of sdids A

block

ring

Kg. 3. Block-ring wear test and specimen size.

L

Fig. 4. Worn groove on test surface of ceramic specimen.

Thus, once the free surface energy and mofecular volumes of the two solids are knawn, the adhesive energy o, can be calculated.

The surface of a solid cannot flow. Hence, the free surface energy of a solid surface is difficult to determine experimentalfy. Therefore, the solution must resort to theoretical calcufation. According to ref. 15 the free surface energy of covalent crystals can be calcufated as follows. If fracture is made along the lattice plane of minimum energy (generally the close-packed plane), half of the sum of the energy of bonds broken on unit area is the total surface energy of each new surface.

For materials with high melting point as ceramics, the above-mentioned energy is approximately the free sur- face energy. For ionic crystals, according to ref. 17, the free surface energy can be calculated by

E6 ‘yi”

-? (4)

where rj is the free surface energy of ionic crystals; E is Young’s elastic modulus; r, is the equifibrium distance between the planes of negative and positive ions.

The bonds in fine ceramics are a mixture of covalent bonds and ionic bonds. The free surface energy can be calculated by the weighted averages of the free surface energy of covalent crystals and ionic crystals. After calculations as mentioned above the results are: the free surface energy of cr-Al@:, is 7.58 J rnb2 and the free surface energy of tetragonal ZrO, is 5.96 J mV2.

For metals, the free surface energy may be estimated using the energy of sublimation (151. Using the energy of sublimation, the energy of interaction between metal atoms can be calculated. Then the total surface energy of the close-packed planes of metals can be obtained in accordance with the crystal structures of the metals. For metals with high melting point, such as iron, the total surface energy is roughly equal to the free surface energy. The energy of sublimation of iron is 415.9 kJ moi-‘, then the surface energy of y-Fe is calculated as 2.00 J mm2 and cu-Fe 1.32 J m-‘.

According to the calculated results of free surface energy and molecular volume of tbe materials, and using eqn, (3), the adhesive energy between ceramics and metals is calculated and listed in Table 7.

From Table 7 it can be seen that the adhesive energy between Al,O, and metals is higher than that between ZrOz and metals. Meanwhile, the adhesive energy of ‘y-Fe against ceramics is higher than that of a-Fe.

4.1.2. Theoretical basis for estimation of the metallic transfer film t~i&k~e~s OS the ceramic surface 31te metallic transfer f&n on ceramic surfaces is very

thin, A precise detestation of its thickness is very difhcult. However, in the present paper, energy-dis- persive analysis by X-rays (EDAX) is adopted to do a rough determination of the relative thickness of the transfer fiims.

TABLE 5. Wear test results of A12Q&ast irons friction couples

Iron designation MI M2 M3 M4 MA A P

Volume loss of &U3 (mm’) 0.053 0.036 0.028 0.067 0.030 0.014 0.039 Volume loss of high chromium cast iron (mrn3~ 224 336 2.21 2.03 1.07 0.87 3.13

44 L. Zho~c et al. / Unlubricated sliding wear mechanism of j%e ceramics

TABLE 6. Wear test results of ZrO&ast irons friction pairs

Iron designation MI M2 M3 M4 MA A P

Volume loss of ZrOz (mm3) 0.285 0.266 0.272 0.232 0.193 0.217 0.300 Volume loss of high chromium cast iron (mm3) 0.72 1.00 0.55 0.71 3.17 3.87 0.78

TABLE 7. Adhesive energy between ceramics and metals (J m-‘)

Coupling materials tr-Fe y-Fe

Ceramic Al,O, 6.04 7.44 Ceramic ZrO, 5.43 6.70

Fig. 5. Pear-shaped region of electronic beam and depth of penetration.

When a high energy electronic beam is injected into a specimen surface, the beam will propagate normally and laterally, forming a pear-shaped region as shown in Fig. 5. The signals received by the detector of EDAX are essentially coming from the entire region. The analysis given by the EDAX is actually the average analysis of the region. In the present paper, the transfer film is formed mainly by cast iron debris which contain predominantly Fe and Cr. However, Fe and Cr are absent in ceramics. There should be a relation between the film thickness 6 and the relative amount of the elements (For Al,O,/cast iron couple, to determine the relative amount of Fe, Cr and Al; for ZrOJcast iron couple, to determine the relative amount of Fe, Cr and Zr) as shown in Fig. 6.

4.2. I~~uence of matrices of cast irorz on the wear behaviour of Al,UJcast iron couple

4.2.1. Influence of matrices on transfer film on A&O, sulfate

I

(b) When A&O3 was coupled with all cast irons, there Fig. 7. Transfer film on ceramic Al,Oa surface: (a) transfer

was always transfer film with similar mo~holo~ as (b) two areas for EDAX.

Thickness of transfer film 6

Fig. 6. Relation between transfer film thickness and EDAX results.

(a>

Mm;

area I

area II

0

cast iron

___

A

-r

shown in Fig. 7(a). The morphology can be divided into two areas, I and II, as shown in Fig. 7(b). EDAX was done in the two areas. The results are shown in Fig. 8,

From Fig. 8 it is seen that the thickness of the transfer film in area II increases with increasing austenite content in the cast iron. In area I, the EDAX results are almost the same for various matrices. Wowever, from the mo~holo~ of the worn surface in Fig. 7(a) it is clear that the thickness of the film in area I is obviously greater than that in area II. It should be noted that the thickness of the film has exceeded S, as shown in Fig. 5. As a result the film thickness in the four cases makes no difference.

From the discussion above, a conclusion can be drawn. In the couple Al@,/cast iron, the austenite in the matrices of the cast irons can promote the formation of transfer film on the A&O3 surface. The effect of marten&tic and pearlitic matrices is nearly the same when the fo~ation of transfer film is concerned. Aus- tenite is ‘y-Fe, while martensite and pearlite are a-Fe.

Therefore, the ability to form transfer film of the matrices is coincident with the calculated results of adhesive energy in Table 7, i.e. the adhesive energy of y-Fe is higher than a-Fe. Therefore, the direct factor controlling the formation af transfer film is the adhesive energy.

4.2.2. Relation between imnsfer film and wear Once a transfer film has formed, it can protect the

ceramic surface, resnlting in a reduction of wear of

-

I^

-

-

-1 MA M P

Fig. 8. EDAX results on ceramic A1203 surface.

ceramics. It is seen that in Fig. 7(a) the transfer film on the A&O, surface is smooth and continuous. The cast iron transfer film is much softer than A1203. This soft, smooth and continuous film can protect the cast iron surface

It is clear from Fig. 9(b) that the wear of both Al&), and cast iron decreases as the amount of austenite in the matrices increases, i.e. the thickness of the transfer film on the A&O3 surface increases. As shown in Fig. 9(a), although the cast irons M,-M, possess different matrix hardness, they are all cr-Fe matrix. Their transfer films are similar. So, the wear of both ceramic and cast iron does not change with the change in the matrix hardness of cast iron. As seen from Table 4 the wear of iron P is nearly the same as irons h/I,--M, as coupled

o’40 r--------------4’ou ---- _-____._-- I

. ----

iron

* - - - - - - “_ - -

‘;; 0.10 ***** Al& -I

1.00 ‘;;

2 g

~__~_____---_----____ ~

L * * * ; .-.. ----_______-_______

i O.O400

I I i

800 900 1uao 1 l-w0 (a) Matrix hardness of cast irons MI_, (HV,,)

% I a-**- cast iron ‘\

0.00 &----&-- W%.OO (b) Amount of austenite in cast iron matrix (% )

Fig. 9. Effect of matrices on wear in A1203/cast iron couple: (a) effect of mark&tic matrix on wear; (b) effect af austenite ammmt on wear.

-_.... .”

fa5

cI----~“mY-.---- - - .,- 1 __-eLY

Fig. 12. iSect of matrices on transfer film on ceramic ZrC% surface: (a) coupled with iron A, (b) couphd with iron MI.

Fig. 13. Spa&g on the surface of ZrU2.

Furthermore, the transfer fihn can somewhat relax the contact stress. So, the transfer film is able to inhibit the phase transformation on the surface of ZrOz and hence the spalling. It is similar to A&O, that the transfer film on ZrOa surface can also reduce the wear of ceramics.

There are some other similarities between ZrOz and A120,. When ZrO, is coupled with martensiti~ iron M,--A&, the wear of ZrOz does not change with the

change of matrix hardness as shown in Fig. M(a), which is similar to Fig. 9(a). When austenite is present in the matrix, the wear of ZrQ, decreases (Fig. 14(b)). From Table 6 it can be seen that the wear of ZrOa coupled with iron P is dearly the same as that with martensitic irons l&&f+ All these results opt that the formation of transfer Glm on ZrOz depends also 5x2 adhesive energy.

The transfer f&n on Zro, is thinner than that on A&O, as Fig. II is compared with Fig. 8 (note that the bars in Fig, II are shorter than those in Fig. 8, especially in area II). Spafling of ZrO, is a severe wear pattern. Then, comparing Table 6 with Table 5, it is clear that the wear of ZrC?, is always greater than that of Al&&+.

5.30 m_ _ _ _ _ _ _ _ _ - - _-r r---- *

I * * a- i i .._t_______-_____--“-.-

N0.20 *****&-0, 2 w

3 E ;rl z

0.10 I ***** cast iron

3 /_____________-;--.____

a.00 A+*- -d$*oo (51 Amount af austenite in 6cast ir:z matrix (% f

Fig. 14. ISect of matrices cm wear in Ai&3&ast iron couple: (a) effect of marten&k matrix on wear; fb) effect of austenite amount on wear.

48 L. Zhou et al. / Unlubricated sliding wear mechanism of fine ceramics

Discontinuous islands of the transfer film make the surface of ZrO, rough, as shown by the surface mor- phology in Fig. 12(a)(~oupled with iron A), This rough surface causes high wear of iron ~~taioiog austenite (Fig. 14(b)). The transfer films on ZrO, coupled with martensitic irons MI-M, and pearlitic iron P are rel- atively thin. As a result, the surface of Zr02 is relatively smooth. The smooth surfaces of ZrO, make the cast irons MI--M4 and P have lower wear as compared with that of cast irons containing austenite (iron A and MA) (Table 5). The transfer films on ZrO, coupled with iron M,M, and P are s~ilarly thin. Therefore, the wear of iron MI-M, and P is also similar.

From the above discussion it can be summar~~d that the wear of 2X& coupled with high chromium cast irons is affected by two factors: (1) phase trans- formation; (2) formation of transfer film. When ZrOz is coupled with irons MI-M, and P, the matrix does not promote the formation of transfer film. The film islands are fewer and thinner. The direct contact be- tween ZrO, and cast iron contributes to the phase transformation, leading to spalling of ZrO, and en- hancing its wear as compared with irons A or MA (Table 6). Meanwhile, the surface of ZrOz is smooth, resulting in low wear of the cast irons. However, when austenite is present in the matrix (iron A and MA), thick transfer films will form on the ZrO, surface. The phase transformation is inhibited and wear of ZrO, is low. However, the transfer films make the ZrO, surface rough, increasing the wear of the cast irons as compared with irons MI-M, and P (Table 6).

5. ConcI~sio~s

(1) When ceramic Al,O, is coupled with high chro- mium cast iron, its wear depends on the fo~ation of transfer film coming from cast iron. The austenite in the matrix of the cast iron promotes the formation of transfer film which can reduce simultaneously the wear of both A&Q, and cast iron.

(2) The direct factor controlling the fo~ation of transfer film on ceramics is the adhesive energy.

(3) When ceramic ZrO, is coupled with high cbro- mium cast iron, its wear depends on the formation of transfer film and phase transformation of ZrO,. Phase transformation leads to spalling of ZrOz and increases its wear. The transfer film can inhibit phase transfor- mation, thus reduces its wear, but increases the wear of cast iron.

(4) The wear couple Alz03/cast iron A is promising because it possesses minimum wear of both sides.

(5) The EDAX analysis can be used to determine roughly the relative thickness of the transfer film.

(6) The matrix hardness of martensitic cast irons does not affect the wear of both ceramics and cast irons.

References

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