Tribological behaviour of colloidally processed sialon ceramics sliding

against steel under dry conditions

P. Reisa, J.P. Davima,*, X. Xub and J.M.F. Ferreirab

aDepartment of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal.bDepartment of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

Received 13 June 2004; accepted 17 October 2004

Advanced structural ceramics are presently used in several tribological applications such as precision instrument bearings,

water pumps, automotive engine parts and cutting tools inserts. In the present work, the tribological behaviours of colloidally

processed and pressureless sintered sialon ceramics with different phases (a and b) have been studied, aiming at increasing the

industrial applications of sialon ceramics. The friction and wear behaviour of sialon ceramics against steel DIN-Ck45K were

investigated using a pin-on-disk tribometer under dry conditions. Scanning electron microscopy (SEM), and energy dispersion

spectroscopy (EDS) were used to analyse the worn surfaces of the sialon ceramics. Under the conditions used, sialon ceramics

exhibited a typical mild wear (10)6 mm3 N)1 m)1) and the dominant wear mechanisms present were adhesive and abrasion.

The results confirmed that colloidal processing and pressureless sintering are effective methods to prepare wear resistant sialon

ceramic components.

KEY WORDS: wear, friction, advanced structural ceramics, sialon, crystalline phases

1. Introduction

Advanced structural ceramics have been the focusof research and development in the last two decadesfor a wide variety of engineering applications owing toits excellent mechanical and thermal properties: highhardness, fracture toughness, compressive strength,stiffness, good wear and corrosion resistance and lowthermal conductivity. These materials (structuralceramics) are widely applied in several applicationssuch as: cutting tools, seal rings, valve seats and in avariety of high efficiency engines where are requiredexcellent tribological characteristics such as wear resis-tance and chemical stability at elevated temperatures[1–3].

As solid solutions of silicon nitride, sialon ceramicshave better high temperature properties and high hard-ness than silicon nitride ceramics due to less amountof intergranular glassy phase[1]. Sialon has many crys-talline forms, among which a- and b-sialon have beenmost widely studied. a-Sialon ceramics exhibit highhardness, but their sintered microstructures is usuallycomposed of equiaxed grains, which confer them lowervalues of fracture toughness in comparison to theb-phase [4]. b-Sialon ceramics have been widely studiedfor their good sinterability and relative high fracture

toughness associated with the typical rod-like grains ofthe sintered microstructures. a-Sialon ceramics are dif-ficult to pressureless sintering due to the low amountof transient liquid phase during the last stage of sinter-ing process, although this confers them a high poten-tial of cleaning up grain boundaries and therefore, forgood strength retention at high temperature [5,6].

It is important to recognise that the phase composi-tion, microstructure and the intrinsic properties ofstructural ceramics vary strongly depending on theprocessing route used in the manufacturing of theproduct [7,8]. Colloidal processing routes have greatadvantages in terms of homogeneity of the green mi-crostructures, high sintering ability of green bodies,large-scale fabrication of advanced ceramic compo-nents of either very simple or very complicated shapes,and the overall processing costs are potentially moder-ate [9,10]. In fact dense and in situ reinforced a-sialonceramics or sheets could be obtained by pressurelesssintering homogeneous and relatively dense greencompacts prepared by colloidal processing fromwell-dispersed and high solids volume fraction suspen-sions [11–13].

It has been reported that the modes of wearadvanced ceramics can be described as mild and severewear and the wear mechanisms as: fracture mecha-nism, ‘‘plastic deformation’’, fatigue-induced wear, ero-sion, adhesive and abrasion wear [14–16]. The widevariety of wear mechanism are due to the fact that the

*To whom correspondence should be addressed.

E-mail: pdavim@mec.ua.pt

1023-8883/05/0300–0295/0 � 2005 Springer Science+Business Media, Inc.

Tribology Letters, Vol. 18, No. 3, March 2005 (� 2005) 295

DOI: 10.1007/s11249-004-2756-5

material removal process in a ceramic tribocontact isdependent on operating parameters (load, slidingvelocity, temperature, contact geometry) materialintrinsic properties (Young’s modulus, thermal expan-sion coefficient) and microstructure or defects (grainsize, pores, microcracks) [16].

Several investigators [16–19] reported that the fric-tion and wear of behaviour of structural ceramics aredepending on the material’s composition and structure,as well as on the test conditions, contact load, slidingvelocity, temperature, and environment. Wang andHsu [16] carried out a study on wear mechanisms ofceramics and realised that the wear mechanisms ofceramics are predominantly dependent on the tribologi-cal contact stresses. Liu et al. [17] investigated the fric-tion and wear behaviour of sialon ceramics slidingagainst steel and lubricated by three fluorine-containinglubricants (PFPE, X-1P and L108) and realised thatthe three lubricants reduced both the friction coefficientand the wear volume. Zhang et al. [18,19] investigatedthe tribological properties of (Ca,Mg)–sialon/GCr15steel couple under dry sliding and lubrication of waterand polyols. They found that all the polyols not onlyreduced the friction coefficient of (Ca,Mg)–sialon slid-ing against GCr15 steel significantly but also reducedthe wear volume by one or two orders of magnitude.

In this paper, friction and wear behaviour of sialonceramics with different phases (a and b), are investi-gated under dry conditions. The sialon ceramics wereprepared by colloidal processing and pressureless sin-tering. Such an approach enables complex shape for-mation, large-scale production, and relatively lowprocessing costs, thee key points to feasibly spreadceramic applications. The experiments were carried outusing sialon ceramics as pins, which were loaded andslid against steel discs. The objective is to confirm thata cheap processing route is effective in preparing wearresistant sialon ceramic components.

2. Experimental procedure

2.1. Materials and manufacturing process

The starting powders used to prepare the sialonceramics were a-Si3N4 (H.C. Stark, Germany, d50 =0.38 lm), Al2O3 (Alcoa Chemicals, USA, d50 =0.38 lm), AlN (H.C. Stark, Germany, d50 = 2 lm)

and Y2O3 (H.C. Stark, Germany, d50 = 0.75 lm). Anazeotropic mixture of 60 vol.% methylethylketon(MEK) (Riedel-de Haen, Germany) and 40 vol.% eth-anol (E) (Merck, Germany) was selected as solvent.Hypermer KD1 (Imperial Chemical Industries PLC,England) was used as dispersant.

Three kinds of sialon ceramics were selected, withthe compositions shown in table 1. The powder mix-tures of the precursor sialon compositions were firstplanetary milled for 4 h in 2-butanol, using Al2O3

jars and Si3N4 balls, dried and sieved through an 80lm sieve. Then, the powders were dispersed in an az-eotropic mixture of 60 vol% methyl ethyl keton(MEK) (Riedel-de Haen, Germany) and 40 vol% eth-anol (E) (Merck, Germany) in the presence of 3wt.% a polyester/polyamine polymer as dispersant(Imperial Chemical Industries PLC, England), fol-lowed by planetary milling for 4 h to prepare a stablesuspension.

Cylindrical green bodies were consolidated by slipcasting by pouring the suspensions (solids loading of50–60 vol%) into teflon rings set on absorbent plasterblocks. Then, the green compacts were put inside a pow-der bed composed by about 30 wt.% BN + 70 wt.%raw sialon powders (composition A) and pressurelesssintered in a graphite furnace under N2 atmosphere.They were initially heated to 1200 �C at a rate of20 �C/min, and then to 1750 �C at a rate of 4 �C/min,followed by holding for 2 h. After sintering, the sam-ples were cooled down to room temperature at a rateof 20 �C/min.

2.2. Tribological tests and surface analysis

A pin-on-disc machine PLINT TE67HT� connectedto a computer was used to evaluate friction and wearbehaviour of sialon ceramics under dry conditions.Flat-ended pins (sialon ceramics) were fixed to a loadarm by a chuck. The pins stayed over the discs (steel)with two freedom degrees: a vertical one, which allowsnormal load application by direct contact with the sur-face of the disc, and a horizontal one, for friction mea-surement as it can be seen in figure 1. The normalload applied on the pin is provided by a pneumaticsystem with a compression load cell. A motor with atachogenerator feedback ensured the stable runningspeeds.

Table 1.

Experimental compositions (wt%).

Composition Si3N4 AlN Y2O3 Al2O3 Crystalline phases after sintering

A* 71.88 16.26 9.33 2.53 100 wt% a-sialonB* 79.15 11.59 6.38 2.88 70 wt% a-sialon/30 wt% b-sialonC* 79.13 4.67 4.78 11.43 100 wt% b-sialon

*Two tests were performed for each composition using new specimens.

296 P. Reis et al./Tribological behaviour of colloidally processed sialon

Tribological (friction & wear) behaviour of sialonceramic/steel pair was performed with a normal loadof 100 N, a sliding speed of 0.5 m/s and a slidingdistance of 2000 m under dry sliding conditions atroom temperature (»22 �C) with a relative humidityof about 50%, in order to eliminate the contributionof lubricant.

Sialon ceramics were machined, ground and pol-ished, to a mean surface roughness (Ra) of 0.12 lm,into a rectangular shape of 7 mm · 5 mm · 5 mm,and then fixed onto an aluminium pin. Before andafter testing, the sialon ceramics were weighted andultrasonically cleaned in an acetone bath.

DIN Ck45K steel discs with 8 mm thickness and 76mm of diameter were machined and ground to a meansurface roughness (Ra) of 0.3 lm. All discs have thefollowing chemical composition (wt.%): 0.45% C,0.25% Si, 0.65% Mn and present a hardness value of230 HB. Before testing the discs were thoroughlycleaned with 96 vol.% ethanol.

The wear was determined by weight loss. All pins(sialon ceramics) were weighted in a Mettler H78ARbalance with 0.1 mg precision.

The specific wear rate (Ws) was determined by theratio of volume lost [mm3] per unit of sliding distance[m] per unit of load [N].

Ws ¼DVd� L

ð1Þ

where, DV is the volume lost in [mm3], d is the slidingdistance in [m] and L is the load applied in [N].

The volume lost was calculated based on theweight loss and the density of each sample of sialon’sceramic.

The worn surfaces of sialon ceramics were charac-terised using a scanning electron microscope (SEM)and the transferred metal layers on the sialon ceramicswere examined using energy-dispersive spectroscopy(EDS) microanalysis.

3. Results and discussion

The experimentally obtained results, friction coeffi-cient, weight loss and specific wear rate, are presentand discussed as function of the different phases (aand b) of sialon ceramics. Figures 2–3 show the evolu-tion of friction coefficient (l) and weight loss, for sia-lon ceramics, as function of the sliding distance (d),respectively, after duplex testing under the same testingconditions.

The frictional behaviour of sialon ceramics withdifferent phases is significantly different as can beseen from figure 2. A large variation in the value ofthe coefficient of friction with the sliding distancewas found for the sialon ceramics with a-phase dur-ing the tests. The initial values of coefficient frictionwere 0.2 and 0.3, respectively, and then increasedwith the sliding distance to values of 0.58 and 0.51,respectively. After 750 m the friction coefficientdecreases and tends to stabilise as function of thesliding distance. In contrast, b-sialon ceramic presentan initial value of the coefficient of friction of 0.56,which remained almost constant during the test. Alsoit can be observed that, after 2000 m, the a- andb-sialon ceramics presents the lower and the highestfriction coefficient, respectively, under dry testingconditions. According to literature [20–22], the lowest

Figure 1. Pin-on-disc tests of Sialon ceramics.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 250 500 750 1000 1250 1500 1750 2000

Sliding distance [m]

sialonsialon

-sialon-

Figure 2. Evolution of friction coefficient (l) as function of the sliding distance for different Sialon ceramics.

P. Reis et al./Tribological behaviour of colloidally processed sialon 297

friction behaviour was expected for a-sialon, asobserved, since the a-phase exhibits higher hardnessand thermal shock resistance compared to b-phase.

The weight loss curves as function of the sliding dis-tance displayed in figure 3 show two regimes: signifi-cant weight loss rates for all the samples in thebeginning of test (up to about 400 m), followed by aplateau for a-sialon, or by linear trend lines as func-tion of sliding distance, the slopes of which increasewith the content of b-sialon, being more significant forb-sialon than a + b sialon. The a-sialon ceramic pre-sents the lowest weight loss as function of the slidingdistance and, as consequence, presents the highestwear resistance.

The friction coefficient (l), and specific wear rate(Ws), for sialon ceramics, after 2000 m under de drytesting conditions, can be seen on figures 4 and 5,

respectively. From figure 4 it can be observed thata-sialon ceramic present the lower friction coefficient(0.39 and 0.43), followed by a+ b-sialon ceramic(0.54 and 0.50) and b-sialon ceramic (0.56 and 0.59).Zhao et al. [23, 24] investigated the wear behaviourof Si3N4 ceramic cutting tools against stainless steelunder dry conditions and the experimental resultsshowed a friction coefficient between 0.3 and 0.6. Thelower friction coefficients of a-sialon ceramic arerelated to the excellent hardness and thermal shockresistance properties of this phase. As a conclusion itcan be said that the different values of friction coeffi-cient between a-and b-sialon ceramics are related tothe intrinsic properties of materials and the adheredfilm on sialon ceramics.

Kato and Adachi [15] investigated the wear ofadvanced ceramics and concluded that wear rate in therange of 10)9 to 10)6 mm3 N)1 m)1 is described asmild wear. From figure 5, it can be observed that allsialon ceramics with different phases (a/b) exhibit atypical mild wear, with approximate values in theorder of 10)6 mm3 N)1 m)1, as consequence present agood wear resistance. Also it can be seen that b-sialonceramic exhibits the highest specific wear rate (3.87 ·10)6 and 4.37 · 10)6 mm3 N)1 m)1) and a-sialon cera-mic the lowest one (1.86 · 10)6 and 2.43 · 10)6 mm3

N)1 m)1). These results are expected because the hard-ness of a-phase is higher than that of b-phase [1,20].In this sense, the material with high a-phase content isexpected to exhibit higher wear resistance.

After the tribological tests, the worn surfaces ofthe pins (sialon ceramics) were examined using ascanning electron microscope (SEM) equipped withan energy dispersive spectroscopy (EDS) microanaly-sis in order analyse and reveal the dominant wearmechanisms.

Figure 6 shows the morphology of the worn sur-face of a- and b-sialon ceramics at different magnifi-cations. The ceramic/steel contact surface of eachsialon (a and b) had different appearance. As can beseen in figure 6(a) and (c), the worn surface of a-sia-lon ceramic presents a higher region of adhesive

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Fri

ctio

n C

oeff

icie

nt (

) Test 1

Test 2

sialon- sialon-sialon-

Figure 4. Friction coefficient (l) after 2000 m of sliding distance.

0.E+00

1.E-06

2.E-06

3.E-06

4.E-06

5.E-06

Spec

ific

Wea

r ra

te (W

s)[m

m³/N

.m]

Test 1

Test 2

sialon- sialon-sialon-

Figure 5. Specific wear rate (Ws) after 2000 m of sliding distance.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 250 500 750 1000 1250 1500 1750 2000

Sliding distance [m]

Wei

ght

loss

[m

g] sialon

- sialon- sialon

Figure 3. Evolution of weight loss as function of the sliding distance for different Sialon ceramics.

298 P. Reis et al./Tribological behaviour of colloidally processed sialon

marks on the surface than b-sialon ceramic. Only afew thicker traces adhered to the worn surface ofb-sialon ceramic can be observed in figure 6(c).

SEM observations revealed that the worn surfacemorphologies of both sialon ceramics (a/b) have thecharacteristics for a typical mild wear (10)6 mm3 N)1

m)1) and present an adhesive and microabrasion wearmechanism [15]. High magnification (1500·) SEMimages given in figure 6(b) and (d) show the higheramount of iron particles transfer from the steel disc tothe surface of a-sialon ceramic.

Figure 7 shows the morphology of the exact regionwhere the energy dispersive spectroscopy (EDS)microanalysis was made, the results of which are

displayed in figure 8. It can be seen that Fe has beentransferred from the steel to the worn surface of thea-sialon ceramic. The intense Si peak in figure 8shows that the wear debris is not only composed ofsteel, but a combination of a-sialon particles andsteel caused by adhesion. So, the wear mechanism ofthe a-sialon ceramic/steel pair is mainly caused byadhesion and microabrasion wear of the rubbing sur-faces.

4. Conclusions

Based on the experimental results presented, the fol-lowing conclusions can be drawn from tribological(friction & wear) behaviour of sialon ceramics withdifferent phases (a/b) under dry conditions:

• The frictional behaviour of sialon ceramics with (a/b)phases is significantly different. a-sialon ceramicpresent less friction coefficient than b-phase underdry conditions.

• The increase of sliding distance promotes an increaseof weight loss. a-Sialon ceramic present the lowestand b-sialon the highest weight loss as function ofthe sliding distance.

• Sialon ceramics with different phases (a/b) presenthigher wear resistance (approximately values in theorder of 10)6 mm3 N)1 m)1). a-Sialon presents thehighest wear resistance.

• The wear of sialon ceramics is caused by adhesionand microabrasion between the rubbing surfaces.

Figure 7. SEM morphology of the exact region of EDS analysis.

Figure 6. SEM morphology of the worn surface of a-sialon ceramic (a, b) and b-sialon ceramic (c, d).

P. Reis et al./Tribological behaviour of colloidally processed sialon 299

• The sialon ceramics produced through colloidalprocessing and pressureless sintering proved to bepotential candidates for wear resistant applications,enabling the wide spread use of sialons.

Acknowledgments

The authors acknowledge the financial support ofthe project SAPIENS, Reference POCTI/CTM/39419/2001, ‘‘Combustion synthesis of one dimension elon-gated a-sialon crystals to be used as reinforcing agentsfor CMC processed by colloidal techniques’’.

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