WATER LUBRICATION OF CERAMICS Koji KATO Tohoku University, Sendai, JAPAN; e-mail: koji@tribo.mech.tohoku.ac.jp ABSTRACT Present fundamental understandings on tribo-properties of ceramics in water lubrication are confirmed in the viewpoints of Stribeck curve, load carrying capacity, seizure, roughness effect, running-in and wear. Silicon carbide and silicon nitride show practically useful properties of low friction, large load carrying capacity and high anti-seizure ability in self-mated contact. These favorable properties are observed as the result of tribochemical wear and resultant smooth wear surfaces covered with tribo-films. Alumina and zirconia show relatively small load carrying capacity and poor anti-seizure ability with high friction in self-mated contact. But equivalent or better tribo-properties can be expected with the material combination of ceramic against metal. By considering these results, ceramics and water lubrication is a promising combination for the future tribo-elements.

Keywords: Ceramics, Water, Lubrication, Tribochemical Wear

1 INTRODUCTION Modern fine ceramics including coatings are being used in a wide spectrum of practical applications under severe conditions as better materials than metals for better tribological performances. Silicon nitride, silicon carbide, alumina and zirconia are representative ceramics. Carbon-graphites, cermets such as tungsten carbides, hard coatings such as amorphous carbon and diamond-like carbon may be included in the term “ceramic” to cover all non-polymeric and non-metallic materials for tribo-elements.

Water lubrication is considered as one of the severe conditions for triboelements in comparison with oil lubrication. Because of the low viscosity of water, which is about 100 ~ 1000 times lower than those of various oil, the film thickness of water becomes much thinner than that of oil under the same hydrodynamic condition. It means that ceramic bearing must be much more sensitive than metal bearing to the surface roughness and microscopic deformation of asperities and substrate in the contact region.

Such sensitiveness of thin water film in lubrication is well modified by high elastic modulus, high hardness and tribochemical wear of ceramics in water. T. E. Fischer and H. Tomizawa clearly showed the tribo-chemical wear of silicon nitride in water which generated smooth enough wear surfaces (roughness below 10 nm) and introduced hydrodynamic lubrication at low speed (0.2 cm/s) and friction coefficient below 0.002 [1, 2]. The estimated water film thickness was 70 nm. Tribochemical wear of silicon carbide and generation of very smooth wear surface for hydrodynamic lubrication with water are similarly observed [4].

Lubricious film is formed on the surface of silicon nitride or silicon carbide which is thought as silicagel. It sticks on the surface strongly and works in air for a long time even after water is removed from the surface by air blow [5].

High hardness and high elastic modulus of silicon nitride help the stability of such a thin film at the

contact interface between smooth wear surfaces of nanometer scale roughness.

Successful applications of ceramics to water lubricated triboelements are increasing in number. Silicon nitride ball bearing is used for the rotary shaft of a seawater pump together with tungsten carbide for the piston and cylinder [4,5]. Silicon carbide journal bearing and thrust bearing are used for water pump [7]. Silicon carbide bush against tungsten carbide is used for vertical water pump [8]. Alumina against DLC coated alumina is used for faucet valve [9]. Silicon carbide is used for water seal [10]. The cost of silicon nitride ball bearing has been reduced by a factor of ten in the past fifteen years. In this way, the cost of ceramic material would not be a major reason for avoiding ceramic application in the future.

Environmental demands at present and in the future are looking for technology of no pollution and savings of resources and energy. Water lubricated ceramic tribo-elements are ideal for the demands. This paper confirms the high potential of such tribo-elements with the present fundamental understandings on triboproperties of ceramics lubricated with water. 2 STRIBECK CURVES Fig. 1 (a) and (b) show Stribeck curves observed with ceramic/ceramic pair in water and with metal/metal pair in oil [10, 11]. Fig. 1 (a) is described with duty parameter G, and Fig. 1 (b) with lubrication number Lb which includes the roughness effect. The friction coefficients of SiC/SiC in water at elasto-hydrodynamic and hydrodynamic lubrication regimes are much smaller than those of metal/metal pairs in oil, which is understood by viscosity effect described in hydrodynamic theory. The exact understanding of water viscosity under various contact conditions becomes very important for theoretical predictions. Other important point for practice is to know the critical conditions for the transitions of lubrication mode from elasto-hydrodynamic lubrication (EHL) to mixed lubrication (ML), from ML to boundary lubrication (BL), and finally from BL to gross seizure (GS).

(a)

0.0001

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2 PB/Steel/Oil 3 Carbon/WC/Water 4 SiC/SiC/Water 5 SiC/Textured SiC/Water

1 LENNING 1960

3 KANAS 1984

2 BRIX 1962 4 WANG et al.

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5 WANG et al. 2000

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1 Cast Iron/Steel/Oil 2 PB/Steel/Oil 3 Carbon/WC/Wa ter 4 SiC/SiC/Water 5 SiC/Textured SiC/Water

1 LENNING 1960

3 KANAS 1984

2 BRIX 1962

4 WANG et al. 2000

5 WANG et al. 2000

10-5 10-4 10-3 10-2 10-1 1 10

Fig. 1: Stribeck curves observed with SiC/SiC in water

and metal/metal in oil, (a) is described with duty parameter G and (b) with lubrication number Lb

3 LOAD CARRYING CAPACITY MAP

Fig. 2: Critical load for the transition from EHL to ML [13]

Fig. 2 shows the quick transition from EHL to ML at a certain critical load Wc [13]. Such a critical load can be increased by introducing surface texture on a sliding surface, and the texture effect would change quanti-tatively depending on the contact materials and lubri-cants.

Fig. 3 shows the effect of pit area ratio r on the normalized critical load Wc/Wc0 for SiC/SiC in water, where optimum pit area ratio for the largest critical load is obtained at around r ≅ 7 % [13].

0

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Crit

ical

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800 rpm1200 rpm

SiC cylinder/SiC diskPit diameter d: 150 µmPit depth h: 3~4 µmWater

Fig. 3: The effect of surface pit area ratio on the critical load Wc for the transition from EHL to ML. Wc0 is the

critical load on the surface of no pit [13]. Fig. 4 shows the effect of combination of pit shape and pit density on the normalized critical load Wc/Wc0 for the transition from EHL to ML by the parameters of h/d (pit depth/pit size) and r (total pit area/apparent contact area) [13]. Since it gives the design standard for the good combination of pit size, pit depth and pit density to increase the load carrying capacity, a figure described in this way is called as “Load Carrying Capacity Map”.

Possibilities are remained to find out better surface texture than those used for Fig. 4.

Wc/Wc0 <1 1.0~1.5 1.5~2.0 2.0~2.5

Fig. 4: Load carrying capacity map of SiC/SiC in water, which shows the regime of good combination of pit

shape and pit density for high critical load for transition from EHL to ML [12]

4 SEIZURE MAP When friction coefficient exceeds well over the value of 0.1 in boundary lubrication, the lubrication condition may be described as “seizure”.

Fig. 5 shows the effect of mean contact pressure and sliding velocity on friction coefficient at SiC/SiC, where the transition from hydrodynamic lubrication regime (µ < 0.03) to seizure (µ > 0.10) through boundary lubrication regime is clearly observed [14]. Fig. 6 shows the boundaries between the hydrodynamic regime and seizure regime including boundary lubrication regime for SiC/SiC, Si3N4/Si3N4, Al2O3/Al2O3 and ZrO2/ZrO2 in water with a rotary cylinder and stationary disk [14].

B: contact width, m η: viscosity, Pa⋅s V: velocity, m/s Ra: equivalent roughness, m W: load, N Pm: contact pressure, Pa Lenning, Brix, Kanas [11], Wang et al. [12]

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SiC cylinder/SiC diskPit diameter: 50~650µmPit depth: 1.9~16.6µmWater

Theoretical hydrodynamic regimes in Fig. 6 is shown for ηN/Pm > 2×10-8, where N is revolutional speed [15].

Fig. 5: The effect of mean contact pressure Pm and

sliding velocity V on friction coefficient µ of SiC cylinder/SiC disk in water [14]

Fig. 6: Seizure map of ceramics in water [14]

It is obvious in Fig. 6 that the friction coefficient below 0.03 is obtainable in water with SiC/SiC, Si3N4/Si3N4, Al2O3/Al2O3 or ZrO2/ZrO2 by choosing good combi-nation of contact pressure and velocity. The practically important differences in these four kinds of ceramics exist in the size of regime for hydrodynamic lubrication determined by the mean contact pressure and sliding velocity; SiC/SiC pair gives the largest regime and ZrO2/ZrO2 the smallest on “Seizure Map”. 5 ROUGHNESS EFFECT ON LUBRI-

CATED FRICTION As shown in Fig. 7 for the repeated sliding contact of SiC/SiC in water, initial surface roughness decreases by wear and smoother surface is generated, which leads friction coefficient from initial high value such as 0.10 to

lower value such as 0.03 where hydrodynamic lubri-cation is performed. In the regime of hydrodynamic lubrication of water, the roughness effect on Stribeck curve is clear in Fig. 8 [16]. Therefore the smoothing mechanism at contact surfaces of ceramics in water becomes next major concern.

Fig. 7: Roughness change of SiC surface by wear in repeated sliding contact in water and its effect on

friction coefficient µ [16]

Fig. 8: The roughness effect on Stribeck curve of

SiC/SiC in water [16]

(a)

(b)

6 RUNNING-IN PROCESS Initial repeated sliding contact at ceramic/ceramic in water generates wear at asperity tips and low friction coefficient is attained in steady state after a certain amount of friction cycles. This process of running-in has more important role in water lubricated ceramics than in oil lubricated metals since the thickness of water film between ceramics is much thinner than that of oil film between metals.

Fig. 9 shows that friction of Si3N4/Si3N4 drops more quickly than that of SiC/SiC and the steady state value of friction coefficient is 0.01 for SiC/SiC and 0.005 for Si3N4/Si3N4 although both of them have the similar initial roughness of about 0.20 µm [4]. Careful analysis of wear scar on SiC shows that mechanical wear is pre-dominant until about 7×104 cycles and then tribo-chemical wear dominates to generate smooth wear surface for hydrodynamic lubrication.

Fig. 9: The running-in curves of SiC/SiC and Si3N4/Si3N4 in water where friction coefficient of SiC/SiC drops to

0.01 and that of Si3N4/Si3N4 to 0.005 [4]

Fig. 10: The effect of initial surface roughness Rrms on the critical friction cycle for running-in of SiC/SiC and

Si3N4/Si3N4 sliding in water [4]. On the other hand, tribochemical wear dominates in Si3N4/Si3N4 soon after starting of sliding and smooth wear surface for hydrodynamic lubrication is formed at 2×104 cycles. Initial surface roughness changes this

critical friction cycle for running-in as shown in Fig. 10, where SiC/SiC takes much more cycles than Si3N4/Si3N4 as the initial surface roughness increases [4].

Fig. 11: Velocity dependency of friction coefficient of SiC/SiC and Si3N4/ Si3N4, sliding in water after

running-in [4] After having smooth enough wear surface by running-in with good conforming, lubrication property of water film becomes the next concern. Fig. 11 shows the velocity dependency of friction coefficients of SiC/SiC and Si3N4/ Si3N4 observed after running-in in water. In the case of SiC/SiC, friction coefficient does not show the hysteresis in the process of velocity change. But in the case of Si3N4/ Si3N4, friction coefficient suddenly rises to a very high value at a certain velocity in the decreasing process of velocity, and it decreases slowly in the process of velocity increase. Such a difference in friction behavior would depend on the wear property of tribo-film formed on the wear surface, and so the understandings on wear mechanisms become important for water lubrication. 7 WEAR MECHANISMS IN WATER

7.1 Mechanical and tribochemical wear in sliding

Fig. 12 shows the wear mode transition of Si3N4 from mechanical wear to tribochemical wear in the running-in process [17]. Mechanical wear takes place at the early stage of running when initial surface asperities are in contact with high contact stresses. Crystalline wear particles are formed by transgranular or intergranular fracture, and pits are generated on wear surface. As the contact surfaces have better conforming by wear, tribo-chemical film grows on wear surface and it is removed from the surface by forming fine rolls or dissolved into water. This tribochemical wear is thought to have the following chemical reactions [2, 3]. Si3N4 + 6H2O → 3SiO2 + 4NH3 (1) SiO2 + 2H2O → Si(OH)4 (2) Generation of NH3 is experimentally confirmed, and the volume of NH3 is proportional to the wear volume [18].

Transgranular fracture

Intergranular fracture

Film delamination

Net-like agglomerate of wear particles

Transition Tribochemical wear

Mechanical wear

Fig. 12: Wear mode transitions of Si3N4 during running-

in of Si3N4/ Si3N4 sliding in water at initial roughness Rrms = 0.058 µm on disk surface [17].

Fig. 13 shows typical wear surface profiles of pin and disk made of Si3N4, where very smooth wear surfaces and good conforming of two surfaces are observed. It is the result of tribochemical wear which enables hydro-dynamic lubrication of water [17].

0.2 µ

m

0.2 µ

m

Fig. 13: Typical wear surface profiles of Si3N4 pin and Si3N4 disk after 2500 m sliding (37500 cycles) in water at room temperature. Nominal contact pressure on the

figure is 5.0 MPa [17]. Wear process of running-in is described as a function of sliding distance in Fig. 14 for SiC/SiC and Si3N4/Si3N4. Wear rates are calculated from the curves in Fig. 13 and shown in Table 1 [4].

The wear rate in the initial wear is mainly determined by mechanical wear, but chemical wear is partly joined. The wear rate in the steady wear is mainly determined by tribochemical wear. If sliding would be continued further, smaller wear rate in the ranges of 10-9 mm3/Nm and 10-10 mm3/Nm would be generated [19].

Fig. 14: Wear curves of SiC pin and Si3N4 pin sliding

against a disk of the same material in water. These wear curves correspond to the friction curves in Fig. 9 [4].

Initial wear

Steady wear

Wear rate of SiC pin against SiC disk in water (mm3/Nm) 2.8×10-6 2.1×10-7

Wear rate of Si3N4 pin against Si3N4 disk in water (mm3/Nm) 4.0×10-6 5.7×10-8

Table 1: Wear rate in running-in Remaining task is to introduce wear models and wear equations. Some challenges have been already made for mechanical wear and tribochemical wear although they are not yet ready for practical design [20, 21, 22, 23]. 7.2 Mechanical and tribochemical wear in

rolling-sliding Tribochemical wear dominates in pure rolling contact even when rolling of Si3N4/Si3N4 is made in humid air at high contact pressure above 1.0 GPa and extremely smooth wear surfaces are generated [24, 25]. Intro-duction of slippage in rolling contact increases the possibility of mechanical wear with crack propagation driven by additional shear stress at the contact interface.

Fig. 15: Wear mode map of Si3N4 in rolling-sliding

contact against Si3N4 with water lubrication. Revolution speed: 800 rpm [26].

The broken line in Fig. 15 shows the boundary between tribochemical wear regime and mechanical wear regime observed in air of 55 % RH [26]. The solid line in the figure shows the boundary of the same meaning observed in air with water lubrication. It is obvious from Fig. 15 that water lubrication extremely expands the regime of tribochemical wear on the wear mode map. 8 MATERIALS COMBINATION

8.1 Self-mated ceramics in water SiC/SiC and Si3N4 / Si3N4 are already practically established combinations for water lubricated ceramic tribo-elements in the past fifteen years. Load carrying capacity of SiC or Si3N4 is much larger than that of Al2O3 or ZrO2 [26]. For further applications and improvements of tribological performances under severe working conditions, there is much to do in funda-mental research.

Al2O3/Al2O3 is other attractive combination for water lubrication. A lubricious trihydroxide (Al(OH)3) layer is formed on a wear surface and it reduces wear and friction in water [28, 29]. Sliding wear rate and friction coefficient start with 2.4×10-9 mm3/Nm and µ ≅ 0.2 in the initial state and they increase to 1.6×10-8 mm3/Nm and µ ≅ 0.3 in the steady state [30].

These wear rate values are low enough to have water lubrication of Al2O3/Al2O3 for practice. It is already shown in Fig. 6 that Al2O3/Al2O3 has hydrodynamic regime under a certain combination of contact pressure and sliding velocity in water.

It is important to notice that Al2O3/Al2O3 (Ra ≅ 0.03 µm) shows smaller wear rate of 3.4×10-10 mm3/Nm in air than in water (3.4×10-9 mm3/Nm) under the contact pressure of 1.0 MPa [31]. Even increase of room humidity increases wear of Al2O3 [32].

ZrO2 / ZrO2 shows similar wear behavior to Al2O3/Al2O3. Wear rate of tetragonal zirconia, which has high fracture toughness of 11.6 MPam1/2, is in the order of 10-7 mm3/Nm at the initial wear in air, but it increases to the order of 10-6 mm-3/Nm in water. Intergranular fracture, which may be induced by water brittleness, is observed as a major wear mechanism for such a high wear rate [33]. In comparative rolling-sliding tests of tough ZrO2 (13.9 MPam1/2) and Si3N4 (4.1 MPam1/2) in water with 10 % slip and 3 GPa contact pressure, ZrO2/ ZrO2 shows the wear rate in the order of 10-5 mm-3/Nm with friction coefficient of 0.4 and Si3N4/ Si3N4 shows 10-8 mm-3/Nm and 0.05 respectively [34].

Because of such tribo-properties of ZrO2/ZrO2, its application in water is rather limited. 8.2 Ceramic against metal in water Hydrodynamic lubrication observed with Si3N4/Si3N4 after running-in in Fig. 8 is similarly observed with Si3N4/Cast Iron [35]. Friction coefficient is 0.7 at the starting point and it is reduced to below 0.02 after 2000 m sliding.

Lubrication number L

Alumina-alumina

Alumina-s. steel

Alumina-c.graphite

Fric

tion

coef

ficie

nt f

Fig. 16: Stribeck curves for alumina sliding in water

against three different materials; Al2O3, steel, graphite [37].

A tribo-film which consists of silica gel, graphite, Fe2O3 and crystal SiO2 is formed on the wear surface of cast iron [36]. This suggests that Si3N4/metal may have a chance to give equivalent or better tribological per-formance than of Si3N4 / Si3N4 in water depending on practical needs as shown in [5] where WC/ Si3N4 is shown as a good combination for lake water pump.

In the case of alumina, the combination of Al2O3/Steel shows much better performance in Stribeck curve than Al2O3/Al2O3 in water as shown in Fig. 16 [37]. These results above introduced strongly suggest that some ceramic/metal combinations may give better tribological performances in water than ceramic/ceramic combi-nations. 9 CONCLUSION Present understanding on fundamental friction and wear properties of ceramics in water are introduced, and the practical usefulness of water lubrication of ceramics is confirmed in the viewpoints of friction coefficient, load carrying capacity, seizure load and wear.

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