WATER LUBRICATION OF CERAMICS

Transcription

1 WATER LUBRICATION OF CERAMICS Koji KATO Tohoku University, Sendai, JAPAN; 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 tribochemical 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 [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 triboelements 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 L b 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 elastohydrodynamic lubrication (EHL) to mixed lubrication (ML), from ML to boundary lubrication (BL), and finally from BL to gross seizure (GS).

2 Friction coefficient µ Friction coefficient µ LENNING WANG et al E E E E E E E -4 (a) (b) 3 KANAS 1984 Duty parameter ηvb/w Lubrication number 1 Cast Iron/Steel/Oil 2 PB/Steel/Oil 3 Carbon/WC/Water 4 SiC/SiC/Water 5 SiC/Textured SiC/Water 2 BRIX WANG et al Cast Iron/Steel/Oil 1 LENNING 2 PB/Steel/Oil Carbon/WC/Wa ter 4 SiC/SiC/Water 5 SiC/Textured SiC/Water 3 KANAS BRIX WANG et al WANG et al E E E E E E E+0 10 ηv/rapm 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 B: contact width, m η: viscosity, Pa s V: velocity, m/s Ra: equivalent roughness, m W: load, N Pm: contact pressure, Pa 3 LOAD CARRYING CAPACITY MAP Friction coefficient µ Lenning, Brix, Kanas [11], Wang et al. [12] SiC cylinder/sic disk Ra = µm Water V = 0.62 m/s (800 rpm) Load W, N 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 quantitatively depending on the contact materials and lubricants. Fig. 3 shows the effect of pit area ratio r on the normalized critical load Wc/Wc 0 for SiC/SiC in water, where optimum pit area ratio for the largest critical load is obtained at around r 7 % [13]. Wc Critical load ratio Wc/Wc SiC cylinder/sic disk Pit diameter d: 150 µm Pit depth h: 3~4 µm Water 400 rpm 800 rpm 1200 rpm Pit area ratio r, % Fig. 3: The effect of surface pit area ratio on the critical load Wc for the transition from EHL to ML. Wc 0 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/Wc 0 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. Depth-diameter ratio h/d SiC cylinder/sic disk Pit diameter: 50~650µm Pit depth: 1.9~16.6µm Water Pit area ratio r, % Wc/Wc 0 <1 1.0~ ~ ~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, Si 3 N 4 /Si 3 N 4, Al 2 O 3 /Al 2 O 3 and ZrO 2 /ZrO 2 in water with a rotary cylinder and stationary disk [14].

3 Theoretical hydrodynamic regimes in Fig. 6 is shown for ηn/pm > , where N is revolutional speed [15]. lower value such as 0.03 where hydrodynamic lubrication 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. 5: The effect of mean contact pressure Pm and sliding velocity V on friction coefficient µ of SiC cylinder/sic disk in water [14] (a) (b) Fig. 7: Roughness change of SiC surface by wear in repeated sliding contact in water and its effect on friction coefficient µ [16] 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, Si 3 N 4 /Si 3 N 4, Al 2 O 3 /Al 2 O 3 or ZrO 2 /ZrO 2 by choosing good combination 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 ZrO 2 /ZrO 2 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 Fig. 8: The roughness effect on Stribeck curve of SiC/SiC in water [16]

4 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 Si 3 N 4 /Si 3 N 4 drops more quickly than that of SiC/SiC and the steady state value of friction coefficient is 0.01 for SiC/SiC and for Si 3 N 4 /Si 3 N 4 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 predominant until about cycles and then tribochemical wear dominates to generate smooth wear surface for hydrodynamic lubrication. Fig. 9: The running-in curves of SiC/SiC and Si 3 N 4 /Si 3 N 4 in water where friction coefficient of SiC/SiC drops to 0.01 and that of Si 3 N 4 /Si 3 N 4 to [4] Fig. 10: The effect of initial surface roughness Rrms on the critical friction cycle for running-in of SiC/SiC and Si 3 N 4 /Si 3 N 4 sliding in water [4]. On the other hand, tribochemical wear dominates in Si 3 N 4 /Si 3 N 4 soon after starting of sliding and smooth wear surface for hydrodynamic lubrication is formed at 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 Si 3 N 4 /Si 3 N 4 as the initial surface roughness increases [4]. Fig. 11: Velocity dependency of friction coefficient of SiC/SiC and Si 3 N 4 / Si 3 N 4, 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 Si 3 N 4 / Si 3 N 4 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 Si 3 N 4 / Si 3 N 4, 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 Si 3 N 4 from mechanical wear to tribochemical wear in the runningin 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, tribochemical 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]. Si 3 N 4 + 6H 2 O 3SiO 2 + 4NH 3 (1) SiO 2 + 2H 2 O Si(OH) 4 (2) Generation of NH 3 is experimentally confirmed, and the volume of NH 3 is proportional to the wear volume [18].

5 Mechanical wear Transition Tribochemical wear Transgranular fracture Intergranular fracture Film delamination Net-like agglomerate of wear particles Fig. 14: Wear curves of SiC pin and Si 3 N 4 pin sliding against a disk of the same material in water. These wear curves correspond to the friction curves in Fig. 9 [4]. Fig. 12: Wear mode transitions of Si 3 N 4 during runningin of Si 3 N 4 / Si 3 N 4 sliding in water at initial roughness Rrms = µm on disk surface [17]. Fig. 13 shows typical wear surface profiles of pin and disk made of Si 3 N 4, where very smooth wear surfaces and good conforming of two surfaces are observed. It is the result of tribochemical wear which enables hydrodynamic lubrication of water [17]. 0.2µm Wear rate of SiC pin against SiC disk in water (mm 3 /Nm) Wear rate of Si 3 N 4 pin against Si 3 N 4 disk in water (mm 3 /Nm) Initial wear Table 1: Wear rate in running-in Steady wear 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 Si 3 N 4 /Si 3 N 4 is made in humid air at high contact pressure above 1.0 GPa and extremely smooth wear surfaces are generated [24, 25]. Introduction of slippage in rolling contact increases the possibility of mechanical wear with crack propagation driven by additional shear stress at the contact interface. 0.2µm Fig. 13: Typical wear surface profiles of Si 3 N 4 pin and Si 3 N 4 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 Si 3 N 4 /Si 3 N 4. 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 mm 3 /Nm and mm 3 /Nm would be generated [19]. Fig. 15: Wear mode map of Si 3 N 4 in rolling-sliding contact against Si 3 N 4 with water lubrication. Revolution speed: 800 rpm [26].

6 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 Si 3 N 4 / Si 3 N 4 are already practically established combinations for water lubricated ceramic tribo-elements in the past fifteen years. Load carrying capacity of SiC or Si 3 N 4 is much larger than that of Al 2 O 3 or ZrO 2 [26]. For further applications and improvements of tribological performances under severe working conditions, there is much to do in fundamental research. Al 2 O 3 /Al 2 O 3 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 mm 3 /Nm and µ 0.2 in the initial state and they increase to mm 3 /Nm and µ 0.3 in the steady state [30]. These wear rate values are low enough to have water lubrication of Al 2 O 3 /Al 2 O 3 for practice. It is already shown in Fig. 6 that Al 2 O 3 /Al 2 O 3 has hydrodynamic regime under a certain combination of contact pressure and sliding velocity in water. It is important to notice that Al 2 O 3 /Al 2 O 3 (Ra 0.03 µm) shows smaller wear rate of mm 3 /Nm in air than in water ( mm 3 /Nm) under the contact pressure of 1.0 MPa [31]. Even increase of room humidity increases wear of Al 2 O 3 [32]. ZrO 2 / ZrO 2 shows similar wear behavior to Al 2 O 3 /Al 2 O 3. Wear rate of tetragonal zirconia, which has high fracture toughness of 11.6 MPam 1/2, is in the order of 10-7 mm 3 /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 ZrO 2 (13.9 MPam 1/2 ) and Si 3 N 4 (4.1 MPam 1/2 ) in water with 10 % slip and 3 GPa contact pressure, ZrO 2 / ZrO 2 shows the wear rate in the order of 10-5 mm -3 /Nm with friction coefficient of 0.4 and Si 3 N 4 / Si 3 N 4 shows 10-8 mm -3 /Nm and 0.05 respectively [34]. Because of such tribo-properties of ZrO 2 /ZrO 2, its application in water is rather limited. 8.2 Ceramic against metal in water Hydrodynamic lubrication observed with Si 3 N 4 /Si 3 N 4 after running-in in Fig. 8 is similarly observed with Si 3 N 4 /Cast Iron [35]. Friction coefficient is 0.7 at the starting point and it is reduced to below 0.02 after 2000 m sliding. Friction coefficient f Alumina-alumina Alumina-s. steel Alumina-c.graphite Lubrication number L Fig. 16: Stribeck curves for alumina sliding in water against three different materials; Al 2 O 3, steel, graphite [37]. A tribo-film which consists of silica gel, graphite, Fe 2 O 3 and crystal SiO 2 is formed on the wear surface of cast iron [36]. This suggests that Si 3 N 4 /metal may have a chance to give equivalent or better tribological performance than of Si 3 N 4 / Si 3 N 4 in water depending on practical needs as shown in [5] where WC/ Si 3 N 4 is shown as a good combination for lake water pump. In the case of alumina, the combination of Al 2 O 3 /Steel shows much better performance in Stribeck curve than Al 2 O 3 /Al 2 O 3 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 combinations. 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. If we would remember that the pioneering study on ceramic ball bearing was made in 1961 [38] and an incubation period for fundamental researches [39, 40, 41] were necessary until 1985 when commercial ceramic ball bearing appeared in the market, we may expect a big progress in the technology of water lubrication of ceramics in the coming twenty years as the environmental demands become stronger. 10 ACKNOWLEDGMENT The author would like to express his sincere appreciation to Dr. Wang Xiaolei at Tribology Laboratory of Tohoku University for his useful discussions and a big help in preparation of materials for this paper. 11 REFERENCES [1] Fischer, T. E. and Tomizawa, H.: Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride. Wear, 105 (1985), [2] Tomizawa, H. and Fischer, T. E.: Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication at low sliding speed obtained by tribochemical wear. 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