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Chemical wear of sintered Si3N4, hBN and Si3N4–hBN composites by water lubrication
Wear 247 (2001) 223–230
Chemical wear of sintered Si3N4 , hBN and Si3N4 –hBN composites by water lubrication Toshiyuki Saito a,∗ , Takeshi Hosoe b , Fumihiro Honda b a b
Toyoda Machine Works Ltd., Kariya, Aichi 448-8652, Japan Toyota Technological Institute, Nagoya, Aichi 468-8511, Japan
Received 7 August 2000; received in revised form 6 October 2000; accepted 6 October 2000
Abstract Chemical/mechanical wear ratios of Si3 N4 , hBN and Si3 N4 –hBN composites were investigated by determining the quantity of ammonia tribochemically produced in water. Chemical wear ratio, defined as an index of the chemical contribution to wear, showed a high value in the low-wear region and was expressed in an exponential curve as a function of the total wear loss. The chemical wear ratio of Si3 N4 depended strongly on sliding speed, but negligibly on surface roughness. A large constant for the ka of the regression curve of the chemical wear ratio showed a large chemical contribution to the wear of the material. A high constant k a = 17 was observed for Si3 N4 and low value of 8 for hBN indicated that wear proceed mainly mechanically. These results indicate that wear proceed chemically in the region of low wear rate, and a high chemical wear was observed on mechanically strong material. The Si3 N4 –hBN composite also showed a high chemical wear ratio in the low wear region, the curve of which is expressed by the linear combination of the equations for Si3 N4 with hBN. Furthermore, a chart of chemical wear ratio as a function of total wear loss, which indicates the chemical contribution to the wear of a material, is presented for practical use as a chemical wear map. © 2001 Published by Elsevier Science B.V. Keywords: Wear; Water; Ceramics; Chemical wear ratio; hBN; Boron nitride; Silicon nitride; Composite; Wear map
1. Introduction Silicon nitride (Si3 N4 ) is reported to provide low friction sliding µ = 0.01 at 0.06–0.20 m/s [1] and low contact pressures [2], and hexagonal boron nitride (hBN) is also reported to show relatively low friction and wear [3]. The friction characteristic of hBN depends on atmospheric conditions [5]. A higher friction coefficient of 0.7 is measured under vacuum, but in water this value decreases one or two orders of magnitude [4] compared with that in dry air [6] or oil [7]. These results suggest that hBN as a solid lubricant shows high performance only at high relative humidity. High tribological performance of sintered hBN is reported to be controlled by a basal plane slip [8] or tribological products like H3 BO3 [9] and B2 O3 [10]. Previous reports confirm that the crystal orientation parallel to a lamination layer (0 0 2) [11] slides between hBN/steel in water [4]. However, another study on hBN in NaCl solutions [12] shows that a low friction coefficient of approximately 0.1 lasts in spite of a thick layer of an NaCl absorbant on the sliding surface. A small friction variation in NaCl so∗ Corresponding author. Fax: +81-566-25-5489. E-mail address: [email protected] (T. Saito).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 0 ) 0 0 5 3 9 - 1
lution suggests a minor lubricant function for the interfacial tribo-products on hBN. On the other hand, a strong correlation between tribochemical reaction products and hBN sliding is observed with the tribochemical reaction product NH3 [4,13] in water. Estimation of the chemical influence on the tribological behavior of hBN provides significant information for practical applications, e.g. chemo-mechanical polishing (CMP) [14,15]. Although, a chemical reaction in water may be investigated indirectly by the oxidation of the worn particle [16] or a theoretical analysis [17], it could be monitored immediately by a quantitative measurement of tribochemical products such as NH3 in the case of nitride ceramics sliding in water. A spectrophotometric method makes it possible to quantitatively measure ammonia and estimate chemical wear [4,13] in water. The concept of chemical wear ratio is proposed [13] as an index of chemical influence on wear. This previously reports that the chemical influence is higher on hBN than on Si3 N4 and ascribe a low friction sliding [18] and mirror-like surface [19] of Si3 N4 to a result of the stronger tribochemical influence of water. As for hBN, the sintering additive CaO has a strong influence on its chemical wear and protects its surface from dissolution in water [4]. However, previous
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reports do not clarify the effect of these individual parameters on the chemical wear ratio, and the influence of friction force and sliding speed [20], which are related closely to the heat of formation [21], has not been investigated. Further approximations of the chemical wear ratio enable us to estimate the chemical influence on various materials. A useful information for the chemical wear ratio of Si3 N4 and hBN might be provided by investigation of Si3 N4 –hBN composites. Some reports have been published on composite ceramics [22], such as Si3 N4 –hBN and Si3 N4 –TiN composites [23]. The Si3 N4 –20% hBN composite slides under unlubricated conditions and shows low friction and wear sliding under 150◦ C [24]. Its improvement in terms of friction is reported to be attained by a tribological H3 BO3 coating on the sliding surface. Lubricating H3 BO3 [25] is produced from the reaction between hBN and water [26], but it dissolves easily in water. Therefore, sliding tests in water provide tribological information on the Si3 N4 –hBN interface layer compared with sliding tests under unlubicated conditions. Since increase in wear with the addition of hBN into a composite should be defused in water [27], sliding tests in water are suitable for the investigation of the tribochemical influence on wear. Furthermore, another report suggests that the coefficient of friction is not decreased following the addition of hBN into Si3 N4 at less than 30% [28], thus the larger amounts of hBN should be investigated. This paper clarifies the chemical influence on wear of nitride ceramics by investigating the individual parameter of sliding speed and provides a regression equation to determine the chemical wear ratio. Furthermore, the chemical influence on the wear of Si3 N4 –hBN composites with varied hBN contents was investigated in water to extend our knowledge of the chemical wear of nitride ceramics. Considerations based on the chemical wear ratio clarify the wear mechanism in terms of the chemical influence and a wear map [29]. 2. Experimental 2.1. Apparatus A traditional pin-on-plate testing device was used for the friction and wear experiments. A schematic assembly of the equipment is shown in Fig. 1. Basically, a normal load was applied on a stationary pin, and the friction force was continuously measured using a strain gauge. The sliding
Fig. 1. Schematics of pin-on-plate friction and wear tester.
surface was immersed in the test water and kept in the vessel during the sliding tests. 2.2. Specimens and solutions A Si3 N4 pin 4 mm in diameter was used to form point contacts. The Si3 N4 pin had a density of 3.20 g/cm3 and a Vickers’ hardness of 1400. The test plate specimens prepared were: (1) HIP silicon nitride (Si3 N4 ) [1]; (2) HIP pure hBN (hBN) [4]; and (3) HIP Si3 N4 –hBN composites with hBN content between 30 and 70 wt.%. The diameter of sintering particles are 5 m or smaller. The characteristics of the plates are shown in Table 1. The dimensions of the plates were 30 mm × 30 mm with a 4 mm thickness. The surfaces of the pin and plates were finished by grinding and lapping. The ground plate had 0.13 m Ra and the lapped one 0.01 m Ra. The solutions used in this test was air-saturated distilled water. 2.3. Procedure The specimens were made to slide in solution at an unidirectional speed of 0.06–0.20 m/s and a load of 3–22 N. The sliding distance was fixed at 700 m for most of the specimens, and only lap-finished Si3 N4 slid for 940 m, specifically. After the experiments, the specimens were rinsed and stored in a vacuum desiccator prior to surface analysis. The reaction products formed on the sliding surfaces were analyzed by a wave-dispersive electron probe microanalyzer (EPMA-WDS) operating at 15 kV acceleration voltage with a beam diameter of 1 m. The chemical states of the products on the sliding surface were analyzed by X-ray diffraction (XRD). The sliding surfaces of the plates were observed by
Table 1 Characteristics of plates of (1) Si3 N4 ; (2–4) Si3 N4 –hBN composites and (5) hBN S. No.
hBN ratio in a composites (wt.%)
Sintering additive
Density (g/cm3 )
Vickers’ hardness
Young’s modulus (GPa)
1 2 3 4 5
0 (silicon nitride) 30 50 70 100 (hexagonal boron nitride)
1 wt.% Y2 O3 –2 wt.% Al2 O3 3 wt.% MgO–0.5 wt.% CaO 2 wt.% MgO–1 wt.% CaO 2 wt.% MgO–2 wt.% CaO No additive
3.24 2.68 2.21 1.96 1.87
1420 316 82 41 8
310 170 115 80 35
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scanning electron microscopy (SEM), and the surface profile was obtained using a highly sensitive stylus profilometer to calculate the specific wear rate. The amount of ammonia formed tribochemically in water was quantitatively determined by a conventional spectrophotometric method using Nessler’s reagent. For the investigation of the chemical influence on wear, the parameter of chemical wear ratio (CR) has been proposed [13]: CR (%) =
100Wc Wtotal
(1)
where Wc is the chemical wear loss (mm3 ) and Wtotal is the total wear loss (mm3 ). Wc of Si3 N4 was calculated from the quantity of ammonia produced from the following reaction (2) [1]: Si3 N4(s) + 6H2 O(l) → 3SiO2(s) + 4NH3(aq.)
(2)
According to reaction (2), 1 mol of ammonia corresponds to the consumption of 1/4 mol Si3 N4 . Ammonia is also produced by hBN sliding due to the following reaction (3) [4]: 2BN(s) + 3H2 O(l) → B2 O3(s) + 2NH3(aq.)
(3)
In reaction (3), 1 mol of ammonia corresponds to the chemical wear loss of 1 mol of hBN, and the chemical wear ratio of hBN is also obtained from Eq. (1).
3. Results and discussion 3.1. Friction behavior and mirror-like surface of Si3 N4 under various solutions, sliding speeds, and roughness To investigate the influence of original surface roughness on friction behavior of Si3 N4 /Si3 N4 in water, the ground and lap-finished specimens were slid under various sliding speeds. Fig. 2 shows the friction coefficient of a ground finished Si3 N4 in water as a function of sliding distance,
Fig. 3. Friction coefficient of lap-finished Si3 N4 sliding in water.
and Fig. 3 shows the same for a lap-finished one. Although, some samples of the ground-finished Si3 N4 required longer sliding distances than lap-finished Si3 N4 specimens to show a low friction sliding of µ = 0.01, the friction coefficient of ground-finished Si3 N4 was stable after low friction sliding appeared. On the other hand, the lap-finished Si3 N4 showed fluctuating functions, indicating that lap-finished surfaces were not advantageous to reach static and low-friction sliding. Figs. 2 and 3 indicate that a low-friction sliding does not only require an initial smooth surface, but also satisfactory running-in or film-formation of hBN. Additional knowledge of the mechanism of low-friction sliding indicates its strong dependence on sliding speed. Low-friction sliding depends on tribochemical products on the sliding surface [1] which are produced under a high friction energy. Therefore, the quantitative determination of tribochemical reaction products as a function of sliding speed is advantageous for the understanding of low-friction sliding, because friction energy with sliding speed [20]. Fig. 4 shows the plate surface of Si3 N4 before and after sliding. A ground track on the surface before sliding (Fig. 4(a)) was worn away and a mirror-finished surface (c) like a lap-finished one (b) was formed after sliding. Previous reports [2,13] suggested that a chemical contribution to wear may be investigated by measuring the quantity of tribochemically produced ammonia. In the following section, the chemical contribution to wear is discussed from the perspective of the chemical wear ratio. 3.2. Tribochemical influence on wear of Si3 N4 sliding in water
Fig. 2. Friction coefficient of ground-finished Si3 N4 sliding in water.
Fig. 5 shows the quantity of ammonia determined spectrophotometrically after Si3 N4 /Si3 N4 sliding tests in water. At a high sliding speed, the quantity of ammonia decreased markedly on both ground- and lap-finished Si3 N4 . The amount of ammonia is independent of the initial surface roughness, and little difference is observed in the quantity of generated ammonia between ground and lap-finished Si3 N4 ,
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Fig. 4. SEM images of (a) surface before sliding of ground-Si3 N4 plate; (b) surface before sliding of a lap-finished Si3 N4 plate and (c) surface after sliding of a Si3 N4 plate in distilled water.
Fig. 5. Quantity of ammonia formed in water as a function of the sliding speed of Si3 N4 /Si3 N4 .
because a difference in wear loss due to the roughness of the sliding surface is small after a sufficiently long sliding distance. Fig. 6 shows the total and chemical wear losses calculated from the quantity of ammonia in Fig. 5 using Eqs. (1) and (2). In Fig. 6, the total wear loss decreases as the sliding speed increases, although, chemical wear loss only de-
Fig. 6. Total and chemical wear losses by Si3 N4 /Si3 N4 sliding in water as a function of sliding speed.
creased slightly. The difference between the total and chemical wear loss increased the chemical influence on wear prominent in the region of low total wear at high sliding speed. Further understanding is attained from the graph of chemical wear ratio plotted as a function of sliding speed (Fig. 7). For the ground-finished Si3 N4 , the chemical wear ratio has a high value at high sliding speed; in contrast, the wear ratio saturated at approximately 40–50% for lap-finished Si3 N4 . The high chemical wear ratio at high sliding speed is not only due to a high heat of friction, but also to a change in the chemical wear loss and a decrease in the total wear loss as shown in Fig. 6. As a result, the chemical wear ratio is independent of sliding speed and original surface roughness in the range studied, but strongly dependent on the total wear loss. 3.3. Approximation of chemical wear ratios of Si3 N4 and hBN From previous results, the chemical wear loss of Si3 N4 was discussed as a function of total wear loss. Moreover, hBN was slid in water to investigate its chemical wear ratio.
Fig. 7. Chemical wear ratio of Si3 N4 /Si3 N4 sliding in water with various sliding speeds.
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Fig. 8. Chemical wear ratios of Si3 N4 and hBN as a function of total wear loss.
hBN is very soft compared with Si3 N4 , hence, its chemical behavior is assumed to be different. Fig. 8 shows chemical wear ratios of nitrides as a function of the total wear loss. In Fig. 8, chemical wear ratios of Si3 N4 and hBN had a strong correlation with the total wear loss and apparently decreased in the region of high wear. A higher chemical influence was observed in the region of little wear, and a lower one in the region of large wear loss. This behavior indicates that wear proceeds chemically in the region of little wear and mechanically in the region of large wear. Fig. 8 indicates that the chemical wear ratio could be plotted against total wear loss along an exponential curve. From the analysis of the chemical wear ratio, a regression curve expressed as Eq. (4) is proposed with a regression coefficient of 0.9: 2/3
Wc = k1 Wtotal
(4)
where k1 is a constant. Eq. (4) shows that the chemical wear loss Wc depends on the surface area on which a tribochemical reaction occurs prominently. From the definition in Eq. (1), the chemical wear ratio CR is CR (%) =
k2 Wc , Wtotal
(5)
where k2 is constant. From Eqs. (4) and (5), CR is derived as: 2/3 k2 k1 Wtotal −1/3 or CR (%) = ka Wtotal , (6) CR (%) = Wtotal where ka is a combined constant, varies with the tribochemical activity of a material. Eq. (6) indicates that CR is exponentially correlated with the total wear loss. In Fig. 8, we can calculate CR using the coefficient ka from Eq. (7) as 17 and that of Eq. (8) as 8.0 by best fit regression analysis as follows: −1/3
Si3 N4 : CR (%) = 17 Wtotal
−1/3
hBN : CR (%) = 8.0 Wtotal
(7) (8)
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Fig. 9. Friction coefficient of Si3 N4 /Si3 N4 –hBN composites with varying hBN content.
Comparing data within the same region of the wear loss, a high chemical influence on wear is observed in the region of high ka . In Fig. 8, a higher chemical wear ratio is observed particularly for Si3 N4 , indicating the differences in the wear behavior of nitrides: chemical wear on the part of hard Si3 N4 and mechanical wear on the part of soft hBN. Furthermore, in the sliding Si3 N4 -contacts, a formation of polished mirror-surface promoted the lubrication of hydrate on sliding surface, so that a mechanical to chemical wear transition was attained smoothly. 3.4. Analysis of chemical wear ratios of Si3 N4 –hBN composites with various hBN ratios The difference between the chemical influence on wear of Si3 N4 and hBN is very clear, hence, the chemical influence on sintered Si3 N4 –hBN composites is a serious concern. In this study, Si3 N4 –hBN composites were sintered to investigate their friction characteristics and chemical wear ratios. Fig. 9 shows the friction coefficient observed for a sintered Si3 N4 pin against sintered Si3 N4 –hBN composite plates in water. The Si3 N4 –hBN composite with a low hBN ratio shows a reduction in friction with sliding, indicating that it inherited the frictional characteristics of Si3 N4 . The mirror-like sliding surface of Si3 N4 was also attained on the Si3 N4 –hBN composite. At 30 wt.% hBN, the friction coefficient at the first stage and the frictional fluctuation were decreased. In contrast, hBN at 50 wt.% or more reduced and apparently stabilized the friction coefficient. These results indicate that Si3 N4 –hBN composites apparently retain the friction characteristics of Si3 N4 and hBN. Fig. 10 shows the results for composites of the chemical wear ratios and total wear losses at a load of 11.4 N and sliding speed of 0.13 m/s. The chemical wear ratios of Si3 N4 –hBN shown in Fig. 10 were calculated from the approximation in Eq. (9), which is generated by the linear combination of Eq. (7) for Si3 N4 and Eq. (8) for hBN:
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3.5. Chemical wear map for Si3 N4 –hBN composites in water
Fig. 10. The calculated chemical wear loss and total wear loss for composites as a function of hBN composition at a load of 11.4 N and a sliding speed of 0.13 m/s.
CR =
[17 (α/100Wtotal )2/3 +8.0 {(100−α)/100Wtotal }2/3 ] Wtotal (9)
where α (%) is the volume percent of hBN in the composite (0 ≤ α ≤ 100). The chemical wear ratio decreases as the hBN ratio increase as shown in Fig. 11. The agreement of the measurement of chemical wear ratio and the analysis based on the approximate Eq. (9) in terms of results obtained indicated that the chemical wear ratio of Si3 N4 –hBN reflected the chemical characteristics of Si3 N4 and hBN. Furthermore, a good correlation between the measurement of chemical wear ratio and Eq. (9) in terms of results obtained suggests that the chemical wear ratio of Si3 N4 –hBN composite has a close relationship with the total wear loss. From Eq. (9), the chemical influence on Si3 N4 –hBN composite sliding could be calculated from the total wear loss.
Fig. 11. Ammonia formed by Si3 N4 –hBN composite in water after sliding 700 m.
In this section, results of Si3 N4 –hBN composites slid under various loads from 3 to 35 N and at speeds from 0.06 to 0.19 m/s are presented and summarized on a chemical wear loss chart as a function of total wear loss, which is valid for the discussion of chemical influence on wear. For the purpose of making the chemical wear ratio–total wear loss plot, the amount of ammonia generated after sliding in water was measured using a spectrometric method and graphed as shown in Fig. 11, which shows a parabolic increase in ammonia as a function of total wear loss. The chemical wear loss of Si3 N4 –hBN composites was derived from Fig. 11 and is shown in Fig. 12. The specific wear rate lies in the range 10−6 –10−4 mm3 /N m, and the chemical wear ratio decreased with increasing total wear loss. The regression curve in Fig. 12 was calculated using Eq. (9) with a coefficient ka between 10 and 50. The coefficient ka indicates the chemical influence on wear loss and is distributed in the narrow range from 10 to 30, as shown in Fig. 12. In the case of the Si3 N4 –hBN composite, CR was maintained at an approximate ka of 17 for hBN to 8.0 for Si3 N4 . As mentioned above, this approximate equation could be used for investigating the chemical influence on wear and for classifying wear characteristics using the order of magnitude of ka . Fig. 13 shows the chemical wear ratio as a function of total wear loss for the 50 wt.% hBN composite for investigating wear behavior in detail. From the coefficient ka , the chemical wear ratio of the 50 wt.% hBN composite was classified into three types: Type 1: region of little chemical wear similar to the level of hBN. Type 2: region of large chemical wear, in the order of wear of Si3 N4 . Type 3: region of dominant chemical wear.
Fig. 12. Chart of chemical wear ratio–total wear loss for the Si3 N4 –hBN composites.
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Si3 N4 and hBN as a function of total wear loss and hBN constant. 5. The map of chemical wear ratio as a function of total wear loss, which includes regression curves, suggests the chemical influence on wear rate.
Acknowledgements
Fig. 13. Chart of chemical wear ratio–total wear loss for the Si3 N4 –hBN composite with 50 wt.% hBN.
Types 1 and 2 simply reflect the wear characteristics of hBN and Si3 N4 . As discussed above, sintered Si3 N4 –hBN composites of types 1 and 2 reflect the friction characteristics of monolithic Si3 N4 and hBN, and Eq. (9) enables us to estimate the order of its chemical influence on wear. On the other hand, type 3 is a complex formation which includes the effect of chemical activity. A higher chemical activity or a transition in wear should be considered in the region of type 3; however, further investigations of the wear process are required. As a result, the chemical wear ratios of composites are distributed in a narrow ka range of 10–30, which includes the values of single phase Si3 N4 and hBN, however, some data showed a higher chemical activity of wear as shown in the chart of chemical wear ratio–total wear loss. The chart of chemical wear ratio–total wear loss, which includes regression curves, is available for an analysis of the wear behavior of binary composite materials. In conclusion, a chart of chemical wear ratio–total wear loss can be considered to be a wear map [29] for investigating chemical influence on wear and wear patterns.
4. Conclusions 1. The low-wear/high wear transitions of Si3 N4 , hBN and Si3 N4 –hBN are governed by the change of mechanism from chemical to mechanical wear. 2. High chemical wear ratios are attained by Si3 N4 /Si3 N4 sliding couples and result in wear reduction in the speed range at 0.2 m/s. On the other hand, surface roughness has little influence on wear ratio after a sufficient sliding distance. 3. Chemical wear ratios are expressed by an exponential function of the total wear loss with ka coefficients of 17 for Si3 N4 and 8.0 for hBN in the valid PV between 0.18–4.4 N m/s. 4. Chemical wear ratios of Si3 N4 –hBN composites were calculated using the approximate equations for simple
We thank Emeritus Professor K. Nakajima and Dr. Y. Imada of Toyota Technological Institute for helpful suggestions. We also thank Dr. Y. Mizutani of Toyota Central R&D Labs., Inc. for instructive support. Toyoda Machine Works, Ltd. is acknowledged for the financial support of this work. References [1] T. Saito, Y. Imada, F. Honda, An analytical observation of the tribochemical reaction of silicon nitride sliding with low friction in aqueous solutions, Wear 205 (1997) 153–159. [2] T.E. Fischer, H. Tomizawa, Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride, Wear 105 (1985) 29–45. [3] S. Jahanmir, T.E. Fischer, Friction and wear of silicon nitride lubricated by humid air, water, hexadecane and hexadecane + 0.5 percent stearic acid, STLE Trans. 31 (1) (1988) 32–43. [4] T. Saito, F. Honda, Chemical contribution to friction behavior of sintered hexagonal boron nitride in water, Wear 237 (2000) 253– 260. [5] G.W. Rowe, Some observations of the frictional behavior of boron nitride and of graphite, Wear 3 (1960) 274–285. [6] E. Rabinowicz, M. Imai, Frictional properties of pyrolytic boron nitride and graphite, Wear 7 (1964) 298–300. [7] Y. Kimura, T. Wakabayashi, K. Okada, T. Wada, H. Nishikawa, Boron nitride as a lubricant additive, Wear 232 (1999) 199–206. [8] J.M. Martin, M.N. Gardos, Friction of hexagonal boron nitride in various environments, Tribol. Trans. 35 (3) (1992) 462–472. [9] A. Erdemir, G.R. Fenske, R.A. Erck, A Study of the formation and self-lubrication mechanisms of boric acid films on boric oxide coatings, Surf. Coating Technol. 43/44 (1990) 588–596. [10] P. Larsson, N. Axen, S. Hogmark, Tribofilm formation on boron carbide in sliding wear, Wear 236 (1999) 73–80. [11] R. Geick, C.H. Perry, Normal modes in hexagonal boron nitride, Phys. Rev. 146 (2) (1966) 543–547. [12] T. Saito, F. Honda, Low friction behavior of sintered hBN sliding in sodium chloride solution, Wear 244 (2000) 133–140. [13] T. Saito, Y. Imada, F. Honda, Chemical influence on wear of Si3 N4 and hBN in water, Wear 236 (1999) 153–158. [14] M. Jiang, N.O. Wood, R. Komanduri, On chemo-mechanical polishing (CMP) of silicon nitride (Si3 N4 ) workmaterial with various abrasives, Wear 220 (1998) 59–71. [15] K. Kato, Tribology of ceramics, Wear 136 (1990) 117–133. [16] J. Xu, K. Kato, T. Hirayama, The transition of wear mode during the running-in process of silicon nitride sliding in water, Wear 205 (1997) 55–63. [17] V.A. Muratov, T. Luangvaranunt, T.E. Fischer, The tribochemistry of silicon nitride: effects of friction, temperature and sliding velocity, Tribol. Int. 31 (10) (1998) 601–611. [18] B. Löffelbein, M. Woydt, K.H. Habig, Sliding friction and wear of ceramics in neutral, acid and basic aqueous solutions, Wear 162/164 (1993) 220–228.
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Chemical wear of sintered Si3N4 , hBN and Si3N4 –hBN composites by water lubrication Toshiyuki Saito a,∗ , Takeshi Hosoe b , Fumihiro Honda b a b
Toyoda Machine Works Ltd., Kariya, Aichi 448-8652, Japan Toyota Technological Institute, Nagoya, Aichi 468-8511, Japan
Received 7 August 2000; received in revised form 6 October 2000; accepted 6 October 2000
Abstract Chemical/mechanical wear ratios of Si3 N4 , hBN and Si3 N4 –hBN composites were investigated by determining the quantity of ammonia tribochemically produced in water. Chemical wear ratio, defined as an index of the chemical contribution to wear, showed a high value in the low-wear region and was expressed in an exponential curve as a function of the total wear loss. The chemical wear ratio of Si3 N4 depended strongly on sliding speed, but negligibly on surface roughness. A large constant for the ka of the regression curve of the chemical wear ratio showed a large chemical contribution to the wear of the material. A high constant k a = 17 was observed for Si3 N4 and low value of 8 for hBN indicated that wear proceed mainly mechanically. These results indicate that wear proceed chemically in the region of low wear rate, and a high chemical wear was observed on mechanically strong material. The Si3 N4 –hBN composite also showed a high chemical wear ratio in the low wear region, the curve of which is expressed by the linear combination of the equations for Si3 N4 with hBN. Furthermore, a chart of chemical wear ratio as a function of total wear loss, which indicates the chemical contribution to the wear of a material, is presented for practical use as a chemical wear map. © 2001 Published by Elsevier Science B.V. Keywords: Wear; Water; Ceramics; Chemical wear ratio; hBN; Boron nitride; Silicon nitride; Composite; Wear map
1. Introduction Silicon nitride (Si3 N4 ) is reported to provide low friction sliding µ = 0.01 at 0.06–0.20 m/s [1] and low contact pressures [2], and hexagonal boron nitride (hBN) is also reported to show relatively low friction and wear [3]. The friction characteristic of hBN depends on atmospheric conditions [5]. A higher friction coefficient of 0.7 is measured under vacuum, but in water this value decreases one or two orders of magnitude [4] compared with that in dry air [6] or oil [7]. These results suggest that hBN as a solid lubricant shows high performance only at high relative humidity. High tribological performance of sintered hBN is reported to be controlled by a basal plane slip [8] or tribological products like H3 BO3 [9] and B2 O3 [10]. Previous reports confirm that the crystal orientation parallel to a lamination layer (0 0 2) [11] slides between hBN/steel in water [4]. However, another study on hBN in NaCl solutions [12] shows that a low friction coefficient of approximately 0.1 lasts in spite of a thick layer of an NaCl absorbant on the sliding surface. A small friction variation in NaCl so∗ Corresponding author. Fax: +81-566-25-5489. E-mail address: [email protected] (T. Saito).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 0 ) 0 0 5 3 9 - 1
lution suggests a minor lubricant function for the interfacial tribo-products on hBN. On the other hand, a strong correlation between tribochemical reaction products and hBN sliding is observed with the tribochemical reaction product NH3 [4,13] in water. Estimation of the chemical influence on the tribological behavior of hBN provides significant information for practical applications, e.g. chemo-mechanical polishing (CMP) [14,15]. Although, a chemical reaction in water may be investigated indirectly by the oxidation of the worn particle [16] or a theoretical analysis [17], it could be monitored immediately by a quantitative measurement of tribochemical products such as NH3 in the case of nitride ceramics sliding in water. A spectrophotometric method makes it possible to quantitatively measure ammonia and estimate chemical wear [4,13] in water. The concept of chemical wear ratio is proposed [13] as an index of chemical influence on wear. This previously reports that the chemical influence is higher on hBN than on Si3 N4 and ascribe a low friction sliding [18] and mirror-like surface [19] of Si3 N4 to a result of the stronger tribochemical influence of water. As for hBN, the sintering additive CaO has a strong influence on its chemical wear and protects its surface from dissolution in water [4]. However, previous
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reports do not clarify the effect of these individual parameters on the chemical wear ratio, and the influence of friction force and sliding speed [20], which are related closely to the heat of formation [21], has not been investigated. Further approximations of the chemical wear ratio enable us to estimate the chemical influence on various materials. A useful information for the chemical wear ratio of Si3 N4 and hBN might be provided by investigation of Si3 N4 –hBN composites. Some reports have been published on composite ceramics [22], such as Si3 N4 –hBN and Si3 N4 –TiN composites [23]. The Si3 N4 –20% hBN composite slides under unlubricated conditions and shows low friction and wear sliding under 150◦ C [24]. Its improvement in terms of friction is reported to be attained by a tribological H3 BO3 coating on the sliding surface. Lubricating H3 BO3 [25] is produced from the reaction between hBN and water [26], but it dissolves easily in water. Therefore, sliding tests in water provide tribological information on the Si3 N4 –hBN interface layer compared with sliding tests under unlubicated conditions. Since increase in wear with the addition of hBN into a composite should be defused in water [27], sliding tests in water are suitable for the investigation of the tribochemical influence on wear. Furthermore, another report suggests that the coefficient of friction is not decreased following the addition of hBN into Si3 N4 at less than 30% [28], thus the larger amounts of hBN should be investigated. This paper clarifies the chemical influence on wear of nitride ceramics by investigating the individual parameter of sliding speed and provides a regression equation to determine the chemical wear ratio. Furthermore, the chemical influence on the wear of Si3 N4 –hBN composites with varied hBN contents was investigated in water to extend our knowledge of the chemical wear of nitride ceramics. Considerations based on the chemical wear ratio clarify the wear mechanism in terms of the chemical influence and a wear map [29]. 2. Experimental 2.1. Apparatus A traditional pin-on-plate testing device was used for the friction and wear experiments. A schematic assembly of the equipment is shown in Fig. 1. Basically, a normal load was applied on a stationary pin, and the friction force was continuously measured using a strain gauge. The sliding
Fig. 1. Schematics of pin-on-plate friction and wear tester.
surface was immersed in the test water and kept in the vessel during the sliding tests. 2.2. Specimens and solutions A Si3 N4 pin 4 mm in diameter was used to form point contacts. The Si3 N4 pin had a density of 3.20 g/cm3 and a Vickers’ hardness of 1400. The test plate specimens prepared were: (1) HIP silicon nitride (Si3 N4 ) [1]; (2) HIP pure hBN (hBN) [4]; and (3) HIP Si3 N4 –hBN composites with hBN content between 30 and 70 wt.%. The diameter of sintering particles are 5 m or smaller. The characteristics of the plates are shown in Table 1. The dimensions of the plates were 30 mm × 30 mm with a 4 mm thickness. The surfaces of the pin and plates were finished by grinding and lapping. The ground plate had 0.13 m Ra and the lapped one 0.01 m Ra. The solutions used in this test was air-saturated distilled water. 2.3. Procedure The specimens were made to slide in solution at an unidirectional speed of 0.06–0.20 m/s and a load of 3–22 N. The sliding distance was fixed at 700 m for most of the specimens, and only lap-finished Si3 N4 slid for 940 m, specifically. After the experiments, the specimens were rinsed and stored in a vacuum desiccator prior to surface analysis. The reaction products formed on the sliding surfaces were analyzed by a wave-dispersive electron probe microanalyzer (EPMA-WDS) operating at 15 kV acceleration voltage with a beam diameter of 1 m. The chemical states of the products on the sliding surface were analyzed by X-ray diffraction (XRD). The sliding surfaces of the plates were observed by
Table 1 Characteristics of plates of (1) Si3 N4 ; (2–4) Si3 N4 –hBN composites and (5) hBN S. No.
hBN ratio in a composites (wt.%)
Sintering additive
Density (g/cm3 )
Vickers’ hardness
Young’s modulus (GPa)
1 2 3 4 5
0 (silicon nitride) 30 50 70 100 (hexagonal boron nitride)
1 wt.% Y2 O3 –2 wt.% Al2 O3 3 wt.% MgO–0.5 wt.% CaO 2 wt.% MgO–1 wt.% CaO 2 wt.% MgO–2 wt.% CaO No additive
3.24 2.68 2.21 1.96 1.87
1420 316 82 41 8
310 170 115 80 35
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scanning electron microscopy (SEM), and the surface profile was obtained using a highly sensitive stylus profilometer to calculate the specific wear rate. The amount of ammonia formed tribochemically in water was quantitatively determined by a conventional spectrophotometric method using Nessler’s reagent. For the investigation of the chemical influence on wear, the parameter of chemical wear ratio (CR) has been proposed [13]: CR (%) =
100Wc Wtotal
(1)
where Wc is the chemical wear loss (mm3 ) and Wtotal is the total wear loss (mm3 ). Wc of Si3 N4 was calculated from the quantity of ammonia produced from the following reaction (2) [1]: Si3 N4(s) + 6H2 O(l) → 3SiO2(s) + 4NH3(aq.)
(2)
According to reaction (2), 1 mol of ammonia corresponds to the consumption of 1/4 mol Si3 N4 . Ammonia is also produced by hBN sliding due to the following reaction (3) [4]: 2BN(s) + 3H2 O(l) → B2 O3(s) + 2NH3(aq.)
(3)
In reaction (3), 1 mol of ammonia corresponds to the chemical wear loss of 1 mol of hBN, and the chemical wear ratio of hBN is also obtained from Eq. (1).
3. Results and discussion 3.1. Friction behavior and mirror-like surface of Si3 N4 under various solutions, sliding speeds, and roughness To investigate the influence of original surface roughness on friction behavior of Si3 N4 /Si3 N4 in water, the ground and lap-finished specimens were slid under various sliding speeds. Fig. 2 shows the friction coefficient of a ground finished Si3 N4 in water as a function of sliding distance,
Fig. 3. Friction coefficient of lap-finished Si3 N4 sliding in water.
and Fig. 3 shows the same for a lap-finished one. Although, some samples of the ground-finished Si3 N4 required longer sliding distances than lap-finished Si3 N4 specimens to show a low friction sliding of µ = 0.01, the friction coefficient of ground-finished Si3 N4 was stable after low friction sliding appeared. On the other hand, the lap-finished Si3 N4 showed fluctuating functions, indicating that lap-finished surfaces were not advantageous to reach static and low-friction sliding. Figs. 2 and 3 indicate that a low-friction sliding does not only require an initial smooth surface, but also satisfactory running-in or film-formation of hBN. Additional knowledge of the mechanism of low-friction sliding indicates its strong dependence on sliding speed. Low-friction sliding depends on tribochemical products on the sliding surface [1] which are produced under a high friction energy. Therefore, the quantitative determination of tribochemical reaction products as a function of sliding speed is advantageous for the understanding of low-friction sliding, because friction energy with sliding speed [20]. Fig. 4 shows the plate surface of Si3 N4 before and after sliding. A ground track on the surface before sliding (Fig. 4(a)) was worn away and a mirror-finished surface (c) like a lap-finished one (b) was formed after sliding. Previous reports [2,13] suggested that a chemical contribution to wear may be investigated by measuring the quantity of tribochemically produced ammonia. In the following section, the chemical contribution to wear is discussed from the perspective of the chemical wear ratio. 3.2. Tribochemical influence on wear of Si3 N4 sliding in water
Fig. 2. Friction coefficient of ground-finished Si3 N4 sliding in water.
Fig. 5 shows the quantity of ammonia determined spectrophotometrically after Si3 N4 /Si3 N4 sliding tests in water. At a high sliding speed, the quantity of ammonia decreased markedly on both ground- and lap-finished Si3 N4 . The amount of ammonia is independent of the initial surface roughness, and little difference is observed in the quantity of generated ammonia between ground and lap-finished Si3 N4 ,
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Fig. 4. SEM images of (a) surface before sliding of ground-Si3 N4 plate; (b) surface before sliding of a lap-finished Si3 N4 plate and (c) surface after sliding of a Si3 N4 plate in distilled water.
Fig. 5. Quantity of ammonia formed in water as a function of the sliding speed of Si3 N4 /Si3 N4 .
because a difference in wear loss due to the roughness of the sliding surface is small after a sufficiently long sliding distance. Fig. 6 shows the total and chemical wear losses calculated from the quantity of ammonia in Fig. 5 using Eqs. (1) and (2). In Fig. 6, the total wear loss decreases as the sliding speed increases, although, chemical wear loss only de-
Fig. 6. Total and chemical wear losses by Si3 N4 /Si3 N4 sliding in water as a function of sliding speed.
creased slightly. The difference between the total and chemical wear loss increased the chemical influence on wear prominent in the region of low total wear at high sliding speed. Further understanding is attained from the graph of chemical wear ratio plotted as a function of sliding speed (Fig. 7). For the ground-finished Si3 N4 , the chemical wear ratio has a high value at high sliding speed; in contrast, the wear ratio saturated at approximately 40–50% for lap-finished Si3 N4 . The high chemical wear ratio at high sliding speed is not only due to a high heat of friction, but also to a change in the chemical wear loss and a decrease in the total wear loss as shown in Fig. 6. As a result, the chemical wear ratio is independent of sliding speed and original surface roughness in the range studied, but strongly dependent on the total wear loss. 3.3. Approximation of chemical wear ratios of Si3 N4 and hBN From previous results, the chemical wear loss of Si3 N4 was discussed as a function of total wear loss. Moreover, hBN was slid in water to investigate its chemical wear ratio.
Fig. 7. Chemical wear ratio of Si3 N4 /Si3 N4 sliding in water with various sliding speeds.
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Fig. 8. Chemical wear ratios of Si3 N4 and hBN as a function of total wear loss.
hBN is very soft compared with Si3 N4 , hence, its chemical behavior is assumed to be different. Fig. 8 shows chemical wear ratios of nitrides as a function of the total wear loss. In Fig. 8, chemical wear ratios of Si3 N4 and hBN had a strong correlation with the total wear loss and apparently decreased in the region of high wear. A higher chemical influence was observed in the region of little wear, and a lower one in the region of large wear loss. This behavior indicates that wear proceeds chemically in the region of little wear and mechanically in the region of large wear. Fig. 8 indicates that the chemical wear ratio could be plotted against total wear loss along an exponential curve. From the analysis of the chemical wear ratio, a regression curve expressed as Eq. (4) is proposed with a regression coefficient of 0.9: 2/3
Wc = k1 Wtotal
(4)
where k1 is a constant. Eq. (4) shows that the chemical wear loss Wc depends on the surface area on which a tribochemical reaction occurs prominently. From the definition in Eq. (1), the chemical wear ratio CR is CR (%) =
k2 Wc , Wtotal
(5)
where k2 is constant. From Eqs. (4) and (5), CR is derived as: 2/3 k2 k1 Wtotal −1/3 or CR (%) = ka Wtotal , (6) CR (%) = Wtotal where ka is a combined constant, varies with the tribochemical activity of a material. Eq. (6) indicates that CR is exponentially correlated with the total wear loss. In Fig. 8, we can calculate CR using the coefficient ka from Eq. (7) as 17 and that of Eq. (8) as 8.0 by best fit regression analysis as follows: −1/3
Si3 N4 : CR (%) = 17 Wtotal
−1/3
hBN : CR (%) = 8.0 Wtotal
(7) (8)
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Fig. 9. Friction coefficient of Si3 N4 /Si3 N4 –hBN composites with varying hBN content.
Comparing data within the same region of the wear loss, a high chemical influence on wear is observed in the region of high ka . In Fig. 8, a higher chemical wear ratio is observed particularly for Si3 N4 , indicating the differences in the wear behavior of nitrides: chemical wear on the part of hard Si3 N4 and mechanical wear on the part of soft hBN. Furthermore, in the sliding Si3 N4 -contacts, a formation of polished mirror-surface promoted the lubrication of hydrate on sliding surface, so that a mechanical to chemical wear transition was attained smoothly. 3.4. Analysis of chemical wear ratios of Si3 N4 –hBN composites with various hBN ratios The difference between the chemical influence on wear of Si3 N4 and hBN is very clear, hence, the chemical influence on sintered Si3 N4 –hBN composites is a serious concern. In this study, Si3 N4 –hBN composites were sintered to investigate their friction characteristics and chemical wear ratios. Fig. 9 shows the friction coefficient observed for a sintered Si3 N4 pin against sintered Si3 N4 –hBN composite plates in water. The Si3 N4 –hBN composite with a low hBN ratio shows a reduction in friction with sliding, indicating that it inherited the frictional characteristics of Si3 N4 . The mirror-like sliding surface of Si3 N4 was also attained on the Si3 N4 –hBN composite. At 30 wt.% hBN, the friction coefficient at the first stage and the frictional fluctuation were decreased. In contrast, hBN at 50 wt.% or more reduced and apparently stabilized the friction coefficient. These results indicate that Si3 N4 –hBN composites apparently retain the friction characteristics of Si3 N4 and hBN. Fig. 10 shows the results for composites of the chemical wear ratios and total wear losses at a load of 11.4 N and sliding speed of 0.13 m/s. The chemical wear ratios of Si3 N4 –hBN shown in Fig. 10 were calculated from the approximation in Eq. (9), which is generated by the linear combination of Eq. (7) for Si3 N4 and Eq. (8) for hBN:
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3.5. Chemical wear map for Si3 N4 –hBN composites in water
Fig. 10. The calculated chemical wear loss and total wear loss for composites as a function of hBN composition at a load of 11.4 N and a sliding speed of 0.13 m/s.
CR =
[17 (α/100Wtotal )2/3 +8.0 {(100−α)/100Wtotal }2/3 ] Wtotal (9)
where α (%) is the volume percent of hBN in the composite (0 ≤ α ≤ 100). The chemical wear ratio decreases as the hBN ratio increase as shown in Fig. 11. The agreement of the measurement of chemical wear ratio and the analysis based on the approximate Eq. (9) in terms of results obtained indicated that the chemical wear ratio of Si3 N4 –hBN reflected the chemical characteristics of Si3 N4 and hBN. Furthermore, a good correlation between the measurement of chemical wear ratio and Eq. (9) in terms of results obtained suggests that the chemical wear ratio of Si3 N4 –hBN composite has a close relationship with the total wear loss. From Eq. (9), the chemical influence on Si3 N4 –hBN composite sliding could be calculated from the total wear loss.
Fig. 11. Ammonia formed by Si3 N4 –hBN composite in water after sliding 700 m.
In this section, results of Si3 N4 –hBN composites slid under various loads from 3 to 35 N and at speeds from 0.06 to 0.19 m/s are presented and summarized on a chemical wear loss chart as a function of total wear loss, which is valid for the discussion of chemical influence on wear. For the purpose of making the chemical wear ratio–total wear loss plot, the amount of ammonia generated after sliding in water was measured using a spectrometric method and graphed as shown in Fig. 11, which shows a parabolic increase in ammonia as a function of total wear loss. The chemical wear loss of Si3 N4 –hBN composites was derived from Fig. 11 and is shown in Fig. 12. The specific wear rate lies in the range 10−6 –10−4 mm3 /N m, and the chemical wear ratio decreased with increasing total wear loss. The regression curve in Fig. 12 was calculated using Eq. (9) with a coefficient ka between 10 and 50. The coefficient ka indicates the chemical influence on wear loss and is distributed in the narrow range from 10 to 30, as shown in Fig. 12. In the case of the Si3 N4 –hBN composite, CR was maintained at an approximate ka of 17 for hBN to 8.0 for Si3 N4 . As mentioned above, this approximate equation could be used for investigating the chemical influence on wear and for classifying wear characteristics using the order of magnitude of ka . Fig. 13 shows the chemical wear ratio as a function of total wear loss for the 50 wt.% hBN composite for investigating wear behavior in detail. From the coefficient ka , the chemical wear ratio of the 50 wt.% hBN composite was classified into three types: Type 1: region of little chemical wear similar to the level of hBN. Type 2: region of large chemical wear, in the order of wear of Si3 N4 . Type 3: region of dominant chemical wear.
Fig. 12. Chart of chemical wear ratio–total wear loss for the Si3 N4 –hBN composites.
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Si3 N4 and hBN as a function of total wear loss and hBN constant. 5. The map of chemical wear ratio as a function of total wear loss, which includes regression curves, suggests the chemical influence on wear rate.
Acknowledgements
Fig. 13. Chart of chemical wear ratio–total wear loss for the Si3 N4 –hBN composite with 50 wt.% hBN.
Types 1 and 2 simply reflect the wear characteristics of hBN and Si3 N4 . As discussed above, sintered Si3 N4 –hBN composites of types 1 and 2 reflect the friction characteristics of monolithic Si3 N4 and hBN, and Eq. (9) enables us to estimate the order of its chemical influence on wear. On the other hand, type 3 is a complex formation which includes the effect of chemical activity. A higher chemical activity or a transition in wear should be considered in the region of type 3; however, further investigations of the wear process are required. As a result, the chemical wear ratios of composites are distributed in a narrow ka range of 10–30, which includes the values of single phase Si3 N4 and hBN, however, some data showed a higher chemical activity of wear as shown in the chart of chemical wear ratio–total wear loss. The chart of chemical wear ratio–total wear loss, which includes regression curves, is available for an analysis of the wear behavior of binary composite materials. In conclusion, a chart of chemical wear ratio–total wear loss can be considered to be a wear map [29] for investigating chemical influence on wear and wear patterns.
4. Conclusions 1. The low-wear/high wear transitions of Si3 N4 , hBN and Si3 N4 –hBN are governed by the change of mechanism from chemical to mechanical wear. 2. High chemical wear ratios are attained by Si3 N4 /Si3 N4 sliding couples and result in wear reduction in the speed range at 0.2 m/s. On the other hand, surface roughness has little influence on wear ratio after a sufficient sliding distance. 3. Chemical wear ratios are expressed by an exponential function of the total wear loss with ka coefficients of 17 for Si3 N4 and 8.0 for hBN in the valid PV between 0.18–4.4 N m/s. 4. Chemical wear ratios of Si3 N4 –hBN composites were calculated using the approximate equations for simple
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