Wear 257 (2004) 89–96
Effect of doping elements on the friction and wear properties of SiC in unlubricated sliding condition V.S.R. Murthy a,∗,1 , H. Kobayashi a , N. Tamari b , S. Tsurekawa a , T. Watanabe a , K. Kato a b
a Division of Mechanical Engineering, Tohoku University, Sendai 980-8579, Japan National Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda, Osaka 563-8577, Japan
Received 13 September 2002; received in revised form 3 September 2003; accepted 8 October 2003
Abstract The effect of doping elements on the friction and wear properties of silicon carbide is investigated in unlubricated sliding under varying humidity conditions. Doping elements (Al, Mg and P) appreciably affect the friction coefficient at lower humidity (30% RH) than at higher humidity (60% RH). The change in the kinetics of tribochemical reactions at lower humidity is attributed to chemisorption and microstructural changes. At higher humidity, coefficient of friction (COF) decreases but the effect of doping elements is rather insignificant. This is attributed to faster kinetics of tribochemical reactions. Continuing adhesion, twisting and rolling of small wear particles during the wear process results in the formation of long needle-like debris. © 2003 Published by Elsevier B.V. Keywords: SiC; Friction; Wear; Doping elements; Humidity; Needle-like wear particles
1. Introduction SiC is one of the promising materials for wear resistant applications. The friction and wear properties of SiC (both in dry and lubricating conditions) have been studied extensively because it is used in applications like bearings, cylinder liners and mechanical seals. Yamamoto and Ura  have studied the dry sliding wear of SiC in different environments (air, nitrogen and argon) and reported that the wear resistance is dependent on the surrounding atmosphere. In inert atmosphere, i.e., in argon or nitrogen, there was no change in the surface chemistry of SiC and the friction coefficient remained high (∼0.8). However, in air, SiC got oxidised to silica and the coefficient of friction (COF) decreased from 0.8 to ∼0.3. Andersson and Blomberg  also carried out wear tests in air and arrived at a similar conclusion. Takadoum et al.  pointed out that the fall of friction coefficient was due to formation of hydrated silica. After the running-in-period, the oxide debris generated was removed and the test was continued. Interestingly, friction coefficient increased again due to removal of silicon oxide debris. Based on this observation, they have concluded that silicon oxide ∗ Corresponding author. Fax: +91-512-597-505. E-mail address: [email protected]
(V.S.R. Murthy). 1 On leave from Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India.
0043-1648/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.wear.2003.10.016
is responsible for lowering the friction coefficient. Investigations by other researchers also confirm this point [4,5]. Additionally, the silica formation in SiC was also found to be sensitive to microstructural features (grain size, chemistry, phases, etc.), atmospheric conditions (humidity, temperature) and local contact stresses [6,7]. The tribochemical reactions (i.e. formation of silica) reported in SiC are similar to that of Si3 N4 . SiC is quite stable and does not undergo oxidation at lower temperatures. However, during wear, under combined mechanical and chemical processes, the degradation process becomes more intense [9,10]. The free surfaces of SiC react with surrounding humid air (SiC + 2H2 O → SiO2 + CH4 ) and get converted to silica [11–16]. The silica further reacts with moisture and hydrated silica (SiO2 +H2 O → Si(OH)4 ) is generated on the worn surface. The silica gel being soft in nature reduces the friction coefficient and progressively the tribo-surfaces become very smooth. All these reactions identified in humidified air are also seen in water. However, water assisted tribochemical reactions and lubrication effects have been widely studied in Si3 N4 than in SiC [12–16]. The trends though similar in these two non-oxides, however, also show certain differences . Although the general nature of these tribochemical reactions have been identified, it is not clear how effectively they can be controlled during sliding. One plausible method is doping of SiC. It is anticipated that these doping elements control the wear process
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Table 1 Mechanical properties of ␣-SiC ball material Density (% of theoretical) Hardness (GPa) √ Fracture toughness (MPa m) Elastic modulus (GPa) Flexural strength (MPa)
99.3 22.85 2.2 410 550
by changing the kinetics of tribochemical reactions. To explore such possibility, in the present study, small quantities of n- and p-type of elements (w.r.t. Si) were added to SiC and the effect has been studied under dry sliding condition by changing relative humidity in air. The other interesting aspect is the formation of long needle-like particles (rolls). Long needle-like debris is generated in self-mated SiC and Si3 N4 pairs when the humidity is reasonably high (RH > 30%) [8,10,12,15,16]. However, it is not clear how these long particles are generated. Based on the SEM observations made in this study, an explanation is offered how the aspect ratio increases during sliding.
2. Experimental A pin-on-disc tester was used to study the dry sliding wear behaviour of different SiC tribo-pairs. All tests were conducted in air (humidity levels varying from 30 to 60%) for a sliding distance of 130 m maintaining a sliding speed of 36 mm/s (100 rpm) and normal load of 1 N. The ball (8 mm diameter) material was a commercially available ␣-SiC (mainly 6H2 ) with small amounts of B (3700 ppm) and C (not known), which were added to densify the material to 99.3% of theoretical density. The average grain size of the ball material was ∼7 m. The surface of the ball was polished to mirror finish with Ra < 10 nm. The mechanical properties of the ball material (as provided by Kyocera Corporation, Japan) are given in Table 1. The discs were of ␤-SiC (mainly 3C (see Footnote 2) form with small traces of ␣) with different p-type (Al, Mg) and n-type (P) doping elements. Doping elements were added to modify tribochemical reactions as well as the grain boundary microstructure of SiC. To avoid the formation of any secondary phases at the grain boundary, only small quantities were added. The disc samples were hot-pressed in air using a graphite die assembly at 2100 ◦ C with a pressure of 40 MPa. High purity SiC powder (C: 32.2, O: 0.47, N 0.018 and balance: Si in wt.% with Ca, Na as major impurities at ≤0.1 ppm) was used for all the hot-pressed disc samples. Boron (0.5 wt.%) was added in all cases as a sin2 ␣ (hexagonal, rhombohedral) and ␤ (zinc blende structure) are two allotropic forms of SiC. 6H and 3C are polytypes of ␣- and ␤-SiC, respectively. SiC exhibits a large number of polytypes. They are designated by Ramsdell notation, where the first number represents the number of atomic layers that get repeated in a primitive cell and the second letter represents the lattice type.
tering aid. The residual doping elements, grain size and mechanical properties of different SiC disc materials are given in Table 2 . Prior to wear tests, all test surfaces were polished to 1 m finish (Ra ≈ 12 nm). After each experiment, wear loss was estimated both in the case of ball and the disc by measuring the diameter of the worn scar on the ball and by taking surface profile traces on the discs. All tribo-surfaces were examined using optical as well as scanning electron microscopy (FE-SEM HITACHI 4700T with EDX facility) techniques.
3. Results The representative microstructures of SiC materials that were used for ball as well as disc materials are shown in Fig. 1. Both microstructures consist of carbon (as graphite) in different forms. In a ball material, the graphite is present as small isolated particles either at the triple junctions or at the grain boundaries. In the SiC disc materials, graphite exists as long needles (average thickness <1.0 m and the length varies from 5 to 35 m) (Fig. 1b). The variation in COF as a function of relative humidity and doping elements is shown in Fig. 2. The COF decreases with the addition of different doping elements. Such an effect is more pronounced at lower humidity (30% RH). Particularly, P addition decreases the friction coefficient from 0.8 (undoped) to 0.4. As the humidity increases, COF decreases and the difference between different doped samples becomes narrow (compare Fig. 2a and c). At 30% RH, COF varies between 0.45 and 0.8, whereas at 45 and 60% RH, COF varies in the range of 0.3–0.5 and 0.2–0.3, respectively. From the surface profiles of wear tracks on different SiC discs as well as from the scar diameter on the ball, the specific wear rates (= wear volume/(sliding distance × load)) were calculated at 30% RH and compared in Fig. 3. As evident from the data, the wear loss of ball is quite high as compared to disc materials. This is expected, as the mechanical properties of SiC ball are inferior to SiC disc materials (see Tables 1 and 2). Some general features of a wear track are shown in Fig. 4a. For different doped SiC’s, variation in width and depth of wear track is observed with difference in doping elements and relative humidity. The depth of wear scar is higher at lower humidity and the scar became shallow with increasing relative humidity. The population of needle-like particles increased with decreasing humidity. On both sides of the track, there is an even distribution of wear debris and a variation in the debris size is also noticed (Fig. 4b). At lower humidity the debris is more isolated, whereas at higher humidity (60% RH) the debris gets agglomerated due to formation of more hydrated silica on the surface. The debris size varied from sub-micron to few tens of microns. While major part of this debris is finer and equiaxed, a small portion is of small rod-shaped particles. EDX analysis revealed that the debris is composed of C, partially oxidised SiC and SiO2 particles.
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Table 2 Mechanical properties of ␤-SiC disc materials with different dopants Dopant (ppm)
Density (% of theoretical)
Average grain size (m)
Average hardness (GPa)
Average fracture √ toughness (MPa m)
Average flexural strengtha (MPa)
– 2100 (Al) 3100 (Mg) 440 (P)
98.9 99.1 99.0 99.4
1.15 1.10 2.50 1.12
28 24 24 28
3.3 3.5 3.8 3.6
1050 760 810 820
Four-point bend test.
This can also be readily recognised from their colour when observed under the optical microscope. Most of the SiC particles generated during sliding underwent oxidation and a SiO2 layer covered these particles. Some of them appear as translucent particles. At the central portion of the track, the surface is very smooth and is covered with fine as well as long needle-like particles (Fig. 4c and d). Especially in Al doped samples, the number of these particles is quite high. Most of these long particles are oriented perpendicular to the sliding direction and at some locations agglomeration of these particles can be seen. Earlier investigators identified these particles as hydrated silica gel particles, which are formed due to tribochemical reactions [6–16]. A detailed examination carried out on these long needle-like particles revealed that the diameter (submicron to ∼6 m) and length (5 m to several tens of microns) varies from particle to particle. The worn surface of SiC gets oxidised and hydrated silica covers the surface. Hydrated silica being soft, it gets rolled into cylindrical particles during sliding and several such particles adhere together to form long needle-like particles (Fig. 4d). At some locations, a gradual decrease in the diameter is also noticed (for example, see Fig. 4d). Fig. 4d also demonstrates the formation of needle-like particle from net-like agglomerated gel particles (marked as A). The most interesting feature is the aspect ratio increment of these particles by adherence and
coalescence of smaller needle-like particles. It is probable that soon after smaller and thinner cylindrical particles are generated, they further join and form a larger particle by sticking to each other (Fig. 4c and d). The joining of two particles can be seen clearly in Fig. 5a. During rolling, these particles get twisted like cotton thread (Fig. 5b) and overall dimensions of the particle increase in length. The length of some of the particles is of the order of several tens of microns. The twining of translucent gel particles is shown in Fig. 5c and d. Another interesting feature of debris formation is the incorporation of carbon particles into twisted hydrated silica gel particles. In SiC, carbon is added as a sintering aid along with boron. The excess (free) carbon is always present in the form of graphite. The graphite particles, fractured (both from disc and ball) during sliding penetrate into these soft gel particles (Fig. 6). As the silica gel is sticky in nature, the adherence of carbon particles appears to be natural. The incorporation of these particles can be seen along the length of needle-like particles. However, it is important to note that the distribution of carbon particles along the length of needle-like gel particle is not uniform. The formation of such carbonaceous, fibrous hydrated silica needle-like particles can influence wear behaviour when sliding is continued for a longer time. Furthermore, the features that are shown in Fig. 6 also reinforce the point that was explained earlier, i.e.
Fig. 1. (a) Fracture surface of a ␣-SiC ball showing the grain size and distribution of graphite as isolated black particles which are smaller than about 1 m. (b) The etched microstructure of Mg doped ␤-SiC disc showing long needles of graphite thinner than about 1 m as indicated by arrows.
1.0 SiC ball / SiC disk Normal load: 1N Sliding speed: 36mm/s (100rpm) Sliding distance: 130m (6000cycles) Room temperature: 24˚C Relative humidity: 60%
P Al Mg No doping
40 60 80 100 Sliding distance L, m (a)
SiC ball / SiC disk Normal load: 1N Sliding speed: 36mm/s (100rpm) Sliding distance: 130m (6000cycles) Room temperature: 24˚C Relative humidity: 45%
P Al Mg No doping
0.2 0 0
40 60 80 100 Sliding distance L, m (b)
Specific wear rate of ball, x10-6mm3/Nm
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5 -SiC ball (3100ppm B) Al
2 1 P
0 Doping elements in disc (a)
Specific wear rate of disc, x10-7mm3/Nm
8 -SiC disc (0.5wt% B) Al
Doping elements in disc (b) Fig. 3. Comparison of specific wear rates of (a) ␣-SiC ball (against ␤-SiC discs having different doping elements) and (b) ␤-SiC discs of different dopants (against ␣-SiC ball) observed in air at 30% RH.
P SiC ball / SiC disk Mg Normal load: 1N Sliding speed: 36mm/s (100rpm) Sliding distance: 130m (6000cycles) Room temperature: 24˚C Relative humidity: 30%
0.2 0 0
P Al Mg No doping
40 60 80 100 Sliding distance L, m (c)
is due to continuous film formation of carbon mixed silica gel. This tribochemical film formation was observed, irrespective of doping element. However, it is more uniform in P doped samples as compared to other samples.
Fig. 2. The combined effect of relative humidity and doping elements on the COF at: (a) 60% RH, (b) 45% RH and (c) 30% RH.
joining and twisting of small needle-like particles. However, it should be pointed out that the concentration of cylindrical particles varied from sample to sample. The number of particles were relatively high at lower humidity and the distribution of these particles is not uniform along the track. Another interesting feature in these long needle-like particles (Fig. 6) is the formation of ‘brush-like’ ends in some needle-like particles (highlighted with arrows). This type of feature is generally seen in fracture of twisted polymer fibres. This indicates that a needle-like gel particle is not a single entity, but is formed by rolling of fine fibrous like gel fibres. At higher humidity, the number of needle-like particles is relatively less. The hydrated silica formed mixed well with solid carbon and this admixture spread uniformly on the surface (Fig. 7). The lower COF observed at higher humidity
4. Discussion The following points emerge from the experimental results: (a) COF of dissimilar SiC pairs decreases with increasing humidity. (b) At lower humidity (30% RH), the friction coefficient of SiC reduces with the addition of doping elements; in undoped condition friction coefficient is 0.8 and it reduces to a lower value with the addition of doping elements (Al, Mg, P). Of all the doping elements, P addition was more effective. (c) Further, the effect of doping elements is more distinctly noticed at lower humidity (30% RH) as compared to higher humidity (60% RH). During unlubricated sliding wear of SiC two important processes viz., mechanical damage and tribochemical reactions occur simultaneously and compete with each other. At lower humidity, fracture followed by attrition play a dominant role, while at higher humidity, tribochemical reactions (oxidation and hydrolysis) play a leading role. At higher humidity, oxidation (SiC + 2H2 O → SiO2 + CH4 ) and hydrolysis (SiO2 + H2 O → Si(OH)4 ) reactions accelerate as more water molecules are readily available for
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Fig. 4. (a) Wear track on ␤-SiC disc (no doping elements) after a sliding distance of 130 m. It shows the formation of fine debris on both sides of the track. At top extreme corner cylindrical particles can be seen; (b) a gradation in the debris; (c) formation of needle-like particle. At lower extreme corner (marked as B) gradual joining coalescence of particles can be observed and (d) spinning of a fibre for hydrated silica network particles.
these reactions. At higher humidity, the difference in COF narrows down because of faster kinetics of tribochemical reactions. The faster kinetics can be evidenced from the continuous spread of silica gel/graphite film. As the friction coefficient decreases, the wear rate decreases at higher humidity. Fischer and Tomizawa  noted a similar propensity in self-mated silicon nitride ceramics. The local contact stresses at different asperities decrease due to formation of soft hydrated silica/graphite film. Consequently, the friction coefficient as well as mechanical wear decrease and the sliding surfaces become smoother. At lower humidity, the moisture driven reactions took place slowly and hence the effect of doping elements is observed more distinctly than at higher humidity levels. The doping elements added to SiC can influence the wear properties mainly in two different ways: (i) they can modify the kinetics of tribochemical reactions through chemisorption and (ii) they can modify the microstructure of SiC. Analytical high-resolution electron microscopic studies carried out by the authors  revealed that the doping elements added to SiC get partitioned during high-temperature processing; partly they dissolve in SiC and form solid solution, and the remaining amount segregates at the grain boundaries. The segregation of these elements affects the solubility of silica in water . For example, the presence of a small amount of Al (even as oxide) reduces the solubility of silica drastically, whereas Mg and P affect the solubility to a lesser extent . The rate of solubility affects the kinetics of hydrated silica formation and hence friction coefficient at the sliding interfaces. It is also likely that the
doping elements affect the spreading of silica gel film on the worn surface by affecting the viscosity hydrated silica. Phosphorus, when present in the form of oxide (P2 O5 ) acts as a network forming oxide, while the other two oxides viz., MgO and Al2 O3 are network modifying and intermediate oxides, respectively. In glasses and gels, P2 O5 additions decrease the viscosity of glass. Further, P2 O5 reacts easily with water . As a result, the gel viscosity decreases rapidly and smearing of gel becomes more uniform and easy. The propensity of these reactions is more distinctly seen at lower humidity (at 30% RH) because of slower kinetics. In any material, degradation reactions such as corrosion and oxidation are microstructure-dependent. In particular, the grain boundary microstructure plays an important role as the degradation reactions get initiated at these locations. The doping elements added to SiC affect not only the grain size but also grain boundary microstructure. For example, Mg promotes the grain growth, whereas other elements like Al or P do not modify the grain size significantly . In the hot-pressed microstructures, the average grain size in all the SiC remained at 1 m, whereas the grain size increased to 2.5 m in Mg doped samples (Table 2). However, grain size alone is not the deciding factor; grain boundary microstructure and its connectivity also play a vital role. Here the doping elements added to SiC modify the grain boundary microstructure. The grain boundaries in materials are broadly classified as low-angle, coincidence and random boundaries. In SiC, Al promotes the concentration of coincidence site lattice (CSL) boundaries and the frequency of coincidence boundaries in Al doped sample is at 41% as
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Fig. 6. The incorporation of carbon particles can be seen in the long needle-like wear particles. Note (a) twisting of particles and (b) fibrous ends (shown with arrow).
Fig. 5. Various needle-like particles observed on wear track of ␤-SiC disc in air at 30% RH. (a) Joining of two needle-like particles; (b) twisting of several fine particles; (c) a part of twisted needle-like particle and (d) a sequence of dynamic twisting process.
compared to undoped sample, where only 35% grain boundaries are of coincidence type. There is a noticeable change in the frequency of coincidence boundaries . It is well known that low energy grain boundaries affect the fracture as well as grain boundary related degradation processes like oxidation  or corrosion . As Al addition promotes the frequency of coincidence boundaries, the degradation reactions become slower. Therefore, the COF drop is lower in Al doped samples (as compared to other dopants) at lower humidity (Fig. 2a). Another reason for variation of wear resistance can be the accumulation of dopant oxide particles on worn surfaces. A detailed examination of worn surfaces using energy depressive X-ray analysis (EDX) did not reveal any build up of these doping elements on the tribo-surfaces.
Further, the proportions of ␣ and ␤ in SiC and free graphite present in the samples also play a major role in tribological behaviour. These two parameters affect mechanical properties, especially toughness and hardness. In the present study, all the experiments were carried out with ␣- (ball) against ␤-phase (disc). The mechanical properties of ball are inferior to disc material, hence the wear resistance of SiC ball is always found to be poor compared to SiC disc materials. In addition to the effect of doping elements, some interesting observations are also made on the formation of needle-like particles. However, no significant difference in the formation mechanism is evident with variation of doping element. The SiC debris generated due to fracture and attrition reacts with moisture and gets converted to hydrated gel (Fig. 4). Many such hydrated silica particles stick together and form net-like clusters (Fig. 4d marked as A). Similar agglomerated network like particles were earlier reported by Xu and Kato  in silicon nitride ceramics. Transmission electron microscopy work on these net-like agglomerates clearly indicated that these are amorphous or microcrystalline in nature [10,12]. In a dynamic sliding process, these networks roll and form small needle-like particles. The formation of needle shape particles is not unique to non-oxides but also reported in oxides (especially under water lubrication) [23,24]. There are two different mechanisms for the formation of long needle shape particles: (a) rolling of thin hydrated silica sheet that is formed on the worn surfaces as shown earlier by Fischer and Tomizawa . A thin layer of hydrated
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eventuality can bring small variations in COF, especially in dry sliding conditions. On the contrary, when humidity is high, the silica gel viscosity decreases and the carbon mixing gives rise to a thin lubricated coating (see Fig. 7). Another important feature is the ends of gel particles. The ends of needle-like particles exhibit ‘brush-like’ features with fine fibrous structure (Fig. 6). This indicates that silica gel polymerises into long fibrous chain-like structure and further rolls into cylinders during sliding. 5. Conclusions
Fig. 7. Formation of a continuous tribochemical layer of hydrated silica and graphite at 60% RH on (a) Al doped surface and (b) P doped worn surface.
silica gel gets delaminated from the surface and it rolls up due to the residual tensile stresses within the sheet. The sliding process further assists in tight rolling of sheet. (b) Joining and twisting of small gel particles to give rise to large aspect ratio needle-like particles as shown in this study. Smaller needle-shaped particles join continuously and increase the aspect ratio by a dynamic joining process. An accumulation of such particles creates a condition similar to that of a third body contact. Gel particles being very soft, the COF does not vary significantly. However, a variation in number of these particles along the wear track can give fluctuations. Here, the incorporation of solid carbon particles has important but possibly contradicting effects. On the one hand, they provide additional lubrication along with soft silica particles. Thus double lubrication may provide synergetic effect. On the other hand, the addition of solid carbon particles makes the soft silica gel particles very rigid and hard, thereby reducing their lubrication effect. Further, the distribution of carbon particles along the length of long silica particles is of considerable importance. A non-uniform distribution of solid carbon gives (as seen in Fig. 6) rise to uneven stress distribution during rolling and the long silica gel particles may again break as rolling continues. Such an
1. At 30% RH in air, a dissimilar SiC pair shows friction coefficient of 0.80. Doping of Al (2100 ppm) reduces the friction coefficient to about 0.60. The other dopants like Mg (3100 ppm) and P (440 ppm) further reduce the friction coefficient to 0.45. At 60% RH in air, the friction coefficient does not change significantly with dopants and the friction coefficient is around 0.25. 2. The doping elements were observed to influence the wear process in two ways: (a) they can directly affect the tribochemical reactions and (b) they can also modify the frequency of coincidence boundaries and can change the kinetics of tribochemical reaction. 3. The reduction in friction coefficient at 60% RH is attributed to faster kinetics of tribochemical reactions. At higher humidity (60% RH), the viscosity of silica gel decreases and mixes well with carbon and spreads uniformly on the surface. As a result, the friction coefficient decreases. As the kinetics is faster, the difference in the friction coefficient is not as high as it is observed at lower humidity (30% RH). 4. The mechanism of long needle-like wear particles is examined. Adhesion, twisting and rolling of small cylindrical particles resulted in formation of long needle-like particles (of the order of several tens of microns). The incorporation of solid carbon particles increases lubrication and rigidity of gel particles. The brush-like ends of these particles suggest that the needles are composed of fine fibrous silica structure.
Acknowledgements This work was supported by a Grant-in-aid for Scientific Research (B) (12450277) and a grant in aid for exploratory research (14655061) from Japan Society for the Promotion of Science (JSPS). The financial support is gratefully acknowledged. References  Y. Yamamoto, A. Ura, Influence of interposed wear particles on the wear and friction of silicon carbide in different dry atmospheres, Wear 154 (1992) 141–150.
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