Tribological characteristics of Si3N4-based composites in unlubricated sliding against steel ball

Tribological characteristics of Si3N4-based composites in unlubricated sliding against steel ball

Materials Science and Engineering A 384 (2004) 299–307 Tribological characteristics of Si3N4 -based composites in unlubricated sliding against steel ...

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Materials Science and Engineering A 384 (2004) 299–307

Tribological characteristics of Si3N4 -based composites in unlubricated sliding against steel ball Chien-Cheng Liu∗ , Jow-Lay Huang Department of Material Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan ROC Received 24 April 2004

Abstract The dry-sliding wear mechanism of Si3 N4 -based composites against AISI-52100 steel ball was studied using a ball-on-disc mode in a reciprocation motion. The addition of TiN particles can increase the fracture toughness of Si3 N4 -based composites. The fracture toughness of Si3 N4 -based composites played an important role for wear behavior. The Si3 N4 -based composites exhibits a small friction and wear coefficient compared to monolithic Si3 N4 . Atomic force microscopy (AFM) studies displayed fine wear grooves along the sliding traces. The subsurface deformation shows that the microcrack propagation extends along the TiN/Si3 N4 grain interface. The wear mechanisms were determined with scanning electron microscopy, transmission electron microscopy, X-ray diffraction and atomic force microscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Silicon nitride; Si3 N4 -based composites; Wear; Friction

1. Introduction Silicon based nonoxide ceramics have received extensive attention owing to their two potential applications: engine components and cutting tools. Both of these applications required good mechanical properties, chemical stability and reliability at elevated temperature. Ceramic reinforced silicon nitride is one of the most promising materials for high temperature structural applications. TiN has been used for cutting tool applications on account of its good thermal conductivity, chemical inertness and wear resistance. The toughness of silicon nitride has been reported to be enhanced with the incorporation of TiN particles [1–5]. Moreover, TiN itself is appreciated as the second phase material having a low friction coefficient together with high hardness. It is often chosen as a surface coating to improve the tribological characteristics of materials [6–10]. Much study ∗ Corresponding author. Present address: Kun Shan University of Technology, No. 949, Da Wan Rd., Yung-Kang City, Tainan Hsien 710, Taiwan ROC. Tel.: +886 6 2050783; fax: +886 6 2726510. E-mail address: [email protected] (C.-C. Liu).

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.06.027

has been carried out on the friction and wear behavior of ceramic against metals and ceramics against ceramics couples in sliding contact. Most of them focused on the effect of environment including temperature, humidity and lubricant on the sliding wear behavior of Si3 N4 against a variety of counterface materials [11–13]. The wear coefficient of Si3 N4 was distinctly reduced by the addition of TiN under dry oscillating sliding conditions [14]. Gomes and coworkers [15,16] have shown that the tribological behavior of Si3 N4 against iron alloys is greatly influenced by the temperature and environment. They concluded that in the range 22–600 ◦ C, ceramic surfaces are progressively protected by a tribolayer of oxidized wear debris. The humidity has opposite trends on the wear resistance of the nitride. The wear coefficient increases with relative humidity due to weak adhesion of debris to the ceramic surfaces. However, tribological behavior of ceramic materials is strongly dependent upon contact load, sliding speed, temperature, humidity, contact geometric configuration, lubricant, as well as microstructure of materials [17–20]. This work investigated the tribological characterization of silicon nitride based composites with secondary phase TiN

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addition, and examines the influence of wear depth for different load under reciprocated sliding wear test. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to provide information of the wear processes on worn surface. The micro worn trace and groove were also examined by atomic force microscopy (AFM).

2. Experimental procedure The Si3 N4 (UBE SN-E10) powder was mixed with 2 wt.% Al2 O3 (16SG, Alcoa, USA, 0.5 ␮m) and 6 wt.% Y2 O3 (5603, Molycorp, USA, 1.8 ␮m) in a polyurethane bottle with highpurity silicon nitride balls and ethanol for 24 h. The ratio of ball, charge and vehicle was 6:1:5 in weight. TiN particles of 3.5 ␮m size (H.C. Stark, 20% and 40% of total volume) were dispersed and added as 20 vol.% and 40 vol.% to the Si3 N4 slurry, milled for a further 2 h and then dried in a rotary evaporator. Dried agglomerates were ground with a mortar and pestle to pulverize aggregates and passed through 100 mesh sieve. Samples were hot-pressed at 1850 ◦ C for 1 h under a uniaxial pressure of 24.5 MPa and nitrogen flow in a graphite furnace (Fuji Dempa High Multi 5000). Some samples were plasma etched (Plasma-Them Inc., Series 70) in mixtures of CF4 and O2 with a gas flow ratio of 93:7 in RF sputtering system for 2 min and then ultrasonically cleaned prior to the examination by SEM. After each test, microstructural modifications and wear volume in the track were observed by SEM coupled with energy dispersive analysis of X-rays. TEM foil preparation was performed by standard techniques, which include diamond cutting, ultra-sound drilling, mechanical grinding, dimpling, argon ion thinning to perforation from the side opposite to the wear scar and a light carbon coating. Friction and wear tests were carried out on a SRV reciprocated sliding wear test machine (Optimal Schwingung Reibungund Verschleiss, Germany), as shown in Fig. 1. The main configuration of the SRV test machine consists of a fixed lower specimen supporter and a mobile replaceable upper specimen holder. The upper specimen can be a ball or a cylinder, depending on the type of holder, while the lower

Fig. 1. Schematic representation of the SRV reciprocating sliding wear testing apparatus.

specimen is a disc. The diameter of the AISI-52100 steel ball is 10 mm, tolerance ±0.1 mm, and the disc of Si3 N4 -based composites is 24 mm in diameter, and 8 mm in thickness. Arrangement of a ball and a disc form a ball-on-disc configuration. Surface roughness and wear depth values after worn tests on specimens were taken using a surface profilometer (Kosaka SE30H, Japan) with a precision of ±0.005 ␮m at a magnification of 106 ×. The mechanical properties of monolithic Si3 N4 and Si3 N4 -based composites are given in Table 1. The steel balls of AISI-52100 were commercial products, usually used in ball bearings. The chemical compositions of steel balls were listed in Table 2. Tribological testing was performed under a normal contact load of 100, 200 and 300 N, and a sliding distance of 120 m at 50 Hz (sliding velocity of 0.1 m s−1 ) for 20 min. All the tests were conducted at room temperature with a relative humidity of 50–55%.

3. Results and discussion 3.1. Friction coefficient Fig. 2 shows the friction coefficient of monolithic Si3 N4 against AISI-52100 steel ball as a function of sliding time

Table 1 Mechanical and physical properties of the monolithic Si3 N4 and TiN/Si3 N4 composites Sample Si3 N4 20 vol.% TiN/Si3 N4 40 vol.% TiN/Si3 N4

Relative density (%)

Resistivity ( cm)

Hv (GPa)

Fracture toughness (MPa m1/2 )

Flexural strength (MPa)

98.7 97.45 97.35

4.2 × 7 × 109 1.25 × 10−3

13.9 ± 0.6 13.2 ± 0.4 12.4 ± 0.4

5.4 ± 0.2 6.5 ± 0.4 6.2 ± 0.35

822 ± 55 663 ± 51 521 ± 30

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Table 2 Chemical compositions of AISI-52100 steel ball Composition (wt.%)

AISI-52100

Fe

C

Si

Mn

P

S

Ni

Cu

Cr

Balance

1.03

0.22

0.31

0.01

0.01

0.07

0.06

1.39

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Fig. 2. Friction coefficient vs. sliding time of silicon nitride against AISI-52100 steel ball in unlubricated sliding tests (300 N load).

Fig. 3. Friction coefficient vs. sliding time of 40 vol.% TiN/Si3 N4 composite against AISI-52100 steel ball in unlubricated sliding tests (300 N load).

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under unlubricated sliding conditions. The monolithic ceramic showed a lower friction coefficient at the beginning of sliding. After a certain distance, the friction coefficient gradually rose from 0.6 to 0.75. The frictional behavior of this ceramic shows that the friction coefficient values are relatively higher (about 0.7–0.75) and fluctuate throughout the tribological interaction. In contrast, the addition of TiN particles to Si3 N4 -based composites shows a lower friction coefficient during sliding. The friction coefficient begins to suddenly rise up to a relatively high level (∼0.62), and then quickly descending to reach a stable level throughout the run, as shown in Fig. 3. The Si3 N4 -based composites were characterized by a friction coefficient in the order of 0.45–0.5 in unlubricated sliding against AISI-52100 steel ball. This result showed that the friction coefficient of Si3 N4 -based composites was relatively low against AISI-52100 steel ball pairs under the high contact pressure (L = 300 N) and reciprocating motion. Skopp et al. [21] have reported that the friction and wear decrease due to the addition of titanium nitride, as a consequence of the formation of relatively soft lubricious oxides like TiO2−x on a hard Si3 N4 matrix. The friction coefficient of silicon nitride is distinctly reduced when secondary phase (TiN) is added to the silicon nitride matrix. The result shows that the addition TiN particles to Si3 N4 -based composites have a stable friction behavior and low friction coefficient values.

Fig. 4. Wear track cross-section of (a) before wear (surface roughness, Ra = 0.084 ␮m) and (b) typical profile crossing wear scar.

3.2. Wear coefficient measurement In order to measure the volume of material removed, the wear surface was recorded by a stylus profilometer. Typical traces of a sliding surface before and after test are illustrated in Fig. 4. The cross-section of the groove was used to measure groove depth (scale 2000×), and the width and length were measured by using optical microscopy. This position

Fig. 5. Wear depth as a function of TiN content in different normal loads for silicon nitride based composites against AISI-52100 steel ball.

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Fig. 6. Wear coefficient of Si3 N4 -based composites as function of fracture toughness.

of each wear surface was measured in at least three different locations and the wear coefficient can be calculated from the mean value of each specimen. Fig. 5 shows the relationships between wear depth and normal load for Si3 N4 -based composites after wear test. The wear depth of TiN/Si3 N4 composites is lower than that of monolithic silicon nitride for all test conditions. At higher contact loading (300 N), the wear depth of Si3 N4 was particularly higher than that of TiN/Si3 N4 composites. The wear coefficient (W) was calculated using the equation: W = V/LD (mm3 (N m)−1 ), where V (mm3 ) is the wear volume, L (N) the normal force and D (mm) is the sliding distance [22]. The obtained results shows that the wear coefficient values of TiN/Si3 N4 composite containing 20 vol.% and 40 vol.% TiN are 0.37 × 10−6 mm3 (N m)−1 and 0.32 × 10−6 mm3 (N m)−1 with 300 N applied load. For monolithic silicon nitride materials, the wear coefficient was 1.39 × 106 mm3 (N m)−1 . The low wear coefficients of Si3 N4 based composites may be caused by the formation of titanium oxide layer [21]. Fig. 6 shows the wear coefficient as a function of fracture toughness for Si3 N4 -based composites in unlubricated sliding wear conditions. The results indicated that the wear coefficient decreased as the fracture toughness increased. The average wear coefficient of monolithic Si3 N4 was about four times higher than that of TiN/Si3 N4 composites. In this case, the fracture toughness of Si3 N4 -based

ceramic composites plays an important role for reduction of wear. Consequently, the addition TiN particles to Si3 N4 based composites improved their wear resistance. Kato [23] has reported that the wear of ceramics was caused by crack propagation due to the hardness and brittleness of the material. The severe wear takes place by tensile stress at crack tips in the sliding contact region, the critical condition for the generation of severe wear being given by the following equation [24]: Sc,m =

(1 + 10µ)Pmax (d)1/2 KIC

(1)

where Sc,m is the mechanical severity of contact, µ the friction coefficient, Pmax the maximum Hertzian contact pressure, d the crack length and KIC is the fracture toughness. The addition of TiN particles to Si3 N4 -based composites has been demonstrated to give higher KIC values than monolithic Si3 N4 [25]. This result corroborates the present study, as the Si3 N4 -based composites have higher wear resistance than monolithic Si3 N4 during sliding wear test. 3.3. Debris analysis After test, debris were collected and stored in a vacuum desiccator for further analysis. These particles were placed

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Fig. 9. SEM micrographs showing the edge wear track of (a) Si3 N4 and (b) 40 vol.% TiN/Si3 N4 after unlubricated sliding tests. Fig. 7. TEM micrograph of debris, showing (a) fine particles and (b) diffraction pattern.

Fig. 8. EDS X-ray spectra from the debris of Si3 N4 -based composites.

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Fig. 12. Representative (a) Si3 N4 and (b) 40 vol.% TiN/Si3 N4 composites AFM image after unlubricated sliding test.

Fig. 10. SEM micrographs showing the center wear track of (a) Si3 N4 and (b) 40 vol.% TiN/Si3 N4 after unlubricated sliding tests.

Fig. 11. EDS X-ray spectra from the debris of Si3 N4 -based composites.

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onto holey copper films for examination in a transmission electron microscope. Fig. 7 shows that the wear debris looks like flakes and small nano-particles. The large flakes always exhibited different amount of iron, oxygen and a small amount of chromium, which was originated from the steel ball. These features have obvious diffraction spots in the diffraction pattern, as shown in Fig. 7b. The diffraction pattern shows that these wear particles are rich in oxygen. This result clearly indicates that the flakes have poly-crystalline diffraction patterns. On the other hand, some fine particles were detected with different amount of titanium and silicon in EDS spectra (Fig. 8). These wear particles may be adhering to the wear scar in the sliding tests. Kalin et al. [26] have reported that the dry fretting test of silicon nitride against bearing steel was characterized by a tribochemical reaction, where the tribo-oxidation controlled the wear behavior of ceramic–steel couples. Gauitier and Kato [27] pointed out the formation of a debris interface layer, which would prevent direct contact between the two bodies. The ceramic–steel pair can provide a lower wear rate than like-on-like Si3 N4 pair.

3.4. Wear surface analysis Under dry sliding conditions, the specimen surfaces presented iron-rich adherent layers on the ceramic surfaces, as evidenced by the scanning electron micrograph of the worn surfaces shown in Fig. 9. The edge surface of Si3 N4 -based composite is fully covered with a layer of transferred metal, which is iron oxide in Fig. 9b. In contrast, the center worn surface was smooth after sliding tests (Fig. 10). The tribological behavior of Si3 N4 and TiN/Si3 N4 composite was compared from the surface morphology. As the action of reciprocated sliding continued, the sliding area was gradually smoothed on the sliding surface. The EDS analysis reveals iron oxide adhered to the ceramic worn surface in unlubricated sliding wear conditions, as shown in Fig. 11. The formation of surface layers certainly changes the tribological behavior in the sliding test conditions. Kalin et al. [28] have reported that the model of wear behavior should contain chemical reaction, spot temperatures, thermal conductivity, and phase transformations for silicon nitride against bearing steel contacts. The friction coefficient of the self-mated couple of TiN/Si3 N4 is

Fig. 13. TEM revealing a crack path at the subsurface in the TiN/Si3 N4 interface after unlubricated sliding test.

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controlled by the nature of the outer reaction layers, which are probably composed of SiNx Oy and TiO2 ·H2 O molecules that can be adsorbed on these layers. The effect of adherent layers of oxidized material on the ceramic surface is the decrease of the ceramic wear [29]. Therefore, this result reveals that the wear resistance of TiN/Si3 N4 composite is better than that of monolithic silicon nitride in sliding wear test. Fig. 12a shows AFM micrographs of the monolithic Si3 N4 worn surface after test. AFM studies displayed fine wear grooves along the sliding traces. The measurement image indicates that the length and the width are 50 ␮m, respectively. On the wear track of monolithic Si3 N4 , grooves are clearly visible in the sliding direction. The average height was about 968 nm for the monolithic Si3 N4 worn surface. Fig. 12b shows the relationship height of AFM micrographs for Si3 N4 -based composites after sliding wear test. For the vertical axis, the average height indicates a worn surface roughness of about 280 nm. This groove wear is probably caused by harder wear particles, which may be ceramic particles or iron oxide product. Some ceramic or iron oxide particles adhered or embedded on the steel ball surface, promoting groove wear. These results show that the wear mechanism is a combination of adhesive wear, plastic deformation and groove wear on this microscale. The average surface roughness of AFM micrographs shows that the monolithic Si3 N4 worn surface is also rougher than that of TiN/Si3 N4 composite worn surfaces. Furthermore, transmission electron microscopy investigated the existence of twins and dislocation in subsurface of TiN/Si3 N4 grains after sliding. Fig. 13 shows the crack propagation around the TiN grains and large elongated ␤Si3 N4 grains boundary at subsurface deformation. Arrows at the micrograph indicates that the crack propagation occurred along grain boundaries in the Si3 N4 matrix after the sliding tests. The crack interaction, such as crack deflection and crack bridging, played an extremely essential role in determining the fracture mechanisms of the particulate reinforced ceramic matrix composite [3]. The crack deflection in Si3 N4 -based composites indicates to have large amplitude. Therefore, the addition of TiN to Si3 N4 -based composites can contribute to improve their fracture toughness. These results show that the high fracture toughness of Si3 N4 -based composites promote good wear resistance compared to monolithic Si3 N4 .

4. Conclusions The following conclusions can be drawn from the results obtained in this investigation: 1. The adding of TiN particles can increase the fracture toughness of Si3 N4 -based composites. The Si3 N4 -based composites were characterized by a relatively low friction coefficient (0.4–0.5) in unlubricated sliding against AISI-52100 steel ball. 2. The fracture toughness of Si3 N4 -based ceramic composites plays an important role in their wear behavior. The

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addition of TiN particles to Si3 N4 -based composites improved their wear resistance. 3. AFM analysis shows that the average surface roughness of monolithic Si3 N4 specimen is greater than that of Si3 N4 based composites. The wear mechanism is a combination of adhesive wear, plastic deformation and groove wear on this micro-scale. 4. The TEM investigation shows the crack deflection at particle/matrix interface after sliding wear tests. The crack deflection increases the fracture toughness and enhances the wear resistance of Si3 N4 -based composites. Acknowledgment The authors would like to thank the National Science Council of the ROC for its financial support under the contract No. NSC89-2216-E-006-034.

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