Friction and wear behaviour of SiAlON ceramics under fretting contacts

Friction and wear behaviour of SiAlON ceramics under fretting contacts

Materials and Engineering A359 (2003) 228 /236 www.elsevier.com/locate/msea Friction and wear behaviour of SiAlON ceramics under fretting contacts B...

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Materials and Engineering A359 (2003) 228 /236 www.elsevier.com/locate/msea

Friction and wear behaviour of SiAlON ceramics under fretting contacts Bikramjit Basu a,*, Jozef Vleugels b, Mitjan Kalin c, Omer Van Der Biest b a

b

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, IIT, Kanpur 208016, India Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, Leuven B-3001, Belgium c University of Ljubljana, Centre for Tribology and Technical Diagnostics, Bogisiceva 8, SI-1000 Ljubljana, Slovenia Received 1 October 2002; received in revised form 16 April 2003

Abstract The goal of this paper is to understand the friction and wear mechanism of monolithic SiAlON ceramics against important engineering materials. Unlubricated fretting tests were performed on monolithic sialon ceramics against sialon, alumina, steel and hardmetal. The wear experiments were carried out in a gross slip fretting regime with identical testing parameters (8 N, 200 mm, 10 Hz and 100,000 cycles) under ambient conditions of temperature (23 /25 8C) and humidity (50 /52% RH). Microstructural characterization of the worn surfaces was performed, and the underlying wear mechanisms in the investigated fretting couples are elucidated. Tribochemical interaction, abrasion and spalling were observed to be the predominant wear mechanisms in most of the studied material combinations. Based on the friction and wear data, a relative ranking of the different wear couples was established. The lowest wear volume was measured for the sialon /hardmetal (WC /Co) combination, whereas the sialon /steel combination showed the highest wear volume. # 2003 Elsevier B.V. All rights reserved. Keywords: Sialon; Alumina; Steel; Hardmetal; Fretting wear; Tribochemical wear; Abrasion

1. Introduction Several structural ceramics, owing to their high hardness and elastic modulus are widely used in tribological applications. Among the non-oxide ceramics, Silicon nitride based ceramics have been intensively investigated over last decades for several potential engineering applications because of their superior combination of properties [1], such as retention of high strength over a wide temperature range, high toughness and hardness. Common engineering applications of sialon ceramics are cutting tools, bearing balls and rollers, refractory parts, engine valves, turbine vanes and blades, turbocharger rotors, ceramic knives, etc. Due to its importance in tribological applications, several studies were carried out to assess the potential of silicon nitride based ceramics as tribomaterial and wear

* Corresponding author. Tel.: /91-512-2597771; fax: /91-512597505. E-mail address: [email protected] (B. Basu). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00349-6

maps of several engineering ceramics, including silicon nitride are now available in the literature [2]. Prakash et al. reported the friction characteristics of hot pressed sialon (HPSN) ceramics against bearing steel and selfmated silicon nitrides under dry sliding conditions [3]. The authors observed that the friction coefficient (COF) is strongly influenced by the hardness and decreases as the a-sialon content decreases. The same group of researchers studied the influence of sintering aids and secondary phase additions on the wear behavior of HPSN ceramics against bearing steel [4]. Liquid phase sintered HPSN with yttria as sintering additive was found to have a superior wear resistance than HPSN sintered with MgO and ceria as sinter additives. Tribochemical reactions, along with abrasive wear, were identified as the major wear mechanisms. Xinghong et al. noted that the wear rate of silicon nitride ceramics against stainless steel increases with increasing load and sliding speed and the ceramic wear is mainly caused by adhesion at the tribocontact [5]. Fischer et al. investigated the tribochemical wear mechanisms of

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silicon nitride materials, which were found to be strongly related to the presence of water [6]. It was also reported that the presence of secondary phases like TiB2 can lead to a substantial improvement in the sliding wear resistance of silicon nitride based ceramics [7]. Kalin et al. investigated the fretting wear of monolithic silicon nitride against steel under oil lubricated conditions and the obtained results were compared with that in unlubricated conditions [8]. The influence of different fretting parameters on the wear behavior of self-mated silicon nitrides was investigated [9,10] and tribochemical reactions are reported to play a major role in influencing the wear behavior. Despite the tribological research carried out on silicon nitride based ceramics, as mentioned above, comparative data on the friction and wear behavior in fretting contact with different engineering-relevant materials under identical contact conditions are lacking. Therefore, due to the growing interest and new potential applications, this paper continues with the tribological study, in particular the fretting wear of commercial monolithic sialon ceramics against important engineering materials, such as sialon, alumina, steel and hardmetal. The fretting experiments were performed under identical testing conditions to assess the relative wear ranking of the different material combinations. The wear mechanisms are investigated and qualitative information on the wear and friction behavior is provided.

2. Materials and experimental procedure 2.1. Materials The mechanical properties of the investigated materials, as obtained from the commercial suppliers are listed in Table 1. Sialon samples were used as stationary flat samples in the fretting test, whereas the oscillating balls were made of sialon, alumina, steel, and hardmetal. The flat samples (as-received sialon discs) were ground and polished to a surface roughness of 0.05 mm. The sialon flats and balls were both of the commercial bearing

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grade (grade TCQ, Toshiba, Japan). The substitution level (z) in the Si6z Alz Oz N8z was 0.3. The other counterbody materials are commercially available bearing grade alumina balls (99.7% pure, grade 10, Ceratec, Germany), steel balls (Bearing grade DIN 100Cr6, Fritsch, Germany) and hardmetal balls (WC-6 wt.% Co, grade H200C, Spheric Engineering, UK), all 10 mm in diameter, with mirror-finished surfaces, i.e. a surface roughness (Ra ) of 0.02 /0.04 mm (data from the suppliers). Representative thermal conductivity literature [11] data for the different material classes involved in the present investigation are provided in Table 2. 2.2. Fretting tests The fretting experiments have been conducted on a computer controlled tribometer under ambient conditions of temperature (23 /25 8C) and humidity (50 /52% RH). A ball-on-plate configuration is used and fretting vibration at the contact is actuated by a linear relative displacement of constant stroke (mode I, linear relative reciprocatory displacement sliding). Prior to the fretting experiment, the materials were ultrasonically cleaned in acetone. The flat sample is placed on a translation table, which oscillates at the required displacement with desired frequency by means of a stepping motor. An inductive displacement transducer monitors the displacement of the sample, and the friction force is recorded with a piezoelectric transducer attached to the holder that supports the counterbody. More details on the experimental set up can be found in Ref. [12]. The COF was obtained from the on-line measurements of tangenTable 2 Literature [11] thermal conductivity data of related material classes as involved in this work Materials

Thermal conductivity (W m 1 K 1)

Sialon 22 Alumina 39 Steel (DIN 100Cr6) 25 Hardmetal WC /Co (6 wt.% Co) 100

Table 1 Mechanical properties of the materials, as obtained from the suppliers, used in the present investigation HV5 (GPa)

KIc (MPa m1/2)

E (GPa)

Flat materials Sialon (Si6z Alz Oz N8z ; z/0.3, grade TCQ)

15.5

4.7

365

Counterbody ball materials Sialon (Si6z Alz Oz N8z ; z/0.3, grade TCQ) Alumina (99.7% pure, grade 10) Steel (DIN 100Cr6, construction grade) Hardmetal WC /Co (6 wt.% Co, grade H200C)

15.5 19.0 7.8 16.4

4.7 4.0 20 10.0

310 300 210 630

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tial force. During the test, fretting loops are recorded at a certain interval of time. A fretting loop represents the evolution of the tangential force as a function of the displacement amplitude during each cycle. The COF is further evaluated from the average of the two plateau values of the tangential force in the fretting loop, as described elsewhere [13]. For comparative reasons, all tests are performed under identical fretting parameters, i.e. a normal load of 8 N, a linear displacement of 200 mm, a frequency of 10 Hz and a duration of 100,000 cycles (see Fig. 1). Preliminary experiments revealed that the selected combination of load and displacement ensure the gross slip conditions at the tribocontact. It is also well recognized that the fretting wear rate steadily increases with load and displacement in the gross slip regime before the transition to reciprocatory sliding occurs [14]. To investigate and compare the fretting wear behaviour of different material combinations, it is necessary to conduct wear tests in gross slip regime and hence all the wear tests in the present case are carried out with the selected fretting parameters (Fig. 1). 2.3. Wear measurement and characterization of the worn surfaces After the fretting tests, the worn surfaces are ultrasonically cleaned prior to the profilometry measurements. Detailed microstructural characterization of the as-worn and cleaned surfaces both on flat and ball were performed with a Reichert-Jung POLYVAR optical microscope (Nomarski contrast), and a scanning electron microscope (FEI, XL-30 FEG) equipped with an energy dispersive spectrometer (EDS) allowing compositional analysis. A Rodenstock laser profilometer (RM600X/Y-100) was used to evaluate the geometry and the wear volumes of the fretting wear tracks on the flat samples. The depth profiles were acquired along the fretting wear scar in the direction perpendicular to the sliding direction. A number of equally spaced depth profiles covering the whole wear pit were used to acquire the volumetric wear. The ball wear was calculated from the measured wear scar diameters (both in the sliding and in the transverse directions) according to the equation proposed by Klaffke [15]. The use of this

Fig. 1. Schematic of the fretting test with experimental parameters. Different counterbodies include commercial bearing steel, alumina, SiAlON, hardmetal balls of 10 mm diameter.

equation is reported to be justified for the present fretting conditions, providing errors less than 5% when the wear scar diameter is larger than twice the Hertzian contact diameter [16], as was the case in our experiments.

3. Results 3.1. Friction and wear data The frictional behavior of the investigated fretting couples is illustrated in Fig. 2. The COF, 0.62, of the self-mated sialon couple was the highest. The steadystate COF of the sialon /alumina and sialon /steel tribocouples is comparable, whereas a significantly lower COF of 0.4 was measured for the sialon/hardmetal material combination. The measured volumetric wear data in the investigated tribocouples is plotted in Fig. 3. The error bars indicate the scatter in the wear data of at least three fretting tests. The wear loss of the sialon flats in the selfmated test is one order of magnitude lower than the wear volume of the sialon flats in the sialon /alumina and sialon /steel fretting couples. The highest sialon flat wear was measured when fretting against steel. The sialon flat wear against hardmetal was of the same order of magnitude but smaller than in the self-mated tests. From the volumetric ball wear data; it is evident that the ball wear loss was higher than the wear of the corresponding sialon flat, except for the sialon/hardmetal combination. The wear of the sialon ball is one order of magnitude higher than that of the flat in the self-mated sialon combination. The alumina ball wears less than the sialon ball, but the volumetric wear is of the same order of magnitude. The highest ball wear under the investigated conditions was observed for the steel ball, which is in accordance with the highest sialon flat

Fig. 2. The evolution of the COF when fretting monolithic sialon flats against sialon, alumina, steel, and hardmetal balls under a load of 8 N for 100,000 cycles with a frequency of 10 Hz and a displacement and 200 mm.

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Fig. 3. Ball and flat wear volumes after fretting of sialon flats against commercial sialon, alumina, steel, and hardmetal balls under a load of 8 N for 100,000 cycles with a frequency of 10 Hz and a displacement and 200 mm.

wear in this particular material combination. In contrast, the lowest ball wear was measured for the hardmetal ball and the observed wear resistance of the hardmetal ball was remarkably higher than the counterbody wear in any other material combination. The wear of the WC /Co hardmetal ball is two orders of magnitude lower than the corresponding flat sialon counterbody. Detailed investigation of the wear data in Fig. 3 reveals that the wear volume of the steel and alumina balls is almost double that of the corresponding sialon flat. Considering the total volumetric wear loss (ball/flat wear) of the tribosystems, it can be stated that the sialon/steel combination experienced the highest wear loss, whereas the sialon/hardmetal combination showed the lowest wear loss under the selected experimental conditions. 3.2. Morphological investigation of the worn surfaces The morphology of the worn surfaces in the selfmated sialon couple is presented in Fig. 4. The appearance of both surfaces was rather similar. The wear pit on both the ball and the flat is covered by an adhering tribolayer. A closer examination of the tribolayer on the sialon flat in Fig. 4b shows occasional spalling, indicating its non-protective nature. Similar localized spalling of the silica layer is evident in Fig. 4d. The worn surfaces of both flat and ball are also characterized by abrasive grooves in the sliding direction. EDS analysis revealed the presence of silica in the tribochemical layer, indicating tribo-oxidation of the sialon phase at the fretting contact, as could be expected from literature reports [2]. Therefore, tribo-oxidation in combination with abrasion of the silica-rich tribolayer is the predominant wear mechanism. The morphology and composition analysis of the worn surfaces in the sialon/alumina couple are shown

in Fig. 5. The worn sialon flat surface is covered by a tribolayer, with abrasion marks and extensive cracking and spalling of the layer, causing appreciable wear of the sialon flat. Compositional analysis revealed an increased Al content in the tribochemical layer when compared to the unworn surface. This indicates material transfer from the alumina ball to the sialon flat during fretting. The worn alumina ball surface shows deep abrasive scratches with the occasional presence of a transfer layer (Fig. 5c). EDS analysis of the transfer layer reveals a considerable amount of Si, along with a strong Al-peak (see Fig. 5d), indicating that the sialon phase oxidizes and is transferred to the alumina ball. From the above, it is clear that tribochemical reactions coupled with spalling and abrasion of the tribolayer are the major wear mechanisms for the sialon /alumina tribocouple. The wear characteristics of the sialon/steel fretting couple are illustrated in Fig. 6. The transfer layer on the worn flat sialon surface is found to be discontinuous (see Fig. 6a). The composition analysis of the layer, as shown in Fig. 6b, indicates the predominant presence of Fe and O. This suggests that the iron oxide layer is transferred to the sialon flat during fretting. The worn steel ball is covered by a thick tribolayer, which is non-protective due to intensively spalling during fretting. The EDS analysis reveals a significant amount of Si in the steel tribolayer (see Fig. 6d). These observations implicate extensive tribo-oxidation of both sialon and steel and mutual transfer between the mating counterbodies during fretting. Similar observations are reported in literature [8]. The morphology of the worn surfaces in the sialon/ hardmetal fretting couple is presented in Fig. 7. The worn sialon flat is found to be covered by a thin tribolayer with a significant amount of wear debris spread at and around the edge of the wear scar. EDS analysis of the wear debris on the sialon reveals a minor amount of W and Co (see Fig. 7b). Mild abrasive wear

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Fig. 4. SEM micrographs showing the morphology of the transfer layer on the worn sialon flat (a, b) and sialon ball (c, d) after sialon is fretted against itself. Fretting parameters are the same as mentioned in Fig. 1. Compositional analysis revealed the predominant presence of a silica-rich tribolayer on both surfaces. Doubly pointed arrows indicate the fretting direction.

scratches on a rather smooth worn hardmetal ball can be observed in Fig. 7c. The compositional analysis did not show any detectable material transfer from the sialon flat. These observations correspond well with the low fretting wear loss of the sialon /hardmetal tribosystem.

4. Discussion Based on the tribological data and morphological investigations of the worn surfaces, the wear mechanisms for the different tribocouples can be summarized. Tribochemical wear, followed by mechanical wear in the form of abrasion or spalling, is found to be the typical wear behavior under the selected experimental conditions. It is well known that tribochemical reactions of ceramic materials are critically dependent on the combined effect of working conditions, environmental conditions, and material properties [17]. The significance of environmental condition, in particular the relative

humidity (RH) on tribological behavior of SiAlON is investigated in details by many researchers [2,6,17] and hence not studied in the present work. Also to exclude the humidity influence, all the wear tests in our work were carried out at ambient humidity (50 /52% RH). A possible explanation for the substantial extent and detrimental effect of tribochemical reactions is their dependency on the contact temperature, which is governed by the sliding speed, COF and thermal properties of the contacting bodies [18]. In general, the contact temperature increases with increasing COF and sliding speed, and decreases with increasing thermal conductivity. Based on the representative thermal conductivity data (Table 2) and the COF data (Fig. 2), it is clear that under identical contact conditions, the contact temperature at the sialon/hardmetal couple will be significantly lower than for the sialon /steel, sialon / sialon and sialon /alumina combinations. This could explain the excessive tribo-oxidation of the sialon phase and silica transfer to the counterbodies in all the tribocouples, except with hardmetal. Moreover, very

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Fig. 5. SEM micrographs of the worn surface of the sialon flat (a) and alumina ball (c) after sialon is fretted against alumina. Fretting parameters are the same as mentioned in Fig. 1. The EDS spectra (b, d) were taken from the transfer layer (spot indicated by arrow) on the sialon flat and alumina ball, respectively. Doubly pointed arrows indicate the fretting direction.

limited oxidation and transfer of the WC /Co is observed in our experiments, as revealed in Fig. 7. Self-mated materials in contact imply a high potential for adhesive wear. Indeed, a distinct oxidized transfer layer is observed to adhere in the tribocontact of the sialon/sialon combination. The adhesion of the tribolayers might also be the reason for the high COF (Fig. 2). It is interesting to note here that although the contact remains practically closed during fretting, the difference in wear loss of the flat and the ball is high for the selfmated sialon couple. For the sialon/alumina fretting couple, tribochemical oxidation followed by mutual material transfer and abrasion was observed. The oxidized layer on the sialon surface experienced delamination and abrasion by the much harder alumina countermaterial (see data in Table 1). The generated alumina wear debris particles is most probably embedded into the softer oxidized sialon tribolayer and in turn caused further abrasion of the

alumina ball surface, as can be seen in Fig. 5c. Such phenomena are often found in tribological situations [2] and cause substantial damage to the harder counterbody in the contact. This is reflected in the relatively high wear loss of the sialon /alumina couple. In the case of the sialon /steel couple, extensive tribochemical reactions are observed. This could be well predicted from the chemical instability of sialon against steel [19,20]. It is reported that sialon is chemically unstable and dissolves into steel at and above 800 8C [21], while for silicon nitride this temperature was found to be between 500 and 700 8C both in contact with oxidized and pristine steel [22]. Such dissolution process is expected to be enhanced under the mechanical stress conditions existing at the fretting contact. In the wear induced dissolution process, the surface area of wear debris particles i.e. debris size can have a considerable influence. In addition, the prevailing thermal conditions on asperity hot spots are reported to

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Fig. 6. SEM micrographs showing the morphology of the transfer layer on the worn sialon flat (a) and steel ball (c), together with EDS spectra taken from the transfer layer on the sialon (b) and the steel ball (d) after sialon is fretted against steel. Fretting parameters are the same as mentioned in Fig. 1. The EDS analyses were performed at the locations on the worn surfaces, marked by the arrows. Doubly pointed arrows indicate the fretting direction.

be critical in fretting of silicon nitride against steel [23]. The chemical instability i.e. dissolution of SiAlON particles on to steel (particle wear) is considered to be the major reason for the high fretting wear loss (see Fig. 3) of the sialon /steel tribosystem observed in the present work. The limited tribochemical reaction of sialon in contact with hardmetal, as mentioned above, results in a lower wear loss of the sialon /hardmetal tribosystem. Besides the favorable thermal conductivity of WC /Co (see Table 2), the COF was also the lowest of the investigated combinations, which additionally reduces the contact flash temperatures. This is considered to limit tribochemical reactions in the sialon /hardmetal tribocouple [24]. A relative ranking of the selected material combinations was made and compared with fretting wear data reported in the literature. The wear volume data, presented in Fig. 3, is normalized with respect to the normal load and the total sliding distance in order to

obtain the volumetric wear rate. Fig. 8 illustrates the wear loss of sialon and silicon nitride flats in contact with different engineering materials. While comparing the wear data, it should be kept in mind that the wear rate critically depends on many factors like the testing parameters (normal load, test duration, environmental conditions) and the microstructure of the materials concerned. Recognizing this, our aim here has been to obtain a qualitative basis for comparing the wear loss of silicon nitride based materials against important engineering materials. It is clear from Fig. 8 that the volumetric wear rate of sialon flats under different contact conditions varies within the same order of magnitude. The self-mated sialon in the present work shows one of the best performances reported on silicon nitride-based ceramics. It is interesting to note here that the wear resistance of the self-mated sialon is much better than the reported wear rate of the self-mated silicon nitride couple. Besides the obvious critical effect of tribochemical reaction, it

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Fig. 7. SEM micrographs showing the morphology of the worn surfaces on the sialon flat (a) and hardmetal ball (c) after sialon is fretted against hardmetal. Fretting parameters are the same as mentioned in Fig. 1. The EDS spectrum (b) was taken in the centre of the transfer layer on the sialon flat (c). Doubly pointed arrows indicate the fretting direction.

was previously reported that one of the most important factors influencing the fretting wear behavior of sialon ceramics is the chemical composition and the amount of binder phase [4]. In our experiments, an even better wear resistance is obtained for the sialon/hardmetal (WC / Co) combination than all other material combinations plotted in Fig. 8. This can be attributed to the chemical

stability of both materials in contact, demonstrated by low friction and limited tribochemical reaction. The wear of sialon flats in contacts with alumina and steel were much higher than the self-mated sialon couple and quite similar to those found in other studies (see Fig. 8). The particularly high wear loss of sialon in contact with steel is considered to be due to the high chemical

Fig. 8. Relative ranking of the fretting wear rate of silicon nitride based flats in tribocontacts with different counterbodies. Sialon* (present work), Sialon (2) /Ref. [13], Sialon (3) /Ref. [16], Si3N4 (4) /Ref. [8], Si3N4 (5) /Ref. [10], Si3N4 (6) /Ref. [15].

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dissolution of SiAlON in steel as a result of extensive tribochemical reactions. Equally high wear rates of sialon/silicon nitride against steel are reported in literature (see Fig. 8). On the other hand, the high wear rate in contact with alumina is suggested to be due to a combination of tribochemical and mechanical wear. For example, the hard alumina surface and wear debris easily abrades the softer tribolayer formed on the sialon surface which is formed in the contact as a results of tribochemical reaction, i.e. tribo-oxidation.

5. Conclusions (a) The steady state COF in a sialon/hardmetal fretting couple was measured to be around 0.4 and is significantly lower than that in a self-mated sialon combination, which in turn was found to be slightly higher than that of the sialon/alumina and sialon /steel tribocouples. (b) The sialon /hardmetal combination shows the best volumetric fretting wear resistance, whereas the sialon / steel shows the highest volumetric wear loss under the selected experimental fretting conditions. In comparison to similar fretting studies reported in the literature, the self-mated sialon and sialon /hardmetal combinations investigated in this paper exhibit a relatively better wear resistance under unlubricated conditions. (c) Tribochemical wear followed by mechanical wear in the form of abrasion or spalling is found to be the predominant wear mechanism under the experimental conditions in the sialon /sialon, sialon /alumina and sialon/steel combinations. The formation of tribochemical layers, in particular by means of oxidation, is suggested to be the fretting wear controlling mechanism of sialon. The thermal conductivity of the counterbody material, which affects the asperity flash temperatures and consequently the extent of tribochemical interaction is found to be an important and indicative parameter with respect to the extent of tribochemical reactions at the fretting contacts. Due to the higher thermal conductivity of the WC /Co hardmetal and reduced friction of the sialon /hardmetal couple, the extent of tribochemical reactions as well as volumetric wear is very limited in the sialon /hardmetal tribocouple.

Acknowledgements This work was supported by the Brite/Euram programme of the Commission of the European Communities under project contract No. BRPR-CT96-0304. B. Basu would like to thank the Research Council of the Katholieke Universiteit Leuven in Belgium for a research fellowship.

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