- Email: [email protected]
Fretting wear behavior of TiB2-based materials against bearing steel under water and oil lubrication
Wear 250 (2001) 631–641
Fretting wear behavior of TiB2 -based materials against bearing steel under water and oil lubrication B. Basu, J. Vleugels, O. Van Der Biest∗ Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, W. De Croylaan 2, B-3001 Leuven (Heverlee), Belgium
Abstract Lubricated fretting tests in water and paraffin oil were performed with a monolithic TiB2 , a TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder, a sialon–TiB2 (60/40) composite and a ZrO2 –TiB2 (70/30) composite against ball bearing grade steel. Based on the measured friction and wear data, the ranking of the investigated fretting couples was evaluated. Furthermore, the morphological investigations of the worn surfaces and transfer layers are carried out and the wear mechanisms for the investigated friction couples are elucidated. While fretting in water, experiments revealed that tribochemical reactions, coupled with mild abrasion, played a major role in the wear behavior of the studied material combinations. ZrO2 –TiB2 (70/30)/steel wear couple has been found to have the highest fretting wear resistance among the different tribocouples under water lubrication. Under oil lubrication, extensive cracking of the paraffin oil at the fretting contacts, caused by tribodegradation, leads to the deposition of a carbon-rich lubricating layer, which significantly reduced friction and wear of all the investigated tribosystems. © 2001 Elsevier Science B.V. All rights reserved. Keywords: TiB2 ; Lubrication; Fretting wear; Tribochemical wear
1. Introduction Ceramics are a promising class of advanced materials, which have a tremendous potential for tribological applications. During the last few decades, much attention has been paid to investigate the wear and friction characteristics of several engineering ceramics [1]. Due to its high hardness (around 25 GPa), TiB2 is considered to be a promising material for tribological applications [2]. The poor sinterability and rather low toughness however restricts the use of the monolithic TiB2 in engineering applications. Different binders are used to fabricate the TiB2 -based technical ceramics. In the present work, the wear behavior of a monolithic TiB2 , a sialon-based 40 vol.% TiB2 composite, a ZrO2 -based 30 vol.% TiB2 composite, and a TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder is investigated. The proposed applications of the investigated materials include ball valves as pump components and grinding quills for high-speed grinding operations, etc. In these applications, fretting wear seems to cause a considerable loss in the functionality of these materials. The unlubricated wear performance of several advanced ceramics (e.g. zirconia, SiC, silicon nitrides, etc.) demonstrated the need for lubrication ∗ Corresponding author. Tel.: +32-16-32-1264; fax: +32-16-32-1992. E-mail address: [email protected] (O. Van Der Biest).
at the tribocontacts for the successful application as structural parts [3]. In this perspective, the present paper reports the influence of different lubrication (distilled water and paraffin oil) on the tribological behavior of the TiB2 -based ceramics and cermets when fretted against ball bearing steel. Because of its tremendous engineering importance, steel is selected as the counterbody material. The influence of different lubricants (water, paraffin oil) on the wear behavior of a range of ceramics including alumina, SiC, yttria-doped zirconia and Si3 N4 have been compared with that under dry sliding conditions [4]. Based on the results, wear maps of these materials in different environments have been established and the transitions in wear behavior as a function of testing parameters (load and sliding speed) are discussed. The same group of researchers studied the mechanism of sliding wear of self-mated yttria-stabilized tetragonal zirconia ceramics (Y-TZP) ceramic in different lubricating media [5]. Oscar Barceinas-Sanchez and Rainforth recently investigated the sliding wear of a 3Y-TZP ceramic against Mg-PSZ in distilled water and dry conditions [6]. Water was found to provide modest lubrication, lowering the sliding wear rate by a factor of three. The role of humidity on the fretting wear of self-mated Y-TZP was recently investigated in our laboratory [7]. Liu and Xue [8] constructed a “wear map” for a zirconia/steel couple sliding in water. In another study, Kalin et al. [9] carried out an extensive investigation to understand the
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 6 7 0 - 6
632
B. Basu et al. / Wear 250 (2001) 631–641
fretting wear mechanisms of silicon nitride ceramics against construction grade steel under water and formulated oil environments. It was observed that the presence of an antiwear phosphorus additive in the formulated oil provided more chemical protection to the worn surfaces compared to the purified paraffin oil. Lubricated wear tests in engine oil revealed a reduction in the wear rate of a Si3 N4 –TiC/steel friction couple by four orders of magnitude as compared to that in dry sliding conditions [10]. It should be noted here that the majority of the published work cited above is based on pin-on-disk or ball-on-plate tribometers under unidirectional sliding mode. Very few literature reports, for example [9], focus on the fretting wear (linear reciprocatory displacement sliding) behavior of ceramics in a lubricating medium. It can be pointed out here that the wear of a given material combination depends on many influential factors like the contact configuration, the mechanical properties (hardness, toughness), testing parameters (normal load, sliding speed), microstructure (grain size, porosity, etc.), the interaction with surrounding atmosphere (relative humidity, water or other lubricants), etc. According to the best of the authors’ knowledge, no fretting tests on TiB2 -based ceramics against steel in lubricating media are reported in the literature. In the present study, the lubricated wear behavior of TiB2 -containing materials fabricated with different binders (Y-TZP, sialon, and intermetallic) is investigated. The investigated materials including the TiB2 monolith have a range of properties: hardness ranging from 13 to 21 GPa, fracture toughness from 5 to 10 MPa m1/2 , and elastic modulus from 258 to 500 GPa. The influence of physico-chemical and mechanical properties of the different materials on the wear behavior, when fretted against steel under water and oil lubrication will be elucidated. This will also assess the suitability of the investigated binders in terms of providing the optimum fretting wear resistance of TiB2 materials.
2. Materials and experimental procedure 2.1. Materials The mechanical properties of the materials used in the present work are listed in Table 1. Commercial bearing grade
steel balls (DIN 100Cr6 grade, Fritsch, Germany), 10 mm diameter with mirror finished surfaces (surface roughness of 0.02 m, data from the supplier) were used as counterbody materials. As provided by the supplier, the nominal composition (wt.%) of the steel ball includes C (2.1), Cr (12.0), Si (0.3), Mn (0.3), and rest Fe. The TiB2 monolith was processed from the finest ESK grade TiB2 with 5 vol.% SiC (grade 059S, Superior Graphite Co.) as sinter additive. According to the supplier, the Fischer particle size of the ESK grade TiB2 powder is <0.5 m and nonmetallic impurities include C, O, and N with their individual maximum amount of 0.5, 2.5, and 1 wt.%, respectively. Hot pressing at 1900◦ C for 4 h in vacuum (≈0.1 Pa) under a mechanical load of 28 MPa resulted in a 95% theoretically dense material. The ZrO2 -based composite with 30 vol.% TiB2 was obtained by hot pressing of a ZrO2 and TiB2 (H.C. Starck grade F) powder mixture. The ZrO2 matrix was processed from a powder mixture of 3 mol% yttria co-precipitated (Tosoh grade TZ-3Y) and monoclinic (Tosoh grade TZ-O) ZrO2 powders with an overall yttria content of 2.5 mol%. According to the supplier, the Fischer particle size of the Starck grade F TiB2 powder is around 0.9 m and nonmetallic impurities include C, O, and N with their individual maximum amount of 0.25, 2.0, and 0.25 wt.%, respectively, metallic impurity of Fe with maximum of 0.25 wt.%, and other metallic impurities of maximum 0.2 wt.%. Full density was obtained by hot pressing at 1450◦ C for 1 h in vacuum (≈0.1 Pa) under a mechanical load of 28 MPa. The sialon-based composite with 40 vol.% TiB2 (H.C. Starck grade F) was obtained by conventional hot pressing at 1700◦ C for 10 min under 30 MPa N2 pressure. The substitution level (z) in the Si6−z Alz Oz N8−z was 0.3. More information on the processing and properties of these composites is given elsewhere [11]. The TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder was processed by the sinter-HIP (Hot Isostatic Pressing) route. TiB2 powder used for the cermet production was from H.C. Starck (grade F). The details of the fabrication and properties can be found elsewhere [12]. Representative SEM micrographs of the microstructures of the investigated materials are shown in Fig. 1. In the monolithic TiB2 material, the grey particles are the SiC grains, added as sinter additive, whereas the black phase represents the residual 3–4% porosity (see Fig. 1a). In
Table 1 Mechanical properties of the ceramics used in the present investigationa Flat material
HV10 (GPa)
Monolithic TiB2 TiB2 -based cermet (84/16) Sialon–TiB2 (60/40) ZrO2 –TiB2 (70/30)
21.3 16.1 16.6 13.0
Steel counterbody (10 mm diameter ball) DIN 100Cr6 grade a
± ± ± ±
0.7 0.4 0.3 0.2
7.8 ± 0.1
The counterbody data are supplied by the commercial supplier.
KIc (10 kg) (MPa m1/2 ) 5.6 9.5 6.2 9.7
± ± ± ±
E (GPa)
0.4 0.7 0.4 0.6
500 476 365 258
20 ± 1.0
210
B. Basu et al. / Wear 250 (2001) 631–641
633
Fig. 1. Representative microstructures of the investigated materials: (a) monolithic TiB2 ; (b) TiB2 –Ni3 (Al, Ti) (84/16); (c) sialon–TiB2 (60/40); (d) ZrO2 –TiB2 (70/30). Detailed description of the phase assemblage is presented in Section 2.
the TiB2 -based cermet material, coarse boride particles (4–5 m, grey color) are dispersed in an intermetallic Ni3 (Al, Ti) binder, bright contrast (see Fig. 1b). The black particles in the cermet microstructure are Al2 O3 phase. The matrix phase of the sialon-based TiB2 composite (dark contrast in Fig. 1c) consists of both ␣-Si3 N4 and -sialon with a low substitution (z) value. The intergranular phase contains yttria. The boride particles are in bright contrast. The different phases that can be distinguished in the zirconia-based composite (see Fig. 1d) on the back-scattered electron micrographs are: ZrO2 (white), TiB2 (grey), and Al2 O3 (black). The presence of alumina is due to the use of alumina milling balls during powder mixing. XRD investigations revealed the presence of a small amount of monoclinic ZrO2 in the Y-TZP-based composite materials. A detailed microstructural characterization and more information on the mechanical properties of the zirconia composite are reported elsewhere [13]. As observed in the microstructures
of the different composites, the homogeneously dispersed TiB2 particles are irregularly shaped. 2.2. Fretting tests The fretting experiments have been performed on a computer-controlled tribometer under ambient temperature (25◦ C) and humidity (50–55% RH) conditions. The details of the experimental set-up can be found elsewhere [14]. The 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 reciprocatory relative displacement sliding). The flat samples are ground and polished until they have an average surface roughness (Ra ) of 0.05 m. The nominal dimensions of the flat sample include a length of 20 mm, a width of 5 mm, and a height of 3 mm. Prior to the fretting experiment, the materials are ultrasonically cleaned in acetone. Two lubricants are used
634
B. Basu et al. / Wear 250 (2001) 631–641
in the present work: laboratory distilled water and commercial paraffin oil (<0.02% residual water and viscosity of 345–355 P at 100 F, Fischer Scientific). The flat sample is mounted inside a small stainless steel vessel filled with the lubricant, so that the top of the sample is fully immersed in the liquid lubricant. The whole set-up is mounted on a translation table, which oscillates at the required displacement with desired frequency by means of a stepping motor. The displacement of the sample is monitored by an inductive displacement transducer, and the friction force is measured with a piezoelectric transducer attached to the holder that supports the counterbody. The friction coefficient is calculated from the on-line measured tangential force. During the test, fretting loops are recorded at a predefined number of cycles. A fretting loop gives the evolution of the tangential force as a function of the displacement amplitude during each cycle. The coefficient of friction (COF) is further evaluated from the average of the two plateau values of the tangential force in the fretting loop, as described elsewhere [15]. For comparative reasons, all the tests are performed with identical fretting parameters: a normal load of 8 N, a total linear displacement of 200 m, a frequency of 10 Hz, and a duration of 100,000 cycles. 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 Met optical microscope (Nomarski contrast) and a scanning electron microscope (Philips XL-30 FEG) equipped with an EDS system for compositional
analysis. In the EDS analysis, light elements are detected with an acceleration voltage of 2–5 kV, while the heavier elements are identified with an acceleration voltage of 20–25 kV. A Rodenstock laser profilometer (RM600X/Y-100) was used to evaluate the geometry and wear volumes of the fretting wear tracks on the flat samples. Since profilometer measurements on the ball wear scar are extremely difficult, the wear scar diameters (both in the sliding and in the transverse directions) are measured from optical micrographs, and wear volumes are computed according to the equation proposed by Klaffke [16]: V = 64π d 3 /a, with ‘d’ the average diameter of the wear scar and ‘a’ the ball radius.
3. Results and discussion 3.1. Water lubrication 3.1.1. Friction and wear data The influence of water and paraffin oil lubrication on the frictional behavior of the investigated TiB2 -based ceramic composites against commercial ball bearing grade steel are shown in Fig. 2a and b, respectively. The friction data obtained in paraffin oil will be discussed in Section 3.2.1. Under water lubrication, COF of the sialon–TiB2 /steel couple, unlike the other friction couples, increases rapidly (see Fig. 2a), showing a maximum COF of 0.58 and then decreases during the running-in-period (first 20,000 cycles), and finally stabilized at the steady state level (COF = 0.48). It is also noted that the sialon–TiB2 /steel couple has the highest friction coefficient of 0.48, whereas the steady state COF of the other investigated friction couples varied between 0.35 and 0.38.
Fig. 2. The evolution of COF of the TiB2 -based materials when fretted against construction grade steel balls under a load of 8 N with a frequency of 10 Hz and a displacement of 200 m under water (a) and paraffin oil (b) lubrication.
B. Basu et al. / Wear 250 (2001) 631–641
635
Fig. 3. The wear volumes of the TiB2 -based flat materials and construction grade steel counterbody when fretted in water (a) and the wear volume of steel balls under oil lubrication (b). The fretting conditions are the same as mentioned in Fig. 2.
The wear volume of the TiB2 -based flat materials fretted against 100Cr6 ball bearing grade steel under water lubrication is shown in Fig. 3a. The error bars represent the standard deviation of the wear data obtained from at least three fretting tests. Both the monolithic TiB2 and TiB2 -based cermet show comparable volumetric wear. The wear volume of the sialon-based composite is a factor of two higher than all the other investigated material combinations, whereas the wear volume of the zirconia composite is the lowest. The steel balls experienced a high wear loss during fretting in water, as revealed in the data presented in Fig. 3a. It should be noted here that the counterbody wear is found to be one order of magnitude higher than that of the flat. The volumetric wear of the steel counterbody follows the same trend as the wear of the flat material. The highest ball wear is observed after testing against the sialon composite, while the lowest with the zirconia composite. Considering the total wear of the tribocouples, it is clear that the sialon–TiB2 /100Cr6 grade steel couple is most prone to fretting wear, whereas the zirconia composite/steel combination exhibits the highest fretting wear resistance under water lubrication conditions. It should be mentioned here that the wear data, measured on the flats, do not show any clear relationship with the mechanical properties (see Table 1 and Fig. 3a). This indicates that tribomechanical wear does not play any dominant role in the present case. 3.1.2. Morphological investigation of the worn surfaces The worn surfaces in the ‘monolithic TiB2 /steel tribocouple’ after fretting in water are illustrated in Fig. 4. A tribolayer is found to adhere to the mild abrasive grooves on the TiB2 material, as shown in Fig. 4a. The presence of numerous cracks on the tribolayer shows its brittle, non-protective nature. EDS spectra acquired from such layer indicates the formation of iron oxides, Ti oxides or mixed (Fe, Ti) oxides (see Fig. 4b). Silicon from the silicon
carbide sinter aid was not detected on the worn surfaces. Considering the water lubricating conditions, the presence of hydroxides of Ti and/or Fe is also possible. The different oxidized species, as will be reported throughout this paper, can also exist in the hydroxide form under water lubricating condition. A tribolayer, adhered to the relatively deep abrasive groves, is observed on the steel counterbody (Fig. 4c). Closer look at Fig. 4c also shows the adherence of the tribochemical layer onto the worn steel surface. Elemental analysis indicated the presence of Fe, Ti, Cr, O in the tribolayer (see Fig. 4d). The presence of Ti on the worn steel surface can be explained in either of two ways. The first one is that TiB2 phase from the flat is oxidized during the fretting process and incorporated in a transfer layer (third body) between the mating couple. Another possibility is that TiB2 particles are spalled off from the flat and then transferred to the steel ball and finally oxidized. Both of these factors seem to be plausible. It can be mentioned here that recent XPS investigation in our laboratory revealed that the material transferred between the mating counterbodies are always oxidized in the monolithic TiB2 /steel tribocouple under unlubricated fretting conditions [17]. Following this, it is more probable that TiB2 is oxidized and incorporated in the iron oxide-rich tribolayer. Therefore, experimental observations indicate the occurrence of tribochemical reactions with the mutual transfer of material between the fretting couple and spalling of the tribochemical layer as the major wear mechanism of the monolithic TiB2 /steel fretting couple. The worn surface in the ‘TiB2 -based cermet/steel tribosystem’ after fretting in water is presented in Fig. 5. The fretted surface is characterized by the presence of strongly embedded thick wear debris particles, as shown in Fig. 5a. Mild abrasive scars are noted in the flat worn surface around the debris particles. EDS spectra obtained from the debris show a strong Ni peak along with peaks of
636
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 4. Worn surfaces and EDS spectra of the tribolayers on the TiB2 monolith ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. Numerous cracks could be readily observed in the tribolayer on the flat worn surface. EDS spectra ((b) and (d)) are acquired from the spots indicated by the arrows in (a) and (c). The arrow in (c) also indicates typical adherence of the tribochemical layer. The tribolayer next to it is predominantly iron oxide. The fretting direction is indicated by a doubly pointed arrow.
Fe, Ti, Cr, Al, and O (see Fig. 5b). The worn steel ball is observed to be covered by a tribolayer, as shown in Fig. 5c. The EDS spectrum of the tribolayer on the steel indicates the presence of Fe, Ni, Cr, and O (see Fig. 5d). Although, Ti is not recorded in the reported spectrum, Ti is found in other investigated locations on the worn steel ball. The fact that Fe is present on the flat indicates that iron oxide is transferred from the steel ball onto the flat. The presence of Ni on the worn steel surface indicates that NiO is dissolved in the iron oxide layer on the worn steel. The formation of NiO indicates the tribochemical oxidation of the intermetallic binder phase in the cermet during the fretting process. As-fretted surfaces in the ‘sialon–TiB2 /steel combination’ after fretting in water is shown in Fig. 6. A thin tribofilm is found to cover the flat worn surface (see Fig. 6a). Closer look at the flat worn surface reveals the presence of numerous microcracks in the tribolayer. This indicates the non-protective nature of the tribofilm. The compositional analysis of the tribofilm revealed the presence of Si, Ti, Fe, Cr, and O, as shown in Fig. 6b. The amount of iron oxide transferred from the steel ball is significantly smaller than that in the
previous cases, as evident from the fairly weak Fe peak. The strong Si peak in combination with the strong O peak suggests the formation of silica. Deep abrasive scars could be seen despite the presence of a tribochemical layer on the worn steel ball (see Fig. 6c). The EDS spectrum of the tribofilm reveals the presence of high amounts of Si and Ti (see Fig. 6d). No significant amounts of Fe and Cr were measured in the tribolayer. This indicates the transfer of the mixed and probably hydrated Si–Ti oxide layer on the worn surface of the steel ball. The compositional analysis, as described above thus shows the possible material transfer and tribochemical oxidation of the sialon binder phase and TiB2 onto steel counterbody. It is reported in the literature [4,9,18] that silica can get dissolved into water forming silicon hydroxide or hydrated silica under the water lubricating conditions at the tribocontact. When silicon nitrides slide in water, the tribochemical reaction is reported to be the dissolution of silica at the contacting surfaces with the formation of a lubricating tribolayer [19]. Thus, the higher wear loss of sialon composite/steel couple in water lubrication, as observed in the present case, could be linked to the mutual material transfer and the formation of a non-protective hydrated silica layer.
B. Basu et al. / Wear 250 (2001) 631–641
637
Fig. 5. Worn surfaces and EDS spectra of the tribolayers on the TiB2 -based cermet with Ni3 (Al, Ti) binder ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. EDS spectra are taken from the arrows indicated in the SEM micrographs. The arrow in (a) also indicates the embedding of the wear debris on a rather smooth flat worn surface. The arrow in (c) implicates localized spalling of the tribochemical layer. The fretting direction is indicated by a doubly pointed arrow.
Fig. 7 shows the surfaces in the ‘zirconia composite/steel’ wear couple after fretting in water. Red iron oxides are occasionally found to stick to the wear scars on the zirconia composite. The worn surface in the central part of the wear pit on the composite was extremely smooth, as shown in Fig. 7a, with iron oxide particles locally adhering to the TiB2 phase. Only in these locations, Fe was detected with EDS analysis (not shown). Mild abrasion marks due to the fretting process could be observed optically (not shown). The tribolayer on the steel ball was fragmented over the worn steel surface, as shown in Fig. 7b. Compositional analysis indicated that the tribolayer is a mixed oxide of Fe and Cr with a small amount of dissolved Ti from the ceramic (Fig. 7c). The experimental observations thus indicate that TiB2 from the flat oxidizes, and transferred onto the steel tribolayer. ZrO2 on the other hand was not observed to be transferred onto steel worn surface, as could be expected from literature [20]. The presence of rather low amount (30 vol.%) of TiB2 phase and the stability of ZrO2 against transfer to steel have resulted in limited tribochemical reactions, compared to that observed with the other investigated tribocouples. This, coupled with the mild abrasion marks on
the flat worn surface, corresponds well with the low fretting wear rate of the ZrO2 –TiB2 (70/30) composite against steel in water. 3.2. Oil lubrication 3.2.1. Friction and wear data In another set of experiments, the same TiB2 -based materials were fretted against ball bearing steel in paraffin oil under the same experimental conditions. When using liquid paraffin as lubricating medium, a drastic reduction in friction coefficient is observed for all the investigated fretting couples (see Fig. 2b). The steady state friction values for all the material combinations are in between 0.08 and 0.12. Comparing with the friction data observed under water lubrication (see Section 3.1.1), it can be stated that paraffin oil as compared to water is more efficient in reducing the friction of the investigated materials against construction steel. After fretting in oil and subsequent ultrasonic cleaning, the worn surfaces on the flats are observed to be very smooth with an adhering tribofilm. Wear volume measurements
638
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 6. Worn surfaces and EDS spectra of the tribolayers on the sialon–TiB2 composite ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. The arrow in (c) indicates the adherence of the tribolayer on the worn steel surface. The worn surface around the adhering tribolayer in (c) is predominantly iron oxide. EDS spectra are taken from the arrows indicated in the SEM micrographs. The fretting direction is indicated by a doubly pointed arrow.
using a standard laser profilometer becomes impossible, as the average depth of the wear pit is found to be comparable to the roughness of the flat surface. However, the wear scars on the steel balls after fretting in oil could be observed in the optical microscope and the wear volume is evaluated from the geometry of the wear scar (see Fig. 3b). The steel counterbody shows a higher wear volume in contact with the TiB2 -based cermet. The wear volume of the steel ball fretted against the monolithic TiB2 and the sialon composite is comparable. The lowest wear is measured when fretting was performed against zirconia composite. It should be mentioned here that the volumetric wear of the steel balls fretted in oil is reduced by two orders of magnitude when compared to that under water lubrication. 3.2.2. Morphological investigation of the worn surfaces Fig. 8 illustrates the tribolayer formed on the worn surface of the TiB2 monolith and steel ball after fretting in liquid paraffin. The surface of the TiB2 material is hardly
worn and the abrasive scars are observed to be of the same depth as the polishing marks on the native surfaces. The worn surfaces on both flat and ball are covered by a thin adherent tribofilm, as shown in Fig. 8a and b. The details of the flat worn surface are shown in Fig. 8c. EDS analysis (see Fig. 8d) of the tribolayer on the flat showed a strong carbon peak along with an O peak. It is interesting to note here that the tribocontact under oil lubrication is not completely free from the access to oxygen, as revealed by the O peak. Additionally, EDS analysis showed the presence of Ti, B, and Fe on the worn surface. The amount of iron oxide is considered small. The presence of Ti and B peak along with an O peak indicates the oxidation of the TiB2 phase. Oxidation of bulk TiB2 has been reported to start at 600◦ C in an oxidizing atmosphere [21], whereas it has been revealed that the oxidation process of TiB2 in air starts even below 400◦ C, with the formation of TiBO3 [22]. However, under the prevailing mechanical stress conditions during the fretting tests, tribo-oxidation process can even start at much lower temperature as often reported in the literature [23]. The
B. Basu et al. / Wear 250 (2001) 631–641
639
Fig. 7. Worn surfaces of the ZrO2 –TiB2 composite (a) and steel ball (b) after fretting in water. Note that the iron oxide particles are locally adhered to TiB2 , as pointed by an arrow in (a). The EDS spectrum (c) was taken from the tribolayer on the steel ball, as indicated by the arrow in (b). The fretting direction is indicated by a doubly pointed arrow.
compositional analysis as reported above reveals that local heating at the fretting contact, as also will be discussed below, promoted the oxidation of the TiB2 phase in the oil lubricated condition. Since the microstructural analysis was carried out without a conductive layer for SEM observation, the compositional analysis strongly indicates the formation of a carbon-rich layer on the worn surface. The carbon deposit at the fretting contact could only be formed by the tribodegradation-induced cracking of oil, which occurs at 500◦ C [9]. The presence of carbon-rich tribolayer indicates that the temperature generated locally at the fretting contact in oil lubrication could be around or above 500◦ C. Thus, local heating and subsequent temperature rise in the contact area is assumed to be the major cause for tribodegradation. The carbon-rich layer serves as a lubricating third body, reducing the friction and wear of the investigated tribosystems. The same carbon deposit was observed on the surfaces of the sialon–TiB2 , the ZrO2 –TiB2 , and the TiB2 -based
cermet composite as well as the steel counterbodies (not shown) when fretted in paraffin oil. The experimental results, presented in this work, are quite significant for the selection of a suitable binder phase to develop TiB2 -based materials with improved fretting wear resistance against steel in the lubricating media, distilled water, and paraffin oil. Bulk TiB2 phase from the flat oxidizes during the fretting wear under the water lubricating medium and tribochemically formed TiO2 is found to transfer onto the tribochemical iron oxide layer. Both the sialon and intermetallic Ni3 (Al, Ti) binder are also found to be oxidized and incorporated into the tribolayer on steel via tribochemical reactions. On the other hand, zirconia compared to the other binders is not found to transfer onto worn steel surface. Therefore, zirconia is assessed as the optimum binder for TiB2 -containing ceramics, which will offer the best fretting wear resistance against steel. It should be noted here that the amount of zirconia phase should be optimized, as with increasing TiB2 , the fretting wear rate of the tribosystem
640
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 8. Overview of the worn surface on the TiB2 monolith (a) and steel ball (b) after fretting in paraffin oil with the details of the tribochemical layer on the TiB2 material (c). The EDS spectrum (d) was taken from the tribolayer, as indicated by the arrow in (c). The fretting direction is indicated by a doubly pointed arrow.
would increase through more tribochemical reactions and transfer of the TiB2 phase onto steel.
4. Conclusions 1. The sialon composite/steel combination showed a higher friction coefficient (0.48) than the other material combinations (COF = 0.35–0.38) during the fretting tests against steel in water. Significant reduction in friction (COF around 0.08–0.12) when compared to that in water for all the investigated materials against steel is observed during the fretting tests in paraffin oil. Thus, liquid paraffin is found to be the more effective lubricant than water in reducing friction.
2. Under water lubrication, sialon–TiB2 /100Cr6 grade steel couple is found to have the highest fretting wear rate, whereas the zirconia composite/steel combination exhibits the best fretting wear resistance among the investigated fretting couples. Wear data measured on the flats do not follow any clear relationship with the mechanical properties. The fretting wear of the flat materials after testing in paraffin oil however is too low to be measured with a standard laser profilometer. 3. Tribochemical reactions along with abrasion are the major mechanisms for fretting wear of investigated materials against bearing grade steel in water. Both the sialon and intermetallic binder have been observed to get oxidized and transferred to the steel counterbody. TiB2 phase in all the investigated materials is found to oxidize during the fretting process and incorporated on the steel tribolayer.
B. Basu et al. / Wear 250 (2001) 631–641
Zirconia, on the other hand, has not been found to transfer on the steel counterbody. This along with mild abrasion marks on a smooth worn surface corresponds well with the lowest fretting wear rate of ZrO2 –TiB2 (70/30) material. 4. The worn surfaces on all the investigated flats and balls are found to be fully covered by the adherent carbon-rich (graphite) tribochemical lubricating layer after the fretting tests in paraffin oil lubrication. Tribodegradation of the paraffin oil is found to be the major source for the carbon layer deposition. This observation corresponds well with the significantly low friction and wear of the investigated tribosystems. Acknowledgements This work was supported by the Brite-Euram programme of the Commission of the European Communities under project contract no. BRPR-CT96-0304. The authors would like to thank the University of Warwick, UK, and Centro de Estudios e Investigaciones Técnicas de Guipúzcoa (CEIT), San Sebastian, Spain, for the supply of the sialon–TiB2 composites and TiB2 -based cermets, respectively. B. Basu thanks the Research Council of the Katholieke Universiteit Leuven in Belgium for a research fellowship. The authors also acknowledge the reviewers for the critical comments. References [1] W.M. Rainforth, Ceram. Int. 22 (1996) 365–372. [2] R. Telle, Boride and carbide ceramics, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology, Vol. 11:
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17]
[18] [19] [20] [21] [22] [23]
641
Structure and Properties of Ceramics, VCH, Weinheim, 1994, pp. 175–266. K.F. Dufrane, J. Am. Ceram. Soc. 72 (4) (1989) 691–695. S.M. Hsu, M.C. Shen, Wear 200 (1996) 154–175. S.W. Lee, S.M. Hsu, M.C. Shen, J. Am. Ceram. Soc. 76 (8) (1993) 1937–1947. J.D. Oscar Barceinas-Sanchez, W.M. Rainforth, J. Am. Ceram. Soc. 82 (6) (1999) 1483–1491. B. Basu, R.G. Vitchev, J. Vleugels, J.-P. Celis, O. Van Der Biest, Acta Mater. 48 (2000) 2461–2471. H. Liu, Q. Xue, Wear 201 (1996) 51–57. M. Kalin, J. Vizintin, S. Novak, G. Drazic, Wear 210 (1997) 27–38. M.F. Wani, B. Prakash, P.K. Das, S.S. Raza, J. Mukerji, Am. Ceram. Soc. Bull. 76 (1997) 65–69. A.H. Jones, R.S. Dodeboe, M.H. Lewis, J. Eur. Ceram. Soc. 21 (7) (2001) 969–980. F. Castro, I. Iturriza, J. Mater. Sci. Lett. 9 (1990) 600. B. Basu, J. Vleugels, O. Van Der Biest, A novel route to engineer the toughness of Y-TZP ceramics, J. Eur. Ceram. Soc., submitted for publication. H. Mohrbacher, J.P. Celis, J.R. Roos, Tribol. Int. 28 (5) (1995) 269– 278. P.Q. Campbell, J.P. Celis, J.R. Roos, O. Van Der Biest, Wear 174 (1994) 47–56. D. Klaffke, Tribol. Int. 22 (2) (1989) 89. B. Basu, J. Vleugels, K.C. Hari Kumar R.G. Vitchev, O. Van Der Biest, Unlubricated fretting wear of TiB2 containing composites against bearing steel, Metallurg. Mater. Trans., submitted for publication. T.E. Fischer, H. Tomizawa, Wear 105 (1985) 29–45. J. Takadoum, H.H. Bennani, D. Mairey, J. Eur. Ceram. Soc. 18 (1998) 553–556. J. Vleugels, O. Van Der Biest, Wear 225–229 (1999) 285–294. A. Tampieri, A. Bellosi, J. Mater. Sci. 28 (1993) 649–653. A. Kulpa, T. Troczynski, J. Am. Ceram. Soc. 79 (1996) 518–520. I.L. Singer, MRS Bull. 6 (1998) 37–40.
Fretting wear behavior of TiB2 -based materials against bearing steel under water and oil lubrication B. Basu, J. Vleugels, O. Van Der Biest∗ Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, W. De Croylaan 2, B-3001 Leuven (Heverlee), Belgium
Abstract Lubricated fretting tests in water and paraffin oil were performed with a monolithic TiB2 , a TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder, a sialon–TiB2 (60/40) composite and a ZrO2 –TiB2 (70/30) composite against ball bearing grade steel. Based on the measured friction and wear data, the ranking of the investigated fretting couples was evaluated. Furthermore, the morphological investigations of the worn surfaces and transfer layers are carried out and the wear mechanisms for the investigated friction couples are elucidated. While fretting in water, experiments revealed that tribochemical reactions, coupled with mild abrasion, played a major role in the wear behavior of the studied material combinations. ZrO2 –TiB2 (70/30)/steel wear couple has been found to have the highest fretting wear resistance among the different tribocouples under water lubrication. Under oil lubrication, extensive cracking of the paraffin oil at the fretting contacts, caused by tribodegradation, leads to the deposition of a carbon-rich lubricating layer, which significantly reduced friction and wear of all the investigated tribosystems. © 2001 Elsevier Science B.V. All rights reserved. Keywords: TiB2 ; Lubrication; Fretting wear; Tribochemical wear
1. Introduction Ceramics are a promising class of advanced materials, which have a tremendous potential for tribological applications. During the last few decades, much attention has been paid to investigate the wear and friction characteristics of several engineering ceramics [1]. Due to its high hardness (around 25 GPa), TiB2 is considered to be a promising material for tribological applications [2]. The poor sinterability and rather low toughness however restricts the use of the monolithic TiB2 in engineering applications. Different binders are used to fabricate the TiB2 -based technical ceramics. In the present work, the wear behavior of a monolithic TiB2 , a sialon-based 40 vol.% TiB2 composite, a ZrO2 -based 30 vol.% TiB2 composite, and a TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder is investigated. The proposed applications of the investigated materials include ball valves as pump components and grinding quills for high-speed grinding operations, etc. In these applications, fretting wear seems to cause a considerable loss in the functionality of these materials. The unlubricated wear performance of several advanced ceramics (e.g. zirconia, SiC, silicon nitrides, etc.) demonstrated the need for lubrication ∗ Corresponding author. Tel.: +32-16-32-1264; fax: +32-16-32-1992. E-mail address: [email protected] (O. Van Der Biest).
at the tribocontacts for the successful application as structural parts [3]. In this perspective, the present paper reports the influence of different lubrication (distilled water and paraffin oil) on the tribological behavior of the TiB2 -based ceramics and cermets when fretted against ball bearing steel. Because of its tremendous engineering importance, steel is selected as the counterbody material. The influence of different lubricants (water, paraffin oil) on the wear behavior of a range of ceramics including alumina, SiC, yttria-doped zirconia and Si3 N4 have been compared with that under dry sliding conditions [4]. Based on the results, wear maps of these materials in different environments have been established and the transitions in wear behavior as a function of testing parameters (load and sliding speed) are discussed. The same group of researchers studied the mechanism of sliding wear of self-mated yttria-stabilized tetragonal zirconia ceramics (Y-TZP) ceramic in different lubricating media [5]. Oscar Barceinas-Sanchez and Rainforth recently investigated the sliding wear of a 3Y-TZP ceramic against Mg-PSZ in distilled water and dry conditions [6]. Water was found to provide modest lubrication, lowering the sliding wear rate by a factor of three. The role of humidity on the fretting wear of self-mated Y-TZP was recently investigated in our laboratory [7]. Liu and Xue [8] constructed a “wear map” for a zirconia/steel couple sliding in water. In another study, Kalin et al. [9] carried out an extensive investigation to understand the
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 6 7 0 - 6
632
B. Basu et al. / Wear 250 (2001) 631–641
fretting wear mechanisms of silicon nitride ceramics against construction grade steel under water and formulated oil environments. It was observed that the presence of an antiwear phosphorus additive in the formulated oil provided more chemical protection to the worn surfaces compared to the purified paraffin oil. Lubricated wear tests in engine oil revealed a reduction in the wear rate of a Si3 N4 –TiC/steel friction couple by four orders of magnitude as compared to that in dry sliding conditions [10]. It should be noted here that the majority of the published work cited above is based on pin-on-disk or ball-on-plate tribometers under unidirectional sliding mode. Very few literature reports, for example [9], focus on the fretting wear (linear reciprocatory displacement sliding) behavior of ceramics in a lubricating medium. It can be pointed out here that the wear of a given material combination depends on many influential factors like the contact configuration, the mechanical properties (hardness, toughness), testing parameters (normal load, sliding speed), microstructure (grain size, porosity, etc.), the interaction with surrounding atmosphere (relative humidity, water or other lubricants), etc. According to the best of the authors’ knowledge, no fretting tests on TiB2 -based ceramics against steel in lubricating media are reported in the literature. In the present study, the lubricated wear behavior of TiB2 -containing materials fabricated with different binders (Y-TZP, sialon, and intermetallic) is investigated. The investigated materials including the TiB2 monolith have a range of properties: hardness ranging from 13 to 21 GPa, fracture toughness from 5 to 10 MPa m1/2 , and elastic modulus from 258 to 500 GPa. The influence of physico-chemical and mechanical properties of the different materials on the wear behavior, when fretted against steel under water and oil lubrication will be elucidated. This will also assess the suitability of the investigated binders in terms of providing the optimum fretting wear resistance of TiB2 materials.
2. Materials and experimental procedure 2.1. Materials The mechanical properties of the materials used in the present work are listed in Table 1. Commercial bearing grade
steel balls (DIN 100Cr6 grade, Fritsch, Germany), 10 mm diameter with mirror finished surfaces (surface roughness of 0.02 m, data from the supplier) were used as counterbody materials. As provided by the supplier, the nominal composition (wt.%) of the steel ball includes C (2.1), Cr (12.0), Si (0.3), Mn (0.3), and rest Fe. The TiB2 monolith was processed from the finest ESK grade TiB2 with 5 vol.% SiC (grade 059S, Superior Graphite Co.) as sinter additive. According to the supplier, the Fischer particle size of the ESK grade TiB2 powder is <0.5 m and nonmetallic impurities include C, O, and N with their individual maximum amount of 0.5, 2.5, and 1 wt.%, respectively. Hot pressing at 1900◦ C for 4 h in vacuum (≈0.1 Pa) under a mechanical load of 28 MPa resulted in a 95% theoretically dense material. The ZrO2 -based composite with 30 vol.% TiB2 was obtained by hot pressing of a ZrO2 and TiB2 (H.C. Starck grade F) powder mixture. The ZrO2 matrix was processed from a powder mixture of 3 mol% yttria co-precipitated (Tosoh grade TZ-3Y) and monoclinic (Tosoh grade TZ-O) ZrO2 powders with an overall yttria content of 2.5 mol%. According to the supplier, the Fischer particle size of the Starck grade F TiB2 powder is around 0.9 m and nonmetallic impurities include C, O, and N with their individual maximum amount of 0.25, 2.0, and 0.25 wt.%, respectively, metallic impurity of Fe with maximum of 0.25 wt.%, and other metallic impurities of maximum 0.2 wt.%. Full density was obtained by hot pressing at 1450◦ C for 1 h in vacuum (≈0.1 Pa) under a mechanical load of 28 MPa. The sialon-based composite with 40 vol.% TiB2 (H.C. Starck grade F) was obtained by conventional hot pressing at 1700◦ C for 10 min under 30 MPa N2 pressure. The substitution level (z) in the Si6−z Alz Oz N8−z was 0.3. More information on the processing and properties of these composites is given elsewhere [11]. The TiB2 -based cermet with 16 vol.% Ni3 (Al, Ti) binder was processed by the sinter-HIP (Hot Isostatic Pressing) route. TiB2 powder used for the cermet production was from H.C. Starck (grade F). The details of the fabrication and properties can be found elsewhere [12]. Representative SEM micrographs of the microstructures of the investigated materials are shown in Fig. 1. In the monolithic TiB2 material, the grey particles are the SiC grains, added as sinter additive, whereas the black phase represents the residual 3–4% porosity (see Fig. 1a). In
Table 1 Mechanical properties of the ceramics used in the present investigationa Flat material
HV10 (GPa)
Monolithic TiB2 TiB2 -based cermet (84/16) Sialon–TiB2 (60/40) ZrO2 –TiB2 (70/30)
21.3 16.1 16.6 13.0
Steel counterbody (10 mm diameter ball) DIN 100Cr6 grade a
± ± ± ±
0.7 0.4 0.3 0.2
7.8 ± 0.1
The counterbody data are supplied by the commercial supplier.
KIc (10 kg) (MPa m1/2 ) 5.6 9.5 6.2 9.7
± ± ± ±
E (GPa)
0.4 0.7 0.4 0.6
500 476 365 258
20 ± 1.0
210
B. Basu et al. / Wear 250 (2001) 631–641
633
Fig. 1. Representative microstructures of the investigated materials: (a) monolithic TiB2 ; (b) TiB2 –Ni3 (Al, Ti) (84/16); (c) sialon–TiB2 (60/40); (d) ZrO2 –TiB2 (70/30). Detailed description of the phase assemblage is presented in Section 2.
the TiB2 -based cermet material, coarse boride particles (4–5 m, grey color) are dispersed in an intermetallic Ni3 (Al, Ti) binder, bright contrast (see Fig. 1b). The black particles in the cermet microstructure are Al2 O3 phase. The matrix phase of the sialon-based TiB2 composite (dark contrast in Fig. 1c) consists of both ␣-Si3 N4 and -sialon with a low substitution (z) value. The intergranular phase contains yttria. The boride particles are in bright contrast. The different phases that can be distinguished in the zirconia-based composite (see Fig. 1d) on the back-scattered electron micrographs are: ZrO2 (white), TiB2 (grey), and Al2 O3 (black). The presence of alumina is due to the use of alumina milling balls during powder mixing. XRD investigations revealed the presence of a small amount of monoclinic ZrO2 in the Y-TZP-based composite materials. A detailed microstructural characterization and more information on the mechanical properties of the zirconia composite are reported elsewhere [13]. As observed in the microstructures
of the different composites, the homogeneously dispersed TiB2 particles are irregularly shaped. 2.2. Fretting tests The fretting experiments have been performed on a computer-controlled tribometer under ambient temperature (25◦ C) and humidity (50–55% RH) conditions. The details of the experimental set-up can be found elsewhere [14]. The 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 reciprocatory relative displacement sliding). The flat samples are ground and polished until they have an average surface roughness (Ra ) of 0.05 m. The nominal dimensions of the flat sample include a length of 20 mm, a width of 5 mm, and a height of 3 mm. Prior to the fretting experiment, the materials are ultrasonically cleaned in acetone. Two lubricants are used
634
B. Basu et al. / Wear 250 (2001) 631–641
in the present work: laboratory distilled water and commercial paraffin oil (<0.02% residual water and viscosity of 345–355 P at 100 F, Fischer Scientific). The flat sample is mounted inside a small stainless steel vessel filled with the lubricant, so that the top of the sample is fully immersed in the liquid lubricant. The whole set-up is mounted on a translation table, which oscillates at the required displacement with desired frequency by means of a stepping motor. The displacement of the sample is monitored by an inductive displacement transducer, and the friction force is measured with a piezoelectric transducer attached to the holder that supports the counterbody. The friction coefficient is calculated from the on-line measured tangential force. During the test, fretting loops are recorded at a predefined number of cycles. A fretting loop gives the evolution of the tangential force as a function of the displacement amplitude during each cycle. The coefficient of friction (COF) is further evaluated from the average of the two plateau values of the tangential force in the fretting loop, as described elsewhere [15]. For comparative reasons, all the tests are performed with identical fretting parameters: a normal load of 8 N, a total linear displacement of 200 m, a frequency of 10 Hz, and a duration of 100,000 cycles. 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 Met optical microscope (Nomarski contrast) and a scanning electron microscope (Philips XL-30 FEG) equipped with an EDS system for compositional
analysis. In the EDS analysis, light elements are detected with an acceleration voltage of 2–5 kV, while the heavier elements are identified with an acceleration voltage of 20–25 kV. A Rodenstock laser profilometer (RM600X/Y-100) was used to evaluate the geometry and wear volumes of the fretting wear tracks on the flat samples. Since profilometer measurements on the ball wear scar are extremely difficult, the wear scar diameters (both in the sliding and in the transverse directions) are measured from optical micrographs, and wear volumes are computed according to the equation proposed by Klaffke [16]: V = 64π d 3 /a, with ‘d’ the average diameter of the wear scar and ‘a’ the ball radius.
3. Results and discussion 3.1. Water lubrication 3.1.1. Friction and wear data The influence of water and paraffin oil lubrication on the frictional behavior of the investigated TiB2 -based ceramic composites against commercial ball bearing grade steel are shown in Fig. 2a and b, respectively. The friction data obtained in paraffin oil will be discussed in Section 3.2.1. Under water lubrication, COF of the sialon–TiB2 /steel couple, unlike the other friction couples, increases rapidly (see Fig. 2a), showing a maximum COF of 0.58 and then decreases during the running-in-period (first 20,000 cycles), and finally stabilized at the steady state level (COF = 0.48). It is also noted that the sialon–TiB2 /steel couple has the highest friction coefficient of 0.48, whereas the steady state COF of the other investigated friction couples varied between 0.35 and 0.38.
Fig. 2. The evolution of COF of the TiB2 -based materials when fretted against construction grade steel balls under a load of 8 N with a frequency of 10 Hz and a displacement of 200 m under water (a) and paraffin oil (b) lubrication.
B. Basu et al. / Wear 250 (2001) 631–641
635
Fig. 3. The wear volumes of the TiB2 -based flat materials and construction grade steel counterbody when fretted in water (a) and the wear volume of steel balls under oil lubrication (b). The fretting conditions are the same as mentioned in Fig. 2.
The wear volume of the TiB2 -based flat materials fretted against 100Cr6 ball bearing grade steel under water lubrication is shown in Fig. 3a. The error bars represent the standard deviation of the wear data obtained from at least three fretting tests. Both the monolithic TiB2 and TiB2 -based cermet show comparable volumetric wear. The wear volume of the sialon-based composite is a factor of two higher than all the other investigated material combinations, whereas the wear volume of the zirconia composite is the lowest. The steel balls experienced a high wear loss during fretting in water, as revealed in the data presented in Fig. 3a. It should be noted here that the counterbody wear is found to be one order of magnitude higher than that of the flat. The volumetric wear of the steel counterbody follows the same trend as the wear of the flat material. The highest ball wear is observed after testing against the sialon composite, while the lowest with the zirconia composite. Considering the total wear of the tribocouples, it is clear that the sialon–TiB2 /100Cr6 grade steel couple is most prone to fretting wear, whereas the zirconia composite/steel combination exhibits the highest fretting wear resistance under water lubrication conditions. It should be mentioned here that the wear data, measured on the flats, do not show any clear relationship with the mechanical properties (see Table 1 and Fig. 3a). This indicates that tribomechanical wear does not play any dominant role in the present case. 3.1.2. Morphological investigation of the worn surfaces The worn surfaces in the ‘monolithic TiB2 /steel tribocouple’ after fretting in water are illustrated in Fig. 4. A tribolayer is found to adhere to the mild abrasive grooves on the TiB2 material, as shown in Fig. 4a. The presence of numerous cracks on the tribolayer shows its brittle, non-protective nature. EDS spectra acquired from such layer indicates the formation of iron oxides, Ti oxides or mixed (Fe, Ti) oxides (see Fig. 4b). Silicon from the silicon
carbide sinter aid was not detected on the worn surfaces. Considering the water lubricating conditions, the presence of hydroxides of Ti and/or Fe is also possible. The different oxidized species, as will be reported throughout this paper, can also exist in the hydroxide form under water lubricating condition. A tribolayer, adhered to the relatively deep abrasive groves, is observed on the steel counterbody (Fig. 4c). Closer look at Fig. 4c also shows the adherence of the tribochemical layer onto the worn steel surface. Elemental analysis indicated the presence of Fe, Ti, Cr, O in the tribolayer (see Fig. 4d). The presence of Ti on the worn steel surface can be explained in either of two ways. The first one is that TiB2 phase from the flat is oxidized during the fretting process and incorporated in a transfer layer (third body) between the mating couple. Another possibility is that TiB2 particles are spalled off from the flat and then transferred to the steel ball and finally oxidized. Both of these factors seem to be plausible. It can be mentioned here that recent XPS investigation in our laboratory revealed that the material transferred between the mating counterbodies are always oxidized in the monolithic TiB2 /steel tribocouple under unlubricated fretting conditions [17]. Following this, it is more probable that TiB2 is oxidized and incorporated in the iron oxide-rich tribolayer. Therefore, experimental observations indicate the occurrence of tribochemical reactions with the mutual transfer of material between the fretting couple and spalling of the tribochemical layer as the major wear mechanism of the monolithic TiB2 /steel fretting couple. The worn surface in the ‘TiB2 -based cermet/steel tribosystem’ after fretting in water is presented in Fig. 5. The fretted surface is characterized by the presence of strongly embedded thick wear debris particles, as shown in Fig. 5a. Mild abrasive scars are noted in the flat worn surface around the debris particles. EDS spectra obtained from the debris show a strong Ni peak along with peaks of
636
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 4. Worn surfaces and EDS spectra of the tribolayers on the TiB2 monolith ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. Numerous cracks could be readily observed in the tribolayer on the flat worn surface. EDS spectra ((b) and (d)) are acquired from the spots indicated by the arrows in (a) and (c). The arrow in (c) also indicates typical adherence of the tribochemical layer. The tribolayer next to it is predominantly iron oxide. The fretting direction is indicated by a doubly pointed arrow.
Fe, Ti, Cr, Al, and O (see Fig. 5b). The worn steel ball is observed to be covered by a tribolayer, as shown in Fig. 5c. The EDS spectrum of the tribolayer on the steel indicates the presence of Fe, Ni, Cr, and O (see Fig. 5d). Although, Ti is not recorded in the reported spectrum, Ti is found in other investigated locations on the worn steel ball. The fact that Fe is present on the flat indicates that iron oxide is transferred from the steel ball onto the flat. The presence of Ni on the worn steel surface indicates that NiO is dissolved in the iron oxide layer on the worn steel. The formation of NiO indicates the tribochemical oxidation of the intermetallic binder phase in the cermet during the fretting process. As-fretted surfaces in the ‘sialon–TiB2 /steel combination’ after fretting in water is shown in Fig. 6. A thin tribofilm is found to cover the flat worn surface (see Fig. 6a). Closer look at the flat worn surface reveals the presence of numerous microcracks in the tribolayer. This indicates the non-protective nature of the tribofilm. The compositional analysis of the tribofilm revealed the presence of Si, Ti, Fe, Cr, and O, as shown in Fig. 6b. The amount of iron oxide transferred from the steel ball is significantly smaller than that in the
previous cases, as evident from the fairly weak Fe peak. The strong Si peak in combination with the strong O peak suggests the formation of silica. Deep abrasive scars could be seen despite the presence of a tribochemical layer on the worn steel ball (see Fig. 6c). The EDS spectrum of the tribofilm reveals the presence of high amounts of Si and Ti (see Fig. 6d). No significant amounts of Fe and Cr were measured in the tribolayer. This indicates the transfer of the mixed and probably hydrated Si–Ti oxide layer on the worn surface of the steel ball. The compositional analysis, as described above thus shows the possible material transfer and tribochemical oxidation of the sialon binder phase and TiB2 onto steel counterbody. It is reported in the literature [4,9,18] that silica can get dissolved into water forming silicon hydroxide or hydrated silica under the water lubricating conditions at the tribocontact. When silicon nitrides slide in water, the tribochemical reaction is reported to be the dissolution of silica at the contacting surfaces with the formation of a lubricating tribolayer [19]. Thus, the higher wear loss of sialon composite/steel couple in water lubrication, as observed in the present case, could be linked to the mutual material transfer and the formation of a non-protective hydrated silica layer.
B. Basu et al. / Wear 250 (2001) 631–641
637
Fig. 5. Worn surfaces and EDS spectra of the tribolayers on the TiB2 -based cermet with Ni3 (Al, Ti) binder ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. EDS spectra are taken from the arrows indicated in the SEM micrographs. The arrow in (a) also indicates the embedding of the wear debris on a rather smooth flat worn surface. The arrow in (c) implicates localized spalling of the tribochemical layer. The fretting direction is indicated by a doubly pointed arrow.
Fig. 7 shows the surfaces in the ‘zirconia composite/steel’ wear couple after fretting in water. Red iron oxides are occasionally found to stick to the wear scars on the zirconia composite. The worn surface in the central part of the wear pit on the composite was extremely smooth, as shown in Fig. 7a, with iron oxide particles locally adhering to the TiB2 phase. Only in these locations, Fe was detected with EDS analysis (not shown). Mild abrasion marks due to the fretting process could be observed optically (not shown). The tribolayer on the steel ball was fragmented over the worn steel surface, as shown in Fig. 7b. Compositional analysis indicated that the tribolayer is a mixed oxide of Fe and Cr with a small amount of dissolved Ti from the ceramic (Fig. 7c). The experimental observations thus indicate that TiB2 from the flat oxidizes, and transferred onto the steel tribolayer. ZrO2 on the other hand was not observed to be transferred onto steel worn surface, as could be expected from literature [20]. The presence of rather low amount (30 vol.%) of TiB2 phase and the stability of ZrO2 against transfer to steel have resulted in limited tribochemical reactions, compared to that observed with the other investigated tribocouples. This, coupled with the mild abrasion marks on
the flat worn surface, corresponds well with the low fretting wear rate of the ZrO2 –TiB2 (70/30) composite against steel in water. 3.2. Oil lubrication 3.2.1. Friction and wear data In another set of experiments, the same TiB2 -based materials were fretted against ball bearing steel in paraffin oil under the same experimental conditions. When using liquid paraffin as lubricating medium, a drastic reduction in friction coefficient is observed for all the investigated fretting couples (see Fig. 2b). The steady state friction values for all the material combinations are in between 0.08 and 0.12. Comparing with the friction data observed under water lubrication (see Section 3.1.1), it can be stated that paraffin oil as compared to water is more efficient in reducing the friction of the investigated materials against construction steel. After fretting in oil and subsequent ultrasonic cleaning, the worn surfaces on the flats are observed to be very smooth with an adhering tribofilm. Wear volume measurements
638
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 6. Worn surfaces and EDS spectra of the tribolayers on the sialon–TiB2 composite ((a) and (b)) and steel ball ((c) and (d)) after fretting in water. The arrow in (c) indicates the adherence of the tribolayer on the worn steel surface. The worn surface around the adhering tribolayer in (c) is predominantly iron oxide. EDS spectra are taken from the arrows indicated in the SEM micrographs. The fretting direction is indicated by a doubly pointed arrow.
using a standard laser profilometer becomes impossible, as the average depth of the wear pit is found to be comparable to the roughness of the flat surface. However, the wear scars on the steel balls after fretting in oil could be observed in the optical microscope and the wear volume is evaluated from the geometry of the wear scar (see Fig. 3b). The steel counterbody shows a higher wear volume in contact with the TiB2 -based cermet. The wear volume of the steel ball fretted against the monolithic TiB2 and the sialon composite is comparable. The lowest wear is measured when fretting was performed against zirconia composite. It should be mentioned here that the volumetric wear of the steel balls fretted in oil is reduced by two orders of magnitude when compared to that under water lubrication. 3.2.2. Morphological investigation of the worn surfaces Fig. 8 illustrates the tribolayer formed on the worn surface of the TiB2 monolith and steel ball after fretting in liquid paraffin. The surface of the TiB2 material is hardly
worn and the abrasive scars are observed to be of the same depth as the polishing marks on the native surfaces. The worn surfaces on both flat and ball are covered by a thin adherent tribofilm, as shown in Fig. 8a and b. The details of the flat worn surface are shown in Fig. 8c. EDS analysis (see Fig. 8d) of the tribolayer on the flat showed a strong carbon peak along with an O peak. It is interesting to note here that the tribocontact under oil lubrication is not completely free from the access to oxygen, as revealed by the O peak. Additionally, EDS analysis showed the presence of Ti, B, and Fe on the worn surface. The amount of iron oxide is considered small. The presence of Ti and B peak along with an O peak indicates the oxidation of the TiB2 phase. Oxidation of bulk TiB2 has been reported to start at 600◦ C in an oxidizing atmosphere [21], whereas it has been revealed that the oxidation process of TiB2 in air starts even below 400◦ C, with the formation of TiBO3 [22]. However, under the prevailing mechanical stress conditions during the fretting tests, tribo-oxidation process can even start at much lower temperature as often reported in the literature [23]. The
B. Basu et al. / Wear 250 (2001) 631–641
639
Fig. 7. Worn surfaces of the ZrO2 –TiB2 composite (a) and steel ball (b) after fretting in water. Note that the iron oxide particles are locally adhered to TiB2 , as pointed by an arrow in (a). The EDS spectrum (c) was taken from the tribolayer on the steel ball, as indicated by the arrow in (b). The fretting direction is indicated by a doubly pointed arrow.
compositional analysis as reported above reveals that local heating at the fretting contact, as also will be discussed below, promoted the oxidation of the TiB2 phase in the oil lubricated condition. Since the microstructural analysis was carried out without a conductive layer for SEM observation, the compositional analysis strongly indicates the formation of a carbon-rich layer on the worn surface. The carbon deposit at the fretting contact could only be formed by the tribodegradation-induced cracking of oil, which occurs at 500◦ C [9]. The presence of carbon-rich tribolayer indicates that the temperature generated locally at the fretting contact in oil lubrication could be around or above 500◦ C. Thus, local heating and subsequent temperature rise in the contact area is assumed to be the major cause for tribodegradation. The carbon-rich layer serves as a lubricating third body, reducing the friction and wear of the investigated tribosystems. The same carbon deposit was observed on the surfaces of the sialon–TiB2 , the ZrO2 –TiB2 , and the TiB2 -based
cermet composite as well as the steel counterbodies (not shown) when fretted in paraffin oil. The experimental results, presented in this work, are quite significant for the selection of a suitable binder phase to develop TiB2 -based materials with improved fretting wear resistance against steel in the lubricating media, distilled water, and paraffin oil. Bulk TiB2 phase from the flat oxidizes during the fretting wear under the water lubricating medium and tribochemically formed TiO2 is found to transfer onto the tribochemical iron oxide layer. Both the sialon and intermetallic Ni3 (Al, Ti) binder are also found to be oxidized and incorporated into the tribolayer on steel via tribochemical reactions. On the other hand, zirconia compared to the other binders is not found to transfer onto worn steel surface. Therefore, zirconia is assessed as the optimum binder for TiB2 -containing ceramics, which will offer the best fretting wear resistance against steel. It should be noted here that the amount of zirconia phase should be optimized, as with increasing TiB2 , the fretting wear rate of the tribosystem
640
B. Basu et al. / Wear 250 (2001) 631–641
Fig. 8. Overview of the worn surface on the TiB2 monolith (a) and steel ball (b) after fretting in paraffin oil with the details of the tribochemical layer on the TiB2 material (c). The EDS spectrum (d) was taken from the tribolayer, as indicated by the arrow in (c). The fretting direction is indicated by a doubly pointed arrow.
would increase through more tribochemical reactions and transfer of the TiB2 phase onto steel.
4. Conclusions 1. The sialon composite/steel combination showed a higher friction coefficient (0.48) than the other material combinations (COF = 0.35–0.38) during the fretting tests against steel in water. Significant reduction in friction (COF around 0.08–0.12) when compared to that in water for all the investigated materials against steel is observed during the fretting tests in paraffin oil. Thus, liquid paraffin is found to be the more effective lubricant than water in reducing friction.
2. Under water lubrication, sialon–TiB2 /100Cr6 grade steel couple is found to have the highest fretting wear rate, whereas the zirconia composite/steel combination exhibits the best fretting wear resistance among the investigated fretting couples. Wear data measured on the flats do not follow any clear relationship with the mechanical properties. The fretting wear of the flat materials after testing in paraffin oil however is too low to be measured with a standard laser profilometer. 3. Tribochemical reactions along with abrasion are the major mechanisms for fretting wear of investigated materials against bearing grade steel in water. Both the sialon and intermetallic binder have been observed to get oxidized and transferred to the steel counterbody. TiB2 phase in all the investigated materials is found to oxidize during the fretting process and incorporated on the steel tribolayer.
B. Basu et al. / Wear 250 (2001) 631–641
Zirconia, on the other hand, has not been found to transfer on the steel counterbody. This along with mild abrasion marks on a smooth worn surface corresponds well with the lowest fretting wear rate of ZrO2 –TiB2 (70/30) material. 4. The worn surfaces on all the investigated flats and balls are found to be fully covered by the adherent carbon-rich (graphite) tribochemical lubricating layer after the fretting tests in paraffin oil lubrication. Tribodegradation of the paraffin oil is found to be the major source for the carbon layer deposition. This observation corresponds well with the significantly low friction and wear of the investigated tribosystems. Acknowledgements This work was supported by the Brite-Euram programme of the Commission of the European Communities under project contract no. BRPR-CT96-0304. The authors would like to thank the University of Warwick, UK, and Centro de Estudios e Investigaciones Técnicas de Guipúzcoa (CEIT), San Sebastian, Spain, for the supply of the sialon–TiB2 composites and TiB2 -based cermets, respectively. B. Basu thanks the Research Council of the Katholieke Universiteit Leuven in Belgium for a research fellowship. The authors also acknowledge the reviewers for the critical comments. References [1] W.M. Rainforth, Ceram. Int. 22 (1996) 365–372. [2] R. Telle, Boride and carbide ceramics, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology, Vol. 11:
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17]
[18] [19] [20] [21] [22] [23]
641
Structure and Properties of Ceramics, VCH, Weinheim, 1994, pp. 175–266. K.F. Dufrane, J. Am. Ceram. Soc. 72 (4) (1989) 691–695. S.M. Hsu, M.C. Shen, Wear 200 (1996) 154–175. S.W. Lee, S.M. Hsu, M.C. Shen, J. Am. Ceram. Soc. 76 (8) (1993) 1937–1947. J.D. Oscar Barceinas-Sanchez, W.M. Rainforth, J. Am. Ceram. Soc. 82 (6) (1999) 1483–1491. B. Basu, R.G. Vitchev, J. Vleugels, J.-P. Celis, O. Van Der Biest, Acta Mater. 48 (2000) 2461–2471. H. Liu, Q. Xue, Wear 201 (1996) 51–57. M. Kalin, J. Vizintin, S. Novak, G. Drazic, Wear 210 (1997) 27–38. M.F. Wani, B. Prakash, P.K. Das, S.S. Raza, J. Mukerji, Am. Ceram. Soc. Bull. 76 (1997) 65–69. A.H. Jones, R.S. Dodeboe, M.H. Lewis, J. Eur. Ceram. Soc. 21 (7) (2001) 969–980. F. Castro, I. Iturriza, J. Mater. Sci. Lett. 9 (1990) 600. B. Basu, J. Vleugels, O. Van Der Biest, A novel route to engineer the toughness of Y-TZP ceramics, J. Eur. Ceram. Soc., submitted for publication. H. Mohrbacher, J.P. Celis, J.R. Roos, Tribol. Int. 28 (5) (1995) 269– 278. P.Q. Campbell, J.P. Celis, J.R. Roos, O. Van Der Biest, Wear 174 (1994) 47–56. D. Klaffke, Tribol. Int. 22 (2) (1989) 89. B. Basu, J. Vleugels, K.C. Hari Kumar R.G. Vitchev, O. Van Der Biest, Unlubricated fretting wear of TiB2 containing composites against bearing steel, Metallurg. Mater. Trans., submitted for publication. T.E. Fischer, H. Tomizawa, Wear 105 (1985) 29–45. J. Takadoum, H.H. Bennani, D. Mairey, J. Eur. Ceram. Soc. 18 (1998) 553–556. J. Vleugels, O. Van Der Biest, Wear 225–229 (1999) 285–294. A. Tampieri, A. Bellosi, J. Mater. Sci. 28 (1993) 649–653. A. Kulpa, T. Troczynski, J. Am. Ceram. Soc. 79 (1996) 518–520. I.L. Singer, MRS Bull. 6 (1998) 37–40.