Wear 315 (2014) 11–16
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Wear behaviour of low IR-emission titanate ceramics under gross slip fretting conditions against steel and α-alumina Rolf Wäsche n, Gabriele Steinborn Federal Institute for Materials Research and Testing (BAM), 12200 Berlin, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 16 October 2013 Received in revised form 14 March 2014 Accepted 15 March 2014 Available online 21 March 2014
The tribological behavior of different low infrared (IR) emission ceramic materials from the titanate group has been investigated at room temperature under gross slip fretting conditions against 100Cr6 ball bearing steel and α-alumina as the counterface materials. For all material pairs, friction and wear depended largely on the relative humidity of the environment. When paired with steel the low IRemission ceramic disks showed higher wear resistance. Under dry conditions, almost no wear was found on the low IR-emission ceramic specimen. The high wear on the steel counter body is caused not only by tribo-oxidation, which is the main wear mechanism, but also by abrasion. In dry conditions a COF of 0.6 and in humid conditions a COF of 0.2 have been measured. When paired with α-alumina the results on wear are just opposite to those with steel counterbody and the low IR-emission ceramic materials show much lower wear resistance under all conditions. This is explained by the predominance of abrasion and the relatively large difference in hardness between the low IR-emission ceramic and α-alumina. In high humidity environments the results point to the formation of stable reaction layers causing lower friction and wear. & 2014 Elsevier B.V. All rights reserved.
Keywords: Gross slip fretting Ceramic wear Yttrium titanate Zirconium titanate Low IR-emission ceramics
1. Introduction In modern machinery, and especially in automotive engineering, effective heat management is often a necessary prerequisite for using low temperature components like electronic systems in direct proximity to components of heat generating systems, i.e. engine and exhaust pipe system. But also when heat losses need to be minimized, for instance when the heat is reused and therefore recuperated, the effective guidance of the heat ﬂow is a must. Thermal engineering in general uses different materials and concepts to optimize heat transport in high temperature environments. The heat ﬂow can be effectively inﬂuenced by materials with low heat conductivity (insulation) but also by materials with low IR-emissivity, so that relatively less heat is radiated from the surface of these materials. In applications where a low total emissivity of IR-radiation is needed, like automotive engines, a ceramic low IR-emission material applied as a coating could effectively reduce the heat loading of the environment and help to shield temperature sensitive components from overheating. It can also be used to increase the efﬁciency level of the engine or to apply new temperature sensitive light weight materials to further
Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.wear.2014.03.006 0043-1648/& 2014 Elsevier B.V. All rights reserved.
improve the overall efﬁciency of the car. The emissivity (ε) of a material is a measure of its capability to emit electromagnetic radiation from its surface. It is therefore deﬁned as the ratio of energy radiated by a material surface to the energy radiated by a black body at the same temperature. Principally, both metals and ceramics can show low emissivity of infrared radiation, but ceramic materials have the advantage that they are more stable than metals at high temperatures, especially in oxidizing environments. Yttrium titanate ceramic is a ceramic material with interesting thermophysical properties with low infrared emission coefﬁcients. It is therefore a candidate material for advanced heat ﬂow management systems. In an earlier paper  the infrared optical properties and the manufacturing process of the material has been described. Since the use of this material in automotive, but also in various other applications  may be prone to all kind of vibrations, the wear resistance especially against steel is important to know. We therefore describe the wear behavior of 3 different kinds of yttrium titanate ceramics with slightly varying composition and one specimen containing zirconium titanate in this paper. These samples were tested under gross slip fretting conditions. Steel has been chosen as a counterbody material because of its high potential as a possible housing material for the low IRemission ceramic material, for instance in the automotive engine environment. Additionally α-alumina has been used as a counter material to see if there is any contrast in the tribological response.
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Table 2 Parameters for tribological testing.
2.1. Materials Zirconium titanate and yttrium titanate ceramics with different yttria to titania ratios have been prepared by wet chemical ceramic processing . The specimens were then sintered under normal atmospheric conditions and subsequently hot isostatically pressed (HIP) in the temperature range from 1350 1C to 1650 1C. After HIPping the density was greater than 97.5% of the theoretical density. The sintered bodies were then shaped into disk like specimens. The surface was polished with surface roughness Ra between 0.0049 μm and 0.0066 μm for tribological testing. The Vickers micro hardness HV0.2 using Fischerscope H100 (Helmut Fischer GmbH, 71069 Sindelﬁngen, Germany) with a load of 200 g has been determined on these specimens. The different ceramic materials with relative density after HIPping, Vickers hardness and phase composition according to x-ray diffraction are compiled in Table 1. Vickers hardness values are the average values out of 5 individual measurements. The brackets show the maximum deviation from average. The phase analysis shows that only P65 is pure yttrium titanate. P18 and P4 contain minor amounts of TiO2 in form of rutile. G714 is zirconium titanate with minor amounts of rutile. The table contains also the measured hardness values for the two counterbody materials.
Steel 100Cr6 or α-Al2O3 Ceramic (see Table 1) None Laboratory air 0.2 mm 20 Hz 10 N 100,000 24 1C 5, 50, 99%
Fig. 1. Schematic of wear proﬁle taken at disk; pw and pd are length dimensions of width and depth of the wear scar.
Tribological testing was carried out at room temperature with a ball on disk contact geometry at a normal load of 10 N and a frequency of 20 Hz. Ball bearing steel 100Cr6 and high purity, high density α-Al2O3 were used as counterbody materials. Relative humidity was adjusted to values of 5, 50 or 99% and kept constant in each case during the entire test. The diameter of the steel and α-Al2O3 counterbodies (spheres) was 10 mm. The test parameters are compiled in Table 2. The tribometer is described in detail in [3,4]. After each test friction and wear quantities are derived from the stored values and the planimetric wear Wq is subsequently determined from proﬁlometry by measuring the wear scar proﬁle. The proﬁle is taken across the wear scar along d ? . The friction force FF is calculated from the area inside the friction loop hysteresis. This area corresponds to the friction energy FE dissipated during one cycle. FF ¼
Ball 10 mm diameter Disk Lubricating medium Ambient Stroke Frequency Normal force Fn Number of cycles Temperature Rel. humid.(dry, medium, moist)
The planimetric wear Wq as determined by a proﬁlogram (see Fig. 1) across the wear scar leads to the wear volume as outlined in detail in . The following Eqs. (2)–(5) apply for determining the wear volume on specimen 1 (ball) and specimen 2 (disk) and the coefﬁcient of wear by summation of these [3–5]. 2 2 π d ? djj 1 1 W V ball ¼ 0 ð2Þ R R 64 2
W V disk ¼
W V ¼ W Vball þ W V disk k¼
FE 2 Δx
Dividing this quantity by the normal load gives the coefﬁcient of friction COF. The mean coefﬁcient of friction represents the average value calculated from the data of the last half of the experiment. Table 1 Composition of ceramic and counter body materials. Sample code
Hardness Vickers [GPa]
XRD phase composition
99% after HIP 98% after HIP
11.2 (0.26) 10.82(0.36)
97.5% after HIP
98.2% after HIP
Steel 100Cr6 α-Alumina
Y2Ti2O7, cubic Y2Ti2O7, cubic TiO2, tetragonal Y2Ti2O7, cubic TiO2, tetragonal TiO2, tetragonal, ZrTiO4, orthorombic Martensite Corundum
π d ? djj 1 0 þ Δx W q 64 R
WV WV ¼ s F n 2 Δx n F n
ð3Þ ð4Þ ð5Þ
WVBall ¼wear volume at ball, WVdisk ¼wear volume at disk, WV ¼wear volume, d ? ¼diameter perpendicular to sliding direction, d|| ¼diameter parallel to sliding direction, R¼radius of the ball, R0 ¼radius of the ball after sliding and occurred wear, Wq ¼planimetric wear, Δx¼ stroke, n¼ number of cycles, Fn ¼ normal force, Wl ¼linear wear, K ¼coefﬁcient of wear or wear rate. The corresponding relationships are schematically shown in Fig. 1.
3. Results and discussion 3.1. 100Cr6 as Counter material Fig. 2 shows the coefﬁcient of friction of the 4 different low IRemission ceramics against steel 100Cr6. The friction shows a slight dependency on relative humidity and a maximum of the friction coefﬁcient at a relative humidity of 50% (medium humidity). The coefﬁcient of friction is in all cases lowest under high humidity conditions.
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Fig. 2. Coefﬁcient of friction for the systems ceramic/steel (ceramics according to Table 1).
Fig. 5. Development of coefﬁcient of friction and linear wear with number of cycles for low IR-emission ceramic P14 against steel in medium humid air.
Fig. 3. Coefﬁcient of wear for the systems ceramic/steel (ceramics according to Table 1).
Fig. 6. Development of coefﬁcient of friction and linear wear with number of cycles for low IR-emission ceramic G714 against steel in high humidity air.
Fig. 4. Wear volumes for ball and disk for the systems ceramic/steel.
Fig. 3 shows the coefﬁcient of system wear against the steel 100Cr6. The system wear is dependent on relative humidity and decreases with increasing humidity. This observation is also made in other steel/ceramic systems . Very low wear rates in alignment with low friction are observed with Zr-titanate containing G714 ceramic in high humidity conditions. Fig. 4 shows the wear volumes separately for ball and disk specimens. It may be seen clearly that in all cases under dry conditions the wear almost always occurs at the steel ball with a difference of several orders of magnitude. For medium and high humidity conditions there is still up to 1 order of magnitude difference in higher wear volume at the steel ball. In Fig. 5 a diagram containing the COF and the linear wear as measured online during the test is shown exemplarily for the pairing of P14 ceramic against steel in medium humidity. For the sake of brevity not all diagrams are presented here. This kind of diagram reveals more about the tribological process occurring during testing. Generally, the difference between the pairings at a given humidity condition is relatively small, in other words, the inﬂuence of the difference in ceramic composition is negligible. This can be understood because of the similar phase composition and manufacturing process of the low IR-emission ceramics.
However, Zr-titanate containing G714 ceramic shows signiﬁcantly lower values for friction as well as for wear in moist conditions. Furthermore, in almost all tests the friction level is relatively stable from the beginning. This fact points to a stable interface during the testing time. In some cases however, as may be seen in the case of Zr-titanate containing ceramic G714, a negative slope of the linear wear curve and a very stable friction curve at low COF values of 0.18 is observed, see Fig. 6. This observation points to the formation and growth of stable reaction layers in the sliding interface during the whole testing period. However, this does not mean that there is no wear. Since the linear wear signal is simply measuring the distance between the two mating surfaces, it reﬂects the wear conditions during sliding. If, as is observed in this case due to the formation of reaction layers, the distance between the mating surfaces increases, the slope becomes negative. The determination of the wear volume after sliding needs a thorough cleaning of the surface including the wear scar so that soft reaction layers are removed and hence wear is observed. 3.2. α-Al2O3 as counterbody material Fig. 7 shows the coefﬁcient of friction for the different pairings of low IR-emission ceramic tested against α-Al2O3 as counter material. For all 4 pairings the friction decreases with increasing
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Fig. 7. Coefﬁcient of friction for the systems ceramic/α-alumina (ceramics according to Table 1).
Fig. 10. Diagram of development of COF and linear wear with number of cycles for low IR-emission ceramic G714 against α-alumina in dry conditions.
Fig. 8. Coefﬁcient of wear for the systems ceramic/α-alumina (ceramics according to Table 1).
shows a discontinuity after about 72,000 test cycles. This is accompanied by an abrupt increase in the coefﬁcient of friction. In the subsequent test cycles the former stable situation was reestablished. Since both measuring signals are electronically independent signals, the observed discontinuity points to a sudden change in the interface during sliding which may be explained by the instability of tribo-chemically formed reaction layers.
3.3. Comparison of wear behavior
Fig. 9. Wear volumes for ball and disk for the systems ceramic/α-alumina (ceramics according to Table 1).
humidity. Both of the lowest friction values and wear rates are observed with Zr-titanate containing G714 ceramic with about 2 orders of magnitude difference in wear between dry and moist conditions. Comparing the wear behavior with that against steel, wear rates against α-alumina are generally lower by about one order of magnitude, see Fig. 8. Fig. 9 reveals that there is a signiﬁcant difference in the wear behavior with α-alumina as counter material. The wear under the most severe, dry conditions occurs almost only on the low IRemission ceramic and not on the α-alumina counter body in all four cases. This observation is also made for conditions with medium and moist humidity, although slightly less pronounced. The online recorded friction and linear wear values reveal that in dry and medium humidity conditions a distinctive running-in period with higher wear rate and a steady state period with low wear rate occurs. In moist conditions the linear wear is negative which means that stable reaction layers have formed during sliding. Fig. 10 shows the development of the coefﬁcient of friction as well as the linear wear signal of low IR-emission ceramic G714 against α-alumina under dry conditions. The linear wear signal
Comparing the two counterbody materials, the coefﬁcient of system wear is signiﬁcantly lower against alumina counterbody material by about a factor of 5. However, when considering the wear on disk and ball, differences are noticed and veriﬁed by SEM investigation. Against steel 100Cr6 under dry conditions the wear occurs almost only at the steel ball, whereas against α-alumina counter material the wear is observed almost only at the low IR-emission ceramic. The reason for this different wear behaviour may be seen in different wear mechanisms taking place. When sliding against steel, abrasion and tribo-oxidation are the main wear mechanisms with tribo-oxidation at the steel counterbody as the predominant wear mechanism, subsequently resulting in a much higher wear rate on the steel ball than on the ceramic disk [7,8]. Furthermore there are signiﬁcant differences in hardness. With regard to abrasion the steel is not as hard as the low IRemission ceramic and therefore the wear resistance of the steel is also lower. This conclusion is only valid for the low humidity conditions; with higher humidity reaction layer formation inﬂuences the wear mechanisms, shifting friction and wear to lower values but still with the main wear volume occurring at the steel counter body due to tribo-oxidation. This can be seen in Fig. 11, which shows the wear scar of steel mated against P4 ceramic at medium humidity. The wear scar is relatively large but only a small area in the middle shows signiﬁcant wear on the P4 ceramic. In the interface, some compacted debris from ceramic and oxidized steel is observed. The middle groove area is pointing to some severe wear at the low IR-emission ceramic material, probably due to the initial high contact pressure phase during the running in period. The area of the groove corresponds reasonably well with the calculated Hertzian initial contact area as estimated below. The otherwise sharp contour of the groove and the predominant ﬂat wear scar point to a relatively undamaged ceramic surface.
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Fig. 11. Wear scar of low IR-emission ceramic P4 mated against steel at medium humidity. The east to west dimension of the wear scar is about 1300 μm. Central groove area due to high initial contact pressure.
modulus has been determined to be 161 GPa . For yttrium titanate Young's modulus is published at 253–265 GPa . Against steel an average initial Hertzian contact pressure is estimated to be around 700 MPa and a maximum Hertzian contact pressure of about 1050 MPa. Due to the higher Young's modulus, against α-alumina the initial contact pressure tends to be slightly higher at 850 MPa on average and 1282 MPa maximum, respectively. The initial Hertzian contact area may be estimated to be 1.40 10 2 mm2, with a radius of the contact area of 0.068 mm for steel. The contact area against α-alumina is 1.17 10 2 mm2 with a radius of 0.061 mm. Since these values are of the same order of magnitude and, in the case of the steel counterbody, the hardness is relatively similar to the hardness values of the low IR-emission ceramics, the observed wear mechanisms should be that of brittle materials like ﬂake or powder formation and/or lateral cracking . This is exactly what is observed and exemplarily shown in the wear scar of G714 ceramic running against steel in medium humidity or against α-alumina in dry conditions. Fig. 13 shows this wear scar. Fig. 14 shows the bottom of the wear scar of G714 ceramic run against steel under medium humidity conditions in higher magniﬁcation. Besides a compacted debris layer there is clearly the obser vation of spalling of ceramic material due to the development
Fig. 12. Wear scar of P 4 mated against α-alumina at medium humidity. The east to west dimension of the wear scar is about 600 μm. Fig. 13. Wear scar of G714 ceramics run against steel in medium humidity conditions.
A different behavior is observed in Fig. 12, showing the wear scar of ceramic P4 mated to α-alumina counterbody under medium humidity conditions, which is typical for all titanate ceramics in this investigation. In the central area of the wear scar the formation of a tribo reaction layer is visible, composed mainly from P4 ceramic material. (The cracks in the layer are artifacts due to desorption of water and shrinking of the layer in the vacuum during SEM observation). The presence of the layer conﬁrms the former results of the higher wear volume on the P4 ceramic when mated to α-alumina. Interestingly the outer area in the wear scar around the deeply altered central part shows relatively little damage, pointing to the fact that the tribolayer formation might be dependent on the contact pressure. When sliding against α-alumina, abrasion may become the predominant wear mechanism and the hardness of the two different ceramic sliding partners becomes the important factor [9,10]. With a measured Vickers hardness of 15.8 GPa α-Al2O3 is much harder than the low IR-emission ceramics (see Table 1) and therefore wear is observed at the less hard titanate ceramic. For calculation of the initial Hertzian contact pressure, mechanical property data are published by He et al. . For titania Young's modulus is about 270 GPa. For Zirconium Titanate the Young's
Fig. 14. Crack formation and spalling in the wear scar of G714 ceramic against steel in medium humidity.
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caused by tribo-oxidation, which is the main wear mechanism, but also by abrasion. 4. With α-alumina as counterface material, however, the results change to the opposite. The low IR-emission ceramics show much lower wear resistance under all conditions. This is explained by the predominance of abrasion and the relatively large difference in hardness between the low IR-emission ceramics and α-alumina. 5. With α-alumina as counterface material, the results also point at the fact that, especially under moist conditions, soft and stable reaction layers are formed in the tribological interface. Under these conditions lower friction and wear is observed. 6. It is also noticed that the differences in friction and in wear behaviour within the group of low IR-emission ceramics are only marginal. This may be explained by the similar phase composition and mechanical properties.
Fig. 15. Wear scar of G714 run against α-alumina at low relative humidity.
Acknowledgement of subsurface cracks and subsequent pitting. This can be clearly seen in the formation of ﬂake like debris due to spalling in Fig. 14. In this regard, pitting formation and spalling of ﬂaky chunks of the ceramic surface are consequences of exceeding the local strength of the ceramic surface due to high Hertzian contact pressure and the formation of subsurface cracks. This view is supported by the relatively low fracture toughness of only 1 MPa m0.5  which relates to a rather high brittleness of the low IR-emission ceramic. In the wear scar of G714 against α-alumina the predominant, abrasive process becomes visible. The wear scar shows the relatively exact form of the α-alumina counterbody. The bottom of the wear scar is covered by ﬁne debris particles (Fig. 15) and a compaction of the debris is not clearly observed. This is conﬁrmed by the observation that against α-alumina counterbody material the wear is mainly on the low IR-emission ceramic side. Under humid conditions with both counterface materials reaction layer formation becomes a factor that inﬂuences the wear rates signiﬁcantly, lowering friction and reducing wear. These kinds of relatively soft layers are known to form under these conditions especially in case of oxide ceramics [14,15]. 4. Summary and conclusion Fretting friction and wear experiments were performed on both yttrium titanate and zirconium titanate using two counterface materials and over a range of relative humidity levels. The following conclusions can be drawn from this work: 1. The relative humidity had a signiﬁcant effect on the fretting wear when using both counterface materials. 2. The composition of the counterface material exerts a strong inﬂuence on both friction and wear. 3. With steel as a counterface material, the low IR-emission ceramic disks show higher wear resistance under the applied conditions. The high wear on the steel counterbody is mainly
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