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Effect of grain size on wear behavior in Y-TZP ceramics
Materials Science and Engineering A 527 (2010) 474–479
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of grain size on wear behavior in Y-TZP ceramics B. Venkata Manoj Kumar a , Won-Sik Kim a , Seong-Hyeon Hong a,∗ , Hung-Tak Bae b , Dae-Soon Lim b a Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM), Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Republic of Korea b Department of Materials Science and Engineering, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 August 2009 Accepted 10 October 2009
Keywords: Zirconia Wear Grain size Nanocrystalline
a b s t r a c t The influence of grain size on the wear behavior of zirconia ceramics was investigated when slided against steel in dry unlubricated conditions. Fully densified Y-TZP ceramics (≥98% of theoretical density) with a wide range of grain size from 75 to 1470 nm were developed using spark plasma sintering (SPS), microwave sintering (MS), and conventional pressureless sintering (CS) methods. SPS was effective in producing fully densified nanocrystalline zirconia at low temperature and short sintering time. The steady state coefficient of friction (COF) varied in a range of 0.35–0.44, but the wear rate reduced from 3.5 to 0.88 × 10−6 mm3 /Nm with decreasing grain size. The plastic deformation and microcracking lead to a mild wear for the nanocrystalline zirconia, whereas the delamination/spalling results in the increased wear for the coarser zirconia. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, significant research has been carried out to develop the ceramics of nano-sized grains with the expectation that the nanocrystalline ceramics exhibit improved mechanical properties, wear resistance, chemical inertness, corrosion resistance, and thermal insulating properties [1–5]. However, the difficulty in developing nanoceramics with full densification basically hinders their adoption in full-range applications [4,5]. Polycrystalline yttria-stabilized tetragonal zirconia ceramics (YTZP) are attractive for various structural applications due to their high hardness, high modulus, and moderate fracture toughness [6–8]. There has been a considerable work reported towards estimating the tribological potential of this important material [9–19]. The determined friction coefficients were relatively high for selfmated zirconia ceramics [10,11], whereas dissimilar materials gave friction coefficients ranging from 0.30 to 0.5 [12]. The wear debris particles are reported to form as a result of internal stresses produced by the mechanically/thermally induced phase transformation [15]. It was suggested to find out the appropriate testing conditions or material compositions that stabilize the tetragonal phase at the tribocontact in order to improve the wear behavior [12–14,17]. The dominant wear mechanisms were abrasion, cracking, plastic deformation, tribochemical wear, and surface fatigue [10–12,15–19]. The plastic deformation, microcracking, and transcrystalline shear fracture of individual grains were dominant for
∗ Corresponding author. Tel.: +82 2 880 6273; fax: +82 2 884 1413. E-mail address: [email protected] (S.-H. Hong). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.10.021
fine grain size (∼0.5 m), while the increased tendency for intercrysrtalline fracture was observed for coarser grain size (∼1.6 m) [12]. Based on the above literature, it can be mentioned that the wear behavior of zirconia ceramics in ambient conditions has been extensively studied in the coarse grain size regime (200–1000 nm), while its behavior in the nanometric scale (<100 nm) has not been fully understood. Moreover, most of the reported wear studies were conducted against zirconia or harder counterbody like SiC, alumina, diamond, etc., while performance of nanocrystalline zirconia against steel was less reported [19]. Considering the significance of steel in structural applications, it is important to estimate the tribological performance of zirconia ceramics when slided against steel. According to authors’ knowledge, very limited information is available on the wear behavior of nanocrystalline Y-TZP ceramics against steel. The high hardness (14 GPa) of nano-zirconia (∼90 nm) was attributed to the improved fretting wear resistance, and the material was primarily removed through intergranular fracture and grain pull-out mechanisms [20]. However, there is no systematic investigation that focuses on the wear properties of zirconia ceramics containing a wide range of grain size. The purpose of the present investigation is to understand the tribological properties of Y-TZP ceramics containing a wide range of grain size from nanometer (<100 nm) to micrometer scale (>1 m) when slided against steel. For this, fully densified Y-TZP ceramics with different average grain sizes were obtained using spark plasma sintering (SPS), microwave sintering (MS), and conventional pressureless sintering (CS) techniques. The friction and wear properties were evaluated against steel when subjected to unidirectional sliding in dry unlubricated testing conditions. The important con-
B.V.M. Kumar et al. / Materials Science and Engineering A 527 (2010) 474–479
tribution is to elucidate the significance of nano-grain size on the micromechanisms of material removal when compared with coarser grain size. 2. Experimental procedures 2.1. Processing Two kinds of commercially available 3 mol% yttria-stabilized ZrO2 powders (TZ-3Y and TZ-3YS, Tosoh Corporation, Japan) were used in this study. The BET surface areas of TZ-3Y and TZ-3YS were 17 and 9 m2 /g, respectively. For CS and MS, the powders were uniaxially pressed into pellets of 10 mm diameter and then cold isostatically pressed at 150 MPa. Sintering was carried out at 1400–1650 ◦ C, and the heating rate was 10 and 100 ◦ C/min for CS and MS, respectively. For SPS, ∼1.5 g of the powder was placed into a 10 mm graphite die and an electric current of ∼1000 A was applied under a pressure of ∼30 MPa in N2 flowing atmosphere. The heating rate was 100 ◦ C/min, and the sintering temperature ranged from 1150 to 1200 ◦ C. The detailed sintering conditions are given in Table 1. The apparent density of the sintered specimens was measured using the Archemedes method in water. Microstructure of the samples was examined by field emission scanning electron microscope (FESEM 7401F, JEOL, Japan) after thermal etching. The average grain size was estimated using a conventional linear intercept method. The elastic modulus (E) was determined by an ultrasonic pulseecho tester (Tektronix TDS 220, Panametrics, Model 5800, Korea). Hardness was measured using Vickers indentation on the polished surfaces at 20 kgf loading for 15 s, and fracture toughness was estimated from the crack length measurement [21]. 2.2. Wear testing A unidirectional ball-on-disc tribometer was used to understand the friction and wear characteristics of sintered specimens in ambient conditions (20 ± 5 ◦ C and 40 ± 10% RH). The details of the tribometer can be found elsewhere [22]. Spherical steel balls (bearing grade; SAE 52100) of 6.35 mm diameter were used as
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the counterbody materials. Both steel ball and zirconia disks were cleaned with acetone prior to wear testing. In order to focus on the influence of material property (grain size) on wear of zirconia, the experimental parameters such as sliding speed, sliding duration, and load were kept constant in this study. Steel balls were fixed in the ball holder so as to make a track radius of 6 mm from the central axis and the tests were conducted at a fixed rotational speed of 2000 rpm for 15 min. Such a combination resulted in a linear sliding speed of 0.63 m/s and a total sliding distance of 565.2 m. The sliding tests were carried out at a 5 N load. During the test runs, frictional forces were recorded using an electronic sensor to generate on-line coefficient of friction (COF) data. Each experiment was repeated at least three times and the average values are reported. In order to understand the wear mechanism and to determine the track width, a detailed microstructural characterization of the as-worn surfaces was conducted using SEM (JSM 5600, JEOL, Japan) or FE-SEM. Surface profiles of the worn surfaces were acquired using an alpha-step profilometer (Model No.: 500, TENCOR, Germany) to measure the depth of the wear tracks. The wear track width and depth were further used in computing the wear volume (and subsequently wear rate) according to following relations. wear volume (mm3 )= 2×track radius×track width × wear depth (1)
wear rate (mm3 /Nm) =
wear volume normal load × sliding distance
(2)
3. Results and discussion The apparent density and average grain size of the developed Y-TZP ceramics are given in Table 1, and the representative microstructures are shown in Fig. 1. A nearly full densification (≥98% of theoretical density) was successfully achieved by three different sintering techniques. All the sintered specimens exhibited equiaxed grain microstructures and the average grain size varied over a wide range from 75 to 1470 nm. Consistent with the previous report [20], SPS was effective in reducing sintering temperature,
Fig. 1. FE-SEM micrographs of polished surfaces of developed Y-TZP ceramics: (A) SPS, 1150 ◦ C, 10 min; (B) MS, 1500 ◦ C, 10 min; (C) CS, 1600 ◦ C, 2 h; (D) CS, 1600 ◦ C, 60 h.
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Table 1 Sintering conditions, apparent density, grain size, and elastic modulus of the developed Y-TZP ceramics. Sintering conditions ◦
SPS 1150 C, 10 min SPS 1200 ◦ C, 5 min MS 1400 ◦ C, 10 min CS 1400 ◦ C, 2 h CS 1450 ◦ C, 2 h CS 1500 ◦ C, 2 h MS 1500 ◦ C, 10 min CS 1550 ◦ C, 2 h CS 1600 ◦ C, 2 h CS 1650 ◦ C, 2 h CS 1550 ◦ C, 60 h CS 1600 ◦ C, 60 h
Apparent density (cm3 /g) 5.97 6.02 5.99 6.04 6.05 6.05 6.07 6.08 6.10 6.10 6.09 5.98
Relative density (%)
Grain size (nm)
Elastic modulus (GPa)
98 99 98 99 99 99 99 99 100 100 100 98
75 98 165 205 235 300 318 375 605 705 1100 1470
200 202 208 209 207 215 213 214 215 217 216 215
sintering time, and grain size. Nanocrystalline Y-TZP ceramics with average grain size of ∼75 nm were obtained at 1150 ◦ C for 10 min by SPS. CS samples revealed an enormous grain growth with an average grain size varying from 200 to 1470 nm. The elastic modulus of the developed zirconia ceramics in the present study varied in the range of 200–217 GPa. Fig. 2 shows the variation of Vickers hardness and indentation fracture toughness with grain size. It is evident that the hardness slightly improved, whereas fracture toughness significantly reduced with decreasing grain size. A maximum hardness of 12.7 GPa was achieved in SPS sample with 75 nm grain size, while a minimum hardness of ∼11 GPa was achieved in CS samples with the largest grain size (1470 nm). The fracture toughness reduced from 9 MPa m1/2 for 1100 nm grain size to 4 MPa m1/2 for 700 nm grain size, and the variation was negligible with further reduction to 75 nm grain size. The reason for the sharp reduction of fracture toughness in the range of 700–1100 nm grain size is not well understood at this stage. It can be noted that a hardness of 12–13 GPa and a fracture toughness of 5–10 MPa m1/2 are commonly reported for the Y-TZP ceramics [6,23]. The frictional behavior of Y-TZP ceramics/steel couple was assessed on the basis of the continuous measurement of the frictional force during the test run. All samples exhibited almost identical evolution of COF with time (Fig. 3(A)). The general observation is that COF increased to ∼0.40 during initial 100 s (running-in-period) and thereafter maintained a steady state. It is believed that surface asperity deformation/fracture results in the formation of hard debris particles in the contact. The continuous process of formation and subsequent removal of debris during sliding give rise to the fluctuations in the friction at interface. However, minimal fluctuations in the present study suggest no difference in the rates of formation and removal of debris. The average steady state COF varied in a narrow range from 0.35 to 0.44 with the inves-
tigated range of grain size (Fig. 3(B)). Therefore, the influence of grain size on the frictional behavior of Y-TZP ceramics is said to be minimal. The variation of the calculated wear rates as a function of grain size is shown in Fig. 4. The wear rate was in the order of 10−6 mm3 /Nm for the coarser samples and decreased to 10−7 mm3 /Nm for the specimens with grain size below 200 nm. A minimum wear rate of 8.8 × 10−7 mm3 /nm was measured for the 75 nm grain-sized sample, while a maximum wear rate of 3.5 × 10−6 mm3 /Nm recorded for the 1470 nm grain-sized specimen. The reported wear rate in Y-TZP ceramics varied with the counterbody and it was in the order of 10−5 , 10−6 , and 10−4 –10−5 mm3 /Nm when slided against diamond [17], SiC [12], and Y-TZP [11], respectively. The observed shifting in the wear rate to the lower value in the present study necessarily indicates the influence of softer steel counterbody [19]. In addition, when Y-
Fig. 2. Variation of Vickers hardness and indentation fracture toughness with grain size in the developed Y-TZP ceramics.
Fig. 3. (A) Typical plot for the evolution of coefficient of friction (COF) with time and (B) average COF with grain size in the developed Y-TZP ceramics.
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Fig. 4. Wear rate with grain size in the developed Y-TZP ceramics.
TZP slided against SiC or diamond, the Hall–Petch relation of wear resistance (inverse of wear rate) with grain size was maintained (with the exponent of −0.5) in the specimens with grain size of 0.2–0.6 m, while wear resistance was proportional to reciprocal (exponent −1.0) for grain size larger than 0.8 m [12,17]. From the experimental data in the present investigation of sliding of zirconia against steel, exponents of grain size can be estimated as −1.0 for YTZP ceramics with grain size less than 200 nm and −0.2 for coarser samples. Such a deviation in the wear behavior can be understood on the basis of steel counterbody wear characteristics. It is well known that soft steel ball is readily abraded during sliding against hard zirconia counterface. Further sliding causes transfer of debris particles from steel ball onto flat zirconia surface (inset in Fig. 5(D)) eventually affecting the wear behavior of the system [19]. Thus, the Hall–Petch relation of wear with the grain size was not maintained in the present system. On the other hand, the significant reduction of wear rate for the samples with grain size below 200 nm can be related to the role of processing route. Particularly, SPS used for fabricating samples of 75 nm grain size and 98 nm grain size (Table 1) probably has the positive influence on the grain bound-
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ary characteristics with regard to wear. Such a postulation is also in agreement with the higher hardness values measured for the respective specimens (Fig. 2). This observation for nano-grain size along with the behavior in the entire range of grain sizes of zirconia suggests the dominant influence of hardness than fracture toughness when slided against steel. The influence of grain size on the wear mechanisms of zirconia ceramics is presented in Figs. 5–7. Fig. 5 represents the SEM images of worn surfaces of samples with nano-grain size. The surfaces are mainly characterized by the mild wear with surface polishing, plastic shear, and microfracture (Fig. 5(A) and (C)). The cross-sectional image of the wear track (Fig. 5(B)) shows a considerable deformation of sub-surface zirconia grains. Fig. 5(D) reveals a mild grain pull-out for the nano-sized zirconia. The significant presence of Fe in the EDS analysis (inset in Fig. 5(D)) of the worn surface indicates the transfer of the counterbody steel material. However, the characteristic features of deformation and microcracking on the worn surface of zirconia indicate the dominance of tribomechanical wear. The Hertzian analysis [24] for the present conditions indicates a mean contact stress of 713–734 MPa, while a tensile strength of 700 MPa was reported for 2.5 mol% yttria-stabilized zirconia [25]. Therefore, it is believed that the contact stress in the present investigation was sufficient to yield deformation of the zirconia ceramics indicating the possible mechanical wear. Furthermore, the estimated initial Hertzian contact diameter of 46–47 m is approximately one to two orders magnitude higher than the average grain size of the zirconia samples used in this study. Also, the final wear track width was found to be at least one order of magnitude higher than the initial Hertzian contact diameter. This can be realized by the fact that initially a few grains experience Hertzian contact stress field and with the progression of wear, number of grains experience deformation-induced damage. The increased grain size of the initial Y-TZP ceramics might efficiently produce micro-regions of large strains across the grains, eventually resulting in severe grain fracture/pull-out (Fig. 6). Similar tendency for increased fracture was also observed in case of coarser grain size (∼1.6 m) Y-TZP ceramics slided against SiC [12]. The increased debris particles are often compacted to form a layer (Fig. 6(A)). However, the appearance of microcracks (inset in Fig. 6(A)) on the
Fig. 5. SEM micrographs of worn surface for nano-Y-TZP ceramics: (A) and (B) SPS, 1150 ◦ C, 10 min, (C) and (D) SPS 1200 ◦ C, 5 min. Inset in (D) is an EDS spectrum of the worn surface (D: shear deformation, P: surface polishing, C: microfracture, G: grain pull-out, and SD: sub-surface deformation).
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Fig. 6. SEM micrographs of worn surface for fine Y-TZP ceramics: (A) MS, 1400 ◦ C, 10 min and (B) CS 1550 ◦ C, 2 h (S: plastic smearing, C: microcracking, G: grain pull-out, and GS: grain smoothening).
Fig. 7. SEM micrographs of worn surface for coarse Y-TZP ceramics: (A) CS, 1650 ◦ C, 2 h; (B) CS 1550 ◦ C, 60 h; (C) CS, 1600 ◦ C, 60 h (DL: cracking-induced delamination, SP: spalling, and SD: sub-surface deformation).
smeared layer indicates its poor load bearing capacity and failure. Fig. 7 represents the worn surface of samples containing coarser grain size. The severity of the wear was increased with delamination of the surface layer (Fig. 7(A)). The delamination results in the exposure of the underneath surface causing further wear of the materials. The appearance of sub-surface was evident in the sample of 1100 nm grain size (Fig. 7(B)). Furthermore, samples of 1470 nm grain size (Fig. 7(C)) showed a spalling indicating the increased wear. In order to estimate the influence of grain size on the tetragonal to monoclinic phase transformation in zirconia ceramics, the worn surfaces were subjected to XRD analysis (Fig. 8). The major peak at 30.2◦ is related to (1 1 1) peak of tetragonal phase, while peaks ¯ and (1 1 1) at 28.2◦ and 31.4◦ correspond to the respective (111) peaks of monoclinic phase in the pattern for 1470 nm grain size (Fig. 8(A)). The intensity of monoclinic phase was reduced with decreasing grain size and it was rather difficult to detect below 375 nm grain size (Fig. 8(C)). No difference was found in the XRD patterns between worn and unworn surfaces of zirconia for 75 nm grain size (Fig. 8(D) and (E)). The monoclinic phase content was estimated according to Toraya et al. [26], and it reduced from 6% for 1470 nm grain size to 0.58% for 705 nm grain size. A maximum of 50–60% monoclinc phase was estimated when zirconia of grain size >900 nm was slided against SiC [12], while 8.5% mono-
clinic phase was recorded for zirconia of grain size >1200 nm worn against diamond [17]. Such a difference in the extent of phase transformation can be attributed to the differences in the experimental conditions as well as thermal conductivity of counterbody. The
Fig. 8. XRD patterns of worn surfaces of zirconia ceramics: (A) CS, 1600 ◦ C, 60 h, (B) CS 1650 ◦ C, 2 h, and (C) CS, 1550 ◦ C, 2 h, (D) SPS, 1150 ◦ C, 10 min, and (E) unworn surface of (D).
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observed transformation causes a compressive layer preventing the propagation of microcracks; however, the tensile stress developed beneath the compressive layer results in internal stresses in local areas, promoting microcrack nucleation at pores or grain boundaries. When these sub-surface cracks reach the contact surface, the material is removed through delamination and spalling [27]. The degree of material removal is enhanced with coarser grain size due to increased amount of zirconia available for phase transformation [12,18]. Thus, the contribution of phase transformation towards wear in the present study is considerable for coarser grain size (>705 nm) and diminished for the fine grain size zirconia ceramics. In summary, zirconia ceramics were removed primarily through the mechanical means, when slided against steel. The transfer of debris from the steel counterbody affects the wear behavior of zirconia ceramics in the investigated range of grain size. The hardness of the sintered ceramics appears to be the influencing mechanical property with regard to wear. The shear deformation in combination with limited cracking led to the lateral flow of the deformed material in case of the nanocrystalline/fine zirconia (<200 nm grain size). On the other hand, the coarse grain-sized zirconia samples were worn through severe delamination and spalling. In other words, the severity of mechanical wear attributes changed from nanocrystalline to coarse crystalline ceramics. The contribution of stress-induced phase transformation towards the degree of material removal increased with the increase in grain size. 4. Conclusions Y-TZP polycrystalline ceramics with a wide range of grain size from nanoscale (75 nm) to micronscale (1.47 m) were produced using CS, MS, and SPS methods. The developed ceramics were subjected to the unidirectional sliding against steel in ambient conditions. The following are the major conclusions: (a) SPS was efficient in producing nanometer grain size (<100 nm) at a lower temperature and short time, while CS produced the coarser grain size. (b) Nanocrystalline (75 nm) zirconia exhibited the superior wear resistance with a wear rate of 8.8 × 10−7 mm3 /Nm, while a maximum wear rate of 3.5 × 10−6 mm3 /Nm was recorded for the coarsest (grain size 1470 nm) sample.
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(c) The hardness of the sintered zirconia ceramics appears to be the influencing mechanical property with regard to wear. (d) The material was removed primarily through deformation and microfracture of the wear track when nanocrystalline zirconia was used, whereas a significant transition in wear mechanism to delamination/spalling of the tribosurfaces was observed in case of coarser zirconia. (e) The contribution of tetragonal to monoclinic phase transformation towards wear was considerable for coarser grain size (>705 nm) and diminished for the nano-grain size zirconia ceramics. Acknowledgement This study was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of grain size on wear behavior in Y-TZP ceramics B. Venkata Manoj Kumar a , Won-Sik Kim a , Seong-Hyeon Hong a,∗ , Hung-Tak Bae b , Dae-Soon Lim b a Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM), Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Republic of Korea b Department of Materials Science and Engineering, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 August 2009 Accepted 10 October 2009
Keywords: Zirconia Wear Grain size Nanocrystalline
a b s t r a c t The influence of grain size on the wear behavior of zirconia ceramics was investigated when slided against steel in dry unlubricated conditions. Fully densified Y-TZP ceramics (≥98% of theoretical density) with a wide range of grain size from 75 to 1470 nm were developed using spark plasma sintering (SPS), microwave sintering (MS), and conventional pressureless sintering (CS) methods. SPS was effective in producing fully densified nanocrystalline zirconia at low temperature and short sintering time. The steady state coefficient of friction (COF) varied in a range of 0.35–0.44, but the wear rate reduced from 3.5 to 0.88 × 10−6 mm3 /Nm with decreasing grain size. The plastic deformation and microcracking lead to a mild wear for the nanocrystalline zirconia, whereas the delamination/spalling results in the increased wear for the coarser zirconia. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, significant research has been carried out to develop the ceramics of nano-sized grains with the expectation that the nanocrystalline ceramics exhibit improved mechanical properties, wear resistance, chemical inertness, corrosion resistance, and thermal insulating properties [1–5]. However, the difficulty in developing nanoceramics with full densification basically hinders their adoption in full-range applications [4,5]. Polycrystalline yttria-stabilized tetragonal zirconia ceramics (YTZP) are attractive for various structural applications due to their high hardness, high modulus, and moderate fracture toughness [6–8]. There has been a considerable work reported towards estimating the tribological potential of this important material [9–19]. The determined friction coefficients were relatively high for selfmated zirconia ceramics [10,11], whereas dissimilar materials gave friction coefficients ranging from 0.30 to 0.5 [12]. The wear debris particles are reported to form as a result of internal stresses produced by the mechanically/thermally induced phase transformation [15]. It was suggested to find out the appropriate testing conditions or material compositions that stabilize the tetragonal phase at the tribocontact in order to improve the wear behavior [12–14,17]. The dominant wear mechanisms were abrasion, cracking, plastic deformation, tribochemical wear, and surface fatigue [10–12,15–19]. The plastic deformation, microcracking, and transcrystalline shear fracture of individual grains were dominant for
∗ Corresponding author. Tel.: +82 2 880 6273; fax: +82 2 884 1413. E-mail address: [email protected] (S.-H. Hong). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.10.021
fine grain size (∼0.5 m), while the increased tendency for intercrysrtalline fracture was observed for coarser grain size (∼1.6 m) [12]. Based on the above literature, it can be mentioned that the wear behavior of zirconia ceramics in ambient conditions has been extensively studied in the coarse grain size regime (200–1000 nm), while its behavior in the nanometric scale (<100 nm) has not been fully understood. Moreover, most of the reported wear studies were conducted against zirconia or harder counterbody like SiC, alumina, diamond, etc., while performance of nanocrystalline zirconia against steel was less reported [19]. Considering the significance of steel in structural applications, it is important to estimate the tribological performance of zirconia ceramics when slided against steel. According to authors’ knowledge, very limited information is available on the wear behavior of nanocrystalline Y-TZP ceramics against steel. The high hardness (14 GPa) of nano-zirconia (∼90 nm) was attributed to the improved fretting wear resistance, and the material was primarily removed through intergranular fracture and grain pull-out mechanisms [20]. However, there is no systematic investigation that focuses on the wear properties of zirconia ceramics containing a wide range of grain size. The purpose of the present investigation is to understand the tribological properties of Y-TZP ceramics containing a wide range of grain size from nanometer (<100 nm) to micrometer scale (>1 m) when slided against steel. For this, fully densified Y-TZP ceramics with different average grain sizes were obtained using spark plasma sintering (SPS), microwave sintering (MS), and conventional pressureless sintering (CS) techniques. The friction and wear properties were evaluated against steel when subjected to unidirectional sliding in dry unlubricated testing conditions. The important con-
B.V.M. Kumar et al. / Materials Science and Engineering A 527 (2010) 474–479
tribution is to elucidate the significance of nano-grain size on the micromechanisms of material removal when compared with coarser grain size. 2. Experimental procedures 2.1. Processing Two kinds of commercially available 3 mol% yttria-stabilized ZrO2 powders (TZ-3Y and TZ-3YS, Tosoh Corporation, Japan) were used in this study. The BET surface areas of TZ-3Y and TZ-3YS were 17 and 9 m2 /g, respectively. For CS and MS, the powders were uniaxially pressed into pellets of 10 mm diameter and then cold isostatically pressed at 150 MPa. Sintering was carried out at 1400–1650 ◦ C, and the heating rate was 10 and 100 ◦ C/min for CS and MS, respectively. For SPS, ∼1.5 g of the powder was placed into a 10 mm graphite die and an electric current of ∼1000 A was applied under a pressure of ∼30 MPa in N2 flowing atmosphere. The heating rate was 100 ◦ C/min, and the sintering temperature ranged from 1150 to 1200 ◦ C. The detailed sintering conditions are given in Table 1. The apparent density of the sintered specimens was measured using the Archemedes method in water. Microstructure of the samples was examined by field emission scanning electron microscope (FESEM 7401F, JEOL, Japan) after thermal etching. The average grain size was estimated using a conventional linear intercept method. The elastic modulus (E) was determined by an ultrasonic pulseecho tester (Tektronix TDS 220, Panametrics, Model 5800, Korea). Hardness was measured using Vickers indentation on the polished surfaces at 20 kgf loading for 15 s, and fracture toughness was estimated from the crack length measurement [21]. 2.2. Wear testing A unidirectional ball-on-disc tribometer was used to understand the friction and wear characteristics of sintered specimens in ambient conditions (20 ± 5 ◦ C and 40 ± 10% RH). The details of the tribometer can be found elsewhere [22]. Spherical steel balls (bearing grade; SAE 52100) of 6.35 mm diameter were used as
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the counterbody materials. Both steel ball and zirconia disks were cleaned with acetone prior to wear testing. In order to focus on the influence of material property (grain size) on wear of zirconia, the experimental parameters such as sliding speed, sliding duration, and load were kept constant in this study. Steel balls were fixed in the ball holder so as to make a track radius of 6 mm from the central axis and the tests were conducted at a fixed rotational speed of 2000 rpm for 15 min. Such a combination resulted in a linear sliding speed of 0.63 m/s and a total sliding distance of 565.2 m. The sliding tests were carried out at a 5 N load. During the test runs, frictional forces were recorded using an electronic sensor to generate on-line coefficient of friction (COF) data. Each experiment was repeated at least three times and the average values are reported. In order to understand the wear mechanism and to determine the track width, a detailed microstructural characterization of the as-worn surfaces was conducted using SEM (JSM 5600, JEOL, Japan) or FE-SEM. Surface profiles of the worn surfaces were acquired using an alpha-step profilometer (Model No.: 500, TENCOR, Germany) to measure the depth of the wear tracks. The wear track width and depth were further used in computing the wear volume (and subsequently wear rate) according to following relations. wear volume (mm3 )= 2×track radius×track width × wear depth (1)
wear rate (mm3 /Nm) =
wear volume normal load × sliding distance
(2)
3. Results and discussion The apparent density and average grain size of the developed Y-TZP ceramics are given in Table 1, and the representative microstructures are shown in Fig. 1. A nearly full densification (≥98% of theoretical density) was successfully achieved by three different sintering techniques. All the sintered specimens exhibited equiaxed grain microstructures and the average grain size varied over a wide range from 75 to 1470 nm. Consistent with the previous report [20], SPS was effective in reducing sintering temperature,
Fig. 1. FE-SEM micrographs of polished surfaces of developed Y-TZP ceramics: (A) SPS, 1150 ◦ C, 10 min; (B) MS, 1500 ◦ C, 10 min; (C) CS, 1600 ◦ C, 2 h; (D) CS, 1600 ◦ C, 60 h.
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Table 1 Sintering conditions, apparent density, grain size, and elastic modulus of the developed Y-TZP ceramics. Sintering conditions ◦
SPS 1150 C, 10 min SPS 1200 ◦ C, 5 min MS 1400 ◦ C, 10 min CS 1400 ◦ C, 2 h CS 1450 ◦ C, 2 h CS 1500 ◦ C, 2 h MS 1500 ◦ C, 10 min CS 1550 ◦ C, 2 h CS 1600 ◦ C, 2 h CS 1650 ◦ C, 2 h CS 1550 ◦ C, 60 h CS 1600 ◦ C, 60 h
Apparent density (cm3 /g) 5.97 6.02 5.99 6.04 6.05 6.05 6.07 6.08 6.10 6.10 6.09 5.98
Relative density (%)
Grain size (nm)
Elastic modulus (GPa)
98 99 98 99 99 99 99 99 100 100 100 98
75 98 165 205 235 300 318 375 605 705 1100 1470
200 202 208 209 207 215 213 214 215 217 216 215
sintering time, and grain size. Nanocrystalline Y-TZP ceramics with average grain size of ∼75 nm were obtained at 1150 ◦ C for 10 min by SPS. CS samples revealed an enormous grain growth with an average grain size varying from 200 to 1470 nm. The elastic modulus of the developed zirconia ceramics in the present study varied in the range of 200–217 GPa. Fig. 2 shows the variation of Vickers hardness and indentation fracture toughness with grain size. It is evident that the hardness slightly improved, whereas fracture toughness significantly reduced with decreasing grain size. A maximum hardness of 12.7 GPa was achieved in SPS sample with 75 nm grain size, while a minimum hardness of ∼11 GPa was achieved in CS samples with the largest grain size (1470 nm). The fracture toughness reduced from 9 MPa m1/2 for 1100 nm grain size to 4 MPa m1/2 for 700 nm grain size, and the variation was negligible with further reduction to 75 nm grain size. The reason for the sharp reduction of fracture toughness in the range of 700–1100 nm grain size is not well understood at this stage. It can be noted that a hardness of 12–13 GPa and a fracture toughness of 5–10 MPa m1/2 are commonly reported for the Y-TZP ceramics [6,23]. The frictional behavior of Y-TZP ceramics/steel couple was assessed on the basis of the continuous measurement of the frictional force during the test run. All samples exhibited almost identical evolution of COF with time (Fig. 3(A)). The general observation is that COF increased to ∼0.40 during initial 100 s (running-in-period) and thereafter maintained a steady state. It is believed that surface asperity deformation/fracture results in the formation of hard debris particles in the contact. The continuous process of formation and subsequent removal of debris during sliding give rise to the fluctuations in the friction at interface. However, minimal fluctuations in the present study suggest no difference in the rates of formation and removal of debris. The average steady state COF varied in a narrow range from 0.35 to 0.44 with the inves-
tigated range of grain size (Fig. 3(B)). Therefore, the influence of grain size on the frictional behavior of Y-TZP ceramics is said to be minimal. The variation of the calculated wear rates as a function of grain size is shown in Fig. 4. The wear rate was in the order of 10−6 mm3 /Nm for the coarser samples and decreased to 10−7 mm3 /Nm for the specimens with grain size below 200 nm. A minimum wear rate of 8.8 × 10−7 mm3 /nm was measured for the 75 nm grain-sized sample, while a maximum wear rate of 3.5 × 10−6 mm3 /Nm recorded for the 1470 nm grain-sized specimen. The reported wear rate in Y-TZP ceramics varied with the counterbody and it was in the order of 10−5 , 10−6 , and 10−4 –10−5 mm3 /Nm when slided against diamond [17], SiC [12], and Y-TZP [11], respectively. The observed shifting in the wear rate to the lower value in the present study necessarily indicates the influence of softer steel counterbody [19]. In addition, when Y-
Fig. 2. Variation of Vickers hardness and indentation fracture toughness with grain size in the developed Y-TZP ceramics.
Fig. 3. (A) Typical plot for the evolution of coefficient of friction (COF) with time and (B) average COF with grain size in the developed Y-TZP ceramics.
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Fig. 4. Wear rate with grain size in the developed Y-TZP ceramics.
TZP slided against SiC or diamond, the Hall–Petch relation of wear resistance (inverse of wear rate) with grain size was maintained (with the exponent of −0.5) in the specimens with grain size of 0.2–0.6 m, while wear resistance was proportional to reciprocal (exponent −1.0) for grain size larger than 0.8 m [12,17]. From the experimental data in the present investigation of sliding of zirconia against steel, exponents of grain size can be estimated as −1.0 for YTZP ceramics with grain size less than 200 nm and −0.2 for coarser samples. Such a deviation in the wear behavior can be understood on the basis of steel counterbody wear characteristics. It is well known that soft steel ball is readily abraded during sliding against hard zirconia counterface. Further sliding causes transfer of debris particles from steel ball onto flat zirconia surface (inset in Fig. 5(D)) eventually affecting the wear behavior of the system [19]. Thus, the Hall–Petch relation of wear with the grain size was not maintained in the present system. On the other hand, the significant reduction of wear rate for the samples with grain size below 200 nm can be related to the role of processing route. Particularly, SPS used for fabricating samples of 75 nm grain size and 98 nm grain size (Table 1) probably has the positive influence on the grain bound-
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ary characteristics with regard to wear. Such a postulation is also in agreement with the higher hardness values measured for the respective specimens (Fig. 2). This observation for nano-grain size along with the behavior in the entire range of grain sizes of zirconia suggests the dominant influence of hardness than fracture toughness when slided against steel. The influence of grain size on the wear mechanisms of zirconia ceramics is presented in Figs. 5–7. Fig. 5 represents the SEM images of worn surfaces of samples with nano-grain size. The surfaces are mainly characterized by the mild wear with surface polishing, plastic shear, and microfracture (Fig. 5(A) and (C)). The cross-sectional image of the wear track (Fig. 5(B)) shows a considerable deformation of sub-surface zirconia grains. Fig. 5(D) reveals a mild grain pull-out for the nano-sized zirconia. The significant presence of Fe in the EDS analysis (inset in Fig. 5(D)) of the worn surface indicates the transfer of the counterbody steel material. However, the characteristic features of deformation and microcracking on the worn surface of zirconia indicate the dominance of tribomechanical wear. The Hertzian analysis [24] for the present conditions indicates a mean contact stress of 713–734 MPa, while a tensile strength of 700 MPa was reported for 2.5 mol% yttria-stabilized zirconia [25]. Therefore, it is believed that the contact stress in the present investigation was sufficient to yield deformation of the zirconia ceramics indicating the possible mechanical wear. Furthermore, the estimated initial Hertzian contact diameter of 46–47 m is approximately one to two orders magnitude higher than the average grain size of the zirconia samples used in this study. Also, the final wear track width was found to be at least one order of magnitude higher than the initial Hertzian contact diameter. This can be realized by the fact that initially a few grains experience Hertzian contact stress field and with the progression of wear, number of grains experience deformation-induced damage. The increased grain size of the initial Y-TZP ceramics might efficiently produce micro-regions of large strains across the grains, eventually resulting in severe grain fracture/pull-out (Fig. 6). Similar tendency for increased fracture was also observed in case of coarser grain size (∼1.6 m) Y-TZP ceramics slided against SiC [12]. The increased debris particles are often compacted to form a layer (Fig. 6(A)). However, the appearance of microcracks (inset in Fig. 6(A)) on the
Fig. 5. SEM micrographs of worn surface for nano-Y-TZP ceramics: (A) and (B) SPS, 1150 ◦ C, 10 min, (C) and (D) SPS 1200 ◦ C, 5 min. Inset in (D) is an EDS spectrum of the worn surface (D: shear deformation, P: surface polishing, C: microfracture, G: grain pull-out, and SD: sub-surface deformation).
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Fig. 6. SEM micrographs of worn surface for fine Y-TZP ceramics: (A) MS, 1400 ◦ C, 10 min and (B) CS 1550 ◦ C, 2 h (S: plastic smearing, C: microcracking, G: grain pull-out, and GS: grain smoothening).
Fig. 7. SEM micrographs of worn surface for coarse Y-TZP ceramics: (A) CS, 1650 ◦ C, 2 h; (B) CS 1550 ◦ C, 60 h; (C) CS, 1600 ◦ C, 60 h (DL: cracking-induced delamination, SP: spalling, and SD: sub-surface deformation).
smeared layer indicates its poor load bearing capacity and failure. Fig. 7 represents the worn surface of samples containing coarser grain size. The severity of the wear was increased with delamination of the surface layer (Fig. 7(A)). The delamination results in the exposure of the underneath surface causing further wear of the materials. The appearance of sub-surface was evident in the sample of 1100 nm grain size (Fig. 7(B)). Furthermore, samples of 1470 nm grain size (Fig. 7(C)) showed a spalling indicating the increased wear. In order to estimate the influence of grain size on the tetragonal to monoclinic phase transformation in zirconia ceramics, the worn surfaces were subjected to XRD analysis (Fig. 8). The major peak at 30.2◦ is related to (1 1 1) peak of tetragonal phase, while peaks ¯ and (1 1 1) at 28.2◦ and 31.4◦ correspond to the respective (111) peaks of monoclinic phase in the pattern for 1470 nm grain size (Fig. 8(A)). The intensity of monoclinic phase was reduced with decreasing grain size and it was rather difficult to detect below 375 nm grain size (Fig. 8(C)). No difference was found in the XRD patterns between worn and unworn surfaces of zirconia for 75 nm grain size (Fig. 8(D) and (E)). The monoclinic phase content was estimated according to Toraya et al. [26], and it reduced from 6% for 1470 nm grain size to 0.58% for 705 nm grain size. A maximum of 50–60% monoclinc phase was estimated when zirconia of grain size >900 nm was slided against SiC [12], while 8.5% mono-
clinic phase was recorded for zirconia of grain size >1200 nm worn against diamond [17]. Such a difference in the extent of phase transformation can be attributed to the differences in the experimental conditions as well as thermal conductivity of counterbody. The
Fig. 8. XRD patterns of worn surfaces of zirconia ceramics: (A) CS, 1600 ◦ C, 60 h, (B) CS 1650 ◦ C, 2 h, and (C) CS, 1550 ◦ C, 2 h, (D) SPS, 1150 ◦ C, 10 min, and (E) unworn surface of (D).
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observed transformation causes a compressive layer preventing the propagation of microcracks; however, the tensile stress developed beneath the compressive layer results in internal stresses in local areas, promoting microcrack nucleation at pores or grain boundaries. When these sub-surface cracks reach the contact surface, the material is removed through delamination and spalling [27]. The degree of material removal is enhanced with coarser grain size due to increased amount of zirconia available for phase transformation [12,18]. Thus, the contribution of phase transformation towards wear in the present study is considerable for coarser grain size (>705 nm) and diminished for the fine grain size zirconia ceramics. In summary, zirconia ceramics were removed primarily through the mechanical means, when slided against steel. The transfer of debris from the steel counterbody affects the wear behavior of zirconia ceramics in the investigated range of grain size. The hardness of the sintered ceramics appears to be the influencing mechanical property with regard to wear. The shear deformation in combination with limited cracking led to the lateral flow of the deformed material in case of the nanocrystalline/fine zirconia (<200 nm grain size). On the other hand, the coarse grain-sized zirconia samples were worn through severe delamination and spalling. In other words, the severity of mechanical wear attributes changed from nanocrystalline to coarse crystalline ceramics. The contribution of stress-induced phase transformation towards the degree of material removal increased with the increase in grain size. 4. Conclusions Y-TZP polycrystalline ceramics with a wide range of grain size from nanoscale (75 nm) to micronscale (1.47 m) were produced using CS, MS, and SPS methods. The developed ceramics were subjected to the unidirectional sliding against steel in ambient conditions. The following are the major conclusions: (a) SPS was efficient in producing nanometer grain size (<100 nm) at a lower temperature and short time, while CS produced the coarser grain size. (b) Nanocrystalline (75 nm) zirconia exhibited the superior wear resistance with a wear rate of 8.8 × 10−7 mm3 /Nm, while a maximum wear rate of 3.5 × 10−6 mm3 /Nm was recorded for the coarsest (grain size 1470 nm) sample.
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(c) The hardness of the sintered zirconia ceramics appears to be the influencing mechanical property with regard to wear. (d) The material was removed primarily through deformation and microfracture of the wear track when nanocrystalline zirconia was used, whereas a significant transition in wear mechanism to delamination/spalling of the tribosurfaces was observed in case of coarser zirconia. (e) The contribution of tetragonal to monoclinic phase transformation towards wear was considerable for coarser grain size (>705 nm) and diminished for the nano-grain size zirconia ceramics. Acknowledgement This study was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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