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Abrasive wear performance of zirconium diboride based ceramic composite
International Journal of Refractory Metals & Hard Materials 79 (2019) 224–232
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Abrasive wear performance of zirconium diboride based ceramic composite ⁎
T
M. Mallik , P. Mitra, N. Srivastava, A. Narain, S.G. Dastidar, A. Singh, T.R. Paul Department of Metallurgical and Materials Engineering, National Institute of Technology, Durgapur, Durgapur 713209, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sliding Surface roughness Wear Wear mechanism Surfaces
The present study focuses on the wear behavior of a pressure-less sintered ZrB2–20 vol% SiC composite in dry sliding interaction against the electroplated diamond disc. Pressure-less sintering was carried out at 2000 °C for 2 h in an argon atmosphere. Sintered composite possesses 98% of the theoretical density, and primarily microstructure contains ZrB2 and SiC phases. The sliding wear of the investigated composite has been studied in pin-on-disc equipment at room temperature at different combinations of loads and sliding speeds to examine the influences of the test parameters on wear mechanism. The results show that the elastic modulus, hardness and fracture toughness for ZrB2 20 vol% SiC are 442 ± 4.5 GPa, 16.5 ± 0.5 GPa, and 5.67 MPa√m, respectively. Results also show that the specific wear rate of the ZrB2-SiC composite increases continuously with increasing the applied loads whereas, it decreases with increasing sliding speed. XRD analyses of worn surfaces suggest that the phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. The specific wear rates are in the order of ~10−6–10−8 cm2/Nm. The post wear test characterization suggests that oxidation dominates mild wear with low roughness values at low load and high sliding speed, whereas at high loads and low sliding speed severe wear mechanism is observed with high roughness values and deep grooves in surfaces. The mixed mode (oxidative–grain pullout-micro-cutting) wear mechanism is controlling the severe wear.
1. Introduction Ultra-high-temperature ceramics (UHTCs) are a category of ceramic materials that retain their refractoriness even at temperatures as high as 2000 °C. These materials are suitable for making a thermal protection system (TPS) materials for future hypersonic vehicles and the heating element for furnaces. UHTCs are generally carbides, borides, and nitrides of transition metals. Among these, zirconium diboride (ZrB2) is essential due to its low density, excellent thermal conductivity, and high melting temperature as well as it has excellent creep resistance and high-temperature strength [1–10]. Zirconium diboride (ZrB2) based UHTCs are the most important for particular applications such as aerospace and nuclear industry, where the components faced extreme environments of both thermal and chemical nature. Pure ZrB2 requires high temperatures and pressure for its densification [11–20]. Further, the fracture toughness and oxidation resistance improve vastly on the addition of silicon carbide. The ZrB2–SiC composite shows more oxidation resistance than pure ZrB2 at elevated temperature because of its silica-rich surface. The excellent properties exhibited by this composite are primarily due to its fine grain size and the uniform distribution of the additive over the matrix. Spark plasma or hot pressing sintering generally densify these composites, but
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nowadays pressure-less sintering has become the most preferred way due to its simplicity and low cost [21–31]. Apart from high-temperature applications, the high hardness values of boride based ceramics and composites make them suitable for potential use in tribological applications. For instance, the inherent properties of borides render them useful in sliding components operating as a hot extrusion dies at high temperature. Also, their low densities and high elastic moduli build these composites prime candidates for armor and different aero-applications. Umeda et al. have reported the influence of sliding speed on wear characteristic of boride ceramics under different conditions and found that the coefficients of friction (COF) of B4C and ZrB2 + B4C + SiC decrease with increasing sliding speed [32]. Mitsui et al. have investigated the elevated temperature tribological response of these composites in the air and found that wear behavior of these materials improved due to the presence of B2O3, which acted as a lubricant. In particulate composites, wear performance depends on various elements like sliding velocity, applied load, formation, nature, and permanence of tribo layer, morphology, and chemistry of wear debris [33]. Micelea et al. [34] reported that the dry sliding wear performance of SiC–MoSi2 composites against Al2O3 decreased with decreasing sliding speed. Sharma et al. [35] investigated the influence of the applied load on the wear rate of SiC-WC
Corresponding author. E-mail address: [email protected] (M. Mallik).
https://doi.org/10.1016/j.ijrmhm.2018.12.008 Received 31 August 2018; Received in revised form 9 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
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composites. Debnath et al. [36] observed that the coefficients of friction (COF) of ZrB2-SiC composites under applied loads of 5 and 10 N were about 0.47–0.50. Limited works are available on the wear performance, of ZrB2-SiC ceramic matrix composites. This work describes a series of sliding wear tests of ZrB2–20 vol% SiC (ZSBC-20) composite in the air of varying applied loads and sliding speeds. 2. Material and methods 2.1. Composite preparation and characterization The high purity (> 99%) powders of ZrB2, SiC, and B4C obtained from Alfaesar have been used to prepare the desired composites. Powders of ZrB2–20 vol% SiC were mixed in a ball mill operated at 250 rpm for 6 h using ethanol as the solvent. Carbon and B4C were utilized as sintering additives and phenolic resin as a source of carbon. After milling, the powder was dried in air for 2 h at a temperature of 350 °C. The dry powders were crushed and compacted using the uniaxial pressure of 100 MPa. Green compact was sintered at 2000 °C for 2 h in argon atmosphere using graphite resistance ASTRO furnace. The pressureless sintering (PLS) schedule described by Mallik et al. [37] has been followed in this study. The bulk density of the pressure-less sintered sample was calculated using the Archimedes principle. Microstructural analyses have been carried out by field emission scanning electron microscopy (FESEM, MERLIN, ZEISS) equipped with energy dispersive X-Ray (EDX) analyzer. Phases present in the composite have been identified by X-ray diffraction (X-Ray Diffractometer, X' Pert PRO, PANalytical, B. V. PW 3040/60 Netherlands) analysis. The hardness of the investigated composite has been measured with a Vickers diamond indenter operated at a load of 4.9 N for 15 s. At least 20 indentations are made to evaluate average hardness value. The fracture toughness (KIC) has been determined through the indentation technique with an applied load of 49 N using the relation proposed by Nihara et al. [38].
Fig. 1. Plot showing XRD pattern of the ZrB2-SiC composite.
and the applied normal force, respectively. Sliding distance S is given by S = ∏DNt, where D, N, and t are the track diameter, rotational speed and time, respectively. 3. Results and discussion 3.1. Microstructure and mechanical properties The relative density of sintered composite is found to be 98% of the corresponding theoretical density that has been enumerated from the ROM (rule of mixtures). The peaks of the constituent phases present in the ZrB2–20 vol% SiC composite are shown in Fig. 1. Results show that the ceramic composite contains ZrB2 and SiC phase, no additional phase could be envisaged in XRD analysis. The microstructure of the ZrB2–20 vol% SiC composite sintered at 2000 °C for 2 h is shown in Fig. 2. The contrast in this image, which depends on the average atomic numbers of the constituent phases, shows the presence of two distinct phases. The ZrB2 and SiC look light grey and dark contrast, respectively. EDX analysis confirms the identity of the major important elements in those constituents phases and EDX spectrums of ZrB2 and SiC are shown in Figs. 2(a) and 3(b). Fig. 2 shows the presence of irregularly shaped ZrB2 grains with the average grain size of 10 ± 2 μm, whereas the average particle size of SiC is about 3.3 ± 0.8 μm. Density, hardness, elastic modulus, fracture toughness and the Poisson's ratio of the investigated composite are summarized in Table 1. After sintering, the sample reached almost full density. The value of elastic modulus shows 90% of the theoretical value determined from the rule of mixtures (considering elastic modulus of ZrB2, and SiC are 500 GPa [7] and 375 GPa [41], respectively) that suggesting good bond integrity between the matrix and the reinforcement. Poisson's ratio has been found to be 0.17 which is typical of the ceramic materials in general. The microhardness of the composite found to be 17.5 ± 0.7 GPa.
2.2. Wear studies Dry sliding wear tests of ZSBC-20 was performed in a pin-on-disc tribometer (Wear and friction monitor-TR-20LE DUCOM, Bangalore, India) at room temperature. The wear test has been carried out according to the ASTM G99-95 standard [39] with some partial modifications to suit the objectives of the experiment. There are two vital parts of this setup, viz., the disc and the specimen acting as the pin. A diamond coated electroplated disc was selected as the counter disc and a parallelepiped pin with the dimensions of 20 mm × 4 mm × 4 mm was cut from a PLS sintered sample. The experimental apparatus is similar to our previous work [40]. The composite pin was subjected to metallographic polishing which included diamond coated disc polishing, followed by abrasive SiC papers and finally on a cloth smeared with diamond lapping paste. The weights of the sample at the beginning and the end of each test were measured to find out the weight loss. The wear tests were conducted under combinations of four different normal loads from 4.9 to 29.4 N and three different sliding speeds varied from 0.5 m/s to 1.57 m/s. A track diameter of 100 mm was used with each test being carried out to a sliding distance of 2010 m in contact with the disc. Worn surfaces of the selected samples were characterized using a FESEM, EDX and XRD analyses. Surtronic 25 has been employed to determine the surface roughness of the polished sample and worn surfaces. The initial surface roughness of samples and electroplated diamond disc were 0.23 mm and 10 mm, respectively. Specific wear rate (k) is the amount of material removed from the surface and is expressed by
k=
∇V SFN
3.2. Specific wear rate The variation of specific wear rates with sliding speed and normal loads are shown in Fig. 3 (a) and (b). At higher sliding speeds, the wear rate decreases since the frequency of contact between the abrasive disc and the fixed sample decreases [40]. Thus, lesser wear takes place whereas, at lower sliding speeds, the wear rate is great since the degree of contact between the rotating abrasive disc and the fixed sample increases. Thus, greater wear rates are seen at lower sliding speeds [40]. Similarly, higher loads give much higher wear rates than lower loads. At higher loads, the applied force is higher, and thus there is the greater intensity of contact between the abrasive disc and the fixed sample.
(1)
where ΔV, S, and FN are the volume loss of the material, sliding distance 225
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Fig. 2. (a) SEM (SE) image showing ZrB2 of light-grey phase, SiC looks dark contrast, EDX spectrum of (b) ZrB2 and (c) SiC.
Thus wear rates are higher at higher loads than at lower loads.
arrows in the Figs. 4(a) and 5(a) represent the direction of sliding of the specimen. Comparison of microstructure clearly shows that the more damage region is observed in Fig. 4(a), and the surface is rougher than higher sliding speeds. Fig. 4(b-d) describes the magnified visuals of the grain pullout regions. These images reveal the presence of a crack that is propagating through the specimen and a region of debris layer from
3.3. Worn surface characteristics Effect of sliding speeds on the microstructures of the worn surfaces of the investigated composites is shown in Figs. 4 and 5. The darkened
Fig. 3. The plot is showing specific wear rate of ZrB2 20 vol% SiC composite with varying (a) sliding speeds and (b) normal loads. 226
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Table 1 Mechanical properties of the pressure-less sintered ZrB2-SiC composite. Composite
ZrB2–20 vol% SiC
Relative density (%)
98
Hardness (GPa)
Young's modulus (GPa)
Experimental
Experimental
ROM
16.5 ± 0.5
442 ± 4.5
475
Poisson's ratio
Fracture Toughness (MPa√m)
0.18
5.67 ± 1.2
much lower wear in the sample. Effect of applied loads on wear behavior of ZrB2-SiC composite is shown in Figs. 6 and 7 which represent the microstructure variation of the worn surfaces due to applied loads of 4.9 N and 29.4 N, respectively. It is found that the wear volume is quite low for the sample at the lower load which leads to the lower amount of abrasion during wear. The depth of wear, as well as the wear volume, is quite low (Fig. 6). The white crystalline particles seen in the structure are suspected to be oxide particles. At higher load, it is found that the volume of wear is much higher and the signs of wear are more pronounced (Fig. 7a). There is a higher degree of bulk deformation on the surface; however, this again has to be confirmed at higher magnifications. Fig. 7(b) shows the presence of brittle cracks and fracture. Hence we can conclude that for a higher load of 29.4 N, the sample is subjected to more wear than for a lower load of 4.9 N, considering the sliding speed remains constant
the grain pullout region. Fig. 4(c) is an image of the crack tip, while Fig. 4(d) shows the presence of debris particles of various shapes and sizes. At lower sliding speeds, the number of revolutions of the disc per minute is quite low. Due to such low revolution speeds, there is a high frequency of contact between the disc and the fixed specimen resulting in a higher degree of wear. At lower sliding speed, the worn surface of the investigated composite exhibits an appreciable amount of grain pullout and microfracture. At higher sliding speeds, the worn surface exhibits grooves and polished regions. A smooth and compact layer with microcracks has also been observed at a higher magnification of selected regions (Fig. 5(c) and (d)). At higher sliding speeds, the number of revolutions per minute for the abrasive diamond disc is very high results low contact between the specimen and disc. Therefore, the high speed of revolution of the disc does not allow it to come in contact too frequently with the fixed specimen. Hence, a lack of contact ensures
Fig. 4. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 19.6 N at Sliding speed of 0.5 m/s (a-d). The dark arrow indicates the sliding direction. 227
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Fig. 5. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 19.6 N at Sliding speed of 1.57 m/s (a-d). The dark arrow indicates the sliding direction.
Fig. 6. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 4.9 N at Sliding speed of 1 m/s (a & b).
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Fig. 7. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 29.4 N at Sliding speed of 1 m/s (a & b).
3.4. Wear debris
at 1 m/s. Fig. 8(a) and (b) as well as Fig. 9(a) and (b) show the X-ray Diffraction analysis after the ZrB2-SiC composite has undergone wear. It is known that when wear occurs, the temperature rises in the sample due to frictional heating. This temperature rise leads to the oxidation of ZrB2 to form ZrO2. The formation of ZrO2 leads to an increase in the number of peaks in Figs. 8 and 9. However, it has already been explained that in the case of Fig. 8(a), the low sliding speed ensures greater frequency of contact between the abrasive disc and the sample, leading to higher wear. Greater wear leads to higher frictional forces, and consequently more vigorous oxidation of ZrB2 to form ZrO2 and this increases the intensity of the peaks in the pattern as compared to Fig. 8(b), which is plotted for a higher value of sliding speed. Higher loads lead to the lower wear resistance of the sample. Similarly, there is more vigorous oxidation of ZrB2 to form ZrO2, and the intensity of the peaks is much higher as compared to Fig. 9(a), which is plotted for a lower value of the load.
Wear debris generated after wear tests at normal applied loads of 4.9 N and 29.4 N at a sliding speed of 1 m/s are shown in Fig. 10 (a) and (b). The debris are mainly composed of large and small fragmented particles. At lower applied normal load, the morphology of fragmented particles is larger due to the less rubbing action. Whereas, higher applied loads produce quite large fragmented particles due to grain pullout. In addition, the particles undergo collision and rubbing action with each other during sliding which leads to the reduction in size and produces fine particles under the action of the high normal load which is evident from the examination of the morphology of particles shown in Fig. 10 (b). The EDX spectrum of the loose wear debris formed during the sliding wear of ZrB2-SiC composite at applied normal loads of 4.9 N and 29.4 N are shown in Fig. 10 (c) and (d), respectively. The EDX spectrum from the wear debris shows the Zr, Si and C peaks which confirm the enrichment of C from the diamond disc. The spectrum also shows the oxygen peak confirming the formation of oxides of Zr and Si which is evident in XRD patterns (Figs. 8 and 9).
Fig. 8. XRD Pattern: (a) load 19.6 N sliding speed 0.5 m/s and (b) load 19.6 N sliding speed 1.57 m/s. 229
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Fig. 9. (a) XRD Pattern for load 4.9 N and sliding speed 1 m/s, (b) XRD Pattern for load 29.4 N and sliding speed 1 m/s.
SiC composite against electroplated diamond disc caused fracture and removal of ZrB2, SiC and diamond particles as well as their oxidation in the investigated sliding conditions. After measuring the weight, the surface finish of the worn specimen has been analyzed under Surtronic 25. Surface roughness in the form of Ra values has been calculated as an average of at least three values. The surface roughness of wear surfaces caused by the normal load at different sliding speed is tabulated in Table 2. Surface roughness is measured along the wear track perpendicular to the sliding distance. Surface roughness or surface quality, also recognized as surface texture are
The result of sliding speeds on the morphology of the wear debris is shown in Fig. 11. Debris particles obtained after wear experiments at sliding speed of 0.5 m/s and at normal applied loads of 19.6 N are large and spherical in nature (Fig. 11a), whereas, debris generated due to higher sliding speed (1.57 m/s) are irregular in shape (Fig. 11b). At lower sliding speeds, the frequency of contact between the abrasive disc and the fixed sample increases which leads to more rubbing action. As a result debris particle become blunt and regular in shape. The presence of oxygen in the EDX spectrum confirms the formation of oxide during sliding (Fig. 11 c&d). Therefore, it is plausible that the sliding of ZrB2-
Fig. 10. SEM (SE) image showing debris collected after wear test at normal loads of (a) 4.9 N and (b) 29.4 N at a sliding speed of 1 m/s. Corresponding bulk EDX patterns of debris (a) and (b) are shown in (c) and (d), respectively. 230
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Fig. 11. SEM (SE) image showing debris collected after wear test at normal load 19.6 N at sliding speed of (a) 0.5 m/s and (b) 1.57 m/s. Corresponding bulk EDX patterns of debris (a) and (b) are shown in (c) and (d), respectively.
bodies, in this case, the disc and the specimen, and the formation of a middle or a ‘third body’ layer or film of debris particles. This layer or film accounts for the velocity difference between the two ‘first body’ asperities. It can be summarized from Figs. 4–8 that, under a given applied load, there is more wear for a lesser velocity than that of one with higher velocity. The reason for such a behavior is the creation and presence of debris particles during the experiment. The effect of debris in wear of ceramic materials has been depicted effectively in the study of Denape et al. [45]. The difference in the observations at the two different velocities at a fixed load is mainly attributed to the existence of the ‘third body’ debris particles. It can be proposed that at the beginning of both the cases, during the first few contacts, the response of both remain same, with both the specimens producing debris particles. In the first case with the disc having lower velocity the debris particles are ejected from the surface due to the various force (centrifugal, contact, etc.) acting on it. Whereas in the latter case with higher velocity, the same particles does not get the time to be ejected and is again subjected to wear action between the two ‘first bodies’, resulting in its breakdown into fine particles, which adheres to the surface of the specimen due to van der Waals forces and by electrostatic attractive forces to some extent. [46].
Table 2 Roughness of the worn surfaces of the ZrB2–20 vol% SiC with different wear parameter (Sliding speeds and normally applied loads). Normal applied load (N)
Roughness (μ m) 0.5 m/s
4.9 9.8 19.6 29.4
1.35 1.72 2.38 2.30
± ± ± ±
1 m/s 0.21 0.26 0.5 0.2
1.0 2.0 2.1 2.2
± ± ± ±
1.57 m/s 0.2 0.21 0.2 0.4
1.13 ± 0.11 1.25 ± 0.10 2.04 ± 0.31 2.1 ± 0.5
terms used to define the overall quality of the worn surface, which is concerned with the geometric irregularities and the quality of a surface after wear test. From the Table 2, we note that an increase of applied load from 4.9 N to 29.4 N leads to an increase of surface roughness from 1.0 ± 0.2 (mm) to 2.30 ± 0.2 (mm). Surface quality decreases (increase in surface roughness) while the sliding speed decreases. Maximum surface roughness is detected at an applied load of 19.6 N and a sliding speed of 0.5 m/s. Minimum surface roughness has been found at the applied load of 4.9 N and sliding velocity of 1 m/s. According to the values of specific wear rate and roughness, it has been considered that surface roughness increases with increasing specific wear rate.
4. Conclusions
3.5. Wear mechanism
The wear resistance of pressure-less sintered ZrB2-SiC-based UHTC composite has been studied against rotating the diamond-coated disc at room temperature. This experiment was done on a pin-on-disc machine. The wear tests have been carried out at different applied loads and different sliding speeds. The influences of the applied loads and the sliding speed on the wear resistance have been investigated. The major conclusions are
There are many types of wear mechanism available for study (e.g., oxidation, abrasion, delamination, etc.) [42,43] but we are mainly concentrated our study to the mechanism of wear by abrasion and partly to the ‘third body’ theory proposed by Godet et al. [44]. The ‘third body’ approach envisages a contact that is made out of two 231
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(1) The sintered sample has 98% of the theoretical density (TD), and microstructure contained ZrB2 and SiC phases. (2) The hardness value and elastic modulus for ZrB2 20 vol% SiC is 17.5 GPa and 441 MPa, respectively. (3) The specific wear rates have been established to possess an order of magnitude of ~10−6–10−8, which indicates an abrasive wear mechanism. (4) Specific wear rate of the investigated composites increased with the rise in applied load. (5) Specific wear rate of the ZrB2 20 vol% SiC composite reduced with the increase of sliding speeds. (6) X-ray diffraction analyses of worn surfaces propose that phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. (7) Microstructural characterization of worn surfaces suggests that wear mechanism in the main controlled by grain fracture, microcracking and grain pullout.
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Nomenclature k = Specific wear rate ΔV = volume loss of the material FN = the applied normal force S = Sliding distance D = Track diameter N = Rotational speed t = Time Ra = surface roughness KIC = Indentation fracture toughness Acknowledgment The financial support from sponsors project [Project Number=DST, SERB, SB/EMEQ-251/2013] is gratefully acknowledged. The authors also express their sincere gratitude to technical personnel at the Central Research Facility of IIT Kharagpur and COE NIT Durgapur. References [1] L. Kaufman, E.V. Clougherty, Investigation of Boride Compounds for Very High Temperature Applications, RTD-TRD-N63–4096, Part III, ManLabs Inc, Cambridge, MA, March 1996. [2] A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, D.T. Ellerby, Characterization of zirconium diboride for thermal protection, Key Eng. Mater. 264–268 (2004) 493–496. [3] A.L. Chamberlain, W.G. Fahrenholtz, G.E. Hilmas, D.T. Ellerby, High-strength zirconium diboride-based ceramics, J. Am. Ceram. Soc. 87 (2004) 1170–1172. [4] M. Mallik, A.J. Kailath, K.K. Ray, R. Mitra, Effect of Si3N4 addition on compressive creep behavior of hot-pressed ZrB2–SiC composites, J. Am. Ceram. Soc. 97 (2014) 2957–2964. [5] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, Mechanical, thermal oxidation properties of refractory hafnium zirconium compounds, J. Eur. Ceram. Soc. 19 (1999) 2405–2414. [6] M. Mallik, A.J. Kailath, K.K. Ray, R. Mitra, Electrical and thermophysical properties of ZrB2 and HfB2 based composites, J. Eur. Ceram. Soc. 32 (2012) 2545–2555. [7] R.A. Cutler, Engineering properties of borides, ASTM Engineered Materials Hbook, Ceramics Glasses, Schneider, S J, Technical Chairman, 1991, pp. 787–803. [8] A.F. Guillermet, G. Grimvall, Phase stability properties of transition metal diborides, Am. Inst. Phys. Conf. Proc. 231 (1991) 423–431. [9] C. Mroz, Annual mineral review; zirconium diboride, Am. Ceram. Soc. Bull. 74 (1995) 165–166. [10] K. Upadhya, J.M. Yang, W.P. Hoffmann, Materials for ultrahigh temperature structural applications, Am. Ceram. Soc. Bull. 76 (1997) 51–56. [11] F. Monteverde, A. Bellosi, S. Guicciardi, Processing properties of zirconium diboride-based composites, J. Eur. Ceram. Soc. 22 (2002) 279–288. [12] I.M. Low, R. McPherson, Fabrication of new zirconium boride ceramics, J. Mat. Sci.
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Abrasive wear performance of zirconium diboride based ceramic composite ⁎
T
M. Mallik , P. Mitra, N. Srivastava, A. Narain, S.G. Dastidar, A. Singh, T.R. Paul Department of Metallurgical and Materials Engineering, National Institute of Technology, Durgapur, Durgapur 713209, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Sliding Surface roughness Wear Wear mechanism Surfaces
The present study focuses on the wear behavior of a pressure-less sintered ZrB2–20 vol% SiC composite in dry sliding interaction against the electroplated diamond disc. Pressure-less sintering was carried out at 2000 °C for 2 h in an argon atmosphere. Sintered composite possesses 98% of the theoretical density, and primarily microstructure contains ZrB2 and SiC phases. The sliding wear of the investigated composite has been studied in pin-on-disc equipment at room temperature at different combinations of loads and sliding speeds to examine the influences of the test parameters on wear mechanism. The results show that the elastic modulus, hardness and fracture toughness for ZrB2 20 vol% SiC are 442 ± 4.5 GPa, 16.5 ± 0.5 GPa, and 5.67 MPa√m, respectively. Results also show that the specific wear rate of the ZrB2-SiC composite increases continuously with increasing the applied loads whereas, it decreases with increasing sliding speed. XRD analyses of worn surfaces suggest that the phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. The specific wear rates are in the order of ~10−6–10−8 cm2/Nm. The post wear test characterization suggests that oxidation dominates mild wear with low roughness values at low load and high sliding speed, whereas at high loads and low sliding speed severe wear mechanism is observed with high roughness values and deep grooves in surfaces. The mixed mode (oxidative–grain pullout-micro-cutting) wear mechanism is controlling the severe wear.
1. Introduction Ultra-high-temperature ceramics (UHTCs) are a category of ceramic materials that retain their refractoriness even at temperatures as high as 2000 °C. These materials are suitable for making a thermal protection system (TPS) materials for future hypersonic vehicles and the heating element for furnaces. UHTCs are generally carbides, borides, and nitrides of transition metals. Among these, zirconium diboride (ZrB2) is essential due to its low density, excellent thermal conductivity, and high melting temperature as well as it has excellent creep resistance and high-temperature strength [1–10]. Zirconium diboride (ZrB2) based UHTCs are the most important for particular applications such as aerospace and nuclear industry, where the components faced extreme environments of both thermal and chemical nature. Pure ZrB2 requires high temperatures and pressure for its densification [11–20]. Further, the fracture toughness and oxidation resistance improve vastly on the addition of silicon carbide. The ZrB2–SiC composite shows more oxidation resistance than pure ZrB2 at elevated temperature because of its silica-rich surface. The excellent properties exhibited by this composite are primarily due to its fine grain size and the uniform distribution of the additive over the matrix. Spark plasma or hot pressing sintering generally densify these composites, but
⁎
nowadays pressure-less sintering has become the most preferred way due to its simplicity and low cost [21–31]. Apart from high-temperature applications, the high hardness values of boride based ceramics and composites make them suitable for potential use in tribological applications. For instance, the inherent properties of borides render them useful in sliding components operating as a hot extrusion dies at high temperature. Also, their low densities and high elastic moduli build these composites prime candidates for armor and different aero-applications. Umeda et al. have reported the influence of sliding speed on wear characteristic of boride ceramics under different conditions and found that the coefficients of friction (COF) of B4C and ZrB2 + B4C + SiC decrease with increasing sliding speed [32]. Mitsui et al. have investigated the elevated temperature tribological response of these composites in the air and found that wear behavior of these materials improved due to the presence of B2O3, which acted as a lubricant. In particulate composites, wear performance depends on various elements like sliding velocity, applied load, formation, nature, and permanence of tribo layer, morphology, and chemistry of wear debris [33]. Micelea et al. [34] reported that the dry sliding wear performance of SiC–MoSi2 composites against Al2O3 decreased with decreasing sliding speed. Sharma et al. [35] investigated the influence of the applied load on the wear rate of SiC-WC
Corresponding author. E-mail address: [email protected] (M. Mallik).
https://doi.org/10.1016/j.ijrmhm.2018.12.008 Received 31 August 2018; Received in revised form 9 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
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composites. Debnath et al. [36] observed that the coefficients of friction (COF) of ZrB2-SiC composites under applied loads of 5 and 10 N were about 0.47–0.50. Limited works are available on the wear performance, of ZrB2-SiC ceramic matrix composites. This work describes a series of sliding wear tests of ZrB2–20 vol% SiC (ZSBC-20) composite in the air of varying applied loads and sliding speeds. 2. Material and methods 2.1. Composite preparation and characterization The high purity (> 99%) powders of ZrB2, SiC, and B4C obtained from Alfaesar have been used to prepare the desired composites. Powders of ZrB2–20 vol% SiC were mixed in a ball mill operated at 250 rpm for 6 h using ethanol as the solvent. Carbon and B4C were utilized as sintering additives and phenolic resin as a source of carbon. After milling, the powder was dried in air for 2 h at a temperature of 350 °C. The dry powders were crushed and compacted using the uniaxial pressure of 100 MPa. Green compact was sintered at 2000 °C for 2 h in argon atmosphere using graphite resistance ASTRO furnace. The pressureless sintering (PLS) schedule described by Mallik et al. [37] has been followed in this study. The bulk density of the pressure-less sintered sample was calculated using the Archimedes principle. Microstructural analyses have been carried out by field emission scanning electron microscopy (FESEM, MERLIN, ZEISS) equipped with energy dispersive X-Ray (EDX) analyzer. Phases present in the composite have been identified by X-ray diffraction (X-Ray Diffractometer, X' Pert PRO, PANalytical, B. V. PW 3040/60 Netherlands) analysis. The hardness of the investigated composite has been measured with a Vickers diamond indenter operated at a load of 4.9 N for 15 s. At least 20 indentations are made to evaluate average hardness value. The fracture toughness (KIC) has been determined through the indentation technique with an applied load of 49 N using the relation proposed by Nihara et al. [38].
Fig. 1. Plot showing XRD pattern of the ZrB2-SiC composite.
and the applied normal force, respectively. Sliding distance S is given by S = ∏DNt, where D, N, and t are the track diameter, rotational speed and time, respectively. 3. Results and discussion 3.1. Microstructure and mechanical properties The relative density of sintered composite is found to be 98% of the corresponding theoretical density that has been enumerated from the ROM (rule of mixtures). The peaks of the constituent phases present in the ZrB2–20 vol% SiC composite are shown in Fig. 1. Results show that the ceramic composite contains ZrB2 and SiC phase, no additional phase could be envisaged in XRD analysis. The microstructure of the ZrB2–20 vol% SiC composite sintered at 2000 °C for 2 h is shown in Fig. 2. The contrast in this image, which depends on the average atomic numbers of the constituent phases, shows the presence of two distinct phases. The ZrB2 and SiC look light grey and dark contrast, respectively. EDX analysis confirms the identity of the major important elements in those constituents phases and EDX spectrums of ZrB2 and SiC are shown in Figs. 2(a) and 3(b). Fig. 2 shows the presence of irregularly shaped ZrB2 grains with the average grain size of 10 ± 2 μm, whereas the average particle size of SiC is about 3.3 ± 0.8 μm. Density, hardness, elastic modulus, fracture toughness and the Poisson's ratio of the investigated composite are summarized in Table 1. After sintering, the sample reached almost full density. The value of elastic modulus shows 90% of the theoretical value determined from the rule of mixtures (considering elastic modulus of ZrB2, and SiC are 500 GPa [7] and 375 GPa [41], respectively) that suggesting good bond integrity between the matrix and the reinforcement. Poisson's ratio has been found to be 0.17 which is typical of the ceramic materials in general. The microhardness of the composite found to be 17.5 ± 0.7 GPa.
2.2. Wear studies Dry sliding wear tests of ZSBC-20 was performed in a pin-on-disc tribometer (Wear and friction monitor-TR-20LE DUCOM, Bangalore, India) at room temperature. The wear test has been carried out according to the ASTM G99-95 standard [39] with some partial modifications to suit the objectives of the experiment. There are two vital parts of this setup, viz., the disc and the specimen acting as the pin. A diamond coated electroplated disc was selected as the counter disc and a parallelepiped pin with the dimensions of 20 mm × 4 mm × 4 mm was cut from a PLS sintered sample. The experimental apparatus is similar to our previous work [40]. The composite pin was subjected to metallographic polishing which included diamond coated disc polishing, followed by abrasive SiC papers and finally on a cloth smeared with diamond lapping paste. The weights of the sample at the beginning and the end of each test were measured to find out the weight loss. The wear tests were conducted under combinations of four different normal loads from 4.9 to 29.4 N and three different sliding speeds varied from 0.5 m/s to 1.57 m/s. A track diameter of 100 mm was used with each test being carried out to a sliding distance of 2010 m in contact with the disc. Worn surfaces of the selected samples were characterized using a FESEM, EDX and XRD analyses. Surtronic 25 has been employed to determine the surface roughness of the polished sample and worn surfaces. The initial surface roughness of samples and electroplated diamond disc were 0.23 mm and 10 mm, respectively. Specific wear rate (k) is the amount of material removed from the surface and is expressed by
k=
∇V SFN
3.2. Specific wear rate The variation of specific wear rates with sliding speed and normal loads are shown in Fig. 3 (a) and (b). At higher sliding speeds, the wear rate decreases since the frequency of contact between the abrasive disc and the fixed sample decreases [40]. Thus, lesser wear takes place whereas, at lower sliding speeds, the wear rate is great since the degree of contact between the rotating abrasive disc and the fixed sample increases. Thus, greater wear rates are seen at lower sliding speeds [40]. Similarly, higher loads give much higher wear rates than lower loads. At higher loads, the applied force is higher, and thus there is the greater intensity of contact between the abrasive disc and the fixed sample.
(1)
where ΔV, S, and FN are the volume loss of the material, sliding distance 225
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Fig. 2. (a) SEM (SE) image showing ZrB2 of light-grey phase, SiC looks dark contrast, EDX spectrum of (b) ZrB2 and (c) SiC.
Thus wear rates are higher at higher loads than at lower loads.
arrows in the Figs. 4(a) and 5(a) represent the direction of sliding of the specimen. Comparison of microstructure clearly shows that the more damage region is observed in Fig. 4(a), and the surface is rougher than higher sliding speeds. Fig. 4(b-d) describes the magnified visuals of the grain pullout regions. These images reveal the presence of a crack that is propagating through the specimen and a region of debris layer from
3.3. Worn surface characteristics Effect of sliding speeds on the microstructures of the worn surfaces of the investigated composites is shown in Figs. 4 and 5. The darkened
Fig. 3. The plot is showing specific wear rate of ZrB2 20 vol% SiC composite with varying (a) sliding speeds and (b) normal loads. 226
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Table 1 Mechanical properties of the pressure-less sintered ZrB2-SiC composite. Composite
ZrB2–20 vol% SiC
Relative density (%)
98
Hardness (GPa)
Young's modulus (GPa)
Experimental
Experimental
ROM
16.5 ± 0.5
442 ± 4.5
475
Poisson's ratio
Fracture Toughness (MPa√m)
0.18
5.67 ± 1.2
much lower wear in the sample. Effect of applied loads on wear behavior of ZrB2-SiC composite is shown in Figs. 6 and 7 which represent the microstructure variation of the worn surfaces due to applied loads of 4.9 N and 29.4 N, respectively. It is found that the wear volume is quite low for the sample at the lower load which leads to the lower amount of abrasion during wear. The depth of wear, as well as the wear volume, is quite low (Fig. 6). The white crystalline particles seen in the structure are suspected to be oxide particles. At higher load, it is found that the volume of wear is much higher and the signs of wear are more pronounced (Fig. 7a). There is a higher degree of bulk deformation on the surface; however, this again has to be confirmed at higher magnifications. Fig. 7(b) shows the presence of brittle cracks and fracture. Hence we can conclude that for a higher load of 29.4 N, the sample is subjected to more wear than for a lower load of 4.9 N, considering the sliding speed remains constant
the grain pullout region. Fig. 4(c) is an image of the crack tip, while Fig. 4(d) shows the presence of debris particles of various shapes and sizes. At lower sliding speeds, the number of revolutions of the disc per minute is quite low. Due to such low revolution speeds, there is a high frequency of contact between the disc and the fixed specimen resulting in a higher degree of wear. At lower sliding speed, the worn surface of the investigated composite exhibits an appreciable amount of grain pullout and microfracture. At higher sliding speeds, the worn surface exhibits grooves and polished regions. A smooth and compact layer with microcracks has also been observed at a higher magnification of selected regions (Fig. 5(c) and (d)). At higher sliding speeds, the number of revolutions per minute for the abrasive diamond disc is very high results low contact between the specimen and disc. Therefore, the high speed of revolution of the disc does not allow it to come in contact too frequently with the fixed specimen. Hence, a lack of contact ensures
Fig. 4. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 19.6 N at Sliding speed of 0.5 m/s (a-d). The dark arrow indicates the sliding direction. 227
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Fig. 5. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 19.6 N at Sliding speed of 1.57 m/s (a-d). The dark arrow indicates the sliding direction.
Fig. 6. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 4.9 N at Sliding speed of 1 m/s (a & b).
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Fig. 7. SEM (SE) image is showing worn surfaces of ZrB2 20 vol% SiC composite after sliding at a normal load of 29.4 N at Sliding speed of 1 m/s (a & b).
3.4. Wear debris
at 1 m/s. Fig. 8(a) and (b) as well as Fig. 9(a) and (b) show the X-ray Diffraction analysis after the ZrB2-SiC composite has undergone wear. It is known that when wear occurs, the temperature rises in the sample due to frictional heating. This temperature rise leads to the oxidation of ZrB2 to form ZrO2. The formation of ZrO2 leads to an increase in the number of peaks in Figs. 8 and 9. However, it has already been explained that in the case of Fig. 8(a), the low sliding speed ensures greater frequency of contact between the abrasive disc and the sample, leading to higher wear. Greater wear leads to higher frictional forces, and consequently more vigorous oxidation of ZrB2 to form ZrO2 and this increases the intensity of the peaks in the pattern as compared to Fig. 8(b), which is plotted for a higher value of sliding speed. Higher loads lead to the lower wear resistance of the sample. Similarly, there is more vigorous oxidation of ZrB2 to form ZrO2, and the intensity of the peaks is much higher as compared to Fig. 9(a), which is plotted for a lower value of the load.
Wear debris generated after wear tests at normal applied loads of 4.9 N and 29.4 N at a sliding speed of 1 m/s are shown in Fig. 10 (a) and (b). The debris are mainly composed of large and small fragmented particles. At lower applied normal load, the morphology of fragmented particles is larger due to the less rubbing action. Whereas, higher applied loads produce quite large fragmented particles due to grain pullout. In addition, the particles undergo collision and rubbing action with each other during sliding which leads to the reduction in size and produces fine particles under the action of the high normal load which is evident from the examination of the morphology of particles shown in Fig. 10 (b). The EDX spectrum of the loose wear debris formed during the sliding wear of ZrB2-SiC composite at applied normal loads of 4.9 N and 29.4 N are shown in Fig. 10 (c) and (d), respectively. The EDX spectrum from the wear debris shows the Zr, Si and C peaks which confirm the enrichment of C from the diamond disc. The spectrum also shows the oxygen peak confirming the formation of oxides of Zr and Si which is evident in XRD patterns (Figs. 8 and 9).
Fig. 8. XRD Pattern: (a) load 19.6 N sliding speed 0.5 m/s and (b) load 19.6 N sliding speed 1.57 m/s. 229
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Fig. 9. (a) XRD Pattern for load 4.9 N and sliding speed 1 m/s, (b) XRD Pattern for load 29.4 N and sliding speed 1 m/s.
SiC composite against electroplated diamond disc caused fracture and removal of ZrB2, SiC and diamond particles as well as their oxidation in the investigated sliding conditions. After measuring the weight, the surface finish of the worn specimen has been analyzed under Surtronic 25. Surface roughness in the form of Ra values has been calculated as an average of at least three values. The surface roughness of wear surfaces caused by the normal load at different sliding speed is tabulated in Table 2. Surface roughness is measured along the wear track perpendicular to the sliding distance. Surface roughness or surface quality, also recognized as surface texture are
The result of sliding speeds on the morphology of the wear debris is shown in Fig. 11. Debris particles obtained after wear experiments at sliding speed of 0.5 m/s and at normal applied loads of 19.6 N are large and spherical in nature (Fig. 11a), whereas, debris generated due to higher sliding speed (1.57 m/s) are irregular in shape (Fig. 11b). At lower sliding speeds, the frequency of contact between the abrasive disc and the fixed sample increases which leads to more rubbing action. As a result debris particle become blunt and regular in shape. The presence of oxygen in the EDX spectrum confirms the formation of oxide during sliding (Fig. 11 c&d). Therefore, it is plausible that the sliding of ZrB2-
Fig. 10. SEM (SE) image showing debris collected after wear test at normal loads of (a) 4.9 N and (b) 29.4 N at a sliding speed of 1 m/s. Corresponding bulk EDX patterns of debris (a) and (b) are shown in (c) and (d), respectively. 230
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Fig. 11. SEM (SE) image showing debris collected after wear test at normal load 19.6 N at sliding speed of (a) 0.5 m/s and (b) 1.57 m/s. Corresponding bulk EDX patterns of debris (a) and (b) are shown in (c) and (d), respectively.
bodies, in this case, the disc and the specimen, and the formation of a middle or a ‘third body’ layer or film of debris particles. This layer or film accounts for the velocity difference between the two ‘first body’ asperities. It can be summarized from Figs. 4–8 that, under a given applied load, there is more wear for a lesser velocity than that of one with higher velocity. The reason for such a behavior is the creation and presence of debris particles during the experiment. The effect of debris in wear of ceramic materials has been depicted effectively in the study of Denape et al. [45]. The difference in the observations at the two different velocities at a fixed load is mainly attributed to the existence of the ‘third body’ debris particles. It can be proposed that at the beginning of both the cases, during the first few contacts, the response of both remain same, with both the specimens producing debris particles. In the first case with the disc having lower velocity the debris particles are ejected from the surface due to the various force (centrifugal, contact, etc.) acting on it. Whereas in the latter case with higher velocity, the same particles does not get the time to be ejected and is again subjected to wear action between the two ‘first bodies’, resulting in its breakdown into fine particles, which adheres to the surface of the specimen due to van der Waals forces and by electrostatic attractive forces to some extent. [46].
Table 2 Roughness of the worn surfaces of the ZrB2–20 vol% SiC with different wear parameter (Sliding speeds and normally applied loads). Normal applied load (N)
Roughness (μ m) 0.5 m/s
4.9 9.8 19.6 29.4
1.35 1.72 2.38 2.30
± ± ± ±
1 m/s 0.21 0.26 0.5 0.2
1.0 2.0 2.1 2.2
± ± ± ±
1.57 m/s 0.2 0.21 0.2 0.4
1.13 ± 0.11 1.25 ± 0.10 2.04 ± 0.31 2.1 ± 0.5
terms used to define the overall quality of the worn surface, which is concerned with the geometric irregularities and the quality of a surface after wear test. From the Table 2, we note that an increase of applied load from 4.9 N to 29.4 N leads to an increase of surface roughness from 1.0 ± 0.2 (mm) to 2.30 ± 0.2 (mm). Surface quality decreases (increase in surface roughness) while the sliding speed decreases. Maximum surface roughness is detected at an applied load of 19.6 N and a sliding speed of 0.5 m/s. Minimum surface roughness has been found at the applied load of 4.9 N and sliding velocity of 1 m/s. According to the values of specific wear rate and roughness, it has been considered that surface roughness increases with increasing specific wear rate.
4. Conclusions
3.5. Wear mechanism
The wear resistance of pressure-less sintered ZrB2-SiC-based UHTC composite has been studied against rotating the diamond-coated disc at room temperature. This experiment was done on a pin-on-disc machine. The wear tests have been carried out at different applied loads and different sliding speeds. The influences of the applied loads and the sliding speed on the wear resistance have been investigated. The major conclusions are
There are many types of wear mechanism available for study (e.g., oxidation, abrasion, delamination, etc.) [42,43] but we are mainly concentrated our study to the mechanism of wear by abrasion and partly to the ‘third body’ theory proposed by Godet et al. [44]. The ‘third body’ approach envisages a contact that is made out of two 231
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(1) The sintered sample has 98% of the theoretical density (TD), and microstructure contained ZrB2 and SiC phases. (2) The hardness value and elastic modulus for ZrB2 20 vol% SiC is 17.5 GPa and 441 MPa, respectively. (3) The specific wear rates have been established to possess an order of magnitude of ~10−6–10−8, which indicates an abrasive wear mechanism. (4) Specific wear rate of the investigated composites increased with the rise in applied load. (5) Specific wear rate of the ZrB2 20 vol% SiC composite reduced with the increase of sliding speeds. (6) X-ray diffraction analyses of worn surfaces propose that phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. (7) Microstructural characterization of worn surfaces suggests that wear mechanism in the main controlled by grain fracture, microcracking and grain pullout.
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