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Wear resistance of ZrB2 based ceramic composites
International Journal of Refractory Metals & Hard Materials 81 (2019) 214–224
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Wear resistance of ZrB2 based ceramic composites a
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Dávid Medveď , Ján Balko , Richard Sedlák , Alexandra Kovalčíková , Ivan Shepa , Annamária Naughton-Duszováb, Elżbieta Bączekb, Marcin Podsiadłob, Ján Duszaa a b
Institute of Materials Research, Slovak Academy of Sciences, Division of Ceramic and Non-Metallic Systems, Watsonova 47, 040 01 Košice, Slovak Republic The Institute of Advanced Manufacturing Technology, Wroclawska 37a St., 30-011 Krakow, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: ZrB2 based composites Tribology Wear mechanisms
The wear resistance and tribological characteristics of spark plasma sintered ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites were investigated under dry sliding conditions at applied loads of 5 N and 50 N in air. The microstructure, deformation and damage characteristics were studied using scanning electron microscopy, confocal electron microscopy, a focused ion beam and atomic force microscopy. The friction coefficient values were very similar for all composites with values ranging from 0.63 to 0.72 and with the lowest value recorded for the ZrB2 + SiC composite at a 5 N applied load. The ZrB2 + ZrC composite was the most wear resistant, with wear rates at a 5 N load of 6.15 × 10−6 mm3/(N·m) and at a 50 N of 7.3 × 10−6 mm3/(N·m). None, or a very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all composites, with similar chemical compositions but different sizes and thicknesses at the 50 N load.
1. Introduction Ultra-high temperature ceramics (UHTCs) are compounds in which boron, carbon or nitrogen combine with one of the early transition metals, such as Zr, Hf, Ti, Nb and Ta, and have melting points above 3000 °C [1,2]. This emerging class of materials has the potential for applications in extreme environments such as aerospace, but also in applications such as advanced nuclear fission reactors, high temperature electrodes for metal refining and many others [3,4]. Zirconium diboride (ZrB2), a transition metal boride compound, is a member of the UHTC family and besides its high melting point, it is also characterised by higher thermal conductivities and lower electrical resistivity at room temperature than carbide or nitride ceramics. The densification of ZrB2 without additives is difficult due to its low intrinsic sinterability, resulting from the strong covalent bonding of ZrB2, which leads to a low volume and grain boundary diffusivities. This is one of the reasons why, over the last decade, different ZrB2 based particulate composites with SiC, B4C, MoSi2, etc., have been developed with enhanced sinterability, mechanical properties and oxidation resistance [5–12]. These ZrB2 ceramics and ZrB2 based composites have been densified by various methods including hot pressing (HP), spark plasma sintering (SPS), reactive hot pressing (RHP) and pressureless
⁎
sintering (PS). ZrB2 + SiC composites have received considerable attention due to their improved oxidation resistance, densification behaviour and mechanical properties including strength and fracture toughness [8,13,14]. Boron carbide (B4C) is a widely used sintering additive for ZrB2, allowing it to reach its full density at a relatively low temperature, improving densification by removing surface oxides from the starting powders and pinning the grain growth as a second phase inclusion [10–12,15,16]. E.W. Neuman et al. [17] produced dense ZrB2–9.5 vol% ZrC – 0.1 vol% C ceramics by hot pressing at 1900 °C and investigated their mechanical properties at up to 2300 °C in an argon atmosphere. The flexure strength and fracture toughness values changed at different temperatures, from 695 MPa to 300 MPa and from 4.8 MPa m1/2 to 3.6 MPa·m1/2, respectively. There are several papers dealing with the characterization of the wear of ZrB2 based or similar ceramics using different tribological methods [18–22]. Among the first is a study by K. Umeda et al. [18], who investigated the tribochemical response of hot pressed ZrB2, B4C, ZrB2 + B4C and ZrB2 + B4C + SiC ceramics in air and in de-ionized water using a reciprocating pin-on-block test. The coefficients of friction were about 0.95 for all systems at a low relative humidity (< 20%), but they decreased with an increasing relative humidity. For
Corresponding author. E-mail address: [email protected] (R. Sedlák).
https://doi.org/10.1016/j.ijrmhm.2019.03.004 Received 14 November 2018; Received in revised form 2 March 2019; Accepted 3 March 2019 Available online 04 March 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
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200 rpm for 2 h, with a WC bowl and milling balls using isopropanol as a milling media to obtain the mixed powders. The dried mixtures were put into a graphite die with an inner diameter of 20 mm with graphite foil and were densified using spark plasma sintering (FCT HP D 5; FCT Systeme GmbH, Germany). The sintering was performed in temperatures ranging from 1800 °C to 2050 °C, 50 MPa for 10 min in argon, with a heating rate of 100 °C/min.
a high r.h. of about 95%, the friction coefficients ranged from 0.2–0.3, except for that of ZrB2 which was about 0.4. S. Chakraborty et al. [19] investigated the influence of direct current pulse on–off patterns on densification, and tribological properties of the spark plasma sintering of ZrB2 using a scratch test. It was found that the maximum relative density (98.65%) is achieved at 50 ms pulse on (ton) and 5 ms pulse off (toff) time conditions, and that the wear volume and wear rate at a 10 N load was 4.05 × 104 μm3 and 1.01 × 10−3 mm3/(N·m), respectively. The scratch resistance and wear parameters of ZrB2 + SiC composites, prepared by hot pressing with different sources of SiC, were investigated by D. Debnath et al. [20]. It was found that an interconnected network, as well better contiguity between grains of ZrB2 + SiC composites and the impurity content of the starting powders, can play significant roles in achieving high tribological properties of the composites. Recently, J. He et al. [13] synthesised ZrB2 + SiC composite ceramics with varying compositions through spark plasma sintered (SPS) for 30 min under an argon atmosphere to produce materials having different inter- and trans-granular fractures that showed different wear loss, friction efficient and tribofilm morphologies. The characteristics of silica/hydride silica revealed the formation of tribofilms with different morphologies, thereby implying that several key factors are involved in determining the efficiency of this process. Chakraborty et al. [21] reported the effect of an addition of submicrometer-sized B4C on the microstructure, phase composition, scratch resistance and wear resistance of hot pressed ZrB2 + B4C composites. They found improved tribological properties due to crack deflection with a homogeneously dispersed submicrometer-sized B4C in the ZrB2 matrix. B. M. Moshtaghiouna et al. [22] investigated the abrasive wear resistance of boron carbide as a function of the grain size and porosity. They found that, in contrast to most ceramics, in the studied B4C the grain size was not directly relevant to the wear resistance, whereas the porosity and hardness played the main roles. Although there is an increased demand for ZrB2 based ceramics for tribological applications, only a few studies have examined their tribological performance. Moreover, to our knowledge, there has been no systematic study exploring the roles of different carbide additions on the microstructure, wear and tribological characteristics of zirconium diboride, which is important for the development and fabrication of the next generation of UHTCs. The aim of the present contribution is to investigate the wear and tribological characteristics of ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites under dry sliding conditions in air at room temperature and to describe the dominant wear mechanisms.
2.2. Investigation methods The bulk density of the specimens was measured by the Archimedes' method, using distilled water as the immersing medium according to ASTM C373–88. The relative density was calculated by dividing the Archimedes' density by the density predicted from the nominal ZrB2, B4C, SiC and ZrC. The microstructure of the materials was studied with a scanning electron microscope (ZEISS AURIGA). Young's modulus of the composites was determined by the ultrasonic wave transition method, by measuring the velocity of ultrasonic sound waves passing through the material using an ultrasonic flaw detector (Panametrics Epoch III). The hardness and the fracture toughness were determined by the Vickers indentation method, by applying a load of 9.81 N (1 kg) and 98 N (10 kg) with a Future Tech FLC-50VX hardness tester. For each sample, 10 indentations were made and the stress intensity factor KIC was calculated from the length of the cracks which developed during a Vickers indentation test using Niihara’s equation [23]. Tribology measurements were carried out on UMT 3 (Bruker) equipment using the ball-on-flat technique. The wear behaviour of the experimental materials was studied during dry sliding in air. The tribological partner was a highly polished (roughness Ra < 0.10 μm according to ISO 3290) SiC ball with a 6.35 mm diameter. Two applied loads, 5 and 50 N, were used with the aim to study the influence of different applied loads on the wear characteristics. The sliding speed was 0.1 m/s and the sliding distance 500 m. The experiments were realized at room temperature at the relative humidity of 40 ± 5%. The material losses (volume of the wear tracks) due to wear were measured by the PLu Neox 3D Optical Profiler, a high precision confocal microscope by SENSOFAR, and the specific wear rates (Ws) were then calculated in terms of the volume loss (V) per distance (L) and applied load (FN), according to the equation:
WS =
V FN ⋅L
The microstructure, fracture and the wear damage were studied using scanning electron microscopy (SEM, JEOL JSM 7000, ZEISS AURIGA) and confocal electron microscopy. The deformation characteristics were studied using atomic force microscopy. A focused ion beam (FIB, ZEISS AURIGA) was used for the section preparation and the investigation below the worn surfaces of the investigated composites Table 1.
2. Experimental materials and methods 2.1. Experimental materials Commercial ZrB2 (1.5–3.0 μm; H.C. Starck), α-SiC (0.75 μm; H.C. Starck), B4C (0.6–1.2 μm; H.C. Starck) and ZrC (3 μm, H.C. Starck) powders were used to fabricate the experimental composites. The investigated zirconium diboride based systems ZrB2 + 10 wt% B4C, ZrB2 + 10 wt% SiC, and ZrB2 + 10 wt% ZrC were prepared in a planetary ball mill (Pulverisette 6, Fritsch Co. Ltd.) at a rotation speed of
3. Results and discussion The characteristic microstructures of the investigated composites are illustrated in Fig. 1. As they were visible, the mean grain size of the ZrB2 grains in the composites ZrB2 + B4C and ZrB2 + SiC was approximately 2 μm, and the grain sizes of SiC and B4C were
Table 1 Apparent density, porosity, hardness, elastic modulus and fracture toughness of the ZrB2 based composites. Experimental materials
Apparent density (g/cm3)
Porosity (%)
Hardness HV1 (GPa)
Elastic modulus (GPa)
Fracture toughness (MPa·m1/2)
ZrB2 + SiC ZrB2 + B4C ZrB2 + ZrC
5.510 5.280 6.100
1.3 0.9 0.3
17.33 ± 1.23 19.02 ± 1.07 14.73 ± 1.12
465 460 484
4.47 ± 0.43 4.4 ± 0.27 5.26 ± 0.69
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Fig. 1. Microstructure of the investigated ceramic composites: a) ZrB2 + 10 wt% B4C, b) ZrB2 + 10 wt% SiC and c) ZrB2 + 10 wt% ZrC.
Fig. 3. Wear rate of the investigated ceramic composites at applied loads of 5 N and 50 N.
Fig. 2. Coefficient of friction of the investigated ceramic composites.
approximately 1 μm. The composite ZrB2 + ZrC was slightly different: the mean grain size of the ZrB2 grains was higher at approximately 3.5 μm; and the ZrC grains were smaller with a size ranging from 0.5 to 1.0 μm. The coefficients of frictions for the investigated composites, obtained from the wear test at loads of 5 N and 50 N, are illustrated in Fig. 2. The average values of the COF were very similar and were between 0.63 and 0.72, with a lowest value of 0.63 in the case of the ZrB2 + SiC composite tested at the 5 N applied load. No significant and systematic influence of the applied load was found on the values of the COF: with increasing load, the COF for ZrB2 + B4C was slightly decreasing; and on the other hand for ZrB2 + SiC and ZrB2 + ZrC it was slightly increasing. The wear rate of the investigated systems is shown in Fig. 3 at different applied loads. The average wear rates of the ZrB2 + SiC and ZrB2 + ZrC tested at the applied load 5 N were very similar and were around 6.5 × 10−6 mm3/(N·m), while the average wear rate of the ZrB2 + B4C system at the same load was higher at around 1.1 × 10−5 mm3/(N·m). With the increasing applied load, the wear rate was increasing for all systems. In the case of ZrB2 + B4C, it showed the highest value of 1.42 × 10−5 mm3/(N·m), and for ZrB2 + SiC it showed the lowest value of approximately 7.7 × 10−6 mm3/(N·m). The characteristic wear track profiles of the composites at loads of 5 N and 50 N are illustrated in Figs. 4 and 5. In the case of the 5 N applied load, the initial track width for all studied systems was approximately 800 μm, while the maximal track depth changed from 5 μm to 12 μm. After the wear test with a 50 N load, the track width ranged from 2.5 to 3.0 mm and the maximal wear track depth was approximately 25 μm. It is interesting to note that the highest wear rate in the case of ZrB2 + B4C during the wear test with the applied load of 5 N exhibited the highest wear depth, but on the other hand, the highest wear track width was recorded during the wear test with the 50 N load.
Fig. 4. Characteristic wear tracks of the investigated ceramic composites after the wear test with an applied load of 5 N.
The characteristic wear tracks of the ZrB2 + B4C system and the corresponding SEM + EDX and FIB+EDX analyses are shown in Figs. 6–8. In Fig. 6. the wear tracks of ZrB2 + 10 wt% B4C are illustrated after the 5 and 50 N applied loads. Based on this study we can say that, at an applied load of 5 N, the wear is mostly in the form of mechanical wear-abrasive grooves connected with a deformation of the matrix grains and additive grains, without any grain pull-outs or grain failure. We frequently detected cracks normal to the wear direction, with a size ranging from 2 μm to 5 μm, intragranularly in the ZrB2 grains. In some cases, we found interconnections of these cracks with a final size of 10–15 μm. 216
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this, it is evident that the thickness of the tribolayer covering both the ZrB2 and B4C grains was up to 0.5 μm, containing mainly zirconium and oxygen. It is important to note that we found no damage below the worn surface in the form of microcrack nucleation/propagation. The characteristic wear tracks of the ZrB2 + SiC system and the corresponding SEM + EDX and FIB+EDX analyses are shown in Figs. 9–11. We can see that at an applied load of 5 N, similarly to the case of ZrB2 + B4C, the wear was mostly in the form of abrasive grooves connected with a deformation of the matrix grains and SiC without grain pull-outs. Similarly, we frequently detected cracks normal to the wear direction, with sizes measuring from 5 μm to 20 μm, intragranularly in the ZrB2 grains. Micro-areas of wear track covered by tribofilms with a size of approximately 3 μm were detected as well. The wear track of this system after the wear with an applied load of 50 N showed elongated areas covered by a tribolayer, with sizes up to 150 μm in a direction parallel to the wear direction. A characteristic “striation” in the tribolayers was found perpendicular to the wear direction. The chemical analyses results of the individual phases on the worn surface of the ZrB2 + SiC system are illustrated in Fig. 10. According to this, the tribolayer contains an average of 38 wt% zirconium, 36 wt% oxygen, 14 wt% silicon and 12 wt% carbon. In Fig. 11, the wear damage and chemical composition of the worn surface and the FIB cut/polished area below the surface in the volume of this ceramic system is shown. From this, it is evident that the thickness of the tribolayer covering both ZrB2 and SiC grains was up to 1.5 μm, containing mainly zirconia and oxygen. Similarly, as in the case of the ZrB2 + B4C composite, we found no damage below the worn surface in the form of microcrack formations. The characteristic wear tracks of the ZrB2 + ZrC system and the corresponding SEM + EDX analyses are shown in Figs. 12–13. At an applied load of 5 N, similarly as in the case of the ZrB2 + B4C and ZrB2 + SiC composites, the wear was mostly in the form of abrasive grooves connected with deformations of the matrix grains and ZrC without grain pull-outs. On the other hand, in this system we found microcracks normal to the wear direction only randomly. Micro-areas of the wear track covered by a tribofilm with a size of approximately
Fig. 5. Characteristic wear tracks of the investigated ceramic composites after a wear test with the applied load of 50 N.
Micro-areas of wear track covered by tribofilms in sizes from 1 μm to 5 μm were detected as well. The wear track of this system after the wear with an applied load of 50 N looked different, mainly as regarding the areas covered by the tribolayer, as the size of these areas was up to 80–100 μm in a direction parallel to the wear direction. In this case, we found fewer microcracks in comparison to the wear track created at the applied load of 5 N. The chemical analyses results of the individual phases on the worn surface of the ZrB2 + B4C system are illustrated in Fig. 7. According to this, the tribolayer contained an average 47 wt% zirconium, 35 wt% oxygen, 12 wt% boron, 5 wt% carbon and 0.5 wt% silicon. The chemical composition of the areas with thin tribolayer, positions 2 and 3, is similar to the composition of the constituents of the tested composite, without a detectable amount of oxygen. In Fig. 8, the wear damage and chemical composition of the worn surface and the FIB cut/polished area below the surface in the volume of the ceramic system is shown. From
Fig. 6. Wear tracks of ZrB2 + 10 wt% B4C after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c). 217
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Fig. 7. EDX analysis of the wear track in the ZrB2 + 10 wt% B4C system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - tribolayer, spectrum 2 - ZrB2 grain, spectrum 3 - B4C/ZrB2 boundary.
10 μm were detected. As a clear difference in this system, in comparison to the previous ones, we found that submicron/nano-sized debris formations occurred during the wear process. Maybe this is the reason that the abrasive grooves are less visible and the wear surface is smooth, in comparison to the wear surface of ZrB2 + B4C and ZrB2 + SiC composites after the wear test at applied load of 5 N. The wear track of this system, after the wear with an applied load of 50 N, showed elongated areas covered by tribolayers, with sizes up to 200 μm in the parallel direction and up to 25 μm wide perpendicular to the wear direction, respectively. Similar “striation” in the tribolayers was found perpendicular to the wear direction as was found in the ZrB2 + SiC system, but in this case it is not so marked. The thickness of the tribolayer covering both ZrB2 and ZrC grains, measured on FIB section, was in the range of 0.5 to 1.2 μm. The results of the chemical analyses of individual phases on the worn surface of the ZrB2 + ZrC system are illustrated in Fig. 13. According to these, the tribolayer contains an average of 55 wt% zirconium, 25 wt% oxygen, 13 wt% boron and 8 wt% carbon. The chemical composition of the areas with thin tribolayer is similar to the composition of the constituents of the tested composite, with presence of oxygen and hafnium. Similarly to the case of the previous ceramic composites, abrasive grooves and microcracks were the most observed characteristic features of wear scar at the lower loads, whereas at a higher load the debris formation, oxidation and tribolayer formation features became more dominant. It is well known that the wear behaviour of ceramics critically depends on numerous factors, including the testing parameters (e.g. normal load, test duration, surface roughness, environmental conditions, tribochemical stability, etc.) and the microstructure and mechanical properties of the tested material. Therefore, it is not
straightforward to compare the wear characteristics, even when testing the same system in different laboratories using different testing methods. But on the other hand, it is interesting to see the tendencies, as well as the influence of the composition, processing route and microstructure on the wear characteristics of the studied systems, in spite of the fact that they have been investigated using different methods. S. Chakraborty et al. [19,21,24], for the tribological characterization of similar zirconium diboride based ceramics as in our study, performed scratch tests at room temperature. They applied dry and unlubricated conditions at a relative humidity level of 60 ± 5%, using a Vickers' type diamond tip at loads of 5 N and 10 N with an indenter velocity of 0.1 mm/s. The effect of DC pulse on–off patterns during the spark plasma sintering of ZrB2 on the densification, mechanical and tribological properties.was investigated, too. During the study, the effect of an addition of submicrometer-sized B4C on the tribological characteristics of hot pressed ZrB2-B4C composites reported lower values of the COF for the composites in comparison to monolithic ZrB2 and an oscillation in the COF dates. This was explained by the creation of an intermediate debris layer on the moving track due to the surface deformation under the moving stylus, which makes the wear track rougher and creates the oscillation. They found that, of all the wear parameters, the best was observed in the case of the ZrB2 + 10 wt% B4C composite. During the investigation of the mechanical, thermal and some tribological properties of the ZrB2 + 20 wt% SiC composite sintered by SPS at different temperatures, they reported the best wear parameters for the system sintered at a temperature of 1900 °C. Microcracks and fractured debris formation were observed on the worn track, mainly in the system prepared at the highest temperature. Among the first studies, K. Umeda [18] investigated the tribochemical responses of hot pressed boride ceramics, such as ZrB2, B4C, ZrB2 + B4C and ZrB2 + B4C + SiC, in air and in de-ionized water. 218
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Fig. 8. EDX map of the worn surface and FIB section below the wear track in the ZrB2 + 10 wt% B4C system after the wear test with applied load of 50 N. a) SEM of the wear track and FIB section below the wear track, b) oxygen EDX map, c) zirconium EDX map and d) boron EDX map.
Fig. 9. Wear tracks of ZrB2 + 10 wt% SiC after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c).
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Fig. 10. EDX analysis of the wear track in the ZrB2 + 10 wt% SiC system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - ZrB2 grain, spectrum 2 - SiC/ZrB2 boundary and spectrum 3 - tribolayer.
different compositions via the polymeric precursor route and investigated the effect of the microstructures in the tribological properties of these materials. The wear tests were performed under conditions similar to those in our tests, using a ball-on-flat method at room temperature and a 35% relative humidity with a SiC ball as a counterpart that slid against the sample discs of 5 and 10 N loads and a radius of 3 mm. The wear tests of ZrB2 + 80 vol% SiC and ZrB2 + 40 vol% SiC showed that, in all cases and in agreement with our results, the wear loss increased with the applied load. It increased rapidly for the ZrB2 + 40 vol% SiC for loads higher than 5 N. On the other hand, this composite showed the lowest frictional coefficient, with a value of approximately 0.5 at the 5 N load, which was even lower than the COF of the systems measured in our study. This system showed a COF value at the 10 N load that was significantly higher, at approximately 0.8. They found also the presence of tribofilms that partially covered the wear track, generated directly during the wear process of the composites. These tribofilms were more continuous in the system with a higher volume fraction of SiC. Such a tribofilm can prevent the surface from wear; however, the morphology, thickness, chemical composition, etc., are considered key factors when estimating the protective efficiency of the tribofilm. T. Murthy et al. [25] investigated the microstructure and wear properties of B4C + 5 wt% ZrB2 composites formed in-situ under dry conditions at room temperature and at a 45 ± 5% relative humidity, using ball-on-flat reciprocative sliding wear tests conducted at loads of 5 N, 10 N and 20 N against a WC-Co ball. Increasing the load from 5 N to 20 N resulted in a decrease in the COF by 37.5% at the 10 Hz frequency, from 0.24 to 0.15. These values of the COF were significantly lower in comparison to the values measured in our experiment, which can be explained by the different chemical composition of the
Sliding tests were made using a reciprocating type pin-on-block machine with a pin size of 4 × 15 mm and a hemispherical tip radius of 2 mm, where the block size was 10 × 15 × 6.5 mm. Both the pin and the block were made of the same material and the tests were made at a load of 7.8 N with a sliding speed of 1.5 mm/s in a glove box, in which the relative humidity was increased in steps from 10% to 95%. Some of these testing conditions are comparable with the conditions used during the present investigation. According to their results, the coefficients of friction were about 0.95 for all the ceramics at low relative humidity (< 20%), but decreased with an increasing relative humidity to 0.2–0.3 for the composites and B4C, and it was higher only for ZrB2 at about 0.4. During the test at a relative humidity comparable to that used in our experiment, their COF results for monolithic ZrB2 were higher in comparison to our results, while the composites ZrB2 + B4C and ZrB2 + B4C + SiC exhibited a similar COF as our composites. A lower COF was found in the case of monolithic B4C. In addition, they found similar Hertzian type cracks on the worn surfaces of ZrB2 during the test at a relative humidity of approximately 40%, as we found in the case of the ZrB2 + B4C and ZrB2 + SiC composites. On the other hand, at this testing condition in the case of B4C and the ZrB2 + B4C and ZrB2 + B4C + SiC composites, such cracks did not occur, similarly to our ZrB2 + ZrC composites. The length of these Hertzian cracks in the case of their experiments had a size of approximately 100 μm, while in our case these cracks were shorter, ranging from several micron to approximately 20 μm according to the applied load. In our case, this was probably connected with the wider worn track width and lower tensile stresses behind the spherical rider. They also reported, contrary to our results, no significant tribolayer formations during their experiments. Recently, J. He et al. [13] prepared ZrB2–SiC composites with three 220
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Fig. 11. EDX map of the worn surface and the FIB section below the wear track in the ZrB2 + 10 wt% SiC system after the wear test with applied load of 50 N. a) SEM of the wear track and FIB section below the wear track, b) oxygen EDX map, c) zirconium EDX map and d) silicon EDX map.
Fig. 12. Wear tracks of ZrB2 + 10 wt% ZrC after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c). 221
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Fig. 13. EDX analysis of the wear track in the ZrB2 + 10 wt% ZrC system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - ZrB2 grain, spectrum 2 - tribolayer, spectrum 3 - ZrC grain.
B2O3 as a liquid layer:
composites, but also by a different counterpart and a higher relative humidity. The reported specific wear rate data, in order of 10−5 mm3/ (N·m), is in agreement with the values measured for the ZrB2 + B4C composite in our experiment. Both abrasive and tribo-chemical reaction wear mechanisms were observed on the worn surface of the flat and the counter-body materials. A dominant tribo-chemical reaction wear mechanism was observed at higher loads, such as 10 N. Similarly, as occurred during our investigation of ZrB2 based composites, they found that the severity of the damage during reciprocative sliding increased with an increasing applied load. This is evidence that, during the sliding wear test, mechanical wear is responsible for the formation of abrasive grooves, and the intensity of these grooves on the wear track is diminished as the load increases due to the formation of a tribo-oxidative chemical layer. The low COF and specific wear rate recorded in this study was probably caused by the formation of a tribo-oxide layer with a lubricant property. The heat generated due to friction between the sliding surfaces was probably sufficient for the oxidation of B4C according to the reaction (1) to form B2O3 [26]. The oxygen required for this reaction was supplied by the surrounding air. During the cooling to room temperature (after the test), the B2O3 phase reacted with the moisture in the air and formed a secondary film of boric acid, H3BO3 (in the reactions: cr – crystal, amph – amorphous phase, l – liquid, g – gas):
B4 C + 7/2 O2 → 2 B2 O3 (l) + CO (g )
(1)
B2 O3 (amph) + 3 H2 O → 2 H3 BO3 (amph/ cr )
(2)
ZrB2 (cr ) + 5/2 O2 → ZrO2 (cr ) + B2 O3 (l)
(3)
The formed B2O3 scale is non-protective since it has a high vapour pressure, so at temperatures above 450 °C it exists in a liquid form that readily evaporates, exposing the formed zirconia grains and the unoxidized underlying bulk. However, in a liquid state it can play the role of a lubricant and can change the COF. The oxidation of the ZrB2 + B4C composite is a far more complex process. Both the components oxidate and reactions 1, 2 and 3 take place. But the B4C can also interact with the formed ZrO2 and reduce it to ZrB2:
7 ZrO2 (cr ) + 5 B4 C (cr ) → 7 ZrB2 + 5 CO (g ) + 3 B2 O3 (l)
(4)
The presence of SiC improves the oxidation resistance of the composite. This effect is well-studied:
SiC (cr ) + 3/2O2 → SiO2 (amph) + CO (g )
(5)
Between 800 and 1200 °C, the reaction (4) is the dominant chemical process, resulting in passive oxidation behaviour due to the continuous formation of the liquid boria layer. At higher temperatures, the SiC begins to oxidize according to the reaction (5) and as a result, it forms the amorphous protective layer of SiO2 and/or borosilicate glass with the B2O3. This glassy layer fills the pores and protects the underlying material from further deep oxidation. In principle, the formation of this glassy layer is possible even during tribotests. In the results of the EDX analysis, boron wasn't detected in the tribolayer. This means that the contact temperature during the test was high enough that the formed B2O3 evaporated. The thermal oxidation process, along with the thermodynamic approach concerning the formation mechanism of the protective layer, was recently described in detail [27,28].
The oxidation of ZrB2, HfB2, ZrC, HfC and the related ceramics has been already described in a number of articles [25–30]. Based on the literature, it is known that at elevated temperatures (above 1000 °C), zirconium diboride, similarly to carbide, is not resistant to oxidation. When exposed to air, ZrB2 reacts with oxygen to form solid ZrO2, with 222
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similar chemical composition, but with different lateral/vertical dimensions at the 50 N load.
Table 2 Summary of the tribosurfaces analysis (SiC ball used). Elements
Wt%
At.%
Composition
Acknowledgements ZrB2 + B4C Zr O B C Si
47 35 12 5 0.5
12.13 51.51 26.14 9.80 0.42
ZrO2, B4C, B2O3/H3BO3, traces of SiO2 (from the ball)
ZrB2 + SiC Zr O B C Si
38 36 0 12 14
10 54.03 – 23.99 11.97
ZrO2, SiC, SiO2
ZrB2 + ZrC Zr O B C Si
55 25 13 8 0
14.95 38.73 29.81 16.51 –
ZrO2, ZrB2, ZrC, B2O3/H3BO3
The authors gratefully acknowledge the financial support from the following projects: APVV-15-0469, APVV-14-0385, VEGA 2/0163/16, VEGA 2/0130/17. This work was realized within the framework of the Research Centre of Advanced Materials and Technologies for Recent and Future Applications “PROMATECH” Project, ITMS 26220220186, supported by the “Research and Development” Operational Programme financed through European Regional Development Fund. The authors acknowledge the support of the FNP Project No. POWROTY/2016. The POWROTY/2016-1/3 Project is carried out within the Powroty / Reintegration Programme of the Foundation for Polish Science and is co-financed by the European Union under the European Regional Development Fund. References [1] W. Fahrenholtz, E. Wuchina, W. Lee, Y. Zhou, Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications, (2014). [2] D. Sciti, L. Silvestroni, V. Medri, F. Monteverde, Sintering and Densification Mechanisms of Ultra-High Temperature Ceramics. Ultra-High Temp. Ceram, John Wiley & Sons, Inc, Hoboken, NJ, 2014, pp. 112–143. [3] W.G. Fahrenholtz, G.E. Hilmas, Ultra-high temperature ceramics: materials for extreme environments, Scr. Mater. 129 (2017) 94–99. [4] M.J. Gasch, D.T. Ellerby, S.M. Johnson, Ultra High Temperature Ceramic Composites. Handb. Ceram. Compos, Springer, US, 2005, pp. 197–224. [5] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Processing, microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics, Ceram. Int. 43 (2017) 6942–6948. [6] S.Q. Guo, Densification of ZrB2-based composites and their mechanical and physical properties: a review, J. Eur. Ceram. Soc. 29 (2009) 995–1011. [7] F. Monteverde, S. Guicciardi, A. Bellosi, Advances in microstructure and mechanical properties of zirconium diboride based ceramics, Mater. Sci. Eng. A 346 (2003) 310–319. [8] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Mechanical behavior of zirconium diboride-silicon carbide ceramics at elevated temperature in air, J. Eur. Ceram. Soc. 33 (2013) 2889–2899. [9] E.J. Cheng, Y. Li, J. Sakamoto, S. Han, H. Sun, J. Noble, et al., Mechanical properties of individual phases of ZrB2-ZrC eutectic composite measured by nanoindentation, J. Eur. Ceram. Soc. 37 (2017) 4223–4227. [10] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Mechanical behavior of zirconium diboride-silicon carbide-boron carbide ceramics up to 2200°C, J. Eur. Ceram. Soc. 35 (2015) 463–476. [11] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, Pressureless sintering of zirconium diboride using boron carbide and carbon additions, J. Am. Ceram. Soc. 90 (2007) 3660–3663. [12] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Pressureless densification of zirconium diboride with boron carbide additions, J. Am. Ceram. Soc. 89 (2006) 1544–1550. [13] J. He, Y. Cao, Y. Zhang, Y. Wang, Mechanical properties of ZrB2–SiC ceramics prepared by polymeric precursor route, Ceram. Int. 44 (2018) 6520–6526. [14] Z. Balak, M. Azizieh, H. Kafashan, M.S. Asl, Z. Ahmadi, Optimization of effective parameters on thermal shock resistance of ZrB2-SiC-based composites prepared by SPS: using Taguchi design, Mater. Chem. Phys. 196 (2017) 333–340. [15] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2-B4C binary composites, Ceram. Int. 41 (2015) 379–387. [16] S. Chakraborty, P.K. Das, D. Ghosh, Spark plasma sintering and structural properties of ZrB2 based ceramics: a review, Rev. Adv. Mater. Sci. 44 (2016) 182–193. [17] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, M. Cinibulk, Ultra-high temperature mechanical properties of a zirconium diboride-zirconium carbide ceramic, J. Am. Ceram. Soc. 99 (2016) 597–603. [18] K. Umeda, Y. Enomoto, A. Mitsui, K. Mannami, Friction and wear of boride ceramics in air and water, Wear 169 (1993) 63–68. [19] S. Chakraborty, A.R. Mallick, D. Debnath, P.K. Das, Densification, mechanical and tribological properties of ZrB2 by SPS: effect of pulsed current, Int. J. Refract. Met. Hard Mater. 48 (2015) 150–156. [20] D. Debnath, S. Chakraborty, A.R. Mallick, R.K. Gupta, A. Ranjan, P.K. Das, Mechanical, tribological and thermal properties of hot pressed ZrB2–SiC composite with SiC of different morphology, Adv. Appl. Ceram. 114 (2015) 45–54. [21] S. Chakraborty, D. Debnath, A.R. Mallick, P.K. Das, Mechanical, tribological, and thermal properties of hot-pressed ZrB2–B4C composite, Int. J. Appl. Ceram. Technol. 12 (2015) 568–576. [22] B.M. Moshtaghioun, D. Gomez-Garcia, A. Dominguez-Rodriguez, R.I. Todd, Abrasive wear rate of boron carbide ceramics: influence of microstructural and mechanical aspects on their tribological response, J. Eur. Ceram. Soc. 36 (2016) 3925–3928. [23] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of KIC of brittle solids by the
The oxidation of ZrC can be described as the following reaction:
ZrC (cr ) + 3/2O2 → ZrO2 (cr ) + CO (g )
(6)
Most of the reactions correspond to the results of thermal oxidation experiments performed at temperatures ranging from 900 to above 1500 °C, in stagnant air, in oxygen and/or in air flows. But the developed models and thermodynamic approaches also describe the oxidation processes very well. According to the results of our experiments and the EDX analysis (summarised in Table 2), we can assume that the tribolayers consisted mostly of ZrO2 with small amounts of the original materials (debris of ZrB2, B4C, SiC, ZrC), as well as SiO2 and hydrated B2O3, formed due to reaction (2). The hydration of B2O3 explains the high amount of oxygen registered by the EDX analyses. The local temperature at the contact point between the ball and the substrate material can probably reach up to 800–1000 °C. It may be not enough for a high oxidation rate, but a mechanical energy activation of the reaction also takes place, which decreases the temperature needed for the formation of the oxide products. 4. Conclusions The aim of the present contribution was to investigate the wear characteristics of ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites under dry sliding conditions in air at room temperature and to describe the dominant wear mechanisms. The main results are as follows:
• The friction coefficient values were very similar for all composites, •
• • •
with values in the range of 0.63 to 0.72, and with the lowest value recorded for the ZrB2 + SiC composites at the 5 N applied load. The highest wear rate was found in the case of ZrB2 + B4C, at 1.42 × 10−5 mm3/(N·m) for 50 N and at 1.095 × 10−5 mm3/(N·m) for the 5 N load, respectively. The ZrB2 + SiC and ZrB2 + ZrC composites show similar wear rates at the 5 N load, of approximately 6.15 × 10−6 mm3/(N·m), while at 50 N the ZrB2 + ZrC composite was more wear resistant with a wear rate value of 7.3 × 10−6 mm3/(N·m). None, or very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all the composites with a 223
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27 (2007) 2495–2501. [27] H.L. Liu, J.X. Liu, H.T. Liu, G.J. Zhang, Changed oxidation behavior of ZrB2-SiC ceramics with the addition of ZrC, Ceram. Int. 41 (2015) 8247–8251. [28] R. He, Z. Zhou, Z. Qu, X. Cheng, High temperature flexural strength and oxidation behavior of hot-pressed B4C-ZrB2 ceramics with various ZrB2 contents at 10001600°C in air, Int. J. Refract. Met. Hard Mater. 57 (2016) 125–133. [29] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds, J. Eur. Ceram. Soc. 19 (1999) 2405–2414. [30] W.G. Fahrenholtz, Thermodynamic analysis of ZrB2-SiC oxidation: formation of a SiC-depleted region, J. Am. Ceram. Soc. 90 (2007) 143–148.
indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [24] S. Chakraborty, D. Debnath, A.R. Mallick, R.K. Gupta, A. Ranjan, P.K. Das, et al., Microscopic, mechanical and thermal properties of spark plasma sintered ZrB2 based composite containing polycarbosilane derived SiC, Int. J. Refract. Met. Hard Mater. 52 (2015) 176–182. [25] T.S.R.C. Murthy, S. Ankata, J.K. Sonber, K. Sairam, K. Singh, A. Nagaraj, et al., Microstructure, thermo-physical, mechanical and wear properties of in-situ formed boron carbide - zirconium diboride composite, Ceramics-Silikáty 62 (2017) 1–10. [26] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Evolution of structure during the oxidation of zirconium diboride-silicon carbide in air up to 1500 °C, J. Eur. Ceram. Soc.
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Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Wear resistance of ZrB2 based ceramic composites a
a
a,⁎
T a
a
Dávid Medveď , Ján Balko , Richard Sedlák , Alexandra Kovalčíková , Ivan Shepa , Annamária Naughton-Duszováb, Elżbieta Bączekb, Marcin Podsiadłob, Ján Duszaa a b
Institute of Materials Research, Slovak Academy of Sciences, Division of Ceramic and Non-Metallic Systems, Watsonova 47, 040 01 Košice, Slovak Republic The Institute of Advanced Manufacturing Technology, Wroclawska 37a St., 30-011 Krakow, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: ZrB2 based composites Tribology Wear mechanisms
The wear resistance and tribological characteristics of spark plasma sintered ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites were investigated under dry sliding conditions at applied loads of 5 N and 50 N in air. The microstructure, deformation and damage characteristics were studied using scanning electron microscopy, confocal electron microscopy, a focused ion beam and atomic force microscopy. The friction coefficient values were very similar for all composites with values ranging from 0.63 to 0.72 and with the lowest value recorded for the ZrB2 + SiC composite at a 5 N applied load. The ZrB2 + ZrC composite was the most wear resistant, with wear rates at a 5 N load of 6.15 × 10−6 mm3/(N·m) and at a 50 N of 7.3 × 10−6 mm3/(N·m). None, or a very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all composites, with similar chemical compositions but different sizes and thicknesses at the 50 N load.
1. Introduction Ultra-high temperature ceramics (UHTCs) are compounds in which boron, carbon or nitrogen combine with one of the early transition metals, such as Zr, Hf, Ti, Nb and Ta, and have melting points above 3000 °C [1,2]. This emerging class of materials has the potential for applications in extreme environments such as aerospace, but also in applications such as advanced nuclear fission reactors, high temperature electrodes for metal refining and many others [3,4]. Zirconium diboride (ZrB2), a transition metal boride compound, is a member of the UHTC family and besides its high melting point, it is also characterised by higher thermal conductivities and lower electrical resistivity at room temperature than carbide or nitride ceramics. The densification of ZrB2 without additives is difficult due to its low intrinsic sinterability, resulting from the strong covalent bonding of ZrB2, which leads to a low volume and grain boundary diffusivities. This is one of the reasons why, over the last decade, different ZrB2 based particulate composites with SiC, B4C, MoSi2, etc., have been developed with enhanced sinterability, mechanical properties and oxidation resistance [5–12]. These ZrB2 ceramics and ZrB2 based composites have been densified by various methods including hot pressing (HP), spark plasma sintering (SPS), reactive hot pressing (RHP) and pressureless
⁎
sintering (PS). ZrB2 + SiC composites have received considerable attention due to their improved oxidation resistance, densification behaviour and mechanical properties including strength and fracture toughness [8,13,14]. Boron carbide (B4C) is a widely used sintering additive for ZrB2, allowing it to reach its full density at a relatively low temperature, improving densification by removing surface oxides from the starting powders and pinning the grain growth as a second phase inclusion [10–12,15,16]. E.W. Neuman et al. [17] produced dense ZrB2–9.5 vol% ZrC – 0.1 vol% C ceramics by hot pressing at 1900 °C and investigated their mechanical properties at up to 2300 °C in an argon atmosphere. The flexure strength and fracture toughness values changed at different temperatures, from 695 MPa to 300 MPa and from 4.8 MPa m1/2 to 3.6 MPa·m1/2, respectively. There are several papers dealing with the characterization of the wear of ZrB2 based or similar ceramics using different tribological methods [18–22]. Among the first is a study by K. Umeda et al. [18], who investigated the tribochemical response of hot pressed ZrB2, B4C, ZrB2 + B4C and ZrB2 + B4C + SiC ceramics in air and in de-ionized water using a reciprocating pin-on-block test. The coefficients of friction were about 0.95 for all systems at a low relative humidity (< 20%), but they decreased with an increasing relative humidity. For
Corresponding author. E-mail address: [email protected] (R. Sedlák).
https://doi.org/10.1016/j.ijrmhm.2019.03.004 Received 14 November 2018; Received in revised form 2 March 2019; Accepted 3 March 2019 Available online 04 March 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
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200 rpm for 2 h, with a WC bowl and milling balls using isopropanol as a milling media to obtain the mixed powders. The dried mixtures were put into a graphite die with an inner diameter of 20 mm with graphite foil and were densified using spark plasma sintering (FCT HP D 5; FCT Systeme GmbH, Germany). The sintering was performed in temperatures ranging from 1800 °C to 2050 °C, 50 MPa for 10 min in argon, with a heating rate of 100 °C/min.
a high r.h. of about 95%, the friction coefficients ranged from 0.2–0.3, except for that of ZrB2 which was about 0.4. S. Chakraborty et al. [19] investigated the influence of direct current pulse on–off patterns on densification, and tribological properties of the spark plasma sintering of ZrB2 using a scratch test. It was found that the maximum relative density (98.65%) is achieved at 50 ms pulse on (ton) and 5 ms pulse off (toff) time conditions, and that the wear volume and wear rate at a 10 N load was 4.05 × 104 μm3 and 1.01 × 10−3 mm3/(N·m), respectively. The scratch resistance and wear parameters of ZrB2 + SiC composites, prepared by hot pressing with different sources of SiC, were investigated by D. Debnath et al. [20]. It was found that an interconnected network, as well better contiguity between grains of ZrB2 + SiC composites and the impurity content of the starting powders, can play significant roles in achieving high tribological properties of the composites. Recently, J. He et al. [13] synthesised ZrB2 + SiC composite ceramics with varying compositions through spark plasma sintered (SPS) for 30 min under an argon atmosphere to produce materials having different inter- and trans-granular fractures that showed different wear loss, friction efficient and tribofilm morphologies. The characteristics of silica/hydride silica revealed the formation of tribofilms with different morphologies, thereby implying that several key factors are involved in determining the efficiency of this process. Chakraborty et al. [21] reported the effect of an addition of submicrometer-sized B4C on the microstructure, phase composition, scratch resistance and wear resistance of hot pressed ZrB2 + B4C composites. They found improved tribological properties due to crack deflection with a homogeneously dispersed submicrometer-sized B4C in the ZrB2 matrix. B. M. Moshtaghiouna et al. [22] investigated the abrasive wear resistance of boron carbide as a function of the grain size and porosity. They found that, in contrast to most ceramics, in the studied B4C the grain size was not directly relevant to the wear resistance, whereas the porosity and hardness played the main roles. Although there is an increased demand for ZrB2 based ceramics for tribological applications, only a few studies have examined their tribological performance. Moreover, to our knowledge, there has been no systematic study exploring the roles of different carbide additions on the microstructure, wear and tribological characteristics of zirconium diboride, which is important for the development and fabrication of the next generation of UHTCs. The aim of the present contribution is to investigate the wear and tribological characteristics of ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites under dry sliding conditions in air at room temperature and to describe the dominant wear mechanisms.
2.2. Investigation methods The bulk density of the specimens was measured by the Archimedes' method, using distilled water as the immersing medium according to ASTM C373–88. The relative density was calculated by dividing the Archimedes' density by the density predicted from the nominal ZrB2, B4C, SiC and ZrC. The microstructure of the materials was studied with a scanning electron microscope (ZEISS AURIGA). Young's modulus of the composites was determined by the ultrasonic wave transition method, by measuring the velocity of ultrasonic sound waves passing through the material using an ultrasonic flaw detector (Panametrics Epoch III). The hardness and the fracture toughness were determined by the Vickers indentation method, by applying a load of 9.81 N (1 kg) and 98 N (10 kg) with a Future Tech FLC-50VX hardness tester. For each sample, 10 indentations were made and the stress intensity factor KIC was calculated from the length of the cracks which developed during a Vickers indentation test using Niihara’s equation [23]. Tribology measurements were carried out on UMT 3 (Bruker) equipment using the ball-on-flat technique. The wear behaviour of the experimental materials was studied during dry sliding in air. The tribological partner was a highly polished (roughness Ra < 0.10 μm according to ISO 3290) SiC ball with a 6.35 mm diameter. Two applied loads, 5 and 50 N, were used with the aim to study the influence of different applied loads on the wear characteristics. The sliding speed was 0.1 m/s and the sliding distance 500 m. The experiments were realized at room temperature at the relative humidity of 40 ± 5%. The material losses (volume of the wear tracks) due to wear were measured by the PLu Neox 3D Optical Profiler, a high precision confocal microscope by SENSOFAR, and the specific wear rates (Ws) were then calculated in terms of the volume loss (V) per distance (L) and applied load (FN), according to the equation:
WS =
V FN ⋅L
The microstructure, fracture and the wear damage were studied using scanning electron microscopy (SEM, JEOL JSM 7000, ZEISS AURIGA) and confocal electron microscopy. The deformation characteristics were studied using atomic force microscopy. A focused ion beam (FIB, ZEISS AURIGA) was used for the section preparation and the investigation below the worn surfaces of the investigated composites Table 1.
2. Experimental materials and methods 2.1. Experimental materials Commercial ZrB2 (1.5–3.0 μm; H.C. Starck), α-SiC (0.75 μm; H.C. Starck), B4C (0.6–1.2 μm; H.C. Starck) and ZrC (3 μm, H.C. Starck) powders were used to fabricate the experimental composites. The investigated zirconium diboride based systems ZrB2 + 10 wt% B4C, ZrB2 + 10 wt% SiC, and ZrB2 + 10 wt% ZrC were prepared in a planetary ball mill (Pulverisette 6, Fritsch Co. Ltd.) at a rotation speed of
3. Results and discussion The characteristic microstructures of the investigated composites are illustrated in Fig. 1. As they were visible, the mean grain size of the ZrB2 grains in the composites ZrB2 + B4C and ZrB2 + SiC was approximately 2 μm, and the grain sizes of SiC and B4C were
Table 1 Apparent density, porosity, hardness, elastic modulus and fracture toughness of the ZrB2 based composites. Experimental materials
Apparent density (g/cm3)
Porosity (%)
Hardness HV1 (GPa)
Elastic modulus (GPa)
Fracture toughness (MPa·m1/2)
ZrB2 + SiC ZrB2 + B4C ZrB2 + ZrC
5.510 5.280 6.100
1.3 0.9 0.3
17.33 ± 1.23 19.02 ± 1.07 14.73 ± 1.12
465 460 484
4.47 ± 0.43 4.4 ± 0.27 5.26 ± 0.69
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Fig. 1. Microstructure of the investigated ceramic composites: a) ZrB2 + 10 wt% B4C, b) ZrB2 + 10 wt% SiC and c) ZrB2 + 10 wt% ZrC.
Fig. 3. Wear rate of the investigated ceramic composites at applied loads of 5 N and 50 N.
Fig. 2. Coefficient of friction of the investigated ceramic composites.
approximately 1 μm. The composite ZrB2 + ZrC was slightly different: the mean grain size of the ZrB2 grains was higher at approximately 3.5 μm; and the ZrC grains were smaller with a size ranging from 0.5 to 1.0 μm. The coefficients of frictions for the investigated composites, obtained from the wear test at loads of 5 N and 50 N, are illustrated in Fig. 2. The average values of the COF were very similar and were between 0.63 and 0.72, with a lowest value of 0.63 in the case of the ZrB2 + SiC composite tested at the 5 N applied load. No significant and systematic influence of the applied load was found on the values of the COF: with increasing load, the COF for ZrB2 + B4C was slightly decreasing; and on the other hand for ZrB2 + SiC and ZrB2 + ZrC it was slightly increasing. The wear rate of the investigated systems is shown in Fig. 3 at different applied loads. The average wear rates of the ZrB2 + SiC and ZrB2 + ZrC tested at the applied load 5 N were very similar and were around 6.5 × 10−6 mm3/(N·m), while the average wear rate of the ZrB2 + B4C system at the same load was higher at around 1.1 × 10−5 mm3/(N·m). With the increasing applied load, the wear rate was increasing for all systems. In the case of ZrB2 + B4C, it showed the highest value of 1.42 × 10−5 mm3/(N·m), and for ZrB2 + SiC it showed the lowest value of approximately 7.7 × 10−6 mm3/(N·m). The characteristic wear track profiles of the composites at loads of 5 N and 50 N are illustrated in Figs. 4 and 5. In the case of the 5 N applied load, the initial track width for all studied systems was approximately 800 μm, while the maximal track depth changed from 5 μm to 12 μm. After the wear test with a 50 N load, the track width ranged from 2.5 to 3.0 mm and the maximal wear track depth was approximately 25 μm. It is interesting to note that the highest wear rate in the case of ZrB2 + B4C during the wear test with the applied load of 5 N exhibited the highest wear depth, but on the other hand, the highest wear track width was recorded during the wear test with the 50 N load.
Fig. 4. Characteristic wear tracks of the investigated ceramic composites after the wear test with an applied load of 5 N.
The characteristic wear tracks of the ZrB2 + B4C system and the corresponding SEM + EDX and FIB+EDX analyses are shown in Figs. 6–8. In Fig. 6. the wear tracks of ZrB2 + 10 wt% B4C are illustrated after the 5 and 50 N applied loads. Based on this study we can say that, at an applied load of 5 N, the wear is mostly in the form of mechanical wear-abrasive grooves connected with a deformation of the matrix grains and additive grains, without any grain pull-outs or grain failure. We frequently detected cracks normal to the wear direction, with a size ranging from 2 μm to 5 μm, intragranularly in the ZrB2 grains. In some cases, we found interconnections of these cracks with a final size of 10–15 μm. 216
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this, it is evident that the thickness of the tribolayer covering both the ZrB2 and B4C grains was up to 0.5 μm, containing mainly zirconium and oxygen. It is important to note that we found no damage below the worn surface in the form of microcrack nucleation/propagation. The characteristic wear tracks of the ZrB2 + SiC system and the corresponding SEM + EDX and FIB+EDX analyses are shown in Figs. 9–11. We can see that at an applied load of 5 N, similarly to the case of ZrB2 + B4C, the wear was mostly in the form of abrasive grooves connected with a deformation of the matrix grains and SiC without grain pull-outs. Similarly, we frequently detected cracks normal to the wear direction, with sizes measuring from 5 μm to 20 μm, intragranularly in the ZrB2 grains. Micro-areas of wear track covered by tribofilms with a size of approximately 3 μm were detected as well. The wear track of this system after the wear with an applied load of 50 N showed elongated areas covered by a tribolayer, with sizes up to 150 μm in a direction parallel to the wear direction. A characteristic “striation” in the tribolayers was found perpendicular to the wear direction. The chemical analyses results of the individual phases on the worn surface of the ZrB2 + SiC system are illustrated in Fig. 10. According to this, the tribolayer contains an average of 38 wt% zirconium, 36 wt% oxygen, 14 wt% silicon and 12 wt% carbon. In Fig. 11, the wear damage and chemical composition of the worn surface and the FIB cut/polished area below the surface in the volume of this ceramic system is shown. From this, it is evident that the thickness of the tribolayer covering both ZrB2 and SiC grains was up to 1.5 μm, containing mainly zirconia and oxygen. Similarly, as in the case of the ZrB2 + B4C composite, we found no damage below the worn surface in the form of microcrack formations. The characteristic wear tracks of the ZrB2 + ZrC system and the corresponding SEM + EDX analyses are shown in Figs. 12–13. At an applied load of 5 N, similarly as in the case of the ZrB2 + B4C and ZrB2 + SiC composites, the wear was mostly in the form of abrasive grooves connected with deformations of the matrix grains and ZrC without grain pull-outs. On the other hand, in this system we found microcracks normal to the wear direction only randomly. Micro-areas of the wear track covered by a tribofilm with a size of approximately
Fig. 5. Characteristic wear tracks of the investigated ceramic composites after a wear test with the applied load of 50 N.
Micro-areas of wear track covered by tribofilms in sizes from 1 μm to 5 μm were detected as well. The wear track of this system after the wear with an applied load of 50 N looked different, mainly as regarding the areas covered by the tribolayer, as the size of these areas was up to 80–100 μm in a direction parallel to the wear direction. In this case, we found fewer microcracks in comparison to the wear track created at the applied load of 5 N. The chemical analyses results of the individual phases on the worn surface of the ZrB2 + B4C system are illustrated in Fig. 7. According to this, the tribolayer contained an average 47 wt% zirconium, 35 wt% oxygen, 12 wt% boron, 5 wt% carbon and 0.5 wt% silicon. The chemical composition of the areas with thin tribolayer, positions 2 and 3, is similar to the composition of the constituents of the tested composite, without a detectable amount of oxygen. In Fig. 8, the wear damage and chemical composition of the worn surface and the FIB cut/polished area below the surface in the volume of the ceramic system is shown. From
Fig. 6. Wear tracks of ZrB2 + 10 wt% B4C after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c). 217
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Fig. 7. EDX analysis of the wear track in the ZrB2 + 10 wt% B4C system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - tribolayer, spectrum 2 - ZrB2 grain, spectrum 3 - B4C/ZrB2 boundary.
10 μm were detected. As a clear difference in this system, in comparison to the previous ones, we found that submicron/nano-sized debris formations occurred during the wear process. Maybe this is the reason that the abrasive grooves are less visible and the wear surface is smooth, in comparison to the wear surface of ZrB2 + B4C and ZrB2 + SiC composites after the wear test at applied load of 5 N. The wear track of this system, after the wear with an applied load of 50 N, showed elongated areas covered by tribolayers, with sizes up to 200 μm in the parallel direction and up to 25 μm wide perpendicular to the wear direction, respectively. Similar “striation” in the tribolayers was found perpendicular to the wear direction as was found in the ZrB2 + SiC system, but in this case it is not so marked. The thickness of the tribolayer covering both ZrB2 and ZrC grains, measured on FIB section, was in the range of 0.5 to 1.2 μm. The results of the chemical analyses of individual phases on the worn surface of the ZrB2 + ZrC system are illustrated in Fig. 13. According to these, the tribolayer contains an average of 55 wt% zirconium, 25 wt% oxygen, 13 wt% boron and 8 wt% carbon. The chemical composition of the areas with thin tribolayer is similar to the composition of the constituents of the tested composite, with presence of oxygen and hafnium. Similarly to the case of the previous ceramic composites, abrasive grooves and microcracks were the most observed characteristic features of wear scar at the lower loads, whereas at a higher load the debris formation, oxidation and tribolayer formation features became more dominant. It is well known that the wear behaviour of ceramics critically depends on numerous factors, including the testing parameters (e.g. normal load, test duration, surface roughness, environmental conditions, tribochemical stability, etc.) and the microstructure and mechanical properties of the tested material. Therefore, it is not
straightforward to compare the wear characteristics, even when testing the same system in different laboratories using different testing methods. But on the other hand, it is interesting to see the tendencies, as well as the influence of the composition, processing route and microstructure on the wear characteristics of the studied systems, in spite of the fact that they have been investigated using different methods. S. Chakraborty et al. [19,21,24], for the tribological characterization of similar zirconium diboride based ceramics as in our study, performed scratch tests at room temperature. They applied dry and unlubricated conditions at a relative humidity level of 60 ± 5%, using a Vickers' type diamond tip at loads of 5 N and 10 N with an indenter velocity of 0.1 mm/s. The effect of DC pulse on–off patterns during the spark plasma sintering of ZrB2 on the densification, mechanical and tribological properties.was investigated, too. During the study, the effect of an addition of submicrometer-sized B4C on the tribological characteristics of hot pressed ZrB2-B4C composites reported lower values of the COF for the composites in comparison to monolithic ZrB2 and an oscillation in the COF dates. This was explained by the creation of an intermediate debris layer on the moving track due to the surface deformation under the moving stylus, which makes the wear track rougher and creates the oscillation. They found that, of all the wear parameters, the best was observed in the case of the ZrB2 + 10 wt% B4C composite. During the investigation of the mechanical, thermal and some tribological properties of the ZrB2 + 20 wt% SiC composite sintered by SPS at different temperatures, they reported the best wear parameters for the system sintered at a temperature of 1900 °C. Microcracks and fractured debris formation were observed on the worn track, mainly in the system prepared at the highest temperature. Among the first studies, K. Umeda [18] investigated the tribochemical responses of hot pressed boride ceramics, such as ZrB2, B4C, ZrB2 + B4C and ZrB2 + B4C + SiC, in air and in de-ionized water. 218
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Fig. 8. EDX map of the worn surface and FIB section below the wear track in the ZrB2 + 10 wt% B4C system after the wear test with applied load of 50 N. a) SEM of the wear track and FIB section below the wear track, b) oxygen EDX map, c) zirconium EDX map and d) boron EDX map.
Fig. 9. Wear tracks of ZrB2 + 10 wt% SiC after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c).
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Fig. 10. EDX analysis of the wear track in the ZrB2 + 10 wt% SiC system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - ZrB2 grain, spectrum 2 - SiC/ZrB2 boundary and spectrum 3 - tribolayer.
different compositions via the polymeric precursor route and investigated the effect of the microstructures in the tribological properties of these materials. The wear tests were performed under conditions similar to those in our tests, using a ball-on-flat method at room temperature and a 35% relative humidity with a SiC ball as a counterpart that slid against the sample discs of 5 and 10 N loads and a radius of 3 mm. The wear tests of ZrB2 + 80 vol% SiC and ZrB2 + 40 vol% SiC showed that, in all cases and in agreement with our results, the wear loss increased with the applied load. It increased rapidly for the ZrB2 + 40 vol% SiC for loads higher than 5 N. On the other hand, this composite showed the lowest frictional coefficient, with a value of approximately 0.5 at the 5 N load, which was even lower than the COF of the systems measured in our study. This system showed a COF value at the 10 N load that was significantly higher, at approximately 0.8. They found also the presence of tribofilms that partially covered the wear track, generated directly during the wear process of the composites. These tribofilms were more continuous in the system with a higher volume fraction of SiC. Such a tribofilm can prevent the surface from wear; however, the morphology, thickness, chemical composition, etc., are considered key factors when estimating the protective efficiency of the tribofilm. T. Murthy et al. [25] investigated the microstructure and wear properties of B4C + 5 wt% ZrB2 composites formed in-situ under dry conditions at room temperature and at a 45 ± 5% relative humidity, using ball-on-flat reciprocative sliding wear tests conducted at loads of 5 N, 10 N and 20 N against a WC-Co ball. Increasing the load from 5 N to 20 N resulted in a decrease in the COF by 37.5% at the 10 Hz frequency, from 0.24 to 0.15. These values of the COF were significantly lower in comparison to the values measured in our experiment, which can be explained by the different chemical composition of the
Sliding tests were made using a reciprocating type pin-on-block machine with a pin size of 4 × 15 mm and a hemispherical tip radius of 2 mm, where the block size was 10 × 15 × 6.5 mm. Both the pin and the block were made of the same material and the tests were made at a load of 7.8 N with a sliding speed of 1.5 mm/s in a glove box, in which the relative humidity was increased in steps from 10% to 95%. Some of these testing conditions are comparable with the conditions used during the present investigation. According to their results, the coefficients of friction were about 0.95 for all the ceramics at low relative humidity (< 20%), but decreased with an increasing relative humidity to 0.2–0.3 for the composites and B4C, and it was higher only for ZrB2 at about 0.4. During the test at a relative humidity comparable to that used in our experiment, their COF results for monolithic ZrB2 were higher in comparison to our results, while the composites ZrB2 + B4C and ZrB2 + B4C + SiC exhibited a similar COF as our composites. A lower COF was found in the case of monolithic B4C. In addition, they found similar Hertzian type cracks on the worn surfaces of ZrB2 during the test at a relative humidity of approximately 40%, as we found in the case of the ZrB2 + B4C and ZrB2 + SiC composites. On the other hand, at this testing condition in the case of B4C and the ZrB2 + B4C and ZrB2 + B4C + SiC composites, such cracks did not occur, similarly to our ZrB2 + ZrC composites. The length of these Hertzian cracks in the case of their experiments had a size of approximately 100 μm, while in our case these cracks were shorter, ranging from several micron to approximately 20 μm according to the applied load. In our case, this was probably connected with the wider worn track width and lower tensile stresses behind the spherical rider. They also reported, contrary to our results, no significant tribolayer formations during their experiments. Recently, J. He et al. [13] prepared ZrB2–SiC composites with three 220
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Fig. 11. EDX map of the worn surface and the FIB section below the wear track in the ZrB2 + 10 wt% SiC system after the wear test with applied load of 50 N. a) SEM of the wear track and FIB section below the wear track, b) oxygen EDX map, c) zirconium EDX map and d) silicon EDX map.
Fig. 12. Wear tracks of ZrB2 + 10 wt% ZrC after a), b) 5 N applied load and c), d) 50 N applied load with marked wear track area at low magnification, a) and c). 221
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Fig. 13. EDX analysis of the wear track in the ZrB2 + 10 wt% ZrC system after the wear test with 50 N applied load. a) SEM of the wear track, spectrum 1 - ZrB2 grain, spectrum 2 - tribolayer, spectrum 3 - ZrC grain.
B2O3 as a liquid layer:
composites, but also by a different counterpart and a higher relative humidity. The reported specific wear rate data, in order of 10−5 mm3/ (N·m), is in agreement with the values measured for the ZrB2 + B4C composite in our experiment. Both abrasive and tribo-chemical reaction wear mechanisms were observed on the worn surface of the flat and the counter-body materials. A dominant tribo-chemical reaction wear mechanism was observed at higher loads, such as 10 N. Similarly, as occurred during our investigation of ZrB2 based composites, they found that the severity of the damage during reciprocative sliding increased with an increasing applied load. This is evidence that, during the sliding wear test, mechanical wear is responsible for the formation of abrasive grooves, and the intensity of these grooves on the wear track is diminished as the load increases due to the formation of a tribo-oxidative chemical layer. The low COF and specific wear rate recorded in this study was probably caused by the formation of a tribo-oxide layer with a lubricant property. The heat generated due to friction between the sliding surfaces was probably sufficient for the oxidation of B4C according to the reaction (1) to form B2O3 [26]. The oxygen required for this reaction was supplied by the surrounding air. During the cooling to room temperature (after the test), the B2O3 phase reacted with the moisture in the air and formed a secondary film of boric acid, H3BO3 (in the reactions: cr – crystal, amph – amorphous phase, l – liquid, g – gas):
B4 C + 7/2 O2 → 2 B2 O3 (l) + CO (g )
(1)
B2 O3 (amph) + 3 H2 O → 2 H3 BO3 (amph/ cr )
(2)
ZrB2 (cr ) + 5/2 O2 → ZrO2 (cr ) + B2 O3 (l)
(3)
The formed B2O3 scale is non-protective since it has a high vapour pressure, so at temperatures above 450 °C it exists in a liquid form that readily evaporates, exposing the formed zirconia grains and the unoxidized underlying bulk. However, in a liquid state it can play the role of a lubricant and can change the COF. The oxidation of the ZrB2 + B4C composite is a far more complex process. Both the components oxidate and reactions 1, 2 and 3 take place. But the B4C can also interact with the formed ZrO2 and reduce it to ZrB2:
7 ZrO2 (cr ) + 5 B4 C (cr ) → 7 ZrB2 + 5 CO (g ) + 3 B2 O3 (l)
(4)
The presence of SiC improves the oxidation resistance of the composite. This effect is well-studied:
SiC (cr ) + 3/2O2 → SiO2 (amph) + CO (g )
(5)
Between 800 and 1200 °C, the reaction (4) is the dominant chemical process, resulting in passive oxidation behaviour due to the continuous formation of the liquid boria layer. At higher temperatures, the SiC begins to oxidize according to the reaction (5) and as a result, it forms the amorphous protective layer of SiO2 and/or borosilicate glass with the B2O3. This glassy layer fills the pores and protects the underlying material from further deep oxidation. In principle, the formation of this glassy layer is possible even during tribotests. In the results of the EDX analysis, boron wasn't detected in the tribolayer. This means that the contact temperature during the test was high enough that the formed B2O3 evaporated. The thermal oxidation process, along with the thermodynamic approach concerning the formation mechanism of the protective layer, was recently described in detail [27,28].
The oxidation of ZrB2, HfB2, ZrC, HfC and the related ceramics has been already described in a number of articles [25–30]. Based on the literature, it is known that at elevated temperatures (above 1000 °C), zirconium diboride, similarly to carbide, is not resistant to oxidation. When exposed to air, ZrB2 reacts with oxygen to form solid ZrO2, with 222
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similar chemical composition, but with different lateral/vertical dimensions at the 50 N load.
Table 2 Summary of the tribosurfaces analysis (SiC ball used). Elements
Wt%
At.%
Composition
Acknowledgements ZrB2 + B4C Zr O B C Si
47 35 12 5 0.5
12.13 51.51 26.14 9.80 0.42
ZrO2, B4C, B2O3/H3BO3, traces of SiO2 (from the ball)
ZrB2 + SiC Zr O B C Si
38 36 0 12 14
10 54.03 – 23.99 11.97
ZrO2, SiC, SiO2
ZrB2 + ZrC Zr O B C Si
55 25 13 8 0
14.95 38.73 29.81 16.51 –
ZrO2, ZrB2, ZrC, B2O3/H3BO3
The authors gratefully acknowledge the financial support from the following projects: APVV-15-0469, APVV-14-0385, VEGA 2/0163/16, VEGA 2/0130/17. This work was realized within the framework of the Research Centre of Advanced Materials and Technologies for Recent and Future Applications “PROMATECH” Project, ITMS 26220220186, supported by the “Research and Development” Operational Programme financed through European Regional Development Fund. The authors acknowledge the support of the FNP Project No. POWROTY/2016. The POWROTY/2016-1/3 Project is carried out within the Powroty / Reintegration Programme of the Foundation for Polish Science and is co-financed by the European Union under the European Regional Development Fund. References [1] W. Fahrenholtz, E. Wuchina, W. Lee, Y. Zhou, Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications, (2014). [2] D. Sciti, L. Silvestroni, V. Medri, F. Monteverde, Sintering and Densification Mechanisms of Ultra-High Temperature Ceramics. Ultra-High Temp. Ceram, John Wiley & Sons, Inc, Hoboken, NJ, 2014, pp. 112–143. [3] W.G. Fahrenholtz, G.E. Hilmas, Ultra-high temperature ceramics: materials for extreme environments, Scr. Mater. 129 (2017) 94–99. [4] M.J. Gasch, D.T. Ellerby, S.M. Johnson, Ultra High Temperature Ceramic Composites. Handb. Ceram. Compos, Springer, US, 2005, pp. 197–224. [5] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Processing, microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics, Ceram. Int. 43 (2017) 6942–6948. [6] S.Q. Guo, Densification of ZrB2-based composites and their mechanical and physical properties: a review, J. Eur. Ceram. Soc. 29 (2009) 995–1011. [7] F. Monteverde, S. Guicciardi, A. Bellosi, Advances in microstructure and mechanical properties of zirconium diboride based ceramics, Mater. Sci. Eng. A 346 (2003) 310–319. [8] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Mechanical behavior of zirconium diboride-silicon carbide ceramics at elevated temperature in air, J. Eur. Ceram. Soc. 33 (2013) 2889–2899. [9] E.J. Cheng, Y. Li, J. Sakamoto, S. Han, H. Sun, J. Noble, et al., Mechanical properties of individual phases of ZrB2-ZrC eutectic composite measured by nanoindentation, J. Eur. Ceram. Soc. 37 (2017) 4223–4227. [10] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, Mechanical behavior of zirconium diboride-silicon carbide-boron carbide ceramics up to 2200°C, J. Eur. Ceram. Soc. 35 (2015) 463–476. [11] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, Pressureless sintering of zirconium diboride using boron carbide and carbon additions, J. Am. Ceram. Soc. 90 (2007) 3660–3663. [12] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Pressureless densification of zirconium diboride with boron carbide additions, J. Am. Ceram. Soc. 89 (2006) 1544–1550. [13] J. He, Y. Cao, Y. Zhang, Y. Wang, Mechanical properties of ZrB2–SiC ceramics prepared by polymeric precursor route, Ceram. Int. 44 (2018) 6520–6526. [14] Z. Balak, M. Azizieh, H. Kafashan, M.S. Asl, Z. Ahmadi, Optimization of effective parameters on thermal shock resistance of ZrB2-SiC-based composites prepared by SPS: using Taguchi design, Mater. Chem. Phys. 196 (2017) 333–340. [15] M. Shahedi Asl, M. Ghassemi Kakroudi, B. Nayebi, A fractographical approach to the sintering process in porous ZrB2-B4C binary composites, Ceram. Int. 41 (2015) 379–387. [16] S. Chakraborty, P.K. Das, D. Ghosh, Spark plasma sintering and structural properties of ZrB2 based ceramics: a review, Rev. Adv. Mater. Sci. 44 (2016) 182–193. [17] E.W. Neuman, G.E. Hilmas, W.G. Fahrenholtz, M. Cinibulk, Ultra-high temperature mechanical properties of a zirconium diboride-zirconium carbide ceramic, J. Am. Ceram. Soc. 99 (2016) 597–603. [18] K. Umeda, Y. Enomoto, A. Mitsui, K. Mannami, Friction and wear of boride ceramics in air and water, Wear 169 (1993) 63–68. [19] S. Chakraborty, A.R. Mallick, D. Debnath, P.K. Das, Densification, mechanical and tribological properties of ZrB2 by SPS: effect of pulsed current, Int. J. Refract. Met. Hard Mater. 48 (2015) 150–156. [20] D. Debnath, S. Chakraborty, A.R. Mallick, R.K. Gupta, A. Ranjan, P.K. Das, Mechanical, tribological and thermal properties of hot pressed ZrB2–SiC composite with SiC of different morphology, Adv. Appl. Ceram. 114 (2015) 45–54. [21] S. Chakraborty, D. Debnath, A.R. Mallick, P.K. Das, Mechanical, tribological, and thermal properties of hot-pressed ZrB2–B4C composite, Int. J. Appl. Ceram. Technol. 12 (2015) 568–576. [22] B.M. Moshtaghioun, D. Gomez-Garcia, A. Dominguez-Rodriguez, R.I. Todd, Abrasive wear rate of boron carbide ceramics: influence of microstructural and mechanical aspects on their tribological response, J. Eur. Ceram. Soc. 36 (2016) 3925–3928. [23] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of KIC of brittle solids by the
The oxidation of ZrC can be described as the following reaction:
ZrC (cr ) + 3/2O2 → ZrO2 (cr ) + CO (g )
(6)
Most of the reactions correspond to the results of thermal oxidation experiments performed at temperatures ranging from 900 to above 1500 °C, in stagnant air, in oxygen and/or in air flows. But the developed models and thermodynamic approaches also describe the oxidation processes very well. According to the results of our experiments and the EDX analysis (summarised in Table 2), we can assume that the tribolayers consisted mostly of ZrO2 with small amounts of the original materials (debris of ZrB2, B4C, SiC, ZrC), as well as SiO2 and hydrated B2O3, formed due to reaction (2). The hydration of B2O3 explains the high amount of oxygen registered by the EDX analyses. The local temperature at the contact point between the ball and the substrate material can probably reach up to 800–1000 °C. It may be not enough for a high oxidation rate, but a mechanical energy activation of the reaction also takes place, which decreases the temperature needed for the formation of the oxide products. 4. Conclusions The aim of the present contribution was to investigate the wear characteristics of ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites under dry sliding conditions in air at room temperature and to describe the dominant wear mechanisms. The main results are as follows:
• The friction coefficient values were very similar for all composites, •
• • •
with values in the range of 0.63 to 0.72, and with the lowest value recorded for the ZrB2 + SiC composites at the 5 N applied load. The highest wear rate was found in the case of ZrB2 + B4C, at 1.42 × 10−5 mm3/(N·m) for 50 N and at 1.095 × 10−5 mm3/(N·m) for the 5 N load, respectively. The ZrB2 + SiC and ZrB2 + ZrC composites show similar wear rates at the 5 N load, of approximately 6.15 × 10−6 mm3/(N·m), while at 50 N the ZrB2 + ZrC composite was more wear resistant with a wear rate value of 7.3 × 10−6 mm3/(N·m). None, or very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all the composites with a 223
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27 (2007) 2495–2501. [27] H.L. Liu, J.X. Liu, H.T. Liu, G.J. Zhang, Changed oxidation behavior of ZrB2-SiC ceramics with the addition of ZrC, Ceram. Int. 41 (2015) 8247–8251. [28] R. He, Z. Zhou, Z. Qu, X. Cheng, High temperature flexural strength and oxidation behavior of hot-pressed B4C-ZrB2 ceramics with various ZrB2 contents at 10001600°C in air, Int. J. Refract. Met. Hard Mater. 57 (2016) 125–133. [29] M.M. Opeka, I.G. Talmy, E.J. Wuchina, J.A. Zaykoski, S.J. Causey, Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds, J. Eur. Ceram. Soc. 19 (1999) 2405–2414. [30] W.G. Fahrenholtz, Thermodynamic analysis of ZrB2-SiC oxidation: formation of a SiC-depleted region, J. Am. Ceram. Soc. 90 (2007) 143–148.
indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [24] S. Chakraborty, D. Debnath, A.R. Mallick, R.K. Gupta, A. Ranjan, P.K. Das, et al., Microscopic, mechanical and thermal properties of spark plasma sintered ZrB2 based composite containing polycarbosilane derived SiC, Int. J. Refract. Met. Hard Mater. 52 (2015) 176–182. [25] T.S.R.C. Murthy, S. Ankata, J.K. Sonber, K. Sairam, K. Singh, A. Nagaraj, et al., Microstructure, thermo-physical, mechanical and wear properties of in-situ formed boron carbide - zirconium diboride composite, Ceramics-Silikáty 62 (2017) 1–10. [26] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Evolution of structure during the oxidation of zirconium diboride-silicon carbide in air up to 1500 °C, J. Eur. Ceram. Soc.
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