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Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites Alexandra Kovalˇcíková a,∗ , Ján Balko a , Csaba Balázsi b , Pavol Hvizdoˇs a , Ján Dusza a a
b
Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Koˇsice, Slovakia Bay Zoltan Nonprofit Ltd for Applied Research, Institute for Materials Science and Technology, Budapest, Hungary
Abstract Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 ◦ C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si3 N4 /BN composites. In addition, the fracture toughness of Si3 N4 /micro BN composites was enhanced comparing to monolithic Si3 N4 because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si3 N4 /BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si3 N4 matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials. © 2014 Elsevier Ltd. All rights reserved. Keywords: Si3 N4 /BN composites; Mechanical properties; Toughening mechanisms; Wear resistance
1. Introduction Silicon nitride has very good combination of mechanical, physical, and chemical properties. The high strength, hardness, and toughness at room and elevated temperatures, the high thermal shock and wear resistance make it suitable for use in many structural and tribological applications.1,2 To obtain superior mechanical properties, a fine-grained microstructure with elongated beta grains is preferred. These in situ grown beta silicon nitride grains can significantly improve the fracture toughness over monolithic ceramics, producing self-reinforced silicon nitrides. However, just like most of the ceramic materials, the machinability of Si3 N4 ceramics is extremely poor.3 It is well known that h-BN offers a number of interesting properties such as lubrication action, low hardness and low friction coefficient.4 Hexagonal boron nitride has also excellent machinability due to a plate-like structure similar to that of graphite. To improve
∗
Corresponding author at: Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Koˇsice, Slovakia. Tel.: +421 55 792 2463; fax: +421 55 792 2408. E-mail address: [email protected] (A. Kovalˇcíková).
the machinability, fracture toughness and tribological properties (by the selective oxidation of the compounds) of Si3 N4 ceramics at room and elevated temperatures, hexagonal boron nitride particles were introduced as second-phase dispersions into the Si3 N4 matrix by many researchers.5−9 In Si3 N4 /BN composite ceramics, the cleavage behaviour of plate-like structured BN particles endowed the material with good machinability together with superior thermal shock resistance.10 Various processing techniques have been developed to fabricate Si3 N4 /BN composites for structural and functional applications. Kusunose et al.11 prepared Si3 N4 /BN nanocomposites with high strength and excellent machinability via chemical route. Gao et al.3 developed Si3 N4 /BN composites via a similar chemical route and subsequent hot pressing. They achieved the Si3 N4 /BN nanocomposites with higher strength due to formation of fine and homogenous microstructure. Li et al.9 fabricated Si3 N4 /BN composites by spark plasma sintering through traditional powder mixing process and by a chemical route to Si3 N4 /BN composite powder, respectively. The SPS-processed Si3 N4 /BN materials showed high bending strength further enhanced in the composites prepared by powder mixing relative to the counterparts obtained via the chemical process. Shen et al.12 found that the microstructures of the
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.021 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
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Table 1 Composition of the starting powders, preparation of powder mixture and density of the Si3 N4 /BN composites. Samples
SN SNB1-nB SNB3-nB SNB5-nB SNB1-nE SNB3-nE SNB5-nE SNB1-mE SNB3-mE SNB5-mE
Starting powders wt%
Additive wt%
Si3 N4
Al2 O3
Y2 O3
h-BN
90 90 90 90 90 90 90 90 90 90
4 4 4 4 4 4 4 4 4 4
6 6 6 6 6 6 6 6 6 6
– 1 3 5 1 3 5 1 3 5
produced Si3 N4 -based ceramics could be tailored by making use of SPS with a rapid heating time. Phase transformation and elongated -Si3 N4 grains formation were greatly enhanced and yielded interlocking microstructures and compact products with improved mechanical properties. Wang et al.13 prepared porous Si3 N4 /BN composite ceramics with homogeneous dispersion of BN by the gel casting technique. With BN contents increasing, the mechanical properties of porous Si3 N4 /BN ceramics partially declined, but the dielectric properties and thermal shock resistance were enhanced. Li et al.14 fabricated Si3 N4 ceramics with 25 vol% h-BN by pressureless sintering with Y2 O3 and Al2 O3 as sintering additives. Cho et al.15 investigated R-curve behaviour of the hot-pressed Si3 N4 /BN composites to understand its machinability. They showed the grain bridging and pullout as possible toughening mechanisms for monolithic Si3 N4 as well as Si3 N4 -hBN composite. Higher hBN content in the composites resulted in more slowly rising R-curves, which could enhance machinability. Both, the silicon nitride and hexagonal boron nitride showed relatively low friction and wear. hBN as a solid lubricant has high performance only at high relative humidity.16 The formation of oxide or hydrated layers (H3 BO3 and BN(H2 O)x ) is reported to have a beneficial effect on the tribological performance of Si3 N4 -BN composites at room temperature, reducing the wear coefficient by one order of magnitude to k = 10−6 mm3 N−1 m−1 , relative to the matrix material.17 High tribological performance of hBN is reported to be controlled by a basal plane slip or tribological products like B2 O3 .18 In this present study we fabricated the Si3 N4 /BN micro/microcomposite and Si3 N4 /BN micro/nanocomposite by hot isostatic pressing. The effect of BN addition on microstructure, basic mechanical properties and wear resistance was investigated and discussed in details. 2. Experimental materials and methods 2.1. Materials In this work two types of materials were prepared. First group were samples when micro sized Si3 N4 particles were mixed with micro sized hBN particles (Si3 N4 /BN micro/microcomposites).
Preparation of powder mixture
Density g/cm3
Monolithic material NanoBN addition at the beginning of milling NanoBN addition at the beginning of milling NanoBN addition at the beginning of milling NanoBN addition at the end of milling NanoBN addition at the end of milling NanoBN addition at the end of milling MicroBN addition at the end of milling MicroBN addition at the end of milling MicroBN addition at the end of milling
3.381 3.361 3.297 3.209 3.368 3.294 3.033 3.344 3.131 2.937
Second type was samples when micro sized Si3 N4 particles were mixed together with nano sized hBN particles (Si3 N4 /BN micro/nanocomposites). The nano BN powders were prepared by mechanical milling. For many applications this process can be a simple and efficient way for nano BN production. In brief, commercial BN powder (Starck) has been milled intensively in high efficient attritor mill equipped with zirconia agitator delta discs and zirconia grinding media in presence of ethanol at 4000 rpm for 10 h. After wet milling a following dry milling at 4000 rpm for 5 h was applied. The 90 wt% Si3 N4 , 4 wt% Al2 O3 , 6 wt% Y2 O3 powder mixtures with 1, 3, 5 wt% nano BN were prepared in two ways. In the first approach the nano BN addition to the mixture was performed at the beginning of milling. In this case the milling lasted 5 h at 4000 rpm in distilled water. In the second method, first the powder mixture was milled for 4.5 h at 4000 rpm in distilled water. After that the nano BN was added to this mixture and milled for 30 minutes at 600 rpm. In the case of Si3 N4 /BN micro/microcomposite the BN addition to the mixture was performed at the end of milling. The substance was dried and sieved with a filter with mesh size of 150 m. Green samples were obtained by dry pressing at 220 MPa. Following to high energy milling procedure the hot isostatic pressing was selected as sintering method of composites. Samples prepared for HIP were oxidized at 400 ◦ C to eliminate the PEG. The heating rate did not exceed 25 ◦ C/min. Hot isostatic pressing (HIP) was employed at 1700 ◦ C in high purity nitrogen using BN embedding powder at 20 MPa, with 3 h holding time. The chemical composition and way of preparation is briefly summarized in Table 1. 2.2. Test procedure The densities of the sintered specimens were measured according to Archimedes’ principle. Phase compositions were determined by X-ray diffractometer (Philips X’Pert Pro) with Cu K␣ radiation. Sintered specimens were cut, polished to a 1 m finish by routine ceramography procedure and chemically etched in molten NaOH at 400 ◦ C for 2 min. The microstructures were then studied using an SEM (JEOL JSM-7000F). Mechanical properties were investigated using indentation methods. Hardness was determined by Vickers indentation
Please cite this article in press as: Kovalˇcíková A, et al. Influence of hBN content on mechanical and tribological properties of Si3 N4 /BN ceramic composites. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.021
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(hardness testers LECO 700AT) under load of 9.81 N with a dwell time of 10 s. In order to determine the indentation toughness at least 10 Vickers indentations per specimen were introduced with the load of 49.05 N. The indentation toughness was calculated from the lengths of radial cracks and indents diagonals using a formula valid for semi-circular crack systems as proposed by Shetty19 :
KIC
HP = 0.0899 4l
0.5 (1)
where H is the hardness P is the indentation load and l = c − a is the length of the indentation crack. Elastic modulus was measured by resonant frequency method on specimens with dimensions 2 mm × 3 mm × 25 mm. The four-point bending strength values for HIP samples were determined by bending tests on a tensile/loading machine (Lloyd LR 5KPlus) using specimens with dimensions 2 mm × 3 mm × 25 mm. They were ground and polished by 15 m diamond grinding wheel before testing. The two edges on the tensile surface were rounded to a radius about 0.15 mm in order to eliminate a failure initiated from an edge of the specimen. The specimens were tested in four point bending fixture (inner span of 20 mm and an outer span of 40 mm) with the crosshead speed of 0.5 mm/min at ambient temperature and atmosphere. Fractographic analysis of broken flexural specimens was used to characterize the processing flaws. Wear testing was carried out at room temperature on a High Temperature Tribometer (CSM Instruments) using the pin-on-disc technique. The surfaces were carefully prepared by polishing down to surface roughness below 0.05 m where possible. Wear behaviour of the prepared materials was studied in dry sliding, where the tribological partner was a highly polished Si3 N4 ball with 6 mm diameter. The normal load of 5 N and sliding speed of 0.1 m/s were applied. Total sliding distance was 300 m. The friction coefficients were continually recorded and wear volume on each specimen was calculated from the surface profile traces across the wear track (and perpendicular to the sliding direction) which were measured using the high precision confocal microscope Plu neox 3D Optical Profiler (SENSOFAR). The specific wear rate is calculated according to the standard ISO 2080820 :
WS =
V FN L
(2)
where Ws is the specific wear rate (mm3 N−1 m−1 ), V the volume of removed material (mm3 ), FN the normal load (N) and L is the total sliding distance (m). The wear tracks created at the surfaces of the investigated materials were studied using SEM to identify the wear mechanisms.
Fig. 1. XRD patterns of the Si3 N4 /BN ceramic composites, (a) SNB5-mE and (b) SNB5-nB.
3. Results and discussion 3.1. Density and XRD analysis The highest density was found in the case of monolithic HIP-ed material with a value of 3.381 g/cm3 , see Table 1. The density of Si3 N4 /BN micro/nanocomposites was between 3.209 and 3.368 g/cm3 and the density of Si3 N4 /BN micro/microcomposites was in interval from 2.937 to 3.344 g/cm3 . The density showed a slight decrease when amount of BN addition increased. The reduction in bulk density with increased BN content could be caused by two reasons: the low density of hBN (2.27 g/cm3 ) and the low-chemically active nature of hBN in Si3 N4 /hBN composites.6 However, the Si3 N4 /BN micro/nanocomposites with a small quantity of BN maintain high relative density. Fig. 1 shows the XRD patterns of Si3 N4 /BN ceramic composites with addition of 5 wt% macro or nano hBN. The XRD results indicated that complete transformation from ␣- to Si3 N4 was achieved in all specimens. The XRD patterns of Si3 N4 /BN micro/microcomposites consisted of -Si3 N4 , hBN and ZrO1.96 . In the case of Si3 N4 /BN micro/nanocomposites, the hBN phase was unidentified by XRD. Absence of secondary crystalline phase in these samples indicated that sintering additives formed intergranular amorphous (glassy) phases (IAP). Also other authors confirmed the existence of IAP in Si3 N4 /BN composites.14,21 Each sample contained also some zirconia as contamination of intensive milling in ZrO2 medium before sintering. 3.2. Microstructure Fig. 2 shows the representative microstructures of the prepared Si3 N4 /BN ceramic composites with different macro or nano BN content. The microstructure of the sintered SNB1-nE (Fig. 2a) consisted mainly of elongated -Si3 N4 grains (several micrometres in length), equiaxed -Si3 N4 grains, hBN particles
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Fig. 2. SEM micrographs of the Si3 N4 /BN ceramic composites, (a) SNB1-nE and (b) SNB5-mE.
and intergranular phase. The formation of elongated Si3 N4 grains can be explained by anisotropic grain growth. Due to the lower boundary energy in the c-direction than in a-direction of hexagonal crystal, the energetically more favourable nucleation takes place on the surface of the basal plane.21 Unlike the Si3 N4 /BN micro/microcomposites, no large BN platelets or agglomerates were found in the micro/nanocomposites. However, the TEM examination is necessary for observation of distribution and grain size of hBN nanoparticles and interface structure of BN/IAP phase and Si3 N4 /IAP in Si3 N4 /BN ceramic composites doped with Y2 O3 and Al2 O3 . In our case the increase of BN content up to 5 wt% had no significant influence on the diameter and aspect ratio of Si3 N4 grains. In contrast, Kusunose et al.11 showed that samples containing BN had a finer and more uniform microstructure than the BN-free sample because of the retarding effect of BN on Si3 N4 grain growth. Wei et al.21 also mentioned that higher content (10%) of h-BN flakes hinders the growth of elongated Si3 N4 grains. The microstructure of SNB5-mE consisted of large hBN platelets (Fig. 2b). The distribution of hBN was quite uniform but the agglomeration of bundles of micro BN were still observed. Agglomeration of bundles of micro BN between the silicon nitride boundaries inhibits the densification of composites. For this reason, micro BN added samples had significantly larger porosity than the reference monolithic sample and micro/nanocomposites.
Fig. 3. Effect of hBN content on the hardness of Si3 N4 /BN composites.
basal plane of h-BN platelets causes hardness and bending strength to decrease with h-BN addition.6 As can be seen from Fig. 4 the elastic modulus of the Si3 N4 /BN composites decreased sharply with increasing BN content, too. But we
3.3. Mechanical properties Fig. 3 shows the hardness of Si3 N4 /BN composites as a function of BN content. The hardness of the composites is lower compared to the hardness of the monolithic material. As expected, the Vickers hardness of the composites significantly decreased with increasing content of the BN. The Vickers hardness of SN was 16.4 GPa. The hardness values of the micro/nanocomposites prepared by the first approach varied from 14.2 to 9.95 GPa, for the micro/nanocomposites prepared by the second approach decreased from 14.9 to 7.7 GPa. The lowest values of hardness had the micro/microcomposites in interval from 14.5 GPa to 7.4 GPa. Easy cleavage of
Fig. 4. Effect of hBN content on the elastic modulus of Si3 N4 /BN composites.
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Fig. 5. Effect of hBN content on the indentation fracture toughness of Si3 N4 /BN composites.
Fig. 6. Variation of indentation fracture toughness with indentation load (Rcurve-like behaviour) of Si3 N4 /BN composites.
could note that hardness values and elastic modulus values of Si3 N4 /BN micro/nanocomposites are little bit higher than those of Si3 N4 /BN micro/microcomposite. The lower hardness and elastic modulus of the composite compared to the monolithic material are also dependent on the residual porosity that remains in the material after the sintering.8 The monolithic Si3 N4 sample had an indentation fracture toughness of 6.5 MPa m1/2 . The fracture toughness values of micro/nanocomposites were comparable with monolithic
Si3 N4 , see Fig. 5. The fracture toughness of Si3 N4 /BN micro/microcomposites was slightly improved, mainly at 5 wt% BN addition, up to 7.5 MPa m1/2 . Many investigators have demonstrated that the fracture toughness of -Si3 N4 ceramic is strongly dependent upon grain morphology.22,23 The high fracture toughness is connected to large number of coarse elongated -Si3 N4 grains with high aspect ratio. In this type of microstructure various mechanisms of toughening such as pull-out of elongated -Si3 N4 grain, grain bridging or crack deflection has
Fig. 7. Toughening mechanisms of Si3 N4 /BN composites, (a) crack deflection in SNB3-mE, (b) crack branching in SNB3-mE and (c) pullout of hBN platelet in SNB5-mE.
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Fig. 8. SEM micrographs of fracture surfaces of experimental materials (a) SN, (b) SNB5-nE and (c) SNB3-mE.
been observed.2,24 Because the addition up to 5 wt% BN in our experimental samples had no significant influence on diameter and aspect ratio on Si3 N4 grains, it should not weaken the toughening mechanisms. Also other investigators have shown that adding a small quantity of h-BN to Si3 N4 can increase the fracture toughness of Si3 N4 ceramic matrix.25 The variation of fracture toughness with indentation load (R-curve-like behaviour) was estimated by changing the indentation load over a range 49.05–294.3 N. Silicon nitride shows a characteristic toughening (R-curve) behaviour with increased crack growth resistance connected with increased crack length.26 In the case of the composites, slight R curve behaviour is evident, Fig. 6. Cho et al.15 showed gently rising R-curve for Si3 N4 /BN composite and grain bridging and pullout as possible toughening mechanisms. Higher content of BN in the composites resulted in less sharply rising R-curve behaviour, which could enhance for the machinability of these composites. The enhanced fracture toughness of the Si3 N4 /BN micro/microcomposites in this investigation is connected with the observed toughening mechanisms on the fracture surface and fracture lines during the crack propagation. Fractographic examination of the SNB3-mE and SNB5-mE composites revealed that the crack deflection (Fig. 7a), crack branching (Fig. 7b) and pullout of hBN platelets were main toughening mechanisms. Crack deflection and crack arrest were observed at the interaction of the crack with larger BN platelets of size approximately 20 m. At such an interaction the process zone of the crack rapidly arises,
and after an increase of the outer applied load, crack branching or/and crack twisting occurs in a different direction to the main crack.27 The pullout mechanism is shown in Fig. 7c where BN with large size pulled out from the matrix with the plane of the sheets nearly perpendicular to the plane of fracture surface. Regarding the fracture toughness values in the present work, our results pointed that the micro BN platelets are effective in toughening the silicon nitride matrix. Microfractographic observations of the fracture surface and fracture profiles, illustrated in Fig. 8, have shown that the crack propagation was mostly controlled by intergranular fracture in all materials. In the case of micro/nanocomposites the pores and porous areas were often observed as main fracture origins, Fig. 9a. Together with the porosity, the large BN platelets and agglomerates were found in the micro/microcomposites, Fig. 9b. An important reason is that the densification of Si3 N4 /BN is difficult due to BN addition, which brings many pores in the microstructure.6 As expected, Fig. 10 confirms that the bending strength of the ceramic composites rapidly decreased with increasing BN content, which is mainly due to weak interface between BN and Si3 N4 grains.4 When samples were bended, less energy was required with the increase of BN content, which caused the strength of the composite decrease.8 Unfortunately, a sharp decrease in fracture strength caused by clusters of the BN particles has been observed when BN content in Si3 N4 /BN composite increased to a certain level.13 Therefore, the homogenous dispersion of BN particles into the matrix is probably an
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Fig. 9. Typical fracture origins of Si3 N4 /BN composites, (a) SNB5-nB and (b) SNB3-mE.
effective way to ensure the mechanical properties of Si3 N4 /BN composite ceramics. We can also conclude that mechanical properties of the Si3 N4 /BN micro/nanocomposites are not affected by different preparation procedures. 3.4. Tribological properties Fig. 11 summarizes the average values of the coefficient of friction for all experimental composites in contact with Si3 N4 ball. COF of composites are similar to the reference Si3 N4 . The COF of monolithic Si3 N4 material was around 0.7, which is similar to other investigation.28 It can be seen that for used amounts of boron nitride (1, 3 and 5 wt%) the COF remained basically the same. The COF of Si3 N4 /BN micro/nanocomposites varied from 0.64 to 0.73 and COF of Si3 N4 /BN micro/microcomposites were between 0.69 and 0.74. hBN has a lower friction coefficient and lubrication action, thereby the COF of Si3 N4 /hBN ceramic composites should decreases with increasing BN content but in our study no significant lubrication effect was observed. Investigation of Wei et al.4 confirmed lower COF of Si3 N4 /BN in the
Fig. 10. Effect of hBN content on the bending strength of Si3 N4 /BN composites.
Fig. 11. Effect of hBN content on the coefficient of friction of Si3 N4 /BN composites.
Fig. 12. Effect of hBN content on the specific wear rate of Si3 N4 /BN composites.
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Fig. 13. Worn surfaces of Si3 N4 /BN composites, (a) and (b) SNB5-nB, (c) EDS analysis of wear. track of SNB5-nB, (d) and (e) SNB5-mE, (f) EDS analysis of wear track of SNB5-mE. The EDS analysis clearly points out the oxygen enrichment of the debris layer.
case of BN addition higher than 10 wt%. Chen et al.29 showed that the addition of h-BN to Si3 N4 resulted in decrease of COF from 0.95 for Si3 N4 against stainless steel to 0.03 for Si3 N4 -30% hBN against stainless steel. Skopp and Woydt30 investigated the tribological performance of Si3 N4 /BN composites under unlubricated sliding conditions. Their results revealed that COF of Si3 N4 is between 0.4 and 0.9. When the addition amount of hBN raised to 20 wt%, the friction coefficient decreased to a range between 0.1 and 0.3 at room temperature. Carrapichano
et al.7 also reported a slight reduction of the friction coefficient from 0.82 for Si3 N4 to 0.67 for Si3 N4 -10 vol% h-BN. Another report suggests that the COF is not decreased following the addition of hBN into Si3 N4 at less than 30%.31 For all experimental materials the wear rates were lower than for the monolithic Si3 N4 , Fig. 12. The Si3 N4 /BN micro/microcomposites had the values of specific wear rate one order of magnitude lower (3.5 × 10−6 –6.7 × 10−6 mm3 /Nm) compared with monolithic Si3 N4 (1.6 × 10−5 mm3 /Nm). The tendency suggests that larger
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BN particles coincide with better wear resistance. The lowest wear resistance had material with 5 wt% nano BN particles prepared by first approach with the wear rate close to that of monolithic Si3 N4 . From Fig. 11 is clearly visible, that the positive effect of BN on friction is accompanied by an increase in wear resistance only when low BN content (up to 5 wt%) are added to the Si3 N4 . At 5 wt% of BN particles the specific wear rates slowly increased. Our results are similar with other authors,7 who showed sharp increase of wear coefficient when BN volume fraction was greater than 10%. Other study claimed that the BN addition has no net effect on COF and wear coefficients.16 In this case, boron nitride is reported to be ineffective in providing a solid lubricating film at room temperature. On the other hand, Chen et al.29 reported that the wear coefficient decreased sharply with increase in hBN volume fraction, e.g. it was lower than 10−6 mm3 /Nm for Si3 N4 -20% hBN. The examples of worn surfaces of experimental materials are illustrated in Fig. 13. Fig. 13a and b shows the wear track of SNB5-nB material which was very small and smooth with some micro scratches. The higher amount of coherent debris has been observed in the wear tracks of micro/microcomposites, i.e. in materials with higher wear resistance, (SNB5-mE, Fig. 13d and e). The main wear mechanism in this work was similar for all studied materials in the form of mechanical failure (micro-fracture) and tribochemical reaction. The tribochemical reactions create a film on the surface of materials and above the critical load the tribochemical film was partially removed, resulting in micro-fracture in discrete regions.32 This tribofilm should protect the wear surface. EDS analysis of wear tracks (Fig. 13c and f) indicated that the coherent layers contain large amount of oxygen. These tribochemical reactions can be due to the effects of humidity according to which silicon nitride and silicon carbide react with oxygen from air to form hydrate SiO2 layer.33,34 Moreover, due to possible high contact temperatures, also oxidation could take place. However, the oxygen content of the debris layer should be mainly attributed to humidity-driven triboreaction more than an oxidation. Skopp et al.5 concluded that the addition of BN to Si3 N4 is only tribologically effective below 100 ◦ C in humid air because a film formation of BN or BN(H2 O)x on the wear surface. Furthermore, Erdemir et al.35 ascribed very low friction coefficients to the formation of self-lubricating boric acid films (H3 BO3 ) on boroncontaining surfaces. As temperature increases, the lubricant layers of BN(H2 O)x and (H3 BO3 ) are destroyed either by vaporization of water, or by thermal decomposition above 150 ◦ C.7 The decrease of COF is attributed to BN hydration, which provides an in situ lubrication. Gangopadhyay et al.36 reported the ineffectiveness of BN to lubricate either alumina or silicon nitride due to the limited formation of transfer films. In this study, the formation of hydrated layers on the worn surfaces was not identified and probably therefore the COF on prepared composites are similar to that of the monolithic silicon nitride. However, the positive effect of BN on friction is accompanied by improved wear resistance, when low amounts of BN (5 wt%) were added to silicon nitride matrix. This improvement can be related to higher fracture toughness of Si3 N4 /BN micro/microcomposites compared with monolithic silicon nitride.
9
4. Conclusion The Si3 N4 /BN composites with various micro or nano hBN have been prepared and the influence of the addition of hBN on mechanical and tribological properties was investigated. The main conclusions are as follows: - The BN platelets are quite well distributed in the Si3 N4 matrix of all prepared ceramics composites, but the agglomerations of micro BN particles were still observed. These large hBN platelets are often connected with the porosity. - The increase of hBN content results in a sharp decrease in the Vickers hardness, bending strength and elastic modulus of Si3 N4 /BN micro/nanocomposites and Si3 N4 /BN micro/microcomposites. - The addition of small quantity of micro-sized BN can increase the fracture toughness of Si3 N4 ceramic matrix because different type of toughening mechanisms can be observed in these microstructures. - For used weight fractions (1%, 3% or 5%) of all types of BN additives no decrease of coefficient of friction was observed. BN phase did not participate in lubricating process. However, introduction of boron nitride did lead to better wear resistance. The best results were found for the materials with 1 wt% micro-sized BN addition whose specific wear rate was 78% lower than that of monolithic silicon nitride. The main wear mechanism is similar for all studied materials in the form of mechanical wear (micro-fracture) and tribochemical reaction. - The mechanical and tribological properties of Si3 N4 /BN micro/nanocomposites are independent on the preparation procedure, i.e. on the milling sequence. Acknowledgement Authors gratefully acknowledge financial support of the VEGA 2/0043/14, VEGA 2/0075/13, APVV-0161-11, APVV0500-10 and APVV-0108-12. References 1. Riley FL. Silicon nitride and related materials. J Am Ceram Soc 2000;83:245–65. ˇ 2. Sajgalík P, Dusza J, Hoffmann MJ. Relationship between microstructure, toughening mechanisms, and fracture toughness of reinforced silicon nitride ceramics. J Am Ceram Soc 1995;78:2619–24. 3. Gao L, Jin X, Li J, Li Y, Sun J. BN/Si3 N4 nanocomposite with high strength and good machinability. Mat Sci Eng A 2006;415:145–8. 4. Wei D, Meng Q, Jia D. Mechanical and tribological properties of hot-pressed h-BN/Si3 N4 ceramic composites. Ceram Int 2006;32:549–54. 5. Skopp A, Woydt M. Ceramic–ceramic composite materials with improved friction and wear properties. Tribol Int 1992:61–70. 6. Ruigang W, Wei P, Mengning J, Jian Ch, Yongming L. Investigation of the physical and mechanical properties of hot-pressed machinable Si3 N4 /h-BN composites and FGM. Mat Sci Eng B 2002;90:261–8. 7. Carrapichano JM, Gomes JR, Silva RF. Tribological behaviour of Si3 N4 -BN materials for dry sliding applications. Wear 2002;253:1070–6. 8. Sun Y, Meng Q, Jia D, Guan Ch. Effect of hexagonal BN on the microstructure and mechanical properties of Si3 N4 ceramics. J Mater Process Technol 2007;182:134–8.
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9. Li YL, Li RX, Zhang JX. Enhanced mechanical properties of machinable Si3N4/BN composites by spark plasma sintering. Mater Sci Eng A 2008;483–484:207–10. 10. Liu HY, Hsu SM. Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J Am Ceram Soc 1996;79:2452–7. 11. Kusunose T, Sekino T, Choa YH, Niihara K. Fabrication and microstructure of silicon nitride/boron nitride nanocomposites. J Am Ceram Soc 2002;85:2678–88. 12. Shen ZJ, Zhao Z, Peng H, Nygren M. Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening. Nature 2002;417:266–8. 13. Wang S, Jia D, Yang Z, Duan X, Tian Z, Zhou Y. Effect of BN content on microstructures, mechanical and dielectric properties of porous BN/Si3 N4 composite prepared by gel casting. Ceram Int 2013;39:4231–7. 14. Li Y, Liu P, Wang X, Jin H, Yang J, Qiao G. Interface structure and formation mechanism of BN/intergranular amorphous phase in pressureless sintered Si3 N4 /BN composites. Scr Mater 2010;63:185–8. 15. Cho MW, Kim DW, Cho WS. Analysis of micro-machining characteristics of Si3 N4 -hBN composites. J Eur Ceram Soc 2007;27:1259–65. 16. Saito T, Hosoe T, Honda F. Chemical wear of sintered Si3 N4 , hBN and Si3 N4 -hBN composites by water lubrication. Wear 2001;247:223–30. 17. Skopp A, Woydt M, Habig KH. Tribological behavior of silicon nitride materials under unlubricated sliding between 22 ◦ C and 1000 ◦ C. Wear 1995;181–183:571–80. 18. Larsson P, Axen N, Hogmark S. Tribofilm formation on boron carbide in sliding wear. Wear 1999;236:73–80. 19. Shetty DK, Wright IG, Mincer PN, Clauer AH. Indentation fracture of WCCo cermets. J Mat Sci 1985;20:1873–82. 20. ISO 20808:2004(E). Fine ceramics (advanced ceramics, advanced technical ceramics) – determination of friction and wear characteristics of monolithic ceramics by ball-on-disc method; 2004. 21. Wei D, Meng Q, Jia D. Microstructure of hot-pressed h-BN/Si3 N4 ceramic composites with Y2 O3 –Al2 O3 sintering additive. Ceram Int 2007;33: 221–6. 22. Becher FP, Sun YE, Plucknett PK. Microstructural design of silicon nitride with improved fracture toughness. I. Effect of grain shape and size. J Am Ceram Soc 1998;81:2821–30.
23. Peillon FC, Thevenot F. Microstructural designing of silicon nitride related to toughness. J Eur Ceram Soc 2002;22:271–8. 24. Becher PF, Sun EY, Plucknett KP, Alexander KB, Hsueh ChH, Lin HT, et al. Microstructural design of silicon nitride with improved fracture toughness. I. Effects of grain shape and size. J Am Ceram Soc 1998;81:2821–30. 25. Petrak DR, Lee J. Silicon nitride/boron nitride composite with enhanced fracture toughness. US Patent 5, 324, 694, 28 June 1994. 26. Kovalˇcíková A, Balázsi Cs Dusza J, Tapasztó O. Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite. Ceram Int 2012;38:527–33. 27. Dusza J, Morgiel J, Duszová A, Kvetková L, Nosko M, Kun P, et al. Microstructure and fracture toughness of Si3 N4 +graphene platelet. J Eur Ceram Soc 2012;32:3389–97. ˇ 28. Kaˇsiarová M, Rudnayová E, Kovalˇcík J, Dusza J, Hnatko M, Sajgalík P, et al. Wear and creep characteristics of a carbon derived Si3 N4 /SiC micro/nanocomposite. Materialwiss Werkstofftech 2003;34:338–42. 29. Chen W, Gao Y, Chen C, Xing J. Tribological characteristics of Si3 N4 -hBN ceramic materials sliding against stainless steel without lubrication. Wear 2010;269:241–8. 30. Skopp A, Woydt M. Ceramic and ceramic composite materials with improved friction and wear properties. Tribol Trans 1995;38:233–42. 31. Iwasa M, Kakiuchi S. Mechanical and tribological properties of Si3 N4 -BN composite ceramics. J Jpn Ceram Soc 1985;93:661–5. ˇ 32. Tatarko P, Kaˇsiarová M, Dusza J, Morgiel J, Sajgalík P, Hvizdoˇs P. Wear resistance of hot-pressed Si3 N4 /SiC micro/nanocomposites sintered with rare-earth oxide additives. Wear 2010;269:867–74. 33. Jones MI, Hirao K, Hyuga H, Yamauchi Y. Effect of rare-earth species on the wear properties of ␣-sialon and -silicon nitride under tribochemical type conditions. J Mater Res 2004;19:2750–8. 34. Zum Gahr KH, Blattner R, Hwang DH, Pöhlmann K. Micro-and macro-tribological properties of SiC ceramics in sliding contact. Wear 2001;250:299–310. 35. Erdemir A, Bindal C. Formation and self-lubricating mechanisms of boric acid on borided steel surfaces. Surf Coat Technol 1995;76–77:443–9. 36. Gangopadhyay A, Jahanmir S, Peterson MB. Self-lubricating ceramic matrix composites. In: Jahanmir S, editor. Friction and wear of ceramics. New York: Marcel Dekker; 1994. p. 163–97.
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Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites Alexandra Kovalˇcíková a,∗ , Ján Balko a , Csaba Balázsi b , Pavol Hvizdoˇs a , Ján Dusza a a
b
Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Koˇsice, Slovakia Bay Zoltan Nonprofit Ltd for Applied Research, Institute for Materials Science and Technology, Budapest, Hungary
Abstract Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 ◦ C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si3 N4 /BN composites. In addition, the fracture toughness of Si3 N4 /micro BN composites was enhanced comparing to monolithic Si3 N4 because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si3 N4 /BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si3 N4 matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials. © 2014 Elsevier Ltd. All rights reserved. Keywords: Si3 N4 /BN composites; Mechanical properties; Toughening mechanisms; Wear resistance
1. Introduction Silicon nitride has very good combination of mechanical, physical, and chemical properties. The high strength, hardness, and toughness at room and elevated temperatures, the high thermal shock and wear resistance make it suitable for use in many structural and tribological applications.1,2 To obtain superior mechanical properties, a fine-grained microstructure with elongated beta grains is preferred. These in situ grown beta silicon nitride grains can significantly improve the fracture toughness over monolithic ceramics, producing self-reinforced silicon nitrides. However, just like most of the ceramic materials, the machinability of Si3 N4 ceramics is extremely poor.3 It is well known that h-BN offers a number of interesting properties such as lubrication action, low hardness and low friction coefficient.4 Hexagonal boron nitride has also excellent machinability due to a plate-like structure similar to that of graphite. To improve
∗
Corresponding author at: Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Koˇsice, Slovakia. Tel.: +421 55 792 2463; fax: +421 55 792 2408. E-mail address: [email protected] (A. Kovalˇcíková).
the machinability, fracture toughness and tribological properties (by the selective oxidation of the compounds) of Si3 N4 ceramics at room and elevated temperatures, hexagonal boron nitride particles were introduced as second-phase dispersions into the Si3 N4 matrix by many researchers.5−9 In Si3 N4 /BN composite ceramics, the cleavage behaviour of plate-like structured BN particles endowed the material with good machinability together with superior thermal shock resistance.10 Various processing techniques have been developed to fabricate Si3 N4 /BN composites for structural and functional applications. Kusunose et al.11 prepared Si3 N4 /BN nanocomposites with high strength and excellent machinability via chemical route. Gao et al.3 developed Si3 N4 /BN composites via a similar chemical route and subsequent hot pressing. They achieved the Si3 N4 /BN nanocomposites with higher strength due to formation of fine and homogenous microstructure. Li et al.9 fabricated Si3 N4 /BN composites by spark plasma sintering through traditional powder mixing process and by a chemical route to Si3 N4 /BN composite powder, respectively. The SPS-processed Si3 N4 /BN materials showed high bending strength further enhanced in the composites prepared by powder mixing relative to the counterparts obtained via the chemical process. Shen et al.12 found that the microstructures of the
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Table 1 Composition of the starting powders, preparation of powder mixture and density of the Si3 N4 /BN composites. Samples
SN SNB1-nB SNB3-nB SNB5-nB SNB1-nE SNB3-nE SNB5-nE SNB1-mE SNB3-mE SNB5-mE
Starting powders wt%
Additive wt%
Si3 N4
Al2 O3
Y2 O3
h-BN
90 90 90 90 90 90 90 90 90 90
4 4 4 4 4 4 4 4 4 4
6 6 6 6 6 6 6 6 6 6
– 1 3 5 1 3 5 1 3 5
produced Si3 N4 -based ceramics could be tailored by making use of SPS with a rapid heating time. Phase transformation and elongated -Si3 N4 grains formation were greatly enhanced and yielded interlocking microstructures and compact products with improved mechanical properties. Wang et al.13 prepared porous Si3 N4 /BN composite ceramics with homogeneous dispersion of BN by the gel casting technique. With BN contents increasing, the mechanical properties of porous Si3 N4 /BN ceramics partially declined, but the dielectric properties and thermal shock resistance were enhanced. Li et al.14 fabricated Si3 N4 ceramics with 25 vol% h-BN by pressureless sintering with Y2 O3 and Al2 O3 as sintering additives. Cho et al.15 investigated R-curve behaviour of the hot-pressed Si3 N4 /BN composites to understand its machinability. They showed the grain bridging and pullout as possible toughening mechanisms for monolithic Si3 N4 as well as Si3 N4 -hBN composite. Higher hBN content in the composites resulted in more slowly rising R-curves, which could enhance machinability. Both, the silicon nitride and hexagonal boron nitride showed relatively low friction and wear. hBN as a solid lubricant has high performance only at high relative humidity.16 The formation of oxide or hydrated layers (H3 BO3 and BN(H2 O)x ) is reported to have a beneficial effect on the tribological performance of Si3 N4 -BN composites at room temperature, reducing the wear coefficient by one order of magnitude to k = 10−6 mm3 N−1 m−1 , relative to the matrix material.17 High tribological performance of hBN is reported to be controlled by a basal plane slip or tribological products like B2 O3 .18 In this present study we fabricated the Si3 N4 /BN micro/microcomposite and Si3 N4 /BN micro/nanocomposite by hot isostatic pressing. The effect of BN addition on microstructure, basic mechanical properties and wear resistance was investigated and discussed in details. 2. Experimental materials and methods 2.1. Materials In this work two types of materials were prepared. First group were samples when micro sized Si3 N4 particles were mixed with micro sized hBN particles (Si3 N4 /BN micro/microcomposites).
Preparation of powder mixture
Density g/cm3
Monolithic material NanoBN addition at the beginning of milling NanoBN addition at the beginning of milling NanoBN addition at the beginning of milling NanoBN addition at the end of milling NanoBN addition at the end of milling NanoBN addition at the end of milling MicroBN addition at the end of milling MicroBN addition at the end of milling MicroBN addition at the end of milling
3.381 3.361 3.297 3.209 3.368 3.294 3.033 3.344 3.131 2.937
Second type was samples when micro sized Si3 N4 particles were mixed together with nano sized hBN particles (Si3 N4 /BN micro/nanocomposites). The nano BN powders were prepared by mechanical milling. For many applications this process can be a simple and efficient way for nano BN production. In brief, commercial BN powder (Starck) has been milled intensively in high efficient attritor mill equipped with zirconia agitator delta discs and zirconia grinding media in presence of ethanol at 4000 rpm for 10 h. After wet milling a following dry milling at 4000 rpm for 5 h was applied. The 90 wt% Si3 N4 , 4 wt% Al2 O3 , 6 wt% Y2 O3 powder mixtures with 1, 3, 5 wt% nano BN were prepared in two ways. In the first approach the nano BN addition to the mixture was performed at the beginning of milling. In this case the milling lasted 5 h at 4000 rpm in distilled water. In the second method, first the powder mixture was milled for 4.5 h at 4000 rpm in distilled water. After that the nano BN was added to this mixture and milled for 30 minutes at 600 rpm. In the case of Si3 N4 /BN micro/microcomposite the BN addition to the mixture was performed at the end of milling. The substance was dried and sieved with a filter with mesh size of 150 m. Green samples were obtained by dry pressing at 220 MPa. Following to high energy milling procedure the hot isostatic pressing was selected as sintering method of composites. Samples prepared for HIP were oxidized at 400 ◦ C to eliminate the PEG. The heating rate did not exceed 25 ◦ C/min. Hot isostatic pressing (HIP) was employed at 1700 ◦ C in high purity nitrogen using BN embedding powder at 20 MPa, with 3 h holding time. The chemical composition and way of preparation is briefly summarized in Table 1. 2.2. Test procedure The densities of the sintered specimens were measured according to Archimedes’ principle. Phase compositions were determined by X-ray diffractometer (Philips X’Pert Pro) with Cu K␣ radiation. Sintered specimens were cut, polished to a 1 m finish by routine ceramography procedure and chemically etched in molten NaOH at 400 ◦ C for 2 min. The microstructures were then studied using an SEM (JEOL JSM-7000F). Mechanical properties were investigated using indentation methods. Hardness was determined by Vickers indentation
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(hardness testers LECO 700AT) under load of 9.81 N with a dwell time of 10 s. In order to determine the indentation toughness at least 10 Vickers indentations per specimen were introduced with the load of 49.05 N. The indentation toughness was calculated from the lengths of radial cracks and indents diagonals using a formula valid for semi-circular crack systems as proposed by Shetty19 :
KIC
HP = 0.0899 4l
0.5 (1)
where H is the hardness P is the indentation load and l = c − a is the length of the indentation crack. Elastic modulus was measured by resonant frequency method on specimens with dimensions 2 mm × 3 mm × 25 mm. The four-point bending strength values for HIP samples were determined by bending tests on a tensile/loading machine (Lloyd LR 5KPlus) using specimens with dimensions 2 mm × 3 mm × 25 mm. They were ground and polished by 15 m diamond grinding wheel before testing. The two edges on the tensile surface were rounded to a radius about 0.15 mm in order to eliminate a failure initiated from an edge of the specimen. The specimens were tested in four point bending fixture (inner span of 20 mm and an outer span of 40 mm) with the crosshead speed of 0.5 mm/min at ambient temperature and atmosphere. Fractographic analysis of broken flexural specimens was used to characterize the processing flaws. Wear testing was carried out at room temperature on a High Temperature Tribometer (CSM Instruments) using the pin-on-disc technique. The surfaces were carefully prepared by polishing down to surface roughness below 0.05 m where possible. Wear behaviour of the prepared materials was studied in dry sliding, where the tribological partner was a highly polished Si3 N4 ball with 6 mm diameter. The normal load of 5 N and sliding speed of 0.1 m/s were applied. Total sliding distance was 300 m. The friction coefficients were continually recorded and wear volume on each specimen was calculated from the surface profile traces across the wear track (and perpendicular to the sliding direction) which were measured using the high precision confocal microscope Plu neox 3D Optical Profiler (SENSOFAR). The specific wear rate is calculated according to the standard ISO 2080820 :
WS =
V FN L
(2)
where Ws is the specific wear rate (mm3 N−1 m−1 ), V the volume of removed material (mm3 ), FN the normal load (N) and L is the total sliding distance (m). The wear tracks created at the surfaces of the investigated materials were studied using SEM to identify the wear mechanisms.
Fig. 1. XRD patterns of the Si3 N4 /BN ceramic composites, (a) SNB5-mE and (b) SNB5-nB.
3. Results and discussion 3.1. Density and XRD analysis The highest density was found in the case of monolithic HIP-ed material with a value of 3.381 g/cm3 , see Table 1. The density of Si3 N4 /BN micro/nanocomposites was between 3.209 and 3.368 g/cm3 and the density of Si3 N4 /BN micro/microcomposites was in interval from 2.937 to 3.344 g/cm3 . The density showed a slight decrease when amount of BN addition increased. The reduction in bulk density with increased BN content could be caused by two reasons: the low density of hBN (2.27 g/cm3 ) and the low-chemically active nature of hBN in Si3 N4 /hBN composites.6 However, the Si3 N4 /BN micro/nanocomposites with a small quantity of BN maintain high relative density. Fig. 1 shows the XRD patterns of Si3 N4 /BN ceramic composites with addition of 5 wt% macro or nano hBN. The XRD results indicated that complete transformation from ␣- to Si3 N4 was achieved in all specimens. The XRD patterns of Si3 N4 /BN micro/microcomposites consisted of -Si3 N4 , hBN and ZrO1.96 . In the case of Si3 N4 /BN micro/nanocomposites, the hBN phase was unidentified by XRD. Absence of secondary crystalline phase in these samples indicated that sintering additives formed intergranular amorphous (glassy) phases (IAP). Also other authors confirmed the existence of IAP in Si3 N4 /BN composites.14,21 Each sample contained also some zirconia as contamination of intensive milling in ZrO2 medium before sintering. 3.2. Microstructure Fig. 2 shows the representative microstructures of the prepared Si3 N4 /BN ceramic composites with different macro or nano BN content. The microstructure of the sintered SNB1-nE (Fig. 2a) consisted mainly of elongated -Si3 N4 grains (several micrometres in length), equiaxed -Si3 N4 grains, hBN particles
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Fig. 2. SEM micrographs of the Si3 N4 /BN ceramic composites, (a) SNB1-nE and (b) SNB5-mE.
and intergranular phase. The formation of elongated Si3 N4 grains can be explained by anisotropic grain growth. Due to the lower boundary energy in the c-direction than in a-direction of hexagonal crystal, the energetically more favourable nucleation takes place on the surface of the basal plane.21 Unlike the Si3 N4 /BN micro/microcomposites, no large BN platelets or agglomerates were found in the micro/nanocomposites. However, the TEM examination is necessary for observation of distribution and grain size of hBN nanoparticles and interface structure of BN/IAP phase and Si3 N4 /IAP in Si3 N4 /BN ceramic composites doped with Y2 O3 and Al2 O3 . In our case the increase of BN content up to 5 wt% had no significant influence on the diameter and aspect ratio of Si3 N4 grains. In contrast, Kusunose et al.11 showed that samples containing BN had a finer and more uniform microstructure than the BN-free sample because of the retarding effect of BN on Si3 N4 grain growth. Wei et al.21 also mentioned that higher content (10%) of h-BN flakes hinders the growth of elongated Si3 N4 grains. The microstructure of SNB5-mE consisted of large hBN platelets (Fig. 2b). The distribution of hBN was quite uniform but the agglomeration of bundles of micro BN were still observed. Agglomeration of bundles of micro BN between the silicon nitride boundaries inhibits the densification of composites. For this reason, micro BN added samples had significantly larger porosity than the reference monolithic sample and micro/nanocomposites.
Fig. 3. Effect of hBN content on the hardness of Si3 N4 /BN composites.
basal plane of h-BN platelets causes hardness and bending strength to decrease with h-BN addition.6 As can be seen from Fig. 4 the elastic modulus of the Si3 N4 /BN composites decreased sharply with increasing BN content, too. But we
3.3. Mechanical properties Fig. 3 shows the hardness of Si3 N4 /BN composites as a function of BN content. The hardness of the composites is lower compared to the hardness of the monolithic material. As expected, the Vickers hardness of the composites significantly decreased with increasing content of the BN. The Vickers hardness of SN was 16.4 GPa. The hardness values of the micro/nanocomposites prepared by the first approach varied from 14.2 to 9.95 GPa, for the micro/nanocomposites prepared by the second approach decreased from 14.9 to 7.7 GPa. The lowest values of hardness had the micro/microcomposites in interval from 14.5 GPa to 7.4 GPa. Easy cleavage of
Fig. 4. Effect of hBN content on the elastic modulus of Si3 N4 /BN composites.
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Fig. 5. Effect of hBN content on the indentation fracture toughness of Si3 N4 /BN composites.
Fig. 6. Variation of indentation fracture toughness with indentation load (Rcurve-like behaviour) of Si3 N4 /BN composites.
could note that hardness values and elastic modulus values of Si3 N4 /BN micro/nanocomposites are little bit higher than those of Si3 N4 /BN micro/microcomposite. The lower hardness and elastic modulus of the composite compared to the monolithic material are also dependent on the residual porosity that remains in the material after the sintering.8 The monolithic Si3 N4 sample had an indentation fracture toughness of 6.5 MPa m1/2 . The fracture toughness values of micro/nanocomposites were comparable with monolithic
Si3 N4 , see Fig. 5. The fracture toughness of Si3 N4 /BN micro/microcomposites was slightly improved, mainly at 5 wt% BN addition, up to 7.5 MPa m1/2 . Many investigators have demonstrated that the fracture toughness of -Si3 N4 ceramic is strongly dependent upon grain morphology.22,23 The high fracture toughness is connected to large number of coarse elongated -Si3 N4 grains with high aspect ratio. In this type of microstructure various mechanisms of toughening such as pull-out of elongated -Si3 N4 grain, grain bridging or crack deflection has
Fig. 7. Toughening mechanisms of Si3 N4 /BN composites, (a) crack deflection in SNB3-mE, (b) crack branching in SNB3-mE and (c) pullout of hBN platelet in SNB5-mE.
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Fig. 8. SEM micrographs of fracture surfaces of experimental materials (a) SN, (b) SNB5-nE and (c) SNB3-mE.
been observed.2,24 Because the addition up to 5 wt% BN in our experimental samples had no significant influence on diameter and aspect ratio on Si3 N4 grains, it should not weaken the toughening mechanisms. Also other investigators have shown that adding a small quantity of h-BN to Si3 N4 can increase the fracture toughness of Si3 N4 ceramic matrix.25 The variation of fracture toughness with indentation load (R-curve-like behaviour) was estimated by changing the indentation load over a range 49.05–294.3 N. Silicon nitride shows a characteristic toughening (R-curve) behaviour with increased crack growth resistance connected with increased crack length.26 In the case of the composites, slight R curve behaviour is evident, Fig. 6. Cho et al.15 showed gently rising R-curve for Si3 N4 /BN composite and grain bridging and pullout as possible toughening mechanisms. Higher content of BN in the composites resulted in less sharply rising R-curve behaviour, which could enhance for the machinability of these composites. The enhanced fracture toughness of the Si3 N4 /BN micro/microcomposites in this investigation is connected with the observed toughening mechanisms on the fracture surface and fracture lines during the crack propagation. Fractographic examination of the SNB3-mE and SNB5-mE composites revealed that the crack deflection (Fig. 7a), crack branching (Fig. 7b) and pullout of hBN platelets were main toughening mechanisms. Crack deflection and crack arrest were observed at the interaction of the crack with larger BN platelets of size approximately 20 m. At such an interaction the process zone of the crack rapidly arises,
and after an increase of the outer applied load, crack branching or/and crack twisting occurs in a different direction to the main crack.27 The pullout mechanism is shown in Fig. 7c where BN with large size pulled out from the matrix with the plane of the sheets nearly perpendicular to the plane of fracture surface. Regarding the fracture toughness values in the present work, our results pointed that the micro BN platelets are effective in toughening the silicon nitride matrix. Microfractographic observations of the fracture surface and fracture profiles, illustrated in Fig. 8, have shown that the crack propagation was mostly controlled by intergranular fracture in all materials. In the case of micro/nanocomposites the pores and porous areas were often observed as main fracture origins, Fig. 9a. Together with the porosity, the large BN platelets and agglomerates were found in the micro/microcomposites, Fig. 9b. An important reason is that the densification of Si3 N4 /BN is difficult due to BN addition, which brings many pores in the microstructure.6 As expected, Fig. 10 confirms that the bending strength of the ceramic composites rapidly decreased with increasing BN content, which is mainly due to weak interface between BN and Si3 N4 grains.4 When samples were bended, less energy was required with the increase of BN content, which caused the strength of the composite decrease.8 Unfortunately, a sharp decrease in fracture strength caused by clusters of the BN particles has been observed when BN content in Si3 N4 /BN composite increased to a certain level.13 Therefore, the homogenous dispersion of BN particles into the matrix is probably an
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Fig. 9. Typical fracture origins of Si3 N4 /BN composites, (a) SNB5-nB and (b) SNB3-mE.
effective way to ensure the mechanical properties of Si3 N4 /BN composite ceramics. We can also conclude that mechanical properties of the Si3 N4 /BN micro/nanocomposites are not affected by different preparation procedures. 3.4. Tribological properties Fig. 11 summarizes the average values of the coefficient of friction for all experimental composites in contact with Si3 N4 ball. COF of composites are similar to the reference Si3 N4 . The COF of monolithic Si3 N4 material was around 0.7, which is similar to other investigation.28 It can be seen that for used amounts of boron nitride (1, 3 and 5 wt%) the COF remained basically the same. The COF of Si3 N4 /BN micro/nanocomposites varied from 0.64 to 0.73 and COF of Si3 N4 /BN micro/microcomposites were between 0.69 and 0.74. hBN has a lower friction coefficient and lubrication action, thereby the COF of Si3 N4 /hBN ceramic composites should decreases with increasing BN content but in our study no significant lubrication effect was observed. Investigation of Wei et al.4 confirmed lower COF of Si3 N4 /BN in the
Fig. 10. Effect of hBN content on the bending strength of Si3 N4 /BN composites.
Fig. 11. Effect of hBN content on the coefficient of friction of Si3 N4 /BN composites.
Fig. 12. Effect of hBN content on the specific wear rate of Si3 N4 /BN composites.
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Fig. 13. Worn surfaces of Si3 N4 /BN composites, (a) and (b) SNB5-nB, (c) EDS analysis of wear. track of SNB5-nB, (d) and (e) SNB5-mE, (f) EDS analysis of wear track of SNB5-mE. The EDS analysis clearly points out the oxygen enrichment of the debris layer.
case of BN addition higher than 10 wt%. Chen et al.29 showed that the addition of h-BN to Si3 N4 resulted in decrease of COF from 0.95 for Si3 N4 against stainless steel to 0.03 for Si3 N4 -30% hBN against stainless steel. Skopp and Woydt30 investigated the tribological performance of Si3 N4 /BN composites under unlubricated sliding conditions. Their results revealed that COF of Si3 N4 is between 0.4 and 0.9. When the addition amount of hBN raised to 20 wt%, the friction coefficient decreased to a range between 0.1 and 0.3 at room temperature. Carrapichano
et al.7 also reported a slight reduction of the friction coefficient from 0.82 for Si3 N4 to 0.67 for Si3 N4 -10 vol% h-BN. Another report suggests that the COF is not decreased following the addition of hBN into Si3 N4 at less than 30%.31 For all experimental materials the wear rates were lower than for the monolithic Si3 N4 , Fig. 12. The Si3 N4 /BN micro/microcomposites had the values of specific wear rate one order of magnitude lower (3.5 × 10−6 –6.7 × 10−6 mm3 /Nm) compared with monolithic Si3 N4 (1.6 × 10−5 mm3 /Nm). The tendency suggests that larger
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BN particles coincide with better wear resistance. The lowest wear resistance had material with 5 wt% nano BN particles prepared by first approach with the wear rate close to that of monolithic Si3 N4 . From Fig. 11 is clearly visible, that the positive effect of BN on friction is accompanied by an increase in wear resistance only when low BN content (up to 5 wt%) are added to the Si3 N4 . At 5 wt% of BN particles the specific wear rates slowly increased. Our results are similar with other authors,7 who showed sharp increase of wear coefficient when BN volume fraction was greater than 10%. Other study claimed that the BN addition has no net effect on COF and wear coefficients.16 In this case, boron nitride is reported to be ineffective in providing a solid lubricating film at room temperature. On the other hand, Chen et al.29 reported that the wear coefficient decreased sharply with increase in hBN volume fraction, e.g. it was lower than 10−6 mm3 /Nm for Si3 N4 -20% hBN. The examples of worn surfaces of experimental materials are illustrated in Fig. 13. Fig. 13a and b shows the wear track of SNB5-nB material which was very small and smooth with some micro scratches. The higher amount of coherent debris has been observed in the wear tracks of micro/microcomposites, i.e. in materials with higher wear resistance, (SNB5-mE, Fig. 13d and e). The main wear mechanism in this work was similar for all studied materials in the form of mechanical failure (micro-fracture) and tribochemical reaction. The tribochemical reactions create a film on the surface of materials and above the critical load the tribochemical film was partially removed, resulting in micro-fracture in discrete regions.32 This tribofilm should protect the wear surface. EDS analysis of wear tracks (Fig. 13c and f) indicated that the coherent layers contain large amount of oxygen. These tribochemical reactions can be due to the effects of humidity according to which silicon nitride and silicon carbide react with oxygen from air to form hydrate SiO2 layer.33,34 Moreover, due to possible high contact temperatures, also oxidation could take place. However, the oxygen content of the debris layer should be mainly attributed to humidity-driven triboreaction more than an oxidation. Skopp et al.5 concluded that the addition of BN to Si3 N4 is only tribologically effective below 100 ◦ C in humid air because a film formation of BN or BN(H2 O)x on the wear surface. Furthermore, Erdemir et al.35 ascribed very low friction coefficients to the formation of self-lubricating boric acid films (H3 BO3 ) on boroncontaining surfaces. As temperature increases, the lubricant layers of BN(H2 O)x and (H3 BO3 ) are destroyed either by vaporization of water, or by thermal decomposition above 150 ◦ C.7 The decrease of COF is attributed to BN hydration, which provides an in situ lubrication. Gangopadhyay et al.36 reported the ineffectiveness of BN to lubricate either alumina or silicon nitride due to the limited formation of transfer films. In this study, the formation of hydrated layers on the worn surfaces was not identified and probably therefore the COF on prepared composites are similar to that of the monolithic silicon nitride. However, the positive effect of BN on friction is accompanied by improved wear resistance, when low amounts of BN (5 wt%) were added to silicon nitride matrix. This improvement can be related to higher fracture toughness of Si3 N4 /BN micro/microcomposites compared with monolithic silicon nitride.
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4. Conclusion The Si3 N4 /BN composites with various micro or nano hBN have been prepared and the influence of the addition of hBN on mechanical and tribological properties was investigated. The main conclusions are as follows: - The BN platelets are quite well distributed in the Si3 N4 matrix of all prepared ceramics composites, but the agglomerations of micro BN particles were still observed. These large hBN platelets are often connected with the porosity. - The increase of hBN content results in a sharp decrease in the Vickers hardness, bending strength and elastic modulus of Si3 N4 /BN micro/nanocomposites and Si3 N4 /BN micro/microcomposites. - The addition of small quantity of micro-sized BN can increase the fracture toughness of Si3 N4 ceramic matrix because different type of toughening mechanisms can be observed in these microstructures. - For used weight fractions (1%, 3% or 5%) of all types of BN additives no decrease of coefficient of friction was observed. BN phase did not participate in lubricating process. However, introduction of boron nitride did lead to better wear resistance. The best results were found for the materials with 1 wt% micro-sized BN addition whose specific wear rate was 78% lower than that of monolithic silicon nitride. The main wear mechanism is similar for all studied materials in the form of mechanical wear (micro-fracture) and tribochemical reaction. - The mechanical and tribological properties of Si3 N4 /BN micro/nanocomposites are independent on the preparation procedure, i.e. on the milling sequence. Acknowledgement Authors gratefully acknowledge financial support of the VEGA 2/0043/14, VEGA 2/0075/13, APVV-0161-11, APVV0500-10 and APVV-0108-12. References 1. Riley FL. Silicon nitride and related materials. J Am Ceram Soc 2000;83:245–65. ˇ 2. Sajgalík P, Dusza J, Hoffmann MJ. Relationship between microstructure, toughening mechanisms, and fracture toughness of reinforced silicon nitride ceramics. J Am Ceram Soc 1995;78:2619–24. 3. Gao L, Jin X, Li J, Li Y, Sun J. BN/Si3 N4 nanocomposite with high strength and good machinability. Mat Sci Eng A 2006;415:145–8. 4. Wei D, Meng Q, Jia D. Mechanical and tribological properties of hot-pressed h-BN/Si3 N4 ceramic composites. Ceram Int 2006;32:549–54. 5. Skopp A, Woydt M. Ceramic–ceramic composite materials with improved friction and wear properties. Tribol Int 1992:61–70. 6. Ruigang W, Wei P, Mengning J, Jian Ch, Yongming L. Investigation of the physical and mechanical properties of hot-pressed machinable Si3 N4 /h-BN composites and FGM. Mat Sci Eng B 2002;90:261–8. 7. Carrapichano JM, Gomes JR, Silva RF. Tribological behaviour of Si3 N4 -BN materials for dry sliding applications. Wear 2002;253:1070–6. 8. Sun Y, Meng Q, Jia D, Guan Ch. Effect of hexagonal BN on the microstructure and mechanical properties of Si3 N4 ceramics. J Mater Process Technol 2007;182:134–8.
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9. Li YL, Li RX, Zhang JX. Enhanced mechanical properties of machinable Si3N4/BN composites by spark plasma sintering. Mater Sci Eng A 2008;483–484:207–10. 10. Liu HY, Hsu SM. Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J Am Ceram Soc 1996;79:2452–7. 11. Kusunose T, Sekino T, Choa YH, Niihara K. Fabrication and microstructure of silicon nitride/boron nitride nanocomposites. J Am Ceram Soc 2002;85:2678–88. 12. Shen ZJ, Zhao Z, Peng H, Nygren M. Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening. Nature 2002;417:266–8. 13. Wang S, Jia D, Yang Z, Duan X, Tian Z, Zhou Y. Effect of BN content on microstructures, mechanical and dielectric properties of porous BN/Si3 N4 composite prepared by gel casting. Ceram Int 2013;39:4231–7. 14. Li Y, Liu P, Wang X, Jin H, Yang J, Qiao G. Interface structure and formation mechanism of BN/intergranular amorphous phase in pressureless sintered Si3 N4 /BN composites. Scr Mater 2010;63:185–8. 15. Cho MW, Kim DW, Cho WS. Analysis of micro-machining characteristics of Si3 N4 -hBN composites. J Eur Ceram Soc 2007;27:1259–65. 16. Saito T, Hosoe T, Honda F. Chemical wear of sintered Si3 N4 , hBN and Si3 N4 -hBN composites by water lubrication. Wear 2001;247:223–30. 17. Skopp A, Woydt M, Habig KH. Tribological behavior of silicon nitride materials under unlubricated sliding between 22 ◦ C and 1000 ◦ C. Wear 1995;181–183:571–80. 18. Larsson P, Axen N, Hogmark S. Tribofilm formation on boron carbide in sliding wear. Wear 1999;236:73–80. 19. Shetty DK, Wright IG, Mincer PN, Clauer AH. Indentation fracture of WCCo cermets. J Mat Sci 1985;20:1873–82. 20. ISO 20808:2004(E). Fine ceramics (advanced ceramics, advanced technical ceramics) – determination of friction and wear characteristics of monolithic ceramics by ball-on-disc method; 2004. 21. Wei D, Meng Q, Jia D. Microstructure of hot-pressed h-BN/Si3 N4 ceramic composites with Y2 O3 –Al2 O3 sintering additive. Ceram Int 2007;33: 221–6. 22. Becher FP, Sun YE, Plucknett PK. Microstructural design of silicon nitride with improved fracture toughness. I. Effect of grain shape and size. J Am Ceram Soc 1998;81:2821–30.
23. Peillon FC, Thevenot F. Microstructural designing of silicon nitride related to toughness. J Eur Ceram Soc 2002;22:271–8. 24. Becher PF, Sun EY, Plucknett KP, Alexander KB, Hsueh ChH, Lin HT, et al. Microstructural design of silicon nitride with improved fracture toughness. I. Effects of grain shape and size. J Am Ceram Soc 1998;81:2821–30. 25. Petrak DR, Lee J. Silicon nitride/boron nitride composite with enhanced fracture toughness. US Patent 5, 324, 694, 28 June 1994. 26. Kovalˇcíková A, Balázsi Cs Dusza J, Tapasztó O. Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite. Ceram Int 2012;38:527–33. 27. Dusza J, Morgiel J, Duszová A, Kvetková L, Nosko M, Kun P, et al. Microstructure and fracture toughness of Si3 N4 +graphene platelet. J Eur Ceram Soc 2012;32:3389–97. ˇ 28. Kaˇsiarová M, Rudnayová E, Kovalˇcík J, Dusza J, Hnatko M, Sajgalík P, et al. Wear and creep characteristics of a carbon derived Si3 N4 /SiC micro/nanocomposite. Materialwiss Werkstofftech 2003;34:338–42. 29. Chen W, Gao Y, Chen C, Xing J. Tribological characteristics of Si3 N4 -hBN ceramic materials sliding against stainless steel without lubrication. Wear 2010;269:241–8. 30. Skopp A, Woydt M. Ceramic and ceramic composite materials with improved friction and wear properties. Tribol Trans 1995;38:233–42. 31. Iwasa M, Kakiuchi S. Mechanical and tribological properties of Si3 N4 -BN composite ceramics. J Jpn Ceram Soc 1985;93:661–5. ˇ 32. Tatarko P, Kaˇsiarová M, Dusza J, Morgiel J, Sajgalík P, Hvizdoˇs P. Wear resistance of hot-pressed Si3 N4 /SiC micro/nanocomposites sintered with rare-earth oxide additives. Wear 2010;269:867–74. 33. Jones MI, Hirao K, Hyuga H, Yamauchi Y. Effect of rare-earth species on the wear properties of ␣-sialon and -silicon nitride under tribochemical type conditions. J Mater Res 2004;19:2750–8. 34. Zum Gahr KH, Blattner R, Hwang DH, Pöhlmann K. Micro-and macro-tribological properties of SiC ceramics in sliding contact. Wear 2001;250:299–310. 35. Erdemir A, Bindal C. Formation and self-lubricating mechanisms of boric acid on borided steel surfaces. Surf Coat Technol 1995;76–77:443–9. 36. Gangopadhyay A, Jahanmir S, Peterson MB. Self-lubricating ceramic matrix composites. In: Jahanmir S, editor. Friction and wear of ceramics. New York: Marcel Dekker; 1994. p. 163–97.
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