Effect of ZnO on the interfacial bonding between Na2O–B2O3–SiO2 vitrified bond and diamond

Solid State Sciences 11 (2009) 1427–1432

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Effect of ZnO on the interfacial bonding between Na2O–B2O3–SiO2 vitrified bond and diamond P.F. Wang, Zh.H. Li, J. Li, Y.M. Zhu* Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, 92, Weijin Road, Tianjin 300072, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2009 Received in revised form 20 April 2009 Accepted 24 April 2009 Available online 3 May 2009

Diamond composites were prepared by sintering diamond grains with low melting Na2O–B2O3–SiO2 vitrified bonds in air. The influence of ZnO on the wettability and flowing ability of Na2O–B2O3–SiO2 vitrified bonds was characterized by wetting angle, the interfacial bonding states between diamond grains and the vitrified bonds were observed by scanning electron microscope (SEM), and the micro-scale bonding mechanism in the interfaces was investigated by means of energy-dispersive spectrometer (EDS), Fourier transform infrared (FTIR) spectrometer and X-ray photoelectron spectroscopy (XPS). The experimental results showed that ZnO facilitated the dissociation of boron/silicon–oxygen polyhedra and the formation of larger amount of non-bridging oxygen in the glass network, which resulted in the increase of the vitrified bonds’ wettability and the formation of –C]O, –O–H and –C–H bonds on the surface of diamond grains. B and Si diffused from the vitrified bonds to the interface, and C–C, C–O, C]O and C–B bond formed on the surface of sintered diamond grains during sintering process, by which the interfacial bonding between diamond grains and the vitrified bonds was strengthened. Crown Copyright Ó 2009 Published by Elsevier Masson SAS. All rights reserved.

Keywords: Interface Vitrified bond Diamond composites Wettability

1. Introduction Diamond abrasive tools are a group of high performance ultrahard composite materials, which have high hardness, excellent abrasion resistance, and good thermal conductivity. These make high demands of diamond abrasive tools for high speed, high efficiency and high precision machining and grinding industry nowadays [1]. These tools are composed of diamond grains and certain type of bonds, including metal, resin and vitrified bonds [2–4], among which vitrified bond diamond abrasive tools show outstanding properties over other types, e.g., higher strength, high elastic modulus, low fracture toughness, good self-dressing and shape-keeping ability, as well as controllable porosity [5]. While, the structural properties of composites greatly depend on the interfacial characteristics in the composites [6–10], and vitrified bond diamond composites are of no exception [11–13]. Especially, when properties including bending strength, fracture mode and heat transfer are concerned, the interfacial characteristics are of much interest and importance. However, only a few studies are reported on the interface between diamond grains and

* Corresponding author. E-mail address: [email protected] (Y.M. Zhu).

vitrified bond [12,13]. To explore interfacial properties of diamond composites is of great importance to open the way to new type and high performance vitrified bond diamond abrasive tools. In this work, Na2O–B2O3–SiO2 vitrified bonds with and without ZnO are prepared to make vitrified bond diamond composites, and the microstructure of interface between diamond grains and vitrified bonds, the element distribution across the interfaces and the chemical bonding states of carbon are investigated to disclose the change of interfacial bonding states between Na2O–B2O3–SiO2 vitrified bond and diamond with the addition of ZnO. 2. Experimental procedures 2.1. Preparation of vitrified bonds and diamond composites The basic vitrified bond made of sodium carbonate, silicon and boric acid was prepared by traditional method, consisting in screening the raw materials, accurate weighing and mixing, and next melting in an alumina crucible at the temperature up to 1300  C and quenching in cold water. The basic vitrified bond had a composition of 16.0Na2O–30.0B2O3–54.0SiO2 (mol%) (hereafter referred to as B-1). With the same method, zinc was introduced to the basic vitrified bond in the form of ZnO by melting 1.5 wt% ZnO powder with the quenched glass. The vitrified bond with 1.5 wt% ZnO was hereafter named as B-2.

1293-2558/$ – see front matter Crown Copyright Ó 2009 Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.04.026

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Commercial diamond (average grain size 200 mm, Henan Funik Ultrahard Material Co. Ltd, China) was also cleaned with ethanol, acetone and deionized water for 5 min using ultrasonic cleaner, respectively. Green samples made of the treated diamond and the vitrified bonds were sintered in electric furnace in air with the holding time in the range 30–120 min. 2.2. Characterization of the vitrified bonds and their interfaces with diamond grains Wetting angle was used to characterize the wettability between vitrified bonds and diamond. Measurements of the wetting angle were obtained using high-temperature microscopy (EM201) by applying the sessile drop method in air. Considering the fact that the presence of defects and gaseous contamination in the graphite surface disturbs the process of wetting [14], we used diamond compact disk as a substitute for graphite disk. Diamond micropowder (Henan Funik Ultrahard Material Co. Ltd, China) with grains of 0.5–1 mm was pressed into compact disk under high temperature and high pressure, which were polished to the mirror finish and cleaned in ultrasonic bath with ethanol, acetone and deionized water, respectively. Observation of interface microstructure and bonding state in sintered diamond composites was performed using scanning electron microscope (Philips XL30E), and the EDS line scan was processed on the diamond/vitrified bond interface. Samples of green and sintered diamond composites were crushed and grinded, and diamond grains with a thin layer of vitrified bonds were chosen for FTIR and XPS measurements. FTIR measurement was carried out on a Nicolet FT-IR 470 instrument and the spectra were recorded in transmission mode in the 400–4000 cm1 wavenumber range. The XPS measurement was performed on a MicroLab 310-F instrument (VG Scientific) using monochromatized Al Ka radiation.

vitrified bonds and diamond. Firstly, when diamond compact disk was used as substrate for wetting angle measurement, the wetting angle reduced greatly from above 110 [15,16] to 70–75 for B-1. Compared with B-1, the wetting angle of B-2 declines by about 10 with addition of 1.5 wt% ZnO. After being treated at 780  C for 120 min, B-2 maintains a value of about 55 . It suggested that the addition of ZnO increased the wettability of basic Na2O–B2O3–SiO2 vitrified bond. Higher wettability is favorable for the vitrified bond to flow between diamond grains and wet diamond in sintering process. 2The surface morphology and chemistry analysis of treated diamond are shown in Fig. 2. The surface (see Fig. 2(a)) is relatively regular and smooth, and no large voids are observed on the surface except few scratches. Data from the EDS scan on the diamond surface (Fig. 2(b)) do not show any inclusion. Note also that other intensive unassigned peaks, especially at about 2.15 keV, are the corresponding peaks of Au deposited on the diamond surface while preparing SEM specimen. The smooth and clean surface of diamond is more prone to form chemical bonding instead of strong mechanical bond between vitreous bonds with improved wettability of the vitrified bond. The change in wettability between diamond and B-2 can be illustrated both by SEM observation of the fracture surface of sintered diamond composite with B-2 and detailed EDS analysis of the interfaces, as shown in Fig. 3. As seen from Fig. 3(a), diamond grains are closely overlaid by vitrified bond and their interfaces are clearly observed. Four points (A–D) at the interface between diamond and vitrified bonds are chosen to quantify the sodium/zinc ratio, which has a tremendous effect on viscosity of molten vitrified bond, and its fluidity and wettability. The presence of Al peak is due to the use of alumina balls as medium during ball milling. It can be seen that little fluctuation in sodium/zinc ratio is observed between A (2.152), B (2.146) and C (2.122), which results in an average ratio of

3. Results and discussions 3.1. Vitrified bonds’ wettability and diamond’s surface morphology The refractoriness of B-1 and B-2 was measured to be about 782  C and 760  C, respectively. In our experiments, the sintering temperature was scheduled to be 800  C and 780  C for B-1 and B-2, at which the wetting angles were measured. Fig. 1 describes the holding time-dependent variations in the wetting angle between the

80 B-1(Basic vitrified bond) B-2 (Vitrified bond with ZnO)

Wetting angle/degree

75

70

65

60

55

50

30

60

90

120

Holding time/min Fig. 1. Change of wetting angle with holding time for the vitrified bonds (B-1, B-2).

Fig. 2. Surface morphology (a) and chemistry analysis (b) of diamond grains.

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Fig. 3. SEM observation of the sintered diamond composite with B-2 (a) and EDS analysis of the interfaces (b) point A, (c) point D.

2.14. At point D, the ratio declines rapidly to 0.55 (see Fig. 3(c)). In addition, the interfaces at A, B and C are apparent, even warped surface appears at point C. By contrast, the interface at point D is transitional and flat. It is known that alkali metal ions have notable influence on reducing the viscosity of borosilicate glass, more dramatic than zinc. But the difference in sodium/zinc ratio seems contradictory with the micro observation on the interface. It is inferred that vitrified bond with high zinc concentration may otherwise have large fluidity and wettability in local region. 3.2. Interfacial bond observation and analysis by SEM and EDS line scan Fig. 4 presents the SEM observation of bonding between diamond grain and B-1, and EDS line scan across the interface. The diamond grain is covered by the vitrified bond except the ledge (see Fig. 4(a)), above which a tilting layer structure is clearly observed in the region. This demonstrates that B-1 does not exhibit very good fluidity and wettability to diamond grain. Fig. 4(b) quantifies the

Fig. 4. EDS line scan across the interface between diamond and B-1: (a) SEM image of the interface; (b) concentration profiles of Na, Si, O, N, C and B element.

element concentration of B, C, O, Si and Na across the interface. It shows large amount of boron but quite bit oxygen, sodium distribute on the uncovered diamond surface and near the tilting layer. The presence of these main components of basic vitrified bond on diamond surface suggests that the diamond surface might have been covered by a thin vitrified bond layer, which might rupture and tilt due to the increase of viscosity during cooling process. The SEM image of the interfacial bonding state between diamond and B-2, together with the element distribution across the interface is given in Fig. 5. The diamond grain is fully covered with vitrified bond, and the vitrified bond bridge shows better densification, which can be attributed to the improved wettability of B-2

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and its lowered viscosity. The element distribution (in Fig. 5(b)) is quite different from that in Fig. 4(b). However, B, C, O, Si and Na show a similar distribution profile except concentration magnitude, and boron and carbon both maintain higher concentration near the interface. Compared with the concentration profile in Fig. 4(b), the concentration of carbon is one-third of its former level, while boron concentration at the interface increases nearly by twice. For zinc, its distribution across the interface is rather homogenous and displays a similarity with O, Na and Si (as seen in Fig. 5(b)). The similarity in concentration of sodium, boron and silicon between sintered and

raw vitrified bond also hints that phase separation did not happen during the sintering process at low temperature. Moreover, the sodium/zinc molar ratio in the vitrified bond at the interface is calculated to be about 2 from EDS line scan, which is near the average ratio, 2.14. The decrease of sodium/zinc ratio might improve the viscosity of glass at the interface to some extent. Even though the wetting angle, about 55 , is quite larger than 30 below which it is assumed that better adhesion of the vitrified bond to the surface of abrasive grains would be obtained [17,18]. However, a relatively high viscosity would also promise a good shape-keeping ability and rigidity for vitrified bond diamond abrasive tools. 3.3. Interfacial bond analysis by FTIR and XPS FTIR analyses of diamond grains with a thin layer of vitrified bonds before and after sintering are compared in Fig. 6. The spectra in Fig. 6(a) are similar in the range of 400–3000 cm1; the same bands occur at similar wavenumbers. Compared with diamond/B-1 system, small blue shift in wavenumber for diamond/B-2 system is observed. The intensities of the particular bands are also highly comparable. In the spectra distinctly developed bands of great half-widths and flat maxima can be observed, especially for diamond/B-1 system in Fig. 6(a) and both spectra in Fig. 6(b), which are characteristic for amorphous compound.

a

2941.4

Transmittance (%)

80

60

2351.2

2943.4

2355.1 1641.4

40 3419.8

1639.4 1396.5

20 diamond/B-1

1423.5

diamond/B-2

0 4000

3500

3000

690.5

1082.1

2500

2000

1500

1053.1

1000

500

Wavenumber (cm-1)

b

80 diamond/B-1

Transmittance (%)

70

diamond/B-2

692.4 1172.7

60

50 2322.3

40

1633.7

2366.6 3612.7

30

20 4000

1072.4

3425.6

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 5. EDS line scan across the interface between diamond and B-2: (a) SEM image of the interface; (b) element distribution of Si, C, B, Na, O and Zn.

Fig. 6. FTIR spectra of diamond/vitrified bond systems: (a) before and (b) after sintering; the black figure is for diamond/B-1 and the grey one diamond/B-2.

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In Fig. 6(a), the following obvious bands for diamond/B-1 system are presented: The band at about 690.5 cm1 corresponds to the bending vibration of B–O with boron in coordination three; the band at about 1082.1 cm1 represents the asymmetric tensile vibration of Si–O–Si, and also the tensile vibration of Si–O–B; the band at about 1423.5 cm1 may be stated to originate from mixing of B–O bonds (including trihedra and tetrahedra). This result is in good agreement with the FTIR features of vitrified bonds described by B. Procyk [14]. In addition, the bands at about 1639.4 cm1, 2355.2 cm1 and 2943.4 cm1 are assigned to the absorption band of –C]O, –O–H and –C––H, respectively. These bands demonstrate the existence of dangling bonds on the diamond surface. The broad bands between 3000 and 4000 cm1 are commonly derived from the hydroxyl group (–O–H) stretching peaks. It falls in and verifies the hypothesis that the speciation of water dissolves in silicate glass by rapid quenching from melt as both molecular water and hydroxyl groups, which can explain the variations in viscosity and phase relationship of the water-bearing glass as described in Ref. [19]. For diamond/B-2 system in Fig. 6(a), similar bands about silica and boron oxygen polyhedra (at about 1053.1 cm1 and 1396.5 cm1) shift to short wavenumber direction by about 30 cm1. Particularly, an obvious band at about 3419.8 cm1 appears for diamond/B-2 system. The observed blue shift is attributed to the relaxation of glass network, because of the modification effect of zinc ion on the basic vitrified bond. The band at about 3419.8 cm1 is also regarded as the characteristic absorption of hydroxyl group. Its large intensity might result from the higher concentration of planar water in B-2. This provides another proof that the glass network is more fragile with the influence of zinc and water quenching. The interaction between the dangling bonds on the diamond surface and the chemically adsorbed hydroxyl group on the vitrified bond particle surface increases the bonding strength and polarity of –O–H, resulting in the dense absorption of –O–H. However, the effect of ZnO on the shift of –C]O and –C–H absorption bands (only about 2–3 cm1 shift) is not as obvious as that on boron/silicon– oxygen polyhedra (about 30 cm1). It should be noted that the characteristic absorption peak of Zn–O, which is assigned at about 490 cm1, is difficult to observe, due to the poor resolution and severe vibronic structure between 400 and 500 cm1. In Fig. 6(b), FTIR spectra of sintered diamond particles show large difference in the wavenumber range of 400–3000 cm1. The absorption intensities of the bands decline and the spectra broaden after diamond composites are sintered. The bands deriving from boron/silicon–oxygen polyhedra are weakened greatly, which illustrates the dissociation of boron/silicon–oxygen polyhedra. Even though, diamond/B-2 system also displays the characteristic broaden bands as the system before sintering. Several bands at about 692.4 cm1, 1633.7 cm1 and 2322.3 cm1 can be found in the spectra with a little blue shift. At the same time, the appearance of broad bands in the spectra illustrates the two vitrified bonds have become vitreous after sintering, which is advantageous to the wetting of diamond grains via physical and/or chemical adsorption, or chemical reactions. Although, it is known that oxides in the composition of some vitrified bonds may react with diamond grains, according to the following mode [20]: Cdiamond þ MxOy / xCO þ yM. When Na2O, B2O3, SiO2 and ZnO were introduced in the formula, the obtained results of thermodynamic potential (DG) are intensively positive, which suggests that these oxides do not react directly with diamond. While, the FTIR spectra of diamond/vitrified bond systems revealed that the introduction of ZnO into basic vitrified bond promoted the formation of non-bridging oxygen in the glass network. Newly formed non-bridging oxygen atoms are active, and

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they may bond with boron, carbon and hydrogen atoms via Van der Waals bonding. Due to the higher wettability and fluidity of B-2, the non-bridging oxygen atoms were easy to form bonding with carbon atom in the interface directly/indirectly. This is favorable for the transfer of oxygen, carbon and boron atoms to the surface of diamond, as depicted by EDS line scan in Figs. 4(b) and 5(b). These dangling bonds adsorbed on the surface of diamond and vitrified bond might react during the sintering process because of thermal etching of diamond surface, which contributed to the formation of new bonds between carbon atoms and ions in boron/silicon– oxygen polyhedra. The chemical bonding states of carbon on the surface of sintered diamond grains with B-1 and B-2 were investigated by means of C1s core-level XPS spectrum, as given in Fig. 7. The broad and asymmetrical C1s spectrum profile indicates that the carbon atoms

Fig. 7. C1s core-level XPS spectrum profiles of sintered diamond grains with B-1 (a) and with B-2 (b).

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are involved in several chemical conditions. Four individual lines had to be used in order to get good agreement between the measured and the fitted C1s spectrum. In Fig. 7(a), the fitted C1s peak centers at 283.03, 284.60, 286.27 and 287.91 eV, respectively. The main energy peak at 284.6 eV is identified as graphitizing C–C bond on the diamond surface, due to the thermal etching of diamond surface. The binding energy of 286.27 eV corresponds to C–O bonds, and that of 287.91 eV derives from C]O bonds. The small component peaked at 283.03 eV of the C1s spectrum is very close to the reported values for C–B bond (283.3–283.6 eV) [21] and C–Si bond [22,23]. This is most likely attributed to the carbon bonded to boron or silicon atom in boron/ silicon–oxygen polyhedra. It was hypothesized that carbon atoms have two bonding states with molecular water, direct bonding with oxygen or hydrogen; the decomposition of molecular water during sintering might resulted in the formation of C–B and/or C–Si bonds between graphitizing carbon and boron/silicon atoms in the thermal and chemical active interface. In that case, it is more reasonable to form C–B bond, because boron is more chemically active in the glass network. And each type of carbon chemical state maintains a ratio of 11.91, 81.28, 5.28 and 1.53% (mol ratio), respectively. For the C1s core-level XPS spectrum profile of sintered diamond with B-2 (as shown in Fig. 7(b)), the fitted C1s spectrum shows similar bands peaked at 283.21, 284.66, 286.32 and 288.17 eV, corresponding to C–B, C–C, C–O and C]O bond, respectively. The ratio of carbon in each band type changes to 16.37, 76.39, 5.39 and 1.85 accordingly. The increase of C–B ratio is attributed to the degradation of boron/silicon–oxygen polyhedra under the effect of ZnO, which results in the formation of larger amount of nonbridging oxygen and also the improvement of wettability. On the other hand, improved wettability and fluidity of B-2 make its interface with diamond grain more active, so that the ratio of C–O and C]O bond increases.

vitrified bond between diamond grains. The microstructural observation and sodium/zinc ratio calculation inferred that the vitrified bond with a little higher sodium concentration in local region has larger fluidity and wettability. Boron/silicon–oxygen polyhedra dissociated to form larger amount of non-bridging oxygen in the glass network due to the addition of ZnO, and –C]O, –O–H and –C–H bonds formed in the presence of non-bridging oxygen and molecular water in the vitrified bonds. What’s more, four types of chemical bonding states, C–C, C–O, C]O and C–B bonds, were proposed for carbon atoms on the surface of diamond grains, due to the thermal etching of melted vitrified bonds towards diamond surface as well as the decomposition of planar water during sintering.

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4. Conclusion The effect of ZnO on the interfacial bonding of Na2O–B2O3–SiO2 vitrified bonds and diamond was investigated. It has been shown that the introduction of ZnO into the basic Na2O–B2O3–SiO2 vitrified bond reduced the wetting angles and improved the wettability, which resulted in the more homogenous flowing and wetting of the

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