- Email: [email protected]
Crystallization and properties of B2O3 doped LZAS vitrified bond for diamond grinding tools
Journal of Non-Crystalline Solids 427 (2015) 69–75
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Crystallization and properties of B2O3 doped LZAS vitrified bond for diamond grinding tools Xiaolin Hu, Lei Cui, Taoyong Liu, Zhanghong Zheng, Yu Tang, Anxian Lu ⁎ School of Material Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2015 Received in revised form 29 June 2015 Accepted 2 July 2015 Available online 24 July 2015 Keywords: Crystallization; TEC; Vitrified bond; B2O3-doped
a b s t r a c t The effects of B2O3 on the crystallization and properties of Li2O–ZnO–Al2O3–SiO2 (LZAS) vitrified bonds were investigated for diamond grinding tool application. The results show that the [BO3] in the glass structure increased with increasing B2O3 content. The glass transition temperature and the first crystallization peak temperature decreased, while the second crystallization peak temperature increased with increasing B2O3 content. The increase in B2O3 content has a little influence on the value of Avrami exponent (n), keeping two-dimensional crystal growth mechanism. Realized composite with 8 mol% B2O3 of LZAS vitrified bond has a higher crystal volume fraction and a uniform sphere-shaped microstructure. Moreover, vitrified bond B8 exhibits a high bending strength (198 MPa), a good chemical durability (20 days, DR = 3.1 × 10−9 g/cm2 min), an appropriate TEC (4.63 × 10−6/°C) and a good wettability to diamond abrasives, suggesting that it would be a promising material for diamond grinding tool. © 2015 Elsevier B.V. All rights reserved.
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. Due to the excellent properties, diamond abrasive tools have been increasingly applied in machining industries with high-speed, high-efficiency and high-precision in recent years [1–3]. These tools are composed of diamond grains and certain type of bonds, including metal, resin and vitrified bonds [4–6], among which vitrified bond diamond abrasive tools show more outstanding properties than other types, e.g., high elastic modulus, low fracture toughness, good thermal stability and high rigidity, as well as controllable porosity [7]. However, it is difficult to balance the grinding performance, wheel safety and product consistency of the vitrified bond systems presently used in diamond abrasives tools. Therefore, it is necessary to further improve the properties of vitrified bonds to make full use of the outstanding potential of diamond grains and satisfy the demands of modern precision machining industry. As we all know, properties of vitrified bonds depend essentially on the composition and concentration of the residual glassy phase and the microstructure, all of which is controlled by glass composition and applied heat treatments [8,9]. To develop a good grinding tool, it is, therefore, necessary to choose a suitable composition. Yang et al. [10] determined the optimum chemical composition of glass bonds in an Al2O3–B2O3–CaO–Na2O system in order to obtain cubic boron nitride (CBN) grinding wheels with high flexural strength. The results suggested that these bonds should include a small amount of alkaline ⁎ Corresponding author. E-mail address: [email protected] (A. Lu).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.005 0022-3093/© 2015 Elsevier B.V. All rights reserved.
oxides and alkaline earths. Jackson et al. [9,11,12] showed that the selection of chemical composition for grinding wheels was made of both conventional alumina grains and CBN. Composition based on Li2O–ZnO–Al2O3–SiO2 (LZAS) glass-ceramics has aroused considerable interest due to its adjustable thermal expansion coefficient, high hardness, creep resistance, good resistance to mechanical and thermal shock, and excellent chemical durability [13–16]. These properties make LZAS systems appropriate for grinding tool applications. It was found that B2O3 has been applied to various ceramic materials since it can low the sintering temperature, increase thermal stability, decrease TEC as well as adjust the dielectric properties of materials [17–19]. According to our knowledge, there is still no report on the crystallization and properties of B2O3 doped LZAS vitrified bond for diamond grinding tools application. Accordingly, the purpose of this work is to study the effect of different amounts of B2O3 on the crystallization and properties of LZAS vitrified bond. Glasses and glass-ceramics were characterized using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The thermal expansion coefficient (TEC), bending strength and chemical durability of the obtained products were also evaluated. The study aimed also at developing suitable varieties of vitrified bonds that can be successfully used in diamond abrasive bodies. 2. Experimental 2.1. Glass preparation The starting materials were of analytical grade: SiO2, Al2O3, ZnO, HBO3, Li2CO3 and NH4H2PO4. Li2CO3 was chosen as the source of Li2O
70
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
while NH4H2PO4 as the source of nucleation agent P2O5. Sb2O3 was added due to its promising clarifying effect. The detailed proportions of the glasses are given in Table 1. The weighted errors of all raw materials were controlled in the range of ±0.1 mol. A glass batch of homogeneous mixture was prepared by ball mixing and then melted in alumina crucibles at 1400 °C for 3 h in an electric furnace and then quenched in distilled water to form frits which were crushed and wet-milled for 30 h. The mean particle sizes of the glass powders were measured with a particle size analyzer to be about 3.8 μm. The glass powders were mixed with paraffin liquid as a binder and pressed by a laboratory uniaxial hydraulic press at a pressure of 50 MPa to obtain 5 mm × 5 mm × 20 mm bars and 10 mm diameter pellets. The bars and pellets were sintered in an electric furnace in air with 2 h holding at 780 °C. The heating rate for the sintering was maintained at 5 °C/min. After sintering, the samples were cooled to room temperature in the furnace. The bond/diamond composite mixture, which consisted of diamond abrasives (monocrystal, average grain size 70#, Henan Funik Ultrahard Material Co. Ltd, China), temporary binder (wax) and vitrified bond, was dry-pressed into the form of pellets (Ø10 × 5 mm) under 50 MPa of pressure. Then the green samples were sintered at 780 °C for 2 h in an electric furnace in air. After sintering, the samples were cooled to room temperature in the furnace. 2.2. Testing and characterization 2.2.1. Infrared spectra (IR) The IR absorption spectra of glasses were carried out on the Thermo scientific Nicolet 6700 FT-IR spectrophotometer in the range of 400–4000 cm−1 with a resolution of 2 cm−1 using the KBr pellet technique. The samples were investigated as fine particles which were mixed with pulverized KBr in the ratio of 1:50 mg glass powder to KBr, respectively. 2.2.2. Differential scanning calorimeter (DSC) In order to optimize the crystallization procedure, the differential scanning calorimeter (DSC, Netzsch 404 PC, Germany) was employed to determine the glass transition temperature (Tg) and crystallization temperature (Tp). The measurements were conducted with finely glass powders in Pt crucible with Al2O3 as the reference material in the temperature from ambient temperature to 1000 °C at the heating rates of 5, 10, 20 and 30 °C/min. The Tg and Tp were determined by the slope intercept method with an accuracy of ±3 K. 2.2.3. X-ray diffraction (XRD) The sintered samples were pulverized into powder in an agate mortar for XRD analysis. The crystalline phases of the sintered samples were analyzed by the X-ray diffractometer (XRD, D/max 2500 model, Rigaku, Japan) which used Cu–Ka radiation (λ = 1.54178 Å), at 40 kV and 50 mA, with 2θ = 10–80° and 8°/min. 2.2.4. Scanning electron microscopy (SEM) Microstructure of the etched surface (by immersion in vol. 4% HF solution for 70–80 s) of sintered vitrified bond and the combination state between vitrified bond and diamond grains were observed with a FEI Quanta 200 scanning electron microscopy. Etched glass ceramic samples were coated with a thin layer of gold.
2.2.5. Thermal expansion coefficient and bending strength Thermal expansion coefficient (TEC) of sintered samples (about 5 mm × 5 mm × 20 mm) was measured in a dilatometer (Netzsch, DIL402PC, Germany) in air at a heating and cooling rate of 5°/min and the thermal expansion coefficient was calculated between 50 and 400 °C. The TEC reported here are accurate to ±2%. Bending strength was measured using a 3-point bending strength with a span of 15 mm at a crosshead speed of 0.5 mm/min. The average value was obtained from measurement of five samples. The reproducibility of the bending strength values was within ±4 MPa. 2.2.6. Chemical durability The chemical durability of sintered samples was determined by the dissolution rates from the calculated weight loss of the samples immersed in distilled water. For these measurements, the samples were cut from the sintered bars using a diamond saw to have dimensions of approximately 1 cm × 1 cm × 1 cm. The samples were polished using 600 grit SiC papers, cleaned with acetone and distilled water, dried and weighted (±0.01 mg) before suspending them in 100 ml distilled water containing polyethylene bottles using plastic threads. The bottles were placed in an oven at 90 °C. The samples were removed, rinsed with distilled water, dried, and weighted at the end of 5, 10, 15 and 20 days. The dissolution rate (DR) was calculated from the measured weight loss (Δw) using the equation DR = Δw (g)/[A (cm2) × t (min)] where A is the surface area of the sample and t is the immersion time. The weight loss per unit surface area measurement was precise to ±0.05 mg/cm2 as determined by combination of the measurement errors. 3. Results and discussions 3.1. FTIR analysis of vitrified bond FTIR spectrum of the glasses with various B2O3 contents is shown in Fig. 1. As shown in Fig. 1, the band located at 462 cm−1 is assigned to Si–O–Si bending vibration mode [20,21]. The intensity of this band increases with the amount of B2O3 content. The bending vibration of B–O–B (in the [BO3] triangles) located near 700 cm−1 gradually increased with the increase of B2O3 content, indicating an increasing amount of [BO3]. The bands at about 775 cm−1 and 801 cm−1 belonging to the six-membered borate rings and six-membered boroxol rings with two or one [BO4] tetrahedral units observed when the B2O3 content above 5 mol% [22,23]. The broad peak near 988 cm−1 in vitrified bond B0 is assigned to Si–O–Si asymmetric stretching vibration [24,25]. However, the peaks in vitrified bonds with B2O3 in the region from 1011 cm−1 to 1030 cm−1 represented the overlapping bands of the antisymmetric stretching vibration of [BO4] and the antisymmetric stretching vibration of Si–O–Si. This peak gradually shifted towards a higher wave number as a result of increased [SiO4] network integrity, as the B–O bond affected on the Si–O–Si, and the [SiO4] increased with increasing amounts of B2O3. The band at about 1085 cm−1 in vitrified bond systems with B2O3 corresponds to the asymmetric tensile vibration of Si-O-Si, and also the tensile vibration of Si–O–B [26]. The peaks in the range of 1403 cm−1–1426 cm−1 are assigned to the antisymmetric stretching vibration of [BO3] [27,28]. This peak's shifting towards lower wave number with the increase of B2O3 revealed that, to a certain degree, the [BO3] existed in a triangular configuration. However, diborate groups
Table 1 Compositions of LZAS glass-ceramics (mol%). Sample
SiO2
Li2O
ZnO
Al2O3
P2O5
B2O3
B0 B4 B8 B12
52.29 ± 0.01 50.18 ± 0.01 48.06 ± 0.01 45.97 ± 0.01
23.62 ± 0.02 22.67 ± 0.02 21.72 ± 0.02 20.77 ± 0.02
14.55 ± 0.01 13.96 ± 0.01 13.38 ± 0.01 12.79 ± 0.01
8.86 ± 0.01 8.51 ± 0.01 8.16 ± 0.01 7.79 ± 0.01
0.68 ± 0.01 0.68 ± 0.01 0.68 ± 0.01 0.68 ± 0.01
0 4 ± 0.02 8 ± 0.02 12 ± 0.02
Note: The errors were caused by converting the content of oxide into that of raw material, weighting of raw materials, volatilizing of oxides and corroding on crucible surface.
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
71
Table 2 Characteristic temperatures for the samples in the study (Tg: glass transition temperature; T1p and T2p: maximum exothermic peak temperatures). Tg/°C
Samples
B0 B4 B8 B12
549 ± 3 521 ± 3 492 ± 3 463 ± 3
T1p/°C
637 ± 3 617 ± 3 595 ± 3 567 ± 3
T2p/°C
678 ± 3 715 ± 3 745 ± 3 782 ± 3
ΔT = Tp − Tg ΔT = T1p − Tg
ΔT = T2p − Tg
88 ± 6 96 ± 6 103 ± 6 104 ± 6
129 ± 6 194 ± 6 253 ± 6 319 ± 6
The effects of B2O3 on the crystallization kinetics of LZAS glasses were measured by a non-isothermal DSC method using the Arrhenius [32], Kissinger [33,34] and Augis–Bennett [35] equation as follows:
Fig. 1. FTIR spectrums of vitrified bonds with different amounts of B2O3 additives.
and tetraborate groups were generated in the vitrified bonds with increasing amounts of B2O3, and the [BO3] connected directly with the [BO4]. Furthermore, the B–O bond strength in the [BO3] was higher than that of the [BO4], which was thought to lead to weak the B–O bond strength of the [BO3] and make the absorption near the 1400 cm−1 shift towards low wave numbers [29].
3.2. Crystallization kinetics DSC curves of four glasses performed at a heating rate of 10 °C/min are shown in Fig. 2. The summarized results of Fig. 2 are illustrated in Table 2. From these curves, it is visible that the glass transition temperature (Tg) and first crystallization peak temperature (T1p) shifted to lower temperatures with increasing B2O3 content. However, the second crystallization temperature (T2p) increased from 637 °C to 751 °C as B2O3 content increased. First crystallization peak temperature (T1p) corresponded to the crystallization of Li2ZnSiO4 and second crystallization peak temperature (T2p) corresponded to the crystallization of β-quartzss. The decrease in Tg and T1p is due to the reduction of viscosity with the addition of B2O3 which resulted in higher mobilities for different ions and ionic complexes operative in crystallization process of glasses. As for T2p, it increased as the B2O3 content increased, suggesting that the number of bridging bonds in silica-based networks would reduce with the B2O3 addition [30]. ΔT = Tp − Tg is an important parameter to estimate the glass stability [31]. The larger the ΔT is, the more stable the glass will be. It can be seen from Table 2 that the increase of B2O3 leads to an increase of ΔT. This means the introduction of B2O3 to the glass system contributes to increasing glass stability.
E k ¼ v exp − RT ln
T 2p β
! ¼
E E þ ln − ln v RT P R
ð1Þ
ð2Þ
2
n¼
2:5 RT p ΔT E
ð3Þ
wherein, E is the activation energy, kJ/mol; R is the gas constant; v is the frequency factor; a is the DSC heating rate; k is the reaction rate constant, which is related to E and v; n is known as the crystallization index, i.e., Avrami exponent depending on the actual nucleation and growth mechanism, ΔT is the half-height width of the crystallization exothermic peak. According to the Johnson–Mehl–Avrami (JMA) theory, n ≌ 2 means that the surface crystallization dominates the overall crystallization; n ≌ 3 means two-dimensional crystallization, and n ≌ 4 means three-dimensional crystallization for bulk materials [36–38]. Table 3 shows the crystallization maximum peak temperatures (Tp) from DSC curves at different heating rates. The relationship between ln(TP/β) and 1/Tp is constructed (Fig. 3 and 4) to calculate the effective activation energy, is shown in Table 3. As shown in Table 3, the first crystallization peak activation energy of crystal growth E decreased from 314 kJ mol−1 to 247 kJ mol−1. However, the activation energy of the second crystallization peak increased from 341 kJ mol− 1 to 399 kJ mol−1. These results are in agreement with the similar systems of glass-ceramics, which shows that the other crystallization index is appropriate [39,40]. The Avrami exponent (n) values, which are calculated by using Eq. (3), are also given in Table 3. The n values obtained from the first crystallization peak (T1p) of B0, B4, B8 and B12 are 2.84, 3.04, 3.23 and 3.34. On the other hand, the n values for second crystallization peak (T2p) were 2.94, 3.03, 3.14 and 3.24. It is clear that the values of n increased with the increase of B2O3 content. Furthermore, the values of n are close to 3, therefore the fact indicates that it is a case of two dimensional crystal growths for the material concerned. 3.3. Phase analysis and microstructure of glass-ceramics
Fig. 2. DSC plots of glass samples with variation in B2O3 contents at a heating rate of 10 K/min.
Fig. 5 shows the XRD patterns of vitrified bonds sintered at 780 °C for 2 h according to DSC curves. From Fig. 5, it was found that predominant phase in vitrified bond with 0 mol% and 4 mol% B2O3 was β-quartzss (LixAlxSi3 − xO6, JCPDF card no.31-707) and the minor crystalline phase was Li2ZnSiO4 (JCPDF card no. 24–682). In vitrified bond with 8 mol% B2O3, β-quartzss was still as the main phase and additional peaks corresponding to SiO2 (JCPDF card no. 12–708) emerge. Moreover, the peak intensity of the β-quartzss increased gradually with the increase of B2O3 content. This is thought to be due to the [BO4] concentration increased initially with the increasing B2O3 quantity which could reinforce the glass network structure compactness [41]. As the B2O3 content increased to 12 mol%, the content of SiO2 increased at the
72
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
Table 3 Values of activation energy (Kissinger [Ek]) and Avrami exponent. Sample no.
B0
B4
B8
B12
Heating rate (β)(°C/min)
5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30
1st crystallization peak temperature (T1p) (K)
2nd crystallization peak temperature (T2p) (K)
T1p (K)
Activation energy (kJ mol−1)
Avrami exponent (n)
bnN
T2p (K)
Activation energy (kJ mol−1)
Avrami exponent (n)
bnN
620 ± 3 637 ± 3 649 ± 3 658 ± 3 600 ± 3 616 ± 3 628 ± 3 641 ± 3 578 ± 3 592 ± 3 603 ± 3 612 ± 3 550 ± 3 567 ± 3 579 ± 3 592 ± 3
314.27 ± 4
2.71 2.81 2.89 2.93 2.92 3.02 3.09 3.13 3.11 3.21 3.29 3.32 3.22 3.32 3.40 3.44
2.84
663 ± 3 678 ± 3 691 ± 3 703 ± 3 696 ± 3 715 ± 3 727 ± 3 736 ± 3 733 ± 3 745 ± 3 762 ± 3 769 ± 3 766 ± 3 783 ± 3 798 ± 3 808 ± 3
340.87 ± 4
2.81 2.91 2.99 3.03 2.93 3.01 3.08 3.12 3.03 3.13 3.17 3.21 3.12 3.22 3.30 3.34
2.94
285.17 ± 4
310.94 ± 4
246.93 ± 4
3.04
3.23
3.34
382.43 ± 4
390.76 ± 4
399.07 ± 4
3.03
3.14
3.24
expense of β-quartzss. The reason is that when the content of B2O3 increased to some certain values, the capacity of oxygen atoms (from ZnO and Li2O) did not satisfy with the generation of [BO4], yet the [BO3] concentration increased. The [BO3] would immiscible with the [SiO4] and combine with [BO4] in the mixed structures, leading to a phase separation in the glass. Thus, B12 vitrified bond formed a boron-rich glass phase and a silicon-rich glass phase of partially crystallized SiO2. Fig. 6 shows SEM micrographs of the vitrified bonds with different amounts of B2O3 addition sintered at 780 °C for 2 h. In vitrified bond B0, the sphere-shaped crystals of β-quartzss can be seen as the major crystalline phase. The size of the crystals is about 0.1 μm. With more B2O3 addition, as in vitrified bond B4 and B8, a homogeneous dispersion of tiny crystallites with an average size about 0.2 μm appeared. Furthermore, vitrified bond B8 exhibits a denser structure compared to that of vitrified bond B4. With further more B2O3 addition, as in vitrified bond B12, the size of grain increases to about 0.3 μm. However, the crystallinity of the vitrified bond is very low, which is in accordance with the results of XRD. SEM investigations confirm that two-dimensional crystallization is the predominant mechanism in all vitrified bonds.
stress forming at the interface, it is preferable to achieve thermal matching as high as possible between vitrified bond and abrasive grains [42]. The TEC (30–400 °C) of samples sintered at 780 °C for 2 h are shown in Fig. 7. The TEC of vitrified bonds B0, B4, B8 and B12 sintered at 780 °C is 6.91 × 10−6/°C, 5.85 × 10−6/°C, 4.63 × 10−6/°C and 5.64 × 10−6/°C, respectively. From the above result, it is indicated that the TEC of vitrified bonds decreased initially with an increase in B2O3 doping amounts and reached the lowest TEC at a certain level of added B2O3. As the amount of B2O3 increases further, the TEC increased. The result is attributed to the content of low-expanding β-quartzss (TEC is 20 × 10−7/°C, 20–400 °C) phase increased with an increase in the B2O3 content (0–8 mol%). Whereas, overfull B2O3 (12 mol%) will greatly inhibit the formation of β-quartzss and finally decrease the TEC. The TEC of vitrified bond B8 is closer to that of diamond grains (4.5 × 10−6/°C). So we can conclude that the better thermal matching between vitrified bond B8 and diamond grains can prevent the abrasives from shedding during grinding.
3.4. TEC measurement The TEC of vitrified bond has significant influence on the performance of vitrified grinding wheels. To obtain good adhesion and avoid thermal
The bending strength of vitrified bonds is often equated with its resistance to abrasion and this characteristic is of practical interest since it determines the strength of abrasive tools. Bending strength of sintered vitrified bond depends not only on the type of precipitating phases
Fig. 3. Variation of ln(T1p/β) vs. 1/Tp of all the glass specimens for first crystallization peak (T1p) temperature.
Fig. 4. Variation of ln(T2p/β) vs. 1/Tp of all the glass specimens for second crystallization peak (T2P) temperature.
3.5. Bending strength analysis
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
Fig. 5. X-ray diffraction patterns of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
73
Fig. 7. TEC of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
3.6. Chemical durability but also on the emergency or absence of internal porosity as well on the glass structure [43,44]. The bending strength (average of five values) of four vitrified bonds sintered at 780 °C is shown in Fig. 8. As seen in Fig. 8, the bending strength of vitrified bonds increased initially with an increase in B2O3 doping amounts and reached the highest bending strength at a certain level of added B2O3. As the amount of B2O3 additive increases further, the bending strength decreased. Compared to the vitrified bond B0, the bending strength of the vitrified bond B4 increased by 30 MPa. The highest bending strength (198 MPa) was obtained when B2O3 content reached 8 mol%. However, when the added amount of B2O3 reached 12 mol%, the bending strength of the vitrified bonds decreased to 126 MPa. Based on the results of FTIR, the [BO4] concentration increased as well as [BO3] initially with the increase of B2O3 content. With the further increase of B2O3, the capacity of oxygen atoms from ZnO and Li2O did not satisfy with the generation of [BO4], yet the [BO3] concentration increased. The [BO4] would reinforce the glass network structure, promoting strength; whereas, the [BO3] which weakens the glass network structure, reducing strength, apparently increased amounts of B2O3. The vitrified bond with 8 mol% B2O3 addition possesses the high bending strength, which is conducive to improving the grinding efficiency of grinding abrasives as well as the service life.
Chemical durability of vitrified bond is a determining factor influencing the mechanical strength of vitrified diamond composites when aqueous solution is used in the grinding process. Chemical durability of sintered vitrified bonds depends not only on the type of precipitating phases but also on its content as well as on the microstructure [45]. The DR of sintered vitrified bonds, calculated from the measured weight losses after 5, 10, 15, and 20 day immersions at 90 °C is given in Fig. 9. As shown in Fig. 9, the DR of vitrified bond samples decreased initially with the increase of B2O3 doping amounts, and reached the lowest DR at a certain level of added B2O3. As the amount of B2O3 addition increases further, the DR increased. The lowest DR was obtained when the content of B2O3 was 8 mol%. The present results revealed that, the chemical durability of the investigated vitrified bond B8 was the highest among those in other studied samples. The reason is that the fine microstructure of the vitrified bond B8 makes it possible to count on better chemical durability compared with porous ceramics, which have a more extensive surface and greater penetrability to liquids and vapors [46]. The vitrified bond with 8 mol% B2O3 addition possesses the high chemical durability, which can guarantee diamond abrasive tool safety and service life.
3.7. Vitrified diamond composite microstructure analysis by SEM An optimized wettability of the vitrified bonds to the diamond abrasives contributes to improve mechanical properties and combination states in abrasive tools. According to the above analysis, the vitrified
Fig. 6. SEM micrographs of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
Fig. 8. Bending strength of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
74
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
of this with increasing B2O3. The present results show that the B8 vitrified bond will be a promising material for diamond abrasive tools. Acknowledgment The work was financially supported by the National Nature Science Foundation of China (No. 51172286). References
Fig. 9. Dissolution rate of the sintered vitrified bonds in distilled water at 90 °C for 5, 10, 15, and 20 days.
bond B8 exhibited a higher bending strength, a better chemical durability and a more appropriate TEC over the other vitrified bonds. So the wettability of the vitrified bond B8 for diamond abrasives was selected to be studied. The microstructure of the B8 vitrified diamond composite was represented in the SEM micrograph in Fig. 10. The micrograph showed that the diamond grain was wetted and intimately covered by the glass matrix, demonstrating that the vitrified bond B8 exhibited an excellent combination state with the diamond abrasives.
4. Conclusions The crystallization mechanism and properties of Li2O–ZnO–Al2O3– SiO2 vitrified bonds by varying the B2O3 content were investigated for diamond grinding tools. The FTIR observations indicated that the [BO3] in the vitrified bond systems increased with increasing content of B2O3. The DSC results showed that the glass transition temperature and first crystallization temperature (T1p) reduced with increase in the B2O3 content, while the second crystallization temperature (T2p) increased. The crystals formed were largely homogeneous and two dimensional in nature for all the vitrified bonds samples. XRD analysis showed the existence of β-quartzss, and Li2ZnSiO4 crystallized phases in the vitrified bond and the content of β-quartzss increased with increasing B2O3 content (from 0 mol% to 8 mol%). Whereas, as the B2O3 content increases to 12 mol%, the content of SiO2 (generated by the glass phase separation) increased at the expense of β-quartzss. SEM observations indicated that the addition of B2O3 has evident effects on the crystal size and crystallinity of LZAS vitrified bonds. The bending strength and chemical durability first increased and then decreased with the increase of B2O3 content; while the TEC showed the inverse
Fig. 10. SEM micrograph of the B8 vitrified diamond composite.
[1] Y. Isono, T. Tanaka, A study on the toughening of bond bridge of Ni–Cu–Sn alloydevelopment of porous metal bonded diamond wheel, J. Jpn. Soc. Precis. Eng. 64 (1998) 923–992. [2] M. Cook, P. Bossom, Trends and recent developments in the material manufacture and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride, Int. J. Refract. Met. Hard Mater. 18 (2000) 147–152. [3] F. Mata, V. Gaitonde, S. Karnik, J.P. Davim, Influence of cutting conditions on machinability aspects of PEEK, PEEK CF 30 and PEEK GF 30 composites using PCD tools, J. Mater. Process. Technol. 209 (2009) 1980–1987. [4] T. Tanaka, N. Ikawa, N. Ueno, S. Okada, A ceramic aspect of vitrified bond for diamond grinding wheel, Bull. Jpn. Soc. Precis. Eng. 19 (1985) 221–223. [5] S.-F. Huang, H.-L. Tsai, S.-T. Lin, Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix, Mater. Chem. Phys. 84 (2004) 251–258. [6] J. Kopac, P. Krajnik, High-performance grinding—a review, J. Mater. Process. Technol. 175 (2006) 278–284. [7] M. Jackson, Sintering and vitrification heat treatment of cBN grinding wheels, J. Mater. Process. Technol. 191 (2007) 232–234. [8] K.-H. Lin, S.-F. Peng, S.-T. Lin, Sintering parameters and wear performances of vitrified bond diamond grinding wheels, Int. J. Refract. Met. Hard Mater. 25 (2007) 25–31. [9] M. Jackson, N. Barlow, B. Mills, The effect of bond composition on the strength of partially-bonded vitrified ceramic abrasives, J. Mater. Sci. Lett. 13 (1994) 1287–1289. [10] J. Yang, D.-Y. Kim, H.-Y. Kim, Effect of glass composition on the strength of vitreous bonded c-BN grinding wheels, Ceram. Int. 19 (1993) 87–92. [11] M. Jackson, B. Mills, Materials selection applied to vitrified alumina & CBN grinding wheels, J. Mater. Process. Technol. 108 (2000) 114–124. [12] M. Jackson, B. Mills, Microscale wear of vitrified abrasive materials, J. Mater. Sci. 39 (2004) 2131–2143. [13] A. Omar, A. El-Shennawi, A. El-Ghannam, Thermal expansion of Li2O–ZnO–Al2O3– SiO2 glasses and corresponding glass-ceramics, J. Mater. Sci. 26 (1991) 6049–6056. [14] E. Demirkesen, E. Maytalman, Effect of Al2O3 additions on the crystallization behaviour and bending strength of a Li2O–ZnO–SiO2 glass-ceramic, Ceram. Int. 27 (2001) 99–104. [15] E. Demirkesen, G. Göller, Effect of Al2O3 additions on the acid durability of a Li2O– ZnO–SiO2 glass and its glass-ceramic, Ceram. Int. 29 (2003) 463–469. [16] E. Demirkesen, Z. Erkmen, N. Yildiz, Effect of Al2O3 additions on the thermal expansion behavior of a Li2O–ZnO–SiO2 glass‐ceramic, J. Am. Ceram. Soc. 82 (1999) 3619–3621. [17] H.-I. Hsiang, S.-H. Chung, Effects of B2O3 addition on the microstructure and microwave dielectric properties of La4Ba2Ti5O18, J. Alloys Compd. 465 (2008) 356–360. [18] H. Takebe, T. Harada, M. Kuwabara, Effect of B2O3 addition on the thermal properties and density of barium phosphate glasses, J. Non-Cryst. Solids 352 (2006) 709–713. [19] M. Karabulut, B. Yuce, O. Bozdogan, H. Ertap, G. Mammadov, Effect of boron addition on the structure and properties of iron phosphate glasses, J. Non-Cryst. Solids 357 (2011) 1455–1462. [20] F. Branda, F. Arcobello-Varlese, A. Costantini, G. Luciani, Effect of the substitution of M2O3 (M = La, Y, In, Ga, Al) for CaO on the bioactivity of 2.5CaO·2SiO2 glass, Biomaterials 23 (2002) 711–716. [21] M. Wang, J. CHeng, M. Li, F. He, Raman spectra of soda–lime–silicate glass doped with rare earth, Phys. B 406 (2011) 3865–3869. [22] B. Meera, J. Ramakrishna, Raman spectral studies of borate glasses, J. Non-Cryst. Solids 159 (1993) 1–21. [23] W. Konijnendijk, H. Verweij, Structural aspects of vitreous PbO·2B2O3 studied by Raman scattering, J. Am. Ceram. Soc. 59 (1976) 459–461. [24] V. Kumar, A. Arora, O. Pandey, K. Singh, Studies on thermal and structural properties of glasses as sealants for solid oxide fuel cells, Int. J. Hydrog. Energy 33 (2008) 434–438. [25] F.-h. Kan, Optical and Spectroscopic Properties of Glass, Springer-Verlag, 1991. [26] P. Wang, Z.H. Li, J. Li, Y. Zhu, Effect of ZnO on the interfacial bonding between Na2O– B2O3–SiO2 vitrified bond and diamond, Solid State Sci. 11 (2009) 1427–1432. [27] E. Kamitsos, M. Karakassides, G.D. Chryssikos, Vibrational spectra of magnesiumsodium-borate glasses. 2. Raman and mid-infrared investigation of the network structure, J. Phys. Chem. 91 (1987) 1073–1079. [28] G.D. Chryssikos, L. Liu, C. Varsamis, E. Kamitsos, Dielectric and structural investigation of alkali triborate glasses, J. Non-Cryst. Solids 235 (1998) 761–765. [29] S.X.Y. Guangzhong, Infrared absorption spectra on the structure of titanium barium borosilicate glasses [J], J. Hunan Univ. 4 (1998). [30] P. James, Glass ceramics: new compositions and uses, J. Non-Cryst. Solids 181 (1995) 1–15. [31] S. Mahadevan, A. Giridhar, A.K. Singh, Calorimetric measurements on As–Sb–Se glasses, J. Non-Cryst. Solids 88 (1986) 11–34. [32] P. McMillan, Glass-ceramics, Academic, London, 1979.
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75 [33] W.A. Johnson, R.F. Mehl, Reaction kinetics in processes of nucleation and growth, Trans. AIME 135 (1939) 396–415. [34] H. Kissinger, J. Res. Natl. Bur. Stand. 57 (1956) 217 (CAS, Web of Science® Times Cited, 1326). [35] J. Augis, J. Bennett, Calculation of the Avrami parameters for heterogeneous solid state reactions using a modification of the Kissinger method, J. Therm. Anal. Calorim. 13 (1978) 283–292. [36] K. Cheng, Evaluation of crystallization kinetics of glasses by non-isothermal analysis, J. Mater. Sci. 36 (2001) 1043–1048. [37] Y.J. Park, J. Heo, Nucleation and crystallization kinetics of glass derived from incinerator fly ash waste, Ceram. Int. 28 (2002) 669–673. [38] L. Perez-Maqueda, J.M. Criado, J. Malek, Combined kinetic analysis for crystallization kinetics of non-crystalline solids, J. Non-Cryst. Solids 320 (2003) 84–91. [39] A. Hu, M. Li, D. Mao, Growth behavior, morphology and properties of lithium aluminosilicate glass ceramics with different amount of CaO, MgO and TiO2 additive, Ceram. Int. 34 (2008) 1393–1397.
75
[40] X. Guo, H. Yang, C. Han, F. Song, Crystallization and microstructure of Li2O–Al2O3– SiO2 glass containing complex nucleating agent, Thermochim. Acta 444 (2006) 201–205. [41] Q. Fu, M.N. Rahaman, H. Fu, X. Liu, Silicate, borosilicate, and borate bioactive glass scaffolds with controllable degradation rate for bone tissue engineering applications. I. Preparation and in vitro degradation, J. Biomed. Mater. Res. A 95 (2010) 164–171. [42] M. Jackson, B. Mills, Thermal expansion of alumino-alkalisilicate and aluminoborosilicate glasses—comparison of empirical models, J. Mater. Sci. Lett. 16 (1997) 1264–1266. [43] S.J. Schneider, Engineered materials handbook, Ceramics and Glasses, vol. 41991. [44] D. Shan, Z. Li, Y. Zhu, H. Ye, K. Gao, Y. Yu, Influence of TiO2 on the physical properties of low-temperature ceramic vitrified bond and mechanical properties of CBN composites, Ceram. Int. 38 (2012) 4573–4578. [45] P.W. McMillan, Glass-ceramics, 1964. [46] A.I. Berezhnoi, Glass-ceramics and Photo-sitalls, 1970.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Crystallization and properties of B2O3 doped LZAS vitrified bond for diamond grinding tools Xiaolin Hu, Lei Cui, Taoyong Liu, Zhanghong Zheng, Yu Tang, Anxian Lu ⁎ School of Material Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2015 Received in revised form 29 June 2015 Accepted 2 July 2015 Available online 24 July 2015 Keywords: Crystallization; TEC; Vitrified bond; B2O3-doped
a b s t r a c t The effects of B2O3 on the crystallization and properties of Li2O–ZnO–Al2O3–SiO2 (LZAS) vitrified bonds were investigated for diamond grinding tool application. The results show that the [BO3] in the glass structure increased with increasing B2O3 content. The glass transition temperature and the first crystallization peak temperature decreased, while the second crystallization peak temperature increased with increasing B2O3 content. The increase in B2O3 content has a little influence on the value of Avrami exponent (n), keeping two-dimensional crystal growth mechanism. Realized composite with 8 mol% B2O3 of LZAS vitrified bond has a higher crystal volume fraction and a uniform sphere-shaped microstructure. Moreover, vitrified bond B8 exhibits a high bending strength (198 MPa), a good chemical durability (20 days, DR = 3.1 × 10−9 g/cm2 min), an appropriate TEC (4.63 × 10−6/°C) and a good wettability to diamond abrasives, suggesting that it would be a promising material for diamond grinding tool. © 2015 Elsevier B.V. All rights reserved.
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. Due to the excellent properties, diamond abrasive tools have been increasingly applied in machining industries with high-speed, high-efficiency and high-precision in recent years [1–3]. These tools are composed of diamond grains and certain type of bonds, including metal, resin and vitrified bonds [4–6], among which vitrified bond diamond abrasive tools show more outstanding properties than other types, e.g., high elastic modulus, low fracture toughness, good thermal stability and high rigidity, as well as controllable porosity [7]. However, it is difficult to balance the grinding performance, wheel safety and product consistency of the vitrified bond systems presently used in diamond abrasives tools. Therefore, it is necessary to further improve the properties of vitrified bonds to make full use of the outstanding potential of diamond grains and satisfy the demands of modern precision machining industry. As we all know, properties of vitrified bonds depend essentially on the composition and concentration of the residual glassy phase and the microstructure, all of which is controlled by glass composition and applied heat treatments [8,9]. To develop a good grinding tool, it is, therefore, necessary to choose a suitable composition. Yang et al. [10] determined the optimum chemical composition of glass bonds in an Al2O3–B2O3–CaO–Na2O system in order to obtain cubic boron nitride (CBN) grinding wheels with high flexural strength. The results suggested that these bonds should include a small amount of alkaline ⁎ Corresponding author. E-mail address: [email protected] (A. Lu).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.005 0022-3093/© 2015 Elsevier B.V. All rights reserved.
oxides and alkaline earths. Jackson et al. [9,11,12] showed that the selection of chemical composition for grinding wheels was made of both conventional alumina grains and CBN. Composition based on Li2O–ZnO–Al2O3–SiO2 (LZAS) glass-ceramics has aroused considerable interest due to its adjustable thermal expansion coefficient, high hardness, creep resistance, good resistance to mechanical and thermal shock, and excellent chemical durability [13–16]. These properties make LZAS systems appropriate for grinding tool applications. It was found that B2O3 has been applied to various ceramic materials since it can low the sintering temperature, increase thermal stability, decrease TEC as well as adjust the dielectric properties of materials [17–19]. According to our knowledge, there is still no report on the crystallization and properties of B2O3 doped LZAS vitrified bond for diamond grinding tools application. Accordingly, the purpose of this work is to study the effect of different amounts of B2O3 on the crystallization and properties of LZAS vitrified bond. Glasses and glass-ceramics were characterized using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The thermal expansion coefficient (TEC), bending strength and chemical durability of the obtained products were also evaluated. The study aimed also at developing suitable varieties of vitrified bonds that can be successfully used in diamond abrasive bodies. 2. Experimental 2.1. Glass preparation The starting materials were of analytical grade: SiO2, Al2O3, ZnO, HBO3, Li2CO3 and NH4H2PO4. Li2CO3 was chosen as the source of Li2O
70
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
while NH4H2PO4 as the source of nucleation agent P2O5. Sb2O3 was added due to its promising clarifying effect. The detailed proportions of the glasses are given in Table 1. The weighted errors of all raw materials were controlled in the range of ±0.1 mol. A glass batch of homogeneous mixture was prepared by ball mixing and then melted in alumina crucibles at 1400 °C for 3 h in an electric furnace and then quenched in distilled water to form frits which were crushed and wet-milled for 30 h. The mean particle sizes of the glass powders were measured with a particle size analyzer to be about 3.8 μm. The glass powders were mixed with paraffin liquid as a binder and pressed by a laboratory uniaxial hydraulic press at a pressure of 50 MPa to obtain 5 mm × 5 mm × 20 mm bars and 10 mm diameter pellets. The bars and pellets were sintered in an electric furnace in air with 2 h holding at 780 °C. The heating rate for the sintering was maintained at 5 °C/min. After sintering, the samples were cooled to room temperature in the furnace. The bond/diamond composite mixture, which consisted of diamond abrasives (monocrystal, average grain size 70#, Henan Funik Ultrahard Material Co. Ltd, China), temporary binder (wax) and vitrified bond, was dry-pressed into the form of pellets (Ø10 × 5 mm) under 50 MPa of pressure. Then the green samples were sintered at 780 °C for 2 h in an electric furnace in air. After sintering, the samples were cooled to room temperature in the furnace. 2.2. Testing and characterization 2.2.1. Infrared spectra (IR) The IR absorption spectra of glasses were carried out on the Thermo scientific Nicolet 6700 FT-IR spectrophotometer in the range of 400–4000 cm−1 with a resolution of 2 cm−1 using the KBr pellet technique. The samples were investigated as fine particles which were mixed with pulverized KBr in the ratio of 1:50 mg glass powder to KBr, respectively. 2.2.2. Differential scanning calorimeter (DSC) In order to optimize the crystallization procedure, the differential scanning calorimeter (DSC, Netzsch 404 PC, Germany) was employed to determine the glass transition temperature (Tg) and crystallization temperature (Tp). The measurements were conducted with finely glass powders in Pt crucible with Al2O3 as the reference material in the temperature from ambient temperature to 1000 °C at the heating rates of 5, 10, 20 and 30 °C/min. The Tg and Tp were determined by the slope intercept method with an accuracy of ±3 K. 2.2.3. X-ray diffraction (XRD) The sintered samples were pulverized into powder in an agate mortar for XRD analysis. The crystalline phases of the sintered samples were analyzed by the X-ray diffractometer (XRD, D/max 2500 model, Rigaku, Japan) which used Cu–Ka radiation (λ = 1.54178 Å), at 40 kV and 50 mA, with 2θ = 10–80° and 8°/min. 2.2.4. Scanning electron microscopy (SEM) Microstructure of the etched surface (by immersion in vol. 4% HF solution for 70–80 s) of sintered vitrified bond and the combination state between vitrified bond and diamond grains were observed with a FEI Quanta 200 scanning electron microscopy. Etched glass ceramic samples were coated with a thin layer of gold.
2.2.5. Thermal expansion coefficient and bending strength Thermal expansion coefficient (TEC) of sintered samples (about 5 mm × 5 mm × 20 mm) was measured in a dilatometer (Netzsch, DIL402PC, Germany) in air at a heating and cooling rate of 5°/min and the thermal expansion coefficient was calculated between 50 and 400 °C. The TEC reported here are accurate to ±2%. Bending strength was measured using a 3-point bending strength with a span of 15 mm at a crosshead speed of 0.5 mm/min. The average value was obtained from measurement of five samples. The reproducibility of the bending strength values was within ±4 MPa. 2.2.6. Chemical durability The chemical durability of sintered samples was determined by the dissolution rates from the calculated weight loss of the samples immersed in distilled water. For these measurements, the samples were cut from the sintered bars using a diamond saw to have dimensions of approximately 1 cm × 1 cm × 1 cm. The samples were polished using 600 grit SiC papers, cleaned with acetone and distilled water, dried and weighted (±0.01 mg) before suspending them in 100 ml distilled water containing polyethylene bottles using plastic threads. The bottles were placed in an oven at 90 °C. The samples were removed, rinsed with distilled water, dried, and weighted at the end of 5, 10, 15 and 20 days. The dissolution rate (DR) was calculated from the measured weight loss (Δw) using the equation DR = Δw (g)/[A (cm2) × t (min)] where A is the surface area of the sample and t is the immersion time. The weight loss per unit surface area measurement was precise to ±0.05 mg/cm2 as determined by combination of the measurement errors. 3. Results and discussions 3.1. FTIR analysis of vitrified bond FTIR spectrum of the glasses with various B2O3 contents is shown in Fig. 1. As shown in Fig. 1, the band located at 462 cm−1 is assigned to Si–O–Si bending vibration mode [20,21]. The intensity of this band increases with the amount of B2O3 content. The bending vibration of B–O–B (in the [BO3] triangles) located near 700 cm−1 gradually increased with the increase of B2O3 content, indicating an increasing amount of [BO3]. The bands at about 775 cm−1 and 801 cm−1 belonging to the six-membered borate rings and six-membered boroxol rings with two or one [BO4] tetrahedral units observed when the B2O3 content above 5 mol% [22,23]. The broad peak near 988 cm−1 in vitrified bond B0 is assigned to Si–O–Si asymmetric stretching vibration [24,25]. However, the peaks in vitrified bonds with B2O3 in the region from 1011 cm−1 to 1030 cm−1 represented the overlapping bands of the antisymmetric stretching vibration of [BO4] and the antisymmetric stretching vibration of Si–O–Si. This peak gradually shifted towards a higher wave number as a result of increased [SiO4] network integrity, as the B–O bond affected on the Si–O–Si, and the [SiO4] increased with increasing amounts of B2O3. The band at about 1085 cm−1 in vitrified bond systems with B2O3 corresponds to the asymmetric tensile vibration of Si-O-Si, and also the tensile vibration of Si–O–B [26]. The peaks in the range of 1403 cm−1–1426 cm−1 are assigned to the antisymmetric stretching vibration of [BO3] [27,28]. This peak's shifting towards lower wave number with the increase of B2O3 revealed that, to a certain degree, the [BO3] existed in a triangular configuration. However, diborate groups
Table 1 Compositions of LZAS glass-ceramics (mol%). Sample
SiO2
Li2O
ZnO
Al2O3
P2O5
B2O3
B0 B4 B8 B12
52.29 ± 0.01 50.18 ± 0.01 48.06 ± 0.01 45.97 ± 0.01
23.62 ± 0.02 22.67 ± 0.02 21.72 ± 0.02 20.77 ± 0.02
14.55 ± 0.01 13.96 ± 0.01 13.38 ± 0.01 12.79 ± 0.01
8.86 ± 0.01 8.51 ± 0.01 8.16 ± 0.01 7.79 ± 0.01
0.68 ± 0.01 0.68 ± 0.01 0.68 ± 0.01 0.68 ± 0.01
0 4 ± 0.02 8 ± 0.02 12 ± 0.02
Note: The errors were caused by converting the content of oxide into that of raw material, weighting of raw materials, volatilizing of oxides and corroding on crucible surface.
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
71
Table 2 Characteristic temperatures for the samples in the study (Tg: glass transition temperature; T1p and T2p: maximum exothermic peak temperatures). Tg/°C
Samples
B0 B4 B8 B12
549 ± 3 521 ± 3 492 ± 3 463 ± 3
T1p/°C
637 ± 3 617 ± 3 595 ± 3 567 ± 3
T2p/°C
678 ± 3 715 ± 3 745 ± 3 782 ± 3
ΔT = Tp − Tg ΔT = T1p − Tg
ΔT = T2p − Tg
88 ± 6 96 ± 6 103 ± 6 104 ± 6
129 ± 6 194 ± 6 253 ± 6 319 ± 6
The effects of B2O3 on the crystallization kinetics of LZAS glasses were measured by a non-isothermal DSC method using the Arrhenius [32], Kissinger [33,34] and Augis–Bennett [35] equation as follows:
Fig. 1. FTIR spectrums of vitrified bonds with different amounts of B2O3 additives.
and tetraborate groups were generated in the vitrified bonds with increasing amounts of B2O3, and the [BO3] connected directly with the [BO4]. Furthermore, the B–O bond strength in the [BO3] was higher than that of the [BO4], which was thought to lead to weak the B–O bond strength of the [BO3] and make the absorption near the 1400 cm−1 shift towards low wave numbers [29].
3.2. Crystallization kinetics DSC curves of four glasses performed at a heating rate of 10 °C/min are shown in Fig. 2. The summarized results of Fig. 2 are illustrated in Table 2. From these curves, it is visible that the glass transition temperature (Tg) and first crystallization peak temperature (T1p) shifted to lower temperatures with increasing B2O3 content. However, the second crystallization temperature (T2p) increased from 637 °C to 751 °C as B2O3 content increased. First crystallization peak temperature (T1p) corresponded to the crystallization of Li2ZnSiO4 and second crystallization peak temperature (T2p) corresponded to the crystallization of β-quartzss. The decrease in Tg and T1p is due to the reduction of viscosity with the addition of B2O3 which resulted in higher mobilities for different ions and ionic complexes operative in crystallization process of glasses. As for T2p, it increased as the B2O3 content increased, suggesting that the number of bridging bonds in silica-based networks would reduce with the B2O3 addition [30]. ΔT = Tp − Tg is an important parameter to estimate the glass stability [31]. The larger the ΔT is, the more stable the glass will be. It can be seen from Table 2 that the increase of B2O3 leads to an increase of ΔT. This means the introduction of B2O3 to the glass system contributes to increasing glass stability.
E k ¼ v exp − RT ln
T 2p β
! ¼
E E þ ln − ln v RT P R
ð1Þ
ð2Þ
2
n¼
2:5 RT p ΔT E
ð3Þ
wherein, E is the activation energy, kJ/mol; R is the gas constant; v is the frequency factor; a is the DSC heating rate; k is the reaction rate constant, which is related to E and v; n is known as the crystallization index, i.e., Avrami exponent depending on the actual nucleation and growth mechanism, ΔT is the half-height width of the crystallization exothermic peak. According to the Johnson–Mehl–Avrami (JMA) theory, n ≌ 2 means that the surface crystallization dominates the overall crystallization; n ≌ 3 means two-dimensional crystallization, and n ≌ 4 means three-dimensional crystallization for bulk materials [36–38]. Table 3 shows the crystallization maximum peak temperatures (Tp) from DSC curves at different heating rates. The relationship between ln(TP/β) and 1/Tp is constructed (Fig. 3 and 4) to calculate the effective activation energy, is shown in Table 3. As shown in Table 3, the first crystallization peak activation energy of crystal growth E decreased from 314 kJ mol−1 to 247 kJ mol−1. However, the activation energy of the second crystallization peak increased from 341 kJ mol− 1 to 399 kJ mol−1. These results are in agreement with the similar systems of glass-ceramics, which shows that the other crystallization index is appropriate [39,40]. The Avrami exponent (n) values, which are calculated by using Eq. (3), are also given in Table 3. The n values obtained from the first crystallization peak (T1p) of B0, B4, B8 and B12 are 2.84, 3.04, 3.23 and 3.34. On the other hand, the n values for second crystallization peak (T2p) were 2.94, 3.03, 3.14 and 3.24. It is clear that the values of n increased with the increase of B2O3 content. Furthermore, the values of n are close to 3, therefore the fact indicates that it is a case of two dimensional crystal growths for the material concerned. 3.3. Phase analysis and microstructure of glass-ceramics
Fig. 2. DSC plots of glass samples with variation in B2O3 contents at a heating rate of 10 K/min.
Fig. 5 shows the XRD patterns of vitrified bonds sintered at 780 °C for 2 h according to DSC curves. From Fig. 5, it was found that predominant phase in vitrified bond with 0 mol% and 4 mol% B2O3 was β-quartzss (LixAlxSi3 − xO6, JCPDF card no.31-707) and the minor crystalline phase was Li2ZnSiO4 (JCPDF card no. 24–682). In vitrified bond with 8 mol% B2O3, β-quartzss was still as the main phase and additional peaks corresponding to SiO2 (JCPDF card no. 12–708) emerge. Moreover, the peak intensity of the β-quartzss increased gradually with the increase of B2O3 content. This is thought to be due to the [BO4] concentration increased initially with the increasing B2O3 quantity which could reinforce the glass network structure compactness [41]. As the B2O3 content increased to 12 mol%, the content of SiO2 increased at the
72
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
Table 3 Values of activation energy (Kissinger [Ek]) and Avrami exponent. Sample no.
B0
B4
B8
B12
Heating rate (β)(°C/min)
5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30
1st crystallization peak temperature (T1p) (K)
2nd crystallization peak temperature (T2p) (K)
T1p (K)
Activation energy (kJ mol−1)
Avrami exponent (n)
bnN
T2p (K)
Activation energy (kJ mol−1)
Avrami exponent (n)
bnN
620 ± 3 637 ± 3 649 ± 3 658 ± 3 600 ± 3 616 ± 3 628 ± 3 641 ± 3 578 ± 3 592 ± 3 603 ± 3 612 ± 3 550 ± 3 567 ± 3 579 ± 3 592 ± 3
314.27 ± 4
2.71 2.81 2.89 2.93 2.92 3.02 3.09 3.13 3.11 3.21 3.29 3.32 3.22 3.32 3.40 3.44
2.84
663 ± 3 678 ± 3 691 ± 3 703 ± 3 696 ± 3 715 ± 3 727 ± 3 736 ± 3 733 ± 3 745 ± 3 762 ± 3 769 ± 3 766 ± 3 783 ± 3 798 ± 3 808 ± 3
340.87 ± 4
2.81 2.91 2.99 3.03 2.93 3.01 3.08 3.12 3.03 3.13 3.17 3.21 3.12 3.22 3.30 3.34
2.94
285.17 ± 4
310.94 ± 4
246.93 ± 4
3.04
3.23
3.34
382.43 ± 4
390.76 ± 4
399.07 ± 4
3.03
3.14
3.24
expense of β-quartzss. The reason is that when the content of B2O3 increased to some certain values, the capacity of oxygen atoms (from ZnO and Li2O) did not satisfy with the generation of [BO4], yet the [BO3] concentration increased. The [BO3] would immiscible with the [SiO4] and combine with [BO4] in the mixed structures, leading to a phase separation in the glass. Thus, B12 vitrified bond formed a boron-rich glass phase and a silicon-rich glass phase of partially crystallized SiO2. Fig. 6 shows SEM micrographs of the vitrified bonds with different amounts of B2O3 addition sintered at 780 °C for 2 h. In vitrified bond B0, the sphere-shaped crystals of β-quartzss can be seen as the major crystalline phase. The size of the crystals is about 0.1 μm. With more B2O3 addition, as in vitrified bond B4 and B8, a homogeneous dispersion of tiny crystallites with an average size about 0.2 μm appeared. Furthermore, vitrified bond B8 exhibits a denser structure compared to that of vitrified bond B4. With further more B2O3 addition, as in vitrified bond B12, the size of grain increases to about 0.3 μm. However, the crystallinity of the vitrified bond is very low, which is in accordance with the results of XRD. SEM investigations confirm that two-dimensional crystallization is the predominant mechanism in all vitrified bonds.
stress forming at the interface, it is preferable to achieve thermal matching as high as possible between vitrified bond and abrasive grains [42]. The TEC (30–400 °C) of samples sintered at 780 °C for 2 h are shown in Fig. 7. The TEC of vitrified bonds B0, B4, B8 and B12 sintered at 780 °C is 6.91 × 10−6/°C, 5.85 × 10−6/°C, 4.63 × 10−6/°C and 5.64 × 10−6/°C, respectively. From the above result, it is indicated that the TEC of vitrified bonds decreased initially with an increase in B2O3 doping amounts and reached the lowest TEC at a certain level of added B2O3. As the amount of B2O3 increases further, the TEC increased. The result is attributed to the content of low-expanding β-quartzss (TEC is 20 × 10−7/°C, 20–400 °C) phase increased with an increase in the B2O3 content (0–8 mol%). Whereas, overfull B2O3 (12 mol%) will greatly inhibit the formation of β-quartzss and finally decrease the TEC. The TEC of vitrified bond B8 is closer to that of diamond grains (4.5 × 10−6/°C). So we can conclude that the better thermal matching between vitrified bond B8 and diamond grains can prevent the abrasives from shedding during grinding.
3.4. TEC measurement The TEC of vitrified bond has significant influence on the performance of vitrified grinding wheels. To obtain good adhesion and avoid thermal
The bending strength of vitrified bonds is often equated with its resistance to abrasion and this characteristic is of practical interest since it determines the strength of abrasive tools. Bending strength of sintered vitrified bond depends not only on the type of precipitating phases
Fig. 3. Variation of ln(T1p/β) vs. 1/Tp of all the glass specimens for first crystallization peak (T1p) temperature.
Fig. 4. Variation of ln(T2p/β) vs. 1/Tp of all the glass specimens for second crystallization peak (T2P) temperature.
3.5. Bending strength analysis
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
Fig. 5. X-ray diffraction patterns of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
73
Fig. 7. TEC of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
3.6. Chemical durability but also on the emergency or absence of internal porosity as well on the glass structure [43,44]. The bending strength (average of five values) of four vitrified bonds sintered at 780 °C is shown in Fig. 8. As seen in Fig. 8, the bending strength of vitrified bonds increased initially with an increase in B2O3 doping amounts and reached the highest bending strength at a certain level of added B2O3. As the amount of B2O3 additive increases further, the bending strength decreased. Compared to the vitrified bond B0, the bending strength of the vitrified bond B4 increased by 30 MPa. The highest bending strength (198 MPa) was obtained when B2O3 content reached 8 mol%. However, when the added amount of B2O3 reached 12 mol%, the bending strength of the vitrified bonds decreased to 126 MPa. Based on the results of FTIR, the [BO4] concentration increased as well as [BO3] initially with the increase of B2O3 content. With the further increase of B2O3, the capacity of oxygen atoms from ZnO and Li2O did not satisfy with the generation of [BO4], yet the [BO3] concentration increased. The [BO4] would reinforce the glass network structure, promoting strength; whereas, the [BO3] which weakens the glass network structure, reducing strength, apparently increased amounts of B2O3. The vitrified bond with 8 mol% B2O3 addition possesses the high bending strength, which is conducive to improving the grinding efficiency of grinding abrasives as well as the service life.
Chemical durability of vitrified bond is a determining factor influencing the mechanical strength of vitrified diamond composites when aqueous solution is used in the grinding process. Chemical durability of sintered vitrified bonds depends not only on the type of precipitating phases but also on its content as well as on the microstructure [45]. The DR of sintered vitrified bonds, calculated from the measured weight losses after 5, 10, 15, and 20 day immersions at 90 °C is given in Fig. 9. As shown in Fig. 9, the DR of vitrified bond samples decreased initially with the increase of B2O3 doping amounts, and reached the lowest DR at a certain level of added B2O3. As the amount of B2O3 addition increases further, the DR increased. The lowest DR was obtained when the content of B2O3 was 8 mol%. The present results revealed that, the chemical durability of the investigated vitrified bond B8 was the highest among those in other studied samples. The reason is that the fine microstructure of the vitrified bond B8 makes it possible to count on better chemical durability compared with porous ceramics, which have a more extensive surface and greater penetrability to liquids and vapors [46]. The vitrified bond with 8 mol% B2O3 addition possesses the high chemical durability, which can guarantee diamond abrasive tool safety and service life.
3.7. Vitrified diamond composite microstructure analysis by SEM An optimized wettability of the vitrified bonds to the diamond abrasives contributes to improve mechanical properties and combination states in abrasive tools. According to the above analysis, the vitrified
Fig. 6. SEM micrographs of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
Fig. 8. Bending strength of vitrified bonds with various B2O3 contents sintered at 780 °C for 2 h.
74
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75
of this with increasing B2O3. The present results show that the B8 vitrified bond will be a promising material for diamond abrasive tools. Acknowledgment The work was financially supported by the National Nature Science Foundation of China (No. 51172286). References
Fig. 9. Dissolution rate of the sintered vitrified bonds in distilled water at 90 °C for 5, 10, 15, and 20 days.
bond B8 exhibited a higher bending strength, a better chemical durability and a more appropriate TEC over the other vitrified bonds. So the wettability of the vitrified bond B8 for diamond abrasives was selected to be studied. The microstructure of the B8 vitrified diamond composite was represented in the SEM micrograph in Fig. 10. The micrograph showed that the diamond grain was wetted and intimately covered by the glass matrix, demonstrating that the vitrified bond B8 exhibited an excellent combination state with the diamond abrasives.
4. Conclusions The crystallization mechanism and properties of Li2O–ZnO–Al2O3– SiO2 vitrified bonds by varying the B2O3 content were investigated for diamond grinding tools. The FTIR observations indicated that the [BO3] in the vitrified bond systems increased with increasing content of B2O3. The DSC results showed that the glass transition temperature and first crystallization temperature (T1p) reduced with increase in the B2O3 content, while the second crystallization temperature (T2p) increased. The crystals formed were largely homogeneous and two dimensional in nature for all the vitrified bonds samples. XRD analysis showed the existence of β-quartzss, and Li2ZnSiO4 crystallized phases in the vitrified bond and the content of β-quartzss increased with increasing B2O3 content (from 0 mol% to 8 mol%). Whereas, as the B2O3 content increases to 12 mol%, the content of SiO2 (generated by the glass phase separation) increased at the expense of β-quartzss. SEM observations indicated that the addition of B2O3 has evident effects on the crystal size and crystallinity of LZAS vitrified bonds. The bending strength and chemical durability first increased and then decreased with the increase of B2O3 content; while the TEC showed the inverse
Fig. 10. SEM micrograph of the B8 vitrified diamond composite.
[1] Y. Isono, T. Tanaka, A study on the toughening of bond bridge of Ni–Cu–Sn alloydevelopment of porous metal bonded diamond wheel, J. Jpn. Soc. Precis. Eng. 64 (1998) 923–992. [2] M. Cook, P. Bossom, Trends and recent developments in the material manufacture and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride, Int. J. Refract. Met. Hard Mater. 18 (2000) 147–152. [3] F. Mata, V. Gaitonde, S. Karnik, J.P. Davim, Influence of cutting conditions on machinability aspects of PEEK, PEEK CF 30 and PEEK GF 30 composites using PCD tools, J. Mater. Process. Technol. 209 (2009) 1980–1987. [4] T. Tanaka, N. Ikawa, N. Ueno, S. Okada, A ceramic aspect of vitrified bond for diamond grinding wheel, Bull. Jpn. Soc. Precis. Eng. 19 (1985) 221–223. [5] S.-F. Huang, H.-L. Tsai, S.-T. Lin, Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix, Mater. Chem. Phys. 84 (2004) 251–258. [6] J. Kopac, P. Krajnik, High-performance grinding—a review, J. Mater. Process. Technol. 175 (2006) 278–284. [7] M. Jackson, Sintering and vitrification heat treatment of cBN grinding wheels, J. Mater. Process. Technol. 191 (2007) 232–234. [8] K.-H. Lin, S.-F. Peng, S.-T. Lin, Sintering parameters and wear performances of vitrified bond diamond grinding wheels, Int. J. Refract. Met. Hard Mater. 25 (2007) 25–31. [9] M. Jackson, N. Barlow, B. Mills, The effect of bond composition on the strength of partially-bonded vitrified ceramic abrasives, J. Mater. Sci. Lett. 13 (1994) 1287–1289. [10] J. Yang, D.-Y. Kim, H.-Y. Kim, Effect of glass composition on the strength of vitreous bonded c-BN grinding wheels, Ceram. Int. 19 (1993) 87–92. [11] M. Jackson, B. Mills, Materials selection applied to vitrified alumina & CBN grinding wheels, J. Mater. Process. Technol. 108 (2000) 114–124. [12] M. Jackson, B. Mills, Microscale wear of vitrified abrasive materials, J. Mater. Sci. 39 (2004) 2131–2143. [13] A. Omar, A. El-Shennawi, A. El-Ghannam, Thermal expansion of Li2O–ZnO–Al2O3– SiO2 glasses and corresponding glass-ceramics, J. Mater. Sci. 26 (1991) 6049–6056. [14] E. Demirkesen, E. Maytalman, Effect of Al2O3 additions on the crystallization behaviour and bending strength of a Li2O–ZnO–SiO2 glass-ceramic, Ceram. Int. 27 (2001) 99–104. [15] E. Demirkesen, G. Göller, Effect of Al2O3 additions on the acid durability of a Li2O– ZnO–SiO2 glass and its glass-ceramic, Ceram. Int. 29 (2003) 463–469. [16] E. Demirkesen, Z. Erkmen, N. Yildiz, Effect of Al2O3 additions on the thermal expansion behavior of a Li2O–ZnO–SiO2 glass‐ceramic, J. Am. Ceram. Soc. 82 (1999) 3619–3621. [17] H.-I. Hsiang, S.-H. Chung, Effects of B2O3 addition on the microstructure and microwave dielectric properties of La4Ba2Ti5O18, J. Alloys Compd. 465 (2008) 356–360. [18] H. Takebe, T. Harada, M. Kuwabara, Effect of B2O3 addition on the thermal properties and density of barium phosphate glasses, J. Non-Cryst. Solids 352 (2006) 709–713. [19] M. Karabulut, B. Yuce, O. Bozdogan, H. Ertap, G. Mammadov, Effect of boron addition on the structure and properties of iron phosphate glasses, J. Non-Cryst. Solids 357 (2011) 1455–1462. [20] F. Branda, F. Arcobello-Varlese, A. Costantini, G. Luciani, Effect of the substitution of M2O3 (M = La, Y, In, Ga, Al) for CaO on the bioactivity of 2.5CaO·2SiO2 glass, Biomaterials 23 (2002) 711–716. [21] M. Wang, J. CHeng, M. Li, F. He, Raman spectra of soda–lime–silicate glass doped with rare earth, Phys. B 406 (2011) 3865–3869. [22] B. Meera, J. Ramakrishna, Raman spectral studies of borate glasses, J. Non-Cryst. Solids 159 (1993) 1–21. [23] W. Konijnendijk, H. Verweij, Structural aspects of vitreous PbO·2B2O3 studied by Raman scattering, J. Am. Ceram. Soc. 59 (1976) 459–461. [24] V. Kumar, A. Arora, O. Pandey, K. Singh, Studies on thermal and structural properties of glasses as sealants for solid oxide fuel cells, Int. J. Hydrog. Energy 33 (2008) 434–438. [25] F.-h. Kan, Optical and Spectroscopic Properties of Glass, Springer-Verlag, 1991. [26] P. Wang, Z.H. Li, J. Li, Y. Zhu, Effect of ZnO on the interfacial bonding between Na2O– B2O3–SiO2 vitrified bond and diamond, Solid State Sci. 11 (2009) 1427–1432. [27] E. Kamitsos, M. Karakassides, G.D. Chryssikos, Vibrational spectra of magnesiumsodium-borate glasses. 2. Raman and mid-infrared investigation of the network structure, J. Phys. Chem. 91 (1987) 1073–1079. [28] G.D. Chryssikos, L. Liu, C. Varsamis, E. Kamitsos, Dielectric and structural investigation of alkali triborate glasses, J. Non-Cryst. Solids 235 (1998) 761–765. [29] S.X.Y. Guangzhong, Infrared absorption spectra on the structure of titanium barium borosilicate glasses [J], J. Hunan Univ. 4 (1998). [30] P. James, Glass ceramics: new compositions and uses, J. Non-Cryst. Solids 181 (1995) 1–15. [31] S. Mahadevan, A. Giridhar, A.K. Singh, Calorimetric measurements on As–Sb–Se glasses, J. Non-Cryst. Solids 88 (1986) 11–34. [32] P. McMillan, Glass-ceramics, Academic, London, 1979.
X. Hu et al. / Journal of Non-Crystalline Solids 427 (2015) 69–75 [33] W.A. Johnson, R.F. Mehl, Reaction kinetics in processes of nucleation and growth, Trans. AIME 135 (1939) 396–415. [34] H. Kissinger, J. Res. Natl. Bur. Stand. 57 (1956) 217 (CAS, Web of Science® Times Cited, 1326). [35] J. Augis, J. Bennett, Calculation of the Avrami parameters for heterogeneous solid state reactions using a modification of the Kissinger method, J. Therm. Anal. Calorim. 13 (1978) 283–292. [36] K. Cheng, Evaluation of crystallization kinetics of glasses by non-isothermal analysis, J. Mater. Sci. 36 (2001) 1043–1048. [37] Y.J. Park, J. Heo, Nucleation and crystallization kinetics of glass derived from incinerator fly ash waste, Ceram. Int. 28 (2002) 669–673. [38] L. Perez-Maqueda, J.M. Criado, J. Malek, Combined kinetic analysis for crystallization kinetics of non-crystalline solids, J. Non-Cryst. Solids 320 (2003) 84–91. [39] A. Hu, M. Li, D. Mao, Growth behavior, morphology and properties of lithium aluminosilicate glass ceramics with different amount of CaO, MgO and TiO2 additive, Ceram. Int. 34 (2008) 1393–1397.
75
[40] X. Guo, H. Yang, C. Han, F. Song, Crystallization and microstructure of Li2O–Al2O3– SiO2 glass containing complex nucleating agent, Thermochim. Acta 444 (2006) 201–205. [41] Q. Fu, M.N. Rahaman, H. Fu, X. Liu, Silicate, borosilicate, and borate bioactive glass scaffolds with controllable degradation rate for bone tissue engineering applications. I. Preparation and in vitro degradation, J. Biomed. Mater. Res. A 95 (2010) 164–171. [42] M. Jackson, B. Mills, Thermal expansion of alumino-alkalisilicate and aluminoborosilicate glasses—comparison of empirical models, J. Mater. Sci. Lett. 16 (1997) 1264–1266. [43] S.J. Schneider, Engineered materials handbook, Ceramics and Glasses, vol. 41991. [44] D. Shan, Z. Li, Y. Zhu, H. Ye, K. Gao, Y. Yu, Influence of TiO2 on the physical properties of low-temperature ceramic vitrified bond and mechanical properties of CBN composites, Ceram. Int. 38 (2012) 4573–4578. [45] P.W. McMillan, Glass-ceramics, 1964. [46] A.I. Berezhnoi, Glass-ceramics and Photo-sitalls, 1970.