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Effects of Y2O3 addition on structure and properties of LZAS vitrified bond for CBN grinding tools application
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 9916–9922 www.elsevier.com/locate/ceramint
Effects of Y2O3 addition on structure and properties of LZAS vitrified bond for CBN grinding tools application Lei Cui, Xiaojun Hao, Xiaolin Hu, Anxian Lun School of Material Science and Engineering, Central South University, Changsha 410083, PR China Received 11 December 2014; accepted 13 April 2015 Available online 20 April 2015
Abstract The effects of Y2O3 addition on the structure and properties of Li2O–ZnO–Al2O3–SiO2 (LZAS) vitrified bonds were firstly investigated for CBN grinding tools application. Glasses and glass-ceramics were characterized using differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy and infrared spectroscopy. The thermal expansion coefficient (TEC), microhardness, bending strength and chemical durability of the obtained products were also evaluated. Results showed that Y2O3 acted as the network former in the track of SiO4 tetrahedrals. Introducing Y2O3 in the glasses increased the glass transition temperature and crystallization temperature. The crystallization of the main βquartzss phase increased with increase of Y2O3 content. The morphology of the crystals was dependent on the Y2O3 content. The TEC (5.15 10 6/1C) of vitrified bond containing 1.0 mol% Y2O3 (Y1.0) was very close to the TEC (5.0 10 6/1C) of CBN grains. Moreover, Y1.0 vitrified bond exhibits a high microhardness (5.98 GPa), a high bending strength (202 MPa) and a good chemical durability (20 days, DR ¼2.8 10 9 g/cm2 min), suggesting that it would be a promising material for CBN grinding tool. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Vitrified bond; Structure; CBN; TEC
1. Introduction CBN abrasive tools are a group of high performance ultrahard composite materials, which have high hard hardness, excellent abrasion resistance, and good thermal conductivity. Due to the excellent properties, CBN 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 CBN grains and certain type of bonds, including metal, resin and vitrified bonds [4–6], among which vitrified bond CBN 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 CBN abrasives. Therefore, it is necessary to further improve the properties of n
Corresponding author. Tel.: þ 86 0731 8830351; fax: þ 86 0731 88303513. E-mail address: [email protected] (A. Lu).
http://dx.doi.org/10.1016/j.ceramint.2015.04.069 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
vitrified bonds to make full use of the outstanding potential of diamond grains and satisfy the demands of modern precise machining industry. As we all know, properties of vitrified bonds depend essentially on the composition and concentration of 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 CBN grinding wheels of high flexural strength. The results they have obtained suggest that these bonds should include a small amount of alkaline 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 LZAS glass-ceramics has aroused considerable interest due to its adjustable thermal expansion coefficient, high hardness, creep resistance, good resistance to
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mechanical and thermal shock, and excellent chemical durability [13–16]. These properties make LZAS systems appropriate candidates for grinding tools applications. Yttria serves as a network modifier, which may either decrease or increase the viscosity on the basis of the additions [17]. Zheng et al. [18,19] investigated the viscosity and structure of LAS glass-ceramics by adding a small content of Y2O3. Their results demonstrated that the small Y2O3 contents can lower the viscosity and improve the crystallization of LAS glass-ceramics. To the best of our knowledge, there is still no report on the structure and properties of Y2O3 doped LZAS vitrified bond for CBN grinding tools application. The aim of the present work is to investigate the structure and properties of LZAS vitrified bond by adding various Y2O3 contents in the glass composition, and to develop suitable varieties of vitrified bonds that can be successfully used in CBN abrasive bodies. 2. Experimental procedure
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Nicolet 6700 FT-IR spectrophotometer was used to obtain the spectra in the wave number range between 400 and 4000 cm 1. 2.2.2. Differential scanning calorimeter (DSC) In order to optimize the crystallization procedure, a 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 glass powders in Pt crucible with Al2O3 as the reference material in the temperature from ambient temperature to 1000 1C with a heating rate of 10 1C/min. 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 an X-ray diffractometer ( XRD, D/max 2500 model, Rigaku, Japan) which used Cu-Kα radiation (λ¼ 1.54178 Å), at 40 kV and 50 mA, with 2θ ¼ 10– 801 and 81/min.
2.1. Sample preparation The starting materials were of analytical grade: SiO2, Al2O3, ZnO, Li2CO3, Y2O3, NH4H2PO4 and Sb2O3. Li2CO3 was chosen as the source of Li2O while NH4H2PO4 as the source of nucleation agent P2O5. The detailed proportions of the glasses are given in Table 1. A glass batch of homogeneous mixture was prepared by ball mixing and then melted in alumina crucibles at 1400 1C 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 using 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 heated at 550 1C for 2 h to eliminate the binder and finally sintered at 900 1C for 2 h. The heating rate for the sintering was maintained at 5 1C/min. After sintering, the samples were cooled to room temperature in the furnace. 2.2. Characterization 2.2.1. Infrared spectra (IR) Infrared spectra of the samples were recorded at room temperature using the KBr disc technique. A Thermo scientific
2.2.4. Scanning electron microscopy (SEM) Microstructure of the sintered samples was observed with a FEI Quanta 200 scanning electron microscopy. For this measurement, samples were prepared with standard metallographic techniques followed by chemical etching in a HF solution (4%) for 70–80 s. Etched glass ceramic samples were coated with a thin layer of gold. 2.2.5. Thermal expansion coefficient measurement 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 51/min and the thermal expansion coefficient was calculated between 50 and 400 1C. 2.2.6. Microhardness and bending strength The microhardness of sintered samples was measured by using a Vickers tester (Model MXT-a 1). The specimens were cut by a low speed diamond saw, dried ground and polished carefully to obtain smooth and flat parallel surfaces of glassceramic samples before indentation testing. At least 10 indentation readings were made and measured for each sample. Testing was made using a load of 100 g; loading time was fixed for all the crystalline samples (15 s). Bending strength was measured using a 3-point bending strength with a span of 15 mm at a crosshead speed of 0.5 mm/
Table 1 Compositions of LZAS glass-ceramics (mol%). Sample
SiO2
Li2O
ZnO
Al2O3
P2O5
Sb2O3
Y2O3
Y0 Y0.5 Y1.0 Y1.5
51.28 51.28 51.28 51.28
23.96 23.96 23.96 23.96
14.89 14.89 14.89 14.89
9.11 9.11 9.11 9.11
0.68 0.68 0.68 0.68
0.08 0.08 0.08 0.08
0 0.5 1.0 1.5
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min. The average value was obtained from measurement of five samples. 2.2.7. Chemical durability The chemical durability of sintered samples was determined by measuring 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 (7 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 1C. 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 ¼ Δw (g)/[A(cm2) t (min)] where A is the surface area of the sample and t is the immersion time.
network former as silicon in the glass, so the asymmetric vibrations of Si–O–Al take part in this band [20]. With the increase of Y2O3 content of samples, some variations occurred in the IR spectra like reduction of peaks intensity and minor shift of peaks to the lower wavelengths. These changes could be attributed to structural changes which are associated with variation of intertetrahedral band angles caused by dopants [27,28]. Reduction of peaks intensity caused by increasing content of Y2O3 illustrates existence of more ionic bonds in compare with covalent ones due to creation of non-bridging oxygens (NBOs). By the way, these little changes illustrate the prominent role of Y3 þ ions as network former in the glass. 3.2. DSC analysis of base glasses
The IR spectra are capable of investigating the variation of glass structure with composition changed and can provide information about molecular vibration or rotation associated with a covalent bond [20]. As shown in Fig. 1, IR measurements have been carried out to investigate the structure of the LZAS glass with different Y2O3 contents. The band located at 427–582 cm 1 is assigned to Si–O–Si bending vibration mode [21,22]. The absorption peak near 752 cm 1 is related to Si– O–Si symmetric stretching vibrations [23]. The band located at 895 cm 1 is assigned to Si–O with two nonbridging oxygens per SiO4 tetrahedron stretching vibration [24]. Existence of wide band at high frequency 1013 cm 1 illustrates asymmetric stretching vibrations of the Si–O–Si bonds with threedimensional network structure [25,26]. In view of alumina, it is preferably located in the tetrahedral network and acts as a
Fig. 2 shows the DSC curves of glass samples with various Y2O3 contents. The summarized results of Fig. 2 are illustrated in Table 2. One endothermic peak and three exothermic peaks in the temperature range of 500–850 1C can be seen from the figure. Based on the XRD results (Fig. 3) described below, the first exothermic peak (Tp1) corresponds to the crystallization of β/II–LZS phase. The second crystallization peak (Tp2) is attributed to the formation of β-quartss phase, whereas the third crystallization peak (Tp3) caused by the β/II–LZS phase was transferred into γ0-LZS phase. As can be seen in Table 2, the glass transformation temperature (Tg) and the crystallization temperature (Tp) of the original Y2O3 free glass increase gradually with the Y2O3 addition. Furthermore, crystallization temperature increases slightly when the amount of Y2O3 increases in the composition. In the present study, the crystallization temperature increased with increase in Y2O3 content. The reason may be that there is a greater strength of the cross-links between the Y3 þ cation and oxygen atoms [29]. The glass transition temperature increased with the Y2O3 addition, which may be due to the increase in the Y2O3/Al2O3 ratio of samples which contribute to the creation of more bridging oxygens (BOs) in the glassy matrix [30]. Al3 þ ions can form the tetrahedrally coordinated AlO4 that acts as the network former by increasing BOs and octahedrally coordinated
Fig. 1. IR spectra of LZAS glass doped with different amounts of Y2O3.
Fig. 2. DSC curves of the Y-doped LZAS glass with various Y2O3 contents.
3. Results and discussion 3.1. IR analysis of base glasses
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Table 2 DSC results of the glass samples. Sample Y0 Y0.5 Y1.0 Y1.5
Tg (1C)
Tp1 (1C)
Tp2 (1C)
Tp3 (1C)
515 523 531 540
624 640 665 682
710 760 770 775
755 825 830 834
Note: Tg—glass transition temperature; Tp1, Tp2, Tp3—first, second and third crystallization temperatures, respectively.
Fig. 3. XRD patterns of the Y-doped LZAS vitrified bond with various Y2O3 contents sintered at 900 1C for 2 h.
AlO6 that acts as the network modifier by increasing NBOs. Yttrium is a larger size metal so it can act as network former. The competition between Y3 þ and Al3 þ ions for structural forming will not permit the formation of octahedrally coordinated AlO6 [31]. Therefore, network formation increases with increase of Y2O3/Al2O3 ratio in the samples. Some authors [30] have reported that Y3 þ ion plays the role of network forming in the presence of tetrahedrally coordinated Al3 þ ions despite of network modification of octahedrally coordinated Al3 þ ions. By taking these into consideration, it would be noted that the more Y2O3/Al2O3 ratio it is, the more network forming will produce which affects the property. Network formation and creation of NBOs in the presence of Y2O3 dopant could be demonstrated [31] as the following: 3.3. Phase analysis and microstructure of glass-ceramics XRD patterns of the vitrified bonds sintered at 900 1C for 2 h are shown in Fig. 3. These patterns indicate that the predominant crystalline phase is β-quartzss (Li2Al2Si3O10, JCPDF card no. 25-1183) and the minor crystalline phases are β/II–LZS (Li2ZnSiO4, JCPDF card no. 24-682) and γ0-LZS (Li2ZnSiO4, JCPDF card no.24-677) for all examined samples. With Y2O3 content increasing, the peak intensity of the βquartzss increases gradually. This may be attributed to the network formation and creation of NBOs in the presence of Y2O3.
Fig. 4 shows SEM micrographs of the LZAS glass-ceramic samples with different amounts of Y2O3. Fig. 4(A) shows interconnected dispersion of coarse dendritic crystals, and the length of grains is about 0.4 μm. XRD analysis (Fig. 3) showed that the crystallization phases in this sample were βquartzss, β/II–LZS and γ0-LZS phase. β-quartzss belongs to a hexangular crystal system, and its ideal shape is symmetrical hexagon like snow flake [32]. Hu et al. [33] had found that mon-β-quartzss crystal phase can also form network structure by a number of tiny spherical crystals. β/II–LZS and γ0-LZS crystal possess the structure feature similar to Li3PO4 crystals, and its shape is dendritic. The crystallinity of the spherical crystal β-quartzss is very low and the development of massive thin dendritic crystal β/II–LZS and γ0-LZS resulted in the formation of honeycomb-shaped network structure as seen in Fig. 4(A). With addition of the amount of Y2O3, as in Fig. 4 (B) and (C), the spherical crystals formed by dendritic crystals with the size of 0.1–0.2 μm formed the network structure. The result showed that dendritic crystals could further gather into smaller pellets in the Y0.5 and Y1.0 samples. Furthermore, vitrified bond Y1.0 exhibits a denser structure compared to that of vitrified bond Y0.5. The reason is that the network formation and creation of NBOs in the presence of Y2O3 caused the crystallization and the development of β-quartzss. It can be seen from Fig. 4(D) that Y1.5 vitrified bond represents a coarse microstructure, which results from the development of crystal β-quartzss in its spatial direction. 3.4. Microhardness analysis The microhardness of vitrified bond is often equated with its resistance to abrasion or wear and this characteristic is of practical interest since it may determine the resistivity of a vitrified bond during use and it may also decide the suitabilility of the vitrified bond for diamond abrasive tools applications. The microhardness of sintered vitrified bond depends not only on the type of precipitating phases but also on its content as well as on the emergence or absence of internal porosity [34]. The microhardness of vitrified bonds sintered at 900 1C for 2 h is shown in Fig. 5. As seen in Fig. 5, the microhardness of vitrified bond samples increased initially with increase of Y2O3 doping amounts, and reached the highest microhardness at a certain level of added Y2O3. As the amount of Y2O3 additive increases further, the microhardness decreased. The microhardness of the vitrified bond Y1.0 (5.98 GPa) was much higher than vitrified bond Y0 (4.32 GPa), vitrified bond Y0.5 (5.12 GPa) and vitrified bond
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Fig. 4. SEM photographs of LZAS vitrified bond with various Y2O3 contents sintered at 900 1C for 2 h. (A) Y0, (B) Y0.5, (C) Y1.0 and (D) Y1.5.
Fig. 5. Microhardness of vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
Fig. 6. Bending strength of LZAS vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
Y1.5 (5.61 GPa). The highest microhardness was obtained when the content of Y2O3 was 1.0 mol%. This may be due to the formation of fine microstructure as indicated from the SEM micrograph of the vitrified bond of Y1.0, as compared with that of microstructure formed in the vitrified bond Y0, Y0.5 and Y1.5. The vitrified bond with 1.0 mol% Y2O3 addition possesses the high microhardness value, which is conducive to improving the grinding efficiency of grinding wheels as well as the service life.
tendencies. Compared to the vitrified bond Y0, the bending strength of the vitrified bond Y0.5 increased by approximately 50 MPa. This may be due to the formation of plate-like microstructure as indicated from the SEM micrograph of the vitrified bond of Y0.5, as compared with that of honeycombshaped microstructure formed in the vitrified bond Y0. The highest bending strength (202 MPa) was obtained when Y2O3 content reached 1.0 mol%. This may result from the formation of spherical grains that are well uniformed with a mean diameter of about 0.2 μm, which is a benefit for the high strength of the Y1.0 vitrified bond. With the further increasing of Y2O3, the bending strength of Y1.5 vitrified bond decreased slightly. The reason is that the coarse spherical grains of Y1.5 vitrified bond lower the bending strength.
3.5. Bending strength analysis The bending strength of vitrified bonds sintered at 900 1C for 2 h is shown in Fig. 6. The curve of the bending strength and microhardness of the samples showed similar variation
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3.6. Chemical durability analysis Chemical durability of vitrified bond is a determining factor influencing the mechanical strength of vitrified CBN 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 [35]. The dissolution rates of sintered vitrified bonds, calculated from the measured weight losses after 5, 10, 15, and 20 day immersions at 90 1C, are shown in Fig. 7. As shown in Fig. 7, the dissolution rates of the vitrified bond samples decreased initially with increase of Y2O3 doping amounts, and reached the lowest dissolution rate at a certain level of added Y2O3. As the amount of Y2O3 additive increases further, the dissolution rate increased. The lowest dissolution rate was obtained when the content of Y2O3 was 1.0 mol%. The present results revealed that, the chemical durability of the investigated vitrified bond Y1.0 was the highest among those in other studied samples. The reason is that the fine microstructure of the vitrified bond Y1.0 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 [36]. The vitrified bond with 1.0 mol% Y1.0 addition possesses the high chemical durability, which can guarantee CBN abrasive tools safety and service life.
Fig. 8. TEC of LZAS vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
increased with increase of Y2O3 concentration. The TEC of CBN in the temperature range 300–1000 1C is reported to be about 5.0 10 6/1C, while the average thermal expansion coefficient of vitrified bond Y1.0 in the temperature range 30–400 1C is 5.15 10 6/1C which is very close to the TEC of CBN grains. So we can conclude that the better thermal matching between vitrified bond Y1.0 and CBN grains can achieve optimal strength of the diamond grinding tools.
3.7. TEC measurement
4. Conclusion
The TEC of vitrified bond has important influence on the performance of vitrified grinding wheels. To obtain good adhesion and avoid thermal stress forming at the interface, it is better to achieve thermal matching as high as possible between vitrified bond and abrasive grains [37]. The TEC (30–400 1C) of samples sintered at 900 1C for 2 h is shown in Fig. 8. The TEC of vitrified bonds Y0, Y0.5, Y1.0 and Y1.5 sintered at 900 1C is 7.42 10 6/1C, 6.21 10 6/1C, 5.15 10 6/1C and 6 4.72 10 /1C, respectively. From the above result, it is indicated that the TEC of vitrified bonds can be decreased by adding Y2O3. The reason is that the content of the lowexpanding β-quartzss. (TEC is 20 10 7/1C, 20–400 1C) phase
The effects of Y2O3 addition on the structure and properties of LZAS vitrified bond were firstly investigated for CBN grinding tools application. The infrared spectra observations indicated that Y2O3 plays a role as a network former in the LZAS glass altering their network structure. The DSC results showed that the glass transition temperature and the crystallization temperature of the resulting glass increase with Y2O3 addition. The XRD results have revealed that the crystallinity degree of the major β-quartzss phase increases with increase of Y2O3 concentration. SEM observations indicated that the addition of Y2O3 has the evident effects on the main crystalline phase and the morphology of the crystals. In addition, the thermal expansion coefficient is decreased with the introduction of Y2O3, while the bending strength, microhardness and chemical durability of the resulting samples first increased and then decreased. Realized composite with 1.0 mol% Y2O3 of LZAS vitrified bond exhibited an appropriate TEC (5.15 10 6/1C), high microhardness (5.98 GPa), a high bending strength (202 MPa) and a good chemical durability (20 days, DR ¼ 2.8 10 9 g/cm2 min), suggesting that it would be a promising material for CBN grinding tool.
Acknowledgment Fig. 7. Dissolution rate of the sintered LZAS vitrified bonds in distilled water at 90 1C for 5, 10, 15, and 20 days.
This work has been supported by the Science and Technology Plans of Hunan (No. 2014GK3105).
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Ceramics International 41 (2015) 9916–9922 www.elsevier.com/locate/ceramint
Effects of Y2O3 addition on structure and properties of LZAS vitrified bond for CBN grinding tools application Lei Cui, Xiaojun Hao, Xiaolin Hu, Anxian Lun School of Material Science and Engineering, Central South University, Changsha 410083, PR China Received 11 December 2014; accepted 13 April 2015 Available online 20 April 2015
Abstract The effects of Y2O3 addition on the structure and properties of Li2O–ZnO–Al2O3–SiO2 (LZAS) vitrified bonds were firstly investigated for CBN grinding tools application. Glasses and glass-ceramics were characterized using differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy and infrared spectroscopy. The thermal expansion coefficient (TEC), microhardness, bending strength and chemical durability of the obtained products were also evaluated. Results showed that Y2O3 acted as the network former in the track of SiO4 tetrahedrals. Introducing Y2O3 in the glasses increased the glass transition temperature and crystallization temperature. The crystallization of the main βquartzss phase increased with increase of Y2O3 content. The morphology of the crystals was dependent on the Y2O3 content. The TEC (5.15 10 6/1C) of vitrified bond containing 1.0 mol% Y2O3 (Y1.0) was very close to the TEC (5.0 10 6/1C) of CBN grains. Moreover, Y1.0 vitrified bond exhibits a high microhardness (5.98 GPa), a high bending strength (202 MPa) and a good chemical durability (20 days, DR ¼2.8 10 9 g/cm2 min), suggesting that it would be a promising material for CBN grinding tool. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Vitrified bond; Structure; CBN; TEC
1. Introduction CBN abrasive tools are a group of high performance ultrahard composite materials, which have high hard hardness, excellent abrasion resistance, and good thermal conductivity. Due to the excellent properties, CBN 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 CBN grains and certain type of bonds, including metal, resin and vitrified bonds [4–6], among which vitrified bond CBN 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 CBN abrasives. Therefore, it is necessary to further improve the properties of n
Corresponding author. Tel.: þ 86 0731 8830351; fax: þ 86 0731 88303513. E-mail address: [email protected] (A. Lu).
http://dx.doi.org/10.1016/j.ceramint.2015.04.069 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
vitrified bonds to make full use of the outstanding potential of diamond grains and satisfy the demands of modern precise machining industry. As we all know, properties of vitrified bonds depend essentially on the composition and concentration of 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 CBN grinding wheels of high flexural strength. The results they have obtained suggest that these bonds should include a small amount of alkaline 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 LZAS glass-ceramics has aroused considerable interest due to its adjustable thermal expansion coefficient, high hardness, creep resistance, good resistance to
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mechanical and thermal shock, and excellent chemical durability [13–16]. These properties make LZAS systems appropriate candidates for grinding tools applications. Yttria serves as a network modifier, which may either decrease or increase the viscosity on the basis of the additions [17]. Zheng et al. [18,19] investigated the viscosity and structure of LAS glass-ceramics by adding a small content of Y2O3. Their results demonstrated that the small Y2O3 contents can lower the viscosity and improve the crystallization of LAS glass-ceramics. To the best of our knowledge, there is still no report on the structure and properties of Y2O3 doped LZAS vitrified bond for CBN grinding tools application. The aim of the present work is to investigate the structure and properties of LZAS vitrified bond by adding various Y2O3 contents in the glass composition, and to develop suitable varieties of vitrified bonds that can be successfully used in CBN abrasive bodies. 2. Experimental procedure
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Nicolet 6700 FT-IR spectrophotometer was used to obtain the spectra in the wave number range between 400 and 4000 cm 1. 2.2.2. Differential scanning calorimeter (DSC) In order to optimize the crystallization procedure, a 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 glass powders in Pt crucible with Al2O3 as the reference material in the temperature from ambient temperature to 1000 1C with a heating rate of 10 1C/min. 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 an X-ray diffractometer ( XRD, D/max 2500 model, Rigaku, Japan) which used Cu-Kα radiation (λ¼ 1.54178 Å), at 40 kV and 50 mA, with 2θ ¼ 10– 801 and 81/min.
2.1. Sample preparation The starting materials were of analytical grade: SiO2, Al2O3, ZnO, Li2CO3, Y2O3, NH4H2PO4 and Sb2O3. Li2CO3 was chosen as the source of Li2O while NH4H2PO4 as the source of nucleation agent P2O5. The detailed proportions of the glasses are given in Table 1. A glass batch of homogeneous mixture was prepared by ball mixing and then melted in alumina crucibles at 1400 1C 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 using 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 heated at 550 1C for 2 h to eliminate the binder and finally sintered at 900 1C for 2 h. The heating rate for the sintering was maintained at 5 1C/min. After sintering, the samples were cooled to room temperature in the furnace. 2.2. Characterization 2.2.1. Infrared spectra (IR) Infrared spectra of the samples were recorded at room temperature using the KBr disc technique. A Thermo scientific
2.2.4. Scanning electron microscopy (SEM) Microstructure of the sintered samples was observed with a FEI Quanta 200 scanning electron microscopy. For this measurement, samples were prepared with standard metallographic techniques followed by chemical etching in a HF solution (4%) for 70–80 s. Etched glass ceramic samples were coated with a thin layer of gold. 2.2.5. Thermal expansion coefficient measurement 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 51/min and the thermal expansion coefficient was calculated between 50 and 400 1C. 2.2.6. Microhardness and bending strength The microhardness of sintered samples was measured by using a Vickers tester (Model MXT-a 1). The specimens were cut by a low speed diamond saw, dried ground and polished carefully to obtain smooth and flat parallel surfaces of glassceramic samples before indentation testing. At least 10 indentation readings were made and measured for each sample. Testing was made using a load of 100 g; loading time was fixed for all the crystalline samples (15 s). Bending strength was measured using a 3-point bending strength with a span of 15 mm at a crosshead speed of 0.5 mm/
Table 1 Compositions of LZAS glass-ceramics (mol%). Sample
SiO2
Li2O
ZnO
Al2O3
P2O5
Sb2O3
Y2O3
Y0 Y0.5 Y1.0 Y1.5
51.28 51.28 51.28 51.28
23.96 23.96 23.96 23.96
14.89 14.89 14.89 14.89
9.11 9.11 9.11 9.11
0.68 0.68 0.68 0.68
0.08 0.08 0.08 0.08
0 0.5 1.0 1.5
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min. The average value was obtained from measurement of five samples. 2.2.7. Chemical durability The chemical durability of sintered samples was determined by measuring 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 (7 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 1C. 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 ¼ Δw (g)/[A(cm2) t (min)] where A is the surface area of the sample and t is the immersion time.
network former as silicon in the glass, so the asymmetric vibrations of Si–O–Al take part in this band [20]. With the increase of Y2O3 content of samples, some variations occurred in the IR spectra like reduction of peaks intensity and minor shift of peaks to the lower wavelengths. These changes could be attributed to structural changes which are associated with variation of intertetrahedral band angles caused by dopants [27,28]. Reduction of peaks intensity caused by increasing content of Y2O3 illustrates existence of more ionic bonds in compare with covalent ones due to creation of non-bridging oxygens (NBOs). By the way, these little changes illustrate the prominent role of Y3 þ ions as network former in the glass. 3.2. DSC analysis of base glasses
The IR spectra are capable of investigating the variation of glass structure with composition changed and can provide information about molecular vibration or rotation associated with a covalent bond [20]. As shown in Fig. 1, IR measurements have been carried out to investigate the structure of the LZAS glass with different Y2O3 contents. The band located at 427–582 cm 1 is assigned to Si–O–Si bending vibration mode [21,22]. The absorption peak near 752 cm 1 is related to Si– O–Si symmetric stretching vibrations [23]. The band located at 895 cm 1 is assigned to Si–O with two nonbridging oxygens per SiO4 tetrahedron stretching vibration [24]. Existence of wide band at high frequency 1013 cm 1 illustrates asymmetric stretching vibrations of the Si–O–Si bonds with threedimensional network structure [25,26]. In view of alumina, it is preferably located in the tetrahedral network and acts as a
Fig. 2 shows the DSC curves of glass samples with various Y2O3 contents. The summarized results of Fig. 2 are illustrated in Table 2. One endothermic peak and three exothermic peaks in the temperature range of 500–850 1C can be seen from the figure. Based on the XRD results (Fig. 3) described below, the first exothermic peak (Tp1) corresponds to the crystallization of β/II–LZS phase. The second crystallization peak (Tp2) is attributed to the formation of β-quartss phase, whereas the third crystallization peak (Tp3) caused by the β/II–LZS phase was transferred into γ0-LZS phase. As can be seen in Table 2, the glass transformation temperature (Tg) and the crystallization temperature (Tp) of the original Y2O3 free glass increase gradually with the Y2O3 addition. Furthermore, crystallization temperature increases slightly when the amount of Y2O3 increases in the composition. In the present study, the crystallization temperature increased with increase in Y2O3 content. The reason may be that there is a greater strength of the cross-links between the Y3 þ cation and oxygen atoms [29]. The glass transition temperature increased with the Y2O3 addition, which may be due to the increase in the Y2O3/Al2O3 ratio of samples which contribute to the creation of more bridging oxygens (BOs) in the glassy matrix [30]. Al3 þ ions can form the tetrahedrally coordinated AlO4 that acts as the network former by increasing BOs and octahedrally coordinated
Fig. 1. IR spectra of LZAS glass doped with different amounts of Y2O3.
Fig. 2. DSC curves of the Y-doped LZAS glass with various Y2O3 contents.
3. Results and discussion 3.1. IR analysis of base glasses
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Table 2 DSC results of the glass samples. Sample Y0 Y0.5 Y1.0 Y1.5
Tg (1C)
Tp1 (1C)
Tp2 (1C)
Tp3 (1C)
515 523 531 540
624 640 665 682
710 760 770 775
755 825 830 834
Note: Tg—glass transition temperature; Tp1, Tp2, Tp3—first, second and third crystallization temperatures, respectively.
Fig. 3. XRD patterns of the Y-doped LZAS vitrified bond with various Y2O3 contents sintered at 900 1C for 2 h.
AlO6 that acts as the network modifier by increasing NBOs. Yttrium is a larger size metal so it can act as network former. The competition between Y3 þ and Al3 þ ions for structural forming will not permit the formation of octahedrally coordinated AlO6 [31]. Therefore, network formation increases with increase of Y2O3/Al2O3 ratio in the samples. Some authors [30] have reported that Y3 þ ion plays the role of network forming in the presence of tetrahedrally coordinated Al3 þ ions despite of network modification of octahedrally coordinated Al3 þ ions. By taking these into consideration, it would be noted that the more Y2O3/Al2O3 ratio it is, the more network forming will produce which affects the property. Network formation and creation of NBOs in the presence of Y2O3 dopant could be demonstrated [31] as the following: 3.3. Phase analysis and microstructure of glass-ceramics XRD patterns of the vitrified bonds sintered at 900 1C for 2 h are shown in Fig. 3. These patterns indicate that the predominant crystalline phase is β-quartzss (Li2Al2Si3O10, JCPDF card no. 25-1183) and the minor crystalline phases are β/II–LZS (Li2ZnSiO4, JCPDF card no. 24-682) and γ0-LZS (Li2ZnSiO4, JCPDF card no.24-677) for all examined samples. With Y2O3 content increasing, the peak intensity of the βquartzss increases gradually. This may be attributed to the network formation and creation of NBOs in the presence of Y2O3.
Fig. 4 shows SEM micrographs of the LZAS glass-ceramic samples with different amounts of Y2O3. Fig. 4(A) shows interconnected dispersion of coarse dendritic crystals, and the length of grains is about 0.4 μm. XRD analysis (Fig. 3) showed that the crystallization phases in this sample were βquartzss, β/II–LZS and γ0-LZS phase. β-quartzss belongs to a hexangular crystal system, and its ideal shape is symmetrical hexagon like snow flake [32]. Hu et al. [33] had found that mon-β-quartzss crystal phase can also form network structure by a number of tiny spherical crystals. β/II–LZS and γ0-LZS crystal possess the structure feature similar to Li3PO4 crystals, and its shape is dendritic. The crystallinity of the spherical crystal β-quartzss is very low and the development of massive thin dendritic crystal β/II–LZS and γ0-LZS resulted in the formation of honeycomb-shaped network structure as seen in Fig. 4(A). With addition of the amount of Y2O3, as in Fig. 4 (B) and (C), the spherical crystals formed by dendritic crystals with the size of 0.1–0.2 μm formed the network structure. The result showed that dendritic crystals could further gather into smaller pellets in the Y0.5 and Y1.0 samples. Furthermore, vitrified bond Y1.0 exhibits a denser structure compared to that of vitrified bond Y0.5. The reason is that the network formation and creation of NBOs in the presence of Y2O3 caused the crystallization and the development of β-quartzss. It can be seen from Fig. 4(D) that Y1.5 vitrified bond represents a coarse microstructure, which results from the development of crystal β-quartzss in its spatial direction. 3.4. Microhardness analysis The microhardness of vitrified bond is often equated with its resistance to abrasion or wear and this characteristic is of practical interest since it may determine the resistivity of a vitrified bond during use and it may also decide the suitabilility of the vitrified bond for diamond abrasive tools applications. The microhardness of sintered vitrified bond depends not only on the type of precipitating phases but also on its content as well as on the emergence or absence of internal porosity [34]. The microhardness of vitrified bonds sintered at 900 1C for 2 h is shown in Fig. 5. As seen in Fig. 5, the microhardness of vitrified bond samples increased initially with increase of Y2O3 doping amounts, and reached the highest microhardness at a certain level of added Y2O3. As the amount of Y2O3 additive increases further, the microhardness decreased. The microhardness of the vitrified bond Y1.0 (5.98 GPa) was much higher than vitrified bond Y0 (4.32 GPa), vitrified bond Y0.5 (5.12 GPa) and vitrified bond
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Fig. 4. SEM photographs of LZAS vitrified bond with various Y2O3 contents sintered at 900 1C for 2 h. (A) Y0, (B) Y0.5, (C) Y1.0 and (D) Y1.5.
Fig. 5. Microhardness of vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
Fig. 6. Bending strength of LZAS vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
Y1.5 (5.61 GPa). The highest microhardness was obtained when the content of Y2O3 was 1.0 mol%. This may be due to the formation of fine microstructure as indicated from the SEM micrograph of the vitrified bond of Y1.0, as compared with that of microstructure formed in the vitrified bond Y0, Y0.5 and Y1.5. The vitrified bond with 1.0 mol% Y2O3 addition possesses the high microhardness value, which is conducive to improving the grinding efficiency of grinding wheels as well as the service life.
tendencies. Compared to the vitrified bond Y0, the bending strength of the vitrified bond Y0.5 increased by approximately 50 MPa. This may be due to the formation of plate-like microstructure as indicated from the SEM micrograph of the vitrified bond of Y0.5, as compared with that of honeycombshaped microstructure formed in the vitrified bond Y0. The highest bending strength (202 MPa) was obtained when Y2O3 content reached 1.0 mol%. This may result from the formation of spherical grains that are well uniformed with a mean diameter of about 0.2 μm, which is a benefit for the high strength of the Y1.0 vitrified bond. With the further increasing of Y2O3, the bending strength of Y1.5 vitrified bond decreased slightly. The reason is that the coarse spherical grains of Y1.5 vitrified bond lower the bending strength.
3.5. Bending strength analysis The bending strength of vitrified bonds sintered at 900 1C for 2 h is shown in Fig. 6. The curve of the bending strength and microhardness of the samples showed similar variation
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3.6. Chemical durability analysis Chemical durability of vitrified bond is a determining factor influencing the mechanical strength of vitrified CBN 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 [35]. The dissolution rates of sintered vitrified bonds, calculated from the measured weight losses after 5, 10, 15, and 20 day immersions at 90 1C, are shown in Fig. 7. As shown in Fig. 7, the dissolution rates of the vitrified bond samples decreased initially with increase of Y2O3 doping amounts, and reached the lowest dissolution rate at a certain level of added Y2O3. As the amount of Y2O3 additive increases further, the dissolution rate increased. The lowest dissolution rate was obtained when the content of Y2O3 was 1.0 mol%. The present results revealed that, the chemical durability of the investigated vitrified bond Y1.0 was the highest among those in other studied samples. The reason is that the fine microstructure of the vitrified bond Y1.0 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 [36]. The vitrified bond with 1.0 mol% Y1.0 addition possesses the high chemical durability, which can guarantee CBN abrasive tools safety and service life.
Fig. 8. TEC of LZAS vitrified bonds with various Y2O3 contents sintered at 900 1C for 2 h.
increased with increase of Y2O3 concentration. The TEC of CBN in the temperature range 300–1000 1C is reported to be about 5.0 10 6/1C, while the average thermal expansion coefficient of vitrified bond Y1.0 in the temperature range 30–400 1C is 5.15 10 6/1C which is very close to the TEC of CBN grains. So we can conclude that the better thermal matching between vitrified bond Y1.0 and CBN grains can achieve optimal strength of the diamond grinding tools.
3.7. TEC measurement
4. Conclusion
The TEC of vitrified bond has important influence on the performance of vitrified grinding wheels. To obtain good adhesion and avoid thermal stress forming at the interface, it is better to achieve thermal matching as high as possible between vitrified bond and abrasive grains [37]. The TEC (30–400 1C) of samples sintered at 900 1C for 2 h is shown in Fig. 8. The TEC of vitrified bonds Y0, Y0.5, Y1.0 and Y1.5 sintered at 900 1C is 7.42 10 6/1C, 6.21 10 6/1C, 5.15 10 6/1C and 6 4.72 10 /1C, respectively. From the above result, it is indicated that the TEC of vitrified bonds can be decreased by adding Y2O3. The reason is that the content of the lowexpanding β-quartzss. (TEC is 20 10 7/1C, 20–400 1C) phase
The effects of Y2O3 addition on the structure and properties of LZAS vitrified bond were firstly investigated for CBN grinding tools application. The infrared spectra observations indicated that Y2O3 plays a role as a network former in the LZAS glass altering their network structure. The DSC results showed that the glass transition temperature and the crystallization temperature of the resulting glass increase with Y2O3 addition. The XRD results have revealed that the crystallinity degree of the major β-quartzss phase increases with increase of Y2O3 concentration. SEM observations indicated that the addition of Y2O3 has the evident effects on the main crystalline phase and the morphology of the crystals. In addition, the thermal expansion coefficient is decreased with the introduction of Y2O3, while the bending strength, microhardness and chemical durability of the resulting samples first increased and then decreased. Realized composite with 1.0 mol% Y2O3 of LZAS vitrified bond exhibited an appropriate TEC (5.15 10 6/1C), high microhardness (5.98 GPa), a high bending strength (202 MPa) and a good chemical durability (20 days, DR ¼ 2.8 10 9 g/cm2 min), suggesting that it would be a promising material for CBN grinding tool.
Acknowledgment Fig. 7. Dissolution rate of the sintered LZAS vitrified bonds in distilled water at 90 1C for 5, 10, 15, and 20 days.
This work has been supported by the Science and Technology Plans of Hunan (No. 2014GK3105).
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