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Effect of hot pressing temperature on microstructure, mechanical properties and grinding performance of vitrified-metal bond diamond wheels
Int. Journal of Refractory Metals and Hard Materials 54 (2016) 289–294
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of hot pressing temperature on microstructure, mechanical properties and grinding performance of vitrified-metal bond diamond wheels Dongdong Song a, Long Wan a,⁎, Xiaopan Liu a, Weida Hu b, Delong Xie a, Junsha Wang a a b
Hunan University, Changsha 410082, China Hunan University of Technology, Zhuzhou 412007, China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Received in revised form 13 August 2015 Accepted 25 August 2015 Available online 1 September 2015 Keywords: Vitrified-metal bond Hot pressing temperature Interface structure Interfacial bending strength TRS Roundness Straightness
a b s t r a c t The self-sharpening vitrified-metal bond diamond wheels added with a 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond were fabricated by hot pressed sintering technique. Using the methods of scan-electroscope, energy spectrum analysis, X-diffraction analysis, XPS analysis, Rockwell hardness test and three-point bending test, the effects of hot pressing temperature on the microstructure, hardness and the transverse rupture strength (TRS) of vitrified-metal bond were investigated. Then the grinding performance of cylinder of the diamond wheels was also studied. The results showed that, when the hot pressing temperature was 850 °C, a thin FeAl2O4 transition layer formed, which enhanced the interfacial bending strength between metal and glass phase, and the TRS of vitrified-metal bond reached the maximum value 826.54 MPa. Comparing with metal bond diamond wheel's, the average value of the roundness and straightness of the 50 cylinders ground by the vitrified-metal diamond wheel reduced from 3.1 μm and 2.5 μm to 2.7 μm and 2.1 μm. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Due to high density and toughness of metal matrix, metal bond diamond grinding wheels have long service life and are widely employed in advanced precision machining of hard, fragile and some metal materials [1–5]. However, the high density and toughness of metal matrix cause poor self-sharpening and frictional heat generated during grinding, which led to poor machining efficiency and quality. So the metal bond diamond wheels need periodic dressing to keep good self-sharpening in grinding process. Two ways can improve its self-sharpening in present. One is increasing the content of brittle phase by changing the constituent of the matrix to decrease its abrasion resistance. The CuSn bond met the requirements of bonds for honing stones by increasing the content of Sn from 20% to 25%, which decreased the hardness of CuSn bond and further decreased its abrasion resistance [3] but the realistic grinding performance wasn't discussed. The other way is introducing the sponge-like structure to the wheels. Alumina bubble particles were used as pore-forming agents in Cu-Sn-Ti bond CBN wheel and decreased grinding forces and specific grinding energy [4] and the porous increased the storage space for chips which could reduce the frictional heat generated during grinding [4–5]. However, this ⁎ Corresponding author at: College of Materials Science and Engineering, Hunan University, Changsha 410082, China. E-mail address: [email protected] (L. Wan).
http://dx.doi.org/10.1016/j.ijrmhm.2015.08.008 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
way substantially decreased the holding force to diamond particles and transverse rupture strength (TRS) of wheels due to the pores in the bond bridge. This method reduced the speed and security of the wheels so a new method needs to be put forward to improve the selfsharpening of the metal bond wheels, meanwhile, the wheels should maintain high holding force to diamond particles and high TRS to meet the high speed and security. Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond has low expansion coefficient (CTE) and good wettability to diamond in high temperature, which can maintain high holding force to diamond particles, so it extensively services in vitrified bond diamond tools [6]. In addition, a thin FeAl2O4 layer formed between Fe and Al2O3 can enhance their interface bonding strength [7], which indicated the maximum high bonding strength between Fe-based bond and Al2O3. Therefore, Febased metal bond diamond wheels added with a small amount brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond [8] will meet the requirements of good self-sharpening and high TRS of wheels in theory. In this paper, the Fe-based metal bond diamond wheels added with a 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond were fabricated by hot pressed sintering technique. The effect of different hot pressing temperatures on the microstructure, interface structure, hardness and the TRS of vitrified bond were discussed. Meanwhile, the grinding performance of the cylinder of the vitrified bond diamond wheels was also studied. It's a new attempt to improve the selfsharpening of metal bond diamond wheels.
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D. Song et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2016) 289–294
Table 1 Chemical compositions of vitrified bond. Composition
SiO2
Al2O3
B2O3
Na2O
Li2O
Mole percentage (mol %)
58–62
5–8
18–21
6–8
3–6
Table 2 Characteristics of vitrified bond. Average particle size
Tg/°C
Tf/°C
CTE/k
Hardness/HRB
TRS/MPa
0.5 μm
670
770
6.14 × 10−6
83
106.75
2. Experimental procedure 2.1. Raw materials Atomized Fe powders (Tianjin FuChen Chemical Reagent Factory, China) and electrolytic Cu, Ni, Sn powders (Guangdong Guanghua Chemical Reagent Factory, China) with the maximum size of 43 μm were used as the metal matrix. Diamond particles (CRIMM superhard materials plant, China) with an average size of 62 μm were chosen. The chemical composition and characteristics of Na2O-B2O3-SiO2Al2O3-Li2O vitrified bond were listed in Tables 1 and 2, respectively. 2.2. Specimen fabrication The metal bond (Fe, 24 wt.% Cu, 14 wt.% Ni and 6 wt.% Sn) and vitrified bond (Fe, 23.28 wt.% Cu, 13.58 wt.% Ni 5.82 wt.% Sn and 3 wt.% Na2O-B2O3-SiO2-Al2O3-Li2O) were hot-pressed in graphite molds for 3 min at 750, 800, 850 and 900 °C under 25 MPa. The sintered specimens were cooled down to room temperature at a cooling rate of 125 °C/min, and then the pressure was relieved. The size of the two rectangular specimens was 40 mm × 11 mm × 5 mm. Then the metal bond diamond specimens (Fe, 24 wt.% Cu, 14 wt.% Ni, 6 wt.% Sn and extra 6 wt.% diamond particles) and vitrified-metal bond diamond specimens (Fe, 23.28 wt.% Cu, 13.58 wt.% Ni 5.82 wt.% Sn, 3 wt.% Na2O-B2O3-SiO2-
Al2O3-Li2O and extra 6 wt.% diamond particles) were sintered with the same process parameters. The size of the two cylindrical wheels was 40 mm × 11 mm × 5 mm. 2.3. Characterization The microstructures and interface structures of specimens were observed by a scanning electron microscope (SEM; FEI QUANTA-200). The element composition of the different color areas was analyzed by the energy disperses spectroscopy (EDS). The crystal structures were identified by an X-ray diffractometer (XRD, SIEMENS D-5000) with a standard CuKα radiation source, 2θ angle from 20° to 80°. The valence analysis of Fe element was observed by X-Ray Photoelectron Spectrometer (XPS, K-Alpha 1063). The transverse rupture strength (TRS) of specimens was observed by a three point bending test machine (DKZ5000, China), according to ASTM B528-12. The TRS can be calculated by the following equation: TRS = 3PL / 2 wt2, where P is the loading force, L is the length of specimen, t is the thickness of the specimen, and w is the width of the specimen. The hardness of the specimen was observed by Rockwell apparatus specimens (HR150DT, China) with a steel ball diameter of 1.588 mm and a load of 100 kg, according to ASTME18-12. The CTEs were measured by a thermal analysis apparatus (NETZSCH-DIL402PC) with a heating rate of 10 °C/min from 25 to 950 °C. The Mo-doped wear resisting cast iron cylinder stators were machining by M2011 grinding machine with a wheel speed of 60 m/s, a workpiece speed of 1500 rpm/min, a feed rate of 0.2–0.4 mm and an axial feed rate of 3 mm/min. The roundness and straightness of the cylinder were measured by RA-120 Pneumatic Roundness Detector. 3. Results and discussion 3.1. Effect of hot pressing temperature on the microstructure and composition of vitrified-metal bond The microstructure and composition of vitrified-metal bond sintered at different temperatures were shown in Fig. 1. According to EDS and XRD analysis (Fig. 2), the white phase was the metal phase due to it
Fig. 1. SEM of vitrified-metal bond hot pressed at different temperatures after polishing: a. 750 °C, b. 800 °C, c. 850 °C, and d. 900 °C.
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Fig. 2. XRD patterns of vitrified-metal bond hot pressed at different temperatures.
Fig. 4. XPS patterns of vitrified-metal sintered at different temperatures.
mainly containing Fe, Cu, Ni and Sn, and the black phase was the glass phase due to it mainly containing Si, Al, Na and O. All figures in Fig. 1 showed that the glass phase uniformly distributed in the metal phase. When the hot pressing temperature was 750 °C, because the hot pressing temperature was lower than the softening point temperature (Tf = 770 °C) of the Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond, the glass phase didn't deform even under pressure and existed as a hard sphere with the size of 0.1–0.6 μm (Fig. 1a). When the hot pressing temperature reached 800 °C which was slightly higher than Tf, the flowability of glass slightly increased and deformed to a spheroid. The macroaxis of the spheroid was perpendicular to the pressure direction (Fig. 1b). When sintered at 850 °C, a higher hot pressing temperature resulted in the lower viscosity, so the glass phase was easy to deform to a rodlike shape with the size of 0.2–1 μm under press and its macroaxis was also perpendicular to pressure direction (Fig. 1c). When sintered at 900 °C, the glass phase further deformed to a lamella (Fig. 1d). XRD patterns of vitrified-metal bond sintered at different temperatures were shown in Fig. 2. The scattering peaks near 26° in all patterns revealed the existence of amorphous glass [9], which came from Na2OB2O3-SiO2-Al2O3-Li2O. When the hot pressing temperatures were 750 °C and 800 °C, strong and sharp diffraction peaks of Fe, (Fe, Ni) solid solution and Cu41Sn11 were identified. When sintered at 850 °C, some weak diffraction peaks of FeAl2O4 appeared. When the hot pressing temperature increased to 900 °C, the intensity of FeAl2O4 diffraction peaks slightly increased, which indicated that the crystalline degree of FeAl2O4 obviously improved [10,12]. Interface structures of vitrified-metal bond sintered at different temperatures were shown in Fig. 3. EDS analysis indicated that the white phase was metal phase, and the black phase was glass phase. When the hot pressing temperature was 800 °C, a clear interface and crack between metal and glass phases were observed in Fig. 3a. The crack indicated poor interface bonding and resulted from large residual stress which owed to large differences in CTEs of metal and glass phases (the CTEs of metal and glass phases were 14.37 × 10−6/k and 6.14 × 10−6/k). When sintered at 850 °C, a 10–20 nm thin transition
layer appeared between metal and glass phases. According EDS and XRD analysis, a thin transition layer was confirmed as FeAl2O4. No crack indicated good interface bonding (Fig. 3b). When sintered at 900 °C, the thickness of transition layer increased to 40–50 nm (Fig. 3c). XPS analysis of Fe element in vitrified-metal bond sintered at different temperatures was shown in Fig. 4. When the hot pressing temperature was 800 °C, a high content of Fe and a few content of Fe3+ existed in vitrified-metal bond. When sintered at 850 °C, except Fe and Fe3+, a small amount of Fe2 + from FeAl2O4 appeared [11]. When sintered at 900 °C, the amount of Fe2+ increased, which indicated that the amount of FeAl2O4 increased. The results of XPS analysis were the same with XRD analysis. The literature [12] considered FeAl2O4 came from the following process: Fe þ 1=2O2 →FeO
ð1Þ
FeO þ Al2 O3 →FeO Al2 O3
ð2Þ
When sintered at 850 °C, the glass phase with low viscosity well wetted the metal phase, which supplied an ideal environment for atomic diffusion. Fe further reacted with free O from glass phase and FeO came out. Then the FeO reacted with Al2O3 in glass phase and FeAl2O4 appeared. When the hot pressing temperature increased to 900 °C, the faster atomic diffusion resulted in more FeAl2O4. 3.2. Effect of hot pressing temperature on the mechanical properties of vitrified-metal bond Hardness and TRS of metal bond and vitrified-metal bond sintered at different temperatures were shown in Fig. 5. Owing to the dispersion strengthening of hard glass phase, the hardness of vitrified-metal bond was slightly higher than metal bond in all specimens. The TRS of vitrified-metal bond was obviously lower than metal bond because of adding the brittle glass phase but it was still higher than in previous
Fig. 3. Interface structure of vitrified-metal hot pressed at different temperatures: a. 800 °C, b. 850 °C, and c. 900 °C.
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Fig. 5. Hardness and TRS of metal bond and vitrified-metal bond sintered at different temperatures: a. hardness, and b. TRS.
research [3–5]. As the hot pressing temperature increases, the hardness and TRS of vitrified-metal bond firstly increased and then came down, and the hardness inappreciably changed. When sintered at 850 °C, TRS of vitrified-metal bond reached the maximum value 826.54 MPa. Fracture surface morphologies of metal bond and vitrified-metal bond sintered at different temperatures were shown in Fig. 6. When the hot pressing temperature was 750 °C and 800 °C, owing
Fig. 7. Roundness and straightness of 50 cylinders ground by metal bond and vitrifiedmetal bond diamond wheels: a. roundness, and b. straightness.
to largely different CTEs between metal and vitrified phases, the cracks and separation between them were observed in Fig. 6a and b, which indicated poor interface bonding between them and led to low TRS. When sintered at 850 °C, the thin transition layer FeAl2O4 appeared and decreased the residual stress between metal and glass phase, which led to good interface bonding between them [7,12]. Crack extension needed more energy to destroy the good interface bonding or spread
Fig. 6. Fracture surface morphologies of metal bond and vitrified-metal bond sintered at different temperatures: a. 750 °C, b. 800 °C, c. 850 °C, and d. 900 °C (vitrified-metal bond); e. 850 °C (metal bond).
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Fig. 8. Roundness and straightness of the fiftieth cylinder ground by metal bond and vitrified-metal bond diamond wheels: a. roundness, and b. straightness (vitrified-metal bond); c. roundness, and d. straightness (metal bond).
in brittle glass phase so the separation between metal and glass phases decreased while the brittle fracture of glass phase increased (Fig. 6c) which led to the increasing of the TRS. The literature [7,12] also insisted that the FeAl2O4 could increase the TRS of Al2O3/Fe composite materials. The granular and lamellar regions appearing on the fracture surface of metal bond indicated that intergranular and transgranular two fracture modes exist (Fig. 6e). When the hot pressing temperature increased to 900 °C, the more brittle FeAl2O4 appeared and increased the volume fraction of brittle phase, which led to the brittle fracture further increasing and the TRS decreasing [13].
3.3. Effect of adding 3 wt.% vitrified bond on the grinding performance of vitrified-metal bond diamond wheels The roundness and straightness of 50 cylinders ground by metal and vitrified-metal bond diamond wheels in one dressing cycle were shown in Fig. 7. The roundness and straightness of the top 20 cylinders ground by metal and vitrified-metal bond diamond wheels were low and almost the same, which indicated good grinding performance. As the numbers of ground cylinders increased, the roundness and straightness of the cylinders ground by two wheels increased. However, the increasing of the roundness and straightness of the cylinders ground by
Fig. 9. SEM of the fiftieth cylinder surface ground by metal bond and vitrified-metal bond diamond wheels: a. metal bond, and b. vitrified-metal bond.
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Fig. 10. SEM of the surface of metal bond and vitrified-metal bond diamond wheels after grinding the fiftieth cylinder: a. metal bond, and b. vitrified-metal bond.
vitrified-metal bond diamond wheels was less than metal bond diamond wheels', which indicated a better grinding performance reached by vitrified-metal bond diamond wheels. Comparing with metal bond diamond wheels', the average values of the roundness and straightness of the 50 cylinders ground by the vitrified-metal diamond wheel were improved from 3.1 μm and 2.5 μm to 2.7 μm and 2.1 μm. The roundness and straightness of the 50th cylinder ground by metal and vitrified-metal bond diamond wheels were shown in Fig. 8. The average values of the roundness and straightness of the 50th cylinder ground by vitrified-metal bond diamond wheels (2.6 μm and 2.1 μm) were less than the 50th cylinder's ground by metal bond diamond wheels (3.8 μm and 3.3 μm). Meanwhile, the deep grinding groove and high upthrow owing to strong stress were clearly observed on the surface of the 50th cylinder ground by metal diamond wheels (Fig. 9a) and the surface ground by vitrified-metal bond diamond wheels was smoother (Fig. 9b). Because of the high abrasive resistance of metal bond, the seriously damaged diamond particles which lost grinding function were hard to separate from the metal, and new diamond particles were hard to come out. The low exposed height of diamond particles could be seen on the surface of metal bond diamond wheels after the 50th cylinder was (Fig. 10a), which led to the nonuniform grinding for the cylinder and poor grinding performance. By adding 3 wt.% brittle Na2O-B2O3SiO2-Al2O3-Li2O vitrified bond, the abrasive resistance of vitrifiedmetal bond decreased, and the self-sharpening of vitrified-metal bond diamond wheels was improved. The new diamond particles easily come out, and the high exposed height of diamond particles could be seen on the surface of vitrified-metal bond diamond wheels (Fig. 10b). The self-sharpening increasing ensured the uniform grinding for the cylinder, which led to a smoother surface, ideal roundness and straightness. 4. Conclusion 1. When the vitrified-metal bond added with 3 wt.% Na2O-B2O3SiO2-Al2O3-Li2O sintered at different temperatures, the glass phase uniformly distributed in the metal phase. As the hot pressing temperature increases, the shape of glass phase changed in the following order: sphere, spheroid, rodlike shape and lamella. When it sintered at 850 °C, a 10–20 nm thin transfer layer FeAl2O4 appeared. When the hot pressing temperature further increased to 900 °C, the thickness of FeAl2O4 increased to 40–50 nm. 2. Owing to the dispersion strengthening of the hard glass phase, the hardness of vitrified-metal bond was slightly higher than metal bond. The TRS of vitrified-metal bond was obviously lower than metal bond, but it was still high. Due to the thin FeAl2O4 transition layer decreasing
the residual stress between metal and glass phases, when vitrifiedmetal bond sintered at 850 °C, the TRS of vitrified-metal bond reached the maximum value 826.54 MPa. When the hot pressing temperature increased to 900 °C, overmuch brittle FeAl2O4 decreased the TRS. 3. Due to the added 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond, the self-sharpening of vitrified-metal bond diamond wheels was improved. Comparing with metal bond diamond wheels, the average values of roundness and straightness of the 50 cylinders ground by vitrified-metal bond diamond wheels were improved to 2.7 μm and 2.1 μm.
Acknowledgments The authors thank National Science Foundation of China (Grant No. 51375157) for the grants that support this research.
References [1] M.T. Lee, C.Y. Chung, S.C. Yen, C. Lu, S. Lin, High thermal conductive diamond/Ag–Ti glass-metals fabricated by low-cost cold pressing and vacuum liquid sintering techniques, Diam. Relat. Mater. 44 (2014) 95–99. [2] L.J.D. Oliveira, G.S. Bobrovnitchii, M. Filgueira, Processing and characterization of impregnated diamond cutting tools using a ferrous metal matrix, Int. J. Refract. Met. Hard Mater. 25 (4) (2007) 328–335. [3] P. Han, F. Xiao, W. Zou, B. Liao, Influence of hot pressing temperature on the microstructure and mechanical properties of 75% Cu–25% Sn alloy, Mater. Des. 53 (2014) 38–42. [4] W.F. Ding, J.H. Xu, Z.Z. Chen, C.Y. Yang, Fabrication and performance of porous metal-bonded CBN grinding wheels using alumina bubble particles as poreforming agents, Int. J. Adv. Manuf. Technol. 67 (2013) 1309–1315. [5] T. Tanaka, New development of metal bonded diamond wheel with pore by the growth of bonding bridge, Jpn Soc. Precis. Eng. 26 (1) (1992) 27–32. [6] P. Wang, Z. Li, Y. Zhu, Effect of CaO on the surface morphology and strength of water soaked Na2O-B2O3-Al2O3-SiO2 vitrified bond, J. Non-Cryst. Solids 354 (2008) 3019–3024. [7] S.V. Pepper, Shear strength of metal–sapphire contacts, J. Appl. Phys. 47 (3) (1976) 801–808. [8] V.V. Krstic, P.S. Nicholson, R.G. Hoagland, Toughening of glasses by metallic particles, J. Am. Ceram. Soc. 64 (9) (1981) 499–504. [9] K. Singh, N. Gupta, O.P. Pandey, Effect of Y2O3 on the crystallization behavior of SiO2MgO-B2O3-Al2O3 glasses, J. Mater. Sci. 42 (15) (2007) 6426–6432. [10] P.R. Kidambi, J.P.E. Cleeton, S.A. Scott, D.S. Dennis, C.D. Bohn, Interaction of iron oxide with alumina in a glass-metal oxygen carrier during the production of hydrogen by chemical looping, Energy Fuel 26 (1) (2012) 603–617. [11] S. Chen, Y. Chen, Y. Tang, B. Luo, Z. Yi, J. Wei, W. Sun, Synthesis and characterization of FeAl nanoparticles by flow-levitation method, J. Cent. South Univ. 20 (2013) 845–850. [12] S. Schicker, T. Erny, D.E. Garcia, R. Janssen, N. Claussen, Microstructure and mechanical properties of Al-assisted sintered Fe/Al2O3 cermets, J. Eur. Ceram. Soc. 19 (13) (1999) 2455–2463. [13] H.J.D. Bruin, A.F. Moodie, C.E. Warble, Ceramic–metal reaction welding, J. Mater. Sci. 7 (1972) 909–918.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of hot pressing temperature on microstructure, mechanical properties and grinding performance of vitrified-metal bond diamond wheels Dongdong Song a, Long Wan a,⁎, Xiaopan Liu a, Weida Hu b, Delong Xie a, Junsha Wang a a b
Hunan University, Changsha 410082, China Hunan University of Technology, Zhuzhou 412007, China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Received in revised form 13 August 2015 Accepted 25 August 2015 Available online 1 September 2015 Keywords: Vitrified-metal bond Hot pressing temperature Interface structure Interfacial bending strength TRS Roundness Straightness
a b s t r a c t The self-sharpening vitrified-metal bond diamond wheels added with a 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond were fabricated by hot pressed sintering technique. Using the methods of scan-electroscope, energy spectrum analysis, X-diffraction analysis, XPS analysis, Rockwell hardness test and three-point bending test, the effects of hot pressing temperature on the microstructure, hardness and the transverse rupture strength (TRS) of vitrified-metal bond were investigated. Then the grinding performance of cylinder of the diamond wheels was also studied. The results showed that, when the hot pressing temperature was 850 °C, a thin FeAl2O4 transition layer formed, which enhanced the interfacial bending strength between metal and glass phase, and the TRS of vitrified-metal bond reached the maximum value 826.54 MPa. Comparing with metal bond diamond wheel's, the average value of the roundness and straightness of the 50 cylinders ground by the vitrified-metal diamond wheel reduced from 3.1 μm and 2.5 μm to 2.7 μm and 2.1 μm. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Due to high density and toughness of metal matrix, metal bond diamond grinding wheels have long service life and are widely employed in advanced precision machining of hard, fragile and some metal materials [1–5]. However, the high density and toughness of metal matrix cause poor self-sharpening and frictional heat generated during grinding, which led to poor machining efficiency and quality. So the metal bond diamond wheels need periodic dressing to keep good self-sharpening in grinding process. Two ways can improve its self-sharpening in present. One is increasing the content of brittle phase by changing the constituent of the matrix to decrease its abrasion resistance. The CuSn bond met the requirements of bonds for honing stones by increasing the content of Sn from 20% to 25%, which decreased the hardness of CuSn bond and further decreased its abrasion resistance [3] but the realistic grinding performance wasn't discussed. The other way is introducing the sponge-like structure to the wheels. Alumina bubble particles were used as pore-forming agents in Cu-Sn-Ti bond CBN wheel and decreased grinding forces and specific grinding energy [4] and the porous increased the storage space for chips which could reduce the frictional heat generated during grinding [4–5]. However, this ⁎ Corresponding author at: College of Materials Science and Engineering, Hunan University, Changsha 410082, China. E-mail address: [email protected] (L. Wan).
http://dx.doi.org/10.1016/j.ijrmhm.2015.08.008 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
way substantially decreased the holding force to diamond particles and transverse rupture strength (TRS) of wheels due to the pores in the bond bridge. This method reduced the speed and security of the wheels so a new method needs to be put forward to improve the selfsharpening of the metal bond wheels, meanwhile, the wheels should maintain high holding force to diamond particles and high TRS to meet the high speed and security. Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond has low expansion coefficient (CTE) and good wettability to diamond in high temperature, which can maintain high holding force to diamond particles, so it extensively services in vitrified bond diamond tools [6]. In addition, a thin FeAl2O4 layer formed between Fe and Al2O3 can enhance their interface bonding strength [7], which indicated the maximum high bonding strength between Fe-based bond and Al2O3. Therefore, Febased metal bond diamond wheels added with a small amount brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond [8] will meet the requirements of good self-sharpening and high TRS of wheels in theory. In this paper, the Fe-based metal bond diamond wheels added with a 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond were fabricated by hot pressed sintering technique. The effect of different hot pressing temperatures on the microstructure, interface structure, hardness and the TRS of vitrified bond were discussed. Meanwhile, the grinding performance of the cylinder of the vitrified bond diamond wheels was also studied. It's a new attempt to improve the selfsharpening of metal bond diamond wheels.
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D. Song et al. / Int. Journal of Refractory Metals and Hard Materials 54 (2016) 289–294
Table 1 Chemical compositions of vitrified bond. Composition
SiO2
Al2O3
B2O3
Na2O
Li2O
Mole percentage (mol %)
58–62
5–8
18–21
6–8
3–6
Table 2 Characteristics of vitrified bond. Average particle size
Tg/°C
Tf/°C
CTE/k
Hardness/HRB
TRS/MPa
0.5 μm
670
770
6.14 × 10−6
83
106.75
2. Experimental procedure 2.1. Raw materials Atomized Fe powders (Tianjin FuChen Chemical Reagent Factory, China) and electrolytic Cu, Ni, Sn powders (Guangdong Guanghua Chemical Reagent Factory, China) with the maximum size of 43 μm were used as the metal matrix. Diamond particles (CRIMM superhard materials plant, China) with an average size of 62 μm were chosen. The chemical composition and characteristics of Na2O-B2O3-SiO2Al2O3-Li2O vitrified bond were listed in Tables 1 and 2, respectively. 2.2. Specimen fabrication The metal bond (Fe, 24 wt.% Cu, 14 wt.% Ni and 6 wt.% Sn) and vitrified bond (Fe, 23.28 wt.% Cu, 13.58 wt.% Ni 5.82 wt.% Sn and 3 wt.% Na2O-B2O3-SiO2-Al2O3-Li2O) were hot-pressed in graphite molds for 3 min at 750, 800, 850 and 900 °C under 25 MPa. The sintered specimens were cooled down to room temperature at a cooling rate of 125 °C/min, and then the pressure was relieved. The size of the two rectangular specimens was 40 mm × 11 mm × 5 mm. Then the metal bond diamond specimens (Fe, 24 wt.% Cu, 14 wt.% Ni, 6 wt.% Sn and extra 6 wt.% diamond particles) and vitrified-metal bond diamond specimens (Fe, 23.28 wt.% Cu, 13.58 wt.% Ni 5.82 wt.% Sn, 3 wt.% Na2O-B2O3-SiO2-
Al2O3-Li2O and extra 6 wt.% diamond particles) were sintered with the same process parameters. The size of the two cylindrical wheels was 40 mm × 11 mm × 5 mm. 2.3. Characterization The microstructures and interface structures of specimens were observed by a scanning electron microscope (SEM; FEI QUANTA-200). The element composition of the different color areas was analyzed by the energy disperses spectroscopy (EDS). The crystal structures were identified by an X-ray diffractometer (XRD, SIEMENS D-5000) with a standard CuKα radiation source, 2θ angle from 20° to 80°. The valence analysis of Fe element was observed by X-Ray Photoelectron Spectrometer (XPS, K-Alpha 1063). The transverse rupture strength (TRS) of specimens was observed by a three point bending test machine (DKZ5000, China), according to ASTM B528-12. The TRS can be calculated by the following equation: TRS = 3PL / 2 wt2, where P is the loading force, L is the length of specimen, t is the thickness of the specimen, and w is the width of the specimen. The hardness of the specimen was observed by Rockwell apparatus specimens (HR150DT, China) with a steel ball diameter of 1.588 mm and a load of 100 kg, according to ASTME18-12. The CTEs were measured by a thermal analysis apparatus (NETZSCH-DIL402PC) with a heating rate of 10 °C/min from 25 to 950 °C. The Mo-doped wear resisting cast iron cylinder stators were machining by M2011 grinding machine with a wheel speed of 60 m/s, a workpiece speed of 1500 rpm/min, a feed rate of 0.2–0.4 mm and an axial feed rate of 3 mm/min. The roundness and straightness of the cylinder were measured by RA-120 Pneumatic Roundness Detector. 3. Results and discussion 3.1. Effect of hot pressing temperature on the microstructure and composition of vitrified-metal bond The microstructure and composition of vitrified-metal bond sintered at different temperatures were shown in Fig. 1. According to EDS and XRD analysis (Fig. 2), the white phase was the metal phase due to it
Fig. 1. SEM of vitrified-metal bond hot pressed at different temperatures after polishing: a. 750 °C, b. 800 °C, c. 850 °C, and d. 900 °C.
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Fig. 2. XRD patterns of vitrified-metal bond hot pressed at different temperatures.
Fig. 4. XPS patterns of vitrified-metal sintered at different temperatures.
mainly containing Fe, Cu, Ni and Sn, and the black phase was the glass phase due to it mainly containing Si, Al, Na and O. All figures in Fig. 1 showed that the glass phase uniformly distributed in the metal phase. When the hot pressing temperature was 750 °C, because the hot pressing temperature was lower than the softening point temperature (Tf = 770 °C) of the Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond, the glass phase didn't deform even under pressure and existed as a hard sphere with the size of 0.1–0.6 μm (Fig. 1a). When the hot pressing temperature reached 800 °C which was slightly higher than Tf, the flowability of glass slightly increased and deformed to a spheroid. The macroaxis of the spheroid was perpendicular to the pressure direction (Fig. 1b). When sintered at 850 °C, a higher hot pressing temperature resulted in the lower viscosity, so the glass phase was easy to deform to a rodlike shape with the size of 0.2–1 μm under press and its macroaxis was also perpendicular to pressure direction (Fig. 1c). When sintered at 900 °C, the glass phase further deformed to a lamella (Fig. 1d). XRD patterns of vitrified-metal bond sintered at different temperatures were shown in Fig. 2. The scattering peaks near 26° in all patterns revealed the existence of amorphous glass [9], which came from Na2OB2O3-SiO2-Al2O3-Li2O. When the hot pressing temperatures were 750 °C and 800 °C, strong and sharp diffraction peaks of Fe, (Fe, Ni) solid solution and Cu41Sn11 were identified. When sintered at 850 °C, some weak diffraction peaks of FeAl2O4 appeared. When the hot pressing temperature increased to 900 °C, the intensity of FeAl2O4 diffraction peaks slightly increased, which indicated that the crystalline degree of FeAl2O4 obviously improved [10,12]. Interface structures of vitrified-metal bond sintered at different temperatures were shown in Fig. 3. EDS analysis indicated that the white phase was metal phase, and the black phase was glass phase. When the hot pressing temperature was 800 °C, a clear interface and crack between metal and glass phases were observed in Fig. 3a. The crack indicated poor interface bonding and resulted from large residual stress which owed to large differences in CTEs of metal and glass phases (the CTEs of metal and glass phases were 14.37 × 10−6/k and 6.14 × 10−6/k). When sintered at 850 °C, a 10–20 nm thin transition
layer appeared between metal and glass phases. According EDS and XRD analysis, a thin transition layer was confirmed as FeAl2O4. No crack indicated good interface bonding (Fig. 3b). When sintered at 900 °C, the thickness of transition layer increased to 40–50 nm (Fig. 3c). XPS analysis of Fe element in vitrified-metal bond sintered at different temperatures was shown in Fig. 4. When the hot pressing temperature was 800 °C, a high content of Fe and a few content of Fe3+ existed in vitrified-metal bond. When sintered at 850 °C, except Fe and Fe3+, a small amount of Fe2 + from FeAl2O4 appeared [11]. When sintered at 900 °C, the amount of Fe2+ increased, which indicated that the amount of FeAl2O4 increased. The results of XPS analysis were the same with XRD analysis. The literature [12] considered FeAl2O4 came from the following process: Fe þ 1=2O2 →FeO
ð1Þ
FeO þ Al2 O3 →FeO Al2 O3
ð2Þ
When sintered at 850 °C, the glass phase with low viscosity well wetted the metal phase, which supplied an ideal environment for atomic diffusion. Fe further reacted with free O from glass phase and FeO came out. Then the FeO reacted with Al2O3 in glass phase and FeAl2O4 appeared. When the hot pressing temperature increased to 900 °C, the faster atomic diffusion resulted in more FeAl2O4. 3.2. Effect of hot pressing temperature on the mechanical properties of vitrified-metal bond Hardness and TRS of metal bond and vitrified-metal bond sintered at different temperatures were shown in Fig. 5. Owing to the dispersion strengthening of hard glass phase, the hardness of vitrified-metal bond was slightly higher than metal bond in all specimens. The TRS of vitrified-metal bond was obviously lower than metal bond because of adding the brittle glass phase but it was still higher than in previous
Fig. 3. Interface structure of vitrified-metal hot pressed at different temperatures: a. 800 °C, b. 850 °C, and c. 900 °C.
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Fig. 5. Hardness and TRS of metal bond and vitrified-metal bond sintered at different temperatures: a. hardness, and b. TRS.
research [3–5]. As the hot pressing temperature increases, the hardness and TRS of vitrified-metal bond firstly increased and then came down, and the hardness inappreciably changed. When sintered at 850 °C, TRS of vitrified-metal bond reached the maximum value 826.54 MPa. Fracture surface morphologies of metal bond and vitrified-metal bond sintered at different temperatures were shown in Fig. 6. When the hot pressing temperature was 750 °C and 800 °C, owing
Fig. 7. Roundness and straightness of 50 cylinders ground by metal bond and vitrifiedmetal bond diamond wheels: a. roundness, and b. straightness.
to largely different CTEs between metal and vitrified phases, the cracks and separation between them were observed in Fig. 6a and b, which indicated poor interface bonding between them and led to low TRS. When sintered at 850 °C, the thin transition layer FeAl2O4 appeared and decreased the residual stress between metal and glass phase, which led to good interface bonding between them [7,12]. Crack extension needed more energy to destroy the good interface bonding or spread
Fig. 6. Fracture surface morphologies of metal bond and vitrified-metal bond sintered at different temperatures: a. 750 °C, b. 800 °C, c. 850 °C, and d. 900 °C (vitrified-metal bond); e. 850 °C (metal bond).
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Fig. 8. Roundness and straightness of the fiftieth cylinder ground by metal bond and vitrified-metal bond diamond wheels: a. roundness, and b. straightness (vitrified-metal bond); c. roundness, and d. straightness (metal bond).
in brittle glass phase so the separation between metal and glass phases decreased while the brittle fracture of glass phase increased (Fig. 6c) which led to the increasing of the TRS. The literature [7,12] also insisted that the FeAl2O4 could increase the TRS of Al2O3/Fe composite materials. The granular and lamellar regions appearing on the fracture surface of metal bond indicated that intergranular and transgranular two fracture modes exist (Fig. 6e). When the hot pressing temperature increased to 900 °C, the more brittle FeAl2O4 appeared and increased the volume fraction of brittle phase, which led to the brittle fracture further increasing and the TRS decreasing [13].
3.3. Effect of adding 3 wt.% vitrified bond on the grinding performance of vitrified-metal bond diamond wheels The roundness and straightness of 50 cylinders ground by metal and vitrified-metal bond diamond wheels in one dressing cycle were shown in Fig. 7. The roundness and straightness of the top 20 cylinders ground by metal and vitrified-metal bond diamond wheels were low and almost the same, which indicated good grinding performance. As the numbers of ground cylinders increased, the roundness and straightness of the cylinders ground by two wheels increased. However, the increasing of the roundness and straightness of the cylinders ground by
Fig. 9. SEM of the fiftieth cylinder surface ground by metal bond and vitrified-metal bond diamond wheels: a. metal bond, and b. vitrified-metal bond.
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Fig. 10. SEM of the surface of metal bond and vitrified-metal bond diamond wheels after grinding the fiftieth cylinder: a. metal bond, and b. vitrified-metal bond.
vitrified-metal bond diamond wheels was less than metal bond diamond wheels', which indicated a better grinding performance reached by vitrified-metal bond diamond wheels. Comparing with metal bond diamond wheels', the average values of the roundness and straightness of the 50 cylinders ground by the vitrified-metal diamond wheel were improved from 3.1 μm and 2.5 μm to 2.7 μm and 2.1 μm. The roundness and straightness of the 50th cylinder ground by metal and vitrified-metal bond diamond wheels were shown in Fig. 8. The average values of the roundness and straightness of the 50th cylinder ground by vitrified-metal bond diamond wheels (2.6 μm and 2.1 μm) were less than the 50th cylinder's ground by metal bond diamond wheels (3.8 μm and 3.3 μm). Meanwhile, the deep grinding groove and high upthrow owing to strong stress were clearly observed on the surface of the 50th cylinder ground by metal diamond wheels (Fig. 9a) and the surface ground by vitrified-metal bond diamond wheels was smoother (Fig. 9b). Because of the high abrasive resistance of metal bond, the seriously damaged diamond particles which lost grinding function were hard to separate from the metal, and new diamond particles were hard to come out. The low exposed height of diamond particles could be seen on the surface of metal bond diamond wheels after the 50th cylinder was (Fig. 10a), which led to the nonuniform grinding for the cylinder and poor grinding performance. By adding 3 wt.% brittle Na2O-B2O3SiO2-Al2O3-Li2O vitrified bond, the abrasive resistance of vitrifiedmetal bond decreased, and the self-sharpening of vitrified-metal bond diamond wheels was improved. The new diamond particles easily come out, and the high exposed height of diamond particles could be seen on the surface of vitrified-metal bond diamond wheels (Fig. 10b). The self-sharpening increasing ensured the uniform grinding for the cylinder, which led to a smoother surface, ideal roundness and straightness. 4. Conclusion 1. When the vitrified-metal bond added with 3 wt.% Na2O-B2O3SiO2-Al2O3-Li2O sintered at different temperatures, the glass phase uniformly distributed in the metal phase. As the hot pressing temperature increases, the shape of glass phase changed in the following order: sphere, spheroid, rodlike shape and lamella. When it sintered at 850 °C, a 10–20 nm thin transfer layer FeAl2O4 appeared. When the hot pressing temperature further increased to 900 °C, the thickness of FeAl2O4 increased to 40–50 nm. 2. Owing to the dispersion strengthening of the hard glass phase, the hardness of vitrified-metal bond was slightly higher than metal bond. The TRS of vitrified-metal bond was obviously lower than metal bond, but it was still high. Due to the thin FeAl2O4 transition layer decreasing
the residual stress between metal and glass phases, when vitrifiedmetal bond sintered at 850 °C, the TRS of vitrified-metal bond reached the maximum value 826.54 MPa. When the hot pressing temperature increased to 900 °C, overmuch brittle FeAl2O4 decreased the TRS. 3. Due to the added 3 wt.% brittle Na2O-B2O3-SiO2-Al2O3-Li2O vitrified bond, the self-sharpening of vitrified-metal bond diamond wheels was improved. Comparing with metal bond diamond wheels, the average values of roundness and straightness of the 50 cylinders ground by vitrified-metal bond diamond wheels were improved to 2.7 μm and 2.1 μm.
Acknowledgments The authors thank National Science Foundation of China (Grant No. 51375157) for the grants that support this research.
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