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Microstructure and performance of diamond abrasive grains brazed in mesh belt furnace with ammonia dissociating atmosphere
Int. Journal of Refractory Metals and Hard Materials 60 (2016) 154–159
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and performance of diamond abrasive grains brazed in mesh belt furnace with ammonia dissociating atmosphere Bao-jun Sun a,b, Bing Xiao a,⁎, Si-xing Liu c a b c
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Mechanical Engineering, Taizhou University, Taizhou, Zhejiang 318000, PR China College of Mechanical Engineering, Yangzhou University, Yangzhou, Jiangsu 225009, PR China
a r t i c l e
i n f o
Article history: Received 10 March 2016 Received in revised form 16 July 2016 Accepted 19 July 2016 Available online 21 July 2016 Keywords: Diamond brazing Mesh belt furnace Ammonia dissociating atmosphere Interfacial microstructure Performance
a b s t r a c t In order to achieve the mass production of the brazing diamond tools, brazing of diamond grains to the matrix using Ni\\Cr active filler alloy was investigated in the mesh belt furnace, which could realize uninterrupted brazing under ammonia dissociating atmosphere. The surface characteristics and interfacial microstructures of brazed diamond in mesh belt furnace were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The residual stress, static compressive strength and impact toughness of the brazing diamond were measured and the corresponding results were compared to the brazing diamond grains in vacuum furnace. Results demonstrated that chemical reaction occurred between the diamond and filler alloy. The compound composed of Cr3C2 and Cr7C3 was formed at diamond interface and a higher bonding strength was obtained. The residual stresses of brazing diamond in mesh belt furnace were lower than that in vacuum furnace in the case of identical protruding height. However the static compressive strength and the impact toughness were higher than that in vacuum furnace. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Diamond with high hardness, wear resistance and chemical stability is widely used to fabricating cutting tools (e.g. diamond grinding wheel, diamond blade saw and diamond drilling bore), which are used to machine the hard and brittle materials, such as cemented carbide, natural stone and concrete, et al. [1]. Nowadays diamond tools are practically manufactured by sintering, electroplating and brazing method. Diamond is difficult to wet because of its chemical inertness, so it is difficult to acquire a strong bonding strength between the diamond and the matrix in sintering condition, even if surface metallization of the diamond and adding active element (e.g. Cr, W, V and Ti) to the substrates are developed [2–6]. Meanwhile, the diamond grains are mechanically embedded by nickel (Ni) in electroplated diamond tools, hence the holding force between the abrasive grains and matrix is not enough as well. The grains of brazed diamond tools are firmly joined to substrates because of strong chemical bonds between the grains and the filler alloys [7–9]. Therefore brazing technique could solve the problem of premature pullout, which is regarded as a major shortcoming of electroplating diamond tools [10]. Moreover, diamonds in single-layer brazed tools can be shallowly buried in the brazing alloy. As a consequence, the protrusion height of diamonds is higher and more space for coolant flow produces, which improves not only the cooling and ⁎ Corresponding author. E-mail address: [email protected] (B. Xiao).
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.016 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
the storage of the chips [11,12], but also the cutting speed and tool service life [13,14]. In recent years, the research upsurge of the brazing diamond was gradually increased due to its excellent cutting property. However, previous studies focused on microstructure formation, interface characterization at the joint of abrasive grains and brazing alloy, or thermal damage on the diamond under the condition of vacuum and induction heating [15–20]. There has been less research on the subject of how to improve the productivity of the brazing diamond tools on the basis of ensuring the quality and in turn realize the mass production of brazing diamond tools. In fact, the process of brazing diamond by vacuum furnace or induction heating has somewhat limitation respectively. The brazing atmosphere can be controlled effectively in vacuum furnace, but it takes a long heating and cooling time. In addition, the volume of vacuum furnace is limited, which results in small batch production. The induction brazing has the advantages of shorter heating time and smaller heat-affected zone, while the brazing temperature is difficult to control. Furthermore, the induction brazing cannot be used to mass production because of its technical feature. Therefore, the mass production of brazing diamond tools cannot be realized through the above methods. In order to solve this problem, this paper intended to explore the brazing process of continuous heating in mesh belt furnace with the ammonia dissociating protective atmosphere. Mesh belt furnace, which has been utilized to brazing of cement carbide and carbon steel extensively [21], can facilitate the large-scale batch production
B. Sun et al. / Int. Journal of Refractory Metals and Hard Materials 60 (2016) 154–159
continuously. However, nowadays there are almost no relevant reports on the application of brazing diamond. In this study, brazing diamond tools were fabricated with Ni\\Cr filler alloys in mesh belt furnace with ammonia dissociating protective atmosphere. Subsequently, the surface characteristics, interfacial microstructures and residual stress of brazing diamond were analyzed. Finally, the static compressive strength and impact toughness for the brazing diamond after chloroazotic acid etching were measured and compared with the diamonds brazed in vacuum furnace. 2. Materials and experimental procedures The commercial Ni\\Cr (78.5 Ni\\12 Cr\\4 Fe\\3 Si\\2.5 B, wt.%) filler alloy powders, diamond and steel substrates composed of 0.45% C were used in the experimental process. Diamond (Huanghe Whirlwind Co. Ltd., China) with sizes ranged from 297 μm to 350 μm were used. The filler alloy powder was made into paste through adding resin glue. The substrate was machined into the trial samples with sizes of 20 mm × 10 mm × 5 mm. The diamond grains were ultrasonically cleaned in acetone to remove the impurities. The paste-shaped filler alloy was covered on the substrate sample evenly with the thickness about 200 μm. Then the diamond grains were arranged on the filler alloys. Thus, the raw materials were assembled into three-layered structure of diamond grits-filler alloy-substrate. The brazing samples were placed in mesh belt furnace with ammonia dissociating atmosphere and heat-treated at 1100 °C for 15 min. The running speed of mesh belt was 82 mm/min, the gas flow of protective atmosphere entering into the heating chamber was 4 m3/h, and the pressure of cooling water was 0.035 MPa. Other samples were put into the vacuum furnace with the vacuum degree of no N10−2 Pa, and heated to 1020 °C for 18 min. Fig. 1 illustrates the schematic of mesh belt furnace, which contains feeding zone, heating zone and cooling zone. The protective atmosphere was used in heating zone includes hydrogen and nitrogen decomposed by liquid ammonia, the equation is: catalyst
2NH3 → N2 þ 3H2
ð1Þ
The dew point could reach −60 °C after the mixed gas of the hydrogen and the nitrogen produced by ammonia decomposition was purified by highly-efficient molecular sieve. The purified gas entered into the heating zone, and excluded the air in the mesh belt furnace. Then the protective atmospheres were ignited, and flame screen came into being at the entrance and the exit of heating zone, which could prevent the outside air from entering the heating chamber and protect the brazing sample against oxidation. Double water jacket structures were made for the cooling zone. When fabricating the diamond, test samples were placed on the mesh belt driven by electric motor and passed through the feeding zone, heating zone and cooling zone successively, thus the brazing processes of the diamond tools were finished. The interfacial microstructure of the diamond and elemental distribution of the joining interface were observed by scan electron microscope (SEM, Hitachi-4800, Tokyo, Japan) coupled with energy-dispersive X-ray spectrometry (EDS). The interfacial resultant of diamonds after
chloroazotic acid etching was detected by X-ray diffraction (XRD, Bruke AXS D8 Advance, Germany). The surface nature of brazing diamond protruding out of the filler alloy was observed and the residual stresses in the diamond after brazing were measured using laser Raman microscopy (Renishaw Invia, Nd-YAG laser type, wave length 532 nm, England). Raman scatter peak reflects the vibration spectra characteristics of materials. When the laser beam of the microscope is focused on the point on the diamond under compressive stress, the Raman-Stokes peak in Raman spectra shifts to a higher wave number compared with the wave number of Raman-Stokes peak under unbrazed and stress-free diamond grits. When the laser beam is focused on the point of diamond grits under tensile stress, the peak shifted to a lower wave number. So the stress state of brazed diamond can be determined by Raman-Stokes peak position [22]. DLY-92 static strength measuring equipment was applied for checking the static strength of diamond grits after chloroazotic acid etching. Forty grits were measured and the mean maximum static load P was calculated. In accordance with the literature [23], the static compressive strength σ (MPa) of the diamonds after chloroazotic acid etching was determined as follows: σ ¼ 1:37P=d
2
ð2Þ
where P was the mean static load of diamond grits after chloroazotic acid etching and d was the mean diameter (mm) of the diamond grits after chloroazotic acid etching. Impact toughness of diamond grits after chloroazotic acid etching was measured using CM-II super-hard abrasive grains impact toughness measuring equipment. The rotating speed of measuring equipment is 2400 r/min. A group of diamond grits weighing 0.4 g was impacted for 2000 times. After impacting, the diamonds were sieved through a 60-mesh screen. The unbroken ratio (TI) of the diamond grains after chloroazotic acid etching was calculated as follows [23]: TI ¼
m1 100% m
ð3Þ
where m1 was the mass of remained diamonds after impacting and sieving (g), and m was the mass of diamonds before impacting (g). The measurement above was repeated three times and a average value of TI was determined as the impact toughness of diamond after etching. 3. Results and discussion 3.1. Surface and interface The surface morphology of diamond grains brazed in mesh belt furnace with ammonia dissolving atmosphere was shown in Fig. 2. It can be seen that the filler alloy climbed well along the diamond surface, which demonstrating good wettability of Ni\\Cr alloy toward diamond at the brazing ambient of ammonia dissociating atmosphere. Each diamond grains had clear edges and smooth surface, it can be inferred that protective atmosphere had no effects on the surface of diamond protruding out of the filler metal. In ammonia dissolving atmosphere, nitrogen was an inert gas, which wound not react with carbon (C). However, hydrogen erosion exists possibly in high temperature, which can be explained as the following reaction may occur: xC þ yH 2 →C x H 2y
Fig. 1. The schematic of mesh belt furnace.
155
ð4Þ
In general, the enthalpy change of chemical reaction is negative (Δ H b 0), so the case for the entropy change (Δ S b 0). The variety of Gibbs free energy for the reaction (ΔG = ΔH − TΔS) has a critical temperature (T0 = | ΔH/ΔS |), which means Δ G = 0 in this temperature.
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Diamond
Filler alloy
(a) Fig. 2. Morphologies of diamonds brazed in mesh belt furnace.
When the brazing temperature is higher than the critical temperature (T N T0), the variety of Gibbs free energy for the chemical reaction is positive (ΔG N 0), the reaction cannot occur spontaneously. Taking the resultant of CH4 for example, the critical temperature was acquired (T0 = 733 °C) by substituted the thermodynamics data into ΔG. And the critical temperatures of other hydrocarbons could be got by the same way. Hydrogen erosion phenomena could not happen because the diamonds brazing in mesh belt furnace were carried out at 1100 °C. The wave number of Raman-Stokes peak of the original and stressfree diamond was determined to be ω0 = 1331.70 cm−1 during the experiment. Fig. 3 was laser Raman spectroscopy of brazed diamond surface protruding out of the filler alloy in mesh belt furnace. As shown in Fig. 3, Raman-Stokes peak for surface of diamond brazed in mesh belt furnace lay in the wave number of 1331.13 cm− 1, which was the characteristic peak of diamond that had been suffered strain stresses. Other characteristic peaks were not detected by laser Raman microscopy. According to the morphology observation, thermodynamic analysis and Raman spectroscopy measurement above, surface nature of diamond were not affected by the protective atmosphere when brazed in mesh belt furnace with ammonia dissociating atmosphere. The SEM of the joining interface of diamond brazed in mesh belt furnace was shown in Fig. 4a, it can be seen that the joining interface micro-structurally bonded well. The element linear distribution of joining interface was shown in Fig. 4b, it can be seen that the curves connecting Ni, Cr and C were verified, suggesting the presence of a slow transition trend at the joining interface: the enriching Cr. The migration of the active element Cr toward the diamond grain region
(b) Fig. 4. Interfacial microstructure of brazing diamond in mesh belt furnace (a) and element linear distribution (b).
implied that the Cr from the Ni\\Cr alloy was actually segregated. This phenomenon was consistent with the high chemical affinity of Cr toward C, which causes beneficial near-interfacial changes that promote wetting and joining [23]. Brazed diamond tools whether in mesh belt furnace or in vacuum furnace were immersed into the chloroazotic acid to remove the filler alloy and the substrate respectively, and the interfacial resultant of diamond grains was investigated. Fig. 5a and b showed the surface morphology and the XRD pattern of mesh belt furnace-brazed diamond grit after deep etching individually. The counterparts of vacuum brazed diamond after deep etching were shown in Fig. 5(c) and (d), it can be seen that the interfacial resultant of diamond brazed in mesh belt furnace mainly contained Cr3C2 and Cr7C3 as well as vacuum brazed diamond, which confirms the chemical combination between the filler alloy and the diamond. Although the phase composition of the interfacial resultant obtained in mesh belt furnace was the same as that obtained in vacuum furnace, there were some differences in the sizes, directions and shapes (cp. Fig. 6a and b). The column-shaped carbides formed in the vacuum furnace were long, continuous and parallel to the crystal plane. However, carbides formed in mesh belt furnace were short, discontinuous, not compact and random in directions. 3.2. Residual stresses
Fig. 3. Raman spectroscopy of diamond surface protruding out of the filler alloy when brazing in mesh belt furnace.
The beam of laser Raman microscopy focused on the different points within the diamond according to the direction of arrow in Fig. 7 and Raman spectra was recorded every 25 μm. The residual stresses were
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157
A
B
(a)
(c)
(b)
(d)
Fig. 5. Surface morphology (a) and the XRD pattern (b) of diamond grit brazed in mesh belt furnace after chloroazotic acid etching and Surface morphology (c) and the XRD pattern (d) of vacuum brazed diamond after chloroazotic acid etching.
measured from the shift in the wavenumber ω of Raman-Stocks peak of a brazed diamond compared to the wavenumber ω0 of Raman-Stocks peak of an unbrazed and stress free diamond. Due to the geometry of specimen (Fig. 7), which allows unconstrained thermal expansion and contraction of diamond in Z direction during brazing process, an equibiaxial stress-state could be assumed. The equibiaxial residual stress σequibiaxial in X-Y plane could be calculated from the shift Δω in the diamond Raman-Stokes peak as: σ ¼ A ðω−ω0 Þ ¼ A Δω
ð5Þ
The proportionality factor A for diamond in this case is [24]: A ¼ 2ω0 =½p ðS11 þ S12 Þ þ q ðS11 þ 3S12 Þ ¼ −0:429 GPa=cm−1
ð6Þ
where the S11 and S12 (S11 = 1.01 TPα−1, S12 = − 0.14 TPα−1) are the elastic compliance constants of diamond and p and q (p = − 2.82 ⋅ ω20, q = − 1.78⋅ ω20) are the diamond phonon deformation potentials [24]. The curve connecting the residual stresses calculated based on Eqs. (5) and (6) to distance (H) from the top of diamond brazed in mesh belt furnace or vacuum furnace to focus points was shown in Fig. 8. From this figure, it becomes evident that in the vicinity of top surface to the depth of approximately 0–25 μm, less tensile stresses were detected; thereafter they become compressive stresses. Meanwhile, the values of compressive stresses increased with the increasing depth
from the top of diamond either brazed in mesh belt furnace or in vacuum furnace, it increased rapidly within the first 50 μm, and then become gradually. A comparison of the residual stresses values formed in two brazing environment implied that compressive residual stresses in vacuum furnace were higher than in mesh belt furnace at the same measuring distance. For example, residual stresses were − 808.6 MPa in vacuum furnace and −600.6 MPa in mesh belt furnace respectively at the depth around 250 μm. The residual stresses during brazing were caused by the combined action of the thermal stresses and the volume change of phase in the filler alloy due to the phase transformation. During brazing, a layer of chromium carbide (Cr\\C) compound was formed between the diamond and filler alloy, which caused the phase transformation in filler alloy and volume change of brazing gaps, and the corresponding residual stresses were generated. On the other hand, the formation of thermal stresses could be attributed to the different coefficients of thermal expansion at substrate/filler alloy, filler alloy/Cr–C, Cr\\C/diamond (1 × 10− 6K− 1 for diamond, 9.7 × 10− 6K− 1 for Cr3C2, 11.5 × 10−6K− 1 for Ni\\Cr filler alloy, 11.59 × 10− 6K− 1 for substrate), among which the coefficient of substrate was the largest. After cooling, the filler alloy starts solidifying and consequently causing thermal stress via Cr3C2 and Cr7C3 into the diamond. Meanwhile the substrate transmits the thermal stresses via layers of filler alloy and Cr\\C to diamond. In addition, a small part of thermal stress was also generated by the resultant of Cr\\C in cooling cycle due to its thinner thickness. So the compressive residual stresses were generated at the bottom of
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Fig. 8. Residual stress-depth graphs inside the diamond.
(a) caused thermal stresses transmitted from the substrate and filler alloy to the diamond discontinuously, thus part of the residual stresses were released.
3.3. Compressive strength and impact toughness
diamond, and tensile residual stresses were generated at the top of the diamond. The compressive residual stresses of diamond brazed in mesh belt furnace were lower than that in vacuum furnace at identical protruding height of diamond. In comparison to vacuum brazed diamond (c.f. Fig. 6), the column-shaped carbides formed in mesh belt furnace were short, not compact and there were gaps between each other, which
The compressive strength and the impact toughness of original diamond (No. 1), brazing diamond in mesh belt furnace (No. 2) and that in vacuum furnace (No. 3) after etching were shown in Fig. 9. It could be seen that the compressive strength and impact toughness of brazing diamonds declined to a certain extent both in mesh belt furnace and in vacuum furnace. The compressive strength and impact toughness of brazing diamond in mesh belt furnace were 2315.7 MPa and 65.2% respectively, which were higher than that in vacuum furnace as 1865.6 MPa and 59.3% correspondingly. This denoted that the diamonds suffered the lesser thermal damage in the process of brazing in mesh belt furnace. High temperature and residual stress during brazing diamond were considered as the main reasons of the thermal damage for the diamonds. The thermal damage caused the stacking faults and microcracks originated from the diamond surface, which could easily induce diamond fragmentation [14]. The compressive strength and impact toughness of diamonds brazed in mesh belt furnace were higher than that in vacuum furnace because of the shorter brazing time and the lower residual stresses (described in Section 3.2) produced during brazing, which illustrate that a better mechanical performance could be acquired for brazing diamond in mesh belt furnace in comparison to that in vacuum furnace.
Fig. 7. Schematic of measuring residual stress using laser Raman spectroscopy.
Fig. 9. Compressive strength and impact toughness of original diamond (No. 1), brazing diamond in mesh belt furnace (No. 2) and that in vacuum furnace (No. 3) after etching.
(b) Fig. 6. Zoom of micro-zone A on Fig. 5a (a) and B on Fig. 5c (b).
B. Sun et al. / Int. Journal of Refractory Metals and Hard Materials 60 (2016) 154–159
4. Conclusions (1) The brazing experiments of diamond were carried out using Ni\\Cr filler alloy in mesh belt furnace with ammonia dissociating protective atmosphere, and the interfacial compounds composed of Cr3C2 and Cr7C3 were formed during brazing, which confirmed the chemical combination between diamond and filler alloys. (2) The residual stresses in diamond brazed in mesh belt furnace were lower than that in vacuum furnace with identical protruding height of diamond due to that the column-shaped carbides formed in mesh belt furnace were short, no compact and gaps between each other, which caused thermal stresses transmitted from the substrate and filler alloy could not deliver to the diamond continuously, and part of residual stresses were released. (3) The compressive strength and impact toughness of diamond brazed in mesh belt furnace after chloroazotic acid etching were higher than that in vacuum furnace because of its shorter brazing time and the lower residual stress.
Acknowledgments The authors gratefully acknowledge the financial support of this research by the Collaborative Innovation Program of Industry University Research of Jiangsu Province (No. BY2015003-02, No. BY2014003-02, No. BY2014003-11, No. BY2014003-18, No. BY2013003-04, No. BY201300314 and BY2013003-15) and industry technology innovation project of Jiangsu Province Industry Technology Research Institute (BO2015005). References [1] Y. Chen, Y.C. Fu, H.H. Su, J.H. Xu, H.J. Xu, The effects of solder alloys on the morphologies and mechanical properties of brazed diamond grits, Int. J. Refract. Met. Hard Mater. 42 (2014) 23–29. [2] Y.Z. Hsieh, S.T. Lin, Diamond tool bits with iron alloys as the binding matrices, Mater. Chem. Phys. 72 (2001) 121–125. [3] Y.H. Wang, J.B. Zang, M.Z. Wang, Y. Guan, Y.Z. Zheng, Properties and applications of Ti-coated diamond grits, J. Mater. Process. Technol. 129 (2002) 369–380. [4] X.P. Xu, X.R. Tie, Y.Q. Yu, The effects of rare earth on the fracture properties of different metal-diamond composites, J. Mater. Process. Technol. 187-188 (2007) 421–428. [5] X.P. Xu, X.R. Tie, H.R. Wu, The effects of a Ti coating on the performance of metalbonded diamond composites containing rare earth, Int. J. Refract. Met. Hard Mater. 25 (2007) 244–249.
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Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and performance of diamond abrasive grains brazed in mesh belt furnace with ammonia dissociating atmosphere Bao-jun Sun a,b, Bing Xiao a,⁎, Si-xing Liu c a b c
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Mechanical Engineering, Taizhou University, Taizhou, Zhejiang 318000, PR China College of Mechanical Engineering, Yangzhou University, Yangzhou, Jiangsu 225009, PR China
a r t i c l e
i n f o
Article history: Received 10 March 2016 Received in revised form 16 July 2016 Accepted 19 July 2016 Available online 21 July 2016 Keywords: Diamond brazing Mesh belt furnace Ammonia dissociating atmosphere Interfacial microstructure Performance
a b s t r a c t In order to achieve the mass production of the brazing diamond tools, brazing of diamond grains to the matrix using Ni\\Cr active filler alloy was investigated in the mesh belt furnace, which could realize uninterrupted brazing under ammonia dissociating atmosphere. The surface characteristics and interfacial microstructures of brazed diamond in mesh belt furnace were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The residual stress, static compressive strength and impact toughness of the brazing diamond were measured and the corresponding results were compared to the brazing diamond grains in vacuum furnace. Results demonstrated that chemical reaction occurred between the diamond and filler alloy. The compound composed of Cr3C2 and Cr7C3 was formed at diamond interface and a higher bonding strength was obtained. The residual stresses of brazing diamond in mesh belt furnace were lower than that in vacuum furnace in the case of identical protruding height. However the static compressive strength and the impact toughness were higher than that in vacuum furnace. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Diamond with high hardness, wear resistance and chemical stability is widely used to fabricating cutting tools (e.g. diamond grinding wheel, diamond blade saw and diamond drilling bore), which are used to machine the hard and brittle materials, such as cemented carbide, natural stone and concrete, et al. [1]. Nowadays diamond tools are practically manufactured by sintering, electroplating and brazing method. Diamond is difficult to wet because of its chemical inertness, so it is difficult to acquire a strong bonding strength between the diamond and the matrix in sintering condition, even if surface metallization of the diamond and adding active element (e.g. Cr, W, V and Ti) to the substrates are developed [2–6]. Meanwhile, the diamond grains are mechanically embedded by nickel (Ni) in electroplated diamond tools, hence the holding force between the abrasive grains and matrix is not enough as well. The grains of brazed diamond tools are firmly joined to substrates because of strong chemical bonds between the grains and the filler alloys [7–9]. Therefore brazing technique could solve the problem of premature pullout, which is regarded as a major shortcoming of electroplating diamond tools [10]. Moreover, diamonds in single-layer brazed tools can be shallowly buried in the brazing alloy. As a consequence, the protrusion height of diamonds is higher and more space for coolant flow produces, which improves not only the cooling and ⁎ Corresponding author. E-mail address: [email protected] (B. Xiao).
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.016 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
the storage of the chips [11,12], but also the cutting speed and tool service life [13,14]. In recent years, the research upsurge of the brazing diamond was gradually increased due to its excellent cutting property. However, previous studies focused on microstructure formation, interface characterization at the joint of abrasive grains and brazing alloy, or thermal damage on the diamond under the condition of vacuum and induction heating [15–20]. There has been less research on the subject of how to improve the productivity of the brazing diamond tools on the basis of ensuring the quality and in turn realize the mass production of brazing diamond tools. In fact, the process of brazing diamond by vacuum furnace or induction heating has somewhat limitation respectively. The brazing atmosphere can be controlled effectively in vacuum furnace, but it takes a long heating and cooling time. In addition, the volume of vacuum furnace is limited, which results in small batch production. The induction brazing has the advantages of shorter heating time and smaller heat-affected zone, while the brazing temperature is difficult to control. Furthermore, the induction brazing cannot be used to mass production because of its technical feature. Therefore, the mass production of brazing diamond tools cannot be realized through the above methods. In order to solve this problem, this paper intended to explore the brazing process of continuous heating in mesh belt furnace with the ammonia dissociating protective atmosphere. Mesh belt furnace, which has been utilized to brazing of cement carbide and carbon steel extensively [21], can facilitate the large-scale batch production
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continuously. However, nowadays there are almost no relevant reports on the application of brazing diamond. In this study, brazing diamond tools were fabricated with Ni\\Cr filler alloys in mesh belt furnace with ammonia dissociating protective atmosphere. Subsequently, the surface characteristics, interfacial microstructures and residual stress of brazing diamond were analyzed. Finally, the static compressive strength and impact toughness for the brazing diamond after chloroazotic acid etching were measured and compared with the diamonds brazed in vacuum furnace. 2. Materials and experimental procedures The commercial Ni\\Cr (78.5 Ni\\12 Cr\\4 Fe\\3 Si\\2.5 B, wt.%) filler alloy powders, diamond and steel substrates composed of 0.45% C were used in the experimental process. Diamond (Huanghe Whirlwind Co. Ltd., China) with sizes ranged from 297 μm to 350 μm were used. The filler alloy powder was made into paste through adding resin glue. The substrate was machined into the trial samples with sizes of 20 mm × 10 mm × 5 mm. The diamond grains were ultrasonically cleaned in acetone to remove the impurities. The paste-shaped filler alloy was covered on the substrate sample evenly with the thickness about 200 μm. Then the diamond grains were arranged on the filler alloys. Thus, the raw materials were assembled into three-layered structure of diamond grits-filler alloy-substrate. The brazing samples were placed in mesh belt furnace with ammonia dissociating atmosphere and heat-treated at 1100 °C for 15 min. The running speed of mesh belt was 82 mm/min, the gas flow of protective atmosphere entering into the heating chamber was 4 m3/h, and the pressure of cooling water was 0.035 MPa. Other samples were put into the vacuum furnace with the vacuum degree of no N10−2 Pa, and heated to 1020 °C for 18 min. Fig. 1 illustrates the schematic of mesh belt furnace, which contains feeding zone, heating zone and cooling zone. The protective atmosphere was used in heating zone includes hydrogen and nitrogen decomposed by liquid ammonia, the equation is: catalyst
2NH3 → N2 þ 3H2
ð1Þ
The dew point could reach −60 °C after the mixed gas of the hydrogen and the nitrogen produced by ammonia decomposition was purified by highly-efficient molecular sieve. The purified gas entered into the heating zone, and excluded the air in the mesh belt furnace. Then the protective atmospheres were ignited, and flame screen came into being at the entrance and the exit of heating zone, which could prevent the outside air from entering the heating chamber and protect the brazing sample against oxidation. Double water jacket structures were made for the cooling zone. When fabricating the diamond, test samples were placed on the mesh belt driven by electric motor and passed through the feeding zone, heating zone and cooling zone successively, thus the brazing processes of the diamond tools were finished. The interfacial microstructure of the diamond and elemental distribution of the joining interface were observed by scan electron microscope (SEM, Hitachi-4800, Tokyo, Japan) coupled with energy-dispersive X-ray spectrometry (EDS). The interfacial resultant of diamonds after
chloroazotic acid etching was detected by X-ray diffraction (XRD, Bruke AXS D8 Advance, Germany). The surface nature of brazing diamond protruding out of the filler alloy was observed and the residual stresses in the diamond after brazing were measured using laser Raman microscopy (Renishaw Invia, Nd-YAG laser type, wave length 532 nm, England). Raman scatter peak reflects the vibration spectra characteristics of materials. When the laser beam of the microscope is focused on the point on the diamond under compressive stress, the Raman-Stokes peak in Raman spectra shifts to a higher wave number compared with the wave number of Raman-Stokes peak under unbrazed and stress-free diamond grits. When the laser beam is focused on the point of diamond grits under tensile stress, the peak shifted to a lower wave number. So the stress state of brazed diamond can be determined by Raman-Stokes peak position [22]. DLY-92 static strength measuring equipment was applied for checking the static strength of diamond grits after chloroazotic acid etching. Forty grits were measured and the mean maximum static load P was calculated. In accordance with the literature [23], the static compressive strength σ (MPa) of the diamonds after chloroazotic acid etching was determined as follows: σ ¼ 1:37P=d
2
ð2Þ
where P was the mean static load of diamond grits after chloroazotic acid etching and d was the mean diameter (mm) of the diamond grits after chloroazotic acid etching. Impact toughness of diamond grits after chloroazotic acid etching was measured using CM-II super-hard abrasive grains impact toughness measuring equipment. The rotating speed of measuring equipment is 2400 r/min. A group of diamond grits weighing 0.4 g was impacted for 2000 times. After impacting, the diamonds were sieved through a 60-mesh screen. The unbroken ratio (TI) of the diamond grains after chloroazotic acid etching was calculated as follows [23]: TI ¼
m1 100% m
ð3Þ
where m1 was the mass of remained diamonds after impacting and sieving (g), and m was the mass of diamonds before impacting (g). The measurement above was repeated three times and a average value of TI was determined as the impact toughness of diamond after etching. 3. Results and discussion 3.1. Surface and interface The surface morphology of diamond grains brazed in mesh belt furnace with ammonia dissolving atmosphere was shown in Fig. 2. It can be seen that the filler alloy climbed well along the diamond surface, which demonstrating good wettability of Ni\\Cr alloy toward diamond at the brazing ambient of ammonia dissociating atmosphere. Each diamond grains had clear edges and smooth surface, it can be inferred that protective atmosphere had no effects on the surface of diamond protruding out of the filler metal. In ammonia dissolving atmosphere, nitrogen was an inert gas, which wound not react with carbon (C). However, hydrogen erosion exists possibly in high temperature, which can be explained as the following reaction may occur: xC þ yH 2 →C x H 2y
Fig. 1. The schematic of mesh belt furnace.
155
ð4Þ
In general, the enthalpy change of chemical reaction is negative (Δ H b 0), so the case for the entropy change (Δ S b 0). The variety of Gibbs free energy for the reaction (ΔG = ΔH − TΔS) has a critical temperature (T0 = | ΔH/ΔS |), which means Δ G = 0 in this temperature.
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Diamond
Filler alloy
(a) Fig. 2. Morphologies of diamonds brazed in mesh belt furnace.
When the brazing temperature is higher than the critical temperature (T N T0), the variety of Gibbs free energy for the chemical reaction is positive (ΔG N 0), the reaction cannot occur spontaneously. Taking the resultant of CH4 for example, the critical temperature was acquired (T0 = 733 °C) by substituted the thermodynamics data into ΔG. And the critical temperatures of other hydrocarbons could be got by the same way. Hydrogen erosion phenomena could not happen because the diamonds brazing in mesh belt furnace were carried out at 1100 °C. The wave number of Raman-Stokes peak of the original and stressfree diamond was determined to be ω0 = 1331.70 cm−1 during the experiment. Fig. 3 was laser Raman spectroscopy of brazed diamond surface protruding out of the filler alloy in mesh belt furnace. As shown in Fig. 3, Raman-Stokes peak for surface of diamond brazed in mesh belt furnace lay in the wave number of 1331.13 cm− 1, which was the characteristic peak of diamond that had been suffered strain stresses. Other characteristic peaks were not detected by laser Raman microscopy. According to the morphology observation, thermodynamic analysis and Raman spectroscopy measurement above, surface nature of diamond were not affected by the protective atmosphere when brazed in mesh belt furnace with ammonia dissociating atmosphere. The SEM of the joining interface of diamond brazed in mesh belt furnace was shown in Fig. 4a, it can be seen that the joining interface micro-structurally bonded well. The element linear distribution of joining interface was shown in Fig. 4b, it can be seen that the curves connecting Ni, Cr and C were verified, suggesting the presence of a slow transition trend at the joining interface: the enriching Cr. The migration of the active element Cr toward the diamond grain region
(b) Fig. 4. Interfacial microstructure of brazing diamond in mesh belt furnace (a) and element linear distribution (b).
implied that the Cr from the Ni\\Cr alloy was actually segregated. This phenomenon was consistent with the high chemical affinity of Cr toward C, which causes beneficial near-interfacial changes that promote wetting and joining [23]. Brazed diamond tools whether in mesh belt furnace or in vacuum furnace were immersed into the chloroazotic acid to remove the filler alloy and the substrate respectively, and the interfacial resultant of diamond grains was investigated. Fig. 5a and b showed the surface morphology and the XRD pattern of mesh belt furnace-brazed diamond grit after deep etching individually. The counterparts of vacuum brazed diamond after deep etching were shown in Fig. 5(c) and (d), it can be seen that the interfacial resultant of diamond brazed in mesh belt furnace mainly contained Cr3C2 and Cr7C3 as well as vacuum brazed diamond, which confirms the chemical combination between the filler alloy and the diamond. Although the phase composition of the interfacial resultant obtained in mesh belt furnace was the same as that obtained in vacuum furnace, there were some differences in the sizes, directions and shapes (cp. Fig. 6a and b). The column-shaped carbides formed in the vacuum furnace were long, continuous and parallel to the crystal plane. However, carbides formed in mesh belt furnace were short, discontinuous, not compact and random in directions. 3.2. Residual stresses
Fig. 3. Raman spectroscopy of diamond surface protruding out of the filler alloy when brazing in mesh belt furnace.
The beam of laser Raman microscopy focused on the different points within the diamond according to the direction of arrow in Fig. 7 and Raman spectra was recorded every 25 μm. The residual stresses were
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A
B
(a)
(c)
(b)
(d)
Fig. 5. Surface morphology (a) and the XRD pattern (b) of diamond grit brazed in mesh belt furnace after chloroazotic acid etching and Surface morphology (c) and the XRD pattern (d) of vacuum brazed diamond after chloroazotic acid etching.
measured from the shift in the wavenumber ω of Raman-Stocks peak of a brazed diamond compared to the wavenumber ω0 of Raman-Stocks peak of an unbrazed and stress free diamond. Due to the geometry of specimen (Fig. 7), which allows unconstrained thermal expansion and contraction of diamond in Z direction during brazing process, an equibiaxial stress-state could be assumed. The equibiaxial residual stress σequibiaxial in X-Y plane could be calculated from the shift Δω in the diamond Raman-Stokes peak as: σ ¼ A ðω−ω0 Þ ¼ A Δω
ð5Þ
The proportionality factor A for diamond in this case is [24]: A ¼ 2ω0 =½p ðS11 þ S12 Þ þ q ðS11 þ 3S12 Þ ¼ −0:429 GPa=cm−1
ð6Þ
where the S11 and S12 (S11 = 1.01 TPα−1, S12 = − 0.14 TPα−1) are the elastic compliance constants of diamond and p and q (p = − 2.82 ⋅ ω20, q = − 1.78⋅ ω20) are the diamond phonon deformation potentials [24]. The curve connecting the residual stresses calculated based on Eqs. (5) and (6) to distance (H) from the top of diamond brazed in mesh belt furnace or vacuum furnace to focus points was shown in Fig. 8. From this figure, it becomes evident that in the vicinity of top surface to the depth of approximately 0–25 μm, less tensile stresses were detected; thereafter they become compressive stresses. Meanwhile, the values of compressive stresses increased with the increasing depth
from the top of diamond either brazed in mesh belt furnace or in vacuum furnace, it increased rapidly within the first 50 μm, and then become gradually. A comparison of the residual stresses values formed in two brazing environment implied that compressive residual stresses in vacuum furnace were higher than in mesh belt furnace at the same measuring distance. For example, residual stresses were − 808.6 MPa in vacuum furnace and −600.6 MPa in mesh belt furnace respectively at the depth around 250 μm. The residual stresses during brazing were caused by the combined action of the thermal stresses and the volume change of phase in the filler alloy due to the phase transformation. During brazing, a layer of chromium carbide (Cr\\C) compound was formed between the diamond and filler alloy, which caused the phase transformation in filler alloy and volume change of brazing gaps, and the corresponding residual stresses were generated. On the other hand, the formation of thermal stresses could be attributed to the different coefficients of thermal expansion at substrate/filler alloy, filler alloy/Cr–C, Cr\\C/diamond (1 × 10− 6K− 1 for diamond, 9.7 × 10− 6K− 1 for Cr3C2, 11.5 × 10−6K− 1 for Ni\\Cr filler alloy, 11.59 × 10− 6K− 1 for substrate), among which the coefficient of substrate was the largest. After cooling, the filler alloy starts solidifying and consequently causing thermal stress via Cr3C2 and Cr7C3 into the diamond. Meanwhile the substrate transmits the thermal stresses via layers of filler alloy and Cr\\C to diamond. In addition, a small part of thermal stress was also generated by the resultant of Cr\\C in cooling cycle due to its thinner thickness. So the compressive residual stresses were generated at the bottom of
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Fig. 8. Residual stress-depth graphs inside the diamond.
(a) caused thermal stresses transmitted from the substrate and filler alloy to the diamond discontinuously, thus part of the residual stresses were released.
3.3. Compressive strength and impact toughness
diamond, and tensile residual stresses were generated at the top of the diamond. The compressive residual stresses of diamond brazed in mesh belt furnace were lower than that in vacuum furnace at identical protruding height of diamond. In comparison to vacuum brazed diamond (c.f. Fig. 6), the column-shaped carbides formed in mesh belt furnace were short, not compact and there were gaps between each other, which
The compressive strength and the impact toughness of original diamond (No. 1), brazing diamond in mesh belt furnace (No. 2) and that in vacuum furnace (No. 3) after etching were shown in Fig. 9. It could be seen that the compressive strength and impact toughness of brazing diamonds declined to a certain extent both in mesh belt furnace and in vacuum furnace. The compressive strength and impact toughness of brazing diamond in mesh belt furnace were 2315.7 MPa and 65.2% respectively, which were higher than that in vacuum furnace as 1865.6 MPa and 59.3% correspondingly. This denoted that the diamonds suffered the lesser thermal damage in the process of brazing in mesh belt furnace. High temperature and residual stress during brazing diamond were considered as the main reasons of the thermal damage for the diamonds. The thermal damage caused the stacking faults and microcracks originated from the diamond surface, which could easily induce diamond fragmentation [14]. The compressive strength and impact toughness of diamonds brazed in mesh belt furnace were higher than that in vacuum furnace because of the shorter brazing time and the lower residual stresses (described in Section 3.2) produced during brazing, which illustrate that a better mechanical performance could be acquired for brazing diamond in mesh belt furnace in comparison to that in vacuum furnace.
Fig. 7. Schematic of measuring residual stress using laser Raman spectroscopy.
Fig. 9. Compressive strength and impact toughness of original diamond (No. 1), brazing diamond in mesh belt furnace (No. 2) and that in vacuum furnace (No. 3) after etching.
(b) Fig. 6. Zoom of micro-zone A on Fig. 5a (a) and B on Fig. 5c (b).
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4. Conclusions (1) The brazing experiments of diamond were carried out using Ni\\Cr filler alloy in mesh belt furnace with ammonia dissociating protective atmosphere, and the interfacial compounds composed of Cr3C2 and Cr7C3 were formed during brazing, which confirmed the chemical combination between diamond and filler alloys. (2) The residual stresses in diamond brazed in mesh belt furnace were lower than that in vacuum furnace with identical protruding height of diamond due to that the column-shaped carbides formed in mesh belt furnace were short, no compact and gaps between each other, which caused thermal stresses transmitted from the substrate and filler alloy could not deliver to the diamond continuously, and part of residual stresses were released. (3) The compressive strength and impact toughness of diamond brazed in mesh belt furnace after chloroazotic acid etching were higher than that in vacuum furnace because of its shorter brazing time and the lower residual stress.
Acknowledgments The authors gratefully acknowledge the financial support of this research by the Collaborative Innovation Program of Industry University Research of Jiangsu Province (No. BY2015003-02, No. BY2014003-02, No. BY2014003-11, No. BY2014003-18, No. BY2013003-04, No. BY201300314 and BY2013003-15) and industry technology innovation project of Jiangsu Province Industry Technology Research Institute (BO2015005). References [1] Y. Chen, Y.C. Fu, H.H. Su, J.H. Xu, H.J. Xu, The effects of solder alloys on the morphologies and mechanical properties of brazed diamond grits, Int. J. Refract. Met. Hard Mater. 42 (2014) 23–29. [2] Y.Z. Hsieh, S.T. Lin, Diamond tool bits with iron alloys as the binding matrices, Mater. Chem. Phys. 72 (2001) 121–125. [3] Y.H. Wang, J.B. Zang, M.Z. Wang, Y. Guan, Y.Z. Zheng, Properties and applications of Ti-coated diamond grits, J. Mater. Process. Technol. 129 (2002) 369–380. [4] X.P. Xu, X.R. Tie, Y.Q. Yu, The effects of rare earth on the fracture properties of different metal-diamond composites, J. Mater. Process. Technol. 187-188 (2007) 421–428. [5] X.P. Xu, X.R. Tie, H.R. Wu, The effects of a Ti coating on the performance of metalbonded diamond composites containing rare earth, Int. J. Refract. Met. Hard Mater. 25 (2007) 244–249.
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