AbstractâThe manifestation of Rebinder effect in abrasive machining of sapphire has been studied versus coolant pH. The change of pH from neutral to acidic ...
ISSN 10634576, Journal of Superhard Materials, 2013, Vol. 35, No. 2, pp. 126–130. © Allerton Press, Inc., 2013. Original Russian Text © A.V. Voloshin, E.F. Dolzhenkova, L.A. Litvinov, A.A. Petukhov, E.V. Slyunin, 2013, published in Sverkhtverdye Materialy, 2013, Vol. 35, No. 2, pp. 86–91.
INVESTIGATION OF MACHINING PROCESSES
The Influence of Coolant pH on Efficiency of Machining Sapphire A. V. Voloshin, E. F. Dolzhenkova, L. A. Litvinov, A. A. Petukhov, and E. V. Slyunin Institute of Monocrystals, National Academy of Sciences of Ukraine, pr. Lenina, 60, Kharkiv, 61178 Ukraine Received March 20, 2012
Abstract—The manifestation of Rebinder effect in abrasive machining of sapphire has been studied versus coolant pH. The change of pH from neutral to acidic results in an 1.8 times higher rate of grinding with a bound abrasive. An alkaline medium also facilitates the workpiece surface attrition by a bound abrasive. The use of an acidic solution as a coolant leads to an increase in efficiency of polishing sapphire, while an alkaline one lowers the polishing efficiency. DOI: 10.3103/S1063457613020093 Keywords: sapphire, grinding, polishing, coolant, Rebinder effect.
1. INTRODUCTION One of the methods for improving machining efficiency for sapphire is to choose an appropriate coolant. Mechanisms of the coolant action has not been adequately explored. Some of the hypotheses that explain the coolant action at the workpiece–liquid interface are based on Rebinder effect. It is known [1, 2] that in an aqueous solution at the sapphire–liquid interface the crystal interacts with H+ and OH– at high and low pH values of the liquid, respectively. This reaction gives rise to the following phases on the sapphire surfaces: positively charged
Al2O3 + 6H+ ' 2Al3+ + 3H2O;
negatively charged Al2O3 + 2OH– ' 2AlO2– + 2H2O. If the number of oppositely charged ions is equal, i.e., the charges of positive and negative ions compensate each other, there occurs an isoelectric state on the surface. The system in this state exhibits no electrokinetic properties and its zeta–potential becomes zero. At ξ = 0, most nonmetallic solids feature maximum hardness. Zeta–potential of sapphire is zero if the pH value of the liquid is between 4.5 and 6 (for various crystal planes) [1, 3]; in the case of polycrystalline state, it is zero for pH ranging from 8 to 9.3 [1]. This difference is attributed to the presence of hydroxyl groups OH– on the polycrystal surface and to its larger affinity to hydrogen cation H+ in comparison to the monocrystal. In [4, 5], it was shown that both the mono and polycrystalline alumina in aqueous solutions with various pH values exhibited maximum hardness and minimum resistance to dia mond drilling when in the isoelectric state. Zero value of the electrokinetic potential features the slowest wear rate of alumina [4]. At present, the most widely accepted explanation of the mechanism of variation of mechanical properties is the assumption made by Westwood and Shchukin [6] implying an increase in the mean free path of dislocations in deformation or, on the contrary, a hindered motion of dislocations. As a result of action by a liquid, there occurs recharging of point defects and dislocation cores in the crystal surface layer. As this takes place, the dislocation mobility changes because the nature of dislocations interaction with the lattice, vacancies, and interstitial atoms has been affected. The differences in the observed behavior of mechanical properties of sapphire in the isoelectric state is explained by differences in fracture modes [4, 5]. Specifically, hardness is governed by the crystal resistance to plastic deformation, the energy in diamond drilling is spent mostly to initiate cracks and brittle fractre in the material, while abrasive machining proceeds with surface fracture and deformation processes occurring simul taneously Alley and Devereux [7] studied the influence of Rebinder effect of variation of pH (between 6 and 12) of a surfaceactive medium on the ground surface quality of αAl2O3 optical ceramics. It was demonstrated that the minimum and maximum values of the surface roughness parameter Ra are observed at ξ = 0 for the coarse and fine grinding, respectively. Also, the pattern of stress variations in the directions normal and tangential to the alumina surface was clarified: in coarse grinding in the isoelectric state the indentation resistance was max 126
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imum and resistance to brittle fracture was minimum, while in the case of fine grinding just an opposite ten dency was observed. Mechanical properties of alumina products greatly depend on the abrasive machining conditions. In par ticular, in [7] the bending strength of specimens of αAl2O3 optical ceramics was shown to vary with pH values of the liquid and depend on the surface roughness. The highest strength values corresponded to the lowest val ues of the surface roughness parameter. Nowadays, the physicalchemical action of a liquid on alumina hardness in the process of wear is still not clearly understood. Therefore, a more detailed study of the mechanism of Rebinder effect has to be under taken to further improve the abrasive machining technology for these crystals. The present work has addressed the influence coolant pH on efficiency of grinding and polishing of sap phire and on the machined surface roughness and fracture toughness. 2. EXPERIMENTAL The investigations were carried out on 13mmdiameter 5mmhigh sapphire workpieces with (0001) ori entation along the cylinder. The workpiece end faces were machined on a diamond scaife of ASM 60/40 micron powder, and using M40 boron carbide grinding powders of grit size 28–40 μm, on a Mod. ZPSh 350M grinder with a spindle speed of 100 rpm and loads from 0.22 to 0.5 kPa. Then, they were polished with a capron polisher using loose ASM 28/20 diamond micron powders on a Mod. 4PD200 machine tool at a speed of 100 rpm and load of 0.22 kPa. Upon each change of machining conditions, we monitored the surface roughness Ra wear rate vwear. After various machining modes the surface roughness was measured by Mod. TR200 profilometerprofilograph. For each workpiece, the surface roughness parameter Ra was deter mined by the results of four to five measurements. The wear rate was monitored by means of a dial test indi cator (scale interval = 1 μm). Standard deviation was 6.3% for the surface roughness parameter and 5.4% for the wear rate. Coolants were water and aqueous solutions of HNO3 acid (0.2 mol/l) and NH4OH alkali (0.1 mol/l). The pH value was 7.53 for water, 0.12 for the acidic solution, and 11.78 for the alkaline solution. The solutions were prepared within 0.5 h before the use. Fracture toughness of sapphire was assessed by the value of critical stress intensity factor KIc during Vickers indentation using a Mod. PMT3 hardness tester with an indenter load P = 3.5 N, by the results of at least 30 measurements. 3. RESULTS AND DISCUSSION Table 1 summarizes the wear rate values of sapphire in grinding on ASM 60/40 diamond faceplate and in grinding with loose M40 boron carbide powder, using coolants with various pH values. It is evident that the change of coolant pH from neutral (pH = 7.53) to acidic (pH ≈ 0.12) has increased the grinding rate by a factor of about 1.8 in the case of the bound abrasive and just 1.1 in the case of the loose abrasive. The alkaline medium (pH ≈ 11.78) also accelerated (though, to a somewhat lesser extent in comparison to he acidic medium—1.4 times) the grinding process on the diamond faceplate and had almost no effect in the case of the loose abrasive. The variation of load between 0.22 and 0.55 kPa has no influence on the pattern of action of coolants with var ious pH values on sapphire machinability. Table 1. The influence of a coolant on the grinding rate of sapphire
Coolant pH
0.12 7.53 11.78
Rate of grinding, μm/h, with the abrasive bound ASM 60/40 diamond grits loose M40 boron carbide grits at a load, × 10–4 MPa 2.2 5 2.2 5 6.12 13.2 1.37 3.2 3.39 7.79 1.29 3.0 4.8 10.8 1.29 2.96
The results obtained correspond to those published elsewhere. The wear rate is inversely proportional to hardness of the workpiece material. In [2, 5], it was demonstrated that both the acidic and alkaline liquids con JOURNAL OF SUPERHARD MATERIALS
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tribute to a decrease in surface hardness of sapphire. Therefore, the observed increase in machining rate with a change in coolant pH is a consequence of manifestation of Rebinder effect. The physicalchemical interaction between the coolant and sapphire work surface facilitated also a change in the machined surface roughness (Table 2). Maximum surface roughness was observed upon the bound abrasive grinding using the acidic coolant. The microrelief height of the workpieces machined in an alkaline medium exceeds also that of the workpiece machined with a neutral coolant. A decrease in hardness of a crys tal is known to result in its lower indentation resistance and higher scratch resistance in the process grinding [7, 8]. Thus, the use of acidic and alkaline coolants leads to an actual increase in contact between the abrasive and the workpiece surface. The depth of propagation of the grindinginduced cracks growth, the rate of abra sive machining goes up. As this takes place, the height of microirregularities in the surface machined in the acidic and alkaline media also undergoes a slight increase in comparison to machining in the neutral medium due to an increase in size of the particles being spalled out. The influence of a coolant on the grinding process shows up in the same manner in the bound and looseabrasive machining, though to a greater extent in the case of the bound abrasive. Table 2. The influence of a coolant on the surface roughness parameter of sapphire in grinding
Coolant pH
0.12 7.53 11.78
Surface roughness parameter Ra, μm, in grinding with the abrasive bound ASM 60/40 diamond grits loose M40 boron carbide grits at a load, × 10–4 MPa 2.2 5 2.2 5 1.324 1.423 0.893 0.949 1.241 1.301 0.727 0.744 1.267 1.388 0.729 0.755
A slightly different pattern was observed in polishing sapphire of (0001) orientation. The use of the acidic medium provided a higher machining rate in comparison to the neutral and alkaline media; this took place during the early 30 minutes of polishing and was also accompanied by a decrease in surface roughness (Figs. 1, 2). Further machining in the acidic medium did not show any significant change in the sapphire surface microrelief height and the polishing rate remained almost unchanged. At an early stage, the rate of polishing in the alkaline medium was lower than that in the neutral one. In the cases of the neutral and alkaline liquids, the surface roughness tended to decrease throughout the entire polishing process but was almost the same upon machining for 60 min. υ, mm/h
1
0.036 0.032
2
0.028 0.024
3
0.020 0.016 0.012 0.008 0.004 10 20 30 40 50 60 70 τ, min Fig. 1. Rate of polishing vs. machining time for coolant pH values of 0.12 (1), 7.53 (2), and 11.7 (3).
Polishing is the final machining operation, the mechanical actions of the polisher on the crystal being machined are small. Plastic deformation becomes the governing factor in the balance of energy consumptions for the surface dispersion even in a material as brittle as sapphire. Due to a decrease in sapphire hardness in the liquid with pH 0.12, less energy is needed to disperse the crystal surface, while the crystal resistance to abrasive indentation grows. Thus, the liquid facilitates decreasing the depth of propagation of polishing induced scratches and cracks and consequently further reduces the size of particles which are formed by mul JOURNAL OF SUPERHARD MATERIALS
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tiple crossing of such cracks and removed from the workpiece surface. The hight of microirregularities of the polished surface reaches its minimum as early as 30 minutes of polishing. Ra, μm
1
0.25
0.20
2
0.15
0.10
3
0.05 10
20
30
40
50
60
70 τ, min
Fig. 2. Surface roughness parameter Ra as a function of machining time for coolant pH values of 11.7 (1), 7.53 (2), and 0.12 (3).
The use of the alkaline solution resulted in a slower rate of polishing and larger surface microrelief height in comparison to the surface machined in the neutral medium. Swain et al. [4] demonstrated that the rate of abrasive machining of sapphire in monohydric alcohols goes down. This was attributed to a fairly high viscosity of the hydroxyl compounds used and to a thicker layer between the workpiece surface and the polisher. Also, a decrease in the rate of polishing of sapphire in NH4OH solution (0.1 mol/l) is most likely associated with a reduction of contact between the abrasive and the workpiece surface. It is known that for (0001) plane the iso electric state is observed at pH ≈ 5.5 [1, 3]. Therefore, at a high pH value (an alkaline medium) the concen tration of counterions in the charged surface turns out to be essentially higher than that at a low pH value (an acidic medium) [9]. The thickness of the double electrical layer turns out to be larger too; therefore, some dia mond grains do not play any active role in polishing. The use of the acidic medium resulted in a higher fracture toughness of the workpiece surface in compar ison to polishing in the neutral medium: the critical stress intensity factor KIc was 3.8 and 3.2 MN m–3/2, respectively. As would be expected, the surface that has the smallest roughness exhibits the highest resistance to crack growth. Machining in the alkaline medium has not changed the surface fracture toughness. In com parison to the neutral medium, the stress intensity factor is within the measurement error. 4. CONCLUSIONS By changing the coolant pH value from neutral to acidic and alkaline, one can improve efficiency of machining of sapphire and reduce its surface roughness. The use of the coolant contributes to an increase in fracture toughness of the workpiece surface. REFERENCES 1. Franks, G.V. and Meagher, L., The Isoelectric Point of Sapphire and AlphaAlumina Powder, Colloids Surfaces A: Physicochem. Eng. Aspects, 2003, vol. 214, pp. 99–110. 2. Zhu, H., Niesz, D.E., and Greenhut, V.A., The Effect of Abrasive Hardness on the ChemicalAssisted Polishing of (0001) Plane Sapphire, J. Mater. Res., 2005, vol. 20, no. 2, pp. 504–520. 3. Fitts, J.P., Shang, X., Flynn, G.W., et al., Electrostatic Surface Charge at Aqueous/αAl2O3 SingleCrystal Inter faces as Probed by Optical SecondHarmonic Generation, J. Phys. Chem., 2005, vol. 109, pp. 7981–7986. 4. Swain, M.V., Latanision, R.M., and Westwood, A.R.C., Further Studies on EnvironmentSensitive Hardness and Machinability of Al2O3, J. Amer. Ceram. Soc., 1975, vol. 58, no. 9–10, pp. 372–376. 5. Alley, D.W. and Devereux, O.F., Coolant pH Control for Optimum Ceramic Grinding. I. Rebinder Effect in Poly crystalline Aluminum Oxide, J. Mater. Sci., 2002, vol. 37, no. 23, pp. 5135–5140.
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6. Shchukin, E.D., The Influence of SurfaceActive Media on the Mechanical Properties of Materials, Adv. Colloid. Interface Sci., 2006, vol. 123–126, pp. 33–47. 7. Alley, D.W. and Devereux, O.F., Coolant pH Control for Optimum Ceramic Grinding. I. Influence of the Rebinder Effect on the Surface Grinding of Aluminum Oxide, J. Mater. Sci., 2003, vol. 38, no. 6, pp. 1353–1358. 8. Ardamatskii, A.L., Almaznaya obrabotka opticheskikh detalei (Diamond Machining of Optical Components), Len ingrad: Mashinostroenie, 1978. 9. Pisarenko, A.P., Pospelova, K.A., and Yakovlev, A.G., Kurs kolloidnoi khimii (A Course of Colloid Chemistry), Mos cow: Vysshaya Shkola, 1964.
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