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An atomic-scale and high efficiency finishing method of zirconia ceramics by using magnetorheological finishing
Applied Surface Science 444 (2018) 569–577
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An atomic-scale and high efficiency finishing method of zirconia ceramics by using magnetorheological finishing Hu Luo a,b, Meijian Guo a,b, Shaohui Yin a,b,c,⇑, Fengjun Chen b,c, Shuai Huang a,b, Ange Lu b, Yuanfan Guo a,b a
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China College of Mechanical and Vehicle Engineering, Hunan University, Changsha, Hunan 410082, China c Key Laboratory for Intelligent Laser Manufacturing of Hunan Province, Hunan University, Changsha, Hunan 410082, China b
a r t i c l e
i n f o
Article history: Received 8 February 2018 Revised 6 March 2018 Accepted 11 March 2018 Available online 12 March 2018 Keywords: Zirconia ceramics Magnetorheological finishing Ultra-smooth surface Surface morphology
a b s t r a c t Zirconia ceramics is a valuable crucial material for fabricating functional components applied in aerospace, biology, precision machinery, military industry and other fields. However, the properties of its high brittleness and high hardness could seriously reduce its finishing efficiency and surface quality by conventional processing technology. In this work, we present a high efficiency and high-quality finishing process by using magnetorheological finishing (MRF), which employs the permanent magnetic yoke with straight air gap as excitation unit. The sub-nanoscale surface roughness and damage free surface can be obtained after magnetorheological finishing. The XRD results and SEM morphologies confirmed that the mechanical shear removal with ductile modes are the dominant material removal mechanism for the magnetorheological finishing of zirconia ceramic. With the developed experimental apparatus, the effects of workpiece speed, trough speed and work gap on material removal rate and surface roughness were systematically investigated. Zirconia ceramics finished to ultra-smooth surface with surface roughness less than Ra 1 nm was repeatedly achieved during the parametric experiments. Additionally, the highest material removal rate exceeded 1 mg/min when using diamond as an abrasive particle. Magnetorheological finishing promises to be an adaptable and efficient method for zirconia ceramics finishing. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Zirconia ceramics, as a new functional material, are widely used in aerospace, biology, precision machinery, military industry and other fields [1–3]. Excellent physical properties of the zirconia such as high hardness, small density, good insulation performance and thermal conductivity make it a valuable crucial material [4,5]. Not only such physical advantages but also mechanical advantages of wear resistance, anti-corrosion, anti-crush and heating resistant are bring a bright application future of zirconia in modern industry [6]. Owing to these excellent properties, zirconia ceramics has been widely used in fabricating component of the aero engine, inertial guidance missile, precision bearings and seals, and so on. This puts forward higher requirements for the surface processing of zirconia ceramic, which demands a high-quality surface with nanoscale roughness and without surface/subsurface damage simultaneously. However, due to the high brittleness and high hardness of ⇑ Corresponding author at: College of Mechanical and Vehicle Engineering, Hunan University, Changsha, Hunan 410082, China. E-mail address: [email protected] (S. Yin). https://doi.org/10.1016/j.apsusc.2018.03.091 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
zirconia ceramic materials, the finishing of such components has been faced a great challenge. Developing finishing methods of zirconia ceramics have been raised numerous interests in recent years. Fiocchi et al. [7] reported that the ultra-precision Ud-lap grinding was capable of processing flat 3Y-TZP surfaces with nanometric finishing. The best surface roughness of Ra 60.63 nm can be obtained by using #300 grinding wheel dressed with Ud=5. Zheng et al. [8] established the cutting force model in ultrasonic vibration assisted grinding of zirconia ceramics. Through theoretical analysis and experiments, they found that cutting force was the main factor that affects the processed surface/subsurface quality. Although the machining efficiency of ultra-precision grinding was satisfactory, however, the nanoscale surface roughness could not be achieved and critical cracks were easy to generate [9]. The ultrasonic vibration assisted grinding can improve the surface quality, however, the grinding related defects such as scratches are still cannot be avoided [4]. Furthermore, the free abrasive assisted lapping is easy to cause surface/subsurface damages due to the application of a certain normal load during machining, and the processing efficiency is unstable [10,11].
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As a typical flexible finishing technology, magnetorheological finishing (MRF) takes advantages of extremely low normal force and very small cut induced by abrasives during machining, which can process out a high quality and damage-free surface [12]. In addition, constrained by the magnetic field, the slurry in the magnetorheological fluid is well-distributed on the interface of workpiece and MR ribbon stably, contributing to the continued and efficient material removal [13]. Hong et al. [14] introduced a wheel based MRF polishing process of alumina-reinforced zirconia ceramics, by using the electromagnet with arc air gap as the excitation unit. The fine roughness of Ra 1.96 nm can be obtained after 60 min finishing. The conventional wheel-based MR process made a significant impact on the fabricating of the nanometric precision surface. Nevertheless, the efficiency would become an issue towards the large surfaces machining due to the tiny material removal rate of the generated ‘‘MR polishing spot” in the process [14,15]. And the cost would be greatly increased for the use of raster tool path strategy to machining the large dimension workpiece. The aim of this study is to develop a high efficient and high quality MRF process for flat zirconia ceramics surfaces. A permanent magnetic (PM) yoke with a straight air gap was employed as the magnetic excitation unit. This new type of the PM yoke can make the instantaneous finishing contact area considerably enlarged. The effects of different abrasives (diamond, ceria oxide, aluminum oxide) on the surface roughness were systematically investigated. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to characterize the finished ground surface of zirconia ceramics. The results indicate that the diamond abrasive exhibited an excellent material removal ability without inducing surface damages in MRF process. 2. MRF apparatus and experimental details 2.1. The principle of MRF and experimental setup As shown in Fig. 1, the workpiece is fixed on the fixture and can rotate with the B-axis, while the polishing trough is connected with the A-axis which rotates reversely with the B-axis. The magnetic excitation device is placed below the workpiece-axis (B-axis) whose distance from the workpiece can be adjusted, thus realizing the control of the strength of formed magnetic field. The whole device is fixed on the worktable of NC machining center, so that the polishing trough can move back and forth with the worktable along the X direction. A polishing pad is attached to the trough to increase the friction between the magnetorheological fluid and trough, making the MR fluid recyclable and updatable in the
polishing disc. When the MR fluid flows over the gradient magnetic field, it will be stiffened to form a rectangular MR fluid ribbon within milliseconds. As a result, the CI particles will be arranged in a chain-like curve along with the magnetic line. Affecting by the magnetic field’s gradient, the abrasives in MR fluid will float upward and assemble gather in the interface of workpiece surface and MR fluid ribbon. Then, the abrasive grains will be pressed into the workpiece surface by the pressure of MR ribbon. Material removal occurs when the abrasive grains move relative to the workpiece surface. The continuous renewed abrasives will continuously wipe workpiece surface, thus ensuring the sustainable and stable material removal during finishing process. In order to increase the contact area between the MR ribbon and workpiece, the PM yoke with a long straight air gap was used in this study, as shown in Fig. 2(a). The novel developed magnetic excitation system mainly consists two permanent magnets made by N50 grade NdFeB and electrical pure iron (DT4). There is a long straight air gap between two N50 magnets, which the straight gradient magnetic field will form above the air gap because of magnetic leakage. The experimental apparatus mainly consists of lapping machining, speed governor, PM yoke and fixture, as shown in Fig. 2(b). 2.2. Materials and characterization The flat zirconia ceramics sized in 40 40 1.1 mm3, purchased from Shenzhen Hard Precision Ceramic Co., Ltd. (China), were used to carry out the parameter experiments. The CIPs with the average particles size of 3.2 lm and density of 7.87 g/cm3 were purchased from BASF (Germany) with OP series. Abrasive powders of diamond, Al2O3 and CeO2 (average particles size: 3.2 lm) were applied by ChangSha Xinhui Technology Co., Ltd. (China). The properties of zirconia ceramics are shown in Table 1. The initial surface roughness of flat zirconia ceramic is about Ra 70–100 nm. All of the finished workpieces were ultrasonically cleaned up with acetone, absolute ethanol and deionized water, successively. Subsequently, the finished specimen was dried up. In order to obtain the material removal rate, the flat zirconia ceramics was weighted before and after finishing by using electronic balancer with a high resolution of 1 mg. Thus, the difference between two weights represents the mass removed during MRF process. Therefore, the material removal rate (MRR) can be calculated according to the following equation:
MRR ¼
103 Dm t
Fig. 1. The schematic diagram of planar magnetorheological finishing of zirconia ceramics.
ð1Þ
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Fig. 2. PM yoke (a) and experimental setup (b).
Table 1 Zirconia ceramic properties. Properties
Typical values 3
Density (g/cm ) Vickers hardness (Gpa) Fracture toughness (MPa/m2) Flexural strength (MPa) Thermal expansion (106/°C)
6.0 11.5 8.0 1200 10
where Dm (g) is the mass loss of the zirconia ceramic before and after finishing, t (min) is the processing time, and MRR (mg/min) is the corresponding removal rate. Subsequently, the roughness of the finished surfaces was measured by a white light interferometer (Zygo Newviewer 7100, USA). The average roughness was obtained by measuring 5 different points on the same surface. Finally, a field-emission scanning electron microscope (SEM, Carl Zessi, Sigma HD) was used to study the distinguishing surface morphology of finished surface by using diamond, Al2O3 and CeO2 as abrasive powder. The XRD diffractograms were obtained using diffractometer D/max-2000/PC (Rigaku, Japan) with Cu radiation. The generator settings were 40 kV and 100 mA. The XRD data were collected in a 2h range of 25–80° with a step width of 0.02° and scanning velocity of 8°/min.
The second set of experiment was set up to study the effect of processing parameters on MRF performance when using diamond as abrasives. So, effects of working gap (dw), workpiece rotation speed (nw) and trough rotation speed (nT) on finishing performance have been parametrically investigated. Three tests were arranged in this parametric study, which were designed based on the onefactor-at-a-time rule, i.e. if one parameter was changed the other parameters were remain unchanged. In Test 1, the effect of the work gap was studied, which was varied from 1.0 to 1.8 mm at an increment of 0.2 mm. The Test 2 was set to investigate the effect of workpiece speed which increases from 100 to 300 rpm at an increment of 50 rpm. While the effect of trough speed was studied in the set of Test 3, which changed from 15 to 35 rpm. These detailed experimental conditions are summarized in Table 3. The MR fluid used in this study mainly contains four parts: CI particles, abrasives, additive agent and deionized water. The active additive consists of dispersion stabilizer and NaCO3, respectively. The dispersion stabilizer contributes to the particles stably dispersed in the MR suspension system. And the NaCO3 can adjust pH value of the solution to 10, which will greatly reduce the oxidation rate of CI particles to prolong the service life of MR fluid. The detailed composition of MR fluid is shown in Table 2.
2.3. Design of experiments
3. Results and discussions
The first set of experiment was conducted to select the most efficient abrasive particles during the finishing process. In order to investigate the finishing performance of different abrasives, diamond, aluminum oxide (Al2O3) and ceria (CeO2) were chosen as abrasive particle in the experiments. The set of these tests were carried out to present the evolution of finished surface roughness and topography with finishing time. All the experiment conditions kept the same in finishing process by using three kinds of abrasives, as presented in Table 2.
3.1. Different abrasive particles finishing performance
Table 2 Experimental conditions. Processing conditions
Parameters
MR fluid components (vol%)
CIPs-40, AIPs-6, DI water-50, Additive agent-4, pH = 10 0.5 3.2 1.5 100 25 1200
AIPs mean size (lm) CIPs mean size (lm) Working gap (mm) Workpiece speed (r/min) Trough speed (r/min) Volume of MR fluid (ml)
The finishing performance of each kind of abrasive particles are quite different [16,17]. As shown in Fig. 3, although the variation trend of zirconia ceramics’ surface roughness with finishing time are similar, the ultimate limit values and reduction rate of surface roughness show a significant difference. The diamond is the most efficient abrasives, shown in Fig. 3(a). The surface roughness decreases rapidly in the first 10 min and reaches a stable state after finishing 30 min. The final surface roughness value is maintained stably at about Ra 0.7 nm. Fig. 3(b) shows the performance of ceria abrasive on reducing the surface roughness. It will cost at least 60 min by using CeO2 as abrasive to reach the ultimate surface roughness during the finishing process. And the final surface roughness keeps around about Ra 5 nm. It can be seen from Fig. 3(c) that the surface roughness decreases slowly in the MRF process when the Al2O3 worked as abrasive. The surface roughness value can reach to a stable state at 10 nm after finishing at least 2 h. Despite the different performances among the three abrasives, the flat zirconia ceramics surface can be finished to be a mirror-face which can reflect objective well, eventually.
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Table 3 Experimental conditions of parametric experiments. Test no.
Work gap (mm)
Workpiece speed (r/min)
Trough speed (r/min)
Finishing time (min)
1 2 3
1.0–1.8 1.4 1.4
100 100–300 100
25 25 15–35
30 30 30
impurity was removed, though some slight scratches and deep pits still remained. Further finishing to 30 min, the surface reaches to ultra-smooth level (Ra<1 nm) and the scratches were fully removed, only left a few pits. After 90 min of finishing, all the surface damages from the previous machining procedure have been removed. The corresponding topographic maps measured over a region of 1.88 1.44 mm2 as shown in Fig. 5(a) definitely support the findings in Fig. 4(a). Furthermore, the situation is quite different when used the CeO2 as an abrasive, shown in Fig. 4(b). After 30 min finishing process, there were no oxide impurity and scratches left on the surface but serious crater-like defect with a high peak around the deep valley still remained. As the MRF process moves on, the high peaks will be flattened. And the single deep valley turns out to be three valleys after finishing for 120 min. However, the surface couldn’t become ultra-smooth and lots of pits were still remained on the zirconia ceramics surface after finishing for 480 min. The topographic maps shown in Fig. 5(b) could certainly support the findings in Fig. 4(b). The performance of Al2O3 is similar to that of CeO2 which can be seen from Fig. 4(c). The oxide impurity and scratches were eliminated during the first 30 min finishing. And the deep valleys and a lot of pits can be observed all over the surface. After finishing for 120 min, the valleys were gone but the pits still remained. Then further finishing till to 720 min, the pits related damages were entirely unremoved. The homologous topographic maps evolution with finishing time shown in Fig. 5(c) certainly afford more details to sustain the findings shown in Fig. 4(c). Fig. 5 shows the evolution of 3d topography with finishing time when using different abrasive particles. It can be seen that all the initial surfaces before finishing are very rough which contains a great deal of peak-valley structures in micro/nano scale. The rough structures will be removed after finishing for 10 min and the surface roughness reduces to 7.197 nm when using diamond as an abrasive in the MRF process, better than that of 10.126 nm with CeO2 and 14.073 nm with Al2O3 after 30 min finishing. Finally, the surface roughness can reach to sub-nanometer level, which is 0.702 nm, in further finishing by diamond abrasive for 30 min. On the contrary, there are still some rough structures remain on the finished surface by using the abrasive of CeO2 and Al2O3, leading to the ultimate roughness values are much bigger than that of diamond. Apparently, the smallest surface roughness and the smoothest finished surface can be obtained by using diamond as an abrasive during the MRF process.
3.2. XRD analyses
Fig. 3. Surface roughness changes with finishing time: (a) Diamond, (b) CeO2, (c) Al2O3.
Fig. 4(a) shows the optical micrographs of the zirconia ceramics surfaces before and after finishing for 10, 30 and 90 min. The initial zirconia ceramics surface was covered with oxide impurity and full of scratches, highly likely caused by the sintering process. After 10 min finishing by using diamond as an abrasive, most of the oxide
The XRD was employed to distinguish the phases of sintered and finished surfaces. According to the previous experiment study (Fig. 3), the finish time of each kind of abrasives is set as 2 h to keep all the samples to reach the smallest surface roughness before Xray diffraction. Moreover, the finishing conditions of three samples keep all the same, which as shown in Table 2. The graph in Fig. 6 shows four different XRD diffraction patterns of sintered and finished surfaces. The analysis of the sintered surface spectrum reveals the presence of tetragonal phase (t-ZrO2) and monoclinic phase (m-ZrO2). Clearly, it can be found that the
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Fig. 4. Time lapse optical microscopy images with finishing time: (a) Diamond, (b) CeO2, (c) Al2O3.
Fig. 5. The 3d micro topography images with different finishing time.
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Fig. 6. X-ray diffraction patterns of sintered and finished surfaces. (The finishing time of b–d samples are set as 2 h).
main phase of the sintered surface is t-ZrO2, which the peaks appear at 2h = 30.2°, 34.9°, 50.4°, 60.2°, 63.5°, and 74.2° [18,19]. The minor peaks at 2h=28.2° respect the m-ZrO2 as the secondary phase [20,21]. The X-ray diffraction patterns of three finished surface keep consistent with sintered surface except that the diffraction peak intensity of m-ZrO2 decreased (indicated by arrows in Fig. 6). Particularly, the X-ray diffraction patterns of diamond finished surface are consistent with standard tetragonal zirconia (PDF#48-0224), indicating that the pure t-ZrO2 was obtained [22]. It also means that the phase of m-ZrO2 is vanished after finishing by diamond. However, the changing intensity of m-ZrO2 of three finished surfaces do not imply that the MRF process will cause the phase transformation. Firstly, the mechanical finishing process will not cause the phase transformation, as widely reported in the literatures [23–26]. Additionally, the XRD confirmed that there is no new phase appeared, thus the chemical reaction is not the reason of m-ZrO2 phase decreased or vanished on finished surfaces. It is more reasonable to say that the m-ZrO2 is mainly formed and concentrated in the few outmost grain layers during sintering process. According to the aforementioned study, the abrasives of CeO2 and Al2O3 are insufficient to eliminate the rough structures in the outmost grain layers of sintered surface. Therefore, the diffraction peak intensity of m-ZrO2 is decreased but not vanished after finished by CeO2 and Al2O3, which agree well with the SEM images in Fig. 7(c) and (d) shown that the residual valley-peak structures are remained. And the diffraction peak intensity of mZrO2 disappeared because the sintered rough structures were fully removed by diamond abrasive [Fig. 7(b)]. As a result, the m-ZrO2 on the finished surface will be declined or vanished because it mainly focuses on the outmost layer of the sintered surface. 3.3. SEM morphologies To better understand the removal mechanism of MRF, SEM observation was performed on the sintered and finished surface. As shown in Fig. 7, each sample was characterized by a low and high range, which the high range is given to show the specific morphology details of the area in blue rectangle. Fig. 7(a) showed that the sintered surface consists of many grains with different sizes and shapes, and which formed a large amount of micro/nano peak-valley structures. The mutual stacking and interleaving behaves of these grains explain the reason why the initial sintered surface is rough. As clearly shown in the high
magnification images in the top right corner in Fig. 7(a), the mutual stacking behaves of the outmost layer grains also lead to form many apertures. As a result, the outmost layer grains are very easy to remove out during the MRF process, which has been verified by our experimental results showing in Figs. 3–5. Fig. 7(b)–(d) shows the surface morphology after finishing for 2 h. As shown in Fig. 6(b), the rough peak-valley structures are eliminated during the finishing process by using diamond as abrasive, the porosities induced by sintering process are random distributed on the finished surface. The higher magnification image reveals slight scratching-induced traces by the diamond particles. Surface defects such as cracks and brittle fractures cannot be detected on the finished surface, indicating that the ductile modes are the dominant material removal mechanism for the MRF process of zirconia ceramic. In the contrary, most of peak structures are removed but the valley structures are still remained when using the CeO2 and Al2O3 as abrasive, as shown in Fig. 6(c) and (d). In addition, the higher magnification in the top right corner shows the details of the rough area containing valleys on the finished surface. Surface defects such as scratch and fracture are distributed in these structures. It should be noted that the fracture was not introduced during MRF process but caused by the sintering process. Since the pressure and temperature during the sintering process are extremely high, it is very easy to induce defect on the sintered surface, particularly in the few outmost grain layers [27]. Moreover, t-m phase transformation often occurs during the sintering process (from high temperature to room temperature cooling process), inducing volume change of grain crystal will eventually lead to cracks and even fragmentation in the sintered surface [28]. According to the experiment results, the MRR of CeO2 is about 2 mg/h while 0.79 mg/h for Al2O3. Therefore, the finishing time (2 h) is insufficient to remove out the rough structures of sintered surface. Thus, the valleys are remained and some fractures are exposed after peaks have been removed out. These SEM images indicate that the ineffectiveness of used abrasives (CeO2 and Al2O3) to remove the rough structures and surface/sub-surface defects produced in the sintered surface. According to the above analyses, it is not difficult to understand why the experimental results shown in Fig. 3 are obtained. The surface roughness was dropping very fast in the first 10 min finishing time by diamond and 30 min by CeO2 and Al2O3, then reducing very slow with further finishing. Firstly, the XRD analyses show that experiment used flat zirconia ceramics is pure t-ZrO2 which contains m-ZrO2 in the outmost layers. It means that the outmost layers are easier to remove out by abrasives since the m-ZrO2 has lower strength and fracture toughness than t-ZrO2 [29,30]. In addition, the SEM images show that the outmost layers are stacked with each other to form lots of gaps. Therefore, both the m-ZrO2 and scattered grain structure are contributing to the high efficient removal in the beginning MRF process. As finishing process is going on, those interlaced grain layers mix with m-ZrO2 and t-ZrO2 are removed and the inner grain layers with pure t-ZrO2 of good crystallinity and high densification are exposed. Due to the exposed crystal layer has a higher strength, the removal effect of abrasive particles on the crystal material will be weakened. Thus, the surface roughness has limit decline with the further finishing.
3.4. Parametric experiment The parameters of working gap, workpiece speed and trough speed are the domain factors affecting the material removal rate and surface quality in MRF process [12,13]. Nevertheless, the different abrasive experimental results show that diamond grits not only have the highest removal efficiency, but also have the best surface quality. Therefore, the diamond abrasive was chosen to
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Fig. 7. SEM images of different samples: (a) sintered; finishing by diamond (b), CeO2 (c) and Al2O3 (d). (The finishing time of b–d samples are set as 2 h).
Fig. 8. Effects of (a) work gap, (b) workpiece speed, (c) trough speed on MRR; effects of (d) work gap, (e) workpiece speed, (f) trough speed on the Ra of finished zirconia ceramics surface. The used finishing conditions are shown in Table 3.
carry out the parametric research. Fig. 8 shows the effect of those parameters mentioned above on MRR and Ra. Fig. 8(a) shows the effect of work gap on the MRR. It is seen that the MRR decreases with the increasing in work gap. And the highest MRR of 0.77 mg/min was obtained when the work gap is set to 1 mm. Since the magnetic excitation device is fixed on the worktable of the finishing device, changing the work gap will vary yield strength and working area of MR ribbon. An increase in work gap leads to the decrease in magnetic flied, thus decreasing in yield strength of MR ribbon. As a result, the shearing action between the MR ribbon and the workpiece surface will be weakened. Furthermore, when the fluid ribbon flows through the work gap, it will be compressed by the workpiece surface which results in the expansion of useful working area of MR ribbon. The decreasing in work gap will lead to the increase of the MR ribbon’s expansion,
thus increasing of the useful working area during the MRF process. The work gap affects the yield strength and working area of MR ribbon simultaneously, resulting in the significant changing on MRR. The effect of work gap on Ra is as shown in Fig. 8(d). The surface roughness remains stable around at 0.8 nm firstly and then increase from 0.8 to 1.26 nm when the work gap increases from 1.4 to 1.8 mm. As stated in the previous analysis, a bigger of the work gap would lead to the smaller yield strength and working area of fluid ribbon, thus the smaller of the squeeze force during the MRF process. As a result, the material’s shear removal by diamond grain will be weakened, therefore the surface roughness value increases when the finishing time remain unchanged. As shown in Fig. 8(b), the MRR increases from 0.6 to 1.03 mg/ min with nw increase from 100 to 300 r/min. The relative speed between the workpiece and the diamond grain is the domain
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reason of the material removal in a MRF process. Therefore, the greater nw will lead to a higher material removal rate. It should be noted that the MRR increases sharply when the nw smaller than 200 r/min and slows down with the nw further increasing to 300r/ min. This is because the high rotation movement of workpiece will destroy the homogeneity of MR ribbon, thus the material removal becomes instable. The effect of nw on Ra is shown in Fig. 8(e). It is seen that the nw has limit effect on Ra. The value of Ra increases slightly from 0.83 to 0.95 nm when the nw increases from 100 to 300 r/min. Since the high rotation movement of workpiece will cause the non-uniform material removal in MRF process. So the bigger Ra is obtained with higher nw. Fig. 8(c) shows the effect of nT on MRR. Similar to that of nw, the MRR increases from 0.3 to 0.8 mg/min with the nT from 15 to 35 r/ min. As mentioned before, the increasing in nT will certainly lead to enlarge the relative speed between MR ribbon and workpiece, thus a bigger MRR is obtained. The effect of nT on the Ra appears different from that of nw and dw [Fig. 8(f)]. The Ra declines from 1.38 to 0.84 nm when the nT increases from 15 to 25 r/min, then enlarges slightly to 0.93 nm with the nT constant increase to 35 r/min. The relative speed between the workpiece and the MR ribbon increases with the increasing nT explains the reason why the Ra declines in the beginning. However, if the nT was too great, a considerable amount of the MR fluid including diamond grains would be pushed by the significant centrifugal force to the outer periphery of the trough. As a consequence, the diamond grains decrease when fluid ribbon passing through the work gap, resulting in a slightly increase in Ra. 4. Conclusions In this work, a high efficiency and high surface quality MRF process based on permanent magnetic yoke excitation for zirconia ceramics finishing was developed. Through the three kind abrasives finishing performance experimental investigation, the highest material removal rate and best surface quality can be obtained by using diamond as abrasive during MRF process. Moreover, the XRD results show that there is no chemical removal occurs during the shear action between the MR ribbon and zirconia ceramics, which indicates that the material removal only caused by mechanical scratching of abrasive particles. Then, the SEM morphologies of finished surface state clearly that the ductile modes are the dominant material removal mechanism for the MRF process of zirconia ceramic. The parametric finishing experiments showed that the material removal rate increases with the increase in workpiece rotation speed and trough speed, but the decrease in work gap. In addition, the surface roughness of finished zirconia ceramic surface increases with work gap and workpiece rotation speed, but decline with the increase in trough speed. In summary, the finishing experiments demonstrated that the zirconia ceramic surface could be improved to atomic-scale level in Ra 0.702 nm after 30 min of finishing in the condition of dw = 1.4 mm, nT = 25 r/min and nw = 100 r/min. The result confirmed that the MRF process using the diamond as the abrasive particle has great potential to fabricate the ultra-smooth zirconia ceramic surface without damage at relatively high efficiency. Acknowledgments This work is financially supported by the National Natural Science Foundation of China [Grant No. 51675171]. And the Science and Technology Planning Project of Hunan Province [No. 2016TP1008].
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Full Length Article
An atomic-scale and high efficiency finishing method of zirconia ceramics by using magnetorheological finishing Hu Luo a,b, Meijian Guo a,b, Shaohui Yin a,b,c,⇑, Fengjun Chen b,c, Shuai Huang a,b, Ange Lu b, Yuanfan Guo a,b a
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China College of Mechanical and Vehicle Engineering, Hunan University, Changsha, Hunan 410082, China c Key Laboratory for Intelligent Laser Manufacturing of Hunan Province, Hunan University, Changsha, Hunan 410082, China b
a r t i c l e
i n f o
Article history: Received 8 February 2018 Revised 6 March 2018 Accepted 11 March 2018 Available online 12 March 2018 Keywords: Zirconia ceramics Magnetorheological finishing Ultra-smooth surface Surface morphology
a b s t r a c t Zirconia ceramics is a valuable crucial material for fabricating functional components applied in aerospace, biology, precision machinery, military industry and other fields. However, the properties of its high brittleness and high hardness could seriously reduce its finishing efficiency and surface quality by conventional processing technology. In this work, we present a high efficiency and high-quality finishing process by using magnetorheological finishing (MRF), which employs the permanent magnetic yoke with straight air gap as excitation unit. The sub-nanoscale surface roughness and damage free surface can be obtained after magnetorheological finishing. The XRD results and SEM morphologies confirmed that the mechanical shear removal with ductile modes are the dominant material removal mechanism for the magnetorheological finishing of zirconia ceramic. With the developed experimental apparatus, the effects of workpiece speed, trough speed and work gap on material removal rate and surface roughness were systematically investigated. Zirconia ceramics finished to ultra-smooth surface with surface roughness less than Ra 1 nm was repeatedly achieved during the parametric experiments. Additionally, the highest material removal rate exceeded 1 mg/min when using diamond as an abrasive particle. Magnetorheological finishing promises to be an adaptable and efficient method for zirconia ceramics finishing. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Zirconia ceramics, as a new functional material, are widely used in aerospace, biology, precision machinery, military industry and other fields [1–3]. Excellent physical properties of the zirconia such as high hardness, small density, good insulation performance and thermal conductivity make it a valuable crucial material [4,5]. Not only such physical advantages but also mechanical advantages of wear resistance, anti-corrosion, anti-crush and heating resistant are bring a bright application future of zirconia in modern industry [6]. Owing to these excellent properties, zirconia ceramics has been widely used in fabricating component of the aero engine, inertial guidance missile, precision bearings and seals, and so on. This puts forward higher requirements for the surface processing of zirconia ceramic, which demands a high-quality surface with nanoscale roughness and without surface/subsurface damage simultaneously. However, due to the high brittleness and high hardness of ⇑ Corresponding author at: College of Mechanical and Vehicle Engineering, Hunan University, Changsha, Hunan 410082, China. E-mail address: [email protected] (S. Yin). https://doi.org/10.1016/j.apsusc.2018.03.091 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
zirconia ceramic materials, the finishing of such components has been faced a great challenge. Developing finishing methods of zirconia ceramics have been raised numerous interests in recent years. Fiocchi et al. [7] reported that the ultra-precision Ud-lap grinding was capable of processing flat 3Y-TZP surfaces with nanometric finishing. The best surface roughness of Ra 60.63 nm can be obtained by using #300 grinding wheel dressed with Ud=5. Zheng et al. [8] established the cutting force model in ultrasonic vibration assisted grinding of zirconia ceramics. Through theoretical analysis and experiments, they found that cutting force was the main factor that affects the processed surface/subsurface quality. Although the machining efficiency of ultra-precision grinding was satisfactory, however, the nanoscale surface roughness could not be achieved and critical cracks were easy to generate [9]. The ultrasonic vibration assisted grinding can improve the surface quality, however, the grinding related defects such as scratches are still cannot be avoided [4]. Furthermore, the free abrasive assisted lapping is easy to cause surface/subsurface damages due to the application of a certain normal load during machining, and the processing efficiency is unstable [10,11].
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As a typical flexible finishing technology, magnetorheological finishing (MRF) takes advantages of extremely low normal force and very small cut induced by abrasives during machining, which can process out a high quality and damage-free surface [12]. In addition, constrained by the magnetic field, the slurry in the magnetorheological fluid is well-distributed on the interface of workpiece and MR ribbon stably, contributing to the continued and efficient material removal [13]. Hong et al. [14] introduced a wheel based MRF polishing process of alumina-reinforced zirconia ceramics, by using the electromagnet with arc air gap as the excitation unit. The fine roughness of Ra 1.96 nm can be obtained after 60 min finishing. The conventional wheel-based MR process made a significant impact on the fabricating of the nanometric precision surface. Nevertheless, the efficiency would become an issue towards the large surfaces machining due to the tiny material removal rate of the generated ‘‘MR polishing spot” in the process [14,15]. And the cost would be greatly increased for the use of raster tool path strategy to machining the large dimension workpiece. The aim of this study is to develop a high efficient and high quality MRF process for flat zirconia ceramics surfaces. A permanent magnetic (PM) yoke with a straight air gap was employed as the magnetic excitation unit. This new type of the PM yoke can make the instantaneous finishing contact area considerably enlarged. The effects of different abrasives (diamond, ceria oxide, aluminum oxide) on the surface roughness were systematically investigated. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to characterize the finished ground surface of zirconia ceramics. The results indicate that the diamond abrasive exhibited an excellent material removal ability without inducing surface damages in MRF process. 2. MRF apparatus and experimental details 2.1. The principle of MRF and experimental setup As shown in Fig. 1, the workpiece is fixed on the fixture and can rotate with the B-axis, while the polishing trough is connected with the A-axis which rotates reversely with the B-axis. The magnetic excitation device is placed below the workpiece-axis (B-axis) whose distance from the workpiece can be adjusted, thus realizing the control of the strength of formed magnetic field. The whole device is fixed on the worktable of NC machining center, so that the polishing trough can move back and forth with the worktable along the X direction. A polishing pad is attached to the trough to increase the friction between the magnetorheological fluid and trough, making the MR fluid recyclable and updatable in the
polishing disc. When the MR fluid flows over the gradient magnetic field, it will be stiffened to form a rectangular MR fluid ribbon within milliseconds. As a result, the CI particles will be arranged in a chain-like curve along with the magnetic line. Affecting by the magnetic field’s gradient, the abrasives in MR fluid will float upward and assemble gather in the interface of workpiece surface and MR fluid ribbon. Then, the abrasive grains will be pressed into the workpiece surface by the pressure of MR ribbon. Material removal occurs when the abrasive grains move relative to the workpiece surface. The continuous renewed abrasives will continuously wipe workpiece surface, thus ensuring the sustainable and stable material removal during finishing process. In order to increase the contact area between the MR ribbon and workpiece, the PM yoke with a long straight air gap was used in this study, as shown in Fig. 2(a). The novel developed magnetic excitation system mainly consists two permanent magnets made by N50 grade NdFeB and electrical pure iron (DT4). There is a long straight air gap between two N50 magnets, which the straight gradient magnetic field will form above the air gap because of magnetic leakage. The experimental apparatus mainly consists of lapping machining, speed governor, PM yoke and fixture, as shown in Fig. 2(b). 2.2. Materials and characterization The flat zirconia ceramics sized in 40 40 1.1 mm3, purchased from Shenzhen Hard Precision Ceramic Co., Ltd. (China), were used to carry out the parameter experiments. The CIPs with the average particles size of 3.2 lm and density of 7.87 g/cm3 were purchased from BASF (Germany) with OP series. Abrasive powders of diamond, Al2O3 and CeO2 (average particles size: 3.2 lm) were applied by ChangSha Xinhui Technology Co., Ltd. (China). The properties of zirconia ceramics are shown in Table 1. The initial surface roughness of flat zirconia ceramic is about Ra 70–100 nm. All of the finished workpieces were ultrasonically cleaned up with acetone, absolute ethanol and deionized water, successively. Subsequently, the finished specimen was dried up. In order to obtain the material removal rate, the flat zirconia ceramics was weighted before and after finishing by using electronic balancer with a high resolution of 1 mg. Thus, the difference between two weights represents the mass removed during MRF process. Therefore, the material removal rate (MRR) can be calculated according to the following equation:
MRR ¼
103 Dm t
Fig. 1. The schematic diagram of planar magnetorheological finishing of zirconia ceramics.
ð1Þ
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Fig. 2. PM yoke (a) and experimental setup (b).
Table 1 Zirconia ceramic properties. Properties
Typical values 3
Density (g/cm ) Vickers hardness (Gpa) Fracture toughness (MPa/m2) Flexural strength (MPa) Thermal expansion (106/°C)
6.0 11.5 8.0 1200 10
where Dm (g) is the mass loss of the zirconia ceramic before and after finishing, t (min) is the processing time, and MRR (mg/min) is the corresponding removal rate. Subsequently, the roughness of the finished surfaces was measured by a white light interferometer (Zygo Newviewer 7100, USA). The average roughness was obtained by measuring 5 different points on the same surface. Finally, a field-emission scanning electron microscope (SEM, Carl Zessi, Sigma HD) was used to study the distinguishing surface morphology of finished surface by using diamond, Al2O3 and CeO2 as abrasive powder. The XRD diffractograms were obtained using diffractometer D/max-2000/PC (Rigaku, Japan) with Cu radiation. The generator settings were 40 kV and 100 mA. The XRD data were collected in a 2h range of 25–80° with a step width of 0.02° and scanning velocity of 8°/min.
The second set of experiment was set up to study the effect of processing parameters on MRF performance when using diamond as abrasives. So, effects of working gap (dw), workpiece rotation speed (nw) and trough rotation speed (nT) on finishing performance have been parametrically investigated. Three tests were arranged in this parametric study, which were designed based on the onefactor-at-a-time rule, i.e. if one parameter was changed the other parameters were remain unchanged. In Test 1, the effect of the work gap was studied, which was varied from 1.0 to 1.8 mm at an increment of 0.2 mm. The Test 2 was set to investigate the effect of workpiece speed which increases from 100 to 300 rpm at an increment of 50 rpm. While the effect of trough speed was studied in the set of Test 3, which changed from 15 to 35 rpm. These detailed experimental conditions are summarized in Table 3. The MR fluid used in this study mainly contains four parts: CI particles, abrasives, additive agent and deionized water. The active additive consists of dispersion stabilizer and NaCO3, respectively. The dispersion stabilizer contributes to the particles stably dispersed in the MR suspension system. And the NaCO3 can adjust pH value of the solution to 10, which will greatly reduce the oxidation rate of CI particles to prolong the service life of MR fluid. The detailed composition of MR fluid is shown in Table 2.
2.3. Design of experiments
3. Results and discussions
The first set of experiment was conducted to select the most efficient abrasive particles during the finishing process. In order to investigate the finishing performance of different abrasives, diamond, aluminum oxide (Al2O3) and ceria (CeO2) were chosen as abrasive particle in the experiments. The set of these tests were carried out to present the evolution of finished surface roughness and topography with finishing time. All the experiment conditions kept the same in finishing process by using three kinds of abrasives, as presented in Table 2.
3.1. Different abrasive particles finishing performance
Table 2 Experimental conditions. Processing conditions
Parameters
MR fluid components (vol%)
CIPs-40, AIPs-6, DI water-50, Additive agent-4, pH = 10 0.5 3.2 1.5 100 25 1200
AIPs mean size (lm) CIPs mean size (lm) Working gap (mm) Workpiece speed (r/min) Trough speed (r/min) Volume of MR fluid (ml)
The finishing performance of each kind of abrasive particles are quite different [16,17]. As shown in Fig. 3, although the variation trend of zirconia ceramics’ surface roughness with finishing time are similar, the ultimate limit values and reduction rate of surface roughness show a significant difference. The diamond is the most efficient abrasives, shown in Fig. 3(a). The surface roughness decreases rapidly in the first 10 min and reaches a stable state after finishing 30 min. The final surface roughness value is maintained stably at about Ra 0.7 nm. Fig. 3(b) shows the performance of ceria abrasive on reducing the surface roughness. It will cost at least 60 min by using CeO2 as abrasive to reach the ultimate surface roughness during the finishing process. And the final surface roughness keeps around about Ra 5 nm. It can be seen from Fig. 3(c) that the surface roughness decreases slowly in the MRF process when the Al2O3 worked as abrasive. The surface roughness value can reach to a stable state at 10 nm after finishing at least 2 h. Despite the different performances among the three abrasives, the flat zirconia ceramics surface can be finished to be a mirror-face which can reflect objective well, eventually.
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Table 3 Experimental conditions of parametric experiments. Test no.
Work gap (mm)
Workpiece speed (r/min)
Trough speed (r/min)
Finishing time (min)
1 2 3
1.0–1.8 1.4 1.4
100 100–300 100
25 25 15–35
30 30 30
impurity was removed, though some slight scratches and deep pits still remained. Further finishing to 30 min, the surface reaches to ultra-smooth level (Ra<1 nm) and the scratches were fully removed, only left a few pits. After 90 min of finishing, all the surface damages from the previous machining procedure have been removed. The corresponding topographic maps measured over a region of 1.88 1.44 mm2 as shown in Fig. 5(a) definitely support the findings in Fig. 4(a). Furthermore, the situation is quite different when used the CeO2 as an abrasive, shown in Fig. 4(b). After 30 min finishing process, there were no oxide impurity and scratches left on the surface but serious crater-like defect with a high peak around the deep valley still remained. As the MRF process moves on, the high peaks will be flattened. And the single deep valley turns out to be three valleys after finishing for 120 min. However, the surface couldn’t become ultra-smooth and lots of pits were still remained on the zirconia ceramics surface after finishing for 480 min. The topographic maps shown in Fig. 5(b) could certainly support the findings in Fig. 4(b). The performance of Al2O3 is similar to that of CeO2 which can be seen from Fig. 4(c). The oxide impurity and scratches were eliminated during the first 30 min finishing. And the deep valleys and a lot of pits can be observed all over the surface. After finishing for 120 min, the valleys were gone but the pits still remained. Then further finishing till to 720 min, the pits related damages were entirely unremoved. The homologous topographic maps evolution with finishing time shown in Fig. 5(c) certainly afford more details to sustain the findings shown in Fig. 4(c). Fig. 5 shows the evolution of 3d topography with finishing time when using different abrasive particles. It can be seen that all the initial surfaces before finishing are very rough which contains a great deal of peak-valley structures in micro/nano scale. The rough structures will be removed after finishing for 10 min and the surface roughness reduces to 7.197 nm when using diamond as an abrasive in the MRF process, better than that of 10.126 nm with CeO2 and 14.073 nm with Al2O3 after 30 min finishing. Finally, the surface roughness can reach to sub-nanometer level, which is 0.702 nm, in further finishing by diamond abrasive for 30 min. On the contrary, there are still some rough structures remain on the finished surface by using the abrasive of CeO2 and Al2O3, leading to the ultimate roughness values are much bigger than that of diamond. Apparently, the smallest surface roughness and the smoothest finished surface can be obtained by using diamond as an abrasive during the MRF process.
3.2. XRD analyses
Fig. 3. Surface roughness changes with finishing time: (a) Diamond, (b) CeO2, (c) Al2O3.
Fig. 4(a) shows the optical micrographs of the zirconia ceramics surfaces before and after finishing for 10, 30 and 90 min. The initial zirconia ceramics surface was covered with oxide impurity and full of scratches, highly likely caused by the sintering process. After 10 min finishing by using diamond as an abrasive, most of the oxide
The XRD was employed to distinguish the phases of sintered and finished surfaces. According to the previous experiment study (Fig. 3), the finish time of each kind of abrasives is set as 2 h to keep all the samples to reach the smallest surface roughness before Xray diffraction. Moreover, the finishing conditions of three samples keep all the same, which as shown in Table 2. The graph in Fig. 6 shows four different XRD diffraction patterns of sintered and finished surfaces. The analysis of the sintered surface spectrum reveals the presence of tetragonal phase (t-ZrO2) and monoclinic phase (m-ZrO2). Clearly, it can be found that the
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Fig. 4. Time lapse optical microscopy images with finishing time: (a) Diamond, (b) CeO2, (c) Al2O3.
Fig. 5. The 3d micro topography images with different finishing time.
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Fig. 6. X-ray diffraction patterns of sintered and finished surfaces. (The finishing time of b–d samples are set as 2 h).
main phase of the sintered surface is t-ZrO2, which the peaks appear at 2h = 30.2°, 34.9°, 50.4°, 60.2°, 63.5°, and 74.2° [18,19]. The minor peaks at 2h=28.2° respect the m-ZrO2 as the secondary phase [20,21]. The X-ray diffraction patterns of three finished surface keep consistent with sintered surface except that the diffraction peak intensity of m-ZrO2 decreased (indicated by arrows in Fig. 6). Particularly, the X-ray diffraction patterns of diamond finished surface are consistent with standard tetragonal zirconia (PDF#48-0224), indicating that the pure t-ZrO2 was obtained [22]. It also means that the phase of m-ZrO2 is vanished after finishing by diamond. However, the changing intensity of m-ZrO2 of three finished surfaces do not imply that the MRF process will cause the phase transformation. Firstly, the mechanical finishing process will not cause the phase transformation, as widely reported in the literatures [23–26]. Additionally, the XRD confirmed that there is no new phase appeared, thus the chemical reaction is not the reason of m-ZrO2 phase decreased or vanished on finished surfaces. It is more reasonable to say that the m-ZrO2 is mainly formed and concentrated in the few outmost grain layers during sintering process. According to the aforementioned study, the abrasives of CeO2 and Al2O3 are insufficient to eliminate the rough structures in the outmost grain layers of sintered surface. Therefore, the diffraction peak intensity of m-ZrO2 is decreased but not vanished after finished by CeO2 and Al2O3, which agree well with the SEM images in Fig. 7(c) and (d) shown that the residual valley-peak structures are remained. And the diffraction peak intensity of mZrO2 disappeared because the sintered rough structures were fully removed by diamond abrasive [Fig. 7(b)]. As a result, the m-ZrO2 on the finished surface will be declined or vanished because it mainly focuses on the outmost layer of the sintered surface. 3.3. SEM morphologies To better understand the removal mechanism of MRF, SEM observation was performed on the sintered and finished surface. As shown in Fig. 7, each sample was characterized by a low and high range, which the high range is given to show the specific morphology details of the area in blue rectangle. Fig. 7(a) showed that the sintered surface consists of many grains with different sizes and shapes, and which formed a large amount of micro/nano peak-valley structures. The mutual stacking and interleaving behaves of these grains explain the reason why the initial sintered surface is rough. As clearly shown in the high
magnification images in the top right corner in Fig. 7(a), the mutual stacking behaves of the outmost layer grains also lead to form many apertures. As a result, the outmost layer grains are very easy to remove out during the MRF process, which has been verified by our experimental results showing in Figs. 3–5. Fig. 7(b)–(d) shows the surface morphology after finishing for 2 h. As shown in Fig. 6(b), the rough peak-valley structures are eliminated during the finishing process by using diamond as abrasive, the porosities induced by sintering process are random distributed on the finished surface. The higher magnification image reveals slight scratching-induced traces by the diamond particles. Surface defects such as cracks and brittle fractures cannot be detected on the finished surface, indicating that the ductile modes are the dominant material removal mechanism for the MRF process of zirconia ceramic. In the contrary, most of peak structures are removed but the valley structures are still remained when using the CeO2 and Al2O3 as abrasive, as shown in Fig. 6(c) and (d). In addition, the higher magnification in the top right corner shows the details of the rough area containing valleys on the finished surface. Surface defects such as scratch and fracture are distributed in these structures. It should be noted that the fracture was not introduced during MRF process but caused by the sintering process. Since the pressure and temperature during the sintering process are extremely high, it is very easy to induce defect on the sintered surface, particularly in the few outmost grain layers [27]. Moreover, t-m phase transformation often occurs during the sintering process (from high temperature to room temperature cooling process), inducing volume change of grain crystal will eventually lead to cracks and even fragmentation in the sintered surface [28]. According to the experiment results, the MRR of CeO2 is about 2 mg/h while 0.79 mg/h for Al2O3. Therefore, the finishing time (2 h) is insufficient to remove out the rough structures of sintered surface. Thus, the valleys are remained and some fractures are exposed after peaks have been removed out. These SEM images indicate that the ineffectiveness of used abrasives (CeO2 and Al2O3) to remove the rough structures and surface/sub-surface defects produced in the sintered surface. According to the above analyses, it is not difficult to understand why the experimental results shown in Fig. 3 are obtained. The surface roughness was dropping very fast in the first 10 min finishing time by diamond and 30 min by CeO2 and Al2O3, then reducing very slow with further finishing. Firstly, the XRD analyses show that experiment used flat zirconia ceramics is pure t-ZrO2 which contains m-ZrO2 in the outmost layers. It means that the outmost layers are easier to remove out by abrasives since the m-ZrO2 has lower strength and fracture toughness than t-ZrO2 [29,30]. In addition, the SEM images show that the outmost layers are stacked with each other to form lots of gaps. Therefore, both the m-ZrO2 and scattered grain structure are contributing to the high efficient removal in the beginning MRF process. As finishing process is going on, those interlaced grain layers mix with m-ZrO2 and t-ZrO2 are removed and the inner grain layers with pure t-ZrO2 of good crystallinity and high densification are exposed. Due to the exposed crystal layer has a higher strength, the removal effect of abrasive particles on the crystal material will be weakened. Thus, the surface roughness has limit decline with the further finishing.
3.4. Parametric experiment The parameters of working gap, workpiece speed and trough speed are the domain factors affecting the material removal rate and surface quality in MRF process [12,13]. Nevertheless, the different abrasive experimental results show that diamond grits not only have the highest removal efficiency, but also have the best surface quality. Therefore, the diamond abrasive was chosen to
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Fig. 7. SEM images of different samples: (a) sintered; finishing by diamond (b), CeO2 (c) and Al2O3 (d). (The finishing time of b–d samples are set as 2 h).
Fig. 8. Effects of (a) work gap, (b) workpiece speed, (c) trough speed on MRR; effects of (d) work gap, (e) workpiece speed, (f) trough speed on the Ra of finished zirconia ceramics surface. The used finishing conditions are shown in Table 3.
carry out the parametric research. Fig. 8 shows the effect of those parameters mentioned above on MRR and Ra. Fig. 8(a) shows the effect of work gap on the MRR. It is seen that the MRR decreases with the increasing in work gap. And the highest MRR of 0.77 mg/min was obtained when the work gap is set to 1 mm. Since the magnetic excitation device is fixed on the worktable of the finishing device, changing the work gap will vary yield strength and working area of MR ribbon. An increase in work gap leads to the decrease in magnetic flied, thus decreasing in yield strength of MR ribbon. As a result, the shearing action between the MR ribbon and the workpiece surface will be weakened. Furthermore, when the fluid ribbon flows through the work gap, it will be compressed by the workpiece surface which results in the expansion of useful working area of MR ribbon. The decreasing in work gap will lead to the increase of the MR ribbon’s expansion,
thus increasing of the useful working area during the MRF process. The work gap affects the yield strength and working area of MR ribbon simultaneously, resulting in the significant changing on MRR. The effect of work gap on Ra is as shown in Fig. 8(d). The surface roughness remains stable around at 0.8 nm firstly and then increase from 0.8 to 1.26 nm when the work gap increases from 1.4 to 1.8 mm. As stated in the previous analysis, a bigger of the work gap would lead to the smaller yield strength and working area of fluid ribbon, thus the smaller of the squeeze force during the MRF process. As a result, the material’s shear removal by diamond grain will be weakened, therefore the surface roughness value increases when the finishing time remain unchanged. As shown in Fig. 8(b), the MRR increases from 0.6 to 1.03 mg/ min with nw increase from 100 to 300 r/min. The relative speed between the workpiece and the diamond grain is the domain
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reason of the material removal in a MRF process. Therefore, the greater nw will lead to a higher material removal rate. It should be noted that the MRR increases sharply when the nw smaller than 200 r/min and slows down with the nw further increasing to 300r/ min. This is because the high rotation movement of workpiece will destroy the homogeneity of MR ribbon, thus the material removal becomes instable. The effect of nw on Ra is shown in Fig. 8(e). It is seen that the nw has limit effect on Ra. The value of Ra increases slightly from 0.83 to 0.95 nm when the nw increases from 100 to 300 r/min. Since the high rotation movement of workpiece will cause the non-uniform material removal in MRF process. So the bigger Ra is obtained with higher nw. Fig. 8(c) shows the effect of nT on MRR. Similar to that of nw, the MRR increases from 0.3 to 0.8 mg/min with the nT from 15 to 35 r/ min. As mentioned before, the increasing in nT will certainly lead to enlarge the relative speed between MR ribbon and workpiece, thus a bigger MRR is obtained. The effect of nT on the Ra appears different from that of nw and dw [Fig. 8(f)]. The Ra declines from 1.38 to 0.84 nm when the nT increases from 15 to 25 r/min, then enlarges slightly to 0.93 nm with the nT constant increase to 35 r/min. The relative speed between the workpiece and the MR ribbon increases with the increasing nT explains the reason why the Ra declines in the beginning. However, if the nT was too great, a considerable amount of the MR fluid including diamond grains would be pushed by the significant centrifugal force to the outer periphery of the trough. As a consequence, the diamond grains decrease when fluid ribbon passing through the work gap, resulting in a slightly increase in Ra. 4. Conclusions In this work, a high efficiency and high surface quality MRF process based on permanent magnetic yoke excitation for zirconia ceramics finishing was developed. Through the three kind abrasives finishing performance experimental investigation, the highest material removal rate and best surface quality can be obtained by using diamond as abrasive during MRF process. Moreover, the XRD results show that there is no chemical removal occurs during the shear action between the MR ribbon and zirconia ceramics, which indicates that the material removal only caused by mechanical scratching of abrasive particles. Then, the SEM morphologies of finished surface state clearly that the ductile modes are the dominant material removal mechanism for the MRF process of zirconia ceramic. The parametric finishing experiments showed that the material removal rate increases with the increase in workpiece rotation speed and trough speed, but the decrease in work gap. In addition, the surface roughness of finished zirconia ceramic surface increases with work gap and workpiece rotation speed, but decline with the increase in trough speed. In summary, the finishing experiments demonstrated that the zirconia ceramic surface could be improved to atomic-scale level in Ra 0.702 nm after 30 min of finishing in the condition of dw = 1.4 mm, nT = 25 r/min and nw = 100 r/min. The result confirmed that the MRF process using the diamond as the abrasive particle has great potential to fabricate the ultra-smooth zirconia ceramic surface without damage at relatively high efficiency. Acknowledgments This work is financially supported by the National Natural Science Foundation of China [Grant No. 51675171]. And the Science and Technology Planning Project of Hunan Province [No. 2016TP1008].
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