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ScienceDirect Procedia CIRP 62 (2017) 293 – 298
10th CIRP Conference on Intelligent Computation in Manufacturing Engineering - CIRP ICME '16
Measurements of surface texture parameters after ultrasonic assisted and conventional grinding of ZrO2 based ceramic material characterized by different states of sintering Roman Wdowika*, Janusz Porzyckia, Marek Magdziaka a
Rzeszów University of Technology, Department of Manufacturing Techniques and Automation, al. Powstańców Warszawy 12, 35-959 Rzeszów, Poland
* Corresponding author. Tel.: +48 17 865 11 32; fax: +48 17 854 25 95; E-mail address:
[email protected]
Abstract The article presents the results of measurements of surface texture parameters after ultrasonic assisted grinding (UAG) and conventional grinding (CG) of ZrO2 based ceramic material. The machined samples are characterized by different states of sintering. Ultrasonic 20 linear machine tool and two different diamond grinding pins were applied to machine ceramic samples. Surface roughness measurements were performed and the values of parameters were computed using Infinite Focus Real 3D microscope (optical focus-variation technique) and 3D scanner Talyscan (contact measurement). The results of measurements were analyzed in order to state the influence of machining conditions on surface texture parameters. © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2017 2016The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the International Scientific Committee of “10th CIRP ICME Conference". Peer-review under responsibility of the scientific committee of the 10th CIRP Conference on Intelligent Computation in Manufacturing Engineering Keywords: grinding, ultrasonic, roughness, ceramic
1. Introduction Ultrasonic assisted grinding (UAG) is a hybrid machining process. There are different variants of ultrasonic assisted grinding. Usually, ultrasonics are applied by the use of vibrations of a grinding pin (a tool) or a workpiece. It is also possible to use ultrasonic vibrations of a tool and a workpiece at the same time. There are lots of scientific investigations which prove the usefulness of ultrasonic assisted grinding of hard and brittle materials (e.g. ceramics) [4, 7, 13 – 15, 17] and ductile materials (e.g. steel [12], nickel based superalloys [1], titanium alloys [8]). Usually, the influence of ultrasonic vibrations on selected quality indicators such as process forces, surface roughness, tool wear and accuracy of parts is investigated by researchers. The quality of surface plays the important role among quality indicators. Denkena et al. [2] stated that, after micromachining of ceramic samples with ultrasonic assistance, values of Ra can be obtained between 20 and 30 nm. Liang et al. [6] stated that there are two components of vibrations in elliptical ultrasonic assisted grinding – radial and axial. They also conclude that axial vibrations have the
positive impact on surface roughness, while radial vibrations lead to lower process forces. The similar investigations were performed by Yanyan et al. [16]. They describe the results of ultrasonic assisted grinding of ZrO2 based ceramic material. The results of experiments, regarding surface roughness, indicate the better surface quality after ultrasonic assisted grinding. Ra parameter was measured in these experiments [16]. The significant investigations were also carried out by the performers of CORNET project which concerned ultrasonic assisted grinding of different hard and brittle materials. Their work is presented in report [13]. In the case of ZrO2 based ceramic material, the authors of report stated the influence of feed rate and mounting torque of a tool on surface roughness of machined slots. The similar results to the report are also presented in papers [4, 5, 14] of the contractors of this CORNET project. The tool wear and the method of cooling also play the important role in shaping of the surface roughness [5, 9]. Gao et al. [3] stated that the critical depth of cut, when the ductile material removal mechanism is the dominant one, was increased in ultrasonic assisted grinding of nano-ZrO2, comparing to the
2212-8271 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 10th CIRP Conference on Intelligent Computation in Manufacturing Engineering doi:10.1016/j.procir.2016.06.049
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conventional grinding. In paper [18] the influence of depth of cut, feed rate, and the cutting speed was investigated in the case of Al2O3 based ceramic material. This article presents the investigations of ultrasonic assisted grinding process with vibrations of a tool. The influence of machining conditions on surface texture parameters was investigated. 2. The need for measurements of surface texture
feed. Figure 3 illustrates the differences in Ra values after ultrasonic assisted grinding and conventional grinding. It can be, inter alia, stated that the values of Ra for the measurement performed perpendicularly to the direction of feed compared to the measurements parallel to the direction of feed are higher. It is a typical observation for conventional grinding but ultrasonic assistance, for specific machining conditions, did not change the results of this observation [15].
There are several reasons for the measurements and analysis of surface texture parameters after ultrasonic assisted grinding process. The first of these reasons is a lack of sufficient data, regarding surface texture parameters measurements after UAG in existing publications. The results presented in the introduction and the results of other work are usually based on the measurements of parameters of a profile (e.g. Ra). The next reason is associated with the results of microscopic investigations of different surfaces machined in ultrasonic assisted and conventional grinding. Ultrasonic assistance leads to the visible change of surface texture but material properties have an influence on the material removal mechanisms. The investigations, regarding alumina and zirconia ceramics, performed at Rzeszów University of Technology indicate the differences in material removal mechanisms for these two materials. Grinding of alumina based ceramic material reveals rather brittle fracture while grinding of zirconia based ceramic material leads to clearly visible plastic deformations [15]. Grinding of Al2O3 based ceramic material leads also to the removal of directional traces of grains after UAG (Fig. 1). Because of more plastic deformations, the differences between the surfaces machined in conventional and ultrasonic assisted grinding are visible in microscopic images in the case of zirconia based ceramic material (Fig. 2).
Fig. 2. Comparison of machined surfaces of ZrO2 based ceramic material (CG – conventional grinding, UAG – ultrasonic assisted grinding)
Fig. 1. Example of machined surfaces of Al2O3 based ceramic material (CG – conventional grinding, UAG – ultrasonic assisted grinding) [15]
The presented results are identical to these which were obtained by other researchers and are presented in selected publications, e.g. [4, 7]. This work is also motivated by the previous research. Investigations in the area of surface roughness were focused on the measurements of parameters of profile (Ra, Rz, Rt) in two directions - parallel and perpendicularly to the direction of feed [15]. The machining process was performed with the use of the lateral surface of grinding pins. The results of investigations usually illustrate the worse surface roughness in the case of hybrid machining of ZrO2 based ceramic material – mainly for the measurements in the direction of
Fig. 3. Comparison of the results of measurements of Ra parameter [15]
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In the case of ZrO2 based ceramic material and grinding pin (D46) the values of Ra are higher for the measurement performed after ultrasonic assisted grinding. More detailed results are presented in Fig. 4. The examples given above were the motivation for searching different methods of description of surface texture. Surface texture parameters (e.g. Sa, Sdr, etc) are used to describe the complexity of surface [10] and they are chosen for these investigations.
in the case of the microscope and the 3D scanner. The results of measurement (values of parameters) were calculated using the software of applied measuring devices. The detailed description of input machining parameters is presented in Table 1. Table 1. Machining parameters and other data Diamond Schott metal bonded grinding pins Φ6 (diameter): grinding wheels D46, D91 (diamond grain sizes) Feed rate, vf
400 mm/min
Spindle speed, n
1000 rev/min, 4000 rev/min, 7000 rev/min, 10000 rev/min
Examples of material properties of ceramic samples
Other parameters
Fig. 4. Comparison of Ra values measured in the direction of feed [15]
3. Experimental setup Experiments were performed using Ultrasonic 20 linear machine tool from Sauer company. There were prepared 2 ceramic samples made of ZrO2 based ceramic material in two different sintering states. The samples were machined with the use of diamond grinding pins based on the strategy presented in Fig. 5.
Fig. 5. Diagram presenting the machining strategy
Measurements of surface roughness parameters were performed using two devices – Infinite Focus Real 3D Microscope from Alicona company and 3D scanner Talyscan 150 from Taylor Hobson. Infinite Focus Real 3D Microscope allows to perform non-contact measurements based on the focus-variation technique. Scanner Talyscan was used for contact measurements. There were performed the measurements for two samples (characterized by different sintering states and material properties), two grinding pins (characterized by different grain sizes) and different spindle speeds. Other machining parameters were not being changed. The important conditions of measurement (measured area, cut-off ratio) were the same
ZrO2 based ceramic material in two sintering states State 1 (softer) Hardness HV: ≈3,8 GPa Water absorption: 5,84 % Porosity: 26,22 % Density : 4,49 g/cm3
State 2 (harder) Hardness HV: ≈12,2 GPa Water absorption: 0 % Porosity: 0,01 % Density: 6,07 g/cm3
ae = 0,008 mm (depth of cut in radial direction of a tool), ap = 4 mm (depth of cut in axial direction of a tool), T = 15 Nm (mounting torque), fUS = 26 500 Hz (frequency of vibrations)
4. Results of experiments Figures 6 and 7 present the results of measurements of Sa parameter. Two replications of machining and measurement were performed for the specific machining conditions (e.g. UAG1 is the first replication, UAG2 is the second one). The results of these measurements illustrate the influence of grain size on the values of Sa parameter. The values of parameter in the case of grain D91 are usually higher than the values after grinding with the use of D46 grain size. The influence of ultrasonic assistance is not clear but it can be assumed that the influence of grain size, for tested grain sizes, is more significant that the influence of ultrasonic vibrations. This phenomenon can be explained by the relatively big size of diamond grains which have the great impact on surface texture parameters. It can be also stated that it is better to use D46 grain size than D91 to achieve smaller values of Sa parameter even if the ultrasonic generator is switched on. It is also difficult to specify the influence of material properties on Sa parameter values. State 1 refers to the pre-sintered ceramic material (softer) while State 2 refers the ZrO2 in the final fired state (harder). Figure 8 and 9 illustrate the values of Sdr parameter (hybrid surface texture parameter) measured using two aforementioned devices. The shape of the plot for the measurements of Sdr performed on the scanner Talyscan 150 is similar to plots illustrating Sa values. The influence of grain size is visible. In the case of measurement of Sdr performed on Infinite Focus Real 3D microscope it is difficult to notice the character of the plot similar to the Sa. Authors computed also the values of other surface texture parameters. The examples are Sp, Sq, Sv, St, etc. The character of plots for all these parameters are similar in the case of scanner Talyscan 150 and reveal the influence of grain size.
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Fig. 6. Results of measurements of Sa parameter using Infinite Focus Real 3D microscope (for different material states, diamond grain sizes (D), spindle speeds (n), CG and UAG)
Fig. 7. Results of measurements of Sa parameter using scanner Talyscan 150 (for different material states, diamond grain sizes (D), spindle speeds (n), CG and UAG)
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Fig. 8. Results of measurements of Sdr parameter using Infinite Focus Real 3D microscope (for different material states, diamond grain sizes (D), spindle speeds (n), CG and UAG)
Fig. 9. Results of measurements of Sdr parameter using scanner Talyscan 150 (for different material states, grain sizes (D), spindle speeds (n), CG and UAG)
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5. Conclusion Surface texture parameters were measured with the use of Infinite Focus Real 3D microscope (Alicona) and 3D scanner Talyscan 150 (Taylor Hobson). Values of selected parameters were computed with the use of software of microscope and 3D scanner after data processing. The measurements were performed for two different grinding wheels characterized by different diamond grain size and zirconia based ceramic material in two different states of sintering. The investigations had a character of initial experiments in the area of surface texture parameters after ultrasonic assisted grinding process (UAG) and conventional grinding (CG). Specific machining conditions were chosen for the experiments. The microscopic images indicate the significant differences in the texture of surface after conventional and hybrid process. Ultrasonic vibrations are clearly visible as the traces of grains similar to the shape of sine function because of plastic deformations. The performed investigations have not revealed the significant differences in the values of surface texture parameters between conventional and ultrasonic assisted grinding but the more clear influence of grain size is reported. Further investigations will be performed for specific machining parameters and many replications of measurement to indicate in details the influence of ultrasonic assistance for specific machining conditions. Also the smaller grain sizes which are available from the producers of grinding pins for Ultrasonic 20 linear machine tool (up to D25) can be applied during future research in order to investigate the influence of machining parameters on surface texture. Acknowledgements The investigations were performed within the Applied Research Programme and the project “Technology of high performance machining with ultrasonic assistance of geometrically complex ceramic parts” (PBS2/B6/17/2013). Project is funded by the National Centre for Research and Development (www.ncbr.gov.pl). Authors would also like to thank Jacek Misiura, MSc from the Department of Manufacturing Techniques and Automation/Rzeszów University of Technology for his help during measurements with the use of microscope and 3D scanner. References [1] Bhaduri D, Soo SL, Aspinwall DK, Novovic D, Harden P, Bohr S, Martin D. A study on ultrasonic assisted creep feed grinding of nickel based superalloys. Procedia CIRP, 1, 2012, 359-364, doi: 10.1016/j.procir.2012.04.064. [2] Denkena B, Hoffmeister HW, Reichstein M, Illenseer S, Hlavac M. Micro–machining processes for microsystem technology. Microsyst Technol, 12, 2006, 659-664, doi: 10.1007/s00542-006-0089-z. [3] Gao GF, Zhao B, Xiang DH, Kong QH. Research on the surface characteristics in ultrasonic grinding nano-zirconia ceramics. Journal of Materials Processing Technology, 209, 2009, 32-37. [4] Lauwers B, Bleicher F, Ten Haaf P, Vanparys M, Bernreiter J, Jacobs T,
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