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Effects of laser-assisted grinding on surface integrity of zirconia ceramic
Ceramics International 46 (2020) 921–929
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of laser-assisted grinding on surface integrity of zirconia ceramic Zhelun Ma a b c
a,b,∗
, Zhao Wang
a,b
c
, Xuezhi Wang , Tianbiao Yu
T
a,b,∗∗
School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China Liaoning Provincial Key Laboratory of High-end Equipment Intelligent Design and Manufacturing Technology, Northeastern University, PR China School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-assisted grinding Zirconia ceramic Surface quality Surface morphology Subsurface damage
Zirconia ceramics has an important position in the field of biomaterials because of excellent mechanical properties. Because of abrasive resistance and hardness, conventional machining of zirconia ceramic is difficult and time-consuming. A nontraditional hybrid laser-assisted grinding (LAG) system combining laser and cubic boron nitride (CBN) grinding wheel was developed for machining zirconia ceramic. This hybrid system utilizes the outstanding thermostability of zirconia ceramic: The material present on the cutting path is rapidly removed by local laser heating and grinding wheel. A combined theoretical and experimental study was performed to evaluate different modes of machining of zirconia ceramic. The machining parameters were predicted by numerical analysis. The surface quality, surface morphology, and subsurface damage of zirconia ceramic specimens were analyzed and compared. The results show that LAG can easily achieve ductile regime grinding in the same machine tool. Compared with conventional grinding, the surface integrity of zirconia ceramic was significantly improved by the LAG process. LAG could achieve ductile regime grinding with a large depth-of-cut. It changes the machinability of zirconia ceramic.
1. Introduction Zirconia ceramic usually consists of ZrO2 and 3 mol% Y2O3. Zirconia ceramic has many practically useful physical properties such as a low thermal conductivity and high fracture toughness [1]. Zirconia ceramic is used in all-ceramic restoration of prosthodontics and shows favorable biocompatibility, chemical stability, flexural strength, and color. Zirconia ceramic has been used in thermal insulation coatings and void media in combustion because of extremely low thermal conductivity. Zirconia is one of the strongest and toughest structural ceramics [2,3]. Therefore, zirconia ceramics have attracted much interest. The majority of zirconia ceramic products which require machining after sintering are grinded. Grinding is often used for the finishing and rough machining of structural ceramics because no other suitable method for rough machining has been proven and accepted by industry [4–6]. However, zirconia ceramic is a hard-to-brittle material owing to the localized covalent ionic bonding and well-organized crystal structure. Machining of zirconia ceramic is difficult and expensive by grinding processes [7]. A large number of surface/subsurface damage is generated in conventional grinding (CG) due to these characteristics
[8,9]. Therefore, the removal mechanism of zirconia ceramic has been extensively studied. Ductile regime grinding has been developed to obtain a better machined surface integrity and tight dimensional tolerance. Ductile regime grinding of difficult-to-machine materials can produce parts with minimum surface damage, leading to a higher strength [10]. Ductile regime grinding is achieved with a depth-of-cut of the order of nanometers that yields a high-strength part but requires a long machining time. Although ductile regime grinding has provided some interesting results, it is difficult to apply in industry due to poor efficiency. In recent years, other processing methods have been combined with CG to solve the abovementioned problems such as ultrasonic-assisted grinding and milling-grinding methods. Xiao et al. [11] developed a grinding force model for ultrasonic-assisted grinding zirconia ceramics to better understand the mechanism of plastic removal and brittle removal on cutting force during ultrasonic-assisted grinding. Liang et al. [12] reported that the critical depth-of-cut of nanocrystal sapphire in ultrasonic-assisted grinding is much larger than that in CG. Chen et al. [13] improved the grinding-milling force and surface roughness by using a hybrid milling-grinding system with proper feedback to control
∗ Corresponding author. School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China. ∗∗ Corresponding author. School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China. E-mail addresses: [email protected] (Z. Ma), [email protected] (Z. Wang), [email protected] (X. Wang), [email protected] (T. Yu).
https://doi.org/10.1016/j.ceramint.2019.09.051 Received 20 May 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 06 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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the machining feed rate. However, hybrid machining involves the control of undeformed chip thickness with abrasive grains during processing by changing the motion and trajectory of abrasive particles. They are not significantly different from CG. Laser-assisted grinding (LAG) is a hybrid machining method: The workpiece material is locally heated and softened using a focused laser beam prior to material removal with a traditional grinding wheel. In contrast with previously proposed combined machining method, LAG can reduce the contact stiffness and impact toughness, thus softening the material. LAG allows the abrasive cutting edge to more easily penetrate the material during grinding, reduce the grinding energy, and improve the removal of processed materials and processing efficiency. The LAG operation is carried out under dry condition without a coolant, making the process more environmentally friendly and reducing the cost. Several studies have focused on laser-assisted machining (LAM) to better understand the processing mechanism. Lee et al. [14] proposed a laser-assisted fillet milling method: The position of laser heat source was controlled using addition axes. Shelton et al. [15] conducted laserassisted micromilling experiments on Ti6–Al–4V and Inconel718 alloy materials. The experimental results show that laser-assisted micromilling resulted in different degrees of improvement in terms of surface quality, tool wear, and edge burrs. Garcí et al. [16] found that LAM can improve the machinability of Inconel 718 because the required cutting force and surface roughness are reduced. Chang et al. [17] found that rotational speed is the most important factor on LAM system performance. The impact ratio is as high as 42.68%, followed by feed, depthof-cut, and pulse frequency. Chang et al. [18] also found that LAM can reduce the required cutting force by 20% (thrust force) and 22% (feed force), producing a workpiece with better surface integrity than conventional method. Ding et al. [19] reported that LAM can improve the machinability of Waspaloy. The specific cutting energy, surface roughness, and ceramic tool life clearly improved during LAM. Ding et al. [20] also found that the axial residual stress of compression surface generated by LAM is ~150 MPa larger than that of hard turning, which reduces the variation in hoop stress. Anderson et al. [21] reported that tool life can be increased by 200–300% in LAM compared to conventional machining by using ceramic inserts. They also reported chip curvature formation and color changes in chips during the LAM of Inconel 718. In this study, cutting and laser parameters for the hybrid LAG cutting of zirconia ceramic are predicted. Using these parameters, experiments were performed. The surface roughness, surface morphology, and subsurface damage were analyzed and compared between CG and LAG. The results provide an experimental reference for the LAM process, and the machinability of difficult-to-machine materials such as zirconia ceramic was also improved.
Fig. 1. Schematic diagram of ductile–brittle transition depth.
t T T0 v xa yb ρ δ
2.1. Prediction of cutting parameters As shown in Fig. 1, Bifano et al. [22] proposed an empirical formula for the depth of brittle plastic transition of hard-to-brittle materials. The ultimate depth of plastic machining is shown in Eq. (1). 2
E K dc = 0.15 ⎛ ⎞ ⎛ c ⎞ ⎝ Hv ⎠ ⎝ Hv ⎠ ⎜
P q r
⎟⎜
⎟
(1)
where E is the elastic modulus, Hv is the hardness, and Kc is the fracture toughness. According to the characteristics of structural ceramics, the elastic modulus, hardness, and fracture toughness are varied with temperature. The temperature-dependent behavior of zirconia ceramics has been studied. The detailed trend is shown in Fig. 2. By substituting Fig. 2 (a)(b)(c) into Eq. (1), the trend of brittle plastic transition depth was obtained. As shown in Fig. 2 (d), the depth of brittle plastic transition of zirconia ceramics is ~890 nm at room temperature. As the temperature increased, the depth morphology increased. When the temperature reached 1000 °C, the depth of brittle plastic transition exceeded by 18 μm. In CG, because of a small depth of brittle plastic transformation, it is difficult to achieve ductile regime grinding for zirconia ceramics at room temperature. Ductile regime grinding has not only poor efficiency, but also requires a very rigid grinding machine with a high horsepower spindle to overcome the larger grinding forces with nanoscale accuracy. However, during LAG, the workpiece is heated locally using a laser beam; the surface temperature increases rapidly; the elastic modulus, hardness, and fracture toughness vary with the increase in temperature. When the temperature of zirconia ceramics reaches 1000 °C, the depth-of-cut rapidly increases during grinding; thus, ductile regime grinding can be easily achieved. In LAG, a point on the cutting path is first heated with the laser beam and then removed by the grinding wheel. Therefore, Laser irradiation also causes local damage, leading to crack nucleation and expansion. Laser-induced damage can cause two types of cracks [1]: channeling crack along the surface and edge crack towards the center (as shown in Fig. 3). At the beginning of LAG, the boundary of specimen is subjected to laser radiation, causing damage. Because channeling crack has the same direction as grinding path, the crack would be
2. Theoretical analysis a as cp dc E G H Hv k Kc Lx , Ly n,m
Time (s) Temperature (K) the initial temperature (K) the workpiece feed (m/s) the semi-width of heating region along the x-direction (m) the length of heating region along the y-direction (m) the density of material (Kg/m3) the Dirac delta function
depth-of-cut (m) absorptivity the specific heat capacity (J/kg K) ultimate depth of plastic machining (m) elastic modulus (GPa) Green's function the Heaviside function Hardness (GPa) the thermal conductivity of material (W/mK) fracture toughness (MPa/m1/2) the length and width of material (m) the index number for Eigen values along the x and y directions the laser power (W) the heating source term (W/m2) the laser spot size (m) 922
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Fig. 2. Temperature-dependent behavior of zirconia ceramics: (a) elastic modulus [23], (b) hardness [24], (c) fracture toughness [25], and (d) plastic transition depth.
in Eq. (2):
q (x , y, t ) ∂T ∂ 2T ∂ 2T ⎞ = a2 ⎛ 2 + + 2 dt ∂ x ∂ y k ⎝ ⎠ ⎜
⎟
(2a)
and
a2 =
cp ρ (2b)
k
where T is the temperature, cp is the specific heat capacity, ρ is the density of material, k is the thermal conductivity of material, t is the time, and q is the heating source term. q can be expressed as Eq. (3):
q (x , y, t ) =
as P H (r − x )[H (vt − y ) − H (vt − y − 2r )] πr 2
(3)
where H is the Heaviside function, v is the workpiece feed, r is the laser spot size, as is the absorptivity, and P is the laser power. The heating source term shows the heat input due to laser irradiation. The integration of differential equations was simplified using the Heaviside function (H). To simplify the calculation, the heat affected zone of laser was treated as a square area with a side length of 2r. The absorptivity as was calculated by considering various losses, i.e., losses due to laser reflection and transmission caused by materials (~5% loss) [1], loss in laser optics (~5% loss) [26], and loss due to the heat emission of material (~4% loss). Therefore, only ~86% of the laser incident energy was absorbed in the zirconia ceramic specimen.
Fig. 3. Schematic of LAG system.
removed during grinding; the depth of plain-strain crack reaches almost the cut depth during LAG. 2.2. Prediction of laser parameters Temperature distributions of different locations of zirconia ceramics in LAM were calculated using Fourier heat conduction equation shown 923
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Laser heating and grinding occur on the top surface, and minimal heat transfer occurs at the bottom of specimen. Eq. (4) shows the boundary conditions of the whole process:
k
∂T
∂T ∂T =k =k =0 ∂yy = yb ∂yy = 0
∂x x =±x a
Table 2 Details of grinding wheel used in this experiment.
(4)
where xa is the semiwidth of heating region along the x-direction; yb is the length of heating region along the y-direction.
Parameter
Value
Size (mm) Abrasive type Abrasive size (#) Hardness Bond type CBN concentration (%)
Φ40 × 10 × 10 CBN 120 N vitrified 150
2.3. The initial condition is
T (x , y, t = 0) = T0
conducting the experiments, the zirconia ceramic surfaces were polished with silicon carbide emery paper with #240, #400, #600, #800, #1000, #1600, and #2000 grain sizes, followed by polishing with a cloth wheel to remove any residual scratch. Then, the zirconia ceramic was mounted on a jig. A hard rubber pad was used to avoid specimen breakage. A grinding wheel was used in all the experiments. A KUKA robot equipped with an YLR-500 W laser generator was used to irradiate the sample. This robot has six degrees of freedom to ensure an accurate laser position. The grinding was conducted along the length of specimen and extended over the entire width of specimen.
(5)
where T0 is the initial temperature. The transient temperature distribution was determined using Green's function approach:
T (x , y, t ) =
t
yb
xa
∫0 ∫0 ∫−x
q (ξ , η , τ ) G (x , y, t; ξ , η , τ ) dξdηdτ + T0
a
(6)
G (x , y, t; ξ , η , τ ) indicates that the field value of space (x, y) at time t is excited by the heat source q located space (ξ, η) at time τ. ∂G ∂ 2G ∂ 2G ⎞ − a2 ⎛ 2 + = δ (x − ξ ) δ (y − η) δ (t − τ ) dt ∂ x ∂y 2 ⎠ ⎝ ⎜
3.3. Experimental parameters
⎟
(7) A numerical analysis was conducted to predict the machining parameters. Based on equipment conditions and material properties, the analysis parameters were used to calculate the machining parameters such as workpiece feed, laser spot size, depth-of-cut, laser power, and distance between the laser beam and grinding wheel. The experiment was carried out with a laser spot size of 2 mm in diameter. The distance between laser focus and grinding wheel is 5 mm. The angle between the laser beam and zirconia ceramic surface is about 57.13. Table 3 shows the other processing parameters.
where δ is the Dirac delta function. the series representation of Green's function G is shown in Eq. (8):
G (x , y, t; ξ , η , τ ) =
2 Lx ×
∞
nπa
∑ e−⎛⎝ Lx ⎞⎠ (t−τ ) sin ⎛ nπ x ⎞ sin ⎛ nπ ξ ⎞ ⎜
2 Ly
⎟
⎝ Lx ⎠
n=1 ∞
∑e m=1
⎜
⎟
⎝ Lx ⎠
−⎛ mπa ⎞ (t − τ ) mπ ⎞ ⎛ mπ ⎞ sin ⎜⎛ y ⎟ sin ⎜ η⎟ ⎝ Ly ⎠ ⎝ Ly ⎠ ⎝ Ly ⎠ ⎜
⎟
(8)
where Lx and Ly are the length and width of material, respectively; n and m are the index number for Eigen values along the x and y directions, respectively.
3.4. Surface quality measurements The surface quality of ceramic is an important factor for practical ceramic applications. To obtain accurate roughness, the specimens were cleaned with ethyl alcohol and deionized water in an ultrasonic cleaner for about 15 min individually; at last, the specimens were dried in a drying baker at 50 °C for 3 h. The average roughness height (Rz) and average roughness profile (Ra) were measured using a 3D laser scanning confocal microscope (OLYMPUS LEXT OLS4100). Rz is the maximum peak-to-valley height. Ra is the average roughness and shows the degree of surface roughness in a simplified manner. A diagram of surface roughness is shown in Fig. 5. The assessment area of surface roughness measurement is 1 mm2. Two random specimens were selected, and ten surfaces of each specimen were measured for reducing the random errors.
3. Experimental 3.1. Materials Hot-pressed sintered zirconia ceramic was examined experimentally. The material properties are shown in Table 1. The specimens have dimensions of 50 × 50 × 1 mm3, and the grinding was conducted on a 50 × 50 mm2 surface. A vitrified bond cubic boron nitride (CBN) grinding wheel was used in this experiment. Before the experiment, the grinding wheel was trimmed using a diamond trimming pen to restore the shape and cutting efficiency of grinding wheel. Details of grinding wheel are shown in Table 2.
3.5. Subsurface damage measurement
3.2. Experimental setup and conditions
The measurement of subsurface damage (SSD) in this study is shown in Fig. 6. The measurement consists of four steps [27]: (1) A slit is introduced from the opposite side of grinded surface using a diamond stylus. (2) Half of the zirconia ceramic is fixed, and another half is impacted, so that the specimen can be divided into two parts and a cross-section can be observed. (3) The cross-section is cleaned with ethyl alcohol and deionized water in an ultrasonic cleaner for about 15 min individually. Then, the specimens are air-dried in a drying baker at 50 °C for 3 h (4) The SSD was observed using a scanning electron microscope (SEM, ULTRA PLUS).
The detailed experimental setup is shown in Fig. 4. Before Table 1 Material properties of ZrO2 workpiece used in the experiment. Parameter
Value
Material Density Hardness Elasticity modulus Thermal conductivity Crush strength Bending toughness Coefficient of thermal expansion
ZrO2 > 6 g cm−3 1500 HV 210–220 GPa 2.2–3 W·m−1 K−1 ≥120 Kgf (1.5 mm O.D.) 8–13 MPa m1/2 9.6–10.3 × 10−6 °C
4. Results and discussion During the grinding, the surface integrity of grinded workpiece is an 924
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Fig. 4. Experimental process.
important indicator used to evaluate the machining method. Therefore, this section describes the surface integrity of zirconia ceramic obtained after LAG. The results were compared with the results of zirconia ceramic surface after CG. The improvement in grinding performance owing to LAG was determined.
Table 3 Machining parameters.
CG LAG
Depth of cut ap (μm)
Wheel speed Vs (m/s)
Workpiece feed Vw (mm/s)
Laser power PL (W)
15 15
12 12
5 5
0 185
925
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Fig. 5. Schematic diagram of 2D surface roughness parameters.
Fig. 6. Illustration of the method for SSD measurement.
4.1. Surface quality Surface quality is an important parameter for evaluating the surface integrity of a machined surface. Table 4 shows the total surface roughness (Rz and Ra) of zirconia ceramics produced using CG and LAG. The surface roughness Rz of zirconia ceramics produced using CG reached 10.17 μm and 9.61 μm, whereas Ra reached 1.72 μm and 1.51 μm. However, the surface roughness Rz of zirconia ceramic produced using LAG reached 6.02 μm and 6.59 μm, whereas Ra reached 1.31 μm and 1.21 μm. A comparison of surface quality is shown in Fig. 7. The maximum improvements in surface roughness Rz and Ra are 40.8% and 29.7%, respectively, whereas the minimum improvement in surface roughness Rz and Ra are 31.4% and 13.2%, respectively. In summary, the surface quality was significantly improved when zirconia was processed using LAG. The poor surface quality is caused by brittle removal when most abrasives are in the chip-formation stage during grinding. Because of the improvement in surface quality, it can be concluded that most abrasives are in the scratching and plowing stage; therefore, plastic
Fig. 7. Surface roughness (Ra and Rz) of zirconia ceramics.
removal is the major removal mode. To study the removal mechanism of zirconia ceramic with different machining modes more extensively, the surface morphologies were analyzed and compared.
4.2. Surface morphology Fig. 8 shows the surface morphologies of zirconia ceramic subjected to different grinding methods. For CG (Fig. 8(a)(b)(e)), brittle fracture is the main removal mode for zirconia ceramic grinding. This is because the depth-of-cut is larger than the threshold. In the brittle material removal mode, the initial cracks generated inside the ceramic start to expand with changes in the expansion direction. Chips are formed when the crack spreads to the surface and inside of material simultaneously. The crack propagation eventually ends, leaving behind a residual crack.
Table 4 Surface roughness comparison of grinded surface of zirconia ceramic. Number
CG
LAG
Rz (μm)
1 2 3 4 5 6 7 8 9 10
Ra (μm)
Rz (μm)
Ra (μm)
Surface A
Surface B
Surface A
Surface B
Surface C
Surface D
Surface C
Surface D
10.229 10.346 10.284 9.997 10.095 9.958 10.151 10.179 10.07 10.426
9.374 9.593 9.481 9.545 9.594 9.655 9.783 9.68 9.776 9.587
1.762 1.686 1.738 1.724 1.776 1.75 1.825 1.773 1.757 1.762
1.483 1.508 1.502 1.512 1.51 1.518 1.524 1.516 1.536 1.528
6.401 6.731 6.67 6.477 6.717 6.876 6.518 6.302 6.286 6.908
5.763 5.781 5.812 5.921 6.16 6.161 6.19 6.044 6.303 6.023
1.309 1.313 1.292 1.302 1.298 1.311 1.33 1.324 1.309 1.298
1.227 1.226 1.208 1.219 1.19 1.196 1.226 1.195 1.205 1.183
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Fig. 8. Surface morphologies of grinded zirconia ceramic.
functional performance of zirconia ceramic after grinding. The subsurface damage morphologies of CG and LAG are shown in Fig. 9. In CG, because of the randomness of brittle fracture, the intersecting grinded surface is not smooth (Fig. 9 (a)). Some unformed chips are present, causing a poor subsurface. Because of the high tenacity of zirconia ceramic, it is difficult to produce cracks. Therefore, some lateral cracks did not extend to the grinding surface to form chips. This is the major cause for the poor surface integrity of machined surface during CG. In LAG, the grinded surface is regular and smooth (Fig. 9(b)). The plastic flow clearly exists. This phenomenon occurs on the side of grinding, and microcracks are rarely present on the subsurface of workpiece. These results show that LAG hardly cause subsurface damage to the workpiece by using reasonable processing parameters.
Fig. 8(a) and (b) show that in CG, successive crushing zones exist on the grinded surface, and a large number of pits appear on the surface, i.e., brittle fracture occurs during CG. Because of the randomness of brittle fractures, it is difficult to control the surface integrity, causing poor surface quality and less tight dimensional tolerance. Fig. 8(c)(d)(f) show that plastic removal is the main removal type of grinding zirconia ceramic during LAG. When the depth-of-cut is 15 μm, almost no removal occurs in the grinded surface. Therefore, the surface is smoother through LAG than CG. The depth of gully observed on the surface of workpiece obtained by LAG is obviously shallow. According to the ductile regime grinding theory [22], while the depth-of-cut is less than the threshold, zirconia ceramic undergoes a plastic material removal mode. In the plastic removal mode, brittle material is removed in the form of plastic deformation chips, which do not form cracks and underground damage.
5. Conclusions and suggestions 4.3. Subsurface damage 5.1. Conclusions Because a brittle crack zone on the subsurface of zirconia ceramic after grinding is obtained by removing the brittle material between abrasive workpieces, the subsurface in this region often have defect structures such as lateral cracks, radial cracks, or microcracks. Therefore, the size of brittle crack region in the subsurface after grinding also mainly determines the mechanical properties and
A hybrid LAG system was established. A combined theoretical and experimental study was performed to evaluate different modes in the machining of zirconia ceramic. The surface roughness, surface morphology, and subsurface damage of zirconia ceramic specimens were analyzed and compared. The results are as follows: 927
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Fig. 9. Sectional morphologies of zirconia ceramic subsurface.
roughness Rz and Ra reached 40.8% and 29.7%, respectively. (2) The surface morphologies of zirconia ceramic after CG and LAG were examined experimentally. In CG, the major removal mode of zirconia ceramic is brittle fracture. A large number of crashing zones was observed on the specimen surface. However, in LAG,
(1) LAG can improve the depth of brittle plastic transformation of zirconia ceramic; thus, ductile regime grinding can be easily achieved. The cutting and laser parameters were predicted by numeral calculations. In LAG, the surface quality of zirconia ceramic was significantly improved. The maximum improvement in surface 928
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plastic flow phenomenon was clearly observed. Thus, plastic removal is the main removal mode of zirconia ceramic. The experimental results are consistent with theoretical predictions. The machinability of zirconia ceramic was improved. (3) The subsurface of zirconia ceramic was observed to study crack propagation. In CG, the grinded surface is not smooth. Some lateral cracks were observed on the subsurface. However, in LAG, almost no cracks were observed, and the grinded surface is smooth. Because of plastic removal, zirconia ceramic piled up on the side of grinding grooves. This indicates that ductile regime grinding with a large depth-of-cut can be achieved with LAG. LAG can improve the grinding efficiency of zirconia ceramic while ensuring good surface integrity.
116279. [6] H.N. Li, Y. Yang, Y.J. Zhao, Z. Zhang, W. Zhu, W. Wang, H. Qi, On the periodicity of fixed-abrasive planetary lapping based on a generic model, J. Manuf. Process. 44 (2019) 271–287, https://doi.org/10.1016/J.JMAPRO.2019.05.036. [7] B. Denkena, A. Wippermann, S. Busemann, M. Kuntz, L. Gottwik, Comparison of residual strength behavior after indentation, scratching and grinding of zirconiabased ceramics for medical-technical applications, J. Eur. Ceram. Soc. 38 (2018) 1760–1768, https://doi.org/10.1016/j.jeurceramsoc.2017.11.042. [8] D. Jia, C. Li, Y. Zhang, M. Yang, X. Zhang, R. Li, H. Ji, Experimental evaluation of surface topographies of NMQL grinding ZrO2 ceramics combining multiangle ultrasonic vibration, Int. J. Adv. Manuf. Technol. 100 (2019) 457–473, https://doi. org/10.1007/s00170-018-2718-y. [9] S. Ji, L. Liu, J. Zhao, C. Sun, Finite element analysis and simulation about microgrinding of SiC, J. Nanomater. 2015 (2015), https://doi.org/10.1155/2015/ 575398. [10] H.N. Li, T.B. Yu, L. Da Zhu, W.S. Wang, Analytical modeling of ground surface topography in monocrystalline silicon grinding considering the ductile-regime effect, Arch. Civ. Mech. Eng. 17 (2017) 880–893, https://doi.org/10.1016/j.acme. 2017.03.010. [11] X. Xiao, K. Zheng, W. Liao, H. Meng, Study on cutting force model in ultrasonic vibration assisted side grinding of zirconia ceramics, Int. J. Mach. Tool Manuf. 104 (2016) 58–67, https://doi.org/10.1016/j.ijmachtools.2016.01.004. [12] Z. Liang, X. Wang, Y. Wu, L. Xie, L. Jiao, W. Zhao, Experimental study on brittle–ductile transition in elliptical ultrasonic assisted grinding (EUAG) of monocrystal sapphire using single diamond abrasive grain, Int. J. Mach. Tool Manuf. 71 (2013) 41–51, https://doi.org/10.1016/j.ijmachtools.2013.04.004. [13] S.-T. Chen, Z.-H. Jiang, A force controlled grinding-milling technique for quartzglass micromachining, J. Mater. Process. Technol. 216 (2015) 206–215, https://doi. org/10.1016/j.jmatprotec.2014.09.017. [14] W.-S. Woo, C.-M. Lee, A study on the optimum machining conditions and energy efficiency of a laser-assisted fillet milling, Int. J. Precis. Eng. Manuf. Technol. 5 (2018) 593–604, https://doi.org/10.1007/s40684-018-0061-2. [15] J.A. Shelton, Y.C. Shin, Comparative evaluation of laser-assisted micro-milling for AISI 316, AISI 422, TI-6AL-4V and Inconel 718 in a side-cutting configuration, J. Micromech. Microeng. 20 (2010) 75012. [16] V. Garcí, A. Navas, I. Arriola, O. Gonzalo, J. Leunda, Mechanisms involved in the improvement of Inconel 718 machinability by laser assisted machining (LAM), Int. J. Mach. Tool Manuf. 74 (2013) 19–28, https://doi.org/10.1016/j.ijmachtools. 2013.06.009. [17] C.-W. Chang, C.-P. Kuo, An investigation of laser-assisted machining of Al2O3 ceramics planing, Int. J. Mach. Tool Manuf. 47 (2007) 452–461, https://doi.org/10. 1016/J.IJMACHTOOLS.2006.06.010. [18] C.-W. Chang, C.-P. Kuo, Evaluation of surface roughness in laser-assisted machining of aluminum oxide ceramics with Taguchi method, Int. J. Mach. Tool Manuf. 47 (2007) 141–147, https://doi.org/10.1016/j.ijmachtools.2006.02.009. [19] H. Ding, Y.C. Shin, Improvement of machinability of Waspaloy via laser-assisted machining, Int. J. Adv. Manuf. Technol. 64 (2013) 475–486, https://doi.org/10. 1007/s00170-012-4012-8. [20] H. Ding, Y.C. Shin, Laser-assisted machining of hardened steel parts with surface integrity analysis, Int. J. Mach. Tool Manuf. 50 (2010) 106–114, https://doi.org/ 10.1016/j.ijmachtools.2009.09.001. [21] M. Anderson, R. Patwa, Y.C. Shin, Laser-assisted machining of Inconel 718 with an economic analysis, Int. J. Mach. Tool Manuf. 46 (2006) 1879–1891, https://doi. org/10.1016/j.ijmachtools.2005.11.005. [22] D.T.A. Bifano T G Scattergood R O, Ductile-regime grinding: a new Technology for machining brittle materials, J. Eng. Ind. 113 (1991) 6. [23] J.W. Adams, R. Ruh, K.S. Mazdiyasni, Young"s modulus, flexural strength, and fracture of yttria-stabilized zirconia versus temperature, J. Am. Ceram. Soc. 80 (1997) 6. [24] R.P. Ingel, D. Lewis, B.A. Bender, R.W. Rice, Temperature dependence of strength and fracture toughness of ZrO2 single crystals, J. Am. Ceram. Soc. 65 (1982) c150–c152. [25] V. Tikare, A.H. Heuer, Temperature-dependent indentation behavior of transformation-toughened zirconia-based ceramics, J. Am. Ceram. Soc. 74 (1991) 593–597. [26] D. Kalyanasundaram, G. Shehata, C. Neumann, P. Shrotriya, P. Molian, Design and validation of a hybrid laser/water-jet machining system for brittle materials, J. Laser Appl. 20 (2008) 127–134, https://doi.org/10.2351/1.2900554. [27] H.N. Li, T.B. Yu, L. Da Zhu, W.S. Wang, Evaluation of grinding-induced subsurface damage in optical glass BK7, J. Mater. Process. Technol. 229 (2016) 785–794, https://doi.org/10.1016/j.jmatprotec.2015.11.003.
5.2. Suggestions In the future, the mechanism of effect of laser on the grinding characteristics (grinding force, grinding temperature, surface integrity) need to be further studied. Materiallo-thermo-mechanical coupled models will be developed to help design the experimental conditions for post-processing and model the critical surface microstructure attributes such as grain size, phase constituents, microhardness, and residual stress. Uniquely graded microstructure types will be generated via LAM processed surfaces. The mechanical performance will be evaluated to test the ability of the predictive framework. Acknowledgment The authors thank the National Natural Science Foundation of China (No. U1508206), Science and Technology Planning Project of Shenyang (No. 18006001). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.051. References [1] D. Kalyanasundaram, P. Shrotriya, P. Molian, Fracture mechanics-based analysis for hybrid laser/waterjet (LWJ) machining of yttria-partially stabilized zirconia (YPSZ), Int. J. Mach. Tool Manuf. 50 (2010) 97–105, https://doi.org/10.1016/j. ijmachtools.2009.09.002. [2] T.S. Frangulyan, I.P. Vasil’ev, S.A. Ghyngazov, Effect of grinding and subsequent thermal annealing on phase composition of subsurface layers of zirconia ceramics, Ceram. Int. 44 (2018) 2501–2503, https://doi.org/10.1016/j.ceramint.2017.10. 234. [3] Z. Yang, L. Zhu, B. Lin, G. Zhang, C. Ni, T. Sui, The grinding force modeling and experimental study of ZrO2 ceramic materials in ultrasonic vibration assisted grinding, Ceram. Int. (2019) 1–17, https://doi.org/10.1016/j.ceramint.2019.01. 216. [4] M. Yang, C. Li, Y. Zhang, D. Jia, X. Zhang, Y. Hou, R. Li, J. Wang, Maximum undeformed equivalent chip thickness for ductile-brittle transition of zirconia ceramics under different lubrication conditions, Int. J. Mach. Tool Manuf. 122 (2017) 55–65, https://doi.org/10.1016/j.ijmachtools.2017.06.003. [5] H.N. Li, K. Ge Xie, B. Wu, W.Q. Zhu, Generation of textured diamond abrasive tools by continuous-wave CO2 laser: laser parameter effects and optimisation, J. Mater. Process. Technol. (2019) 116279, https://doi.org/10.1016/J.JMATPROTEC.2019.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of laser-assisted grinding on surface integrity of zirconia ceramic Zhelun Ma a b c
a,b,∗
, Zhao Wang
a,b
c
, Xuezhi Wang , Tianbiao Yu
T
a,b,∗∗
School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China Liaoning Provincial Key Laboratory of High-end Equipment Intelligent Design and Manufacturing Technology, Northeastern University, PR China School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-assisted grinding Zirconia ceramic Surface quality Surface morphology Subsurface damage
Zirconia ceramics has an important position in the field of biomaterials because of excellent mechanical properties. Because of abrasive resistance and hardness, conventional machining of zirconia ceramic is difficult and time-consuming. A nontraditional hybrid laser-assisted grinding (LAG) system combining laser and cubic boron nitride (CBN) grinding wheel was developed for machining zirconia ceramic. This hybrid system utilizes the outstanding thermostability of zirconia ceramic: The material present on the cutting path is rapidly removed by local laser heating and grinding wheel. A combined theoretical and experimental study was performed to evaluate different modes of machining of zirconia ceramic. The machining parameters were predicted by numerical analysis. The surface quality, surface morphology, and subsurface damage of zirconia ceramic specimens were analyzed and compared. The results show that LAG can easily achieve ductile regime grinding in the same machine tool. Compared with conventional grinding, the surface integrity of zirconia ceramic was significantly improved by the LAG process. LAG could achieve ductile regime grinding with a large depth-of-cut. It changes the machinability of zirconia ceramic.
1. Introduction Zirconia ceramic usually consists of ZrO2 and 3 mol% Y2O3. Zirconia ceramic has many practically useful physical properties such as a low thermal conductivity and high fracture toughness [1]. Zirconia ceramic is used in all-ceramic restoration of prosthodontics and shows favorable biocompatibility, chemical stability, flexural strength, and color. Zirconia ceramic has been used in thermal insulation coatings and void media in combustion because of extremely low thermal conductivity. Zirconia is one of the strongest and toughest structural ceramics [2,3]. Therefore, zirconia ceramics have attracted much interest. The majority of zirconia ceramic products which require machining after sintering are grinded. Grinding is often used for the finishing and rough machining of structural ceramics because no other suitable method for rough machining has been proven and accepted by industry [4–6]. However, zirconia ceramic is a hard-to-brittle material owing to the localized covalent ionic bonding and well-organized crystal structure. Machining of zirconia ceramic is difficult and expensive by grinding processes [7]. A large number of surface/subsurface damage is generated in conventional grinding (CG) due to these characteristics
[8,9]. Therefore, the removal mechanism of zirconia ceramic has been extensively studied. Ductile regime grinding has been developed to obtain a better machined surface integrity and tight dimensional tolerance. Ductile regime grinding of difficult-to-machine materials can produce parts with minimum surface damage, leading to a higher strength [10]. Ductile regime grinding is achieved with a depth-of-cut of the order of nanometers that yields a high-strength part but requires a long machining time. Although ductile regime grinding has provided some interesting results, it is difficult to apply in industry due to poor efficiency. In recent years, other processing methods have been combined with CG to solve the abovementioned problems such as ultrasonic-assisted grinding and milling-grinding methods. Xiao et al. [11] developed a grinding force model for ultrasonic-assisted grinding zirconia ceramics to better understand the mechanism of plastic removal and brittle removal on cutting force during ultrasonic-assisted grinding. Liang et al. [12] reported that the critical depth-of-cut of nanocrystal sapphire in ultrasonic-assisted grinding is much larger than that in CG. Chen et al. [13] improved the grinding-milling force and surface roughness by using a hybrid milling-grinding system with proper feedback to control
∗ Corresponding author. School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China. ∗∗ Corresponding author. School of Mechanical Engineering and Automation, Northeastern University, NO. 3-11, Wen hua Road, He ping District, Shenyang, 110819, PR China. E-mail addresses: [email protected] (Z. Ma), [email protected] (Z. Wang), [email protected] (X. Wang), [email protected] (T. Yu).
https://doi.org/10.1016/j.ceramint.2019.09.051 Received 20 May 2019; Received in revised form 5 September 2019; Accepted 5 September 2019 Available online 06 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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the machining feed rate. However, hybrid machining involves the control of undeformed chip thickness with abrasive grains during processing by changing the motion and trajectory of abrasive particles. They are not significantly different from CG. Laser-assisted grinding (LAG) is a hybrid machining method: The workpiece material is locally heated and softened using a focused laser beam prior to material removal with a traditional grinding wheel. In contrast with previously proposed combined machining method, LAG can reduce the contact stiffness and impact toughness, thus softening the material. LAG allows the abrasive cutting edge to more easily penetrate the material during grinding, reduce the grinding energy, and improve the removal of processed materials and processing efficiency. The LAG operation is carried out under dry condition without a coolant, making the process more environmentally friendly and reducing the cost. Several studies have focused on laser-assisted machining (LAM) to better understand the processing mechanism. Lee et al. [14] proposed a laser-assisted fillet milling method: The position of laser heat source was controlled using addition axes. Shelton et al. [15] conducted laserassisted micromilling experiments on Ti6–Al–4V and Inconel718 alloy materials. The experimental results show that laser-assisted micromilling resulted in different degrees of improvement in terms of surface quality, tool wear, and edge burrs. Garcí et al. [16] found that LAM can improve the machinability of Inconel 718 because the required cutting force and surface roughness are reduced. Chang et al. [17] found that rotational speed is the most important factor on LAM system performance. The impact ratio is as high as 42.68%, followed by feed, depthof-cut, and pulse frequency. Chang et al. [18] also found that LAM can reduce the required cutting force by 20% (thrust force) and 22% (feed force), producing a workpiece with better surface integrity than conventional method. Ding et al. [19] reported that LAM can improve the machinability of Waspaloy. The specific cutting energy, surface roughness, and ceramic tool life clearly improved during LAM. Ding et al. [20] also found that the axial residual stress of compression surface generated by LAM is ~150 MPa larger than that of hard turning, which reduces the variation in hoop stress. Anderson et al. [21] reported that tool life can be increased by 200–300% in LAM compared to conventional machining by using ceramic inserts. They also reported chip curvature formation and color changes in chips during the LAM of Inconel 718. In this study, cutting and laser parameters for the hybrid LAG cutting of zirconia ceramic are predicted. Using these parameters, experiments were performed. The surface roughness, surface morphology, and subsurface damage were analyzed and compared between CG and LAG. The results provide an experimental reference for the LAM process, and the machinability of difficult-to-machine materials such as zirconia ceramic was also improved.
Fig. 1. Schematic diagram of ductile–brittle transition depth.
t T T0 v xa yb ρ δ
2.1. Prediction of cutting parameters As shown in Fig. 1, Bifano et al. [22] proposed an empirical formula for the depth of brittle plastic transition of hard-to-brittle materials. The ultimate depth of plastic machining is shown in Eq. (1). 2
E K dc = 0.15 ⎛ ⎞ ⎛ c ⎞ ⎝ Hv ⎠ ⎝ Hv ⎠ ⎜
P q r
⎟⎜
⎟
(1)
where E is the elastic modulus, Hv is the hardness, and Kc is the fracture toughness. According to the characteristics of structural ceramics, the elastic modulus, hardness, and fracture toughness are varied with temperature. The temperature-dependent behavior of zirconia ceramics has been studied. The detailed trend is shown in Fig. 2. By substituting Fig. 2 (a)(b)(c) into Eq. (1), the trend of brittle plastic transition depth was obtained. As shown in Fig. 2 (d), the depth of brittle plastic transition of zirconia ceramics is ~890 nm at room temperature. As the temperature increased, the depth morphology increased. When the temperature reached 1000 °C, the depth of brittle plastic transition exceeded by 18 μm. In CG, because of a small depth of brittle plastic transformation, it is difficult to achieve ductile regime grinding for zirconia ceramics at room temperature. Ductile regime grinding has not only poor efficiency, but also requires a very rigid grinding machine with a high horsepower spindle to overcome the larger grinding forces with nanoscale accuracy. However, during LAG, the workpiece is heated locally using a laser beam; the surface temperature increases rapidly; the elastic modulus, hardness, and fracture toughness vary with the increase in temperature. When the temperature of zirconia ceramics reaches 1000 °C, the depth-of-cut rapidly increases during grinding; thus, ductile regime grinding can be easily achieved. In LAG, a point on the cutting path is first heated with the laser beam and then removed by the grinding wheel. Therefore, Laser irradiation also causes local damage, leading to crack nucleation and expansion. Laser-induced damage can cause two types of cracks [1]: channeling crack along the surface and edge crack towards the center (as shown in Fig. 3). At the beginning of LAG, the boundary of specimen is subjected to laser radiation, causing damage. Because channeling crack has the same direction as grinding path, the crack would be
2. Theoretical analysis a as cp dc E G H Hv k Kc Lx , Ly n,m
Time (s) Temperature (K) the initial temperature (K) the workpiece feed (m/s) the semi-width of heating region along the x-direction (m) the length of heating region along the y-direction (m) the density of material (Kg/m3) the Dirac delta function
depth-of-cut (m) absorptivity the specific heat capacity (J/kg K) ultimate depth of plastic machining (m) elastic modulus (GPa) Green's function the Heaviside function Hardness (GPa) the thermal conductivity of material (W/mK) fracture toughness (MPa/m1/2) the length and width of material (m) the index number for Eigen values along the x and y directions the laser power (W) the heating source term (W/m2) the laser spot size (m) 922
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Fig. 2. Temperature-dependent behavior of zirconia ceramics: (a) elastic modulus [23], (b) hardness [24], (c) fracture toughness [25], and (d) plastic transition depth.
in Eq. (2):
q (x , y, t ) ∂T ∂ 2T ∂ 2T ⎞ = a2 ⎛ 2 + + 2 dt ∂ x ∂ y k ⎝ ⎠ ⎜
⎟
(2a)
and
a2 =
cp ρ (2b)
k
where T is the temperature, cp is the specific heat capacity, ρ is the density of material, k is the thermal conductivity of material, t is the time, and q is the heating source term. q can be expressed as Eq. (3):
q (x , y, t ) =
as P H (r − x )[H (vt − y ) − H (vt − y − 2r )] πr 2
(3)
where H is the Heaviside function, v is the workpiece feed, r is the laser spot size, as is the absorptivity, and P is the laser power. The heating source term shows the heat input due to laser irradiation. The integration of differential equations was simplified using the Heaviside function (H). To simplify the calculation, the heat affected zone of laser was treated as a square area with a side length of 2r. The absorptivity as was calculated by considering various losses, i.e., losses due to laser reflection and transmission caused by materials (~5% loss) [1], loss in laser optics (~5% loss) [26], and loss due to the heat emission of material (~4% loss). Therefore, only ~86% of the laser incident energy was absorbed in the zirconia ceramic specimen.
Fig. 3. Schematic of LAG system.
removed during grinding; the depth of plain-strain crack reaches almost the cut depth during LAG. 2.2. Prediction of laser parameters Temperature distributions of different locations of zirconia ceramics in LAM were calculated using Fourier heat conduction equation shown 923
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Laser heating and grinding occur on the top surface, and minimal heat transfer occurs at the bottom of specimen. Eq. (4) shows the boundary conditions of the whole process:
k
∂T
∂T ∂T =k =k =0 ∂yy = yb ∂yy = 0
∂x x =±x a
Table 2 Details of grinding wheel used in this experiment.
(4)
where xa is the semiwidth of heating region along the x-direction; yb is the length of heating region along the y-direction.
Parameter
Value
Size (mm) Abrasive type Abrasive size (#) Hardness Bond type CBN concentration (%)
Φ40 × 10 × 10 CBN 120 N vitrified 150
2.3. The initial condition is
T (x , y, t = 0) = T0
conducting the experiments, the zirconia ceramic surfaces were polished with silicon carbide emery paper with #240, #400, #600, #800, #1000, #1600, and #2000 grain sizes, followed by polishing with a cloth wheel to remove any residual scratch. Then, the zirconia ceramic was mounted on a jig. A hard rubber pad was used to avoid specimen breakage. A grinding wheel was used in all the experiments. A KUKA robot equipped with an YLR-500 W laser generator was used to irradiate the sample. This robot has six degrees of freedom to ensure an accurate laser position. The grinding was conducted along the length of specimen and extended over the entire width of specimen.
(5)
where T0 is the initial temperature. The transient temperature distribution was determined using Green's function approach:
T (x , y, t ) =
t
yb
xa
∫0 ∫0 ∫−x
q (ξ , η , τ ) G (x , y, t; ξ , η , τ ) dξdηdτ + T0
a
(6)
G (x , y, t; ξ , η , τ ) indicates that the field value of space (x, y) at time t is excited by the heat source q located space (ξ, η) at time τ. ∂G ∂ 2G ∂ 2G ⎞ − a2 ⎛ 2 + = δ (x − ξ ) δ (y − η) δ (t − τ ) dt ∂ x ∂y 2 ⎠ ⎝ ⎜
3.3. Experimental parameters
⎟
(7) A numerical analysis was conducted to predict the machining parameters. Based on equipment conditions and material properties, the analysis parameters were used to calculate the machining parameters such as workpiece feed, laser spot size, depth-of-cut, laser power, and distance between the laser beam and grinding wheel. The experiment was carried out with a laser spot size of 2 mm in diameter. The distance between laser focus and grinding wheel is 5 mm. The angle between the laser beam and zirconia ceramic surface is about 57.13. Table 3 shows the other processing parameters.
where δ is the Dirac delta function. the series representation of Green's function G is shown in Eq. (8):
G (x , y, t; ξ , η , τ ) =
2 Lx ×
∞
nπa
∑ e−⎛⎝ Lx ⎞⎠ (t−τ ) sin ⎛ nπ x ⎞ sin ⎛ nπ ξ ⎞ ⎜
2 Ly
⎟
⎝ Lx ⎠
n=1 ∞
∑e m=1
⎜
⎟
⎝ Lx ⎠
−⎛ mπa ⎞ (t − τ ) mπ ⎞ ⎛ mπ ⎞ sin ⎜⎛ y ⎟ sin ⎜ η⎟ ⎝ Ly ⎠ ⎝ Ly ⎠ ⎝ Ly ⎠ ⎜
⎟
(8)
where Lx and Ly are the length and width of material, respectively; n and m are the index number for Eigen values along the x and y directions, respectively.
3.4. Surface quality measurements The surface quality of ceramic is an important factor for practical ceramic applications. To obtain accurate roughness, the specimens were cleaned with ethyl alcohol and deionized water in an ultrasonic cleaner for about 15 min individually; at last, the specimens were dried in a drying baker at 50 °C for 3 h. The average roughness height (Rz) and average roughness profile (Ra) were measured using a 3D laser scanning confocal microscope (OLYMPUS LEXT OLS4100). Rz is the maximum peak-to-valley height. Ra is the average roughness and shows the degree of surface roughness in a simplified manner. A diagram of surface roughness is shown in Fig. 5. The assessment area of surface roughness measurement is 1 mm2. Two random specimens were selected, and ten surfaces of each specimen were measured for reducing the random errors.
3. Experimental 3.1. Materials Hot-pressed sintered zirconia ceramic was examined experimentally. The material properties are shown in Table 1. The specimens have dimensions of 50 × 50 × 1 mm3, and the grinding was conducted on a 50 × 50 mm2 surface. A vitrified bond cubic boron nitride (CBN) grinding wheel was used in this experiment. Before the experiment, the grinding wheel was trimmed using a diamond trimming pen to restore the shape and cutting efficiency of grinding wheel. Details of grinding wheel are shown in Table 2.
3.5. Subsurface damage measurement
3.2. Experimental setup and conditions
The measurement of subsurface damage (SSD) in this study is shown in Fig. 6. The measurement consists of four steps [27]: (1) A slit is introduced from the opposite side of grinded surface using a diamond stylus. (2) Half of the zirconia ceramic is fixed, and another half is impacted, so that the specimen can be divided into two parts and a cross-section can be observed. (3) The cross-section is cleaned with ethyl alcohol and deionized water in an ultrasonic cleaner for about 15 min individually. Then, the specimens are air-dried in a drying baker at 50 °C for 3 h (4) The SSD was observed using a scanning electron microscope (SEM, ULTRA PLUS).
The detailed experimental setup is shown in Fig. 4. Before Table 1 Material properties of ZrO2 workpiece used in the experiment. Parameter
Value
Material Density Hardness Elasticity modulus Thermal conductivity Crush strength Bending toughness Coefficient of thermal expansion
ZrO2 > 6 g cm−3 1500 HV 210–220 GPa 2.2–3 W·m−1 K−1 ≥120 Kgf (1.5 mm O.D.) 8–13 MPa m1/2 9.6–10.3 × 10−6 °C
4. Results and discussion During the grinding, the surface integrity of grinded workpiece is an 924
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Fig. 4. Experimental process.
important indicator used to evaluate the machining method. Therefore, this section describes the surface integrity of zirconia ceramic obtained after LAG. The results were compared with the results of zirconia ceramic surface after CG. The improvement in grinding performance owing to LAG was determined.
Table 3 Machining parameters.
CG LAG
Depth of cut ap (μm)
Wheel speed Vs (m/s)
Workpiece feed Vw (mm/s)
Laser power PL (W)
15 15
12 12
5 5
0 185
925
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Fig. 5. Schematic diagram of 2D surface roughness parameters.
Fig. 6. Illustration of the method for SSD measurement.
4.1. Surface quality Surface quality is an important parameter for evaluating the surface integrity of a machined surface. Table 4 shows the total surface roughness (Rz and Ra) of zirconia ceramics produced using CG and LAG. The surface roughness Rz of zirconia ceramics produced using CG reached 10.17 μm and 9.61 μm, whereas Ra reached 1.72 μm and 1.51 μm. However, the surface roughness Rz of zirconia ceramic produced using LAG reached 6.02 μm and 6.59 μm, whereas Ra reached 1.31 μm and 1.21 μm. A comparison of surface quality is shown in Fig. 7. The maximum improvements in surface roughness Rz and Ra are 40.8% and 29.7%, respectively, whereas the minimum improvement in surface roughness Rz and Ra are 31.4% and 13.2%, respectively. In summary, the surface quality was significantly improved when zirconia was processed using LAG. The poor surface quality is caused by brittle removal when most abrasives are in the chip-formation stage during grinding. Because of the improvement in surface quality, it can be concluded that most abrasives are in the scratching and plowing stage; therefore, plastic
Fig. 7. Surface roughness (Ra and Rz) of zirconia ceramics.
removal is the major removal mode. To study the removal mechanism of zirconia ceramic with different machining modes more extensively, the surface morphologies were analyzed and compared.
4.2. Surface morphology Fig. 8 shows the surface morphologies of zirconia ceramic subjected to different grinding methods. For CG (Fig. 8(a)(b)(e)), brittle fracture is the main removal mode for zirconia ceramic grinding. This is because the depth-of-cut is larger than the threshold. In the brittle material removal mode, the initial cracks generated inside the ceramic start to expand with changes in the expansion direction. Chips are formed when the crack spreads to the surface and inside of material simultaneously. The crack propagation eventually ends, leaving behind a residual crack.
Table 4 Surface roughness comparison of grinded surface of zirconia ceramic. Number
CG
LAG
Rz (μm)
1 2 3 4 5 6 7 8 9 10
Ra (μm)
Rz (μm)
Ra (μm)
Surface A
Surface B
Surface A
Surface B
Surface C
Surface D
Surface C
Surface D
10.229 10.346 10.284 9.997 10.095 9.958 10.151 10.179 10.07 10.426
9.374 9.593 9.481 9.545 9.594 9.655 9.783 9.68 9.776 9.587
1.762 1.686 1.738 1.724 1.776 1.75 1.825 1.773 1.757 1.762
1.483 1.508 1.502 1.512 1.51 1.518 1.524 1.516 1.536 1.528
6.401 6.731 6.67 6.477 6.717 6.876 6.518 6.302 6.286 6.908
5.763 5.781 5.812 5.921 6.16 6.161 6.19 6.044 6.303 6.023
1.309 1.313 1.292 1.302 1.298 1.311 1.33 1.324 1.309 1.298
1.227 1.226 1.208 1.219 1.19 1.196 1.226 1.195 1.205 1.183
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Fig. 8. Surface morphologies of grinded zirconia ceramic.
functional performance of zirconia ceramic after grinding. The subsurface damage morphologies of CG and LAG are shown in Fig. 9. In CG, because of the randomness of brittle fracture, the intersecting grinded surface is not smooth (Fig. 9 (a)). Some unformed chips are present, causing a poor subsurface. Because of the high tenacity of zirconia ceramic, it is difficult to produce cracks. Therefore, some lateral cracks did not extend to the grinding surface to form chips. This is the major cause for the poor surface integrity of machined surface during CG. In LAG, the grinded surface is regular and smooth (Fig. 9(b)). The plastic flow clearly exists. This phenomenon occurs on the side of grinding, and microcracks are rarely present on the subsurface of workpiece. These results show that LAG hardly cause subsurface damage to the workpiece by using reasonable processing parameters.
Fig. 8(a) and (b) show that in CG, successive crushing zones exist on the grinded surface, and a large number of pits appear on the surface, i.e., brittle fracture occurs during CG. Because of the randomness of brittle fractures, it is difficult to control the surface integrity, causing poor surface quality and less tight dimensional tolerance. Fig. 8(c)(d)(f) show that plastic removal is the main removal type of grinding zirconia ceramic during LAG. When the depth-of-cut is 15 μm, almost no removal occurs in the grinded surface. Therefore, the surface is smoother through LAG than CG. The depth of gully observed on the surface of workpiece obtained by LAG is obviously shallow. According to the ductile regime grinding theory [22], while the depth-of-cut is less than the threshold, zirconia ceramic undergoes a plastic material removal mode. In the plastic removal mode, brittle material is removed in the form of plastic deformation chips, which do not form cracks and underground damage.
5. Conclusions and suggestions 4.3. Subsurface damage 5.1. Conclusions Because a brittle crack zone on the subsurface of zirconia ceramic after grinding is obtained by removing the brittle material between abrasive workpieces, the subsurface in this region often have defect structures such as lateral cracks, radial cracks, or microcracks. Therefore, the size of brittle crack region in the subsurface after grinding also mainly determines the mechanical properties and
A hybrid LAG system was established. A combined theoretical and experimental study was performed to evaluate different modes in the machining of zirconia ceramic. The surface roughness, surface morphology, and subsurface damage of zirconia ceramic specimens were analyzed and compared. The results are as follows: 927
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Fig. 9. Sectional morphologies of zirconia ceramic subsurface.
roughness Rz and Ra reached 40.8% and 29.7%, respectively. (2) The surface morphologies of zirconia ceramic after CG and LAG were examined experimentally. In CG, the major removal mode of zirconia ceramic is brittle fracture. A large number of crashing zones was observed on the specimen surface. However, in LAG,
(1) LAG can improve the depth of brittle plastic transformation of zirconia ceramic; thus, ductile regime grinding can be easily achieved. The cutting and laser parameters were predicted by numeral calculations. In LAG, the surface quality of zirconia ceramic was significantly improved. The maximum improvement in surface 928
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plastic flow phenomenon was clearly observed. Thus, plastic removal is the main removal mode of zirconia ceramic. The experimental results are consistent with theoretical predictions. The machinability of zirconia ceramic was improved. (3) The subsurface of zirconia ceramic was observed to study crack propagation. In CG, the grinded surface is not smooth. Some lateral cracks were observed on the subsurface. However, in LAG, almost no cracks were observed, and the grinded surface is smooth. Because of plastic removal, zirconia ceramic piled up on the side of grinding grooves. This indicates that ductile regime grinding with a large depth-of-cut can be achieved with LAG. LAG can improve the grinding efficiency of zirconia ceramic while ensuring good surface integrity.
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5.2. Suggestions In the future, the mechanism of effect of laser on the grinding characteristics (grinding force, grinding temperature, surface integrity) need to be further studied. Materiallo-thermo-mechanical coupled models will be developed to help design the experimental conditions for post-processing and model the critical surface microstructure attributes such as grain size, phase constituents, microhardness, and residual stress. Uniquely graded microstructure types will be generated via LAM processed surfaces. The mechanical performance will be evaluated to test the ability of the predictive framework. Acknowledgment The authors thank the National Natural Science Foundation of China (No. U1508206), Science and Technology Planning Project of Shenyang (No. 18006001). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.051. References [1] D. Kalyanasundaram, P. Shrotriya, P. Molian, Fracture mechanics-based analysis for hybrid laser/waterjet (LWJ) machining of yttria-partially stabilized zirconia (YPSZ), Int. J. Mach. Tool Manuf. 50 (2010) 97–105, https://doi.org/10.1016/j. ijmachtools.2009.09.002. [2] T.S. Frangulyan, I.P. Vasil’ev, S.A. Ghyngazov, Effect of grinding and subsequent thermal annealing on phase composition of subsurface layers of zirconia ceramics, Ceram. Int. 44 (2018) 2501–2503, https://doi.org/10.1016/j.ceramint.2017.10. 234. [3] Z. Yang, L. Zhu, B. Lin, G. Zhang, C. Ni, T. Sui, The grinding force modeling and experimental study of ZrO2 ceramic materials in ultrasonic vibration assisted grinding, Ceram. Int. (2019) 1–17, https://doi.org/10.1016/j.ceramint.2019.01. 216. [4] M. Yang, C. Li, Y. Zhang, D. Jia, X. Zhang, Y. Hou, R. Li, J. Wang, Maximum undeformed equivalent chip thickness for ductile-brittle transition of zirconia ceramics under different lubrication conditions, Int. J. Mach. Tool Manuf. 122 (2017) 55–65, https://doi.org/10.1016/j.ijmachtools.2017.06.003. [5] H.N. Li, K. Ge Xie, B. Wu, W.Q. Zhu, Generation of textured diamond abrasive tools by continuous-wave CO2 laser: laser parameter effects and optimisation, J. Mater. Process. Technol. (2019) 116279, https://doi.org/10.1016/J.JMATPROTEC.2019.
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