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TiC micro-composite and micro-nano-composite ceramics
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Biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nanocomposite ceramics Xiaobin Cuia,∗, Kai Yana, Jingxia Guob, Dong Wangc a
School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China c School of Mechanical Engineering, Xi'an Technological University, Xi'an, 710021, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Biomimetic micro-textures Mechanical behaviors Intermittent turning performance Al2O3/TiC ceramics
This work was conducted to investigate biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nano-composite ceramics. Chip characteristics and the geometry features of the structures on the cuticle of Procambarus clarkia were considered in the preparation of the laser-induced biomimetic micro-textures on the composite ceramic surfaces. Characteristics of the biomimetic micro-textures were revealed in terms of geometry, morphology and chemical composition. The connection between the thermal stress resulting from laser pulse and the fractal dimension of the micro-crack on the micro-textures was analyzed and identified. The correlation between the mechanical behaviors (damage and fracture toughness) of the textured composite ceramics and the fractal dimension of the micro-crack was fitted and revealed quantitatively. The performance of the textured composite ceramic tools was pre-evaluated by means of a proposed indicator. Damage, fracture toughness and tool stress were incorporated in the indicator. The indicator and the experimental tool wear were compared for validating the effectiveness of the proposed indicator. It was found that the micro-composite ceramic exhibited greater sensitivity to laser than the micro-nano-composite ceramic did. The damage of the textured micro-composite ceramic was larger than that of the textured micro-nano-composite ceramic when the same laser parameters were utilized in the micro-texture preparation. On the contrary, the fracture toughness of the textured micro-composite ceramic was found to be smaller. There was a negative correlation between the damage of the textured composite ceramic and the fractal dimension of the micro-crack. Conversely, there was a positive correlation between the fracture toughness and the fractal dimension. The performance of the textured micro-nano-composite ceramic tool was better than that of the textured micro-composite ceramic tool. The proposed indicator can be used to predict the combination of laser parameters that resulted in the optimum performance of the textured composite ceramic tool.
1. Introduction Previous study [1] indicated that Al2O3-based ceramics were suitable for cutting hardened steel. Al2O3-based ceramic tools fail abruptly in intermittent turning due to severe mechanical and thermal impacts. The performance of Al2O3-based ceramic tools has been improved by adding secondary phase particles with varying sizes to the tool material [2]. However, intrinsic defects such as relatively low strength and relatively low fracture toughness still limit the intermittent turning performance of Al2O3-based ceramic tools to some extent. The structures on the cuticle of some biological species were found to be capable of reducing external loads and enhancing movement performance [3].
∗
They provided inspiration for the preparation of surface micro-textures on the Al2O3-based ceramic tool. The micro-textures are expected to further improve the intermittent turning performance of Al2O3-based ceramic tools. Laser surface texturing (LST) is widely utilized to fabricate surface micro-textures on various tool materials because it has advantages such as high processing accuracy, no contact force and no pollution. Previous researches on the design of laser-induced micro-textures mainly focused on pattern [4,5], size [6] (width, depth, height, diameter and spacing), distance between micro-textures and cutting edge [7] and integration of micro-textures and lubricant [8]. The existing studies provided much valuable information for the design of tool surface micro-textures. The
Corresponding author. E-mail address: [email protected] (X. Cui).
https://doi.org/10.1016/j.ceramint.2019.06.251 Received 15 June 2019; Received in revised form 23 June 2019; Accepted 24 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiaobin Cui, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.251
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Table 1 Composition (wt%) of the two kinds of Al2O3/TiC composite ceramics. Material
α-Al2O3
TiC (0.4 μm)
TiC (80 nm)
Mo
Sintering aids
Micro-composite ceramic Micro-nano-composite ceramic
55 55
37 17
0 20
5 5
3 3
micro-textures on the tool surface interact with the chip in cutting process. The interaction between the micro-textures and the chip substantially affect the cutting loads and subsequently influence the tool performance. Thus, it is critical to consider the chip characteristics in the design of tool surface micro-textures. However, it was found that few researches were conducted with the characteristics of chip considered. Biomimetics has been extensively employed to provide guide for surface micro-texture design [3]. Previous studies rarely referred to the structures on the cuticle of biological species in the design of the micro-textures on tool surface. The laser changes the properties of the tool material during the LST process. The laser-induced micro-textures prepared on tool material surface were examined by the researchers from different perspectives. Analysis of the laser-induced micro-textures concentrated on geometric dimensions [9], surface quality [10], chemical composition [10], wetting characteristics [11] and friction behaviors [12]. Laser pulses resulted in fierce thermal stress field within the tool material and microcracks arose on the micro-textures. Mechanical behaviors such as damage and fracture toughness of the tool material alter as a result of the actions of laser pulses. These mechanical behaviors had substantial effects on the tool performance in intermittent turning. It can be found from the former studies that scant quantitative analysis of the mechanical behaviors was performed for the textured tool material. Various indicators such as friction coefficient [13], cutting force [13], cutting temperature [13] and tool wear [13–15] were employed in the existing studies for evaluating the textured tool performance. Preevaluation of performance is important for the efficient use of the textured tool. Most of the indicators used in the previous studies were obtained through cutting tests. Pre-evaluation of the textured tool performance is difficult to achieve with the existing indicators. Not only the cutting loads but also the mechanical behaviors of the textured tool greatly influence the tool performance in intermittent turning. Thus, a new indicator should be proposed for pre-evaluating the textured tool performance. Moreover, the cutting loads and the mechanical behaviors of the textured tool should be incorporated in the indicator. This work is conducted to study biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nano-composite ceramics. The structures on the cuticle of the chela of Procambarus clarkia are analyzed to acquire the geometry features. Biomimetic micro-textures are prepared using different combinations of laser parameters on the surfaces of the Al2O3/TiC micro-composite and micro-nano-composite ceramics according to the chip characteristics and the geometry features of the biological structures. Geometry, morphology and chemical composition are examined for the laser-induced biomimetic microtextures. The thermal stress caused by laser pulse and the fractal dimension of the micro-crack on the micro-textures are studied to reveal the connection between them. The mechanical behaviors such as the damage and the fracture toughness of the textured composite ceramics are analyzed to quantitatively identify the correlation between the mechanical behaviors and the micro-crack's fractal dimension. An indicator is proposed for pre-evaluating the intermittent turning performance of the composite ceramic tools with biomimetic micro-textures. Damage, fracture toughness and tool stress are integrated in it. The indicator and the tool wear obtained from turning test are compared to validate the effectiveness of the proposed indicator.
2. Materials and methods 2.1. Preparation of the Al2O3/TiC micro-composite and micro-nanocomposite ceramics The Al2O3/TiC micro-composite and micro-nano-composite ceramics studied in this work were prepared using hot-pressing sintering. The initial powders used in the preparation were particles of α-Al2O3, TiC and Mo. The size of α-Al2O3 particle (purity: 99.9%) was 0.5 μm and the size of Mo particle (purity: 99.5%) was 1.2 μm. TiC particles with a purity of 99.9% and a size of 0.4 μm were used for fabricating the micro-composite and the micro-nano-composite ceramics. TiC particles with a purity of 99.9% and a size of 80 nm existed in the initial powder of the micro-nano-composite ceramic besides the micro-particles of TiC. Sintering aids, MgO and Y2O3, were employed to ensure the densification of the composite ceramics in hot-pressing sintering. The hybrid slurries of the initial powders were ball-milled and dried. Compositions of the two kinds of composite ceramics are presented in Table 1. The hybrid powders were compacted after sieving. The powders were sintered at a temperature of 1700 °C in nitrogen atmosphere. The polished surfaces and the fractured surfaces of the prepared composite ceramics were observed by means of scanning electron microscope (SEM) (Merlin Compact, Germany). The enlarged images of the polished surfaces of the composite ceramics are shown in Fig. 1(a) and (b). It was found that the average grain size of the micro-nanocomposite ceramic was much smaller. SEM images of the fractured surfaces of the micro-composite ceramic and the micro-nano-composite ceramic are presented in Fig. 1(c) and (d), respectively. Intergranular fracture and tansgranular fracture were found to be the main fracture modes of the micro-composite ceramic and the micro-nano-composite ceramic, respectively. A microscopic hardness tester (HV-1000, China) was used to analyze the composite ceramics. Fracture toughness KIC (MPa*m1/2) of the composite ceramics can be determined according to the experimental results acquired from the microindentation tests [16]. The load applied in the test was 9.8 N. The dwelling time was 10 s. Nanoindentation tests were carried out using a nanoindentation system (G200, USA) at room temperature. A Berkovich indenter was used in the tests. The peak load was set to be 100 mN. The loading rate was 10 mN/s. The dwelling time at the peak load was 10 s. Elastic modulus of the composite ceramics can be determined based on the nanoindentation tests [17]. Each test was replicated 6 times. The fracture toughness and the elastic modulus of the micro-composite ceramic were found to be 4.6 MPa*m1/2 and 426 GPa, respectively. These mechanical property parameters of the micro-nano-composite ceramic were 5.1 MPa*m1/2 and 445 GPa, respectively. 2.2. Examination of the cuticle of procambarus clarkia Fig. 2 shows the typical images of Procambarus clarkia. Procambarus clarkia survives and prospers in environment full of sludge which can be considered as non-Newtonian fluid. Previous study indicated that chip which arose in cutting process exhibited features of non-Newtonian fluid [18]. Taking into account the similarity between sludge and chip, the cuticle of Procambarus clarkia was examined using digital microscope (VHX-1000E, Japan). Chela is important to the
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Fig. 1. The polished surfaces and the fractured surfaces of the prepared composite ceramics.
movement of Procambarus clarkia. The zone of the chela cuticle denoted in Fig. 2(c) was focused on in the examination process. The geometry features of the structures on chela cuticle provided critical information for the design of the biomimetic micro-textures on the composite ceramic surfaces.
frequency, scanning speed and the number of scans were set to be constant. The values of them were 70 kHz, 70 mm/s and 3, respectively. SEM, energy dispersive X-ray (EDX) (X-MAX80, UK) and X-ray diffraction (XRD) (D8 Advance, Germany) were employed to analyze the textured composite ceramics. Fracture toughness KIC (MPa*m1/2), elastic modulus of the textured composite ceramics were obtained on the basis of microscopic hardness tests and nanoindentation tests. The test parameters were the same to those utilized for the composite ceramics with no laser-induced micro-textures.
2.3. Laser surface texturing Biomimetic micro-textures were fabricated on the surfaces of the two kinds of composite ceramics considering the geometry features of the structures on chela cuticle. The laser pulse duration was 100 ns The wavelength of the laser was equal to 1064 nm. The power of the laser was set in the range of 2 W to 4 W in the laser surface texturing process. The angle αl of the laser beam varied from 50° to 90° as shown in Fig. 3. The value of the laser beam angle was defined considering the relative movement direction of the composite ceramic tool to chip. Pulse
2.4. Intermittent turning tests AISI 52100 steel (60 HRC) was turned intermittently by means of tools made of the Al2O3/TiC micro-composite and micro-nano-composite ceramics as presented in Fig. 4. Biomimetic micro-textures were manufactured on the tool surface by laser surface texturing. The
Fig. 2. Typical images of Procambarus clarkia. 3
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Fig. 3. Angle αl of the laser beam.
the heat source on the composite ceramics changed from a circle to an ellipse as the angle αl varied from 90° to the other values. The area of the heat source changed correspondingly due to the variation of the angle αl. It should be noted that the diameter of the circle and the minor axis of the ellipse had the same magnitude. Thus, the effects induced by the major axis of the ellipse were focused on in the analysis. The intensity distribution of the heat source was dominated by both the geometry (shape and area) of the heat source and the energy of the Gaussian laser pulse. The pulse frequency and the laser power were used for the calculation of the energy of the Gaussian laser pulse. The action of a single laser pulse on the two kinds of composite ceramics was simulated based on the geometry and the intensity distribution of the heat source. Taking the characteristics of microstructures into account, the two-dimensional models of the composite ceramics analyzed in the simulation was established according to the work by Wang et al. [20]. The two-dimensional models were built with Voronoi diagram [20–22] and random method used. The heat source was set at edge 1. Heat radiation and convection took place at edges 2, 3 and 4. The ablated zone of the two-dimensional models was determined according to the temperature distribution and the melting point of the composite ceramic. The stresses in the remaining zone were critical for the generation of microcracks. Thus, the stress components of each element in the remaining zone were obtained and recorded for subsequent analysis. Fig. 6 shows the schematic of the turning simulation. The geometries and the materials of the textured composite ceramic tools and the workpiece studied in the simulation were the same to those used in turning tests. The textured composite ceramic tool was considered to be an elastic body. The workpiece behavior was investigated using Johnson–Cook model [23] which incorporated the influences caused by temperature and strain rate. The essential parameters in the Johnson–Cook model were determined on the basis of the study by Ramesh and Melkote [23]. For the purpose of ensuring the simulation accuracy, local refining technology was employed to refine the textured composite ceramic tool and the workpiece as shown in Fig. 6. The movements of the workpiece and the textured composite ceramic tool were set according to the turning tests.
Fig. 4. Schematic of intermittent turning.
macroscopic geometry of the composite ceramic tools was the same. The rake angle of the composite ceramic tools was manufactured to be −6°. The cutting edge angle had the value of 90°. The clearance angle was set as 6°. Non-cutting length ln and cutting length lc were denoted in Fig. 4. The ratio of ln to lc was 14.7. Cutting speed was 56 m/min. Feed rate was 0.12 mm/r. Depth of cut (ap) was 1 mm. Each test was performed 6 times for each group of the biomimetic micro-texture and the composite ceramic tool. The cutting length was set to be invariable for all the turning tests and the value was 600 m. An optical microscope was employed to examine the flank wear VB of the textured composite ceramic tools. SEM was used to analyze the textured composite ceramic tools after turning tests. 2.5. Numerical simulations Numerical simulations of laser surface texturing and intermittent turning were conducted for the acquisition of the data difficult to obtain through tests. Laser surface texturing can be considered as a process determined by a certain number of laser pulses. The effects of a single laser pulse on the composite ceramics were concentrated on in the numerical simulation. Since the duration of the nanosecond laser pulse is much larger than the electron-phonon-relaxation time [19], the action of a single laser pulse on the composite ceramics can be modeled as a heat diffusion problem. The heat diffusion problem was analyzed as presented in Fig. 5. It has been mentioned that the angle αl of the laser beam was in the range of 50° to 90° and the laser power P was in the range of 2 W to 4 W. The shape of
3. Results and discussion 3.1. Characteristics of the laser-induced biomimetic micro-textures on the composite ceramic surfaces The micro-textures designed with the chip characteristics considered are expected to efficiently enhance the tool performance. 4
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Fig. 5. Analysis of the action of laser on the composite ceramics.
Taking the high temperature, high strain and high strain rate into consideration, the chip which interact with the tool surface can be thought to be non-Newtonian fluid [18]. Procambarus clarkia is often found to move in sludge which can also be considered as non-Newtonian fluid. The structures on the cuticle of Procambarus clarkia provide inspiration for the design of the micro-textures on composite ceramic tool surface. The cuticle of the chela of Procambarus clarkia was analyzed. Fig. 7 shows the typical images of the structures on the cuticle. Fig. 7 indicates that there existed many crescent-shaped micropits on the cuticle. There was an obvious correspondence between the shape of the micro-pits and the movement direction of the chela. The “crescent” consisted of a large arc and a small arc. Theoretically, the large arc of the “crescent” first contacted with the sludge as the chela moved forward. It should be noted that the sloping direction of the crescent-shaped micro-pit was the same to the movement direction of the chela. The analysis results of the structures on the cuticle of Procambarus clarkia provided the basis for the design of the biomimetic micro-textures on composite ceramic surfaces. Biomimetic micro-textures, namely crescent-shaped micro-pits, were prepared on the surfaces of Al2O3/TiC micro-composite and micro-nano-composite ceramics. Fig. 8 presents the typical morphologies of the biomimetic micro-textures. The biomimetic micro-textures well captured the geometry features of the structures on the cuticle of Procambarus clarkia. There was also a correspondence between the shape of the biomimetic micro-texture and the relative movement direction of the composite ceramic tool. The
Fig. 6. Schematic of the turning simulation.
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Fig. 7. Typical images of the structures on the cuticle of the chela of Procambarus clarkia.
sloping direction of the biomimetic micro-texture was set to be the same to the relative movement direction of the composite ceramic tool. The surface area A and the sloping angle φ denoted in Fig. 8 were analyzed for the biomimetic micro-textures. Fig. 9 shows the values of the surface area A and the sloping angle φ obtained at different combinations of laser parameters such as laser beam angle αl and laser power P. The surface area A of the micro-texture became smaller with the increase of αl. Conversely, larger P resulted in larger surface area A. The decrease of αl or the increase of P led to larger sloping angle φ. The values of A and φ of the biomimetic micro-texture on the micro-
composite ceramic surface were larger than those on the micro-nanocomposite ceramic surface at the same group of laser parameters. Moreover, the variations of A and φ were found to be greater when the biomimetic micro-texture were manufactured on the micro-composite ceramic. These results indicated that the micro-composite ceramic was more sensitive to laser than was micro-nano-composite ceramic. Typical SEM images of the biomimetic micro-textures on the microcomposite ceramic surface and the micro-nano-composite ceramic surface are presented in Fig. 10. These biomimetic micro-textures were prepared at the same laser parameters. Comparisons were carried out
Fig. 8. Typical morphologies of the biomimetic micro-textures (Al2O3/TiC micro-nano-composite ceramic, laser beam angle αl = 70°, laser power P = 3W). 6
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Fig. 9. Surface area A and sloping angle φ of the micro-textures obtained at different combinations of laser parameters such as laser beam angle αl and laser power P.
for the biomimetic micro-textures on the surfaces of different composite ceramics. The depth of the destroyed zone was relatively larger when the biomimetic micro-textures were manufactured on the micro-composite ceramic surface. This phenomenon further revealed the higher sensitivity of the micro-composite ceramic to the action of laser. It can be observed from Fig. 10(g) and (h) that micro-cracks appeared on the biomimetic micro-texture. It should be noted that the geometry characteristics of the micro-cracks varied as the ceramic material changed from the micro-composite ceramic to the micro-nano-composite ceramic. The appearance of these micro-cracks was accompanied by the change of the mechanical behaviors of the composite ceramics. It was inferred that a close correlation existed between the mechanical behaviors of the textured composite ceramics and the geometry characteristics of these micro-cracks. XRD analysis was performed for the textured micro-composite ceramic and the textured micro-nano-composite ceramic as presented in Fig. 11(a) and (b). A new phase of Ti2O arose on the two kinds of composite ceramics after laser surface texturing. EDX analysis results of point 1 and point 2 denoted in Fig. 10 are illustrated in Fig. 11(c) and (d), respectively. The relatively high oxygen content indicates the oxidation of the composite ceramics resulting from laser surface texturing.
ceramic was larger than that in the micro-nano-composite ceramic at the same laser parameters. The value of Sm increased when αl or P became larger. The geometry characteristics of the micro-cracks caused by laser pulses were studied in terms of fractal dimension. Image enhancement was performed for the typical images of the micro-cracks in Fig. 13(a) and (b). The micro-cracks edges were obtained as presented in Fig. 13(c) and (d) after image binaryzation. Box counting method [24] was used in the fractal analysis of the micro-cracks on the basis of Fig. 13(c) and (d). The fractal image of the micro-crack can be covered by a certain amount Cs of squares with a length Ls. Ls can be presented as: (1)
Ls = n
where n is a positive integer and δ is the pixel size. The value of Cs changed as the value of Ls varied. The correlation between Cs and Ls can be defined using the following equation:
Cs = Con Ls
(2)
Fra
In Eq. (2), Con is used to represent a constant and the micro-crack's fractal dimension was represented by Fra. Eq. (2) can be derived as:
log Cs = log Con
3.2. Mechanical behaviors of the textured Al2O3/TiC micro-composite and micro-nano-composite ceramics
Fra logLs
(3)
Eq. (3) indicates that the micro-crack's fractal dimension can be fitted based on the data of logCs and logLs. Fig. 13(e) and (f) show the data fitting and the results of Fra for the typical micro-cracks. When the laser parameters were fixed, it was found that Fra of the micro-crack obtained for the textured micro-composite ceramic was relatively smaller than that acquired for the textured micro-nano-composite ceramic. Fig. 13(g) and (h) present the values of the fractal dimension Fra obtained at varying combinations of laser parameters such as beam angle αl and laser power P. It can be found that relatively smaller Fra arose at relatively larger values of αl and P. The evolving trend of fractal dimension Fra with laser parameters was opposite to that of Sm. Therefore, it can be deduced that larger Sm resulted in smaller value of Fra. Fracture is the dominant failure mechanism of ceramic tools in intermittent turning [2]. Mechanical behaviors such as damage and fracture toughness were critical to the failure of the composite ceramic
The mechanical behaviors of the composite ceramic tools changed because of the formation of the biomimetic micro-textures. High temperature gradient emerged within the composite ceramics due to the action of laser pulses. The high temperature gradient led to great stress gradient which substantially influenced the generation of micro-cracks and the mechanical behaviors of the textured composite ceramics. The maximum principal stress of the elements in the remaining zone denoted in Fig. 5 was obtained based on the finite element simulation of the action of a single laser pulse. The present work focused on the maximum principal stress that arose at the highest temperature. The largest value of the maximum principal stress in the remaining zone was represented by Sm. Fig. 12 shows the values of Sm acquired at different combinations of laser parameters. The laser parameters were laser beam angle αl and laser power P. Sm in the micro-composite 7
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Fig. 10. Typical SEM images of the biomimetic micro-textures on the micro-composite ceramic surface and the micro-nano-composite ceramic surface (laser beam angle αl = 70° and laser power P = 3W).
tools with biomimetic micro-textures. Taking these into consideration, the damage D and the fracture toughness KIC of the textured composite ceramics were obtained. The correlation between the mechanical behaviors and the micro-crack's fractal dimension was identified quantitatively. It was assumed that the damage of the textured composite ceramics was isotropic and it could be represented by a scalar. It was considered in this work that there was no damage within the composite ceramics before laser surface texturing. The degradation of elastic modulus was employed to acquire the damage [25]. The relationship between the elastic modulus ED of the textured ceramic and the elastic modulus E of the untextured ceramic can be expressed as:
ED = E (1
D)
The value of D was obtained based on the values of ED and E acquired from nanoindentation tests. Fig. 14 presents the scatter plot and the data fitting for the fractal dimension Fra and the damage D. It was found from the scatter plot in Fig. 14(a) and (b) that the damage of the textured micro-composite ceramic was larger than that of the textured micro-nano-composite ceramic when using the same laser parameters. The relatively large absolute values of Pearson correlation coefficient indicate that a close correlation appeared between the damage D of the textured composite ceramic and the micro-crack's fractal dimension Fra. Moreover, there was a negative correlation between them. The correlation between D and Fra was fitted as presented by Fig. 14(c) and (d). The largest value of R2 arose when this correlation was evaluated in terms of quadratic polynomial. This correlation can be defined for the textured micro-composite ceramic using the following equation:
(4)
According to Eq. (4), the damage D of the textured composite ceramic can be derived as:
D=1
ED/ E
D = 0. 91Fra 2
(5)
2.26Fra + 1.50
(6)
As for the textured micro-nano-composite ceramic, this correlation 8
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Fig. 11. XRD analysis and EDX analysis of the textured composite ceramics.
can be expressed as:
D = 0. 26Fra 2
0.84Fra + 0.70
be used to define the correlation for the textured micro-composite ceramic and the textured micro-nano-composite ceramic, respectively. Eq. (9) can be established as:
(7)
Fracture toughness KIC of the textured composite ceramic was calculated using the following formula [16]:
KIC = 0.203 HV (d i/2)1/2 (d i/2ci)3/2
KIC =
13. 69Fra 2 + 37.57Fra
(9)
20.87
Eq. (10) can be expressed as:
(8)
KIC = 3. 56Fra 2
where HV is microscopic hardness, di is used to represent the diagonal length of the microindentation mark, ci is half the crack length caused by microindentation. Fig. 15 presents the scatter plot and the data fitting for the fractal dimension Fra and the fracture toughness KIC. The data in the scatter plot shown in Fig. 15(a) and (b) were analyzed. KIC of the textured micro-composite ceramic was found to be smaller than that of the textured micro-nano-composite ceramic when the same laser parameters were applied. Fig. 15(a) and (b) show that the values of Pearson correlation coefficient were relatively large. This meant that there was also a close correlation between the fracture toughness KIC of the textured composite ceramics and the micro-crack's fractal dimension Fra. It should be noted that there was a positive correlation between them. Fig. 15(c) and (d) present the fitting of the correlation between KIC and Fra. Quadratic polynomial was found to be the most suitable formula for describing this correlation. Eq. (9) and Eq. (10) can
0.17Fra
0.36
(10)
The results and discussions presented above validated the inference that a close correlation existed between the mechanical behaviors of the textured composite ceramics and the geometry characteristics of the micro-cracks. 3.3. Intermittent turning performance of the textured Al2O3/TiC microcomposite and micro-nano-composite ceramics The formation and the existence of the laser-induced biomimetic micro-textures substantially affected damage D, fracture toughness KIC and tool stress σ in cutting process, which subsequently influenced the intermittent turning performance of the composite ceramic tools. Performance pre-evaluation is critical to the efficient application of the textured composite ceramic tools. Thus, it is important to propose an
Fig. 12. Values of Sm acquired at different combinations of laser parameters such as laser beam angle αl and laser power P. 9
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Fig. 13. Fractal analysis of the micro-cracks on the micro-textures.
indicator capable of pre-evaluating the intermittent turning performance. Moreover, D, KIC and σ should be incorporated in the proposed indicator. In this work, a new indicator was established to pre-evaluate the tool performance considering the high strain rate in the textured composite ceramic tools during intermittent turning. Damage equivalent stress σ∗ can be defined for the textured composite ceramic tools by means of Eq. (11) [25]:
{
= (1 + )
+:
+
tr
2
+
1 D [(1+ ) 1 hD
:
tr
2]
=
11
12
21
22
13 23
31
32
33
(12)
For the purpose of reducing the complexity, the tri-axial stress was simplified as bi-axial compressive stress. The material element shown in Fig. 16(a) was employed to describe the bi-axial stress state. In this case, σ can be defined as:
}
1/2
t11 ( t )
=
(11)
0 0
0 t22 ( t )
0
0 0 0
(13)
In Eq. (13), σt11 and σt22 are equal to σt and they have negative values. The tri-axial tool stress was acquried from cutting simulation. Eq. (11) to Eq. (13) can be utilized to obtain σt according to the hypothesis that the tri-axial stress and the bi-axial stress resulted in the same value of σ∗. An array of intrinsic wing cracks was thought to
where σ is tool stress, ν is used to represent the Poisson's ratio of the composite ceramic, h is 0.2 [25]. The textured composite ceramic tools were under a tri-axial stress state during cutting. The representative volume element (RVE) shown in Fig. 16(a) was used to illustrate the triaxial stress state. In this case, tool stress σ can be expressed as: 10
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Fig. 14. Scatter plot and data fitting of the damage D and the fractal dimension Fra.
Fig. 15. Scatter plot and data fitting of the fracture toughness KIC and the fractal dimension Fra.
appear in the material element. There existed a spacing 2w between the cracks. The model of a single wing crack [26] is presented in Fig. 16(b). As for the crack array, the dynamic stress intensity factor KID is calculated as [27]:
KID = kIDa (l ) KIDa + kIDb (l ) KIDb
K
Da
= F sin
w sin
(l + l ) w
1/2
(15)
where θ can be considered to have a value of 0.392π [26], l is the initial tensile crack length which has a value of zero, w is equivalent to 4c [28], l∗ is 0.27c [26], c is a half of the average grain size and F can be defined as:
(14)
where l is the growth rate of tensile crack. Both the concentrated load and the uniform load are considered in Eq. (14). KIDa in Eq. (14) can be expressed as:
F = 2c
11
(16)
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Fig. 16. RVE, material element, crack model and Ip of the textured composite ceramic tools prepared at varying combinations of laser parameters such as laser beam angle αl and laser power P.
where τ∗ is given by:
1 ( 2
=
t11
t22 )sin
2
+
1 µ[ 2
t11
+
t22
(
t11
t22)cos
during a cutting cycle was represented by KMID. A new indicator Ip for the performance pre-evaluation of the textured composite ceramic tools is proposed as:
2 ] (17)
Ip =
KIDb in Eq. (14) can be calculated as:
K
Db
=
t22
2w tan
l 2w
(18)
Cr l Cr 0.75l
(19)
where Cr is used to represent the Rayleigh wave speed and it can be calculated using Eq. (20) [29]:
Cr =
0.862 + 1.14 1+
E 2 (1 + )
(20)
where ρ is the density of the composite ceramic under consideration. kIDb (l ) can be given as:
kIDb (l ) =
Cr l Cr 0.5l
(21)
The growth rate l of tensile crack can be calculated as [26]:
l=
Cr (1.5KIDa + 1.75KIDb
1.25KIC
KIDa + 1.5KIDb
0.75KIC
) (22)
where χ is given by:
= (1.5KIDa + 1.75KIDb (0.5KIDa + 0.75KIDb
1.25KIC)2 0.375KIC)
4(KIDa + KIDb
(24)
The value of the indicator Ip can be acquried based on Eq. (1) to Eq. (24). Smaller Ip corresponds to lower fracture risk of the textured composite ceramic tool. Thus, lower tool wear is likely to arise when the value of Ip is relatively smaller. It can be deduced from Eq. (24) that smaller Ip corresponds to lower tool wear. Moreover, damage D, fracture toughness KIC and tool stress σ in cutting process are all incorporated in the indicator Ip. Fig. 16(c) shows the values of Ip for the textured micro-composite ceramic tool prepared at varying combinations of laser parameters such as laser beam angle αl and laser power P. Fig. 16(d) shows the values of Ip for the textured micro-nano-composite ceramic tool. The value of Ip acquried for the textured micro-composite ceramic tool was found to be relatively larger at each group of laser parameters. Thus, relatively better performance was expected for the textured micro-nano-composite ceramic tool. Indicator Ip decreased first and then increased as αl or P grew larger. The smallest Ip of the textured micro-composite ceramic tool appeared when αl was 60° and P was 2.5W. The optimum performance can be expected for the textured micro-composite ceramic tool at this combination of laser pamaters. The smallest Ip of the textured micro-nano-composite ceramic tool arose when αl was 70° and P was 3W. These laser parameters were considered to result in the best performance. The typical images of the flank face are presented in Fig. 17(a) and Fig. 17(b) for the two kinds of textured composite tools which have been tested in intermittent turning. The main tool failure mechanism was found to be fracture that occurred at different scales. The wear of the untextured composite ceramic tools were found to be larger than those of the textured composite ceramic tools at most of the laser parameter combinations. Fig. 17(c) and (d) show the wear VB of the
1/2
kIDa (l ) and kIDb (l ) in Eq. (14) can be determined according to work by Nemat-Nasser and Deng [27]. kIDa (l ) can be presented as: kIDa (l ) =
KMID KIC
KIC) (23)
The maximum value of the dynamic stress intensity factor KID 12
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Fig. 17. Typical failure images and wear VB of the textured composite ceramic tools manufactured at different combinations of laser parameters such as laser beam angle αl and laser power P.
textured composite ceramic tools manufactured at different combinations of laser parameters such as laser beam angle αl and laser power P. Comparisons of Fig. 17(c) and (d) indicate that the textured micronano-composite ceramic tool exhibited better performance. The evolving trend of tool wear VB with laser parameters was the same to that of the indicator Ip. Furthermore, the optimum intermittent turning performance of the textured composite ceramic tool can be acquired at the combination of laser parameters that led to the smallest Ip. These validated the effectiveness of the indicator Ip.
micro-nano-composite ceramic when the same laser parameters were used in the LST process. The fracture toughness of the textured micro-composite ceramic was smaller than that of the textured micro-nano-composite ceramic at the same laser parameter combination. A negative correlation existed between the damage of the textured composite ceramic and the micro-crack's fractal dimension. There was a positive correlation between the fracture toughness of the textured composite ceramic and the micro-crack's fractal dimension. 3. An indicator incorporating damage, fracture toughness and tool stress was established for pre-evaluating the intermittent turning performance of the textured micro-composite and micro-nanocomposite ceramic tool. The indicator of the textured micro-composite ceramic tool was relatively larger at each laser parameter combination. The value of the indicator decreased first and then increased as the laser beam angle αl or the laser power P grew larger. The smallest indicator of the micro-composite ceramic tool appeared when αl and P were 60° and 2.5 W respectively. The smallest indicator of the textured micro-nano-composite ceramic tool arose when αl and P were 70° and 3 W respectively. The performance of the textured micro-nano-composite ceramic tool was relatively better. The evolving trend of the tool wear with the composite ceramic micro-structure or the laser parameters was the same to that of the indicator. The combination of laser parameters that led to the smallest value of the indicator resulted in the optimum performance of the textured composite ceramic tool. The effectiveness of the proposed indicator was validated.
4. Conclusions The present work led to the following conclusions: 1. The laser-induced biomimetic micro-textures designed considering the chip characteristics well captured the geometry features of the structures on the cuticle of Procambarus clarkia. The surface area of the micro-texture grew larger as the laser beam angle decreased or the laser power increased. The decrease of laser beam angle or the increase of laser power led to larger sloping angle of the microtexture. The Al2O3/TiC micro-nano-composite ceramic was less sensitive to laser than was the micro-composite ceramic. The geometry of the micro-cracks on the biomimetic micro-texture exhibited different characteristics when the ceramic material varied from the micro-composite ceramic to the micro-nano-composite ceramic. A new phase of Ti2O arose on the textured composite ceramics. Laser surface texturing led to the oxidation of the composite ceramics. 2. When the laser parameters were fixed, the largest value Sm of the maximum principal stress in the micro-composite ceramic was larger than that in the micro-nano-composite ceramic during the action of a single laser pulse. Sm increased as the laser beam angle or the laser power became larger. Compared with Sm, the fractal dimension of the micro-crack on the micro-structure exhibited opposite evolving trend as the composite ceramic micro-structure or the laser parameters changed. Larger Sm resulted in smaller value of the micro-crack's fractal dimension. The damage of the textured microcomposite ceramic was found to be larger than that of the textured
Acknowledgements This project is sponsored by National Natural Science Foundation of China (Grant No. 51505132), the Fundamental Research Funds for the Universities of Henan Province (Grant No. NSFRF170304), the Science Research Funds for the Universities of Henan Province (Grant No. J2018-3) and the Natural Science Basic Research Plan in Shannxi Province of China (Grant No. 2018JQ5127).
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nanocomposite ceramics Xiaobin Cuia,∗, Kai Yana, Jingxia Guob, Dong Wangc a
School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China c School of Mechanical Engineering, Xi'an Technological University, Xi'an, 710021, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Biomimetic micro-textures Mechanical behaviors Intermittent turning performance Al2O3/TiC ceramics
This work was conducted to investigate biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nano-composite ceramics. Chip characteristics and the geometry features of the structures on the cuticle of Procambarus clarkia were considered in the preparation of the laser-induced biomimetic micro-textures on the composite ceramic surfaces. Characteristics of the biomimetic micro-textures were revealed in terms of geometry, morphology and chemical composition. The connection between the thermal stress resulting from laser pulse and the fractal dimension of the micro-crack on the micro-textures was analyzed and identified. The correlation between the mechanical behaviors (damage and fracture toughness) of the textured composite ceramics and the fractal dimension of the micro-crack was fitted and revealed quantitatively. The performance of the textured composite ceramic tools was pre-evaluated by means of a proposed indicator. Damage, fracture toughness and tool stress were incorporated in the indicator. The indicator and the experimental tool wear were compared for validating the effectiveness of the proposed indicator. It was found that the micro-composite ceramic exhibited greater sensitivity to laser than the micro-nano-composite ceramic did. The damage of the textured micro-composite ceramic was larger than that of the textured micro-nano-composite ceramic when the same laser parameters were utilized in the micro-texture preparation. On the contrary, the fracture toughness of the textured micro-composite ceramic was found to be smaller. There was a negative correlation between the damage of the textured composite ceramic and the fractal dimension of the micro-crack. Conversely, there was a positive correlation between the fracture toughness and the fractal dimension. The performance of the textured micro-nano-composite ceramic tool was better than that of the textured micro-composite ceramic tool. The proposed indicator can be used to predict the combination of laser parameters that resulted in the optimum performance of the textured composite ceramic tool.
1. Introduction Previous study [1] indicated that Al2O3-based ceramics were suitable for cutting hardened steel. Al2O3-based ceramic tools fail abruptly in intermittent turning due to severe mechanical and thermal impacts. The performance of Al2O3-based ceramic tools has been improved by adding secondary phase particles with varying sizes to the tool material [2]. However, intrinsic defects such as relatively low strength and relatively low fracture toughness still limit the intermittent turning performance of Al2O3-based ceramic tools to some extent. The structures on the cuticle of some biological species were found to be capable of reducing external loads and enhancing movement performance [3].
∗
They provided inspiration for the preparation of surface micro-textures on the Al2O3-based ceramic tool. The micro-textures are expected to further improve the intermittent turning performance of Al2O3-based ceramic tools. Laser surface texturing (LST) is widely utilized to fabricate surface micro-textures on various tool materials because it has advantages such as high processing accuracy, no contact force and no pollution. Previous researches on the design of laser-induced micro-textures mainly focused on pattern [4,5], size [6] (width, depth, height, diameter and spacing), distance between micro-textures and cutting edge [7] and integration of micro-textures and lubricant [8]. The existing studies provided much valuable information for the design of tool surface micro-textures. The
Corresponding author. E-mail address: [email protected] (X. Cui).
https://doi.org/10.1016/j.ceramint.2019.06.251 Received 15 June 2019; Received in revised form 23 June 2019; Accepted 24 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiaobin Cui, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.251
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Table 1 Composition (wt%) of the two kinds of Al2O3/TiC composite ceramics. Material
α-Al2O3
TiC (0.4 μm)
TiC (80 nm)
Mo
Sintering aids
Micro-composite ceramic Micro-nano-composite ceramic
55 55
37 17
0 20
5 5
3 3
micro-textures on the tool surface interact with the chip in cutting process. The interaction between the micro-textures and the chip substantially affect the cutting loads and subsequently influence the tool performance. Thus, it is critical to consider the chip characteristics in the design of tool surface micro-textures. However, it was found that few researches were conducted with the characteristics of chip considered. Biomimetics has been extensively employed to provide guide for surface micro-texture design [3]. Previous studies rarely referred to the structures on the cuticle of biological species in the design of the micro-textures on tool surface. The laser changes the properties of the tool material during the LST process. The laser-induced micro-textures prepared on tool material surface were examined by the researchers from different perspectives. Analysis of the laser-induced micro-textures concentrated on geometric dimensions [9], surface quality [10], chemical composition [10], wetting characteristics [11] and friction behaviors [12]. Laser pulses resulted in fierce thermal stress field within the tool material and microcracks arose on the micro-textures. Mechanical behaviors such as damage and fracture toughness of the tool material alter as a result of the actions of laser pulses. These mechanical behaviors had substantial effects on the tool performance in intermittent turning. It can be found from the former studies that scant quantitative analysis of the mechanical behaviors was performed for the textured tool material. Various indicators such as friction coefficient [13], cutting force [13], cutting temperature [13] and tool wear [13–15] were employed in the existing studies for evaluating the textured tool performance. Preevaluation of performance is important for the efficient use of the textured tool. Most of the indicators used in the previous studies were obtained through cutting tests. Pre-evaluation of the textured tool performance is difficult to achieve with the existing indicators. Not only the cutting loads but also the mechanical behaviors of the textured tool greatly influence the tool performance in intermittent turning. Thus, a new indicator should be proposed for pre-evaluating the textured tool performance. Moreover, the cutting loads and the mechanical behaviors of the textured tool should be incorporated in the indicator. This work is conducted to study biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nano-composite ceramics. The structures on the cuticle of the chela of Procambarus clarkia are analyzed to acquire the geometry features. Biomimetic micro-textures are prepared using different combinations of laser parameters on the surfaces of the Al2O3/TiC micro-composite and micro-nano-composite ceramics according to the chip characteristics and the geometry features of the biological structures. Geometry, morphology and chemical composition are examined for the laser-induced biomimetic microtextures. The thermal stress caused by laser pulse and the fractal dimension of the micro-crack on the micro-textures are studied to reveal the connection between them. The mechanical behaviors such as the damage and the fracture toughness of the textured composite ceramics are analyzed to quantitatively identify the correlation between the mechanical behaviors and the micro-crack's fractal dimension. An indicator is proposed for pre-evaluating the intermittent turning performance of the composite ceramic tools with biomimetic micro-textures. Damage, fracture toughness and tool stress are integrated in it. The indicator and the tool wear obtained from turning test are compared to validate the effectiveness of the proposed indicator.
2. Materials and methods 2.1. Preparation of the Al2O3/TiC micro-composite and micro-nanocomposite ceramics The Al2O3/TiC micro-composite and micro-nano-composite ceramics studied in this work were prepared using hot-pressing sintering. The initial powders used in the preparation were particles of α-Al2O3, TiC and Mo. The size of α-Al2O3 particle (purity: 99.9%) was 0.5 μm and the size of Mo particle (purity: 99.5%) was 1.2 μm. TiC particles with a purity of 99.9% and a size of 0.4 μm were used for fabricating the micro-composite and the micro-nano-composite ceramics. TiC particles with a purity of 99.9% and a size of 80 nm existed in the initial powder of the micro-nano-composite ceramic besides the micro-particles of TiC. Sintering aids, MgO and Y2O3, were employed to ensure the densification of the composite ceramics in hot-pressing sintering. The hybrid slurries of the initial powders were ball-milled and dried. Compositions of the two kinds of composite ceramics are presented in Table 1. The hybrid powders were compacted after sieving. The powders were sintered at a temperature of 1700 °C in nitrogen atmosphere. The polished surfaces and the fractured surfaces of the prepared composite ceramics were observed by means of scanning electron microscope (SEM) (Merlin Compact, Germany). The enlarged images of the polished surfaces of the composite ceramics are shown in Fig. 1(a) and (b). It was found that the average grain size of the micro-nanocomposite ceramic was much smaller. SEM images of the fractured surfaces of the micro-composite ceramic and the micro-nano-composite ceramic are presented in Fig. 1(c) and (d), respectively. Intergranular fracture and tansgranular fracture were found to be the main fracture modes of the micro-composite ceramic and the micro-nano-composite ceramic, respectively. A microscopic hardness tester (HV-1000, China) was used to analyze the composite ceramics. Fracture toughness KIC (MPa*m1/2) of the composite ceramics can be determined according to the experimental results acquired from the microindentation tests [16]. The load applied in the test was 9.8 N. The dwelling time was 10 s. Nanoindentation tests were carried out using a nanoindentation system (G200, USA) at room temperature. A Berkovich indenter was used in the tests. The peak load was set to be 100 mN. The loading rate was 10 mN/s. The dwelling time at the peak load was 10 s. Elastic modulus of the composite ceramics can be determined based on the nanoindentation tests [17]. Each test was replicated 6 times. The fracture toughness and the elastic modulus of the micro-composite ceramic were found to be 4.6 MPa*m1/2 and 426 GPa, respectively. These mechanical property parameters of the micro-nano-composite ceramic were 5.1 MPa*m1/2 and 445 GPa, respectively. 2.2. Examination of the cuticle of procambarus clarkia Fig. 2 shows the typical images of Procambarus clarkia. Procambarus clarkia survives and prospers in environment full of sludge which can be considered as non-Newtonian fluid. Previous study indicated that chip which arose in cutting process exhibited features of non-Newtonian fluid [18]. Taking into account the similarity between sludge and chip, the cuticle of Procambarus clarkia was examined using digital microscope (VHX-1000E, Japan). Chela is important to the
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Fig. 1. The polished surfaces and the fractured surfaces of the prepared composite ceramics.
movement of Procambarus clarkia. The zone of the chela cuticle denoted in Fig. 2(c) was focused on in the examination process. The geometry features of the structures on chela cuticle provided critical information for the design of the biomimetic micro-textures on the composite ceramic surfaces.
frequency, scanning speed and the number of scans were set to be constant. The values of them were 70 kHz, 70 mm/s and 3, respectively. SEM, energy dispersive X-ray (EDX) (X-MAX80, UK) and X-ray diffraction (XRD) (D8 Advance, Germany) were employed to analyze the textured composite ceramics. Fracture toughness KIC (MPa*m1/2), elastic modulus of the textured composite ceramics were obtained on the basis of microscopic hardness tests and nanoindentation tests. The test parameters were the same to those utilized for the composite ceramics with no laser-induced micro-textures.
2.3. Laser surface texturing Biomimetic micro-textures were fabricated on the surfaces of the two kinds of composite ceramics considering the geometry features of the structures on chela cuticle. The laser pulse duration was 100 ns The wavelength of the laser was equal to 1064 nm. The power of the laser was set in the range of 2 W to 4 W in the laser surface texturing process. The angle αl of the laser beam varied from 50° to 90° as shown in Fig. 3. The value of the laser beam angle was defined considering the relative movement direction of the composite ceramic tool to chip. Pulse
2.4. Intermittent turning tests AISI 52100 steel (60 HRC) was turned intermittently by means of tools made of the Al2O3/TiC micro-composite and micro-nano-composite ceramics as presented in Fig. 4. Biomimetic micro-textures were manufactured on the tool surface by laser surface texturing. The
Fig. 2. Typical images of Procambarus clarkia. 3
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Fig. 3. Angle αl of the laser beam.
the heat source on the composite ceramics changed from a circle to an ellipse as the angle αl varied from 90° to the other values. The area of the heat source changed correspondingly due to the variation of the angle αl. It should be noted that the diameter of the circle and the minor axis of the ellipse had the same magnitude. Thus, the effects induced by the major axis of the ellipse were focused on in the analysis. The intensity distribution of the heat source was dominated by both the geometry (shape and area) of the heat source and the energy of the Gaussian laser pulse. The pulse frequency and the laser power were used for the calculation of the energy of the Gaussian laser pulse. The action of a single laser pulse on the two kinds of composite ceramics was simulated based on the geometry and the intensity distribution of the heat source. Taking the characteristics of microstructures into account, the two-dimensional models of the composite ceramics analyzed in the simulation was established according to the work by Wang et al. [20]. The two-dimensional models were built with Voronoi diagram [20–22] and random method used. The heat source was set at edge 1. Heat radiation and convection took place at edges 2, 3 and 4. The ablated zone of the two-dimensional models was determined according to the temperature distribution and the melting point of the composite ceramic. The stresses in the remaining zone were critical for the generation of microcracks. Thus, the stress components of each element in the remaining zone were obtained and recorded for subsequent analysis. Fig. 6 shows the schematic of the turning simulation. The geometries and the materials of the textured composite ceramic tools and the workpiece studied in the simulation were the same to those used in turning tests. The textured composite ceramic tool was considered to be an elastic body. The workpiece behavior was investigated using Johnson–Cook model [23] which incorporated the influences caused by temperature and strain rate. The essential parameters in the Johnson–Cook model were determined on the basis of the study by Ramesh and Melkote [23]. For the purpose of ensuring the simulation accuracy, local refining technology was employed to refine the textured composite ceramic tool and the workpiece as shown in Fig. 6. The movements of the workpiece and the textured composite ceramic tool were set according to the turning tests.
Fig. 4. Schematic of intermittent turning.
macroscopic geometry of the composite ceramic tools was the same. The rake angle of the composite ceramic tools was manufactured to be −6°. The cutting edge angle had the value of 90°. The clearance angle was set as 6°. Non-cutting length ln and cutting length lc were denoted in Fig. 4. The ratio of ln to lc was 14.7. Cutting speed was 56 m/min. Feed rate was 0.12 mm/r. Depth of cut (ap) was 1 mm. Each test was performed 6 times for each group of the biomimetic micro-texture and the composite ceramic tool. The cutting length was set to be invariable for all the turning tests and the value was 600 m. An optical microscope was employed to examine the flank wear VB of the textured composite ceramic tools. SEM was used to analyze the textured composite ceramic tools after turning tests. 2.5. Numerical simulations Numerical simulations of laser surface texturing and intermittent turning were conducted for the acquisition of the data difficult to obtain through tests. Laser surface texturing can be considered as a process determined by a certain number of laser pulses. The effects of a single laser pulse on the composite ceramics were concentrated on in the numerical simulation. Since the duration of the nanosecond laser pulse is much larger than the electron-phonon-relaxation time [19], the action of a single laser pulse on the composite ceramics can be modeled as a heat diffusion problem. The heat diffusion problem was analyzed as presented in Fig. 5. It has been mentioned that the angle αl of the laser beam was in the range of 50° to 90° and the laser power P was in the range of 2 W to 4 W. The shape of
3. Results and discussion 3.1. Characteristics of the laser-induced biomimetic micro-textures on the composite ceramic surfaces The micro-textures designed with the chip characteristics considered are expected to efficiently enhance the tool performance. 4
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Fig. 5. Analysis of the action of laser on the composite ceramics.
Taking the high temperature, high strain and high strain rate into consideration, the chip which interact with the tool surface can be thought to be non-Newtonian fluid [18]. Procambarus clarkia is often found to move in sludge which can also be considered as non-Newtonian fluid. The structures on the cuticle of Procambarus clarkia provide inspiration for the design of the micro-textures on composite ceramic tool surface. The cuticle of the chela of Procambarus clarkia was analyzed. Fig. 7 shows the typical images of the structures on the cuticle. Fig. 7 indicates that there existed many crescent-shaped micropits on the cuticle. There was an obvious correspondence between the shape of the micro-pits and the movement direction of the chela. The “crescent” consisted of a large arc and a small arc. Theoretically, the large arc of the “crescent” first contacted with the sludge as the chela moved forward. It should be noted that the sloping direction of the crescent-shaped micro-pit was the same to the movement direction of the chela. The analysis results of the structures on the cuticle of Procambarus clarkia provided the basis for the design of the biomimetic micro-textures on composite ceramic surfaces. Biomimetic micro-textures, namely crescent-shaped micro-pits, were prepared on the surfaces of Al2O3/TiC micro-composite and micro-nano-composite ceramics. Fig. 8 presents the typical morphologies of the biomimetic micro-textures. The biomimetic micro-textures well captured the geometry features of the structures on the cuticle of Procambarus clarkia. There was also a correspondence between the shape of the biomimetic micro-texture and the relative movement direction of the composite ceramic tool. The
Fig. 6. Schematic of the turning simulation.
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Fig. 7. Typical images of the structures on the cuticle of the chela of Procambarus clarkia.
sloping direction of the biomimetic micro-texture was set to be the same to the relative movement direction of the composite ceramic tool. The surface area A and the sloping angle φ denoted in Fig. 8 were analyzed for the biomimetic micro-textures. Fig. 9 shows the values of the surface area A and the sloping angle φ obtained at different combinations of laser parameters such as laser beam angle αl and laser power P. The surface area A of the micro-texture became smaller with the increase of αl. Conversely, larger P resulted in larger surface area A. The decrease of αl or the increase of P led to larger sloping angle φ. The values of A and φ of the biomimetic micro-texture on the micro-
composite ceramic surface were larger than those on the micro-nanocomposite ceramic surface at the same group of laser parameters. Moreover, the variations of A and φ were found to be greater when the biomimetic micro-texture were manufactured on the micro-composite ceramic. These results indicated that the micro-composite ceramic was more sensitive to laser than was micro-nano-composite ceramic. Typical SEM images of the biomimetic micro-textures on the microcomposite ceramic surface and the micro-nano-composite ceramic surface are presented in Fig. 10. These biomimetic micro-textures were prepared at the same laser parameters. Comparisons were carried out
Fig. 8. Typical morphologies of the biomimetic micro-textures (Al2O3/TiC micro-nano-composite ceramic, laser beam angle αl = 70°, laser power P = 3W). 6
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Fig. 9. Surface area A and sloping angle φ of the micro-textures obtained at different combinations of laser parameters such as laser beam angle αl and laser power P.
for the biomimetic micro-textures on the surfaces of different composite ceramics. The depth of the destroyed zone was relatively larger when the biomimetic micro-textures were manufactured on the micro-composite ceramic surface. This phenomenon further revealed the higher sensitivity of the micro-composite ceramic to the action of laser. It can be observed from Fig. 10(g) and (h) that micro-cracks appeared on the biomimetic micro-texture. It should be noted that the geometry characteristics of the micro-cracks varied as the ceramic material changed from the micro-composite ceramic to the micro-nano-composite ceramic. The appearance of these micro-cracks was accompanied by the change of the mechanical behaviors of the composite ceramics. It was inferred that a close correlation existed between the mechanical behaviors of the textured composite ceramics and the geometry characteristics of these micro-cracks. XRD analysis was performed for the textured micro-composite ceramic and the textured micro-nano-composite ceramic as presented in Fig. 11(a) and (b). A new phase of Ti2O arose on the two kinds of composite ceramics after laser surface texturing. EDX analysis results of point 1 and point 2 denoted in Fig. 10 are illustrated in Fig. 11(c) and (d), respectively. The relatively high oxygen content indicates the oxidation of the composite ceramics resulting from laser surface texturing.
ceramic was larger than that in the micro-nano-composite ceramic at the same laser parameters. The value of Sm increased when αl or P became larger. The geometry characteristics of the micro-cracks caused by laser pulses were studied in terms of fractal dimension. Image enhancement was performed for the typical images of the micro-cracks in Fig. 13(a) and (b). The micro-cracks edges were obtained as presented in Fig. 13(c) and (d) after image binaryzation. Box counting method [24] was used in the fractal analysis of the micro-cracks on the basis of Fig. 13(c) and (d). The fractal image of the micro-crack can be covered by a certain amount Cs of squares with a length Ls. Ls can be presented as: (1)
Ls = n
where n is a positive integer and δ is the pixel size. The value of Cs changed as the value of Ls varied. The correlation between Cs and Ls can be defined using the following equation:
Cs = Con Ls
(2)
Fra
In Eq. (2), Con is used to represent a constant and the micro-crack's fractal dimension was represented by Fra. Eq. (2) can be derived as:
log Cs = log Con
3.2. Mechanical behaviors of the textured Al2O3/TiC micro-composite and micro-nano-composite ceramics
Fra logLs
(3)
Eq. (3) indicates that the micro-crack's fractal dimension can be fitted based on the data of logCs and logLs. Fig. 13(e) and (f) show the data fitting and the results of Fra for the typical micro-cracks. When the laser parameters were fixed, it was found that Fra of the micro-crack obtained for the textured micro-composite ceramic was relatively smaller than that acquired for the textured micro-nano-composite ceramic. Fig. 13(g) and (h) present the values of the fractal dimension Fra obtained at varying combinations of laser parameters such as beam angle αl and laser power P. It can be found that relatively smaller Fra arose at relatively larger values of αl and P. The evolving trend of fractal dimension Fra with laser parameters was opposite to that of Sm. Therefore, it can be deduced that larger Sm resulted in smaller value of Fra. Fracture is the dominant failure mechanism of ceramic tools in intermittent turning [2]. Mechanical behaviors such as damage and fracture toughness were critical to the failure of the composite ceramic
The mechanical behaviors of the composite ceramic tools changed because of the formation of the biomimetic micro-textures. High temperature gradient emerged within the composite ceramics due to the action of laser pulses. The high temperature gradient led to great stress gradient which substantially influenced the generation of micro-cracks and the mechanical behaviors of the textured composite ceramics. The maximum principal stress of the elements in the remaining zone denoted in Fig. 5 was obtained based on the finite element simulation of the action of a single laser pulse. The present work focused on the maximum principal stress that arose at the highest temperature. The largest value of the maximum principal stress in the remaining zone was represented by Sm. Fig. 12 shows the values of Sm acquired at different combinations of laser parameters. The laser parameters were laser beam angle αl and laser power P. Sm in the micro-composite 7
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Fig. 10. Typical SEM images of the biomimetic micro-textures on the micro-composite ceramic surface and the micro-nano-composite ceramic surface (laser beam angle αl = 70° and laser power P = 3W).
tools with biomimetic micro-textures. Taking these into consideration, the damage D and the fracture toughness KIC of the textured composite ceramics were obtained. The correlation between the mechanical behaviors and the micro-crack's fractal dimension was identified quantitatively. It was assumed that the damage of the textured composite ceramics was isotropic and it could be represented by a scalar. It was considered in this work that there was no damage within the composite ceramics before laser surface texturing. The degradation of elastic modulus was employed to acquire the damage [25]. The relationship between the elastic modulus ED of the textured ceramic and the elastic modulus E of the untextured ceramic can be expressed as:
ED = E (1
D)
The value of D was obtained based on the values of ED and E acquired from nanoindentation tests. Fig. 14 presents the scatter plot and the data fitting for the fractal dimension Fra and the damage D. It was found from the scatter plot in Fig. 14(a) and (b) that the damage of the textured micro-composite ceramic was larger than that of the textured micro-nano-composite ceramic when using the same laser parameters. The relatively large absolute values of Pearson correlation coefficient indicate that a close correlation appeared between the damage D of the textured composite ceramic and the micro-crack's fractal dimension Fra. Moreover, there was a negative correlation between them. The correlation between D and Fra was fitted as presented by Fig. 14(c) and (d). The largest value of R2 arose when this correlation was evaluated in terms of quadratic polynomial. This correlation can be defined for the textured micro-composite ceramic using the following equation:
(4)
According to Eq. (4), the damage D of the textured composite ceramic can be derived as:
D=1
ED/ E
D = 0. 91Fra 2
(5)
2.26Fra + 1.50
(6)
As for the textured micro-nano-composite ceramic, this correlation 8
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Fig. 11. XRD analysis and EDX analysis of the textured composite ceramics.
can be expressed as:
D = 0. 26Fra 2
0.84Fra + 0.70
be used to define the correlation for the textured micro-composite ceramic and the textured micro-nano-composite ceramic, respectively. Eq. (9) can be established as:
(7)
Fracture toughness KIC of the textured composite ceramic was calculated using the following formula [16]:
KIC = 0.203 HV (d i/2)1/2 (d i/2ci)3/2
KIC =
13. 69Fra 2 + 37.57Fra
(9)
20.87
Eq. (10) can be expressed as:
(8)
KIC = 3. 56Fra 2
where HV is microscopic hardness, di is used to represent the diagonal length of the microindentation mark, ci is half the crack length caused by microindentation. Fig. 15 presents the scatter plot and the data fitting for the fractal dimension Fra and the fracture toughness KIC. The data in the scatter plot shown in Fig. 15(a) and (b) were analyzed. KIC of the textured micro-composite ceramic was found to be smaller than that of the textured micro-nano-composite ceramic when the same laser parameters were applied. Fig. 15(a) and (b) show that the values of Pearson correlation coefficient were relatively large. This meant that there was also a close correlation between the fracture toughness KIC of the textured composite ceramics and the micro-crack's fractal dimension Fra. It should be noted that there was a positive correlation between them. Fig. 15(c) and (d) present the fitting of the correlation between KIC and Fra. Quadratic polynomial was found to be the most suitable formula for describing this correlation. Eq. (9) and Eq. (10) can
0.17Fra
0.36
(10)
The results and discussions presented above validated the inference that a close correlation existed between the mechanical behaviors of the textured composite ceramics and the geometry characteristics of the micro-cracks. 3.3. Intermittent turning performance of the textured Al2O3/TiC microcomposite and micro-nano-composite ceramics The formation and the existence of the laser-induced biomimetic micro-textures substantially affected damage D, fracture toughness KIC and tool stress σ in cutting process, which subsequently influenced the intermittent turning performance of the composite ceramic tools. Performance pre-evaluation is critical to the efficient application of the textured composite ceramic tools. Thus, it is important to propose an
Fig. 12. Values of Sm acquired at different combinations of laser parameters such as laser beam angle αl and laser power P. 9
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Fig. 13. Fractal analysis of the micro-cracks on the micro-textures.
indicator capable of pre-evaluating the intermittent turning performance. Moreover, D, KIC and σ should be incorporated in the proposed indicator. In this work, a new indicator was established to pre-evaluate the tool performance considering the high strain rate in the textured composite ceramic tools during intermittent turning. Damage equivalent stress σ∗ can be defined for the textured composite ceramic tools by means of Eq. (11) [25]:
{
= (1 + )
+:
+
tr
2
+
1 D [(1+ ) 1 hD
:
tr
2]
=
11
12
21
22
13 23
31
32
33
(12)
For the purpose of reducing the complexity, the tri-axial stress was simplified as bi-axial compressive stress. The material element shown in Fig. 16(a) was employed to describe the bi-axial stress state. In this case, σ can be defined as:
}
1/2
t11 ( t )
=
(11)
0 0
0 t22 ( t )
0
0 0 0
(13)
In Eq. (13), σt11 and σt22 are equal to σt and they have negative values. The tri-axial tool stress was acquried from cutting simulation. Eq. (11) to Eq. (13) can be utilized to obtain σt according to the hypothesis that the tri-axial stress and the bi-axial stress resulted in the same value of σ∗. An array of intrinsic wing cracks was thought to
where σ is tool stress, ν is used to represent the Poisson's ratio of the composite ceramic, h is 0.2 [25]. The textured composite ceramic tools were under a tri-axial stress state during cutting. The representative volume element (RVE) shown in Fig. 16(a) was used to illustrate the triaxial stress state. In this case, tool stress σ can be expressed as: 10
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Fig. 14. Scatter plot and data fitting of the damage D and the fractal dimension Fra.
Fig. 15. Scatter plot and data fitting of the fracture toughness KIC and the fractal dimension Fra.
appear in the material element. There existed a spacing 2w between the cracks. The model of a single wing crack [26] is presented in Fig. 16(b). As for the crack array, the dynamic stress intensity factor KID is calculated as [27]:
KID = kIDa (l ) KIDa + kIDb (l ) KIDb
K
Da
= F sin
w sin
(l + l ) w
1/2
(15)
where θ can be considered to have a value of 0.392π [26], l is the initial tensile crack length which has a value of zero, w is equivalent to 4c [28], l∗ is 0.27c [26], c is a half of the average grain size and F can be defined as:
(14)
where l is the growth rate of tensile crack. Both the concentrated load and the uniform load are considered in Eq. (14). KIDa in Eq. (14) can be expressed as:
F = 2c
11
(16)
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Fig. 16. RVE, material element, crack model and Ip of the textured composite ceramic tools prepared at varying combinations of laser parameters such as laser beam angle αl and laser power P.
where τ∗ is given by:
1 ( 2
=
t11
t22 )sin
2
+
1 µ[ 2
t11
+
t22
(
t11
t22)cos
during a cutting cycle was represented by KMID. A new indicator Ip for the performance pre-evaluation of the textured composite ceramic tools is proposed as:
2 ] (17)
Ip =
KIDb in Eq. (14) can be calculated as:
K
Db
=
t22
2w tan
l 2w
(18)
Cr l Cr 0.75l
(19)
where Cr is used to represent the Rayleigh wave speed and it can be calculated using Eq. (20) [29]:
Cr =
0.862 + 1.14 1+
E 2 (1 + )
(20)
where ρ is the density of the composite ceramic under consideration. kIDb (l ) can be given as:
kIDb (l ) =
Cr l Cr 0.5l
(21)
The growth rate l of tensile crack can be calculated as [26]:
l=
Cr (1.5KIDa + 1.75KIDb
1.25KIC
KIDa + 1.5KIDb
0.75KIC
) (22)
where χ is given by:
= (1.5KIDa + 1.75KIDb (0.5KIDa + 0.75KIDb
1.25KIC)2 0.375KIC)
4(KIDa + KIDb
(24)
The value of the indicator Ip can be acquried based on Eq. (1) to Eq. (24). Smaller Ip corresponds to lower fracture risk of the textured composite ceramic tool. Thus, lower tool wear is likely to arise when the value of Ip is relatively smaller. It can be deduced from Eq. (24) that smaller Ip corresponds to lower tool wear. Moreover, damage D, fracture toughness KIC and tool stress σ in cutting process are all incorporated in the indicator Ip. Fig. 16(c) shows the values of Ip for the textured micro-composite ceramic tool prepared at varying combinations of laser parameters such as laser beam angle αl and laser power P. Fig. 16(d) shows the values of Ip for the textured micro-nano-composite ceramic tool. The value of Ip acquried for the textured micro-composite ceramic tool was found to be relatively larger at each group of laser parameters. Thus, relatively better performance was expected for the textured micro-nano-composite ceramic tool. Indicator Ip decreased first and then increased as αl or P grew larger. The smallest Ip of the textured micro-composite ceramic tool appeared when αl was 60° and P was 2.5W. The optimum performance can be expected for the textured micro-composite ceramic tool at this combination of laser pamaters. The smallest Ip of the textured micro-nano-composite ceramic tool arose when αl was 70° and P was 3W. These laser parameters were considered to result in the best performance. The typical images of the flank face are presented in Fig. 17(a) and Fig. 17(b) for the two kinds of textured composite tools which have been tested in intermittent turning. The main tool failure mechanism was found to be fracture that occurred at different scales. The wear of the untextured composite ceramic tools were found to be larger than those of the textured composite ceramic tools at most of the laser parameter combinations. Fig. 17(c) and (d) show the wear VB of the
1/2
kIDa (l ) and kIDb (l ) in Eq. (14) can be determined according to work by Nemat-Nasser and Deng [27]. kIDa (l ) can be presented as: kIDa (l ) =
KMID KIC
KIC) (23)
The maximum value of the dynamic stress intensity factor KID 12
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Fig. 17. Typical failure images and wear VB of the textured composite ceramic tools manufactured at different combinations of laser parameters such as laser beam angle αl and laser power P.
textured composite ceramic tools manufactured at different combinations of laser parameters such as laser beam angle αl and laser power P. Comparisons of Fig. 17(c) and (d) indicate that the textured micronano-composite ceramic tool exhibited better performance. The evolving trend of tool wear VB with laser parameters was the same to that of the indicator Ip. Furthermore, the optimum intermittent turning performance of the textured composite ceramic tool can be acquired at the combination of laser parameters that led to the smallest Ip. These validated the effectiveness of the indicator Ip.
micro-nano-composite ceramic when the same laser parameters were used in the LST process. The fracture toughness of the textured micro-composite ceramic was smaller than that of the textured micro-nano-composite ceramic at the same laser parameter combination. A negative correlation existed between the damage of the textured composite ceramic and the micro-crack's fractal dimension. There was a positive correlation between the fracture toughness of the textured composite ceramic and the micro-crack's fractal dimension. 3. An indicator incorporating damage, fracture toughness and tool stress was established for pre-evaluating the intermittent turning performance of the textured micro-composite and micro-nanocomposite ceramic tool. The indicator of the textured micro-composite ceramic tool was relatively larger at each laser parameter combination. The value of the indicator decreased first and then increased as the laser beam angle αl or the laser power P grew larger. The smallest indicator of the micro-composite ceramic tool appeared when αl and P were 60° and 2.5 W respectively. The smallest indicator of the textured micro-nano-composite ceramic tool arose when αl and P were 70° and 3 W respectively. The performance of the textured micro-nano-composite ceramic tool was relatively better. The evolving trend of the tool wear with the composite ceramic micro-structure or the laser parameters was the same to that of the indicator. The combination of laser parameters that led to the smallest value of the indicator resulted in the optimum performance of the textured composite ceramic tool. The effectiveness of the proposed indicator was validated.
4. Conclusions The present work led to the following conclusions: 1. The laser-induced biomimetic micro-textures designed considering the chip characteristics well captured the geometry features of the structures on the cuticle of Procambarus clarkia. The surface area of the micro-texture grew larger as the laser beam angle decreased or the laser power increased. The decrease of laser beam angle or the increase of laser power led to larger sloping angle of the microtexture. The Al2O3/TiC micro-nano-composite ceramic was less sensitive to laser than was the micro-composite ceramic. The geometry of the micro-cracks on the biomimetic micro-texture exhibited different characteristics when the ceramic material varied from the micro-composite ceramic to the micro-nano-composite ceramic. A new phase of Ti2O arose on the textured composite ceramics. Laser surface texturing led to the oxidation of the composite ceramics. 2. When the laser parameters were fixed, the largest value Sm of the maximum principal stress in the micro-composite ceramic was larger than that in the micro-nano-composite ceramic during the action of a single laser pulse. Sm increased as the laser beam angle or the laser power became larger. Compared with Sm, the fractal dimension of the micro-crack on the micro-structure exhibited opposite evolving trend as the composite ceramic micro-structure or the laser parameters changed. Larger Sm resulted in smaller value of the micro-crack's fractal dimension. The damage of the textured microcomposite ceramic was found to be larger than that of the textured
Acknowledgements This project is sponsored by National Natural Science Foundation of China (Grant No. 51505132), the Fundamental Research Funds for the Universities of Henan Province (Grant No. NSFRF170304), the Science Research Funds for the Universities of Henan Province (Grant No. J2018-3) and the Natural Science Basic Research Plan in Shannxi Province of China (Grant No. 2018JQ5127).
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