Int. Journal of Refractory Metals and Hard Materials 45 (2014) 125–129
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Al2O3/(W,Ti)C/CaF2 multi-component graded self-lubricating ceramic cutting tool material C.H. Xu ⁎, G.Y. Wu, G.C. Xiao, B. Fang School of Mechanical and Automotive Engineering, Qilu University of Technology, Jinan 250353, PR China Key Laboratory of Advanced Manufacturing and Measurement & Control Technology for Light Industry in Universities of Shandong, Qilu University of Technology, Jinan 250353, PR China
a r t i c l e
i n f o
Article history: Received 25 November 2013 Accepted 8 April 2014 Available online 18 April 2014 Keywords: Self-lubricating cutting tool Functionally graded ceramic Mechanical property Cutting performance
a b s t r a c t Since the existed compositional distribution functions are not applicable to the design of functionally graded materials consisting of more than two main components, a multi-component gradient distribution model was proposed during the design process. An advanced Al2O3/(W,Ti)C/CaF2 multi-component graded self-lubricating ceramic cutting tool material was subsequently developed. The flexural strength, hardness, and fracture toughness of the as-developed material were increased by about 25%, 19%, and 6%, respectively, as compared with the corresponding homogeneous material. The cutting tests show that the Al2O3/(W,Ti)C/CaF2 multi-component graded self-lubricating ceramic cutting tool has better wear resistance than the homogeneous cutting tool. © 2014 Elsevier Ltd. All rights reserved.
Introduction Ceramic cutting tool materials are superior to conventional tool materials such as high speed steel and cemented carbide because of their excellent performance in terms of their high hot hardness, chemical inertness, and high thermal stability [1–3]. Meanwhile, dry cutting is widely popularized as an environment-friendly and cost-saving machining technology [4–6]. However, the relatively high friction coefficient at the tool–workpiece interface of ceramic cutting tools often generates excess cutting heat during dry cutting, which eventually leads to serious thermal wear and the reduction of the tool's lifespan. One of the effective means to improve the tribological properties of ceramic cutting tools is to prepare self-lubricating cutting tool materials by adding solid lubricants to the ceramic matrix [7]. Previous studies reported that adding solid lubricants could both positively and negatively influence the properties of ceramic materials [8–10]. The added solid lubricants can reduce the friction coefficient by forming a lubricating film in the working areas. However, the dispersed solid lubricants can limit the mechanical properties of the ceramic matrix. Consequently, the wear resistance of the self-lubricating ceramic materials worsens because of their lower mechanical properties [11]. To provide ceramic cutting tool materials with the desired friction-reducing property and considerable wear resistance, an idea of graded self-lubrication was proposed for the design of ceramic cutting tool materials [12]. The graded self-lubricating ceramic cutting tool material was designed with one compositional distribution that ⁎ Corresponding author at: No. 3501, University Road, Changqing District, Jinan 250353, PR China. Tel.: +86 531 89631131. E-mail address:
[email protected] (C.H. Xu).
http://dx.doi.org/10.1016/j.ijrmhm.2014.04.006 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
changes along the direction of its thickness. However, the existed compositional distribution functions are not applicable to the design of functionally graded materials consisting of more than two main components. In the present study, a multi-component gradient distribution model was proposed and used in the fabrication of Al2O3/ (W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool material. Its mechanical properties and machining performance were studied in detail. Material and methods The Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool material was prepared using a layer stacking method and hot pressing technique. The starting powders α-Al2O3, CaF2 and (W,Ti)C were commercially available, with the average particle size of about 1 μm and purity higher than 99.8%. The symmetrical seven-layer structure required four types of mixed powders with different constituent ratios (Table 1). Each of them was separately prepared using the wet ballmilling technique. The mixed powders were laminated into a graphite mold in the order of their layer numbers, from the first to the seventh layer. The amount of mixed powders in each layer was derived from the layer's designed thickness which was determined by the compositional distribution exponent n1 and n2. After padding, the seven-layer mixed powders were uniaxially compacted into a green body by cold pressing, and then hot-pressed in a vacuum at 1550 °C for 20 min under 30 MPa. The obtained ceramic disk was approximately 42 mm in diameter and 6 mm in thickness. For comparison, a homogeneous Al2O3/(W,Ti)C/CaF2 self-lubricating ceramic disk was prepared using the same ceramic powders as the surface layers under the same fabricating conditions.
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Table 1 Compositions and thermal expansion coefficients of graded layers. Layer number
Content of CaF2 (vol.%)
Volumetric ratio of (W,Ti)C:α-Al2O3 in the matrix (vol.%)
Thermal expansion coefficient (×10−6/K)
4th 3rd;5th 2nd;6th 1st;7th
0 3.3 6.7 10
30:70 40:60 50:50 60:40
7.660 7.516 7.386 7.262
The ceramic disks were cut into bars and made into test specimens by grounding and polishing. The flexural strength was measured using the three-point-bending technique with a span of 20 mm at a crosshead speed of 0.5 mm/min. The Vickers hardness was measured on the polished surface using a Vickers hardness tester with a load of 196 N and a loading time of 15 s. The fracture toughness was evaluated by the indentation method. The crack length was measured under optical microscope. Five specimens for each ceramic disk were tested to obtain the respective average values of flexural strength, Vickers hardness and fracture toughness. Cutting tests were carried out on a CDE6140A lathe under dry cutting condition. The specification of the tool holder is CSSNR2525M12-MN7 H1BM I10. Both the graded and homogeneous Al2O3/(W,Ti)C/CaF2 cutting tool materials were made into tool inserts with the type of ISO SNGN120604 and a chamfered cutting edge (− 20° × 0.2 mm). The workpiece material was the 45# hardened carbon steel with a hardness of HRC25–30. The machining process was performed at cutting speeds of 90 m/min and 150 m/min, a constant feed rate of 0.102 mm/rev and a constant depth of cut of 0.2 mm. The average flank wear VB was measured by an optical microscope. The gradient microstructure and wear morphologies of the developed ceramic tools were observed using environmental scanning electron microscopy (ESEM; FEI-quanta 200). Theory/calculation The graded self-lubricating ceramic cutting tool material was designed to be disk-shape, with a compositional distribution that changes along the direction of its thickness. A symmetrical power-law distribution function was adopted as basic function so that the top and bottom surfaces of the composite can work as rake faces of the cutting tool with the same cutting performance [13]. In the multi-component distribution model, pores and sintering additives are not taken into account because of their very small amounts. First, by regarding α-Al2O3 and (W,Ti)C as a matrix component, the volume fraction of CaF2 in Al2O3/(W,Ti)C/CaF2 material along the thickness of the material is given as:
V CaF2
8 0:5−ξn1 CF CF CF > > þ f0 < f 1 −f 0 0:5 ¼ f 1 ðξÞ ¼ ξ−0:5 n1 > CF > : f 1 CF −f 0 CF þ f0 0:5
0 ≤ξ≤0:5
ð1Þ
0:5≤ξ≤1
where ξ is the ratio of arbitrary coordinate value to the total thickness of the material; f1CF and f0CF are the volume fractions of CaF2 in the surface layers and the middle layer, respectively; and n1 is the compositional distribution exponent of CaF2. Then the composition profile of the Al2O3/(W,Ti)C matrix component is determined as V Al2 O3 =ðW;TiÞC ¼ 1−V CaF2 . The volume fraction of (W,Ti)C in the matrix along the thickness of the Al2O3/(W,Ti)C/CaF2 material is given as:
V ðW;TiÞC
8 0:5−ξn2 WT WT WT > > þ f0 < f 1 −f 0 0:5 ¼ f 2 ðξÞ ¼ ξ−0:5n2 > WT > : f 1 WT −f 0 WT þ f0 0:5
0≤ξ ≤0:5 0:5 ≤ξ≤1
ð2Þ
where f1WT and f0WT are the volume fractions of (W,Ti)C in Al2O3/(W,Ti) C matrixes of the surface layers and the middle layer, respectively; and n2 is the compositional distribution exponent of (W,Ti)C. The volume fraction of (W,Ti)C and Al2O3 in Al2O3/(W,Ti)C/CaF2 material along the thickness of the material can then be given respectively as follows: V ðW;TiÞC ¼ ð1−f 1 ðξÞÞf 2 ðξÞ
ð3Þ
V Al2 O3 ¼ ð1−f 1 ðξÞÞð1−f 2 ðξÞÞ ¼ 1−V CaF2 −V ðW;TiÞC :
ð4Þ
The combinations of nonnegative values of n1 and n2 can determine the compositional distributions of Al2O3/(W,Ti)C/CaF2 material and then its gradients of microstructure as well as properties. According to values of n1 and n2, Al2O3/(W,Ti)C/CaF2 material can be categorized into five types: (a) n1 = n2 = 0; (b) n1 = 0, n2 ≠ 0; (c) n1 ≠ 0, n2 = 0; (d) n1 = n2 ≠ 0 and (e) n1 ≠ n2 ≠ 0. The proper values of n1 and n2 can be selected from the five types according to the following two design rules. First, the CaF2 content gradually decreases from the surface layers to the middle layer of the material. Second, residual compressive stresses are formed in the surface layers after the sintering process, which requires the thermal expansion coefficient of the material decreasing from the middle layer to the two surface layers [14]. Types (a) and (b) are firstly rejected because the CaF2 content keeps constant along the thickness direction, which does not conform to the first design rule. The three remaining types are assessed under the second design rule. Instead of continuously graded distribution, a symmetrically laminated structure was adopted in the study. To obtain each layer's component ratio and designed thickness of Al2O3/(W,Ti)C/CaF2 graded material, first to use the golden section method [13] to divide the material into k layers (k is the designed layer number) on the distribution curve of CaF2 (corresponding to the formula (1) in this paper) and the distribution curve of (W,Ti)C (corresponding to the formula (2) in this paper), respectively, and then combine all of the layers' boundaries. According to the golden section method, the layer thickness on each distribution curve is determined by the compositional distribution exponent. For the graded system of type (c), the Al2O3/(W,Ti)C matrix can be regarded as one component because of n2 = 0, and the layer division needs only be conducted on the distribution curve of CaF2, thus the actual layer number of the graded material is equal to k. For the graded system of type (d), the corresponding layer boundaries on the two distribution curves are completely coincident because of n1 = n2, thus the actual layer number of the graded material is equal to k. For the graded system of type (e), the corresponding layer boundaries on the two distribution curves are not completely coincident because of n1 ≠ n2, thus the actual layer number of the graded material is more than k, which means that the material has more complex structure. Therefore, the selection priorities of the graded systems are types (c) and (d) in consideration of the structural complexity of the material. The physical properties of the starting powders are listed in Table 2. It seems that thermal expansion coefficient of CaF2 (18.85 × 10−6/K) is much greater than that of (W,Ti)C (5.8 × 10 − 6 /K) and α-Al 2 O 3 (8.5 × 10− 6/K). To achieve the gradient transition from the surface layers with good self-lubricating properties to the middle layer
Table 2 Physical properties of the starting powders. Starting powder
Density ρ/g·cm−3
Elastic modulus E/GPa
Poisson's ratio ν
Thermal expansion coefficient α/×10−6/K
α-Al2O3 (W,Ti)C CaF2
3.99 9.56 3.18
380 550 75.8
0.26 0.194 0.26
8.5 5.8 18.85
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with high mechanical properties, the CaF2 content was designed to decrease from 10 vol.% in the surface layers to 0 vol.% in the middle layer. For the graded system with n1 ≠ 0 and n2 = 0 (type (c)), the volumetric ratio of Al2O3 and (W,Ti)C in the Al2O3/(W,Ti)C matrix keeps constant. Under this condition, the Al 2 O 3 /(W,Ti)C matrix will always have a lower thermal expansion coefficient than CaF2, regardless of the volumetric ratio of Al2O3 and (W,Ti)C. Thus, thermal expansion coefficient of the Al2O3/(W,Ti)C/CaF2 graded material in the graded system of type (c) increases from the middle layer to the surface layers, which does not conform to the second design rule. However, the graded system with n1 = n2 ≠ 0 (type (d)), in which the volumetric ratio of (W,Ti)C and Al2O3 in the matrix changes along the material thickness, may be able to meet the second design rule. Due to the thermal expansion coefficient of (W,Ti)C is lower than that of Al2O3, the volume fraction of (W,Ti)C in the Al2O3/(W,Ti)C matrix was designed to increase from 30 vol.% in the middle layer to 60 vol.% in the surface layers. The thermal expansion coefficient of Al2O3/(W,Ti)C/CaF2 material as a function of ξ can be obtained by successively substituting the gradient distribution functions of (W,Ti)C and CaF2 into Kerner's equation [15]. It can be seen from Fig. 1(a) that type (c) fails to meet the second design rule but type (d) is competent. Therefore, the graded system with n1 = n2 ≠ 0 (type (d)) was selected in the study.
Fig. 1. Parameter curves of Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic tool material: (a) thermal expansion coefficient as a function of ξ in conditions of n1 ≠ 0, n2 = 0 and n1 = n2 ≠ 0 and (b) thermal residual stresses versus ξ when compositional distribution exponents n1 = n2 = 2.0.
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According to our previous report [16], the layer number has an obvious effect on the residual stresses of gradient self-lubricating ceramic tool materials through the FEM calculation. By means of the same method with the consideration of both thermal residual stresses and Von Mises stresses, the layer number was determined as seven. The composition of each layer and its thermal expansion coefficient are listed in Table 1. With the calculation of the finite element method (FEM), the compositional distribution exponents were determined to be n1 = n2 = 2.0, in which case the maximum Von Mises stresses of the material is the lowest. The thermal residual stresses of the continuously graded distribution were calculated with the analytical method by using the calculation formula for thermal stress of functionally graded materials [17], while that of the symmetrically laminated structure were calculated by means of the FEM. As shown in Fig. 1(b), the stress curve with the analytical method changes smoothly, while the stress curve with the FEM has a stair-step shape. Both the stress curves have a consistent variation trend in which they transit from compressive stress in the surface layers to tensile stress in the middle layer. It coincides well with the second design rule.
Fig. 2. Micrographs of cross section of Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic tool material: (a) general view of graded layers and (b) an indentation on the surface layer.
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Results and discussion As shown in Fig. 2(a), a symmetrical seven-layer structure can be easily observed with the brightness of the micrograph increasing layer by layer from the interior layer to the exterior layer. Fig. 2(b) shows a Vickers indentation on the cross-section of surface layer, using a load of 49 N. It can clearly be observed that the indentation cracks perpendicular to the direction of graded layers are clearly shorter and narrower than those parallel to the direction of graded layers. According to the literature [18,19], the kind of case indicates the presence of compressive stresses in the surface layers of the graded self-lubricating ceramic tool material. It further proves that the developed graded self-lubricating ceramic tool material conforms well to the design rules. The significant improvement in the mechanical properties can be attributed to the multi-component graded distribution of both (W,Ti)C and CaF2. First, the solid lubricant CaF2 has a detrimental effect on the mechanical properties of ceramic materials. For example, the flexural strength of Al2O3/TiC ceramic material was decreased by 26.3% with the addition of 10 vol.% CaF2 [20]. Thus, the decreasing gradient of the CaF2 content from the surface layers to the middle layer of the graded material can improve the flexural strength of the whole material due to the substantially higher flexural strength of the middle layer where no CaF2 was added. Second, previous studies have shown that the presence of residual compressive stresses can cause the surface layers of functionally graded ceramic materials to possess higher hardness and fracture toughness as compared with the corresponding homogeneous materials [21–24]. For Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool material, the increasing gradient of the (W,Ti)C content from the middle layer to the surface layers leads to the formation of residual compressive stresses in the surface layers. As a result, the mechanical properties including flexural strength, Vickers hardness and fracture toughness of the graded self-lubricating ceramic tool material were all enhanced. Fig. 3 illustrates the flank wear VB of the two kinds of ceramic cutting tools in regard to the cutting length when machining the 45# hardened carbon steel. The flank wear of both ceramic cutting tools increases with the cutting length at different cutting speeds. It is evident, however, that the flank wear of Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool is lower than that of the corresponding homogeneous ceramic cutting tool under the same cutting conditions no matter at a cutting speed of 150 m/min or 90 m/min. It suggests that wear resistance of the former ceramic cutting tool is higher than that of the latter. When VB = 0.29 mm, the cutting length of the graded self-lubricating ceramic tool is 1.12 times of that of the homogeneous ceramic tool which means 12% improvement in cutting performance.
Fig. 3. Flank wear versus cutting length of the two kinds of ceramic cutting tools at different cutting speeds.
Fig. 4 shows the SEM micrographs of the worn flank faces of the two cutting tools after machining for 25 min at a cutting speed of 90 m/min. It reveals that the grooves in the flank wear area of the homogeneous cutting tool are dense and sharp with obvious notch wear (Fig. 4(a)), while that of the graded cutting tool are sparse and mild only with small notch wear (Fig. 4(b)). It proves that the graded cutting tool exhibited better cutting performance especially in notch wear resistance than the homogeneous cutting tool. It was reported that wear rate of ceramic material when abrasion is a primary wear mechanism is inversely proportional to the product of its fracture toughness and hardness, i.e., 1/2 K3/4 [11]. Hence, the reason for the better flank wear resistance of IC H the graded cutting tool can attributed to the improvement of material properties especially the hardness and fracture toughness. Furthermore, micro-chippings are found near the cutting edge of the homogeneous cutting tool (Fig. 4(a)), while the cutting edge of the graded cutting tool was kept in good state. It can reasonably be related to the exclusive existence of residual compressive stresses on the surface layers of the graded cutting tool. The favorable stresses can partially counteract the tensile stresses that resulted from the cutting forces and then increase
Fig. 4. SEM micrographs of the flank wear area: (a) homogeneous self-lubricating ceramic tool and (b) graded self-lubricating ceramic tool.
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the resistance to the tool chipping or breakage near the cutting edge [13]. Conclusions In summary, an Al2O3/(W,Ti)C/CaF2 multi-component graded selflubricating ceramic cutting tool material was developed through the combination of functionally graded material and self-lubricating ceramic. The following conclusions were drawn: (1). An Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool material was designed by using the proposed multi-component gradient distribution model and then fabricated with the hot pressing technique. (2). The Al2O3/(W,Ti)C/CaF2 graded self-lubricating ceramic cutting tool material has notably better mechanical properties as compared with the corresponding homogeneous material. (3). During dry cutting of the 45# hardened steel, the Al2O3/(W,Ti) C/CaF2 multi-component graded self-lubricating ceramic cutting tool exhibited higher wear resistance than the homogeneous cutting tool. It suggests that the developed multi-component graded self-lubricating ceramic tool material may have further application in the metal cutting field working as an effective green cutting technology. Acknowledgments The authors acknowledge the support provided by the National Natural Science Foundation of China (Grant No. 51075248), the Shandong Provincial Natural Science Foundation for Distinguished Young Scholars, China (Grant No. JQ201014) and the Key Project of the Chinese Ministry of Education (Grant No. 212097). References [1] Ai X, Li ZQ, Deng JX. Development and perspective of advanced ceramic cutting tool materials. Key Eng Mater 1995;108:53–66. [2] Senthil Kumar A, Raja Durai A, Sornakumar T. Wear behaviour of alumina based ceramic cutting tools on machining steels. Tribol Int 2006;39:191–7. [3] Qiu Like, Li Xikun, Peng Yang, Ma Weimin, Qiu Guanming, Sun Yanbin. Types, performance and application of Al2O3 system ceramic cutting tool. J Rare Earth 2007;25:322–6. [4] Sreejith PS, Ngoi BKA. Dry machining: machining of the future. J Mater Process Technol 2000;101:287–91.
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