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Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool
Journal of Materials Processing Technology 129 (2002) 161±166
Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool J. Zhao*, X. Ai, X.P. Huang School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, PR China
Abstract Based on a deep understanding of the requirements of cutting conditions on ceramic tools, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution is presented in this paper, according to which an Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by a powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al2O3±TiC functionally gradient ceramics FG-1 was evaluated by water quenching and subsequent three-point bending tests of ¯exural strength diminution. Comparisons were made with results from parallel experiments conducted using a homogeneous Al2O3±TiC ceramics. The functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to the homogeneous ceramics, indicating their higher thermal shock resistance. The experimental results were supported by the calculation of the transient thermal stress ®eld. The cutting performance of the Al2O3±TiC functionally gradient ceramic tool FG-1 was also investigated in the rough turning of the cylindrical surface of an exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The results indicated that the tool life of FG-1 increased by 50% over that of LT55. The tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock, whilst tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of functionally gradient ceramics than that of common ceramics. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Functionally gradient materials; Ceramic tool materials; Thermal shock resistance; Transient thermal stress; Cutting performance
1. Introduction As cutting tool materials, ceramics posses high hardness, wear resistance, heat resistance and chemical stability, with less deformation or dissolution wear in cutting processes. In spite of the advances in the strengthening and toughening of ceramic tool materials as well as the improved processing techniques, their applications in intermittent cutting operations are still restricted by their intrinsic drawbacks. Due to their lower thermal conductivity, higher thermal expansion coef®cient and hence lower thermal shock resistance, severe temperature gradients and thermal stress gradients are present in the interior of a ceramic cutting tool under cyclic mechanical and thermal shock in an intermittent cutting process, which may lead to the fracture of the cutting edge. Unpredictable failure by fracture can lead to damage to the part being machined or to the machine tool, both of which can be several orders of magnitude more valuable than the tool itself in modern industry with the increased use of CNC machine tools. * Corresponding author. E-mail address: [email protected] (J. Zhao).
The introduction of the concept of functionally gradient materials (FGM) into the fabrication of ceramic cutting tool materials provided a new approach to improving their thermal and mechanical properties especially their thermal shock resistance [1,2]. If a ceramic tool material is designed and fabricated with compositional distribution, with the microstructure varying continuously in the thickness direction in an optimum way to achieve improved thermal and mechanical properties especially thermal shock resistance, it is expected to relax the temperature gradients and thermal stress gradients in the interior of the ceramic tool and hence to improve the fracture resistance especially the thermal fracture resistance of ceramic tool materials so as to meet the critical requirements of the cutting process. In the present work a design model for functionally gradient ceramic tool materials with symmetrical composition distribution was developed based on a deep understanding of the requirements of the cutting conditions of ceramic tools, according to which an Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by the powderlaminating and uniaxially hot-pressing technique. The thermal shock behavior of the Al2O3±TiC functionally gradient
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Table 1 Data of the physical properties of Al2O3 and TiC Materials
Specific gravity, r (g cm 3)
Thermal conductivity k at 20 8C (W(m K) 1)
Specific heat c at 20 8C (cal(g K) 1)
Thermal expansion coefficient a at 20 8C (10 6 K 1)
Young's modulus, E (GPa)
Poisson's ratio, n
a-Al2O3 TiC
3.99 4.93
40.37 24.28
0.185 0.134
8.5 7.6
380 450
0.26 0.19
ceramic tool material was evaluated by water quenching and subsequent three-point bending tests of ¯exural strength diminution. Parallel reference experiments were conducted with homogeneous Al2O3±TiC ceramics. The experimental results were discussed in terms of the calculation of transient thermal stress ®eld using the perturbation method. The cutting performance of the Al2O3±TiC functionally gradient ceramic tool FG-1 was also investigated in rough turning the cylindrical surface of an exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The cutting performance and especially the failure characteristics of the functionally gradient ceramic tool were correlated to its thermal shock behavior. 2. Design and fabrication of functionally gradient ceramic material Different from the unidirectional composition distribution used in the design of heat-shielding FGMs, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution is presented. Consider an Al2O3±TiC functionally gradient ceramics, with data of the physical properties of Al2O3 and TiC listed in Table 1. By not taking into account pores and very small amounts of additives, the volume fraction of TiC in the thickness direction is of the following exponential form [1,2] (Fig. 1): j1 j0 ^zn j0 ; 1:0 ^z 0 jTiC ^z (1) n j1 j0 ^z j0 ; 0 ^z 1:0
where ^z is a dimensionless coordinate variable in the thickness direction, n the distribution exponent which determine the compositional distribution of the material, and j1 and j0 the volume fractions of TiC of two surfaces (^z 1:0) and the middle position (^z 0), respectively. Five different volume fractions of TiC (30, 40, 50, 60, 70 vol.%) were selected in designing the Al2O3±TiC functionally gradient ceramic tool material with a nine-layer structure, with the volume fractions of TiC in the middle layer and surface layers being 30 and 70 vol.%, respectively. Thus the thermal expansion coef®cient of the material increases from the surface to the middle position continuously (referring to Table 1). This may lead to the formation of residual compressive stresses in the surface region of the compact during the fabricating process (cooling from the sintering temperature to room temperature), which is favorable for an insert made of this material to resist external loading in cutting process. The optimum distribution exponent n 1:2 was determined with the aim of the securing the highest structural integrity of the compact, i.e. the lowest residual thermal stress (the Von Mises stress calculated by means of the ®nite element method) in fabricating process. However, the details are omitted here for brevity. The starting materials were a-Al2O3 powder with average grain size of approximately 0.5 mm, purity 99.9%, and TiC powder with average grain size of 1.0 mm, purity 99.8%. Five Al2O3±TiC composite powders of different mixture ratios were prepared with the addition of some sintering additives in very small amounts, and then laminated into the mould according to the pre-determined compositional distribution exponent n 1:2. The sample was then hotpressed in ¯owing nitrogen for 20 min at 1700 8C temperature and 30 MPa pressure. This material was named FG-1. 3. Thermal shock experiments and calculation of the transient thermal stresses 3.1. Experimental procedure and results
Fig. 1. Symmetrical composition distribution.
In addition to the functionally gradient ceramics FG-1, a homogeneous Al2O3±TiC ceramics (30%Al2 O3 70 vol.%TiC) was synthesized by using the same technique for comparison. Sintered specimens were cut and machined into bend bars with dimensions of 3:5 mm 4:5 mm 36 mm for strength measurements. All specimen surfaces were ground ¯at to a 10 mm ®nish. Flexural strength was
J. Zhao et al. / Journal of Materials Processing Technology 129 (2002) 161±166
163
Fig. 2. Retained flexural strength versus temperature difference.
measured using a three-point bend ®xture with a 30 mm span width at a loading cross-head rate of 0.5 mm/min, the surface perpendicular to the hot-pressing direction selected as the tensile surface. Measurements were performed on at least ®ve test bars of each materials. The bend bars were heated to the desired temperature for 20 min in a furnace and then quenched into a container of water at 20 8C. Thermal quenching was conducted at temperatures 200, 250, 300, 400, 600, 800 and 1000 8C. The shocked specimens were dried before their retained strength was measured. The variation of retained ¯exural strength of functionally gradient ceramics FG-1 and homogeneous ceramics with temperature difference is shown in Fig. 2. It can be seen that the retained ¯exural strength of quenched bars of FG-1 are generally higher than those of the homogeneous ceramics under all thermal shock temperature differences. The two materials exhibit different strength degradation behavior. The strength of homogeneous ceramics remains approximately constant up to 230 8C and then drops precipitously in a range of temperature difference from 230 to 280 8C followed by moderate strength degradation, whereas the functionally gradient ceramics exhibits a slower strength degradation compared to homogeneous ceramics, indicating its higher thermal shock resistance.
Fig. 3. Functionally gradient ceramic plate with symmetrical structure.
of the functionally gradient ceramic plate are all variables, i.e. functions of z, the calculations of transient temperature ®elds were carried out by using the perturbation method [3]. The transient thermal stress ®elds were calculated subsequently by using mechanics of elasticity. Some laws of mixtures were employed to calculate the thermo-mechanical properties of Al2O3±TiC composites with different mixture ratios, thermal conductivities k and speci®c heats c obtained by the theory of Kingery et al. [4], thermal expansion coef®cients a obtained by Kerner's equation [5], and Young's modulus E and Poisson's ratio n obtained by Mori±Tanaka's equation [6]. In addition to the dimensionless variable in the thickness direction ^z, time t was also normalized as t for simplicity. Because of the lengthiness and complexity of the calculation, the details are omitted here for brevity. The temperature distribution under surface cooling (initial temperature T0 300 8C and ambient temperature Ts 20 8C are shown in Fig. 4. It can be seen that the shorter is the dimensionless time t is, the higher is the temperature gradient near to the surface; the longer is t, the lower is the temperature gradient near to the surface.
3.2. Calculations of the transient thermal stresses As shown in Fig. 3(a), consider an in®nite functionally gradient ceramic plate with symmetrical structure, with its thickness being 2rm. It is assumed that initially the medium is at a uniform temperature T0 and is suddenly subjected to an uniform temperature Ts by the surrounding medium. Because of the symmetrical structure and symmetrical thermal loading conditions, the model was simpli®ed into the form shown in Fig. 3(b) with only the upper half being studied. Because the thermo-mechanical properties (speci®c heat c, thermal conductivity k, speci®c gravity r, thermal expansion coef®cient a, Young's modulus E and Poisson's ratio n)
Fig. 4. Temperature distribution under surface cooling.
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4. Cutting experiments 4.1. Experimental procedure and results
Fig. 5. Thermal stress distribution under surface cooling.
The calculation results of the transient thermal stress distribution under surface cooling (Fig. 5) revealed clearly that tensile stress is generated in the surface region (^z 1:0), while the middle region (^z 0) is subjected to compressive stress when the dimensionless time t is shorter. It also can be seen that the shorter is the dimensionless time t, the higher is the tensile stress in the surface region; the longer is t, the lower is the tensile stress. In order to investigate the effect of the compositional distribution on the thermal stress distribution, the transient thermal stress distribution of a homogeneous Al2O3±TiC ceramic plate (30%Al2 O3 70 vol.%TiC) under the same cooling conditions was calculated also using the same method. The calculation results for t 0:1 are listed in Table 2. It can be seen that the values of tensile stresses in the surface region of the homogeneous ceramic plate are higher than those of the gradient ceramic plate. The thermal stress distributions for t 0:01 and t 1:0 exhibited the similar characteristics. The lower thermal stresses of the functionally gradient ceramic plate should be attributed to both the higher thermal conductivity and the higher thermal expansion coef®cient of the middle region than those of the surface region (referring to Table 1). The former could relax the temperature gradient and the later could partially release the tensile stress in the surface region during the cooling process. Therefore the combined effect of the optimum distributions of the thermo-mechanical properties of functionally gradient ceramics led to the improved resistance to thermal shock loading, and consequently the higher retained ¯exural strength of quenched specimens than that of the homogeneous ceramics.
The cutting behavior of the functionally gradient ceramic tool material FG-1 in the rough turning of the cylindrical surface of the big end of an exhaust valve for a diesel engine was investigated practically in comparison with that of a homogeneous Al2O3±TiC ceramic tool LT55. The exhaust valve was made of 4Cr14Ni14W2Mo with its big end (7 mm in width) built-welded with a chromium-based alloy (48±52 HRC). The machining experiments were carried out on a Y2-1022 lathe equipped with a 458 lead angle, 68 negative inclination, and a 68 negative rake tool holder, under a cutting speed of v 195 m min 1, with the ®xed feed rate of 0.1 mm rev 1 at a 2.5 mm depth of cut. The geometry of the tool inserts was SNG15083, with an edge chamfer of 0.2 mm at 458. Edge fracture was taken as the tool life criterion. The tests were conducted on ®ve edges for each tools. Results revealed that the average tool life of LT55 and FG-1 tools were 120 parts per edge and 180 parts per edge, respectively, i.e. the tool life of FG-1 increased by 50% over that of LT55. It was found that both LT55 and FG-1 tools were prone to depth-of-cut notch wear, with less damage to the tool ¯ank faces before edge fracture occurred. However, LT55 and FG-1 tools exhibited different fracture characteristics when the parts machined increased to a certain number. The failure of FG-1 tool was characterized by a typical fatigue crack extension, with a fatigue crack nucleation region (located at the depth-of-cut line), a fan-shaped fatigue extension region and a fast crack extension region being clearly observed, as shown in Fig. 6(a). By contrast, bulk fracture of tool nose occurred for the LT55 tool (Fig. 6(b)). 4.2. Discussion The rough turning of the cylindrical surface of the big end of the exhaust valve showed intermittent cutting characteristics by virtue of the alternation of a very short machining time for each part (only 4.4 s) and an interval of 3 s for replacing the part, as well as the unevenness of the stock. Because the surface to be machined was built-welded with a chromiumbased alloy (5 mm in thickness) of high hardness, the insert was subjected to severe mechanical and thermal shock when cutting into the workpiece, the greatest mechanical and thermal stress gradients acting on the depth-of-cut line where cutting speed is the highest. Thus both LT55 and FG-1 tools
Table 2 Distribution of thermal stress s ^z; t (MPa) under surface cooling for t 0:1 Materials
^z 0.0
FG-1 Homogeneous ceramics
326.3 326.9
0.2 292.0 290.8
0.4 175.7 170.7
0.6
0.8
1.0
44.5 56.5
371.1 392.5
775.9 808.5
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combined effect of thermal crack extension and the accompanying mechanical shock. Therefore, tool life of LT55 was mainly controlled by thermal shock cracking, which was accompanied by mechanical shock. This was not the case for FG-1, which exhibited a higher resistance to thermal crack initiation and extension as a result of its higher thermal shock resistance than that of common ceramics. However, with the increase of the number of parts machined and the aggravation of depth-of-cut notch wear as well as the accumulated damage, a macroscopic fatigue crack parallel to the rake face formed in the depth-of-cut region (the fatigue crack nucleation region) and extended under the cyclic fatigue loading, whereas thermal stresses contributed less to tool fracture. Thus the life of FG-1 was mainly controlled by the mechanical fatigue crack growth rate. 5. Conclusions An Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by a powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al2O3±TiC functionally gradient ceramic tool material FG-1 was evaluated relative to that of a homogeneous Al2O3±TiC ceramics by water quenching, subsequent three-point bending tests of strength diminution and the calculation of the transient thermal stress ®eld. Its cutting performance was also investigated in the rough turning of the cylindrical surface of the exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The major ®ndings from the study are as follows:
Fig. 6. Fracture modes of ceramic tools.
were susceptible to depth-of-cut notch wear rather than ¯ank wear before tool fracture occurred. The damage resulting from the alternating thermal stresses (the alternation of tensile and compressive stresses caused by heating and cooling of the tool) and mechanical stresses were accumulated with the increase of tool wear, and eventually led to tool fracture. In the case of the rough turning of the cylindrical surface of the big end of an exhaust valve by using an LT55 tool, the alternating thermal stresses would be more effective than the cyclic mechanical loading in producing damage to the tool because of the high cutting speed and the lower thermal shock resistance of LT55. The cutting process was mainly controlled by the alternating thermal stresses and the resultant thermal shock damage, with thermally induced cracks perpendicular to the rake face initiating, linking and extending along the direction of the maximum energy release rate. The catastrophic failure of the tool would occur under the
1. Functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to homogeneous ceramics, indicating the higher thermal shock resistance. 2. The calculated results of transient thermal stress fields revealed that the values of tensile stresses in the surface region of the homogeneous ceramic plate are higher than those of the gradient ceramic plate under the same cooling conditions. 3. The cutting experiment results indicated that tool life of FG-1 increased by 50% over that of LT55. The tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock, whilst tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of the functionally gradient ceramics compared with that of common ceramics. Acknowledgements This work was supported by the National Natural Science Foundation of China (through grant nos. 59505014 and 50105011).
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References [1] J. Zhao, X. Ai, J.H. Zhang, C.Z. Huang, Chin. J. Mech. Eng. 34 (4) (1998) 32±36 (in Chinese). [2] X. Ai, J. Zhao, C.Z. Huang, J.H. Zhang, Mater. Sci. Eng. A 248 (1) (1998) 125±131.
[3] J. Zhao, X. Ai, J.X. Deng, J.H. Zhang, C.Z. Huang, Chin. J. Mech. Eng. 37 (11) (2001) 22±27 (in Chinese). [4] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, Wiley, New York, 1976. [5] E.H. Kerner, Proc. Phys. Soc. B 69 (1956) 808. [6] T. Mori, K. Tanaka, Acta Metall. 21 (1973) 571.
Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool J. Zhao*, X. Ai, X.P. Huang School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, PR China
Abstract Based on a deep understanding of the requirements of cutting conditions on ceramic tools, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution is presented in this paper, according to which an Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by a powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al2O3±TiC functionally gradient ceramics FG-1 was evaluated by water quenching and subsequent three-point bending tests of ¯exural strength diminution. Comparisons were made with results from parallel experiments conducted using a homogeneous Al2O3±TiC ceramics. The functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to the homogeneous ceramics, indicating their higher thermal shock resistance. The experimental results were supported by the calculation of the transient thermal stress ®eld. The cutting performance of the Al2O3±TiC functionally gradient ceramic tool FG-1 was also investigated in the rough turning of the cylindrical surface of an exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The results indicated that the tool life of FG-1 increased by 50% over that of LT55. The tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock, whilst tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of functionally gradient ceramics than that of common ceramics. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Functionally gradient materials; Ceramic tool materials; Thermal shock resistance; Transient thermal stress; Cutting performance
1. Introduction As cutting tool materials, ceramics posses high hardness, wear resistance, heat resistance and chemical stability, with less deformation or dissolution wear in cutting processes. In spite of the advances in the strengthening and toughening of ceramic tool materials as well as the improved processing techniques, their applications in intermittent cutting operations are still restricted by their intrinsic drawbacks. Due to their lower thermal conductivity, higher thermal expansion coef®cient and hence lower thermal shock resistance, severe temperature gradients and thermal stress gradients are present in the interior of a ceramic cutting tool under cyclic mechanical and thermal shock in an intermittent cutting process, which may lead to the fracture of the cutting edge. Unpredictable failure by fracture can lead to damage to the part being machined or to the machine tool, both of which can be several orders of magnitude more valuable than the tool itself in modern industry with the increased use of CNC machine tools. * Corresponding author. E-mail address: [email protected] (J. Zhao).
The introduction of the concept of functionally gradient materials (FGM) into the fabrication of ceramic cutting tool materials provided a new approach to improving their thermal and mechanical properties especially their thermal shock resistance [1,2]. If a ceramic tool material is designed and fabricated with compositional distribution, with the microstructure varying continuously in the thickness direction in an optimum way to achieve improved thermal and mechanical properties especially thermal shock resistance, it is expected to relax the temperature gradients and thermal stress gradients in the interior of the ceramic tool and hence to improve the fracture resistance especially the thermal fracture resistance of ceramic tool materials so as to meet the critical requirements of the cutting process. In the present work a design model for functionally gradient ceramic tool materials with symmetrical composition distribution was developed based on a deep understanding of the requirements of the cutting conditions of ceramic tools, according to which an Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by the powderlaminating and uniaxially hot-pressing technique. The thermal shock behavior of the Al2O3±TiC functionally gradient
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 0 2 - 7
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Table 1 Data of the physical properties of Al2O3 and TiC Materials
Specific gravity, r (g cm 3)
Thermal conductivity k at 20 8C (W(m K) 1)
Specific heat c at 20 8C (cal(g K) 1)
Thermal expansion coefficient a at 20 8C (10 6 K 1)
Young's modulus, E (GPa)
Poisson's ratio, n
a-Al2O3 TiC
3.99 4.93
40.37 24.28
0.185 0.134
8.5 7.6
380 450
0.26 0.19
ceramic tool material was evaluated by water quenching and subsequent three-point bending tests of ¯exural strength diminution. Parallel reference experiments were conducted with homogeneous Al2O3±TiC ceramics. The experimental results were discussed in terms of the calculation of transient thermal stress ®eld using the perturbation method. The cutting performance of the Al2O3±TiC functionally gradient ceramic tool FG-1 was also investigated in rough turning the cylindrical surface of an exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The cutting performance and especially the failure characteristics of the functionally gradient ceramic tool were correlated to its thermal shock behavior. 2. Design and fabrication of functionally gradient ceramic material Different from the unidirectional composition distribution used in the design of heat-shielding FGMs, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution is presented. Consider an Al2O3±TiC functionally gradient ceramics, with data of the physical properties of Al2O3 and TiC listed in Table 1. By not taking into account pores and very small amounts of additives, the volume fraction of TiC in the thickness direction is of the following exponential form [1,2] (Fig. 1): j1 j0 ^zn j0 ; 1:0 ^z 0 jTiC ^z (1) n j1 j0 ^z j0 ; 0 ^z 1:0
where ^z is a dimensionless coordinate variable in the thickness direction, n the distribution exponent which determine the compositional distribution of the material, and j1 and j0 the volume fractions of TiC of two surfaces (^z 1:0) and the middle position (^z 0), respectively. Five different volume fractions of TiC (30, 40, 50, 60, 70 vol.%) were selected in designing the Al2O3±TiC functionally gradient ceramic tool material with a nine-layer structure, with the volume fractions of TiC in the middle layer and surface layers being 30 and 70 vol.%, respectively. Thus the thermal expansion coef®cient of the material increases from the surface to the middle position continuously (referring to Table 1). This may lead to the formation of residual compressive stresses in the surface region of the compact during the fabricating process (cooling from the sintering temperature to room temperature), which is favorable for an insert made of this material to resist external loading in cutting process. The optimum distribution exponent n 1:2 was determined with the aim of the securing the highest structural integrity of the compact, i.e. the lowest residual thermal stress (the Von Mises stress calculated by means of the ®nite element method) in fabricating process. However, the details are omitted here for brevity. The starting materials were a-Al2O3 powder with average grain size of approximately 0.5 mm, purity 99.9%, and TiC powder with average grain size of 1.0 mm, purity 99.8%. Five Al2O3±TiC composite powders of different mixture ratios were prepared with the addition of some sintering additives in very small amounts, and then laminated into the mould according to the pre-determined compositional distribution exponent n 1:2. The sample was then hotpressed in ¯owing nitrogen for 20 min at 1700 8C temperature and 30 MPa pressure. This material was named FG-1. 3. Thermal shock experiments and calculation of the transient thermal stresses 3.1. Experimental procedure and results
Fig. 1. Symmetrical composition distribution.
In addition to the functionally gradient ceramics FG-1, a homogeneous Al2O3±TiC ceramics (30%Al2 O3 70 vol.%TiC) was synthesized by using the same technique for comparison. Sintered specimens were cut and machined into bend bars with dimensions of 3:5 mm 4:5 mm 36 mm for strength measurements. All specimen surfaces were ground ¯at to a 10 mm ®nish. Flexural strength was
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Fig. 2. Retained flexural strength versus temperature difference.
measured using a three-point bend ®xture with a 30 mm span width at a loading cross-head rate of 0.5 mm/min, the surface perpendicular to the hot-pressing direction selected as the tensile surface. Measurements were performed on at least ®ve test bars of each materials. The bend bars were heated to the desired temperature for 20 min in a furnace and then quenched into a container of water at 20 8C. Thermal quenching was conducted at temperatures 200, 250, 300, 400, 600, 800 and 1000 8C. The shocked specimens were dried before their retained strength was measured. The variation of retained ¯exural strength of functionally gradient ceramics FG-1 and homogeneous ceramics with temperature difference is shown in Fig. 2. It can be seen that the retained ¯exural strength of quenched bars of FG-1 are generally higher than those of the homogeneous ceramics under all thermal shock temperature differences. The two materials exhibit different strength degradation behavior. The strength of homogeneous ceramics remains approximately constant up to 230 8C and then drops precipitously in a range of temperature difference from 230 to 280 8C followed by moderate strength degradation, whereas the functionally gradient ceramics exhibits a slower strength degradation compared to homogeneous ceramics, indicating its higher thermal shock resistance.
Fig. 3. Functionally gradient ceramic plate with symmetrical structure.
of the functionally gradient ceramic plate are all variables, i.e. functions of z, the calculations of transient temperature ®elds were carried out by using the perturbation method [3]. The transient thermal stress ®elds were calculated subsequently by using mechanics of elasticity. Some laws of mixtures were employed to calculate the thermo-mechanical properties of Al2O3±TiC composites with different mixture ratios, thermal conductivities k and speci®c heats c obtained by the theory of Kingery et al. [4], thermal expansion coef®cients a obtained by Kerner's equation [5], and Young's modulus E and Poisson's ratio n obtained by Mori±Tanaka's equation [6]. In addition to the dimensionless variable in the thickness direction ^z, time t was also normalized as t for simplicity. Because of the lengthiness and complexity of the calculation, the details are omitted here for brevity. The temperature distribution under surface cooling (initial temperature T0 300 8C and ambient temperature Ts 20 8C are shown in Fig. 4. It can be seen that the shorter is the dimensionless time t is, the higher is the temperature gradient near to the surface; the longer is t, the lower is the temperature gradient near to the surface.
3.2. Calculations of the transient thermal stresses As shown in Fig. 3(a), consider an in®nite functionally gradient ceramic plate with symmetrical structure, with its thickness being 2rm. It is assumed that initially the medium is at a uniform temperature T0 and is suddenly subjected to an uniform temperature Ts by the surrounding medium. Because of the symmetrical structure and symmetrical thermal loading conditions, the model was simpli®ed into the form shown in Fig. 3(b) with only the upper half being studied. Because the thermo-mechanical properties (speci®c heat c, thermal conductivity k, speci®c gravity r, thermal expansion coef®cient a, Young's modulus E and Poisson's ratio n)
Fig. 4. Temperature distribution under surface cooling.
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4. Cutting experiments 4.1. Experimental procedure and results
Fig. 5. Thermal stress distribution under surface cooling.
The calculation results of the transient thermal stress distribution under surface cooling (Fig. 5) revealed clearly that tensile stress is generated in the surface region (^z 1:0), while the middle region (^z 0) is subjected to compressive stress when the dimensionless time t is shorter. It also can be seen that the shorter is the dimensionless time t, the higher is the tensile stress in the surface region; the longer is t, the lower is the tensile stress. In order to investigate the effect of the compositional distribution on the thermal stress distribution, the transient thermal stress distribution of a homogeneous Al2O3±TiC ceramic plate (30%Al2 O3 70 vol.%TiC) under the same cooling conditions was calculated also using the same method. The calculation results for t 0:1 are listed in Table 2. It can be seen that the values of tensile stresses in the surface region of the homogeneous ceramic plate are higher than those of the gradient ceramic plate. The thermal stress distributions for t 0:01 and t 1:0 exhibited the similar characteristics. The lower thermal stresses of the functionally gradient ceramic plate should be attributed to both the higher thermal conductivity and the higher thermal expansion coef®cient of the middle region than those of the surface region (referring to Table 1). The former could relax the temperature gradient and the later could partially release the tensile stress in the surface region during the cooling process. Therefore the combined effect of the optimum distributions of the thermo-mechanical properties of functionally gradient ceramics led to the improved resistance to thermal shock loading, and consequently the higher retained ¯exural strength of quenched specimens than that of the homogeneous ceramics.
The cutting behavior of the functionally gradient ceramic tool material FG-1 in the rough turning of the cylindrical surface of the big end of an exhaust valve for a diesel engine was investigated practically in comparison with that of a homogeneous Al2O3±TiC ceramic tool LT55. The exhaust valve was made of 4Cr14Ni14W2Mo with its big end (7 mm in width) built-welded with a chromium-based alloy (48±52 HRC). The machining experiments were carried out on a Y2-1022 lathe equipped with a 458 lead angle, 68 negative inclination, and a 68 negative rake tool holder, under a cutting speed of v 195 m min 1, with the ®xed feed rate of 0.1 mm rev 1 at a 2.5 mm depth of cut. The geometry of the tool inserts was SNG15083, with an edge chamfer of 0.2 mm at 458. Edge fracture was taken as the tool life criterion. The tests were conducted on ®ve edges for each tools. Results revealed that the average tool life of LT55 and FG-1 tools were 120 parts per edge and 180 parts per edge, respectively, i.e. the tool life of FG-1 increased by 50% over that of LT55. It was found that both LT55 and FG-1 tools were prone to depth-of-cut notch wear, with less damage to the tool ¯ank faces before edge fracture occurred. However, LT55 and FG-1 tools exhibited different fracture characteristics when the parts machined increased to a certain number. The failure of FG-1 tool was characterized by a typical fatigue crack extension, with a fatigue crack nucleation region (located at the depth-of-cut line), a fan-shaped fatigue extension region and a fast crack extension region being clearly observed, as shown in Fig. 6(a). By contrast, bulk fracture of tool nose occurred for the LT55 tool (Fig. 6(b)). 4.2. Discussion The rough turning of the cylindrical surface of the big end of the exhaust valve showed intermittent cutting characteristics by virtue of the alternation of a very short machining time for each part (only 4.4 s) and an interval of 3 s for replacing the part, as well as the unevenness of the stock. Because the surface to be machined was built-welded with a chromiumbased alloy (5 mm in thickness) of high hardness, the insert was subjected to severe mechanical and thermal shock when cutting into the workpiece, the greatest mechanical and thermal stress gradients acting on the depth-of-cut line where cutting speed is the highest. Thus both LT55 and FG-1 tools
Table 2 Distribution of thermal stress s ^z; t (MPa) under surface cooling for t 0:1 Materials
^z 0.0
FG-1 Homogeneous ceramics
326.3 326.9
0.2 292.0 290.8
0.4 175.7 170.7
0.6
0.8
1.0
44.5 56.5
371.1 392.5
775.9 808.5
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combined effect of thermal crack extension and the accompanying mechanical shock. Therefore, tool life of LT55 was mainly controlled by thermal shock cracking, which was accompanied by mechanical shock. This was not the case for FG-1, which exhibited a higher resistance to thermal crack initiation and extension as a result of its higher thermal shock resistance than that of common ceramics. However, with the increase of the number of parts machined and the aggravation of depth-of-cut notch wear as well as the accumulated damage, a macroscopic fatigue crack parallel to the rake face formed in the depth-of-cut region (the fatigue crack nucleation region) and extended under the cyclic fatigue loading, whereas thermal stresses contributed less to tool fracture. Thus the life of FG-1 was mainly controlled by the mechanical fatigue crack growth rate. 5. Conclusions An Al2O3±TiC functionally gradient ceramic tool material FG-1 was synthesized by a powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al2O3±TiC functionally gradient ceramic tool material FG-1 was evaluated relative to that of a homogeneous Al2O3±TiC ceramics by water quenching, subsequent three-point bending tests of strength diminution and the calculation of the transient thermal stress ®eld. Its cutting performance was also investigated in the rough turning of the cylindrical surface of the exhaust valve of a diesel engine in comparison with that of a common Al2O3±TiC ceramic tool LT55. The major ®ndings from the study are as follows:
Fig. 6. Fracture modes of ceramic tools.
were susceptible to depth-of-cut notch wear rather than ¯ank wear before tool fracture occurred. The damage resulting from the alternating thermal stresses (the alternation of tensile and compressive stresses caused by heating and cooling of the tool) and mechanical stresses were accumulated with the increase of tool wear, and eventually led to tool fracture. In the case of the rough turning of the cylindrical surface of the big end of an exhaust valve by using an LT55 tool, the alternating thermal stresses would be more effective than the cyclic mechanical loading in producing damage to the tool because of the high cutting speed and the lower thermal shock resistance of LT55. The cutting process was mainly controlled by the alternating thermal stresses and the resultant thermal shock damage, with thermally induced cracks perpendicular to the rake face initiating, linking and extending along the direction of the maximum energy release rate. The catastrophic failure of the tool would occur under the
1. Functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to homogeneous ceramics, indicating the higher thermal shock resistance. 2. The calculated results of transient thermal stress fields revealed that the values of tensile stresses in the surface region of the homogeneous ceramic plate are higher than those of the gradient ceramic plate under the same cooling conditions. 3. The cutting experiment results indicated that tool life of FG-1 increased by 50% over that of LT55. The tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock, whilst tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of the functionally gradient ceramics compared with that of common ceramics. Acknowledgements This work was supported by the National Natural Science Foundation of China (through grant nos. 59505014 and 50105011).
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