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The effects of MgO addition on microstructure, mechanical properties and wear performance of zirconia-toughened alumina cutting inserts
Journal of Alloys and Compounds 497 (2010) 316–320
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effects of MgO addition on microstructure, mechanical properties and wear performance of zirconia-toughened alumina cutting inserts Ahmad Zahirani Ahmad Azhar a , Hasmaliza Mohamad a , Mani Maran Ratnam b , Zainal Arifin Ahmad a,∗ a b
School of Materials and Mineral Resources, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia School of Mechanical, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 2 March 2010 Accepted 2 March 2010 Available online 9 March 2010 Keywords: Microstructure Mechanical properties MgO Wear
a b s t r a c t The microstructure, mechanical properties and wear performance of ceramic cutting inserts produced from Al2 O3 , yttria stabilized zirconia and magnesium oxide system were investigated. The MgO weight percent was varied from 0 wt% to 3.5 wt%. Each batch of composition was mixed, uniaxially pressed into rhombic 80◦ cutting inserts with an average 0.6 mm tip radius and sintered at 1600 ◦ C for 4 h in pressureless conditions. Studies on the effects of the inserts’ microstructures on their mechanical and physical properties such as nose wear, Vickers hardness and fracture toughness were carried out. Mild steel (AISI 1018) was used as the workpiece in the machining test. Results show that an addition of 0.7 wt% of MgO produces the minimum wear area. When the amount of MgO was increased to more than 0.7 wt%, the wear area increased from 0.019 mm2 to 0.065 mm2 . Thus, ZTA cutting inserts fabricated with MgO additives show 50% improvement of wear compared to ZTA cutting inserts without MgO addition. Furthermore, microstructural observations show that the Al2 O3 grain size is significantly dependent on the amount of MgO additives. Results of the Vickers hardness test is directly related to the result of wear area, where the cutting insert with the minimum wear area also showed the highest hardness. The increase of hardness of the cutting insert is mainly contributed by small sized Al2 O3 grains. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The potential of alumina (Al2 O3 ) as a cutting insert was studied by the Germans in 1905 [1]. These cutting inserts were prepared using the hot pressing method with a small amount of sintering agent (such as MgO, NiO, Cr2 O3 and TiO2 ), namely white porcelain [1]. Al2 O3 cutting inserts are suitable for use in machining cast iron of hardness lower than HB235, carbide steel of hardness lower than HRC38 and alloy steel. These Al2 O3 cutting inserts consist of fine grains (less than 5 m), relatively high density and contain less than 2% porosity. Cutting inserts fabricated from pure alumina are abundantly used due to their inherent properties of high hot hardness, abrasion resistance and chemical stability [2]. Furthermore, alumina ceramics can maintain up to 90% or their strength even at 1100 ◦ C [3]. Unfortunately, applications of these pure Al2 O3 as cutting inserts under mechanical loads and thermal shock conditions is limited application due to their brittleness and low strength [4]. As a result, monolithic Al2 O3 cutting inserts sometimes experience premature failure since the metal cutting process is one of the most severe applications of ceramics materials. The cutting edge and faces of an insert are exposed to high stress and elevated tem-
∗ Corresponding author. Tel.: +60 4 599 6128; fax: +60 4 594 1011. E-mail address: [email protected] (Z.A. Ahmad). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.03.054
peratures, as well as to thermal shock, especially in an intermittent cutting process [5,6]. The introduction of a second phase has proved to efficiently improve its toughness by making use of the transformation strengthening process through phase transformation of a certain amount of yttria stabilized zirconia (YSZ) [4,6–9]. By adding YSZ in the alumina matrix, zirconia-toughened Al2 O3 (ZTA) is produced. It is a family of ceramic cutting inserts that has become firmly established as a good alternative to metal carbide inserts, especially due to its lower wear rate. The increase of wear resistance of ZTA can be explained as a result of a transformation toughening mechanism that originates from YSZ when surrounded by alumina matrix [6]. The mechanism of this process is based on the polymorphic transformation of ZrO2 (t) tetragonal phase into ZrO2 (m) monoclinic phase during cooling from sintering temperature to room temperature, enabling an increase of the strength and/or fracture toughness of alumina ceramics [4]. The phase transformations of ZrO2 from tetragonal (t) to monoclinic (m) has been widely used to improve the toughness brittle ceramics materials. The improvement is understood as a result of volume expansion during the t → m transformation of ZrO2 dispersed in the matrix. In an Al2 O3 matrix, t-ZrO2 grains undergo the t → m transformation (stress-induced phase transformation) and microcracks form around pretransformed m-ZrO2 grains. The stress-induced phase transformation toughening and microcrack toughening are the major toughening
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Fig. 1. Result of XRD analysis on ZTA–MgO sintered body with different MgO wt%.
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mechanism in Al2 O3 –ZrO2 composite [10].Previous work done by Smuk et al. [4] and Azhar et al. [6] showed that a 20 wt% of YSZ is the amount needed to obtain optimum wear performance. Further addition of YSZ will lead to decreased hardness and will eventually reduce the overall wear performance. Besides the improvement of toughness by the introduction of YSZ, the hardness of bulk Al2 O3 can be improved by introducing a small amount of magnesium oxide (MgO) in alumina matrix. Previous work by Coble [11] in 1961 indicated that the presence of MgO in Al2 O3 matrix will have a significant effect on the mechanical and electrical properties of bulk Al2 O3 [12]. The microstructure pinning effects are widely known to hinder the grain growth of Al2 O3 by (i) lowering the grain boundary mobility; (ii) increasing the surface diffusivity and therefore increasing the pore mobility; (iii) increasing densification rate by promoting lattice and boundary diffusions and (iv) decreasing the grain boundary anisotropy and surface energy of grains [13]. Furthermore, MgO is a traditional additive for Al2 O3 since it can reduce the sintering temperature
Fig. 2. Microstructural images of cutting insert surface (a) 0 wt% MgO, (b) 0.3 wt% MgO, (c) 0.5 wt% MgO, (d) 0.7 wt% MgO, (e) 0.9 wt% MgO, (f) 1.0 wt% MgO.
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and grain size. Addition of small amounts of MgO (<0.25 wt%) enabled Al2 O3 to sinter to near theoretical density, which dense and low porous cutting insert is a requirement for wear-protection application [14]. Furthermore, previous work by Wang et al. also shows that MgO efficiently improved the sintered density of Al2 O3 [13]. Even though the study of ZTA and Al2 O3 –MgO has been extensively, but separately, studied for the past 20 years, studies on Al2 O3 –YSZ–MgO mechanical properties and the wear performance of this ceramic composite system is rarely reported elsewhere, until now. In this study, the microstructural evolution, mechanical properties and wear performances of MgO doped ZTA were investigated and the influence of MgO addition on its microstructure and mechanical properties is discussed. 2. Experimental details A monolithic Al2 O3 (average particle size 0.5 m, supplier Martinswerck), YSZ (average particle size 1.5 m, supplier Goodfellow) and MgO (average particle size 0.5 m, supplier Strem Chemicals) were used as the starting materials. Ceramic cutting inserts with an average of 0.60 mm tip radius were fabricated via the solid state processing route. Samples with an 80/20 ratio for Al2 O3 /YSZ respectively were prepared with different MgO wt% ranging from 0 wt% to 3.5 wt%. Hardness (HV) and fracture toughness (KIc ) were determined by means of Vickers indentation with a 30 kgf load. To calculate fracture toughness, the formula proposed by Niihara for Palmqvist crack was used [15]:
3KIc = 0.035(Ha1/2 )
3E H
0.4 −0.5 l a
(1)
Fabricated ceramic cutting insert was tested to machined mild steel 1018 with 1-in. diameter in dry environment. Cutting speed of 540 rpm, feed rate 0.2 mm/rev, cutting length of 40 mm and 4 mm depth of cut were used as the cutting parameter. In addition, on-line measurements of nose wear of the cutting tool inserts were carried out using high-resolution CCD camera with vision system [16]. Detailed processing route, characterization techniques, machining parameter and wear area measurement are described elsewhere [6,17].
Table 1 Densities and shrinkage of the ZTA–MgO cutting inserts with different MgO contents. MgO wt%
Density (g/cm3 )
Shrinkage (%)
0 0.1 0.3 0.5 0.7 0.9 1.0 1.5 2.0
4.24 4.29 4.34 4.39 4.47 4.42 4.35 4.28 4.20
15.77 15.80 15.84 15.97 15.12 15.03 15.82 15.71 15.37
ilar microstructural characteristic were observed in these samples i.e. uniformly sized grains with high degree of grain close packing. Almost no abnormal grain growth was observed. The SEM micrographs indicate that the role of MgO is to inhibit grain growth of Al2 O3 ceramic, in agreement with other studies [12,21,22]. It is seen that the grain size decreases gradually with the content of MgO. As a result, it is proven that the content of MgO is an important parameter for the development of ceramic microstructures. From the SEM images, it is shown that MgO did not significantly affect the YSZ grain sizes. On the other hand, alumina grains are greatly affected by the addition of MgO, as shown in Fig. 2. The grains of Al2 O3 gradually decreased from samples with 0 wt% MgO to 0.7 wt% MgO, indicating that the amount of MgO plays an important role in determining Al2 O3 grains size. But with more
3. Results and discussion 3.1. XRD XRD analysis shows both tetragonal and monoclinic phases as present in YSZ. In addition, the maximum amount of monoclinic volume fraction present in YSZ is 0.20. The monoclinic volume fraction was determined using the polymorph method and the integrated intensity ratio equation [18]. Al2 O3 XRD result indicates the phase present is corundum. Presence of MgO was recorded as MgAl2 O4 and detectable only when the amount of MgO is more than 1.0 wt% as shown in Fig. 1. These results are similar with previous research [6]: powders after mixing and the sintered body show no significant change in the raw materials and the presence of monoclinic phase is present both before and after the sintering process [19].
Fig. 3. Vickers hardness of the ZTA cutting inserts as a function of MgO wt%.
3.2. Densification and microstructure Densities of the sintered cutting inserts were determined using the Archimedes principle. Table 1 contains the data on densities and shrinkage of the ZTA–MgO cutting inserts with different MgO content. Maximum density was obtained in the cutting inserts with 0.7 MgO wt%. Densities increased with increasing MgO wt% up until 0.7 wt%. Moreover, linear shrinkage was shown to have a similar trend with densities. FESEM micrographs for the polished surface of the cutting inserts are shown in Fig. 2. Consistent with previous studies [4,6,9,15,20], YSZ and Al2 O3 grains are well distributed among each other but minor agglomeration was unavoidable. EDX analysis indicates the white areas as representing YSZ grains and the dark areas as representing Al2 O3 and MgO grains. In general, sim-
Fig. 4. Fracture toughness of the ZTA cutting inserts as a function of MgO wt%.
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 497 (2010) 316–320
addition of MgO (more than 0.7 wt% MgO), Al2 O3 grain size gradually increased, indicating that the MgO microstructure pinning effect is diminished. 3.3. Mechanical properties The effect of MgO addition on Vickers hardness of the cutting inserts is shown in Fig. 3 (Y-error bars indicate the standard deviation value). Vickers hardness gradually increases approximately linearly up to 0.7 wt% MgO addition. After that, it decreases gradually. The increase in Vickers hardness can be explained to have happened due to the small grain size of the sample microstructures,
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which resulted from the microstructure pinning effect introduced by MgO. The increase in the hardness of ZTA–MgO ceramics is mainly due to the addition of MgO which inhibits grain growth of ZTA–MgO ceramic, giving rise to homogeneous and dense ceramics. However, as the Vickers hardness results show, the microstructure pinning effect is only effective up to 0.7 wt%. Addition of more than 0.7 wt% MgO will decrease the hardness of the cutting insert. The effect of MgO addition on fracture toughness of the cutting insert is shown in Fig. 4 (Y-error bars indicate the standard deviation value). The result of fracture toughness is obtained by using Eq. (1). The result is also including the measured fracture toughness of pure Al2 O3 (2.85 MPa m1/2 ). This measured value is smaller
Fig. 5. Images of cutting inserts (a) unworn, (b) example of a worn cutting insert, (c) 0 wt%, (d) 0.1 wt%, (e) 0.2 wt%, (f) 0.5 wt%, (g) 0.6 wt%, (h) 0.7 wt%, (i) 0.8 wt% and (j) 1.0 wt%.
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Even though the property of fracture toughness is important for metal cutting applications, the wear performance of the fabricated cutting insert did not seem to be significantly affected, compared to its hardness. 4. Conclusion
Fig. 6. Wear area of cutting inserts as a function of MgO wt%.
compared to the theoretical value (3.50 MPa m1/2 for pure Al2 O3 ) is due to the different measurement conditions and few assumptions made by the mathematical model employed. The fracture toughness gradually decreases with increasing MgO addition. The fracture toughness decreases at a higher rate for samples containing small amounts of MgO wt% and continues to gradually decrease afterwards. The drastic decrease of fracture toughness is basically related to the small grain size of the grain boundary for samples 0–0.7 wt%. According to Casellas et al. [23], smaller grain size will result in lower intrinsic toughness due to reduced load bridging capability of smaller grain bridges [20]. A previous study by Rittidech et al. stated that the addition of MgO will significantly reduce the fracture toughness [12], which is consistent with current the study. Addition of more than 0.7 MgO wt% resulted in a slower rate of decrease due to the intrinsic nature of low fracture toughness of MgO. The result of fracture toughness correlates with the SEM observations, where sizes of Al2 O3 grains decrease when the property of fracture toughness decreases. This trend is obvious for samples with 0 MgO wt% to 1.0 MgO wt%. With addition of 1.0 MgO wt% and above, the property of fracture toughness no longer depends on the grain size. Instead, it follows the rules of mixtures for ceramic composites. Due to the intrinsic nature of low fracture toughness of MgO, the result of fracture toughness for samples with more than 1.0 MgO wt% decreases gradually, depending on the MgO composition. 3.4. Wear area Images of the cutting tips before and after machining were captured as shown in Fig. 5. These two images were aligned automatically before subtraction. The software subsequently produces images shown in Fig. 5a and b depending on the final condition of the inserts. Moreover, the software is able to exactly calculate the area differences between the two images, which are indicated by the black colored area in Fig. 5c–j. A larger black area indicates that the inserts have experienced a greater amount of wear, i.e. more material loss has occurred due to machining. Analysis of wear area (Fig. 6, Y-error bars indicate the standard deviation value) clearly indicates that the cutting inserts with 0.7 wt% shows the lowest wear area of 0.017 mm2 , over 50% of wear improvement when cutting mild steel 1018, compared to the cutting insert with 0 wt% of MgO addition. The results of wear area are closely related to the results of hardness, in which the cutting insert with the highest result of hardness shows the lowest wear area.
The introduction of fine MgO particles into ZTA increased the hardness by reducing the grain size of Al2 O3 , thereby greatly improving its cutting performance when compared to ZTA cutting inserts without MgO additives. Thus, it is proven that the effect of MgO as an effective additive is valid not only in monolithic Al2 O3 systems, but also in the ZTA system. In addition, ZTA cutting inserts with MgO additives give a 50% improved wear performance on machining mild steel 1018. The present results indicate that the ZTA–MgO cutting inserts are a promising material for machining applications. Acknowledgements This works has been funded by Universiti Sains Malaysia (USM) under grant 1001/BAHAN/811074 and USM Fellowship Scheme. The authors are grateful to Mr. Sharul, Dr. Shamsul (JMG) and Mr. Rashid for their technical support. References [1] Q. Like, L. Xikun, P. Yang, M. Weimin, Q. Guanming, S. Yanbin, Journal of Rare Earths 25 (2007) 322–326. [2] A.K. Dutta, A.B. Chattopadhyaya, K.K. Ray, Wear 261 (2006) 885–895. [3] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, International Journal of Refractory Metals and Hard Materials 22 (2004) 17–20. [4] B. Smuk, M. Szutkowska, J. Walter, Journal of Materials Processing Technology 133 (2003) 195–198. [5] A. Xing, Z. Jun, H. Chuanzhen, Z. Jianhua, Materials Science and Engineering A 248 (1998) 125–131. [6] A.Z.A. Azhar, M.M. Ratnam, Z.A. Ahmad, Journal of Alloys and Compounds 478 (2009) 608–614. [7] T. Sornakumar, M.V. Gopalakrishnan, V.E. Annamalai, R. Krishnamurthy, C.V. Gokularathnam, International Journal of Refractory Metals and Hard Materials 13 (1995) 181–185. [8] P.M. Kelly, L.R. Francis Rose, Progress in Materials Science 47 (2002) 463–557. [9] B. Basu, J. Vleugels, O. Van Der Biest, Journal of Alloys and Compounds 372 (2004) 278–284. [10] M. Szutkowska, Journal of Materials Processing Technology 153–154 (2004) 868–874. [11] R.L. Coble, Journal of Applied Physics 32 (1961) 793–799. [12] A. Rittidech, L. Portia, T. Bongkarn, Materials Science and Engineering A 438–440 (2006) 395–398. [13] J. Wang, S.Y. Lim, S.C. Ng, C.H. Chew, L.M. Gan, Materials Letters 33 (1998) 273–277. [14] E. Medvedovski, Wear 249 (2001) 821–828. [15] G. Magnani, A. Brillante, Journal of the European Ceramic Society 25 (2005) 3383–3392. [16] H. Shahabi, M. Ratnam, The International Journal of Advanced Manufacturing Technology 38 (2008) 718–727. [17] J.K.C. Hao, A.Z.A. Azhar, M.M. Ratnam, Z.A. Ahmad, Materials Science and Technology 26 (2010) 95–103. [18] V.V. Mishra, A.K. Garg, D.C. Agrawal, Bull Materials Science 21 (1998) 81–86. [19] V. Sergo, V. Lughi, G. Pezzotti, E. Lucchini, S. Meriani, N. Muraki, G. Katagiri, S. Lo Casto, T. Nishida, Wear 214 (1998) 264–270. [20] R.K. Sadangi, V. Shukla, B.H. Kear, Processing and properties of ZrO2 (3Y2 O3 )–Al2 O3 nanocomposites, International Journal of Refractory Metals and Hard Materials, 23, 363–368. [21] Y. Ji, J.A. Yeomans, Journal of the European Ceramic Society 22 (2002) 1927–1936. [22] C.T. Fu, J.M. Wu, A.K. Li, Journal of Materials Science 29 (1994) 2671–2677. [23] D. Casellas, M.M. Nagl, L. Llanes, M. Anglada, Journal of Materials Processing Technology 143 (2003) 148–152.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effects of MgO addition on microstructure, mechanical properties and wear performance of zirconia-toughened alumina cutting inserts Ahmad Zahirani Ahmad Azhar a , Hasmaliza Mohamad a , Mani Maran Ratnam b , Zainal Arifin Ahmad a,∗ a b
School of Materials and Mineral Resources, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia School of Mechanical, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 2 March 2010 Accepted 2 March 2010 Available online 9 March 2010 Keywords: Microstructure Mechanical properties MgO Wear
a b s t r a c t The microstructure, mechanical properties and wear performance of ceramic cutting inserts produced from Al2 O3 , yttria stabilized zirconia and magnesium oxide system were investigated. The MgO weight percent was varied from 0 wt% to 3.5 wt%. Each batch of composition was mixed, uniaxially pressed into rhombic 80◦ cutting inserts with an average 0.6 mm tip radius and sintered at 1600 ◦ C for 4 h in pressureless conditions. Studies on the effects of the inserts’ microstructures on their mechanical and physical properties such as nose wear, Vickers hardness and fracture toughness were carried out. Mild steel (AISI 1018) was used as the workpiece in the machining test. Results show that an addition of 0.7 wt% of MgO produces the minimum wear area. When the amount of MgO was increased to more than 0.7 wt%, the wear area increased from 0.019 mm2 to 0.065 mm2 . Thus, ZTA cutting inserts fabricated with MgO additives show 50% improvement of wear compared to ZTA cutting inserts without MgO addition. Furthermore, microstructural observations show that the Al2 O3 grain size is significantly dependent on the amount of MgO additives. Results of the Vickers hardness test is directly related to the result of wear area, where the cutting insert with the minimum wear area also showed the highest hardness. The increase of hardness of the cutting insert is mainly contributed by small sized Al2 O3 grains. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The potential of alumina (Al2 O3 ) as a cutting insert was studied by the Germans in 1905 [1]. These cutting inserts were prepared using the hot pressing method with a small amount of sintering agent (such as MgO, NiO, Cr2 O3 and TiO2 ), namely white porcelain [1]. Al2 O3 cutting inserts are suitable for use in machining cast iron of hardness lower than HB235, carbide steel of hardness lower than HRC38 and alloy steel. These Al2 O3 cutting inserts consist of fine grains (less than 5 m), relatively high density and contain less than 2% porosity. Cutting inserts fabricated from pure alumina are abundantly used due to their inherent properties of high hot hardness, abrasion resistance and chemical stability [2]. Furthermore, alumina ceramics can maintain up to 90% or their strength even at 1100 ◦ C [3]. Unfortunately, applications of these pure Al2 O3 as cutting inserts under mechanical loads and thermal shock conditions is limited application due to their brittleness and low strength [4]. As a result, monolithic Al2 O3 cutting inserts sometimes experience premature failure since the metal cutting process is one of the most severe applications of ceramics materials. The cutting edge and faces of an insert are exposed to high stress and elevated tem-
∗ Corresponding author. Tel.: +60 4 599 6128; fax: +60 4 594 1011. E-mail address: [email protected] (Z.A. Ahmad). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.03.054
peratures, as well as to thermal shock, especially in an intermittent cutting process [5,6]. The introduction of a second phase has proved to efficiently improve its toughness by making use of the transformation strengthening process through phase transformation of a certain amount of yttria stabilized zirconia (YSZ) [4,6–9]. By adding YSZ in the alumina matrix, zirconia-toughened Al2 O3 (ZTA) is produced. It is a family of ceramic cutting inserts that has become firmly established as a good alternative to metal carbide inserts, especially due to its lower wear rate. The increase of wear resistance of ZTA can be explained as a result of a transformation toughening mechanism that originates from YSZ when surrounded by alumina matrix [6]. The mechanism of this process is based on the polymorphic transformation of ZrO2 (t) tetragonal phase into ZrO2 (m) monoclinic phase during cooling from sintering temperature to room temperature, enabling an increase of the strength and/or fracture toughness of alumina ceramics [4]. The phase transformations of ZrO2 from tetragonal (t) to monoclinic (m) has been widely used to improve the toughness brittle ceramics materials. The improvement is understood as a result of volume expansion during the t → m transformation of ZrO2 dispersed in the matrix. In an Al2 O3 matrix, t-ZrO2 grains undergo the t → m transformation (stress-induced phase transformation) and microcracks form around pretransformed m-ZrO2 grains. The stress-induced phase transformation toughening and microcrack toughening are the major toughening
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 497 (2010) 316–320
Fig. 1. Result of XRD analysis on ZTA–MgO sintered body with different MgO wt%.
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mechanism in Al2 O3 –ZrO2 composite [10].Previous work done by Smuk et al. [4] and Azhar et al. [6] showed that a 20 wt% of YSZ is the amount needed to obtain optimum wear performance. Further addition of YSZ will lead to decreased hardness and will eventually reduce the overall wear performance. Besides the improvement of toughness by the introduction of YSZ, the hardness of bulk Al2 O3 can be improved by introducing a small amount of magnesium oxide (MgO) in alumina matrix. Previous work by Coble [11] in 1961 indicated that the presence of MgO in Al2 O3 matrix will have a significant effect on the mechanical and electrical properties of bulk Al2 O3 [12]. The microstructure pinning effects are widely known to hinder the grain growth of Al2 O3 by (i) lowering the grain boundary mobility; (ii) increasing the surface diffusivity and therefore increasing the pore mobility; (iii) increasing densification rate by promoting lattice and boundary diffusions and (iv) decreasing the grain boundary anisotropy and surface energy of grains [13]. Furthermore, MgO is a traditional additive for Al2 O3 since it can reduce the sintering temperature
Fig. 2. Microstructural images of cutting insert surface (a) 0 wt% MgO, (b) 0.3 wt% MgO, (c) 0.5 wt% MgO, (d) 0.7 wt% MgO, (e) 0.9 wt% MgO, (f) 1.0 wt% MgO.
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A.Z.A. Azhar et al. / Journal of Alloys and Compounds 497 (2010) 316–320
and grain size. Addition of small amounts of MgO (<0.25 wt%) enabled Al2 O3 to sinter to near theoretical density, which dense and low porous cutting insert is a requirement for wear-protection application [14]. Furthermore, previous work by Wang et al. also shows that MgO efficiently improved the sintered density of Al2 O3 [13]. Even though the study of ZTA and Al2 O3 –MgO has been extensively, but separately, studied for the past 20 years, studies on Al2 O3 –YSZ–MgO mechanical properties and the wear performance of this ceramic composite system is rarely reported elsewhere, until now. In this study, the microstructural evolution, mechanical properties and wear performances of MgO doped ZTA were investigated and the influence of MgO addition on its microstructure and mechanical properties is discussed. 2. Experimental details A monolithic Al2 O3 (average particle size 0.5 m, supplier Martinswerck), YSZ (average particle size 1.5 m, supplier Goodfellow) and MgO (average particle size 0.5 m, supplier Strem Chemicals) were used as the starting materials. Ceramic cutting inserts with an average of 0.60 mm tip radius were fabricated via the solid state processing route. Samples with an 80/20 ratio for Al2 O3 /YSZ respectively were prepared with different MgO wt% ranging from 0 wt% to 3.5 wt%. Hardness (HV) and fracture toughness (KIc ) were determined by means of Vickers indentation with a 30 kgf load. To calculate fracture toughness, the formula proposed by Niihara for Palmqvist crack was used [15]:
3KIc = 0.035(Ha1/2 )
3E H
0.4 −0.5 l a
(1)
Fabricated ceramic cutting insert was tested to machined mild steel 1018 with 1-in. diameter in dry environment. Cutting speed of 540 rpm, feed rate 0.2 mm/rev, cutting length of 40 mm and 4 mm depth of cut were used as the cutting parameter. In addition, on-line measurements of nose wear of the cutting tool inserts were carried out using high-resolution CCD camera with vision system [16]. Detailed processing route, characterization techniques, machining parameter and wear area measurement are described elsewhere [6,17].
Table 1 Densities and shrinkage of the ZTA–MgO cutting inserts with different MgO contents. MgO wt%
Density (g/cm3 )
Shrinkage (%)
0 0.1 0.3 0.5 0.7 0.9 1.0 1.5 2.0
4.24 4.29 4.34 4.39 4.47 4.42 4.35 4.28 4.20
15.77 15.80 15.84 15.97 15.12 15.03 15.82 15.71 15.37
ilar microstructural characteristic were observed in these samples i.e. uniformly sized grains with high degree of grain close packing. Almost no abnormal grain growth was observed. The SEM micrographs indicate that the role of MgO is to inhibit grain growth of Al2 O3 ceramic, in agreement with other studies [12,21,22]. It is seen that the grain size decreases gradually with the content of MgO. As a result, it is proven that the content of MgO is an important parameter for the development of ceramic microstructures. From the SEM images, it is shown that MgO did not significantly affect the YSZ grain sizes. On the other hand, alumina grains are greatly affected by the addition of MgO, as shown in Fig. 2. The grains of Al2 O3 gradually decreased from samples with 0 wt% MgO to 0.7 wt% MgO, indicating that the amount of MgO plays an important role in determining Al2 O3 grains size. But with more
3. Results and discussion 3.1. XRD XRD analysis shows both tetragonal and monoclinic phases as present in YSZ. In addition, the maximum amount of monoclinic volume fraction present in YSZ is 0.20. The monoclinic volume fraction was determined using the polymorph method and the integrated intensity ratio equation [18]. Al2 O3 XRD result indicates the phase present is corundum. Presence of MgO was recorded as MgAl2 O4 and detectable only when the amount of MgO is more than 1.0 wt% as shown in Fig. 1. These results are similar with previous research [6]: powders after mixing and the sintered body show no significant change in the raw materials and the presence of monoclinic phase is present both before and after the sintering process [19].
Fig. 3. Vickers hardness of the ZTA cutting inserts as a function of MgO wt%.
3.2. Densification and microstructure Densities of the sintered cutting inserts were determined using the Archimedes principle. Table 1 contains the data on densities and shrinkage of the ZTA–MgO cutting inserts with different MgO content. Maximum density was obtained in the cutting inserts with 0.7 MgO wt%. Densities increased with increasing MgO wt% up until 0.7 wt%. Moreover, linear shrinkage was shown to have a similar trend with densities. FESEM micrographs for the polished surface of the cutting inserts are shown in Fig. 2. Consistent with previous studies [4,6,9,15,20], YSZ and Al2 O3 grains are well distributed among each other but minor agglomeration was unavoidable. EDX analysis indicates the white areas as representing YSZ grains and the dark areas as representing Al2 O3 and MgO grains. In general, sim-
Fig. 4. Fracture toughness of the ZTA cutting inserts as a function of MgO wt%.
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addition of MgO (more than 0.7 wt% MgO), Al2 O3 grain size gradually increased, indicating that the MgO microstructure pinning effect is diminished. 3.3. Mechanical properties The effect of MgO addition on Vickers hardness of the cutting inserts is shown in Fig. 3 (Y-error bars indicate the standard deviation value). Vickers hardness gradually increases approximately linearly up to 0.7 wt% MgO addition. After that, it decreases gradually. The increase in Vickers hardness can be explained to have happened due to the small grain size of the sample microstructures,
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which resulted from the microstructure pinning effect introduced by MgO. The increase in the hardness of ZTA–MgO ceramics is mainly due to the addition of MgO which inhibits grain growth of ZTA–MgO ceramic, giving rise to homogeneous and dense ceramics. However, as the Vickers hardness results show, the microstructure pinning effect is only effective up to 0.7 wt%. Addition of more than 0.7 wt% MgO will decrease the hardness of the cutting insert. The effect of MgO addition on fracture toughness of the cutting insert is shown in Fig. 4 (Y-error bars indicate the standard deviation value). The result of fracture toughness is obtained by using Eq. (1). The result is also including the measured fracture toughness of pure Al2 O3 (2.85 MPa m1/2 ). This measured value is smaller
Fig. 5. Images of cutting inserts (a) unworn, (b) example of a worn cutting insert, (c) 0 wt%, (d) 0.1 wt%, (e) 0.2 wt%, (f) 0.5 wt%, (g) 0.6 wt%, (h) 0.7 wt%, (i) 0.8 wt% and (j) 1.0 wt%.
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Even though the property of fracture toughness is important for metal cutting applications, the wear performance of the fabricated cutting insert did not seem to be significantly affected, compared to its hardness. 4. Conclusion
Fig. 6. Wear area of cutting inserts as a function of MgO wt%.
compared to the theoretical value (3.50 MPa m1/2 for pure Al2 O3 ) is due to the different measurement conditions and few assumptions made by the mathematical model employed. The fracture toughness gradually decreases with increasing MgO addition. The fracture toughness decreases at a higher rate for samples containing small amounts of MgO wt% and continues to gradually decrease afterwards. The drastic decrease of fracture toughness is basically related to the small grain size of the grain boundary for samples 0–0.7 wt%. According to Casellas et al. [23], smaller grain size will result in lower intrinsic toughness due to reduced load bridging capability of smaller grain bridges [20]. A previous study by Rittidech et al. stated that the addition of MgO will significantly reduce the fracture toughness [12], which is consistent with current the study. Addition of more than 0.7 MgO wt% resulted in a slower rate of decrease due to the intrinsic nature of low fracture toughness of MgO. The result of fracture toughness correlates with the SEM observations, where sizes of Al2 O3 grains decrease when the property of fracture toughness decreases. This trend is obvious for samples with 0 MgO wt% to 1.0 MgO wt%. With addition of 1.0 MgO wt% and above, the property of fracture toughness no longer depends on the grain size. Instead, it follows the rules of mixtures for ceramic composites. Due to the intrinsic nature of low fracture toughness of MgO, the result of fracture toughness for samples with more than 1.0 MgO wt% decreases gradually, depending on the MgO composition. 3.4. Wear area Images of the cutting tips before and after machining were captured as shown in Fig. 5. These two images were aligned automatically before subtraction. The software subsequently produces images shown in Fig. 5a and b depending on the final condition of the inserts. Moreover, the software is able to exactly calculate the area differences between the two images, which are indicated by the black colored area in Fig. 5c–j. A larger black area indicates that the inserts have experienced a greater amount of wear, i.e. more material loss has occurred due to machining. Analysis of wear area (Fig. 6, Y-error bars indicate the standard deviation value) clearly indicates that the cutting inserts with 0.7 wt% shows the lowest wear area of 0.017 mm2 , over 50% of wear improvement when cutting mild steel 1018, compared to the cutting insert with 0 wt% of MgO addition. The results of wear area are closely related to the results of hardness, in which the cutting insert with the highest result of hardness shows the lowest wear area.
The introduction of fine MgO particles into ZTA increased the hardness by reducing the grain size of Al2 O3 , thereby greatly improving its cutting performance when compared to ZTA cutting inserts without MgO additives. Thus, it is proven that the effect of MgO as an effective additive is valid not only in monolithic Al2 O3 systems, but also in the ZTA system. In addition, ZTA cutting inserts with MgO additives give a 50% improved wear performance on machining mild steel 1018. The present results indicate that the ZTA–MgO cutting inserts are a promising material for machining applications. Acknowledgements This works has been funded by Universiti Sains Malaysia (USM) under grant 1001/BAHAN/811074 and USM Fellowship Scheme. The authors are grateful to Mr. Sharul, Dr. Shamsul (JMG) and Mr. Rashid for their technical support. References [1] Q. Like, L. Xikun, P. Yang, M. Weimin, Q. Guanming, S. Yanbin, Journal of Rare Earths 25 (2007) 322–326. [2] A.K. Dutta, A.B. Chattopadhyaya, K.K. Ray, Wear 261 (2006) 885–895. [3] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, International Journal of Refractory Metals and Hard Materials 22 (2004) 17–20. [4] B. Smuk, M. Szutkowska, J. Walter, Journal of Materials Processing Technology 133 (2003) 195–198. [5] A. Xing, Z. Jun, H. Chuanzhen, Z. Jianhua, Materials Science and Engineering A 248 (1998) 125–131. [6] A.Z.A. Azhar, M.M. Ratnam, Z.A. Ahmad, Journal of Alloys and Compounds 478 (2009) 608–614. [7] T. Sornakumar, M.V. Gopalakrishnan, V.E. Annamalai, R. Krishnamurthy, C.V. Gokularathnam, International Journal of Refractory Metals and Hard Materials 13 (1995) 181–185. [8] P.M. Kelly, L.R. Francis Rose, Progress in Materials Science 47 (2002) 463–557. [9] B. Basu, J. Vleugels, O. Van Der Biest, Journal of Alloys and Compounds 372 (2004) 278–284. [10] M. Szutkowska, Journal of Materials Processing Technology 153–154 (2004) 868–874. [11] R.L. Coble, Journal of Applied Physics 32 (1961) 793–799. [12] A. Rittidech, L. Portia, T. Bongkarn, Materials Science and Engineering A 438–440 (2006) 395–398. [13] J. Wang, S.Y. Lim, S.C. Ng, C.H. Chew, L.M. Gan, Materials Letters 33 (1998) 273–277. [14] E. Medvedovski, Wear 249 (2001) 821–828. [15] G. Magnani, A. Brillante, Journal of the European Ceramic Society 25 (2005) 3383–3392. [16] H. Shahabi, M. Ratnam, The International Journal of Advanced Manufacturing Technology 38 (2008) 718–727. [17] J.K.C. Hao, A.Z.A. Azhar, M.M. Ratnam, Z.A. Ahmad, Materials Science and Technology 26 (2010) 95–103. [18] V.V. Mishra, A.K. Garg, D.C. Agrawal, Bull Materials Science 21 (1998) 81–86. [19] V. Sergo, V. Lughi, G. Pezzotti, E. Lucchini, S. Meriani, N. Muraki, G. Katagiri, S. Lo Casto, T. Nishida, Wear 214 (1998) 264–270. [20] R.K. Sadangi, V. Shukla, B.H. Kear, Processing and properties of ZrO2 (3Y2 O3 )–Al2 O3 nanocomposites, International Journal of Refractory Metals and Hard Materials, 23, 363–368. [21] Y. Ji, J.A. Yeomans, Journal of the European Ceramic Society 22 (2002) 1927–1936. [22] C.T. Fu, J.M. Wu, A.K. Li, Journal of Materials Science 29 (1994) 2671–2677. [23] D. Casellas, M.M. Nagl, L. Llanes, M. Anglada, Journal of Materials Processing Technology 143 (2003) 148–152.