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YSZ microstructures on wear and mechanical properties of cutting inserts
Journal of Alloys and Compounds 478 (2009) 608–614
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Al2 O3 /YSZ microstructures on wear and mechanical properties of cutting inserts Ahmad Zahirani Ahmad Azhar a , M.M. Ratnam b , Zainal Arifin Ahmad a,∗ a b
School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia School of Mechanical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 16 September 2008 Received in revised form 19 November 2008 Accepted 21 November 2008 Available online 13 December 2008 Keywords: Cutting inserts Wear Microstructure Microcracks
a b s t r a c t Wear and mechanical properties of ceramic cutting inserts produced from Al2 O3 /yttria stabilized zirconia (YSZ) system has been investigated. The YSZ compositions were varied from 0 wt% to 100 wt%. Each Al2 O3 and YSZ composition was mixed, uniaxially pressed into rhombic 80◦ cutting inserts and sintered at 1600 ◦ C for 4 h in pressureless condition. Study on the effect of the microstructure of the inserts on the mechanical and physical properties such as nose wear, Vickers hardness and fracture toughness has been carried out. Mild steel (AISI 1018) was used as the workpiece in the machining tests using the cutting inserts. The results show that 20 wt% of YSZ produced the minimum wear area. When the amount of YSZ was increased from 20 wt% to 100 wt%, the wear area also increased from 0.039 mm2 to 0.182 mm2 . However, the Vickers hardness of the inserts decreased with the increase of YSZ, while the fracture toughness of the cutting inserts shows a continuous increase up to 60 wt% YSZ. Above 60 wt% of YSZ, the microstructure of the polished samples started to show microcracks and formed larger grain sizes of YSZ, thus hindering the transformation toughening mechanism from functioning effectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The advances in ceramic composites have resulted in newer materials with improved toughness. The control of microstructure has led to the development of ceramic composite cutting tool materials like titanium carbide added alumina, zirconia toughened alumina (ZTA) and silicon carbide whisker reinforced alumina which are successfully used for cutting tool applications [1]. Alumina is one of the materials that exhibits high hot-hardness and very high abrasion resistance, thus making it suitable for highspeed machining. Unfortunately, alumina based cutting inserts lack toughness, therefore resulting in premature tool failure caused by chipping or breakage [2]. Significant improvements have been made in the past to increase the toughness of ceramics but brittleness continues to keep ceramics from more widespread use [3–5]. Due to the brittle nature of alumina itself, a second composition is introduced to increase the toughness of the cutting tools. Materials used as second composition are such as yttria stabilized zirconia (YSZ) [3,6–12], whiskers silicon carbide [7,13], titanium carbide [6], molybdenum [14], titanium carbide [7], silver [8] and ceria [15]. By adding YSZ in alumina matrix, ZTA is produced as one family of
∗ Corresponding author. Tel.: +60 604 5996128; fax: +60 604 5941011. E-mail address: [email protected] (Z.A. Ahmad). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.156
ceramic cutting inserts that has firmly established as a good alternative to metal carbide inserts, especially due to their lower wear rate [9]. The increase of wear resistance of ZTA can be explained as a result of transformation toughening mechanism that originated from YSZ when surrounded by alumina matrix. Smuk et al. [3] conducted a research on determining the best amount of zirconia needed to obtain the optimum mechanical properties of Al2 O3 /YSZ. However, the research only involves between 20 wt% and 30 wt% YSZ compositions. This study investigates the wear and mechanical properties of Al2 O3 /YSZ system cutting inserts for the complete range of Al2 O3 /YSZ compositions with the main focus on the effect of their microstructures on their physical and mechanical properties. 2. Experimental details A monolithic Al2 O3 (average particle size 0.5 m) and YSZ (average particle size 1.5 m) powders were mixed using ball mill for 8 h and uniaxially pressed at 295 MPa into rhombic 80◦ cutting inserts using a hydraulic press. The specimens were then sintered in a Lenton electric furnace at 1600 ◦ C for 4 h with 5 ◦ C/min sintering rate. Samples were prepared with different ratios of Al2 O3 /YSZ, ranging from 0 wt% to 100 wt% YSZ. X-ray diffraction analyses (XRD) for YSZ mixed with Al2 O3 before and after sintering are presented in Fig. 1. The sintered samples were subjected to Vickers Hardness (HV30), indentation toughness (Kc(HV) ), wear area, density and porosity tests. Point indentation technique as explained by Palmquivst crack for Vickers hardness indentation was used to determine the Vickers Hardness and fracture toughness
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614 for the sintered ceramic cutting tips. Kc(HV30) was calculated using Eq. (1) [1]:
3Kc(HV30) = 0.035(Ha1/2 ) ×
3E H
0.4 −0.5 ×
l a
(1)
609
where H is the hardness, a is half distance of the indent diagonal, E is modulus of Young and l is the length of the crack. Field emission scanning electron microscopy (FESEM) was employed to study the microstructure of the polished samples. The samples were thermally etched in the same furnace used for sintering at 1400 ◦ C for 2 h. Machining tests were carried out using conventional lathe machine to cut 25 mm mild steel rods (AISI 1018).
Fig. 1. Results of XRD for (A) Al2 O3 (ICDD10-0173), (B) YSZ (ICDD 89-9068 and ICDD 78-1807), (C) after mixing (Al2 O3 + YSZ), and (D) sintered body.
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Fig. 1. (Continued ).
Table 1 Cutting test parameter used to cut mild steel (AISI 1018).
3. Results and discussion
Parameter
Value
3.1. XRD
Cutting speed (rpm) Feed rate (mm/rev) Depth of cut (mm) Cutting environment Cutting length (mm) Cutting time (s)
540 0.2 4 Dry 40 22
Data obtained by XRD analysis show that YSZ has both tetragonal and monoclinic ZrO2 crystal structure together with the presence of yttria (Fig. 1(a)). On the other hand, Fig. 1(b) shows that Al2 O3 is present as corundum phase. Powders after mixing and the sintered
Table 1 shows the cutting condition used. Although the AISI 1018 steel is an easyto-cut material, the cutting parameters were selected so that sufficient tool wear occurs and can be used for comparing the performance of cutting inserts of various compositions. The volume of the workpiece material before cutting was 73,631 mm3 . Wear analysis were carried out by capturing the images of the cutting tips before and after machining. These two images (before and after machining) were superimposed to calculate the difference in area of the overlapped images. Fig. 2 shows a schematic diagram of the cutting process. Fig. 3 shows the arrangement of the cutting tip and the camera capturing the images for wear measurement. Detail information and further mechanisms of the techniques is elaborated elsewhere [16].
Fig. 2. A schematic diagram of the cutting process.
Fig. 3. A schematic for capturing the inserts for wear measurement: (a) side view and (b) top view.
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614
611
Fig. 4. Images of cutting tips (a) unworn (used as the reference to subtract worn images), (b) example of a worn cutting insert, (c) image showing wear areas for 0 wt%, (d) 20 wt%, (e) 40 wt%, (f) 60 wt%, (g) 80 wt%, and (h) 100 wt% of YSZ.
body show no sign of change in the raw materials. Though XRD analysis indicates the presence of monoclinic ZrO2 crystal structure, it is an unavoidable phenomenon and the small amount of monoclinic ZrO2 would not affect the mixture as a whole. Moreover, a research conducted by Sergo et al. [17] stated that at least 15% of monoclinic phase will always be present, even in the commercial cutting tools [18]. 3.2. Wear area Images of the cutting tips before and after machining were captured as shown in Fig. 4. These two images were aligned Fig. 5. Wear area of cutting tools as a function of percentage amount of zirconia.
Fig. 6. Vickers hardness of the sintered cutting tips as a function of YSZ composition.
Fig. 7. The fracture toughness of the sintered cutting tips as function of YSZ wt%.
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Fig. 8. Microstructural images of cutting tools surface (a) 0 wt% YSZ, (b) 5 wt% YSZ, (c) 10 wt% YSZ, (d) 15 wt% YSZ, (e) 20 wt% YSZ, (f) 30 wt% YSZ, (g) 40 wt% YSZ, (h) 50 wt% YSZ, (i) 60 wt% YSZ, (j) 70 wt% YSZ, (k) 80 wt% YSZ, (l) 90 wt% YSZ, (m) 95 wt% YSZ, and (n) 100 wt% YSZ.
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614
613
Fig. 8. (Continued ).
automatically by the software before subtraction. The software subsequently produces the images shown in Fig. 4(c)–(h) depending on the final condition of the inserts. In addition, the software is also able to calculate exactly the area differences between the two images, which are indicated by the black color in Fig. 4(c)–(h). Larger area for black color indicates that the inserts have experienced a greater amount of wear, i.e. more material loss has occurred due to the machining. Analysis of wear area (Fig. 5) clearly indicates that the cutting tips with 20 wt% YSZ shows the lowest wear area of 0.039 mm2 . A similar observation was made by Smuk et al. [3] where the same composition showed the best mechanical properties. Even though samples with 0 wt% YSZ (i.e. pure Al2 O3 ) possess the highest hardness, the sample basically lacks toughness, which makes it brittle and chips easily during machining [19]. With further addition of YSZ, their Vickers hardness decreased continuously. This is due to the loss of corundum phase which was replaced by ZrO2 -
tetragonal and ZrO2 -monoclinic phases that originated from YSZ. However, the inserts with addition of YSZ up to 20 wt% experienced less wear compared to those with higher amounts. Above 20 wt% YSZ the hardness of the cutting tips reduced, thus their wear performance also reduced. The highest wear area is 0.182 mm2 , produced from 100 wt% YSZ addition. 3.3. Mechanical properties The effect of YSZ addition on Vickers hardness of the cutting inserts is shown in Fig. 6. The hardness gradually decreases with increasing YSZ content. The hardness decreases at higher rate for samples containing more than 20 wt% YSZ. Since cutting inserts require reasonable hardness, therefore the amount of YSZ should be on the minimum side, i.e. a minimum of 20 wt%. The hardness of the cutting tips basically depends on the amount of corundum phase presents [7].
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The fracture toughness of the cutting tips is shown in Fig. 7. The fracture toughness increases approximately linearly up to 60 wt% YSZ addition and after that it decreases linearly. The increase of fracture toughness can be explained due to the presence of ZrO2 monoclinic phase in the ceramic cutting tips. Therefore, the cutting tips are getting softer and less wear resistant. The decreasing Kc(HV) for cutting tips added with more than 60 wt% YSZ is basically related to the formation of microcracks in their microstructures (as indicated in Fig. 8).
4. Conclusion
3.4. Microstructural
References
FESEM micrographs and their EDX analysis for the unpolished surface of the cutting tips are shown in Fig. 8. YSZ and Al2 O3 grains are well distributed among each other but minor agglomeration was unavoidable. EDX analysis indicates that the dark areas represent alumina grains and white areas represent YSZ grains. The evolution of microstructure with the increasing YSZ additions is shown in Fig. 8. Cutting tip with 0 wt% YSZ contained the highest porosity (open pores) and large grain size (more than 3 m). With increasing amount of YSZ, these open pores were gradually filled by smaller YSZ particles and reached the optimum at about 20 wt% YSZ additions. Therefore, with less number of open pores, the cutting tip is able to reduce the wear rate to act as the best composition as a cutting tip. But with further addition of YSZ, the ceramic cutting tips itself become softer and are not be able to withstand or resist the shear stress experienced while cutting. During sintering process, Al2 O3 and YSZ particles expanded depending on their coefficient of thermal expansion (CTE). CTE value for Al2 O3 and YSZ are 8.1 × 10−6 ◦ C−1 and 10.3 × 10−6 ◦ C−1 , respectively. Due to their thermal mismatch, YSZ will expand slightly more compared to Al2 O3 . As a result, the expansion of Al2 O3 grains will be prohibited by the YSZ grains. The compressive stress generated by YSZ becomes larger for higher YSZ wt%. As an alternative way to release the high compressive stress, microcracks started to appear in the microstructure. With addition of more than 70 wt% YSZ, microcracks can clearly be observed in the microstructure (identified with circles) and became more noticeable with further additions of YSZ.
[1] A.K. Kumar, A.R. Durai, T. Sornakumar, International Journal of Refractory Metals & Hard Materials 21 (2003) 109–117. [2] S. Kalpakjikan, S. Schmid, Manufacturing Process for Engineering Materials, 4th ed., Pearson Education, USA, 2003, pp. 449. [3] B. Smuk, M. Szutkowska, J. Walter, Journal of Material Processing Technology 133 (2003) 195–198. [4] G.E. D’Erico, S. Bugliosi, R. Calzavarini, D. Cuppini, Journal of Wear 225–229 (1999) 267–272. [5] M. Eugene, Journal of Wear 249 (2001) 821–828. [6] G. Magnani, A. Brillante, Journal of the European Ceramic Society 25 (2005) 3383–3392. [7] A. Gatto, Journal of Materials Processing Technology (2006) 67–73. [8] A.K. Dutta, A.B. Chattpadhyaya, K.K. Ray, Journal of Wear 261 (2006) 885–895. [9] S. Valter, L. Vanni, P. Giuseppe, L. Elio, M. Sergoio, M. Naoki, K. Gen, C.S. Lo, N. Toshihiko, Journal of Wear 214 (1998) 264–270. [10] M. Balassubramaniam, S.K. Malhotra, C.V. Gokularathnam, Journal of Materials Processing Technology 67 (1997) 67–70. [11] M. Szutkowska, Journal of Materials Processing Technology 153–154 (2004) 868–874. [12] B. Basu, Institute of Materials, Minerals and Mining and ASM International, 2005. [13] A. Gatto, L. Iuliano, International Journal of Machine Tools and Manufacture 34 (8) (1994) 1147–1161. [14] E. Lucchini, S. Lo Casto, O. Sbaizero, Materials Science and Engineering A 357 (2003) 369–375. [15] A.S. Kumar, A.R. Durai, T. Sornakumar, International Journal of Refractory Metals & Hard Materials 25 (2007) 214–219. [16] H.H. Shahabi, M.M. Ratnam, International Journal of Advanced Manufacturing Technology 38 (2008) 718–727. [17] V. Sergo, L. Vanni, P. Giuseppe, L. Elio, M. Sergoio, M. Naoki, K. Gen, L.C. Salvatore, N. Toshihiko, Journal of Wear 214 (1998) 264–270. [18] S. Lo Casto, E. Lo Valvo, E. Lucchini, S. Maschio, V.F. Ruisi, Journal of Wear 208 (1997) 67–72. [19] X. Chonghai, A. Xing, H. Chuanzhen, Wear 249 (2001) 503–508.
The microstructures of Al2 O3 /YSZ of cutting inserts are dependent on the wt% YSZ additions. Wear resistance performance, Vickers hardness and fracture toughness of the cutting inserts are greatly affected by these microstructures. The appearance of microcracks in the microstructure due to the thermal mismatch becomes the reason to weaken the cutting inserts for above 60 wt% YSZ additions. The optimum addition of YSZ is around 20 wt%.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Al2 O3 /YSZ microstructures on wear and mechanical properties of cutting inserts Ahmad Zahirani Ahmad Azhar a , M.M. Ratnam b , Zainal Arifin Ahmad a,∗ a b
School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia School of Mechanical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 16 September 2008 Received in revised form 19 November 2008 Accepted 21 November 2008 Available online 13 December 2008 Keywords: Cutting inserts Wear Microstructure Microcracks
a b s t r a c t Wear and mechanical properties of ceramic cutting inserts produced from Al2 O3 /yttria stabilized zirconia (YSZ) system has been investigated. The YSZ compositions were varied from 0 wt% to 100 wt%. Each Al2 O3 and YSZ composition was mixed, uniaxially pressed into rhombic 80◦ cutting inserts and sintered at 1600 ◦ C for 4 h in pressureless condition. Study on the effect of the microstructure of the inserts on the mechanical and physical properties such as nose wear, Vickers hardness and fracture toughness has been carried out. Mild steel (AISI 1018) was used as the workpiece in the machining tests using the cutting inserts. The results show that 20 wt% of YSZ produced the minimum wear area. When the amount of YSZ was increased from 20 wt% to 100 wt%, the wear area also increased from 0.039 mm2 to 0.182 mm2 . However, the Vickers hardness of the inserts decreased with the increase of YSZ, while the fracture toughness of the cutting inserts shows a continuous increase up to 60 wt% YSZ. Above 60 wt% of YSZ, the microstructure of the polished samples started to show microcracks and formed larger grain sizes of YSZ, thus hindering the transformation toughening mechanism from functioning effectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The advances in ceramic composites have resulted in newer materials with improved toughness. The control of microstructure has led to the development of ceramic composite cutting tool materials like titanium carbide added alumina, zirconia toughened alumina (ZTA) and silicon carbide whisker reinforced alumina which are successfully used for cutting tool applications [1]. Alumina is one of the materials that exhibits high hot-hardness and very high abrasion resistance, thus making it suitable for highspeed machining. Unfortunately, alumina based cutting inserts lack toughness, therefore resulting in premature tool failure caused by chipping or breakage [2]. Significant improvements have been made in the past to increase the toughness of ceramics but brittleness continues to keep ceramics from more widespread use [3–5]. Due to the brittle nature of alumina itself, a second composition is introduced to increase the toughness of the cutting tools. Materials used as second composition are such as yttria stabilized zirconia (YSZ) [3,6–12], whiskers silicon carbide [7,13], titanium carbide [6], molybdenum [14], titanium carbide [7], silver [8] and ceria [15]. By adding YSZ in alumina matrix, ZTA is produced as one family of
∗ Corresponding author. Tel.: +60 604 5996128; fax: +60 604 5941011. E-mail address: [email protected] (Z.A. Ahmad). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.156
ceramic cutting inserts that has firmly established as a good alternative to metal carbide inserts, especially due to their lower wear rate [9]. The increase of wear resistance of ZTA can be explained as a result of transformation toughening mechanism that originated from YSZ when surrounded by alumina matrix. Smuk et al. [3] conducted a research on determining the best amount of zirconia needed to obtain the optimum mechanical properties of Al2 O3 /YSZ. However, the research only involves between 20 wt% and 30 wt% YSZ compositions. This study investigates the wear and mechanical properties of Al2 O3 /YSZ system cutting inserts for the complete range of Al2 O3 /YSZ compositions with the main focus on the effect of their microstructures on their physical and mechanical properties. 2. Experimental details A monolithic Al2 O3 (average particle size 0.5 m) and YSZ (average particle size 1.5 m) powders were mixed using ball mill for 8 h and uniaxially pressed at 295 MPa into rhombic 80◦ cutting inserts using a hydraulic press. The specimens were then sintered in a Lenton electric furnace at 1600 ◦ C for 4 h with 5 ◦ C/min sintering rate. Samples were prepared with different ratios of Al2 O3 /YSZ, ranging from 0 wt% to 100 wt% YSZ. X-ray diffraction analyses (XRD) for YSZ mixed with Al2 O3 before and after sintering are presented in Fig. 1. The sintered samples were subjected to Vickers Hardness (HV30), indentation toughness (Kc(HV) ), wear area, density and porosity tests. Point indentation technique as explained by Palmquivst crack for Vickers hardness indentation was used to determine the Vickers Hardness and fracture toughness
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614 for the sintered ceramic cutting tips. Kc(HV30) was calculated using Eq. (1) [1]:
3Kc(HV30) = 0.035(Ha1/2 ) ×
3E H
0.4 −0.5 ×
l a
(1)
609
where H is the hardness, a is half distance of the indent diagonal, E is modulus of Young and l is the length of the crack. Field emission scanning electron microscopy (FESEM) was employed to study the microstructure of the polished samples. The samples were thermally etched in the same furnace used for sintering at 1400 ◦ C for 2 h. Machining tests were carried out using conventional lathe machine to cut 25 mm mild steel rods (AISI 1018).
Fig. 1. Results of XRD for (A) Al2 O3 (ICDD10-0173), (B) YSZ (ICDD 89-9068 and ICDD 78-1807), (C) after mixing (Al2 O3 + YSZ), and (D) sintered body.
610
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614
Fig. 1. (Continued ).
Table 1 Cutting test parameter used to cut mild steel (AISI 1018).
3. Results and discussion
Parameter
Value
3.1. XRD
Cutting speed (rpm) Feed rate (mm/rev) Depth of cut (mm) Cutting environment Cutting length (mm) Cutting time (s)
540 0.2 4 Dry 40 22
Data obtained by XRD analysis show that YSZ has both tetragonal and monoclinic ZrO2 crystal structure together with the presence of yttria (Fig. 1(a)). On the other hand, Fig. 1(b) shows that Al2 O3 is present as corundum phase. Powders after mixing and the sintered
Table 1 shows the cutting condition used. Although the AISI 1018 steel is an easyto-cut material, the cutting parameters were selected so that sufficient tool wear occurs and can be used for comparing the performance of cutting inserts of various compositions. The volume of the workpiece material before cutting was 73,631 mm3 . Wear analysis were carried out by capturing the images of the cutting tips before and after machining. These two images (before and after machining) were superimposed to calculate the difference in area of the overlapped images. Fig. 2 shows a schematic diagram of the cutting process. Fig. 3 shows the arrangement of the cutting tip and the camera capturing the images for wear measurement. Detail information and further mechanisms of the techniques is elaborated elsewhere [16].
Fig. 2. A schematic diagram of the cutting process.
Fig. 3. A schematic for capturing the inserts for wear measurement: (a) side view and (b) top view.
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614
611
Fig. 4. Images of cutting tips (a) unworn (used as the reference to subtract worn images), (b) example of a worn cutting insert, (c) image showing wear areas for 0 wt%, (d) 20 wt%, (e) 40 wt%, (f) 60 wt%, (g) 80 wt%, and (h) 100 wt% of YSZ.
body show no sign of change in the raw materials. Though XRD analysis indicates the presence of monoclinic ZrO2 crystal structure, it is an unavoidable phenomenon and the small amount of monoclinic ZrO2 would not affect the mixture as a whole. Moreover, a research conducted by Sergo et al. [17] stated that at least 15% of monoclinic phase will always be present, even in the commercial cutting tools [18]. 3.2. Wear area Images of the cutting tips before and after machining were captured as shown in Fig. 4. These two images were aligned Fig. 5. Wear area of cutting tools as a function of percentage amount of zirconia.
Fig. 6. Vickers hardness of the sintered cutting tips as a function of YSZ composition.
Fig. 7. The fracture toughness of the sintered cutting tips as function of YSZ wt%.
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Fig. 8. Microstructural images of cutting tools surface (a) 0 wt% YSZ, (b) 5 wt% YSZ, (c) 10 wt% YSZ, (d) 15 wt% YSZ, (e) 20 wt% YSZ, (f) 30 wt% YSZ, (g) 40 wt% YSZ, (h) 50 wt% YSZ, (i) 60 wt% YSZ, (j) 70 wt% YSZ, (k) 80 wt% YSZ, (l) 90 wt% YSZ, (m) 95 wt% YSZ, and (n) 100 wt% YSZ.
A.Z.A. Azhar et al. / Journal of Alloys and Compounds 478 (2009) 608–614
613
Fig. 8. (Continued ).
automatically by the software before subtraction. The software subsequently produces the images shown in Fig. 4(c)–(h) depending on the final condition of the inserts. In addition, the software is also able to calculate exactly the area differences between the two images, which are indicated by the black color in Fig. 4(c)–(h). Larger area for black color indicates that the inserts have experienced a greater amount of wear, i.e. more material loss has occurred due to the machining. Analysis of wear area (Fig. 5) clearly indicates that the cutting tips with 20 wt% YSZ shows the lowest wear area of 0.039 mm2 . A similar observation was made by Smuk et al. [3] where the same composition showed the best mechanical properties. Even though samples with 0 wt% YSZ (i.e. pure Al2 O3 ) possess the highest hardness, the sample basically lacks toughness, which makes it brittle and chips easily during machining [19]. With further addition of YSZ, their Vickers hardness decreased continuously. This is due to the loss of corundum phase which was replaced by ZrO2 -
tetragonal and ZrO2 -monoclinic phases that originated from YSZ. However, the inserts with addition of YSZ up to 20 wt% experienced less wear compared to those with higher amounts. Above 20 wt% YSZ the hardness of the cutting tips reduced, thus their wear performance also reduced. The highest wear area is 0.182 mm2 , produced from 100 wt% YSZ addition. 3.3. Mechanical properties The effect of YSZ addition on Vickers hardness of the cutting inserts is shown in Fig. 6. The hardness gradually decreases with increasing YSZ content. The hardness decreases at higher rate for samples containing more than 20 wt% YSZ. Since cutting inserts require reasonable hardness, therefore the amount of YSZ should be on the minimum side, i.e. a minimum of 20 wt%. The hardness of the cutting tips basically depends on the amount of corundum phase presents [7].
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The fracture toughness of the cutting tips is shown in Fig. 7. The fracture toughness increases approximately linearly up to 60 wt% YSZ addition and after that it decreases linearly. The increase of fracture toughness can be explained due to the presence of ZrO2 monoclinic phase in the ceramic cutting tips. Therefore, the cutting tips are getting softer and less wear resistant. The decreasing Kc(HV) for cutting tips added with more than 60 wt% YSZ is basically related to the formation of microcracks in their microstructures (as indicated in Fig. 8).
4. Conclusion
3.4. Microstructural
References
FESEM micrographs and their EDX analysis for the unpolished surface of the cutting tips are shown in Fig. 8. YSZ and Al2 O3 grains are well distributed among each other but minor agglomeration was unavoidable. EDX analysis indicates that the dark areas represent alumina grains and white areas represent YSZ grains. The evolution of microstructure with the increasing YSZ additions is shown in Fig. 8. Cutting tip with 0 wt% YSZ contained the highest porosity (open pores) and large grain size (more than 3 m). With increasing amount of YSZ, these open pores were gradually filled by smaller YSZ particles and reached the optimum at about 20 wt% YSZ additions. Therefore, with less number of open pores, the cutting tip is able to reduce the wear rate to act as the best composition as a cutting tip. But with further addition of YSZ, the ceramic cutting tips itself become softer and are not be able to withstand or resist the shear stress experienced while cutting. During sintering process, Al2 O3 and YSZ particles expanded depending on their coefficient of thermal expansion (CTE). CTE value for Al2 O3 and YSZ are 8.1 × 10−6 ◦ C−1 and 10.3 × 10−6 ◦ C−1 , respectively. Due to their thermal mismatch, YSZ will expand slightly more compared to Al2 O3 . As a result, the expansion of Al2 O3 grains will be prohibited by the YSZ grains. The compressive stress generated by YSZ becomes larger for higher YSZ wt%. As an alternative way to release the high compressive stress, microcracks started to appear in the microstructure. With addition of more than 70 wt% YSZ, microcracks can clearly be observed in the microstructure (identified with circles) and became more noticeable with further additions of YSZ.
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The microstructures of Al2 O3 /YSZ of cutting inserts are dependent on the wt% YSZ additions. Wear resistance performance, Vickers hardness and fracture toughness of the cutting inserts are greatly affected by these microstructures. The appearance of microcracks in the microstructure due to the thermal mismatch becomes the reason to weaken the cutting inserts for above 60 wt% YSZ additions. The optimum addition of YSZ is around 20 wt%.