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Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining
RMHM-03882; No of Pages 5 Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining R. Landfried ⁎, F. Kern, R. Gadow Institut für Fertigungstechnologie keramischer Bauteile, Universität Stuttgart, Allmandring 7b, D-70569 Stuttgart, Germany
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 2 August 2014 Accepted 3 August 2014 Available online xxxx Keywords: EDM ZTA TiC Ceramics Particle size
a b s t r a c t Processing of highly abrasive materials via powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High performance ceramics which exhibit high hardness, bending strength and toughness show the perfect combination of properties for these applications. However they also have the usual drawback that they cannot be economically customized in complex shapes and low quantities, as they are required for tool and mold design. Recent material development enabled EDM of electrically conductive oxide ceramics, the most widespread machining process for machining of hard materials, as an alternative to conventional ceramic manufacturing and hard machining technologies. This study focuses on the influence of TiC particle sizes on material properties and EDM machinability of ZTA–TiC ceramics with 24 vol.% TiC, 17 vol.% ZrO2 and 59 vol.% Al2O3. Fracture toughness, bending strength and electrical conductivity were analyzed for samples produced from TiC powders with particle sizes varying from 0.43 μm to 2.54 μm. Surface integrity of wire cut samples and feed rate during machining were investigated. It was shown that reducing the size of electrical conductive grains strongly increases the electrical conductivity and slightly decreases mechanical properties. Therefore also the machining characteristics are influenced by TiC grain size. The feed rate increases with decreasing particle size to a maximum at d50 = 1–1.3 μm. Reduction of TiC particle size also leads to significantly decreasing surface roughness after the main cut. Additionally the necessary number of trimming steps to achieve a distinct surface roughness is also minimized for low particle sizes. © 2014 Elsevier Ltd. All rights reserved.
Introduction High performance ceramics are well known for their high strength, toughness and outstanding wear resistance. This combination of properties not only is beneficial for a wide range of industrial applications, but also prohibits shape cutting of sintered ceramics and significantly increases the cost of hard machining. Therefore the use of wear resistant ceramics in industrial applications is frequently inhibited, because the demands concerning the production costs, high accuracy and geometrical complexity cannot be met by conventional ceramic manufacturing techniques. Especially in tool and mold design, where complex customized parts are required, the material properties of high performance ceramics are beneficial to avoid frequent reproduction and repair of the tribologically loaded molds and dies. Electrical discharge machining (EDM) processes, such as die sinking or wire cutting, are established technologies to machine materials with high hardness. Due to thermally induced material removal, these processes are independent of the wear resistance of the work piece. In order to machine high wear resistant ceramics by EDM a mandatory threshold of 1 S m−1 electrical conductivity has to be overcome [1]. ⁎ Corresponding author.
Research on zirconia with addition of electrically conductive hard materials has shown the feasibility of EDM to machine high performance ceramics [2,3]. In order to use EDM processes to machine ceramics that combine high strength with high wear resistance, electrically conductive ceramics based on alumina and zirconia with addition of TiC were investigated in recent work of the authors [4,5]. Results have shown that ZTA (zirconia toughened alumina) based compositions with 24 vol.% TiC and 17–19 vol.% ZrO2 exhibit best combination of mechanical properties and machinability by EDM processes. The electrical conductivity of multiphase materials strongly depends on the distribution, form and size of the electrically conductive phase, which is embedded in an isolating matrix [6–8]. In order to improve the electrical conductivity of mixtures, the relation of D/d has to be maximized. Here D is the grain size of the isolating matrix and d is the grain size of the conductive phase. In this study the influence of particle size of TiC starting powders on electrical and mechanical properties as well as on EDM machinability of ZTA based composites was analyzed. Experimental Electrically conductive ceramics with 24 vol.% TiC, 17 vol.% 0.8 Y-TZP and 59 vol.% Al2O3 were investigated. Yttria content within
http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
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R. Landfried et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
stabilized zirconia phase was adjusted by coating monoclinic zirconia (0Y-TZP) with yttria as described by Sommer et al. [9]. Grain size of titanium carbide phase was varied in order to investigate their influence on electrical and mechanical properties as well as on EDM performance. Particle sizes of TiC starting powders were reduced by ball milling. Size distributions of 5 separately prepared TiC powders before (sample 1) and after milling (sample 2–5) were measured by laser granulometry (see Table 1). TiC, Al2O3 and ZrO2 powders were attrition milled in order to achieve homogeneous phase distribution and to reduce the amount of aggregates in sintered samples. For material characterization disks with a diameter of 45 mm and a thickness of ca. 2 mm were hot pressed under an axial pressure of 40 MPa at a temperature of 1525 °C for 2 h. For machining tests disks with a height of 5.5 mm were hot pressed under the same conditions. Samples were cut into bars with a cross section of 2 × 4 mm and a minimal length of 25 mm. Edges were beveled; surfaces were lapped and polished for mechanical characterization. Bending strength was measured in a 4-point setup with 10/20 mm span. Fracture toughness was determined by the ISB method [10] in the same bending setup. Density of hot pressed samples was determined by the principle of Archimedes. Electrical conductivity was measured according to the 4-point method. Machining tests were performed on a wire-cutting machine (AgieCharmilles, Cut 1000, OilTech) in oil based dielectric (IonoPlus, Oelheld) starting with the main cut, followed by 3 consecutive trimming steps. Material removal rate was tracked and gap width was measured with a microscope at a cross section after machining. Surface integrity was analyzed by SEM (Jeol, Japan) and subsurface region was observed on polished cross sections of machined samples. Tactile surface measurements (Mahr, Germany) revealed surface roughness after each machining step for all samples. Results Mechanical and electrical properties Results of bending strength and fracture toughness measurements can be seen in Fig. 1. Both properties show a slight trend to decrease with decreasing particle size of TiC starting powders. Fracture toughness values show typical low standard deviation and drop linearly from 5.06 MPam1/2 (d 50,TiC = 2.54 μm) to 4.39 MPam− 1 (d50,TiC = 0.43 μm). Bending strength values exhibit higher deviation and the average strength decreases from 732 MPa to 687 MPa for the reduction of TiC particle sizes from d50 = 2.54 μm to d50 =0.43 μm respectively. Electrical conductivity of the hot pressed samples is shown in Fig. 2. The curve progression shows an interaction of different mechanisms. By decreasing the particle size of the electrically conductive inclusion within the composite the electrical conductivity increases [6–8]. This correlation can be seen for particle sizes of TiC powders ranging between d50 = 2.54 μm and d50 = 0.72 μm. Further reduction of TiC particle size leads to a significant drop in electrical conductivity to a minimum of 477 S m−1.
Fig. 1. Bending strength and fracture toughness of ZrO2–TiC ceramics derived from TiC powders with particle sizes (d50) varying between 2.54 μm and 0.43 μm.
Maximum MRR of 2.5 mm/min is obtained for a TiC particle size of d50 = 1.31 μm. Further reduction of particle size, especially from d50 = 0.72 μm to d50 = 0.43 μm, leads to a significant reduction of MRR. The gap width does not show any local maximum for distinct particle size but decreases with decreasing TiC particle size. Fig. 4 shows the influence of the TiC powder particle size on the surface roughness created by each machining step. As expected, Fig. 4 reveals a decreasing mean surface roughness index for consecutive machining steps. The highest drop of surface roughness appears for TiC particle sizes of d50 = 0.72–2.54 μm between the 3rd and the 4th machining step. Solely samples produced from the finest TiC powder show a different behavior and trimming steps appear to be unable to further reduce the surface roughness after the main cut. A decreasing TiC particle size leads to a significant reduction of surface roughness after the main cut and simultaneously decreases the minimal surface roughness that can be achieved by 3 consecutive trimming steps. While TiC powder with d50 = 2.54 μm leads to surface roughness of Ra = 2.48 μm and Ra = 0.94 μm after the main cut and the 3rd trimming step respectively, TiC powder with d50 = 1.31 μm already reduces the roughness to Ra = 1.86 μm and Ra = 0.56 μm respectively. Below a TiC particle size of d 50 = 0.5 μm trimming steps have no significant influence on the tactile measured surface roughness.
EDM Average material removal rates (MRR) that were achieved during the main cut as well as the resulting gap widths are shown in Fig. 3. Table 1 Particle sizes of TiC powders before (sample 1) and after milling (samples 2–5). [μm]
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
d10 d50 d90
0.87 2.54 7.56
0.60 1.31 2.73
0.49 0.92 1.54
0.40 0.72 1.23
0.18 0.43 0.95
Fig. 2. Electrical conductivity of samples for varying TiC particle size [11].
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
R. Landfried et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
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Fig. 3. Material removal rate (MRR) and gap width observed for the main cut [11].
After each machining step microscope images of cross sections were made for all samples in order to evaluate surface integrity. Fig. 5 shows cross sections after the main cut for samples produced from TiC starting powder with particle sizes of d50 = 2.54 μm (a), d50 = 0.72 μm (b) and d50 = 0.43 μm (c). Images of cross sections reveal the size and homogeneous dispersion of TiC inclusions and confirm that tactile roughness measurements of samples with TiC particles sizes N 0.5 μm provide an accurate description of the surface roughness. Only for finest TiC powder tactile measurements seem to underestimate the roughness on the surface. Especially roughness valleys with high aspect ratios, which frequently occur on these machined samples after the main cut, cannot be adequately measured. Fig. 6 shows cross sections for the same samples after the 3rd trimming step and therefore exhibits the smoothest obtainable surface structure by wire cutting of these materials. Comparing images of Figs. 5 and 6 visualizes the amount of surface roughness that can be removed by consecutive trimming steps. The cross section of samples with finest TiC inclusion also shows that – in contrast to findings by tactile measurements – trimming does lead to a significant improvement of surface integrity. Deep surface roughness valleys with high aspect
Fig. 5. Cross sections of machined samples with TiC particle sizes of (a) d50 = 2.54 μm, (b) d50 = 0.72 μm and (c) d50 = 0.43 μm after the main cut [11].
ratios, which are particularly critical for crack activation, are removed after the 3rd trimming step. Fig. 7 shows SEM images of surface structures after the 3rd trimming step. The machined surface of the sample that was produced from coarsest TiC powder shows a foamy structure caused by vaporization of work piece material as well as rounded roughness tips that indicate molten and resolidified material. With decreasing TiC particle size the structure fades from this foamy structure to a flat and smooth topology built by predominantly molten and resolidified material. Discussion
Fig. 4. Mean roughness index of samples after each machining step [11].
The reduction of particle size of TiC powders leads to a decrease of strength and toughness due to the decreasing contribution of crack deflection and bridging to these mechanical properties. Moreover a finer dispersion should also lead to a grain refinement of the matrix and zirconia dispersion, thereby reducing the toughness [12]. The progression of the electrical conductivity can be explained for the coarse branch of TiC particle sizes by the correlation of grain sizes of isolating and conductive phases as described before [6–8]. By milling TiC powders and reducing the particle size the specific surface area increases and TiC particles tend to oxidize during milling or drying, which in turn reduces the electrical conductivity. Therefore at a TiC particle size of d50 = 0.42 μm oxidation seems to have occurred during powder preparation — most probably during drying. It is known that titania stabilizes zirconia in a tetragonal t′ phase that is not capable of transforming into monoclinic structure and therefore does not contribute to transformation toughness [13]. Therefore the appearance of TiO2 in the composites also decreases toughness and strength of the material.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
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Progression of material removal rate depending on TiC particle size is predominantly influenced by the electrical conductivity of the work piece. The comparison of the MRR and the gap width (Fig. 3) shows that obviously the gap width is more directly correlated to the TiC particle size than to the MRR. Considering the fact that higher removal rates generally cause higher amount of debris and therefore result in larger gap widths, the observed interaction between MRR and gap width indicates that the form of the debris and the effort needed to remove it from the working gap significantly change with TiC particle size. Combined with the absence of resolidified molten TiC on the machined surfaces, this observation suggests that TiC particles, which have a melting point of 3017 °C and a vaporization point of 4820 °C, are predominantly removed from the work piece, without melting or vaporization, by removing their surrounding matrix.
Summary and conclusion It was shown that the particle size of TiC starting powders has a strong influence on the material properties as well as the machinability by EDM. Besides a slight trend to reduce mechanical properties such as bending strength and toughness, the reduction of TiC particle size also significantly increases the electrical conductivity. Therefore a decreasing TiC particle size improves the performance during EDM and optimizes the surface integrity of machined samples. However for best machinability the TiC particle size should not be reduced below a threshold of about d50 = 0.5 μm. Otherwise oxidation of TiC during powder preparation leads to a decrease of electrical conductivity and significantly worsens MRR.
Acknowledgments Fig. 6. Cross sections of machined samples with TiC particle sizes of (a) d 50 = 2.54 μm, (b) d 50 = 0.72 μm and (c) d 50 = 0.43 μm after the 3rd trimming step [11].
The support of Graveurbetrieb Leonhardt (Hochdorf, Germany) is gratefully acknowledged by the authors.
Fig. 7. SEM images of machined samples with TiC particle sizes of (a) d50 = 2.54 μm, (b) d50 = 0.72 μm and (c) d50 = 0.43 μm after the 3rd trimming step [11].
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
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References [1] König W, Dauw DF, Levy G, Panten U. EDM — future steps towards the machining of ceramics. CIRP Ann Manuf Technol 1988;37(2):623–31. [2] Vleugels J, Van der Biest O. Development and characterization of Y2O3-stabilized ZrO2 (Y-TZP) composites with TiB2, TiN, TiC and TiC0.5N0.5. J Am Ceram Soc 1999; 82(10):2717–20. [3] Basu B, Vleugels J, Van der Biest O. Microstructure and mechanical properties of ZrO2–TiB2 composites. J Mater Sci 2004;39:6389–92. [4] Landfried R, Kern F, Gadow R. Electrical discharge machining of alumina–zirconia–TiC Composites with varying zirconia content. Key Eng Mater 1916–1921;2013:554–7. [5] Landfried R, Kern F, Gadow R, Burger W, Leonhardt W. Development of electrical discharge machinable ZTA ceramics with 24 vol% of TiC, TiN, TiCN, TiB2 and WC as electrically conductive phase. Int J Appl Ceram Technol 2013;10(3):509–18. [6] Bruggeman DAG. Berechnung verschiedener physikalischerKonstanten von heterogenen Substanzen – I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen. Ann Phys (Leipzig) 1935;5(24):636–64. [7] Lux F. Review — models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials. J Mater Sci 1993;28:285–301.
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[8] Ran S, Gao L. Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering. Ceram Int 2012;38:4923–8. [9] Sommer F, Landfried R, Kern F, Gadow R. Mechanical properties of zirconia toughened alumina with 10–24 vol.% 1.5 mol% Y-TZP reinforcement. J Eur Ceram Soc 2012;3(15):3905–10. [10] Chantikul P, Anstis GR, Lawn BR, Marshall DB. A critical evaluation of indentation techniques for measuring fracture toughness: II strength method. J Am Ceram Soc 1981;64(9):539–43. [11] Landfried R. Funkenerosiv bearbeitbare Keramiken für den Werkzeug- und Formenbau. Aachen: Shaker; 2014. [12] Kim D-K, Kriven WM. Processing and characterization of multiphase ceramic composites, part II: triplex with composites with a wide sintering temperature range. J Am Ceram Soc 2008;91(3):793–8. [13] Vleugels J, Van der Biest O. Development and characterization of Y2O3-stabilized ZrO2 (Y-TZP) composites with TiB2, TiN, TiC, and Ti0.5N0.5. J Am Ceram Soc 1999; 82(10):2717–20.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining R. Landfried ⁎, F. Kern, R. Gadow Institut für Fertigungstechnologie keramischer Bauteile, Universität Stuttgart, Allmandring 7b, D-70569 Stuttgart, Germany
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 2 August 2014 Accepted 3 August 2014 Available online xxxx Keywords: EDM ZTA TiC Ceramics Particle size
a b s t r a c t Processing of highly abrasive materials via powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High performance ceramics which exhibit high hardness, bending strength and toughness show the perfect combination of properties for these applications. However they also have the usual drawback that they cannot be economically customized in complex shapes and low quantities, as they are required for tool and mold design. Recent material development enabled EDM of electrically conductive oxide ceramics, the most widespread machining process for machining of hard materials, as an alternative to conventional ceramic manufacturing and hard machining technologies. This study focuses on the influence of TiC particle sizes on material properties and EDM machinability of ZTA–TiC ceramics with 24 vol.% TiC, 17 vol.% ZrO2 and 59 vol.% Al2O3. Fracture toughness, bending strength and electrical conductivity were analyzed for samples produced from TiC powders with particle sizes varying from 0.43 μm to 2.54 μm. Surface integrity of wire cut samples and feed rate during machining were investigated. It was shown that reducing the size of electrical conductive grains strongly increases the electrical conductivity and slightly decreases mechanical properties. Therefore also the machining characteristics are influenced by TiC grain size. The feed rate increases with decreasing particle size to a maximum at d50 = 1–1.3 μm. Reduction of TiC particle size also leads to significantly decreasing surface roughness after the main cut. Additionally the necessary number of trimming steps to achieve a distinct surface roughness is also minimized for low particle sizes. © 2014 Elsevier Ltd. All rights reserved.
Introduction High performance ceramics are well known for their high strength, toughness and outstanding wear resistance. This combination of properties not only is beneficial for a wide range of industrial applications, but also prohibits shape cutting of sintered ceramics and significantly increases the cost of hard machining. Therefore the use of wear resistant ceramics in industrial applications is frequently inhibited, because the demands concerning the production costs, high accuracy and geometrical complexity cannot be met by conventional ceramic manufacturing techniques. Especially in tool and mold design, where complex customized parts are required, the material properties of high performance ceramics are beneficial to avoid frequent reproduction and repair of the tribologically loaded molds and dies. Electrical discharge machining (EDM) processes, such as die sinking or wire cutting, are established technologies to machine materials with high hardness. Due to thermally induced material removal, these processes are independent of the wear resistance of the work piece. In order to machine high wear resistant ceramics by EDM a mandatory threshold of 1 S m−1 electrical conductivity has to be overcome [1]. ⁎ Corresponding author.
Research on zirconia with addition of electrically conductive hard materials has shown the feasibility of EDM to machine high performance ceramics [2,3]. In order to use EDM processes to machine ceramics that combine high strength with high wear resistance, electrically conductive ceramics based on alumina and zirconia with addition of TiC were investigated in recent work of the authors [4,5]. Results have shown that ZTA (zirconia toughened alumina) based compositions with 24 vol.% TiC and 17–19 vol.% ZrO2 exhibit best combination of mechanical properties and machinability by EDM processes. The electrical conductivity of multiphase materials strongly depends on the distribution, form and size of the electrically conductive phase, which is embedded in an isolating matrix [6–8]. In order to improve the electrical conductivity of mixtures, the relation of D/d has to be maximized. Here D is the grain size of the isolating matrix and d is the grain size of the conductive phase. In this study the influence of particle size of TiC starting powders on electrical and mechanical properties as well as on EDM machinability of ZTA based composites was analyzed. Experimental Electrically conductive ceramics with 24 vol.% TiC, 17 vol.% 0.8 Y-TZP and 59 vol.% Al2O3 were investigated. Yttria content within
http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
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R. Landfried et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
stabilized zirconia phase was adjusted by coating monoclinic zirconia (0Y-TZP) with yttria as described by Sommer et al. [9]. Grain size of titanium carbide phase was varied in order to investigate their influence on electrical and mechanical properties as well as on EDM performance. Particle sizes of TiC starting powders were reduced by ball milling. Size distributions of 5 separately prepared TiC powders before (sample 1) and after milling (sample 2–5) were measured by laser granulometry (see Table 1). TiC, Al2O3 and ZrO2 powders were attrition milled in order to achieve homogeneous phase distribution and to reduce the amount of aggregates in sintered samples. For material characterization disks with a diameter of 45 mm and a thickness of ca. 2 mm were hot pressed under an axial pressure of 40 MPa at a temperature of 1525 °C for 2 h. For machining tests disks with a height of 5.5 mm were hot pressed under the same conditions. Samples were cut into bars with a cross section of 2 × 4 mm and a minimal length of 25 mm. Edges were beveled; surfaces were lapped and polished for mechanical characterization. Bending strength was measured in a 4-point setup with 10/20 mm span. Fracture toughness was determined by the ISB method [10] in the same bending setup. Density of hot pressed samples was determined by the principle of Archimedes. Electrical conductivity was measured according to the 4-point method. Machining tests were performed on a wire-cutting machine (AgieCharmilles, Cut 1000, OilTech) in oil based dielectric (IonoPlus, Oelheld) starting with the main cut, followed by 3 consecutive trimming steps. Material removal rate was tracked and gap width was measured with a microscope at a cross section after machining. Surface integrity was analyzed by SEM (Jeol, Japan) and subsurface region was observed on polished cross sections of machined samples. Tactile surface measurements (Mahr, Germany) revealed surface roughness after each machining step for all samples. Results Mechanical and electrical properties Results of bending strength and fracture toughness measurements can be seen in Fig. 1. Both properties show a slight trend to decrease with decreasing particle size of TiC starting powders. Fracture toughness values show typical low standard deviation and drop linearly from 5.06 MPam1/2 (d 50,TiC = 2.54 μm) to 4.39 MPam− 1 (d50,TiC = 0.43 μm). Bending strength values exhibit higher deviation and the average strength decreases from 732 MPa to 687 MPa for the reduction of TiC particle sizes from d50 = 2.54 μm to d50 =0.43 μm respectively. Electrical conductivity of the hot pressed samples is shown in Fig. 2. The curve progression shows an interaction of different mechanisms. By decreasing the particle size of the electrically conductive inclusion within the composite the electrical conductivity increases [6–8]. This correlation can be seen for particle sizes of TiC powders ranging between d50 = 2.54 μm and d50 = 0.72 μm. Further reduction of TiC particle size leads to a significant drop in electrical conductivity to a minimum of 477 S m−1.
Fig. 1. Bending strength and fracture toughness of ZrO2–TiC ceramics derived from TiC powders with particle sizes (d50) varying between 2.54 μm and 0.43 μm.
Maximum MRR of 2.5 mm/min is obtained for a TiC particle size of d50 = 1.31 μm. Further reduction of particle size, especially from d50 = 0.72 μm to d50 = 0.43 μm, leads to a significant reduction of MRR. The gap width does not show any local maximum for distinct particle size but decreases with decreasing TiC particle size. Fig. 4 shows the influence of the TiC powder particle size on the surface roughness created by each machining step. As expected, Fig. 4 reveals a decreasing mean surface roughness index for consecutive machining steps. The highest drop of surface roughness appears for TiC particle sizes of d50 = 0.72–2.54 μm between the 3rd and the 4th machining step. Solely samples produced from the finest TiC powder show a different behavior and trimming steps appear to be unable to further reduce the surface roughness after the main cut. A decreasing TiC particle size leads to a significant reduction of surface roughness after the main cut and simultaneously decreases the minimal surface roughness that can be achieved by 3 consecutive trimming steps. While TiC powder with d50 = 2.54 μm leads to surface roughness of Ra = 2.48 μm and Ra = 0.94 μm after the main cut and the 3rd trimming step respectively, TiC powder with d50 = 1.31 μm already reduces the roughness to Ra = 1.86 μm and Ra = 0.56 μm respectively. Below a TiC particle size of d 50 = 0.5 μm trimming steps have no significant influence on the tactile measured surface roughness.
EDM Average material removal rates (MRR) that were achieved during the main cut as well as the resulting gap widths are shown in Fig. 3. Table 1 Particle sizes of TiC powders before (sample 1) and after milling (samples 2–5). [μm]
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
d10 d50 d90
0.87 2.54 7.56
0.60 1.31 2.73
0.49 0.92 1.54
0.40 0.72 1.23
0.18 0.43 0.95
Fig. 2. Electrical conductivity of samples for varying TiC particle size [11].
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
R. Landfried et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
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Fig. 3. Material removal rate (MRR) and gap width observed for the main cut [11].
After each machining step microscope images of cross sections were made for all samples in order to evaluate surface integrity. Fig. 5 shows cross sections after the main cut for samples produced from TiC starting powder with particle sizes of d50 = 2.54 μm (a), d50 = 0.72 μm (b) and d50 = 0.43 μm (c). Images of cross sections reveal the size and homogeneous dispersion of TiC inclusions and confirm that tactile roughness measurements of samples with TiC particles sizes N 0.5 μm provide an accurate description of the surface roughness. Only for finest TiC powder tactile measurements seem to underestimate the roughness on the surface. Especially roughness valleys with high aspect ratios, which frequently occur on these machined samples after the main cut, cannot be adequately measured. Fig. 6 shows cross sections for the same samples after the 3rd trimming step and therefore exhibits the smoothest obtainable surface structure by wire cutting of these materials. Comparing images of Figs. 5 and 6 visualizes the amount of surface roughness that can be removed by consecutive trimming steps. The cross section of samples with finest TiC inclusion also shows that – in contrast to findings by tactile measurements – trimming does lead to a significant improvement of surface integrity. Deep surface roughness valleys with high aspect
Fig. 5. Cross sections of machined samples with TiC particle sizes of (a) d50 = 2.54 μm, (b) d50 = 0.72 μm and (c) d50 = 0.43 μm after the main cut [11].
ratios, which are particularly critical for crack activation, are removed after the 3rd trimming step. Fig. 7 shows SEM images of surface structures after the 3rd trimming step. The machined surface of the sample that was produced from coarsest TiC powder shows a foamy structure caused by vaporization of work piece material as well as rounded roughness tips that indicate molten and resolidified material. With decreasing TiC particle size the structure fades from this foamy structure to a flat and smooth topology built by predominantly molten and resolidified material. Discussion
Fig. 4. Mean roughness index of samples after each machining step [11].
The reduction of particle size of TiC powders leads to a decrease of strength and toughness due to the decreasing contribution of crack deflection and bridging to these mechanical properties. Moreover a finer dispersion should also lead to a grain refinement of the matrix and zirconia dispersion, thereby reducing the toughness [12]. The progression of the electrical conductivity can be explained for the coarse branch of TiC particle sizes by the correlation of grain sizes of isolating and conductive phases as described before [6–8]. By milling TiC powders and reducing the particle size the specific surface area increases and TiC particles tend to oxidize during milling or drying, which in turn reduces the electrical conductivity. Therefore at a TiC particle size of d50 = 0.42 μm oxidation seems to have occurred during powder preparation — most probably during drying. It is known that titania stabilizes zirconia in a tetragonal t′ phase that is not capable of transforming into monoclinic structure and therefore does not contribute to transformation toughness [13]. Therefore the appearance of TiO2 in the composites also decreases toughness and strength of the material.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
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Progression of material removal rate depending on TiC particle size is predominantly influenced by the electrical conductivity of the work piece. The comparison of the MRR and the gap width (Fig. 3) shows that obviously the gap width is more directly correlated to the TiC particle size than to the MRR. Considering the fact that higher removal rates generally cause higher amount of debris and therefore result in larger gap widths, the observed interaction between MRR and gap width indicates that the form of the debris and the effort needed to remove it from the working gap significantly change with TiC particle size. Combined with the absence of resolidified molten TiC on the machined surfaces, this observation suggests that TiC particles, which have a melting point of 3017 °C and a vaporization point of 4820 °C, are predominantly removed from the work piece, without melting or vaporization, by removing their surrounding matrix.
Summary and conclusion It was shown that the particle size of TiC starting powders has a strong influence on the material properties as well as the machinability by EDM. Besides a slight trend to reduce mechanical properties such as bending strength and toughness, the reduction of TiC particle size also significantly increases the electrical conductivity. Therefore a decreasing TiC particle size improves the performance during EDM and optimizes the surface integrity of machined samples. However for best machinability the TiC particle size should not be reduced below a threshold of about d50 = 0.5 μm. Otherwise oxidation of TiC during powder preparation leads to a decrease of electrical conductivity and significantly worsens MRR.
Acknowledgments Fig. 6. Cross sections of machined samples with TiC particle sizes of (a) d 50 = 2.54 μm, (b) d 50 = 0.72 μm and (c) d 50 = 0.43 μm after the 3rd trimming step [11].
The support of Graveurbetrieb Leonhardt (Hochdorf, Germany) is gratefully acknowledged by the authors.
Fig. 7. SEM images of machined samples with TiC particle sizes of (a) d50 = 2.54 μm, (b) d50 = 0.72 μm and (c) d50 = 0.43 μm after the 3rd trimming step [11].
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003
R. Landfried et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2014) xxx–xxx
References [1] König W, Dauw DF, Levy G, Panten U. EDM — future steps towards the machining of ceramics. CIRP Ann Manuf Technol 1988;37(2):623–31. [2] Vleugels J, Van der Biest O. Development and characterization of Y2O3-stabilized ZrO2 (Y-TZP) composites with TiB2, TiN, TiC and TiC0.5N0.5. J Am Ceram Soc 1999; 82(10):2717–20. [3] Basu B, Vleugels J, Van der Biest O. Microstructure and mechanical properties of ZrO2–TiB2 composites. J Mater Sci 2004;39:6389–92. [4] Landfried R, Kern F, Gadow R. Electrical discharge machining of alumina–zirconia–TiC Composites with varying zirconia content. Key Eng Mater 1916–1921;2013:554–7. [5] Landfried R, Kern F, Gadow R, Burger W, Leonhardt W. Development of electrical discharge machinable ZTA ceramics with 24 vol% of TiC, TiN, TiCN, TiB2 and WC as electrically conductive phase. Int J Appl Ceram Technol 2013;10(3):509–18. [6] Bruggeman DAG. Berechnung verschiedener physikalischerKonstanten von heterogenen Substanzen – I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen. Ann Phys (Leipzig) 1935;5(24):636–64. [7] Lux F. Review — models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials. J Mater Sci 1993;28:285–301.
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[8] Ran S, Gao L. Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering. Ceram Int 2012;38:4923–8. [9] Sommer F, Landfried R, Kern F, Gadow R. Mechanical properties of zirconia toughened alumina with 10–24 vol.% 1.5 mol% Y-TZP reinforcement. J Eur Ceram Soc 2012;3(15):3905–10. [10] Chantikul P, Anstis GR, Lawn BR, Marshall DB. A critical evaluation of indentation techniques for measuring fracture toughness: II strength method. J Am Ceram Soc 1981;64(9):539–43. [11] Landfried R. Funkenerosiv bearbeitbare Keramiken für den Werkzeug- und Formenbau. Aachen: Shaker; 2014. [12] Kim D-K, Kriven WM. Processing and characterization of multiphase ceramic composites, part II: triplex with composites with a wide sintering temperature range. J Am Ceram Soc 2008;91(3):793–8. [13] Vleugels J, Van der Biest O. Development and characterization of Y2O3-stabilized ZrO2 (Y-TZP) composites with TiB2, TiN, TiC, and Ti0.5N0.5. J Am Ceram Soc 1999; 82(10):2717–20.
Please cite this article as: Landfried R, et al, Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining, Int J Refract Met Hard Mater (2014), http://dx.doi.org/10.1016/j.ijrmhm.2014.08.003