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The coating of TiC layer on the surface of nickel by electric discharge coating (EDC) with a multi-layer electrode
Journal of Materials Processing Technology 210 (2010) 642–652
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
The coating of TiC layer on the surface of nickel by electric discharge coating (EDC) with a multi-layer electrode Yu-Lung Hwang ∗ , Chia-Lung Kuo, Shun-Fa Hwang Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, 123 University Road, Sec. 3, Douliu, Yunlin 640, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 8 August 2009 Received in revised form 26 October 2009 Accepted 21 November 2009
Keywords: Titanium carbide (TiC) Surface modification Abrasion resistance Multi-layer electrode (MLE) EDC EDM
a b s t r a c t Multi-layer electrodes (MLEs) are proposed in this study to coat a titanium carbide (TiC) layer on the surface of a nickel workpiece by electric discharge coating (EDC). The MLE is composed of titanium (Ti) and graphite (Gr) layers with the same dimensions stacking alternately. Both of this new type of electrode and the conventional electrode, the bulk type, are compared in this study. The experimental results indicate that the Gr layer of the MLE may enhance the concentration of carbon element locally. Also, carbon element with high concentration could increase the combination of Ti and carbon (C) to become TiC, enhance surface hardness of the coated layer, decrease surface roughness of the coated layer, reduce formation of microcracks, and enhance the stability of electric discharge and coating speed.
1. Introduction Surface modification of workpiece has been broadly applied to tooling, molding and implements, and it may not only increase the life span of the workpiece but also reduce manufacturing cost. Ti compounds have been applied extensively as a material for surface modification due to its features of high hardness, high abrasion resistance, high melting point and low coefficient of friction (COF) (Matweb, 1998; Tang et al., 1998; Dobrzanski and Mikula, 2005). Coating a TiC layer onto the surface of workpiece can be accomplished by several ways. First, laser is used to coat TiC powder on the surface of the workpiece and the coated layer has the surface hardness of HV1200 (Yang et al., 2004). Second, powder (TiC and other elements) is coated on the surface of the workpiece by high-energy electron-beam irradiation to get the surface hardness of HV2060 (Yun et al., 2004). Next, unbalanced magnetron sputtering (UBMS) is applied to coat TiC layer onto the workpiece. For example, Deng and Braun (1994) obtained the coated layer with the hardness of HV3511 and COF of 0.2–0.3, while Pei et al. (2005) acquired the coated layer with 5–35 GPa hardness and 0.04 COF. The fourth way is the EDC technology. In this technology, Ti element released from the electrode and C element decomposed from dielectric oil are combined together to create TiC, which will be coated onto the surface of tooling to prolong the life span of the tool (Moro et al.,
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2004). Besides, instead of titanium bulk, Ti powder could be used to make the electrode used in EDC technology. By this way, the coated TiC layer has the hardness of HV2250 (Wang et al., 2002). However, all the above four methods need expensive equipment, complicated processing procedures, and tedious powder preparation. Hence, it is desired to improve these disadvantages. In this work, MLE applied to EDC is proposed to simplify the complicated processing procedures, increase the combination of Ti and C to become TiC, enhance surface hardness of the coated layer, decrease the surface roughness of the coated layer, reduce the formation of microcracks, and enhance the stability of electric discharge and coating speed. 2. Experimental equipment and method MLE is presented in this study to coat a TiC layer by EDC on the surface of a nickel (Ni) workpiece that has the hardness of HV250. Element analysis, surface hardness and surface roughness tests at room temperature, and abrasion resistance tests at high and room temperatures are conducted to verify that MLE indeed enhances the combination of Ti and C, increases surface hardness of the coated layer, reduces surface roughness of the coating layer, and increases coating speed. 2.1. Electrodes
∗ Corresponding author. Tel.: +886 5 5342601x4336; fax: +886 5 5312062. E-mail address: [email protected] (Y.-L. Hwang). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.11.013
Electric discharge machining (EDM) produces sparks of high temperature, which motivates kerosene/working fluid to release C
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Fig. 1. Three types of electrodes, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
Fig. 2. Three types of electrodes fastened by the jig.
atoms. As long as Ti exists either in the electrode or the workpiece, C and Ti would be combined forming a TiC layer on the surface of the workpiece (Chen et al., 1999). Consequently, C atoms play a critical role in the EDC process. Generally, the electrode for EDC has two types. One is the bulk type and the other is the type of multi-layer structure. The bulk electrode is made of a Ti block with the width of 16 mm and the depth of 6 mm and called Type 6 electrode in this work as shown in Fig. 1(a). MLE is composed of Ti and Gr layers with the same dimensions stacking alternately. There are also two types, Type 1
Fig. 4. Diagram of scanning coating process by Type 1 MLE.
and Type 2, for the MLE. For Type 2 MLE, one Gr layer is imbedded between two Ti layers as shown in Fig. 1(b), and all three layers have the same thickness of 2 mm. For Type 1 MLE, three Ti layers and two Gr layers with 1.2 mm thickness are stacked alternately as shown in Fig. 1(c). By reducing the thickness of each layer, the overall dimensions of these two types of MLE are the same as those of Type 6. MLE is assembled and fastened by a fixture as shown in Fig. 2. All three types of electrodes have the same end surface of 6 mm × 16 mm for electric discharge.
Fig. 3. Diagram of coated TiC layer on the surface of Ni workpiece by Type 2 MLE.
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Y.-L. Hwang et al. / Journal of Materials Processing Technology 210 (2010) 642–652 Table 1 Machining conditions for static experiment. Parameter
Condition
Electrical
Polarity Voltage (V) Pulse on time (s) Pulse off time (s) Discharge current (A) Capacitance (F)
Workpiece (+) 90 20–150 with three levels (20, 80 and 150) 20–150 with three levels (20, 80 and 150) 8–20 with three levels (8, 12 and 20) 0–0.2 with three levels (0, 0.1 and 0.2)
Working fluid
Pressure (MPa) Dielectric
0.1 CASTROL (SE FLUID 180)
Table 2 Parameter combinations for static experiment. Experiment
1 2 3 4 5 6 7 8 9 a
L9
Parameters
A
B
C
D
IPa (A)
On time (s)
Off time (s)
C (F)
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
8 8 8 12 12 12 20 20 20
20 80 150 20 80 150 20 80 150
20 80 150 80 150 20 150 20 80
0 0.1 0.2 0.2 0 0.1 0.1 0.2 0
IP is discharge current, on time is pulse on time, off time is pulse off time, and C is capacitance.
2.2. Static experiment A static experiment is defined in this work as machining the surface of the workpiece along the vertical direction for a specific depth. To obtain the desired depth, the origin point of processing needs to be defined. By using the discharge inspection of an electric discharge machine (Sodick, AP1L), the end surface of the electrode can be set to touch the surface of the workpiece, and then this point is set to be the origin/home point of the following processing along vertical direction. The machining depth is the feed length
of machine below the home point. The machining depth of static experiments is 40 m as shown in Fig. 3. In addition to the polarity set-up for the workpiece, the processing parameters for EDC include voltage, pulse on time, pulse off time, discharge current, and capacitance. The selection of appropriate values for these parameters may depend on the material of the electrode, the material of the workpiece, and the working area. Since both the bulk type and the multi-layer type of electrode are considered in this work and the material of the workpiece is Ni, which is seldom investigated, the range of the above parame-
Fig. 5. Surface photos of coated layer by three types of electrodes after static experiment described in Table 2.
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ters will be chosen to be as wide as possible, as listed in Table 1. The working fluid in this work is CASTROL (SE FLUID 180). Both the flushing method and the dielectric bath are simultaneously used in the static and dynamic experiments. Because of these four parameters and their large ranges of values, the number of possible combinations for static experiments is too large to execute. To provide representative combinations for comparing the effect of different types of electrodes, the idea of experiment design with orthogonal array is considered. For four parameters at three levels, nine static experiments by Taguchi L9 orthogonal array as shown in Table 2 are chosen. 2.3. Dynamic experiment In addition to machining the surface of the workpiece along the vertical direction similar to the static experiment, dynamic experiment will also machine the surface of the workpiece along the horizontal direction. Similar to the static experiment, the home point needs to be set before the process. After setting the home point, the dynamic experiment will just machines the surface for 5 m along the vertical direction and then machines (or scans) the surface along the horizontal direction (the length of the workpiece) in each routine. After completing one routine, the electrode moves upward for 2.0 mm to be away from the surface of the workpiece and goes back to the start point for next machining as shown in Fig. 4. This routine will be repeated until the machining depth is 40 m that is the same as that in the static experiment. For dynamic experiments, the processing conditions will be choose from the best selection of the above nine combinations in static experiments. Also all three types of electrodes will be tested in dynamic experiments. 2.4. Property testing of the coated layer To evaluate the properties of the coated layer on the workpiece after the static or dynamic experiments, several tests are executed. First, element analysis is executed by Scanning Electron Microscope (JOEL JSM-6300) and Energy Dispersive System (OXFORD). Then, surface hardness is tested by SHIMADZU HMV-2 and surface roughness is measured by Talyor Hobson Talysurf Series 2. Furthermore, the mechanical and tribological test equipment (CETR UMT-3 and S33HE-1000) is used to evaluate the abrasion resistance of the coated layer, and an electronic scale (AND ER-120A) is used to measure the weight loss of the workpiece. To roughly estimate the C distribution on the surface of the coated layer, the surface is observed by optical microscope (OLYMPUS STM). Finally, the consumption situation of the electrode is examined by 3DProfilometrie equipment (OM Engineering GmbH Scan AF 2000). 3. Results and discussions 3.1. Static experiment The surface of the workpiece after experiment was observed and filmed by an optical microscope and the results are shown in Fig. 5. The results indicate the surface of the workpiece can be considered as a duplicate of the electrode, which is obvious in particular when Type 2 and Type 1 MLEs are applied in experiments 5, 8, and 9. Besides, measurements were made on the working surfaces of Type 2 and Type 1 MLEs (experiment 5) by 3D-Profilometrie equipment and the results are shown in Fig. 6(a) and (b). The surface level of the Gr layer is relatively lower than that of the Ti layer with a difference of about 50 m providing evidence that the Gr layer is consumed more than the Ti layer (the surface levels of both the Gr layer and the Ti layer are the same before experiment). Since Gr is composed of C atom planes and these planes are formed together by van der Waals force, the Gr layer of the MLE is easily broken into C powders
Fig. 6. Working surface profile of MLEs after static experiment by Scan, (a) Type 2 MLE and (b) Type 1 MLE.
during the EDC process. As a result, the Gr layer produces more C powders locally during the EDC process. The measurement zone of the coated layer for surface hardness is showed in Fig. 7(a). In this figure a code, Type n-m, is used to represent the coated TiC layer by using Type n electrode under experiment m. For example, Type 6-5 layer is the coated layer obtained by Type 6 electrode and under the conditions of experiment 5. For Type 6-5 layer, surface hardness was measured at the center of the layer along the width direction. For Type 1-5 or 2-5 layer, surface hardness was tested at one side of the layer along the width direction. The measurement results, as shown in Fig. 7(b), indicate that the average surface hardness of the coated layer by using Type 1 MLE is the best among the three types of electrodes for most of nine static tests. Furthermore, for all three types of electrodes the greatest hardness can be achieved under experiment 5 among the nine static tests. The average hardness values of the coated layer obtained by Type 6, Type 2 and Type 1 are HV1196.2, HV1481.6 and HV1710.6, respectively. For Type 6 electrode, coating is accomplished by sparks of high temperature to disintegrate kerosene/working fluid and produce C, which is further combined with Ti and coated onto the surface of the workpiece in Zone 1 to form a TiC coated layer as shown in Fig. 8(a). The average surface hardness of Type 6-5 layer is the
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Fig. 7. Measurement of average surface hardness on the coated layer after static experiment, (a) measurement zone and (b) results of average surface hardness of coated layer by three types of electrodes.
lowest and just about 4.78 times of that of the substrate, because there is no Gr layer on the electrode. For Type 2 MLE, not only does the Gr layer coats a highly concentrated C layer onto the surface of the workpiece in Zone 2, but C powders scatter to Zone 1 on both sides, as shown in Fig. 8(b). Consequently, the C element in Zone 1 includes both C disintegrated from kerosene/working fluid and C powders leading to a higher concentration, which increases the combination possibility of C and Ti and improves the surface hardness. For Type 1 MLE, as shown in Fig. 8(c), the total width of the Gr layer is 2.4 mm and larger than that of Type 2 MLE, which is 2 mm. This will make the former be capable of providing more C powders during the coating process. Besides, the two Gr layers of Type 1 MLE are distributed evenly in the electric discharge zone and the C powders released from the Gr layer in Zone 2 can uniformly spread to Zone 1 and Zone 3 resulting in a average surface hardness of HV1710.6 in Zone 1 of the coated layer (6.84 times more than the hardness of the substrate, HV250). Hence, Type 1 MLE has the best performance on TiC composition among the three types of electrodes. The results of element analysis reveal that there are five major elements appearing on the coated layer, for example as shown in Fig. 9 for the surfaces under different parts of Type 2 MLE. Two of them, oxygen (O) and sulfur (S), are contributed from the working fluid, and it is believed that their effects on the surface hardness of the coated layer are minor. Hence, the other three elements, Ni, Ti, and C, are focused. For experiment 5, the weight percentage of these three elements for the coated layer by using the three types of electrode are shown in Fig. 10. Since the other elements are neglected, the total weight percentage of these three elements is 100% in this figure. As shown, there are Ti and C elements on the coated layer. Since the hardness of Ti, Gr and Ni element is HV257, Mohs2.0 (Matweb, 1998) and HV250, respectively, and their hardness is lower than that of the coated layer, this may imply the
Fig. 8. Diagram of mechanism of coating TiC layer using three types of electrodes in static experiment, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
existence of TiC on the surface of the workpiece. Furthermore, the Gr layer of Type 2 and Type 1 MLEs can coat a layer with highly concentrated carbon on the surface of the workpiece and the C content can reach 65.28% and 62.08%, respectively. This layer with highly concentrated C will be critically important for the scanning coating process. The inhomogeneity in the coated layer may be concerned because of the different wear of the electrode materials. From the measured data of surface hardness and EDS analysis (not shown in this study because average value is used), the data variation on different locations is small. This may indicate that the inhomogeneity in a layer is not so clear. Since Ni, C, and TiC exist in the coated layer, the lubrication and abrasion resistance will be good and it could be applied to tooling or molding. 3.2. Dynamic experiment Based on the result of static experiments, there are two points to be noted for MLE in the dynamic scanning coating. First, as shown in Fig. 11, T0 stands for the beginning of coating process, and after T0 and before T1 electric discharge occurs simultaneously in Zone 1, Zone 2 and Zone 3. Before T1, the Gr layer of the Type 2 MLE is
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Fig. 9. Element analysis of Type 2-5 layer.
consumed more than the Ti layer causing that the electric discharge gap h2 gradually becomes larger than h1. Hence, after T1, electric discharge only occurs in Zone 1 and Zone 3, not in Zone 2. Before T2 and after T1, the consumption of Ti layer in Zone 1 and Zone 3 makes h1 equal h2 again. After T2, electric discharge occurs simultaneously in all three zones. This cycle will repeat as the dynamic process continues. Because of this cycling, the Gr layer of MLE has
Fig. 10. The results of element analysis of three types of coated layers by EDS.
the chance to periodically release highly concentrated carbon powders. Second, the highly concentrated C powders produced in Zone 2 not only spread to Zone 1 and Zone 3 enhancing the combination of C and Ti, but also coat a layer of highly concentrated C on the surface of the workpiece in Zone 2, as shown in Fig. 12(b). Later, due to the movement of the electrode, Zone 2 will be under the Ti layer and a TiC coating procedure will be created. By the way, the surface of the workpiece in Zone 1 will be also coated a layer of highly concentrated C due to the movement of the electrode. For Type 1 MLE, this coating process may repeat twice because of five layers in the electrode, as shown in Fig. 12(c). The result of the static experiment indicates that the optimum coating condition for the three types of electrodes is experiment 5 as shown in Table 2. Thus, only experiment 5 is applied to the dynamic experiment illustrated in Fig. 13. The photograph of the surface of the coated layer obtained by the three types of electrodes is shown in Fig. 14. In this figure the scanning length of the dynamic experiments is 25 mm. To test the surface roughness, a line with 3 mm is measured. As shown in Fig. 14, the surface roughness of Type 2-5 layer and Type 1-5 layer is Ra 1.6850 m and Ra 1.8826 m, respectively, and they are lower than that of Type 6-5 layer, which is Ra 2.0074 m. This means that Type 2-5 and Type 1-5 layers can reduce the surface roughness by 16.06% and 6.22%, respectively, as compared to Type 6-5 layer. Furthermore,
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Fig. 11. Diagram of mechanism of coating TiC layer by Type 2 MLE for dynamic experiment.
the results from SEM reveal that there are microcracks on the surfaces of the coated layers obtained by all three electrodes, as shown in Fig. 15 for Type 6-5 and Type 2-5 layers. However, the surface of Type 6-5 layer has the dentist and biggest of microcracks. The measured surface hardness of the three types of layers and the pure Ni layer is compared in Fig. 16. It is evident that the average surface hardness of the layer obtained by both types of MLE is greater than that obtained by the bulk electrode. This is because Ti of Type 6 electrode compounds only with C dissolved from kerosene/working fluid during the coating process and coats TiC onto the surface of the workpiece in Zone 1 as shown in Fig. 12(a). Nevertheless, the Gr layer of Type 1 and Type 2 MLEs not only provide C powders of high concentration to the local area, but also offer the repetitive coating mechanism for once (Type 2 MLE) or twice (Type 1 MLE) on the surface of the workpiece. In Fig. 12(b), the Gr layer of Type 2 MLE produces C powders of high concentration in Zone 2 and these highly concentrated C powders spread to Zone 1 and Zone 3 enhancing the composition of C and Ti and coating a C layer on the surface of the workpiece in Zone 2, which is beneficial to the one-time repetitive coating mechanism in Zone 3. Likewise, Type 1 MLE can also produce C powders of high concentration in Zone 2 and Zone 4 spreading to Zone1, Zone 3 and Zone 5, enhancing the composition of carbon and Ti and coating a C layer on the surface of the workpieces in Zone 2 and Zone 4, which helps the one-time and two-time repetitive coating mechanism in Zone 3 and Zone 5, respectively, as shown in Fig. 12(c). However, the hardness achieved by dynamic scanning coating is lower than that accomplished by static coating under the same conditions because
electrode scanning stirs kerosene/working fluid and reduces the composition of C and Ti. The machining time for Type 2-5 and Type 1-5 layers is 40.2 min and 38.1 min, respectively, which reduces about 135 min as compared to that of Type 6-5 layer (174.6 min), as shown in Fig. 17. Consequently, MLE can not only decrease roughness and microcracks on the surface of the coated layer, but also reduce machining time to a great extent. The main reason is due to the C powder of high concentration locally produced by the Gr part of MLE during the electric discharge process. The C powder of high concentration in a local area is like powder mixed electric discharge machining (PMEDM), and it can accelerate processing speed, reduce the formation of microcracks, reduce surface roughness, and enhance electrical discharge stability (Kansal et al., 2007; PeC¸as and Henriques, 2008; Abbas et al., 2007; Tzeng and Chen, 2005; Han et al., 2007; Kumer et al., 2009). Therefore, using the multi-layer electrode is better than using the tradition electrode for coating TiC in the surface of Ni. Furthermore, it will be very interesting to compare the present results with the ones by using powder (Gr) additives in the dielectric Ti-electrodes. However, since PMEDM is also a big issue and a lot of parameters need to be suitably decided in advance, this interesting comparison will be explored in the future. A scanning experiment (with experiment 5) is conducted on Type 2 and Type 1 MLEs at different machining depths (10 m, 20 m, 30 m and 40 m) for the understanding of the relationship between the machining depth and thickness of the coated layer. The photograph of the cross-section of the coated layer is shown in Fig. 18(a), and the relationship between the scanning
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Fig. 12. Diagram of mechanism of coating TiC layer using three types of electrode in dynamic experiment, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
depth and thickness of the coated layer is illustrated in Fig. 18(b). The thicknesses of Type 2-5 and Type 1-5 layers are the same at a machining depth of 10 m, while they have different thicknesses at other machining depths. Also the thickness variation of Type 1-5 (a min. of 11.50 m and a max. of 15.50 m) is higher than that of Type 2-5. Therefore, machining depth plays a significant part on the obtained layer thickness for Type 1 MLE. 3.3. Abrasion resistance testing Abrasion resistance testing is conducted on the surfaces of pure Ni layer, Type 2-5 layer, and Type 1-5 layer (a machining depth of
Fig. 14. Photograph of surface of coated layer using three types of electrodes.
40 m for Type 2-5 and Type 1-5 layers) with testing conditions as listed in Table 3. The measurements of the scratch widths as shown in Fig. 19 indicate that the scratch widths of Type 2-5 and Type 1-5 layer are less than that of Ni layer under the same testing conditions. The average hardness of Type 2-5 and Type 1-5 layers is HV1216.0 and HV1131.0, respectively, which is higher than HV250.0 of Ni layer, and higher hardness has higher abrasion resistance. Besides, the scratch widths of the three layers increase with temperature. As a result, the coated layer with high hardness is capable of abrasion resistance at a high temperature. Some of the results are missing in this figure because the edge of the scratch could not be identified. For low temperature at 30 ◦ C, COF of Ni layer increases with the increasing of normal force as shown Fig. 20(a). The average COF increases from 0.190 at 10 N to 0.572 at 100 N as shown in Fig. 21(a). Type 2-5 and Type 1-5 layers have excellent abrasion resistance performance at a normal force of 100 N as shown in Fig. 20(b). The average COF of Type 2-5 and Type 1-5 layers slightly increase Table 3 Conditions for abrasion resistance testing.
Fig. 13. Set-up of dynamic experiment.
Parameter
Condition
Test head Rotate speed Test time Workpiece surface Test temperature Loading force
Ball (tungsten carbide, Ø 4 cm) 300 (rpm) 60 (s) Dry 30, 400 (◦ C) 10, 50, 100 (N)
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Fig. 17. Machining time for Type 6-5, Type 2-5, and Type 1-5 layer.
Fig. 15. SEM micrographs for surface of coated layer, (a) Type 6-5 layer and (b) Type 2-5 layer.
from 0.116 and 0.132 to 0.193 and 0.166, respectively, as shown in Fig. 21(a). Furthermore, weight loss is illustrated in Fig. 21(b). Ni layer loses weight tremendously with an increase in the normal force from 0 mg (10 N) to 101 mg (100 N). This is because the shear modulus and hardness of Ni are only 76 GPa (Matweb, 1998) and HV250.0. A greater normal/loading force easily results in plastic flow causing tearing off the material. On the other hand, Type 2-5 layer and Type 1-5 layer only lose 4 mg, which is due to their high hardness and high shear modulus (110–193 GPa (Matweb, 1998)) from the coated TiC layer. Besides, this coated layer has C content and solid C element is effective in lubrication. Consequently, a
Fig. 16. Results of surface hardness measurement on the layer of Ni, Type 6-5, Type 2-5 and Type 1-5.
Fig. 18. Cross-section of the coated layer of Type 2-5 and Type 1-5, (a) photos of cross-section of the coated layer at different machining depth and (b) measurement results of the coated layer thickness.
Fig. 19. Photos of width of groove worn on the surface of the three types of coated layer after abrasion resistance testing.
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Fig. 20. Results of abrasion resistance testing: (a) Ni layer under different forces, (b) the three layers under 100 N and 30 ◦ C, (c) the three layers under 10 N and 400 ◦ C and (d) the three layers under 50 N and 400 ◦ C.
workpiece with a TiC layer is good at abrasion resistance at room temperature. For high temperature at 400 ◦ C, the results of abrasion resistance testing are illustrated in Fig. 20(c) and (d). The COFs of Type 2-5 layer and Type 1-5 layer increase slowly with the testing time at a loading force of 10 N, while they are near the maximum value
of Ni layer at the 28th second under a loading force of 50 N. The COF of Ni layer is the highest with an average value of 0.715 and 0.664 at the loading force of 10 N and 50 N, respectively, as shown in Fig. 22(a). From Fig. 22(b), under a loading force of 10 N, Ni layer loses 82 mg, while Type 2-5 layer and Type 1-5 layer only lose 29 mg and 21 mg, respectively. However, under the loading force of 50 N, the three layers lose a similar weight of 390 mg. Accordingly, the
Fig. 21. Results of abrasion resistance testing at temperature of 30 ◦ C: (a) COFavg and (b) weight loss.
Fig. 22. Results of abrasion resistance testing at temperature of 400 ◦ C: (a) COFavg and (b) weight loss.
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workpiece with the TiC layer still has abrasion resistance under the loading force of 10 N since the layer does not tear off completely; nevertheless, the TiC layer is almost striped off at the 28th second under a loading force of 50 N causing the following COF consistent with that of Ni layer. The main reason that the coated layer is torn off is because the shearing force first occurs in the softer substrate (Ni) under the coated layer and the high temperature of 400 ◦ C generates plastic flow easily. Thus, the coated layer moves with the plastic flow of the substrate causing microcracks in the coated layer or breaks at thinner places. The coated layer adhering to the substrate (Ni) is torn off. In addition, this mechanism could be confirmed from the testing under 30 ◦ C and 100 N loading force since such a great loading force still cannot make the coated layer adhering to the substrate (Ni) be torn off. Consequently, stripping occurs in the substrate (Ni) instead of the TiC layer at a high temperature. 4. Conclusions 1. MLE indeed decreases the complicated machining procedures, enhances the composition of Ti and C, enhances surface hardness of the coated layer, reduces surface roughness and microcracks, and increases electric discharge stability and coating speed when applying to electric discharge coating. Therefore, MLEs should be extensively applied to the composition of different materials. 2. The abrasion resistance of a coated layer is excellent at room temperature (30 ◦ C) and such competence still maintains at high temperature (400 ◦ C). However, this ability reduces with the increase in the loading force, which mainly results from the stripping of the substrate (Ni) under the coated layer instead of the coated layer itself. Acknowledgements The authors would like to thank the China Steel Corporation, Taiwan for financial support under Contract No. 95-336.
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The coating of TiC layer on the surface of nickel by electric discharge coating (EDC) with a multi-layer electrode Yu-Lung Hwang ∗ , Chia-Lung Kuo, Shun-Fa Hwang Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, 123 University Road, Sec. 3, Douliu, Yunlin 640, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 8 August 2009 Received in revised form 26 October 2009 Accepted 21 November 2009
Keywords: Titanium carbide (TiC) Surface modification Abrasion resistance Multi-layer electrode (MLE) EDC EDM
a b s t r a c t Multi-layer electrodes (MLEs) are proposed in this study to coat a titanium carbide (TiC) layer on the surface of a nickel workpiece by electric discharge coating (EDC). The MLE is composed of titanium (Ti) and graphite (Gr) layers with the same dimensions stacking alternately. Both of this new type of electrode and the conventional electrode, the bulk type, are compared in this study. The experimental results indicate that the Gr layer of the MLE may enhance the concentration of carbon element locally. Also, carbon element with high concentration could increase the combination of Ti and carbon (C) to become TiC, enhance surface hardness of the coated layer, decrease surface roughness of the coated layer, reduce formation of microcracks, and enhance the stability of electric discharge and coating speed.
1. Introduction Surface modification of workpiece has been broadly applied to tooling, molding and implements, and it may not only increase the life span of the workpiece but also reduce manufacturing cost. Ti compounds have been applied extensively as a material for surface modification due to its features of high hardness, high abrasion resistance, high melting point and low coefficient of friction (COF) (Matweb, 1998; Tang et al., 1998; Dobrzanski and Mikula, 2005). Coating a TiC layer onto the surface of workpiece can be accomplished by several ways. First, laser is used to coat TiC powder on the surface of the workpiece and the coated layer has the surface hardness of HV1200 (Yang et al., 2004). Second, powder (TiC and other elements) is coated on the surface of the workpiece by high-energy electron-beam irradiation to get the surface hardness of HV2060 (Yun et al., 2004). Next, unbalanced magnetron sputtering (UBMS) is applied to coat TiC layer onto the workpiece. For example, Deng and Braun (1994) obtained the coated layer with the hardness of HV3511 and COF of 0.2–0.3, while Pei et al. (2005) acquired the coated layer with 5–35 GPa hardness and 0.04 COF. The fourth way is the EDC technology. In this technology, Ti element released from the electrode and C element decomposed from dielectric oil are combined together to create TiC, which will be coated onto the surface of tooling to prolong the life span of the tool (Moro et al.,
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2004). Besides, instead of titanium bulk, Ti powder could be used to make the electrode used in EDC technology. By this way, the coated TiC layer has the hardness of HV2250 (Wang et al., 2002). However, all the above four methods need expensive equipment, complicated processing procedures, and tedious powder preparation. Hence, it is desired to improve these disadvantages. In this work, MLE applied to EDC is proposed to simplify the complicated processing procedures, increase the combination of Ti and C to become TiC, enhance surface hardness of the coated layer, decrease the surface roughness of the coated layer, reduce the formation of microcracks, and enhance the stability of electric discharge and coating speed. 2. Experimental equipment and method MLE is presented in this study to coat a TiC layer by EDC on the surface of a nickel (Ni) workpiece that has the hardness of HV250. Element analysis, surface hardness and surface roughness tests at room temperature, and abrasion resistance tests at high and room temperatures are conducted to verify that MLE indeed enhances the combination of Ti and C, increases surface hardness of the coated layer, reduces surface roughness of the coating layer, and increases coating speed. 2.1. Electrodes
∗ Corresponding author. Tel.: +886 5 5342601x4336; fax: +886 5 5312062. E-mail address: [email protected] (Y.-L. Hwang). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.11.013
Electric discharge machining (EDM) produces sparks of high temperature, which motivates kerosene/working fluid to release C
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Fig. 1. Three types of electrodes, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
Fig. 2. Three types of electrodes fastened by the jig.
atoms. As long as Ti exists either in the electrode or the workpiece, C and Ti would be combined forming a TiC layer on the surface of the workpiece (Chen et al., 1999). Consequently, C atoms play a critical role in the EDC process. Generally, the electrode for EDC has two types. One is the bulk type and the other is the type of multi-layer structure. The bulk electrode is made of a Ti block with the width of 16 mm and the depth of 6 mm and called Type 6 electrode in this work as shown in Fig. 1(a). MLE is composed of Ti and Gr layers with the same dimensions stacking alternately. There are also two types, Type 1
Fig. 4. Diagram of scanning coating process by Type 1 MLE.
and Type 2, for the MLE. For Type 2 MLE, one Gr layer is imbedded between two Ti layers as shown in Fig. 1(b), and all three layers have the same thickness of 2 mm. For Type 1 MLE, three Ti layers and two Gr layers with 1.2 mm thickness are stacked alternately as shown in Fig. 1(c). By reducing the thickness of each layer, the overall dimensions of these two types of MLE are the same as those of Type 6. MLE is assembled and fastened by a fixture as shown in Fig. 2. All three types of electrodes have the same end surface of 6 mm × 16 mm for electric discharge.
Fig. 3. Diagram of coated TiC layer on the surface of Ni workpiece by Type 2 MLE.
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Y.-L. Hwang et al. / Journal of Materials Processing Technology 210 (2010) 642–652 Table 1 Machining conditions for static experiment. Parameter
Condition
Electrical
Polarity Voltage (V) Pulse on time (s) Pulse off time (s) Discharge current (A) Capacitance (F)
Workpiece (+) 90 20–150 with three levels (20, 80 and 150) 20–150 with three levels (20, 80 and 150) 8–20 with three levels (8, 12 and 20) 0–0.2 with three levels (0, 0.1 and 0.2)
Working fluid
Pressure (MPa) Dielectric
0.1 CASTROL (SE FLUID 180)
Table 2 Parameter combinations for static experiment. Experiment
1 2 3 4 5 6 7 8 9 a
L9
Parameters
A
B
C
D
IPa (A)
On time (s)
Off time (s)
C (F)
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
8 8 8 12 12 12 20 20 20
20 80 150 20 80 150 20 80 150
20 80 150 80 150 20 150 20 80
0 0.1 0.2 0.2 0 0.1 0.1 0.2 0
IP is discharge current, on time is pulse on time, off time is pulse off time, and C is capacitance.
2.2. Static experiment A static experiment is defined in this work as machining the surface of the workpiece along the vertical direction for a specific depth. To obtain the desired depth, the origin point of processing needs to be defined. By using the discharge inspection of an electric discharge machine (Sodick, AP1L), the end surface of the electrode can be set to touch the surface of the workpiece, and then this point is set to be the origin/home point of the following processing along vertical direction. The machining depth is the feed length
of machine below the home point. The machining depth of static experiments is 40 m as shown in Fig. 3. In addition to the polarity set-up for the workpiece, the processing parameters for EDC include voltage, pulse on time, pulse off time, discharge current, and capacitance. The selection of appropriate values for these parameters may depend on the material of the electrode, the material of the workpiece, and the working area. Since both the bulk type and the multi-layer type of electrode are considered in this work and the material of the workpiece is Ni, which is seldom investigated, the range of the above parame-
Fig. 5. Surface photos of coated layer by three types of electrodes after static experiment described in Table 2.
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ters will be chosen to be as wide as possible, as listed in Table 1. The working fluid in this work is CASTROL (SE FLUID 180). Both the flushing method and the dielectric bath are simultaneously used in the static and dynamic experiments. Because of these four parameters and their large ranges of values, the number of possible combinations for static experiments is too large to execute. To provide representative combinations for comparing the effect of different types of electrodes, the idea of experiment design with orthogonal array is considered. For four parameters at three levels, nine static experiments by Taguchi L9 orthogonal array as shown in Table 2 are chosen. 2.3. Dynamic experiment In addition to machining the surface of the workpiece along the vertical direction similar to the static experiment, dynamic experiment will also machine the surface of the workpiece along the horizontal direction. Similar to the static experiment, the home point needs to be set before the process. After setting the home point, the dynamic experiment will just machines the surface for 5 m along the vertical direction and then machines (or scans) the surface along the horizontal direction (the length of the workpiece) in each routine. After completing one routine, the electrode moves upward for 2.0 mm to be away from the surface of the workpiece and goes back to the start point for next machining as shown in Fig. 4. This routine will be repeated until the machining depth is 40 m that is the same as that in the static experiment. For dynamic experiments, the processing conditions will be choose from the best selection of the above nine combinations in static experiments. Also all three types of electrodes will be tested in dynamic experiments. 2.4. Property testing of the coated layer To evaluate the properties of the coated layer on the workpiece after the static or dynamic experiments, several tests are executed. First, element analysis is executed by Scanning Electron Microscope (JOEL JSM-6300) and Energy Dispersive System (OXFORD). Then, surface hardness is tested by SHIMADZU HMV-2 and surface roughness is measured by Talyor Hobson Talysurf Series 2. Furthermore, the mechanical and tribological test equipment (CETR UMT-3 and S33HE-1000) is used to evaluate the abrasion resistance of the coated layer, and an electronic scale (AND ER-120A) is used to measure the weight loss of the workpiece. To roughly estimate the C distribution on the surface of the coated layer, the surface is observed by optical microscope (OLYMPUS STM). Finally, the consumption situation of the electrode is examined by 3DProfilometrie equipment (OM Engineering GmbH Scan AF 2000). 3. Results and discussions 3.1. Static experiment The surface of the workpiece after experiment was observed and filmed by an optical microscope and the results are shown in Fig. 5. The results indicate the surface of the workpiece can be considered as a duplicate of the electrode, which is obvious in particular when Type 2 and Type 1 MLEs are applied in experiments 5, 8, and 9. Besides, measurements were made on the working surfaces of Type 2 and Type 1 MLEs (experiment 5) by 3D-Profilometrie equipment and the results are shown in Fig. 6(a) and (b). The surface level of the Gr layer is relatively lower than that of the Ti layer with a difference of about 50 m providing evidence that the Gr layer is consumed more than the Ti layer (the surface levels of both the Gr layer and the Ti layer are the same before experiment). Since Gr is composed of C atom planes and these planes are formed together by van der Waals force, the Gr layer of the MLE is easily broken into C powders
Fig. 6. Working surface profile of MLEs after static experiment by Scan, (a) Type 2 MLE and (b) Type 1 MLE.
during the EDC process. As a result, the Gr layer produces more C powders locally during the EDC process. The measurement zone of the coated layer for surface hardness is showed in Fig. 7(a). In this figure a code, Type n-m, is used to represent the coated TiC layer by using Type n electrode under experiment m. For example, Type 6-5 layer is the coated layer obtained by Type 6 electrode and under the conditions of experiment 5. For Type 6-5 layer, surface hardness was measured at the center of the layer along the width direction. For Type 1-5 or 2-5 layer, surface hardness was tested at one side of the layer along the width direction. The measurement results, as shown in Fig. 7(b), indicate that the average surface hardness of the coated layer by using Type 1 MLE is the best among the three types of electrodes for most of nine static tests. Furthermore, for all three types of electrodes the greatest hardness can be achieved under experiment 5 among the nine static tests. The average hardness values of the coated layer obtained by Type 6, Type 2 and Type 1 are HV1196.2, HV1481.6 and HV1710.6, respectively. For Type 6 electrode, coating is accomplished by sparks of high temperature to disintegrate kerosene/working fluid and produce C, which is further combined with Ti and coated onto the surface of the workpiece in Zone 1 to form a TiC coated layer as shown in Fig. 8(a). The average surface hardness of Type 6-5 layer is the
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Fig. 7. Measurement of average surface hardness on the coated layer after static experiment, (a) measurement zone and (b) results of average surface hardness of coated layer by three types of electrodes.
lowest and just about 4.78 times of that of the substrate, because there is no Gr layer on the electrode. For Type 2 MLE, not only does the Gr layer coats a highly concentrated C layer onto the surface of the workpiece in Zone 2, but C powders scatter to Zone 1 on both sides, as shown in Fig. 8(b). Consequently, the C element in Zone 1 includes both C disintegrated from kerosene/working fluid and C powders leading to a higher concentration, which increases the combination possibility of C and Ti and improves the surface hardness. For Type 1 MLE, as shown in Fig. 8(c), the total width of the Gr layer is 2.4 mm and larger than that of Type 2 MLE, which is 2 mm. This will make the former be capable of providing more C powders during the coating process. Besides, the two Gr layers of Type 1 MLE are distributed evenly in the electric discharge zone and the C powders released from the Gr layer in Zone 2 can uniformly spread to Zone 1 and Zone 3 resulting in a average surface hardness of HV1710.6 in Zone 1 of the coated layer (6.84 times more than the hardness of the substrate, HV250). Hence, Type 1 MLE has the best performance on TiC composition among the three types of electrodes. The results of element analysis reveal that there are five major elements appearing on the coated layer, for example as shown in Fig. 9 for the surfaces under different parts of Type 2 MLE. Two of them, oxygen (O) and sulfur (S), are contributed from the working fluid, and it is believed that their effects on the surface hardness of the coated layer are minor. Hence, the other three elements, Ni, Ti, and C, are focused. For experiment 5, the weight percentage of these three elements for the coated layer by using the three types of electrode are shown in Fig. 10. Since the other elements are neglected, the total weight percentage of these three elements is 100% in this figure. As shown, there are Ti and C elements on the coated layer. Since the hardness of Ti, Gr and Ni element is HV257, Mohs2.0 (Matweb, 1998) and HV250, respectively, and their hardness is lower than that of the coated layer, this may imply the
Fig. 8. Diagram of mechanism of coating TiC layer using three types of electrodes in static experiment, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
existence of TiC on the surface of the workpiece. Furthermore, the Gr layer of Type 2 and Type 1 MLEs can coat a layer with highly concentrated carbon on the surface of the workpiece and the C content can reach 65.28% and 62.08%, respectively. This layer with highly concentrated C will be critically important for the scanning coating process. The inhomogeneity in the coated layer may be concerned because of the different wear of the electrode materials. From the measured data of surface hardness and EDS analysis (not shown in this study because average value is used), the data variation on different locations is small. This may indicate that the inhomogeneity in a layer is not so clear. Since Ni, C, and TiC exist in the coated layer, the lubrication and abrasion resistance will be good and it could be applied to tooling or molding. 3.2. Dynamic experiment Based on the result of static experiments, there are two points to be noted for MLE in the dynamic scanning coating. First, as shown in Fig. 11, T0 stands for the beginning of coating process, and after T0 and before T1 electric discharge occurs simultaneously in Zone 1, Zone 2 and Zone 3. Before T1, the Gr layer of the Type 2 MLE is
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Fig. 9. Element analysis of Type 2-5 layer.
consumed more than the Ti layer causing that the electric discharge gap h2 gradually becomes larger than h1. Hence, after T1, electric discharge only occurs in Zone 1 and Zone 3, not in Zone 2. Before T2 and after T1, the consumption of Ti layer in Zone 1 and Zone 3 makes h1 equal h2 again. After T2, electric discharge occurs simultaneously in all three zones. This cycle will repeat as the dynamic process continues. Because of this cycling, the Gr layer of MLE has
Fig. 10. The results of element analysis of three types of coated layers by EDS.
the chance to periodically release highly concentrated carbon powders. Second, the highly concentrated C powders produced in Zone 2 not only spread to Zone 1 and Zone 3 enhancing the combination of C and Ti, but also coat a layer of highly concentrated C on the surface of the workpiece in Zone 2, as shown in Fig. 12(b). Later, due to the movement of the electrode, Zone 2 will be under the Ti layer and a TiC coating procedure will be created. By the way, the surface of the workpiece in Zone 1 will be also coated a layer of highly concentrated C due to the movement of the electrode. For Type 1 MLE, this coating process may repeat twice because of five layers in the electrode, as shown in Fig. 12(c). The result of the static experiment indicates that the optimum coating condition for the three types of electrodes is experiment 5 as shown in Table 2. Thus, only experiment 5 is applied to the dynamic experiment illustrated in Fig. 13. The photograph of the surface of the coated layer obtained by the three types of electrodes is shown in Fig. 14. In this figure the scanning length of the dynamic experiments is 25 mm. To test the surface roughness, a line with 3 mm is measured. As shown in Fig. 14, the surface roughness of Type 2-5 layer and Type 1-5 layer is Ra 1.6850 m and Ra 1.8826 m, respectively, and they are lower than that of Type 6-5 layer, which is Ra 2.0074 m. This means that Type 2-5 and Type 1-5 layers can reduce the surface roughness by 16.06% and 6.22%, respectively, as compared to Type 6-5 layer. Furthermore,
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Fig. 11. Diagram of mechanism of coating TiC layer by Type 2 MLE for dynamic experiment.
the results from SEM reveal that there are microcracks on the surfaces of the coated layers obtained by all three electrodes, as shown in Fig. 15 for Type 6-5 and Type 2-5 layers. However, the surface of Type 6-5 layer has the dentist and biggest of microcracks. The measured surface hardness of the three types of layers and the pure Ni layer is compared in Fig. 16. It is evident that the average surface hardness of the layer obtained by both types of MLE is greater than that obtained by the bulk electrode. This is because Ti of Type 6 electrode compounds only with C dissolved from kerosene/working fluid during the coating process and coats TiC onto the surface of the workpiece in Zone 1 as shown in Fig. 12(a). Nevertheless, the Gr layer of Type 1 and Type 2 MLEs not only provide C powders of high concentration to the local area, but also offer the repetitive coating mechanism for once (Type 2 MLE) or twice (Type 1 MLE) on the surface of the workpiece. In Fig. 12(b), the Gr layer of Type 2 MLE produces C powders of high concentration in Zone 2 and these highly concentrated C powders spread to Zone 1 and Zone 3 enhancing the composition of C and Ti and coating a C layer on the surface of the workpiece in Zone 2, which is beneficial to the one-time repetitive coating mechanism in Zone 3. Likewise, Type 1 MLE can also produce C powders of high concentration in Zone 2 and Zone 4 spreading to Zone1, Zone 3 and Zone 5, enhancing the composition of carbon and Ti and coating a C layer on the surface of the workpieces in Zone 2 and Zone 4, which helps the one-time and two-time repetitive coating mechanism in Zone 3 and Zone 5, respectively, as shown in Fig. 12(c). However, the hardness achieved by dynamic scanning coating is lower than that accomplished by static coating under the same conditions because
electrode scanning stirs kerosene/working fluid and reduces the composition of C and Ti. The machining time for Type 2-5 and Type 1-5 layers is 40.2 min and 38.1 min, respectively, which reduces about 135 min as compared to that of Type 6-5 layer (174.6 min), as shown in Fig. 17. Consequently, MLE can not only decrease roughness and microcracks on the surface of the coated layer, but also reduce machining time to a great extent. The main reason is due to the C powder of high concentration locally produced by the Gr part of MLE during the electric discharge process. The C powder of high concentration in a local area is like powder mixed electric discharge machining (PMEDM), and it can accelerate processing speed, reduce the formation of microcracks, reduce surface roughness, and enhance electrical discharge stability (Kansal et al., 2007; PeC¸as and Henriques, 2008; Abbas et al., 2007; Tzeng and Chen, 2005; Han et al., 2007; Kumer et al., 2009). Therefore, using the multi-layer electrode is better than using the tradition electrode for coating TiC in the surface of Ni. Furthermore, it will be very interesting to compare the present results with the ones by using powder (Gr) additives in the dielectric Ti-electrodes. However, since PMEDM is also a big issue and a lot of parameters need to be suitably decided in advance, this interesting comparison will be explored in the future. A scanning experiment (with experiment 5) is conducted on Type 2 and Type 1 MLEs at different machining depths (10 m, 20 m, 30 m and 40 m) for the understanding of the relationship between the machining depth and thickness of the coated layer. The photograph of the cross-section of the coated layer is shown in Fig. 18(a), and the relationship between the scanning
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Fig. 12. Diagram of mechanism of coating TiC layer using three types of electrode in dynamic experiment, (a) Type 6 electrode, (b) Type 2 MLE and (c) Type 1 MLE.
depth and thickness of the coated layer is illustrated in Fig. 18(b). The thicknesses of Type 2-5 and Type 1-5 layers are the same at a machining depth of 10 m, while they have different thicknesses at other machining depths. Also the thickness variation of Type 1-5 (a min. of 11.50 m and a max. of 15.50 m) is higher than that of Type 2-5. Therefore, machining depth plays a significant part on the obtained layer thickness for Type 1 MLE. 3.3. Abrasion resistance testing Abrasion resistance testing is conducted on the surfaces of pure Ni layer, Type 2-5 layer, and Type 1-5 layer (a machining depth of
Fig. 14. Photograph of surface of coated layer using three types of electrodes.
40 m for Type 2-5 and Type 1-5 layers) with testing conditions as listed in Table 3. The measurements of the scratch widths as shown in Fig. 19 indicate that the scratch widths of Type 2-5 and Type 1-5 layer are less than that of Ni layer under the same testing conditions. The average hardness of Type 2-5 and Type 1-5 layers is HV1216.0 and HV1131.0, respectively, which is higher than HV250.0 of Ni layer, and higher hardness has higher abrasion resistance. Besides, the scratch widths of the three layers increase with temperature. As a result, the coated layer with high hardness is capable of abrasion resistance at a high temperature. Some of the results are missing in this figure because the edge of the scratch could not be identified. For low temperature at 30 ◦ C, COF of Ni layer increases with the increasing of normal force as shown Fig. 20(a). The average COF increases from 0.190 at 10 N to 0.572 at 100 N as shown in Fig. 21(a). Type 2-5 and Type 1-5 layers have excellent abrasion resistance performance at a normal force of 100 N as shown in Fig. 20(b). The average COF of Type 2-5 and Type 1-5 layers slightly increase Table 3 Conditions for abrasion resistance testing.
Fig. 13. Set-up of dynamic experiment.
Parameter
Condition
Test head Rotate speed Test time Workpiece surface Test temperature Loading force
Ball (tungsten carbide, Ø 4 cm) 300 (rpm) 60 (s) Dry 30, 400 (◦ C) 10, 50, 100 (N)
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Fig. 17. Machining time for Type 6-5, Type 2-5, and Type 1-5 layer.
Fig. 15. SEM micrographs for surface of coated layer, (a) Type 6-5 layer and (b) Type 2-5 layer.
from 0.116 and 0.132 to 0.193 and 0.166, respectively, as shown in Fig. 21(a). Furthermore, weight loss is illustrated in Fig. 21(b). Ni layer loses weight tremendously with an increase in the normal force from 0 mg (10 N) to 101 mg (100 N). This is because the shear modulus and hardness of Ni are only 76 GPa (Matweb, 1998) and HV250.0. A greater normal/loading force easily results in plastic flow causing tearing off the material. On the other hand, Type 2-5 layer and Type 1-5 layer only lose 4 mg, which is due to their high hardness and high shear modulus (110–193 GPa (Matweb, 1998)) from the coated TiC layer. Besides, this coated layer has C content and solid C element is effective in lubrication. Consequently, a
Fig. 16. Results of surface hardness measurement on the layer of Ni, Type 6-5, Type 2-5 and Type 1-5.
Fig. 18. Cross-section of the coated layer of Type 2-5 and Type 1-5, (a) photos of cross-section of the coated layer at different machining depth and (b) measurement results of the coated layer thickness.
Fig. 19. Photos of width of groove worn on the surface of the three types of coated layer after abrasion resistance testing.
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Fig. 20. Results of abrasion resistance testing: (a) Ni layer under different forces, (b) the three layers under 100 N and 30 ◦ C, (c) the three layers under 10 N and 400 ◦ C and (d) the three layers under 50 N and 400 ◦ C.
workpiece with a TiC layer is good at abrasion resistance at room temperature. For high temperature at 400 ◦ C, the results of abrasion resistance testing are illustrated in Fig. 20(c) and (d). The COFs of Type 2-5 layer and Type 1-5 layer increase slowly with the testing time at a loading force of 10 N, while they are near the maximum value
of Ni layer at the 28th second under a loading force of 50 N. The COF of Ni layer is the highest with an average value of 0.715 and 0.664 at the loading force of 10 N and 50 N, respectively, as shown in Fig. 22(a). From Fig. 22(b), under a loading force of 10 N, Ni layer loses 82 mg, while Type 2-5 layer and Type 1-5 layer only lose 29 mg and 21 mg, respectively. However, under the loading force of 50 N, the three layers lose a similar weight of 390 mg. Accordingly, the
Fig. 21. Results of abrasion resistance testing at temperature of 30 ◦ C: (a) COFavg and (b) weight loss.
Fig. 22. Results of abrasion resistance testing at temperature of 400 ◦ C: (a) COFavg and (b) weight loss.
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workpiece with the TiC layer still has abrasion resistance under the loading force of 10 N since the layer does not tear off completely; nevertheless, the TiC layer is almost striped off at the 28th second under a loading force of 50 N causing the following COF consistent with that of Ni layer. The main reason that the coated layer is torn off is because the shearing force first occurs in the softer substrate (Ni) under the coated layer and the high temperature of 400 ◦ C generates plastic flow easily. Thus, the coated layer moves with the plastic flow of the substrate causing microcracks in the coated layer or breaks at thinner places. The coated layer adhering to the substrate (Ni) is torn off. In addition, this mechanism could be confirmed from the testing under 30 ◦ C and 100 N loading force since such a great loading force still cannot make the coated layer adhering to the substrate (Ni) be torn off. Consequently, stripping occurs in the substrate (Ni) instead of the TiC layer at a high temperature. 4. Conclusions 1. MLE indeed decreases the complicated machining procedures, enhances the composition of Ti and C, enhances surface hardness of the coated layer, reduces surface roughness and microcracks, and increases electric discharge stability and coating speed when applying to electric discharge coating. Therefore, MLEs should be extensively applied to the composition of different materials. 2. The abrasion resistance of a coated layer is excellent at room temperature (30 ◦ C) and such competence still maintains at high temperature (400 ◦ C). However, this ability reduces with the increase in the loading force, which mainly results from the stripping of the substrate (Ni) under the coated layer instead of the coated layer itself. Acknowledgements The authors would like to thank the China Steel Corporation, Taiwan for financial support under Contract No. 95-336.
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