3.1 Hardness test
Figure 3 shows hardness and standard deviation values of prepared AISI 440C substrate, Ni and Cr-C deposits. The hardness values of as-purchased AISI 440C remain relatively identical after annealing at 500oC for 30 min. This implies that as-purchased AISI 440C rods had been quenched and tempered at a relatively high temperature to obtain a martensitic structure with a high toughness. The Ni deposit became softer after annealing at 500oC. On the other hand, the hardness of as-plated Cr-C deposit increased from 597 Hv to 1636 Hv after annealing at 500oC. According to our previous study [12,15], the anneal-hardening mechanism of Cr-C deposit could be attributed to the presence of diamond-like membranes. That is, prepared electroplated diamond tools could be firmly enclosed within the anneal-hardened Cr-C deposit. At the same time, the soft Ni-undercoating with an annealed hardness of 147 Hv could act as a buffer layer to absorb grinding impact between extruded diamond particles and the Al2O3 plate. It can be expected that the extruded diamond particles will gradually be compressed into the Ni-undercoating and prevent delamination, resulting an increased ground length.
3.2 Preparation of electroplated diamond tool
As shown in Fig. 2, the diamond-contained composite deposits of electroplated diamond tools were prepared through a three-step electroplating processes. In this study, the effect of Ni-undercoating thickness varying from 30 to 150 µm on the grinding performances of prepare electroplated diamond tools was investigated. After Ni-undercoating, Ni-diamond co-electrodeposition was conducted on the Ni-coated AISI 440C rod. As shown in Fig. 4(a), a high diamond density layer could be plated onto the AISI 440C rod surface after Ni-diamond co-electrodeposition. The diamond particles were fixed onto the Ni deposit with a thickness of about 30 µm. Upon strengthening with Cr-C electroplating at a plating current density of 10 A/dm2, it was found that some extruded diamond particles were lost in the trivalent Cr plating bath. This is likely due to the evolution of hydrogen bubbles between extruded diamond particles during Cr-C electroplating, leading to dislodgement of some diamond particles from the tool surface. Thus, Ni electroplating with a thickness of 40 µm was further performed prior to Cr-C strengthening electroplating, as shown in Fig. 4(b). After increasing the Ni-deposit thickness to about 70 µm, a Cr-C deposit with a thickness of about 60 µm was successfully electroplated to strengthen diamond particles in the composite deposit, as shown in Fig. 4(c). The extruded diamond particles with an average height of about 20 µm from the tool surface allow for material cutting and removing Al2O3 powder-chips during slot grinding.
As shown in Fig. 4(c), the Cr-C strengthening deposit with a typical nodular surface can be seen on the tool surface after electroplating and annealing. Based on our previous study [11,12], some through-deposit cracks could be easily developed in the Cr-C deposit after annealing at temperature above 200oC. However, surface cracks were not observed from the Cr-C deposit on the prepared tools after annealing at 500oC for 30 min. This is because, according to our previous study , surface cracks can be significantly reduced through Cr-C-inert particles composite electroplating. Therefore, it can be expected that the crack-free Cr-C deposit provides sufficient strength to reinforce diamond particles on the tool surface.
As shown in Fig. 5(a), an obvious outward wash can be seen around the end of AISI 440C rod, owing to the charge concentration at the end corner during electroplating. To trim the outward wash, an electrical discharge machining (EDM) was adopted. Figure 5(b) shows the end side of tool trimmed with EDM. Figures 5(c) and (d) show cross sections of ground slots in Al2O3 plates. A rectangular slot was ground with the tool after EDM trimming; on the other hand, a bottom-enlarged slot was seen after grinding with untrimmed tool. To avoid the effect of outward wash around the end of the tool on the grinding performance, trimmed tools were used for the grinding tests in this study.
3.3 Grinding test
Figure 6 shows maximum ground lengths of prepared electroplated diamond tools with different Ni-undercoating thickness varying from 30 to 150 µm. The results indicate that the grinding performance of an electroplated diamond tool depends strongly on the Ni-undercoating thickness. A relatively short ground length of 357 mm was detected from the electroplated diamond tool with Ni-undercoating thickness of 30 µm. By further increasing the Ni-undercoating thickness to 60 µm and above, the maximum ground lengths of longer than 1450 mm can be achieved, which is more than four times than using 30 µm Ni-undercoating, as shown in Fig. 6. Overall, these ground lengths are much longer than those with electroplated diamond tools with multilayered 50-µm-sized diamond particles prepared in our previous works [4,11]. That is, with larger diamond particles, a higher grinding performance of tool was recognized. Due to anneal-softening, the annealed Ni-undercoating could be used as a buffer layer to absorb the grinding impact and force during grinding. The extruded diamond particles on the tool surface will be impacted and gradually compressed with increasing ground length. Therefore, it is crucial to have an optimal undercoating thickness with respect to the size of diamond particles. As the results indicated, the 30-µm-thick Ni-undercoating in the electroplated diamond tool cannot sufficiently bear the grinding impact, leading to low grinding performance. Thus, a thickness of 30 µm is not suitable for the proposed tool fabrication with 150-µm-sized diamond particles. On the other hand, with the highest maximum ground length of 2231 mm, the Ni-undercoating thickness of 90 µm is desirable.
3.4 Fractographic study
To clarify the effect of Ni-undercoating thickness on grinding performances, surface morphologies and cross sections of electroplated diamond tools with Ni-undercoating thickness of 30, 90 and 120 µm were examined after grinding to failure. Cr-Kα-line scanning with EDS along the cross sections of failed tools are shown in Figs. 7–9. Figures 7(a) and (b) depict a Cr-C deposit with a nodular surface on the electroplated diamond tool prepared with a 30-µm-thick Ni-undercoating after grinding test. As shown in Fig. 7(a), some wide cracks were observed from the tool surface. Moreover, obvious delamination between the diamond-contained composite deposit and the Ni-undercoating was seen from the cross section of the failed tool shown in Fig. 7(b). The Cr-C strengthening deposit can be realized from the result of Cr-Kα-line scanning with EDS from site A to site B shown in Fig. 7(c), from which the Cr-Kα radiation along a deposit thickness of about 50 µm was detected. This means that most of the Cr-C deposit did not wear out upon tool failure at the ground length of 357 mm.
As shown in Figs. 8(a) and (b), cracking of the Cr-C strengthening deposit was not found from the tool with Ni-undercoating thickness of 90 µm, which was evidenced to be an optimal thickness for obtaining the highest tool performance. Moreover, delamination between diamond-contained composite deposit and the Ni-undercoating was not observed, as shown in Fig. 8(b). The surface morphology of the tool was flattened, and extruded diamond particles were almost compressed into the Ni-undercoating. From the result of Cr-Kα-line scanning with EDS from site A to site B shown in Fig. 8(c), Cr-Kα radiation was not detected in the range of the Cr-C strengthening deposit. This indicates Cr-C deposit was almost worn out after grinding to failure. It can be reasonably assumed that the extruded diamond particles on the tool surface were compressed into the Ni-undercoating quickly when the anneal-hardened Cr-C deposit layer was depleted.
As shown in Fig. 9(a), a network of thin cracks and diamond particles leveled together with the Ni-undercoating were seen from the tool surface with a Ni-undercoating thickness of 120 µm. Similar to the tool with 90-µm-thick Ni-undercoating, interfacial delamination and thin cracks were not seen from the cross section of the tool shown in Fig. 9(b). From the result of Cr-Kα-line scanning with EDS from site A to site B shown in Fig. 9(c), Cr-Kα radiation was not found on the surface of the tool. This means that the anneal-hardened Cr-C deposit wore out after grinding to failure, leading to embedding of diamond particles into the Ni-undercoating. The extruded diamond particles on the tool with a relatively thick Ni-undercoating thickness of 120 µm could be embedded after Cr-C deposit was fully worn out. A thin net of surface cracks was developed when grinding proceeded.
Figure 10a) and b) depict failed tools with 30- and 60-µm-thick Ni-undercoating from grinding tests, respectively. It can been seen from Fig. 10a) that the diamond-contained composite deposit peeled off from the tool surface. This fully agrees with the observation on the surface and the cross section of the failed tool shown in Fig. 7(a) and (b), in which interfacial delamination and cracking were seen. Once the diamond-contained composite deposit peeled off from the tool surface, the AISI 440C substrate was apparently worn because of different hardness between the Al2O3 plate and the AISI 440C rod. Because Al2O3 plates were produced through powder metallurgy, the removal of Al2O3-powder chips by the extruded diamond particles are essential. Owing to difficulty in removing Al2O3-powder chips, a high grinding force is experienced when the diamond particles are fully embedded into the Ni-undercoating. Because the tool rod was bent during grinding with a grinding feed of 2 mm/min and a rotational speed of 15,000 rpm, the rod was subjecting a cyclic stress. The stress amplitude increased greatly when the extruded diamond particles were fully embedded into the Ni-undercoating. As higher stress amplitude was applied, a shorter fatigue life was recognized. Fatigue failures of AISI 440C rod were found from the tools with Ni-undercoating thickness varying from 60 to 150 µm after extruded diamond particles were leveled with the Ni-undercoating. Figure 10(b) shows the fatigue fracture of failed AISI 440C rod from the tool with Ni-undercoating thickness of 90 µm. The fatigue fracture can be characterized by almost no plastic deformation and flat fracture morphology as shown in the figure.