Experimental Investigation on Conventional and Non-Conventional Machining of Sintered Advanced Ceramics Using CVD Diamond Coated End Mills and Electroplated Diamond Grinding Points

Abstract

The superior properties of advanced ceramics of ZrO2, Al2O3, Si3N4, and SiC, such as high hardness, strength, functioning under high temperatures of up to 2500°C, wear resistance, and corrosion resistance, have made them good alternative for metallic materials. Due to better durability and functionality that could exhibit compared to metals, in several applications, such as dental implants, hip joint replacement, body armours, and break discs. However, machining advanced ceramics to its final shape with high dimension precision and without defects is a challenging process, because of their brittleness and high hardness that makes them difficult to cut. Therefore, ceramic components are often machined before sintering, but this remains challenging because sintering causes unpredictable volume shrinkage of up to 5%. This study investigated the machinability of the four commonly used advanced ceramics, using two different tooling. The assessment criteria included the tool life, surface roughness, material removal mechanism, and cutting forces. In addition, the effect of cutting fluid was investigated, where in phase 1, Al2O3, Si3N4, and SiC were benchmarked against ZrO2 using a 4-mm-diameter chemical vapor-deposited (CVD) diamond-coated (15µm thick) end mill at the following parameters: cutting speed V, 60 m/min; feed fz, 0.5 µm/tooth; cutting depth, 0.25 mm; and cutting width, 0.5 mm. In phase 2, for benchmarking, 4-mm-diameter D126 diamond grinding point to machine slot was used at the following parameters: V, 1 m/s; F, 1 mm/ min; and cutting depth, 0.1 mm. Moreover, the machinability was investigated using a non-conventional machining technique: ultrasonic assisted milling/grinding that referred improving the machinability of hard materials. This study is the first to benchmark the four advanced ceramics using CVD diamond-coated end mills and diamond grinding points by conventional and non-conventional machining technique. The findings from phase 1 show that end milling in a wet environment increased the tool life five times. Among the four ceramics, ZrO2 showed the best machinability, where the cutter machined for 1500 mm. For Al2O3, Si3N4, and SiC, the tool lives were 100, 40, and 10 mm, respectively. The average surface roughness Ra achieved for the four ceramics were 0.33, 0.47, 0.24, and 0.28 µm, respectively. Compared with those of the other ceramics, the resulting surface of ZrO2 was dominated by ductile fractures. The cutting forces were proportional to the tool life, as the forces increased as the tool life decreased, where the obtained Euclidean norm F values of ZrO2, Al2O3, Si3N4, and SiC were 51, 61, 96, and 118 N, respectively. Tool life was markedly improved using the ultrasonic assisted machining method. The tool life of used cutters that machined Al2O3, Si3N4, and SiC were improved by 1900%, 25%, and 900%, respectively. However, the cutter that machined ZrO2 did not show any improvement in tool life, as it was reduced from 1500 mm to 1300 mm, because of what believed thermal softening of the cutter. In phase 2, the findings show that a lower cutting speed is better for grinding ZrO2. Also, the benchmarking showed that the grinding points markedly reduced the influence of hardness on the grinding forces. SiC, the hardest ceramic, obtained the lowest grinding force, with Euclidean norm F value of 31.5 N, whereas ZrO2, Al2O3, and Si3N4 obtained F values of 107, 63, and 339 N, respectively. These were clearly reflected on the tool wear of the four diamond points, where the SiC cutter showed the lowest wear rate. The obtained Ra values of ZrO2, Al2O3, Si3N4, and SiC were 0.33, 0.53, 0.16, and 0.08 µm, respectively. None of the resulting surfaces of the four ceramics were dominated by ductile fractures. However, the reduction in grinding force caused by the use of a non-conventional method (ultrasonic assisted grinding [UAG]) was noticeable. The F values for ZrO2, Al2O3, and Si3N4 were reduced by 30%, 26%, and 24%, respectively. However, the F value for SiC was almost similar to that with the conventional method. However, grinding using the UAG method resulted in a shorter tool life.