1. Introduction
The applications of advanced ceramics such as alumina and zirconia are increasing gradually in the medical and engineering fields due to the significant advancements in the fabrication techniques [
1]. Moreover, advanced ceramics offer excellent properties such as high-temperature resistance, high strength to weight ratio and anti-corrosive properties comparing to their competitor materials such as titanium and Inconel alloys. Recently, advanced ceramics were used in the manufacturing of microcomponents such as micro-molds, micro-dies, micro-fluidics and micro-nozzles [
1]. Micro-features such as microchannels and micro-pockets with sound surface integrity and high dimensional accuracy are often required in the miniaturized/micro-components for their functioning with ever-increasing stringent specifications [
2]. Alumina (Al
2O
3) is mostly employed material in biomedical applications due to its high conformance to biocompatibility standards [
3]. Like other advanced ceramics, Al
2O
3 is classified as difficult to fabricate material due to its poor thermal and electrical conductivities and high hardness [
4]. The traditional machining processes such as turning, milling and drilling are not desirable for the economical machining of the Al
2O
3 due to excessive tool wear, poor surface integrity and high edge chipping of the machined parts [
5]. Non-traditional processes such as chemical machining and electric discharge machining have their own limitation in the machining of ceramic materials as these processes depend on the chemical and electrical properties of the processed materials. On the other hand, advanced machining processes such as ultrasonic machining (USM), laser machining and waterjet machining have the potential to machine the Al
2O
3 ceramic components. Several attempts have been reported on machining micro/macro features in Al
2O
3 by using laser machining. For example, the fabricating of pockets in Al
2O
3 by using laser ablation was performed by Umer et al. [
6]. The authors applied the multi-objective optimization tool in order to optimize the MRR and the surface roughness (Ra) of the fabricated pockets. Results showed that the optimal values of Ra and MRR of the produced pockets were 4.1 μm and 0.7 mm
3/min respectively. Laser milling was also applied by Loen et al. [
7] for fabricating square pockets (5 × 5 mm
2) in Al
2O
3. The effects of fiber laser process parameters and scanning strategy on surface roughness (Ra) and MRR were analyzed. Results indicated that the best value of Ra = 3 μm was achieved under a certain range of the selected parameters. Many limitations such as high heat-affected zones, poor surfaces, a lack of geometrical accuracy, etc., that can be associated with laser ablation [
5,
8,
9,
10]. Therefore, the machining of microchannel on advanced ceramics, like Al
2O
3, ZrO
2, etc., is a difficult task as documented in earlier the studies. As a result, there is an essential need for a new machining technique, which can efficiently fabricate the ceramic components.
Rotary ultrasonic machining (RUM) is a non-traditional hybrid process, which combines the mechanisms of diamond grinding and USM [
11]. It can machine brittle and hard materials regardless of their mechanical, electrical and thermal properties. Moreover, RUM can be used to produce components with high surface quality and high dimensional accuracy without altering their physical properties [
11,
12,
13]. It can be used for micro/macro machining both brittle materials such as glass and hard to machine materials [
14,
15,
16]. For example, Jain and Pandey [
14] applied RUM for drilling micro-holes (300 µm dia.) in borosilicate glass. Results revealed that in addition to the tool vibration frequency, the thickness and grain size, the used RUM tool had a significant effect on tool wear. RUM was also applied for micro-drilling and milling of BK7 and Zerodur glasses in the research documented in [
15]. Results found that, by using RUM the cutting forces, the edge chipping and the holes taper can be minimized by using RUM comparing to the results of micro grinding for the drilling operation. Recently, RUM was applied for milling microchannels and drilling microholes in Al2O3 materials. For example, Abdo et al. [
17,
18] used RUM to fabricate microchannels on Al
2O
3. The authors investigated the significance of RUM input parameters on the surface roughness, edge chipping, and dimensional accuracy of the fabricated microchannels. Results revealed that high dimensional accuracy (width and depth error ≥ 5%) with smooth surface roughness (Ra = 0.27 µm) of the machined microchannels could be obtained under optimal conditions of RUM parameters. RUM was also applied for drilling of Al
2O
3 to investigate the effects of the parameters on cutting force, MRR and chipping size of the drilled holes [
19,
20]. Tool wear and MRR were the outputs investigated while side milling of Al
2O
3 by using RUM in the research documented in [
21]. Another study was documented on studying tool wear [
22] and surface roughness [
23] while grinding of Al
2O
3 by using RUM. Mathematical models were also developed for predicting cutting force [
24] and tool wear [
21] while processing Al
2O
3 by using RUM. Other research studies were also investigated in applying RUM for machining other types of advanced ceramics such as zirconia [
25,
26,
27] and silicon carbide [
28] and silicon nitride [
29]. For instance, an experimental investigation had been carried out for fabricating stepped hole on zirconia bioceramics in the study documented in [
25]. Results indicated that the maximum MRR could be achieved at coarser grain sizes, higher feed rates and power ratings. Churi et al. [
28] studied the effects of RUM input parameters on the surface roughness, edge chipping and cutting force of the fabricated holes in silicon carbide materials. The authors found that the spindle speed and the feed rate have the most significant effect on all the outputs.
It can be concluded from the above literature review that very few research studies have been conducted on the micro-milling of Al
2O
3. According to the author’s knowledge, no research has been found on milling pockets in Al
2O
3 by using RUM. Moreover, the most of available studies have been limited to investigate the effects of RUM input parameters such as feed rate, spindle speed, depth of cut, frequency, amplitude and coolant pressure on the cutting force, edge chipping and tool wear of the fabricated holes and/or channels. Furthermore, the effect of tool overlapping and tool path strategies on the surface quality and dimensional accuracy of the machined parts have never been investigated in all studies conducted for the RUM of Al
2O
3 ceramic. Moreover, the effect of the tool overlap and tool path were not considered in the studies documented on the fabrication of the microchannels through RUM [
17,
18]. This is because, in those investigations, the selected sizes (widths) of the fabricated channels were equal to the diameter of the used RUM tools. On the contrary, the tool overlap and the tool path strategy are important parameters while machining the pocket. In contrast to the existing literature, the current study is aimed to analyze the significance of tool overlapping and tool path strategies on the surface roughness, the profiles, surface morphology and the MRR of the fabricated pockets. Scanning electron microscopy (SEM) is used to investigate the types of RUM tools wear.
2. Materials and Method
DMG ultrasonic 20 linear is employed in this study to conduct the experiments. It is a five-axis process in which both precision milling and RUM can be achieved (see
Figure 1a). The main characteristics of the used machine are listed in
Table 1. The schematic diagram of the used RUM is shown in
Figure 1b.
In order to remove the chamfer edges from the workpiece, face milling was previously conducted the surface flatness was measured by the dial gauge indicator (see
Figure 2a). RUM tools provided by Schott Company, Germany are applied in the experiments as shown in
Figure 2b. The used tools are nickel-based materials bonded with diamond abrasives with a mean diameter size of ≈44 µm (D46, mesh size of 325/400), which are recommended by the Schott Company for machining smooth surfaces on ceramic-based materials. All specifications of the used tools can be found in
Table 2. The size of the micro-pockets milled in this research was 6 mm (length) × 6 mm (width) × 0.3 mm (depth), as shown in
Figure 1b. The material adopted in the current study was alumina (Al
2O
3 99.99%) from CeramTec, Germany. It is a polycrystalline high purity alumina, which is mainly used in medical applications [
30]. Other applications of the used alumina ceramic can include hybrid circuits, high-strength ceramic tools, wear-resistant, corrosion-resistant parts, etc. [
31]. Each of the alumina samples was under the form of block dimension 50 × 50 × 10 mm
3 in size. The specifications of the used Al
2O
3 material are listed in
Table 3.
2.1. Experimental Procedure
The experiments performed on the alumina (Al
2O
3) substrate were divided into two main tests, (i) tool overlapping test and (ii) tool path strategies test. In the tool overlapping test, the effect of several tool overlapping percentages (0%, 5%, 10%, 15%, 20% and 25%) on the surface quality and profile accuracy of the milled micro-pockets were examined. The tool overlapping was calculated as a percentage of the RUM tool diameter (2 mm). For example, the 10% overlap was equal to 0.2 mm ((10% × 2 mm)/100).
Figure 3 shows the schematic diagram of the tool overlaps used in this test. It should be noted that surface roughness in terms of Ra and Rt, surface morphology and pockets profiles were the three outputs considered during the overlapping test because all the three outputs play a vital role in the micro-pockets functioning [
32,
33].
Based on the results of the overlapping test, the experiments of the tool strategies test were carried out by fixing the tool overlapping at 20%, which results in the best surface roughness and morphology as will be explained in the results and discussion section. In this test, the effects of tool path strategies on the surface quality, tool wear and material removal rate (MRR) of the milled pockets were analyzed. The tool path strategies considered in this study are illustrated in
Figure 4. As mentioned earlier, the pocket’s depth = 0.3 mm can be achieved by removing two passes of 0.15 mm each (depth of cut = 0.15 mm). In Uni-X and Uni-Y strategies, the tool always moves in one direction (
X-axis or
Y-axis) from point a to point b for the first pass and repeat the same path movement for the second pass. Zigzag strategy means the tool moves in bi-direction (both X and
Y-axes). In cross strategy, the tool moves in the
X-axis direction for the first pass (from point a to point b) and then moves in the
Y-axis direction for the second pass (from a1 to b1). In the Mix2 strategy, the first pass of the tool follows the zigzag path (from a to b) while the second pass follows a uni-x strategy (from a1 to b1). The tool movement in the Mix3 strategy follows zigzag X for the first pass (from a to b) and Zigzag Y for the second pass (from a1 to b1). Machine strategy is the strategy built in the RUM Siemens controller for milling the pocket feature. The tool paths of the remaining strategies (Spiral and Mix-1) can be observed by following the direction of the arrow in schematic diagrams in
Figure 4. The range of the machining conditions (see
Table 4) was selected based on trial experiments and guidance taken from previous studies reported in the machining of alumina materials by using RUM [
14,
15,
17].
2.2. Measurement Procedure
In order to measure the dimensions of the fabricated pockets, optical microscopic images of the milled pockets were captured by using mikroskop technik rathenow from ASKANIA company- Germany (see
Figure 5a). The pockets profiles and surface roughness were measured by using Talysurf 120 from Taylor-Hobson, Japan. Surface roughness (Ra and Rt) was measured across the tool feed direction of each pocket surface at six different locations with 4mm tracing length and cut–off 0.8 mm according to the ISO 4287:1997. The average of the six readings was used for analysis.
Figure 5b depicts the measuring setup of the pockets profiles and the surface roughness. The surface morphology of the pockets was observed using SEM as shown in
Figure 5c. SEM images were also taken for the new and used RUM tools to investigate the mechanisms of the tool wear. The MRR was measured by calculating the volume of the material removed divided by the actual machining time.
Figure 6 shows the research methodology followed in the current study.