Bulletproof ceramics are usually hard and brittle with high elastic modulus, high compressive strength, and low tensile strength. While machining bulletproof ceramics, severe tool wear makes it difficult to obtain desired machining quality and efficiency, especially in hole drilling. In this work, an intensive experimental study on the overall wear rate of the sintering diamond thin-wall core bit during the hole drilling of Al2O3 bulletproof ceramics (99 wt.%) has been carried out. The quality loss of the bit after each hole drilled was selected for representing the overall wear rate of the bit. Based on experimental data, the influences of the main bit performance and machining process parameters on the overall wear rate of the bit have been analyzed. According to the results discussed, under the test conditions, finer diamond grit, higher diamond concentration, lower number of water gaps, thinner wall thickness, or lower bit load all can decrease the wear rate of the bit. However, within a certain range, the spindle speed has little influence on the overall wear resistance of the bit, but when the spindle speed increases, the machining efficiency can be significantly improved. The results obtained in this work can offer a valuable reference for the use of sintering diamond thin-wall core bits in the hole drilling of bulletproof ceramics.
Engineering ceramics used in the field of armor protection (bulletproof ceramics) are usually hard and brittle with a very high elastic modulus and have high compressive strength and low tensile strength. During machining engineering ceramics, which are typical difficult-to-machine materials, severe tool wear makes it difficult to guarantee the machining quality and efficiency, especially in the hole machining. This greatly limits the widespread application and popularization in the field of armor protection.
At present, special processing technologies are usually adopted for the machining of hard and brittle materials such as engineering ceramics, including laser processing [
However, special processing technologies are mainly suitable for machining of microholes or microstructures, when it turns to larger holes of about ten mm or even tens of millimeters in diameter, the machining efficiency is very low. When machining a relatively large hole, it is very difficult to drill engineering ceramics with solid bits; therefore, diamond core bits are commonly used [
The above analysis indicates that, for large holes machining of engineering ceramics, a higher machining efficiency and better machining quality can be achieved through the optimization of processing parameters and bit performance parameters. Thus, core drilling with diamond bit is an efficient method for machining of holes in engineering ceramics. However, few studies have been reported about the tool wear during the machining of engineering ceramics when employing diamond core bits. Noticeably, Zhang et al. [
According to the existing literature, investigations on the wear of diamond tools during the machining of engineering ceramics are mainly focused on the abrasive machining of diamond grinding wheels and diamond burs. Kizaki et al. [
In this study, a typical high-purity alumina bulletproof ceramics (99 wt.% Al2O3) was selected as a machining object. Taking the drill quality variation measured as the indicator for reflecting the overall wear degree of the drill, a systematical experimental investigation on the wear rate of the sintering diamond thin-wall core drill has been conducted. The influences of the main bit performance and machining process parameters on the overall wear rate of the bit have been analyzed. The results obtained in the present paper can offer a valuable reference regarding the use of sintering diamond thin-wall core bits to machine bulletproof ceramics.
According to the experimental research on the ceramic composite components [
The sintering diamond core bit.
In order to reflect the overall wear loss variation of the diamond bit accurately, the bit was taken down after every hole was drilled on the bulletproof ceramics and then was washed cleanly and dried for weighing with a precise analytical balance. The overall wear rate of the bit was represented by the quality loss of the bit after each hole was drilled, which reflected the whole wear loss of the diamond grains and the matrix binding agent of the bit. When the drilling operation became significantly difficult or the bit sintering body broke up considerably, the normal drilling operation could not be continued and the bit should be scrapped.
The test was conducted on a ZXL-20 vertical drilling-milling machine (with a spindle power of 750 W) equipped with water cooling device. A constant pressure feed mode was adopted. The bulletproof ceramics used was a hexagonal high-purity alumina engineering ceramic brick (99 wt.% Al2O3) with a thickness of 10 mm. One Al2O3 bulletproof ceramic brick drilled is presented in Figure
Al2O3 bulletproof ceramic brick drilled.
The TG-328B analytical balance.
With the given matrix formula and diamond grade, the grit size, grit concentration, number of water gaps at the crown, and bit wall thickness are the main bit performance parameters which can affect the wear conditions of the tools. Under the constant pressure feed mode, the bit load and the spindle speed are two important process parameters that affect the wear conditions of the bit. According to the previous experimental studies on the Al2O3 ceramic composite armors [
Test conditions.
Test no. | Bit performance parameters | Number of water gaps | Wall thickness (mm) | Technological parameters | Number of machined holes | ||
---|---|---|---|---|---|---|---|
Grit size (mesh) | Concentration (%) | Bit load (N) | Spindle speed (rpm) | ||||
1 | 35/40 | 75 | 3 | 2 | 352 | 3200 | 23 |
2 | 50/60 | 75 | 3 | 2 | 352 | 3200 | 13 |
3 | 35/40 | 125 | 3 | 2 | 352 | 3200 | 26 |
4 | 35/40 | 75 | 2 | 2.5 | 278 | 3200 | 33 |
5 | 35/40 | 75 | 4 | 2.5 | 278 | 3200 | 21 |
6 | 45/50 | 125 | 4 | 2 | 390 | 3200 | 15 |
7 | 45/50 | 125 | 4 | 2.5 | 390 | 3200 | 9 |
8 | 35/40 | 75 | 3 | 1.5 | 352 | 3200 | 9 |
9 | 45/50 | 125 | 4 | 2.5 | 278 | 3200 | 23 |
10 | 35/40 | 75 | 3 | 2 | 352 | 1750 | 21 |
Figure
Wear rates of the bit under different diamond grits.
As can be seen from Figure
In addition, the cutting load on a single diamond grit increases, which makes the diamond easy to break or fall off. Thus, new diamond grits can be constantly exposed to the working surface, and the performance of diamond bit can be fully taken use of. When the grit size becomes smaller, diamond grits exposed on the lip surface of the drill is increased, the cutting depth of a single diamond grit decreases (under a constant drilling load), the size of ceramic abrasive debris becomes smaller, and the wear of bit also becomes slighter; meanwhile, the cutting load on a single diamond grit decreases, and the diamond is easy to be worn into a planar shape, which results in the occurrence of the skidding phenomenon [
Figure
Wear rates of the bit under different diamond concentrations.
Further, it can be seen from Figure
When the concentration is relatively low, the cutting load acting on a single diamond grit becomes higher and the cutting and breaking of the ceramics becomes easier. Accordingly, the diamond grits on the bit crown are more prone to fragmentation wear, and the abrasion wear of diamond grits reduces. In this way, the cutting ability of the bit can be preserved. However, if the concentration is too low, the massive fragmentation of diamond grits will increase rapidly, which may cause severe wear and premature scrap of the bit. However, if the concentration is too high, the holding strength of the matrix binding agent to the diamond grits is not high enough and the diamonds are prone to fall off prematurely during the machining process, where the wear resistance of the bit is reduced.
Figure
Wear rates of the bit under different number of water gaps.
According to Figure
In addition, when the number of water gaps increases, the difference between the cutting loads on the diamonds at the inner and outer cutting circular lines increases, which further aggravates the uneven wear of the bit crown and degrades the wear resistance of the bit. In contrast, when there are fewer water gaps, the work layer surface area of the bit crown increases and the exposed diamond grits on the crown increases; the cutting load of a single diamond grit is thus reduced, and the proportion of diamond abrasion wear is increased. This probably leads to the skidding phenomenon, for example, the bit in Test 4. Therefore, the number of water gaps should be determined according to specific processing conditions. According to the machining tests on bulletproof ceramics carried out in this investigation, the number of water gaps should be set to 2-3 (when the bit diameter is Ф 24 mm).
Figure
Wear rates of the bit under different wall thicknesses.
As can be seen in Figure
On the whole, with the increase of wall thickness, the difference between the feed load and cutting load of the diamond at the inner and outer cutting circular lines is increased, so the uneven wear of the bit crown becomes more severe, which leads to a faster bit wear. In addition, when the wall thickness is relatively thin, the strength of the bit sintering body is reduced, so that cracking and even breakup of the diamond layer are easy to occur during machining, such as the bit in Test 6.
It is noticed that the bits in Test 3 (bit load is 352 N, and wall thickness is 2 mm) and Test 8 (bit load is 352 N, and wall thickness is 1.5 mm) are also scrapped prematurely because of the massive breakup of the sintering body. Therefore, the appropriate wall thickness and bit pressure has a crucial impact on the normal use of the bit. According to the machining tests of the bulletproof ceramics, the appropriate wall thickness should not be more than 2.5 mm and not less than 2 mm (when the bit diameter is Ф 24 mm). Too thick a wall thickness would reduce the service life of the bit due to the seriously uneven wear of the crown. Too thin a wall thickness would lead to a poor strength and rigidity of the bit sintering body, which would result in the breakup of the work layer during machining process, finally causing the scrap of the bit prematurely. As a result, the wall thickness should match the bit load to obtain a longer bit service life.
Figure
Wear rates of the bit under different bit loads.
From Figure
It can be observed that when the wall thickness is 2.5 mm (Tests 4, 5, 7, and 9), an inner trumpet-shaped wear of different degrees appears on the matrix crown during the machining process [
Therefore, the increase of wall thickness does not always enhance the wear resistance or extend the service life of the bit; the key is to match the wall thickness with the bit load. Only a reasonable bit load can make full use of the bit work layer and thereby achieve a longer service life.
Figure
Wear rates of the bit under different spindle speeds.
It can be seen from Figure
In addition, according to the study [
The cumulative drilling depth of the bit can be represented by the total number of machined holes. Hence, Figures
This special wear characteristic of the sintering diamond bit is closely related to the periodical layer-changing characteristic of the diamond grits on the crown surface of the impregnated diamond tools. Since the diamond grits are randomly distributed in the matrix, with the continuous wear of the matrix body, the diamond grits are exposed layer by layer and act as microcutting edges to cut the ceramic material and are worn gradually, broken, and finally fall off, namely, continuously working in the form of periodical layer-changing [
In general, if the wear loss of the bit after machining a certain hole is large, the bit should be in the layer-changing process of the diamond grits on the crown surface when the hole is being machined, whereas when the wear loss of the bit after machining a certain hole is small, the bit should be at the abrasion wear stage or microfragmentation wear stage. Thus, during the hole machining process, the bit is sometimes at the abrasion wear stage, sometimes at the microfragmentation stage, sometimes at the massive fragmentation or fall-off stage, and sometimes in the layer-changing process of the diamond grits on the crown surface. Hence, in the machining process of each hole, the state of the diamond grits on the crown surface is different, which leads to differences in the wear loss after machining different holes. Since the wear loss after machining each hole is used to represent the overall wear rate of the bit, the wear rate curve of the bit sometimes does not cover a complete cycle, such as in Test 7 (Figure
In this work, an intensive experimental study on the wear rate of the sintering diamond thin-wall core bit during drilling Al2O3 bulletproof ceramics (99 wt.%) has been conducted. The influences of the main bit performance and machining process parameters on the overall wear rate of the bit have been analyzed. According to the experimental results, the main conclusions can be summarized as the following: Under the test conditions, finer diamond grit, higher diamond concentration, lower number of water gaps, thinner wall thickness, or lower bit load all can decrease the wear rate of the bit. When the diamond grits turns finer, the diamond concentration turns higher, or the number of water gaps is lowered, the bit can skid easily, whereas thinner wall thickness and higher diamond concentration can make the diamond layer to crack easily or even break up. The increase in wall thickness does not always increase the wear resistance or extend the service life of the bit, and the key is to ensure the matching of bit load and wall thickness. From the perspective of wear resistance, the number of water gaps should be 2-3, and the wall thickness should be kept between 2 and 2.5 mm (when the bit diameter is Ф 24 mm). Within a certain range, the spindle speed has little influence on the overall wear resistance of the bit, but when the spindle speed increases, the machining efficiency can be significantly improved. The wear rate of the bit shows an approximately periodical variation with the cumulative drilling depth. This is closely related to the periodical layer-changing characteristic of the diamond grains on the working surface of the impregnated diamond tools.
The wear rate data of the sintering diamond core bit under different bit performance and machining process parameters involved in this paper can be found in the figures drawn with Origin software. The data can be sent by e-mail.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by National Natural Science Foundation of China (51575470), Jiangsu Provincial Six-Big-Talent-Peak High Level Personnel Project of China (JXQC-029), Qing Lan Project of Jiangsu Higher Education of China (Document 15th of Jiangsu Education Department in 2016), Postgraduate Research & Practice Innovation Program of Jiangsu Province of China (SJCX17-0444 and SJCX17-YG01), and Shandong Provincial Natural Science Foundation of China (ZR2012EEL04). Thanks to all study participants for their contributions.