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The objective of this study is to systematically examine the drilling efficiency and performance of various core drill bits in lunar rock formation using the discrete element method (DEM) and drilling experiments conducted in a lunar vacuum environment. This research aims to establish a scientific foundation for selecting core drill bits for lunar deep drilling operations. To achieve this, four distinct core drill bits were designed. Subsequently, a numerical model of lunar rock was constructed and the load characteristics and drilling efficiency of each bit during the drilling process were analyzed using DEM. Drilling and coring tests were then performed in both atmospheric and lunar vacuum environments, thereby validating the numerical simulation results and providing a comprehensive evaluation of the actual performance of the core drill bits. The study revealed that the carbide-tipped core drill bit with octagonal prisms design resulted in the core disking due to a significant rise in temperature, underscoring the critical importance of temperature control in maintaining core integrity. While the carbide-tipped core drill bit with cutting edges demonstrates exceptional drilling efficiency and coring quality, its inherent fragility and rapid wear of the cutting edges present considerable challenges for practical application. The diamond-impregnated core drill bit is unsuitable for drilling operations under lunar loads and power limitations due to its high weight-on-bit (WOB) requirements. In contrast, the PDC core drill bit exhibits excellent drilling stability, low rotary torque requirements, minimal temperature-rise effects, and significantly enhanced penetrating speed in the lunar vacuum environment, making it a recommended choice for lunar rock drilling. This study provides substantial theoretical and experimental support for the development of lunar drilling equipment and the formulation of effective drilling strategies.
Highlights
An improved HMB contact model was used in the numerical simulation calculations.
Simulated lunar rock drilling tests were conducted in both atmospheric and vacuum environments, and a comparative analysis was performed with the numerical simulation results.
Among the four self-designed drill bits, the PDC bit was identified as the most suitable for lunar rock drilling through comparative selection.
Introduction
The Moon, Earth's only natural satellite, has been at the forefront of humanity's exploration of the cosmos since ancient times. Since the 1960s, when humans first set foot on the lunar surface, scientific research on the Moon has made significant progress. The Moon not only contains abundant resources and energy sources, but its geological structure and material composition are also crucial for understanding the early history of the solar system. With advancements in deep-space exploration technology, lunar drilling has emerged as a key method for obtaining subsurface information about the Moon and facilitating resource development.
The lunar surface is covered by a layer of regolith composed of fine particles, beneath which lies a layer of breccia consisting of regolith and rock particles of varying grain sizes, followed by rock strata at different depths (Fa et al. 2015; Huang et al. 2018; Li et al. 2020). Obtaining samples of regolith and rocks from various depths through coring drilling facilitates the study of the Moon's geological structure, resource endowment, and evolutionary history. Furthermore, the presence of water ice resources in the polar regions of the Moon offers potential support for future lunar exploration and the establishment of long-term lunar bases (Li et al. 2018). This situation necessitates in-depth research and exploration of the lunar poles to understand the distribution, reserves, and extractability of these water ice resources. The design of lunar drilling equipment must take into account the limitations of payload mass and energy, the extreme lunar environment, the uncertainty of geological conditions, and the automation of the drilling process (Zhang et al. 2019). To this end, scientific teams have conducted a series of laboratory tests using a 2-m spiral coring drill in a lunar simulation environment to verify the feasibility of automatic drilling missions by lunar robots (Zhang et al. 2021; Xu et al. 2022), which has supported the Chang'e-5 mission in sampling the lunar regolith (Zheng et al. 2023).
The core drill bit is a crucial piece of equipment for cutting and crushing extraterrestrial regolith and rock to obtain samples, which directly influences drilling efficiency and the quality of the samples (Liu et al. 2021a). The design of a core drill bit requires comprehensive consideration of the physical properties of the drilling target, the interaction between the bit and the regolith or rock, the distribution of forces during the drilling process, and the material characteristics of the bit (Zacny et al. 2008, 2013a). For the feasibility exploration of future deep lunar drilling, an in-depth analysis of the impact of bit types and drilling parameters on drilling efficiency and power consumption is essential (Zou et al. 2023). Research institutions, such as Honeybee Robotics in the United States, have developed various types of cutting bits, beginning with the design of cutting tool configurations and material selection, with the cutting performance of the bit as the primary design goal for different drilling targets, including extraterrestrial soil, ice layers, and rocks (Zacny et al. 2013b; Hironaka and Stanley 2010). To ensure drilling stability, Liu et al. (2015) designed a cone body core drill bit suitable for the lunar environment. However, a study based on DEM suggests that the cone body is not conducive to the forces acting on the drill tool (Liu et al. 2021a). Focusing on simulated lunar regolith, Deng et al. (2013) analyzed the impact of bit structural parameters on the core recovery rate and designed a new type of core drill bit featuring a blocking ring structure, which can effectively enhance the core recovery rate of extraterrestrial regolith and has been applied in China's lunar exploration project.
The above-mentioned core drill bit described above primarily utilizes a straight-flute cutting tool structure. Research indicates that the chip removal channel of this bit is not conducive to effective chip evacuation, resulting in a higher likelihood of bit clogging (Shi et al. 2014). Additionally, lunar rock particles of critical size within the lunar regolith may further obstruct the bit, rendering coring impossible (Liu et al. 2021b). To address the challenges associated with chip removal, a core drill bit designed with a spiral chip removal channel has been developed. Studies suggest that this configuration is more effective in facilitating chip evacuation (Zhang et al. 2016). Drilling load is a crucial metric for assessing the performance of core drill bits. Accurate prediction of drilling load is essential for the effective design of these tools. Researchers have conducted extensive studies on predicting the drilling load of straight-edge core drill bits, progressively establishing and refining models to represent drilling load. They have also examined the impact of structural parameters, motion parameters, and mechanical parameters on drilling load (Liu et al. 2015; Shi et al. 2014).
In the foreseeable future, lunar drilling operations are anticipated to advance towards the extraction of deep bedrock. However, current research primarily focuses on core drill bits designed for lunar regolith, with limited comprehensive assessments of their drilling performance in lunar rock. This study utilizes the discrete element method (DEM) to simulate the drilling behavior of various core drill bit configurations within lunar rock. Building on this simulation, drilling tests were conducted in a lunar-based pseudo-real environment to analyze increases in drilling temperature, as well as the efficiency and performance of different core drill bits. A systematic evaluation of the advantages and disadvantages of these configurations was performed by comparing their performance during the drilling process. The design and testing of various core drill bit configurations provide a theoretical foundation and technical support for future deep drilling and coring endeavors on the Moon, facilitating the acquisition of lunar bedrock samples and advancing the exploration of lunar resources and scientific research.
Selection and verification of lunar rock simulant
Lunar mare basalt is a significant component of lunar geology. Research indicates that it covers approximately 17% of the lunar surface area while constituting about 1% of the lunar crust's volume (Head 1976). This material is of considerable value for the exploration, development, and utilization of in-situ lunar resources (Zhang 2018). Due to the impracticality of conducting drilling and coring experiments on large lunar rock samples on Earth, researchers typically select terrestrial rocks with similar mineral compositions and physical properties to lunar mare basalt as simulants for experimental purposes. For example, Chifeng basalt, located in Inner Mongolia, is considered an exemplary lunar rock simulant (Hao et al. 2023; Wu et al. 2023a).
A series of systematic physical and mechanical tests were conducted on the simulant, with each experiment replicated three times to ensure the reliability of the results. The average values obtained from these tests are summarized in Table 1. The simulant demonstrates an average porosity of 0.755%, indicating a relatively dense structure. Its mechanical properties include an average uniaxial compressive strength (UCS) of 184.247 MPa, an elastic modulus of 45.723 GPa, a tensile strength (TS) of 11.762 MPa, and a notably high tensile-to-compressive strength ratio of 15.665. Under confining pressures of 5, 7, and 9 MPa, the measured triaxial compressive strength (TCS) values were 230.504, 254.492, and 271.042 MPa, respectively. A comparative analysis with physical–mechanical data from lunar mare basalt samples obtained during the Apollo missions indicates a close correspondence: authentic lunar mare basalts exhibit porosity ranging from 0 to 10%, compressive strength of approximately 200 MPa, and estimated tensile strength between 6.1 and 24.18 MPa, as derived from predictive methods, characterizing them as highly brittle rocks (Warren et al. 1973; Cohn et al. 1981). These comparisons suggest that the physical–mechanical properties of the simulant closely align with those of genuine lunar mare basalts, indicating its potential suitability as a substitute for lunar bedrock in future in situ investigations.
Table 1. Physical and mechanical properties of lunar rock simulant
Porosity | UCS | TS | E | TCS (MPa) | ||
|---|---|---|---|---|---|---|
(%) | (MPa) | (MPa) | (GPa) | σ3 = 5 | σ3 = 7 | σ3 = 9 |
0.755 | 184.247 | 11.762 | 45.723 | 230.504 | 254.492 | 271.042 |
Furthermore, a numerical model utilizing DEM has been developed for the core drill bit and lunar rock simulant, aiming to thoroughly investigate their interaction. A preliminary analysis of the drill bit's performance has been conducted. By employing this methodology, researchers can replicate complex physical phenomena that occur during the actual drilling process, including rock fragmentation and fracture propagation. This approach provides essential data and insights for optimizing the drill bit, ultimately enhancing drilling efficiency.
Drilling numerical analysis of lunar rock simulant
Design and parameters of core drill bits
In the context of lunar exploration, the design of coring tools requires a dual focus on efficient cutting performance and effective rock fragmentation to meet the technical requirements for deep hard rock sampling. This study presents a comparative analysis of four distinct coring bit designs (Fig. 1). Previous Chinese lunar exploration missions, specifically Chang'e-5 and Chang'e-6, primarily targeted the sampling of lunar regolith. The drilling and production systems employed in these missions utilized a coring tool with an outer diameter of 38 mm, which was integrated with a flexible sample packaging device measuring 15 mm in diameter (Jiang et al. 2022; Hu et al. 2024). However, the small core diameter of this configuration does not comply with the standardization requirements for rock mechanics analysis as stipulated by the International Society for Rock Mechanics (ISRM), which specifies that cylindrical specimens should have dimensions of 25 mm in diameter and 50 mm in height. Consequently, the newly designed coring bit standardizes the outer diameter to 42 mm and adjusts the coring diameter to 25 mm. This enhancement not only ensures the standardization and comparability of samples but also provides a solid foundation for subsequent standardized testing of rock mechanical parameters.
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Fig. 1
Core drill bits with different configurations: a diamond-impregnated core drill bit; b PDC core drill bit; c carbide-tipped core drill bit with cutting edges; d carbide-tipped core drill bit with octagonal prisms
Specifically, Fig. 1a illustrates a diamond-impregnated core drill bit, renowned for its exceptional wear resistance and effective crushing capabilities in hard rock. The uniform distribution of diamond particles enhances its cutting efficiency, allowing it to maintain a consistent drilling rate in the challenging lunar hard rock environment. Figure 1b depicts a polycrystalline diamond compact (PDC) core drill bit, favored for its high wear resistance and versatility across various rock types. Figure 1c presents a carbide-tipped core drill bit characterized by a sharp cemented carbide cutting edge, designed to exhibit robust rock-breaking capabilities. Finally, Fig. 1d features a carbide-tipped core drill bit with octagonal prisms design, which provides an expanded cutting area and improved chip removal efficiency due to its distinctive shape (Liu et al. 2021a, 2021b).
The matrix material of the core drill bit (a) is cast tungsten carbide, with the elastic modulus of 710 GPa, Poisson 's ratio of 0.31, and density of 15,630 kg/m3, respectively. The elastic modulus of diamond particles is 1000 GPa, Poisson 's ratio is 0.2, and density is 3500 kg/m3. The matrix materials of core drill bits (b), (c) and (d) are 40CrNiMoA alloy steel, whose elastic modulus is 210 GPa, Poisson 's ratio is 0.29, and density is 7850 kg/m3. The elastic modulus of PDC is 890 GPa, Poisson 's ratio is 0.07, and density is 11,500 kg/m3. The cutting-edge material is YG8 cemented carbide, whose elastic modulus is 530 GPa, Poisson 's ratio is 0.29, and density is 14,800 kg/m3.
DEM numerical model of lunar rock simulant
During the processes of drilling and coring, the core drill bit exerts both pressure and torque on the rock, resulting in fragmentation. The tensile strength of the rock is a crucial factor that influences both drilling efficiency and the longevity of the drill bit. However, numerous studies have shown that the Hertz-Mindlin with bonding (HMB) contact model tends to significantly overestimate tensile strength while accurately representing compressive strength (Moon et al. 2012; Van et al. 2014).
In developing a discrete element model for the lunar rock simulant, this study employed the modified HMB contact model, considering the high brittleness characteristics of lunar mare basalt. Drawing on existing discrete element simulation methodologies and the physical properties of lunar rock simulant, the discrete element parameters for the lunar rock simulant were established (Wu et al. 2023a, 2023b), as detailed in Table 2. The parameters indicate that the uniaxial compressive strength and tensile strength are 184.822 MPa and 11.847 MPa, respectively, resulting in a compression-tension ratio of 15.601. The elastic modulus is measured at 45.848 MPa, which aligns within 1% of the test results. This evidence suggests that the modified HMB contact model effectively replicates the brittleness characteristics of lunar rock simulant, thereby providing a more accurate representation of the mechanical response of lunar rock during drilling operations (Wu et al. 2023b).
Table 2. DEM parameters of lunar rock simulant
Elastic modulus of particles | Stiffness ratio | The bond radius | Elastic modulus of the bond | Angle of internal friction | Cohesion |
|---|---|---|---|---|---|
45.723 GPa | 2.373 | 0.8625 mm | 29.055 GPa | 54.66° | 29.17 MPa |
To further validate the alignment between the failure characteristics observed in the numerical model of lunar rock simulant and the experimental findings (Liu et al. 2023), industrial computed tomography (CT) scans were conducted on the lunar rock simulant following uniaxial compression failure, accompanied by three-dimensional reconstruction (Fig. 2). The analysis revealed that the failure characteristics under uniaxial compression for the lunar rock simulant samples predominantly exhibit an "X"-shaped conjugate shear failure pattern, while splitting failure primarily occurs along the normal reference direction of the disk. The principal fracture surface extends through the center of the disk, radially bisecting the sample, which retains a significant degree of structural integrity. Additionally, secondary cracks were observed near the main fracture surface, which can be attributed to tensile failure within the matrix (Fig. 3). Under varying loading conditions, the stress distribution and failure modes within the numerical model of the lunar rock simulant closely mirror the actual experimental results, thereby affirming the reliability of the numerical model.
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Fig. 2
Uniaxial compressive failure characteristics of lunar rock simulant (left: test results; right: numerical simulation results)
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Fig. 3
Tensile failure characteristics of lunar rock simulant (left: test results; right: numerical simulation results)
The variable particle size modeling approach was employed to enhance computational efficiency. This methodology segments the particle system into distinct regions, assigning varying particle sizes to each region, thereby minimizing the total number of particles while maintaining simulation accuracy and significantly increasing computational speed (Cui et al. 2023; Zhao et al. 2023). In this study, the discrete element model was categorized into two primary regions. The particles within the internal region were densely arranged around the drilling tool, with a maximum diameter of 1.5 mm and a minimum diameter of 0.6 mm. The particle size distribution followed a normal distribution. Conversely, the external region utilized larger particles with a diameter of 2.5 mm.
The load characteristics of drilling tools during the drilling process represent a complex issue influenced by various factors. These characteristics are not only intricately linked to the quantity of particles and the drilling depth within the model but are also significantly affected by the model's boundary conditions. Prior research has indicated that the stress variations of particles near the core drill bit exhibit a high sensitivity to boundary effects. It has been established that to mitigate the influence of these boundary effects, the ratio of the sample diameter to the outer diameter of the core drill bit should exceed 1.38 (Liu et al. 2020). Consequently, this study selected a cylindrical sample with a height of 50 mm and a diameter of 80 mm (Fig. 4), resulting in a ratio of 1.9 between the sample diameter and the outer diameter of the core drill bit.
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Fig. 4
Discrete element model of lunar rock simulant
Simulation result analysis
In the conducted simulation analysis, a consistent set of drilling parameters was employed across all tests, specifically a weight-on-bit (WOB) of 800 N, a rotary speed of 120 revolutions per minute (rpm), and a percussive frequency of 4 Hz. Figure 5 illustrates the relationship between rotary torque and drilling depth for various core drill bits. It was observed that rotary torque generally increases with drilling depth before eventually stabilizing. The diamond-impregnated core drill bit exhibited the highest drilling resistance, with a maximum rotary torque of 17.09 N·m. In comparison, the maximum rotary torque values for PDC core drill bit, the carbide-tipped core drill bit with cutting edges, and the carbide-tipped core drill bit with octagonal prisms were relatively similar, measuring 10.95 N·m, 11.19 N·m, and 10.88 N·m, respectively. Notably, the PDC core drill bit displayed greater fluctuations in rotary torque, while the other two cemented carbide core drill bits demonstrated more stable performance.
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Fig. 5
Rotary torque curves of different core drill bits in DEM
To further evaluate the drilling efficiency of each core drill bit, the total simulation time required for each core drill bit to reach the same drilling depth was compared and analyzed. The diamond-impregnated core drill bit completed the designated depth in 7.37 s, while the PDC core drill bit and the carbide-tipped core drill bit with octagonal prisms exhibited similar performance, taking 5.30 s and 5.83 s, respectively. In contrast, the carbide-tipped core drill bit with cutting edges demonstrated the highest drilling efficiency, achieving the same depth in only 2.97 s.
This variation in drilling efficiency can be attributed to the grinding characteristics of the diamond-impregnated core drill bit, which is better suited for high WOB drilling conditions. The other three core drill bits employ cutting methods. A comprehensive evaluation of the drilling efficiency and stability of the four types of core drill bits indicates that the carbide-tipped core drill bit with cutting edges performs the best, followed by the PDC core drill bit and the carbide-tipped core drill bit with octagonal prisms, while the diamond-impregnated core drill bit ranks last.
Figure 6 illustrates the distribution of force chains within cores obtained using various core drill bits. It is evident that the internal force chain distribution of the core acquired with the carbide-tipped core drill bit with cutting edges is the most uniform and consistent. This uniformity suggests that this drill bit inflicts the least damage on the core during the drilling process, thereby preserving the core's integrity. Following this, the diamond-impregnated core drill bit exhibits a relatively uniform force chain distribution; however, the phenomenon of particle aggregation is observable in certain areas (indicated by the red circle in Fig. 6). This observation may imply that these regions experience heightened stress concentrations during the drilling operation. In contrast, the cores obtained using the PDC core drill bit and the carbide-tipped core drill bit with octagonal prisms not only display particle aggregation but also reveal significant blank areas (highlighted by the black rectangular box in Fig. 6). These blank areas suggest the potential formation of cracks within the cores, which could adversely affect both the integrity and quality of the cores. In terms of core integrity, the four core drill bits can be ranked as follows: the carbide-tipped core drill bit with cutting edges demonstrates the best performance, followed by the diamond-impregnated core drill bit, then the PDC core drill bit, and finally, the carbide-tipped core drill bit with octagonal prisms.
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Fig. 6
The force chain distribution of cores: a diamond-impregnated core drill bit; b PDC core drill bit; c carbide-tipped core drill bit with cutting edges; d carbide-tipped core drill bit with octagonal prisms
It is important to note that all core drill bits are modeled as rigid bodies in DEM, which precludes any damage. However, in practical applications, core drill bits are subject to wear and potential breakage due to various influencing factors. Consequently, to provide a comprehensive assessment of the practical efficiency of these core drill bits, this study conducted drilling tests in both atmospheric and lunar vacuum environments to obtain more accurate and reliable data.
Drilling and coring tests of lunar rock simulant
Utilizing lunar-based simulation environment fidelity coring test and analysis system (Fig. 7) designed and developed independently by the Institute of Deep Earth Sciences and Green Energy of Shenzhen University, a series of drilling and coring tests were conducted. The primary research medium was lunar rock simulant. The study focused on testing the drilling load characteristics of various core drill bits and evaluating their performance. The analysis of the data concerning several parameters, including rotational torque, WOB, penetrating speed, temperature, and coring quality. Furthermore, it discusses the coring efficacy of different core drill bits, thereby providing a scientific foundation for efficient coring operations of deep lunar rocks in future endeavors.
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Fig. 7
lunar-based simulation environment fidelity coring test and analysis system
Drilling tests in an atmospheric environment
In evaluating the performance of various core drill bits, a series of drilling and coring tests were conducted in an atmospheric environment. To prevent potential damage to the drilling tools caused by excessive penetration speed, the constant WOB control mode was implemented during the tests to ensure precise regulation of the drilling process.
Based on the numerical simulation results, the WOB was set at 800 N, with a rotation speed of 120 rpm, and a percussive frequency of 4 Hz. It was observed that the diamond-impregnated core drill bit was largely ineffective under these drilling parameters, making it difficult to achieve the cutting and fragmentation of lunar rock simulant. Consequently, so it is no longer necessary to carry out follow-up testing and performance evaluation.
For the PDC core drill bit and the carbide-tipped core drill bit with octagonal prisms, rotary-percussive drilling tests were conducted to comprehensively assess the performance of the bits under composite loading conditions. In the case of the carbide-tipped core drill bit with cutting edges, since its design is not conducive to the application of percussive loads, only rotary drilling tests were performed to evaluate its drilling efficiency and stability. The results of these tests are anticipated to provide valuable reference data.
PDC core drill bit
Figure 8 shows the test results of the PDC core drill bit under different drilling parameters. At a WOB of 800 N, both the rotary torque and real-time WOB exhibited significant fluctuations throughout the drilling operation. The peak value of the real-time WOB reached 1033 N, with a fluctuation range of approximately 200 N, indicating instability in the drilling process. The rotary torque gradually increased to a maximum of 20 N m, with an average value of 12 N m. Upon reaching a drilling depth of 20 mm, the PDC core drill bit cut the rock with a harsh sharp sound, and remained unable to break through the rock for several minutes, ultimately necessitating a cessation of drilling. The total duration of the drilling operation was recorded at 900 s. Additionally, the temperature of the core drill bit exhibited an initial rapid increase followed by gradual stabilization, with a maximum temperature rise of 114 °C observed. Furthermore, under the condition of a given WOB increased to 1000 N, the PDC core drill bit successfully penetrated to a depth of 240 mm. The maximum rotary torque increased to 25 N m. During the drilling process, fluctuations in rotary torque and real-time WOB become more pronounced, with the fluctuation range of the real-time WOB expanding to approximately 500 N. This variability may be attributed to the heterogeneity of the rock sample or dynamic changes in the contact interface between the core drill bit and the rock. The dynamic temperature changes of the core drill bit warranted attention, as the temperature rose rapidly to around 120 °C before exhibiting slight fluctuations around this value. The maximum recorded temperature increase was 101 °C.
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Fig. 8
Drilling tests of the PDC core drill bit in an atmospheric environment
Throughout the drilling operation, significant vibrations were observed, closely associated with pronounced fluctuations in rotary torque and real-time WOB. This instability not only compromises the reliability of the drilling process but also contributes to inaccuracies in the recorded drilling depth data. This assertion is further substantiated by Fig. 9, which indicates that under a WOB of 1000 N, the measured actual drilling depth was 290 mm, with a calculated penetration rate of 1.38 mm/min. The core extracted by the PDC core drill bit exhibited fractures; however, the fractured segments remained largely intact.
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Fig. 9
The coring quality of the PDC core drill bit in an atmospheric environment
Carbide-tipped core drill bit with cutting edges
For the carbide-tipped core drill bit with cutting edges, this study conducted rotary drilling tests to evaluate its performance under different WOB conditions. Under the constant WOB of 800 N, there were fluctuations in real-time WOB and rotary torque, which remained within controllable ranges as shown in Fig. 10a. The peak rotary torque reached 25 N m with an average torque of 19 N·m. The maximum real-time WOB was 910 N, exhibiting a fluctuation range of approximately 110 N. The drill bit temperature increased rapidly before stabilizing, reaching a maximum of 151 °C (a temperature increase of 120 °C). At a drilling depth of 240 mm, the process required only 967 s, demonstrating the drill bit's high efficiency and rock-breaking capability. Additional tests at 700 N WOB showed slightly reduced rotary torque (Fig. 10b), with a peak value of 19 N m and an average of 15 N m. The peak real-time WOB decreased to 849 N but exhibited greater fluctuation (approximately 150 N). For the same 240 mm depth, drilling time increased to 2835 s while maintaining stable penetrating speed. The temperature profile followed a similar trend to the 800 N condition, but reached 205 °C (a 173 °C increase), potentially attributable to prolonged drilling duration.
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Fig. 10
Drilling tests of the carbide-tipped core drill bit with cutting edges in an atmospheric environment
During the tests, minor vibrations were observed in the sample chamber and drilling assembly. The carbide-tipped core drill bit with cutting edges demonstrated greater stability compared to the PDC core drill bit. As shown in Fig. 11, the cores obtained from both drilling tests exhibited high structural integrity. Under a WOB of 800 N, the actual drilling depth reached 250 mm with an actual penetrating speed of 15.52 mm/min. When the WOB was reduced to 700 N, the drilling actual depth was 277 mm, but the actual penetrating speed dropped significantly to 5.92 mm/min. These results indicate that increased WOB enhances both drilling stability and penetrating speed for the carbide-tipped core drill bit with cutting edges.
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Fig. 11
The coring quality of the carbide-tipped core drill bit with cutting edges in an atmospheric environment
Carbide-tipped core drill bit with octagonal prisms
The drilling test results of the carbide-tipped core drill bit with octagonal prisms revealed its performance across different drilling parameters (Fig. 12). At a WOB of 800 N, rotary torque and real-time WOB exhibited high-frequency fluctuations while maintaining overall stability. The peak real-time WOB reached 915 N with a fluctuation range of approximately 100 N, indicating periodic load variations during drilling. The maximum rotary torque was 16 N m, with an average of 13 N m. Upon reaching 20 mm depth, a sharp acoustic signal similar to that of the PDC core drill bit was detected, suggesting the presence of high-hardness minerals in this rock layer. Although the penetrating speed declined, the core drill bit successfully traversed the zone before drilling was terminated at 62 mm depth. The core drill bit's temperature underwent rapid heating followed by slight cooling and stabilization, peaking at 260 °C (a 246 °C increase). At a WOB of 1000 N, the maximum rotary torque increased to 21 N m (average 16 N m), while the peak real-time WOB rose to 1118 N. The penetrating speed followed a linear trend, achieving a final depth of 125 mm. Concurrently, the drill bit temperature surged to 340 °C, then gradually decreases and stabilizes at about 300 °C, and the maximum temperature appreciation is 305 °C, indicating that a large amount of heat is generated during the drilling process.
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Fig. 12
Drilling tests of the carbide-tipped core drill bit with octagonal prisms in an atmospheric environment
Minor vibrations in the sample chamber and drilling assembly were observed, indicating the stability of drilling, though some depth measurement inaccuracies persisted. Figure 13 compares actual drilling depths and coring quality under different WOB conditions. At 800 N WOB, the achieved depth was 84 mm with a penetrating speed of 1.27 mm/min. When the WOB was increased to 1000 N, the actual depth was 140 mm but the penetrating speed dropped to 0.80 mm/min. The increase of WOB does not seem to bring the expected increase of penetrating speed, which may be related to the geological inhomogeneity of the samples.
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Fig. 13
The coring quality of the carbide-tipped core drill bit with octagonal prisms in an atmospheric environment
It is particularly noteworthy that the high temperature phenomenon of carbide-tipped core drill bit with octagonal prisms in drilling process. At a depth of 40 mm or more, the core was completely broken into cakes due to excessive temperature for a long time, which significantly reduces the integrity of the core. Combined with the test results in Fig. 12b, the core drill bit reached the highest temperature in this area, and the high temperature above 300 °C for a long time. This discovery emphasizes the importance of drilling temperature to coring quality. In the drilling process, the temperature appreciation must be strictly controlled to ensure that the complete core with good bedding information is obtained.
Comprehensive evaluation and optimization of various core drill bits
Research demonstrates that polycrystalline diamond (PCD) materials initiate internal damage at 600 °C, the wear rate of the polycrystalline diamond material increases sharply beyond 700 °C, resulting in a significant decrease in its cutting ability (Appl et al. 1993; Deng et al. 2011). Of particular concern for lunar exploration, prolonged drilling operations risk inducing these critical temperatures through frictional heat accumulation. Consequently, the service life of the lunar coring drill bit under the influence of temperature is an important evaluation indicator.
After a series of drilling tests on different core drill bits, the damage of each core drill bit is shown in Fig. 14. The results show that PDC core drill bit and carbide-tipped core drill bit with octagonal prisms show excellent durability and reliability after two drilling tests, and no obvious damage signs are found. This shows that the design of these two bits can effectively resist the wear and impact during drilling, thus ensuring long-term service life and stable drilling performance. In contrast, although carbide-tipped core drill bit with cutting edges shows the highest drilling efficiency and rock breaking ability, the cutting edge fracture occurs after each drilling test. In order to ensure continuous drilling operations, damaged core drill bits need to be replaced after each test, which increases the maintenance cost and time cost of drilling tasks.
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Fig. 14
Damage states of different core drill bits
Integrated analysis of numerical simulations and drilling tests enabled systematic evaluation of lunar drill bit performance, providing selection criteria for mission planning (Table 3). This study constructs an evaluation system, which adopts a hierarchical quantitative standard and ranks the evaluation indicators from high to low in order of importance. Among them, Grade A indicates that the performance of core drill bit is the best in specific evaluation indicators, and Grade B indicates that the performance of core drill bit is at a high level. Although it is not as good as grade A, the overall performance is still reliable. Grade C indicates that there are obvious deficiencies in the evaluation index of core drill bit, and its performance may need to be improved by improving its design or material. In particular, in terms of drilling temperature, grade A corresponds to 50 ~ 150 ℃, grade B corresponds to 150 ~ 250 ℃, and grade C corresponds to > 250 ℃.
Table 3. Evaluation indexes of different core drill bits
PDC core drill bit | Carbide-tipped core drill bit with cutting edges | Carbide-tipped core drill bit with octagonal prisms | |
|---|---|---|---|
Drilling temperature | A | B | C |
Coring quality | A | A | C |
Damage state | A | C | A |
Penetrating speed | C | A | C |
Drilling stability | C | B | A |
For the drilling task of lunar deep rock, the influence of drilling temperature on the core is particularly significant. The temperature is directly related to the integrity of the core and the fidelity of geological information. In addition, drilling stability, penetrating speed and service life of core drill bits are also important indicators for evaluating bit performance. These together constitute a multi-dimensional performance evaluation system, which aims to fully reflect the performance of core drill bit in the actual drilling process. It can be seen from the Table 3 that PDC core drill bit is the first choice for drilling and coring tasks in lunar deep rock.
Drilling tests of PDC core drill bit in a lunar vacuum environment
For future lunar deep drilling missions, thermal management and core sample integrity demand primary focus. Therefore, the PDC core drill bit was subjected to lunar rock simulant drilling tests under a lunar vacuum environment. In this study, the environmental vacuum inside the equipment was reduced to 10–2 Pa to simulate the special environment of lunar deep rock. The drilling parameters were set as WOB = 800 N, rotation speed = 120 rpm, and percussive frequency = 4 Hz.
In the drilling tests conducted in a vacuum environment, although real-time WOB and rotary torque exhibited fluctuations, they were generally kept within a controllable range (Fig. 15). In two tests, the maximum real-time WOB reached 941 N and 1043 N, respectively. After stable drilling, the fluctuation range was maintained at approximately 150 N. The temperature variation pattern of the core drill bit exhibited a similar trend to that under atmospheric conditions: it rose rapidly in the initial stage and then gradually stabilized. In two tests, the highest temperature reached 99 °C and 105 °C, respectively, further demonstrating the temperature control capability of the PDC core drill bit in different environments. It should be emphasized that the rotary torque of the PDC core drill bit was substantially reduced in a vacuum environment. The peak values of the rotary torque were only 9.8 N m and 11.2 N m, respectively. Furthermore, the average torques were also reduced to 5.4 N m and 5.76 N m, respectively. When compared to drilling tests under atmospheric conditions, this phenomenon demonstrates the superior performance of the PDC core drill bit in a vacuum environment. The significant decrease in the required load is likely attributable to the vacuum environment reducing the friction and bonding between rock particles.
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Fig. 15
Drilling tests of the PDC core drill bit in a lunar vacuum environment: a first test; b second test
Additionally, based on the penetrating speed derived from the actual drilling depth shown in Fig. 16, it is evident that the drilling stability and penetrating speed of the PDC core drill bit in a lunar vacuum environment have been substantially enhanced. This conclusion is strongly supported by two independent test results, which exhibit a high degree of consistency and repeatability. Specifically, in the first test, the PDC core drill bit took 2785 s, achieving a penetrating speed of 5.17 mm/min; in the second test, despite a slight increase in time consumption to 3270 s, the penetrating speed remained at a high level of 4.39 mm/min. These results demonstrate the excellent stability and high efficiency of the PDC core drill bit during the drilling process, providing critical insights for its application in extreme environments, such as lunar deep drilling missions. Through a comprehensive analysis of the drilling test results conducted under atmospheric environment and vacuum environment, it is apparent that the vacuum environment may exert a significant influence on the mechanical response of the drilling process.
[See PDF for image]
Fig. 16
The coring quality of the PDC core drill bit in a lunar vacuum environment
Conclusion and prospect
Utilizing the discrete element method (DEM) in conjunction with multi-environment test verification, a comprehensive assessment of the drilling performance of four distinct lunar coring bits was conducted. The findings indicate that drilling efficiency and core integrity are significantly influenced by the type of bit used, the environmental conditions, and the drilling parameters. Based on these conclusions, several targeted recommendations and future prospects for lunar deep drilling missions are proposed:
The analysis indicates that as drilling depth increases, the rotating torque demonstrates a nonlinear increase before stabilizing. Given the torque management challenges expected in future lunar deep drilling projects, it is suggested to learn from the mature wireline coring technology from terrestrial drilling. Implementing drill pipe cycle replacement may help mitigate torque accumulation, potentially resulting in reduced energy consumption and improved efficiency in drilling and coring operations.
Given the load and power limitations inherent to lunar drilling missions, the diamond-impregnated core drill bit are ineffective for drilling high-strength lunar rock under low WOB conditions (≤ 1000 N). To address the challenge of insufficient WOB in a microgravity environment, it is recommended to incorporate an anchoring device on the drilling rig to increase the reaction force. Additionally, utilizing a power device to generate impact motion at the bottom of the borehole could further enhance drilling efficiency.
The carbide-tipped core drill bit with cutting edges exhibit superior drilling efficiency; however, its cutting edge is easily damaged. The carbide-tipped core drill bit with octagonal prisms can lead to significant increase in temperature, resulting in the formation of core cake, which shows the importance of temperature control. Future deep drilling initiatives can explore ultrasonic-assisted rock-breaking technologies, which utilize ultrasonic vibrations to reduce the strength of rock, reduce the conversion of mechanical energy into thermal energy, thereby reducing bit wear and preserving core integrity.
Taking into account factors such as drilling temperature, coring quality, damage state, and rate of penetration (ROP), the PDC core drill bit demonstrate several advantageous characteristics, including high ROP, low torque, and minimal temperature rise in a vacuum environment. These features make PDC bits the preferred choice for lunar deep drilling. Future efforts could focus on the integration of thin-wall drilling tools to further enhance drilling efficiency.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (No. 52225403, 52434004), the National Key R&D Program of China (2023YFF0615401), the Shenzhen National Science Fund for Distinguished Young Scholars (RCJC20210706091948015), and the Scientific Instrument Developing Project of Shenzhen University.
Authors’ contribution
X. W.Data curation, Writing-Original draft preparation, Writing -Review & Editing; Y. W. Data curation, Writing-Original draft preparation, Investigation, Formal analysis; Z. G. Supervision, Methodology; H. H. Data Curation; L. Z. Data Curation, Investigation; J. L. Data Curation; M. G. Project administration, Conceptualization, Resources.
Date availability
All data generated or analysed during this study are included in this published article.
Declarations
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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