A Study on Laser-Assisted Cylindrical Grinding of Superhard Diamond Composite (DSiC) Materials: Surface Integrity and Efficiency

Abstract

A novel laser-assisted cylindrical grinding process has been developed to enhance the machining of silicon-carbide-bonded diamond composites (DSiCs), critical for improving the performance and durability of components in subsea pump applications. DSiCs, containing approximately 50% diamond by volume, exhibit excellent mechanical and thermal properties. The conventional grinding of these superhard materials presents challenges such as high grinding forces, elevated temperatures, and significant tool wear. To overcome these difficulties, a laser-assisted cylindrical grinding process has been developed, utilizing ultra-short-pulse laser radiation to induce material ablation with controlled structural damages, thereby reducing grinding forces, temperatures, and tool wear. This research investigates the influence of grinding wheel specifications and grinding parameters on surface quality and tool life. The results indicate modest enhancements in surface integrity, achieving damage-free ground surfaces, and notable improvements in grinding ratio (G-ratio) by up to 247% and actual removal depth by up to 99% compared to conventional grinding. The laser-assisted cylindrical grinding process using vitrified-bonded diamond wheels holds significant promise for advancing subsea pump technology by enabling the use of DSiCs and achieving plateau ground surfaces.

1. Introduction

Diamond materials bonded with silicon carbide (DSiC) are a novel class of cost-effective, superhard composites with exceptional mechanical and thermal properties. These materials contain approximately 50% diamond by volume within a three-dimensional SiC structure. DSiCs exhibit high hardness, wear resistance, and thermal conductivity, making them ideal for demanding applications in challenging environments. These materials are particularly significant for subsea pump systems where their high wear resistance ensures a long service life under extreme conditions such as high pressure, salinity, and abrasive particles. Beyond subsea environments, DSiCs find applications in automotive engine components, industrial bearings, and cutting tools where their exceptional tribological properties minimize friction and wear, thereby improving performance and durability. These characteristics also position DSiCs as promising materials for high-speed machining and other precision manufacturing applications [1,2,3,4,5].

Notably, DSiCs are manufactured by infiltrating a diamond-containing preform with silicon, resulting in a surface layer of SiC-bonded diamond. This approach aims to create reliable, high-performance components for subsea environments, enhancing efficiency and safety in deep-sea resource extraction [4,5].

However, conventional grinding methods pose challenges during the grinding of DSiCs, leading to high grinding forces, elevated temperatures, and significant tool wear [6,7]. Hybrid machining techniques, such as ultrasonic-assisted grinding (UAG) and laser-assisted grinding (LAG), have been developed to address these issues. UAG employs high-frequency vibrations superimposed on the grinding process, resulting in intermittent cutting phenomena, reduced grinding forces, and improved lubrication. While UAG has proven effective in enabling ductile material removal and minimizing subsurface damage, it can face issues such as uneven material removal in brittle materials and difficulties managing thermal effects in high-conductivity materials. Furthermore, the integration of ultrasonic vibrations introduces additional complexity, necessitating advanced control systems and specialized equipment [8]. LAG with nanosecond lasers modifies the material surface prior to grinding by introducing thermal damages, reducing grinding forces, and improving material removal. However, the longer pulse duration of nanosecond lasers causes significant thermal damage, such as microcracking and phase transformations, and heat diffusion extends beyond the desired machining zone, compromising precision and surface integrity [9,10].

Laser-assisted grinding (LAG) with ultra-short-pulse lasers is a cutting-edge strategy in manufacturing, offering reduced grinding forces, lower temperatures, and decreased tool wear. In this process, laser structuring is performed prior to grinding on a separate machine, creating controlled subsurface damage in radial and lateral directions. These controlled microstructural modifications enhance the grinding process by improving chip formation, reducing grinding forces, and minimizing thermal effects during subsequent grinding. This approach not only enhances surface quality but also significantly improves material removal rates and tool life. Moreover, the laser structuring of the workpiece surface before grinding improves cutting fluid delivery, and the process can be conducted independently of grinding, with the cutting mechanism primarily governed by brittle fracture and crack propagation [11,12,13,14,15,16,17,18,19].

This study aims to establish cause–effect relationships between process variables and output parameters to improve the efficiency of cylindrical grinding of DSiCs. The key innovation lies in the integration of ultra-short-pulse laser structuring to precondition DSiCs, addressing challenges such as high grinding forces, elevated temperatures, and excessive tool wear. Laser-induced damages were confined to the grinding depth of cut, with no remaining damages or cracks on the ground workpiece surface. LAG resulted in smaller grinding forces, sometimes achieving smaller surface roughness compared to conventional grinding. Additionally, LAG enhanced the grinding ratio (G-ratio) by up to 247% and increased actual removal depth by 99% compared to conventional processes. Vitrified-bonded grinding wheels induced plateau ground surfaces, lower grinding forces, and higher removal rates compared to other types of wheels.

2. Materials and Methods

Figure 1 illustrates the experimental setup including the DSiC shaft bearing sleeve, grinding wheel, hard fabric–plastic composite, and coolant nozzle. The external cylindrical grinding tests were conducted in the up grinding-mode on an EMAG KARSTENS HG-204S cylindrical grinding machine (Emag GmbH & Co. KG., Salach, Germany), utilizing oil as the process fluid. Different clamping methods were employed, as shown in Figure 2. Three-point internal clamping (Figure 2b) resulted in a workpiece breakout due to the localized point forces. Additionally, glue clamping (Figure 2b) on a steel shaft led to some cracks in the workpiece due to the prolonged loading and excessive force from the grinding wheel. Consequently, a hard fabric–plastic composite (Figure 2c) was successfully employed as the clamping material due to the fully contact condition produced, which minimizes stress concentrations and prevents surface marks on the DSiC material. To ensure repeatability, each test condition was replicated, and standard deviation values were calculated to confirm data consistency.

The laser-structured pattern, oriented perpendicular to the grinding direction, is intended to enhance the effect of laser structuring on reducing grinding forces [20]. Accordingly, the superhard shaft bearing sleeves were laser-structured using a GF Femto Flexipulse P400 U 5-axis laser machine (GF Machining Solutions GmbH, Schorndorf, Germany), as depicted in Figure 3.

The workpieces were subjected to laser structuring at two different percentages: 50% and 170%, as shown in Figure 4. The laser pattern consisted of circumferential lines, with material removal being highest at the center of the beam due to the Gaussian distribution of laser beam intensity as shown in the section view in Figure 4a. To completely remove the surface, a laser structuring percentage of 170% was applied (Figure 4b), requiring a pulsed overlap ratio of 70%. This overlap ensures a thorough removal of the workpiece surface by positively overlapping laser lines.

Ultra-short-pulse laser structuring may cause minor subsurface damage on a micro-scale, necessitating an appropriate grinding allowance to mitigate such damage. This allowance is essential for preventing unintended subsurface damages, such as micro-cracks, ensuring a damage-free ground surface and subsurface. The SEM image in Figure 5 reveals lateral and radial micro-cracks on a laser-structured DSiC sample, with subsurface damage extending up to approximately 50 µm.

Three types of 400 mm diameter grinding wheels were employed: a galvanic-bonded, a metal-bonded, and a vitrified-bonded diamond grinding wheel. The morphology of these grinding wheels after the grinding process of DSiCs, measured by a Keyence digital microscope (VHX5000) (KEYENCE DEUTSCHLAND GmbH, Neu-Isenburg, Germany) with a magnification of X100, is illustrated in Figure 6. The morphologies indicate that the effectiveness of galvanic-bonded and metal-bonded diamond grinding wheels was unsatisfactory due to pulled-out grains and wear flattened grains, respectively, being the main mechanisms of wheel wear. However, in the vitrified-bonded diamond grinding wheel, the primary wheel wear mechanism was microfractured grains, which contribute to the self-sharpening of the grinding wheel [20].

After selecting the vitrified-bonded diamond grinding wheel as the preferred option due to its superior performance, the parameter study was conducted on flat workpieces during surface grinding to determine the optimal conditions, primarily because of the high-cost factor associated with the shaft bearing sleeve. These optimal conditions were then applied to the cylindrical grinding process. In surface grinding, the specific material removal rate ($Q_w^{\prime }$) was constantly maintained at 0.03 mm²/s, while the cutting speed varied within the range of 20 to 70 m/s. To achieve the same specific material removal rate in cylindrical grinding, the grinding parameters were adjusted, resulting in the following optimal values: a cutting speed (v_c) of 50 m/s, a radial feed rate (v_fr) of 5 µm/min, and a speed ratio (q_s) of 50.

The 3D topology and characteristics of the ground surfaces were measured using a confocal microscope (Nanofocus Mobile µsurf) (NanoFocus AG, Oberhausen, Germany) and analyzed with µsoft analysis premium 7.2.

3. Results

3.1. Actual Removal Depth

Figure 7 presents the impact of 50% laser structuring on the actual removal depth per grinding pass using a vitrified-bonded diamond grinding wheel at various cutting speeds. Laser-structured workpieces consistently exhibited higher actual depths compared to non-structured ones across all cutting speed ranges.

For both laser-structured and non-structured workpieces, the actual depth decreased with increasing cutting speed from 20 to 50 m/s and then increased at 70 m/s probably due to impact forces overcoming factors like elastic deformation and spring back. As the cutting speed increases, the beneficial effects of laser structuring only partially offset the decrease in the actual removal depth, without altering the overall decreasing trend. This could imply that the influences of vibrations, spring back, and elastic deformation, which contribute to the reduction in actual removal depth with increased cutting speed, cannot be fully compensated for by the effects of laser structuring alone.

3.2. Grinding Ratio (G-Ratio)

The investigation of the G-ratio, a critical parameter in the grinding of exceptionally hard materials, as shown in Figure 8, presented laser structuring as a promising approach for enhancing the G-ratio and grinding performance. Measurements were conducted using a Keyence digital microscope (VHX7000) at 100× magnification to determine the actual grinding area and depth for material removal calculation.

Increasing the cutting speed resulted in increased wheel wear for non-structured workpieces, reducing the actual removal depth and material removal volume while potentially increasing process temperature and vibrations. Laser-structured workpieces experienced intermittent cutting, leading to an increased actual removal depth despite reduced grinding forces, minimizing grinding wheel wear. Laser structuring increased the G-ratio by up to 247% compared to non-structured workpieces.

It is worth mentioning that, owing to the superior hardness of DSiCs, the G-ratio is significantly low. The impact of this low G-ratio is evident in Figure 7, where, among all tests, the maximum actual removal depth per pass is approximately 1.5 µm, representing a 25% reduction compared to the adjusted depth per pass (2 µm).

3.3. Ground Surface Roughness

Figure 9 presents the influence of cutting speed on surface roughness parameters (Sa, Sq, and Sz) for both laser-structured and non-structured workpieces. While increasing the cutting speed had minimal effect on the surface roughness for the non-structured workpieces, a slight deterioration was observed for the laser-structured workpieces when the cutting speed increased from 20 to 30 m/s. However, further increasing the cutting speed to 70 m/s led to a deterioration in the surface quality. This deterioration is attributed to enhanced intermittent cutting, increased process vibrations, and the self-sharpening of the grinding wheel at higher speeds, causing a microfracture wear of the grains and grain pull-out, which negatively affected surface quality.

3.4. Subsurface Damage

Figure 10 illustrates the subsurface damage evaluation of ground surface on both laser-structured and non-structured workpieces. This assessment was conducted using scanning electron microscopy (SEM) equipped with a focus ion beam (FIB) system. Cross-sections were cut by FIB and subsequently observed by SEM to any subsurface damage. The figure reveals that, under identical grinding conditions, the laser-structured workpiece exhibits no subsurface damage after the complete removal of laser structuring. In contrast, the non-structured workpieces display significant subsurface damage. This finding highlights the effectiveness of prior laser structuring in achieving a damage-free subsurface workpiece during grinding.

The mechanism of LAG process is closely linked to the surface morphology observed in Figure 10. During ultra-short-pulse laser structuring, controlled microstructural damage is introduced into the workpiece material, primarily in the form of micro-cracks and weakened zones. These laser-induced features facilitate crack propagation and localized brittle fracture during grinding, which reduces the energy required for material removal. Consequently, grinding forces are significantly reduced, minimizing the likelihood of subsurface damage such as residual stresses or plastic deformation.

Moreover, the grinding allowance is precisely calibrated to ensure that the laser-damaged layer is completely removed. This allowance prevents any residual damage from affecting the ground surface and subsurface integrity. As demonstrated through SEM imaging, the resulting ground surface is free of cracks and structural discontinuities.

In contrast, non-structured workpieces experience higher grinding forces, leading to substantial subsurface damage. Laser-structured workpieces, however, maintain superior subsurface integrity, as evidenced in Figure 10.

3.5. Cylindrical Grinding Surface Analysis

The external cylindrical grinding and laser structuring processes were used to create plateau surfaces on shaft bearing sleeves, employing controlled parameters as shown in Figure 11. This led to a distinctive surface morphology where diamond grains were flattened, and the surrounding SiC matrix was more extensively ground. The deeper grinding of the SiC matrix could be primarily due to the grinding wheel’s cutting edges penetrating these areas more deeply. The process of lapping, caused by diamond debris from both the workpiece and the grinding wheel in the contact zone, likely contributed to the observed outcome. This action leads to the formation of a plateau-like surface where diamond grains are flattened, and deeper sections of the SiC matrix create small pockets, or micro-reservoirs. These features enhance the tribological properties of the surface, meaning they help to reduce friction and wear. Such advantages are particularly beneficial for applications like shaft bearing sleeves where minimizing friction extends the component’s lifespan. This technology finds applications in automotive engine components, industrial bearings, and cutting tools, enhancing their performance and durability [21,22].

4. Conclusions

This study investigated the effects of laser structuring on the cylindrical grinding of silicon-carbide-bonded diamond materials (DSiCs), emphasizing its potential to address the challenges associated with grinding of superhard materials. The key findings demonstrated that laser-assisted grinding (LAG) using vitrified-bonded diamond grinding wheels effectively enhanced surface integrity, increased the G-ratio by up to 247%, and improved the actual material removal depth by 99% compared to conventional grinding methods.

The innovative application of ultra-short-pulse laser technology enabled the precise preconditioning of the material surface, facilitating controlled material removal mechanisms during grinding and achieving ground surfaces with damage-free subsurface conditions. The resulting plateau-like surfaces, characterized by reduced friction and enhanced tribological properties, hold significant promise for critical applications in subsea pump components, automotive engines, and industrial bearings.

Despite these advancements, challenges such as the low G-ratio inherent to DSiCs highlight the need for further research into more advanced grinding wheel materials and bonding technologies. Future investigations should focus on optimizing laser structuring parameters and assessing the long-term performance of plateau-ground surfaces under operational conditions.

By addressing these challenges, this research establishes a robust foundation for the industrial adoption of LAG, providing a sustainable and efficient solution for the grinding of superhard materials.