Study on the Rate of the Removal of Nano-Crystalline Diamond—Coated Materials by Femtosecond Laser Etching

Abstract

The rate of the removal of materials coated with nano−crystalline diamonds by femtosecond laser etching was examined by adjusting the repetition rate of the femtosecond laser, the energy−flux density of the concentrated spot, and the scanning speed. The observational results of the white-light interferometer and the numerical fitting approach were used to develop the removal rate function model of the nano-crystalline diamond-covered material etched by the femtosecond laser. The findings demonstrated that the rate of material removal was not greatly affected by the repetition frequency and that the amount of laser energy accumulated over time on the coated surface is steady. The processing outcomes under different laser scanning speeds are different, and the material removal rate tends to increase and then decrease with an increase in scanning speed. The greater the energy−flux density of the focused spot, the greater the etching intensity, and the greater the material removal rate. With an increase in scanning speed, the rate at which the material is removed often rises initially before falling.

1. Introduction

Diamond is widely used in a variety of applications, including precision cutting tools, optical devices for microelectronic devices, and biological fields, due to its excellent properties, such as high hardness, high strength, high thermal conductivity, extremely high electron mobility, excellent optical properties, stable chemical properties, and good biocompatibility. As a result, there is a growing need for diamond to be processed with extreme precision [1,2,3].

Currently, grinding, chemical–mechanical, thermochemical, ion beam, and laser processing are the primary procedures utilized to extract diamonds. The removal of materials occurs during the classic grinding process by friction between the workpiece and diamond particles fused to the grinding wheel. Low removal efficiency and significant grinding wheel wear are issues with diamond grinding because of the small hardness differential between the workpiece and the grinding wheel [4]. Chemical and mechanical action are used in chemical–mechanical machining to remove materials. Although it can produce a superior machining surface, grinding is more efficient. It only applies to the machining of shaped workpieces and has stringent standards for the workplace [5]. Although the processing efficiency of thermochemical processing has significantly increased, this technology still requires further optimization due to its low stability, high cost, and sophisticated equipment as well as its sensitivity to temperature and ambient gas [6]. The ion beam sputtering technique can produce finely machined surfaces, but its promotion and use are constrained by its expensive equipment and slow material removal rate [7]. Because of its low cost, straightforward procedure, and high etching rate, laser processing is commonly employed in the production of diamonds. A femtosecond laser differs from conventional lasers in that it has a short pulse duration and high instantaneous power. Due to its small heat−affected zone, high accuracy, and lack of shock wave cracks in the processing area, femtosecond laser processing is widely employed in precision and special machining applications [8,9].

In recent years, there has been an increase in the use of femtosecond laser processing technology in the bulk processing, surface microstructure processing, and surface modification of various materials, including metal materials. This is due to its unique machining mechanism, with high-quality machining results that can be used for the machining of non−metallic organic materials [10,11,12], polymeric substances [13], and semiconductor materials [14], and excellent properties such as high machining accuracy [15,16,17]. Due to the femtosecond laser machining technique’s extremely high spatial resolution, femtosecond laser machining of diamond and other super-hard materials has great efficiency, good processing quality, and high flexibility.

In Shrestha et al.’s study [18], although CVD diamond microtools can be made by precision grinding, they do not have high machining capacity. Laser machining is one of the most important methods for diamond machining because it offers the advantages of precision directivity, capability, and a small heat-impacted zone [19,20]. Diamond arrays were brazed onto a stainless-steel substrate by Dong et al. [21] using 40 fs and 5 ns laser pulses. The test results showed that the femtosecond laser’s threshold for processing diamond is lower than that of a nanosecond laser because of its thermal inhibition. In a study by a team from Keio University in Japan [22], an adhesive-free polycrystalline diamond micro ball end milling cutter was successfully developed using a circularly polarized femtosecond pulse laser. A study team at Hunan University investigated the mechanisms of the femtosecond laser etching of single-crystal diamond materials and the use of a micro-texture array in the end face of a single crystal diamond grinding tool [23,24]. By utilizing the best laser average power and scanning times identified through numerical fitting, the team was able to successfully create a new type of bond-free diamond end grinding tool with good surface qualities and high profile accuracy, with the top inclination angle of the abrasive grains being less than 90°, and the arrangement of an abrasive grain array. In addition to being used directly as the material of micro tools and abrasive tools, diamond’s ultra-high hardness can be employed as a protective coating to improve the friction performance and wear resistance of the working surfaces of tools and abrasive tools. Diamond film production prices, film−forming quality, and preparation efficiency have all drastically increased as chemical vapor deposition (CVD) technology has progressed. Providing protective coatings to a range of precisely shaped instruments using CVD diamond films has proven to have excellent performance and a wealth of R&D potential [25] and wire drawing dies [26,27]. In order to further improve the tribological characteristics and wettability of coated or super-hard tools, reduce the cutting force and temperature, and shorten the chip contact length, there has been an increase in research into the use of the femtosecond laser modification principle [28,29]. Numerous studies have been conducted on the output power and scanning speed of femtosecond lasers. Positive application outcomes have been achieved by studying the impact of scanning durations [30,31], target surface micro-textures [32,33,34], and even the microstructure scale on aspects of tool processing.

With the in-depth study of femtosecond laser etching of diamond and the wide application of diamond−coated tools in recent years, it is of profound significance to study the influence law of femtosecond laser processing parameters on the diamond surface. Therefore in order to investigate the material removal mechanism under various process parameters, the effects of parameters such as the laser repetition frequency, energy−flux density of the focused spot, and scanning speed on the material removal rate of the coated surface were examined in this paper. Nano-crystalline diamond coating material was etched by a femtosecond laser to produce a coating.

2. Materials and Methods

2.1. Preparation of Diamond Coating

The Key Open Laboratory of Precision Manufacturing Technology and Engineering at Henan University of Technology donated the diamond coating samples. YG6 is the substrate, while WCo is 6%. The hot filament chemical vapor deposition procedure was used to coat the substrate with a layer of nano−crystalline diamond (NCD), and the deposition parameters are shown in

Table 1. Deposition parameters of NCD.

Index	Deposition Parameters
Deposition pressure of chemical vapor (KPa) Acetone/hydrogen/argon (nmL · min−1)	1.8 50/250/250
Applied voltage of tantalum wire (V) Additional bias current (A) Tantalum wire–substrate spacing (mm) Temperature of vapor deposition (°C) Substrate temperature (°C) Time of vapor deposition (h)	20 2 10 450 700 7

. The dimensions of the full sample are 16 × 16 × 4 mm. According to Figure 1, which shows the diamond coating surface and cross-sectional microscopic morphology, the diamond’s thickness is 8 μm and its average grain size is about 100 nm.

2.2. Surface Roughness of the Coating

A 3D white-light interferometric surface profiler was used to quantify the diamond coating’s surface roughness over a 240 × 300 μm² scanning area. The outcomes are displayed in Figure 2. The average roughness Sa of the diamond–coated surface is approximately 0.5686 μm.

2.3. Processing Equipment and Process

The laser has a core wavelength of 800 nm, a pulse width of 104 fs, and a spot diameter of 30 μm. It is a titanium-doped sapphire femtosecond laser from Coherent in the CA, USA. Figure 3 presents the laser processing equipment. The confocal microprocessing system receives the femtosecond laser pulse after it has passed through the half-wave, attenuator, high-climbing reflector, and shutter. The laser pulse is then focused by the objective lens and irradiated vertically onto the diamond coating surface. By adjusting the laser’s repetition frequency (100 Hz–10 kHz), spot energy density (14.2–42.5 J/cm²), and scanning speed (0.1–2 mm/s), the diamond-coated surface can be etched. We cleaned the laser–treated specimen by dipping it into acetone solution. We cleaned the surface of the sample of any remaining dirt. A ContourGTK white-light interferometer from the American Bruker Company (DE, USA) was then used to measure the etching depth and width of the coating surface, and the effects of various parameters on the rate of material removal from the diamond coating surface and its change rule were investigated.

2.4. Modelling of Material Removal Rates and Research on Material Removal Methods

2.4.1. Investigation of the Material Removal Process

The energy of a single femtosecond laser pulse follows a Gaussian distribution (as depicted in Figure 4), with a central region where it is concentrated and an edge region where it is less evenly distributed. When a coating material is etched with a femtosecond laser, some of the thermal radiation will be reflected by the material’s surface, while the remainder will pass through the material’s surface into its interior and gradually absorb. The thermal radiation that is not completely absorbed will penetrate the material and create a transmission. The propagation of heat radiation in a material has a defined penetration depth, or thermal penetration depth, for materials with a particular volume and form [35]. The femtosecond laser can be used to selectively fine–process the coating without affecting the coating substrate if the diamond coating thickness is more than its own thermal penetration depth. Without affecting the coating substrate, the femtosecond laser can be utilized to selectively refine the coating.

Due to the narrow pulse width of the femtosecond laser, the diamond coating can be assumed not to transfer energy to the lattice and the lattice temperature remains constant under the action of the femtosecond laser [36]. However, temperature impacts cannot be avoided, and the coating’s inside component grows. Inside the coating, multiphoton ionization and free electron heating take place. On the surface and bottom layers, laser pulses excite electrons by multiphoton ionization, the initial formation of electrons in a few femtoseconds to obtain a high kinetic energy, with a certain kinetic energy of free electrons can diffuse in the material to a certain depth, causing strong ionization, which turns the substance into a dense plasma [37]. The material can quickly acquire a significant number of high−temperature free electrons due to multiphoton ionization. Some surface−ejected material collects in the vicinity as a result of a shock wave or high pressure produced by a fast rise in temperature in a confined volume. The area of the boiling zone and the volume of material ejected both grow as the incident laser energy rises, along with the production of plasma. The interaction between the laser and plasma causes a phase transition, which results in the removal of the material.

The ablation threshold for the femtosecond laser etching of diamond coatings is clearly defined, and local energy deposition can be carried out fast and effectively while the material is being treated. When the actual energy−flux density exceeds the threshold energy density and the energy deposition reaches a particular level, ablation takes place, irreversibly altering the structure of the material.

Therefore, it is crucial to evaluate the correlation between the femtosecond laser energy density and ablation threshold. A femtosecond laser pulse’s energy has a Gaussian distribution, and the energy−flux F at various points inside a pulse can be written as

$$\mathrm{F}={\mathrm{F}}_0\exp \left(-\frac{8{\mathrm{r}}^2}{{\mathsf{\omega }}_{\mathrm{d}}^2}\right)$$

(1)

where r is the distance from the measured position to the center of the light spot and F₀ is the peak energy−flux. Compared to single-pulse ablation [38], the energy density F_th (N) of multi-pulse ablation has a cumulative impact and is denoted by

$${\mathrm{F}}_{\mathrm{t}\mathrm{h}}\left(\mathrm{N}\right)=\mathrm{F}·{\mathrm{N}}^{\mathrm{S}-1}$$

(2)

where S is the accumulation factor and N is the number of pulses. In the scan path of the etched diamond coating, the actual energy−flux density F_a [39] can be represented as

$${\mathrm{F}}_{\mathrm{a}}=\frac{8\mathrm{P}}{\mathsf{\pi }\mathrm{f}{\mathrm{w}}_{\mathrm{d}}^2}$$

(3)

where P stands for laser power and f stands for laser repetition frequency.

2.4.2. Material Removal Rate Modelling

The number of laser pulses given to the same location determines the laser’s ablation threshold, as has been demonstrated [40]. The ratio of the spot diameter to the length d of the overlapping region between two adjacent pulses along a linear scan is known as the spot overlap ratio. The number of laser pulses per etched region at once increases as the spot overlap ratio of the laser increases; as a result, the overlap ratio of the pulses might affect the laser’s ablation threshold. A schematic illustration of the spot overlap rate is shown in Figure 5. The time between two adjacent pulses can be roughly calculated as the reciprocal of the repetition frequency since the duration of a femtosecond laser pulse is significantly shorter than the space between adjacent pulses.

$$\mathrm{v}·\frac{1}{\mathrm{f}}={\mathsf{\omega }}_{\mathrm{d}}(1-\mathsf{\phi })$$

(4)

where v is the scanning speed of the laser and φ is the spot overlap rate. From Equation (4), the spot overlap rate can be obtained:

$$\mathsf{\phi }=(1-\frac{\mathrm{v}}{{\mathsf{\omega }}_{\mathrm{d}}\mathrm{f}})·100\%$$

(5)

Equation (5) demonstrates how the laser repetition frequency and scanning speed might affect the laser ablation threshold. It takes a significant amount of work to optimize the laser processing parameters to acquire the desired material removal effect. By adjusting the repetition frequency, energy−flux density, and scanning speed of the femtosecond laser, this experiment examines the impact of each femtosecond laser parameter on the rate of material removal from nano−crystalline diamond coatings.

The amount of material removed per unit of time is known as the material removal rate. A mathematical model can be created to determine the material removal rate under various laser processing parameters in order to analyze the rate of material removal caused by a femtosecond laser, as shown in Figure 6 below:

The removal rate for the material under various laser processing settings is calculated by mathematically modelling the morphology of the laser ablation kerf, and the fitted functional model is integrated over the width. The expression for the removal rate of the material is provided below:

$$\mathrm{f}=[\mathrm{a}\mathrm{b}-2{\int }_0^{\frac{\mathrm{a}}{2}}\mathrm{f}\left(\mathrm{x}\right)\mathrm{d}\mathrm{x}]\mathrm{v}$$

(6)

where f is the material removal rate; a, b is the width of the ablation area and the ablation depth; $\mathrm{f}$($\mathrm{x}$) is the fitting function for the cut-out profile; and v is the laser scanning speed.

3. Results

3.1. Effect of Laser Repetition Frequency on the Material Removal Rate

The nano−crystalline diamond coating was etched using repetition frequencies of 100 Hz, 500 Hz, 1000 Hz, and 10 kHz when the energy−flux density of the focused spot was 14.2 J/cm² and 56.6 J/cm². The morphology of the white−light interferometer scan is shown in Figure 7.

As shown in Figure 7, the effect of repetition frequency on the etch profile is minimal. As the repetition frequency increases from 100 Hz to 10 kHz, there is no appreciable change in the etch width, but there is a slight increase in the etch depth, and the center of the ablation pits becomes rougher rather than more uniform. This conclusion is supported by the data in the bar chart, which show that the etch depth slowly increases while the etch breadth maintains a steady value at various repetition frequencies. The etching depth rose from 2.09 μm to 2.25 μm at a 14.2 J/cm² energy density and from 2.81 μm to 3.08 μm at a 56.6 J/cm² energy density. Due to the laser beam’s Gaussian distribution, as the repetition frequency rises, the ablation in the core region becomes more pronounced and the etching depth increases correspondingly. When the femtosecond laser interacts with the material at high repetition frequencies (>200 kHz), a thermal accumulation effect is created [41], which has a significant impact on the morphology of femtosecond−laser−induced micro and nanostructures but is less noticeable at lower repetition frequencies. This is brought on by the lengthy delay between femtosecond laser pulses with low repetition frequencies. The temperature in the laser focus zone has already decreased to room temperature by the time the following pulse reaches the material, and this little thermal accumulation effect also causes the etching depth to significantly deepen with increased repetition frequency.

The material removal rate at various repetition frequencies is calculated using the data in Figure 8 and the mathematical model for the material removal rate; the resulting curves are displayed in Figure 9.

The etch depth increases gradually as the femtosecond laser’s repetition frequency rises. However, as shown in Figure 9, the repetition frequency’s overall impact on the material removal rate is negligible, and the rate is still rather steady. Approximately 1.4 × 10⁵ μm³/s of material is removed at an energy density of 14.2 J/cm², whereas 2.25 × 10⁵ μm³/s of material is removed at an energy density of 56.6 J/cm². The amount of energy that the various materials absorb from the laser varies. There are fewer effective pulses per unit area and higher single pulse energies at lower repetition frequencies. A dynamic ablation equilibrium can occur as the repetition frequency rises, which results in a continual drop in the single-pulse impulse energy but an increase in the effective number of pulses per unit area. This process maintains the energy received by the material per unit of time constant, reducing the impact of the laser’s repetition frequency on the rate of material removal.

In order to study the effect of laser repetition frequency on the material removal rate, the data in Figure 9 were nonlinearly fitted to make the Asymptotic function in Exponential type, where the independent variable is the repetition frequency and the dependent variable is the material removal rate. The corresponding fitting equations are shown in Equation (7) when the energy−flux density is 56.6 J/cm² and in Equation (8) when the energy−flux density is 14.2 J/cm².

$$\mathrm{y}=\mathrm{240,099.899}-\mathrm{20,449.467}\times {0.999}^{\mathrm{x}}$$

(7)

$$\mathrm{y}=\mathrm{153,442.415}-\mathrm{25,098.606}\times {0.999}^{\mathrm{x}}$$

(8)

From the fitting curve in Figure 9, it can be seen that when the energy−flux density of the focused spot and the scanning speed are certain, due to the ablation equilibrium, the changes in the ablation width and ablation depth of the diamond film at different repetition frequencies are not obvious, and the removal rate of the material tends to be stabilized.

3.2. Effect of the Focused Spot’s Energy−Flux Density on the Pace of Material Removal

For etching of the nano-crystalline diamond coatings, energy−flux densities of 14.2 J/cm², 19.8 J/cm², 22.7 J/cm², 28.3 J/cm², 34.0 J/cm², and 42.5 J/cm² were chosen when the laser repetition frequency was 1 kHz and the scanning speed was 1 mm/s.

From Equation (3), the relationship between the focused spot energy−flux density and the laser output power can be calculated, and the results are shown in

Table 2. Laser output powers under different energy−flux densities of focusing spot.

Power (mW)	50	70	80	100	120	150
Energy−flux density (J·cm−2)	14.2	19.8	22.7	28.3	34.0	42.5

.

Figure 10 depicts the morphology of the white−light interferometer scan made possible by the femtosecond laser etching of diamond coatings with various energy−flux densities. Figure 11 displays the appropriate etches’ widths and depths. As demonstrated in Figure 10, the area of the etched area grows as the energy−flux density rises, and the border between the etched area and the surrounding area becomes increasingly evident, exhibiting deeper and deeper ablation holes. Figure 11 displays the etching. With a rise in energy density, the etching’s width and depth considerably increase. When the energy−flux density reaches 42.5 J/cm², the material’s etching width and depth considerably increase. The material has an etching breadth and depth of 128.2 μm and 4.32 μm, respectively. These values are considerably higher than those of the other energy densities. The amount of material accumulating in the laser scanning path is minimal when the concentrated spot energy−flux density (14.2–22.7 J/cm²) is low. The material removal mechanism at this point is primarily a Coulomb explosion [42]; when the focused spot energy−flux density is low (14.2–22.7 J/cm²), the material accumulates less energy in the laser scanning path, resulting in a smaller volume of diamond reaching the ablation threshold per unit time and, therefore, a smaller etch width and depth. The electrostatic field on the coated surface becomes unstable as a result of femtosecond excitation, and a Coulomb explosion happens if the electric field strength is greater than the binding energy of the target atoms. Electron–hole pair excitation trajectories with a charge imbalance are also produced. The mass is only slightly reduced by the typical melting and vaporization of the etched area. As the energy−flux density rises, the laser power density rises, the etching intensity rises, and the etching depth and etching width rise significantly. The role of phase explosion and vaporization predominates at greater energy−flux densities [43]. The laser-irradiated region enters the phase diagram sub−stable region when the critical temperature is approached and the thermodynamic stability limit is met.

The superheated liquid changes into a mixture of liquid and vapor and evaporates pretty quickly when the liquid temperature reaches a critical threshold. After evaporation removes a small amount of material, the phase explosion continues with thermal evaporation, leading to significant material removal and disruption of the nanoscale structure.

The material removal rates for various energy−flux densities were calculated using the data in Figure 11 and the material removal rate mathematical model; the resulting curves are presented in Figure 12. As shown in Figure 12, the rate of material removal rises as the energy−flux density rises, and the rise in the rate progressively accelerates. The diamond coating material is removed at a rate of 3.64 × 10⁵ μm³/s when the energy−flux density reaches 42.5 J/cm².

In order to investigate the effect of energy−flux density on material removal rate, the data in Figure 12 were nonlinearly fitted using the Stirling function of Exponential type, where the independent variable is energy−flux density and the dependent variable is material removal rate, and the resulting functional equation of the fit is shown in Equation (9).

$$\mathrm{y}=\mathrm{65,826.701}+\frac{\mathrm{10,623.867}\times ({\mathrm{e}}^{0.428\times \mathrm{x}}-1)}{0.428}$$

(9)

From the fitting curve in Figure 12, it can be seen that when the femtosecond laser repetition frequency and scanning speed are stable, with the increase of the focused spot energy−flux density, the intensity of the femtosecond laser etching increases, the width of the etching and the ablation depth increase significantly, and the material removal rate also improves, and the growth rate becomes faster.

3.3. Effect of Laser Scanning Speed on Material Removal Rate

At repetition frequencies of 1 KHz and scanning speeds of 0.1 mm/s, 0.3 mm/s, 0.5 mm/s, 1.0 mm/s, 1.3 mm/s, 1.5 mm/s, 1.7 mm/s, and 2.0 mm/s, the nano-crystalline diamond coatings were etched at energy−flux densities of 14.2 J/cm² and 56.6 J/cm². Figure 13 displays the white-light interferometer scans of the diamond covering at various scanning speeds. Figure 14 displays the appropriate etches’ widths and depths.

As shown in Figure 13, as the scanning speed is increased, the area of the ablation crater gradually shrinks. The bottom of the ablation crater exhibits a clear wave structure when the scanning speed is modest (0.1−0.5 mm/s) and the energy−flux density is 14.2 J/cm². This is because the rate of excessive spot overlap causes the coating surface to gather too much laser energy, the area of the effective spot is larger, causing the phenomenon of over-melting, and the speed of laser scanning cannot keep up with the rate of material ablation. Additionally, the morphological characteristics of ablation are more obvious, with uneven edges.

From Figure 14, it can be inferred that when the energy−flux density is 14.2 J/cm² and 56.6 J/cm², the etch width does not significantly change with the scanning speed. This is because the laser’s energy has a Gaussian distribution, and because changing the scanning speed has no effect on the laser’s energy density, the width of the ablation pits in the coating cross-section remains constant. The laser spot overlap rate is high when the scanning speed is slow. The number of pulses the laser emits to the coating surface when scanning the same path is high, and the accumulated energy in the same region conducts deep within the coating, producing a deep etching depth. By using a femtosecond laser to etch the diamond coating, increasing the scanning speed will increase the temperature of the diamond coating’s surface. This will also hasten the vaporization of graphite on the diamond coating’s surface, which will absorb some of the energy and ionize to create a dense plasma with a shielding effect on the film’s surface [44,45]. The shielding of the plasma effect causes the absorption of the pulse by the material to decrease as the plasma concentration increases; hence, as the scanning velocity rises, the etching depth of the material tends to decrease.

From Figure 14, it can be inferred that when the energy−flux density is 56.6 J/cm² and the scanning speed is 0.1 mm/s and 0.3 mm/s, the laser has penetrated the coating and etched into the substrate and the diamond coating is being etched to a depth greater than 15 μm. As a result, the etching results for these two instances are invalid for calculating the material removal rate. The material removal rates at various scanning speeds were calculated using the valid data in Figure 14 and the material removal rate mathematical model, and the resulting curves are displayed in Figure 15.

The area of the incised shape and the scanning speed must be used to calculate the material removal rate. The ablation area is not the highest when the speed is 1 mm/s, but the product of the area and the speed is larger, leading to a higher rate of material removal. The material removal rates were 3.26 × 10⁵ μm³/s and 1.7 × 10⁵ μm³/s at 1 mm/s when the energy−flux density was 14.2 J/cm² and 56.6 J/cm², respectively, which were faster than the results at other scanning speeds. The best removal effect was attained at a speed of 1 mm/s, as illustrated in Figure 15. The material removal rate tends to increase and then drop as the scanning speed increases.

In order to investigate the effect of scanning speed on material removal rate, the data in Figure 15 were nonlinearly fitted using the GaussAmp function of Gauss type, where the independent variable is the scanning speed and the dependent variable is the material removal rate. The corresponding fitting equations are shown in Equation (10) when the laser power is 200 mW and in Equation (11) when the laser power is 100 mW.

$$\mathrm{y}=\mathrm{247,082.764}+\mathrm{81,225.199}\times {\mathrm{e}}^{{(-0.5\times \frac{\mathrm{x}-1.071}{0.353})}^2}$$

(10)

$$\mathrm{y}=\mathrm{80,761.213}+\mathrm{93,213.034}\times {\mathrm{e}}^{{(-0.5\times \frac{\mathrm{x}-0.971}{0.392})}^2}$$

(11)

From the fitting curve in Figure 15, it can be seen that when the energy−flux density of the focused spot and the repetition frequency is certain, with the increase in scanning speed, the volume of material removal per unit of time increases and then decreases, and the material removal rate is maximum when the scanning speed is around 1 mm/s.

4. Conclusions

The single-factor variable test method was used to examine the impacts of the repetition frequency of the femtosecond laser, the energy−flux density of the focused spot, and the scanning speed on the material removal rate for the diamond film surface.

(1)

The diamond film’s ablation width and depth at various repetition frequencies do not fluctuate much due to the ablation equilibrium, and the material removal rate tends to be constant when the energy−flux density of the focused point and the scanning speed are known.

(2)

After determining the femtosecond laser repetition frequency and scanning speed, the focused spot energy−flux density increases, increasing the femtosecond laser’s etching intensity. This increases the etching width and ablation depth significantly, as well as the rate at which material is removed.

(3)

The spot overlap rate reduces as the scanning speed increases, but the laser energy density does not change; therefore, the etching width does not change significantly when the energy−flux density and repetition frequency of the focused spot are known. Due to the plasma shielding effect and the decreased spot overlap rate, the etching depth of the film is greatly lowered. The volume of material removed increases and subsequently falls as the scanning speed increases; the material removal rate is highest at 1 mm/s.