CN105181616B - A kind of measuring method of laser ablation process plasma absorptivity - Google Patents

Abstract

The invention provides a method for measuring plasma absorptivity in the process of laser ablation, which converts the measurement of plasma absorptivity into the measurement of the shape size of micro-cavities obtained by laser ablation. First, on the flat and clean target surface, a series of micro-cavities are processed with a specific pulse laser, and the laser energy density is uniformly changed during the processing; then, the diameter and depth of the micro-cavities are measured by a three-dimensional topography instrument. The laser ablation threshold F _th is obtained from the diameter-laser energy density curve; then combined with the depth of the micro-cavity, the effective coefficient α of the surface material is obtained by theoretical calculation, and then the plasma absorption rate b during the laser ablation process is obtained; this method is easy to operate, The measured result is stable and reliable, and has a wide range of applications.

Description

A method for measuring plasma absorption rate in laser ablation process

technical field

The invention relates to a method for measuring plasma absorptivity during laser ablation, in particular to a method for measuring laser plasma absorptivity by measuring the size of a micro-cavity.

Background technique

Since the advent of lasers, lasers have been widely used in various industries. Among them, the characteristics and rules of the interaction between laser and various materials have always been the research hotspot of many scholars. When the high-power laser beam is incident on the surface of the target, the surface of the target absorbs the laser energy, the temperature rises rapidly, reaches the boiling point, and the gasification phenomenon suddenly intensifies. As the temperature continues to rise, the gasified material continues to absorb the laser energy to complete ionization. Then a high-temperature and high-density plasma is formed. High-temperature and high-density plasma has a strong absorption effect on incident laser light. For most laser processing, this absorption effect will prevent part of the laser energy from reaching the target surface, shielding the energy coupling between the laser beam and the target surface, reducing its processing efficiency, and even leading to processing failure. Accurate and reliable measurement of the laser absorption rate of plasma is very important for the study of the law of interaction between light and matter, as well as the development and application of laser processing technology.

However, the interaction between plasma and laser is a very complicated process. Different laser parameters (such as wavelength, pulse width, power, etc.), targets, and ambient gases will affect the interaction between laser and target, and further affect the plasma's effect on the laser. absorption rate. At the same time, the absorption of laser by plasma is an instantaneous process. The above factors make it extremely difficult to obtain an accurate plasma absorption rate. In order to solve the above problems, we invented a simple and feasible method for experimental measurement of plasma absorption rate in the process of laser ablation.

Contents of the invention

The purpose of the present invention is to provide a simple, reliable and accurate measurement method of laser plasma absorption rate in order to solve the deficiencies in the prior art.

The technical solution of the present invention is: a method for measuring plasma absorption rate in the process of laser ablation, comprising the following steps:

S1. Select a target material and smooth and clean the surface of the target material, so that the surface roughness Ra of the target material surface is 0.05-0.1 μm;

S2. Using pulsed laser to ablate a series of micro concave cavities on the surface of the target. During the processing, the laser energy density F increases or decreases regularly;

S3. Using a three-dimensional shape analyzer to measure the diameter D and depth h of the micro-cavity obtained by the ablation in S2;

S4. In a two-dimensional coordinate system where the abscissa is the laser energy density F and the ordinate is the cavity diameter D, plot the data points of the energy density F and the corresponding micro-cavity diameter D, and use the logarithmic relationship Fitting a function curve; the intersection point of the function curve and the abscissa is the ablation threshold F _th corresponding to the pulse laser ablation of the target; at the same time, calculate the target surface material in the pulse laser Effective coefficient α in the ablation process;

S5. Deduce the plasma absorption rate b during the laser ablation process according to the depth h of the micro-cavity, the ablation threshold F _th , the effective coefficient α, and the laser energy density.

In the above solution, the step S1 specifically includes the following steps:

1), using metallographic sandpaper to polish the target material, so that the surface roughness Ra of the target material reaches 0.05-0.1 μm;

2) Wipe the surface of the target block to be processed with a cotton ball soaked in absolute alcohol.

In the above solution, the step S2 specifically includes the following steps:

3), fix the target on the workbench, focus the pulsed laser on the surface of the target to be processed, the focused laser spot diameter ω0 is _0.001-1mm , the The wavelength of the pulse laser is 193nm-10.6μm, the pulse width is 1fs-1ms, and the single pulse energy E is 1μJ-100J;

4), by changing the single pulse energy E of the pulse laser, changing the laser energy density F, the relationship between the single pulse energy E and the laser energy density F is: A series of micro-recessed cavities are ablated on the surface of the target by using the laser pulse with the change of the laser energy density F. During the processing, the value of the laser energy density F conforms to a geometric sequence or an arithmetic sequence Distribution; the number of values of the laser energy density F is equal to the number of a series of micro-cavities; the number of the series of micro-cavities is greater than or equal to 5;

5), post-treatment of the target: removing the slag on the surface of the target and polishing;

6) Put the polished target block into a measuring cup filled with absolute ethanol, place it in an ultrasonic cleaning machine for cleaning, take it out and dry it for testing after cleaning.

In the above solution, the step S3 specifically includes the following steps:

7) Before measuring the shape of the micro-cavity, first wipe the surface of the target block with a cotton ball soaked in absolute alcohol;

8) Using a WYKO-NT1100 surface three-dimensional topography measuring instrument to measure the geometric topography of the surface texture of the micro-cavity, including the diameter D and depth h of the micro-cavity.

In the above scheme, in the step S4, the method for obtaining the effective coefficient α is: through the obtained laser ablation threshold F _th , determine the energy density F ₀ that does not generate ablation plasma, and the energy density F ₀ The value range of is (F _th ,3F _th ]; the micro-cavity is processed by pulsed laser with energy density F ₀ , and the depth of the micro-cavity is measured as h ₀ , according to the formula Obtain the effective coefficient α of the surface material of the target.

In the above scheme, the formula for deriving the ion absorption rate b in the step S5 is:

Among them, b is the laser plasma absorption rate;

F _th is the laser ablation threshold;

α is an effective coefficient, and the effective coefficient is an optical absorption coefficient or a thermal permeability coefficient;

F is the laser energy density;

h is the depth of the micro-cavity corresponding to the laser energy density.

In the above solution, the energy density distribution at the focused spot of the pulsed laser is a two-dimensional Gaussian distribution.

The present invention is compared with the prior art in that the present invention converts the measurement of the absorption rate into the measurement of the appearance size of the micro concave cavity obtained by ablation. First, on the flat and clean target surface, a series of micro-cavities are processed with a specific pulse laser, and the laser energy density F is uniformly changed during the processing; then, the diameter D and Depth h, the laser ablation threshold F _th is obtained through the diameter-laser energy density curve; then combined with the depth h of the micro-cavity, the effective coefficient α of the surface material is obtained by theoretical calculation, and then the plasma absorption rate b during the laser ablation process is obtained; The method is easy to operate, the measured results are stable and reliable, and has a wide range of applications, and is very suitable for the monitoring and control of laser processing in industrial applications and the research of photoinduced plasma in scientific research.

Description of drawings

FIG. 1 is a schematic flowchart of a measuring method according to an embodiment of the present invention.

Fig. 2 is a fitting curve diagram of the micro-cavity diameter D and the laser energy density F of an embodiment of the present invention.

Fig. 3 is a graph showing the variation of laser plasma absorption rate b with laser energy density F according to an embodiment of the present invention.

detailed description

The present invention will be described in further detail below in conjunction with the specific embodiments of the accompanying drawings, but the protection scope of the present invention is not limited thereto.

The laser micromachining equipment used in this embodiment includes a pulsed Nd:YAG laser, a collimating beam expander, a 45° total reflection mirror, a focusing lens and a workbench. The laser beam emitted from the pulsed Nd:YAG laser passes through a collimating beam expander, a 45° mirror, and a focusing lens in sequence, and finally focuses on the target surface on the workbench. The shape measuring equipment is WYKO-NT1100 surface three-dimensional shape measuring instrument.

Taking the 45 ^# steel target as an example, the measurement method of laser plasma is shown in Figure 1, including the following steps:

S1. Select 45 ^# steel as the target material and smooth and clean the surface of the target material so that the surface roughness Ra of the target material surface is 0.05-0.1 μm. The specific steps are:

Select 45 ^# steel and use 250 ^# , 600 ^# , 800 ^# , 1200 ^# and 1500 ^# metallographic sandpaper to polish the surface of the target material so that the surface roughness Ra of the target material surface is 0.05-0.1 μm; Then wipe the surface of the target block with a cotton ball soaked in absolute alcohol to be processed.

S2. Use a specific pulse laser to ablate a series of micro-cavities on the surface of 45 ^# steel. During the processing, the laser energy density F increases or decreases regularly, and the number of micro-cavities is greater than or equal to 5. The specific steps are as follows: :

1) Fix the 45 ^# steel on the workbench, focus the specific pulse laser on the surface of the 45 ^# steel to be processed, the laser spot diameter _ω0 after focusing is 60μm, the selected pulse laser wavelength is 532nm, and the pulse width is 200ns, single pulse energy is 0.1mJ-5mJ;

2) By changing the single pulse energy E of the pulsed laser, the laser energy density F is changed, and the relationship between the two is: Specifically, the method of changing the single pulse energy E of the pulsed laser is to adjust the pulse frequency or change the average output power; select a group of single pulse energy E to be 0.2mJ, 0.4mJ, 0.7mJ, 1.1mJ, 1.6mJ, 2.2mJ , 2.9mJ, 3.7mJ, 4.6mJ, according to the formula It can be calculated that the laser energy density F is 14.15J/cm ² , 28.31J/cm ² , 49.54J/cm ² , 77.85J/cm ² , 113.23J/cm ² , 155.70J/cm ² , 205.24J/cm ² , 261.85J/cm ² , 325.55J/cm ² ; then turn on the laser micromachining equipment, and process micron-scale concave cavity morphology on the surface of 45 ^# steel;

3) Post-treatment of the target: firstly use 1500 ^# metallographic sandpaper to polish the surface of the 45 ^# steel to remove the surface slag, then use a diamond polishing agent with a particle size of 0.5um, and polish it on a polishing machine for 10-20min;

4) Put the polished target block into a measuring cup filled with absolute ethanol, place it in an ultrasonic cleaner for 20 minutes, take it out after cleaning, blow dry it, number it, and place it in a drying dish for detection.

S3. Measure the diameter D and depth h of the micro-cavity obtained by ablation in step S2 with a shape analyzer. The specific steps are:

5) Before measuring the shape of the micro-recessed cavity, first wipe the surface of the 45 ^# steel target block with a cotton ball soaked in absolute alcohol;

6) Using a WYKO-NT1100 surface three-dimensional topography measuring instrument to measure the geometric topography of the surface texture of the micro-cavity, including the diameter D and depth h of the micro-cavity.

S4. In the two-dimensional coordinate system where the abscissa is the laser energy density F and the ordinate is the cavity diameter D, plot the energy density F and the corresponding data points of the micro-cavity diameter D, and use the logarithmic relationship Fitting function curve; The intersection point of described function curve and abscissa is the ablation threshold F _th of described pulse laser ablation 45 ^# steel; Calculate simultaneously the effective effect of 45 ^# steel surface material in described pulse laser ablation process Coefficient α, the specific steps are as follows:

7) Calculating the laser ablation threshold _Fth of 45 ^# steel: In the two-dimensional coordinate system where the abscissa is the laser energy density F and the ordinate is the cavity diameter D, draw the energy density F in the step S3 and the corresponding The cavity diameter D corresponds to each point, and the function curve is fitted through the logarithmic relationship. The fitting curve is shown in Figure 2, and the equation corresponding to the curve is as follows:

D=25.161ln(F)-48.467 (1)

The intersection point of the curve and the abscissa is the laser ablation threshold F _th of 45 ^# steel = 6.86J/cm ² ;

8) Take the single pulse energy E ₀ =0.2mJ, the laser energy density F ₀ =14.15J/cm ² , and the ablation micro-cavity depth h ₀ =1.89μm for 45 ^# steel at the energy density F ₀ =14.15J/cm2. Since the energy density F ₀ is small, it can be considered that no plasma is formed on the surface of the material. At this time, it can be considered that the plasma has no significant absorption loss for the incident laser energy, and the actual energy density incident on the material surface is the energy density F ₀ , so The relationship between the depth h ₀ of the ablation micro-cavity of 45 ^# steel at the energy density F ₀ and the actual energy density F ₀ incident on the surface of the material is:

Wherein α is an effective coefficient, and the effective coefficient is an optical absorption coefficient or a thermal permeability coefficient. When the pulse width is long and the thermal effect is obvious, it is expressed by the thermal permeability coefficient, and when an ultrashort pulse is used, it is expressed by the optical absorption coefficient.

Substituting the energy density F ₀ and the micro-cavity depth h ₀ into the formula (2), the effective coefficient α=0.493μm ^-1 of the 45 ^# steel material is obtained.

S5. Deduce the plasma absorptivity during the laser ablation process under a specific laser energy density through the micro-cavity depth h, the ablation threshold F _th and the effective coefficient α, and the specific laser energy density conditions b. The specific steps are:

When the laser energy density F is greater than 3F _th , an obvious plasma will be formed on the surface of the material. Therefore, the energy density of the laser penetrating the plasma to reach the surface of the material is:

F _inc = (1-b)F (3)

Wherein, F _inc is the energy density of the pulsed laser incident on the surface of the material, b is the absorption rate of the laser by the plasma, and the relationship between the ablation depth h of the target and the actual energy density F _inc incident on the surface of the material is:

Wherein α is an effective coefficient, and the effective coefficient is an optical absorption coefficient or a thermal permeability coefficient.

Combining formulas (3) and (4), the relationship between the plasma absorption rate b and the depth h of the micro-cavity can be obtained as:

Among them, b is the laser plasma absorption rate;

F _th is the laser ablation threshold;

α is an effective coefficient, and the effective coefficient is an optical absorption coefficient or a thermal permeability coefficient;

F is the laser energy density;

h is the depth of the micro-cavity corresponding to the laser energy density.

Substitute the micro-cavity depth h, laser ablation threshold F _th , and effective coefficient α into the formula The absorptivity b of laser plasma at different energy densities F can be obtained, and the variation curve of laser plasma absorptivity b with laser energy density F is shown in Fig. 3 .

The described embodiment is a preferred implementation of the present invention, but the present invention is not limited to the above-mentioned implementation, without departing from the essence of the present invention, any obvious improvement, replacement or modification that those skilled in the art can make Modifications all belong to the protection scope of the present invention.

Claims (6)

1. a method for measuring plasma absorptivity in a laser ablation process, is characterized in that, comprises the following steps: S1. Select a target material and smooth and clean the surface of the target material, so that the surface roughness Ra of the target material surface is 0.05-0.1 μm; S2. Using pulsed laser to ablate a series of micro concave cavities on the surface of the target. During the processing, the laser energy density F increases or decreases regularly; S3. Using a three-dimensional shape analyzer to measure the diameter D and depth h of the micro-cavity obtained by the ablation in S2; S4. In a two-dimensional coordinate system where the abscissa is the laser energy density F and the ordinate is the cavity diameter D, plot the data points of the energy density F and the corresponding micro-cavity diameter D, and use the logarithmic relationship Fitting a function curve; the intersection point of the function curve and the abscissa is the ablation threshold F _th corresponding to the pulse laser ablation of the target; at the same time, calculate the target surface material in the pulse laser Effective coefficient α in the ablation process; S5, deduce the plasma absorption rate b during the laser ablation process through the depth h of the micro-cavity, the ablation threshold _Fth , the effective coefficient α, and the laser energy density, The formula of the ion absorption rate b is: <mrow><mi>b</mi><mo>=</mo><mn>1</mn><mo>-</mo><mfrac><msub><mi>F</mi><mrow><mi>t</mi><mi>h</mi></mrow></msub><mi>F</mi></mfrac><mi>exp</mi><mrow><mo>(</mo><mi>&amp;alpha;</mi><mi>h</mi><mo>)</mo></mrow></mrow> Among them, b is the laser plasma absorption rate; F _th is the laser ablation threshold; α is effective coefficient; F is the laser energy density; h is the depth of the micro-cavity corresponding to the laser energy density. 2. the method for measuring plasma absorptivity in the laser ablation process according to claim 1, is characterized in that, described step S1 specifically comprises the following steps: 1), using metallographic sandpaper to polish the target material, so that the surface roughness Ra of the target material reaches 0.05-0.1 μm; 2) Wipe the surface of the target block to be processed with a cotton ball soaked in absolute alcohol. 3. the method for measuring plasma absorptivity in the laser ablation process according to claim 1, is characterized in that, described step S2 specifically comprises the following steps: 3), fix the target on the workbench, focus the pulsed laser on the surface of the target to be processed, the focused laser spot diameter ω0 is _0.001-1mm , the The wavelength of the pulse laser is 193nm-10.6μm, the pulse width is 1fs-1ms, and the single pulse energy E is 1μJ-100J; 4), by changing the single pulse energy E of the pulse laser, changing the laser energy density F, the relationship between the single pulse energy E and the laser energy density F is: A series of micro-recessed cavities are ablated on the surface of the target by using the laser pulse with the change of the laser energy density F. During the processing, the value of the laser energy density F conforms to a geometric sequence or an arithmetic sequence Distribution; the number of values of the laser energy density F is equal to the number of a series of micro-cavities; the number of the series of micro-cavities is greater than or equal to 5; 5), post-treatment of the target: removing the slag on the surface of the target and polishing; 6) Put the polished target block into a measuring cup filled with absolute ethanol, place it in an ultrasonic cleaning machine for cleaning, take it out and dry it for testing after cleaning. 4. the method for measuring plasma absorptivity in the laser ablation process according to claim 1, is characterized in that, described step S3 specifically comprises the following steps: 7) Before measuring the shape of the micro-cavity, first wipe the surface of the target block with a cotton ball soaked in absolute alcohol; 8) Using a WYKO-NT1100 surface three-dimensional topography measuring instrument to measure the geometric topography of the surface texture of the micro-cavity, including the diameter D and depth h of the micro-cavity. 5. The method for measuring plasma absorptivity in the laser ablation process according to claim 1, characterized in that, in the step S4, the method for obtaining the effective coefficient α is: through the obtained laser ablation The erosion threshold F _th determines the energy density F ₀ that does not generate ablation plasma, and the value range of the energy density F ₀ is (F _th , 3F _th ]; the micro-recessed cavity is processed by a pulsed laser with an energy density F ₀ , Measure the depth of the dimple cavity as h ₀ , according to the formula Obtain the effective coefficient α of the surface material of the target. 6 . The method for measuring plasma absorptivity during laser ablation according to claim 1 , wherein the energy density distribution at the focused spot of the pulsed laser is a two-dimensional Gaussian distribution.