Conventional forms of laser machining for metals and other types of surfaces use gas to pull the evaporated material away from the surface being machined. Thus, use of gas in conventional machining is merely passive and does not contribute in modifying/enhancing the characteristics of the machining process itself.
It would be beneficial to have a laser machining process that can be enhanced and/or modified using a gas or a mixture of gases.
The present invention is generally related to machining of various surfaces. Specifically, embodiments of the present invention relate to gas-assisted laser machining in which a gas (or a mixture of gases) serves to enhance certain aspects of the laser machining process and/or modify certain characteristics of the laser machining process. In a particular embodiment, the gas helps to increase the evaporation/etching rate of the material at a lower temperature than would be needed by conventional process to achieve the same evaporation rate. The type and amount of gas used in the process depends on the type of surface or item being machined worked on.
Gas-assisted laser machining techniques as described herein can impact the surface finish/roughness/quality, by melting, flow, or surface molecular relaxation, even without any significant evaporation (for the duration of the heating). The surface finish, roughness effect can occur because of (a) modification of the surface chemistry and therefore of the interfacial energy, e.g., the tendency for a rough surface to flatten out is greater for greater interfacial energies, (b) modification of the temperature dependence of the interfacial energy driving the Marangoni flow, (c) modification of the local material viscosity, e.g., modification of the OH content of glass due to reaction with Hydrogen, which can diffuse in the bulk and react, and (d) lowering evaporation temperature increases viscosity and reduces material flow, thus reducing rim formation.
Some embodiments of the present invention provide a method for treating a work piece. The method includes providing a work piece having a surface. The method further includes impinging a gas jet on a portion of the surface. In some embodiments, the gas jet includes a reactive gas. Thereafter the method further includes focusing a laser beam on the portion of the surface for a predetermined duration and heating the portion of the surface to a first temperature. The method finally includes removing a predetermined amount of material from the portion of the surface.
In some embodiments, removing the predetermined amount of material from the portion of the surface further includes breaking the bonds between the material molecules due to the heating and evaporating material due to a reaction between the reactive gas and the material. In some embodiments, the method further includes turning off the laser beam upon expiration of the predetermined duration, impinging the gas jet on another portion of the surface, and focusing the laser beam on the other portion of the surface. In a particular embodiment, the work piece is a silica based optical component.
Certain embodiments of the present invention provide a method that includes impinging a gas jet on a surface of a work piece. The gas jet includes a gas that has higher diffusivity than air. The method further includes focusing a laser beam having a first power on the surface for a first duration, heating the surface to a first temperature to remove material from the surface, and moving the removed material away from the surface using the gas. In an embodiment, the gas includes Helium.
An embodiment of the present system provides a system for treating a work piece. The system includes a substrate holder configured to hold a work piece having a surface. The system also includes a nozzle positioned adjacent to the work piece and configured to impinge a gas jet on a desired area of the surface. In a particular embodiment, the gas jet is positioned orthogonal to a plane occupied by the surface of the work piece. The system further includes a laser source configured to emit a laser beam that can be focused at the desired area of the surface. In a particular embodiment, the laser beam passes through the nozzle before impinging on the desired area of the surface. The system also includes a gas delivery mechanism coupled to the nozzle to provide the gas jet. The system is configured to impinge the gas jet on the desired area of the surface, heat the desired area to a first temperature using the laser beam, and remove predetermined amount of material from the desired area. In an embodiment, the gas jet may include Nitrogen, Hydrogen, Helium, air, water vapor, or combinations thereof.
The following detailed description, together with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention.
Certain embodiments of the present invention provide techniques for machining various surfaces using a laser and one or more gases. In some embodiments, the techniques described herein use a laser to heat the surface or area of the surface being machined. A gas or a mixture of gases is used in conjunction with the laser beam to control evaporation rate, etching characteristics, surface shape, and amount of re-deposited material onto the surface being machined. In a particular embodiment, a gas jet is co-incident with the laser beam.
Other embodiments of the present invention provide a system for performing gas-assisted laser machining. The system includes a laser source, a gas delivery system for delivering gas to the area being machined, diagnostic equipment to perform in-situ monitoring of the machining process, material removal sub-system, and a gas source.
Lasers can be used for various machining activities such as drilling, cutting, removing coating of one material from another material, marking/engraving, surface finishing/smoothing, etc. Embodiments of the present invention relate to using a laser to remove a material from a surface of an item. In addition, embodiments of the present invention may be used to in melting, flowing, or surface finishing of material without removal of material. However, the techniques disclosed herein are applicable to any other applications of laser machining. Specifically, embodiments described below relate to removing material from fused-silica based optics components. One skilled in the art will realize that the techniques disclosed herein are equally applicable to laser machining of metals, ceramics, and other types of material.
Silica is used in many industrial applications such as raw material in refractory linings, fiber optics, optical substrates and, in general, as a component in devices requiring inertness and toughness. However, silica is difficult to process. High temperatures above the glass working point (˜2400° K) are used for molding of fused silica, while very reactive species are needed for chemical etching of silica. Furthermore, many of silica's processing properties depend greatly on temperature. In particular, evaporative etching of silica uses extreme temperatures approaching the boiling point of silica, e.g., 3000° K. Such temperatures are not practical for machining under ambient conditions. In applications where localized heating is used for machining glass in air these high temperature requirements often cause unwanted increases in residual stresses, formation of rim structures, and redeposit defects of the glass. A reduction in the treatment temperature for material removal greatly improves thermal processing by reducing and/or eliminating these unwanted factors. In one embodiment of the present invention the laser-driven vapor pressure of silica decomposition products is increased by using reactive gases to assist evaporation.
Until now a systematic study silica behavior near the boiling point of silica was never performed because most containment vessels degrade above □ about 2000° K. Moreover, in-situ measurements of such a process are difficult due to both high blackbody radiation background and high fluxes of heated material. Embodiments of the present invention provide techniques for laser heating a surface to reach surface temperatures of up to 3100° K at the gas-solid interface, and using selected gas reactivities on the evaporation kinetics of silica control and/or modify the etching process. In some embodiments of the present invention, the gases used in the laser machining process include air, water vapor (e.g., humidified air), 100% Hydrogen, a 5% Hydrogen-95% Nitrogen mixture, 100% Nitrogen, 100% Helium, a mixture of Hydrogen and Helium, and combinations thereof. In some embodiments, the etching can be performed in an oxidizing, a reducing, or an inert atmosphere.
Conventional laser machining relies on laser-based evaporation of the material and on the velocity of escaped species within the Knudsen layer close to a hot surface. However, conventional techniques do not include any chemical reactions from a reacting gas, or any shift in the equilibrium of the evaporation reactions from the presence of a gas phase product. In addition in conventional techniques, the gas used for material removal does not react directly with the material during the evaporation process. Embodiments of the present invention provide a laser-based evaporation technique that assumes near-equilibrium conditions within a boundary layer where most of the variation in the species concentration occurs. The equilibrium concentration in the vicinity of the gas-solid interface establishes the driving force for the rate of diffusive transport within the boundary layer before mixing and removal in the bulk of the gas stream. The boundary layer thickness, in turn, depends on the gas properties, flow rate, and flow configuration, and determines the transport kinetics via the mass transport coefficient, hm˜D/δ where D is the gas species diffusivity and δ is the boundary layer thickness. In some embodiments, the laser-based evaporation rates for the methods described herein can be obtained through determination of the hm and equilibrium constants, Kp, from which equilibrium concentrations can be calculated.
Embodiments of the present invention provide methods for laser-based machining in which specific gas phase components are added to enhance the process of evaporation during the heating of a material. In other embodiments, the gas phase components may also help with smoothing of the surface and with the flow of the surface material. The gases are selected so as to lower the evaporation temperature of the material and reduce the laser energy deposited in the material thereby reducing stress on the material.
Many advantages are realized by using the embodiments of the present invention. For example, techniques described herein lower the evaporation temperature for a given evaporation rate of the material and thus etching of material can be performed at reduced temperatures. This lowering in the amount of laser deposited energy as expressed by the temperature of the material, along with the corresponding reduction in the structural modifications of the material helps in reducing stress and residual stress after cooling of the material and increase the materials lifetime, while reducing the extent to which the material will damage in case of failure (e.g. reduced fracture size from smaller stress fields) and also helps in reducing material flow. Another advantage is that reduced laser energy is needed to evaporate/etch the material for a desired etch rate compared to conventional processes. In addition, techniques disclosed herein also help to reduce the amount of the apparent re-deposited material on the surface thus reducing structural and optical defects of the machined surface. Additionally, using reactive gases during the laser evaporation process results in reduction or even elimination of rim formations and curvatures due to Marangoni flow at the heated site edges. This helps to preserve a flatter surface with fewer features that can act to intensify propagated light when the material is used to steer light in optical applications. Similar surface topology and process improvements can be obtained for other materials such as metals, ceramics, etc.
The following embodiments of the present invention are described primarily in relation to fused silica-based material. However, it is to be understood that the embodiments described below are equally applicable to other types of materials such as metals, ceramics, etc. as well.
Nozzle 110 includes a laser window (not shown) to allow passage of laser beam 104 through the nozzle, while also forcing the flow through the nozzle front opening where the laser exits. In some embodiments, nozzle 110 may have a 3 mm opening on one end for dispensing the gas or a mixture of gases. The gas jet is impinged normal/orthogonal to the surface plane of work piece 106 and submerges the treated area of work piece 106 well beyond the boundaries of the heated site by displacing the ambient air at the surface of work piece 106 before onset of laser heating. The laser beam passes through the transparent laser window mounted on the backside of nozzle 110 and focuses on the surface of work piece 106. The gas (or mixture of gases) is delivered to the surface of work piece 106 via nozzle 110. In one embodiment, nozzle 110 has a side opening to receive the gas from gas source 114. Temperature measurements can be obtained from infrared imaging of the blackbody radiation emitted during the evaporation process using camera 116. The amount of evaporated silica can be determined from the surface shape profiles obtained by interferometry measurements following treatment of the surface. In some embodiments, gas source 114 may include compressed gas cylinders or a central gas supply cabinet. In some embodiments, the gases used may include dry air (78% Nitrogen, 21% Oxygen, 1% trace gases), 100% Nitrogen, 5% Hydrogen+95% Nitrogen, 5% Hydrogen+95% Helium, 100% Hydrogen, and 100% Helium. Gas flow controller 112 can be used to set the volumetric flow rate of the gases, which can range between 0.2 L/min to about 10 L/min. In some embodiments, the gas flow is started before laser exposure of work piece 106 to insure that all the dead volume is removed from the gas delivery lines and that surface gas submersion of work piece 106 is at steady state.
In an embodiment, laser source 102 can be a Carbon Dioxide (CO2) laser that emits laser beam 104 having a wavelength of between 10 μm and 12 μm with a maximum output power of 20 W and power stability of about 1% over the duration of the exposure. The diameter of laser beam 104 can be about 1 mm. The laser power delivered to the surface of silica work piece 106 can be between 6.5 W and 7.2 W. In some embodiments, laser beam 104 can be impinged on work piece 106 for about 5 seconds at a time. When laser beam 104 is impinged on surface of work piece 106, the temperature of the surface increases thereby evaporating material at and/or near the location where laser beam 104 is impinged. This results in formation of a pit on the surface of material 106.
The temperature and composition dependent evaporation rate, R(T, Ci), can be estimated based on the measurement of the depth profile, e.g., as illustrated in
νf=(dγ/dT)ΔT/μ (1)
where dγ/dT represents the rate of change of the surface tension with temperature, ΔT, is the temperature drop from the center of the pit to the edge of the pit (
In addition, the amount of material removed from drag associated with the gas flow is very low because the gases lack inertia at atmospheric pressure and superficial velocities are small, e.g., <25 m/s. Contributions of vapor-induced shear forces and recoil pressure in shaping laser produced cavities in solids have a negligible impact on the cavity axial depth produced for the relatively slow evaporation conditions. None of the surface profiles display roughening within the pit that would normally occur if explosive boiling had taken place and irradiances are well below the phase explosion threshold, e.g., 10″ W/cm2. Therefore, in attributing the axial depth, d, solely to the evaporation of material at that location, the measure of the temperature dependent evaporation rate is given by
R(Tp)˜ρ′d/Δt, (2)
where ρ is the fused silica density, Δt is related to the laser exposure time, and Tρ is the peak temperature measured at the center of the pit. Center depth, d, is used because the location of that spot can easily be found from the surface and temperature spatial profiles. Furthermore, restricting the analysis to that location circumvents any ambiguity arising from the non-uniform heating of the Gaussian shaped laser beam. In one embodiment, the effective exposure time, Δt, may be about 4 seconds, since the thermal diffusion time needed to approach peak temperatures with thermal diffusivity D=8×10−7 m2/s is approximately a2/D=0.98 sec, where a is the beam diameter. The resulting error based on the time-integrated experimental evaporation rates extrapolated to lower temperatures is <3% of the bottom pit depth. Therefore small variations, δ, in the effective exposure time will have negligible impact. Peak temperatures are within 5% of the final peak temperatures reached right before laser turn off for exposure durations greater than the thermal diffusion time, and may increase asymptotically at the rate determined by D and as the heat losses from the work piece balance out the heat input from laser heating.
In some embodiments, the etching/evaporation rate may depend on whether the process is transport limited or based on reaction kinetics control, or both. If the evaporation rate is transport limited, the mass transfer coefficient (hm) and the reaction equilibrium constant (Kp) are the controlling parameters. If the evaporation rate is not transport limited, the rate constants for the evaporation and condensation reactions are the controlling parameters. If the rate of evaporation (R) is not dependent on the flow rate of the gases, then it can be concluded that the evaporation process is not transport limited.
As can be seen from
In a particular embodiment, where the treated surface is silica-based, the type of gas used significantly influences the evaporation rate.
The main endothermic reactions that occur at the temperatures illustrated in
SiO2(l)SiO(g)+½O2(g) (3)
SiO2(l)+H2(g)SiO(g)+H2O(g) (4)
Reaction (3) is the main decomposition reaction that occurs when any of the gases illustrated in
As can be seen from
R=P
sat(T)√(2πmkBT) (5)
where Psat is the vapor pressure of SiO in reaction (3), in is the molecular mass, and kB is the Boltzmann constant, are compared to the rates illustrated in
In an embodiment, Helium can be used instead of Nitrogen as the carrier gas along with the same Hydrogen fraction of 5%. Thus the gas combination in this embodiment would be 95% Helium and 5% Hydrogen. The magnitude of the rate of evaporation R in Helium is larger because the gas phase diffusivity in Helium is larger than in Nitrogen. This results in a greater hm and R in Helium in the case where mass transport is the limiting transport mechanism. As illustrated in
R˜h
m[SiO]eq (6)
In order to perform a quantitative analysis of the evaporation rates illustrated in
K
p=exp(−ΔGi°/RcT) (7)
Where Rc is the gas constant and T is the temperature for reaction i. Thus, for the overall system, the reaction equilibrium constant can be determined as
K
p1(T)=((ni-sio+ξ+α)/nT)*((ni-02+½ξ)/nT)0.5*P3/2 (8)
K
p2(T)=((ni-h2−α)/nT)−1*((ni-sio+ξ+α)/nT)*((ni-H2O+α/)nT)+P (9)
The terms in parenthesis represent mole fractions. P is the total pressure taken at 1 atm in the system, ni are the initial species quantity in moles, α and ξ are the extent of reaction for each reaction, nT represents the total number of moles calculated based on the ni, α and ξ. Standard free energies, ΔG°, are generally known in the art and may be found in thermodynamic databases.
In embodiments where Hydrogen is used in the gas mixture, there is a difference in the experimental versus calculated ratios of R (e.g., H2 mixture vs. pure N2). Using an initial 5% Hydrogen fraction, the calculated Hydrogen concentration equilibrates locally to values between 0.5% and 2.5% for temperatures ranging from 2600° K to 3000° K, respectively. The mass transport of Hydrogen is fast enough to maintain a bulk Hydrogen concentration throughout the gas-solid interface where it is being consumed in reaction (4). This is consistent with the finding that the transport of the products, and not the reactants, is rate limiting. Thus a fixed 5% Hydrogen concentration, along with the derived ΔG° can be used to determine R and to calculate the predicted ratio. In contrast with the RN2(T)/Rair(T) ratio in
A complete expression of the absolute R includes the determination of not only the equilibrium SiO concentrations, but also of the mass transport kinetics expressed in the hm. The hm can be extracted from the data by fitting across the two process variables on which R depends, e.g., temperature and flow rate. Equation (6) can now be written as
R(T,V)=hm(T,V)′[SiO]eq(T) (10)
where V is the gas volumetric flow rate and the [SiO]eq(Y) is determined for each gas from the fitted reaction free energies. For this purpose, generalized expressions describing the kinetics of transport using a boundary layer approximation are useful. Typically used are the dimensionless Sherwood number, Sh, which relates Sh to the Reynolds, Re, and the Schmidt number, Sc. Sh is defined such that
Sh=h
m
L/D=f(Sc=μ/ρD,Re=ρVL/μ) (11)
where L is a characteristic length (taken as the beam diameter), μ is the dynamic viscosity, D is the species diffusivity, and ρ is the gas density. All the temperature dependent gas properties can be calculated from (a) available data and empirical models to extrapolate the viscosity, diffusivity, and (b) from the ideal gas law for density. The particular form of the empirical expression for hm is given by:
Sh=□C*Sc
m
*Re
n (12)
where C, in, and n represent a single set of fitting parameters applicable for all the gases described herein.
Thus, using the determined hm and the equilibrium concentration calculated from Kp, the laser-based evaporation behavior of silica can be determined, which accounts for the temperature dependent gas properties, the thermodynamics of the reaction of the gas phase reactant, and the mass transport configuration in the flow system. The methods described herein are applicable to a broad range of materials exposed to both steady state heating with lasers and to gases with selected reactivities. As described above, the laser-based evaporation is a process that is mass transport limited and therefore dependent on the thermodynamics of the reactions through the free energy. The techniques described herein can enable the derivation of thermodynamic properties of gas-solid phase reactions at extremes temperatures, provided that accurate measurement of the evaporation rates and temperatures are made. The techniques can also help understand the mechanism by which specific gases interact with the solid during reactive etching and can improve control of thermal etching processes in general.
In some embodiments, adding certain gases during the evaporation process can significantly help to enhance the evaporation process as described above. In a particular embodiment, the gas jet can be impinged normal to the surface plane of the work piece. The gas jet is impinged before beginning the evaporation/etching process such that the portion of the surface being treated is submerged in the gas well beyond the boundaries of the heated site by displacing and replacing the ambient air at the surface being treated. The laser beam passes through the nozzle and is focused on a portion of the work piece surface. Laser exposure time and surface temperature can be controlled by controlling the laser pulse length, laser power level, laser operating mode, beam shape, etc. and gas composition. In some embodiments, the laser exposure time can be between 0.5 seconds to about 5 seconds with a power of up to 20 W.
As discussed above, a variety of gases or combinations thereof may be used to submerge the surface being treated. Using a mixture of 5% Hydrogen (or any other reducing gas such as carbon monoxide (CO), hydrogen fluoride (HF), some organic compounds, etc.) in a carrier gas such as Nitrogen or Helium results in a 5-10 fold increase in the evaporation rate compared to ambient air. In the instance where 100% Hydrogen is used, an additional 10 fold increase in evaporation rate at a given temperature and transport conditions may be realized relative to air. In addition, the increase in the evaporation rate is achieved at temperatures lower than that required for ambient air. As described above, the rate of removal depends on the type of gas or gas mixture used and the type of environment that a given gas mixture creates at or near the surface to treated. Using Helium increases the evaporation rate compared to Nitrogen due to the greater diffusivity of product and/or reactants in Helium relative to other carrier gases such as Nitrogen, thus increasing mass transport through boundary layer resistance near the surface being treated. Similarly, using a reducing gas increases the evaporation rate compared to using an inert gas or an oxidizing gas because the additional solid silica reactive pathway from the reducing agent increases the evaporation product gas concentration within the boundary layer near the surface, increasing thus the driving force (concentration gradient) and the diffusive mass transport through the boundary layer.
In addition, using techniques described herein results in reduced amounts of small particles that may re-deposit onto the surface being treated. One reason for this is that since the majority of the removed material is carried away from the surface being treated rapidly by convective transport, there is very little material left that can be re-deposited on to the treated surface. Also, gas, induced changes in the surface chemistry may re-melt the re-deposited material on the surface more easily to produce a less rough surface.
Initially a work piece is provided (602). In an embodiment, the work piece can be silica based optical component. Thereafter a gas jet is impinged on a surface of the work piece (604). In some embodiments, the gas jet may include a mixture of a reducing gas and a carrier gas. A laser beam is then focused on an area of the surface for a pre-determined duration (606). In some embodiments, the gas jet and the laser beam are co-incident. Subsequently, the area of the surface is heated to a first temperature (608). In some embodiments, the first temperature can be between 2000° K and 3100° K. The increase in temperature results in the breaking of the bonds of the silica material and the gas reacts with the silica material to evaporate the material as described above (610). After a predetermined amount of material is removed and/or after the pre-determined duration has elapsed, the laser beam is turned off (612). Thereafter the laser beam is focused on another area of the surface (614) and the process is repeated.
It should be appreciated that the specific steps illustrated in
A work piece having a surface is provided (702). A gas jet is impinged on a portion of the surface that is to be treated (704). The gas jet includes a gas that has higher diffusivity than air and/or is lighter than air, e.g., Helium. The portion of the surface is then heated using a laser beam for a predetermined duration (706). As the material is removed from the surface, the gas jet helps to carry the removed material away from the surface. Upon expiration of the predetermined duration, the laser beam is turned off (708).
It should be appreciated that the specific steps illustrated in
In an embodiment, the techniques disclosed herein can be used for damage mitigation of silica-based optical components. Silica-based optical components that are used in conjunction with lasers suffer from optical damage due to prolonged exposure to laser fluence. In particular, optical components, such as lenses, windows, etc. are prone to damage when exposed to high-power, high-energy laser irradiation. All optical materials will ultimately damage at sufficiently high laser intensities through processes intrinsic to the optical material. Such intrinsic optical damage is the result of high-energy deposition through multi-photon ionization and is determined by the material's bulk electronic structure. Such damage normally occurs at intensities in excess of 200 GW/cm2. In practice, however even the highest quality optical components can damage at fluences well below their intrinsic damage threshold.
Photoactive impurities in the near surface layer of the silica-based optical component can absorb high-intensity light thus transferring energy from the beam into near surface of the optical component raising the local temperature. If the combination of the intensity of the laser beam and the strength of the absorption are sufficient, a small local plasma can ignite on the surface of the optical component. Such plasma may itself absorb energy from the light beam further raising the local temperature until the end of the termination of the light pulse. Physically, the damage is a manifestation of the plasma including melted material, ejecta, and thermally induced fractures. This results in pitting of the surface thereby degrading the optical component. Techniques described herein can be used to treat the damaged sites of such optical components.
For example, a gas jet including a reducing gas can be impinged on the surface of a damaged optical component and a laser beam applied to remove the material from the damaged area and generally smooth out the damaged area. Every optical component has certain tolerance level for such mitigation of damaged areas. However, as described in relation to
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application No. 61/466,382, filed on Mar. 22, 2011, the contents of which are incorporated by reference herein in their entirety for all purposes.
The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US12/28943 | 3/13/2012 | WO | 00 | 8/27/2014 |
Number | Date | Country | |
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61466382 | Mar 2011 | US |