Laser drilling

Last updated

Laser drilling is the process of creating thru-holes, referred to as “popped” holes or “percussion drilled” holes, by repeatedly pulsing focused laser energy on a material. The diameter of these holes can be as small as 0.002” (~50 μm). If larger holes are required, the laser is moved around the circumference of the “popped” hole until the desired diameter is created; this technique is called “trepanning”.

Contents

Applications

Laser drilling is one of the few techniques for producing high-aspect-ratio holes—holes with a depth-to-diameter ratio much greater than 10:1. [1]

Laser-drilled high-aspect-ratio holes are used in many applications, including the oil gallery of some engine blocks, aerospace turbine-engine cooling holes, laser fusion components, [1] and printed circuit board micro-vias. [2] [3] [4] [5]

Engine block Part of an internal combustion engine

An engine block is the structure which contains the cylinders, and other parts, of an internal combustion engine. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was attached. Modern engine blocks typically have the crankcase integrated with the cylinder block as a single component. Engine blocks often also include elements such as coolant passages and oil galleries.

Manufacturers of turbine engines for aircraft propulsion and for power generation have benefited from the productivity of lasers for drilling small (0.3–1 mm diameter typical) cylindrical holes at 15–90° to the surface in cast, sheet metal and machined components. Their ability to drill holes at shallow angles to the surface at rates of between 0.3 and 3 holes per second has enabled new designs incorporating film-cooling holes for improved fuel efficiency, reduced noise, and lower NOx and CO emissions.

Laser Device which emits light via optical amplification

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

Sheet metal metal formed by an industrial process into thin, flat pieces

Sheet metal is metal formed by an industrial process into thin, flat pieces. Sheet metal is one of the fundamental forms used in metalworking and it can be cut and bent into a variety of shapes. Countless everyday objects are fabricated from sheet metal. Thicknesses can vary significantly; extremely thin sheets are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate steel or "structural steel."

Fuel efficiency is a form of thermal efficiency, meaning the ratio from effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.

Incremental improvements in laser process and control technologies have led to substantial increases in the number of cooling holes used in turbine engines. Fundamental to these improvements and increased use of laser drilled holes is an understanding of the relationship between process parameters and hole quality and drilling speed.

Drilling cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials

Drilling is a cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials. The drill bit is usually a rotary cutting tool, often multi-point. The bit is pressed against the work-piece and rotated at rates from hundreds to thousands of revolutions per minute. This forces the cutting edge against the work-piece, cutting off chips (swarf) from the hole as it is drilled.

Theory

Following is a summary of technical insights about the laser drilling process and the relationship between process parameters and hole quality and drilling speed.

Physical phenomena

Laser drilling of cylindrical holes generally occurs through melting and vaporization (also referred to as "ablation") of the workpiece material through absorption of energy from a focused laser beam.

Melting material phase change

Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increases the substance's temperature to the melting point. At the melting point, the ordering of ions or molecules in the solid breaks down to a less ordered state, and the solid melts to become a liquid.

Vaporization phase transition from the liquid phase to vapor (either through evaporation or boiling)

Vaporization of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon.

Ablation removal of material from the surface of an object by vaporization, chipping, or other erosive processes

Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. Examples of ablative materials are described below, and include spacecraft material for ascent and atmospheric reentry, ice and snow in glaciology, biological tissues in medicine and passive fire protection materials.

The energy required to remove material by melting is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is often favored.[ citation needed ]

Whether melting or vaporization is more dominant in a laser drilling process depends on many factors, with laser pulse duration and energy playing an important role. Generally speaking, ablation dominates when a Q-switched Nd:YAG laser is used.[ citation needed ] On the other hand, melt expulsion, the means by which a hole is created through melting the material, dominates when a flashtube pumped Nd:YAG laser is used.[ citation needed ] A Q-switched Nd:YAG laser normally has pulse duration in the order of nanoseconds, peak power on the order of ten to hundreds of MW/cm2, and a material removal rate of a few micrometres per pulse. A flash lamp pumped Nd:YAG laser normally has a pulse duration on the order of hundreds of microseconds to a millisecond, peak power in the order of sub MW/cm2, and material removal rate of ten to hundreds of micrometers per pulse. For machining processes by each laser, ablation and melt expulsion typically coexist.[ citation needed ]

Melt expulsion arises as a result of the rapid build-up of gas pressure (recoil force) within a cavity created by evaporation. For melt expulsion to occur, a molten layer must form and the pressure gradients acting on the surface due to vaporization must be sufficiently large to overcome surface tension forces and expel the molten material from the hole. [6]

The "best of both worlds" is a single system capable of both "fine" and "coarse" melt expulsion. "Fine" melt expulsion produces features with excellent wall definition and small heat-affected zone while "coarse" melt expulsion, such as used in percussion drilling and trepanning, removes material quickly.

The recoil force is a strong function of the peak temperature. The value of Tcr[ clarification needed ] for which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion. For instance, liquid expulsion from titanium can take place when the temperature at the center of the hole exceeds 3780 K.

In early work (Körner, et al., 1996), [7] the proportion of material removed by melt expulsion was found to increase as intensity increased. More recent work (Voisey, et al., 2000) [8] shows that the fraction of the material removed by melt expulsion, referred to as melt ejection fraction (MEF), drops when laser energy further increases. The initial increase in melt expulsion on raising the beam power has been tentatively attributed to an increase in the pressure and pressure gradient generated within the hole by vaporization.

A better finish can be achieved if the melt is ejected in fine droplets.[ citation needed ] Generally speaking, droplet size decreases with increasing pulse intensity. This is due to the increased vaporization rate and thus a thinner molten layer. For the longer pulse duration, the greater total energy input helps form a thicker molten layer and results in the expulsion of correspondingly larger droplets. [9]

Previous models

Chan and Mazumder (1987) [10] developed a 1-D steady state model to incorporate liquid expulsion consideration but the 1-D assumption is not suited for high aspect ratio hole drilling and the drilling process is transient. Kar and Mazumder (1990) [11] extended the model to 2-D, but melt expulsion was not explicitly considered. A more rigorous treatment of melt expulsion has been presented by Ganesh, et al. (1997), [12] which is a 2-D transient generalized model to incorporate solid, fluid, temperature, and pressure during laser drilling, but it is computationally demanding. Yao, et al. (2001) [13] developed a 2-D transient model, in which a Knudsen layer is considered at the melt-vapor front, and the model is suited for shorter pulse and high peak power laser ablation.

Laser energy absorption and melt-vapor front

At the melt-vapor front, the Stefan boundary condition is normally applied to describe the laser energy absorption (Kar and Mazumda, 1990; Yao, et al., 2001).

(1)

where is the absorbed laser intensity, β is the laser absorption coefficient depending on laser wavelength and target material, and I(t) describes temporal input laser intensity including pulse width, repetition rate, and pulse temporal shape. k is the heat conductivity, T is the temperature, z and r are distances along axial and radial directions, p is density, v the velocity, Lv the latent heat of vaporization. The subscripts l, v and i denote liquid phase, vapor phase and vapor-liquid interface, respectively.

If the laser intensity is high and pulse duration is short, the so-called Knudsen layer is assumed to exist at the melt-vapor front where the state variables undergo discontinuous changes across the layer. By considering the discontinuity across the Knudsen layer, Yao, et al. (2001) simulated the surface recess velocity Vv distribution, along the radial direction at different times, which indicates the material ablation rate is changing significantly across the Knudsen layer.[ citation needed ]

Melt expulsion

After obtaining the vapor pressure pv, the melt layer flow and melt expulsion can be modeled using hydrodynamic equations (Ganesh et al.,1997). Melt expulsion occurs when the vapor pressure is applied on the liquid free surface which in turn pushes the melt away in the radial direction. In order to achieve fine melt expulsion, the melt flow pattern needs to be predicted very precisely, especially the melt flow velocity at the hole’s edge. Thus, a 2-D axisymmetric transient model is used and accordingly the momentum and continuity equations used.

Ganesh’s model for melt ejection is comprehensive and can be used for different stages of the hole drilling process. However, the calculation is very time consuming and Solana, et al. (2001), [14] presented a simplified time dependent model that assumes that the melt expulsion velocity is only along the hole wall, and can give results with a minimum computational effort.

The liquid will move upwards with velocity u as a consequence of the pressure gradient along the vertical walls, which is given in turn by the difference between the ablation pressure and the surface tension divided by the penetration depth x.

Assuming that the drilling front is moving at a constant velocity, the following linear equation of liquid motion on the vertical wall is a good approximation to model the melt expulsion after the initial stage of drilling.

(2)

where p is the melt density, μ is the viscosity of the liquid, P(t)=(ΔP(t)/x(t)) is the pressure gradient along the liquid layer, ΔP(t) is the difference between the vapor pressure Pv and the surface tension .

Pulse shape effect

Roos (1980) [15] showed that a 200 µs train consisting of 0.5 µs pulses produced superior results for drilling metals than a 200 µs flat shaped pulse. Anisimov, et al. (1984) [16] discovered that process efficiency improved by accelerating the melt during the pulse.

Grad and Mozina (1998) [17] further demonstrated the effect of pulse shapes. A 12 ns spike was added at the beginning, middle, and the end of a 5 ms pulse. When the 12 ns spike was added to the beginning of the long laser pulse, where no melt had been produced, no significant effect on removal was observed. On the other hand, when the spike was added at the middle and the end of the long pulse, the improvement of the drilling efficiency was 80 and 90%, respectively. The effect of inter-pulse shaping has also been investigated. Low and Li (2001) [18] showed that a pulse train of linearly increasing magnitude had a significant effect on expulsion processes.

Forsman, et al. (2007) demonstrated that a double pulse stream produced increased drilling and cutting rates with significantly cleaner holes. [1]

Conclusion

Manufacturers are applying results of process modeling and experimental methods to better understand and control the laser drilling process. The result is higher quality and more productive processes that in turn lead to better end products such as more fuel efficient and cleaner aircraft and power generating turbine engines.

See also

Related Research Articles

Cavitation Formation of vapour-filled low-pressure voids in a liquid

Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is relatively low.

Distillation method of separating mixtures based on differences in volatility of components in a boiling liquid mixture

Distillation is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation. Distillation may result in essentially complete separation, or it may be a partial separation that increases the concentration of selected components in the mixture. In either case, the process exploits differences in the relative volatility of the mixture's components. In industrial chemistry, distillation is a unit operation of practically universal importance, but it is a physical separation process, not a chemical reaction.

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium. It is that temperature and pressure at which the sublimation curve, fusion curve and the vaporisation curve meet. For example, the triple point of mercury occurs at a temperature of −38.83440 °C and a pressure of 0.2 mPa.

Magma Mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth

Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has historically relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, and once in Hawaii.

Electrical discharge machining

Electrical discharge machining (EDM), also known as spark machining, spark eroding, burning, die sinking, wire burning or wire erosion, is a manufacturing process whereby a desired shape is obtained by using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the "tool" or "electrode," while the other is called the workpiece-electrode, or "work piece." The process depends upon the tool and work piece not making actual contact.

Sublimation (phase transition) transition of a substance directly from the solid to the gas phase

Sublimation is the transition of a substance directly from the solid to the gas without passing through the intermediate liquid. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid. The reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase. Sublimation has also been used as a generic term to describe a solid-to-gas transition (sublimation) followed by a gas-to-solid transition (deposition). While a transition from liquid to gas is described as evaporation if it occurs below the boiling point of the liquid, and as boiling if it occurs at the boiling point, there is no such distinction within the solid-to-gas transition, which is always described as sublimation.

Laser cutting technology that uses a laser to cut materials

Laser cutting is a technology that uses a laser to cut materials, and is typically used for industrial manufacturing applications, but is also starting to be used by schools, small businesses, and hobbyists. Laser cutting works by directing the output of a high-power laser most commonly through optics. The [laser optics] and CNC are used to direct the material or the laser beam generated. A commercial laser for cutting materials involved a motion control system to follow a CNC or G-code of the pattern to be cut onto the material. The focused laser beam is directed at the material, which then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high-quality surface finish. Industrial laser cutters are used to cut flat-sheet material as well as structural and piping materials.

Pulsed laser deposition

Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target which deposits it as a thin film on a substrate. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.

Selective laser sintering additive manufacturing technique used for the low volume production of prototype models and functional components

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material, aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to Selective Laser Melting (SLM); the two are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered, allowing different properties. SLS is a relatively new technology that so far has mainly been used for rapid prototyping and for low-volume production of component parts. Production roles are expanding as the commercialization of AM technology improves.

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

Laser beam welding

Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications using automation, as in the automotive industry. It is based on keyhole or penetration mode welding.

Laser ablation process that removes material from an object by heating it with a laser

Laser ablation or photoablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm.

GeSbTe (germanium-antimony-tellurium or GST) is a phase-change material from the group of chalcogenide glasses used in rewritable optical discs and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing bitrates of up to 35 Mbit/s to be written and direct overwrite capability up to 106 cycles. It is suitable for land-groove recording formats. It is often used in rewritable DVDs. New phase-change memories are possible using n-doped GeSbTe semiconductor. The melting point of the alloy is about 600 °C (900 K) and the crystallization temperature is between 100 and 150 °C.

Vacuum deposition

Vacuum deposition is a family of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure. The deposited layers can range from a thickness of one atom up to millimeters, forming freestanding structures. Multiple layers of different materials can be used, for example to form optical coatings. The process can be qualified based on the vapor source; physical vapor deposition uses a liquid or solid source and chemical vapor deposition uses a chemical vapor.

Melting-point depression is the phenomenon of reduction of the melting point of a material with reduction of its size. This phenomenon is very prominent in nanoscale materials, which melt at temperatures hundreds of degrees lower than bulk materials.

Vapor–liquid–solid method

The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. The growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

In thermodynamics, explosive boiling or phase explosion is a method whereby a superheated metastable liquid undergoes an explosive liquid-vapor phase transition into a stable two-phase state because of a massive homogeneous nucleation of vapor bubbles. This concept was pioneered by M. M. Martynyuk in 1976 and then later advanced by Fucke and Seydel.

References

  1. 1 2 3 Forsman, A; et al. (June 2007). "Superpulse A nanosecond pulse format to improve laser drilling" (PDF). Photonics Spectra. Retrieved 2014-07-20.
  2. Bovatsek, Jim; Tamhankar, Ashwini; Patel, Rajesh (November 1, 2012). "Ultraviolet lasers: UV lasers improve PCB manufacturing processes". Laser Focus World . Retrieved 20 July 2014.
  3. Meier, Dieter J.; Schmidt, Stephan H. (2002). "PCB Laser Technology for Rigid and Flex HDI – Via Formation, Structuring, Routing" (PDF). LPKF Laser and Electronics. Retrieved 20 July 2014.
  4. Gan, E.K.W.; Zheng, H.Y.; Lim, G.C. (7 Dec 2000). Laser drilling of micro-vias in PCB substrates. Proceedings of 3rd Electronics Packaging Technology Conference. IEEE. doi:10.1109/eptc.2000.906394. ISBN   0-7803-6644-1.
  5. Kestenbaum, A.; D'Amico, J.F.; Blumenstock, B.J.; DeAngelo, M.A. (1990). "Laser drilling of microvias in epoxy-glass printed circuit boards". IEEE Transactions on Components, Hybrids, and Manufacturing Technology. Institute of Electrical and Electronics Engineers (IEEE). 13 (4): 1055–1062. doi:10.1109/33.62548. ISSN   0148-6411.
  6. Basu, S.; DebRoy, T. (1992-10-15). "Liquid metal expulsion during laser irradiation". Journal of Applied Physics. AIP Publishing. 72 (8): 3317–3322. doi:10.1063/1.351452. ISSN   0021-8979.
  7. Körner, C.; Mayerhofer, R.; Hartmann, M.; Bergmann, H. W. (1996). "Physical and material aspects in using visible laser pulses of nanosecond duration for ablation". Applied Physics A Materials Science & Processing. Springer Science and Business Media LLC. 63 (2): 123–131. doi:10.1007/bf01567639. ISSN   0947-8396.
  8. Voisey, K.T.; Cheng, C.F.; Clyne, T.W. (2000). "Quantification of Melt Ejection Phenomena During Laser Drilling". MRS Proceedings. San Francisco: Cambridge University Press (CUP). 617. doi:10.1557/proc-617-j5.6. ISSN   0272-9172.
  9. Voisey, K. T.; Thompson, J. A.; Clyne, T. W. (14–18 Oct 2001). Damage caused during laser drilling of thermal spray TBCs on superalloy substrates. ICALEO 2001. Jacksonville FL: Laser Institute of America. p. 257. doi:10.2351/1.5059872. ISBN   978-0-912035-71-0.CS1 maint: Date format (link)
  10. Chan, C. L.; Mazumder, J. (1987). "One‐dimensional steady‐state model for damage by vaporization and liquid expulsion due to laser‐material interaction". Journal of Applied Physics. AIP Publishing. 62 (11): 4579–4586. doi:10.1063/1.339053. ISSN   0021-8979.
  11. Kar, A.; Mazumder, J. (1990-10-15). "Two‐dimensional model for material damage due to melting and vaporization during laser irradiation". Journal of Applied Physics. AIP Publishing. 68 (8): 3884–3891. doi:10.1063/1.346275. ISSN   0021-8979.
  12. Ganesh, R.K.; Faghri, A.; Hahn, Y. (1997). "A generalized thermal modeling for laser drilling process—I. Mathematical modeling and numerical methodology". International Journal of Heat and Mass Transfer. Elsevier BV. 40 (14): 3351–3360. doi:10.1016/s0017-9310(96)00368-7. ISSN   0017-9310.
  13. Zhang, W.; Yao, Y.L.; Chen, K. (2001-09-01). "Modelling and Analysis of UV Laser Micromachining of Copper". The International Journal of Advanced Manufacturing Technology. Springer Science and Business Media LLC. 18 (5): 323–331. doi:10.1007/s001700170056. ISSN   0268-3768.
  14. Solana, Pablo; Kapadia, Phiroze; Dowden, John; Rodden, William S.O.; Kudesia, Sean S.; Hand, Duncan P.; Jones, Julian D.C. (2001). "Time dependent ablation and liquid ejection processes during the laser drilling of metals". Optics Communications. Elsevier BV. 191 (1–2): 97–112. doi:10.1016/s0030-4018(01)01072-0. ISSN   0030-4018.
  15. Roos, Sven‐Olov (1980). "Laser drilling with different pulse shapes". Journal of Applied Physics. AIP Publishing. 51 (9): 5061–5063. doi:10.1063/1.328358. ISSN   0021-8979.
  16. Anisimov, V. N.; Arutyunyan, R. V.; Baranov, V. Yu.; Bolshov, L. A.; Velikhov, E. P.; et al. (1984-01-01). "Materials processing by high-repetition-rate pulsed excimer and carbon dioxide lasers". Applied Optics. The Optical Society. 23 (1): 18. doi:10.1364/ao.23.000018. ISSN   0003-6935.
  17. Grad, Ladislav; Možina, Janez (1998). "Laser pulse shape influence on optically induced dynamic processes". Applied Surface Science. Elsevier BV. 127-129: 999–1004. doi:10.1016/s0169-4332(97)00781-2. ISSN   0169-4332.
  18. Low, D.K.Y; Li, L; Byrd, P.J (2001). "The influence of temporal pulse train modulation during laser percussion drilling". Optics and Lasers in Engineering. Elsevier BV. 35 (3): 149–164. doi:10.1016/s0143-8166(01)00008-2. ISSN   0143-8166.