Figure 1. Magnified image of patterned hemispherical depressions created with laser surface texturing on a material's surface.
Location data
Loading map...
{"minzoom":false,"maxzoom":false,"mappingservice":"leaflet","width":"auto","height":"200px","centre":false,"title":"","label":"","icon":"","lines":[],"polygons":[],"circles":[],"rectangles":[],"copycoords":false,"static":false,"zoom":1,"defzoom":14,"layers":["OpenStreetMap"],"image layers":[],"overlays":[],"resizable":false,"fullscreen":false,"scrollwheelzoom":true,"cluster":false,"clustermaxzoom":20,"clusterzoomonclick":true,"clustermaxradius":80,"clusterspiderfy":true,"geojson":"","clicktarget":"","imageLayers":[],"locations":[{"text":"","title":"","link":"","lat":44.2306870000000031950548873282968997955322265625,"lon":-76.481323000000003275999915786087512969970703125,"icon":""}],"imageoverlays":null}
Location Kingston, Canada

Laser surface texturing (LST) is a surface engineering process used to improve tribologicalW characteristics of materials. Using a laserW to create patterned microstructuresW on the surface of the materials can improve load capacity, wear rates, lubrication lifetime, and reduce friction coefficients. The use of surface irregularities to improve tribological properties was first discussed in the 1960's and is implemented in several manufacturing techniques.[1] While surface texture engineering has been studied for many years, the use of laser patterned surface microstructures for tribological improvements emerged in the 1990's and continues to undergo significant technological advancements. Lasers offer unparallel control of the surface microstructure and a low environmental impact compared to other surface etching processes.[2] Given that frictionW creates unavoidable losses and wear in countless processes and devices, the opportunities for improving efficiency and lifespan with LST technologies are extensive. As well, LST offers possibilities to overcome stictionW, for example in microelectromechanical systemsW.[3]

Basic Underlying Principles

LST is a materials process used to create patterned microstructures on the contact surface of the work piece. While different patterns can be used, the common microstructures are linear grooves, crossed grooves, and circular dimple-like depressions (Figure 1). These microstructures function to improve tribological characteristics in several manners. The effects listed below act in different magnitudes dependant on several application-specific properties (lubricant viscosityW, micropore geometry, relative contact velocity, load pressure, etc.).[4]

Microhydrodynamic Bearings

Figure 2. Pressure gradient created in each microcavity when relative velocity exists between the contact surfaces. The pressure gradient is a function of Reynolds equation and contributes to each microcavity acting as a microhydrodynamic bearing.

Each microcavity, whether it is a groove or dimple, acts as a miniature hydrodynamicW bearing during relative motion between the two contact surfaces.[1]This hydrodynamic effect is a result of the pressure gradient that forms in each pore, which can be modelled using the Reynold's averaged Navier-Stokes equationW.[5]With the relative motion between the two contact surfaces and the shear forces acting on the lubricant at the surfaces, a pressure profile is created due to the wedge effect (Figure 2). Microhydrodynamic bearings reduce the friction and wear between sliding surfaces. Lubrication is necessary for the microhydrodynamic bearing effect to become influential, as air has a very low viscosity relative to liquid lubricants. Non-lubricated surfaces required high relative speeds for this bearing effect to become relevant.

Debris Traps

The microcavities provide a sink for debris particles to fit into and reduce the associated additional friction of debris in the contact zone. The function of the pores as debris traps is found in both lubricated and non-lubricated applications, and is the main positive effect of surface texturing for non-lubricated applications.[4]

Lubricant Reservoirs

If an area of surface contact loses lubrication, the microcavities can provide additional lubricant that is drawn to the starved area through capillary actionW.[4]The patterned geometry allows for countless miniature lubricant reservoirs, providing direct and immediate lubricant relief for starved areas. In order for these reservoirs to exist, the geometries of the pattern microstructure must be closed to prevent lubricant from being forced out through channels.[6]

Applying the Technology

The creation of specific patterned surface microstructures can be achieved in many ways, for example abrasive blasting, reactive-ion etchingW, and ultrasonicW machining. However, laser technology offers the most control and precision over the produced geometry. Also, laser ablation does not use chemical reagents or produce significant waste. See the section below for alternative surface texturing methods.

To implement laser surface texturing, several technological decisions regarding the equipment and application need to be considered. These include the laser characteristics, the use of scanning or interference patterns, pore geometry and frequency, and full versus partial LST.

Lasers

In order to manufacture pattern surface microstructures, the laser used needs to be suitable for the workpiece material and capable of either melting or ablating the material. In practice, some of the main types of lasers utilized in LST are Nd: YAG laserW, carbon dioxide laserW, and excimerW lasers. The lasers are usually pulsed, often using a Q-switching setupW, to produce one dimple per pulse. With high repetition rates (pulse durations of μs to fs), thousands of dimples can be created in very short processing times. With possible speeds of well over 1000 microcavities produced per second, LST can be scaled to large areas.[6]

Nd: YAG and CO2 are more common than excimer lasers, as excimer lasers have comparatively low ablation rates and take excessive time for LST. On the other hand, due to the low abolition of excimer lasers, they can be used to create microstructures within micrometer precision. As well, excimer lasers are often used with lithography methods and etching procedures to create surface textures.

In laser machining, the material is removed either through ablation, melting, or a combination of both. Often, a high-pressure gas is introduced next to the laser focal point, which blasts away molten and sublimated material. Cutting and removal rates depend on the laser power, wavelength, pulse duration, and on the material properties of the workpiece; characteristics such as absorption, thermal conductivity, and thermal capacity. The hardness of the material does not affect its laser cutting rate, making LST effective on typically difficult substances like carbides and ceramics.

Direct Beam, Scanning, and Interference Patterns

In order to produce the patterned microstructures on the material's surface, the laser beam position relative to the surface has to be manipulated. There are three main techniques for controlling the laser ablation to create the desired patterns: Direct beam movement, scanning, and the use of interference patterns. The technique to be used depends on the workpiece geometry, the scale of production, and the budget.

Direct Beam

The direct beam methodology applies the laser beam directly to the material's surface. An ablation head focuses the laser and a high pressure gas line blows away melted and sublimated material. A mechanized system allows for the workpiece or the ablation head to be moved in controlled increments. By manipulating the relative position of the ablation head and the workpiece, along with the pulse duration of the laser, the frequency of surface depressions can be controlled.

Scanning

Another method of controlling the laser patterning is to utilize a scanning system. By reflecting the laser beam with a series of motorized mirrors, the laser beam can be quickly moved to the desired position for each microcavity to be created. These motorized mirrors, usually referred to as a mirror galvanometerW, can precisely and rapidly direct the laser beam to the desired contact point. To ensure the beam does not strike the surface on an angle, a flat field lens is utilized to ensure the beam and surface are perpendicular at the contact point.[6]

Interference Patterns

To create patterned grooves and crossed grooves, optical interferenceW patterns can also be used.[2]The interference pattern covers the size of the beam diameter and creates many microcavities at once. When two beams interfere with each other, constructive and destructive interference occur, creating parallel linear lines of light. If this pattern of laser light is directed at a material's surface for a given time, ablation and melting will occur, creating parallel linear grooves. The period of the linear grooves is given by the formula:

Period = λ/(2sin(α/2))

Where: λ is the laser wavelength, α is the angle between the laser beams.[2]

To create a crossed groove microstructure, a sample can be exposed in one orientation to create linear grooves, and then rotated 90° and exposed again to the light to create crossed grooves.

Pore Geometry, Size, and Frequency

The optimum patterning and geometry of micropores for the best tribological characteristics depend on several application factors and is not an exact science. With dimple-like depressions, it is found that the exact geometry is not significant compared to the ratio between pore diameter and depth, and the area fraction covered by pores.[1]The application factors that influence the optimum pore ratio and area fraction include:

  • Load capacity
  • Pressure
  • Material
  • Sliding Velocity
Figure 3. A 3D optical profilometer scan of the face of a piston ring treated with partial laser surface texturing.

In general, the current studies report optimum pore area coverage of between 10% and 15%.[7], [8] As well, the optimum pore diameter in the general case is around 10 μm.[8]At larger pore diameters, the lubrication film clearance drops and a sharp rise in friction is found.

In certain high pressure applications, LST is found to be most effective when only partial areas ofthe contact surface are textured.[9] By only texturing a partial area, the load capacity between the surfaces can be much higher.[9]An image of an engine piston ring with partial LST is shown in Figure 3.

Finishing Procedures

Laser surface texturing relies on the target material being melted, ablated, and blown away by high pressured gas. After LST has been applied to a surface, excess melt rims and slag are present on the surface as an artifact from the laser processing.[4]These melt rims and excess resolidified material need to be removed through polishing in order for the intended surface geometry to function as expected. This polishing can be done with gentle abrasives and standard mechanical polishing techniques.

Coatings can also be applied after LST to improve surface characteristics. Films of materials like diamond-like carbonW and alloys of Ti can be applied to improve certain surface characteristics, such as hardness and wear.[4]

Benefits

激光表面纹理有许多好处,可以潜在地节省大量能源并提高许多机械系统的效率。最明显的好处是减少摩擦。由于该领域相对较新,LST 应用中的变量数量允许一系列报告的摩擦减少。目前的研究表明,摩擦力大幅降低了 20% – 65%。[2] , [6] , [10]摩擦力的确切减少取决于许多变量,包括负载能力、微孔几何形状、速度和使用的材料。这种摩擦减少对于润滑和干燥情况都是真实的,非润滑应用的摩擦减少较低 (15%-40%)。[10]固体润滑剂,如 MoS 2和石墨,也可用于 LST 表面并受益于减少摩擦。[11]摩擦的减少有几个好处。首先,因热损失而节省的能量可以降低应用的能源需求。其次,较低的摩擦产生较少的热量,因此减少了表面的热应力和应变。最后,较低的摩擦系数会减少静摩擦,从而允许某些设备(例如MEMS W)使用较小的力来启动运动。[12]

另一个好处是疲劳寿命的提高。微腔充当碎片陷阱,防止小的松散颗粒引发微裂缝和损坏。据观察,经过 LST 处理的部件的磨损寿命比标准部件的疲劳寿命提高了三倍以上。[2] , [10]当应用 LST 时,由重复的小表面运动引起的磨损,称为微动W磨损,可以大大减少。实验发现,应用 LST 后微动疲劳寿命翻了一番。[13]

LST 应用的这些令人印象深刻的结果显示了这项技术的潜力。几乎所有的机械系统都以某种方式对抗摩擦,LST 的应用也在不断扩大。

应用

当前

LST作为一个相对较新的领域,主要处于研究和小规模应用阶段。目前,LST已有一些商业应用,如计划在汽车发动机生产线上使用LST,以及在磁存储驱动器中使用LST。[6] , [14]此外,还有几家专业公司将对提供的零件和密封件进行纹理处理。许多调查正在进行中,涉及不同的商业用途应用,包括:

未来应用

如上所述,LST 的潜力是巨大的,因为设备的体积会导致大量摩擦损失。未来 LST 应用的一些想法包括:

  • 直线轴承、旋转轴承
  • 飞轮W储能
  • 休闲活动和运动(滑雪板、溜冰鞋、游戏、雪橇)
  • 通过管子的弹丸加速(即电磁/轨道炮W卫星发射)

案例示例 -

为了强调通过 LST 应用节省的能源,可以研究一个具体的案例。活塞环W适用于 LST 应用,因为活塞环和气缸壁之间存在明显的重复相对接触。此能量计算中使用了以下保守参数:

  • 5 kW CO 2激光器
  • 100 次脉冲/秒,每次脉冲产生酒窝状印象
  • 4缸发动机
  • 每个活塞 3 个活塞环
  • 活塞环宽 2.5 毫米,半径 100 毫米,外径向表面有纹理
  • 凹坑之间间距为 0.5 mm 的凹坑方阵

使用这些参数,激光操作对所有 12 个活塞环进行纹理化所消耗的能量等于3.8 MJ为了提供此 LST 应用所节省的能量的近似值,可以检查涉及 LST 应用于福特全顺 4 缸柴油发动机的研究结果。[15]研究发现,与标准活塞环相比,使用 LST 活塞环时油耗降低了大约 4%。为了比较这些数字,柴油的能量含量为42 MJ/kg(大约 80% 的能量含量可转化为热能)。[17]这个例子没有考虑在 LST 的应用过程中会产生的许多其他小的能量损失,只有激光能量消耗,但它仍然表明能量回收期非常短。

替代表面纹理方法

与表面纹理相关的摩擦学改进也可以通过其他纹理技术而不是 LST 来实现。虽然激光提供了无与伦比的精度和控制,但正在使用和研究以下技术用于表面纹理化应用:[3]

References

  1. Jump up to: 1.0 1.1 1.2 D.B. Hamilton, J.A Walowit, C.M. Allen, "A Theory of Lubrication by Micro-Irregularities", ASME Journal of Basic Engineering 88(1), 177-185.
  2. Jump up to: 2.0 2.1 2.2 2.3 2.4 M. Duarte, A. Lasagni, R. Giovanelli, J. Narciso, E. Louis, F. Mücklich, "Increasing Lubricant Film Lifetime by Grooving Periodical Patterns Using Laser Interference Metallury", Advanced Engineering Materials 10(6), 554-558, 2008.
  3. Jump up to: 3.0 3.1 3.2 3.3 I. Etsion, "State of the Art in Laser Surface Texturing", Journal of Tribology 127 (248). 2005
  4. Jump up to: 4.0 4.1 4.2 4.3 4.4 C. Donnet, A. Erdemir, "Tribology of Diamond-Like Carbon Films, Chapter 3: Laser Processing", Springer US Publishing. 2008.
  5. L. Burstein, D. Ingman, "Effect of Pore Ensemble Statistics on Load Support of Mechanical Seal with Pore-Covered Faces", ASME Journal of Tribology 121(1), 927-932, 1999.
  6. Jump up to: 6.0 6.1 6.2 6.3 6.4 6.5 6.6 A. Hoppermann, M. Kordt, "Tribological Optimisation Using Laser-Structured Contact Surfaces", Oelhydraulik und Pneumatik 46(4), 2002.
  7. H.L. Costa, I.M Hutchings, "Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions", Tribology International 40(8), 1227-1238, 2007.
  8. Jump up to: 8.0 8.1 M. Mahbubur Razzaque, M. Tanvir Rahman Faisal, "Performance of Mechanical Face Seals with Surface Micropores", Journal of Mechanical Engineering 37(1), 2008.
  9. Jump up to: 9.0 9.1 Y. Kligerman, I. Etsion, A. Shinkarenko, "Improving Tribology Performace of Piston Rings by Partial Surface Texturing", Journal of Tribology 127(3), 632-639, 2005.
  10. Jump up to: 10.0 10.1 10.2 I. Etsion, "Improving Tribological Performance of Mechanical Components by Laser Surface Texturing", Tribology Letters 17(4), 733-737. 2005.
  11. .A. Voevodin, J.S. Zabinski, "Laser Surface Texturing for Adaptive Solid Lubrication", Wear 261(11), 1285-1292. 2006.
  12. Jump up to: 12.0 12.1 K. Komvopoulos, "Adhesion and friction forces in microelectromechanical systems: mechanisms, measurement, surface modification techniques, and adhesion theory", Journal of Adhesion Science & Technology 17(4), 477-517, 2003.
  13. A. Volchok, G. Halperin, I. Etsion, "The effect of surface regular microtopography on fretting fatigue life", Wear 253(3), 509-515, 2002.
  14. Jump up to: 14.0 14.1 B. Raeymaekers, I. Etsion, F.E. Talke, "Enhancing tribological performance of the magnetic tape/guide interface by laser surface texturing", Tribology Letters 27(1), 89-95, 2007.
  15. Jump up to: 15.0 15.1 I. Etsion, E. Sher, "Improving fuel efficiency with laser surface textured piston rings", Tribology International (In Press), Available online April 15, 2008 at: http://web.archive.org/web/20100306192703/http://www.sciencedirect.com:80/science/article/B6V57-4S92XSP-1/2/fd392ac261846c9ae6e6be66d42c7dfd
  16. M. Marticorena, G. Corti, D. Olmedo, M.B. Guglielmotti, S. Duhalde, "Laser surface modifications of Ti implants to improve osseointegration",Journal of Physics: Conference Series 59(1), 662-665, 2007.
  17. Food and Agriculture Organization of the United Nations, "Utilization of renewable energy sources and energy-saving technologies by small-scale milk plants and collection centres", FAO Animal Production and Health PApers 93(1), 662-665, 1992. Available online at: http://www.fao.org/docrep/004/t0515e/T0515E08.htm
Page data
Part of MECH370
Type Location
Keywords materials processing, nanotechnology, technology
SDG Sustainable Development GoalSDG09 Industry innovation and infrastructure
Published 2008
License CC-BY-SA-4.0
隶属关系女王大学
影响Number of views to this page and its redirects. Updated once a month. Views by admins and bots are not counted. Multiple views during the same session are counted as one.13,708
Cookies help us deliver our services. By using our services, you agree to our use of cookies.