Introduction

Surface texturing – also referred to as surface structuring – is the process of applying a specific roughness onto a surface in order to change its properties. Ultrashort pulse lasers offer many new possibilities to tailor surface textures and control their properties in a way that cannot be matched by other techniques. Textures with features on multiple scales can be applied, starting in the nanometer range and extending up to the millimeter range. The technology furthermore can be applied on any material and can be used for texturing 3D curved surfaces.

Ultrashort pulse laser texturing enables many new applications of engineered surfaces, where specific surface textures can be used to influence the functional properties, e.g., the wetting, tribologic, or optical properties. Such surfaces are also referred to as functional surfaces.

Technology

We can distinguish two general approaches of ultrashort pulse laser surface texturing: machining using direct-write processes (closely related to laser milling) and a method using self-organizing effects that result from the laser-material interaction when using ultrashort pulses.

Self-Organizing Textures

Self-organizing effects caused by the laser material interaction can be used to create patterns in an implicit process. The dimensions of the textures are independent of the focused laser spot size; typical sizes can even be much smaller than the used laser wavelength. The textures appear homogeneously over larger areas that are scanned by the laser spot. Self-organizing textures can be fabricated on different materials, e.g., on metals, ceramics, semiconductors, and glass. The important laser parameters for such patterns are the wavelength, polarization, and the laser dose; the latter can be understood as the accumulated energy per unit area.

Several different types of self organizing textures can be made. LIPSS, an acronym for Laser Induced Periodic Surface Structures, are regular ripple structures with a typical period of a few hundred nanometers. The three SEM pictures below illustrate different stages of LIPSS that have been investigated in collaboration with the University of Twente, Chair of Applied Laser Technology, in the TULP project. The material used here is a high temperature steel.

At a very low laser dose spheres with diameters of 10–50 nm appear on the surface. After increasing the dose – in this case by using a higher pulse energy – a fine pattern of ripples emerges parallel to the laser polarization direction. Such patterns are called high spatial frequency LIPSS. The third SEM picture shows the normal LIPSS pattern where the ripples are perpendicular to the laser polarization and the spacing is in the order of the laser wavelength.

By applying special techniques it is also possible to generate very uniform LIPSS, as presented in the SEM picture below.

The use of a higher laser dose enables creating chaotic structures. The LIPSS pattern with a period of a few hundred nanometers is still visible in horizontal direction, but it is interrupted by deep, irregular vertical grooves.

After applying even more laser dose another type of self-organizing texture can appear on some materials. These so called cone-like-protrusions (CLP), here demonstrated in a nickel-chromium alloy, are characterized by high aspect ratios combined with high spatial frequencies. The properties of CLP's can be influenced to some extent by the laser parameters.

Three different variants of CLP's on the same material.

Direct-Write Textures

As with laser drilling or milling, the focused laser spot can be used to remove material in a defined pattern for the production of surface textures. The textures resulting from this method are defined by the laser scan pattern and the focused spot size; we also refer to such textures as explicit, while the self-organizing textures are implicit processes.

Typical distances between the peaks of direct-write textures are starting at about 10 μm and the structure depth is typically in the same order of magnitude. A texture like the ones shown here illustrate the accuracy that can be obtained. Here precisely shaped pillars are machined in stainless steel.

 By selective removal of material a texture of free standing pillars can be created with accurate control on the pillar height and shape, as shown in this 3D measurement.

An often used texture type is the dimple texture, which consists of a pattern of laser drilled dimples or holes. Typical hole diameters range from below ten micrometer to several tens of micrometers. The aspect ratios of the holes can be up to about 10, but for most applications typical aspect ratios are close to 1.

It is possible to create a self-organizing sub-micrometer texture superimposed on the direct-write texture in a single processing step. This example shows a pillar texture superimposed with LIPSS.

Fabrication of Textured Surfaces

Lightmotif has developed the technology to apply textures to large and 3D curved surfaces. For this a high precision 5-axis manipulator was designed and integrated. This machine can handle large workpieces with a weight of up to 300 kg.

To apply the textures to large or curved parts the surface is first divided into tiles, based on a surface description like a CAD file. These tiles can be textured one after the other using our step-and-scan approach. The textured steel ball shown here illustrates this technique. The upper part is fully covered with a texture that has been distributed into each single tile. On the lower part only the tile boundaries are marked to demonstrate the tiling approach.

A special application of this technique is texturing of injection molds. Such molds can be used to transfer the micro-nano textures into the injection molded polymer product. This method can be used as a cost effective solution for producing plastic parts with improved properties or new functionality.

Part of this technology was developed in the European FP7 project NANOCLEAN.

Application Areas

Textures on the micro- and nanoscale have a large effect on various material properties such as wetting, reflection and absorption of radiation, friction, (cell) adhesion and many more. Lightmotif is not specialized in these applications; instead we concentrate on the fabrication of the required textures. By working together with our customers and using their application knowledge we can realize their applications. To sketch the possibilities a couple of applications are explained below.

Wetting Properties

The wetting properties of a surface are dependent on its surface tension and surface texture. The texture amplifies the basic wetting property of a material. A hydrophobic material can be changed to obtain super-hydrophobic properties; and a hydrophilic surface gets even more hydrophilic after applying a texture. This can be used for example to become super-wetting surfaces.

The images below show super-hydrophobic surfaces. Here a water droplet is only in contact with the tops of the texture and displays a very high contact angle – instead of spreading out on the surface the water forms a round sphere. Water easily runs off such surfaces; small drops can even bounce, as illustrated in the compilation of a high speed recording.

A nice application, where a needle for contact angle measurements was textured and subsequently coated with a monolayer of a fluor containing coating, is illustrated below. This coating in combination with the texture rendered the needle's surface super-hydrophobic. This way water was prevented from wetting the needle surface while performing measurements with the needle.

Optical Properties

Surface textures can change the absorption and reflection of light, for example, to reduce glare of a surface. The self organizing LIPSS can be used to create optical gratings, as shown in this example.

Friction and Lubrication

Textures can also be used to alter the tribological properties of materials, for example, to reduce or increase the friction, or to reduce the wear between sliding surfaces. Dimples are frequently used to improve lubrication in sliding contacts, as they can create a hydrodymanic pressure, or function as an oil reservoir or wear particle trap.

Besides technical applications, medical or consumer products can also benefit from altered friction properties. Skin friction of a surface can be drastically reduced for example. By applying a pillar texture the haptics of surfaces can be changed into a silky smooth touch. This effect can also be applied to injection molded products, where Lightmotif's 3D texturing technology is applied to fabricate the required textures into the mold. The picture below shows a typical texture that can be used to obtain such effect on a molded product.