Laser implantation of ceramics particles for enhanced surface textures on tool steel
Bart Ettema is a PhD student in the Department Surface Technology and Tribology. (Co)Promotors are prof.dr.ir. prof.dr.ir. G.R.B.E. Römer and dr. D.T.A. Matthews from the Faculty of Engineering Technology.
The innovative additive manufacturing technique known as “laser implantation texturin” allows to deposit a pattern of hard “implant” on a (metal) substrate. In this process, first a powder paste, typically about 100 micrometers thick, consisting of micrometer-sized ceramic powder particles and a binder, such as polyvinylbutyral, is deposited onto the metal substrate. In the subsequent step, a pulsed laser beam is directed at the powder paste layer. The absorbed laser energy causes the binder to evaporate and the metal substrate material to liquefy, allowing the solid ceramic particles to sink into the laser-induced melt pool. After the laser pulse, that is after solidification of the melt pool, the particles are either anchored or dissolved within the resolidified substrate, forming protruding geometries on the surface of the substrate. These protruding features are referred to as implants. As a last step surface is cleaned by removing excess powder paste around the implants. When repeating the above steps deterministic implant patterns on the substrate, with a large design freedom, could be created.
The ceramic particles are chosen for their high melting temperature and high hardness, resulting in hard and possibly highly wear resistant surface textures. Hence, LITex could for example be exploited to manufacture wear resistant textures on e.g. skin-pass rolls. This thesis studies scientific and technical challenges, that need to be resolved before the industrial applications of laser implantation texturing can be fully adopted by industry.
To that end, a millisecond pulsed Nd:YAG laser source, with laser pulse powers ranging from 10 W to 100 W, and laser spot diameters ranging from 54 micrometer to 105 micrometer, is used to process powder layers, ranging in thickness from 50 micrometer to 100 micrometer micrometer, consisting of either WC, TiB2 or TiC particles in a polyvinylbutyral (PVB) binder. Implants with dimensions smaller than the state-ofthe art—i.e. diameters smaller than 150 μm and implant heights between 1 μm and 15 μm— are needed for industrial adoption. Through extensive experiments, found that reducing the height of the powder paste layer to 50 micrometer is essential in order to reduce the diameter of implants.
As a result of absorbed laser energy, the binder in the powder paste evaporates, dragging powder particles in its wake, leaving a crater—i.e. a powder affected zone (PAZ), in the paste layer. Hence, creating an implant results in a loss (or lack) of powder surrounding the implant. The latter is limiting the distance (pitch) between subsequent implants, which is a limiting factor in the industrial application of LITex. To address this issue, an innovative coat-and-recoat approach is proposed and evaluated. That is, first a series of implants with a relatively large pitch is created. Then, after the substrate is cleaned, it is recoated with a second powder layer. Then, a second a series of implants, with again a relatively large pitch, is created in between the first series of implants. It is shown that this coat-and-recoat approach would allow patterns of densly packed patterns of implants, with a pitch as small as 150 micrometer.
Also, the effect of the laser-intensity profile on the morphology of the powder affected zone (PAZ) was studied. Using an optical beam shaping device, the laser beam emitted by the laser source was reshaped into a tophat, ring shape and a Gaussian beam profile. From the analysis of the powder layer it was found that the averaged intensity threshold at which the PVB binder evaporates equals 4.75 ± 2.25 kW/cm2. Unfortunately, the used beam shaping device introduces parasitic side lobes in the reshaped intensity profiles, with relatively high intensity levels from 2 kW/cm2 up to 48 kW/cm2. The latter are well over the intensity threshold at which binder evaporates. Hence, in order to reduce the diameter of the PAZ, the intensity in the slide lobes need to be significantly reduced. Alternatively, a binder material with a higher evaporation threshold should be chosen.
High-resolution (1.02 μm/pixel) and high-speed (frame rate 54000 fps) was employed in order to unveil the temporal behavior of the laser-powder layer interaction. From the recorded frames it was found that, the laser-powder interaction is highly dynamic. That is, in a period as short as the first millisecond after the start of the laser pulse, not only a plume of evaporated binder material forms, which dissolves quickly after about 0.35 ms, but that also the crater in the powder layer forms (at 0.52 ms), the powder affected zone (PAZ) grows, and a rim of powder particles forms on the edge of the PAZ forms (at 0.77 ms). Then, up to the end of the laser pulse (at 5 ms), the diameter of the crater, PAZ and rim do not change significantly. The melt pool was found to solidify in about 0.15 ms after the end of the laser pulse. Also, from these results it is concluded that the choice of the binder material in the powder paste is critical.
Finally preliminary tribological assessments of various densly packed LITex patterns consisting of TiC implants on AISI A2 tool steel substrates were carried out. That is, samples with densly packed LITex patterns, with a surface roughness Sa between 1.94 μm and 4.48 μm, were subjected to mechanical embossing and sliding-shear resistance tests. When embossing a mild steel strip, using the sample with the LITex patterns at representative loads in skin-pass rolling, it was found that the transferred patterns in the steel strip met the targeted surface roughness of Sa between 0.8 μm and 2.1 μm, which are typical for the automotive industry. The tribological shear tests comprised the sliding of a AISI 52100 steel cylinder repeatedly at representative loads, over the LITex textured substrate. Volumetric wear analysis of the LITex substrate after the tests show a small pattern volume decrease (ranging from 10 % to 35 % depending on the pattern type) after the first initial slide according to typical running-in behaviour. After the first few slides no significant further volume decrease is observed. All-in-all, from the findings and conclusions in this thesis, it can be concluded that this research has opened new possibilities for laser implantation texturing (LITex) as a promising technique for creating wear-resistant surface textures with new levels of design freedom by local additive manufacturing. The process is therefore suitable for up-scaling to applications in skin-pass rolling.
