In the field of surface technology, enhancing the wear resistance of materials used in industrial processes is of paramount importance. Bart Ettema’s doctoral research at the University of Twente focuses on an advanced technique called Laser Implantation Texturing, LITex, which has the potential to revolutionize how we design and manufacture durable surfaces for high-performance tools. The process combines laser technology with additive manufacturing to embed hard ceramic particles into metal substrates, creating highly wear-resistant surface textures. This innovative approach could be applied to critical components like skin-pass rolls in the steel industry, which experience heavy wear during manufacturing processes.
LITex begins with a powder paste containing fine ceramic particles, such as tungsten carbide WC, titanium diboride TiB2, or titanium carbide TiC, suspended in a binder material like polyvinyl butyral PVB. The paste is deposited onto the metal substrate, and a pulsed laser beam is directed at the surface. As the laser pulse interacts with the powder layer, it evaporates the binder, melting the substrate material and allowing the ceramic particles to sink into the molten metal. Upon cooling, these particles become embedded in the solidified metal, forming protruding features known as implants. These implants create textured surfaces that are highly resistant to wear, making them suitable for high-stress applications like metal forming and machining.
The research delves deeply into optimizing the LITex process for industrial applications. One key challenge in applying this technique is controlling the size and density of the implants. To achieve this, Ettema explored the effect of powder layer thickness on the diameter of the implants. By reducing the powder layer thickness to as little as 50 micrometers, he was able to create smaller implants with finer details, making the process more suitable for high-precision applications. However, the creation of implants also results in the loss of surrounding powder, which can limit the distance (pitch) between adjacent implants. This poses a challenge for achieving dense, uniform patterns on the substrate.
To overcome this limitation, Ettema introduced a novel coat-and-recoat approach. In this method, a series of implants with a relatively large pitch is first created. After cleaning the substrate, a second layer of powder is applied, and a second series of implants is generated in the spaces between the first series. This technique allows for densely packed implant patterns with a pitch as small as 150 micrometers, making it possible to produce intricate, wear-resistant textures with higher resolution. The coat-and-recoat method significantly enhances the versatility of LITex, enabling the creation of more complex surface patterns that can be customized for specific applications.
Another critical aspect of LITex involves the laser intensity and the resulting heat profile. Ettema's research studied the effect of different laser intensity profiles on the morphology of the powder-affected zone (PAZ), the region where the laser interacts with the powder and substrate. Using an optical beam-shaping device, Ettema tested several beam profiles, including Gaussian, tophat, and ring-shaped patterns. His findings revealed that controlling the laser intensity was crucial to achieving the desired implant shapes. Specifically, the intensity threshold required to evaporate the binder material in the powder was found to be approximately 4.75 kW/cm². However, the beam-shaping device introduced parasitic side lobes with intensity levels that exceeded this threshold, potentially causing unwanted heating and distortion in the surrounding material. Addressing this issue required reducing the intensity of the side lobes or selecting a binder material with a higher evaporation threshold.
High-speed, high-resolution imaging techniques were employed to study the dynamics of the laser-powder interaction in real time. Ettema used a high-speed camera with a frame rate of 54,000 frames per second to capture the millisecond-scale events occurring during the laser pulse. This allowed him to observe the rapid formation of the crater and the powder-affected zone, as well as the behavior of the evaporated binder material. The results showed that the laser interaction is highly dynamic, with the formation of a plume of evaporated material and the subsequent growth of the PAZ occurring within fractions of a millisecond. This precise understanding of the laser-powder interaction was essential for refining the process and improving the quality of the resulting surface textures.
Ettema also conducted tribological tests on the LITex-processed substrates to evaluate their wear resistance. Samples with densely packed implants were subjected to mechanical embossing and sliding-shear resistance tests, simulating the conditions they would experience in industrial applications. The results indicated that the LITex patterns were highly effective in improving the wear resistance of the substrates. In embossing tests with a mild steel strip, the transferred patterns achieved the targeted surface roughness levels typical for industries like automotive manufacturing. Volumetric wear analysis showed a small but consistent decrease in the implant volume after initial sliding, with little further wear after the first few cycles, indicating that the textured surfaces maintained their durability over time.
The implications of this research extend beyond the laboratory, offering a pathway to the widespread industrial adoption of LITex as a tool for enhancing the durability of critical components. By providing a high degree of design freedom and enabling the creation of wear-resistant textures with intricate patterns, LITex could transform the way industries approach the design and maintenance of high-performance tools and machinery. With further refinement, the technique has the potential to be scaled up for use in large-scale manufacturing, particularly in sectors like automotive, aerospace, and steel production.
