Effect of triangular texture on the tribological performance of die steel with TiN coatings under lubricated sliding condition
Graphical abstract
Introduction
Stamping die has been used widely in the modern industrial production, and the life of stamping die is the most crucial factor for stamping production [1]. Since the friction and wear of a stamping die are a major reason to cause die invalidation, there is an increasing demand to reduce and control the friction and wear, in order to extend their lifetime, to improve their efficiency and reliability, to conserve scarce material resources and energy, and to improve the safety [2], [3].
The laser surface texturing (LST) is a relatively effective and novel technique to improve the tribological performance of mechanical components by creating patterned microstructures on the mechanical contact surfaces [4], [5]. In LST technology, a pulsated laser beam is used to create patterned microstructures on the surface by a material ablation process, which can improve anti-seizing ability by reserving lubricant and trapping wear debris to prevent further abrasive wear [6], [7], decreasing the contact area to reduce adhesion, and generating hydrodynamic pressure to improve additional lift [8], [9]. In previous work, the authors demonstrated that the dimension parameters of surface textures had an effect on the tribological performance of the contact surfaces, and that increased the lifetime of tribo-systems sliding under different conditions with optimal parameters [10], [11]. In addition, the influence of surface texturing shapes on the tribological performance was also widely studied. The effect of the triangle-shaped dimples array on the tribological performance through ball-disk point contacts was investigated, and found that the coverage ratio of approximately 10% and the dimple depth of approximately 10–15 μm resulted in the lowest coefficient of friction [12]. Compared with the regular circular texturing surface, the gourd-shaped surface texture with positive direction obtained the lowest coefficient of friction [7].
Surface coating is another effective way to improve the tribological performance of components, such as metal forming tools. Here, TiN coatings have been widely used in industry as wear resistant coating, especially for forming tools due to their high hardness, favorable corrosion resistance and good chemical stability [13], [14], [15], [16], [17]. Previous researches studied the tribological behavior of TiN coatings and suggested that the tribological performance was closely related to the microstructure, residual stress and mechanical property of TiN coatings [15]. Moreover, the coatings process has an effect on the tribological performance of the TiN coatings, and the results suggested that the hardness, plastic deformation resistance and wear behavior of TiN coatings had increased remarkably after ion implantation [16], and the element content of coatings also has a certain influence, and the frictional coefficient of TiN coatings decreased with the increase of chlorine contents [17].
To further enhance the tribological performance of TiN coatings, quite a few researches applied surface texturing on the coatings, and the results indicated that textured TiN coatings have an excellent tribological performance in different applications [18], [19], [20], [21]. The PVD-TiN coatings of tool steel had discontinuous surface architecture, which significantly decreased the friction, wear and the wearing speed of tool steels and improved its lifetime [19]. Textured TiN coatings also have been applied to die steels, but the studies are extremely limited, the results stated that in-lined and staggered spot-island patterned coatings possessed relatively better wear behaviors than the fully coated discs [21].
Most of the researchers investigated the tribological performance of cycle-textured TiN coatings on the surfaces of cutting tools or alloy steel. However, limited studies have analyzed the tribological behaviors of triangular-textured TiN coatings on the surface of stamping die steel. Considering the stamping process is unidirectional and the convergence speed of triangular texturing from a vertex angle to the side is fast under oil lubrication [22], the present work aims to studying the effect of triangular surface texture on the tribological performance of the die steel substrate with TiN coatings by carrying out the oil lubrication sliding tests on a pin-on-disk tribometer.
Section snippets
Laser surface texturing and coating deposition
In this work, 50Cr die steel, extensively used in stamping die with a nominal hardness of 50–55 HRC, was selected as the test sample. Before the laser surface texturing and coating process, all samples were polished using SiC papers with a grit size range of 800–3000 to reduce surface roughness. After polishing, the samples were cleaned with alcohol in an ultrasonic bath to achieve a surface roughness of Ra 0.1 μm.
To compare and analyze the influence of texturing, coating, as well as the
Surface morphology of the test samples
The two and three dimensional morphology and the two dimensional cross profile of a single triangular texture were measured and shown in Fig. 3. It can be seen that the regular triangle texture was about 550 μm in side length and about 4.6 μm in depth. There were a large amount of metal oxide particles on the sample surface and many burrs around the triangle dimple after laser surface texturing process, which indicated that part of the materials removed by laser were oxidized in air and splashed
Summary and conclusions
The effect of the triangular surface texture on reducing the friction and wear of the stamping die steel surface with TiN coatings was experimentally investigated under oil lubrication conditions. The test samples, processed in different ways, exhibited different tribological performance.
Both the laser surface texturing and TiN coatings showed good effectiveness in reducing friction and wear. Compared with the none-t/c samples, the t/c samples exhibited an enhanced tribological performance,
Acknowledgements
The authors would like to thank National Natural Science Foundation of China (Grant no. 51305023) and the Fundamental Research Funds for the Central Universities of China (FRF-BR-15-037A) for the financial support. In addition, the authors would like to thank Hongfei Shang (State Key Laboratory of Tribology, Tsinghua University) for technical support, and express special thanks to Lei Zhang (University of Alaska Fairbanks) for writing assistance.
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