| As one of failures of materials, the damage of material and loss of energy caused by wear and/or abrasion are tremendous for all kinds of mechanical components. In order to reduce the loss and damage, countries in the world have paid much attention to the research on wear or abrasion failure, researchers, engineers and technicians are constantly looking for all kinds of newly anti-frictional methods, developing novelty materials with excellent wear resistant property, exploiting innovative surface processing technology and also, having obtained great achievement in both theoretical study and practical application.From time immemorial, numerous inventions and creations made by human beings that were originated from powerful nature also provide newly breakthrough for developing novelty materials with excellent wear resistant and innovating surface preparation for new materials. Having investigated nature organisms thoroughly, our research group has found that the organism surface obtain some powerful ability by evolution over thousands of millions of years, including distinctive surface morphology,novel materials,sensible structure and unique functions, which can wonderfully adapt to nature. For instance, geobionts such as dung beetles, earthworms and pangolins et al.exhibits excellent wear or abrasion resistance when shuttling through soil and/or sandstone; dragonflies can take a long range flight, even through its wings sustained alternative stress over a long period of time and conflicted with all kinds of airflow. The results show that the function coupling with biological surface is similar to the failure of wear and/or abrasion.In this paper, according to actual requirement, we chose crimp elytrum of dung beetle as biological prototype, and intensively study its mechanism of wear resistance, design optimized biomimetic tribological specimens hoping for greatly improving wear resistance of metal materials. Air and de-ionized water were used as processing medium, respectively. An Nd: YAG pulsed laser with a wavelength of 1064nm was chosen for processing the strip biomimetic units with nano-grains on the surface of specimens underwater, which were cut from different materials, including vermicular cast iron, ball bearing steel and hot die steel. We study influence of different processing medium on characteristic dimension and effect of different thickness of water film on microstructure of biomimetic units, deduce temperature field expression of laser surface melting and cooling rate of molten pool under different thickness of water film, calculate the cooling rate of molten pool that processed in air and under different thickness of water film, investigate formation mechanism of nano-sized grains fabricated by laser surface melting under water film and detailed analyze the influence of air and different thickness of water film on the wear /abrasion resistance of vermicular cast iron, ball bearing steel and hot die steel.The present study shows that processing medium play a positive role in improving the wear resistance of three kinds of materials mentioned above. The important conclusions emerging out of this study are the following.(1) After the laser surface melting under water film, there are two zones in the biomimetic units: the melted zone and the transition zone. Along with water film thickness increasing, both the molten pool widths and that of depths decrease resulted from Marangoni convection, which is caused by absorbed and dissolved O2 that generated under the irradiation of laser beam.(2) Laser surface melting generates local liquid metal at the surface of specimen, which results in the formation of the cellular austenite structure during solidification, and simultaneous under cooling by a water film can lead to the formation of nano-sized grains in the form of rotational ellipsoids. Large grains are formed on the surfaces of GCr15 and H13 specimens by laser melting treatment under water with a 1 mm thickness of water film, and the average diameter decreases greatly to 50 nm when a 3 mm thickness of water film is employed. An increase in the thickness of water film is accompanied by a reduction of the mean size of the produced nano-sized grains. However, laser surface melting of CGI specimens cannot produce austenite nano-sized grains under the same conditions. During solidification of CGr15 and H13, austenite precipitates as a cellular structure, which is much different from the dendrite structures in CGI. The laser melted surface layers have not the columnar grain structure typical of epitaxial regrowth, but a fine, equiaxed grain structure.(3) It is concluded that the chemical segregation of chromium and the growth mechanism of austenite in liquid metal play a key role in the formation of nano-sized structures under a very large degree of supercooling. GCr15 ball bearing steel and H13 hot die steel both have high chromium content which became nucleation sites easily during the rapid solidification of molten pool. Droplets from the melting pool were solidified in water and the primary austenite forms and then crystallize due to high cooling rate of 1011m/s, and degree of nucleation that was greatly suppressed have distinctly effect on the growth of grains, therefore, nano-sized grains were formed under such experimental conditions. However, the primary austenite forms and then crystallizes into the dendritic structure perpendicularly to the solid-liquid interface due to a relatively lower alloy content that vermicular cast iron has. Although there is on nano-sized grains, the microstructure of vermicular cast iron formed under water film is much finer than that processed in air.(4) The results show that laser surface melting with water film can significantly increase microhardness of the three kinds of materials mentioned above. Under the same experimental conditions, the microhardness of H13 hot die steel is the highest, then the GCr15 ball bearing steel and that of vermicular cast iron is the lowest. In relation to every kind material, the more thickening the water film is, the higher the microhardness is.(5) Wear tests show that the wear resistance biomimetic units of vermicular cast iron against the 45 steel rings under dry sliding condition is significantly improved due to ultra– fined microstructure, increased hardness and strengthening phases. The wear loss is reduced by 20.51%, 41.02% and 51.28%, respectively. In comparison to the specimen processed in air, the wear loss of GCr15 ball bearing steel processed in air is reduced by 14.53%, 36.62% and 50.58%, respectively. In the case of H13 hot die steel, compared with the specimen processed in air, the wear loss of this steel processed in air is reduced by 40.84%,53.52% and 83.09%, respectively. Steady frictional coefficient of units processed under water film is much lower than that of units processed in air. Concerning the biomimetic units, these intensified unites decrease the contact area of the sliding pairs so that the wear rate reduces. According to SEM micrograph analysis, the specimen processed in are has obvious characters of adhesive wear and tear, but only there is adhesive wear for that of specimens processed in water film. Wear mechanism was changed due to application of water film. Under the same experimental conditions, the wear loss and frictional coefficient of H13 hot die steel were the lowest, then GCr15 ball bearing steel and that of vermicular cast iron were the highest.(6) The specimen processed in are has obvious characters of adhesive wear and tear, but only there is adhesive wear for that of specimens processed in water film. During the abrasion process, retained austenite would transform to martensite phase by phase transformation, therefore, it has a beneficial effect on wear resistance of the steel. Wear mechanism was changed due to nano-sized grains, ultra-high microhardness and martensitic transformation induced by retained austenite.(7) Nano-structure with high microhardness and strength can effectively prevent specimen surface from plastic deformation, also, material transition was difficult to happen during wear/abrasion process; initiation of cracks was effectively inhibited and grain breaking was alleviated by nano-sized grains, also, abrasive dust and oxide can not be easily plunged into surface under normal load. Therefore, it is concluded that nano-sized grains plays an active role in improving the wear resistance of ball bearing steel and hot die steel.(8) Biomimetic specimens with soft and hard alternated structure, where the relatively soft substrate can act as a buffer zone to absorb energy of stress and units with high hardness and strength can inhibit development of cracks, play key roles in preventing delamination from wear, and reduce wear loss.The main advantage of the laser melting process under water has the ability to prepare uniform nano-sized grains without any pre or post treatments. This technique could lead to the formation of nano grains with proper experimental parameters. Moreover, compared with other chemical or physical methods, this technique has the following advantages: simple procedures, controllable parameters, composition purity and no requirement for extreme temperature, pressure or a high vacuum system. This method could be extended to other metals, and these results have the basic and important meanings for further understanding the growth of nano materials. |
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