| Nanocrystalline materials have attracted considerable interest because of their novel mechanical, physical and chemical properties originating from a large volume fraction of grain boundaries or interfaces with respect to the conventional coarse-grained polycrystalline materials. Most failures of materials occur on surface, including fatigue fracture, corrosion and wear etc., which are very sensitive to microstructure and properties of the surface. It is expected to achieve surface modification by the generation of a nanostructured surface layer, referring as surface nanocrystallization(SNC), and there is not obvious interface between the layer of nanostructures and the matrix, thus the nanostructures layer don't separate and appear break-crust because of the change of external condition, so that the properties and behavior of materials are significantly improved.Gas nitriding is widely used in industrial production to improve the surface properties of materials. The surface nanocrystalization can accelerate the process of chemical heat treatment. Nanocrystal materials have abundant interface and dislocation, these defects supply ideal entryway for the diffuseness of chemical elements, and can markedly lower the temperature and time of chemical treatment and enhance infiltration concentration and deepness.A nanostructured surface layer was fabricated on a 40Cr and 20Cr steel by using supersonic fine particles bombarding (SFPB) technique. The microstructure and mechanics property of the surface layer of sample was characterized by means of X-ray diffraction, transmission electron microscope ,optical microscope, microhardness testing machine and friction and wear testing machine. The development mechanism and mechanics property of the surface layer was essentially probed. The nanocrystalized samples are nitridized respectively at the temperature of 300℃, 350℃,400℃,450℃and characterized by multiform test methods, such as optical microscope, microhardness and XRD. Then we bring forward the principium of low temperature nitriding. Experimented evidence show that:1) After SFPB, equiaxed nanocrystallines with random crystallographic orientations were obtained in the surface layer of 20Cr and 40Cr steel, the average grain size of the nanocrystallines approximates to10nm. Under the pressure of 0.4MPa, the prime duration of 40Cr steel ranges from 60s to 480s; Under the pressure of 0.2MPa, the prime duration of 20Cr steel ranges from 480s to 3600s. The depth of the nanocrystal layer is about 30μm.With the increase of the distance from the top surface, the crystal dimension increases and the plastic deformation decreases.2) After SFPB, the surface microhardness is obviously increased, is about twice of the original sample before SDFPB; the abrasion-resistance of the surface layer is significantly enhanced compared with that of original sample, which can be primarily attributed to the grain refinement. The friction coefficient decreases and the weight of wear reduces.3) The result shows that the nitriding temperature of the nanocrystalized samples could be as low as 300℃, which was much lower than the traditional nitriding temperature. The critical nitrogen potential to formεphase from SFPB was obviously reduced with respect to that of the coarse-grained form. And the depth of the nitriding layer was increased obviously.4) The principium of the low temperature nitriding treatment is: the surface naocrystallization supplies plenty of entryway for the diffuseness of nitrogen elements; abundant interface and defects supply interface deposited energy for lowing the formation free energy of nitride; the coarseness of the sample is increased after SFPB, which increase the contact area for nitrogen atom. These three factors benefit for nitriding treatment.
|
PDF Full Text Request