Patent Grant
6926780
1. Field of the Invention
The present invention relates to a method of surface treatment of metallic materials, more particularly, to a method of nanocrystallizing the surface of metallic materials by bombarding of supersonic fine particles.
2. General Background and State of the Art
It is well known that material failures mostly occur on the surface of materials. Most of material failures, for example, such as fatigue fracture, fretting fatigue, wear, corrosion and the like, are very sensitive to the structure and properties of the material surface. Optimization of surface structures and properties may effectively improve combined properties of the material. As a result, the surface-modification of engineering materials is used in more and more industrial applications to greatly improve the behavior of materials. With the increasing of evidences of novel properties in nanocrystalline materials, it is necessary to provide a method for surface-modifying by the generation of a nanostructured surface layer, through which the combined properties and behavior of the materials can be significantly improved. This kind of surface modification, i.e., surface nanocrystallization (SNC), will greatly enhance the surface properties without changing the chemical composition. For example, the material which surface is nanocrystallized may have a strong and broad-spectrum absorption property. It is also a flexible method which is likely to fulfill specific requirements for structure/property of the surface of the sample. Surface nanocrystallization of materials can be carried out by using various processes. Among them, one is based on various coating and depositing technologies such as PVD, CVD and spraying methods. The coated materials can be either nanometer-sized isolated particles or polycrystalline powders with nano-sized grains. The coated layers and the matrix can be made of different or same materials. The predominant factor of this process is the bonding of the coated layer with the matrix. Another type of surface nanocrystallization is to transform the surface layers of the materials into nanocrystalline states while maintaining the overall composition and/or phases unchanged, and such a method is so-called surface self-nanocrystallization (SSNC).
Most of conventional mechanical surface treatment methods can be used for the SSNC. For example, ultrasonic shot peening (USSP) method has been used in surface self-nanocrystallization of metallic materials [J.Mater.Sic.Technol. 1999, Vol. 15(3): 193.]. In this paper, a concept of surface nanocrystallization (SNC) of metallic materials and surface self-nanocrystallization of metallic materials by ultrasonic shot peening was introduced. French patent No. 2689431 disclosed a method, in which, ultrasonic shot peening is carried out by the vibration of spherical shots (diameter thereof is 2 mm) created by high power ultrasound. The shots are placed in a reflecting chamber (including an ultrasonic concentrator) that is vibrated by a supersonic generator, after which the shots are resonated. Because of the high frequency of the system (20 kHz), the entire surface of the component to be treated is peened with a large number of impacts over a short period of time to obtain nanostructure layer. But this method is not suitable for the treatment of surface of complicated or larger parts.
Russian patent No. 1391135 disclosed a gas-dynamic spraying method for applying a coating. Upon which, the level of thermal stress can substantially reduced, and the thermal chemical action upon coated surface and powder particles can be weakened, and initial structure of the powder material can be preserved and there are no phase transformations, over-saturated structures or evaporation during application and formation of coatings. The aim of the patent only is to form an additional over-layer on the surface of materials but not to realize surface self-nanocrystallization of metallic materials.
It is an object of the present invention to provide a method for the surface self-nanocrystallization of metallic materials, which needs no coating materials and can effectively change the normal surface of metallic materials into nanocrystallized surface.
The present invention provides a method for the surface self-nanocrystallization of metallic materials, comprising the step of bombarding the surface of metallic substrate material with fine particles at supersonic speed of 300-1200 m/s carried by a compressed gas.
A more complete appreciation of the invention will be readily obtained by reference to the following description of the preferred embodiments and the accompanying drawings in which numerals in different figures represent the same structures or elements, wherein:
The term “supersonic” used herein means a speed of 300-1200 m/s.
The surface self-nanocrystallization of metallic materials needs no coating materials and can effectively change the normal surface of metallic materials into nanocrystallized surface.
The present invention provides a method for the surface self-nanocrystallization of metallic materials, comprising a step of bombarding the surface of metallic substrate material with fine particles at supersonic speed of 300-1200 m/s carried by a compressed gas which ejected from a nozzle.
According to the present invention, the substrate can be any metallic materials. Among them, carbon steel and 316L stainless steel are the most preferred.
According to the present invention, the metallic substrate surface can be pre-treated by any conventional methods before bombarding. The preferred method is to polish the surface and then wash it with acetone and/or alcohol.
Many fine particles can be used in the present method. Preferably, the fine particles is at least one selected from the group consisting of α-Al2O3, SiO2, BN and WC., et al. Among them, α-Al2O3 and SiO2 are more prefered, α-Al2O3 is the most prefered. The average size of fine particles is depending upon the finished surface desired. Preferably, the average particle size of fine particles is from about 50 nm to about 200 μm.
Any safe gases can be used in the present invention. Preferably, the compressed gas is air or nitrogen gas.
The bombarding can be carried out continuously, intermittently or half-continuously, preferably continuously.
Many kinds of nozzles can be used to eject the compressed gas. Preferably, the nozzle is Laval.
According to one preferred embodiment of the present invention, the operating parameters are as follows:
According to another preferred embodiment, the present method comprises the following steps:
Although the present invention is not bounded by any theory, it is believed that bombarding of a mass of particles with high speed continually and effectively on the surface creates localized severe plastic deformation, which further creates dislocation, twin crystalline structure and subcrystalline structure, as a result, crystal structure is refined and finally nanometer regime is obtained.
The present method has the following advantages:
The substrate material used is 316L stainless steel tube; the used particles are α-Al2O3 (about 50 μm). The nanocrystallization was carried out as the following:
Microstructural analysis has showed that the average crystalline grain size of the surface of the 316L tube is to be refined from 18 μm to 14 nm by means of X-ray diffraction and atomic power microscopic techniques.
The equipment for applying surface self-nanocrystallization of metallic materials showed as
Firstly, the compressed gas controlled by the control console 2 is fed into the particles feeder 3 where the particles are accelerated to a supersonic speed and carried by the compressed gas to pass through the supersonic nozzle 6 to bombard the surface of the substrate material in the bombarding chamber 4 continuously, as a result, plastic deformation of the surface is created and then a mass of dislocation, twin crystal structure or subcrystalline structure are produced. Finally, the crystal grains are refined and a nanocrystalline layer is formed. The crystalline grain size thickness varies in the range of 0.5˜50 μm. The residual particles are recovered by the DESC system 5. The control console 2 controls the all process.
The principle of design of supersonic nozzle was given by the equation of fluid mechanics. As to one-dimensional steady fluid, considering the compress fluid, the equation should be:
v2/2+(K/K−1)·P/ρ=constant (1)
ρ·v·S=constant (2)
P/ρk=constant (3)
The following equation can be calculated from the above equations:
ds/s=(M2−1)dv/v (4)
In the four above equations: S is cross sectional area of the nozzle; M=v/vS (Mach number, wherein vS is velocity of sound); ρ is gas density; K is gas constant; P is gas pressure; v is gas velocity. It can be known from formula (4) that, when v>vS, both dv and ds are positive or both are negative. That is to say, the velocity of gas flow increases as the cross sectional area of a tube increases (ds is positive number). However, when v<vS, one is positive and the other is negative, that is, the velocity of gas flow increases while the cross sectional area of a tube decreases (ds is negative number). Therefor, after the section area of a tube had been contracted fully, the velocity of gas flow is accelerated to a velocity of sound at one cross section of throat after has passed this section, the velocity of gas flow will reach an supersonic speed.
The same procedure was employed in the same manner as in Example 1, except for the following:
Microstructural analysis has showed that the average crystalline grain size of the surface of the carbon steel plate is to be refined from 12 μm to 20 nm by means of X-ray diffraction and atomic power microscopic techniques.
The device used is shown as in
| Number | Date | Country | Kind |
|---|---|---|---|
| 01128225 A | Sep 2001 | CN | national |
| Number | Date | Country |
|---|---|---|
| 2 689 431 | Oct 1993 | FR |
| Number | Date | Country | |
|---|---|---|---|
| 20030127160 A1 | Jul 2003 | US |
