PhD A. Brover,Y. Morozova, D. Nefedov, T. Volkova

Don State Technical University, Russia

LASER PROCESSING OF COATINGS

OBTAINED BY ELECTRIC SPARK ALLOYING

AND ION-PLASMA SPRAYING

Analysis of the problem of improving the reliability and durability of products for various functional purposes has shown that nowadays it is impossible to decide the question of increasing the product life only when using the expensive high-alloy steels, because in most cases it is not justified economically.

So the way to increase the durability of products of carbon and economically alloyed steels at the expense of thermostrengthening and microalloying of their effective parts using the laser radiation becomes extremely actual and important.

First of all, the following characteristic features of coatings obtained on steels by the methods of electric spark alloying (ESA) and ion-plasma spraying should be noted.

Coatings obtained by electric spark alloying, for example, graphite and hard alloy VK6, have relatively high bond strength with the substrate material. The specific properties of the surface layer of the product after the ESA are determined by two phenomena: directed migration of the anode material onto a substrate and pulse influence on microvolumes of the surface layer of high pressures and temperatures. However a large degree of composition heterogeneity as well as structural nonequilibrium and irregularity of the layers formed on the surfaces of products should be noted.

The technology of coating by the method of condensation with ionic bombardment (CIB) is characterized by high productivity. Coatings, in particular TiN and ZrN, have high hardness (more than 20 GPa), passivity towards the external environment as well as heat resistance in combination with the sufficiently high adhesion of coatings to metal substrates. But coatings have an extremely small thickness (3-5 µm), possess the fragility and in the process of their formation the substrate heating up to temperatures of 350-500°C is required that limits the range of coated steels. In addition, in the coatings there are considerable internal stresses caused by a high rate of their condensation when applied (0,1-0,5 µm/min). During the relaxation of these stresses the peeling of coatings from the metal substrate may occur.

To improve the quality of coatings obtained by means of ESA and the IRB this paper proposes the use of laser surface processing of steels.

Studies have shown that despite the extremely short duration of the laser allying process, in case of laser alloying produced by hard alloys of type VK, a significant number (30-35%) of fused carbides WC, W₂C having a high hardness is marked. These effects have a positive influence on the basic performance properties of irradiated steels.

During the laser processing of ion-plasma coatings there is an opportunity to increase the depth of the alloyed layer while maintaining the basic performance properties of coatings, to ensure a smooth transition of the properties from the coating to the substrate and to reduce the localization of stresses at the interface "coating – substrate" at the expense of diffusion processes with stirring the metal melted by laser radiation.

It has been established experimentally that the laser radiation with q = 125 MW/m² allowing the submelting of coating with thickness of 5 µm without evaporation and violation of surface microgeometry should be considered as an optimal regime of laser processing. This increases the adhesion strength of the coating to the steel substrate and the fracture toughness of irradiated products with coatings.

Studies on the scanning probe microscope of the TiN coatings before and after the laser processing have shown that the topography of the coating surface can be decreased to values of 0.1-0.9 µm in the original topography of 0.2-1.3 µm due to the laser submelting of the coating surface and the fusion of refractory element nitrides into the surface layers of the steel substrate.

It has been established that when the laser processing of coatings obtained by the ESA or the IRB method is carried out, there is a significant increase in the depth of the hardened layer at its sufficiently high hardness and wear resistance. It is also noticed that the coatings applied onto the sample surface increase the absorption capacity and thus the depth of the hardened layer by 10-35% as compared with pulsed laser processing.

On the basis of conducted research we can draw the following conclusions:

1.The combination of laser heating with alloying of steel surface layers of different coating composition obtained by ESA and by the CIB method is an effective means of improving the basic performance properties of the hardened products.

2. The structure and properties of laser alloyed surface layers on steels depend on the method of alloy coatings. The friction coefficients of the samples after laser alloying are reduced as compared to their values in the base metal in 1,5-3 times, and in the metal subjected only to laser hardening - 1.5 times.

3. The coating composition should be determined based on the hardened product properties required in the operating conditions.

Literature:

1. Brover, A.V. Structural features of the process of surface hardening of steel with the help of concentrated energy fluxes // Materials technology.- 2005.- No. 9.- P. 18-23

2. Brover A. V. Complex of mechanisms of hardening of metallic materials by pulsed laser processing // Advanced materials.- 2008.- No. 1.- P. 63-69

3. Brover A. V., Brover G. I., Dyachenko L. D. Some features of the structural state of steels in the areas of laser processing // News of higher educational institutions. Ferrous metallurgy.- 2007.- No. 6.- P. 37-40

4. Brover G. I., Dyachenko L. D., A. V. Brover. The improvement of operational characteristics of chemical coatings on steels by means of laser processing // Hardening technologies and coatings.- 2007.- No. 5.- P. 11-14.

5. Brover A. V., Brover G. I., Shevtsova, O. V. Structural self-organization of surface layers of steels by laser microalloying of powder coatings // Hardening technologies and coatings.- 2015.- №2 (122).- P. 21-28