B. A. Lyashenko

On the Reduction of Residual Stresses in Plasma-Vacuum-Deposited Coatings

Residual stresses in a plasma-vacuum-deposited titanium-nitride coating, which vary between –780 and –2800 MPa depending on the coating thickness, have been determined. It has been shown that deposition of a pure-chromium sublayer and substitution of a discontinuous (discrete) coating for a continuous one allow residual stresses to be reduced appreciably. It has been found that residual compression stresses can affect favorably the characteristics of strength of both the coating itself and the coated part as a whole.

ON THE HARDENING EFFECT OF PLASMA-DEPOSITED COATINGS

We studied the influence of plasma-deposited titanium-nitride coatings on the characteristics of short-term static strength of the matrix material. In the process of deposition of these coatings, we observed an increase in the conventional yield limit and ultimate strength of the material accompanied by a decrease in the characteristics of plasticity.

Strength and deformation properties of compositions with plasma-sprayed coatings

Results, which make it possible to compare the strength properties of the group of compositions investigated with plasma-sprayed coatings, are obtained. It is established that the deformability characteristic εcrc correlates better with strength characteristics as compared with the universally adopted deformability characteristic εcrb. It is demonstrated that based on the set of parameters determined, compositions with a thin dysprosium oxide coating and compositions with a thick yttrium oxide exhibit the best mechanical characteristics of the coatings compared.

Improvement of Strength and Wear Resistance of Metal Products with Ion-Plasma Nitride Coatings by Pulse Technique Implementation*

The article covers the innovative technology developed at Pisarenko Institute of Problems of Strength, National Academy of Sciences of Ukraine, i.e., the ion-plasma coating deposition pulse technique, which offers a considerable opportunity for producing diffusion layers of a desired structure by controlling the process of diffusion saturation and its optimization depending on the specific engineering requirements. The diffusion layers with nitrided and non-nitrided phases can be obtained by adjusting the composition of ionized gases and glow charge intensity. The proposed innovation, which was nationally patented in 2013, is currently used for small-scale industrial implementation in domestic aircraft and machinery repair shop-and-plants. A brief review of the state-of-the-art industrial developments in this area implies that the proposed technique can be utilized in available commercial applications for PVD coating deposition. The results of experimental studies on static strength, fatigue, and wear resistance in corrosive environments of 40Kh13 steel specimens treated by the proposed pulse technique confirm its positive effect. 2TK]

Fatigue Strength of Metals with Hardening Coatings (Review)

Investigations on the fatigue strength of structural materials with coatings are analyzed. The results of validation of both some theoretical approaches and numerous experimental data are presented for materials with coatings obtained by ion nitriding methods or by a physical vapor deposition method.

Distribution of temperature in a plate with a single-layer coating subjected to intense heating

We establish the distribution of temperature formed under conditions of abrupt changes in the surface temperature near the adhesional contact between the base and the coating in a plate of infinite length made of molybdenum or niobium both sides of which are covered with silicide coatings. The problem of heat conduction for a multilayer plate subjected to thermal cycling is solved by the method of finite integral transformations. It is shown that a silicide coating with a thickness of 60–100 μm leads to the formation of a significant temperature gradient in the base material under conditions of cyclic variation of temperature.

Effect of Poisson's Ratio on the Limiting Stressed State of a Coating

We present relations for the calculation of equivalent stresses in a coating. Plots of equivalent stresses against the difference in Poissons ratio between the substrate and the coating have been constructed. It has been shown that the calculated curves give a good fit to the magnitudes of the maximum normal and equivalent stresses for different coatings. Analysis of the results obtained shows that it is necessary to take into account the effect of Poissons ratio of the coating on the equivalent stresses and, accordingly, on the limiting stressed state of the coating in the strength analysis of coated structural elements and in the experimental determination of cohesive strength in laboratory tests. When designing coated structural elements, one should seek the combination of the elastic properties of the substrate and coating at which the magnitude of the equivalent stresses in the coating achieves its minimum value.

Determination of Discrete-Structure Parameters of Coatings with Allowance for Residual Stresses

The authors suggest that the dimensions of a discrete section of a coating should be chosen on the basis of calculation of the critical crack spacing in cohesion cracking. This approach allows for residual stresses in the coating. The reliability of the calculation method has been verified experimentally by direct measurement of the crack spacing.

Effect of Coatings on the Fatigue Characteristics of Steels Under Contact Load

A new type of technological process is proposed for increasing the life of structural elements under rolling friction. Results of comparative experimental studies on increasing the life of steels under rolling friction, using hydrogen-free integral techniques for the application of gradient coatings with optimal properties, residual stresses and substrate hardness and using ion nitriding and heat treatment.

Determination of Residual Stresses in Surface Layers Upon Ion Nitriding by Microhardness Values

We present the results of investigations on the value and distribution of residual stresses in surface layers of 20Kh13 steel upon ion nitriding in hydrogen-free media. The effect of nitriding parameters on the depth and hardness of the nitrided layer as well as on residual stresses in the surface layer is analyzed. The procedure for the determination of residual stresses at the arbitrary point of the nitrided layer depth by the microhardness values at the given point is developed.