N. A. Dolgov

Analytical Methods to Determine the Stress State in the Substrate–Coating System Under Mechanical Loads

The paper presents the analysis of analytical models to determine the stress state in the substrate–coating system, when a load is applied to the substrate. The results obtained with these models are shown to differ. It is noted that the advantage of the analytical methods as compared to the numerical analysis is the possibility to calculate stresses in the singularity region in the vicinity of the free edge of the coating. The character of the shear stress distribution is analyzed using the analytical models. Examples of using different approaches to determine the critical shear stresses that define the adhesive strength of coatings are given. The analysis of literature sources shows that both simple and more sophisticated analytical models are employed in the investigation of the adhesive strength of coatings.

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.

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.

Mechanical Characteristics of Epoxy Nanocomposite Coatings with Ultradisperse Diamond Particles

The mechanical characteristics of epoxy nanocomposites with ultradisperse diamond particles as a filler are investigated. It is shown that the addition of the particles of ultradisperse diamond at a concentration of 0.05 part by weight (pt.wt.) per 100 pt.wt. of the epoxy binder, increases the most the bending strength and reduces residual stresses in the composite. Moreover, the elastic modulus of the composite material increases. The chemical activity of the nanofiller particle surface resulting in the enhancement of physical and mechanical properties in the cross-linking of epoxy-based composite materials is determined by IR-spectrum analysis. The fracture surfaces of nanocomposite materials were investigated using optical microscopy. Based on the analysis of the topology of fracture surfaces of nanocomposite materials, it was revealed that the structure is ordered, with no visible inclusions, which is indicative of the maximum degree of cross-linking of composite materials with a concentration of ultradisperse diamond of 0.05 pt.wt. A uniform stress distribution in the volume of the investigated materials is indicative of the thermodynamic equilibrium in systems after cross-linking.

Adhesive Pull and Shear Strength of Epoxy Nanocomposite Coatings Filled with Ultradispersed Diamond

In this work, epoxy diane oligomer ÉD-20 is used to form nanocomposite materials, polyethylene polyamine is used as a hardener, ultradispersed diamond with a particle size of 4–6 nm is used as a filler. The epoxy diane resin ÉD-20 was heated to 353 K. Then the ÉD-20 oligomer and the nanofiller were subjected to hydrodynamic mixing and subsequent ultrasonic treatment of the obtained composition. The effect of the concentration of diamond nanoparticles on the adhesion properties of composites was studied. The adhesion pull strength of materials was studied by measuring breaking stresses of cylindrical specimens glued by the nanocomposite adhesive. The adhesive shear strength was evaluated by the breaking stress of the lap joints of flat metal specimens glued by the nanocomposite adhesive. In both types of specimens, the glued sections had the same area. It has been found that the incorporation of nanoparticles in the concentration from 0.010 to 0.025 parts by weight per 100 parts by weight of ÉD-20 epoxy oligomer yields the adhesive pull strength of 25.0–26.3 MPa. The optimal concentration increases the latter value by a factor of 1.5 (to 34.3 MPa) and the adhesive shear strength – by a factor of 1.3 (to 10.8 MPa). The further increase in the concentration (0.075–1.0 parts by weight) is shown to deteriorate the adhesion strength. The nanocomposite coating–metal substrate bond breaking surface has been investigated. It is experimentally proved that the coatings with the cohesive nature of failure have the maximum adhesive pull strength, while those with the mixed (adhesive-cohesive) mode of failure exhibit an improved adhesive shear strength.

Effect of the Elastic Modulus of a Coating on the Serviceability of the Substrate–Coating System

We study the effect of the elastic modulus of a coating on its strength and strain characteristics, as well as on the residual stresses in it. The tangential and normal stresses that occur in the substrate and coating depend on the elastic modulus of the latter. It has been concluded that when designing substrate–coating systems, one should try to seek the optimum ratio of their elastic properties. An original approach has been proposed to the deposition of protective structurally inhomogeneous coatings, in which, unlike functionally gradient materials, not the composition but the structure is varied continuously.

Effect of a difference between the elasticity characteristics of the base and the coating on the stress-strain state of a composition. Part 3. Distribution of tangential and normal stresses in the coating

We present expressions for numerical evaluation of normal stresses in coatings and tangential stresses in the plane of adhesive contact formed under conditions of uniaxial tension of the base. These expressions take into account the difference between the Poisson ratios of the coating and the base. The indicated stresses are functions of the level of strains in the base and elastic and geometric characteristics of the base and the coating. We present plots of distributions of stresses in the plane of adhesive contact for gas-thermal coatings. The influence of fields of additional stresses must be taken into account in strength analysis of products with coatings and in experimental evaluation of the mechanical characteristics of base-coating compositions.