Cohesive Strength Of Coatings Research Articles

Determination of Adhesive and Cohesive Strength in Metal Coatings Deposited by Hypersonic Metallization

. It is known that at present, methods of thermal spraying are widely used to restore and strengthen various worn-out machine parts. As a rule, metal coatings applied by thermal spraying have lower strength characteristics than solid materials. It is believed that the strength of coatings is proportional to their adhesive and cohesive strength. The value of adhesive and cohesive strength depends on various factors, including the nature of the materials and the technology of coating. An important factor characterizing the possibility of using metal coatings in various industries is the strength of adhesion of coatings to the base metal. The paper presents the determination of the adhesive and cohesive strength of coatings from different materials, applied by the method of hypersonic metallization. The results of testing the strength of metal coatings made of ER316LSi-grade wire, nichrome (Cr20Ni80) and molybdenum wire are given in the paper. Based on the results of metallographic studies, the proportion of the participation of cohesive and adhesive components in the strength of coatings has been determined, and some features of coating destruction have been described. It has been found that the participation of the cohesive and adhesive components of the coating strength differs depending on the material used. The cohesive component prevails in the strength of coatings made of high-alloy wire of the ER316LSi-grade, at which destruction mainly occurs along the coating-base boundary. For nichrome coatings and especially for coatings made of molybdenum, the cohesive component is predominant, in which the destruction of the coating occurs not along the coating-base boundary, but between the coating layers.

Estimation of the Probability of Cracking of Facade Coatings

Information on the stress state of protective and decorative coatings during the curing process, in particular on the cohesive state of destruction, is given. The influence of the type of substrate on the change in internal stresses in the coating is considered. It was revealed that the greatest value of shear stresses is observed in coatings on a heavyweight concrete substrate. The subsequent increase in temperature after curing to 50°C leads to an increase in the value of the normal stresses. The probability of cracking of coatings during thermal aging is estimated. It was revealed that during aging there is an exponential decrease in the cohesive strength of coatings and an increase in internal stresses. Aging tends to increase the likelihood of cracking of coatings. The change in stresses in coatings as a result of seasonal fluctuations in air temperature is considered.

Restoration of internal surfaces of cylindrical parts of small diameters by cold gas dynamic spraying

Wear of internal surfaces of cylindrical parts of small diameters is a common defect of equipment used in the agro-industrial complex. There are problems with low powder utilization and lack of a design solution for coating the inner surfaces of cylindrical parts with small diameters. (Research purpose) The research purpose is in evaluating the adhesive and cohesive strength and developing a technology for restoring the internal surfaces of cylindrical parts of small diameters by cold gas-dynamic spraying. (Materials and methods) Authors applied the method of cold gas dynamic spraying. The article presents a device for gas-dynamic coating on the inner surfaces of cylindrical parts and determines the optimal deposition modes for powder consumption and heating temperature. A portable DIMET-403 unit, aluminum oxide powder for surface preparation, a-20-11 aluminum powder, and a steel plate were used for the study. The weight was measured on analytical scales with an accuracy of 0.001 grams. Tests were performed on the adhesive and cohesive strength of the coating on cylindrical samples prepared by mechanical processing, under the most favorable conditions with a duration of deposition equal to 60 seconds. Samples were processed after sputtering on a lathe to obtain a cylindrical belt. (Results and discussion) The values of adhesive and cohesive strength of coatings fit into the range of values suitable for operation, and meet the conditions for obtaining a wear-resistant ceramic coating. The article presents a technology for obtaining a sublayer for the manufacture of ceramic wear-resistant surfaces in a pair of friction piston ring-cylinder. (Conclusions) The developed restoration technology of internal surfaces of cylindrical parts of small diameters by cold gas-dynamic spraying solved the problem of their wear.

Estimation of Rheological Properties of Ash Silicate Paints

Information provided on the color of paint coatings on the mortar substrate, depending on the type of pigment in accordance with the RAL color file. It is shown that presence of glycerin in the composition of the sol of silicate paint promotes an increase in the cohesive strength of coatings, an improvement in the filling of paint, and an improvement in the quality of the appearance of coatings. Information is given on the rheological properties of a silicate paint sol with the addition of glycerin depending on the pigment content. It has been established, that the introduction of glycerin into the paint formulation leads to an increase in the values of the critical volume concentration of the pigment (filler). A model for changing the viscosity of paint from a pigment volume concentration (filler) is proposed.

Evaluation Methods of Cohesive Strength of Coatings

Based on the test results of the coatings applied by gas-dynamic deposition, the paper evaluates the possibility of using the “pin” and “ring separation” methods to examine the cohesive strength of coatings on tensile test. There is a limitation of the “pin” method application for coatings with porosity due to a significant scatter in results (≤50%). Using the method of the “ring separation” with a high reliability of the results (scatter ≤5%) makes it possible to evaluate the effect of technological parameters of deposition, as well as changes in the phase composition of the coating on cohesion. For example, the cohesive strength of the aluminum coating decreases from ≈180 MPa to ≈50 MPa with rising deposition temperature from 180°C to 540°C; a sharp drop in cohesion of the applied metal layer from 78 MPa to 44 MPa is observed for the coating based on copper and zinc particles on formation of βʹ - and γ phases.

Cohesive strength of nanocrystalline ZnO:Ga thin films deposited at room temperature

In this study, transparent conducting nanocrystalline ZnO:Ga (GZO) films were deposited by dc magnetron sputtering at room temperature on polymers (and glass for comparison). Electrical resistivities of 8.8 × 10-4 and 2.2 × 10-3 Ω cm were obtained for films deposited on glass and polymers, respectively. The crack onset strain (COS) and the cohesive strength of the coatings were investigated by means of tensile testing. The COS is similar for different GZO coatings and occurs for nominal strains approx. 1%. The cohesive strength of coatings, which was evaluated from the initial part of the crack density evolution, was found to be between 1.3 and 1.4 GPa. For these calculations, a Young's modulus of 112 GPa was used, evaluated by nanoindentation.

Characterization of surface coatings by the scratch adhesion test and by indentation measurements

Functional coating and surface modified layers can be characterized by engineering related criteria such as the ultramicrohardness test and the scratch adhesion test. The ultramicrohardness test is based on penetration depth measurements during a loading and unloading cycle. The depth sensing indentation provides a means of evaluating the elastic and plastic deformations of the tested material, from which, by calculation, the microhardness, HV, and the elasticity modulus, E, can be derived as illustrated on a number of examples of hard and soft materials and coating composites. The scratch test (REVETESTR) and microscratch test (MSTR) has been applied to determine the adhesive and/or cohesive strength of coatings deposited on softer or tougher substrates. A Rockwell C diamond cone is used to scratch the surface of the coated or surface-modified specimen at a constant speed and under a continuously increasing load. The smallest load at which the coating is damaged, called the Critical Load, Lc, is determined by optical, electron optical, acoustic emission detection and/or by frictional force measurement, i.e. by the sudden increase of the driving force. This is illustrated on hard, brittle, and on soft coatings.

Cohesive strength with the base of coatings of pseudoalloy materials

The cohesive strength of pseudoalloy coatings depends substantially upon the volume content of the low-melting component, forming a maximum at a completely definite value of it. The extreme character of the cohesive strength of the coatings is the result of partial relaxation of temperature residual stresses in the plastic low-melting filler. One of the effective means of increasing the cohesive strength of coatings (by 1.3–1.4 times) is addition to the powder mixture of a low-melting component with a volume concentration of 0.2–0.5.