Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5370_Библиотеки_им_академика_М_И_Перельмана

increase in time. The main reason for the lack of improvement in corrosion resistance of the LSM ZE41 Mg alloy was because of the formation of surface cracks following the high thermal stress created by the high input energy of the laser irradiation. This was substantiated by the smaller extent of surface cracking at a laser power of 820 rather than 1856 W. Another major issue in LSM is the change in microstructural features and surface roughness in the overlapping tracks during treatment. Coy et al.

the substrate surface. Upon impact with the substrate, the particles undergo severe plastic deformation, resulting in flattened splats, which is also accompanied by peen­ing of the substrate. As the temperature of the expanded gas stream that exits the spray nozzle is relatively lower than the conventional thermal spray processes, it is called a “cold spray” process. Hence, it is particularly suitable for coatings and substrate ma­terials that are sensitive to heat or oxidation and that would transform or react under the conditions of a conventional thermal spray process (Champagne, 2007). The perfor­mance of CS coatings is largely a function of bond strength between the coating and the substrate and the residual stress, porosity, and density of the coating. To achieve a better bond strength, it is important that the mean velocity at which the par­ticles are accelerated exceeds the minimum critical velocity so that the particles have sufficient kinetic energy to generate extensive plastic deformation of the substrate and to promote interlocking of the spray splats (Champagne, 2007). Because the particles are sprayed at relatively lower temperatures, there is little temperature-driven dimen­sional change, and hence the resultant coatings usually are in a state of compressive residual stress, which is beneficial to prevent delamination of the coating. The porosity of CS coatings is generally low, on the order of 1e 5%, and most of them are identified on the top surface. The following factors, either individually or in combination, are considered responsible for the occurrence of porosity in CS coatings: (1) rebound of particles with a velocity lower than the critical velocity; (2) large particles that do not undergo enough plastic deformation upon impact, generating porosity at the particlee particle boundaries; (3) the absence of a significant peening effect on the particles that are already deposited, particularly at the top of the coating; and (4) the sudden incompatibility in terms of localized heating, stresses, and plastic deformation (Balani,