Tham khảo tài liệu 'the materials science of coatings and substrates part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Porosity 251 Figure 2 The influence of porosity on a the yield stress b the elongation to failure and c the percent reduction in area for chemically pure titanium and T1-6A1-4V. Adapted from Reference 10. of high pore content which are preferred sites for flow localization and fracture 10 . Table 1 shows the influence of porosity on various mechanical and physical properties of thin films 1 . Point defects such as pinholes laid down during deposition and generated during thermal cycling may act as starting points for severe film cracking at high temperature. Tests carried out on evaporated coatings of chromium copper and nickel showed that cracks radiated from pinholes in the films. This effect was attributed to shess concenhation in the neighborhood of the pinhole 12 . Pre-existing Table 1 Effects of Voids on the Properties of Thin Films Properties Mechanical properties Electrical properties Corrosion properties Dielectric properties Effects of Voids Ductility decrease Hydrogen embrittlement Creep resistance Reduced elastic modulus Decrease in adhesion interfacial void Resistivity increase Reduced corrosion resistance through-pores Dielectric constant From reference 3. 252 Eiectrodeposition voids along with hydrogen are responsible for the reduced ductility of electroless copper deposits 13 . This is discussed in more detail in the chapter on hydrogen embrittlement. Chromate coatings on copper and nickel-phosphorus films prepared by electrodeposition also contain a high density of voids with a structure similar to that of a crack network. The presence of these voids contributes significantly to brittleness in these films 14 15 . GOOD ASPECTS ABOUT POROSITY There are occasions where porosity is desired in a coating. Pores in anodized aluminum provide the opportunity to provide a wide range of colors when they are sealed to eliminate the path between the aluminum and the environment and pores in phosphoric acid anodized aluminum provide for adhesion of subsequent .
