Adhesion of Coatings to the Substrate

Over the years, a large variety of different test have been devised to measure bond strengths in an accurate and reproducible way (see Heimann, 2008 Mohseni, Zalnezhad and Bushroa, 2014). Despite those efforts, however, to date there is no reliable method available that can be generally applied to any kind of coating on any kind of substrate. An older review by Davies and Whittaker (1967) identified 

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The microhardness of coatings was 18-19 GPa for molybdenum carbide [6], 29-31 GPa for tungsten carbide, and 31-32 GPa for zirconium diboride. The presence of a diffusion zone that ensured the adhesion of coatings to the substrate, was confirmed by qualitative and semiquantitative X-ray spectral microanalyses of microsections which were performed with a MS-46 Cameca electronic probe. The continuity of the coating-substrate transition was confirmed by stereoscopic images of the cross-sections of the electroplated samples. 

FIGURE 1.1 Physical and chemical causes for the adhesion of coatings to the substrate [11]. 

The process has been improved over a period so that now very small particles of palladium may be deposited very evenly, and this has been demonstrated to give improved adhesion of the layers of coating to the substrate. 

Often, the surface modification of biomedical devices is required to maximize performance. The surface can be modified in many ways, including plasma modification and application of coatings to the substrate. Surface modifications can be used to affect surface energy, cell adhesion, mechanical performance, chemical inertness, lubricity, degradation, bioactive agents release, and hydrophilicity. 

Glass fibers have also been coated with conductive polymers. Deposition of polyaniline on glass-fiber fabric provided a method for the production of conductive fabrics [75]. The coating was achieved by several cycles of immersion of the fibers in an acid aniline solution followed by drying at 60°C and immersion in an oxidant solution. The mean growth rate of the deposit thickness was reported to be 150 nm per cycle. The conductivity of the coated fabrics reached 0.23 S cm. According to the authors, the adhesion of polyaniline to the substrate seems to be sufficient, since the mechanical properties of composites do not change in comparison to those of untreated fabric. 

X-cut According to ASTM D 3359, method A, an X is cut into the film to the substrate, pressure tape (TESAPACK 4287) is applied over the X and then removed, and adhesion is evaluated by comparison with descriptions and pictures. The method is used to establish whether the adhesion failure of the coating to the substrate may occur between the coats (adhesive break) or in the coating (cohesive break). 

Adhesion of polyimides to inorganic substrates is of great importance to the microelectronics industry [1, 2]. The polyimide films are deposited most often by spin coating the polyamic acid (PAA) usually from a TV-methylpyrrolidone (NMP) solution onto the substrate surface followed by thermal imidization at temperatures up to 400<>C. The most studied polyimide is the pyromellitic dianhydride-oxydianiline (PMDA-ODA), which exhibits excellent mechanical and dielectric properties, but not so good adhesion characteristics. The latter has been generally overcome by application of an adhesion promoter, such as y-aminopropyltriethoxysilane [3-7]. The reactions of APS (coated from water solution) with the silicon dioxide surface as well as with polyamic acid have been well characterized by Linde and Gleason [4] however, we do not have such detailed information available on APS interaction with other ceramic surfaces. 

In surface coating, it has been customary to coat metal with a primer to effect better adhesion of a polymeric coating to the substrate. W. J. Jackson and J. R. Caldwell report that a single coat is sufficient if car-boxylated polyesters are added to the polymeric coating. These carbox-ylated polyesters are soluble in volatile lacquer type solvents and improve the adhesion of the common polymeric lacquers and varnishes. 

It is generally accepted that coating endurance, especially in moist air, is very dependent on the strength of adhesion of coating to substrate. Fleischauer and Bauer ° came to the conclusion that, just as slight oxidation of the lower layers of... 

Figure 24.14 Schematic representation of water diffusion through a coating with and without water-insensitive adhesion of the coating to the substrate.

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