Handbook of Modern Coating Technologies

Protective coatings

Protective coatings are widely used in many fields to avoid the materials being damaged by the environment. Usually the protective coatings are thin or ultrathin, and SE with high sen­sitivity is very suitable to characterize them.

Kaneko et al. [112] prepared films of Al₂O₃—Ta₂O₅, a well-known highly corrosion— resistant material and analyzed the deposition process of films with in situ SE. It was revealed that thicknesses of the films changed linearly with the deposition time (exclude the short induction periods). They also found that the increased tantalum cationic fraction would increase refractive indices of the film, and the extinction coefficients of the composite films were larger than those of single Al₂O₃ and Ta₂O₅ films.

Titanium diboride (TiB₂) is a high-performance metallic ceramic with high electrical conductivity, excellent corrosion, and wear resistance. TiB₂ has been widely used in microelectronics, superconducting, and other fields. Kumar et al. [113] monitored the nucleation and growth of TiB₂ films during thermal chemical vapor deposition on highly conducting n-type Si(100) substrate by using in situ SE. The SE data were deconvoluted with a multilayer model, in which a surface roughness layer was involved. The surface roughness layer was regarded as a mixture of 50% TiB₂ and 50% void based on the Bruggeman EMA. The results showed the deposition temperature must be higher 170°C, and an increase in the surface roughness to 2.6 nm at the onset of film growth, indicating the nucleation phase. After nucleation, the film thickness increased linearly with the increase in the film roughness.

Rahman et al. [114] studied the formation of a silica protection layer on top of the Ni and Ti silicide under three oxidation atmospheres (dry, wet, and microwave plasma) and experi­mental conditions (time and temperature). By using SE, XRD, and SEM techniques, they measured the properties of the silicides and oxides, including the roughness, thickness, and density, and they found that NiSi would be a promising protective material in the future and the microwave plasma oxidation would be an excellent oxidation atmosphere.

The evolution of sol—gel based silicon-zirconia wear-resistant coatings during annealing process were investigated by Uhlmann et al. [115] with SE, SEM, and other modern techni­ques. The thickness results determined by SE were consistent with them measured by SEM when the annealing temperature was above 200°C. Moreover, when the preparation temper­ature is higher than 600°C, the film density would be 4.64 g/cm³ while the mean refractive index within the range of 1.88 to 1.93, which was very helpful to guide the preparation of high-quality protective coatings.

To enhance the hardness of the systems, Gioti group [116] deposited ultrathin (~10nm) protective hard optical coatings of amorphous carbon (a-C), carbon nitride (CN_x), and boron nitride (BN_x) on the top surface of optical systems, respectively. SE was applied to investigate the dielectric response and optical properties of those protective hard optical coatings. All the three coatings had low absorption indices in the visible energy region while the BN_x and CN_x film had the lowest the highest absorption, respec­tively. It could be concluded that films of a-C and BN_x were promising candidates for protective hard optical coatings compared with CN_x films. Another carbon wear—resistant coating, diamond-like carbon (DLC) film, was prepared on Ti—6Al—4V substrate and measured using VASE by Wu et al. [117]. The results showed that the thicknesses of DLC coatings were usually several-hundred nanometers and the thickness values were directly related to the hardness values but had no influence on the adhesion between the DLC coating and substrate. Moreover, SE could act as a rapid assessment technique of film quality by extracting the thickness and optical constants of film. Corbella et al. [118] found that the optical, electric and mechanical properties of DLC films could be improved by adding transition metal atoms (Mo, Nb, Ti, and W) into the DLC matrix. They used UV—vis SE to reveal the influence of the composition on the optical constants and found that optical gap increased upon the metal content reduced. Similarly the optical constants of titanium chromium nitride nanocomposite protective coatings were measured by Aouadi et al. [119] using SE. The absorption coefficient and elemental/ phase composition were correlated as the former increased with the increase of Cr con­tent in the films.

Since SE is a nondestructive and noninterfering technique, it has been employed in chemical or electrochemical environments to study the preparation and protective properties of the protecting coatings.

Chromate conversion coatings (CCCs) technique is a traditional protective technique for metals and has been used for many years. Zhang et al. [120] investigated the CCCs formed on metal of zinc in Cr(VI) and Cr(III) treatment baths by SE. The results showed, for the same immersion period, the CCCs of Cr(VI) was thicker than that of Cr(III). The latter inhib­ited effectively the corrosion of zinc, but the inhibition effect was worse than that of the former. There were two reasons for the difference of these corrosion protection coatings: (1) the Cr(VI) coating was thicker and (2) the Cr(VI) species could repassivate the defects of the coating. However, chromate containing materials are extremely toxic, so some environmental-friendly nonchromate protection treatments are developed. Liu et al. [121] investigated the formation mechanism of nontoxic self-assembled monolayers (SAMs) on Mg surface. They measured the thicknesses of the alkylcarboxylate (C_nH_2n-₁O₂Na, n = 12, 14, and 18) SAMs with SE and found that the thickness increased upon increasing chain length of carboxylate. Organosilane pretreatment is another approach to replace hexavalent chro­mium conversion treatment. SE and electrochemical quartz crystal microbalance were used to elucidate the formation mechanism of organosilane film by Pen et al. [122].

In some situations, metals corrode in solutions and the products form protective film on the metal surface, which could prevent the metal from corroding further. Hara et al. [123] reported this kind of protective surface films generated on AZ91D Mg alloy in 0.1 M NaCl solution. Based on the in situ SE results, as shown in Fig. 2—13, the experimental plot moved from the theoretical curve of N₂ = 1.510 — 0.010i to that of N₂ = 1.415 — 0.016i within 0—25 s (the initial period of immersion), the authors inferred that the air—formed MgCO₃ film chan­ged into Mg(OH)₂ film during this period. Then, the SE experimental spots moved along the theoretical curve of N₂ = 1.415 — 0.016i, which meant that the Mg(OH)₂ film grew homo­geneously. This formed Mg(OH)₂ film could passivate Mg in the NaCl solution so that it could be regarded as a protective coating. The authors also studied the thicknesses variation of surface films on AZ91D alloy and 6N—Mg with the immersion time in 0.1 M NaCl solution by using in situ SE. The results are shown in Fig. 2—14. Obviously the films grew quickly in two continuous stages: at stage I, the film thickness increased linearly with the immersion time, indicating the film grew at a constant rate; at stage II, the film growth rate decreased with the lapse of time.

SE was also applied to monitor the growth of the passive film potentiostatically formed on 304 stainless steel in a 0.1-M sodium sulfate solution by Mohanmmadi et al. [124]. The results indicated that the thickness of the passive film was 2—2.6 nm. Another example of application of SE was reported by Pen et al. [125], they found that a fresh uncured organosi- lane film of BTSE cannot provide effective protection for aluminum substrate. Then they

monitored the evolution of the film during curing with SE. The results showed that the thick­ness decreased and the film refractive index increased during curing, which indicates that the curing treatment could modify the structure of the BTSE film and thus enhance the pro­tection ability. Similarly Chen et al. [126] studied 2-mercaptobenzothiazole corrosion inhibi­tor protective film on copper with in situ SE, IR, and Raman spectroscopies.