Coating Performance_5

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9.2 Adhesion and Adhesion Strength 485 Table 9.15 Effect of abrasive type on the adhesion of sprayed aluminium to blast cleaned lowcarbon steel (Varacalle et al., 2006) Abrasive type Pull-off strength in MPa Steel grit HG-16 Steel grit HG-18 Steel grit HG-25 Steel grit HG-40 Copper slag Coal slag Chilled iron grit 8.21 8.07 7.82 8.48 6.83 7.07 3.62 influence on the pull-off strength values. Even for equal roughness values (say RZ = 50 μm), pull-off strength of sprayed aluminium dropped from σA = 10 MPa for basalt or furnish slag to σA = 4 MPa for nickel slag. Similar was the situation for a roughness of RZ = 80 μm. Yankee et al. (1991) could also show that abrasive type can play a decisive role in adhesion. For equivalent roughness values and equal cleaning procedures, it was the profile characteristics (thus, the abrasive shape), that determined the adhesion strength. These effects are illustrated in Fig. 9.27. Blast cleaning with aluminium oxide delivered notably higher adhesion strengths. The distinct profile characteristics generated during the blast cleaning with aluminium Fig. 9.25 Effects of surface preparation methods on the adhesion of adhesive epoxy joints to steel (Mannelqvist and Groth, 2001). Preparation methods: 1 – degreasing; 2 – Scotch-Brite; 3 – steel brushing; 4 – blast cleaning with grit; 5 – blast cleaning with glass beads; 6 – water blasting 486 9 Coating Performance Fig. 9.26 Effects of abrasive materials on the adhesion of metal-sprayed coatings to steel (Feist et al., 1988). Abrasive types: (1) – basalt; (2) – furnace slag; (3) – VV–slag; (4) – electric-furnace slag; (5) – nickel–iron slag; (6) – lead–tin slag. Parameters: p = 0.6 MPa; dP = 630–1,250 μm; dN = 8 mm; nozzle type: Laval nozzle oxide seemed to promote a good bond of the thermally sprayed hydroxylapatite coating to the titanium substrate. However, as also shown in Fig. 9.27, intense postcleaning of the substrate surfaces with ultrasound reduced the differences in adhesion strength, which pointed to additional contamination effects (see Sect. 8.5.3). Brewis et al. (1999) could prove that the preparation of aluminium substrates with carbon dioxide particles (“dry ice blasting”) could notably improve the strength of single-lap shear joints. Joint strength increased from 1,859 N for a degreased surface up to 4,420 N for carbon dioxide blast cleaned surfaces. Apps (1967) found that the individual influence of the abrasive type to the adhesion of thermally sprayed coatings depended on the blasting angle. Results of his study are plotted in Fig. 9.28. The effect of abrasive deterioration, for example, is much more distinguished if the abrasives were propelled at normal angle. At this angle, even worn steel grit performed better than steel grit that was just used once. Tests performed on the adhesion of enamel coatings to steel by Sorokin et al. (1977) verified notable effects of particle shape. Steel shot abrasives delivered better bonding conditions than steel grit particles. The coatings were unevenly distributed 9.2 Adhesion and Adhesion Strength 487 Fig. 9.27 Effects of abrasive type and substrate fine-cleaning on the adhesion of a plasma-sprayed coating to titanium (Yankee et al., 1991) relative adhesion to substrate in % 120 90 abrasive type G 24 new G 24 used G 24 worn S 240 shot 60 30 0 30 90 blasting angle in degrees Fig. 9.28 Effects of blasting angle and abrasive quality on the relative adhesion strength of thermally sprayed coatings (Apps, 1967) 488 9 Coating Performance over the grit blasted surfaces and peeled during application. Beitelman (2003) applied the abrasive shape designations according to Fig. 2.8 and measured the adhesion performance of organic and metallic coatings to the prepared substrates. Results of this study are provided in Table 9.16. Abrasive particle shape had an influence on the adhesion strength of the metallic coatings, whereas it was insensitive to the adhesion strength of the organic coating. This difference in the coating types was also found for the failure behaviour during the adhesion tests. Particle shape affected the failure type for the metal-sprayed coating, but not that of the organic coating. The amount of cohesion failure of the metal-sprayed coating dropped with a decrease in the angularity of the abrasive particles. 9.2.3.3 Effects of Air Pressure Apps (1969) performed a systematic study into the effects of blasting pressure variations on the adhesion of metal-sprayed coatings to steel substrates. Although the author noted a certain trend that adhesion increased with an increase in blasting pressure, blasting pressure did not always show a distinct relationship to the adhesion of coatings to steel substrates. An example is illustrated in Fig. 9.29. It seemed that optimum blasting pressure ranges existed which depended on abrasive quality. Worn abrasive materials deteriorated adhesion strength. Sofyan et al. (2005) found an increase in bond between WC-Co coatings and steel if blast cleaning pressure increased. 9.2.3.4 Effects of Stand-off Distance Apps (1969) performed a systematic study into the effects of changes in stand-off distance on the adhesion of metal-sprayed coatings on steel plates and steel bars, and he did not find any notable trends. The pull-off strength was unaffected by variations in the stand-off distance in the range between x = 15 and 90 mm. 9.2.3.5 Statistical Assessment Models Day et al. (2005) and Varacalle et al. (2006) performed statistical analyses into the effects of numerous process parameters on the adhesion of thermally sprayed coatings to steel substrates. Day et al. (2005) derived the following relationship: Table 9.16 Abrasive shape effects on the adhesion of coatings to blast cleaned substrates (Beitelman, 2003) Particle shapea Pull-off strength in MPa Cohesion failure in % Adhesion failure in % Very angular Angular Sub-angular Sub-rounded Rounded a Organicb Metallicc Organic Metallic Organic Metallic 13.3 13.2 13.0 13.3 13.3 9.8 9.4 9.7 8.8 8.3 100 100 100 100 100 81 83 86 45 44 0 0 0 0 0 19 17 14 55 56 Crushed steel grit (G-59); see Fig. 2.9 for shape designations Zinc-rich organic coating (DFT = 93–130 μm) c Metal-sprayed Zn/Al (DFT = 340–500 μm) b 9.3 Mechanical Behaviour of Coatings 489 Fig. 9.29 Effects of air pressure and abrasive type on the adhesion of adhesive epoxy joints to steel (Mannelqvist and Groth, 2001) σA = 38.7 · GN + 76.9 · p + 414.2 · nS + 182.6 · x + 20.6 · ϕ (9.4) In this equation, the adhesion strength is given in psi, the grit number is given in mesh, the pressure is given in psi, the stand-off distance is given in), and the blasting angle is given in degrees. Varacalle et al. (2006) derived the following relationship for aluminium, sprayed on a low-carbon steel substrate: σA = 3, 518.2 + 457.3 · x + 59.55 · CG + 8.6 · pS − 13.78 · x2 − 0.113 · C2G − 0.012 · pS 2 (9.5) In that equation, the adhesion strength is given in kPa, the stand-off distance is given in cm, the spray pressure is given in kPa, and the spray gun current is given in A. The equation holds for the use of HG 16 steel grit (dP = 1,000–1,700 μm). 9.3 Mechanical Behaviour of Coatings Surface preparation methods can affect the mechanical behaviour of coatings, mainly those of sprayed metal or ceramic coatings. Examples are provided in Fig. 9.30. In the two graphs, four sections can be distinguished as functions of coating thickness and power density: (1) fusion of the inconel substrate, (2) segmentation 490 9 Coating Performance Fig. 9.30 Effects of surface preparation methods on coating failure types during thermal shock tests (Bordeaux et al., 1991). Left: blast cleaning; right: macro-roughening; Failure types: 1 – substrate fusion; 2 – segmentation of coating; 3 – delamination; 4 – no damage of the coating, (3) delamination and (4) no damage. The no-damage region was not notably affected by the surface preparation method, but the coatings were more sensitive to segmentation if blast cleaning was performed. For blast cleaning, the limit for segmentation was at a coating thickness of about 400 μm, whereas it was at a coating thickness of about 950 μm for the machined substrate. These relationships are of importance for the application of thermal barrier coatings. Sobiecki et al. (2003) measured the microhardness of tungsten carbide coatings applied to substrates which were prepared with different surface preparation methods, but they did not record any effect. Bochenin (2005) performed investigation on aluminium coatings deposited by diffusion metallisation on steel substrates. The abrasive material used was iron shot (dP = 1,000 μm); the air pressure was p = 0.6 MPa. The coated specimens were placed into a furnace, heated to 1,000◦C and held for 90 h. The specific weight loss of the coating was defined as a measure for its heat resistance. It was found that heat resistance of the coating could be affected notably due to blast cleaning of the substrate. The heat resistance depended on several blast cleaning parameters. Results are displayed in Fig. 9.31. For the stand-off distance, an optimum value could be detected, whereas heat resistance was highest for a perpendicular blast cleaning angle. Tolpygo et al. (2001) investigated the behaviour of thermal barrier coatings deposited on (Ni, Pt) Al bond-coat substrates. The samples were cyclically oxidised. Each cycle consisted of 10 h of exposure at 1,150◦C, with heating and cooling rates of about 200◦C/min. The authors noted that blast cleaning of the substrate with aluminium oxide particles promoted a very high growth rate of the oxide scale. The high oxidation rate was explained by impurities from the blast cleaning 9.3 Mechanical Behaviour of Coatings 491 specific weight loss in 10 – 4 g/cm2 18 12 6 abrasive type: iron shot abrasive size: 1.0 mm air pressure: 0.6 MPa (a) 0 0 40 80 120 160 stand-off distance in mm 200 specific weight loss in 10 – 4 g/cm2 18 12 6 (b) 0 40 abrasive type: iron shot abrasive size: 1.0 mm air pressure: 0.6 MPa 80 60 angle of impingement in degrees 100 Fig. 9.31 Effects of blast cleaning parameters on the heat resistance of aluminium coatings deposited to a steel substrate (Bochenin, 2005). (a) Effect of stand-off distance; (b) Effect of blasting angle 492 9 Coating Performance process (alkali and titanium), which became incorporated into the growing scale and significantly accelerated the oxide growth. The high grow rate resulted in cracking and spalling of the scale followed by a mass decrease after only 30 tenhour cycles at 1,150◦C. The scale formed on the aluminised surfaces had much lower impurity content and a slower growth rate, and they showed an excellent spalling resistance during cyclic oxidation. Based on these results, blast cleaning is expected to have a detrimental effect on the durability of thermal barrier coatings. A study into the performance of sol-gel-derived coatings over blast cleaned aluminium alloys was conducted by You et al. (2001). The substrate was blast cleaned with aluminium oxide powder (dP = 20 and 100 μm). The authors found that the coating layer was more uniform for the sampled blast cleaned with the finer abrasives. These coatings also showed a smaller number of cracks after firing compared with the coatings applied over the substrate blast cleaned with the coarser abrasives. The authors attributed these results to effects of substrate roughness. 9.4 Corrosion Protection Performance of Coatings The corrosion protection performance of organic coatings can be evaluated by means of electrochemical methods. One method that became notably involved in coating testing during the recent years is EIS. The physical and chemical background is complex and beyond the scope of this book. The reader may refer to Baboian (1986). One assessment parameter, however, is the electrical resistance of a coating. If this parameter has high values, corrosion protection capability of the coating is high as well. Studies where effects of different surface preparation methods on the corrosion protection performance of coatings were investigated by means of EIS were conducted by Lin et al. (1992), Santaga et al. (1998), Vesga et al. (2000) and Elsner et al. (2003). Lin et al. (1992) investigated the effects of surface preparation methods on the electrical resistance of rather thin organic coatings. Results of their study are plotted in Fig. 9.32. It appeared that for the coating systems studied, blast cleaning deteriorated the performance of the coatings. The authors contributed this result to areas of “deficient” coating coverage (e.g. peaks of a rough substrate), which occurred at the blast cleaned surfaces. Figure 9.33 shows results of EIS measurements performed by Vesga et al. (2000) on organic primers applied to blast cleaned steel substrates. The primer behaviour was subdivided into three stages, denoted “R” (resistive), “CR” (capacitive/resistive) and “C” (capacitive) in Fig. 9.33. The value for the resistance at a given time can indicate the state of the primer degradation. The lower this value, the more severe degradation took place. It can be seen that the resistance had rather low values for the primer applied to the wet blast cleaned steel. The resistance of the primer applied to the dry blast cleaned steel was one order of magnitude higher. It was demonstrated that the pore resistance of the primers showed the same qualitative trend over the
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