1. INTRODUCTION
⌅Alumina (Al2O3) is a commercially available ceramic widely employed in industry to overcome wear and corrosion issues; coatings developed with Al2O3 offer a reasonable cost alternative to surface protection. In particular, Al2O3 + 13 wt.-% TiO2 alloy coatings provide acceptable sliding, abrasion and erosion wear resistance owe to a combination of high hardness and reliable toughness. The addition of TiO2 has several advantages; for instance, the melting temperature and hardness decrease as the content of TiO2 increases; on the contrary, the toughness increase by increasing TiO2 content, thus improving the overall wear properties of the coatings (Wang et al., 2000Wang, Y., Jiang, S., Wang, M., Wang, S., Xiao, T.D., Strutt, P.R. (2000). Abrasive wear characteristics of plasma sprayed nanostructured alumina/titania coatings. Wear 237 (2), 176-185. https://doi.org/10.1016/S0043-1648(99)00323-3.; Habib et al., 2006Habib, K.A., Saura, J.J., Ferrer, C., Damra, M.S., Giménez, E., Cabedo, L. (2006). Comparison of flame sprayed Al2O3/TiO2 coatings: Their microstructure, mechanical properties and tribology behavior. Surf. Coat. Technol. 201 (3-4), 1436-1443. https://doi.org/10.1016/j.surfcoat.2006.02.011.; Yılmaz, et al., 2007Yılmaz, R., Kurt, A.O., Demir, A., Tatlı, Z. (2007). Effects of TiO2 on the mechanical properties of the Al2O3 - TiO2 plasma sprayed coating. J. Eur. Ceram. Soc. 27, 1319-1323. https://doi.org/10.1016/j.jeurceramsoc.2006.04.099.; Di Girolamo et al., 2014Di Girolamo, G., Brentari, A., Blasi, C., Serra, E. (2014). Microstructure and mechanical properties of plasma sprayed alumina-based coatings. Ceram. Int. 40 (8), 12861-12867. https://dx.doi.org/10.1016/j.ceramint.2014.04.143.; Matikainen et al., 2014Matikainen, V., Niemi, K., Koivuluoto, H., Vuoristo, P. (2014). Abrasion, erosion and cavitation erosion wear properties of thermally sprayed alumina based coatings. Coatings 4 (1), 18-36. https://doi.org/10.3390/coatings4010018.; Ghazali et al., 2016Ghazali, M.J., Forghani, S.M., Hassanuddin, N., Muchtar, A., Daud, A.R. (2016). Comparative wear study of plasma sprayed TiO2 and Al2O3-TiO2 on mild steels. Tribol. Int. 93, 681-686. https://doi.org/10.1016/j.triboint.2015.05.001.).
Flame thermal spraying is a cost-effective method for protecting or refurbishing components and structures because: a) it is easy to handle and quite adaptable to manufacturing processing and the automation is not needed; b) it is portable and easy to use as the operators do not require high-level of training; c) it produces lower dust and fume levels and the security equipment for operation is not highly sophisticated as compared to other thermal spraying processes; d) it is easy to coat parts or components with complex geometries due to the possibility of utilizing manual operation; e) it inputs lower distortion to substrate due low heat transfer; and f) it is suitable to be employed in environments which are less demanding. Flame spraying is often used to repair and/or protect industrial components subjected to sliding wear such as shafts, bearings, gears, or similar surfaces, by utilizing an ample variety of materials as feedstock (Oerlikon Metco, 1945Oerlikon Metco (1945). Metallizing Handbook. Long Island City, N.Y.; Romero, 1990Romero, R.R. (1990). Machinery Repairman. NAVEDTRA 12204-A. Naval Education and Training Program.; ASTM Committee, 1994ASTM Committee (1994). ASM Handbook “Surface Engineering”. Vol 5, ASM International, Materials Park, OH, USA.; Davis, 2004Davis, J.R. (2004). Handbook Thermal Spray Technology. ASM International, Materials Park, OH, USA.).
Adhesion is one of the most important characteristic of thermally sprayed coatings that strongly influence on the wear performance; thus, the surface roughness of the substrate is of critical importance since this guarantees suitable mechanical anchorage or mechanical interlocking (Davis, 2004Davis, J.R. (2004). Handbook Thermal Spray Technology. ASM International, Materials Park, OH, USA.; Wang et al., 2005Wang, Y.Y., Li, C.J., Ohmori, A. (2005). Influence of substrate roughness on the bonding mechanisms of high velocity oxy-fuel sprayed coatings. Thin Solid Films 485 (1-2), 141-147. https://doi.org/10.1016/j.tsf.2005.03.024.). In low velocity thermal spraying processes i.e. flame spraying and/or in processes where exist un-melted particles; mechanical interlocking of the splat within the substrate irregularities is the principal bonding mechanism influencing on the adhesion strength in coatings (Paredes et al., 2006Paredes, R.S.C., Amico, S.C., d’Oliveira, A.S.C.M. (2006). The effect of roughness and pre-heating of the substrate on the morphology of aluminium coatings deposited by thermal spraying. Surf. Coat. Technol. 200 (9), 3049-3055. https://doi.org/10.1016/j.surfcoat.2005.02.200.). Roughening is a critical step in preparing the surface before the coating application as the former ensures proper adhesion. Therefore, in order to obtain adequate roughening; the substrate surface must be correctly prepared and various common methods can be utilized viz. rough threading (used for cylindrical surfaces), grit blasting, and/or a combination of the previously mentioned (ASTM Committee, 1994ASTM Committee (1994). ASM Handbook “Surface Engineering”. Vol 5, ASM International, Materials Park, OH, USA.).
Thick coatings are found in various situations such as: built-up or reconditioning of machine parts, repairmen of components, thermal barrier, wear protection, corrosion protection, among others. Nevertheless, processing of thick coatings is not an easy task and several problems must be faced: i.e. weak mechanical anchoring (poor adhesion) (Berndt and McPherson, 1979Berndt, C.C., McPherson, R. (1979). The Adhesion of Flame and Plasma Sprayed Coatings - A Literature Review. Australas. Weld. Res. 6 (January), pp. 75-85.), presence of edge delamination failure (Bordeaux et al., 1991Bordeaux, F., Saint Jacques, R. G., Moreau, C. (1991). Study of surface preparation for enhanced resistance to thermal shocks of plasma-sprayed TiC coatings. Surf. Coat. Technol. 49 (1-3), 50-56. https://doi.org/10.1016/0257-8972(91)90030-Z.), increasing of residual stresses with thickness (Tucker, 1974Tucker, R.C. (1974). Structure property relationships in deposits produced by plasma spray and detonation gun techniques. J. Vac. Sci. Technol. 11 (4), 725-734. https://doi.org/10.1116/1.1312743.), generation of microcracks (Steffens et al., 1999Steffens, H.D., Babiak, Z., Gramlich, M. (1999). Some aspects of thick thermal barrier coating lifetime prolongation. J. Therm. Spray Tech. 8 (4), 517-522. https://doi.org/10.1361/105996399770350197.).
Macro-roughening (an easier to handle technique, faster, and economical) is commonly applied for restoring greater dimensions (thick coatings) in damaged surfaces. Macro-roughening can be performed on shafts through a lathe machine operation by cutting a narrow, low-pitch, shallow groove or thread in the surface (Romero, 1990Romero, R.R. (1990). Machinery Repairman. NAVEDTRA 12204-A. Naval Education and Training Program.). It is worth mentioning that macro-roughening is an effective technique for residual stress reduction by inducing folds in the coatings thus promoting low shrinkage. In addition, macro-roughening can prevent shear or edge delamination, and further reduce cracking and/or spalling in coatings (Tucker, 1974Tucker, R.C. (1974). Structure property relationships in deposits produced by plasma spray and detonation gun techniques. J. Vac. Sci. Technol. 11 (4), 725-734. https://doi.org/10.1116/1.1312743.; James, 1984James, D.H. (1984). A review of Experimental Findings in surface preparation for thermal spraying. J. Mech. Work. Technol. 10 (2), 221-232. https://doi.org/10.1016/0378-3804(84)90069-X.; Bordeaux et al.,1992Bordeaux, F., Jacques, R.G.S., Moreau, C., Dallaire, S., Lu, J. (1992). Thermal shock resistance of TiC coatings plasma sprayed onto macroroughened substrates. Surf. Coat. Technol. 53 (1), 49-56. https://doi.org/10.1016/0257-8972(92)90102-G.; Matějíček et al., 2007Matějíček, J., Chráska, P., Linke, J. (2007). Thermal spray coatings for fusion applications - Review. J. Therm. Spray Tech. 16 (1), 64-83. https://doi.org/10.1007/s11666-006-9007-2.; Hollis et al., 2007Hollis, K.J., Bartram, B.D., Roedig, M., Youchison, D., Nygren, R. (2007). Plasma-sprayed beryllium on macro-roughened substrates for fusion reactor high heat flux applications. J. Therm. Spray Tech. 16 (1), 96-103. https://doi.org/10.1007/s11666-006-9011-6.). However, there is a lack of information in the literature relating the cylindrical surface macro-roughness viz. knurling and grooving, to the resultant microstructure and sliding wear performance upon flame spraying of thick coatings.
This study aims at evaluating the effect of the cylindrical surface macro-roughness on the overall microstructure and sliding wear behaviour of Al2O3 + 13 wt.-% TiO2 thick coatings deposited on a AISI/SAE 1045 steel bar, using two different machining methods as anchorage viz. spiral grooving and knurling pattern.
2. MATERIALS AND METHODS
⌅A commercial (Oerlikon MetcoTM), agglomerated and sintered Al2O3 + 13 wt.-% TiO2 powder alloy with a particle size distribution of -45+15 µm (d50=30.9 µm) was used as feedstock in this work (Fig. 1). An Oerlikon MetcoTM Ni-5Mo-5.5Al powder alloy with size distribution of -90+45 μm (d50=59.6 μm) was additionally employed as bond-coat in order to improve the adhesion of alumina to the substrate.
An AISI/SAE 1045 cylindrical steel bar with diameter of 31.75 mm and 256 HV0.5 in hardness was used as substrate. A drilled hole (15 mm in diameter) was performed in the center of the cylinder; afterwards, the steel bar was cross-sectioned with the purpose of forming a ring specimen of 5 mm in thickness.
Four different macro-roughened surfaces were prepared at the outer surface of the ring specimens: two diamond pattern knurling (DKA and DKB), and two spiral pattern grooving (SGA and SGB). Geometry and dimensions of the surface profile for both DK and SG patterns are indicated in the schematic of Fig. 2.
Before the coating deposition, all specimens were properly cleaned in an ultrasonic bath. The coating deposition was conducted towards the outer surface of the ring specimen, as it is shown in Fig. 3a, by using a CastoDyn DS8000™ thermal spray gun. Just after eight preliminary trials; suitable deposition parameters for Al2O3 + 13 wt.-% TiO2 powders were obtained as it is listed in Table 1, the deposition parameters were constant for all conditions. Multi-pass torch (6 passes in total) was performed to obtain thick (1000-1200 µm) Al2O3 + 13 wt.-% TiO2 coatings.
Material | Powder feeding rate (Kg/h) | Air Pressure (Bar) | Oxygen Pressure (Bar) | Acetylene Pressure (Bar) | Spraying Distance (mm) | Flame type | Noozle diameter (mm) |
---|---|---|---|---|---|---|---|
Al2O3 + 13 wt.-% TiO2 | 7.4 | 3.0 | 4.0 | 0.7 | 200 | Neutral | 1.25 |
Ni-5Mo-5.5Al | 7.4 | 3.0 | 4.0 | 0.7 | 150 | Neutral | 2.4 |
The coated specimens were sectioned in the transverse direction. After performing the usual metallographic procedures all micrographs were taken in a Tescan Mira3™ scanning electron microscope. By following the ASTM E384 (2017)ASTM E384 (2017). Standard Test Method for Microindentation Hardness of Materials. ASTM International, West Conshohocken, USA. hardness standard and employing a Shimadzu tester, Vickers hardness was measured with 500 g (4.905 N) of applied load and a dwell time of 15 s. Three linear paths of indentations parallel to each other with a minimum number of 8 indentations (located 200 µm apart as a minimum to each other) were obtained at the substrate and throughout the coating thickness in all specimens, it is important to declare that porosity was avoided when placing the hardness indentations. The coatings were evaluated by X-ray diffraction (XRD) using Cu-Kα radiation on a D8Advance Bruker diffractometer by setting up a scan step of 0.002° (2q) and a time per scan of 1 s. The standard test method ASTM E2109−01 (2021)ASTM E2109−01 (2021). Standard Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings. ASTM International, West Conshohocken, USA. was used for determining the area percentage porosity in the thermal sprayed coatings using image analysis.
Sliding wear testing was conducted as per the configuration illustrated in Fig. 3b. An Al2O3 disk with an initial roughness of Ra=18 µm and hardness within a range of 2110-2450 HV was utilized as counterface. Sliding wear testing was conducted by setting up a constant load of 16 N, a rotational speed of 450 rpm, and a fixed sliding distance of 1979 m. All samples were cleaned with an ultrasonic acetone bath and weighed before and after the trial in ±0.01 mg AUW220D Shimadzu™ precision balance with the purpose of evaluating mass loss of the specimen. Four replicas per condition were subjected to sliding wear testing at room temperature.
3. RESULTS
⌅The corresponding cross-sectioned microstructure of the Al2O3 + 13 wt.-% TiO2 coatings deposited onto four different macro-roughened surfaces is shown in Fig. 4. The Ni-5Mo-5.5Al bond coat (hereafter named as “Ni bonding layer”) with an average layer thickness of 177 µm was properly anchored to the steel substrate. The subsequent flame sprayed region is composed of various layers of Al2O3 + 13 wt.-% TiO2 that form the final thick coating structure.
The porosity (as pointed by black arrows) was revealed throughout the thickness of the coating in both spiral grooving surface conditions (SGA and SGB); it is worth mentioning that higher density of porosity is located about the tip of the threads (Fig. 4a-b). Porosity is clearly correlated to the sharp tip and the slope of the groove. Due to the grooving surface is not parallel to the particles stream; there is poor adhesion along the slope of the grooves, and additionally, few particles are trapped around the sharp tip, thus promoting porosity. On the other hand, porosity is also observed upon the knurling conditions (DKA and DKB) as shown in Fig. 4c-d; however, there is a clear reduction in porosity content as compared to the SGA and SGB conditions. The absence of a sharp tips as well as the reduced slope of the grooves over the substrate actually creates an extensive active zone, leaving a larger amount of deposited particles onto the surface with reduced porosity. The porosity percentage is clearly reduced from 8.2% in spiral grooving pattern up to 3.5% upon knurling pattern due to the macro-roughness effect.
Based on the abovementioned results, the surface topography viz. macro-roughness has a clear effect on the distribution and density of porosity; for instance, the absence of sharp tips and the reduced slope in the knurling conditions, leaded to a reduction in porosity; an indirect evidence of acceptable adhesion conditions. Indeed, the top surface of the coating takes the original shape of the initial substrate macro-roughness, for this reason a smoother surface can be obtained when employing the knurling pattern.
A laminar structure formed by the particle impact and the subsequent flattening of splats commonly found in flame spraying processes is observed in all Al2O3 + 13 wt.-% TiO2 coatings (Fig. 5). A relatively dense microstructure with distinguishable microcracks is revealed; however, the extension of cracking depended on macro-roughness condition, hence, visibly dense and coarser microcracks are observed upon the SG pattern (Fig. 5a-b); whereas, a reduced fraction of thinner microcracks are revealed upon the DK condition as shown in Fig. 5c-d. It is clear that microcracks are minimized upon the pattern knurling DKA and DKB.
Microcracks are the product of the shrinkage due to rapid solidification of the deposited particles on the surface. Both spiral grooving conditions resulted in coarser and larger microcracks due to abrupt surface geometry changes (i.e. sharp tips, with high slope within the groove shape), thus causing distortion during the splat disposition affecting the shrinkage pattern, and hence, the inter-splat adhesion. On the other hand, a properly defined pattern such as knurling, ensures uniform surface geometry changes, free of sharp-tips, and reduced slope, the splats are properly trapped to the substrate with smooth geometry changes, having an effective deposition, thus facilitating the stacking (coating build up), and promoting good inter-splat adhesion. Therefore, it is again inferred that the surface topography greatly affects the characteristics of microcracks present in the coatings. Predominantly melted regions are encounter in all coatings; however, partially melted and/or un-melted round particles are also revealed.
XRD analysis of the feedstock powder provided in Fig. 6a, shows the presence of stable α- Al2O3 and TiO2 (rutile) phases along with a reduced fraction of Al2TiO5 which can be ascribed to the sintering reaction of Al2O3 + 13 wt.-% TiO2 powder mixture during the manufacturing process of the powder.
No differences in terms of phase transformation were found among SGA, SGB, DKA and DKB conditions as shown in Fig. 6b. Transformation from α-Al2O3 phase to the metastable γ-Al2O3 phase during flame spraying processing resulted in all specimens as it is seen in Fig. 6b, not evidence neither of TiO2 nor Al2TiO5 was recorded. The transformation from α-Al2O3 phase to γ-Al2O3 phase during solidification is attributed to its lower critical free energy for nucleation; on this basis, γ-Al2O3 would be nucleated rather than α-Al2O3 at low temperatures (McPherson, 1973McPherson, R. (1973). Formation of metastable phases in flame- and plasma-prepared alumina. J. Mater.s Sci. 8, 851-858. https://doi.org/10.1007/BF02397914.; R. McPherson, 1980McPherson, R. (1980). On the formation of thermally sprayed alumina coatings. J. Mater. Sci. 15, 3141-3149. https://doi.org/10.1007/BF00550387.; Fervel et al., 1999Fervel, V., Normand, B., Coddet, C. (1999). Tribological behavior of plasma sprayed Al2O3 -based cermet coatings. Wear 230 (1), 70-77. https://doi.org/10.1016/S0043-1648(99)00096-4.; Normand et al., 2000Normand, B., Fervel, V., Coddet, C., Nikitine, V. (2000). Tribological properties of plasma sprayed alumina-titania coatings: role and control of the microstructure. Surf. Coat. Technol. 123 (2-3), 278-287. https://doi.org/10.1016/S0257-8972(99)00532-0.; Yılmaz et al., 2007Yılmaz, R., Kurt, A.O., Demir, A., Tatlı, Z. (2007). Effects of TiO2 on the mechanical properties of the Al2O3 - TiO2 plasma sprayed coating. J. Eur. Ceram. Soc. 27, 1319-1323. https://doi.org/10.1016/j.jeurceramsoc.2006.04.099.; Di Girolamo et al., 2014Di Girolamo, G., Brentari, A., Blasi, C., Serra, E. (2014). Microstructure and mechanical properties of plasma sprayed alumina-based coatings. Ceram. Int. 40 (8), 12861-12867. https://dx.doi.org/10.1016/j.ceramint.2014.04.143.).
Interestingly, a semi-quantification analysis made to the multilayered coating (obtained with more than one torch pass) resulted in an average fraction of approximately 25% of α-Al2O3 and 755 of γ-Al2O3. In further analysis, XRD was executed in a single layer coating (obtained with only one torch pass) thus resulting in 5 to 10% content of α-Al2O3; hence, the presence of α-Al2O3 is correlated to the partially melted and/or the un-melted particles in single-layered coatings (Normand et al., 2000Normand, B., Fervel, V., Coddet, C., Nikitine, V. (2000). Tribological properties of plasma sprayed alumina-titania coatings: role and control of the microstructure. Surf. Coat. Technol. 123 (2-3), 278-287. https://doi.org/10.1016/S0257-8972(99)00532-0.; Lou et al., 2003Lou, H., Goberman, D., Shaw, L., Gell, M. (2003). Identation fracture behavior of plasma-sprayed nanostructured Al2O3-13wt.%TiO2 coatings. Mater. Sci. Eng. A 346 (1-2), 237-245. https://doi.org/10.1016/S0921-5093(02)00523-3.; Yılmaz et al., 2007Yılmaz, R., Kurt, A.O., Demir, A., Tatlı, Z. (2007). Effects of TiO2 on the mechanical properties of the Al2O3 - TiO2 plasma sprayed coating. J. Eur. Ceram. Soc. 27, 1319-1323. https://doi.org/10.1016/j.jeurceramsoc.2006.04.099.; Di Girolamo, et al., 2014Di Girolamo, G., Brentari, A., Blasi, C., Serra, E. (2014). Microstructure and mechanical properties of plasma sprayed alumina-based coatings. Ceram. Int. 40 (8), 12861-12867. https://dx.doi.org/10.1016/j.ceramint.2014.04.143.; Yang et al., 2015Yang, Y., Wang, Y., Tian, W., Yan, D-ran, Zhang, J.-xin, Wang, L. (2015). Influence of composite powders’ microstructure on the microstructure and properties of Al2O3-TiO2 coatings fabricated by plasma spraying. Mater. Des. 65, 814-822. https://doi.org/10.1016/j.matdes.2014.09.078.). However, the remained fraction of α-Al2O3 phase in the multilayered specimens, is absolutely related to re-transformation of γ-Al2O3 → α-Al2O3 during the multi-pass torch spray processing (reheating). For instance, during the deposition process; the melted particles resulted in full transformation from α-Al2O3 to γ-Al2O3 phase; nevertheless, with the subsequent passing of the torch on the already deposited layer, a localized heat treatment is developed with enough temperature for causing γ-Al2O3 → α-Al2O3 reversed transformation. It is well known that transformation from γ-Al2O3 to α-Al2O3 phase is possible when heating the substrate approximately about 1100 °C (McPherson, 1980McPherson, R. (1980). On the formation of thermally sprayed alumina coatings. J. Mater. Sci. 15, 3141-3149. https://doi.org/10.1007/BF00550387.; Di Girolamo, et al., 2014Di Girolamo, G., Brentari, A., Blasi, C., Serra, E. (2014). Microstructure and mechanical properties of plasma sprayed alumina-based coatings. Ceram. Int. 40 (8), 12861-12867. https://dx.doi.org/10.1016/j.ceramint.2014.04.143.). It is important to mention that approximately 15 to 20% of the preliminary layer is re-transformed to α-Al2O3 due to the fact that the maximum temperature for transformation penetrates few microns within the previous layer. The subsequent torch passes created a layering which combines the presence of thinner α-Al2O3 phase and thicker γ-Al2O3 phase layers as schematically depicted in Fig. 7a. It is worth mentioning that spraying parameters (nozzle, flame type, spraying distance, etc.) were constant for all specimens; then it is deduced that the surface geometry has no effect on the phase transformation of the coatings as no differences are observed according to XRD.
Figure 7b, shows the micro-hardness profile taken from the substrate throughout the coating thickness. A schematic depicting the paths of hardness indentations on the various regions viz. substrate and the Al2O3 layering is provided in Fig. 7a. The hardness of the substrate averaged 256 HV0.5, whereas the mean value hardness of the alumina layering was 972 HV0.5. A clear fluctuation of the hardness values is observed along the coating in all specimens which is markedly related to the distinct α- and γ-Al2O3 layers throughout the coating. Therefore, as some indentations lay into α-Al2O3 and others into γ-Al2O3 along the distance from the substrate/coating line over the coating; the higher hardness values are correlated to α-Al2O3 because of its highest hardness as compared with γ-Al2O3 phase (Yilmaz, 2009Yilmaz, Ş. (2009). An evaluation of plasma-sprayed coatings based on Al2O3 and Al2O3-13 wt.% TiO2 with bond coat on pure titanium substrate. Ceramics International 35 (5), 2017-2022. https://doi.org/10.1016/j.ceramint.2008.11.017.; Singh et al., 2011Singh, V.P., Sil, A., Jayaganthan, R. (2011). A study on sliding and erosive wear behaviour of atmospheric plasma sprayed conventional and nanostructured alumina coatings. Mater. Des. 32 (2), 584-591. https://doi.org/10.1016/j.matdes.2010.08.019.; Islak et al., 2012Islak, S., Buytoz, S., Orhan, N., Stokes, J. (2012). Effect on microstructure of TiO2 rate in Al2O3-TiO2 composite coating produced using plasma spray method. Optoelectron. Adv. Mater. Rapid Commun. 6 (9), 844-849.).
Wear track of multi-layered Al2O3 + 13 wt.-% TiO2 coatings are shown in Fig. 8. Scratched surfaces are revealed for all specimens. Severe wear damage is clearly observed upon SGB condition followed by SGA. Coating defects (i.e. cracks) located parallel to the sliding wear track are clearly distinguishable in Fig. 8b. On the contrary, coating defects in SGA, DKA and DKB conditions are perpendicular to the wear tracks. The initial contact area between the counter disk (alumina) and the coating is minimum; in fact, the actual contact area is supported by the coating tips inherited by the substrate geometry. Once the sliding friction displacement has begun; coalescence of existing cracks as well as nucleation of new ones are generated, following this, particles are detached out from coatings (debris), it is worth mentioning that the presence of porosity and microcracks also generate splats exfoliation (as shown in the SEM micrograph in Fig. 9) (Normand et al., 2000Normand, B., Fervel, V., Coddet, C., Nikitine, V. (2000). Tribological properties of plasma sprayed alumina-titania coatings: role and control of the microstructure. Surf. Coat. Technol. 123 (2-3), 278-287. https://doi.org/10.1016/S0257-8972(99)00532-0.; Psyllaki et al., 2001Psyllaki, P.P., Jeandin, M., Pantelis, D.I. (2001). Microstructure and wear mechanisms of thermal-sprayed alumina coatings. Mater. Lett. 47 (1-2), 77-82. https://doi.org/10.1016/S0167-577X(00)00215-9.; Michalak et al., 2021Michalak, M., Sokołowski, P., Szala, M., Walczak, M., Łatka, L., Toma, F.-L., Björklund, S. (2021). Wear Behavior Analysis of Al2O3 Coatings Manufactured by APS and HVOF Spraying Processes Using Powder and Suspension Feedstocks. Coatings 11 (8), 879. https://doi.org/10.3390/coatings11080879.), which is conducted by the weak splat unions and microcracks, the above mentioned produce a progressive increment in contact area by the constant material removing from the tips. Additionally, the hard debris trapped at the inter-face between the disk and the coatings promotes the abrasion mechanism. Actually, debris is generated from both: the detached splats coming from coatings and the separated material from the counter disk. Scratching scar demonstrates an abrasion mechanism involved in the wear process.
Figure 10, shows the relationship between the wear ratio (coating mass loss/ substrate Mass loss) and porosity. It is seen that the specimens with the knurling pattern possess a higher wear resistance as compared to specimens with the spiral grooving pattern. It is clear that the wear ratio is higher when the porosity increases; for instance, SGB specimen having the largest amount of porosity and microcracks (Fig. 8b) had a higher wear ratio. Therefore, the porosity influenced on the wear resistance by weakening the splats joining; if porosity the increases, splats adhesion is logically reduced by promoting splats detachment, which clearly affects the wear resistance. Furthermore, microcracks limit splats contact, influencing on splats adhesion, and consequently on the coating toughness. If splats joint is deficient, then the toughness is reduced thus facilitating splats decohesion during wear.
4. CONCLUSIONS
⌅-
The type of cylindrical surface macro-roughness pattern had a clear effect on the distribution and density of the porosity, the absence of sharp tips and the reduced slope of the threads upon the knurling pattern leaded to a reduction in the porosity content. In addition, the top surface of the coating inherits the initial shape of the substrate leading to a smoother final surface when employing the knurling pattern.
-
The amount and extension of the microcracks are clearly larger in the spiral grooving surface patterns, whereas upon he knurling surface patterns these are minimized because the splats adapted better to the reduced geometry changes with effective deposition, then promoting the stacking and good inter-splat adhesion.
-
Hard α-Al2O3 phase was consistently found throughout the coating in all specimens coming from both: a) the presence of partially melted and melted particles and, b) the formation of α-Al2O3 interlayers due to the multi-pass torch and the subsequent surface re-heating that produced reversed phase transformation from γ-Al2O3 to α-Al2O3 phase.
-
The improved sliding wear resistance in the specimens with the knurling pattern was promoted by the combination of γ-Al2O3 (toughness) and α-Al2O3 (hardness) phases through the thickness of the coating and, predominantly, by the reduced amount of porosity and microcracks that strengthen the inter-splat anchorage.