Production of Al12Si matrix with SiCp reinforced composite coatings by cold gas dynamic spray coating method and investigation of tribological behaviour of the coatings in water

The Cold gas dynamic spray ( CGDS) coating method was first discovered in mid of 1980s at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk by Dr. Anatolii Papyrin and his colleagues. The main difference of cold gas dynamic spray method from the other thermal spray coatings is; the coating powder particles do not melt during the coating process and can be coated with supersonic velocity at low temperature, therefore it is possible to obtain intensive coating with cold gas dynamic spray coatings. Cold gas dynamic cold spray can be achieved with station or portable type coating machines. Main differences of these systems are; gas pressure they use and coating powder flow rate during coating process. Portable systems are used manually and controlled by operator, while station types of machines are connected by robotic systems. CGDS method provides many benefits in comparison to the other thermal spray coating types, such as avoidance of undesirable phases, oxidation and fluctuation in coated substrate material or in coatings; possibility to select and use different type of substrate and coating materials; increase of deposition efficiency as a result of coatings produced with high speed. It also prevents radiation, explosive gas generated from high temperature and decreases oxides, porosity and discontinuity to almost non-existing amount, in the case of coating in high temperature when temperature of substrate materials are likely to rise at very low level. As a result of above mentioned advantageous of the cold gas dynamic spray coating, it has a big utilization in the industry such as; automotive, aerospace, manufacturing, glass, nuclear, Electronics Energy, Medical, Metal, Agricultural Chemical, Construction, Fishery, Food, Furniture, Mold and tooling repair, Forestry, Marine, Mining, Oil and Gas, Paper, Water treatment. Due to the fact that efficient coatings could be formed with cold dynamic gas spray processes, this method is promoted to be used in production of metal matrix composites. Aluminium composite coating processes are likely to have some complexities because of its melting rate, reaction of melted Aluminium and probabilities of ceramic particles, therefore it is difficult to produce homogeneous coatings with thermal spray coatings. CGDS method can be achieved efficiently, eliminating other potential problematic process results. Today, CGDS method is very reliable and environment friendly coating technology, this contributes many opportunities to be used in industrial areas. Through the CGDS process, a high level of deposition velocity and bond strength can be achieved. Despite these benefits of the method, some undesirable results such as; oxides, porosity and discontinuity occur with very little amounts. These coating processes are studied in room temperature (without higher temperatures); so it is called cold gas dynamic spray coating. In CGDS processes, metallic powder particles accelerated rapidly and the coating can be formed through the deformation ability of powder particles. It is possible to increase wear resistance of coatings and prepare them to tribo-systems by using CGDS with suitable parameters. In tribo-systems, where high friction and wear are undesirable, lubricant usage is necessary. The idea of lubrication involves the separation of moving surfaces by a lubricant film. In these systems, generally synthetic oils are preferred to be use as lubricants. However, as a result of processing of the oils during discharging and storage leads to water, soil and air pollution directly by transferring ground water. Thus, it harms to the natural environment. Because of the fact that oils have these hazardous effects to environment and in the lubrication of sliding areas are required to be fire-resistant, water can be used as lubricant in these systems. Water can eliminate detrimental effects of oils which are summarized above. When metal is used to to be operated in environments in which water is present, the problem of rusting occurs, so ceramic materials which have no danger of rusting are suitable for these kinds of applications. According to the patents related water lubricity effect of seramics, they create a thin film on to wear surfaces by their reaction with water. Because of this thin films, direct contacts of wear surfaces can be eliminated. SiCp has unique properties such as; low density, high hardness and elasticity modulus, chemical inertness, thermal stability which leads it to be use in tribological systems. Through these properties of SiCp, it can be highly use in valve components, piston rings, mechanical seals, etc. In this master study, it is aimed to obtain homogeneous non-porous structured composite coatings with no discontinuity, by utilizing cold dynamic gas spray method and reinforcement with SiCp in the Al12Si matrix of coating powders, it is intended to get more hardness and wear resistance of the coatings. Objective of using different volumes of SiCp reinforcement was to analyze ratio of SiCp effect on the hardness, wear rates, wear areas and friction coefficients of the coated samples. Wear tests were realized both in water and air conditions, so it was aimed to investigate lubricity effect of SiCp materials in water and compare both conditions in terms of the test results. In the direction of these aims, CGDS method was used in order to produce composite coatings. Feedstocks were prepared with 100 % Al12Si , 5 vol. % SiCp +95 vol. Al12Si, 10 vol. % SiCp + 90 vol. Al12Si, 15 vol. % SiCp + 85 vol. Al12Si, 20 vol. % SiCp + 80 vol. Al12Si, 25 vol. % SiCp + 75 vol. Al12Si, 30 vol. % SiCp + 70 vol. Al12Si were deposited on 1050 Aluminium substrate. During the coating, air was used in the conditions of 6 bar pressure as process gas. Coating powders were preferred to be used with different volumes in order to analyze effect of reinforcement material SiCp in the coatings. Characterization of the coatings were analyzed with microscopic investigations, X-Ray diffraction, microhardness measurements, Scanning electron microscope, wear tests and FTIR measurements. Wear tests were conducted in air and in distilled water against alumina ball under loads of 1, 2, 3 and 5 N on a reciprocating wear tester at room temperature. For all coated samples, wear tests were realized in water. 25 SiCp % coated samples were tested in air conditions, as well. Conducted test results were summarized as belows; Al12Si particles which were used as matrix and SiCp particles which were used as reinforcement materials were bonded to Al 1050 substrate coherently. As a result of microscopic analysis, any discontinuity, spaces and porosity were not observed in the coated samples. Microvickers values which were found after hardness tests, were compared in terms of ratio of SiCp % involvement of coatings. Results show that, while the composition of the SiCp materials increased, the microhardness values of the coated samples were increased, as well. As the SiCp involvement increased in the composition of the coatings, friction coefficients and wear areas of coated samples were decreased. In air conditions, it was resulted that; friction coefficients and wear areas were higher than the water test conditions. As a result of wear tests of SiCp reinforced coatings in water, lubricity effect was observed due to reaction of the SiCp with water. Because of the lubricity, wear resistance of coatings in water tested are higher than the 25 % vol. SiCp involved coatings tested in air conditions. The findings of the wear tests revealed potentiality of water to be used as a lubricant for the SiCp reinforced composite coatings.