Formation of titanium aluminium intermetallic coatings by different methods

Cold gas dynamic spray is an emerging coating technology based on the use of supersonic gas jet to accelerate (up to 1000 m/s) and to impact a powder, with the size ranging from 1 to 50 µm in diameter, on a substrate. Due to the high speed, during the impact, the powder undergoes a severe plastic deformation such that it adheres on the substrate. Thanks to this method, it is possible to produce up to fully dense metallic coatings on substrates of different materials. With this technology, different kinds of powder (metals, polymers, ceramics, composite materials and nanocrystalline powders) or their mixing can be deposited. Among the various possible powders that can be deposited by cold gas dynamic spray, titanium is one of the most attractive materials thanks to its potential applications. Titanium and its alloys are widely used in automobile, aerospace, marine and chemical industries. Such a wide range of applications is related to high strength to weight ratio, excellent corrosion resistance and reasonably good toughness of titanium alloys. However, titanium and titanium alloys exhibit poor frictional resistance due to high friction coefficient and adhesive welds that are formed on their surface and insuficcient resistance to oxidation at elevated temperatures. In order to improve wear resistance and to reduce absorption of oxygen, titanium and its alloys are subjected to various surface treatments. The available experimental data show that the aluminide phases of the Ti-Al system can be used as protective coatings for titanium alloys. Intermetallic compunds of the Ti-Al system have been considered extensively for high-temperature aplications because they offer good oxidation resistance and high mechanical properties at temperatures higher than those possible with titanium and titanium alloys. The aim of this research is to form a Ti-Al intermetallic layer on titanium substrate with pure aluminum and titanium-aluminum cold gas dynamic sprayed coatings by heat treatment and laser surface melting.. Commercially pure titanium substrates were used in this study. The coating technique used in this study is cold gas dynamic spray process. Heat treatment samples were annealed in vacuum of 10-3 bar and 750oC temperature then cooled in the furnace. In all cases annealing time was 24h. Laser surface melting is performed using continuous CO2 laser under 77W, 5Hz, 100mm/s and 6 pass as parameters. Parameters of laser surface melting process were investigated experimentally in order to form a better coating layer on the substrates. Within the context of this study, 6 different group of samples were prepared. But separation of coating from substrate after laser process is observed on the laser surface melted sample with pure Al coating,. Also in the case of heat treated then laser melted sample with Ti+Al mixture coating, extreme crack propagation through thickness is observed. These 2 specimens could not be used in the further stages of this study As for characterization of the coated samples; cross-sectional microstructures of coatings were studied by optical microscopy. Phase compositions of coatings determined via X-ray diffraction (XRD) analysis. Micro hardness investigations both from surface and cross-section are held using micro hardness tester. Then wear tests were performed and the tribological properties of coatings were compared. Wear rates are calculated from the wear track depth, length and width using profilometer. Phase identification was carried out by a GBC MMA 027 X-ray diffractometer (XRD) using CuKα radiation. The samples were scanned over 1o angles of 10-90o at a step of 0.02o and a scanning speed of 2o/min. Results of the analysis showed that the intermetallic transformation occurred in all coatings. Annealing of both pure Al and Ti-Al mixed coatings resulted in the formation of the TiAl3 compund. As for the laser remelted coatings, other Ti-Al intermetallic compunds such as TiAl, TiAl2, Ti3Al ve Ti3Al2 with low intensities were formed aside from TiAl3. Formation of these intermetallic phases were caused by the high temperatures of laser process. Microstructures of the coatings were investigated by LEICA type optical microscopy. Cross sectional pictures of coatings showed that homogeneous microstructures with low porosity were obtained in all cases except for one specimen with laser surface melted Ti-Al coating which laser could not penetrate throughtout the coating from surface to substrate. Also an eutectic solidification region was observed on several parts of the surface of laser surface melted Ti-Al coating. Additionally, it was shown that there were some micro cracks lying throughout the coating from surface to interface. These cracks are interpreted as the result of thermal expansion coefficient differences and cooling rate. Moreover in the cross-section images of the heat treated sample with pure Al coating, cracks parallel to coating surface were observed in the mid-thickness region where the amount of porosity is increased. Both cross-sectional and surface hardness tests conducted out with a Vickers pyramid identer using BUEHLER TUKON 1102 type micro-hardness tester. Hardness measurements were performed under 10 gr load and hardness values were taken as the average of a minimum of 10 measurements. As a result, it was measured that hardness is averaged 558 VHN on the surface and 553 on the cross-section of coatings. Heat treated sample with pure Al coating showed the highest hardness on both the surface and the cross-section of coating. Furthermore, in order to form an opinion about brittleness of coatings, hardness measurements were performed on the surface under 10, 25, 50, 100, 200, 300, 500 and 1000g loads then the loads that formation of cracks occurred are determined. Finally wear tests were conducted under 1N and 3N loads via TRIBOTECH reciprocating tribotester device. To compare the wear resistances of coatings, wear tracks were scanned using Veeco Dektak 6M type profilometer and the wear rates were calculated. Also SEM images of wear tracks were examined along with the optical microscopy images of the Al2O3 balls. Micro abrasion and cracks were observed in all the samples except laser surface melted Ti-Al coating which has the lowest friction coefficient. This out come is tought to be the result of the eutectic solidification regions acting like a lubricant. Crack formation in the structure of the coatings and failing to penetrate throughtout the coating from surface to interface with laser are thought to be the paramount deficiency of this study. In all the studies using these material couples, this type of cracks are observed. There are certain treatments that are suggested to prevent crack formation in the literature. Penetration throughout the coating can be achieved by using a higher power laser. Also, in the case with the same parameters used in this study, forming thinner coatings is recommended.