Investigation of mechanical and thermal properties of TiB2 coating grown on Ti-6Al-4V VIA CRTD-Bor

Abstract

Surface modification methods are extensively used in modern engineering to promote surface properties. Various surface modification methods and agents are utilized to conduct the process of surface treatment. The agents and methods vary in each other via their different advantages and disadvantages. Hence, among all surface modification methods and agents titanium diboride and CRTD-Bor technique possess the uttermost advantages. TiB2 is considered a desired transition metal boride for most engineering enthusiasts since it performs advanced electrical, thermal, and mechanical properties. The main advantage of the TiB2 coatings are well known as their excellent hardness and wear resistance against sliding surfaces. Moreover, the high-temperature stability of titanium diboride coatings makes it required for high-temperature applications. The boriding process is conveniently conducted by governing the pack and past process techniques. However, in modern days these techniques have several drawbacks, mostly about time and hazardous by-products. When compared with the convenient techniques, the CRTD-Bor method offers; ▪ Shortened process times ▪ Thicker coating layer ▪ Elimination of hazardous by-products ▪ Aligned stoichiometric compliance with the desired coating compound. Hence, when the process advantages and outstanding properties of TiB2 are considered, the application of titanium diboride coatings via the CRTD-Bor technique may be useful for aerospace applications where high-temperature stability and excellent mechanical properties are desired to be combined. In this study, a thin film layer of TiB2 is coated onto Grade-5 titanium (Ti-6Al-4V) substrate material. After several condition and coating trials, 15 minutes of electrolysisxii and 30 minutes of electrolytic holding were implemented to the substrate specimens at 1000° C. Afterwards, solution treatment and aging and heat treatment were implemented to the specimens to eliminate the drawbacks of high-temperature electrolysis application. The attained coating on the substrate material was near 3 microns thick TiB2 and well aligned with the stoichiometric patterns. To obtain the effects of titanium diboride coating on Grade-5 titanium alloy a series of test campaigns was held. In particular, roughness measurements, tensile tests, high cycle fatigue tests, high-temperature oxidation tests, thermal radiation emittance/reflectance tests, and wettability tests were executed with the borided and non-borided (bare, lean) specimens. Roughness measurements of borided specimens showed 0.461µm for Ra (average roughness) compared with the bare specimen roughness of 0.479 µm for Ra. In addition to the average roughness values, RZ values of both types of specimens have been obtained as 2.5076 µm for borided and 2.883 µm for bare specimens. Thus, 15 minutes of electrolytic coating and 30 minutes of holding in the molted salt bath process generated no adverse effect on roughness values. Tensile tests have been carried out at 3 different temperature ranges by the specific aerospace regulations. No adverse effect of boriding on the tensile properties is obtained at the end of the campaign with ultimate tensile strength results of 1112 MPa- 1155 Mpa for -55°C, 1014 MPa, and 1014 MPa for 23° C, and 849 MPa-873 MPa for 180°C respectively for bare and borided specimens. High cycle fatigue test has been carried out by implementing various stress amplitudes with a stress ratio of R 0.1. Borided specimens showed a significant amount of decrease when compared with the bare specimens. This event was linked with columnar-like microstructure and dendritic morphology of the coating. High-temperature oxidation tests were executed at 700 C°, 800 C°, and 1000 C° respectively for 150, 28, and 24 hours. For the 700 C° test, a 5.07 rate of weight change for bare specimen/coated specimen has been examined. For 800 C° and 1000 C° tests, 1.4 and 1.6 rates of weight change for bare specimen/coated specimen have been found respectively. In addition to that, the overall reaction energy between the implemented temperature ranges was evaluated as -255.12 kJ for bare and -456.46 kJ for coatedxiii specimens. Hence 1.8 times better performance of titanium diboride coating is discussed for the overall reaction energy. Thermal emissivity test has been executed to characterize the radiation dissipation form of the borided specimens. Three types of specimens have been used; bare, Type 1 borided (unwashed) with a darker surface color, and Type 2 borided (washed) with a lighter surface color in comparison with the Type 1 borided specimen. Resulted revealed that, for the average of 20° and 60° incidence angle, Type 1 specimen showed an emissivity value of 0.91, Type 2 specimen showed an emissivity value of 0.42 and bare specimen showed an emissivity value of 0.28 between the wavelength of 3-5 µm which is named as MWIR (mid-wave infrared wavelength). This wavelength was discussed as important for jet-engine interior radiance dissipation and remarked as vital for low observability requirements of aerospace applications. The optimum value which combines emissivity with low observability is found for the Type-2 specimen. Wettability tests have been conducted to see the reaction of the coating for possible secondary surface treatments such as dying or secondary coatings. Results showed a contact angel of nearly 50 degrees for bare specimen and nearly 20 degrees for the coated specimen. Cross-checks have been done from different locations of specimens to validate the results. The hydrophilicity of TiB2 was way higher than that of bare specimen which can be correlated to better capacity for secondary operations. The driving force for the above test group has been sourced from the huge literature gap for TiB2 coatings. The whole test group generated unique findings and hopefully will be beneficial for later studies for coating candidates on aerospace applications.