TiB2 coating on different substrates via dual process:CA-PDV and CRTD-Bor
TiB2 coating on different substrates via dual process:CA-PDV and CRTD-Bor
Dosyalar
Tarih
2024-01-22
Yazarlar
Karimzadehkhoei, Mehran
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Graduate School
Özet
Boriding process is a surface modification technique that enhances the mechanical properties of industrial components including stiffness, hardness, strength, and wear/corrosion resistance, resulting in enhancements in their lifetime and efficiency. Among the transitional metal borides, titanium diboride (TiB2) is one of the hardest metallic-ceramic phases with its excellent electrical and thermal conductivities, good thermal oxidation resistance, high melting point, and good chemical stability. In bulk form, titanium diborides are used as thermal evaporation crucibles, armors, and as a cathode in electrochemically reducing alumina to aluminum metal. TiB2 is also applied as a coating for cutting and forming aluminum alloys and improving titanium alloys' oxidation and wear resistance. Various boriding methods such as pack/paste boriding, salt-bath boriding, fluidized bed boriding, plasma boriding, and gas phase are used to form TiB2 layers on titanium-based substrates. In addition, the TiB2 layer can be grown as a coating on metallic substrates using vacuum-based techniques (CVD and PVD). However, all these procedures have at least one of these drawbacks: the high cost of chemicals and materials, the requirement of expensive equipment, long processing time, toxic emissions and by-products, and the consumption of a large amount of energy. In the current study, to overcome the above-mentioned problems, a dual process consisting of a very common and undemanding PVD coating technique, cathodic arc-physical vapor deposition (CA-PVD), and rapid, environmentally friendly, and cost-effective cathodic reduction and thermal diffusion-based boriding (CRTD-Bor) is used. Ti layers on different substrates are produced with CA-PVD that are then converted into titanium borides with CRTD -Bor process. The advantages of the preferred route to form TiB2 on different substrates are as follows: Eliminate the use of expensive TiB2 cathodes Stoichiometric TiB2 coatings growth Overcome the adhesion problems of TiB2 coatings on substrate materials Eliminate the need for expensive equipment and starting materials Use of stabile oxide-based chemicals No toxic raw materials Eliminate greenhouse gas emissions Multiple useability of stable electrolytes with periodic chemical additions Shortening the boriding process time resulting in an increased TiB2 formation rate In the first part of the study, the growth of the TiB2 layer on the Cu substrate, which is used as spot welding electrode material, was targeted. Naked copper electrodes do not function properly and create serious problems such as welded material and electrode tip adhesion and electrode deterioration, resulting in reduced electrode life and weld quality during spot welding of galvanized steel and aluminum because of the high chemical reactivity of zinc and aluminum with copper. Relying on the limited reactivity of TiB2 with Zn and Al, copper electrode surfaces were initially Ti coated using CA-PVD and then borided via CRTD-Bor to convert the Ti layer into a TiB2 structure. Because the boriding temperature is above the eutectic reaction temperature between Ti and Cu (at 875 °C), an Nb interlayer was applied between Ti coating and Cu substrate to prevent local melting and disbonding problems that may occur at the Cu-Ti interface. The coating architecture consisted of 2.6 - 3.2 micron thick Nb and 4 - 5 micron Ti. Boriding experiments were carried out at different temperatures (950 and 900 °C) for various durations (10-90 min.) at a consistent current density (200 mA/cm2) in the molten salt containing 90% sodium tetraborate and 10% sodium carbonate. As a result, the proposed ~1.3 m Ti-boride layers were produced at idealized conditions (i.e., 900 °C and 30 minutes) with high adhesion strength to the substrate (HF1). Lab scale spot welding tests showed the positive role of the boride layer and spot welding performance of copper electrodes. In the second part of the study, improving the wear resistance issues associated with M2 high-speed steel (HSS) was aimed. This was achieved by surface modification through the formation of a TiB2/TiC multilayer via the combined process. CRTD-Bor was applied to the CA-PVD Ti-deposited HSS substrate. During the boriding process, TiB2 growth occurred at the top layer due to boron diffusion at the interface of the substrate (cathode) and the electrolyte. Simultaneously, the TiC layer formation took place as carbon in the steel diffused from the substrate to the Ti layer, ultimately resulting in the production of a TiB2/TiC multilayer. To determine the effects of boron and carbon, diffusion on the nature of grown multilayer, boriding experiments were conducted at various temperatures (from 900 to 1000°C) for 15-60 minutes with a constant current density (200 mA/cm2). According to the characterization results, both the boride and carbide layers' thickness increased as the boriding times and temperatures increased and it obeyed the Parabolic Law. Empirical equations are derived for estimating the thickness of layers at different times and temperatures. These equations could be used in the temperature range of 1173 to 1273 K at the current density of 200 mA/cm2 with the Ti layer having a thickness of 8.5 µm. d_(TiB2 )=86.60√(exp(-17573/T).t) d_TiC=425.44√(exp(-22529/T).t) Where d is the thickness of the modified layer's thickness (µm), T is the boriding temperature (K), and t is the boriding time (sec.). Moreover, based on the kinetic investigations, the activation energies (Q) and pre-exponential factors (K0) were calculated as 146.10 kJ/mol and 7.50 × 10−9 m2/s for the formation of TiB2 as well as 187.31 kJ/mol and 1.81 × 10−7 m2/s for TiC layers respectively. Furthermore, according to the micro-indentation investigations as a function of the penetration depth, the surface hardness value was measured as 41 ± 5 GPa which is consistent with the reported value for the TiB2 hardness. However, in the mixed TiB and Ti regions (at deeper penetration depth), the hardness values declined to 24 ± 2 GPa and 13 ± 1 GPa and then raised to 20 ± 1 GPa because of the TiC contribution to the total hardness. Also, all treated samples exhibited excellent adhesion properties (HF1) as determined by the Daimler-Benz Rockwell C test. In the final stage, ball-on-disk sliding wear tests were conducted for the samples against an alumina ball to assess and compare the tribological performance of the borided sample and M2 HSS. The outcomes of the tribological investigation demonstrated an eightfold enhancement in the wear resistance of the boride sample compared to the untreated sample.
Açıklama
Thesis (Ph.D.) -- Istanbul Technical University, Graduate School, 2024
Anahtar kelimeler
titanium diboride,
titanyum diborür,
coating,
kaplama,
boriding,
borlama