Investigation of high temperature oxidation and glass interaction bahaviors of electroless Ni-P, Ni-B ve Ni-W-B coatings

Mold materials, which are used in glass container production industry, are continuously in contact with molten glass during production processes. Because this contact takes place in air atmosphere at elevated temperatures, the mold surfaces are subjected to oxidation, mechanical stresses, thermal cycling and chemical or physical interactions with the glass. Potential failures of the mold surfaces lead to several problems related to production process and product quality. In other words, molds can be defined as one of the most important components of a glass container production process and directly affect the whole process. Cast iron is the most commonly used mold material, which meets the requirements of a glass container production process with a low cost. However, the usage of cast iron molds creates problems especially for gravured bottles, that show itself as cracking of the bottles in the close vicinity of gravures. Studies about enhancing the properties of mold materials, which mainly aims increasing the service life of the molds and decreasing the number of mold related problems are always in the center of interest for glass production plants. The main topics are generally based on finding an alternative mold material and/or improving the surface characteristics of the current materials. In the scope of these investigations, favorable results were obtained for enhancing the surface properties of mold materials by using several techniques such as carburising, nitriding, nitrocarburising and boronizing. Especially boronizing has found a wide application for molds in glass production industry in recent years. However, the surfaces of pack borided molds should be subjected to mechanical grinding and polishing processes for removing the remnants of boriding process and also the surface roughness created by the process. When working with gravure molds, which contain more detailed surfaces, these post treatments causes the deterioration of the gravure details on mold surfaces. Thus, classical boronizing applications are not appropriate for these kind of molds used in the production. Therefore, another coating or surface treatment process, that does not need post mechanical treatments and able to give appropriate surface finish and even thickness distribution is required for such applications. All of these requirements can be satisfied with electroless plating. Electroless nickel coatings can be classified in two main categories, Ni-P and Ni-B. The co-deposition of a third element within the growing film has led to a new generation of electroless alloy coatings. The co-deposited metal is mainly chosen from transition metals such as W, Co, Mo, Re, etc. because of their superior specific properties. These coatings can also be customized for desired properties by selecting the composition of the coating to obtain specific requirements. In the MSc thesis study which was conducted in 2010, electroless Ni-W-B coatings were obtained by the addition of sodium tungstate into plating solution and wear behavior of these coatings were investigated beside their general characteristics. As a result of this study, it was seen that obtained Ni-W-B coatings showed better behavior than the commercial Ni-B coatings in terms of wear and corrosion. Based on these previous results, three different electroless coatings (Ni-P, Ni-B and Ni-W-B) were used in this study. Cast iron and mild steel were used as the substrate material for deposition of the coatings. Commercial hypophospide and DMAB based plating solutions were used for the Ni-P and Ni-B deposits, respectively. Besides, Ni-W-B bath used was obtained by adding 5 g/L Na2WO4.2H2O into the DMAB based solution. Before pre-treatment, the specimens were mechanically ground with SiC paper up to 1200 grit and polished with 1 μm Al2O3 paste. Samples were degreased in hot alkaline solution for 5 min and deoxidized in dilute inhibited hydrochloric acid solution for 2 min. After each step, deionized water was used for rinsing and then the specimens were immediately immersed in the plating solutions for deposition. The duration of the coating processes were tuned to give an equivalent thickness of 11±1 µm. For determining the behavior of electroless coatings in the glass container production industry, two main issues must be taken into consideration. First one is the oxidation of the coatings in air atmosphere at elevated temperatures and the second one is the interaction of the coating surface with molten glass. For this purpose, the study was conducted in two main parts. In the first part, the prepared Ni-P, Ni-B and Ni-W-B coated steel specimens were oxidized in a temperature controlled tube furnace in a wide temperature range for different oxidation times. Oxide thicknesses and the composition of diffusion layer between the coating and the substrate were determined from GDOES depth profile measurements (Horiba JY, GD Profiler HR). Thicknesses of the sputtered layer were determined from the sputter crater by 3D optical profilometer (Veeco Wyko NT1100) and sputter depth vs. thickness values were calculated by assuming a homogenous sputter rate during GDOES measurements. Surface morphologies and compositions of the oxidized samples were observed using scanning electron microscopy with an energy dispersive X-ray spectroscopy (Jeol 5410/Noran). The phase structures of the oxide scales formed on the samples were identified with X-ray diffraction (Philips PW3710 attached with Xpert Pro) using Cu-K radiation generated at 40 kV and 40 mA, using glancing angle configuration. Angle of incidence was adjusted to 2° in order to get information from the near surface. Micro Raman spectroscopy measurements were also conducted on oxidized Ni-B and Ni-W-B samples for further verifying of the structure of compounds that are formed on the surface. For this purpose, micro-Raman spectrometer (Horiba JY HR 800) was used with 632 nm wavelength HeNe laser and 10 mW power. In the second part, for investigating and comparing the interaction between coating surface and molten glass, a new and simple methodology was developed. In this method, coated steel samples and soda lime glass balls were used in CETR UMT system. The coated sample was fixed in the furnace and the soda lime glass ball was put in the furnace again by the help of a ball holder. For these set of experiments, the test temperature was adjusted to 600°C, representing the real production conditions. After stabilization of the temperature of the samples, the glass ball was brought into contact with the surface of coating by applying a pre determined normal load (Fz) of 20N. After different exposure times, the holder was pulled back with a lower Fz force (10N) and the change in the Fz values was monitored during the detachment. By this way, the adhesion between the coating material and glass ball could be evaluated. The coating and ball surfaces after the experiments were also investigated by SEM, EDS and 3D optical profilometer. Results of these investigations clearly showed that oxidation properties of Ni-P, Ni-B and Ni-Wi-B deposited on steel are controlled not only by the oxidation properties of the coating itself but also by the diffusion and interdiffusion processes that take place between the substrate and the coating. Thus, it can be stated that high temperature oxidation behavior of these coatings is a system property that shows strong dependence with the interaction of the coating with the substrate and structural changes that may take place within the coating. The results obtained from oxidation tests conducted on Ni-P gave similar results to the previous studies. Drastic degradation of the coatings became evident after diffusion of iron to the coating surface and its subsequent oxidation after treatment at 700°C for 10 hours. An interdiffusion layer that grew up to 8 micron with substantial formation of Kirkendall voids was observed after the same treatment. P present in the coating did not take part in the interdiffusion layer. For Ni-B coatings, the most important result was the depletion of the B content of the coatings. After 10 hour treatment at 700°C, almost no B was left in the coating. For samples treated at 700 and 800°C, nickel borates were formed on the surfaces as indicated both by GDOES and XRD measurements. For samples treated at 600°C, crack formation was observed at the coating-substrate interface. This cracking was not present for samples treated at 700 and 800°C, only Kirkendall voids were observable. Interdiffusion layers also formed in these coatings. However, their thicknesses were lower than the ones observed for Ni-P coatings (6 microns after treatment at 700°C for 10 hours). Diffusion of iron to the surface also took place at 700°C for these coatings. However, oxidation of the coating was not as drastic as Ni-P deposited steel at this temperature. Dramatic degradation of the coating commenced at 800°C with extensive cracking and thick oxide layer formation. Ni-W-B coatings exhibited similar oxidation behavior when compared to Ni-B coatings. Most important contribution of the presence of W was on the thickness of the interdiffusion layer. Only 2.7 microns thick interdiffusion layer formed after treatment of 10 hours at 700°C. Presence of the W drastically decreased the diffusion of iron within the coating and delayed the iron oxide formation on the surface. This led to a better oxidation behavior of these coatings at 700°C. However, although the oxidation rate was lower, degradation of the coatings started at 800°C similar to Ni-B coatings. Results of the experiments, concerning the coating – glass interaction showed us that the Ni-B and Ni-W-B coatings performed better adhesion behavior than Ni-P coating. During the detachment, Fz values were lower for Ni-W-B coating than Ni-B, whereas for Ni-P no separation was observed under applied force conditions. This result for Ni-P can be related to the formation of phosphorus oxides at the coating – glass interface. Highly probably, by the effect of phosphorus, the wettability of the glass is increased and a strong adhesion was observed between glass and coating in all the surface area. Thus, the applied force was not enough for the separation of these two surfaces. In boron-based coatings, a glassy phase was observed at the interface, which shows us the formation of boron oxides at the interface. Material transfer both on the coating itself and glass ball was clearly seen after electron microscopy investigations. Formation of tungsten oxides, as a difference in Ni-W-B coatings, may be the reason of lower detachment forces compared to Ni-B. The most interesting result of the experiments was the change in the detachment forces by increasing time in Ni-B and Ni-W-B coatings. The boron, after forming boron oxides, evaporates from the coating structure with increasing oxidation time. This finding was also shown and explained in detail in high temperature oxidation experiments. During the coating – glass interaction experiments, it was seen that the detachment force was decreased by the increase in the evaporation of boron from the structure. Therefore, it can be said that the evaporating boron in the Ni-B and Ni-W-B gives us more advantage during applications. The results of the study clearly showed the high potential of Ni-W-B electroless coatings for increasing the useful life of gravured glass molds. By the comparative studies conducted on high temperature oxidation and glass-coating interaction behavior of three different types of electroless nickel coatings, it became possible to explain the role of alloying elements (P, B and W) on high temperature oxidation and also coating-glass adhesion behavior.