| Proposing a new coating fabrication technique, which includes the developing and perfecting the conventional coating fabrication technique or exploiting much more advanced coating fabrication technique, and kowning the degradation mechanism of oxidation-resistant coating and establishing its degradation formula for oxidation-lifetime prediction were always the hotspot and difficulty in the field of coating fabrication, thus, it was much necessary and potential to develop the relevant research works.Based on these questions, a new fusing technique, low oxygen partial pressure fusing (LOPPF) technique, was proposed and the primary powder system used in fusing coating were also breached and broadened. To know and understand the LOPPF technique completely, three kind of fabrication methods were divided based on the number of elements that pasted on the surface of substrate alloy, which involved the fabrication methods within one element, within two elements and within multi-elements; Meanwhile, according to the interaction between the elements in the powder and the substrate, the detailed fusing mechanism of every kind of corresponding fabrication methods and the relationship among them were presented in theories; Finally, To greatest decrease the fusing temperature, the Al element was acted as the mainly component in the present powder system, which includes the powder systems within one element Al, within two elements Al-Si,Al-Cr and Al-B4C, and within three elements Al-Si-TiC were proposed; the Ti-6Al-4V alloy, which has been been limited its further development and commercial application for its poor oxidation resistance and insufficient wear resistance in industrial development, especially in aerospace, ocean engineering, petrochemical industry, was selected as the substrate alloy and corresponding every kind of them was fabricated on the Ti-6Al-4V alloy by the furnace heating method under low oxygen partial pressure condition.Besides, a novel approach to prediction of oxidation-lifetime has generally concentrated on the oxidation-induced degradation in the oxidation-resistant coating system based on the CET and the corresponding degradation tm (T )=ε×N2(T)×t formula for its oxidation-lifetime prediction was also further established.In the present research about the fabrication of the LOPPF coating formed on Ti-6Al-4V alloy and oxidation-lifetime prediction for the oxidation-resistant coating, the major research efforts of this present study were as follows:(1) Classification, fabrication and fusing mechanism of LOPPF coating layerThe classification and corresponding fusing mechanism of LOPPF coating were pointed out and the detailed fusing mechanism and the relationship among them were also presented, it could be statistically treated by transforming the fusing mechanism of LOPPF coating within multi-elements into the fusing mechanism of LOPPF coatings one element or two elements. The representative powder systems, which include the powder systems within one element Al, within two elements Al-Si, Al-Cr and Al-B4C and within three elements Al-Si-TiC were proposed and corresponding fusing coatings have been fabricated on Ti-6Al-4V alloy by the furnace heating under low oxygen partial pressure condition and the detailed fusing mechanism and the relationship among them were also further proved.(2) Influencing factors of kinetics for the fabrication of LOPPF coating layerInfluencing factors of kinetics of the LOPPF coating layer with different elements system were detailedly investigated by experimental research:(a) It has been found by experimental research that the influencing factors of kinetics, which involved the fusing temperature, fusing time and Si content, affected greatly the fabrication and phase composition of LOPPF Al-Si coating. It could favor the formation of Ti5Si3 phase and the increase of Si-rich layer by improving the fusing temperature and prolonging the fusing time; and the improvement of Si content favored not only the formation of Ti5Si3 phase, but also the forming continuity of the coating. (b) It has been found when the content of B4C was 5 wt. %, a desirable MMCs coating was formed; as the content was 10 wt. %, although this coating could be formed on the Ti alloy, some large cavities began to appear at the interface; while the content was raised up to 20 wt. %, only a TiAl3 layer was formed and the outer MMCs layer was desquamated after the fusing treatment during the fabrication of TiC-TiB2 particulates reinforced MMCs coatings. The molten aluminum served not only as a diluent agent but also as a reactant and participates in the exothermic reaction. The addition of aluminum provided a much easier route for the TiC and TiB2 formation in the MMCs coating.(c) The addition of diluent TiC powder in the in the Al-B4C system not only decreased the explosive exothermic reaction (4-14), improved the interfacial bonding strength and favored the formation of MMCs coating, but also increased the volume fraction of the reinforcements TiC in the coating. The thermal shock tests showed the addition of a small amount of Y2O3 in the Al-B4C system not only improved vastly the adhesion of the MMCs coating, but also changed the separation style, which made the stress centralized from the interface of coating /substrate to the inner of the coating.(d) It has been found the increasing of Al content could favor the interfacial bonding strength, but not the improvement of the volume fraction of Ti3SiC2 and the thickness of Si-rich layer; by using the Al-Si-B4C, instead of Al-Si-TiC, the in situ TiB2 has been successfully fabricated in the Ti3SiC2 particulate reinforced MMCs coating. Besides, the aluminum served not only as a diluent agent but also as an activator agent, reactant assistant agent and deoxidation agent, and the addition of aluminum provided a much easier route for the Ti3SiC2 formation in the composite coating.(3) Oxidation property and sliding wear performance of LOPPF coatingThe oxidation or sliding wear test showed that the oxidation or wear resistance of Ti-6Al-4V alloy was vastly improved for the formation of LOPPF coating layer and the relationship between the coating performance and its influencing factors of kinetics was also optimized and presented.(4) A study of oxidation-induced degradation in the oxidation-resistant coating system and its working-lifetime predictionA one-dimensional oxidation-induced degradation model for the oxidation- resistant coating system was developed in this paper. According to the dependent relation among the coating effective thickness, the experimenting oxidation time and the decrease of the coating effective thickness resulting from surface oxidation and coating-substrate inter-diffusion, the degradation formula for predicting the coating oxidation lifetime was established.(a) A one-dimensional oxidation-induced degradation model for the ideal oxidation-resistant coating system within one element was developed and the degradation formula for the coating was proposed according to the degradation mechanism and on the basis of Fick's law. By means of testing a short-term oxidation and measuring the CET and the changes resulting from surface oxidation and inter-diffusion, the oxidation lifetime could be predicted. Using equivalence ratio, the equivalent concentration of oxidation-resistant element in the coating within two or more elements could be statistically treated by transforming the concentration of other elements into the concentration of the oxidation-resistant element; the quivalent oxidation temperature of coating oxidized at different temperatures could also be statistically treated by transforming the oxidation temperatures into an equivalent oxidation temperature and by transforming the area of the porosity and crack into an effective area, the equivalent effective area of the coating within porosity and crack could be statistically transformed into the effective area of the dense coating.(b) The coating oxidation-lifetime under different oxidizing atmosphere, external factors (wear and odditional load) and generalized high-temperature oxidation were discussed and analyzed. It showed that these factors only changed the equivalence degradation rate by surface oxidation and the proportion of consumption by the surface oxidation in the total consumption, and its calculation of the oxidation lifetime was consistent with the former.(c) The errors for predicting the oxidation lifetime resulting from experimenting a short-term oxidation time was analyzed and discussed. For the difference between the degradation rate in the chosen oxidation time and the average degradation rate, the error would be inescapable and it could be reduced by increase of the experimenting oxidation time. |
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