Preparation And Properties Of Mo-based Alloys Oxidation Resistance Coatings At Elevated Temperature

Abstract:Due to its high strength and creep properties at high temperatures, high electrical and thermal conductivities, excellent corrosion resistance, and better thermal shock and fatigue resistance, molybdenum is widely applied. However, high volatile oxide vapor MoO3easily forms in Mo and its alloys at high temperatures in oxidizing environments, which leads to loss of their excellent-properties at elevated temperatures and restricts their wider application. Development of a suitable protective coating is considered as an effective method for improving their oxidation resistance at high temperatures. MoSi2is regarded as a promising material against oxidation at temperatures beyond1200?as a result of a continuous, protective SiO2layer formed on its surface. There are advantages and disadvantages in preparation methods of oxidation resistant coatings of Mo in the present studies. And the diffusion between MoSi2coating and Mo substrate at high temperatures results in a drastic reduction of service life. Employing high effective preparation methods and postponing the diffusion between MoSi2coating and Mo substrate are very important for developments of theory and applications.MoSi2was used as protective material, and air plasma spraying (APS) technology and in-situ chemical vapor deposition (CVD) method were used to prepare MoSi2coating and MoSi2/MoB composite coating on Mo substrate in the current thesis. The relation between APS or CVD process conditions and microstructure of coating was discussed. The mechanisms of oxidation resistance and diffusion of coating were studied. Based on the kinetic bias of diffusion, the coating structure was adjusted. The diffusion of Si element was postponed at elevated temperatures, which increased the lifetime of coating.MoSi2powder used in APS was prepared by spray drying and vacuum heat treatment. The technique puzzle of oxidation of small MoSi2particles was solved. A slight oxidation of agglomerated MoSi2during heat treatment caused a compositional change from single MoSi2phase to composite phases, including the major MoSi2phase and the minor Mo5Si3phase. The flowablity and apparent density of the MoSi2powder were38.5s/50g and1.65g/cm3, respectively. Effect of APS conditions and critical plasma spraying parameter (CPSP) on phase composition, microstructure and properties were examined. A better oxidation resistance of MoSi2coating at high temperatures was achieved by adjusting process parameters.Molybdenum boride coating was prepared on Mo by in-situ chemical vapor deposition method. Effect of depositional conditions on phase compositon, microstructure and growth kinetics of MoB coating was studied. It's shown that the single MoB coating could be prepared at temperatures over900?with the content of B above0.2wt.%. With the increase of temperature and time, the thickness of MoB coating increased. The effect of temperature on the thickness was more significant than that of time. The activation energy obtained from the slope ofln(T1/2h) vs1/T plot was271.74±32.41kJ/mol.MoSi2coating and MoSi2/MoB composite coating were prepared by in-situ chemical vapor deposition method. Effect of conditions of deposition on phase compositon, microstructure and growth kinetics of MoSi2layer was discussed. The mechanisms of oxidation resistance and diffusion of coating were studied.The MoSi2/MoB composite coating formed on the surface of Mo substrate in granular fashion. The grains were arranged in parallel fashion to each other. The hardness and elastic modulus of MoB layer was higher than those of MoSi2layer.With increasing temperature and time, the thickness of MoSi2layer increased. The effect of temperature on the thickness was obviously stronger than that of time. The activation energy obtained from the slope ofln(T1/2h) vs1/7plot was194.45±20.78kJ/mol.MoSi2/MoB composited coating showed a better oxidation resistance at high temperatures. The weight gain, oxidation rate and parabolic oxidation rate constant of MoSi2/MoB composite coating at1200and1300?were0.270mg/cm2and0.499mg/cm2,7.487×10-7mg/(cm2·s) and1.733×10-6mg/(cm2·s),7.80×10-4mg2/(cm4·h) and3.14×10-3mg2/(cm4·h).The growth kinetic of Mo5Si3layer obeyed a parabolic law at high temperatures with two different steps. The thickness of Mo5Si3layer in the composite coating showed a fast growth (5.48×10-11cm2/s) at the first step (0?20h), which was similar to the growth of M05S13layer in single MoSi2coating (7.66×10-11cm2/s). However, the growth of Mo5Si3layer in the composite coating dramatically decreased at the second step (more than20h), which was two order of magnitude lower than that in single MoSi2coating. The diffusion of Si element varied in two types of coatings at high temperatures. The diffusivity of Si element in single MoSi2coating was faster than that in the MoSi2/MoB composite coating. MoB layer in MoSi2/MoB composite coating played a role of diffusion barrier for Si, and prolonged the service life of coating at high temperatures.