An Investigation On The Properties And Surface Laser Treatment Of TIB₂-TIC_0.3N_0.7 Based Composited Coating Deposited By Reactive Plasma Spraying

The best advantage of surface engineering is that it can prepare good properties of surface coatings than substrate using different kinds of methods. The thermal spray technology can be used to deposit ceramic and composite coatings with the properties of wear resistance, corrosion resistance, high temperature resistance, oxidation resistance, thermal insulation and heat radiation resistance. It has been used extensively due to its diversity of methods and coatings as well as economic efficiency. However, the ceramic phases (TiB₂?TiC) in traditionally thermal sprayed cermet coatings are added into the thermal spray powders. The disadvantages of this method are large particle size, easy pollution in the interface between ceramic and metal and different control of the ceramic composition during thermal spraying. Reactive thermal spray technology is developed by combining the self-propagating high temperature synthesis (SHS) and thermal spray technology. The well distributed hard ceramic phases in the metal substrate can be obtained with decreasing supplied heat due to in situ reaction synthesis of ceramic phases during reactive thermal spraying. However, there are some pores among the particles in the coatings due to the typical layer structure of thermal sprayed coatings. Laser surface treatment can solve such problems in the coatings, such as pores and layer structure. It will improve the density and properties of coatings after laser remelting for the thermal sprayed coatings.SHS reaction of Ti-B₄C system with metallic binder of Co and Cr addition was investigated in the dissertation. TiB₂-TiC_0.3N_0.7 based composite coatings was deposited using Ti-B₄C-Co/Ti-B₄C-Cr powder with 25wt.%NiCoCrAlY addition by atmosphere plasma spraying. Laser partially remelted sprayed composite coatings was studied. Advanced ARES rheometer and Zetaplus cataphoric facility was used in the paper. The effect on the rheological change of Ti-B₄C-Co slurry was investigated with the content change of A15 disperser, PVA binder and solid loading. The thermal spray powders of Ti-B₄C-Co and Ti-B₄C-Cr were prepared using spray-dried method based on the optimal addition of A15, PVA and solid loading determined by measurement of fluidity and loose density of spray-dried powders. The reaction mechanism of Ti-B₄C system was discussed from reactive thermodynamics and dynamics during the plasma spraying. The dry wear tests for the plasma sprayed and laser remelted TiB₂-TiC_0.3N_0.7 based coatings were conducted under different applied loads and sliding speeds in the air. The main results are as follows:Ti-B₄C-Co composite powders are dispersed with A15 disperser addition and their slurry possesses the character of shear thinning which is helpful to spray-dried. The spray-dried powders have optimal flowability and loose density with 0.5wt.% and 2.5wt.% of A15 and PVA addition of the total powders as well as 35wt.% solid loading. It is better for reactive plasma spraying with even particle size distribution of the thermal spray powders under heat treatment in the protection of argon. Non-equilibrium thermodynamics and dynamics were involved in the reactions of Ti-B₄C system in air during plasma spraying. So all the equations based on the Ti-B₄C system will react in the non-equilibrium system under the heat provided by plasma flame and exothermic from SHS reactions. The of the equation between Ti and B₄C still meets the requirement of SHS developed by A.G.Merzhanov and the reaction can last by itself.The titanium powder is oxidized during preparation of Ti-B₄C-Co/Ti-B₄C-Cr thermal spray powders. However, Ti₆O and Ti₂O with strong reducibility are unstable oxide and able to react for the target products. Spray-dried Ti-B₄C thermal spray powders have good sphericity and flowability. It is very useful to the formation of TiC_0.3N_0.7 and TiB₂ after powder sintering during reactive plasma spraying due to increment of bond strength between powders. The main composition of reactive plasma sprayed coatings using spray-dried Ti-B₄C-Co powder is TiB₂ and TiC_0.3N_0.7. There are some cracks in the coating due to insufficient metallic binder and bad wettability between phases. The coating deposited using spray-dried Ti-B₄C-Co powder with 25wt.% NiCoCrAlY is much denser. The effect on the phase composition could be neglected with addition of metallic binder Co and Cr in the spray-dried Ti-B₄C powders. Some unmelted particles, pores and microcracks exist in the composite coatings prepared using reactive plasma spraying. Partially non-homogeneous phases concentrate in the coating due to complicated reactions during plasma spraying.The composite coatings deposited using Ti-B₄C-Co/Ti-B₄C-Cr+25wt.% NiCoCrAlY powders exhibit anisotropy on the mechanical property. The Vickers microhardness on the cross section of coatings is higher than that on the surface. The mean Vickers microhardness on the cross section of the coating deposited using Ti-B₄C-Cr and 25wt.% NiCoCrAlY powders is higher than that deposited using Ti-B₄C-Co and 25wt.% NiCoCrAlY powders. Furthermore, Weibull distribution displays that the Vickers microhardness values on the cross section of both coatings are binary distribution. The dispersion of the coating deposited using Ti-B₄C-Cr and 25wt.% NiCoCrAlY powders is lower than that of the coating deposited using Ti-B₄C-Co and 25wt.% NiCoCrAlY Tad powders. The elastic module of the coating prepared using Ti-B₄C-Cr and 25wt.% NiCoCrAlY powders is 236.3±42.5 GPa. Its main pore size distribution is around 0.2?m and the content of remaining mercury in the coating is 0.18 ml/g with 10.46% porosity of the coating. And porosity of the coating deposited using Ti-B₄C-Co and 25wt.% NiCoCrAlY powders is about 12.27% with main pore size distribution of 1?m and remaining mercury of 0.25 ml/g in the coating. The bond strength of both composite coating decreases with the increment of thickness of the coating. However, the bond strength of the coating prepared using Ti-B₄C-Cr+25wt.% NiCoCrAlY powders is stronger with the same coating thickness.Laser remelted plasma sprayed coatings does not change the phase composition of the as-sprayed coating. The surface of as-sprayed coating becomes smooth with the porosity and unmelted particles decreasing after laser remelting. The Vickers microhardness on the cross section and surface of TiB₂-TiC_0.3N_0.7 based composite coatings deposited using different thermal spray powders increases significantly with laser surface treatment. However, laser remelted as-sprayed coatings still have anisotropy and the hardness on the cross section of the laser remelted coatings is higher than that on the surface of the laser remelted coatings. The Weibull distribution of both laser remelted coatings is binary and the disperse of the laser remelted as-sprayed coating deposited using Ti-B₄C-Cr+25wt.% NiCoCrAlY powders is lower with higher uniform structure. There are some microcracks distributed on the surface of laser remelted coatings due to rapid heating and cooling of laser.The wear mechanisms of abrasion wear, adhesive wear and oxidation wear act on the as-sprayed coatings at the same time during the dry wear test in air. One of the wear mechanisms is primary during the course of wear test. However, the wear mechanism changes to fatigue failure when the high sliding speed and applied load work on the as-sprayed coating at the same time. The wear resistance of as-sprayed TiB₂-TiC_0.3N_0.7 composite coating improves significantly after laser remelting. The wear mechanism of laser remelted coating also varies with the changeable of normal loads, from oxidation wear at low normal load to abrasion wear at middle normal load and then changing to adhesive wear at high normal lord. But all of them influence the wear tests of laser remelted coatings. The main wear mechanism for the laser remelted coating is fatigue failure under extremely high sliding speed and normal load operated at the same time.