Microstructure And Performances Of Ultrafine Ceramic Compound Coating Prepared By Laser Cladding

As a high-performance coating preparation method, the hybrid laser surface strengthening technology has become a research focus, because it shows great application prospects in manufacturing key components for locomotive, magnitude equipment, aeroplane, giant tools and molds. In this paper, the high-performance ceramic composite coatings were prepared by combining laser cladding technology, chemical deposition technology, sol-gel chemical synthesis and nano-ceramics together. The hybrid laser surface strengthening technique is expected to obtain the complementary advantages of each technology, and to avoid the shortcomings of the traditional surface strengthening technology and laser cladding technology, which is considered as the inheritance and development of the laser surface strengthening technology.The research in this paper was carried on around the following aspects. At first, the three kinds of powder preparation methods:presetting, electroless plating and sol-gel method, were adopted to assemble micro-and nano-materials effectively on the surface of the sample, and the pulsed Nd:YAG laser was used to produce hardening coatings on the metallic substrate. Secondly, with the help of the process parameters optimization model, the ultrafine ceramic reinforced composite coatings with high performances were prepared. Thirdly, the relations between the microstructure, microhardness distribution and the wear properties were investigated, the function of ceramic phases and the strengthening mechanism of the coatings were analyzed, and the characteristics and applications of different preparation methods were discussed as well. The main results are as follows:(1) The Al2O3ceramic composite coatings were achieved by laser cladding preseted Ni-coated nano-Al2O3. The optimal processing parameters for crack-free coating with high hardness are under the condition of50%Ni+50%Al2O3, current intensity of150A, scanning speed of300mm/min, pulse width of2ms and pulse frequency of30Hz. With the optimal processing parameter, the coating with maximum hardness of672Hv0.2and hardening layer thickness of0.6mm can be obtained, and its wear resistance increases by1.5times with respect to that of the substrate. This method is convenient and flexible, which is suitable for local strengthening of the complex parts, such as the cavity and inner wall of mold, and tools.(2) The Ni-P-Al2O3electroless plating coatings were strengthened by the pulsed Nd:YAG laser. A process parameters prediction model based on neural network was established, and the optimal strengthening parameters for single track are under the conditions including current intensity of200to240A, pulse width of2.1to2.3ms, pulse frequency of21to23Hz, scanning speed of450mm/min. With the optimal processing parameters, the coating with maximum hardness of840Hv0.2and hardening layer thickness of0.2-0.3mm can be obtained, and its wear resistance increases by6times relative to that of the substrate. This method is featured with nice homogeneity and is suitable for local strengthening of the key components with high propertied requirement, such as the cutting edge of cutters, turbine blades and shaft neck.(3) The titanium compound (TiC/TiN/TiB2) strengthening coatings were achieved by chemical synthesis reaction via laser action and sol-gel. The optimal processing parameters are under the condition of TiO2and C molar ratio of1:5, the preseted layer thickness of0.2mm, the current intensity of250A, the pulse width of2.5ms, the pulse frequency of18Hz, and the laser scanning speed of50mm/min. With the optimal processing parameters, the coating with maximum hardness of1150Hv0.2and hardening layer thickness of0.3~0.4mm can be obtained and its wear resistance increases by12.5times from that of the substrate. This method is convenient and flexible, and can produce coatings with high hardness and wear resistance, which is suitable for the hardening of tools blade.(4) The research results on the function of ceramic phases and the hardening mechanism show that, due to the presence of nano-Al2O3heterogeneous phase particles, the melt supercooling degree increases, and thus changes the crystal growth morphology as a consequence. The columnar grains change to cellular dendrite in the upper layer of the coating, which greatly promotes the formation of the fine grain structures. The grain refinement and the generation of highly dispersed hard phases such as Al2O3, Al5FeNi, FeNi and Fe0.64Ni0.36are the main factor causing the coating strengthening. As to the sol-gel compound laser hardening method, it is the synthesis reactions of laser and TiO2·2H2O+C+BN sol-gel to produce the hardening coating with TiC, TiB2and TiN ceramic reinforced phases. In conclusion, different enhanced coatings with different functional characteristics were obtained thought the three different composite methods mentioned above. The three hybrid methods are suitable for the local hardening of complex parts, tools blade and local strengthening of the critical parts, respectively, which provides a new way to obtain high-performance strengthening coating. The preliminary application tests in the medical devices and mechanized equipment show that the technology has broad application prospects and great potential for the application in medical minimally invasive surgical instruments and the fields of tool and die.