Situ Authigenic Particles Reinforced Ni Matrix Laser Cladding Coating

Laser cladding of ceramics particulate reinforced metal matrix composites (MMCs) on various traditional substrates is an advanced superficial modification technology, which can change the material surface performances, such as resistance to wear, erosion resistance, anti-oxidation and so on. In this technology, the reinforcements are usually formed by directly adding ceramics particulates into the coating materials. However, it is well known that there is great difference in the thermo-mechanical properties between the additional reinforcement ceramics particles and the matrix. The bad compatibility results in lower bond strength on the reinforcements/matrix interface that become the latent source of the microcrack and other flaws and eventually affects the coating's strength. However, in the in-situ synthesis of composite coatings by laser cladding, one or more kinds of ceramics reinforcements with high strength and high elasticitic coefficient are formed by reacting between added elements or compounds under the laser illumination, which consequently strengthens bonding to the substrate. Because this kind of enhancement body is the thermodynamics steady phase that nucleates and grows from the matrix. The surface is pollution-free. The reinforcements are more compatible with the matrix and the interface bond strength is high. Therefore, in this thesis several kinds of in-situ synthesized particulates reinforced Ni-based composite coatings were produced by laser cladding and their microstructure and resistance to wear and so on were systematically investigated.The main results and conclusions are as the following:1. Investigation on in-situ synthesis of TaC particulate reinforced Ni-based composite coatings by laser cladding:(1) Good finish and in situ synthesized TaC particulate reinforced Ni-based composite coatings can be successfully achieved on A3 steel by laser cladding by employing a proper amount of Ni60+( Ta₂O₅+C)-doping.. The optimum conditions are 20wt.% content of (Ta₂O₅+C)-doping, 1.6 kW laser power, 2 mm/s scanning velocity and 47mm defocusing length. (2) The microstructure of the coating is mainly composed of in-situ synthesized TaC white particulate phases and the dendrite phases consisting of mainly chromium carbide dispersed in the gray dendriteγ(Ni) solid solution and intercrystalline eutectic biphase matrix.(3) The composite coating gives very high average hardness of Hv_0.31100, which is 37.5 percent higher than the average hardness of Hv_0.3800 of pure Ni60 coatings, and excellent wear resistance which is five times as high as that of pure Ni60 coatings. In-situ Synthesis and uniform distribution of TaC particulates are the key factors of the coatings excellence hardness and wear resistance.2. Investigation of WC particulate reinforced Ni-based composite coatings by laser cladding:(1) An excellent Ni-based composite coating by laser cladding can be successfully achieved by laser cladding on A3 steel by employing a proper amount of Ni60+(WO₃+C)-doping. The optimum conditions are 15wt.% content of (wO₃+C)-doping, 1.6kW laser power, 2mm/s scanning velocity and 50mm defocusing amount.(2) The microstructure of the coating is composed of mainly WC particulate phases and dendrite phases dispersed in the gray dendriteγ(Ni) matrix. At first, WO₃ reacts with C to form W₂C, and then W₂C reacts with C to form WC. Since the laser cladding process is a rapid heating and cooling process, the content of WO3 is not easily controlled to form WC so that the content of WC is very small in the laser cladding.(3) Because of the combined action of grain strengthening, solidsolution strengthening and martensitic phase transformation strengthening, the composite coating under 15% content of (WO₃+C)-doping gives a very high average hardness of HV_0.31100 and excellent wear resistance which is 1.7 times as high as that of pure Ni60 coatings.3. Investigation on in-situ synthesis of boride particulate reinforced Ni-based composite coatings by laser cladding:(1) An excellent coating can be achieved by cladding proper content of Ni60+(MoO₃+B₂O₃)-doping into Ni60 under 1.6kW laser power, 2mm/s scanning velocity, 15% content of (MoO₃+B₂O₃)-doping and 50mm defocusing length.(2) The microstructure of the coating is composed of mainly reticulation eutectic of borides and carbides dispersed inγ(Ni) matrix.(3) The wear resistance is considerably improved at little expense of a decrease in hardness. It is due to the disappearance of larger brittle phases and increase of tough phases with proper content of (MoO₃+B₂O₃ )-doping.