Microstructures And Properties Of NbC-Doped And TiH₂-Doped Tungsten-Based Coatings Fabricated By Plasma Spraying

In order to reduce the brittleness of tungsten and improve its comprehensive properties,tungsten-based composite coatings doped with 1.5 wt.%NbC and 1.5wt.%NbC+1.5wt.%TiH₂ were prepared on steel surface by atmospheric plasma spraying（APS）and supersonic atmospheric plasma spraying（SAPS）,respectively.As a comparison,pure tungsten coatings without doping were prepared by the above spraying methods.Microstructure、porosity、nanoindentation hardness（H）、nanoindentation elastic modulus（E*）and thermal diffusivity of tungsten and its composite coatings were studied by OM,SEM+EDS,XRD,TEM,nanoindentation instrument and laser thermal conductivity instrument.The resistance of pure tungsten coating and NbC-doped tungsten-based composite coating prepared by APS technology to high-energy thermal shock was evaluated by high-energy laser beam.The results show that,as a whole,tungsten and its composite coatings prepared by APS and SAPS technology have typical flake characteristics of thermal spraying coatings,both APS and SAPS coatings have certain porosity.For pure tungsten（APS-W）coatings and NbC doped tungsten（APS-W+NbC）coatings prepared by APS technology,APS-W mainly consists of body-centered cubic structure ofα（W）.In addition toα（W）,banded NbC phases distributed in the gap between tungsten layers in APS-W+NbC coatings and tungsten grains are smaller than those in APS-W coatings.The porosity,thermal diffusivity at room temperature and nanoindentation toughness(H³/E^*2)of APS-W+NbC coatings are about 42%lower,14%higher and 7.4%higher than pure tungsten coating.The surface-centered cubic structure ofγ（W）was formed in both the peeling and non-peeling areas of APS-W coatings,while onlyγ（W）was found in the peeling area of APS-W+NbC coating.,and the NbC distributes at tungsten grain boundary in granular form.For pure tungsten coating,single doping of NbC coating and composite doping of NbC and TiH₂ coating prepared by SAPS technology,the main structures of the three coatings areα（W）.The second phase NbC and TiH₂ in the two composite coatings are also distributed in the gap of tungsten lamellae,and the grain size of W in APS-W+NbC coatings is 2.55±0.92 um,which is about 10%lower than that of pure W coatings.The porosity,thermal diffusivity and nanoindentation toughness of APS-W+NbC coatings are 49%lower,26%higher and 7.1%higher than pure tungsten coating.The porosity and thermal diffusivity of APS-W+NbC+TiH2 coatings are 37%lower and 17%higher than those of pure tungsten coatings,respectively,and plasticity is the worst of the three coatings.In summary,SAPS-W+NbC coating has lower porosity,higher thermal diffusivity and better nanoindentation toughness among the tungsten and its composite coatings involved in this research.The comprehensive properties of SAPS-W+NbC coating are the best.It is found that the filling effect of NbC and TiH₂on the gap of tungsten-based coatings and the refinement of tungsten grains by the introduction of second phase are the main reasons for reducing the porosity of tungsten-based composite coatings and improving their nanoindentation toughness and comprehensive properties.