Preparation, Wear Resistance And Corrosion Resistance Of The Nanocrystalline Ni60-TiB₂ Coating Sprayed By High Velocity Oxygen Fuel

As a potential attractive alternative to Cr3C2 and WC, titanium diboride （TiB₂） withexcellent mechanical properties, wear resistance and corrosion resistance properties, whichmakes it become a promising reinforced material for metal matrix composite coatings. Inrecent work, TiBB₂-based coatings such as Fe（Cr）-TiB₂ have been deposited and exhibitedbetter abrasive resistance than that of TiC-the conventional C-NiCr and Cr3C2-NiCrHowever,coatings.the high brittleness and low fatigue resistance for TiB₂ have become the mainrestrictions for the conventional TiB₂BB -based coatings. Nanotechnology especiallynanostructured coating can solve this problem perfectly by improving the performanceproperties such as hardness, toughness, membrane-based adhesion, fatigue resistance andwear-corrosion resistance, which is the frontier and research hotspot in thermal spraying.In this thesis, nanostructured Ni60-TiBB₂ composite coating was prepared by High VelocityOxygen Fuel （HVOF） spraying using ball milled nanocrystalline powders. The microstructureand performances of the milled powders and HVOF coatings were also studied.Nanostructured Ni60-TiB₂ B composite coating exhibited better mechanical properties andwear-corrosion resistance than that of its conventional counterpart, then the comprehensiveproperties of the nanostructured coating was greatly improved as well. This work is expectedto play an important role in the preparation and application of high performancenanostructured coating.The results show that nanocrystalline Ni60-TiBB₂ feedstock with qualified fluidity and particlesize distribution for thermal spraying is successfully synthesized using mechanical alloying.After 20 h mechanical alloying, nanocrystalline TiB₂B particles are evenly dispersed in the ballmilled powders and their average particle crystal size approachs a constant value of 38 nmwith equilibrium. Most of the milled powders are above 5μm and are in approximatelyspherical shape with excellent fluidity, which are suitable for preparing nanostructuredcomposite coating by HVOF spraying with sized.Nanostructured Ni60-TiBB₂ composite coating with excellent properties is successfullyfabricated via HVOF technique using high energy ball milled powders. The nanostructuredcomposite coating exhibits a compact and uniform structure with the average grain size of about 45.7 nm. The micro-hardness and fracture toughness of the nanostructured coating are1102 kgf. mm and 3.5 MPa.m respectively, which are much higher than those of theconventional Ni60-TiB₂-2 1/2B composite coating.Both conventional and nanostructured Ni60-TiBB₂ composite coating exhibt excellentcycle oxidation resistance at 600 and 800 . Their cycle oxidation kinetic behaviours allapproximately follow the parabolic law. This indicates that the oxidation processes arecontrolled by a diffusion mechanism. The nanostructured coating has better cycle oxidationresistance than that of the conventional coating under the same oxidation condition. Thereason for this improvement can be attributed to the information of the intact SiO℃℃2-Cr2O3protective layer, and the enhanced adhesion between oxide film and nanostructure coating.The sliding wear properties of HVOF-sprayed conventional and nanostructuredNi60-TiBB₂ composite coating were studied comparatively at room temperature under ambientpressure and with applied load varied from 20 to 60 N.nanostructured composite coating possesses lower sliding wear coefficient and smallervolume loss at all applied load than that of the conventional composite coating, whichindicate that the nanostructured coating has better sliding wear resistance. The reason for thisimprovement can be attributed to the microstructual homogenization and the well preservednanostructure characteristic of the ball milled powders. Adhesive and abrasive wears arefound to be responsible for the wear down mechanisms of the nanostructured Ni60-TiB₂The results show that theBcomposite coating.The results of hot corrosion behaviours indicate that the hot corrosion resistances ofcomposite coatings coated Na2SO4-60% V2O5 are much better than that of composite coatingscoated with Na2SO4-30% K2SO4. The reason is that the SiO2 layer formed during hotcorrosion tests can dissolve in Na2SO4-30% K2SO4 with alkaline environment to form aporous and non-protective Ni-Ti-Cr-Si-O oxide film, but it has excellent chemical stability inNa2SO4-60% V2O5 with acidic environment to form an intact Si-Cr-O protective film finally.