Effect Of Y₂O₃on Microstructure And Properties Of Metal Matrix Composite Coatings Reinforced By In-situ Synthesized Particles By Laser Cladding

In this paper, high hardness and high chemical stability of ceramic materials,high ductility and high strength of Titanium alloy is combined by laser cladding Ni-basedcomposite coatings reinforced by in-situ synthesized TiB2-TiC and Ti-based compositecoatings reinforced by in-situ synthesized TiB-TiC in Ti6Al4V surface to increase itsfriction and wear properties. The ceramic reinforcements in situ synthesized have bettercompatibility to matrix, and the interface is cleaner and more solid, which would ensurethe substrate has enough strength to transfer stress. Rare earth in the laser cladding layercan play roles in micro-alloying, purifying grain boundaries, grain refining, improving thegrain boundary state, inhibiting columnar grain growth and so on. This article introducedthe rare earth oxide Y₂O₃to the process of laser cladding, which would effectively improvethe microstructure and properties of the coatings. This research work embodied in thefollowing main contents:1. Selected four reaction systems: Ti-B₄C-Al, Ti-B₄C-Al+（1-3wt%）Y₂O₃,Ni-Ti-B-Cr, Ni-Ti-B-Cr+（nano/sub-micron）Y₂O₃. Which reactions can occur and whichphases can synthesized in the coatings were predicted by calculation of thermodynamicdata for these cladding systems, and then laser cladding experiments were done under thetest conditions of optimized process parameters（power, spot diameter and scanning speed）;2. By X-ray diffraction （XRD）, energy dispersive spectroscopy （EDS）, scanningelectron microscopy （SEM）, transmission electron microscopy （TEM）, etc. The phasecomposition of the coatings, microstructure of matrix, morphology and distribution ofreinforcements, also the changes of composition, distribution and dislocation density,interface type （coherent, semicoherent and incoherent）, mismatch and orientation relationship of the reinforcements in different regions of the coating and the interfaceregion. Combined with thermodynamics and kinetics, the characteristics and solidificationlaw of the melt, this paper deeply analyzed the growth mechanism of in-situ reinforcements,the process parameters of impacting reinforcements’ mircostructure, type, content, form ofadded rare-earth and the ratio of other components;3. The tribological behaviors of coating and substrate were tested with hightemperature friction and wear tester at room temperature. The hardness distribution of thespecimen cross-section （including dilution zone, heat affected zone and substrate） weremeasured by the Vickers hardness tester and the micro-hardness profile along the depthdirection of the coating was analysized. The fracture toughness in different regions of thecoating was tested by means of indentation method. Finally, based on the above analysis,mechanical properties of composite coatings were compared with that of the matrix, thestrengthening and toughening mechanisms of reinforcements were initially revealed;4. The paper emphasizes on analyzing the effect of added Y₂O₃with differentparticle size and percentage composition on microstructure and mechanical property of thecoatings, which reveals the in-situ growth and strengthening and toughening mechanismsof reinforcements under the effect of added rare earth. Research reveals on condition thatthe hardness of coating is not reduced by added appropriate rare earth, to enhance thefracture toughness, reduce cracking sensitivity, and get proper match of hardness andfracture toughness,can be realized by refining matrix grain, reducing the solubility ofsolute elements in the matrix,etc, which make titanium alloy have excellent comprehensivewear resistance.