Microstructure And Wear Behavior Of Laser Induced In-situ Formation Of Titanium Composite Coatings

To increase the wear resistance of the titanium alloy surface, according to the tendency of the development of composite materials, B4C powder and different B4C+TC4powder mixture were used as the laser cladding materials on TC4titanium alloy substrates. Wear resistant composite coatings were prepared by laser induced in-situ formation. Influence of the powder composite and the processing parameters on the microstructure and properties of the coating were studied.In this experiment, sigle-path laser alloying coatings were prepared by adding B4C powder prepasted or synchronized fed on the TC4substrate. Single, multi-path and milti-layer coatings were prepared by laser cladding of prepasted30%B4C+70%TC4(wt.%) powder. Composite graded coatings were prepared layer by layer though adding10%B4C+90%TC4,20%B4C+80%TC4and30%B4C+70%TC4(wt.%) each layer separatel. OM, SEM,TEM, and XRD were used to investgate the microstructure and composite of the deposited layers The microhardness of all the specimens and the wear resistant properties of multi-path coatings produced by laser cladding layers were measured.Experimental results show that two and more kinds of ceramic reinforcements were in-situ formatted in the matrix of TC4, such as TiC, TiB and (or) TiB2. TiC ceramic was formed evenly with the morphology of dendrites and disperse particles. TiB and TiB2were rod-shaped. TiB2was thick while TiB was more like tiny needles. The size of rod-shaped TiB2is thicker for synchronized B4C powder feed coating layers than that of the prepasted ones which lead to the hardness of the layer is up to1800HV, which is over five times that of the TC4substrate (320HV).The microstructure of muti-layer and composite graded coatings becomes finer with the increase of the layer thickness.More TiB and TiB2were found in the composite graded coatings. The microhardness of the top layer of the composite graded coating (1200HV) is higher than that of non-gradient coating (900HV). The gradient coating is smooth with no crack or pore on the surface or inside. From the bottom to the top of the graded coating the microstructure exhibited as β oriented growth of columnar crystals, disperse spherical particles and tiny rod-shaped reinforcements, dendrites and thick rod-shaped reinforcements.The laser cladding coatings have the low friction coefficient (0.18-0.3), which is only1/2that of the TC4(0.4-0.5). Along with attrition skidding distance’s increase, the speed of wear loss increases, and ceramic coating’s wear-resisting performance are assumes the graded distribution, the surface layer wear-resisting performance is best. The wear resistance property of the graded coating is more excellent than that of the non-graded coating. The sliding wear properties of the titanium composite coatings were greatly improved compared with that of the substrate because of the in-situ formation of the TiB and TiC hard and wear-resistant reinforced phases. The wear mechanism of the layer was the mixture of adhesive wear and oxidation wear. TiBx and TiC ceramic which were formed in-situ by laser scanning evenly reinforced in the matrix of TC4and increased the surface wear-resistance of the TC4substrates.