Study On Microstructure And Properties Of Laser Cladding Wollastonite Derived Bioceramic Coating On Titanium Alloy

The demand for biomedical materials grows daily with the increasing aging population and trauma, the research and development of biomedical materials has become the focus of medical research. Biomedical titanium alloys have excellent mechanical properties and biocompatibility, which has been widely used in the field of medical implant materials. However, titanium alloy cannot bone-bond with bone tissue and was thought to be bioinert. Hydroxyapatite has high biological activity, and it is widely used in clinic. However, it cannot be used in load bearing applications due to its high brittleness and low intensity. Hydroxyapatite/metal composite coating combines good mechanical properties of metal materials and nice biological properties of bioceramics. However, there is difference in thermal expansion coefficient between the coating and substrate, which is easy to cause the peeling between the coating and substrate. Wollastonite has good bioactivity, and its thermal expansion coefficient is close to that of titanium alloy, which is a kind of ideal material for preparation biocoating on titanium alloy.The interface bonding strength between the coating and substrate prepared by laser cladding is high, and the coating thickness can be controlled. Laser cladding has found wide application in the preparation of bio-ceramic coating. To solve the lack of strength of bulk biomaterials for load-bearing applications and improve the bioactivity of titanium alloy (Ti-6A1-4V), the CaO-SiO2 coatings on titanium alloy were fabricated by laser cladding technique. In this paper, firstly, wollastonite ceramics powders were prepared by sol-gel method. Then wollastonite derived bioceramic coating was successfully fabricated on the surface of titanium alloy by laser cladding. The effect of laser process parameters, Na20, MgO, ZnO, ZrO2, CeO2 and Y2O3 on microstructure and properties of laser cladding coating was analyzed. In addition, the formation of CaTiO3 in the ceramic layer was analyzed through thermodynamics. The mechanism of the bioactivity of cladding layer (the formation of apatite on the cladding layer) in simulated body fluid (SBF) was discussed. The biocompatibility of cladding layer was evaluated by the hemolysis test, in vitro cytotoxicity test and systemic toxicity test.The main phase of wollastonite prepared by sol-gel method is β-CaSiO3.10mol% Na20, MgO, ZnO and ZrO2 do not change the main phase of wollastonite. The main phase of wollastonite contained 15 mol% MgO is Ca3Mg(SiO4)2 and CaMgSi2O6. When ZrO2 was added, t-ZrO2 (Tetragonal-ZrO2) and a-CaSiO3 (pseudowollastonite) appear. a-CaSiO3 is the high temperature phase of CaSiO3. ZrO2 reduces the crystallization temperature of pseudowollastonite. When the increase of ZrO2 addition amounts, X ray diffraction peak intensity of t-ZrO2 increases gradually.Bioactivity and biodegradability of wollastonite are associated with the speed of ion exchange rate. After soaking in SBF, the apatite layer is formed, indicating that the ceramic is bioactive. The addition of 10 mol% Na2O destroys the integrity of wollastonite structure, and accelerates the ion exchange rate between wollastonite and simulated body fluid (SBF) or Tris-HCl buffer solution, which improves the apatite deposition rate of wollastonite in simulated body fluid (SBF), and increases the degradation rate of wollastonite in Tris-HCl solution. While the addition of 10 mol% MgO, ZnO and ZrO2 make the structure of wollastonite more compact, and decrease the ion exchange rate between wollastonite and the solution, which reduces the apatite deposition rate and the degradation rate of wollastonite.Wollastonite derived bioceramic coating on titanium Alloy (Ti-6A1-4V) was successfully prepared by laser cladding. A transition layer was between the ceramic layer and the substrate fron the cross-section microstructure of the cladding sample. There are no obvious defects in the bonding interface between ceramic layer and transition layer, transition layer and substrate, when the laser output power is 500W, the scanning speed is 2.5mm·s-1 or 5.0mm·s-1, or the laser output power is 600W, the scanning speed is 7.5mm·s-1(Beam diameter is 3mm). When the laser output power is 500 W, the scanning speed is 2.5mm·s-1, the proportion of the ceramic layer is very small. The cross-section microstructure of the cladding sample is mainly divided into ceramic layer, transition layer and substrate. The micro-hardness from the ceramic layer to substrate is increased firstly then reduced, and the maximum hardness appears in the transition layer. The average hardness of the ceramic layer prepared on conditions of different process parameters is 550HV0.2-800HV0.2-The average hardness is significantly higher than that of substrate, and the wear resistance of the cladding layer is better than that of substrate. The cladding layers prepared on the conditions (The output power 500 W and scanning speed 2.5 mm/s, the output power 500 W and scanning speed 5 mm/s, the output power 600 W and scanning speed 5 mm/s) exhibite better performance. Based on the microstructure observation, microhardness and wear resistance analysis in different process parameters, the optimum process parameters are as follows:the output power 500W, scanning speed 5 mm/s and light spot diameter 3 mm.The cladding coating with various surface morphologies (mesh, coral, petals, honeycomb) has been prepared, and the coating surface is coarse, accidented and microporous. The solidification structure is related to the temperature gradient and solidification rate in the molten pool. The cross-section microstructure of ceramic layer from bottom to top gradually changes from cellular crystal, fine cellular-dendrite structure to underdeveloped dendrite crystal, and part equiaxed crystal. The morphology of cladding layer from the bottom region to the upper region from compact becomes loose. The mutual diffusion of elements occurs between the coating and substrate. The ceramic layer is mainly composed of CaTiO3, CaO, a-Ca2(SiO4), SiO2 and TiO2. Thermodynamic calculations show that, the change of gibbs free energy of the reaction of CaTiO3 is lower than that of CaSiO3, when temperature is above 1200℃, and CaTiO3 is easily formed compared with CaSiO3, which is consistent with the experimentation results.0.5 wt.% CeO2 and Y2O3 do not change the main phase of the ceramic layer, and Y2O3 inhibit the formation of CaO. CaO is toxic to cells, and the addition of Y2O3 could stimulate proliferation and differentiation activity of cells. CeO2 and Y2O3 refine the microstructure of the ceramic layer in the middle and upper regions, especially Y2O3. Ce and Y are surface active elements, which can reduce the critical nucleation radius, increase the number of crystal nucleus, and then refine microstructure in the process of spontaneous nucleation. The addition of CeO2 and Y2O3 increases the average hardness of the ceramic layer and improves the wear resistance of the cladding layer, and they also reduce the weight loss of the cladding layer in Tris-HCl buffer solution, and improve the corrosion resistance.The main phase of the ceramic layer contained with Na20, MgO, ZnO, ZrO2 are all CaTiO3. When MgO, ZnO or ZrO2 was added, some new phases appear. The ceramic layer contained ZrO2 contains o-ZrO2 (Orthorhombic-ZrO2), CaZrO3 and β-Ca2(SiO4). o-ZrO2 is a high pressure phase of ZrO2, and its appearance attributes to the phase transformation between t-ZrO2 to o-ZrO2 caused by the thermal mismatch between t-ZrO2 and the surrounding phase.10mol% Na20 has no obvious impacts on the microstructure, MgO and ZnO refine the microstructure of the ceramic layer, and when ZrO2 was added, the microstructure becomes coarse.Na2O has no obvious impacts on the average hardness of the ceramic layer, and it improves the wear resistance of the cladding layer. MgO reduce the average hardness of the ceramic laye. With the increase of the addition amounts of MgO, the average hardness becomes smaller. ZnO reduces the average hardness of the ceramic laye, and decreases the wear resistance of the cladding layer. The average hardness increases with the increase of the addition amounts of ZrO2, and the wear resistance becomes better and better, due to the phase transformation between t-ZrO2 to o-ZrO2.After soaking 21 days in simulated body fluid (SBF), the apatite layer is formed on the coating surface, indicating the coating has the ability to form apatite. When the cladding layer is soaked in simulated body fluid (SBF), SBF enters the cladding layer mainly through surface defects. The ion exchange between the cladding and SBF happens, and itleads to SiO2-rich layer formation on the surface of the cladding layer, which can provide favorable conditions for the nucleation of apatite. Once apatite nuclei formed on the cladding layer surface, calcium ions in SBF are first migrated onto the SiO2-rich layer and then the ionic activity products (Ca2+, HPO42-, PO43-, OH- and CO32-) of apatite are migrated onto the SiO2-rich layer to grow into apatite.The hemolysis ratio of cladding layer is 0.32%, which is smaller than the judgement standard (5%).There are no toxic symptoms and death during the whole observation in the mouse acute systemic toxicity test, indicating that the cladding layer have no toxicity. The relative proliferation rate of L-929 fibroblast cells in experimental group is 97.20%, and the cells have good activity of adherence and proliferation. The hemolysis test, in vitro cytotoxicity test and systemic toxicity test demonstrate that the cladding coating not cause hemolysis reaction, and have no toxicity to cell and living animal, which shows the cladding coating has good biocompatibility.