Study On Fatigue Properties Of Zirconia Toughened Hydroxyapatite Bioceramic Coating

In order to overcome the defects of mechanics properties of bioceramic hydroxyapatite（HA,for short）,composite coatings were prepared by adding the toughened zirconia in HA powder.To improve the biological and the physical properties of the ceramic coating,but not to reduce the biocompatibility of HA biological ceramic coating.But in clinical application,more reliable and intuitive reference was still insufficient when the material workers and patients selected artificial joint coating,so studying the fracture failure properties and estimating the life of coating had a certain significance.The pure HA coating and HA/ZrO₂ composite coating containing 30% stabilized zirconia on titanium alloy by employing plasma spraying technology（named HA0,HA30）were prepared.The relation curves of load – displacement for HA0 and HA30 coating were obtained with three point bending method,and obtained the slope about the straight fitting line.Then use the composite beam theory to calculate the elastic modulus of the both coatings.The fracture toughness of HA0 and HA30 was calculated combined with micro-hardness fracture toughness.Static fatigue,cycle fatigue life for HA0 and HA30 was estimated by using fatigue crack propagation theory.Establish the model for the bending strength,residual bending strength between failure probability by using Weibull analysis.Finally observed the fracture morphology of the two coatings after their failure by using electron microscope,and then analyzed the toughening principles for zirconia.In this research,the main work and achievements were as follows:（1）Considering the shape and size of the samples,the clamp diameter and height of Instron universal testing machine,the fixture and pressure head of the three-point-bending test were designed with Q345 steel which had a good comprehensive performance.（2）The samples HA0 and HA30 were loaded respectively with the loading rate of 0.01 mm/min,0.1 mm/min,and 1 mm/min.The average slope k of fitting line for HA0 and HA30 were 6.4963,6.7248 respectively.The elastic moduli of HA0,HA30 which were calculated according to the theory of composite beam respectively were 30.41 GPa and 38.86 GPa.It proved that the addition of zirconia can improve the elastic modulus of the coating.According to the elastic modulus calculated above and the data obtained by micro-hardness test,the calculation about the fracture toughness of HA0 and HA30 were 0.68MPa·mm1/2,1.139MPa·mm1/2 respectively,the results showed that HA30 had a better fracture toughness than HA0.The three point bending strength of HA0 coating under the three loading rate were 963.61 MPa,1007.41 MPa,1063.30 MPa.And the one of HA30 were 1064.53 MPa,1102.69 MPa,and 1159.39 MPa,higher than the bending strength of HA0.The study showed that three point bending strength increased with the increase of loading stress rate.In contrast,HA30 showed a better fatigue property.Finally the last stress corrosion factors for HA0,HA30 measured were 40.67 and 51.14.（3）The coatings inside had already occurred fatigue behavior,reflecting the bending strength obviously dropped after 104 cycles,the bending strength decreased from 1007.41 to 705.87 MPa,the strength degradation rate was 29.93%.While the bending strength of HA30 was from 1073.39 MPa to 851.19 MPa,declined by 20.7%,explaining that HA30 coating showed a more reliable mechanical performance and fatigue resistance.Weibull analysis function showed Weibull modulus about HA30 appeared a downward trend after a rise first with the increase of cycling times,this may be the reason of toughening phase zirconia in HA30.（4）The fracture of coating started with tiny cracks on the surface,then expanded and converged,eventually formed a main crack throughout the coating surface,leading to the final failure.The crack of HA0 coating was a smooth and straight path,mainly caused transgranular fracture;it appeared slip band similar to the "river" in the fracture.While the crack paths of HA30 were crooked,.The toughening mechanism of zirconia for HA mainly included: 1)phase transformation toughening: the zirconia troostite turned into martensite,it achieved the toughening aim by volume expansion and shearing strain which absorbed energy during the phase transformation,2)particles toughening: zirconia bridged the crack so as to absorb energy,and the thermal stress leaded to the crack "pinning" while expanding into zirconia particles,hindering the crack continued to expand.