| Hydroxyapatite (HA) and titanium alloy are the most widely used materials forthe replacement of artificial bone in medical. But the tensile strength and toughnessof HA are low; it’s easy to failure in dynamic environment. And the titanium alloycan’t induce the growth of bone, it can release metal ions after implantation in thehuman body, may cause harm to human body. In order to integrate the advantages ofthese two kinds of materials, people spray HA on titanium surface, formed the HAcoatings based on titanium, which has good bioactivity.Usually, the HA coating is fabricated by plasma spraying, sintering method,electrochemical method etc. But during the fabricating of these methods, the HA isin the molten state because of the high processing temperature, it can result in thedecomposition of HA, reduce the biochemical of the coating.As a new type of spraying method, the cold spraying has obtained rapiddevelopment in recent years, because of its unique advantages. The processingtemperature is low in the cold spraying, so the HA can combine with the titanium inthe solid state, avoid the phase change of HA. This paper analyses the process of coldspraying and the residual stress of coating in numerical simulation, and detects theproperty of the coating which is fabricated by cold spraying.Simulate the process of the HA particles impact the titanium at high speed incold spraying, reconstruct the spraying process, describe the deformation and stress ofHA particles and titanium during the process of cold spraying. The orthogonal showsthat the most influential factor to the result of impact is the particle velocity, followedby spraying temperature and particle diameter. Illustrate the influence of differentprocess parameters on the impact process by single factor experiment. The larger theparticle velocity is, the larger the contact stress is, and the deformation is moreobvious; The contact pressure increases as the spraying temperature increases, but thecontact stress begins to decline when the spraying temperature exceeds350.Analyse the bonding mechanism of HA coating. The combination of HA particles and titanium depends on the accumulation of particles, it’s mainly themechanical combination; the combination of subsequent particles and coating dependson the impact of particles, it’s mainly the micro-forging mechanism.The residual stress in coating can lead to the coating appear the phenomenon ofcrack, fall off and buckling; it could affect the performance of the coating. Due to thetraditional methods of measuring residual stress are not applicable, so simulate theresidual stress of coating with the finite element method. This paper mainly analysesthe influence of the coating thickness and spraying temperature to the residual stressof coating, the result shows that, with the increase of coating thickness and sprayingtemperature, the radial stress, axial stress and shear stress of coating increaseaccordingly.HA coating is fabricated in different process parameters, and the performance istested. The thickness of coating is measured by coordinate measuring machine, theresult shows that the coating thickness increases with the increase of gas pressureand spraying time; as the spraying distance increases, the coating thickness isgradually increases, but the coating thickness declines sharply when the sprayingdistance exceeds the critical value; the spraying temperature changes have no effecton coating thickness.XRD diffraction experiment and scanning electron microscopy experiments arecarried out to analyse the phase composition and surface topography of the coating.The phase transition reaction does not occur in HA coating; it avoids the phasetransition reaction in thermal spraying and could keep the biochemical activity ofcoating. There are many micro cracks in the coating, when the gas pressure isincreased, the coating will be more compact, and the number of cracks decreasessignificantly; while the increase of spraying temperature will cause the increase ofcracks. |
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