Li H, Khor K A, Cheang P
School of Mechanical and Production Engineering, Advanced Materials Research Center (AMRC), Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.
Biomaterials. 2004 Aug;25(17):3463-71. doi: 10.1016/j.biomaterials.2003.10.051.
Microstructure of thermal sprayed hydroxyapatite (HA) splats was characterized using transmission electron microscopy (TEM), and the formation mechanisms of micropores within the splats were investigated with the aid of simulated body fluids (SBF). High-velocity oxy-fuel and direct current (DC) plasma spray techniques were both utilized for the splats' deposition. The microstructure features of individual HA splats were revealed through TEM observation of as-sprayed, and ion-milled splats. Amorphous calcium phosphate and tricalcium phosphate phases were observed at the splats' fringes, which indicated that extensive decomposition of HA had occurred at these locations. The fringes of the HA splats are essentially nanostructured ( approximately 20-50 nm grains), while calcium phosphate grains up to 5 microm, depending on flattening state, are present at the center of the splats. Morphological observation classified the pores within the HA splats into three main categories according to distinctive features in their microstructure: open pores, sealed pores and through-thickness pores. It was found that particle velocity with which the particle impinged on the substrate surface, particle melt state, and structure of starting particle (mainly porosity) are the key variables in determining the formation and morphology of the micropores within the flattened splats. Influence of subsequent splats on the pores of prior deposited splat was also studied using an in vitro incubation test in SBF. Obvious pore-sealing action on the open pores was revealed, which was achieved through liquid filling of subsequent droplets. It was postulated that the overall porosity of a bulk coating could be attributed primarily to the sealed pores and flaws among the splats, and, it could be adequately governed through appropriate particle melt state and optimized velocity of the particles during coating formation.
利用透射电子显微镜（TEM）对热喷涂羟基磷灰石（HA）薄片的微观结构进行了表征，并借助模拟体液（SBF）研究了薄片内微孔的形成机制。高速氧燃料喷涂和直流（DC）等离子体喷涂技术均用于薄片的沉积。通过对喷涂态和离子研磨后的HA薄片进行TEM观察，揭示了单个HA薄片的微观结构特征。在薄片边缘观察到非晶态磷酸钙和磷酸三钙相，这表明在这些位置HA发生了广泛分解。HA薄片的边缘基本上是纳米结构（晶粒约20 - 50 nm），而根据扁平化状态，薄片中心存在尺寸达5微米的磷酸钙晶粒。形态学观察根据HA薄片内微孔微观结构的独特特征将其分为三大类：开孔、封孔和贯穿厚度的孔。研究发现，颗粒撞击基体表面的速度、颗粒的熔化状态以及起始颗粒的结构（主要是孔隙率）是决定扁平化薄片内微孔形成和形态的关键变量。还使用SBF中的体外孵育试验研究了后续薄片对先前沉积薄片孔隙的影响。结果表明，后续液滴的液体填充对开孔有明显的封孔作用。据推测，整体涂层的总孔隙率主要可归因于封孔和薄片之间的缺陷，并且可以通过适当的颗粒熔化状态和涂层形成过程中颗粒的优化速度来充分控制。
