The Effect of Space Holder Size on the Mechanical Properties of Porous Titanium

Titanium and its alloys are widely known by orthopedic and dentistry communities for their exceptional biocompatibility for their use as bone implants due to their chemical resistance, low density, and suitable mechanical properties [1, 2]. Young’s modulus of titanium and its alloys are between 100 - 110 GPa, while Young’s modulus of bone is around 20 GPa [2]. Stress shielding occurs when the prosthesis has a higher Young's modulus than the bone, resulting in bone degradation and, subsequently, prosthetic replacement [3]. A Porous titanium material retains excellent biocompatibility, low density, surface area, and mechanical properties. In addition, it is also possible to control the mechanical properties of porous titanium by varying the pore size and volume fraction [4]. In this work, open-cell titanium foams were developed using different sizes and concentrations of sodium chloride (NaCl) as a space holder. For this purpose, powders of Ti (99.5%, 44 µm, from Alfa Aesar) and NaCl (99.9%, 400 and 595 µm from Fermont) were mixed during 1 h in a Spex 8000M high-energy ball mill (without milling media) in volume concentrations of 60%, 65% and 70% NaCl. The mixtures were compacted under the uniaxial pressure of 1000 MPa to make cylinders of 6.4 mm in diameter and 13 mm in height. Distilled water at a temperature between 60 and 70 °C with constant agitation was used to remove the NaCl space holder from the green samples. The samples were dried at 100 °C for 4 h and subsequently sintered at 1250 °C for 1.5 h under a high vacuum. The morphology and microstructure of the as-received materials and the sintered samples were studied using an optical Dino-Lite digital microscope and a scanning electron microscope (SEM) Hitachi SU3500 operated at 10 and 15 kV. The density was obtained by using the Archimedes method with distilled water and the compression tests were performed using a Universal Testing Machine Instron 3382 according to the ASTM: E9-09. The powder surface morphologies of the as-received materials (Ti and NaCl) are displayed in Fig. 1a. In both titanium and sodium chloride, the powder particles are irregular and have angular shapes. Fig 2b shows the axial section of the sintered porous titanium sample with a composition of 40 Vol.% NaCl-595 µm. Based on the features shown in Fig. 1b, high porosity was achieved by using NaCl as a space holder. Micro-pores and interconnected macro-pores have been obtained in the samples: the macro-pores are produced by the space holder, and the micro-pores are produced by sintering Ti powder on the pore walls. Moreover, a uniform Ti matrix is formed by the coalescence of titanium powders. Table 1 displays the density and the compressive Young’s modulus of the samples. The density and Young's modulus decrease while the volume percentage of the space holder increases, indicating an increase in porosity. Additionally, it is observed that the size of the space holder also influences both mechanical properties and density, resulting in lower compression stresses. The mechanical properties of titanium foams can be modified by varying the volume concentration and size of the space holder. The compressive Young's modulus in this study was observed to be significantly reduced, suggesting that it is possible to obtain a compressive Young's modulus like that of human bone by correctly controlling the size and quantity of the space holder. Density and mechanical properties of sintered porous titanium samples. Optical and SEM micrographs of (a) as-received powders (SEM) and (b) sintered porous titanium with 60% NaCl, optical image on the left and SEM images on the right.