Naoyuki Nomura

Mechanical Properties of Forged Low Ni and C-Containing Co-Cr-Mo Biomedical Implant Alloy

In order to examine the influence of variation in grain size and microstructure on mechanical properties of a low Ni and C-containing Co-29Cr-6Mo alloy for biomedical implant materials, the tensile properties and the dry friction wear characteristics of the wrought Co-29Cr-6Mo alloy without addition of Ni and C were investigated. The microstructure of as-forged alloys, consisting of fcc phase mixed with athermal hcp uf020martensite, is found to become finer as the reduction during forging increases. The hot-forging processes for fabricating the fine microstructure with different grain sizes of 43, 11 and 3 μm has been successfully established under the condition that forging temperature of higher than 1273 K is strictly kept during forging. The tensile properties, such as the yield stress, the tensile stress and the elongation, are improved with decreasing the grain size and thereby increasing the volume fraction of the fcc phase in the microstructure. The dry wear resistance against the alumina ball is enhanced by decreasing the grain size.

Microstructure and Mechanical Properties of Hot-Pressed Co-Cr-Mo Alloy Compacts

The microstructure and mechanical properties of compacts produced from hot-pressing the Co-29Cr-6Mo alloy powder, fabricated by gas atomization, were investigated in this study. The average diameter of the alloy powder is 41 μm. Electron probe micro analysis revealed that the concentration scattering of chromium and molybdenum is relatively small between the powders. The compact hot-pressed at 1273 K in Ar with an applied pressure of 80 MPa possesses the relative density of 97% and the fine grain (2.7 μm). The ultimate tensile strength of the compact is 1240 MPa, which is comparable to that of the forged Co-29Cr-6Mo alloy.

Osteoconductivity of Porous Titanium Having Young’s Modulus Similar to Bone and Surface Modification by OCP

The present study was designed to investigate whether porous titanium (Ti) having Young’s modulus similar to bone has osteoconductive characteristics in rat critical-sized calvarial bone defect. The effect of coating by octacalcium phosphate (OCP) was also examined. OCP is known as a precursor of initial mineral crystals of biological apatite in bones and teeth. Ti powder was prepared by plasma rotating electrode process in an Ar atmosphere. Then, porous Ti disks, 8 mm in diameter with 1 mm thick, were obtained using the particles ranging from 300 to 500 +m, by sintering at 1573 K without applied pressure. The disks had about 35 vol% in porosity and about 10 GPa in Young’s modulus which corresponds to that of human cortical bone. Newly formed bone was observed so as to fill the pore up at 12 weeks, confirming the ability to conduct the ingrowths of the bone tissue. Although in vitro study showed that proliferation of mouse bone marrow stromal ST-2 cells was inhibited on the dishes coated by OCP rather than the control dish, OCP coating on porous Ti seemed to stimulate the bone formation in vivo. Taken together, it seems likely that porous Ti having Young’s modulus similar to bone shows osteoconductive characteristics to conduct bone ingrowths. OCP could be a potential coating agent to assist bone regeneration on porous Ti.

Mechanical Properties of Porous Titanium Compacts Reinforced by UHMWPE

Porous Ti compacts reinforced by ultra-high molecular weight polyethylene (UHMWPE) were fabricated and their mechanical properties were evaluated. Ti powder atomized by plasma rotating electrode process (PREP) was sintered at temperatures ranging from 1473 K to 1673 K for 7.2 ks in a vacuum. The porous Ti compacts contain the porosity of about 40%, irrespective of the sintering temperature. Porous Ti/UHMWPE composites were successfully fabricated by compressing UHMWPE powder into the porous Ti compacts. The compacts exhibit open pore structure and enables the penetration of UHMWPE into pores in the compacts. Young’s modulus of the composites is higher than that of the porous Ti compacts. The increment in Young’s modulus is not simply explained by the rule of mixture because Young’s modulus of the UHMWPE is approximately 1.3 GPa. Three-point bending strength of the composites is improved, presumably due to the local stress relief by UHMWPE in the vicinity of neck in the composites.

Effect of Nitrogen on Mechanical Properties of Porous Titanium Compacts Prepared by Powder Sintering

Nitrogen-doped and -undoped Ti powders were obtained by Plasma Rotating Electrode Process (PREP) in an Ar atmosphere using Ar-2 vol.%N2 and pure Ar, respectively, for plasma flame. Auger electron spectroscopy (AES) revealed that nitrogen is enriched at the surface of nitrogen-doped Ti powder and decreases along with the depth direction. Microstructure of a compact prepared by hot pressing the nitrogen-doped Ti powder shows a shell structure. The outer area is considered to form by the diffusion of nitrogen. This microstructure disappears after annealing at 1373 K for 3.6 ks. It is found that the compressive yield strength of porous Ti compacts can be improved by introducing nitrogen in Ti powder and is superior to that of human cortical bone.

Effect of Nitrogen on Microstructure of Porous CoCrMo Alloy Compacts for Biomedical Applications

Porous Co-29Cr-6Mo alloy compacts were fabricated and their microstructures were examined. The Co-29Cr-6Mo alloy powders were atomized by using Plasma Rotating Electrode Process (PREP) in an Ar atmosphere. These powders were hot-pressed at 1223 K in Ar or nitrogen and annealed at 1473 K in vacuum or nitrogen. X-ray analysis revealed that the γ phase (f.c.c.) mostly exists in the compact hot-pressed in nitrogen although the ε phase (h.c.p.) is dominant in the compact hot-pressed in Ar. In addition, precipitates are observed at the interdendritic region in the compact hot-pressed in nitrogen. These results suggest that the Co-29Cr-6Mo alloy powders react with nitrogen to stabilize the γ phase. Accordingly, the constituent phase in the Co-29Cr-6Mo alloy is strongly affected by the atmosphere during hot-pressing.

Effect of Sigma Phase in Co-29Cr-6Mo Alloy on Corrosion and Mechanical Properties

Effect of the sigma (σ) phase in Co-29Cr-6Mo alloy on corrosion and mechanical behavior has been investigated. The area fraction of the σ phase varies depending on the aging time at 1023 K. The area fraction of the sigma phase increases with increasing aging time and reaches 0.6 % after aging at 1023 K for 21.6 ks. Inductively coupled plasma atomic emission spectrometer (ICP-AES) analysis revealed the quantity of released Co ion shows almost the same values regardless of different area fraction of the σ phase. In addition, plastic elongation and 0.2% proof strength exhibit almost the same values, although the alloys have different area fraction of the σ phase. These results suggest that a small amount of the σ phase (<0.6%) hardly affect the corrosion and mechanical properties in the Co-29Cr-6Mo alloy.