Yongzhong Jin

In vitro studying corrosion behavior of porous titanium coating in dynamic electrolyte

Porous titanium (PT) is considered as a promising biomaterials for orthopedic implants. Besides biocompatibility and mechanical properties, corrosion resistance in physiological environment is the other important factor affecting the long stability of an implant. In order to investigate the corrosion behavior of porous titanium implants in a dynamic physiological environment, a dynamic circle system was designed in this study. Then a titanium-based implant with PT coating was fabricated by plasma spraying. The corrosion resistance of PT samples in flowing 0.9% NaCl solution was evaluated by electrochemical measurements. Commercial pure solid titanium (ST) disc was used as a control. The studies of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) show that the pores in the PT play a negetive part in corrosion resistance and the flowing electrolyte can increase the corrosive rate of all titanium samples. The results suggest that pore design of titanium implants should pay attention to the effect of dynamic process of a physiological environment on the corrosion behavior of implants.

TOPOGRAPHICAL EVOLUTION OF MAGNETRON SPUTTERING Ti THIN FILMS DURING OXIDATION OBSERVED BY AFM

Titanium films of 120 nm thickness were magnetron sputtered on glass substrates at room temperature, and subsequently they were annealed under flowing oxygen atmosphere at different temperatures and time. Atomic force microscopy (AFM) was used to study topographic characteristics of the films, including nucleation, crystalline feature, grain size, clustering and roughening. The initial nucleation of titanium oxides has almost completed during annealing at 300°C for 120 min or 400°C for 30 min. Especially, we have already observed the preferential nucleation and grain growth of titanium oxides on locations that protrude from the surface, as opposed to deep grooves. It is confirmed by AFM characterization that both of annealing temperature and time can hasten the nucleation and grain growth of titanium oxides, but annealing time is less influential than its temperature. The typical crystal transfer from amorphous-like to crystalline state occurs at 300–400°C for 120 min during annealing, but the too low temperature of 200°C does not contribute to the crystal transfer. In addition, higher annealing temperature (600°C) leads to the transformation of crystal texture from globular-like to flaky type. Generally, higher annealing temperature or time can lead to higher film surface roughness through the grooving effect, but the roughness decreases with the increase of annealing time (at 400°C for 90–120 min).

INFLUENCE OF SUBSTRATE TOPOGRAPHY ON THE NUCLEATION AND GROWTH OF 321 STAINLESS STEEL THIN FILMS PRODUCED BY MAGNETRON SPUTTERING

We have investigated the effects of substrate topography on the nucleation and growth behavior of 321 stainless steel (ss) films, introducing textured surfaces into mica substrates by the abrasion treatment. In this study, two groups of samples were prepared at three different sputtering time within 12 s using radio frequency (r.f.) magnetron sputtering and characterized by atomic force microscopy. Good nucleation uniformity has been obtained on the unabraded mica substrates due to the statistical distribution of nucleation sites. Especially, we have already observed an interesting unusual nucleation phenomenon, the island–rim structure on the abraded mica substrates after 4 s, where the island is fractal-like and the rim around the island was only occupied by few grains for nucleation. These ramified islands were located at the wide grooves introduced as predominate nucleation sites. The island–rim structure formed in initial nucleation process is closely associated with x, y velocity components of surface atoms and the distribution of active sites (related intimately to the surface free energy σ and strain energy e) for nucleation. With the increasing of the sputtering time, voids and overlarge grains have been observed in the island–rim region after the sputtering time of 8 s and 12 s, respectively.

INITIAL GROWTH PROCESS OF MAGNETRON SPUTTERING 321 STAINLESS STEEL FILMS OBSERVED BY AFM

To investigate the initial morphological evolution of 321 stainless steel (SS) films, we examined the effect of sputtering time on the morphology of 321 SS film. In this study, a group of samples were prepared at nine different sputtering times within 20 s using radio-frequency (r.f.) magnetron sputtering and characterized by atomic force microscopy (AFM). Only globular-like grains were formed on mica substrates within 6 s, whose average grain size is ~ 21–44 nm. Meanwhile, few grains with larger size are subject to settle at the defect sites of mica substrates. At 8 s, we found large columnar crystallites with the average grain size of 61 nm. From 10 to 14 s, islands grew continuously and coalesced in order to form an interconnected structure containing irregular channels or grooves, with a depth of ~ 3.5–5 nm. Up to 16 s, a nearly continuous film was formed and some new globular-like grains were again present on the film. Study of the AFM image at 20 s suggests that the watercolor masking method designed by us is an effective method, by which we can prepare thin films with steps for the measurement of the thickness of continuous thin films. It is also found that the coverage rate of films increases with the increase in sputtering time (from 2 to 16 s). On the other hand, the increase in root mean square (RMS) roughness is much more significant from 6 to 10 s, and there is a maximum value, 2.81 nm at 10 s due to more islands during deposition. However, RMS roughness decreases with the decrease in length and width of channels or grooves from 10 to 16 s. Especially, a lower RMS roughness of 0.73 nm occurs at 16 s, because of the continuous film produced with a large coverage rate of 98.43%.

A NOVEL BLACKENING METHOD OF PREPARING ULTRA-BLACK Ni–P COATINGS: EFFECT OF PROCESS PARAMETERS ON MORPHOLOGY AND OPTICAL PROPERTY

Ni–P ultra-black coatings were prepared by a novel blackening method of anodic oxidation in nonoxidizing acid media. The effect of the applied voltage, concentration of H3PO4 and anodization time on the microstructure and optical property of blackened coatings was investigated. The results show that the applied voltage plays the most important role during the formation of the ultra-black coatings. The concentration of H3PO4 is the next-important factor, then anodization time. The optimum process parameter for obtaining ultra-black coatings is at 0.9 V in 3 mol/L for 40 min, in which the reflectance is only 0.14–0.21% in the visible region. The low reflectance is mainly attributable to the unique array structure of conical cavities ranging 1–3 μm in size, in which the innumerable tiny pits distribute on the cavity wall. In contrast, the black coatings etched by chemical etching method by strong oxidizing acid (HNO3) have larger conical cavities (10–20 μm) with smooth cavity wall, and thus lead to higher reflectance of 0.69–2.44%.