I. Gotman

Corrosion behavior of titanium nitride coated Ni-Ti shape memory surgical alloy.

Nickel-titanium (NiTi, nitinol) shape memory alloy was nitrided using an original powder immersion reaction assisted coating (PIRAC) method in order to modify its surface properties. PIRAC nitriding method is based on annealing the samples in the atmosphere of highly reactive nitrogen supplied by decomposition of unstable nitride powders or, alternatively, by selective diffusion of the atmospheric nitrogen to the sample surface. Being a non-line-of-sight process, PIRAC nitriding allows uniform treatment of complex shape surgical implants. Hard two-layer titanium nitride (TiN)/Ti2, Ni coatings were obtained on NiTi surface after PIRAC anneals at 900 and 1000 degrees C. PIRAC coating procedure was found to considerably improve the corrosion behavior of NiTi alloy in Ringers solution. In contrast to untreated nitinol, no pitting was observed in the samples PIRAC nitrided at 1000 degrees C, 1 h up to 1.1 V. The coated samples were also characterized by very low anodic currents in the passive region and by an exceedingly low metal ion release rate. The research results suggest that PIRAC nitriding procedure could improve the in vivo performance of NiTi alloys implanted into the human body.

Fabrication of Al matrix in situ composites via self-propagating synthesis

Abstract Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

TiN coating improves the corrosion behavior of superelastic NiTi surgical alloy

The key to the biocompatibility of NiTi surgical devices resides in the improvement of the materials corrosion resistance. To protect the surface of NiTi from corrosion, an original PIRAC nitriding method was proposed. In the present work, the corrosion and electrochemical properties of NiTi samples annealed under a low pressure of highly reactive nitrogen at 900 and 1000°C were studied. The microstructure of PIRAC coatings was characterized employing XRD and SEM/EPMA and was found to consist of a hard TiN outer layer followed by a Ti2Ni layer. Electrochemical tests were performed in Ringers solution, both upon immersion in deaerated solution and after long term exposure (400 h) under aeration at open circuit potential (OCP). PIRAC nitriding treatment was found to significantly improve the corrosion resistance of NiTi alloy. Samples nitrided at 900°C, 6 h and at 1000°C, 0.5–5 h were passive in a wide range of potentials with no signs of activation up to 1.1 V. A very low anodic current in the passive region and an exceedingly low metal ion release rate were measured on PIRAC coated samples. The relationship between the nitriding parameters and protective properties of PIRAC coatings has been established and the role of the outer and inner surface layers in the improvement of NiTi corrosion resistance is discussed.

Titanium nitride coatings on surgical titanium alloys produced by a powder immersion reaction assisted coating method: residual stresses and fretting behavior

Abstract Titanium and Ti–6Al–4V alloy samples were coated using a Powder Immersion Reaction Assisted Coating (PIRAC) nitriding method in order to modify their surface properties. Depending on the processing temperature, strongly adherent single(TiN)- or double(Ti2N/TiN)-layer coatings were obtained on both substrates. Several characteristics of PIRAC-coated Ti alloys relevant to their applications in total joint replacements were studied. Residual stresses in PIRAC coatings measured by sin2ψ X-ray diffraction method were found to be compressive in nature and were significantly lower than those reported for PVD TiN layers on similar substrates. In vitro fretting tests of PIRAC nitrided Ti–6Al–4V-to-Ti–6Al–4V couples simulating in vivo conditions at the interface of modular orthopedic implants demonstrated a major reduction in fretted areas, as well as a remarkable reduction of the corrosion potential drop at the initial stages of fretting as compared to the uncoated alloy. In addition, a 25% reduction of fretting-induced dissolved Ti ions concentration in testing solution was measured by EAAS. The results of the research suggest that titanium nitride PIRAC coatings can provide surgical titanium alloys with the longed-for fretting wear and corrosion resistant surface thereby minimizing the ion- and particulate-generating potential of modular orthopedic implants.

Ti2AlC ternary carbide synthesized by thermal explosion

Ti2AlC ternary carbide was fabricated from 2Ti–Al–C powder blend employing thermal explosion (TE) mode of selfpropagating high-temperature synthesis (SHS). The formation of Ti2AlC was found to occur during cooling from the high combustion temperature (>2000 jC) by a peritectic reaction between the earlier formed liquid Ti aluminides and a solid Ti carbide, TiC1� x. The application of a moderate 30-MPa pressure during TE at 800 jC yielded a near fully dense material containing, in addition to the Ti2AlC ternary phase, an appreciable amount of TiC1� x. D 2002 Elsevier Science B.V. All rights reserved.

Bonelike apatite formation on niobium metal treated in aqueous NaOH

The essential condition for a biomaterial to bond to the living bone is the formation of a biologically active bonelike apatite on its surface. In the present work, it has been demonstrated that chemical treatment can be used to create a calcium phosphate (CaP) surface layer, which might provide the alkali treated Nb metal with bone-bonding capability. Soaking Nb samples in 0.5 M NaOH, at 25 °C for 24 h produced a nano-porous ∼40 nm thick amorphous sodium niobate hydrogel layer on their surface. Immersion in a simulated body fluid (SBF) lead to the deposition of an amorphous calcium phosphate layer on the alkali treated Nb. The formation of calcium phosphate is assumed to be a result of the local pH increase caused by the cathodic reaction of oxygen reduction on the finely porous surface of the alkali-treated metal. The local rise in pH increased the ionic activity product of hydroxyapatite and lead to the precipitation of CaP from SBF that was already supersaturated with respect to the apatite. The formation of a similar CaP layer upon implantation of alkali treated Nb into the human body should promote the bonding of the implant to the surrounding bone. This bone bonding capability could make Nb metal an attractive material for hard tissue replacements.

Alumina–Ti aluminide interpenetrating composites: microstructure and mechanical properties

Abstract The microstructure and mechanical properties of dense interpenetrating phase Al2O3–TiAl–Ti3Al composites fabricated by pressure-assisted thermal explosion of TiO2–Al powder blend have been studied. The unusual wavy morphology of aluminide–Al2O3 interface matching the lamellar γ–α2 aluminide structure was formed due to big difference between oxygen solubility in γ and α2. Crack deflection by alumina grains and crack bridging by the more ductile intermetallic may control the fracture toughness of these composites.

Reactive formation of coatings at boron carbide interface with Ti and Cr powders

Abstract Boron carbide, B4C, is an attractive candidate material for reinforcement in metal matrix composites, whose application is severely hampered by its reactions with most engineering alloys at the high processing or service temperatures. The reactivity of B4C with some of the metals, however, may be made use of to create protective coatings on its surface. In the present research, the microstructure of coatings obtained by the interaction of B4C with Ti and Cr powders at 1000–1200 °C was investigated employing X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy. Coatings obtained by treating B4C in Ti powder were found to contain Ti carbide, TiC1− x, and Ti borides (TiB2 and TiB). A relatively thin inner layer of the coating was carbide-free and contained only borides, while the major part of the coating was a mixture of TiC1− x and TiB. In contrast to this, coatings formed by reaction of B4C with Cr powder contained no carbides, and were shown to consist of Cr borides (CrB2, CrB, Cr5B3 and Cr2B) and amorphous carbon. A thick outer layer of the coating was carbon-free and consisted almost entirely of CrB. In both cases, the growth of the coatings was controlled by diffusion, the activation energy for the growth of B 4 C Ti coating being approximately 175 KJ mol . The phase composition, layer sequence and morphology of the coatings obtained were interpreted on the basis of kinetic and thermodynamic data of the ternary systems involved. A good agreement between the experimental results and theoretical predictions was obtained.

Synthesis of dense Ti3SiC2-based ceramics by thermal explosion under pressure

Self-propagating high-temperature synthesis (SHS) of Ti3SiC2 from the elemental powders employing different SHS modes: wave propagation and pressureless or pressure-assisted thermal explosion, has been studied. SHS reaction was ignited in the solid state below the melting point of either constituent, and started with the formation of Ti5Si3 and Ti-rich TiC1� x. The formation of Ti3SiC2 took place at the later stage by crystallization from the liquid concurrent with precipitation of the stoichiometric TiC. In neither approach, a single-phase Ti3SiC2 material was obtained, the combustion products containing appreciable amounts of TiC. An additional ‘phase’ with the approximate composition of 35Ti–52Si–14C (at.%), supposedly the Ti3SiC2–TiSi2–SiC eutectic mixture, was detected in all types of samples. A short 1 min application of a moderate 80 MPa pressure during thermal explosion (reactive forging) yielded 595% dense samples containing � 45 vol.% Ti3SiC2. As the samples do not contain open porosity, they can be further used for HIPing without encapsulation at around 1500 � C to produce fully dense single-phase Ti3SiC2. # 2002 Elsevier Science Ltd. All rights reserved.

Pressure-assisted SHS synthesis of MgAl2O4–TiAl in situ composites with interpenetrating networks

Self-propagating high-temperature synthesis (SHS) of compacted 2TiO2–Mg–4Al powder blends was used to fabricate in situ magnesium aluminate spinel–TiAl/Ti3Al interpenetrating phase composites. The combustion wave propagation and thermal explosion modes of SHS, as well as thermal explosion under uniaxial pressure (reactive forging) were investigated, with the samples’ temperature closely monitored in the course of processing. SHS in 2TiO2–Mg–4Al blend was found to proceed in two stages, the leading reaction being the reaction between Mg and TiO2 with the formation of MgTiO3. Wave-like propagation of both leading and lagging reactions was observed during thermal explosion. The application of a moderate 60–80 MPa pressure during reactive forging at 800–900 °C yielded near fully dense (up to 98% of the theoretical density) MgAl2O4 spinel–TiAl/Ti3Al composites with fine micrometer/submicrometer scale interpenetrating ceramic and intermetallic networks. The aluminide component had a very fine γ+α2 lamellar structure that should beneficially affect the toughness of the material synthesized.