Elazar Y. Gutmanas

Dense in situ TiB2/TiN and TiB2/TiC ceramic matrix composites: reactive synthesis and properties

Abstract In the present research, near-fully dense in situ composites were fabricated from cold sintered B4C–3Ti and 2BN–3Ti powder blends with and without the addition of Ni. Two reactive synthesis techniques were employed: combustion consolidation (pressure-assisted thermal explosion) and reactive hot pressing (displacement reaction under pressure) RHP. In both approaches, the processing or preheating temperature (≤1100°C) was considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructure characterization of the materials obtained was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive analysis (EDS). Mechanical properties were evaluated by measuring microhardness, fracture toughness and three-point bending strength. Full conversion of reagents into products was achieved in B4C–3Ti, B4C–3Ti–1.5Ni and 2BN–3Ti–1.5Ni blends during combustion consolidation, and a moderate external pressure of 150 MPa was sufficient to ensure full density of the final products. Unlike this, no thermal explosion occurred in 2BN–3Ti samples at 1100°C under pressure. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The RHP processing route yielded dense materials with finer microstructures, however full conversion of reagents into products has not been achieved. The addition of Ni to the powder blends was shown to enhance densification, as well as improve the fracture toughness of the composites synthesized.

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.

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.

Surface modification of titanium alloy orthopaedic implants via novel powder immersion reaction assisted coating nitriding method

Abstract There is increasing interest in using surface modification technology to improve the wear properties of titanium alloys used in total joint replacements. In the present work, a simple and original Power Immersed Reaction Assisted Coating (PIRAC) nitriding method suitable for surface modification of large complex shape orthopaedic implants has been developed. CP Ti and Ti-6Al-4V alloy samples were annealed at 850–1100°C in sealed stainless steel containers that allow selective diffusion of nitrogen atoms from the atmosphere. The relationship between the microstructure, nitrogen concentration and microhardness was studied employing X-ray diffraction (XRD), scanning electron microscopy (SEM)/electron probe X-ray microanalysis (EPMA) and high resolution SEM (HRSEM). PIRAC nitrided surfaces were found to have a layered structure with a TiN/Ti 2 N coating followed by nitrogen-stabilized α-Ti. In contrast to previous investigations of surface nitrided Ti-6Al-4V alloy, a Ti 3 Al intermetallic phase was detected at the Ti 2 N/α–Ti interface acting as a barrier for nitrogen diffusion. Importantly for biomedical applications, no toxic Al or V were detected in the surface layer of PIRAC nitrided Ti-6Al-4V alloy.

Synthesis ofin situ TiB2/TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends

In the present research, near-net-shapein situ TiB2/TiN and TiB2/TiN/Ni composites were fabricated from cold-sintered BN/Ti and BN/Ti/Ni powder blends by pressureless displacement reaction synthesis or thermal explosion under pressure. In both approaches, the processing or preheating temperatures (≤1200 °C) were considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructural characterization of the materials obtained was performed using X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical properties were evaluated by measuring microhardness, fracture toughness, and three-point bending strength. Application of a moderate external pressure (≤250 MPa) during self-propagating synthesis (SHS) synthesis was shown to be sufficient to ensure full density of the TiB2/TiN/Ni composite. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The high fracture toughness of thein situ synthesized TiB2/TiN/Ni composite (20.5 MPa√m) indicated that the finely dispersed ductile Ni phase was effective in dissipating the energy of cracks propagating in the ceramic matrix.

Coating of non-oxide ceramics by interaction with metal powders

Abstract Non-oxide ceramics, such as Si3N4, SiC, BN and B4C, interact at elevated temperatures with powders of metals having a high affinity for carbon and nitrogen with the formation of reaction product coatings on the ceramic surface. The diffusion of metal atoms along the ceramic surface plays an important role in the process of coating; starting temperatures for reactions with the formation of coatings correlate with surface self-diffusion rates of the corresponding metals. The coatings obtained on Si3N4 and SiC have a layered structure and their growth is diffusion controlled. The composition of the inner layer of the coatings is in agreement with the presented thermodynamic calculations for the corresponding Si3N4− and SiC-metal interfaces. The composition and morphology of the surface layer depend on the environment and can be modified by changing the treatment conditions. Based on the experimental observations and thermodynamic calculations, a model for the described powder immersion reaction assisted coating (PIRAC) is proposed.

A constitutive model for densification of metal compacts: the case of copper

A hybrid constitutive model for densification of metallic powders is proposed. It is based on a plasticity model for porous materials combined with a constitutive description of dislocation density evolution. The model was applied to the case of the uniaxial die compaction of copper powders. Densification behaviour during powder compaction, including the dislocation density evolution and the variation of the relative density of the compact, was simulated. The model was gauged by comparing the simulation results with experimental data generated by the cylindrical die compaction tests on copper powder.

Mechanical properties of Al2O3Si composites fabricated by pressureless infiltration technique

Abstract Two-phase interpenetrating systems with residual microstresses have been shown to exhibit high fracture toughness and strength. In this work, the microstress-induced strengthening concept was applied to Al 2 O 3 Si composites. The composites were fabricated by pressureless infiltration of porous Al 2 O 3 preforms with a molten Si. The materials obtained exhibited superior fracture toughness, strength and hardness when compared with technical aluminas having the same volume fraction of Al 2 O 3 but containing aluminosilicate glass phases instead of silicon. For example, an Al 2 O 3 Si composite with 30 vol% Si had K 1 c ≈ 4.8 MPa √m and a bending strength of ~ 320 MPa compared to ~ 3.5 MPa √m and ~ 230 MPa, respectively for the technical alumina AD-85 (‘Coors’). It is believed that the strengthening effect is caused by compressive residual microstresses in an inherently weak Si phase generated as a result of the solidification-related expansion of silicon.

Reactive synthesis of ceramic matrix composites under pressure

Abstract Near fully dense in situ ceramic matrix composites were fabricated from blends of fine Ti–B 4 C, Ti–BN, Ti–SiC, Ti–B 6 Si and Al–TiO 2 powders without or with the addition of Ni. Two reactive synthesis techniques were employed: thermal explosion/TE (SHS) under pressure and reactive hot pressing/RHP. In both approaches, the processing or preheating temperature (⩽1100°C) was considerably lower than those typical of current methods used for processing of ceramic matrix composites. Partial to full conversion of reagents into products was achieved during TE, and a moderate external pressure of 150 MPa was sufficient to ensure full density of the final products. RHP processing yielded dense materials with finer microstructures, however full conversion of reagents into products was not achieved.

Consolidation, microstructure and mechanical properties of nanocrystalline metal powders

Abstract Nickel, iron and aluminum nanocrystalline powders prepared by the evaporation-condensation method were high pressure consolidated to densities close to the theoretical at room temperature and at 300°C. Microstructures of the materials obtained were characterized by employing scanning (SEM) and transmission (TEM) electron microscopy. Microhardness and yield stress of the materials fabricated from the elemental nanocrystalline powders were measured and compared to those of Ni, Fe and Al high pressure consolidated from the micron/submicron elemental powders, and of a nanocrystalline Ni-20 TiC composite processed via attrition milling of a nickel oxide (NiO)/TiC powder blend followed by the reduction of NiO and high pressure consolidation to full density.