B.Z. Weiss

Direct observation of porous SiC formed by anodization in HF

A process for forming porous SiC from single‐crystal SiC wafers has been demonstrated. Porous SiC can be fabricated by anodizing n‐type 6H‐SiC in HF under UV illumination. Transmission electron microscopy reveals pores of sizes 10–30 nm with interpore spacings ranging from ≊5 to 150 nm. This is the first reported direct observation of porous SiC formation.

Characterization of nanocrystallites in porous p-type 6H-SiC

We report the formation of porous p‐type 6H‐SiC. The existence of uniformly dispersed pores was confirmed by transmission electron microscopy, with interpore spacings in the range of 1–10 nm. The porous film as a whole is a single crystal. Luminescence peaks above the normal band gap of 6H‐SiC have been observed in the porous layer, but were not distinguished in the bulk SiC substrate. Quantum confinement is discussed as a possible mechanism for the luminescence effects.

Structure and mechanical properties of vacuum arc-deposited NbN coatings

Abstract Thin NbN coatings were deposited using a vacuum arc plasma gun connected to a straight plasma duct, with an imposed axial magnetic field. The substrates were cemented carbide bars having a composition of 90% WC, 1.8% TaC, 0.2% NbC, and 8% Co. The influence of the nitrogen pressure in the deposition system, which was in the range of 0.13–2 Pa, on the structure, phase composition, microhardness, and scratch critical load of the coatings was studied. It was shown that for nitrogen in the pressures range of 0.13–0.4 Pa the coating is composed of a mixture of two phases: hexagonal β-Nb2N and cubic δ-NbN, whereas at pressures of 0.67 Pa and above single-phase δ-NbN with a NaCl type structure was obtained. In most cases the coatings consisted of randomly oriented equiaxial grains. A maximum microhardness of 42 GPa was obtained for the two-phase coatings deposited at a nitrogen pressure of 0.4 Pa. However the maximal critical load of 95 N was obtained with the homogeneous δ-NbN coatings, deposited at a nitrogen pressure of 0.93 Pa, while the coatings with the hexagonal β-Nb2N phase had a much lower critical load (50 N). The NbN coatings with the highest critical load exhibited an average Vickers microhardness of 38 GPa.

Multicomponent Ti–Zr–N and Ti–Nb–N coatings deposited by vacuum arc

Abstract A triple-cathode vacuum arc plasma gun was used to deposit Ti–Zr–N and Ti–Nb–N multicomponent coatings onto cemented carbide (90% WC, 8% Co, 1.8% TaC, and 0.2% NbC) substrates. The coatings were deposited at a bias voltage of −40 V relative to the anode, and a substrate temperature of 400°C. The influence of the nitrogen background pressure, which was in the range of 0.67–2 Pa, on the structure, phase composition, and microhardness was studied. It was shown that a solid solution (Ti,Zr)N was formed in the Ti–Zr–N coatings, in which the elements Ti, Zr, and N were distributed homogeneously. The films had a fine structure. The (Ti,Zr)N grains had an average diameter of 30 nm and were {111} orientated. The nitrogen concentration in the solid solution was not affected by the nitrogen pressure in the range studied. However, increasing the nitrogen pressure to 2 Pa increased the Zr concentration, while that of Ti decreased and a less dense structure is formed. The formation of a (Ti,Nb)N solid solution was observed in the Ti–Nb–N coatings. The (Ti,Nb)N grains were randomly oriented. A maximum microhardness of 51.5 GPa was obtained for the Ti–Nb–N film deposited at a nitrogen pressure of 1.33 Pa. Increasing the nitrogen pressure to 2 Pa decreased the microhardness to 31.5 GPa.

Bias voltage and incidence angle effects on the structure and properties of vacuum arc deposited TiN coatings

TiN coatings were deposited on WC–Co bar substrates using a vacuum arc plasma gun connected to a cylindrical plasma duct in which an axial magnetic field was imposed. During deposition, the cathode arc current was 200 A, nitrogen pressure was 0.67 Pa, and the substrate temperature was 420°C. Substrate bias voltage (Vbias) was varied in the range of −40 to −600 V. The coating structure and properties were studied both on the sample face surface, i.e. normal to the plasma flux, and on its side surfaces. The structure and phase composition were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness and scratch critical load were studied using Vickers micro-indentation and scratch tests, respectively. The TiN coatings had a single-phase cubic δ-TiN structure and consisted of oriented columnar grains. No difference was observed in the preferred grain orientation and grain size at the substrate–coating interface for the coatings deposited on the face and the substrate side surface for every studied Vbias. The deposition rate decreased both on the face and the side surfaces, while the ratio between deposition rates on the face and the side surfaces increased from three to seven times when |Vbias| increased from 40 to 600 V. With increasing |Vbias|, the preferred orientation of the columnar grains changed from a mixture of (200) and (111) at −40 V to a strong (111) at −200 and −400 V. At −600 V, (111) remained dominant, while the (220) orientation also appeared. Increasing |Vbias| increased the grain size on the coating surface. A zone of equiaxed grains was observed near the substrate–coating interface, whose thickness increased with increasing |Vbias|. Possibly, the grain size growth was a thermal effect due to an increase in ion beam heating with increased |Vbias|. The grain size on the side surfaces was smaller than that on the face. The coating surface roughness and friction coefficient were smaller on the side surfaces than those on the face, while no differences in microhardness were observed.

Phase identification in titanium‐rich Ti‐Fe system by Mössbauer spectroscopy

Various phases of the titantium rich part of the Ti‐Fe binary system were obtained by thermal treatments and were identified by Mossbauer spectroscopy and x‐ray diffraction. Room‐temperature Mossbauer parameters were derived for the following phases: αm, α, ϑ, ω, β, and TiFe. The range of concentrations for which an athermal ω phase appears upon quenching was found to be 2.7⩽CFe⩽5 wt%. The athermal ω phase disappears during aging at 285 °C for 15 h, when the alloy contains CFe⩽4.0 wt%. Martensite αm was found to form when Fe concentrations were ⩽2.7 wt.%. After quenching from the β field, the β phase retains when the Fe content of the alloy is ⩾2.7 wt.%. The ϑ phase forms during aging at temperatures above 280 °C, for alloys with a Fe content ⩽2.7 wt.%. Room‐temperature Mossbauer parameters are given to enable phases to be indentified and analyzed by means of Mossbauer spectroscopy.

Vacuum arc deposition and microstructure of ZrN-based coatings

Abstract Thin coatings of ZrN, and bilayer coatings TiN/ZrN and ZrN/TiN of up to 3 μm thickness were deposited using a triple-cathode vacuum arc plasma gun connected to a straight plasma duct, where an axial magnetic field was imposed. The substrates were cemented carbide bars, having a composition of 90% WC, 1.8% TaC, 0.2% NbC, and 8% Co. The coatings were deposited with an arc current of 200 A, background nitrogen pressure of 0.4–2 Pa, substrate temperatures of 200 to 600 °C, and substrate bias voltages in the range of 0 to −200 V. The magnetic field in the duct was in the range of 1 to 10 mT. The structure and composition of the coating and interface morphology were studied by means of X-ray diffraction, Auger electron spectroscopy, transmission electron microscopy and scanning electron microscopy combined with energy dispersive spectroscopy analyses. It was shown that for nitrogen pressures higher than 0.4 Pa a single-phase ZrN coating with a NaCl-type structure was obtained. The microstructure of the ZrN coatings and ZrN and TiN layers of the bilayer coatings was found to be composed of (111) oriented columnar grains, although near the coating-substrate interface randomly oriented grains were also observed. In the bilayer coatings a sharp interface without intermixing between the TiN and ZrN layers was observed. The preferred grain orientation was independent of the substrate bias voltage and temperature. However, the coating grain size increased with the substrate temperature and decreased with the substrate bias voltage. It was shown that Co diffused from the cemented carbide substrate to the free surface of the coating, and its concentration there increased with the deposition temperature.

Interaction of Ni90Ti10 alloy thin film with 6H‐SiC single crystal

Interfacial reactions, phase formation, microstructure, and composition, as functions of heat treatments (400–800 °C) were investigated in Ni90Ti10 alloy thin film coevaporated on an n‐type 6H‐SiC (0001) single‐crystal substrate. The study was carried out with the aid of Auger electron spectroscopy, x‐ray diffraction, and analytical transmission electron microscopy. The interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of Ni‐rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third consists of Ni2Si. This composite structure, consisting of the silicide as a low resistivity ohmic contact, and of the carbide as a diffusion barrier, promises high‐temperature stability crucial to ohmic contact development for SiC technology. Factors controlling phase formation in the Ni–Ti/SiC system are discussed.Interfacial reactions, phase formation, microstructure, and composition, as functions of heat treatments (400–800 °C) were investigated in Ni90Ti10 alloy thin film coevaporated on an n‐type 6H‐SiC (0001) single‐crystal substrate. The study was carried out with the aid of Auger electron spectroscopy, x‐ray diffraction, and analytical transmission electron microscopy. The interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of Ni‐rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third consists of Ni2Si. This composite structure, consisting of the silicide as a low resistivity ohmic contact, and of the carbide as a diffusion barrier, promises high‐temperature stability crucial to ohmic contact development for SiC technology. Factors controlling phase formation in the Ni–Ti/SiC system are discussed.

Dopant-selective etch stops in 6H and 3C SiC

A novel photoelectrochemical etching process for 6H– and 3C–SiC is described. This method enables n-type material to be etched rapidly (up to 25 μm/min), while a buried p-type layer acts as an etch stop. Dissolution of SiC takes place through hole–catalyzed surface dissolution. The holes are supplied either from the bulk (e.g., p-SiC) or by UV photogeneration (in n- or p-SiC). The differing flatband potentials of n- and p-type SiC in HF solutions allow the selection of a potential range for which hole current injection occurs only in n-type materials, facilitating dopant-selective etching. This process can be utilized in controlled etching of deep features, as well as in precise patterning of multilayer films.

Explosive cladding of Cu/Cu systems: An electron microscopy study and a thermomechanical model

Abstract Transmission electron microscopy investigations were carried out on three explosively cladded Cu/Cu systems: equiaxial small grains, elongated large grains and single crystals. The investigation has shown that the cladding process leads to the formation of a ‘bond zone’ rather than to a ‘planar interface’. The ‘bond zone’ independent of the cladding system could be divided into a few ‘cladding affected regions’ symmetrically extending on both sides of the colliding surfaces. Each of the regions is characterized by a definite structural morphology. Selected area diffraction patterns show that in all three systems, regions characterized by similar structures exhibit the same zone axis, namely 〈111〉 or 〈011〉. The ‘metallurgical bond’ achieved in the cladding process is constituted of small equiaxed recrystallized grains which form the center of the ‘bond zone’. The array of structures in the ‘bond zone’ as well as the dislocation densities result from a combination of three basic processes taking place during explosion cladding: mechanical (deformation), thermal and mass transfer. A theoretical model has been suggested for the treatment of the effects of the thermomechanical process on the annihilation of dislocations and their density distribution after cladding and on the extent of mass transport during transient annealing. The theoretically predicted results were compared with the experimental results obtained by means of transmission electron microscopy. For the Cu/Cu systems the agreement was found to be reasonable. The model presented should enable thermomechanical processes in other similar explosion clad systems to be treated.