Surface erosion of TiC coatings under 4He+ and D+ ion irradiation

The surface damage and erosion of chemically vapor-deposited TiC coatings irradiated by 20, 40 and 60 keV D⁺ and ⁴He⁺ were studied for as-deposited and polished surfaces. Scanning electron micrographs of irradiated TiC reveal surface damage and erosion due to blistering and surface exfoliation. The erosion yield increases with increasing ion energy for both D⁺ and He⁺ irradiations, and it is generally larger for D⁺ than for He⁺ irradiations at a given energy. The erosion yield is larger for polished than for as-deposited surfaces for both D⁺ and He⁺ irradiations. The relationship between the blister diameter and the skin thickness agrees well with a relationship derived from a gas pressure model but disagrees with a relationship derived from a model of integrated lateral stresses as a prime mechanism for blister formation.     

Carbon coatings for fusion applications

The use of low atomic number material coatings in a fusion reactor plasma chamber may ease concern over maintaining plasma purity. A deposition process involving the discharge activated deposition of carbon coatings from methane at about 1 Pa pressure has been investigated. A coating of thickness 10 μm on copper has survived 1000 cycles of pulsed thermal heating at 37 MW m^-2 with only minor flaking. The surface damage of similar coatings on stainless steel surfaces has been investigated for irradiation with 20, 40, 60 and 120 keV D⁺ and He⁺ ions at ambient temperature. Scanning electron microscopy of irradiated surfaces revealed no significant surface damage for either D⁺ or He⁺ irradiation with energies of 40 and 60 keV for doses of 4 × 10²² and 8.1 × 10²² ions m^-2 respectively. For 120 keV D⁺ and He⁺ irradiations for a dose of 2.2 × 10²³ ions m^-2, surface damage in the form of ridges was observed. A comparison of the results for carbon coatings with those obtained for ATJ graphite reveals that this type of graphite shows surface damage for all irradiations performed, while the carbon coating appears to be more resistant to damage for many of these irradiations. These results reflect favorably on the possible use of these coatings on various components in the plasma chamber of a fusion reactor.     

The effect of biological environment on the surface of titanium and plasma-sprayed layer of hydroxylapatite

The solubility of titanium samples with different surface coatings, i.e., hydroxylapatite (HA) powders, a two-layer coating of ZrO₅+HA on a titanium substrate in solution and of tooth implants after long-term functioning in the human organism, was studied. A minimum difference in solubility of titanium samples with different surface finishes (polished or grit blasted) was established. For the HA powders and coatings, the lowest solubility was observed with a coarse-grained HA–B powder and a coating made of that powder. Clinical tests of tooth implants after long implantation times were performed. A titanium implant (implantation 12 y), a titanium implant with a two-layer coating of ZrO₅+HA–A (implantation time 4 y) and a titanium implant with a two-layer coating of Al₅O₃+3% TiO₂)+ HA–A (implantation time 6 y) were studied. The results show that the titanium surface and HA–A layers were dissolved. Nevertheless, after 6 y implantation, total removal of HA–A coating from that part of implant set into the bone, was not observed.     

Plasma-sprayed coatings for fusion reactor applications

A series of plasma-sprayed coatings has been given a preliminary evaluation to assess the potential of this class of materials in fusion reactor applications. TiC, TiB₂, beryllium and VBe₁₂ coatings on copper and stainless steel (SS) were tested for coating adherence, ion erosion resistance and susceptibility to arc erosion. The coatings, in general, display a good resistance to thermal shock failure. The TiC and TiB₂ coatings exhibit favorable ion erosion characteristics and the resistance of the coatings to arc erosion was, in general, superior to that of SS.     

Preparation of the TiB2 coatings by electroplating in molten salts

The improvement of titanium diboride (TiB₂) coatings over the molybdenum substrate was investigated by comparing the electrochemical techniques of continuous current plating (CCP) and periodically interrupted current (PIC). The solvent used was a eutectic Flinak mixture (LiF-NaF-KF, 46.5:11.5:42 molar ratio) with solutes K₂TiF₆ and KBF₄ with the electrochemically-active components in the molar ratio of one to five. The coatings produced by PIC show improvements in morphology and microstructure for the suitable conditions: i = 0.5 A/cm², frequency = 100 Hz, the time ratio t_c/t_off (current on/current off) = 4/1. X-ray diffraction (XRD) analyses indicated that the coatings are composed of the relatively pure TiB₂ and the preferred orientation is [001] + [110], which is in accordance with the prediction of the two-dimensional crystal nuclei theory.     

Preparation and electrical erosion resistance of TiB2/Cu nanocomposites

A procedure is described for producing nanocomposite TiB₂/Cu powders containing up to 57 vol % TiB₂. Using shock compression of composite powders, we have prepared electrode materials offering enhanced electrical erosion resistance at high arc discharge currents. The effect of titanium diboride nanoparticles embedded in the copper matrix on the erosion behavior of the nanocomposites is examined. The nanoparticles are shown to suppress the copper droplet entrainment during the service of the electrode. TiB₂/Cu nanocomposite electrodes containing more than 10 vol % TiB₂ retain their shape and dimensions in the course of electrical erosion tests and offer enhanced service life.     

Correlation between surface damage and micro-defects in Si covered with insulating layer by implantation of He and H ions

Cz n-type Si (100) wafers covered with a 220 nm SiO₂ layer or a 170 nm Si₃N₄ layer were singly implanted with 160 keV He ions at a dose of 5 × 10¹⁶/cm² or successively implanted with 160 keV He ions at a dose of 5 × 10¹⁶/cm² and 110 keV H ions at a dose of 1 × 10¹⁶/cm². Surface morphologies together with defect microstructures have been studied by means of several techniques, including optical microscopy, atomic force microscopy, and cross-sectional transmission electron microscopy (XTEM). Only surface blistering has been observed for He and H sequentially implanted SiO₂/Si samples after annealing in temperature range up to 1000 °C. However, as for the He and H implanted Si₃N₄/Si samples, surface features including blistering and the localized exfoliation of both the top Si₃N₄ layer and the implanted Si layer have been well demonstrated during subsequent annealing. XTEM observations reveal quite different defect morphologies in two kinds of materials under the same implantation and annealing conditions. The possible mechanisms of surface damage in two kinds of materials have been discussed and presented based on the XTEM results.     

Fabrication of Co-Based Coatings on Titanium Alloy by Laser Cladding with CeO2 Addition

In this study, Co-based laser cladding coatings reinforced by multiple phases were fabricated on titanium alloy. Co42 Co-based self-fluxing alloy, B₄C, and CeO₂ mixed powders were used as the precursor materials. The coatings were mainly composed of γ-Co/Ni, CoTi₂, CoTi, NiTi, TiC, Cr₇C₃, TiB₂, and TiB phases. A typical TiB₂/Cr₇C₃/TiC composite structure was chosen. It was found that CeO₂ did not influence the phase types of the coating significantly, but was effective in refining the microstructure and enhancing the microhardness and dry sliding wear resistance. Compared with the Ti-6Al-4V titanium alloy, the microhardness and wear resistance of the composite coatings were enhanced by 3.44–4.21 times and 14.26–16.87 times, respectively.     

Enhanced corrosion resistance of discontinuous anodic film on in situ TiB<Subscript>2p</Subscript>/A356 composite by cerium electrolysis treatment

The agglomerates of TiB₂ particulates and Si phases badly break the continuity of anodized film of in situ TiB_2p/A356 composite, which will restrict the improvement of corrosion resistance. In this study, cerium conversion coatings were successfully deposited on anodized TiB_2p/A356 composite by electrolysis treatment. Scanning electron microscope observations show that the conversion coatings effectively cover the whole surface of anodized composite. The conversion coatings on continuous porous anodic film are composed of many spherical nano-particulates; however, at the regions without anodic film the conversion coatings present a planar structure. The different morphologies are attributed to the different formation characteristics of cerium conversion coatings at different regions of anodized composite. X-ray photoelectron spectroscopy analysis indicates that the conversion coatings consist of CeO₂, Ce₂O₃, Ce(OH)₄, and Ce(OH)₃. The potentiodynamic polarization results testify that the integrated surface coatings of anodic film and cerium conversion coating provide a higher degree of protection for in situ TiB_2p/A356 composite in a chloride-containing environment.     

Plasma spray synthesis of TiB2-Fe coatings

A limiting feature of the plasma spray process is the need for the powder to melt during its passage through the plasma flame. It is quite impossible to obtain coatings with materials that are difficult to melt. However, metal borides, particularly titanium boride, are attractive. Because of their high melting point, satisfactory coatings based on these materials have not been achieved.To overcome this problem, a process for making TiB₂-Fe coatings was studied. The TiB₂-based coatings were produced by the reaction of ferrotitanium with boron. The TiB₂ formation was first studied by thermal differential analysis and X-ray diffraction. It was observed that TiB₂ is formed at low temperature by an exothermic reaction. The characteristics of the reaction products obtained at different reaction temperatures are described.Agglomeration techniques were used to prepare the reagents: fine powders of ferrotitanium alloy and boron. TiB₂-Fe coatings were produced by plasma spraying the agglomerated powders. The influence of the plasma spray process parameters and the powder preparation techniques on the coating microstructure is discussed.Thick hard coatings comprising compounds of the reagent materials are produced during spraying by this synergetic process. Such coatings may be suitable for wear resistance applications.     

Ion beam modifications of defect sub-structure of calcite cleavages

Experimental investigations on the defect sub-structure and surface modifications, brought about by He⁺ ion-bombardment of calcite cleavages (100), have been carried out. Optical and scanning electron microscopic investigations revealed drastic modifications on the surface morphology, local symmetry and defect concentration. Additional structural defects on ion-bombardment of calcite surfaces also have been observed. Changes in shape and form of chemical etch pits are found to be a function of ion-beam energy, as studied by optical microscopy. Radiation damage in calcite has been attributed mainly due to desorption of CO₃⁻² ions from the calcite surfaces, on irradiation. Measurements of surface conductivity on irradiated calcite surfaces have been made employing a four-probe technique. Enhancement of surface conductivity has been considered to be due to an increase in concentration of CO₃⁻² ions formed, on ion irradiation and subsequent thermal stimulation. Planar plastic anisotropy has been studied on irradiated calcite cleavages by measurement of microhardness.     

Studies of carbon behavior in tungsten under carbon and hydrogen mixed irradiation

The erosion of W coatings with the different content of C was investigated in Ar and Ar + H₂ plasmas. The enhanced erosion was registered in Ar + H₂ plasma for coatings containing 40 at.% of C and more, which might be owing to chemical erosion of carbon. The erosion kinetics was analyzed together with the studies of coatings microstructure, phase composition and Vicker hardness properties.The C distribution profiles in W were investigated in dependence on the redeposition rate of sputtered C and simultaneous 0.3 keV Ar⁺ ion irradiation. It is shown that C penetration depth depends on the coverage of W by C during ion irradiation. Deep C penetration was registered when W was partially covered by redeposited carbon.The computer simulation was used to extrapolate obtained results to experimental investigations performed by Ueda Y, Fukumoto M, Sawamura I, Sakizono D, Shimada T, Nishikawa M. [Fusion Eng Des 2006;81:233–9]. It is revealed that the ballistic relocation of implanted C atoms is the dominant process which explains the deep penetration of C and formation of WC compound in the near-surface region of W.     

Characterization of novel borides in Ti–Nb–Zr–Ta + 2B metal-matrix composites

Metal-matrix composites consisting of a complex quaternary Ti-35Nb-7Zr-5Ta alloy reinforced by borides have been successfully deposited from a powder feedstock consisting of a blend of elemental titanium, niobium, zirconium, tantalum, and, titanium diboride (TiB₂) powders, using the laser engineered net-shaping (LENS?)¹ process. The microstructures of the as-deposited composites have been characterized using scanning electron microscopy, orientation microscopy, and, transmission electron microscopy. Both primary and eutectic boride precipitates, exhibiting the orthorhombic B27 structure, are observed in these as-deposited composites. The complex primary borides exhibit an unusual compositional variation within the same precipitate, which has been investigated in detail using site-specific characterization with a transmission electron microscope. The ability to form near-net shape components using the Laser Engineered Net Shaping process makes these laser-deposited composites promising candidates for wear-resistant applications in biomedical implants.     

Phase formation and modification of corrosion property of nitrogen implanted Ti-Al-V alloy

In the present investigation, polished samples were implanted with nitrogen ion at an energy of 60 keV and implantation doses were 1×10¹⁶, 5×10¹⁶, 1×10¹⁷ and 6×10¹⁷ ions/cm². Glancing incidence X-ray diffraction was employed on the implanted specimens to understand the phases formed with increasing dose. The valence states of nitrogen, titanium and carbon on the sample surfaces were analyzed by X-ray photoemission spectroscopy. The corrosion resistance was examined by the electrochemical methods in a solution with pH=10 at room temperature in order to determine the optimum dose that can give good corrosion resistance in a simulated nuclear reactor condition. Scanning electron microscopy was used to observe the topographies of nitrogen-implanted Ti-Al-Zr after potentiodynamic measurement. It was found that implanted nitrogen dissolved in titanium matrix with increasing dose and the resultant nitrides such as TiN and Ti₂N precipitated. Implantation of nitrogen ions into the surface of Ti-Al-V alloy improves its corrosion resistance, and the increase of the corrosion resistance depends on the nitrogen dose employed; the maximum improvement of the corrosion resistance was observed at a dose of 1×10¹⁷ N⁺/cm².     

Microstructure and electrochemical properties of Zircaloy-4 under Fe ion irradiation

To investigate the microstructure and electrochemical properties of Zircaloy-4 induced by Fe ion irradiation with the energy of 150 keV at liquid nitrogen temperature, transmission electron microscope analysis (TEM) was employed to analyze the surface layer of the samples irradiated at a dose ranging from 1×10¹³ to 1×10¹⁶ ions/cm², and potentiodynamic polarization measurements were used to evaluate the corrosion resistance of Zircaloy-4 in a 1 N H₂SO₄ solution at room temperature. TEM analyses show that Fe ion irradiation lead to a structural change and amorphous phase formation on the surface of the samples. Moreover, it is indicated from the corrosion tests that with an increase of the irradiation dose, the passive current density increases at first and then decreases rapidly, while the natural corrosion potential goes down at first and then up rapidly. The critical point, where the corrosion properties are transformed from a damaging stage to protecting stage, is at the damage level of 3.19 dpa. Finally, the mechanism for the change of corrosion resistance of the Zircaloy-4 samples is discussed.     

Synthesis of Boride Coating on Steel using High Energy Density Processes: Comparative Study of Evolution of Microstructure

Ultrahard titanium diboride (TiB₂) coatings are deposited on plain carbon steel substrate using two high energy density processes, viz. pulsed electrode surfacing (PES) and laser surface engineering (LSE). These two processes are entirely different in physical nature and hence result in dissimilar microstructures. In the present investigation, a comparative study has been made between the evolved microstructures. Both processes produced a surface layer which is adherent and metallurgically bonded to the substrate. PES produced relatively thinner and less uniform coating than LSE process. The PES coating was, however, homogeneous and very fine grained. The laser-assisted coating was “composite” in nature with TiB₂ particles embedded in Fe matrix. Mechanical characterization of these coatings has been performed using microhardness measurements.     

Self-propagating high temperature synthesis and dynamic compaction of titanium diboride/titanium carbide composites

Self-propagating High-temperature Synthesis (SHS) of titanium and boron carbide (B₄C) combined with explosively driven Dynamic Compaction (DC) was employed for the fabrication of composite TiB₂/TiC compacts. A 2³ factorially designed experiment set was used to examine the effects of the TiB₂/TiC ratio, delay time and C/M ratio on the consolidation and properties of the compacts. The delay time is the time between completion of the SHS reaction and compaction. The C/M ratio, the ratio of the explosive mass to that of the flyer plate, influences the pressure applied to the samples during compaction. Composites with molar TiB₂/TiC ratios of 2:1 or 1:2 were prepared using Ti and B₄C or Ti, C and B₄C, respectively, as reactants. The SHS/DC of Ti and B₄C resulted in high quality, near fully dense TiB₂/TiC composite compacts. Under best conditions, the densities were greater than 98% of the theoretical maximum. While the microhardness and densities of the compacts with TiB₂/TiC ratio of 2:1 were comparable to those of monolithic TiB₂ and TiC, compacts with TiB₂/TiC ratios of 1:2 were poorly consolidated and contained extensive cracks. Given the high energy and time efficiency, high product quality and inexpensive reactants, the SHS/DC of Ti and B₄C represents an attractive technique for the economical fabrication of TiB₂/TiC composites.     

Structure and morphology of TiB2 duplex coatings deposited over X40 CrMoV 5-1-1 steel by DC magnetron sputtering

The deposition of titanium diboride (TiB₂) films over tool steel substrates (AISI H13 premium/EN X40 CrMoV 5-1-1) is being investigated due to its excellent corrosion resistance and chemical stability against liquid aluminium. The use of nitrided steels as substrates for TiB₂ deposition may contribute to increase its adhesion and the overall steel resistance in applications such as forging, extrusion and die casting of aluminium. Duplex coatings were obtained by the PVD deposition of TiB₂ films over heat treated and nitrided steel using non-reactive DC magnetron sputtering from a TiB₂ target, varying the substrate bias voltage. Well structured and crystalline TiB₂ films were obtained for the selected deposition conditions, the best crystalline coatings being obtained for the positively biased substrates. Selected films produced over die-casting pins at a bias voltage of +50 V were tested for resistance to liquid aluminium soldering by immersion tests, and compared with the nitrided steel. The duplex TiB₂ coating has a much larger chemical resistance to attack by molten aluminium alloy than the just nitrided steel. Where there is soldering, steel is rapidly attacked and a complex Al-Fe-Si intermetallic forms.     

TiB2-coated cathodes for aluminum smelting cells

Pipes consisting of coatings 700 μm thick of chemically vapor-deposited TiB₂ on carbon were produced and tested for corrosion and/or erosion resistance in a laboratory aluminum smelting cell for 100 h. The projected lifetime of the coating was approximately 30 days, a factor of 50 shorter than that projected by assuming uniform saturation dissolution of TiB₂ in molten aluminum. The corrosion and/or erosion as measured from scanning electron micrographs pointed to grain boundary attack with subsequent crystalline pull-out as the mechanism for the shorter than expected lifetimes.     

Comparison of the coatings deposited using Ti and B4C powder by reactive plasma spraying in air and low pressure

The coatings were deposited by reactive plasma spraying (RPS) in air and low-pressure plasma spraying (LPPS) based on the reaction between Ti and B₄C powder, respectively. The thermal spray powder of Ti and B₄C added with powder Cr (metallic binder) in air is compared with that without powder Cr addition in the low pressure. (Prior to deposition, the powder was screened and separated for RPS whereas spray drying, sintering and sieving were done for LPPS.) The phase composition and the microstructure of coatings were studied by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The anti-corrosion property of coatings was also investigated. It is found that the coating prepared by RPS, which is more densification, is composed of TiN, TiB₂, and a small phase fraction of titanium oxides. The composition of the coating deposited by reactive LPPS is TiB₂, Ti(C, N), Ti₄N_3−x and impurity phase of Ti₅Si₃. There is no appearance of titanium oxides in low pressure. The coatings have the typical lamellar structure and adhere to the bond coating well. The mean Vickers microhardness value of the coating deposited by RPS is higher than that of the coating deposited by LPPS. Furthermore, the corrosion resistance of the coating deposited by RPS is superior to that of the coating prepared by LPPS in near neutral 3.5 wt% NaCl electrolyte.