G.J. van der Kolk

Investigation on the formation of tungsten carbide in tungsten-containing diamond like carbon coatings

A series of tungsten-containing diamond-like carbon (Me-DLC) coatings have been produced by unbalanced magnetron sputtering using a Hauzer HTC-1000 production PVD system. Sputtering from WC targets has been used to form W-C:H coatings. The metal to carbon ratio has been varied to study changes in the metal carbide formation and distribution within the amorphous hydrocarbons (a-C:H) matrix. The difference in the formation of the metal carbide is then linked to changes in the mechanical and tribological properties of the coatings. Detailed high-resolution cross-section TEM has been carried out to analyze the microstructure of the coatings. By changing the amount of a-C:H in the W-C:H coatings, the coefficient of friction could be varied between 0.129 and 0.312. The hardness was found to vary between 8 and 27.5 GPa by using different acetylene gas flows. It was observed that all coatings did have a pronounced multilayered structure.

Properties and characterization of multilayers of carbides and diamond-like carbon

Abstract Metal containing diamond-like carbon (Me-DLC) coatings are widely applied in industrial applications. Normally, the coatings are produced with small inclusions of carbide forming elements like the 3d, 4d or 5d metals, or Si or B. The small carbide islands have sizes of approximately 2–20 nm. The effect of the nano inclusions is a reduction of internal compressive stress, a lowering of Youngs modulus, a lower hardness, and a higher coefficient of friction as compared to pure diamond-like carbon (DLC). The contents of metallic elements are typically between 10 and 20 at.%; for B containing diamond-like carbon a higher percentage is observed (up to 50% B). In this work the effect of replacing the nano inclusions by a stack of extremely thin carbide layers, separated by carbon layers, is tested. Properties like adhesion, hardness, E-modulus, fatigue resistance, coefficient of friction and wear resistance were studied. Furthermore, detailed high resolution TEM was performed to observe the effects of layer integrity. The results show a clear difference in wear and fatigue behavior when the multilayer structure was altered. Short periods within the multilayer structure of W-C:H promotes the wear resistance, while long periods promote the fatigue resistance.

Argon defect complexes in low energy Ar irradiated molybdenum

Abstract Thermal desorption spectrometry has been used to study the defects created in Mo irradiated along the 〈110〉 direction with Ar ions ranging in energy from 0.1 to 2 keV. In addition to monitoring the release of the implanted Ar, additional information has been obtained by decoration of the defects with low energy helium and subsequent monitoring of the helium release. These studies show evidence that the Ar can be trapped in both substitutional sites and in a configuration in which the Ar is associated with vacancies (ArV n , n ⩾2). Most of the Ar implanted at high energy is released at ≈ 1500 K by thermal vacancy assisted diffusion. Argon trapped closer to the surface is released at lower temperatures via at least three different surface related release mechanisms. Additional results are presented on the interaction of self interstitial atoms (introduced by 100 eV Xe bombardment) with the Ar defects. Substitutional Ar is found to convert to interstitial Ar which seems to be mobile at room temperature. The Ar-vacancy complexes are found to be reduced to substitutional Ar. The results of atomistic calculations of the release mechanisms will also be presented.

Self-interstitials generated by low energy heavy ion bombardment of metals

Abstract Thermal helium desorption spectrometry (THDS) has been used to study the ion bombardment conditions for creating stable self-interstitial atoms (SIA) below the surface of monocrystalline Mo, W and Ni. The SIA are observed by the relase of helium atoms when SIA recombine with HeV defects. Threshold energies for SIA generation are found to be 80 eV for Ar → Mo (110), 50 eV for Xe → W(100) and 25 eV for Ar → Ni(110). The measured SIA capture rate coefficient for deep HeV defects (mean depth > 20 nm) is of the order of 10 −15 cm 2 per ion. Results of MARLOWE calculations are used to analyse the contribution of different replacement collision sequences to the SIA generating process.

Vacancy binding to substitutional silver in tungsten observed with thermal helium desorption spectrometry

Abstract Low doses silver with energies between 5 and 25 keV were implanted in a (100) tungsten single crystal. The silver-vacancy complexes remaining after subsequent annealing were studied with thermal helium desorption spectrometry. The results show that the binding energy of one vacancy to substitutional silver is 0.8±0.3 eV, and that additional vacancies are more strongly bound. The binding energy of He to substitutional silver increased with increasing filling degree.

Thermally activated clustering of silver in tungsten studied with THDS

Abstract Silver and copper were implanted in a tungsten {100} single crystal with energies of 5–20 keV and doses up to 5×1013 ions/cm2. Subsequently the crystal was annealed at temperatures ranging from RT to 2400 K, and Thermal Helium Desorption Spectrometry (THDS) was used to monitor the dissociation and clustering reactions of the formed defect-complexes. Evidence was found that at doses of 1011–1012 ions/cm2 the implants are surrounded by a few vacancies, which dissociate at temperatures between 1000 and 1400 K, leaving silver or copper in substitutional positions. Binding of helium to those substitutional defects was observed. It is found that at higher Ag doses (∼ 1013 ions/cm2) other defect complexes are formed which apparently bind He stronger. We attribute this to He binding to a complex of two or three Ag atoms in a divacancy or trivacancy. A similar effect was found earlier for krypton clustering in tungsten. In the case of copper under similar conditions no stronger He binding was observed, indicating that clustering of copper did not take place. Depth dependent trapping calculations of He to defects will also be presented to interpret the experimental results quantitatively.

Clustering phenomena of implants in tungsten observed with THDS

Abstract A W(100) single crystal was irradiated with 5–10 keV of different metallic species (Ag, Cu, Cr, Mn, Al and In). Subsequently the crystal was annealed at temperatures ranging from room temperature to 2400 K. Thermal helium desorption spectrometry (THDS) was applied to monitor the dissociation and clustering reactions of defect complexes either formed at room temperature during implantation or during partial annealing at a higher temperature. Strong evidence is found for clustering of Ag if implantation doses of Ag exceed 6 × 1012 Ag+ cm−2. For the implants Al, Cr, Cu and Mn, substantial clustering was not detected. For In, a conclusion could not be made concerning clustering since He does not bind to In in W at room temperature. Atomistic calculations are presented which indicate that clustering of the implants other than Ag would certainly be detected if the impurity atoms in the cluster all occupy lattice positions. Therefore we believe that thermally activated radiation enhanced diffusion is much more efficient for Ag than for Cu and Mn in W. For Cr and Al we cannot exclude enhanced diffusion, since the binding energies of Cr and Al to Cr- and Al-clusters in W, respectively, will be rather small. Of the implants Cu, Mn and Ag, the latter is the only one being oversized. A rather strong binding of vacancies to Ag is envisaged, which enables transport of the Ag atom, being part of a moving vacancy cluster.

Cascade Annealing of Tungsten Implanted with 5 keV Noble Gas Atoms : A Computer Simulation

The trapping of vacancies by implanted atoms is calculated. After low energy implantation (5 keV) of tungsten with heavy noble gas atoms most of the implanted atoms are in substitutional position with one or two vacancies closer than two lattice units. Under the influence of the lattice distortion around the implanted atoms the vacancies follow a preferential migration path towards the implant during annealing. With lattice relaxation simulations migration energies close to the implanted atom are calculated. Monte Carlo theory is applied to obtain trapping probabilities as a function of implant-vacancy separation and temperature. An estimate of the initial implant-vacancy separation follows from collision cascade calculations. The results show that nearby vacancies are trapped by the implanted atoms.

Effects of Vacancies near Substitutional Implants on Trapping and Desorption of Helium - A Simulation

Trapping of He by vacancies and drainage of He from substitutional implants (Ag and Kr in W) to nearby vacancies are investigated using static lattice calculations. The calculations indicate that drainage of He will occur to vacancies within a radius of 2.5 lattice units from the implant. Furthermore the trapping probability of substitutional and interstitial random walkers on a bcc lattice by substitutional traps or vacancies is calculated. When implantation-produced vacancies are present in the vicinity of the observed trap a shielding effect occurs. Trapping constants are calculated with two random walk models for both the unshielded and the shielded defect. For the latter several configurations were taken. The results show that shielding of a defect by one vacancy at a distance of three lattice units leads already to a reduction of He trapping by that defect of 30% to 40%.