Thomson scattering using a Bragg grating notch filter is used to determine the electron properties of a pulsed, kHz-driven, non-thermal atmospheric pressure plasma jet in helium expanding in air. The plasma jet is allowed to freely expand or interact with targets with different electrical properties, i.e. glass, copper and water. With the same setup, Raman scattering is used to determine spatially- and time-resolved the densities and rotational temperatures of oxygen and nitrogen molecules entrained into the jet. Fast imaging is used to determine the development of the discharge in the plasma jet as well as its behavior in the plasma-target interaction zone. As the discharge approaches the target, the rise of electron density was followed by the fall of electron temperature. The discharge is influenced only over a few millimeters before it hits the target. The electron density and temperature during the spreading of the discharge on the low-permittivity target are measured to be resp. 2 × 1019 m−3 and ≈1 eV. During the return stroke on the high-permittivity and the metallic target the densities rise with a factor 1.5 resp. 2.2, and the temperature with a factor 2.5 for both cases. The discharges on the high- and low-permittivity targets extinguished soon after the initial impact of the ionization front, while the diffuse discharge on the metallic target extinguished only after the end of the voltage pulse (with a duration of 1 μs). In the diffuse discharge the electron temperature reaches 3.4 eV, the gas temperature increases by approximately 100 K and the electron density increases by approximately a factor three with respect to before its formation.

https://iopscience.iop.org/article/10.1088/1361-6595/aad4d7/pdf

How dielectric, metallic and liquid targets influence the evolution of electron properties in a pulsed He jet measured by Thomson and Raman scattering

A Stellite alloy mixture hardfacing consisting of 70% Stellite 3 and 30% Stellite 21, is created via laser cladding for control valve seat sealing surfaces, aiming at enhancing hardness and wear resistance compared with Stellite 6, and improving cracking in laser cladding compared with Stellite 3. The Stellite alloy mixture hardfacing is made on 316 stainless steel substrate and does not show any cracking in liquid penetrant testing. The microstructure of the hardfacing is analyzed using SEM, EDS and XRD. The hardness, dry sliding wear resistance, cavitation-erosion resistance in NaOH solution and corrosion resistance in morpholine solution at pH 9.5 to simulate the amine environment of boiler feedwater service in power generation plants, are evaluated. The Stellite 6 hardfacing prepared with the same laser process parameters is also analyzed and tested under the same conditions for comparison. The experimental results and real industrial test demonstrate superior performance of the Stellite alloy mixture hardfacing to the Stellite 6 hardfacing for control valve seat sealing application.

Stellite alloy mixture hardfacing via laser cladding for control valve seat sealing surfaces

Low-carbon Stellite alloys such as Stellite 21 exhibit good high-temperature and corrosion properties but need improved wear resistance in some applications. In this research, two low-carbon Stellite alloys with highly increased molybdenum content are produced via laser cladding. The microstructures of the laser cladding hardfacings are studied using scanning electron microscopy (SEM) with an energy dispersive X-ray (EDS) spectroscopy, and X-ray diffraction. The wear resistance of the hardfacings is evaluated using a pin-on-disc tribometer. The corrosion performance of the hardfacings is investigated under electrochemical tests in 3.5 wt.% sodium chloride (NaCl) solution and in Green Death solution. The experimental results show that the presence of Mo-rich intermetallic compounds enhances the wear resistance of the alloy hardfacings significantly. Since Stellite alloys are all able to form protective oxide films due to high chromium content against corrosion of the substrates, the bonding strength and repair ability of the oxide films dominate the corrosion resistance of the hardfacings in the corrosive environments. Stellite 21 hardfacing is also studied under the same testing conditions for comparison.

Wear and corrosion performance of laser-clad low-carbon high-molybdenum Stellite alloys

Effects of DLC/TiAlN-coated die on friction and wear in sheet-metal forming under dry and oil-lubricated conditions: Experimental and numerical studies

The results of a high-resolution Raman scattering study of a diamond crystal with a high content of single substitutional nitrogen impurities (550 ppm) in the temperature range from 50 to 673 K are presented and compared with the data for defect-free diamond. It is established that the increase of the nitrogen concentration in diamond leads to the temperature-independent increase of the Raman line width. Analysis of the experimental data allows us to conclude that this broadening should be attributed to the defect-induced shortening of the Raman phonon lifetime. We believe that this mechanism is responsible for the increase of the Raman line width caused by most point-like defects in diamond. No pronounced effects of the nitrogen defects on the Raman line position and phonon anharmonicity are observed.

https://www.mdpi.com/2073-4352/7/8/239/pdf

Effect of Nitrogen Impurities on the Raman Line Width in Diamond, Revisited

Lubricants are commonly used in metal forming processes to reduce the friction between the workpiece and the forming tool, to protect semi-finished products and goods against corrosion and to reduce the load on the tool. One aim of environmentally friendly production technologies is to achieve dry forming without the use of lubricants. An oscillating ball-on-plate tribometer (load: 10 N, frequency: 2.5 Hz, ball diameter: 10 mm, 99,900 cycles) was used to compare the suitability of different carbon coatings for dry contact against the aluminium alloy EN AW 5083. As candidates a graded and multilayered hydrogenated amorphous carbon coating (a-C:H) system by physical vapour deposition (PVD) as well as a polycrystalline diamond coating by chemical vapour deposition (CVD) were studied. The CVD diamond coatings were tested in two surface conditions, a rough as-deposited (roughness Sa 1.19 μm) and a subsequent polished (Sa 0.01 μm) variant. An uncoated, hardened steel plate of ledeburitic cold working steel (X153CrMoV12-1) was used as reference material. Worn surfaces were examined using laser scanning and atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine their wear rates and to characterize the degree of adhesion, the change in nano-topography and the type and quantitative change of chemical bindings. The tests showed that polishing of CVD-diamond coatings is mandatory for the use in regards to dry forming. Sliding against the hydrogenated amorphous carbon (a-C:H) coating and polished polycrystalline CVD-diamond coating (pCVDD) resulted in the lowest sliding friction coefficients (COF) of 0.13 respectively 0.12. The polished polycrystalline diamond coating showed the lowest wear rates at the round-ended aluminium pin of 4 · 10−9 mm3/Nm and at the coating of 1 · 10−8 mm3/Nm. Cycle-dependent friction coefficient transitions were exhibited by the a-C:H-coating. For the pCVDD-coating as well as for the a-C:H-coating the formation of a transfer film was detected, which explains the small COFs. Due to the nearly negligible wear rate the pCVDD-coating is the most promising one of the tested carbon coatings to enable dry aluminium forming with a long lifetime of the coated tool.

Friction and wear performance of different carbon coatings for use in dry aluminium forming processes

Picosecond-laser polishing of CVD-diamond coatings without graphite formation

The aim of this paper was to characterize different classes of materials, light metals, steels, cobalt–chromium-based and ceramic materials under tribological stress and to determine the reproducibility of the measurement results by monitoring the production, processing and analysis of the test materials. The results were used to ensure pre-selection of materials for use in the dry forming process. To depict real conditions at the edges of a forming process surface pressures of 10–200 N/mm2 need to be adjusted. For this purpose, the friction and wear properties were determined in a dry ball-on-disc test. The functional surfaces were produced by laser-cladding, selective laser melting, chemical vapor deposition (CVD) and laser hardening. The tested and selected classes of materials were steel (42CrMo4, X110CrMoVAl 8-2), light metals (aluminum 6082, C95300 aluminum-bronze), cobalt–chromium alloys (Stellite 12, Stellite 21), ceramic (S3N4) and polycrystalline CVD diamond. The test setup of the dry ball-on-disc test included 10 N contact force, 10 mm traversing distance, 10 mm/s average speed, 40 % humidity and 24 °C ambient temperature. The resulting wear map includes five orders of magnitude of different wear rates and one order of magnitude of different coefficient of friction. An eligibility number is introduced for a better differentiation of the combined properties of the tribosystem of a tool material.

M. Prieske

Papers

Friction and wear performance of different carbon coatings for use in dry aluminium forming processes

Picosecond-laser polishing of CVD-diamond coatings without graphite formation

Pre-selection of laser-processed materials for dry forming tools by means of a dry oscillating ball-on-disc test

In situ incorporation of silicon into a CVD diamond layer deposited under atmospheric conditions

Comparison of analytical methods to determine the electron density and temperature for a laser-based atmospheric plasma jet