ICMCTF 2023 Session B4-2-MoA: Properties and Characterization of Hard Coatings and Surfaces II

For long distance land and overseas transport of large quantities natural gas is usually liquefied. Methane (CH₄), the main component of natural gas, has a boiling temperature of -161,5°C, which is much lower than the pour point of any liquid lubricant. Therefore, only dry running tribosystems can be employed in this temperature range, which is a challenge for frictionally stressed mechanical components.

Also for long distance transport of hydrogen liquefaction is an option. With a temperature of -253°C for liquefied hydrogen (LH₂) the conditions for tribosystems are even more severe. Furthermore, also for the utilization in gaseous form, in many cases purity requirements impede lubrication by oils or greases. Another requirement is that technical systems containing natural gas or hydrogen need to be inertized from time to time. Therefore, the function of the moving parts must be ensured not only in the reactive media, but also in inert environment.

It is well known that among the solid lubricants graphite and most amorphous carbon coatings show good lubricity in humid air and hydrogen environment, but the question arises if this is also true for methane and very low temperatures.

For identifying suitable carbon-based solid lubricant coatings, ball-on-disk model tests with several variants of a-C:H and ta-C were carried out in gaseous hydrogen, methane and nitrogen. Some coatings were also tested in liquid methane. The results show friction coefficients below 0.1, with lowest values of about 0.01 in CH₄ at room temperature. Also wear coefficients in the order of 10^-8 mm³N^-1m^-1 for most coatings indicate that amorphous carbon coatings are suitable for frictionally stressed components in hydrogen and natural gas environment.

Thermochemical treatments are used for increasing the surface mechanical properties of iron and steels. The wear on mechanical components caused by sliding contact is an important parameter in the field of tribology. In this study, the wear resistance of ARMCO pure iron hardened by the powder-pack boriding process was evaluated, using unidirectional sliding. The boriding treatment was carried out at temperatures of 1123 K for 1, 2 and 3 h of exposure time. Surface properties such as hardness and Young’s modulus were obtained by the nanoindentation technique. Fracture toughness of the three layer/substrate systems were obtained by Vickers indentation.The wear test was performed by multi-pass scratch using an Al₂O₃ ball (6 mm diameter) as a counterpart under normal loads of 20 N. The sliding distances were 125, 250, 375 and 500 mm (25, 50, 75 and 100 passes). Archard´s model was used to obtain the wear coefficient. Finite element method applying mesh nonlinear adaptivity was used to evaluate the surface wear on the borided samples. It was found that wear resistance was mainly influenced by both the fracture toughness value and thickness of the boride layers.

This study focused on microstructure evolution and mechanical behavior of binary refractory metal nitride systems films, including (MoHf)N, (WHf)N and (MoW)N. The phase, adhesion, and wear behavior variations were discussed in terms of material selection. With a radio frequency, RF, magnetron dual gun co-sputtering system, the binary refractory metal nitride thin films were fabricated at a fixed Ar/N₂ inlet gas ratio of 12/8 sccm/sccm. The structure of (MoHf)N thin film exhibited MoN, Mo₂N, and MoN₂ phases, while the (WHf)N thin film possessed WN and Hf₄N₃ phases. As for the (MoW)N thin film multiple phases including WN, MoN, and Mo₂N were observed. The (MoW)N and (WHf)N possessed adhesion level of HF3 and HF4, respectively, indicating a poor adhesion due to the exist of tungsten element. On the other hand, (MoHf)N thin film showed an index of HF2. The wear results reflected a similar trend, For the (MoHf)N coating, the track maintained a smooth surface and the film kept intact after a wear length of 100m. On the contrary, the wear tracks of (MoW)N and (WHf)N coatings showed cracking and pealing after the wear test, indicated a weaker tribological behavior.

Keywords: Microstructure; Refractory metal nitride; Multiple phase; Adhesion; Wear.

Boron carbide (B4C) is a material with outstanding mechanical properties, chemical inertness, and low density. It is currently the leading candidate for use as the next generation amorphous ablator for inertial confinement fusion (ICF). Several approaches to deposit B4C ablators have been explored with promising recent results. However, the doping B4C for ICF applications has not been investigated. In this work, we investigate the feasibility of incorporating high concentrations of Si, Ge, and W dopants (i.e., hyper-doping) via combinatorial magnetron sputter deposition of ultrathick B4C films. We focus on the influence of the dopant type and concentration on film growth mode and major film properties of relevance to ICF, including residual stress, crystallinity, density and its uniformity, and chemical stability.

High-entropy alloy nitride coatings (HEANs) characterize superior mechanical strength, high oxidation resistance, thermal stability, corrosion resistance and wear resistance. As a result, it is a potential candidate in the protective hard coating field in recent years.

In this study, the (AlCrNbSiTi)N coatings were co-sputtered by radio-frequency magnetron sputtering. The power for the silicon target was varied from 0 watts to 120 watts and the silicon concentration of coatings varied from 0 (at. %) to 10.6 (at. %). As the silicon content in coatings increased to 4.4 (at. %), the coatings exhibited the maximum hardness of 31 (GPa) due to the mechanism of dislocation pile-up. However, as the silicon content in coatings exceeded 4.4 (at. %), the hardness of coatings started to decrease, which was attributed to the appreciable amounts of softer amorphous segregation. Furthermore, the incorporation of silicon interrupted the growth of the column structure due to the spinodal decomposition of amorphous SiN_x and (AlCrNbTi)N. Thus coating structure changes from a loose column structure to a dense featureless structure.

Last but not the least, the coating density was calculated from the data measured by XRR analysis. As the silicon content in the coating exceeded 4.4 (at. %), the lack of long-range ordering and free volume in amorphous structure lowered the packing density of the coatings, which led to the poor mechanical properties of the coatings. Nevertheless, the results exhibited that the coating characterized the highest packing density as the silicon content in coatings is 4.4 (at. %), exhibiting the optimal mechanical properties, which is strongly correlated to the crystallinity of the coatings.