Centre for Mechanical Enginnering, Materials and Processes

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Ceramic-reinforced HEA matrix composites exhibiting an excellent combination of mechanical properties

Publication . Mehmood, M. Adil; Shehzad, Khurram; Mujahid, M.; Yaqub, Talha Bin; Godfrey, Andy; Fernandes, Filipe; Muhammad, F. Z.; Yaqoob, Khurram

CoCrFeNi is a well-studied face centered cubic (fcc) high entropy alloy (HEA) that exhibits excellent ductility but only limited strength. The present study focusses on improving the strength-ductility balance of this HEA by addition of varying amounts of SiC using an arc melting route. Chromium present in the base HEA is found to result in decomposition of SiC during melting. Consequently, interaction of free carbon with chromium results in the in-situ formation of chromium carbide, while free silicon remains in solution in the base HEA and/or interacts with the constituent elements of the base HEA to form silicides. The changes in microstructural phases with increasing amount of SiC are found to follow the sequence: fcc → fcc + eutectic → fcc + chromium carbide platelets → fcc + chromium carbide platelets + silicides → fcc + chromium carbide platelets + silicides + graphite globules/flakes. In comparison to both conventional and high entropy alloys, the resulting composites were found to exhibit a very wide range of mechanical properties (yield strength from 277 MPa with more than 60% elongation to 2522 MPa with 6% elongation). Some of the developed high entropy composites showed an outstanding combination of mechanical properties (yield strength 1200 MPa with 37% elongation) and occupied previously unattainable regions in a yield strength versus elongation map. In addition to their significant elongation, the hardness and yield strength of the HEA composites are found to lie in the same range as those of bulk metallic glasses. It is therefore believed that development of high entropy composites can help in obtaining outstanding combinations of mechanical properties for advanced structural applications.

2022Journal article

Open access

Tribological properties of TiSiVN coatings: The effect of the counterpart on the wear performance

Publication . Fernandes, Filipe; AL-Rjoub, A; Yaqub, TB; Ferreira, F; Serra, R; Cavaleiro, A

Self-lubricant coatings are a subject of continual research and improvements. In this research, the tribological performance of TiSiVN coatings produced by increasing V concentration is evaluated. The coatings were deposited by high-power impulse magnetron sputtering in deep oscillation mode, and were tested against three different counterparts (Al2O3, stainless steel and Titanium). The wear mechanisms were identified and the tribolayer was characterized to correlate with different specific wear rates and friction coefficient values. The coatings tested against Al2O3 balls displays a polishing type wear mechanism with some material adhered to the film wear track surface. When V is added to the reference film, V-O formed on the wear track has a beneficial influence by decreasing the friction and the wear of the coatings. For stainless steel balls, the wear track displays a mixture of severe abrasion and adhesion wear. Enclosure of hard particles at the softer stainless steel ball is the cause for the severe abrasion of the coating surface. This harsh abrasion removes the V-O from the contact and consequently decreases the efficiency of reducing friction coefficient and wear when compared to the tests against Al2O3 balls. For tests conducted against Ti ball, a strong adhesion of the material to the counterpart is observed. The adhered material protects the coatings from wear by giving rise to similar friction coefficient values irrespective of the coatings’ chemical composition.

2022Journal article

Restricted access

Vacuum Tribological Properties of W-S-N Coatings Synthesized by Direct Current Magnetron Sputtering

Publication . Yaqub, Talha Bin; Yaqoob, Khuram; Mukhtar, Amir; Fernandes, Filipe; Bondarev, Andrey; Ferreira, Fabio; Al-Rjoub, Abbas; Cavaleiro, Albano

This work deals with the investigation of the tribological performance of DC magnetron sputteredW-S-N coatings under vacuum atmosphere, as part of the exploration of multi-environment sliding properties of W-S-N solid lubricants. This study is part of the systematic testing of W-S-N solid lubricants in different environments, especially vacuum, which is often ignored. The trend is to test sliding properties in dry N2 by considering it as replacement of vacuum environment testing. This approach is not appropriate. In this work, a set of coatings was synthesized with N-alloying content in the range of 0–25.5 at.%. A maximum S/W ratio of 1.47 was observed for the pure WSx coating. A maximum hardness of 8.0 GPa was observed for 23 at.% of N-alloying. The coating with the lowest N content (14.6 at.%) displayed the lowest friction, specific wear rate and wear scar depth under vacuum conditions. Despite superior sliding performance at room temperature (35% humidity), 200 C and dry nitrogen conditions, the performance of the WSN12.5 coating deteriorated vacuum environment.

2022Journal article

Open access

Multilayer Mo2N-Ag/SiNx films for demanding applications: Morphology, structure and temperature-cycling tribological properties

Publication . Ju, Hongbo; Zhou, Rui; Luan, Jing; Yu, Lihua; Xu, Junhua; Zuo, Bin; Yang, Junfeng; Geng, Yaoxiang; Zhao, Lijun; Fernandes, Filipe

Nowadays there is the need to avoid the excessive consumption of liquid lubricant oils, as they are harmful to the environment and hard to disposal. Self-lubricant films have been seen as the sustainable solution to achieve a long-term lubrication under high temperature-cycling conditions. In this manuscript, multilayer Mo2N-Ag/SiNx films with a fixed modulation ratio (thickness of Mo2N-Ag to SiNx) of 3:1, with changing modulation period (Λ, thickness of Mo2N-Ag and SiNx) from 8 to 200 nm were produced to achieve an effective balance between the lubricious phase diffusion control and the adequate formation of the low friction tribo-layers. Results showed that a dual-phase of fcc-Mo2N and fcc-Ag co-existed in Mo2N-Ag layers, while the SiNx layer exhibited an amorphous character. Both room temperature (RT) lubricant and wear-resistance properties of the films were improved by increasing Λ from 8 to 64 nm, while a further increase of Λ degraded the wear-resistance properties. The multilayer film at Λ = 64 nm exhibited an excellent RT-500 °C temperature-cycling tribological properties. Mechanical properties and the synergistic effect of both modulation layers were the cause for the improvement of the tribological properties.

2022Journal article

Open access

Effect of Annealing Heat Treatment on the Composition, Morphology, Structure and Mechanical Properties of the W-S-N Coatings

Publication . Yaqub, Talha Bin; Al-Rjoub, Abbas; Khalid, Hafiza Ayesha; Yaqoob, Khurram; Fernandes, Filipe; Cavaleiro, Albano

Alloyed-transition metal dichalcogenide (TMD) coatings have been under investigation as multi-environment lubricants for the past few decades. These coatings display very low coefficient of friction properties at elevated temperatures. Studies on the annealing of these low-friction coatings are missing in the literature. For the first time, in this study, the annealing of the W-S-N dry lubricant coatings was carried out to study its effects on the composition, morphology, crystal structure and hardness of the coatings. The W-S-N coatings were deposited by direct current (DC) reactive magnetron sputtering. The analysis was carried out for as-deposited, 200 °C and 400 °C annealed coatings. The as-deposited coatings have N content in the range of 0–25.5 at. %. The coatings are compact and the densification increased with the increase in N-alloying. All the coatings are crystalline except the highest N-alloyed coating which is X-ray amorphous. A maximum hardness of 8.0 GPa was measured for the coating alloyed with 23 at. % N. Annealing did not affect the composition and morphology of the coatings, while some variations were observed in their crystal structure and hardness. The maximum hardness increased from 8 GPa to 9.2 GPa after 400 °C annealing of the 23 at. % N-alloyed coating.

2022Journal article

Open access