Research Articles

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Corrosion, oxidation and high-temperature tribological properties of Ti–B–N coatings
(Surface Engineering, 2019) Asempah, I.; Yu, L.; Ju, H.; Yu, D.; Xu, J.; Miao, R.
Ti–B–N (nc-TiN/ a-BN) composite coatings with various boron contents were deposited by magnetron sputtering. nano-indentation, ball-on-disc dry sliding X-ray diffraction, and, Raman spectroscopy was used in evaluating the compositional, mechanical and tribological properties of these coatings. The corrosion behaviour of the Ti–B–N coatings was assessed by the electrochemical potentiodynamic polarisation method in a 3 mol-% of NaCl solution. The coefficient of friction (CoF) of the coatings initially increased to 1.0 when the test temperature was 200°C, but a further increase in temperature to 700°C resulted in a decrease in the CoF value to ∼0.18 which was due to the presence of Magneli-type TinO2n−1 oxides. In contrast to the CoF, there was a consistent increase in wear rate when the temperature was increased; the highest wear rate was obtained at 700°C. The B incorporated coatings exhibited improvement in both oxidation and corrosion resistance properties.
Crystal structure and high temperature tribological behavior of niobium aluminum nitride films.
(Materialia, 2018) Ju, H.; Jia, P.; Xu, J.; Yu, L.; Asempah, I.; Geng, Y.
Niobium aluminum nitride (Nbsingle bondAlsingle bondN) films with various aluminum content were deposited using a reactive magnetron system and the film with 22.3 at% aluminum exhibiting the highest hardness, lowest wear rate and friction coefficient at room temperature was chosen to investigate the tribological properties at elevated testing temperatures. The results showed that from room temperature to 200 °C, the absence of oxide-based tribo-phases led to the reduction in wear rate, but friction coefficient remained stable. As the testing temperature increased from 200–500 °C, there was reemergence of γ-alumina tribo-phase, this resulted in intensified interaction between the wear track and the counterpart leading to an increase in both the wear rate and friction coefficient. A further increase in testing temperature resulted in a phase transition from a porous γ-alumina to a dense α-alumina. This transition aided in the reconstruction of the worn area in the wear track and the improvement in the friction and wear resistance properties.
Crystal structure and the improvement of the mechanical and tribological properties of tungsten nitride films by addition of titanium.
(Surface and Coatings Technology, 2018) Ju, H.; Ding, N.; Xu, J.; Yu, L.; Asempah, I.; Xu, J.; Ma, B.
Tungsten titanium nitride films were synthesized by the reactive magnetron sputtering system and the influence of titanium content on the crystal structure, mechanical and tribological properties of the films were investigated. The results showed that the tungsten titanium nitride films at<6.7 at.% titanium were substitution solid solution of (W1−xTix)2−yNy and exhibited a single face-centered cubic (fcc) W2N structure, while further increasing in titanium content induced the appearance of other fcc-TiN phase and the films was consisted of (W1−xTix)2−yNy and (Ti1−xWx)1−yNy. Both the sub-stoichiometric nitrogen content and solid solution strengthening led to the hardness increase from ~26 GPa at 0 at.% titanium to ~39 GPa at 6.7 at.% titanium, while the fcc-TiN phase dropped the hardness with a further increase in the titanium content. The incorporation of titanium below 12.3 at.% dropped the room temperature wear rate significantly. This could be attributed to the increase of hardness to elastic modulus ratio, elastic recovery and hardness. Tungsten titanium nitride film at 12.3 at.% titanium was chosen to investigate the tribological properties at elevated temperatures, and it exhibited the higher working temperature than the binary W2N film.
Effect of boron concentration on the mechanical, tribological and corrosion properties of Ta–B–N films by reactive magnetron sputtering
(Ceramics International, 2019) Asempah, I.; Xu, J.; Yu, L.; Wang, L.
A practical design approach in improving the mechanical and corrosion characteristics of nitride-based films is by the synthesis of nanocomposites, and we have fabricated tantalum boron nitride (Ta–B–N) nanocomposite films by a reactive magnetron sputtering process. The deposited Ta–B–N coatings were found to exhibit a nanocomposite structure consisting of nanocrystalline TaN and amorphous BN phase coexisting. The incorporation of boron into the films modified the microstructure from columnar to fine grain with a face-centred cubic (fcc) TaN (200) preferred orientation. However, the hardness increased to a maximum value of ∼33.3 GPa when the boron concentration was 1.5 at.%. The increase in hardness was due to both grain refinement and the increase in compressive residual stress. More so, the wear rate of the film initially improved upon the addition of boron but later deteriorated when the boron concentration was further increased; however, the coefficient of friction values decreased monotonically when the content of boron was increased. The initial improvement in the wear performance was due to the increased hardness. Furthermore, potentiodynamic polarisation analysis shows that the film with the highest boron content had the best corrosion resistance properties. This improvement is as a result of the evolution of the film's microstructure by the introduction of boron.
Effect of Different Ni Contents on Thermal Stability of Cu(Ni) Alloy Film
(Springer, 2020) Li, X.; Cheng, B.; Asempah, I.; Shi, Q.; Long, A.-Q.; Zhu, Y.-L.; Wang, Q.; Li, Y.-L.; Wang, L.; Jin, L.
The effect of doping different contents of Ni on the thermal stability of Cu(Ni) alloy films has been investigated. Cu(Ni) films with different Ni contents were deposited on SiO2/Si substrates by magnetron sputtering, then annealed in vacuum at 350°C to 650°C for 0.5 h. X-ray diffraction analysis and resistance measurements revealed that high-resistance copper silicide was formed after annealing at 450°C for the Cu(Ni, 1.66 at.%) and Cu(Ni, 9.16 at.%) samples. However, no copper silicide was observed for Cu(Ni, 3.59 at.%) even after annealing at 650°C. Transmission electron microscopy provided evidence for a ∼ 25-nm self-formed barrier layer at the Cu/SiO2 interface with Cu(Ni, 3.59 at.%). The failure to form a diffusion barrier for the Cu(Ni, 1.66 at.%) sample resulted from its low Ni doping concentration, which was insufficient to produce such a self-formed layer during annealing. The barrier failure was caused by grain refinement due to the increased Ni content, providing diffusion channels for atom diffusion. The results clearly suggest that addition of an appropriate amount of Ni can improve the thermal stability of Cu(Ni)/SiO2/Si interconnect structure materials.
The enhancement of fracture toughness and tribological properties of the titanium nitride films by doping yttrium.
(Surface and Coatings Technology, 2017) Ju, H.; Yu, L.; He, S.; Asempah, I.; Xu, J.; Hou, Y.
In this paper, yttrium was incorporated into titanium nitride matrix to deposit Tisingle bondYsingle bondOsingle bondN films using a reactive magnetron sputtering system and the microstructure, mechanical and tribological properties were investigated. The results showed that solid solution face centered cubic (fcc) Tisingle bondYsingle bondN was the major phase of the film when the yttrium content was < 7.8 at.%. When the yttrium content was further increased, the film exhibited mixed phases of fcc-Tisingle bondYsingle bondN and amorphous yttrium oxide, this induced grain refinement and appearance of lots of grain boundaries. Hardness of the films first rose from 21 GPa at 0 at.% Y to 26 GPa at 10.2 at.% Y, and remained stable as the yttrium content was above 10.2 at.%. The hardness was influenced by the amorphous yttrium oxide phase, solid solution strengthening, grain boundaries and residual stress. Between 0 at.% to 7.8 at.% of yttrium, the fracture toughness (KIC) rose slowly, KIC rose sharply to 0.85 MPa.m1/2 as the yttrium content was 10.2 at.% due to the formation of amorphous yttrium oxide phase, it rose slowly again with a further increase in yttrium content. For the film at < 7.8 at.% Y, average friction coefficient (μ) and wear rate were influenced by the load significantly and there was an obvious rise in their values with an increase in load. A further increase in yttrium content in the films, improved the tribological properties, μ and wear rate were almost independent on the load. Fracture toughness enhancement attributed to the improved load capacity and tribological properties.
The improvement of oxidation resistance, mechanical and tribological properties of W2N films by doping silicon
(Surface and Coatings Technology, 2017) Ju, H.; He, S.; Yu, L.; Asempah, I.; Xu, J.
W-Si-N composite films were deposited on both stainless steel and silicon wafers using a reactive magnetron system. The silicon content of the film was varied to evaluate the effect of silicon on films' elemental composition, microstructure, oxidation resistance, mechanical and tribological properties. These series of test were conducted using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) analytical tools. The W-Si-N films showed a two-phase structure of solid solution face-centered cubic (fcc) W(Si)Nx and an amorphous Si3N4. The thermal stability and the oxidation resistant temperature of W-Si-N films increased from about 380 °C at 0 at% Si to about 650 °C at 31.2 at% Si with the incorporation of Si into W2N matrix. The hardness of the films increased continuously from 26 GPa at 0 at% Si to 40 GPa at 23.5 at% Si which was due to solid strengthening and fine-grained strengthening. The silicon content affected the tribological properties of the film at room temperature but this was mainly influenced by the yield pressure, H/E ratio and elastic recovery, since lubricant tribo-film WO3 was detected on all surfaces of the W-Si-N films. There was an initial gradual decrease in average coefficient of friction and wear rate until a minimum, but a further increase in silicon content resulted in increase in both friction coefficient and wear rates. The least coefficient of friction and wear rate values were 0.30 and 8.7 × 10− 8 mm3/N·mm at 23.5 at% Si.
Indentation size effect in aqueous electrophoretic deposition zirconia dental ceramic.
(Journal of Materials Research,, 2019) Asempah, I.; Xu, L.; Zang, S. Q.; Yan-Fei, Z.; Ding, J.; Jin, L.
Highly dense zirconia dental ceramic coatings were fabricated by aqueous electrophoretic deposition (EPD) and subsequently sintered between 1250 and 1450 °C. Microstructural examination revealed that aqueous EPDZrO2 coatings possessed a tetragonal phase structure and the grain size increased with increasing sintering temperature. Nanoindentation study proved that the aqueous EPDZrO2 coating also had excellent mechanical properties. The effect of different applied loads on hardness and elastic modulus of the 1350 °C-sintered sample at room temperature was investigated by the method of progressive multicycle measurement nanoindentation. The simulative experiment proved that hardness of aqueous EPDZrO2 exhibited reverse indentation size effect (ISE) behavior and then displayed the normal ISE response. The analysis indicates that the reverse ISE is attributed to the relaxation of surface stresses resulting from indentation cracks at small loads and normal ISE is caused by geometrically necessary dislocations. The tetragonal—monoclinic stress-induced phase transformation during nanoindentation is the primary cause of dental zirconia failures.
Influence of Ag content on microstructure, mechanical and tribological properties of WNbN-Ag composite films.
(Protection of Metals and Physical Chemistry of Surfaces, 2018) Yu, L.; Lin, Y.; Ju, H.; Geng, Y.; Asempah, I.; Xu, J.
A series of TaVN–Ag nanocomposite films were deposited using a radio-frequency magnetron sputtering system. The microstructure, mechanical properties, and tribological performance of the films were investigated. The results showed that TaVN–Ag films were composed of face-centered cubic (fcc) TaVN and fcc-Ag. With increasing Ag content, the hardness of TaVN–Ag composite films first increased and then decreased rapidly. The maximum hardness value was 31.4 GPa. At room temperature, the coefficient of friction (COF) of TaVN–Ag films decreased from 0.76 to 0.60 with increasing Ag content from 0 to 7.93at%. For the TaVN–Ag films with 7.93at% Ag, COF first increased and then decreased rapidly from 0.60 at 25°C to 0.35 at 600°C, whereas the wear rate of the film increased continuously from 3.91 × 10−7 to 19.1 × 10−7 mm3/(N·mm). The COF of the TaVN–Ag film with 7.93at% Ag was lower than that of the TaVN film, and their wear rates showed opposite trends with increasing temperature.
Influence of V content on properties of Ti–W–V–N films
(Surface Engineering, 2019) He, S.; Yu, L.; Ju, H.; Geng, Y.; Asempah, I.; Xu, J.
Ti–W–V–N composite films with different vanadium contents were synthesised by multi-target reactive magnetron sputtering. The microstructure, mechanical and tribological properties of Ti–W–V–N films were investigated by X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy, nanoindentation and high-temperature ball-on-disc tribometer. The results showed that all the Ti–W–V–N composite films were face-centred cubic (fcc) structure, consisting of fcc-(Ti,W,V)N substitutional solid solution. When the V content ≥21.5 at.-%, the films consisted of fcc-(Ti,W,V)N and fcc-VN. The hardness of the films first increased and then decreased with the maximum value of 32.4 GPa, at 21.5 at.-% V. The test temperature had a significant effect on the tribological properties of Ti–W–V–N film with 21.5 at.-% V, the coefficient of friction (CoF) first increased and then decreased, with a minimum value of 0.304 at 700°C.While the wear rate increased gradually with increasing temperature.
Microstructure, mechanical and tribological properties of magnetron sputtered Ti-BN films.
(Surface Engineering, 2019) Asempah, I.; Xu, J.; Yu, L.; Ju, H.; Wu, F.; Luo, H.
Titanium boron nitride (Ti-B-N) films with various boron contents were deposited using titanium and boron targets in a reactive magnetron sputtering system. The boron content in the film was varied from 0 at.-% to 8.7 at.-%, to evaluate its properties. Various analytical techniques such as X-ray diffraction, high-resolution electron microscopy, nanoindentation and ball-on-disk dry sliding etc. were used. Incorporation of B into the film influenced the microstructure, mechanical and room-temperature tribological properties. At a B content of 0.2 at.-%, the film exhibited the highest hardness of ∼27 GPa. It also presented the lowest wear rate of ∼2.9 × 10−7 mm3 N−1 mm−1. However, there was a gradual decrease in coefficient of friction (CoF) values of the film when the B content was increased, with a minimum of 0.2 attained at 8.7 at.-% boron. The increased volume fraction of amorphous boron nitride phase contributed to the decrease in coefficient of friction.
Microstructure, mechanical and tribological properties of TiN-Ag films deposited by reactive magnetron sputtering
(Vacuum, 2017) Ju, H.; Yu, L.; Yu, D.; Asempah, I.; Xu, J.
Composite TiN-Ag films with various Ag content (Ag/(Ti + Ag)) were deposited using reactive magnetron sputtering and the influence of Ag content on the crystal structure, mechanical and tribological properties were investigated. The result showed that face-centered cubic (fcc) TiN and fcc-Ag co-existed in the films and TiN had a columnar-growth and the Ag nanoparticles embedded in the boundary of columnar crystal. The hardness of TiN-Ag films initially increased gradually and reached an optimum value, and then decreased with an increase in Ag content in the films. The maximum hardness value was 29 GPa at 0.8 at.% Ag. The addition of Ag into TiN film could enhance the fracture toughness (KIC) and critical load (LC1) of the films because the nanoparticle Ag provides a large volume fraction of grain boundaries. As Ag was added into TiN film, the average friction coefficient decreased from 0.78 at 0 at.% Ag to 0.20 at 41.1 at.% Ag; however, the wear rate of TiN-Ag films initially decreased and then increased, after reaching the minimum value of about 1.3 × 10−7 mm3/(mm.N), at 0.8 at.% Ag.
Microstructure, mechanical and tribological properties of VCN-Ag composite films by reactive magnetron sputtering
(Elsevier B.V., 2020) Yu, D.; Yu, L.; Asempah, I.; Ju, H.; Xu, J.; Koyama, S.; Gao, Y.
A series of VCN-Ag films with different Ag content were deposited by the magnetron sputtering system. The microstructure, mechanical and tribological properties of VCN-Ag films were characterized by X-ray diffractometer (XRD), Raman spectrometer, transmission electron microscopy (TEM), scanning electron microscopy (SEM), nano-indentation and ball-on disc tribo-meter. The results showed that the deposited VCN-Ag films consisted of face-centered cubic (fcc) Ag, fcc-VCN, tetragonal (t) V5CN and amorphous graphite and CNx phases, and nano-crystalline Ag was dispersed in the films. The addition of Ag below 0.72 at.% into the film increased the hardness, however, the hardness decreased rapidly when the Ag content was increased furtherly. The initial introduction of Ag into the film led to the lowering of the film's room-temperature wear rate, nonetheless, the wear resistance deteriorated when the Ag content was further increased. However, the friction coefficient of the films at room temperature increased monotonically as the content of the Ag increased. As for elevated temperature tribological properties of the film at 3.03 at.% Ag, there was an initial increase in frictional values but, when the temperature was above 300 °C the frictional values declined. More so, the wear rate increased monotonically when the temperature increased from 100 °C to 500 °C.
Quantitative studies of electric field intensity on atom diffusion of Cu/Ta/Si stacks during annealing
(Applied Surface Science, 2017) Wang, L.; Asempah, I.; Dong, S. T.; Yin, P. P.; Jin, L.
It has been shown that enhanced electric field intensity (0–4.0 kV/cm) has an obvious effect on accelerating atom diffusion in Cu/Ta/Si interconnect stacks at 650 °C. The theoretical deduction proves that diffusion coefficient is accelerated proportional to an acceleration factor (1 + a·αE/0.8)2. The analysis indicates that the accelerating effect is mainly attributed to the perturbation of the electric state of the defects and enhanced vacancy and dislocation densities.
The role of copper incorporation on the microstructure, mechanical and tribological properties of TiBN-Cu films by reactive magnetron sputtering.
(Journal of Alloys and Compounds, 2019) Asempah, I.; Xu, J.; Yu, L.; Luo, H.; Liu, J.; Yu, D.; Ding, N.
An experimental study was conducted on a series of Ti-B-N-Cu films via magnetron sputtering of titanium, boron and copper targets. This study aims at assessing the microstructure, mechanical and tribological properties of the Ti-B-N-Cu films by varying the concentration of copper. The deposited film revealed a face-centred cubic TiN structure with a (111) preferred orientation, and its grain size decreased with increasing content of copper. More so, the film exhibited the highest hardness and elastic modulus values of ∼33.2 GPa and ∼288.8 GPa respectively when the content of copper in the film was 1.8 at.% which also coincides with the best wear resistance value of the films. The initial increase in hardness is due to grain refinement and an increase in compressive residual stress in the film. Even though the incorporation of copper improved the high-temperature coefficient of friction, the wear rate deteriorates as the content of copper and temperature increases. The presence of tribo-oxides and copper within the wear track acted as a lubricating medium for the reduction of the coefficient of friction whiles the reduced hardness of the films as the copper content in increased resulted in the increased wear rate. Also, the wear rate of the films with the same copper content increases as the test temperature increases from RT to 700 °C. Hence caution must be taken when applying copper doped films for high temperature wear resistance operations.
Self-Formed Diffusion Layer in Cu (Re) Alloy Film for Barrierless Copper
(Coatings, 2022) Cheng, B.; Chen, H.; Asempah, I.; Wang, J.; Zhu, Y.; Wan, J.; Qiao, Y.
The barrier properties and diffusion behavior of Cu(Re) alloy films were studied. The films were deposited onto barrierless SiO2/Si by magnetron sputtering. X-ray diffraction patterns and electric resistivity results proved that the Cu(Re) alloy films without a barrier layer were thermally stable up to 550 °C. Transmission electron microscopy images and energy-dispersive spectrometry employing scanning transmission electron microscopy provided evidence for a self-formed Re-enriched diffusion layer between the Cu(Re) alloy and SiO2/Si substrate. Furthermore, the chemical states of Re atoms at the Cu(Re)/SiO2 interface were analyzed by X-ray photoemission spectroscopy. The self-formed diffusion layer was found to be composed of Re metal, ReO, ReO2 and ReO3. At 650 °C, the Cu(Re) layer was completely destroyed due to atom diffusion. The low electrical resistivity in combination with the high thermal stability suggests that the Cu(Re) alloy could be the ultimate Cu interconnect diffusion barrier.
Structural, mechanical and tribological properties of NbCN-Ag nanocomposite films deposited by reactive magnetron sputtering.
(Coatings,, 2018) Wu, F.; Yu, L.; Ju, H.; Asempah, I.; Xu, J.
spectroscopy (EDS), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscopy (HRTEM), Raman spectrometry, nano-indentation, and high-temperature sliding wear tests. Results indicated that face-centered cubic (fcc) NbN, hexagonal close-packed (hcp) NbN and fcc Ag, amorphous C and amorphous CNx phase co-existed in the as-deposited NbCN-Ag films. After doping with 2.0 at.% Ag, the hardness and elastic modulus reached a maximum value of 33 GPa and 340 GPa, respectively. Tribological properties were enhanced by adding Ag in NbCN-Ag films at room temperature. When the test temperature rose from 300 to 500 °C, the addition of Ag was found beneficial for the friction properties, showing a lowest friction coefficient of ~0.35 for NbCN-12.9 at.% Ag films at 500 °C. This was mainly attributed to the existence of AgOx, NbOx, and AgNbOx lubrication phases that acted as solid lubricants to modify the wear mechanism.

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Browsing Research Articles by Author "Asempah, I."