Influence of the N2 partial pressure on the mechanical properties and tribological behavior of zirconium nitride deposited by reactive magnetron sputtering

Zirconium nitride was deposited by reactive unbalanced magnetron sputtering at different N₂ partial pressures, on an AISI 316L stainless steel substrate. The mechanical properties of the coatings were evaluated by means of nanoindentation tests employing a Berkovich indenter and loads which varied between 120-9000 µN. The sliding wear behavior of the substrate-coating systems was studied under a normal load of 2 N using a ball-on-disc tribometer, with an AISI 52100 ball (6 mm diameter) as counterpart. It has been found that N₂ partial pressure has a significant effect both on the hardness and corresponding Young's modulus of the coatings. As the N₂ partial pressure increases from 1 × 10^− 4 Torr to 10 × 10^− 4 Torr, the hardness and Young's modulus of the coatings decrease from 26 to 20 GPa and 360 to 280 GPa, respectively. The nanoindentation tests revealed the presence of a third oxide layer (10 nm thick, approximately) on the surface of the coating. Scanning electron microscopy (SEM) analysis performed on the worn triboelements indicated that both abrasive and adhesive wear mechanisms could take place in addition to the substrate plastic deformation. The deposition conditions and coating mechanical integrity determine the predominant wear mechanism.     

Hard and superhard TiAlBN coatings deposited by twin electron-beam evaporation

Superhard nanostructured coatings, prepared by plasma-assisted chemical vapour deposition (PACVD) and physical vapour deposition (PAPVD) techniques, such as vacuum arc evaporation and magnetron sputtering, are receiving increasing attention due to their potential applications for wear protection. In this study nanocomposite (TiAl)B_xN_y (0.09 ≤ x ≤ 1.35; 1.07 ≤ y ≤ 2.30) coatings, consisting of nanocrystalline (Ti,Al)N and amorphous BN, were deposited onto Si (100), AISI 316 stainless steel and AISI M2 tool steel substrates by co-evaporation of Ti and hot isostatically pressed (HIPped) Ti-Al-B-N material from a thermionically enhanced twin crucible electron-beam (EB) evaporation source in an Ar plasma at 450 °C. The coating stoichiometry, relative phase composition, nanostructure and mechanical properties were determined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), in combination with nanoindentation measurements. Aluminium (∼ 10 at.% in coatings) was found to substitute for titanium in the cubic TiN based structure. (Ti,Al)B_0.14N_1.12 and (Ti,Al)B_0.45N_1.37 coatings with average (Ti,Al)N grain sizes of 5-6 nm and either ∼ 70, or ∼ 90, mol% (Ti,Al)N showed hardness and elastic modulus values of ∼ 40 and ∼ 340 GPa, respectively. (Ti,Al)B_0.14N_1.12 coatings retained their ‘as-deposited’ mechanical properties for more than 90 months at room temperature in air, comparing results gathered from eight different nanoindentation systems. During vacuum annealing, all coatings examined exhibited structural stability to temperatures in excess of 900 °C, and revealed a moderate, but significant, increase in hardness. For (Ti,Al)B_0.14N_1.12 coatings the hardness increased from ∼ 40 to ∼ 45 GPa.     

Thermal stability of superhard Ti-B-N coatings

Ternary transition-metal boron nitride Ti-B-N offers outstanding hardness and thermal stability, which are increasingly required for wear resistant applications, as the protective coatings are subjected to high temperature, causing thermal fatigue. Ti-B-N coatings with chemical compositions close to the quasibinary TiN-TiB₂ tie line and boron contents below ∼ 18 at.% contain a crystalline supersaturated NaCl structure phase, where B substitutes for N. Annealing above the deposition temperature causes precipitation of TiB₂, which influence dislocation mobility and hence the hardness of TiB_0.40N_0.83 remains at a very high level of ∼ 43 GPa with annealing temperature T_a up to 900 °C. Growth of Ti-B-N coatings with B contents above ∼ 18 at.% results in the formation of nm sized TiN and TiB₂ crystallites embedded in a high volume fraction of disordered boundary layer. The compaction of this disordered phase during annealing results in a hardness increase of TiB_0.80N_0.83 coatings from the as-deposited value of ∼ 37 GPa to ∼ 42 GPa at T_a = 800 °C. Excess B during growth of TiB_2.4 coatings causes the formation of bundles of ∼ 5 nm wide TiB₂ subcolumns encapsulated in a B-rich tissue phase. This nanocolumnar structure is thermally stable up to temperatures of ∼ 900 °C, and consequently the hardness remains at the very high level of ~ 48 GPa, as nucleation and growth of dislocations is inhibited by the nm sized columns. Furthermore, the high cohesive strength of the B-rich tissue phase prevents grain boundary sliding.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate

Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5-40 μm). Young's modulus E_f of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300-600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo E_f = 40 GPa and for 32 wt.% of Mo E_f = 23 GPa.     

Microstructure and properties of W-ZrC composites prepared by the displacive compensation of porosity (DCP) method

Tungsten-zirconium carbide composites were fabricated at different temperatures by the displacive compensation of porosity (DCP) method, the microstructure, mechanical properties, and ablation resistance were investigated. It was found that no WC phase was left in the composites prepared at 1400 °C, and a few residual W₂C particles were surrounded in W product. Microstructure analyses revealed that zirconium atoms diffused into tungsten carbide to form ZrC and W₂Zr besides carbon diffused into the Zr₂Cu melt. Composites fabricated at 1400 °C had a flexural strength of 356.7 ± 15.2 MPa, an elastic modulus of 193.7 ± 9.8 GPa, a fracture toughness of 7.0 ± 0.7 MPa m^1/2, and a hardness of 13.6 ± 0.7 GPa. After ablated by an oxyacetylene flame for 30 s, the higher temperature prepared composites had a better ablation resistance, the linear ablation rate was 0.0033 ± 0.0004 mm/s, and the mass ablation rate was 0.0012 ± 0.0001 g/s.     

Mechanical and tribological properties of multilayered TiSiN/CrN coatings synthesized by a cathodic arc deposition process

The monolayered TiSiN and multilayered TiSiN/CrN were synthesized by cathodic arc evaporation. The Ti/Si (80/20 at.%) and chromium targets were used as the cathodic materials. With the different I_[TiSi]/I_[Cr] cathode current ratios of 1.8, 1.0, and 0.55, the multilayered TiSiN/CrN coatings possessed different multilayer periods (Λ) of 8.3 nm, 6.2 nm, and 4.2 nm. From XRD and TEM analyses, both the monolayered TiSiN and multilayered TiSiN/CrN revealed a typical columnar structure and B1-NaCl crystalline, no peaks of crystalline Si₃N₄ were detected. Among the multilayered TiSiN/CrN coatings, the multilayered coating with Λ = 8.3 nm possessed higher hardness of 37 ± 2 GPa, higher elastic modulus of 396 ± 20 GPa and the lower residual stress of − 1.60 GPa than the monolayered (Ti_0.39Si_0.07)N_0.54 coating(− 7.25 GPa). Due to the higher Cr/(Ti +Cr + Si) atomic ratio, the multilayered TiSiN/CrN with Λ = 5.5 nm possessed the lowest friction coefficient. But the lowest of wear rate was obtained by the multilayered TiSiN/CrN with Λ = 8.3 nm, because of higher H³/E^?2 ratio of 0.323 GPa. The monolayered TiSiN possessed the highest wear rate of 2.87 μm²/min. Therefore, the mechanical and tribological property can be improved by the design of multilayered coating.     

Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering

Nanocomposite coatings of CrN/Si₃N₄ and CrAlN/Si₃N₄ with varying silicon contents were synthesized using a reactive direct current (DC) unbalanced magnetron sputtering system. The Cr and CrAl targets were sputtered using a DC power supply and the Si target was sputtered using an asymmetric bipolar-pulsed DC power supply, in Ar + N₂ plasma. The coatings were approximately 1.5 μm thick and were characterized using X-ray diffraction (XRD), nanoindentation, X-ray photoelectron spectroscopy and atomic force microscopy. Both the CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings exhibited cubic B1 NaCl structure in the XRD data, at low silicon contents (< 9 at.%). A maximum hardness and elastic modulus of 29 and 305 GPa, respectively were obtained from the nanoindentation data for CrN/Si₃N₄ nanocomposite coatings, at a silicon content of 7.5 at.%. (cf., 24 and 285 GPa, respectively for CrN). The hardness and elastic modulus decreased significantly with further increase in silicon content. CrAlN/Si₃N₄ nanocomposite coatings exhibited a hardness and elastic modulus of 32 and 305 GPa, respectively at a silicon content of 7.5 at.% (cf., 31 and 298 GPa, respectively for CrAlN). The thermal stability of the coatings was studied by heating the coatings in air for 30 min in the temperature range of 400-900 °C. The microstructural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data of the heat-treated coatings in air indicated that CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings, with a silicon content of approximately 7.5 at.% were thermally stable up to 700 and 900 °C, respectively.     

Structural and mechanical stability upon annealing of arc-deposited Ti-Zr-N coatings

Ti-Zr-N coatings were formed by the method of vacuum arc deposition using combined Ti and Zr plasma flows in a N₂ atmosphere at different ratios of arc currents of Ti and Zr cathodes. After deposition, obtained samples were annealed in vacuum at the temperature of 850 °C. The element and phase composition, residual stresses and nanohardness were studied by Auger-Electron Spectroscopy, X-ray diffraction (XRD) and nanoindentation, respectively.XRD analysis reveals the formation of ternary Ti-Zr-N nitride coatings with the structure of solid solutions. It is shown that Ti-Zr-N coatings possess high hardness in comparison with TiN and ZrN binary nitrides. An increase in hardness is observed with increasing Zr content. However, it is established that after annealing coatings keep better stability of hardness with decrease of Zr content. The intrinsic stress in the as-deposited coatings is found to be largely compressive (− 4 GPa) and almost independent of Zr content, but much higher than in ZrN and TiN binary nitrides (− 2 GPa). After annealing, a significant stress relaxation is observed in all coatings due to relief of growth-induced point defects. Stress analysis on as-grown and annealed samples enabled us to determine the stress-free lattice parameter a₀. This latter is expanded by ∼ 0.4-0.7% as compared to Vegard's law.The thermal stability of Ti-Zr-N coatings will be discussed in terms of evolution and interdependence between structure, composition and hardness after annealing.     

Non-linear finite element constitutive modeling of indentation into super- and ultrahard materials: The plastic deformation of the diamond tip and the ratio of hardness to tensile yield strength of super- and ultrahard nanocomposites

Using a non-linear constitutive material model for super- (H ≥ 40 GPa) and ultrahard (H ≥ 80 GPa) materials that accounts for the pressure enhancement of elastic moduli and of plastic resistance, we present the effect of plastic deformation and resultant blunting of the diamond indenter, and discuss the limitations to the measurement on ultrahard materials by means of nano-indentation. We further show that the ratio of the hardness H to plastic resistance Y in tension amounts to about 2.4 for the super- and 2.84 for ultrahard nanocomposites, although the ratio of hardness to Young's modulus is relatively high. These results are briefly discussed in terms of the expanding cavity model and the elastic-plastic transition.     

Elastic properties of γ′-Fe4N probed by nanoindentation and ab initio calculation

We studied the elastic properties of γ′-Fe₄N experimentally and theoretically, as the difference between calculated and measured literature data is up to 31%. A phase-pure γ′-Fe₄N thin film and a bulk-like sample were synthesized using a reactive magnetron sputtering technique and a two-step ammonolysis reaction, respectively. The elastic moduli were measured to be 157 ± 11 and 159 ± 17 GPa, respectively, by nanoindentation. For nanoindentation on bulk-like γ′-Fe₄N, the correction of structural compliance was applied to remove the influence of the porosity as well as of mounting. These measured elastic moduli are consistent with the value of 162 GPa obtained from ab initio calculations.     

Formation and properties of SrB6 single crystals synthesized under high pressure and temperature

Pure SrB₆ single crystals are synthesized under high pressure (5 GPa) and temperature (1300 °C). The structure and morphology of the SrB₆ single crystals are characterized by X-ray diffraction and field emission scanning electron microscope. The lattice constant of the SrB₆ crystals with a space group of Pm-3m is a = 4.1975 Å. The dependence of electric resistivity and Hall coefficient on temperatures from 2 to 300 K show that the SrB₆ single crystals are conductive materials with semi-metallic behavior and can be considered as electronic current carriers. The results of the band structure calculation show that the conduction and valence bands meet at the X point at the Fermi Level, which is consistent with the experimentally measured conducting behavior of SrB₆ single crystals. The total and partial density of states show that the states at the Fermi level come from the 2p orbitals of the B atoms and the 4d orbital of the Sr atom. The magnetization measurement shows the diamagnetic nature of the SrB₆. The nanoindentation measurement indicates that the hardness of SrB₆ reached 33 GPa.     

Properties of magnetron sputtered Al-Si-N thin films with a low and high Si content

The article reports on properties of Al-Si-N films with a low (≤ 10 at.%) and high (≥ 25 at.%) Si content reactively sputtered using a closed magnetic field dual magnetron system operated in ac pulse mode. The films were sputtered from a composed target (a Si plate fixed by an Al ring with inner diameter Ø_i = 15 or 26 mm). Main attention was devoted to the investigation of a relationship between the structure of the films and their mechanical properties, thermal stability of hardness, and oxidation resistance. It was found that (1) while the films with a low (≤ 10 at.%) Si content are crystalline (c-(Al-Si-N)), those with a high (≥ 25 at.%) Si content are amorphous (a-(Al-Si-N)) when sputtered at the substrate temperature T_s = 500 °C, (2) both groups of the films exhibit (i) a high hardness H = 21 and 25 GPa, respectively, and high values of the oxidation resistance exceeding 1000 °C; 1100 °C (Δm = 0 mg/cm²) and 1300 °C (Δm ≈ 0.003 mg/cm²), respectively, (3) the hardness of a-(Al-Si-N) does not vary with increasing annealing temperature T_a up to 1100 °C even after 4 h, and (4) a high oxidation resistance of c-(Al-Si-N) film with a low (< 10 at.%) Si content is due to the formation of a dense, nearly amorphous Al₂O₃ surface layer which is formed in reaction of free Al atoms with ambient oxygen and prevents the fast penetration of oxygen into bulk of the film. Obtained results contribute to understand the effect of Al and Si in the Al-Si-N thin film on its mechanical properties, thermal stability and oxidation resistance.     

Nanoindentation study of amorphous-Co79Zr13Nb8/Cr multilayers

The mechanical properties of Co₇₉Zr₁₃Nb₈/Cr multilayers were investigated using nanoindentation. The hardness is higher than the average value calculated by rule-of-mixture. The hardness and the resistance to plastic deformation characterized by the ratio of H³/E² vary similarly with periodicity (Λ). They all arrive to the maximum at Λ = 8 nm and decrease subsequently when the Λ increases. The hardness dependence on the Λ is fitted by Hall-Petch relation. The fitted index n is much lower than the normal value (~ 0.5) in many crystalline multilayers. The mutual restriction of shear band and dislocation in amorphous/crystalline structure, which is named structure barrier strengthening, should be main mechanism for the hardness enhancement. The SEM study of indents shows that the shear bands are distorted significantly at the smaller Λ (4 nm) and disappear at the larger Λ (> 20 nm). This morphology variation implies a potential improvement of plasticity caused by the restriction effect of the Cr crystalline layers on the shear bands propagation.     

Lateral gradients of phases,residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal

The influence of a local thermal treatment on the properties of Ti–Al–N coatings is not understood. In the present work, a Ti_0.52Al_0.48N coating on a WC–Co substrate was heated with a diode laser up to 900 °C for 30 s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti–Al–N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47 GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about − 200 to 500 MPa down to a depth of 6 μm is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti_0.52Al_0.48N coating on a WC–Co substrate was performed in the range of 25–1003 °C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure–property relationships in complex materials.     

Density, stress, hardness and reduced Young's modulus of W-C:H coatings

Density, hardness and compressive stress of tungsten contained in an amorphous-hydrogenated-carbon matrix (W-C:H) have been studied as a function of composition and bias voltage. W-C:H coatings were deposited by reactive sputter deposition from a tungsten-carbide (WC) target on silicon substrate in an argon-acetylene plasma. W-C:H coatings obtained at different acetylene flow rates and substrate bias voltages, were characterized by scanning electron microscopy, X-ray diffraction, nanoindentation and substrate curvature method. It has been observed that compressive stress, hardness and reduced Young's modulus decrease when the acetylene flow is increased from 0 to 10 sccm. Also, compressive stress and hardness increases with the substrate bias voltage. In particular, for W-C:H coatings obtained at 5 sccm of acetylene flow, the compressive stress and hardness increase from − 1.6 GPa to − 3.2 GPa and from 19 GPa to 24 GPa, respectively, when increasing the substrate bias from 0 to 200 V. The variation of the internal stress, hardness and density of the coatings is discussed in terms of composition and structure of the W-C:H coatings.     

Thermal stability of Ta-Si-N nanocomposite thin films at different nitrogen flow ratios

Ta-Si-N thin films were applied as diffusion barriers for Cu interconnections or hard coatings in mechanical application. The resistivity, hardness and thermal stability were the important issues in the interconnections and hard coatings, respectively. In this paper, we investigated the relationship between the microstructures, resistivity, nanohardness and thermal stability of the Ta-Si-N thin films at different nitrogen flow ratios of 0-30% (N₂% = N₂ / (Ar + N₂) × 100%) by magnetron reactive co-sputtering. The Ta-Si-N films were annealed at 600, 750 and 900 °C at about 6 × 10⁻³ Pa for 1 h, respectively, to examine their thermal stability. The microstructures of Ta-Si-N films at low N₂% of 2-10% still retained the amorphous-like phase with nanocrystalline grains in an amorphous matrix at annealing of 600-900 °C. The nanohardness of amorphous-like Ta-Si-N film at N₂% of 3% was measured to be 15.2 GPa much higher than that of polycrystalline film of 10.1 GPa at N₂% of 20%. The average nanohardness of both films is stable up to 900 °C and varied in the range of 0.43-0.83 GPa. The resistivity of the as-deposited Ta-Si-N films increase with increasing N₂ flow rate. It is small around 220-540 μΩ cm for low N₂% of 2-10% while it increases abruptly to about 7700-43,000 μΩ cm at high N₂% of 20-30%. The best thermal stability of resistivity of Ta-Si-N film occurs at the N₂% of 2% in the range of 220 to 250 μΩ cm from RT to 900 °C.     

Nanoindentation on the surface of thermally sprayed coatings

The ability to quantify surface mechanical properties is valuable for assessing the quality of thermal spray coatings. This is especially important for prostheses where loading is placed directly on the surface. Hydroxyapatite was classified to small (20-40 μm), medium (40-60 μm) and large (60-80 μm) particle sizes and thermal sprayed to produce a coating from spread solidified hydroxyapatite droplets. It was revealed for the first time, that nanoindentation can be successfully used to determine the hardness and elastic modulus on the surface of well spread solidified droplets at the hydroxyapatite coating surface. Comparison with indentation results from polished cross-section exhibited comparable values and statistical variations. The hardness was 5.8 ± 0.6, 5.4 ± 0.5 and 5.0 ± 0.6 GPa on coatings produced from small, medium and large sized powder. Similarly, the elastic modulus decreased from 121 ± 7, 118 ± 7 to 114 ± 7 GPa, respectively. Use of several indentation loads gave comparable results with sintered hydroxyapatite suggesting good inter-splat bonding within the coating. MicroRaman spectroscopy and X-ray diffraction confirmed a larger degree of dehydroxylation for the smaller particles also revealing a lower elastic modulus. This shows the influence of particle size and possibly dehydroxylation of hydroxyapatite on the mechanical properties of the coating surface.     

Comparative investigation of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets

A comparative investigation of the structure and properties of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti_2 − хCr_хAlC targets (where x = 0, 0.5, 1.5, and 2) in an Ar atmosphere or in a gaseous mixture of Ar + N₂ is presented. The coatings were characterized in terms of their structure, elemental and phase composition, hardness, elastic modulus, elastic recovery, thermal stability, friction coefficient, wear rate, corrosion, and high-temperature oxidation resistance. The structure of the coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum or in air at temperatures 600-1200 °C. The results obtained show that the TiAlCN coatings possess high hardness of 32-35 GPa, low friction coefficient against WC-Co well below 0.25, high thermal stability up to 1200 °C, and superior performance in dry milling tests against high Cr steel. Meanwhile, the coatings with high Cr content demonstrated improved oxidation resistance up to 1000 °C and superior electrochemical behavior, but their mechanical and tribological properties were deteriorated.     

A novel technique for development of A356/Al2O3 surface nanocomposite by friction stir processing

A356/Al₂O₃ surface nanocomposite was produced by friction stir processing (FSP) method. X-ray diffractometery, optical and scanning electron microscopy, microhardness and nanoindentation tests were used to characterize the samples. The results indicated that the uniform distribution of Al₂O₃ particles in A356 matrix by FSP process can improve the mechanical properties of specimens. The hardness and elastic modulus of the as-received A356, the sample treated by the FSP without Al₂O₃ particles, surface micro- and nanocomposite specimens were about 75 Hv and 74 GPa, 69 Hv and 73 GPa, 90 Hv and 81 GPa, 110 Hv and 86 GPa, respectively.