Study of strontium ferrites substituted by lanthanum on the structural and magnetic properties

M-type strontium ferrites, Sr_0.8La_0.2Fe₁₂O₁₉ have been synthesized by conventional ceramic process. The effects of lanthanum addition and sintering temperature on microstructures and magnetic properties of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ samples were investigated. Microstructural analysis of the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ specimens, sintered at different temperatures revealed that average grain sizes of SrFe₁₂O₁₉ ferrites were larger than that of Sr_0.8La_0.2Fe₁₂O₁₉ ferrite and increased with increasing sintering temperature. The X-ray diffraction (XRD) results confirmed the strontium hexagonal ferrite phase of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ compounds. A maximum coercivity value of 4850 Oe and maximum saturation magnetization value of 102 emu/g were obtained for the SrFe₁₂O₁₉ ferrite sintered at 1150 °C and for the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ ferrites sintered at 1300 °C, respectively. The remanence (Mr) of Sr_0.8La_0.2Fe₁₂O₁₉ sample sintered at 1200 °C possesses the maximum value of 60 emu/g.     

Facile synthesis of metal-doped Ni-Zn ferrite from treated Zn-containing electric arc furnace dust

Metal-doped Ni-Zn ferrite with a spinel structure was directly synthesized from treated zinc-containing electric arc furnace dust (Zn-containing EAFD) by solid state reaction method, realizing the transformation of solid waste to high value-added material. First, NiCl₂·6H₂O was added to the treated Zn-containing EAFD before calcination. Then, the effects of the mass ratio of treated Zn-containing EAFD to NiCl₂·6H₂O (R_TZE/N, g g⁻¹) and the calcination temperature on the synthesis and magnetic properties of as-synthesized samples were systematically investigated by X-ray diffraction, Raman spectroscopy and physical property measurement. It was found that R_TZE/N and calcination temperature significantly influenced the synthesis of single-phase spinel ferrite and their magnetic properties. Pure metal-doped Ni-Zn ferrite, exhibiting good magnetic properties of higher saturation magnetization (Ms, 60.5 emu g⁻¹) and lower coercivity (Hc, 49.8 Oe), was obtained when the calcination temperature was controlled at 1100 °C for 2 h with a R_TZE/N of 1:0.9.     

Synthesis of nanocrystalline Mg0.6Cd0.4Fe2O4 ferrite by glycine-nitrate auto-combustion method and investigation of its microstructure and magnetic properties

Nanocrystalline Mg_0.6Cd_0.4Fe₂O₄ ferrite powders were produced by the glycine-nitrate auto-combustion method for the first time. The influence of the different molar ratios of glycine-to-nitrate G.N⁻¹) on the characteristics of the prepared powders was systematically investigated by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometry (VSM). Thermodynamic calculations revealed that the adiabatic flame temperature changes from 598.79 K to 1757.97 K by increasing the G.N⁻¹ ratios from 0.30 to 0.85. The results confirmed that under fuel-lean combustion (G.N⁻¹ = 0.30), Mg_0.6Cd_0.4Fe₂O₄ nanoparticles can be obtained at a significantly lower temperature and shorter synthesis time, compared to other preparation methods like standard ceramic and co-precipitation. The XRD and ICP results showed that the crystallite size of the powders changes in the range of 8–43 nm, and their Cd content notably decreases with increasing the G.N⁻¹ values. The FE-SEM results proved that the porosity and size of the as-prepared ferrite nanoparticles drastically change with variations in the G.N⁻¹ ratio. The evolution of phase, crystallite/particle size, and magnetic properties after annealing was discussed in detail. At the optimized annealing condition, the synthesized Mg_0.6Cd_0.4Fe₂O₄ ferrite offered a high saturation magnetization of 41.70 Am² kg⁻¹ and a coercivity of 1.92 kA m⁻¹, indicating noticeably better soft magnetic properties compared to the same ferrite produced by the other wet chemical methods.     

Synthesis and characterization of S2O82 −/ZnFexAl2 − xO4 solid acid catalysts for the esterification of acetic acid with n-butanol

New spinel-types of S₂O₈^2 −/ZnFe_xAl_2 − xO₄ solid acid catalysts were prepared by sol–gel method. Their catalytic performances for the synthesis of n-butyl acetate were investigated. The catalysts were characterized by means of XRD, IR, XPS, FT-IR of adsorbed pyridine and NH₃-TPD. The experimental results showed that S₂O₈^2 −/ZnFe_xAl_2 − xO₄ solid acid catalysts maintained the spinel structure as well as the support of ZnFe_xAl_2 − xO₄. Fe^3 + ions were well incorporated and highly dispersed into the spinel lattice. S₂O₈^2 −/ZnFe_0.15Al_1.85O₄ exhibited the maximum conversion of acetic acid with 98.2%. Moreover, S₂O₈^2 −/ZnFe_0.15Al_1.85O₄ showed better reusability, which remained above 72.7% conversion of acetic acid even after being used five times.     

First μ3-oxalato-bridged cis and trans configurations nickel macrocyclic complex: Synthesis,structure and magnetic properties

A novel nickel macrocyclic complex {[Ni₂(cyclam)₂(μ₃-C₂O₄)]}_n · (ClO₄)₂ · H₂O has been synthesized and structural characterized by single-crystal X-ray diffraction. The complex forms a chain-like one-dimensional structure linked through a novel μ₃-oxalato-bridging mode, which has never been reported for nickel. The complex adopts both trans-III configuration with the R, R, S, S arrangement of the four chiral nitrogen centers and cis-V conformation with the R, R, R, R nitrogen configuration. Variable-temperature magnetic susceptibility studies indicate the existence of weak antiferromagnetic (zj = −1.07 cm⁻¹) coupling.     

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr_1−xCa_xFe_1−yAl_yO_3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO₂ gradient of 0.21/0.021 atm vary from 3.7×10⁻¹⁰ mol s⁻¹ cm⁻² to 1.5×10⁻⁷ mol s⁻¹ cm⁻² at 950 °C. The maximum solubility of Al³⁺ cations in the perovskite lattice of SrFe_1−yAl_yO_3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr_1−xCa_xFe_0.5Al_0.5O_3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO₃-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe_0.5Al_0.5O_2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe_0.7Al_0.3O_3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO₂ stability limits of Sr_1−xCa_xFe_1−yAl_yO_3−δ seem essentially independent of composition, varying between that of LaFeO_3−δ and the Fe/Fe_1−γO boundary. Thermal expansion coefficients of Sr_1−xCa_xFe_1−yAl_yO_3−δ ceramics in air are 9×10⁻⁶ K⁻¹ to 16×10⁻⁶ K⁻¹ at 100–650 °C and 12×10⁻⁶ K⁻¹ to 24×10⁻⁶ K⁻¹ at 650–950 °C. Doping of SrFe_1−yAl_yO_3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.     

Heat generation properties in AC magnetic field for Y3Fe5O12 powder material synthesized by a reverse coprecipitation method

Ferrimagnetic Y₃Fe₅O₁₂ powder was synthesized by a reverse coprecipitation method in order to study its heat generation property in an AC magnetic field. An orthorhombic YFeO₃ phase having a small particle size (<100 nm) was obtained for the samples calcined at a low temperature. The maximum heat generation ability in an AC magnetic field was obtained for the Y₃Fe₅O₁₂ ferrite powder by calcination at 1100 °C. The heat generation ability was reduced for the samples calcined at a higher temperature. The particle growth with the formation of the cubic single phase might influence the heat generation ability. The heat generation ability and the hysteresis loss value were proportional to the cube of the magnetic field (H³), because the coercivity value of the B–H curve was proportional to the square of the amplitude of the AC magnetic field (H²). The heat generation ability (W g⁻¹) of the Y₃Fe₅O₁₂ sample sintered at 1100 °C can be expressed by the equation 2.2×10⁻⁴∙f∙H³ using the frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which has the highest heat generation ability of the reported magnetic materials. The hysteresis loss value for the B–H curve agreed with the heat generation ability of the samples calcined at 1100 °C and lower temperatures.     

Influence of sintering aids and excess Lithium on the conductivity of perovskite-type Li3/8Sr7/16Zr1/4Nb3/4O3 solid electrolyte

Perovskite-structured Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ solid-state Lithium-conductors were prepared by conventional solid-state reaction method. Influence of sintering aids (Al₂O₃, B₂O₃) and excess Lithium on structure and electrical properties of Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ (LSNZ) has been investigated. Their crystal structure and microstructure were characterized by X-ray diffraction analysis and scanning electron microscope, respectively. The conductivity and electronic conductivity were evaluated by AC-impedance spectra and potentiostatic polarization experiment. All sintered compounds are cubic perovskite structure. Optimal amount of excess Li₂CO₃ was chosen as 20 wt% because of the total conductivity of LSNZ-20% was as high as 1.6×10⁻⁵ S cm⁻¹ at 30 °C and 1.1×10⁻⁴ S cm⁻¹ at 100 °C, respectively. Electronic conductivity of LSNZ-20% is 2.93×10⁻⁸ S cm⁻¹, nearly 3 orders of magnitude lower than ionic conductivity. The density of solid electrolytes appears to be increased by the addition of sintering aids. The addition of B₂O₃ leads to a considerable increase of the total conductivity and the enhancement of conductivity is attributed to the decrease of grain-boundary resistance. Among these compounds, LSNZ-1 wt%B₂O₃ has lower activation energy of 0.34 eV and the highest conductivity of 1.98×10⁻⁵ S cm⁻¹ at 30 °C.     

Magnetocrystalline anisotropy study of Co-substituted M-type strontium hexaferrite single crystals

The study on the magnetocrystalline anisotropy (MA) of La–Co co-substituted strontium hexaferrite (La–Co SrM) shows a joint effect of Fe²⁺ and Co²⁺ ions in the enhancement of MA. Since the role of Fe²⁺ single ion has been studied with La-substituted strontium ferrite, in this work, Co-substituted strontium hexaferrite SrFe_12-xCo_xO₁₉ (Co-SrM) single crystals were successfully grown for 0 ≤ x ≤ 0.31 by the Na₂CO₃ flux method to elucidate the anisotropy of Co²⁺ single ions. Co-substitution in this preparation condition has a limit solubility of 0.31 and enhances uniaxial magnetic anisotropy field H_A by 19% for 0.03 = x ≤ 0.11, with a mere loss of 7% of saturation magnetization M_S at 5 K. The enhanced H_A of Co-SrM is reported for the first time, even higher than that of La–Co SrM, which is suitable to be used as permanent magnets in this concentration range. But with the further substitution of Co, the planar anisotropy of x = 0.31 was observed at 5 K. The potential nonlinear magnetic structure of Co-SrM remains to be discovered for magnetoelectric effects. This work is also of great significance as a complement to the magnetocrystalline anisotropy study of La–Co SrM.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method

The purpose of the research was to improve the intrinsic magnetic properties of strontium ferrite by substituting lanthanum and cobalt for strontium and iron. The salt-assisted ultrasonic spray pyrolysis (SA-USP) following calcination process were used to from La-Co substituted strontium ferrite particles (La_xSr_1-xFe_12-yCo_yO₁₉), and their compositional dependent magnetic properties systemically investigated. All the samples were calcined at 1050 °C for 1 h in an air atmosphere to yield single-phased hexagonal particles several hundred nanometers to microns in size. A saturation magnetization of 70.76 emu/g and a coercivity 7265 Oe were obtained at a composition of La_0.25Sr_0.75Fe_11.75Co_0.25O₁₉. The amount of Co was reduced to obtain an optimized saturation magnetization of 71.40 emu/g and a coercivity of 7572 Oe at a composition of La_0.25Sr_0.75Fe_11.8Co_0.2O₁₉.     

A strategy for fabricating textured silicon nitride with enhanced thermal conductivity

C-axis textured Si₃N₄ with a high thermal conductivity of 176 W m⁻¹ K⁻¹ along the grain alignment direction was fabricated by slip casting raw α-Si₃N₄ powder seeded with near-equiaxed β-Si₃N₄ particles and Y₂O₃–MgSiN₂ as sintering additives in a rotating strong magnetic field of 12 T, followed by gas pressure sintering at 1900 °C for 12 h at a nitrogen pressure of 1 MPa. The green material reached a relative density of 57%, with slip casting and the sintered material exhibited a relative density of 99% and a Lotgering orientation factor of 0.98. The morphology of the β-Si₃N₄ seeds had little effect on the texture development and thermal anisotropy of textured Si₃N₄. The technique developed provides highly conductive Si₃N₄ with conductivity to the thickness direction, which is a major advantage in practical use. The technique is also simple, inexpensive and effective for producing textured Si₃N₄ with high thermal conductivity of over 170 W m⁻¹ K⁻¹.     

Binuclear cyano-bridged complex derived from [MnIII(salpn)] and [FeIII(CN)6]: Synthesis,structure and magnetic properties

The reaction of the neutral [Mn(salpn)C(CN)₃(H₂O)] (salpn^2 − = N,N^′-1,3-propylenebis(salicylideneiminato) dianion) with [Fe^III(CN)₆]^3 − in the presence of strong oxidizer (NH₄)₂S₂O₈ yields a binuclear anion complex [NH₃CH₂CH₂CH₂NH₃]^2 +{[Mn^III(salpn)(H₂O)][Fe^III(CN)₆]}^2 − (1). Its structure, DC and AC susceptibility have been studied. Frequency dependence of the AC susceptibility characteristic for single-molecule magnets has been found.     

Fast synthesis of SrFe12O19 hexaferrite in a single-mode microwave cavity

The M-type SrFe₁₂O₁₉ hexaferrite has been synthesized by a microwave solid state reaction process – a fast heating process – in a home-made 2.45 GHz single-mode microwave cavity. Starting from SrCO₃ and Fe₂O₃ mixtures (1:6 ratio), cold pressed samples have been heated up in the microwave electric field without needing any susceptor, demonstrating the good coupling of the precursors with this microwave mode in our experimental setup.After optimization of the experimental conditions, the properties of the obtained ceramics, including structure, microstructure and magnetic properties, are compared with those of ceramics synthesized by conventional solid state reaction. With that microwave process, it is found that SrFe₁₂O₁₉ ceramics prepared in less than 30 min exhibit magnetic properties similar to those of the same compound produced by a conventional process. This highlights the potentialities of the technique to synthesize hexaferrite ceramics.     

Hidden value in low-cost inorganic pigments as potentially valuable magnetic materials

Inorganic pigments are substances that develop colour in organic solids such as ceramics and glazes, and are usually a complex mixture of oxides, and relatively low-cost. Their chromatic properties have been extensively studied, yet surprisingly the magnetic and electrical properties of these economic and common materials have been neglected, despite the fact many are based on ferrite spinels. Therefore, we investigated these properties in commercial black and brown pigments, to assess their potential as magnetic materials. The brown and black pigments were found to be spinel ferrites, with estimated formulas of Fe_1.34Cr_0.62Mn_0.66Zn_0.22Ni_0.10Co_0.06O₄ and Fe_1.02Cr_0.97Co_0.57Mn_0.23Ni_0.21O₄, respectively. The brown pigment also contained a higher amount of SiO₂ compared to the black pigment (~7 mol% vs. ~2 mol%), which appeared as a second phase of crystalline quartz, and adversely affected its porosity, magnetisation and electrical ac conductivity, compared to the black pigment. However, both were very magnetic and very soft ferrites. The brown pigment had M_s=11.7 A m² kg⁻¹ and H_c of 1.5 kA m⁻¹, with a high electrical conductivity (σ) of 4×10⁻⁴−7×10⁻³ Ω⁻¹ m⁻¹ between 100 Hz and 1 MHz. The black pigment was equally magnetically soft, but had a much greater magnetisation and lower electrical conductivity, with M_s=18.7 A m² kg⁻¹, H_c=2.4 kA m⁻¹, and σ=5×10⁻⁶−8×10⁻⁵ Ω⁻¹ m⁻¹ between 100 Hz and 1 MHz.This work has revealed the potential hidden value of low-cost commercial inorganic pigments based on spinel ferrites as magnetic materials. This demonstrates their potential at low-cost alternative materials for applications such as in power supply transformers, switching materials and sensors, where soft magnetisation is especially important.     

Microwave absorption study of carbon nano tubes dispersed hard/soft ferrite nanocomposite

Nickel substituted strontium hexaferrite, SrNi₂Fe₁₀O₁₉·(SrFe₁₂O₁₉/NiFe₂O₄) nanoparticles have been synthesized by low combustion method by citrate precursors using sol to gel (S–G) followed by gel to nano crystalline (G–N) conversion. The resulting ‘as-synthesized’ powder is heat treated (HT) at 800 and 1000 °C for 4 h in nitrogen atmosphere. The hysteresis loops show an increase in saturation magnetization from 27.443 to 63.706 emu/g with increasing HT temperatures. The multiwalled carbon nano tubes (CNTs) were synthesized by thermal decomposition of acetylene gas over iron-catalyst deposited on silicon wafer in the temperature range of 750–800 °C. A microwave absorbing medium is prepared by adding CNTs in the nickel substituted strontium hexaferrite nanoparticles. Addition of certain mass of CNTs improves the microwave absorption properties and wave band of SrFe₁₂O₁₉/NiFe₂O₄ absorbent. When 10 wt% CNTs is mixed with SrFe₁₂O₁₉/NiFe₂O₄ nanoparticles to fabricate a composite with 2 mm thickness, the maximum reflection loss reaches to ?36.817 dB at 9.292 GHz and ?10 dB bandwidth reaches 3.27 GHz.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

Synthesis,structure and magnetic properties of spinel ferrite (Ni,Cu, Co)Fe2O4 from low nickel matte

Spinel ferrite (Ni, Cu, Co)Fe₂O₄ was synthesized from the low nickel matte by using a co-precipitation-calcination method for the first time. The influences of the added amount of NiCl₂·6H₂O, calcination temperature and time on the structure and magnetic properties of the as-prepared ferrites were studied in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy, and Vibrating sample magnetometer (VSM). It is indicated that pure (Ni, Cu, Co)Fe₂O₄ with cubic phase could be obtained under the experimental conditions (NiCl₂·6H₂O added amount of 3.0: 100 g mL⁻¹, calcination temperature from 800 to 1000 °C and calcination time from 1 to 3 h). With increasing calcination temperature and time, saturation magnetization (M_S) of the synthesized (Ni, Cu, Co)Fe₂O₄ increased and the coercivity (H_C) decreased. Under the optimum conditions (i.e. NiCl₂·6H₂O added amount of 3.0: 100 g mL⁻¹, 1000 °C, 3 h), the M_S and H_C values of the product were approximately 46.1 emu g⁻¹ and 51.0 Oe, respectively, which were competitive to those of other nickel ferrites synthesized from pure chemical reagents. This method explores a novel pathway for efficient and comprehensive utilization of the low nickel matte.     

Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

The amorphous carbon nitride coatings (a-CN_x) were deposited on Si₃N₄ disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CN_x coatings' hardness was measured by nano-indentation and the friction and wear property of the a-CN_x coatings sliding against Si₃N₄, SiC, Al₂O_3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CN_x coatings contained 12 at.% nitrogen and the major chemical bonding was sp² C = N and sp³C–N. The nano-hardness of the a-CN_x coatings was 29 GPa, higher than those of balls. Among five kinds of tribo-systems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si₃N₄ balls, the largest wear rate of the a-CN_x coating of 1.77 × 10^− 7 mm³/Nm was obtained as sliding against Al₂O₃ ball, while the smallest wear rate of a-CN_x coating of 1.44 × 10^− 8 mm³/Nm was gotten as sliding against Si₃N₄ ball. However, SUJ2 ball showed the highest wear rate of 7.0 × 10^− 7 mm³/Nm, whereas Al₂O₃ ball exhibited the lowest wear rate of ball of 3.55 × 10^− 9 mm³/Nm. The XPS analysis on the worn surface for the a-CN_x coatings displayed that the nitrogen concentration decreased and the sp²-bonding-rich structure was formed after sliding tests in water.