Mechanical and tribological properties of coatings sputtered from SiC target in the presence of CH4 gas

Amorphous hydrogen-free silicon carbide (a-SiC) coatings demonstrate good adhesion to different steel substrates, low intrinsic stress and high hardness however show quite high coefficient of friction in comparison with carbon-based coatings. Some addition of carbon to SiC can promote the decrease of friction coefficient.In the present work the amorphous hydrogenated silicon-carbide (a-SiC:H) films with different C/Si ratio were prepared at room temperature using DC magnetron sputtering in two ways: (i) sputtering of silicon target; (ii) sputtering of SiC target, both in the gas mixture of Ar and CH₄. In the latter case the films contained less hydrogen at the same C/Si ratio. The mechanical and tribological properties of these films were studied to find their optimum combination.The hardness, elastic modulus (nanoindentation), intrinsic stress (Stoney's formula) and coefficient of friction (pin on disc tribometer) were examined in dependence on the technological parameters, film structure and composition (Raman spectra, electron probe microanalysis). An increase of carbon in the films from 50 to 70 at.% resulted in decrease of hardness and friction coefficient. In the first case (i) the hardness decreased from 13 to10 GPa and in the second case (ii) from 23 to 16 GPa. Thus sputtering of SiC target in the gas mixture of Ar and CH₄ allows obtaining at room temperature the films with C/Si > 1 in which relatively high hardness (16-18 GPa) and low friction coefficient (~ 0.15) are combined.     

Effect of small concentrations of oxygen and nitrogen on the structure and mechanical properties of sputtered titanium films

The results of a study of small additions of oxygen and nitrogen on the structure and mechanical properties of nanocrystalline titanium films obtained by magnetron sputtering are presented.     

A comparative study of magnetron co-sputtered nanocrystalline titanium aluminium and chromium aluminium nitride coatings

A comparative study of magnetron co-sputtered (Ti,Al)N and (Cr,Al)N coatings was made. It was found that while both coatings followed similar development pattern with increasing nitrogen pressure, the (Cr,Al)N coatings achieved much higher deposition rate and hardness, suggesting the coatings had a great potential for many industrial applications.     

On the amorphous and nanocrystalline Zr–Cu and Zr–Ti co-sputtered thin films

In the current study, we examined and compared the mixing and vitrification behavior of the Zr–Cu and Zr–Ti binary systems in the form of co-sputtered thin films with or without post-annealing. The co-sputtered Zr–Cu films are all amorphous under various co-sputtering conditions, suggesting the high vitrification tendency. The amorphous Zr–Cu thin film will start to crystallize into nano-crystalline Zr₂Cu and Zr₇Cu₁₀ phases upon long exposure at temperatures above 350 °C. On the other hand, it is difficult to form amorphous film with the Zr–Ti system, except at a low sputtering power of 30–50 W. The low powers enable the co-sputtered Zr–Ti thin film to exhibit the diffuse hump in the X-ray diffraction. Examination by high resolution transmission electron microscopy reveals numerous fine nano-crystalline phases around 2 nm in the amorphous matrix. Upon exposure at 700 °C, the Zr–Ti films transform into crystalline hexagonal close-packed α and body-centered cubic β phases.     

Mechanical properties of nanocrystalline nickel films deposited by pulse plating

This paper reports the mechanical properties of Ni films fabricated by pulse electrodeposition. Transmission electron microscope revealed that the prepared films had an average grain size of 25 nm with a narrow size distribution and the absence of dislocations. Small grain size leads to an increasing hardness as high as 7.8 GPa while Young's moduli keep a constant bulk value of 215 GPa, resulting in an increasing ratio of hardness (H) to elastic modulus (E). Interestingly, the wear resistance was also improved significantly. Under a constant normal load of 500 μN, the penetration depths of indenter slightly increased from 25 nm to 30 nm and the coefficient of friction varied from 0.12 to 0.20, depending on sliding scans. Depth sensing instrumented indentation experiments performed at different loading rates on specimens revealed an increasing rate-sensitivity of hardness, which concerns with a significantly small activation volume for plastic flow.     

Annealing effects in Ag-doped amorphous carbon films deposited by dc magnetron sputtering

Thin carbon films containing about 11 at.% Ag were deposited by dc magnetron sputtering of composite graphite/silver target. The stability of film microstructure upon annealing at 600 °C in a vacuum has been studied by transmission electron microscopy and electron diffraction. The as-deposited C/Ag films consisted of silver nanoparticles distributed in an amorphous carbon matrix. Upon annealing, the tendency was revealed towards coalescence within the set of particles, i.e. increase in the particle average diameter and decrease in the density of particles with time. The above changes occurred faster than it is predicted by the theories for three-dimensional and two-dimensional diffusion coalescence. The direct collisions and fusion of particles along with the diffusion transport of Ag atoms is suggested to cause the above effect.     

Nanomechanical and nanotribological properties of carbon-based thin films: A review

Carbon-based thin films possess unique and adjustable combination of properties such as high hardness and wear resistance, chemical resistance and good tribological performances. Among critical variables to tailor a-C film’s properties for specific application is the distribution of the carbon hybridization states (sp¹, sp² and sp³ bonds), the atomic H content, the content in dopants such as Si, F, N, B and O. Here we focus on: (i) a-C and hydrogenated amorphous carbon (a-C:H) films with a mixture of sp² and sp³ bonding, highly sp³-boned material (ta-C) and sp²-bonded carbon, (ii) carbon nitride (CN_x) coatings and (iii) metal/amorphous carbon (a-C:M) composite films.The study is focused on the review of the nanomechanical properties and analysis of the nanoscratching processes at low loads to obtain quantitative analysis, the comparison of their elastic/plastic deformation response, and nanotribological behavior of the a-C, ta-C, a-C:H, CN_x, and a-C:M films. For ta-C and a-C:M films new data are presented and discussed.     

Effect of air annealing on mechanical properties and structure of amorphous B4C films

Amorphous B₄C films were prepared by magnetron sputtering of the hot-pressed B₄C target in different regimes. Hardness, intrinsic stress and film structure were investigated in dependence on the annealing temperature in air.Changes in the film structure and composition were investigated by Raman spectroscopy, confocal microscopy, and electron probe microanalysis. It has been shown that an annealing at 500 °C for 1 h leads to stress reduction, slight thickness decrease and increase of film hardness. However already at 600 °C the film oxidation proceeds very intensively with formation of the phases of boron oxide and amorphous carbon in the surface layer. The thickness of the film decreases quickly.The film oxidation is accompanied by formation of numerous carbon hillocks and redistribution of film material after annealing in furnace at 500 and especially 600 °C. The oxidation of a-B₄C films as well as of the crystalline bulk samples starts in some locations and has clearly pronounced heterogeneous character that indicates heterogeneous structure of amorphous films as well as of bulk crystalline samples.Annealing in air for a long period shifts down the onset of formation of hillocks to 400 °C and changes in film morphology to 300 °C. Thus the upper temperature limit for application of a-B₄C films in air depends also on the exposure time at the operation temperature.     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.     

Microstructure evolution and hardening mechanisms of Ni-P electrodeposits

Ni-P alloy coatings with different phosphorus contents were prepared by electroplating in a nickel sulfate bath containing phosphorous acid (H₃PO₃). Hardening mechanism, such as dispersion hardening of Ni nano crystals in amorphous matrix of the as-deposited Ni-P coating and coarsening weakening of Ni₃P for the high P coating after 1 h of heat treatment at 400 °C were concluded from the experimental data. Hardening mechanism of Ni-P alloys were further discussed based on the microstructure evolution with increasing deposit P content and during the heat treatment by using high resolution TEM (HR-TEM). A maximum hardness was observed for the as-deposited and heat-treated Ni-P alloys with 4 wt.% and 6 wt.% P, respectively. These composition ranges corresponding to the microstructure with high hardness proposed are believed to be useful for the industrial applications and further study.     

溅射技术在SiC薄膜沉积中的应用和工艺研究进展

近年来,国内外SiC薄膜材料制备工艺研究迅速发展,由此带来SiC薄膜性能方面的研究也获得长足的进步,新技术、新工艺、新性能不断涌现.溅射SiC技术相对于其它沉积技术(CVD、PIP等)有许多独特的优点:沉积温度低、结合性和致密性好、表面平整、硬度高、光电性能优异以及工艺安全环保等,因此越来越受到重视,且已经成为沉积高性能SiC薄膜的重要技术方法.主要论述了几种不同溅射技术,着重介绍了溅射技术在SiC薄膜制备中的研究进展及SiC薄膜性能方面的研究进展和应用,并且展望了溅射技术制备SiC薄膜的发展前景和当前热点研究领域.     

Stress induced crystallization of amorphous materials and mechanical properties of nanocrystalline materials: a molecular dynamics simulation study

The deformation behavior of amorphous and nanocrystalline pure Ni thin films has been investigated using a molecular dynamics simulation study based on a semi-empirical interatomic potential (MEAM). It was observed that a tensile stress introduced to an amorphous material can enhance crystallization which eventually serves as an important deformation mechanism. After completion of crystallization, grains grow mainly by the rotation and coalescence, and with increasing grain size, the flow stress also increases. It was also found that when the grain size is small (below about 3 nm), the dominant deformation mechanisms are the grain rotation and the grain boundary sliding, the former being more active for smaller grains. The dependence of these observations on the interatomic potential used in the simulation is also discussed.     

Crystallization kinetics of amorphous Cr2AlC thin films

Amorphous Cr₂AlC thin films were produced by room temperature magnetron sputtering on NaCl substrates with subsequent dissolution of the NaCl. The crystallization kinetics of Cr₂AlC was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Two exothermal reactions are observed during DSC up to 1200 °C. Comparing lattice parameters obtained from XRD and ab initio calculations it is suggested that the first reaction is associated with the formation of hexagonal (Cr_,Al)₂C_x, while after the second reaction Cr₂AlC is formed. The activation energy for the phase transformations are 426 and 762 kJ/mol, respectively.     

奥氏体不锈钢上功能梯度SiC薄膜的制备和性能

用磁控溅射法在奥氏体不锈钢基片上制备了SiC单层膜和Ti/TiN双层膜以及Ti/TiN/SiC功能梯度薄膜。采用XRD和显微硬度计对薄膜的结晶质量和硬度进行表征;用AFM和SEM对薄膜的表面和截面形貌进行了表征。结果表明:Ti/TiN双层膜在氩氮流量比为15∶15时,薄膜的结晶质量最好,硬度最高,达到15.6 GPa,最适合作为钢基SiC薄膜的缓冲层。另外,功能梯度SiC薄膜比SiC单层膜的结晶质量好;不同退火温度下功能梯度SiC薄膜的硬度高于SiC单层膜,同时功能梯度SiC薄膜的表面结晶质量也优于SiC单层膜。     

Ion bombardment-induced nanocrystallization of magnetron-sputtered chromium carbide thin films

Nearly stoichiometric chromium carbide thin films (Cr₃C₂) were deposited by unbalanced r.f. magnetron sputtering of a chromium carbide target in a pure argon discharge. Their microstructure and hardness are shown to be strongly influenced by the working pressure and substrate bias during deposition. Further correlation with the plasma parameters, i.e. the energy and flux of argon ions as well as the flux of film-forming particles, is illustrated, which indicates the momentum transferred by bombarding ions per film-forming particle P_dens which creates a densification and simultaneously does not cause a mobility enhancement to be dominant for the film structure. Below a threshold value of P_dens of about 0.91 u^0.5eV^0.5, the resulting films exhibited an amorphous structure with a relatively low hardness of 1200 HV0.02. A nanocrystalline structure emerged only for larger values of P_dens, and was responsible for a hardness enhancement to 3500 HV0.02.     

On plastic strain recovery in freestanding nanocrystalline metal thin films

In a recent article [J. Rajagopalan, J.H. Han, M.T.A. Saif, Science 315 (2007) 1831–1834], we have reported substantial (50–100%) plastic strain recovery in freestanding nanocrystalline metal films (grain size 50–65 nm) after unloading. The strain recovery was time dependent and thermally activated. Here we model the time evolution of this strain recovery in terms of a thermally activated dislocation propagation mechanism. The model predicts an activation volume of ≈42b³ for the strain recovery process in aluminum.     

纳米压痕法测定微铸件硬度及弹性模量

利用新的金属型微精密铸造工艺制备微米尺度的ZnAl4微齿轮铸件,并用纳米压痕仪测试微铸件的硬度和弹性模量。结果表明,微齿轮铸件不同部位的硬度表现出梯度性,随晶粒尺寸的增加而减小,弹性模量的值比较分散。与常规尺寸铸件的显微硬度相比,微铸件硬度明显提高,最大达到1．70倍,而弹性模量则下降约50％。分析表明快冷造成的非平衡凝固组织是性能改变的主要原因。     

Effects of milling and subsequent consolidation treatment on the microstructural properties and hardness of the nanocrystalline chromium carbide powders

The influence of milling and subsequent consolidation treatments on the microstructural properties and hardness of the fabricated Cr₃C₂, Cr₇C₃ and Cr₂₃C₆ ceramic powders are investigated. For this reason, the elemental powders of Cr and C were mixed with proper ratio and then milled to the nanometer crystallite sizes (between 6 and 20 nm) and then were consolidated by using uniaxial cold press and subsequent heat treatment (at 1100 °C for 1 h) in Argon atmosphere. Microstructures of consolidated samples were characterized using X-ray diffraction (XRD) and microhardness measurements. A drastic increase in crystallite size of the samples was observed due to the effect of heat treatment. However, the as-consolidated samples still maintained their nanocrystalline characteristic with an average grain size of less than 100 nm. Besides, a very high hardness of 25 GPa was achieved for the Cr₃C₂ composition. This high hardness is attributed to the formation of carbide phases in the consolidated samples.     

Origins of microstructure and stress gradients in nanocrystalline thin films: The role of growth parameters and self-organization

The development of depth gradients of texture, morphology and stresses in thin nanocrystalline films was experimentally demonstrated for a nanocrystalline CrN film by means of position-resolved synchrotron X-ray nanodiffraction and explained by atomistic processes at the growing film surface and the effect of interfaces, both controlled by the deposition conditions. Controllable changes in the energy of incident particles adjusted by bias voltages ranging from ?40 to ?120 V affect the competitive growth of grains with different orientations, induce disruption of grain growth and thus give rise to structural variations across the film thickness. Subsequent changes in the volume fraction of grain boundaries and film texture were found to be responsible for changes in the residual stress state as defect generation proceeds to different extents in the interior of differently oriented grains and in the interfacial area. While the defect density predominantly affects the development of intrinsic stress, the variation in the number of weakly bonded atoms of grain boundaries determines the thermal stress component. The structural dependence of both stress components thus contributes to the characteristic development of stress gradients in thin nanocrystalline films.