C. Fernández-Ramos

Bonding structure in amorphous carbon nitride: A spectroscopic and nuclear magnetic resonance study

Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquired by NMR, enables the description of a structure model for the studied amorphous-CNx phase prepared by dc-magnetron sputtering and to revise the interpretation found in the literature.Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquire...

Study of the thermal stability of carbon nitride thin films prepared by reactive magnetron sputtering

Abstract CN x amorphous films have been prepared by reactive magnetron sputtering in a pure N 2 discharge. The films grown on NaCl have been characterised by Fourier transform infrared spectroscopy (IR), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). C/N atomic ratios have been determined by EELS with values in the range 2.0–1.2 for samples grown under different conditions. The thermal stability of the films upon heating in vacuum was followed ‘in situ’ at the transmission electron microscope by EELS. This study has been completed by a thermogravimetric and mass spectrometer analysis of evolved gases upon heating in nitrogen flow and vacuum, respectively. Under these conditions the films are stable up to 1023 K. Above this temperature the films decompose by elimination of nitrogen remaining a carbonaceous residue. The thermal stability of the films upon annealing in air was studied by following the evolution of the X-ray photoelectron spectroscopy (XPS) peaks during heating in air of films grown on steel. Deconvolution analysis of the XPS spectra allows to determine the evolution of the different type of bonds. In particular pure carbon in the films appears more reactive to oxygen than CN and C–N bonds.

Preparation, microstructural characterisation and tribological behaviour of CNx coatings

Abstract A review is presented on the main results obtained during the last years on the synthesis and microstructural characterisation of CN x and Si-doped CN x coatings prepared mainly by the magnetron sputtering technique. An exhaustive characterisation by XPS, EELS, IR, XAS and NMR has allowed us to determine the microstructure of the films obtained under different experimental conditions and with different compositions. Thermal stability studies in vacuum and air atmosphere provided also a temperature limit for the practical use of the films. Finally, the relationships between the nature of the films and the friction behaviour are highlighted and discussed.

Tribochemical effects on CNx films

Abstract CN x thin coatings have been deposited by dc magnetron sputtering using a graphite target in nitrogen atmosphere under different experimental conditions. X-Ray photoelectron spectroscopy, electron-energy loss spectroscopy, nuclear magnetic resonance and Fourier-transformed infrared spectroscopy were used to elucidate the structural chemistry of each film. The tribological behavior has been investigated using a reciprocating pin-on-flat tribometer in a wide range of environmental conditions: ambient air, dry air and nitrogen. Tribochemical effects in relation to the nature of the surrounding atmosphere during friction tests are presented. Strong relationships between the N/C ratio, the nature of C–N bonds and the friction behavior are highlighted and discussed. An optimum in the friction and wear properties was found for a maximum in the CN/CC ratio for the studied set of CN x samples. The XPS/AES analysis of the sliding counterfaces support a destabilization of the CN x network under friction and build-up of a carbon rich tribolayer under steady-state.

Tribological behaviour and chemical characterisation of Si-free and Si- containing carbon nitride coatings

Abstract In the present paper, we compare the tribological behaviour of Si-free and Si-containing carbon nitride films grown on high speed steel substrates against steel counterfaces. The CN x coatings have been prepared by magnetron sputtering of a carbon target in a N 2 atmosphere while the SiCN x films were obtained by the same method, but adding a vapour pressure of Si(CH 3 ) 3 Cl. In the case of pure CN x , the presence of water molecules in the gas phase produces a negative effect in the tribological response while the Si-containing film is able to maintain a low friction value (0.12) even under humid atmosphere similarly to Si-DLC coatings. To achieve a better understanding of the friction mechanism, both Si-free and Si-containing films were characterised by Energy Filtered Transmission Electron Microscopy (EFTEM), X-Ray Photoelectron Spectroscopy (XPS) and Infrared Spectroscopy (IR). The buffer effect of silicon-doped CN x , decreasing the moisture sensitivity of the friction coefficient, is attributed to the adsorption of water molecules on SiO 2 domains formed in the Si-containing films. This adsorbed water may lubricate the contact in humid atmosphere allowing the shear strength to diminish.

Successive ion implantation of high doses of carbon and nitrogen on steels

Abstract HSS and stainless steel samples were successively implanted with high doses of carbon and nitrogen. Different sequences of implantation (C + , C + +N 2 + , N 2 + +C + and N 2 + +C + +N 2 + ) were carried out in order to achieve C x N y layers of ultra-high hardness. The bonded states of carbon and nitrogen were studied by XPS spectroscopy. The tribological behaviour was measured by means of ball-on-disc and microhardness tests. The microhardness tests showed that increases in hardness are highly influenced by the implantation order. None of the sequences tested led to a hardness increase higher than that for simple carbon implantation. When nitrogen was implanted into carbon-implanted steel, the result was a huge decrease in surface hardness. Ion implantation of high doses of nitrogen and carbon brought about a decrease in the friction and wear coefficients. The samples for which carbon was implanted in the last step showed the lowest friction and wear coefficients.

Tribological and mechanical properties of CNx- and SiCNx-TiN/Ti multilayered systems grown onto steel

One of the major problems of CN x -based hard coatings for mechanical applications is their poor adhesion to steel substrates. In the present case, a TiN/Ti gradient underlayer has been deposited between the carbon nitride films and a high-speed steel substrate by means of magnetron sputtering. The same method was used to grow different CN x top layers in which various deposition parameters (Ar/N 2 ratio, DC bias) were varied. A silicon-doped CN x film was also prepared by dosing a certain amount of the gaseous precursor Si(CH 3 ) 3 Cl in the reaction chamber. Scratch tests have shown that the introduction of the underlayer leads to an improvement of the adhesion in all cases. Tribological measurements clearly indicate that a good adhesion between the substrate and the coating is always needed in order to obtain low and stable friction coefficients. The wear rate and nanohardness of the samples have been also studied in relation with the different growing conditions. The results show an improvement of the adhesion, hardness and wear resistance without an increase of the friction coefficient for the samples synthesised using Ar/N 2 mixtures and DC-biasing of the substrate during deposition.

Structural modifications of silicon-doped carbon nitride films during post-deposition annealing

Abstract The incorporation of silicon in the composition of carbon nitride coatings has been demonstrated to improve their tribological behaviour by diminishing the negative influence of ambient moisture [1] . In this work, a detailed study by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) concerning the thermal stability of these silicon-containing amorphous carbon nitride films (SiCN x ) is presented. Such films are grown at room temperature by reactive direct current magnetron sputtering of a graphite target in a nitrogen discharge while a volatile silicon precursor is added. The structural and compositional modifications induced upon heating in air and vacuum environments are followed by XPS and in situ EELS measurements, respectively. The thermal stability is not observed to be improved by incorporation of silicon in the CN network compared to similar treatments on free-silicon CN x films [2] . Annealing in ambient air led to the loss of nitrogen for temperatures above 350 °C and partial oxidation of carbon and silicon species. TEM observation combined with EDX nanoprobe analysis revealed the formation of separate morphological domains with different chemical composition. A strong differential charge effect in XPS core level spectra of Si 2p and O 1s confirmed this phase segregation. The stability in vacuum is much higher, showing a significant decrease of the nitrogen content above 700 °C and a partial reduction of oxidised silicon species.