ICMCTF2013 Session EP: Syposium E Poster Session

Due to its superior mechanical properties, titanium nitride (TiN) has been widely applied in industry, especially in hard and protective coatings. For the deposition of TiN coatings, people usually focused on adjusting the deposition parameters, such as bias voltage, nitrogen flow rate and substrate temperature, to produce harder and thicker coatings. However, as the film thickness increases to a few μm, the residual stress may increase concomitantly and thereby causing film spallation. The purpose of this study was to develop a deposition method to produce TiN coatings with a thickness larger than 3 μm and above without using Ti interlayer, in other words, to produce a thick TiN coating with low residual stress.

TiN was deposited on p-type (100) Si by unbalanced magnetron sputtering (UBMS). The controlling deposition parameter was either the opening of gate valve between turbomolecular pump (TP) and deposition chamber or the pumping speed of TP. The flux of argon and nitrogen was adjusted at a fixed ratio to maintain the same pressure in the chamber. The film thickness was measured by scanning electron microscopy (SEM). The Hardness of the film was measured using nanoindentation (NIP). X-ray photoelectron spectroscopy (XPS) was used to characterize the bonding state and compositions of the coatings. The preferred orientation of TiN coating was determined using X-ray diffraction (XRD). Laser curvature method was used to measure the residual stress of the TiN film. Using the proposed method, we could successfully produce TiN films with a thickness more than 4 μm and the residual stress was less than -2 GPa. By lowering pumping speed, the lifetime of TP can be effectively prolonged, which would be beneficial to the hard coating industry.

Wear is the progressive damage, which occurs on the surface of a component as a result of its motion relative to the adjacent working parts. To prevent from wear damage, solid lubricants have been introduced to protect the surface of materials and to prolong the service life of element. In the past 40 years, many studies have been done on them, especially for the MoS₂ coating. The experimental studies described in the previous works are all based on the observed trend using the coated flat substrate. Experiments on the comparison of MoS₂ based coating lifetime under one side coated or two side coated counterbodies is, however, lacking, which is popular problems in the linear guidance system and ball screw drives.

Therefore, it is interesting to do some research to explore the effect of coating position on the coating lifetime and friction coefficient. In this study, the coating was sprayed on the ball or cylinder and flat substrate. The FE model was chosen to model the stress distribution for different conditions, to get a better understanding of the wear mechanism of the coating.

In this study, an aerosol sprayed MoS₂ based varnish was deposited on a cleaned and polished 304 stainless steel flat surface and AISI52100 ball or cylinder surface at room temperature. The thickness of the coating is about 10 µm (flat) and 12 µm (ball and cylinder). The normal force is varied from 100 N to 1000 N and displacement amplitude is from ± 10 µm to ± 40 µm.

Some results could be drawn from this study. Firstly, the coating sprayed on substrate will lower contact pressure and enlarge the contact area. The coating on the flat has the similar maximum contact pressure as coating on the cylinder or ball, but they have different contact areas. Secondly, under lower contact pressure of cylinder-on-flat, there is a significant stress gradient between substrate and coating, especially at the interface of coating and substrate. Hence, coating failure occurs easier at the interface, which leads to a lower coating lifetime. In this low contact stress situation, patches of debris appears to be extruded back and forth in steady-state sliding, while in the high contact stress, this debris movement is delayed. The plastic shearing of the junctions forms plucks off the tips of the softer asperities leaving them adhering to the harder surface in the high contact stress, thereby forming the tribo-film and bringing a longer lifetime. Thirdly, the coating on the ball/cylinder has always higher coating lifetime than coating on flat, because the debris generated between two contact bodies could form more easily the transfer film or stay on the wear track.

Internal stress in plated deposits has been a common problem that may affect the functionality of coatings. Coatings may develop cracks, blisters, distortions and peel away from the substrate material when the stress levels are high. Electrodeposition parameters, such as; current density, coating thickness, pH and temperature of the bath affect the levels and the characteristics of internal stress of coatings. Furthermore, insoluble particles in electrocodeposition are also expected to make contributions. The influence of the electrocodeposition parameters and their interaction effects on the internal stress during the electrodeposition of Ni and Ni-MoS₂ composite coatings were studied by fractional factorial design. The parameters studied and their ranges were; MoS₂ particle concentration (0-5 g/l), temperature (30-50ºC), pH (2-4), current density (1.2-4.8 A/dm²), and coating thickness (25-50µm). MoS₂ addition into Watts bath resulted in the decrease of the tensile internal stress values of the composite coatings or even changed the stress character from tensile to compressive. Moreover; low stress values were obtained when pH was low and coating thickness was high. The temperature of the Watts bath did not show a significant effect. However, the effect of current density was seen to depend on other electroplating parameters.

The effective indenter concept was introduced by Bolshakov et al in 1995 [1]. It's well known that this concept is the theoretical basis of the standard analysis method for nanoindentation measurements, the Oliver & Pharr method [2]. With some extensions this concept is also applicable to more complex mechanical contact experiments on layered or viscose materials for normal and multi-axial loading conditions. This was shown in many papers in the last 15 years [e.g. 3,4].

This work will focus on presenting examples for these extensions, mainly the extension for time dependent material behaviour. So analysis results for viscose materials (like polymers or soft metals) for different test methods (indentation, scratch and tribotests) will be presented.

[1] A. Bolshakov, W. C. Oliver, G. M. Pharr, MRS Symp. Proc 356, p 675 (1995)

[2] W. C. Oliver, G. M. Pharr, J. Mat. Res. 7 (1992) 1564-1583.

[3] N. Schwarzer, J. Mater. Res., Vol. 24, No. 3, March 2009, 1032 – 1036

[4] N. Schwarzer, Phil. Mag. 86 (33-35) 21 Nov – 11 Dec 2006, 5153 – 5767

Hydrogenated amorphous carbon (a-C:H) thin films are among the best coating solutions for the reduction of automotive fuel consumption because of its very good tribological properties. Quite high values for hardness are required to withstand high contact pressure occurring for example in the cam-tappet contact in automotive motorsport engine. Commercial coatings exist with adapted sub-layers to obtain a good adhesion of highly stressed DLC (Diamond-Like Carbon) on the steel substrate. Since wear can occur, the development of wear-resistant coating is still necessary.

Each deposition process lead to a DLC film with specific properties correlated with hydrogen content and sp³/sp² bonded carbon ratio. Many elements have been introduced in the a-C:H matrix to modify and improve some properties of this material. For fluorine containing DLC, friction and wear properties improvement can be observed [1] but hardness is strongly decreasing with fluorine introduction [2]. Objective is thus to obtain hard a-C:H:F with enhanced tribological properties compared to standard a-C:H without changing deposition parameters.

DLC films are produced by radio-frequency PECVD with different combinations of precursors in order to explore various F/H ratios. ERDA-RBS measurements are performed to determine the chemical composition of the films. Chemical bonding is observed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Then, Young modulus and hardness are measured by nanoindentation. To conclude, tribological tests are performed with a ball-on-disc tribometer to measure wear resistance and friction coefficient in dry conditions. Fluorine introduction led to a decrease of hydrogen content and hardness. Coatings with less than 6 at.% of F kept a hardness superior to 28 GPa and wear resistance was improved.

[1] Donnet C., Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review, Surface and Coatings Technology, 1998, vol. 100-101, p. 180-186

[2] Bottani C.E., Lamperti A., Nobili L., Ossi P.M., Structure and mechanical properties of PACVD fluorinated amorphous carbon film, Thin Solid Films, 2003, vol. 433, p. 149-154

To improve the tribological properties of materials for use in various fields, the application of solid lubricant films that lower the friction coefficient is desirable. Among solid lubricant materials, although soft metals have a higher friction coefficient and poorer durability than other solid lubricants such as graphite and MoS₂, these metals are currently used in electrical and mechanical parts such as electrical contacts owing to their high electrical conductivity. Therefore, low-friction, wear-resistant electroconductive solid lubricant films are desired. In contrast, diamond-like carbon (DLC) films have low friction and high wear resistance. Coating with hydrogen-free DLC films is highly advantageous. Lubricants such as polyalphaolefin (PAO) with and without glycerol monooleate (GMO) are capable of significantly reducing the friction of hydrogen-free DLC films under boundary lubrication conditions. Research on the development of nanometer period multilayer films for improving film hardness has thus been carried out. To apply these multilayer films as solid lubricants, we proposed a nanoperiod multilayer solid lubricant film with friction lower than that of conventional solid lubricants. This study aims to realize lower friction and electroconductive solid lubricant films on the basis of a nanoperiod multilayer solid lubricant model. To develop electroconductive solid lubricant films, multilayer and mixed films composed of gold and DLC layers were deposited using bias radio frequency sputtering. Then, the electroresistivity and tribological properties of the deposited films were evaluated. The electroresistivities of 80 at% gold-DLC multilayer and mixed films were low at values less than 0.03 Ω cm, while the electroresistivities of 50 at% gold content multilayer and mixed films were low at 15–18 Ω cm. The 50 at% gold content multilayer films had a higher Young’s modulus and hardness than the gold monolayer films and values similar to those of DLC films.

The friction coefficient of the nanoperiod multilayer film was the lowest of all the tested samples. In addition, the wear depths of the multilayer films were smaller than those of the gold and DLC mixed and monolayer films in air. The friction coefficient and wear durability were considerably improved for the multilayer films among the samples tested under water boundary lubrication conditions. Under PAO boundary lubrication conditions with and without GMO, the wear durability of the multilayer films was superior, and the damage to the multilayer films was less than that for the gold monolayer film and similar to that for the DLC film.

Bismuth is a soft semimetal with a rhombohedral graphite-like quasi-layered structure and a low melting point of 271^oC. These characteristics point to the use of Bi or Bi-compounds for self lubrication applications where low friction and low wear rates are required. However, pure Bi and its compounds are very soft. Therefore our proposal is the inclusion of Bismuth into a hard coating using the methodologies developed for nanocomposite thin films; either as nano-sized particles into a hard matrix or as a lamellar structure, alternating a bismuth layer with a hard-coating layer. Then, the excellent frictional properties of Bismuth could be exploited in combination with materials that provide mechanical resistance. Bismuth sputtered films for mechano-tribological applications and its addition into metal nitride coatings have been poorly studied, therefore there is a strong need to define the deposition conditions adequate to achieve a stable and adhered coating. In this work, we report the deposition of Bismuth containing niobium nitride coatings using a confocal dual sputtering system. Both targets (2”) were ignited simultaneously, but using different powers; 400 W direct current for the Nb target (99.95 %) and 6,10 and 20 W radio frequency for the Bi target (99.999%). The coatings were deposited on Silicon at 200 ^oC using a mixture a fixed Ar/N₂ (14/6) flow ratio. The deposition pressure was 3 mTorr starting from a backing pressure below 5x10^-6 Torr. The composition of the coatings was obtained by both X-ray photoelectron, to clearly detect the presence of oxidized phases, and energy dispersive spectroscopy, in order to detect the presence of segregated bismuth. The Bi segregation and NbN structure were also studied by X-ray diffraction. The coefficient of friction of the coatings was measured by pin-on-disk tests using a load of 5 N and a sliding distance of 500 m.

Acknowledgement: The research leading to these results has received funding from the European Community Seven Framework Program (FP7-NMP-2010-EU-MEXICO) and CONACYT under grant agreements Nº 263878 and 125141, respectively.

A report on the design and construction of an inexpensive, compact triboluminescence measuring setup is presented. The system simply composes of a rotating stage and a stainless steel sliding pin. The counter pin is interchangable. An optical fiber, being connected to spectrophotometer, is used for detecting the generated light signal from the luminescent materials. A visible emission is measured from the bottom of the contact area. Any experimental parameters, such as an external load or rotation speed, are able to be adjusted. ZnS-based phosphor powders are selected as a reference sample. A characteristic emission from substitutional Mn⁺² ions at wavelength of 580 nm is able to be recorded even the load is 0.5N. Further information of the equipment with the results on triboluminescence of the phosphors are also included and discussed.

Elastic properties of anisotropic metallic thin films were investigated experimentally by several complementary techniques, namely in-situ tensile testing under X-ray diffraction (XRD), nanoindentation, and Brillouin light scattering (BLS). Specimens were probed along different directions to reveal the strong effects of elastic anisotropy at the (local) grain and (global) film scales. XRD allows the investigation of both local and global anisotropies, while BLS and nanoindentation are limited to global analyses. A micro-mechanical model, based on the Self-Consistent scheme, and accounting for the actual microstructure of the films, is applied to interpret experimental data. Although different kind of elastic constants can be determined with the used experimental techniques (static/dynamic, local/global), a good agreement is obtained, showing that comparison of these techniques is feasible when carried out carefully.

In the case of nanometric multilayers, a three-scale transition model is proposed for estimating the effective elastic constants from the intrinsic properties of elementary constituents (grains), taking into account the thickness ratio of each sublayer (in the case of multilayers) and the grains orientation distribution in each kind of layers. We show that the elastic anisotropy at the sample scale can be greater in the case of multilayers than in the case of single-layers, as confirmed experimentally by Brillouin light scattering.

The Oliver & Pharr method [1] is probably the most widely used methodology for the analysis of nanoindentation measurements in order to mechanically characterize thin films and coatings. It is based on the implicit assumption of the concept of an effectively shaped indenter which was introduced later by Bolshakov [2]. This concept implies certain model assumptions (e.g. a flat surface and a monolithic half space) which may not be met by sophisticated coating structures. Nevertheless, the Oliver & Pharr method is frequently used in literature to analyze such sophisticated coating structures (e.g. multi-layer coatings, thin films, gradient coatings), even though extensions to this method allowing a proper analysis of such sophisticated surface structures have already been developed by Schwarzer [3]. This work will explain the model assumptions in order to help understanding them by means of adapted schemata of the “effective indenter” concept and examples which illustrate the significant differences in the analysis results (e.g. elastic modulus E, hardness H, yield strength Y, stress profiles, etc.) between the original Oliver & Pharr method and the extended one by Schwarzer.

[1] W. C. Oliver, G. M. Pharr, J. Mat. Res. 7 (1992) 1564-1583.

[2] A. Bolshakov, W. C. Oliver, G. M. Pharr, MRS Symp. Proc 356, p 675 (1995).

[3] N. Schwarzer, J. Phys. D: Appl. Phys., 37 (2004) 2761-2772.

Microstructural characterization of WC - based coatings obtained by standard and modified HVOF method was showed in this article. Two different standard feedstock powders of WC-Co 88-12 and 83-17 (by Amperit) type was used to deposition of coating o steel substrate. The aim of investigation was related to comparison of microstructure and basic mechanical properties of coatings depending of applied method of deposition. The range of investigations included characterization of feedstock powders by SEM, EDS, XRD and EBSD method and their technological properties as well. In second step the characterization of deposited coatings were made, especially evaluation of theirs overall quality, porosity, micro-hardness distribution, adhesion of coatings to substrate alloys and theirs tendency to cracks. To characterization of coatings microstructure the same methods were used. Adhesion to substrate alloy and tendency to crack of coatings were characterized by bend test and Brinell hardness measurement on polished top surface of carbide coatings.

Financial support of Structural Funds in the Operational Program - Innovative Economy (IE OP ) financed from the European Regional Development Fund - Project No POIG.0101.02-00-015/09 is gratefully acknowledged.

This research was conducted as a part of Innotech program founded by The National Centre for Research and Development in Warsaw (Poland).