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Structure, Mechanical Properties and Tribology of W–N and W–O Coatings

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Structure, Mechanical Properties and Tribology of W–N and W–O Coatings Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM Structure, mechanical properties and tribology of W–N and W–O coatings T. Polcar a,*, A. Cavaleiro b a Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague 6, Czech Republic b SEG-CEMUC – Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos, P-3030 788 Coimbra, Portugal article info abstract Article history: The tribological properties of nitrides and oxides of transition metal thin films deposited by reactive mag- Received 21 February 2009 netron sputtering have been thoroughly studied for three decades. Nevertheless, there are still several Accepted 13 July 2009 gaps in knowledge. The majority of studies are focused on a limited number of metals, namely Ti, Al and Cr, while other potentially attractive compounds are aside the main attention. Even in case of TiN, probably the most studied hard thin film, the frictional and wear behaviour brings many controversies. Keywords: Despite significant progress of analytical and computational methods, the analysis of the wear behaviour High temperature tribology is still a great challenge. Tungsten nitride We are presenting here a summary of our recent work on tungsten nitride (nitrogen content 0–58 at.%) Tungsten oxide Protective coatings and tungsten oxide (oxygen content 0–75 at.%) coatings deposited by reactive magnetron sputtering. Our aim has been the analysis of the connection of fundamental properties of these films, such as chemical composition, structure, hardness, Young’s modulus and residual stress, with their tribological properties – friction coefficient and wear rate. We have been focused mainly on the description of the dominant wear mechanisms influencing the tribological properties. The tribological tests have been carried out both at room and elevated temperature; the temperature was increased in steps until immediate coatings failure. The tungsten nitride coatings with the ‘‘worst” parameters generally considered as vital for high wear resistance, such as hardness, were considered to have the best tribological performance. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction W–N coatings were studied for mechanical applications [2]; nevertheless, the main attention was paid to ternary systems pre- Coatings of transition metals nitrides are known for their high pared by the addition of different elements (e.g., W–Ti–N [3] and hardness and excellent wear resistance, being titanium nitride W–Si–N [4]), with the final scope of improving the mechanical the best known member of this family. However, its tribological properties of the coatings. Tribological properties of tungsten ni- properties are significantly diminished when the coating is ex- tride coatings at room [2,5] and high [6] temperature were thor- posed to elevated temperatures, particularly due to rapid oxida- oughly studied by present authors. It has been shown that the tion. In this respect, chromium nitride possesses significant tribological properties of tungsten nitride coatings are strongly advantages compared to TiN, mainly higher oxidation resistance, influenced by the formation of a tungsten oxide tribolayer even thermal stability and, particularly, corrosion resistance. On the at room temperature. Further increase of environment tempera- other hand, as main drawback CrN exhibits a significantly higher ture led to the oxidation of the coatings, particularly in the area friction coefficient, which is not desired in many applications [1]. of the contact. A great amount of research work was also carried Other nitrides, such as MoN, VN or AlN, could outperform TiN or out by the authors in the W–O system [7,8]. Besides a complete CrN in many parameters; however, their commercial use is very and detailed basic characterization of the coatings, particular inter- limited. Tungsten nitride stays still aside of attention despite some est was paid to their thermal stability in both protective and significant advantages, such as extremely high hardness and excel- oxidant atmospheres [9]. In some specific cases, the overall tribo- lent adhesion on typical steel substrates. logical behaviour of the coatings was assessed by pin-on-disk against different counterbody materials [5–7]. This paper summarizes our work dealing with the tribological properties of tungsten nitrides deposited by magnetron sputtering. Additional results concerned with the W–O system will be also * Corresponding author. Tel./fax: +420 224 312 439. presented as a tool for interpreting and understanding the global E-mail address: [email protected] (T. Polcar). tribological behaviour of these systems. 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.07.013 16 T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 2. Experimental details + + Tungsten oxide and nitride coatings were deposited by DC reac- W25O75 + * tive magnetron sputtering from a tungsten target in a Ar + O2 or N2 # atmosphere, respectively, onto high speed steel AISI M2 substrates W45N55 # * heat treated to have a hardness close to 9 GPa. The deposition runs º W88N12 º º for both tungsten compounds were performed keeping constant º * the following parameters: total working pressure (0.3 Pa), target W81O13 º º À2 º current density (10 mA cm ), substrate at floating potential, no & W90N10 substrate rotation, inter-electrode distance (65 mm) and substrate Intensity (arb. units) & temperature (<350 °C, with no external heating). In order to W100 achieve different N and O contents in the films, the coatings were deposited with different flow ratios of the reactive gas, N2/Ar from 0 to 3 and O2/Ar from 0 to 1, respectively. Before any deposition, an 10 30 50 70 ultimate vacuum pressure better than 5 Â 10À4 Pa was reached 2 Theta (deg) and the substrates surface was ion cleaned with an ion gun. The chemical composition and the structure were determined Fig. 1a. Typical X-ray diffraction patterns of W–N and W–O sputtered coatings. by a Cameca SX-50 electron probe microanalysis apparatus (EPMA) (* – Substrate; + – WO3;o–b-W, # – W2N, and – a-W). and an X-Pert Philips X-ray diffractometer (XRD), respectively. The oxides microstructure was analyzed by high resolution transmis- sion electron microscope (HR-TEM – JEOL 2010-FEG, operated at 200 kV). The mechanical properties of the coatings were evaluated by depth-sensing indentation technique using a Fischer Instru- ments-Fischerscope, with a maximum load of 50 mN and following the testing procedure described elsewhere [10]. The adhesion/ cohesion of the coatings was evaluated by scratch-testing tech- nique using a Revetest, CSM Instruments. The load was increased linearly from 0 to 50 N, using a Rockwell C 200 lm radius indenter tip, loading rate of 100 N/min, and scratching speed of 10 mm/min. Wear testing was done using a high temperature pin-on-disc tribometer (CSEM Instruments). Al2O3 balls with a diameter of 6 mm were used as counter-parts. All measurements were pro- vided with a load of 5 N and a linear speed of 0.05 m/s; the relative humidity of the air was kept constant at (25 ± 5)%. The maximum static Hertzian pressure for the elastic contact between the ball and the coating was about 1.5 GPa. The worn volume of the balls was evaluated by optical microscopy. The morphology of the coat- ing surface, ball scars, wear tracks and wear debris were examined by scanning electron microscopy (SEM); the chemical analysis of the wear tracks and the wear debris was obtained by energy-dis- persive X-ray analysis (EDX). The profiles of the wear tracks were measured by mechanical profilometer. The wear rates of the ball Fig. 1b. HR-TEM image of W26O74 coating. and the coating were calculated according to [6] as the worn mate- rial volume per sliding distance and normal load. The average value of five profiles measured on each wear track was used to calculate Typical XRD diffractograms for W–N and W–O are shown in the coating wear rate. Fig. 1a [5,8]. The structure of the W–N and W–O coatings can be correlated with their chemical composition: (i) for low-nitrogen or oxygen contents (N,O < 9 at.%), the coatings exhibited the equi- librium bcc a-W phase with a (1 1 0) preferential orientation; (ii) 3. Results and discussion for coatings with intermediate nitrogen and oxygen contents up to 30 at.% (e.g., W88N12,W85N15 or W81O19) the b-W phase is de- 3.1. Coating characterization at room temperature tected mixed with the a-W; in the N-containing films for at.% N > 15at.% fcc. NaCl-type b-W2N is also occurring (iii) for coatings 3.1.1. Chemical composition and structure with higher reactive element contents (N,O > 30 at.%) the structure The increase of the nitrogen partial pressure ratio p =p from is either formed by the NaCl-type nitride compound, b-W N, with ð N2 ArÞ 2 0 to 3 led to a linear increase of the nitrogen content in the films (2 2 0) preferential orientation, in W–N coatings or by a low crys- from 0 to 55 at.%. The oxygen content originating from the residual tallinity order phase. It should be pointed out that an increase of atmosphere and the chamber leaks was lower than 5 at.%; when nitrogen content over 55 at.% was followed by the formation of higher nitrogen flows were used, the oxygen content decreased. hexagonal d-WN phase [6] whereas the structure of the W–O coat- Similar results were obtained for the W–O system, but being pos- ings with high oxygen content can be described as nanocrystalline sible to reach much higher O contents (75 at.%) even if the max- or quasi-amorphous.
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