ICMCTF2017 Session EP: Symposium E Poster Session

Transition metal nitride coatings are very attractive materials because of their excellent mechanical properties. Especially, the CrAlN coatings have been paid much attention to cutting tool’s film due to their high hardness, low surface roughness and excellent thermal stability.In this work the influence of Cr content and various interlayers on mechanical properties of CrAlN coating was investigated. In order to control the Cr content the pulsed DC current was adjusted between 0.4 and 2.0 A, and various interlayers such as CrN, CrZrN, CrN/CrZrSiN were synthesized between the CrAlN coating and the WC substrate. The microstructure, residual stress, hardness and elastic modulus, and friction coefficient were evaluated by field-emission scanning electron microscopy (FE-SEM), laser reflectance system, nano-indentation, and ball-on-disc type wear tester, respectively.

When the Cr content in the CrAlN coatings increased from 0.11 to 0.24 at.%, the hardness and compressive residual stress were measured to be in a range from 31 to 41 GPa, and from 4.3 to 5.7 GPa, respectively. Hardness enhancement could be attributed to the solid solution hardening, in that with Cr insertion. The lattice distortion in the coating developed, and this leads a compressive residual stress enhancement. Therefore, the dislocations became more and more difficult to move, and the hardness of coatings gradually increased. After the scratch test, the critical load of the CrAlN coatings gradually decreased from 48 to 41 N. Generally, the high residual stress causes the low adhesion, and the compressive residual stress could be considered as a factor on the adhesion decreasement. During the wear test, the friction coefficient of the CrAlN coatings with the CrN and CrN/CrZrN interlayer exhibited improved values of 0.34 compared to that of the CrAlN coating with the CrZrN interlayer (COF 0.41). These improved friction coefficient could be attributed to the H/E ratio of the interlayer between the CrAlN coating and the WC substrate. In view of the coating structure, there exists a gradual increase in the H/E ratio from the WC substrate (H/E, 0.040), to the CrN interlayer (H/E, 0.076), and CrZrSiN interlayer (H/E, 0.083), and the CrAlN coating (H/E, 0.089). The CrN and CrZrSiN interlayers induced a smooth transition of the stress effectively under loading conditions, and wear properties could be improved significantly by structuring the coating with an optimal gradient of the H/E ratio of the coating/interlayer/substrate.

The present work studied the effect of boriding coatings on hydrogen embrittlement on AISI 8620 by means the mechanical behavior. The formation of boride were carry out at three different temperatures (1173, 1223, and 1273K) for 6 hours of exposure time by dehydrated paste pack method. After boronizing, the presence of the boride coatings were observed scanning electron microscopy (SEM), X-ray diffractometer and energy dispersive spectroscopy (EDS) analysis.

Hydrogen was introduce into samples with boride coating through cathodic charging applying a current density of 50 mA/cm² by 0.5 M sulphuric acid solution kept at a room temperature.

The mechanical behavior of boride coating with hydrogen diffusion were used the following experimental techniques: Vickers micro-hardness, Daimler-Benz Rockwell-C indentation and three-point bend test. As a result of the hydrogen diffusion on sample boride, the borided layer thickness decrease and microhardness tests showed a significant increase in the surface hardness caused by the increased boronizing temperature, moreover the adhesion strength in all condition obtained sufficient cohesion. Finally, three point bend tested show a drastic reduction in ductility and increase the fracture stress value.

CoCrMoSi alloys were developed for high temperature applications particularly to resist liquid metal corrosion, due to the distribution of the Laves phase. However, the range of properties of this alloy system allows for uses well beyond the original scope. The metallurgical stability of CoCrMoSi coatings exposed to temperature has been shown to be associated with the stability of Laves phase. The successful use of coatings for high temperature applications requires the understanding of oxidation behavior and influence of the oxide layer on wear. This research focuses on the study of the abrasive wear behavior of CoCrMoSi coatings exposed at 450C and 750C for 6h in an air furnace. The aim is to characterize the oxides formed at the surface of coatings and their role on wear. The CoCrMoSi alloy was deposited by Plasma Transferred arc on AISI304 stainless steel plates (120mmx100mmx12mm). Abrasive wear tests were carried out on a rotating ball apparatus with applied loads ranging from 0.2N to 0,45N. The low loads intended to magnify the impact of surface oxides on wear behavior. For each surface a set of wear tests was carried out with small increments on the applied load until the oxide film was broken and the CoCrMoSi coating started to worn. Raman spectroscopy and X-ray diffraction identified oxides at the surface. Confocal microscopy and scanning electron microscopy analysis characterized the wear scar. At the oxidation temperature used of 450ºC Co and Fe oxides form. The kinects of Co₃O_4, Fe₃O₄ and Fe₂O₃ allow for the fast oxidation of Co and Fe even at low temperatures. Under these conditions scratches were identified on the wear scar associated with the removal of oxide particles, suggesting a low adherence to the coating surface. Exposure at 750ºC resulted on a continuous oxide film of Cr₂O_{3 ,} analysis of the wear scar reveals rolling to be the predominant mechanism. Correlation with non-oxidized CoCrMoSi coatings shows that oxides formed at 450ºC do not impact on the wear performance of coatings. Oxidation at 750ºC resulted on a reduction of friction coefficient, leading to an increase on wear resistance.

The increasing interest for new coatings with higher properties, opened the doors to research of carbon nitrides (CNx); these coatings are attractive for industrial applications due to their its high recovery rates, low friction coefficients and wear rates, in addition to their self-lubricating properties. In this research, CNx coatings were deposited by DC magnetron sputtering, using a Graphite target and a power density of 2,4W/cm², deposition temperature was 250°C, working pressure of 6-7x10^-3 mbar and a BIAS voltage of -70V, prior to the deposition an ionic cleaning was carried out to clean the surface of the substrates, the percentage of nitrogen was varied between 10% and 50% (N₂/(Ar+N₂)) in the gas mixture, in order to evaluate the effect of nitrogen incorporation on microstructure, composition, mechanical and tribological properties. The thicknesses around 2.0 µm were obtained for all coatings; SEM images revealed homogeneous, compact and columnar coatings; the XRD analysis showed that all coatings are completely amorphous. The micro-Raman spectra are characteristic of carbon-rich sample; these clearly show D band (disordered aromatic rings) and G band (graphite). It was also possible to identify the band associated to C-N triple bonds. The mechanical and tribological properties are affected by the incorporation of nitrogen, by increasing the nitrogen in the gas mixture, increasing the compressive residual stress and hardness; all coatings show similar tribological behavior, with smooth friction records, their low friction coefficients and wear rates are significantly low compared to others self-lubricating coatings like, VN, VSiN, CrVN, etc.

The reduction of the coefficient of friction combined with reduced wear rates is a major topic in many different industrial applications. Extreme-pressure (EP) and anti-wear (AW) additives as well as low friction coatings have been intensively investigated for several years to achieve this behavior. Lately, the addition of WS₂ or MoS₂ nanoparticles has been proven to be beneficial to the reduction of friction in various tribological contacts. Investigations into the application of sulphur containing lubricants in combination with tungsten functionalized or doped surfaces like W-DLC coatings revealed an additional decrease in the coefficient of friction and wear rates compared to base oils. This effect was proposed to be related to the in-situ formation of a WS₂ carrying low friction tribofilm.

To obtain more information about the suggested formation of WS₂, tungsten based carbide coatings have been deposited applying physical vapour deposition (PVD) and were tested in a linear oscillation SRV testing system. The influence of the applied normal load and EP additive concentration was investigated. Energy dispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were carried out to investigate the interface near regions within the tribocontact. An emphasis was placed on the distinction whether the reduction of friction and wear is contributed by the in-situ formation of WS₂ rather than by the generation of Magnéli phase oxides.

It will be shown how the classical Oliver and Pharr method [1] has to be extended in order to make it fit for the performance and analysis of mixed loading nanoindentation tests. While the classical Oliver and Pharr method can only deal with pure normal loads and allows the extraction of hardness and Young’s modulus for a given Poisson’s ratio [1, 2, 3], the extended method principally allows for the simultaneous parameter identification of hardness, yields strength in two directions, Young’s modulus and Poisson’s ration. Under proper experimental conditions, also the extraction of intrinsic stresses seems to be possible. The author will present the method, the theoretical background and a few experimental examples (data from T. Chudoba, ASMEC GmbH, with thanks).

[1] W.C. Oliver and G.M. Pharr, J. Mater. Res. 7, 1564 (1992)

[2] N. Schwarzer, G. M. Pharr, Thin Solid Films, Vol. 469-470C pp. 194-200

[3] N. Schwarzer, T. Chudoba, F. Richter: „Investigation of ultra thin coatings using Nanoindentation”, Surface and Coatings Technology, Vol 200/18-19 pp 5566-5580

Aluminium Metal Matrix Composites are a special class of metal matrix composites with immense potential which open up countless possibilities to enhance properties of materials that are needed in aerospace, military and automotive applications. The potential of these materials lies into their ability to be tailored to fulfil the expectations of the designer. In this study, microstructure and wear properties of aluminium metal matrix composite (AMMC) reinforced with titanium diboride (TiB₂)were investigated. The composite was fabricated through liquid state processing by incorporating 3, 6, 9, 12 and 15 wt% of titanium diboride into aluminium matrix. Uniform distribution of the reinforcement particles into the metal matrix was observed by the microscopic examination of the composite. To determine the friction and wear properties of the composite, experiments were conducted on a pin-on-disc tribometer, under dry condition, by varying applied load and sliding velocity while keeping the sliding distance constant. Loads of 10N, 30N, 50N and velocities of 100 m/min, 200 m/min, 300 m/min were employed over a constant sliding distance. Results obtained from the test revealed that the friction coefficient and overall wear rate increase with the increasing load and sliding velocity. However, the Al/TiB₂ composite shows lower wear rate in contrast to the unreinforced aluminium. Analysis of the worn out surface of the composite under scanning electronic microscope reveals the domination of abrasive wear. The details presented in the current paper form a basis for materials engineers to switch over to AMMCs from monolithic metals and alloys which offer superior wear properties.

Keywords: Aluminium metal matrix composite, Al-TiB₂, Friction, Wear, Dry sliding

The purpose of this work is to conduct a tribological characterization of thin films based on residual stresses, micro-abrasive wear modes, volume of wear (V) and coefficient of friction (μ). Initially, the residual stresses of thin films of TiN, CrN, TiAlN, ZrN, TiZrN, TiHfC and TiHfCN were analysed by X-ray diffraction; after ball-cratering wear experiments were performed using a ball of AISI 52100 steel and abrasive slurries prepared with black silicon carbide (SiC) particles and glycerine. The normal force (N) and the tangential force (T) were monitored throughout the tests and the coefficient of friction was calculated as μ = T/N. The results showed that the abrasive slurry concentration affected the volume of wear, the occurrence of micro-abrasive wear modes (grooving abrasion or rolling abrasion) and, consequently, the magnitude of the coefficient of friction: i) a low abrasive slurry concentration was related with low volume of wear, action of grooving abrasion and a relatively high coefficient of friction; ii) a high abrasive slurry concentration was related with high volume of wear, action of rolling abrasion and a relatively low coefficient of friction. In general, the compressive residual stresses measured were relatively low (< 1 GPa).

Keywords: Micro-scale abrasion, residual stress, two-body abrasion, three-body abrasion, PVD coatings.

The aim of this study was to analyze the frictional behavior of a bismuth-based soft coating, developed using a novel, eco-friendly and economically competitive synthesis. Bismuth has a non-toxic nature, making it attractive for the development of new tribological applications such as coatings or as oil additives.

Bismuth sulfide (Bi₂S₃) nanoparticles were synthesized in-house by means of an eco-friendly reaction in an aqueous medium under mild reaction conditions, in presence of a surfactant. The obtained particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Bi₂S₃ nanoparticles were mixed with a commercial organic varnish in order to generate a soft coating. The coating had a weight fraction of nanoparticles of 17.4 wt% and was manually applied on a SAE 4140 steel disk . The frictional response of the soft coating was evaluated using a pin on disc test (ν_l=0.02 m/s; p₀=1100 MPa; 6 m of sliding speed), using an AISI 52100 steel ball with a diameter of 6 mm as the counterpart. The same test procedure was employed using a commercially available molybdenum disulfide varnish to serve as a reference.

Both coatings exhibited similar friction coefficients during the test, with an initial low value (<0.1) that increased slightly towards the end of the test. The bismuth based soft coating showed an average friction coefficient ~30% higher than the molybdenum disulfide coating used as reference.