Terms Of Critical Load Research Articles

Parameters study of carbon bombardment step for enhanced adhesion of thick DLC coating deposited by cathodic arc evaporation

The main limitation for the deposition of thick (> 5 μm) DLC coatings is to achieve adhesion to the substrate. In the present work, the adhesion of thick (~20 μm) amorphous carbon coatings was analyzed considering the combination of metallic chromium as bond layer and a carbon bombardment step before the deposition of the functional coating. The carbon bombardment step was included to generate an interfacial structure composed by a mixing layer of Cr and C and a carbon transition layer rich in sp2 bonds. The complete deposition process was made by cathodic arc and the substrate was nitrided steel. Different sets of parameters were used during the carbon bombardment to change the interfacial structure and to analyze the effect on adhesion. The parameters evaluated were the bias voltage, duration, cathode current and temperature. The adhesion of a-C layer was assessed by scratch tests and the interface structure was evaluated by scanning transmission electron microscopy (STEM). Results indicate that a lower bias voltage possibly increased the sp3 content of the carbon transition layer, mitigating the negative impact of a thicker carbon transition layer. Moreover, an increase in the carbon cathode current may increase the adhesion, possibly by an increase in the thickness of the mixing layer. Finally, improved adhesion was achieved when the temperature of the bombardment was reduced. A diagram of expected behavior for the adhesion (in terms of critical load during scratch tests) as a function of carbon transition layer thickness and sp-bonds was proposed based on the available results.

Derivatives of pyrimidine-2,4,6-triones as components of additives to increase the lubricating properties of motor fuels

One of the important tasks of modern petrochemistry is the development of new additives that can improve the performance of motor fuels. With the expansion of the number of automotive and agricultural machinery and increasing its technical level, the demand for a wider range of petroleum products is increasing and, as well, the requirements for their quality are increasing. As the environmental and operational requirements for motor fuels are constantly increasing, it is necessary to find new, more efficient additives. The paper describes the synthesis of spirocyclic pyrimidine-2,4,6-trionеs by ring-closing metathesis reactions (RCM). This type of transformation is actively used in petrochemical and organic syntheses and allows to obtain compounds that are difficult to synthesize by conventional methods of organic chemistry. RCM conversion was performed using a Grubbs-Hoveyda catalyst in an amount of 5 mol. %. The compounds formed during the reaction were investigated as additives to motor fuels. Studies of the effect of synthesized spiropyrimidine-2,4,6-trione derivatives as additives on the tribological characteristics of ethanol and diesel fuel were evaluated on a four-ball tribometer in terms of critical load. At least three experiments were performed at each load. The error of the experiment was less than 5%. It was found that substances 2c and 2b increase the lubricating properties of ethanol - the critical load to the fuel burr when adding the synthesized compounds increased by 25%. Spiropyrimidine-2,4,6-trione 2b also had a positive effect on the lubricity of diesel fuel and had the effect of increasing the load to burr by 12% at a concentration of 0.032%. Thus, derivatives of spiropyrimidine-2,4,6-triones have proven to be promising compounds for the manufacture of motor fuels.

Research of N,N-diallyl (3-arylisoxasol-5-yl)-methylenesulfonylamides as additives for increasing the load carryng capacity of syntetic oil based on the pentaerythritol esther and butyric acid

The object of research is N,N-diallyl (3-arylisoxasol-5-yl)-methylenesulfonylamides (Ar: C6H5 (1); C6H4–4–CH3 (2); C6H4–4–OC2H5 (3)) as antiwear additives to oils that are derived from the corresponding sulfonyl chloride and dialylamine. The industrial additive DF-11 (zinc dialkyldithiophosphate) (4) was used as a reference in action, and the 2-mercaptobenzthiazole allyl ether was used as a structural standard (5). As a synthetic oil, pentaerythritol and n-butyric acid ester, which is obtained by the esterification reaction, is used.Some physical characteristics (relative viscosity and refractive index) of the obtained oil were studied with and without the addition of sulfonylamides (1)–(3).The effect of the addition of N, N-diallyl (3-arylisoxasol-5-yl)-methylenesulfonylamides (Ar: C6H5 (1); C6H4–4–CH3 (2); C6H4–4–OC2H5 (3)) on the dynamic strength of the test oil was evaluated by ASTM D2783 (GOST 9490-75) on the four ball friction machine in terms of critical load. The tests were carried out by friction in the corresponding liquids of standardized metal balls made of ShKh 15 steel (microhardness – 64–66 HRC, stiffness parameter – Ra<0.25 μm). The rotation frequency of the upper loaded balls in relation to three stationary balls is 1500 × 1, the oil temperature is 20 °C. The test time at each load is 10 s, the experiment repeatability is three tests for each load.The study of changes in the diameter of the wear spot Dw of metal balls during friction in the initial oil without adding compound (3) and in accordance with the addition was carried out at revolutions of 1500 rpm, an initial temperature of 25 °C, a load of 98 N, and a study time of 1:00. The results obtained indicate that Dw of the oil without making the specified compound was 0.75 mm, and when it was added (0.1 wt. %) – 0.67 mm, that is, the reduction in wear is 10.67 %.It is found that the presence of the studied additives (1)–(3) in low concentrations in a synthetic oil based on pentaerythritol and n-butyric acid can significantly increase its bearing capacity. The most effective compound (3), which at a concentration of 0.1 % of the mass exceeds the bearing capacity in comparison with (4) by 1.38 times, and with (5) – by 1.37 times. The specified compound is more effective in concentration, 10–20 times less in comparison with known additives.Thus, the use of N,N-diallyl (3-arylisoxasol-5-yl)-methylenesulfonylamides as additives to increase the bearing capacity of synthetic oils based on pentaerythritol and synthetic fatty acids, allows to increase the antiwear activity of lubricants. Therefore, they can be used to create new effective compositions for oils and petroleum products.

Upheaval buckling of pipelines due to internal pressure: A geometrically nonlinear finite element analysis

This work presents an analysis of upheaval buckling of pipelines triggered by internal pressure. It discusses the existing relationship between internal pressure and equivalent compressive axial force in the buckling context. The main focus is on the relative influence of prop imperfections and soil friction coefficients in critical load prediction and post-buckling configuration. To perform the analyses, numerical models are developed using geometrically-exact finite element of beams, undergoing large displacements and finite rotations. Contact between the pipeline and the soil is also included in the models. As a result, the work shows the equivalence of applying the internal pressure as a distributed load dependent on pipe curvature and as a follower compressive axial force, both in terms of critical load and post-buckling configuration. Varying prop imperfections and soil friction coefficients, it is concluded that the first parameter has more influence in critical load prediction than the last one. The same occurs in terms of post-buckling configuration: for the same increase of internal pressure from critical load, the imperfections have more influence in the post-buckling displacements than the friction between the pipeline and the soil.

Numerical modeling and experimental verification of ductile damage in boron steel hot stamping process

Abstract High Strength Steel (HSS) sheets formed by hot stamping is more and more widely used in automotive industry. Compared to cold formed components, hot stamped parts show significantly improved material formability as well as the desired microstructure and mechanical property distribution. However, the description of the material formability by means of Forming Limit Diagrams (FLDs), which are usually implemented in FEM dedicated codes, appears strongly limiting when applied to hot stamping, as the material behavior is dependent on temperature and strain rate. A general Lemaitre based damage model was developed and implemented in the FE model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550 oC and 750 oC, at different strain rates, to identify the necessary parameters for the calibration of the Lemaitre damage model. Nakajima-type experiments were conducted and numerically simulated to test the validity of the proposed criterion. The comparison between numerical and experimental results in terms of critical load at crack growth initiation shows that the proposed criterion is able to predict the ductile fracture onset in hot stamping.

An improved damage evolution model to predict fracture of steel sheet at elevated temperature

Sheet forming processes of High Strength Steel (HSS) at elevated temperature are being increasingly applied to produce parts of the car body-in-white, which were previously produced at room temperature. The proper selection of the temperature-related parameters is one of the most critical aspects in the process design with the numerical simulation. In particular, the description of the material formability by Forming Limit Diagrams (FLDs), which are usually implemented in dedicated FEM codes, appears strongly limited when applied to the hot stamping process, as the material behavior is dependent on both the temperature and strain rate. In this paper, a damage model based on the Continuum Damage Mechanics (CDM) was modified to account for the elevated temperature influence, enabling a comprehensive description of the coupled thermo-mechanical–microstructural events that interact during the hot stamping process. The modified damage model was then implemented in the numerical model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550°C and 850°C, at different strain rates, to identify the material parameters necessary for the calibration of the proposed damage model. Nakajima-type experiments at elevated temperature were conducted and numerically simulated to validate the proposed approach. The comparison between numerical and experimental results in terms of critical load at crack growth initiation and fracture location shows that the proposed model is able to predict the ductile fracture onset in hot stamping process.

Effect of bio‐based monomers on the scratch resistance of acrylate photopolymerizable coatings

ABSTRACTPhotopolymerizable clear coatings based on bio‐sourced acrylates, dedicated to the protection of polycarbonate substrates, were studied. The bio‐sourced compounds were not based on triglycerides but were smaller, industrially available molecules similar to classical petro‐based monomers. Their polymerization kinetics was studied by photo‐DSC and was shown to allow high acrylate conversions even at 25 °C. Closely related coatings enriched in alkyl segments, or in monoacrylates to decrease the crosslinking density, were compared. The material composition affects its nanomorphology deduced from X‐ray diffraction. Although these changes in composition can slightly shift the mechanical relaxation, it remains wide, and the elastic modulus remains high (>108 Pa) for all the tested materials. Microscratch experiments highlighted the efficiency of all the new coatings in terms of protection against scratches. Incorporating a monoacrylate, particularly isobornyl acrylate, can improve the scratch resistance especially in terms of critical load (up to 175% increase compared with a classical petro‐based coating). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 379–388

A Comparative Study of Wear and Oxidation Behaviors of End Mill Coated by PVD Coatings

The objective of this research is to study wear behaviors of TiN, nanolaminated AlCrN and nanocomposite TiAlSiN coated on cemented carbide end mill deposited by cathodic arc physical vapor deposition methods in comparison with uncoated end mill. Wear behaviors were investigated by nanoindentation hardness test, scratch test and cutting test. Oxidation test was also done in air at temperatures of 700°- 900°C in order to evaluate resistance of oxidation. In the nanoindentation hardness and scratch tests, nanocomposite TiAlSiN coating exhibited higher hardness than TiN and nanolaminated AlCrN coatings. The nanolaminated AlCrN coating represented the highest adhesion ability in terms of critical load and the lowest coefficient of friction in comparison with the TiAlSiN and TiN coatings, respectively. The cutting performance, represented in terms of maximum flank wear as a function of cutting length, was found to be highest in the AlCrN coating. Oxides of these coatings, i.e., TiO2 for TiN, TiO2 for TiAlSiN and Cr2O3 for AlCrN, generated at different temperatures of 700°, 800° and 900°C, respectively. From all of results, it is obvious that the AlCrN coating exhibited more excellent wear resistance and oxidation resistance than the uncoated end mill, TiN coating and TiAlSiN coating.

Mechanical properties and phase structure of (TiAlZr)N films deposited by multi arc ion plating

A series of Ti-Al-Zr alloy targets with the atomic ratio of (Al + Zr)/(Ti + Al + Zr) from 0.29 to 0.40 were used directly to prepare (Ti,Al,Zr)N multi component hard reactive films on high speed steel substrates by multi arc ion plating (MAIP) technology. The surface morphology, the cross-fracture microstructure, the surface compositions and the phase structure of the (Ti,Al,Zr)N films were investigated by scanning electronic microscope (SEM) and X-ray diffraction (XRD). The dense columnar microstructure was obtained in all of the (Ti,Al,Zr)N films, though micro-droplets evidently existed on the surface of the films. The XRD analysis revealed f.c.c. structure only existing in all of the (Ti,Al,Zr)N films. The lattice parameter was changed with varying the Al and Zr contents in alloy targets. The micro-hardness of film surface and the adhesive property of film/substrate were measured. All the (Ti,Al,Zr)N films displayed excellent mechanical properties. The adhesive strength, in terms of critical load, was larger than 100N and the hardness was bigger than 3000Kg(f)/mm 2. The (Ti,Al,Zr)N film displayed the highest micro-hardness as the atomic ratio of (Al + Zr)/(Ti + Al + Zr) in Ti-Al-Zr alloy target reached 0.40. The present results suggest the expectant substitution of (Ti,Al,Zr)N films for TiN, (Ti,Al)N and (Ti,Zr)N hard films in industrial application and the technical advantage of Ti-Al-Zr alloy targets in preparing (Ti,Al,Zr)N hard films by multi arc ion plating.

Effect of bilayer period on properties of Cr/CrN multilayer coatings produced by laser ablation

Multilayers of alternate Cr/CrN coatings deposited on AISI 301 steel by a pulsed Nd:YAG laser have been tested by microindentation to measure both hardness and Young's modulus. Critical load L C has been determined by scratch test. The total thickness of multilayers was 1 µm, and the bilayer period Λ varied from between 60 nm and 1000 nm. TEM and EDS techniques were used for investigating and analyzing the microstructure of the multilayers. The results of these measurements show that the mechanical properties of Cr/CrN multilayers depend on the bilayer period. The maximum improvement in performance over a single layer of CrN has been found to be Λ at 250 nm, particularly in terms of critical load and Young's modulus.

Strength measurement of a brittle coating with a trilayer structure using instrumented indentation and in situ observation techniques

A mechanical-testing method, devised to measure the strength of brittle thin coatings with a trilayer structure, was investigated. A semi-empirical relationship among radial cracking and associated parameters was derived by introducing a buffer and coating layer with an effective thickness concept, extending the established bilayer equation. Adjustable parameters of the proposed trilayer equation were determined for various ratios of coating thicknesses and moduli. The validity of the FEA-based relationship was analyzed by an experimental method utilizing a graphite/glass/substrate structure. An in situ observation of radial cracks through the transparent substrate during sphere indentation enabled the determination of the strength of the thin coating layer. Chemical vapour-deposited silicon carbide films were used to evaluate the strength and Weibull modulus using the present technique, in a case study format. The validity of the trilayer equation in terms of critical load for radial cracking and the strength of the thin brittle coating are discussed. This study can contribute to knowledge in the area of evaluating brittle thin coating systems on a compliant substrate.

Experimental and numerical study of buckling response of composite shells compressed transversely between rigid platens

This paper aims at describing the buckling behaviour of transversely loaded composite shell structures. Two distinctive glass fabrics, woven and knitted, embedded in the same thermoplastic matrix, polypropylene, were utilised to produce the composites to be used as subject materials. Curved strips with fixed curvature and span, but with different number of plies and orientations, were compressed between two rigid flat platens. Load–displacement responses and deformed shapes were examined using experiments and numerical simulations based on a general finite element code, LUSAS ®. As the nonlinear collapse is the buckling mode evident in the experimental tests, a nonlinear stress analysis technique has been adopted to simulate the buckling responses. Single-ply strips, both for woven and knitted, display limit point symmetrical buckling failure that shows good agreement with the predictions in terms of critical load and buckling mode. In the cases of multi-ply construction, the modes of failure for woven fabric specimens are by means of delaminations while the multi-ply knitted shells still exhibit similar behaviours to that of the single-ply specimens.

AN EMPIRICAL RELATION FOR CRITICAL LOAD OF DLC COATINGS PREPARED ON SILICON SUBSTRATES

The adhesion, of a coating to the underlying substrate, in terms of critical load (using scratch test) is complex and is a function of many parameters such as coating thickness, interfacial stress, coefficient of friction, intermixing, hardness, structural parameters, etc. The present study examines the relationship of critical load to other properties such as film thickness, compressive stress, coefficient of friction, etc. of diamond-like carbon coatings prepared under floating condition (no substrate bias), on silicon substrates. The results show that the compressive stress increases with increasing thickness. However, the adhesive strength in terms of critical load increases with increasing the film thickness up to a certain value and then it decreases. The decrease in critical load with thickness of the films is due to the dominant effect of compressive stress. In order to obtain the parameters that affect the adhesion, a factor analysis has been carried out on the experimental data using SPSS — a statistical software package. Thickness, stress, and critical load emerged as one of the factors and hence it suggests that thickness and stress are two primary parameters that affect adhesion. None of the parameters of Raman study were emerged in the factor analysis along with critical load and thickness. This suggests that the variation of the sp3-bonded carbon with thickness is relatively small and that may not have any direct impact on the adhesion. In this case, stress shows a strong linear relationship with thickness. Moreover, it is not an operational parameter. Hence inclusion of stress in the model becomes unnecessary. Therefore, an empirical relation has been obtained for critical load as a function of thickness.