Jiawen He

Improvement of the fretting fatigue and fretting wear of Ti6Al4V by duplex surface modification

Abstract Ion-beam-enhanced deposition (IBED) was investigated as a way to increase the fretting fatigue resistance of Ti6Al4V. Both hard CrN coatings with good toughness and soft CuNiIn coatings of low friction have been applied on the base material, and shot peening combined with coatings has also been studied in order to improve the fretting fatigue (FF) resistance. Since these IBED coatings exhibit a good bonding strength even after shot peening, they do not spall off during fretting fatigue and fretting wear tests. When the contact stress is not severe and gross slip contact conditions are operative, both CrN and CuNiIn show a better fretting fatigue resistance than that of shot-peened Ti6Al4V. As the contact stress concentration of fretting fatigue is high, coatings combined with shot peening achieved high levels. The fretting fatigue lifetime is largely dependent on the sliding contact conditions such as contact geometry, sliding distance and contacting materials. Under partial slip, cracks initiate at an early stage limiting the fretting fatigue lifetime, while under gross slip, a much higher fretting fatigue limit is achieved. Compressive residual stresses are particularly important to improve the fretting fatigue lifetime, when crack propagation is predominant during the failure progress.

Effect of crystallographic orientation on fretting wear behaviour of MoSx coatings in dry and humid air

The tribological behaviour of MoS2 coatings in air of high humidity is critical for their application in air. An improved friction and wear resistance of sulfur-deficient MoSx coatings in humid air is reported based on fretting wear tests. For random-oriented MoSx coatings, the coefficient of friction and the wear volume are still significantly dependent on the relative humidity. On the contrary, basal-oriented MoSx coatings show an interesting low sensitivity to humidity. MoO3 and SO42− have been detected in the wear track and the debris. The amount of MoO3 and SO42− is related to the crystallographic orientation of the coatings and the relative humidity in the fretting wear tests. However, the structure and grain size of the MoO3 debris are not dependent on the relative humidity and the crystal orientation of the coatings. The effect of humidity on the friction and wear of these sulfur-deficient MoSx coatings with different crystallographic orientations is discussed.

Friction and wear behaviour and abradability of abradable seal coating

The friction and wear behaviour of several kinds of middle temperature abradable seal coatings used in aircraft turbine engine was investigated. Their abradability was evaluated by sliding worn volume. The mechanisms of the sliding wear of the coatings are abrasive wear, adhesive wear and oxidation wear, but the weight of the adhesive wear and abrasive wear is different in different coatings and under different test loads. The results show that the abradability decreases with the increase of the hardness for a given kind of coating. Even if the hardness is close, the abradability is very different in different kinds of coatings. So, only by the hardness can the level of abradability not be not judged and the coating not be chosen and designed. The abradability of M313 type of coating is the best, M310 is close to M601 and M307 with low hardness is fairly good, but M307 with high hardness is the worst.

Improving the anticorrosion and mechanical behaviour of PACVD TiN

Abstract In the d.c. power supply plasma-assisted chemical vapor deposition TiN coating process, Si is introduced to form a (Ti,Si)N coating layer. Auxiliary heating is used and some of the metal chloride in the precursor is replaced by SiH 4 gas; the chlorine content in the coating layer is reduced. The (Ti,Si)N with 16 wt.% Si exhibits a higher hardness than that of binary TiN coating. The bonding strengths are evaluated with scratch tests and contact rolling tests. When the composition of the ternary layer is tailored to set the TiN in the interface region, the interfacial fatigue strengths of TiN and (Ti,Si)N have the same magnitude. The oxidization resistance at elevated temperatures for (Ti,Si)N is higher than that of TiN. The aqueous corrosion behavior of the (Ti,Si)N-coated material is also better than that of the TiN-coated material as shown by electrochemical measurement.

Development and characterization of CrN films by ion beam enhanced deposition for improved wear resistance

Chromium nitride (CrN) films were synthesized by ion beam enhanced deposition (IBED) process which involves using an argon ion beam to sputter the chromium target while using a nitrogen ion beam to bombard the deposited film. The surface and cross-section morphologies of CrN films were studied by scanning electron microscope (SEM) and atomic force microscope (AFM). The distribution of chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). The crystalline structures of thin films were identified by X-ray diffraction (XRD). The adhesion strength of film-substrate system was evaluated by the scratch test. Results showed that the ion bombardment energy and ion beam current density had significant effects on surface morphology, crystalline structure and adhesion strength of the deposited CrN films. With an increase in bombardment energy, the microhardness of the IBED CrN films increased, but the adhesion strength of the films decreased. Ion beam current density had a significant sputtering effect on the deposited CrN films. Wear tests were performed on a block-on-ring system under lubricated conditions and a ball-on-flat system under dry sliding condition. The data from both experiments showed that: (1) the wear resistance of GCr15 steel has been improved significantly after being coated with IBED CrN film: (2) thin films synthesized at lower bombardment energy or lower ion beam current density had better tribological properties.

Evaluation of the mechanical properties of thin metal films

Abstract The nano-indentation method has been developed to evaluate the mechanical properties of metal films. In this study an indentation loading curve is employed to determine the yield strength and hardening index of an Al film on a Si substrate. The result is compared with that measured by a uniaxial tensile test with an Al film deposited on an Al foil. The yield strength of the Al film can be determined according to the testing results of this compound material and it agrees reasonably with that evaluated by the nano-indentation method. In addition, X-ray diffraction measurements of the four point bending specimen is used to measure the stress–strain curve of Cu films on steel substrate. The samples are prepared by ion beam enhanced deposition and magnetron-sputtering deposition. The results show that the strength of Cu film depends on the deposition technology and is much higher than that of the bulk material.

Ab-initio calculation of elastic constants of TiN

Elastic constants are important for internal stress measurement, yet the value for thin films is difficult to test by mechanical methods. In this paper the elastic constants of TiN crystal were calculated ab initio. The cohesion energy curves in three deformation directions were calculated. The results of the second-order partial differential at the equilibrium point of the cohesion energy curve lead to the elastic constants of the single crystal. The effect of Poissons ratio or the deformation of the lattice must be taken into consideration otherwise large errors are involved. The theoretical elastic model of the polycrystalline, Kroner model, was employed to obtain the elastic constant of the polycrystalline aggregate. This study shows that the calculated elastic constants are in good agreement with the experimental data. For the X-ray diffraction method the residual stress is calculated from strain data measured and the elastic constant of a designated (hkl) plane must be known. Four-point bending was used to measure the X-ray elastic constants of (422) and (331) planes of TiN film. The calculated values were in good agreement with the experimental results. For the measurement of the elastic constants of thin films, texture and columnar grain size of the deposited film may result in a large scatter of the X-ray diffraction results.

Structure and tribological properties of MoSx coatings prepared by bipolar DC magnetron sputtering

Abstract MoS x coatings were prepared by bipolar pulse DC (balanced and unbalanced) magnetron-sputtering systems. Deposition parameters, cathode current density, argon pressure, bias voltage and magnetron sputtering conditions, were varied in order to obtain low friction and wear-resistant coatings. Composition and surface morphology were determined by energy dispersive X-rays and scanning electron microscopy, while the structure was investigated by X-ray diffraction. The friction and wear properties were investigated by fretting tests in ambient air of x coatings characterised by a strong (002) basal plane orientation parallel to the substrate were obtained in a reproducible way. Such coatings deposited under optimised conditions exhibit very good lubricity and high wear-resistance, even in ambient air with a relative humidity of 50%. Conversely, when deposition parameters such as higher argon pressure and larger cathode current density result in the deposition of a MoS x coatings with a needle-like structure and preferential (100) and (110) plane orientations, a lower wear resistance is achieved especially in air of 50% relative humidity. Ion bombardment, achieved by applying a negative voltage bias or unbalanced sputtering conditions, improves the friction and wear performance. Conversely, a positive bias voltage deteriorates the tribological performance of the coatings. Experimental data on the friction and sliding wear resistance for MoS x coatings indicate that the best tribological properties are obtained with low sulfur and featureless MoS x coatings.

Comparison of fretting wear of Cr-rich CrN and TiN coatings in air of different relative humidities

Abstract The CrN and TiN coatings deposited on tool steel substrates by ion beam assisted deposition (IBAD) and triode ion-plating techniques, respectively, were investigated. The fretting wear of these coatings has been determined in air of different relative humidities (RHs). The results show that the coefficient of friction of the CrN coatings versus corundum at 50 and 85% RH is about 0.3, slightly higher than that of the TiN coatings. However, the TiN coatings exhibit a high coefficient of friction of about 0.8 at RH

Improvement in the load-bearing capacity and adhesion of TiC coatings on TiAl6V4 by duplex treatment

Abstract It is shown that the use of nitrogen or carbon as alloying elements in plasma diffusion treatment of TiAl6V4 is beneficial for the load-supporting properties of the base material. The plasma diffusion treatment produces a compound layer consisting of nitrides or carbides and an inner diffusion zone, essentially characterized by the presence of nitrogen- or carbon-rich α-Ti crystals which are embedded in the metal matrix. This results in a hardness gradient from the interior to the surface. To avoid hydrogen embrittlement, plasma nitriding is performed in a pure nitrogen atmosphere. The formation of soot by carburizing could be avoided by optimizing the carbon flow during plasma carburizing. The influence of plasma nitriding and carburizing on the adhesion of the TiC coating depends on the plasma diffusion pretreatment and the conditions of the TiC coating. Plasma-nitrocarburizing pretreatment leads to the best adhesion of the TiC coating; the critical load of failure is 80% higher than in the case of TiC on base material. A critical load of approximately 33 N was found under the optimum parameters. We conclude that the combination of plasma diffusion treatment and TiC coating extends the tribological applicability of TiAl6V4 in many industrial sectors. Fretting fatigue and wear could be increased by a duplex plasma treatment combined with shot peening.