超音速火焰喷涂微米和纳米结构WC-12Co涂层及其性能

以纳米和微米级WC-12Co粉末为原料,采用超音速火焰喷涂(HVOF)方法在16Mn基体上制备了两种涂层.利用X射线衍射仪对喷涂粉末及涂层进行了相结构分析,用扫描电镜对喷涂粉末、磨粒磨损前后的涂层表面形貌进行了观察,探讨了粉末结构、涂层的组织和结构以及抗磨粒磨损的性能.结果表明:WC-12Co粉末结构对涂层的组织结构影响非常显著,微米WC-12Co粉末中的WC的分解基本上得到了抑止,而纳米结构的粉末由于出现了WC的部分分解,导致了纳米涂层的抗磨粒磨损性能相对于微米涂层提高不多,但是与基体16Mn相比,两种涂层均表现出优异的抗磨粒磨损性能.     

Evaluation of surface residual stresses in HVOF sprayed WC-12Co coatings by XRD and ED-hole drilling

Thermally sprayed coatings are inherently associated with residual stresses in the coatings. These stresses have a noticeable effect on the physical and mechanical properties of coatings. The high speed hole drilling method is widely used to measure the residual stress. Due to the nature of the thermally sprayed coatings, the application of this method for WC/Co coatings has some limitations. In the current study, WC-12Co coatings were deposited using HVOF thermal spraying. The electro discharge hole drilling method was developed to measure the through thickness residual stress in WC-Co thermally sprayed coatings. Morphological studies were conducted using optical microscopy and scanning electron microscopy (SEM) to evaluate the powder and coating characteristics. The sin²ψ method was used to evaluate the surface residual stress by means of XRD. The residual stress at the surface using EDM and XRD was approximately ?32.54 MPa and ?40.6 MPa respectively. The experimental results reveal that the stress curves are not uniform through the coating thickness. It has been found that the mean residual stress is of approximately ?126 MPa. Obtained results are in good agreement with the reported values from literatures. The developed method confirms the feasibility of residual stresses measurement for HVOF thermally sprayed WC-Co coatings.     

Effect of heat treatment on wear behavior of HVOF thermally sprayed WC-Co coatings

A WC-17Co coating was deposited onto ST37 mild steel substrate using HVOF spray technique and then heat treated at different temperatures in a vacuum chamber. The coatings were then evaluated in the as sprayed and heat treated conditions. Inspections by SEM and phase analysis by XRD indicated that some brittle eta (η) phases were produced at high temperature heat treatments. Generation of these phases increased the coating's hardness and decreased fracture toughness of the coating. Tribological properties were studied under dry condition by using pin on disc machine and diamond metal matrix composite disc as counterface. Wear test results showed that as sprayed deposit had the best wear resistance and its wear mechanism was sharp cutting abrasion. The weight loss in heat treated samples increased by increasing heat treatment temperature and the wear mechanisms gradually changed from cutting to gouging abrasion.     

Friction and wear properties of high-velocity oxygen fuel sprayed WC-17Co coating under rotational fretting conditions

Rotational fretting which exist in many engineering applications has incurred enormous economic loss. Thus, accessible methods are urgently needed to alleviate or eliminate damage by rotational fretting. Surface engineering is an effective approach that is successfully adopted to enhance the ability of components to resist the fretting damage. In this paper, using a high-velocity oxygen fuel sprayed(HVOF) technique WC-17 Co coating is deposited on an LZ50 steel surface to study its properties through Vickers hardness testing, scanning electric microscope(SEM), energy dispersive X-ray spectroscopy(EDX), and X-ray diffractrometry(XRD). Rotational fretting wear tests are conducted under normal load varied from 10 N to 50 N, and angular displacement amplitudes vary from 0.125° to 1°. Wear scars are examined using SEM, EDX, optical microscopy(OM), and surface topography. The experimental results reveal that the WC-17 Co coating adjusted the boundary between the partial slip regime(PSR) and the slip regime(SR) to the direction of smaller amplitude displacement. As a result, the coefficients of friction are consistently lower than the substrate's coefficients of friction both in the PSR and SR. The damage to the coating in the PSR is very slight. In the SR, the coating exhibits higher debris removal efficiency and load-carrying capacity. The bulge is not found for the coating due to the coating's higher hardness to restrain plastic flow. This research could provide experimental bases for promoting industrial application of WC-17 Co coating in prevention of rotational fretting wear.     

Sliding and abrasive wear behaviour of boride coatings

Polyphase boride coatings constituted by an inner layer of Fe₂B and an outer layer of FeB were thermochemically grown on iron and medium carbon steel by a pack cementation process. The tribological behaviour of borided samples was investigated under both sliding and abrasion testing conditions. Considerably different values of wear rate were found in different regions of the coatings. The differences were explained on the basis of the crystallographic order of iron borides. The resistance to both types of wear was initially poor due to the presence on the coatings of a thin, friable layer constituted by disordered crystals. Then the resistance increased to a maximum value in regions constituted by compact, highly ordered crystals of Fe₂B. The resistance to dry sliding of borided samples was better than that displayed by samples submitted to alternative surface treatments (e.g. gas nitriding) and lower that that measured for a WC-Co hard metal coating.     

Relating contact conditions to abrasive wear

Damage caused by particles within rolling/sliding contacts can severely reduce the operational life of machinery such as roller bearings, gears and pumps.Abrasive wear of spherical roller thrust bearings has been studied using a stylus apparatus and scanning electron microscopy (SEM). Both a standard bearing and a bearing with rollers coated with metal mixed amorphous carbon (Me-C:H) were studied. The SEM measurements were performed systematically across the contact surfaces so that surfaces with gradually different contact situations could be examined. These measurements were compared to the measured wear depth of the components of the roller bearing. Also, the calculated contact conditions in terms of creep, contact size and surface separation have been related to the observed wear pattern at various locations.To attempt to understand the wear behaviour of the bearing with coated rollers, the coating as well as the material content of the surfaces were examined using both SEM and energy dispersive X-ray spectrometry (EDS). This revealed that the coating did not flake off but rather was scratched off.It is possible to link the abrasive wear behaviour to the contact conditions. It is crucial to understand this relationship when building a simulation model of abrasive wear.     

Erosion-corrosion of candidate HVOF aluminium-based marine coatings

This paper describes gravimetric results of HVOF sprayed commercially pure Al and Al/12% Si eutectic alloy coatings obtained under erosion-corrosion conditions. The performance of these coatings are compared to a hot-dipped zinc coating. These coatings have been applied to AISI 1020 carbon steel substrates and tested in a free jet impingement rig with a variety of fluids and slurries to enable the erosion and corrosion contributions to the total erosion-corrosion removal of the surface to be determined. Tests have been conducted at 30° and 90° jet impingement angles at jet velocities of 3.5 m/s.Erosion damage mechanisms are identified and the erosion-corrosion mass loss results discussed in terms of the contributions made by erosion and corrosion. Zinc was found to suffer severely from flow corrosion, while the other targets did not. All target materials under sand erosion lose mass in a way directly proportional to kinetic energy, and inversely related to hardness. Corrosion losses are relatively small while the generation of solid products sometimes results in net weight gains. There are greater mass losses at nominally normal incidence than at nominally oblique incidence, due to particle interference effects. Synergy is found to be both positive and negative and appears to be angle dependent, but is difficult to determine precisely.     

Influence of the normal force,abrasive slurry concentration and abrasive wear modes on the coefficient of friction in ball-cratering wear tests

The purpose of this work is to study the influence of the normal force (N), abrasive slurry concentration (C) and abrasive wear modes on the coefficient of friction in ball-cratering wear tests. Experiments were conducted with balls of AISI 52100 steel, an AISI H10 tool-steel specimen and abrasive slurries prepared with black silicon carbide (SiC) particles+distilled water. The tangential (T) and normal loads were monitored throughout the tests and the results have shown that: (i) the coefficient of friction behavior was independent of the normal force and (ii) both the concentrations of abrasive slurries and the subsequent action of the abrasive wear modes, generally, did not affect the behavior or magnitude of the coefficient of friction.     

An investigation of sliding wear behaviour of WC-Co coating

Dry sliding wear tests on specimens of mild steel (MS) and WC coated mild steel (MSC) specimens were performed against a hardened EN32 steel (EN32) and a WC coated EN32 steel (EN32C) discs. Four different combinations of specimen and counter surface were tested under dry sliding conditions. Results suggest that wear mechanisms differ depending on the combination of materials under sliding contact. Expectedly the MS specimen suffered high wear loss, but the MSC specimen showed interesting results. When slid against EN32, MSC specimens showed negative wear results whereas positive wear results occurred against EN32C. Steady wear rate was attained after a critical sliding distance.     

Microstructure and abrasive wear in silicon nitride ceramics

It is well known that abrasive wear resistance is not strictly a materials property, but also depends upon the specific conditions of the wear environment. Nonetheless, characteristics of the ceramic microstructure do influence its hardness and fracture toughness and must, therefore, play an active role in determining how a ceramic will respond to the specific stress states imposed upon it by the wear environment. In this study, the ways in which composition and microstructure influence the abrasive wear behavior of six commercially-produced silicon nitride based ceramics are examined. Results indicate that microstructural parameters, such as matrix grain size and orientation, porosity, and grain boundary microstructure, and thermal expansion mismatch stresses created as the result of second phase formation, influence the wear rate through their effect on wear sheet formation and subsurface fracture. It is also noted that the potential impact of these variables on the wear rate may not be reflected in conventional fracture toughness measurements.     

Using parallel scratches to simulate abrasive wear

Abrasive wear is currently classified according to different particle dynamics: (a) the sliding of active particles on the sample surface and (b) the rolling of abrasive particles between the surfaces. In this paper, instrumented laboratory tests are used to present a new methodology for the simulation of abrasive wear. The rolling of the abrasives is represented by a sequence of indentations, and the sliding of the active particle by a sequence of scratches. A new piece of equipment was especially developed to reproduce the action of an abrasive particle. Two high resolution sliders drive the sample horizontally while the indenter is moved vertically by another slider. Besides this, a high resolution piezoelectric translator is used to control the indenter movement while a 3D load cell controls the intensity of the process. A worn surface produced in a rubber wheel abrasive wear test was used as the reference for the simulation. Its topography was assessed by using laser interferometry and scanning electron microscopy and showed that the prevailing wear mechanism was parallel scratches. The results showed that the superimposition of scratches is the basis which makes it possible to correlate topographical parameters of the reference to the controlling variables used in the simulation. A special method to describe the average depth of the scratches in function of the distance between them (superimposition) was developed. Wear occurs when superimposition is greater than 80%. The average depth of the scratches increased according to an elevation in the degree of superimposition and to the augmentation of normal load. This simulation methodology produced a surface topographically and morphologically similar to that of the reference.     

The particle size effect on abrasive wear of high-chromium white cast iron mill balls

Granite grinding tests, under dry and wet conditions, were performed to assess the influence of abrasive particle size to the wear behavior of martensitic high-chromium white cast iron mill balls. The tests were performed, at first, using raw granite particle sizes between 0.074 and 19.1 mm, and then with coarse and fine granite fractions obtained after screening the raw granite in a 3.36 mm sieve. It is demonstrated that the relative particle/ball size relationship is the determining parameter to ball wear. The highest ball wear rates were observed for fine granite grinding under dry (120 mg/cycle) and wet (129 mg/cycle) conditions. The lowest wear rate (ca. 50 mg/cycle) was observed for coarse granite grinding (dry and wet). These different results were attributed to the different size relationships between grinding body diameter and granite particles size. For wet-grinding of raw granite, the mineral components may influence significantly the wear behavior. Feldspar can act as a bonding agent, gluing fine quartz particles to the coarse granite and to the balls surface and turning the dependence of the relationship between the relative sizes of ball and granite particle less important to the wear process. This explains why wet-grinding of raw granite results in a ball wear two times greater (106 mg/cycle) than dry-grinding (51 mg/cycle).     

Feature extraction method of abrasive belt wear state for rail grinding

Abstract

Abrasive belt has the advantages of high removal efficiency and high waste collection; furthermore, it does not easily damage the rail when applied in rail grinding. However, the poor wear consistency and short service life restrict its development in this field. It is important to explore the theory and technology of monitoring the abrasive belt wear condition to evaluate the means of improving the grinding performance of the belt. This article discusses the contact stress state between the belt and the rail surface. In addition, the study establishes the belt rail grinding force model during operation using the abrasive grain distribution function. The analysis of the force model proposes that the belt abrasion condition be monitored by using the force ratio parameter of the abrasive belt in the grinding process. The extraction method of the force ratio information is proposed in accordance with the grinding environment. The results of experiments successfully verified the validity of the method.     

An investigation on three-body abrasive wear test at elevated temperature

A high temperature three-body abrasive wear tester has been designed and made. In order to exhibit the effect of oxidation on the wear amount of specimen and to simulate various service conditions, the oxygen content in atmosphere, the time of exposing specimen in atmosphere and the intensity of abrasion can be controlled in the tester. In the temperature range 20–900 °C, both the discriminability of wear resistances of materials and the data reproducibility of the tester are quite satisfactory. Moreover, an intermittent oxidation-abrasion test procedure has been established for investigating the interaction of oxidation and abrasion at elevated temperature. By comparing the result of test performed in argon atmosphere with that in ambient atmosphere, the respective effects of oxidation and abrasion on volume loss of specimen can be clearly distinguished.     

Effect of surface nanocrystallization on abrasive wear properties in Hadfield steel

A nanocrystalline surface layer was synthesized on a Hadfield steel by shot peening treatment. The microstructure evolvement of the surface layer of the shot peening treated sample was characterized by optical microscopy, X-ray diffraction and high-resolution transmission electron microscopy. It has been shown that the grain sizes in surface layer were decreased to 11.1-17.4 nm after 60 min shot peening duration. Surface hardness was also increased greatly. Two- and three-body abrasive wear experiments were carried out for work hardening and original specimens, separately. The results showed that the wear resistance of the nanocrystallized Hadfield steel has distinctly been improved in case of soft particles used as two-body wear abrasives or light impact load applied for impact abrasive wear.     

Influence of WC size and HVOF process on erosion wear performance of WC-10Co4Cr coatings

In the present study, based on the velocity and temperature measurements of in-flight particles and parameter optimization, multimodal and conventional WC-10Co4Cr cermet coatings were sprayed by high velocity oxygen gas fuel spraying (HVOGF) and high velocity oxygen liquid fuel spraying (HVOLF). The coatings’ structure, porosity, microhardness and fracture toughness were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) etc. Furthermore, erosion resistance of the coatings to solid sand was tested, followed by the exploration of the material failure mechanisms. Results show that the WC-10Co4Cr coatings deposited by HVOLF are mainly composed of WC with no obvious decarburization and amorphous CoCr binder. The mechanical properties of the coatings deposited by HVOLF are much more superior to those deposited by HVOGF. Multimodal WC-10Co4Cr coating deposited by HVOLF possesses the highest microhardness and fracture toughness, the lowest porosity and the most excellent resistance to sand solid erosion wear, which was enhanced by 15 and 40% than that of HVOLF conventional coatings at 30° and 90° impact angles. The improvement is even greater in comparison with multimodal coating deposited by HVOGF. These results have provided important reference for WC-CoCr anti-erosion coating design and optimization of high velocity oxygen fuel (HVOF) process.     

Experimental investigation of oil pockets effect on abrasive wear resistance

The experiments were carried out using a block-on-ring tester. The stationary blocks were modified by a burnishing technique in order to obtain surfaces with oil pockets of spherical shape. The area density of oil pockets varied in order to explore their effect on wear resistance and wear intensity. Specimen surfaces had dimples with depths 45-60 μm and diameters 1-1.2 mm. The area density of oil pockets S_p was in the range 4-20%. The block samples were made from bronze B101 (CuSn10P) of 138 HB hardness. The rotated rings were made from 42CrMo4 steel, hardness of 40 HRC obtained after heat treatment. The tested assembly was lubricated by mineral oil L-AN 46. The experiment was carried out under artificially increased dustiness conditions. The dust added to oil consists mainly of SiO₂ (74%) and Al₂O₃ (15%) particles. During the test friction force and temperature of block sample were registered. The tendencies of block surface topography changes during wear were analysed. It was found that sliding pairs with textured specimens were not superior to a system with a turned block with regard to abrasive wear resistance.     

Micro-scale abrasive wear testing of PVD coatings on curved substrates

A method is presented which enables a micro-scale abrasion test to be used to measure the wear performance of a coating over a small region, typically of millimetre dimensions, on a curved surface. The method is also applicable to studies of the wear resistance of any bulk material with a surface having complex curvature. The technique is illustrated by measurement of the intrinsic abrasion resistance of thin PVD coatings of TiZrN, ZrNbN and TiNbN on both flat and cylindrical tool steel and flat stainless steel substrates. The ability to measure the wear resistance of both a coating and its substrate, independently of each other and by a single test, is confirmed by experiment.     

High temperature fretting wear behavior of WC-25Co coatings prepared by D-gun spraying on Ti-Al-Zr titanium alloy

Detonation gun (D-gun) spraying is one of the most promising spraying techniques for producing wear-resistance coatings. A thick layer (about 0.3 mm thickness) of WC-25Co with high hardness was covered on Ti-Al-Zr titanium alloy by D-gun spraying and the fretting wear behavior of WC-25Co coatings was studied experimentally on a high precision hydraulic fretting wear test rig. An experimental layout was designed to perform fretting wear tests at elevated temperatures from room temperature (25 °C) to 400 °C in ambient air. In the tests, a sphere (Si₃N₄ ceramic ball) was designed to rub against a plane (Ti-Al-Zr titanium alloy with or without WC-25Co coatings). It was found that the fretting running regimes of WC-25Co coatings were obviously different from those of Ti-Al-Zr titanium alloy. The mixed fretting regime disappeared in WC-25Co coatings, and the boundaries in the running condition fretting map (RCFM) showed hardly any change as temperature increased. The worn scars were examined using a laser confocal scanning microscope (LCSM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the coefficients of friction (COF) of WC-25Co coatings at elevated temperatures were nearly constant in the partial slip regime and very low in the steady state. The fretting damage of the coatings was very slight. In the slip regime, the WC-25Co coatings exhibited a good wear resistance, and the wear volume of the coatings obviously decreased with increasing tested temperature. The fretting wear mechanisms of WC-25Co coatings were delamination, abrasive wear and oxidation wear at elevated temperature. The oxide debris layer formed at higher temperature was denser and thicker on top of WC-25Co coatings, thus providing more surface protection against fretting wear, which played an important role in the low fretting wear of the coatings.