The role of lubrication and corrosion in abrasion of materials in aqueous environments

A rubber wheel type test apparatus has been constructed which allows abrasion testing to be conducted in slurry or dry environments in otherwise identical conditions. Abrasion tests of a steel, a sintered tungsten carbide–cobalt hardmetal and an HVOF sprayed nickel chrome–chromium carbide cermet coating have been performed in dry and aqueous slurry conditions, the latter with both neutral and acidic carriers. It has been shown that the aqueous carrier acts as an effective lubricant and thus significantly reduces the abrasion rate over that observed in dry conditions. However, enhancement of corrosion by use of an acid slurry lead to an increase in the rate of material removal over that of the neutral aqueous conditions in all cases. Increases were small for the corrosion resistant cermet coating and moderate for the steel. Significant enhancement of wear was observed for the sintered WC–Co hardmetal where rapid removal of the cobalt binder by the acid resulted in a change in dominant mechanism of carbide removal from attritive wear to pullout.     

A contribution to the surface analysis and characterisation of HVOF coatings for petrochemical application

The appropriate selection of bulk materials and coatings of valve components is an important factor for the economic success of oil and gas production activities in the petrochemical field. Materials and coatings are important because particle erosion and surface wear are associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problems and cost increases. The most common solution of these problems is the deposition of hard materials as tungsten carbide or chromium carbide by thermal spray. These coatings are deposited by high velocity oxygen fuel (HVOF) thermal spray process to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt–chromium based coating, chromium carbide nickel–chromium coating as well as Inconel 625 have been adopted in the specifications of petrochemical companies and their behaviour and wear, erosion and corrosion properties are reported in the literature.

This paper addresses the experimental study, surface analysis and functional characterisation of HVOF coatings innovative for the specific application such as NiAl and composite material WC/intermetallic compounds containing Ni, Cr, Co and Mo. These coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the coatings in H₂S and CO₂ atmosphere and to wear and erosion according to standard ASTM G75-95 (slurry test); material loss and surface damage have been determined; the coatings have been completely characterised from the point of view of the structure (morphology, porosity, hardness, wear) and of the surface properties by means of a prototype 3-dimensional (3-D) stylus micro-geometrical surface analysis system; their corrosion and functional behaviour have been compared with the behaviour of the above mentioned coatings.

The slurry test allows a clear discrimination among the performances of analysed coatings. Namely, WC/Mo compound, because of its carbide content, shows fairly good behaviour in an erosive environment and higher erosion resistance than Inconel 625 and NiAl; all the tested coatings show similar behaviour in a corrosive environment.     

钴基合金-碳化钨复合涂层材料耐磨性能的研究

采用真空熔烧法制得钴基合金—碳化钨复合涂层材料,借助扫描电子显微镜、X射线衍射仪等先进的测试手段对涂层的组织结构和表面形貌进行观察分析。应用盘销式摩擦磨损试验机对不同碳化钨质量分数的复合涂层材料和淬火态45钢进行了磨损试验。结果表明:在相同试验条件下,复合涂层的耐磨性显著高于淬火钢,且其耐磨性随碳化钨质量分数的增加而提高:淬火钢的耐磨性随着载荷的增加迅速降低,而复合涂层的耐磨性则变化不大。     

超音速火焰喷涂碳化钨钴合金涂层对橡胶密封件的磨损研究

通过磨合试验,比较了超音速火焰(HVOF)喷涂碳化钨钴合金涂层与传统的含氰镀铬镀层与橡胶密封件对磨时的磨损情况.结果表明:HVOF喷涂碳化钨钴合金涂层具有良好的耐磨性和致密性,始终保持较良好的表面状况,对非金属密封件具有良好的适应性.     

Microstructure and failure modes during scratch testing of laser cladded WC–NiCrBSi coatings with spherical and angular carbides

Abstract

Laser cladded coatings have been used extensively to extend the service life of components exposed to severe abrasive wear. One of the main wear resistant materials used in laser cladding is ceramic–metallic composite. Despite extensive use of this class of material, there is very limited knowledge regarding mechanical degradation mechanisms, such as cracking and plastic deformation, under different wear conditions. In this investigation a mixture of nickel alloy and tungsten carbide powders were used to deposit the coating. Two types of tungsten carbide powders with spherical and angular carbides were employed. The microstructures of the coatings were analysed thoroughly by optical microscopy, electron probe microanalysis and wavelength dispersive spectrometry. Failure and cracking mechanisms of laser cladded coatings under normal and tangential loading were systematically investigated using scratch testing. In the nickel alloy matrix, fine mixed secondary carbides were formed due to partial dissolution and formation of the secondary tungsten carbide during laser cladding. These secondary carbides were rich in chromium, tungsten and nickel and had a blocky and/or bar-like shape. Failure mechanisms associated with scratch testing were dependent on the microstructure and carbide morphology, applied stress and location of carbide particles with regard to the scratch groove. Owing to the high binder mean free path between the carbide particles, plastic deformation of the binder was the dominant failure mechanism. Additionally, partial or whole fragmentation of carbides, carbide/binder interface cracking and limited binder fracture were observed.     

Amorphous diamond coating of tungsten carbide and titanium carbonitride for erosive slurry pump component service

Tungsten carbide spray coatings have become well established for resisting abrasion and erosion in pumps used in conventional oil production and in oil sands operations. To achieve additional benefits from the extreme wear resistance of tungsten carbide its use is being extended to solid forms for some critical components. Slightly harder titanium carbonitride-based cermets have much lower density and coefficient of thermal expansion and for these reasons are being considered as an alternative to tungsten carbide. PVD amorphous diamond coating also has potential to further increase the service life of selected pump parts fabricated from solid cermets. Micro-abrasion testing and single scratch and nano-indentation evaluation have been carried out on non-coated and PVD amorphous diamond coated WC-4.8% TaC–4.5% TiC/6% Co–1% Cr and TiCN-17% WC/8% Mo materials. Data obtained illustrate that the tungsten–carbide based product has superior wear properties to the titanium carbonitride material and that PVD amorphous diamond coating of both cermets enhanced wear resistance significantly and displayed potential for successful service application.     

Abrasion of cemented carbides by small grits

The abrasion properties of a series of cemented carbides with different carbide grain sizes, different amounts and types of binder phases have been investigated under varied conditions. Abrasion results from other works are also incorporated for comparison reasons. The results are interpreted in the light of a previously published model for the abrasion properties of multiphase materials, although this is the first time this model is applied to materials with very high amounts of hardphase. It is confirmed that the abrasion resistance of tungsten carbide–cobalt materials may vary considerably, also for fixed amounts of metal and hardphase. Not only the wear resistance level but also the ranking depends both on the test conditions and on the microstructure. It is further showed that some nano-crystalline materials posses a wear resistance superior to those of the pure carbide material.     

A scanning electron microscope study of attrition wear in tungsten carbide taper pin reamers

The wear of tungsten carbide taper pin reamers used to ream holes in a composite structure of aluminium alloy plate overlaying a plate of ultra-high strength steel was examined under the scanning electron microscope. The predominant process of wear was by attrition, which involved the mechanical detachment of individual or groups of tungsten carbide grains from the reamers by both the swarf and workpiece material.This process of attrition wear was initiated by the removal of the cobalt binder phase, resulting in the undermining and subsequent removal of tungsten carbide grains. A mechanism is described for explaining how the cobalt binder phase is initially removed, and the means by which tungsten carbide grains are then removed are discussed. Consequent on the removal of tungsten carbide grains some cobalt binder phase attached to these grains is also removed, and this subsequently becomes an important mechanism of cobalt binder phase removal.     

Brush plated cobalt-molybdenum and cobalt-tungsten alloys for wear resistant applications

Brush plating solutions have been developed that enable sound cobalt alloy coatings containing up to 12% molybdenum or tungsten to be deposited at thicknesses up to 13 μm. The alloy deposits had a very fine grain size but their crystallographic structure could not be determined. The cobalt deposit alone had a mixed cph and fcc structure at room temperature. Evaluation of the wear resistant properties of the cobalt alloys was carried out in the laboratory using a pin and disc technique and a simulated hot forging test. Both tests showed the alloys to have relatively good wear characteristics. Extensive industrial trials have been undertaken to assess the value of these cobalt alloy coatings to enhance the performance of hot forging dies. The results have indicated that die life can be increased by up to 100%     

The wear of ultrafine WC–Co hard metals

The wear performance of ultrafine-grained tungsten carbide–cobalt (WC–Co) hard metals during three-body abrasion and particle erosion has been evaluated and compared to that of similar conventional coarser grained hard metals. The tungsten carbide grain size varied between 0.5 and 3 μm with cobalt contents ranging from 6 to 15%. Silica particles were used in both forms of testing. Erosion was carried out at 60 ms⁻¹ at an impact angle of 75° and abrasion at a velocity of 0.5 ms⁻¹ and a load of 50 N.

The wear resistance of the ultrafine grades was found to be at least double that of the closest conventional fine grained hard metals. These increases in wear performance are considerably higher than any corresponding increase in hardness which is, at most, 25% and is not achieved at the expense of fracture toughness which is maintained at a similar level to that of conventional fine grained hard metals. The increase in wear resistance coincides with a change in the mechanism of material removal. Sub-micron materials experience ductile deformation and bulk removal of material whilst coarser grades display more localised response with extensive fragmentation of the WC grains.     

等离子喷涂WC/Co Fe基涂层摩擦与磨损性能

以普通铸铁为基体,碳化钨陶瓷粉末WC 12Co为热喷涂材料,采用大气等离子法制备WC/Co Fe复合涂层.通过SEM、EDS、XRD等手段对WC/Co Fe涂层微观组织与结构进行表征,并对WC/Co Fe复合涂层耐磨损性能进行测试.结果表明,等离子喷涂制备的WC/Co Fe涂层物相以WC相为主;WC涂层摩擦因数波动小于铸铁材料摩擦因数,表明WC复合涂层具有良好的抗摩擦性能.WC涂层耐磨损性能高于铸铁,主要归因于WC颗粒韧性好、硬度高、抗冲击及抗磨损性能强,与基体金属的结合性好.     

Comparative Study of Thermally Sprayed Coatings Under Different Types of Wear Conditions for Hard Chromium Replacement

The tribological properties of part surfaces, namely their wear resistance and friction properties, are decisive in many cases for their proper function. To improve surface properties, it is possible to create hard, wear-resistant coatings by thermal spray technologies. With these versatile coating preparation technologies, part lifetime, reliability, and safety can be improved. In this study, the tribological properties of the HVOF-sprayed coatings WC–17%Co, WC–10%Co4%Cr, WC–15% NiMoCrFeCo, Cr₃C₂–25%NiCr, (Ti,Mo)(C,N)–37%NiCo, NiCrSiB, and AISI 316L and the plasma-sprayed Cr₂O₃ coating were compared with the properties of electrolytic hard chrome and surface-hardened steel. Four different wear behavior tests were performed; the abrasive wear performance of the coatings was assessed using a dry sand/rubber wheel test according to ASTM G-65 and a wet slurry abrasion test according to ASTM G-75, the sliding wear behavior was evaluated by pin-on-disk testing according to ASTM G-99, and the erosion wear resistance was measured for three impact angles. In all tests, the HVOF-sprayed hardmetal coatings exhibited superior properties and can be recommended as a replacement for traditional surface treatments. Due to its tendency to exhibit brittle cracking, the plasma-sprayed ceramic coating Cr₂O₃ can only be recommended for purely abrasive wear conditions. The tested HVOF-sprayed metallic coatings, NiCrSiB and AISI 316L, did not have sufficient wear resistance compared with that of traditional surface treatment and should not be used under more demanding conditions. Based on the obtained data, the application possibilities and limitations of the reported coatings were determined.     

Dynamic SEM Wear Studies of Tungsten Carbide Cermets

Dynamic friction and wear experiments were conducted in a scanning electron microscope. The wear behavior of pure tungsten carbide and composite with 6 and 15 weight percent cobalt binder was examinded. Etching of the binder was done to selectively determine the role of the binder in the wear process. Dynamic experiments were conducted as the WC and bonded WC cermet surfaces were transversed by a 50 micron radiused diamond stylus. These studies show that the predominant wear process in WC is fracture initiated by plastic deformation. The wear of the etched cermets is similar to pure WC. The presence of the cobalt binder reduces both friction and wear. The cementing action of the cobalt reduces granular separation and promotes a dense polished layer because of its low shear strength film-forming properties. The wear debris generated from unetched surface is approximately the same composition as the bulk.     

Friction and dry sliding wear behaviour of cermets

The friction and wear behaviour of cermets/steel rubbing pairs were investigated. Friction and wear tests were carried out using three different crèmets on the base of tungsten, titanium and chromium carbides under dry sliding conditions against steel disk (0.45% C). Sliding wear tests were carried out using modified block-on-ring equipment at a sliding speed of 2.2 m/s and normal load 40 N.It is shown that wear resistance and coefficient of friction depend on the type and chemical composition of the cermets. The WC–Co cermets have the highest wear resistance. The wear rate of WC–Co and TiC–NiMo cermets increased with increasing binder content in the cermets. The wear of Cr₃C₂–Ni cermets is more complicated and depends on the composition of cermets. The wear of WC–Co cermets is caused mainly by preferential removal of the cobalt binder, followed by fracture of the intergranular boundaries and fragmentation of the carbide grains. The main wear mechanism in the TiC–NiMo cermets is polishing (micro-abrasion) and adhesion, resulting in a low wear rate. The main wear mechanism of Cr₃C₂–Ni cermets involves thermal cracking and fatigue-related crushing of large carbide grains and carbide framework and also adhesion.     

Abrasive wear behavior of thermally sprayed diamond reinforced composite coating deposited with both oxy-acetylene and HVOF techniques

Diamond and diamond-based coatings have long been studied for their exceptional properties. Although a great deal of research has been carried out in this field, little is known about their tribological wear behavior. In the present work, diamond reinforced composite (DRC) coatings of varying diamond content was deposited on mild steel substrates using both oxy-acetylene (OA) and high velocity oxy fuel (HVOF) thermal spraying techniques. The high stress abrasive wear behavior of these coatings is studied by performing two body abrasion tests for varying experimental parameters. It is observed that the HVOF-sprayed coatings suffered abrasion at a relatively low wear rate. The reasons for variations observed in the wear rate as a function of displacement during abrasion and grit size could be attributed to the deterioration of abrasive particles and the particle size effect respectively. While the disparity in the wear rates with respect to composition of the coatings was primarily controlled by the diamond content in the coating. The abrasive wear mechanism was found to be the same in both the coatings except that the coating deposited by HVOF spray technique, offered better abrasion resistance and therefore abraded at a slower rate. This is possibly due to lower porosity in the coating and higher bond strength between reinforced diamond particulates and the bronze matrix in HVOF-sprayed specimens.     

Wear prevention in molds used to mold boron filled elastomers

An investigation was conducted to identify surface coatings which would improve the wear characteristics of compression molds used to mold contoured parts from elastomers filled with crystalline boron particles. An analysis was made of the mode of wear present on the surfaces of a production mold, followed by selection and modification of a capillary rheometer as the test device. Test specimens were coated by electroplating, electroless plating, plasma spraying, chemical vapor deposition, sputtering and a fused salt process. Testing was conducted under conditions simulating those encountered in the production molding operation.Techniques used in evaluating the results included optical microscopy, scanning electron microscopy, transmission electron microscopy and surface profilometry. The results are expressed as a ratio of volumetric wear of the tested coating to that of unprotected mold steel.Results indicate that titanium diboride (TiB₂) applied by chemical vapor deposition provides wear resistance superior to plasma sprayed aluminum oxide mixed with titanium dioxide, plasma sprayed chromium oxide, electrodeposited chromium, electroless nickel containing synthetic diamond powder, chemical vapor deposited tungsten carbide (W/W₂C), aluminum oxide and aluminum oxide over chromium carbide. Sputtered titanium diboride was also superior to these coatings and to sputtered titanium carbide and boron carbide.     

Braking performance and noise in excessive worn brake discs coated with HVOF thermal spray process

In this paper, two excessive worn brake discs of a light commercial vehicle were coated with High velocity oxygen fuel (HVOF) thermal spray process for reuse. The coated discs were compared with uncoated Original equipment manufacturer (OEM) D1 disc in terms of braking performance and noise. The D2 disc was coated with tungsten-carbide-cobalt (88 %WC-12 %Co) powder and NiCr (80/20) binder, and 500 Gm thick coating was obtained. The D3 disc was coated with Colmonoy-88 (Ni-W-Cr-B-Si) powder and NiCr (80/20) binder and 600 Gm thick coating was obtained. Several test procedures (e.g. bedding, vibration, and structural-strength tests) was applied to three discs using inertia brake dynamometer. Results showed that coated D2 disc with tungsten carbide cobalt provided lower brake noise and higher brake performance compared with OEM disc. Considering the coefficient of friction and temperature, coated D3 disc has approximately equal braking performance with OEM D1 disc despite the high braking noise value.     

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.     

The influence of surface roughness on friction and wear of machine element coatings

The use of low friction coatings like amorphous carbon or metal-doped carbon coatings on machine elements is constantly increasing. Most often, a surface treatment, e.g. grinding and polishing or honing, is required for optimal performance of the coated machine element. This can be time consuming and costly.In this study, the effect of surface roughness on friction and sliding wear of two different coatings, one tungsten containing and one chromium containing coating, were examined using a ball-on-disc test. Ball bearing steel plates were grinded to different surface roughnesses and coated with the two different coatings.The friction was found to depend on surface roughness where the rougher surfaces gave higher friction coefficients. The wear rate for the a-C:W coating was found to be independent of the roughness, whereas the roughness had a strong influence on the wear rate for the a-C:Cr coating. This could partly be explained by a difference in wear mechanism, where fatigue wear was observed for the a-C:Cr coating but not for the a-C:W coating.     

Wear mechanisms in coated carbide tools

Micrographic evidence is presented which shows the retention of thin, 5μm carbide coating even when the ‘crater’ depth far exceeds the original coating thickness. Whereas earlier authors have attributed this retention to chip smearing, the present authors have shown that cratering can occur by plastic deformation with the coating remaining almost intact. X-ray images from the electron microprobe analyser and from the energy dispersive X-ray analyser support the micrographic findings. Plastic deformation results in flank bulging which causes the coating to wear rapidly by abrasion: the substrate, however, is able to withstand further deterioration. Chip notching on the rake face and grooving wear on the clearance faces also cause removal locally. Severe grooving wear on the end cutting edge as a possible result of oxidation and catalytic action is highlighted