A study on erosive wear behavior of HVOF sprayed nanostructured WC-CoCr coatings

WC-CoCr cermet coatings were deposited on stainless steel substrate using high-velocity oxy-fuel (HVOF) thermal spray process. The coatings were developed with two different thermal spray powders: one has WC grains of conventional micron size and the other is composed of nanosized (near-nanostructured) grains. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control the process parameters that have resulted in quality coatings. Cavitation erosion testing was performed using a vibratory test apparatus based on ASTM standard G32-98. Surface morphology of powders and coatings was examined using the FESEM images, and phase identification was performed by XRD analysis. The erosion behavior of coatings and mechanism of material removal was discussed by examining the microstructure images of worn-out surfaces. WC-CoCr cermet coating deposited with nanosized WC grains exhibited higher cavitation erosion resistance as compared to conventional coating.     

A method to estimate the metallic material erosion resistance according to energy criterion

We simulate how the external environment influences solid shielding surfaces and how materials with heterogeneous structure and protective coatings react to repeated external pulse impacts with different intensity, and the results of simulation are presented. The structural-energy model that describes the wearing process of materials and coatings, and the physical-mechanical and time parameter criteria of wear resistance are presented. We also present the methods for numerical predicting wear resistance and durability of a material. The practical application of the theoretical model describing the cavitation erosion is shown; in particular, the way to use the erosion-resistant coating, the way to generate relief that ensures units wearlessness, how to change the working medium parameters in order to decrease the wear activity, etc.     

硬密封球阀耐冲蚀陶瓷涂层研究

利用多功能超音速火焰喷涂技术，在金属硬密封球阀的阀芯上制备了WC-12Co金属陶瓷涂层，用于提高球阀的耐冲蚀磨损性能。试验表明，涂层机械力学性能好，与基体结合强度超过70MPa，孔隙率小于2％，硬度超过HV1000，耐冲蚀磨损性能比基体提高5倍以上，有效地提高了球阀的使用寿命和密封可靠性。     

Cavitation erosion resistance of Cr–N coating deposited on stainless steel

Results of investigation on cavitation-erosion resistance of Cr–N coating deposited on stainless steel X6CrNiTi18-10 (1H18N9T) by means of the cathodic-arc method are presented. The evaluation of Cr–N coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The investigation was performed at variable-cavitation intensity and the estimated cavitation resistance parameters of coatings were the incubation period of damage and the instantaneous erosion rate after exposure of specified duration. It has been confirmed that the incubation period of the Cr–N coating damage is approximately 50% longer than that of the uncoated X6CrNiTi18-10 steel, and the instantaneous erosion rate after exposure of specified duration is comparable in both cases. The scanning microscope analysis indicates that the damage of Cr–N coating is due mainly to its delamination, while the erosion of deeper parts of the coating is of minor importance. The character of the coating and substrate damage in multiple locations indicates that the hard coating microparticles torn-off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness are subject to cavitation erosion and to solid particle erosion with the hard torn-off microparticles of coating. The results of the investigation and the analysis indicate that the factors mainly responsible for a long incubation period and low cavitation erosion rate of the steel substrate/hard coating systems are the gained high hardness of substrate and high level of coating adhesion.     

Resistance to wear and cavitation erosion of bearing alloys

Sliding wear and vibratory cavitation erosion tests in paraffin oil were carried out on bearing alloys, i.e. tin-based and lead-based white metals, Cu-Pb alloy and leaded bronze. In lubricated wear under mild conditions the surface is worn smooth and a slight difference exists between the wear resistances of the four alloys. In cavitation erosion an eroded surface which is much rougher than the worn surface is formed. Cavitation erosion is affected strongly by the composition and crystal structure of the alloy and thus the erosion resistances of the four alloys differ greatly, the ranking of resistance being lead-based white metal < Cu-Pb alloy < tin-based white metal < leaded bronze. The surface damage, which is caused by the joint action of cavitation erosion and wear, was also investigated by rubbing the eroded surfaces which had been exposed to cavitation erosion for various times. This damage becomes larger with increasing cavitation damage. The resistance to this damage differs much more in the four alloys tested and tends to correlate with the results of the erosion tests rather than those of the wear tests. Therefore, it is clear that the cavitation erosion resistance should be considered in the selection of bearing materials.     

Evaluation of erosive wear resistance of TiN coatings by a slurry jet impact test

In this paper, it is proposed to use a new type of solid particle impact test (slurry jet) to swiftly evaluate wear properties of thin, single layered or multilayered coatings. By the slurry jet, 1.2 μm alumina particles were impacted at high velocity perpendicular to thin PVD coatings of TiN deposited on high speed steel substrate materials under various substrate temperatures. Since the coatings have a much higher wear resistance than the substrate material, the wear rate increases significantly to the higher level of the HSS material when the coatings are penetrated. This is utilized in the quantification of the assessment of coating wear. A ranking of wear resistance and correlations to the coating surface hardness measured by nano-indentation tests, and coating morphology and structures are given and discussed. The TiN deposited under the highest substrate temperature proved to have the highest wear resistance although it had a relatively low hardness. The wear rate of the TiN coatings varies with the orientation of grains, that is, the {1 1 1} orientation that dominates for the high temperature deposition shows a higher wear resistance than the {1 0 0} orientation, which corresponds with the cleavage fracture behavior. Thus, it can be recommended as a screening test when evaluating coatings and coated materials.     

Cavitation erosion characteristics of nickel-based alloy-composite coatings obtained by plasma spraying

Nickel-based alloy and composite coatings were obtained on an 18-8 stainless steel substrate by a plasma spraying technique. They were sintered in a vacuum furnace (10^?2 Pa) to improve toughness and to reduce hardness. The cavitation erosion resistance of these coatings was evaluated using a rotating disc apparatus. The results of the investigation reveal that sintered coatings are several times better than “as-sprayed” plasma coatings. The sintered coatings can help in increasing the length of the incubation period.     

Cavitation and high-velocity slurry erosion resistance of welded Stellite 6 alloy

The cavitation and slurry erosion resistances of Stellite 6 coatings and 13-4 stainless steel were compared in laboratory. The Cavitation Resistance (CR) was measured according to ASTM G32 standard and the Slurry Erosion Resistance (SER) was tested in a high-velocity erosion tester under several impact angles. The results showed that the coatings improved the CR 15 times when compared to bare stainless steel. The SER of the coatings was also higher for all the impingement angles tested, the highest erosion rate being observed at 45°. The main wear mechanisms were micro-cracking (cavitation tests), and micro-cutting and micro-ploughing (slurry erosion tests).     

Solid Particle Erosion Behavior of WC Coating Obtained by Electrospark Technique and Detonation Spraying

Solid particle erosion is an important material degradation process. One way of improving the erosion resistance of a material is to suitably modify the surface. Electrospark deposition (ESD) is a well-known surface modification process. Operational simplicity, low capital cost, and low operational cost of the ESD process have made it attractive for high-technology areas in engineering industries. Tungsten carbide (WC) is considered a potential hard material for erosion-resistant application. This material can be deposited by ESD. The present investigation has been undertaken to evaluate the room-temperature erosion response of WC coating deposited by ESD and to compare the erosion behavior of this coating with that of detonation-sprayed WC-Co coating. WC coatings were deposited on mild steel (MS) and aluminum substrate by ESD. Similarly, WC-12% Co coatings were deposited on MS and Al by detonation spraying. The microstructural features and mechanical properties of these coatings were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and microhardness testing. The solid particle erosion rate was determined using an erosion test rig. The morphology of the eroded surfaces and the areas beneath the eroded surfaces were examined by means of SEM. The results showed that the WC coating by ESD improves erosion resistance. Although most coatings exhibit a ductile erosion response, WC coating by ESD on Al substrate exhibits a brittle erosion response. Material loss from ESD coating on Al occurs due to the joining of preexisting cracks and the removal of chunk of material.     

A comparative study of tribological behavior of microarc oxidation and hard-anodized coatings

Microarc oxidation (MAO), a novel coating technique capable of depositing dense, hard ceramic composite coatings on aluminium and its alloys, has the potential to replace conventional hazardous anodizing techniques. However, the emergence of such a scenario depends strongly on the properties and performance of MAO coatings in comparison to hard-anodized coatings. In order to facilitate such a comparative investigation, a 6061 T6 aluminium alloy was employed as the substrate and the coatings were deposited through microarc oxidation (MAO) and hard anodizing techniques. The tribological performance of the coatings was evaluated using dry-sand abrasive wheel tests at different normal loads and solid-particle erosion wear tests at different particle velocities and impact angles by employing silica as erodent. The hard-anodized coatings reduced the abrasive wear rate of 6061 Al alloy by a factor of 2, while the MAO coatings reduced the wear rate by a factor of 12-30. Under erosion conditions, the overall wear rate of MAO coatings is identical to that of bare alloy, whereas the hard-anodized coatings exhibit 10 times higher erosion rate.     

Cavitation erosion in silicon nitride: Experimental investigations on the mechanism of material degradation

Test samples from three silicon nitride materials used for potential rolling element bearing applications were experimentally studied at different conditions to understand their mechanisms of cavitation erosion. High and low powered ultrasonic vibratory systems were adapted for this study. Variation in the properties and microstructure of test materials helped to identify the mechanisms of wear and the factors that provide resistance to cavitation. Multiple intergranular and transgranular fractures were noted as the initial stages of erosion. Dislodging of grains from the surface led to the formation of small scale pits, resulting in a rough surface, forming a favourable condition for bubble nucleation, which accelerated the damage, eventually dislodging more grains with noticeable pit proliferation and coalescence. Erosion formed a centre wear scar surrounded by pits due to the variation in fluid film thickness. Detailed surface analysis on eroded samples revealed the influence of microstructure on damage initiation and progression. Erosion rate decelerated and remained at a low level of material removal as wear pits grew deeper, creating a bubble cushioning effect.     

采煤机滑靴激光熔覆铁镍基涂层的抗磨性能*

为提高采煤机滑靴在无油工况下的耐磨性,采用激光熔覆技术在45钢为基体上分别制备FeNiMo和FeNiMoSi涂层,并对其物相组成及硬度等进行分析。结果发现：制备的涂层结构致密,与基底保持了良好的冶金结合;FeNiMoSi涂层的平均硬度为438HV,分别约为基体（153HV）的2.8倍以及FeNiMo涂层（385HV）的1.1倍。通过往复式摩擦磨损试验机研究涂层的干摩擦磨损性能,并探讨其磨损机制。结果表明：随着载荷和滑动速度的增大,涂层的摩擦因数均呈现出减小的趋势;随着载荷的增大,涂层的磨损率逐渐升高;随着滑动速度的增大,FeNiMo涂层的磨损率出现先下降后上升的趋势,而FeNiMoSi涂层的磨损率则逐渐下降;涂层的磨损机制主要为磨粒磨损、塑性变形以及轻微的氧化磨损。总体来说,FeNiMoSi涂层相比FeNiMo涂层表现出更好的耐磨性能,这是因为涂层中Si元素的添加,不仅起到细晶强化作用,而且促进了FeSi金属间化合物相的生成。     

Development of a new type micro slurry‐jet erosion (MSE) tester for evaluation of wear properties of hard thin coatings

Versatile and reliable techniques for evaluation of hard thin coatings are necessary for the development and tribological assessment of new coatings. We have proposed a new type of micro slurry‐jet erosion (MSE) test, i.e. a solid particle impact erosion test for swift evaluation of wear properties of hard thin coatings. We are using a new type of MSE test apparatus (pot type tester) that makes it possible to obtain the wear loss per unit mass of erodent, which in this test was alumina particles with an average size of 1.2 µm. Its performance was evaluated by using a Si wafer plate under various test condition. In addition, the MSE tester was demonstrated by evaluating the wear resistance of TiN on high‐speed steel substrate. The new MSE test generates highly reproducible results and is very sensitive to the quality of the coatings. Copyright © 2009 John Wiley & Sons, Ltd.     

Two-body free-abrasive wear of polyethylene, nylon1010, expoxy and polyurethane coatings

Two-body free-abrasive wear behavior of polyethylene (PE) coatings, nylon1010 (PA1010) coatings, polyurethane (PU) coatings, and expoxy (EP) coatings was investigated. Abrasive wear tests were performed on a turnplate abrasive wear test machine JMM. Hardened and low-temperature tempered steel 45 was used as a reference material. The results shown that in abrasive wear resistance, the PU coatings are approximately 120%, about 5 times larger than the PE coatings and 3 times larger than the PA1010 coatings and the EP coatings, the EP coatings and the PA1010 coatings approach to 40%, and the PE coatings about 20% compared with the reference material (steel 45). Of all the tested polymer coatings, the PU coatings are the best, the PE coatings the worst, and the PA1010 coatings and the EP coatings intermediate. Wear morphologies of the polymer coatings were observed by scanning electron microscope (SEM). Main wear mechanisms of the tested polymer coatings include ploughing wear for the PE coatings and the PA1010 coatings, fatigue wear for the PU coatings and fatigue fracture as well as some brittle fracture and somewhat ploughing wear for the EP coatings.     

Effect of heat treatment on the structure,cavitation erosion and erosion–corrosion behavior of Fe-based amorphous coatings

In this study, high-velocity oxygen-fuel sprayed amorphous coatings have been heat treated at various temperatures to form microstructures with crystalline phases. The structure, micro-hardness, cavitation erosion resistance and erosion–corrosion resistance of these coatings are compared. Crystalline phases are discovered in the coatings after heat treatments at 650 °C and 750 °C. The coating heat treated at 750 °C exhibits the poorest cavitation erosion resistance in 3.5 wt% NaCl solution among all coatings due to the degraded corrosion resistance. However, the hardness of the crystallized coating can reach 1000 Hv and the erosion–corrosion resistance of the heat treated coating is better than the untreated one.     

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.     

水力机械抗汽蚀材料研究新进展

对抗汽蚀新材料的研究与应用现状进行了综述,包括形状记忆合金、加工硬化型合金（Ni3Al、Ti基合金）、金属涂层材料及非金属涂层材料等,并对未来抗汽蚀新材料的研究进行了展望.     

Predictive modelling of dry sliding wear in sealed plasma-sprayed Mo coating using response surface methodology

Plasma spraying is used to produce wear resistant coatings. However, the primary problem is the poor bonding strength between the coating and the substrate. The secondary problem is the high porosity in the as-sprayed coatings, which reduces the wear resistance of coating. In order to overcome these problems, the sealing of plasma-sprayed coating by electrodeposition has been used. The sealing of plasma-sprayed coatings alters the wear mechanism and wear resistance. The wear mechanism and wear resistance largely depends on the applied load, sliding speed and sliding distance. Hence, an effort has been made in the present work to study the effects of these parameters on wear volume loss using response surface methodology (RSM)-based mathematical models. The experiments were conducted as per Central Composite Design (CCD). It reveals that the applied load was the most predominant factor affecting the wear volume loss of the coating material. The sliding speed is the next most important parameter influencing the wear volume loss. The wear volume loss of the sealed plasma-sprayed molybdenum coating occurs mainly due to the formation of grooves, surface tribo films, fracture of splats and delamination of the coating.     

The method of hardening soil rippers by surfacing and technical-economic assessment

The wear resistance and feasibility of applying soil rippers have been studied after surfacing for use in the clayey soil. The study illustrates laboratory and production research of standard and novel (with large grain tungsten carbide) material for manual electric arc surfacing. Hardness and wear resistance in the clayey soil have been determined. Rippers (and the components of soil cultivating machinery yield the least wear by using wear-resistant materials and the comprehensive surfacing of tailing and lateral surfaces. It is shown that hard-alloy tips of rippers are better suited for clayey soil, which is the toughest to cultivate. The best wear resistance in clayey soil is found in hard-alloyed layers of Fe-C-Cr-Si-Mo-Nb-W and Fe-C-Mn-B alloys. The wear rate depends on the depth of cultivation and varies exponentially.     

冲击形式产生的冲蚀、空蚀和腐蚀联合磨损试验研究

舰船等海洋运输装备在海水中行驶时,船体表面受到含沙海水的冲蚀磨损,舰船行驶速度变化引起的空蚀磨损,以及海水的腐蚀磨损,因此舰船表面材料的破坏形式是冲击式的冲蚀、空蚀和腐蚀联合磨损。为了研究联合磨损的影响因素,研制冲蚀、空蚀和腐蚀联合磨损试验台,并实验研究沙粒浓度、沙粒尺寸、含盐浓度以及冲蚀速度对45钢试件的联合磨损影响规律。实验结果表明:冲蚀、空蚀和腐蚀联合磨损程度强于冲蚀和空蚀交互磨损或单纯腐蚀磨损,在较低的沙粒浓度和较低的含盐浓度条件下,金属材料联合磨损质量损失与沙粒浓度、沙粒尺寸、含盐浓度和冲蚀速度均成正比关系。磨损面形貌分析表明:随着沙粒浓度、沙粒尺寸、含盐浓度、冲蚀速度的增加,试件表面的冲蚀沟和空蚀孔的磨痕深度和面积增大,而在人造海水中,试件表面不仅存在冲蚀沟和空蚀孔,还产生了腐蚀坑。