Accumulated damage process of thermal sprayed coating under rolling contact by acoustic emission technique

The accumulated damage process of rolling contact fatigue (RCF) of plasma-sprayed coatings was investigated. The influences of surface roughness, loading condition, and stress cycle frequency on the accumulated damage status of the coatings were discussed. A ball-ondisc machine was employed to conduct RCF experiments. Acoustic emission (AE) technique was introduced to monitor the RCF process of the coatings. AE signal characteristics were investigated to reveal the accumulated damage process. Result showed that the polished coating would resist the asperity contact and remit accumulated damage. The RCF lifetime would then extend. Heavy load would aggravate the accumulated damage status and induce surface fracture. Wear became the main failure mode that reduced the RCF lifetime. Frequent stress cycle would aggravate the accumulated damage status and induce interface fracture. Fatigue then became the main failure mode that also reduced the RCF lifetime.     

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

Al₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al₂O₃ coating was superior to that of the Cr₂O₃ coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al₂O₃ coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al₂O₃ coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr₂O₃ coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation.     

Improvement in tribological properties of plasma-sprayed Cr3C2–NiCr coating followed by electroless Ni-based alloy plating

Electroless-plated Ni-based alloy coatings, Ni, Ni–Co and Ni–Mo coatings with thickness less than 5 μm were deposited on surfaces of plasma-sprayed Cr₃C₂–NiCr coating. The tribological properties of these electroless-plated coatings against the as-sprayed Cr₃C₂–NiCr coating as sliding pairs were investigated with a block-on-ring arrangement in air at room temperature. It was found that all the Ni-based alloy coatings effectively improved the tribological properties of the Cr₃C₂–NiCr coating. Especially when the Cr₃C₂–NiCr coatings plated with Ni–Co and Ni–Mo coatings were against the as-sprayed Cr₃C₂–NiCr coating as sliding pairs, friction coefficients of 0.10 to 0.13 and coefficients wear coefficients less than 10⁻⁶ mm³·N⁻¹·m⁻¹ were achieved. Through examination and analysis of the worn surfaces employing scanning electron microscopy and X-ray photoelectron spectrometer, the improvement in tribological properties of the Cr₃C₂–NiCr coating may be attributed to the transformation of wear mechanism and the formation of CrO₃ on the worn surfaces.     

纳米Fe-Al/Cr_3C_2复合涂层及其抗高温腐蚀性能

制备纳米金属间化合物Fe-Al/Cr_3C_2复合涂层并测试其抗腐蚀性能,为利用热喷涂技术治理火电站易损部件腐蚀问题提供有效手段。运用自主研发的造粒系统,成功对高活性的纳米Fe-Al/Cr_3C_2复合喷涂粉体实施团聚造粒;运用高速火焰喷涂方法,在结构材料表面制备出了纳米Fe-Al/Cr_3C_2复合涂层,对比测试了微米、纳米Fe-Al/Cr_3C_2复合涂层的抗高温腐蚀性能,分别采用抛物线型和幂函数型对腐蚀动力学曲线进行拟合。纳米Fe-Al/Cr_3C_2复合喷涂材料的粒径由原始的50nm团聚到最终的114~178μm,团聚后的纳米颗粒呈圆形或椭圆形,各成分比例保持原始比例,团聚颗粒内部仍然保持纳米粉体状态;纳米Fe-Al/Cr_3C_2复合涂层表面致密、铺展均匀,截面元素过渡平缓、层片细小;运用幂函数方程对腐蚀动力学曲线的拟合效果更好。通过对腐蚀动力学拟合方程进行求导运算可推算出各复合涂层的腐蚀速率。团聚后的纳米颗粒满足热喷涂材料的相关要求,纳米Fe-Al/Cr_3C_2复合涂层的抗高温腐蚀性能显著高于微米Fe-Al/Cr_3C_2复合涂层。纳米Al、Cr优先氧化生成具有保护作用的氧化膜机理解释了纳米涂层抗高温腐蚀性能优异的原因。     

A rolling ball test for establishing contact fatique behaviour of bulk materials, surface layers and coatings

This paper describes a new method for characterising contact fatique behaviour of bulk materials, surface layers and coatings. It is based on contact between a tungsten-carbide ball and a cylinder. In principle most ‘pin-on-ring’ type apparatus can be adapted to meet the test requirements. The method seems particularly attractive for charaterising the performance of surface layers and coatings under dynamic loading conditions. Examples relate to carburised steel surfaces and to a series of six hard chromium coatings with different bond strengths, obtained by applying different plating conditions.The test with the chromium coatings were performed under conditions of substantial initial plastic deformation of the steel substrate material. They were supposed to yield information on the ‘dynamic bond strength’ of the coatings.It was found to be possible to distinguish four quality classes, in agreement with expectations expressed by a plating expert.     

Tribological properties of plasma sprayed and laser remelted 75/25 Cr3C2/NiCr coatings

A CO₂ laser was used to fuse based Ni–Cr Cr₃C₂ coatings for the purpose of homogenizing their microstructures and eliminating their porosity. Tests of layer control and wear resistance were carried out on the samples treated with the laser. The results have shown that laser remelting improves the microstructure of the coatings, increases the coating microhardness, and improves coating–substrate adherence. The dry sliding wear behaviour was characterized by the existence of two periods. During the first period the square of the wear volume is proportional to the sliding distance. During the second, the wear volume is proportional to the sliding distance.     

Wear behaviour of laser-treated plasma-sprayed ZrO2 coatings

The effect of laser remelting on the wear behaviour of plasma-sprayed ZrO₂ ceramic coatings was studied. The results showed that the porosity and roughness of the coatings were reduced significantly after laser treatment, and the bonding strength was apparently increased by the remelting process. However, there were extensive network cracks, as well as a few large bubbles, in the laser-treated coatings. Unlubricated pin-on-disc wear tests revealed that, when compared with as-sprayed ceramic coatings, the wear resistance was improved significantly after laser treatment. The wear resistance of laser-treated specimens increased with increasing laser power, and the minimum weight loss of the coating specimen occurred at a specific travel speed in laser processing. The main wear mechanism of the as-sprayed coatings was spallation of the coating, whereas the wear of laser-remelted specimens was dominated by ploughing and gouging (scratching).     

Cr2N/CrN multilayer coatings for wood machining tools

The present paper describes results from a recent research project aimed at forming a wear resistant coating based on chromium on tools to wood machining. Cr₂N/CrN multilayer coatings deposited on HS6-5-2 steel substrates using cathodic arc evaporation were tested. These coatings were formed from 7 bilayers being ca. 340 nm thick and equally thick Cr₂N and CrN layers. For comparison, Cr₂N and CrN monolayer coatings were also prepared. Hardness measurements, indentation and scratch tests, friction and wear were performed to characterize the mechanical properties. The wear tracks and Rockwell indentations enable to assess wear mechanisms of the coatings. The results of the Cr₂N/CrN coatings investigated show high hardness: ca. about 22 GPa and a critical force being higher than 95 N and a low wear rate.The industrial tests of planer knives with Cr₂N/CrN multilayer coatings were carried out on a down-spindle milling machine to determine the durability of tools with wear resistant coatings for woodworking. These tools show increase of “life time” two times. Another positive feature of the use of such tools is the increase of the quality of wood surface machined when compared with uncoated tools.     

Friction and wear behaviour of plasma-sprayed Cr2O3 coatings against steel in a wide range of sliding velocities and normal loads

This study investigates the influence of sliding speed and normal load on the friction and wear of plasma-sprayed Cr₂O₃ coatings, in dry and lubricated sliding against AISI D2 steel. Friction and wear tests were performed in a wide speed range of 0.125–8 m/s under different normal loads using a block-on-ring tribometer. SEM, EDS and XPS were employed to identify the mechanical and chemical changes on the worn surfaces. A tangential impact wear model was proposed to explain the steep rising of wear from the minimum wear to the maximum wear. The results show that the wear of Cr₂O₃ coatings increases with increasing load. Secondly, there exist a minimum-wear sliding speed (0.5 m/s) and a maximum-wear sliding speed (3 m/s) for a Cr₂O₃ coating in dry sliding. With the increase of speed, the wear of a Cr₂O₃ coating decreases in the range 0.125–0.5 m/s, then rises steeply from 0.5 m/s to 3 m/s, followed by a decrease thereafter. The large variation of wear with respect to speed can be explained by stick-slip at low speeds, the tangential impact effect at median speeds and the softening effect of flash temperature at high speeds. Thirdly, the chemical compositions of the transfer film are a-Fe₂O₃ in the speed range 0.25–2 m/s, and FeO at 7 m/s. In addition, the wear mechanisms of a Cr₂O₃ coating in dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a mixture of abrasion and brittle fracture at high speeds. Finally the lubricated wear of Cr₂O₃ coating increases sharply from 1 to 2.8 m/s.     

Effects of oil hydraulic pressure on surface crack growth in rolling/sliding contact

Rolling/sliding contact fatigue tests were conducted using a pair of transparent discs containing a hole or a surface crack produced artificially on the disc surface. The crack initiation from the hole or the growth of the surface crack were compared with predictions by numerical analyses based on fracture mechanics. Experiments and analyses showed that Way's hypothesis on pitting should be accepted as a plausible mechanism. The experimental results verified several important theoretical predictions on crack growth under rolling and/or contact conditions.     

Friction and wear behaviour of Thordon XL and LgSn80 in sliding against plasma-sprayed Cr2O3 coatings

This paper studies the friction and wear behaviour of two important bearing materials, Thordon XL and LgSn80, in dry and lubricated sliding vs. plasma-sprayed Cr₂O₃ coatings. As a reference, AISI 1043 steel is also studied under the same conditions. SEM, EDS and surface topography were employed to study the wear mechanisms. The results indicate that the Thordon XL/Cr₂O₃ coating pair gives the lowest dry friction coefficient (0.16) under a normal load of 45.3 N (pressure 0.453 MPa) at a velocity of 1 m/s. The dry friction coefficient of Thordon XL/Cr₂O₃ coating increases to 0.38 under a normal load of 88.5 N (pressure 0.885 MPa). The dry friction coefficients of the LgSn80/Cr₂O₃ coating are in the range of 0.31–0.46. Secondly, both dry wear rate under low normal load (45.3 N) and lubricated wear rate under a load of 680 N for Thordon XL are lower than those of LgSn80 in sliding against plasma-sprayed Cr₂O₃ coatings at a speed of 1 m/s. However, under a normal load of 88.5 N the dry wear rate of Thordon XL is much higher than that of LgSn80. Thirdly, a high viscosity lubricant (SAE 140) leads to lower wear for Thordon XL and LgSn80 than a low viscosity lubricant (SAE 30). Finally, the dominating wear mechanism for Thordon XL is shear fracture when against the plasma-sprayed Cr₂O₃ ceramic coating. For LgSn80 against plasma-sprayed Cr₂O₃ ceramic coating, abrasive wear is the governing failure mechanism.     

Effect of contact configuration on the durability and friction coefficient of pressure‐sprayed MoS2 coatings under fretting conditions

Fretting wear is often found at the contact surfaces of a tight assembly where small‐amplitude oscillatory movement occurs, which can be the concealed origin of some enormous accidents. Employment of solid lubrication coatings, as one of effective measurements to palliate the fretting damage, has been widely acknowledged. The present work studied the fretting behaviour of a molybdenum disulphide coating on SUS 316 stainless steel substrate by a relatively cheap and easy‐to‐use process: pressure spraying. Two contact configurations (cylinder‐on‐flat and ball‐on‐flat) were used in the tests with different displacement amplitudes (from 5 to 75 µm) and normal loads (from 100 to 400 N for ball‐on‐flat and from 400 to 1000 N for cylinder‐on‐flat). The results showed that large displacement amplitude is adverse to friction coefficient and coating lifetime and that under a critical contact pressure, coating endurance is improved contact pressure increases. Contact configuration influences friction coefficient by changing contact area and distribution of contact pressure. One master curve of average dissipated energy per cycle in initial stable stage was obtained for two contact configurations, which can be employed to approximately predict coating lifetime. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of undercoating on rolling contact fatigue performance of Fe-based coating

The aim of this paper was to address the influence of undercoating on the rolling contact fatigue (RCF) performance of coatings. The Fe-based alloy and Ni/Al alloy were deposited on substrates as surface coating and undercoating by plasma spraying. The failure modes were investigated for two kinds of coatings. RCF performances were compared using Weibull distribution plot. The results showed undercoating played an important role on increasing RCF life of coating for the higher bond strength, and decreasing the scatter of RCF life data. The deposited undercoating could influence the failure mode of coating and prevent the occurrence of catastrophic delamination.     

Tribological behavior of plasma sprayed Cr2O3-3%TiO2 coatings

Tribological behaviors of plasma-sprayed conventional and nanostructured Cr₂O₃-3%TiO₂ ceramic coatings (i.e., CC3T and NC3T, respectively) using pin on disc type dry sliding and pot type slurry erosion test were investigated in the present work. The experimental results indicated that there were two main wear mechanisms, plastic smearing and adhesive tearing, in the worn coatings under dry sliding. Plastic smearing corresponded to a lower average friction coefficient and wear rate, while adhesive tearing corresponded to higher values. The erosive environment selected for the slurry erosion experiments include 10, 20 and 30% of SiO₂ slurry concentrations in water with particle size 75-106 μm. The main damage mechanism observed in all the coatings submitted to slurry erosion were the formation and propagation of brittle cracks resulting in the detachment of coating surface material. Microstructural investigation was also carried to investigate the wear and erosion mechanism of the coatings using FE-SEM and EDS analysis. Properties like microhardness and porosity were also investigated for these coatings. Tribological performance of NC3T was better as compared to CC3T as observed in the present work.     

Effect of surface texturing on rolling contact fatigue within mixed lubricated non-conformal rolling/sliding contacts

The rolling contact fatigue (RCF) life of highly loaded machine components is significantly influenced by the surface roughness features so that there is a continuous effort to design the topography of rubbing surfaces to enhance lubrication efficiency and prolong the operation of machine components. It can be suggested from the recent experimental results that lubricant emitted from shallow micro-dents could effectively lift off the real roughness features and reduce the asperities interactions within rolling/sliding mixed lubricated contacts. Thereby the additional supply of lubricant from surface features could help to reduce the risk of surface damage through the reduction of the interaction of rubbing surfaces during start-up or starvation. However, the introduction of such roughness features into the rubbing surfaces of highly loaded non-conformal contacts should consider not only the effects on lubrication film thickness but also on RCF.That is why this study is focused on the effects of surface texturing on RCF within non-conformal rolling/sliding contacts operated under mixed lubrication conditions. The principal task has been whether possible beneficial effect on film thickness is not accompanied by the reduction in RCF life. Textures with various sizes of micro-dents and their arrangement within the contacts have been considered. It has been found that results obtained with textured surfaces have exhibited no obvious reduction in RCF. Conversely, some increase in RCF using textured surfaces was observed that could be attributed to the positive contribution of micro-dents working as lubricant micro-reservoirs that reduce asperities interactions. Nevertheless, further experiments are necessary to confirm this possible beneficial contribution of surface texturing on RCF.     

A separation of experimental study on coatings failure signal responses under rolling contact

The objective of present study is to address the signal responses of the acoustic emission (AE), vibration and torque in rolling contact fatigue (RCF) tests of the coatings. The Fe-Cr alloy coatings were deposited using plasma spraying technique. The AE, vibration and torque sensors were assembled to detect the coating failures during the RCF tests, respectively. The results show the signal responses of the vibration and torque accurately indicate the RCF failures of the coatings, but based on the coating losses. The AE signals can sensitively monitor fatigue fractures prior to the formation of the final failure.     

Functional behaviour of PVD coatings under sliding and rolling stress conditions

Whereas wear-resistant PVD coatings are well established in the field of metalcutting, and the functional and tribological behaviour of these coatings is well known under such conditions, PVD coatings are used only occasionally in mechanical engineering. The reason for this seems to be the lack of information concerning the functional behaviour of these coatings in closed tribosystems. To evaluate new areas of application together with optimised coating compounds, model wear tests were performed under sliding, rolling and slip-rolling stress conditions. In addition, the test parameters, such as sliding speed, load, ambient temperature, and number of revolutions were varied, as were the coating compounds and their thicknesses. The results obtained show that friction and wear of PVD coatings are both strongly influenced by the kind of stress and the test parameters themselves. Coatings that perform well under certain test conditions can break down quickly under some other stress conditions. TiN coatings, for example, which display low friction and wear under sliding friction, fail under rolling conditions very shortly afterwards.     

Rolling contact fatigue of alumina ceramics sprayed on steel roller under pure rolling contact condition

The rolling contact fatigue of sprayed alumina ceramics with a nominal composition of Al₂O₃–2.3 mass% TiO₂ was studied with a two-roller test machine under a pure rolling contact condition with oil lubricant. The influence of undercoating of sprayed Ni-based alloy on the rolling contact fatigue was investigated. The failure mode of all sprayed rollers was spalling caused by subsurface cracking. The undercoating did not contribute to the improvement of the rolling contact fatigue life. The elastic modulus of the alumina sprayed layer evaluated with the nano-indentation method was around 85 GPa. The depths of the observed subsurface cracks corresponded approximately to the depths where the orthogonal shear stress or the maximum shear stress calculated with two-dimensional FEM became maximum.     

Behavior of perfluoropolyalkylethers under simulated dynamic bearing conditions

Perfluoropolyalkylether (PFPAE) fluids are of interest to the United States Air Force as potential high temperature liquid lubricants in gas turbine engines. PFPAE fluids have desirable thermal and oxidative stability, and favorable temperature/viscosity characteristics. However, their performance depends on the specific base fluids, additives, bearing material used as well as contact conditions and environments. Screening tests using a modified ball‐on‐rod type rolling contact fatigue (RCF) tester were conducted to study the effects of the above variables and lubricant circulation on fatigue life, wear and performance of PFPAE. Post test lubricant samples were analyzed for changes in physical and chemical properties. Traditional testing for viscosity, acid number and weight changes was performed. Fluid degradation was studied using Fourier transform infrared spectroscopy (FTIR), gas chromatography with atomic emission detector (GC‐AED) and supercritical fluid chromatography (SFC). Elemental analysis of the deposits formed at the tribocontact were determined by Auger electron spectroscopy (AES). This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of a solid additive on rolling fatigue life: Part 2: Behaviour in a water-accelerated test

In this paper, as the second part of another previously published in this journal, the behaviour of a non-stoichiometric inorganic compound (nsic-bp1 additive) in a water-accelerated rolling fatigue test is studied.The results show the remarkable efficacy of the additive against the detrimental action of water present in lubricating oil. The efficacy of additive to inhibit the pernicious effect of water is explained by its reaction capacity.