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.     

Durability of plasma-sprayed Cr3C2-NiCr coatings under rolling contact conditions

The aim of this paper was to address the rolling contact fatigue (RCF) failure mechanisms of plasma-sprayed Cr3C2-NiCr coatings under different tribological conditions of contact stress. Weibull distribution plots of fatigue lives of the coated specimens at different contact stresses were obtained. The failure modes of coatings were identified on the basis of wore surface observations of the failed coatings. Results showed that the RCF failure modes can be classified into four main categories, i.e., surface abrasion, spalling, cohesive delamination, and interfacial delamination. The probabilities of the surface abrasion and spalling type failures were relatively high at low contact stress. When the coatings were subjected to abrasion and spalling type failures, the failure of the coating was depended on the microstrcture of the coating. The stress concentration near the micro-defects in the coating may be the may reason for the formation of spall. The coatings were prone to fail in delamination under higher contact stresses. However, the delamination of coating may be related to distribution of shear stress amplitude within coating. The location of maximum shear stress amplitude can be used as a key parameter to predict the initiation of subsurface cracks within coating in rolling contact.     

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.     

Thermal shock/fatigue evaluation of FGM by AE technique

This study has been carried out to analyze the thermal shock and fatigue characteristics of functionally gradient material (FGM). The thermal shock/fatigue tests were conducted at a specific temperature and fracture patterns were studied by SEM and AE. Also, thermal fracture behavior of plasma-sprayed FGM and conventional coating material (NFGM) was examined by acoustic emission technique under heating and cooling. Furnace cooling and air cooling tests were used to examine the effect of temperature change under various conditions. The conventional and FGM coatings were compared to heat-resistant property and fracture surface of these materials for each temperature history. Based on these results, some critical temperature at the onset of coating failure can be evaluated to characterize the thermal resistance of the materials. It turned out to be that FGM gives higher thermal property compared with NFGM by AE signal and fracture surface analysis.     

Acoustic emission monitoring and failure mechanism analysis of rolling contact fatigue for Fe-based alloy coating

The aim of present study is to monitor rolling contact fatigue (RCF) behavior and analyze failure mechanism of the coating by acoustic emission (AE) technology. The results show that AE amplitude and counts are sensitive to RCF damage process, and variation regularities are different during different damage stages. Spalling is the key failure mode. RCF damage process is composed of five stages, such as elastic deformation, plastic deformation, crack initiation and propagation, crack stable growth, and unstable growth. Fatigue cracks are liable to initiate and grow along the direction of defects within the coating, such as pores and cracks.     

Rolling contact fatigue performance of detonation gun coated elements

Rolling contact fatigue performance of thermal spray coatings has been investigated using an experimental approach. A modified four ball machine which simulates a rolling element bearing was used to examine the coating performance and failure modes in a conventional steel ball bearing and hybrid ceramic bearing configurations. Tungsten carbide (WC-15% Co) and aluminium oxide (Al₂O₃) were thermally sprayed using a super D-Gun (SDG2040) on M-50 bearing steel substrate in the geometrical shape of a cone. A coated cone replaced the upper ball that contacts with three lower balls. The rolling contact fatigue (RCF) tests were performed under immersed lubricated conditions using two different lubricants. Fatigue failure modes were observed using a scanning electron microscope. Microhardness measurements of the coating and the substrate and elastohydrodynamic fluid film thickness results are included. The results show the requirement for significant optimization of the coating before use in rolling element bearing applications. The coating was fractured in a delamination mode. Test results show an optimization in coating process is required before these coatings can be used for rolling contact applications. WC-Co coatings perform better than Al₂O₃ coatings in rolling contact.     

Effects of Service Condition on Rolling Contact Fatigue Failure Mechanism and Lifetime of Thermal Spray Coatings——A Review

The service condition determines the Rolling Contact Fatigue(RCF) failure mechanism and lifetime under ascertain material structure integrity parameter of thermal spray coating. The available literature on the RCF testing of thermal spray coatings under various condition services is considerable; it is generally difficult to synthesize all of the result to obtain a comprehensive understanding of the parameters which has a great effect on a thermal spray coating’s resistance of RCF. The effects of service conditions(lubrication states, contact stresses, revolve speed, and slip ratio) on the changing of thermal spray coatings’ contact fatigue lifetime is introduced systematically. The effects of different service condition on RCF failure mechanism of thermal spray coating from the change of material structure integrity are also summarized. Moreover, In order to enhance the RCF performance, the parameter optimal design formula of service condition and material structure integrity is proposed based on the effect of service condition on thermal spray coatings’ contact fatigue lifetime and RCF failure mechanism. The shortage of available literature and the forecast focus in future researches are discussed based on available research. The explicit result of RCF lifetime law and parameter optimal design formula in term of lubrication states, contact stresses, revolve speed, and slip ratio, is significant to improve the RCF performance on the engineering application.     

等离子喷涂NiCrBSi涂层接触疲劳过程中残余应力变化规律研究

采用超声速等离子喷涂技术在45钢基体上制备NiCrBSi合金涂层,涂层整体结构致密,含有少量微缺陷。使用球盘式接触疲劳试验机在接触应力为1.86 GPa,转速为1 500 r/min的条件下对涂层试样进行接触疲劳试验,得到涂层的接触疲劳寿命为9.0×105周次;同时建立Weibull失效概率图,通过该图可以直观地得到在同一工作条件下,涂层任意循环次数的失效概率。测量涂层不同疲劳阶段的残余应力,结果表明:一旦进行接触疲劳试验,其残余应力形式由拉应力迅速转化为压应力,在整个疲劳过程的前半段,应力增强较为缓慢,在疲劳过程的后半段,应力值迅速增大,疲劳过程的中期成为了应力变化的转折点。根据这一规律,可以有效地判定涂层处于疲劳过程的哪个阶段,从而为预防涂层的突然失效提供了一种新的手段。     

The use of artificial defects in the 5-ball-rod rolling contact fatigue experiments

The fatigue performance of steels under rolling contact loading is normally evaluated using time consuming rigs, though accelerated tests are also used. The present work reports the analysis carried out on tests accelerated by using artificial defects (ADs) over the wear track. Two modifications were introduced to the 3-ball-rod testing protocol. The first modification reduces the time required to produce a spall by substituting 5-ball for the traditional 3-ball mounted on a retainer. The second modification is intended to study surface resistance to rolling contact fatigue (RCF) by furnishing the wear track with artificial defects which promote surface nucleated failures. Three different types of pedigreed artificial defects, i.e. Knoop, Vickers and Rockwell-C indentations were used. The experiments were conducted on specimens of AISI 440C. The failures obtained were optically analyzed and the exact location for spall nucleation was determined to be inside the AD rather than outside. An attempt was then made to correlate the results with the specific oil film parameter (lambda) but life changes could not be predicted by this criteria. The results indicate that the combination of ADs in conjunction with the five balls methodology does not bias the results, but it considerably reduces the time required to test materials.     

Investigation of Fatigue Damage of 20Kh13 Structural Steel and its Compositions with Fused Coatings by the Free Vibration Method

The frequency of free vibrations of samples of 20Kh13 steel and its compositions with fused coatings is measured during fatigue tests. The dynamics of the frequency spectrum of the samples is investigated during the development of a fatigue crack. It is shown that the dynamics of variation of the free vibration frequency of the samples is tightly related to the kinetics of fatigue damage. Features of variation of the frequency spectrum of free vibrations in samples with fused coatings which are characterized by complex kinetics of the development of a fatigue crack are noted. Results of an analysis of the possibilities of diagnostics of fatigue damage of pieces from metallic materials and compositions with coatings obtained with the use of the free vibration method are given.     

Incipient damage detection and its propagation monitoring of rolling contact fatigue by acoustic emission

The acoustic emission (AE) technique was applied to rolling contact fatigue tests using a test-rig running under constant load and speed for detecting the incipient damage and damage location. This incipiently-damaged roller was investigated in detail and monitored by further running to determine the damage severity and to understand the surface damage propagation process by applying the AE techniques. The conventional AE parameters and AE signal features were studied, and their relation with the AE source locator hit count rate were correlated. The results demonstrated the successful use of the AE measurement unit, which is principally, consists of the AE data analyzer and the AE source locator as a new system for detecting incipient damage produced by fatigue. Moreover, the system is able to forecast the position of the damage in the roller, capable of providing an indication of the severity of damage i.e. damage size, and thus it could allow the user to monitor the rate of further degradation of the rolling elements.     

结构疲劳损伤的声发射检测可靠性研究

针对声发射技术对金属结构损伤的检测可靠性问题,以金属疲劳裂纹检测试验为基础,利用声发射特征参数的趋势分析和关联分析法,首先根据贝叶斯理论,计算了声发射对一定长度疲劳裂纹的检测概率;其次,研究了检测结构起裂时对应声发射幅度参数的阈值及其对损伤判断的影响。该研究可为减少声发射检测中的金属疲劳损伤信号误判,提高检测效率,以及确定复杂航空结构疲劳寿命提供支持。     

A Study on Incipient Damage Monitoring in Rolling Contact Fatigue Process Using Acoustic Emission

Acoustic emissions (AE) technique was applied to rolling contact fatigue tests of two radially loaded rollers running under constant load and velocity to detect the incipient damage and damage location. Signals detected from contacts were processed using signal conditioning and enhancement techniques by an AE source locator to bring out the difference between the signals from the sound and damaged rollers. It was found that AE hit count pulse observations by the AE source locator can provide an indication of the damage at its initial stage. The conventional AE parameters and the AE signal features were studied and correlated with the AE source locator counts. The results demonstrated the successful use of the combination of the AE monitoring and the AE source locator as a new technique for detecting the incipient damage and forecasting the position of the damage in the roller, and this technique could allow the user to monitor the rate of deterioration of the rolling elements.     

接触式机械密封启停过程摩擦特性实验研究

为探讨接触式机械密封启停过程摩擦状态的转变特性,采用自主研发的摩擦磨损实验装置,对碳石墨-碳化硅配副材料的摩擦特性进行实验研究。通过采集启停过程中的声发射信号以及摩擦扭矩数据,分析二者之间的关联性,研究启停过程中端面摩擦状态的转变特性。结果表明：在启停阶段,声发射信号均方根值与摩擦扭矩值变化趋势相同,均为先增大后减小,最后再增大至一稳定值,二者具有很强的相关性;研究的配副材料在7 s左右出现声发射信号峰值,表明此时发生从干摩擦状态到混合摩擦状态的转变;稳定阶段的声发射信号以及摩擦扭矩值分别与转速、压力呈正比关系。     

Coatings for fretting prevention

The purpose of the program to study coatings to prevent fretting fatigue was to determine the optimum fretting fatigue resistant coatings for titanium/steel mating surfaces found in helicopter rotor hub assemblies. Here several problems have been encountered during component accelerated fatigue tests of the hub assembly where fatigue failures of the titanium hub initiated at areas of severe fretting. In order to address this problem a program was developed to evaluate candidate coatings in terms of their ability to reduce or eliminate fretting of Ti-6Al-4V when in contact with 17-4 PH steel.This paper describes the new fretting test apparatus, the results obtained thus far on screening selected coatings which might have potential for fretting protection and an evaluation of bend bar test specimen configurations designed to achieve fretting fatigue under conditions comparable to those experienced in the actual rotor hub assemblies.     

In situ Acoustic Emission for wear life detection of DLC coatings during slip-rolling friction

Different diamond-like carbon (DLC) coatings on a steel substrate (100Cr6) were tested under slip-rolling friction conditions against uncoated counter bodies of the same steel. The initial maximum Hertzian pressure was varied in a range of P₀ = 1.5–2.3 GPa. The friction tests were carried out under dry conditions and with an unadditivated paraffin oil as lubricant. It could be shown that the thickness of the coatings affects the respective wear life. Further, a very important factor for the wear life of a coating under lubricated slip-rolling conditions is the roughness of the surface of the respective counterbody. The wear life tests were monitored by recording in situ the Acoustic Emission (AE) signals. Some causes for a high AE activity could be identified.     

Dry sliding wear response of some industrial powder coatings

This investigation analyses the sliding wear response of polyester and polyphthalamide powder coatings deposited by electrostatic spraying and ‘hot dipping’ fluidised bed. Tribological tests were conducted under dry conditions in a pin-on-disc arrangement, using a spherical counterpart. The experimental results showed that the deposition technology, the coating material and the thickness of the coating play key roles in determining the wear response of powder coatings. In particular, polyester coatings are deposited by a fluidised bed offer superior wear endurance, along with a low abrasion volume and a low wear rate. Conversely, polyphthalamide coatings are susceptible to faster wear by local cutting and plastic fatigue mechanisms.     

Sonic monitoring of the development of fretting fatigue

Investigations were carried out to determine whether the loosening of a knuckle joint under the influence of an alternating vibration with subsequent fretting fatigue at the interface could be remotely detected by studying the trend in an acoustic signal recorded at a microphone. The results indicate that air-borne sonic monitoring can be successfully applied to a basic system and they are adequate to explore the possibility of monitoring fretting fatigue failures.     

钻削监控传感及诊断

对有关钻削加工中的刀具磨损和失效的监控试验和报告作了综述.对监控中用到的间接测量方法有所涉及,如轴向切削力和扭矩、刀具振动、电机电流和功率以及加工中的声发射等.对所用的传感器和信号分析技术,包括统计方法和参数、快速傅里叶和小波变换以及倒谱技术均有所提及.对结果的诊断多数采用人工神经网络技术.     

Effect of Formulation and Temperature on Boundary Lubrication Performance of Polyphenylethers (5P4E)

Two formulations of polyphenylether (PPE) (5P4E) were evaluated in boundary lubrication regime using a ball-on-rod type rolling contact fatigue (RCF) tester. RCF tests were conducted using VIM-VAR M50 bearing elements at a stress of 4.8 GPa. To study the lubricant's dynamic thermal stability and tribological characteristics, tests were conducted at temperatures ranging from 66 to 343 °C for a 24 h period. As expected, both formulations showed higher oxidative stability compared to the base stock by 100 °C. The dynamic thermal stability of formulation A (with tin based additive) in rolling contact tests surpassed formulation B (with an alkaline additive) by over 50 °C. At higher temperatures, the PPEs showed heavy build-up of friction polymers in the contact zone. Viscosity showed only small changes for all the three PPE evaluated. Auger electron microscopy (AES) of tested specimens showed definite presence of tin in the tribofilm. Rolling Contact Fatigue (RCF) tests were conducted at 288 °C on formulation A (PPE-A) and formulation B (PPE-B) in order to compare them to the state-of-the-art gas turbine engine lubricant MIL-PRF-7808L tested at 177 °C. The L₅₀ fatigue life and wear of PPE-B at 288 °C are comparable to that of MIL-PRF-7808L at 177 °C.