A method for the tribological testing of thin,hard coatings

A new method has been developed for tribological testing of thin, hard antiwear coatings, using a ball‐on‐disc tribosystem, under conditions of dry sliding. In this, an Al₂O₃ ball is pressed against a coated steel disc. Wear debris is removed from the contact zone by a stream of dry argon in this novel method. This improves the stability of the tribological properties and the repeatability of the test results. All test conditions are precisely defined, in particular: the type of motion, air relative humidity, ambient temperature, sliding speed, load, tribosystem spatial configuration, substrate material, substrate hardness and roughness, and coating thickness. The method developed has been used to test various physical vapour deposition coatings (deposited by the vacuum arc method), i. e., single‐layer TiN, Ti(C,N), CrN, and Cr(C,N), and multilayer Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr₂N)/CrN/Cr. It is shown that CrN coatings exhibit the best antiwear properties, and Ti(C,N) the worst. Friction coefficients for CrN and Cr(C,N) coatings are much lower than for the more commonly used TiN. Multilayer coatings have better antiwear properties than single‐layer ones.     

MECHANICAL PROPERTIES AND ANTI-WEARABILITY STUDIES OF MULTILAYER THIN COATINGS ON CUTTING TOOLSS

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MECHANICAL PROPERTIES AND ANTIWEARABILITY STUDIES OF MULTILAYER THIN COATINGS ON CUTTING TOOLS

The nanoindentation fracture of multilayer hard coatings, such as TiN, TiN/Ti(C,N)/TiC, TiN/Ti(C,N)/TiC/Ti(C,N)/TiC and TiN/Ti(C,N)/TiC/Ti(C,N)/TiC/Ti(C,N)/TiC coatings, deposited on cemented carbide using a CVD technique are studied. It is found that these coatings have high hardness. Based on the analysis of the energy release in cracking, the fracture toughness of these coatings are calculated. The observations clearly establish a step occurs in the forcedisplacement curves at the onset of coating fracture and a straigh t line segment in the loadpenetration depth squared curves to identify the interfacial failure of coatings. The hardness, fracture toughness and antiwearability of these coatings are clearly compared. The results show that with the layes increasing, the fracture toughness and antiwearability are getting larger.     

Investigation of antiwear coatings deposited by the pvd process

The tribological properties of various PVD‐deposited coatings (vacuum arc method) have been tested, both single‐layer coatings (TiN, CrN, Ti(C,N), and Cr(C,N)) and multilayer coatings (Cr(C,N)/CrN/Cr and CR(C,N)/(CrN+Cr₂N)/CrN/Cr). An unlubricated ball‐on‐disc tribosystem was used in which an Al₂O₃ ball is pressed against a coated steel disc rotating in the horizontal plane. A novelty of the method is the removal of wear debris from the contact zone using a draught of dry argon. This improves the repeatability of the test results and the stability of the tribological characteristics. It is shown that CrN coatings exhibit the best antiwear properties and Ti(C,N) the worst. Multilayer coatings have better antiwear properties than single‐layer ones. The friction coefficients for CrN and Cr(C,N) coatings are much smaller than for the commonly used TiN. A correlation has also been found between the physical properties of the coatings tested (adhesion of the coating to the substrate assessed in scratch tests, and coating hardness) and their antiwear properties. An improvement in coating‐substrate adhesion results in wear reduction, while greater hardness (causing a coating embrittlement increase and a change in the wear mechanism) brings about greater wear. There is no correlation between the physical properties and the friction coefficients of the coatings tested.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Monitoring of Wear Characteristics of TiN and TiAlN Coatings at Long Sliding Distances

TiN and TiAlN thin hard coatings have been widely applied on machine components and cutting tools to increase their wear resistance. These coatings have different wear behaviors, and determination of their wear characteristics in high-temperature and high-speed applications has great importance in the selection of suitable coating material to application. In this article, the wear behavior of single-layer TiN and TiAlN coatings was investigated at higher sliding speed and higher sliding distances than those in the literature. The coatings were deposited on AISI D2 cold-worked tool steel substrates using a magnetron sputtering system. The wear tests were performed at a sliding speed of 45 cm/s using a ball-on-disc method, and the wear area was investigated at seven different sliding distances (36–1,416 m). An Al₂O₃ ball was used as the counterpart material. The wear evolution was monitored using a confocal optical microscope and surface profilometer after each sliding test. The coefficient of friction and coefficient of wear were recorded with increasing sliding distance. It was found that the wear rate of the TiAlN coating decreases with sliding distance and it is much lower than that of TiN coating at longer sliding distance. This is due to the Al₂O₃ film formation at high temperature in the contact zone. Both coatings give similar coefficient of friction data during sliding with a slight increase in that of the TiAlN coating at high sliding distances due to the increasing alumina formation. When considering all results, the TiAlN coating is more suitable for hard machining applications.     

Ti(C,N)基金属陶瓷在海水润滑下的摩擦磨损特性研究

在材料端面摩擦磨损试验机上,分别对Ti(C,N)金属陶瓷及表面沉积TiN的Ti(C,N)金属陶瓷2种材料与碳纤增强聚醚醚酮(CFRPEEK)组合在海水润滑下的摩擦磨损特性进行实验研究,探讨接触压力和滑动速度对摩擦因数的影响规律。结果表明:在速度较低时摩擦因数随速度增加下降较快,但速度较高时,摩擦因数趋向稳定,速度影响不大;接触压力对摩擦因数的影响较小。通过对试件磨损表面微观形貌的分析,认为CFRPEEK的磨损机制主要是机械切削和机械犁耕引起的材料塑性变形和脱落,并在摩擦过程中发生向陶瓷表面的转移。Ti(C,N)金属陶瓷较表面沉积TiN的Ti(C,N)金属陶瓷具有较低的摩擦因数。通过与其他材料研究结果的比较,认为Ti(C,N)金属陶瓷与CFRPEEK组成的材料副应用于纯水液压泵或马达中的关键摩擦副具有较好的可行性。     

Surface Modification of Polyether Ether Ketone (PEEK) with a Thin Coating of UHMWPE for Better Tribological Properties

The effect of normal load and sliding speed on the tribological properties of a thin film of ultra-high-molecular-weight polyethylene (UHMWPE) coated onto a polyether ether ketone (PEEK) substrate sliding against a stainless steel ball in dry conditions are investigated. Wear tests are carried out with a ball-on-disc configuration to evaluate the tribological properties of the plasma-treated PEEK samples coated with UHMWPE film at different normal loads (5, 7, and 9 N) and linear speeds (0.1, 0.2, and 0.5 m/s). The coated samples exhibited a very low coefficient of friction of ～0.09 compared to that of uncoated PEEK samples, which showed a coefficient of friction of ～0.3.     

Tribological characterisation of hard coatings with and without DLC top layer in fretting tests

The potential of coatings to protect components against wear and to reduce friction has led to a large variety of protective coatings. In order to check the success of coating modifications and to find solutions for different purposes, initial tests with laboratory tribometers are usually done to give information about the performance of a coating. Different Ti‐based coatings (TiN, Ti(C,N), and TiAlN) and NiP were tested in comparison to coatings with an additional diamond‐like carbon (DLC) top coating. Tests were done in laboratory air at room temperature with oscillating sliding (gross slip fretting) with a ball‐on‐disc arrangement against a ceramic ball (Al₂O₃). Special attention was paid to possible effects of moisture (relative humidity). The coefficient of friction was measured on line, and the volumetric wear at the disc was determined after the test from microscopic measurements of the wear scar and additional profiles. The friction and wear behaviour is quite different for the different coatings and depends more or less on the relative humidity. The DLC coating on top of the other coatings reduces friction and wear considerably. In normal and in moist air the coefficient of wear of the DLC top‐layer coating is significantly less than 10⁻⁶ mm³/Nm and the coefficient of friction is below 0.1. In dry air, however, there is a certain tendency to high wear and high friction. Copyright © 2006 John Wiley & Sons, Ltd.     

Wear Behavior of (TiB2–TiC)–Ni/TiAl/Ti Gradient Materials Prepared by the FAPAS Process

(TiB₂–TiC)–Ni/TiAl/Ti functionally gradient materials were prepared by field-activated pressure-assisted synthesis processes. (TiB₂–TiC)–Ni composite ceramic, the top layer of the functional gradient materials, was prepared in situ by the combustion synthesis process using Ti and B₄C powders as raw materials. Scanning electron microscope (SEM) images of the ceramic layer revealed that the TiB₂ and TiC particles in the composite were fine and homogeneously dispersed in the Ni matrix. The friction and wear properties of the (TiB₂–TiC)–Ni ceramic were evaluated by sliding against a GCr15 disk at temperatures from ambient up to 400 °C. The experimental results showed that the friction coefficient of the (TiB₂–TiC)–Ni ceramic decreased with the increasing testing temperature, load, and sliding speed. However, the loss rate decreased at higher temperature and increased at higher load and higher sliding speed. The wear mechanisms of (TiB₂–TiC)–Ni ceramic mainly depend upon thermal oxidation at higher temperature, load, and sliding speed. The worn topography and phase component of the worn surfaces were analyzed using SEM, energy dispersive spectroscopy, and X-ray diffraction. The oxide films of Fe₂O₃, TiO₂, and B₂O₃ formed during the friction process play an important role in lubrication, which results in a smaller friction coefficient.     

Tribological Behavior of Aluminum Alloys Implanted with Nitrogen,Titanium Then Acetylene

The composition depth profiles, structure and ball-on-disk frictional characteristics of aluminum alloys 2024 plasma-based ion implanted with nitrogen, titanium and nitrogen then acetylene were investigated. The layers implanted with nitrogen then with nitrogen and titanium and finally with acetylene included three zones: a top DLC (diamond-like carbon) zone, a C, Ti and N coexisting intermediate zone which undergoes chemical changes forming TiC, Ti(C,N), TiN, (Ti, Al)N and AlN second phases, and the bottom zone of the substrate. The micro-hardness and nano-hardness of these layers are HK7.8 GPa and 22 GPa, respectively. The layers showed lower friction coefficient and higher wear resistance. The Raman spectra for worn tracks after sliding for different numbers of cycles showed that when the loading was 1 N after sliding 10,000 cycles, a slight graphitization phenomenon of the DLC film is found. If the loading was 20 N, the graphitization phenomenon of the DLC film is more obvious after sliding 2000 cycles. The SEM morphologies of the wear tracks showed that when the load was 1 N, after sliding 7200 cycles the wear is from rubbing and abrasive wear. When the load was 20 N, after sliding 2000 cycles, delamination wear is dominant.     

Formation of a Metallic Amorphous Layer During the Sliding Wear of Ti/TiN Nanolaminates

This paper describes experimental studies of metallic/ceramic nanolaminate performance under sliding contact and identifies the formation of an amorphous layer between the nanolaminate and counterface. Nanolaminates used for this study had either 20- or 100-nm-thick alternating layers of Ti and TiN, resulting in a total thickness of ~1-μm films. The structure of the Ti and TiN layers was confirmed using X-ray diffraction [(111)_TiN and (002)_Ti], and compositions were determined using electron energy loss spectroscopy (EELS)—Ti and TiN_0.7. Variation of the individual layer thicknesses within Ti/TiN nanolaminates was shown to influence both the deformation observed through the nanolaminate thickness and also the friction coefficient between the nanolaminate and 440C steel counterface during linear reciprocating wear. During sliding, the 100-nm-layered nanolaminate had a lower coefficient of friction (0.25 ± 0.01) than the 20-nm-layered nanolaminate (0.56 ± 0.06). An amorphous titanium layer developed during sliding at the interface between the 100-nm nanolaminate and steel counterface. EELS confirmed that this layer did not contain any nitrogen and recrystallization occurred near the in-contact surface. While phase changes under compressive loading have been reported for other systems, this is the first report to indicate this response within a titanium layer.     

Wear and performance of coated carbide and ceramic tools

The essential properties of modern high production cutting tools include high wear resistance, toughness, chemical stability at high temperature and under high sliding forces and a sufficiently high flow strength. It is difficult to achieve all these properties in a single tool material and techniques have been developed for coating a thin layer of a highly wear-resistant and friction-reducing material such as TiC, TiN, Ti(C,N), Al₂O₃ and Ti(C,N,O) onto a tough and strong substrate such as cemented carbides.The performance of such coated tools and their wear mechanisms were investigated.     

Influence of low friction coatings on the scuffing load capacity and efficiency of gears

The influence of multilayer composite surface coatings on gear scuffing load carrying capacity, gear friction coefficient and gearbox efficiency is discussed in this work.The deposition procedures of molybdenum disulphide/titanium (MoS₂/Ti) and carbon/chromium (C/Cr) composite coatings are described.Tests reported in the literature, such as Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear, confirm the excellent adhesion to the substrate and the tribological performance of these coatings, suggesting they can be applied with success in heavy loaded rolling–sliding contacts, such as those found in gears.FZG gear scuffing tests were performed in order to evaluate the coatings anti-scuffing performance, which both improved very significantly in comparison to uncoated gears. These results in conjunction with the friction power intensity (FPI) scuffing criterion allowed the determination of a friction coefficient factor X_SC to include the coating influence on the friction coefficient expression.The composite coatings were also applied to the gears of a transfer gearbox and its efficiency was measured and compared at different input speeds and torques with the uncoated carburized steel gears. Significant efficiency improvement was found with the MoS₂/Ti coating.     

WEAR PERFORMANCE OF MULTILAYER-COATED CARBIDE TOOLS

Three multilayer-coated carbides [two trigon-shaped inserts: Ti(C,N)/TiC/Al₂O₃ (T1), Ti(C,N)/ Al₂O₃/TiN (T2) and one 80°-rhomboid shaped insert: TiC/Al₂O₃/TiN (T3)] were used to machine a martensitic stainless steel at various combinations of cutting speed and feed rate without coolant to assess their wear performance. Significant nose wear and chipping/fracture of the cutting edge were the predominant failure modes affecting tool performance at higher speed conditions. Plucking of tool materials was the main rake face wear phenomenon observed on T1 grade insert with alumina as the top-layer coating when machining at the lower speed conditions. Attrition and plastic flow were the main wear mechanisms observed on the ceramic coating layers, with dissolution-diffusion being the probable wear mechanism of the tool grades where tungsten carbide substrate had direct contact with the flowing chip. The fitted statistical wear models revealed T3 grade insert with 80°-rhomboid shape as having the highest speed-feed capability, resulting in the highest material removal rate relative to T1 and T2 grade inserts with trigon shapes.     

Exploitation of wear- and corrosion-resistant cvd-coatings

Hard, friction reducing, wear- and corrosion-resistant coatings, in particular TiC, TiN, Ti(C,N), Cr₇C₃, borides and combinations of these substances in composite layers, can be applied to steel, Ni(Co) alloys and cemented carbides by cvd and pvd. This paper discusses friction and wear in ambient and more hostile environments, notably ultra-high vacuum and at about 350°C in a He-atmosphere. The corrosion behaviour of TiC in sulphuric acid and in sea water is also covered. Applications of these coatings in machine elements and tooling are described.     

Sliding Wear of Ni–17.5Si–29.3Cr Alloy under Water Lubrication

The tribological properties of Ni–17.5Si–29.3Cr alloy against Si₃N₄ under water lubrication conditions were studied on a ball-on-disc reciprocating 1tribotester. The effects of load and sliding speed on tribological properties of the alloy were investigated. The worn surfaces of the alloy were examined with SEM, TEM and an X-ray photoelectron spectroscope (XPS). It was found that the tribological properties of the alloy were closely dependent on the sliding conditions. Wear rate with the load of the alloy increased slightly at low and moderate load and increased dramatically at high load. Wear rate with the sliding speed of the alloy increased slightly at low and moderate sliding speed and increased dramatically at high sliding speed, which showed the same trend as that with the load. The friction coefficient increased with the load (especially at high load), and decreased with sliding speed at low sliding speed and increased significantly at high sliding speed. Wear mechanism of the alloy was mainly microploughing and delamination at low and moderate load and transformed to microfracture and delamination at high load.     

Effect of counter materials on coefficients of friction of TiN coatings with preferred grain orientations

In order to investigate the effect of counter materials on the coefficients of friction of TiN coatings with preferred grain orientations, the coefficients of friction against six counter materials (ball) were measured. The ball materials were aluminum (A1050), stainless steel (SUS304, SUS440C), bearing steel (SUJ2), carburized steel (SWRM10), cemented tungsten carbide (WC-Co) and alumina (Al₂O₃). After tests, the worn flat surfaces of balls and the wear tracks of TiN coatings were analyzed by the electron probe micro analysis (EPMA) and the auger electron spectroscopy (AES) to observe the Ti oxide film on each ball material and the adhesion of ball materials to the TiN coatings. The Ti oxide film was observed on the worn flat surfaces of the ball materials and the ball materials did not adhere to the TiN coatings in case that the low coefficients of friction were obtained.     

Tribological properties of a Fe3Al material in sulfuric acid corrosive environment

The tribological properties of a Fe₃Al material in an aqueous solution of 1 mol/l H₂SO₄ corrosive environment sliding against a Si₃N₄ ceramic ball are studied using an Optimol SRV oscillating friction and wear tester in a ball-on-disc contact configuration. We investigate the effects of load and sliding speed on tribological properties of the Fe₃Al material. The worn surfaces of the Fe₃Al material are examined by a scanning electron microscope (SEM) and an X-ray photoelectron spectroscope (XPS). It is found that the Fe₃Al material exhibits better wear resistance than 1Cr18Ni9Ti stainless steel in the sulfuric acid corrosive environment. The wear rate of the Fe₃Al material is on the order of 10^?13 m³/m and increases with increasing load, but does not vary below the sliding speed of 0.08 m/s then dramatically increases with increasing sliding speed. The friction coefficient of the Fe₃Al material is in the range of 0.1–0.28, and slightly increases with increasing load, and does not vary with the increase of sliding speed. The Fe₃Al material occurs tribochemical reaction with the H₂SO₄ aqueous solution in the friction process. Wear mechanism of the Fe₃Al material is dominated by microploughing and corrosive wear.     

TiN/Ti涂层在波箔轴承中的摩擦磨损性能

采用真空离子镀的方法在304不锈钢基体上喷涂厚度为3μm的TiN/Ti薄层,利用硬度计、三维形貌仪、划痕试验仪对涂层基本力学性能进行分析,通过球盘试验机分析涂层试样的摩擦磨损性能,根据波箔轴承性能测试实验台的测试结果:研究TiN/Ti涂层对基体表面耐磨减摩性能的影响。研究结果表明:TiN涂层硬度可达HV1500,是基材硬度的5.5倍;TiN/Ti涂层平均摩擦因数为0.23,相对不锈钢304基材的平均摩擦因数0.71,降低了68%,磨损量也仅为基材的18.75%;GCr15与PTFE对磨的最大摩擦力矩可达2.4 N·mm,而TiN/Ti与PTFE对磨的最大摩擦力矩仅为1 N·mm,仅为GCr15的41.7%。TiN/Ti涂层表现出了优异的承载能力和耐磨减摩性能。