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。     

General aspects for tribological applications of hard particle coatings

Hard coatings, consisting of WC, TiC or Cr₃C₂ particles with a nickel or cobalt matrix were compared with conventional wear-resistant materials like hardened steel 100 Cr6, Ferro TiC P143, WC-Co hard metal and a widely used thermal spray layer NiCrBSi. The coating procedure was flame spraying and diffusion welding. Some layers were remelted using an electron beam to improve their microstructural properties, porosity and binding to the bulk material.

Wear tests were performed under different degrees of severity to qualify the resistance of the coating, using abrasive, sliding and impact test methods representing different wear mechanisms. It is shown that the benefit of the hard particle content depends on the acting loading situation. Under abrasive and sliding conditions the advantage of a high hardness level, i.e. a high concentration of hard phases, could be demonstrated. For impact loading, causing severe surface fatigue, homogeneous materials with high toughness, such as martensitic steels, are beneficial; followed by coatings with a high concentration of ductile matrix. In some cases, the weaknesses, such as brittleness and limited strength of binding to the bulk, could be improved by electron beam remelting.     

Y2O3改性石墨/CaF2/TiC/镍基合金复合涂层微观组织与摩擦学性能研究

为提高石墨/CaF₂/TiC/镍基合金（GCTN）复合涂层的力学性能和摩擦学性能,运用等离子喷涂技术在45钢表面制备了Y₂O₃改性GCTN复合涂层,研究了Y₂O₃对复合涂层的微观组织、显微硬度、断裂韧性和摩擦磨损性能的影响。结果表明：Y₂O₃改性GCTN复合涂层主要由γ-Ni、CrB、Cr7C₃、TiC、CaF₂和石墨等物相组成。Y₂O₃在等离子火焰加热作用下与C元素反应生成活性元素Y,Y净化了复合涂层的微观组织,并细化了CrB、Cr₃C7等硬质相晶粒,提高了其致密性。当Y₂O₃质量分数为0.5%时,复合涂层的显微硬度和断裂韧性分别为593.3MPa和6.82MPa·m^1/2,比不含Y₂O₃的复合涂层分别增大了8%和22%,其机理主要是Y₂O₃细化了CrB、Cr₃C₇等硬质相晶粒,起到了细化强化作用。由于GCTN-0.5Y₂O₃复合涂层的显微硬度和断裂韧性显著提高,减少了其黏着磨损和微观断裂磨损,因而GCTN-0.5Y₂O₃复合涂层的摩擦因数和磨损率最小,分别为0.085和0.39×10-3mm3/m。     

超声辅助电火花粉末沉积WC-Ni金属陶瓷涂层的微观结构及摩擦学性能

针对传统电火花沉积工艺中工具电极预制成本高、工艺复杂、材料选择范围受限等问题,提出了一种超声辅助电火花粉末沉积（Ultrasonic-assisted electro-spark powder deposition,UEPD）的新方法。利用UEPD工艺成功地在316L不锈钢基材上制备了WC-Ni金属陶瓷涂层。所制备的WC-Ni金属陶瓷涂层的厚度为89~159 μm,表面粗糙度约为3.672 μm,并且与基材呈现良好的冶金结合。超声振动的引入能够有效改善涂层的成形质量。涂层的微观组织主要由亚微米级细小枝晶组成,主要物相包括FeNi、Cr₃Ni、WC、W₂C、Cr₂₃C₆和Cr₃C₂等。这些细小的晶粒和强化相使金属陶瓷涂层的硬度明显增加,平均硬度达到980.68 HV,约为基材的4.1倍。摩擦磨损性能测试表明,金属陶瓷涂层的磨损率相比基材和不含WC的Ni基合金涂层分别降低了50.7%和37.7%,并且还表现出明显低于二者的摩擦因数。WC-Ni金属陶瓷涂层的主要磨损机理为疲劳磨损和磨粒磨损,其中高硬度表面和具有颗粒流润滑效果的磨屑层是金属陶瓷涂层实现高耐磨、低摩擦的主要原因。UEPD工艺相比于传统的电火花沉积工艺省却了复杂的工具电极预制过程,其工艺更简单,成本更低廉、材料选择更广泛,并且所制备的涂层也表现出良好的成形质量和性能。这为电火花沉积技术的发展提供了一种新的思路。     

Analysis of the friction and wear mechanisms of multilayered plasma-sprayed ceramic coatings

Wear tests were conducted on a rotor-vane-disk adaptor where three rotating vanes were pressed against a disk. Vanes were coated by WC and used as the upper specimen while the disk was coated by Cr₂O₃ and used as the lower specimen. A buffer layer of various thicknesses and contents was placed between the top coating and the bulk steel of the disk to alleviate the effects of the large difference in thermal properties of the two materials. The experimental results reveal that correct placement of a buffer layer can indeed improve the wear resistance. Factors such as the temperature to which the specimen was heated before testing, the proportion by weight of each individual constituent in the buffer layer, and the thickness of each coating layer, were also important for the volume of wear of the lower specimen. As the specimens were heated to higher temperatures, the wear volume decreased with increasing proportions of Cr₂O₃ in the buffer layer. Elevating the preheating temperature of the specimens can diminish the wear volume but increases the friction coefficient. The steady-state wear rate is not much influenced by the constituents of the buffer layer and the coating thickness. Brittle fracture, abrasion, adhesion and oxidation were found to be the primary wear mechanisms in the tests.     

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF_x with x = 1.85) and chromium-carbon (Cr₃C₂) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF_x single layers as well as Ag/CaF_x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF_x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF_x/Cr₃C₂ thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF₂-CaF₂ eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.     

Counterface effects on the wear of a composite dry-bearing liner

Plastics-based dry-bearing liners used for flight control bearings in aircraft are usually mated against counterfaces of 440C stainless steel hardened to about 700 HV and finished to R_a ≈ 0.05 μm. In this paper experiments to examine the possibility of reducing liner wear by modifications to the counterface are described. Accelerated (pin-on-disc) tests were made against 440C stainless steel of varying hardness and roughness, electroplated with copper and cadmium, ion implanted with nitrogen, copper and cadmium, vacuum deposited with TiN and TiC, diffusion treated with nitrogen, boron, sulphur, Sn-Cu and Sn-Sb and coated with ceramics-cermets (Al₂O₃, Cr₂O₃, (Cr₂C₃)-Ni-Cr and WC-Co). The most important counterface properties influencing liner wear are the hardness and surface roughness, and for ceramic and cermet coatings, the harder and smoother the surface, the lower is the liner wear. No evidence was found to indicate that the chemical nature of the counterface has a major affect on the liner wear.     

Laser cladding of ti-6al-4v with bn for improved wear performance

Titanium and its alloys suffer from galling, seizing, ploughing and adhesion during sliding contacts. A laser-cladding method, to enhance the wear performance of titanium, was investigated. A 1.5 kW continuous-wave CO₂ gas laser was used to clad hexagonal BN powder with and without the addition of NiCrCoAlY on a Ti-6Al-4V alloy substrate. X-ray diffraction, scanning electron microscopy, optical metallography, and Vickers' micro-hardness tests were employed to characterize the clad layers. A pin-on-block reciprocating wear machine was used to evaluate the sliding wear characteristics of age-hardened, laser surface-melted and laser-clad Ti-6Al-4V alloys. Results indicated that the clad layers consisted of TiN, TiB₂, and various alloy phases. Claddings with excellent adhesion and thicknesses up to 600 μm, with a maximum hardness of 1600 HV, were obtained. Wear tests showed a substantial improvement (10–200 times) in wear resistance of claddings over age-hardened and surface-melted layers. The improved wear performance is attributed to the high hardness and low friction properties of clad layers.     

Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6

The development of wear surfaces formed during limited debris retention sliding wear of Incoloy MA956 against Stellite 6 between room temperature and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ (7 N applied load, 4522 m sliding distance) were investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to oxidation of Stellite 6-sourced debris and transfer to the Incoloy MA956; this debris separated the Incoloy MA956 and Stellite 6 wear surfaces. Between room temperature and 450 °C, the debris mainly took the form of loose particles with limited compaction, whilst between 510 °C and 750 °C the debris were compacted and sintered together to form a Co–Cr-based, wear protective ‘glaze’ layer. The behaviour was identical to that previously observed on sliding Nimonic 80A versus Stellite 6 at 0.314 m s⁻¹.At 0.905 m s⁻¹, mild oxidational wear was only observed at room temperature and 270 °C and dominated by Incoloy MA956-sourced debris. At 390 and 450 °C, the absence of oxide debris allowed ‘metal-to-metal’ contact and resulted in intermediate temperature severe wear; losses in the form of ejected metallic debris were almost entirely Incoloy MA956-sourced. This severe wear regime was also observed from 510 up to 630 °C, but increasingly restricted to the early stages of wear by development of a wear protective Incoloy MA956-sourced ‘glaze’ layer. This ‘glaze’ layer formed so rapidly at 690 °C and 750 °C, that severe wear was all but eliminated and wear levels were kept low.The behaviour observed for Incoloy MA956 versus Stellite 6 at 0.905 m s⁻¹ contrasts sharply with that previously observed for Nimonic 80A versus Stellite 6, in that the Incoloy MA956-sourced high Fe–Cr debris formed a protective oxide ‘glaze’, whilst the Nimonic 80A-sourced Ni and Cr oxides formed an abrasive oxide that at high sliding speeds assisted wear. The data indicates that the tendency of oxide to form a ‘glaze’ is readily influenced by the chemistry of the oxides generated.     

高温耐磨球阀密封面涂层强化工艺的研究

介绍了采用超音速火焰喷涂(HVOF)工艺结合物理气相沉积(PVD)技术在Cr₃C₂-NiCr涂层上沉积CrN涂层形成CrN/Cr₃C₂-NiCr复合涂层的强化工艺,并对复合涂层从薄膜微观结构设计、摩擦、腐蚀及减磨防腐机理方面进行了研究。     

Studies of high temperature sliding wear of metallic dissimilar interfaces

The evolution of microstructures in the glaze layer formed during limited debris retention sliding wear of Nimonic 80A against Stellite 6 at 750 °C and a sliding speed of 0.314 m s⁻¹ (7 N applied load, 4522 m sliding distance) was investigated using scanning electron microscopy (SEM), energy dispersive analysis by X-ray (EDX), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The collected data indicate the development of a wear resistant nano-structured glaze layer. The process of ‘fragmentation’ involving deformation, generation of dislocations, formation of sub-grains and their increasing refinement causing increasing misorientation was responsible for the formation of nano-structured grains. The rapid formation of this glaze layer from primarily cobalt–chromium debris transferred from (and also back to) the surface of the Stellite 6, kept wear of both the Nimonic 80A and Stellite 6 to very low levels.However, increasing the sliding speed to 0.905 m s⁻¹ (750 °C) suppressed glaze formation with only a patchy, unstable glaze forming on the Stellite 6 counterface and an absence of glaze development on the Nimonic 80A sample (the Nimonic 80A surface was covered with at most, a very thinly smeared layer of oxide). The high levels of oxide debris generated at 0.905 m s⁻¹ instead acted as a loose abrasive assisting wear of especially the Nimonic 80A. This behaviour was attributed to a change in oxide chemistry (due to the dominance of nickel and chromium oxides generated from the Nimonic 80A) resulting in poor oxide sintering characteristics, in combination with increased mobility and reduced residency of the oxide debris at 0.905 m s⁻¹.     

Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered M_n+1AC_n ternary carbides – MAX phases – Ta₂AlC, Ti₂AlC, Cr₂AlC and Ti₃SiC₂ were tested under dry sliding conditions against alumina at 550 °C and 3 N load (for a stress of ≈0.08 MPa) using a pin-on-disk tribometer. Ta₂AlC and Ti₂AlC exhibited low specific wear rates, SWRs, (≤1 × 10⁻⁶ mm³/N m), while the coefficients of friction, μ, were 0.9 and 0.6, respectively. At 0.4, μ of Ti₃SiC₂ was the lowest measured, but the SWR, at ≈2 × 10⁻⁴ mm³/N m, was high. With a μ of 0.44 and a SWR of 6 × 10⁻⁵ mm³/N m the Cr₂AlC sample was in between. No visible wear of Al₂O₃ counterparts was observed in all the tribocouples. Tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. The correlations between observed tribological properties and tribofilm characteristics are discussed.     

基于销-盘材料互换试验的钴基合金摩擦磨损特性研究

以销-盘互换试验为基础,通过对比销-盘材料分别为42CrMoV-Stellite6和Stellite6-42CrMoV时,摩擦因数与磨损体积随速度、磨程的变化规律,研究钴基合金Stellite6的摩擦磨损性能。试验结果表明,Stellite6的摩擦因数随滑动速度的增大而减小,随磨程的增加逐渐增大之后达到稳定状态;磨损体积随速度和磨程的增大而增大,跑合阶段与稳定磨损阶段的分界点随速度的增大而减小。销-盘材料为42CrMoV-Stellite6较之销-盘材料为Stellite6-42CrMoV,摩擦因数与磨损体积差别很大,在工程应用中,需要根据实际工况,设计与之具有相同接触特征的试验以获得材料的摩擦磨损性能。     

A discussion of oxidation, oxide thickness and oxide transfer in wear

Wear is defined as a set of processes of which adhesion, transfer, abrasion, fatigue and oxidation may be some of the constituent processes; these effect transformations of matter from one state to another. The final transformation in wear is attrition in which free debris particles are formed.

Oxidation in the wear of metals is discussed, in particular for circumstances in which an oxide layer thick enough for the oxidation rate to be inversely dependent on oxide thickness is formed i.e. the standard relationship (outside the wear situation) between the oxide thickness and elapsed time is parabolic.

It is shown that the basic factors determining the wear process are the mechanical characteristics of the oxide layer and that the oxidation process, while important, is a secondary influence.

Theoretical considerations show that the oxidation temperature may be lower than the contact temperature, even for oxidation nominally at the contact regions. Also, an oxidation rate in wear may decrease with increasing oxidation temperature, as is sometimes observed in practice. This may be due to an increase in the thickness to which the oxide layer can grow in wear, when the oxidation temperature increases.     

The wear performance of yttrium-modified Stellite 712 at elevated temperatures

Cobalt-based alloys are often used for bearing applications, especially at elevated temperatures. One of the newly developed 700 series cobalt-based alloys, Stellite 712, has been demonstrated to possess high resistance to wear and corrosion in aggressive environments. Continuous efforts have been made to further improve this alloy for enhanced resistance to high-temperature wear involving oxidation. Recent studies showed that the improvement of the oxide scale on Co-base alloys by alloying with yttrium was an effective way to diminish wear of the alloys at elevated temperatures.In this work, sliding wear performances of yttrium-free and yttrium-containing Stellite 712 samples at elevated temperatures were evaluated. The mechanism responsible for changes in its wear performance was investigated by studying the effects of alloying yttrium on microstructure and mechanical properties of the bulk alloy and its oxide scale, employing various experimental methods including micro- and nano-mechanical probing, XRD, SEM-EDS, AFM and high-temperature pin-on-disc wear testing. The research demonstrated that alloying a small amount of yttrium (e.g. less than 1%Y) rendered the oxide scale on Stellite 712 stronger with higher adherence to the substrate, which was largely beneficial to the wear performance of the alloy at elevated temperatures. Mechanisms involved are discussed in this article.     

Effects of dissolved oxygen in saline on corrosive wear of steel

Wear behavior in a corrosive electrolyte where the corrosion reaction is controlled by the diffusion of oxygen cannot be explained only by the removal of corrosion products. The thickness of the oxide layer which varies with test conditions greatly affects the wear rate as well as the corrosion rate. In saline containing O₂ of 1 p.p.m. surface activation was confirmed. Adhesive wear or fatigue wear was found to have a predominant influence on wear behavior. Lubrication by a small amount of oxidation product can be expected to be beneficial. In O₂-saturated saline damage caused by corrosion fatigue is important owing to the intensely corrosive environment. Under O₂-saturated conditions the wear rate is constant under a load P < 3 4 kg, whilst w increases with increasing P above 3 4 kg, which may be considered to be due to the change of contact conditions. Under conditions of O₂-1 p.p.m. wear rate decreases slightly with increase of load because of the more effective lubrication by corrosion products.     

Evaluation of wear and tribological properties of coatings rubbing against copper

A new crossed-cylinders tribo-tester is proposed. This tribo-tester can decrease the tendency of the chatter vibration. The tribological properties of coatings against copper is evaluated with this tribo-tester. The wear rate of TiN, TiC and TiCN rubbing against copper is higher than the substrate high speed tool steel: SKH51 (JIS). The catalytic action of copper for oxidation of Ti-based coatings is a main reason of this high wear rate of TiN, TiC and TiCN rubbing against copper. The wear rate of CrN rubbing against copper is in a very low level because CrN shows the excellent oxidation resistance and Cr₂O₃ film formation decreases the wear loss of CrN coating.     

Effects of a second phase on the tribological properties of Al2O3 and ZrO2 ceramics

The tribological properties of four different materials are investigated, tetragonal zirconia (Y-ZTP), Al₂O₃ dispersed in Y-TZP (ADZ), ZrO₂ dispersed in Al₂O₃ (ZTA) and Al₂O₃ (with 300 ppm MgO). These materials are used as a cylinder sliding against a plate of Y-TZP (TZ-3Y)). Compared to Y-TZP, the wear resistance of ADZ composites is increased by a factor of 4–10. At a contact pressure of 230 MPa, a wear transition for Y-TZP is observed from plastic deformation to microchipping and microfracture due to the high interfacial temperature (450°C–550°C) generated by frictional heating. Because of the higher elastic modulus, hardness and fracture toughness at high temperature, ADZ composites show better wear resistance and a higher transition contact pressure (over 400 MPa) under the present conditions. For Al₂O₃, the transition from mild to severe wear occurs when the contact pressure is changed from 250 to 400 MPa. For ZTA ceramics, the wear behaviour does not change because of the presence of a compressive layer due to the zirconia phase transformation during sliding.

In water the wear resistance for ADZ and ZY5 is almost two orders of magnitude higher than the results under dry conditions. Reduction of the interfacial temperature by using water and the formation of a hydroxide layer at the contact surface by the tribochemical reaction of water with the ceramic, as observed by XPS, gives a positive effect on wear resistance.     

Solidification and abrasion wear of white cast irons alloyed with 20% carbide forming elements

Three different white cast irons with compositions of Fe–3%C–10%Cr–5%Mo–5%W (alloy no. 1), Fe–3%C–10%V–5%Mo–5%W (alloy no. 2) and Fe–3.5%C–17%Cr–3%V (alloy no. 3) were prepared in order to study their solidification and abrasion wear behaviors. Melts were super-heated to 1873 K in a high frequency induction furnace, and poured at 1823 K into Y-block pepset molds. The solidification sequence of these alloys was investigated. The solidification structures of the specimens were found to consist of austenite dendrite (γ); (γ+M₇C₃) eutectic and (γ+M₆C) eutectic in the alloy no. 1; proeutectic MC; austenite dendrite (γ); (γ+MC) eutectic and (γ+M₂C) eutectic in the alloy no. 2, and proeutectic M₇C₃ and (γ+M₇C₃) eutectic in the alloy no. 3, respectively.

A scratching type abrasion test was carried out in the states of as-cast (AS), homogenized (AH), air-hardened (AHF) and tempered (AHFT) using the abrasive paper with 120 mesh SiC and 10 N application load. In all the specimens, the abrasion wear loss was found to decrease in the order of AH, AS, AHFT and AHF states. Abrasion wear loss was lowest in the specimen no. 2 and highest in the specimen no. 1 except for the as-cast and homogenized states in which the specimen no. 3 showed the highest abrasion wear loss. The lowest abrasion wear loss of the specimen no. 2 could be attributed to the fact that it contained proeutectic MC carbide, eutectic MC and M₂C carbides having extremely high hardness. The matrix of each specimen was fully pearlitic in the as-cast state but it was transformed by heat-treatments to martensite, tempered martensite and austenite. From these results, it becomes clear that MC carbide is a significant phase to improve the abrasion wear resistance of white cast iron.