Correlations between microstructural parameters, micromechanical properties and wear resistance of plasma sprayed ceramic coatings

The micromechanical integrity of a ceramic plasma sprayed (PS) coating is determined by the size and distribution of the defects found in the coating, such as porosity, the inter-lamellar microcrack density, the intra-lamellar microcrack density as well as the lamellar, or splat, dimensions. In this work, several micromechanical tests were used to advance our understanding of the relationships between the different microstructural parameters found in PS ceramic coatings. The tests included depth sensing indentation, micro and macrohardness testing, and controlled scratch testing. Abrasive and erosive wear tests were performed on the same set of coatings, including plasma sprayed alumina and chromia, as well as sintered alumina as a reference material. The best correlations were found between the material hardness (H), the level of porosity (P) and the abrasive wear volume (W). Knoop hardness measurements provided the best correlation with wear data, followed by scratch hardness and Vickers hardness. An exponential function of the type W=k/Hⁿ was found, where k and n are constants. A similar function describes the correlation of wear volume with the elastic modulus of the coating. Fracture toughness could only be correlated with wear volume when combined with hardness in a function of the type W=k/H^0.5K_c^0.5. The incorporation into this function of a “microstructural factor” M=Pⁿ improves the correlation.     

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT

The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr₃C₂-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.     

Synthesis of boride coatings on steel using plasma transferred arc (PTA) process and its wear performance

Boriding of the surface of a tool steel using boron powder and the plasma transferred arc process was investigated. It was shown that this method is an easy and effective technique in producing uniform alloyed layers with a thickness of about 1.5 mm and a hardness between 1000 and 1300 HV.The microstructure of the borided surfaces consists of primary Fe₂B-type borides and a eutectic mixture of borides and martensite. Some cracks are observed in the eutectic regions but they do not seem to critically affect the behaviour of the coatings in sliding wear.The wear rate of pin on disc tests is primarily affected by the applied load and it lies between 10⁻⁵ mm³/m for low loads and 10⁻² mm³/m for high loads. Two distinct regimes of mild and severe wear are obtained separated by a critical load. Mild wear is due to the load supporting effect of borides and severe wear is due to their breakage above a critical load. The wear rate is not significantly affected by the sliding velocity and is consistent with the friction coefficient.The friction coefficient varies from 0.13 to 0.23 and depends strongly on the oxidation status of the wear track. The sliding velocity affects the sliding distance where the coefficient of friction reaches equilibrium.     

Tribological behaviour of plasma sprayed Al2O3 and TiO2 ceramic hard coatings under dry contact

Plasma sprayed ceramic coatings are used in a number of industries in which surface modification of components to compare tribological properties is important: so hence, are evaluations of their tribological properties. This paper presents a study on the wear behaviour of three ceramic coatings — Al₂O₃, TiO₂ and Al₂O₃-TiO₂combination — in the load and speed ranges of 5 to 50 N, and 0.3 to 10 m/s, respectively, on which few data are available in the literature. The tests were carried out using a standard dry sand rubber wheel abrasion test and a pin-on-disc machine under dry sliding conditions. It was found that a stick-slip effect seems to occur at low sliding speeds, and transition takes place at a sliding speed of around 4 m/s. Of the three ceramic coatings, TiO₂ was found to be the most wear resistant, with the least friction coefficient, although it is less hard than the Al₂O₃ coatings. Scanning electron microscopy of the surface shows evidence of wear mechanisms such as plastic deformation, transfer-film formation, micro cracks, and grain pull-out in the coatings.     

三偏心蝶阀蝶板等离子喷焊过程PLC控制系统研究

介绍了三偏心蝶阀自动跟踪等离子喷焊设备。控制系统采用CPM2A-60CDT—D型PLC（可编程控制器）为控制核心，实现喷焊工艺过程控制及喷焊面实时跟踪控制。给出了PLC控制系统的硬件实现及软件设计。     

Sliding wear resistance of reactive plasma sprayed Ti–TiN coatings

The reactive plasma spraying (RPS) of titanium powders in a nitrogen containing plasma gas produces thick coatings characterised by microdispersed titanium nitride phases in a titanium matrix. In this paper, the wear resistance properties of Ti–TiN coatings deposited on carbon steel substrates by means of RPS technique are studied. Wear tests were performed in block-on-ring configuration and dry sliding conditions, at different applied loads (45 and 100 N) and sliding velocities (in the range 0.4–2.0 m s⁻¹) by using hardened and stress relieved AISI O2 disks as counterpart. At low applied load the wear volumes are low, and tend to slightly increase as the sliding velocity increases. At high applied load and low sliding velocities the highest wear volumes for the coated samples are observed, due to adhesion in the contact area with the tool steel counterpart and decohesion of coating particles. As the sliding velocity is increased, the wear volume of the coated samples tends to decrease owing to oxidation phenomena.     

Wear and oxidation behaviour of shrouded plasma sprayed fly ash coatings

In a wide variety of applications, mechanical components are subjected to severe conditions, such as high load, speed or temperature and hostile chemical environment. Thus, their surface modification is necessary in order to protect them against various types of degradation. Thermal spraying has emerged as an important tool of increasingly sophisticated surface engineering technology. Plasma spraying is a rather simple process from a practical point of view. In the present investigation, the fly ash coating has been obtained by shrouded plasma spray process on carbon steel. The coating was characterized with relative to important behavioural parameters. Wear, oxidation and salt corrosion behaviour have also been evaluated. The coating has found to posses much higher hardness values than base steel and its porosity is slightly higher than the range of porosity for plasma spray coatings. The coating was effective to increase the oxidation and salt corrosion resistance of the given carbon steel. However, the wear resistance of fly ash coated steel was observed to be lesser than the bare steel probably due to coarse grain size.     

Studies on erosion behaviour of plasma sprayed coatings on a Ni-based superalloy

In the present investigation NiCrAlY, Ni-20Cr and Ni₃Al metallic coatings were deposited on a Ni-based superalloy (18.5Fe-19Cr-0.15Cu-0.5Al-3.05Mo-0.18Mn-0.9Ti-0.18S-0.04C-5.13 (Ta + Cb)-balance Ni). NiCrAlY was used as bond coat in all the cases. Erosion studies were conducted on uncoated as well as plasma spray coated superalloy specimens at room temperature. The erosion experiments were carried out using an air-jet erosion test rig at a velocity of 40 m/s and impingement angles of 30 and 90°. Silica sand particles of size ranging between 150 and 212 μm were used as erodent. The coatings have been characterised by scanning electron microscope (SEM), optical microscope, microhardness tester and X-ray diffractometer (XRD). Scanning electron microscope (SEM), equipped with an energy dispersive X-ray analyser (EDAX) was used to analyse the eroded surfaces. Possible erosion mechanisms are discussed. The phases revealed by XRD of the coatings have shown the formation of solid solutions. Out of the three plasma sprayed coatings, the Ni₃Al coating gave the lowest erosion rate regardless of the impact angle, and the Ni-20Cr coating gave the highest erosion rate.     

Abrasive wear behavior of sprayed hydroxyapitite coatings by gas tunnel type plasma spraying

Hydroxyapatite (HA) coatings were sprayed using gas tunnel type plasma spraying at different arc currents. Abrasive wear test was carried out for the coatings sprayed at different arc currents under unlubricated conditions in air atmosphere. The abrasive wear rate was measured at different coatings thickness to study the effect of coating thickness on the anti-abrasion resistance of HA coatings. The results showed that the abrasive wear resistance of HA coatings increases as the operating arc current of the plasma torch increases. On the other hand, the abrasive wear rate reaches a minimum value near the substrate with coating thickness less than 50 μm. The results showed that the coating hardness increases in the region near the substrate and increases as the arc current increases. The experimental results indicated that there is a relation between the abrasion resistance and hardness properties of HA coatings.     

The erosion and abrasion characteristics of alumina coatings plasma sprayed under different spraying conditions

A series of plasma sprayed alumina coatings was evaluated regarding their erosion and abrasion characteristics. The coatings were deposited under different spraying conditions, using a commercial axial injection plasma spray system, and with powders of different grit sizes and crystallinity. A sintered bulk alumina and a conventionally sprayed coating, produced by a radial injection air plasma spray technique, were tested as reference materials. To evaluate the importance of energy input the coatings were produced using two different torch nozzle sizes and gas mixtures with a varied amount of hydrogen. The erosion and abrasion results indicate that hydrogen concentration, nozzle size and precursor powder type and size influence the tribological characteristics of the coatings. The wear resistance of the coatings seems to benefit from an increase in hydrogen concentration or torch nozzle size. The effect of precursor powder size on the wear resistance was more complex but indicates that sapphire powders of medium precursor sizes are advantageous to ordinary plasma spraying powders of alumina.     

Sliding wear behavior of ceramic,plasma sprayed on casting aluminum alloy against SiC ball

The wear behavior of Al₂O₃–40% TiO₂ and Cr₂O₃ deposited on a casting aluminum alloy (ASTM A356) by plasma spray against an SiC ball was investigated. It was found that the voids and porosities of the coating surface generated cracks. As the tensile stresses in the coating increased with an increased friction coefficient, the columnar grain of the coating fractured at the critical stress point. It was also found that the cohesiveness of the splats and porosity of the surface both played a role in the wear characteristics. It is suggested that the thermal expansion mismatch of the substrate and coating plays an important role in the wear performance. Tensile, compressive, and thermo-mechanical stress may also occur due to this same thermal expansion mismatch of the substrate and coating. Crack propagation above the interface was observed with a SEM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Measurement and optimization of atmospheric plasma sprayed CoMoCrSi coatings parameters on Ti-6Al-4V substrates affecting microstructural and properties using hybrid abductor induction mechanism

A hybrid artificial intelligence method (AIM) is used to optimize the atmospheric plasma spraying (APS) processing parameters for CoMoCrSi coatings deposited on Ti-6Al-4V substrates. In the proposed approach, the Taguchi design method is used to obtain an initial solution for the optimal set of APS parameters. The true optimal values of the spray distance, chamber pressure, current, argon gas flow rate and hydrogen gas flow rate are then obtained using an artificial neural network (ANN) and genetic algorithm (GA). The coatings deposited using the processing parameters determined using the Taguchi design method alone have a porosity of 8.5%. However, following the ANN/GA optimization procedure, the porosity reduces to just 5.6%. The XRD analysis results show that the as-sprayed coatings contain Cr₃Si phase as a result of the high-temperature plasma flame. In addition, it is shown that the structure of the Ti phase in the coating transforms from a α-Ti crystal structure (i.e., HCP) to a β-Ti crystal structure (i.e., BCC) during thermal treatment at temperatures to 1200 °C. Finally, it is shown that the hardness of the annealed coatings increases with both an increasing annealing time and an increasing annealing temperature. The higher hardness under higher temperature conditions is attributed to the precipitation of Cr₃Si phase. Overall, the coatings obtained in the present study using the optimal spray-coating parameters have a low porosity, a high hardness and a good thermal stability at high temperatures.     

Erosion-corrosion of plasma as sprayed and laser remelted Stellite-6 coatings in a coal fired boiler

Unacceptable levels of surface degradation of metal containment walls and heat exchanger tubing by a combined erosion-corrosion (E-C) mechanism have been experienced in some boilers. The recent use of coatings to protect the heat exchanger tubes of fluidized bed combustor from E-C problems has been suggested by many authors. The laser remelting of the surface coating is suggested as a promising technique to improve its physical properties.Aim of the present investigation is to evaluate the erosion-corrosion (E-C) behaviour of plasma as sprayed and laser remelted Stellite-6 (St-6) coatings on boiler tube steels in the actual coal fired boiler environment. The cyclic experimental studies were performed in the platen superheater zone of a coal fired boiler where the temperature was around 755 °C and the study was carried out upto 10 cycles each of 100 h duration followed by 1 h cooling at ambient temperature. Coated steels were found to possess higher resistance to E-C than the uncoated steels. The highest degradation resistance has been indicated by the T11 steel coated and subsequently laser remelted.     

Erosion-corrosion of thermal sprayed coatings in FBC boilers

Varieties of bed ash and fly ash were retrieved from operating fluidized bed combustor (FBC) boilers firing different fuels in North America and Europe. Using these ashes, the relative erosion-corrosion resistances of HVOF Cr₃C₂NiCr coating and several other thermal sprayed coatings were determined in an elevated temperature blast nozzle erosion tester. Test conditions attempted to simulate erosive conditions found at the refractory—waterwall interface and in the convection pass region in tubular heat exchangers of FBC boilers. Erosion-corrosion (E-C) wastage mechanisms of the structural metals (AISI 1018, ASTM SA213-T22) were discussed and compared with the E-C wastage of HVOF Cr₃C₂NiCr cermet coatings. The relatively different erosivities of ashes retrieved from North America and from Europe were also discussed.     

Tool strengthening by surface modification in low-temperature plasma

A method is proposed for strengthening cutting tools by modification in low-temperature discharge plasma.     

Tribological behaviour of thermally sprayed silicon carbide coatings

SiC coatings have been successfully deposited using thermal spray detonation technique with a newly patented feedstock. Their tribological performance was compared to bulk SiC for dry and lubricated conditions (polyalphaolefin and 3.5 wt% NaCl solution). The lowest coefficient of friction (CoF=0.10) and wear-rate were detected with polyalphaolefin lubricant regardless of the test pair due to mixed fluid-film lubrication. Contradicting results were recorded under other test conditions. The coatings show low CoF of 0.20 in comparison to four times higher CoF of bulk SiC under dry sliding. Oppositely, SiC coatings in NaCl solution record five times higher CoF compared to bulk SiC CoF of 0.20. Such behaviour is associated with tribochemical reaction and tribo-corrosion mechanisms occurring in dry and NaCl sliding, respectively.     

等离子体技术在金属材料表面改性中的应用

介绍了等离子体的有关概念及其在材料表面改性的基本原理,阐述了低温等离子体技术在金属材料表面改性中的应用研究,探讨了该技术在金属材料表面改性中的应用前景及其存在的问题。     

纳米陶瓷涂层磨削表面残余应力的研究进展

纳米陶瓷涂层材料具有高硬度和高耐磨性,采用超硬磨料的金刚石砂轮磨削是其最主要的加工方法,在磨削时容易出现表面残余应力而导致表面裂纹.目前,国外在纳米结构陶瓷涂层磨削表面残余应力的研究很少,我国正在对纳米陶瓷涂层材料超精密磨削后表面残余应力方面进行研究.介绍了纳米结构陶瓷涂层的研究背景,阐述了纳米结构陶瓷喷涂材料性能特点,分析了纳米结构陶瓷涂层的磨削研究动态和磨削表面残余应力的研究现状.分析研究表明,纳米结构陶瓷涂层的开发与研究将会受到越来越广泛的重视,其后续研究将是下一步的工作重点.     

Residual stresses in aluminium phosphate sealed plasma sprayed oxide coatings and their effect on abrasive wear

Effect of residual stresses on plasma sprayed alumina and chromia coatings sealed with aluminium phosphate were studied as a function of the temperature of the sealing treatment. Stresses were measured by X-ray stress analysis and high-speed circular microhole drilling method. Residual stress states were correlated with other coating properties such as microhardness, porosity, microstructure and dry abrasion wear resistance. Correlations were found between sealing treatment temperature, residual stress state and wear resistance. Wear resistance of the oxide coatings was increased at all sealing temperatures. Sealing treatment affected coatings by two mechanisms. Aluminium phosphate sealing induced compressive stresses to coatings and simultaneously bonded coating lamellar structure.