Behaviour of ferritic–martensitic steel and aluminium base coatings under demanding elevated temperature conditions

Abstract

Ferritic–martensitic steels are found in many elevated temperature applications due to their excellent strength properties and thermal conductivity. However, their resistance to elevated temperature corrosion, wear and combination of these is typically not at a desired level. One solution is to improve the surface properties by the application of a coating. In this study, aluminium diffusion coatings were deposited on 9Cr–1Mo steel and characterised in terms of microstructure and elevated temperature corrosion and erosion–corrosion behaviours. The two behaviours are then compared to those of an uncoated steel. The results from the tests indicate that aluminising shows great potential under the studied demanding elevated temperature conditions. The benefits and challenges of the deposition and use of aluminised coatings are discussed in this paper.     

Nanoindentation and scratch behaviour of Ni–P electroless coatings

ABSTRACT

In the present paper, the mechanical properties and the scratch failure mechanisms of Ni–P electroless coatings are described. The material microstructure was studied in as-deposited and annealed conditions through SEM and EDS analyses. Nanoindentation measurements on the coatings showed a remarkable hardening due to the crystallization and precipitation behaviour produced by annealing. The scratch tests, conducted by increasing the load during scratch, revealed the coating failure mechanisms in a broad range of applied stresses up to delamination.     

Performance evaluation of multilayer thin film coatings under mixed rolling–sliding dry contact conditions

This paper focuses on the application of advanced finite element analysis to simulate the response of complex multilayer coatings when subjected to mixed rolling–sliding dry contact conditions, typical of those experienced by heavily loaded gear tooth flanks. In this study, a versatile model is developed to investigate the response of a variety of complex coatings. Through the investigation of three advanced surface coatings the developed model is shown to offer detailed subsurface stress and strain information, quantifying the benefits provided by the application of each coating.A number of rolling contact fatigue tests were also conducted to complement the computational simulations. An adapted twin disc testing machine was used to investigate the progressive wear rates of the three advanced surface coatings when subjected to mixed rolling–sliding contact at high load and in dry conditions. The result is a clear indication that advanced surface coating techniques can provide significant improvements in wear resistance and surface durability. From the rolling contact fatigue experiments conducted in this study, a multilayer tungsten carbide/carbon coating was shown to provide the most significant increase in surface durability with a 16-fold reduction in measured weight loss compared to the uncoated substrate over the 145,000 cycle test duration investigated.     

Comparison of self-mated hardmetal coatings under dry sliding conditions up to 600 °C

Hardmetal coatings prepared by high velocity oxy-fuel (HVOF) spraying represent an advanced solution for surface protection against wear. In the current systematic study the high-temperature oxidation and unidirectional sliding wear in dry and lubricated conditions were studied. Results for a series of experiments on self-mated pairs in dry conditions as part of that work are described in this paper. Coatings with nominal compositions WC-10%Co4%Cr, WC-(W,Cr)₂C-7%Ni, Cr₃C₂-25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co were prepared with an ethylene-fuelled DJH 2700 HVOF spray gun. Electrolytic hard chromium (EHC) coatings and bulk (Ti,Mo)(C,N)-15%NiMo (TM10) hardmetal specimens were studied for comparison. The wear behaviour was investigated at room temperature, 400 and 600 °C. For the coatings sliding speeds were varied in the range 0.1–1 m/s for a wear distance of 5000 m and a normal force of 10 N. In some cases the WC- and (Ti,Mo)(C,N)-based coatings showed total wear rates (sum of wear rates of the rotating and stationary samples) of less than 10^?6 mm³/Nm, i.e., comparable to values typically measured under mixed/boundary conditions. Coefficients of friction above 0.4 were found for all test conditions. The P × V values as an engineering parameter for coating application are discussed. The microstructures and the sliding wear behaviour of the (Ti,Mo)(C,N)-based coatings and the (Ti,Mo)(C,N)-15%NiMo hardmetal are compared.     

Tribological studies of polymer based ceramic–metal composites processed at ambient temperature

Use of composite material is increasing due to economical processing of complicated shapes in large quantities. Addition of fiber/particulates improves the composite strength. In the current study, the tribological characterization of polymer based particulate composites which are processed at room temperature are investigated. The friction and wear behavior of polystyrene reinforced with steel powder (polymer–metal), alumina powder (polymer–ceramic) and a mix of steel and alumina powders (polymer–metal–ceramic) have been investigated under dry sliding conditions using a pin-on-disc tribometer. Tests were conducted at different normal loads and sliding velocities at room temperature. Coefficient of friction and wear loss during the wear tests are determined. Presence of metal and ceramic particulates affects the tribological behavior of the composite. The rise in temperature of the pin during sliding was measured. The rise in contact temperature is influenced by the composition which in turn influences the wear behaviour. The polymer–ceramic composite exhibits the lowest wear rate among the materials investigated.     

Tribological characterisation of epoxy–graphene–liquid filler composite coatings on steel under base oil external lubrication

In our earlier study, epoxy-based composites with graphene (10?wt-%) and in situ liquid fillers (base oil SN150 or perfluoropolyether at 10?wt-%) were found to provide low friction and highly wear durable as thin coatings on the steel substrate in dry interfacial state. In this present work, we have tested this composite in the presence of an external lubricant (base oil SN150). The lowest coefficient of friction was recorded as 0.04 and the lowest specific wear rate was measured as 9.8?×?10^?7?mm^3?Nm^?1 for the composites without any failure of the coating up to 200,000 sliding cycles. It is shown that such polymeric coatings can be an excellent boundary film in both dry and lubricated conditions for various bearings.     

The Preparation and Tribological Investigation of Ni–P Amorphous Alloy Nanoparticles

Amorphous Ni–P alloy nanoparticles were synthesized by chemical reduction of nickel acetate in water reacted with sodium hypophosphite under stirring. The nanoparticles were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Results of XRD and TEM showed that nanoparticles have an average diameter 100 nm. And XPS analysis indicated that part of the surface of Ni–P amorphous alloy nanoparticles was oxidized. The tribological properties of the prepared Ni–P nanoparticles as an additive in lithium grease were evaluated with a four-ball friction and wear tester. The worn surfaces of the lubricated GCr15 steel were analyzed by means of XPS and scanning electron microscopy (SEM). The lubricating mechanisms were discussed on the basis of XPS and SEM analyses of the worn steel surfaces. The results show that these nanoparticles as a grease additive can effectively enhance the friction-reduction and antiwear ability of lithium grease. Tribochemical reactions were involved for steel–steel frictional pair lubricated with the lithium grease containing amorphous Ni–P alloy nanoparticles, with the formation of a boundary lubricating and protecting film composed of additives of lithium grease and tribochemical reaction products (iron phosphate, iron oxides, nickel oxide, nickel, etc.) of the lubricants. This contributes to improve the tribological properties of the lithium grease.     

Tribological behavior of Cu20%Nb and Cu15%Cr in situ composites under dry sliding conditions

The tribological behavior of Cu20%Nb and Cu15%Cr in situ composites was studied on a pin-on-disk tester. Composite pins were slid against a hardened AISI 52100 steel disk under dry ambient conditions. Comparison of coefficient of friction, wear rate and bulk temperature was made between the two composites in terms of effects of normal pressure and sliding speed. Microstructural changes in the composites due to sliding were studied and correlated with the change of tribological behavior. The wear rate increased with increasing normal pressure and decreased with increasing sliding speed in the studied range of normal pressure (0.06–0.56 MPa) and sliding speed (2.50–5.83 m s⁻¹) for both composites. Cu20%Nb showed a much better wear resistance and a lower coefficient of friction than Cu15%Cr. The coefficient of friction slightly increased for Cu20%Nb and slightly decreased for Cu15%Cr with increasing normal pressure. The coefficient of friction for both composites decreased with increasing sliding speed. The bulk temperature was higher and subsurface deformation layer thickness was larger for Cu15%Cr than for Cu20%Nb. Both of them increased with increasing normal pressure and sliding speed. Scanning electron microscopy analysis showed plastic deformation flow on the wear surface at an early stage for both composites. However, the formation of a surface film led Cu20%Nb composite to reach a steady state much sooner and therefore to have a lower wear rate than Cu15%Cr.     

Tribological behavior of Cu–6Sn–6Zn–3Pb under sea water,distilled water and dry-sliding conditions

Tribological behaviors of Cu–6Sn–6Zn–3Pb alloy sliding against AISI321 stainless steel under sea water, distilled water and dry sliding conditions are studied on a pin-on-disc tester. Generally, the friction coefficient in distilled water is the largest and the smallest in dry-sliding. However, the wear rate is in the opposite case. The wear mechanism is microplough and plastic deformation in distilled water and under dry-sliding, but much severe in the latter case. In sea water environment, the wear mechanism is microplough, plastic deformation and corrosion. Sea water and distilled water show a cooling effect in comparison with dry sliding, additionally, sea water also has lubricating and corrosive effects.     

Tribological behavior of electrodeposited Zn,Zn–Ni,Cd and Cd–Ti coatings on low carbon steel substrates

The tribological behavior of electrodeposited Zn–Ni alloy coatings was investigated for its suitability to replace Zn- and Cd-based coatings. An in situ tribometry technique with a transparent sapphire hemisphere as a counter face on a pin on flat tribometer was utilized to examine the contribution of third bodies in friction and wear behavior. Wear mechanisms and tribo/transfer film morphology were also studied with the X-ray diffraction and electron microscopy. In situ tribometry and additional ex situ analyses revealed that Zn–Ni coatings had superior resistance to adhesive wear compared to cadmium coatings. Microhardness of Zn–Ni coatings was higher than Zn and Cd coatings. Hardness on the wear track of Zn–Ni coatings showed the formation of a strain hardened tribo layer.     

Tribological performance of PFPE and X-1P lubricants at head–disk interface. Part II. Mechanisms

This paper, with the concepts of hydrogen bonding interaction and tribo-emission, develops a new approach of the mechanism of perfluoropolyether (PFPE) lubricant degradation at the head–disk interface. The role of lubricant X-1P in tribological performance is also described. The mechanism is as follows: (1) at the interface, there exist hydrogen atoms with partial positive charge and oxygen atoms with partial negative charge; (2) hydrogen bonding interactions at the sliding interface result in high friction which depletes the lubricant film at some sites; (3) low energy electrons are emitted from the sites with solid–solid asperity contact, inducing C–O bond scission through the interaction of low-energy electrons with PFPE lubricant molecules. Carbon overcoat on Al₂O₃–TiC surface passivates the interaction between water and PFPE lubricant molecules. Hydrogen bonding interactions are minimized during the presence of lubricant X-1P. The new approach well explains experimental results in part I of the paper. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tribological performance of PFPE and X-1P lubricants at head–disk interface. Part I. Experimental results

X-1P, a cyclic phosphazene lubricant, is studied and compared with polar perfluoropolyether (PFPE) lubricant Z-Dol. Contact angles of lubricants are measured on different solid surfaces. Contact start-stop (CSS), drag, and ball-on-flat tests are performed and the results are discussed. Drag tests under high vacuum are also performed and discussed. Experimental results show that lubricant X-1P exhibits lower static friction and higher durability than lubricant Z-Dol, especially at high humidity. Higher durability is also observed for X-1P under the high vacuum condition compared with lubricant Z-Dol. Diamond-like carbon (DLC) overcoat on the Al₂O₃–TiC slider surface lowers friction and prolongs durability, especially for lubricant Z-Dol at high humidity, whereas for X-1P, there is no benefit of DLC. X-1P as an additive shows some improvement in durability at high humidity as compared to lubricant Z-Dol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Erosion-oxidation of uncoated,aluminized and chromized-aluminized 9Cr–1Mo steels under fluidized-bed conditions at 550–700 °C

Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of many components in the energy production sector, such as heat exchangers. In this paper, some results are presented for uncoated, aluminized and chromized-aluminized 9Cr–1Mo steel, subjected to air and to impacts by 200 μm silica particles at angles of 30° and 90° and speeds of 7.0–9.2 m s^?1 at 550 –700 °C, in a laboratory fluidized-bed rig, to determine whether or not aluminized and chromized-aluminized diffusion coatings could protect the steel under such conditions. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy.Under most conditions, the coatings provided some protection to the substrate: under 30° impacts, up to 650 °C, and under 90° impacts, at 700 °C, both coatings were effective, whereas under 90° impacts, up to 650 °C, only the chromized-aluminized coating gave significant protection. However, for 30° at 700 °C, the oxide scale on the substrate was protective and the coatings were not needed. Explanations for these observations are presented in this paper, in terms of interactions between the erosion and oxidation processes for the materials.     

Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum–silicon alloys under cylinder/bore wear conditions

summary  Understanding the lubrication of aluminum–silicon (Al–Si) alloys (>18 Si) under conditions similar to those in the cylinder/bore system is vital to determining their applicability to current engine designs. A novel investigation of the location of zinc-dialkyl-dithiophosphate (ZDDPs) antiwear (AW) film formation on an Al–Si alloy has been performed using X-ray absorption near edge structure (XANES) analysis, X-ray photoelectron emission spectroscopy (X-PEEM), and imaging nanoindentation techniques. A study of the initial stages of wear (10 min) to prolonged rubbing (60 min) was performed. The findings show that the film forms primarily on the raised silicon grains and is consistent with a zinc polyphosphate glass. The film has an elastic modulus of ~70 GPa and a similar elastic response to a ZDDP AW film formed on steel under the same conditions. This provides the first direct observation and characterization of a ZDDP antiwear film on Al–Si alloys using spatially resolved chemical and mechanical techniques at the nanoscale.     

Experimental investigations of mechanical properties and slurry erosion behaviour of high velocity oxy fuel and plasma sprayed Cr2O3–50%Al2O3 coatings on CA6NM turbine steel under hydro accelerated conditions

Slurry erosion behaviour of HVOF (High Velocity Oxy Fuel) and plasma sprayed coatings on CA6NM hydraulic turbine steel has been investigated at different levels of various parameters. The Cr₂O₃–50%Al₂O₃ composite powder was prepared and deposited on CA6NM steel samples to get the uniform thickness coatings. The surface roughness, porosity and microhardness of as-coated samples were measured. The as-coated samples were subjected to SEM/EDS analysis to evaluate the surface microstructure of the developed coatings. Erosion tests were performed on self made erosion test rig under hydro accelerated conditions. The study reveals that the velocity, impact angle and slurry concentration were the most significant parameters, influencing the erosion rate of these coatings. The average particle size had least affect on the erosion rate. HVOF-coated samples showed better corrosion resistance as compared to plasma-coated samples due to high hardness of HVOF-coated CA6NM samples.