S.J. Harris

Microstructure formation and corrosion behaviour in HVOF-sprayed Inconel 625 coatings

The nickel-based alloy Inconel 625 was thermally sprayed by two different variants of the high velocity oxy-fuel process. In this study, coatings deposited by a liquid-fuelled gun were compared with those produced by a gas-fuelled system; in general, the former generates higher particle velocities but lower particle temperatures. Investigations into the microstructural evolution of the coatings, using scanning electron microscopy and X-ray diffraction, are presented along with results on their aqueous corrosion behaviour, obtained from salt spray and potentiodynamic tests. It is inferred from coating microstructures that, during spraying, powder particles generally comprised three separate zones as follows: fully melted regions; partially melted zones; and an unmelted core. However, the relative proportions formed in an individual powder particle depended on its size, trajectory through the gun, the gas dynamics (velocity/temperature) of the thermal spray gun and the type of gun employed. Cr2O3 was the principal oxide phase formed during spraying and the quantity appeared to be directly related to the degree to which particles were melted. The salt spray test provides a sensitive means of determining the presence of interconnected porosity in coatings and those produced with the liquid-fuelled gun exhibited reduced interconnected porosity and increased corrosion resistance compared with deposits obtained from the gas-fuelled system. In addition, potentiodynamic tests revealed that passive current densities are 10–20 times lower in liquid-fuel coatings than in those sprayed with the gas-fuelled gun.

The elastic modulus of aluminium-lithium alloys

Youngs modulus measurements have been made on Al-Li alloys containing up to 32 at % lithium, in an attempt to determine the cause of the high modulus that characterizes this potentially important alloy system. In alloys of commercial interest (7–11 at %, 2–3 wt % lithium) the modulus is in the range 79 to 83 GPa, the actual value depending on heat-treatment conditions. The major contribution to this increased modulus arises from lithium in solid solution. The Youngs moduli of the Al3 Li and AlLi intermetallic phases are estimated to be 96 GPa and 105 GPa respectively. Additions of magnesium to the Al-Li system produce a small decrease of the modulus, e.g. 4.5 at % (4 wt %) magnesium reduces the modulus by approximately 2 GPa.

Microstructural characterisation of a Ni-Cr-B-C based alloy coating produced by high velocity oxy-fuel thermal spraying

Abstract High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings, approximately 200 μm thick, of a commercially available Ni-Cr based alloy containing boron and carbon [Ni-23.5Cr-3.8Cu-0.8Fe-5.9Mo-3.4W-2.8B-4.3C (at.%)]. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy. The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M 23 C 6 particles ∼50 nm in size. The Ni-rich matrix had a predominantly crystalline structure with only a small fraction of amorphous phase regions. Cr 2 O 3 and NiCr 2 O 4 oxide phases occurred in the form of either intersplat lamellae or globules, with Cr 2 O 3 being the predominant oxide. Coating microhardness values were found to be ∼6.0 GPa which is significantly higher than that of B- and C-free nickel-based alloys similarly deposited.

Lubricated fretting wear of a high-strength eutectoid steel rope wire

Abstract The fretting wear behaviour of heavily work-hardened eutectoid steel wire is an important issue in the construction and usage of locked coil steel ropes. These ropes, which are used for high duty applications in aggressive environments, such as those encountered in the mining industries, can exhibit a variable service life. Earlier papers examined the design features of this class of rope and aspects of the operating regime, which may contribute to this variable life. The as-drawn surface of the wire was determined to play a significant role in the fretting behaviour, initially suppressing wear and friction. In this paper, the influence of low viscosity oils, with and without graphite additions, on the fretting behaviour of the as-drawn wire are examined. Oil bath lubrication suppresses effectively wear and friction throughout the tests. Of greater significance, however, a smear of oil, more typical of service conditions, is also effective, at least during the early stages of testing. The addition of graphite to this smear of oil aids its retention, and its ability to lubricate and form a shield around the fretting interface. The presence of oil inhibits the ingress of oxygen and consequently, at higher normal forces, when significant breakdown of the oil film occurs, micro-welding and tearing of the clean metal surfaces follows. Even so, wear and friction are still suppressed effectively.

Stress corrosion cracking of GRP pultruded rods in acid environments

Stress corrosion cracking of GRP pultruded rods has been investigated in 0.0001 to 5.0 N hydrochloric acid environments under bending and tensile loading modes. Crack initiation takes place at exposed glass fibres in the surface of the rod, and crack propagation is planar and at right angles to the rod axis. Leaching of calcium and aluminium from the fibres takes place during the cracking process, and time-to-failure is dependent on the acid concentration, the stress level and the ease of access of the acid to the glass fibre surface. Possible mechanisms of crack propagation through the glass fibres and resin are discussed.

Microstructure formation in high velocity oxy-fuel thermally sprayed Ni-Cr-Mo-B alloys

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni‐Cr‐Mo‐B alloy coatings, approximately 200 mm thick, onto mild steel substrates. Gas atomised powders of three different compositions were used in the experiments. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy and differential thermal analysis (DTA). The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M3B2 particles 10‐30 nm in size and with a tetragonal crystal structure. The Ni-rich matrix comprised both crystalline and amorphous regions. The former was predominantly nanocrystalline with a grain size of ca. 50 nm and DTA showed that the crystallization temperature of the amorphous phase varied from 800 to 860 K depending on the alloy constitution. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules. The thin lamellar oxides exhibited the a-Cr2O3 crystal structure whereas globular oxides, up to 1 mm in size, were found with both a-Cr2O3 and NiCr2O4 structures.

Nanostructured Co–W coatings produced by electrodeposition to replace hard Cr on aerospace components

Abstract Co–W alloys with 12–26 at.-%W (25–45 wt-%W) can be produced by electrodeposition from complexed aqueous plating baths. The deposits can be in the amorphous or nanostructured conditions, depending on the plating bath operations. Most recent attention has been focused on the nanocrystalline conditions which have hardness values in excess of 850 kg mm−2, which makes them competitive with hard engineering chromium. This may allow the coatings to replace hard Cr which is produced from hexavalent chromium baths having associated health and safety problems. Co–W coatings, 2–120 μm thick, can be deposited with a low incidence of cracking, with good corrosion and wear properties (dry and lubricated) when deposited on a variety of steel substrates. The underlying reasons, i.e. the amount of tungsten in solution, the density of grain boundaries and the reduction in bonding between coating and counterface under dry sliding conditions, which account for these enhanced properties are discussed. Potential applications are in many sectors of engineering including aerospace, hydraulics and automobile.

The corrosion behavior and microstructure of high-velocity oxy-fuel sprayed nickel-base amorphous/nanocrystalline coatings

The corrosion characteristics of two Ni-Cr-Mo-B alloy powders sprayed by the high-velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5 M H2SO4. The deposits were also microstructurally characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) (utilizing both secondary electron and backscattered electron modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline face centered cubic (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths.Apparent recrystallization of the amorphous matrix has also been observed in the form of cellular crystals with a fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr2O3, though regions of the spinel oxide NiCr2O4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5 M H2SO4, as revealed by potentiodynamic tests. They both had rest potentials approximately equal to −300 mV saturated calomel electrode (SCE) and passive region current densities of ∼1 mA/cm2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit occurred.

Metallographic investigation of the damage caused to GRP by the combined action of electrical, mechanical and chemical environments

The use of glass fibre-reinforced polymers in electrical insulator components has gradually been taking place. Problems may arise where such insulators are in service at very high voltage, e.g. 200 kV and above, are under significant mechanical loads, and the environment (rain, and various pollutants) is able to gain access to the surface of the GRP. With the aid of optical and scanning electron microscope techniques, a detailed examination has been carried out on the nature of damage which has taken place in GRP pultruded rods that have operated for various periods of time in the above service conditions. These pultruded rods can receive significant levels of damage under the action of electrical fields, and the attendant environment; this takes the form of erosion, melting, burning and displacement of both glass and polymer phases. When a mechanical stress, which may be less than 10% of the breaking stress of the rod, is applied in conjunction with the above conditions a different form of insulator breakdown can take place. Instead of material displacement on the scale mentioned above, brittle failure of the GRP takes place. Such a failure mode can be compared with the process of stress corrosion which takes place when GRP is tested in 0.1 N acid solutions. It is suggested that the combined action of electrical activity and the presence of minor amounts of pollutants are able to influence the surface of glass fibres and promote stress corrosion in an analogous fashion to that described for concentrated acid solutions.

Failure of high voltage electrical insulators with pultruded glass fibre-reinforced plastic cores

Abstract A group of ten 275 kV insulators exhibiting various states of electrical and mechanical damage were withdrawn from service, and subjected to comprehensive optical and scanning electron microscopy to determine the failure sequence. The insulators were of two types, suspension and tension. Both types suffered progressive electrical damage starting at the end fitting and progressing along the interface between the core and the shedding, eventually producing electrical punctures between sheds. Tension insulators, which carried higher mechanical loads, also suffered cracking between sheds which appears to admit moisture. The combined effect of electrical activity and moisture appears to be similar to acid stress corrosion in producing brittle fracture of the pultruded rod.